Simultaneous measurement of Pb, Cd and Zn using differential pulse anodic stripping voltammetry at a bismuth/poly(p-aminobenzene sulfonic acid) film electrode

A novel sensor was developed for simultaneous detection of Pb, Cd and Zn, based on the differential pulse anodic stripping response at a bismuth/poly(p-aminobenzene sulfonic acid) (Bi/poly(p-ABSA)) film electrode. This electrode was generated in situ by depositing simultaneously bismuth and the metals by reduction at −1.40 V on the poly(p-ABSA) modified electrode. Compared with the bismuth film electrode, the Bi/poly(p-ABSA) film electrode can yield a larger stripping signal for Pb, Cd and Zn. Under the optimum conditions, a linear response was observed for Cd and Zn in the range from 1.00 to 110.00 μg L⁻¹ and for Pb in the range from 1.00 to 130.00 μg L⁻¹. The detection limits of Pb(II), Cd(II) and Zn(II) were 0.80, 0.63 and 0.62 μg L⁻¹, respectively. Finally this sensor had been applied to the simultaneous determination of Pb(II), Cd(II) and Zn(II) in river water samples and the results were quite corresponding to the value obtained by atomic absorption spectrometry.     

Gold nanowire array electrode for non-enzymatic voltammetric and amperometric glucose detection

The non-enzymatic voltammetric and amperometric detection of glucose using a gold nanowire array electrode is described. The voltammetric detection of glucose was performed by cyclic and differential-pulse voltammetry. The detection of glucose by partial and direct oxidation of glucose during the anodic and cathodic potential sweeps was shown in cyclic voltammetry. An unusual decrease in overpotential for partial oxidation of glucose on a Au NW array electrode was observed. A linear differential-pulse voltammetric response for partial oxidation of glucose was observed up to a glucose concentration of at least 20 mM with a sensitivity of 41.9 μA mM⁻¹ cm⁻² and detection limit below 30 μM (signal-to-noise ratio of 3) for glucose oxidation at low potentials, where the influence of possible intermediates can be avoided. The amperometric response was also linear up to a glucose concentration of 10 mM with a sensitivity of 309.0 μA mM⁻¹ cm⁻². The wide dynamic range and high sensitivity, selectivity and stability, as well as good biocompatibility of the Au NW electrode make it promising for the fabrication of non-enzymatic glucose sensors.     

Voltammetric sensor for amoxicillin determination in human urine using polyglutamic acid/glutaraldehyde film

A voltammetric sensor for determination of amoxicillin (AMX) was developed based on a glutaraldehyde cross-linked polyglutamic acid modified glassy carbon electrode. The proposed method is based on pre-concentration of AMX by cathodic accumulation as its oxidative product before being determined indirectly at potential as low as +0.23 V by square wave voltammetry. Linear response range, sensitivity and limit of detection were 2.0–25.0, 1.06 and 0.9.2 μmol L⁻¹, respectively, for AMX in 0.1 mol L⁻¹ acetate buffer pH 5.2, pre-accumulation time of 60 s and accumulation potential of +1.0 V. It was demonstrated that the glassy carbon electrode modified with PGA/GLU could be used for AMX determination in human urine without any separation step.     

Ultra-sensitive polysilicon wire glucose sensor using a 3-aminopropyltriethoxysilane and polydimethylsiloxane-treated hydrophobic fumed silica nanoparticle mixture as the sensing membrane

In this paper a highly sensitive glucose biosensor is proposed based on a polysilicon (poly-Si) wire structure coated with 3-aminopropyltriethoxysilane (γ-APTES) mixed with polydimethylsiloxane-treated hydrophobic fumed silica nanoparticles (NPs) as the sensing membrane. The γ-APTES and fumed silica NPs mixture was directly transferred to and coated onto the poly-Si wire region with the help of a focus-ion-beam (FIB) processed capillary atomic-force-microscope (C-AFM) tip. After the necessary curing and UV illumination processes, the resultant sensor showed an extremely wide linear detection range from 0.1 μM to 10 mM with a channel current sensitivity as high as 5.33 A mM⁻¹ cm⁻² (or a channel conductance sensitivity of 70 μS mM⁻¹), and a detection limit as low as 10 nM can be achieved. Our experimental results showed that the poly-Si wire sensor has good selective analysis and operational stability on glucose detection under a 10:1 concentration ratio of glucose and uric acid. Its linear range and lowest detection limit remain virtually unimpaired in the presence of uric acid.     

