A urea electrochemical sensor based on molecularly imprinted chitosan film doping with CdS quantum dots

An improved imprinted film-based electrochemical sensor for urea recognition was developed using CdS quantum dots (QDs) doped chitosan as the functional matrix. The microstructure and composition of the imprinted films depicted by scanning electron microscopy (SEM), attenuated total reflection infrared (ATR-IR), X-ray diffraction (XRD), and electrochemical impedance spectroscopy (EIS) indicated the fabricated feasibility of the nanoparticle doped films via in situ electrodeposition. Differential pulse voltammetric responses under the optimal fabrication conditions showed that the sensitivity of CdS QDs-MIP (molecularly imprinted polymer) electrochemical sensor was enhanced from the favorable electron transfer and magnified surface area of CdS QDs with a short adsorption equilibrium time (7 min), wide linear range (5.0 × 10(-12) to 4.0 × 10(-10) M and 5.0 × 10(-10) to 7.0 × 10(-8) M), and low detection limit (1.0 × 10(-12) M). Meanwhile, the fabricated sensor showed excellent specific recognition to template molecule among the structural similarities and coexistence substances. Furthermore, the proposed sensor was applied to determine the urea in human blood serum samples based on its good reproducibility and stability, and the acceptable recovery implied its feasibility for practical application.     

Electrochemical imprinted sensor for determination of oleanic acid based on poly (sodium 4-styrenesulfonate-co-acrylic acid)-grafted multi-walled carbon nanotubes-chitosan and cobalt hexacyanoferrate nanoparticles

A novel sensitive and selective imprinted electrochemical sensor for the determination of oleanic acid was constructed on a carbon electrode by stepwise modification of functional multi-walled carbon nanotubes, cobalt hexacyanoferrate nanoparticles and a thin imprinted sol-gel film. The fabrication of a homogeneous porous poly (sodium 4-styrenesulfonate-co-acrylic acid)-grafted multi-walled carbon nanotubes/SiO(2)-chitosan nanocomposite film was conducted by controllable electrodeposition technology. The surface morphologies of the modified electrodes were characterized by scanning electron microscope. The performance of the imprinted sensor was investigated by cyclic voltammetry, square wave voltammetry and electrochemical impedance spectroscopy in detail. The imprinted sensor displayed high sensitivity and selectivity towards oleanic acid. A linear relationship between the sensor response signal and the logarithm of oleanic acid concentrations ranging from 1.0×10(-8) to 1.0×10(-3) mol L(-1) was obtained with a detection limit of 2.0×10(-9) mol L(-1). It was applied to the determination of oleanic acid in real capsule samples successfully.     

Electrochemical sensor based on nitrogen doped graphene: simultaneous determination of ascorbic acid, dopamine and uric acid

Nitrogen doped graphene (NG) was prepared by thermally annealing graphite oxide and melamine mixture. After characterization by atomic force microscopy and X-ray photoelectron spectroscopy etc., the electrochemical sensor based on NG was constructed to simultaneously determine small biomolecules such as ascorbic acid (AA), dopamine (DA) and uric acid (UA). Due to its unique structure and properties originating from nitrogen doping, NG shows highly electrocatalytic activity towards the oxidation of AA, DA and UA. The electrochemical sensor shows a wide linear response for AA, DA and UA in the concentration range of 5.0×10(-6) to 1.3×10(-3)M, 5.0×10(-7) to 1.7×10(-4)M and 1.0×10(-7) to 2.0×10(-5)M with detection limit of 2.2×10(-6)M, 2.5×10(-7)M and 4.5×10(-8)M at S/N=3, respectively. These results demonstrate that NG is a promising candidate of advanced electrode material in electrochemical sensing and other electrocatalytic applications.     

