Hetero atoms doped carbon dots modified electrodes for the sensitive and selective determination of phenolic anti-oxidant in coconut oil

Carbon dots (CDs) and CDs doped with nitrogen (N-CDs) and nitrogen-sulphur (N,S-CDs) were prepared by ultrasonication using different precursors. The HR-TEM images showed the average particle size of CDs, N-CDs and N,S-CDs was found to be 1.6, 2.0 and 1.9 nm, respectively. The synthesized different CDs were modified on glassy carbon (GC) electrode by immersing it on the respective CDs for 8 h. The modification of CDs was confirmed by attenuated total reflectance (ATR)-FT-IR, scanning electron microscopy (SEM), X-ray diffraction, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS) techniques. The ATR-FT-IR spectrum of GC substrate modified with CDs shows characteristic carboxyl stretching at 1723 cm−1 whereas NH stretching at 3345 cm−1 for N-CDs and 3384 cm−1 for N,S-CDs. The heterogeneous rate constant (ket) calculated for 1 mM Ru (NH3)6Cl3 at bare GC, GC/CDs, GC/N-CDs and GC/N,S-CDs electrodes was found to be 0.92 × 10−4, 1.79 × 10−3, 2.1 × 10−3 and 3.05 × 10−3 cm s−1, respectively. The higher ket value obtained at GC/N,S-CDs electrode indicates that the electron transfer was faster at this electrode compared to other electrodes. The electrocatalytic activity of GC/CDs, GC/N-CDs and GC/N,S-CDs electrodes towards the oxidation of tert-butylhydroquinone (TBHQ) was studied. It was found that the GC/N,S-CDs electrode showed higher oxidation current than other electrodes. It showed 4-fold higher oxidation current with 177 mV less positive potential for TBHQ when compared to bare GC electrode. This was attributed to higher electroactive surface area, hydrogen bonding and π-π interactions between N,S-CDs and TBHQ. Under optimized conditions, N,S-CDs modified electrode exhibited a linear response in a wide concentration range from 50 × 10−9-10 × 10−6 M TBHQ and a limit of detection was found to be 0.68 ng L−1 (S/N = 3). Finally, the present modified electrode was successfully utilized for the determination of TBHQ in coconut oil samples and the results were validated with GC–MS method.