Amperometric glucose sensor based on enhanced catalytic reduction of oxygen using glucose oxidase adsorbed onto core-shell Fe3O4@silica@Au magnetic nanoparticles

Monodisperse Fe₃O₄ magnetic nanoparticles (NPs) were prepared under facile solvothermal conditions and successively functionalized with silica and Au to form core/shell Fe₃O₄@silica@Au NPs. Furthermore, the samples were used as matrix to construct a glucose sensor based on glucose oxidase (GOD). The immobilized GOD retained its bioactivity with high protein load of 3.92 × 10^? 9 mol·cm^? 2, and exhibited a surface-controlled quasi-reversible redox reaction, with a fast heterogeneous electron transfer rate of 7.98 ± 0.6 s^? 1. The glucose biosensor showed a broad linear range up to 3.97 mM with high sensitivity of 62.45 μA·mM^? 1 cm^? 2 and fast response (less than 5 s).     

Highly selective electrochemical biosensor for the determination of folic acid based on DNA modified-pencil graphite electrode using response surface methodology

An electrochemical DNA biosensor was proposed as a screening device for the rapid analysis of folic acid using a pencil graphite electrode modified with salmon sperm ds-DNA. At first, immobilization of the ds-DNA on pencil graphite electrode was optimized using response surface methodology. Solution pH, DNA concentration, time of DNA deposition and potential of deposition was optimized each at three levels. The optimum combinations for the reaction were pH 4.8, DNA concentration of 24 μg mL^? 1, deposition time of 304 s, and deposition potential of 0.60 V, by which the adenine signal was recorded as 3.04 μA. Secondly the binding of folic acid to DNA immobilized on a pencil graphite electrode was measured through the variation of the electrochemical signal of adenine. Folic acid could be measure in the range of 0.1–10.0 μmol L^? 1 with a detection limit of 1.06 × 10^? 8 μmol L^? 1. The relative standard deviations for ten replicate differential pulse voltammetric measurements of 2.0 and 5.0 μmol L^? 1 folic acid were 4.6% and 4.3%, respectively. The biosensor was successfully used to measure folic acid in different real samples.     

Electrochemical horseradish peroxidase biosensor based on dextran–ionic liquid–V2O5 nanobelt composite material modified carbon ionic liquid electrode

Direct electrochemistry of horseradish peroxidase (HRP) was realized in a dextran (De), 1-ethyl-3-methylimidazolium ethylsulphate ([EMIM]EtOSO₃) and V₂O₅ nanobelt composite material modified carbon ionic liquid electrode (CILE). Spectroscopic results indicated that HRP retained its native structure in the composite. A pair of well-defined redox peaks of HRP appeared in pH 3.0 phosphate buffer solution with the formal potential of ?0.213 V (vs. SCE), which was the characteristic of HRP heme Fe(III)/Fe(II) redox couple. The result was attributed to the specific characteristics of De–IL–V₂O₅ nanocomposite and CILE, which promoted the direct electron transfer rate of HRP with electrode. The electrochemical parameters of HRP on the composite modified electrode were calculated and the electrocatalysis of HRP to the reduction of trichloroacetic acid (TCA) was examined. Under the optimal conditions the reduction peak current increased with TCA concentration in the range from 0.4 to 16.0 mmol L^?1. The proposed electrode is valuable for the third-generation electrochemical biosensor.     

A new immobilized glucose oxidase using SiO2 nanoparticles as carrier

Using SiO₂ nanoparticles as a carrier, a novel immobilized glucose oxidase (GOD) (EC1.1.3.4) was prepared via crosslinking with glutaraldehyde (GA). The optimal immobilization condition was achieved with 1% (v/v) GA, 2% (v/v) 3-aminopropiltrietoxysilane (APTS), 2.5 mg GOD (in 34 mg carrier) and solution pH of 6.5. The immobilized GOD showed maximal catalytic activity at pH 7.0 and 60 °C, and more than 85% of initial activity at the temperature from 20 °C to 80 °C. After immobilization, the enzyme exhibited improved thermal, storage and operation stability. The immobilized GOD still maintained 85% of its initial activity after the incubation at 45 °C for 360 min, whereas free enzyme had only 23% of initial activity after the same incubation. After kept at 4 °C for 30 days, the immobilized and free enzyme retained 84% and 60% of initial activity, respectively. The immobilized GOD also preserved 87% of its initial activity after six consecutive operations.     

