Full Paper Evaluation of Meldola Blue-Carbon Nanotube-Sol-Gel Composite for Electrochemical NADH Sensors and Their Application for Lactate Dehydrogenase-Based Biosensors Adina Arvinte, a,b Adama M. Sesay , b Vesa Virtanen, b Camelia Bala a * a LaborQ, Department of Analytical Chemistry, University of Bucharest, 4-12 Regina Elisabeta Blvd., 030018 Bucharest-3, Romania *e-mail:camelia.bala@g.unibuc.ro b Laboratory of Biotechnology , University of Oulu, Salmelantie 43, 88600 Sotkamo, Finland Received: June 3, 2008 Accepted: August 15, 2008 Abstract A novel MB-SWNT-sol-gel nanocomposite material was prepared by the sol-gel process incorporating a redox mediator and carbon nanotubes. The electrocatalytic properties of the nanomaterial based sensor toward NADH oxidation were studied by electrochemical measurements. Significant enhancement of oxidation current is obtained at electrodes modified by MB-SWNT-sol-gel in comparison with the analogous carbon black and/or graphite composite modified electrode. The usefulness of the nanocomposite material as a matrix for immobilizing enzymes is also demonstrated. Analytical parameters of d-lactate biosensors with and without SWNT in the hybrid film were compared demonstrating that performance of the biosensor was significantly improved when introducing SWNT. Keywords: Carbon-nanotubes, Nanocomposite, NADH, d-Lactate, Sol-gel DOI: 10.1002/elan.200804332 1. Introduction Among the many deposition techniques used for prepara- tionofbiosensingfilms,thesol–gelmethodhasbeenshown to be an interesting and adaptable way to prepare modified electrodesformanyyears[1–11].Itprovidesalow-costand efficient way to produce homogeneous films with a variety of adjustable chemical compositions. Generally, sol-gel reactionsproceedbythehydrolysisofanalkoxideprecursor under acidic or basic conditions, and subsequent condensa- tion of the hydroxylated monomers to form a porous gel. Most of the useful sol – gel matrices are organic-inorganic orthosilicateshavingasolfollowedbyawetgelphase,which can be dried to form a xerogel with controlled porous surface area [2, 5, 8]. Theencapsulationofactivebiomoleculessuchasenzymes in sol-gel materials has been widely applied to the develop- ment of electrochemical biosensors [12 – 19]. By encapsu- lating the enzyme in sol-gel can improve some of their properties, such as operational stability and activity com- pared to immobilization in other matrices or covalent cross- linking with glutaraldehyde [13, 14, 18]. Since such electro- des with encapsulated enzymes still present interference problems due to high operating potentials at different electrode substrates, mediators were also included in the composition of sol-gel for the construction of composite- based biosensors, for example ferrocene [13, 16, 17, 19], tetrathiafulvalene[13],Meldola)sblue[20],methylviologen [21, 22] and cobalt-phtalocyanine [23]. Exploring the encapsulation ability of sol-gel, many other nanomaterials were immobilized on to the electrode surface, such as carbon nanotubes (CNTs) [24 – 26] and carbon nanofibers (CNFs) [27] demonstrating that sol-gel nanocomposites can provide new capabilities for the development of electro- chemicaldevices.Todevelopasensitiveandstablemediated biosensor, sol-gel method could be explored as a suitable immobilization technique for enzyme as well as for the mediator and CNTs. Recently, the integration of CNTs and redox mediators into polymeric matrix has been explored to catalyze some molecules. Li et all [28] report the integration of Prussian blue and multiwalled carbon nanotubes (MWNT) in a polymeric matrix of poly(4-vinylpyridine) for constructing electrochemical sensors with improved sensitivity towards the electro-reduction of hydrogen peroxide. Zhang and Gorskireportedanewelectrodesystemthatutilizessynergy betweenCNTandredoxmediatorstofacilitatetheelectron- transfer processes of NADH [29, 30]. However, in this case, redoxmediators,suchasToluidineblueOandAzureAwere covalently attached to a polymer chitosan with good film- forming ability by the formation of Schiff bases using glutaric dialdehyde as a tether and then interspersed with CNT to form composite film for amperometric determina- tion of NADH. Obviously, this covalent route involving a multistepped procedure was complicated and time-consum- ing. In the present work, a simple strategy was used to immobilize a phenoxazine redox mediator (Meldola)s blue) 2355 Electroanalysis 20, 2008, No.21, 2355–2362 # 2008 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim