Determination of vanillin in different food samples by using SMM/Au@ZIF-67 electrochemical sensor.

Vanillin is a popular flavoring agent in many food products. Simple, fast, and reliable quantification of this compound is crucial for the food industry. In this work, we have developed a new electrochemical sensor for accurate detection of vanillin in various real samples. The composite electrode was made of sodium montmorillonite nanoclay (SMM) and gold nanoparticles modified ZIF-67 (Au@ZIF-67), in which SMM contributes to the large adsorption capacity of the analyte, ZIF-67 and SMM supply more sensing active sites, and gold nanoparticles provide high electrical conductivity. The sensing electrode was comprehensively characterized using Brunauer-Emmett-Teller, EDS, XRD, SEM, FTIR, and TEM, and its electrochemical behavior for determination of vanillin including the electrooxidation mechanism of vanillin and different parameters such as scan rate and pH value was investigated. The result revealed that a two electron-two proton process was involved in the electrooxidation of vanillin, which takes place more readily due to the lower potential on the surface of SMM/Au@ZIF-67/carbon paste electrode. The new composite electrode was also more sensitive to vanillin detection with an anodic peak current almost 2.6 times more than that of the bare electrode. A linear sensing concentration range was established between 1 and 1200 nM with a detection limit of 0. 3 nM and a limit of quantitation of 1 nM. For real samples, the sensor demonstrated excellent recovery rates and reliability that was comparable to the standard high-performance liquid chromatography method.

Modification of gold surface by electrosynthesized mono aza crown ether substituted catechol-terminated alkane dithiol and its application as a new electrochemical sensor for trace detection of cadmium ions.

Among the toxic metals, cadmium is a very dangerous pollutant because it can extremely damage organs in humans and animals. This toxic metal is introduced into water from different industries such as metal plating, batteries, and alloys. Cadmium bioaccumulates in vital organs and unlike organic pollutants does not show any biological degradation. In this study, an electroactive self-assembled monolayer (SAM) was developed by covalent attachment of a novel mono aza-crown ether substituted catechol-terminated hexane dithiol onto the gold surface. The electrochemical behavior of the fabricated SAM electrode was investigated using voltammetry techniques and electrochemical impedance spectroscopy (EIS). The obtained results from voltammetric experiments revealed that the crown ether moiety of SAM forms a selective complex with cadmium ion. Under optimal conditions, Cd2+ could be detected in the range of 15 µM to 65 µM with a detection limit of 4.5 µM. Selectivity measurements reveal that the sensor is specific for Cd2+ even in the presence of high concentrations of other metal ions. The proposed sensor was applied to the determination of cadmium ion in water samples with high sensitivity and good selectivity.

Aptamer-based electrochemical biosensor by using Au-Pt nanoparticles, carbon nanotubes and acriflavine platform.

Herein, an ultrasensitive electrochemical aptasensor for quantitative detection of bisphenol A (BPA) was fabricated based on a novel signal amplification strategy. This aptasensor was developed by electrodeposition of gold-platinum nanoparticles (Au-PtNPs) on glassy carbon (GC) electrode modified with acid-oxidized carbon nanotubes (CNTs-COOH). In this protocol, acriflavine (ACF) was covalently immobilized at the surface of glassy carbon electrode modified with Au-PtNPs/CNTs-COOH nanocomposite. Attachment of BPA-aptamer at the surface of modified electrode was performed through the formation of phosphoramidate bonds between the amino group of ACF and phosphate group of the aptamer at 5'end. By interaction of BPA with the aptamer, the conformational of aptamer was changed which lead to retarding the interfacial electron transfer of ACF as a probe. Sensitive quantitative detection of BPA was carried out by monitoring the decrease of differential pulse voltammetric (DPV) responses of ACF peak current with increasing the BPA concentration. The resultant aptasensor exhibited good specificity, stability and reproducibility, indicating that the present strategy was promising for broad potential application.

Construction of a sensitive and selective sensor for morphine using chitosan coated Fe3O4 magnetic nanoparticle as a modifier.

A simple and sensitive sensor based on carbon paste electrode (CPE) modified by chitosan-coated magnetic nanoparticle (CMNP) was developed for the electrochemical determination of morphine (MO). The proposed sensor was characterized with scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The electrooxidation of MO was studied on modified carbon paste electrode using cyclic voltammetry, chronoamperometry and differential pulse voltammetry as diagnostic techniques. The oxidation peak potential of morphine on the CMNP/CPE appeared at 380 mV which was accompanied with smaller overpotential and increase in oxidation peak current compared to that obtained on the bare carbon paste electrode (CPE). Under optimum conditions the sensor provides two linear DPV responses in the range of 10-2000 nM and 2-720 µM for MO with a detection limit of 3 nM. The proposed sensor was successfully applied for monitoring of MO in serum and urine samples and satisfactory results were obtained.

