(139c) Functionalization and Characterization of Gold Nanoparticles

Gold nanoparticles (AuNPs) have a high percentage of surface atoms that gives them special electronic properties and reactivities. Since these properties are dependent on AuNP size, shape and surface chemistry one can tune the AuNPs for optimal performance in nanomedicine, microarray and biosensor applications. In this research, AuNPs 14, 25 and 40 nm in diameter were synthesized via the citrate reduction method and then functionalized with various chain length (6, 8, 11, and 16 CH₂ units) carboxylic acid terminated alkyl thiols by direct displacement of the citrate ions. The size, shape and size distribution of the AuNPs were characterized with transmission electron microscopy (TEM) and ImageJ. The size distribution increased with increasing particle size, going from a 3 σ value of 2.5 nm for the 14 nm AuNPs to a 3 σ value of 20nm for the 40 nm AuNPs. Surface chemistries of the alkyl thiol self assembled monolayer (SAMs) were characterized with X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The XPS and ToF-SIMS results were used to optimize the synthesis and functionalization procedures so contaminants (e.g., Na) and unbound thiols were not detected. In particular, it was found dialysis was the best method for removing unbound thiol molecules. Comparison for SAMs of 16-mercaptohexadecanoic acid on the three different AuNPs sizes and flat Au surface were done to investigate effect of particle size and surface curvature on the SAM surface chemistry. In addition, 14nm AuNPs and flat Au surfaces were functionalized with the four different chain length COOH-SAMs to investigate effect of SAMs chain length on the measured SAMs chemistry. In the first case, changing the substrate while holding the SAM constant, the XPS C/Au atomic ratio decreased as the AuNPs size increased and was the smallest on the flat Au. The changing C/Au atomic ratio could be due several effects, including the curvature of the AuNP surfaces and the packing of the thiol molecules on those surfaces. Work is currently in progress to develop XPS data analysis methods to account for AuNP surface curvature effects. In the second case, changing SAMs chain length while holding substrate constant, the XPS C/Au atomic ratio decreased as the chain length decreased for both AuNPs and flat Au surfaces, as expected since the Au signal will be more attenuated as the thickness of the SAM layer increases. The ToF-SIMS data were analyzed using principal component analysis (PCA) and the results supported the XPS results. Intensity ratios of hydrocarbons ions to gold containing ions exhibited similar trends as the XPS C/Au atomic ratios for all samples. We are also investigating the SAM packing with FTIR and the AuNP stability with UV/VIS.