Charge Transfer Mediator Research Articles

Z-scheme and multipathway photoelectron migration properties of a bayberry-like structure of BiOBr/AgBr/LaPO4 nanocomposites: Improvement of photocatalytic performance using simulated sunlight

In this paper, bayberry-like BiOBr/AgBr/LaPO4 nanocomposites were synthesized by microwave-assisted hydrothermal treatment. The composition, structure, and morphology of BiOBr/AgBr/LaPO4 nanocomposites were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV−visible diffuse reflectance (UV−vis/DRS), high-power transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), and N2 adsorption−desorption measurements. The results show that the light absorption capacity of BiOBr/AgBr/LaPO4 is clearly enhanced and that a bayberry-like structure is more likely to produce diffraction and reflection of light on the surfaces of composites. AgBr in the composite is capable of reducing to Ag0 after light irradiation. Ag0, acting as a charge-transfer mediator, can be the medium, thus forming a Z-scheme structure. At the same time, the surface plasmon resonance (SPR) effect of Ag0 exhibits higher activity in the simulated sunlight photocatalysis process in BiOBr/AgBr/LaPO4. The photoelectrochemical methods further indicate that BiOBr/AgBr/LaPO4 possesses the best electron−hole pair separation ability. Furthermore, two different interface electron transfer modes and multipathway photoelectron migration in the composite are determined.

Construction and mechanism of a novel Z-scheme photocatalyst α-Fe2O3/P3HT with O-Ti-O for organic pollutant degradation under visible light

There is a great need to design a new method for fabricating efficient and cheap Z-scheme photocatalysts. For this reason, we synthesized a novel Z-scheme photocatalyst α-Fe2O3/P3HT with O-Ti-O charge transfer mediator. The effective compounding of each part was confirmed by different characterization methods. The charge transfer mechanism was confirmed to obey the Z-scheme mechanism by measuring the valence band value and by ESR experiments. Moreover, the Z-scheme mechanism in this system was further discussed based on the observed changes in the chemical structure and the theoretical calculation. Based on the characterization of the optical properties, this novel catalyst exhibited better charge transfer and separation abilities under visible light conditions due to its unique architecture. Additionally, this novel Z-scheme photocatalyst showed enhancement in the bisphenol A degradation effect, the degradation reaction was pseudo-first-order kinetics, a removal rate of up to 90% was maintained after four cycles. The BPA degradation pathway was also discussed in this study. This study provides a new approach for the construction of Z-scheme photocatalysts.

In-situ growth of Zn–AgIn5S8 quantum dots on g-C3N4 towards 0D/2D heterostructured photocatalysts with enhanced hydrogen production

Photocatalytic water splitting for hydrogen production represents an ideal pathway for solar energy harvesting and conversion, for which narrow bandgap multinary sulfides play an important role. Here, series of Zn–AgIn5S8/g-C3N4 0D/2D nanocomposites were prepared by in-situ growth of the Zn–AgIn5S8 quantum dots (QDs) on g-C3N4 nanosheets for improved charge separation. To our surprise, rather than a photoactive component, here g-C3N4 nanosheets act as a charge transfer mediator, where only a relatively low ratio is required. The as-fabricated Zn–AgIn5S8/g-C3N4 nanocomposites were systematically studied. When the mass ratio of g-C3N4 was 10%, the hydrogen production rate was maximized, which was 1.39 times higher than pure Zn–AgIn5S8 QDs and 138.6 times higher than g-C3N4. The enhanced photocatalytic activity of the Zn–AgIn5S8/g-C3N4 nanocomposites is attributed to the intimate interface contact, which results in the effective separation and transfer of the photogenerated charge carriers as proved by the PL lifetime, transient photocurrent and electrochemical impedance spectra measurements. The Zn–AgIn5S8/g-C3N4 nanocomposites also exhibit excellent cycle stability. A plausible mechanism was proposed for the 0D/2D Zn–AgIn5S8/g-C3N4 composite photocatalysts. This work provides a relatively simple method for constructing high-quality 0D/2D heterostructure of QDs/nanosheets, as well as new insight for the efficiency improvement of narrow-bandgap sulfide photocatalysts.

