Formation Process of Self-Assembled Monolayer on Gold Nanosphere Probed by Second Harmonic Generation

Abstract

The formation process of a hemicyanine-terminated alkanethiolate self-assembled monolayer (SAM) on gold nanospheres immobilized on a glass substrate was studied by absorption spectroscopy, optical second harmonic generation, and Monte Carlo simulation. It was found that hemicyanine thiolate SAMs mainly form in the upper hemisphere region of the gold nanospheres in the early stage, followed by the additional SAM formation in the lower region of gold nanospheres. The hemicyanine SAM does not homogeneously form over the nanosphere surface and does not fully cover the nanospheres even after long exposure to the hemicyanine solution. This is because of the narrow space under the nanosphere, where the binding of the alkane disulfide to the gold surface is considered to be a diffusion-controlled reaction. The orientation of the hemicyanine molecules on gold nanospheres is similar to that of hemicyanine on a flat gold substrate. These results are important to understand the surface chemistry at nanostructure surfaces as well as the localized surface plasmon biosensing using metallic nanostructures.

Angular-Resolved Polarized Surface Enhanced Raman Spectroscopy

“…This means that the molecules do not fully cover the SIGNs, even at low coverage, because there is a narrow space below the nanosphere where the molecules are difficult to intrude. This result is consistent with our exploration based on SHG and Monte Carlo simulation . For sample B, a unique lower θ cap = 50–60° is matched, in which part of the upper hemisphere is covered with R6G molecules and molecules are absent in the lower hemisphere.…”

“…The measured η profile of sample C shown in Figure (c) also matches two profiles at θ cap = 60–75° and θ cap = 160–175°, supposing that the molecules are well ordered on the SIGNs. Because of the same reason in sample A, the lower range is excluded, so that sample C has θ cap = 160–175°, which is consistent with our exploration based on SHG . The η profile of sample D matches a unique profile at θ cap = 55–60°.…”

“…The gap distance can be controlled by varying the thickness of the supporting gap layer, and the number of gold nanospheres (including their images) in a single structure is exactly two, that is, a dimer. Hence, we can calculate theoretically the enhanced electric fields around the SIGN, ,, and it is possible to predict and measure the linear and nonlinear optical responses from the SIGN. ,, To determine the mechanism of SERS, it is necessary to know how the molecules are adsorbed on the SIGN. Recently, we have solved this problem, using polarized SHG.…”

Effects of Self-Assembled Monolayers with Different Chemical Groups on Ovarian Cancer Cell Line BehaviorIn Vitro

“…Many molecules can be assembled onto these substrates, forming SAMs with different functional groups that affect the integral properties of the SAM surfaces [4][5][6]. A unique feature of SAMs compared with traditional culture substrates, however, is the fact that functionalized alkane thiolates (terminated with a wide variety of chemical functionalities) can be readily synthesized and efficiently packed into SAMs on Au [7]. This chemical variability has provided a unique tool to probe the influence of biomaterial surface chemistry on protein adsorption and thus cell interactions with adsorbed proteins.…”

SHG studies of self-assembled monolayers of subphthalocyanines on gold

“…This technique is widely used for adsorption kinetics studies of SAMs [19]. The nonlinear optical response of the metal surfaces is known to be dominated by the free-electron density at the surface.…”