S. Upadhyay

A new glucose sensor based on encapsulated glucose oxidase within organically modified sol–gel glass

Abstract A non-mediated glucose biosensor is reported based on encapsulated glucose oxidase (GOD) within the composite sol–gel glass, which is prepared using optimum concentrations of 3-aminopropyltriethoxy silane, 2-(3, 4-epoxycyclohexyl)-ethyltrimethoxy silane, GOD dissolved in double distilled water and HCl. A white, smooth film of sol–gel glass with controlled thickness is also prepared at the surface of a Pt disk electrode without GOD to study the electrochemistry of ferrocene monocarboxylic acid at the surface of the modified electrode. The electrochemistry of ferrocene monocarboxylic acid at composite sol–gel glass electrode with varying thickness is reported. The GOD-immobilized film over the Pt disk surface shows a yellow colour. The new sol–gel glass in the absence and the presence of GOD is characterized by scanning electron microscopy (SEM). The enzyme-immobilized film of different thickness is made using varying concentrations of soluble sol–gel components applied to the well of the Pt disk electrode. The enzyme is cross-lined with the 3-aminopropyltriethoxysilane, one of the composite component of sol–gel glass using glyoxal at 4°C for 4 h. The response of non-mediated enzyme sensor is studied based on cyclic voltammetry and amperometric measurements. A typical amperometric response of the enzyme sensor having varying thickness of the modified sol–gel glass film is reported. The variation of the response time as a function of the film thickness is reported. The stability of cross-linked GOD to sol–gel glass is found to be more than a month without loss of enzymatic activity when the enzyme sensor is stored at 4°C.

Studies on the electrochemical performance of glucose biosensor based on ferrocene encapsulated ORMOSIL and glucose oxidase modified graphite paste electrode

The electrochemical performance of a new glucose biosensor is reported. The glucose biosensor is developed using glucose oxidase (GOD) and ferrocene encapsulated palladium (Pd)-linked organically modified sol-gel glass (ORMOSIL) material incorporated within graphite paste electrode. The ORMOSIL material incorporated within graphite paste electrode behaves as an excellent electrocatalyst for the oxidation of enzymatically reduced GOD. The electrochemical behavior of new glucose biosensor has been examined by cyclic volammetry and amperometric measurements. The bioelectrocatalysis of ORMOSIL embedded within graphite paste as a function of storage time and varying concentration of ORMOSIL is reported. The initial amperometric response on glucose sensing is recorded to be 145 microA at 15% (w/w) concentration of the ORMOSIL which is decreased to 20 microA at 5% of the same keeping GOD concentration constant. The variation of electrochemical behavior of the ORMOSIL embedded within graphite paste as a function of time has also been studied based on cyclic voltammetry. The voltammograms showing the reversible electrochemistry of ORMOSIL encapsulated ferrocene is changed into a plateau shape as a function of time, however, the electrocatalytic behavior is still retained. The practical usability of new glucose sensor has been compared with earlier developed glucose sensor. The sensitivity, response time and linearity of the new glucose biosensor are found to be excellent over earlier reported glucose biosensor. The amperometric response, calibration curve and practical applications of new glucose sensor are reported.

Acetylthiocholine/acetylcholine and thiocholine/choline electrochemical biosensors/sensors based on an organically modified sol–gel glass enzyme reactor and graphite paste electrode

