Harnessing Graphene-Modified Electrode Sensitivity for Enhanced Ciprofloxacin Detection.

Increased evidence has documented a direct association between Ciprofloxacin (CFX) intake and significant disruption to the normal functions of connective tissues, leading to severe health conditions (such as tendonitis, tendon rupture and retinal detachment). Additionally, CFX is recognized as a potential emerging pollutant, as it seems to impact both animal and human food chains, resulting in severe health implications. Consequently, there is a compelling need for the precise, swift and selective detection of this fluoroquinolone-class antibiotic. Herein, we present a novel graphene-based electrochemical sensor designed for Ciprofloxacin (CFX) detection and discuss its practical utility. The graphene material was synthesized using a relatively straightforward and cost-effective approach involving the electrochemical exfoliation of graphite, through a pulsing current, in 0.05 M sodium sulphate (Na2SO4), 0.05 M boric acid (H3BO3) and 0.05 M sodium chloride (NaCl) solution. The resulting material underwent systematic characterization using scanning electron microscopy/energy dispersive X-ray analysis, X-ray powder diffraction and Raman spectroscopy. Subsequently, it was employed in the fabrication of modified glassy carbon surfaces (EGr/GC). Linear Sweep Voltammetry studies revealed that CFX experiences an irreversible oxidation process on the sensor surface at approximately 1.05 V. Under optimal conditions, the limit of quantification was found to be 0.33 × 10-8 M, with a corresponding limit of detection of 0.1 × 10-8 M. Additionally, the developed sensor's practical suitability was assessed using commercially available pharmaceutical products.

Analytical Applicability of Graphene-Modified Electrode in Sunset Yellow Electrochemical Assay.

Due to the recent increase in average living standards, food safety has caught public attention. It is necessary to conduct a qualitative and quantitative rapid test of prohibited food additives since the inclusion of food additives or the improper usage of synthetic dyes can negatively impact on the human health. Herein, a highly sensitive method for Sunset Yellow detection based on a glassy carbon electrode modified with few-layer graphenes was proposed. The electrochemical behavior of SY at the GR-exf/GCE modified surface was investigated by Cyclic Voltammetry, Square Wave Voltammetry, Electrochemical Impedance Spectroscopy and Amperometry. The influences of pH, scan rate, and interfering species were studied. Under optimized conditions, the developed sensor shows good linearity over a broad SY concentration range, e.g., 0.028-30 µM, with a low limit of detection (LOD = 0.0085 µM) and quantification (LOQ = 0.028 µM) (data obtained by amperometric technique). Furthermore, the modified electrode shows good selectivity, precision and sensitivity and has been successfully applied for SY quantification from commercially available pharmaceutical formulation as well as from candy bars and orange juice.

Graphene-Based Electrochemical Sensing Platform for Rapid and Selective Ferulic Acid Quantification.

Due to the multitude of physiological functions, ferulic acid (FA) has a wide range of applications in the food, cosmetic, and pharmaceutical industries. Thus, the development of rapid, sensitive, and selective detection tools for its assay is of great interest. This study reports a new electroanalytical approach for the quantification of ferulic acid in commercial pharmaceutical samples using a sulphur-doped graphene-based electrochemical sensing platform. The few-layer graphene material (exf-SGR) was prepared by the electrochemical oxidation of graphite, at a low applied bias (5 V), in an inorganic salt mixture of Na2S2O3/(NH4)2SO4 (0.3 M each). According to the morpho-structural characterization of the material, it appears to have a high heteroatom doping degree, as proved by the presence of sulphur lines in the XRD pattern, and the C/S ratio was determined by XPS investigations to be 11.57. The electrochemical performances of a glassy carbon electrode modified with the exf-SGR toward FA detection were tested by cyclic voltammetry in both standard laboratory solutions and real sample analysis. The developed modified electrode showed a low limit of detection (30.3 nM) and excellent stability and reproducibility, proving its potential applicability as a viable solution in FA qualitative and quantitative analysis.

Stochastic microsensors based on modified graphene for pattern recognition of maspin in biological samples.

