Electrochemical Sensors Based on a Composite of Electrochemically Reduced Graphene Oxide and PEDOT:PSS for Hydrazine Detection

Fluorescent probe for imaging N2H4 in plants, food, and living cells and for quantitative detection of N2H4 in soil and water using a smartphone

Hydrazine is volatile and highly toxic, causing severe harm to water, soil, air, and organisms. Therefore, real-time detection and long-term monitoring of hydrazine are crucial for environmental protection and human health. Herein, an "OFF-ON" fluorescent probe 5-((10-ethyl-2-methoxy-10 H-phenothiazin-3-yl)methylene)-2,2-dimethyl-1,3-dioxane-4,6-dione (MPD) for hydrazine detection through a nucleophilic addition reaction was developed. MPD could exclusively identify hydrazine through colorimetric and fluorescent dual-channel responses within 30 s, which also demonstrated high sensitivity (detection limit, 12 nM) and a wide pH range (6 -12). The sensing mechanism of MPD was confirmed using theoretical calculations, where fluorescence was emitted following the recognition of hydrazine because of the disappearance of the photoinduced electron transfer (PET) process. Using a smartphone, MPD enabled the quantitative detection of hydrazine in real water samples and sandy soil. Notably, in the process of detecting hydrazine in actual water samples, the establishment of analytical methods and the completion of rapid quantitative detection only required a smartphone and built-in apps. Additionally, we showed that MPD could recognize hydrazine in various environmental samples, including plants, food, hydrazine vapors, and cells. We believe that the fluorescent probe MPD developed in this study and the established smartphone visualization platform will provide a convenient and effective tool for detecting hydrazine in environmental monitoring, food safety assessment, biological system safety, and other fields.

Single laser synthesis of gold nanoparticles-polypyrrole-chitosan on laser-induced graphene for ascorbic acid detection

The simultaneous synthesis of gold nanoparticles (AuNPs) and graphene by laser ablation was demonstrated. The in-situ synthesis was performed by laser ablation of a polymer substrate covered with a gold precursor dispersion. The gold precursor was prepared in a copolymer solution of pyrrole (Py) and chitosan (Chi) to improve the nucleation of gold embedded on the laser-induced graphene electrode (LIGE). The morphology of AuNPs-pPy-Chi/LIGE was studied by scanning electron microscopy and characterized electrochemically by cyclic voltammetry. A comprehensive investigation of the electrochemical and physical features of the AuNPs-pPy-Chi/LIGE was carried out. The parameters of differential pulse voltammetry were adjusted to enhance the response to ascorbic acid (AA). The AuNPs-pPy-Chi/LIGE produced two linear ranges: from 0.25 to 5.00 and 5.00-25.00 mmol L-1. The limit of detection was 0.22 mmol L-1. Hundreds of electrodes were tested to demonstrate the excellent reproducibility of the AuNPs-pPy-Chi/LIGE fabrication. Overall, the proposed electrode allows the successful detection of AA in orange juice products with acceptable accuracy (recoveries = 97 ± 2 to 109.1 ± 0.7). The preparation strategy of the proposed AuNPs-pPy-Chi/LIGE could be adapted to detect other compounds or biomarkers.

Electrocatalysis Using Cobalt Porphyrin Covalently Linked with Multi-Walled Carbon Nanotubes: Hydrazine Sensing and Hydrazine-Assisted Green Hydrogen Synthesis

Acid-treated multi-walled carbon nanotube (MWCNT) covalently functionalized with cobalt triphenothiazine porphyrin (CoTriPTZ-OH) A3B type porphyrin, containing three phenothiazine moieties (represented as MWCNT-CoTriPTZ) is synthesized and characterized by various spectroscopic and microscopic techniques. The nanoconjugate, MWCNT-CoTriPTZ, exhibits a pair of distinct redox peaks due to the Co2+/Co3+ redox process in 0.1 M pH 7.0 phosphate buffer. Further, it electrocatalytically oxidizes hydrazine at a low overpotential with a high current. This property is advantageously utilized for the sensitive determination of hydrazine. The developed electrochemical sensor exhibits high sensitivity (0.99 µAµM-1cm-2), a low limit of detection (4.5 ppb), and a broad linear calibration range (0.1 µM to 3.0 mM) for the determination of hydrazine. Further, MWCNT-CoTriPTZ is exploited for hydrazine-assisted green hydrogen synthesis. The high efficiency of hydrazine oxidation is confirmed by the low onset potential (0.45 V (vs RHE)) and 0.60 V (vs RHE) at the current density of 10 mA.cm-2. MWCNT-CoTriPTZ displays a high current density (77.29 mA.cm-2) at 1.45 V (vs RHE).

