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.     

Response time of nanostructured relative humidity sensors

Capacitive relative humidity (RH) sensors were fabricated by coating countersunk interdigitated electrode substrates with nanostructured TiO₂ films produced using glancing angle deposition. Areal capacitance increased from 1 nF cm⁻² to 800 nF cm⁻² as relative humidity was increased from 2% RH and 95% RH. For films deposited at 81° and with a thickness below 4 m, response time was (162±4) ms m⁻¹. Response times increased from 64 ms to 1440 ms as film thickness increased from 280 nm to 8.5 m. The linear dependence of response time with film thickness indicates that device response time is dominated by surface adsorption. Response time decreased with increasing deposition angle, with a slope of (−15.2±1.6) ms degree⁻¹ for the adsorption data, and (−17.3±2.5) ms degree⁻¹ for the desorption data. The optimum operating range of the sensors depends on deposition angle, and can be tuned to different ranges to match application needs.     

Direct amperometric determination of l-ascorbic acid (Vitamin C) at octacyanomolybdate-doped-poly(4-vinylpyridine) modified electrode in fruit juice and pharmaceuticals

The glassy carbon electrode (GCE) modified with Mo(CN)₈⁴⁻-incorporated-poly(4-vinylpyridine) (PVP/Mo(CN)₈⁴⁻), which has been recently shown to possess several attractive attributes as an efficient electrocatalytic electrode for l-ascorbic acid oxidation and its estimation, is used for l-ascorbic acid estimation directly in orange fruit juice and Celin tablet in a 0.1 M H₂SO₄ acid solution without any special treatment. Constant potential amperometry at 570 mV (saturated calomel electrode, SCE) in stirred solutions is used for this purpose. A good correlation is attained with the official titrametric method. To understand the possible electrocatalytic reaction mechanism for the electro-oxidation of l-ascorbic acid, calibration graphs over the range 1 × 10⁻⁵ to 1 × 10⁻² mol dm⁻³ l-ascorbic acid are compared for the three electrodes, ca. PVP/Mo(CN)₈⁴⁻, undoped PVP, and GCE; the curvature at high ascorbic acid concentration for the PVP/Mo(CN)₈⁴⁻ electrode is explained in terms of Michaelis–Menten (MM) saturation kinetics. The apparent MM constant (K_M), the maximum catalytic current (i_M), the complex decomposition rate constant (k_c), and the heterogeneous modified electrode rate constant (k^′_ME) are calculated from three different approaches. A reasonably high value of ≈1 × 10⁻² cm s⁻¹ is obtained for k^′_ME, indicating efficient l-ascorbic acid mediation at the PVP/Mo(CN)₈⁴⁻ electrode, thus accounting for quite a high sensitivity of this modified film electrode compared to several other modified electrodes.     

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.     

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).     

Using electric actuation and detection of oscillations in microcantilevers for pressure measurements

Response characteristics of a microcantilever, such as resonant frequency, amplitude, phase and quality factor, can be used for absolute pressure measurements in the range of 10⁻⁴ to 10³ Torr. To this end, it would be very convenient to have the resonance of the microcantilever actuated and detected electrostatically. Herein, we report the nonlinear dynamics of microcantilevers under varying pressure and different gases using the harmonic detection of resonance (HDR) technique [J. Gaillard, M.J. Skove, R. Ciocan, A.M. Rao, Electrical detection of oscillations in 340 microcantilevers and nanocantilevers, Rev. Sci. Instrum. 77 (2006) 073907]. The HDR technique exploits nonlinearities in the cantilever-counter electrode system to allow electrostatic actuation and detection of the responses of the microcantilever to the pressure and gas composition. In particular, the 2nd and 3rd harmonics of the measured charge on the cantilever are investigated. The microcantilever demonstrates a quality factor of 10,000 at 10⁻³ Torr, and a usable response in the range from 10⁻³ to 10³ Torr. The use of different harmonics can enable us to adjust the range of pressures over which the sensor has an efficacious response, enhancing its sensitivity to a particular environment. The experimental results are in reasonable agreement with theoretical calculations, despite the nonlinearities involved.     

