Order–Disorder Transition in Doped Microgel ... - ACS Publications

Letter Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Order−Disorder Transition in Doped Microgel Colloidal Crystals and Its Application for Optical Sensing Siyu Jia,† Zhuo Tang,† 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 S Supporting Information *

ABSTRACT: Hydrogel photonic crystal-based optical sensors usually can only be used as free-standing films. Here, a doped microgel colloidal crystal film was developed as glucose sensor, which exploits structural order−disorder transition, instead of change in lattice constant, to report an analyte. Changing glucose concentration induces a change in structural order degree in the crystal and hence a change in the intensity of the stop band, and thus reports glucose concentration in the media. The response is fast and reversible. As the overall swelling degree of the gel does not change, it can be used as substrate-attached film.

KEYWORDS: hydrogel, colloidal crystals, glucose sensor

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their order degree.13−15 For doped microgel CCs, applying an external stimulus can not only induce or erase a defect state in the crystals,16 but also changes the order degree and thus the intensity of the stop band. As a proof of concept, poly(Nisopropylacrylamide) (PNIPAM) microgel CC doped with poly(N-isopropylacrylamide-co-3-acrylamidophenyl-boronic acid) (P(NIPAM-3-AAPBA)) microgel was designed as a glucose sensor. Unlike the previously designed PC-based sensors, the overall swelling degree of the crystal does not change, thus allowing the sensor to be attached on rigid substrate. In addition, its response to glucose is fast and reversible. The synthesis of the doped microgel CC films is schematically depicted in Scheme 1A. First, a mixed solution containing PNIPAM microgel (the host), poly(N-isopropylacrylamide-coacrylic acid) (P(NIPAM-AAc)) microgel (the dopant), monomer AAm, cross-linker BIS, and photoinitiator DEAP was prepared. Both microgels are monodisperse and their size is the same. Therefore, they self-assemble into doped colloidal crystal,16 just like one-component microgel dispersions.10,17,18 The crystallized sample was then exposed to UV light to form PAAm hydrogel matrix to stabilize the ordered structure of the crystal.19−21 Figure 1 shows the reflection spectra of the resulting P(NIPAM-AAc) microgel-doped PNIPAM microgel CC film, from which a sharp Bragg diffraction peak was

hotonic crystals (PCs) have been widely exploited for optical sensing. The periodic spatial modulation of refractive index in PCs allows Bragg reflection effects to occur, inhibits the propagation of light of certain wavelengths through the medium, and thus creates a so-called partial or full band gap or stop band.1 Up to now, many stimuli-responsive PCs, for example polymerized crystalline colloidal arrays,2−4 inverse opal hydrogels,5−9 and polymerized microgel colloidal crystals,10 have been designed for this purpose. These materials are usually fabricated from hydrogels, which respond to an external stimulus or an analyte and change its degree of swelling. As a result, the lattice constant of the crystals will be changed, and the position of the stop band will shift. Using this principle, researchers have designed optical sensors for various analytes, including pH,3 temperature,10 ionic strength,10,11 humidity,12 metal ions,3 glucose,2,6 L-3,4dihydroxyphenyalanine,8 and proteins.9 These sensors are highly attractive as they are label-free and in many cases allow the visual determination of the chemical species and their concentrations.3,10 However, since the sensing principle relies on the size change of the hydrogels, they can only be used as free-standing films, as the attachment of the hydrogel films to a substrate will hamper their swelling. In addition, the hydrogel PCs are usually extremely soft, making the free-standing films extremely difficult to handle. Here we propose a new sensing principle based on the order−disorder transition in doped microgel colloidal crystals (CCs).(Scheme 1) It is well-known that the appearance of the stop band is a result of the highly ordered structure of the crystals, and their photonic properties dramatically depend on © XXXX American Chemical Society

Received: January 24, 2018 Accepted: April 19, 2018

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DOI: 10.1021/acsami.8b01326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Scheme 1. (A) Synthesis of P(NIPAM-3-AAPBA) Microgel-Doped PNIPAM Microgel CC;a (B) Introduction of Defects Leads to Local Structural Disorder and Order−Disorder Transition Induced by Glucose;b (C) Reaction between Glucose and PBA Groups in the Gel

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First P(NIPAM-AAc) microgel-doped PNIPAM microgel CC is fabricated by self-assembly of the two microgels with same size. The ordered structure is then stabilized by embedding within a PAAm hydrogel matrix. P(NIPAM-3-AAPBA) microgel-doped CC is finally obtained by treating with 3-APBA and EDC. bThe microgel CCs are close packed. For the sake of clarity, a non-close packed structure is depicted here.