Maspin is a novel serine protease inhibitor differentially expressed in several types of human cancers. It proved to be a key biomarker in the assessment of gastric cancer. Therefore, we design, characterize, and validate two stochastic microsensors based on graphene co-doped with N and S, and modified with α-cyclodextrin and maltodextrin, for the pattern recognition and quantification of maspin in whole blood, gastric tumor tissue, saliva, and urine. While the sensitivities were comparable with magnitude order, the variations were in the wideness of the linear concentration range, when measurements were performed at a pH of 7.40. Very low limits of quantification were recorded at both working pHs: 7.40, and 3.00. High recoveries of maspin in whole blood, gastric tissue tumor, saliva, and urine were also recorded.

N,S-Decorated graphenes modified with 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine manganese(III) chloride-based 3D needle stochastic sensors for enantioanalysis of arginine: a key factor in the metabolomics and early detection of gastric cancer.

Arginine has an important role in the metabolomics of gastric cancer. Two 3D enantioselective needle stochastic sensors based on the physical immobilization of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine manganese(III) chloride (solution, 10-3 mol L-1) in graphene paste matrices decorated with N and S atoms were designed, characterized and validated for the enantioanalysis of arginine in whole blood and tissue samples. The signature values obtained for the enantiomers of arginine confirmed that the stochastic sensors are enantioselective. The lowest limit of quantification obtained for both enantiomers of arginine was 1 fmol L-1, while sensitivity of up to 1011 s-1 mol-1 L was recorded for the stochastic sensors. High recoveries were obtained for the determination of one enantiomer in the presence of the other one; moreover, very good correlation was found between the results obtained with the two 3D enantioselective needle stochastic sensors.

Electrochemical L-Tyrosine Sensor Based on a Glassy Carbon Electrode Modified with Exfoliated Graphene.

In this study, a graphene sample (EGr) was synthesized by electrochemical exfoliation of graphite rods in electrolyte solution containing 0.1 M ammonia and 0.1 M ammonium thiocyanate. The morphology of the powder deposited onto a solid substrate was investigated by the scanning electron microscopy (SEM) technique. The SEM micrographs evidenced large and smooth areas corresponding to the basal plane of graphene as well as white lines (edges) where graphene layers fold-up. The high porosity of the material brings a major advantage, such as the increase of the active area of the modified electrode (EGr/GC) in comparison with that of bare glassy carbon (GC). The graphene modified electrode was successfully tested for L-tyrosine detection and the results were compared with those of bare GC. For EGr/GC, the oxidation peak of L-tyrosine had high intensity (1.69 × 10-5 A) and appeared at lower potential (+0.64 V) comparing with that of bare GC (+0.84 V). In addition, the graphene-modified electrode had a considerably larger sensitivity (0.0124 A/M) and lower detection limit (1.81 × 10-6 M), proving the advantages of employing graphene in electrochemical sensing.

Highly Sensitive Electrochemical Detection of Azithromycin with Graphene-Modified Electrode.

An electrochemical cell containing two graphite rods was filled with the appropriate electrolyte (0.2 M ammonia + 0.2 M ammonium sulphate) and connected to the exfoliation system to synthesize graphene (EGr). A bias of 7 V was applied between the anode and cathode for 3 h. After synthesis, the morphology and structure of the sample was characterized by SEM, XRD, and FTIR techniques. The material was deposited onto the surface of a glassy carbon (GC) electrode (EGr/GC) and employed for the electrochemical detection of azithromycin (AZT). The DPV signals recorded in pH 5 acetate containing 6 × 10-5 M AZT revealed significant differences between the GC and EGr/GC electrodes. For EGr/GC, the oxidation peak was higher and appeared at lower potential (+1.12 V) compared with that of bare GC (+1.35 V). The linear range for AZT obtained with the EGr/GC electrode was very wide, 10-8-10-5 M, the sensitivity was 0.68 A/M, and the detection limit was 3.03 × 10-9 M. It is important to mention that the sensitivity of EGr/GC was three times higher than that of bare GC (0.23 A/M), proving the advantages of using graphene-modified electrodes in the electrochemical detection of AZT.

Enhanced Acetaminophen Electrochemical Sensing Based on Nitrogen-Doped Graphene.

