Oxidation-Assisted Secondary Ion Mass Spectrometry Methodology to Quantify Mixed Alkylthiol Self-Assembled Monolayers on Gold:  Applications to Competitive Chemical Adsorption

Abstract

A novel ozone oxidation assisted secondary ion mass spectrometry (SIMS) methodology was developed. The method provides a direct and simple approach to quantify mixed alkylthiol self-assembled monolayers (SAMs) on gold with higher detection sensitivity. Ozone oxidation of alkylthiol SAMs on gold causes the rupture of thiol−gold covalent bonds and the conversion of alkanethiolate to alkyl sulfonate. With negative ion SIMS measurement, the secondary ion yields of alkyl sulfonate are much higher than those of any molecular ions which originate from covalently bound alkylthiols. Using the intensity of molecular ions originating from intact alkyl sulfonate molecules from oxidized mixed SAMs to quantify the original mixed SAMs on gold can improve detection sensitivity. In this paper, we report results on the optimization of conditions for ozone oxidation of mixed alkylthiol SAMs on gold. Optimization using a combination of X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry data demonstrated that a small amount of oxidation was sufficient to produce strong SIMS signals to quantify the composition of mixed alkylthiol SAMs on gold. We applied the optimized method to a case study of steric effects on competitive chemical adsorption of alkylthiols to gold. The results from the case study indicated that by conversion of alkanethiolate to alkyl sulfonate using ozone oxidation pretreatment, mixed SAMs on gold can be straightforwardly quantified by SIMS analysis. Further, steric effects controlling the relative adsorption probability and surface coverage of a component from a mixture can be quantified. The competitive adsorption probabilities with respect to 1-decanethiol are 0.2 for 1-butanethiol, 5 × 10-3 for 1-methyl-1-propanethiol, and 9 × 10-4 for 2-methyl-2-propanethiol. The molar concentration ratio that is needed to form 1:1 mixed short/long SAMs is 5:1 for 1-butanethiol/decanethiol, 200:1 for 1-methyl-1-propanethiol/decanethiol, and 1000:1 for 2-methyl-2-propanethiol/decanethiol.