A comprehensive review of the application of Zr-based metal-organic frameworks for electrochemical sensors and biosensors.

A family of inorganic-organic hybrid crystalline materials made up of organic ligands and metal cations or clusters is known as metal-organic frameworks (MOFs). Because of their unique stability, intriguing characteristics, and structural diversity, zirconium-based MOFs (Zr-MOFs) are regarded as one of the most interesting families of MOF materials for real-world applications. Zr-MOFs that have the ligands, metal nodes, and guest molecules enclosed show distinct electrochemical reactions. They can successfully and sensitively identify a wide range of substances, which is important for both environmental preservation and human health. The rational design and synthesis of Zr-MOF electrochemical sensors and biosensors, as well as their applications in the detection of drugs, biomarkers, pesticides, food additives, hydrogen peroxide, and other materials, are the main topics of this comprehensive review. We also touch on the current issues and potential future paths for Zr-MOF electrochemical sensor research.

Electrochemical reduction of Cr(VI) in the presence of sodium alginate and its application in water purification.

Chromium (Cr) is used in many manufacturing processes, and its release into natural waters is a major environmental problem today. Low concentrations of Cr(VI) are toxic to human health and living organisms due to the carcinogenic and mutagenic nature of this mineral. This work examined the conversion of Cr(VI) to Cr(III) via electrochemical reduction using gold electrode in an acidic sodium alginate (SA) solution and subsequent removal of the produced Cr(III)-SA by the polymer-enhanced ultrafiltration (PEUF) technique. A solution of SA in nitric acid was used both as an electrolytic medium during the voltammetric measurements and bulk electrolysis and as an extracting agent during the PEUF technique. The electroanalysis of Cr(VI) was performed by linear sweep voltammetry in the presence of acidic SA solution to study its voltammetric behavior as a function of the Cr(VI) concentration, pH, presence of Cr(III), SA concentration and scan rate. In addition, the quantitative reduction of Cr(VI) to Cr(III) was studied through the bulk electrolysis technique. The results showed efficient reduction with well-defined peaks at approximately 0.3 V vs. Ag/AgCl, using a gold working electrode. As the pH increased, the reduction signal strongly decreased until its disappearance. The optimum SA concentration was 10 mmol/L, and it was observed that the presence of Cr(III) did not interfere in the Cr(VI) electroanalysis. Through the quantitative reduction by bulk electrolysis in the presence of acidic SA solution, it was possible to reduce all Cr(VI) to Cr(III) followed by its removal via PEUF.

Adsorption of 4,4'-Dithiodipyridine Axially Coordinated to Iron(II) Phthalocyanine on Au(111) as a New Strategy for Oxygen Reduction Electrocatalysis.

The coordination of PySSPy to FePc was monitored by UV/Vis spectroscopy while the adsobed FePc, anchored by PyS-Au(111), was examined by in situ STM in 0.1 M HClO4 and X-ray photoelectron spectroscopy (XPS). Rotating-disc-electrode (RDE) and linear-sweep-voltammetry (LSV) studies on the resulting FePc-modified Au(111) electrodes in an oxygen-saturated 0.1 M NaOH electrolyte exhibit excellent electrocatalytic properties for the oxygen reduction reaction (ORR), with a smaller overpotential than that observed for Au(111) with FePc deposited by direct adsorption from a benzene solution.

Surface Structure of 4-Mercaptopyridine on Au(111): A New Dense Phase.

4-Mercaptopyridine (4MPy) self-assembled on Au(111) has been studied by in situ electrochemical scanning tunneling microscopy (EC-STM) in HClO4, cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Samples prepared by varying the immersion time at constant concentration named short time (30 s) and long time (3 min) adsorption have been studied. Cyclic voltammetry and XPS showed that the chemistry of the adsorbed molecules does not depend on the adsorption time resulting in a well established chemisorbed thiol self-assembled monolayer on Au(111). EC-STM study of the short time adsorption sample revealed a new self-assembled structure after a cathodic desorption/readsorption sweep, which remains stable only if the potential is kept negative to the Au(111) zero charge potential (EPZC). DFT calculations have shown a correlation between the observed structure and a dense weakly adsorbed phase with a surface coverage of θ = 0.4 and a (5 × âˆš3) lattice configuration. At potentials positive to the EPZC, the weakly adsorbed state becomes unstable, and a different structure is formed due to the chemisorption driven by the electrostatic interaction. Long time adsorption experiments, on the other hand, have shown the typical (5 × âˆš3) structure with θ = 0.2 surface coverage (chemisorbed phase) and are stable over the whole potential range. The difference observed in long time and short time immersion can be explained by the optimization of molecular interactions during the self-assembly process.

