Voltammetric determination of 4-nitrophenol using a glassy carbon electrode modified with a gold-ZnO-SiO2 nanostructure

Experimental and theoretical approaches to unveil the interaction mechanisms of carbon dots with 4-nitrophenol

Developing a non-toxic, cost-effective, biocompatible, selective, and sensitive optical sensor for the optical detection of 4-nitrophenol (4-NP) has been challenging yet significant. Among carbon-based materials, the selectivity of carbon dots toward 4-NP is a key area of research, and a comprehensive understanding is crucial to broaden its practical application. Our unique contribution to this field involves synthesizing and testing different N-doped carbon dots (CDs) for selective 4-NP detection through absorbance and photoluminescence. We have also theoretically provided the interaction mechanism between 4-NP and N-doped CDs (H-NCDs). The optimized hydrothermally synthesized nitrogen-doped CDs demonstrate excellent specificity towards 4-NP through simultaneous quenching of the PL (98 %) and redshift of the signal (34 nm) in an aqueous medium. The density functional theory (DFT) model that was performed to understand the electronic structure properties behind the interaction of 4-NP and CD could be a predictive tool for developing CD-based materials to enhance 4-NP capture. Our study provides a mechanistic understanding of the analyte-specific CD design. It offers promising implications for the practical application of our findings, thereby contributing a unique perspective to the field and benefiting various industries and environmental monitoring efforts, sparking optimism for the future.

A bifunctional MoS2/SGCN nanocatalyst for the electrochemical detection and degradation of hazardous 4-nitrophenol

Herein, we reported the dual functions of molybdenum disulfide/sulfur-doped graphitic carbon nitride (MoS2/SGCN) composite as a sensing material for electrochemical detection of 4-NP and a catalyst for 4-NP degradation. The MoS2 nanosheet, sulfur-doped graphitic carbon nitride (SGCN) and MoS2/SGCN were characterized using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) spectroscopy and X-ray photoelectron spectroscopy (XPS). Electrochemical characterization of these materials with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) in 1 mM K4[Fe(CN)6]3-/4- show that the composite has the lowest charge transfer resistance and the best electrocatalytic activity. The limit of detection (LOD) and the linear range of 4-nitrophenol at MoS2/SGCN modified glassy carbon electrode (MoS2/SGCN/GCE) were computed as 12.8 nM and 0.1 - 2.6 μM, respectively. Also, the percentage recoveries of 4-NP in spiked tap water samples ranged from 97.8 - 99.1 %. The electroanalysis of 4-NP in the presence of notable interferons shows that the proposed electrochemical sensor features outstanding selectivity toward 4-NP. Additionally, the results of the catalytic degradation of 4-NP at MoS2/SGCN show that the nanocatalyst catalyzed the transformation of 4-NP to 4-aminophenol (4-AP) with a first-order rate constant (k) estimated to be 4.2 ×10-2 s-1. The results of this study confirm that the MoS2/SGCN nanocatalyst is a useful implement for electroanalytical monitoring and catalytic degradation of the hazardous 4-NP in water samples.

Advances in 4-Nitrophenol Detection and Reduction Methods and Mechanisms: An Updated Review

This review emphasizes the progress in identifying and eliminating para-nitrophenol (4-NP), a toxic organic compound. It covers various strategical methods and materials, including organic and inorganic nanomaterials, for detecting and reducing 4-NP. Detection techniques such as electrochemical methods. Optical fiber-based surface plasmon resonance and photoluminescence, as well as the mechanisms of Förster Resonance Energy Transfer (FRET) and Inner Filter Effect (IFE) in fluorescence detection, are presented. Removal techniques for this contaminant include homogeneous catalysis, electrocatalysis, photocatalysis, and thermocatalysis, and their reaction mechanisms are also discussed. Further, the theoretical perspectives of 4-NP detection and reduction, parameters influencing the activities, and future perspectives are also reviewed in detail.

