Hybridization biosensor based on the covalent immobilization of probe DNA on chitosan-mutiwalled carbon nanotubes nanocomposite by using glutaraldehyde as an arm linker

A label-free DNA biosensor for hybridization detection of short DNA species related to the transgenic plants gene fragment of cauliflower mosaic virus (CaMV) 35S promoter was developed in this paper. The nanocomposite containing chitosan (CS) and mutiwalled carbon nanotubes (MWNTs) was first coated on a glassy carbon electrode. Then a highly reactive dialdehyde reagent of glutaraldehyde (GTD) was applied as an arm linker to covalently graft the 5′-amino modified probe DNA to the CS-MWNTs surface via the facile aldehyde-ammonia condensation reaction. The hybridization capacity of the developed biosensor was monitored with electrochemical impedance spectroscopy (EIS) using [Fe(CN)₆]^3−/4− as an indicating probe, and the experimental results showed that the biosensor had fast hybridization rate and low background interference. A wide dynamic detection range (1.0 × 10⁻¹³-5 × 10⁻¹⁰ M) and a low detection limit (8.5 × 10⁻¹⁴ M) were achieved for the complementary sequence. In addition, the hybridization specificity experiments showed that the sensing system can accurately discriminate complementary sequence from mismatch and noncomplementary sequences.     

A novel bienzyme glucose biosensor based on three-layer Au-Fe3O4@SiO2 magnetic nanocomposite

The magnetic core-shell Au-Fe₃O₄@SiO₂ nanocomposite was prepared by layer-by-layer assembly technique and was used to fabricate a novel bienzyme glucose biosensor. Glucose oxidase (GOD) and horseradish peroxidase (HRP) were simply mixed with Au-Fe₃O₄@SiO₂ nanocomposite and cross-linked on the ITO magnetism-electrode with nafion (Nf) and glutaraldehyde (GA). The modified electrode was designated as Nf-GOD-HRP/Au-Fe₃O₄@SiO₂/ITO. The effects of some experimental variables such as the pH of supporting electrolyte, enzyme loading, the concentration of the mediator methylene blue (MB) and the applied potential were investigated. The electrochemical behavior of the biosensor was studied using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and chronoamperometry. Under the optimized conditions, the biosensor showed a wide dynamic range for the detection of glucose with linear ranges of 0.05-1.0 mM and 1.0-8.0 mM, and the detection limit was estimated as 0.01 mM at a signal-to-noise ratio of 3. The biosensor exhibited a rapid response, good stability and anti-interference ability. Furthermore, the biosensor was successfully applied to detect glucose in human serum samples, showing acceptable accuracy with the clinical method.     

Preparation of an electrochemical PNA biosensor for detection of target DNA sequence and single nucleotide mutation on p53 tumor suppressor gene corresponding oligonucleotide

Development of an electrochemical biosensor based on peptide nucleic acid (PNA) probe for detection of target DNA sequence and single nucleotide mutation in p53 tumor suppressor gene corresponding oligonucleotide using methylene blue (MB) as an electrochemical indicator is described. The interaction between MB and short sequence of p53, one of the most important tumor suppressor genes due to its dysfunction in the majority of human cancers, was studied by differential pulse voltammety (DPV). Probe modified electrode was prepared by self-assembled monolayer (SAM) formation of thiolated PNA molecules on the surface of gold electrode (GE). The hybridization of PNA probe with target DNA was performed in solution to form PNA-DNA hybrid on the surface of the GE. A significant increase in the reduction signal of MB was observed upon hybridization of the probe with the complementary DNA. The selectivity of the biosensor was studied using noncomplementary oligonucleotides. Furthermore, our results confirmed the ability of the sensor to detect single base mismatch in the sample oligonucleotide. The influence of probe concentration on the effective discrimination against noncomplementary sequence and point mutation was also investigated. Diagnostic performance of the biosensor is described and the detection limit is found 6.82 × 10⁻¹⁰ M. The electrochemical impedance spectroscopy was also employed to further investigate the sensor function.     