Comparison of the catalytic activity for O2 reduction of Fe and Co MN4 adsorbed on graphite electrodes and on carbon nanotubes.

We have compared the electrocatalytic activity of several substituted and unsubstituted Co and Fe N4-macrocyclic complexes (MN4) for the electro-reduction of oxygen with the complexes directly adsorbed on the edge plane of pyrolytic graphite or adsorbed on carbon nanotubes (CNTs). In the presence of CNTs, one order of magnitude higher surface concentrations of MN4 catalysts per geometric area unit could be adsorbed leading to a lower overpotential for the oxygen electro-reduction and activities in the same order of magnitude as the commercially available Pt/C catalysts in basic pH. From Koutecky-Levich regression analysis, the total number of electrons transferred was approximately 2 for all the Co complexes and 4 for all the Fe ones, both in the presence and in the absence of the carbon nanotubes. Furthermore, the Tafel slopes did not vary due to the presence of the CNTs and presented values in the range of -0.06 V decade-1 for the CoN4 compounds and in the range of -0.04 V decade-1 for FeN4. When plotting the log of kinetic current densities (i.e. log jk) at a constant potential for each complex divided by the surface concentration Γ, and the number of electrons transferred n for the ORR for each catalyst, versus the difference between the redox potential of the metal active site of the Co(ii)/(i) or Fe(iii)/(ii) catalyst and the reversible potential of the reaction they promote, the catalytic activity increases when the formal potential of the complex becomes more positive and the data obtained with complexes adsorbed on graphite are in agreement with the data obtained when using CNTs indicating that the increase in jk when CNTs are present is only due to an increase in the number of active sites per geometric area of the electrode.

A disposable immunosensor for the detection of salivary MMP-8 as biomarker of periodontitis.

This work describes the development of a novel voltammetric immunosensor for the detection of salivary MMP-8 at the point-of-care. The electrochemical platform was based on a graphene (GPH) screen-printed electrode (SPE) functionalized by gold-nanospheres (AuNSs) and antibodies against MMP-8 protein (anti-MMP-8). The functionalization with anti-MMP-8 was realized by using 11-mercaptoundecanoic acid (11-MUA), thanks to its ability to give strong sulfur bonds with its -SH end, and to cross-link the -NH2 groups of the antibody molecule with the other -COOH end, using the traditional EDC-NHS method. The voltammetric sensor showed good performances with a linear range of 2.5-300 ng mL-1, a LOD value of 1.0 ± 0.1 ng mL-1 and a sensitivity of 0.05 µA mL cm-2 ng-1. Moreover, the proposed immunosensor was tested in real saliva samples, showing comparable results to those obtained with the conventional ELISA method. The biosensor was single-use and cost-effective and required a small quantity of test medium and a short preparation time, representing a very attractive biosensor for MMP-8 detection in human saliva.

Spectroscopic and crystallographic characterization of "alternative resting" and "resting oxidized" enzyme forms of bilirubin oxidase: implications for activity and electrochemical behavior of multicopper oxidases.

While there is broad agreement on the catalytic mechanism of multicopper oxidases (MCOs), the geometric and electronic structures of the resting trinuclear Cu cluster have been variable, and their relevance to catalysis has been debated. Here, we present a spectroscopic characterization, complemented by crystallographic data, of two resting forms occurring in the same enzyme and define their interconversion. The resting oxidized form shows similar features to the resting form in Rhus vernicifera and Trametes versicolor laccase, characterized by "normal" type 2 Cu electron paramagnetic resonance (EPR) features, 330 nm absorption shoulder, and a short type 3 (T3) Cu-Cu distance, while the alternative resting form shows unusually small A(||) and high g(||) EPR features, lack of 330 nm absorption intensity, and a long T3 Cu-Cu distance. These different forms are evaluated with respect to activation for catalysis, and it is shown that the alternative resting form can only be activated by low-potential reduction, in contrast to the resting oxidized form which is activated via type 1 Cu at high potential. This difference in activity is correlated to differences in redox states of the two forms and highlights the requirement for efficient sequential reduction of resting MCOs for their involvement in catalysis.

