Amperometric L-cysteine sensor based on a carbon paste electrode modified with Y2O3 nanoparticles supported on nitrogen-doped reduced graphene oxide

Advanced design of chemically modified electrodes for the electrochemical analysis of uric acid and xanthine

This study reviews advances in chemical detection methods applied to the metabolic products known as uric acid (UA) and xanthine (XA), which are residues of purine metabolism, with XA being an important intermediate preceding UA. UA and XA play crucial roles in maintaining physiological homeostasis in organisms. Chemical modification of electrodes is a widely used method to address the issues of poor sensitivity and selectivity encountered with bare electrodes. This article reviews various materials commonly used to modify electrode surfaces for the detection of uric acid and xanthine, focusing on properties that enhance electrocatalytic activity. We highlight recent trends in detecting these compounds using electrochemical methods with microfabricated devices and explore cutting-edge modification techniques involving novel nanomaterials, carbon derivatives, metallic nanoparticles, and polymers. The review includes a comparative analysis of these materials, addressing their strengths, limitations, and recent advancements, such as in carbon-based materials and metal-organic frameworks (MOFs). Finally, we critically examine the challenges and future prospects of electrochemical detection of UA and XA in real samples, offering strategies to address these issues. The challenges associated with determination of UA and XA in real samples are also discussed.

Electrochemical Detection of Uric Acid Based on a Carbon Paste Electrode Modified with Ta2O5 Recovered from Ore by a Novel Method

Except for well-known commercial production procedures, this study demonstrates that Ta2O5 particles can be produced. Through a series of steps, highly pure Ta2O5 particles (99.45%) were produced from the raw ore. We have electrochemically detected one of the important nitrogenous compounds present in urine, "uric acid", by a Ta2O5 particle-modified carbon paste electrode (Ta2O5-MCPE) using cyclic voltammetry. The prepared electrode has shown excellent current sensitivity at a pH of 6.0 phosphate-buffered solution. We have found that 4 mg Ta2O5-MCPE has recorded the highest current sensitivity of 75.75 μA. The oxidation peak current was varied with the uric acid concentration in the range from 1 to 5 mM at 4 mg Ta2O5-MCPE. We have calculated the electrode-active surface area for a bare carbon paste electrode and 4 mg Ta2O5-MCPE using the Randles-Sevcik equation, and the values were found to be 0.0202 and 0.0450 cm2, respectively. On the other hand, the calculated values of limit of detection and limit of quantification were reported as 0.5937 × 10-8 M and 1.9791 × 10-8 M, respectively, for the prepared 4 mg Ta2O5-MCPE. The interfere studies revealed that the variation in the electrochemical signal of uric acid in the presence of different metal ions was found to be less than ±5%.

ZnO/Cu2O heterojunction integrated fiber-optic biosensor for remote detection of cysteine

Indium tin oxide, semiconductor nanomaterial ZnO, and Cu₂O were first loaded on the surface of the optical fiber to form an optical fiber probe. Large-volume macroscopic spatial light is replaced by an optical fiber path, and remote light injection is implemented. Based on the optical fiber probe, a photoelectrochemical biosensor was constructed and remote detection of cysteine was realized. In this tiny device, the optical fiber probe not only acts as a working electrode to react with the analyte but also directs the light exactly where it is needed. Simultaneously, the electrochemical behavior of cysteine on the surface of the working electrode is dominated by diffusion-control, which provides strong support for quantitative detection. Then, under the bias potential of 0 V, the linear range of the fiber-optic-based cysteine biosensor was 0.01∼1 μM, the regression coefficient (R²) value was 0.9943. In spiked synthetic urine, the detection of cysteine was also realized by the integrated biosensor. Moreover, benefiting from the low optical fiber loss, the new structure also possesses a unique remote detection function. This work confirms that photoelectrochemical biosensors can be integrated via optical fibers and retain comparable sensing performance. Based on this property, different materials can also be loaded on the surface of the optical fiber for remote detection of other analytes. It is expected to facilitate the research on fiber-optic-based integrated biosensors and show application prospects in diverse fields such as biochemical analysis and disease diagnosis.