Towards microbial biofuel cells: Improvement of charge transfer by self-modification of microoganisms with conducting polymer – Polypyrrole

In this research, we are reporting the application of microorganisms modified by a conducting polymer – polypyrrole (Ppy) – for the improvement of microbial biofuel cells. The synthesis of polypyrrole was assisted by metabolic and other biochemical processes that occur in microorganisms. Therefore, in this research the reported Ppy formation method can be recognized as environmentally friendly. During herein reported synthesis of Ppy, the cell walls and some other structures of Aspergillus niger and Rhizoctania sp selected for this research were modified by the formed Ppy, which facilitated the transfer of electrical charge generated by microorganisms towards an electrode. Two electrochemical methods: (i) amperometric measurements at constant potential and (ii) scanning electrochemical microscopy (SECM) were applied for the evaluation of charge transfer efficiency and for the determination of electrochemical activity of Ppy-modified and non-modified cells. Two mediators (1,10-phenanthroline-5,6-dione (PD) or 9,10-phenanthrenequinone (PQ) and potassium ferricyanide (K3[Fe(CN)6])) based redox system was applied for the assessment of charge transfer from above mentioned living cells. The amperometric signals registered for electrodes based on Ppy modified cells were over six times higher in comparison to that of electrodes based on non-modified cells. This effect is assigned to advanced electrical conductivity of Ppy-modified cell wall and/or to better permeability of charge transfer mediators through the membrane and/or cell wall of Ppy-modified cells. Fungi of Aspergillus niger, which showed few times higher electrochemical activity in comparison to Rhizoctania sp., were investigated by SECM. Electrochemical differences between Ppy-modified and non-modified cells have been confirmed by SECM results. Therefore, this eco-friendly Ppy-formation technique seems effective in improving the performance of electrochemical devices such as microbial fuel cells and/or cell-based biosensors.

Electrochemical Synthesis of Cu-Cu2o-CNT/CNF Composite Coatings

The main scientific objective of performed studies was development of methodology for electrochemical synthesis of ternary composite materials in the Cu-Cu2O-CNT/CNF systems (CNT – carbon nanotubes, CNF – carbon nanofibers). Copper is the only metal that exhibits very high selectivity for electrochemical conversion of CO2 to ethylene (1). Ren et al. observed similar effect in case of p-type Cu2O (2). Reported effects allows on assumption that combination of both materials can be great solution in the frame of light supported electrochemical conversion of carbon dioxide to ethylene. Additionally application of carbon structures: CNT or CNF as charge transfer mediators can limit photodegradation of Cu2O, improving simultaneously its chemical stability (3) and catalytic activity as well as selectivity for electrochemical conversion of CO2 to ethylene (2). Presence of highly hydrophobic CNT/CNF phase in Cu matrix may significantly improve selectivity through limiting adsorption of polar water molecules favoring simultaneously adsorption of nonpolar CO2 molecules. Present studies describing optimization of electrochemical synthesis process towards synthesis of composite coatings of wide composition range. Synthesis conditions were investigated starting from thermodynamic analysis of the electrolyte. In next step, based on zeta potentials values stability of colloidal electrolyte containing carbon nanotubes was analyzed. Current-voltage conditions allowing on deposition of Cu with Cu2O with simultaneous incorporation of CNT/CNF structures were determined based on voltammetric and elctrogravimetric tests. Synthesized coatings were characterized towards their composition, structure and morphology. References Y. Kwon, Y. Lum, E. L. Clark, J. W. Ager and A. T. Bell, ChemElectroChem, 3, 1012 (2016).D. Ren, Y. Deng, A. D. Handoko, C. S. Chen, S. Malkhandi and B. S. Yeo, ACS Catalysis, 5, 2814 (2015).P. D. Tran, S. K. Batabyal, S. S. Pramana, J. Barber, L. H. Wong and S. C. J. Loo, Nanoscale, 4, 3875 (2012).