An amperometric ethanol sensor based on a Pd–Ni/SiNWs electrode

A new amperometric ethanol sensor has been developed. The sensor uses the silicon nanowires covered with co-deposited palladium–nickel (Pd–Ni/SiNWs) as the working electrode. The detection of ethanol concentration is based on the response currents resulted from the electro-catalytic oxidation of ethanol. The performance of the sensor was characterized by cyclic voltammetry and fixed potential amperometry techniques. In 1 M KOH solution containing different ethanol concentrations, the sensor shows a good sensitivity of 7.48 mA mM⁻¹ cm⁻² and the corresponding detection limit (signal-to-noise ratio = 3) of 6 μM for cyclic voltammetry. Meanwhile, it also displays a sensitivity of 0.76 mA mM⁻¹ cm⁻² and the corresponding detection limit of 10 μM for fixed potential amperometry. The results demonstrate that the Pd–Ni/SiNWs electrodes are potential as the electrochemical integrated sensors for ethanol detection.     

Bean sprout peroxidase biosensor based on l-cysteine self-assembled monolayer for the determination of dopamine

A new bean sprout peroxidase was immobilized on a gold electrode modified with self-assembled monolayers (SAM) of l-cysteine for the determination of dopamine in pharmaceutical samples using square wave voltammetry. In the bean sprout–(SAM)–Au electrode, the peroxidase, in the presence of hydrogen peroxide, catalyzes the oxidation of dopamine to the corresponding quinone, which is electrochemically reduced back to dopamine at +0.15 V vs. Ag/AgCl. The performance and the factors influencing the response of this biosensor were studied in detail. The best performance was obtained using 0.1 mol L⁻¹ phosphate buffer solution (pH 6.0), 6.0 × 10⁻⁵ mol L⁻¹ hydrogen peroxide, frequency of 100 Hz, pulse amplitude of 80 mV and scan increment of 4.0 mV. The analytical curve was linear for dopamine concentrations from 9.91 × 10⁻⁶ to 2.21 × 10⁻⁴ mol L⁻¹ and the detection limit was 4.78 × 10⁻⁷ mol L⁻¹. The recovery of dopamine ranged from 98.0 to 111.8% and the relative standard deviation was 3.1% for a solution containing 1.30 × 10⁻⁵ mol L⁻¹ dopamine (n = 6). The lifetime of this biosensor was 15 days (at least 300 determinations). The results obtained for dopamine determination in pharmaceutical formulations using the proposed bean sprout–SAM–Au electrode were in agreement with those obtained with the standard method at the 95% confidence level.     

A novel frequency method for quantitative analysis of fluorescence dye concentration by using series photodetector frequency circuit system

This study reported the frequency characteristics of series photodetector frequency circuit system for detection of fluorescence dye concentration. In the condition of the same fluorescence intensity, the series photodetector frequency circuit system with higher responsivity of photodetector had higher frequency shift. The 100 MHz series photodetector frequency circuit system was applied to determine the fluorescence dye concentration of HEX by frequency shift. The correlation curve showed that the frequency shift was linearly related to the logarithm of fluorescence dye concentration from 100 pmol 3 μl⁻¹ to 10 amol 3 μl⁻¹. The proposed method can be applied simply and the detection limit of fluorescence dye concentration was lower than the conventional fluorescence technique by 2–3 orders.     

