Highly sensitive and selective electrochemical paper-based device using a graphite screen-printed electrode modified with molecularly imprinted polymers coated Fe3O4@Au@SiO2 for serotonin determination.

Herein, we propose a highly sensitive and selective three-dimensional electrochemical paper-based analytical device (3D-ePAD) to determine serotonin (Ser). It uses a graphite-paste electrode modified with nanoparticles coated with molecularly imprinted polymer (MIP). Fe3O4@Au nanoparticles were encapsulated with silica to create novel nano-sized MIP. Morphology and structural characterization reveal that silica imprinted sites (Fe3O4@Au@SiO2) synthesized via sol-gel methods provide excellent features for Ser detection, including high porosity, and greatly improve analyte diffusion and adsorption to provide a faster response by the MIP sensor. The template molecule was effectively removed by solvent extraction to provide a greater number of specific cavities that enhance analyte capacity and sensitivity. The 3D-ePAD was fabricated by alkyl ketene dimer (AKD)-inkjet printing of a circular hydrophobic detection zone on filter paper for application of aqueous samples, coupled with screen-printed electrodes on the paper, which was folded underneath the hydrophobic zone. The sensor was constructed by drop coating of Fe3O4@Au@SiO2-MIP nanocomposites on the graphite electrode (GPE) surface. The MIP sensor (Fe3O4@Au@SiO2-MIP/GPE) was used in the detection of Ser by linear-sweep voltammetry (LSV) in 0.1 M phosphate buffer at pH 8.0. The device exhibits high sensitivity toward Ser, which we attribute to synergistic effects between catalytic properties, electrical conductivity of Fe3O4@Au@SiO2, and significantly increased numbers of imprinted sites. Ser oxidation was observed at +0.39 V. Anodic peak currents for Ser show linearity from 0.01 to 1000 µM (y = 0.0075 ± 0.0049 x + 0.4071 ± 0.0052, r2 = 0.993), with a detection limit of 0.002 µM (3S/N). The device provides good repeatability (%relative standard deviations; RSD) = 4.23%, calculated from the current responses of ten different MIP sensors). The device also exhibits high selectivity and reproducibility (%RSD = 8.35%, obtained from five calibration plots). The analytical performance of the device is suitable for the determination of Ser in pharmaceutical capsules and urine samples.

Novel three-Dimensional molecularly imprinted polymer-coated carbon nanotubes (3D-CNTs@MIP) for selective detection of profenofos in food.

A new and facile method for selective measurement of profenofos (PFF) using a simple flow-injection system with a molecularly-imprinted-polymer-coated carbon nanotube (3D-CNTs@MIP) amperometric sensor is proposed. The 3D-CNTs@MIP was synthesized by successively coating the surface of carboxylated CNTs with SiO2 and vinyl end groups, then terminating with molecularly imprinted polymer (MIP) shells. MIP was grafted to the CNT cores using methacrylic acid (MAA) monomer, ethylene glycol dimethacrylate (EGDMA) as cross linker, and 2,2'-azobisisobutyronitrile (AIBN) as initiator. We constructed the PFF sensor by coating the surface of a glassy carbon electrode (GCE) with 3D-CNTs@MIP and removed the imprinting template by solvent extraction. Morphological and structural characterization reveal that blending of the MIP on the CNT surface significantly increases the selective surface area, leading to greater numbers of imprinting sites for improved sensitivity and electron transfer. The 3D-CNTs@MIP sensor exhibits a fast response with good recognition when applied to PFF detection by cyclic voltammetry and amperometry. The PFF oxidation current signal appears at +0.7 V vs Ag/AgCl using 0.1 M phosphate buffer (pH 7.0) as the carrier solution. The designed 3D-imprinted sensor provides a linear response over the range 0.01-200 µM (r2 = 0.995) with a low detection limit of 0.002 µM (3σ). The sensor was successfully applied to detection of PFF in vegetable samples.

Selective amperometric flow-injection analysis of carbofuran using a molecularly-imprinted polymer and gold-coated-magnetite modified carbon nanotube-paste electrode.

Herein, we propose a new approach for selective determination of carbofuran (CBF) in vegetables, based on a simple flow-injection system using a molecularly-imprinted amperometric sensor. The sensor design is based on a carbon-paste electrode decorated with carbon nanotubes and gold-coated magnetite (CNTs-Fe3O4@Au/CPE) coated with a molecularly-imprinted polymer (MIP) for CBF sensing. The MIP was synthesized on the electrode surface by electropolymerization using a supramolecular complex, namely 4-ter-butylcalix [8] arene-CBF (4TB[8]A-CBF), as the template. We used o-phenylenediamine as the functional monomer. Our results demonstrate that incorporation of the MIP coating improves the electrochemical catalytic properties of the electrode, increases its surface area, and increases CBF selectivity by modulating the electrical signal through elution and re-adsorption of CBF. The imprinted sensor (MIP-CNTs-Fe3O4@Au/CPE) was used in a flow-injection analysis (FIA) system. Experimental conditions were investigated in amperometric mode, with the following optimized parameters: phosphate buffer solution (0.1M, pH 8.0) as the carrier, flow rate 0.5mLmin-1, applied potential +0.50V. When used in the FIA system, the designed imprinted sensor yields a linear dynamic range for CBF from 0.1 to 100µM (r2 = 0.998) with a detection limit of 3.8nM (3Sb), and a quantification limit of 12.7nM (10Sb). The sensor exhibits acceptable precision (%RSD = 4.8%) and good selectivity toward CBF. We successfully applied the electrode to detect CBF in vegetable samples.

