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A novel poly(styrene‐‐ε‐caprolactone)/multiwalled carbon nanotubes/cholesterol oxidase film‐coated glassy carbon electrode was designed for cholesterol detection by square wave voltammetry (SWV). The biosensor responded to cholesterol with a measurement concentration range between 1 and 130 μM, a relative standard deviation of only 0.095% and accuracy of 100.42% ±2.85 with the SWV technique in the potential range from −0.6 to +0.6 V. The limit of detection was calculated to be 0.63 μM. The biosensor was preserved 91 and 84% of its initial response at the end of the 9st and 25st days, respectively. Human serum from human male AB plasma was analyzed without pretreatment except for dilution to investigate the performance of the biosensor in a complex medium. [ABSTRACT FROM AUTHOR]

The present study describes the effect of hydrolysis of polyethyleneimines in water/ethanol mixture on the morphology of the cobalt oxide (Co3O4), used as the main sensor component. The structure of the generated Co3O4 nanocrystals is consistent with a well-defined cubic phase crystallography, having only cobalt and oxygen elements. Developing simple, low-cost, sensitive, and selective cholesterol biosensors is essential for accurate monitoring of cholesterol to avoid cardiovascular diseases. These nanocrystals exhibit large surfaces suitable for facile and high loading of cholesterol oxidase enzyme through the physical adsorption method. Then, the fabricated cholesterol oxidase/ Co3O4 nanocrystals composite was implemented for potentiometric detection of cholesterol in 10 mM phosphate buffer of pH 7.3. Importantly, the presented cholesterol biosensor revealed a wide linear range of 0.005 mM to 3.0 mM with a limit of detection (LOD) of 0.001 mM. Additionally, the sensitivity of biosensor was estimated around 60 mVdec−1. The selectivity, stability, reproducibility, and repeatability were also observed as satisfactory. The dynamic response of the proposed method demonstrated a fast response time of less than 1 s. Furthermore, the successive addition method confirmed a remarkably stable response towards various cholesterol concentrations. Thus, the developed cholesterol oxidase/ Co3O4 nanocomposite may be used as an efficient alternative method to monitor low cholesterol concentrations form real samples. [ABSTRACT FROM AUTHOR]

Synthesis and functionalization of magnetite nanoparticles (Fe3O4) was achieved with the view to covalently bind both cholesterol oxidase and cholesterol esterase biorecognition agents for the development of free and total cholesterol biosensors. Prior to enzyme attachment, Fe3O4 was functionalized with 3-aminopropyltriethoxysilane (APTES) and polyamidoamine (PAMAM) dendrimer. Characterization of the material was performed by FT-IR and UV spectroscopy, SEM/EDX surface analysis and electrochemical investigations. The response to cholesterol and its palmitate ester was examined using cyclic voltammetry. Optimum analytical performance for the free cholesterol biosensor was obtained using APTES-functionalized magnetite with a sensitivity of 101.9 μA mM−1 cm−2, linear range 0.1–1 mM and LOD of 80 μM when operated at 37 °C. In the case of the total cholesterol biosensor, the best analytical performance was obtained using PAMAM dendrimer-modified magnetite with sensitivity of 73.88 μA mM−1 cm−2 and linear range 0.1–1.5 mM, with LOD of 90 μM. A stability study indicated that the free cholesterol biosensors retained average activity of 98% after 25 days while the total cholesterol biosensors retained 85% activity upon storage over the same period. [ABSTRACT FROM AUTHOR]

Abstract A facile one-step electrochemical synthesis of a platinum/reduced graphene oxide/poly(3-aminobenzoic acid) (Pt/rGO/P3ABA) nanocomposite film on a screen-printed carbon electrode (SPCE) and its application in the development of sensitive amperometric biosensors was successfully demonstrated herein. The electropolymerization of P3ABA together with co-electrodeposition of rGO and Pt was conducted by cyclic voltammetry, as was the GO reduction to rGO. A Pt/rGO/P3ABA-modified SPCE exhibited excellent electrocatalytic oxidation towards hydrogen peroxide (H 2 O 2) and can be employed as an electrochemical platform for the immobilization of glucose oxidase (GOx) and cholesterol oxidase (ChOx) to fabricate glucose and cholesterol biosensors, respectively. Under the optimized conditions at a working potential of +0.50 V, the proposed biosensors revealed excellent linear responses to glucose and cholesterol in the concentration ranges of 0.25–6.00 mM and 0.25–4.00 mM, respectively, with high sensitivities of 22.01 and 15.94 μA mM−1 cm−2 and low detection limits (LODs) of 44.3 and 40.5 μM. Additionally, the Michaelis-Menten constant (K m) of GOx was 3.54 mM, while the K m of ChOx was 3.82 mM. Both biosensors displayed a good anti-interference ability and clearly exhibited acceptable recoveries for the detection of glucose and cholesterol in a human serum sample (98.2–104.1%). Graphical abstract Unlabelled Image Highlights • Deposition of platinum/reduced graphene oxide/poly(3-aminobenzoic acid) was achieved. • The nanocomposite provided a versatile sensing platform. • A nanocomposite electrode was constructed as glucose and cholesterol biosensors. • This system enabled glucose and cholesterol assay at biological levels. [ABSTRACT FROM AUTHOR]

Highlights • Nanofibers for simple colorimetric detection of cholesterol. • Chitosan nanofibers as a biocompatible support for cholesterol oxidase. • Colour gradient scale for detecting varying cholesterol concentrations. Abstract Recent developments in biosensor-related research have provided economic and highly sensitive biosensors for numerous biological analytes. Several strategies have evolved for developing a sensitive biosensor for cholesterol detection. In this study, we have developed a point-of-care, chitosan nanofiber-based cholesterol biosensor, involving colorimetric detection of the analyte. Chitosan nanofibers, fabricated using electrospinning, were utilized for immobilizing cholesterol oxidase and peroxidase enzymes. A uniform and bead-free chitosan nanofibers were obtained using chitosan and polyvinyl alcohol (Cs: PVA), at the ratio of 0.7:1 w/w, in a solvent system containing 10%v/v of methanol in 63% acetic acid. The nanofibers were characterized using scanning electron microscopy (SEM), which revealed the formation of smooth fibers, with an average diameter of 60–90 nm. The fabricated nanofibers offered a greater surface area for immobilizing high quantities of enzymes and demonstrated the potential to be developed into a strip-based intervention, based on a simple, visual detection system. A colorimetric detection method was developed using the chromogenic substrate, 3,3́,5,5́-tetramethylbenzidine hydrochloride. Analysis by UV–vis spectrophotometry demonstrated a linear increase in the absorbance, with increasing concentrations of cholesterol. The intensity of color change, as a function of cholesterol concentration was used for developing a color gradient scale for the investigated biosensor. [ABSTRACT FROM AUTHOR]

