Gold Disk Research Articles

The development of micro–sized enzyme sensor based on direct electron transfer type open circuit potential sensing principle

One long term goal for biosensor development is towards implantable, long–term sensing for diagnostics, and monitoring of chronic health. The most common electrochemical sensors employ amperometric principles, although there is a fundamental challenge when trying to minimize the sensor size since the measurement current is directly proportional to the size of the working electrode. An alternative principle to amperometry for enzyme sensor is potentiometry, measuring open circuit potential (OCP). In this study, we demonstrate the size independency of an OCP based enzyme sensor focusing on direct electron transfer (DET) type– and quasi–DET–type oxidoreductases for, glucose and L–lactate, respectively. For glucose detection, we employed DET type flavin adenine dinucleotide glucose dehydrogenase, and the engineered L–lactate oxidase derived from Aerococcus viridans (AvLOx) conjugated with amine–reactive phenazine ethosulfate (arPES) was used as the quasi–DET type enzyme for the L–lactate detection. First, both enzymes were immobilized on 3 mm diameter gold disc electrode (GDE), and glucose and L–lactate were monitored based on OCP and transient potentiometric measurement, dOCP/dt. Then, L–lactate and glucose sensors were constructed using 10 µm GDEs, showing L–lactate and glucose concentration dependent OCP and dOCP/dt changes. The sensitivity was nearly equivalent for ΔOCP and dOCP/dt between the sensor using 3 mm and 10 µm GDE even though amperometric signal was entirely lost when measuring the arPES modified AvLOx on the 10 µm electrode. These results clearly demonstrate the size independency of an OCP based enzyme sensor which will be applied for the implantable and continuous monitoring system for metabolites in vivo.

Amperometric Detection of Ethanol Vapors by Screen Printed Electrodes Modified by Paper Crowns Soaked with Room Temperature Ionic Liquids

AbstractA convenient assembly recently proposed for screen printed gold electrodes (SPEs) suitable for measurements in gaseous samples is here tested for the analysis of the ethanol content in alcoholic drinks. This assembly involves the use of a circular crown of filter paper, soaked in the room temperature ionic liquid (RTIL) 1‐butyl‐3‐methylimidazolium hydrogen sulfate, which is simply placed upon a disposable screen printed cell, so as to contact the outer edge of the gold disc working electrode, as well as peripheral counter and reference electrodes. The electrical contact between the paper crown soaked in RTIL and the SPE electrode is assured by a gasket and all components are installed in a polylactic acid holder. This assembly provides a portable and disposable electrochemical platform, assembled by the easy immobilization onto a porous and inexpensive supporting material such as paper of a RTIL characterized by profitable electrical conductivity and negligible vapor pressure. The electroanalytical performance of this device was assayed for the flow injection analysis of the ethanol concentration in some real samples of wine and beer and the results obtained are compared with the alcoholic degree reported in the relevant bottle‐labels, thus highlighting a substantially satisfactory agreement. Repeatable sharp peaks (RSD=6–8 %) were detected for ethanol over a wide linear range (1–20 % v/v in water) and a detection and quantitation limit of 0.55 % v/v and 1.60 % v/v were inferred for a signal‐to‐noise ratio of 3 and 10, respectively.

EIS biosensor based on a novel Myoviridae bacteriophage SEP37 for rapid and specific detection of Salmonella in food matrixes

