Mathematical modeling of the action of biosensor possessing variable parameters

Electrochemical biosensor containing flat semi-permeable membrane covering enzyme-containing layer has been investigated. Mathematical modeling of the action modes of electrochemical biosensors with outer diffusion membrane was performed. Operation of the biosensor under the conditions when the permeability of the membrane and the activity of the biocatalytic layer depend on the parameters of the probe has been examined. The pH and temperature were selected as the main parameters which often affect the action of biosensors. A set of parameters was selected when the biosensor operates in kinetic and diffusion modes of action. The response time of the biosensor was shown to be sensitive to the mode of the biosensor action especially in the boundary region of the biosensor action. The linearity of the biosensor (the linear dependence of the biosensor response on the substrate concentration) in the deep diffusion mode can be increased by several magnitudes, whereas the response time increases only several times.     

Miniaturised enzymatic conductometric biosensor with Nafion membrane for the direct determination of formaldehyde in water samples

A new conductometric enzyme-based biosensor was developed for the determination of formaldehyde (FA) in aqueous solutions. The biosensor was prepared by cross-linking formaldehyde dehydrogenase from Pseudomonas putida with bovine serum albumin in saturated glutaraldehyde vapours (GA) at the surface of interdigitated gold microelectrodes. Nicotinamide adenine dinucleotide cofactor (NAD⁺) was added in solution at each measurement to maintain enzyme activity. Addition of a Nafion layer over the enzyme modified electrode resulted in a significant increase of biosensor signal due to enhanced accumulation of protons generated by enzymatic reaction at the electrode surface. Different parameters affecting enzyme activity or playing a role in ionic transfer through the Nafion membrane were optimised. In optimal conditions (0.045 mg enzyme, 30 min exposure to GA, 0.3 μL of a 1 % (v/v) Nafion solution deposit, measurement in 5 mM phosphate buffer pH 7 containing 20 μM NAD⁺), the biosensor signal was linear up to 10 mM FA, and the detection limit was 18 μM. Relative standard deviations calculated from five consecutive replicates of FA solutions were lower than 5 % in the 1–10 mM range. The biosensor was successfully applied to the determination of FA in spiked water samples (tap water and Rhone river water), with recoveries in the 95–110 % range.

α-MoO3 nanowire-based amperometric biosensor for l-lactate detection

Large-scale orthorhombic single-crystalline molybdenum trioxide nanowires were synthesized using a facile one-pot hydrothermal method. Lactate oxidase enzyme was immobilized on the nanowires to produce a highly sensitive electrochemical biosensor for l-lactate detection. At an applied potential of 0.5 V, the sensor exhibited a high sensitivity of 0.87 μA/mM with a fast response to l-lactate (90% of response times within 10 s). A linear response was obtained over a concentration range from 0.5 to 8 mM with a detection limit of 0.15 mM (S/N?=?3). The developed biosensor showed excellent reproducibility and operational stability, as well as the ability to be stored long term.     

A voltammetric sensor based on graphene-modified electrode for the determination of trace amounts of l-dopa in mouse brain extract and pharmaceuticals

l-Dopa is the intermediate precursor of the neurotransmitter dopamine. Unlike dopamine, l-dopa easily enters the central nervous system. l-Dopa, as one of the catecholamines, is widely used as a source of dopamine in the treatment of most patients with Parkinson’s disease and epilepsy. Graphene (GR) is ideally suited for implementation in electrochemical applications due to its reported large electrical conductivity, large surface area, unique heterogeneous electron transfer rate, and low production costs. This work reports the synthesis of GR using a modified Brodie method and its application for the electrochemical determination of l-dopa in real samples. Electrochemical measurements were performed at glassy carbon electrode modified with graphene (GR/GCE) via drop casting method. Cyclic voltammograms of l-dopa at GR/GCE showed an increased current intensity compared with GCE. All the measurements were done in phosphate buffer solution 0.1 M (pH 6.2) and the oxidation peak was observed at 0.27 V vs. Ag/AgCl. The effect of scan rate showed that oxidation of l-dopa on GR/GCE was surface controlled. The oxidation peak current of l-dopa gradually increased with increasing accumulation time from 0 to 300 s and accumulation potential from 0.0 to 0.3 V and reached the maximum current response at 240 s and 0.2 V for the accumulation time and accumulation potential, respectively. Voltammetric peak currents showed a linear response for l-dopa concentration in the range of 0.04 to 79 μM and a detection limit of 0.022 μM (22 nM). The relative standard deviation for five determinations of 50 μM l-dopa was 0.52 %.     

