Co-immobilization of cholesterol oxidase and horseradish peroxidase in a sol–gel film

Buffer Effects in Zirconium-Based UiO Metal-Organic Frameworks (MOFs) That Influence Enzyme Immobilization and Catalytic Activity in Enzyme/MOF Biocatalysts

Novel biocatalysts that feature enzymes immobilized onto solid supports have recently become a major research focus in an effort to create more sustainable and greener chemistries in catalysis. Many of these novel biocatalyst systems feature enzymes immobilized onto metal-organic frameworks (MOFs), which have been shown to increase enzyme activity, stability, and recyclability in industrial processes. While the strategies used for immobilizing enzymes onto MOFs can vary, the conditions always require a buffer to maintain the functionality of the enzymes during immobilization. This report brings attention to critical buffer effects important to consider when developing enzyme/MOF biocatalysts, specifically for buffering systems containing phosphate ions. A comparative analysis of different enzyme/MOF biocatalysts featuring horseradish peroxidase and/or glucose oxidase immobilized onto the MOFs UiO-66, UiO-66-NH₂, and UiO-67 using a noncoordinate buffering system (MOPSO buffer) and a phosphate buffering system (PBS) show that phosphate ions can have an inhibitory effect. Previous studies utilizing phosphate buffers for enzyme immobilization onto MOFs have shown Fourier transform infrared (FT-IR) spectra that have been assigned stretching frequencies associated with enzymes after immobilization. Analyses and characterizations using zeta potential measurements, scanning electron microscopy, Brunauer-Emmett-Teller surface area, powder X-ray diffraction, Energy Dispersive X-ray Spectroscopy, and FT-IR show concerning differences in enzyme loading and activity based on the buffering system used during immobilization.

The Peroxidase-like Nanocomposites as Hydrogen Peroxide-Sensitive Elements in Cholesterol Oxidase-Based Biosensors for Cholesterol Assay

Catalytically active nanomaterials, in particular, nanozymes, are promising candidates for applications in biosensors due to their excellent catalytic activity, stability and cost-effective preparation. Nanozymes with peroxidase-like activities are prospective candidates for applications in biosensors. The purpose of the current work is to develop cholesterol oxidase-based amperometric bionanosensors using novel nanocomposites as peroxidase (HRP) mimetics. To select the most electroactive chemosensor on hydrogen peroxide, a wide range of nanomaterials were synthesized and characterized using cyclic voltammetry (CV) and chronoamperometry. Pt NPs were deposited on the surface of a glassy carbon electrode (GCE) in order to improve the conductivity and sensitivity of the nanocomposites. The most HRP-like active bi-metallic CuFe nanoparticles (nCuFe) were placed on a previously nano-platinized electrode, followed by conjugation of cholesterol oxidase (ChOx) in a cross-linking film formed by cysteamine and glutaraldehyde. The constructed nanostructured bioelectrode ChOx/nCuFe/nPt/GCE was characterized by CV and chronoamperometry in the presence of cholesterol. The bionanosensor (ChOx/nCuFe/nPt/GCE) shows a high sensitivity (3960 A·M^-1·m^-2) for cholesterol, a wide linear range (2-50 µM) and good storage stability at a low working potential (-0.25 V vs. Ag/AgCl/3 M KCl). The constructed bionanosensor was tested on a real serum sample. A detailed comparative analysis of the bioanalytical characteristics of the developed cholesterol bionanosensor and the known analogs is presented.

