Design and application of proton gradient-based pH-responsive nanomaterials in the tumor microenvironment

Abstract

The pH of the tumor microenvironment (TME) is a fundamental physicochemical parameter, and pH-responsive nanoparticles for the TME generally rely on two mechanisms: (1) protonation-induced ionization of functional groups and (2) cleavage of acid-labile chemical bonds. Based on these principles, a wide range of pH-responsive drug-delivery nanoplatforms have been developed, including inorganic nanoparticles, lipid-based nanoparticles (LNPs), polymeric micelles, metal–organic frameworks (MOFs), and protein/peptide-based nanoparticles. This review summarizes the recent progress in the design of traditional pH-responsive nanomaterials, emphasizing the molecular strategies employed in mesoporous silica nanoparticles (MSNs), liposomes, LNPs, polymeric micelles, MOFs, and protein/peptide-based nanoparticles. However, current pH-responsive nanoparticles still suffer from limited tumor selectivity, poorly defined thresholds and overreliance on simplified subcutaneous tumor models. It's necessary for comprehensive studies on pH-responsive nanoparticles, particularly using orthotopic tumor models to mimick the tumor microenvironment. Traditional pH responsiveness is largely passive, responding to acidic environments. Future developments may exploit the proton gradients not only as triggers but also as energy sources to actively drive nanoparticle targeting, providing a new paradigm for pH-based tumor nanomedicine.

---

Fangzhou Li

Dr Fangzhou Li is a Professor and Principal Investigator at the Institute of Biomedical Engineering, Chinese Academy of Medical Sciences & Peking Union Medical College. He received his Ph.D. from Peking University Health Science Center and completed his postdoctoral training with Prof. Xing-Jie Liang at the National Center for Nanoscience and Technology. He was selected for the Young Elite Scientists Sponsorship Program by CAST. His research group focuses on the development of precision nanomedicines based on individual differences. He has published over 40 papers in prestigious journals, including Nature Communications and Advanced Materials.

---

1. Introduction

Global cancer incidence is projected to reach 34 million new cases by 2070, nearly doubling the number reported in 2018.¹ This trend reflects not only the remarkable progress made by mankind in controlling infectious diseases, but also the growing challenge posed by chronic non-communicable diseases to global public health. Among these, cancer has emerged as the leading threat to human health, characterized by high incidence, high mortality, and biological complexity.² Driven by lifestyle changes, environmental pollution and population aging, the global cancer burden continues to escalate.^3–5 Moreover, cancer development involves multifactorial pathogenesis, including genetic mutations, dysregulated signaling pathways, and tumor microenvironmental (TME) modulation. Therefore, elucidating the molecular basis of cancer and developing safe, effective, and precisely targeted therapies have become major goals in biomedical research.^6–8

Cancer is a group of diseases caused by genetic mutations, with various factors contributing to DNA alterations. These factors can be intrinsic, such as immune responses and hormonal influences, or extrinsic, including smoking, infections, environmental exposures, and unhealthy lifestyles.⁹ However, tumor development follows a complex and dynamic process involving both tumor cells and surrounding normal cells within the tumor tissue. Together, these cells create the environment that drives tumor initiation and progression, known as the tumor microenvironment (TME).¹⁰ In recent years, more and more studies have shown that the occurrence of tumors not only depends on the genetic changes of cancer cells themselves, but is also profoundly affected by the interactions between cells and molecules in the surrounding TME. Given the critical role of the TME in cancer progression, studying the TME and developing anti-cancer therapies targeting it have become important strategies in cancer treatment.¹¹ The TME differs significantly from normal tissues, featuring unique characteristics such as a special immune environment, acidic conditions, and hypoxia, which can promote tumor cell proliferation and inhibit normal cell function.¹² Therefore, targeted therapy targeting the unique physiological characteristics of these tumors, especially the design of drug delivery systems based on environmental response mechanisms, has become an effective way to improve treatment specificity and safety. By harnessing certain unique properties of the TME, it is possible to develop TME-responsive drug delivery systems that reduce the toxicity of anticancer drugs while achieving targeted delivery. For example, drug delivery systems responsive to hypoxia,¹³ acidity,¹⁴ or reactive oxygen species (ROS)¹⁵ can be designed to enable precise drug targeting and minimize side effects.

Among these differential features, the acidic condition of the TME is particularly prominent. The acidity of the TME is more prominent and common than other characteristics such as hypoxia and high ROS, and is closely related to the deterioration, loss of control and metastasis of tumors. Therefore, it is important to develop sensitive nanomaterials that respond to the acidic condition of the TME.¹⁶ The pH of normal human tissues ranges from 7.3 to 7.4, and in tumor tissues the pH can be as low as 5.6, but mostly it's between 6.3 and 7.0.¹⁷ The pH is lower at the subcellular level, with the pH in endosomes and lysosomes ranging from 4.5 to 5.5.¹⁸ These highly acidic environments are an important research direction to improve the targeting of anti-tumor drugs. In addition, due to tumor vasculature leakage and impaired lymphatic drainage, nanoscale particles exhibit enhanced permeability and retention (EPR) effects, enhancing the accumulation of nanoparticles in the TME.¹⁹ It is an important research direction in pharmacy to reduce the toxicity of anticancer drugs and improve the efficacy of anticancer drugs by using environmentally responsive materials to identify one (acidic) or multiple (acidic and redox) properties in the TME and prepare nanodrug delivery systems (NDDSs) to deliver anticancer drugs such as doxorubicin (DOX).

2. Mechanism of pH responsiveness

pH-Responsive NDDSs have been applied to cancer, bacterial, inflammatory, autoimmune, and neurological diseases.²⁰ Various NDDSs like polymer micelles, liposomes, silica nanoparticles, and nanohydrogels show promising targeted applications.²¹ Based on the pH of the TME, the response should be stable at pH 7.4 but trigger drug release at pH ≤ 6.5.²² The mechanisms include (1) protonation-induced charge changes and (2) rupture of pH-sensitive bonds.

2.1. Accepting proton and charge changes

Polymers with acceptable proton groups can be affected by pH values in the environment. After receiving protons, the charge of the group changes, thereby changing the physical and chemical properties of the compound.²³ The increased charge of the polymer often causes electrostatic repulsion leading to unstable depolymerization and drug release, while the decrease in charge reverts to a relatively stable aggregation state.²⁴ Such pH-responsive polymerization and depolymerization are relatively reversible, so repeated pH-dependent polymerization and depolymerization can be observed when they are prepared into NDDSs. It is ideal that the pH response range matches the pH of the tumor, which depends mainly on the pK_a value of the polymer. The number of proton acceptor groups on the polymer can effectively adjust the pK_a value to match the tumor pH.²⁵ Commonly used electrically acceptable proton groups include amino groups, imidazole, pyridine, sulfonic acid groups, carboxyl groups, etc. By accepting protons and changing the charge, the polymer changes from hydrophobic to hydrophilic, and can change from a polymerized state to a dispersed state to release drugs.²⁶ The polymers containing ethylenediamine have two pK_a values: 6.3 and 9.0, whereby the ethylenediamine moiety is mono-protonated at pH 7.4 towards the progress of the protonation as the pH decreases.²⁷ pH-Responsive polycarboxybetaine polymers containing two amino groups were also successfully developed for the delivery system of drug and genes in tumor tissues.²⁸ The coarse-grain dissipative particle dynamics method can also be used to study polymer micelles (PDEA) containing tertiary amine groups. Its micelle formation, drug loading and drug release can be simulated. The hydrophobic–hydrophilic transition of PDEA to PDEAH⁺ occurs in an acidic environment, leading to rapid paclitaxel (PTX) release.²⁹ Lysine–histidine copolymers also possess such zwitterionic properties and are used in the shell of pH-responsive nanoparticles.³⁰ Imidazolyl groups were introduced into starch to achieve pH triggering.³¹ The above zwitterionic polymers that can accept protons are shown in Fig. 1.

Fig. 1 The two mechanisms of pH-responsiveness. On the left are ionizable functional groups (zwitterions, secondary amines, tertiary amines, quaternary amines, imidazoles, and pyridines). On the right are pH-labile chemical bonds (hydrazone bonds, Schiff bases (imine bonds), acetals (ketals), ortho esters, cis-aconityl bonds, and β-thiopropionate bonds).

2.2. pH-Sensitive chemical bond rupture

pH-Sensitive chemical bonds are relatively stable in weakly alkaline in vivo environments and break in weakly acidic tumor tissues or intracellularly. This property can be used to develop pH-responsive DDSs. Currently, pH-sensitive chemical bonds used in the field of drug delivery mainly include hydrazone,³² Schiff base (imine bond), acetal, ortho ester,^34–36cis-aconityl and β-thiopropionate (Fig. 1).³⁷

2.2.1 Hydrazone and Schiff base. Hydrazone bonds are covalent bonds formed by aldehydes or ketones and hydrazine compounds. Under acidic conditions, the imine nitrogen (the C=N part) in the hydrazone bond is first protonated to form an iminium ion. This step makes the carbon atoms more electrically positive and more susceptible to attacks by nucleophiles (water molecules). Water molecules attack the protonated C atoms to form an unstable tetrahedral intermediate. After the molecular rearrangement, the tetrahedral intermediate breaks, eventually forming the original aldehyde or ketone and hydrazine derivatives.^38,39 The pH response characteristics of hydrazone bonds were analyzed using ¹H NMR. The proton signal of the hydrazone bond is intact at pH 7.4, while the proton peak of the hydrazone bond almost disappears at acidic pH 5 and 6.5. This suggests that hydrazone bonds are good in vivo pH-responsive chemical bonds.⁴⁰ Antineoplastic drugs such as DOX can be attached directly to polymers via pH-sensitive chemical bonds, such as hydrazone bonds. It enters tumor cells and breaks hydrazone bonds at low acidity, enabling pH-responsive drug release.³³

Schiff bases, also known as imines, have carbon–nitrogen double bonds, so they are chemically similar to hydrazone bonds that are unstable to acid. They are synthesized typically by condensation between aldehydes or ketones and primary amines.⁴¹ The pH sensitivity of a Schiff base is mainly ascribed to the lone pair of electrons on the imine nitrogen atom in its structure. In an acidic environment, imine nitrogen is protonated to form imine ions (C=N⁺), which reduces its nucleophilicity and thus affects its reactivity with other molecules. This protonation process changes the stability and reactivity of the Schiff base under different pH conditions, giving it pH responsiveness.^42,43

2.2.2. Acetal and ortho ester. Acetal bonds (ketal bonds) are structures formed by the condensation reaction of an aldehyde or ketone with two alcohol molecules.⁴⁴ Under acidic conditions, an ether oxygen atom in the acetal is first protonated to form an oxygen positive ion, which increases the positive electrical properties of adjacent carbon atoms and makes it easy to dealcoholize. After dehydration, an oxocarbenium (carboxonium) ion intermediate is formed, and water molecules nucleophilically attack the carbo-positive ion and remove one proton to form a hemiacetal. The above process is repeated and finally the acetal hydrolyzes into two molecules of alcohol and one molecule of ketone or aldehyde.⁴⁵ There are other electron-removing groups such as amide groups in the molecule, which will greatly affect the rate of their degradation. But the length of the alkyl chain substituents in the acetal carbon did not have a substantial influence on the stability of the oxocarbenium ion and thus the rates of acetal hydrolysis.⁴⁶ For the purpose of encapsulating hydrophobic drugs, cyclic benzylene acetals as a composition of block copolymers have several advantageous properties, such as low toxicity and adjustable hydrolysis rates (depending on the substituent effect on the aromatic ring).⁴⁷

Ortho ester is another pH-sensitive chemical bond that is commonly used as a pH-responsive group in hydrogels.⁴⁸ Compared to acetal, ketal, and hydrazine, ortho ester is more susceptible to a mildly acidic environment and the hydrolysis rate of ortho ester may improve 1–4 orders of magnitude, which offers a new strategy to design drug delivery systems with rapid responsiveness.⁴⁹ Ortho ester in hydrogels generally contains a 1,3-dioxolane structure. It can be obtained by reacting a compound containing an ortho-diol structure with trimethyl orthoformate.^50,51 The acidic hydrolysis of ortho ester is similar to the acidic hydrolysis of acetals, both of which undergo the reaction steps of protonation of oxygen atoms, nucleophilic attack of carbon atoms by water molecules, and departure of alcohol groups.^52,53

2.2.3. cis-Aconityl. cis-Aconityl is a pH-responsive amide bond formed by the reaction of cis-aconitic anhydride and amino groups.^54,55 Under acidic conditions, the double bonds and carboxyl groups of the cis-aconityl moiety are protonated to form an electrophilic active center, which induces protons to attack amide bonds to cause hydrolysis reactions, and linker cleavage to cause drug release or carrier depolymerization.⁵⁶ During the degradation process under acidic conditions, the double bonds will isomerize to trans structures, thus preventing the complete degradation of the polymer and affecting the pH-responsive degradation rate.⁵⁷ When aconitic anhydride and an amino group are used to synthesize a pH-sensitive linker, the competitive decarboxylation, double bond isomerization and hydrolysis reactions of aconitic anhydride will reduce the yield of the target product pH-sensitive linker.⁵⁸

2.2.4. β-Thiopropionate. The pH-sensitive hydrolysis of β-thiopropionate originates from the sulfur atom at its β position. In an acidic environment, the sulfur atom at the β position is protonated, and the newly formed S–H bond forms a hydrogen bond with the carbonyl group, inducing water molecules to attack the carbonyl group and cause a hydrolysis reaction. At present, the synthesis method of β-thiopropionate for pH-sensitive degradation mainly involves the addition reaction of 2-mercaptoethanol with double bonds.^59,60 It can also be started from a carboxylate containing a β-disulfide bond, which is cleaved to produce a thiol group and then added to the double bond to form β-thiopropionate.⁶¹ In recent years, it has been reported that palladium-catalyzed carbonylation of disulfide and ethylene produces β-thiopropionate thioesters, which are expected to be used in the synthesis of pH-sensitive chemical bonds.⁶²

3. Inorganic nanoparticles

There are many types of inorganic materials with excellent performance. It is commonly used to synthesize nanoparticles. Among them, silica-based nanoparticles, gold nanoparticles, metal oxide nanoparticles, and quantum dots (QD) have been extensively and thoroughly investigated.⁶³ Inorganic nanoparticles have been extensively studied and applied in the biomedical field due to their distinct physicochemical properties. Among silica-based nanoparticles, mesoporous silica nanoparticles (MSNs) stand out due to their exceptional drug-loading capacity, attributed to their mesoporous structure. Additionally, the abundance of surface silanol groups allows for facile chemical functionalization, making MSNs an excellent platform for intelligent drug delivery systems.^64,65 Gold nanoparticles, owing to their unique photothermal properties and facile surface functionalization, have been widely utilized in biomedical imaging and in combined photothermal–chemotherapy treatments.⁶⁶ Metal oxide nanoparticles and QDs are primarily employed in biomedical imaging and diagnostics, as well as in applications such as magnetic targeting and nanozyme-based therapies.^67,68 For the pH-responsive inorganic drug delivery systems, MSNs are considered the preferred materials.