Using silver nanoparticle-decorated whey protein isolate amyloid fibrils to modify the electrode surface used for electrochemical detection of para-nitrophenol

Due to their organized structures, remarkable stiffness, and nice biocompatibility and biodegradability, amyloid fibrils serve as building blocks for versatile sustainable materials. Silver nanoparticles (AgNPs) are commonly used as the nano-catalysts for various electrochemical reactions. Given their large specific surface area and high surface energy, AgNPs exhibit high aggregation propensity, which hampers their electrocatalytic performance. Food protein wastes have been identified to be associated with climate change and environmental impacts, and a surplus of whey proteins in dairy industries causes high biological and chemical demands, and greenhouse gas emissions. This study is aimed at constructing sustainable electrode surface modifiers using AgNP-deposited whey protein amyloid fibrils (AgNP/WPI-AFs). AgNP/WPI-AFs were synthesized and characterized via spectroscopic techniques, electron microscopy, and X-ray diffraction. Next, the electrocatalytic performance of AgNP/WPI-AF modified electrode was assessed via para-nitrophenol (p-NP) reduction combined with various electrochemical analyses. Moreover, the reaction mechanism of p-NP electrocatalysis on the surface of AgNP/WPI-AF modified electrode was investigated. The detection range, limit of detection, sensitivity, and selectivity of the AgNP/WPI-AF modified electrode were evaluated accordingly. This work not only demonstrates an alternative for whey valorization but also highlights the feasibility of using amyloid-based hybrid materials as the electrode surface modifier for electrochemical sensing purposes.

Copper based metal organic framework decorated with gold nanoparticles as a new electrochemical sensor material for the detection of L-Cysteine in milk samples

A facile electrochemical sensor based on carbon felt electrode (CFE) modified with gold nanoparticles decorated copper based metal organic framework (AuNPs@Cu-MOF) was achieved for the electrochemical sensing of L-Cysteine (L-Cys). For this purpose, AuNPs@Cu-MOF was synthesized and characterized. The electrochemical behaviors of L-Cys at plain and modified CFEs were investigated via cyclic voltammetry (CV). According CV results, AuNPs@Cu-MOF structure showed a catalytic effect on the oxidation of L-Cys as well as increasing the active electrode surface area by 206% compared to bare CFE. In addition, the pH effect on electrochemical determination of L-Cys at AuNPs@Cu-MOF/CFE was widely examined, and it was determined that the best oxidation peak current of L-Cys was obtained in pH 5 acetate buffer. Moreover, a linear detection range of 30-400 µM for L-Cys with a limit of detection value of 2.21 µM (n = 3) was achieved with the proposed electrochemical sensor. The developed L-Cys sensor was also applied for L-Cys detection in various milk samples and acceptable recovery values were obtained ranging from 100.05 to 108.45%.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05866-1.

A flexible electrochemical sensor for bisphenol A detection based on photoinitiated molecular imprinting on CdS functionalized carbon felt

Long-term and excessive exposure to bisphenol A (BPA) has an extremely detrimental effect on human health and ecological system. Hence, there is an urgent need to develop a sensitive and selective sensor for precisely monitoring BPA levels. In this work, a flexible and tailor-made electrochemical sensor for BPA has been fabricated based on in situ photopolymerization of molecular imprinting on cadmium sulfide (CdS) modified carbon felt (MIP@CdS-CF). It is worth nothing that CdS acts as a photocatalyst to enhance the capacity of photopolymerization, accordingly upgrading imprinting efficiency. Meanwhile, carbon felt (CF) exhibits attractive merits in term of superior electrical conductivity, enlarged electrochemical active areas and unique flexibility. Consequently, the novel MIP@CdS-CF electrochemical sensor shows superior sensitivity, high selectivity, extraordinary reproducibility and stability to detect BPA. The detection limit is 1.5 nM, which is lower than those of previously reported electrochemical sensors for the detection of BPA. More importantly, this newly developed electrochemical sensor can be utilized for detecting BPA in plastic bottles with satisfactory results.