An electrochemical biosensor for determination of hydrogen peroxide using nanocomposite of poly(methylene blue) and FAD hybrid film

An electrochemical biosensor for determination of hydrogen peroxide (H₂O₂) has been developed by the hybrid film of poly(methylene blue) and FAD (PMB/FAD). The PMB/FAD hybrid film was performed in PBS (pH 7) containing methylene blue and FAD by cyclic voltammetry. Repeatedly scanning potential range of −0.6-1.1 V, FAD was immobilized on the electrode surface by electrostatic interaction while methylene blue was electropolymerized on electrode surface. This modified electrode was found surface confined and pH dependence. It showed good electrocatalytic reduction for H₂O₂, KBrO₃, KIO₃, and NaClO as well as electrocatalytic oxidation for NADH. At an applied potential of −0.45 V vs. Ag/AgCl, the sensor showed a rapid and linear response to H₂O₂ over the range from 0.1 μM to 960 μM, with a detection limit of 0.1 μM and a significant sensitivity of 1109 μA mM⁻¹ cm⁻² (S/N = 3). It presented excellent stability at room temperature, with a variation of response current less than 5% over 30 days.     

Gas sensing properties of SnO2 nanowires on micro-heater

The SnO₂ nanowires (NWs) network gas sensors were fabricated on a micro-electrode and heater suspended in a cavity. The sensors showed selective detection to C₂H₅OH at a heater power during sensor operation as low as 30-40 mW. The gas response and response speed of the SnO₂ NWs sensor to 100 ppm C₂H₅OH were 4.6- and 4.7-fold greater, respectively, than those of the SnO₂ nanoparticles (NPs) sensor with the same electrode geometry. The reasons for these enhanced gas sensing characteristics are discussed in relation to the sensing materials and sensor structures.     

Fabrication and application of a new DNA biosensor based on on-substrate PCR and electrochemistry

DNA probes immobilized on a gold electrode (AuE) were employed as the primers of asymmetric PCR on the AuE. In the asymmetric PCR process, the DNA probes extended in the presence of target strands in the PCR solution. After PCR the dsDNAs were denaturalized and the target DNAs were eliminated and only the extended probes maintained on the AuE. At last the electrochemical indicator of methylene blue combined to the extended probes and the electrochemical signal of indicator was measured. This signal was higher than that of the AuE modified only by original probe. When there was no target in the PCR solution, the probe did not extend and the signal did not increase. The specific sequences of chitinase gene were detected successfully from four sorts of target with different length: oligonucleotide acid, PCR products, molecule cloning vector DNA and total genome DNA of transgenic capsicum, and the estimated detection limit were 7.3 × 10⁻¹², 3.2 × 10⁻¹¹, 5.4 × 10⁻¹¹ and 4.1 × 10⁻¹⁰ mol l⁻¹ respectively. The regeneration of the biosensor was also tested and the results indicated that its half life was 6 times.     

Direct electrochemistry and electrocatalysis of hemoglobin immobilized into poly (lactic-co-glycolic acid)/room temperature ionic liquid composite film

A promising material of poly(lactic-co-glycolic acid) (PLGA) and, room temperature ionic liquid (ILs) (1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF₄) was firstly used as an immobilization matrix to entrap proteins and its bioelectrochemical properties were studied. Direct electrochemistry and electrocatalytic behaviors of hemoglobin (Hb) entrapped in the PLGA/ILs composite film on the surface of glass carbon electrode were investigated. UV-vis spectroscopy, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the composite film. The obtained results demonstrated that the Hb molecule in the film kept its native structure and showed its good electrochemical behavior. A pair of well-defined redox peaks of Hb was obtained at the Hb/PLGA/ILs composite film-modified GC electrode through direct electron transfer between the protein and the underlying electrode. The proposed biosensor showed good reproducibility and high sensitivity to H₂O₂ with the detection limit of 2.37 × 10⁻⁷ M (S/N = 3). In the range of 5.0 × 10⁻⁶ to 8.05 × 10⁻³ M, the catalytic reduction current of H₂O₂ was proportional to its concentration. The apparent Michaelis-Menten constant of Hb in the PLGA/ILs composite film was estimated to be 0.069 mM, showing its high affinity.     

