Selective functionalization of In2O3 nanowire mat devices for biosensing applications

A strategy to covalently attach biological molecules to the electrochemically active surface of indium oxide nanowire (In2O3 NW) mat devices is presented. A self-assembled monolayer (SAM) of 4-(1,4-dihydroxybenzene)butyl phosphonic acid (HQ-PA) was generated on an indium tin oxide (ITO)-coated glass and In2O3 NWs surface. The chemical steps required for surface derivatization were optimized on an ITO surface prior to modifying the In2O3 NWs. The hydroquinone group contained in the HQ-PA SAM was electrochemically oxidized to quinone (Q-PA) at +330 mV. The monolayer of Q-PA was allowed to react with a thiol-terminated DNA. The DNA was paired to its complementary strand tagged with a fluorescence dye. Attachment of DNA was verified using fluorescence microscopy. A device was subsequently prepared on a SiO2-supported mat of In2O3 NWs by depositing gold electrodes on the mat surface. The reaction strategy optimized on ITO was applied to this In2O3 NW-based device. Arrays of In2O3 NWs on a single substrate were electrochemically activated in a selective manner to Q-PA. Activated In2O3 NWs underwent reaction with HS-DNA and gave a positive fluorescence response after pairing with the dye-DNA. The unactivated In2O3 NWs gave no response, thus demonstrating selective functionalization of an In2O3 NW array. This can be considered a key step for the future fabrication of large-scale, inexpensive, nanoscale biosensors.     

Electrochemical Characterization of Horse Spleen Ferritin Covalently Immobilized on Self‐Assembled Monolayer Modified Gold Electrodes

Horse spleen ferritin was covalently attached to SAM‐modified gold electrodes using cross‐linking agents. Reduction of ferritin occurs at negative potentials and is electrochemically irreversible. The voltammetry reveals the presence of a new electrochemical couple that has been determined to be a dissolved iron species released upon the reduction of ferritin. Covalently attached ferritin retains its ability to release iron as evidenced by the absence of the dissolved couple peaks when ferritin is reduced in the presence of nitrilotriacetate. As the SAM chain length increases, the reduction potential becomes more negative, suggesting a tunneling mechanism is involved in the electron transfer.     

Self-assembled monolayers of a hydroquinone-terminated alkanethiol onto gold surface. Interfacial electrochemistry and Michael-addition reaction with glutathione

An electroactive self-assembled monolayer (SAM) was fabricated by covalent attachment of a novel hydroquinone-terminated dodecanethiol onto the gold surface and its electrochemical behavior was investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The capability of the designed SAM in immobilization of organic molecules onto the gold surface was studied utilizing the Michael-addition as a model reaction. The results obtained from cyclic voltammetry, electrochemical impedance and grazing incidence Fourier transform infrared (GI-FTIR) spectroscopy revealed that, upon applying an anodic potential to the Au-SAM electrode system in the presence of glutathione, the electrochemically generated p-quinone participated in a Michael-addition reaction with glutathione and the corresponding Michael adduct was formed at the solid–liquid interface. The kinetic parameters were then derived for this interfacial Michael-addition reaction.     

Epimeric monosaccharide-quinone hybrids on gold electrodes toward the electrochemical probing of specific carbohydrate-protein recognitions

Carbohydrates represent one of the most significant natural building blocks, which govern numerous critical biological and pathological processes through specific carbohydrate-receptor interactions on the cell surface. We present here a new class of electrochemical probes based on gold surface-coated epimeric monosaccharide-quinone hybrids toward the ingenious detection of specific epimeric carbohydrate-protein interactions. Glucose and galactose, which represent a pair of natural monosaccharide C4 epimers, were used to closely and solidly conjugate with the 1,4-dimethoxybenzene moiety via a single C-C glycosidic bond, followed by the introduction of a sulfhydryl anchor. The functionalized aryl C-glycosides were sequentially coated on the gold electrode via the self-assembled monolayer (SAM) technique. X-ray photoelectron spectroscopy (XPS) was used to confirm the SAM formation, by which different binding energies (BE) between the glucosyl and the galactosyl SAMs on the surface, probably rendered by their epimeric identity, were observed. The subsequent electrochemical deprotection process readily furnished the surface-confined quinone/hydroquinone redox couple, leading to the formation of electrochemically active epimeric monosaccharide-quinone SAMs on the gold electrode. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) used for the detection of specific sugar-lectin interactions indicated that the addition of specific lectin to the corresponding monosaccharide-quinone surface, i.e., concanavalin A (Con A) to the glucosyl SAM and peanut agglutinin (PNA) to the galactosyl SAM, resulted in an obvious decrease in peak current, whereas the addition of nonspecific lectins to the same SAMs gave very minor current variations. Such data suggested our uniquely constructed gold surface coated by sugar-quinone hybrids to be applicable as electrochemical probes for the detection of specific sugar-protein interactions, presumably leading to a new electrochemistry platform toward the study of carbohydrate-mediated intercellular recognitions.     