In this study, an amperometric cholesterol biosensor was constructed based on cholesterol oxidase immobilized on a conducting 4‐(4H‐dithienol[3,2‐b : 2′,3′‐d]pyrrole‐4)aniline polymer, (DTP(aryl)aniline). Glassy carbon electrodes were covered with P(DTP(aryl)aniline) which is used for the wiring of enzyme to the electrode surface by using electro‐polymerization. The electron transfer was successfully made by the bio‐catalytic activity and possession of the unique morphology of the polymer allowed efficient immobilization of the cholesterol oxidase enzyme. Analytical performances; linear range, detection limit, limit of quantification and the Michaelis‐Menten constant (Km) of biosensor electrodes were obtained 2.0 μM–23.7 μM, 0.27 μM, 0.82 μM, 17,81 μM respectively. Biosensor optimization parameters: optimum pH, optimum temperature, stability test and response time were evaluated. The real sample and recovery studies were also performed in order to show applicability of the biosensing electrodes. [ABSTRACT FROM AUTHOR]

Cholesterol is an essential ingredient in mammals, and serum cholesterol is a major component of atherosclerotic plaques. The level of cholesterol in human serum has become an important index for clinical diagnosis and prevention of cardiovascular disease. In this paper, a simple and ultrasensitive cholesterol biosensor based on graphene oxide (GO) and gold nanoparticles (Au NPs) co-mediated enzymatic silver deposition was designed by immobilizing cholesterol oxidase (CHOD), cholesterol esterase (CHER) and GO onto the surface of Au NPs modified screen-printed carbon electrode (SPE). Under the synergistic effect of CHER, CHOD and GO, the cholesterol was hydrolyzed to generate hydrogen peroxide, which can reduce the silver (Ag) ions in the solution to metallic Ag which deposited on the surface of Au NPs modified SPE. The ultrasensitive detection of cholesterol was achieved by anodic stripping voltammetry measurement of the enzymatically deposited Ag. Under optimal conditions, the anodic stripping peak current of Ag increased with the increasing cholesterol concentration in the range from 0.01 μg/mL to 5000 μg/mL with a limit of detection of 0.001 μg/mL (S/N = 3). In addition, the ultrasensitive cholesterol biosensor exhibited higher specificity, acceptable reproducibility and excellent recoveries for cholesterol detection. [ABSTRACT FROM AUTHOR]

Herein, we demonstrate the exfoliation of bulk graphitic carbon nitrides (g-C 3 N 4 ) into ultra-thin (~3.4 nm) two-dimensional (2D) nanosheets and their functionalization with proton (g-C 3 N 4 H + ). The layered semiconductor g-C 3 N 4 H + nanosheets were doped with cylindrical spongy shaped polypyrrole (CSPPy-g-C 3 N 4 H + ) using chemical polymerization method. The as-prepared nanohybrid composite was utilized to fabricate cholesterol biosensors after immobilization of cholesterol oxidase (ChOx) at physiological pH. Large specific surface area and positive charge nature of CSPPy-g-C 3 N 4 H + composite has tendency to generate strong electrostatic attraction with negatively charged ChOx, and as a result they formed stable bionanohybrid composite with high enzyme loading. A detailed electrochemical characterization of as-fabricated biosensor electrode (ChOx-CSPPy-g-C 3 N 4 H + /GCE) exhibited high-sensitivity (645.7 µAmM −1 cm −2 ) in wide-linear range of 0.02–5.0 mM, low detection limit (8.0 μM), fast response time (~3 s), long-term stability, and good selectivity during cholesterol detection. To the best of our knowledge, this novel nanocomposite was utilized for the first time for cholesterol biosensor fabrication that resulted in high sensing performance. Hence, this approach opens a new prospective to utilize CSPPy-g-C 3 N 4 H + composite as cost-effective, biocompatible, eco-friendly, and superior electrocatalytic as well as electroconductive having great application potentials that could pave the ways to explore many other new sensors fabrication and biomedical applications. [ABSTRACT FROM AUTHOR]

Cholesterol is one of the essential structural constituents of cell membranes. Determination of cholesterol is of great importance in clinical analysis because the level of cholesterol in serum is an indicator in the diagnosis and prevention of heart diseases. In this work, a simple and ultrasensitive cholesterol biosensor based on enzymatic silver deposition was designed by immobilizing cholesterol oxidase (CHOD) and cholesterol esterase (CHER) onto the surface of gold nanoparticles (Au NPs) modified screen-printed carbon electrode (SPE). By the catalytic action of CHER and CHOD, the cholesterol was hydrolyzed to generate hydrogen peroxide (H 2 O 2 ) which can reduced the silver (Ag) ions in the solution for the deposition of metallic Ag on the surface of Au NPs modified SPE. The ultrasensitive detection of cholesterol was achieved by anodic stripping voltammetry (ASV) measurement of the enzymatically deposited Ag. The influence of relevant experimental variables was optimized. The anodic stripping peak current of Ag depended linearly on the concentration of cholesterol in the range of 5–5000 μg/mL with the regression correlation coefficient of 0.9983. A detection limit of 3.0 μg/mL was attained by 3 sigma-rule. In addition, the ultrasensitive cholesterol biosensor exhibited higher specificity, acceptable reproducibility and excellent recoveries for cholesterol detection. [ABSTRACT FROM AUTHOR]

