Mediating electron transfer from bacteria to a gold electrode via a self-assembled monolayer

Numerous bacterial genera are known to respire anaerobically using macroscopic electrodes as electron acceptors. Typically, inexpensive graphite electrodes, which are readily colonized, are used to monitor electrogenic bacterial metabolism for microbial fuel cell and bioelectronics studies. We compare current production by electrogenic bacteria on gold electrodes coated with various alkanethiol self-assembled monolayers to current production on glassy carbon electrodes. Current production is correlated to chain length and headgroup of the monolayer molecules as expected. Relative to graphite, the coated gold electrodes achieve more reproducible experimental conditions and certain headgroups enhance electronic coupling to the bacteria.     

On the origin of the efficient nanoparticle mediated electron transfer across a self-assembled monolayer

Recent advances in bioelectrochemistry came from the elaboration of conducting electrodes modified by an organic layer onto which nanoparticles are adsorbed. Self-assembled monolayers on noble-metal electrodes are known to hinder the electrochemical kinetics of fast-transfer redox systems. Surprisingly, fast kinetics are recovered when metal nanoparticles are deposited on top of the monolayer. We show that this surprising behavior can be fully accounted for when realizing that electron transfer from metal to metal is intrinsically easier than transfer between metal and redox system by many orders of magnitude.     

Improvement of alternariol monomethyl ether detection at gold electrodes modified with a dodecanethiol self-assembled monolayer

The electro-oxidation of alternariol monomethyl ether (AME), one of the main metabolites of the Alternaria genus mycotoxins, is studied at 1-dodecanethiol (DDT)-modified gold electrodes, in acetonitrile (ACN) – aqueous phosphate buffer solutions of different pH values, by using cyclic (CV) and square-wave (SWV) voltammetries. The AME voltammetric response at the bare electrode suffers from two drawbacks: it appears at potentials close to the onset of gold oxide formation, and it is hampered by a fouling of the electrode surface due to the accumulation of oxidized products. These shortcomings are circumvented by the use of DDT-coated electrodes, since the intervening monolayer inhibits gold oxide formation and surface passivation by the electrochemical products, without affecting the oxidation kinetics of AME significantly. Diagnostic criteria based on the voltammetric peak parameters show that the electrochemical behavior of AME at the modified electrode is mainly controlled by reactant diffusion from solution, with a weak adsorption of both the mycotoxin and its oxidation products at monolayer defects. Calibration curves were constructed from the AME square-wave voltammetric response and a detection limit of 9.1 × 10⁻⁸ mol dm⁻³ was determined, which is about three times smaller than a previous estimate at platinum and glassy carbon electrodes, and about fifty times smaller than the limit derived from measurements carried out at a polyphenol oxidase-modified carbon paste electrode.     

Electron transfer across the interface gold/self-assembled organic monolayer. Comparison of single- and two-component systems

Single- and two-component self-assembled organic monolayers (SAM) formed of HS-C₁₀H₂₀-COOH and the HS-C₁₀H₂₀-COOH + HS-C₆H₁₂-OH mixture and applied on gold electrodes of different shapes and roughness are studied. The resulting monolayers are characterized in 1 M NaNO₃ solutions by the methods of electrochemical impedance spectroscopy in the frequency range from 1 Hz to 100 kHz and cyclic voltammetry in the potential range from 0 to −0.4 V (SCE). Using these method, the behavior of modified single-crystal and polycrystalline gold electrodes in electrolyte solutions of different acidity is assessed and the standard rate constants for the Ru[(NH₃)₆]^3+/2+ redox reaction are determined. The SAM film formed from the ethanol solution of the HS-C₁₀H₂₀-COOH + HS-C₆H₁₂-OH mixture (0.025 M) on the Au(210) singlecrystal face lowers down the rate of the heterogeneous Ru[(NH₃)₆]^3+/2+ reaction from 1.5 to 4.02 × 10⁻⁴ cm/s in the electrolyte solution with pH from 2. In solutions with higher pH, the rate constant is higher by almost an order of magnitude (2.74 × 10⁻³ cm/s). The distribution of micropores in SAM films is studied within the framework of a model of micro-array electrodes. The results of studies are compared with the literature data on the gold electrode in solutions of inactive electrolytes in the absence and in the presence of SAM films formed by alkylmercaptans with equal chain lengths.     

