Electrochemical synthesis of carbon dots with a Stokes shift of 309 nm for sensing of Fe³⁺ and ascorbic acid

Preparation of carbon dots (C dots) with strong emission at long wavelengths is an ongoing goal to their advanced applications, especially in bioanalysis. For sensing Fe³⁺ and ascorbic acid (AA), we prepared C dots with a large Stokes shift using o-phenylenediamine (OPD) as the carbon source through an electrochemical method. During the electrolysis, OPD undergoes polymerization, carbonization, and passivation at the platinum anode to form C dots. These C dots were characterized by UV–Vis spectroscopy, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and FT-IR spectroscopy. They are composed of C, N, O, H elements, having a maximum of emission at 570 nm and a supreme excitation at 261 nm, with a Stokes shift of 309 nm. A new emission at 350 nm emerged with their exposure to Fe³⁺ and the intensity rises upon increasing Fe³⁺, while the emission intensity at 570 nm decreases. Thus, dissimilar to the usual quenching strategy, a novel ratiometric fluorescent sensor based on the fluorescence intensity ratio (F₃₅₀/F₅₇₀) of the C dots at 350 nm and 570 nm has been developed for quantitation of Fe³⁺ with a limit of detection (LOD) of 0.16 μM and high selectivity toward Fe³⁺ against K⁺, Cu²⁺, and Fe²⁺. With evidence of decreased C–N[dbnd]C and C–O/N–O in XPS and increased content of C[dbnd]O/N[dbnd]O and N–H, we suggested the complexation of Fe³⁺ with the surface groups led to the changes in chemical bonds accounting for their optical property variation. Furthermore, AA can quench the fluorescence of C dots at 570 nm and Fe³⁺-treated C dots (incubated with 50 μM Fe³⁺) at 350 nm by a static mechanism. Accordingly, two assays toward AA were developed with LODs of 0.12 μM and 0.32 μM by using C dots and Fe³⁺-treated C dots, respectively. Especially, both AA assays exhibited excellent selectivity under various substances including those with strong reducing ability such as pyrogallic acid, sodium citrate, tea polyphenol, and dithiothreitol. The applications of the assays were validated with the quantitation of Fe³⁺ in urine and AA in beverages. Electrochemical method is a significant approach to develop novel C dots even using the same carbon sources as in other methods.