Gold Surface Area Research Articles

Label-free electrochemical detection of glucose and glycated hemoglobin (HbA1c)

Measuring glucose together with glycated hemoglobin (HbA1c) could provide short-term and long-term information of blood glycemic condition after a single treatment. However, it has been a challenge to quantify glucose and HbA1c from single sample drop and strip of electrodes with suitable sensitivity, selectivity, and efficiency. In this paper, we demonstrated a label free & single sample drop electrochemical detection method of glucose and HbA1c by modifying carbon electrodes. Glucose oxidase and capture antibodies (C-Ab) against HbA1c were immobilized onto the first working electrode (WE1) and the second working electrode (WE2) of dual working screen-printed carbon electrodes (DWSPCEs), respectively. WE2 was modified with Gold Nano Flower. After that, 3-mercaptopropionic acid was coated as a linker. Finally, C-Ab was bonded by the linker. The relationship between gold surface area and concentration of HgAuCl4 was evaluated to optimize HbA1c incubation time. Linear calibration curves for glucose concentration (0.02–35 mM), HbA1c concentration (0.01 to 1mgml−1), and HbA1c percentage solution (0–14%) were obtained, with correlation coefficients of 0.99. Sensitivities of the biosensor for glucose and HbA1c were 0.5 μAmm−2mM−1 and 0.09 μAmm−2μg−1ml, respectively. The biosensor also showed proper stability (>93%, 45days) and selectivity (>92%). The efficiency of the proposed biosensor was also compared with those of commercial kits using whole blood samples of diabetic and normal cases. Results of this study demonstrate that this biosensor can measure both glucose and HbA1c electrochemically using label-free methods to overview glycemic conditions of blood samples using cheap and commercially viable dual carbon electrode-based biosensors.

Shape-Dependent Catalytic Activity of Gold and Bimetallic Nanoparticles in the Reduction of Methylene Blue by Sodium Borohydride

In this study the catalytic activity of different gold and bimetallic nanoparticle solutions towards the reduction of methylene blue by sodium borohydride as a model reaction is investigated. By utilizing differently shaped gold nanoparticles, i.e., spheres, cubes, prisms and rods as well as bimetallic gold–palladium and gold–platinum core-shell nanorods, we evaluate the effect of the catalyst surface area as available gold surface area, the shape of the nanoparticles and the impact of added secondary metals in case of bimetallic nanorods. We track the reaction by UV/Vis measurements in the range of 190–850 nm every 60 s. It is assumed that the gold nanoparticles do not only act as a unit transferring electrons from sodium borohydride towards methylene blue but can promote the electron transfer upon plasmonic excitation. By testing different particle shapes, we could indeed demonstrate an effect of the particle shape by excluding the impact of surface area and/or surface ligands. All nanoparticle solutions showed a higher methylene blue turnover than their reference, whereby gold nanoprisms exhibited 100% turnover as no further methylene blue absorption peak was detected. The reaction rate constant k was also determined and revealed overall quicker reactions when gold or bimetallic nanoparticles were added as a catalyst, and again these were highest for nanoprisms. Furthermore, when comparing gold and bimetallic nanorods, it could be shown that through the addition of the catalytically active second metal platinum or palladium, the dye turnover was accelerated and degradation rate constants were higher compared to those of pure gold nanorods. The results explore the catalytic activity of nanoparticles, and assist in exploring further catalytic applications.

