ORIGINAL PAPER Electrochemical investigation of a glassy carbon electrode modified with carbon nanotubes decorated with (poly)crystalline gold Antonio Radoi & Simona-Carmen Litescu & Sandra A. V. Eremia & Mihaela Miu & Mihai Danila & Adrian Dinescu & Gabriel-Lucian Radu Received: 29 March 2011 /Accepted: 1 July 2011 /Published online: 13 July 2011 # Springer-Verlag 2011 Abstract Multiwalled carbon nanotubes with nanosized sputtered gold were used to modify a glassy carbon electrode (GCE). The substrate was characterized by scanning electron microscopy (SEM), X-ray diffraction, cyclic voltammetry and amperometry. SEM micrographs indicated an uniform coverage of the carbon nanotubes with nanosized (poly) crystalline gold. Cyclic voltammetry reveals that peak separation of the unmodified GCE in the presence of 1 mM ferricyanide is 131 mV, but 60 mV only for the modified GCE. In addition, the oxidation of NADH (1 mmol L -1 solution) begins at negative potentials (around -100 mV vs. Ag/AgCl), and the anodic peak potential (corresponding to the irreversible oxidation of NADH) is found at +94 mV. The effect of pH on the electrocatalytic activity was studied in the range from 5.4 to 8.0. The relationship between the anodic peak potential and the pH indicated a variation of -33.5 mV/pH which is in agreement with a two-electron and one-proton reaction mechanism. Amperometry, performed at either -50 or +50 mV vs. an Ag/AgCl reference electrode, indicates that the modified electrode is a viable amperometric sensor for NADH. At a working potential of +50 mV, the response to NADH is linear in the concentration range from 1 to 100 μmol L -1 , with an RSD of 6% (n =4). Keywords Carbon nanotubes . (Poly)crystalline gold . Glassy carbon electrode . NADH Introduction Carbon nanotubes have been used as chemically affordable carbon materials [1] for electrodes with improved electro- catalytic effects towards β-nicotinamide adenine dinucleo- tide (NADH) electrooxidation [2, 3]. The importance of NADH as a cofactor in naturally occurring enzymatic reactions [4, 5], as well as the potential applications in NADH based biosensors are relevant for the bio-electro- analytical research field [6–16]. In heterogeneous catalysis the catalytic activity is correlated with the (electro)active surface area, i.e. to the number of catalytically active atoms on the surface which are available for the targeted molecules [17]. Even if some restrictions apply [16], supported catalytic particles on substrates with large surface area and reducing the catalysts’ particle size are precursors for an enhanced catalytic activity [18]. The electrochemistry of the redox couple NAD + /NADH has been studied extensively by Gorton [19]; the formal redox potential (E°′) of the NAD + /NADH is known to be -560 mV vs. SCE [4] meanwhile a variation of E with pH of -30.3 mV/pH was determined by Rodkey [20]. Even if it is still controversial, however, a sequential electrochem- ical–chemical–electrochemical (ECE) mechanism is widely accepted to be the path undertaken in NADH oxidation. Electronic supplementary material The online version of this article (doi:10.1007/s00604-011-0658-4) contains supplementary material, which is available to authorized users. A. Radoi (*) : M. Miu : M. Danila : A. Dinescu National Institute for Research and Development in Microtechnology (IMT-Bucharest), 126A Erou Iancu Nicolae, 077190 Bucharest, Romania e-mail: radoiantonio@yahoo.com S.-C. Litescu : S. A. V. Eremia : G.-L. Radu Centre of Bioanalysis, National Institute for Biological Sciences, 296 Spl. Independentei, Bucharest, Romania Microchim Acta (2011) 175:97–104 DOI 10.1007/s00604-011-0658-4