A fluorescent turn-on nanosensor for selective detection of L-morphine using D-cysteine-functionalized graphene quantum dots

Here, L- and D-cysteine-functionalized graphene quantum dots (L-/D-cys-GQDs) were designed with the aim of obtaining a selective fluorescent nanosensor to detect L-morphine. Citric acid was pyrolyzed to synthesize the GQDs, which were then functionalized with chiral L- and D-cys species using a thiol-ene click reaction between sulfur group of cysteine species and CC double bonds of GQDs. Energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopies (XPS) provides elemental analysis data which approved the presence of sulfur and nitrogen elements of L- and D-cysteine species on the surface of GQDs. Transmission electron microscopy (TEM) showed that the particle size of the modified GQDs ranges from 2 to 4.2 nm. The results of fluorescence spectroscopy showed that upon functionalization of GQDs with L-/D-cys the fluorescence intensity decreases as a result of Forster resonance energy transfer (FRET) mechanism. Interestingly, in the presence of L-morphine, the fluorescence intensity of D-cys-GQDs was selectively turned on as the FRET mechanism is ceased between the cysteine species and GQDs. Additional tests demonstrated that this nanosensor cannot interact with other drugs like methamphetamine or ibuprofen. As a result, it can serve as a cheap and precise nanosensor for identifying low quantities of L-morphine.