Probing the Surface of Noble Metals Electrochemically by Underpotential Deposition of Transition Metals

The advances in material science have led to the development of novel and various materials as nanoparticles or thin films. Underpotential deposition (upd) of transition metals appears to be a very sensitive method for probing the surfaces of noble metals, which is a parameter that has an important effect on the activity in heterogeneous catalysis. Underpotential deposition as a surface characterization tool permits researchers to precisely determine the crystallographic orientations of nanoparticles or the real surface area of various surfaces. Among all the work dealing with upd, this review focuses specifically on the main upd systems used to probe surfaces of noble metals in electrocatalysis, from poly‒ and single-crystalline surfaces to nanoparticles. Cuupd is reported as a tool to determine the active surface area of gold‒ and platinum‒based bimetallic electrode materials. Pbupd is the most used system to assess the crystallographic orientations on nanoparticles’ surface. In the case of platinum, Bi and Ge adsorptions are singled out for probing (1 1 1) and (1 0 0) facets, respectively.

Versatile Micropatterns of N-Heterocyclic Carbenes on Gold Surfaces: Increased Thermal and Pattern Stability with Enhanced Conductivity.

Patterned monolayers of N-heterocyclic carbenes (NHCs) on gold surfaces were obtained by microcontact printing of NHC-CO2 adducts and NHC(H)[HCO3 ] salts. The NHC-modified areas showed an increased conductivity compared to unmodified gold surface areas. Furthermore, the remaining surface areas could be modified with a second, azide-functionalized carbene, facilitating further applications and post-printing modifications. Thorough elucidation by a variety of analytical methods offers comprehensive evidence for the viability of the methodology reported here. The protocol enables facile access to versatile, microstructured NHC-modified gold surfaces with highly stable patterns, enhanced conductivity, and the option for further modification.

Rapid colorimetric sensing platform for the detection of Listeria monocytogenes foodborne pathogen

Listeria monocytogenes is a serious cause of human foodborne infections worldwide, which needs spending billions of dollars for inspection of bacterial contamination in food every year. Therefore, there is an urgent need for rapid, in-field and cost effective detection techniques. In this study, rapid, low-cost and simple colorimetric assay was developed using magnetic nanoparticles for the detection of listeria bacteria. The protease from the listeria bacteria was detected using D-amino acid substrate. D-amino acid substrate was linked to the carboxylic acid on the magnetic nanoparticles using EDC/NHS chemistry. The cysteine residue at the C-terminal of the substrate was used for the self-assembled monolayer formation on the gold sensor surface, which in turn the black magnetic nanobeads will mask the golden color. The color will change from black to golden color upon the cleavage of the specific peptide sequence by the Listeria protease. The sensor was tested with serial dilutions of Listeria bacteria. It was found that the appearance of the gold surface area is proportional to the bacterial concentrations in CFU/ml. The lowest detection limit of the developed sensor for Listeria was found to be 2.17×102 colony forming unit/ml (CFU/ml). The specificity of the biosensor was tested against four different foodborne associated bacteria (Escherichia coli, Salmonella, Shigella flexnerii and Staphylococcus aureus). Finally, the sensor was tested with artificially spiked whole milk and ground meat spiked with listeria.

Ensembles-of Gold Nanowires for the Anodic Stripping Voltammetric Determination of Inorganic Arsenic.

3D-ensembles of gold nanowires electrodes (3D-NEEs) are produced by electroless gold deposition in track-etched polycarbonate (PC) membranes, followed by partial etching (plama or chemical) of the polymeric membrane. These electrodes are applied to the anodic stripping voltammetric determination of inorganic As. The controlled etching of the PC template increased the gold surface area, widening the linear range of the analytical response with respect to ensembles of gold nanodisk electrodes (2D-NEEs). 3D-NEEs prepared using a chemical etching time of 10 s allows the anodic stripping determination of As(III) with a detection limit of 0.08 μg/L and a linear range extended up to 20 μg/L. The speciation of inorganic As (As(III) and (As(V)) in river water is possible by difference between As(III) and total inorganic As, determined after reduction of As(V) with cysteine. The proposed method is successfully validated by comparison with lCP-MS determination.