Effect of deglycosylation of cellobiose dehydrogenases on the enhancement of direct electron transfer with electrodes.

Cellobiose dehydrogenase (CDH) is a monomeric extracellular flavocytochrome composed of a catalytic dehydrogenase domain (DH(CDH)) containing flavin adenine dinucleotide (FAD), a cytochrome domain (CYT(CDH)) containing heme b, and a linker region connecting the two domains. In this work, the effect of deglycosylation on the electrochemical properties of CDH from Phanerochaete chrysosporium (PcCDH) and Ceriporiopsis subvermispora (CsCDH) is presented. All the glycosylated and deglycosylated enzymes show direct electron transfer (DET) between the CYT(CDH) and the electrode. Graphite electrodes modified with deglycosylated PcCDH (dPcCDH) and CsCDH (dCsCDH) have a 40-65% higher I(max) value in the presence of substrate than electrodes modified with their glycosylated counterparts. CsCDH trapped under a permselective membrane showed similar changes on gold electrodes protected by a thiol-based self-assembled monolayer (SAM), in contrast to PcCDH for which deglycosylation did not exhibit any different electrocatalytical response on SAM-modified gold electrodes. Glycosylated PcCDH was found to have a 30% bigger hydrodynamic radius than dPcCDH using dynamic light scattering. The basic bioelectrochemistry as well as the bioelectrocatalytic properties are presented.

Evidence of carbon-supported porphyrins pyrolyzed for the oxygen reduction reaction keeping integrity.

Fe(III) 5,10,15,20-(tetraphenyl)porphyrin chloride (FeTPP) and Co(III) 5,10,15,20-(tetraphenyl)porphyrin chloride (CoTPP) were adsorbed on carbon Vulcan and studied as electrocatalysts for the oxygen reduction reaction (ORR) before and after pyrolysis. The pyrolysis process was also simulated through ab initio molecular dynamic simulations and the minimum energy path for the O2 dissociation after the interaction with the metal center of the FeTPP and CoTPP were calculated. After the pyrolysis the FeTPP showed the best performances reducing O2 completely to H2O with increased limiting current and lower overpotential. Tafel slops for the various catalysts did not change after the pyrolytic process suggesting that the mechanism for the ORR is not affected by the heat treatment. TEM images, X-ray diffraction, XPS spectroscopy, 57Fe Mössbauer, and DFT simulations, suggest that there is no breakdown of the macrocyclic complex at elevated temperatures, and that the macro cyclic geometry is preserved. Small variations in the Metal-O2 (M-O2) binding energies and the M-N bond length were observed which is attributed to the dispersive interaction between the macrocycles and the irregular surface of the Vulcan substrate induced by the heat treatment and causing better interaction with the O2 molecule. The theoretical strategy herein applied well simulate and explain the nature of the M-N-C active sites and the performances towards the ORR.

Disposable Voltammetric Immunosensor for D-Dimer Detection as Early Biomarker of Thromboembolic Disease and of COVID-19 Prognosis.

In this work, we report on the development of a simple electrochemical immunosensor for the detection of D-dimer protein in human plasma samples. The immunosensor is built by a simple drop-casting procedure of chitosan nanoparticles (CSNPs) as biocompatible support, Protein A (PrA), to facilitate the proper orientation of the antibody sites to epitopes as a capture biomolecule, and the D-dimer antibody onto a carboxyl functionalized multi-walled carbon nanotubes screen printed electrode (MWCNTs-SPE). The CSNPs have been morphologically characterized by Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS) techniques. Successively, the electrochemical properties of the screen-printed working electrode after each modification step have been characterized by differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The resulting MWCNTs-CSNPs-PrA-D-dimer Ab immunosensor displays an optimal and promising platform for antibody immobilization and specific D-dimer detection. DPV has been used to investigate the antigen/antibody interaction at different D-dimer concentrations. The proposed voltammetric immunosensor allowed a linear range from 2 to 500 µg L-1 with a LOD of 0.6 µg L-1 and a sensitivity of 1.3 µA L µg-1 cm-2. Good stability and a fast response time (5 s) have been reported. Lastly, the performance of the voltammetric immunosensor has been tested in human plasma samples, showing satisfactory results, thus attesting to the promising feasibility of the proposed platform for detecting D-dimer in physiological samples.

