Synergistic Effect in a Graphene Quantum Dot-Enabled Luminescent Skinlike Copolymer for Long-Term pH Detection

The alluring properties of a luminescent graphene quantum dot (GQD)-based nanocomposite are unquestionable to realize many advanced applications, such as sweat pH sensors. The well-suited hydrophilic polymers to host GQDs can face an unavoidable swelling behavior, which deteriorates the mechanical stability, whereas the hydrophobic polymers can prevent swelling but at the same time barricade the analyte pathways to GQDs. To resolve the two aforementioned obstacles, we develop a nanocomposite film containing nitrogen-doped GQDs (NGQDs) incorporated into a transparent, elastic, and self-healable polymer matrix, composed of a hydrophobic n-butyl acrylate segment and a hydrophilic N-(hydroxymethyl)acrylamide segment for wearable healthcare pH sensors on the human body. Besides serving as the fluorescence source, NGQDs are also designed as a nano-cross-linker to promote abundant chemical and physical interactions within the nanocomposite network. This synergetic effect gives rise to a 10-fold higher mechanical strength, 7-fold increment in Young's modulus, 4-fold increment in toughness, and 15-fold more sensitivity in pH detection (pH 3-10) compared to those of the pristine copolymer and NGQDs, respectively. Moreover, the mechanically enhanced nanocomposite possesses a high self-healing efficiency (94%) at room temperature even under water and demonstrates a stable sensing performance after repetitive usage for 30 days. Our work provides insights into the simple preparation of human skinlike nanocomposite elastomers usable for wearable pH sensors.