Interface Electrostatics Dictates the Electron Transport via Bioelectronic Junctions.

Different batches of Si wafers with nominally the same specifications were found to respond differently to identical chemical surface treatments aimed at regrowing Si oxide on them. We found that the oxides produced on different batches of wafer differ electrically, thereby affecting solid-state electron transport (ETp) via protein films assembled on them. These results led to the another set of experiments, where we studied this phenomenon using two distinct chemical methods to regrow oxides on the same batch of Si wafers. We have characterized the surfaces of the regrown oxides and of monolayers of linker molecules that connect proteins with the oxides and examined ETp via ultrathin layers of the protein bacteriorhodopsin, assembled on them. Our results illustrate the crucial role of (near) surface charges on the substrate in defining the ETp characteristics across the proteins. This is expressed most strikingly in the observed current's temperature dependences, and we propose that these are governed by the electrostatic landscape at the electrode-protein interface rather than by intrinsic protein properties. This study's major finding, relevant to protein bioelectronics, is that protein-electrode coupling in junctions is a decisive factor in ETp across them. Hence,surface electrostatics can create a barrier that dominates charge transport and controls the transport mode across the junction. Our findings' wider importance lies in their relevance to hybrid junctions of Si with (polyelectrolyte) biomolecules, a likely direction for future bioelectronics. A remarkable corollary of presented results is that once an electron is injected into the protein, transport within the proteins is so efficient that it does not encounter a measurable barrier down to 160 K.

Protein electronic conductors: hemin-substrate bonding dictates transport mechanism and efficiency across myoglobin.

Electron transport (ETp) across met-myoglobin (m-Mb), as measured in a solid-state-like configuration between two electronic contacts, increases by up to 20 fold if Mb is covalently bound to one of the contacts, a Si electrode, in an oriented manner by its hemin (ferric) group, rather than in a non-oriented manner. Oriented binding of Mb is achieved by covalently binding hemin molecules to form a monolayer on the Si electrode, followed by reconstitution with apo-Mb. We found that the ETp temperature dependence (>120 K) of non-oriented m-Mb virtually disappears when bound in an oriented manner by the hemin group. Our results highlight that combining direct chemical coupling of the protein to one of the electrodes with uniform protein orientation strongly improves the efficiency of ET across the protein. We hypothesize that the behavior of reconstituted m-Mb is due to both strong protein-substrate electronic coupling (which is likely greater than in non-oriented m-Mb) and direct access to a highly efficient transport path provided by the hemin group in this configuration.

Electron Transfer Proteins as Electronic Conductors: Significance of the Metal and Its Binding Site in the Blue Cu Protein, Azurin.

Electron transfer (ET) proteins are biomolecules with specific functions, selected by evolution. As such they are attractive candidates for use in potential bioelectronic devices. The blue copper protein azurin (Az) is one of the most-studied ET proteins. Traditional spectroscopic, electrochemical, and kinetic methods employed for studying ET to/from the protein's Cu ion have been complemented more recently by studies of electrical conduction through a monolayer of Az in the solid-state, sandwiched between electrodes. As the latter type of measurement does not require involvement of a redox process, it also allows monitoring electronic transport (ETp) via redox-inactive Az-derivatives. Here, results of macroscopic ETp via redox-active and -inactive Az derivatives, i.e., Cu(II) and Cu(I)-Az, apo-Az, Co(II)-Az, Ni(II)-Az, and Zn(II)-Az are reported and compared. It is found that earlier reported temperature independence of ETp via Cu(II)-Az (from 20 K until denaturation) is unique, as ETp via all other derivatives is thermally activated at temperatures >≈200 K. Conduction via Cu(I)-Az shows unexpected temperature dependence >≈200 K, with currents decreasing at positive and increasing at negative bias. Taking all the data together we find a clear compensation effect of Az conduction around the Az denaturation temperature. This compensation can be understood by viewing the Az binding site as an electron trap, unless occupied by Cu(II), as in the native protein, with conduction of the native protein setting the upper transport efficiency limit.

Electron-transfer quenching of nucleic acid-functionalized CdSe/ZnS quantum dots by doxorubicin: a versatile system for the optical detection of DNA, aptamer-substrate complexes and telomerase activity.

The optical detection of DNA or the sensing of low-molecular-weight substrates or proteins by aptamer nucleic acids is a long term challenge in the design of biosensors. Similarly, the detection of the telomerase activity, a versatile biomarker of cancer cells, is important for rapid cancer diagnostics. We implement the luminescence quenching of the CdSe/ZnS quantum dots (QDs) as a versatile process to develop DNA sensors and aptasensors, and to design an analytical platform for the detection of telomerase activity. The formation of nucleic acid duplexes on QDs, or the assembly of aptamer-substrate complexes on the QDs (substrate=cocaine or thrombin) is accompanied by the intercalation of doxorubicin (DB) into the duplex domains of the resulting recognition complexes. The intercalated DB quenches the luminescence of the QDs, thus leading to the detection readout signal. Similarly, the telomerase-induced formation of the telomere chains on the QDs is followed by the hybridization of nucleic-acid units complementary to the telomere repeat units, and the intercalation of DB into the resulting duplex structure. The resulting luminescence quenching of the QDs provides an indicating signal for the activity of telomerase.

Nano-engineered flavin-dependent glucose dehydrogenase/gold nanoparticle-modified electrodes for glucose sensing and biofuel cell applications.

A three-dimensional composite consisting of the oxygen-insensitive flavin-dependent glucose dehydrogenase, GDH, and Au nanoparticles (NPs) is assembled on a Au surface using an electropolymerization process. The bis-aniline-cross-linked GDH/Au NPs composite reveals effective electrical contact with the electrode (ket=1100 s(-1)), and the effective bioelectrocatalyzed oxidation is driven by the enzyme/NPs matrix. The GDH/Au NPs-functionalized electrode is implemented as an amperometric glucose sensor, and it reveals superior functions when compared to an analogous glucose oxidase/Au NPs system. The O2-insensitive GDH/Au NPs composite electrode was further used as an anode in a membraneless glucose/O2 biofuel cell. The cathode in this system was composed of bilirubin oxidase cross-linked onto a carbon nanotube-modified glassy carbon electrode. The power output of the cell was 32 µW cm(-2).