Ferrocenylalkylthiolates as a probe of heterogeneity in binary self-assembled monolayers on gold

The oft-cited complexity of tethered ferrocene electrochemistry in single component (FcRS-) or binary (FcRS-/CH3R'S-) self-assembled monolayers (SAMs) on gold has been investigated. The complex voltammetry is shown to be linked to local electrostatics caused by the formation of the ferrocenium ion. This conclusion is reached by studying model effects in binary SAMs, where a cationic alkylthiolate (H3N+ C11S-Au) is mixed with FcC12S-Au. A fitting procedure involving both a Gaussian and a Lorentzian distribution is used for deconvolution of the two peaks which are consistently observed in the SAMs when chi(Fc)surf > or = 0.2. The lower-potential (E degrees ' = 250 mV) and higher-potential (E degrees ' = 350 mV) voltammetric peaks are assigned to Fc moieties in "isolated" and "clustered" states, respectively. Use of this method to better understand SAM structure is demonstrated by distinguishing the degree of homogeneity in two binary SAMs of similar composition.     

Ferrocenylalkylthiolate labeling of defects in alkylthiol self-assembled monolayers on gold

Coverage defects in alkylthiol self-assembled monolayers (SAMs) are critically important to function related to electron transfer from soluble redox probes. There is therefore a need for an accurate and direct measurement of the number and type of coverage defect in a range of SAMs. Ferrocenyldodecanethiol (FcC(12)SH) has been assessed as an electrochemically-addressable label of coverage defects. It is shown that short time exposure of a SAM to FcC(12)SH leads to a quantifiable Fc coverage (Gamma(Fc)), with Gamma(Fc) < 1% readily measurable. The voltammetric signature of FcC(12)SH label is also able to differentiate types of defect in a given SAM. A number of SAM preparation conditions are assessed for the density and type of coverage defect. This labeling method therefore will be a useful tool for research into SAM property-function relationships.     

Electrochemical Oxidative Formation and Reductive Desorption of a Self-Assembled Monolayer of Decanethiol on a Au(111) Surface in KOH Ethanol Solution

Electrochemical oxidative adsorption and reductive desorption of a self-assembled monolayer (SAM) of decanethiol on a Au(111) single crystal electrode were examined in 0.1 M KOH ethanol solution containing various concentrations of decanethiol ranging from 1 muM to 1 mM. Anodic and cathodic current peaks corresponding to the adsorption and desorption of decanethiol, respectively, were observed in cyclic voltammograms of a Au(111) single crystal electrode obtained in 0.1 M KOH ethanol solution containing more than 10 muM of decanethiol. Positions of both peaks depended on the concentration of decanethiol, and they shifted negatively by ca. 0.057 V/decade with increase in decanethiol concentration. This result confirms that the adsorption and desorption of decanethiol is a one-electron process. The reductive charge, which consists of desorption charge and capacitive charge, increased when the sweep rate was decreased and the decanethiol concentration was increased and reached the saturated value of 103 (+/-5%) muC cm-2, which corresponds to the reductive charge of thiol SAM of full coverage with a ( radical3 x radical3)R30 degrees structure. Potentiostatic SAM formation was also investigated by holding the potential at +0.1 V. The reductive charge, i.e., the coverage of the SAM, increased with time and reached the saturated value of 103 (+/-5%) muC cm-2, corresponding to full coverage, after holding the potential at +0.1 V for a certain period of time. The time when the amount of adsorbed thiolate reached full coverage depended on the concentration of decanethiol. The higher the concentration was, the faster full coverage was reached. The desorption peak shifted negatively as the holding time at +0.1 V was increased even after the adsorbed amount had reached full coverage. These results suggest that the ordering of decanethiol SAMs requires a much longer time than the time required for full coverage adsorption. The position of the reductive desorption peak was independent of the thiol concentration if the electrode was kept at +0.1 V for long enough so that a highly ordered SAM was formed. The cathodic peak shifted negatively as the sweep rate was increased, showing that reductive desorption of the SAM was rather slow. The rate constant for the reductive desorption was determined from the potential dependent peak shift to be 0.24 s-1, which is in good agreement with the value obtained for a SAM prepared without potential control, indicating that the quality of the electrochemically prepared SAM is as good as that of the SAM prepared nonelectrochemically.     