Detection of anticancer drug tamoxifen using biosensor based on polyaniline probe modified with horseradish peroxidase

Amperometric biosensor based on horseradish peroxidase immobilized via glutaraldehyde on the polyaniline modified platinum electrode shows evidenced promising characteristics in detecting anticancer drug tamoxifen. The sensor was fabricated simply by adsorbing horseradish peroxidase enzyme on the electrode surface for which Cyclic Voltammetry was used to monitor the electro-catalytic reduction of tamoxifen under diffusion-adsorption controlled conditions. Fourier Transform Infrared Spectroscopy, Cyclic Voltammetry and Electrochemical Impedance Spectroscopic techniques are used to characterize the electrochemical interfacial properties of surface modified electrodes. The first-hand effort on modified biosensor within Platinum/Polyaniline/Horseradish peroxidase biosensor system has demonstrated excellent electro-analytical properties with biosensor sensitivity of 1.6 μA ng mL^? 1. The optimum limit of detection and limit of quantification are 0.07 ng mL^? 1 and 0.29 ng mL^? 1 respectively for the determination of anticancer drug tamoxifen. It is felt that the present study will help in improving our knowledge of cost-effective quantitative determination of tamoxifen in metabolized biological fluids and other pharmaceutical formulations.     

Potentiometric urea biosensor based on multi-walled carbon nanotubes (MWCNTs)/silica composite material

A novel potentiometric urea biosensor has been fabricated with urease (Urs) immobilized multi-walled carbon nanotubes (MWCNTs) embedded in silica matrix deposited on the surface of indium tin oxide (ITO) coated glass plate. The enzyme Urs was covalently linked with the exposed free –COOH groups of functionalized MWCNTs (F-MWCNTs), which are subsequently incorporated within the silica matrix by sol–gel method. The Urs/MWCNTs/SiO₂/ITO composite modified electrode was characterized by Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA) and UV–visible spectroscopy. The morphologies and electrochemical performance of the modified Urs/MWCNTs/SiO₂/ITO electrode have been investigated by scanning electron microscopy (SEM) and potentiometric method, respectively. The synergistic effect of silica matrix, F-MWCNTs and biocompatibility of Urs/MWCNTs/SiO₂ made the biosensor to have the excellent electro catalytic activity and high stability. The resulting biosensor exhibits a good response performance to urea detection with a wide linear range from 2.18 × 10^? 5 to 1.07 × 10^? 3 M urea. The biosensor shows a short response time of 10–25 s and a high sensitivity of 23 mV/decade/cm².     

Biopolymer-clay nanoparticles composite system (Chitosan-laponite) for electrochemical sensing based on glucose oxidase

Nanocomposite matrix based on chitosan/laponite was successfully utilized to construct a new type of amperometric glucose biosensor. This hybrid material combined the merits of organic biopolymer, chitosan, and synthesized inorganic clay, laponite. Glucose oxidase (GOD) immobilized in the material maintained its activity well as the usage of glutaraldehyde was avoided. The composite films were characterized by Fourier transform infrared (FT-IR). The parameters affecting the fabrication and experimental conditions of biosensors were optimized. The sensitivity of the proposed biosensor (33.9 mA M^− 1 cm^− 2) permitted the determination of glucose in the concentration range of 1 × 10^− 6–5 × 10^− 5 M with a detection limit of 0.3 μM based on S/N = 3. The apparent Michaelis–Menten constant (K_M^app) for the sensor was found to be 15.8 mM.     

Comparative study of conductometric glucose biosensor based on gold and on magnetic nanoparticles

The aim of this study was to show the feasibility and the performances of nanoparticle biosensing. A glucose conductometric biosensor was developed using two types of nanoparticles (gold and magnetic), glucose oxidase (GOD) being adsorbed on PAH (poly(allylamine hydrochloride)) modified nanoparticles, deposited on a planar interdigitated electrode (IDEs). The best sensitivities for glucose detection were obtained with magnetic nanoparticles (70 μM/mM and 3 μM of detection limit) compared to 45 μM/mM and 9 μM with gold nanoparticles and 30 μM/mM and 50 μM with GOD directly cross-linked on IDEs. When stored in phosphate buffer (20 mM, pH 7.3) at 4 °C, the biosensor showed good stability for more than 12 days.     

