Multitechnique Characterization of Self-Assembled Carboxylic Acid-Terminated Alkanethiol Monolayers on Nanoparticle and Flat Gold Surfaces

Gold nanoparticles (AuNPs) with 14, 25, and 40 nm diameters were functionalized with different chain length (C6, C8, C11, and C16) carboxylic acid terminated alkanethiol self-assembled monolayers (COOH-SAMs). X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to examine the changes in surface chemistry as both AuNP diameter and SAM chain length were varied. COOH-SAMs on flat gold surfaces were also examined and compared to the COOH-SAM on AuNP results. For a given surface, as the COOH-SAM chain length increased the XPS C/Au atomic ratio increased due to an increased number of carbon atoms per molecule in the overlayer and an increased attenuation of the Au substrate signal. For the C16 COOH-SAMs, as the size of AuNPs decreased the XPS C/Au atomic ratio and the apparent SAM thickness increased due to the increased curvature of the smaller AuNPs. The C16 COOH-SAMs on the flat Au had the lowest XPS C/Au atomic ratio and apparent SAM thickness of any C16 COOH-SAM covered Au surface. The effective takeoff angles of the COOH-SAMs were also calculated by comparing the apparent thickness of COOH-SAMs with literature values. The effective takeoff angle for C16 COOH-SAM on 14-, 25-, and 40-nm diameter AuNPs and flat Au were found to be 57 degrees, 53 degrees, 51 degrees, and 39 degrees, respectively, for data acquired in a mode that collects a wide range of photoelectron takeoff angles. The effective takeoff angle for C16 COOH-SAM on 14 nm AuNP and flat Au decreased to 52 and 0 degrees, respectively, for data acquired in a mode that collects a narrow range of photoelectron takeoff angles. The ToF-SIMS results showed similar changes in surface chemistry with COOH-SAM chain length and AuNP size. For example, the ratio of the sum of the C1-4HxOy), positive ion intensities to the sum of the Au-containing positive ions intensities increased with decreasing AuNP size and increasing COOH-SAM chain length. Fourier transform IR spectroscopy in the attenuated total reflectance mode (FTIR-ATR) was used to characterize the crystallinity of the COOH-SAMs. The CH2 stretching frequencies decreased with increasing COOH-SAM chain length on flat Au. The C16 COOH-SAM on the 14 nm AuNPs exhibited a crystalline-like CH2 stretching frequency. The size, size distribution, shapes, and solution stability of AuNPs were investigated with transmission electron microscopy (TEM) and UV/vis spectroscopy. As the average diameter of the AuNPs decreased, the size distribution became narrower, and the shape became more spherical.