Immobilization of a Bienzymatic System via Crosslinking to a Metal‐Organic Framework

A leading biotechnological advancement in the field of biocatalysis is the immobilization of enzymes on solid supports to create more stable and recyclable systems. Metal-organic frameworks (MOFs) are porous materials that have been explored as solid supports for enzyme immobilization. Composed of organic linkers and inorganic nodes, MOFs feature empty void space with large surface areas and have the ability to be modified post-synthesis. Our target enzyme system for immobilization is glucose oxidase (GOx) and chloroperoxidase (CPO). Glucose oxidase catalyzes the oxidation of glucose and is used for many applications in biosensing, biofuel cells, and food production. Chloroperoxidase is a fungal heme enzyme that catalyzes peroxide-dependent halogenation, oxidation, and hydroxylation. These two enzymes work sequentially in this enzyme system by GOx producing peroxide, which activates CPO that reacts with a suitable substrate. This study focuses on using a zirconium-based MOF, UiO-66-NH2, to immobilize the enzyme system via crosslinking with the MOF’s amine group on the surface of the MOF. This study investigates two different crosslinkers: disuccinimidyl glutarate (DSG) and 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC)/N-hydroxysuccinidimide (NHS), providing stable crosslinking of the MOF to the enzymes. The two crosslinkers are used to covalently bond CPO and GOx onto UiO-66-NH2, and a comparison of the recyclability and enzymatic activity of the single immobilization of CPO and the doubly immobilized CPO and GOx is discussed through assays and characterization analyses. The DSG-crosslinked composites displayed enhanced activity relative to the free enzyme, and all crosslinked enzyme/MOF composites demonstrated recyclability, with at least 30% of the activity being retained after four catalytic cycles. The results of this report will aid researchers in utilizing CPO as a biocatalyst that is more active and has greater recyclability.

Optical enzymatic formaldehyde biosensor based on alcohol oxidase and pH-sensitive methacrylic-acrylic optode membrane

Optical biosensor for the detection of formaldehyde has been developed based on the transparent enzymatic stacked membranes system on the glass substrate, and employing optical absorption transducer with H⁺ ion-selective Nile Blue chromoionophore (NBCM) dye-doped methacrylic acrylic (MB28) copolymer membrane as the optode membrane. Alcohol oxidase (AOx) enzymes were entrapped within the biocompatible sol-gel matrix and deposited on top of the pH optode membrane. As the uppermost catalytic membrane catalyzes the oxidative conversion of formaldehyde to formic acid and hydrogen peroxide, the immobilized NBCM undergoes protonation reaction and forms HNBCM⁺, the dark blue ion-chromoionophore complex via H⁺ ion transfer reaction within the soft and flexible MB28 polymeric membrane. This rendered the enzymatic optode membrane absorbed a high yellow light intensity from the light source and exhibited maximum absorption peaks at 610 and 660 nm. Optical evaluation of formaldehyde by means on UV-vis absorption transduction of the enzymatic stacked membranes demonstrated rapid response time of 10 min with high sensitivity, good linearity and high reproducibility across a wide formaldehyde concentration range of 1 × 10^-3-1 × 10³ mM (R² = 0.9913), and limit of detection (LOD) at 1 × 10^-3 mM, which could be useful for formaldehyde assay in industrial, agricultural, environmental, food and beverages as well as medical samples. The formaldehyde concentration in snapper fish, pomfret fish and threadfin fish samples determined by the proposed optical enzymatic biosensor were very much close to the formaldehyde concentration values determined by the UV-vis spectrophotometric NASH standard method based on the statistical t-test. This suggests that the optical biosensor can be used as a reliable method for quantitative determination of formaldehyde levels in food samples.

Electrochemical Biosensors Employing Natural and Artificial Heme Peroxidases on Semiconductors

Heme peroxidases are widely used as biological recognition elements in electrochemical biosensors for hydrogen peroxide and phenolic compounds. Various nature-derived and fully synthetic heme peroxidase mimics have been designed and their potential for replacing the natural enzymes in biosensors has been investigated. The use of semiconducting materials as transducers can thereby offer new opportunities with respect to catalyst immobilization, reaction stimulation, or read-out. This review focuses on approaches for the construction of electrochemical biosensors employing natural heme peroxidases as well as various mimics immobilized on semiconducting electrode surfaces. It will outline important advances made so far as well as the novel applications resulting thereof.