Amperometric biosensor for catechol using electrochemical template process

A novel amperometric biosensor for the determination of catechol was developed accordingly to the electrochemical template procedure. The optimum fabricating conditions of the biosensor were studied. The resulting biosensor with the limit of less than 0.05 μM can be used for detection of catechol in the linear range of 2.5-140 μM. The maximum response current (I_max) and the Michaelis-Menten constant (k′_m) are 3.08 μA and 77.52 μM, respectively. The activation energy (E_a) of the polyphenol oxidase (PPO) catalytic reaction is 25.56 kJ mol⁻¹ in the B-R buffer. The stability of the PANI-CA biosensor fabricated with the electrochemical template process (retains 86% of the original activity after four months) is much higher than that fabricated with one-step and two-step processes (retains 75% of the original activity after four months). The effects of potential and pH on the response current of the biosensor are also described.     

High performance polymer actuator based on carbon nanotube-ionic liquid gel: Effect of ionic liquid

In this study, we investigated a dependence of anionic species of ionic liquids (ILs) (IL: perfluoroalkyltrifluoroborate anions ([C_nF_2n+1BF₃]⁻ (n = 0, 1, 2) and bis(perfluoroalkylsulfonyl)imide anions ([(C_mF_2m+1SO₂)(C_nF_2n+1SO₂)N]⁻ (m, n = 0, 1, 2)) on electrochemical and electromechanical properties. 1-Ethyl-3-methylimidazolium (EMI⁺) was selected as a cation for ILs. 1-Ethyl-3-methylimidazolium trifluoromethyltrifluoroborate (EMI[CF₃BF₃]), 1-ethyl-3-methylimidazolium pentafluoroethyltrifluoroborate (EMI[CF₃CF₂BF₃]), 1-ethyl-3-methylimidazolium fluorosulfonyl(trifluoromethylsulfonyl)imide (EMI[FTA]) and 1-ethyl-3-methylimidazolium pentafluoroethylsulfonyl(trifluoromethylsulfonyl)imide (EMI[C₁C₂]) were synthesized according to the literatures. The generated strains of the bucky-gel electrodes of the actuators containing EMI[CF₃BF₃] (in the high frequency range: 10-0.5 Hz) and EMI[CF₃CF₂BF₃] (in the high frequency range of 1-0.5 Hz) are larger than that containing EMI[BF₄] (that is to say the quick response). For low frequencies (0.1-0.005 Hz), the generated strain containing EMI[CF₃CF₂BF₃] was larger than those containing other ILs (EMI[C_nF_2n+1BF₃] (n = 0, 1) and EMI[(C_mF_2m+1SO₂)(C_nF_2n+1SO₂)N] (m, n = 0, 1, 2)). The Young's modulus of actuators containing EMI[CF₃BF₃] and EMI[CF₃CF₂BF₃] were 145 and 110 MPa, respectively. The melting points of EMI[CF₃BF₃] and EMI[CF₃CF₂BF₃] are lower than that of EMI[BF₄].Therefore, trifluoromethyltrifluoroborate ([CF₃BF₃]⁻) and pentafluoroethyltrifluoroborate ([CF₃CF₂BF₃]⁻) anions performed much better as the actuator using the polymer-supported bucky-gel electrode containing the IL. These results are considered to be the actuator enough to apply actual applications (e.g. tactile display).     

Carbon dioxide sensing properties of bismuth cobaltite

Bismuth cobaltite with sillenite-type structure was prepared from Co(OH)₂ and Bi(NO₃)₃·6H₂O through solid state reaction at 600 °C. Neutron powder diffraction (NPD) data and X-ray absorption spectroscopy revealed the existence of mixed oxidation states for cobalt in this compound, the chemical formula being Bi₁₂(Bi_0.55Co_0.45)O_19.6. The gas sensing properties of Bi₁₂(Bi_0.55Co_0.45)O_19.6 were characterized by alternating current, at 200, 300 and 400 °C. The optimal response was observed at 400 °C, using a frequency of 100 kHz.     