Recently, using bacteriophages as new molecular probes in reliable platforms for the detection of bacterial pathogens has attracted more and more increasing attentions. In this paper, a novel isolated Myoviridae bacteriophage SEP37 was covalently immobilized onto gold nanoparticles (AuNPs) modified gold disk electrode (GDE) surfaces using cysteamine (Cys) as a crosslinker. Substrates of GDE-AuNPs-Cys-Phage SEP37 and specific capture of Salmonella cells had been characterized using scanning electron microscopy (SEM) separately. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to study the electrochemical response of the biosensor interface manufacturing and bacterial capture process. Under the optimal experimental conditions, this phage-based EIS biosensor was able to detect Salmonella with a wide linear range from 2 × 101 to 2 × 106 colony forming unit (CFU)/mL within 30 min in spiked lake water and lettuce samples, with a limit of detection (LOD) of 17 CFU/mL. The detection linear range of spiked chicken samples was 2 × 102 to 2 × 105 CFU/mL, with a LOD of 1.3 × 102 CFU/mL. In combination with a pre-enrichment process for 3.5 h, this assay could reach a LOD of 1 CFU/mL in chicken breast meat samples. Besides, this phage-based EIS biosensor provided good reproducibility and stability. This phage-based EIS biosensor opens a new opportunity for the detection of pathogenic bacteria using the inherent selectivity of bacteriophage recognition.

Bibliometric analysis on nanoparticle modified screen-printed carbon electrode for E.coli detection using scopus database

Foodborne and waterborne infections are major causes of worry around the world. Escherichia coli (E. coli) pathogenic strains like O157:H7 are a leading cause of food and water-borne illness epidemics all over the world. E. coli is a pathogen present in the lower intestinal of warm-blooded species, it is used to detect fecal contaminants in specimens collected. Human infections can be caused by even low levels of E. coli O157:H7 (10–100 viable organisms). The standard techniques for food assessment are mainly conducted in centralized laboratories and involve long analysis times and complex instrumentation. Sensors based on screen-printed electrodes (SPEs) had grown in popularity in this context due to their favorable properties, such as ease of use and portability, which allow for quick analysis in point-of-need circumstances. The application of SPE-based sensors in the management of food contamination as beneficial analytical techniques to traditional techniques offers for quick inspection at any step in the food manufacturing process, reducing food waste and preventing foodborne infections. However, a screen-printed electrode (SPE) may not be as durable as a traditional electrode, such as a glassy carbon or gold disc, and the surface of its working electrode is not as perfect as that of a mirror-like polished solid electrode, the advantages of SPEs in terms of cost and size have led to an increase in their use as (bio) sensing transducers in recent years. The field of research for detection of E.coli using sensors is very extensive and actively studied by researchers in this area of interest. Here, publication for E.coli and nanoparticle sensor growth research were analyzed from Scopus databases. The keywords were biosensor, E.coli, screen printed electrode and nanoparticle reported from 2012 to 2021. This bibliometric analysis concluded that E.coli biosensors development involved multidisciplinary collaborations of biology, analytical chemistry, optoelectronics, material science, engineering and data science and each subject area plays an important role in establishing biosensors for E. coli detection.

Broadband terahertz metamaterial absorber and modulator based on hybrid graphene-gold pattern

We have numerically demonstrated a tunable broadband metamaterial absorber (MA) by introducing a graphene pattern and a gold disk, of which the bandwidth is up to 3.4 THz. When the chemical potential of graphene changes from 0 eV to 0.5 eV, the proposed MA structure can also be used as a modulator with the minimum and maximum modulate depth being about 69.5% and 92.5%, respectively. By tuning the relaxation time of the graphene from 0.1 ps to 0.5 ps, several obvious absorption peaks will occur in the broad and flat absorption band , which will deteriorate the broadband absorption characteristics and make it show a trend of multi-band absorption characteristics. To further increase the absorption bandwidth, a new graphene pattern and an additional gold disk are added into the proposed MA structure to form a new type of MA structure. Simulation result shows that the newly designed MA structure can achieve a wider absorption bandwidth of more than 4.0 THz. Meanwhile, it still has the performance of excellent modulation characteristic, polarization insensitive and wide tolerance of incident angles. All these optimal characteristics of the proposed MA structures will undoubtedly benefit the use of metamaterial absorber, high speed communication, terahertz detection devices, optoelectronic devices and so on. • Both graphene and gold patterns are used in the design of proposed metamaterial absorber. • The proposed structure can achieve broadband absorption with a bandwidth of 3.4 THz. • It has the characteristics of both polarization insensitivity and wide incident angles. • By adjusting the chemical potential of the graphene, dynamic tunability of the absorption performances can be achieved.

