Spontaneous grafting: a novel approach to graft diazonium cations on gold nanoparticles in aqueous medium and their self-assembly on electrodes.

The spontaneous grafting of aminophenyl groups on gold nanoparticles (AuNPs) by reaction with in situ generated 4-aminophenyl diazonium cations (APD) in an aqueous medium was described. The spontaneous grafting was likely to proceed by transfer of electrons from AuNPs to the APD cations to form an aminophenyl radical and subsequent attachment with AuNPs. The aminophenyl (AP) functionalized gold nanoparticles (AP-AuNPs) were characterized by UV-visible spectroscopy, high resolution-transmission electron microscopy (HR-TEM), X-ray diffraction, FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS) and surface-enhanced Raman spectroscopy (SERS). The absence of characteristic vibrational bands corresponding to diazonium group in the FT-IR spectrum confirmed the reduction of the aminophenyl diazonium cations at the surface of AuNPs. The spontaneous attachment of AP on AuNPs was confirmed by XPS from the observed binding energy values for -NH2 at 399.4 eV and -N=N- at 400.2 eV. The SERS spectrum reveals the presence Au-C (437 cm(-1)) bond on AP-AuNPs. Further, the AP-AuNPs were self-assembled on GC/ITO electrode (AP-AuNPs modified electrode) with the aid of free amine groups present on the surface of AP-AuNPs via Michael's nucleophilic addition reaction. The AP-AuNPs modified electrode was characterized by cyclic voltammetry, impedance spectroscopy, UV-visible spectroscopy and scanning electron microscopy. Impedance studies show that the electron transfer reaction of Fe(CN)6 was higher at the AP-AuNPs modified electrode (1.81×10(-4) cm s(-1)) than at bare (3.77×10(-5) cm s(-1)) GC electrode. Finally, the electrocatalytic activity of the AP-AuNPs modified electrode was demonstrated by studying the oxidation of dopamine (DA).