Ligand Engineered MnO Catalytic Nanoparticles As an Efficient Charge Transfer Mediator for Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) water splitting is regarded as one of the most promising approaches for the generation of hydrogen as a renewable and carbon-free fuel. Bismuth vanadate (BiVO4) has been recognized a highly promising photoanode material for solar water splitting. However the PEC performance of BiVO4 remains significantly poorer than the theoretical value due to the severe recombination of photogenerated charge carrier near the solid/electrolyte interface. A novel strategy is therefore essential for the progress of highly efficient PEC water splitting. The band edge positions of semiconductors determine their functionality in photoelectrochemical water splitting. While surface modification on the catalytic surface is known to enable to facilitate modification of the band structure, its crucial role in water splitting efficiency has not yet been fully understood. Here, we firstly demonstrate the ligand engineering effect of manganese oxide Co-catalyst nanoparticles (MnO NPs) on bismuth vanadate (BiVO4)-based cascade anodes, and achieve a remarkably enhanced photocurrent density of 6.25 mA/cm2 at the 1.23 V versus reversible hydrogen electrode. It is the highest value among the previously reported BiVO4-based anodes. We verified that improved photoactivity is closely related to the substantial shifts in band edge energies that originate from both the induced dipole at the ligand/MnO interface and intrinsic dipole of the ligand. In particular, combined spectroscopic analysis and electrochemical study revealed the clear relationship between the surface structure driven by the ligand treatment and the band edge positions of MnO NPs for photoelectrochemical water oxidation. Our systematic study provides general strategy for enabling new active semiconductor photocatalyst and is applicable to solar water splitting system.

Boosting Charge-Transfer Efficiency by Simultaneously Tuning Double Effects of Metal Nanocrystal in Z-Scheme Photocatalytic Redox System

Integrating individual functional materials into elegant nanoarchitectures holds great promise for creating high-efficiency photosynthesis systems with unique structure-directing merits. Herein, an all-solid-state metal-based Z-scheme photocatalytic system consisting of well-defined one-dimensional WO3@Au@CdS core–shell heterostructure has been progressively and rationally designed by a green and facile two-step wet-chemistry approach. Significantly, it was uncovered that Au ingredient sandwiched in between the interfacial domain of WO3 and CdS layer plays simultaneous dual roles in boosting the visible-light-driven photoactivities of core–shell ternary heterostructure, that is, as interfacial charge-transfer mediator to expedite vectorial Z-scheme electron transfer between CdS and WO3 and plasmonic photosensitizer to trigger the generation of plasmon-induced hot electrons, thereby substantially augmenting the photoelectron density in a photoredox catalytic system. Such cooperative concurrent dual roles o...

Evaluation of Kefir as a New Anodic Biocatalyst Consortium for Microbial Fuel Cell.

Kefir, a combined consortium of bacteria and yeast encapsulated by a polymeric matrix of exopolysaccharides, was used as anodic biocatalyst in a two-chamber microbial fuel cell (MFC). Fermentation was followed during 72h and polarization curves were obtained from linear sweep voltammetry. The effect of methylene blue as charge-transfer mediator in the kefir metabolism was evaluated. UV/Vis spectrophotometry and cyclic voltammetry were applied to evaluate the redox state of the mediator and to characterize the electrochemical activity, whereas current interruption was used for internal resistance determination. Aiming to establish a relationship between the microbial development inside the anodic chamber with the generated power in the MFC, total titratable acidity, pH, viscosity, carbohydrate assimilation, and microbial counting were assayed. The kefir-based MFC demonstrated a maximum power density of 54mWm-2 after 24h fermentation, revealing the potential use of kefir as a biocatalyst for microbial fuel cells.

