An Effective Electrochemical Platform for Chloramphenicol Detection Based on Carbon-Doped Boron Nitride Nanosheets.

Currently, accurate quantification of antibiotics is a prerequisite for health care and environmental governance. The present work demonstrated a novel and effective electrochemical strategy for chloramphenicol (CAP) detection using carbon-doped hexagonal boron nitride (C-BN) as the sensing medium. The C-BN nanosheets were synthesized by a molten-salt method and fully characterized using various techniques. The electrochemical performances of C-BN nanosheets were studied using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results showed that the electrocatalytic activity of h-BN was significantly enhanced by carbon doping. Carbon doping can provide abundant active sites and improve electrical conductivity. Therefore, a C-BN-modified glassy carbon electrode (C-BN/GCE) was employed to determine CAP by differential pulse voltammetry (DPV). The sensor showed convincing analytical performance, such as a wide concentration range (0.1 µM-200 µM, 200 µM-700 µM) and low limit of detection (LOD, 0.035 µM). In addition, the proposed method had high selectivity and desired stability, and can be applied for CAP detection in actual samples. It is believed that defect-engineered h-BN nanomaterials possess a wide range of applications in electrochemical sensors.

Defect-enhanced electrochemical property of h-BN for Pb2+ detection.

A new strategy has been developed for the determination of trace lead ions (Pb2+) based on hexagonal boron nitride (h-BN) laden with point defect. The defect-laden boron nitride (D-BN) was synthesized by a thermal polymerization route, in which melamine borate was used as a precursor. The defect microstructure was confirmed by photoluminescence (PL) and x-ray diffraction (XRD) techniques. As compared with h-BN, the D-BN-modified glassy carbon electrode (GCE) showed an enhanced electrochemical response towards Pb2+ peaking at - 0.551 V (vs. SCE), which was evidenced by linear sweep anodic stripping voltammetry (LSASV) results. The point defect plays a pivotal role in the electrocatalytic reaction process, which can mediate the electronic structure and surface properties of h-BN. Accordingly, the sensor presented a low detection limit of 0.15 µg/L towards Pb2+ and a wide linear response concentration range from 0.5 to 400 µg/L (correlation coefficient = 0.995). In view of its superior selectivity, stability, and reproducibility, the proposed method was applied for Pb2+ determination in real samples and exhibited satisfactory results. This work provides insight for the construction of electrochemical sensor with high-performance by engineering defects of modifying materials. Defect-loaden h-BN exhibited enhanced electrocatalytic redox reaction towards lead ions and thus a novel Pb2+ sensor with high performances was constructed.

Dynamical Casimir-Polder force on a two-level atom with superposition state in a cavity comprising a dielectric.

We study the dynamical Casimir-Polder force on a two-level atom with different initial states in the one-dimensional dielectric cavity with output coupling, and obtain the analytical expression of the expectation value of dynamical Casimir-Polder force. Results show that the expectation values of dynamical Casimir-Polder force may be affected by the initial states of the atom. Moreover, the expectation value of Casimir-Polder force may vanish at some special atomic positions by properly selecting the initial state of the system. The effects of different relative dielectric constants and the cavity size on the expectation value of Casimir-Polder force are also discussed.

Phytoextraction of cadmium-contaminated soils: comparison of plant species and low molecular weight organic acids.

To select suitable plants for phytoextraction of Cd-contaminated soils, we evaluated the phytoextraction potential of five local Cd-accumulators: Amaranthus hypochondriacus L., Solanum nigrum L., Phytolacca acinosa Roxb., Celosia argentea L., and Sedum spectabile Boreau. The plants were grown in three naturally contaminated soils with different total Cd levels (1.57, 3.89, and 22.4 mg kg-1). Throughout the experimental period, no plants showed any visible symptoms of metal toxicity. The Cd uptake of C. argentea was the greatest in the S-YS soil (105 µg plant-1) and among the greatest in the S-HC soil and S-TJ soil. Besides, C. argentea exhibited the highest bioconcentration factor (12.3) in three soils. To improve the phytoextraction efficiency of C. argentea, we applied four low molecular weight organic acids (LMWOAs): tartaric acid, malic acid, oxalic acid, and citric acid. Malic acid was more effective in enhancing Cd uptake by C. argentea than the other LMWOAs. Therefore, C. argentea may be a potential choice in actual remediation projects. Moreover, application of malic acid is an effective way to increase the phytoextraction efficiency of C. argentea.

