Synthesis of Silver Nanoparticle‐Graphene Composites for Electroanalysis Applications using Chemical and Electrochemical Methods

An electrochemical device was developed for the simultaneous determination of sulfamethoxazole (SMX) and trimethoprim (TMP) using differential pulse voltammetry and glassy carbon (GC) electrodes modified with reduced graphene oxide (rGO) and silver nanoparticle (AgNP) composites, synthesised using both chemical and electrochemical methods. The morphology and electrochemical behaviour of the GC electrodes modified with the rGO/AgNP (chemical method) and rGO‐AgNP (electrochemical method) composites were characterised by scanning electron microscopy and cyclic voltammetry. These techniques demonstrated that, in both methods, the graphene oxide was modified by the AgNPs, and the composite synthesised by the electrochemical method showed a better dispersion of the nanoparticles, resulting in an increase in the surface area compared to the rGO/AgNP composite. The GC/rGO‐AgNP electrode was evaluated and optimised for the simultaneous determination of SMX and TMP, achieving detection limits of 0.6 μmol L⁻¹ for the SMX and 0.4 μmol L⁻¹ for the TMP. The proposed GC/rGO‐AgNP electrochemical device was successfully applied to the simultaneous determination of SMX and TMP in wastewaters samples.     

Electrochemical Sensor‐Based Ruthenium Nanoparticles on Reduced Graphene Oxide for the Simultaneous Determination of Ethinylestradiol and Amoxicillin

The composite material of reduced graphene and Ru nanoparticles (rGO/RuNP) was obtained by electrochemical oxidation of ruthenium nanoparticles immobilised on the glassy carbon electrode (GCE) surface and used for simultaneous electroanalysis of drugs. There are many discussions in the scientific community about the decrease in therapeutic efficacy of contraceptives when associated with antibiotics. The antibiotic effect of the antagonist can increase the contraceptive excretion levels in urine, indicating a reduction in the body and thus a decrease in the desired effect. Using the DPV technique, it was possible to quantify ethinylestradiol (EE2) and amoxicillin (AMX) with a linear response range, 5.50x10⁻⁸ – 1.20x10⁻⁶ mol L⁻¹, and LOD, 2.04 nmol L⁻¹ and 1.63 nmol L⁻¹, respectively. The results for the electrochemical behaviour of EE2 and AMX using GCE/rGO/RuNP were compared with GCE/rGO and GCE. The GCE/rGO/RuNP showed greater stability and separation anodic peak currents, suitable for the quantification of organic molecules in samples of environmental interest, as well as clinical and food samples.     

A Simple,sensitive, and selective electrochemical aptasensor for cortisol based on rGO-AuNPs

Cortisol is a steroid hormone naturally produced by the adrenal glands. It participates in and controls several processes in the body and is considered an important physiological biomarker. Due to its very low concentrations in body fluids, its detection requires high sensitivity and specificity. Here, we present a simple electrochemical biosensor based on reduced graphene oxide (rGO) modified with gold nanoparticles (AuNPs) for the immobilization of the cortisol-specific aptamer (Ap) (MCH-Ap-AurGO/GCE). Important analytical parameters for identifying the target analyte were optimized, such as conditions and amount of immobilized Ap, the influence of the concentration and nature of the supporting electrolyte, pH of the medium, and incubation time. The optimized conditions for the aptasensor were: concentration of Ap 1.0 × 10⁻⁶ mol L⁻¹, support electrolyte Tris/HCl 50 mmol L⁻¹, MgCl₂ 10 mmol L⁻¹, and NaCl 10 mmol L⁻¹, at pH 5.0 and incubation time of 15 min. A linear response range was obtained from 1 × 10⁻¹⁸ up to 1 × 10⁻¹¹ mol L⁻¹ of cortisol with a detection limit (LOD) of 1.0 × 10⁻¹⁸ mol L⁻¹. A curve adjusted for operational purposes in the saliva sample was fitted for the concentration range between 0.5 × 10⁻¹⁴ and 1 × 10⁻¹¹ mol L⁻¹, with a linear regression equation ΔRtc/Rtc₁ = 2.70 + 0.17 × log([Cortisol]). The aptasensor demonstrated a great potential for detecting cortisol in a simple, fast, and highly sensitive way, opening its path for application in real samples, which present levels below the concentration in which cortisol is commonly found in body fluids.     

