Electrochemiluminescence of nitrogen- and sulfur-doped graphene quantum dots

Nitrogen- and sulfur-doped graphene quantum dots (NS-GQDs) have been synthesized via one-pot pyrolysis approach using citric acid and l-Cysteine as precursors. The obtained NS-GQDs have a size around 2.8 ± 1.0 nm. The fluorescence quantum yield of NS-GQDs was calculated to be 84 times that of Ru(bpy)3Cl2, which is 9.3-fold higher than that of undoped GQDs. Electrochemiluminescence (ECL) of NS-GQDs was found to be robust with K2S2O8 as a coreactant and an ECL efficiency of 32% relative to the Ru(bpy)3Cl2/K2S2O8 was reached, which is also 5.8-fold enhanced when compared with the same system of undoped GQDs. By means of spooling ECL spectroscopy, a potential-dependent ECL emission was discovered in the ECL processes of both NS-GQDs and GQDs. Scanning the applied potential to different values changed actually the driving force, and thus various emissive excited states were produced, leading to variable emission wavelengths. Importantly, because much more surface capturing centers of excitons for NS-GQDs than GQDs created by nitrogen and sulfur doping effect, the NS-GQDs can capture more holes from SO4− and thereby give higher ECL efficiency. The ECL and electron transfer mechanisms proposed here provide insight into development of new GQD for light emitting devices and bioanalytical applications.