Influence of gold nanoparticle loading in Au/C on the activity towards electrocatalytic glycerol oxidation

The influence of gold loading on the electrocatalytic oxidation of glycerol over carbon supported gold nanoparticles has been investigated. Over the range of 1–20wt% Au/C, the gold particle size distributions were similar with an overall average particle diameter of 4.6nm and an average specific gold surface area of 43±4m2g−1. As expected, cyclic voltammetry measurements in 1M KOH revealed that the number of electrochemically active gold sites were directly proportional to the gold loading. This observation, together with the particle size analysis, strongly suggests that the inherent electrocatalytic behaviour of the gold nanoparticles should be independent of gold loading. Despite this, very clear evidence was found to suggest that the electrocatalytic glycerol oxidation behaviour is influenced by gold loading: both the drop-off and onset potential are functions of gold loading. These findings are explained by the differences in mass transfer and subsequent accumulation of oxidation products within the electrocatalytic layers, with layers containing higher gold nanoparticle loadings having higher levels of accumulation. This increased accumulation is speculated to inhibit gold oxide formation thus shifting the characteristic drop-off in anodic current during the forward sweep of the voltammograms to higher potentials. While this accumulation allows glycerol oxidation to occur over a wider potential range, the specific activity for glycerol oxidation is reduced, presumably due to partial blocking the glycerol oxidation reaction by the adsorbed oxidation products. These findings have important implications not only for evaluating the specific activity electrocatalytic materials but also understanding the electrocatalytic oxidation of glycerol over gold nanoparticles.

The effect of MnO2 loading on the glycerol electrooxidation activity of Au/MnO2/C catalysts

Manganese oxides have shown promise as co-catalysts for platinum- and palladium-based alcohol electrooxidation catalysts. In this investigation, the effect of adding MnO2 to a gold-based catalyst system was explored for the glycerol electrooxidation reaction. It was found that modification of the carbon support with MnO2 had the effect of stabilising gold nanoparticles, giving rise to smaller gold particles with a more uniform particle size distribution than on the carbon alone. All gold-based catalysts gave very high currents for glycerol oxidation (1–5Amg−1cat) and catalysts with MnO2 contents of 5 and 9wt% yielded the highest mass activities (35–37Amg−1Au). This is believed to be partly due to MnO2 preventing gold agglomeration, creating a larger gold surface area for reaction. In addition, it is believed that MnO2 may act as an active co-catalyst, especially at higher potentials, by means of oxygen spillover onto gold – leading to higher activities and lower peak potentials. MnO2 was also found to activate the glycerol oxidation reaction earlier, giving lower onset potentials. Stability and impedance studies showed that MnO2-modified catalysts were less stable at a low potential (0V) than Au/C. This could be due to blocking of mixed-valence manganese oxide sites or poisoning of the smaller gold particles on these catalysts. However, at a higher potential of 0.2V, Au/5% MnO2/C demonstrated the best activity and stability overall. All our catalysts are more active than the palladium- and platinum-based catalysts reported in literature, with our best-performing catalyst on a mass activity basis (Au/9% MnO2/C) being at least 12 times more active. This effectively demonstrates the advantage of using a gold-based catalyst for glycerol oxidation, especially when supported by MnO2 which aids in lowering the overpotential for glycerol oxidation over gold.

