Amplification of amperometric biosensor responses by electrochemical substrate recycling. 3. Theoretical and experimental study of the phenol-polyphenol oxidase system immobilized in Laponite hydrogels and layer-by-layer self-assembled structures

Electrochemical Biosensor for Markers of Neurological Esterase Inhibition

A novel, integrated experimental and modeling framework was applied to an inhibition-based bi-enzyme (IBE) electrochemical biosensor to detect acetylcholinesterase (AChE) inhibitors that may trigger neurological diseases. The biosensor was fabricated by co-immobilizing AChE and tyrosinase (Tyr) on the gold working electrode of a screen-printed electrode (SPE) array. The reaction chemistry included a redox-recycle amplification mechanism to improve the biosensor's current output and sensitivity. A mechanistic mathematical model of the biosensor was used to simulate key diffusion and reaction steps, including diffusion of AChE's reactant (phenylacetate) and inhibitor, the reaction kinetics of the two enzymes, and electrochemical reaction kinetics at the SPE's working electrode. The model was validated by showing that it could reproduce a steady-state biosensor current as a function of the inhibitor (PMSF) concentration and unsteady-state dynamics of the biosensor current following the addition of a reactant (phenylacetate) and inhibitor phenylmethylsulfonylfluoride). The model's utility for characterizing and optimizing biosensor performance was then demonstrated. It was used to calculate the sensitivity of the biosensor's current output and the redox-recycle amplification factor as a function of experimental variables. It was used to calculate dimensionless Damkohler numbers and current-control coefficients that indicated the degree to which individual diffusion and reaction steps limited the biosensor's output current. Finally, the model's utility in designing IBE biosensors and operating conditions that achieve specific performance criteria was discussed.

Integrated Experimental and Theoretical Studies on an Electrochemical Immunosensor

Electrochemical immunosensors (EIs) integrate biorecognition molecules (e.g., antibodies) with redox enzymes (e.g., horseradish peroxidase) to combine the advantages of immunoassays (high sensitivity and selectivity) with those of electrochemical biosensors (quantitative electrical signal). However, the complex network of mass-transfer, catalysis, and electrochemical reaction steps that produce the electrical signal makes the design and optimization of EI systems challenging. This paper presents an integrated experimental and modeling framework to address this challenge. The framework includes (1) a mechanistic mathematical model that describes the rate of key mass-transfer and reaction steps; (2) a statistical-design-of-experiments study to optimize operating conditions and validate the mechanistic model; and (3) a novel dimensional analysis to assess the degree to which individual mass-transfer and reaction steps limit the EI's signal amplitude and sensitivity. The validated mechanistic model was able to predict the effect of four independent variables (working electrode overpotential, pH, and concentrations of catechol and hydrogen peroxide) on the EI's signal magnitude. The model was then used to calculate dimensionless groups, including Damkohler numbers, novel current-control coefficients, and sensitivity-control coefficients that indicated the extent to which the individual mass-transfer or reaction steps limited the EI's signal amplitude and sensitivity.

Protein-based polyelectrolyte multilayers

The immobilization of proteins to impart specific functions to surfaces is topical for chemical engineering, healthcare and diagnosis. Layer-by-Layer (LbL) self-assembly is one of the most used method to immobilize macromolecules on surfaces. It consists in the alternate adsorption of oppositely charged species, resulting in the formation of a multilayer. This method in principle allows any charged object to be immobilized on any surface, from aqueous solutions. However, when it comes to proteins, the promises of versatility, simplicity and universality that the LbL approach holds are unmet due to the heterogeneity of protein properties. In this review, the literature is analyzed to make a generic approach emerge, with a view to facilitate the LbL assembly of proteins with polyelectrolytes (PEs). In particular, this review aims at guiding the choice of the PE and the building conditions that lead to the successful growth of protein-based multilayered self-assemblies.

