Advancing biomedical applications of polyoxometalate-based metal–organic frameworks: from design to therapeutic potential

Abstract

Polyoxometalate-based metal–organic frameworks (POMOFs) are novel materials composed of polyoxometalates (POMs) and metal–organic frameworks (MOFs), which are widely used in biomedical research. Synthesis strategies for POMOF materials predominantly utilise liquid-phase self-assembly via diverse methodologies and solid-phase self-assembly facilitated by mechanical grinding. Additionally, POMOF materials can be modified to enhance their efficacy through enhancements in catalytic activity and structural refinement. This review article summarises the common design, synthesis, and modification strategies of POMOF materials suitable for biomedical applications. The review further elucidates the unique properties of POMOF materials, regulatability, stability, and multifunctionality, which distinctly exceed those of other biomedical materials. Subsequently, the review is centred on applying POMOF biosensors classified by colourimetric and electrochemical methods and their multiple roles in biomedical therapy fields. Finally, we introduce the challenges faced by POMOFs in terms of synthesis methods and, using conditions, propose potential solutions and summarise the future development potential of POMOFs in the field of biomedical detection and therapy.

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Lijin Wang

Lijin Wang received her B.S. degree in Chemistry from the Northeast Forestry University in 2023 under the guidance of Prof. Jinsong Peng. Her research focuses on the biomedical application of POMOF nanomaterials based on probe and catalysis, and photothermal therapy (PTT) combined with starvation therapy in collaborative diagnosis and treatment and prescription of multiple tumours, as well as the structure–activity relationship of camptothecin and its derivatives in tumour therapy.

Pengyu Dai

Pengyu Dai received his B.S. and B.Eng. degrees in Chemistry and Computer Science from the Northeast Forestry University in 2023. His research work mainly involves improving the catalytic activity of nanozymes and enhancing the targeting of nanozymes for the diagnosis and therapy of diseases. His most recent research project is materials research on black phosphorus nanoribbons.

Hongli Ma

Hongli Ma received her master's degree in the Clinical Major at Jilin University in 2013. Following her academic achievement, she undertook her professional engagement at the Obstetrics and Gynecology Department of the First Hospital of Suihua. In October 2021, she began her teaching duties, and in February 2022, she became the deputy director of the Second Gynecology Department. She primarily engages in the research of anti-tumour drugs and is skilled in diagnosing and treating common gynaecological diseases.

Tiedong Sun

Tiedong Sun received his Ph.D. degree from the Harbin Institute of Technology in 2016. He studied at Yale University as an international exchange student and then joined Northeast Forestry University as a faculty member. He was promoted to associate professor in 2017 and then promoted to professor in 2023, now at the College of Chemistry, Chemical Engineering and Resource Utilization, Northeast Forestry University. His research work focuses on the biomimetic synthesis of nanozymes and their biomedical applications.

Jinsong Peng

Jinsong Peng graduated from Southwest University for Nationalities in 2002 and received his Ph.D. in 2007 from the Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences. He became an associate professor at Northeast Forestry University in 2007 and was promoted to full professor in 2018. His current research interest is focused on the design, synthesis and discovery of novel anti-tumor drugs.

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1 Introduction

MOFs represent a burgeoning class of porous functional materials wherein metal ions (predominantly transition metal ions) intricately coordinate with organic ligands to form a robust framework.¹ The establishment of these coordination bonds results in a highly versatile and regulatable structure, rendering MOFs promising candidates for diverse applications in advanced materials.² MOFs exhibit precise regulation of pore sizes within the micron to nanometer scale range.³ The remarkable attributes of MOFs include an exceedingly high specific surface area, customisable pore volume, regulatable pore sizes, and adjustable surface properties, imparting great promise for multifaceted applications in adsorption, separation, and catalysis.⁴ The extraordinary array of properties exhibited by MOFs has led to their burgeoning applications in the field of biomedicine.⁵ To propel the advancement of MOFs in biomedicine, several challenges and complexities persist in MOF materials research.⁶ Notably, enhancing the stability of MOFs remains imperative to meet application demands across diverse conditions. At the same time, achieving successful integration and synergy with computational and other disciplines will enable MOFs for practical applications.⁷ Furthermore, the ongoing development and refinement of synthesis and modification methodologies are essential to expand the prospects of MOFs in biomedicine further.⁸

POMs are a subclass of metal oxides consisting of multiple metal oxide clusters with unique physical and chemical properties that generate dynamic micron to nanometer-scale structures.⁹ According to the structure of POMs, POMs can be divided into six main types: Keggin, Wells–Dawson, Silverton, Waugh, Lindqvist, and Anderson. Various methods can tune the structure and properties of POMs.¹⁰ The regulatable acidity and solubility of POMs make them an important catalyst for a wide range of organic reactions.¹¹ The acidic and redox properties of POMs can be fine-tuned by changing their constituent elements, thereby tuning their catalytic activity according to different scenarios.¹² POMs can also be combined with organic molecules to form organic–inorganic hybrid materials, which can achieve the modulation of their optical, electrical, and magnetic properties, thus expanding their applications in materials science and nanotechnology.¹³ POMs also have good multi-electron redox properties and can undergo redox transformations under mild conditions, making them adaptable to various applications. POM anions are a group of inorganic building blocks with Lewis and Brønsted acidity that can be used as efficient solid acid catalysts for various organic reactions.¹⁴ The multi-electron redox properties of POMs also make them an ideal electrocatalyst for photoelectrochemistry, capacitors, and other applications.¹⁵ The regulatability and multi-electron redox attributes of POMs are now making notable strides in the realm of antibacterial and antitumour biomedical treatments, exemplifying their burgeoning applications in this field.¹⁶

The convergence of POM and MOF materials has emerged as a prominent and compelling research frontier, driven by their individual outstanding properties.¹⁷ MOFs with high porosity provide a platform for good dispersion of POMs and improve the exposure of active sites. Various organic ligands can be selected to design MOFs with the desired structure and fixed pore size to meet their requirements for encapsulating POMs.¹⁸ The multicomponent synergistic effect between the two components can produce excellent composite properties. Currently, POMOF materials exhibit exceptional adsorption properties, rendering them ideal loading materials for sensor applications. Primarily, they facilitate the selective loading of specific substance components, enabling the sensing of various physical quantities, including pH and temperature. Additionally, POMOF materials exhibit enzyme-like activity for colourimetric detection and manifest electrochemical catalytic activity for electrochemical sensing purposes. This multifaceted functionality positions POMOF materials as a promising candidate for advancing sensor technologies.¹⁹ POMOF materials are also used in biomedical therapy because the vigorous enzyme-like activity can control the balance of reactive oxygen species (ROS) in the body, thus treating tumours and inflammation. Moreover, the POMOFs platform presents commendable mechanical properties, making it an appealing medium for drug encapsulation and delivery while also serving as an effective enzyme carrier to augment therapeutic interventions. In summary, the unique properties (regulatability, stability, and multifunctionality) of POMOF materials warrant continued scrutiny and exploration within the realm of biomedical research, accentuating their potential for transformative advancements in drug delivery and medical therapies.²⁰Table 1 demonstrates the biomedical applications of POMOF materials. Fig. 1 encapsulates a visual overview of POMOF materials deployed in the biomedical therapy field.

Fig. 1 Summary illustration of POMOF materials used in biomedical fields.

Table 1 Representative biomedical applications for POMOF materials

Applications	POMOFs	Abbreviations	Types (POMs)	Activities	Synthesis	Ref.
pH	{Mn(3-dpye)0.5[CrMo6(OH)6O18](H2O)}·(3-H2dpye)0.5	C14H23CrMnMo6N4O27	Anderson	Carrier	One-pot	21
Temperature	Eu, TbPOM@MOF	Eu, TbPOM@MOF	Keggin	Carrier	Hydrothermal	22
Temperature	EuW10@[Tb16(TATB)16(DMA)24]·91DMA·108H2O	EuW10@Tb-TATB-DMA	Weakley	Carrier	Impregnation	23
H2O2	[Cu5(pz)6Cl][SiW12O40]	CuSiW12	Keggin	POD	One-pot	24
L-Cysteine	[Cu18(trz)12Cl3(H2O)2][PW12O40]/SWNTs	Cu18PW12/SWNTs	Keggin	POD	Ultrasound	25
AA&H2O2	[Ag3(FKZ)2(H2O)2][H3SiW12O40]@PPy	AgFKZSiW12@PPy	Keggin	POD	One-pot	26
UA	Ag5[bimt]2[PMo12O40]·2H2O@PPy	Ag5PMo12@PPy	Keggin	POD	One-pot	27
AA&H2O2	[Cu9(FKZ)12(H2O)8][H3P2W18O62]2·4H2O/PPy	CuFKZP2W18/PPy	Wells–Dawson	POD	One-pot	28
CA&H2O2	[Ni4(Trz)6(H2O)2][SiW12O40]·4H2O/PDDA-rGO	Ni4SiW12/PDDA-rGO	Keggin	POD	One-pot	29
L-Cysteine	NiMo6@ZIF-67	NiMo6@ZIF-67	Anderson	POD	Solvothermal	30
Glc	MIL-100(Fe)@PMo12@3DGO	MIL-100@PMo12	Keggin	POD	One-pot	31
Glc	MIL-101(Fe)@P2W18@SWNT	MIL-101@P2W18	Wells–Dawson	POD	One-pot	32
DA	[Ag5(trz)4]2·[PMo12O40]@SWCNTs-COOH	PAZ@SWCNTs-COOH	Keggin	EA	One-pot	33
DA	V10O28@NU-902	V10O28@NU-902	Keggin	EA	Impregnation	34
XA	CuMOFP2W18	CuMOFP2W18	Dawson	EA	One-pot	35
L-Cysteine	Mo-POM@HKUST-1	Mo-POM@HKUST-1	Dawson	EA	One-pot	36
H2O2	[Cu2(BTC)4/3(H2O)2 ]6[H3PMo12O40]-KB	NENU5-KB	Keggin	EA	One-pot	37
H2O2	[(Ag7bpy7Cl2){As(W^V)2(W^VI) 10O40}]·H2O	AsW12@MOF	Keggin	EA	Grind	38
H2O2	{Ag5BW12O40}@[Ag3(μ-Hbtc)(μ-H2btc)]n	Ag5[BW12O40]@Ag-BTC-2	Keggin	EA	Grind	39
H2O2	{Co3Mo7O24}@[Ag4(μ-Hbtc)(μ-H2btc)]1/2	Co3Mo7O24@Ag-BTC-2	Keggin	EA	Grind	40
H2O2	{Ag4K2P2W18O62}@[Ag3(μ-Hbtc)(μ-H2btc)]n	Ag4K2P2W18O62@Ag-BTC-n	Wells–Dawson	EA	Grind	41
H2O2	CoK4[P2W18O62]@Co3(btc)2	P2W18@Co-BTC	Wells–Dawson	EA	Grind	42
H2O2	[Ag5(pz)6(H2O)4][BW12O40]	[Ag5(pz)6(H2O)4][BW12O40]	Keggin	EA	One-pot	43
H2O2	Ni3P2W18O62-Ni3(BTC)2	Ni3P2W18O62-Ni3(BTC)2	Wells–Dawson	EA	Grind	44
H2O2	Cu3[P2W18O62]@HKUST-1	HRBNU-7	Wells–Dawson	EA	Grind	45
Antitumour	W-POM NCs@HKUST-1	W-POM NCs@HKUST-1	Keggin	POD	Impregnation	46
Antitumour	{ZnW11CuO40}@ZIF-8 NPs	POM@ZIF-8	Keggin	POD	Hydrothermal	47
Antitumour	POM-CaO2@ZIF-8	POM-CaO2@ZIF-8	Keggin	POD	One-pot	48
Antitumour	ZIF-8/CQ/POM/HA NPs	ZCPH NPs	Keggin	POD	Solvothermal	49
Antibacterial	[Ag3(H2O)(L)2](H2PMo12O40)·H2O	PMo12@MOF	Keggin	POD	One-pot	50
Enzyme therapy	PW12@UiO-67	PW12@UiO-67	Keggin	Absorbent	One-pot	51
Macrophage	H7[β-P(Mo^V)4(Mo^VI)8O40]@MIL-101	POM@MIL-101	Keggin	Carrier	One-pot	52
Degradation of antibiotics	H3PMo12O40·nH2O@ZIF-67	PMA@ZIF-67	Keggin	Carrier	Impregnation	53
Degradation of antibiotics	PMo12O40@MIL-101	POM@MIL-101	Keggin	Carrier	Solvothermal	54
Cyt-C	Na6[TeW6O24]·22H2O@ZIF-8	TeW6@ZIF-8	Anderson	Carrier	Impregnation	55