Label-free detection of aflatoxin M1 with electrochemical Fe3O4/polyaniline-based aptasensor

The selective detection of ultratrace amounts of aflatoxin M1 (AFM1) is extremely important for food safety since it is the most toxic mycotoxin class that is allowed to be present on cow milk with strictly low regulatory levels. In this work, Fe₃O₄ incorporated polyaniline (Fe₃O₄/PANi) film has been polymerized on interdigitated electrode (IDE) as sensitive film for AFM1 electrochemical biosensor. The immobilized aptamers as an affinity capture reagent and magnetic nanoparticles for signal amplification element have been employed in the sensing platform. Label-free and direct detection of the aptamer-AFM1 on Fe₃O₄/PANi interface were performed via electrochemical signal change, acquired by cyclic and square wave voltammetries. With a simplified strategy, this electrochemical aptasensor shows a good sensitivity to AFM1 in the range of 6–60 ng·L^? 1, with the detection limit of 1.98 ng·L^? 1. The results open up the path for designing cost effective aptasensors for other biomedical applications.     

Electrosynthesis of pinecone-shaped Pt–Pb nanostructures based on the application in glucose detection

The pinecone-shaped Pt–Pb nanostructures were synthesized by electrochemical deposition. The morphology and composition of the pinecone-shaped Pt–Pb nanostructures were characterized by scanning electron microscopy, energy dispersive X-ray detector, transmission electron microscopy and X-ray photoelectron spectroscopy. Cyclic voltammetry and differential pulse voltammetry were used to evaluate the electrocatalytic performance of the pinecone-shaped Pt–Pb nanostructures electrode toward glucose oxidation in neutral media. As a result, the pinecone-shaped Pt–Pb nanostructures electrode exhibited strong current responses to glucose at a negative potential of ? 0.1 V, where the interference from the oxidation of ascorbic acid was effectively avoided. The sensitivity of the sensor was 10.71 μA mM^?1 cm^?2 with a linearity up to 12 mM and a detection limit of 8.4 μM. In addition, the as-prepared nonenzyme glucose sensor exhibited acceptable stability and reproducibility for determination of glucose. The simple preparation method and good analytical performance can potentially pave the way for effective and highly sensitive non-enzyme glucose sensors.     

Modified gold surfaces by 6-(ferrocenyl)hexanethiol/dendrimer/gold nanoparticles as a platform for the mediated biosensing applications

An electrochemical biosensor mediated by using 6-(Ferrocenyl) hexanethiol (FcSH) was fabricated by construction of gold nanoparticles (AuNPs) on the surface of polyamidoamine dendrimer (PAMAM) modified gold electrode. Glucose oxidase (GOx) was used as a model enzyme and was immobilized onto the gold surface forming a self assembled monolayer via FcSH and cysteamine. Cyclic voltammetry and amperometry were used for the characterization of electrochemical response towards glucose substrate. Following the optimization of medium pH, enzyme loading, AuNP and FcSH amount, the linear range for the glucose was studied and found as 1.0 to 5.0 mM with the detection limit (LOD) of 0.6 mM according to S/N = 3. Finally, the proposed Au/AuNP/(FcSH + Cyst)/PAMAM/GOx biosensor was successfully applied for the glucose analysis in beverages, and the results were compared with those obtained by HPLC.     

Multi-walled carbon nanotubes-based glucose biosensor prepared by a layer-by-layer technique

The loading of multi-walled carbon nanotubes (MWNTs) and glucose oxidase (GOx) in the alternate layers of a glucose biosensor was first optimized based on a layer-by-layer construction on the surface of a graphite disk electrode. With the increasing of MWNTs/GOx layers, the response current to glucose was changed regularly and the response current reached a maximum value when the number of MWNTs/GOx layers was 6. Owing to a good electrical conductivity, strong adsorption and excellent bioconsistency of MWNTs, the (MWNTs/GOx)₆ films-coated glucose biosensor had an excellent electrochemical properties. The response current of the (MWNTs/GOx)₆ films-coated biosensor to 3 × 10^− 2 M glucose was 1.63 μA while the response time was only 6.7 s. The linear range and the lowest detectable concentration of this biosensor was 5 × 10^− 4∼1.5 × 10^− 2 M and 0.9 × 10^− 4 M, respectively.     

Application of graphene–pyrenebutyric acid nanocomposite as probe oligonucleotide immobilization platform in a DNA biosensor

A stable and uniform organic–inorganic nanocomposite that consists of graphene (GR) and pyrenebutyric acid (PBA) was obtained by ultrasonication, which was characterized by scanning electron microscopy (SEM) and UV–vis absorption spectra. The dispersion was dropped onto a gold electrode surface to obtain GR–PBA modified electrode (GR–PBA/Au). Electrochemical behaviors of the modified electrode were characterized by cyclic voltammetry and electrochemical impedance spectroscopy using [Fe(CN)₆]^3 ?/4 ? as the electroactive probe. A novel DNA biosensor was constructed based on the covalent coupling of amino modified oligonucleotides with the carboxylic group on PBA. By using methylene blue (MB) as a redox-active hybridization indicator, the biosensor was applied to electrochemically detect the complementary sequence, and the results suggested that the peak currents of MB showed a good linear relationship with the logarithm values of target DNA concentrations in the range from 1.0 × 10^? 15 to 5.0 × 10^? 12 M with a detection limit of 3.8 × 10^? 16 M. The selectivity experiment also showed that the biosensor can well distinguish the target DNA from the non-complementary sequences.     

