Simultaneous detection of uric acid and glucose on a dual-enzyme chip using scanning electrochemical microscopy/scanning chemiluminescence microscopy

Scanning electrochemical microscopy (SECM) and scanning chemiluminescence microscopy (SCLM) were used for imaging an enzyme chip with spatially-addressed spots for glucose oxidase (GOD) and uricase microspots. For the SECM imaging, hydrogen peroxide generated from the GOD and/or uricase spots was directly oxidized at the tip microelectrode in a solution containing glucose and/or uric acid (electrochemical (EC) detection). For the SCLM imaging, a tapered glass capillary (i.d. of 1∼2 μm) filled with luminol and horseradish peroxidase (HRP) was used as the scanning probe for generating the chemiluminescence (CL). The inner solution was injected from the capillary tip at 78 pl s⁻¹ while scanning above the enzyme-immobilized chip. The CL generated when the capillary tip was scanned above the enzyme spots was detected using a photon-counter (CL detection). Two-dimensional mapping of the oxidation current and photon-counting intensity against the tip position affords images of which their contrast reflects the activity of the immobilized GOD and uricase. For both the EC and CL detections, the signal responses were plotted as a function of the glucose and uric acid concentrations in solution. The sensitivities for the EC and CL detection were found to be comparable.     

On the diffusion of ferrocenemethanol in room-temperature ionic liquids: an electrochemical study

The electrochemical behaviour of ferrocenemethanol (FcMeOH) has been studied in a range of room-temperature ionic liquids (RTILs) using cyclic voltammetry, chronoamperomery and scanning electrochemical microscopy (SECM). The diffusion coefficient of FcMeOH, measured using chronoamperometry, decreased with increasing RTIL viscosity. Analysis of the mass transport properties of the RTILs revealed that the Stokes-Einstein equation did not apply to our data. The "correlation length" was estimated from diffusion coefficient data and corresponded well to the average size of holes (voids) in the liquid, suggesting that a model in which the diffusing species jumps between holes in the liquid is appropriate in these liquids. Cyclic voltammetry at ultramicroelectrodes demonstrated that the ability to record steady-state voltammograms during ferrocenemethanol oxidation depended on the voltammetric scan rate, the electrode dimensions and the RTIL viscosity. Similarly, the ability to record steady-state SECM feedback approach curves depended on the RTIL viscosity, the SECM tip radius and the tip approach speed. Using 1.3 μm Pt SECM tips, steady-state SECM feedback approach curves were obtained in RTILs, provided that the tip approach speed was low enough to maintain steady-state diffusion at the SECM tip. In the case where tip-induced convection contributed significantly to the SECM tip current, this effect could be accounted for theoretically using mass transport equations that include diffusive and convective terms. Finally, the rate of heterogeneous electron transfer across the electrode/RTIL interface during ferrocenemethanol oxidation was estimated using SECM, and k(0) was at least 0.1 cm s(-1) in one of the least viscous RTILs studied.     

Electrochemistry in deep eutectic solvents

We report the cyclic voltammetry, chronoamperometry, and scanning electrochemical microscopy of ferrocene dissolved in deep eutectic solvents (DES), consisting of choline chloride (ChCl) and either trifluoroacetamide (TFA) or malonic acid as the hydrogen-bond donor. Despite the use of ultramicroelectrodes, which were required due to the modest conductivities of the DES employed, linear diffusion behavior was observed in cyclic voltammetric experiments. The high viscosity of 1:2 ChCl/TFA relative to non-aqueous electrochemical solvents leads to a low diffusion coefficient, 2.7 x 10(-8) cm2 s(-1) for ferrocene in this medium. Because of the difficulties in achieving steady-state conditions, SECM approach curves were tip velocity dependent. Under certain conditions, SECM approach curves to an insulating substrate displayed a positive-feedback response. Satisfactory simulation of this unexpected behavior was obtained by including convection terms into the mass transport equations typically used for SECM theory. The observance of positive-feedback behavior at an insulating substrate can be described in terms of a dimensionless parameter, the Peclet number, which is the ratio of the convective and diffusive timescales. Fitting insulator approach curves of ferrocene in 1:2 ChCl/TFA shows an apparent increase in the diffusion coefficient with increasing tip velocity, which can be explained by DES behaving as a shear thinning non-Newtonian fluid.     