Sensitive Voltammetric Determination of Methyl Parathion Using a Carbon Paste Electrode Modified with Mesoporous Zirconia

A mesoporous zirconia modified carbon paste electrode was developed for electrochemical investigations of methyl parathion (MP, Phen‐NO₂). The significant increase of the peak currents and the improvement of the redox peak potential indicate that mesoporous zirconia facilitates the electronic transfer of MP. The oxidation peak current was proportional to the MP concentration in the range from 1.0×10⁻⁸ to 1.0×10⁻⁵ mol L⁻¹ with a detection limit of 4.6×10⁻⁹ mol L⁻¹ (S/N=3) after accumulation under open‐circuit for 210 s. The proposed method was successfully applied to the determination of MP in apple samples.     

Graphdiyne and Ionic Liquid Composite Modified Gold Electrode for Sensitive Voltammetric Analysis of Rutin

It is significant to develop a point-of-care testing (POCT) method for rapid detection of medicinal molecules. In this paper, a graphdiyne (GDY)-ionic liquid (IL) composite was prepared via one-step facile ultrasound preparation process and then modified on gold (Au) electrode surface by simple casting method. Scanning electron microscopy and transmission electron microscopy were used to characterize the morphology of GDY-IL composite. Cyclic voltammetric results proved that GDY-IL composite on the electrode surface could effectively improve electron transfer rate, which meant that GDY-IL composite had high conductivity with big surface area. Finally, the modified electrode exhibited excellent performances for rutin detection with wider linear range (8.0×10⁻⁹ mol L⁻¹–2.0×10⁻⁶ mol L⁻¹ and 2.0×10⁻⁶ mol L⁻¹–1.5×10⁻⁴ mol L⁻¹) and lower detection limit (2.7 nmol L⁻¹, 3S₀/S). The Nafion/GDY-IL/Au electrode showed good sensitivity and high selectivity, which was satisfactory in analytical application to real samples. Therefore, the GDY-IL composite modified electrode has the potential applications in the POCT for electrochemical analysis of various medicinal molecules.     

Electropolymerization of Methylene Blue on Carbon Ionic Liquid Electrode and Its Electrocatalysis to 3,4‐Dihydroxybenzoic Acid

In this paper an ionic liquid modified carbon paste electrode (CILE) was prepared and methylene blue (MB) was electropolymerized on the CILE by using the cyclic voltammetric technique in the potential range from −1.0 V to 0.8 V (vs. SCE). A stable polymer film was obtained and exhibited a pair of redox peaks. The morphology and characteristics of poly(methylene blue) (PMB) film was studied by the techniques such as scanning electron microscopy and electrochemical impedance spectroscopy. This PMB modified CILE (PMB/CILE) showed excellent electrocatalytic response to 3,4‐dihydroxybenzoic acid with the increase of the electrochemical responses. The oxidation peak current had a linear relationship with 3,4‐dihydroxybenzoic acid concentration in the range of 5.0 × 10⁻⁴ ∼ 3.0 × 10⁻² mol L⁻¹ and the detection limit was 1.72 × 10⁻⁴ Mol L⁻¹ (3 σ).     

Fast and Simple Preparation of a Sensor Based on Electrochemically Reduced Graphene Oxide (rGO) for the Determination of Zopiclone in Pharmaceutical Dosage by Square Wave Adsorptive Stripping Voltammetry (SWAdSV)

The interactions of zopiclone with electrochemically reduced graphene oxide (rGO) modified electrode were examined. A comparison of GC/rGO and glassy carbon electrode (GC) by electrochemical impedance spectroscopy and scanning electrochemical microscopy (SECM) shows that the modified surface is much less conductive than GC. The role of rGO is to act as a site of specific adsorption of the analyte. Molecular dynamics showed that the monoanionic form of zopiclone presents more interactions with defects of rGO. The analytical methodology allowed obtaining a linearity of 10–130 μg L⁻¹, with a limit of detection of 2.14 μg L⁻¹ using SWAdSV at pH 10.0.     