A voltammetric sensor for simultaneous determination of hydroquinone and catechol by using a heterojunction prepared from gold nanoparticle and graphitic carbon nitride.

An electrochemical sensor is described for the simultaneous determination of hydroquinone (HQ) and catechol (CC) based on a nanocomposite consisting of gold nanoparticles and graphitic carbon nitride (g-C3N4). The nanocomposite was synthesized via one-step thermal polymerization route and characterized by X-ray diffraction, transmission electron microscopy, and Fourier transform infrared techniques. The results confirmed the close contact between gold nanoparticles and g-C3N4. The nanocomposites exhibited the enhanced electrocatalytic redox towards HQ and CC. A glassy carbon electrode was modified with the nanocomposite to obtain a sensor that exhibited favorable analytical properties in the simultaneous detection of HQ and CC, with voltammetric peaks typically near -0.14 and - 0.02 V (vs. saturated calomel electrode). Linear responses are found between 1.0 and 320 µM for HQ (with a 0.3 µM detection limit; at S/N = 3), and between 0.1 and 320 µM for CC (with a 0.04 µM detection limit; at S/N = 3). The sensor was applied for the simultaneous determination of HQ and CC in spiked water samples, and acceptable recoveries were achieved. The superior sensing properties of the electrode are attributed to the synergy between the microstructure (heterojunction and porosity) and the π interactions between phenolic isomers and g-C3N4. Graphical abstractA novel electrochemical sensor is demonstrated for the simultaneous determination of hydroquinone and catechol based on a nanocomposite consisting of gold nanoparticles (AuNPs) and graphitic carbon nitride (g-C3N4).

In situ decoration of Au nanoparticles on carbon nitride using a single-source precursor and its application for the detection of tetracycline.

Optimizing heterostructure of nanocomposites holds great potential for making full use of their ability. Herein, gold nanoparticles (AuNPs) were in situ synthesized over the surface of graphitic carbon nitride (g-C3N4) via one-step pyrolyzation route using a single source precursor. The precursor of melamine chloroauric (C3H6N6H+⋅AuCl4-) was obtained through chemical precipitation reaction between melamine and chloroauric acid. The morphological analysis confirmed the compact contact between Au nanoparticles and g-C3N4. Then, the Au-g-C3N4 nanocomposites were employed to fabricate electrochemical sensor by modifying glassy carbon electrode (GCE). Electrochemical experiments showed that the Au-g-C3N4 exhibited enhanced electrocatalytic activity towards tetracycline oxidation as compared with either pure g-C3N4 or Au nanoparticles. Based on cyclic voltammetry (CV) method, the sensor was applied in the detection of tetracycline with a low detection limit of 0.03 µM (S/N = 3) and the linear range of concentration were 0.1-20 µM and 20-200 µM, respectively. Moreover, such an electrochemical sensor demonstrated high stability and good selectivity. Finally, the electrochemical sensor was applied to drug assays and exhibited sufficient precision and accuracy. Therefore, this work paves a new way of preparing g-C3N4-based heterostructures and provides an efficient method for the detection of tetracycline in clinical analysis and quality control.

Immobilization of horseradish peroxidase on amino-functionalized carbon dots for the sensitive detection of hydrogen peroxide.

