Multitechnique Characterization of Self-Assembled\nCarboxylic Acid-Terminated Alkanethiol Monolayers on Nanoparticle\nand Flat Gold Surfaces

Abstract

Gold nanoparticles (AuNPs) with 14, 25, and 40 nm diameters were\nfunctionalized with different chain length (C6, C8, C11, and C16)\ncarboxylic acid terminated alkanethiol self-assembled monolayers (COOH-SAMs).\nX-ray photoelectron spectroscopy (XPS) and time-of-flight secondary\nion mass spectrometry (ToF-SIMS) were used to examine the changes\nin surface chemistry as both AuNP diameter and SAM chain length were\nvaried. COOH-SAMs on flat gold surfaces were also examined and compared\nto the COOH-SAM on AuNP results. For a given surface, as the COOH-SAM\nchain length increased the XPS C/Au atomic ratio increased due to\nan increased number of carbon atoms per molecule in the overlayer\nand an increased attenuation of the Au substrate signal. For the C16\nCOOH-SAMs, as the size of AuNPs decreased the XPS C/Au atomic ratio\nand the apparent SAM thickness increased due to the increased curvature\nof the smaller AuNPs. The C16 COOH-SAMs on the flat Au had the lowest\nXPS C/Au atomic ratio and apparent SAM thickness of any C16 COOH-SAM\ncovered Au surface. The effective takeoff angles of the COOH-SAMs\nwere also calculated by comparing the apparent thickness of COOH-SAMs\nwith literature values. The effective takeoff angle for C16 COOH-SAM\non 14-, 25-, and 40-nm diameter AuNPs and flat Au were found to be\n57°, 53°, 51°, and 39°, respectively, for data\nacquired in a mode that collects a wide range of photoelectron takeoff\nangles. The effective takeoff angle for C16 COOH-SAM on 14 nm AuNP\nand flat Au decreased to 52° and 0°, respectively, for data\nacquired in a mode that collects a narrow range of photoelectron takeoff\nangles. The ToF-SIMS results showed similar changes in surface chemistry\nwith COOH-SAM chain length and AuNP size. For example, the ratio of\nthe sum of the C_1–4H_<i>x</i>O_<i>y</i> positive ion intensities to the sum\nof the Au-containing positive ions intensities increased with decreasing\nAuNP size and increasing COOH-SAM chain length. Fourier transform\nIR spectroscopy in the attenuated total reflectance mode (FTIR-ATR)\nwas used to characterize the crystallinity of the COOH-SAMs. The CH₂ stretching frequencies decreased with increasing COOH-SAM\nchain length on flat Au. The C16 COOH-SAM on the 14 nm AuNPs exhibited\na crystalline-like CH₂ stretching frequency. The size,\nsize distribution, shapes, and solution stability of AuNPs were investigated\nwith transmission electron microscopy (TEM) and UV/vis spectroscopy.\nAs the average diameter of the AuNPs decreased, the size distribution\nbecame narrower, and the shape became more spherical.