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

#### **Shimo Yu, Shun Dong, Xiuling Jiao, Cheng Li \* and Dairong Chen \***

National Engineering Research Center for Colloidal Materials and School of Chemistry and Chemical Engineering, Shandong University, Ji'nan 250100, China; 201411530@mail.sdu.edu.cn (S.Y.); shundong1995@mail.sdu.edu.cn (S.D.); jiaoxl@sdu.edu.cn (X.J.)

**\*** Correspondence: chengli@sdu.edu.cn (C.L.); cdr@sdu.edu.cn (D.C.); Tel.: +86-531-88364280 (C.L. & D.C.)

Received: 8 March 2019; Accepted: 17 March 2019; Published: 21 March 2019

**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:** polymer gel; colloidal crystals; optical film; pH sensor
