45128-G5
Investigation of the Fundamental Properties of Partition Layers in Surface-Enhanced Raman Scattering
The goal of this project is to use partition layers to enable detection of non-traditional SERS molecules and complex mixtures of analytes. During year 2, significant progress has been made towards defining the self-assembled monolayer (SAM) molecular properties that yield effective partition layers and exploring the partitioning and orientation of non-traditional SERS molecules, specifically polychlorinated biphenyls (PCBs). The packing characteristics of a SAM have significant effect on the partitioning of an analyte. This phenomenon was explored by using four SAMs of varying alkyl chain lengths (octanethiol, decanethiol, hexadecanethiol, and octadecanethiol)with varied assembly times. Packing density was analyzed using SERS band analysis of the C-S and C-C gauche and trans band ratios. The C-S band ratio indicates the packing density of the SAM near the surface of the SERS substrate. The C-C band ratio indicates the packing density of the alkane chains further from the SERS substrate; an increase in gauche conformers indicates a less crystalline, and less well-ordered, SAM. As expected, the crystallinity of the SAM is dependent on assembly time in the ethanolic SAM solution and chain length; it takes longer for long chain SAMs to form crystalline monolayers. For example, the C-S ratio for octanethiol is 14.7±1.3 after one hour in the SAM solution, while the C-S ratio for octadecanethiol is 7.8±1.3, indicating that the octanethiol is a more crystalline monolayer than octadecanethiol. The C-S ratio also changes significantly as the assembly time is varied for each alkanethiol SAM. To further explore how to control crystallinity of partition layers, SAMs were forced to assemble by application of a modest potential for 15 minutes. The applied potential induces dense packing and ordering of SAMs, as previously proven by Lennox and coworkers. Comparison of the C-S stretches of alkanethiols assembled with and without the applied potential demonstrates a trend of increased crystallinity when the potential is used for SAM assembly. The partitioning of PCBs into the potential packed SAMs was more effective than open circuit packed substrates of the same alkanethiol chain. This new technique of SAM assembly will lead to more homogeneous SAMs, can lend some insight into the analyte partitioning, and is more efficient than open circuit assembly, which requires up to 72 hours for complete monolayer formation. Further analyte partitioning experiments were conducted with the four different alkanethiol chain lengths to clarify the mechanism of analyte partitioning. If the analyte dwells at the surface of the SAM, the analyte SERS signal should decrease as chain length increases; however, if the analyte partitions into the alkanethiol SAM, the analyte signal would not be dependent on the length of the SAM. Our results show that the analyte signal was the greatest in the decanethiol monolayer, which does not fit either partitioning mechanism, and suggests a combination of the two is in effect. Experiments were conducted to probe the partitioning behavior of two chemically similar compounds that have different structures using a non-planar and planar PCB. PCB-47 and 77 have distinguishable vibrational spectra due to the different positions of their chlorine substituents. If the PCB is partitioning deeply into the decanethiol monolayer, there should be a shift in the C-S stretch Raman shift (normally appearing at 716 cm-1). PCB-47 causes the C-S stretch to shift to 700 cm-1 shift, but PCB-77 causes no disturbance in the C-S stretch. This lack of change in the C-S stretch as the PCB-77 Raman bands appear could be related to the planarity of PCB-77, allowing it to partition into the SAM without disturbance of the C-S bond. Further analysis of C-S and C-C bands may yield more information about analyte partitioning. The kinetics of analyte partitioning were also studied, where a SAM was exposed to a analyte for 0.5 min, 1 min, 30 min, 60 min or 4 hours, and the extent of partitioning was determined by the intensity of the aromatic breathing stretch of the PCB. We found that the majority of the analyte partitions into the SAM after one minute, since the intensity of the breathing stretch levels off after this time period. In addition, departitioning of the analyte was achieved when the SAM was exposed to a hydrophobic solvent; for PCBs, exposure to 1-octanol induced departitioning, as evidenced by the disappearance of PCB Raman bands.
