Scanning tunneling spectroscopy investigation of self-assembled plastocyanin mutants onto gold substrates under controlled environment

Distance and Potential Dependence of Charge Transport Through the Reaction Center of Individual Photosynthetic Complexes

Charge separation and transport through the reaction center of photosystem I (PSI) is an essential part of the photosynthetic electron transport chain. A strategy is developed to immobilize and orient PSI complexes on gold electrodes allowing to probe the complex's electron acceptor side, the chlorophyll special pair P700. Electrochemical scanning tunneling microscopy (ECSTM) imaging and current-distance spectroscopy of single protein complex shows lateral size in agreement with its known dimensions, and a PSI apparent height that depends on the probe potential revealing a gating effect in protein conductance. In current-distance spectroscopy, it is observed that the distance-decay constant of the current between PSI and the ECSTM probe depends on the sample and probe electrode potentials. The longest charge exchange distance (lowest distance-decay constant β) is observed at sample potential 0 mV/SSC (SSC: reference electrode silver/silver chloride) and probe potential 400 mV/SSC. These potentials correspond to hole injection into an electronic state that is available in the absence of illumination. It is proposed that a pair of tryptophan residues located at the interface between P700 and the solution and known to support the hydrophobic recognition of the PSI redox partner plastocyanin, may have an additional role as hole exchange mediator in charge transport through PSI.

Molecular sensing with the tunnel junction of an Au nanogap in solution

The tunnel junction of a gold nanogap was fabricated electrochemically for a molecular sensing device in solution. The tunnel junction was sensitive enough to detect the variation of a potential barrier within the nanogap, such as the chemical adsorption of molecules. By monitoring the variation of the tunneling current, which represents the change of a potential barrier due to molecular adsorption, the molecules could be detected.

Functional Metalloproteins Integrated with Conductive Substrates: Detecting Single Molecules and Sensing Individual Recognition Events

In the past decade, there has been significant interest in the integration of biomaterials with electronic elements: combining biological functions of biomolecules with nanotechnology offers new perspectives for implementation of ultrasensitive hybrid nanodevices. In particular, great attention has been devoted to redox metalloproteins, since they possess unique characteristics, such as electron-transfer capability, possibility of gating redox activity, and nanometric size, which make them appealing for bioelectronics applications at the nanoscale. The reliable connection of redox proteins to electrodes, aimed at ensuring good electrical contact with the conducting substrate besides preserving protein functionality, is a fundamental step for designing a hybrid nanodevice and calls for a full characterization of the immobilized proteins, possibly at the single-molecule level. Here, we describe how a multitechnique approach, based on several scanning probe microscopy techniques, may provide a comprehensive characterization of different metalloproteins on metal electrodes, disclosing unique information not only about morphological properties of the adsorbed molecules but also about the effectiveness of electrical coupling with the conductive substrate, or even concerning the preserved biorecognition capability upon adsorption. We also show how the success of an immobilization strategy, which is of primary importance for optimal integration of metalloproteins with a metal electrode, can be promptly assessed by means of the proposed approach. Besides the characterization aspect, the complementary employment of the proposed techniques deserves major potentialities for ultrasensitive detection of adsorbed biomolecules. In particular, it is shown how sensing of single metalloproteins may be optimized by monitoring the most appropriate observable. Additionally, we suggest how the combination of several experimental techniques might offer increased versatility, real-time response, and wide applicability as a detection method, once a reproducible correlation among signals coming from different single-molecule techniques is established.

Tip to substrate distances in STM imaging of biomolecules

STM images of single biomolecules adsorbed on conductive substrates do not reproduce the expected physical height, which generally appears underestimated. This may cause the tip to interfere with the soft biological sample during the imaging scans. Therefore, a key requirement to avoid invasive STM imaging is the knowledge, and the control, of the initial tip to substrate distance. This is connected to the setting of the tunnelling current and applied voltage, which define a tunnelling resistance. The height of the STM tip was measured by calibrating the tunnelling resistance, as a function of its vertical displacement until establishing a mechanical contact. At a tunnelling resistance of 4 x 10(9)Omega, distances of about 3 and 6 nm are estimated when flat Au substrates are imaged in water and in air, respectively. On such a ground, the relevance of the starting tip-substrate distance in determining a non-invasive imaging has been investigated for a plastocyanin mutant chemisorbed on Au(111) electrodes. At tunnelling distances sufficient to overcome the physical height of the imaged biomolecules, their lateral dimensions are found to be consistent with the crystallography, whereas they are significantly broadened for smaller distances.