A Newly Competitive Electrochemical Sensor for Sensitive Determination of Chrysin Based on Electrochemically Activated Ta2O5 Particles Modified Carbon Paste Electrode

A novel electrochemically activated doped Ta₂O₅ particles modified carbon paste electrode (EA‐Ta₂O₅‐CPE) was prepared and applied for selective and sensitive determination of chrysin. X‐ray diffraction (XRD), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) techniques and cyclic voltammetry (CV) were used to characterize the Ta₂O₅ particles and investigate the electrochemical response of the sensor. Compared with bare CPE, the doped Ta₂O₅ modified electrode got much more porous by electrochemical treatment and exhibited larger effective surface area, more reactive site and excellent electrochemical catalytic activity toward the oxidation of chrysin. Under optimum conditions by LSV, the oxidation peak currents responded to chrysin linearly over a concentration range from 5.0×10⁻⁸ to 7.0×10⁻⁶ mol L⁻¹ with a detection limit of 2.0×10⁻⁸ mol L⁻¹ (5.08 ng mL⁻¹). The fabricated sensor showed anti‐interference ability against the biological common interferents (i.e. baicalein, baicalin) and provided to be reliable for the determination of chrysin in Chinese medicinal herb Oroxylum indicum and chrysin capsules samples with satisfactory results.     

Voltammetric Determination of Sophoridine Based on Gold Nanoparticles/L‐cysteine/ Graphene Modified Glassy Carbon Electrode

A new strategy to make the electrochemical sensor was presented, through combining gold nanoparticles (GNPs) with reduced graphene oxide (rGO) via L‐cysteine (L‐cys) as crosslinker. The resulting electrodes were characterized by scanning electron microscopy (SEM) and electrochemical methods. And it was applied to develop a high‐sensitive electrochemical sensor for determination of sophoridine. Compared with the bare GCE and reduced graphene oxide modified electrode, the resulting electrodes exhibited excellent response toward the oxidation of sophoridine by significantly enhancing the oxidation peak currents and decreasing the overpotential of sophoridine. Under the selected conditions, there exist the linear relation between the oxidation peak currents and sophoridine concentration in the range of 1.0 x 10^‐6～1.0 x 10^‐4 mol L^‐1, with detection limit of 4.0 x 10^‐7 mol L^‐1. At the same time, the method can be successfully applied to the quantitative determination of sophoridine in injection samples and its result is satisfactory.     

Graphite Oxide and Gold Nanoparticles as Alternative Materials in the Design of a Highly Sensitive Electrochemical Sensor for the Simultaneous Determination of Biological Species

New aspects related to electrochemical performance of modified carbon paste electrodes (CPE) of high analytical performance are presented in this work. We studied whether the functionalisation of graphite powder with oxygenated functional groups (graphite oxide, GrO) could affect the electrochemical features of a classical CPE. By introducing oxygen termination over the graphite surface, a remarkable improvement in electrochemical performance was verified, including enhancement of analytical signals and charge transfer kinetics, as demonstrated from electrochemical characterisation assays conducted by cyclic voltammetry and electrochemical impedance spectroscopy towards the potassium hexa‐cyanoferrate (II/III) probe. In addition, a positive effect was noted from the anchoring of Au nanoparticles on GrOPE. In analytical terms, two biologically relevant molecules were simultaneously determined using the proposed modified electrode based on GrO and AuNPs: epinephrine (EP) and uric acid (UA). Using differential pulse voltammetry (DPV), wide linear concentration ranges were founded for the analytical curves of EP and UA, and the limits of detection of 1.0×10⁻⁷ mol L⁻¹ (EP) and 5.0×10⁻⁸ mol L⁻¹ (UA) were predicted, respectively. The designed electrochemical sensor showed excellent precision of measurement and appropriate applicability for the analysis of biological fluids.     

