Surface charge influence on the surface plasmon absorbance of electroactive thiol-protected gold nanoparticles

Gold nanoparticles (3.1-5.0 nm in size) surface-derivatized with both electroactive and nonelectroactive self-assembled monolayers were synthesized. The surface-derivatized electroactive particles can be easily oxidized/reduced at an electrode surface based on the diffusion-controlled current-voltage curve observed in cyclic voltammetry measurements. Spectroelectrochemical investigation demonstrated that the maximum absorbance of the nanoparticles in their oxidized state red-shifted compared with their reduced state to a different extent according to their size distribution. In the case of the particles surface-derivatized with nonelectroactive monolayers, much less shift was observed. This study showed that surface plasmon absorbance of gold nanoparticles was not only related to core charge states but was also influenced by surface charge states as well.

Co-assembly of ferrocene-terminated and alkylthiophene thiols on gold and its redox chemistry modulated by surfactant adsorption

Coadsorption of ferrocene-terminated alkanethiols (FcCO(2)(CH2)(8)SH, Fc=(mu(5)-C5H5)Fe(mu(5)-C5H4)) with alkylthiophene thiols (2-mercapto-3-n-octylthiophene) yields stable, electroactive self-assembled monolayers on gold. The resulting mixed monolayer provides an energetically favorable hydrophobic surface for the adsorption of the surfactant aggregates in aqueous solution. The adsorptions have been characterized via their effect on the redox properties of ferrocenyl alkanethiols immobilized as minority components in the monolayers and on the interfacial capacitance of the electrode. Surfactant adsorption causes a decrease in the overall capacitance at the electrode and dramatically shifts the redox potential for ferrocene oxidation in a positive or negative direction depending on the identity of the surfactant employed. A structural model is proposed in which the alkane chains of the adsorbed surfactants interdigitate with those of the underlying self-assembled monolayer, leading to the formation of a hybrid bilayer membrane.

pH-dependent conformational changes of ferricytochrome c induced by electrode surface microstructure

pH-dependent processes of bovine heart ferricytochrome c have been investigated by electronic absorption and circular dichroism (CD) spectra at functionalized single-wall carbon 'nanotubes (SWNTs) modified glass carbon electrode (SWNTs/ GCE) using a long optical path thin layer cell. These methods enabled the pH-dependent conformational changes arising from the heme structure change to be monitored. The spectra obtained at functionalized SWNTs/GCE reflect electrode surface microstructure-dependent changes for pH-induced protein conformation, pK(a) of alkaline transition and structural microenvironment of the ferricytochrome c heme. pH-dependent conformational distribution curves of ferricytochrome c obtained by analysis of in situ CD spectra using singular value decomposition least square (SVDLS) method show that the functionalized SWNTs can retain native conformational stability of ferricytochrome c during alkaline transition.

Polyamidoamine dendrimers-assisted electrodeposition of gold-platinum bimetallic nanoflowers

Novel Au-Pt bimetallic flower nanostructures fabricated on a polyamidoamine dendrimers-modified surface by electrodeposition are reported. These polyamidoamine dendrimers were stable, and they assisted the formation of Au-Pt bimetallic nanoflowers during the electrodeposition process. These nanoflowers were characterized by field-emitted scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and electrochemical methods. FE-SEM images showed that the bimetallic nanoflower included two parts: the "light" and the "pale" part. The two parts consisted of many small bimetallic nanoparticles, which was attributed to the progressive nucleation process. Moreover, the "light" part contained more bimetallic nanoparticles. The morphologies of bimetallic nanoflowers depended on the electrodeposition time and potential and the layer number of assembled dendrimers. The average size of nanoflowers increased with the increase in electrodeposition time. The layer number of assembled dendrimers obviously affected the size and morphologies of the "pale" parts of deposited nanoflowers.