DNA biosensor for detection of Neisseria gonorrhoeae causing sexually transmitted disease

A sequence-specific electrochemical sexually transmitted disease (STD) sensor based on self-assembled monolayer of thiolated DNA probe specific to target opa gene for detection of Gonorrhoea, a sexually transmitted disease has been fabricated. 6-Mercapto-1-hexanol (MCH) has been used as a blocking agent to facilitate oligos "stand" up at the surface, a configuration favoring subsequent DNA hybridization and to repel non-specific adsorption of undesired DNA. The results of differential pulse voltammetric studies of this STD sensor reveal low detection limit (1.0 × 10(-18)M) and a wide dynamic range (from 1.0 × 10(-6)M to 0.5 × 10(-18)M) arising due to the stable hybridization using methylene blue as an electro-active DNA hybridization indicator. The experimental results with genomic DNA, clinical patient sample of Neisseria gonorrhoeae, culture of non-N. gonorrhoeae Neisseria species (NgNS) and gram negative bacteria indicate that the fabricated sensor is specific to this STD.     

Electrochemical sensor based on gold nanoparticles fabricated molecularly imprinted polymer film at chitosan-platinum nanoparticles/graphene-gold nanoparticles double nanocomposites modified electrode for detection of erythromycin

A molecularly imprinted electrochemical sensor was fabricated based on gold electrode decorated by chitosan-platinum nanoparticles (CS-PtNPs) and graphene-gold nanoparticles (GR-AuNPs) nanocomposites for convenient and sensitive determination of erythromycin. The synergistic effects of CS-PtNPs and GR-AuNPs nanocomposites improved the electrochemical response and the sensitivity of the sensor. The molecularly imprinted polymers (MIPs) were prepared by HAuCl(4), 2-mercaptonicotinic acid (MNA) and erythromycin. Erythromycin and MNA were used as template molecule and functional monomer, respectively. They were first assembled on the surface of GR-AuNPs/CS-PtNPs/gold electrode by the formation of Au-S bonds and hydrogen-bonding interactions. Then the MIPs were formed by electropolymerization of HAuCl(4), MNA and erythromycin. The sensor was characterized by cyclic voltammetry (CV), scanning electron microscope (SEM), UV-visible (UV-vis) absorption speactra and amperometry. The linear range of the sensor was from 7.0×10(-8)mol/L-9.0×10(-5)mol/L, with the limit of detection (LOD) of 2.3×10(-8)mol/L (S/N=3). The sensor showed high selectivity, excellent stability and good reproducibility for the determination of erythromycin, and it was successfully applied to the detection of erythromycin in real spiked samples.     

Determination of three major catecholamines in human urine by capillary zone electrophoresis with chemiluminescence detection

A sensitive simple method is presented for the determination of three major catecholamines in human urine by capillary electrophoresis (CE) with on-line chemiluminescence (CL) detection. This was also the first time that the luminol-Ag(III) complex CL system was used for CE detection. This method was based on the enhancing effect of epinephrine (EP), norepinephrine (NE), and dopamine (DA) on the CL reaction between luminol and the Ag(III) complex in alkaline solution. The separations and determinations were performed with an electrophoretic buffer consisting of 20.0mM sodium borate and 1.0mM luminol. Under optimized conditions, the three catecholamines were baseline separated and detected in less than 8 min. Detection limits of 7.9 × 10(-8)M, 1.0 × 10(-7)M, and 6.9 × 10(-8)M were observed for EP, NE, and DA, respectively. Relative standard deviation (RSD) values for the peak height were 4.7% to 5.4% (n = 5). Our proposed method was applied to the determinations of the catecholamines in urine samples from 12 healthy individuals and 26 pheochromocytoma patients. Our results suggest that this method might be useful to monitor the catecholamine levels in routine screening and to diagnose pheochromocytoma.     

Biomimetic sensor for certain catecholamines employing copper(II) complex and silver nanoparticle modified glassy carbon paste electrode