The local electrochemical behavior of the AA2098-T351 and surface preparation effects investigated by scanning electrochemical microscopy

In this work, scanning electrochemical microscopy (SECM) measurements were employed to characterize the electrochemical activities on polished and as-received surfaces of the 2098-T351 aluminum alloy (AA2098-T351). The effects of the near surface deformed layer (NSDL) and its removal by polishing on the electrochemical activities of the alloy surface were evaluated and compared by the use of different modes of SECM. Confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) were also employed to characterize the morphology of the surfaces. The surface chemistry was analyzed by X-ray photoelectron spectroscopy (XPS). The surface generation/tip collection (SG/TC) and competition modes of the SECM were used to study hydrogen gas (H₂) evolution and oxygen reduction reactions, respectively. H₂ evolution and oxygen reduction were more pronounced on the polished surfaces. The feedback mode of SECM was adopted to characterize the electrochemical activity of the polished surface that was previously corroded by immersion in a chloride-containing solution, in order to investigate the influence of the products formed on the active/passive domains. The precorroded surface and as-received surfaces revealed lower electrochemical activities compared with the polished surface showing that either the NSDL or corrosion products largely decreased the local electrochemical activities at the AA2098-T351 surfaces.     

Electrochemical characterization of self-assembled thiol-porphyrin monolayers on gold electrodes by SECM.

Herein, the scanning electrochemical microscopy (SECM) approach is applied to study the formation of thiol-porphyrin self-assembled monolayer (SAMs). Using cyclic voltammetry (CV), the formation process is characterized adopting different probe molecules. The observed phenomena indicate that the formation process is affected by solution properties and the molecular structure of the probe molecules. In K(3)Fe(CN)(6) , the SAMs show a strong electron-transfer (ET) blocking effect on a pure porphyrin-modified electrode. However, addition of metal ions to the porphyrin molecules leads to ET. A consistent tendency is observed throughout the modification process using CV and SECM methods. Furthermore, k(eff) values, the apparent heterogeneous rate constants, obtained for different modification periods affirm the validity of these results. SECM images are used to collect surface information in the course of the modification process when the substrate potential is 0.5 V versus Ag/AgCl. The effect of the substrate potential indicates that the oxidation of the porphyrin molecules is supported by more positive potentials because of the similar bimolecular reaction of the porphyrin ring with positive charge and the probe molecules with negative charge.     

Scanning electrochemical microscopy study of ion transfer process across water/2-nitrophenyloctylether interface supported by hydrophobic carbon ceramic electrode

A carbon ceramic electrode (CCE) modified with the redox probe—decamethylferrocene solution in hydrophobic organic solvent—2-nitrophenyloctyl ether and immersed into an aqueous solution was studied by scanning electrochemical microscopy (SECM). After the electrochemical oxidation of decamethylferrocene, its cations were detected near the electrode surface in the aqueous phase. This indicates that some fraction of the redox-active cations electrochemically produced in the organic phase is transferred across the liquid/liquid interface. They are reduced at the SECM tip and form a solid deposit. The amount of deposited decamethylferrocene was estimated by the anodic reaction at the tip. It is affected by the substrate–tip distance, deposition time, and electrolyte concentration. The SECM images of unmodified and modified CCEs are consistent with their heterogeneous structure.     

Tip current/positioning close-loop mode of scanning electrochemical microscopy for electrochemical micromachining

Scanning electrochemical microscopy (SECM) has been approved as a prospective electrochemical micromachining (ECMM) technique soon after its birth. However, it still remains challenge for SECM to fabricate arbitrary three-dimensional (3D) microstructures because of the limitation of positioning system. To solve this problem, we proposed a tip current signal/positioning close-loop mode in which the tip current signal is fed back to the positioning system in order to program the motion trial of SECM tip. Both the triedge-cone and sinusoidal microstructures were obtained by the close-loop positioning mode. The static-state etching process was demonstrated not to be disturbed by the slow motion rate of SECM tip. The unique positioning mode would be significant for both ECMM and electrochemical imaging.     