3.1. Preparation method of MSNs

MSNs are silica-based nanoparticles with uniform pores (2–50 nm in diameter), high surface area, tunable sizes, and easy surface functionalization.⁶⁹ Due to their surface chemistry, colloidal stability, and high dispersity, MSNs are widely used in drug delivery, catalysis, and sensing.⁷⁰ Their mesopores can encapsulate small molecules or biomacromolecules, and their surfaces can be functionalized for improved biocompatibility, stimuli-responsiveness, targeting, and controlled release. MSNs are prepared through methods like self-assembly, soft and hard templating, and the Stöber method, with soft templating being preferred for biomedical applications.⁷¹ The soft templating method uses nanoparticles (e.g., micelles) in solution, with CTAB (a surfactant) as a structure-directing agent (SDA) to form cylindrical micelle templates (2–10 nm). The MSNs produced are part of the M41S family, typically used for small molecule drug delivery.^72,73 Other MSN families, like the SBA family, use polymer micelles as SDAs, with mesopore diameters of 5–30 nm, making them suitable for biomacromolecule delivery.^74,75 The silicon source, like tetraethoxysilane (TEOS), is hydrolyzed to produce Si–OH groups, which condense to form uniform silica, a process known as the Stöber method (or sol–gel method), first reported by Stöber in 1968.⁷⁶ After aging to form Si–O–Si bonds and removing the template, hard silica nanoparticles with mesopores are formed.⁷⁷ In some cases, metal nanoparticles serve as the core, with mesoporous silica on the surface. MSNs with gold nanorods exhibit photothermal effects,⁷⁸ while those with iron oxide (Fe₃O₄) nanocrystals enable MRI imaging⁷⁹ and magnetic targeting.⁸⁰

3.2. Overall design strategies of MSNs for tumor targeting

Various trimethoxysilane-modified molecules can be easily grafted onto MSNs, providing functional groups for further modification. For example, (3-aminopropyl)triethoxysilane (APTES) links amino groups,^81,82 3-(triethoxysilyl)propylsuccinic acid (TEPSA) links carboxyl groups,⁸³ and methacryloxypropyl trimethoxysilane (MPS) introduces double bonds for grafting acrylate-containing polymers.^84–86 (3-Glycidyloxypropyl)trimethoxysilane (GPTMS) attaches ethylene oxide for further functionalization.^80,87

MSN-based DDSs can achieve responses to stimuli of TME such as temperature, pH, redox, ultrasound, magnetic field, enzymes, sugars, and ATP through surface modifications.⁸⁸ Responsiveness to TMEs gives DDSs less premature release and controlled release. Functional molecules on the MSN surface form a molecular gate that encloses drugs, which opens or degrades upon stimulation to release the drug.^89,90 Many environment-responsive drug delivery platforms use the TME and external factors^84,91 to co-stimulate or enable multi-stage drug release,⁹² rather than relying solely on pH-responsiveness. An important trend is introducing biomacromolecules with recognition and targeting effects, rather than simple chemicals.^93,94

pH-Responsive materials dissolve over time, with dissolution accelerating as the acidity increases. Normal tissues (approximately pH 7.4), the TME (approximately pH 6.5), and endosomes/lysosomes (approximately pH 4.5 to 5.5) exhibit pH gradients, which can be used for stimuli-triggered endocytosis and drug release.⁹³ To improve biocompatibility and increase circulation time, NPs are coated with biocompatible materials and negatively charged surfaces.^78,95 The EPR effect is used to passively target DDSs to the TME, but further research is needed.⁹⁶ Upon reaching the TME, NPs enhance endocytosis through ligand–receptor mediation.^85,93 The pH of the TME can reverse the surface charge of NPs from negative to positive, promoting internalization.^92,95,97 In the cell, endosome acidity dissolves gatekeepers, releasing drugs to kill cells or induce apoptosis.^87,93 MSNs can be modified with nuclear-targeted peptides for enhanced delivery into the nucleus.^93,94

3.3. Design of MSNs for acidic TME targeting

The following will introduce the design of pH response based on biogenic materials, synthetic polymer materials, and metal ion complexes as gatekeepers and direct drug connection to MSNs (Table 1).

Table 1 Selected examples of pH-responsive MSNs

Name	Responsive/targeting molecules/groups	The loaded drug	Experimental model	Ref.
MSN-HAgel	Hyaluronic acid, disulfide bond, aptamer (HER2)	DOX, HER2-aptamer	SK-BR-3	81
DOX@MSN-PEI-AA	Polyethyleneimine, anisamide (sigma receptor)	DOX	MDA-MB-231, MCF-7, 4T1, Balb/c bearing 4T1	98
MSN-H1[Ru] MSN-H2[Ru]	(2-Thienylmethyl)hydrazine, (5,6-dimethylthieno[2,3-d]pyrimidin-4-yl)hydrazine	Dichloro(p-cymene) ruthenium(II)	B16F1	99
ConA-MSN@ DOX	3,9-Bis(3-aminopropyl)-2,4,8,10-tetraoxaspiro [5.5] undecane, lectin concanavalin A (sialic acid glycans), polyacrylic acid	DOX	MC3T3-E1, HOS	83
FA-CH-MSN@ATZ	Chitosan, folic acid	Anastrozole	MCF-7, Balb/c bearing Ehrlich ascites carcinoma (EAC)	82
PLL-MSN/PLG-MSN @DOX	Poly(L-lys), poly(L-Glu)	DOX	HeLa	100
DG@NPs	Polydopamine (pH, enzymes, redox), folic acid	DOX, glucose oxidase	HPDE6C7, SW1990, nude♀ bearing SW1990	79
D/B-PDA-AuNRs @MSN	PDA, boron–oxygen bond, gold-nanorod	DOX, Bortezomib	4T1, mice bearing 4T1	78
DOX/ICG@MSN-p(NIPAM-co-MA)	Methacrylic acid, N-isopropylacrylamide (thermal)	DOX, Indocyanine green	HeLa	84
R-P-MSN@DTX	N-(2-Hydroxypropyl)methacrylamide, 3-aminopropylmethacryl amide and 2,3-dimethylmaleic anhydride co-polymer	Docetaxel (DTX)	HeLa, nude bearing HeLa	95
MOP-hyd-MSN@DOX	Morpholin-4-yl-acetyl-poly(ethylene glycol)-b-poly(lactic acid)	DOX	HepG2	92
MSN-pNIPAm/MAA-HER2	Methacrylic acid, N-isopropylacrylamide (thermal), anti-HER2 scFv	DOX	SK-BR-3, Balb/c♀ nude bearing SK-BR-3	85
Polymer-MSN@DOX	Poly(poly(ethylene glycol) methylether methacrylate-co-p-(2-methacryloxyethoxy)benzaldehyde)	DOX	HepG2	101
PDA-MSN@CTS	Polydopamine (pH, photothermal)	Crytotanshinone	AGS, Balb/c♂ nude bearing AGS	102
PDA-MSN@DOX	Polydopamine (pH, ultrasound)	DOX	HeLa	91
Fu-MSN@MTX-NMOF	Zeolitic imidazolate framework-8	5-Fu, methotrexate	MCF-7, albino mice♀ bearing EAC	103
FITC-HA-MSN@DOX	Hyaluronic acid (pH, enzymes)	DOX	SK-BR-3	104
MSN-PAA-AGA	Poly(acrylic acid), glucosamine, 4,4-(ethylene dicarbamoyl)phenylboronic acid, (pH, glucose)	Rhodamine 6G	NA	105
HA-hMSN@DOX	Hyaluronic acid (pH, CD44)	DOX	HeLa	87
β-CD-MSN@DOX	β-Cyclodextrin and p-anisidine	DOX	NA	106
CSNP@DOX	Galactose modified poly(allylamine hydrochloride)-citraconic anhydride, HIV TAT peptide (nuclear)	DOX	QGY-7703, mice bearing H-22	94
MSN-Por-CA-PEG	cis-Aconitic anhydride, coordinate bond histidine, tetraphenylporphyrin zinc (photo)	DOX	HeLa, MCF-7	97
MSN-Tf@CPT	Glutaraldehyde, transferrin	Camptothecin	NA	107
Cyt/MSN-NH2/SH/DOX/SF/O-CMC	Cysteine, sodium hyaluronate, silk fibroin, oxidized-CMC	Cytarabine (Cyt), DOX	MDA-MB-231, MCF-7	108
Cyt/NH2-MSN/HA/MTX/CS/oxCMC	Cysteine, sodium hyaluronate, chitosan, oxidized-CMC	Cyt, MTX	HepG2	109
PCD@SCU@MSN @Fe3O4	Poly(ε-lysine)-cyclodextrin, 2-(benzo[d]thiazol-2-yl)phenyl-4-aminobenzoate (pH, H2O2), Fe3O4 (magnet)	Scutellarin	Huh7, HCT116, nude bearing H22	80
MSN@Mela@TTM	Tetrathio-maleimide, melamide	DOX	MDA-MB-231	110
BBR@MSN@p(NIPAM-co-MA)@ EVO-LB	Methacrylic acid, N-isopropylacrylamide (temperature)	Evodiamine, berberine	HepG2, HCT-8, HeLa, HUVEC, nude bearing Experimental Mammary Tumour-6 (EMT-6)	86
CP/CQ@MSN-HtB-Cu^2+	HtB peptide (containing His, Arg, Asn), Cu^2+ (chemodynamic therapy)	Cis-platin, chloroquine	B16, C57bl/6 mice bearing B16	111
TAT-MSN-DOX/PAH-Cit/GTC/siRNA	Poly(allylamine hydrochloride)-citraconic anhydride, HIV-1 TAT peptide(nuclear), galactose-modified trimethyl chitosan-cysteine (loading siRNA, GSH)	DOX, siVEGF	QGY-7703, Balb/c nude♀ bearing QGY-7703	93
DOX@MSN-WS2-HP	4-Adamantane carboxylate benzaldehyde, WS2 QDs, tLyp-1 peptide (homing/-penetrating), cyclodextrin and adamantane	DOX	4T1, 4T1 tumor sheroids, Balb/c♀ bearing 4T1	112
DOX@CS-CMSN	Chitosan, L-Pro modified MSNs	DOX	4T1	113
SP-FS-USMSN cluster	Spiropyran, flourinated silane	DOX, curcumin	HepG2, nude♀ bearing HepG2	114

---

3.3.1. Biogenic materials as gatekeepers. Hyaluronic acid (HA) is an effective gatekeeper for mesopores due to its pH sensitivity, biocompatibility, and CD44 targeting.^{81,87,104,108,109} Disulfide bonds in HA enable GSH-responsive disassembly,^81,108,109 and HER2 aptamers enhance targeting specificity.⁸¹ HA can also be degraded by Hyal-1 in the TME to release drugs.¹⁰⁴ Chitosan (CS), a biocompatible polymer, exhibits pH-responsiveness due to its amino groups, making it suitable for MSN gatekeeping.^82,113 The amino groups of CS crosslink with the aldehyde groups of oxidized carboxymethyl cellulose (oxCMC) to form pH-sensitive imine bonds, producing MSN-loaded nanogels.¹⁰⁹ Folic acid is conjugated to CS to target folate receptors.⁸² Introducing D-Pro onto MSNs loaded with CS imparts chirality to otherwise non-chiral anticancer drugs, enabling targeting of the chiral environments within the body.¹¹³ Silk fibroin (SF) can replace CS for cross-linking with oxCMC to achieve pH-sensitive degradation.¹⁰⁸ Cyclodextrins (CD) serve as effective gatekeepers by encapsulating molecules that de-encapsulate upon acquiring a positive charge in acidic environments.^80,106p-Anisidine on MSNs becomes protonated under acidic conditions, causing CD caps to detach.¹⁰⁶ CD can also be conjugated to 2-(benzo[d]thiazol-2-yl)phenyl-4-aminobenzoate (BTPA) derivatives for pH-responsive decapping. H₂O₂-sensitive phenyl benzoate groups on BTPA provide additional H₂O₂ responsiveness (Fig. 2a).⁸⁰ Poly(ε-Lys) can be used to link CDs to form polymers containing a large number of CDs (PL-CD).¹¹⁵ The self-assembly properties of CDs add functional molecules to MSNs. Adamantane conjugated via imine bonds triggers pH-dependent drug release at the tumor surface. Tungsten disulfide quantum dots (WS₂ QDs) (approximately 5 nm) modified with CD encapsulate adamantane, self-assemble to seal mesopores, and penetrate deeper into the tumor tissue with tLyP-1 for QD-PTT.¹¹² Transferrin (Tf) targets tumor cell TfR and seals MSN mesopores. Glutaraldehyde (GA) links MSN-NH₂ and Tf via imine bonds, enabling drug release in acidic environments.¹⁰⁷

Fig. 2 Selected representative gatekeepers of MSN designs. (a) Cyclodextrins from natural sources form inclusion complexes with ionizable groups, and ionization can cause the cyclodextrin to dissociate. (b) Synthetic polymer material polyacrylic acid with pH-sensitive acetal bonds as gatekeepers of MSNs. (c) Metal-ion complex metal–organic framework ZIF-8 with ionizable 2-methylimidazole acts as a gatekeeper of MSNs to achieve pH response. (d) The anticancer drug dichloro(p-cymene) ruthenium(II) was directly linked to the surface of MSNs via pH-sensitive hydrazone bonds. a. Reprinted from ref. 80. © 2024 Elsevier B.V. All rights reserved. b. Reprinted from ref. 83. © 2018 Elsevier Inc. All rights reserved. c. Reprinted from ref. 103. © 2023 Published by Elsevier B.V. d. Reprinted from ref. 99. Copyright © 2021 by the authors. Published by MDPI under the Creative Commons Attribution (CC BY) license.

3.3.2. Synthetic polymer materials as gatekeepers. Polyethylenimine (PEI), with its amino groups, is a pH-responsive polymer that degrades in lysosomes. Incorporating anisamide (AA) targets sigma receptors highly expressed in tumor cells.⁹⁸ Polyacrylic acid (PAA) and 2,4,8,10-tetraoxspirocyclo[5,5]undecane with acid-degradable acetal structures form pH-responsive gates for drug release (Fig. 2b).⁸³ Polydopamine (PDA) offers excellent biocompatibility, pH sensitivity, and enzymatic degradation. It is used in photothermal therapy (PTT) due to its melanin-like structure, capable of absorbing infrared light and converting it into heat for enhanced antitumor effects, especially when combined with gold nanoparticles.⁷⁸ PDA-coated MSNs enable pH-responsive,⁷⁹ dual pH/photothermal,^78,102 or pH/ultrasound⁹¹ responsive delivery. Glucose oxidase (Gox) loaded onto MSNs with PDA generates glucuronic acid and H₂O₂ in cancer cells, inducing oxidative stress and apoptosis.⁷⁹ Poly(methacrylic acid) (pMAA) enables acid-targeted delivery, while poly(N-isopropylacrylamide) (pNIPAM) targets tumor thermogenesis. Their copolymer (p(NIPAM-co-MAA)) conjugated to MSNs via MPS acts as a gatekeeper, imparting both pH and temperature responsiveness.^84–86 2,3-Dimethylmaleic anhydrides (DMA), N-(2-hydroxypropyl)methacrylamide (HPMA) and the 3-aminopropyl methacrylamide (APMA) copolymer acts as a pH-responsive gatekeeper, forming two different cleavable amide bonds. It enables two-stage decoating: charge reversal in the TME for cellular internalization, followed by complete decoating in endosomes to release the drug.⁹⁵ Poly(poly(ethylene glycol)methylether methacrylate-co-p-(2-methacryloxyethoxy)benzaldehyde) [P(PEGMA-co-MAEBA)] grafted onto MSNs forms pH-sensitive imine bonds to released drugs mainly within the endosomes of cells.¹⁰¹ Morpholin-4-yl-acetyl-polyethylene glycol-b-polylactic acid (MOP), containing morpholine groups, seals mesopores on MSNs via acid-sensitive hydrazone bonds. In the TME, morpholine accepts protons, reversing the surface charge to promote internalization, while hydrazone bond cleavage in endosomes releases the drug.⁹² Melamine (Mela) grafted on MSNs interacts with tetrathio-maleimide (TTM) to seal the drug via non-covalent bonds. Under acidic conditions, protonation imparts positive charges to enhance cellular uptake and triggers decapping via an electrostatic force.¹¹⁰ Poly(acrylic acid) (PAA) grafted onto MSNs binds to glucosamine (AGA). 4,4′-(Ethylene bis(aminomethyl))diphenylboronic acid (EPBA) crosslinks PAA-AGA with boronate ester bonds, forming a gatekeeper that responds to both acidic conditions and glucose for controlled drug release.¹⁰⁵ Synthetic peptides, like poly-L-lysine (PLL) and poly-L-glutamic acid (PLG), with free amine and carboxyl groups, self-assemble through electrostatic interactions to act as pH-responsive gatekeepers for MSNs.¹⁰⁰ Spiropyran, a pH-responsive compound, undergoes ring-opening and ring-closing transitions. When linked to long-chain perfluoroalkane on ultra-small MSNs (approximately 12 nm), it self-assembles into clusters (approximately 110 nm) under neutral conditions. In acidic environments, ring-opening increases hydrophilicity, disassembling the clusters and releasing the drug.¹¹⁴

3.3.3. Metal ion complexes as gatekeepers. Metal–organic frameworks (MOFs), like ZIF-8 (formed by Zn²⁺ and 2-methyl imidazole), are used as coatings for nanoparticles and exhibit pH sensitivity. MSNs, loaded with 5-fluorouracil (Fu), are encapsulated by ZIF-8 with methotrexate (MTX) inside, enabling sequential drug release (Fig. 2c).¹⁰³ Histidine (His) and zinc porphyrin (Zn-Por) complexes serve as pH-responsive gatekeepers. His, anchored on MSNs, coordinates with Zn²⁺ in Zn-Por. In acidic environments, His protonation causes Zn–Por dissociation, triggering drug release. PEG conjugated with Por via pH-sensitive cis-aconitic anhydride (CA) enables charge reversal in the TME, promoting cellular uptake. Zn–Por also acts as a photosensitizer, generating ROS under light to target the tissue.⁹⁷ A His-tag (His)₆ sequence modifies the integrin β3-targeting peptide B3int, forming the HtB peptide on MSNs. Cu²⁺ binds to imidazole groups in the His-tag, blocking mesopores. Under acidic conditions, protonation dissociates Cu²⁺, triggering drug release and catalyzing a Fenton-like reaction for ROS generation in chemodynamic therapy (CDT).¹¹¹

3.3.4. Drugs directly attached to the carrier surface. The anti-tumor drug can be conjugated to the MSN surface via pH-sensitive bonds. For example, DOX is linked to MSNs using a hydrazone bond and coated with HA.⁸⁷ Similarly, cytotoxic ruthenium(II) complexes are attached to the MSN surface via a hydrazone bond (Fig. 2d).⁹⁹