银电极表面异亮氨酸单分子膜结构和表面性质的表面增强拉曼光谱研究

利用表面增强拉曼光谱(SERS)技术研究了在粗糙化银电极表面吸附的异亮氨酸自组装单层膜结构及其表面性质随溶液酸碱性和电极电位改变的特征.研究结果表明溶液pH值的变化并没有显著改变异亮氨酸分子在银电极表面以去质子化羧基吸附为主的特征.借助于高氯酸根离子这一SERS光谱探针,对异亮氨酸单分子膜的表面酸碱性质进行了表征和分析.而就电位改变对该单分子膜结构的影响而言,在所研究的电位范围内,单分子膜中的异亮氨酸分子是通过去质子化羧基与氨基两个位点而吸附的,且吸附作用随电位负移而呈现有规律的变化.     

Enantioselectivity of redox reaction of DOPA at the gold electrode modified with a self-assembled monolayer of homocysteine

The enantioselectivity of the self-assembled monolayer (SAM) of homocysteine formed on the (111)-oriented gold surface was investigated. We analyzed the redox behavior of 3,4-dihydroxyphenylalanine (DOPA), which is an electrochemically active chiral molecule, by means of cyclic voltammetry at a gold electrode modified with one enantiomeric form of homocysteine. It was demonstrated that the homocysteine SAM of one enantiomeric form blocked the redox reaction of only one enantiomer of DOPA, with cross inversion for the other enantiomer, in acidic solution.     

Tunable wetting mechanism of polypyrrole surfaces and low-voltage droplet manipulation via redox

This paper presents the experimental results and analyses on a controlled manipulation of liquid droplets upon local reduction and oxidation (redox) of a smart polymer-dodecylbenzenesulfonate doped polypyrrole (PPy(DBS)). The electrochemically tunable wetting property of PPy(DBS) permitted liquid droplet manipulation at very low voltages (-0.9 to 0.6 V). A dichloromethane (DCM) droplet was flattened upon PPy(DBS) reduction. It was found that the surface tension gradient across the droplet contact line induced Marangoni stress, which caused this deformation. Further observation of PPy(DBS)'s color change upon the redox process confirmed that the surface tension gradient was the driving force for the droplet shape change.     

3,4-二羟基苯甲酸在自组装结构中的电化学行为

电极与有确定取向的电活性基团之间的电子传递是电化学领域的研究热点．利用二维有序薄膜固定电活性官能团是一个成熟的方法［１］,主要包括ＬＢ技术和自组装技术,这两者都存在着样品合成困难的问题．近年来,通过表面逐层反应来固定电活性官能团已有研究,但是反应过程...     

Impact of surface immobilization and solution ionic strength on the formal potential of immobilized cytochrome C

Four different self-assembled monolayer (SAM) electrode systems were examined electrochemically in order to better understand surface charge effects on the redox thermodynamics of immobilized horse heart cytochrome c (cyt c). Neutralization of protein surface charge upon adsorption on anionic COOH-terminated SAMs was found to cause substantial changes in the formal potential, as determined by cyclic voltammetry. For cyt c immobilized on negatively charged surfaces, the formal potential shifted to more negative values as the ionic strength was decreased, which is opposite to the trend displayed by solution cyt c. In contrast, immobilization to uncharged interfaces resulted in an ionic strength dependence for cyt c that is similar to its solution behavior. The results provide insight into the importance of surface charge on the formal potential of cyt c.     