Modification of electrode surface through electrospinning followed by self-assembly multilayer film of polyoxometalate and its photochromic

Electrospun poly (vinyl alcohol) (PVA) nanofibers mat was collected on indium tin oxide (ITO) substrate. Heat crosslinked nanofibers mat became water-insoluble and firmly fixed on ITO substrate even in water. Oppositely charged poly (allylamine hydrochloride) (PAH) and Dawson-type polyoxometalate (POM), Na6P2Mo18O62 (P2Mo18), were alternately assembled on PVA nanofibers-coated ITO substrate to construct multilayer film through an electrostatic layer-by-layer (LBL) technique. The scanning electron microscope (SEM) images showed that P2Mo18 multilayer film was selectively deposited on PVA nanofibers while the unoccupied space by nanofibers on bare ITO was acted as substrate at the same time because the electrospun nanofibers have larger surface area and surface energy than the flat substrate. The cyclic voltammograms current responses of the P2Mo18 multilayer film on PVA/ITO electrode showed three well-defined redox couples of P2Mo18, but very small because P2Mo18 multilayer film was selectively deposited on PVA nanofibers with poor conductivity. In addition, the photochromic behavior of P2Mo18 multilayer film on PVA/ITO was investigated through UV-vis spectra and electron spin resonance (ESR). Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) proved that the charge-transfer complex was formed between PAH and P2Mo18 after UV irradiation.

Electropolyrnerization of polypyrrole on PFIL-PSS-modified electrodes without added support electrolytes

A novel polyelectrolyte-functionalized ionic liquid (PFIL)/poly(4-styrene sulfonate sodium) (PSS) modified electrode composed of the coaxial and coplanar working, reference and counter electrodes, was used to electropolymerize the polypyrrole. The PFIL/PSS was modified on the integrated electrode (IE) and connected by the working, reference and counter electrodes, resulting in an available charge transfer and lower Ohmic potential drop between the working and counter electrodes. Then polypyrrole (PPy) film was successfully prepared electrochemically without any participation of supporting electrolytes, only in a pyrrole monomer solution. The resulting PPy film in PFIL/PSS matrix exhibited a preferable electroactivity. Subsequently, influence of the modifications on the formation of PPy was further discussed. The results indicated that the synergetic cooperation of PFIL and PSS components accomplished such a successful electropolymerization of PPy.

Assembly of multilayer films containing iron(III)- substituted Dawson-type heteropolyanions and its electrocatalytic properties: cyclic voltammetry, electrochemical impedance spectroscopy and UV-Vis spectrometry

Ultrathin multilayer films have been prepared by means of alternate adsorption of iron(Ill)-substituted heteropolytungstate anions and a cationic redox polymer on the 4-aminobenzoic acid modified glassy carbon electrode surface based on electrostatic layer-by-layer assembly. Cyclic voltammetry, electrochemical impedance spectroscopy and UV-Vis absorption spectrometry have been used to easily monitor the uniformity of thus-formed multilayer films. Especially, the electrochemical impedance spectroscopy is successfully used to monitor the multilayer deposition processes and is a very useful technique in the characterization of multilayer films because it provides valuable information about the interfacial impedance features. All these results reveal regular film growth with each layer adsorption. The resulting multilayer films can effectively catalyze the reduction of H2O2,NO2- and BrO3-.

Electrochemical study of 4-ferrocene thiophenol monolayers assembled on gold nanoparticles

In this paper, 4-ferrocene thiophenol was employed as a novel capping agent to synthesize electroactive gold nanoparticles. Transmission electron microscopy showed an average core diameter of 2.5 nm. The optical and electrochemical properties of the 4-ferrocene thiophenol capped gold nanoparticles were characterized by UV-Vis spectroscopy and cyclic voltammograms. Surface plasmon absorbance was detected at 522 nm. Cyclic voltammograms revealed the adsorbed layer reaction controlled electrode process, and the formal potential of electroactive ferrocene centers shifted anodically compared with ferrocene in solution, which could be attributed to the electron-withdrawing phenyl moiety linked to ferrocene.