Enhanced sensing of bacteria biomarkers by ZnO and graphene oxide decorated PEDOT film

Developing a biofilm biomarker detector and inhibitor will immensely benefit efforts geared at curbing infectious diseases and microbiologically induced corrosion of medical implants, marine vessels and buried steel pipelines. N-Acyl homoserine lactones (AHLs) are important biomarkers gram-negative bacteria use for communication. In this work, we investigated the interactions between three AHL molecules and graphene oxide (GO) and ZnO nanomaterials embedded in conjugated poly(3,4-ethylenedioxythiophene) (PEDOT) film. The results show that PEDOT/GO/ZnO detected AHLs to a considerable extent with adsorption enthalpies of -4.02, -4.87 and -4.97 KJ/mol, respectively, for N-(2-oxotetrahydrofuran-3-yl)heptanamide (AHL1), 2-hydroxy-N-(2-oxotetrahydrofuran-3-yl)nonanamide (AHL2) and (E)-3-(3-hydroxyphenyl)-N-(2-oxotetrahydrofuran-3-yl)acrylamide (AHL3) molecules. The ZnO nanoparticles facilitated charge redistribution and transfer, thereby enhancing the conductivity and overall sensitivity of the substrate toward the AHLs. The adsorption distance and sites of interactions further tuned the charge migration and signal generation by the substrate, thus affirming the suitability of the modeled thin film as a sensor material. Excellent stability and conductivity were maintained before and after the adsorption of each AHL molecule. Moreover, the desorption time for each AHL molecule was calculated, and the result affirmed that the modeled film materials are promising for developing highly sensitive biosensors.Communicated by Ramaswamy H. Sarma.

Engineering a "turn-on" NIR fluorescent sensor-based hydroxyphenyl benzothiazole with a cinnamoyl unit for hydrazine and its environmental and in-vitro applications

Hydrazine (N2H4), a chemical compound widely used in various industrial applications, causes significant environmental and biological hazards. Therefore, it is crucial to develop methodologies for the visualization and real time tracking of N2H4. In this regard, we have constructed a novel near-infrared fluorescent probe (HBT-Cy) that can effectively detect N2H4 in various samples. HBT-Cy contains 2-(2'-hydroxyphenyl)benzothiazole (HBT), cinnamoyl (Cy), and pyridinium (Py) moieties. Importantly, HBT-Cy exhibits a rapid, selective, and highly sensitive response to N2H4. This response results in the release of HBT-Py and the generation of considerable colorimetric changes along with a significant NIR (near infrared) fluorescence signal, peaking at 685 nm. Advantages of this system include turn on NIR fluorescence with large Stokes shift, (approximately 171 nm), low limit of detection (LOD = 0.11 μM) and quantum yield (0.211). The probe with low cytotoxic behavior demonstrates strong NIR fluorescence imaging capabilities to visualize endogenous and exogenous N2H4 in live cells. This mitochondria-targetable probe shows effective subcellular localization. These results suggest that HBT-Cy is a valuable probe for tracking and investigating the behavior of N2H4 in biological systems and environmental samples.

Electrochemical Sensors based on Gold-Silver Core-Shell Nanoparticles Combined with a Graphene/PEDOT:PSS Composite Modified Glassy Carbon Electrode for Paraoxon-ethyl Detection

Herein, a nonenzymatic detection of paraoxon-ethyl was developed by modifying a glassy carbon electrode (GCE) with gold-silver core-shell (Au-Ag) nanoparticles combined with the composite of graphene with poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). These core-shell nanoparticles (Au-Ag) were synthesized using a seed-growth method and characterized using UV-vis spectroscopy and high-resolution transmission electron microscopy (HR-TEM) techniques. Meanwhile, the structural properties, surface morphology and topography, and electrochemical characterization of the composite of Au-Ag core-shell/graphene/PEDOT:PSS were analyzed using infrared spectroscopy, field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and electrochemical impedance spectroscopy (EIS) techniques. Moreover, the proposed sensor for paraoxon-ethyl detection based on Au-Ag core-shell/graphene/PEDOT:PSS modified GCE demonstrates good electrochemical and electroanalytical performance when investigated with cyclic voltammetry (CV), differential pulse voltammetry (DPV), and chronoamperometry techniques. It was found that the synergistic effect between Au-Ag core-shell nanoparticles and the composite of graphene/PEDOT:PSS provides a higher conductivity and enhanced electrocatalytic activity for paraoxon-ethyl detection at an optimum pH of 7. At pH 7, the proposed sensor for paraoxon-ethyl detection shows a linear range of concentrations from 0.2 to 100 μM with a limit of detection of 10 nM and high sensitivity of 3.24 μA μM-1 cm-2. In addition, the proposed sensor for paraoxon-ethyl confirmed good reproducibility, with the possibility of being further developed as a disposable electrode. This sensor also displayed good selectivity in the presence of several interfering species such as diazinon, carbaryl, ascorbic acid, glucose, nitrite, sodium bicarbonate, and magnesium sulfate. For practical applications, this proposed sensor was employed for the determination of paraoxon-ethyl in real samples (fruits and vegetables) and showed no significant difference from the standard spectrophotometric technique. In conclusion, this proposed sensor might have a potential to be developed as a platform of electrochemical sensors for pesticide detection.

