In Situ Kinetics of Self-Assembly by Surface Plasmon Resonance Spectroscopy

Abstract

Using surface plasmon resonance (SPR) spectroscopy in the Kretschmann configuration, we followed the self-assembly of organic ultrathin films which resulted from exposure of gold surfaces to solutions of CH3(CH2) n - 1SH (n = 8, 12, 16, and 18) in ethanol and heptane. We monitored film growth in situ and continuously for up to 72 h with an overall thickness resolution of <1 Å. Film dielectric constants, necessary for accurately calculating average film thicknesses from SPR spectra, were determined unambiguously for fully formed films by comparing spectra from organic films in different solvents. In addition, we introduce a novel two-color SPR experiment with which we can obtain both film thickness and film dielectric constant without changing solvents. We studied the chain length dependent and concentration dependent kinetics of film formation in ethanol and found that there are at least three distinct kinetics steps. The kinetics of the first, most rapid, step and the third, slowest, step can be described well with Langmuir adsorption models. The kinetics of the second step are zeroth order and depend on alkanethiol chain length, concentration, and partial film thickness. The formation kinetics in heptane can be described with a single step Langmuir adsorption model. We find that films formed by continuous self-assembly in solution are always thicker than films for which the process of self-assembly has been interrupted by rinsing with neat solvents. We also present evidence that, by rinsing the partially formed film before reimmersion into thiol solution, we can change the final film thickness and structure of a hexadecanethiolate film. Our results are most consistent with a formation mechanism involving adsorption of both chemisorbed and physisorbed molecules during film formation in ethanol, with the overall formation kinetics determined by the relative solubility of the alkanethiol in the solvent.

Quantifying thiol–gold interactions towards the efficient strength control

“…However, the reaction time for the formation of SAMs can be different. Depending on the type of thiol molecules (for example, with long or short alkyl chain), the concentration as well as the cleanness of the gold surface, the formation time for a 'full monolayer' can vary from seconds to minutes, up to hours and days 2,55,64 . The weakened strength of the thiol-gold contact in SAMs compared with that of isolated thiols can be ascribed to both the reduction effect of the thiols and the further weakening effect of the self-assembled thiols on the Au-Au interactions.…”

Thiol Diffusion and the Role of Humidity in “Dip Pen Nanolithography”

“…Note that this halo is not apparent when the tip is sharp, presumably because the much greater contact pressure produced by the sharper tip causes it to penetrate through the ODT to the gold surface. The "standing" ODT region has a topographic height of 1.1 nm, lower than that of a fully formed SAM (ϳ3 nm), indicating that the thiols are standing but not as densely packed [15]. As shown in Fig.…”

Electrostatic surface plasmon resonance: Direct electric field-induced hybridization and denaturation in monolayer nucleic acid films and label-free discrimination of base mismatches

“…The SPR optical instrumentation used for these experiments has been described previously (8,23,24). The electrochemical experiments use a potentiostat (EG & G, Salem, MA, 263A) to control the potential of a standard three-electrode cell, as described previously (14).…”

“…Preparation of the SPR gold substrate and subsequent self assembly of the ssDNA monolayer film was carried out as described previously (8). Gold substrates were cleaned with piranha solution before use, as described previously (23).…”