Effects of polymerization potential on the permselectivity of poly(o-phenylenediamine) coatings deposited on Pt–Ir electrodes for biosensor applications

Design and Evaluation of a Lactate Microbiosensor: Toward Multianalyte Monitoring of Neurometabolic Markers In Vivo in the Brain

Direct in vivo measurements of neurometabolic markers in the brain with high spatio-temporal resolution, sensitivity, and selectivity is highly important to understand neurometabolism. Electrochemical biosensors based on microelectrodes are very attractive analytical tools for continuous monitoring of neurometabolic markers, such as lactate and glucose in the brain extracellular space at resting and following neuronal activation. Here, we assess the merits of a platinized carbon fiber microelectrode (CFM/Pt) as a sensing platform for developing enzyme oxidase-based microbiosensors to measure extracellular lactate in the brain. Lactate oxidase was immobilized on the CFM/Pt surface by crosslinking with glutaraldehyde. The CFM/Pt-based lactate microbiosensor exhibited high sensitivity and selectivity, good operational stability, and low dependence on oxygen, temperature, and pH. An array consisting of a glucose and lactate microbiosensors, including a null sensor, was used for concurrent measurement of both neurometabolic substrates in vivo in the anesthetized rat brain. Rapid changes of lactate and glucose were observed in the cortex and hippocampus in response to local glucose and lactate application and upon insulin-induced fluctuations of systemic glucose. Overall, these results indicate that microbiosensors are a valuable tool to investigate neurometabolism and to better understand the role of major neurometabolic markers, such as lactate and glucose.

Nitric oxide permselectivity in electropolymerized films for sensing applications

The presence of biological interferents in physiological media necessitates chemical modification of the working electrode to facilitate accurate electrochemical measurement of nitric oxide (NO). In this study, we evaluated a series of self-terminating electropolymerized films prepared from one of three isomers of phenylenediamine (PD), phenol, eugenol, or 5-amino-1-naphthol (5A1N) to improve the NO selectivity of a platinum working electrode. The electrodeposition procedure for each monomer was individually optimized using cyclic voltammetry (CV) or constant potential amperometry (CPA). Cyclic voltammetry deposition parameters favoring slower film formation generally yielded films with improved selectivity for NO over nitrite and l-ascorbate. Nitric oxide sensors were fabricated and compared using the optimized deposition procedure for each monomer. Sensors prepared using poly-phenol and poly-5A1N film-modified platinum working electrodes demonstrated the most ideal analytical performance, with the former demonstrating the best selectivity. In simulated wound fluid, platinum electrodes modified with poly-5A1N films proved superior with respect to the NO sensitivity and detection limit.

Low electro-synthesis potentials improve permselectivity of polymerized natural phenols in biosensor applications

First-generation amperometric biosensors are often based on the electro-oxidation of oxidase-generated H₂O₂. At the applied potential used in most studies, other molecules such as ascorbic acid or dopamine can be oxidized. Phenylenediamines are commonly used to avoid this problem: when these compounds are electro-deposited onto the transducer surface in the form of poly-phenylenediamine, a highly selective membrane is formed. Although there is no evidence of toxicity of the resulting polymer, phenylenediamine monomers are considered carcinogenic. An aim of this work was to evaluate the suitability of natural phenols as non-toxic alternatives to the ortho isomer of phenylenediamine. Electrosynthesis over Pt-Ir electrodes of 2-methoxy phenols (guaiacol, eugenol and isoeugenol), and hydroxylated biphenyls (dehydrodieugenol and magnolol) was achieved. The potentials used in the present study are significantly lower than values commonly applied during electro-polymerization. Polymers were obtained by means of constant potential amperometry, instead of cyclic voltammetry, in order to achieve multiple polymerizations, hence decreasing the time of realization and variability. Permselective properties of natural phenols were significantly improved at low polymerization potentials. Among the tested compounds, isoeugenol and magnolol, polymerized respectively at +25mV and +170mV against Ag/AgCl reference electrode, proved as permselective as poly-ortho-phenylenediamine and may be considered as effective polymeric alternatives. The natural phenol-coated electrodes were stable and responsive throughout 14 days. A biosensor prototype based on acetylcholine esterase and choline oxidase was electro-coated with poly-magnolol in order to evaluate the interference-rejecting properties of the electrosynthesized film in an amperometric biosensor; a moderate decrease in ascorbic acid rejection was observed during in vitro calibration of biosensors.

