Simultaneous electrochemical sensing of ascorbic acid and uric acid under biofouling conditions using nanoporous gold electrodes

Antifouling strategies for electrochemical sensing in complex biological media

Surface fouling poses a significant challenge that restricts the analytical performance of electrochemical sensors in both in vitro and in vivo applications. Biofouling resistance is paramount to guarantee the reliable operation of electrochemical sensors in complex biofluids (e.g., blood, serum, and urine). Seeking efficient strategies for surface fouling and establishing highly sensitive sensing platforms for applications in complex media have received increasing attention in the past. In this review, we provide a comprehensive overview of recent research efforts focused on antifouling electrochemical sensors. Initially, we present a detailed illustration of the concept about biofouling along with an exploration of four key antifouling mechanisms. Subsequently, we delve into the commonly employed antifouling strategies in the fabrication of electrochemical sensors. These encompass physical surface topography (micro/nanostructure coatings and filtration membranes) and chemical surface modifications (PEG and its derivatives, zwitterionic polymers, peptides, proteins, and various other antifouling materials). The progress in antifouling electrochemical sensors is proposed concerning the antifouling mechanisms as well as sensing capability assessments (e.g., sensitivity, stability, and practical application ability). Finally, we summarize the evolving trends in the field and highlight some key remaining limitations.

Impedimetric nano-collision Escherichia coli analysis based on Silver-Gold bimetallic nanoparticles

An impedimetric detection of E. coli was developed using chemically synthesised bimetallic Ag-Au (1:2) nanoparticles (NPs). The UV-visible spectra of the NPs had absorption bands at 470 and 580 nm for Ag NPs and Au NPs, respectively. In the presence of E. coli, a negative potential shift and a blue shift was observed in the voltammograms and spectra respectively. The complex formed had an oxidation potential at + 0.95 V. Technique choice was based on sensitivity comparison of Differential pulse voltammetry, cyclic voltammetry and impedance spectroscopy in 0.1 M PBS with Impedance being the best choice. Optimum sensing conditions of the NPs-E. coli complex for NPs concentration, incubation period, method modulation amplitude and applied potential were 5 mM, 20 min, 10 mV and + 0.5 V, respectively. The sensor's linearity range, lower limits of detection and quantification were found to be 10¹-10⁷, 1.88 × 10¹, 2.34 × 10² cells/mL, respectively. The sensor's applicability was validated by repeatability, stability and selectivity studies showing minimum changes in signal. Potential usage of the sensor in real samples was demonstrated by standard addition analysis of sea and River water samples as well as recovery of spiked water and fruit juices with acceptable % RSD < 2%.

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

Antifouling Electrochemical Biosensor Based on Conductive Hydrogel of DNA Scaffold for Ultrasensitive Detection of ATP

As an energy supplier, ATP plays an important role in various life activities, and there is an urgent need to develop an effective means of detecting ATP. However, the traditional sensors face serious nonspecific adsorption. In this work, an antifouling electrochemical biosensor based on the interpenetrating network of Y-DNA scaffold and polyaniline hydrogel was designed for ATP detection. The polyaniline hydrogel was conducive to the transport of electrons and ions, the structure of Y-DNA cross-linked by ATP aptamers in the polyaniline hydrogel achieved the effect of signal amplification. Super hydrophilic cellulose nanocrystals (CNCs) and zwitterion polypeptide sequence (Pep) were doped to play a synergistic antifouling effect. The hydrogel sensor we have built has a wide linear range of 0.1 pM-1 μM for ATP detection and a low detection limit of 0.025 pM (S/N = 3). For ATP detection in actual serum samples, the recovery of this sensor was 99.5%-106%, and the relative standard deviation was 0.4%-2.88%. It is proven that the sensor has good ATP detection performance, and it will provide a certain reference value for the detection of other biological small molecules.