∼4.2 and ∼8.2 for AAc and AAPBA, respectively). As shown in Figure 2A, upon addition of glucose into the solution, the transmission window becomes less distinct. The phenomenon can be explained by the glucose-induced swelling of P(NIPAM3-AAPBA) microgel spheres, which results in a reduced size difference between the host and the dopant. As shown in Figure 2C, the size of free P(NIPAM-3-AAPBA) microgel increases with increasing glucose concentration, while the size of PNIPAM microgel remains unchanged. The glucose-sensitive behavior of P(NIPAM-3-AAPBA) microgel was reported previously.21−23,25 As shown in Scheme 1C, PBA groups in the microgel can bind with 1,2-diols, such as glucose. The glucose binding converts hydrophobic, neutral PBA groups to hydrophilic, charged phenylboronates, and thus builds up a Donnan potential for gel swelling.26,27 It is expected that the P(NIPAM-3-AAPBA) microgel spheres embedded in the CC films also swell in the presence of glucose, resulting in a decreased size difference with the host PNIPAM microgel. When glucose concentration increases to be 30 mM, only a narrow dip can be seen within the CC’s stop band. When further increasing glucose concentration to 40 mM, the sizes of the two microgels become comparable. The P(NIPAM-3AAPBA) microgel spheres cannot act as real defect any longer, therefore the defect state disappears totally. The defect state could not only be erased by increasing the glucose concentration but also be induced again by decreasing the glucose concentration. As shown in Figure 2B, when the glucose concentration was lowered to be 30 mM, the defect state appears again. The reappearance of the defect state could be explained by the fact that the P(NIPAM-3-AAPBA) microgel spheres shrink with lowering glucose concentration, become smaller than the host PNIPAM microgel spheres, (Figure 2C) and act as real defects again. Further lowering the glucose concentration induces a larger size difference between the host and the dopant. Therefore, the defect state becomes more distinct. Reversible erasing and inducing the defect state

Figure 1. Reflection spectra of a P(NIPAM-AAc) microgel-doped PNIPAM microgel CC and the corresponding P(NIPAM-3-AAPBA) microgel-doped PNIPAM microgel CC. The films were soaked in 0.020 M pH3.5 HAc-NaAc buffer at 26 °C.

observed, suggesting a highly ordered crystalline structure of the film. It is noteworthy that no defect state was observed from the film, because the size and refraction index of the two microgels is similar.16 The P(NIPAM-AAc) microgel-doped PNIPAM microgel CC films were then treated with 3-APBA and EDC. In this way, the P(NIPAM-AAc) microgel spheres in the films were transformed into P(NIPAM-3-AAPBA) microgel spheres,21−24 and the P(NIPAM-AAc) microgel-doped PNIPAM microgel CC films were transformed into P(NIPAM-3AAPBA) microgel-doped PNIPAM microgel CC films. At room temperature, the size of the P(NIPAM-3-AAPBA) microgel spheres is smaller than the host PNIPAM microgel spheres. Therefore, they can act as real defects, and, as a result, a transmission window (defect state) appears within the stop band.16 (Figure 1) The defect state of the doped CC can be erased and induced reversibly by glucose.16 For this purpose, a free-standing CC film was soaked in pH 8.5 phosphate buffer, as P(NIPAM-3AAPBA) microgel shows glucose sensitivity at this pH22 (pKa is B

DOI: 10.1021/acsami.8b01326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

beads into 3D CCs and found that doping the crystals with smaller beads leads to disorder of the lattices and therefore attenuation of the stop band. Palacios-Lidon et al.15 also observed that the Bragg peak intensity of PMAA CCs decreases as the concentration of the dopant, larger size PS spheres, increases. The ability of impurities to induce structural disorder depends on not only their concentration, but also their size difference to the host. As an example, Palacios-Lidón et al.15 found that, in PMAA CCs doped with larger size PS spheres, the detrimental effect of the impurities on the intensity of the stop band increases dramatically with increasing size of the impurities. Introducing defects in microgel CCs will also lead to structural disorder in the crystals, and hence a decrease in stop band intensity. This is evidenced when converting the latent defects, P(NIPAM-AAc) microgel spheres, to real defects, P(NIPAM-3-AAPBA) microgel spheres, as shown in Figure 1. Note here the defects were induced in situ via a chemical reaction, whereas in the previous works, they were introduced during the self-assembly process of the colloids.13−15 More importantly, the defect-induced structural disorder can be tuned by external stimuli. Addition of glucose causes the swelling of the dopant P(NIPAM-3-AAPBA) microgel spheres, reduces the size difference between the dopant and the host, and hence the structural disorder of the crystal caused by the defects. Therefore, the stop band intensity increases gradually with increasing glucose concentration.(Figure 2A) Oppositely, decreasing glucose concentration leads to shrinkage of the dopant P(NIPAM-3-AAPBA) microgel spheres and increased structural disorder of the crystals. Therefore, the intensity of diffraction band decreases gradually with decreasing glucose concentration, as Figure 2B shows. The external stimulus-induced change in the order degree of the doped crystals and accordingly change in stop band intensity makes the material potential for optical sensing. Interestingly the band position does not shift under the experimental conditions, suggesting the overall size of the film does not change. This is because the overall structure of the crystal is stabilized by the surrounding hydrogel matrix, and only 10% of the microgel spheres change their size. However, the presence of the hydrogel matrix still allows local order− disorder transition to occur in response to external stimuli. This is highly desirable for its application as sensor, because the analyte concentration can be simply reported by the stop band intensity at a certain wavelength. In addition, the property allows for attachment of the film to a rigid substrate, which is more convenient for its application. Further discussion on the shift of stop band of doped microgel CC can be found in the Supporting Information. The response of a P(NIPAM-3-AAPBA) microgel-doped PNIPAM microgel CC film attached on glass slide to glucose was shown in Figure 3A and 3B. Just like the free-standing film shown in Figure 2, increasing glucose concentration erases the defect and increases the stop band intensity, while decreasing glucose concentration induces the defect and decreases the stop band intensity. The result suggests the attachment of the film to a rigid substrate does not affect its optical response. Figure 3C shows that the intensity of the stop band at 709 nm increases with increasing glucose concentration in the media. Particularly in the clinically important glucose concentration range from 0 to 30 mM, a linear relationship between the band intensity and glucose concentration was found.

