Microfluidic-enzymatic biosensor with immobilized tyrosinase for electrochemical detection of pipemidic acid in pharmaceutical samples

A perspective on the biotechnological applications of the versatile tyrosinase

Tyrosinase (E.C. 1.14.18. 1) is a type of Cu-containing oxidoreductase which has bifunctional activity for various phenolic substrates: ortho-hydroxylation of monophenols to diphenols (a cresolase activity) and oxidation of diphenols to quinones (a catecholase activity). Based on the broad substrate spectrum, tyrosinase has been used in bioremediation of phenolic pollutants, constructing biosensors for identifying phenolic compounds, and L-DOPA synthesis. Furthermore, not only tyrosinase has been used to produce useful polyphenol derivatives, but also it is recently revealed that the promiscuous activity of tyrosinase is closely related with delignification in the biorefinery. Accordingly, tyrosinase might be a potential biocatalyst for industrial applications (e.g., electroenzymatic L-DOPA production, but its long-term stability and reusability should be further explored. In this review, we emphasize the versatility of tyrosinase, which includes conventional applications, and suggest new perspectives as an industrial biocatalyst (e.g., electroenzymatic L-DOPA production). Especially, this review focuses on and comprehensively discusses recent innovative studies.

Influence of antibody immobilization strategy on carbon electrode immunoarrays

We report here the influence of antibody immobilization strategy for protein immunosensors on screen printed carbon electrode arrays in terms of antibody binding activity, analytical sensitivity, limit of detection, and stability. Horseradish peroxidase (HRP) was the model analyte with anti-HRP immobilized on the sensors, and HRP activity was used for detection. Covalently immobilized anti-HRP antibodies on electrodes coated with chitosan, electrochemically reduced graphene oxide (rGO), and dense gold nanoparticle (AuNP) films had only 20-30% of the total immobilized antibodies active for binding. Active antibodies increased to 60% with passively adsorbed antibodies on bare electrodes, to 85% with oriented antibodies using protein A covalently immobilized on AuNP-coated carbon electrode, and to 98% when attached to protein A passively adsorbed onto bare electrodes. Passively adsorbed antibodies on bare electrodes lost activity in 1-2 days, but antibodies immobilized using other strategies remained relatively stable after 5 days. Covalent immobilization gave limits of detection (LOD) of 40 fg mL-1, while passively adsorbed antibodies or protein A on carbon electrodes had LODs 4-8 fg mL-1, but were unstable. Sensitivity was highest for antibodies covalently attached to AuNP electrodes (2.40 nA per log pg per mL) that also had highest antibody coverage, and decreased slightly when protein A on AuNP was used to orient antibodies. Passively adsorbed antibodies and oriented antibodies on protein A gave slightly lower sensitivities. Immobilization strategy or antibody orientation did not have a significant effect on LOD, but dynamic range increased as the number of active antibodies on sensor surfaces increased.

Recent Advances in Electrochemical Biosensors Based on Enzyme Inhibition for Clinical and Pharmaceutical Applications

A large number of enzyme inhibitors are used as drugs to treat several diseases such as gout, diabetes, AIDS, depression, Parkinson's and Alzheimer's diseases. Electrochemical biosensors based on enzyme inhibition are useful devices for an easy, fast and environment friendly monitoring of inhibitors like drugs. In the last decades, electrochemical biosensors have shown great potentials in the detection of different drugs like neostigmine, ketoconazole, donepezil, allopurinol and many others. They attracted increasing attention due to the advantage of being high sensitive and accurate analytical tools, able to reach low detection limits and the possibility to be performed on real samples. This review will spotlight the research conducted in the past 10 years (2007-2017) on inhibition based enzymatic electrochemical biosensors for the analysis of different drugs. New assays based on novel bio-devices will be debated. Moreover, the exploration of the recent graphical approach in diagnosis of reversible and irreversible inhibition mechanism will be discussed. The accurate and the fast diagnosis of inhibition type will help researchers in further drug design improvements and the identification of new molecules that will serve as new enzyme targets.

Immobilized Biocatalyst for Detection and Destruction of the Insensitive Explosive, 2,4-Dinitroanisole (DNAN)

Accurate and convenient detection of explosive components is vital for a wide spectrum of applications ranging from national security and demilitarization to environmental monitoring and restoration. With the increasing use of DNAN as a replacement for 2,4,6-trinitrotoluene (TNT) in insensitive explosive formulations, there has been a growing interest in strategies to minimize its release and to understand and predict its behavior in the environment. Consequently, a convenient tool for its detection and destruction could enable development of more effective decontamination and demilitarization strategies. Biosensors and biocatalysts have limited applicability to the more traditional explosives because of the inherent limitations of the relevant enzymes. Here, we report a highly specific, convenient and robust biocatalyst based on a novel ether hydrolase enzyme, DNAN demethylase (that requires no cofactors), from a Nocardioides strain that can mineralize DNAN. Biogenic silica encapsulation was used to stabilize the enzyme and enable it to be packed into a model microcolumn for application as a biosensor or as a bioreactor for continuous destruction of DNAN. The immobilized enzyme was stable and not inhibited by other insensitive munitions constituents. An alternative method for DNAN detection involved coating the encapsulated enzyme on cellulose filter paper. The hydrolase based biocatalyst could provide the basis for a wide spectrum of applications including detection, identification, destruction or inertion of explosives containing DNAN (demilitarization operations), and for environmental restorations.

