Au Microelectrodes Research Articles

Enhanced Electrochemiluminescence at the Gas/Liquid Interface of Bubbles Propelled into Solution.

Electrochemiluminescence (ECL) is typically confined to a micrometric region from the electrode surface. This study demonstrates that ECL emission can extend up to several millimeters away from the electrode employing electrogenerated chlorine bubbles. The mechanism behind this bubble-enhanced ECL was investigated using an Au microelectrode in chloride-containing and chloride-free electrolyte solutions. We discovered that ECL emission at the gas/solution interface is driven by two parallel effects. First, the bubble corona effect facilitates the generation of hydroxyl radicals capable of oxidizing luminol while the bubble is attached to the surface. Second, hypochlorite generated from chlorine sustains luminol emission for over 200 s and extends the emission range up to 5 mm into the solution, following bubble detachment. The new approach can increase the emission intensity of luminol-based assays 5-fold compared to the conventional method. This is demonstrated through a glucose bioassay, using a midrange mobile phone camera for detection. These findings significantly expand the potential applications of ECL by extending its effective range in time and space.

A home-made nanoporous gold microsensor for lead(II) detection in seawater with high sensitivity and anti-interference properties.

A nanoporous gold microelectrode (NPG-μE) was fabricated and used for Pb(II) detection in seawater samples via square wave anodic stripping voltammetry (SWASV). The Au microelectrode (Au-μE) was fabricated by embedding a gold microfiber into a Pasteur pipette, and its surface was further modified by an anodization-electrochemical reduction (A-ECR) method, yielding the NPG-μE. The fabricated electrodes were characterized by cyclic voltammetry (CV) and field emission scanning electron microscopy (FE-SEM) for electrochemical and structural morphological investigations. SWASV results show a Pb(II) stripping peak at around -0.05 V vs. Ag/AgCl, sat. KCl, which is unusual for common Pb(II) detection (typically occurring at around -0.40 V) in anodic stripping voltammetry (ASV) analysis. The Pb(II) detection at less negative stripping potential is more beneficial. Hence, it exhibited anti-interference properties with Cd(II), which is attributed to the preferential deposition and stripping of the target analyte on the low-indexed crystal planes of the NPG structure. The calibration plot obtained by SWASV was linear in the concentration range of 0.1-10 μM, and the detection limit was found to be 57 nM (correlation coefficient of 0.9974). The NPG microsensor presented a 15-fold enhanced current response compared to Au-μE, with excellent sensitivity (27.2 μA μM-1 cm-2). The application of the NPG microsensor was examined by detecting Pb(II) in seawater samples, and a satisfactory performance was obtained.

Monolayer of microgel particles based on poly(acrylic acid) modified with dopamine for electrochemical pH sensing

A novel electroactive and pH-responsive microgel was successfully synthesized. The obtained microgel was based on acrylic acid, dopamine methacrylamide and N,N′-bis(acryloyl)cystamine as the crosslinker. Using the distillation precipitation polymerization method, spherical, smooth-surface microgel particles of ca. 400 nm in diameter were obtained. The presence of dopamine groups in the polymer network was confirmed with cyclic voltammetry and NMR spectroscopy. The dependence of microgel hydrodynamic diameter on pH was investigated using the DLS technique. The presence of -S–S- groups in the microgel particle was used to modify the Au electrode surface by chemisorption. The attachment was monitored with the QCM-D technique. The obtained microgel was successfully anchored to the Au regular- and microelectrode surfaces. The electrochemical properties of the modified electrodes were promising. It was found that modified electrodes exhibited potentially useful pH-sensing properties. Modification of Au microelectrode surface with the environmentally sensitive and electroactive microgel monolayer is a novel, innovative approach.

