In situ stabilization of hydroxylamine via electrochemical immobilization of 4-nitrophenol on GCE/MWCNT electrodes: NADH electrocatalysis at zero potential†
Electrochemical immobilization of 4-nitrophenol (4-NP) was conducted on a purified multi-walled carbon nanotube (p-MWCNT) modified glassy carbon electrode (GCE/p-MWCNT) in pH 7 phosphate buffer solution (PBS). The electrochemical reduction of 4-NP to stable in situ electrogenerated hydroxylamine (ØNHOH) intermediate species within the p-MWCNT matrix may be the underlying mechanism of immobilization. The ØNHOH-stabilized p-MWCNT modified electrode, GCE/ØNHOH@p-MWCNT, showed stable and well-defined surface-confined redox peaks at −0.11 V (A1/C1) and 0.080 V (A2/C2) vs. Ag/AgCl over other CNT-modified GCEs. The modified electrode system suggested quasi-reversible and reversible electron transfer mechanisms for the A1/C1 and A2/C2 redox couples. TEM analysis of the ØNHOH@p-MWCNT hybrid powder demonstrated the presence of ØNHOH species on the surfaces, as well as on the inner walls of the p-MWCNTs. The XRD peaks exhibited shifts in 2θ values for the hybrid material compared with the unmodified materials, which confirmed the stabilization of ØNHOH within the p-MWCNT via π–π interactions. Electrochemical characterization of the GCE/ØNHOH@p-MWCNT revealed two electron transfer mechanisms with adsorption-controlled and Nernstian behaviours. A highly sensitive electrocatalytic oxidation of dihydronicotinamide adenine dinucleotide (NADH) at 0.02 V vs. Ag/AgCl was achieved with the GCE/ØNHOH@p-MWCNT. Furthermore, the hybrid electrode successfully sensed NADH amperometrically with 2.9 nA μM−1 sensitivity, a 0.043 μM limit of detection and a linear detection range from 100 μM to 1 mM for ten successive additions of 100 μM NADH at an applied potential of 0.02 V vs. Ag/AgCl in pH 7 PBS.