pH-modulated double LCST behaviors with diverse aggregation processes of random-copolymer grafted silica nanoparticles in aqueous solution

Abstract

Thermo-responsive hybrid nanoparticles composed of silica-core and poly(N,N-dimethylaminoethyl methacrylate-co-N-isopropylacrylamide) P(DMAEMA-co-NIPAM) copolymer-shell were prepared through a one-pot surface-initiated atom transfer radical polymerization (ATRP) technique. The well-defined core–shell hybrid nanoparticles with copolymer shell of uniform thickness were revealed by transmission electron microscopy (TEM). The thermo-responsive behavior of the hybrid nanoparticles in aqueous solutions was evaluated through combined techniques, including ultraviolet-visible spectrophotometer (UV-vis) and dynamic light scattering (DLS) analysis. Interestingly, pH-modulated LCST behavior with diverse aggregation processes can be observed. Specifically, the random copolymer grafted nanoparticles presented an LCST behavior with one-step transition process of shrink in acidic solution, a double LCST behavior with three-step transition process of shrink–shrink–aggregation in neutral solution, and a double LCST behavior with two-step transition process of assembly-shrink/aggregation in basic solution. The difference was attributed to the pH-modulated imbalances of electrostatic repulsion and hydrophobic interaction of the attached copolymer chains. Overall, these results disclose that pH and temperature can act as efficient modulators for the programmable control and fine-tuning of the morphology and aggregate size of the core–shell functionalized nanoparticles.

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1 Introduction

Thermo-responsive polymers, regarded as a class of smart materials, which can tune their physical or chemical properties to the changes of the environmental temperature, have become one of the research hotspots. The characteristic property of these polymers is that they can undergo the phase transition from a soluble to an insoluble state in solvent at the lower critical solution temperature (LCST) or the upper critical solution temperature (UCST).^1,2 The functionalization of inorganic nanoparticles with thermo-responsive polymers has aroused great interest since it can provide the composite materials unique properties incorporated the properties of both organic and inorganic components.³ The development of these smart hybrid materials has a great impact on the applications in drug delivery,^4–7 interfacial transport,^8–10 chemical/physical sensing,^11–13 catalysis^14–16 and other fields.

Incorporating different monomers which have thermo-responsive properties with LCST and/or UCST to be copolymers will give an opportunity to form new multi-responsive systems with largely extended diversity and flexibility of the thermo-responsive behavior. These multi-thermo-responsive systems usually contain polymers that exhibit different LCSTs,^17–20 or both LCST and UCST^21–23 in aqueous solutions. Usually, the phase transition behavior of thermo-responsive systems can be tuned by the chain length and the chemical composition of polymers, the ionic strength, the solvent polarity and the solution pH.^24–29 If pH sensitive moieties contained, the system is expected to present pH-tunable phase transition behavior. Changing pH is a simple and effective way to tune the thermo-responsive behavior and it may have potential value in biological applications.^30–32 Large reports have focused on the pH-tunable phase transition behavior of bulk polymers in solution. Change of phase transition temperature^33,34 or phase transition type³⁵ and multiple phase transitions³⁶ by simply adjusting the solution pH have been achieved. Savoji et al.³⁷ studied the thermo-responsive behavior of random copolymers poly(N,N-propylacrylamide-co-2-(diethylamino)ethyl methacrylate) and found that the cloud point in the range of 68–13 °C was strongly dependent on the solution pH. Jiang et al.³⁸ demonstrated that a multiple and reversible phase transition of poly(ethylene oxide)-b-poly(methoxydi(ethylene glycol) methacrylate-co-methacrylic acid) block copolymers can be realized through adjustment of the solution pH. In our previous work³⁵ the thermo-response of block copolymers, poly(ethylene glycol) methyl ether-b-poly(N,N-dimethylaminoethyl methacrylate) (mPEG-PDMAEMA), from no response in acidic media to an LCST transition in neutral media and an LCST and UCST transition in basic media was observed. While rare example about pH-modulated phase transition behavior of surface grafted polymers was reported, especially for the diverse aggregation behaviors derived from the synergistic effect of pH and temperature. Several groups^39–41 reported the size of PDMAEMA-modified nanoparticles in aqueous solution was highly pH-dependent. PDMAEMA grafted hollow mesoporous silica nanoparticles was synthesized by Yu et al.⁴² and a good pH-controlled release behavior was observed with PDMAMEA chain as a gatekeeper. Yang et al.⁴³ achieved the “on–off” switchable drug release system by utilizing the pH-responsive property of poly(2-(diethylamino)ethyl methacrylate)-b-poly(N-isopropylacrylamide) (PDEAEMA-b-PNIPAM) coated mesoporous silica nanoparticles. Matsumoto et al.⁴⁴ reported a pH-tunable thermo-responsive aggregation behavior of a poly(N-isopropylacrylamide-co-acrylic acid) conjugated protein nanoparticle system, which may be useful in drug delivery. Zhang et al.⁴⁵ synthesized asymmetric polymer grafted silica nanoparticles with one hemisphere coated with PNIPAM and the other coated with block copolymer PNIPAM-b-PDMAEMA. They claimed this polymer/inorganic nanocomposite presented UCST in pH 4 to LCST transition in pH 7 and pH 9. But the phase transitions were less obvious through UV-vis spectroscopy and there needs a detailed discussion about the reason. A pH-tunable thermo-responsive behavior of PDMAEMA grafted silica nanoparticles was reported by Dong et al.⁴⁶ They found that the LCST of the system decreased with the increasing of the solution pH, which was explained by the electrostatic repulsion of protonated PDMAEMA in low pH.

