Rafael Madueño

A voltammetric study of 6-mercaptopurine monolayers on polycrystalline gold electrodes

Abstract 6-Mercaptopurine (6MP) monolayers were formed spontaneously on different gold surfaces by chemisorption from 6MP solutions. The present cyclic voltammetric study reveals that the monolayer formed is stable in a wide potential range up to values where the oxidative desorption occurs. The reductive desorption of 6MP at a polyfaceted gold electrode shows a multi-wave response. This has been analysed by comparing the observed peak potentials with those obtained at low-index single crystal gold electrodes. The film has no apparent effect on the electron transfer rate constant of Ru(NH 3 ) 6 3+/2+ and Fe(CN) 6 3−/4− at the substrates studied, but modifies the electron transfer rate of the benzoquinone redox couple. This behaviour is typical of thin films where only the slower reactions should display a larger apparent decrease of the rate constant. In contrast, the 6MP-monolayer shows a strong ability to inhibit electroreduction of dioxygen at polyfaceted gold while it remains stable.

The direct electrochemistry of cytochrome c at a hanging mercury drop electrode modified with 6-mercaptopurine

Abstract The modification of a mercury electrode with 6-mercaptopurine (6MP) has been performed at pH 6 in acetate buffer and the electrode behaviour has been checked in the absence of the modifier in solution. Direct electrochemistry of cytochrome c has been observed on a 6MP-Hg modified electrode. At high coverage of 6MP, the monolayer acts as an effective promoter for the redox exchange of cyt c , which is not observed on a bare mercury electrode. The film is stable for more than 200 scans and must play a role on avoiding irreversible adsorption and denaturation of the protein. At pH 7, 25 mM phosphate buffer and 0.1 M NaClO 4 , cyt c exhibits a quasi-reversible electron exchange similar to that reported with gold modified electrodes, the electron rate constant being 4.1×10 −3 cm s −1 . As far as we know it is the first time that such a response has been observed on mercury.

Characterization of 6-mercaptopurine monolayers on Hg surfaces

Abstract The formation and organization of self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) has been studied on mercury surfaces. The organization process has been examined by cyclic voltammetry and chronoamperometry. The study confirms the occurrence of a two-dimensional (2D) condensation at more positive potentials than those of the cathodic/anodic adsorption peaks at about −0.2 V. The condensed phase is destroyed at more negative potentials. Charging current measurements as well as electron transfer experiments indicate the formation of a thin film monolayer. This film has no effect on the electron transfer rate constant of an outer-sphere redox couple but it shows a strong ability to inhibit electroreduction of dioxygen as it is characteristic of condensed organic monolayers. Cu 2+ is able to permeate the 6MP film, giving a current only three-fold lower than that at a bare hanging mercury drop electrode (HMDE) and with no-linear diffusion characteristics.

Formation of a 1,8-Octanedithiol Self-Assembled Monolayer on Au(111) Prepared in a Lyotropic Liquid-Crystalline Medium

A characterization of the 1,8-octanedithiol (ODT) self-assembled monolayer (SAM) formed from a Triton X-100 lyotropic medium has been conducted by electrochemical techniques. It is found that an ODT layer of standing-up molecules is obtained at short modification time without removing oxygen from the medium. The electrochemical study shows that the ODT layer formed after 15 min of modification time has similar electron-transfer blocking properties to the layers formed from organic solvents at much longer modification times. On the basis of XPS data, it is demonstrated that the inability to bind gold nanoparticles (AuNPs) is due to the presence of extra ODT molecules either interdigited or on top of the layer. Treatment consisting of an acid washing step following the formation of the ODT-Au(111) SAM produces a layer that is able to attach AuNPs as demonstrated by electrochemical techniques and atomic force microscopy (AFM) images.

Hemoglobin bioconjugates with surface-protected gold nanoparticles in aqueous media: The stability depends on solution pH and protein properties

The identification of the factors that dictate the formation and physicochemical properties of protein-nanomaterial bioconjugates are important to understand their behavior in biological systems. The present work deals with the formation and characterization of bioconjugates made of the protein hemoglobin (Hb) and gold nanoparticles (AuNP) capped with three different molecular layers (citrate anions (c), 6-mercaptopurine (MP) and ω-mercaptoundecanoic acid (MUA)). The main focus is on the behavior of the bioconjugates in aqueous buffered solutions in a wide pH range. The stability of the bioconjugates have been studied by UV-visible spectroscopy by following the changes in the localized surface resonance plasmon band (LSRP), Dynamic light scattering (DLS) and zeta-potential pH titrations. It has been found that they are stable in neutral and alkaline solutions and, at pH lower than the protein isoelectric point, aggregation takes place. Although the surface chemical properties of the AuNPs confer different properties in respect to colloidal stability, once the bioconjugates are formed their properties are dictated by the Hb protein corona. The protein secondary structure, as analyzed by Attenuated total reflectance infrared (ATR-IR) spectroscopy, seems to be maintained under the conditions of colloidal stability but some small changes in protein conformation take place when the bioconjugates aggregate. These findings highlight the importance to keep the protein structure upon interaction with nanomaterials to drive the stability of the bioconjugates.

Hemoglobin becomes electroactive upon interaction with surface-protected Au nanoparticles

In this work, we report on the electrochemical behavior of bioconjugates prepared with gold nanoparticles (AuNP) capped with three different molecular layers (citrate anions, 6-mercaptopurine and ω-mercaptoundecanoic acid) and the protein hemoglobin (Hb). Freshly formed bioconjugates are deposited on a glassy carbon electrode and assayed for electroactivity. A pair of redox peaks with formal potential at -0.37V is obtained, in contrast with the free Hb protein that is inactive on the glassy carbon substrate. The redox response is typical for quasi-reversible processes allowing the determination of the electron transfer rate constant for the three bioconjugates. Additional evidence of the structural integrity of protein upon forming the bioconjugate is obtained by monitoring the electrochemical response of the Hb heme Fe(III)/Fe(II) redox couple as a function of solution pH. Moreover, the Hb forming the protein corona around the AuNPs show good electrocatalytic activity for the reduction of hydrogen peroxide and oxygen. It has been found that only the first layer of Hb surrounding the AuNPs are electroactive, although some part of the second layer also contribute, pointing to the role of the AuNP in the electrochemical response.

Modification of metal substrates and its application to the study of redox proteins

Abstract This work deals with a comparative study of the surface modification of metal substrates (gold, mercury and platinum) by chemisorption of 6-mercaptopurine (6MP). Experimental conditions for film formation were defined and tested in ex-situ voltammetric experiments. A self-assembled monolayer (SAM) seems to be formed by judging from desorptive reduction peaks and supression of hydrogen adsorption region. The 6MP-Pt substrate allows to observe promoted quasi-reversible stable electrochemistry of metalloprotein cyt c similar to that reported on 6MP-Au and 6MP-Hg electrodes. Differences in metal-S bonding strength are evidenced but lateral interactions of 6MP-adsorbed molecules seems to play a main role on the behavior of the modified substrate.