Influence of the major pilA transcriptional regulator in electrochemical responses of Geobacter sulfureducens PilR-deficient mutant biofilm formed on FTO electrodes.

Geobacter sulfurreducens is a model organism for understanding the role of bacterial structures in extracellular electron transfer mechanism (EET). This kind of bacteria relies on different structures such as type IV pili and over 100 c-type cytochromes to perform EET towards soluble and insoluble electron acceptors, including electrodes. To our knowledge, this work is the first electrochemical study comparing a G. sulfurreducens PilR-deficient mutant and wild type biofilms developed on fluorine-doped tin oxide (FTO) electrodes. Open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV), were used to evaluate the electroactive properties of biofilms grown without externally imposed potential. Parallel studies of Confocal Laser Scanning Microscopy (CLSM) correlated with the electrochemical results. PilR is a transcriptional regulator involved in the expression of a wide variety of genes, including pilA (pilus structural protein) relevant c-type cytochromes and some other genes involved in biofilm formation and EET processes. Our findings suggest that PilR-deficient mutant forms a thinner (CLSM analysis) and less conductive biofilm (EIS analysis) than wild type, exhibiting different and irreversible redox processes at the interface (CV analysis). Additionally, this work reinforces some of the remarkable features described in previous reports about this G. sulfurreducens mutant.

Gold nanoparticles/4-aminothiophenol interfaces for direct electron transfer of horseradish peroxidase: Enzymatic orientation and modulation of sensitivity towards hydrogen peroxide detection.

Hydrogen peroxide electrochemical detection by horseradish peroxidase has been widely studied. The use of gold nanoparticles to prepare electrode/enzyme bioconjugates has attracted attention due to their catalytic properties. In this work, it is reported the use of gold nanoparticles and 4-aminothiophenol as a scaffold to obtain a suitable matrix for enzyme bioconjugation with horseradish peroxidase. A critical factor in biosensors design and development is the enzymatic electrochemical activity understanding. Comparison of voltammetric studies of the heme prosthetic group showed a reversible electrochemical behavior when the enzymes were immobilized in a well-dispersed gold deposit; on the other hand, a discrete redox response was observed on a randomly deposited gold electrode. These results show that the distance between enzymes is essential. Hydrogen peroxide catalysis and the enzymatic behavior were analyzed considering two types of nanoparticles dispositions. The catalytic behavior observed in the well-dispersed nanoparticles configuration suggests a preserved enzyme folding, a decrease of steric impediments, and appears to be a better immobilization strategy. In contrast, the randomly electrodeposited gold electrode decreased the enzyme orientation and the electrochemical activity. The advantages of this methodology are the electrode fabrication affordable cost and the enzymatic direct electron transfer response improvement.