In Situ Formation of Bi2MoO6-Bi2S3 Heterostructure: A Proof-Of-Concept Study for Photoelectrochemical Bioassay of l-Cysteine

A novel signal-increased photoelectrochemical (PEC) biosensor for l-cysteine (L-Cys) was proposed based on the Bi₂MoO₆–Bi₂S₃ heterostructure formed in situ on the indium–tin oxide (ITO) electrode. To fabricate the PEC biosensor, Bi₂MoO₆ nanoparticles were prepared by a hydrothermal method and coated on a bare ITO electrode. When L-Cys existed, Bi₂S₃ was formed in situ on the interface of the Bi₂MoO₆/ITO electrode by a chemical displacement reaction. Under the visible light irradiation, the Bi₂MoO₆–Bi₂S₃/ITO electrode exhibited evident enhancement in photocurrent response compared with the Bi₂MoO₆/ITO electrode, owing to the signal-increased sensing system and the excellent property of the formed Bi₂MoO₆–Bi₂S₃ heterostructure such as the widened light absorption range and efficient separation of photo-induced electron–hole pairs. Under the optimal conditions, the sensor for L-Cys detection has a linear range from 5.0 × 10⁻¹¹ to 1.0 × 10⁻⁴ mol L⁻¹ and a detection limit of 5.0 × 10⁻¹² mol L⁻¹. The recoveries ranging from 90.0% to 110.0% for determining L-Cys in human serum samples validated the applicability of the biosensor. This strategy not only provides a method for L-Cys detection but also broadens the application of the PEC bioanalysis based on in situ formation of photoactive materials.

SPR based gold nano-probe as optical sensor for cysteine detection via plasmonic enhancement in the presence of Cr3

A selective and sensitive detection of L-cysteine (Cys) is an important tool for various biological studies. Here, Au nanoparticles (NPs) were prepared by chemical reduction technique. The probe was developed to detect and quantify Cys in the presence of Cr³⁺ ions which acts as a cross linker. The citrate capped Au NPs probe was analyzed by UV-visible spectrophotometry, TEM, EDAX, FTIR, DLS, XPS and zetasize. The zeta potential and effective size of Au NPs were -41.22 mV and 12 nm, respectively. The Cys interaction with Au NPs showed drastic colour variation from red to purple and colourless with rapid response time of 1 min. The limit of detection (LOD) of Au NPs probe was as low as 0.012 nM. The TEM image of Au NPs after Cys interaction verified the aggregation that resulted in colour change. The XPS core level scans of Au 4f showed 0.3 eV red shift when Cyswas interacted. The Au NPs sensor is highly selective for Cys with excellent reproducibility. Acidic pH slightly favored Cys detection. Further, the probe was applied to estimate Cys quantity from milk, urine, blood and environmental augmented samples in the presence of other amino acids . The study suggests that the proposed Au NPs could detect Cys with high accuracy from various biological samples.

Electrochemical Amino Acid Sensing: A Review on Challenges and Achievements

The rapid growth of research in electrochemistry in the last decade has resulted in a significant advancement in exploiting electrochemical strategies for assessing biological substances. Among these, amino acids are of utmost interest due to their key role in human health. Indeed, an unbalanced amino acid level is the origin of several metabolic and genetic diseases, which has led to a great need for effective and reliable evaluation methods. This review is an effort to summarize and present both challenges and achievements in electrochemical amino acid sensing from the last decade (from 2010 onwards) to show where limitations and advantages stem from. In this review, we place special emphasis on five well-known electroactive amino acids, namely cysteine, tyrosine, tryptophan, methionine and histidine. The recent research and achievements in this area and significant performance metrics of the proposed electrochemical sensors, including the limit of detection, sensitivity, stability, linear dynamic range(s) and applicability in real sample analysis, are summarized and presented in separate sections. More than 400 recent scientific studies were included in this review to portray a rich set of ideas and exemplify the capabilities of the electrochemical strategies to detect these essential biomolecules at trace and even ultra-trace levels. Finally, we discuss, in the last section, the remaining issues and the opportunities to push the boundaries of our knowledge in amino acid electrochemistry even further.