Electrochemical biosensors based on multienzyme systems: Main groups, advantages and limitations - A review

In the review, the principles and main purposes of using multienzyme systems in electrochemical biosensors are analyzed. Coupling several enzymes allows an extension of the spectrum of detectable substances, an increase in the biosensor sensitivity (in some cases, by several orders of magnitude), and an improvement of the biosensor selectivity, as showed on the examples of amperometric, potentiometric, and conductometric biosensors. The biosensors based on cascade, cyclic and competitive enzyme systems are described alongside principles of function, advantages, disadvantages and practical use for real sample analyses in various application areas (food production and quality control, clinical diagnostics, environmental monitoring). The complications and restrictions regarding the development of multienzyme biosensors are evaluated. The recommendations on the reasonability of elaboration of novel multienzyme biosensors are given.

Laponite-based Nanomaterials for Biomedical Applications: A Review

Laponite based nanomaterials (LBNMs) are highly diverse regarding their mechanical, chemical, and structural properties, coupled with shape, size, mass, biodegradability and biocompatibility. These ubiquitous properties of LBNMs make them appropriate materials for extensive applications. These have enormous potential for effective and targeted drug delivery comprised of numerous biodegradable materials which results in enhanced bioavailability. Moreover, the clay material has been explored in tissue engineering and bioimaging for the diagnosis and treatment of various diseases. The material has been profoundly explored for minimized toxicity of nanomedicines. The present review compiled relevant and informative data to focus on the interactions of laponite nanoparticles and application in drug delivery, tissue engineering, imaging, cell adhesion and proliferation, and in biosensors. Eventually, concise conclusions are drawn concerning biomedical applications and identification of new promising research directions.

CF, Gnanamani A. Preparation, characterization and reusability efficacy of amine-functionalized graphene oxide-polyphenol oxidase complex for removal of phenol from aqueous phase

The present study explores the preparation, characterization and reusability efficacy of an amine-functionalized graphene oxide and polyphenol oxidase complex for the removal of phenol from aqueous phase. In brief, graphene oxide (GO) is synthesized according to modified Hummer's method using graphite powder and functionalized with amine using the Bucherer's method (GO-NH₂). Partially purified polyphenol oxidase (PP-PPO) enzyme extracted from Solanum tuberosum is used for the preparation of the complex. The resultant GO-NH₂-(PP-PPO) complex is used for the phenol degradation studies. The samples of GO, GO-NH₂, and GO-NH₂-(PP-PPO) complex are characterized using various instrumental techniques. Spectral UV data and FTIR and XRD diffraction patterns confirm the amine functionalization on GO. Raman spectrum, SEM micrograph and thermogravimetric (TG) analyses authenticate the linked enzyme on GO-NH₂. GO-NH₂-(PP-PPO) complex demonstrates >90% enzyme stability at all the studied temperatures (4 °C, −20 °C, RT and 37 °C). Phenol degradation studies show >99% removal of 1000 ppm of phenol within 5 hours from the start of the experiment at the optimized pH of 5.0 and temperature of 30 °C, as inferred from HPLC analysis. Catechol and hydroquinone compounds are identified as intermediates during the removal of phenol. Furthermore, studies on the reuse of GO-NH₂-(PP-PPO) complex suggest that the complex can be used effectively for the removal of phenol up to maximum 7 cycles. In conclusion, the observations made in the present study show that the complex containing amine-functionalized graphene oxide and phenoloxidase is effective for the removal of phenol with appreciable reusability.

Schematic representation of the present study describing amine functionalization followed by enzyme immobilization and degradation of phenol.

Analytical Expressions Pertaining to the Concentration of Substrates and Product in Phenol-Polyphenol Oxidase System Immobilized in Laponite Hydrogels: A Reciprocal Competitive Inhibition Process

Theoretical analysis corresponding to the diffusion and kinetics of substrate and product in an amperometric biosensor is developed and reported in this paper. The nonlinear coupled system of diffusion equations was analytically solved by Homotopy perturbation method. Herein, we report the approximate analytical expressions pertaining to substrate concentration, product concentration, and current response for all possible values of diffusion and kinetic parameters. The numerical solution of this problem is also reported using Scilab/Matlab program. Also, we found excellent agreement between the analytical results and numerical results upon comparison.