A novel Z-scheme visible light driven Cu2O/Cu/g-C3N4 photocatalyst using metallic copper as a charge transfer mediator

The novel Cu2O/Cu/g-C3N4 composites were successfully synthesized via one step reduction method in the presence of g-C3N4. Under visible light irradiation, the Cu2O/Cu/g-C3N4 composites exhibited higher photocatalytic activity than g-C3N4 and Cu/Cu2O, and the 20at% Cu2O/Cu/g-C3N4 photocatalyst displayed the highest photocatalytic activity. The enhanced photcatalytic mechanism was investigated by radical-trapping and electron spin resonance experiments. A Z-scheme charge transfer mechanism was proposed, in which metallic Cu particles acted as charge separation center. This work could provide some insight into the design of novel Z-scheme photocatalyst with metallic Cu particles as a charge transfer mediator.

Charge Transfer Catalysis Across Semiconductor/Electrolyte Interfaces By Metalloporphyrin/Polymer Matrices

Polycrystalline metal chalcogenide (CdS, CdSe, CdTe, CuS, CuSe and others) thin-film electrodes are being considered as alternative for the more costly and advanced-tech demanding photovoltaic (PV) systems. Despite their ease of preparation and economy in starting materials, the thin film electrodes have major drawbacks. Due to lack of surface uniformity, low carrier mobility and high resistance at the solid/liquid interface, such electrodes exhibit low conversion efficiency in PEC processes. Moreover, the electrode surfaces are unstable and tend to undergo photo-corrosion. These shortcomings are a real challenge for using thin film electrodes in PEC technology, and should be overcome. Porphyirinato manganese (MnTPyP) complexes embodied inside different polymeric materials have been used as charge transfer mediators across solid/liquid junctions in PEC processes. The MnTPyP/Polymer coating (~80 nm thick) can be cast onto surfaces of the polycrystalline metal chalcogenide thin film electrodes in simple methods. The coating showed remarkable enhancement in the PEC conversion efficiency of different electrodes. Both short-circuit current and fill factor values have been remarkably enhanced. Conversion efficiency values of as high as 19% have been observed here. Such results have not been preceded before in metal chalchogenide thin-film electrode processes. Moreover, the coating enhanced the semiconductor electrode stability to photo-corrosion under PEC conditions. The MnTPyP/Polymer matrix behaved as charge transfer mediator across the solid/liquid interface. Upon exciting an n-type semiconductor electrode, electrons are excited from the valence band to the conduction band and move to the back contact wiring. The remaining holes must move to the redox couple to make photo-current. In polycrystalline semiconductor electrodes, such a transfer is not fast enough, which lowers the value of the short circuit current, and consequently the conversion efficiency. Therefore, holes accumulate in the space charge layer and cause electrode corrosion. The MnTPyP/Polymer matrix enhances both efficiency and stability of the electrode by behaving as a charge transfer mediator. The MnTPyP complex involves both MnIII and MnII ions together. The holes in the valence band oxidize the MnII ions into MnIII ions. The MnIII ions in turn oxidize the reduced phase of the redox couple. By this way, the MnTPyP complex catalyzes the hole-transfer and enhances both efficiency and stability of the electrode. The charge transfer mediation formalism is summarized in the attached Scheme. Results and discussions will be presented in detail together with effect of the coating on the electrode band edge positions. Figure 1

Impact of substituent effects on the Raman spectra of structurally related N-substituted diketopyrrolopyrroles

Control over vibrational frequency modes is important in optimising the performance and behaviour of conjugated organic materials employed as charge transfer mediators and optical components in optoelectronic devices. Raman spectroscopy represents a powerful technique that can be employed to determine the structural implications of molecular substitution on photophysical properties in such conjugated organic environments. Herein, we report for the first time, the optimised geometries for a series of eight systematically varied N-substituted diketopyrrolopyrroles as well as their experimental and computed Raman spectra, with special emphasis placed upon their spectral band assignments. Clear out-of-plane structural re-arrangements, including pyramidalisation of the lactam nitrogens arising from intramolecular H-bonding interactions were observed upon N-substitution in the reported systems, leading to significant vibrational frequency shifts for ν(NC) and ν(CO) modes. In addition, mode scaling factors were determined and found to be comparable with those reported previously, employed using the same density functional. The following study addresses the implications of structural variation on the progression of those intense Raman modes which play a key role in tuning the photophysical properties of N-substituted diketopyrrolopyrrole systems and as such should be of broad interest to those developing functional materials based upon this molecular motif.