Sensitive voltammetric detection of clomipramine at 16-mercapto-hexadecanoic acid self-assembled monolayer modified gold electrode

Clomipramine, an effective and important antipsychotic drug with low redox activity and poor hydrophilicity, was found to effectively accumulate on hydrophobic 16-mercaptohexadecanoic acid (i.e. MHA) self-assembled monolayer (SAM) modified gold electrode (i.e. MHA/Au) and generating a sensitive anodic peak at about 0.86 V (vs. SCE) in 0.05 M Tris–HCl (pH = 8.1) buffer solution. Thus, quantitative measurement of clomipramine was established with high sensitivity under optimum conditions. The anodic peak current was linear to clomipramine concentration in the range from 1 × 10⁻⁶ to 5 × 10⁻⁵ M, with a detection limit of 6 × 10⁻⁹ M. This method was successfully applied to the detection of clomipramine in drug tablets and proved to be reliable compared to UV. The spectral features, electrochemical characteristics and wettability of MHA-SAM were also studied. Correspondence: Jilie Kong, Department of Chemistry, Fudan University, Shanghai 200433, P.R. China     

Enzymatic nanolithography of a self-assembled oligonucleotide monolayer on gold

This paper describes a simple strategy to biochemically manipulate a surface at the nanoscale by enzyme dip-pen nanolithography using an endonuclease (DNase I) that is directly patterned on a self-assembled monolayer presenting a terminal oligonucleotide. Physisorbed nanopatterns of DNase I carried out nanoscale enzymology at the surface creating oligonucleotide patterns with the fidelity of the patterned enzyme because of the affinity of the enzyme for the immobilized, oligonucleotide substrate.     

Development of a myoglobin impedimetric immunosensor based on mixed self-assembled monolayer onto gold

Immunosensors rely on antibody-antigen binding with a range of possible detection methodologies. In this study, electrochemical impedance spectroscopy was used to monitor the sensor surface assembly and recognition of the analyte (myoglobin). Myoglobin is rapidly released into the circulatory system after an acute myocardial infarction and rapidly rising levels make it the first biochemical marker of myocardial damage. The immunosensor fabrication steps comprised the steps of (a) formation of mixed self-assembled monolayers (mSAM) on gold electrodes using a mixture of biotinyl-phospholipid and mer captohexadecanoic acid; (b) neutravidin functionalisation and (c) attachment of biotinyl anti-myoglobin antibodies. A range of analyte concentration (10⁻¹²–10⁻⁶ M) was successfully detected in phosphate buffered saline and in serum concentration ranging from 10% (v/v) to 100% (v/v) serum. Quartz crystal microbalance and atomic force microscopy studies were carried out to study each step of fabrication to elucidate binding characteristics and surface topography. Correspondence: Morsaline Billah, Institute of Membrane and Systems Biology, Garstang Building, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom     

Electrochemical detection of copper(II) at a gold electrode modified with a self-assembled monolayer of penicillamine

A penicillamine (PCA) self-assembled monolayer (SAM) was prepared on a gold electrode. It has been found that the modified electrode exhibited a selective response to copper ions. As demonstrated by cyclic voltammetric experiments, the SAM-based electrode showed an attractive ability to preconcentrate efficiently traces of copper(II) from solutions. Under optimum conditions, the anodic peak current was proportional to the concentration of Cu(II) in the range from 8.0 × 10⁻⁷ to 1.0 × 10⁻⁴ M with a detection limit of 4.0 × 10⁻⁷ M. Moreover, this modified gold electrode is also characterized by excellent repeatability, showing a relative standard deviation of 3.2% for nine successive measurements of 1.0 × 10⁻⁵ M Cu(II). The PCA/Au SAM gold electrode was used for the determination of Cu(II) in a tap water sample and the results showed a good agreement with the data obtained by atomic emission spectrometry. The text was submitted by the authors in English.     

多巴胺在DTNB自组装膜上的电催化研究

在金电极表面制备了DTNB(5,5′ Di thiobis(2 nitrobenzoicacid))自组装单分子层膜(DTNB/AuSAM)。多巴胺在DTNB自组装膜上有一对可逆性良好的氧化还原峰,其氧化峰电流与多巴胺的浓度在5.0×10-6mol/L～1.0×10-4mol/L的范围内呈线性关系,检出限为1.0×10-6mol/L。在pH3.5的缓冲溶液中,在DTNB自组装膜上多巴胺和抗坏血酸的电化学响应可以明显区分,氧化峰电位分离达276mV。可用于抗坏血酸存在下多巴胺的检测。测定了盐酸多巴胺注射液中多巴胺的含量,其平均回收率为104%。     

Electroanalysis of dopamine at a gold electrode modified with N-acetylcysteine self-assembled monolayer

Voltammetric behavior of dopamine (DA) on a gold electrode modified with the self-assembled monolayer (SAM) of N-acetylcysteine has been investigated, and one pair of well-defined redox peaks of dopamine is obtained at the SAM modified gold electrode. The oxidation peak current increases linearly with the concentration of dopamine in the range of  mol l⁻¹. The detection limit is 8.0×10⁻⁷ mol l⁻¹. This method will be applicable to the determination of dopamine in injection of dopamine hydrochloride, and the good recovery of dopamine is obtained. Furthermore, The SAM modified gold electrode can resolve well the voltammetric responses of dopamine and ascorbic acid (AA), so it can also be applied to the determination of dopamine in the presence of ascorbic acid.     