Thermally reconfigurable extraordinary terahertz transmission using vanadium dioxide

We numerically demonstrate a reconfigurable extraordinary terahertz transmission based on a phase-change material of vanadium dioxide ( V O 2 ). The proposed hybrid metasurface is composed of an array of subwavelength apertures perforated on a gold film. The holes are partially filled with annular V O 2 and gold disks to control the effective aperture area and the modes inside the aperture. Switching between the insulator and the metallic phase of V O 2 provides a convenient way to shift the transmission window. We present two designs offering redshift or blueshift of the extraordinary terahertz transmission. Upon phase transition from the insulator to the metallic phase, in the first design, the transmission peak redshifts from 1.02 to 0.82 THz while in the second design the transmission peak blueshifts from 0.71 to 0.77 THz. Furthermore, the transmission level and resonance frequency can be modulated by controlling the partial phase transition of the V O 2 . The potential applications for the proposed structures are terahertz modulators and reconfigurable filters.

In Vitro Continuous 3 Months Operation of Direct Electron Transfer Type Open Circuit Potential Based Glucose Sensor: Heralding the Next CGM Sensor.

While continuous glucose monitoring (CGM) systems allow precise and real-time blood glucose control, current electrochemicalbased CGM technologies inherently harbor enzyme instability issues. The direct electron transfer (DET) type open circuit potential (OCP) based enzyme sensing principle can minimize the catalytic turnover of the enzyme reaction, thereby providing longer-term operational stability in future CGM glucose sensors. DET-type OCP based glucose sensors were constructed using gold disk electrodes with glucose dehydrogenase capable of DET which was immobilized using a self-assembled monolayer (SAM). The single enzyme layer prepared on the gold electrode was operated in the presence of glucose, using in vitro buffer solution, continuously for over 3 months with the OCP sensor signal monitored every 10 seconds at 25°C. The DET-type OCP glucose sensor was continuously operated for more than 3 months without a significant decrease of the sensor signal and sensitivity (slope). These results suggest that the DET-type OCP glucose sensor is far more stable than the sensor constructed based on the amperometric principle. The long-term stability of DET-type OCP glucose sensor is attributed to the enzyme's minimized catalytic reaction during the operation, thereby extending the lifetime of enzyme. The DET-type OCP glucose sensor can be continuously operated for more than 3 months at 25 °C, in vitro without significant decreases in sensor signal and sensitivity. While the further investigation will be required for in vivo validation, the DET-type OCP glucose sensor is ideal for next generation CGM's, especially in long duration implantable use cases.

Chiral biosensing using terahertz twisted chiral metamaterial.

Subwavelength chiral metamaterials with tunable geometries and compositions are essential to advance the development of chiral biochemical samples detection. Here, we report a spatial symmetry breaking chiral terahertz (THz) metamaterial structure with stacked layers of L-shape arranged gold disks as the periodic unit cell. The chiroptical response can be adjusted on-demand by manipulating the number of stacking layers and the twisted angle of the periodic unit between adjacent array layers. We reveal that the chiroptical response originates from the optical resonances of the gold disks and the adjacent gold disks array layers via experiments and numerical simulation analysis. Furthermore, we find that this chiral metamaterial can realize label-free detection of proline in biological samples and label-free enantio-discrimination of chiral molecules. The change of the analyte concentration can also regulate the transmission circular dichroism (TCD) intensity of the chiral metamaterials. Our results not only provide new ideas into the design of functional chiral metamaterials, but also bring new strategies to develop chiroptical biosensing devices.