Synthesis and electrocatalytic effect of Ag@Pt core-shell nanoparticles supported on reduced graphene oxide for sensitive and simple label-free electrochemical aptasensor

Bimetallic Ag@Pt core-shell nanoparticles supported on reduced graphene oxide nanosheets (Ag@Pt-GRs) was synthesized and used as novel desirable sensor platform and electrocatalyst for catechol as probe in aptasensor. Gold screen-printed electrodes modified with Ag@Pt-GRs and applied to advance enzyme-free and label-free electrochemical aptasensor for detection of protein biomarker tumor necrosis factor-alpha (TNF-α). The morphology of the Ag@Pt-GRs could be characterized by transmission electron microscopy, X-ray diffraction and UV-vis spectra. The results showed that these nanocomposite exhibited attractive electrocatalytic activity and also yielded large surface area, which improve the amount of immobilized TNF-α aptamer. Due to the excellent electrocatalytic activity of Ag@Pt-GRs towards the oxidation of catechol, determination of TNF-α antigen was based on its obstruction to the electrocatalytic oxidation of catechol by Ag@Pt-GRs after binding to the surface of electrode through interaction with the aptamer. The calibration curve was obtained by differential pulse voltammetry and square wave voltammetry. Under optimum conditions, the results demonstrated that this electrochemical aptasensor possessed a dynamic range from 0.0 pg/mL to 60 pg/mL with a low detection limit of 2.07 pg/mL for TNF-α. The analytical usefulness of the aptasensor was finally demonstrated analyzing serum samples. The simple fabrication method, high sensitivity, specificity, good reproducibility and stability as well as acceptable accuracy for TNF-α detection in human serum samples are the main advantages of this aptasensor, which might have broad applications in protein diagnostics and bioassay.

A Simple Microfluidic Electrochemical HPLC Detector for Quantifying Fenton Reactivity from Welding Fumes

Development and characterization of a simple microfluidic electrochemical flow cell that can be coupled with HPLC to enable dual absorbance/electrochemical detection is described. Coupling absorbance and electrochemical detection increases the information that can be gathered from a single injection, but a second (typically expensive) detection system is required. Here, an inexpensive, customizable microfluidic electrochemical detector is coupled in series with a commercial HPLC/UV system. The microfluidic device is made from poly(dimethylsiloxane) and contains carbon paste electrodes. To demonstrate the utility of this dual-detection system, the reaction products of the radical scavenging agent salicylic acid and hydroxyl radical generated by Fenton chemistry were analyzed. The dual-detection system was used to quantify 2,5-dihydroxybenzoic acid, 2,3-dihydroxybenzoic acid, and catechol produced by the addition of H₂O₂ to filter samples of welding fumes. Measurement recovery was high, with percent recoveries between 97-102%, 92-103%, and 95-103% for 2,5-dihydroxybenzoic acid, 2,3-dihydroxybenzoic acid, and catechol, respectively, for control samples. The methods described in this work are simple, reliable, and can inexpensively couple electrochemical detection to HPLC-UV systems.

Continuous-flow step gradient mass spectrometry based method for the determination of kinetic parameters of immobilized mushroom tyrosinase in equilibrating conditions: comparison with free enzyme

A mass spectrometry (MS)-based methodology for enzymatic assay in equilibrium conditions was designed and evaluated. This on-line assay involves the introduction of a continuous-flow step gradient (CFSG) of a substrate solution in the column containing immobilized enzyme and the simultaneous tracking of the product formation. We showed that the constant concentration of substrate in the entire bioreactor for an appropriate duration ensures the equilibration of the studied enzyme (mushroom tyrosinase). Under these conditions, it was demonstrated also that the kinetic and enzymatic parameters (Michaelis-Menten constant, K(M) , the maximal specific activity, SA(max)) are independent of the flow rate of the mobile phase. The feasibility of the mentioned approach for inhibitory tests was also investigated. The coupling of the mass spectrometer to the bio-reactor allows the selective monitoring of the enzymatic reaction products and increases their detection level. Very high sensitivity, 500 pmol/min/column, and selective monitoring of the products of the enzymatic reaction are allowed by MS detection. The methodology developed here constitutes a sensitive analytical tool to study enzymes requiring long equilibration times.