Glucose-Induced Transition among Three States of a Doped Microgel

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Glucose-induced transition among three states of a doped microgel colloidal crystal Zhuo Tang, Siyu Jia, Lijuan Yao, Ying Guan, and Yongjun Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01341 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 24, 2018

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Glucose-induced transition among three states of a doped microgel colloidal crystal Zhuo Tang,† Siyu Jia,† Lijuan Yao,† Ying Guan*,† and Yongjun Zhang*,†,‡

† Key Laboratory of Functional Polymer Materials and State Key Laboratory of Medicinal Chemical Biology, Institute of Polymer Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China ‡ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, China.

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ABSTRACT

For the first time here, we report a colloid crystal capable of undergoing transition among three states in response to external stimuli. The colloidal crystal was assembled from poly(Nisopropylacrylamide) (PNIPAM) microgel and doped with poly(N-isopropylacrylamide-co-2acrylamido-phenylboronic acid) (P(NIPAM-2-AAPBA)) microgel. The ordered structure was locked by in situ photo-polymerization. Taking advantage of the different response of the two microgels to external stimuli, defect state can be induced and erased reversibly. Particularly because the dopant, i.e., P(NIPAM-2-AAPBA) microgel sphere, shrinks with increasing glucose concentration, its size changes from larger than the host, i.e., PNIPAM microgel sphere, to equal to the host, and finally smaller than the host. Therefore, upon addition of glucose, the crystal undergoes transition from a state with acceptor-type defect, to no defect state, and then to a state with donor-type defect. The transition among the three states is fully reversible. In addition, the response of the doped crystal to glucose is relatively fast.

Keywords: artificial defects; colloidal crystals; defect states; microgels; glucose-sensitive


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Introduction Photonic crystals (PhCs) possess spatial periodicity in their dielectric constant on order of the wavelength of light and therefore exhibit a partial or full photonic bandgap. Analogy to the doping of semiconductors, controlled introduction of artificial defects is required to fulfill the functions of PhCs.1 Colloidal crystals (CCs) self-assembled from monodisperse colloids have been widely studied as PhCs due to their ease of fabrication and low cost.1 Unlike the doping of semiconductor, however, introducing artificial defects into CCs is not compatible with the selfassembly of colloids and therefore remains a big challenge.1-3 The techniques developed up to now, for example lithography,4 multi-photon photopolymerization,5 and electron-beam writing,6 are usually time-consuming, limited to small areas, and possibly not applicable to mass production. In addition, once the artificial defects are introduced, it is generally impossible to tune the optical properties of the crystals, which severally limited their applications. The only exceptions may be CCs with smart planar defects (polyelectrolyte multilayer) designed by Ozin et al.7-10 By applying an external stimulus such as light,7 a small shift in defect-state wavelength could be achieved. Usually colloidal crystals were assembled from hard colloids, such as SiO2, polystyrene (PS), and poly(methyl methacrylate) (PMMA) microspheres. Indeed they can also be assembled from soft hydrogel microspheres, for example poly(N-isopropylacrylamide) (PNIPAM) microgels.11-16 The soft nature of the hydrogel microspheres makes the resulting microgel colloidal crystals intrinsically defect-tolerant.17 Therefore they are promising candidates for the fabrication of large arrays of colloidal crystals.18 In addition, the size of microgel spheres can change in response of certain external stimulus.19-21 However, like CCs from hard colloids, doping a microgel CC is difficult too, because the introduction of microgel spheres with a different size (dopant) will


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severely lower the ordering of the resulting structure.22 To circumvent the problem, we recently demonstrated that doped microgel CCs can be facilely assembled from two microgels with the same size. By applying an external stimulus, such as temperature and pH, defect state can be induced and erased reversibly into the doped microgel CCs.23-25 When inducing and erasing defect state in the doped crystals using external stimuli, the crystals are switched between two states, i.e., a state with defect and a state without defect.23 Here a new doped microgel CCs was designed. Glucose, a biological stimulus, can be used to induce and erase defect state in the crystal. More importantly, the crystal can be switched among three states reversibly, i.e., a state with acceptor-type defect, a state without defect, and a state with donor-type defect, instead of just between two states as reported before.

