Reactivity of nanocolloidal particles gamma-Fe2O3 at charged interfaces. Part 2. Electrochemical conversion. Role of the electrode material

In this paper we are interested in the reactivity of magnetic nanoparticles at the electrode involved in the electrochemical synthesis of magnetic and conductive liquids. The reactivity of charged colloidal particles occurs in two steps, first the approach toward the electrode with a possible adsorption phenomenon and secondly the electron transfer. In this paper we focus on the electrochemical behaviour of well-defined gamma-Fe(2)O(3) nanoparticles at a gold and at a mercury electrode. Particles can be electrochemically reduced at the two electrodes and can be dispersed into mercury at a highly negative potential. Here, we probe in particular the properties of nanoreactor of the particles, that is to say, the possible conservation of their size after they have undergone the electrochemical process. By correlating complementary techniques (here atomic force microscopy (AFM) observations, Raman spectroscopy and cyclic voltammetry on gold electrode) and by studying the magnetic properties of the material obtained after reduction of the particles on a mercury electrode, we are able to probe both the chemical nature and the physical state of the particles once transformed. Experimental results show that under specific conditions, the particles are individually converted into iron, which justifies their use for preparing a liquid with both magnetic properties and properties of electron conduction.     

Gold-conductive polymer nanoparticles: a hybrid material with enhanced photonic reactivity to environmental stimuli

We have designed a simple synthetic procedure to encapsulate colloidal gold nanoparticles by electrostatic adsorption with water-soluble poly(aniline-2-carboxylic acid). The composite nanoparticles are stable in aqueous buffer and retain the respective optical reactivity of the gold colloid to refractive index increases, and of the conductive polymer to pH changes and oxidoreduction. The new composite displays, however, significant enhancements in photonic performance when compared to the individual components, which seem to result from electronic interplay between the two materials in the hybrid structure. The enhanced photonic reactivity of the composite structure offers new opportunities for biosensing application.     

Model electrodes with defined mesoscopic structure

Model electrodes with defined mesoscopic structure were either generated by adsorption of surfactant stabilized metal clusters from colloidal solution on a support of gold or by electrochemical deposition of platinum on gold substrates. Both types of model electrodes were characterized by STM (scanning tunnelling microscopy), cyclic voltammetry and electrooxidation of adsorbed CO. The supported colloidal Pt as well as the electrochemically deposited Pt revealed different reactivities regarding the CO monolayer electrooxidation as compared to a polycrystalline Pt bulk electrode. In addition, in-situ FTIR (Fourier transformed infrared) spectroscopy was applied to characterize CO adsorbed on electrochemically deposited Pt on gold. Combined with the structural information from STM it seems likely that the differences regarding the catalytic properties of the model electrodes are due to different coverages of the substrate with catalyst particles. Received: 24 June 1996 / Revised: 29 November 1996 / Accepted: 4 December 1996     

Time-dependent complexation of glucose-reduced gold nanoparticles with octadecylamine Langmuir monolayers

We report on the reduction of aqueous chloroaurate ions by glucose to form gold nanoparticles of uniform size. We further demonstrate the complexation of these particles with octadecylamine (ODA) monolayers at the air-water interface. Pressure-area (pi-A) isotherms as a function of time of complexation revealed a significant expansion of the monolayer. Surface pressure variation with time for constant areas after spreading of the monolayer was carried out to observe the kinetics of complexation of the colloidal particles at the interface. The kinetics of complexation of the particles at the interface was also monitored by Brewster angle microscopy (BAM) measurements. Langmuir-Blodgett films of the particles complexed with ODA were formed at a subphase pH of 9 onto different substrates. Quartz crystal microgravimetry (QCM) was used to quantify the amount of particles deposited per immersion cycle of the quartz crystal. The LB films were further characterized by UV-vis and transmission electron microscopy (TEM) measurements. TEM measurements indicate a close packed and equidistant arrangement of colloidal particles in the LB film, probably due to hydrogen-bonding interactions.     