Quantification of photoelectrogenerated hydroxyl radical on TiO(2) by surface interrogation scanning electrochemical microscopy

The surface interrogation mode of scanning electrochemical microscopy (SI-SECM) was used for the detection and quantification of adsorbed hydroxyl radical ˙OH((ads)) generated photoelectrochemically at the surface of a nanostructured TiO(2) substrate electrode. In this transient technique, a SECM tip is used to generate in situ a titrant from a reversible redox pair that reacts with the adsorbed species at the substrate. This reaction produces an SECM feedback response from which the amount of adsorbate and its decay kinetics can be obtained. The redox pair IrCl(6)(2-/3-) offered a reactive, selective and stable surface interrogation agent under the strongly oxidizing conditions of the photoelectrochemical cell. A typical ˙OH((ads)) saturation coverage of 338 μC cm(-2) was found in our nanostructured samples by its reduction with the electrogenerated IrCl(6)(3-). The decay kinetics of ˙OH((ads)) by dimerization to produce H(2)O(2) were studied through the time dependence of the SI-SECM signal and the surface dimerization rate constant was found to be ～k(OH) = 2.2 × 10(3) mol(-1) m(2) s(-1). A radical scavenger, such as methanol, competitively consumes ˙OH((ads)) and yields a shorter SI-SECM transient, where a pseudo-first order rate analysis at 2 M methanol yields a decay constant of k'(MeOH) ～ 1 s(-1).     

Diverse redox-active molecules bearing identical thiol-terminated tripodal tethers for studies of molecular information storage

To examine the effects of molecular structure on charge storage in self-assembled monolayers (SAMs), a family of redox-active molecules has been prepared wherein each molecule bears a tether composed of a tripodal linker with three protected thiol groups for surface attachment. The redox-active molecules include ferrocene, zinc porphyrin, ferrocene-zinc porphyrin, magnesium phthalocyanine, and triple-decker lanthanide sandwich coordination compounds. The tripodal tether is based on a tris[4-(S-acetylthiomethyl)phenyl]-derivatized methane. Each redox-active unit is linked to the methane vertex by a 4,4'-diphenylethyne unit. The electrochemical characteristics of each compound were examined in solution and in SAMs on Au. Redox-kinetic measurements were also performed on the SAMs (with the exception of the magnesium phthalocyanine) to probe (1) the rate of electron transfer in the presence of an applied potential and (2) the rate of charge dissipation after the applied potential is disconnected. The electrochemical studies of the SAMs indicate that the tripodal tether provides a more robust anchor to the Au surface than does a tether with a single site of attachment. However, the electron-transfer and charge-dissipation characteristics of the two tethers are generally similar. These results suggest that the tripodal tether offers superior stability characteristics without sacrificing electrochemical performance.     

Characterization of Fe/N-doped graphene as air-cathode catalyst in microbial fuel cells

This work proposed a simple and efficient approach for synthesis of durable and efficient non-precious metal oxygen reduction reaction(ORR) electro-catalysts in MFCs. The rod-like carbon nanotubes(CNTs)were formed on the Fe–N/SLG sheets after a carbonization process. The maximum power density of1210 ± 23 m W·m~(-2) obtained with Fe–N/SLG catalyst in an MFC was 10.7% higher than that of Pt/C catalyst(1080 ± 20 mW ·m~(-2)) under the same condition. The results of RDE test show that the ORR electron transfer number of Fe–N/SLG was 3.91 ± 0.02, which suggested that ORR catalysis proceeds through a four-electron pathway. The whole time of the synthesis of electro-catalysts is about 10 h, making the research take a solid step in the MFC expansion due to its low-cost, high efficiency and favorable electrochemical performance. Besides, we compared the electrochemical properties of catalysts using SLG, high conductivity graphene(HCG, a kind of multilayer graphene) and high activity graphene(HAG, a kind of GO) under the same conditions, providing a solution for optimal selection of cathode catalyst in MFCs.The morphology, crystalline structure, elemental composition and ORR activity of these three kinds of Fe–N/C catalysts were characterized. Their ORR activities were compared with commercial Pt/C catalyst.It demonstrates that this kind of Fe–N/SLG can be a type of promising highly efficient catalyst and could enhance ORR performance of MFCs.     