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This review presents a comprehensive review of the burgeoning applications of POMOF materials in biomedicine. Firstly, the synthesis strategies, encompassing liquid-phase self-assembly and solid-phase self-assembly methods, are thoroughly elucidated. Subsequently, the modification strategies applied to POMOFs, involving catalytic activity and structural refinement, are detailed. This review also introduces the unique properties of POMOFs based on regulatability, stability, and multifunctionality. POMOFs in biosensing are classified according to their colourimetric and electrochemical detection capabilities, which are rooted in their electrocatalytic activity (EA). Moreover, the potential of POMOFs in biomedical treatment is explored, encompassing their applications in antibacterial and antitumour aspects. Lastly, the therapeutic prospects of POMOFs in biomedical applications are astutely anticipated, highlighting promising avenues for future research and development.

2 Synthesis strategies

As innovative synthesis technologies rapidly advance, POMOF preparation methodologies have diversified significantly.⁵⁶ This review focuses on the strategic synthesis of POMOFs, delivering a systematic elucidation of their developmental trajectory. For POMOF materials with biomedical applications, two main types can be distinguished, the first with POMs as part of a framework and the second with POMs encapsulated within the cavities of MOFs. This objective is realised by employing a self-assembly strategy.⁵⁷ The self-assembly strategy is pivotal in POMOF synthesis, grounded on the principle of molecules or atoms autonomously organising into ordered structures via local interactions.⁵⁸ This approach is advantageous due to its operational simplicity, yet it exhibits considerable dependence on reaction conditions, including the concentration and type of metal oxide anions, heteroatoms, pH, ionic strength, reducing agents, ligands, and temperature.⁵⁹ Achieving full control of the synthesis of POMOF structures continues to be a formidable challenge within POMOF applications. In POMOF synthesis, self-assembly is primarily categorised into two types: solid-phase self-assembly and liquid-phase self-assembly, both of which are delineated subsequently.⁶⁰

2.1 Liquid-phase self-assembly strategy

Within the liquid-phase self-assembly process for POMOFs, metal precursors and organic ligands coalesce in solution, utilising intermolecular interactions present in the solvent like “hydrogen bonding, coordination bonds, and van der Waals forces” to facilitate self-assembly.⁶¹ This method's merit lies in the precise controllability of POMOF's structure and properties by altering the solvent, temperature, pH, and other reaction conditions.⁶² Conversely, the drawback is that the reaction kinetics of POMOF liquid-phase self-assembly can be intricate, necessitating extensive experimentation and theoretical calculations to preclude side reactions. Liquid-phase self-assembly is crucial for synthesising POMOFs with tailored pore structures and functionalities, finding extensive applications in catalysis, targeted transport, drug release, and various other domains.⁶³ For biomedical applications of POMOFs, liquid-phase self-assembly predominantly occurs via hydrothermal or solvothermal synthesis methods.⁶⁴ The production of specially functionalised POMOF materials necessitates hydrothermal or solvothermal synthesis under high-pressure conditions as a component of the one-pot process.⁶⁵

2.1.1 Hydrothermal or solvothermal method. The primary approaches are the conventional hydrothermal and solvothermal synthesis methods, frequently employed in laboratory syntheses.⁶⁶ Xu et al. prepared NiMo₆@ZIF-67 materials for the detection of L-cysteine, introducing pre-existing ZIF-67 into a methanol solution containing Co(NO₃)₂·6H₂O and NiMo₆, and stirred it gently with heat until NiMo₆ was undetectable by UV-Vis in the solution, indicating completion of self-assembly.³⁰ While these hydrothermal and solvothermal techniques facilitate rapid crystal growth, they necessitate subsequent modifications before application.

2.1.2 Ultrasonic drive method. In the synthesis of Cu₁₈PW₁₂/SWNTs materials applied to L-cysteine, Li et al. employed an ultrasonically driven functionalisation approach to enhance liquid-phase self-assembly.²⁵ Initially, two novel [PW₁₂O₄₀]³⁻ based Cu-triazole (trz) MOFs were synthesised successfully. Subsequently, POMs, MOFs, and SWNTs (single-walled carbon nanotubes) were amalgamated in methanol utilising a straightforward ultrasonic-driven periodic functionalisation strategy, followed by drying to procure the final product. This strategy was also adopted by Zhou et al. in the creation of PAZ@SWCNTs-COOH materials aimed at dopamine detection.³³ The utilisation of ultrasonic-driven techniques facilitates a more efficacious material integration, thereby augmenting the performance of the resultant POMOF composite.

2.1.3 One-pot method. Wang et al. also prepared Ag₅PMo₁₂@PPy materials for detecting uric acid through one-pot strategic liquid-phase self-assembly.²⁷ Dissolve H₃PMo₁₂O₄₀, silver nitrate, and NaVO₃ in water and agitate for thirty minutes until homogeneity is achieved. Adjust the pH to roughly 1.75, transfer the blend into a reaction kettle, elevate the temperature to 170 °C, and maintain for four days. Following ancillary post-treatment steps, including filtration, deionised water rinsing, and air-drying, the nascent POMOF product is procured, which, post-enhancement and refinement, is ready for use. Furthermore, numerous POMOF syntheses employ this methodology.

Murinzi et al.'s Mo-POM@HKUST-1³⁶ and Wang et al.'s NENU5-KB³⁷ materials were crafted following a proposed efficacious pre-synthesis treatment, where the precursors were blended into a gel-like substance before one-pot synthesis. Addressing the three-dimensional network structure engendered by polymerisation or cross-linking in such a manner not only effectually governs the architecture of the synthesised material but also diminishes reliance on organic solvents during the reaction. In the aforementioned synthetic protocol, the system's pH is maintained between 1.75 and 3.4, with the reaction temperature regulated between 120 °C and 180 °C. Although the one-pot approach for liquid-phase self-assembly is straightforward and apt for intricate systems, the matters of raw material utilisation and product purity necessitate additional investigation.⁶⁷

2.1.4 Impregnation method. Ho et al. employed the dipping technique for liquid-phase self-assembly in fabricating V₁₀O₂₈@NU-902 materials aimed at dopamine detection, introducing V₁₀O₂₈ into NU-902 within a liquid medium.³⁴ Specifically, V₁₀O₂₈ has initially dissolved in an acidic aqueous solution at a pH of 4.5, followed by the immersion of pre-existing NU-902 to acquire the POMOFs product, post-impregnation, cleansing, solvent exchange, and drying stages. Impregnation-driven liquid-phase self-assembly facilitates the tailoring of materials to various application demands. However, a drawback is that POMOFs synthesised via the impregnation approach exhibit subpar mechanical and chemical stabilities.

Liquid-phase self-assembly is pivotal in the synthesis strategies for POMOF materials, encompassing hydrothermal or solvothermal techniques, ultrasonic facilitation, impregnation procedures, and one-pot approaches. This strategy permits exact manipulation of POMOF's structure and properties by modulating the solvent, temperature, and pH, among other variables. Nevertheless, the recovery rate of raw materials and the reaction kinetics within this POMOFs synthesis strategy require additional exploration.⁶⁸

2.2 Solid-phase self-assembly strategy

Solid-phase self-assembly constitutes a principal synthesis technique for POMOF materials within the biomedical treatment domain. It involves the formation of an ordered structure through interplay among solid substances.⁶⁹ This assembly process accentuates physical means, with diverse methods applicable to foster or regulate self-organizational behaviours.⁷⁰ Techniques such as grinding, thermal treatment, high-pressure lamination, and templating are instrumental in achieving this, with grinding being the primary method employed.⁷¹

Zhou's group frequently employs grinding methods to facilitate the self-assembly of POMOF materials, primarily for colourimetric or electrochemical detection of H₂O₂ in serum. The process involves synthesising POM and MOF materials separately and then grinding and assembling them in an agate mortar in a specified ratio. This is followed by characterisation to identify POMOFs that fulfil structural criteria before delving into the study of their properties and precise synthetic steps. Notably, in the synthesis of AgFKZSiW₁₂@PPy, a liquid phase is introduced to aid in grinding and assembly.²⁶ Despite the addition of a liquid phase, the assembly is fundamentally achieved through physical means, offering an integrated solid-phase assembly approach. The benefit of this solid-phase self-assembly strategy is its operational simplicity and broad applicability. However, challenges include difficulty in achieving precise structural and compositional control of the final product, particularly with complex hybrid materials, and potential compromises in product purity and uniformity compared to liquid-phase self-assembly.

3 Modification strategies

The POMOF modification strategies aim to augment the attributes of the synthesised materials via certain techniques to enhance their aptness and stability for targeted applications. It typically encompasses specific approaches such as ion exchange, physical adsorption, or encapsulation. Employing these techniques enables researchers to tailor POMOF materials to the exigencies of distinct applications.⁷² This article predominantly delineates the enhancement of POMOF materials from two aspects: the amplification of catalytic efficacy and the refinement of structural integrity.

