Researchers at Université de Sherbrooke and at The Open University in Milton Keynes, UK, have used x-ray photoelectron spectroscopy to analyze gold surfaces for contaminants after the removal of thiolated DNA oligomers by UV/ozone treatment. “We are interested in problems related to radiobiology and cancer therapy,” said postdoctoral fellow Nasrin Mirsaleh-Kohan, who conducted the investigation with two other researchers. “We are basically working to improve radiotherapy by making patients’ tumors more sensitive to radiation. Our hope is that self-assembled monolayers of DNA will serve as good models for cellular DNA, which will allow us to test the suitability of various drugs as radiosensitizers.” Professor Léon Sanche, Mirsaleh-Kohan’s supervisor, is a well-known contributor to the field of low-energy electron-induced processes. Mirsaleh-Kohan explained that an important part of their research is to understand the mechanism by which low-energy electrons, such as those generated in human tissue by ionizing radiation, can induce molecular damage to DNA. “One way to do this is to prepare thin films of DNA, irradiate them under vacuum with beams of monoenergetic electrons and study the resulting damage in situ using various surface-sensitive techniques,” she said. “As a consequence, we’ve become very interested in different methods of preparing DNA films. In theory, molecular self-assembly of thiolated DNA on gold substrates should allow us to prepare reproducibly pure, ordered films for our experiments. We’ve often heard that it is possible to reuse gold substrates, and this [research] is an attempt to test this claim.” The gold substrate represents a significant fraction of the total cost of preparing the films, she added. “Several properties of gold make it a suitable substrate for preparing films of DNA. It has been well-characterized by many spectroscopic techniques and does not itself react with DNA. Sulfur atoms do, however, react with gold, and DNA strands can thus stick to a gold surface by attaching a sulfur atom at one end of the molecule,” Mirsaleh-Kohan said. When gold surfaces are exposed to solutions containing such thiolated DNA, the DNA attaches to the gold via the sulfur atom. Given enough time, this film can become highly dense and ordered. “If we attempt to reuse the gold substrate, but are unable to ensure its cleanliness, then the properties such as order and density will change between samples,” she said. UV/ozonolysis is a widely used technique for cleaning gold surfaces prior to their exposure to solutions containing thiolated molecules. It also has been used to remove self-assembled monolayers of linear alkanethiols from gold. The researchers used x-ray photoelectron spectroscopy to quantitatively analyze the chemical composition of the DNA films as they were cleaned. They placed samples in an ultrahigh-vacuum chamber, where they were exposed to a monochromatic x-ray source that ionizes atoms in the sample, ejecting electrons from the film. “The kinetic energies of these ‘photoelectrons’ are measured and are characteristic of the elements from which they are ejected,” Mirsaleh-Kohan said, adding that the intensity of peaks in the resulting photoelectron spectrum is indicative of the number of atoms of each type in the sample. The researchers monitored the intensities of photoelectron peaks associated with nitrogen (indicative of DNA), sulfur, gold, carbon and oxygen for samples subjected to various degrees of cleaning. “We were initially interested in finding the optimum conditions for cleaning DNA from our gold slides but were unable to find any conditions that could remove DNA films completely from gold substrates,” Mirsaleh-Kohan said.