Selective reductive desorption of a SAM-coated gold electrode revealed using fluorescence microscopy

The reductive desorption of a self-assembled monolayer (SAM) of a fluorescent thiol molecule (BodipyC10SH) from Au was characterized using electrochemistry and epi-fluorescence microscopy. Molecular luminescence is quenched near a metal surface, so fluorescence was only observed for molecules reductively desorbed and then separated from the electrode surface. Fluorescence imaging showed that reductive desorption was selective, with desorption occurring from different regions of the Au electrode depending on the extent of the negative potential excursion. When desorbed, the molecules were sufficiently mobile, diffusing away from the electrode surface, thereby preventing oxidative readsorption. At sufficiently negative desorption potentials, all of the thiol was desorbed from the electrode surface, resulting in fluorescence at the air/solution interface. The selective removal of the thiol monolayer from distinct regions was correlated to features on the electrode surface and was explained through potential-dependent interfacial energies. This in situ electrofluorescence microscopy technique may be useful in sensor development.     

Ferrocenylundecanethiol self-assembled monolayer charging correlates with negative differential resistance measured by conducting probe atomic force microscopy

Electrical and mechanical properties of metal-molecule-metal junctions formed between Au-supported self-assembled monolayers (SAMs) of electroactive 11-ferrocenylundecanethiol (FcC(11)SH) and a Pt-coated atomic force microscope (AFM) tip have been measured using a conducting probe (CP) AFM in insulating alkane solution. Simultaneous and independent measurements of currents and bias-dependent adhesion forces under different applied tip biases between the conductive AFM probe and the FcC(11)SH SAMs revealed reversible peak-shaped current-voltage (I-V) characteristics and correlated maxima in the potential-dependent adhesion force. Trapped positive charges in the molecular junction correlate with high conduction in a feature showing negative differential resistance. Similar measurements on an electropassive 1-octanethiol SAM did not show any peaks in either adhesion force or I-V curves. A mechanism involving two-step resonant hole transfer through the occupied molecular orbitals (MOs) of ferrocene end groups via sequential oxidation and subsequent reduction, where a hole is trapped by the phonon relaxation, is proposed to explain the observed current-force correlation. These results suggest a new approach to probe charge-transfer involving electroactive groups on the nanoscale by measuring the adhesion forces as a function of applied bias in an electrolyte-free environment.     

Total and free charge densities on mercury coated with self-assembled phosphatidylcholine and octadecanethiol monolayers and octadecanethiol/phosphatidylcholine bilayers

The thermodynamic ‘total’ charge density is the charge to be supplied to the electrode to keep the applied potential constant when the electrode surface is increased by unity, while the extrathermodynamic ‘free’ charge density is the charge actually experienced by the diffuse layer ions. The total charge density at dioleoylphosphatidylcholine (DOPC) and octadecanethiol (ODT) monolayers and mixed ODT/DOPC bilayers self-assembled on mercury from aqueous solutions was determined from chronocoulometric single potential steps to a final potential negative enough to cause complete desorption of the film. The effect of different alkali metal ions and of tetramethylammonium on DOPC desorption was examined. The total charge for ODT monolayers and ODT/DOPC bilayers, +56±3 μC cm⁻², agrees with the value obtained by integration of the current under the reductive desorption voltammetric peaks, only provided the scan rate is higher than 100 mV s⁻¹. An approximate model of the interface of the ODT-coated electrode, which accounts for partial charge transfer from sulfur to mercury and for the degree of dissociation of the sulfhydryl group upon self-assembly, was employed to estimate the free charge density.     

