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

Abstract

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 1.9nm, respectively. The synthesized different CDs were modified on glassy carbon (GC) electrode by immersing it on the respective CDs for 8h. 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 1723cm−1 whereas NH stretching at 3345cm−1 for N-CDs and 3384cm−1 for N,S-CDs. The heterogeneous rate constant (ket) calculated for 1mM 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 3.05×10−3cms−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 177mV 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 50×10−9-10×10−6M TBHQ and a limit of detection was found to be 0.68ngL−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.