Microneedle-based nanoporous gold electrochemical sensor for real-time catecholamine detection

Dopamine (DA), epinephrine (EP), and norepinephrine (NEP) are the main catecholamine of clinical interest, as they play crucial roles in the regulation of nervous and cardiovascular systems and are involved in some brain behaviors, such as stress, panic, anxiety, and depression. Therefore, there is an urgent need for a reliable sensing device able to provide their continuous monitoring in a minimally invasive manner. In this work, the first highly nanoporous gold (h-nPG) microneedle-based sensor is presented for continuous monitoring of catecholamine in interstitial fluid (ISF). The h-nPG microneedle-based gold electrode was prepared by a simple electrochemical self-templating method that involves two steps, gold electrodeposition and hydrogen bubbling at the electrode surface, realized by sweeping the potential between + 0.8 V and 0 V vs Ag/AgCl for 25 scans in a 10 mM HAuCl₄ solution containing 2.5 M NH₄Cl, and successively applying a fixed potential of − 2 V vs Ag/AgCl for 60 s. The resulting microneedle-based h-nPG sensor displays an interference-free total catecholamine detection expressed as NEP concentration, with a very low LOD of 100 nM, excellent sensitivity and stability, and fast response time (< 4 s). The performance of the h-nPG microneedle array sensor was successively assessed in artificial ISF and in a hydrogel skin model at typical physiological concentrations.

A Micro Electrochemical Sensor for Multi-Analyte Detection Based on Oxygenated Graphene Modified Screen-Printed Electrode

Electrode interfaces with both antibiofouling properties and electrocatalytic activity can promote the practical application of nonenzymatic electrochemical sensors in biological fluids. Compared with graphene, graphene oxide (GO) possesses unique properties such as superior solubility (hydrophilicity) in water, negative charge, and abundant oxygenated groups (oxo functionalities) in the plane and edge sites, which play an essential role in electrocatalysis and functionalization. In this work, a micro electrochemical sensor consisting of GO-modified screen-printed electrode and PDMS micro-cell was designed to achieve multi-analyte detection with excellent selectivity and anti-biofouling properties by electrochemically tuning the oxygen-containing functional species, hydrophilicity/hydrophobicity, and electrical conductivity. In particular, the presented electrodes demonstrated the potential in the analysis of biological samples in which electrodes often suffer from serious biofouling. The interaction of proteins with electrodes as well as uric acid was investigated and discussed.

An ultrasensitive molecularly imprinted polymer-based electrochemical sensor for the determination of SARS-CoV-2-RBD by using macroporous gold screen-printed electrode

Herein, a novel molecularly imprinted polymer (MIP) based electrochemical sensor for the determination of the receptor-binding domain of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2-RBD) has been developed. For this purpose, first, a macroporous gold screen-printed electrode (MP-Au-SPE) has been fabricated. The MIP was then synthesized on the surface of the MP-Au-SPE through the electro-polymerization of ortho-phenylenediamine in the presence of SARS-CoV-2-RBD molecules as matrix polymer, and template molecules, respectively. During the fabrication process, the SARS-CoV-2-RBD molecules were embedded in the polymer matrix. Subsequently, the template molecules were removed from the electrode by using alkaline ethanol. The template molecules removal was studied using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), and attenuated total reflectance spectroscopy (ATR). The fabricated MIP film acted as an artificial recognition element for the measurement of SARS-CoV-2-RBD. The EIS technique was used for the measurement of the SARS-CoV-2-RBD in the saliva solution. The electron transfer resistance (R_et) of the MIP-based sensor in a ferri/ferrocyanide solution increased as the SARS-CoV-2-RBD concentration increased due to the occupation of the imprinted cavities by the SARS-CoV-2-RBD. The MIP-based sensor exhibited a good response to the SARS-CoV-2-RBD in the concentration range between 2.0 and 40.0 pg mL^-1 with a limit of detection of 0.7 pg mL^-1. The obtained results showed that the fabricated MIP sensor has high selectivity sensitivity, and stability.

A novel electrochemical sensor for determination of uric acid in the presence of ascorbic acid and dopamine based on a carbon paste electrode modified with an electrochemically reduced para-nitrobenzoic acid/graphene oxide nanocomposite

In this study, a highly sensitive electrochemical sensor based on a carbon paste electrode was modified by an electrochemically reduced para-nitrobenzoic acid/graphene oxide nanocomposite to measure uric acid.