Multi-wavelength metamaterial absorber for retrieving complex refractive index of thin-film materials at infrared regimes

Multi-wavelength absorption in uncomplicated system is pursued continuously due to its advantage on measuring thin-film material dispersion. Here, we experimentally demonstrate a single-periodic metamaterial with polarization-insensitive multi-wavelength absorption for the infrared thin-film material measurement. The meta-atom of metamaterial absorber is constructed by holed cylindrical gold disk and gold ground separated by a ZnS substrate. Simulation shows that the multi-wavelength absorption is related to the multiple resonant eigenmodes. The metamaterial absorber is proposed through photolithography process and multi-wavelength absorptions occur at 3.95 μm, 4.27 μm and 10.90 μm, respectively, in the experiment. We show that the multi-wavelength absorption is very sensitive to the dielectric property of the coating thin-film material, which indicates the proposed absorber can be used to retrieve the complex refractive index of coating thin film at various wavelengths. Our work provides a convenient way to measure refractive indices at both mid-infrared and far-infrared regimes.

Experimental observation of ohmic impedance

Impedance spectroscopy measurements are often confounded by electrode-geometry-induced high-frequency phenomena associated with a frequency-dependent complex ohmic impedance. The ohmic impedance was obtained by Levenberg-Marquardt regression of a process model that used the Havriliak-Negami equation to account for the ohmic impedance. Experimental results obtained for the ferri/ferrocyanide redox species on a gold disk used as a model system were interpreted using a physical description expressed in terms of an ohmic impedance and an expression that accounted for the influence of a local constant-phase element on the faradaic reaction and the convective diffusion impedance. Regressions were weighted by the experimentally determined error structure of the data, and the resulting parameters characteristic of the ohmic impedance were in excellent agreement with numerical simulations.

Improving the Thermosphere Ionosphere in a Whole Atmosphere Model by Assimilating GOLD Disk Temperatures

AbstractGlobal‐Scale Observations of Limb and Disk (GOLD) disk measurements of far ultraviolet molecular nitrogen band emissions are used to retrieve column integrated disk temperatures (Tdisk), which are representative of the lower‐and‐middle thermosphere. The present work develops a new approach to assimilate the Tdisk in the whole atmosphere community climate model with thermosphere‐ionosphere eXtension using the data assimilation research testbed ensemble adjustment Kalman filter. Nine days of data, 1–9 November 2018, are assimilated. Analysis state variables such as thermospheric effective temperature (Teff, airglow layer integrated temperature), ratio of atomic oxygen to molecular nitrogen column densities (O/N2), and column electron content are compared with a control simulation that is only constrained up to ∼50 km. It is observed that assimilation of the GOLD Tdisk improves the analysis states when compared with the control simulation. The analysis and model states, particularly, Teff, O/N2, and electron column density (ECD) are compared with their measurement counterparts for a validation of the assimilation. Teff and O/N2 are compared with GOLD Tdisk and O/N2. While, the ECD is compared with ground based total electron content measurements from global navigational satellite system receivers. Root mean square error (RMSE) improvements in Teff and O/N2 are about 10.8% and 22.6%, respectively. The RMSE improvement in analyses ECD is about 10% compared to the control simulation.

Substrate influence on the vibrational response of gold nanoresonators: Towards tunable acoustic nanosources

The development of applications based on metallic nano-objects requires an extensive knowledge of their vibrational properties. Its size, shape, materials, or even crystallinity have a strong influence on the vibrational frequencies of a nano-object. It is also known that the coupling with environment has a strong effect on the lifetime of its resonating modes. However, the impact of the contact between a nano-object and its substrate still requires further investigation. Here we report on the vibrational response of gold disks for different contact area with a silicon substrate. The investigation of the quality factor of the first thickness mode of the disks highlights an important energy transfer from the disks to the substrate. The impact of the size of the contact area on the frequencies of the radial eigenmodes is also reported both experimentally and numerically. Finally, it is shown that such objects can be excited by acoustic waves propagating from the substrate. These findings display the possibility to use nanodisks to develop acoustic microscopy at the nanoscale.