The authors describe the preparation of amino-functionalized carbon dots (NH2-CDs) via a one-step hydrothermal process using silver nitrate and chitosan as the precursors. The NH2-CDs have a fairly consistent size distribution with an average size of 2.8 ± 0.5 nm. This is attributed to the introduction of Ag(I) both as a catalyst and a precipitant. The NH2-CDs are highly crystalline. Their surface carries amino groups and carboxy groups which is confirmed by transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy. Horseradish peroxidase (HRP) was immobilized in the NH2-CDs and then placed on a glassy carbon electrode (GCE). Spectroscopic and electrochemical analyses evidenced the stability and good bioactivity of the immobilized HRP. This reveals that NH2-CD is a desirable matrix for enzyme immobilization. The modified GCE exhibits enhanced electro-catalytic activity towards hydrogen peroxide (H2O2) reduction as compared to that of plain CDs. The effects of pH value and loading on the performances of the modified GCEs were studied. Under optimized conditions, the biosensor has a linear response in the 5 to 590 nM H2O2 concentration range, with a 1.8 nM defection limit (at an S/N ratio of 3). The sensor is stable, reproducible and selective. Finally, the sensor was applied to determine H2O2 in real samples, and satisfactory recoveries were achieved. Graphical abstract Amino-functionalized carbon dots (NH2-CDs) can provide more active sites and a friendly microenvironment for horseradish peroxidase (HRP) immobilization. Thus, a novel sensitive H2O2 biosensor has been developed.

Graphene-like carbon nitride nanosheet as a novel sensing platform for electrochemical determination of tryptophan.

In this paper, a new and facile strategy has been demonstrated for the electrochemical determination of tryptophan (Trp), based on graphite-like carbon nitride (g-C3N4) nanosheets modified glassy carbon (CNNS/GC) electrode. The g-C3N4 nanosheets were obtained via exfoliating bulk graphitic carbon nitride (bg-C3N4), which was synthesized using a thermal poly-condensation process. The obtained g-C3N4 nanosheets were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM). The results confirmed graphite-like structure with thickness of about 6-8nm. The as-synthesized g-C3N4 nanosheets were closely attached to the surface of GC electrode to construct electrochemical sensor without needing any film-forming agents. The CNNS/GC electrode exhibited good electrocatalytic activity towards Trp, and hereby some parameters, including scan rate and pH effect on Trp determination were investigated. Under the optimum experimental conditions, the oxidation peak currents had good linear relationship with Trp concentrations in the range of 0.1-110µM and a detection limit of 0.024µM (S/N=3) was achieved. In addition, the obtained sensor showed good sensitivity, favorable repeatability, and long-term stability. Finally, the proposed electrochemical sensor has been successfully applied for the determination of Trp concentration in real samples with satisfactory results.

Enhanced photocatalytic properties of α-SnWO4 nanosheets modified by Ag nanoparticles.

Decoration of silver nanoparticles (Ag-NPs) on surface of α-SnWO4 nanosheets has been achieved by a microwave-assisted deposition method. The as-synthesized products are structurally characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results illustrate that Ag-NPs are evenly anchored onto α-SnWO4 surface to form close heterojunction and the amount of Ag nanoparticles grown on α-SnWO4 nanosheets can be well controlled by tuning Ag+ concentration. The photocatalytic properties of Ag-NPs/α-SnWO4 composites are evaluated by degrading methyl orange (MO) under visible-light irradiation. Ag-NPs/α-SnWO4 composites exhibit better photocatalytic properties than that of pure α-SnWO4, and Ag-NPs/α-SnWO4 (5mol% Ag) presents the best photocatalytic activity, whose photodegradation efficiency of MO is about 97% within 70min. In addition, the obtained samples demonstrate good recyclability. The enhanced photocatalytic properties was attributed to synergistic effect between Ag-NPs and α-SnWO4 nanosheets, which can increase absorption of visible light enabled by surface plasma resonance (SPR) of Ag-NPs and facilitate the separation of photogenerated electron-hole pairs.