Cellobiose dehydrogenase aryl diazonium modified single walled carbon nanotubes: enhanced direct electron transfer through a positively charged surface.

One of the challenges in the field of biosensors and biofuel cells is to establish a highly efficient electron transfer rate between the active site of redox enzymes and electrodes to fully access the catalytic potential of the biocatalyst and achieve high current densities. We report on very efficient direct electron transfer (DET) between cellobiose dehydrogenase (CDH) from Phanerochaete sordida (PsCDH) and surface modified single walled carbon nanotubes (SWCNT). Sonicated SWCNTs were adsorbed on the top of glassy carbon electrodes and modified with aryl diazonium salts generated in situ from p-aminobenzoic acid and p-phenylenediamine, thus featuring at acidic pH (3.5 and 4.5) negative or positive surface charges. After adsorption of PsCDH, both electrode types showed excellent long-term stability and very efficient DET. The modified electrode presenting p-aminophenyl groups produced a DET current density of 500 µA cm(-2) at 200 mV vs normal hydrogen reference electrode (NHE) in a 5 mM lactose solution buffered at pH 3.5. This is the highest reported DET value so far using a CDH modified electrode and comes close to electrodes using mediated electron transfer. Moreover, the onset of the electrocatalytic current for lactose oxidation started at 70 mV vs NHE, a potential which is 50 mV lower compared to when unmodified SWCNTs were used. This effect potentially reduces the interference by oxidizable matrix components in biosensors and increases the open circuit potential in biofuel cells. The stability of the electrode was greatly increased compared with unmodified but cross-linked SWCNTs electrodes and lost only 15% of the initial current after 50 h of constant potential scanning.

A third generation glucose biosensor based on cellobiose dehydrogenase from Corynascus thermophilus and single-walled carbon nanotubes.

A third generation glucose biosensor working under physiological conditions with a linear range of 0.1-30 mM, a detection limit of 0.05 mM, and a sensitivity of 222 nA µM(-1) cm(-2) has been developed by co-adsorption of cellobiose dehydrogenase (CDH) from the ascomycete Corynascus thermophilus (CtCDH) and oxidatively shortened single-walled carbon nanotubes (SWCNTs).

Novel Nanoarchitectures Based on Lignin Nanoparticles for Electrochemical Eco-Friendly Biosensing Development.

Novel nanoarchitectures based on lignin nanoparticles (LNPs) were designed and realized for electrochemical eco-friendly biosensing development. Two types of lignin nanoparticles were utilized for the modification of a gold bare electrode, namely organosolv (OLNPs) and kraft lignin (KLNPs) nanoparticles, synthetized from a sulfur-free and a sulfur lignin, respectively. The electrochemical behavior of LNP-modified electrodes was studied using two electrochemical techniques, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Compared to the gold bare electrode, an evident decrease in the faradaic current and increase of the ΔEp were observed in cyclic voltammograms. In addition, larger semicircles were registered in Nyquist plots. These results suggest a strong inhibition effect of the electron transfer reaction by LNPs layer, especially in the case of KLNPs. The modified electrodes, properly assembled with concanavalin A (ConA) and glucose oxidase (GOx), were successively tested as biosensing platforms for glucose, showing a sensitivity of (4.53 ± 0.467) and (13.74 ± 1.84) µA mM-1 cm2 for Au/SAMCys/OLNPs/ConA/GOx and Au/KLNPs/ConA/GOx biosensors, respectively. Finally, different layers of the KNLPs/ConA/GOx-modified Au electrode were tested, and the three-layered Au(KNLPs/ConA/GOx)3 showed the best analytical performance.

Cellobiose dehydrogenase: a versatile catalyst for electrochemical applications.

Cellobiose dehydrogenase catalyses the oxidation of aldoses--a simple reaction, a boring enzyme? No, neither for the envisaged bioelectrochemical applications nor mechanistically. The catalytic cycle of this flavocytochrome is complex and modulated by its flexible cytochrome domain, which acts as a built-in redox mediator. This intramolecular electron transfer is modulated by the pH, an adaptation to the environmental conditions encountered or created by the enzyme-producing fungi. The cytochrome domain forms the base from which electrons can jump to large terminal electron acceptors, such as redox proteins, and also enables by that path direct electron transfer from the catalytically active flavodehydrogenase domain to electrode surfaces. The application of electrochemical techniques to the elucidation of the molecular and catalytic properties of cellobiose dehydrogenase is discussed and compared to biochemical methods. The results lead to valuable insights into the function of this cellulose-bound enzyme, but also form the basis of exciting applications in biosensors, biofuel cells and bioelectrocatalysis.