Ratiometric Electrochemical Immunosensor Based on L-cysteine Grafted Ferrocene for Detection of Neuron Specific Enolase

In order to realize the ultra sensitive detection of Neuron specific enolase (NSE) in human serum, we chose electrochemical immunosensor as a simple analytical method. During the experiment, we found that the peak value signals of Cu-MOFs-Au and Fc-_L-Cys were significantly changed at -0.20 V and 0.20 V potentials by DPV. So a ratiometric electrochemical immunosensor for quantitative analysis of NSE was prepared for Cu-MOFs-Au as the electrode sensing surface and Fc-_L-Cys as the label of Ab₂. The data and performances of the immunosensor were tested and analyzed by DPV. Cu-MOFs not only provide the required signal for the immunosensor, but also have a large specific surface area, which can provide more sites for the placement of Au nanoparticles. _L-cysteine (_L-Cys) can prevent a large amount of Fc-COOH leakage, so that Fc⁺ can stably provide another required signal. With the beefing up of NSE concentration, redox peak of Cu-MOFs-Au decreased and that of Fc-_L-Cys raised. The ratio (ΔI=ΔI_Cu/ΔI_Fc) of two different signals was linear with the logarithm of NSE concentration in a certain value range. In brief, with the optimized experimental conditions, the immunosensor showed excellent performance in the concentration range of 1 pg/mL to 1 μg/mL, and the detection limit was 0.011 pg/mL. Compared with other immunosensors, it showed an unexpected high sensitivity. This method also provided a new idea for the ultra sensitive quantitative detection of other biomarkers.

Recent developments in voltammetric and amperometric sensors for cysteine detection

This review article aims to provide an overview of the recent advances in the voltammetric and amperometric sensing of cysteine (Cys). The introduction summarizes the important role of Cys as an essential amino acid, techniques for its sensing, and the utilization of electrochemical methods and chemically modified electrodes for its determination. The main section covers voltammetric and amperometric sensing of Cys based on glassy carbon electrodes, screen printed electrodes, and carbon paste electrodes, modified with various electrocatalytic materials. The conclusion section discusses the current challenges of Cys determination and the future perspectives.

Enhancement of the Peroxidase-Like Activity of Iodine-Capped Gold Nanoparticles for the Colorimetric Detection of Biothiols

A colorimetric assay was developed for the detection of biothiols, based on the peroxidase-like activity of iodine-capped gold nanoparticles (AuNPs). These AuNPs show a synergetic effect in the form of peroxidase-mimicking activity at the interface of AuNPs, while free AuNPs and iodine alone have weak catalytic properties. Thus, iodine-capped AuNPs possess good intrinsic enzymatic activity and trigger the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), leading to a change in color from colorless to yellow. When added to solution, biothiols, such as cysteine, strongly bind to the interface of AuNPs via gold-thiol bonds, inhibiting the catalytic activity of AuNPs, resulting in a decrease in oxidized TMB. Using this strategy, cysteine could be linearly determined, at a wide range of concentrations (0.5 to 20 μM), with a detection limit of 0.5 μM using UV-Vis spectroscopy. This method was applied for the detection of cysteine in diluted human urine.

Developments and applications of nanomaterial-based carbon paste electrodes

This review summarizes the progress that has been made in the past ten years in the field of electrochemical sensing using nanomaterial-based carbon paste electrodes. Following an introduction into the field, a first large section covers sensors for biological species and pharmaceutical compounds (with subsections on sensors for antioxidants, catecholamines and amino acids). The next section covers sensors for environmental pollutants (with subsections on sensors for pesticides and heavy metal ions). Several tables are presented that give an overview on the wealth of methods (differential pulse voltammetry, square wave voltammetry, amperometry, etc.) and different nanomaterials available. A concluding section summarizes the status, addresses future challenges, and gives an outlook on potential trends.

Enhanced photoelectrochemical sensing based on novel synthesized Bi2S3@Bi2O3 nanosheet heterostructure for ultrasensitive determination of L-cysteine

The design of a low-cost and highly efficient photoactive heterojunction material for sensing is still a challenging issue. On the basis of the formation of sheet-like Bi₂O₃ via coating Bi₂S₃, a novel Bi₂O₃@Bi₂S₃ heterostructure is controllably synthesized via a facile water bath approach. The prepared Bi₂O₃@Bi₂S₃ nanosheets show a superior photoelectrochemical (PEC) performance for the detection of L-cysteine (L-Cys), and the photocurrent signal is three and four times higher than those of Bi₂S₃ and Bi₂O₃ under visible irradiation, respectively. Also, the heterostructure presents an outstanding linear range for the detection of L-Cys: 0.1-10,000 μM. In addition, the mechanism of improved PEC response of Bi₂O₃@Bi₂S₃ nanosheets is investigated according to the estimated energy band positions. Thus, the integration of the novel heterostructure and the photoelectrochemical technique demonstrates a rapid photocurrent response, showing a great effect on the performance of the sensing system and a new PEC method for highly selective and sensitive chemical detection. Graphical abstract.