Gold Nanoparticles/Carbon Nanotubes and Gold Nanoporous as Novel Electrochemical Platforms for L-Ascorbic Acid Detection: Comparative Performance and Application

Herein, the effects of nanostructured modifications of a gold electrode surface in the development of electrochemical sensors for L-ascorbic acid detection have been investigated. In particular, a bare gold electrode has been modified by electrodeposition of gold single-walled carbon nanotubes (Au/SWCNTs) and by the formation of a highly nanoporous gold (h-nPG) film. The procedure has been realized by sweeping the potential between +0.8 V and 0 V vs. Ag/AgCl for 25 scans in a suspension containing 5 mg/mL of SWCNTs in 10 mM HAuCl4 and 2.5 M NH4Cl solution for Au/SWCNTs modified gold electrode. A similar procedure was applied for a h-nPG electrode in a 10 mM HAuCl4 solution containing 2.5 M NH4Cl, followed by applying a fixed potential of −4 V vs. Ag/AgCl for 60 s. Cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the properties of the modified electrodes. The developed sensors showed strong electrocatalytic activity towards ascorbic acid oxidation with enhanced sensitivities of 1.7 × 10−2 μA μM−1cm−2 and 2.5 × 10−2 μA μM−1cm−2 for Au/SWCNTs and h-nPG modified electrode, respectively, compared to bare gold electrode (1.0 × 10−2 μA μM−1cm−2). The detection limits were estimated to be 3.1 and 1.8 μM, respectively. The h-nPG electrode was successfully used to determine ascorbic acid in human urine with no significant interference and with satisfactory recovery levels.

Antifouling Strategies for Electrochemical Biosensing: Mechanisms and Performance toward Point of Care Based Diagnostic Applications

Although there exist numerous established laboratory-based technologies for sample diagnostics and analyte detection, many medical and forensic science applications require point of care based platforms for rapid on-the-spot sample analysis. Electrochemical biosensors provide a promising avenue for such applications due to the portability and functional simplicity of the technology. However, the ability to develop such platforms with the high sensitivity and selectivity required for analysis of low analyte concentrations in complex biological samples remains a paramount issue in the field of biosensing. Nonspecific adsorption, or fouling, at the electrode interface via the innumerable biomolecules present in these sample types (i.e., serum, urine, blood/plasma, and saliva) can drastically obstruct electrochemical performance, increasing background "noise" and diminishing both the electrochemical signal magnitude and specificity of the biosensor. Consequently, this review aims to discuss strategies and concepts used throughout the literature to prevent electrode surface fouling in biosensors and to communicate the nature of the antifouling mechanisms by which they operate. Evaluation of each antifouling strategy is focused primarily on the fabrication method, experimental technique, sample composition, and electrochemical performance of each technology highlighting the overall feasibility of the platform for point of care based diagnostic/detection applications.

Measurement of Neuropeptide Y Using Aptamer-Modified Microelectrodes by Electrochemical Impedance Spectroscopy

Aptamer-modified microelectrodes for Neuropeptide Y measurement by electrochemical impedance spectroscopy was described here. The advantages of using carbon fiber or platinum microelectrodes are because they are promising materials with high electrical conductivity, chemical stability, and high surface area that can be easily modified on their surface. The immobilization and biofouling were studied and compared using EIS. Moreover, the adsorption of NPY to the aptamer-modified microelectrodes was also demonstrated by EIS. Changes of -ω*Zimag, an impedance factor that gives information of the capacitance, is directly correlated with concentrations. A widely linear range was obtained from 10 to 1000 ng/mL of NPY. This method was able to detect NPY without performing a redox reaction by adsorption at the surface of the microelectrodes, with the specificity provided by aptamer functionalization of the microelectrode surface.

Metal-free graphene-based nanoelectrodes for the electrochemical determination of ascorbic acid (AA) and p-nitrophenol (p-NP): implication towards biosensing and environmental monitoring

Herein, tyramine functionalized graphene oxide electrocatalyst is used for the electrochemical determination of ascorbic acid and p-nitrophenol in 1 M phosphate buffer solution at pH-7  with long term current/potential stability and reproducibility.