Controlled adsorption of cytochrome c to nanostructured gold surfaces

Controlled electrostatic physisorption of horse heart cytochrome c (Cyt c) onto nanostructured gold surfaces was investigated using Quartz-Crystal Microbalance measurements in planar gold surfaces with or without functionalization using a self-assembled monolayer (SAM) of the alkanethiol mercaptoundecanoic acid (MUA). MUA is a useful functionalization ligand for gold surfaces, shedding adsorbed biomolecules from the excessive electron density of the metal. A parallel analysis was conducted in the corresponding curved surfaces of 15 nm gold nanoparticles (AuNPs), using zeta-potential and UV– visible spectroscopy. Atomic Force Microscopy of both types of functionalized gold surfaces with a MUA SAM, allowed for visualization of Cyt c deposits on the nanostructured gold surface. The amount of Cyt c adsorbed onto the gold surface could be controlled by the solution pH. For the assays conducted at pH 4.5, when MUA SAM- functionalized planar gold surfaces are positive or neutral, and Cyt c has a positive net charge, only 13 % of the planar gold surface area was coated with protein. In contrast, at pH 7.4, when MUA SAM-functionalized planar gold surfaces and Cyt c have opposite charges, a protein coverage of 28 % could be observed implying an adsorption process strongly governed by electrostatic forces. Cyt c adsorption on planar and curved gold surfaces are found to be greatly favored by the presence of a MUA-capping layer. In particular, on the AuNPs, the binding constant is three times larger than the binding constant obtained for the original citrate-capped AuNPs.

Functional Binary Micropattern of Hyperbranched Poly(ether amine) (hPEA-AN) Network and Poly(ether amine) (PEA) Brush for Recognition of Guest Molecules

A binary micropattern of anthracene-contained hyperbranched poly(ether amine) (hPEA-AN) network and poly(ether amine) (PEA) brush on gold surface was developed and explored. First, a micropatterned hPEA-AN network array on gold surface was fabricated by photolithography via photodimerization of anthracene moieties, and a PEA brush was subsequently immobilized on the remaining free gold surface areas by chemical adsorption of thiol groups. The patterned hPEA-AN network exhibits selectivity with respect to the adsorption of hydrophilic dyes: Methyl orange is strongly adsorbed, but rhodamine 6G is not, as indicated by the fluorescence response. The PEA brush domain exhibits excellent protein adsorption repellency, whereas the hPEA-AN network layer readily adsorbs protein. These characteristics make the binary hPEA-AN network and PEA brush array sensitive to different kinds of dyes and proteins, which open up pathways to potential applications as microsensors, biochips, and bioassays.

Facile approaches to control catalytic activity of viral-templated palladium nanocatalysts for dichromate reduction

We present two facile procedures to control catalytic activity of viral-templated palladium (Pd) nanocatalysts. Specifically, tunable and selective surface assembly of genetically modified Tobacco Mosaic Virus (TMV1cys) templates is exploited to control the loading of Pd nanocatalysts on solid substrates via preferential electroless deposition. In the first method, the surface density of Pd–TMV complexes is controlled by varying the concentration of TMV solution for surface assembly. Apparent surface density of Pd–TMV complexes from AFM images correlated well with the Pd loading observed via XPS and the catalytic activity in dichromate reduction, an important environmental remediation reaction for detoxification of abundant inorganic wastes. In the second method, the Pd catalyst loading is controlled by varying the gold surface area on patterned silicon chips. The catalytic activity of these chips correlated well with the surface area, further indicating control over the location of active Pd catalysts via selective surface assembly of viral templates. We envision that our approach to control the TMV surface density and specific location of catalysts by exploiting biologically derived template materials can be extended to other biotemplates and/or reaction systems via a wide range of tunable surface patterning/assembly techniques.

Feasibility of Methyl Mercaptane as Probe Molecule for Supported Gold Nanoparticle Surface Area Determination

Abstract: Gold nanoparticles supported on TiO2 were probed by adsorption of methyl mercaptane (MM), and the process was quantified gravimetrically. This method allowed discrimination between weakly adsorbed (physisorbed) and strongly bound (chemisorbed) methyl mercaptane. Strong adsorption of MM occured on exposed Au faces, while low-temperature pre-treatment (30 degrees C) completely suppressed adsorption of MM on the TiO2 support. The thus obtained high selectivity of MM adsorption on Au enabled characterization of the gold surface area and the resulting values are comparable with other noble metal systems of similar average particle size. The estimated adsorption stoichiometry indicates that the entire Au surface is probed.