Electrochemical immobilization of redox active molecule based ionic liquid

The electrochemical immobilization of redox active molecule based ionic liquid onto glassy carbon electrode has been performed. 1-Nitrophenylethyl-3-methylferrocenylimidazolium bis(trifluoromethylsulfonyl)imide was synthesized and characterized by electrochemistry showing the presence of two redox couples. Following that, the electrochemical reduction of this molecule in acidic media containing sodium nitrite leads to the in situ formation of the corresponding diazonium, in the vicinity of the electrode, and subsequently the grafting of redox based ionic liquid molecule onto the electrode surface. The surface analysis of the attached layer confirms the formation of organic thin film strongly attached to the electrode surface, and evidences the presence of the components of the imidazolium ring, ferrocenyl unit, and TFSI anion. In addition, the modified electrode was electrochemically characterized by following the electrochemical signal of the attached ferrocenyl unit. Finally, the electrochemical reversible wettability of the modified electrode upon oxidation and reduction process was demonstrated.     

Scanning Droplet Cell for Chemoselective Patterning through Local Electroactivation of Protected Quinone Monolayers

A reagentless strategy for template‐free patterning of uniformly inert surfaces is suggested. A layer of p‐hydroquinone (HQ) protected by the tert‐butyldimethylsilyl (TBDMS) group is electrografted onto glassy carbon electrodes. Chemoselective activation is performed through electrochemically controlled cleavage of the TBDMS group, which yields the redox‐active surface‐confined quinone moieties. The latter are shown to undergo electrochemically induced Michael addition, which serves for subsequent functionalization of the electrode surface. Patterning of the TBDMS–quinone‐modified surface is accomplished by using selective localized cleavage of the protecting group. State‐of‐the‐art direct‐mode scanning electrochemical microscopy (SECM) patterning fails to yield the anticipated interfacial reaction; however, the electrochemical scanning droplet cell (SDC) is capable of conducting the localized chemoselective reaction. In a small area, dictated by the dimensions of the droplet, electrochemically induced cleavage of the protecting group can be performed locally to give rise to arrays of active quinone spots. Upon deprotection, the redox signals, attributed to the hydroquinone/benzoquinone couple, provide the first direct evidence for chemoselective electrochemical patterning of sensitive functionalities. Subsequent SECM studies of the resulting modified areas demonstrate spatial control of the proposed patterning technique.     

Adsorption-induced asymmetric assembly from an achiral adsorbate

Symmetry breaking in the self-assembled monolayer (SAM) structure of 1-octadecanol on highly ordered pyrolytic graphite (HOPG) is observed. Due to the slight mismatch of the octadecanol molecule with the graphite lattice, the alkane chain undergoes distortion upon adsorption on the surface. The asymmetric distortion of the octadecanol SAM unit cell pair is observed by scanning tunneling microscopy at the liquid/solid interface. Asymmetric distortion is due to the requirement for planarity of the hydrogen bond connecting the two octadecanol molecules in the chevron-shaped unit cell. This very simple structure provides the first example of an adsorption-induced distortion to form a supramolecular asymmetric structure, which is formed by achiral molecules adsorbed on an achiral surface. What makes this system interesting and different from other examples of adsorption-induced chirality is that the adsorbate itself undergoes asymmetric distortion due to the existence of the substrate and the adsorbate conformation is different from the molecule in solution.     

Electrochemical study of self-assembled monolayer adsorption

The electrochemical behaviour of self-assembled monolayer (SAM) of aliphatic hexadecanethiol was studied by cyclic voltammetry (CV), elimination voltammetry with linear scan (EVLS) and crystal quartz microbalance (QCM). SAMs were electrochemically created on gold-coated QCM crystal through the sulphur in 1-hexadecanethiol molecule head group. The effect of thiol concentration and potential scan rate on the SAM formation was studied. Formation of SAM was confirmed by CV and QCM. EVLS results revealed the kinetically controlled process followed with electrode reaction in adsorbed state characteristic for SAM formation at lower concentration. The electrode reaction of a totally adsorbed electroactive species was indicated by means of a peak-counter peak signal at higher thiol concentration.     