Fabrication of CeO2 Nanoparticle Modified Glassy Carbon Electrode for Ultrasensitive Determination of Trace Amounts of Uric Acid in Urine

The preparation of a glassy carbon electrode modified by CeO2 nanoparticles was described, which was characterized by cyclic voltammetry and electrochemical impedance spectroscopy. In pH 6.0 buffer, the CeO2 nanoparticle modified electrode （CeO2 NP/GC） gave an excellent electrocatalytic activity for the oxidation of uric acid （UA）. The catalytic current of UA versus its concentration had a good linear relation in the range of 2.0 × 10^-7-5.0 × 10^- 4 mol/L, with the correlation coefficient of 0.9986 and detection limit of 1.0 ×10^-7 mol/L. The modified electrode can be used for the determination of UA in urine, which can tolerate the interference of ascorbic acid up to 1000-fold. The method was simple, quick and sensitive.     

Electrochemical determination of nitrate on Ag nanoparticles-decorated poly (direct blue 15) electrodes

In this study, for the first time, the electro-polymerization of Direct blue15 (DB15), an azo dye, was carried out on the surface of ITO. Furthermore, the poly(DB15) surface was electrochemically decorated with Ag nanoparticles (AgNPs), and the fabricated AgNPs/PDB15 electrodes were examined as nitrate sensors. Compared to unmodified ITO electrode, the AgNPs/PDB15 electrode had greatly improved electrochemical response to nitrate reduction. The nitrate determination in a linear range from 1.0×10⁻⁵ mol L⁻¹ to 2.27×10⁻³ mol L⁻¹ was performed with a detection limit of 9.66 μM. The synthesized electrode is a promising sensor for the electrochemical detection of nitrate pollutants in water.     

Nanostructured Hexacyanoferrate Intercalated Ni/Al Layered Double Hydroxide Modified Electrode as a Sensitive Electrochemical Sensor for Paracetamol Determination

An electrochemical sensor for paracetamol (PC) based on the hexacyanoferate(III) intercalated Ni−Al layered double hydroxide (Ni−Al−HCF) was presented. The as‐prepared LDH structurally and morphologically was characterized by scanning electron microscopy, X‐ray diffraction, and Fourier transform IR. Electrochemical studies revealed that Ni−Al−HCF film modified glassy carbon (GC) electrode exhibited remarkable electrocatalytic activity toward the oxidation of paracetamol. The electrochemical behavior of PC on the Ni−Al−HCF film was investigated in detail. Under optimum experimental conditions, the electrocatalytic response of the modified GC electrode was linear in the PC concentration range 3×10⁻⁶⁻–1.5×10⁻³ mol L⁻¹, with a detection limit of 8×10⁻⁷ mol L⁻¹ (S/N=3), using hydrodynamic amperometry. In addition, the modified electrode exhibited good reproducibility, long‐term stability and anti‐interference property. The fabricated sensor was successfully applied to determination of PC in various pharmaceutical preparations such as tablets, oral solution, and oral drops. Finally, the method was validated by the analysis of paracetamol spiked human serum samples, and good recoveries were obtained in the range of 99.2–103 %.     

A Novel Electrochemical Glassy Carbon Electrode Modified with Carbon Black and Glyceline Deep Eutectic Solvent within a Crosslinked Chitosan Film for Simultaneous Determination of Acetaminophen and Diclofenac

Glyceline, reline, or ethaline deep eutectic solvents and carbon black nanoparticles within a crosslinked chitosan film are investigated as glassy carbon electrode modifiers for the first time. The selected 5 mg mL⁻¹ glyceline modified GCE was characterized by cyclic voltammetry, electrochemical impedance spectroscopy, and scanning electron microscopy. Simultaneous determination of acetaminophen and diclofenac by differential pulse adsorptive stripping voltammetry (DPAdSV) presented limits of detection of 2.6×10⁻⁸ and 5.2×10⁻⁸ mol L⁻¹ for acetaminophen and diclofenac, respectively, in pharmaceutical and biological samples. The obtained results were compared with those obtained by HPLC at a confidence level of 95 %.     