In this paper, a novel amperometric cholesterol biosensor with immobilization of cholesterol oxidase on electrochemically polymerized polypyrrole–polyvinylsulphonate (PPy–PVS) films has been accomplished via the entrapment technique on the surface of a platinum electrode. Electropolymerization of pyrrole and polyvinylsulphonate on the Pt surface was carried out by cyclic voltammetry between -1.0 and +2.0 V (vs. Ag/AgCl) at a scan rate of 100 mV upon the Pt electrode with an electrochemical cell containing pyrrole and polyvinylsulphonate. The amperometric determination is based on the electrochemical detection of H2O2 generated in the enzymatic reaction of cholesterol. Determination of cholesterol was carried out by the oxidation of enzymatically produced H2O2 at 0.4 V vs. Ag/AgCl. The effects of pH and temperature were investigated and optimum parameters were found to be 7.25 and 35 °C, respectively. The storage stability and operational stability of the enzyme electrode were also studied. The results show that 32% of the response current was retained after 19 activity assays. The prepared cholesterol biosensor retained 43% of initial activity after 45 days when stored in 0.1 M phosphate buffer solution at 4 °C.

This paper reports the development of a highly sensitive and selective amperometric cholesterol biosensor based on the bismuth subcarbonate (Bi 2 O 2 CO 3 ) nanoplates synthesized by the facile hydrothermal process at low temperature. The detail characterization of as-synthesized material in terms of their morphological, structural and compositional properties revealed that the nanoplates are synthesized in large quantity with well-crystallinity. The fabricated biosensors exhibit a very high and reproducible sensitivity of 139.5 μAmM −1 cm −2 , wide linear range from 0.05 mM to 7.4 mM, fast response time of∼4s and low detection limit of 10 μM (S/N=3) for cholesterol sensing. The anti-interference ability, reproducibility, and long-term stability were also assessed. To the best of our knowledge; this is the first report which demonstrates the use of Bi 2 O 2 CO 3 nanostructures for the fabrication of highly sensitive and selective cholesterol biosensor. [ABSTRACT FROM AUTHOR]

This work demonstrates effective biosensing of cholesterol with the help of an efficient inorganic H 2 O 2 transducer based on Pt-Bi combined with the organic enzyme platform. It could be shown that the Bi (bismuth) adatoms modified Pt (platinum) surface displays enhanced catalytic oxidation of H 2 O 2 at neutral pH and the catalytic oxidation of H 2 O 2 occurs at a lower potential of 0.25 V vs NCE (normal calomel electrode). The sensing platform is highly sensitive and shows linear response towards [H 2 O 2 ] in the absence of any redox mediator or enzyme. The H 2 O 2 sensing platform, further modified with cholesterol oxidase led to cholesterol biosensing with a sensitivity of 3.41 μA mM − 1 cm − 2 . The apparent Michaelis-Menten constant ( K m app ) was calculated to be 0.43 mM which indicates high binding affinity with the substrate. The cholesterol biosensor does not suffer from the interferences due to other common electroactive species and is highly stable. [ABSTRACT FROM AUTHOR]

This study reports the fabrication of an amperometric cholesterol biosensor based on cholesterol oxidase (ChOx), SnO2NPs and Nafion-modified carbon paste enzyme electrodes (CPE/SnO2NPs-ChOx/Naf). The electrochemical characterisations of BCPE and CPE/SnO2NPs were performed using CV and EIS. The determination of cholesterol was carried out by electrochemical oxidation of H2O2 at 0.6 V vs. Ag/AgCl. The CPE/SnO2NPs-ChOx/Naf presented a linear range from 0.20 μmol L−1 to 4.95 μmol L−1 with a low limit of detection (0.04 μmol L−1 ). In addition, the optimal values for pH and temperature were found to be 7.5 and 35°C, respectively. The CPE/SnO2NPs-ChOx/Naf was used for the determination of cholesterol in serum samples and good results were obtained [ABSTRACT FROM AUTHOR]

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 K 3 Fe(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. [ABSTRACT FROM AUTHOR]

A simple and robust cholesterol biosensor was designed by immobilizing cholesterol oxidase (ChOx) onto a conducting polymer modified graphite electrode. For this purpose, monomer, (Z)-4-(4-(9 H -carbazol-9-yl) benzylidene)-2-(4-nitrophenyl) oxazol-5(4 H )-one (CBNP), was synthesized and electrochemically polymerized on an electrode to achieve an effective immobilization platform for enzyme immobilization. After electropolymerization of the monomer (CBNP), electrochemical and spectroelectrochemical properties were investigated. Through the presence of nitro group on the polymer backbone hydrogen-bonding between enzyme molecules and polymer was achieved. Moreover, strong π–π stacking between aromatic moities in the polymer and aromatic residues of the enzyme enables a sensitive and reliable biosensor by conserving the crucial structure of biological molecules during the enzymatic reaction. The efficient interaction of the enzyme with the polymer coated surface brings easy and long-life detection of the substrate, cholesterol. After successful immobilization of ChOx with the help of glutaraldehyde as the crosslinking agent, amperometric biosensor responses were recorded at −0.7 V vs Ag wire in phosphate buffer (pH 7.0). K M app (37.3 μM), I max (3.92 μA), LOD (0.4063 μM) and sensitivity (1.49 μA μM −1 cm −2 ) values were determined. Finally, the prepared biosensor was successfully applied for determination of cholesterol content in real blood samples. [ABSTRACT FROM AUTHOR]

A novel cholesterol biosensor (ChOx/CS–GR/GCE) with enhanced sensitivity and low detection limit was investigated. The biosensor based on direct electrochemistry of cholesterol oxidase with an apparent rate constant ( k s ) of 2.69 s −1 was fabricated by immobilizing cholesterol oxidase (ChOx) on Glassy carbon electrode (GCE) functionalized by chitosan–graphene (CS–GR) nanocomposites. Graphene oxide (GO) was synthesized by a novel microwave method and CS–GR was prepared by simple situ reduction of chitosan–graphene oxide (CS–GO). The results of transmission electron microscopy (TEM) and FT-IR spectroscopy showed that the graphene oxide (GO) was successfully prepared and deoxygenized. The presence of the CS–GR nanocomposites not only accelerated direct electron transfer from the enzyme to the electrode surface, but also enhanced the immobilized amount and stability of cholesterol oxidase (ChOx). The fabricated electrode exhibited a linear response to cholesterol in the range of 0.005–1.0 mM with a detection limit of 0.715 μM ( S / N = 3). The Michaelis–Menten constant ( k m app ) was found as 17.39 μM. In addition, the biosensor also exhibited excellent reproducibility, stability and very high specificity to cholesterol with complete elimination of interference from UA, AA, DA and glucose. [ABSTRACT FROM AUTHOR]