Colorimetric detection of DNA at the nanomolar level based on enzyme-induced gold nanoparticle de-aggregation

The authors describe a colorimetric method for the determination of DNA based on the deaggregation of gold nanoparticles (AuNPs) induced by exonuclease III (Exo III). DNA amplification is accomplished by Exo III to generate large quantities of the residual DNA. Residual DNA tethers onto the surfaces of AuNPs which prevents their aggregation. Hence, the color of the solution is red. However, in the absence of DNA, salt-induced aggregation is not prevented, and the bluish-purple color of the aggregated AuNPs is observed. The ratio of absorbances at 525 and 625 nm increases up to 150 nM DNA concentrations, and the LOD is as low as 3.0 nM. It is shown that the presence of 300 nM concentrations of random DNA (with a mass up to 10-fold that of target DNA) does not interfere. The method was successfully applied to the analysis of DNA in spiked serum samples. The method is simple, reliable, and does not require complicated amplification steps and expensive instrumentation.

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Graphical abstract Schematic of a sensing strategy for DNA detection by exonuclease III-induced deaggregation of gold nanoparticles. DNA concentrations as  low as 3 nM can be detected via colorimetric monitoring of the color change from red to purple-blue.

     

Proton coupled electron transfer of ubiquinone Q2 incorporated in a self-assembled monolayer

We present a complete study of the reduction of ubiquinone Q(2) (UQ(2)) in simpler aqueous medium, over a pH range of 2.5 to 12.5. The short isoprenic chain ubiquinones (UQ(2)) were incorporated in a self-assembled monolayer. Under these conditions, the global 2e(-) electrochemical reaction can be described on the basis of a nine-member square scheme. The thermodynamic constants of the system were determined. The global 2e(-) process is controlled by the uptake of the second electron. The elementary electrochemical rate constants obtained by fitting of the experimental rate constant were k(s4) = 1.5 s(-1) for QH˙(+)(2)? QH(2), k(s5) = 1.5 s(-1) for QH˙? QH(-) and k(s6) = 1 s(-1) for Q˙(-)? Q(2-). The three electrochemical reactions QH˙(+)(2)? QH(2), QH˙? QH(-) and Q˙(-)? Q(2-) are successively involved when increasing the pH. Protonations can occur or not, before or after the electron uptake and the reaction paths are, from low to high pH: e(-), H(+)e(-), e(-)H(+), H(+)e(-)H(+), H(+)e(-) and e(-)H(+).     

Enzymatic fabrication of DNA nanostructures: extension of a self-assembled oligonucleotide monolayer on gold arrays

Nucleic acid nanostructures are useful as templates for bionanofabrication of composite molecular nanostructures in materials science, molecular electronics, and biosensing. Here, we demonstrate that terminal deoxynucleotidyl transferase, which repetitively adds mononucleotides to the 3' end of a short DNA initiator, can be used to rapidly fabricate DNA nanostructures up to 121 nm high with lateral dimensions from 0.1 to 4 mum in 2 h. These programmable scaffolds can potentially be employed to build more complex nanostructures consisting of natural or unnatural nucleotides with selective docking sites along the single-stranded DNA.     

Stereo-electrochemistry by a self-assembled monolayer of sulfur-bridged calixthiophene on gold

Sulfur-bridged calixthiophene formed a self-assembled mono-molecular layer on polycrystalline gold, and it regulated an electrochemical electron transfer by the host–guest interaction between the cavity and reactants. 1,7,13,19,25-Tetrathia[1.₅](2,5)thiophenophane (thiacalix[5]thiophene) perfectly passivated the gold electrode for relatively large reversible metal complexes: [Fe(CN)₆]^4−/3− and [IrCl₆]^3−/2−. However, for mono-atomic ions, such as silver and some of the halogen ions, the electrode behaved reversibly. For copper reduction, a large activation overpotential was observed to induce an initial copper reduction in the cavity.     