Detecting the cross-linking process of hybrid large-compound vesicles by an electrochemical method

A simple electrochemical method of cyclic voltammetry (CV) was exploited to detect the cross-linking process of hybrid large-compound vesicles (LCVs) bearing electrochemically reactive ferrocene group inside. Results indicated that the CV behavior of LCVs deposited electrode was relative to the cross-linking degree of LCVs. It was found that oxidation peak potential became more positive and diffusion rates of electroactive species became smaller after the addition of triethylamine into LCVs solution with increasing time in few days. This showed that the increasing of cross-linking degree for LCVs hindered the electron transport of charge transfer between the neighboring active sites. Our studies will benefit the understanding and the control of the gelation process of hybrid LCVs for potential application in encapsulation and drug delivery.      

Pt-polyaniline nanocomposite on boron-doped diamond electrode for amperometic biosensor with low detection limit

Boron-doped diamond electrodes covered with a nanostructured Pt nanoparticle-polyaniline composite have been fabricated and employed as sensitive amperometric sensors with low detection limit. A highly conductive boron-doped diamond thin film (BDD) was prepared by chemical vapor deposition, and its morphology was characterized by scanning electron microscopy and transmission electron microscopy. The nanostructured composite layer was grown on the BDD electrode by electrochemical deposition of polyaniline and Pt nanoparticles. Glucose oxidase (GOx) was then adsorptively immobilized on the modified BDD electrode. The biosensor displays a large surface area, high catalytic activity of the Pt nanoparticles, efficient electron mediation through the conducting polymer, and low background current of the electrode. The biosensor exhibits an excellent response to glucose, with a broad linear range from 5.9 μM to 0.51 mM, a sensitivity of 5.5 μA·mM^?1, a correlation coefficient (R) of 0.9947, and a detection limit of 0.10 μM. The apparent Michaelis-Menten constant (K _M ^app ) and the maximum current density of the electrode are 4.1 mM and 0.021 mA, respectively. This suggests that the immobilized GOx possesses a higher affinity for glucose at the lower K _M ^app , and that the enzymatic reaction rate constitutes the rate-limiting step of the response.     

Mathematical Modelling of a Non‐enzymatic Amperometric Electrochemical Biosensor for Cholesterol

Thickness of the electro‐polymerized layer grown on a substrate and used as the recognition element for the analyte is critical to measuring the response of a biosensor, with high sensitivity and accuracy. However, it is difficult to control the thickness during synthesis. A mathematical model is developed in this study that considers thickness of the electro‐polymerized layer in simulating the electrochemical response of a non‐enzymatic biosensor for cholesterol in blood. The model includes transient kinetics and one‐dimensional diffusion of the analyte in the poly‐methyl orange (PMO) recognition layer electrochemically grown on the electrode. The governing partial differential equations resulting from the species conservation balances in the PMO layer are numerically solved. Time and spatial concentration profiles of the analyte in the PMO layer are determined. Model predictions are calibrated with the experimental data for different PMO thicknesses. Interestingly, model predictions show a linear response over the calibrated concentration range of cholesterol for all PMO layer thicknesses. Based on the chronoamperometry measurements, the model predictions for the cholesterol concentrations measured in the laboratory samples were also found to be remarkably accurate. This is the first mathematical model developed to understand the transport and kinetics of an analyte in the electro‐polymerized layer used as the recognition element of a non‐enzymatic biosensor.     