Electrochemical studies on the interaction of heparin with crystal violet and its analytical application

In this paper, crystal violet (CV) was used to determine heparin concentration by linear sweep voltammetry on a dropping mercury electrode (DME). In Britton-Robinson (B-R) buffer solution, pH 3.0, CV had a well-defined second-order derivative linear sweep voltammetric reductive wave at −0.74 V (vs. SCE). After the addition of heparin to the CV solution, the reductive peak current decreased greatly with the positive movement of the peak potential and without appearance of new peaks in the scanning potential range. Based on the decrease in the reductive peak current, a new voltammetric method for the determination of heparin was established. The conditions for the interaction and the electrochemical detection were optimized, and interfering substances were investigated. Under the optimal conditions, the decrease in reductive peak currents of CV was proportional to heparin concentration in the range 0.1–8.0 mg/L with the linear regression equation Δip″(nA) = 400.42 + 1563.11c (mg/L), (n = 14, γ = 0.993). The detection limit was 0.092 mg/L. This new method was further successfully applied to the determination of heparin content in heparin sodium injection samples with satisfactory results. The binding ratio and binding mechanism were also studied by the electrochemical method. The text was submitted by the authors in English.     

The [Ru(CN)5(pyS)](4-) complex, an efficient self-assembled monolayer for the cytochrome c heterogeneous electron transfer studies

The organothiol 4-mercaptopyridine (pyS) has been used extensively as facilitator for the assessment of heterogeneous electron transfer reaction of cytochrome c (cyt c). Its efficiency, however, is strongly affected by the instability of the adlayer due to the C-S bond cleavage. The K(4)[Ru(CN)(5)(pyS)].3H(2)O complex was synthesized and characterized aiming its utilization as an inorganic self-assembled monolayer (SAM) that would enhance the gold adlayer stability. The SAM formed by this complex onto gold (RupySAu) was characterized by spectroscopic (FTIRRAS and SERS) and electrochemical (LSV) techniques. The ex situ vibrational SERS and FTIRRAS spectra data of this SAM formed onto gold suggest a sigma interaction between the gold and sulfur atoms of the complex, inducing a perpendicular arrangement in relation to the surface normal. Additionally, SERS and FTIRRAS spectra performed for freshly prepared RupySAu adlayer and for large immersion times in the precursor solution have not shown any significant change that would reflect the degradation of the adlayer. The LSV desorption curves of this SAM indicate an enhancement in the C-S bond strength of the pyS ligand when coordinated to the [Ru(CN)(5)](3-) moiety. Comparatively to the data obtained for the desorption process of the pyS monolayer, the reductive desorption potential, E(rd), of the RupySAu presents a shift of -17 mV. This bond strength intensification leads to an increase in the stability of the monolayer. The voltammetric curves of cyt c carried out with the RupySAu electrode showed electrochemical parameters consistent with those reported for the native protein, as well as the maintenance of the electrochemical kinetic data after repetitive cycles. The results all together suggest that the pi back-bonding effect from the [Ru(CN)(5)](3-) metal center plays an important role in the stability of the RupySAu adlayer, improving the assessment of the cyt c heterogeneous electron transfer reaction.     

Monolayer assemblies of a de novo designed 4-alpha-helix bundle carboprotein and its sulfur anchor fragment on Au(111) surfaces addressed by voltammetry and in situ scanning tunneling microscopy