3.1 Enhancement of catalytic performance

Primarily, doping synthetic composite materials serves as a means to elevate catalytic efficacy or bolster catalytic stability.⁷³ This principal approach alters the electron density and energy level distribution of POMOF materials by incorporation of distinct compounds or ions and by varying the ligand and heteroatom types. Such alterations impact the redox potential, thus enhancing electron transfer and catalytic activity. Furthermore, catalytic performance can be augmented via a synergistic interplay among different constituents. The maxim “the whole is greater than the sum of its parts” can be realised by integrating a cocatalyst, which steers the catalytic reaction towards a specific, precise trajectory. Modulating solubility facilitates versatility across various milieus, reinforcing catalytic steadfastness.

Sha's group introduced PPy (polypyrrole) materials when preparing AgFKZSiW₁₂@PPy²⁶ and Ag₅PMo₁₂@PPy.²⁷ As a prototypical and widely used conductive polymer, PPy is characterised by its expansive specific surface area and rapid electron transport properties. These attributes render it conducive for enhancing catalytic efficacy in conjunction with POMOF materials. Ultimately, the integration of PPy functionalisation imparts a pivotal role to POMOF materials in the detection of UA (uric acid) and AA (ascorbic acid), substantially enhancing the results’ resilience against interference and yielding improved detection accuracy.

Zhou et al. additionally executed the functionalisation of SWCNTs (single-walled carbon nanotubes) for the synthesis of PAZ@SWCNTs-COOH.³³ SWCNTs boast an extensive electrochemical potential window, exceptional electrochemical traits, and biomolecule congruence. This endows them with the capacity to modulate the redox potential of the product and bolster electron transfer in catalytic reactions. Concurrently, the incorporation of SWCNTs can amplify both the mechanical robustness and biocompatibility of the product. The PAZ@SWCNTs-COOH material, derived via SWCNT functionalisation, demonstrated promising detection capabilities in DA (dopamine) analysis.

Incorporating compounds into POMOF materials can modify their structure to an extent, augment the exposure of active sites, and elevate catalytic performance.⁷⁴ Enhancing catalytic activity can refine the electronic, optical, and thermal properties of materials in line with the application context, broadening their scope of applications and enhancing the performance of functional materials. Strategies aimed at augmenting catalytic activity stand as a cornerstone in the optimisation of POMOF materials' performance.⁷⁵

3.2 Refinement of structure

Structural optimisation of POMOF materials constitutes a critical endeavour to enhance their performance and extend their applicability. This optimisation entails modulating POMOFs' composition, morphology, and pore architecture. The reaction enhancement is achieved by altering the morphology of POMOFs via structural modifications, which in turn modify their catalytic performance by adjusting the surface charge distribution and active sites.⁷⁶ Tailored structures predicated on application demands are engineered to enhance POMOFs' catalytic efficacy.⁷⁷

Initially, reactants are gelled before the liquid-phase self-assembly of Mo-POM@HKUST-1³⁶ and NENU5-KB³⁷ materials. Pore size and distribution are tuned to yield a three-dimensional network within the POMOF, enhancing the material's adsorption capabilities. Furthermore, gelled POMOFs may exhibit improved thermal and mechanical resilience, which is crucial for specific applications. Yet, a notable drawback is the complexity of processing, with preservation posing a significant challenge.

In the synthesis of P₂W₁₈@Co-BTC for H₂O₂ detection, Yu et al. employed a templated modification strategy.⁴² The resulting POMOF possesses a core–shell architecture, reinforcing mechanical integrity and adsorption capacity. Subsequently, the POMOF particles are systematically affixed to a nanoscale nickel foam template, modifying the surface structure. This adjustment facilitates reactant contact with POMOFs, rendering active sites more accessible and enhancing reaction kinetics.

During the one-pot synthesis of CuMOFP₂W₁₈, Zhang et al. utilised a physical grinding approach to reassembling CuMOFP₂W₁₈ with acetylene black (XC-72R), creating a network that encapsulates POMOFs in a relatively uniform system.³⁵ This effectively augments electron transfer within the regular network and boosts electrocatalytic performance. The spatial optimisation of POMOF materials heralds the tremendous potential for applications in sensing and detection.⁷⁸

4 Unique properties of POMOF materials

POMOF materials, a combination of POMs and MOFs, inherit the exemplary attributes of both constituents, including the high porosity of POMs and the structural diversity of MOFs.⁷⁹ POMOF materials further demonstrate their unique properties. The unique properties of POMOF materials significantly enhance their applications in the biomedical field. The unique properties of POMOFs are discussed below from three aspects: the regulatability of POMOFs, stability, and their multifunctionality.

4.1 Regulatability

The regulatability of POMOF materials is mainly manifested in two aspects: the regulatability of catalytic activity and the regulatability of structure. In terms of catalytic activity, the type and ratio of metal centres or organic ligands in MOF materials can be customised, enabling them to exhibit a variety of enzyme-like activities, such as catalase-like (CAT-like) and peroxidase-like (POD-like) activities. Nanoscale POMs also exhibit exceptional properties in catalysing redox reactions.⁸⁰ POMOF materials, as a combination of both, not only integrate a variety of enzyme-like and catalytic activities but also demonstrate an enhancement in catalytic strength to a certain extent.⁸¹ Ji et al. successfully synthesised a POMOF material by combining H₃PMo₁₂O₄₀, 3DGO, and MIL-100(Fe), exhibiting outstanding POD-like activity.³¹ The synergistic effect of these components resulted in enhanced electrical conductivity and surface catalytic activity, surpassing that of each component. When employed for glucose (Glc) detection, this innovative material achieved a remarkably lower detection limit, and concurrently, its chemical stability was significantly improved. POMOF materials with specific catalytic properties can be tailored according to specific application environments.⁸² The diverse and efficient catalytic characteristics of POMOFs make them significantly valuable in the fields of anti-inflammatory and cancer therapy.

In terms of structural regulatability, based on the various preparation and modification methods mentioned above, precise control of the structure of POMOF can be achieved by altering precursors and reaction conditions. The variety of POMs, the diverse spatial structures of MOFs, and the adjustable pore size and distribution of POMOFs together provide a blueprint for the structural diversity of POMOFs. Structurally modified POMOFs demonstrate outstanding performance in drug loading, targeted drug delivery, and controlled drug release. An et al. created a specific structure by encapsulating PW₁₂ into the UiO-67 framework, where POMs act akin to sponges, providing ample electrons to facilitate the stable adsorption of MP-11 enzymes for enzymatic therapy.⁵¹ POMOFs can also be synthesised by incorporating functional groups, thus endowing the materials with specific chemical and physical properties. This enhancement facilitates interactions with specific molecules, achieves surface functionalisation, and improves the adsorption capacity for certain gases or biomolecules. Composite materials can be created by combining specific materials with POMOFs, resulting in POMOFs with distinct structural functions that exhibit excellent performance even under extreme conditions. These composite POMOF materials hold a pivotal significance in advancing sensory technology.⁸³ The potential shown by POMOFs in terms of regulatable catalytic and structural properties provides researchers with a broad scope for developing POMOFs for biomedical applications.⁸⁴

4.2 Stability

The stability of POMOFs refers to their ability to function effectively under extreme external conditions. In the field of biomedicine, this translates to maintaining high efficiency under various biophysical conditions.⁸⁵ Firstly, POMOFs exhibit commendable functional stability, meaning they can maintain robust catalytic activity towards target substances even under interfering conditions. Li et al. combined PPy with POMOFs to form rod-like spiral structures, resulting in enhanced catalytic properties. Moreover, under the interference of factors such as Glc, Mg²⁺, Na⁺, Cl⁻, NO₃⁻, and K⁺, as well as under harsh conditions of pH and temperature, they demonstrated exceptional detection specificity for AA.²⁶ Secondly, there is structural stability, where under biomedical conditions, POMOFs can maintain their porous structure largely intact. Although POMOFs tend to aggregate under physiological conditions, this effect can be mitigated to a certain extent through modification and regulation. Liu et al. synthesised W-POM NCs@HKUST-1 for combined chemodynamic therapy (CDT) and photothermal therapy (PTT).⁴⁶ The resultant encapsulated structure exhibited good dispersibility in RPMI-1640 aqueous cell culture medium containing fetal bovine serum for seven days, with no significant macroscopic aggregation observed. This can be attributed, to a certain extent, to the synergistic effect of gallic acid and the charge effect stability of HKUST-1, which enhances the structural stability of POMOFs. The combined stability of catalytic performance and structure lays a foundation for the application of POMOFs in the field of biomedicine.

4.3 Multifunctionality

In addition to their exceptional regulatability and good stability, POMOFs also possess multifunctional characteristics. POMs, known for their photothermal effects, are commonly used in PTT. MOFs, while providing a stable framework, also introduce various enzyme-like activities, endowing POMOFs with excellent CDT characteristics in the biomedical field.⁸⁶ Consequently, POMOF materials exhibit multifunctional features by enabling simultaneous multiple therapeutic approaches. Liu and colleagues synthesised W-POM NCs@HKUST-1 materials, under NIR light irradiation, and combined photothermal therapy with dynamic therapy, producing a synergistic treatment effect on tumour cells.⁴⁶ Our group synthesised POM@ZIF-8 NPs, combining electrocatalytic and dynamic properties to conduct Electrodynamic therapy (EDT) and CDT.⁴⁷ The functional diversity of POMOFs offers broad prospects for material development, making it an exciting area of research in the field of biomedicine.⁸⁷

5 Biomedical applications

POMOFs, as popular materials in recent years, have a wide range of applications in the biomedical field. The main mechanism of action is that POMOFs have good POD-like, electrochemical catalytic activity, and good adsorption properties. At the same time, POMOF materials have good biocompatibility and can be used in the biomedical detection field as potential materials for supercapacitors. Its potential catalytic activity also enables its application in the field of biomedical therapy. The following focuses on the biomedical applications of POMOF materials from two aspects: biodetectors and biomedical therapies.

5.1 Biosensors

POMOF materials have POD-like and electrochemical catalytic activities for different substrate reactions. Due to the perfect performance of these activities, POMOF materials can be used for applications in the field of biosensors.⁸⁸ POMOF materials biosensors have the advantages of high selectivity against interference and low detection limits. Their biocompatibility and nature as a promising material for supercapacitors allow them to be used for colourimetric and electrochemical detections, and the application of POMOF materials to the sensing of physical quantities and the detection of different substrates by these two methods is presented below. Table 2 demonstrates the linear detection range and LOD (limit of detection) of POMOF biosensors.