A novel biosensor using entangled carbon nanotubes layer grown on an alumina substrate by CCVD of methane on FeOx–MgO

Glucose oxidase (GOx) was immobilized on entangled and high surface area carbon nanotubes (CNTs) grown on an alumina substrate, and direct electron transfer reaction between GOx and the electrode was revealed. Fe/MgO catalyst layer was spin-coated on the insulating alumina substrate and the CNT layer was grown on the catalyst by chemical vapor deposition of methane at 950 °C for 15 min. About 20–30 nm bundles of about 1 nm single-wall as well as 10 nm multiwall CNTs are formed. The redox process was surface-controlled and electron transfer coefficient and the rate constant were estimated to be 0.35 and 0.64 s⁻¹, respectively. In addition GOx immobilized on CNT layer showed a linear response range between 12 and 62 μM of glucose concentration. A detection limit and sensitivity of 0.1 μM and 635 μA mM⁻¹ cm⁻², respectively, were obtained for the biosensor.     

Direct electrochemistry of glucose oxidase on screen-printed electrodes through one-step enzyme immobilization process with silica sol–gel/polyvinyl alcohol hybrid film

A one-step enzyme immobilization process with silica sol–gel/polyvinyl alcohol was described to achieve direct electrochemistry of glucose oxidase on screen-printed electrodes. The immobilized glucose oxidase displays a couple of stable and well-defined redox peaks with an electron transfer rate constant of 8.38 s⁻¹ and a formal potential of −460 mV (versus SCE) in phosphate buffer (0.05 M, pH 7.0) at a scan rate of 300 mV s⁻¹. The results suggested that conformation and bioactivity of glucose oxidase could be retained efficiently using the proposed immobilization method and the porous structure of screen-printed electrode surface was helpful for electron communication of glucose oxidase and the electrode. Furthermore, the modified electrode was used as a glucose biosensor, exhibiting a linear response to glucose concentration ranging from 0 to 4.13 mM and a sensitivity of 3.47 μA mM⁻¹ cm⁻² at an applied potential of −0.5 V. The detection limit of the biosensor is 9.8 μM, based on a signal-to-noise ratio of 3. The present work provided a promising strategy for fabricating a novel and disposable mediatorless glucose biosensor, which could be mass-produced through further development.     

Designing of MIP-based QCM sensor for the determination of Cu(II) ions in solution

A novel quartz crystal microbalance (QCM) sensor with a high selectivity and sensitivity has been developed for the determination of Cu(II) ions, based on the modification of Cu(II) ion-imprinted polymer (Cu(II)-IIP) film onto a quartz crystal. The performance of the developed MIP-QCM sensor was evaluated and the results indicated that a sensitive MIP-QCM sensor could be fabricated. The obtained MIP-QCM sensor presents high-selectivity monitoring of Cu(II) ions, better reproducibility, shorter response time (6 min), wider linear range (0.001–50 μM) and lower detection limit (8 × 10⁻⁴ μM). The practical analysis of the MIP-QCM sensor confirms the feasibility of Cu(II) determination in wastewater.     

Electrochemical sensor for hydroperoxides determination based on Prussian blue film modified electrode

An electrochemical sensor for hydroperoxides determination was investigated. The sensor was based on the electrocatalytic reduction of hydroperoxides on Prussian blue (PB)-modified glassy carbon electrode. The modified electrode possesses a high electrocatalytic effect towards all studied peroxides with the highest effect obtained with H₂O₂ followed by tert-butyl hydroperoxide (TBH), cumene hydroperoxide (CH) and linoleic acid hydroperoxide (LAH). In addition, the modified electrode showed a good stability and a fast response time (<20 s). The lower detection limits of H₂O₂, TBH, CH and LAH were found to be 10⁻⁷ mol L⁻¹, 2 × 10⁻⁷ mol L⁻¹, 3.5 × 10⁻⁷ mol L⁻¹ and 4 × 10⁻⁷ mol L⁻¹, respectively. The electrochemical sensor was then applied for amperometric determination of peroxide value (PV) in edible oil at an applied potential of 50 mV (vs. Ag/AgCl (1 M KCl)). A good linearity has been found in the range 0.02–1.0 mequiv. O₂/kg, with a detection limit (S/N = 3) of 0.001 mequiv. O₂/kg. The precision of the method (R.S.D., n = 9) for within and between-days is better than 1.9% and 2.7%, respectively at 0.1 mequiv. O₂/kg. The method was successfully applied to the determination of PV in real edible oil samples with an excellent agreement with results obtained with the official standard procedure. The proposed method is accurate, simple, cheap and could be used to control edible oil rancidity with a high sample throughputs (more than 120 samples/h).     