Abstract Electrochemical sensors for acetylthiocholine and acetylcholine are described. The non-mediated electrochemistry of acetylthiocholine and thiocholine is studied on the surface of graphite paste electrode and results show that acetylthiocholine is directly oxidized/reduced at >0.32 V vs. Ag/AgCl in both acidic and basic medium. In basic medium, both cathodic and anodic peak currents are less as compared to that of the same amount in acidic medium, which shows that the kinetics of non-enzymatic hydrolysis of acetylcholine into electroactive thiocholine is faster in acidic medium and slower in basic medium. Thiocholine is directly oxidized/reduced at >0.35 V vs. Ag/AgCl with relatively larger anodic current compared to cathodic peak current similar to that of acetylcholine results recorded in acidic medium (pH 6.0). The electrochemical sensor/biosensors for acetylthiocholine/acetylcholine and thiocholine/choline are developed using two enzyme reactors: (1) acetylcholinesterase (AChE) encapsulated organically modified sol–gel glass, and (2) choline oxidase (ChO) immobilized within mediators (tetracyanoquinodimethane (TCNQ), tetrathiafulvalene (TTF), and dimethyl ferrocene (dmFc))-modified graphite paste electrodes. The AChE-immobilized into organically modified sol–gel glass behaves as the reactor for enzymatic hydrolysis of acetylthiocholine/acetylcholine into thiocholine/choline, whereas mediator- and ChO-modified paste electrodes are used for the detection of thiocholine/choline through mediated mechanism. The electrochemistry of AChE-generated thiocholine is studied at the mediator-modified electrodes in the presence and absence of ChO. It is observed that thiocholine undergoes both mediated and non-mediated oxidation in the absence of ChO as well as oxidation through enzyme-catalyzed mediated reactions. The results based on cyclic voltammetry on the oxidation of thiocholine at the surface of mediator-modified electrodes in the presence and absence of ChO are reported. In the presence of the ChO large anodic current is observed near the mediators redox potentials as compared to the anodic current in the absence of enzyme, which shows mediated bioelectrochemistry of thiocholine. The typical response curves for the detection of thiocholine/choline using mediators and ChO-modified electrodes below 0.24 V vs. Ag/AgCl in 0.1 M Tris–HCl buffer pH 8.0 are reported. Comparative analytical performance on the mediated electrochemical responses of the biosensors is discussed.

A New Glucose Biosensor Based on Sandwich Configuration of Organically Modified Sol-Gel Glass

A new glucose biosensor was developed based on the sandwich configuration of organically modified sol-gel glasses. The new sol-gel glass was developed using 3-aminopropyltrimethoxy silane and 2-(3,4-epoxycyclohexyl)-ethyltrimethoxy silane. Two types of sol-gel glasses were used to develop glucose biosensors that differ in absence (A) and the presence of graphite powder [particle size 1–2 μ] (B). An additional additive (polyethylene glycol, Mol. wt. 6000) was also incorporated in both types of the upper sol-gel glass layer. The new sol-gel matrix with immobilized glucose oxidase was analyzed by scanning electron microscopy (SEM).The sandwich configuration was developed using a bilayer of sol-gel glasses having a layer of glucose oxidase in between the bilayer. This electrode with special configuration was used to form a layer of sol-gel glass of ca. 0.2 mm thickness. The performance of sol-gel glasses (A & B) was analyzed based on cyclic voltammetry using ferrocene monocarboxylic acid. The results show a diffusion limited condition of ferrocene across the sol-gel matrix. The characterization of sol-gel glass based biosensor was recorded based on the cyclic voltammograms in absence and presence of glucose. The results show an increase in anodic current which is also characteristic of hydrogen peroxide oxidation in both cases (A & B). The responses of the sol-gel glasses based biosensors were analyzed based on chronoamperometric measurements. An amplified signal on the addition of the same concentrations of glucose was recorded with the B-type sol-gel glass electrode which was attributed to its relatively high porosity and better conductivity of the graphite loaded sol-gel glass. These observations were in accordance with the results on the diffusion of ferrocene and the magnitude of anodic current resulting from hydrogen peroxide oxidation. The calibration plots for glucose analysis using both type of sensors are reported. Data on the mediated electrochemical oxidation of glucose oxidase using soluble ferrocene were also reported based on cyclic voltammograms and amperometric measurement.