Table 2 The linear detection range and LOD of POMOFs biosensors

Applications	Abbreviations	Linear range	LOD
H2O2	CuSiW12	1–60 μM	0.10 μM
L-Cysteine	Cu18PW12/SWNTs	1–80 μM	0.103 mM
AA&H2O2	AgFKZSiW12@PPy	1–80 μM AA	2.7 μM AA
		1–100 μM H2O2	0.12 μM H2O2
UA	Ag5PMo12@PPy	1–50 μM	0.47 μM
AA&H2O2	CuFKZP2W18/PPy	5–10 μM AA	0.627 μM AA
		5–100 μM H2O2	0.72 μM H2O2
CA&H2O2	Ni4SiW12/PDDA-rGO	1–60 μM CA	2.07 μM AA
		1–100 μM H2O2	0.49 μM H2O2
L-Cysteine	NiMo6@ZIF-67	1–20 μM	0.018 μM
Glc	MIL-100@PMo12	1–100 μM	0.14 μM
Glc	MIL-101@P2W18	1–150 μM	0.2 μM
DA	PAZ@SWCNTs-COOH	0.05–100 μM	8.6 nM
DA	V10O28@NU-902	25–400 μM	2.1 μM
XA	CuMOFP2W18	0.5–240 μM	0.26 μM
L-Cysteine	Mo-POM@HKUST-1	3–10 mM	0.307 μM
H2O2	NENU5-KB	10–50 mM	1.03 μM
H2O2	AsW12@MOF	1.43–1890 μM	0.48 μM
H2O2	Ag5[BW12O40]@Ag-BTC-2	0.4–270 μM	0.19 μM
H2O2	Co3Mo7O24@Ag-BTC-2	1–430 μM	0.33 μM
H2O2	Ag4K2P2W18O62@Ag-BTC-n	1.5–1500 μM	0.206 μM
H2O2	P2W18@Co-BTC	1.9–1670 μM	0.633 μM
H2O2	[Ag5(pz)6(H2O)4][BW12O40]	5–270 μM	2.2 μM
H2O2	Ni3P2W18O62-Ni3(BTC)2	0.5–250 μM	0.17 μM
H2O2	HRBNU-7	0.5–300 μM	0.17 μM

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5.1.1 Colourimetric biosensors. The colourimetric method is a method to determine the content of the components of the substance to be measured by comparing or measuring the depth of the colour change of the coloured substance solution.⁸⁹ In addition, the colour change of the excitation spectrum of the effective component adsorbed in POMOFs can be caused by the change in some physical quantities. The colourimetric method has the advantages of simple operation, a wide range of uses, low cost, etc. It is often widely used.⁹⁰ The colourimetric method can be divided into the visual colourimetric method, and the photoelectric colourimetric method, and the photoelectric colourimetric method is often used in biological or medical fields.⁹¹ POMOF materials for substrate colourimetric detection mechanism: often through the reaction of substrate by-products and colour source reagents to react and then detect the change in colour absorbance to determine the concentration of the substrate, this reaction is usually based on H₂O₂-mediated reaction, colour source reagents are usually selected TMB (3,3′,5,5′-tetramethylbenzidine) and so on. The POMOF materials colourimetric detector includes the following two presentations: 1 colour change of the system due to changes in the physical properties of the system. This paper is about sensing pH and temperature. 2 Quantitative detection of the target substance by detecting changes in colour absorbance.
5.1.1.1 pH sensing. pH is an important scale for indicating the strength of acids and bases and has been used in a wide variety of scientific applications since its introduction. It plays a vital role in the design of batteries, in the field of food, and in the diagnosis and treatment of diseases.⁹² The level of pH in the body is also an essential gauge of the abnormalities of the internal environment of human tissues.⁹³ Currently, pH test strips and pH meter methods are the main methods for detecting pH.⁹⁴ The redox potential of POMOF materials is highly sensitive to pH and can be used as a potentiometric pH sensor.

Wang et al. synthesised the {Mn(3-dpye)_0.5[CrMo₆(OH)₆O₁₈] (H₂O)}·(3H₂dpye)_0.5 (3-dpye = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane) by using one-pot method can be used for sensing of pH detection.²¹ The detection mechanism was that the POMOFs material modified electrode was used for the electrochemical detection of pH and showed redox activity by cyclic voltammetry scanning to know that pH was correlated with electrochemical behaviour and showed an electrochemical response. Therefore, POMOF materials can make sensor material as a potentiometric pH sensor and exhibit high sensitivity and interference resistance.

5.1.1.2 Temperature sensing. Temperature is a physical quantity that indicates the degree of heat or cold of an object and is an important indicator in industry, agriculture, and medicine.⁹⁵ In the field of biomedicine, low body temperature not only represents a system disorder but also causes cardiovascular and cerebrovascular diseases. At the same time, high body temperature can also cause abnormalities in the function of the body's internal organs, so temperature detection is also an important area of biosensing.⁹⁶ The POMOF materials also provide a colourimetric method for temperature detection.

Kaczmarek synthesised POM@MOF doped with Eu³⁺ and Tb³⁺, whose emission light colour changes significantly with temperature.²² The principle is the excitation spectrum of Eu. The temperature is related to the frequency of the excitation spectrum. When the temperature increases the emission colour of the compound from blue-violet (60 K) to orange (360 K), POMOF materials effectively increase the sensitivity of temperature detection. The method shows a good detection width of 60–360 K and 0.71% K⁻¹ sensitivity, so POMOF materials can be used to make sensor material used as a colourimetric temperature sensor.

Viravaux et al. similarly synthesised POMOF materials doped with 9.1 wt% and 19.5 wt% EuW₁₀@Tb-TATB-DMA (TATB = 1,3,5-triamino-2,4,6-trinitrobenzenet, DMA = dimethylacetamide), which can be used for the detection of temperature.²³ The principle is also the excitation spectrum of Eu, where the temperature is related to the frequency of the excitation spectrum and exhibits good thermal sensitivity. This method displays a good detection width of 200–320 K, 9.1 wt% EuW₁₀@Tb-TATB Showing a sensitivity of 2.68% K⁻¹, 19.5 wt% EuW₁₀@Tb-TATB Showing a sensitivity of 2.37% K⁻¹.

5.1.1.3 Detection of substrates. Jingquan Sha's group has made significant contributions in the field of colourimetric detection of POMOF materials, and their work is summarised and reviewed in chronological order below.

Li et al. synthesised the POMOF materials [Cu₅(pz)₆Cl][SiW₁₂O₄₀] (pz = pyrazine) for the colourimetric detection of H₂O₂.²⁴ The detection mechanism is CuSiW₁₂ exerting POD-like enzyme activity and then producing the colour change with TMB colour source reagent, which can be used for the quantitative determination of H₂O₂ by detecting absorbance into the standard curve. The same method is used to determine the H₂O₂ content by measuring the absorbance of the unknown sample after the standard curve is made. The linear detection limit of H₂O₂ is 1–60 μM, and the LOD is 0.10 μM.

Li et al. synthesised POMOF materials by one-pot method for the colourimetric detection of AA and H₂O₂.²⁶ The POMOF materials were first synthesised by the one-pot method of [Ag₃(FKZ)₂(H₂O)₂][H₃SiW₁₂O₄₀] (HFKZ = 1-(2,4-difluorophenyl)-1,1-bis[(1H-1,2,4-triazol-1-yl)methyl] ethanol) of POMOF materials. Then, the surface structure of the POMOF material was modified to get the AgFKZSiW₁₂@PPy composites for detection. The detection mechanism is that AgFKZSiW₁₂@PPy exerts POD-like enzyme activity on AA and H₂O₂ and later produces colour change with TMB colour source reagent, which can be used for quantitative detection of AA and H₂O₂ by detecting absorbance. The detection was performed by making standard absorbance curves for AA and H₂O₂ in the presence of acetate buffer and then measuring the absorbance of unknown samples under the same conditions to determine the unknown concentration of AA and H₂O₂ solutions. The linear detection limit of AA was 1–80 μM with a LOD of 2.7 μM. The linear detection limit of H₂O₂ was 1–100 μM with a LOD of 0.12 μM.

Li et al. also synthesised Nano-POMOF materials [Cu₁₈(trz)₁₂Cl₃(H₂O)₂] [PW₁₂O₄₀] (trz = 1,2,4-triazole) by ultrasonic drive self-assembly strategy and then combined with SWNTs by ultrasonic centrifugation to form Cu₁₈PW₁₂NC/SWNTs composites for colourimetric detection of L-cysteine.²⁵ The detection mechanism is that Cu₁₈PW₁₂NC/SWNTs can exert a specific POD-like enzymatic activity on L-cysteine and later produce a colour change with TMB chromogenic reagent, which can be used for the quantitative detection of L-cysteine by detecting the absorbance. Fig. 2 presents the detailed absorbance standard curve for colourimetry alongside a comparative graph. The linear detection limit of the method is 1–80 μM, and the LOD is 0.103 mM.

Fig. 2 Applications of the Cu₁₈PW₁₂NC/SWNTs in the detection of L-cysteine. (a) UV–vis absorbance curves of the different reaction systems (pH = 3.5, 40 °C, 5 min). (b) UV–vis absorbance curves and the pertinent photographs of absorption mode with different catalysts. (c) The relationship of the POD-like activity of Cu₁₈PW₁₂NC/SWNTs with different pH values. (d) UV-vis absorption maps and related photographs of Cu₁₈PW₁₂NC/SWNTs with different ratios in the presence of fixed TMB (150 μM) and H₂O₂ (100 μM).²⁵ Copyright 2019, American Chemical Society.

A colourimetric detection for UA without uricase was also developed by the one-pot method.²⁷ The research group synthesised Ag₅[bimt]₂[PMo₁₂O₄₀]·2H₂O (Ag₅PMo₁₂) (Hbimt = 3,5-bis((1H-imidazol-1-yl)methyl)-1H-1,2,4-triazole) by one-pot method and synthesised Ag₅PMo₁₂@PPy composite for colourimetric detection of UA by precipitation method for modification of POMOF materials. The detection mechanism (Fig. 3): Ag₅PMo₁₂@PPy exerted POD-like activity carried out by H₂O₂-mediated TMB colour source reagent to produce a colour change. The detection method: a standard absorbance curve is developed through the reaction of a standard uric acid solution. The concentration of an unknown sample of a certain concentration can be obtained by detecting the absorbance. The detection demonstrates excellent selectivity and reproducibility for UA detection under the interference of AA, urea, triglyceride, cholesterol, grape, and other metal ions. The linear detection limit was 1–50 μM, and the LOD was 0.47 μM.

Fig. 3 Applications of the Ag₅PMo₁₂@PPy in the detection of UA. (a) Absorption spectra of the Ag₅PMo₁₂@PPy system with or without UA after the reaction. (b) Schematic illustration of the reaction. (c) Absorption spectra of the Ag₅PMo₁₂@PPy system with or without UA after the reaction. (d) Schematic illustration of the corresponding process. (e) Schematic illustration of the synthesis of Ag₅PMo₁₂@PPy and the corresponding colourimetric UA biosensor.²⁷ Copyright 2020, American Chemical Society.

Li et al. synthesised Wells–Dawson-type POMOF material CuFKZP₂W₁₈/PPy by one-pot method for the colourimetric detection of AA and H₂O₂.²⁸ Through the incorporation of PPy as a modification agent, a profound alteration is instilled within the electrochemical activity of the POMOFs. This transformative intervention engenders an expansion of its reaction contact area, thereby conferring a distinct advantage to the POMOFs in realising an enhanced catalytic performance. The detection mechanism is that CuFKZP₂W₁₈/PPy exerts high POD-like activity and electrochemical activity on AA and H₂O₂. The detection method is similar to the above assay. The linear detection limit was 5–100 μM for AA with a LOD of 0.627 μM.