Immobilization of Cholesterol Oxidase: An Overview

Background:

Cholesterol oxidases are bacterial oxidases widely used commercially for their application in the detection of cholesterol in blood serum, clinical or food samples. Additionally, these enzymes find potential applications as an insecticide, synthesis of anti-fungal antibiotics and a biocatalyst to transform a number of sterol and non-sterol compounds. However, the soluble form of cholesterol oxidases are found to be less stable when applied at higher temperatures, broader pH range, and incur higher costs. These disadvantages can be overcome by immobilization on carrier matrices.

Methods:

This review focuses on the immobilization of cholesterol oxidases on various macro/micro matrices as well as nanoparticles and their potential applications. Selection of appropriate support matrix in enzyme immobilization is of extreme importance. Recently, nanomaterials have been used as a matrix for immobilization of enzyme due to their large surface area and small size. The bio-compatible length scales and surface chemistry of nanoparticles provide reusability, stability and enhanced performance characteristics for the enzyme-nanoconjugates.

Conclusion:

In this review, immobilization of cholesterol oxidase on nanomaterials and other matrices are discussed. Immobilization on nanomatrices has been observed to increase the stability and activity of enzymes. This enhances the applicability of cholesterol oxidases for various industrial and clinical applications such as in biosensors.

Cholesterol-oxidase-magnetic nanobioconjugates for the production of 4-cholesten-3-one and 4-cholesten-3, 7-dione

Cholesterol oxidase(ChOx) enzyme isolated from Pseudomonas aeruginosa PseA(ChOxP) and Rhodococcus erythropolis MTCC 3951(ChOxR) strains as well as a commercial variant produced by Streptomyces sp.(ChOxS) were immobilized on silane modified iron(II, III)oxide magnetic nanoparticles(MNP) by covalent coupling methods. The nanobiocatalysts in case of ChOxP, ChOxR and ChOxS, retained 71, 91 and 86% of cholesterol oxidase activity respectively, as compared to their soluble counterparts. The catalytic efficiency of the immobilized enzymes on nanoparticles was more than 2.0 times higher than the free enzyme. They also showed enhanced pH and thermal stability. After 10 cycles of operation, the MNP-bioconjugates retained 50, 52 and 51% of residual activity in case of ChOxP, ChOxR and ChOxS respectively. The presence of enzyme on nanoparticles was confirmed by FTIR, SEM and TEM. The nanobiocatalysts were used for the biotransformation of cholesterol and 7-ketocholesterol to 4-cholesten-3-one and 4-cholesten-3, 7-dione respectively, which are industrially and medically important steroid precursors.

Zinc Oxide-Based Self-Powered Potentiometric Chemical Sensors for Biomolecules and Metal Ions

Advances in the miniaturization and portability of the chemical sensing devices have always been hindered by the external power supply problem, which has focused new interest in the fabrication of self-powered sensing devices for disease diagnosis and the monitoring of analytes. This review describes the fabrication of ZnO nanomaterial-based sensors synthesized on different conducting substrates for extracellular detection, and the use of a sharp borosilicate glass capillary (diameter, d = 700 nm) to grow ZnO nanostructures for intracellular detection purposes in individual human and frog cells. The electrocatalytic activity and fast electron transfer properties of the ZnO materials provide the necessary energy to operate as well as a quick sensing device output response, where the role of the nanomorphology utilized for the fabrication of the sensor is crucial for the production of the operational energy. Simplicity, design, cost, sensitivity, selectivity and a quick and stable response are the most important features of a reliable sensor for routine applications. The review details the extra- and intra-cellular applications of the biosensors for the detection and monitoring of different metallic ions present in biological matrices, along with the biomolecules glucose and cholesterol.