Mapping and control of proteins and oligonucleotides on metallic and nonmetallic surfaces are important in many respects. Electrochemical techniques based on single-crystal electrodes and scanning probe microscopies directly in aqueous solution (in situ SPM) have recently opened perspectives for such mapping at a resolution that approaches the single-molecule level. De novo design of model proteins has evolved in parallel and holds promise for testing and controlling protein folding and for new tailored protein structural motifs. In this report we combine these two strategies. We present a scheme for the synthesis of a new 4-alpha-helix bundle carboprotein built on a galactopyranoside derivative with a thiol anchor aglycon suitable for surface immobilization on gold. The carboprotein with thiol anchor in monomeric and dimeric (disulfide) form, the thiol anchor alone, and a sulfur-free 4-alpha-helix bundle carboprotein without thiol anchor have been prepared and investigated for comparison. Cyclic and differential pulse voltammetry (DPV) of the proteins show desorption peaks around -750 mV (SCE), whereas the thiol anchor desorption peak is at -685 mV. The peaks are by far the highest for thiol monomeric 4-alpha-helix bundle carboprotein and the thiol anchor. This pattern is supported by capacitance data. The DPV and capacitance data for the thiolated 4-alpha-helix bundle carboproteins and the thiol anchor hold a strong Faradaic reductive desorption component as supported by X-ray photoelectron spectroscopy. The desorption peak of the sulfur-free 4-alpha-helix bundle carboprotein, however, also points to a capacitive component. In situ scanning tunneling microscopy (in situ STM) of the thiol anchor discloses an adlayer with small domains and single molecules ordered in pin-striped supramolecular structures. In situ STM of thiolated 4-alpha-helix bundle carboprotein monomer shows a dense monolayer in a broad potential range on the positive side of the desorption potential. The coverage decreases close to this potential and single-molecule structures become apparent. The in situ STM contrast is also strengthened, indicative of a new redox-based tunneling mechanism. The data overall suggest that single-molecule mapping of natural and synthetic proteins on well-characterized surfaces by electrochemistry and in situ STM is within reach.     

Partial stripping of Ag atoms from silver bilayer on a Au(111) surface accompanied with the reductive desorption of hexanethiol SAM

The interfacial structures of Ag bilayer prepared by underpotential deposition on Au(111) (Ag(2ML)/Au(111)) were determined by ex situ scanning tunneling microscopy and in situ surface X-ray scattering measurements before and after oxidative adsorption and after reductive desorption of a self-assembled monolayer (SAM) of hexanethiol (C₆SH) in alkaline ethanol solution. While no structural change was observed after oxidative formation of C₆SH SAM on the Ag(2ML)/Au(111) in an ethanol solution containing 20 mM KOH and 0.1 mM C₆SH, some of the Ag atoms in the bilayer were stripped when the SAM was reductively desorbed. Dedicated to Professor J. O’M. Bockris on the occasion of his 85th birthday.     

Polycrystalline Gold Electrodes: A Comparative Study of Pretreatment Procedures Used for Cleaning and Thiol Self‐Assembly Monolayer Formation

The influence of different surface pretreatment procedures on the electrochemical response of a polycrystalline gold electrode was evaluated. Mechanical polishing with slurry alumina (M), chemical oxidation with H₂SO₄/H₂O₂ (C), electrochemical polishing (potential cycling between ?0.1 V and 1.2 V vs. SCE) (E), chemical reduction with ethanol, and combinations among these treatments were employed to change the surface electrode characteristics. The efficiency of the proposed pretreatments was evaluated by electrochemical responses towards the redox couple ferri(II/III)‐ammonium sulfate and by the formation of a self‐assembly monolayer of 3‐mercaptopropionic acid (3 MPA SAM) on gold electrodes. The procedure (C) allowed important gold surfaces activation. Using procedures (C) and (E) the roughness of polycrystalline gold surfaces was significantly minimized and more reproducible surfaces could be obtained. From the profile of reductive desorption of 3 MPA SAM it was possible to verify that reduced gold surfaces generated better packed monolayers than oxidized ones and a comparative study using CV and DPV techniques showed that between the two desorption peaks, the one localized at more negative potential values corresponds to the cleavage of Au‐S bond from the chemisorbed thiol. In general, the improvement in the studied electrochemical responses could not only be attributed to an increase in the real surface area of the electrode, but to the chemical surface states set off by the pretreatment procedure.     

Microdetermination of double-stranded DNA by linear sweep voltammetry with phenosafranine.