Binding studies of the anti-retroviral drug, efavirenz to calf thymus DNA using spectroscopic and voltammetric techniques.

Interactions between efavirenz (EFZ) with calf thymus DNA (CT-DNA) were investigated in vitro under stimulated physiological conditions using multispectroscopic techniques, cyclic voltammetry viscosity measurement, and gel electrophoresis. Methylene blue and acridine orange dyes were used as spectral probes by fluorescence spectroscopy. Hypochromicity was observed in ultra-violet (UV) absorption band of EFZ. Considerable fluorescence enhancement of EFZ was observed in the presence of increasing amounts of DNA solution and the binding constants (Kf ) and corresponding numbers of binding sites (n) were calculated at different temperatures. Thermodynamic parameters including enthalpy change (ΔH) and entropy change (ΔS) were calculated to be -304.78 kJ mol(-1) and -924.52 J mol(-1) K(-1) according to the van 't Hoff equation, which indicated that reaction is predominantly enthalpically driven. In addition, UV/vis absorption titration of DNA bases confirmed that EFZ interacted with guanine and cytosine preferentially. Gel electrophoresis of DNA with EFZ demonstrated that EFZ also has the ability to cleave supercoiled plasmid DNA. Circular dichroism study showed stabilization of the right-handed B form of CT-DNA. All results suggest that EFZ interacts with CT-DNA via an intercalative mode of binding.

Silver ion imprinted polymer nanobeads based on a aza-thioether crown containing a 1,10-phenanthroline subunit for solid phase extraction and for voltammetric and potentiometric silver sensors.

A new nano-sized silver(I) ion-imprinted polymer (IIP) was prepared via precipitation copolymerization using ethyleneglycol dimethacrylate, as a cross-linking agent in the presence of Ag(+) and an aza-thioether crown containing a 1,10-phenanthroline subunit as a highly selective complexing agent. The imprint silver(I) ion was removed from the polymeric matrix using a 1.0M HNO3 solution. The resulting powder material was characterized using IR spectroscopy and scanning electron microscopy. The SEM micrographs showed colloidal nanoparticles of about 52 nm and 75 nm in diameter and slightly irregular in shape for leached and unleached IIPs, respectively. The optimal pH for quantitative enrichment was 6.0 and maximum sorbent capacity of the prepared IIP for Ag(+) was 18.08 µmol g(-1). The relative standard deviation and limit of detection (LOD=3Sb/m) for flame atomic absorption spectrometric determination of silver(I) ion, after its selective extraction by the prepared IIP nanobeads, were evaluated as 2.42% and 2.2×10(-8) M, respectively. The new Ag(+)-IIP was also applied as a suitable sensing element to the preparation of highly selective and sensitive voltammetric and potentiometric sensors for ultra trace detection of silver(I) ion in water samples, with limits of detection of 9.0×10(-10) and 1.2×10(-9) M, respectively.

Development of a selective and sensitive voltammetric sensor for propylparaben based on a nanosized molecularly imprinted polymer-carbon paste electrode.

The design and construction of a selective voltammetric sensor for propylparaben (PP) in cosmetics by using a molecularly imprinted polymer (MIP) as recognition element was introduced. The MIP was synthesized by using PP as template and methacrylic acid as functional monomer and then incorporated in the carbon paste electrode as PP sensor. The molecularly imprinted polymer-carbon paste electrode (MIP-CPE) showed very high recognition ability in comparison to non-imprinted polymer-carbon paste electrode (NIP-CPE). It was shown that electrode washing after PP extraction, led to enhanced selectivity, without noticeably decreasing the sensitivity. Some parameters affecting sensor response were optimized, and a calibration curve was then plotted using differential pulse voltammetric (DPV) technique. A dynamic linear range of 1 nM to 100 nM was obtained. The detection limit of the sensor was calculated to be equal to 0.32 nM. The imprinted electrode also displayed good selectivity for PP and selectivity coefficients were 2.29 and 1.66 for methylparaben (MP) and ethylparaben (EP) respectively. Structural analogs, such as phenol and p-hydroxybenzoic acid had almost no response. This sensor was used successfully for propylparaben determination in cosmetic sample.