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Monitoring the presence of ionic mercury in environmental water by plasmon-enhanced infrared spectroscopy.

Hoang CV, Oyama M, Saito O, Aono M, Nagao T - Sci Rep (2013)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: WPI Center for Materials NanoArchitectonics-MANA, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan. Hoang.ChungVu@nims.go.jp

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.

No MeSH data available.


Experimental and simulated spectra on the optical property of the plasmonic Au nanogap network.On the left: three typical scanning electron microscope (SEM) images of the gold nanostructures, taken at different growth stages, scale bar: 200 nm. Those SEM pictures are used as models for the rigorous coupled wave analysis (RCWA) simulations. (a): A comparison of the simulated spectra (adopting the blue frame SEM) and the measured IR reflectance of the samples whose morphology is similar to that of the SEM pictures in the left. (b): Experimental time evolution of the IR spectra measured in-situ during the growth of the AuNP. (c): RCWA simulations of the three growth stages (SEM images in the left) in the presence of water in the nanogaps. The increase of the spectral coupling between the water vibration and the plasmonic excitation of the Au nanostructure could be identified accordingly from the distorted feature of the water bands. The color of each spectrum in (b) and (c) corresponds to each growth stage represented by the SEM images with the same color on the frame.
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f2: Experimental and simulated spectra on the optical property of the plasmonic Au nanogap network.On the left: three typical scanning electron microscope (SEM) images of the gold nanostructures, taken at different growth stages, scale bar: 200 nm. Those SEM pictures are used as models for the rigorous coupled wave analysis (RCWA) simulations. (a): A comparison of the simulated spectra (adopting the blue frame SEM) and the measured IR reflectance of the samples whose morphology is similar to that of the SEM pictures in the left. (b): Experimental time evolution of the IR spectra measured in-situ during the growth of the AuNP. (c): RCWA simulations of the three growth stages (SEM images in the left) in the presence of water in the nanogaps. The increase of the spectral coupling between the water vibration and the plasmonic excitation of the Au nanostructure could be identified accordingly from the distorted feature of the water bands. The color of each spectrum in (b) and (c) corresponds to each growth stage represented by the SEM images with the same color on the frame.

Mentions: First, to understand the infrared absorption of the plasmonic structure used in this study, we combined the results of in-situ IR experiments (see Method) and the numerical electromagnetic (EM) simulations. The rigorous coupled wave analysis method (RCWA method, DiffractMOD, RSoft Design)12 was used to simulate the influence of the morphological development of nano-particles on their IR response, as well as the spectral coupling of the plasmonic substrate and stretching vibrational signal of water. To precisely simulate the experimental situation, we used typical scanning electron microscopy (SEM) images taken in real experiments in accordance with the growth stages of the plasmonic substrate to simulate the spectra. They are shown in the left of Fig. 2, framed by red, green and blue colors; corresponding height: 15 nm, 25 nm and 40 nm; filling factor: 11%, 52% and 81%, respectively. The dielectric functions in the IR region are taken from the measurements by Johnson and Christy (for Au)13, and by W. Theiss (for Si and water)14. The impinging, polarized (s or p polarization) IR radiation is illuminated from air to the Si substrate, inclined 30° relative to the normal incident of Si. Figure 2a shows the relative reflectance IR spectra of the measured and simulated plasmonic substrates, performed in air. A clear development of the reflectance spectra in the mid-IR is observed as the island's lateral size and the filling factor increase. This evolution shows good accordance with the reports on the lithographic plasmonic objects1516: the red-shift in IR plasmon is induced by the development in the object size and the electromagnetic (EM) coupling between them. In order to verify this simulation result, two typical reflectance spectra are shown. The lower reflectance curve (with circle) corresponds to the film at percolation, showing a good matching with the simulation data. The data at higher reflectance corresponds to the film at the stage slightly before the percolation. It should be noted that the growth conditions between each experiment cannot be exactly the same, a slight variation of the reflectance is often recognized within this range of experiment. A small discrepancy between the simulation and measured data from 3000 cm−1 toward higher energy is found which might be either assigned to the surface roughness and roundish corners of real AuNP gap structures, or the much smaller area of the simulated structure than in experiment.


