Fabrication of SERS Substrates for the Detection of Metal Pollutants via Directed Self-Assembly of Hydrophobic Gold Nanoparticles at the Air-Water Interface.
Harnish, Sarah M.
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Plasmonic nanostructures, such as gold nanoparticles (AuNPs), have promising applications in surface enhanced Raman scattering (SERS)-based sensing of environmental pollutants. Two-dimensional nanoparticle arrays give Raman enhancements necessary for ultrasensitive detection, but are difficult and costly to produce by common techniques such as nanosphere lithography. This study explores the effectiveness of a lipid monolayer spread at the air-water interface in directing the self-assembly of hydrophobic gold nanoparticles into 2D structures. Palmitic acid monolayer and AuNP composite layer films were deposited by the Langmuir-Blodgett (LB) method onto SiO2/Si wafer substrates, and thoroughly characterized by FTIR spectroscopy, light microscopy, and scanning electron microscopy. Increasing the pH to 8 and introducing Ca2+ions to the subphase condensed palmitic acid leading to greater surface coverage and ordering of deposited monolayer films than achieved with a subphase of pure water. Composite films of AuNPs and d31-palmitic acid showed AuNP multilayer formation when spread in a concentrated solution of chloroform. Spreading AuNPs in dilute solutions of toluene reduced AuNP aggregation, but the lipid film disassembled. Substrates were functionalized with selfassembled monolayers (SAMs) of trichloro(phenethyl)silane (PETCS) and trichloro(octadecyl)- silane (OTS) in order to deposit films by the Langmuir-Schaefer (LS) method. Si wafers were more effectively functionalized than SiO2/Si because they had a greater density of OH groups along the substrate surface. The hydrocarbon chains of OTS functionalized substrates interacted more strongly with palmitic acid, giving better monolayer depositions than the aromatic rings of PETCS substrates. LS depositions of d31-PA and AuNP composite films produced substrates that could function as SERS sensors. However, breakdown of the palmitic acid monolayer and AuNP aggregation remain persistent issues that may inhibit the Raman enhancement.