Co-deposition motif for constructing inverse opal photonic crystals with pH sensing

Abstract

Polyacrylamide (PAM) hydrogel photonic crystals with an inverse opal structure were elaborated using a modified vertical deposition approach involving a co-deposition motif. Free-standing inverse opal hydrogel (IOH) films of different thicknesses that could be peeled from the support were first fabricated. A three-dimensional ordered structure without over-layers could easily be acquired by the co-deposition approach. Moreover, the bi-continuous structure was not affected by the increase in film thickness. All these IOH films showed a fascinating pH response property after hydrolysis, and the color change corresponding to the shift of the Bragg diffraction peak of the acquired hydrogel photonic crystal films could be easily distinguished by the naked eye. Three types of IOH films with different thicknesses showed similar pH response properties. The co-deposition approach is found to be suitable for the construction of thicker IOH film without over-layers.

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Introduction

Hydrogel photonic crystals (HPCs) have attracted considerable interest due to their sensing ability for external stimuli such as temperature,^1,2 pH,^3,4 ionic strength,⁵ humidity,^6,7 mechanic forces,^8,9 metal ions^10,11 and molecules.^12,13 Generally, there are two fundamental types of HPCs, namely, polymerized crystalline colloidal array (PCCA) hydrogel photonic crystals¹⁴ and inverse opal hydrogel (IOH) film deposited on a substrate.¹⁵ PCCA hydrogel photonic crystals are formed through the immobilization of non-close-packed, three-dimensional, and highly ordered suspension crystalline colloidal arrays (CCAs) in the polymer network via an in situ polymerization. The PCCA system has the advantage of a simpler fabrication process; however, it results into poor sensitivity and slow response because of the mass transfer resistance of the polymer network, which may limit its application in real-world situations. Furthermore, the PCCA system could be easily destroyed when charged monomers are introduced into the pre-polymerization solution because the CCAs need to be stabilized by charge.¹⁶ Fortunately, these issues can be easily addressed using an IOH with a three-dimensional highly ordered macroporous structure, which provides merits of highly sensitive and rapid response and allows the employment of various types of charged monomers.^17–20

It is well known that the conventional fabrication method of IOH is generally conducted in a three-step templating process, namely, assembly of the colloidal crystal template, infiltration of precursors to the colloidal crystal template and removal of colloidal crystal template.²¹ Compared with PCCA, IOH possesses a more complicated fabrication process, and a thin film of IOH deposited on a substrate cannot be peeled off easily. However, the adhesion to the substrate can affect the swelling or shrinking behavior of the IOH film and thereby its excellent responsive property, which is imparted by the macroporous structure.²² Therefore, it is highly desirable and useful to obtain a free-standing IOH film in accordance with the formation of PCCA. It can be envisioned that a free-standing IOH film could be acquired when the thickness of the colloidal crystal template reaches a certain level. Nevertheless, to date, a free-standing IOH film has never been demonstrated, probably due to the following difficulties. First, the infiltration of precursors to the template cannot be easily controlled when a thick template is employed. Excessive infiltration could occur in this situation, resulting in an overlap of the surface of the colloidal crystal template, and the inverse opal structure cannot be prepared in this case owing to the failure of the removal of the colloidal particles. The overlapping area of IOH film can almost be considered as PCCA, where lattice spacing is approximately equal to the particle diameter itself. In such a case, the excellent sensing property of IOH film with a complex fabrication process is no longer obtained. On the other hand, colloidal crystal templates produced via conventional methods, such as sedimentation,²³ centrifugation,²⁴ spin-coating²⁵ and evaporative deposition,²⁶ bear many cracks, colloidal vacancies and other defects. Moreover, many more defects are formed with the increase in the thickness of the template, which are harmful for the formation of an ordered macroporous structure. Furthermore, the three-step method for producing the IOH film is much more sophisticated than that used for producing PCCA. Therefore, there is an urgent need to develop a simple and efficient method that allows the formation of a free-standing IOH film. Co-assembly method has been proven to be effective for the preparation of silica inverse opal with crack-free and high-order over centimeter length scales by Hatton's group.²⁷ Moreover, our group has prepared organic inverse opal materials based on the co-deposition approach, combining the in situ polymerization of PCCA with the colloidal crystal template method.²⁸ However, though the fabricated inverse opal photonic crystals were deposited on a substrate, the sensing property of this substrate has not yet been investigated. It is necessary for practical applications to study the response of co-deposition based on photonic crystals.

In this study, free-standing IOH films with different thicknesses were first synthesized based on a co-deposition motif, and without the phenomenon of overlap for various situations. The pH sensing property of free-standing IOH films with various thicknesses were investigated. The pH sensing behavior of the free-standing IOH film can be accomplished between 5 and 10 seconds. Moreover, the reciprocal sensing of a free-standing IOH film was also elaborated, evidencing the good physical stability and efficient mechanical strength of the IOH film.