Fundamental Calculations on the Surface Area Determination of Supported Gold Nanoparticles by Alkanethiol Adsorption

Surface area determination is a crucial step for the characterization of the activity of noble metal catalysts. Not only the development of useful determination methods but foremost the understanding of surface properties and their conversion into mathematical expressions are essential to obtain reliable results. A selective method to gain access to the specific surface area of gold on oxidic supports is the chemisorption of alkanethiol from suspensions. Therefore, the concentration of a 1-dodecanthiol solution before and after immersion of supported gold catalysts was determined by gas chromatography. To convert the concentration information into a specific surface area, the surface coverage, the surface atom concentration, the interatomic Au-Au distance, and the particle morphology were considered. Further calculations afforded the determination of a mean particle diameter. A good agreement was found between gold particle sizes obtained from transmission electron microscopy and thiol adsorption. The given mathematical expressions are highly valuable for a broad range of chemisorption methods and noble metal catalysts.

Gold line array electrodes increase substrate affinity and current density of electricity-producing G. sulfurreducens biofilms

During growth of Geobacter species able to transfer electrons to electrodes, biofilms consisting of multiple cell layers accumulate on surfaces. These biofilms require pathways for efficient electron relay towards the electrode, and diffusion of protons and end products away from the electrode. We hypothesized that altering the geometry of current-collecting electrodes would improve diffusion of substrates into electricity-producing biofilms, and allow testing of hypotheses related to the limits of long-range electron transfer. Two designs exposing equal gold surface areas to cultures of Geobacter sulfurreducens were compared: one consisting of a rectangular gold electrode and the other an array of 10 µm wide lines separated by 100 µm of non-conductive material. In all experiments, the line array electrode stabilized at a current density 4-fold higher (per unit electrode surface area) after 140 h of growth (1600 µA cm−2vs. 400 µA cm−2). Confocal imaging and cyclic voltammetry analysis demonstrated that because cells could grow at least 15 µm outward in a semicylinder from the gold lines, 4-fold more biomass could share each line electrode, compared to the rectangular geometry. The semicylinder-shaped biofilms did not fill in gaps between the electrodes after 300 h of growth, suggesting a limitation to the distance of useful between-cell electron transfer. The wider spacing of biofilms also improved the affinity of cells for acetate, especially under quiescent conditions.

Branched nanotrees with immobilized acetylcholine esterase for nanobiosensorapplications

A novel lab-on-a-chip nanotree enzyme reactor is demonstrated for the detection of acetylcholine.The reactors are intended for use in the RISFET (regional ion sensitive field effect transistor)nanosensor, and are constructed from gold-tipped branched nanorod structures grown onSiNx-covered wafers. Two different reactors are shown: one with simple, one-dimensionalnanorods and one with branched nanorod structures (nanotrees). Significantly higherenzymatic activity is found for the nanotree reactors than for the nanorod reactors, mostlikely due to the increased gold surface area and thereby higher enzyme bindingcapacity. A theoretical calculation is included to show how the enzyme kinetics andhence the sensitivity can be influenced and increased by the control of electricalfields in relation to the active sites of enzymes in an electronic biosensor. Thepossible effects of electrical fields employed in the RISFET on the function ofacetylcholine esterase is investigated using quantum chemical methods, which showthat the small electric field strengths used are unlikely to affect enzyme kinetics.Acetylcholine esterase activity is determined using choline oxidase and peroxidase bymeasuring the amount of choline formed using the chemiluminescent luminol reaction.

