Characterization of Self-Assembled Monolayers of Octadecanethiol and Dodecanethiol Joya Cooley, Brian Toney, Marion Martin Department Of Chemistry, Furman University, Greenville SC Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS) Contact Angle Introduction Table 2: Average νa(CH3)/νa(CH2) ratios As a preliminary step to characterizing alginate-based composite polymers for use in bionic devices, we examined properties of self-assembled monolayers (SAMs) of alkanethiolsoctadecanethiol (ODT) and dodecanethiol (DDT) on gold substrates. There is considerable interest in developing composite materials of alginic acid with highly conducting polymers to bridge metal electrodes to biological cells. These alginate composites have shown great potential as “smart” materials, where chemical or biological stimuli have been employed to change the composites’ physical and chemical properties. The particular goals of this study were determining whether or not a SAM had formed on a gold substrate, and determining ways to differentiate between SAMs grown from alkanethiols of different chain lengths. Optical tensiometry measured the contact angle of a drop of deionized water with the gold (Au) substrate surface in order to determine the relative hydrophobicity or hydrophilicity of the modified surface. Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS) was used as way to characterize the films present on the substrate after deposition. • Used to determine whether or not a thin film has formed on the surface of a substrate. • Places a drop of water on the surface and measures the contact angle. • Resulting angle yields information about hydrophobicity or hydrophilicity and, to some extent, degree of coverage. • IRRAS technique can detect thin films less than 200 nm in thickness by measuring surface absorption with light beams polarized parallel and perpendicular to the reflection surface. • Adding Polarization Modulation (PM) to IRRAS yields a more accurate result by measuring the reflections of the two polarizations simultaneously. • Spectral analysis included determining the area under the curve, as well as the intensity and the position of the CH2 asymmetric stretching [νa(CH2)] and CH3 asymmetric stretching [νa(CH3)] peaks. Table 2: Average νa(CH3)/νa(CH2) ratios at each step; ratio closer to 0.5 indicates a more well-ordered monolayer Conclusions • Both ODT and DDT can successfully be coated on a gold slide reproducibly. • Cleaning the slides prior to coating may create an oxide layer that prevents a film from growing. • An alkanethiol SAM that is not very soluble in the solution’s solvent should be rinsed in the solvent for best results. • DDT coated slides’ spectra gave much less signal/less defined peaks we believe this is due to less order because of the shorter chain and lesser van der Waals interactions. Figure 1: A drop of water on a blank gold slide; contact angle: 85.68° νa(CH2) νa(CH3) νa(CH3) νa(CH2) Alkanethiol Film Growth Tail group Absorbance • SAMs arrange themselves in an ordered fashion on a surface; the head group (sulfur) binds covalently to the gold substrate and interactions between the alkane chains stabilize the monolayer. The tail group provides the functionality of the SAM. Future Research Alkane chain • Employing PM-IRRAS to distinguish SAMs grown with different tail groups (e.g. carboxylic acids vs. alcohols). • Eventual characterization of alginate composites. Head group (thiol) Figure 3: PM-IRRAS spectrum of ODT a on gold coated glass slide. substrate References Chechik, V.; Schönherr, H.; Vancso, G.J.; Stirling, C.J.M. Langmuir1998, 14, 3003-3010. Frey, B. L.; Robert, M. C.; Weibel, S. C. In Handbook of VibrationalSpectroscopy; Griffiths, P. R., Ed.; John Wiley&Sons: New York, 2002; Vol. 2; p 1042. Figure 2: A drop of water on a gold slide immersed in 1 mM ODT solution for 15 hours; contact angle: 107.65° Table 1: Average Contact Angle Data Absorbance Acknowledgements • Au slide dipped in 1 mM solution of ODT or DDT in ethanol for varying amounts of time. • Best growth was observed when a blank slide was immersed in SAM solution for 30 minutes to an hour. • PM-IRRAS scans and contact angle data taken at each step to monitor progress of SAM growth. • NSF Cooperative Agreement Number EPS – 0903795. • Grants for Exploratory Academic Research (GEAR). Figure 4: PM-IRRAS spectrum of DDT on a gold coated glass slide Table 1: Average contact angle before and after coating in ODT and DDT. A lower contact angle indicates a more hydrophilic surface.