An interferometric biosensor composed of a prism-chamber assembly and a composite waveguide with a Ta2O5 nanometric layer

A highly sensitive integrated polarimetric interferometer biosensor with improved long-time stability and simple operation was prepared by using a novel prism-chamber assembly and an inexpensive waveguide made by sputtering a tapered nanometric layer of Ta₂O₅ on a single-mode glass waveguide. By comparing the measured refractive-index (RI) sensitivities with those simulated based on a four-layer homogeneous waveguide, both the equivalent thicknesses (T_eq) for the tapered Ta₂O₅ layers and a severe dependence of RI sensitivity on T_eq were obtained. Addition of 1 g of water in 100 g of a Chinese liquor (alcohol concentration = 46% (v/v)) was easily detected by the sensor. Monitoring of anti-human IgG adsorption with a waveguide of T_eq = 31.99 nm indicates that the antibody coverage required for inducing a phase-different change of Δ? = π is less than 0.012 monolayer. The same waveguide presents a quasi-linear dependence of Δ? on water temperature with the slope of d(Δ?)/dT = −28.50°/°C to which the contribution by the thermo-optical effect of the waveguide is 4.24°/°C, equivalent to a liquid RI change of Δn_c = 1.41 × 10⁻⁵. The interferometer exhibits the promising potential for chemical and biological analyses because of its outstanding characteristics.     

A stable sandwich-type amperometric biosensor based on poly(3,4-ethylenedioxythiophene)-single walled carbon nanotubes/ascorbate oxidase/nafion films for detection of L-ascorbic acid

In this paper, a stable sandwich-type amperometric biosensor based on poly(3,4-ethylenedioxythiophene) (PEDOT)-single walled carbon nanotubes (SWCNT)/ascorbate oxidase (AO)/Nafion films for detection of l-ascorbic acid (AA) was successfully developed. PEDOT-SWCNT nanocomposite and Nafion films were used as inner and outer films, respectively. AO was immobilized between these two films. The PEDOT-SWCNT nanocomposite films were characterized by electrochemical impedance spectroscopy and scanning electron microscopy. The influence of detection potential and temperature on the biosensor performance was examined in detail. Despite the multilayer configuration, the biosensor exhibited a relatively fast response (less than 10 s) and a linear range from 1 μM to 18 mM (a correlation coefficient of 0.9974). The sensitivity of the biosensor was found to be 28.5 mA M⁻¹ cm⁻². Its experimental detection limit was 0.7 μM (S/N = 3) and the apparent Michaelis-Menten constant (K_m) was calculated to be 18.35 mM. Moreover, the biosensor exhibited good anti-interferent ability and excellent long-term stability. All the results showed that such sandwich-type PEDOT-SWCNT/AO/Nafion films could provide a promising platform for the biosensor designs for AA detection.     

Amperometric glucose biosensor based on integration of glucose oxidase with platinum nanoparticles/ordered mesoporous carbon nanocomposite

This article reports a new amperometric glucose biosensor based on ordered mesoporous carbon (OMC) supported platinum nanoparticles (Pt/OMC) modified electrode. The Pt/OMC nanocomposite modified electrode exhibited excellent electrocatalytic activities towards the reduction and oxidation of H₂O₂ as well. This feature allowed us to use it as bioplatform on which glucose oxidase (GOD) was immobilized by entrapment in electropolymerized pyrrole film for the construction of the glucose biosensor. The biosensor showed good analytical performances in terms of low detection (0.05 mM), high sensitivity (0.38 μA/mM) and wide linear range (0.05-3.70 mM). In addition, the effects of pH value, applied potential, electroactive interference and the stability of the biosensor were discussed. The applicability to blood analysis was also evaluated.     