4. Lipid-based nanoparticles

4.1. Preparation of lipid-based nanoparticles

The lipid-based nanoparticle technology has evolved to include five major types of nanoparticles: liposomes, lipid nanoemulsions (LNEs), solid lipid nanoparticles (SLNs), nanostructured lipid carriers (NLCs), and lipid nanoparticles (LNPs).^63,116 The primary differences among these nanoparticles lie in the composition of their constituent materials, while their fabrication methods are largely similar. The earliest liposome synthesis techniques were characterized by their simplicity in preparation but suffered from poor product uniformity. There are three primary methods for liposome preparation: the film dispersion method, the solvent injection method, and the reverse-phase evaporation method. Nanoliposomes with improved homogeneity can be obtained by applying particle size control techniques such as sonication, freeze–thaw cycles, high pressure or high shear homogenization, or extrusion.¹¹⁷ The synthesis of LNEs typically involves the initial preparation of a coarse emulsion, followed by high-pressure or high-shear homogenization, microfluidics, or ultrasonication to form nanoemulsions. The preparation of SLNs and NLCs can be achieved through various techniques, including high-pressure homogenization, high-shear homogenization, ultrasonication, the membrane contractor method, the film-ultrasonic method, solvent emulsification and evaporation, and the use of supercritical fluids.¹¹⁸ LNPs can also be synthesized using the aforementioned methods; however, microfluidic technology has become the predominant approach for their production. Originally developed for small-scale fabrication of LNPs, microfluidic systems are inherently scalable, making them suitable for both research purposes and potential industrial applications.^119,120 Liposomes are hollow vesicles composed of phospholipids, cholesterol, and PEG-conjugated lipids.¹¹⁷ LNEs and SLNs are formulated from lipids (either liquid or solid) in combination with surfactants, whereas NLCs are prepared using a mixture of liquid and solid lipids together with surfactants.^118,121,122 LNPs are generally composed of ionizable lipids (approximately 50%), phospholipid helpers (approximately 10%), cholesterol (approximately 30%) and PEG-conjugated lipids (approximately 1–2%).^123,124

The first FDA-approved nanomedicine was Doxil®,¹⁴⁵ a liposomal formulation of doxorubicin used in cancer treatment. Consequently, the use of conventional liposomes to encapsulate small-molecule drugs, proteins, and nucleic acids, along with the incorporation of environmentally responsive features such as pH sensitivity, has become a central focus in the development of lipid-based nanoparticles for anti-tumor applications. With the approval of COVID-19 mRNA vaccines, mRNA therapeutics have emerged as a major topic in biomedical research.¹⁴⁶ Owing to their potent nucleic acid delivery capabilities, LNPs have become a key platform for the development of nucleic acid-based anti-cancer therapies. Among the essential components of LNPs, ionizable lipids are specifically designed to promote endosomal escape and are inherently pH-responsive, meaning that LNPs themselves possess intrinsic pH-sensitive properties. Selected examples of pH-responsive liposomes and LNPs will be introduced as follow (Table 2).

Table 2 Selected examples of pH-responsive liposomes and LNPs

Name	Responsive/targeting molecules/groups	The loaded drug	Experimental model	Ref.
DC-liposome	3β-[N-(N′,N′-Dimethylaminoethane)-carbamoyl]cholesterol	DOX	K7M2, NIH/3T3	125
Virus-mimicking liposome	L-Phenylalanine grafted onto poly(L-lysine iso-phthalamide)	DOX	HeLa	126
Virus-mimicking liposome	Hydrophobic decylamine grafted onto poly(L-lysineiso-phthalamide)	DOX	HeLa, A549, MES-SA, MES-SA/DX5	127
DCPA–H2O liposome	Zwitterionic lipid 2-(4-((1,5-bis(octadecyloxy)-1,5-dioxopentan-2-yl)carbamoyl)pyridin-1-ium-1-yl)acetate	Rhodamine	Nude bearing HepG2	128
DPRP liposome	Polyanionic shielding domain (ehG)n, cells-penetrating domain (octaarginine)	siPLK-1 and DTX	MCF-7, MCF-7 tumors spheroid, mice bearing MCF-7	129
GC-liposome	Glycol chitosan	DOX	HT1080, nude mice bearing T6-17	130
DOX-PSL-H7K(R2)2	Imidazole(H7), guanidine((R2)2)	DOX	C6, nude mice bearing C6, orthotopic U87-MG	131
TR-Lip	Imidazole, pH-responsive cells-penetrating peptides(TH), c(RGDfK)(target for integrin αvβ3)	PTX	B16F10, L02, MCF-7, C57BL/6 bearing B16F10	132
AL	Polyanionic shielding domain (ehG)n, cells-penetrating domain (octaarginine)	siPLK-1	MCF-7, A549	133
D-Lip	Imidazole([D]-H6L9-Cys)	Antagomir-10, PTX	4T1, mice bearing 4T1	134
LNPssPalmE	Tertiary amines, disulfide bond, vitamin E (for hydrophobic scaffold)	sFlt-1 pDNA	Balb/c AJcl-nu/nu mice bearing OS-RC-2	135
4A3-SCC-PH LNPs	Tertiary amines	Fluc mRNA, tdTomato Cre mRNA	Mice bearing metastasis 4T1, B16F10	136
DOP-DEDA LNPs	DOP-DEDA (diethylenediamine)	siPLK-1	HT1080-EGFP, MDA-MB-231	137
NLS-(-30)GFP-LNPs	DOP-DEDA (diethylenediamine)	NLS-(-30)GFP	HeLa	138
PGlu(DET-Car)30 LNPs	Ethylenediamine-based polycarboxybetaine zwitterion [PGlu(DET-Car)]	siPLK-1	SKOV3-luc, CT26, mice bearing CT26	139
βGlus/mOVA@LNPs	β-Glucans	Ovalbumin (OVA) mRNA	C57BL/6 bearing E.G7-OVA	140
XP-LNPs	Succinoyl tetraethylene pentamine, lipoamino fatty acid	VEGFR-2 siRNA	Nude mice bearing Huh7	141
ssPalm-LNPs	Tertiary amines, disulfide bond	GFP mRNA	hCMEC/D3	142
DODAP LNPs	1,2-Dioleoyl-3-dimethylammonium propane	MCT4 siRNA	4T1	143
LAF-Stp carriers	Succinoyl tetraethylene pentamine, lipoamino fatty acid	Cas9/sgGFP/ssDNA	Hepa 1-6 Pcsk9^tdTomato	144

---

4.2. pH-Responsive liposomes

The mainstream approach for imparting pH-responsive properties to liposomes involves surface modification with structures capable of pH-triggered activation. For example, influenza virus gains cellular entry through a complex trimeric hemagglutinin (HA) complex, which responds to the acidification of the endosome by destabilizing the endosomal membrane, thereby facilitating endosomal escape.¹⁴⁷ Inspired by this mechanism, a series of novel biocompatible pseudopeptides mimicking viral pH-responsive endosomal escape functions have been developed and incorporated onto the surface of liposomes for targeted delivery of DOX to tumors.^126,127 These pseudopeptides are derived from the backbone of poly(L-lysine iso-phthalamide) (PLP), which is further functionalized by conjugation with hydrophobic moieties such as hydrophobic decylamine (NDA)¹²⁷ and L-phenylalanine.¹²⁶ Cell-penetrating peptides (CPPs) are a class of specialized peptide sequences capable of facilitating cellular uptake via endocytosis and delivering cargo into the cytoplasm. CPPs were initially identified in the Tat protein of HIV-1 as arginine-rich sequences, which led to the development of a wide range of CPP derivatives for drug delivery applications. Due to their content of ionizable amino acids, many CPPs, especially those that are strategically designed, exhibit both pH-responsive behavior and cell-penetrating ability, making them extremely versatile tools in targeted delivery systems.^148,149 A multistage pH-responsive co-delivery liposomal platform was developed by modifying the liposome surface with a composite structure called DPRP. It is composed of a CPP (octaarginine (R₈)), and a polyanionic shielding domain, linked via pH-sensitive imine bonds. This complex was anchored to the liposomal surface using the post-insertion method. Upon exposure to acidic environments, the imine bonds and the octaarginine moiety gradually become protonated, thereby enabling the system's multistage pH-responsiveness. This platform was designed for the co-delivery of DOX and siPLK-1, enhancing targeted delivery and therapeutic efficacy (Fig. 3a).^129,133 The peptide H₇K(R₂)₂, composed of hepta-histidine (H₇) and di-arginine (R₂), possesses both pH-responsive and CPP properties. It has been conjugated to the surface of liposomes for the targeted delivery of DOX in the treatment of glioma (Fig. 3b).¹³¹ A histidine-rich peptide (TH) with both pH-responsive and CPP functionalities was conjugated with the integrin α_vβ₃-targeting peptide c(RGDfK) and anchored onto the surface of liposomes for the delivery of PTX in melanoma treatment. This design enabled the liposomal system to achieve dual functionality, combining tumor-targeting specificity with pH-responsive drug release.¹³² A pH-dependent, cell-penetrating antimicrobial peptide, [D]-H₆L₉, was used to modify the surface of liposomes for the co-delivery of PTX and a metastasis inhibitor, antagomir-10b, in the treatment of metastatic breast cancer.¹³⁴ Liposomes can also be endowed with pH-responsive properties through modification with naturally occurring pH-sensitive materials. For example, native glycol chitosan (GC) exhibits inherent pH-responsiveness. By incorporating COOH-terminated phospholipids into the liposomal formulation and employing carbodiimide chemistry, GC can be covalently linked to the liposome surface, resulting in the formation of pH-responsive liposomes.¹³⁰ An alternative strategy to impart pH-responsiveness to liposomes involves the chemical modification of their constituent components. For example, cholesterol can be modified into 3β-[N-(N′,N′-dimethylaminoethane)-carbamoyl]cholesterol, an ionizable derivative that enables endosomal escape by responding to the acidic environment within endosomes.¹²⁵ Another approach for introducing pH-responsiveness is the incorporation of novel components into conventional liposomal formulations. For instance, the addition of a new zwitterionic lipid, 2-(4-((1,5-bis(octadecyloxy)-1,5-dioxopentan-2-yl)carbamoyl)pyridin-1-ium-1-yl) acetate, endows the liposomes with pH-sensitive properties.¹²⁸

Fig. 3 Selected representative designs of liposomes and LNPs. (a) Multistage pH-responsive co-delivery liposome and its tumor-targeted delivery strategy. The acidic pH TME splits DPRP at the imine site and detaches the shielding domain from the CPP section. (b) A pH-responsive peptide composed of seven ionizable histidines was loaded onto the surface of liposomes, endowing the liposomes with pH-responsive function. (c) DOP-DEDA as a pH-sensitive, charge-reversible lipid is used as the main component of LNPs. (d) LAF-Stp carriers as cationizable carriers for polyplex formulation. a. Reprinted from ref. 129. Copyright © 2022, The Author(s). Published by Springer Nature under a Creative Commons Attribution License (CC BY). b. Reprinted from ref. 131. Copyright © 2015 Elsevier B.V. All rights reserved. c. Reprinted from ref. 138. Copyright © 2021, The Author(s). Published by Springer Nature under a Creative Commons Attribution License (CC BY). d. Reprinted from ref. 144. Copyright © 2024 The Authors. Published by Elsevier B.V. under the CC BY.

4.3. pH-Responsive LNPs

pH-Responsive liposomes have been utilized for the delivery of both small-molecule anticancer drugs and anticancer nucleic acids, whereas pH-responsive LNPs are primarily employed for the delivery of anticancer nucleic acids. Similar to liposomes, the pH-responsiveness of LNPs is mainly achieved through surface modification or formulation adjustment. A notable example is the design of a SS-cleavable Proton-Activated Lipid-like Material (ssPalm), specifically engineered to facilitate pH-triggered endosomal escape. This material features a tertiary amine and a disulfide bond as its core functional moieties, while its hydrophobic region can be composed of myristic acid, retinoic acid, or α-tocopherol succinate, making it highly suitable for nucleic acid delivery.^135,142 Ionizable lipids are essential components of LNPs, conferring pH-responsive endosomal escape capability, which is critical for enhancing the transfection efficiency of nucleic acids.^150,151 LNPs developed using the ionizable lipid dioleoylglycerophosphate-diethylenediamine (DOP-DEDA) as a key component have demonstrated efficient endosomal escape, enabling high transfection efficiency for nucleic acid delivery (Fig. 3c).^137,138 Ionizable lipids can also be modularly designed with three distinct components: a tertiary amine headgroup, a linker, and hydrophobic tails.¹⁵² By synthesizing a large library of tertiary amine headgroups, incorporating disulfide bonds within the linker region, and using hydrophobic tails composed of linear or branched alkyl chains of varying lengths, a wide array of ionizable lipids can be generated through combinatorial assembly. These diverse ionizable lipids are then formulated into LNPs, and the candidates exhibiting the highest transfection efficiency are selected for further development in nucleic acid delivery.¹³⁶ A polyzwitterion-conjugated lipid, composed of a hydrophobic DSPE moiety and a hydrophilic PGlu(DET-Car)₃₀ segment (DSPE-PGlu(DET-Car)₃₀), has been utilized as the ionizable lipid component in LNPs. The PGlu(DET-Car)₃₀ segment, representing poly(N-{N′-[N″-(2-carboxyethyl)-2-aminoethyl]-2-aminoethyl}glutamide), enables efficient endosomal escape, thereby enhancing the delivery efficiency of nucleic acids.¹³⁹ Taking advantage of effective intracellular delivery mechanisms of both cationizable lipids and polymers, highly potent double pH-responsive nucleic acid carriers are generated by combining at least two lipoamino fatty acids (LAFs) as hydrophobic cationizable motifs with hydrophilic cationizable aminoethylene units into novel sequence-defined molecules.¹⁵³ This class of specialized molecules, referred to as lipoamino xenopeptides (XP), represents a distinct category of modularly designed ionizable lipids, differing from those described above. These lipids exhibit dual pH-responsiveness and, when used as the primary component in LNP formulations, are capable of facilitating efficient nucleic acid delivery. The most commonly used aminoethylene unit is succinoyl tetraethylene pentamine (Stp), whereas the types of LAFs are more diverse. By varying the ratio between Stp and different LAFs, a wide range of structurally tunable and functionally diverse ionizable lipids can be rationally designed.^154,155 After screening this lipid system, the optimal formulation was selected for nucleic acid delivery, achieving high transfection efficiency for both VEGFR-2 siRNA and the Cas9/sgGFP system (Fig. 3d).^141,144 1,2-Dioleoyl-3-dimethylammonium propane (DODAP) is the earliest prototypes of ionizable cationic lipids and has been successfully used to deliver various nucleic acid cargos.^156,157 Notably, the delivery of MCT4 siRNA using DODAP-based formulations has been shown to effectively suppress the activity of the aggressive 4T1 breast cancer cell line.¹⁴³ Yeast-derived β-glucans have been employed as both adjuvants and protective agents in an oral delivery system, in combination with Moderna's mRNA-1273 LNP formulations as the delivery vehicle for ovalbumin (OVA) mRNA. The β-glucans, which exhibit acid-responsive properties, form β-glucan-complexed mRNA structures that protect the mRNA from degradation in the acidic environment of the stomach.¹⁴⁰

5. Polymer micelles

Polymer nanoparticles can be classified into polymer micelles, polymer nanocapsules, polymer nanogels/hydrogels, etc.^158–160 Applicable polymer materials include poly(amino acids), polysaccharides, glycopolymers, polyesters, vinyl polymers, polyethyleneimine, and poly(ethylene glycol).¹⁶¹ The preparation methods of polymer nanoparticles include solvent evaporation/displacement (the polymer and hydrophobic drug are dissolved in an organic solvent and then the evaporating solvent is added to promote the formation of micelles), salting-out (high concentration of salt ions reduces the solubility of water for hydrophobic segments), dialysis (organic solvent replaces water, and the hydrophobic segment loses its solvent effect and aggregates, while the hydrophilic segment stretches), supercritical fluid (green and environmentally friendly, high drug loading, controllable particle size), interfacial polymerization and electrostatic compounding methods (interactions between charged groups in polymers and molecules of opposite charge and suitable for nucleic acid or protein drug delivery).¹⁶² In terms of tumor environment stimulus responsiveness and drug loading, polymer micelles have the most powerful advantages among polymeric nanoparticles. Micelles are thermodynamically stable nanoscale ordered assemblies that spontaneously form from amphiphilic molecules in solution above a critical concentration, called the critical micelle concentration (CMC). Micelles are small in size and have a core–shell structure. The core–shell nanostructure is stabilized by hydrophobic interactions, van der Waals forces, hydrogen bonds, π–π stacking and electrostatic interactions, and its formation process is essentially the result of free energy minimization. They can be used as effective carriers for drug encapsulation to enhance the stability and targeting ability of therapeutic agents.¹⁶³ The foundational concept of micelles dates back over a century. The evolution of block copolymer micelles represents an extension and structural evolution of micellar self-assembly principles into polymeric systems. This progression originated from a fundamental understanding of the thermodynamics governing micelle formation in small-molecule surfactants. Advances in precision polymer synthesis subsequently enabled the construction of well-defined amphiphilic block copolymers with controlled nanoscale topology.^164,165

Genexol-PM™ is the first FDA-approved polymeric micellar injectable formulation, composed of poly(ethylene glycol) (PEG) and poly(D,L-lactic acid) (PDLLA),¹⁶⁶ encapsulating the chemotherapeutic agent paclitaxel. In contrast to conventional paclitaxel injection formulations, Genexol-PM™ eliminates the need for Cremophor EL.¹⁶⁷ Consequently, premedication to prevent allergic responses is not required prior to administration of Genexol-PM™. Globally, polymeric micellar drugs are in the early stages of development. Although many nanoscale drug-loaded polymer micelles are undergoing clinical trials or patent applications,^168,169 their stable and controllable large-scale production still faces challenges, which is a major obstacle to their industrialization. Furthermore, safety concerns constitute a critical bottleneck for clinical translation, resulting in a sparse number of approved products worldwide. Current research on polymeric micellar drug carriers predominantly focuses on anti-tumor applications. Achieving precise tumor targeting is paramount for both therapeutic efficacy and safety assurance.¹⁷⁰ Among strategies to enhance micellar targeting, exploiting the pH-responsive properties of these systems to the acidic tumor microenvironment is a key approach. Selected examples of pH-responsive polymer micelles are tabulated in Table 3.

