**4. Conclusions**

In summary, UPPGF using non-close-packed monolayer colloidal crystals (ncp MCCs) as template were prepared by spin-coating of qP2VP and subsequent thermal cross-linking. The UPPGF showed bright structural colors depending on the thickness of the film. The swelling of P2VP gel in acidic solution caused changes in the film thickness, thus resulting in a change in the visible color. Compared with those templated by cp MCCs, the UPPGF not only increased the wavelength shift of the reflection peak upon pH changes, but also improved the linearity of response, while maintaining the advantages of fast response speed, high cycle stability and small hysteresis loop. The packing density of ncp MCCs could be adjusted by controlling the time of oxygen plasma etching to effectively regulating the volume and filling factor of P2VP, which could be used to improve the responsiveness and linearity. The present strategy of fine-tuning the volume of filling fraction of polymeric gel in a sensor device can be feasibly extended to other responsive materials/systems to improve the responsiveness and linearity, as well as makes it simple to construct sensor arrays to enable more accurate detections.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4360/11/3/534/s1, Figure S1: SEM images of UPPGF-0 after equilibration in pH 5.08, pH 4.20, pH 3.39, and pH 2.57 buffer solution; Figure S2: SEM images of UPPGF-2 after equilibration in pH 5.08, pH 4.20, pH 3.39, and pH 2.57 buffer solution; Figure S3: SEM images of UPPGF-6 after equilibration in pH 5.08, pH 4.20, pH 3.39, and pH 2.57 buffer solution; Figure S4: SEM images of UPPGF-8 after equilibration in pH 5.08, pH 4.20, pH 3.39, and pH 2.57 buffer solution; Figure S5: Corresponding shifts of the reflectance peak of UPPGF-x (x = 0 min, 2 min, 4 min, 6 min, 8 min) after equilibration in pH buffer solution.

**Author Contributions:** Conceptualization, C.L.; methodology, C.L. and S.Y.; software, S.Y. and S.D.; validation, C.L., S.Y., D.C. and X.J.; formal analysis, C.L. and D.C.; investigation, C.L. and S.Y.; resources, D.C. and X.J.; data curation, S.Y. and C.L.; writing—original draft preparation, C.L. and S.Y.; writing—review and editing, C.L. and S.Y.; visualization, C.L., S.Y. and S.D.; supervision, C.L., D.C. and X.J.; and project administration, C.L., D.C. and X.J.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors acknowledge support from the National Natural Science Foundation of China (NSFC 21771118), and the Taishan Scholars Climbing Program of Shandong Province (Grant tspd20150201).

**Conflicts of Interest:** The authors declare no conflict of interest.