Because of the widespread acetaminophen usage and the danger of harmful overdosing effects, developing appropriate procedures for its quantitative and qualitative assay has always been an intriguing and fascinating problem. A quick, inexpensive, and environmentally friendly approach based on direct voltage anodic graphite rod exfoliation in the presence of inorganic salt aqueous solution ((NH4)2SO4-0.3 M) has been established for the preparation of nitrogen-doped graphene (exf-NGr). The XRD analysis shows that the working material appears as a mixture of few (76.43%) and multi-layers (23.57%) of N-doped graphenes. From XPS, the C/O ratio was calculated to be 0.39, indicating a significant number of structural defects and the existence of multiple oxygen-containing groups at the surface of graphene sheets caused by heteroatom doping. Furthermore, the electrochemical performances of glassy carbon electrodes (GCEs) modified with exf-NGr for acetaminophen (AMP) detection and quantification have been assessed. The exf-NGr/GCE-modified electrode shows excellent reproducibility, stability, and anti-interfering characteristics with improved electrocatalytic activity over a wide detection range (0.1-100 µM), with a low limit for AMP detection (LOD = 3.03 nM). In addition, the developed sensor has been successfully applied in real sample analysis for the AMP quantification from different commercially available pharmaceutical formulations.

Hydrothermal Synthesis of Nitrogen, Boron Co-Doped Graphene with Enhanced Electro-Catalytic Activity for Cymoxanil Detection.

A sample of nitrogen and boron co-doped graphene (NB-Gr) was obtained by the hydrothermal method using urea and boric acid as doping sources. According to XRD analysis, the NB-Gr sample was formed by five-layer graphene. In addition, the XPS analysis confirmed the nitrogen and boron co-doping of the graphene sample. After synthesis, the investigation of the electro-catalytic properties of the bare (GC) and graphene-modified electrode (NB-Gr/GC) towards cymoxanil detection (CYM) was performed. Significant differences between the two electrodes were noticed. In the first case (GC) the peak current modulus was small (1.12 × 10-5 A) and appeared in the region of negative potentials (-0.9 V). In contrast, when NB-Gr was present on top of the GC electrode it promoted the transfer of electrons, leading to a large peak current increase (1.65 × 10-5 A) and a positive shift of the peak potential (-0.75 V). The NB-Gr/GC electrode was also tested for its ability to detect cymoxanil from a commercial fungicide (CURZATE MANOX) by the standard addition method, giving a recovery of 99%.

Enantioanalysis of glutamine-a key factor in establishing the metabolomics process in gastric cancer.

Gastric cancer is the second leading cause of death in the world. Early detection will facilitate early treatment and full recovery of the patients. Metabolomics facilitated the detection of few amino acids able to be used as biomarkers. It was always assumed that the L-enantiomer of the chiral amino acid is part of the process, forgetting the twisting of DNA molecules which may also produce the D-enantiomer of the amino acid. Therefore, an enantioanalysis of amino acids such as glutamine which are part of the gastric cancer metabolomics is absolutely necessary. Four stochastic sensors based on immobilization of protoporphyrin IX, ß-cyclodextrin, and 2,2-diphenyl-1-picrylhydrazyl on graphene materials were proposed for the enantioanalysis of glutamine in whole blood samples of patients with gastric cancer. Different signatures were obtained for the enantiomers for each stochastic sensor, making possible the enantioanalysis of glutamine in large concentration ranges-from fmol/L to mmol/L; these ranges facilitating the enantioanalysis of glutamine in healthy patients as well as in patients were found in early and later stages of gastric cancer. Graphical abstract.

Myoglobin-silver reduced graphene oxide nanocomposite stochastic biosensor for the determination of luteinizing hormone and follicle-stimulating hormone from saliva samples.

A silver reduced graphene oxide (Ag-rGO) nanocomposite paste was prepared for the assay of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in children's saliva. The paste was modified with a solution of myoglobin (Myb) to form the stochastic biosensor, to improve the sensitivity of the method. The Ag-rGO powder and Myb-Ag-rGO paste were morphologically and structurally characterized by SEM, TEM and XRD measurements. For the molecular recognition of the hormones, the biosensor based on Myb-Ag-rGO reached determination limits of 8.5 × 10-3 UI L-1 (2.0 × 10-12 g/mL) for luteinizing hormone and 1.4 × 10-2 UI L-1 (1.0 × 10-12 g/mL) for follicle-stimulating hormone. The determination of both hormones in saliva samples collected from children was done with high reliability. Graphical Abstract.

