5-(4-pyridinyl)-1,3,4-oxadiazole-2-thiol on gold: SAM Formation and electroactivity

J. Braz. Chem. Soc., Vol. 19, No. 4, 711-719, 2008. Printed in Brazil - ©2008 Sociedade Brasileira de Química 0103 - 5053 $6.00+0.00 Tercio de F. Paulo,a Maria A. S. da Silva,a Solange de O. Pinheiro,a Emerson Meyer,a Lucidalva S. Pinheiro,b José A. Freire,g,b Auro A. Tanaka,c Pedro de Lima Neto,d Ícaro de S. Moreiraa and Izaura C. N. Diógenes*,a a Departamento de Química Orgânica e Inorgânica, Centro de Ciências, Universidade Federal do Ceará, CP 12200, 60455-960 Fortaleza-CE, Brazil b c d Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, CP 6030, 60455-960 Fortaleza-CE, Brazil Departamento de Química, Centro de Ciências Exatas e Tecnologia. Universidade Federal do Maranhão, Avenida dos Portugueses, s/n, 65085-580 São Luís-MA, Brazil Departamento de Química Analítica e Físico Química, Centro de Ciências, Universidade Federal do Ceará, CP 6035, Fortaleza-CE, Brazil O composto 5-(4-pyridinyl)-1,3,4-oxadiazole-2-thiol (Hpyt) adsorve espontaneamente sobre ouro formando SAMs (“Self-Assembled Monolayers”) que, de acordo com os resultados eletroquímicos e de STM (“Scanning Tunneling Microscopy”), contêm poros através dos quais as moléculas dos complexos [Fe(CN)6]4– e [Ru(NH3)6]3+ acessam a superfície livre de modificação. Para o complexo [Fe(CN)6]4–, a dependência da corrente faradáica com o pH da solução eletrolítica permitiu o cálculo do pKa da molécula de Hpyt adsorvida sobre ouro (4,2). Os parâmetros termodinâmicos, $Hads and $Gads, para o processo de adsorção desta molécula foram estimados em –20,01 e –39,39 kJ mol-1, respectivamente, utilizando-se o modelo de Langmuir. O processo redox da metaloproteína citocromo c foi estudado utilizando-se a SAM de Hpyt. A constante de velocidade heterogênea de transferência de elétrons foi calculada em 2,29 r 10–3 cm s-1. 5-(4-pyridinyl)-1,3,4-oxadiazole-2-thiol (Hpyt) spontaneously adsorbs on gold forming SAMs (self-assembled monolayers) that, based on STM (Scanning Tunneling Microscopy) and electrochemical data, contain pinholes through which [Fe(CN)6]4– and [Ru(NH3)6]3+ probe molecules access the underlying gold electrode. For the former molecule, the dependence of the faradaic current on the electrolyte solution pH value allowed the evaluation of the surface pKa as 4.2. The thermodynamic parameters $Hads and $Gads for the Hpyt adsorption process could be described by the Langmuir model and were calculated as –20.01 and –39.39 kJ mol-1, respectively. Electrodic redox reaction of cytochrome c metalloprotein was accessed by using the Hpyt SAM with a heterogeneous electron transfer rate constant of 2.29 r 10–3 cm s-1. Keywords: oxadiazole, thiol, SAMs, STM, kinetic of adsorption, Cyt c Introduction Supramolecular assemblies constructed by using single electroactive molecules as building blocks, offer an interesting way to create electrically conducting materials whose organized architectures make them suitable for developing molecular electronic devices. In such a way, thiolate molecules have been used so far because both the g In memorian * e-mail: strong affinity of sulfur to gold surfaces and formations of highly ordered self-assembled monolayers (SAMs).1-3 Progress toward this strategic goal demands not only the development of new synthetic approaches that yield highly ordered materials, but also careful attention to those elementary processes that dictate the heterogeneous electron transfer (hET) through metal/monolayer interfaces. One aspect that affects this process is the thermodynamic stability of a monolayer. The magnitude of the adsorption free energy ($G ads) gives a quantitative measure of Article 5-(4-Pyridinyl)-1,3,4-oxadiazole-2-thiol on Gold: SAM Formation and Electroactivity 712 5-(4-Pyridinyl)-1,3,4-oxadiazole-2-thiol on Gold: SAM Formation and Electroactivity adsorption strength, allowing insight into the chemical process of monolayer formation. For instance, results reported for the adsorption of H 2Q(CH 2) nSH, where H 2Q = hydroquinone, on gold 4 have shown a small dependence of $Gads with n, the alkyl spacer length, indicating that this parameter is mostly dominated by the gold-thiolate bond. Pyridinic oxiadiazole compounds constitute an interesting class of modifier species since they have been systematically studied due to their luminescent properties and potential application in biological activities.5,6 Despite these aspects, there are no data in the literature concerning the use of this type of compound as electrochemical biological sensor. Within this compound class, 5-(4-pyridinyl)1,3,4-oxadiazole-2-thiol (Hpyt) species is particularly interesting because of the possibility to study the effect of SAM protonation equilibrium, which is one of the processes that influence hET. In fact, it is well known that the charge of SAMs may limit the electron transfer rate within biological systems where subtle changes in the chemical microenvironment surrounding the redox site can dramatically alter the ET rates.7 In this context, the objective of this work is to determine the thermodynamic parameters concerning the Hpyt adsorption process on gold. Also, by using Cyt c metalloprotein, [Fe(CN)6]4–, and [Ru(NH3)6]3+ complexes as probe molecules, to obtain a better understanding of the factors that influence hET. Experimental Apparatus The electrochemical experiments were performed on a BAS 100W Electrochemical Analyzer (Bioanalytical Systems-BAS, West Lafayette, IN). A Nanoscope IIIA equipped with a standard Scanning Tunneling Microscopy (STM) head, able to read tunneling currents from 30 pA, was used to run the STM experiments. Tungsten wire 0.25 mm mechanically cut was used as tip. The high resolution STM images were obtained in the constant height mode. The electrochemical experiments with probe molecules were carried out by using a three-electrode configuration cell. For acquisition of the reductive desorption curves, a Teflon cell was used to prevent KOH chemical attack. Polycrystalline gold surfaces (BAS, A = 0.07 cm2) modified with Hpyt species and a gold flag were used as working and auxiliary electrodes, respectively. The polishing procedure of the gold surfaces was made as described by Qu et al.8 These electrodes were mechanically polished, rinsed and sonicated (10 min) in Milli-Q water. The electrode was J. Braz. Chem. Soc. then immersed in a freshly prepared “piranha solution” (3:1 concentrated H2SO4/30% H2O2; CAUTION: Piranha solution is a high oxidant solution that reacts violently with organic compounds), rinsed exhaustively with water, and sonicated again. The cleanness was evaluated by comparison of the i-E curve obtained in a 0.5 mol L–1 H2SO4 solution with the well-established curve for a clean gold surface.9 The electroactive area of this electrode was determined by means of cyclic voltammetry technique based on Randles-Sevick equation.10 For this purpose, a 0.5 mol L–1 K2SO4 solution at 25 oC containing 0.1 mmol L–1 [Fe(CN)6]4– complex as redox active (...truncated)