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 6–8 nm. 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 0.1–110 μM and a detection limit of 0.024 μM (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.Download high-res image (123KB)Download full-size image

This work reports a sensitive sensor for the electrochemical determination of trace mercury (Hg2+) by employing ultrathin graphitic carbon nitride (utg-C3N4) nanosheet as enhanced sensing platform. The utg-C3N4 nanosheets were obtained by exfoliating the bulk g-C3N4, which was synthesized via a thermal polycondensation process. The as-prepared samples were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM), which confirmed graphite-like structure with thickness of about 8 nm. The g-C3N4 nanosheets can be easily attached on the surface of glassy carbon (GC) electrode free of any film-forming agent. It was found that Utg-C3N4 modified GC electrode showed enhanced electrochemical response to Hg2+ in comparison with bulk g-C3N4, which could be ascribed to the strong affinity between utg-C3N4 and Hg2+ through its NH and NH2 groups. This allows detection of Hg2+ in aqueous solutions with high sensitivity and selectivity. Under the optimized experimental conditions, the anodic stripping voltammetric (ASV) currents are linearly responsible to Hg2+ concentrations in the range of 0.1–15 μg/L with a detection limit of 0.023 μg/L (S/N = 3). The sensitivity of the as-constructed sensor is about 6.8 μA(μg/L)−1 cm−2. In addition, the proposed sensor was applied in determining Hg2+ in practical samples and the results are comparable to those obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES) method.

•Enhanced direct electrochemistry of GOD was achieved at Ag@C modified electrode.•A novel glucose biosensor based on Ag@C core–shell structure was developed.•The designed GOD-Ag@C/Nafion/GCE biosensor showed favorable analysis properties.•The biosensor is easy to prepare and can be applied for real sample assay.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 0.05–2.5 mM, with a detection limit of 0.02 mM (S/N = 3). The apparent Michaelis–Menten constant (KMapp) of the biosensor is calculated to be 1.7 mM, 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.

•We report a facile method to selectively detect Cu2+ based on NH2-MCM-41 as sensing platform.•NH2-MCM-41 has good affinity toward Cu2+.•Detection limit of 0.9 ng L−1 and linear concentration range of 5–1000 ng L−1 are achieved.•The method is successfully applied to detect Cu2+ in real samples.This paper described a facile and direct electrochemical method for the determination of ultra-trace Cu2+ 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 Cu2+ 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 Cu2+ 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 Cu2+ in real samples and the accuracy of the results were comparable to those obtained by inductively coupled plasma optical emission spectrometry (ICP-OES) method.NH2-MCM-41 modified glassy carbon electrode was prepared and it exhibited enhanced anodic stripping response toward Cu (II), which could result from the large surface area of MCM-41 and the good chelating ability of amine-group. The as-constructed electrochemical sensor showed excellent analytical properties in the determination of Cu2+ and was successfully used for real sample assays.

In this work, a new strategy for the simultaneous determination of Pb2+ and Cd2+ was described based on amino-functionalized mesoporous silica (NH2-MCM-41) as sensing mediator. NH2-MCM-41 was prepared using a post-grafting process and the successful amino-functionality was confirmed by Fourier transform infrared (FTIR) and X-ray energy dispersive (EDS) spectra. It was found that both MCM-41- and NH2-MCM-41-modified glassy carbon (GC) electrode exhibit simultaneous response to Pb2+ and Cd2+. However, the NH2-MCM-41 modified electrode showed higher sensitivity than that of the MCM-41-modified one, which was attributed to the good chelating ability of amino groups to metal ions besides the high surface area and special mesoporous morphology of MCM-41. As a result, simultaneous assay of Pb2+ and Cd2+ was realized using anodic stripping voltammetric (ASV) method. Under the optimum experimental conditions, the linear response ranges for Pb2+ and Cd2+ ions are 0.5-250 μgL−1 and 50-450 μgL−1, respectively. The corresponding detection limits are 0.2 μgL−1 for Pb2+ and 1.0 μgL−1 for Cd2+, with good electrode renewability, which is defined as the complete removal of the accumulated metals from the electrode surface. In addition, the NH2-MCM-41 modified electrode was demonstrated for the successful determination of Pb2+ and Cd2+ in real samples, including tap water, lake water and tea.

•A novel HRP biosensor based on Ag@C nano-composite has been developed for H2O2 detection.•The Ag@C core–shell structure has high electron transport capacity and good biocompatibility.•Wide linear range and low detection limit of HRP–Ag@C/ITO biosensor for H2O2 were obtained.•The biosensor is easy to be prepared and exhibits good reproducibility and long term stabilityAg@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 (KappM) 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.