Immobilization of magnetic iron oxide nanoparticles on laponite discs - an easy way to biocompatible ferrofluids and ferrogels

Magnetic nanocomposites containing iron oxide (maghemite) nanoparticles, well embedded in a synthetic clay matrix (laponite) were prepared by a new one step chemical route and characterized by TEM, XRD, magnetization measurements, Mössbauer spectroscopy, DLS, and MRI measurements. The synthetic procedure leads to non-stoichiometric γ-Fe(2)O(3) with a controllable content in the nanocomposite. Magnetic nanoparticles incorporated in the diamagnetic clay matrix exhibit a mean diameter of 13 nm, superparamagnetic behavior with a high saturation magnetization achievable at low applied magnetic fields. In-field Mössbauer spectra and ZFC/FC magnetization curves reveal a perfect ferrimagnetic ordering within nanoparticles with negligible spin frustration and interparticle interactions due to the complete coating of maghemite surfaces by the nanocrystalline laponite matrix. Magnetic iron oxide nanoparticles embedded in laponite matrix exhibit strong T(2) weighted MRI contrast. The maghemite/laponite composite particles have 200 nm hydrodynamic diameter and form very stable hydrosols and/or hydrogels depending on their concentration in water.

Biosensing applications of clay-modified electrodes: a review

Two-dimensional layered inorganic solids, such as cationic clays and layered double hydroxides (LDHs), also defined as anionic clays, have open structures which are favourable for interactions with enzymes and which intercalate redox mediators. This review aims to show the interest in clays and LDHs as suitable host matrices likely to immobilize enzymes onto electrode surfaces for biosensing applications. It is meant to provide an overview of the various types of electrochemical biosensors that have been developed with these 2D layered materials, along with significant advances over the last several years. The different biosensor configurations and their specific transduction procedures are discussed.

Expression of esophageal cancer related gene 4 (ECRG4), a novel tumor suppressor gene, in esophageal cancer and its inhibitory effect on the tumor growth in vitro and in vivo

The ECRG4 gene was initially identified and cloned in our laboratory from human normal esophageal epithelium (GenBank accession no. AF325503). We revealed the expression of ECRG4 protein was downregulated in 68.5% (89/130) ESCC samples using tissue microarray. The low ECRG4 protein expression was significantly associated with regional lymph node metastasis, primary tumor size, and tumor stage in ESCC (p < 0.05). ECRG4 mRNA expression was downregulated in ESCC due to the hypermethylation in the gene promoter. The treatment with 5-aza-2'-deoxycytidine, which is a DNA methyltransferase inhibitor restored ECRG4 mRNA expression in ESCC cells. The result indicated that promoter hypermethylation may be 1 main mechanism leading to the silencing of ECRG4. The high expression of ECRG4 in patients with ESCC was associated with longer survival compared with those with low ECRG4 expression by Kaplan-Meier survival analysis (p < 0.05). ECRG4 protein was an independent prognostic factor for ESCC by multivariable Cox proportional hazards regression analysis (p < 0.05). The restoration of ECRG4 expression in ESCC cells inhibited cell proliferation, colony formation, anchorage-independent growth, cell cycle progression and tumor growth in vivo (p < 0.05). The transfection of ECRG4 gene in ESCC cells inhibited the expression of NF-kappaB and nuclear translocation, in addition to the expression of COX-2, a NF-kappaB target gene, was attenuated. Taken together, ECRG4 is a novel candidate tumor suppressor gene in ESCC, and ECRG4 protein is a candidate prognostic marker for ESCC.

