Dopamine (DA) is an important catecholamine neurotransmitter in the mammalian central nervous system that influences several physiological functions. The impact of DA levels within the human body significantly affects the body functions. Maintaining DA level is essential and the electrochemical detection methods are often used to detect the DA level to regulate the body function. In this review, graphene (functionalized graphene and N-doped graphene) and its composites (metal, metal oxide, polymer, carbonaceous materials, clay, zeolite, and metal-organic framework based graphene composites) modified electrodes with their improved sensing performance towards DA along with several interfering species are described. Further, recent developments on the fabrication of various graphene based composite modified electrodes are also presented. Some important strategies to improve the selectivity and sensitivity towards DA with graphene based composite modified electrodes are also described.

We report a simple, facile, and reproducible method for the fabrication of electrochemically reduced graphene oxide (ERGO) films on glassy carbon electrode (GCE) by the self-assembly method. The graphene precursor, graphene oxide (GO), was self-assembled on GCE through a diamine linker which was preassembled on GCE by electrostatic interaction between the positively charged amine and the negatively charged layers of graphene oxide (GO). The oxygen functional groups present on the surface of GO were electrochemically reduced to retain the aromatic backbone of graphene. The attachment of GO followed by its electrochemical reduction was confirmed by ATR-FT-IR spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Raman spectra show that the intensity ratio of D and G bands was increased after the electrochemical reduction of GO. XPS results reveal that the carbon-to-oxygen ratio was increased after the electrochemical reduction of electrostatically assembled GO. Further, Raman and XPS results confirm the removal of oxygen functional groups present on the surface of GO after electrochemical reduction. Impedance spectral studies show that the electron transfer reaction was facile at ERGO modified GCE. Finally, the electrocatalytic activity of ERGO was examined by studying the oxidations of ascorbic acid (AA), dopamine (DA), and uric acid (UA). It enhanced the oxidation currents of AA, DA, and UA when compared to bare GCE. The electrocatalytic activity of the present modified electrode was highly stable.

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Gold nanoparticles (AuNPs) stabilized with mercaptothiadiazole ligands, 2,5-dimercapto-1,3,4-thiadiazole (DMT), 5-amino-2-mercapto-1,3,4-thiadiazole (AMT) and 5-methyl-2-mercapto-1,3,4-thiadiazole (MMT), were prepared by the reaction of the respective ligands with HAuCl(4) and NaBH(4) in an aqueous medium. TEM images show that the average size of AuNPs was 6.5 ± 0.5 nm, irrespective of the capping ligands. The colloidal solution of both DMT-capped AuNPs (DMT-AuNPs) and AMT-capped AuNPs (AMT-AuNPs) were highly stable for several months. However, several changes were noticed for MMT-capped AuNPs (MMT-AuNPs) after 2 h from its formation. The SPR band intensity at 518 nm decreases and the narrow SPR absorption band slowly changes into a flat absorption pattern with a broad peak from 518 to 1000 nm which was accompanied by a colour change of the solution from red to purple and then blue and thereafter unchanged. The TEM image of MMT-AuNPs after 96 h shows that most of the spherical shape of the AuNPs assembled to form a nanowire-like structure. The observed changes may be due to the absence of a strong stabilizing force on the surface of the MMT-AuNPs. The amino and thiolate groups on the surface of the AMT-AuNPs and DMT-AuNPs, respectively, were directly self-assembled on Au electrodes. They exhibit excellent electrocatalytic activity towards the oxidation of AA by enhancing its oxidation current twice in addition to more than 200 mV negative shift in the oxidation potential in contrast to bare Au electrode.