Effect of Aspect Ratio of a Gold-Nanorod-Modified Screen-Printed Carbon Electrode for Carbaryl Detection in Three Different Samples of Vegetables

In this study, three different sizes of gold nanorods (AuNRs) were synthesized using the seed-growth method by adding various volumes of AgNO3 as 400, 600, and 800 μL into the growth solution of gold nanoparticles. Three different sizes of AuNRs were then characterized using UV-vis spectroscopy, high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) patterns, and atomic force microscopy (AFM) to investigate the surface morphology, topography, and aspect ratios of each synthesized AuNR. The aspect ratios from the histogram of size distributions of three AuNRs as 2.21, 2.53, and 2.85 can be calculated corresponding to the addition of AgNO3 volumes of 400, 600, and 800 μL. Moreover, each AuNR in three different aspect ratios was drop-cast onto the surface of a commercial screen-printed carbon electrode (SPCE) to obtain three different SPCE-modified AuNRs (SPCE-A400, SPCE-A600, and SPCE-A800, respectively). All SPCE-modified AuNRs were then evaluated for their electrochemical behavior using cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques and the highest electrochemical performance was shown as the order of magnitude of SPCE-A400 > SPCE-A600/SPCE-A800. The reason for the highest electrocatalytic activity of SPCE-A400 might be due to the smallest particle size and uniform distribution of AuNRs ∼ 2.2, which enhanced the charge transfer, thus providing the highest electroactive surface area (0.6685 cm2) compared to other electrodes. These results also confirm that the sensing mechanism for all SPCE-modified AuNRs is controlled by diffusion phenomena. In addition, the optimum pH was obtained as 4 for carbaryl detection for all SPCE-modified AuNRs with the highest current shown by SPCE-A400. Furthermore, SPCE-A400 has the highest fundamental parameters (surface coverage, catalytic rate constant, electron transfer rate constant, and adsorption capacity) for carbaryl detection, which were investigated using cyclic voltammetry and chronoamperometric techniques. The electroanalytical performances of all SPCE-modified AuNRs for carbaryl detection were also investigated with SPCE-A400 displaying the best performance among other electrodes in terms of its linearity (0.2-100 μM), limit of detection (LOD) ∼ 0.07 μM, and limit of quantification (LOQ) ∼ 0.2 μM. All SPCE-modified AuNRs were also subsequently evaluated for their stability, reproducibility, and selectivity in the presence of interfering species such as NaNO2, NH4NO3, Zn(CH3CO2)2, FeSO4, diazinon, and glucose and show reliable results as depicted from %RSD values less than 3%. At last, all SPCE-modified AuNRs have been employed for carbaryl detection using a standard addition technique in three different samples of vegetables (cabbage, cucumber, and Chinese cabbage) with its results (%recovery ≈ 100%) within the acceptable analytical range. In conclusion, this work demonstrates the great potential of a disposable device based on an AuNR-modified SPCE for rapid detection and high sensitivity in monitoring the concentration of carbaryl as a residual pesticide in vegetable samples.

Air-enclosed pores in graphene aerogel inhibit the adsorption of bisphenol A but accelerate the adsorption of naphthalene

Graphene hydrogel (GH) and aerogel (GA) have great application potential as highly effective adsorbents, but the accessibility of their adsorption sites have not yet been identified, restricting our understanding on the adsorption mechanisms and manufacturing. This study comparatively studied the adsorption characteristics of bisphenol A (BPA) and naphthalene (NAP) on GH and GA, focussing on the accessibility of the adsorption sites. The adsorption of BPA on GA was much lower but faster than that on GH. NAP adsorption on GA was very close to that on GH but faster than that on the latter. Considering that NAP is volatilisable, we speculate that some unwetted sites in the air-enclosed pores are available to it, but not to BPA. We applied ultrasonic and vacuum treatments to remove the air in GA pores, which was verified using a CO₂ replacement experiment. BPA adsorption was greatly enhanced but slowed, while that of NAP was not enhanced. This phenomenon suggested that some inner pores became accessible in the aqueous phase after air removal from pores. The enhanced accessibility of air-enclosed pores was verified by the increased relaxation rate of surface-bounded water on GA, based on a ¹H NMR relaxation analysis. This study highlights that the accessibility of adsorption site plays a crucial role for the adsorption properties of carbon-based aerogel. The volatile chemicals may be quickly adsorbed in the air-enclosed pores, which be useful for immobilizing volatile contaminants.