In addition, a study of the detailed phase transition process was important to gain a better understanding of the thermo-responsive behavior. It was found that the thermo-responsive behavior of surface modified polymers was not so coincident with that of corresponding bulk polymers. That may be accounted for the confinement of the polymer chains on the surface and the steric hindrance among the crowded polymer chains.^47,48 Chakraborty et al.⁴⁹ reported that gold nanoparticles grafted PNIPAM presented slower aggregation kinetics compared with the bulk PNIPAM. The phase transition process of bulk polymers with pH-modulated thermo-responsive property was relatively simple. It was about the transition between unimers, micelles and aggregates through changes in the degree of ionization altered by pH. While the pH-modulated transition processes of polymers on surface with more flexibility and diversity need investigation.

In this paper, two thermo-responsive polymers, PDMAEMA and PNIPAM, were used to decorate the silica nanoparticles by one-pot surface-initiated atom transfer radical polymerization (ATRP) technique. As PDMAEMA is a pH-dependent thermo-responsive polymer and PNIPAM has an LCST around 32 °C, the hybrid nanoparticles are expected to present a pH-modulated thermo-responsive behavior. This study may help us gain a better understanding of the phase behavior of copolymer grafted silica nanoparticles and their tunable thermo-responsive behavior with pH.

2 Experimental section

2.1 Materials

Tetraethoxysilane (TEOS, A.R, Ling Feng Chemical Reagent Co. Ltd.) was purified through reduced pressure distillation. Copper(I) chloride (CuCl, C.P, Sinopharm Chemical Reagent Co. Ltd.) and copper(I) bromide (CuBr, C.P, Sinopharm Chemical Reagent Co. Ltd.) were stirred in acetic acid and dried before use. N,N-Dimethylaminoethyl methacrylate (DMAEMA, 97%, Alfa Aesar) was purified through an alumina column to remove the inhibitor, and N-isopropylacrylamide (NIPAM, 98%, Alfa Aesar) was purified by recrystallization from n-hexane for the removal of inhibitor. Toluene, triethylamine and anisole were distilled prior to use. Reagents, 3-aminopropyltriethoxysilane (APTES, 98%, Sinopharm Chemical Reagent Co. Ltd.), 2-bromoisobutyryl bromide (BIBB, 98%, Alfa Aesar), 2,2′-bipyridine (bpy, 98%, Alfa Aesar) and N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA, A.R, Shanghai Chemical Industry Development Co. Ltd.) were used as received. Polymer solution with different pHs was adjusted using hydrochloric acid (HCl, A.R, Sinopharm Chemical Reagent Co. Ltd.) and sodium hydroxide (NaOH, A.R, Sinopharm Chemical Reagent Co. Ltd.).