A sensitive glucose biosensor based on Ag@C core-shell matrix.

Nano-Ag particles were coated with colloidal carbon (Ag@C) to improve its biocompatibility and chemical stability for the preparation of biosensor. The core-shell structure was evidenced by transmission electron microscope (TEM) and the Fourier transfer infrared (FTIR) spectra revealed that the carbon shell is rich of function groups such as -OH and -COOH. The as-prepared Ag@C core-shell structure can offer favorable microenvironment for immobilizing glucose oxidase and the direct electrochemistry process of glucose oxidase (GOD) at Ag@C modified glassy carbon electrode (GCE) was realized. The modified electrode exhibited good response to glucose. Under optimum experimental conditions the biosensor linearly responded to glucose concentration in the range of 0.05-2.5mM, with a detection limit of 0.02mM (S/N=3). The apparent Michaelis-Menten constant (KM(app)) of the biosensor is calculated to be 1.7mM, suggesting high enzymatic activity and affinity toward glucose. In addition, the GOD-Ag@C/Nafion/GCE shows good reproducibility and long-term stability. These results suggested that core-shell structured Ag@C is an ideal matrix for the immobilization of the redox enzymes and further the construction of the sensitive enzyme biosensor.

Amino-functionalized mesoporous silica modified glassy carbon electrode for ultra-trace copper(II) determination.

This paper described a facile and direct electrochemical method for the determination of ultra-trace Cu(2+) by employing amino-functionalized mesoporous silica (NH2-MCM-41) as enhanced sensing platform. NH2-MCM-41 was prepared by using a post-grafting process and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and fourier transform infrared (FTIR) spectroscopy. NH2-MCM-41 modified glassy carbon (GC) electrode showed higher sensitivity for anodic stripping voltammetric (ASV) detection of Cu(2+) than that of MCM-41 modified one. The high sensitivity was attributed to synergistic effect between MCM-41 and amino-group, in which the high surface area and special mesoporous morphology of MCM-41 can cause strong physical absorption, and amino-groups are able to chelate copper ions. Some important parameters influencing the sensor response were optimized. Under optimum experimental conditions the sensor linearly responded to Cu(2+) concentration in the range from 5 to 1000 ng L(-1) with a detection limit of 0.9 ng L(-1) (S/N=3). Moreover, the sensor possessed good stability and electrode renewability. In the end, the proposed sensor was applied for determining Cu(2+) in real samples and the accuracy of the results were comparable to those obtained by inductively coupled plasma optical emission spectrometry (ICP-OES) method.

Core-shell structured Ag@C for direct electrochemistry and hydrogen peroxide biosensor applications.

Ag@C core-shell nano-composites have been prepared by a simple one-step hydrothermal method and are further explored for protein immobilization and bio-sensing. The electrochemical behavior of immobilized horseradish peroxidase (HRP) on Ag@C modified indium-tin-oxide (ITO) electrode and its application as H2O2 sensor are investigated. Electrochemical and UV-vis spectroscopic measurements demonstrated that Ag@C nano-composites provide excellent matrixes for the adsorption of HRP and the entrapped HRP retains its bioactivities. It is found that on the HRP-Ag@C/ITO electrode, HRP exhibited a fast electron transfer process and good electrocatalytic reduction toward H2O2. Under optimum experimental conditions the biosensor linearly responds to H2O2 concentration in the range of 5.0×10⁻7-1.4×10⁻4 M with a detection limit of 2.0×10⁻7 M (S/N=3). The apparent Michaelis-Menten constant (K(app)(M)) of the biosensor is calculated to be 3.75×10⁻5 M, suggesting high enzymatic activity and affinity toward H2O2. In addition, the HRP-Ag@C/ITO bio-electrode shows good reproducibility and long-term stability. Thus, the core-shell structured Ag@C is an attractive material for application in the fabrication of biosensors due to its direct electrochemistry and functionalized surface for efficient immobilization of bio-molecules.

