Yellow-Emissive Carbon Dot-Based Optical Sensing Platforms: Cell Imaging and Analytical Applications for Biocatalytic Reactions.

Carbon dots (CDs) have attracted increasing interest in bioimaging and sensing recently. Herein, we present a simple synthetic strategy to prepare yellow-emissive CDs (λem = 535 nm) by one-pot hydrothermal treatment of p-phenylenediamine and aspartic acid. The as-prepared CDs possess outstanding optical features, excellent biocompatibility, and low cytotoxicity, especially for fluorescence (FL) cellular imaging. Interestingly, by combining the quenching and recognition ability of silver nanoparticles (AgNPs) with the optical capacity of CDs, a label-free strategy for specifically monitoring H2O2-generated biocatalytic processes was proposed, such as glucose oxidase-induced conversion of glucose, cholesterol oxidase-catalyzed oxidization of cholesterol, and bienzyme of acetylcholinesterase and choline oxidase-mediated reaction of acetylcholine. In this process, AgNPs act as a "nanoquencher" to decrease the FL intensity of CDs via surface plasmon-enhanced energy-transfer mechanism. The enzymatic oxidation product (H2O2) subsequently etches the AgNPs to silver ions, thus recovering the FL of CDs, which enabled this proposed nanosensor to sensitively detect H2O2-generated biocatalytic processes. The above results pave the way to implement CDs as FL labels for biosensors and biological imaging.