One-step electrochemical detection of cholesterol in the presence of suitable K₃Fe(CN)₆/phosphate buffer mediator by an electrochemical approach

One-step approach of cholesterol biosensor was fabricated onto smart micro-chips based on cholesterol oxidase (ChOx) co-immobilized thioglycolic acid self-assembled monolayer (TGA-SAM) for biomedical applications. The selective cholesterol biosensor was investigated with modified tiny micro-chip (Au/SAM/ChOx) by the facile and reliable cyclic voltammetric (CV) method in a K3Fe(CN)6/phosphate buffer (PB) system. The modified micro-chip displayed a large dynamic range (1.0 nmol L(-1) to 1.0 mmol L(-1)), lower detection limit (~0.49 nmol L(-1), based on S/N~3), higher sensitivity (~93.75 µA µmol L(-2) cm(-2)), good linearity (correlation coefficient r(2), 0.9995), lower sample volume (<50.0 μL), and good stability as well as reproducibility. The Au/TGA system was implemented for a facile and simple approach to the immobilization of ChOx onto micro-chip, which can offer analytical access to a large group of enzymes for a wide range of bio-molecule applications in health-care and biomedical fields. This integrated microchip provides a promising low-cost platform for the sensitive and rapid detection of biomolecules using miniatured samples.

Reusable and mediator-free cholesterol biosensor based on cholesterol oxidase immobilized onto TGA-SAM modified smart bio-chips

A reusable and mediator-free cholesterol biosensor based on cholesterol oxidase (ChOx) was fabricated based on self-assembled monolayer (SAM) of thioglycolic acid (TGA) (covalent enzyme immobilization by dropping method) using bio-chips. Cholesterol was detected with modified bio-chip (Gold/Thioglycolic-acid/Cholesterol-oxidase i.e., Au/TGA/ChOx) by reliable cyclic voltammetric (CV) technique at room conditions. The Au/TGA/ChOx modified bio-chip sensor demonstrates good linearity (1.0 nM to 1.0 mM; R = 0.9935), low-detection limit (∼0.42 nM, SNR∼3), and higher sensitivity (∼74.3 µAµM⁻¹cm⁻²), lowest-small sample volume (50.0 μL), good stability, and reproducibility. To the best of our knowledge, this is the first statement with a very high sensitivity, low-detection limit, and low-sample volumes are required for cholesterol biosensor using Au/TGA/ChOx-chips assembly. The result of this facile approach was investigated for the biomedical applications for real samples at room conditions with significant assembly (Au/TGA/ChOx) towards the development of selected cholesterol biosensors, which can offer analytical access to a large group of enzymes for wide range of biomedical applications in health-care fields.

An InN/InGaN quantum dot electrochemical biosensor for clinical diagnosis

Low-dimensional InN/InGaN quantum dots (QDs) are demonstrated for realizing highly sensitive and efficient potentiometric biosensors owing to their unique electronic properties. The InN QDs are biochemically functionalized. The fabricated biosensor exhibits high sensitivity of 97 mV/decade with fast output response within two seconds for the detection of cholesterol in the logarithmic concentration range of 1 × 10⁻⁶ M to 1 × 10⁻³ M. The selectivity and reusability of the biosensor are excellent and it shows negligible response to common interferents such as uric acid and ascorbic acid. We also compare the biosensing properties of the InN QDs with those of an InN thin film having the same surface properties, i.e., high density of surface donor states, but different morphology and electronic properties. The sensitivity of the InN QDs-based biosensor is twice that of the InN thin film-based biosensor, the EMF is three times larger, and the response time is five times shorter. A bare InGaN layer does not produce a stable response. Hence, the superior biosensing properties of the InN QDs are governed by their unique surface properties together with the zero-dimensional electronic properties. Altogether, the InN QDs-based biosensor reveals great potential for clinical diagnosis applications.