The use of templates to assist and possess a control over the synthesis of nanomaterials has been an attractive option to achieve this goal. Here we have used sodium dodecyl sulfate (SDS) to act as a template for the low temperature synthesis of cobalt oxide (Co3O4) nanostructures. The use of SDS has led to tune the morphology, and the product was in the form of "cotton-like" nanostructures instead of connected nanowires. Moreover, the variation of the amount of the SDS used was found to affect the charge transfer process in the Co3O4. Using Co3O4 synthesized using the SDS for sensing of cholesterol was investigated. The use of the Co3O4 synthesized using the SDS was found to yield an improved cholesterol biosensor compared to Co3O4 synthesized without the SDS. The improvement of the cholesterol sensing properties upon using the SDS as a template was manifested in increasing the sensitivity and the dynamic range of detection. The results achieved in this study indicate the potential of using template assisted synthesis of nanomaterials in improving some properties, e.g., cholesterol sensing. [ABSTRACT FROM AUTHOR]

A simple and cheap cholesterol biosensor was designed by immobilizing cholesterol oxidase (ChOx) and horseradish peroxidase (HRP) onto a poly(thionine)-modified glassy carbon electrode (GCE/PTH). Being mediated by hydroquinone (HQ), the immobilized HRP exhibited excellent electrocatalytic activity in reducing H2O2, which was produced from cholesterol by the enzymatic reaction of ChOx. The linear detection range for cholesterol was 25-125 µM, with a detection limit (S/N = 3) and a sensitivity of 6.3 µM and 0.18 µA/cm²/µM, respectively, under optimal conditions. The highly reproducible and sensitive GCE/PTH/ChOx/HRP sensor exhibited an interference-free signal for cholesterol detection with excellent recoveries for real sample analysis. [ABSTRACT FROM AUTHOR]

This paper reports the fabrication of highly-sensitive and selective voltammetric/amperometric cholesterol biosensor based on cholesterol oxidase co-immobilized with α-Fe2O3 micro-pine shaped hierarchical structures. The α-Fe2O3 micro-pine shaped hierarchical structures were synthesized by facile hydrothermal process in large quantity and characterized in detail using various techniques. The detailed studies demonstrated that the as-synthesized α- Fe2O3 hierarchical structures are grown in very high density exhibiting well-crystallinity and demonstrating rhombohedral α-Fe2O3 crystal structures. The fabricated cholesterol biosensors based on α-Fe2O3 hierarchical structures exhibited a very high and reproducible sensitivity of 78.56 μA/mMcm2 and detection limit (based on S/N ratio) of 0.018 mM. The biosensor exhibited a linear dynamic range from 0.1-8.0 mM and correlation coefficient of R = 0.9951. A lower value of apparent Michaelis-Menten constant (Kmapp), of 0.007 mM, exhibited a high affinity between the cholesterol and ChOx immobilized on α-Fe2O3 micro-pine shaped hierarchical structures. To the best of our knowledge, this is the first report in which α- Fe2O3 micro-pine shaped hierarchical structures are used for the fabrication of highly sensitive and selective cholesterol biosensor. [ABSTRACT FROM AUTHOR]

Iron oxide nanoparticles of size ∼10 nm have been encapsulated into four nanometer thick shells of Prussian blue and were then electrophoretically deposited onto an indium tin oxide substrate. The immobilization of cholesterol oxidase has been done onto the nanostructured film to investigate the kinetic parameters and biosensing characteristics. The fabricated bioelectrode exhibits an electron transfer coefficient and a charge transfer rate constant of 0.45 and 45.15 s−1, respectively. Direct electron transfer properties of the nanostructured film result in 3rd generation cholesterol biosensor. The bioelectrode exhibits high sensitivity (2.15 mAM−1 cm−2), a low Kmapp value (0.07 mM), good stability and high selectivity towards cholesterol. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we developed a facile approach to synthesize multiwall carbon nanotubes-graphene oxide-thionine-Au (MWCNTs-GO-Thi-Au) nanocomposites with the synergistic effect of GO and Thi, resulting in reducing AuCl4 − to Au nanoparticles (AuNPs). Meanwhile, the AuNPs reduced by GO and Thi promoted the electrochemiluminescence (ECL) of luminol-H2O2 system and offered active sites for immobilization of numerous enzymes. Additionally, it was also found that the synergetic interactions of Thi with MWCNTs and GO was employed for enhancing ECL of luminol-H2O2, which was applied to develop an ECL biosensor for the first time. Using cholesterol oxidase as model enzyme, the proposed biosensor employed by MWCNTs-GO-Thi-Au nanocomposites showed a high sensitivity for cholesterol in a concentration range of 0.15-828μM with a detection limit of 50nM (signal-to-noise ratio of 3). In addition, the proposed cholesterol biosensor exhibited high sensitivity, good selectivity and excellent stability. Taking into account the integrated advantages of MWCNTs-GO-Thi-Au nanocomposites and ECL detection, we confidently expect that this biosensing approach would have potential applications in clinical diagnosis. [Copyright &y& Elsevier]

A new type of amperometric cholesterol biosensor was fabricated to improve the biosensor characteristics such as sensitivity and reliability. For this purpose, a novel immobilization matrix 2-(4-fluorophenyl)-4,7-di(thiophene-2-yl)-1H-benzo[d]imidazole (BIPF) was electrochemically deposited on a graphite electrode and used as a matrix for the immobilization of cholesterol oxidase (ChOx). Due to strong π–π stacking of aromatic groups in the structures of polymer backbone and enzyme molecule, one can easily achieve a sensitive and reliable biosensor without using any membrane or covalent bond formation between the enzyme molecules and polymer surface. Moreover, through pendant fluorine group of the polymer, H-bond formation between with enzyme molecules and polymer was generated. Cholesterol was used as the substrate and amperometric response was measured in correlation with cholesterol amount, at −0.7V vs. Ag/AgCl in phosphate buffer (pH 7.0). Consequently, optimum conditions for this constructed biosensor were determined. K Mapp, I max, LOD and sensitivity values were investigated and calculated as 4.0nM, 2.27µA, 0.404µM and 1.47mA/mMcm2, respectively. A novel and accurate cholesterol biosensor was developed for the determination of total cholesterol in food samples. [ABSTRACT FROM AUTHOR]

