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Speedy Surface Explorations with Principal Component Analysis proaches. The new method described Surface chemistry is involved in a wide in Begue and Simpson’s paper prorange of important biological interacvides faster data analysis by turning a tions, such as those on cellular memnonlinear problem into a linear one. branes, as well as in industrial and pharmaceutical applicationsOyet probing the subtle structural qualities of surfaces has remained a challenge. But now, in a new AC paper (DOI 10.1021/ ac901832u), Nathan Begue and Garth Simpson of Purdue University combine a technique called nonlinear optical Stokes ellipsometry (NOSE), which can acquire data rapidly, with an equally speedy analysis method that merges linear curve fitting with principal component analysis (PCA); this Separation of dyes in principal component space (PC1 versus PC2). overall approach can quickly decipher subtle The researchers first validated the characteristics of surfaces with unprecnew method on a Z-cut quartz crystal, edented precision. which has well-characterized nonlinear Nonlinear optical ellipsometry ofoptical properties with respect to rotafers a unique approach for polarization angle. The experimental setup intion-dependent surface analysis that may add information beyond that from volved directing a time-varying polarization state of light at the sample from an nonlinear optical imaging techniques ultrafast laser; 80 initial polarization that rely on signal intensity alone. Elstates were generated. Then, the nonlinlipsometry data can be converted into ear elements from second harmonic tensor elements that describe in detail generationOlight with double the initial the nonlinear optical properties of a frequencyOwere collected on two orsample and provide structural insight. thogonal detectors. Yet limitations such as the need to “What we’ve done in this work is physically adjust samples and optical developed methods to do some of the elements to maintain sign and phase most precise polarization analysis we can information have limited the method’s conceive of for second harmonic generaapplicability. tion,” says Simpson. “When you inThe development of NOSE, an excrease the precision, you increase the tension of the nonlinear optical ellipinformation content.” sometry technique, allowed, for the The crystal was examined with several first time, lightning-fast measurements different rotation angles, and then the data without the need for physical movement. This shifted the bottleneck from was analyzed using both nonlinear and linear methodologies to determine which data acquisition to analysis; it still remethod could better back-calculate the quired onerous nonlinear fitting ap764
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rotation angles. “From tracking the polarization changes, we can extract five parameters” using the linear analysis, says Simpson. “These parameters are related to the tensor elements, which are in turn related to the nonlinear optical properties of the surface.” The linear methodology in combination with PCA outperformed the nonlinear curve-fitting methodology, not only by speed of analysis, but also in terms of the precision of the crystal’s back-calculated rotation angle. “I think this is really good because usually when you gather signal in nonlinear optical detection and you want to reduce to a structure, you have to apply nonlinear fitting. That type of fitting is not very easy and is time consuming,” says Zhan Chen of the University of Michigan Ann Arbor. In a more challenging test of the method, Begue and Simpson collected data from four monolayer dye films and analyzed it with PCA. The four dyes are very similar in structureOall are achiral charge-transfer dyes. NOSE traces of incident polarization versus signal intensity appear similar among the dyes, but a comparison in principal component space reveals a clear separation. “By using PCA and applying it to the new technique, they’ve made the method more accessible,” says John Conboy of the University of Utah. “Doing nonlinear regression on a dataset has inherent problems. The use of PCA has found fruit in analytical chemistry and allows you to reduce your data to the important things and pull those things out.” Simpson intends to use the method to study crystal polymorphs, which oc-
10.1021/AC902860K 2010 AMERICAN CHEMICAL SOCIETY
Published on Web 12/30/2009
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cur if a molecule orients itself in multiple arrangements in a lattice and can be difficult to rapidly detect. “Polymorph screening is routinely done in pharmaceuticals to determine all the polymorphs that can exist under reasonable conditions,” notes Simpson. “These
methods are fantastic for selectively identifying subtle differences in data sets. If that’s the question you want to ask, then this is the toolkit for doing it.” “I think this is an excellent paper,” says Chen. “They are trying to develop a data analysis method to make this kind
of tool as friendly as possible and also provide some unique information that other techniques cannot not provide to understand important problems.” —Erika Gebel
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