Experimental

Materials

Acrylamide (AM), acrylic acid (AA), ammonium persulfate (APS), N,N,N′,N'-tetramethylethylenediamine (TEMED) and styrene were purchased from Sinopharm Chemical Reagent Co., Ltd. N,N′-Methylene bisacrylamide (BIS) was obtained from Aladdin Chemistry Co., Ltd. (China). Styrene and AA were purified by distillation under vacuum. All the other affiliated chemicals not mentioned here were used without further purification. The glass slides (76.2 × 24.5 × 1 mm) for the colloidal crystal growth were well-cleaned using acetone, anhydrous ethanol and double-distilled water in an ultrasonic bath.

Preparation of free-standing inverse opal hydrogel (IOH)

The monodispersed poly(styrene-co-acrylic acid) (PSA) colloids (∼200 nm) were synthesized by emulsifier-free emulsion polymerization according to our previous study. Prior to the fabrication of IOH, a suspension solution was first obtained by adding 2 mL of pre-polymerization solution into 20 mL of x wt% colloidal suspensions, which take the value of 0.5, 1 or 2. The standard pre-polymerization solution was composed of AM (5.0 g), BIS cross-linker (0.5 g), APS (0.05 g) and H₂O (30.0 mL). Then, the aforementioned glass slide was vertically dipped into the suspension solution prepared in a beaker, and the beaker was then placed in an oven at 50 °C for 28 h, accompanied by the co-deposition of PSA microspheres and the AM precursor on the substrates and polymerization of the AM precursor in a single step. Eventually, an uncoated composite opal film was formed. Subsequently, the composite opal film could be easily peeled off from the substrate through immersion in H₂O. Then, the free-standing IOH film (2.4 × 1.5 cm) with a bright color was finally obtained through selective removal of the PSA colloids by sintering in air at 100 °C for 0.5 h and by subsequently dissolving in xylem solution (24 h) at room temperature. Eventually, the free-standing IOH film was immersed in a mixture of 0.1 M sodium hydroxide and 10 wt% TEMED. After 1 min of hydrolysis, the film was thoroughly washed with deionized water. Three types of IOH film with thicknesses of (a) ∼25 μm, (b) ∼50 μm and (c) ∼90 μm, were fabricated by changing the mass fraction of the colloids suspended in the suspension solution.

Characterization

Surface morphologies of the composite opal film and IOH film were characterized by field emission scanning electron microscopy (SEM, Hitachi, S-4800). The pH-dependent Bragg diffraction wavelength of IOH was recorded by a miniature fiber optic spectrometer (FLA 4000+, China). The corresponding color changes were photographed using a common digital camera under a daylight lamp.

Results and discussion

Co-deposition-based IOH film

A simple and novel co-deposition approach was employed to prepare a free-standing IOH film, as shown in Fig. 1. The co-deposition approach consists of only two steps: the co-deposition of PSA microspheres and AM precursor on the substrates and polymerization of the AM precursor in a single process, followed by the removal of PSA colloids. In the first step, owing to the hydrogen effects between the –CONH₂ groups of AM and the –COOH groups of the PSA particles, the suspension solution exists in the form of AM surrounding the PSA particles. Accordingly, during evaporation-induced colloidal assembly, AM of a certain concentration can homogeneously penetrate into the interstitial spaces between the PSA colloids. Polymerization of the precursor, up to the level of enabling polymerization, proceeded simultaneously with the assembly of the colloids due to the continuous evaporation of the solvent. Second, free-standing IOH film was obtained by the removal of PSA colloids. The thickness of free-standing IOH film can be modulated by changing the mass fraction of colloids suspended in the suspension solution. The higher the mass fraction of the colloids, the thicker the film would be acquired.

Fig. 1 Schematic illustration of the (a) co-deposition process. (b) Composite opals consisting PSA colloids and PAM hydrogel. (c) PAM inverse opals.

Fig. 2 shows the typical SEM images of the resultant IOH film with different thicknesses. As can be clearly seen, 3D, highly ordered and interconnected macroporous arrays were successfully fabricated. Moreover, the ordered macroporous structure was relatively unaffected by the thickness. Herein, it can be asserted that the co-deposition method is extraordinary suitable for the fabrication of nearly overlayer-free free-standing IOH film. In a regular inverse opal structure, there are three pores that can be seen in each void;²⁹ however, in our study, the void had three or less pores, as shown in Fig. 2, and the problem of partial etching was not improved by increasing the etching time. The construction of a perfect inverse opal structure would be an important part of our next study. Although perfect inverse opal structures were not guaranteed, the uncoated IOH film in the first layer could be easily fabricated by the co-deposition approach. Moreover, an IOH film based on a co-deposition motif addresses many other defects such as cracks and overlapping.