Voltammetric Detection of Captopril in a Commercial Drug Using a Gold-Copper Metal-organic Framework Nanocomposite Modified Electrode

We report the application of an electrochemical sensor based on gold-copper metal-organic framework immobilized on the surface of a glassy carbon electrode to the detection of captopril (CAP), an angiotensin-converting enzyme inhibitor. Cyclic voltammetric studies showed that the joint action of gold nanoparticles and copper-1,3,5-benzenetricarboxylate (Cu−BTC) enhanced the electrochemical response to the Cu-captopril complex that is adsorbed onto the surface of the electrode. Release of gold nanoparticles from Au@Cu−BTC not only increased the conductivity of the electrode but also provided a more favorable environment for the deposition of reduced Cu that is catalytically renewed on the electrode surface. The anodic current of the Cu^(II)−CAP oxidation peak varied linearly within two concentration ranges, namely 0.5 to 7.0 μmol L⁻¹ and 10 to 2500 μmol L⁻¹, with a limit of detection of 0.047 μmol L⁻¹. The mean recovery for the determination of captopril in commercial tablets was 100.3 % suggesting that the method has considerable potential for future industrial applications.     

Magnetite Nanoparticles Bonded Carbon Quantum Dots Magnetically Confined onto Screen Printed Carbon Electrodes and their Performance as Electrochemical Sensor for NADH

Hybrid magnetite/carbon quantum dots (MagNP/C‐dots) were prepared and their characterization performed by high resolution transmission electron microscopy (HR‐TEM), X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS). Because of their suitable magnetization and electrochemical properties, they were used as versatile electrode modifiers after magnetically confining onto screen printed carbon electrodes (SPE), with the aid of a miniature external magnet. The reported strategy introduces a convenient procedure for assembling modified electrodes, since the nanoparticles can be easily released by removing the magnet. The non‐enzymatic magnetic biosensor showed excellent performance in the determination of NADH at the concentration range 2×10⁻⁷ to 5×10⁻⁶ mol L⁻¹, exhibiting a sensitivity of 0.15 μmol L⁻¹ and detection limit of 20 nmol L⁻¹. The MagNP/C‐dots/SPE sensor was also successfully applied for the determination of NADH in serum samples. The interference of typical biological molecules has also been investigated.     

Functionalization of Reduced Graphene Oxide with β‐cyclodextrin Modified Palladium Nanoparticles for the Detection of Hydrazine in Environmental Water Samples

A sensitive and selective hydrazine sensor was developed by β‐cyclodextrin modified palladium nanoparticles decorated reduced graphene oxide (PdNPs‐β‐CD/rGO) nanocomposite. The PdNPs‐β‐CD/rGO hybrid material was prepared by simple electrochemical method. The hydrophobic cavity of β‐CD ineracts with palladium nanoparticles by hydrophobic interaction and further it is uniformly assembled on the rGO surface through hydrogen bond formation, which is clearly confirmed by FT‐IR, FESEM and TEM. The high electrocatalytic activity of hydrazine oxidation was observed at −0.05 V (vs. Ag/AgCl) on PdNPs‐β‐CD/rGO modified electrode; due to the excellent stabilization, high catalytic activity and large surface area of the PdNPs‐β‐CD/rGO composite. The PdNPs‐β‐CD/rGO fabricated hydrazine sensor exhibited an excellent analytical performance, including high sensitivity (1.95 μA μM⁻¹ cm⁻²), lower detection limit (28 nM) and a wide linear range (0.05 to 1600 μM). We also demonstrated that the PdNPs‐β‐CD/rGO nanocomposite modified electrode is a highly selective and sensitive sensor towards detection of hydrazine among the various interfering species. Hence, the proposed hydrazine sensor is able to determine hydrazine in different water samples.     