Self-assembled ZnO microspheres coated with carbon dot-doped CoNi LDH wrinkled films as electrochemical sensors for highly sensitive detection of hydrazine

In this study, a 3D surface-folded composite was prepared in situ as a hydrazine sensor by loading a hybrid film of CoNi-layered double hydroxides (LDHs) with nitrogen-doped carbon dots on self-assembled ZnO microspheres. The nanocomposite was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), and the electrochemical behavior of the sensors was investigated by cyclic voltammetry (CV), amperometry and electrochemical impedance spectroscopy (EIS). The results showed that ZnO microspheres with nitrogen-doped carbon dots strongly coupled with LDHs can significantly reduce the charge transfer resistance, accelerate the oxidation kinetics of hydrazine, and effectively increase the electrochemically active surface area (ECSA). The sensor achieved ultra-sensitive (13 040 μA mM-1 cm-2 (S/N = 3)) detection of hydrazine in the concentration range of 0.7 μM to 4 mM, exhibited excellent selectivity, reproducibility and high stability, and was successfully applied to the determination of hydrazine in actual environmental water samples and landfill leachate samples.

A sensing platform based on Cu-MOF encapsulated Dawson-type polyoxometalate crystal material for electrochemical detection of xanthine

A promising sensing platform based on polyoxometalate-based metal-organic framework (POMOF) was established for sensitive electrochemical detection of xanthine (XA). In the unique structure of POMOF, the Dawson polyoxoanions P₂W₁₈ were encapsulated into 3D open copper-mixed ligand nanotube framework Cu-MOF, in which the cavity of the metal-organic framework provides a specific shelter to prevent the aggregation and loss of polyoxometalate in electrocatalytic reactions; meanwhile, unsaturated Cu(II) active sites of Cu-MOF can also serve as electrocatalytic active center. The POMOF-based sensor (CuMOFP₂W₁₈/XC-72R) was fabricated by using acetylene black (XC-72R) as a support material to enhance the conductivity of POMOF. The performances of the POMOF-based sensor were studied by using different electrochemical testing methods. The composite displayed remarkable electrocatalytic activity for the oxidation of XA due to the synergistic effect of polyoxometalate (POM) and metal-organic framework (MOF). The electrochemical sensor demonstrated a wide linear range (0.5 μM-240 μM), low detection limit (0.26 μM), and excellent selectivity for detecting XA. Furthermore, the composite further demonstrated excellent reproducibility and great stability. More importantly, the proposed sensor was utilized to detect XA in real samples, which may provide a new way for early disease diagnosis.

Sensitive Electrochemical Detection of 4-Nitrophenol with PEDOT:PSS Modified Pt NPs-Embedded PPy-CB@ZnO Nanocomposites

In this study, a selective 4-nitrophenol (4-NP) sensor was developed onto a glassy carbon electrode (GCE) as an electron-sensing substrate, which decorated with sol-gel, prepared Pt nanoparticles- (NPs) embedded polypyrole-carbon black (PPy-CB)/ZnO nanocomposites (NCs) using differential pulse voltammetry. Characterizations of the NCs were performed using Field Emission Scanning Electron Microscopy (FESEM), Energy-Dispersive Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), Ultraviolet-visible Spectroscopy (UV-vis), Fourier Transform Infrared Spectroscopy (FTIR), High Resolution Transmission Electron Microscopy (HRTEM), and X-ray Diffraction Analysis (XRD). The GCE modified by conducting coating binders [poly(3,4-ethylenedioxythiophene) polystyrene sulfonate; PEDOT:PSS] based on Pt NPs/PPy-CB/ZnO NCs functioned as the working electrode and showed selectivity toward 4-NP in a phosphate buffer medium at pH 7.0. Our analysis of 4-NP showed the linearity from 1.5 to 40.5 µM, which was identified as the linear detection range (LDR). A current versus concentration plot was formed and showed a regression co-efficient R² of 0.9917, which can be expressed by i_p(µA) = 0.2493C(µM) + 15.694. The 4-NP sensor sensitivity was calculated using the slope of the LDR, considering the surface area of the GCE (0.0316 cm²). The sensitivity was calculated as 7.8892 µAµM^-1cm^-2. The LOD (limit of detection) of the 4-NP was calculated as 1.25 ± 0.06 µM, which was calculated from 3xSD/σ (SD: Standard deviation of blank response; σ: Slope of the calibration curve). Limit of quantification (LOQ) is also calculated as 3.79 µM from LOQ = 10xLOD/3.3. Sensor parameters such as reproducibility, response time, and analyzing stability were outstanding. Therefore, this novel approach can be broadly used to safely fabricate selective 4-NP sensors based on nanoparticle-decorated nanocomposite materials in environmental measurement.