Electrodeposition of gold particles on aluminum substrates containing copper

Electrodeposition of adhesive metal films on aluminum is traditionally preceded by the zincate process, which activates the aluminum surface. This paper presents an alternative approach for activation of aluminum by using films containing 99.5% aluminum and 0.5% copper. Aluminum/copper films are made amenable for subsequent electrodeposition by anodization followed by chemical etching of aluminum oxide. The electrodeposition of gold is monitored with electrochemical impedance spectroscopy (EIS). Analysis of EIS data suggests that electrodeposition of gold increases the interfacial capacitance from values typical for electrodes with thin oxide layers to values typical for metal electrodes. Scanning electron microscopy examination of aluminum/copper films following gold electrodeposition shows the presence of gold particles with densities of 10(5)-10(7) particles cm(-2). The relative standard deviation of mean particle diameters is approximately 25%. Evaluation of the micrographs suggests that the electrodeposition occurs by instantaneous nucleation followed by growth of three-dimensional semispherical particles. The gold particles, which are electrically connected to the conductive aluminum/copper film, support a reversible faradaic process for a soluble redox couple. The deposited gold particles are suitable for subsequent metallization of aluminum and fabrication of particle-type films with interesting catalytic, electrical, and optical properties.     

Effect of electron tunneling on the crystallization of nanostructured metal sols

Regularities of the self-organization of bimodal ensembles of nanoparticles into crystalline structures in metal lyosols are studied by the Brownian dynamics method. Differences in the thicknesses of adsorption layers, van der Waals, elastic, and electrostatic interactions, and random and dissipative forces, as well as variations in interparticle electrostatic interactions due to electron tunneling are taken into account in the model. Pair potentials for polydisperse ensembles of particles are analyzed and the correlation between the pattern of potential curves and the type of forming structures is revealed. The possibility of the selective coagulation of particles with different sizes by virtue of dissimilar charging at the moment of their approach is demonstrated. The kinetics of this process is investigated. Specific features of the crystallization of bimodal sols are discussed using real gold organosols as examples.     

Electrostatic interactions between amphoteric latex particles and proteins

The electrostatic interactions between amphoteric polymethyl methacrylate latex particles and proteins with different pI values were investigated. These latex particles possess a net positive charge at low pH, but they become negatively charged at high pH. The nature and degree of interactions between these polymer particles and proteins are primarily controlled by the electrostatic characteristics of the particles and proteins under the experimental conditions. The self-promoting adsorption process from the charge neutralization of latex particles by the proteins, which have the opposite net charge to that of the particles, leads to a rapid reduction in the zeta potential of the particles (in other words colloidal stability), and so strong flocculation occurs. On the other hand, the electrostatic repulsion forces between similarly charged latex particles and the proteins retard the adsorption of protein molecules onto the surfaces of the particles. Therefore, latex particles exhibit excellent colloidal stability over a wide range of protein concentrations. A transition from net negative charge to net positive charge, and vice versa (charge reversal), was observed when the particle surface charge density was not high enough to be predominant in the protein adsorption process.     

Lamellar multilayer hexadecylaniline-modified gold nanoparticle films deposited by the Langmuir-Blodgett technique

Organization of hexadecylaniline (HDA)-modified colloidal gold particles at the air-water interface and the formation thereafter of lamellar, multilayer films of gold nanoparticles by the Langmuir-Blodgett technique is described in this paper. Formation of HDA-capped gold nanoparticles is accomplished by a simple biphasic mixture experiment wherein the molecule hexadecylaniline present in the organic phase leads to electrostatic complexation and reduction of aqueous chloroaurate ions, capping of the gold nanoparticles thus formed and phase transfer of the now hydrophobic particles into the organic phase. Organization of gold nanoparticles at the air-water interface is followed by surface pressure—area isotherm measurements while the formation of multilayer films of the nanoparticles by the Langmuir-Blodgett technique is monitored by quartz crystal microgravimetry, UV-Vis spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy.     