Electroactive gold nanoparticles protected by 4-ferrocene thiophenol monolayer

Numerous reports have focused on ferrocene-terminated electroactive self-assembled monolayers (SAMs) on a flat An surface but only a few on ferrocene SAMs on An colloid. In this paper, we employ 4-ferrocene thiophenol as a novel capping agent to produce electroactive gold nanoparticles in consideration of the peculiar pi-conjugated structure. Transmission electron microscopy shows the narrow-dispersed gold core with an average core diameter of ca. 2.5 nm. UV/vis spectra examine the pi-conjugated structure of 4-ferrocene thiophenol and surface plasmon absorbance of the indicated gold nanoparticles. X-ray photoelectron spectroscopy reveals electronic properties of the An core and thiol ligands. Electrochemical measurement shows that the oxidation peak current is proportional to the scan rate, indicating the electrode process is controlled by adsorbed layer reaction. The formal potential of the Fc-MPCs is compared with that of free ferrocene in MeCN solution and the Fc-SAMs. The shifts are attributed to the phenyl moiety in the 4-ferrocene thiophenol and dielectric constant of the solvation environment.

Probing UPD-induced surface atomic rearrangement of polycrystalline gold nanofilms with surface plasmon resonance spectroscopy and cyclic voltammetry

Silver underpotential deposition (UPD)-induced surface atomic rearrangement of polycrystalline gold nanofilms was probed with use of surface plasmon resonance spectroscopy (SPRs) as a novel probe tool in combination with cyclic voltammetry. Interestingly, upon repetitive electrochemical UPD and stripping of Ag, the surface structure of the resulting bare Au film is rearranged due to strong adatom-substrate interactions, which causes a large angle shift of SPR R-theta curves, in a good linear relationship with the number of UPDs, to a lower SPR angle. The n, K values of the surfacial Au monolayers before and after the repetitive Ag UPD and stripping for 27 times are found to be 0.133, 3.60 and 0.565, 9.39, respectively, corresponding to the huge shift of 1.61degrees to the left of the SPR minima. Cyclic voltammetry experiments in 0.10 M H2SO4 are carried out before and after the UPD treatment to examine the quality of the whole electrode surface and confirmed this change. To correlate the angle change in SPRs with the profile change in the cyclic voltammogram, the UPD treatment was also performed on a Au(111) textured thin film. It was therefore confirmed that the resonance position of the SPR spectrum is very sensitive to the surface crystallographic orientation of the bare Au substrates. Some surface atomic rearrangement can cause a pronounced SPR angle shift.

Preparation of multilayer films containing pt nanoparticles on a glassy carbon electrode and application as an electrocatalyst for dioxygen reduction

In this paper, a simple route for the preparation of Pt nanoparticles is described. PtCl62- and [tetrakis-(N-methylpyridyl)porphyrinato] cobalt (CoTMPyP) were assembled on a 4-aminobenzoic acid modified glassy carbon electrode through the layer-by-layer method. The three-dimensional Pt nanoparticle films are directly formed on an electrode surface by electrochemical reduction of PtCl62- sandwiched between CoTMPyP layers. Regular growth of the multilayer films is monitored by UV-vis spectroscopy. X-ray photoelectron spectroscopy verifies the constant composition of the multilayer films containing Pt nanoparticles. Atomic force microscopy proves that the as-prepared Pt nanoparticles are uniformily distributed with average particle diameters of 6-10 nm. The resulting multilayer films containing Pt nanoparticles on the modified electrode possess catalytic activity for the reduction of dissolved oxygen. Rotating disk electrode voltammetry and rotating ring-disk electrode voltammetry confirm that Pt nanoparticle containing films can catalyze an almost four-electron reduction of O-2 to water in 0.5 M H2SO4 solution.