Enhanced C2H5OH sensing characteristics of nano-porous In2O3 hollow spheres prepared by sucrose-mediated hydrothermal reaction

In₂O₃ hollow spheres with shell thicknesses of ∼150 nm and ∼300 nm were prepared by the one-pot synthesis of indium-precursor-coated carbon spheres via hydrothermal reaction and subsequent removal of core carbon by heat treatment. The gas response (R_a/R_g, R_a: resistance in air, R_g: resistance in gas) of the thin hollow spheres to 100 ppm C₂H₅OH was 137.2 at 400 °C, which was 1.86 and 3.84 times higher than that of the thick hollow spheres and of the nanopowders prepared by precipitation, respectively. The gas sensing characteristics are discussed in relation to the shell configuration of the hollow spheres. The enhanced gas response of the hollow spheres was attributed to the effective diffusion of analyte gas toward the entire sensor surface via very thin and nano-porous shells.     

Effect of surface defects on biosensing properties of TiO2 nanotube arrays

In this paper, highly ordered titania nanotube (TNT) arrays fabricated by anodization were annealed at different temperatures in CO to create different concentrations of surface defects. The samples were characterized by SEM, XRD and XPS. The results showed different concentrations of Ti³⁺ defects were doped in TNT arrays successfully. Furthermore, after co-immobilized with horseradish peroxidase (HRP) and thionine chloride (Th), TNT arrays was employed as a biosensor to detect hydrogen peroxide (H₂O₂) using an amperometric method. Cyclic voltammetry results and UV-Vis absorption spectra presented that with an increase of Ti³⁺ defects concentration, the electron transfer rate and enzyme adsorption amount of TNT arrays were improved largely, which could be ascribed to the creation of hydroxyl groups on TNT surface due to dissociative adsorption of water by Ti³⁺ defects. Annealing in CO at 500 °C appeared to be the most favorable condition to achieve desirable nanotube array structure and surface defects density (0.27%), thus the TNT arrays showed the largest adsorption amount of enzyme (9.16 μg/cm²), faster electron transfer rate (1.34 × 10⁻³ cm/s) and the best response sensitivity (88.5 μA/mM l⁻¹).     

Electrocatalytic reduction of hydrogen peroxide and its determination in antiseptic and soft-glass cleaning solutions at phosphotungstate-doped-glutaraldehyde-cross-linked poly-l-lysine film electrodes

The present work describes the electrocatalytic behavior of phosphotungstate-doped glutaraldehyde-cross-linked poly-l-lysine (PLL-GA-PW) film electrode towards reduction of hydrogen peroxide (H₂O₂) in acidic medium. The modified electrode was prepared by means of electrostatically trapping the phosphotungstate anion into the cationic PLL-GA coating on glassy carbon electrode. The PLL-GA-PW film electrode showed excellent electrocatalytic activity towards H₂O₂ reduction in 0.1 M H₂SO₄. Under the optimized conditions, the electrochemical sensor exhibited a linear response for H₂O₂ concentration over the range 2.5 × 10⁻⁶ to 6.85 × 10⁻³ M with a sensitivity of 1.69 μA mM⁻¹. The curvature in the calibration curve at high concentration is explained in terms of Michaelis-Menten (MM) saturation kinetics, and the kinetics parameters calculated by three different methods were compared. The PLL-GA-PW film electrode did not respond to potential interferents such as dopamine, ascorbic acid and uric acid. This unique feature of PLL-GA-PW film electrode allowed selective determination of H₂O₂. Finally, the proposed electrochemical sensor was successfully applied to determine H₂O₂ in commercially available antiseptic solution and soft-contact lenses cleaning solution and the method has been validated using independent estimation by classical potassium permanganate titration method. Major advantages of the method are simple electrode fabrication, stability and high selectivity towards hydrogen peroxide.     

