Electrochemically deposited palladium as a substrate for self-assembled monolayers

The vast majority of reports of self-assembled monolayers (SAMs) on metals focus on the use of gold. However, other metals, such as palladium, platinum, and silver offer advantages over gold as a substrate. In this work, palladium is electrochemically deposited from PdCl2 solutions on glassy carbon electrodes to form a substrate for alkanethiol SAMs. The conditions for deposition are optimized with respect to the electrolyte, pH, and electrochemical parameters. The palladium surfaces have been characterized by scanning electron microscopy (SEM) and the surface roughness has been estimated by chronocoulometry. SAMs of alkane thiols have been formed on the palladium surfaces, and their ability to suppress a Faradaic process is used as an indication for palladium coverage on the glassy carbon. The morphology of the Pd deposit as characterized by SEM and the blocking behavior of the SAM formed on deposited Pd delivers a consistent picture of the Pd surface. It has been clearly demonstrated that, via selection of experimental conditions for the electrochemical deposition, the morphology of the palladium surface and its ability to support SAMs can be controlled. The work will be applied to create a mixed monolayer of metals, which can subsequently be used to create a mixed SAM of a biocomponent and an alkanethiol for biosensing applications.     

Attachment of Amine‐ and Maleimide‐Containing Ferrocene Derivatives onto Self‐Assembled Alkanethiol and Alkanedithiol Monolayers: Voltammetric Evaluation of Cross‐Linking Efficiencies and Surface Coverage of Electroactive Groups

Ferrocene derivatives containing primary amines and maleimide groups were attached covalently onto N‐hydrosuccinimidyl (NHS)‐terminated alkanethiol self‐assembled monolayers (SAMs) and SAMs of alkanedithiol. The surface coverage and efficiencies of the two cross‐linking reactions were evaluated with cyclic voltammetry. All the ferrocene derivatives attached onto the alkanethiol or alkanedithiol SAMs exhibit reversible redox waves. The surface coverage of the aminated ferrocene groups was compared to that of N‐hydrosuccinimidyl (NHS)‐terminated alkanethiol SAM. The covalent attachment of β‐ferrocenylethylamine onto a 11,11′‐dithio‐bis(succinimidylundecanoate) SAM yielded an efficiency as high as 63.1%. The cross‐linking efficiency of this reaction was found to increase with the nucleophilicity of the amino groups. SAMs of longer alkyl chains favor the attachment of a greater number of ferrocene derivatives. As for the Michael‐type electrophilic addition between the sulfhydryl groups of the alkanedithiol SAMs and the ferrocenyl maleimide, the cross‐linking efficiencies were found to range from 6.5% to 25.7%, depending on the alkanedithiol chain length. The difference in the efficiencies between the two types of cross‐linking reactions might be partially attributable to the steric hindrance imposed by the SAMs and the relative sizes of the functional groups.     

Electroactive gold nanoparticles protected by 4-ferrocene thiophenol monolayer

Numerous reports have focused on ferrocene-terminated electroactive self-assembled monolayers (SAMs) on a flat Au surface but only a few on ferrocene SAMs on Au 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 Au 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.     

Electrochemical Studies of Chemically Modified Nanometer‐Sized Electrodes

Self‐assembled monolayers (SAMs) of 4‐aminothiophenol (4‐ATP) has been successfully deposited onto nanometer‐sized gold (Au) electrodes. The cyclic voltammograms obtained on a 4‐ATP SAMs modified electrode show peaks and the peak height is proportional to the scan rate, which is similar to that on an electroactive SAMs modified macro electrode. The electrochemical behavior and mechanism of outer‐sphere electron transfer reaction on the 4‐ATP SAMs modified nanometer‐sized electrode has also been studied. The 4‐ATP SAMs modified electrode is further modified with platinum (Pt) nanoparticles. Electrochemical behaviors show that there is electrical communication between Pt nanoparticles and Au metal on the Pt nanoparticles/4‐ATP SAMs/Au electrode. However, scanning electron microscopic image shows that the Pt nanoparticles are not evenly covered the electrode.     

功能化纳米金增强的谷胱苷肽电化学检测和巯基识别

以冠有大量二茂铁的纳米金微粒/抗生蛋白链菌素结合物为标记物,将其标记于生物素修饰的巯基识别试剂上,制成了具有电化学活性和纳米金放大作用的传感器.首先将双官能团的羟基琥珀酰亚胺酯自组装于电极表面上,借助两步交联反应固定含巯基的蛋白质,并且引入巯基识别试剂标记生物素的马来酰亚胺,随后利用生物素与链霉抗生物素之间的特异性吸附作用,引入功能化的纳米金.采用伏安法测定修饰在纳米金上的二茂铁,可识别和测定表面固定的蛋白质,还原型谷胱苷肽在5μmol/L～0.1mmol/L浓度范围内存在线性关系,检测限可达到1nmol/L.     