Biocide Activity of Green Quercetin-Mediated Synthesized Silver Nanoparticles.

The development of new nanomaterials is gaining increasing attention due to their extensive applications in fields ranging from medicine to food and cultural heritage. Green nanoparticles provide advantages compared to conventional nanoparticles as their synthesis is environmentally-friendly and does not require the use of high temperatures, pressure, or toxic chemicals. In this paper, green silver nanoparticles (AgNPs) have been synthesized according to a new method using quercetin as a reducing agent at room temperature. The synthesized AgNPs were characterized using UV-Vis spectroscopy, transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and dynamic light scattering (DLS) techniques and successively tested for biocide activity by studying their effects in the inhibition of bacterial growth. The results demonstrated that the smaller the AgNPs size, the greater their biocide activity. In particular, AgNPs with a diameter of 8 nm showed a minimum inhibitory concentration (MIC) value of 1.0 µg/mL against Streptococcus sp., Escherichia coli and Candida sp. microorganisms, while AgNPs with a larger diameter of about 20 nm were able to inhibit microbial of all selected pathogens at a higher MIC value of 2.5 µg/mL.

Use of a Thermophile Desiccation-Tolerant Cyanobacterial Culture and Os Redox Polymer for the Preparation of Photocurrent Producing Anodes.

Oxygenic photosynthesis conducted by cyanobacteria has dramatically transformed the geochemistry of our planet. These organisms have colonized most habitats, including extreme environments such as the driest warm desert on Earth: the Atacama Desert. In particular, cyanobacteria highly tolerant to desiccation are of particular interest for clean energy production. These microorganisms are promising candidates for designing bioelectrodes for photocurrent generation owing to their ability to perform oxygenic photosynthesis and to withstand long periods of desiccation. Here, we present bioelectrochemical assays in which graphite electrodes were modified with the extremophile cyanobacterium Gloeocapsopsis sp. UTEXB3054 for photocurrent generation. Optimum working conditions for photocurrent generation were determined by modifying directly graphite electrode with the cyanobacterial culture (direct electron transfer), as well as using an Os polymer redox mediator (mediated electron transfer). Besides showing outstanding photocurrent production for Gloeocapsopsis sp. UTEXB3054, both in direct and mediated electron transfer, our results provide new insights into the metabolic basis of photocurrent generation and the potential applications of such an assisted bioelectrochemical system in a worldwide scenario in which clean energies are imperative for sustainable development.

Oxygen Reduction Reaction at Penta-Coordinated Co Phthalocyanines.

From the early 60s, Co complexes, especially Co phthalocyanines (CoPc) have been extensively studied as electrocatalysts for the oxygen reduction reaction (ORR). Generally, they promote the 2-electron reduction of O2 to give peroxide whereas the 4-electron reduction is preferred for fuel cell applications. Still, Co complexes are of interest because depending on the chemical environment of the Co metal centers either promote the 2-electron transfer process or the 4-electron transfer. In this study, we synthetized 3 different Co catalysts where Co is coordinated to 5 N atoms using CoN4 phthalocyanines with a pyridine axial linker anchored to carbon nanotubes. We tested complexes with electro-withdrawing or electro-donating residues on the N4 phthalocyanine ligand. The catalysts were characterized by EPR and XPS spectroscopy. Ab initio calculations, Koutecky-Levich extrapolation and Tafel plots confirm that the pyridine back ligand increases the Co-O2 binding energy, and therefore promotes the 4-electron reduction of O2. But the presence of electron withdrawing residues, in the plane of the tetra N atoms coordinating the Co, does not further increase the activity of the compounds because of pull-push electronic effects.

Comparison of direct and mediated electron transfer for cellobiose dehydrogenase from Phanerochaete sordida.