Preparation of gold nanoislands on various functionalized polymer-modified glass and ITO for electrochemical characterization of monolayer assembly of alkanethiols

Nanometer dimension of citrate-capped gold nanoparticles can be firmly bound with various functionalized polymer-modified glass plate and indium tin oxide (ITO) substrates. Herein we report 3-aminopropyltriethoxysilane, polyvinyl pyridine, polyethylene imines, etc. as binding agents to modify these substrates to stabilize the charged colloidal gold nanoparticles through electrostatic stabilization of gold nanoparticles. When gold nanoparticles pretreated substrate are exposed into the seeding growth solution, the preadsorbed gold nanoparticles grow further and then form nanoislands of gold on glass and ITO substrates. The formation of nanoislands on microscope glass slide and ITO was monitored with UV-visible spectroscopy, cyclic voltammetry, and atomic force microscopy methods. The gold nanoislands and gold nanoparticles pretreated substrates can be used as platform to study the self-assembling behavior of long chain alkanethiols such as C₁₂SH, C₁₆SH, and C₁₈SH. The binding, coverage, and electron transfer characteristics of monolayer assembly on modified gold nanoisland and nanoparticles modified substrates are studied using electrochemical studies. The gold substrates can be prepared by this method, which is simple and reproducible and can be applied to various sensor and electrocatalytic applications.     

Colloidal stability of dextran-modified latex particles toward adsorption of concanavalin A

The colloidal stability of the dextran-modified poly(methyl methacrylate) (PMMA) latex particles toward adsorption of a carbohydrate-binding protein, concanavalin A (Con A), is primarily controlled by the charge neutralization mechanism. Formation of a crosslinked network structure via the specific affinity interactions between the dimeric Con A molecules and the dextran molecules anchored onto different latex particles may also have an impact on the coagulation kinetics. Judging from the data of coagulation kinetics, the colloidal stability of the latex particles toward added Con A in the decreasing order is: latex particles without dextran modification>latex particles with a dextran content of 2.15%>latex particles with a dex-tran content of 1.24% based on total polymer weight (PMMA+grafted dextran). The coagulation mechanisms involved in the adsorption of Con A onto the latex particles have been proposed to explain these experimental data. Charge neutralization of the negatively charged latex particles by adsorption of the positively charged Con A is the predominant destabilization mechanism. The ratio of the number of dextran active sites to that of Con A molecules plays an important role in the formation of the crosslinked network structure. The electrolytes in water cause a reduction in the electrostatic repulsion force among the interactive latex particles, but this ionic strength effect is not significant in comparison with charge neutralization. Received: 22 October 1997 Accepted: 24 December 1997     

Gold–carbon three dimensional film electrode prepared from oppositely charged conductive nanoparticles by layer-by-layer approach

Gold–carbon three dimensional film electrode was prepared from oppositely charged conductive particles by layer-by-layer approach. This was done by alternative immersion of indium tin oxide plate into suspension of positive (gold) and negative (carbon) particles. The film formation is confirmed by scanning electron microscopy. The nanoparticulate film is stable and the obtained electrode exhibits catalytic current of dioxygen reduction connected with the presence of gold nanoparticles. After adsorption of bilirubin oxidase similar effect is observed with substantial decrease of overpotential – typical for bioelectrocatalytic reaction. Both catalytic currents are proportional to the number of immersion and withdrawal steps.     

Immobilization of peroxidase glycoprotein on gold electrodes modified with mixed epoxy-boronic Acid monolayers

The development of bioelectronic enzyme applications requires the immobilization of active proteins onto solid or colloidal substrates such as gold. Coverage of the gold surface with alkanethiol self-assembled monolayers (SAMs) reduces nonspecific adsorption of proteins and also allows the incorporation onto the surface of ligands with affinity for complementary binding sites on native proteins. We present in this work a strategy for the covalent immobilization of glycosylated proteins previously adsorbed through weak, reversible interactions, on tailored SAMs. Boronic acids, which form cyclic esters with saccharides, are incorporated into SAMs to weakly adsorb the glycoprotein onto the electrode surface through their carbohydrate moiety. To prevent protein release from the electrode surface, we combine the affinity motif of boronates with the reactivity of epoxy groups to covalently link the protein to heterofunctional boronate-epoxy SAMs. The principle underlying our strategy is the increased immobilization rate achieved by the weak interaction-induced proximity effect between slow reacting oxyrane groups in the SAM and nucleophilic residues from adsorbed proteins, which allows the formation of very stable covalent bonds. This approach is exemplified by the use of phenylboronates-oxyrane mixed monolayers as a reactive support and redox-enzyme horseradish peroxidase as glycoprotein for the preparation of peroxidase electrodes. Quartz crystal microbalance, atomic force microscopy, and electrochemical measurements are used to characterize these enzymatic electrodes. These epoxy-boronate functional monolayers are versatile, stable interfaces, ready to incorporate glycoproteins by incubation under mild conditions.     

