Quantitative analysis of the surface composition of mixed self-assembled monolayers using ToF-SIMS and FT-IR analyses

Mixed self-assembled monolayers (SAMs) consisting of different molecules have been used to modify surface wettability, functionalize end groups for chemical binding, and control adsorption of proteins, DNA, or cells. In these applications, control over the quantification of the surface composition of a mixed SAM is especially important when designing biochips and biosensors. Previously, we presented a method by which ToF-SIMS can be used to quantify the composition of two adsorbates with acetylene and propene functional groups from their secondary ion yield ratio in mixed SAMs on gold substrates. We concluded that although the ion yield ratio of the two adsorbates in mixed SAM was not equal to the mole fraction of Diyne (-acetylene) and Diene (-propene) in a solution, this was due to a different secondary ion formation, and not to the difference in the amount of adsorbates on the surface. In this study, we revisit the same sample system, this time applying a FT-IR technique to test the accuracy of our previous findings. We found that the absorbances of the acetylene and propene functional groups neatly and linearly correlated with the mole fraction in the solution, which were consistent with our previous conclusions. We also used this methodology to quantify mixed silane SAMs on a silica surface. Our study shows that the ToF-SIMS and FT-IR techniques can complement one another and be useful to quantify mixed SAMs on various substrates.