A biological colourimetric method for the determination of CA (citric acid) and H₂O₂ was first designed by Sha et al.²⁹ POMOF material was synthesised by firstly synthesising MOF framework Ni₄SiW₁₂ by one-pot method, and then one-pot self-assembled with PDDA-rGO (poly (diallyldimethylammonium chloride) functionalised reduced graphene oxide) to synthesise Ni₄SiW₁₂/PDDA-rGO for detection of CA and H₂O₂. Its detection mechanism (Fig. 4) is that PMPG-n catalyses the decomposition of H₂O₂ with POD-like activity and then reacts with chromogenic reagent TMB to change the colour. The system changes from colourless to green, and the system changes from green to colourless after adding CA. The detection procedure is as follows: add POMOF, TMB, H₂O₂, and an unknown concentration of CA in acetate buffer solution at pH = 2.5, filter and transfer to a quartz test tube, and detect the concentration of CA and H₂O₂ by using UV-Vis spectrophotometer to detect the absorbance change of the system into the standard curve. The linear range of CA was from 1–60 μM with a LOD of 2.07 μM. The linear range of H₂O₂ was from 1–100 μM with a LOD of 0.49 μM, and the POMOF materials showed high interference resistance for both substrates.

Fig. 4 Applications of the Ni₄SiW₁₂/PDDA-rGO in the detection of CA. (a) Schematic illustration of synthesising Ni₄SiW₁₂/PDDA-rGO. (b) Schematic illustration of detecting CA using Ni₄SiW₁₂/PDDA-rGO.²⁹ Copyright 2020, Elsevier.

Ji et al. crafted MIL-100(Fe)@PMo₁₂@3DGO for the colourimetric detection of Glc.³¹ This composite was engineered by encasing Keggin-type H₃PMo₁₂O₄₀ within the MIL-100 scaffold, followed by a coating with three-dimensional graphene (3DGO) to enhance conductivity, surface area, porosity, and chemical robustness. The resulting nanocomposite exhibited superior POD-like activity and provided a linear detection scope from 1 to 100 μM, achieving a low Glc detection LOD of 0.14 μM.

The researcher synthesised and assembled porous NiMo₆@Co₃O₄ materials for the detection of L-cysteine by the colourimetric method.³⁰ The detection mechanism (Fig. 5) is that NiMo₆@ZIF-67 can play a certain POD-like enzyme activity on L-cysteine for the H₂O₂-mediated L-cysteine reaction and the reaction produces a colour change with TMB, which can be used for the quantitative detection of L-cysteine by detecting the absorbance. The detection is based on the POMOF-catalyzed L-cysteine action followed by H₂O₂-mediated binding action with TMB for absorbance detection to obtain the L-cysteine content. The linear detection limit of L-cysteine was 1–20 μM, and the LOD was 0.018 μM.

Fig. 5 Applications of the NiMo₆@Co₃O₄ in the detection of L-cysteine. (a) The absorbance changes at 661 nm of NiMo₆@Co₃O₄, Co₃O₄, and commercial Co₃O₄. (b) Schematic illustration of biosensing L-cysteine.³⁰ Copyright 2021, John Wiley and Sons.

Liu et al. fabricated MIL-101(Fe)@P₂W₁₈@SWNT for the colourimetric detection of H₂O₂ and Glc.³² This nanocomposite exhibits superior peroxidase-mimicking properties and stability thanks to the collective and optimally synergistic effects of its components. Fig. 6 shows the specific characterisation and detection standard curve of MIL-101(Fe)@P₂W₁₈@SWNT. Leveraging the remarkable catalytic prowess of MIL-101(Fe)@P₂W₁₈@SWNT, dual colourimetric biosensors were developed to detect H₂O₂ and Glc, achieving linear detection ranges of 0–80 μM (H₂O₂) and 0–150 μM (Glc), with detection limits of 0.3 μM (H₂O₂) and 0.2 μM (Glc), respectively.

Fig. 6 Applications of the MIL-101(Fe)@P₂W₁₈@SWNT in the detection of H₂O₂ and Glc. (a) UV–vis absorbance curves of MIL-101(Fe)@P₂W₁₈@SWNT + H₂O₂ + TMB, MIL-101(Fe)@P₂W₁₈@SWNT + H₂O₂, MIL-101(Fe)@P₂W₁₈@SWNT + TMB, H₂O₂ + TMB. (b) UV–vis absorbance curves of different MIL-101(Fe)@P₂W₁₈@SWNT systems (pH = 3.5, 100 μM H₂O₂, 0.6 mg mL⁻¹ catalyst, 0.5 mM TMB, at 45 °C for 9 min). Dependence of the POD-like activity on (c) pH. (d) MIL-101(Fe)@P₂W₁₈@SWNT dosage. (e) temperature. (f) Time. (g) SEM images of MIL-101(Fe)@P₂W₁₈@SWNT. (h) Schematic illustration of the synthesis routes of ternary MIL-101(Fe)@P₂W₁₈@SWNT. (i) Elemental mapping images of MIL-101(Fe)@P₂W₁₈@SWNT.³² Copyright 2023, Springer Nature.

5.1.2 Electrochemical biosensors. The electrochemical detection method is based on the electrochemical properties of the substances in solution and their change regulation.⁹⁷ The electrochemical detection method is based on the qualitative and quantitative analysis of components based on the stoichiometric relationship between electrical quantities such as potential, conductance, current, and electricity and certain quantities of the measured substance.⁹⁸ The electrochemical detection methods contain conductivity methods, potentiometric titration, electrolytic analysis, voltammetry, dissolution voltammetry, and coulometric analysis.⁹⁹ The electrochemical detection method has the advantages of high sensitivity, high accuracy, and wide measurement range and is widely used for electrochemical testing of POMOF material applications.¹⁰⁰ The following are electrochemical detection applications for POMOF materials.

Zhou et al. used DPV (differential pulse voltammetry) to conduct electrochemical detection of DA through modified POMOF materials.³³ The POMOF material of [Ag₅(trz)₄]₂·[PMo₁₂O₄₀] (PAZ) was synthesised by the one-pot method, and then the POMOF material was modified by adding new compounds to make the composite modified electrode of PAZ@SWCNTs-COOH/GCE. The detection mechanism is that the PAZ@SWCNTs-COOH/GCE material exerts electrochemical activity with DA, producing potential changes that can be used for DPV detection.¹⁰¹ The detection was performed by sequentially dispersing the PAZ solution into deionised water to obtain a suspension. The quantitative determination of DA in serum is performed by DPV. The pulse amplitude, pulse width, pulse period, and amplitude are selected after the pretreatment of human serum samples. The linear range of the DA was 0.05–100 μM with a LOD of 8.6 nM.

Ho et al. prepared V₁₀O₂₈@NU-902 as a non-homogeneous electrocatalyst for electrochemical DA sensors by impregnation method.³⁴ The detection mechanism is that V₁₀O₂₈@NU-902 exerts redox activity with dopamine, producing potential changes that can be used for cyclic voltammetry detection. The detection was performed by placing the V₁₀O₂₈@NU-902 non-homogeneous electrocatalyst film in an electrolyte. The method has a linear detection range of 25–400 μM for DA and a LOD of 2.1 μM.

Zhang et al. provided a method for the cyclic voltammetry detection of XA (xanthine) in POMOF materials.³⁵ The MOF framework of CuMOFP₂W₁₈ was synthesised by the one-pot method, and then the POMOF material of CuMOFP₂W₁₈/XC-72R/GCE (XC-72R = acetylene black) was self-assembled by grinding and sonication. The detection mechanism was that the POMOF materials exerted significant electrocatalytic activity to promote XA oxidation. The detection mechanism is that different volumes of standard XA solutions were added to diluted human serum samples. After each addition, voltammograms were recorded over a range of potentials, peak currents were estimated, calibration curves were calibrated based on the standard samples, and the concentration of XA in the serum samples was calculated. The linearity range was 0.5–240 μM, with a LOD of 0.26 μM and good selectivity, and stability.

A method for the cyclic voltammetry detection of L-cysteine in POMOF materials was provided by Murinzi et al.³⁶ The process involved the synthesis of the MOF framework HKUST-1 via one-pot methods, followed by further modification with Mo-POM using a one-pot technique. Subsequently, the modified Mo-POM was integrated onto a doped Mo-POM-GCE electrode to enable electrochemical detection of L-cysteine. The mechanism of detection relied on the electrochemical catalytic activity of Mo-POM towards L-cysteine, with alterations in its potential being quantified via cyclic voltammetry. A standard curve was generated utilising concentrations of known samples, allowing for the analysis of unknown samples. The assay demonstrated a linear detection range of 3–10 mM and a LOD of 0.307 μM.

Wang et al. provided an enzyme-free method for detecting H₂O₂ by POMOF.³⁷ This approach presents a novel enzyme-free concept for catalysing reactions and their application in detection. As the field of nanozyme research continues to advance, the potential synergy between POMOF materials and nanozymes could lead to significant advancements in enzyme-free detection methodologies. The POMOF materials were synthesised by first synthesizing [Cu₂(BTC)_4/3(H₂O)₂]₆ [H₃PMo₁₂O₄₀] (NENU5) (H₃BTC = 1,3,5-benzenetricarboxylic acid), by a solvothermal method, and NENU5-KB materials were synthesised by applying compounds modification with KB (ketjenblack). The detection mechanism is that the POMOF material exhibits a unique redox activity that can act as an efficient electrochemical activity to promote H₂O₂ decomposition, and H₂O₂ was detected using cyclic voltammetry analysis. The detection method is the prepared NENU5-KB material-modified GC electrode for cyclic voltammetry detection of standard H₂O₂ solution to make a standard curve and then measure the unknown sample potential to get the unknown H₂O₂ concentration (Fig. 7). This method has a linear detection range of 10–50 mM for H₂O₂, with a LOD of 1.03 μM, and exhibits high sensitivity.

Fig. 7 Applications of the NENU5-KB in the detection of L-cysteine. (a) Amperometric current-time curves of different systems with the addition of H₂O₂ (system 1 = GCE, system 2 = HKUST-1/GCE, system 3 = HKUST-1-KB-3/GCE, and system 4 = NENU5-KB-3/GCE). (b) Current curves of different systems with the addition of H₂O₂. (c) Stability of current response of systems 1 and 2. (d) Current–time curve of system 4 with the addition of 0.5 mM H₂O₂, 0.2 mM AA, 0.2 mM APAP (acetaminophen), 0.2 mM DA, and 0.2 mM Glc. The electrolyte in systems 1, 3, and 4 was N₂-saturated 0.1 M PBS (pH = 7.4). The applied potential in systems 1, 3, and 4 was 0.47 V.³⁷ Copyright 2018, John Wiley and Sons.

Baibin Zhou's group has made significant contributions to the field of electrochemical biosensors of POMOF materials, and their related work is summarised and reviewed in chronological order below.