Experimental details Materials N-Isopropylacrylamide (NIPAM) was purchased from Tokyo Chemical Industry Co. 2Hydroxyethyl methacrylate (HEMA) and 2-aminophenylboronic acid (2-APBA) was purchased from ACROS. N-(3-Dimethylaminopropyl)-N’-ethyl-carbodiimide hydrochloride (EDC) and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich. N, N’-methylenebis (acrylamide) (BIS) and 2, 2-diethoxyacetophenone (DEAP) were purchased from Alfa Asear. Acrylic acid (AAc), potassium persulfate (KPS), acryloyl chloride and 4-dimethylaminopyridine (DMAP) were purchased from Tianjin Chemical Reagent Company. NIPAM was purified by recrystallization from hexane/acetone mixture and dried in a vacuum. AAc and HEMA were distilled under reduced pressure. Other reagents were used as received.


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Microgel Synthesis To synthesize poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAM-AAc)) copolymer microgel, 1.314 g of NIPAM, 0.032 g of BIS, 0.150 g of AAc and 0.029 g of SDS were first dissolved in 95 mL of water. The solution was transferred to a three-neck round-bottom flask equipped with a condenser and a nitrogen inlet and was heated to 70oC under a gentle stream of nitrogen. After stabilizing for 1 h, 0.081 g of KPS (dissolved in 5 mL of water) was added to initiate the reaction. The reaction was allowed to proceed at 70oC for 4 h. The resulting microgel was purified by dialysis (cutoff: 8000–15 000 Da) against water, with water change at least twice daily, for 2 weeks. The purified microgel was then lyophilized and stored in a refrigerator. Poly(N-isopropylacrylamide) (PNIPAM) microgel was synthesized in the same way. In this case, 1.402 g of NIPAM, 0.023 g of BIS and 0.058 g of SDS were fed. To modify PNIPAM microgel with surface vinyl groups, a thin poly(N-isopropylacrylamideco-2-hydroxyethyl methacrylate) (P(NIPAM-HEMA)) shell was first added onto the PNIPAM core. For this purpose, 100 mL of PNIPAM microgel solution was added into a three-neck round-bottom flask equipped with a condenser and a nitrogen inlet and was heated to 70oC under a gentle stream of nitrogen. A preheated stock shell solution, which was prepared by dissolving 1.241 g of NIPAM, 0.023 g of BIS, 0.365 g of HEMA and 0.058 g of SDS in 95 mL of water, was then added. After purging with N2 for another 1 h, 0.081 g of KPS (dissolved in 5 mL of water) was added to initiate the polymerization. The resulting core-shell microgel was again purified by dialysis, lyophilized and stored in a refrigerator. Vinyl groups were then introduced by reaction with acryloyl chloride. In a typical experiment, 0.45 g of core-shell microgels with active hydroxyl groups was suspended in 15 mL of DI water in a 100 mL flask. Then, 0.5 mL of acryloyl chloride and 0.05 g of DMAP were added to the


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flask and the reaction was carried out at 0oC for 4 h. The resultant products were purified by dialysis against water with frequent water change for three days, lyophilized and stored in a refrigerator. Preparation of P(NIPAM-AAc) microgel-Doped PMCC (polymerized microgel colloidal crystal) films 0.0643 g of lyophilized vinyl-modified PNIPAM microgel and 0.0072 g of lyophilized P(NIPAM-AAc) microgel were re-dispersed in 2 mL of deionized water, to which 10 µL of DEAP (0.24 mol/L in DMSO) was added. The resulting solution was then added into the space between a glass substrate and a quartz slide separated by 4 layers of Parafilm. The samples were kept at room temperature and became iridescent in a few hours. They were then photopolymerized by exposure to UV light for 24 h (λ = 365 nm). The resulting films were released from the cuvettes and stored as freestanding films in water. Preparation of P(NIPAM-2-AAPBA) microgel-Doped PMCC films To convert the P(NIPAM-AAc) microgel spheres in P(NIPAM-AAc) microgel-doped PMCC film, a mixture solution was prepared by dissolving 0.233 g of 2-APBA and 0.239 g of EDC in 50 mL of water. The solution was then cooled to 0 oC, to which a P(NIPAM-AA) microgel doped-PMCC film was added. The reaction was allowed to proceed at 0 oC for 4 h. The resulting PMCC film was washed extensively in DI water and stored as freestanding film in pH 7.4 phosphate buffer. Characterizations The hydrodynamic radius (Rh) of the microgel particles was measured by dynamic light scattering with a Brookhaven 90Plus laser particle size analyzer. All the measurements were carried out at a scattering angle of 90°. The sample temperature was controlled with a build-in


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Peltier temperature controller. Reflection spectra of the PMCC films were measured with an AvaSpec-2048 Fiber Optic spectrometer. The temperature of the sample cell was controlled with a refrigerated circulator. The experimental setup was shown in Figure S1.