Table 3 Selected examples of pH-responsive polymer micelles

Name	Responsive/targeting molecules/groups	The loaded drug	Cells/tumors type	Ref.
HNP(αPD-L16.9, Dox5.3)	PEO-b-PC7A, PEO-b-PDBA	αPD-L1, DOX	MC38, 4T1, B16F10, C57BL/6 bearing MC38, B16F10, Balb/c bearing 4T1	171
HySTINGIACS-8803	PEG-b-PC7A, PEG-b-PDPA, PEG-b-PDBA, PEG-b-PD5A	Cyclic dinucleotide	4T1, CT26, mice bearing CT26	172
GFL@PM	Poly(ethylene)glycol-poly(β-amino esters)	Fe^2+, glucose oxidase, LET-1052 (probe)	4T1, mice bearing 4T1	173
P(OGMA)m-b-P(DPA)n	2-(Diisopropylamino)ethyl methacrylate	ASA404, HCFU	CT26, MC38, mice bearing CT26	174
PSC7A	Cyclic tertiary amine of 7-membered ring	Stimulator of interferon genes	C57BL/6 bearing TC-1, B16F10	175
P(C6-Bn20)	Tertiary amine	NA	Panc02, A549/DDP, CAL27/DDP, DA-MB-231, A549, 4T1, HeLa, CT26, B16-F10, A2780, U2OS, HepG2, C57BL/6 bearing Panc02, B16F10, CT26	176
IPMs	Ionizable oligomers	siHSP47, siHMGB1	HSCs	177
EK-D-DOX	Zwitterionic peptide EK7, mercaptopropionate bonds	DOX	A549, HeLa, KM ♀ bearing murine cervical tumors	178
PSBP-6-PEG-PDLLA/PEG-PHIS	Poly(L-histidine)	PTX	HeLa, mice bearing U14	179
Trp2/CpG-NPs	Poly(L-histidine)	Trp2(melanoma antigen peptide)	B16F10, C57BL/6 ♀ bearing B16F10	180
Dox/siRNA/DNMs	pH-Responsive triplex DNA	Dox, ALK-specific siRNA	Karpas 299, mice bearing ALCL	181
mPEG5 kDa-b-[(doxo-hydGlu)6-r-Leu10]	Hydrazone bond	DOX	CT26, 4T1, mice bearing CT26, 4T1	182
P(ADH-DOX-Fc)-PEG	Hydrazone bond	DOX, ferrocene	HepG2	183
PPD/MPC	Polyethylene glycol-polylysine-dimethylmaleic anhydride	CpG (adjuvant), mannose	B16F10, 4T1, mice bearing B16F10	184
Ce6-ELP@TM/MM	Tannic acid/Mn^2+, macrophage membranes, chlorin e6 (sonosensitizer)	Chlorin e6 (Ce6)	C6, mice bearing orthotopic glioma	185
PCPA&PPM@TR	PLGA-diamino ketal-PEG-Angiopep-2, PLGA-PEG-p-aminophenyl-α-D-mannopyranoside	Temozolomide, resiquimod	bEnd.3, GL261, C57BL/6 bearing GL261	186

---

5.1. Acceptable proton micelles

One mechanism to endow micelles with pH-responsive properties involves pH-responsive groups, which could utilize the pK_a differences between two polymers to enable “a stepwise dissociation”. For instance, a polymeric micelle system is formed via the self-assembly of two amphiphilic polymers (PC7A and PDBA), whose side chains contain tertiary amine groups with acidic dissociation constants (pK_a) of 6.9 and 5.3, respectively. In a neutral environment, the tertiary amine groups are deprotonated, leading to the formation of a core–shell micellar structure that encapsulates anti-PD-L1 antibodies (αPD-L1) and the chemotherapeutic drug Dox. Within the tumor microenvironment, the PC7A chains undergo protonation, resulting in the initial release of αPD-L1 to block PD-L1 checkpoints on the cell membrane. Subsequently, in the lysosomes, Dox is released to induce immunogenic cell death of tumor cells.¹⁷¹ Another system exemplifying stepwise polymer dissociation utilizes micelles self-assembled from PEG-b-PC7A and a secondary carrier polymer exhibiting a lower pK_a. Notably, these micelles demonstrate ultra-pH-sensitive, stepwise dissociation triggered sequentially at pH values corresponding to the distinct pK_a values of the constituent polymers. This cascade-like dissociation behavior, analogous to transistor logic circuits, allows the micelles to respond discretely to varying pH thresholds encountered in different physiological microenvironments. Consequently, while maintaining stability during systemic circulation (thereby prolonging blood residence time), the micelles achieve sequential payload release specifically within the more acidic tumor microenvironment. This pH-gated, stepwise release mechanism enhances tumor-targeted delivery efficiency.¹⁷² Another strategy utilizing amino protonation is the application of poly(β-amino ester)s. A decrease in pH can promote the charge reversal and cationization of micelles, thereby enhancing their accumulation in tumor tissues (Fig. 4a).¹⁷³ Similarly, research has utilized the tertiary amine group in the hydrophobic block, 2-(diisopropylamino)ethylmethacrylate (DPA). Under acidic conditions, this group undergoes protonation, converting into a hydrophilic cation. This transformation induces the disintegration of the nanostructure, consequently facilitating drug release.¹⁷⁴ Furthermore, an elegant design leverages the pH-responsive behavior of a polycarbonate backbone incorporating ionizable tertiary amine groups. This polymer undergoes a reversible micelle-unimer transition at a critical pH corresponding to its pK_a (6.9). Specifically, an increase in the protonation degree induces self-assembly into micelles. Intriguingly, the micellization process itself induces cooperative deprotonation of proximal amine groups. This creates a broad pH plateau region, effectively amplifying minute pH fluctuations into pronounced physical state transitions.¹⁷⁵ Moreover, leveraging the pH-dependent protonation transition of an ionizable tertiary amine moiety (C6) within a membranolytic block (MB), researchers have engineered pH-responsive nanomaterials termed pTNTs. Under physiological pH conditions, the predominantly deprotonated C6 moieties confer hydrophobicity, driving the self-assembly of pTNTs into neutral nanoparticles. In this state, the stealth PEG corona effectively shields the embedded membrane-lytic activity. Upon encountering the acidic tumor microenvironment, however, a dramatic increase in C6 protonation occurs. This protonation shift triggers a morphological transition, converting the nanoparticles into cationic species and concurrently exposing the MB.¹⁷⁶ Inspired by LNPs, ionizable oligomers can self-assemble with polylactide-polyethyleneglycol (PLA-PEG) to form ionizable polymeric micelles (IPMs). Specifically, the ionizable head groups and hydrophobic segments assemble into a core–shell micellar structure, which is capable of encapsulating small interfering RNA (siRNA). Following lysosomal escape, the nucleic acid is released, enabling efficient gene delivery.¹⁷⁷

Fig. 4 Selected representative designs of polymer micelles. (a) Strategy of comprehensively optimizing Fenton reaction factors for traceable multistage augmented CDT by charge-reversal theranostics (GFL@PM). The designed PEG-PAE for the preparation of charge-reversal theranostics. (b) pH-Responsive polymer micelles composed of PCPA with pH-sensitive ketal bonds and PPM with pH-sensitive hydrazone bonds. a. Reprinted from ref. 173. Copyright © 2023, American Chemical Society. b. Reprinted from ref. 186. Copyright © 2024 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Another system involves the conjugation of a zwitterionic polypeptide (EK7) with dodecyl acrylate via a simple click reaction, resulting in the synthesis of a zwitterionic molecule (EK-D) with pH-responsiveness and biodegradability. Its pH-responsiveness is based on the changes in the ionization states of the carboxyl groups of glutamic acid and the amino groups of lysine, as well as the cleavage of mercaptopropionate bonds.¹⁷⁸ In addition, the nitrogen atom on the imidazole ring of histidine possesses a lone pair of electrons, enabling it to accept protons, and thus can be utilized for the preparation of pH-responsive micelles.^179,180 Moreover, cytosine (C) within DNA structures also exhibits pH-responsiveness. Under acidic conditions, cytosine becomes protonated to form C⁺. This protonation facilitates the binding of double-stranded DNA with a third strand via Hoogsteen hydrogen bonding, forming a stable C–G·C⁺ triplex structure. This structural transition triggers a conformational change in the delivery carrier, enabling the precise release of chemotherapeutic agents. Conversely, under neutral or alkaline conditions, the triplex dissociates, reverting to the double-stranded form. Consequently, the therapeutic payload remains stably encapsulated and is prevented from leakage.¹⁸¹

5.2. Based on pH-sensitive chemical bond micelles

Furthermore, pH-sensitive chemical bonds can also be employed to achieve such responsiveness. For instance, the DOX molecule undergoes a condensation reaction with the hydrazide groups derived from the polymer block through its carbonyl group, forming a hydrazone bond. This bond undergoes hydrolysis in an acidic environment, thereby releasing the free drug.^182,183 Furthermore, the amide bond can also serve as a pH-responsive linkage. For instance, dimethylmaleic anhydride (DMMA) can be conjugated to the polymer backbone via amide bonds. This conjugation enables the conversion of negatively charged carboxyl groups on the polymer shell into positively charged amine groups within the acidic tumor microenvironment. The resultant electrostatic repulsion thereby facilitates the dissociation of the polymeric micelle shell.¹⁸⁴ pH-Responsiveness can also be achieved by exploiting the coordination chemistry in metal–phenolic networks. Specifically, under acidic conditions, the phenolic hydroxyl groups of polyphenols undergo protonation, leading to a significant decrease in their coordination ability with metal ions, leading to the disintegration of the metal–phenolic network, resulting in the release of the encapsulated drug (Fig. 4a).¹⁸⁵ Furthermore, the acid-labile nature of acetal or ketal bonds, which undergo hydrolysis under acidic conditions, provides another strategy for pH-responsive delivery. For example, one study used a pH-sensitive diamino ketal (DAK) linker to conjugate angiopoietin-2, a peptide known for its ability to cross the blood–brain barrier (BBB), to a micellar carrier. Following receptor-mediated binding of Angiopep-2 to LRP1 on BBB endothelial cells and subsequent endocytosis, the micelles are internalized into lysosomes. Within the acidic lysosomal environment, the DAK linker is cleaved. This linker cleavage facilitates lysosomal escape, preventing micellar entrapment and degradation, and ultimately promotes BBB transcytosis, enabling the micelles to reach the brain parenchyma (Fig. 4b).¹⁸⁶

6. Metal–organic framework nanomaterials

Metal–organic frameworks (MOFs) or porous coordination polymers (PCPs), which are assembled from metal ions or clusters of metal and organic linkers through metal–ligand coordination bonds, have attracted widespread scientific attention due to their high crystallinity, excellent porosity, adjustable pore size, high modularity, and diverse functionality.¹⁸⁷ The pH responsiveness of MOFs primarily stems from the sensitivity of their metal–ligand coordination bonds to acidic and alkaline environments. Under acidic conditions, ligands such as carboxyl groups and imidazoles are easily protonated, weakening the metal–ligand interaction and causing the framework to gradually disintegrate or pores to open, thereby triggering the release of drugs or biomolecules. This mechanism enables MOFs to achieve targeted and controlled release in acidic environments such as the tumor microenvironment and intracellular lysosomes.¹⁸⁸ In addition, pH-responsiveness can also be achieved by using pH-sensitive chemical bonds based on MOFs.¹⁸⁹ Various MOFs can be flexibly prepared for drug delivery through different methods, including hydro/solvothermal synthesis, microwave and ultrasonic-assisted synthesis, microemulsion synthesis, mechanochemistry synthesis, continuous flow synthesis, electrochemical synthesis, diffusion synthesis, spray-drying synthesis, solvent evaporation and ionothermal synthesis.¹⁹⁰ The pH-responsive MOFs have developed several families, such as ZIF-n family, MIL-n family, UiO-n family, HKUST-n family, DUT-n family and so on.^188,191 In addition to the traditional MOF family, many novel MOF materials have been developed, such as cyclodextrin-based MOF materials.¹⁹² The organic part of MOFs is often aromatic compounds with strong encapsulation capacity for fat-soluble drugs. Therefore, MOF-based drug delivery systems have broad prospects in substrate development, functional modification and application. The following introduces some newly developed pH-responsive MOF-based tumor drug delivery systems (Table 4).

Table 4 Selected examples of pH-responsive MOF nanomaterials

Name	Responsive/targeting molecules/groups	The loaded drug	Experimental model	Ref.
DOX@Fc-MOFs-Mann	Ferrocene nano-MOFs, Yb^3+, mannose	DOX	HL7702, HepG2	193
MOF@SMON/DOX-HA	Cu^2+, 3-amino-1,2,4-triazole, tetra-sulfide organosilicon, hyaluronic acid	DOX	HepG2	194
MIL-101(Fe)	2-Aminoterephthalic acid, Fe^3+	CPT	HeLa, SH-SY5Y, 3T3	195
ZIF@SLN#L	2-Methylimidazole, Zn^2+, oleylamine-3-(bromomethyl)-4-methyl-2,5-furandione – PEG	Apilimod	Panc02, C57BL/6J bearing Panc02, 4T1	196
HA-MAF@DOX	Cu^2+, Zn^2+, 3-amino-1,2,4-triazole, hyaluronic acid	DOX	HepG2, HL7702	197
CAB/Ce6@ZIF-8@PEG-FA	2-Methylimidazole, Zn^2+, PEG, FA	Cabozantinib, Ce6	HepG2, Huh7, Hep3B, nude bearing HepG2	198
DOX@UiO-67-SiBI@CDP	Zr^4+, biphenyl-4,4′-dicarboxylic acid, benzimidazole, cyclodextrin polymer	DOX	HeLa, Balb/c nude bearing HeLa	199
ZIF-8/DMPP	2-Methylimidazole, Zn^2+, polydopamine, Mn^2+, PEG	DOX	PC-3, HUVEC, Balb/c nude bearing PC-3	200
SMnFeCGH	Mn^2+, Fe^2+, hyaluronic acid, syringic acid	Cis-platin, Au NPs	B16F10, mice bearing B16F10	201
Fe3O4@C@MIL-88-DOX-FC	Fe3O4, maltose, terephthalic acid, Fe^3+, folic acid, chitosan	DOX	MCF-7, MCF-10A	202
ZIF@GOx@PBNPs	2-Methylimidazole, Zn^2+	Glucose oxidase, Prussian blue NPs	SK-MEL28, Balb/c nude bearing SK-MEL28	203
HAMD	Ti^4+, Fe^3+, 2-aminoterephthalic acid, dopamine-grafted sodium alginate, hydroxypropyl chitosan, 2-formylphenylboronic acid	DOX, Au NPs	4T1, Balb/c nude bearing 4T1	204
ZDOS NPs	2-Methylimidazole, Zn^2+, DL-1,4-dithiothreitol, TEOS, CTAB, TESPD	DOX	HeLa	205
CPP44-PEG@ORI@IRMOF-1	Zn^2+, terephthalic acid, PEG, cell-penetrating peptides	Oridonin	HepG2, L02, Balb/c nude bearing HepG2	206
DSF@Z-NPs	Silk fibroin, 2-methylimidazole, Zn^2+	DOX	MCF-7, Balb/c nude bearing MCF-7	207