Hydrogen sensors with double dipole layers using a Pd-mixture-Pd triple-layer sensing structure

This paper reports on new GaN sensors using a Pd-mixture-Pd triple-layer sensing structure to enhance their sensitivity to hydrogen at the tens of ppm level. The proposed hydrogen sensor biased with a constant voltage produced relatively high sensing responses of 4.84 × 10⁵% at 10,100 ppm and 8.7 × 10⁴% at 49.1 ppm H₂ in N₂. The corresponding barrier height variations are calculated to be 220 and 168 mV. When the sensor is biased by a constant current with maximum power consumption of 0.4 mW, a sensing voltage as an output signal showed a voltage shift of more than 17 V (the highest value ever reported) at 49.1 ppm H₂ in N₂. By comparison to Pd-deposited GaN sensors, the improvement in static-state performance is likely attributed to double dipole layers formed individually at the Pd–GaN interface and inside the mixture. Moreover, voltage transient response and current transient response to various hydrogen-containing gases were experimentally studied. The new finding is that the former response time is shorter than the latter one.     

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.     

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.     

A novel microbial biosensor based on Circinella sp. modified carbon paste electrode and its voltammetric application

Microbial biosensors have been developed for voltammetric determination of various substances. This paper describes the development of a new biosorption based microbial biosensor for determination of Cu²⁺. The developed biosensor is based on carbon paste electrode consisting of whole cells of Circinella sp. Cu²⁺ was preconcentrated on the electrode surface at open circuit and then cathodically detected with the reduction of Cu²⁺. The voltammetric responses were evaluated with respect to percentage cell loading in the carbon paste, preconcentration time, pH of preconcentration solution, scan rate and interferences. The optimum response was realized by biosensor constructed using 5 mg of dry cell weight per 100 mg of carbon paste in pH 5.5 preconcentration solution. Under the optimum experimental conditions, the developed microbial biosensor exhibited an excellent current response to Cu²⁺ over a linear range from 5.0 × 10⁻⁷ to 1.0 × 10⁻⁵ M (r² = 0.9938) with a detection limit of 5.4 × 10⁻⁸ M (S/N = 3). The microbial biosensor had good sensitivity and reproducibility (R.S.D. 4.3%, n = 6). Finally, the applicability of the proposed microbial biosensor to voltammetric determination of Cu²⁺ in real sample was also demonstrated and validated with atomic absorption spectrophotometric (AAS) method.     

Wireless sensor network based wearable smart shirt for ubiquitous health and activity monitoring

Nanofibers of a composite of a silicon-containing polyelectrolyte, polyethylene oxide and polyaniline (PANI) were obtained by electrospinning and heat treatment. The morphologies of the composite nanofibers were characterized by scanning electron microscopy, which showed that the nanofibers with a diameter of 250–500 nm formed a non-woven mat with highly porous structure. It was found that the polymer composite nanofibers showed impedance change from 6.3 × 10⁶ to 2.5 × 10⁴ Ω with the increment of relative humidity (RH) from 22 to 97% at room temperature, exhibiting high sensitivity and good linearity on a semi-logarithmic scale. In addition, they exhibited fast and highly reversible response characterized by very small hysteresis of 2% RH and short response time (t_90%: 7 s and 19 s for adsorption and desorption between 33 and 97%RH, respectively). The modification of the electrode with poly(diallyldimethylammonium chloride) prior to the deposition of nanofibers improved their humidity response, which may be related to the enhanced contact between the nanofibers and underlying substrate. The effect of PANI on the humidity response of the composite nanofibers was also investigated, and it was found that PANI effectively decreased the impedance of the nanofibers. The electrospun polymer composite nanofibers may find applications in the preparation of high performance humidity sensors.     

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.     

A sensitive and selective sensor for dopamine determination based on a molecularly imprinted electropolymer of o-aminophenol

A simple and reliable method was proposed for preparing a selective dopamine (DA) sensor based on a molecularly imprinted electropolymer of o-aminophenol. The sensor is selective for the determination of DA in the presence of high concentrations of ascorbic acid (AA), with a maximum molar ratio of 1/1000. The molecular imprinted (MIP) sensor was tested by cyclic voltammetry (CV) as well as differential pulse voltammetry (DPV) to verify the changes in oxidative currents of ferricyanide. In optimized conditions, DA at concentrations of 2 × 10⁻⁸ to 0.25 × 10⁻⁶ mol/L could be determined with a detection limit of 1.98 × 10⁻⁹ mol/L (S/N = 3). The MIP sensor showed high selectivity, sensitivity, and reproducibility. Determination of DA in simulated samples of dopamine hydrochloride showed good recovery.     

