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Enzymatic biocathodes have the potential to replace platinum as an expensive catalyst for the oxygen reduction reaction in microbial fuel cells (MFCs). However, enzymes are fragile and prone to loss of activity with time. This could be circumvented by using suitable immobilization techniques to maintain the activity and increase longevity of the enzyme. In the present study, laccase from Trametes versicolor was immobilized using three different approaches, i.e., crosslinking with electropolymerized polyaniline (PANI), entrapment in copper alginate beads (Cu-Alg), and encapsulation in Nafion micelles (Nafion), in the absence of redox mediators. These laccase systems were employed in cathode chambers of MFCs for decolourization of Acid orange 7 (AO7) dye. The biocatalyst in the anode chamber was Shewanella oneidensis MR-1 in each case. The enzyme in the immobilized states was compared with freely suspended enzyme with respect to dye decolourization at the cathode, enzyme activity retention, power production, and reusability. PANI laccase showed the highest stability and activity, producing a power density of 38 ± 1.7 mW m−2 compared to 25.6 ± 2.1 mW m−2 for Nafion laccase, 14.7 ± 1.04 mW m−2 for Cu-Alg laccase, and 28 ± 0.98 mW m−2 for the freely suspended enzyme. There was 81% enzyme activity retained after 1 cycle (5 days) for PANI laccase compared to 69% for Nafion and 61.5% activity for Cu-alginate laccase and 23.8% activity retention for the freely suspended laccase compared to initial activity. The dye decolourization was highest for freely suspended enzyme with over 85% decolourization whereas for PANI it was 75.6%, Nafion 73%, and 81% Cu-alginate systems, respectively. All the immobilized laccase systems were reusable for two more cycles. The current study explores the potential of laccase immobilized biocathode for dye decolourization in a microbial fuel cell.

A new approach to deposition of electroactive ZnO thin films have been carried out, by one-pot chemical bath deposition with Al dopant and incorporation of neutral red as organic mediator. The morphological, structural and functional characterization of the neutral red incorporated, Al-doped ZnO (NR-AZO) film was carried out using electron microscopy, FTIR, XRD and EIS respectively. The incorporated neutral red was found to induce strain in the crystal of AZO proportional to the concentration used in depositing solution which further affected the charge transfer resistance of the films in solution. One mM neutral red was found to be the optimum concentration for both conductivity and response to NADH/NADPH. The response of the films was further validated by immobilizing NAD+ dependent alcohol dehydrogenase (ADH) and NADP+ dependent glucose dehydrogenase (GDH) independently. The ADH/NR-AZO showed a sensitivity of 3.2 μA cm−2 mM−1 with a LoD of 1.7 μM of ethanol in the range 5.6 μM–7 mM, whereas GDH/NR-AZO showed a sensitivity of 4.33 μA cm−2 mM−1 with a LoD of 27 μM of glucose in the range 90 μM–4 mM. This method serves as a simple alternative to immobilize the organic redox dyes into the inorganic thin films in a single step making it electroactive towards specific biomolecules.

Ti/ML-TNT/HRP/Nafion© sensor device is fabricated using TiO2 multileg nanotubes (ML-TNT). This device is used for photoelectrochemical enzymatic detection of hydrogen peroxide for different concentrations and the electrochemical surface phenomenon was studied using electrochemical impedance spectroscopy. The device shows a photo-induced enhancement of biosensing performance towards increasing concentration of the analyte, which is analyzed with cyclic voltammetry and enzyme kinetics. The non-turn over condition cyclic voltammetry shows reversible one-electron transfer reaction due to Fe2+/3+ reaction of the heme co-factor while the turn over shows two electrons reaction because of the reduction of H2O2. Further analysis shows an enhanced sensitivity of 3.302 mA/mM.cm2, selectivity towards H2O2, range of detection 41.2 × 10−9– 6.0 × 10−3 M and limit of detection 12.0 × 10−9 M. The limitation of this electrode towards high concentration of H2O2 is analysed with Michaelis-Menten fit of the enzymatic reaction and Randles circuit of electrochemical impedance spectra.

More than 80 per cent of wastewater from industries is discharged into receiving water bodies without any pollution control. Microbial fuel cells (MFCs) are a promising technology for the simultaneous treatment of wastewater and electricity production. With regard to azo-dye containing wastewater (e.g. from textile manufacturing), the dye may be fed via the anode chamber containing electrochemically active bacteria or via the cathode chamber containing laccase enzyme as catalyst for oxygen reduction. This study investigated which of the two approaches is the best with regard to rate of decolourization of the dye (Acid orange 7), COD reduction and electricity production. The power density was higher for the MFCDye cathode (50±4 mW m-2, COD reduction 80.4±1.2%) compared with 42.5±2.6 mW m-2 (COD reduction 69±2%) for MFCDye anode. The time required for decolourization was longer in the MFCDye anode (Shewanella oneidensis) where only 20% decolourization was obtained after 24 h compared to 80% for the MFCDye cathode. The anodic dye degradation products were unstable when exposed to air resulting in regaining of colour. In case of degradation by laccase in the cathode chamber, the decolourization products were stable and simpler in chemical structure as determined by GC-MS. This work suggests that feeding azo dyes in cathode chambers of MFCs containing laccase is a better way of treating the dyes compared to the commonly used approach of feeding the dye in the anode chamber provided enzyme activity can be sustained.

Biosensing of NADH on bare electrodes has drawbacks such as high over-potential and poisoning during the oxidation reaction. To overcome this challenge a different approach has been undertaken by incorporating neutral red (NR) in Al doped ZnO (AZO) thin films using one-pot chemical bath deposition (CBD). The surface morphology of the films was hexagonal nanorods along the c-axis, perpendicular to the substrate. The thickness of the thin films were ranging from 400 to 3000 nm varying dependent on time of deposition (30 to 150 min). The average diameter of the nanorods was larger in the presence of neutral red (NR-AZO) with ~ 300 nm in contrast to its absence (AZO) with ~ 200 nm. The density of the packing of nanorods was dependent on the citrate concentration used during deposition. Control over the dopant concentration in the films was achieved by varying the area of Al foil used in the deposition solution. The selected area diffraction (SAED) and X-ray diffraction (XRD) indicated 002 plane of orientation in the nanorods. FTIR and FT-Raman analysis revealed conserved structure of NR and AZO. Chronoamperometric (CA) analysis showed a sensitivity of 0.45 μA cm− 2 mM− 1 and LoD of 22 μM within the range 0.075–4 mM of NADH. The biological sensing of NADH was validated by physical adsorption of NAD+ dependent-lactate dehydrogenase (LDH) on NR-AZO. CA showed sensitivity of 0.56 μA cm− 2 mM− 1 and LoD for lactate was 27 μM in the range of 0.1–1 mM of lactate. Further validation with real-time serum sample shows that LDH/NR-AZO correlates with the clinical values. The distinction in this study is that the organic mediator like neutral red has been incorporated into the grain structure of the ZnO thin film whereas other study with the mediators have only attempted surface functionalization. This article is part of a Special Issue entitled "Recent Advances in Bionanomaterials" Guest Editor: Dr. Marie-Louise Saboungi and Dr. Samuel D. Bader.