Simultaneous intercalated assembly of mesostructured hybrid carbon nanofiber/reduced graphene oxide and its use in electrochemical sensing

Polyacrylonitrile nonwovens intercalated with graphene oxide (GO) sheets were prepared by a simultaneous electrospinning-spray deposition system. These hybrid nonwovens were carbonized in a two-stage process to obtain a mesostructured hybrid carbon containing carbon nanofibers (CNF) and reduced GO sheets (CNF/RGO). During the carbonization process, the CNF act as spacers between the RGO layers to prevent their compactation and restacking resulting in a three-dimensional structure. The presence of RGO increases the electrical conductivity in the CNF/RGO material. The resulting hybrid carbon is nitrogen-doped as indicated by x-ray photoelectron spectroscopy and Fourier transformed infrared spectroscopy. This N-doped porous carbon was used to prepare electrodes with improved sensitivity for the electrochemical detection of L-cysteine.

A glassy carbon electrode modified with hollow cubic cuprous oxide for voltammetric sensing of L-cysteine

This paper reports on an electrochemical sensing system for L-cysteine. It is based on the use of hollow cubic Cu₂O particles that were prepared in two steps. First, the Cu₂O/ polystyrene (PS) composites were prepared by a surface ion exchange strategy for in-situ reductive deposition on the surface of carboxy-capped PS particles. Thereafter, the PS particles were removed from the Cu₂O/PS composites by treatment with tetrahydrofuran (THF). The resulting hollow cubic Cu₂O particles were placed in a Nafion matrix on a glassy carbon electrode (GCE) which exhibits high surface area, good site accessibility and excellent electrocatalytic activity for L-cysteine. The cyclic voltammetric response of the modified GCE to L-cysteine is about 2.8-fold stronger than when using a GCE modified with pure Cu₂O. The detection limit for L-cysteine is lower by about 1 order of magnitude, and the working voltage is rather low (-0.08 V vs. Ag/AgCl). An excellent electrochemical selectivity for L-cysteine over other amino acids was also achieved. The method was successfully applied to the determination of L-cysteine in pharmaceutical samples. Graphical abstract An electrochemical sensing system for the detection of L-cysteine in amino acid injections has been established by using the hollow cubic Cu₂O particles as recognition element.

One-Step Synthesis of CuO-Cu2O Heterojunction by Flame Spray Pyrolysis for Cathodic Photoelectrochemical Sensing of l-Cysteine

CuO-Cu₂O heterojunction was synthesized via a one-step flame spray pyrolysis (FSP) process and employed as photoactive material in construction of a photoelectrochemical (PEC) sensing device. The surface analysis showed that CuO-Cu₂O nanocomposites in the size less than 10 nm were formed and uniformly distributed on the electrode surface. Under visible light irradiation, the CuO-Cu₂O-coated electrode exhibited admirable cathodic photocurrent response, owing to the favorable property of the CuO-Cu₂O heterojunction such as strong absorption in the visible region and effective separation of photogenerated electron-hole pairs. On the basis of the interaction of l-cysteine (l-Cys) with Cu-containing compounds via the formation of Cu-S bond, the CuO-Cu₂O was proposed as a PEC sensor for l-Cys detection. A declined photocurrent response of CuO-Cu₂O to addition of l-Cys was observed. Influence factors including CuO-Cu₂O concentration, coating amount of CuO-Cu₂O, and applied bias potential on the PEC response toward l-Cys were optimized. Under optimum conditions, the photocurrent of the proposed sensor was linearly declined with increasing the concentration of l-Cys from 0.2 to 10 μM, with a detection limit (3S/N) of 0.05 μM. Moreover, this PEC sensor displayed high selectivity, reproducibility, and stability. The potential applicability of the proposed PEC sensor was assessed in human urine samples.