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Self-assembled monomolecular films of 1,8,15,22-tetraaminophthalocyanatocobalt(II) (4alpha-CoIITAPc) and 2,9,16,23-tetraaminophthalocyanatocobalt(II) (4beta-CoIITAPc) on Au surfaces were prepared by spontaneous adsorption from solution. These films were characterized by cyclic voltammetry and Raman spectroscopy. Both the surface coverage (Gamma) and intensity of the in-plane stretching bands obtained from Raman studies vary for these monomolecular films, indicating different orientations adopted by them on Au surfaces. The 4alpha-CoIITAPc-modified electrode exhibits an E1/2 of 0.35 V, while the 4beta-CoIITAPc-modified electrode exhibits an E1/2 of 0.19 V, corresponding to the CoII/CoIII redox couple in 0.1 M H2SO4. The Gamma estimated from the charge associated with the oxidation of Co(II) gives (2.62 +/- 0.10) x 10-11 mol cm-2 for 4alpha-CoIITAPc and (3.43 +/- 0.14) x 10-10 mol cm-2 for 4beta-CoIITAPc. In Raman spectral studies, the intensity ratio between in-plane phthalocyanine (Pc) stretching and the Au-N stretching was found to be 6.6 for 4beta-CoIITAPc, while it was 1.6 for 4alpha-CoIITAPc. The obtained lower Gamma and intensity ratio values suggest that 4alpha-CoIITAPc adopts nearly a parallel orientation on the Au surface, while the higher Gamma and intensity ratio values suggest that 4beta-CoIITAPc adopts a perpendicular orientation. The electrochemical reduction of dioxygen was carried out using these differently oriented Pc's in phosphate buffer solution (pH 7.2). Both the Pc's catalyze the reduction of dioxygen; however, the 4alpha-CoIITAPc-modified electrode greatly reduces the dioxygen reduction overpotential compared to 4beta-CoIITAPc-modified and bare Au electrodes.

This paper reports the synthesis of gold-silver bimetallic nanoparticles (Au-AgNPs) with different Ag : Au compositions in an aqueous medium and their attachment on a glassy carbon electrode (GCE) via a 1,6-hexadiamine (HDA) linker for the electrochemical reduction of hydrogen peroxide (HP) and nitrobenzene (NB). Initially, silver nanoparticles (AgNPs) were synthesized by the reduction of silver nitrate using trisodium citrate as a capping agent and sodium borohydride as a reducing agent. Then, the Au-AgNPs were prepared by the galvanic displacement of Ag (0) by AuCl 4 À ions. The composition of the Au-AgNPs was varied by changing the mole ratio of Ag : Au in the range of 1 : 0 to 1 : 0.16. TEM images show that the Au-AgNPs were spherical in shape with a diameter of $16 nm. The prepared colloidal solution of Au-AgNPs were then attached on a HDA modified GCE through the Michael's addition reaction and were confirmed by UV-vis diffuse reflectance spectroscopy (DRS), atomic force microscopy (AFM), line scanning analysis, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The AFM image shows that the Au-AgNPs were densely packed on the electrode surface. The Au-AgNPs modified electrode exhibits a higher heterogeneous electron transfer rate constant of 2.77 Â 10 À7 cm s À1 when compared to Ag and AuNPs modified electrodes. Furthermore, the electrocatalytic activity of the Au-AgNPs modified electrode was examined by studying the reduction of HP and NB. It was found that the Au-AgNPs with the Ag : Au mole ratio of 1 : 0.12 showed excellent electrocatalytic activity towards the reduction of both HP and NB by not only shifting their reduction potentials toward less negative potentials but also enhanced their currents compared to the bare GCE, Ag and AuNPs modified electrodes and Au-AgNPs of other molar ratios. The present modified electrode shows the limit of detection of 0.12 and 0.23 mM (S/N ¼ 3) for HP and NB, respectively.Fig. 3 AFM images obtained for (A and B) 2-D and (C and D) 3-D views of AgNPs and Au-AgNPs, modified substrates. (E and F) Line spectra obtained for Au-AgNPs and CV obtained for GC/HDA/Au-AgNPs (Ag : Au ¼ 1 : 0.12) electrode in 0.2 M PB solution (pH 7.2) at a scan rate of 50 mV s À1 . 63438 | RSC Adv., 2016, 6, 63433-63444This journal is

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