Electrochemical surface plasmon resonance investigation of dodecyl sulfate adsorption to electroactive self-assembled monolayers via ion-pairing interactions

The redox-induced assembly of amphiphilic molecules and macromolecules at electrode surfaces is a potentially attractive means of electrochemically modulating the organization of materials and nanostructures on solid substrates via ion-pairing interactions or charge-transfer complexation. In this regard, we have investigated the potential-induced adsorption and aggregation of dodecyl sulfate, a common anionic surfactant, at a ferrocenylundecanethiolate (FcC11SAu) self-assembled monolayer (SAM)/aqueous solution interface by electrochemical surface plasmon resonance (ESPR) spectroscopy. The surfactant anions adsorb onto the electroactive SAM by specific ion-pairing interactions with the oxidized ferricinium species. The ferricinium charge density (QFc+) obtained by cyclic voltammetry and surface coverage measured by SPR indicate that the dodecyl sulfate forms an interdigitated monolayer, where half of the surfactant molecules have their sulfate headgroups paired to the surface and half have their headgroups exposed to the aqueous solution. The surface coverage of dodecyl sulfate was found to depend on both the ferricinium surface concentration and the surfactant aggregation state in solution. A maximum coverage of dodecyl sulfate on the ferricinium surface is obtained below the critical micelle concentration (cmc), in contrast to dodecyl sulfate adsorption to SAM surfaces of static positive charge. This marked difference in adsorption behavior is attributed to the dynamic generation of ferricinium by potential cycling and the specific nature of the ion-pairing interactions versus pure electrostatic ones. The results presented point to a new way of organizing molecules via electrical stimulus.     

Multivalent dendrimers at molecular printboards: influence of dendrimer structure on binding strength and stoichiometry and their electrochemically induced desorption

A fundamental understanding of multivalency can have a profound influence on bottom-up nanofabrication. For this purpose, three different types of ferrocenyl (Fc) functionalized dendrimers of generations 1-5 with various spacer groups were adsorbed at self-assembled monolayers (SAMs) of heptathioether-functionalized beta-cyclodextrin (betaCD) on gold. The dendrimers formed kinetically stable supramolecular assemblies at the betaCD host surface having up to eight multivalent supramolecular interactions, but could be efficiently removed from the host surface by electrochemical oxidation of the Fc end groups. Dendrimer desorption and re-adsorption could be repeated a number of times without significant decomposition of the system. The stoichiometries of the dendrimers at the surface were determined using cyclic voltammetry (CV). These were quantitatively confirmed for the lower generations by surface plasmon resonance (SPR) titrations of the dendrimers to the betaCD SAM. Measuring CV and SPR simultaneously gave crucial mechanistic information on the electrochemically induced desorption of the dendrimers from the host surface. The redox-active dendrimers effectively blocked the host surface for binding other molecules, e.g. adamantyl-functionalized dendrimers, but electrochemically induced release of the blocking layer revealed the host surface to which the adamantyl dendrimers could then bind.     

Thioether‐Functionalized Quinone‐Based Resorcin[4]arene Cavitands: Electroswitchable Molecular Actuators

The utility of molecular actuators in nanoelectronics requires activation of mechanical motion by electric charge at the interface with conductive surfaces. We functionalized redox‐active resorcin[4]arene‐quinone cavitands with thioethers as surface‐anchoring groups at the lower rim and investigated their propensity to act as electroswitchable actuators that can adopt two different conformations in response to changes in applied potential. Molecular design was assessed by DFT calculations and X‐ray analysis. Electronic properties were experimentally studied in solution and thin films electrochemically, as well as by X‐ray photoelectron spectroscopy on gold substrates. The redox interconversion between the oxidized (quinone, Q ) and the reduced (semiquinone, SQ ) state was monitored by UV‐Vis‐NIR spectroelectrochemistry and EPR spectroscopy. Reduction to the SQ state induces a conformational change, providing the basis for potential voltage‐controlled molecular actuating devices.     