Au Micro‐ and Nanoelectrodes as Local Voltammetric pH Sensors During Oxygen Evolution at Electrocatalyst‐Modified Electrodes

The scarcity of state‐of‐the‐art oxygen evolution reaction (OER) electrocatalysts has led to intensive research on alternative viable electrocatalytic materials. While activity and cost are the main factors to be sought after, the catalyst stability under harsh acidic conditions is equally crucial. Considering that OER is a proton‐coupled electron‐transfer reaction that involves local acidification of the reaction environment by liberation of H+, the catalyst stability can be largely compromised in such conditions. Consequently, probing the pH value near the catalyst surface under operation leads to a deeper understanding of this process. The applicability of bare Au microelectrodes and nanoelectrodes as sensitive local pH probes during OER is shown in this work by using scanning electrochemical microscopy (SECM). Two case studies are presented, including the state‐of‐the‐art OER catalyst (IrO2) in acidic media and a ZnGa2O4 catalyst in alkaline buffered solution, demonstrating the suitability of the Au probe to accurately determine the local pH value in a wide pH range.

Carbon nanowall-based gas sensors for carbon dioxide gas detection

Carbon nanowalls (CNWs) have attracted significant attention for gas sensing applications due to their exceptional material properties such as large specific surface area, electric conductivity, nano- and/or micro-porous structure, and high charge carrier mobility. In this work, CNW films were synthesized and used to fabricate gas sensors for carbon dioxide (CO2) gas sensing. The CNW films were synthesized using an inductively-coupled plasma (ICP) plasma-enhanced chemical vapor deposition (PECVD) method and their structural and morphological properties were characterized using Raman spectroscopy and electron microscopy. The obtained CNW films were used to fabricate gas sensors employing interdigitated gold (Au) microelectrodes. The gas sensors were fabricated using both direct synthesis of CNW films on interdigitated Au microelectrodes on quartz and also transferring presynthesized CNW films onto interdigitated Au microelectrodes on glass. The CO2 gas-sensing properties of fabricated devices were investigated for different concentrations of CO2 gas and temperature-ranges. The sensitivities of fabricated devices were found to have a linear dependence on the concentration of CO2 gas and increase with temperature. It was revealed that devices, in which CNW films have a maze-like structure, perform better compared to the ones that have a petal-like structure. A sensitivity value of 1.18% was obtained at 500 ppm CO2 concentration and 100 °C device temperature. The CNW-based gas sensors have the potential for the development of easy-to-manufacture and efficient gas sensors for toxic gas monitoring.

Ultrasensitive detection of dopamine using Au microelectrodes integrated with mesoporous silica thin films.

An electrochemical method was developed for ultrasensitive and selective detection of dopamine in human serum using mesoporous silica thin film modified gold microelectrodes. Vertically aligned mesoporous silica thin films were deposited onto Au microelectrodes by electrochemically assisted self-assembly (EASA). The mesochannels have uniform pore sizes of 2.1 nm in diameter and a negatively charged wall surface. Cyclic voltammetry reveals effective charge permselectivity through the negatively charged mesoporous channels. By using differential pulse voltammetry, the mesoporous silica thin film modified Au microelectrode can be employed for the ultrasensitive detection of dopamine with a detection limit as low as 0.084 μM. In addition, thanks to the electrostatic and steric effects of the silica mesochannels, excellent anti-interference and anti-fouling properties of the electrochemical sensors are demonstrated.

Selective Deposition of N‐Heterocyclic Carbene Monolayers on Designated Au Microelectrodes within an Electrode Array (Small 2/2024)

Selective Deposition of <i>N</i>‐Heterocyclic Carbene Monolayers on Designated Au Microelectrodes within an Electrode Array (Small 2/2024)

Evaluation of the Applicability of Voltammetric Modes in Scanning Electrochemical Microscopy for In Situ Corrosion Characterisation of Copper-Based Materials