Preparation and properties of silica–gold core–shell particles

Silica-metal core–shell particles, as for instance those having siliceous core and nanostructured gold shell, attracted a lot of attention because of their unique properties resulting from combination of mechanical and thermal stability of silica and magnetic, electric, optical and catalytic properties of metal nanocrystals such as gold, silver, platinum and palladium. Often, the shell of the core–shell particles consists of a large number of metal nanoparticles deposited on the surface of relatively large silica particles, which is the case considered in this work. Namely, silica particles having size of about 600 nm were subjected to surface modification with 3-aminopropyltrimethoxysilane. This modification altered the surface properties of silica particles, which was demonstrated by low pressure nitrogen adsorption at ?196 °C. Next, gold nanoparticles were deposited on the surface of aminopropyl-modified silica particles using two strategies: (i) direct deposition of gold nanoparticles having size of about 10 nm, and (ii) formation of gold nanoparticles by adsorption of tetrachloroauric acid on aminopropyl groups followed by its reduction with formaldehyde.The overall morphology of silica–gold particles and the distribution of gold nanoparticles on the surface of modified silica colloids were characterized by scanning electron microscopy. It was shown that direct deposition of colloidal gold on the surface of large silica particles gives more regular distribution of gold nanopartciles than that obtained by reduction of tetrachloroauric acid. In the latter case the gold layer consists of larger nanoparticles (size of about 50 nm) and is less regular. Note that both deposition strategies afforded silica–gold particles having siliceous cores covered with shells consisting of gold nanoparticles of tunable concentration.     

Interaction forces between colloidal particles in a solution of like-charged, adsorbing nanoparticles

We have measured the force between a weakly charged micron-sized colloidal particle and flat substrate in the presence of highly charged nanoparticles of the same sign under solution conditions such that the nanoparticles physically adsorb to the colloidal particle and substrate. The objective was to investigate the net effect on the force profile between the microparticle and flat substrate arising from both nanoparticle adsorption and nanoparticles in solution. The experiments used colloidal probe atomic force microscopy (CP-AFM) to measure the force profile between a relatively large (5 μm) colloidal probe glass particle and a planar glass substrate in aqueous solutions at varying concentrations of spherical nanoparticles. At very low nanoparticle concentrations, the primary effect was an increase in the electrostatic repulsion between the surfaces due to adsorption of the more highly charged nanoparticles. As the nanoparticle concentration is increased, a depletion attraction formed, followed by longer-range structural forces at the highest nanoparticle concentrations studied. These results suggest that, depending on their concentration, such nanoparticles can either stabilize a dispersion of weakly-charged colloidal particles or induce flocculation. This behavior is qualitatively different from that in nonadsorbing systems, where the initial effect is the development of an attractive depletion force.     

Biotin-doped porous polypyrrole films for electrically controlled nanoparticle release

A novel method for the preparation of biotin-doped porous conductive surfaces has been suggested for a variety of applications, especially for an electrically controlled release system. Well-ordered and three-dimensional porous conductive structures have been obtained by the electrochemical deposition of the aqueous biotin-pyrrole monomer mixture into particle arrays, followed by subsequent removal of the colloidal particles. Advantageously, direct incorporation of biotin molecules enhances the versatility by modifying surfaces through site-directed conjugate formation, thus facilitating further reactions. In addition, the porosity of the surfaces provides a significant impact on enhanced immobilization and efficient release of streptavidin-tagged gold nanoparticles. Biotinylated porous polypyrrole (Ppy) films were characterized by several techniques: (1) scanning electron microscopy (SEM) to evaluate surface topography, (2) X-ray photoelectron spectroscopy (XPS) to assess the potential-dependent chemical composition of the films, (3) four-point probe evaluation to measure the conductivity, cyclic voltammetry to observe surface eletroactivity, and contact angle measurement to evaluate the surface wettability, and (4) fluorescence microscopy to image and quantify the adsorption and release of gold nanoparticles. Overall, our results demonstrate that these biotinylated porous Ppy films, combined with electrical stimulation, permit a programmable release of gold nanoparticles by altering the chemical strength of the Ppy-biotin interaction.     

