Issue Date: March 1, 2004
SHINY LABELS FOR SURFACE MOLECULES
Chemists at the University of Pittsburgh have developed a selective fluorescence-based technique capable of detecting very low concentrations of analytes on surfaces. Compared with established analysis methods, the new procedure boosts detection sensitivity by a factor of 100 for some species and can be applied to a large variety of molecules [J. Am. Chem. Soc., 126, 2260 (2004)].
Fast-paced research in self-assembling molecular films has led to a large number of synthesis methods and applications. From chemical and biological sensing to fundamental studies of surfaces and interfaces, researchers in several disciplines often need to characterize multicomponent monolayers.
For densely packed homogeneous films in which surface concentrations are on the order of 1014 molecules per cm2, common surface analysis methods such as X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) are generally adequate. But when the concentration of surface species is just a few percent of a monolayer or less--roughly 1012 molecules per cm2--standard detection methods often tend to be ineffective.
Demonstrating the method, a team of Pittsburgh researchers, including assistant chemistry professors Eric Borguet and Stéphane Petoud and coworkers Eric A. McArthur, Tao Ye, and Jason P. Cross, prepared monolayer films of octadecylsiloxane on silicon and treated the films to form a mixture of oxygen-containing molecules. Then, using various chromophores and selective derivatization reactions, the group labeled the molecules such that each functional group was tagged with a specific chromophore. For example, aldehydes were labeled with 1-pyrenemethylamine, carboxylic acid groups were labeled with 2-naphthaleneethanol, and OH groups were tagged with triphenylmethyl chloride. In a final step, the team measured characteristic fluorescence spectra, which indicated the identity and concentration of the labeled molecules.
Based on a series of detection limit studies, the Pittsburgh group determined that molecules containing carboxylic acids or aldehydes can be detected in the low 1011-molecules-per-cm2 range. And alcohols can be detected in even lower concentrations--1010 molecules per cm2, a quantity that corresponds to just one ten-thousandth of a monolayer.
"This work is important because FTIR and XPS are so limited in terms of sensitivity," remarks Jillian M. Buriak, a chemistry professor at the University of Alberta, Edmonton. She adds that the technique "is easy to apply and useful for detecting functional groups that are absolutely ubiquitous."
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