Amperometric flow injection analysis of glucose using immobilized glucose oxidase on nano-composite carbon nanotubes-platinum nanoparticles carbon paste electrode.

We report a novel amperometric glucose biosensor based on glucose oxidase (GOx) immobilized on a carbon nanotube (CNTs)-poly(diallyldimethyl-ammonium chloride) (PDDA)-platinum nanoparticle (PtNPs) modified carbon-paste electrode (CNTs-PDDA-PtNPs/CPE). The CNTs-PDDA-PtNPs composite materials were characterized by TEM and electrochemical techniques. Cyclic voltammetric results reveal direct electron transfer of the immobilized GOx, indicated by two quasi-reversible redox peaks at a potential of 0.37V (vs. Ag/AgCl) in phosphate buffered solution (PBS) (0.10M, pH 7). The biosensor provides good glucose oxidation activity and retention of GOx electrocatalytic activity due to CNTs-PDDA-PtNPs enhancement of the redox response. The carbon paste electrode was installed as working electrode in a flow through electrochemical cell of a flow injection (FI) system. Glucose was quantified using amperometric measurements at 0.5V vs. Ag/AgCl and PBS carrier (0.10M, pH 7.0) at a flow rate of 1.0mLmin-1. The linear working ranges for glucose measurements were 0.1-3mM (r2=0.995) and 5-100mM (r2=0.997), with corresponding sensitivities of 0.127 and 0.060 (µAs) mM-1, respectively. The system provides good precision of 2.8% R.S.D with a calculated detection limit (3S/N) of 15µM. The proposed method was successfully applied to determination of glucose in food and pharmaceutical samples with throughput of 200 samplesh-1.

A sensitive and selective on-line amperometric sulfite biosensor using sulfite oxidase immobilized on a magnetite-gold-folate nanocomposite modified carbon-paste electrode.

We describe a novel amperometric sulfite biosensor, comprising a carbon-paste electrode (Fe3O4@Au-Cys-FA/CPE) modified with immobilized sulfite oxidase (SOx) on a gold-coated magnetite nanoparticle core, encased within a conjugated folic acid (FA) cysteine (Cys) shell. The biosensor electrode was fabricated using a polydimethylsiloxane (PDMS) and mineral oil mixture as binder, which also enhances the physical stability and sensitivity of the electrode. The developed biosensor displays good electrocatalytic activity toward oxidation of H2O2, which occurs by an enzymatic reaction between SOx and sulfite. The Fe3O4@Au-Cys-FA electrode exhibits good electrocatalytic activity, and has good retention of chemisorbed SOx on the electrode because of its large surface area. Sulfite was quantified using amperometric measurements from the Fe3O4@Au-Cys-FA/CPE biosensor, and using an in-house assembled flow cell at +0.35V (vs. Ag/AgCl), with a phosphate buffer carrier (0.10M, pH 7.0) at a flow rate of 0.8mLmin(-1). The system detects sulfite over the range 0.1-200mgL(-1) (r(2)=0.998), with a detection limit of 10µgL(-1) (3σ of blank). The system exhibits acceptable precision (%R.S.D.=3.1%), rapid sample throughput (109samplesh(-1)), and good stability (2w). The developed biosensor shows satisfactory tolerance to potential interferences, such as sugars, anions, ascorbic acid, and ethanol. We applied the developed method to the determination of sulfite content in wines and pickled food extracts, and our results are in good agreement with those obtained by the standard iodometric method.

Simple flow injection for determination of sulfite by amperometric detection using glassy carbon electrode modified with carbon nanotubes-PDDA-gold nanoparticles.

A new approach is presented for sensitive and selective measurement of sulfite (SO3(2-)) in beverages based on a simple flow injection system with amperometric detection. In this work, the sulfite sensor was a glassy carbon electrode modified with multiwall carbon nanotubes-poly(diallyldimethylammonium chloride)-gold nanoparticles composites (CNTs-PDDA-AuNPs/GC). Electrochemical oxidation of sulfite with this electrode was first studied in 0.1M phosphate buffer (pH 7.0) using cyclic voltammetry. The results indicated that the CNTs-PDDA-AuNPs/GC electrode possesses electrocatalytic activity for the oxidation of sulfite with high sensitivity and selectivity. Sulfite was quantified using amperometric measurement with the new sensor at +0.4V vs Ag/AgCl in conjunction with flow injection. The linear working range for the quantitation of sulfite was 2-200 mg L(-1) (r(2)=0.998) with a detection limit of 0.03 mg L(-1) (3σ of blank) and an estimated precision of 1.5%.The proposed method was successfully applied to the determination of sulfite in fruit juices and wines with a sample throughput of 23 samples per hour.