Thermally reduced graphene oxide as green and easily available adsorbent for Sunset yellow decontamination.

The release of synthetic food dyes, like Sunset yellow, into industrial effluents can cause serious environmental and health problems. Due to its aromatic structure, it is recalcitrant towards degradation into non-toxic intermediates and its removal by efficient adsorption represents a cheap and efficient method. Herein we propose the use of thermally reduced graphene oxide (TRGO) as effective Sunset yellow dye adsorbent with an adsorption maximum capacity comparable with other sophisticated, chemically synthesized carbon-based nanomaterials. The reduced graphene oxide and the Sunset yellow adsorbed one were characterized by FT-IR, XPS and XRD spectroscopy, N2 adsorption-desorption isotherm and TGA analysis. BET surface area reduced from 274.1 m2/g (for TRGO) to 39.9 m2/g (for TRGO-SY) showing that Sunset Yellow molecules occupied the corresponding active sites while the number of sheets resulted from the XRD spectra - from 3 to 8 in TRGO to 5 in TRGO-SY indicates the ordered intercalations in the graphene structure. The adsorption isotherm experimental data were better fitted with the Langmuir model than the Freundlich model, with the maximum adsorption capacity of the SY dye monolayer of 243.3 mg/g at pH = 6.0 and 189.0 mg/g from synthetic wastewater. The kinetic study revealed a perfect fit following the Pseudo-second order model with an equilibrium achieved within 30 min. The lack of adsorption on the starting graphene oxide is indicative for π-π interactions between the adsorbate and adsorbent.

Enantioanalysis of tryptophan in whole blood samples using stochastic sensors-A screening test for gastric cancer.

Tryptophan is a key amino acid related to metabolomics in gastric cancer. To date, methods were developed only for the assay of l-tryptophan, the role of d-tryptophan being not yet established. Therefore, four stochastic sensors based on different graphene materials modified with ß-cyclodextrins, 2,2-diphenyl-1-picrylhydrazyl, and protoporphyrin IX were designed and used for enantioanalysis of tryptophan in whole blood samples. High sensitivities, and reliabilities were recorded when the stochastic sensors were used for the enantioanalysis of tryptophan in whole blood samples. The paper opened a new chapter in early detection of gastric cancer, based on establishing the role of d-tryptophan in metabolomics, and in early diagnosis of gastric cancer.

Nitrogen-Doped Graphene: The Influence of Doping Level on the Charge-Transfer Resistance and Apparent Heterogeneous Electron Transfer Rate.

Three nitrogen-doped graphene samples were synthesized by the hydrothermal method using urea as doping/reducing agent for graphene oxide (GO), previously dispersed in water. The mixture was poured into an autoclave and placed in the oven at 160 °C for 3, 8 and 12 h. The samples were correspondingly denoted NGr-1, NGr-2 and NGr-3. The effect of the reaction time on the morphology, structure and electrochemical properties of the resulting materials was thoroughly investigated using scanning electron microscopy (SEM) Raman spectroscopy, X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), elemental analysis, Cyclic Voltammetry (CV) and electrochemical impedance spectroscopy (EIS). For NGr-1 and NGr-2, the nitrogen concentration obtained from elemental analysis was around 6.36 wt%. In the case of NGr-3, a slightly higher concentration of 6.85 wt% was obtained. The electrochemical studies performed with NGr modified electrodes proved that the charge-transfer resistance (Rct) and the apparent heterogeneous electron transfer rate constant (Kapp) depend not only on the nitrogen doping level but also on the type of nitrogen atoms found at the surface (pyrrolic-N, pyridinic-N or graphitic-N). In our case, the NGr-1 sample which has the lowest doping level and the highest concentration of pyrrolic-N among all nitrogen-doped samples exhibits the best electrochemical parameters: a very small Rct (38.3 Ω), a large Kapp (13.9 × 10-2 cm/s) and the best electrochemical response towards 8-hydroxy-2'-deoxyguanosine detection (8-OHdG).