Sorption of Bi3+ from acidic solutions using nano-hydroxyapatite extracted from Persian corals

Nano-crystallite hydroxyapatite (nano-HAp) synthesized from Persian corals was used for removing Bi³⁺ from acidic aqueous solutions. The effects of initial concentration, adsorbent dosage, contact time and temperature were studied in batch experiments. The sorption of Bi³⁺ by nano-HAp increased as the initial concentration of bismuth ion increased in the medium. The pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models were applied to study the kinetics of the sorption processes. The pseudo-second-order kinetic model provided the best correlation (R ² > 0.999) of the used experimental data compared to the pseudo-first-order and intraparticle diffusion kinetic models. Various thermodynamic parameters, such as $ \Updelta G^\circ $ , $ \Updelta H^\circ $ and $ \Updelta S^\circ $ were calculated. Thermodynamics of Bi³⁺ cation sorption onto nano-HAp system pointed at spontaneous and endothermic nature of the process. The maximum Bi³⁺ adsorbed was found to be 3,333.33 mg g^?1. It was found that the sorption of Bi³⁺ on nano-HAp correlated well (R ² = 0.979) with the Langmuir equation as compared to Freundlich and Dubinin–Kaganer–Radushkevich (D-K-R) isotherm equations under the concentration range studied. This study indicated that nano-HAp extracted from Persian corals could be used as an efficient adsorbent for removal of Bi³⁺ from acidic aqueous solution.     

Design of poly-l-methionine–gold nanocomposit/multi-walled carbon nanotube modified glassy carbon electrode for determination of amlodipine in human biological fluids

The present paper describes a sensitive electrochemical detection of amlodipine (AMLO) at the poly-l-methionine–gold nanoparticles/multi-walled carbon nanotube modified glassy carbon electrode (PLM–GNPs/MWCNTs/GCE) by differential pulse voltammetry (DPV) technique at physiological pH 7.12. Cyclic voltammetry results demonstrate that the proposed electrode shows excellent electrocatalytic activity toward oxidation of AMLO. Kinetic parameters of the electrochemical reaction are calculated, and analytical variables such as MWCNT volumes, drug accumulation time, electropolymerization cycles and pH values are also optimized. Under optimal conditions, the linear range covering from 5 nM to 2.5 μM along with detection limit of 1 nM is obtained. Moreover, this method is successfully used to detect AMLO in pharmaceutical samples and biological fluids of a dosage received by the volunteer.     

An improved FIA biosensor for the determination of aspartame in dietary food products

A flow injection analysis (FIA) biosensor system was developed for the determination of the artificial sweetener aspartame (l-aspartyl-l-phenylalanine methyl ester). The system consisted of an enzyme column of pronase immobilized on activated arylamine glass beads and al-amino acid oxidase electrode connected in series. The dipeptide bond of aspartame was cleaved by immobilized pronase to release phenylalanine, which was in turn monitored by the enzyme electrode that usedl-amino acid oxidase, immobilized on a preactivated nylon membrane in combination with an amperometric electrode (platinum vs silver/silver chloride, 700 mV). The response of the FIA biosensor was linear up to 1 mM aspartame with a lower detection limit of 25 μM and had good reproducibility (rsd 0.3%). The FIA biosensor was stable for at least 30 h of continuous use atT _r .Each assay takes 4 min giving a sample throughput of 15 h^?1 When applied to aspartame in dietary food products the results obtained agreed well with those reported by the product manufacturers.     

Degradation of substrate and/or product: mathematical modeling of biosensor action

Mathematical model for evaluation of the multilayer heterogeneous biocatalytic system has been elaborated. The model consists of nonlinear system of partial differential equations with initial values and boundary conditions. An algorithm for computing the numerical solution of the mathematical model has been applied. Two cases: when product diffuses out of the biosensor and when the outer membrane is impermeable for product (product is trapped inside the biosensor) have been dealt with by adjusting boundary conditions in the mathematical model. Profiles of the impact of the substrate and product degradation rates on the biosensor response have been constructed in both cases. Value of the degradation impact has been analyzed as a function of the outer membrane thickness. The initial substrate concentration also affects influence of the degradation rates on the biosensor response. Analytical formulae, defining approximate values of relationships between the degradation rates and the outer membrane thickness or the initial substrate concentration, have been obtained. These formulae can be employed for monitoring of the biosensor response.     