Scheme 1 (A) Synthesis of polymerized PNIPAM microgel colloidal crystal doped with P(NIPAM-2-AAPBA) microgel. (B) P(NIPAM-2-AAPBA) microgel spheres shrink with increasing [Glu], leading to the erasing of defect state in the crystal. Further increasing [Glu] results in the appearance of a new defect state. The microgel colloidal crystals are close packed. For the sake of clarity, a non-close packed structure was depicted here.

Results and Discussion The synthesis of the doped microgel CCs was depicted in Scheme 1A. We used poly(Nisopropylacrylamide)

(PNIPAM)

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poly(N-


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isopropylacrylamide-co-acrylic acid) (P(NIPAM-AAc)) microgel as host and dopant, respectively. Both microgels are highly monodisperse, and their sizes are comparable at room temperature (Figure S2). Therefore, mixed dispersions of the two microgels (the weight ratio of PNIPAM microgel to P(NIPAM-AAc) microgel is 9:1) crystallize just like pure microgel dispersions. It is well-known that the introduction of impurities usually prevents a colloid dispersion from crystallization.22,26,27 Here the problem was successfully circumvented by using two colloids with the same size. The ordered structure of the resulting doped microgel CCs is very fragile, as it can be easily destroyed by heating or shearing.28 To stabilize the ordered structure, the samples were irradiated with UV light in the presence of a photo-initiator 2, 2diethoxyacetophenone (DEAP). With the polymerization of the surface-bonded vinyl groups, the microgel spheres were bonded together in situ, resulting in polymerized microgel colloidal crystals (PMCC) which can exist as free-standing hydrogel films.25,29 The P(NIPAM-AAc) microgel-doped PMCC films were then treated with 2-aminophenylnoronic acid (2-APBA) and EDC.30,31 In this way, the P(NIPAM-AAc) microgel spheres in the PMCC films were converted to poly(N-isopropylacrylamide-co-2-acrylamido-phenylboronic acid) (P(NIPAM-2-AAPBA)) microgel spheres,30,31 and P(NIPAM-AAc) microgel-doped PMCC films were converted to P(NIPAM-2-AAPBA) microgel-doped PMCC films. As shown in Figure S3, the P(NIPAM-2AAPBA) microgel-doped PMCC film remains to be iridescent with crystallites range from hundreds of micrometers to millimeters in diameter. Figure S4 shows its reflection spectra, from which a sharp diffraction peak was observed. These results confirm a highly ordered crystalline structure of the film, because the microgel spheres were bonded in situ,29,32-36 and the P(NIPAMAAc) microgel spheres in the PMCC films were converted to P(NIPAM-2-AAPBA) microgel spheres in situ.


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It is well-known that PNIPAM-based microgels are thermosensitive,11-13 so we first checked if defect state can be induced and erased in the crystals by changing temperature. For this purpose, the reflection spectra of a doped PMCC film were measured when it was cooled from 40oC to 32oC. As shown in Figure 1A, the film presents a sharp diffraction peak at 40oC, suggesting no defect state at this temperature. From Figure 1C one can see the sizes of the host and dopant microgels are comparable at this temperature. They are indistinguishable for light; therefore, no defect state was observed. The film does not exhibit defect state at 38oC either, however, when further cooled to 36oC, a narrow dip (transmission window) appears within the stop band frequency, indicating the successful inducing of defect state in the crystal.7-9,37 At this temperature, the dopant P(NIPAM-2-AAPBA) microgel swells, while the size of the host PNIPAM microgel remains unchanged, resulting in a size difference between the two microgels. Therefore, the P(NIPAM-2-AAPBA) dopant spheres become real defects in the crystal and a transmission window appears. Continuing cooling to 34 and 32oC, the P(NIPAM-2-AAPBA) microgel swells to an even larger degree. The size difference between the two microgels becomes even larger. Therefore, the defect state becomes even pronounced. In agreement with previous observations,7-9,23,37 besides the appearance of a transmission window, the stop band is also significantly widened and becomes asymmetric. In addition, the intensity of the diffraction band decreases with decreasing temperature, which may be mainly attributed to an increased degree of disordering in the crystal.24 The stop band also shifts to a longer wavelength, which could be explained by the swelling of the P(NIPAM-2-AAPBA) microgels.