Multiple Fano resonances in gold split ring disk dimers

A plasmonic dimer composed of a gold split ring and a gold disk is proposed. The scattering spectra and electromagnetic field distributions on the surface of the dimer are calculated by the finite difference time domain method, and the effects of split gap orientation and structural symmetry breaking of the ring on Fano resonance characteristics are theoretically investigated in detail. The results show the multiple Fano resonances that can be formed due to the destructive interference between the electric dipole mode of the disk and multiple multipolar modes of the split ring, and the red shift when the symmetry breaking of the split ring is broken. When the split gap of the ring is parallel to the interparticle axis of the dimer, multiple Fano resonances based on the odd-order mode and the even-order mode of the split ring can be generated, and more even-order Fano resonances can be formed due to the further symmetry breaking of the split ring. In addition, a more refined scattering spectrum can be obtained as the ring internal surface moves far away from the disk along the interparticle axis. On the other hand, Fano resonances based on the lower order multipolar modes of the ring can be increased as they move away from the split gap. When the split gap of the ring is perpendicular to the interparticle axis of the dimer, only the even-order Fano resonances can be excited, and these resonances increase with the ring internal surface going away from the disk no matter whether the split gap of the ring faces the disk or not. As the structural symmetry of the dimer is further broken due to the ring internal surface moving along the split gap direction, the odd-order Fano resonance can be successfully produced in the dimer with the split gap coming back to the disk, at the same time, the even-order Fano resonances are gradually weaken perhaps due to the complicated competitive behaviors of spectral overlapping between the dipole mode of the disk and multipolar mode of the ring in energy. However, there is no odd-order Fano resonance appearing in the dimer with the split gap facing the disk except for a slightly increased even-order Fano resonance, as the structural symmetry of the dimer is further broken. These results are expected to be useful in designing multiple Fano controllable split ring disk dimers and also developing the application of multiwavelength micro-nano photonics.

Highly Efficient Multi-Step Oxidation Bioanode Using Microfluidic Channels.

With the rapid decline of fossil fuels, various types of biofuel cells (BFCs) are being developed as an alternative energy source. BFCs based on multi-enzyme cascade reactions are utilized to extract more electrons from substrates. Thus, more power density is obtained from a single molucule of substrate. In the present study, a bioanode that could extract six electrons from a single molecule of L-proline via a three-enzyme cascade reaction was developed and investigated for its possible use in BFCs. These enzymes were immobilized on the electrode to ensure highly efficient electron transfer. Then, oriented immobilization of enzymes was achieved using two types of self-assembled monolayers (SAMs). In addition, a microfluidic system was incorporated to achieve efficient electron transfer. The microfluidic system, in which the electrodes were arranged in a tooth-shaped comb, allowed for substrates to be supplied continuously to the cascade, which resulted in smooth electron transfer. Finally, we developed a high-performance bioanode which resulted in the accumulation of higher current density compared to that of a gold disc electrode (205.8 μA cm−2: approximately 187 times higher). This presents an opportunity for using the bioanode to develop high-performance BFCs in the future.

Periodic metal resonator chains for surface enhanced Raman scattering (SERS)

A periodic arrangement of chains of gold disks shows pronounced plasmonic grating resonances. These have a clear impact on the surface enhanced Raman-scattering (SERS) signal from 4-methylbenzenethiol molecules, which form self-assembled monolayers on the gold surface: Besides a clear polarization dependence, the SERS-spectra also exhibit a maximum when the excitation laser wavelength matches the plasmonic grating resonance. These features are explained by a combined near and far field coupling of the individual plasmonic dipoles allowing the design of optimized nanostructures for effective SERS substrates in the future.