Molecular valve consisting of poly(acrylic acid) gel

A molecular valve, consisting of poly(acrylic acid) gel-coated Au mesh, was developed based on volume change of the gel in response to cation concentration. The valve closed when concentration of cations such as H⁺, Na⁺, K⁺, Ca²⁺, Cu²⁺, or Al³⁺ was low, whereas opened upon increase in its concentration. The valve re-closed when water was flowed. The concentration where the valve opens was found to increase in the order of Al³⁺, Ca²⁺, and Na⁺ (2 × 10⁻⁴, 5 × 10⁻⁴, and 6 × 10⁻³ M, respectively). The response to Cu²⁺ ion showed similar behaviour, but the opening concentration was ca. 2 × 10⁻⁴ M, which is lower than that of Ca²⁺ ion. The valve appeared to close over the pH range from 3 to 12, whereas to open below and above it. The fastest response time to open the valve (less than 1 min) was obtained for a solution of pH 1–2. The valve showed repeatability at least 25 cycles upon successive loading of a solution of pH 2 and water. Effects of anions and pressure were also studied.     

Electrochemical DNA biosensor for the detection of interaction between di[azino-di(5,6-azafluorene)-κ-NN′]dichlormanganous and DNA

A new Mn(II) complex of MnL₂Cl₂ (L = azino-di(5,6-azafluorene)-κ²-NN′) was synthesized and utilized as an electrochemical indicator for the determination of hepatitis B virus (HBV) based on its interaction with MnL₂Cl₂. The electrochemical behavior of interaction of MnL₂Cl₂ with salmon sperm DNA was investigated on glassy carbon electrode (GCE). In the presence of salmon sperm DNA, the peak current of [MnL₂]²⁺ was decreased and the peak potential was shifted positively without appearance of new peaks. The binding ratio between [MnL₂]²⁺ and salmon sperm DNA was calculated to be 2:1 and the binding constant was 3.72 × 10⁸ mol² L⁻². The extent of hybridization was evaluated on the basis of the difference between signals of [MnL₂]²⁺ with probe DNA before and after hybridization with complementary sequence. Control experiments performed with non-complementary and mismatch sequence demonstrated the good selectivity of the biosensor. With this approach, a sequence of the HBV could be quantified over the range from 1.76 × 10⁻⁸ to 1.07 × 10⁻⁶ mol L⁻¹, with a linear correlation of r = 0.9904 and a detection limit of 6.80 × 10⁻⁹ mol L⁻¹. Additionally, the binding mechanism was preliminarily discussed. The mode of interaction between MnL₂Cl₂ and DNA was found to be primary intercalation binding.     

Potentiometric urea biosensor based on BSA embedded surface modified polypyrrole film

A potentiometric biosensor based on bovine serum albumin (BSA) embedded surface modified polypyrrole has been developed for the quantitative estimation of urea in aqueous solution. The enzyme, urease (Urs), was covalently linked to free amino groups present over the BSA embedded modified surface of the conducting polypyrrole film electrochemically deposited onto an indium–tin-oxide (ITO) coated glass plate. The biosensor has been characterized by UV–visible, infrared spectroscopy and SEM. Potentiometric and spectrophotometric response of the enzyme electrode (Urs/BSA-PPy/ITO) were measured as a function of urea concentration in Tris–HCl buffer (pH 7.0). It has been found that the electrode responds to low urea concentration with wider range of detection. The electrode showed a linear response range of 6.6 × 10⁻⁶ to 7.5 × 10⁻⁴ M urea. The response time is about 70–90 s reaching to a 95% steady-state potential value and 75% of the enzyme activity is retained for about 2 months. These results indicate an efficient covalent linkage of enzyme to free amino groups of the BSA molecules over the surface of polypyrrole film, which leads to high enzyme loading, an increased lifetime stability of the electrode and an improved wide range of detection of low urea concentration in aqueous solution.     

