ORIGINAL PAPER Electrochemical study of Meldola’s blue, methylene blue and toluidine blue immobilized on a SiO 2 /Sb 2 O3 binary oxide matrix obtained by the sol-gel processing method Received: 29 October 2002 / Accepted: 14 February 2003 / Published online: 29 March 2003 Ó Springer-Verlag 2003 Abstract SiO 2 /Sb 2 O 3 (SiSb), having a specific surface area, S BET , of 788 m 2 g )1 , an average pore diameter of 1.9 nm and 4.7 wt% of Sb, was prepared by the sol-gel processing method. Meldola’s blue (MeB), methylene blue (MB) and toluidine blue (TB) were immobilized on SiSb by an ion exchange reaction. The amounts of the dyes bonded to the substrate surface were 12.49, 14.26 and 22.78 lmol g )1 for MeB, MB and TB, respectively. These materials were used to modify carbon paste elec- trodes. The midpoint potentials (E m ) of the immobilized dyes were )0.059, )0.17 and )0.18 V vs. SCE for SiSb/ MeB, SiSb/MB and SiSb/TB modified carbon paste electrodes, respectively. A solution pH between 3 and 7 practically did not affect the midpoint potential of the immobilized dyes. The electrodes presented reproducible responses and were chemically stable under various ox- idation-reduction cycles. Among the immobilized dyes, MeB was the most efficient to mediate the electron transfer for NADH oxidation in aqueous solution at pH 7. In this case, amperometric detection of NADH at an applied potential of 0 mV vs. SCE gives linear responses over the concentration range of 0.1– 0.6 mmol L )1 , with a detection limit of 7 lmol L )1 . Keywords Chronoamperometry Æ NADH Æ Redox dyes Æ SiO 2 /Sb 2 O 3 mixed oxide Æ Sol gel Introduction The binary oxides SiO 2 /M x Oy obtained by the sol-gel processing method have found many applications in recent years [1, 2, 3, 4, 5, 6, 7, 8, 9]. The materials obtained have combined the mechanical properties of the silica matrix with the chemical properties of the bulk metal oxides. The sol-gel process permits obtaining a solid with controlled porosity and the metal oxide can be obtained as highly dispersed particles in the matrices [8]. Basically, the procedure consists of a reaction between the reagents tetraethyl orthosilicate, Si(OEt) 4 , and the metal oxide precursor, M(OR’) 4 : Si OEt ð Þ 4 þ xM OR 0 ð Þ y ! solv þ acid SiO 2 =M x O y þ 4EtOH þ xy R 0 OH ð1Þ The growing interest in using these materials as a porous substrate to immobilize electroactive species is the possibility to prepare a series of electrochemical sensors (chemically modified electrodes) [1, 2, 10, 11, 12, 13]. Studies have demonstrated that Meldola’s blue (MeB) and toluidine blue (TB) can be immobilized on a SiO 2 matrix grafted with TiO 2 and, furthermore, used to prepare modified carbon paste electrodes for use as electrochemical sensors for NADH [14] or oxalate ion [15]. Methylene blue (MB) and MeB have also been immobilized on bulk zirconium phosphate [16, 17], on silica-titanium phosphate [13] or on silica-zirconia-anti- monia [2] prepared by the sol-gel processing method. In these three cases the main characteristic was that mod- ified carbon paste electrodes made with the materials with immobilized dyes, immersed in solutions with a wide range of pH, showed a constant midpoint poten- tial, in contrast to those observed for the dyes in the solution phase [18] or when immobilized on a graphite surface [19, 20]. The shift of E m toward more positive values is desirable to enhance the redox reaction between NADH and mediator, resulting in more efficient elect- rocatalysis [21]. Antimony(III) oxide has been used in numerous ap- plications: as a flame retardant [22, 23, 24, 25], as the main component for glass formation [26], as a catalyst [27, 28, 29, 30] and as an ion exchanger [31, 32]. Within these applications, its use as an ion exchanger is of J Solid State Electrochem (2003) 7: 665–670 DOI 10.1007/s10008-003-0368-x E. S. Ribeiro Æ S. S. Rosatto Y. Gushikem Æ L. T. Kubota E. S. Ribeiro Æ S. S. Rosatto Æ Y. Gushikem (&) L. T. Kubota Instituto de Quimica, UNICAMP, Caixa Postal 6154, 13084-862 Campinas, SP, Brazil E-mail: gushikem@iqm.unicamp.br Fax: +55-19-37883023