Abstract: Zinc oxide nanotube (ZNT) arrays were grown on Si/Ag substrate by one-step chemical process in an aqueous solution and further used as a working electrode to fabricate an enzyme-based cholesterol biosensor through immobilization of cholesterol oxidase (ChOx). The fabricated biosensors exhibit high and reproducible sensitivity of 79.40μA/mM/cm2, wide linear range from 1.0μM to 13.0mM, fast response time of ~2s and ultra-low detection limit of 0.5nM (S/N=3) for cholesterol sensing. The anti-interference ability and long-term stability of the biosensor were also assessed. Finally, the biosensor was applied to analyze cholesterol concentration in human serum samples. [Copyright &y& Elsevier]

Highlights: [•] A conducting polymer modified with sepiolite biosensor for successful biosensor applications. [•] Adsorption technique for beneficial enzyme immobilization. [•] Sensitive, reliable and long-life cholesterol biosensor. [•] Successful application for the determination of cholesterol in food samples. [ABSTRACT FROM AUTHOR]

The nanoparticle (NP) aggregates of commercial cholesterol oxidase (ChOx) were prepared by desolvation method. The formation and characterization of ChOxNP aggregates were studied by transmission electron microscopy and scanning electron microscopy. NP aggregates were more stable, active and had a higher shelf life than that of free enzyme. An amperometric cholesterol biosensor was constructed by immobilizing ChOxNPs onto Au electrode. The biosensor showed optimum response within 8 s at pH 6.0 and 35 °C, when polarized at +0.27 V versus Ag/AgCl. The biosensor possesses high sensitivity and measures cholesterol concentrations as low as 1.56 mg/dl. The working linear range was 12.5-700 mg/dl for cholesterol. The biosensor was evaluated and employed for measurement of total cholesterol in human serum. The enzyme electrode lost 50 % of its initial activity during its regular use for 180 times over a period of 90 days when stored in 0.1 M sodium phosphate buffer, pH 7.0 at 4 °C. [ABSTRACT FROM AUTHOR]

Chronoamperometry was employed to prepare polyaniline-p-toluene sulphonic acid-silver (PANI-pTSA-Ag) nanocomposite film on indium tin oxide–coated glass plate to fabricate a cholesterol bioelectrode. The authors observed the high sensitivity of 36.3 µA/mg/dL with a fast response time of 10 s in the range of 20–400 mg/dL for the bioelectrode. The bioelectrode exhibits uniform activity for 50 days under refrigerated conditions. Attempts have been made to utilize this bioelectrode for estimation of total cholesterol in blood serum samples. [ABSTRACT FROM PUBLISHER]

Abstract: A high-performance cholesterol sensor based on solution-gated field-effect-transistor (FET) was fabricated by using the vertically aligned ZnO nanorods (ZnO NRs) grown selectively on pre-patterned substrate in solution. The structural characterization showed that the as-grown ZnO NRs are vertically aligned, high purity single crystalline. The active layer of ZnO NRs between source and drain electrodes was immobilized with cholesterol oxidase (ChOx) enzyme. The performance of the fabricated FET sensor has been examined with the cholesterol solutions with and without electroactive species, the human serum (H4522), and the freshly drawn blood sample. The FET sensor provided a real-time response towards a wide range of cholesterol concentration (0.001–45mM) with high sensitivity (10μAcm−2 mM−1) and selectivity. [Copyright &y& Elsevier]

Highlights: [•] A novel conducting copolymer for biosensor fabrication. [•] Combination of two surface characteristics for beneficial enzyme immobilization. [•] Sensitive, reliable and long-life cholesterol biosensor. [Copyright &y& Elsevier]

Abstract: A simple and sensitive electrogenerated chemiluminescence (ECL) cholesterol biosensor was prepared based on cerium oxide–graphene (CeO2–graphene) composites as an efficient matrix. CeO2–graphene composites were prepared by depositing CeO2 onto graphene and were characterized by scanning electron microscopy. The experimental results demonstrated that CeO2–graphene could catalyze the ECL of a luminol–H2O2 (hydrogen peroxide) system to amplify the luminol ECL signal greatly. In addition, the use of CeO2–graphene provided a better biocompatible microenvironment for the immobilized enzyme, resulting in excellent stability and a long lifetime of the ECL biosensor. The surface assembly process, ECL behaviors, and electrochemistry of the biosensor were investigated in detail. The quantity of cholesterol was in the linear range from 12μM to 7.2mM with a detection limit of 4.0μM (signal/noise=3). In addition, the biosensor exhibited outstanding reproducibility, long-term stability, and selectivity. Moreover, this cholesterol biosensor offers an alternative analytical method with low cost and high speed. [Copyright &y& Elsevier]

Abstract: A new electrochemical biosensor with enhanced sensitivity was developed for detection of cholesterol by using platinum–palladium–chitosan–graphene hybrid nanocomposites (PtPd–CS–GS) functionalized glassy carbon electrode (GCE). An electrodeposition method was applied to form PtPd nanoparticles-doped chitosan–graphene hybrid nanocomposites (PtPd–CS–GS), which were characterized by scanning electron microscopy (SEM) and electrochemical methods. The presence of the PtPd–CS–GS nanocomposites not only accelerated direct electron transfer from the redox enzyme to the electrode surface, but also enhanced the immobilized amount of cholesterol oxidase (ChOx). Under optimal conditions, the fabricated biosensor exhibited wide linear ranges of responses to cholesterol in the concentration ranges of 2.2×10−6 to 5.2×10−4 M, the limit of detection was 0.75μM (S/N=3). The response time was less than 7s and the Michaelis–Menten constant ( ) was found as 0.11mM. In addition, the biosensor also exhibited excellent reproducibility and stability. Along with these attractive features, the biosensor also displayed very high specificity to cholesterol with complete elimination of interference from UA, AA, and glucose. [Copyright &y& Elsevier]