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.Graphical abstractAg@C and Colloidal carbon sphere modified glassy carbon electrodes were prepared. It was clear that the Ag@C/GCE exhibited enhanced electrocatalytic activity towards Trp, which could result from the synergistic effect between Ag core and carbon shell. The Ag@C/GCE showed excellent analytical properties in the determination of Trp.Highlights► The electrochemical behavior of Ag@C core–shell nanocomposite was firstly proposed. ► Ag@C/GC electrode exhibited favorable electrocatalytic properties towards Trp. ► The good electrocatalysis was due to the synergistic effect of Ag-core and C-shell. ► The Ag@C/GC electrode displayed excellent analytical properties in determining Trp.

An electrochemical sensor was prepared using a glassy carbon electrode modified with Si-doped nano-TiO2 (TS/GC) for the detection of tryptophan (Trp). The electrochemical behavior of Trp at the TS/GC electrode was investigated by cyclic voltammetry (CV). It was found that the anodic peak current of Trp oxidation at the TS/GC electrode was higher than that at the bare electrode or TiO2 nanoparticles modified GC (T/GC) electrode. This finding suggested the enhanced electrochemical properties of the TS/GC electrode, and the improvement of the electrochemical properties was attributed to the presence of oxygen functional groups on the surface of the TS and the easy transfer of electrons and holes. Thereby, the TS/GC electrode was employed to determine Trp and the effects of scan rate, pH and interferents on the response of Trp oxidation were examined. Under the optimum experimental conditions the TS/GC electrode displayed a good analytical performance. The oxidation peak current of Trp showed a linear relationship with concentration over the range of 1.0 × 10−6 to 4.0 × 10−4 M, and the detection limit was estimated to be 5.0 × 10−7 M (S/N = 3). The proposed method was simple and exhibited favorable resistance against interferences. Moreover, the electrochemical sensor was successfully employed to determine Trp in pharmaceutical samples.

•A novel g-C3N4/ZnAl2O4 composite was developed.•ZnAl2O4 particles are successfully coupled with g-C3N4.•The g-C3N4/ZnAl2O4 composites exhibited excellent photocatalytic activities.•The heterojunction between g-C3N4 and ZnAl2O4 can facilitate charge separation.A series of g-C3N4/ZnAl2O4 composites were prepared using a conventional calcination method and the heterostructures were systematically characterized. It was found that the combination of g-C3N4 with ZnAl2O4 significantly improve their photocatalytic activities. The optimum photocatalyst of composite is at 5% (wt%) of ZnAl2O4, whose degradation efficiency for methyl orange (MO) was 96% within 120 min under visible-light irradiation. The formation of heterojunction between g-C3N4 and ZnAl2O4 can facilitate efficient charge separation of photogenerated electron-hole pairs, which were confirmed by electrochemical impedance spectroscopy (EIS). As a result, the photocatalytic properties of composites were enhanced.G-C3N4/ZnAl2O4 composites were prepared using a conventional calcination method and they exhibited excellent photocatalytic activity for degrading MO.Download high-res image (199KB)Download full-size image

An electrochemical sensor was prepared using a glassy carbon electrode modified with Si-doped nano-TiO2 (TS/GC) for the detection of tryptophan (Trp). The electrochemical behavior of Trp at the TS/GC electrode was investigated by cyclic voltammetry (CV). It was found that the anodic peak current of Trp oxidation at the TS/GC electrode was higher than that at the bare electrode or TiO2 nanoparticles modified GC (T/GC) electrode. This finding suggested the enhanced electrochemical properties of the TS/GC electrode, and the improvement of the electrochemical properties was attributed to the presence of oxygen functional groups on the surface of the TS and the easy transfer of electrons and holes. Thereby, the TS/GC electrode was employed to determine Trp and the effects of scan rate, pH and interferents on the response of Trp oxidation were examined. Under the optimum experimental conditions the TS/GC electrode displayed a good analytical performance. The oxidation peak current of Trp showed a linear relationship with concentration over the range of 1.0 × 10−6 to 4.0 × 10−4 M, and the detection limit was estimated to be 5.0 × 10−7 M (S/N = 3). The proposed method was simple and exhibited favorable resistance against interferences. Moreover, the electrochemical sensor was successfully employed to determine Trp in pharmaceutical samples.