Activated Glassy Carbon Electrode as an Electrochemical Sensing Platform for the Determination of 4-Nitrophenol and Dopamine in Real Samples

Glassy carbon electrode (GCE) was electrochemically activated using a repetitive cyclic voltammetric technique to develop an activated glassy carbon electrode (AGCE). The developed AGCE was optimized and utilized for the electrochemical assay of 4-nitrophenol (4-NP) and dopamine (DA). Cyclic voltammetry (CV) was employed to investigate the electrochemical behavior of the AGCE. Compared to the bare GCE, the developed AGCE exhibits a significant increase in redox peak currents of 4-NP and DA, which indicates that the AGCE significantly improves the electrocatalytic reduction of 4-NP and oxidation of DA. The electrochemical signature of the activation process could be directly associated with the formation of oxygen-containing surface functional groups (OxSFGs), which are the main reason for the improved electron transfer ability and the enhancement of the electrocatalytic activity of the AGCE. The effects of various parameters on the voltammetric responses of the AGCE toward 4-NP and DA were studied and optimized, including the pH, scan rate, and accumulation time. Differential pulse voltammetry (DPV) was also utilized to investigate the analytical performance of the AGCE sensing platform. The optimized AGCE exhibited linear responses over the concentration ranges of 0.04-65 μM and 65-370 μM toward 4-NP with a lower limit of detection (LOD) of 0.02 μM (S/N = 3). Additionally, the AGCE exhibited a linear responses over the concentration ranges of 0.02-1.0 and 1.0-100 μM toward DA with a lower limit of detection (LOD) of 0.01 μM (S/N = 3). Moreover, the developed AGCE-based 4-NP and DA sensors are distinguished by their high sensitivity, excellent selectivity, and repeatability. The developed sensors were successfully applied for the determination of 4-NP and DA in real samples with satisfactory recovery results.

Three-dimensional MoS2-graphene aerogel nanocomposites for electrochemical sensing of quercetin

A facile and novel electrochemical sensing platform is reported for quercetin determination with MoS₂ nanoflowers-3D graphene aerogel (3D MoS₂-GA) nanocomposite as signal amplified material. The 3D MoS₂-GA nanocomposite was synthesized through a two-step hydrothermal method, in which MoS₂ nanoflowers were prepared in advance. Characterizations of 3D MoS₂-GA were performed by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The 3D MoS₂-GA-modified glassy carbon electrode (3D MoS₂-GA/GCE) was used to investigate the electrochemical behaviors of quercetin with electrochemical parameters calculated, reaction mechanism discussed, and experimental conditions optimized. Notably, the redox peak current densities of quercetin on 3D MoS₂-GA/GCE raised 5.14 and 6.40 times compared with those on a bare GCE. Furthermore, a novel electroanalytical approach was proposed for the sensitive determination of quercetin within the concentration range 0.01 ~ 5.0 μmol/L, accompanied by a low detection limit of 0.0026 μmol/L (at a working potential of 0.38 V vs. Ag/AgCl). The recovery for practical sample analysis ranges from 97.0 to 105%, and the relative standard deviation is less than 4.2%. This established method shows reliable performance in determination of quercetin in tablets and urine samples.

A Status Update on the Development of Polymer and Metal-Based Graphene Electrochemical Sensors for Detection and Quantitation of Bisphenol A

The detection and quantitation of bisphenol A (BPA) in the environment and food products has been a subject of considerable interest. BPA, a diphenylmethane derivative is a well-known industrial raw material with wide range of applications. It is a well-known endocrine disruptor and acts as an estrogen mimic. BPA is an environmental health concern and its accumulation in hydro-geological cycles is a matter of serious ecological peril. This review basically assesses various chemically modified electrodes composed of diverse components that have been employed to recognize BPA in different matrices. Electrochemical sensors prepared using graphene materials in combination with metals and polymers for selective detection of BPA have been discussed extensively. The emphasis is on detection of BPA in various samples encountered in routine use such as plastic bottles, receipts, baby feed bottles, milk samples, mineralized water, tissue paper, DVDs, and others. Although research in this field is in the exploratory stage, deeper insights into fundamental studies of sensing systems, fast analysis of real samples and validation of sensors are some of the factors that need major impetus. It is expected that chemically modified electrode-based sensing systems will soon take over as a viable option for monitoring diverse pollutants.