Synthesis, characterization and fabrication on a glassy carbon electrode of a tetra-iron substituted sandwich-type pentadecatungstodiarsenate heteropolyanion

A novel sandwich-type compound, Na-12[Fe-4(H2O)(2)(As2W15O56)2].41H(2)O, has been synthesized. The compound was well-characterized by means of IR, UV-vis, W-183 NMR and elemental analyses. The compound crystallizes in the triclinic, P (1) over bar symmetry group. The structure of the compound is similar to that of Na-16[M-4(H2O)(2)(As2W15O56)(2)].nH(2)O (M = Cu, Zn, Co, Ni, Mn, Cd), and consists of an oxo-aqua tetranuclear iron core, [(Fe4O14)-O-III(H2O)(2)], sandwiched by two trivacant alpha-Wells-Dawson structural moieties, alpha-[As2W15O56]. Redoxelectrochemistry of the compound has been studied in buffer solutions at pH = 4.7 using polarography and cyclic voltammetry ( CV). The compound exhibited four one-electron couples associated with the Fe(III) center followed by three four-electron redox processes attributed to the tungsten-oxo framework. The compound-containing monolayer and multilayer films have been fabricated on a 4-aminobenzoic acid modified glassy carbon electrode surface by alternating deposition with a quaternized poly(4-vinylpyridine) partially complexed with [Os(bpy)(2)Cl](2+/-). CV, X-ray photoelectron spectroscopy (XPS), UV-vis spectroscopy and atomic force microscopy (AFM) have been used to characterize the multilayer films.

Organically modified sol-gel/chitosan composite based glucose biosensor

A new type of organically modified sol-gel/chitosan composite material was developed and used for the construction of glucose biosensor. This material provided good biocompatibility and the stabilizing microenvironment around the enzyme. Ferrocene was immobilized on the surface of glassy carbon electrode as a mediator. The characteristics of the biosensor were studied by cyclic voltammetry and chronoamperometry. The effects of enzyme-loading, buffer pH, applied potential and several interferences on the response of the enzyme electrode were investigated. The simple and low-cost glucose biosensor exhibited high sensitivity and good stability.

Site-selective self-assembly of MPA-bridged CuHCF multilayers on APTMS-supported gold colloid electrodes

(3-Aminopropyl)trimethoxysilane (APTMS)-supported gold colloid electrode was constructed by virtue of a recently developed solution-based self-assembly strategy. The preparing procedure of 3-mercaptopropionic acid (MPA)-bridged copper hexacyanoferrate (CuHCF) multilayers on a planar macroelectrode (Bharathi et al. Langmuir 2001, 17, 7468) was copied to the as-prepared colloid electrode. The optical spectra, atomic force microscopy, and electrochemistry demonstrate successful copy of the multilayer system on a macroelectrode to the as-prepared colloid electrode. Remarkably, it was found that multilayer growth is highly selective to the nanoscale sites where gold nanoparticles are immobilized, and multilayer growth does not take place on the sites without nanoparticles. Interestingly, a preliminary electrochemical investigation indicates that electrochemical properties of multilayers systems on the colloid electrode are different from their counterparts on a planar macroelectrode, which might be due to high curvature effects of the gold nanoparticles. This indicates a different motif of multilayers on the colloid electrode from that on a planar macroelectrode.

Chitosan mediated assembly of gold nanoparticles multilayer

We initiate a systematic exploration of a natural polymer, chitosan, as a structural material for designing functional layers on electrode surfaces in this work. Au colloid films are organized on chitosan layer by adsorption. We have successfully constructed a multilayer An nanoparticle assembly through electrostatic interactions on chitosan functionalized quartz substrates by the alternate treatment of the substrate with solution of citrate-stabilized gold nanoparticles (negatively charged) and chitosan solution (positively charged). The resulting substrates were characterized by UV-Vis spectrometry, atomic force microscopy (AFM), and electrochemical impedance spectroscopy (EIS) measurements. These assemblies of colloid An multilayer are highly stable, and can be kept for a long time in distilled water, only being removed by scratching or extreme electrochemical conditions.