Green synthesis of carbon dots using expired agar for a label-free fluorescence signal-amplified detection of ferric ion utilizing oxalate functionalization

Surface passivation strategies for functional carbon-based nanoparticles can provide unrivalled performance whilst fine-tuning their optical properties in addition to giving routes for large-scale syntheses. Herein, the synthesis of highly fluorescent agar-derived and oxalate-functionalized carbon dots (ag-oxCDs) is presented. We deployed a facile hydrothermal protocol, using expired potato dextrose agar and oxalate as "green" precursors to prepare fluorescent ag-oxCDs with a relative fluorescence (FL) quantum yield of ∼32% (emission/excitation wavelengths: 445/340 nm). The switchable fluorescence properties of the prepared ag-oxCDs was used for developing a sensitive nanosensor for ferric ion [Fe(iii)] detection. Through Fe(iii) coordination to the oxalate passivated surface of ag-oxCDs, the FL of ag-oxCDs was enhanced by an aggregation-induced emission enhancement mechanism. The tested and optimized concentration of Fe(iii) was within a broad linear range of 0.5-1500 μM, with a detection limit of 75 nM (s/N = 3). The practical application of the ag-oxCDs-based FL nanosensor for real-time quantitative monitoring of Fe(iii) was demonstrated by detecting up to 0.15 μM of Fe(iii) in spiked human serum and water samples.