Quaternary ammonium functionalized clay film electrodes modified with polyphenol oxidase for the sensitive detection of catechol

Naturally occurring Cameroonian smectite clay has been grafted with trimethylpropylammonium (TMPA) groups and the resulting organoclay has been deposited onto a glassy carbon electrode surface as a suitable immobilization matrix for polyphenol oxidase (PPO). High sensitivity of the electrochemical device to catechol biosensing can be achieved when the enzyme was impregnated within the organoclay film subsequent to its deposition due to favorable electrostatic interaction between PPO and the TMPA-clay layer. The bioelectrode preparation method was also compatible with the use of a mediator (i.e., ferrocene) and the best performance was obtained with a three-layer configuration made of glassy carbon coated with a first layer of ferrocene (Fc), which was then covered with the PPO-impregnated TMPA-clay layer, and finally overcoated with an enzyme-free TMPA-clay film acting as a protecting overlayer to avoid leaching of the biomolecule in solution. The electrochemical behavior of the modified film electrodes was first characterized by cyclic voltammetry and, then, they were evaluated for the amperometric biosensing of the model analyte catechol in batch conditions and in flow injection analysis. Various experimental parameters likely to influence the biosensor response have been investigated, including the electrode preparation mode (composition configuration, thickness), the usefulness of a mediator, the operating potential and pH of the medium, as well as the advantageous features of the TMPA-clay in comparison to related film electrodes based on non-functionalized clays. The organoclay was found to provide a favorable environment to enzyme activity and the multilayer configuration of the film electrode to provide a biosensor with good characteristics, such as an extended linear range for catechol detection (2 x 10(-8) to 1.2 x 10(-5)M) and a detection limit in the nanomolar range (9 x 10(-9)M).

Nanostructured biosensor for measuring neuropathy target esterase activity

Neuropathy target esterase (NTE) is a membrane protein found in human neurons and other cells, including lymphocytes. Binding of certain organophosphorus (OP) compounds to NTE is believed to cause OP-induced delayed neuropathy (OPIDN), a type of paralysis for which there is no effective treatment. Mutations in NTE have also been linked with serious neurological diseases, such as motor neuron disease. This paper describes development of the first nanostructured biosensor interface containing a catalytically active fragment of NTE known as NEST. The biosensor was fabricated using the layer-by-layer assembly approach, by immobilizing a layer of NEST on top of multilayers consisting of a polyelectrolyte (poly-L-lysine) and an enzyme (tyrosinase). The biosensor has a response time on the order of seconds and gives a concentration-dependent decrease in sensor output in response to a known NEST (and NTE) inhibitor. Potential applications of the biosensor include screening OP compounds for NTE inhibition and investigating the enzymology of wild-type and mutant forms of NTE. Although the development of a NEST biosensor was the primary purpose of this study, we found that the approach developed for NEST could also be extended to measure the activity of other esterases involved in neural processes, such as acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). On the basis of measured sensitivities, phenyl valerate was the preferred substrate for NEST and BChE, whereas phenyl acetate was better for AChE.

High Amplification Rates from the Association of Two Enzymes Confined within a Nanometric Layer Immobilized on an Electrode: Modeling and Illustrating Example

Electrochemical responses (e.g., chronoamperometric) obtained with an immobilized enzyme that produces an electroactive species may be used to quantitate the amount of enzyme or the concentration of its substrate. It is shown, on theoretical and experimental bases, that product-to-substrate coupling with a second enzyme co-immobilized with the first within one or within a small number of monolayers, allows high amplification rates (higher than 1000), avoids membrane transport limitations, and lends itself to precise kinetic analyses that provide guidelines for optimization of the analytical sensitivity. Very large amplification factors, as large as several thousands, can be reached experimentally, in agreement with appropriately derived theoretical predictions, thus opening the route to the rational design of high-performance substrate sensing or affinity assays applications.