Voltammetry of Organic Microparticles

 Solid microparticles of several different insoluble organic compounds were mechanically immobilized on the surface of graphite electrodes and immersed into a liquid electrolyte in order to study their electrochemical reactions. Cyclic staircase voltammetry and square-wave voltammetry were used. Quinhydrone was found to be a stable intermediate in the reversible redox reaction of solid quinone and hydroquinone on the electrode surface. The reaction occurs on the surface of the solid particle which is in contact with water. Indigo can be reduced to leucoindigo and oxidized to dehydroindigo in two separate reversible redox reactions. In strongly basic medium indigo dissolves in water upon electroreduction. A hydroacridine radical was detected as a stable intermediate in the electrochemically irreversible redox reaction of acridine and dihydroacridine. Famotidine can be electrooxidized and the product of this reaction can be electroreduced in two separate chemically irreversible reactions. Probucol is oxidized to a semiquinone radical which can be re-reduced in an electrochemically irreversible redox reaction. Propyl- thiouracil can be also oxidized to an unknown product which can be re-reduced in a chemically reversible, but slow solid state surface redox reaction. Reductions of solid thionicotinoylanilide and nicotinoylanilide are totally irreversible. Received September 22, 1998. Revision March 19, 1999.     

In Situ Synthesis of a Novel Quinone Imine Self‐Assembled Monolayer and Consideration of Its Reactivity with L‐Arginine

In situ functionalization of a 4‐aminothiophenol (4ATP) self‐assembled monolayer (SAM) on a Au electrode (4ATP/Au SAM) by the Michael addition reaction is considered. Under optimized conditions, the nucleophilic attack of the amino group of 4ATP/Au SAM to give an electrogenerated ortho‐quinone produced a novel electroactive SAM (ESAM). The ESAM could be oxidized to quinone‐imine SAM (QI SAM) for the covalent immobilization of L ‐arginine monolayers. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Fourier transform infrared (FTIR) spectroscopy are employed to characterize these systems.. The apparent heterogeneous rate constant (k_s^app) for ESAM/Au and the rate constant (k′) of the pseudo‐first order Michael addition reaction of L ‐arginine and ESAM/Au are calculated.     

Electrochemical transduction of DNA hybridization at modified electrodes by using an electroactive pyridoacridone intercalator

A synthetic redox probe structurally related to natural pyridoacridones was designed and electrochemically characterised. These heterocycles behave as DNA intercalators due to their extended planar structure that promotes stacking in between nucleic acid base pairs. Electrochemical characterization by cyclic voltammetry revealed a quasi-reversible electrochemical behaviour occurring at a mild negative potential in aqueous solution. The study of the mechanism showed that the iminoquinone redox moiety acts similarly to quinone involving a two-electron reduction coupled with proton transfer. The easily accessible potential region with respect to aqueous electro-inactive window makes the pyridoacridone ring suitable for the indirect electrochemical detection of chemically unlabelled DNA. Its usefulness as electrochemical hybridization indicator was assessed on immobilised DNA and compared to doxorubicin. The voltamperometric response of the intercalator acts as an indicator of the presence of double-stranded DNA at the electrode surface and allows the selective transduction of immobilised oligonucleotide hybridization at both macro- and microscale electrodes.     

Reactive landing of peptide ions on self-assembled monolayer surfaces: an alternative approach for covalent immobilization of peptides on surfaces

Soft landing of mass-selected peptide ions onto reactive self-assembled monolayer surfaces (SAMs) was performed using a newly constructed ion deposition apparatus. SAM surfaces before and after soft landing were characterized ex situ using time-of-flight secondary-ion mass spectrometry (TOF-SIMS) and infrared reflection-absorption spectroscopy (IRRAS). We demonstrate that reactive landing (RL) results in efficient covalent linking of lysine-containing peptides onto the SAM of N-hydroxysuccinimidyl ester-terminated alkylthiol on gold (NHS-SAM). Systematic studies of the factors that affect the efficiency of RL revealed that the reaction takes place upon collision and is promoted by the kinetic energy of the ion. The efficiency of RL is maximized at ca. 40 eV collision energy. At high collision energies the RL efficiency decreases because of the competition with scattering of ions off the surface. The reaction yield is independent of the charge state of the projectile ions, suggesting that peptide ions undergo efficient neutralization upon collision. Chemical and physical properties of the SAM surface are also important factors that affect the outcome of RL. The presence of chemically reactive functional groups on the SAM surface significantly improves the reaction efficiency. RL of mass- and energy-selected peptide ions on surfaces provides a highly specific approach for covalent immobilization of biological molecules onto SAM surfaces.