Simple Preparation of Au Nanoparticles by Arc Plasma Deposition (APD) and Its Application for SERS Sensing
Authors: Takebayashi Masahiro , Shimizu Tomohiro , Shingubara Shoso , Ito Takeshi 
Published: December 2024

Nanoparticles express many beneficial properties. For examples, large specific surface area and high catalytic activity. These properties are used in fuel cell catalysis and other applications such as biosensing since porous nanoparticles have a high adsorption area for substances. Nanoparticles for biosensing technology are used to not only bind with biomolecules but also observe the behavior of the biomolecules, such as cells. In this study, we focused on Au nanoparticles with plasmon resonance, i.e., one of the optical properties. When metal nanoparticles, such as Au or Ag, are irradiated with light, free electrons of metal nanoparticles are affected by the electric field of the light and cause plasmon resonance, which produces a unique optical phenomenon. On the plasmon resonance, the light with a specific wavelength is strongly absorbed and scattered, resulting in vivid colors in the near-infrared range. This phenomenon is called Localized Surface Plasmon Resonance (LSPR). This phenomenon is used in sensors (to detect changes in refractive index and use as markers) as well as Surface-Enhanced Raman Scattering (SERS), which is used to evaluate material identification. In this study, we focused on SERS sensing. SERS is powerful tool to identify and quantify the materials since Raman spectrum reflect chemical bonds of substances and concentration. Because Raman spectrometers can non-destructively measure substances through glass, bags, or opaque containers, they are used to investigate explosives, toxic chemicals, and other counter-terrorism measures. Therefore, a SERS sensor with a simple fabrication process is required.
There are two methods for producing the nanoparticles: a physical method (pulverization method), and a chemical method (agglomeration method). Physical methods create nanoparticles by crushing powder, but it is difficult to control the particle size. By contrast, chemical methods can be mainly classified into wet methods and dry methods. The wet method is reducing ions in liquid therefore it requires waste liquid treatment, which places a burden on the environment. Therefore, we focused on Arc Plasma Deposition (APD), which is one of dry methods and a low environmental impact. APD allows nanoparticles to be easily produced in one step. Nanoparticles were fabricated using APD and applied to SERS sensor. APD can control the deposition molecules by digital, meaning the number of pulses. The parameters of APD are capacitance of trigger capacitor, applied voltage, the number of pulses, and substrate temperature. At first, we investigated its vapor deposition characteristics when the substrate temperature is kept at room temperature.
The amount of deposited molecules is proportional to the square of the discharge voltage (V), directly proportional to the capacitance (C), and dependent on the number of pulses (n) :(E=1/2×nCV2). Applied volage was ragged from 70 to 100 V, a capacitance was set as 2200 μF, and the number of pulses were from 5 to 80 times. The pressure during vapor deposition was 1.3×10-3 Pa. The average particle diameter and coverage ratio were calculated by observing the prepared sample via a Scanning Transmission Electron Microscope (STEM). It was clear that particle diameter and coverage ratio increased with the applied voltage and the number of pulses. We evaluated the fabricated sensor exhibited SERS activity using rhodamine 6G (R6G), a fluorescent substance. In this study, we evaluated analytical enhancement factor (AEF) to quantify our SERS The experimental results show that theAEF was determined as 2.13×10³. We also created a calibration curve for R6G ranging from 10-7 to 10-4 M. As the results, R6G-derived peaks were not identified at 10⁻⁷ M, but we identified them from 10-6 to 10-4 M (Fig. 1) and were able to create the linear calibration curve.

Figure 1