2.2 Synthesis of initiator modified silica nanoparticles

Initiator modified silica nanoparticles were synthesized as described in a previous report.³ Briefly, ethanol (500 mL) and ammonium hydroxide (25 wt% in water, 25 mL) were added into a flask, a mixture of tetraethoxysilane (TEOS, 12.5 mL) and ethanol (14 mL) was added dropwise to the solution under vigorous magnetic stirring. After stirring for 20 h at room temperature, the solvent was removed by rotary evaporation. The formed silica nanoparticles, denoted as SiO₂, were centrifuged and washed with ethanol for several times and then dried under vacuum. The obtained SiO₂ were dispersed in ethanol (45 mL) and heated up to 60 °C under magnetic stirring. Ammonium hydroxide (25 wt% in water, 50 μL) and APTES (0.4 mL) was added respectively. The mixture solution was allowed to react at 60 °C for 12 h. The formed nanoparticles denoted as SiO₂–NH₂, were centrifuged and washed with toluene for several times and then dried under vacuum. The obtained SiO₂–NH₂ were re-dispersed in a mixture of anhydrous toluene (45 mL) and triethylamine (0.5 mL) and cooled to 0 °C. And then, 2-bromoisobutyryl bromide (0.25 mL) was added dropwise under stirring. The solution was stirred at 0 °C for 0.5 h and then at room temperature for 12 h to obtain the initiator modified silica nanoparticles (SiO₂–Br). The nanoparticles were then centrifuged and washed with toluene for several times and then dried under vacuum.

2.3 Synthesis of PNIPAM/PDMAEMA grafted silica nanoparticles

The modification of silica nanoparticles with PNIPAM and PDMAEMA was conducted via an ATRP reaction. The 2-bromoisobutyrate-functionalized silica nanoparticles (SiO₂–Br, 300 mg), bpy (42.7 mg), MeOH/H₂O (4 mL, v/v = 1 : 1), NIPAM and DMAEMA (total 5.5 mmol at various mole ratios) were added to a Schlenk flask. Dispersed by ultrasonication, the mixture was degassed via three freeze–pump–thaw cycles. CuCl was added under an N₂ flow. Finally, the mixture was degassed by an additional freeze–pump–thaw cycle and then stirred at room temperature for several hours. The solution was exposed to air to terminate the polymerization and dialyzed against distilled water to remove copper(II) ion. After lyophilization, white products were obtained, denoted as SiO₂-g-P(NIPAM-co-DMAEMA). Synthesis of PDMAEMA grafted silica nanoparticles (SiO₂-g-PDMAEMA) was conducted in a similar way, except for the polymerization temperature of 60 °C and the use of CuBr, PMDETA and anisole instead of CuCl, bpy and MeOH/H₂O, respectively. The as-prepared products were labeled as SiO₂-D100, SiO₂-N50D50 and SiO₂-N83D17, where N and D denoted PNIPAM and PDMAEMA, respectively. The numbers represent the mole percent of each monomer in the feed (see ESI, Table S1†). For example, SiO₂-N83D17 means the amount of NIPAM and DMAEMA used in the polymerization was at a mole ratio of 83/17.

2.4 Characterization

Fourier transform infrared (FTIR) spectrum was recorded on a Nicolet Magna-IR 550 FTIR spectrometer in the range of 4000–400 cm⁻¹. Thermogravimetric analysis (TGA) was carried out on a Netzsch STA449F3 TG-DSC between 25 to 600 °C at a heating rate of 10 °C min⁻¹ under the nitrogen atmosphere. Transmission electron microscopy (TEM) analysis was performed with a JEOL JEM-1400 electron microscope. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Kratos Axis Ultra DLD spectrometer with a monochromatic Al Kα X-ray source and a constant pass energy of 40 eV. Ultraviolet-visible spectrophotometer (UV-vis) was recorded using a Shimdzu UV-2450 at 500 nm. The aqueous solution of polymer grafted nanoparticles with acidic pH, neutral pH and basic pH was heated from 25 °C to 79 °C at a heating rate of 0.2 °C min⁻¹. Dynamic light scattering (DLS) was performed with a ZEM4228 Malvern Instruments Zetasizer NanoZS Instrument. The measurements were conducted in the temperature range of 20–80 °C with an increment of 2 °C and an equilibration time of 5 min.

3 Results and discussion

3.1 Synthesis and characterization of polymer-functionalized silica nanoparticles

The approach of the preparation of PNIPAM and PDMAEMA grafted silica nanoparticles is shown in Scheme 1. Silica nanoparticles (SiO₂) were synthesized by the Stöber process and the amino-functionalized silica nanoparticles (SiO₂–NH₂) were prepared through the self-assembly of APTES on the silica surface. The amino groups then reacted with BIBB to obtain initiator-functionalized silica nanoparticles (SiO₂–Br), which initiated ATRP of NIPAM and DMAEMA from the surface.