Chemical imaging of corrosion processes involving copper species using scanning electrochemical microscopy has been hampered by the lack of soluble oxidation states for copper that can be achieved by amperometric conversion at the tip. Indeed, the only possibility is to reduce the corrosion products at the tip, thus modifying the chemical response of the electrode material and requiring subsequent redissolution of the copper deposits. Consequently, the limitations arising from the system prevented a full-scale quantification, requiring the development of new methodologies or the optimisation of those currently available, as we pursued with the present work. Therefore, the voltammetric behaviours of gold macro- and microelectrodes were evaluated with respect to the collection and redissolution of Cu2+ ions, with the aim of using them as sensing probes in scanning electrochemical microscopy (SECM) to investigate the activity of copper surfaces in acidic chloride-containing environments. Cyclic and square-wave voltammetric techniques were explored for copper collection and subsequent stripping on Au microelectrode tips in SECM with the objective to capture in situ image electrochemical reactivity distributions across copper surfaces undergoing corrosion.

Selective Deposition of N-Heterocyclic Carbene Monolayers on Designated Au Microelectrodes within an Electrode Array.

The incorporation of organic self-assembled monolayers (SAMs) in microelectronic devices requires precise spatial control over the self-assembly process. In this work, selective deposition of N-heterocyclic carbenes (NHCs) on specific electrodes within a two-microelectrode array is achieved by using pulsed electrodeposition. Spectroscopic analysis of the NHC-coated electrode arrays reveals that each electrode is selectively coated with a designated NHC. The impact of NHC monolayers on the electrodes' work function is quantified using Kelvin probe force microscopy. These measurements demonstrate that the work function values of each electrode can be independently tuned by the adsorption of a specific NHC. The presented deposition method enables to selectively coat designated microelectrodes in an electrode array with chosen NHC monolayers for tuning their chemical and electronic functionality.

Electrochemical Determination of Iron(III) in River Water by Differential Pulse Voltammetry (DPV) Using a Poly(3,4-Ethylenedioxythiophene)–Polystyrene Sulfonate co-Polymer/Gold Nanoparticle Modified Gold Microelectrode

An electrochemical sensor based upon a poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT-PSS) co-polymer film with gold nanoparticles (AuNPs) modified microelectrode is reported for the determination of Fe(III). Scanning electron microscopy images showed that the AuNPs were immobilized on gold (Au) microelectrode by formation of a nanofilm with PEDOT-PSS. The individual particles and their cluster-like aggregates provide a larger active electrochemical area and higher conductivity, which facilitates the determination of Fe(III). Electrochemical impedance spectroscopy (EIS) shows the AuNPs/PEDOT-PSS/Au microelectrode possesses high conductivity. A low detection limit of 0.01 µM Fe(III) was obtained using the modified microelectrode by differential pulse voltammetry (DPV) due to its large specific surface area, high conductivity, and the presence of more active sites. The optimized pH of the supporting acetate buffer was 2.0. The AuNPs/PEDOT-PSS/Au provided a linear range from 0.0119 μM to 7.14 μM. The modified electrode was successfully employed for the the determination of total dissolved iron in river water from Three Gorges with satisfactory results.

Extended-Gate Field-Effect Transistor Consisted of a CD9 Aptamer and MXene for Exosome Detection in Human Serum.

Cancer progresses silently to the terminal stage of the impossible operable condition. There are many limitations in the treatment options of cancer, but diagnosis in an early stage can improve survival rates and low recurrence. Exosomes are the biomolecules released from cancer cells and are promising candidates for clinical diagnosis. Among them, the cluster of differentiation 9 (CD9) protein is an important exosomal biomarker that can be used for exosome determination. Therefore, here, a CD9 aptamer was first synthesized and applied to an extended-gate field-effect transistor (EGFET)-type biosensor containing a disposable sensing membrane to suggest the possibility of detecting exosomes in a clinical environment. Systematically evaluating ligands using the exponential enrichment (SELEX) technique was performed to select nucleic acid sequences that can specifically target the CD9 protein. Exosomes were detected according to the electrical signal changes on a membrane, which is an extended gate using an Au microelectrode. The fabricated biosensor showed a limit of detection (LOD) of 10.64 pM for CD9 proteins, and the detection range was determined from 10 pM to 1 μM in the buffer. In the case of the clinical test, the LOD and detection ranges of exosomes in human serum samples were 6.41 × 102 exosomes/mL and 1 × 103 to 1 × 107 exosomes/mL, respectively, showing highly reliable results with low error rates. These findings suggest that the proposed aptasensor can be a powerful tool for a simple and early diagnosis of exosomes.

