Controllable silver nanostructure synthesis in a microfluidic channel for real-time SERS system

Abstract

Raman spectroscopy is a label-free detecting technique and it provides fingerprint spectra for identifying molecules and intensity variation as concentration varying. Ag or Au nanoparticle clusters in a thin film form are used as SERS substrate due to its characteristics of enhancing Raman signals. For SERS, silver nanostructures are normally fabricated by chemical vapor deposition (CVD), physical vapor deposition (PVD) and E-beam sputtering. These are performed at high temperature (over 900°C) and high vacuum conditions. The annealing process is needed after the deposition to form nanostructure called islands. In this study, we present a manageable and low-cost method to fabricate Ag nanostructured thin film by using microfluidic system for SERS application. Under atmospheric pressure, Ag thin film formation process is based on polyol method in microfluidic channel at 150°C. For fabrication, ethylene glycol (EG, HO(CH2) 2OH) was used as a solvent. As a precursor, 10 mL of 0.1 M silver nitrate (AgNO3), 10 mL of 0.15 M polyvinylpyrrolidone (PVP, (C6H9NO)n), and 200 μL 4 mM copper chloride (CuCl2). When EG oxidizes and generates electrons, Ag ions form into a nucleus of silver cluster and nuclei of silver molecules are stabilized by PVP. The solution was injected into a polydimethylsiloxane (PDMS) microfluidic channel and heated at 150°C. During the synthesis, EG reduces Ag ions into Ag nanoparticles at the bottom surface of the substrate. The size of Ag nanoparticles constituting Ag thin film was increased as increasing the synthesis time. After Ag thin film formed, the microfluidic channel was cleaned by ethanol (C2H3OH) injection to remove PVP from Ag thin film surface. The optimum region and synthesis time for SERS detection were defined. For SERS detection, adenosine solution was prepared in various concentrations and loaded on the Ag thin film formed inside of microfluidic channel. The fingerprint spectrum of adenosine was obtained and intensity of spectru...