A novel voltammetric method for the determination of microamounts of fish sperm double-stranded (ds) DNA based on its interaction with phenosafranine (PSF) is proposed in this paper. In a pH 3.5 Britton-Robinson (B-R) buffer solution, PSF had a well-defined second-order derivative linear-sweep voltammetric reductive peak at -0.32 V (vs. SCE) on a mercury electrode. After the addition of dsDNA into the PSF solution, the reductive peak current decreased significantly without a shift of the peak potential, and no new peak appeared. The experiment results showed that a new supramolecular complex was formed after the interaction of dsDNA with PSF, which resulted in a decrease of the diffusion coefficient, and then a decrease of the reductive peak current. The interaction conditions and the electrochemical detection conditions were carefully investigated. Under the optimal conditions, the decrease of the peak current was proportional to the dsDNA concentration in the range 1.0 - 40.0 microg/mL with the linear regression equation DeltaI(p)'(nA) = 32.59C(microg/mL) - 4.03 (n = 13, gamma = 0.998) and a detection limit of 0.25 microg/mL (3 sigma). The interaction mechanism was considered based on the aggregation of the dsDNA-PSF supramolecular complex; the stoichiometry of this supramolecular complex was calculated based on voltammetric data with a binding number of 3 and a binding constant of 2.76 x 10(12). This method was successfully applied to the determination of synthetic samples and the polymerase chain reaction (PCR) product of the nopaline synthase gene (NOS) DNA from genetically modified organisms (GMOs) with satisfactory results.     

铁氰化钆修饰电极的制备及表征

应用电化学循环扫描法于玻碳电极表面沉积并形成铁氰化钆修饰电极(GdHCF/GC),扫描电镜(SEM)显示,有两种大小和外形明显不同的颗粒状GdHCF附着在电极表面.红外光谱表明,GdCHF的C≡N弯曲振动吸收峰出现在2062.5 cm-1处.循环伏安法测试表明,在0.2 mol/L NaC l溶液中,GdHCF/GC电极出现两对氧化还原峰,扫速为20 mV/s时,其氧化还原峰的式量电位分别为E0’(I)=192.5 mV和E0’(II)=338.5 mV.研究了不同支持电解质对GdHCF/GC电极电化学性能的影响,GdHCF对Na+离子有优先选择性.     

Electron and ion transfer through multilayers of gold nanoclusters covered by self-assembled monolayers of alkylthiols with various functional groups

The electrochemical characteristics of various kinds of multilayers of gold nanoclusters (GNCs) were investigated. Two types of gold nanoclusters, one covered by self-assembled monolayers (SAMs) of mercaptoundecanoic acid (MUA), hexanethiol (C6SH), and ferrocenylhexanethiol (FcC6SH), MHF-GNC, and the other with MUA and C6SH, MH-GNC, were used. The multilayers were constructed on a Au(111) surface based on a carboxylate/metal cation (Cu++)/carboxylate or carboxylate/cationic polymer (poly(allylamine hydrochloride):PAH)/carboxylate electrostatic interaction. While the multilayers constructed by the former method were stable only in nonaqueous solutions, those constructed by the latter method were stable even in aqueous solutions. Electrochemical measurements of the multilayers of MHF-GNCs showed a pair of waves corresponding to the redox of the ferrocene group around 350-480 mV and the charge of these peaks, i.e., the amount of adsorbed GNC, increased linearly with the construction cycle up to 6 cycles in the former and to 18 cycles in the latter. A rather reversible redox response of the ferrocene moiety was observed even at the gold electrodes with five GNC layers of two different sequences in which MHF-GNC exists as the layer closest to the gold electrode, ie., the first layer, or as the outermost layer with MH-GNC in the other layers. These results show the facile transfer of electrons and ions through the multilayers of the SAM-covered GNCs and electron transfer between the ferrocene moiety and the Au(111) electrode takes place through the GNC cores by hopping.     