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Figure 1 (A) Reflection spectra of a PMCC film doped with P(NIPAM-2-AAPBA) microgel changes when cooled from 40 to 32oC. The film was immersed in 10 mM pH7.4 phosphate buffer. (B) Reflection spectra of a PMCC doped with P(NIPAM-2-AAPBA) microgels changes when heated from 32 to 40 oC. (C) Hydrodynamic radius (Rh) of the free microgel spheres dispersed in 10 mM pH7.4 phosphate buffer as a function of temperature. The sample was then heated to erase the defect state. As shown in Figure 1B, when heated to 34 and 36oC, there is still a transmission window within the stop band, however, the defect state disappears completely when further heated to 38 and 40oC. In addition, the stop band is restored to be sharp and symmetric. These results suggest that the defect state is successfully erased by raising temperature. As shown in Figure 1C, when heated from 32 to 40oC, the P(NIPAM-2AAPBA) microgel spheres shrink gradually. Their size becomes comparable to the size of the


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host PNIPAM microgel spheres. Therefore the two kinds of microgel spheres become indistinguishable for light again, and the defect state is erased.

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Figure 2 (A) Reflection spectra of a PMCC film doped with P(NIPAM-2-AAPBA) microgel changes with increasing [Glu] from 0 to 60 mM. The film was immersed in 10 mM pH7.4 phosphate buffer at 30oC. (B) Reflection spectra of the film changes with decreasing [Glu] from 60 to 0 mM. (C) Hydrodynamic radius (Rh) of the free microgel spheres dispersed in 10 mM pH7.4 phosphate buffer at 30oC as a function of [Glu]. Previous study reveals that the dopant P(NIPAM-2-AAPBA) microgel is glucose-sensitive. The binding of glucose with phenylboronic acid (PBA) groups in the microgel results in the shrinkage of the microgel.30,31 We therefore used glucose as a second external stimulus to induce


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and erase defect state. As Figure 2A shows, the film exhibits a defect state when soaked in pH 7.4 buffer at 30oC. Under the experimental conditions, the dopant P(NIPAM-2-AAPBA) microgel has a larger size than the host PNIPAM microgel, and therefore can act as real defect.(Figure 2C) We then added glucose into the media. When [Glu] increases to 10 and 20 mM, a distinct transmission window was still observed. When further increasing [Glu] to 30 and 40 mM, however, the transmission window disappears from the stop band. From Figure 2C one can see the P(NIPAM-2-AAPBA) microgel shrinks with increasing [Glu] in the media, while the size of PNIPAM microgel remains unchanged. When [Glu] increases to 40 mM, the sizes of the two microgel becomes comparable. Therefore, the P(NIPAM-2-AAPBA) microgel spheres in the crystal cannot act as real defect and the defect state disappears. This result indicates that defect state in the crystal can be erased by increasing [Glu] in the media. We continued to increase [Glu]. At [Glu]=50 mM the stop band remains sharp. No transmission window appears. Interestingly, when further increasing [Glu] to 60 mM a transmission window appears in the stop band. From Figure 2C one can see the P(NIPAM-2AAPBA) microgel continues shrinking with increasing [Glu]. At [Glu]=60 mM, its size becomes smaller than that of PNIPAM microgel. Therefore, a defect state appears again. The new defect state appears at [Glu]=60 mM is different from the one appears at [Glu]=0 mM. In doped colloidal crystals, the defect can be either “acceptor” or “donor” impurity, depending on its size and refractive index.37-40 For microgel spheres, the refractive index factor dominates over the size factor in determining microgel spheres’ scattering efficiency,41 therefore the type of microgel impurities is depended by their refractive index. At [Glu]=0 mM, the dopant P(NIPAM-2-AAPBA) microgel is larger than the host PNIPAM microgel in size, but smaller than PNIPAM microgel in refractive index. Therefore, it should be regarded as an “acceptor”-