Studies on Glucose Biosensors Based on Nonmediated and Mediated Electrochemical Oxidation of Reduced Glucose Oxidase Encapsulated Within Organically Modified Sol-Gel Glasses

A new, organically modified sol-gel glass electrode is reported using 3-aminopropyltriethoxy silane and 2-(3,4-epoxycyclohexyl)-ethyltrimethoxy silane as sol-gel precursors for the construction of electrochemical biosensors. Four different systems of new sol-gel glass modified glucose electrodes are made in acidic medium having common sol-gel precursors and: 1) glucose oxidase, 2) glucose oxidase along with polyethylene glycol, 3) glucose oxidase and graphite powder, and 4) glucose oxidase along with polyethylene glycol and graphite powder. Both nonmediated and mediated electrochemical regeneration of immobilized glucose oxidase within sol-gel glasses are studied in these four systems. The nonmediated regeneration is achieved in the presence of oxygen as electron donor whereas mediated regeneration involves soluble ferrocene monocarboxylic acid as electron donor in each system. The electrochemical performance of sol-gel glass based biosensors is compared on the basis of cyclic voltammetry and amperometry. This leads to the observations: i) all four systems reach a diffusion limited condition associated with the transport of soluble ferrocene monocarboxylic acid as well as for dissolved oxygen within the sol-gel matrix, ii) the relative rate of diffusion of these analytes increases from system 1 to system 4, iii) both nonmediated and mediated amperometric responses at suitable potentials are based on the oxidation of H2O2 and enzymatically reduced soluble ferrocene with relatively amplified electrochemical signal of system 4. Data on the reduction of oxygen at conventional graphite disk electrode and at typical sol-gel glass modified electrode are reported.

A Novel Ferrocene-Encapsulated Palladium-Linked Ormosil-Based Electrocatalytic Biosensor. The Role of the Reactive Functional Group

A novel palladium-linked ormosil material with encapsulated ferrocene is reported along with its application in bioelectrocatalysis. The Pd-glycidoxypropyltrimethoxysilane is made by mixing an aqueous solution of palladium chloride and glycidoxypropyltrimethoxysilane. The linkage of palladium with glycidoxypropyltrimethoxysilane is confirmed by UV-vis, mass, and 13C spectroscopy. It is suggested that Pd is sandwiched between two molecules of glycidoxypropyltrimethoxysilane replacing oxygen. The new ormosil is made using Pd-linked silane precursor containing ferrocene monocarboxylic acid, trimethoxysilane and HCl. The formation of ormosil at two different temperatures (10 and 30 °C) is also studied, with the result that the ormosil formed at 10 °C does not show electrocatalysis of glucose oxidase whereas the ormosil made at 30 °C is found to be an efficient bioelectrocatalyst. The cyclic voltammetry results show peak separation of 57–59 mV of encapsulated ferrocene made at 30 °C and relatively large peak separation of the one made at 10 °C. The performance, stability, and reproducibility of the new ormosil based glucose biosensor are discussed. Another important investigation in support of the above outcome is reported showing the self-assembly of palladium on the reactive solid state ormosil surface. The reactive ormosil is developed using a mixture of trimethoxysilane and 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane in acidic medium.

Studies on Ferrocene Immobilized Sol‐Gel Glasses and Its Application in the Construction of a Novel Solid‐State Ion Sensor

Two new sol-gel glass systems with immobilized ferrocene are developed using two different types of sol-gel precursors. System (1) is developed using ferrocene carboxaldehyde and a mixture of two silanes (3-aminopropyltriethoxy silane and 2-(3,4-epoxycyclohexyl)-ethyltrimethoxy silane). System (2) is developed using ferrocene monocarboxylic acid and a mixture of 3-glycidoxypropyltrimethoxysilane and trimethoxysilane. The surfaces of the sol-gel glasses are analyzed based on scanning electron microscopy (SEM). The electrochemistry of ferrocene in system 1 and system 2 is characterized based on cyclic voltammetry. System 1 shows capacitive CV at slow scan rate whereas system 2 shows good reversible electrochemistry of ferrocene. The potentiometric response of the ferrocene immobilized sol-gel glass (system 1) is studied in 10 mM Tris-HCl buffer pH 7.0. A reproducible potential difference of system 1 to the order of –30 mV is recorded for 2 months with respect to a double junction SCE reference electrode. An ion sensing membrane is assembled over the ferrocene immobilized sol-gel glass using plasticized PVC matrix membrane containing dibenzo-18-crown-6. A typical potentiometric response of the ion sensor is reported. The response of the ion-sensor shows better response time, high reproducibility and relatively better slope for potassium analysis as compared to earlier reported solid-state K+ ion-sensor based on dibenzo-18-crown-6 neutral carrier. The reproducibility, detection limit and relative response of the ion sensor to Na+ and NH4+ ions are reported.