A Novel Salivary Sensor with Integrated Au Electrodes and Conductivity Meters for Screening of Diabetes.

The rise in diabetes cases is a growing concern due to the aging of populations. This not only places a strain on healthcare systems but also creates serious public health problems. Traditional blood tests are currently used to check blood sugar levels, but they are invasive and can discourage patients from regularly monitoring their levels. We recently developed nano-sensing probes that integrate Au microelectrodes and conductivity meters, requiring only 50 μL of saliva for measurement. The usage of the co-planar design of coating-free Au electrodes makes the measurement more stable, precise, and easier. This study found a positive correlation between the participant's fasting blood sugar levels and salivary conductivity. We observed a diabetes prevalence of 11.6% among 395 adults under 65 years in this study, using the glycated hemoglobin > 6.5% definition. This study found significantly higher salivary conductivity in the diabetes group, and also a clear trend of increasing diabetes as conductivity levels rose. The prediction model, using salivary conductivity, age, and body mass index, performed well in diagnosing diabetes, with a ROC curve area of 0.75. The study participants were further divided into low and high groups based on salivary conductivity using the Youden index with a cutoff value of 5.987 ms/cm. Individuals with higher salivary conductivity had a 3.82 times greater risk of diabetes than those with lower levels, as determined by the odds ratio calculation. In conclusion, this portable sensing device for salivary conductivity has the potential to be a screening tool for detecting diabetes.

Preparation of boron-doped diamond microelectrodes to determine the distribution size of platinum nanoparticles using current transient method

Boron-doped diamond (BDD) microelectrodes were prepared to investigate the correlation of hydrazine oxidation current responses with Pt nanoparticle (Pt NP) size distribution. The BDD film was grown on the surface of a tungsten needle with a diameter of 25 µm. An average particle size of around 5 µm BDD crystalline was successfully synthesized using a microwave plasma-assisted chemical vapor deposition technique. The Raman spectrum confirmed the presence of diamond formation as indicated by peaks corresponding to C-C sp3 bonds, while X-ray photoelectron spectroscopy spectrum showed the presence of C-H and C-OH bonds on the surface of the BDD microelectrode. Meanwhile the Pt nanoparticles was synthesized through reduction reaction of PtCl62- solution using NaBH4 with citric acid as the capping agent. Particles size between 4.46 to 4.78 nm were observed by using TEM measurements. The BDD microelectrodes were utilized to investigate the relationship between Pt nanoparticle size distribution and the current generated from the oxidation reaction of 15 mM hydrazine in a 50 mM phosphate buffer solution pH 7.4 in the presence of 1.0 mL nanoparticle solutions. A current range of 5 and 6 nA with a noise level of 0.15 nA was observed showing a good correlation with the particle sizes of Pt NPs. Comparison was also performed with the measurements using Au microelectrodes, indicated that the prepared BDD microelectrodes is promising for the measurements of nanoparticle sizes distribution, especially Pt NPs.

Iridium Oxides Based Potentiometric Sensor for pH Monitoring in Biological Samples

The fabrication and long-term application of a pH Au microelectrode based on an iridium oxide film are reported. A uniform iridium oxide film with a typical thickness of around 1 µm was coated on the microelectrode surface through a 2-step procedure involving electrodeposition at constant potential and further continuous voltammetric scans. A super-Nernstian slope of around 77 mV per pH unit was found from open circuit potential measurements in a broad pH range (2 to 10) in 0.01 mol L-1 phosphate buffer. It was demonstrated experimentally that the short-term pH precision of the IrOx sensor is ± 0.1 pH. The response stability was maintained in the physiological pH range, and the sensor exhibited excellent reproducibility, long-term stability, and a short response time of &lt; 10 s. The results reported in this work confirmed that iridium oxide showed very promising pH sensing performance and can serve as an electrode material for detecting local pH changes in samples of increased complexity, such as juice fruits, culture medium, synthetic urine, and blood.