Template-directed hierarchical self-assembly of graphene based hybrid structure for electrochemical biosensing

A template-directed self-assembly approach, using functionalised graphene as a fundamental building block to obtain a hierarchically ordered graphene-enzyme-nanoparticle bioelectrode for electrochemical biosensing, is reported. An anionic surfactant was used to prepare a responsive, functional interface and direct the assembly on the surface of the graphene template. The surfactant molecules altered the electrostatic charges of graphene, thereby providing a convenient template-directed assembly approach to a free-standing planar sheet of sp(2) carbons. Cholesterol oxidase and cholesterol esterase were assembled on the surface of graphene by intermolecular attractive forces while gold nanoparticles are incorporated into the hetero-assembly to enhance the electro-bio-catalytic activity. Hydrogen peroxide and cholesterol were used as two representative analytes to demonstrate the electrochemical sensing performance of the graphene-based hybrid structure. The bioelectrode exhibited a linear response to H2O2 from 0.01 to 14 mM, with a detection limit of 25 nM (S/N=3). The amperometric response with cholesterol had a linear range from 0.05 to 0.35 mM, sensitivity of 3.14 µA/µM/cm(2) and a detection limit of 0.05 µM. The apparent Michaelis-Menten constant (Km(app)) was calculated to be 1.22 mM. This promising approach provides a novel methodology for template-directed bio-self-assembly over planar sp(2) carbons of a graphene sheet and furnishes the basis for fabrication of ultra-sensitive and efficient electrochemical biosensors.

Medium optimization by orthogonal array and response surface methodology for cholesterol oxidase production by Streptomyces lavendulae NCIM 2499

This paper deals with the optimization of culture conditions for the production of cholesterol oxidase (COD) by Streptomyces lavendulae NCIM 2499 using the one-factor-at-a-time method, orthogonal array method and response surface methodology (RSM) approaches. The one-factor-at-a-time method was adopted to investigate the effects of medium components (i.e. carbon and nitrogen) and environmental factors (i.e. initial pH) on biomass growth and COD production. Subsequently, an L12 orthogonal matrix was used to evaluate the significance of glycerol, soyabean meal, malt extract, K2HPO4, MgSO4 and NaCl. The effects of media components were ranked according to their effects on the production of COD as malt extract > soyabean meal > K2HPO4 > NaCl > MgSO4 > glycerol. The subsequent optimization of the four most significant factors viz. malt extract, soyabean meal, K2HPO4 and NaCl, was carried out by employing a central composite rotatable design (CCRD) of RSM. There was a 2.48-fold increase in productivity of COD as compared to the unoptimized media by using these statistical approaches.

Wireless biosensor system for real-time cholesterol monitoring in fish "Nile tilapia"

The rapidly increasing demand for cultured fish as a food resource requires simple, effective methods for controlling fish health in culture conditions. Plasma total cholesterol levels are significantly related to fish mortality following bacterial challenge, and are thus a good indicator of the general health of fish. We developed a wireless biosensor system to continuously monitor the total cholesterol concentration in fish (Nile tilapia, Oreochromis niloticus). The biosensor was constructed with Pt-Ir wire (phi0.178 mm) as the working electrode and Ag/AgCl paste as the reference electrode. Cholesterol oxidase and cholesterol esterase were immobilized on the working electrode using glutaraldehyde. The sensor output was linear and strongly correlated with the cholesterol level (R=0.9970) in the range of 2.65-403 mg dl(-1). This range covers the range of total cholesterol levels in fish. To avoid blood coagulation and proteins coalescing on the sensor, we implanted the sensor in the fluid under the scleral surface of the eyeball (EISF). The EISF is presumed to reflect the levels of most blood components and does not include the substances contained in blood that inhibit sensor measurement. Total cholesterol concentrations in blood and EISF were strongly correlated (R=0.8818, n=72) in the blood total cholesterol range of 74-480 mg dl(-1). Therefore, we used EISF as an alternative to blood and performed continuous in vivo-monitoring of the total cholesterol concentration in fish. We also investigated the application of the calibration method and wireless monitoring system. These applications enabled us to securely monitor total cholesterol levels in free-swimming fish in an aquarium for over 40 h. Thus, our newly developed sensor provided a rapid and convenient method for real-time monitoring of total cholesterol concentrations in free-swimming fish.