---

Ferrocene (Fe(C₅H₅)₂, Fc) is a stable, reversible redox metallo-organic unit. The MOF constructed using Fc-(COOH)₂ and the rare earth metal ion Yb³⁺ exhibits pH-responsiveness. DOX is loaded into the structure via π–π stacking of Fc. Mannose is attached to the surface via an amide bond, enhancing its liver-targeting ability. Furthermore, the Fenton reaction of Fe²⁺ induces oxidative cell death in cancer cells.¹⁹³ MOFs composed of proton-accepting 3-amino-1,2,4-triazole (3-AT) and Cu²⁺ or Zn²⁺ exhibit pH-responsive properties. 3-AT's inhibitory effect on catalase can enhance the Fenton reaction activity of Cu²⁺. Sulfur-containing mesoporous silica and hyaluronic acid are coated on the surface to load DOX, consuming GSH and enhancing liver targeting.^194,197 MILs are typically prepared using Fe³⁺ and terephthalic acid or benzene trimeric acid. A solvothermal procedure yields a family of MILs, including MIL-101 and MIL-88. MOFs are linked to magnetic Fe₃O₄@maltose to achieve dual pH/magnetic targeting. Surface coating with folic acid or chitosan further enhances their biocompatibility.²⁰² The organic part was aminated to obtain 2-aminoterephthalic acid, and Camptothecin (CPT) was loaded into it via a covalent ester bond to obtain pH responsiveness.²⁰⁵ ZIF-14, formed by 2-methylimidazole and Zn²⁺, exhibits excellent drug-loading properties and was used to load Apilimod. pH-Sensitive 3-(bromomethyl)-4-methyl-2,5-furandione was linked to hydrophobic oleylamine and hydrophilic PEG to form the amphiphilic and zwitterionic compound L. L and S100 were then microfluidically coated to form an SLN encapsulating ZIF-14, achieving pH-responsive drug release.²⁰⁶ 2-Methylimidazole and Zn²⁺ can also be used to synthesize another structure, ZIF-8, which is used to encapsulate the tyrosine kinase inhibitor Cabozantinib and the photosensitizer chlorin e6. Surface coating with PEG-FA chains can also enhance its liver targeting ability.¹⁹⁸ ZIF-8 can also be loaded with DOX and coated with polydopamine (PDA) and Mn²⁺-mediated PEG. ZIF-8 and PDA achieve dual pH/photothermal responses.²⁰⁰ DOX is loaded inside ZIF-8, and a thin layer of silica is deposited using DL-1,4-dithiothreitol as a medium to achieve pH/GSH dual response.²⁰⁵ In addition, natural glucose oxidase and Prussian blue NPs encapsulated with ZIF-8 can achieve starvation therapy and ROS killing therapy for tumors.²⁰³ ZIF-8 can also serve as a coating material. DOX is loaded onto silk fibroin to form nanoparticles. These nanoparticles are coated with a pH-responsive ZIF-8 coating to achieve acid targeting of tumors.²⁰⁷ Zr⁴⁺ and biphenyl-4,4′-dicarboxylic acid can form a UiO-67-structured MOF and load DOX into it. Silane groups can then be used to modify the MOF surface with proton-accepting benzimidazole, which can then be used to achieve pH response through host–guest interactions with cyclodextrin polymers.¹⁹⁹ A simple one-pot biomimetic mineralization method using Mn²⁺, Fe²⁺, hyaluronic acid, and syringic acid can yield a series of pH-responsive Mn/Fe-based MOFs. Encapsulating cisplatin prodrugs with pH-sensitive ester bonds within these MOFs enhances their acid targeting ability. Furthermore, loading glucose oxidase-like Au NPs within these MOFs can achieve tumor starvation therapy.²⁰¹ HAMD is a hydrogel platform formed by dopamine-grafted sodium alginate, hydroxypropyl chitosan, and 2-formylphenylboronic acid. The hydrogel contains pH-responsive imides and borate bonds for loading MOF nanosheets and DOX. The MOF nanosheets are special Ti/Fe bimetallic MOF tetragonal nanosheets formed by Ti⁴⁺, Fe³⁺ and 2-aminoterephthalic acid and bound to Au NPs. This platform exhibits sonodynamic–chemodynamic–starvation–chemotherapy properties.²⁰⁴ Zn²⁺ and terephthalic acid can form a unique pH-responsive isoreticular MOF for oridonin encapsulation. Cell-penetrating peptides and PEG are coated on its surface to enhance its targeting ability and circulation time.²⁰⁶

7. Protein and peptides

Peptides are chains of amino acids linked by peptide bonds. Proteins are large molecules composed of peptides and have many different functions, such as regulating biological processes at the cellular, systemic, and organismal levels, transporting molecules, catalyzing metabolic reactions, responding to stimuli, and replicating DNA.²⁰⁸ Peptides generally consist of fewer than 50 amino acids, while peptides with complex, higher-order structures are called proteins. These two types differ significantly in their properties and each has its own unique characteristics in the field of nano-DDSs. In the field of drug delivery, proteins are classified primarily based on their solubility and can be broadly divided into six categories: albumins, globulins, prolamines, glutelins, protamines, and histones.²⁰⁹ Albumin (derived from serum or eggs) is the most studied protein for the preparation of protein-based drug delivery nanostructures. Modification of albumin can enhance its ability to target specific cells and tissues.²¹⁰ The first drug to be marketed using a protein-based delivery system was Abraxane® (albumin-bound paclitaxel), which enhanced the solubility and active targeting capabilities of paclitaxel.²¹¹ Globulins are most widely used in the field of drug delivery as antibody–drug conjugates (ADCs), which are also becoming increasingly mature in the anti-tumor field.²¹² Glutelins are rich in glutamic acid, which can carry a charge, and thus have pH-responsive properties. They can self-assemble under physiological conditions and dissolve under low pH conditions. They have good application prospects in acid-targeted drug delivery systems.²¹³ Protamines and histones are similar in properties to glutelins and have pH response. As a drug carrier, proteins have excellent biocompatibility and strong functional editability, and have good cycle time and targeting ability.²¹⁴ Unlike traditional proteins that rely on lipid solubility to load drugs, a novel class of protein nanocages is being studied and developed by academia as nano-DDSs, E2 protein nanocages and P22 protein nanocages.²¹⁵ In terms of environmentally responsive drug delivery, peptides are more widely used than proteins. The side chains of peptides contain a variety of active functional groups, including carboxylic acids, hydroxyl groups, amino groups, and thiol groups, so they can be easily modified with a variety of chemical modifications. This versatility makes them extremely valuable in stimulus response systems.²¹⁶ Among them, pH-responsive peptides are widely used, including peptides similar to amphiphilic block copolymers that achieve pH-responsive self-assembly, and the pH-low insertion peptide (pHLIP) family that change conformation according to the environmental pH.²¹⁷ The following introduces some novel pH-responsive protein and peptide nanomaterials (Table 5).

Table 5 Selected examples of pH-responsive protein and peptide nanomaterials

Name	Responsive/targeting molecules/groups/sequence	The loaded drug	Experimental model	Ref.
FER-8	FEFEFRFK	PTX	HepG2, H22, Balb/c bearing H22	218
PL/Pep1	Aspartic acid	PTX, lapatinib	HUVEC, MCF-7	219
H15-ZR-ELP	Histidine, [VPGHGVPGHGVPGFGVPGHGVPGVG]5	NA	NA	220
KD-1	KKKHHHH-Acp-LLLLLLLLGSPDRGD (Acp stand for 6-aminocaproic acid)	siRNA (cyclin B1 gene)	HeLa	221
K^DH^D-C^DC^D-RGD	K^DK^DK^DC^DH^DW^DL^DL^DL^DL^DH^DC^D-C6-H^DH^DH^DRGD (C6 stand for 6-aminocaproic acid)	siRNA (cyclin B1 gene)	HeLa, Balb/c bearing HeLa	222
RFH	RFRHRHRFR	Luciferase mRNA	HeLa, 293T	223
KD peptide	AcNH-KGSFSIQYTYHVD-CONH2	C4 scFv nanobodies	SH-SY5Y	224
E2-GALA	WEAALAEALAEALAEHLAEALAEALEALAA	NA	NA	225
Ce6-Ns	Human serum albumin, poly-L-lysine, PEG	Ce6, protoporphyrin IX, Verteporfin	HeLa, B16, MCF-7, Balb/c bearing MCF-7	226
PRGD/NLG919	PWGGGRGD (P stand for porphyrin), GWPWG (P stand for porphyrin)	NLG919 (indoleamine 2,3-dioxygenase inhibitor)	HeLa, MCF-7	227
NP-Pep	(nap-RAGLQFPVGRLLRRLLRRLLR)(HCO3)n (nap stand for naphthalene imide)	NA	MCF-7, L929, Balb/c nude bearing MCF-7	228
LMMP NPs	BP-FFVLK(OEG-CREKA)-His6 (BP stand for bispyrene, OEG stand for oligo(ethylene glycol))	NA	Nude bearing MDA-MB-231	229

---

An octapeptide (FEFEFRFK) can self-assemble into a pH-responsive hydrogel. The abundance of Phe residues can effectively load the lipid-soluble anti-tumor drug PTX. Its rich Lys, Arg, and Glu residues are ionizable, enabling gelation under neutral conditions and dissolution at low pH, achieving tumor-specific acid targeting.²¹⁸ An Asp-rich peptide self-assembles into spherical NPs at pH 7.4 and can effectively encapsulate PTX and lapatinib. At pH 5, it transforms into nanofibers to release the loaded drugs.²¹⁹ ELP is a tunable protein composed of n pentapeptide repeats of the amino acid sequence (VPGXG)_n, where the guest residue X can be any amino acid except Pro. ELP is then linked to repeating His residues to create H₁₅-Z_R-ELP, which forms vesicles under neutral conditions and ruptures under acidic conditions. H₁₅-Z_R-ELP can be obtained through plasmid expression.²²⁰ A pH-responsive peptide-based nanocarrier (KKKHHHH-Acp-LLLLLLLLGSPDRGD, Acp stands for 6-aminocaproic acid) efficiently and controllably releases nucleic acid drugs through dynamic assembly within cancer cells. Tumor suppression is achieved through the inhibition of the Cyclin B1 gene.²²¹ When peptides composed of L-amino acids self-assemble into nanoparticles for the in vivo delivery of nucleic acid drugs, their circulation time is short. Therefore, the development of chiral peptides composed of D-amino acids for delivery significantly extends their circulation time. Furthermore, the addition of the classic cell-targeting tripeptide sequence RGD enhances the targeting of this delivery system.²²² Researchers designed a unique peptide with alternating polarity. This design allows the Phe and His residues in adjacent peptide molecules to face each other in parallel and antiparallel arrangements, promoting their self-assembly into ordered nanostructures through aromatic ring interactions, thereby enhancing the stability of the complex. The Phe residues impart robust nucleic acid delivery capabilities to the system, while the His residues contribute to pH-responsive properties.²²³ Using microfluidics technology, KD peptide (AcNH-KGSFSIQYTYHVD-CONH₂) and oil can be prepared into pH-tunable peptide microcapsules. These microcapsules can be triggered to disintegrate in response to slight changes in pH, thereby enabling controlled release of the encapsulated drug under physiological conditions.²²⁴ The E2 protein is the core domain (dihydrolipoamide acetyltransferase) of the pyruvate dehydrogenase (PDH) complex from Geobacillus stearothermophilus. A classical pH-responsive α helix peptide GALA (WEAALAEALAEHLAEALAEAEALEALAA) replaces the α helix of the C-terminus of the E2 protein, enabling the release of pH-triggered drugs from the protein cage.²²⁵ Human serum albumin and poly-Lys can be self-assembled into pH-responsive protein nanospheres, and the nanospheres are surface-modified with PEG. These nanospheres are loaded with photosensitizers (Ce6, protoporphyrin IX, and Verteporfin), which are released in response to changes in pH, GSH, and proteases, achieving photodynamic therapy for tumor ablation.²²⁶ Two peptides modified with porphyrins were co-assembled with the indoleamine 2,3-dioxygenase (IDO) inhibitor NLG919 to form a pH-responsive nanosheet called PRGD/NLG919. The RGD sequence enhances its targeting, while the porphyrins contribute to the photodynamic therapy effect.²²⁷ Many peptides have been identified as high-potential drugs for the treatment of various diseases, among which cationic antimicrobial peptides are able to kill cancer cells. A cationic antimicrobial peptide (RAGLQFPVGRLLRRRRRRRRRR) modified by naphthalene imide disrupts the α-helical structure of the original peptide under the action of bicarbonate and self-assembles to form nanoparticles. Under acidic conditions, it releases carbon dioxide, produces free guanidine groups and leads to α-helix recovery. This allows it to recognize the overexpression of gangliosides on tumor cell membranes and transport them across the membrane into the cell, interacting with DNA within the nucleus to induce tumor cell death.²²⁸ A unique peptide (BP-FFVLK(OEG-CREKA)-His₆ (BP stands for bispyrene, OEG stands for oligo(ethylene glycol)) self-assembles into nanoparticles under neutral conditions. In the acidic environment of tumors, these peptides transform into laminin-like nanofibers that attach to microthrombi, further capturing bloodstream cells and rapidly occluding tumor blood vessels. This biomimetic strategy can specifically and efficiently block tumor blood vessels and inhibit tumor growth, making it a promising vascular-based tumor therapy.²²⁹

8. Design of pH-responsive nanoparticles for different tumors

We conducted a meta-analysis of animal model applications from the five categories of nanomaterials listed above, categorizing them by the animal tumor models used (Table 6). The animal models summarized, ranked by number of cases, are: breast cancer, melanoma, liver cancer, colon cancer, cervical cancer, pancreatic cancer, glioma, and other cancers (Ehrlich ascites carcinoma, gastric cancer, renal cancer, lymphoma, and prostate cancer). Breast cancer is the most commonly used model in cancer research, and a wide variety of nanoparticles have been developed for this tumor, including MSNs, liposomes, polymeric micelles, MOF nanoparticles, and protein/peptide nanoparticles. Applicable drug delivery strategies range from classical drugs (DOX, PTX) to proteins/nucleic acids. This model is effective for most pH-responsive nanoparticles. Further targeting of breast cancer phenotypes can be achieved by using ligands or antibodies (folate, HER2, and estrogen receptors). The strategies for constructing delivery vehicles for melanoma are similar to those for breast cancer, and the tumor is responsive to a variety of delivery vehicles. Regarding drugs, melanoma treatment is increasingly favoring protein/nucleic acid drugs over classical chemotherapeutics. In terms of targeting strategies, RGD peptides are used to target integrins. Liver cancer responds to pH-responsive nanoparticles from all five carrier types. Their targeting strategy utilizes sugars and fatty acids as ligands. Because liver cancer cells are redox-active, dual-responsive strategies often incorporate redox responsiveness in addition to pH responsiveness. In situ colon cancer is located in the intestine, so nanoparticles are often designed using polymers or other materials as base materials, then coated with polymers to adapt to the digestive tract environment. Folic acid is often used as a target in targeted strategies. There are not many restrictions on nano-delivery carriers for cervical cancer, and various materials can be used as substrates for further design. Targeting strategies also often use folic acid as a target. Pancreatic cancer is known as the king of cancers. Its early insidiousness, low vascular perfusion, and high fibrosis contribute to its extremely high mortality rate. Given its high fibrosis, the choice of substrate tends to be solid particles or inorganic particles, such as polymer micelles and MSNs. Given its low vascular density, nanoparticles are preferably small in size. Its targeting strategy is to use peptides with penetrating function (RGD) to promote their penetration into dense matrices. The treatment of glioma requires nanoparticles to cross the blood–brain barrier. Therefore, polymers or polypeptides are mainly used in the design of nanoparticles, and penetrating peptides or targeting peptides are especially needed to enhance their ability to cross the blood–brain barrier.