Surface-enhanced mid-infrared absorption spectroscopy using miniaturized-disc metasurface

Surface-enhanced infrared spectroscopy is an important technique for improving the signal-to-noise ratio of spectroscopic material identification measurements in the mid-infrared fingerprinting region. However, the lower bound of the fingerprinting region receives much less attention due to a scarcity of transparent materials, more expensive sources, and weaker plasmonic effects. In this paper, we present a miniaturized metasurface unit cell for surface-enhanced infrared spectroscopy of the 15-upmum vibrational band of CO_{2}. The unit cell consists of a gold disc, patterned along the edge with fine gaps/wires to create a resonant metamaterial liner. In simulation, our plasmonic metamaterial-lined disc achieves greater than 4times the average field intensity enhancement of a comparable dipole array and a miniaturized size of lambda _0/5 using complex, 100-nm features that are patterned using 100-kV electron-beam lithography. In a simple experiment, the metamaterial-lined disc metasurface shows a high tolerance to fabrication imperfections and enhances the absorption of CO_{2} at 15 upmum. The resonant wavelength and reflection magnitude can be tuned over a wide range by adjusting the liner feature sizes and the metasurface array pitch to target other vibrational bands. This work is a step toward low-cost, more compact on-chip integrated gas sensors.

Arsenic(III) Stripping Electroanalysis Enhanced by AC Electrokinetics at Hot Microelectrodes

Inorganic arsenic is regarded as a human carcinogen and is highly toxic to the environment. In this work, we investigated the detection of As(III) at a 25 μm bare gold disk microelectrode polarized by a high-frequency (ca. 100 MHz) alternating current (hot microelectrode). Square wave voltammetry method was employed to determine low arsenic concentration (nanomolar range), which was below the concentration limit set by the US Environmental Protection Agency (130 nM). Nitric acid was used as a supporting electrolyte; a deposition potential was −0.6 V and the deposition time 120 s. A linear relationship from 0 to 60 nM was observed and the sensitivity of detection was 0.0064 nA nM−1 with a limit of detection (LOD) of 8.8 nM, which can be further lowered by increasing the deposition time. For a real-sample analysis, filtered tap water was spiked with known concentrations of arsenic, and a calibration curve was obtained.

A numerical study on the closed packed array of gold discs as an efficient dual mode plasmonic tweezers

In this report, we propose the closed pack array of gold discs on glass, as a dual mode plasmonic tweezers that benefits from two trapping modes. The first trapping mode is based on leaky surface plasmon mode (LSPM) on the gold discs with a longer penetration depth in the water and a longer spatial trapping range, so that target nanoparticles with a radius of 100 nm can be attracted toward the gold surface from a vertical distance of about 2 µm. This trapping mode can help to overcome the inherent short range trapping challenge in the plasmonic tweezers. The second trapping mode is based on the dimer surface plasmonic mode (DSPM) in the nano-slits between the neighboring gold discs, leading to isolated and strong trapping sites for nanoparticles smaller than 34 nm. The proposed plasmonic tweezers can be excited in both LSPM and DSPM modes by switching the incident wavelength, resulting in promising and complementary functionalities. In the proposed plasmonic tweezers, we can attract the target particles towards the gold surface by LSPM gradient force, and trap them within a wide half widthhalfmaximum (HWHM) that allows studying the interactions between the trapped particles, due to their spatial proximity. Then, by switching to the DSPM trapping mode, we can rearrange the particles in a periodic pattern of isolated and stiff traps. The proposed plasmonic structure and the presented study opens a new insight for realizing efficient, dual-mode tweezers with complementary characteristics, suitable for manipulation of nanoparticles. Our thermal simulations demonstrate that the thermal-induced forces does not interefe with the proposed plasmonic tweezing.

Gold Nanoparticles; Synthesis, Characterization and Comparative Studies of Their Antimicrobial Activities