Figure 2. (A) Erasing defect state in a free-standing P(NIPAM-3AAPBA) microgel-doped PNIPAM microgel CC film by increasing glucose concentration from 0 to 40 mM. Inset shows the picture of the film. (B) Introducing defect state into the doped CC by decreasing glucose concentration from 40 to 0 mM. The film is soaked in 0.020 M pH8.5 phosphate buffer at T = 26 °C. (C) Hydrodynamic radii (Rh) of free microgels as a function of glucose concentration, measured in 0.020 M pH8.5 phosphate buffer at T = 26 °C.

in the doped CCs can also be achieved by changing temperature and pH (Figures S1 and S2). Along with the disappearance and reappearance of the transmission window, a gradual increase and decrease in the intensity of the stop band was observed (Figure 2A, B). The results reflect that changing glucose concentration also leads to a structural order−disorder transition in the crystal as shown in Scheme 1B. It is well-known that impurities induce structural disorder in CCs and the intensity of the stop band of a CC is highly sensitive to disorder.13−15 Previously Rengarajan et al.14 assembled CCs from silica spheres with various size distributions. They found that the optical reflectivity at the center of the stop band drops rapidly when size distribution exceeds a critical value (∼6%). Gates and Xia13 organized PS C

DOI: 10.1021/acsami.8b01326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 4. (A) Reflection spectra of a P(NIPAM-3-AAPBA) microgeldoped PNIPAM microgel CC film (on glass slide) when [Glu] changes repeatedly between 0 and 10 mM. Inset is the band intensity at 723 nm. The film is soaked in 0.020 M pH8.5 phosphate buffer at T = 26 °C. (B) Reflection spectra of a P(NIPAM-3-AAPBA) microgeldoped PNIPAM microgel CC film (on glass slide) changes with time upon addition of 40 mM glucose. Inset is the band intensity at 673.8 nm as a function of time. The film is soaked in 0.020 M pH8.5 phosphate buffer at T = 26 °C.

sensor. Unlike previously developed PC-based optical sensors, the new sensor exploits structural order−disorder transition in the crystal, instead of change in lattice constant, to report an analyte. Adding glucose erases the defect state of the doped CC. Meanwhile the impurities-induced structural disorder is eliminated, resulting in an increased stop band intensity. When lowering glucose concentration, the defect state can be induced again. The accompanying structural disorder in the CC leads to a decreased stop band intensity. The sensor’s response is fast and reversible. Since the overall swelling degree of the crystal does not change, the doped CC film can be attached on rigid substrate. With these new features, we expect the new sensing mechanism will pave the way for real applications of photonic sensing materials.

Figure 3. (A, B) Evolution of reflection spectra of a P(NIPAM-3AAPBA) microgel-doped PNIPAM microgel CC film (on glass slide) upon (A) increasing and (B) decreasing glucose concentration. The film is soaked in 0.020 M pH8.5 phosphate buffer at T = 26 °C. Inset shows picture of the film. (C) Reflection intensity at 709 nm as a function of glucose concentration.

The result shown in Figure 3C also suggests a good reversibility of the film’s response to glucose. To further study its reversibility, glucose concentration was repeatedly changed between 0 mM and 10 mM. As shown in Figure 4A, the film’s response to glucose is completely reversible over 3 cycles. A big problem for many hydrogel-based sensors is that their response is usually rather slow.28,29 To measure the response kinetics of a doped microgel CC film, the evolution of its reflection spectra upon addition of 40 mM glucose was followed. As shown in Figure 4B, the defect state was fully erased in about 200s. The fast response of the sensor should be attributed to the small size of the microgel spheres, which allows them to respond to external stimuli quickly.19,25,30,31 In conclusion, P(NIPAM-3-AAPBA) microgel-doped PNIPAM microgel CC film was developed as a new optical glucose

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01326. Experimental details, erasing and inducing defect state by changing temperature and pH, and more discussion on the shift of stop band (PDF) D

DOI: 10.1021/acsami.8b01326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.G.). *E-mail: [email protected] (Y.Z.). ORCID

Yongjun Zhang: 0000-0002-1079-4137 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank financial support for this work from the National Natural Science Foundation of China (Grantss 21374048 and 51625302), Tianjin Committee of Science and Technology (Grant 16JCZDJC32900), and the National Key Research and Development Program of China (2017YFA1103501).

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DOI: 10.1021/acsami.8b01326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX