Efficient Colorimetric pH Sensor Based on Responsive Polymer-Quantum Dot Integrated Graphene Oxide

In this paper, we report the development of a versatile platform for a highly efficient and stable graphene oxide (GO)-based optical sensor that exhibits distinctive ratiometric color responses. To demonstrate the applicability of the platform, we fabricated a colorimetric, GO-based pH sensor that responds to a wide range of pH changes. Our sensing system is based on responsive polymer and quantum dot (QD) hybrids integrated on a single GO sheet (MQD-GO), with the GO providing an excellent signal-to-noise ratio and high dispersion stability in water. The photoluminescence emissions of the blue and orange color-emitting QDs (BQDs and OQDs) in MOD-GO can be controlled independently by different pH-responsive linkers of poly(acrylic acid) (PAA) (pK(a) = 4.5) and poly(2-vinylpyridine) (P2VP) (pK(a) = 3.0) that can tune the efficiencies of Forster resonance energy transfer from the BQDs to the GO and from the OQDs to the GO, respectively. As a result, the color of MOD-GO changes from orange to near-white to blue over a wide range of pH values. The detailed mechanism of the pH-dependent response of the MQD-GO sensor was elucidated by measurements of time-resolved fluorescence and dynamic light scattering. Furthermore, the MOD-GO sensor showed excellent reversibility and high dispersion stability in pure water, indicating that our system is an ideal platform for biological and environmental applications. Our colorimetric GO-based optical sensor can be expanded easily to various other multifunctional, GO-based sensors by using alternate stimuli-responsive polymers.