Table 6 In vivo models of different tumors

Responsive/targeting molecules/groups/sequence		The loaded drug	Ref.
Breast cancer
Polyethyleneimine, anisamide (sigma receptor)		DOX	98
PDA, boron–oxygen bond, gold-nanorod		DOX, Bortezomib	78
4-Adamantane carboxylate benzaldehyde, WS2 QDs, tLyp-1 peptide (homing/penetrating), cyclodextrin and adamantane		DOX	112
Imidazole([D]-H6L9-Cys)		Antagomir-10, PTX	134
Tertiary amines		Fluc mRNA, tdTomato Cre mRNA	136
Poly(ethylene)glycol-poly(β-amino esters)		Fe^2+, glucose oxidase, LET-1052	173
Hydrazone bond		DOX	182
PEO-b-PC7A, PEO-b-PDBA		αPD-L1, DOX	171
2-Methylimidazole, Zn^2+, oleylamine-3-(bromomethyl)-4-methyl-2,5-furandione-PEG		Apilimod	196
Ti^4+, Fe^3+, 2-aminoterephthalic acid, dopamine-grafted sodium alginate, hydroxypropyl chitosan, 2-formylphenylboronic acid		DOX, Au NPs	204
Polyanionic shielding domain (ehG)n, cells-penetrating domain (octaarginine)		siPLK-1 and DTX	171
Silk fibroin, 2-methylimidazole, Zn^2+		DOX	207
Human serum albumin, poly-L-lysine, PEG		Ce6, protoporphyrin IX, Verteporfin	226
(nap-RAGLQFPVGRLLRRLLRRLLR)(HCO3)n (nap stand for naphthalene imide)		NA	228
BP-FFVLK(OEG-CREKA)-His6		NA	229
Methacrylic acid, N-isopropylacrylamide (for thermal), anti-HER2 scFv		DOX	85
Methacrylic acid, N-isopropylacrylamide (for temperature)		Evodiamine, berberine	86
Glycol chitosan		DOX	130
Melanoma
HtB peptide (containing His, Arg, Asn), Cu^2+ (for chemodynamic therapy)		Cis-platin, chloroquine	111
Imidazole, pH-responsive cells-penetrating peptides(TH), c(RGDfK)		PTX	132
Tertiary amines		Fluc mRNA, tdTomato Cre mRNA	136
PEO-b-PC7A, PEO-b-PDBA		αPD-L1, DOX	171
Cyclic tertiary amine of 7-membered ring		Stimulator of interferon genes	175
Tertiary amine		NA	186
poly(L-histidine)		Trp2(melanoma antigen peptide)	180
Polyethylene glycol-polylysine-dimethylmaleic anhydride		CpG(adjuvant), mannose	184
Mn^2+, Fe^2+, hyaluronic acid, syringic acid		Cis-platin, Au NPs	201
2-Methylimidazole, Zn^2+		Glucose oxidase, Prussian blue NPs	203
Liver cancer
Spiropyran, flourinated silane		DOX, curcumin	114
Zwitterionic lipid 2-(4-((1,5-bis(octadecyloxy)-1,5-dioxopentan-2-yl)carbamoyl)pyridin-1-ium-1-yl)acetate		Rhodamine	128
2-Methylimidazole, Zn^2+, PEG, FA		Cabozantinib, Ce6	198
Zn^2+, terephthalic acid, PEG, cell-penetrating peptides		Oridonin	206
Galactose modified poly(allylamine hydrochloride)-citraconic anhydride, HIV TAT peptide		DOX	93
Poly(ε-lysine)-cyclodextrin, 2-(benzo[d]thiazol-2-yl)phenyl-4-aminobenzoate (pH, H2O2), Fe3O4 (magnet)		Scutellarin	80
FEFEFRFK		PTX	218
Poly(allylamine hydrochloride)-citraconic anhydride, HIV-1 TAT peptide, galactose-modified trimethyl chitosan-cysteine (loading siRNA, GSH)		DOX, siVEGF	93
Succinoyl tetraethylene pentamine, lipoamino fatty acid		VEGFR-2 siRNA	141
Colon cancer
Ethylenediamine-based polycarboxybetaine zwitterion [PGlu(DET-Car)]		siPLK-1	139
PEG-b-PC7A, PEG-b-PDPA, PEG-b-PDBA, PEG-b-PD5A		Cyclic dinucleotide	172
2-(Diisopropylamino)ethyl methacrylate		ASA404, HCFU	174
Tertiary amine		NA	176
Hydrazone bond		DOX	182
PEO-b-PC7A, PEO-b-PDBA		αPD-L1, DOX	171
Cervical cancer
N-(2-Hydroxypropyl)methacrylamide, 3-aminopropylmethacryl amide and 2,3-dimethylmaleic anhydride co-polymer		Docetaxel (DTX)	95
Zr^4+, biphenyl-4,4′-dicarboxylic acid, benzimidazole, cyclodextrin polymer		DOX	199
K^DK^DK^DC^DH^DW^DL^DL^DL^DL^DH^DC^D-C6-H^DH^DH^DRGD (C6 stand for 6-aminocaproic acid)		siRNA (cyclin B1 gene)	222
Zwitterionic peptide EK7, mercaptopropionate bonds		DOX	178
Poly(L-histidine)		PTX	179
Cyclic tertiary amine of 7-membered ring		Stimulator of interferon genes	175
Pancreatic cancer
Tertiary amine		NA	176
2-Methylimidazole, Zn^2+, oleylamine-3-(bromomethyl)-4-methyl-2,5-furandione-PEG		Apilimod	196
Polydopamine (pH, enzymes, redox), folic acid		DOX, glucose oxidase	79
Glioma
Imidazole(H7), guanidine((R2)2)		DOX	131
Tannic acid/Mn^2+, macrophage membranes, chlorin e6 (sonosensitizer)		Chlorin e6 (Ce6)	185
PLGA-diamino ketal-PEG-Angiopep-2, PLGA-PEG-p-aminophenyl-α-D-mannopyranoside		Temozolomide, resiquimod	186
Other cancer
Chitosan, folic acid	Anastrozole	Balb/c bearing EAC	82
Zeolitic imidazolate framework-8	5-Fu, methotrexate	Albino mice♀ bearing EAC	103
Polydopamine (pH, photothermal)	Crytotanshinone	Balb/c♂ nude bearing AGS	102
Tertiary amines, disulfide bond, vitamin E (hydrophobic scaffold)	sFlt-1 pDNA	Balb/c AJcl-nu/nu mice bearing OS-RC-2	135
β-Glucans	Ovalbumin (OVA) mRNA	C57BL/6 bearing E.G7-OVA	140
pH-Responsive triplex DNA	Dox, ALK-specific siRNA	Mice bearing ALCL	181
2-Methylimidazole, Zn^2+, polydopamine, Mn^2+, PEG	DOX	Balb/c nude bearing PC-3	200

---

9. Conclusion

This review systematically summarizes the design strategies and research progress of pH-responsive nanocarriers in cancer therapy. First, from a mechanistic perspective, we divide pH-responsive behavior into two major categories: (1) drug delivery systems based on the cleavage of pH-sensitive chemical bonds, such as hydrazone bonds, Schiff base bonds, and ester bonds. These chemical bonds can be selectively cleaved under the acidic conditions of the tumor microenvironment, thereby achieving precise drug release and (2) systems based on the protonation of ionizable functional groups, such as materials containing carboxyl, imidazole, or amine groups. These undergo protonation or deprotonation reactions in an acidic environment, causing changes in the carrier structure or surface charge, thereby triggering drug release or enhancing cellular endocytosis efficiency. On this basis, we further sorted out and compared the five major categories of pH-responsive nanomaterials currently used for cancer therapy: inorganic nanoparticles (represented by MSNs), lipid-based nanoparticles (including traditional liposomes and emerging LNPs), polymer micelle nanoparticles, MOF nanoparticles, and protein/peptide nanoparticles. Each of these materials offers advantages: MSNs offer exceptional surface area and structural tunability; liposomes and LNPs exhibit high biocompatibility and clinical-grade preparation; polymeric micelles excel in drug encapsulation and controlled release; MOFs combine high drug loading capacity with designability; and protein and peptide carriers demonstrate unique potential due to their inherent bioactivity and target recognition abilities. In summary, we strongly recommend prioritizing MSNs, liposomes, LNPs, polymeric micelles, MOFs, and protein/peptide materials as core carriers in the future design of pH-responsive nanomedicines. By rationally combining pH-responsive mechanisms with material properties, precise response to the tumor microenvironment and controlled drug release can be achieved, providing a solid material foundation and design strategy for improving anti-tumor efficacy and reducing systemic toxicity.

pH-Responsive nanomaterials are intelligent systems because they trigger drug release or function only under specific acidic conditions. Our in-depth discussion of the vast array of pH-responsive nanoparticles mentioned above revealed that most examples of these nanomaterials initially trigger their release within a low-acid range. The further the nanomaterial enters the cancer cell's endosome, the more it responds due to a further decrease in pH. In many cases, researchers have not only tested their designed nanoparticles on a single tumor type, but also on multiple tumor types. They have found that their nanoparticles have an inhibitory effect on a wide range of solid tumors. Tumors can exhibit the following characteristics: (1) different pH values within different tumors; (2) different pH values at different stages of progression within the same tumor; (3) different pH values within the same tumor at the same stage; and (4) different drugs have different pK_a values and (5) different EPR effects across different tumors. Current pH-responsive nanomaterials do not precisely respond to pH values. Nanoparticles made from various types of materials vary in hardness, density, and size. Existing research indicates that permeability and interstitial pressure vary across different tumors, necessitating customized materials, sizes, and response strategies. Currently, the systematic and precise design of nanoparticles for treating various tumors has not yet been achieved, and our understanding of various tumor types is limited. Many studies overly conservatively select subcutaneous tumors rather than orthotopic or induced tumors as animal models. Subcutaneous tumors may reduce the heterogeneity of different tumors, resulting in the same delivery vector being effective for various tumors. However, various orthotopic tumors face different microenvironments, different substrate barriers, and different delivery methods. Therefore, we advocate for more systematic research on pH-responsive and even other environmentally responsive nanoparticles.

Smart drug delivery systems have been developed based on these two mechanisms. For instance, MSNs can be engineered with pH-sensitive molecular gates to regulate drug release; liposomes can be surface-functionalized with pH-responsive moieties; and LNPs can incorporate ionizable lipids into their formulation, endowing them with pH-triggered release and endosomal escape capabilities. The groups in the polymer micelles receive protons to ionize and depolymerize, or the pH-sensitive chemical bonds are broken to trigger functional effects. To achieve the next breakthrough in pH-responsive nanomedicine, it will be essential to move beyond these two traditional, passive trigger-based mechanisms. The initial discussion in this review on the concept of pH and its significance in life aims to inspire new hypotheses and research directions. Current pH-responsive strategies rely on the passive diffusion of nanoparticles into acidic environments, where pH triggers drug release. However, the proton gradient itself represents a biologically relevant form of stored energy. To date, no reported studies have exploited this proton-driven energy potential to actively enhance nanoparticle delivery. Harnessing this energy source could redefine the concept of active targeting, leading to a truly intelligent and energy-coupled targeting strategy. The above discussion on evolution aims to arouse the academic community's awareness of the deep intersection between materials science and life science. The understanding of evolution can improve the understanding of life science concepts such as tumors and provide inspiration for material and functional design.

Author contributions

Conceptualization: Fangzhou Li; methodology: Jianjun Jiang, Mingmei Li, and Pengli Gao; investigation: Jianjun Jiang, Zengkai Zhao, and Limin Jin; supervision: Fangzhou Li; writing – original draft: Jianjun Jiang and Zengkai Zhao; writing – review and editing: Jianjun Jiang, Limin Jin, and Fangzhou Li.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

This work is supported by the National Key Research and Development Program of China (2021YFA1201000), the CAMS Innovation Fund for Medical Sciences (2024-I2M-3-007), and the Young Elite Scientists Sponsorship Program by CAST (2024-2026QNRC001).

