Electron transport through noncovalent interaction is of fundamental and practical importance in nanomaterials and nanodevices. Recent single-molecule studies employing singlemolecule junctions have revealed unique electron transport properties through noncovalent interactions, especially those through a π−π interaction. However, the relationship between the junction structure and electron transport remains elusive due to the insufficient knowledge of geometric structures. In this article, we employ surface-enhanced Raman scattering (SERS) synchronized with current−voltage (I−V) measurements to characterize the junction structure, together with the transport properties, of a single dimer and monomer junction of naphthalenethiol, the former of which was formed by the intermolecular π−π interaction. The correlation analysis of the vibrational energy and electrical conductance enables identifying the intermolecular and molecule−electrode interactions in these molecular junctions and, consequently, addressing the transport properties exclusively associated with the π−π interaction. In addition, the analysis achieved discrimination of the interaction between the NT molecule and the Au electrode of the junction, i.e., Au−π interactions through-π coupling and though-space coupling. The power density spectra support the noncovalent character at the interfaces in the molecular junctions. These results demonstrate that the simultaneous SERS and I−V technique provides a unique means for the structural and electrical investigation of noncovalent interactions.

Charge transfer at metal–molecule interfaces plays a decisive role in nanomaterials especially for electronic applications; however, its knowledge at the single-molecule level remains elusive. Herein, we found a highly conductive adsorption site in a single-molecule junction (SMJ) of naphthalenedithiol by a combination of surface-enhanced Raman scattering (SERS) and current–voltage measurements. The vibrational energy and conductance reveal that the high conductivity originates from the adsorption site with large charge transfer. The present study demonstrates that SERS combined with transport measurements reveals not only structures of SMJs but also charge transfer at the metal–molecule interfaces.

Single-molecule measurements of biomaterials bring novel insights into cellular events. For almost all of these events, post-translational modifications (PTMs), which alter the properties of proteins through their chemical modifications, constitute essential regulatory mechanisms. However, suitable single-molecule methodology to study PTMs is very limited. Here we show single-molecule detection of peptide phosphorylation, an archetypal PTM, based on electrical measurements. We found that the phosphate group stably bridges a nanogap between metal electrodes and exhibited high electrical conductance, which enables specific single-molecule detection of peptide phosphorylation. The present methodology paves the way to single-molecule studies of PTMs, such as single-molecule kinetics for enzymatic modification of proteins as shown here.

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