Abstract: An electrochemical cholesterol biosensor based on the direct electron transfer of cholesterol oxidase (ChOx) immobilized on gold nanoparticles-decorated multiwalled carbon nanotubes (GNPs-MWCNTs) was fabricated. GNPs-MWCNTs were prepared based on the reduction of HAuCl4 in the presence of carboxyl group functionalized MWCNTs. Transmission electron microscopy image shows that rounded gold nanoparticles with diameters of 6–10nm were decorated on carbon nanotube surfaces. ChOx was directly adsorbed on the nanocomposite modified glassy carbon electrode and protected by a Nafion film. Direct electrochemistry of ChOx on the electrode surface was obtained, proved by one pair of well-defined, quasi-reversible redox peaks in phosphate buffered saline. Under optimized conditions, the fabricated electrode displayed a linear response in the cholesterol concentration range from 0.0100 to 5.00mmolL−1 with a detection limit of 4.3μmolL−1 estimated at a signal-to-noise ratio of 3. The apparent Michaelis–Menten constant was measured to be 0.29mmolL−1, indicating that the immobilized ChOx on GNPs-MWCNTs matrix retained its native activity. The developed biosensor presented good selectivity, repeatability, reproducibility and stability. The concentration of free cholesterol in a human serum sample, detected by using the developed biosensor, is in good agreement with that determined by the well-established spectrophotometric method. [Copyright &y& Elsevier]

An amperometric cholesterol biosensor was fabricated using polyaniline-coated polyester films. Polyaniline was dissolved in chloroform with camphorsulfonic acid, and polystyrene was added to this solution. Using this mixed solution, the coating was placed onto polyester films. Cholesterol oxidase was immobilized onto these films using an electrostatic layer-by-layer adsorption technique. Poly(diallyldimethylammonium chloride) was used as the counter ion source. The level of adsorption was examined and evidence of layer-by-layer adsorption was investigated using a quartz crystal microbalance (QCM). A cholesterol biosensor was fabricated from these films as a working electrode, and it was used to measure the cholesterol concentration. [ABSTRACT FROM AUTHOR]

Abstract: The polyaniline (PANI)/Au nanocomposite was successfully fabricated through a seed-mediated strategy. And the hybrid nanostructure evaluated as a new material for cholesterol biosensor was demonstrated. UV–vis spectra proved the production of conductive PANI and the increase of the particle size of the Au nano-seeds. Scanning electron microscopic measurements displayed that the synthesized PANI/Au exhibited a spherical structure with dimensions of about 300nm in diameter. Energy dispersive X-ray spectrogram demonstrated the ingredient of the composite. Cyclic voltammetry and electrochemical impedance spectroscopy investigation of the PANI/Au modified electrode indicated the good conductivity of the composite. Direct electron transfer of cholesterol oxidase (ChOx) was obtained in pH 7.0 phosphate buffer solution when ChOx was further immobilized on the PANI/Au modified electrode. This result showed that the PANI/Au nanocomposite was a good candidate for the development of the cholesterol biosensor. The biosensor displayed a response time of 3s. Some common interferents like glucose and ascorbic acid did not cause interference due to the use of a low operating potential. [Copyright &y& Elsevier]

Background & objectives: The use of epoxy resin membrane as a support for immobilization of enzyme has resulted into improved sensitivity and stability of biosensors for uric acid, ascorbic acid and polyphenols. The present work was aimed to prepare an improved amperometric biosensor for determination of serum cholesterol required in the diagnostics and management of certain pathological conditions. Methods: Epoxy resin membrane with immobilized cholesterol oxidase was mounted on the cleaned platinum (Pt) electrode with a parafilm to construct a working electrode. This working electrode along with Ag/AgCl as reference and Ag wire as an auxiliary electrode were connected through a three terminal electrometer to construct a cholesterol biosensor. Results: The sensor showed optimum response within 25 sec at pH 7.0 and 45°C. The linear working range of biosensor was 1.0 to 8.0 mM cholesterol. Km and Imax for cholesterol were 5.0 mM and 9.09 µA, respectively. The biosensor measured serum cholesterol. The minimum detection limit of the sensor was 1.0 mM. The mean analytical recoveries of added cholesterol in serum (2.84 and 4.13 mM) were 91.4±2.8 and 92.3±3.1 per cent (n=6), respectively. Within and between assay coefficient of variation (CV) were <2 and <4 per cent, respectively. Biosensor had a storage life of 6 months at 4°C. Interpretation & conclusions: The use of epoxy resin membrane as a support for immobilization of cholesterol oxidase has resulted into an improved amperometric cholesterol biosensor. The present biosensor had an advantage over the existing biosensors as it worked at comparatively lower potential. [ABSTRACT FROM AUTHOR]

Abstract: We have fabricated highly sensitive and wide linear-range detecting cholesterol biosensors based on zinc oxide nanorods (ZnO NRS). We report, for the first time, the use of aspect-ratio (AR) controlled ZnO NRs grown directly on silver (Ag) electrodes in solution at 90°C for the fabrication of cholesterol biosensors. More immobilization of enzymes and faster electron conduction with larger AR ZnO NRs were obtained. The biosensor with AR=60 ZnO NRs exhibits a high reproducible sensitivity of 74.10μA/mM/cm2, a wide linear range up to 16.0mM, and a fast response time less than 2s, which is the best reported to date for cholesterol biosensors using ZnO nanostructures. Furthermore, the fabricated biosensors show an excellent anti-interference ability against electroactive species such as glucose, ascorbic acid, l-cysteine, and uric acid. [Copyright &y& Elsevier]