Anodic stripping voltammetric determination of copper(II) using a functionalized carbon nanotubes paste electrode modified with crosslinked chitosan

The development and application of a functionalized carbon nanotubes paste electrode (CNPE) modified with crosslinked chitosan for determination of Cu(II) in industrial wastewater, natural water and human urine samples by linear scan anodic stripping voltammetry (LSASV) are described. Different electrodes were constructed using chitosan and chitosan crosslinked with glutaraldehyde (CTS-GA) and epichlorohydrin (CTS-ECH). The best voltammetric response for Cu(II) was obtained with a paste composition of 65% (m/m) of functionalized carbon nanotubes, 15% (m/m) of CTS-ECH, and 20% (m/m) of mineral oil using a solution of 0.05 mol L⁻¹ KNO₃ with pH adjusted to 2.25 with HNO₃, an accumulation potential of −0.3 V vs. Ag/AgCl (3.0 mol L⁻¹ KCl) for 300 s and a scan rate of 100 mV s⁻¹. Under these optimal experimental conditions, the voltammetric response was linearly dependent on the Cu(II) concentration in the range from 7.90 × 10⁻⁸ to 1.60 × 10⁻⁵ mol L⁻¹ with a detection limit of 1.00 × 10⁻⁸ mol L⁻¹. The samples analyses were evaluated using the proposed sensor and a good recovery of Cu(II) was obtained with results in the range from 98.0% to 104%. The analysis of industrial wastewater, natural water and human urine samples obtained using the proposed CNPE modified with CTS-ECH electrode and those obtained using a comparative method are in agreement at the 95% confidence level.     

Electrochemical sensor for simultaneous detection of ascorbic acid, uric acid and xanthine based on the surface enhancement effect of mesoporous silica

The measurement of ascorbic acid (AA), uric acid (UA) and xanthine (XA) is very important in the clinical diagnosis because many diseases have been found to be associated with their concentrations. Herein, an electrochemical sensor using mesoporous SiO₂ as sensing material was firstly developed for the simultaneous detection of AA, UA and XA. With distinctive properties such as uniform porous networks, large surface area and high sorption ability, the mesoporous SiO₂ sensor exhibits remarkable surface enhancement effect, and greatly increases the response signals of AA, UA and XA. In addition, the electrochemical responses of coexistence of AA, UA and XA were studied, and three well-shaped oxidation peaks were observed at 0.00, 0.25 and 0.63 V. Further studies suggest that their oxidation takes place independently and has no mutual interference. This sensor possesses high sensitivity, and the limit of detections are 3.0 × 10⁻⁶ mol L⁻¹, 1.0 × 10⁻⁷ mol L⁻¹ and 7.5 × 10⁻⁷ mol L⁻¹ for AA, UA and XA. Finally, the mesoporous SiO₂ sensor was successfully employed to detect AA, UA and XA in the urine and blood serum samples.     