Cui et al. proposed a method for grafted POMOF to detect H₂O₂ by cyclic voltammetry and chronoamperometry.³⁸ The POMOF material synthesis method is to synthesise (AsW₁₂@MOF) [(Ag₇bpy₇Cl₂){As(W^V)₂(W^VI)₁₀O₄₀}]·H₂O (bpy = 4,4′-bipyridyl) on MOF by solid-phase self-assembly method. The detection mechanism is the catalytic redox activity of the POMOF material for H₂O₂. The detection method is to make an AsW₁₂@MOF-GCE electrode to detect H₂O₂. AsW₁₂@MOF-GCE electrode for cyclic voltammetry detection of H₂O₂. Construct the standard curve by plotting the concentrations of the known samples and subsequently analyse the unknown sample. H₂O₂ was detected linearly in the range of 1.43–1890 μM with a LOD of 0.48 μM.

Yu et al. also provided a method for cyclic voltammetry detection of H₂O₂ by core–shell POMOF.³⁹ POMOF material synthesis method Ag₅[BW₁₂O₄₀] was synthesised by mixing AgNO₃ with K₅[BW₁₂O₄₀]·15H₂O. Then Ag₅[BW₁₂O₄₀] and Ag-BTC were ground in a mortar to synthesise Ag₅[BW₁₂O₄₀]@Ag-BTC-2. The catalytic mechanism is the highly potent redox activity of POMOF by the action of H₂O₂, with a linear detection range of 0.4–270 μM for H₂O₂ and a LOD of 0.19 μM.

Yu et al. provided a method for detecting H₂O₂ by chronoamperometry and cyclic voltammetry for POMOF materials.⁴⁰ The POMOF material synthesis method was modified and synthesised by solid-phase self-assembly method with Co₃Mo₇O₂₄@Ag-BTC-2, and the POM material was injected into MOFs. The detection mechanism is electrochemical catalysis of H₂O₂, and the response current is gradually enhanced with increasing H₂O₂ concentration in electrochemical characterisation. The detection process: using the material as the working electrode, using the platinum sheet as the counter electrode, using the saturated calomel electrode as the reference electrode, using the chronoamperometry and cyclic voltammetry calibration, detecting the unknown sample potential to obtain the unknown sample concentration. The linear detection range of H₂O₂ is 1–430 μM, and the LOD is 0.33 μM.

Liang et al. provided a method for cyclic voltammetry detection of H₂O₂ in POMOFs (Ag₄K₂P₂W₁₈O₆₂@Ag-BTC-n).⁴¹ POMOFs material was synthesised through a liquid-assisted grinding method, with a coating structure designed to increase surface area and electron transfer efficiency. Its detection mechanism is that Ag₄K₂P₂W₁₈O₆₂@Ag-BTC-n assembly enhances the catalytic play of the electrochemically active role of H₂O₂, and it can play electrocatalytic activity. Based on this electrocatalytic activity, there is a certain mathematical relationship between current and voltage. The response current is gradually improved with increasing H₂O₂ concentration in electrochemical characterisation. Fig. 8 shows some characterisation and electrochemical detection of POMOF materials in this case. The linear range of H₂O₂ detection was 1.5–1500 μM, and the LOD was 0.206 μM with a high recovery of 97.54–99.61%.

Fig. 8 Applications of the Ag₄K₂P₂W₁₈O₆₂@Ag-BTC-n in the detection of H₂O₂. (a) Schematic illustration of the synthesis of Ag₄K₂P₂W₁₈O₆₂@Ag-BTC-n. (b) Schematic illustration of a symmetrical two-electrode system for detecting H₂O₂. (c) SEM of Ag₄K₂P₂W₁₈O₆₂@Ag-BTC-n. (d) TEM of Ag₄K₂P₂W₁₈O₆₂@Ag-BTC-n. (e) EDX plots of Ag, P, C, K, W and O. (f) CV curves of four materials. (g) GCD curves of four materials. (h) CV curves for different amounts of H₂O₂ added to the test solution. (i) Current response obtained when H₂O₂ is added. (j) Linear current data graph of H₂O₂ concentration. (k) The influence of five interferents on the selectivity of detection.⁴¹ Copyright 2022, Elsevier.

The symmetrical two-electrode system made of Dawson-type POMOF materials also provided a method for detecting H₂O₂.⁴² POMOF materials are synthesised by hydrothermal synthesis of MOF materials followed by solid phase grinding method to combine POM and MOF to synthesise CoK₄[P₂W₁₈O₆₂]@Co₃(btc)₂ material. The detection mechanism is to make CoK₄[P₂W₁₈O₆₂]@Co-BTC-GCE electrode to make a CV curve. POMOF material has strong redox activity and electrochemical activity of H₂O₂, the response is current and H₂O₂ concentration to make the standard curve. The concentration of H₂O₂ in the unknown serum can then be obtained from the standard curve. The linear range of H₂O₂ was 1.9–1670 μM, and the LOD was 0.633 μM, showing high selectivity and stability.

5.2 Biomedical therapies

The basic mechanism of POMOF materials therapy is that it has POD-like activity. When the body is exposed to various harmful stimuli, sometimes endogenous ROS may be insufficient, and the oxidative and antioxidant systems may be imbalanced, leading to tissue damage.¹⁰² POMOF materials not only provide H₂O₂ but also provide enough ROS with POD-like activity to destroy DNA molecules of foreign substances and thus produce a defence effect, thus effectively regulating the balance of ROS in the body.¹⁰³ POMOF materials can be used as adsorbent and loading materials for biomedical therapy, and their good biocompatibility allows them to be widely used in living organisms.¹⁰⁴

5.2.1 Anti-tumours. Tumours represent a prevailing and life-threatening health concern, posing a significant biomedical challenge in contemporary times.¹⁰⁵ Secondary bacterial infections are a common issue associated with tumours, often necessitating the simultaneous administration of anti-inflammatory and anti-tumour therapies in clinical settings.¹⁰⁶ There are many ways to treat cancer: CDT, PDT (photodynamic therapy), PTT, RT (radiation therapy), EDT, etc. In recent years, the advancement of POMOF materials has introduced these novel approaches to tumour treatments.
5.2.1.1 PTT and CDT. PTT is the use of materials with high photothermal conversion efficiency or strong ROS yield exposed to a specific wavelength of the light source and its target recognition technology to allow the material to produce phototoxicity to specific tumour cells to achieve therapeutic effects. At the same time, phototherapy has been shown to kill a variety of microorganisms.¹⁰⁸ The development and utilisation of POMOF materials combine PTT and CDT to work together to treat tumours.

Our group synthesised W-POM NCs@HKUST-1 by impregnation method to bring a new means of tumour therapy.⁴⁶ First, phosphotungstic acid and gallic acid are combined to synthesise W-POM NCs, which are then incorporated into the HKUST-1 framework. The resulting encapsulated structure allows the POM materials to exhibit excellent PTT therapeutic performance, while the MOF framework enhances the overall system's POD-like activity and strengthens its physical structure. In subsequent simulations under simulated physiological conditions, it was found that there was no significant aggregation in fetal bovine serum culture medium even after seven days, indicating the exceptional stability of the newly synthesized W-POM NCs@HKUST-1 and suggesting its excellent biocompatibility. W-POM NCs@HKUST-1, due to its excellent POD-like activity, exhibits superior targeted therapeutic properties. When combined with PTT and CDT, it shows high stability and an impressive photothermal conversion efficiency of up to 39.38% under near-infrared light irradiation. Fig. 9 shows the mechanism of POMOFs. It also effectively catalyzes the production of hydroxyl radicals within cells, an outstanding effect attributed to the excellent performance of W-POM NCs. The composite material demonstrates greater efficacy in killing cells compared to single therapy strategies, and the HKUST-1 framework additionally reduces the photodegradation of the photothermal materials. The composite material exhibits higher lethality than single therapeutic strategies. The utilisation of POMOF materials significantly amplifies cancer cell treatment efficacy and targeting precision. This study extends the POMOF composite nanomaterials and provides a reliable and promising means to improve tumour therapy.

Fig. 9 Schematic illustration of the PTT and CDT mechanism of W-POM NCs@HKUST-1.⁴⁶ Copyright 2022, John Wiley and Sons.

Wang et al. engineered ZCPH nanoparticles (NPs), a versatile nanoplatform integrating PTT, anti-inflammatory, and anti-autophagy attributes for ovarian cancer intervention.⁴⁹ The synthesis strategy is to encapsulate POMs and chloroquine phosphate (CQ) into the ZIF-8 framework by solvothermal method, and then wrap it in hyaluronic acid (HA) to form an inclusion structure, targeting effective ovarian cancer therapy and inflammation mitigation. The study underscores the potential of non-invasive PTT as a tumour-targeting modality, inflicting nominal collateral damage to tissue via controllable irradiation. The nanocomposite exhibits a photothermal conversion therapeutic efficacy of up to 30.8% under an 808 nm near-infrared laser and has excellent anti-inflammatory effects based on POD-like activity in scavenging ROS in the tumour environment. In terms of biocompatibility, for SKOV3 cells treated at different concentrations, due to the synergistic effect of CQ, more than 80% of normal cells can survive even at concentrations as high as 100 μg mL⁻¹, while tumour cells will absorb more CQ enabling ZCPHs to exert targeted cytotoxicity on tumour cells with high uptake of CQ. While performing targeted elimination of tumour cells, it also enhances the biocompatibility of the material. This study also used pH-responsive CQ to inhibit autophagy in lysosomes, cut off the self-defence mechanism “autophagy” induced by cell necrosis, and achieve collaborative treatment of tumours.

5.2.1.2 EDT and CDT. CDT, a new class of oncology treatment techniques based on endogenous chemical product transformation reactions in tumours, is often used in conjunction with PTT for tumours as an effective treatment.¹⁰⁷ Its mechanism of action involves catalysing the transformation of weakly oxidizing H₂O₂ into potent oxidizing ROS, which escalates intracellular oxidation, provokes DNA necrosis, deactivates proteins, oxidizes lipids, and culminates in the apoptotic demise of cancer cells. CDT has been a prominent research subject in cancer therapy in recent years because of its specificity and independence and its applicability to treating tumours deep in tissues.¹⁰⁹ The disadvantages of conventional ROS therapy and its application are also affected by TME (tumour microenvironment) due to the lack of endogenous H₂O₂ and GSH (glutathione) overexpression.¹¹⁰ POMOF materials not only provide endogenous H₂O₂ but also have strong POD-like activity, thus enabling the development of novel enhanced chemokinetic therapies based on POMOF materials.

