Red emitting carbon dots – gold nanostar fluorescence resonance energy transfer based fluorescent probe for turn-on sensing of melamine

Fluorescent carbon dots (CDs), as a class of newly discovered carbon nanomaterial, have received increasing attention as substitutes for organic fluorophores and conventional semiconductor quantum dots (QDs) [1, 2]. However, emission wavelengths of most reported CDs which are mainly located in the blue-light region hinders their application in the bioimaging and biosensing fields due to the commonly blue auto-fluorescence of biological matrix [3]. In this work a novel probe was prepared for determination of melamine. The sensing approach is based on fluorescence resonance energy transfer (FRET) between red emitting carbon dots (R-CDs) and gold nanostars (AuNSs), which act as energy donor and acceptor, respectively. Amino-functionalized CDs could be readily adsorbed onto the surface of AuNSs through Au–N interactions, leading to the aggregation of AuNSs and the quenching the florescence of CDs due to fluorescence resonance energy transfer. Upon addition of melamine, melamine molecules can interact with AuNSs via covalent binding of the amino group (NH2–) of melamine to the surface of AuNSs, which prevents the interaction of R-CDs and AuNSs, thus reduces the FRET effect, resulting in the increase of the fluorescence intensity. We optimized some important factors including incubation time, AuNSs concentration and media pH, which would affect the efficiency of the FRET system. Under the optimized experimental conditions, melamine could be detected based on fluorescence intensity of C-dots. We could get a linear relationship between 30 nM and 1000 nM and the detection limit was 9 nM. The proposed method was applied to the determination of melamine in raw and pasteurized milk samples with satisfactory results.