References

1. I. Soerjomataram and F. Bray, Nat. Rev. Clin. Oncol., 2021, 18, 663–672.
2. S. Das, H. Mazumdar, K. R. Khondakar and A. Kaushik, BMEMat, 2024, 3, e12117.
3. C. Seale, Soc. Sci. Med., 2000, 51, 917–930.
4. K. Christensen, G. Doblhammer, R. Rau and J. W. Vaupel, Lancet, 2009, 374, 1196–1208.
5. R. Lalji, L. Koh, A. Francis, R. Khalid, C. Guha, D. W. Johnson and G. Wong, Cochrane Database Syst. Rev., 2024, 10, CD014688.
6. N. Namazi, S. S. Moghaddam, S. Esmaeili, M. Peimani, Y. S. Tehrani, F. Bandarian, P. Shobeiri, E. Nasli-Esfahani, M. Malekpour, N. Rezaei, B. Arjmand, B. Larijani and F. Farzadfar, BMC Public Health, 2024, 24, 98.
7. M. Miravitlles, C. Murio, T. Guerrero and R. Gisbert, Chest, 2002, 121, 1449–1455.
8. K. C. Koskinas, E. M. Van Craenenbroeck, C. Antoniades, M. Blüher, T. M. Gorter, H. Hanssen, N. Marx, T. A. McDonagh, G. Mingrone, A. Rosengren and E. B. Prescott, Eur. Heart J., 2024, 45, 4063–4098.
9. C. E. Weeden, W. Hill, E. L. Lim, E. Grönroos and C. Swanton, Cell, 2023, 186, 1541–1563.
10. J. M. Pitt, A. Marabelle, A. Eggermont, J.-C. Soria, G. Kroemer and L. Zitvogel, Ann. Oncol., 2016, 27, 1482–1492.
11. Y. Xiao and D. Yu, Pharmacol. Ther., 2021, 221, 107753.
12. M. Jin and W. Jin, Signal Transduction Targeted Ther., 2020, 5, 166.
13. X. Jing, F. Yang, C. Shao, K. Wei, M. Xie, H. Shen and Y. Shu, Mol. Cancer, 2019, 18, 157.
14. R. Yang, L. Chen, Y. Wang, L. Zhang, X. Zheng, Y. Yang and Y. Zhu, Front. Immunol., 2023, 14, 1237361.
15. B. Guo, F. Yang, L. Zhang, Q. Zhao, W. Wang, L. Yin, D. Chen, M. Wang, S. Han, H. Xiao and N. Xing, Adv. Mater., 2023, 35, 2212267.
16. F. Gong, N. Yang, X. Wang, Q. Zhao, Q. Chen, Z. Liu and L. Cheng, Nano Today, 2020, 32, 100851.
17. E. Boedtkjer and S. F. Pedersen, Annu. Rev. Physiol., 2020, 82, 103–126.
18. M. Ashrafizadeh, A. Zarrabi, A. Bigham, A. Taheriazam, Y. Saghari, S. Mirzaei, M. Hashemi, K. Hushmandi, H. Karimi-Maleh, E. N. Zare, E. Sharifi, Y. N. Ertas, N. Rabiee, G. Sethi and M. Shen, Med. Res. Rev., 2023, 43, 2115–2176.
19. Q. Lu, D. Kou, S. Lou, M. Ashrafizadeh, A. R. Aref, I. Canadas, Y. Tian, X. Niu, Y. Wang, P. Torabian, L. Wang, G. Sethi, V. Tergaonkar, F. Tay, Z. Yuan and P. Han, J. Hematol. Oncol., 2024, 17, 16.
20. H. Ding, P. Tan, S. Fu, X. Tian, H. Zhang, X. Ma, Z. Gu and K. Luo, J. Controlled Release, 2022, 348, 206–238.
21. J. Liu, Y. Huang, A. Kumar, A. Tan, S. Jin, A. Mozhi and X. Liang, Biotechnol. Adv., 2014, 32, 693–710.
22. V. P. Torchilin, Nat. Rev. Drug Discovery, 2014, 13, 813–827.
23. Z. Wang, X. Deng, J. Ding, W. Zhou, X. Zheng and G. Tang, Int. J. Pharm., 2018, 535, 253–260.
24. S. Bazban-Shotorbani, M. M. Hasani-Sadrabadi, A. Karkhaneh, V. Serpooshan, K. I. Jacob, A. Moshaverinia and M. Mahmoudi, J. Controlled Release, 2017, 253, 46–63.
25. S. Bhattacharya, B. G. Prajapati and S. Singh, Crit. Rev. Oncol. Hematol., 2023, 185, 103961.
26. M. Kanamala, W. R. Wilson, M. Yang, B. D. Palmer and Z. Wu, Biomaterials, 2016, 85, 152–167.
27. A. Ranneh, H. Takemoto, S. Sakuma, A. Awaad, T. Nomoto, Y. Mochida, M. Matsui, K. Tomoda, M. Naito and N. Nishiyama, Angew. Chem., Int. Ed., 2018, 57, 5057–5061.
28. X. Shen, A. Dirisala, M. Toyoda, Y. Xiao, H. Guo, Y. Honda, T. Nomoto, H. Takemoto, Y. Miura and N. Nishiyama, J. Controlled Release, 2023, 360, 928–939.
29. Q. Li, W. Yao, X. Yu, B. Zhang, J. Dong and Y. Jin, Colloids Surf., B, 2017, 158, 709–716.
30. H. Tian, Z. Guo, L. Lin, Z. Jiao, J. Chen, S. Gao, X. Zhu and X. Chen, J. Controlled Release, 2014, 174, 117–125.
31. Y. Xu, Y. Zi, J. Lei, X. Mo, Z. Shao, Y. Wu, Y. Tian, D. Li and C. Mu, Carbohydr. Polym., 2020, 233, 115858.
32. F. Wang, Y. Wang, S. Dou, M. Xiong, T. Sun and J. Wang, ACS Nano, 2011, 5, 3679–3692.
33. Y. Shi, X. Pan, S. Xu, H. Zhu, B. Zhao, Z. Sun, R. Dong, N. Li, X. Hou and X. Yang, Eur. J. Med. Chem., 2023, 260, 115715.
34. F. Seidi, Y. Zhong, H. Xiao, Y. Jin and D. Crespy, Chem. Soc. Rev., 2022, 51, 6652–6703.
35. T. Mena-Barragán, A. Narita, D. Matias, G. Tiscornia, E. Nanba, K. Ohno, Y. Suzuki, K. Higaki, J. M. G. Fernández and C. Ortiz Mellet, Angew. Chem., Int. Ed., 2015, 54, 11696–11700.
36. L. Li, Y. Xu, I. Milligan, L. Fu, E. A. Franckowiak and W. Du, Angew. Chem., Int. Ed., 2013, 52, 13699–13702.
37. X. Pang, Y. Jiang, Q. Xiao, A. W. Leung, H. Hua and C. Xu, J. Controlled Release, 2016, 222, 116–129.
38. X. Su and I. Aprahamian, Chem. Soc. Rev., 2014, 43, 1963–1981.
39. J. Kalia and R. T. Raines, Angew. Chem., Int. Ed., 2008, 47, 7523–7526.
40. S. J. Sonawane, R. S. Kalhapure and T. Govender, Eur. J. Pharm. Sci., 2017, 99, 45–65.
41. Z. Cao, W. Li, R. Liu, X. Li, H. Li, L. Liu, Y. Chen, C. Lv and Y. Liu, Biomed. Pharmacother., 2019, 118, 109340.
42. E. H. Cordes and W. P. Jencks, J. Am. Chem. Soc., 1963, 85, 2843–2848.
43. E. H. Cordes and W. P. Jencks, J. Am. Chem. Soc., 1962, 84, 832–837.
44. M. R. Capeletti, L. Balzano, G. de la Puente, M. Laborde and U. Sedran, Appl. Catal., A, 2000, 198, L1–L4.
45. E. H. Cordes and H. G. Bull, Chem. Rev., 1974, 74, 581–603.
46. B. Liu and S. Thayumanavan, J. Am. Chem. Soc., 2017, 139, 2306–2317.
47. R. Gannimani, P. Walvekar, V. R. Naidu, T. M. Aminabhavi and T. Govender, J. Controlled Release, 2020, 328, 736–761.
48. J. Wang, X. Wang, G. Yan, S. Fu and R. Tang, J. Colloid Interface Sci., 2017, 508, 282–290.
49. J. Wang, Y. Lu, S. Li, X. Wang, Y. Huang and R. Tang, Mater. Sci. Eng., C, 2019, 94, 169–178.
50. L. Hu, P. Zhang, X. Wang, X. Cheng, J. Qin and R. Tang, Carbohydr. Polym., 2017, 178, 166–179.
51. J. Gao, Y. Xu, Y. Zheng, X. Wang, S. Li, G. Yan, J. Wang and R. Tang, Carbohydr. Polym., 2019, 225, 115237.
52. M. Ahmad, R. G. Bergstrom, M. J. Cashen, Y. Chiang, A. J. Kresge, R. A. McClelland and M. F. Powell, J. Am. Chem. Soc., 1979, 101, 2669–2677.
53. M. Ahmad, R. G. Bergstrom, M. J. Cashen, A. J. Kresge, R. A. McClelland and M. F. Powell, J. Am. Chem. Soc., 1977, 99, 4827–4829.
54. X. Guan, Y. Li, Z. Jiao, J. Chen, Z. Guo, H. Tian and X. Chen, Acta Biomater., 2013, 9, 7672–7678.
55. J. Qin, Y. Zhu, D. Zheng and Q. Zhao, Bio-Des. Manuf., 2021, 4, 612–626.
56. T. Srinophakun and J. Boonmee, Int. J. Mol. Sci., 2011, 12, 1672–1683.
57. M. Zloh, E. Dinand and S. Brocchini, Theor. Chem. Acc., 2003, 109, 206–212.
58. E. Dinand, M. Zloh and S. Brocchini, Aust. J. Chem., 2002, 55, 467–474.
59. L. Qiu, Q. Liu, C. Hong and C. Pan, J. Mater. Chem. B, 2016, 4, 141–151.
60. M. Oishi, S. Sasaki, Y. Nagasaki and K. Kataoka, Biomacromolecules, 2003, 4, 1426–1432.
61. J. Zou, F. Zhang, S. Zhang, S. F. Pollack, M. Elsabahy, J. Fan and K. L. Wooley, Adv. Healthcare Mater., 2014, 3, 441–448.
62. J. Xu, Z. Bao and X. Wu, Org. Lett., 2022, 24, 1848–1852.
63. M. J. Mitchell, M. M. Billingsley, R. M. Haley, M. E. Wechsler, N. A. Peppas and R. Langer, Nat. Rev. Drug Discovery, 2021, 20, 101–124.
64. S. Theivendran, S. Lazarev and C. Yu, Exploration, 2023, 3, 20220086.
65. B. Xu, S. Li, R. Shi and H. Liu, Signal Transduction Targeted Ther., 2023, 8, 435.
66. P. Kesharwani, R. Ma, L. Sang, M. Fatima, A. Sheikh, M. A. S. Abourehab, N. Gupta, Z.-S. Chen and Y. Zhou, Mol. Cancer, 2023, 22, 98.
67. T. T. Truong, S. Mondal, V. H. M. Doan, S. Tak, J. Choi, H. Oh, T. D. Nguyen, M. Misra, B. Lee and J. Oh, Adv. Colloid Interface Sci., 2024, 332, 103263.
68. Q. Wang, J. Liu, L. He, S. Liu and P. Yang, Nanoscale, 2023, 15, 12455–12463.
69. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 1992, 359, 710–712.
70. R. K. Kankala, Y. Han, J. Na, C. Lee, Z. Sun, S. Wang, T. Kimura, Y. S. Ok, Y. Yamauchi, A. Chen and K. C. Wu, Adv. Mater., 2020, 32, 1907035.
71. S. Wu, C. Mou and H. Lin, Chem. Soc. Rev., 2013, 42, 3862–3875.
72. F. Farjadian, A. Roointan, S. Mohammadi-Samani and M. Hosseini, Chem. Eng. J., 2019, 359, 684–705.
73. Y. Feng, N. Panwar, D. J. H. Tng, S. C. Tjin, K. Wang and K.-T. Yong, Coord. Chem. Rev., 2016, 319, 86–109.
74. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548–552.
75. M. Manzano and M. Vallet-Regí, Adv. Funct. Mater., 2020, 30, 1902634.
76. P. P. Ghimire and M. Jaroniec, J. Colloid Interface Sci., 2021, 584, 838–865.
77. F. Tang, L. Li and D. Chen, Adv. Mater., 2012, 24, 1504–1534.
78. J. Wang, Y. Wu, K. Liu, W. Yang, W. Zeng, X. Gao, S. Liu and B. Zhang, APL Bioeng., 2023, 7, 016115.
79. G. Qi, G. Shi, S. Wang, H. Hu, Z. Zhang, Q. Yin, Z. Li and L. Hao, Int. J. Nanomed., 2023, 18, 7133.
80. S. Yi, R. Liao, W. Zhao and Z. Liu, Int. J. Biol. Macromol., 2024, 269, 132134.
81. Y. Liu, M. Chen, G. Li, S. Xu and H. Liu, RSC Adv., 2022, 12, 19063–19071.
82. D. Bhavsar, J. Gajjar and K. Sawant, Microporous Mesoporous Mater., 2019, 279, 107–116.
83. M. Martínez-Carmona, D. Lozano, M. Colilla and M. Vallet-Regí, Acta Biomater., 2018, 65, 393–404.
84. Y. Shu, R. Song, A. Zheng, J. Huang, M. Chen and J. Wang, Talanta, 2018, 181, 278–285.
85. J. Zhuang, L. Zhou, W. Tang, T. Ma, H. Li, X. Wang, C. Chen and P. Wang, Mater. Sci. Eng., C, 2021, 118, 111361.
86. Y. Feng, N. Li, H. Yin, T. Chen, Q. Yang and M. Wu, Mol. Pharm., 2019, 16, 422–436.
87. J. Lin, J. Du, Y. Yang, L. Li, D. Zhang, C. Liang, J. Wang, J. Mei and G. Wang, Mater. Sci. Eng., C, 2017, 81, 478–484.
88. T. T. H. Thi, V. D. Cao, T. N. Q. Nguyen, D. T. Hoang, V. C. Ngo and D. H. Nguyen, Mater. Sci. Eng., C, 2019, 99, 631–656.
89. E. Rastegari, Y. Hsiao, W. Lai, Y. Lai, T. Yang, S. Chen, P. Huang, S. Chiou, C. Mou and Y. Chien, Pharmaceutics, 2021, 13, 1067.
90. R. Casasús, M. D. Marcos, R. Martínez-Máñez, J. V. Ros-Lis, J. Soto, L. A. Villaescusa, P. Amorós, D. Beltrán, C. Guillem and J. Latorre, J. Am. Chem. Soc., 2004, 126, 8612–8613.
91. X. Li, C. Xie, H. Xia and Z. Wang, Langmuir, 2018, 34, 9974–9981.
92. X. Yuan, S. Peng, W. Lin, J. Wang and L. Zhang, J. Colloid Interface Sci., 2019, 555, 82–93.
93. L. Han, C. Tang and C. Yin, Biomaterials, 2015, 60, 42–52.
94. L. Han, C. Tang and C. Yin, ACS Appl. Mater. Interfaces, 2016, 8, 23498–23508.
95. L. Li, W. Sun, L. Li, Y. Liu, L. Wu, F. Wang, Z. Zhou, Z. Zhang and Y. Huang, Nanoscale, 2016, 9, 314–325.
96. J. W. Nichols and Y. H. Bae, J. Controlled Release, 2014, 190, 451–464.
97. X. Yao, X. Chen, C. He, L. Chen and X. Chen, J. Mater. Chem. B, 2015, 3, 4707–4714.
98. J. Chang, L. Mo, J. Song, X. Wang, H. Liu, C. Meng and Y. Wu, J. Mater. Chem. B, 2022, 10, 3375–3385.
99. M. Mladenović, I. Morgan, N. Ilić, M. Saoud, M. V. Pergal, G. N. Kaluđerović and N. Ž. Knežević, Pharmaceutics, 2021, 13, 460.
100. C. Yang, Z. Shi, C. Feng, R. Li, S. Luo, X. Li and L. Ruan, Macromol. Biosci, 2020, 20, 2000034.
101. S. Peng, X. Yuan, W. Lin, C. Cai and L. Zhang, Colloids Surf., B, 2019, 176, 394–403.
102. D. Wu, M. Chen, N. Zheng, Y. Lu, X. Wang, C. Jiang and H. Xu, Mol. Carcinog, 2024, 63, 2346–2362.
103. A. Mansour, M. Fytory, O. M. Ahmed, F. E. S. A. Rahman and I. M. El-Sherbiny, Int. J. Pharm., 2023, 648, 123608.
104. X. Chen and Z. Liu, Macromol. Rapid Commun., 2016, 37, 1533–1539.
105. L. Tan, M. Yang, H. Wu, Z. Tang, J. Xiao, C. Liu and R. Zhuo, ACS Appl. Mater. Interfaces, 2015, 7, 6310–6316.
106. L. Bai, Q. Zhao, J. Wang, Y. Gao, Z. Sha, D. Di, N. Han, Y. Wang, J. Zhang and S. Wang, Nanotechnology, 2015, 26, 165704.
107. N. Jackson, A. C. Ortiz, A. Jerez, J. Morales and F. Arriagada, Pharmaceutics, 2023, 15, 1590.
108. L. Zhu and S. Wang, J. Biomater. Sci., Polym. Ed., 2023, 34, 147–165.
109. D. Shao, Q. Gao, Y. Sheng, S. Li and Y. Kong, Int. J. Biol. Macromol., 2022, 202, 37–45.
110. M. S. Moorthy, S. Bharathiraja, P. Manivasagan, K. D. Lee and J. Oh, MedChemComm, 2017, 8, 1797–1805.
111. Y. Zhang, J. Lou, G. R. Williams, Y. Ye, D. Ren, A. Shi, J. Wu, W. Chen and L. Zhu, Pharmaceutics, 2022, 14, 1200.
112. Q. Lei, S. Wang, J. Hu, Y. Lin, C. Zhu, L. Rong and X. Zhang, ACS Nano, 2017, 11, 7201–7214.
113. K. Gou, W. Xin, J. Lv, Z. Ma, J. Yang, L. Zhao, Y. Cheng, X. Chen, R. Zeng and H. Li, Colloids Surf., B, 2023, 221, 113027.
114. Y. He, L. Shao, I. Usman, Y. Hu, A. Pan, S. Liang and H. Xu, Biomater. Sci., 2020, 8, 3418–3429.
115. R. Liao, D. Jiang, Y. Liu and P. Lv, Adv. Powder Technol., 2022, 33, 103473.
116. S. Sheoran, S. Arora, R. Samsonraj, P. Govindaiah and S. Vuree, Heliyon, 2022, 8, e09403.
117. S. Shah, V. Dhawan, R. Holm, M. S. Nagarsenker and Y. Perrie, Adv. Drug Delivery Rev., 2020, 154–155, 102–122.
118. P. Graván, A. Aguilera-Garrido, J. A. Marchal, S. A. Navarro-Marchal and F. Galisteo-González, Adv. Colloid Interface Sci., 2023, 314, 102871.
119. Md. A. Haque, A. Shrestha, C. M. Mikelis and G. Mattheolabakis, Int. J. Pharm.: X, 2024, 8, 100283.
120. Y. Ma, S. Li, X. Lin and Y. Chen, Exploration, 2024, 4, 20230147.
121. S. V. Khairnar, P. Pagare, A. Thakre, A. R. Nambiar, V. Junnuthula, M. C. Abraham, P. Kolimi, D. Nyavanandi and S. Dyawanapelly, Pharmaceutics, 2022, 14, 1886.
122. Q. Li, T. Cai, Y. Huang, X. Xia, S. P. C. Cole and Y. Cai, Nanomaterials, 2017, 7, 122.
123. L. J. Kubiatowicz, A. Mohapatra, N. Krishnan, R. H. Fang and L. Zhang, Exploration, 2022, 2, 20210217.
124. F. Zhou, L. Huang, S. Li, W. Yang, F. Chen, Z. Cai, X. Liu, W. Xu, V. Lehto, U. Lächelt, R. Huang, Y. Shi, T. Lammers, W. Tao, Z. P. Xu, E. Wagner, Z. Xu and H. Yu, Exploration, 2024, 4, 20210146.
125. S. Rayamajhi, J. Marchitto, T. D. T. Nguyen, R. Marasini, C. Celia and S. Aryal, Colloids Surf., B, 2020, 188, 110804.