Two-dimensional crystal structure of a quaterthiophene-alkanethiol self-assembled monolayer on gold

We investigated the fine structure of a self-assembled monolayer of dodecanethiol functionalized by alpha-quaterthiophene on gold (alpha-4TC 12H 24SH). The molecular orientation, quantified using polarization modulation infrared reflection-absorption spectroscopy, was studied as a function of the adsorption time. The alpha-4T moieties arrange in the upright position on the surface as the adsorption time increases, while the alkyl chain organization remains poor. Here we quantify the orientation of the self-assembled monolayer and, more significantly, reveal through surface X-ray diffraction that after a long incubation period (12 h) the alpha-4T on the gold surface adopts a 2D crystal structure.     

Separation of pinhole and tunneling electron transfer processes at self-assembled polymeric monolayers on gold electrodes

Self-assembled monolayers of poly(3-alkylthiophene) on gold electrodes are examined by cyclic voltammetry in solutions containing electroactive species. Two well-separated electron transfer processes, namely, electron tunneling through the monolayer and electron exchange at pinholes (defects) of the monolayer are observed. The voltammetric responses of the pinhole electron transfer process take place around the standard potential of the electroactive species and resemble those of a nanoelectrode ensemble of independent individual nanoelectrodes. The voltammetric characteristics of the electron tunneling agree well with predictions of the Marcus theory. Satisfactory values of tunneling coefficient, standard rate constant and organization energy are derived from the voltammetric data.     

Label-free detection of DNA molecules on the dendron based self-assembled monolayer by electrochemical impedance spectroscopy

We report preparation of a novel platform for effective DNA hybridization and its application to the detection of single mismatched DNA. Cone-shaped dendrimer molecules have been immobilized on the gold surface at equidistance, 3.1 nm, from each other with a probe DNA molecule attached to the top of each dendrimer so that enough space would be secured for effective hybridization. This arrangement allows each probe DNA molecule to form a natural DNA double helix upon hybridization with a target DNA molecule. The single nucleotide polymorphism at either the central or end position of the 25-mer target DNA has been shown to be effectively discriminated against on this platform from each other as well as from a complementary DNA by electrochemical impedance measurements. We also report adverse effects exerted by probe ions, Fe(CN)₆^3−/4−, on DNA hybridization reactions. The significance of the results for the use in DNA analysis is discussed.     

Pulsed coulometric detection of carbohydrates at a constant detection potential at gold electrodes in alkaline media

A significant increase in the signal-to-noise ratio for the pulsed amperometric detection (PAD) of carbohydrates at gold electrodes is obtained by increasing the length of the current integration period (t_i) from the traditional value of 16.7 ms (i.e., $\text{1}\text{60}$ Hz). For t_i > 16.7 ms, the integrated response (q, coulombs) is plotted as the signal. This pulsed coulometric detection (PCD) is applied in a flow-injection system. For t_i = 500 ms, the detection limit with the instrumentation used is 1 μM (S/N = 2) for glucose which is a significant improvement on the value 35 μM found with PAD. The absolute detection limits for glucose and sucrose are ca. 50 pmol and 125 pmol, respectively, in 50-μl samples. Calibration plots (q_p vs. C^b) for PCD are linear over significantly larger dynamic ranges than those observed for PAD because of the lower detection limits.     

Sensitive detection of clozapine using a gold electrode modified with 16-mercaptohexadecanoic acid self-assembled monolayer

Clozapine, an effective antipsychotic drug, was found generating a pair of redox peaks at about 0.33-0.4 V (versus SCE) at 16-mercaptohexadecanoic acid (i.e. MHA) self-assembled monolayer (SAM) modified gold electrode (i.e. MHA/Au) in 0.05 mol L⁻¹ Tris-HCl (pH 8.1) buffer solution. Sensitive and quantitative measurement of clozapine based on anodic peak was established under optimum conditions. The anodic peak current was linear to clozapine concentration in the range from 1 × 10⁻⁶ to 5 × 10⁻⁵ mol L⁻¹ with the detection limit of 7 × 10⁻⁹ mol L⁻¹. This method was successfully applied to the detection of clozapine in drug tablets and proved to be reliable compared with ultraviolet spectrophotometry (UV). The MHA SAM was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), contact angle goniometry, electrochemical impedance spectroscopy (EIS) and electrochemical probe.     

2-巯基苯并噻唑自组装膜电极同时测定米吐尔和对苯二酚

在环境污染物中酚类物质由于其难降解和有毒性而引起人们普遍的关注。测定米吐尔和对苯二酚的常用方法有光度法、铈量法和电化学法。但在电化学法测定中较低倍含量的米吐尔会对对苯二酚的测定带来影响,所以同时准确测定较难。本文利用2-巯基苯并噻唑自组装电极对米吐尔和对苯二酚进行同时测定。应用于显影废水测定结果满意,方法简便快速,有实际应用前景。