Highly sensitive and selective LPG sensor based on α-Fe2O3 nanorods

The α-Fe₂O₃ nanorods were successfully synthesized without any templates by calcining the α-FeOOH precursor in air at 300 °C for 2 h and their LPG sensing characteristics were investigated. The α-FeOOH precursor was prepared through a simple and low cost wet chemical route at low temperature (40 °C) using FeSO₄·7H₂O and CH₃COONa as starting materials. The formation of α-FeOOH precursor and its topotactic transformation to α-Fe₂O₃ upon calcination was confirmed by X-ray diffraction measurement (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis. The α-Fe₂O₃ nanorods exhibited outstanding gas sensing characteristics such as, higher gas response (∼1746-50 ppm LPG at 300 °C), extremely rapid response (∼3-4 s), relatively slow recovery (∼8-9 min), excellent repeatability, good selectivity and lower operating temperature (∼300 °C). Furthermore, the α-Fe₂O₃ nanorods are able to detect up to 5 ppm for LPG with reasonable response (∼15) at the operating temperature of 300 °C and they can be reliably used to monitor the concentration of LPG over the range (5-60 ppm). The experimental results clearly demonstrate the potential of using the α-Fe₂O₃ nanorods as sensing material in the fabrication of LPG sensors. Plausible LP G sensing mechanism of the α-Fe₂O₃ nanorods is also discussed.     

Improved crystalline structure and H2S sensing performance of CuO-Au-SnO2 thin film using SiO2 additive concentration

In situ SiO₂-doped SnO₂ thin films were successfully prepared by liquid phase deposition. The influence of SiO₂ additive as an inhibitor on the surface morphology and the grain size for the thin film has been investigated. These results show that the morphology of SnO₂ film changes significantly by increasing the concentration of H₂SiF₆ solution which decreases the grain size of SnO₂. The stoichiometric analysis of Si content in the SnO₂ film prepared from various Si/Sn molar ratios has also been estimated. For the sensing performance of H₂S gas, the SiO₂-doped Cu-Au-SnO₂ sensor presents better sensitivity to H₂S gas compared with Cu-Au-SnO₂ sensor due to the fact that the distribution of SiO₂ particles in grain boundaries of nano-crystallines SnO₂ inhibited the grain growth (<6 nm) and formed a porous film. By increasing the Si/Sn molar ratio, the SiO₂-doped Cu-Au-SnO₂ gas sensors (Si/Sn = 0.5) exhibit a good sensitivity (S = 67), a short response time (t_90% < 3 s) and a good gas concentration characteristic (α = 0.6074). Consequently, the improvement of the nano-crystalline structures and high sensitivity for sensing films can be achieved by introducing SiO₂ additive into the SnO₂ film prepared by LPD method.     

Effect of La2CuO4 electrode area on potentiometric NOx sensor response and its implications on sensing mechanism

The intent of this work is to look at the effects of varying the La₂CuO₄ electrode area and the asymmetry between the sensing and counter electrode in a solid state potentiometric sensor with respect to NO_x sensitivity. NO₂ sensitivity was observed at 500-600 °C with a maximum sensitivity of ∼22 mV/decade [NO₂] observed at 500 °C for the sensor with a La₂CuO₄ electrode area of ∼30 mm². The relationship between NO₂ sensitivity and area is nearly parabolic at 500 °C, decreases linearly with increasing electrode area at 600 °C, and was a mixture of parabolic and linear behavior 550 °C. NO sensitivity varied non-linearly with electrode area with a minima (maximum sensitivity) of ∼−22 mV/decade [NO] at 450 °C for the sensor with a La₂CuO₄ electrode area of 16 mm². The behavior at 400 °C was similar to that of 450 °C, but with smaller sensitivities due to a saturation effect. At 500 °C, NO sensitivity decreases linearly with area.We also used electrochemical impedance spectroscopy (EIS) to investigate the electrochemical processes that are affected when the sensing electrode area is changed. Changes in impedance with exposure to NO_x were attributed to either changes in La₂CuO₄ conductivity due to gas adsorption (high frequency impedance) or electrocatalysis occurring at the electrode/electrolyte interface (total electrode impedance). NO₂ caused a decrease in high frequency impedance while NO caused an increase. In contrast, NO₂ and NO both caused a decrease in the total electrode impedance. The effect of area on both the potentiometric and impedance responses show relationships that can be explained through the mechanistic contributions included in differential electrode equilibria.