Covalent organic framework modified carbon cloth for ratiometric electrochemical sensing of bisphenol A and S

A novel ratiometric electrochemical sensor was developed based on a carbon cloth electrodeposited with silver nanoparticles and drop-coated by covalent organic framework (COF-LZU1) for simultaneous determination of bisphenol A (BPA) and bisphenol S (BPS). Carbon cloth exhibited a significantly larger electrochemical active area than common glassy carbon electrodes (27.5 times). Silver nanoparticles not only provided a stable reference signal but also enhanced electroactivity for the oxidation of BPA and BPS. COF-LZU1 with good adsorption performance and large periodic π-arrays promoted the enrichment of BPA and BPS to further increase the current response. Compared with the traditional single-signal electrochemical sensor, the developed ratiometric sensor exhibited better reproducibility and a wider linear range for BPA and BPS from 0.5 to 100 μM with a limit of detection of 0.15 μM. Furthermore, the developed sensor showed excellent stability and superior anti-interference ability. The real sample analysis for BPA and BPS has been successfully carried out in mineral water, electrolyte drink, tea, juice, and beer with recoveries of 88.3-111.7%. The developed ratiometric sensor is expected to be a candidate for the preparation of other electrochemical sensors and the analysis of additional practical samples.

Bi-functional renewable biopolymer wrapped CNFs/Ag doped spinel cobalt oxide as a sensitive platform for highly toxic nitroaromatic compound detection and degradation

Nanomolar-level detection of priority toxic pollutant 4-nitrophenol (4-NP) in environment using a novel ternary nanocomposite based electrochemical sensor and its photocatalytic degradation is reported in this paper. A non-toxic and renewable natural biopolymer, chitosan wrapped carbon nanofibers was embedded with Ag doped spinel Co₃O₄ to prepare the bi-functional ternary nanocomposite. Economical and ecofriendly sonochemical method was employed in preparation of this porous nanocomposite. We used one-pot aqueous solution approach to synthesize Ag-Co₃O₄ nanoflowers and ultrasound-assisted method was utilized to prepare CS-CNFs. Morphological and structural properties of synthesized materials were analyzed using different characterization techniques. Electrochemical investigations using cyclic voltammetry and differential pulse voltammetry carried out with prepared ternary nanocomposite modified carbon electrode revealed its outstanding electrocatalytic activity in 4-NP quantification. The developed 4-NP sensor showcased excellent sensitivity of 55.98 μAμM^-1cm^-2 and nanomolar detection limit of 0.4 nM. Moreover, reproducibility, repeatability, stability, and selectivity were evaluated to confirm reliability of developed sensor. Further, real sample analyses were conducted using domestic sewage, underground water, and tomato to affirm the practical feasibility of 4-NP detection using the proposed sensor.

Covalent organic framework DQTP modified pencil graphite electrode for simultaneous determination of bisphenol A and bisphenol S

The sensitivity and selectivity of electrochemical analysis are challenging due to the materials used for electrode modification as well as electrical conductivity, catalytic activity and recognition ability of the working electrode. In this work, a portable 3D-printed electrochemical electrode clamp was designed and applied in combination with the developed covalent organic framework (COF DQTP)-modified pencil graphite electrode (DQTP/PGE). The β-ketoenamine-linked COF DQTP synthesized by 1,3,5-triformylphloroglucinol (TP) and 2,6-diaminoanthraquinone (DQ) through solvothermal method is a porous crystalline with excellent conductivity and large periodic π-arrays, coupled with commercial available pencil graphite electrode to fabricate a disposable sensor for simultaneous determination of environmental endocrine disruptors bisphenol A and bisphenol S. The DQTP/PGE sensor exhibited high electrical conductivity and catalytic activity, and a good linearity was obtained in a range of 0.5-30 μM for two bisphenols with a detection limit of 0.15 μM (S/N = 3). Moreover, the sensor showed a reproducible and stable response over one month with negligible interference, and an accepted recovery with real food packaging samples.