Gold nanoparticles of various morphologies have been synthesized using different synthetic approaches. Sonochemical synthesis results in the formation of 25 nm sized gold nano discs and 30 nm sized polyhedral structures with surface area of 179.5 m2/g and 150.5 m2/g respectively[1]. The UV-Visible spectrum shows the SPR peaks due to in plane and out of plane excitation modes. Microemulsion synthesis using Tergitol NP-9 surfactant resulted in the production of hexagonal gold nanoparticles (avg. size: 20 nm) at 0.05 M HAuCl4 (aq) and spherical morphologies with an average size of 25 nm and 15 nm at molar concentrations of 0.04 M and 0.03 M HAuCl4 (aq) respectively[2]. Gold nanoparticles show the surface plasmon resonance peaks at 540 nm and 580 nm along with peaks corresponding to higher order plasmon resonance modes at shorter wavelengths. The surface area was found to increase with decrease in particle size. An ecofriendly synthetic route was also employed to synthesize gold nanoparticles using candida albicans fungal cell free extract[3]. Gold nanocrystals of 5 nm have been obtained with specific surface area of 18.9 m2/g. The encapsulation of gold nanoparticle by biomolecules of the cell free extract was confirmed by FTIR and TGA studies. Gold nanoparticle show surface plasmon resonance (SPR) peak at 540 nm. Gold nanoparticles have been tested for antimicrobial activity against fungal and bacterial species. Gold nano discs displayed higher antimicrobial action against fungus candida albicans as compared to gold polyhedral structures by inhibiting H+-ATPase activity. Biosynthesized gold nanoparticles showed good antimicrobial activity against Staphylococcus aureus and Escherichia coli. However, Escherichia coli, a gram-negative bacterium was found to be more susceptible to gold nanoparticles as compared to Staphylococcus aureus, a gram-positive bacterium. The microemulsion synthesized gold nanoparticles were tested against Candida albicans both alone and in combination with fluconazole. The gold nanoparticles in combination with fluconazole showed maximum inhibition and the antifungal activity increased with decrease in particle size.

In Situ Differential Reflectance Spectroscopy Monitoring the Asynchronous Transient Response at the Edge and Center of the Disk Electrode

The resistance for the current flow to a disk electrode in an electrochemical cell is a function of radius according to the pioneer work by Newman,(1) which results in the varying RC constant along the electrode surface. Theoretical calculations by Oldhan showed a spatial dispersion of the RC constant from zero at the edge to 2 RC at the center for the disk electrode.(2) In this work, the asynchronous transient responses at the edge and center of a gold disk electrode are monitored by in situ differential reflectance spectroscopy (DRS). The DRS spectrum collected in 10mM perchloric acid shows that the edge of the electrode achieves the equilibrium sooner than the center does in chronoamperometry.The cyclic voltammetry of the gold electrode sweeping at 1V/s in 0.1M HClO4 electrolyte and the corresponding reflectance spectrum are shown in panels A and B, Figure 1. The DRS intensity is defined asΔR/R=|Rs-Rref|/Rref whereis Rs the intensity of reflectance at any potential and Rref is the intensity of reflectance at the reference potential (0.4V). Overall, ΔR/R varies in the range of 2.5% while scanning the potential of the electrode. In the forward scan, ΔR/R decreases linearly with potential from 0.4 to 1.3V. A sharper decrease starts at 1.3V, i.e., the onset of gold oxidation, until the end of gold oxidation reaction at 1.5V in the backward scan. ΔR/R resumes back to 0 at 0.4V after the reduction of gold oxide, indicating the surface is recovered back to the initial state. ΔR/R at 1 V and 0.4V are chosen as Rs and Rref in the chronoamperometry since only the double layer charging/discharging occurs, i.e., no redox reactions, resembling a circuit of capacitor and resistor.Shown in Figure 2 is the chronoamperometry and in situ DRS of the gold electrode collected in a more resistive electrolyte, 10 mM HClO4. The RC constant of the electrode is determined to be 68 ms by fitting the i-t curve (Panel A) with the exponential function. The reflectance spectrum without any smoothing is shown in Panel B, Figure 2. The RC constant at the center (red curve) is 142 ms, which is comparable to 2RC. The RC constant at the edge (black curve) is 51 ms, much smaller than that at the center. RC constants at the edge in both 0.1M and 10mM HClO4 electrolytes are not zero because the laser covers a large area of the electrode. J. Newman, J Electrochem Soc, 113, 501 (1966).K. B. Oldham, Electrochem Commun, 6, 210 (2004). Figure 1