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.

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.

Highly Sensitive Graphene-Based Electrochemical Sensor for Nitrite Assay in Waters.

The importance of nitrite ions has long been recognized due to their extensive use in environmental chemistry and public health. The growing use of nitrogen fertilizers and additives containing nitrite in processed food items has increased exposure and, as a result, generated concerns about potential harmful health consequences. This work presents the development of an electrochemical sensor based on graphene/glassy carbon electrode (EGr/GC) with applicability in trace level detection of nitrite in water samples. According to the structural characterization of the exfoliated material, it appears as a mixture of graphene oxide (GO; 21.53%), few-layers graphene (FLG; 73.25%) and multi-layers graphene (MLG; 5.22%) and exhibits remarkable enhanced sensing response towards nitrite compared to the bare electrode (three orders of magnitude higher). The EGr/GC sensor demonstrated a linear range between 3 × 10-7 and 10-3 M for square wave voltammetry (SWV) and between 3 × 10-7 and 4 × 10-4 M for amperometry (AMP), with a low limit of detection LOD (9.9 × 10-8 M). Excellent operational stability, repeatability and interference-capability were displayed by the modified electrode. Furthermore, the practical applicability of the sensor was tested in commercially available waters with excellent results.

Antibacterial Enhancement of High-Efficiency Particulate Air Filters Modified with Graphene-Silver Hybrid Material.

Bacterial infections are a major concern as antibiotic resistance poses a great threat, therefore leading to a race against time into finding new drugs or improving the existing resources. Nanomaterials with high surface area and bactericidal properties are the most promising ones that help combating microbial infections. In our case, graphene decorated with silver nanoparticles Gr-Ag (5 wt% Ag) exhibited inhibitory capacity against S. aureus and E. coli. The newly formed hybrid material was next incubated with high-efficiency particulate air (HEPA) filter, to obtain one with bactericidal properties. The modified filter had greater inhibitory action against the tested strains, compared to the control, and the effect was better against the Gram-negative model. Even if the bacteria remained attached to the filters, their colony forming unit capacity was affected by the Gr-Ag (5 wt% Ag) hybrid material, when they were subsequently re-cultured on fresh agar media. Therefore, the HEPA filter modified with Gr-Ag (5 wt% Ag) has high antibacterial properties that may substantially improve the existing technology.

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.

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.

Carbon nanomaterial functionalization with pesticide-detoxifying carboxylesterase.

Four carbon materials, spent coffee-ground biochar, carbon black, short CNTs, and nitrogen-doped few-layer graphene (N-graphene) were tested for their functionalization with a commercial carboxylesterase. Their robustness to variations in time and key physicochemical parameters (temperature and pH) was analysed. In general, carbon nanomaterials showed better performance than biochar, both in terms of binding capacity and resilience in harsh conditions, at statistically significant levels. Among the tested materials, functionalized N-graphene also showed the highest level of inhibition of carboxylesterase by pesticide exposure. Therefore, N-graphene was selected for biotechnological application of pesticide scavenging toxicity in T. thermophila, a ciliate bioindicator of water quality. While immobilization of the enzyme was not effective in the case of carbaryl, a methyl carbamate, in the case of the organophosphorus dichlorvos, a 1- or 30-min contact time with a water solution containing 5 times the LC100 - 0.5 mM - allowed 50% and 100% rescue of ciliate survival, respectively. These results suggest that functionalization with carboxylesterase may be of additional benefit compared to bare carbon in water clean-up procedures, especially for highly hydrophilic pesticides such as dichlorvos.

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.

Eco-friendly synthesis of sulphur-doped graphenes with applicability in caffeic acid electrochemical assay.

A new electrode based on glassy carbon modified with a sulphur-doped graphene material was successfully developed and applied for caffeic acid (CA) voltammetric detection and quantification. The structural features of sulphur-doped graphene (exfGR-S) characterized by different physicochemical and analytical techniques are presented. Cyclic voltammetry (CV) technique was employed to evaluate the electrochemical behavior of both bare glassy carbon (GCE) and modified GCE/exfGr-S electrodes towards CA oxidation. The study revealed that the modified electrode exhibits superior electrochemical performances compared to the bare electrode, with a broad CA detecting range (from 0.1 to 100.0 µM), a low detection limit 3.03 × 10-8 M), excellent anti-interference capabilities, as well as good stability and repeatability. The developed electrochemical sensor appears to be a promising candidate for real sample quality control analysis since it successfully displayed its ability to directly detect CA in commercially available coffee product without any pretreatment.