A dimeric Cu(II) complex [Cu(μ(2)-hep)(hep-H)](2)·2ClO(4) (1) containing bidentate (hep-H=2-(2-hydroxyethyl)pyridine) ligand was synthesized and characterized by single crystal X-ray diffraction studies. Each Cu-ion in 1 is in a distorted square pyramidal geometry. Further 1 along with silver nanoparticles (SNPs) have been used as modifier in the construction of a biomimetic sensor (1-SNP-GCPE) for determining certain catecholamines viz., dopamine (DA), levodopa (l-Dopa), epinephrine (EP) and norepinephrine (NE) using cyclic voltammetry, chronocoulometry, electrochemical impedance spectroscopy and adsorptive stripping square wave voltammetry (AdSSWV). Finally, the catalytic properties of the sensor were characterized by chronoamperometry. Employing AdSSWV, the calibration curves showed linear response ranging between 10(-6) and 10(-9)M for all the four analytes with detection limits (S/N=3) of 8.52×10(-10)M, 2.41×10(-9)M, 3.96×10(-10)M and 3.54×10(-10)M for DA, l-Dopa, EP and NE respectively. The lifetime of the biomimetic sensor was 3 months at room temperature. The prepared modified electrode shows several advantages such as simple preparation method, high sensitivity, high stability, ease of preparation and regeneration of the electrode surface by simple polishing along with excellent reproducibility. The method has been applied for the selective and precise analysis of DA, l-Dopa, EP and NE in pharmaceutical formulations, urine and blood serum samples.     

A highly selective fluorescent sensor for mercury ion (II) based on azathia-crown ether possessing a dansyl moiety

An intramolecular charge transfer (ICT) fluorescent sensor 1 using a dansyl moiety as the fluorophore and an azathia-crown ether as the receptor was designed, synthesized and characterized. The ions-selective signaling behaviors of the sensor 1 were investigated in CH(3) CN-H(2) O (1:1, v/v) by fluorescence spectroscopy. It exhibited remarkable fluorescence quenching upon addition of Hg(2+), which was attributed to the 1:1 complex formation between 1 and Hg(2+), while other selected metal ions induced basically no spectral changes. The sensor 1 showed a rapid and linear response towards Hg(2+) in the concentration range from 5.0 × 10(-7) to 1.0 × 10(-5) mol L(-1) with the detection limit of 1.0 × 10(-7) mol L(-1). Furthermore, the whole process could be carried out in a wide pH range of 2.0-8.0 and was not disturbed by other metal ions. Thus, the sensor 1 was used for practical determination of Hg(2+) in different water samples with satisfactory results.     

Construction of DNA biosensor at glassy carbon surface modified with 4-aminoethylbenzenediazonium salt

A simple, label-free electrochemical impedance-spectroscopy method for sequence-specific detection of DNA using a 4-aminoethylbenzenediazonium (AEBD) salt as a binder for amino-modified probe DNA is reported. This novel method simplifies the anchoring of DNA at the GC surface and opens new ways for the detection of hybridization. The hybridization of target DNA, without and with mismatches, with the probe DNA anchored at the GC surface modified with AEBD, greatly increases the interfacial electron transfer resistance at the double-stranded DNA modified electrodes for the redox couple Fe(CN)(6)(3-/4-). The resistance was measured using electrochemical impedance spectroscopy. The sensor response increased linearly with logarithm of concentration of target DNA in the range 2×10(-12)÷2×10(-6) M. The obtained quantification limit was circa 6.5×10(-17) mole in a 7 μL droplet and corresponded to a concentration of 9.2×10(-12) M of target DNA in the sample. This limit is equivalent to the detection of circa 4×10(7) copies of DNA in a 7 μL droplet or circa 5.7×10(12) DNA copies in one litre of sample.     

Electrochemical impedimetric immunosensor for insulin like growth factor-1 using specific monoclonal antibody-nanogold modified electrode

An electrochemical impedimetric immunosensor was developed for ultrasensitive determination of insulin-like growth factor-1 (IGF-1) based on immobilization of a specific monoclonal antibody on gold nanoparticles (GNPs) modified gold electrode. Self-assembly of colloidal gold nanoparticles on the gold electrode was conducted through the thiol groups of 1,6-hexanedithiol (HDT) monolayer as a cross linker. The redox reactions of [Fe(CN)(6)](4-)/[Fe(CN)(6)](3-) on the electrode surface was probed for studying the immobilization and determination processes, using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The interaction of antigen with grafted antibody recognition layer was carried out by soaking the modified electrode into antigen solution at 37°C for 3 h. The immunosensor showed linearity over 1.0-180.0 pg mL(-1) and the limit of detection was 0.15 pg mL(-1). The association constant between IGF-1 and immobilized antibody was calculated to be 9.17×10(11) M(-1). The proposed method is a useful tool for screening picogram amounts of IGF-1 in clinical laboratory as a diagnostic test.     