Scheme 1 Synthesis of SiO₂-g-P(NIPAM-co-DMAEMA) silica nanoparticles by one-pot surface initiated ATRP.

FTIR spectra and TGA were conducted to confirm the successful polymerization process. Amide moieties introduced by BIBB were verified by the absorption at 1536 cm⁻¹ (Fig. 1b), as compared to that of bare silica nanoparticles (Fig. 1a). The characteristic absorptions at 1732 cm⁻¹ (ester carbonyl group) (Fig. 1c–e) and 1665 (amide I band), 1536 cm⁻¹ (amide II band) (Fig. 1d and e), suggested the presence of PDMAEMA and PNIPAM respectively. TGA results were shown in Fig. 2A. The difference in the weight retentions between SiO₂, SiO₂–NH₂ and SiO₂–Br indicates the successful functionalization of each step. A more direct comparison on the organic content of various samples is shown in Fig. 2B, which is obtained from the weight loss in the temperature range of 115–600 °C in Fig. 2A. After polymerization, an obvious increase in the organic content of 40.8%, 55.3%, and 64.8% for SiO₂-D100, SiO₂-N50D50 and SiO₂-N83D17 systems, respectively, can be seen. It is suggested that NIPAM is prone to be initiated from the silica surface through ATRP as compared to DMAEMA.

Fig. 1 FTIR spectra of (a) SiO₂, (b) SiO₂–Br, (c) SiO₂-D100, (d) SiO₂-N50D50 and (e) SiO₂-N83D17.

Fig. 2 (A) TGA curves for (a) SiO₂, (b) SiO₂–NH₂ (c) SiO₂–Br, (d) SiO₂-D100, (e) SiO₂-N50D50 and (f) SiO₂-N83D17. (B) Organic content of the resultant silica nanoparticles.

The morphology of the SiO₂-g-P(NIPAM-co-DMAEMA) nanoparticles was investigated and observed using XPS and TEM. After polymerization, an obvious increase in N and C content, and a disappearance of Si peak were observed (Fig. 3a), indicated the coverage of polymers on the surface. For polymer functionalized silica nanoparticles, the C 1s spectra were fitted with the expected four contributions where four peaks at band energy of about 284.14 eV, 284.78 eV, 285.5 eV and 287.61 eV were assigned to C̲–C, C̲–N, C̲–O and C̲=O, respectively (Fig. 3b). The core–shell structure with a polymer shell thickness of approximately 10 nm of polymer grafted silica nanoparticles is clearly observed from the TEM images (Fig. 4b–d), in comparison with the TEM image of SiO₂–Br (Fig. 4a).

Fig. 3 (a) XPS spectra and (b) XPS high resolution C 1s spectra of SiO₂-D100, SiO₂-N50D50 and SiO₂-N83D17.

Fig. 4 TEM images of (a) SiO₂–Br, (b) SiO₂-D100, (c) SiO₂-N50D50 and (d) SiO₂-N83D17. The scale bar in the inset is 50 nm.

3.2 pH induced LCST behavior with diverse aggregation processes in aqueous solution

Since both PNIPAM and PDMAEMA are thermo-responsive polymers where PNIPAM shows an LCST of approximately 32 °C in aqueous solution and PDMAEMA shows a pH-dependent thermo-responsive behavior, diverse LCST behaviors of the PNIPAM/PDMAEMA grafted silica nanoparticles in different pH solutions are expected. In this study, we prepared a series of aqueous solution containing SiO₂-D100, SiO₂-N50D50 or SiO₂-N83D17, respectively (5 mg mL⁻¹) at three pHs in order to investigate their pH-tunable LCST behaviors. For acidic and basic media, the solution pHs were set to be 5.0 and 10.0, respectively; while for the neutral solution, there was a little difference for each system and the solution pHs were 7.3, 8.5, and 6.8 for SiO₂-D100, SiO₂-N50D50 and SiO₂-N83D17, respectively.