A Comparative Study of Modifying Gold and Carbon Electrode with 4‐Sulfophenyl Diazonium Salt

A comparison of the reductive adsorption behavior of 4‐sulfophenyl diazonium salt and subsequent electrochemical reactivity on gold relative to carbon was studied with some significant differences observed. The ability of the 4‐sulfophenyl layer adsorbed onto gold to block access of the redox probe ferricyanide to the underlying electrodes, as determined via cyclic voltammetry was inferior to the same layers formed on glassy carbon electrodes thus indicating a more open, porous layer formed on gold. More significantly, the 4‐sulfophenyl layers are shown to be far less electrochemically stable on gold than on glassy carbon. Electrochemical and X‐ray photoelectron spectroscopy (XPS) evidence suggests the instability is due to cleavage of the bond between sulfonate functional group and phenyl ring. These results provide further evidence that although aryl diazonium salt layers are relatively stable on gold surfaces compared with alkanethiol based self‐assembled monolayer (SAMs), the stability is not as high as is observed on carbon.     

Bioelectrocatalysis by Microperoxidase‐11 in a Multilayer Architecture of Chitosan Embedded Gold Nanoparticles

We report on the redox behaviour of the microperoxidase‐11 (MP‐11) which has been electrostatically immobilized in a matrix of chitosan‐embedded gold nanoparticles on the surface of a glassy carbon electrode. MP‐11 contains a covalently bound heme c as the redox active group that exchanges electrons with the electrode via the gold nanoparticles. Electroactive surface concentration of MP‐11 at high scan rate is between 350±50 pmol cm^?2, which reflects a multilayer process. The formal potential (E°′) of MP‐11 in the gold nanoparticles‐chitosan film was estimated to be ?(267.7±2.9) mV at pH 7.0. The heterogeneous electron transfer rate constant (k_s) starts at 1.21 s^?1 and levels off at 6.45 s^?1 in the scan rate range from 0.1 to 2.0 V s^?1. Oxidation and reduction of MP‐11 by hydrogen peroxide and superoxide, respectively have been coupled to the direct electron transfer of MP‐11.     

Heterogeneous electron transfer at Au/SAM junctions in a room-temperature ionic liquid under pressure

Proven electrochemical approaches were applied to study heterogeneous electron transfer (ET) between selected redox couples and gold electrodes modified with alkanethiol self-assembled monolayers (SAMs), using the room-temperature ionic liquid (RTIL) [bmim][NTf2] as reaction medium; ferrocene as freely diffusing redox probe in the RTIL was tested for ET through both thin (butanethiol) and thick (dodecanethiol) assemblages at pressures up to 150 MPa; well behaved kinetic patterns and reproducibility of data were demonstrated for ET within the unique Au/SAM/RTIL arrays.     

QCM and EC-SPR Studies of Cytochrome cSelf-assembled on Au Electrode and Enhancement of SPR Signal by Au Nanoparticles

Quartz crystal microbalance(QCM) and cyclic voltammetry(CV) were used to characterize the monolayer of cytochrome c(Cyt c), which was adsorbed on gold film modified with alkanethiol mixed monolayer. A direct comparison of protein surface coverages calculated from QCM and cyclic voltammetric measurements illustrates that the ratio of the electroactive Cyt c to the total surface-confined Cyt cis 34%, which suggests that the orientation is a main factor affecting the electroactivity of Cyt c. Moreover, surface plasmon resonance(SPR) measurement combined with CV “in situ” was used to investigate the conformational change of Cyt c in the redox process. Besides, Au nanoparticles(Au NPs) were adsorbed on the surface of Cyt c. The result indicates that Au NPs promote electron transfer between Cyt c and the gold electrode, and SPR result suggests Au NPs enhance SPR signal.     

Nitroxyl radical self-assembled monolayers on gold: Experimental data vs. Laviron’s interaction model

Mixed SAMs of nitroxyl radical derivative and alkanethiol have been studied by cyclic voltammetry in both aqueous and non-aqueous solvents. Cyclic voltammograms exhibit shapes as a function of surface coverage deviating from an “ideal system”. Confronted to Laviron’s interaction model, the agreement observed between theory and experiments provides evidence of a random distribution of electroactive centers on surface and indicates that the local ionic environment of charged redox centers plays a major role in the electrochemical behaviour of SAMs.     