Synthesis and electrochemical properties of nanosized LiFeO2 particles with a layered rocksalt structure for lithium batteries

Layered rocksalt-type LiFeO₂ particles (O3-LiFeO₂) with average particle sizes of ca. 40 and 400 nm were synthesized by an ion exchange reaction from α-NaFeO₂ precursors. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images confirmed the formation of nanosized O3-LiFeO₂. 40-nm LiFeO₂ exhibited a higher discharge capacity (115 mAh g^?1) than 400-nm LiFeO₂ (80 mAh g^?1), and also had better rate characteristics. The downsizing effect and cation disorder between the lithium and iron layers may have improved the electrochemical activity of the LiFeO₂ particles. Transmission electron microscopy (TEM) observation indicated a phase transition from O3-LiFeO₂ to a cubic lattice system during the electrochemical process. The cubic lithium iron oxide exhibited stable electrochemical reactions based on the Fe²⁺/Fe³⁺ and Fe²⁺/Fe⁰ redox couples at voltages between 4.5 and 1.0 V. The discharge capacities of 40-nm LiFeO₂ were ca. 115, 210, and 390 mAh g^?1 under cutoff voltages of 4.5–2.0 V, 4.5–1.5 V, and 4.5–1.0 V, respectively.     

Determination of sulfamethoxazole in foods based on CeO2/chitosan nanocomposite-modified electrodes

An electrochemical immunosensor based on nanocomposite-modified glass carbon (GC) electrode has been developed. The biospecific surface was a CeO₂-chitosan (CHIT)-modified nanocomposite to which anti-sulfamethoxazole (SMX) polyclonal antibody (Ab) was immobilized. The assay was based on competition of SMX and horseradish peroxidase (HRP)–SMX to the antibody immobilized. Electrochemical voltammetry and impedance spectroscopy studies revealed that the presence of CeO₂-CHIT nanocomposite significantly enhanced conductivity of the electrode. The large electro-active surface area of nanoCeO₂-CHIT/GC electrode resulted in the high loading of anti-SMX polyclonal antibody. The electrochemical signals of the immunosensor mainly resulted from the HRP catalyzed hydrogen peroxide reduction in the presence of thionine. The immunosensor showed high sensitivity for the detection of SMX. The electrochemical response signals of the immunosensor were found to be linearly proportional to SMX concentration in the range from 5 × 10^? 7 to 5 × 10^? 4 mg mL^? 1 with a regression coefficient of 0.9935 and a detection limit of 3.25 × 10^? 7 mg mL^? 1. No cross-reactivity of antibodies with other antibiotics of sulfonamide family was found. Under optimal conditions, the immunosensor was successfully applied to the electrochemical determination of SMX in milk, honey and egg samples, showing excellent stability and anti-interference ability.     

Carbon nanotube/chitosan/gold nanoparticles-based glucose biosensor prepared by a layer-by-layer technique

A new amperometric glucose biosensor was constructed, based on the immobilization of glucose oxidase (GOx) with cross-linking in the matrix of chitosan on a glassy carbon electrode, which was modified by layer-by-layer assembled carbon nanotube (CNT)/chitosan (CHIT)/gold nanoparticles (GNp) multilayer films. With the increasing of CNT/CHIT/GNp layers, the response current to H₂O₂ was changed regularly and the response current reached a maximum value when the number of CNT/CHIT/GNp layers was 8. The assembling process of multilayer films was simple to operate. With GOx as an enzyme model, a new glucose biosensor was fabricated. The excellent electocatalytic activity and special structure of the enzyme electrode resulted in good characteristics. The linear range was 6 × 10^? 6 ～ 5 × 10^? 3 M, with a detection limit of 3 × 10^? 6 M estimated at a signal-to-noise ratio of 3, fast response time (less than 6 s). Moreover, it exhibited good reproducibility and stability.     