The transmittance of polymer grafted silica nanoparticles with changes in temperature in acidic solution is shown in Fig. 5A. As can be seen, all samples presented a small and smooth increase in transmittance upon heating, indicating the LCST behavior. In this condition, the PDMAEMA moieties were highly protonated due to its pK_a of around 7.3.³³ The grafted polymer chains were highly extended in the solution due to the strong inter/intra-chain electrostatic repulsion and the strong hydrogen bonding between polymer chains and water. With temperature rising, the extended chains started to shrink to be less stretched ones, ascribed to the intra- and interchain hydrophobic interactions. This can be proved by the DLS results as shown in Fig. 6A and B. Similar results about PDMAEMA grafted silica nanoparticles were reported elsewhere.⁴⁶ Note that there was only a gentle increase in transmittance, which can be implied that the electrostatic repulsion limited the shrink, thus leading to a less obvious LCST transition. This transition cannot be identified solely from PNIPAM or PDMAEMA, but resulted from the thermo-response of both polymers. In addition, the sample with higher PDMAEMA content displayed a smaller change in transmittance, consistent with the stronger electrostatic repulsive interactions.

Fig. 5 Temperature dependent transmittance of different polymer grafted silica nanoparticles in (A) acidic, (B) neutral and (C) basic solutions. The “1^st” means the first phase transition and the “2^nd” means the second phase transition.

Fig. 6 Temperature dependent hydrodynamic diameters and derived count rates of SiO₂-N50D50 (A, C and E) and SiO₂-N83D17 (B, D and F) in acidic (A and B), neutral (C and D) and basic solutions (E and F).

From Fig. 5C, SiO₂-D100 in basic solution exhibited a typical LCST behavior with the LCST of about 64 °C, while the other two samples containing PNIPAM showed a broad and slight transition in a lower temperature range of 25–45 °C. Since the transmittance just reflects the collective behavior of all particles in macroscopic appearance, DLS measurement can sensitively monitor the statistical changes of the particles. Fig. 6E and F about the hydrodynamic diameters (D_h) and derived count rate as a function of temperature for SiO₂-N50D50 and SiO₂-N83D17 proved that there existed a two-stage transition in both systems ascribed to the collapse of PNIPAM at approximately 38 °C and that of PDMAEMA at approximately 58 °C, respectively. The significant increase in the derived count rate above 38 °C suggested the changes in interchain/interparticle association in this temperature range⁵⁰ and a transition from multimodal to monomodal distributions in the Z-average size of the nanoparticles revealed a self-assembly process. Additionally, a sharp increase in the D_h and a slight decrease in the derived count rate were observed at temperature above 54 °C, indicating the collapse of PDMAEMA accompanied by immediate aggregation of nanoparticles. These results confirmed the double LCST behavior of PNIPAM and PDMAEMA grafted silica nanoparticles in basic solution with a two-step transition from assembly to shrink and aggregation.

A typical LCST behavior at about 75 °C in SiO₂-D100 neutral solution was observed, which was similar to that in basic solution with the LCST of about 64 °C. However, P(NIPAM-co-DMAEMA) grafted silica nanoparticles showed very different behaviors in neutral solutions from those in basic solutions. Interestingly, a two-step drop in transmittance was clearly observed upon heating in SiO₂-N83D17 solution (Fig. 5B). It indicated the double LCST thermo-responsive behaviors of polymer grafted silica nanoparticles resulted from the collapse of the PNIPAM and that of PDMAEMA. The first step in the temperature range of 35–46 °C was assigned to the collapse of PNIPAM while the second step in the temperature range of 65–71 °C corresponded to the collapse of PDMAEMA. Although it was not so evident in SiO₂-N50D50 solution to give a narrow transition temperature range, there still existed a two-step drop in transmittance upon heating. The difference may be attributed to the random architecture and the mole ratio of the two components as reported in the block copolymer systems.^51,52 Specifically, random copolymer with equivalent PNIPAM and PDMAEMA content resulted in more homogenous dispersion of both PNIPAM and PDMAEMA segments which led to the shortened segment length and the suppressed aggregation of each component. Thus, the broader and weaker phase transition of SiO₂-N50D50 than that of SiO₂-N83D17 was obtained. The DLS results give a detailed information about the transition process. Fig. 6C and D presented the three-step transition processes of SiO₂-N50D50 and SiO₂-N83D17 with two slight decreases and one evident increase in D_h. This transition can be identified to the collapse of PNIAPM, collapse of PDMAEMA and aggregation of the nanoparticles, respectively. Furthermore, a more obvious transition in D_h upon heating of SiO₂-N83D17 was coincident with the results from turbidity measurement as shown in Fig. 5B.