High PEC conversion efficiencies from CuSe film electrodes modified with metalloporphyrin/polyethylene matrices

Electrodeposited CuSe film electrodes have been prepared onto FTO/glass by a facile method based on earlier methods described for other systems. The films were characterized, modified by annealing and further characterized. The films were then modified by coating with tetra(-4-pyridyl) pophyrinato-manganese (MnTPyP) complexes embedded inside commercial polyethylene (PE) matrices. The effects of modifications on different film properties, such as X-ray diffraction (XRD) patterns, surface morphology, photoluminescence (PL) spectra and electronic absorption spectra were investigated. Compared with other thin film electrode systems, very high photoelectrochemical (PEC) conversion efficiency values have been observed here. Pre-annealing the CuSe films at 150°C for 2h, followed by attaching the MnTPyP/PE matrices remarkably enhanced their PEC characteristics. The conversion efficiency was significantly enhanced, from less than 1.0% to more than 15%. Fill factor (FF) was also enhanced from ∼30% to ∼80%. Values of open-circuit potential (VOC) and short-circuit current (JSC) were significantly enhanced. While annealing affects uniformity, particle inter-connection and surface texture of the CuSe films, the MnTPyP complex species behaves as an additional charge-transfer mediator across the film/electrolyte junction. Optimization of PEC characteristics, using different deposition times, different annealing temperatures, different annealing times and different redox couples, was also investigated.

Optimization of plastic crystal ionic liquid electrolyte for solid-state dye-sensitized solar cell

An efficient plastic crystal-based electrolyte is prepared by employing succinonitrile as solid solvent and binary ionic liquids as charge transfer mediators for dye-sensitized solar cells (DSSCs). It was found that combining succinonitrile with ionic liquids could improve the mechanical property of electrolyte and avoid the fluidity of ionic liquid, and the fabricated solid-state solar cell shows an overall conversion efficiency of 2.14%, which can be further improved to 5.50% after optimizing the addition amount of lithium perchlorate (LiClO4) as charge transfer promoter into the plastic crystal electrolyte, and the role of LiClO4 in the plastic crystal ionic liquid electrolyte is evaluated through electrochemical and photoelectrochemical measurements. The knowledge obtained from this plastic crystal-based electrolyte exhibits new eyeshot to find solid-state electrolyte for DSSCs with high performance.

1.2 Volt manganese oxide symmetric supercapacitor

Capacitance fading of MnO 2 supercapacitor electrode under negative polarization below 0.0 V (versus Ag/AgCl/sat. KCl (aq)) arises from extensive reduction of Mn(IV) to form inactive Mn(II) species, and this has typically limited the operating voltage window of an aqueous symmetric MnO 2 supercapacitor to be no greater than 0.8 V. As this lower potential limit is close to the onset potential of MnO 2-catalyzed oxygen reduction reaction (ORR), the fading problem can be alleviated by effectively passing the accumulated electrons in the oxide electrode to the dissolved oxygen molecules in electrolyte in order to avoid the formation of the surface Mn(II) species. This has been demonstrated by either increasing the dissolved oxygen content or using the Ti(IV)/Ti(III) redox couple in the electrolyte as a charge-transfer mediator to enhance the electrocatalytic activity of MnO 2 for ORR. Therefore, a MnO 2 symmetric supercapacitor showing remarkable cycling stability over an operating voltage window of 1.2 V has been achieved by using Ti(IV)-containing neutral electrolyte (1 M KCl (aq)).