Oligonucleotide-stabilized fluorescent silver nanoclusters for turn-on detection of melamine

Fluorescence nanoclusters have been used for the determination of melamine for the first time. The method is based on the fluorescence turn-on of oligonucleotide-stabilized silver nanoclusters (DNA-Ag NCs) by melamine. The enhancement factors (I-I(0))/I(0) increase linearly with melamine concentrations over the range 5.0×10(-8)-7.0×10(-6) M (R(2)=0.998). The detection limit is 1.0×10(-8) M, which is approximately 2000 times lower than the US Food and Drug Administration estimated melamine safety limit of 20.0 μM. Furthermore, the milk samples spiked with melamine are analyzed with excellent recoveries.     

A novel electrochemical sensor for determination of dopamine based on AuNPs@SiO(2) core-shell imprinted composite

A novel core-shell composite of gold nanoparticles (AuNPs) and SiO(2) molecularly imprinted polymers (AuNPs@SiO(2)-MIPs) was synthesized through sol-gel technique and applied as a molecular recognition element to construct an electrochemical sensor for determination of dopamine (DA). Compared with previous imprinting recognition, the main advantages of this strategy lie in the introduction and combination of AuNPs and biocompatible porous sol-gel material (SiO(2)). The template molecules (DA) were firstly adsorbed at the AuNPs surface due to their excellent affinity, and subsequently they were further assembled onto the polymer membrane through hydrogen bonds and π-π interactions formed between template molecules and silane monomers. Cyclic voltammetry (CV) was carried out to extract DA molecules from the imprinted membrane, and as a result, DA could be rapidly and effectively removed. The AuNPs@SiO(2)-MIPs was characterized by ultraviolet visible (UV-vis) absorbance spectroscopy, transmission electron microscope (TEM) and Fourier transform infrared spectrometer (FT-IR). The prepared AuNPs@SiO(2)-MIPs sensor exhibited not only high selectivity toward DA in comparison to other interferents, but also a wide linear range over DA concentration from 4.8×10(-8) to 5.0×10(-5)M with a detection limit of 2.0×10(-8)M (S/N=3). Moreover, the new electrochemical sensor was successfully applied to the DA detection in dopamine hydrochloride injection and human urine sample, which proved that it was a versatile sensing tool for the selective detection of DA in real samples.     

Electrocatalytic oxidation of salicylic acid by a cobalt hydrotalcite-like compound modified Pt electrode

In this paper a study of the electrocatalytic oxidation of salicylic acid (SA) at a Pt electrode coated with a Co/Al hydrotalcite-like compound (Co/Al HTLC coated-Pt) film is presented. The voltammetric behaviour of the modified electrode in 0.1M NaOH shows two different redox couples: Co(II)/Co(III) and Co(III)/Co(IV). The electrocatalysis occurs at the same potential of the latter couple, showing that Co(IV) centers act as the oxidant. The CV investigation demonstrates that the process is controlled both by mass and charge transfer and that the Co(IV) centers involved in the oxidation are two for each SA molecule. The estimated value of the catalytic constant is 4×10(4) M(-1) s(-1). The determination of salicylic acid was performed both by DPV and chronoamperometry. The linearity ranges and the LOD values resulted 1×10(-5) to 5×10(-4), 5×10(-7) to 1×10(-4), 6×10(-6) and 2×10(-7) M, respectively. The Co/Al HTLC electrode has been used for SA determination in BAYER Aspirina? and the obtained results are consistent with an independent HPLC analysis.     