Direct and mediated electron transfer (DET and MET) between the enzyme and electrodes were compared for cellobiose dehydrogenase (CDH) from the basidiomycete Phanerochaete sordida (PsCDH). For DET, PsCDH was adsorbed at pyrolytic graphite (PG) electrodes while for MET the enzyme was covalently linked to a low potential Os redox polymer. Both types of electrodes were prepared in the presence of single walled carbon nanotubes (SWCNTs). DET requires the oxidation of the heme domain, while MET occurs partially via the heme and the flavin domain at pH 3.5. At pH 6 MET occurs solely via the flavin domain. Most probably, the interaction of the domains decreases from pH 3.5 to 6.0 due to electrostatic repulsion of deprotonated amino acid residues, covering the surfaces of both domains. MET starts at a lower potential than DET. The midpoint potentials at pH 3.5 for the flavin (40 mV) and the heme domain (170 mV) were determined with spectroelectrochemistry. The electrochemical and spectroelectrochemical measurements presented in this work are in conformity. The pH dependency of DET and MET was investigated for PsCDH. The optimum was observed between pH 4 and 4.5 pH for DET and in the range of pH 5-6 for MET. The current densities obtained by MET are 1 order of magnitude higher than by DET. During multicycle cyclic voltammetry experiments carried out at different pHs, the PsCDH modified electrode working by MET turned out to be very stable. In order to characterize a PsCDH modified anode working by MET with respect to biofuel cell applications, this electrode was combined with a Pt-black cathode as model for a membraneless biofuel cell. In comparison to DET, a 10 times higher maximum current and maximum power density in a biofuel cell application could be achieved by MET. While CDH modified electrodes working by DET are highly qualified for applications in amperometric biosensors, a much better performance as biofuel cell anodes can be obtained by MET. The use of CDH modified electrodes working by MET for biofuel cell applications results in a less positive onset of the electrocatalytic current (which may lead to an increased cell voltage), higher current and power density, and much better long-term stability over a broad range of pH.

Tryptophan repressor-binding proteins from Escherichia coli and Archaeoglobus fulgidus as new catalysts for 1,4-dihydronicotinamide adenine dinucleotide-dependent amperometric biosensors and biofuel cells.

The tryptophan (W) repressor-binding proteins (WrbA) from Escherichia coli (EcWrbA) and Archaeoglobus fulgidus (AfWrbA) were investigated for possible use in 1,4-dihydronicotinamide adenine dinucleotide (NADH) dependent amperometric biosensors and biofuel cells. EcWrbA and AfWrbA are oligomeric flavoproteins binding one flavin mononucleotide (FMN) per monomer and belonging to a new family of NAD(P)H:quinone oxidoreductases (NQOs). The enzymes were covalently linked to a low potential Os redox polymer onto graphite in the presence of single-walled carbon nanotube (SWCNT) preparations of varying average lengths. The performance of the enzyme modified electrodes for NADH oxidation was strongly depending on the average length of the applied SWCNTs. By blending the Os redox polymer with SWCNTs, the electrocatalytic current could be increased up to a factor of 5. Results obtained for AfWrbA modified electrodes were better than those for EcWrbA. For NADH detection, a linear range between 5 microM and 1 mM, a lower limit of detection of 3 microM, and a sensitivity of 56.5 nA microM(-1) cm(-2) could be reached. Additionally spectroelectrochemical measurements were carried out in order to determine the midpoint potentials of the enzymes (-115 mV vs NHE for EcWrbA and -100 mV vs NHE for AfWrbA pH 7.0). Furthermore, an AfWrbA modified electrode was used as an anode in combination with a Pt black cathode as a biofuel cell prototype.

Increasing amperometric biosensor sensitivity by length fractionated single-walled carbon nanotubes.

In this work the sensitivity-increasing effect of single-walled carbon nanotubes (SWCNTs) in amperometric biosensors, depending on their average length distribution, was studied. For this purpose the SWCNTs were oxidatively shortened and subsequently length separated by size exclusion chromatography. Transmission electron micrographs of different fractions of SWCNTs were collected. Diaphorase "wired" to an osmium redox polymer was blended with the shortened SWCNTs of different lengths. Depending on the average length of the SWCNTs the sensitivity of the amperometric biosensor model system towards oxidation of 1,4-dihydronicotinamide adenine dinucleotide (NADH) was increased by a factor of five. The best performance was achieved with SWCNTs of medium length. The linear range for NADH detection was between 5 microM and 7 mM, the maximum sensitivity was 47 nA microM(-1) cm(-2), and the detection limit was 1 microM. The biosensor exhibited excellent electrocatalytic properties. Even at relatively high NADH concentrations the oxidative current was limited by the diffusion rate of NADH.