Electrochemical Detection of Self-Assembled Viologen Modified Electrode as Mediator of Glucose Sensor

An amperometric glucose biosensor has been developed using viologen derivatives as a charge transfer mediator between a glucose oxidase (GOD) and a gold electrode. A highly stable self-assembled monolayer (SAM) of thiol-based viologen was immobilized onto the gold electrode of a quartz crystal microbalance (QCM) and GOD was immobilized onto the viologen modified electrode. This biosensor response to glucose was evaluated amperometrically in the potential of -300mV. Upon immobilization of the glucose oxidase onto the viologen modified electrode, the biosensor showed rapid response towards glucose. Experimental conditions influencing the biosensor performance, such as pH potential, were optimized and assessed. This biosensor offered excellent electrochemical responses for glucose concentration below <TEX>${\mu}$</TEX> mol level with high sensitivity and selectivity and short response time. The levels of the RSDs (<5%) for the entire analyses reflected the highly reproducible sensor performance. A linear calibration range between the current and the glucose concentration was obtained up to <TEX>$4.5{\times}10^{-4}M$</TEX>. The detection limit was determined to be <TEX>$3.0{\times}10^{-6}M$</TEX>.

Amperometric bioelectrode for specific human immunoglobulin G determination: Optimization of the method to diagnose American trypanosomiasis

Bioelectrodes to detect immunoglobulin G (IgG) antibodies occurring in sera of patients suffering from American trypanosomiasis were assembled. The device consisted of a gold electrode modified with a thiol sensitized with parasite proteins. The assemblage was accomplished by adsorbing IgG antibodies from confirmed infected patients followed by adsorption of anti-human IgG labeled with a redox enzyme. The appliance was used as a working electrode in a three-electrode cell containing a soluble charge-transfer mediator, also behaving as enzyme cosubstrate. The method is based on the measurement of the catalytic current after addition of the enzyme substrate, occurring when a positive serum is used to build up the biosensor. The discrimination efficiency between positive and negative sera was 100% for the samples studied. A 0.9525 correlation coefficient was obtained for results acquired by using this approach and one commercial diagnostic kit. The reproducibility, evaluated by the percentage coefficient of variation, varied between 7 and 20%. The sensitivity was 12.4 ng mL −1 IgG, which is in the same order as that obtained with the commercial kit. Stability of the device was studied for a 7-day period and the results showed no significant change ( p = 0.218). Leishmaniasic sera showed cross-reactivity when total parasite homogenate was used as antigen.

A comparative study of immobilization methods of a tyrosinase enzyme on electrodes and their application to the detection of dichlorvos organophosphorus insecticide

The use of several designs of amperometric enzymatic biosensors based on the immobilized tyrosinase enzyme (Tyr) for determining dichlorvos organophosphate pesticide are described. The biosensors are based on the reversible inhibition of the enzyme and the chronocoulometric measurement of the charge due to the charge-transfer mediator 1,2-naphthoquinone-4-sulfonate (NQS). Tyr becomes active when reducing the quinone form of the mediator molecule (NQS) to the reactive o-diol form substrate of Tyr (H 2NQS) at the working electrode, thus permitting modulation of the catalytic activity of the enzyme and measurement of the inhibition produced by the pesticide. The full activity of the enzyme reversibly recovers after removal of the pesticide and re-oxidation of H 2NQS. Tyr was immobilized onto electrodes using different procedures: (i) entrapment within electropolymerized conducting and non-conducting polymers, (ii) covalent attachment to self-assembled monolayers (SAM), (iii) cross-linking with glutaraldehyde (and nafion covering) and (iv) dispersion within carbon-paste electrodes. The mediator was co-immobilized onto the working electrode next to the enzyme and reagentless biosensors were subsequently constructed. In the SAM design (ii) NQS was added to the solution. The analytical properties of the different biosensors based on the competitive inhibition produced by dichlorvos were then compared. A detection limit of about 0.06 μM was obtained for dichlorvos with entrapment of NQS and Tyr within electropolymerized poly( o-phenylenediamine) polymer ( oPPD), which was the design that proved to have the best analytical performance.