Sensitive electrochemical sensor of tryptophan based on Ag@C core-shell nanocomposite modified glassy carbon electrode.

We here reported a simple electrochemical method for the detection of tryptophan (Trp) based on the Ag@C modified glassy carbon (Ag@C/GC) electrode. The Ag@C core-shell structured nanoparticles were synthesized using one-pot hydrothermal method and characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), and Fourier transform-infrared spectroscopy (FTIR). The electrochemical behaviors of Trp on Ag@C/GC electrode were investigated and exhibited a direct electrochemical process. The favorable electrochemical properties of Ag@C/GC electrode were attributed to the synergistic effect of the Ag core and carbon shell. The carbon shell cannot only protect Ag core but also contribute to the enhanced substrate accessibility and Trp-substrate interactions, while nano-Ag core can display good electrocatalytic activity to Trp at the same time. Under the optimum experimental conditions the oxidation peak current was linearly dependent on the Trp concentration in the range of 1.0×10(-7) to 1.0×10(-4) M with a detection limit of 4.0×10(-8) M (S/N=3). In addition, the proposed electrode was applied for the determination of Trp concentration in real samples and satisfactory results were obtained. The technique offers enhanced sensitivity and may trigger the possibilities of the Ag@C nanocomposite towards diverse applications in biosensor and electroanalysis.

Binding studies of L-tryptophan to human serum albumin with nanogold-structured sensor by piezoelectric quartz crystal impedance analysis.

Nanogold-modified sensor was constructed and applied to study the binding of L-tryptophan to human serum albumin (HSA) in situ by piezoelectric quartz crystal impedance (PQCI) analysis. It was interesting that the as-prepared nanogold modified sensor was more sensitive and biocompatible than bare gold electrode. The frequency changes due to protein adsorption on the nanogold-modified sensor might be described as a sum of two exponential functions and detailed explanation was given. Additionally, the kinetics of the binding process was also investigated. The binding constant (K) and the number of binding site (n) for the binding process without competitor are fitted to be 1.07 x 10(4) (mol l(-1))(-1) s(-1) and 1.13, respectively, and 2.24 x 10(3) (mol l-(1))(-1) s(-1) and 1.18, respectively for the binding process with competitor.

Molecularly imprinted thin film self-assembled on piezoelectric quartz crystal surface by the sol-gel process for protein recognition.

A novel method of combining sol-gel and self-assembly technology to prepare a human serum albumin (HSA)-imprinted film on the surface of piezoelectric quartz crystal (PQC) Au-electrode modified with thioglycolic acid was described in this paper. The imprinting process was characterized by using the piezoelectric quartz crystal impedance (PQCI) technique and electrochemical impedance technique. Scanning electron microscope (SEM) was employed to characterize the surface morphology of the resultant imprinted film. The piezoelectric technique and electrochemical impedance technique were also employed to investigate the binding performance of the sol-gel-imprinted film with the template protein. The results showed that the imprinted PQC film can give selective recognition to the template protein. The effects of salts and solvents on the binding capacity of the imprinted film with protein were discussed in detail. Other influencing factors (temperature and pH) have also been investigated. This self-assembly sol-gel imprinting technique was proved to be an alternative method for the preparation of biomacromolecule-imprinted thin film.

Effect of the size of molecularly imprinted polymers sensing materials on piezoelectric quartz crystal sensor performance.

The effect of the size of the molecularly imprinted polymers (MIPs) on the piezoelectric quartz crystal (PQC) sensor performance was investigated. Erythromycin imprinted polymers microspheres with different sizes were synthesized by precipitation polymerization. The size of the MIPs was characterized by using transmission electron microscope (TEM) analysis. Being coated with a poly(vinyl chloride) (PVC) membrane containing MIPs, the proposed PQC sensor can selectively adsorb the template molecule. Investigation of the performance of sensors modified with different sizes of MIPs showed that PQC sensor modified with smaller size MIPs exhibited better performance and excellent selectivity. Other influencing factors on sensor functions modified with different sizes MIPs were also investigated.