Impact of two iron(III) chelators on the iron, cadmium, lead and nickel accumulation in poplar grown under heavy metal stress in hydroponics

Poplar (Populus jacquemontiana var. glauca cv. Kopeczkii) was grown in hydroponics containing 10 μM Cd(II), Ni(II) or Pb(II), and Fe as Fe(III) EDTA or Fe(III) citrate in identical concentrations. The present study was designed to compare the accumulation and distribution of Fe, Cd, Ni and Pb within the different plant compartments. Generally, Fe and heavy-metal accumulation were higher by factor 2-7 and 1.6-3.3, respectively, when Fe(III) citrate was used. Iron transport towards the shoot depended on the Fe(III) chelate and, generally, on the heavy metal used. Lead was accumulated only in the root. The amounts of Fe and heavy metals accumulated by poplar were very similar to those of cucumber grown in an identical way, indicating strong Fe uptake regulation of these two Strategy I plants: a cultivar and a woody plant. The Strategy I Fe uptake mechanism (i.e. reducing Fe(III) followed by Fe(II) uptake), together with the Fe(III) chelate form in the nutrient solution had significant effects on Fe and heavy metal uptake. Poplar appears to show phytoremediation potential for Cd and Ni, as their transport towards the shoot was characterized by 51-54% and 26-48% depending on the Fe(III) supply in the nutrient solution.     

A novel enzymatic hydrogen peroxide biosensor based on Ag/C nanocables

A novel enzymatic hydrogen peroxide sensor was successfully fabricated based on the nanocomposites containing of Ag/C nanocables and gold nanoparticles (AuNPs). Ag/C nanocables have been synthesized by a hydrothermal method and then AuNPs were assembled on the surface of Ag/C nanocables. The nanocomposites were confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS). The above nanocomposites have satisfactory chemical stability and excellent biocompatibility. Cyclic voltammetry (CV) was used to evaluate the electrochemical performance of the Ag/C/Au nanocomposites at glassy carbon electrode (GCE). The results indicated that the Ag/C/Au nanocomposites exhibited excellent electrocatalytic activity to the reduction of H(2)O(2). It offered a linear range of 6.7×10(-9) to 8.0×10(-6) M, with a detection limit of 2.2×10(-9) M. The apparent Michaelis-Menten constant of the biosensor was 51.7×10(-6) M. These results indicated that Ag/C/Au nanocomposites have potential for constructing of a variety of electrochemical biosensors.     

Ultra trace analysis of small molecule by label-free impedimetric immunosensor using multilayer modified electrode

A multilayer electrode modified with a self-assembled thiourea monolayer (SATUM) followed by gold nanoparticles (AuNPs), mercaptosuccinic acid (MSA) and antibody was investigated for the detection of ultra trace amount of a small molecule (chloramphenicol) in an impedimetric system. The formation of the antibody-antigen complex at the electrode surface caused the impedance to increase. Under optimum conditions three modified electrodes were compared the SATUM/AuNPs/MSA electrode provided a wide linear range (0.50-10) × 10?1? M, and a very low determination limit of 1.0 × 10?1? M. This determination limit was much lower than the SATUM/AuNPs electrode, 1.0 × 10?1? M, and SATUM electrode, 4.7 × 10?1? M. The modified electrode provided good selectivity for chloramphenicol detection and can be reused up to 45 times with a relative standard deviation of lower than 4%. When applied to determine chloramphenicol in shrimp samples, the results agreed well with those obtained by the high-performance liquid chromatography coupled with a photo diode array detector (P > 0.05). The developed system can be applied to detect other small molecules using appropriate affinity binding pairs.     

Electrochemical sensor for dopamine based on a novel graphene-molecular imprinted polymers composite recognition element