Fig. 2 SEM images of inverse opals film fabricated by various mass fractions of colloidal suspension: (a) 0.5%, (b) 1%, and (c) 2%.

pH response of IOH film

Fig. 3 presents the pH dependence of the Bragg diffraction wavelength of IOH film with thicknesses of 90 μm in buffer solutions. It can be clearly seen that the Bragg diffraction peak of IOH film shifted markedly from 467 nm to 550 nm when the pH was increased from 2.67 to 8.43. This resulted from the electrostatic repulsion among the carboxylate anions on the polymer chains and the increase in osmotic pressure. When the IOH film was hydrolyzed, some amide groups hydrolyzed to carboxyl groups, which ionize as a function of the pH. As the pH increased from 2.67 to 8.43, ionization of the carboxyl groups increased, resulting in an increase in the carboxylate anions. The carboxylate anions on the polymer chains brought about an electrostatic repulsion, which tended to expand the polymer network. Furthermore, osmotic pressure is also a key reason for the pH response of IOH film. According to the Flory–Rehner theory,^30–32 the total osmotic pressure Π for ionic hydrogels is given as the sum of the pressures due to polymer–solvent mixing (mix), deformation of the network chains to a more elongated state (el), and the nonuniform distribution of mobile counterions between the hydrogel and the external solution (ion):

Π = Π_mix + Π_el + Π_ion

Fig. 3 (a) Optical response of IOH film upon soaking in buffer solutions at different pH values. (b) Photograph of IOH film under different pH conditions.

In this study, the pH-dependent optical properties of IOH film are mainly related to Π_ion. With an increase in pH, the concentration differences of the counterions between the hydrogel and the outer solution become larger. Therefore, the value of Π_ion increases, resulting in an increase in the total osmotic pressure Π. The increase in osmotic pressure and electrostatic repulsion among the molecular chains swell the gel and induce a red-shift of the Bragg diffraction peak. It was noteworthy that the Bragg diffraction peak is blue-shifted with further pH increases. Because ionization was complete by pH 9, further pH increase would only increase the ionic strength, which leads to a decrease in the osmotic pressure, and would shrink the hydrogel. In addition, deionization of the acylamino group occurred when the pH value was greater than pK_a (9.6) of the acylamino group. This would also shrink the hydrogel and cause a blue-shift of the Bragg diffraction peak. In particular, the pH response of IOH film with different thicknesses showed little difference. Therefore, thicker IOH films should be fabricated for their convenient employment in real-world situations.

Response speed and recoverability of IOH film

Fig. 4 shows the pH responsive time of the IOH film with thickness of 90 μm upon soaking in solutions with pH between 2.67 and 8.43. The response process was complete within 10 s. As the film thickness increased, the response speed was slightly slower. However, a relatively fast response could still be accomplished. The fast response may due to the 3D ordered interconnected macroporous structure and the ultrathin hydrogel walls. In summary, the pH response and response speed were not obviously influenced by the film thickness. However, the film thickness exerted an impact effect on the recoverability of the IOH film. The IOH film would be mechanically more robust due to the increase of film thickness. Fig. 5 shows the pH dependence of the Bragg diffraction wavelength of IOH film based on 2 wt% colloidal suspensions with the pH varying from 2.67 to 10.38 with one cycle. The significant reciprocal response to pH was certified. Although a similar phenomenon had been observed for other IOH films with different thicknesses, the operation process needed to be carried out very careful owing to the risk of the films being easily damaged. The thicker film, which should not be destroyed in the test process, is convenient for practical employment.

Fig. 4 Kinetic response of IOH film upon soaking in buffer solutions with pH between 2.67 and pH 8.43.

Fig. 5 Reciprocal sensing of PAM IOH film upon soaking in buffer solutions at different pH values.

Conclusions

In this study, PAM inverse opal films with different thicknesses were successfully prepared by a simple co-deposition method. It was proved that the co-deposition approach is suitable for the construction of a thicker IOH film without overlayers. Moreover, the thickness did not weaken the pH response property of the IOH film, including the respond speed and sensitivity. All the IOH films showed fascinating pH response properties after being hydrolyzed. The Bragg diffraction peak red-shift was noted with the increase in pH in the range of 2.67–8.43. This is in contrast, with the continuous increase in pH, where the Bragg diffraction peak would be blue-shifted. The pH sensing behavior of the free-standing IOH film can be accomplished between 5 and 10 seconds. In summary, a thicker IOH film without overlayers can be acquired by the co-deposition approach. Moreover, the sensing property of IOH film is still extraordinary. Use of the co-deposition method in the fabrication of polymer hydrogel photonic crystal would accelerate the practical applications of photonic crystals.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 51173072), the Fundamental Research Funds for the Central Universities (JUSRP51408B), and MOE & SAFEA for the 111 Project (B13025).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08046k

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