Determination of rutin using a CeO2 nanoparticle-modified electrode

CeO₂ nanoparticles approximately 12 nm in size were synthesized and subsequently characterized by XRD, TEM and UV-vis spectroscopy. Then, a gold electrode modified with CeO₂ nanoparticles was constructed and characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The modified electrode demonstrated strong catalytic effects with high stability towards electrochemical oxidation of rutin. The anodic peak currents (measured by differential pulse voltammetry) increased linearly with the concentration of rutin in the range of 5.0 × 10⁻⁷–5.0 × 10⁻⁴ mol · L⁻¹. The detection limit (S/N = 3) was 2.0 × 10⁻⁷ mol · L⁻¹. The relative standard deviation (RSD) of 8 successive scans was 3.7% for 5.0 × 10⁻⁶ mol · L⁻¹ rutin. The method showed excellent sensitivity and stability, and the determination of rutin in tablets was satisfactory.     

Simultaneous Electrochemical Determination of Hydroquinone and Bisphenol A using a Carbon Paste Electrode Modified with Silver Nanoparticles

In this paper, a rapid and sensitive modified electrode for the simultaneous determination of hydroquinone (HQ) and bisphenol A (BPA) is proposed. The simultaneous determination of these two compounds is extremely important since they can coexist in the same sample and are very harmful to plants, animals and the environment in general. A carbon paste electrode (CPE) was modified with silver nanoparticles (nAg) and polyvinylpyrrolidone (PVP). The PVP was used as a reducing and stabilizing agent of nAg from silver nitrate in aqueous media. The nAg‐PVP composite obtained was characterized by transmission electron microscopy and UV‐vis spectroscopy. The electrochemical behavior of HQ and BPA at the nAg‐PVP/CPE was investigated in 0.1 mol L⁻¹ B−R buffer (pH 6.0) using cyclic voltammetry (CV) and square wave voltammetry (SWV). The results indicate that the electrochemical responses are improved significantly with the use of the modified electrode. The calibration curves obtained by SWV, under the optimized conditions, showed linear ranges of 0.09–2.00 μmol L⁻¹ for HQ (limit of detection 0.088 μmol L⁻¹) and 0.04–1.00 μmol L⁻¹ for BPA (limit of detection 0.025 μmol L⁻¹). The modified electrode was successfully applied in the analysis of water samples and the results were comparable to those obtained using UV‐vis spectroscopy.     

A Biosensor for Genetic Modified Soybean DNA Determination via Adsorption of Anthraquinone‐2‐sulphonic Acid in Reduced Graphene Oxide

An electrochemical DNA biosensor for DNA determination of genetically modified (GM) soybean (CaMV 35S target genes) was developed utilizing a new detection concept based on the adsoption of anthraquinone‐2‐sulphonic acid (AQMS) on the reduced graphene oxide nano‐particles (rGO) during DNA hybridization events. The aminated DNA probe for CaMV 35S was immobilized onto poly(n‐butyl acrylate) film modified with succinimide functional groups [poly(nBA‐NAS)] via peptide covalent bond. Nanosheets of rGO were entrapped in the poly(nBA‐NAS) film to form a conducting [poly(nBA‐NAS)‐rGO] film of the DNA biosensor. Besides facilitating the electron transfer reactions, the rGO also functioned as an adsorbent for AQMS. The sensing mechanism of the proposed DNA biosensor involved measuring the oxidation current of the AQMS adsorbed on the electrode surface at −0.50 V using differential pulse voltammetry (DPV) before and after a DNA hybridization event. Under optimum conditions, the DNA biosensor demonstrated a linear proportionality between AQMS oxidation signal and logarithm cDNA concentration from 1.0×10⁻¹⁵ M to 1.0×10⁻⁸ M target DNA with a detection limit of 6.3×10⁻¹⁶ M. The electrochemical DNA biosensor possessed good selectivity and a shelf life of about 40 days with relative standard deviation of reproducibility obtained in the range of 3.7–4.6% (n=5). Evaluation of the DNA biosensor using GM soybean DNA extracts showed excellent recovery percentages of 97.2–104.0.