An Enzyme-Based Biosensor for the Detection of Organophosphate Compounds Using Mutant Phosphotriesterase Immobilized onto Reduced Graphene Oxide

Enzymatic detection of organophosphate (OP) compounds can be tailored using highly sensitive and selective enzymes in the development of biosensors. Previously, mutant (YT) phosphotriesterase (PTE) was reported to efficiently hydrolyze Sp and Rp enantiomers of phosphotriester. This study reports the use of phosphotriesterase mutant YT (YT-PTE) immobilized onto reduced graphene oxide (rGO) and fabricated onto a screen-printed carbon electrode (SPCE) for electrochemical detection of OP compounds. Immobilization of YT-PTE onto rGO was secured using N-hydroxysuccinimide (NHS) and N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide (EDC) cross-linker, and the resulting immobilized enzyme was able to retain up to 90% of its activity. Electrochemical analysis of the SPCE/rGO/YT-PTE showed detection of paraoxon in a linear range of 1 mM–0.005 μM with its limit of detection as low as 0.11 μM. SPCE/rGO/YT-PTE exhibited high selectivity towards paraoxon and parathion and have good reproducibility. Furthermore, detection of paraoxon was also possible in a real water sample with only minor interferences.

Electrochemical detection of bisphenols in food: A review

Bisphenols (BPs) are worldwide used organic compounds in plastics, belonging to the group of endocrine disrupting chemicals (EDCs) which exhibits endocrine disruption to beings. Migration of BPs from food contact materials like plastic containers, epoxy coatings in metal cans and thermal papers, would results in bioaccumulation of BPs in human beings, causing adverse health effects. Therefore, sensitive and selective determination of BPs in food is needed. Among different strategies have been explored for the detection of BPs, electrochemical sensors with relatively high sensitivity and fast response are promising. This paper is devoted to comprehensively review the developed electrochemical methods for BPs sensing in food, so that to find a direction for developing low cost, high accuracy and compatibility sensors toward the sensitive and selective detection of BPs. Different electrochemical technologies categorized by recognition agents, aptamers, enzymes, molecularly imprinted polymers and nanomaterials are discussed and summarized in their mechanisms, usages, merits and limitations. The challenges and further perspectives in the development of electrochemical sensors is also discussed.

Gold Nanoparticle Embedded on a Reduced Graphene Oxide/polypyrrole Nanocomposite: Voltammetric Sensing of Furazolidone and Flutamide

A new electrochemical sensor has been constructed based on the in situ preparation of gold nanoparticle embedded on reduced graphene oxide and polypyrrole nanotube (AuNP@rGO/PPyNT) composite through a nanosecond laser-induced heating technique. The as-prepared composite is used for individual as well as the simultaneous electrochemical determination of chemotherapy drug (furazolidone, FU) and anticancer drug (flutamide, FLT). The composite was analyzed by X-ray Diffraction, scanning electron microscopy/energy-dispersive X-ray analysis, transmission electron microscopy, Raman spectrometry, and X-ray photoelectron spectroscopy analysis, thus confirming the successful synthesis of this composite and its physical features. The modified AuNP@rGO/PPyNT electrode was examined through cyclic voltammetry and differential pulse voltammetry (DPV) methods in pH 7.0 for the determination of FU and FLT in individual, simultaneous, and mixed systems. The fabricated sensor showed wide linear responses (0.01-1080.11 μM and 0.01-1214.11 μM) of analytes, with the lower limits of detection of 2.3 and 2.45 nM and higher sensitivity of 53.75 and 50.06 μA μM^-1 cm^-2, respectively. Furthermore, the constructed sensor demonstrates higher stability, reproducibility, and repeatability, and is effectively applied for the analysis of FU and FLT content in the human serum sample analysis with satisfactory recovery.