A nanocomposite of NiFe2O4-PANI as a duo active electrocatalyst toward the sensitive colorimetric and electrochemical sensing of ascorbic acid

A non-enzymatic ascorbic acid sensor using a nickel ferrite/PANI (NF-PANI) nanocomposite and based on colorimetric and electrochemical sensing methods was investigated in this study. The nanocomposite was prepared by an in situ polymerization and utilized as an electrocatalyst to sense ascorbic acid (AA) through the peroxidase mimic sensing of H₂O₂ in the presence of 3,5,3,5-tetramethylbenzidine (TMB) as a coloring agent. It was also utilized to detect AA present in real samples prepared from fruit extracts, commercial beverages, and vitamin-C tablets. The limit of detection (LoD) for AA sensing by the peroxidase mimic method was found to be 232 nM. The relative standard deviation (RSD) calculated for analysis of the real samples analysis ranged from 1.7-3.2%. Similarly, the electrochemical sensing of AA by NF-PANI was examined by cyclic voltammetric, chronoamperometric, and differential pulse voltammetric analyses. The LoD for the electrochemical method applied to AA sensing was 423 nM. The nanocomposite functioned as an effective electrocatalytic sensing agent in both methods to selectively detect AA due to the combined effect of NF and PANI. Thus, it was shown that the nanocomposites could be utilized for the laboratory-based detection of AA by various methods and could give rapid results.

One-step rapid synthesis of Ni6(C12H25S)12 nanoclusters for electrochemical sensing of ascorbic acid

Metal nanoclusters (NCs) are highly desirable as active catalysts due to their highly active surface atoms. Among the reported metal clusters, nickel nanoclusters (Ni NCs) have been less well developed than others, such as gold, silver and copper. Herein a simple method is developed to synthesize atomically precise Ni clusters with the molecular formular of Ni₆(C₁₂H₂₅S)₁₂. Moreover, the single crystal of the Ni₆(C₁₂H₂₅S)₁₂ cluster is also obtained. The composition, morphology and optical properties of the prepared Ni₆ clusters are characterized by X-ray crystallography, XPS, XRD, SEM, HRTEM, FTIR and UV-Vis spectroscopy. The Ni cluster is composed of six nickel atoms that form a hexagonal ring with an exterior 1-dodecanethiol shell, resembling a double crown. Meanwhile, the Ni₆ NCs can be self-assembled into nanosheets due to their uniform size. It was found that the Ni₆(C₁₂H₂₅S)₁₂ clusters loaded on carbon black exhibit higher electrocatalytic activity than Ni nanoparticles towards ascorbic acid (AA) oxidation. The Ni₆ clusters show high sensing performance for AA with a wide linear range (1-3212 μM) and a low detection limit of 0.1 μM (S/N = 3). The significantly enhanced catalytic activity can be ascribed to the high fraction of surface Ni atoms with low coordination in the sub-nanometer clusters. The present work not only provides a straightforward method for synthesizing atomically precise metal clusters but also indicates that ultrasmall Ni clusters can be used as highly efficient catalysts for the electrochemical detection of AA.

Antifouling strategies in advanced electrochemical sensors and biosensors

A review presented recent development of antifouling strategies in electrochemical sensors and biosensors based on the modification methods.

Amperometric microsensor based on nanoporous gold for ascorbic acid detection in highly acidic biological extracts

Tuning the electrocatalytic properties of high surface area porous metallic frameworks like Nanoporous Gold (NPG) by tailoring the structure is a convenient strategy to design electrochemical sensors. Accordingly, an NPG-based sensitive, selective and robust electroanalytical platform was designed for the detection of ascorbic acid (AA) in acidic extracts of Aspergillus fumigatus fungus and Arabidopsis thaliana leaves. NPG films were electrodeposited on a gold microelectrode by potentiostatic electrodeposition and characterized by electron microscopy techniques, which confirmed the morphology and highly porous structure resembling nanowires-type pure gold fractals. The electrodeposition parameters, particularly deposition potential and time, were optimized to achieve large and selective amperometric detection of AA on the NPG modified electrodes. Faster electron transfer kinetics was manifested on the 0.3 V shift in overpotential and remarkable enhancement of the oxidation peak current as compared with bare gold electrode. Amperometric measurements were performed at 0.3 V vs. Ag/AgCl_{(sat. KCl)} in the highly acidic electrolyte solution employed to extract ascorbate from biological samples and minimize its autoxidation. The sensitivity of conventional Au-microelectrodes was increased about one thousand-fold upon modification with NPG film, reaching 2 nA μmol^-1 L^-1. The detection limit for AA based on a linear current-concentration calibration plot was found to be 2 μmol L^-1. The NPG-based microsensor was demonstrated to be selective, reproducible and stable, and was employed for determinations of AA concentration in highly acidic biological extracts.