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.

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.

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%.

Investigation of L-Tryptophan Electrochemical Oxidation with a Graphene-Modified Electrode.

A graphene sample (EGr) was prepared by electrochemical exfoliation of graphite rods in solution containing 0.05 M (NH4)2SO4 + 0.1 M H3BO3 + 0.05 M NaCl. The exfoliation was performed by applying a constant voltage (12 V) between the graphite rods, while the temperature was kept constant (18 °C) with a temperature-controlled cryostat. The structural investigation of the graphene sample, performed by X-ray powder diffraction (XRD), revealed that the sample consists of a mixture of few-layer (69%), multi-layer graphene (14%) and graphene oxide (17%). In addition, XPS analysis proved that the sample was triple-doped with heteroatoms such as nitrogen (1.7 at%), sulfur (2.5 at%), and boron (3 at%). The sample was deposited onto the surface of a clean, glassy carbon electrode (GC) and investigated for the non-enzymatic electrochemical detection of L-tryptophan (TRP). The electrocatalytic properties of the EGr/GC electrode led to a considerable decrease in the oxidation potential from +0.9 V (bare GC) to +0.72 V. In addition, the EGr/GC electrode has higher sensitivity (two times) and a lower detection limit (ten times) in comparison with the bare GC electrode.

Stochastic biosensors based on N- and S-doped graphene for the enantioanalysis of aspartic acid in biological samples.

Metabolomics plays a very important role in cancer mechanism and also in the early diagnosis of cancer. The enantioanalysis of aminoacids found in biological samples may favor the development of new screening tests for the early diagnosis of cancer. Two stochastic biosensors, based on the immobilization of α-hemolysin and hemin in exfoliated reduced graphene modified with nitrogen and sulphur, were proposed for the enantioanalysis of aspartic acid in biological samples (whole blood, urine, saliva, and tissue). The proposed biosensors showed low limits of quantification, high sensitivity, and large linear concentration ranges. The recoveries of d- and l-aspartic acid in biological samples were higher than 95.00%, with a relative standard deviation lower than 1.00%.

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.

Photocatalytic and Electrocatalytic Properties of NGr-ZnO Hybrid Materials.

N-doped graphene-ZnO hybrid materials with different N-doped graphene:ZnO wt% ratios (1:10; 1:20; 1:30) were prepared by a simple and inexpensive sol-gel method. The materials denoted NGr-ZnO-1 (1:10), NGr-ZnO-2 (1:20), and NGr-ZnO-3 (1:30) were investigated with advanced techniques and their morpho-structural, photocatalytic, and electrocatalytic properties were reported. Hence, pure N-doped graphene sample contains flakes with the size ranging from hundreds of nanometers to micrometers. In the case of all NGr-ZnO hybrid materials, the flakes appear heavily decorated with ZnO nanoparticles, having a cauliflower-like morphology. The X-ray powder diffraction (XRD) investigation of N-doped graphene sample revealed that it was formed by a mixture of graphene oxide, few-and multi-layer graphene. After the ZnO nanoparticles were attached to graphene, major diffraction peaks corresponding to crystalline planes of ZnO were seen. The qualitative and quantitative compositions of the samples were further evidenced by X-ray photoelectron spectroscopy (XPS). In addition, UV photoelectron spectroscopy (UPS) spectra allowed the determination of the ionization energy and valence band maxima. The energy band alignment of the hybrid materials was established by combining UV-Vis with UPS results. A high photocatalytic activity of NGr-ZnO samples against rhodamine B solution was observed. The associated reactive oxygen species (ROS) generation was monitored by electron paramagnetic resonance (EPR)-spin trapping technique. In accordance with bands alignment and identification of radical species, the photocatalytic mechanism was elucidated.

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.

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).

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.