Imaging of enzyme activity by scanning electrochemical microscope equipped with a feedback control for substrate-probe distance

The enzymatic activity of diaphorase (Dp) immobilized on a solid substrate was characterized using a scanning electrochemical microscope (SECM) with shear force feedback to control the substrate-probe distance. The shear force between the substrate and the probe was monitored with a tuning fork-type quartz crystal and used as the feedback control to set the microelectrode probe close to the substrate surface. The sensitivity and the contrast of the SECM image were improved in the constant distance mode (distance, 50 nm) with the shear force feedback compared to the image in the constant height mode without the feedback. By using this system, the SECM and topographic images of the immobilized diaphorase were simultaneously measured. The microelectrode tip used in this study was ground aslant like a syringe needle in order to obtain the shaper topographic images. This shape was also effective for avoiding the interference during the diffusion of the enzyme substrates.     

Densely packed overlayer of iron phthalocyanine molecules grown on single-layer graphene

The FePc molecules form a series of order superstructures on single-layer graphene grown on Ru(0001) with increasing molecular coverage.     

基于环蕃类分子的铁氰化钾电化学存储和释放

通过利用合成的环蕃类化合物1与单壁碳纳米管(SWNTs)间的π-π共轭相互作用,将化合物1固定在SWNTs的表面,制备了1-SWNT修饰电极.利用化合物1氧化态和还原态与铁氰化钾分子之间不同强度的主客体相互作用,实现了铁氰化钾分子在1-SWNT修饰电极表面的电化学可控吸附和解吸.循环伏安和XPS实验结果表明,在本研究采用的实验条件下,铁氰化钾分子在电极表面20s内即可达到吸附平衡;当电极在0.70V下极化1000s后,大多数吸附的铁氰化钾可从电极表面解吸.基于此,制备了铁氰化钾的可控存储和释放的电化学器件,该器件不但可以重复进行铁氰化钾分子的存储和释放,而且多次重复操作表现出较好的稳定性和重现性.本研究在发展具有特殊用途的电化学纳米器件,例如分子搬运器、电化学开关等研究中具有重要意义.     

Scanning electrochemical microscope observation of defects in a hexadecanethiol monolayer on gold with shear force-based tip-substrate positioning

Scanning electrochemical microscopy (SECM) was used for imaging of n-hexadecanethiol-modified Au surfaces. In these studies, small defects were observed in the monolayer when a submicrometer electrode was used as an SECM tip, although a cyclic voltammogram of a Au disk electrode showed that the surface of the Au was completely covered with n-hexadecanethiol. The dependence of the SECM images on the potential of the Au electrode was also examined. A comparison of the current at the Au electrode and the tip current in the SECM images showed that direct electron transfer through the monolayer was dominant, rather than electron transfer at the defects. The size of the defects was estimated from the tip current to be 1-100 nm, under the assumption that the defects were small compared to the SECM probe.     

Novel coupling mechanism-based imaging approach to scanning electrochemical microscopy for probing the electric field distribution at the microchannel end

A novel coupling mechanism-based imaging approach to scanning electrochemical microscopy (SECM) was used to image the distribution of electric field at the end channel of a poly(dimethylsiloxane) (PDMS) capillary electrophoresis (CE) microchip in the absence of redox species. The coupling imaging mechanism was systematically investigated and qualitatively illustrated. It was proved that the distribution of solution potentials within the scanning plane caused a different reduction rate of water at the tip electrode, which led to the variation in tip current. Within the scanning plane, the solution potentials measured in the central area of the microchannel were usually higher than those measured outside. The SECM images showed a strong dependence on tip potential, tip-to-channel distance, and separation potential. According to the Tafel equation, SECM images were converted to parameters that directly showed the distribution of solution potential. Change in the solution potential along the central axial line of the microchannel was also continuously sensed by allowing the tip to approach the microchannel in the presence of high voltage. Using dopamine as a model compound, the effect of solution potential on electrochemical detection was estimated by detecting separation parameters.     

Electro‐oxidation and reduction of H2 on platinum studied by scanning electrochemical microscopy for the purpose of local detection of H2 evolution

Electrochemical detection of H₂ using scanning electrochemical microscopy (SECM) has shown to hold great promise as a sensitive characterization method with high spatial resolution for active surfaces generating H₂. Herein, the factors contributing to the current that is measured by SECM in generation/collection mode for H₂ detection are studied. In particular, the concentration gradient of H₂ at the substrate, the H₂/H⁺ recycling between the SECM tip and substrate and hemispherical profile of H₂ diffusion has been discussed. It was postulated that H₂/H⁺ recycling plays a dominant role in the oxidative current measured in generation/collection mode of SECM when the microelectrode is positioned in close vicinity of substrate. Copyright © 2015 John Wiley & Sons, Ltd.     