Determination of the surface acidity of a free-base corrole in a self-assembled monolayer

Mixed self-assembled monolayers (SAMs) containing a corrole moiety have been prepared to examine their electrochemical properties and surface acidity, with the eventual goal of biosensor development. Mixed SAMs consisting of 6-mercapto-hexanol (6-MHO) and 8-amino-1-octanethiol (8-AOT) in varying ratios were modified with a free-base corrole and characterized via Osteryoung square-wave voltammetry and contact angle measurements. The surface acidity of the free-base corrole was determined using an electrochemical titration method, with pK _a values established at 6.4 when using Fe(CN) ₆ ^4? /Fe(CN) ₆ ^3? as a redox probe and at 6.7 when using iodide, and assigned to the CorH₄ ⁺ ? CorH₃ + H⁺ equilibrium.     

Reagentless d-sorbitol biosensor based on d-sorbitol dehydrogenase immobilized in a sol–gel carbon nanotubes–poly(methylene green) composite

A reagentless d-sorbitol biosensor based on NAD-dependent d-sorbitol dehydrogenase (DSDH) immobilized in a sol–gel carbon nanotubes–poly(methylene green) composite has been developed. It was prepared by durably immobilizing the NAD⁺ cofactor with DSDH in a sol–gel thin film on the surface of carbon nanotubes functionalized with poly(methylene green). This device enables selective determination of d-sorbitol at 0.2 V with a sensitivity of 8.7?μA?mmol^?1?L?cm^?2 and a detection limit of 0.11 mmol?L^?1. Moreover, this biosensor has excellent operational stability upon continuous use in hydrodynamic conditions.

Biosensor for bisphenol A leaching from baby bottles using a glassy carbon electrode modified with DNA and single walled carbon nanotubes

We have developed a biosensor for highly sensitive and selective determination of the endocrinic disruptor bisphenol A (BPA). It is based on glassy carbon electrode modified with calf thymus DNA and a composited prepared from single walled carbon nanotubes (SWNT) and Nafion. The interaction between BPA and DNA was studied by voltammetry. The binding constant was determined to be 3.55?×?10³ M^?1, and the binding site has a length of 4.3 base pairs. These electrochemical studies provide further information for a better understanding of the toxicity and carcinogenicity of BPA. Under optimal conditions, the biosensor displays a linear electrochemical response to BPA in the 10 nM to 20 μM concentration range, with a detection limit as low as 5.0 nM (at an S/N of 3). The method was successfully applied to the quantification of BPA in leachates from plastic baby bottles. Recoveries range from 94.0 % to 106.0 % which underpins the excellent performance of this SWNT-based DNA sensor.

Label-free electrochemical DNA biosensor based on a glassy carbon electrode modified with gold nanoparticles, polythionine, and graphene

We describe the fabrication of a sensitive label-free electrochemical biosensor for the determination of sequence-specific target DNA. It is based on a glassy carbon electrode (GCE) modified with graphene, gold nanoparticles (Au-NPs), and polythionine (pThion). Thionine was firstly electropolymerized on the surface of the GCE that was modified with graphene by cyclic voltammetry. The Au-NPs were subsequently deposited on the surface of the pThion/graphene composite film by adsorption. Scanning electron microscopy and electrochemical methods were used to investigate the assembly process. Differential pulse voltammetry was employed to monitor the hybridization of DNA by measuring the changes in the peak current of pThion. Under optimal conditions, the decline of the peak current is linearly related to the logarithm of the concentration of the target DNA in the range from 0.1 pM to 10 nM, with a detection limit of 35 fM (at an S/N of 3). The biosensor exhibits good selectivity, acceptable stability and reproducibility.

Application of the log-normal model for long term high affinity antibody/antigen interactions using Bio-Layer Interferometry

For more than 50 years, optical biosensors have been used to measure bio-molecular interactions. The most frequently applied binding model to fit biosensor data is the simple 1:1 binding model which requires the stabilization of the association phase to the equilibrium Req and the stabilization of the dissociation phase to the equilibrium zero. However, due to technical limitations many published biosensor measurements are finished before these requirements are fulfilled. In the present study, a long term binding interaction analysis with a monoclonal antibody, namely IgG 2F5 and UG37 a specific antigen with a promising biosensor platform, the Bio-Layer Interferometry, was performed. Data fitting with the simple 1:1 binding model to the association phase was inappropriate and the fitted parameters varied with the concentration and time, which contradicts the theory of the simple 1:1 binding model. Furthermore, extrapolation of the fits with individual times spans compared to 100 % of the obtained data systematically underestimated the actual observed binding curve. Interestingly, an alternative model based on the cumulative distribution function of the log-normal probability distribution remedied the aforementioned problems allowing $\hbox {K}_\mathrm{L}$ (which is the analog to the affinity constant $\hbox {K}_\mathrm{D}$ ) to be estimated. We further demonstrate that this model fits the biosensor data far better and is essentially less affected by the stabilization of the association phase to the equilibrium (Req) and the stabilization of the dissociation phase to the equilibrium zero. Finally, extrapolation with the log-normal model predicts the actually observed binding curve in a proper manner.     