Characterization of Biosensors Based on Recombinant Glutamate Oxidase: Comparison of Crosslinking Agents in Terms of Enzyme Loading and Efficiency Parameters

Amperometric l-glutamate (Glu) biosensors, based on both wild-type and a recombinant form of l-glutamate oxidase (GluOx), were designed and characterized in terms of enzyme-kinetic, sensitivity and stability parameters in attempts to fabricate a real-time Glu monitoring device suitable for future long-term detection of this amino acid in biological and other complex media. A comparison of the enzyme from these two sources showed that they were similar in terms of biosensor performance. Optimization of the loading of the polycationic stabilization agent, polyethyleneimine (PEI), was established before investigating a range of crosslinking agents under different conditions: glutaraldehyde (GA), polyethylene glycol (PEG), and polyethylene glycol diglycidyl ether (PEGDE). Whereas PEI-free biosensor designs lost most of their meager Glu sensitivity after one or two days, configurations with a 2:5 ratio of dip-evaporation applications of PEI(1%):GluOx(400 U/mL) displayed a 20-fold increase in their initial sensitivity, and a decay half-life extended to 10 days. All the crosslinkers studied had no effect on initial Glu sensitivity, but enhanced biosensor stability, provided the crosslinking procedure was carried out under well-defined conditions. The resulting biosensor design based on the recombinant enzyme deposited on a permselective layer of poly-(ortho-phenylenediamine), PoPD/PEI₂/GluOx₅/PEGDE, displayed good sensitivity (LOD < 0.2 μM), response time (t90% < 1 s) and stability over a 90-day period, making it an attractive candidate for future long-term monitoring of Glu concentration dynamics in complex media.

Screen-printed carbon electrodes modified by rhodium dioxide and glucose dehydrogenase

The described glucose biosensor is based on a screen-printed carbon electrode (SPCE) modified by rhodium dioxide, which functions as a mediator. The electrode is further modified by the enzyme glucose dehydrogenase, which is immobilized on the electrode's surface through electropolymerization with m-phenylenediamine. The enzyme biosensor was optimized and tested in model glucose samples. The biosensor showed a linear range of 500–5000 mg L⁻¹ of glucose with a detection limit of 210 mg L⁻¹ (established as 3σ) and response time of 39 s. When compared with similar glucose biosensors based on glucose oxidase, the main advantage is that neither ascorbic and uric acids nor paracetamol interfere measurements with this biosensor at selected potentials.

Effects of applied potential on the mass of non-conducting poly(ortho-phenylenediamine) electro-deposited on EQCM electrodes: comparison with biosensor selectivity parameters

An electrochemical quartz-crystal microbalance (EQCM) was used to determine the mass of poly-(o-phenylenediamine) (PoPD) layers electro-deposited at different applied potentials in neutral buffered monomer solution, conditions that produce the insulating form of the polymer used as a permselective membrane in biosensor applications. There was a systematic increase in the total, steady state PoPD mass deposited for fixed applied potentials from 0.05 to 0.6 V vs. SCE, followed by a plateau up to 0.8 V. Comparison of PoPD mass and permselectivity parameters indicates that the ability of the passivating form of PoPD to block interference species in biosensor applications is not related in a simple way to the mass of material deposited on the surface. Instead, effects of the applied electropolymerisation potential in driving the electro-oxidation of oPD dimers and oligomers formed during the electro-deposition process are likely to have a more direct impact on the selectivity characteristics of the PoPD layer. The results highlight the usefulness of apparent permeabilities, especially of ascorbic acid, in revealing differences between PoPD layers electro-deposited under different conditions.

Enzyme immobilization strategies and electropolymerization conditions to control sensitivity and selectivity parameters of a polymer-enzyme composite glucose biosensor

In an ongoing programme to develop characterization strategies relevant to biosensors for in-vivo monitoring, glucose biosensors were fabricated by immobilizing the enzyme glucose oxidase (GOx) on 125 μm diameter Pt cylinder wire electrodes (Pt(C)), using three different methods: before, after or during the amperometric electrosynthesis of poly(ortho-phenylenediamine), PoPD, which also served as a permselective membrane. These electrodes were calibrated with H(2)O(2) (the biosensor enzyme signal molecule), glucose, and the archetypal interference compound ascorbic acid (AA) to determine the relevant polymer permeabilities and the apparent Michaelis-Menten parameters for glucose. A number of selectivity parameters were used to identify the most successful design in terms of the balance between substrate sensitivity and interference blocking. For biosensors electrosynthesized in neutral buffer under the present conditions, entrapment of the GOx within the PoPD layer produced the design (Pt(C)/PoPD-GOx) with the highest linear sensitivity to glucose (5.0 ± 0.4 μA cm(-2) mM(-1)), good linear range (K(M) = 16 ± 2 mM) and response time (< 2 s), and the greatest AA blocking (99.8% for 1 mM AA). Further optimization showed that fabrication of Pt(C)/PoPD-GOx in the absence of added background electrolyte (i.e., electropolymerization in unbuffered enzyme-monomer solution) enhanced glucose selectivity 3-fold for this one-pot fabrication protocol which provided AA-rejection levels at least equal to recent multi-step polymer bilayer biosensor designs. Interestingly, the presence of enzyme protein in the polymer layer had opposite effects on permselectivity for low and high concentrations of AA, emphasizing the value of studying the concentration dependence of interference effects which is rarely reported in the literature.