The size of the polymer grafted silica nanoparticles in aqueous solution was not only temperature dependent but also pH dependent. In order to verify the effect of pH, sample SiO₂-N33D67 with more PDMAEMA content was chosen to examine the pH dependent hydrodynamic diameters at 25 °C, as shown in Fig. 7. The D_h decreased from 800 nm to 180 nm with the increase of pH value from 3.0 to 9.0. Similar to the reasons discussed above, it was also attributed to the competition between the inter/intra-chain electrostatic repulsion of the highly protonated PDMAEMA moieties at low pH and the hydrophobic interaction of the gradually deprotonated PDMAEMA at higher pH. For the samples with less PDMAEMA component, similar trend can be observed with smaller decrement (see ESI, Fig. S1†). This was consistent with the PDEAEMA-coated mesoporous silica nanoparticles reported by Sun et al.³⁹

Fig. 7 pH-dependent hydrodynamic diameters of SiO₂-N33D67 at 25 °C.

A proposed model for the aggregation behavior of P(NIPAM-co-DMAEMA) random copolymer grafted silica nanoparticles is shown in Fig. 8. Different transition behaviors of SiO₂-g-P(NIPAM-co-DMAEMA) can be observed in aqueous media with different pH values. The pH-modulated variation of electrostatic repulsive interactions resulted in the interchain/interparticle association and disassociation and the delicate balance between them. In acidic solution, copolymer chains with high charge densities resulted in the strong electrostatic interchain repulsions and interparticle repulsions which led to a LCST behavior with only one-step transition process of the shrink of copolymers. It should be noted that the LCST was indistinguishable from that of PNIPAM or PDMAEMA and it was ascribed to the compromise of the phase transition of both polymer segments. A competition of the reduced interchain and interparticle electrostatic repulsion and the stronger intrachain association in neutral solution allowed a stable intermediate state of the two component segments to be bead-necklace morphology⁵³ which led to the two-step collapse individually and then a further increase in temperature caused aggregation. While in basic solution, there was no electrostatic repulsion to disperse and stabilize nanoparticles as both PNIPAM and PDMAEMA collapsed upon heating and the hydrophobic interactions between dehydrated copolymers resulted in an immediate aggregation as soon as the PDMAEMA collapsed. SiO₂-g-P(NIPAM-co-DMAEMA) in neutral and basic solution presented double LCST behaviors but underwent different transition processes, corresponding to a three-step transition from shrink to shrink to aggregation and a two-step process of assembly to shrink/aggregation, respectively. The former is attributed to the electrostatic repulsion and hydrophilic–hydrophobic balance, while the latter is attributed to only the hydrophilic–hydrophobic competitions.

Fig. 8 Schematic illustration of the proposed aggregation behavior of modified silica nanoparticles SiO₂-g-P(NIPAM-co-DMAEMA) in (a) acidic, (b) neutral and (c) basic solutions.

4 Conclusions

PNIPAM/PDMAEMA grafted silica nanoparticles (SiO₂-D100, SiO₂-N50D50 and SiO₂-N83D17) with different polymer contents were prepared through a one-pot ATRP technique. The hybrid nanoparticles with well-defined core–shell nanostructure were successfully obtained confirmed by a series of characterization methods including TEM, FTIR, TGA and XPS. The phase aggregation behaviors of hybrid nanoparticles in aqueous media with different pH values were systematically explored by combined techniques of UV-vis and DLS measurements. Interestingly, the SiO₂-g-P(NIPAM-co-DMAEMA) presented pH-tunable LCST behaviors. Specifically, a one-step transition process of shrink in acidic solution, a double LCST behavior with three-step transition processes of shrink–shrink–aggregation in neutral solution, and a double LCST behavior with two-step transition processes of assembly-shrink/aggregation in basic solution were observed. Besides, the pH-modulated electrostatic repulsive interactions and the hydrophobic–hydrophilic balance were attributed to the different interchain/interparticle aggregation behaviors. This intriguing diverse pH-modulated aggregation properties of such simple system will provide new idea and easy way in the design and applications of smart hybrid nanoparticles.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21176065, 21376073, 91334203) and the 111 Project of Ministry of Education of China (No. B08021) and the Fundamental Research Funds for the Central Universities of China.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13557e

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