Localized Surface Plasmon Resonance (LSPR) is gaining interest as a highly sensitive analytical technique to detect chemical and biological molecules in low concentrations. Plasma treatment is utilized to clean patterned substrates, specifically designed for LSPR, and has been found to significantly enhance signal response.
LSPR relies on the coupling of electromagnetic radiation (typically incident laser light) to surfaces covered with metallic nanostructures and nanoparticles (NPs), resulting in collective excitation and coherent oscillation of surface conduction electrons in the metal. The resulting LSPR spectrum plots optical extinction (light that is absorbed and elastically scattered) as a function of wavelength and gives rise to sharp peaks at distinct wavelengths (resonance frequency). As the resonance frequency greatly depends on the size, shape, and material of the nanostructured surface, the LSPR wavelength can be tuned by varying these parameters [1,2].
In addition, the LSPR signal is sensitive to changes in the refractive index surrounding the nanostructures . As such, this technique can be used to probe chemical change in the local environment and detect the presence of adsorbed molecules with high spatial resolution.
With recent advances in the ability to fabricate periodic nanostructures with greater control and precision, researchers have fabricated substrates with metallic nanostructures and produced chemical and biological sensors that exploit the LSPR phenomenon .
Ag or Au nanostructures are commonly used in these specialized substrates because they exhibit strong resonance frequency (extinction peak) in the visible to near IR spectrum . However, one limitation of Ag is its reaction with the ambient environment, resulting in formation of silver sulfide tarnish (AgS2), which is more pronounced in nanoparticles because of the high surface area to volume ratio. Absorption of impurities alters the refractive index of the surrounding substrate environment and can result in weakening and broadening of the LSPR signal along with a peak shift to longer wavelengths, thus limiting the use of Ag for LSPR.
Recent publications have reported using argon (Ar) plasma to clean Ag nanostructures while further improving the LSPR plasmonic response.
Takahiro’s group found that brief Ar plasma exposures (10 s) can remove impurities and tarnish from Ag nanoparticles and thus recover the metallic properties of Ag without oxidation. At the same time, they observed sharpening of the LSPR extinction peak with a blue shift to shorter wavelength (Figure 1).
Figure 1. Extinction spectra of Ag nanoparticles on SiO2 substrate before (—) and after Ar plasma treatment for 10 (○), 20 (●),30 (□), 40 (■), 50 (△), 100 (▲), 200 (▽), 300 (▼), 400 (◇), 500 s (◆). Source: Hosomi K, Ozaki K, Takahiro K, Nishiyama F and Yokoyama S. Jpn. J. Appl. Phys. (2020) 59: 015002. DOI: 10.7567/1347-4065/ab5c29 © (2019) The Japan Society of Applied Physics
X-ray Photoelectron Spectroscopy (XPS) spectra showed the reduction of C, N, S elements without significant compositional change to the plasma-treated Ag or to the underlying SiO2 substrate. Scanning Electron Microscopy (SEM) imaging showed little change to the NP size and areal density, indicating that the morphology was not significantly affected by the short plasma treatment. These results strongly suggest that the improved LSPR response was because of impurity removal and reduction of Ag2S to metallic Ag. However, because the impurity removal changed the local optical properties, weakening of the LSPR signal did occur (Figure 1).
Longer plasma exposures (up to 300-500 s) again did not change the composition of the Ag NPs or SiO2 substrate but led to an increase in particle size with a more spherical shape and greater distance between particles. Figure 2 is a representation of the Ag particle coarsening with Ar plasma treatment. The authors suggest localized heating during plasma treatment (up to 67°C) contributed to particle coarsening and Ostwald ripening. The morphological change led to increased plasmonic sensitivity to the probed analyte (Takahiro group used ethanol as the analyte in their study) and significant improvement of the LSPR spectrum. Figure 1 shows increased peak intensity, narrower peak width, and blue shift to shorter wavelengths with longer plasma exposures.
Figure 2. A schematic representation of the Ag tarnish removal and particle coarsening with Ar plasma treatment. Recreated from Ozaki K, Nishiyama F and Takahiro K. Appl. Surf. Sci. (2015) 357: 1816-1822. DOI: 10.1016/j.apsusc.2015.10.017
Optical emission spectra showed emission peaks associated with Ar, as expected. In addition, hydrogen and oxygen emission peaks were detected along with UV emissions, most likely from excited Ar returning to the ground state. The mechanism for contamination removal is likely from reactive oxygen species and UV radiation, both of which are capable of breaking organic chemical bonds (CH, CC, CO, CN). Hydrogen may also contribute to plasma formation and H2 plasma has been found to be effective in the reduction of oxidized metal.
Takahiro group’s research demonstrates that, through the combination of impurity removal; reduction of Ag tarnish; and morphological changes (particle coarsening), Ar plasma can enhance LSPR peak signals to improve detection sensitivity to chemical analytes.
The benefits of plasma treatment to clean and recovered tarnished metal surfaces may also be applied to other analytical techniques or research areas that utilize metallic nanostructures with high surface area, such as surface enhanced Raman spectroscopy (SERS) or catalytic applications.
Relevant Articles from Harrick Plasma Users
- Hosomi K, Ozaki K, Takahiro K, Nishiyama F and Yokoyama S. “Plasma induced enhancements in plasmonic sensitivity of sputter-deposited silver nanoparticles to ethanol vapor”. Jpn. J. Appl. Phys. (2020) 59: 015002. DOI: 10.7567/1347-4065/ab5c29
- Hosomi K, Ozaki K, Nishiyama F and Takahiro K. “Enhancement in volatile organic compound sensitivity of aged Ag nanoparticle aggregates by plasma exposure”. Appl. Surf. Sci. (2018) 427: 848-853. DOI: 10.1016/j.apsusc.2017.07.155
- Ozaki K, Nishiyama F and Takahiro K. “Plasma-induced brightening and coarsening of tarnished Ag nanoparticles”. Appl. Surf. Sci. (2015) 357: 1816-1822. DOI: 10.1016/j.apsusc.2015.10.017
- Kawaguchi K, Saito M, Takahiro K, Yamamoto S and Yoshikawa M. “Blueshift and Narrowing of Localized Surface Plasmon Resonance of Silver Nanoparticles Exposed to Plasma”. Plasmonics (2011) 6: 535-539. DOI: 10.1007/s11468-011-9233-4
Supplemental References (Do not report using Harrick Plasma Instruments)
 Willets KA and Van Duyne RP. “Localized Surface Plasmon Resonance Spectroscopy and Sensing.” Annu. Rev. Phys. Chem. (2007) 58: 267-297.
 Hutter E and Fendler JH. “Exploitation of Localized Surface Plasmon Resonance.” Adv. Mater. (2004) 16: 1685-1706.
 Mayer KM and Hafner JH. “Localized Surface Plasmon Resonance Sensors.” Chem. Rev. (2011) 111: 3828–3857.