Self-assembled AuNPs on sulphur-doped graphene: a dual and highly efficient electrochemical sensor for nitrite (NO2-) and nitric oxide (NO)

A simple strategy for the synthesis of self-assembled gold nanoparticles (AuNPs) on sulphur-doped graphene (S-Gr) to form AuNPs-S-Gr nanohybrids is presented. Structural, chemical and morphological characterization of AuNPs-S-Gr nanohybrids through UV-Vis spectroscopy, the X-ray diffraction technique (XRD), Raman spectroscopy, force field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution TEM (HR-TEM) and electrochemical techniques establishes the AuNPs-S-Gr nanohybrid as a monolayer of AuNPs assembled on S-Gr. The lead underpotential deposition experiments ascertain that nano Au in AuNPs-S-Gr nanohybrids is better exposed for faradic processes than in citrate-stabilized AuNPs, with the faradic response in the former being dominated by the Au-(111) planes. The AuNPs-S-Gr nanohybrids are demonstrated to be electrochemically stable, reusable and with an excellent prospect for the sensitive and selective electrochemical detection of two biologically and environmentally important oxides of nitrogen viz. nitrite (NO2-) and nitric oxide (NO). The electrocatalytic electron transfer rate constants (k(cat)) of the AuNPs-S-Gr nanohybrid towards electro-oxidation of NO2- and NO were observed to be 0.98 (+/- 0.03) x 10(5) M-1 s(-1/2) and 0.51 (+/- 0.02) x 10(4) M-1 s(-1/2), respectively. The AuNPs-S-Gr nanohybrid was demonstrated to ensure selective and sensitive electrochemical quantification of NO2- and NO with a very high sensitivity (20766.17 and 21046.72 mu A M-1 cm(-2)), a wide linear range (12.5-680.92 and 24.9-680.93 mu M) and very low detection limits (0.003 and 0.009 mu M), respectively. Importantly, the AuNPs-S-Gr nanohybrid exhibited appreciable selectivity towards the detection of NO2- and NO even in the presence of a very high concentration of common interfering ions. The stability and reproducibility of faradic signals for NO2- and NO detection at the AuNPs-S-Gr nanohybrid were found to be excellent for standard laboratory and real samples. The electrocatalytic activity and sensing results of the AuNPs-S-Gr nanohybrid for NO2- and NO are much better than those recently reported over surfaces purposefully designed for electrocatalytic oxidation and electrosensing of these analytes.