Validated, Stability-Indicating LC Method for Analysis of Pipenzolate Bromide and Its Hydrolysis Products

A liquid chromatographic method has been developed and validated for quantitative analysis of pipenzolate bromide (PP), its hydrolysis products, and phenobarbitone, sodium benzoate, and sodium saccharine. A 5-μm particle ODS column was used with acetonitrile–KH₂PO₄ (10 mm, pH 3.5) 40:60 (v/v), containing 5 mm heptanesulfonic acid sodium salt, as mobile phase. Quantitation was achieved by UV detection at 210 nm, on the basis of peak area. Forced degradation studies were performed on a bulk sample of PP using 0.1 M hydrochloric acid, 0.01 M sodium hydroxide, 0.33% hydrogen peroxide, heat (70 °C), and photolytic degradation. The proposed LC method was used to study the kinetics of acidic hydrolysis and pH-rate profiles of hydrolysis of PP in Britton–Robinson buffer solutions.     

Dopamine sensor based on a glassy carbon electrode modified with a reduced graphene oxide and palladium nanoparticles composite

We report on a sensitive electrochemical sensor for dopamine (DA) based on a glassy carbon electrode that was modified with a nanocomposite containing electrochemically reduced graphene oxide (RGO) and palladium nanoparticles (Pd-NPs). The composite was characterized by scanning electron microscopy, energy dispersive spectroscopy, and electrochemical impendence spectroscopy. The electrode can oxidize DA at lower potential (234 mV vs Ag/AgCl) than electrodes modified with RGO or Pd-NPs only. The response of the sensor to DA is linear in the 1–150 μM concentration range, and the detection limit is 0.233 μM. The sensor was applied to the determination of DA in commercial DA injection solutions.

A lipase-based electrochemical biosensor for target DNA

A lipase-based electrochemical biosensor has been fabricated for the quantitative determination of target DNA. It is based on a stem-loop nucleic acid probe labeled with ferrocene containing a butanoate ester that is hydrolyzed by lipase. The other end of the probe DNA is linked, via carboxy groups, to magnetic nanoparticles. The binding of target DNA transforms the hairpin structure of the probe DNA and causes the exposure of ester bonds. This results in the release of electro-active ferrocene after hydrolysis of the ester bonds, and in an observable electrochemical response. The quantity of target DNA in the concentration range between 1?×?10^?12 mol·L^?1 and 1?×?10^?8 mol·L^?1 can be determined by measuring the electrochemical current. The method can detect target DNA with rapid response (30 min) and low interference.

Direct electrodeposition of a biocomposite consisting of reduced graphene oxide, chitosan and glucose oxidase on a glassy carbon electrode for direct sensing of glucose

We have electrodeposited a composite film consisting of graphene oxide, chitosan and glucose oxidase directly on a glassy carbon electrode (GCE) through electrochemical reduction of a solution of the 3 components under controlled direct electrical potential. The procedure takes only several minutes, and the thickness of the resulting film is uniform and controllable. The GOx has uncompromised bioactivity and exhibits reversible 2-proton and 2-electron transfer in presence of glucose. It therefore can be used amperometric sensing of glucose. The biosensor has a fast response (<3 s), a detection limit of 0.4 μM (which is 50-fold lower compared to the biosensor prepared by drop-casting solutions of the same materials onto an GCE), and a linear response in the 0.4 μM to 2 mM concentration range (which again is much better than that of the biosensor prepared by the drop-casting method). Other features include high reproducibility, long-time storage stability, and satisfactory selectivity. We presume that the direct single-step electrodeposition of this nanocomposite offers a promising approach towards novel types of highly sensitive and stable electrochemical biosensors.