Direct Nanomachining on Semiconductor Wafer By Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) is one of the most important instrumental methods of modern electrochemistry due to its high spatial and temporal resolution. We introduced SECM into nanomachining by feeding the electrochemical modulations of the tip electrode back to the positioning system, and we demonstrated that SECM is a versatile nanomachining technique on semiconductor wafers using electrochemically induced chemical etching. The removal profile was correlated to the applied tip current when the tip was held stationary and when it was moving slowly (<20 μm s⁻¹), and it followed Faraday's law. Both regular and irregular nanopatterns were translated into a spatially distributed current by the homemade digitally controlled SECM instrument. The desired nanopatterns were “sculpted” directly on a semiconductor wafer by SECM direct-writing mode. The machining accuracy was controlled to the sub-micrometer and even nanometer scales. This advance is expected to play an important role in electrochemical nanomachining for 3D micro/nanostructures in the semiconductor industry.     

Local electron transfer rate measurements on modified and unmodified glassy carbon electrodes

A natural and artificial distribution of electron transfer activity on glassy carbon electrodes can be observed and quantified by the use of scanning electrochemical microscopy (SECM). A large (sevenfold) spread in rate constant is found for randomly sampled sites on polished, untreated glassy carbon surfaces. Direct-mode oxidation with the SECM tip was used to produce small regions of oxidized carbon on a polished surface. A large increase in electron transfer rate for the Fe(II/III) ion is observed on the locally oxidized carbon surface in comparison to the unoxidized region. Rate constant measurements made along a line profiles the transition from unoxidized to oxidized surfaces. SECM images of defect sites show reaction–rate variations. Rate constants measured at several locations of the defective surface allows discrimination between the kinetic and topographic components of the SECM image. Dedicated to the 80th birthday of Keith B. Oldham     

Scanning electrochemical microscopy coupled with intracellular standard addition method for quantification of enzyme activity in single intact cells

This paper uses scanning electrochemical microscopy (SECM) coupled with an intracellular standard addition method to quantify enzyme activity in single intact cells. In this work, peroxidase (PO) inside human neutrophils is chosen as the model system. Cells immobilized onto a silanized coverslip are perforated with digitonin to form micropores on the cell membrane. Hydroquinone (H(2)Q) and hydrogen peroxide (H(2)O(2)) as the enzyme substrates diffuse through the micropores into the cell interior. There, H(2)Q is converted into benzoquinone (BQ) by intracellular PO. BQ diffuses with a steady flux through the micropores from the cell interior onto the cell surface. The BQ near the cell surface is detected by the Au tip of SECM held at -0.3 V. When the tip is scanned laterally along the central line over the cell, a 2-D scan curve is obtained. Then, the intracellular standard addition method using ultramicroinjection with a submicrometer-sized micropipette tip is performed. After ultramicroinjection of a standard solution, another 2-D scan curve is recorded. The intercellular enzyme activity can be calculated from both peak current on two scan curves. This method to quantify PO activity in the cell environment has several obvious advantages: high sensitivity due to signal amplification via intracellular enzyme-catalyzed reaction and no sample dilution, no electrode fouling from adsorption of intracellular biological molecules, and no interference from electro-active compounds that can be directly oxidized at the SECM tip or from oxygen in the detected solution.     

Scanning Electrochemical Microscopy for Electrochemical Detection of Single‐base Mismatches by Tagging Ferrocenecarboxylic Acid as a Redox Probe to DNA

Scanning electrochemical microscopy (SECM) was employed for sensitive detection of single base mismatches (SBMs) in a sandwiched dsDNA. Ferrocenecarboxylic acid (Fc), covalently conjugated to the dsDNA, was oxidized to Fc⁺ via the DNA‐mediated charge transfer from the underlying gold substrate, and reduced back to Fc by SECM tip generated ferrocyanide. The electrocatalytic oxidation of SECM tip‐generated ferrocyanide was sensitive to presence, as well as the type of SBMs. Apparent standard rate constants (k⁰_app) values for different SBMs, both near the electrode surface and far from it, were evaluated by SECM. The method can detect SBMs independent of their position in dsDNA.