Facile Synthesis of Silver-Doped Zinc Oxide Nanostructures as Efficient Scaffolds for Detection of p-Nitrophenol

In this paper, silver-doped zinc oxide nanoparticles were synthesized by using a solution combustion technique, in which zinc nitrate is used as an oxidizer and tartaric acid as a fuel. The phase composition, morphology and structural properties of the as-synthesized zinc oxide and silver-doped zinc oxide were established by using powdered X-ray diffraction, field emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy studies. Due to well-defined morphologies and crystallinity, the pure zinc oxide and silver-doped zinc oxide nanostructures can be used as efficient chemical sensors for the detection of p-nitrophenol (PNP). ZnO was found to show a low value of the limit of detection (LOD), i.e., 2.175 µM/L, for p-nitrophenol sensing; moreover, a sharp decrease in the limit of detection was observed with an increase in the concentration of silver ions, and the LOD value decreased to 0.669 µM/L for 10 mol % silver-doped zinc oxide. It is therefore concluded that Ag-doped ZnO shows a lower limit of detection as compared to pure ZnO for p-nitrophenol sensing.

In situ formed zinc oxide/graphitic carbon nitride nanohybrid for the electrochemical determination of 4-nitrophenol

The electrochemical determination of 4-nitrophenol using a nanohybrid consisting of glassy carbon (GC) and zinc oxide/graphitic carbon nitride (ZnO/g-CN nanosheet), is described. The ZnO/g-CN nanohybrid was in situ synthesized by chemical method and well characterized using absorption spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopic analysis. It was observed that the nanosized ZnO particles were present inside the sheet-like g-CN nanostructure. The nanohybrid-modified electrode showed an enhanced electrocatalytic response for 4-nitrophenol reduction compared with the bare GC electrode. The assay exhibited linear ranges of 13.4-100 μM and 100-1000 μM for 4-NP determination. The limit of detection and limit of quantification were 4.0 and 13.4 μM, respectively, at the working potential of - 0.85 V. An appreciable precision was found towards the stability of the assay in the determination. It provides selectivity against inorganic and organic substances such as calcium chloride, potassium chloride, nitrobenzene, uric acid, 1-chloro,2,4-dinitrobenzene, 1-bromo,2-nitrobenzene and 1-iodo,2-nitrobenzene. The practical applicability of the assay was also checked in the analysis of real water samples and satisfactory recovery of 4-NP was found. Schematic representation of the synthesis of zinc oxide (ZnO) nanostructures incorporated graphitic carbon nitride nanosheets (g-C₃N₄ NSs) and its application in the voltammetric determination of 4-nitrophenol (4-NP) is presented. The nanohybrid assay showed selectivity among coexisting compounds and good recovery in real sample analysis.

A flower-like ZnO–Ag2O nanocomposite for label and mediator free direct sensing of dinitrotoluene

2,4-Dinitrotoluene (2,4-DNT) is a nitro aromatic compound used as a raw material for trinitrotoluene (TNT) explosive synthesis along with several other industrial applications. Easy, rapid, cost-effective, and selective detection of 2,4-DNT is becoming essential due to its hepato carcinogenic nature and presence in surface as well as ground water as a contaminant. Keeping this in view, this research, for the first-time, reports the synthesis of novel ZnO–Ag₂O composite nanoflowers on a gold (Au) substrate, to fabricate an electrochemical sensor for label-free, direct sensing of 2,4-DNT selectively. The proposed ZnO–Ag₂O/Au sensor exhibits a sensitivity of 5 μA μM⁻¹ cm⁻² with a low limit of detection (LOD) of 13 nM, in a linear dynamic range (LDR) of 0.4 μM to 40 μM. The sensor showed reasonably high re-usability and reproducibility, with reliable results for laboratory and real-world samples.

2,4-Dinitrotoluene (2,4-DNT) is a nitro aromatic compound used as a raw material for trinitrotoluene (TNT) explosive synthesis along with several other industrial applications.