Abstract: A novel cholesterol biosensor was prepared based on gold nanoparticles-catalyzed luminol electrogenerated chemiluminescence (ECL). Firstly, l-cysteine-reduced graphene oxide composites were modified on the surface of a glassy carbon electrode. Then, gold nanoparticles (AuNPs) were self-assembled on it. Subsequently, cholesterol oxidase (ChOx) was adsorbed on the surface of AuNPs to construct a cholesterol biosensor. The stepwise fabrication processes were characterized with cyclic voltammetry and atomic force microscopy. The ECL behaviors of the biosensor were also investigated. It was found that AuNPs not only provided larger surface area for higher ChOx loading but also formed the nano-structured interface on the electrode surface to improve the analytical performance of the ECL biosensor for cholesterol. Besides, based on the efficient catalytic ability of AuNPs to luminol ECL, the response of the biosensor to cholesterol was linear range from 3.3μM to 1.0mM with a detection limit of 1.1μM (S/N=3). In addition, the prepared ECL biosensor exhibited satisfying reproducibility, stability and selectivity. Taking into account the advantages of ECL, we confidently expect that ECL would have potential applications in biotechnology and clinical diagnosis. [Copyright &y& Elsevier]

A new amperometric cholesterol biosensor was prepared by immobilizing cholesterol oxidase by a glutaraldehyde crosslinking procedure on polypyrrole-polyaniline (ppy-pani) composite film on the surface of a platinum electrode. In order to prepare a biosensor for the determination of cholesterol, electropolymerization of pyrrole and aniline on Pt surface was performed with an electrochemical cell containing pyrrole and aniline in sulphuric acid by cyclic voltammetry between 0.0 and 0,7 V (vs.Ag/AgCl) at a scan rate of 50 mV upon Pt electrode. The amperometric determination is based on the electrochemical detection of H2O2, which is generated in enzymatic reaction of cholesterol. The cholesterol determined by the oxidation of enzymatically generated H2O2 at 0.7 V vs. Ag/AgCl. The optimized cholesterol oxidase biosensor displayed linear working range and a response time of 300 s. The effects of pH and temperature were investigated and optimum parameters were found to be 7.0, 25°C, respectively. In addition to this, the stability and reproducibility of biosensor were tried. Operational stability of the proposed cholesterol biosensor was obtained by periodical measurements of the biosensor response. Biosensor at optimum activity conditions was used in 30 activity assays in one day to determine the operational stability. The results show that 82% of the response current was retained after 30 activity assays. The electrode was stored in a refrigerator at 4 °C after the measurements. The storage stability of the biosensor was determined by performing activity assays within 23 days. The results demonstrate that 60% of the response current was retained after 23 days. Preparing biosensor is used for the analysis of cholesterol in serum. [ABSTRACT FROM AUTHOR]

Abstract: Metal oxide [cerium oxide (NanoCeO2)]–chitosan (CH) nanocomposite film has been fabricated onto indium-tin-oxide (ITO) coated glass plate to immobilize cholesterol oxidase (ChOx) via physiosorption for cholesterol detection. Electrochemical studies reveal that the presence of NanoCeO2 in CH–CeO2 nanocomposite results in increased electroactive surface area for ChOx loading resulting in enhanced electron transport between ChOx and electrode. The ChOx/CH–NanoCeO2/ITO bioelectrode exhibits interesting characteristics such as detection range of 10–400mg/dL, detection limit of 5mg/dL, response time of 10s, low K m value of 3.5mg/dL and value of regression coefficient of 0.994. [Copyright &y& Elsevier]

In this paper, a novel amperometric cholesterol biosensor with immobilization of cholesterol oxidase on electrochemically polymerized polypyrrole-polyvinylsulphonate (PPy-PVS) films has been accomplished via the entrapment technique on the surface of a platinum electrode. Electropolymerization of pyrrole and polyvinylsulphonate on the Pt surface was carried out by cyclic voltammetry between -1.0 and +2.0 V (vs. Ag/AgCl) at a scan rate of 100 mV upon the Pt electrode with an electrochemical cell containing pyrrole and polyvinylsulphonate. The amperometric determination is based on the electrochemical detection of H2O2 generated in the enzymatic reaction of cholesterol. Determination of cholesterol was carried out by the oxidation of enzymatically produced H2O2 at 0.4 V vs. Ag/AgCl. The effects of pH and temperature were investigated and optimum parameters were found to be 7.25 and 35 °C, respectively. The storage stability and operational stability of the enzyme electrode were also studied. The results show that 32% of the response current was retained after 19 activity assays. The prepared cholesterol biosensor retained 43% of initial activity after 45 days when stored in 0.1 M phosphate buffer solution at 4 °C. [ABSTRACT FROM AUTHOR]

Cholesterol biosensor has been fabricated by covalently coupling cholesterol oxidase (ChOx) via glutaraldehyde onto electrochemically prepared polyaniline film in presence of TritonX-100 [4-(1,1,3,3-tetramethylbutyl) phenyl polyethylene glycol], a non-ionic surfactant onto indium-tin-oxide (ITO) glass substrate. These ChOx/PANI-TX-100/ITO bioelectrodes have been characterized using Fourier transform infrared (FTIR) spectroscopy, cyclic voltammetry (CV) and scanning electron microscopy (SEM) techniques. The results of response measurements carried out on ChOx/PANI-TX-100/ITO bioelectrodes using amperometric and photometric techniques, reveal detection limit as 5 mg/dl, linearity from 5 to 400 mg/dl of cholesterol and sensitivity as 131 μA/(mg/dl cm−2). These biosensing electrodes are thermally stable up to 65 °C, can be used about 20 times and have a shelf-life of about 10 weeks when stored at 4 °C. Attempts have also been made to utilize the ChOx/PANI-TX-100/ITO bioelectrodes for estimation of free cholesterol concentration in serum samples. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports the fabrication of highly-sensitive cholesterol biosensor based on cholesterol oxidase (ChOx) immobilization on well-crystallized flower-shaped ZnO structures composed of perfectly hexagonal-shaped ZnO nanorods grown by low-temperature simple solution process. The fabricated cholesterol biosensors reported a very high and reproducible sensitivity of 61.7μAμM−1 cm−2 with a response time less than 5s and detection limit (based on S/N ratio) of 0.012μM. The biosensor exhibited a linear dynamic range from 1.0–15.0μM and correlation coefficient of R =0.9979. A lower value of apparent Michaelis–Menten constant (K m app), of 2.57mM, exhibited a high affinity between the cholesterol and ChOx immobilized on flower-shaped ZnO structures. Moreover, the effect of pH on ChOx activity on the ZnO modified electrode has also been studied in the range of 5.0–9.0 which exhibited a best enzymatic activity at the pH range of 6.8–7.6. To the best of our knowledge, this is the first report in which such a very high-sensitivity and low detection limit has been achieved for the cholesterol biosensor by using ZnO nanostructures modified electrodes. [Copyright &y& Elsevier]

