Newly synthesized Fe-doped CDs for colorimetric and fluorometric nanozyme-based levodopa sensing

A simple single-pot hydrothermal method was used to fabricate a Fe, N, and S co-doped carbon dots (Fe-CDs) nanozyme using ferric chloride and sunset yellow as precursors. The fabricated Fe-CDs exhibited intense green fluorescence at 460 nm with excitation-independent properties and a high quantum yield of 40.23%. This nanozyme mimics peroxidase by catalyzing the oxidation of tetramethylbenzidine (TMB) by H2O2 to yield a blue-coloured TMBox product at 652 nm. Dual detection methods were established for determining levodopa (l-dopa) by taking advantage of the high nanozyme activity and the distinct fluorescence aspect. Both determination methods are based on the oxidation of l-dopa by H2O2 in the presence of Fe-CDs and fading of the blue colour of the TMBox. The colorimetric method monitors the amount of colour fading of TMBox. In the fluorometric method, the formed blue TMBox absorbs the emission light of the Fe-CDs; when l-dopa is present, this effect decreases and the intensity of the emission light increases. The nanozyme-based detection procedures exhibit good linearity in the ranges 2.17 x 10(-3) to 34.78 x 10(-3) mM [limit of detection (LOD) = 0.84 x 10(-3) mM] and 0.85 x 10(3) to 16.95 x 10(3) nM (LOD = 0.102 x 10(3) nM) for colorimetric and fluorometric methods, respectively.