Design and fabrication of a new miniaturized capacitive accelerometer

A novel mercury-based capacitive accelerometer has been designed and fabricated. The accelerometer features a highly symmetrical cubic structure and capacitive coupling of the high-frequency input voltage, which uses a mercury drop for spring material and flexible interconnection layer between the capacitor plates. The device is mounted on a standard IC package with dimensions of 5 mm × 5 mm × 5 mm. The structure, working principle, fabrication, and mathematical model of the accelerometer are presented. Since the accelerometer uses a mercury drop as its sensitive electrode instead of a solid, which is commonly used in traditional accelerometers, the conflict between the requirements of high shock and high sensitivity is solved. The measurement results show a sensitivity of 0.2 mV (m s⁻²)⁻¹ with a corresponding resolution of 0.01 ms⁻², off-axis sensitivity of <5% and good linearity in the output voltage for accelerations up to at least 10 m s⁻².     

Direct electrochemistry and electrocatalysis with horseradish peroxidase immobilized in polyquaternium-manganese oxide nanosheet nanocomposite films

A novel biocompatible polyquaternium (QY)-manganese oxide nanosheet (MNS) nanocomposite has been prepared and shown to be a promising matrix for horseradish peroxidase (HRP) immobilization. The resulting HRP-QY-MNS film was characterized by Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopy, which indicated that HRP retained its native structure in the nanocomposite film. An HRP-QY-MNS film-modified glassy carbon electrode exhibited a pair of well-defined and quasi-reversible cyclic voltammetric peaks centered at −0.272 V (vs. Ag/AgCl) in pH 7.0 phosphate buffer solution. The direct electrochemical behavior of HRP was greatly enhanced in the QY-MNS nanocomposite film compared with that in single-component QY or MNS films. The immobilized HRP showed excellent electrocatalysis in the reduction of hydrogen peroxide (H₂O₂), which was exploited in the construction of an H₂O₂ biosensor. The linear range of the biosensor for H₂O₂ was found to be from 1.0 × 10⁻⁷ to 3.2 × 10⁻⁵ M with a correlation coefficient of 0.998. The detection limit was 7.8 × 10⁻⁸ M at a signal-to-noise ratio of 3. The biosensor exhibited rapid response and good long-term stability.     

Potassium ion sensing using a self-assembled calix[4]crown monolayer by surface plasmon resonance

The precise detection of K⁺ ion is crucial because K⁺ ion plays a leading role in membrane transport. Current K⁺ ion detection methods suffered low resolution and detection limit. Calix[4]crown-5 derivatives are well known as K⁺ ionophores. We described here a K⁺ ion-sensing system using a self-assembled monolayer of calix[4]crown-5 derivative (calix[4]crown) modified gold chip based on surface plasmon resonance (SPR). The calix[4]crown sensing layer was characterized by atomic-force microscopy (AFM), SPR, Fourier transform infrared reflection absorption spectroscopy (FTIR-RAS) and cyclic voltammetry (CV). It was found calix[4]crown was assembled as a monolayer on Au surface. The SPR angle was found to be modulated by various concentrations of K⁺ ion due to the interaction between the calix[4]crown and K⁺ ion. This calix[4]crown monolayer showed a more sensitive and selective binding toward potassium ion over other alkali and alkaline earth metal ions. From the simple SPR spectroscopic analysis, we were able to monitor K⁺ ion concentration with a wide range of 1.0 × 10⁻¹² to 1.0 × 10⁻² M in an aqueous solution with a pH 6–8. These experimental results showed a useful method for the design of simple and precise potassium ion biosensors.     

In vivo monitoring of oxidative burst induced by ultraviolet A and C stress for oilseed rape by microbiosensor

In this paper, poly-o-phenylenediamine and Pt microparticle modified Pt electrode (POPD/Pt-MP/Pt) as microbiosensor has been developed to monitor in vivo oxidative burst induced by ultraviolet A (UV-A) and ultraviolet C (UV-C) radiation stress in oilseed rape (Brassica napus L.). Twice oxidative bursts were detected, appearing at 2 and 25 h, respectively. According to the peak area of amperometry, the amount of H₂O₂ induced by UV stress was estimated to be 6.2 × 10⁻¹⁰ and 3.1 × 10⁻¹⁰ mol for such two oxidative bursts in detected leafstalk, respectively. This novel microbiosensor provides an effective tool for studying the defense reaction of the plant in the situation of UV stress and probing the antioxidative mechanism of the antioxidative enzyme.