SECM investigation on pH changes in cellular environment induced by caffeine

This work shows the results of the fabrication and application of a potentiometric microsensor for real-time pH monitoring in the vicinity of C2C12 cell line muscle cells using Scanning Electrochemical Microscopy (SECM). This was achieved by electrodeposition of an iridium oxide film onto the surface of a dual sensor containing two microelectrodes (Au and Pt). The Au microelectrode was coated with an iridium oxide film and used as a potentiometric pH sensor. The sensor exhibited a super Nernstian response (around 70 mV pH−1), and its precise positioning above a single C2C12 cell was accomplished by means of an approach curve recorded with the Pt microelectrode. pH changes were induced by changing the cellular metabolism through the addition of caffeine to the culture medium, and an average decrease of up to 0.5 units in pH was noticed by measuring the pH at 10 µm from the cell surface. Much smaller pH changes were observed in control experiments, confirming that caffeine induces acidosis and that such a phenomenon can be monitored using the fabricated dual sensor coupled to SECM.

Influence of Charged Self-Assembled Monolayers on Single Nanoparticle Collision.

Studying nanoparticle (NP)-electrode interactions in single nanoparticle collision events is critical to understanding dynamic processes such as nanoparticle motion, adsorption, oxidation, and catalytic activity, which are abundant on electrode surfaces. Herein, NP-electrode electrostatic interactions are studied by tracking the oxidation of AgNPs at Au microelectrodes functionalized with charged self-assembled monolayers (SAMs). Tuning the charge of short alkanethiol-based monolayers and selecting AgNPs that can be partially or fully oxidized upon impact enabled probing the influence of attractive and repulsive NP-electrode electrostatic interactions on collision frequency, electron transfer, and nanoparticle sizing. We find that repulsive electrostatic interactions lead to a significant decrease in collision frequency and erroneous nanoparticle sizing. In stark difference, attractive electrostatic interactions dramatically increase the collision frequency and extend the sizing capability to larger nanoparticle sizes. Thus, these findings demonstrate how NP-monolayer interactions can be studied and manipulated by combining nanoimpact electrochemistry and functionalized SAMs.

Correlation of hierarchical porosity in nanoporous gold with the mass transport of electron transfer-coupled-chemical reactions

Optimization of mass transfer within a porous material is a highly promising strategy to improve the efficiency of electrode reactions. Herein, nanoporous gold (NPG) modified gold electrode is investigated to study the mass transport of ascorbic acid (AA), which oxidation process is characterized by an electrochemical coupled 1st order chemical reaction, commonly termed as EC1 reaction. The template-assisted synthesis of NPG results into the formation of a highly pure and porous film of gold. However, the surface porosity of NPG depends on the choice of electrodeposition parameters, such as deposition time (td) and potential (Ed) and the size of the substrate. Such porosity variation of NPG strongly influences the voltammetric profile of AA anodic reaction, displaying sigmoidal, non-symmetric and symmetric peak features. The analysis of mass transport behaviour of AA reveals a combination of diffusion and thin layer EC1 mechanism, predominance of which is determined by the Ed and td selected for NPG synthesis, as well as the size of the Au substrate. The mass transport of AA on NPG prepared on Au microelectrodes experienced a significant diffusion from bulk solution, owing to the larger pores, which permits the easier exchange of redox species between the NPG volume and the bulk solution. On the contrary, mass transport of AA on NPG deposited on big Au electrode has a significant contribution of thin layer diffusion, attributed to the smaller surface pores of NPG, which limits the exchange of AA and its oxidized form from the bulk solution.