Structured thin films as functional components within biosensors

This review provides an introduction to the field of thin films formed by Langmuir-Blodgett or self-assembly techniques and discusses applications in the field of biosensors. The review commences with an overview of thin films and methods of construction. Methods covered will include Langmuir-Blodgett film formation, formation of self-assembled monolayers such as gold-thiol monolayers and the formation of multilayers by the self-assembly of polyelectrolytes. The structure and forces governing the formation of the materials will also be discussed. The next section focussed on methods for interrogating these films to determine their selectivity and activity. Interrogation methods to be covered will include electrochemical measurements, optical measurements, quartz crystal microbalance, surface plasmon resonance and other techniques. The final section is dedicated to the functionality of these films, incorporation of biomolecules within these films and their effect on film structure. Species for incorporation will include antibodies, enzymes, proteins and DNA. Discussions on the location, availability, activity and stability of the included species are included. The review finishes with a short consideration of future research possibilities and applications of these films.

Structural characterization of (3-mercaptopropyl)sulfonate monolayer on gold surfaces

We have investigated the structure of (3-mercaptopropyl)sulfonate (MPS) monolayer self-assembled onto gold surfaces by quartz crystal microbalance with energy dissipation monitoring (QCM-D) and various electrochemical methods. QCM-D experiments show that the MPS monolayer behaves as a thin rigid film with a surfacic mass of 166 ng cm(-2). Interfacial capacitance measurements demonstrate that the MPS monolayer is a rather open structure that can be penetrated by the ionic species of the phosphate buffer electrolyte. From MPS reductive desorption experiments, MPS surface concentration corresponds to 4.6 x 10(-10) mol cm(-2), which represents 60% of the coverage reported for a densely packed thiol monolayer. Despite this low packing density, oxidation of catechol is strongly inhibited leading to voltammograms that are free ofdiffusional contribution. This unique behavior has been exploited to show that the MPS monolayer covers the entire gold surface with a surface coverage at least equal to theta = 0.9981, which means a very low number of MPS-free pinholes and/or defects. Kinetics of electron transfer toward soluble redox species has been studied using catechol as a neutral hydrophilic probe, but also ferrocyanide as hydrophilic anion and ferrocenemethanol as neutral hydrophobic molecule. It is proposed that the MPS monolayer provides a high kinetic barrier toward permeation of these species and that electron transfer mainly occurs by electron tunneling through the MPS monolayer.

Subnanomolar cyanide detection at polyphenol oxidase/clay biosensors

A novel, inexpensive, and simple amperometric biosensor based on immobilization of polyphenol oxidase (PPO) into Zn-Al layered double hydroxides, also called anionic clays, is applied for determination of cyanide. The detection of cyanide was performed via its inhibiting action on the PPO electrode. Measurement was carried out with 3,4-dihydroxyphenylacetic acid as enzyme substrate, the enzymatically generated quinoid products being electroreduced at -0.2 V. An extremely sensitive detection limit (0.1 nM) was obtained for cyanide. Enzyme immobilization into an anionic exchanger clay seems to cause an increase in cyanide inhibition effects because of anion accumulation in the clay matrix.

Correlation of Analyte Structures with Biosensor Responses Using the Detection of Phenolic Estrogens as a Model