Gas-Directed Production of Noble Metal-Magnetic Heteronanostructures in Continuous Fashion: Application in Catalysis

Complex nanomaterials produced by scale-up batch processes lack suitable control of shape, size distribution, chemical composition, and quality, because heat and mass transfer are seriously affected as the reactor volume increases. Here we use a novel continuous synthesis procedure, the active gas-liquid segmented flow, to produce noble metal-magnetic heteronanostructures with enormous interest in the fields of catalysis, biomedicine, environmental sensors, food monitoring, and chemical analysis. The microreactor technology proposed scales down the reaction volume to gain advantage of the large surface area to volume ratio with respect to conventional batch-type reactors, improving heat and mass transport and, consequently, promoting a uniform heating and mixing. The gas phase was introduced in the chemical reactor as gas slugs of nanoliter scale with a dual role: (1) passive mixing and (2) chemical directing agent to tune the crystallization of nanostructures in a continuous fashion. The shape, size, and magnetic properties of the resulting heteronanostructures, as well as the density, size, and composition of noble metal nanoparticles were tuned to show the versatility of the proposed approach in a timeline of 4 min. We demonstrated that the produced nanostructures provide excellent catalytic properties in the catalyzed hydrogenation of nitrophenols to aminophenols. Electron microscopy, UV-vis spectroscopy, and cyclic voltammetry studies showed the remarkable catalytic performance of the produced heteronanostructures.

Superlattice stacking by hybridizing layered double hydroxide nanosheets with layers of reduced graphene oxide for electrochemical simultaneous determination of dopamine, uric acid and ascorbic acid

A self-assembled periodic superlattice material was obtained by integrating positively charged semiconductive sheets of a Zn-NiAl layered double hydroxide (LDH) and negatively charged layers of reduced graphene oxide (rGO). The material was used to modify a glassy carbon electrode which then is shown to be a viable sensor for the diagnostic parameters dopamine (DA), uric acid (UA) and ascorbic acid (AA). The modified GCE displays excellent electrocatalytic activity towards these biomolecules. This is assumed to be due to the synergistic effects of (a) excellent interfacial electrical conductivity that is imparted by direct neighboring of conductive rGO to semiconductive channels of LDHs, (b) the superb intercalation feature of LDHs, and (c) the enlarged surface with an enormous number of active sites. The biosensor revealed outstanding electrochemical performances in terms of selectivity, sensitivity, and wide linear ranges. Typically operated at working potentials of -0.10, +0.13 and + 0.27 V vs. saturated calomel electrode, the lower detection limits for AA, DA and UA are 13.5 nM, 0.1 nM, and 0.9 nM, respectively, at a signal-to-noise ratio of 3. The sensor was applied to real-time tracking of dopamine efflux from live human nerve cells. Graphical abstract Schematic of the preparation of a superlattice self-assembled material by integrating positively charged semiconductive sheets of Zn-NiAl layered double hydroxide (LDH) with negatively charged reduced graphene oxide (rGO) layers. It was applied to simultaneous electrochemical detection of dopamine (DA), uric acid and ascorbic acid.

Self-stacking of exfoliated charged nanosheets of LDHs and graphene as biosensor with real-time tracking of dopamine from live cells

This study introduces a new strategy for periodic stacking of positively charged NiAl layered double hydroxides (LDHs) nanosheets with negatively charged monolayers of graphene (G) by systematically optimizing several parameters in a controlled co-feeding fashion and resultant heterostacked NiAl LDH/G LBL nanocomposites have been practically applied in sensitive detection of dopamine released from live cells as early Parkinson's disease (PD) diagnostic tool. PD is the second most chronic neurodegenerative disorder with gradual progressive loss of movement and muscle control causing substantial disability and threatening the life seriously. Unfortunately majority of dopaminergic neurons present in substantia nigra of PD patients are destroyed before it is being clinically diagnosed, so early stages PD diagnosis is essential. Because of direct neighboring of extremely conductive graphene to semiconductive LDHs layers, enhanced intercalation capability of LDHs, and huge surface area with numerous active sites, good synergy effect is harvested in heteroassembled NiAl LDH/G LBL material, which in turn shows admirable electrocatalytic ability in DA detection. The interference induced by UA and AA is effectively eliminated especially after the modifying the electrode with Nafion. The outstanding electrochemical sensing performance of NiAl LDH/G LBL modified electrode has been achieved in terms of broad linear range and lowest real detection limit of 2 nM (S/N = 3) towards DA oxidation. Benefitting from superior efficiency, biosensor has been successfully used for real-time in-vitro tracking of DA efflux from live human nerve cell after being stimulated. We believe that our biosensing platform of structurally integrated well-ordered LBL heteroassembly by inserting graphene directly to the interlayer galleries of LDHs material will open up new avenue in diseases determination window.