Real-time investigation of the interaction between primaquine phosphate and bovine serum albumin (BSA) by piezoelectric quartz crystal impedance analysis.

Real-time investigation of the interaction between primaquine phosphate and bovine serum albumin by the piezoelectric quartz crystal impedance (PQCI) analysis was carried out for the first time. Three kinds of electrodes were investigated. Compared with bare gold (Au) electrode, the gold electrode self-assembled of nanogold colloids exhibits maintained biocompatibility, increased capacity and more bioactivity. Additionally, on the basis of the multi-dimensional information provided by the PQCI analysis, the real-time interaction information and the kinetics of the binding process was investigated and a response model was deduced. At 37 degrees C, the binding rate (k1), dissociation rate (k(-1)) and equilibrium constants (Ka) were 4.19x10(2) (mol l(-1))(-1) s(-1), 1.01x10(-3) s(-1) and 4.15x10(5) (mol l(-1))(-1) for the electrode modified by nanogold particles; 3.83x10(2) (mol l(-1))(-1) s(-1), 9.70x10(-4) s(-1) and 3.95x10(5) (mol l(-1))(-1) for the bare gold electrode, respectively.

Piezoelectric quartz crystal impedance and electrochemical impedance study of HSA-diazepam interaction by nanogold-structured sensor.

Human serum albumin (HSA) was immobilized on the surface of colloidal Au and exposed to diazepam. Colloidal Au were at first self-assembled on the gold electrode through the thiol groups of a 1,6-hexanedithiol monolayer. The real-time course of the resonant frequency and equivalent circuit parameters of the sensor during the protein-diazepam binding was determined for the first time by piezoelectric quartz crystal impedance (PQCI). On the basis of the multidimensional information provided by the PQCI analysis, it was concluded that the decrement of the observed frequency was mainly ascribable to the mass loading on the sensor surface. Compared with a bare gold electrode, the gold electrode self-assembled from nanogold colloids exhibits maintained biocompatibility, increased capacity, and more bioactivity. Cyclic voltammetry and electrochemical impedance techniques were used to investigate the immobilization of HSA and the interaction between HSA and diazepam. Results testified that gold colloid could play the role of an efficient electron-conducting tunnel and have a very high ratio of surface to volume. Additionally, the kinetics of the binding process was investigated. The estimated binding constant (K) and the number of binding site (n) on one HSA molecule were 1.66 x 10(6) mol l(-1) and 1.28, respectively.

Study of the immobilization of alcohol dehydrogenase on Au-colloid modified gold electrode by piezoelectric quartz crystal sensor, cyclic voltammetry, and electrochemical impedance techniques.

The immobilization of alcohol dehydrogenase (ADH) on Au-colloid modified gold electrodes has been investigated. Colloidal Au was first self-assembled onto gold electrodes through the thiol groups of an 1,6-hexanedithiol monolayer. Piezoelectric quartz crystal sensor, cyclic voltammetry, and electrochemical impedance techniques were used to investigate the immobilization of ADH on Au colloids. The cyclic voltammogram tends to be more irreversible with increased ADH concentration. In the impedance spectroscopic study, an obvious difference of the electron transfer resistance between the Au-colloid modified electrode and the bare gold electrode was observed. Using the piezoelectric quartz crystal sensor, the Michaelis constant, K(m), and the maximum initial rate, V(max), of the immobilized ADH were estimated as 6.03 x 10(-4) M and 0.63 Hzs (-1), respectively. The binding constant of ADH with nicotinamide adenine dinucleotide (NAD) was also determined as 1.87 x 10(4) M(-1). Experimental results showed that colloidal Au can be used as a biocompatible matrix for enzyme immobilization.