Our group synthesised POM@ZIF-8 NPs by hydrothermal method. POM@ZIF-8 NPs combine EDT and CDT for tumour treatment.⁴⁷ The structure involves POMs being incorporated into the ZIF-8 framework to exert their effect, where the mechanism involves W⁵⁺ decomposing H₂O₂ into ˙OH via the Fenton-like reaction, collectively targeting tumour cells. The mechanism (Fig. 10) is the presence of strong POD-like activity of POM@ZIF-8 NPs and the fact that it protects normal tissues and avoids side effects due to the reaction of ZIF-8 to acid. As a result, a high degree of tumour suppression was observed in vitro and in vivo. With the influence of HeLa cells in a simulated biological microenvironment, POM@ZIF-8, even at concentrations as high as 100 μg mL⁻¹, showed no significant damage to the cells. After 24 hours, the drug release effect remained pronounced, reflecting the exceptional biocompatibility of POMOF materials. This study provides an alternative method for a combination treatment modality with specific efficacy.

Fig. 10 Applications of the POM@ZIF-8 NPs in EDT and CDT. (a) Schematic illustration of the synthesis of POM@ZIF-8 NPs. (b) Schematic illustration of the anticancer mechanism of POM@ZIF-8 NPs.⁴⁷ Copyright 2022, American Chemical Society.

Our group also synthesised POM-CaO₂@ZIF-8 by one-pot method for electro-enhanced CDT.⁴⁸ The synthesised POMs and CaO₂ NPs are co-encapsulated within MOF frameworks, forming an integrated encapsulated structure. The interaction of CaO₂ with water generates H₂O₂, addressing the issue of endogenous oxygen deficiency. W⁵⁺ ions' main function is to convert H₂O₂ into W⁶⁺ and ˙OH based on electrical response, with the hydroxyl radicals acting upon tumour cells. Concurrently, W⁶⁺ can also decompose GSH into GSSH, thereby creating a synergistic therapeutic effect. POMOFs, when interacting with HeLa cells for more than 48 hours, continue to demonstrate effective drug release. Moreover, when interacting with mouse tumour cells, due to the supportive structure of the MOF framework, they exhibit superior stability compared to the action of POMs, and a noticeable reduction in tumour cell size is observed. The mechanism (Fig. 11) of POMOFs is the presence of strong POD-like activity of POM-CaO₂@ZIF-8, while the simultaneous modulation of endogenous (H₂O₂ produced by CaO₂ NPs and GSH consumed by POM) and exogenous Fenton-like responses enhanced by electrical stimulation will have better therapeutic results. This therapeutic strategy has shown excellent tumour cell-killing effects in vitro and in vivo studies.

Fig. 11 Schematic illustration of the synthesis of POM-CaO₂@ZIF-8 NPs and the endogenous and exogenous enhanced CDT therapy of the tumour.⁴⁸ Copyright 2022, John Wiley and Sons.

5.2.2 Anti-bacterial. Bacterial infections are a significant cause of human disease.¹¹¹ Using natural products or chemically modified natural antibiotics that disrupt bacterial cell membranes, prevent protein synthesis and expression, and hinder DNA replication is the traditional approach to treating bacterial infections and diseases.¹¹² Unfortunately, due to the misuse of antibiotics, viruses have significantly increased their ability to resist antibiotics, and even super viruses have emerged.¹¹³Escherichia coli is the most common Gram-negative bacterium.¹¹⁴M. albican (Monilia albican) is the most common human pathogen, causing mucosal and systemic infections with high mortality.¹¹⁵ Axial Xanthomonas campestris is the pathogen of citrus ulcers, which can cause leaf and premature fruit drop and cause significant production losses worldwide.¹¹⁶ POMOF materials offer a novel approach for treating these three bacterial strains.

Zheng et al. synthesised novel POMOF materials by one-pot self-assembly method [Ag₃(H₂O)(L)₂](H₂PMo₁₂O₄₀)·H₂O (HL = 4-(1H-1,2,4-triazole-1-methyl) benzoic acid) for antibacterial treatment.⁵⁰ The structure of POMOFs is formed by the interaction of Ag ions with organic ligands, creating a U-shaped linkage that constitutes a unique structural model (Fig. 12). The mechanism is that the POMOFs better stimulate the active component of Ag to play POD-like activity to trigger the oxidative stress of the cell and cause the increase of intracellular ROS level. The increased ROS will damage the DNA molecules of bacteria and thus play an antibacterial role. POMOF, as a functionalised material, can effectively stimulate the action of the active component, and therefore, POMOF material is a potential antibacterial material. In the realm of antibacterial applications, POMOF materials exhibit substantial potential as medical components. Moving forward, these materials could find utility as portable medical adjuncts, a perspective that will be further explored in the subsequent prospects.

Fig. 12 Applications of the PMo₁₂@MOF in anti-bacterial. (a) The coordination modes of organic ligands. (b) The U-shaped structural channels of POMOFs materials. (c) The U-shaped channels are arranged to form rows in turn. (d) the coordination modes of {PMo₁₂}. (e) μ6-{PMo₁₂} ions connect the adjacent U-shaped channels to form a 3D framework.⁵⁰ Copyright 2022, Elsevier.

5.2.3 Enhanced enzyme therapy. Enzymes are excellent therapeutic agents. They can react precisely and rapidly in tiny amounts at physiological pH and body temperature.¹¹⁷ To date, enzyme therapy has been widely used in biomedical therapeutic areas, with high-purity enzymes playing an important role in cancer treatment, inflammation therapy, gastrointestinal diseases, and kidney dysfunction.¹¹⁸ However, due to the relatively limited conditions under which enzyme therapy operates, it is challenging to use it more effectively in combination with therapies such as PTT and CDT for treating diseases.¹¹⁹ In recent years, the therapeutic properties of enzymes have often improved by (1) soluble chemical modification. (2) Insoluble chemical modification or immobilisation on a surface. (3) Encapsulation of enzymes into capsules of biodegradable or inert substances. POMOF materials provide a method as carriers for enzyme therapeutics, which can effectively improve thermal stability and recoverability.

An et al. developed a POMOF material as an adsorbent for enzyme (MP-11) encapsulation, providing an effective strategy to enhance enzyme therapy.⁵¹Fig. 13 shows the comparison of storage, release and enzyme activity of POMOF materials under different conditions. In the end, the MP-11&PW₁₂@UiO-67 system performed best. In this study, Keggin-type phosphotungstic acid (PW₁₂) encapsulated in a zirconium metal–organic framework (PW₁₂@UiO-67) was used as a non-homogeneous adsorbent for enzyme encapsulation in that study. POMs are orderly filled into MOF metal frameworks and exert a charge attraction similar to magnets on the enzymes filled in other frameworks. This greatly enhances the adsorption of enzymes and facilitates the release of MP-11 at specific sites. The experimental results demonstrated that POMOF complex clusters could enhance enzyme adsorption, and the stability of MP-11 was significantly improved after immobilisation.

Fig. 13 Applications of the PW₁₂@UiO-67 in enzyme therapy. (a) Colour variations of PW₁₂@UiO-67. (b) Catalysis trace diagram. (c) Storage stability of the system. (d) Thermal stability of the system. (e) Organic solvent stability diagram. (f) Recyclable stability diagram.⁵¹ Copyright 2022, American Chemical Society.

5.2.4 Enhanced macrophage action. Macrophages are widely present in vertebrates involved in non-specific defence (innate immunity) and specific defence (cellular immunity).¹²⁰ Their main functions are phagocytosis (i.e., phagocytosis as well as digestion) of cellular debris and pathogens in the form of fixed or free cells and activation of lymphocytes or other immune cells to respond to pathogens. Some drug delivery can be targeted through macrophages.¹²¹ POMOF materials provide a promising method of acting on macrophages that can be applied for drug delivery.

Roch-Marchal et al. prepared the material POM@MIL-101 by one-pot self-assembly method.⁵² The structure of POMOFs is created by embedding POM materials into MOF frameworks, forming an integrated filled structure. Moreover, in comparison with the control group, even after a prolonged period of interaction, the internalization effect was well-distributed with no significant aggregation observed. This study found that the POM@MIL-101 complex exhibited excellent biostability and was rapidly internalised in macrophages, as observed in fluorescence confocal microscopy. The capability to be observed via fluorescence confocal microscopy enhances the potential of these POMOF materials as therapeutic diagnostic agents, ideal for exploring drug delivery pathways and applications. Although this study does not provide specific case studies, it offers a promising strategy for drug delivery targeting macrophages.

5.2.5 Degradation of antibiotics. Antibiotics are antimicrobial agents with antibacterial or bactericidal effects. The mechanism is to inhibit bacterial cell wall synthesis, enhance bacterial cell membrane permeability, interfere with bacterial protein formation, and inhibit bacterial nucleic acid replication and transcription to achieve the effect of antibacterial and bactericidal.¹²² However, the misuse of antibiotics has been particularly serious in recent years, especially in farm animal feeds, resulting in reduced disease resistance in farm animals, increased drug resistance in pathogenic bacteria, drug residues in products, and food safety hazards, so effective degradation of antibiotics has an essential position in the biomedical field. POMOF materials provide a new strategy for degrading antibiotics.

Yang et al. synthesised PMA@ZIF-67 for the degradation of levofloxacin by impregnation method.⁵³ The structure of POMOF materials has led to the formation of a layered, hollow bimetallic oxide nanocage. The mechanism lies in the fact that PMA@ZIF-67 exerts POD-like activity to promote the production of ROS substances, which can effectively promote the degradation of levofloxacin and improve the catalytic activity. This study provides the design of POM@MOF hetero-derived hierarchical hollow nanocages with high peroxymonosulfate activation capacity for antibiotic removal.¹²³ Even after six cycles of reuse, POMOFs still exhibit a degradation rate of 84.7%, thereby demonstrating exceptional stability in terms of efficacy. It gave a new strategy of POMOF materials for the degradation of antibiotics.

Lan et al. synthesised the POM@MIL-101 material to effectively adsorb and degrade the cationic antibiotic tetracycline by solvothermal method.⁵⁴ The synthesised POMOFs are structured with POMs encapsulated within MOFs, forming an integrated whole. The mechanism involves POM@MIL-101 exhibiting POD-like activity to promote the degradation of tetracycline, thereby enhancing its catalytic activity. This study found that the POMOFs exhibited high degradability with a high adsorption capacity of 912.5 mg g⁻¹. The antibiotics adsorbed can be photodegraded, thus allowing POM@MIL-101 to be reused. POMOFs offer a novel approach to the adsorption and photodegradation of antibiotics.

5.2.6 Separation of cytochrome C. Cytochrome C (Cyt-C) is commonly used clinically as an adjunct to emergency treatment of hypoxia in various tissues, such as carbon monoxide poisoning, hypnotic poisoning, cyanide poisoning, neonatal asphyxia, hypoxia during severe shock, cerebrovascular accidents, post-concussion, respiratory distress due to anaesthesia and pulmonary diseases, and myocardial hypoxia due to various cardiac disorders.¹²⁴ Cyt-C is generally prepared by centrifugal separation of Cyt-C from the porcine heart. POMOF materials developed a new method to isolate and purify Cyt-C because of its stronger adsorption properties.

