Applications  
     
  Application Note #1: Studies of Self-Assembled Monolayers Using PA-IR
 


Self-assembled monolayers (SAMs) have received increased attention both as model systems for highly organized biological structures such as biomembranes, vesicles, and micelles and as candidates for a variety of technological applications. In the past thirty years, FT-IR has been used as a major tool to investigate their composition, chain orientation and intermolecular packing. Generally FT-IR studies of SAMs can take anywhere from 1-4 hours to obtain spectra with an acceptable S/N ratio. Using a PA-IR instrument with an In-Sb focal plane array, we have recently obtained spectra of SAMs in as little as 1-2 seconds. Two examples are illustrated below.

  
  Example 1: Octadecyltrichlorosilane SAMs on Glass Substrates
  
  In order to study the effect of solvent on SAM deposition, glass slides (which transmit IR radiation >2500 cm-1) were dipped into different octadecyltrichlorosilane (OTS) solutions. PA-IR transmission spectra were then obtained using 1600 coadds (one pixel row from 1600 frames), collected with a total integration time of 2.4 seconds (1.5ms x 1600), and a total acquisition time of 27.2 seconds (17ms x 1600). Fig.1 contains the spectra of double layers (one SAM on each side of substrate) of OTS. The peaks at 2920 cm-1 and 2852 cm-1 are the CH2 asymmetric and the CH2 symmetric stretching vibrations respectively. Comparing various solvents (benzene, toluene, hexane, bicyclohexane (BCH)) for the deposition of OTS, it was found that hexane is the best solvent for promoting OTS self-assembly onto the surface of the glass substrates.
  
 
 
 

Fig.1. PA-IR spectra of OTS deposited on glass surfaces using different solvents.

  
 
 
  Example 2: Octadecylthiol SAMs on Gold
  PA-IR spectroscopy employing grazing incidence reflection was used to investigate SAMs of octadecylthiol (ODT) on gold surfaces. The data collection conditions were the same as those described above. In order to compare the PA-IR spectrum with that obtained by FT-IR reflection-absorption infrared spectroscopy (Fig.2) a comparison between the spectra obtained from ODT on gold using 1600 coadds from our PA-IR instrument was compared with that obtained from an FTIR with 1000 scans using a resolution of 4 cm-1. IR bands are observed at 2918 and 2850 cm-1 and are assignable to the CH2 asymmetric and symmetric stretching vibrations respectively. It is clear from the frequency position of these bands that the ODT chains are conformationally ordered on the surface. The bands found at 2964 cm-1 and 2880 cm-1 are attributable to the CH3 asymmetric and symmetric stretching vibrations. Additional studies on the orientation of SAMs relative to the surface is currently under way.
  
 
 

Fig.2. Comparison of PA-IR and FT-IR spectra of ODT on gold surfaces.
 
 

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