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Liquid crystals provide new twist on patterned surface analysis As genomic and proteomic chips make 4-cyanobiphenyl. The liquid crystal ina gentle washing, leaving the analytical inroads in a variety of clinical and reteracted with the two parallel surfaces surface undamaged. search applications, developers struggle and assumed a twisted conformation as On a uniform surface, the map to ensure that their manufacturing proa result of the orientation of the gold should show no variation in twist angle cesses churn out consistently high-qualfilm on the reference surface in relation across that surface, whereas on a patity microarrays. In a recent paper in AC to the analytical surface. terned surface, the twist angles should (2008, 80, 2637–2645), collabochange where the chemical comporators Aaron Lowe, Paul Bertics, sition and nanoscale features of the and Nicholas Abbott at the Unisurface change. To confirm this versity of Wisconsin Madison dehypothesis, the researchers created scribe the use of twisted nematic mock arrays consisting of squares liquid crystals—the same technolof various patterned chemistries, ogy that drives high-definition including pentadecanethiol on a LCD televisions—to provide a background of hexadecanethiol, rapid, nondestructive quality check and compiled twist-angle maps. of an array’s surface. “We can Small changes in chemical comspot defects on an array, as well as position showed up clearly on the many other surfaces, in just a couresulting twist-angle map obtained ple of minutes,” says Abbott. with a 4× objective lens. With a In a typical LCD, a nematic 50× objective lens, irregularities in liquid crystal is anchored to two the pentadecanethiol deposition parallel glass plates. One glass sur- Twist-angle map of a model array indicates the location showed up vividly as sharp boundand orientation of the 1 mm array spots (gold squares) face is twisted 90° relative to the aries in the otherwise homogeas well as imperfections in the resulting chip. other. Two polarizing filters, also neous squares. Abbott estimates oriented 90° relative to each other, that the resolution of this techsandwich the plates. Because the liquid The degree to which the liquid crysnique is ~1 μm. crystal is birefringent, it rotates polartal twists, known as the twist angle, Next, Abbott’s team examined an ized light that passes through the first depends on the strength of the noncoantibody array that they prepared with filter and allows it to pass through the valent interactions that occur between affinity microcontact printing. Twistsecond filter. Applying a voltage to the the liquid crystal and each of the two angle mapping showed a complex patanchoring plates torques the liquid crys- anchoring surfaces. “Since the comtern that radiated over the array from a tal, changing the extent to which the position of the reference surface is uncentral point, whereas the background polarization of the incident light is rochanging, any variation in twist angle surface showed no irregularities. The tated and decreasing the intensity of the must arise from changes in the chemical investigators hypothesized that this patlight passing through the second polarmakeup or nanoscale topography of the tern results from shear forces that develizing filter by an amount proportional analytical surface,” explains Abbott. oped when the array was dried under a to the applied voltage. After placing the cuvette between stream of nitrogen. After the processing Abbott and his colleagues simplicrossed polarizing filters, the researchconditions were changed to eliminate fied this setup by doing away with ers recorded the transmission of white shear, the radial pattern vanished in the the applied voltage, relying instead on light through the twisted liquid crystal twist-angle map. slight changes in the chemistry of an by using a polarized light microscope Abbott, a chemical engineer, and analytical surface, such as a microarray, fitted with a digital camera. They then Bertics, a biomolecular chemist, are to alter the twist of the liquid crystal. rotated one of the two polarizing filters now using this technique to quanThey first made what is essentially a in 10° increments through 170°, acquir- tify epidermal growth factor receptor temporary cuvette by pairing a pentaing an image after each rotation. Com(EGFR) expression, detect mutations in decanethiol-treated, gold-coated glass puterized analysis of each pixel in the the EGFR gene, and assess EGFR activslide with the analytical surface. Thin 18 images yielded a 2D map of the twist ity. “If we can perfect the methodology, Mylar strips keep the two surfaces 13 angle of the liquid crystal as a function the result could be a quick assay that μm apart. Using a syringe or micropiof location on the analytical surface. would inform EGFR-based cancer repette, they then pumped in a layer of Once image acquisition was complete, search or therapy,” says Abbott. a —Joe Alper nematic liquid crystal such as 4′-pentylthe liquid crystal could be removed by 3068
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