Application of Principal Component Analysis to Ellipsometric Spectra

Optical properties of poly(ferrocenylsilane) multilayer thin films

Spectroscopic ellipsometry has been used to investigate the optical properties of poly(ferrocenylsilane) polyion multilayer thin films in the visible and near-infrared range of the spectrum. The thin films were deposited using the layer-by-layer assembly process. Films with thicknesses of up to 55 nm were fabricated stepwise from polyelectrolyte solutions with a controlled ionic strength. These films allow an accurate characterization of the optical properties of poly(ferrocenylsilane) polyion layers. We show that the complex refractive index can be described by a simple Cauchy model. Refractive index values vary over the spectral range from 1.53 (near-infrared) to 1.8 (ultraviolet).

Principal component analysis based interconversion between infrared and near-infrared spectra for the study of thermal-induced weak interaction changes of poly(N-isopropylacrylamide)

The use of a novel spectral interconversion scheme, principal component analysis (PCA) based spectral prediction, to probe weak molecular interactions of a polymer film is reported. A PCA model is built based on a joint data matrix by concatenating two related spectral data matrices (such as infrared (IR) and near-infrared (NIR) spectra) along the variable direction, then the obtained loading matrix of the model is split into two parts to predict the desired spectra. For a better PCA-based prediction, it is suggested that the samples whose spectra are to be predicted should be as similar as possible to those used in the model. Based on the PCA model, the thermal-induced changes in the weak interaction of poly(N-isopropylacrylamide) (PNiPA) film is revealed by the interconversion between selected spectral ranges measured between 40 and 220 degrees C. The thermal-induced weak interaction changes of PNiPA, expressed as either the band shift or intensity changes at a specific region, have been probed properly. Meanwhile, the robustness of the spectral prediction is also compared with that achieved by a partial least squares (PLS2) model in detail, illustrating its advantages in predicting more subtle structural changes such as C-H groups.

Binding of 1-methylimidazole to heme determined by factor analysis

We report the binding of 1-methylimidazole to ferric octaethylporphyrin chloride in methylene chloride. Factor analysis of a series of UV-vis titration spectra indicated there were three species in solution. These species are postulated as being the ferric porphyrin chloride species, the ferric porphyrin(1-MeIm)chloride species, and the ferric (1-MeIm)2chloride species. A coupled nonlinear least squares analysis of absorbance changes at two wavelengths yielded estimates of the binding constants. The binding constants for the first and second 1-MeIm are K 1 = 278 ± 7 M -1 and K 2 = 161 ± 25 M -1. It is suggested that displacing the chloride ion is the reason for having K 2 < K 1.