A novel composite of graphene sheets/Congo red-molecular imprinted polymers (GSCR-MIPs) was synthesized through free radical polymerization (FRP) and applied as a molecular recognition element to construct dopamine (DA) electrochemical sensor. The template molecules (DA) were firstly absorbed at the GSCR surface due to their excellent affinity, and subsequently, selective copolymerization of methacrylic acid (MAA) and ethylene glycol dimethacrylate (EGDMA) was further achieved at the GSCR surface. Potential scanning was presented to extract DA molecules from the imprinted polymers film, and as a result, DA could be rapidly and completely removed by this way. With regard to the traditional MIPs, the GSCR-MIPs not only possessed a faster desorption and adsorption dynamics, but also exhibited a higher selectivity and binding capacity toward DA molecule. As a consequence, an electrochemical sensor for highly sensitive and selective detection of DA was successfully constructed as demonstration based on the synthesized GSCR-MIPs nanocomposites. Under experimental conditions, selective detection of DA in a linear concentration range of 1.0 × 10(-7)-8.3 × 10(-4)M was obtained, which revealed a lower limit of detection and wider linear response compared to some previously reported DA electrochemical MIPs sensors. The new DA electrochemical sensor based on GSCR-MIPs composites also exhibited excellent repeatability, which expressed as relative standard deviation (RSD) was about 2.50% for 30 repeated analyses of 20 μM DA.     

Electrochemical sensor using methimazole imprinted polymer sensitized with MWCNTs and Salen-Co(III) as recognition element

This study describes the development of a sensor with molecularly imprinted polymer (MIP) sensitized with MWCNTs and Salen-Co(III) as recognition element for methimazole (MMI) determination. This is the first report of MWCNTs and Salen-Co(III) in MIP being used to enhance its conductivity and catalytic activity in the electrochemical oxidation process. The electrocatalytic mechanism of MMI was explained in detail by evaluating the obtained voltammograms at various potential sweep rates and pH of buffer solutions. A stable, sensitive analytical method of differential pulse voltammetry (DPV) was developed using the prepared MWCNTs-Salen-Co(III)-MIP electrode for the determination of amounts of MMI, resulting in a linear range of 0.5-6.0 mg L(-1) with detection limit of 0.048 mg L(-1). It was also successfully applied for MMI determination in tablets and spiked urine sample. At three concentration levels, the recoveries for two samples were achieved to 94.0-100.1% and 87.8-101.8%, respectively. Analytical reproducibility and stability of the developed method were also evaluated by RSD, which were calculated as 4.6-6.6% and 2.0-6.1% (n=3), respectively.     

Biomolecule-stabilized Au nanoclusters as a fluorescence probe for sensitive detection of glucose

In this work, biomolecule-stabilized Au nanoclusters were demonstrated as a novel fluorescence probe for sensitive and selective detection of glucose. The fluorescence of Au nanoclusters was found to be quenched effectively by the enzymatically generated hydrogen peroxide (H(2)O(2)). By virtue of the specific response, the present assay allowed for the selective determination of glucose in the range of 1.0×10(-5) M to 0.5×10(-3) M with a detection limit of 5.0×10(-6) M. The absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and fluorescence decay studies were then performed to discuss the quenching mechanism. In addition, we demonstrated the application of the present approach in real serum samples, which suggested its great potential for diagnostic purposes.     

Electrochemical immunosensor for casein based on gold nanoparticles and poly(L-Arginine)/multi-walled carbon nanotubes composite film functionalized interface

In this paper, a novel electrochemical immunosensor for the determination of casein based on gold nanoparticles and poly(L-Arginine)/multi-walled carbon nanotubes (P-L-Arg/MWCNTs) composite film was proposed. The P-L-Arg/MWCNTs composite film was used to modify glassy carbon electrode (GCE) to fabricate P-L-Arg/MWCNTs/GCE through electropolymerization of L-Arginine on MWCNTs/GCE. Gold nanoparticles were adsorbed on the modified electrode to immobilize the casein antibody and to construct the immunosensor. The stepwise assembly process of the immunosensor was characterized by cyclic voltammetry and differential pulse voltammetry. Results demonstrated that the peak currents of [Fe(CN)(6)](3-/4-) redox pair decreased due to the formation of antibody-antigen complex on the modified electrode. The optimization of the adsorption time of gold nanoparticles, the pH of supporting electrolyte and the incubation time were investigated in details. Under optimal conditions, the peak currents obtained by DPV decreased linearly with the increasing casein concentrations in the range from 1 × 10(-7) to 1 × 10(-5) g mL(-1) with a linear coefficiency of 0.993. This electrochemical immunoassay has a low detection limit of 5 × 10(-8) g mL(-1) and was successfully applied to the determination of casein in cheese samples.