Stone Paper as a New Substrate to Fabricate Flexible Screen-Printed Electrodes for the Electrochemical Detection of Dopamine.

Flexible screen-printed electrodes (HP) were fabricated on stone paper substrate and amperometrically modified with gold nanoparticles (HP-AuNPs). The modified electrode displayed improved electronic transport properties, reflected in a low charge-transfer resistance (1220 Ω) and high apparent heterogeneous electron transfer rate constant (1.94 × 10-3 cm/s). The voltammetric detection of dopamine (DA) was tested with HP and HP-AuNPs electrodes in standard laboratory solutions (pH 6 phosphate-buffered saline (PBS)) containing various concentrations of analyte (10-7-10-3 M). As expected, the modified electrode exhibits superior performances in terms of linear range (10-7-10-3 M) and limit of detection (3 × 10-8 M), in comparison with bare HP. The determination of DA was tested with HP-AuNPs in spiked artificial urine and in pharmaceutical drug solution (ZENTIVA) that contained dopamine hydrochloride (5 mg/mL). The results obtained indicated a very good DA determination in artificial urine without significant matrix effects. In the case of the pharmaceutical drug solution, the DA determination was affected by the interfering species present in the vial, such as sodium metabisulfite, maleic acid, sodium chloride, and propylene glycol.

Detection of 8-Hydroxy-2'-Deoxyguanosine Biomarker with a Screen-Printed Electrode Modified with Graphene.

In this work we present the preparation of graphene material by exfoliation of graphite rods via pulses of current in electrolyte, containing a mixture of boric acid (0.05 M) and sodium chloride (0.05 M). The material was morphologically and structurally characterized by SEM/TEM/HR-TEM, XRD and FTIR techniques. TEM investigation of graphene flakes deposited onto carbon-coated grids allowed the visualization of thin and transparent regions, attributed to few-layer graphene (FLG), as well as thick and dark regions attributed to multi-layer graphene (MLG). The mixed composition of the material was additionally confirmed by XRD, which further indicated that the amount of FLG within the sample was around 83%, while MLG was around 17%. The performance of a screen-printed electrode (SPE) modified with graphene (SPE-Gr) was tested for 8-hydroxy-2'-deoxyguanosine detection. The graphene-modified electrode had a higher sensitivity in comparison with that of SPE, both in standard laboratory solutions (phosphate buffered saline-PBS) and in human saliva.

Sensitive detection of hydroquinone using exfoliated graphene-Au/glassy carbon modified electrode.

Graphene nanosheets (EGr) were electrochemically prepared through one-step exfoliation of a graphite rod in a mixture of H2SO4:HNO3 (3:1) at low bias (4 V). Subsequently, gold nanoparticles were attached to the graphene surface (EGr-Au) by the reduction of the metal precursor (HAuCl4) in aqueous solution containing dispersed graphene sheets. According to the XRD investigation, the synthesized material consists of a mixture of few-layer (86%) and multi-layer (14%) graphene. The interlayer distance was found to be in the range of 0.466-0.342 nm, which is larger than the interlayer distance in graphite (0.335 nm). The average size of gold nanoparticles in the EGr-Au sample was 24 nm, in excellent agreement with the TEM results. The synthesized material was then employed to modify a glassy carbon (GC) substrate, in order to obtain a modified electrode (GC/EGr-Au). Next, the electrochemical behavior of hydroquinone (HQ) in the presence and absence of interfering species, catechol (CAT) and bisphenol A (BPA) was studied and the corresponding calibration curves were plotted. Thus, in solutions without interfering species, the GC/EGr-Au electrode has a wide linear range (3 × 10-7-10-4 M), high sensitivity (0.089 A M-1) and low detection limit (LOD = 10-7 M; S/N = 3). The presence of either catechol or bisphenol A leads to the increase of LOD to 2 × 10-7 M, and in addition changes the electrode sensitivity, up to 0.146 A M-1.

Pattern recognition of 8-hydroxy-2'-deoxyguanosine in biological fluids.