Reconstitution of Apo-Glucose Dehydrogenase on Pyrroloquinoline Quinone-Functionalized Au Nanoparticles Yields an Electrically Contacted Biocatalyst

An electrically contacted glucose dehydrogenase (GDH) enzyme electrode is fabricated by the reconstitution of the apo-GDH on pyrroloquinoline quinone (PQQ)-functionalized Au nanoparticles (Au-NPs), 1.4 nm, associated with a Au electrode. The Au-NPs functionalized with a single amine group were attached to the Au surface by 1,4-benzenedithiol bridges, and PQQ was covalently linked to the Au-NPs. The apo-GDH was then reconstituted on the PQQ cofactor sites. The surface coverage of GDH corresponded to 1.4 x 10(-12) mol cm(-2). The reconstituted enzyme revealed direct electrical contact with the electrode surface, and the bioelectrocatalytic oxidation of glucose occurred with a turnover number of 11,800 s(-1). In contrast, a system that included the covalent attachment of GDH to the PQQ-Au-NPs monolayer in a random, nonaligned, configuration revealed lack of electrical communication between the enzyme and the electrode, albeit the enzyme existed in a bioactive structure. The bioelectrocatalytic function of the later system was, however, activated by the diffusional electron mediator 2,6-dichlorophenol-indophenol. The results imply that the alignment of GDH on a Au-NP through the reconstitution process leads to an electrically contacted enzyme-electrode, where the Au-NP acts as a charge-transfer mediator.

Amperometric cholesterol biosensor based on in situ reconstituted cholesterol oxidase on an immobilized monolayer of flavin adenine dinucleotide cofactor

A new amperometric biosensor for determining cholesterol based on deflavination of the enzyme cholesterol oxidase (ChOx) and subsequent reconstitution of the apo-protein with a complexed flavin adenine dinucleotide (FAD) monolayer is described. The charge transfer mediator pyrroquinoline quinone (PQQ) was covalently bound to a cystamine self-assembled monolayer (SAM) on an Au electrode. Boronic acid (BA) was then bound to PQQ using the carbodiimide procedure, and the BA ligand was complexed to the FAD molecules on which the apo-ChOx was subsequently reconstituted. The effective release of the FAD from the enzyme and the successful reconstitution were verified using molecular fluorescence and cyclic voltammetry. The optimal orientation of FAD toward the PQQ mediator and the distances between FAD and PQQ and between PQQ and electrode enhance the charge transfer, very high sensitivity (about 2500 nA mM −1 cm −2) being obtained for cholesterol determination. The biosensor is selective toward electroactive interferents (ascorbic acid and uric acid) and was tested in reference serum samples, demonstrating excellent accuracy (relative errors below 3% in all cases). The biosensor activity can be successfully regenerated in a simple process by successive reconstitution with batches of recently prepared apo-ChOx on the same immobilized Au/SAM–PQQ–BA–FAD monolayer (it was tested five times); the lifetime of the biosensor is about 45–60 days.

Recent Advances in Electropolymerized Conducting Polymers in Amperometric Biosensors

Conducting electroactive polymers (CPs) are materials discovered just over 20 years ago which have aroused considerable interest on account of their electronic conducting properties and unique chemical and biochemical properties. Consequently, they have numerous (bio)analytical and technological applications. CPs are easily synthesized and deposited onto the conductive surface of a given substrate from monomer solutions by electrochemical polymerization with precise electrochemical control of their formation rate and thickness. Coating electrodes with CPs under mild conditions opens up enormous possibilities for the immobilization of biomolecules and bioaffinity or biorecognizing reagents, the improvement of their electrocatalytic properties, rapid electron transfer and direct communication to produce a range of analytical signals and new analytical applications. Co-immobilization of other molecules (enzymatic co-factors or charge-transfer mediators) by entrapment within electropolymerized films or by covalent binding on these films permits straightforward fabrication of reagentless biosensors. The characteristics of CPs and their uses, mainly in amperometric biosensors, are reviewed. The most recent applications and lines of research related to CP films are summarized in the different sections of the paper, and probable future trends are discussed.