High performance multi-purpose nanostructured thin films by inkjet printing: Au micro-electrodes and SERS substrates.

Nanostructured thin metal films are exploited in a wide range of applications, spanning from electrical to optical transducers and sensors. Inkjet printing has become a compliant technique for sustainable, solution-processed, and cost-effective thin films fabrication. Inspired by the principles of green chemistry, here we show two novel formulations of Au nanoparticle-based inks for manufacturing nanostructured and conductive thin films by using inkjet printing. This approach showed the feasibility to minimize the use of two limiting factors, namely stabilizers and sintering. The extensive morphological and structural characterization provides pieces of evidence about how the nanotextures lead to high electrical and optical performances. Our conductive films (sheet resistance equal to 10.8 ± 4.1 Ω per square) are a few hundred nanometres thick and feature remarkable optical properties in terms of SERS activity with enhancement factors as high as 107 averaged on the mm2 scale. Our proof-of-concept succeeded in simultaneously combining electrochemistry and SERS by means of real-time tracking of the specific signal of mercaptobenzoic acid cast on our nanostructured electrode.

In-situ synthesis of ZnO onto nanoporous gold microelectrode for the electrochemical sensing of arsenic(III) in near-neutral conditions

The exploration of advanced electrochemical sensors towards highly toxic arsenic(III) has attracted growing interest in recent times. Herein, a ZnO decorated nanoporous Au (NPG) microelectrode was fabricated via a facile electrochemical alloying/dealloying method followed by thermal treatment, and developed for the electrochemical sensing of toxic As(III). Inspired by the electrochemical deposition of Zn and the formation of Au-Zn nanostructure on the surface of the Au microelectrode during the alloying/dealloying process, the in-situ evolution of the formed Au-Zn nanostructure to ZnO nanoparticles wrapped on the nanoporous Au surface was achieved. The ZnO/NPG microelectrode exhibited excellent electrochemical performance towards As(III) detection in near-neutral conditions (0.1 M phosphate buffer solution (PBS), pH: 5.0). A high sensitivity of 1.366 μA·ppb−1·cm−2 with a low limit of detection of 0.30 ppb (S/N = 3) was achieved at concentrations ranging from 1.0 ppb to 260 ppb. Moreover, the prepared ZnO/NPG microelectrode was successfully applied in the analysis of As(III) in tap and lake water samples with satisfactory recovery rates. The ZnO/NPG microelectrode presented exciting prospects for the electrochemical analysis of As(III) in near-neutral conditions.

RGO-PANI composite Au microelectrodes for sensitive ECIS analysis of human gastric (MKN-1) cancer cells

Microelectrode-based cell chip studies for cellular responses often require improved adhesion and growth conditions for efficient cellular diagnosis and high throughput screening in drug discovery. Cell-chip studies are often performed on gold electrodes due to their biocompatibility, and stability, but the electrode–electrolyte interfacial capacitance is the main drawback to the overall sensitivity of the detection system. Thus, here, we developed reduced graphene oxide-polyaniline-modified gold microelectrodes for real-time impedance-based monitoring of human gastric adenocarcinoma cancer (MKN-1) cells. The impedance characterization on modified electrodes showed 28-fold enhanced conductivity than the bare electrodes, and the spectra were modeled with proper equivalent circuits to extrapolate the values of circuit elements. The impedance of both time-and frequency-dependent measurements of cell-covered modified electrodes with equivalent model circuits was analyzed to achieve cellular behavior, such as adhesion, spreading, proliferation, and influence of anti-cancer agents. The normalized impedance at 41.5 kHz (|Z|norm 41 kHz) was selected to monitor the cell growth analysis, which was found linear with the proliferation of adherent cells along with the influence of the anticancer drug agent on the MKN-1 cells. The synergistic effects and biocompatible nature of PANI-RGO modifications improved the overall sensitivity for the cell-growth studies of MKN-1 cells.