Ultrathin Photonic Polymer Gel Films Templated by Non-Close-Packed Monolayer Colloidal Crystals to Enhance Colorimetric Sensing

Abstract

Responsive polymer-based sensors have attracted considerable attention due to their ability to detect the presence of analytes and convert the detected signal into a physical and/or chemical change. High responsiveness, fast response speed, good linearity, strong stability, and small hysteresis are ideal, but to gain these properties at the same time remains challenging. This paper presents a facile and efficient method to improve the photonic sensing properties of polymeric gels by using non-close-packed monolayer colloidal crystals (ncp MCCs) as the template. Poly-(2-vinyl pyridine) (P2VP), a weak electrolyte, was selected to form the pH-responsive gel material, which was deposited onto ncp MCCs obtained by controlled O₂ plasma etching of close-packed (cp) MCCs. The resultant ultrathin photonic polymer gel film (UPPGF) exhibited significant improvement in responsiveness and linearity towards pH sensing compared to those prepared using cp MCCs template, achieving fast visualized monitoring of pH changes with excellent cyclic stability and small hysteresis loop. The responsiveness and linearity were found to depend on the volume and filling fraction of the polymer gel. Based on a simple geometric model, we established that the volume increased first and then decreased with the decrease of template size, but the filling fraction increased all the time, which was verified by microscopy observations. Therefore, the responsiveness and linearity of UPPGF to pH can be improved by simply adjusting the etching time of oxygen plasma. The well-designed UPPGF is reliable for visualized monitoring of analytes and their concentrations, and can easily be combined in sensor arrays for more accurate detection.

Keywords: colloidal crystals; optical film; pH sensor; polymer gel.

Scheme 1

Steps to fabricate UPPGF. (a) Monolayer colloidal crystals of PS nanoparticles were etched through oxygen plasma etching for different times. (b) The ncp MCCs was infiltrated with a polymer precursor (qP2VP) solution by spin coating. (c) P2VP was crosslinked by annealing.

Figure 1

SEM images of the ncp MCCs after etching: (a,f) top views of ncp MCCs-0; (b,g) top views of ncp MCCs-2; (c,h) top views of ncp MCCs-4; (d,i) top views of ncp MCCs-6; and (e,j) top views of ncp MCCs-8.

Figure 2

SEM images of the fabricated structures: (a–e) cross-sectional views of the qP2VP-infiltrated ncp MCCs-x (x = (a) 0 min; (b) 2 min; (c) 4 min; (d) 6 min; and (e) 8 min); and (f–j) cross-sectional views of UPPGF-x (x = (f) 0 min; (g) 2 min; (h) 4 min; (i) 6 min; and (j) 8 min) obtained after thermal annealing.

Figure 3

Optical properties of the fabricated structures:(a) reflectance spectra of the ncp MCCs-x obtained at different etching times (x = 0, 2, 4, 6, 8 min); (b) reflectance spectra of the qP2VP-infiltrated ncp MCCs-x; and (c) reflectance spectra of UPPGF-x obtained after thermal annealing.

Figure 4

(a–e) pH dependence of the reflectance spectra of the UPPGF-x (x = (a) 0 min; (b) 2 min; (c) 4 min; (d) 6 min; (e) 8 min) sensors after equilibration in pH buffer solution; and (f) the corresponding shifts of the reflectance peak of UPPGF-x.

Scheme 2

(a–e) Side views of UPPGF-0, UPPGF-4, and UPPGF-8; and (d–f) top views of UPPGF-0, UPPGF-4, and UPPGF-8.

Figure 5

SEM images of UPPGF-4 after equilibration in buffer solution at: pH 5.08 (a); pH 4.20 (b); pH 3.39 (c); and pH 2.57 (d).

Figure 6

(a) Linear relation between UPPGF-x (x = 0, 2, 4, 6, 8 min) and pH; (b) response kinetics of UPPGF-x to a pH 3.39 buffer solution; (c) reversibility of UPPGF-x over 10 cycles of exposure to pH 9.17–3.39; and (d) hysteresis loops of the UPPGF-x sensors between pH 9.17 and 2.57.

Figure 7

Optical micrographs showing the pH dependence of the different response colors of UPPGF-x (x = 0, 2, 4, 6, 8 min) (the size of each optical micrograph: 1.2 mm × 0.9 mm).