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type impurity under this condition. In contrast at [Glu]=60 mM, because of the glucose-induced shrinkage of the P(NIPAM-2-AAPBA) microgel, its refractive index becomes larger than PNIPAM microgel. Therefore, it becomes a “donor”-type impurity. One can see that, when adjusting [Glu] from 0 to 60 mM, the doped crystal is tuned from a state with acceptor-type defect to a state without defect, and further to a state with donor-type defect. We then adjusted [Glu] back to 0 mM. As shown in Figure 2B, when [Glu] was lowered from 60 mM to 40 mM, the transmission window disappeared again from the stop band, indicating the donor-type defect state was erased. With further decreasing of [Glu], a transmission window appeared, indicating the inducing of an acceptor-type defect state. The result indicates the crystal can transit among the three states reversibly. We further adjusted [Glu] repeatedly from 0 to 60 mM. As shown in Figure 3A, in each cycle, when [Glu] increases from 0 to 60 mM, the doped crystal is tuned from a state with acceptor defect to a state without defect and then to a state with donor defect. Oppositely, when [Glu] decreases from 60 mM back to 0 mM, the crystal transit from a state with donor defect to a state without defect and then back to a state with acceptor defect. In addition, the stop band always shifts back to the same position when [Glu] is adjusted back.


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Figure 3 (A) Reflection spectra of a PMCC film doped with P(NIPAM-2-AAPBA) microgel when the glucose concentration was repeatedly switched between 0, 30mM and 60mM. The film was immersed in 10 mM pH7.4 phosphate buffer at 30oC. (B) Reflection spectra showing the response of a doped PMCC film with time upon adding 10 mM glucose. The film was immersed in 10 mM pH7.4 phosphate buffer at 30oC. The spectra were recorded every 200 s. (C) Shift of the diffraction peak position with time. The solid line shows the best single exponential fit to the data. To study the response kinetics of the doped PMCC films to glucose, the evolution of the reflection spectra of a film upon addition of 10 mM was followed. As shown in Figure 3B, the stop band of the crystal shifts towards lower wavelength gradually with time. Figure 3C shows


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the time course of the shift of the diffraction peak. The kinetic curve can be well-fitted with a single-exponential function, suggesting the swelling of the gel film can be well-described by Tanaka-Fillmore theory.42 From the fitting, the characteristic response time, τ, was determined to be 547.8s, and the collective diffusion coefficient of the gel film was estimated to be ~1×10-4 cm2/s.29 Compared to ordinary PNIPAM hydrogels (collective diffusion coefficient on the order of 10-7 cm2/s),43 the response of the film to glucose is quite fast. The fast response of a PMCC hydrogel can be attributed to its small structural unit (the microgel spheres) and its interconnected porous structure.29,44 Conclusions In conclusion, colloidal crystals of PNIPAM microgel doped with P(NIPAM-AAc) microgel were fabricated from the two microgels of the same size. The ordered structure of the crystals was then locked by in situ photo-polymerization of the surface vinyl groups on PNIPAM microgel spheres. Finally, the P(NIPAM-AAc) microgel spheres in the PMCC films were converted to P(NIPAM-2-AAPBA) microgel spheres by reaction with 2-APBA. Because of the different response of the two microgels to external stimuli, such as temperature and glucose, defect state can be erased and induced reversibly. Particularly when [Glu] increases from 0 to 60 mM, the crystal transfers sequentially from a state with acceptor-type defect, to a state without defect, and finally a state with donor-type defect, because the dopant, P(NIPAM-2-AAPBA) microgel spheres, shrinks with increasing [Glu], and its size changes from larger than the host, to equal to the host, and finally smaller than the host. This is the first report that a colloid crystal can transit among three states. The transition is fully reversible. In addition, the response of the doped crystal to glucose is relatively fast.


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ASSOCIATED CONTENT Supporting Information. Experimental setup for optical measurement, size of microgels, photograph and reflection spectra of PMCC film. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.G.). *E-mail: [email protected] (Y.Z.).

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank financial support for this work from the National Natural Science Foundation of China (Grants Nos: 21374048 and 51625302), Tianjin Committee of Science and Technology (Grants No: 16JCZDJC32900), and the National Key Research and Development Program of China (2017YFA1103501).


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Glucose-Induced Transition among Three States of a Doped Microgel

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