This study reports on the oxygen sensitivity of quantum dot electrodes modified with CdSe/ZnS nanocrystals. The photocurrent behavior is analyzed for dependence on pH and applied potential by potentiostatic and potentiodynamic measurements. On the basis of the influence of the oxygen content in solution on the photocurrent generation, the enzymatic activity of glucose oxidase is evaluated in solution. In order to construct a photobioelectrochemical sensor which can be read out by illuminating the respective electrode area, two different immobilization methods for the fixation of the biocatalyst have been investigated. Both covalent cross-linking and layer-by-layer deposition of GOD by means of the polyelectrolyte polyallylamine hydrochloride show that a sensor construction is possible. The sensing properties of this type of electrode are drastically influenced by the amount and density of the enzyme on top of the quantum dot layer, which can be advantageously adjusted by the layer-by-layer technique. By depositing four bilayers [GOD/PAH](4) on the CdSe/ZnS electrode, a fast-responding sensor for the concentration range of 0.1-5 mM glucose can be prepared. This study opens the door to multianalyte detection with a nonstructured sensing electrode, localized enzymes, and spatial read-out by light.

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Molecular modeling, electrochemical methods, and quartz crystal microbalance were used to characterize immobilized hexameric tyrosine-coordinated heme protein (HTHP) on bare carbon or on gold electrodes modified with positively and negatively charged self-assembled monolayers (SAMs), respectively. HTHP binds to the positively charged surface but no direct electron transfer (DET) is found due to the long distance of the active sites from the electrode surfaces. At carboxyl-terminated surfaces, the neutrally charged bottom of HTHP can bind to the SAM. For this "disc" orientation all six hemes are close to the electrode and their direct electron transfer should be efficient. HTHP on all negatively charged SAMs showed a quasi-reversible redox behavior with rate constant ks values between 0.93 and 2.86 s(-1) and apparent formal potentials ${E{{0{^{\prime }}\hfill \atop {\rm app}\hfill}}}$ between -131.1 and -249.1 mV. On the MUA/MU-modified electrode, the maximum surface concentration corresponds to a complete monolayer of the hexameric HTHP in the disc orientation. HTHP electrostatically immobilized on negatively charged SAMs shows electrocatalysis of peroxide reduction and enzymatic oxidation of NADH.

The multilayer formation of two different sulfonated polyanilines with cytochrome c is presented and mechanistic aspects of the contributions of the polyelectrolytes' properties to the characteristics of the assemblies are discussed. These two modified polymers, PASA1 and PASA2 are chemically synthesized and differ in the grade of sulfonation, substitution, and the chain length of the polymer. The influence of these properties on the multilayer assembly with cytochrome c is studied in detail by Quartz Crystal Microbalance (QCM) technique and Cyclic Voltammetry (CV). It is shown that the multilayer formation is successful, however, the redox activity of polyanilines itself has to be taken into account. In the case of a strong redox activity (PASA2) voltammetric analysis allows the separation of redox processes addressed to the polyelectrolyte and cyt c. For multilayers with PASA1 as building block electroactivity can be predominantly attributed to cyt c ensuring a high amount of electroactive protein and a low probability for interfering redox reaction, making this system suitable for biosensor applications.

The development of a new surface architecture for the efficient direct electron transfer of positively charged redox proteins is presented. For this reason different kinds of polyaniline terpolymers consisting of aminobenzoic acid (AB), aminobenzenesulfonic acid (ABS) and aniline (A) with different monomer ratios were synthesized. The P(AB‐ABS‐A) were grafted to the surface of multiwalled carbon nanotubes (MWCNTs). FTIR measurements prove the covalent binding to the carboxylic groups of the MWCNTs while conductivity tests show an increase in the conductivity of the nanohybrid in comparison to the polymers. The [MWCNT‐P(AB‐ABS‐A)] nanohybrids were used for the immobilization of redox active cytochrome c (cyt.c). The positively charged protein can electrostatically interact with the negatively charged nanohybrid. Cyclic voltammetry (CV) shows an increase in the protein loading on [MWCNT‐P(AB‐ABS‐A)] coupled to cysteamine modified gold electrodes in comparison to non‐grafted MWCNTs. A further increase in the sulfonation degree of P(AB‐ABS‐A) leads to an enhanced current output of the modified electrodes. The redox activity of the polymer decreases after the immobilization of the cyt.c on the nanohybrid. For the first time polymers covalently grafted to the surface of MWCNTs are used in a biosensor.