A New Solid‐State pH Sensor and Its Application in the Construction of all Solid‐State Urea Biosensor

A new solid-state pH sensor is developed using neutral poly(3-cyclohexyl thiophene) assembled over a Pt disk electrode. The new sensor is developed following two different approaches; 1) the neutral poly(3-cyclohexyl thiophene) dissolved in chloroform and subsequent coating on to a Pt disk electrode; 2) the neutral polymer is incorporated into plasticized poly(vinyl chloride) matrix membrane. In both cases the polymer modified electrode is sensitive to pH and a reversible super Nernstian behavior is observed. The typical response of the pH sensor and its reversibility are reported. The polymer coated electrode is subsequently used to construct an all solid-state urea sensor. The construction of this new urea sensor involves the following two major steps; a) 20 µL of urease solution (40 mg /mL) is allowed to assemble overnight at 4 °C over neutral poly (3-cyclohexyl thiophene) modified electrode; b) an organically modified sol-gel layer is allowed to form over the urease adsorbed polymer modified electrode. The new solid-state urea sensor provides excellent reproducibility of the measurements and is stable for 3 months when stored at 4 °C under dry condition. The typical response of the solid-state urea sensor and the calibration plot of urea analysis are reported.

Functionalized Ormosils-Based Biosensor Probing a Horseradish Peroxidase-Catalyzed Reaction

We report herein the electrochemistry of redox materials encapsulated within organically modified sol-gel glasses (ormosil). The ormosils that encapsulated ferrocene monocarboxylic acid and potassium ferricyanide are made using palladium-linked 3-glycidoxypropyltrimethoxysilane, trimethoxysilane, HCI, and an aqueous solution of potassium ferricyanide or ferrocene monocarboxylic acid followed by gelation of the same for 24 h at 30°C. The requirement of palladium linkage with ormosil is also examined and data on the electrochemistry of (i) only potassium-ferricyanide encapsulated ormosil without palladium, (ii) potassium-ferricyanide encapsulated ormosil with varying concentrations of palladium are reported. The ormosil made with optimum concentrations of palladium shows better redox electrochemistry as compared to that made without palladium. The ormosils are converted into fine powder followed by incorporation together with horseradish peroxidase within a graphite paste electrode. The peroxide biosensors based on modified-paste electrodes are characterized by cyclic voltammetry and chronoamperometry. Results on electrochemical sensing of hydrogen peroxide close to the cathodic peak potential of potassium ferricyanide and ferrocene monocarboxylic acid are reported. The performance of these peroxide biosensors is discussed and compared to those reported earlier.

Sensitivity, Selectivity and Reproducibility of Some Mediated Electrochemical Biosensors/sensors

Abstract Comparative studies on the mediated electrochemical response of tetracyanoquinodimethane (TCNQ), tetrahiafulvalene (TTF) and dimethyl ferrocene (dmFc) modified paste electrodes are reported based on cyclic voltammetry and amperometric measurements. The studies have been conducted on the graphite paste electrodes of different composition mainly; i) graphite and mediators (TCNQ/TTF/dmFc) paste only, ii) graphite, glucose oxidase and mediators (TCNQ/TTF/dmFc) modified paste, and iii) graphite, mediators (TCNQ/TTF/dmFc) and peroxidase modified paste. The results on the mediated electrochemical oxidation of NADH and glucose on the systems i–ii are reported. The cyclic voltammetry response of the systems i and ii for subsequent 15 cycles between -0.2 to 0.5 V Vs Ag/AgCl has been examined at a scan rate of 5 mV/s. The electrochemical oxidation of NADH has been studied on system i, whereas mediated electrochemical oxidation of glucose has been studied on system ii. The results suggest that TCNQ is a bett...