Monitoring the presence of ionic mercury in environmental water by plasmon-enhanced infrared spectroscopy.

Hoang CV, Oyama M, Saito O, Aono M, Nagao T - Sci Rep (2013)

Experimental and simulated spectra on the optical property of the plasmonic Au nanogap network.On the left: three typical scanning electron microscope (SEM) images of the gold nanostructures, taken at different growth stages, scale bar: 200 nm. Those SEM pictures are used as models for the rigorous coupled wave analysis (RCWA) simulations. (a): A comparison of the simulated spectra (adopting the blue frame SEM) and the measured IR reflectance of the samples whose morphology is similar to that of the SEM pictures in the left. (b): Experimental time evolution of the IR spectra measured in-situ during the growth of the AuNP. (c): RCWA simulations of the three growth stages (SEM images in the left) in the presence of water in the nanogaps. The increase of the spectral coupling between the water vibration and the plasmonic excitation of the Au nanostructure could be identified accordingly from the distorted feature of the water bands. The color of each spectrum in (b) and (c) corresponds to each growth stage represented by the SEM images with the same color on the frame.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3569543&req=5

f2: Experimental and simulated spectra on the optical property of the plasmonic Au nanogap network.On the left: three typical scanning electron microscope (SEM) images of the gold nanostructures, taken at different growth stages, scale bar: 200 nm. Those SEM pictures are used as models for the rigorous coupled wave analysis (RCWA) simulations. (a): A comparison of the simulated spectra (adopting the blue frame SEM) and the measured IR reflectance of the samples whose morphology is similar to that of the SEM pictures in the left. (b): Experimental time evolution of the IR spectra measured in-situ during the growth of the AuNP. (c): RCWA simulations of the three growth stages (SEM images in the left) in the presence of water in the nanogaps. The increase of the spectral coupling between the water vibration and the plasmonic excitation of the Au nanostructure could be identified accordingly from the distorted feature of the water bands. The color of each spectrum in (b) and (c) corresponds to each growth stage represented by the SEM images with the same color on the frame.
Mentions: First, to understand the infrared absorption of the plasmonic structure used in this study, we combined the results of in-situ IR experiments (see Method) and the numerical electromagnetic (EM) simulations. The rigorous coupled wave analysis method (RCWA method, DiffractMOD, RSoft Design)12 was used to simulate the influence of the morphological development of nano-particles on their IR response, as well as the spectral coupling of the plasmonic substrate and stretching vibrational signal of water. To precisely simulate the experimental situation, we used typical scanning electron microscopy (SEM) images taken in real experiments in accordance with the growth stages of the plasmonic substrate to simulate the spectra. They are shown in the left of Fig. 2, framed by red, green and blue colors; corresponding height: 15 nm, 25 nm and 40 nm; filling factor: 11%, 52% and 81%, respectively. The dielectric functions in the IR region are taken from the measurements by Johnson and Christy (for Au)13, and by W. Theiss (for Si and water)14. The impinging, polarized (s or p polarization) IR radiation is illuminated from air to the Si substrate, inclined 30° relative to the normal incident of Si. Figure 2a shows the relative reflectance IR spectra of the measured and simulated plasmonic substrates, performed in air. A clear development of the reflectance spectra in the mid-IR is observed as the island's lateral size and the filling factor increase. This evolution shows good accordance with the reports on the lithographic plasmonic objects1516: the red-shift in IR plasmon is induced by the development in the object size and the electromagnetic (EM) coupling between them. In order to verify this simulation result, two typical reflectance spectra are shown. The lower reflectance curve (with circle) corresponds to the film at percolation, showing a good matching with the simulation data. The data at higher reflectance corresponds to the film at the stage slightly before the percolation. It should be noted that the growth conditions between each experiment cannot be exactly the same, a slight variation of the reflectance is often recognized within this range of experiment. A small discrepancy between the simulation and measured data from 3000 cm−1 toward higher energy is found which might be either assigned to the surface roughness and roundish corners of real AuNP gap structures, or the much smaller area of the simulated structure than in experiment.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: WPI Center for Materials NanoArchitectonics-MANA, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan. Hoang.ChungVu@nims.go.jp

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.

No MeSH data available.