126. S. Chen and R. Chen, ACS Appl. Mater. Interfaces, 2016, 8, 22457–22467.
127. S. Chen, G. Morrison, W. Liu, A. Kaur and R. Chen, Biomater. Sci., 2022, 10, 6718–6730.
128. D.-Y. Wang, G. Yang, H. C. van der Mei, Y. Ren, H. J. Busscher and L. Shi, Angew. Chem., Int. Ed., 2021, 60, 17714–17719.
129. T. Zhao, C. Liang, Y. Zhao, X. Xue, Z. Ma, J. Qi, H. Shen, S. Yang, J. Zhang, Q. Jia, Q. Du, D. Cao, B. Xiang, H. Zhang and X. Qi, J. Nanobiotechnol., 2022, 20, 177.
130. L. Yan, S. H. Crayton, J. P. Thawani, A. Amirshaghaghi, A. Tsourkas and Z. Cheng, Small, 2015, 11, 4870–4874.
131. Y. Zhao, W. Ren, T. Zhong, S. Zhang, D. Huang, Y. Guo, X. Yao, C. Wang, W. Zhang, X. Zhang and Q. Zhang, J. Controlled Release, 2016, 222, 56–66.
132. K. Shi, J. Li, Z. Cao, P. Yang, Y. Qiu, B. Yang, Y. Wang, Y. Long, Y. Liu, Q. Zhang, J. Qian, Z. Zhang, H. Gao and Q. He, J. Controlled Release, 2015, 217, 138–150.
133. B. Xiang, X. Jia, J. Qi, L. Yang, W. Sun, X. Yan, S. Yan, D. Cao, Q. Du and X. Qi, Int. J. Nanomed., 2017, 12, 2385–2405.
134. Q. Zhang, R. Ran, L. Zhang, Y. Liu, L. Mei, Z. Zhang, H. Gao and Q. He, J. Controlled Release, 2015, 197, 208–218.
135. H. Akita, R. Ishiba, R. Togashi, K. Tange, Y. Nakai, H. Hatakeyama and H. Harashima, J. Controlled Release, 2015, 200, 97–105.
136. Z. Chen, Y. Tian, J. Yang, F. Wu, S. Liu, W. Cao, W. Xu, T. Hu, D. J. Siegwart and H. Xiong, J. Am. Chem. Soc., 2023, 145, 24302–24314.
137. Y. Hirai, R. Saeki, F. Song, H. Koide, N. Fukata, K. Tomita, N. Maeda, N. Oku and T. Asai, Int. J. Pharm., 2020, 585, 119479.
138. Y. Hirai, H. Hirose, M. Imanishi, T. Asai and S. Futaki, Sci. Rep., 2021, 11, 19896.
139. Y. Sung, H. Guo, A. Ghasemizadeh, X. Shen, W. Chintrakulchai, M. Kobayashi, M. Toyoda, K. Ogi, J. Michinishi, T. Ohtake, M. Matsui, Y. Honda, T. Nomoto, H. Takemoto, Y. Miura and N. Nishiyama, Cancer Sci., 2022, 113, 4339–4349.
140. P. Luo, H. Ho, M. Chiang, L. Chu, Y. Chuang, P. Lyu, I. Hu, W. Chang, S. Peng, J. Jayakumar, H. Chen, M. Huang and H. Sung, Adv. Mater., 2024, 36, 2404830.
141. M. Yazdi, J. Pöhmerer, M. H. Kafshgari, J. Seidl, M. Grau, M. Höhn, V. Vetter, C. C. Hoch, B. Wollenberg, G. Multhoff, A. B. Dezfouli and E. Wagner, Small, 2024, 20, 2400643.
142. Y. Sakurai, H. Watanabe, K. Nishio, K. Hashimoto, A. Harada, M. Gomi, M. Suzuki, R. Oyama, T. Handa, R. Sato, H. Takeuchi, R. Taira, K. Tezuka, K. Tange, Y. Nakai, H. Akita and Y. Uchida, Pharmaceutics, 2022, 14, 1560.
143. A. A. H. Abdellatif, A. Bouazzaoui, H. M. Tawfeek and M. A. Younis, Colloids Surf., B, 2024, 238, 113930.
144. J. Germer, A.-L. Lessl, J. Pöhmerer, M. Grau, E. Weidinger, M. Höhn, M. Yazdi, M. A. Cappelluti, A. Lombardo, U. Lächelt and E. Wagner, J. Controlled Release, 2024, 370, 239–255.
145. Y. Barenholz, J. Controlled Release, 2012, 160, 117–134.
146. Y. Zong, Y. Lin, T. Wei and Q. Cheng, Adv. Mater., 2023, 35, 2303261.
147. S. G. Jadhav, R. L. Setten, C. Medina, X. Cui and S. F. Dowdy, AAPS J., 2023, 26, 10.
148. M. Lindgren, M. Hällbrink, A. Prochiantz and Ü. Langel, Trends Pharmacol. Sci., 2000, 21, 99–103.
149. G. Guidotti, L. Brambilla and D. Rossi, Trends Pharmacol. Sci., 2017, 38, 406–424.
150. S. Xu, Z. Hu, F. Song, Y. Xu and X. Han, Mol. Ther. – Methods Clin. Dev., 2025, 33, 101463.
151. D. Sun and Z.-R. Lu, Pharm. Res., 2023, 40, 27–46.
152. A. Akinc, A. Zumbuehl, M. Goldberg, E. S. Leshchiner, V. Busini, N. Hossain, S. A. Bacallado, D. N. Nguyen, J. Fuller, R. Alvarez, A. Borodovsky, T. Borland, R. Constien, A. de Fougerolles, J. R. Dorkin, N. K. Jayaprakash, M. Jayaraman, M. John, V. Koteliansky, M. Manoharan, L. Nechev, J. Qin, T. Racie, D. Raitcheva, K. G. Rajeev, D. W. Y. Sah, J. Soutschek, I. Toudjarska, H. Vornlocher, T. S. Zimmermann, R. Langer and D. G. Anderson, Nat. Biotechnol., 2008, 26, 561–569.
153. S. Thalmayr, M. Grau, L. Peng, J. Pöhmerer, U. Wilk, P. Folda, M. Yazdi, E. Weidinger, T. Burghardt, M. Höhn, E. Wagner and S. Berger, Adv. Mater., 2023, 35, 2211105.
154. Y. Lin, X. Luo, T. Burghardt, S. Dorrer, M. Höhn, E. Wagner and U. Lächelt, J. Am. Chem. Soc., 2023, 145, 15171–15179.
155. R. C. Steffens, S. Thalmayr, E. Weidinger, J. Seidl, P. Folda, M. Höhn and E. Wagner, Nanoscale, 2024, 16, 13988–14005.
156. C. H. Albertsen, J. A. Kulkarni, D. Witzigmann, M. Lind, K. Petersson and J. B. Simonsen, Adv. Drug Delivery Rev., 2022, 188, 114416.
157. E. Samaridou, J. Heyes and P. Lutwyche, Adv. Drug Delivery Rev., 2020, 154–155, 37–63.
158. A. L. Lima, T. Gratieri, M. Cunha-Filho and G. M. Gelfuso, Methods, 2022, 199, 54–66.
159. R. T. Chacko, J. Ventura, J. Zhuang and S. Thayumanavan, Adv. Drug Delivery Rev., 2012, 64, 836–851.
160. M. Elsabahy and K. L. Wooley, J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 1869–1880.
161. M. A. Beach, U. Nayanathara, Y. Gao, C. Zhang, Y. Xiong, Y. Wang and G. K. Such, Chem. Rev., 2024, 124, 5505–5616.
162. B. L. Banik, P. Fattahi and J. L. Brown, Wiley Interdiscip. Rev.: Nanomed. Nanobiotechnol., 2016, 8, 271–299.
163. L. Tomasini, M. Ferrere and J. Nicolas, Nat. Rev. Bioeng., 2024, 2, 501–520.
164. Z. Tong, Y. Xie, M. C. Arno, Y. Zhang, I. Manners, R. K. O'Reilly and A. P. Dove, Nat. Chem., 2023, 15, 824–831.
165. L. Zhao, Z. Yang, J. Wang, Y. Zhou, P. Cao, J. Zhang, P. Yuan, Y. Zhang and Q. Li, Chem. Eng. J., 2023, 451, 138676.
166. C. Oerlemans, W. Bult, M. Bos, G. Storm, J. F. W. Nijsen and W. E. Hennink, Pharm. Res., 2010, 27, 2569–2589.
167. F. Lv, J. Cao, J. Zhang, J. Qian, W. Peng, S. Sun, W. Li, W. Zhang, W. Guo and J. Li, Cancer Chemother. Pharmacol., 2014, 73, 1173–1179.
168. C. J. F. Rijcken, F. De Lorenzi, I. Biancacci, R. G. J. M. Hanssen, M. Thewissen, Q. Hu, F. Atrafi, R. M. J. Liskamp, R. H. J. Mathijssen, I. H. C. Miedema, C. W. Menke-van der Houven van Oordt, G. A. M. S. van Dongen, D. J. Vugts, M. Timmers, W. E. Hennink and T. Lammers, Adv. Drug Delivery Rev., 2022, 191, 114613.
169. T. Hamaguchi, Y. Matsumura, M. Suzuki, K. Shimizu, R. Goda, I. Nakamura, I. Nakatomi, M. Yokoyama, K. Kataoka and T. Kakizoe, Br. J. Cancer, 2005, 92, 1240–1246.
170. I. Negut and B. Bita, Pharmaceutics, 2023, 15, 976.
171. S. Wang, L. Liu, L. Tian, P. Xu, S. Li, L. Hu, Y. Xia, Y. Ding, J. Wang and S. Li, Adv. Mater., 2024, 36, 2403296.
172. Z. T. Bennett, A. Krishnamurthy, S. Ye, V. S. Basava, Q. Feng, G. Huang, B. D. Sumer and J. Gao, J. Am. Chem. Soc., 2025, 147, 19547–19558.
173. Y. Zhang, P. Chen, B. Li, H. Guo, J. Zhu, Z. Dang, S. Lei, P. Huang and J. Lin, ACS Nano, 2023, 17, 16743–16756.
174. X. Ma, Q. Yang, N. Lin, Y. Feng, Y. Liu, P. Liu, Y. Wang, H. Deng, H. Ding and H. Chen, J. Controlled Release, 2024, 370, 230–238.
175. X. Wang, J. Wilhelm, W. Li, S. Li, Z. Wang, G. Huang, J. Wang, H. Tang, S. Khorsandi, Z. Sun, B. Evers and J. Gao, Nat. Commun., 2020, 11, 5828.
176. M. Liu, L. Huang, W. Zhang, X. Wang, Y. Geng, Y. Zhang, L. Wang, W. Zhang, Y. Zhang, S. Xiao, Y. Bao, M. Xiong and J. Wang, Nat. Nanotechnol., 2022, 17, 541–551.
177. Z. Zhou, Y. Feng, M. Jiang, Z. Yao, J. Wang, F. Pan, R. Feng, C. Zhao, Y. Ma, J. Zhou, L. Sun, X. Sun, C. Zhan, X. He, K. Jiang, J. Yu and Z. Yan, Nat. Commun., 2025, 16, 360.
178. L. Zhang, Y. Shen, T. Zhang, X. Jiang, L. Wang, B. Wang, X. Lan, J. Tian and X. Zhang, Biomater. Adv., 2025, 167, 214082.
179. S. Guan, Q. Zhang, J. Bao, T. Duan, R. Hu, T. Czech and J. Tang, Eur. J. Pharm. Biopharm., 2020, 147, 87–101.
180. D. Jiang, T. Gao, S. Liang, W. Mu, S. Fu, Y. Liu, R. Yang, Z. Zhang, Y. Liu and N. Zhang, ACS Appl. Mater. Interfaces, 2021, 13, 22213–22224.
181. Y. Li, S. Yue, J. Cao, C. Zhu, Y. Wang, X. Hai, W. Song and S. Bi, Theranostics, 2020, 10, 8250–8263.
182. S. Brunato, F. Mastrotto, F. Bellato, C. Bastiancich, A. Travanut, M. Garofalo, G. Mantovani, C. Alexander, V. Preat, S. Salmaso and P. Caliceti, J. Controlled Release, 2021, 335, 21–37.
183. X. Zhao and P. Liu, Chin. J. Polym. Sci., 2025, 43, 730–737.
184. S. Meng, H. Du, X. Li, X. Zheng, P. Zhao, Z. Yuan, S. Huang, Y. Zhao and L. Dai, ACS Nano, 2024, 18, 3134–3150.
185. Z. Wang, Y. Guo, G. Li, H. Guo, Y. Li, J. Wang, K. Li, S. Mignani, M. Shen, E. Garanger, S. Lecommandoux and X. Shi, Nano Today, 2025, 64, 102810.
186. J. Li, J. Yang, S. Jiang, Y. Tian, Y. Zhang, L. Xu, B. Hu, H. Shi, Z. Li, G. Ran, Y. Huang and S. Ruan, Biomaterials, 2024, 311, 122708.
187. W. Gong, Z. Chen, J. Dong, Y. Liu and Y. Cui, Chem. Rev., 2022, 122, 9078–9144.
188. Y. Wang, J. Yan, N. Wen, H. Xiong, S. Cai, Q. He, Y. Hu, D. Peng, Z. Liu and Y. Liu, Biomaterials, 2020, 230, 119619.
189. M. Nejabat, A. Samie, A. Khojastehnezhad, F. Hadizadeh, M. Ramezani, M. Alibolandi, K. Abnous, S. M. Taghdisi and M. Siaj, ACS Appl. Mater. Interfaces, 2024, 16, 51837–51859.
190. J. Yang and Y. Yang, Small, 2020, 16, 1906846.
191. F. Xing, J. Xu, Y. Zhou, P. Yu, M. Zhe, Z. Xiang, X. Duan and U. Ritz, Nanoscale, 2024, 16, 4434–4483.
192. N. Yang, L. Wei, Y. Teng, P. Yu, C. Xiang and J. Liu, Eur. J. Med. Chem., 2024, 274, 116546.
193. J. Dong, K. Ma, J. Ding, Y. Pei and Z. Pei, Bioorg. Med. Chem., 2022, 69, 116885.
194. J. Dong, K. Ma, Y. Pei and Z. Pei, Chem. Commun., 2022, 58, 12341–12344.
195. A. Cabrera-García, E. Checa-Chavarria, E. Rivero-Buceta, V. Moreno, E. Fernández and P. Botella, J. Colloid Interface Sci., 2019, 541, 163–174.
196. R. Cheng, L. Jiang, H. Gao, Z. Liu, E. Mäkilä, S. Wang, Q. Saiding, L. Xiang, X. Tang, M. Shi, J. Liu, L. Pang, J. Salonen, J. Hirvonen, H. Zhang, W. Cui, B. Shen and H. A. Santos, Adv. Mater., 2022, 34, 2203915.
197. J. Dong, Y. Yu, Y. Pei and Z. Pei, J. Colloid Interface Sci., 2022, 607, 1651–1660.
198. W. Jia, B. Jin, W. Xu, S. Liu, X. Mao, H. Peng and Y. Zhang, ACS Appl. Mater. Interfaces, 2023, 15, 50069–50082.
199. X. Luan, Z. Xiang, J. Dong, C. Wang, X. Li, Q. Shi and X. Du, ACS Appl. Mater. Interfaces, 2023, 15, 27264–27276.
200. H. Guo, Y. Xia, K. Feng, X. Qu, C. Zhang and F. Wan, Int. J. Nanomed., 2020, 15, 3235–3250.
201. W. J. Chen, D. Gupta, M. Yang, F. Yang, N. Feng, J. Song, M. J. A. Wood, L. Qiu and J. Chen, Small, 2023, 19, 2303403.
202. S. Karimi and H. Namazi, Int. J. Pharm., 2023, 634, 122675.
203. W. Hur, Y. Park, E. Seo, S. E. Son, S. Kim, H. Seo and G. H. Seong, J. Colloid Interface Sci., 2023, 645, 663–675.
204. S. Xu, H. Zhang, Z. Qian and W. Yuan, J. Colloid Interface Sci., 2024, 675, 746–760.
205. S. Ren, D. Zhu, X. Zhu, B. Wang, Y. Yang, W. Sun, X. Wang, P. Lv, Z. Wang and H. Zhu, ACS Appl. Mater. Interfaces, 2019, 11, 20678–20688.
206. M. Cai, W. Liang, K. Wang, D. Yin, T. Fu, R. Zhu, C. Qu, X. Dong, J. Ni and X. Yin, ACS Appl. Mater. Interfaces, 2022, 14, 36366–36378.
207. Y. Chen, H. Wu, T. Yang, G. Zhou, Y. Chen, J. Wang, C. Mao and M. Yang, ACS Appl. Mater. Interfaces, 2021, 13, 47371–47381.
208. A. Yıldız, A. A. Kara and F. Acartürk, Int. J. Biol. Macromol., 2020, 148, 1084–1097.
209. A. L. Martínez-López, C. Pangua, C. Reboredo, R. Campión, J. Morales-Gracia and J. M. Irache, Int. J. Pharm., 2020, 581, 119289.
210. M. Karimi, S. Bahrami, S. B. Ravari, P. S. Zangabad, H. Mirshekari, M. Bozorgomid, S. Shahreza, M. Sori and M. R. Hamblin, Expert Opin. Drug Delivery, 2016, 13, 1609–1623.
211. D. A. Yardley, J. Controlled Release, 2013, 170, 365–372.
212. C. Dumontet, J. M. Reichert, P. D. Senter, J. M. Lambert and A. Beck, Nat. Rev. Drug Discovery, 2023, 22, 641–661.
213. S. Voci, M. Fresta and D. Cosco, J. Controlled Release, 2021, 329, 385–400.
214. L. Sun, H. Liu, Y. Ye, Y. Lei, R. Islam, S. Tan, R. Tong, Y.-B. Miao and L. Cai, Signal Transduction Targeted Ther., 2023, 8, 418.
215. B. A. Richards, A. G. Goncalves, M. O. Sullivan and W. Chen, Curr. Opin. Biotechnol., 2024, 86, 103070.
216. M. Hou and S. Liu, Biomacromolecules, 2024, 25, 5402–5416.
217. D. Berillo, A. Yeskendir, Z. Zharkinbekov, K. Raziyeva and A. Saparov, Medicina, 2021, 57, 1209.
218. F. Raza, Y. Zhu, L. Chen, X. You, J. Zhang, A. Khan, M. W. Khan, M. Hasnat, H. Zafar, J. Wu and L. Ge, Biomater. Sci., 2019, 7, 2023–2036.
219. Z. Gong, H. Zhao and J. Bai, Colloids Surf., B, 2024, 239, 113954.
220. D. R. Dautel, W. T. Heller and J. A. Champion, Biomacromolecules, 2022, 23, 3678–3687.
221. Z. Wang, X. Zhang, M. Han, X. Jiao, J. Zhou, X. Wang, R. Su, Y. Wang and W. Qi, J. Mater. Chem. B, 2023, 11, 8974–8984.
222. Z. Wang, J. Zhang, Y. Wang, J. Zhou, X. Jiao, M. Han, X. Zhang, H. Hu, R. Su, Y. Zhang and W. Qi, ACS Nano, 2024, 18, 10324–10340.
223. R. T. Pirposhteh, N. Mohajel, A. Arashkia, K. Azadmanesh and M. Masoumi, BMC Biotechnol., 2025, 25, 54.
224. U. Shimanovich, A. Levin, D. Eliaz, T. Michaels, Z. Toprakcioglu, B. Frohm, E. De Genst, S. Linse, K. S. Åkerfeldt and T. P. J. Knowles, Small, 2021, 17, 2007188.
225. T. Peng, H. Lee and S. Lim, Biomater. Sci., 2015, 3, 627–635.
226. N. Zhang, F. Zhao, Q. Zou, Y. Li, G. Ma and X. Yan, Small, 2016, 12, 5936–5943.
227. B. Sun, H. Yang, Y. Li, J. F. Scheerstra, M. H. M. E. van Stevendaal, S. Li and J. C. M. van Hest, Biomacromolecules, 2024, 25, 3044–3054.
228. G. Wang, J. Yang, D. Hou, R. Zheng, M. Mamuti, M.-J. Guo, Z. Fan, H.-W. An and H. Wang, Adv. Healthcare Mater., 2021, 10, 2100333.
229. K. Zhang, P. Yang, P. He, S. Wen, X. Zou, Y. Fan, Z. Chen, H. Cao, Z. Yang, K. Yue, X. Zhang, H. Zhang, L. Wang and H. Wang, ACS Nano, 2020, 14, 7170–7180.

---

Footnote

† These authors contributed equally as co-first authors.

---