8-Hydroxy-2'-deoxyguanosine (8-OHdG), a product of oxidative DNA damage, which has been used as a sensitive and reliable marker of oxidative stress and carcinogenesis, is found in high levels in biological fluids of leukemia patients. A reliable screening method based on pattern recognition of 8-OHdG using stochastic sensors designed with graphene materials decorated with nanoparticles of TiO2Ag or TiO2Au was developed. 5,10,15,20-Tetraphenyl-21H,23H porphyrin (P), 2,6-bis((E)-2-(furan-2-yl)vinyl)-4-(4,6,8-trimethylazulen-1-yl)pyridine (Py1), and 2,6-bis((E)-2-(thiophen-3-yl)vinyl)-4-(4,6,8-trimethylazulen-1-yl)pyridine (Py2) were used as modifiers of the graphene paste in the design of sensors. The screening method used for pattern recognition was developed for two pH values accordingly with the nature of the biological fluid to be screened: pH = 3.02 for urine samples and pH = 7.49 for whole blood samples. High sensitivities and low limits of determination for 8-OHdG obtained at both pH values favored the early detection of leukemia. Recoveries over 98.00% with RSD (%) values lower than 1.00% proved the reliability of the proposed screening method. Graphical abstract Graphene based sensors detect 8-hydroxy-2'-deoxyguanosine in biological fluids.

Electrochemical platform based on nitrogen-doped graphene/chitosan nanocomposite for selective Pb2+ detection.

A novel nanocomposite was developed and used for trace determination of Pb2+ cations from aqueous solutions. The nanocomposite was obtained by the association of N-doped graphene (N-Gr) with a biocompatible polymer, namely chitosan (Ch). The characterization of the new nanocomposite material (Ch-N-Gr) was performed using TEM, STEM-EDX, SEM, XRD and XPS techniques. Compared with the bare gold electrode (GE) a remarkable enhancement of the voltammetric response of the modified electrode (Ch-N-Gr/GE) was always observed. Using the Ch-N-Gr/GE, the Pb2+ voltammetric response showed a pair of well defined, quasi-reversible anodic and cathodic peaks, with the peak potentials located at about -0.59 V and -0.69 V, respectively. The calibration curves were obtained over a large linear range, from 10-7 to 10-4 M Pb2+ concentration. Under optimized conditions, the detection limit was found to be 6.64 × 10-8 M. The effect of several interfering species (such as other metallic cations or organic compounds of various concentrations) on the determination of Pb2+ concentration was also studied, and the results proved the selectivity of the proposed modified electrode. The validity and effectiveness of the method was further confirmed by trace determination of Pb2+ in real samples.

Graphene-bimetallic nanoparticle composites with enhanced electro-catalytic detection of bisphenol A.

This study brings for the first time novel knowledge about the synthesis by catalytic chemical vapor deposition with induction heating of graphene-bimetallic nanoparticle composites (Gr-AuCu and Gr-AgCu) and their morphological and structural characterization by transmission electron microscopy, Raman spectroscopy, and x-ray powder diffraction. Gold electrodes modified with the obtained materials exhibit an enhanced electro-catalytic effect towards one of the most encountered estrogenic disruptive chemicals, bisphenol A (BPA). The BPA behavior in varying pH solutions was investigated using the electrochemical quartz crystal microbalance, which allowed the accurate determination of the number of molecules involved in the oxidation process. The modified electrodes promote the oxidation of BPA at significantly lower potentials (0.66 V) compared to bare gold (0.78 V). In addition, the peak current density recorded with such electrodes greatly exceeded that obtained with bare gold (e.g. one order of magnitude larger, for a Au/Gr-AgCu electrode). The two modified electrodes have low detection limits, of 1.31 × 10-6 M and 1.91 × 10-6 M for Au/Gr-AgCu and Au/Gr-AuCu, respectively. The bare gold electrode has a higher detection limit of 5.1 × 10-6 M. The effect of interfering species (e.g. catechol and 3-nitrophenol) was also investigated. Their presence influenced not only the BPA peak potential, but also the peak current. With both modified electrodes, no peak currents were recorded below 3 × 10-5 M BPA.