Bioluminescent sensor for naphthalene in air: Cell immobilization and evaluation with a dynamic standard atmosphere generator

A dynamic atmosphere generator with a naphthalene emission source has been constructed and used for the development and evaluation of a bioluminescence sensor based on the bacteria Pseudomonas fluorescens HK44 immobilized in 2% agar gel (10⁷ cell mL⁻¹) placed in sampling tubes. A steady naphthalene emission rate (around 7.3 nmol min⁻¹ at 27 °C and 7.4 mL min⁻¹ of purified air) was obtained by covering the diffusion unit containing solid naphthalene with a PTFE filter membrane. The time elapsed from gelation of the agar matrix to analyte exposure (“maturation time”) was found relevant for the bioluminescence assays, being most favorable between 1.5 and 3 h. The maximum light emission, observed after 80 min, is dependent on the analyte concentration and the exposure time (evaluated between 5 and 20 min), but not on the flow rate of naphthalene in the sampling tube, over the range of 1.8–7.4 nmol min⁻¹. A good linear response was obtained between 50 and 260 nmol L⁻¹ with a limit of detection estimated in 20 nmol L⁻¹ far below the recommended threshold limit value for naphthalene in air.     

Modified glassy carbon electrode with multiwall carbon nanotubes as a voltammetric sensor for determination of noscapine in biological and pharmaceutical samples

A simple and highly sensitive method is described for direct voltammetric determination of noscapine in blood and pharmaceutical sample. Glassy carbon electrode with effective method is modified with multiwall carbon nanotubes (MWNTs) to cause activation of multiwall carbon nanotubes structures for electrocatalyzes of noscapine oxidation. The cyclic voltammetric (CV) results indicated that MWNTs remarkably enhances electrocatalytic activity toward the oxidation of noscapine, which is leading to considerable improvement of anodic peak current for noscapine, and allows the development of a highly sensitive voltammetric sensor for detection of noscapine in pharmaceutical and clinical samples. Under the optimum condition, the calibration curve was linear in the concentration range 4.0 × 10⁻⁷–1.0 × 10⁻⁴ mol L⁻¹ with the detection limit of 8.0 × 10⁻⁸ mol L⁻¹ and relative standard deviation (R.S.D.%) lower than 1.0% (n = 5). Finally, some kinetic parameters were determined and multistep mechanism for oxidation of noscapine for first time was proposed.     

Application of carbon ionic liquid electrode for the electrooxidative determination of catechol

A carbon ionic liquid electrode (CILE) was constructed using graphite powder mixed with N-butylpyridinium hexafluorophosphate (BPPF₆) in place of paraffin as the binder, which showed strong electrocatalytic activity to the direct oxidation of catechol. In pH 3.0 phosphate buffer solution (PBS) a pair of redox peaks appeared on the CILE with the anodic and the cathodic peak potential located at 387 and 330 mV (vs. SCE), respectively. The electrochemical behaviors of catechol on the CILE were carefully investigated, and the electrochemical parameters were calculated with the results of the electrode reaction standard rate constant k_s as 1.27 s⁻¹, the charge-transfer coefficient α as 0.58 and the electron transferred number n as 2. Under the selected conditions, the anodic peak current increased linearly with the catechol concentration over the range from 1.0 × 10⁻⁶ to 8.0 × 10⁻⁴ mol L⁻¹ by cyclic voltammetry at the scan rate of 100 mV s⁻¹. The detection limit was calculated as 6.0 × 10⁻⁷ mol L⁻¹ (3σ). The CILE showed good ability to separate the electrochemical responses of catechol and ascorbic acid (AA) with the anodic peak potential separation as 252 mV (vs. SCE). The proposed method was further applied to the synthetic samples determination with satisfactory results.     

Highly sensitive label-free immunosensor for ochratoxin A based on functionalized magnetic nanoparticles and EIS/SPR detection

A label-free immunosensor for the detection of ochratoxin A (OTA) based on use of magnetic nanoparticles (MNPs) was developed. A gold electrode was modified using bovine serum albumin conjugate with a glutaraldehyde-thiolamine linker, creating a layer that prevents non-specific binding of OTA on gold. The OTA antibodies were attached to MNPs using the carbodiimide chemistry and afterwards were immobilized on the modified gold electrode using a strong magnetic field. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and surface plasmon resonance (SPR) were used to characterize each step in immunosensor development. The impedance variation due to the specific antibody-OTA interaction was correlated with the OTA concentration in the samples. The increase in electron-transfer resistance values was proportional to the concentration of OTA on a linear range between 0.01 and 5 ng/mL, with a detection limit of 0.01 ng/mL. SPR measurements showed a larger response range (1-50 ng/mL) with a detection limit of 0.94 ng/mL. Analytical results were in accordance with standard ELISA test kit.     

