Ratiometric fluorescent carbon dots for enantioselective sensing of L-lysine and pH discrimination in vivo and in vitro

Ratiometric fluorescent probes with wavelength-resolved dual target sensing capacity are highly desirable for the simultaneous recognition of target objects. A novel fluorescent carbon dots (CDs) with a yield of 32% was simply synthesized by hydrothermal treatment of neutral red and urea, resulting in discrete dual-emission bands (440 nm/542 nm) under single-wavelength excitation of 380 nm. It was discovered that the fluorescence (FL) signal of CDs at 440 nm enhances with the addition of L-Lysine, whereas the FL intensity at 542 nm remains invariable. Surprisingly, no significant luminescence response is observed after the addition of D-Lysine, implying that as-prepared CDs can serve as fluorescent nanoprobes for chiral recognition of Lysine. On the contrary, it was discovered that the FL band at 542 nm is highly sensitive to pH in linear ranges of 2.0–3.4 and 3.4–8.0, while the FL band at 440 nm is inert to it, providing an effective tool for the ratiometric detection of pH in a “turn-on” mode. Confocal imaging experiments further verify that this nanoprobe can monitor L-Lysine and pH fluctuations in biological systems in real time using ratiometric methods. We believe that our research will provide new insight into the suitable construction of prompt and straightforward ratiometric fluorescent probes for the simultaneous differentiation of two analytes.