Red-to-blue colorimetric probe based on biomass carbon dots for smartphone-integrated optosensing of Cu(II) and L-cysteine.

We constructed a smartphone-integrated optosensor with inexpensive, reversible, environmental friendly, and rapid adsorption to detect Cu(II) and L-cysteine (L-Cys). The key part of this study was to prepare a red-to-blue colorimetric probe from herbaceous andrographis paniculata using one-pot polymerization at room temperature. When Cu(II) existed, the red fluorescence on the surface of the core-shell probe was quenched, while the blue fluorescence of the core did not respond, because the colorimetric probe interacted with the Cu(II) on the surface of red CDs. After L-Cys added, it interacted with the Cu(II) to strip it from the surface of red CDs, resulting in the recovery of fluorescence response. Under optimal conditions, the detection limits of this method for Cu(II) and L-Cys were 71 nM and 12 nM, respectively. Further, the red-to-blue colorimetric probe was integrated into smartphone with a software application to convert fluorescent color images into specific red (R), green (G), and blue (B) values. The spiked recovery of Cu(II) and L-Cys in lake water was verified the feasibility of the developed optosensors with a recovery of 98.2-101.6 % and 103.3-121.6 %. This method for detecting Cu(II) and L-Cys can not only recognize metal ions from actual samples, but also effectively protect CDs from quenching and restore fluorescence.