TeW₆@ZIF-8 was synthesised by the impregnation method (Fig. 14) by Zhang et al.⁵⁵ The synthesis relies on the self-assembly of POMs and MOF materials, with the POMs being dispersed around the MOF framework to form a composite. TeW₆@ZIF-8 composite had good biocompatibility, the Cyt-C adsorption behaviour was consistent with the Langmuir adsorption model, the theoretical adsorption capacity was 232.56 mg g⁻¹, and the retained Cyt-C was easily recovered. The method achieved a recovery of 65.6% and a high adsorption efficiency of 94.01%. Therefore, the TeW₆@ZIF-8 complex can be used as a novel adsorbent for the isolation and purification of Cyt-C from porcine hearts. This study gives a new research direction for POMOF materials as adsorbent materials.

Fig. 14 Applications of the TeW₆@ZIF-8 in separation of Cyt-C. (a) Schematic illustration of the synthesis of TeW₆@ZIF-8. (b) Interaction mechanism between cytochrome C and TeW₆@ZIF-8. (c) TEM images of ZIF-8. (d) TEM images of TeW₆@ZIF-8. (e) SEM images of ZIF-8. (f) SEM images of TeW₆@ZIF-8. (g) The adsorption isotherm of Cyt-C on the TeW₆@ZIF-8. (h) The recoveries of the adsorbed Cyt-C from TeW₆@ZIF-8 using various buffers. (i) pH-dependent adsorption behaviours on TeW₆@ZIF-8 surface. (j) Effect of ionic strength on TeW₆@ZIF-8 adsorption efficiency.⁵⁵ Copyright 2022, Elsevier.

6 Challenges and prospects

6.1 Challenges

For the extensive integration of POMOF materials into the biomedical realm, a meticulous examination of the pivotal constraints obstructing their transition from laboratory investigations to clinical trials is imperative. These constraints centre on the aspects of production and utilisation conditions.¹²⁵ Only through meticulous control over the synthesis and modification of POMOF materials can their industrial-scale production be advanced, paving the way for their formal implementation within the realm of biomedicine.¹²⁶ The constraints imposed by utilisation conditions are equally crucial. If POMOF materials fail to manifest catalytic activity within regular physiological parameters and lack inherent biodegradability, their viability in the realm of biomedicine will be considerably curtailed. The subsequent discussion delineates the challenges confronted by POMOF materials about the aforementioned aspects, followed by suggested avenues for enhancement.

6.1.1 Challenges in the synthesis and modification strategies. The primary synthesis and modification techniques of POMOF materials, as presented earlier, predominantly encompass liquid-phase reactions. In upcoming endeavours, a significant portion of these reactions aimed at accelerating synthesis rates will be conducted under conditions involving elevated pressure and temperature. The synthesis of POMOF materials grapples with the limited solubility of ligands within organic solvents (such as acetonitrile, methanol, pyridine, etc.).¹²⁷ Additionally, the formation of crystal phases in inorganic–organic hybrid polyacetal-based materials at higher temperatures poses challenges.¹²⁸ Concurrently, the reduction in viscosity and increase in ion product within the heated solvent enhances reactant diffusion and solubility. However, this phenomenon culminates in the development of interpenetrating structures, which adversely impact the formation of porous materials. Consequently, methods of synthesis and modification impose obstacles upon the efficient production of POMOF materials.¹²⁹

The ionic thermal method involves the utilisation of ionic liquids as a reaction medium in non-organic solvents for the synthesis of crystalline solids.¹³⁰ Comprising solely of ions, ionic liquids possess a significantly high enthalpy of evaporation, rendering them suitable for POM synthesis.¹³¹ The initial ionic thermal synthesis approach offers an environmentally friendly and effective avenue for crafting porous MOF materials based on POMs. Ionic liquids exhibit exceptional attributes, including remarkable thermal stability and a broad liquid temperature range, making the ionic thermal method an optimal strategy for enhanced porous material synthesis. The ionic thermal synthesis techniques present an exciting and promising trajectory, poised to yield improved self-assembly synthesis and modification of POMOFs.

The microwave method, as a common method for synthesising nanomaterials, is also promising to be utilised to synthesise POMOF materials.¹³² Its high-frequency electromagnetic wave oscillation can eliminate the utilisation of organic solvents and obtain POMOF materials in a greener way.¹³³ Simultaneously, the utilisation of the microwave method facilitates rapid nucleation, enhances the reversibility of coordination bonds, and optimises crystalline structure formation.

The aforementioned two approaches primarily explore the synthesis methodologies of POMOF materials, which can also be adapted for specific modifications. However, currently, the predominant focus of POMOF material modification is on surface structure alterations or the introduction of new compounds. Moving forward, the enhancement of POMOF material modification can be pursued by delving into the selection of diverse organic ligands.¹³⁴

6.1.2 Challenges in using conditions. The potential application of POMOF materials in the biomedical field is promising. However, there are certain limitations regarding their usage conditions. The stability of POMOF materials within biological environments must be considered as a critical factor.¹³⁵ Given the intricate chemical reactions and physiological settings in living organisms, material stability is pivotal to preventing adverse reactions during prolonged usage. Furthermore, comprehending the degradation and metabolic pathways of POMOF materials within the body is essential to ensure their safe, controlled decomposition and elimination.¹³⁶ Evaluating material toxicity and potential side effects is imperative prior to clinical application, ensuring no risks to human health. Therefore, as POMOF materials advance into biomedical applications, considering these usage condition constraints is crucial for their secure and effective integration into clinical practice.¹³⁷ Additionally, further research is needed to understand the POD-like activity and electrochemical behaviour of POMOF materials under varying physical and chemical circumstances.¹³⁸ Investigating the adsorption performance of POMOF materials is also essential.¹³⁹ Exploring the selectivity of POMOF materials towards reaction substrates and conditions could lead to enhanced performance through adjustments such as temperature, pH, and reaction time. In summary, further research is required to comprehensively address the utilisation parameters of POMOF materials.¹⁴⁰

6.2 Prospects

POMOF materials exhibit substantial potential for biomedical applications owing to their exceptional POD-like activity, high-efficiency electrocatalytic capabilities, and effective adsorption performance. Based on the perspective of the future application direction of POMOF materials, the “prospects” part is divided into biomedical testing and biomedical therapies according to the above.

6.2.1 Prospects in biomedical detection. Primarily, POMOF materials exhibit promising prospects in the domains of biomedical detection.¹⁴¹ Their structural and functional tunability enables the design of multifunctional agents for bioimaging, thereby paving novel paths for precision medicine. Notably, prevalent detection techniques employing POMOF materials predominantly leverage their POD-like activity. Hence, the possibility of substituting POMOF materials for POD enzyme-based detection emerges as a crucial consideration. Currently, the detection involving POMOF materials predominantly revolves around H₂O₂ detection, independent of enzymatic involvement. Introducing specific oxidases, however, opens doors for decomposing substances such as cholesterol, urea, and Glc into H₂O₂.¹⁴² Consequently, collaborative interactions with other enzyme-mimicking substances are anticipated to advance the detection of biologically active compounds significantly.¹⁴³

The special properties of POMOF materials make them suitable for in vitro diagnostics, such as biosensors. Temperature, pH, and Cyt-C are specifically analysed above. In the future, the detection of more disease markers may also be combined with POMOF materials to detect abnormal internal environments and make sensors with specific functions. In this regard, there are development prospects for nano-robots and wearable sensors.¹⁴⁴ In the future, these materials may play an important role in early disease diagnosis and biomarker detection.

6.2.2 Prospects in biomedical therapy. POMOF materials exhibit remarkable potential as biotherapeutic agents, particularly in various therapeutic modalities, including CDT, PDT, PTT, RT, and EDT.¹⁴⁵ Furthermore, their robust adsorption and catalytic capabilities underscore their significant contributions to forthcoming drug delivery and controlled release strategies. Moreover, these materials can be seamlessly integrated into targeted therapies to enable precise treatment, mitigating the negative impact on healthy cells during tumour intervention. The subsequent analysis delves into specific insights gleaned from the preceding discourse.¹⁴⁶

The first prospect pertains to targeted drug delivery. Currently, the utilisation of POMOF materials for targeted drug delivery remains an underexplored area.¹⁴⁷ The remarkable porous structure and high surface area of POMOF materials offer substantial potential for efficient drug encapsulation and controlled release, rendering them pivotal in the realm of drug delivery.¹⁴⁸ These materials can serve as adept drug carriers to facilitate directed delivery and regulated drug release, thereby enhancing therapeutic efficacy while mitigating toxic and adverse effects. In the context of site-specific targeted drug delivery, the manipulation of surface properties and functionalities of POMOF materials holds promise for achieving precise and focused therapeutic interventions.¹⁴⁹ This advancement is poised to enhance treatment precision, optimise therapeutic outcomes, and minimise collateral harm to healthy cells, ultimately leading to an improved therapeutic experience for patients.

The specific approach to achieve this involves initiating targeted drug delivery using POMOF materials under external stimuli such as magnetic fields and acoustic waves. By modulating the drug release rate based on specific conditions, precise control over drug administration can be attained. This approach can also be integrated with smart wearable devices to enhance the convenience of targeted drug delivery.¹⁵⁰ Furthermore, nanoparticle technology can be leveraged to fabricate nano-sized POMOF drug carriers, such as nanoparticles and nanosheets. These carriers can efficiently transport drugs to specific lesions through passive or active targeting mechanisms, enhancing drug accumulation at the intended site.¹⁵¹

In the context of biomedical treatment, POMOF materials exhibit promising potential. Firstly, remarkable redox performance, biocompatibility, and mechanical strength position POMOFs as potential wound suture materials to facilitate healing processes. Furthermore, POMOF materials can find application in various tumour treatments such as PDT, PTT, and RT.¹⁵² Through the modulation of their optical and thermal properties, POMOF materials can effectively concentrate light or heat energy onto tumour regions, achieving targeted and selective tumour destruction.¹⁵³ Moreover, POMOF materials hold great promise as radiation therapy sensitisers, augmenting tumour cell sensitivity to radiation and enhancing the efficacy of radiation therapy. The realisation of this objective can be synergistically coupled with PTT and PDT through the utilisation of nanorobots.¹⁴⁴ This integration is anticipated to inaugurate a novel domain of nanorobot manipulation, propelling the emergence of cutting-edge medical apparatuses characterised by organelle-level resolution.¹⁵⁴ Such advancements herald a new era of medical machinery, facilitating the attainment of personalised medicine tailored to the distinctive requirements of individual patients.¹⁵⁵ This encompasses tailored drug delivery systems and therapeutic regimens, thereby offering a comprehensive and adaptable approach. POMOF materials may also extend to wearable technology and health monitoring, furnishing a more exact and sensitive platform for health surveillance.

Author contributions

Lijin Wang: investigation, visualization; Pengyu Dai: data curation, writing-original draft; Hongli Ma: conceptualization, software; Tiedong Sun: writing-reviewing and editing; Jinsong Peng: supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (No. 52372264) and the Natural Science Foundation of Heilongjiang Province (No. LH2023B002).

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Footnote

† These authors contributed equally to this work and should be considered co-first authors.

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