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 H₂ O₂ to yield a blue-coloured TMB_ox 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 H₂ O₂ in the presence of Fe-CDs and fading of the blue colour of the TMB_ox . The colorimetric method monitors the amount of colour fading of TMB_ox . In the fluorometric method, the formed blue TMB_ox 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 × 10^-3 to 34.78 × 10^-3  mM [limit of detection (LOD) = 0.84 × 10^-3  mM] and 0.85 × 10³ to 16.95 × 10³  nM (LOD = 0.102 × 10³  nM) for colorimetric and fluorometric methods, respectively.