We demonstrate the ppt-level single-step selective monitoring of the presence of mercury ions (Hg2+) dissolved in environmental water by plasmon-enhanced vibrational spectroscopy. We combined a nanogap-optimized mid-infrared plasmonic structure with mercury-binding DNA aptamers to monitor in-situ the spectral evolution of the vibrational signal of the DNA induced by the mercury binding. Here, we adopted single-stranded thiolated 15-base DNA oligonucleotides that are immobilized on the Au surface and show strong specificity to Hg2+. The mercury-associated distinct signal is located apart from the biomolecule-associated broad signals and is selectively characterized. For example, with natural water from Lake Kasumigaura (Ibaraki Prefecture, Japan), direct detection of Hg2+ with a concentration as low as 37 ppt (37 × 10−10%) was readily demonstrated, indicating the high potential of this simple method for environmental and chemical sensing of metallic species in aqueous solution.

We developed a procedure for the fabrication of sub 1 nm gap Au electrodes via electromigration. Self-aligned nanogap formation was achieved by applying a bias voltage, which causes electromigration during metal evaporation. We also demonstrated the application of this method for the formation of nanogaps as small as 1 nm in width, and we found that the gap size can be controlled by changing the magnitude of the applied voltage. On the basis of the electric conductance and surface-enhanced Raman scattering (SERS) measurements, the fabricated gap size was estimated to be nearly equal to the molecular length of 1,4-benzenedithiol (BDT). Compared with existing electromigration methods, the new method provides two advantages: the process currents are clearly suppressed and parallel or large area production is possible. This simple method for the fabrication of a sub 1 nm gap electrode is useful for single-molecule-sized electronics and opens the door to future research on integrated sub 1 nm sized nanogap devices.

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