Synthesis and Characterization of Accessible Metal Surfaces in Calixarene-Bound Gold Nanoparticles

Use of organic ligands to partially passivate nanoparticles against sintering yet retain a degree of small molecule accessibility to the metal surface has been a lofty goal in functional materials synthesis, which in principle also enables the design of preferred electronic and steric environments on a nanoparticle surface. Catalysis using gold in particular requires donor ligands that facilitate an electron-rich metal surface and generalizable strategies for dealing with deactivation due to sintering. Here, synthesis and characterization of gold nanoparticles postsynthetically modified with the chelating ligand cone-5,11,17,23,29,35-hexa(tert-butyl)-37,39,41-tris(diphenylphosphinomethoxy)-38,40,42-trimethoxycalix[6]arene (1) is reported. In solution as well as when supported on the surface of TiO2, nanoparticles modified with tripodal calix[6]arene phosphine ligand 1 demonstrate enhanced protection against sintering relative to unmodified, tetraoctylammonium bromide-surfactant-stabilized gold nanoparticles. In between adsorbed calixarene ligands, there is accessible gold surface area in these nanoparticles, and this is measured quantitatively for the first time for a calixarene-modified nanoparticle, using a newly developed fluorescence methodology involving 2-naphthalenethiol as a relevant chemisorption probe molecule. Ligand steric bulk critically influences amount of accessible surface on the metal nanoparticle since the use of a smaller calix[4]arene ligand (MBC) results in a 7-fold lower accessible surface area relative to using 1 under otherwise similar conditions. In addition, surface coverage of 1 controls accessible surface area in an unintuitive fashion: a 4-fold increase in accessible metal surface area is observed upon increasing the surface coverage of 1 to be 1.5-fold higher than the minimum required for surface saturation. This is presumably the result of a more open ligand packing of 1 at higher surface coverages, which allows greater accessibility to 2-napthalenethiol.

Electrochemical Fabrication of Nanostructured Surfaces for Enhanced Response

The objective of this work is to explore approaches to enhance electrochemical signals through sequential deposition and capping of gold particles. Gold nanoparticles are electrodeposited from KAuCl(4) solution under potentiostatic conditions on glassy carbon substrates. The number density of the nanoparticles is increased by multiple deposition steps. To prevent secondary nucleation processes, the nanoparticles are isolated after each potentiostatic deposition step by self-assembled monolayers (SAMs) of decanethiol or mercaptoethanol. The increasing number of particles during five deposition/protection rounds is monitored by assembling electroactive SAMs using a ferrocene-labeled alkanethiol. A precise estimation of the surface area of the gold nanoparticles by formation of an oxide layer on gold is difficult due to oxidation of the glassy carbon surface. As an alternative approach, the charge flow of the electroactive SAM is used for surface measurement of the gold surface area. A sixfold increase in the redox signal in comparison to a bulk gold surface is observed, and this increase in redox signal is particularly notable given that the surface area of the deposited nanoparticles is only a fraction of the bulk gold surface. After five rounds of deposition there is a gold loading of 1.94 mug cm(-2) of the deposited nanoparticles as compared to 23.68 mug cm(-2) for the bulk gold surface. Remarkably, however, the surface coverage of the ferrocene alkanethiol on the bulk material is only 10 % of that achieved on the deposited nanoparticles. This enhancement in signal of the nanoparticle-modified surface in comparison to bulk gold is thus demonstrated not to be attributable to an increase in surface area, but rather to the inherent properties of the surface atoms of the nanoparticles, which are more reactive than the surface atoms of the bulk material.

A review of effects of silver, lead, sulfide and carbonaceous matter on gold cyanidation and mechanistic interpretation

