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Integrating proteins in molecular electronic devices requires control over their solid-state electronic transport behavior. Unlike "traditional" electron transfer (ET) measurements of proteins that involve liquid environments and a redox cycle, no redox cofactor is needed for solid-state electron transport (ETp) across the protein. Here we show the fundamental difference between these two approaches by macroscopic area measurements, which allow measuring ETp temperature dependence down to cryogenic temperatures, via cytochrome C (Cyt C), an ET protein with a heme (Fe-porphyrin) prosthetic group as a redox centre. We compare the ETp to electrochemical ET measurements, and do so also for the protein without the Fe (with metal-free porphyrin) and without porphyrin. As removing the porphyrin irreversibly alters the protein's conformation, we repeat these measurements with human serum albumin (HSA), 'doped' (by non-covalent binding) with a single hemin equivalent, i.e., these natural and artificial proteins share a common prosthetic group. ETp via Cyt C and HSA-hemin are very similar in terms of current magnitude and temperature dependence, which suggests similar ETp mechanisms via these two systems, thermally activated hopping (with ~0.1 eV activation energy) >190 K and tunneling by superexchange <190 K. Also, ET rates to and from the Fe redox centres (Fe(2+) <=> Fe(3+) + e(-)), measured by electrochemistry of HSA-hemin are only 4 times lower than those for Cyt C. However, while removing the Fe redox centre from the porphyrin ring markedly affects the ET rate, it hardly changes the ETp currents through these proteins, while removing the macrocycle (from HSA, which retains its conformation) significantly reduces ETp efficiency. These results show that solid-state ETp across proteins does not require the presence of a redox cofactor, and that while for ET the Fe ion is the main electron mediator, for ETp the porphyrin ring has this function.

Electrons can migrate via proteins over distances that are considered long for nonconjugated systems. The nanoscale dimensions of proteins and their enormous structural and chemical flexibility makes them fascinating subjects for exploring their electron transport (ETp) capacity. One particularly attractive direction is that of tuning their ETp efficiency by "doping" them with small molecules. Here we report that binding of retinoate (RA) to human serum albumin (HSA) increases the solid-state electronic conductance of a monolayer of the protein by >2 orders of magnitude for RA/HSA ≥ 3. Temperature-dependent ETp measurements show the following with increasing RA/HSA: (a) The temperature-independent current magnitude of the low-temperature (<190 K) regime increases significantly (>300-fold), suggesting a decrease in the distance-decay constant of the process. (b) The activation energy of the thermally activated regime (>190 K) decreases from 220 meV (RA/HSA = 0) to 70 meV (RA/HSA ≥ 3).

Semiconducting single walled carbon nanotubes (SWCNTs) are promising materials for biosensing applications with electrolyte‐gated transistors (EGT). However, to be employed in EGT devices, SWCNTs often require lengthy solution‐processing fabrication techniques. Here, we introduce a simple solution‐based method that allows fabricating EGT devices from stable dispersions of SWCNTs/bovine serum albumin (BSA) hybrids in water. The dispersion is then deposited on a substrate allowing the formation of a SWCNTs random network as the semiconducting channel. We demonstrate that this methodology allows the fabrication of EGT devices with electric performances that allow their use in biosensing applications. We demonstrate their application for the detection of cortisol in solution, upon gate electrode functionalization with anti‐cortisol antibodies. This is a robust and cost‐effective methodology that sets the ground for a SWCNT/BSA‐based biosensing platform that allows overcoming many limitations of standard SWCNTs biosensor fabrications. [ABSTRACT FROM AUTHOR]

It is reported that spiropyran-a widely investigated molecular photoswitch-can be stabilized in aqueous environments in the presence of a variety of proteins, including human serum albumin, insulin fibrils, lysozyme, and glucose oxidase. The optical properties of the complexed photoswitch are protein dependent, with human serum albumin providing the spiropyran with emission features previously observed for a photoswitch confined in media of high viscosity. Despite being bound to the protein molecules, spiropyran can undergo a ring-opening reaction upon exposure to UV light. This photoisomerization process can affect the properties of the proteins: here, it is shown that the electrical conduction through human serum albumin to which the spiropyran is bound increases following the ring-opening reaction. [ABSTRACT FROM AUTHOR]