Ideal nonideality in adsorption of 2-aminoethanethiol and 2-mercaptoethane sulfonic acid to form electrostatically stabilized binary self-assembled monolayers on Au(111)

In formation of binary self-assembled monolayers (SAMs) composed of 2-aminoethanethiol (AET) and 2-mercaptoethane sulfonic acid (MES) by adsorption from an ethanol solution on Au(111), the adsorption shows nearly ideal nonideality in that the surface ratio of MES to AET in the SAM is unity and does not depend on the mixing ratio of MES to AET in the bathing ethanol solution used for preparing SAMs, chi(soln)MES, over the wide range of chi(soln)MES between 0.01 and 0.95. X-ray photoelectron spectroscopy confirms that at least 80% of AET molecules adsorbed are protonated in this range of chi(soln)MES, indicating that the electrostatic interaction between positively charged AET and negatively charged MES is responsible to the observed nonideality. Correspondingly, there appears only one cathodic peak in a linear-sweep voltammogram of the reductive desorption of the SAM, having a narrow full width at half-maximum of about 20 mV. This suggests the presence of strong lateral attractive interaction between the adsorbed thiolates.     

Rigid molecular tripod with an adamantane framework and thiol legs. Synthesis and observation of an ordered monolayer on Au111

Tripod-shaped trithiols 1-3, containing CH2SH groups at the three bridgehead positions of the adamantane framework and a halogen-containing group [Br (1), p-BrC6H4 (2), or p-IC6H4 (3)] at the fourth bridgehead, were synthesized, and self-assembled monolayers (SAMs) were prepared on atomically flat Au111 surfaces. The three-point chemisorption of these tripods was confirmed by polarization modulation infrared reflection absorption spectroscopy, which showed the absence of a S-H stretching band. Scanning tunneling microscopy of the SAM of 1 exhibited a hexagonal arrangement of the adsorbed molecule with a lattice constant of 8.7 angstroms. A unidirectionally oriented, head-to-tail array of 1, which allows the close approach of neighboring molecules, is proposed as a reasonable model of the two-dimensional crystal, where the adsorbed sulfur atoms form a quasi-(radical3 x radical3)R30 degrees lattice. The charge of the electrochemical reductive desorption of the SAM of 1 was in good agreement with the expected surface coverage, while the SAMs of 2 and 3 showed somewhat less (ca. 70%) charge. The large negative reduction peak potentials, observed for the SAM of 1, are taken to indicate a tight anchoring of this tripod by three sulfur atoms.     

Adsorption energies, inter-adsorbate interactions, and the two binding sites within monolayer benzene on Ag(111)

The adsorption of monolayer and multilayer benzene on the Ag(111) surface was characterized using temperature programmed desorption (TPD). TPD spectra revealed two broad peaks at approximately 205 and approximately 150 K at submonolayer coverage and a sharper, multilayer peak at 140 K. Analysis of the coverage-dependent shape and shift of the two submonolayer peaks has resulted in their assignment to desorption from two different binding geometries on threefold-hollow sites with symmetries C(3v)(sigma d) and C(3v)(sigma v). The TPD peak analysis incorporated inter-adsorbate repulsive interaction that resulted from the local dipole moment at the adsorption site induced by the adsorbate-surface charge transfer bonding. The analysis has yielded desorption energies of 54.9 +/- 0.8 and 50.4 +/- 0.4 kJ/mol for the C(3v)(sigma d) and C(3v)(sigma v) configurations, respectively. The interface dipole and polarizability of the benzene-silver complex have been determined to be 5.4 +/- 1.8 D and 14 +/- 10 A3, respectively. Repulsive interactions in the monolayer were found to lower the desorption energy from the zero-coverage value by 14.8 kJ/mol. Leading edge analysis of the multilayer peak yielded a desorption energy of 40.9 +/- 0.7 kJ/mol.     