Fabricating an Amperometric Cholesterol Biosensor by a Covalent Linkage between Poly(3-thiopheneacetic acid) and Cholesterol Oxidase

In this study, use of the covalent enzyme immobilization method was proposed to attach cholesterol oxidase (ChO) on a conducting polymer, poly(3-thiopheneacetic acid), [poly(3-TPAA)]. Three red-orange poly(3-TPAA) films, named electrodes A, B and C, were electropolymerized on a platinum electrode by applying a constant current of 1.5 mA, for 5, 20 and 100 s, respectively. Further, 1-ethyl-3-(3-dimethylamiopropyl)carbodiimide hydrochloride (EDC · HCl) and N-hydroxysuccinimide (NHS) were used to activate the free carboxylic groups of the conducting polymer. Afterwards, the amino groups of the cholesterol oxidase were linked on the activated groups to form peptide bonds. The best sensitivity obtained for electrode B is 4.49 mA M(-1) cm(-2), with a linear concentration ranging from 0 to 8 mM, which is suitable for the analysis of cholesterol in humans. The response time (t(95)) is between 70 and 90 s and the limit of detection is 0.42 mM, based on the signal to noise ratio equal to 3. The interference of species such as ascorbic acid and uric acid increased to 5.2 and 10.3% of the original current response, respectively, based on the current response of cholesterol (100%). With respect to the long-term stability, the sensing response retains 88% of the original current after 13 days.

Covalent attachment of cholesterol oxidase and horseradish peroxidase on perlite through silanization: Activity, stability and co-immobilization

In the present work, co-immobilization of cholesterol oxidase (COD) and horseradish peroxidase (POD) on perlite surface was attempted. The surface of perlite were activated by 3-aminopropyltriethoxysilane and covalently bonded with COD and POD via glutaraldehyde. Enzymes activities have been assayed by spectrophotometric technique. The stabilities of immobilized COD and POD to pH were higher than those of soluble enzymes and immobilization shifted optimum pH of enzymes to the lower pH. Heat inactivation studies showed improved thermostability of the immobilized COD for more than two times, but immobilized POD was less thermostable than soluble POD. Also activity recovery of immobilized COD was about 50% since for immobilized POD was 11%. The K(m) of immobilized enzymes was found slightly lower than that of soluble enzymes. Immobilized COD showed inhibition in its activity at high cholesterol concentration which was not reported for soluble COD before. Co-immobilized enzymes retained 65% of its initial activity after 20 consecutive reactor batch cycles.

Entrapment of horseradish peroxidase in sugar-modified silica monoliths: Toward the development of a biocatalytic sensor

A miniaturized HRP-entrapped bioreactor was prepared by a one-step enzyme immobilization method using a biocompatible sol-gel processing method employing either diglycerylsilane (DGS) or sodium silicate (SS) as precursors and a covalently tethered sugar, N-(3-triethoxysilylpropyl)gluconamide (GLS) as a silica modifier. Factors such as leaching, catalytic efficiency and long-term stability were examined to assess the role of the precursor and modifier in influencing enzyme performance. The results showed that sodium silicate derived materials modified with covalently bound sugars at a level of 10 mol% were optically transparent and provided the highest catalytic turnover rate for entrapped HRP. The stability and reusability of the entrapped HRP was found to be satisfactory for at least 1 month in the GLS-doped SS materials, and the entrapped HRP was able to respond linearly to the presence of peroxide over the concentration range of 0-750 microM with a detection limit of 6 microM, demonstrating the potential of this material for the development of a reusable optical biosensor.

