Monitoring the Presence of Ionic Mercury in Environmental Water by Plasmon-Enhanced Infrared Spectroscopy

Abstract

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.

Versatile Direct Laser Writing Lithography Technique for Surface Enhanced Infrared Spectroscopy Sensors

“…Two notable examples for such detection methods are surface enhanced Raman scattering (SERS) 1-4 , and surface enhanced infrared absorption (SEIRA) [4][5][6][7][8][9][10][11][12][13][14][15][16] . In SEIRA spectroscopy, metallic and dielectric nanoantennas that exhibit plasmonic resonances in the Infra-Red (IR) regime are used to intensify the IR absorption signal of known vibrational modes of molecules, so that even small amount of analytes can be identified [4][5][6][7][8][9][10][11][12][13][14][15][16] . The improved detection originates from the excitation of localized surface plasmon resonances (LSPRs) -charge oscillations on the surface of the nano-structured surface -which give rise to a strong local electric field at the vicinity of the nanostructures and enhanced coupling between the textured surface and the vibrational modes of the adsorbed molecules.…”

Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy: a Powerful Technique for Bioanalysis

“…In addition, there exist some other types of metal film substrates, like gapcontaining metal films, which confine EM fields in the metal gaps and enhance IR signals. These systems can also be simulated even with polarized light [21]. With these substrates, IR EF of 10-1000 can be achieved, which enables sensitive detection at a single molecular layer level.…”

“…In electroless deposition, chemical reactions between solutes and prisms take place on the prism surface, forming large area of metal films with islands or gaps [48,49]. A derivate method is to disperse nanocrystal seeds on the prism surface and control the chemical deposition extending from the seeds [21,50]. These films adhere strongly to prism surface due to the chemical interaction, making it suitable for aqueous systems even under high electric field.…”

“…Unlike metal film substrates fabricated for UV-Visible region optical enhancement, materials used in ATR-SEIRAS are not limited in noble metals Au [16][17][18][19][20][21][22]26], Ag [27][28][29][30] and Cu [31]. Metals meet the condition that the real part of the dielectric function is larger than the imaginary component [15] like Al [32], Pt [33], Pd [33,34], Ru [33], Rh [33], Sn [13], Ni [35,36], Pb [37] can also be used as SEIRAS substrates.…”

“…Physical methods, normally referring to vacuum evaporation [10,21,29] and vacuum sputtering [43][44][45], are the first and extensively used approaches. With physical methods, metal films containing island nanoparticles can be directly deposited on the prism.…”