A sensitive nonenzymatic hydrogen peroxide sensor based on DNA–Cu complex electrodeposition onto glassy carbon electrode

In this paper, DNA–Cu²⁺ complex was electrodeposited onto the surface of glassy carbon (GC) electrode, which fabricated a DNA–Cu²⁺/GC electrode sensor to detect H₂O₂ with nonenzyme. Cyclic voltammogram of DNA–Cu²⁺/GC electrode showed a pair of well-defined redox peaks for Cu²⁺/Cu⁺. Moreover, the electrodeposited DNA–Cu²⁺ complex exhibited excellent electrocatalytic behavior and good stability for the detection of H₂O₂. The effects of Cu²⁺ concentration, electrodeposition time and determination conditions such as pH value, applied potential on the current response of the DNA–Cu²⁺/GC electrode toward H₂O₂ were optimized to obtain the maximal sensitivity. The linear range for the detection of H₂O₂ is 8.0 × 10⁻⁷ M to 4.5 × 10⁻³ M with a high sensitivity of −40.25 μA mM⁻¹, a low detection limit of 2.5 × 10⁻⁷ M and a fast response time of within 4 s. In addition, the sensor has good reproducibility and long-term stability and is interference free.     

A novel microreactor with 3D rotating flow to boost fluid reaction and mixing of viscous fluids

Based on a concept encompassing splitting and recombination (SAR) and chaotic advection, an efficient microreactor, called a SAR μ-reactor, has been designed to mix fluids at Reynolds numbers from 0.01 to 100 and to be suitable for mixing fluids with viscosity over a wide range (μ = 0.000855–0.186 kg m⁻¹ s⁻¹). This SAR μ-reactor was compared, numerically and experimentally, with a slanted-groove micromixer (SGM) for reaction or mixing of fluids. Results of simulations characterized the designed structure with inducing a 3D rotating flow involving a strong lateral component to stretch intensely the contact interface in the SAR μ-reactor. Chemical colorimetry of two kinds – involving reactions of phenolphthalein with sodium hydroxide and of ascorbic acid with diiodine – revealed that the SAR μ-reactor provided a smaller mixing length, reaction length and period than the SGM; the mixing performance of the SAR μ-reactor was much better than that of the SGM. We assessed the mixing behavior of fluorescent proteins (C-phycocyanin and R-phycoerythrin) in viscous fluids with a confocal microscope. Experimental results and simulations showed that the effect of fluid viscosity on the mixing efficiency of the SAR μ-reactor is less than for the SGM; the SAR mechanism effectively augmented the contact interface even though the intrinsic diffusivity of fluids was diminished.     

Amperometric phenol biosensor based on polyaniline

The catechol biosensor is constructed by cross-linking between polyphenol oxidase (PPO) and polyaniline (PANI) using glutaraldehyde as a cross-linking agent. The PANI, which is electrochemically synthesized in a solution containing ionic liquid, 1-ethyl-3-methylimidazolium ethyl sulfate, possesses good electroactivity and high conductivity above pH 6. In the presence of catechol as a substrate, the biosensor exhibits a linear range from 0.2 to 80 μmol dm⁻³. The maximum response current (I_max) and the Michaelis–Menten constant (k^′_m) are 9.44 μA and 117 μmol dm⁻³, respectively. The effects of pH and operating potential are also explored to optimize measurement conditions. The activation energy (E_a) of the PPO catalytic reaction is 30.23 kJ mol⁻¹ in the B–R buffer. Electrochemical impedance spectroscopy (EIS), UV–vis and SEM are used to characterize the PANI–PPO biosensor. The biosensor exhibits good long-term stability.