Gold Nanoparticle-based Layer-by-Layer Enhancement of DNA Hybridization Electrochemical Signal at Carbon Nanotube Modified Carbon Paste Electrode

Nanoporous materials have been widely applied to biosensor investigation. Recently, Guo et al. have investigated the mesoporous materials modified carbon paste electrode for rapid cTnI (cardiac troponin I) detection with enhanced sensitivity1-3. However, …     

Gold nanoparticle/alkanedithiol conductive films self-assembled onto gold electrode: Electrochemistry and electroanalytical application for voltammetric determination of trace amount of catechol

This paper describes novel electrochemical properties of gold nanoparticles/alkanedithiol conductive films and their electroanalytical applications for voltammetric determination of trace amount of one kind of environmental pollutants, catechol. The conductive films are prepared by closely packing 12-nm diameter gold nanoparticles (Au-NPs) onto Au electrodes modified with the self-assembled monolayers (SAMs) of alkanedithiols (i.e., HS(CH₂)_nSH, n = 3, 6, 9). The assembly of the Au-NPs onto the SAM-modified electrodes essentially restores the heterogeneous electron transfer between Au substrate and redox species in solution phase that is almost totally blocked by the SAMs and, as a result, the prepared Au-NP/SAM-modified electrodes possess a good electrode reactivity without a remarkable barrier toward the heterogeneous electron transfer. Moreover, the prepared Au-NP/SAM-modified electrodes are found to exhibit a largely reduced interfacial capacitance, compared with bare Au electrode. These electrochemical properties of the Au-NP/SAM-modified electrodes essentially make them very useful for electroanalytical applications, which is illustrated by voltammetric determination of trace amount detection of environmental pollutant, catechol.     

Stabilization of oil-in-water emulsions by colloidal particles modified with short amphiphiles

Emulsions stabilized through the adsorption of colloidal particles at the liquid-liquid interface have long been used and investigated in a number of different applications. The interfacial adsorption of particles can be induced by adjusting the particle wetting behavior in the liquid media. Here, we report a new approach to prepare stable oil-in-water emulsions by tailoring the wetting behavior of colloidal particles in water using short amphiphilic molecules. We illustrate the method using hydrophilic metal oxide particles initially dispersed in the aqueous phase. The wettability of such particles in water is reduced by an in situ surface hydrophobization that induces particle adsorption at oil-water interfaces. We evaluate the conditions required for particle adsorption at the liquid-liquid interface and discuss the effect of the emulsion initial composition on the final microstructure of oil-water mixtures containing high concentrations of alumina particles modified with short carboxylic acids. This new approach for emulsion preparation can be easily applied to a variety of other metal oxide particles.     

Generation and detection of single metal nanoparticles using scanning electrochemical microscopy techniques

Different pathways towards the generation and detection of a single metal nanoparticle (MNP) on a conductive carbon support for testing as an electrocatalyst are described. Various approaches were investigated including interparticle distance enhancement, electrochemical and mechanical tip-substrate MNP transfer onto macroscopic surfaces, scanning electrochemical microscopy (SECM)-controlled electrodeposition, and the use of selective binding monolayers on carbon fiber electrodes (CFEs) for solution-phase-selective adsorption. A novel SECM technique for electrodepositing MNPs on CFE tips immersed 100-200 nm below the electrolyte level was developed and used to generate single Pt and Ni nanoparticles. Following their generation, we demonstrate electrocatalytic detection of Fe3+ on individual Pt particles with the CFE in a Fe3+/H2SO4 solution. We also describe an approach of attaching MNPs to CFEs by controlling the composition of monolayers bonded to the CFE. By employing a monolayer with a low ratio of binding (e.g., 4-aminopyridine) to nonbinding molecules (e.g., aniline) and controlling the position of the CFE in a colloidal Pt solution with a SECM, we attached a single 15 nm radius Pt nanoparticle to the CFE. Such chemisorbed Pt particles exhibited a stronger adhesion on surface-modified CFEs and better mechanical stability during proton reduction than MNPs electrodeposited directly on the CFE.