Immobilization of acetylcholinesterase in lipid membranes deposited on self-assembled monolayers

Human red blood cell acetylcholinesterase was incorporated into planar lipid membranes deposited on alkanethiol self-assembled monolayers (SAMs) on gold substrates. Activity of the protein in the membrane was detected with a standard photometric assay and was determined to be similar to the protein in detergent solution or incorporated in lipid vesicles. Monolayer and bilayer lipid membranes were generated by fusing liposomes to hydrophobic and hydrophilic SAMs, respectively. Liposomes were formed by the injection method using the lipid dimyristoylphosphatidylcholine (DMPC). The formation of alkanethiol SAMs and lipid monolayers on SAMs was confirmed by sessile drop goniometry, ellipsometry, and electrochemical impedance spectroscopy. In this work, we report acetylcholinesterase immobilization in lipid membranes deposited on SAMs formed on the gold surface and compare its activity to enzyme in solution.     

Competitive self-assembly and electrochemistry of some ferrocenyl-n-alkanethiol derivatives on gold

Three ferrocenyl-alkanethiol derivatives with different functional groups linking ferrocene to an alkanethiol chain have been synthesized and characterized electrochemically in bulk solution and in self-assembled monolayer films on gold electrodes. Relative affinities of the ferrocenyl-alkanethiols and of the corresponding n-alkanethiols for the electrode surface were evaluated by the competitive self-assembly method. The affinity of the ferrocenyl-alkanethiols for the surface, relative to that of the corresponding alkanethiols, is a function of the polarity of the functional group linking ferrocene to the alkanethiol chain. In general, nonpolar linking groups (methylene) show a stronger affinity for the surface than do polar groups (carboxamides) and especially charged groups (quaternary ammonium salts). It is postulated that electrostatic effects are critically important during self-assembly. Redox potentials for the three ferrocenyl-alkanethiol derivatives scale approximately with the electron donating/withdrawing effects of the functional groups on the cyclopentadiene rings. However, redox potentials for the surface-confined molecules are consistently more positive than for the identical molecules in bulk solution.     

Electrochemical Aptasensor for the Determination of Cocaine Incorporating Gold Nanoparticles Modification

A novel electrochemical aptasensor incorporating a signal enhancement for the determination of cocaine was designed. Gold nanoparticles were self‐assembled onto the surface of a gold electrode through 1,6‐hexanedithiol. A bifunctional derivative of the 32‐base cocaine‐binding aptamer with a redox‐active ferrocene moiety and a thiol linker group at the termini of the strand was self‐assembled onto the surface of gold nanoparticles. The oxidation peak current is linearly related to the concentration of cocaine from 1.0 to 15.0 μM with a detection limit of 0.5 μM. It was found that the sensitivity of the aptasensor with gold nanoparticles modification was ca. 10‐fold higher than that of the aptasensor without gold nanoparticles modification. This work demonstrates that gold nanoparticles‐assembled gold electrode provides a promising platform for immobilizing aptamer and enhancing the sensitivity.     

Electroactive self-assembled monolayers on gold via bipodal dithiazepane anchoring groups

Novel dithiazepane-functionalized ferrocenyl-phenylethynyl oligomers 1 and 2 have been synthesized. Self-assembled monolayers (SAMs) of these ferrocene derivatives have been studied by X-ray photoelectron spectroscopy, ellipsometry, and cyclic voltammetry. It has been shown by XPS that monolayers of the dithiazepane-anchored molecules on gold electrodes contain gold-thiolate species. Cyclic voltammetry of the SAMs were characteristic of stable electroactive monolayers even for single-component SAMs of 1 and 2, with the more ideal responses recorded for the two-component SAMs diluted with undecanethiol. The small variation in peak splittings at progressively higher scan rates in these SAMs makes dithiazepane-bridged redox species promising candidates for further studies on molecular wires with bipodal anchoring.     

An Exfoliated Graphite Based Electrochemical Sensor for As(III) in Water

In this work, we present the application of an exfoliated graphite electrode modified with gold nanoparticles (AuNPs) for the detection of As(III) in acidic media. Gold nanoparticles were deposited on the surface of an exfoliated graphite electrode by electrodeposition at a potential window of ?0.2 V to 1.2 V. This was followed by activation in 0.5 M H₂SO₄ with 10 cycles from 0.6 V to 1.4 V. The modification of exfoliated graphite (EG) showed an increased electroactive surface area of the electrode and improved peak current output in a Fe(CN)₆^3?/4? redox probe. EG‐AuNPs electrode was used to detect As(III) in 1.0 M HNO₃ using square wave anodic stripping voltammetry (SWASV) technique at optimum conditions of pH 3, deposition potential of ?0.8 V, deposition time of 180 s, frequency of 5 Hz and pulse amplitude of 50 mV. The EG‐AuNPs electrode detected As(III) in solution to a limit of 0.58 ppb with regression of 0.9993. The method reported is simple, cheap and possesses good reproducibility. The developed electrochemical sensor was applied in the detection of As (III) in an industrial real water sample. The results of the real water sample analysis from the developed method are comparable with the inductively coupled plasma – optical emission spectroscopy (ICP‐OES) results.     

