Detection of layer-by-layer self-assembly multilayer films by low-temperature plasma mass spectrometry

The detection of layer-by-layer self-assembly multilayer films was carried out using low-temperature plasma (LTP) mass spectrometry (MS) under ambient conditions. These multilayer films have been prepared on quartz plates through the alternate assembling of oppositely charged 4-aminothiophenol (4-ATP) capped Au particles and thioglycolic acid (TGA) capped Ag particles. An LTP probe was used for direct desorption and ionization of chemical components on the films. Without the complicated sample preparation, the structure information of 4-ATP and TGA on films was studied by LTP-MS. Characteristic ions of 4-ATP (M) and TGA (F), including [M](+•), [M-NH(2)](+), [M-HCN-H](+), and [F + H](+), [F-H](+), [F-OH](+), [F-COOH](+) were recorded by LTP-MS on the films. However, [M-CS-H](+) and [F-SH](+) could not be observed on the film, which were detected in the neat sample. In addition, the semi-quantitative analysis of chemical components on monolayer film was carried out, and the amounts of 4-ATP and TGA on monolayer surface were 45 ng/mm(2) and 54 ng/mm(2), respectively. This resulted the ionization efficiencies of 72% for 4-ATP and 54% for TGA. In order to evaluate the reliability of present LTP-MS, the correlations between this approach and some traditional methods, such as UV-vis spectroscopy, atomic force microscope and X-ray photoelectron spectroscopy were studied, which resulted the correlation coefficients of higher than 0.9776. The results indicated that this technique can be used for analyzing the films without any pretreatment, which possesses great potential in the studies of self-assembly multilayer films.