Abstract: Cholesterol esterase (ChEt) and cholesterol oxidase (ChOx) have been immobilized via glutaraldehyde as a cross-linker onto sol–gel-derived silica (SiO2)/chitosan (CHIT)/multi-walled carbon nanotubes (MWCNT)-based nanobiocomposite film deposited onto indium-tin-oxide (ITO) glass for estimation of esterified cholesterol. The ChEt–ChOx/MWCNT/SiO2–CHIT/ITO bioelectrode characterized using Fourier transform infrared (FTIR), scanning electron microscopy (SEM) and electrochemical techniques and shows response time of 10s, linearity as 10–500mg/dL for esterified cholesterol (cholesterol oleate), sensitivity as 3.8μA/mM and shelf-life of about 10 weeks under refrigerated conditions. The value of Michaelis–Menten constant (K m) estimated as 0.052mM using Lineweaver–Burke plot indicates high affinity of ChEt and ChOx to cholesterol oleate. Attempts have been made to utilize this electrode for estimation of total cholesterol in blood serum samples. [Copyright &y& Elsevier]

Abstract: A prototype chronoamperometric biosensor for the determination of total cholesterol was developed that consists of a homemade potentiostat and disposable strips immobilized with Fe3O4, cholesterol oxidase (ChOx), and cholesterol esterase (ChE). The principle of sensing cholesterol is based on the detection of reduction signal of hydrogen peroxide generated in two enzymatic reactions. The co-immobilization of ChE and ChOx allows the sensor to detect both concentrations of esterified and free cholesterol. The effects of biosensor on catalyst, enzymes, applied potential, and buffer pH was investigated, and the operation conditions were optimized. The detection of cholesterol can be accomplished in one step, a 10μL of sample was dropped onto the area of sensing strip and the reduction signal was obtained at an applied potential of −200mV (vs. Ag/Ag+). The pre-reaction time was set at 15s before applying potential on the strip and the sampling time was 5s. The sensing device displays a linear response over the range of 100–400mg/dL (R 2 =0.999) for cholesteryl oleate. The coefficient variation was determined as 5.06% (N =20) for 100mg/dL cholesteryl oleate and the detection limit is 19.4mg/dL (S/N=3). The probable interferences in bio-matrix were selected to test the selectivity and no significant response was observed in the biosensor. [Copyright &y& Elsevier]

Abstract: A high-sensitive cholesterol amperometric biosensor based on the immobilization of cholesterol oxidase (ChOx) onto the ZnO nanoparticles has been fabricated which shows a very high and reproducible sensitivity of 23.7μAmM−1 cm−2, detection limit (based on S/N ratio) 0.37±0.02nM, response time less than 5s, linear range from 1.0 to 500.0nM and correlation coefficient of R =0.9975. A relatively low value of enzyme’s kinetic parameter (Michaelis–Menten constant) ∼4.7mM has been obtained which indicates the enhanced enzymatic affinity of ChOx to Cholesterol. To the best of our knowledge, this is the first report in which such a very high-sensitivity and low detection limit has been achieved for the cholesterol biosensor by using ZnO nanostructures modified electrodes. [Copyright &y& Elsevier]

Abstract: A cholesterol biosensor based on direct electron transfer of a hemoglobin-encapsulated chitosan-modified glassy carbon electrode has been developed for highly sensitive and selective analysis of serum samples. Modified by films containing hemoglobin and cholesterol oxidase, the electrode was prepared by encapsulation of enzyme in chitosan matrix. The hydrogen peroxide produced by the catalytic oxidation of cholesterol by cholesterol oxidase was reduced electrocatalytically by immobilized hemoglobin and used to obtain a sensitive amperometric response to cholesterol. The linear response of cholesterol concentrations ranged from 1.00×10−5 to 6.00×10−4 mol/L, with a correlation coefficient of 0.9969 and estimated detection limit of cholesterol of 9.5μmol/L at a signal/noise ratio of 3. The cholesterol biosensor can efficiently exclude interference by the commonly coexisting ascorbic acid, uric acid, dopamine, and epinephrine. The sensitivity to the change in the concentration of cholesterol as the slope of the calibration curve was 0.596 A/M. The relative standard deviation was under 4.0% (n =5) for the determination of real samples. The biosensor is satisfactory in the determination of human serum samples. [Copyright &y& Elsevier]

Abstract: The development of a cholesterol biosensor by co-immobilization of cholesterol esterase (ChEt) and cholesterol oxidase (ChOX) on oxygen electrode is described. The electrode consists of gold cathode and Ag/AgCl anode. The enzymes were immobilized by cross-linking with glutaraldehyde and Bovine Serum Albumin (BSA). The immobilized enzymatic membrane was attached to the tip of the electrode by a push cap system. The optimum pH and temperature of the sensor was determined, these are 6 and 25 °C respectively. The developed sensor was calibrated from 1–75 mg/dl of cholesterol palmiate and found linear in the range of 2–50 mg/dL. The calibration curve was drawn with V i (ppm/min)(initial velocity) vs different concentrations of cholesterol palmiate (mg/dL). The application of the sensor to determine the total cholesterol in different real food samples such as egg, meat was investigated. The immobilized enzymatic layer can be reused over 30 times and the stability of the enzymatic layer was studied up to 9 weeks. [Copyright &y& Elsevier]

Abstract: 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°C, response time as 180s, linearity up to 780mgdL−1 (12mM), shelf life of 1 month, detection limit of 12mgdL−1 and sensitivity as 5.4×10−5 Abs.mg−1 dL−1. [Copyright &y& Elsevier]