The apparent increase in hormone-induced cancers and disorders of the reproductive tract in wildlife and humans has led to a search for an accurate and reliable method for monitoring endocrine-disrupting chemicals (EDCs). This study presents a generic approach that may allow researchers to establish screening procedures for potential EDCs by correlating the analyte structures with biosensor responses and explain possible reaction mechanisms. A simple amperometric tyrosinase-based biosensor (Tyr-CPE) has been developed for the detection of phenolic EDCs. The investigation of the enzymatic oxidation of selected phenolic estrogens was first carried out using UV-vis spectroscopy. The result was used to correlate sensor responses to enzymatic activity. Natural phytoestrogen polyphenols, including resveratrol (RES), genistein (GEN), and quercetin (QRC), were compared with synthetic estrogens, for example, bisphenol A (BPhA), nonylphenol (NPh), and diethylstilbestrol (DES). The Tyr-CPE biosensor resulted in rapid, simple, and accurate measurement of phenolic estrogens with varying degrees of sensitivity, selectivity, and response times. The sensor responses have been evaluated for the detection of binary and tertiary mixtures of EDCs and natural estrogens. The results showed that BPhA could be successfully discriminated in a composite mixture containing NPh and DES at various ratios. In the case of natural phenolic estrogens GEN, RES, and QRC, the sensor allows the determination of a total phenolic content. The sensor was also validated for the detection of BPhA in a real environmental water sample, and the results was compared with standard ASTM method 9065. Mechanistically, our results indicated that the number of OH groups, the nature and the position of aryl ring substituents, or both could affect the detection limit and the biosensor sensitivity.

Effect of the polycation nature on the structure of layer-by-layer electrostatically self-assembled multilayers of polyphenol oxidase

Self-assembled multilayers comprised of alternate layers of polyphenol oxidase (PPO) and poly(allylamine) (PAH) or PPO and poly(diallyldimethylamine) (PDDA), deposited on a 3-mercaptopropanesulfonic acid (MPS)-modified gold surface, were studied "in-situ" (under water) by means of ellipsometry and quartz crystal microbalance (QCM), and "ex-situ" (in open air) by ellipsometry and fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS). Optical ellipsometric properties of (PAH)(n)(PPO)(n) and (PDDA)(n)(PPO)(n) multilayers were obtained at two wavelengths, employing an isotropic single-layer model with the substrate parameters measured after thiol adsorption. Film thickness as well as ellipsometric mass values based on the de Feijter equation were also evaluated. The quartz crystal impedance analysis showed that self-assembled multilayers behaved as acoustically thin films, and therefore, the shifts observed in the film inductive impedance parameter were interpreted in terms of gravimetric mass. The enzyme mass up-take in each adsorption step was determined on PAH or on PDDA topmost layer. A comparative study between the ellipsometric thickness and acoustic mass values allowed us to obtain average values of "apparent" densities of (2.1 +/- 0.1) and (2.4 +/- 0.1) g cm(-3) for (PAH)(n)(PPO)(n) and (PDDA)(n)(PPO)(n) multilayers, respectively. The content of water included in the open polymer-enzyme structure was evaluated by comparison of QCM and ellipsometric mass values. FT-IRRAS spectra of each layer in (PAH)(n)(PPO)(n) and (PDDA)(n)(PPO)(n) films were recorded, and the intensity ratio of the amide bands was evaluated to obtain information about layer-by-layer enzyme conformation. An enzyme/polycation distribution model for (PAH)(n)(PPO)(n)and (PDDA)(n)(PPO)(n) multilayer structures is presented on the basis of combined ellipsometric, QCM, and FT-IRRAS results.

Electrochemical biosensors for environmental monitoring

Highly sensitive electrochemical biosensors offer precision, sensitivity, rapidity, and ease of operation for on-site environmental analysis. An electrochemical biosensor is an analytical device in which a specific biological recognition element (bioreceptor) is integrated within or intimately associated with an electrode (transducer) that converts the recognition event to a measurable electrical signal for the purpose of detecting a target compound (analyte) in solution. The signal generated allows both qualitative and quantitative measurements of an analyte in real time. In most cases, a miniaturized electrochemical cell contains a low volume of analyte, which is vital when dealing with hazardous materials and makes such devices ideal for environmental monitoring. This approach not only provides the means for on-site analysis but also removes the time delay and sample alteration that can occur during transport to a centralized laboratory. We first address the basic principles of electroanalytical measurement and the merger of electrochemistry and biology into a biosensing system, and then we discuss various environmental monitoring strategies involving this technology.