Cholesterol biosensor based on N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane self-assembled monolayer

Cholesterol oxidase (ChOx) has been covalently immobilized onto two-dimensional self-assembled monolayer (SAM) of N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (AEAPTS) deposited on the indium-tin oxide (ITO) coated glass plates using N-ethyl-N'-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide (EDC/NHS) chemistry. These ChO x/AEAPTS/ITO bioelectrodes are characterized using contact angle (CA) measurements, UV-visible spectroscopy, atomic force microscopy (AFM), electrochemical impedance technique, and Fourier transform infrared (FT-IR) technique. The covalently immobilized ChOx-modified AEAPTS bioelectrodes are used for the estimation of cholesterol in solution using UV-visible technique. These cholesterol sensing bioelectrodes show linearity as 50 to 500 mg/dl for cholesterol solution, detection limit as 25mg/dl, sensitivity as 4.499 x 10(-5) Abs (mg/dl)(-1), K(m) value as 58.137 mg/dl (1.5mM), apparent enzyme activity as 1.81 x 10(-3) U cm(-2), shelf life of approximately 10 weeks, and electrode reusability as 10 times.

Immobilization of cholesterol esterase and cholesterol oxidase onto sol–gel films for application to cholesterol biosensor

Cholesterol oxidase (ChOx) and cholesterol esterase (ChEt) have been covalently immobilized onto tetraethylorthosilicate (TEOS) sol-gel films. The tetraethylorthosilicate sol-gel/ChEt/ChOx enzyme films thus prepared have been characterized using scanning electron microscopic (SEM), UV-vis spectroscopic, Fourier-transform-infrared (FTIR) spectroscopic and amperometric techniques, respectively. The results of photometric measurements carried out on tetraethylorthosilicate sol-gel/ChEt/ChOx reveal thermal stability up to 55 degrees C, response time as 180 s, linearity up to 780 mg dL(-1) (12 mM), shelf life of 1 month, detection limit of 12 mg dL(-1) and sensitivity as 5.4 x 10(-5) Abs. mg(-1) dL(-1).

Selective sample preparation with bioaffinity columns prepared by the sol–gel method

Bioaffinity materials prepared by entrapping highly selective bioligands in the pores of a sol-gel glass offer unique advantages for the clean-up of complex sample matrices. The inclusion of sol-gel bioaffinity columns frequently allows the efficient elimination of interfering matrix components and enrichment of analytes in a simple step. After introducing the basic principles the paper reviews the methods for the production of protein-doped sol-gel materials and discusses the characteristics of sol-gel affinity columns by comparing their stability, selectivity, binding capacity and reusability. The potential of sample clean-up with sol-gel affinity columns is demonstrated giving applications in environmental, food and clinical analysis.

Platinum nanoparticles-doped sol–gel/carbon nanotubes composite electrochemical sensors and biosensors

Platinum nanoparticle-doped sol-gel solution is prepared and used as a binder for multi-walled carbon nanotubes (CNT) for the fabrication of electrochemical sensors. Amine group containing sol-gel solution is selected to utilize the affinity of -NH(2) groups toward metal nanoparticles for stabilization the nanoparticles in solution. The resulting CNT-silicate material brings new capabilities for electrochemical devices by using the synergistic action of the electrocatalytic activity of Pt nanoparticles and CNT. The combined electrocatalytic activity permits low-potential detection of hydrogen peroxide with remarkably improved sensitivity. With the incorporation of glucose oxidase within the Pt-CNT-silicate matrix, a Pt-CNT paste-based biosensor has been constructed that responds more sensitively to glucose than CNT-based biosensor. The influences of the composite of the sol-gel solution, the quantity of the solution and the Pt nanoparticles loading are examined. In pH 6.98 phosphate buffer, almost interference free determination of glucose is realized at 0.1 V versus SCE with a linear range from 1 to 25 mM, a response time <15s, and the sensitivity is 0.98 microA mM(-1)cm(-2). The sensitivity of the Pt-CNT paste-based biosensor is almost four times larger than that of the CNT-based biosensor (0.27 microA mM(-1)cm(-2) at 0.1 V). The improved electrocatalytic activity and surface renewability made the Pt-CNT-silicate system a potential platform to immobilize different enzymes for other bioelectrochemical applications.