Fluorescence imaging of the oxidative desorption of a BODIPY-alkyl-thiol monolayer coated Au bead

The reductive and oxidative desorption of a BODIPY labeled alkylthiol self-assembled monolayer (SAM) on Au was studied using electrochemical methods coupled with fluorescence microscopy and image analysis procedures to monitor the removal of the adsorbed layer. Two SAMs were formed using two lengths of the alkyl chain (C10 and C16). The BODIPY fluorescent moiety used is known to form dimers which through donor-acceptor energy transfer results in red-shifted fluorescence. Fluorescence from the monomer and dimer were used to study the nature of the desorbed molecules during cyclic step changes in potential. The reductive desorption was observed to occur over a small potential window (0.15 V) signified by an increase in capacitance and in fluorescence. Oxidative readsorption was also observed through a decrease in capacitance and a lack of total removal of the fluorescent layer. Removal by oxidative desorption occurred at positive potentials over a broad potential range near the oxidation of the bare Au. The resulting fluorescence showed that the desorbed molecules remained near the electrode surface and were not dispersed over the 20 s waiting time. The rate of change of the fluorescence for oxidative desorption was much slower than the reductive desorption. Comparing monomer and dimer fluorescence intensities indicated that the dimer was formed on the Au surface and desorbed as a dimer, rather than forming from desorbed monomers near the electrode surface. The dimer fluorescence can only be observed through energy transfer from the excited monomer suggesting that the monomers and dimers must be in close proximity in aggregates near the electrode. The fluorescence yield for longer alkyl chain was always lower presumably due to its decreased solubility in the interfacial region resulting in a more efficient fluorescence quenching. The oxidative desorption process results in a significantly etched or roughened electrode surface suggesting the coupling of thiol oxidative removal and Au oxide formation which results in the removal of Au from the electrode.     

在抗坏血酸存在下用L-赖氨酸修饰玻碳电极测定多巴胺

采用电化学氧化法制备了L-广赖氮酸单分子层修饰玻碳电极,研究了多巴胺（DA）和抗坏血酸（AA）在该电极上的电化学行为。结果表明,L-广赖氨酸单分子层修饰玻碳电极不仅能改善多巴胺和抗坏血酸的电化学行为,而且能将多巴胺和抗坏血酸二者在裸电极上的完全重叠的单氧化峰分开成为两个完全独立的氧化峰,循环伏安（CV）图上峰间距为507mV,差分脉冲伏安（DPV）图上峰间距为460mV,由此可实现在AA的共存下对样品中的DA进行选择性测定。     

The redox chemistry of the covalently immobilized native and low-pH forms of yeast iso-1-cytochrome c

Cyclic voltammetry experiments were carried out on native Saccharomyces cerevisiae iso-1-cytochrome c and its C102T/N62C variant immobilized on bare polycrystalline gold electrode through the S-Au bond formed by a surface cysteine. Experiments were carried out at different temperatures (5-65 degrees C) and pH values (1.5-7). The E degrees ' value at pH 7 (+370 mV vs SHE) is approximately 100 mV higher than that for the protein in solution. This difference is enthalpic in origin and is proposed to be the result of the electrostatic repulsion among the densely packed molecules onto the electrode surface. Two additional electrochemical waves are observed upon lowering the pH below 5 (E degrees ' = +182 mV) and 3 (E degrees ' = +71 mV), which are attributed to two conformers (referred to as "intermediate" and "acidic", respectively) featuring an altered heme axial ligation. This is the first determination of the reduction potential for low-pH conformers of cytochrome c in the absence of denaturants. Since the native form of cytochrome c can be restored, bringing back the pH to neutrality, the possibility offered by this transition to reversibly modulate the redox potential of cytochrome c is appealing for bioelectronic applications. The immobilized C102T/N62C variant, which differs from the native protein in the orientation of the heme group with respect to the electrode, shows very similar reduction thermodynamics. For both species, the rate constant for electron transfer between the heme and the electrode increases for the acidic conformer, which is also found to act as a biocatalytic interface for dioxygen reduction.