Trypsin immobilization on discs of polyvinyl alcohol glutaraldehyde/polyaniline composite

Discs of polyvinyl alcohol cross-linked with glutaraldehyde (PVAG) were synthesized and covered with polyaniline activated with glutaraldehyde (PANIG). Trypsin was covalently immobilized on this composite yielding a preparation containing 21.1 units per disc. The FT-IR spectra of the discs showed bands of PVA (3300 cm^?1, 2930 cm^?1 and 1440 cm^?1) and PANI (1594 cm^?1 and 1100 cm^?1). The best immobilization conditions were: trypsin concentration at 0.2 mg mL^?1, pH 7.6 and 60 min of incubation, similar to polyaniline–trypsin systems reported in the literature. The PVAG–PANIG–trypsin derivative showed an optimal pH and an optimal temperature of 7.0 and 35 °C, respectively. Hydrolysis of casein showed variations in the size of the products, revealing differences between the immobilized enzyme and the mechanism catalyzed by the native enzyme.     

Enzyme distribution and secondary structure of sol–gel immobilized glucose oxidase by micro-attenuated total reflection FT-IR spectroscopy

Glucose oxidase (GOD) immobilized into sol–gel matrices was studied by using Micro-Attenuated Total Reflection Fourier Transform Infrared (micro-ATR FT-IR) spectroscopy in order to characterize enzyme distribution and secondary structure in systems with valuable potentialities in amperometric and optical biosensing. Spectra were acquired in the 4000–600 cm^? 1 frequency region and the analysis of specific fingerprints in the FT-IR spectra evidenced that the enzyme was actually immobilized in the matrix. The enzyme spatial distribution was obtained by examining the amide I and amide II band region of spectra from defined sample positions. The deconvolution of the amide I band in terms of lorentzian functions provided information on the secondary structure of the immobilized GOD. By this approach a macroscopic preservation of GOD activity upon immobilization was evidenced along with the existence of some matrix sites with locally inactivated GOD. To our knowledge this is the first example of point-by-point characterization of conformational changes of immobilized enzyme by means of micro-ATR infrared spectroscopy, thus confirming that this technique can be usefully employed for a non- or minimally-invasive detailed micro-characterization of catalytic supports in order to improve their functionality.     

Highly sensitive amperometric sensor for micromolar detection of trichloroacetic acid based on multiwalled carbon nanotubes and Fe(II)–phtalocyanine modified glassy carbon electrode

A highly sensitive electrochemical sensor for the detection of trichloroacetic acid (TCA) is developed by subsequent immobilization of phthalocyanine (Pc) and Fe(II) onto multiwalled carbon nanotubes (MWCNTs) modified glassy carbon (GC) electrode. The GC/MWCNTs/Pc/Fe(II) electrode showed a pair of well-defined and nearly reversible redox couple correspondent to (Fe(III)Pc/Fe(II)Pc) with surface-confined characteristics. The surface coverage (Γ) and heterogeneous electron transfer rate constant (k_s) of immobilized Fe(II)–Pc were calculated as 1.26 × 10^? 10 mol cm^? 2 and 28.13 s^? 1, respectively. Excellent electrocatalytic activity of the proposed GC/MWCNTs/Pc/Fe(II) system toward TCA reduction has been indicated and the three consequent irreversible peaks for electroreduction of CCl₃COOH to CH₃COOH have been clearly seen. The observed chronoamperometric currents are linearly increased with the concentration of TCA at concentration range up to 20 mM. Detection limit and sensitivity of the modified electrode were 2.0 μM and 0.10 μA μM^? 1 cm^? 2, respectively. The applicability of the sensor for TCA detection in real samples was tested. The obtained results suggest that the proposed system can serve as a promising electrochemical platform for TCA detection.     

Electrochemical sensor for uric acid based on a molecularly imprinted polymer brush grafted to tetraethoxysilane derived sol-gel thin film graphite electrode

Determination of uric acid in human serum and urine is useful to provide treatment guidelines to hyperuricemic patients. An electrochemical sensor was developed for selective and quantitative recognition of uric acid by using a preanodised sol-gel coated graphite electrode with a molecularly imprinted polymer brush of poly(melamine–co-chloranil) grafted to its exterior surface. During a preconcentration step at (+ 2.0 V versus saturated calomel electrode), the encapsulated analyte recapture involved hydrophobically induced hydrogen-bondings in outwardly exposed MIP cavities in aqueous environment (pH 7.0), instantly oxidised as dications, and then cathodically stripped off as corresponding lactam responding differential pulse, cathodic stripping voltammetric signal. The uric acid was selectively detected without any cross reactivity in the windows of 14.56–177.42 µg mL^? 1 (aqueous medium), 4.78–106.96 µg mL^? 1 (blood serum), and 7.81–148.42 µg mL^? 1 (urine) indicating detection limits in the range of 3.71–4.10 µg mL^? 1 (3σ, RSD = 1.9%).