Voltammetric study of sodium hypochlorite using dendrimer-stabilized gold nanoparticles

Electrochemical activity of dendrimer-stabilized gold nanoparticles on a glassy carbon electrode (GCE) was studied by means of cyclic voltammetry of sodium hypochlorite. Dendrimer-stabilized gold nanoparticles were deposited onto a glassy carbon surface by electrophoresis and the deposition was observed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Both anodic and cathodic peak currents were found to increase after the deposition of dendrimer-stabilized gold nanoparticles. In the case of dendrimers deposited onto the GCE surface, the magnitude of the current was smaller than for dendrimer-stabilized-gold-nanoparticle-modified GCE. This indicates the enhancement in the rate of electrolysis due to gold nanoparticles.     

A study of carbon surfaces in molten LiCl-KCl eutectic-currentless deposition of precious metals on glassy carbon substrates

Currentless deposition of precious metals, Au, Pt, Ir, Ru, Rh and Pd, on glassy carbon surfaces in LiCl-KCl eutectic at 450°C has been reported. When the electrodes are simply immersed in the melt solutions containing the metal ions, metal deposits equivalent to several atomic layers are obtained within a few minutes. Chronopotentiometric stripping of deposited metals and the decays of open-circuit potentials during the soaking experiments strongly suggest that the deposition is related to the surface redox properties of carbon substrates. Interpretation of the deposition process based on the mixed potential concept is presented, and the substrate reactivity derived from the deposited amount is ascribed to the surface redox reactions involving bulk O^2? ions, superficial water and surface functional groups.     

Functional silver nanostructured surfaces applied in SERS and SIMS

Present article deals with functionality of silver nanostructured surfaces prepared by potentiostatic electrochemical deposition on the paraffin impregnated graphite electrode as template‐free substrates. The effect of the electrodeposition conditions on two silver surface functions: analytical signal enhancement in Surface‐enhanced Raman spectroscopy and pre‐ionization function, applied in secondary ion mass spectrometry (SIMS) is reported. Functional silver nanostructured substrate was prepared at a potential ?850 mV with a deposition duration of 20 min. Analytical signal enhancement factors of 3.2 ×10⁵ for Raman peak at 649 cm^?1, 3.0×10⁵ for peak at 810 cm^?1 and 2.7×10⁵ for peak at 1539 cm^?1 were determined for Rhodamine 6G at deposited surface. Slight pre‐ionization effect has been observed in SIMS, and 1.2×10⁵ fold signal enhancement was established for fragment of Rhodamine 6G with m/z 429 (M‐CH₃‐Cl). Electrochemical preparation of nanostructures represents a step towards surface integration directly into miniaturized systems and sensors. Copyright © 2013 John Wiley & Sons, Ltd.     

The Effect of Ionic Liquid Covalent Bonding to Sol‐Gel Processed Film on Ion Accumulation and Transfer

Accumulation of electroactive anions into a silicate film with covalently bonded room temperature ionic liquid film deposited on an indium tin oxide electrode was studied and compared with an electrode modified with an unconfined room temperature ionic liquid. A thin film containing imidazolium cationic groups was obtained by sol‐gel processing of the ionic liquid precursor 1‐methyl‐3‐(3‐trimethoxysilylpropyl)imidazolium bis(trifluoromethylsulfonyl)imide together with tetramethylorthosilicate on the electrode surface. Profilometry shows that the obtained film is not smooth and its approximate thickness is above 1 μm. It is to some extent permeable for a neutral redox probe – 1,1′‐ferrocene dimethanol. However, it acts as a sponge for electroactive ions like Fe(CN)₆^3?, Fe(CN)₆^4? and IrCl₆^3?. This effect can be traced by cyclic voltammetry down to a concentration equal to 10^?7 mol dm^?3. Some accumulation of the redox active ions also occurs at the electrode modified with the ionic liquid precursor, but the voltammetric signal is significantly smaller compare with the bare electrode. The electrochemical oxidation of the redox liquid t‐butyloferrocene deposited on silicate confined ionic liquid film is followed by the expulsion of the electrogenerated cation into an aqueous solution. On the other hand, the voltammetry obtained with the electrode modified with t‐butyloferrocene solution in the ionic liquid precursor exhibits anion sensitive voltammetry. This is explained by anion insertion into the unconfined ionic liquid deposit following t‐butylferricinium cation formation.     

Underpotential deposition of copper on electrodes modified with colloidal gold

This communication is concerned with the electrochemical addressability of gold colloidal particles deposited on a conducting substrate. Cyclic voltammetry of electrodes modified with gold colloid layers indicates that an underpotential deposition of copper onto the gold surface takes place. Analysis of the charge associated with the underpotential deposition permits the electroactive gold area to be calculated. The total gold area may be determined from transmission electron microscopy (TEM) images. Comparison of the geometric and electroactive areas obtained indicates that electrochemically all the gold particles are addressable and the entire colloid surface is accessible.