Despite the wealth of published data on the beneficial or detrimental effects of silver, lead, sulfide, and carbonaceous matter on the rate of gold cyanidation at an anode or by dissolved oxygen, the lack of comparative studies on relative effects has hampered rationalisation of the role of these activators or passivators of gold. In the present study, the published rate data per unit surface area of gold, silver, and gold–silver alloys based on electrochemical or chemical dissolution of rotating discs or foils of constant surface area in aerated cyanide solutions at ambient temperatures are analysed on the basis of the Levich equation. The current status of the reaction mechanism is also reviewed and updated on the basis of species distribution and potential–pH diagrams, stoichiometric factors, and interim chemical species of gold(I), silver(I), and lead(II). The anodic peak potentials of reported voltammograms closely follow the potential–pH lines of Au(I)/Au(0) and Pb(II)/Pb(0) couples. Despite the formation of stable complexes between lead(II), nitrate, and hydroxide ions, the total calculated soluble lead(II) in alkaline solutions of pH range 10–11 saturated with lead hydroxide is shown to be < 0.1 g/m 3. A comparison of the reported diffusion coefficients of cyanide ions and dissolved oxygen with the values based on the Levich plots of reported rates reveals the rate-controlling stoichiometric M/CN or M/O 2 molar ratios. The difference between some of these ratios and the generally accepted ratios of M/CN = 1/2 and M/O 2 = 1/0.5 or 1/0.25 based on the formation of M(CN) 2 −, H 2O 2 or OH − in the overall cyanidation reaction is attributed to the oxidation of cyanide to cyanate and passivation due to the formation of gold hydroxides/oxides. The alloyed or dissolved silver and lead eliminate passivation due to the involvement of mixed hydroxo–cyano complexes of silver and lead ions in the surface reaction. Gold dissolution by oxygen in cyanide-rich solutions is limited by oxygen diffusion, but enhanced by the presence of a low concentration of sodium sulfide due to the involvement of hydrosulfide ion in the surface reaction. However, excess lead or sulfide retards gold cyanidation due to surface blockage by metallic lead, lead hydroxide, or due to passivation by Au 2S/S. Even low concentrations of hydrosulfide passivate gold–silver alloys due to the formation of Ag 2S. This can be eliminated by adding stoichiometric quantities of lead(II) to precipitate sulfide as PbS. Large stoichiometric ratios of O 2/M for the cyanidation of graphite coated gold appears to be a result of the enhanced oxidation of cyanide by oxygen or hydrogen peroxide, leading to a cyanide deficiency at the surface and passivation of gold by hydroxide/oxide. The presence of excess cyanide or lead(II) does not override this effect.

Particle-size effects on the photocurrent efficiency of nanostructured assemblies consisting of gold nanoparticles and a ruthenium complex–viologen linked thiol

Photoelectrochemical assemblies consisting of gold nanoparticles and a tris(2,2′-bipyridine)ruthenium(II)–viologen linked disulfide derivative ([RuVS] 2) were constructed on gold electrodes, by a combination of salting-out and self-assembly processes. Structural characterization of the nanostructured assemblies was carried out by scanning electron microscopy and cyclic voltammetry. The adsorbed amount of the tris(2,2′-bipyridine)ruthenium(II)–viologen linked moiety (RuVS) was fairly well correlated with the ratio of the total gold surface area of the nanostructured electrode. Nanostructured assemblies consisting of ∼50 to ∼<100 nm particles showed high photocurrent efficiencies.

Labelling and binding of poly-( l-lysine) to functionalised gold surfaces. Combined FT-IRRAS and XPS characterisation

We compare herein the interfacial reactivity of self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA), 1-undecanethiol (UDT) and 11-mercaptoundecanol (MUD) on gold surfaces towards aqueous solutions of poly-( l-lysine) (PL). Liquid-phase labelling of PL with the alkyne dicobalt hexacarbonyl cluster 1 combined with analysis of the substrates by Fourier transform infrared reflection–absorption spectroscopy (FT-IRRAS) and X-ray photoelectron spectroscopy (XPS) revealed that irreversible binding of PL occurred in all cases. However, the mechanism of binding involved differed markedly from one monolayer to the other. The main mode of interaction of PL to MUA SAM was of electrostatic nature between the terminal carboxylate of MUA and the ammonium groups of PL. For a similar number of bound thiolate molecules, the UDT adsorbed layer was found less continuous than the MUA one, allowing a higher fraction of PL to directly bind to the gold surface. As for MUD, very little thiolate molecules were adsorbed, leaving bare gold surface areas for non specific adsorption of PL.