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.

Plasmon-induced hot-electron generation has recently received considerable interest and has been studied to develop novel applications in optoelectronics, photovoltaics and green chemistry. Such hot electrons are typically generated from either localized plasmons in metal nanoparticles or propagating plasmons in patterned metal nanostructures. Here we simultaneously generate these heterogeneous plasmon-induced hot electrons and exploit their cooperative interplay in a single metal-semiconductor device to demonstrate, as an example, wavelength-controlled polarity-switchable photoconductivity. Specifically, the dual-plasmon device produces a net photocurrent whose polarity is determined by the balance in population and directionality between the hot electrons from localized and propagating plasmons. The current responsivity and polarity-switching wavelength of the device can be varied over the entire visible spectrum by tailoring the hot-electron interplay in various ways. This phenomenon may provide flexibility to manipulate the electrical output from light-matter interaction and offer opportunities for biosensors, long-distance communications, and photoconversion applications.

Plasmons in metallic nanomaterials exhibit very strong size and shape effects, and thus have recently gained considerable attention in nanotechnology, information technology, and life science. In this review, we overview the fundamental properties of plasmons in materials with various dimensionalities and discuss the optical functional properties of localized plasmon polaritons in nanometer-scale to atomic-scale objects. First, the pioneering works on plasmons by electron energy loss spectroscopy are briefly surveyed. Then, we discuss the effects of atomistic charge dynamics on the dispersion relation of propagating plasmon modes, such as those for planar crystal surface, atomic sheets and straight atomic wires. Finally, standing-wave plasmons, or antenna resonances of plasmon polariton, of some widely used nanometer-scale structures and atomic-scale wires (the smallest possible plasmonic building blocks) are exemplified along with their applications.