A far-red FRET fluorescent probe for ratiometric detection of l-cysteine based on carbon dots and N-acetyl-l-cysteine-capped gold nanoparticles

A novel far-red fluorescence resonance energy transfer (FRET) fluorescent probe for ratiometric detection of l -cysteine ( l -Cys) has been designed. The system was established a FRET assembly by positively charged carbon dots (CDs) and negatively charged N -acetyl- l -cysteine capped gold nanoparticles (NAC-AuNPs). The fluorescence of CDs at 539 nm could be effectively quenched in the presence of NAC-AuNPs owing to FRET process, while the emission of NAC-AuNPs at 630 nm was appeared. Subsequently, the interactions between l -Cys and NAC-AuNPs resulted in the decreased emission intensity of NAC-AuNPs, but the emission intensity of CDs kept almost constant due to the continuous FRET efficiency. The ratio of emission intensities at 539 and 630 nm ( I 539 / I 630 ) exhibited a linear correlation to the l -Cys concentration in the range of 1.0–110 μM with the detection limit of 0.16 μM. Moreover, this far-red ratiometric sensor also revealed excellent selectivity toward l -Cys over other amino acids , which showed very high potential in the practical application for diagnosing of cysteine-related disease.