for determining DNA-cation binding constants. In both methods the organic phase (1,2-dichloroethane, DCE) is contained inside the micropipet. In the nonfacilitated approach, the cation of interest (/V-methylphenanthroline, MP+) is present in the aqueous phase and transferred to the organic phase by applying sufficiently negative voltage. When DNA is added to the aqueous phase, a sharp decrease in diffusion current is observed. The binding constant is calculated based on the normalized steady-state current as a function of DNA concentration. Because the experiments are performed under diffusion-limiting conditions and at steady state the binding constants can be determined without knowing the micropipet size binding kinetics or diffusion coefficient values Measurements of DNA binding to electrochemically active ions are common, says Mirkin. "What is new here is that the technique is done with micropipets based on ion transfer rather than on oxidation/reduction," he says. In the facilitated approach the cation is present in the organic phase and is transferred to the aqueous phase containing DNA "To my knowledge, there have been no previous reports on facilitated ion transfer from an organic phase to water," says Mirkin, "and there have been no publications on facilitated transfer involving DNA" In principle, the ion can be transferred from the organic phase into water in the absence of a complexing agent; however, it requires significant voltage. "We do not observe the transfer of M P without adding DNA to the aqueous phase because it is outside our potential window," says Mirkin. The DNA acts as a facilitating agent, lowering the potential necessary for the ion transfer to occur. Binding constants are estimated based on the difference between the half-wave potentials with and without DNA Because M P is a known DNA intercalator, the mechanism of ion transfer is believed to involve insertion of the cation's planar aromatic rings into DNA duplexes at the DCE/water interface. Stripping of the cation from the DNA complex produced a symmetrical surface wave. Further work is underway to investigate the nature of the interfacial film. The possibility of developing a method for stripping DNA duplexes that may lead to detection of hybridization schemes is also being explored. Britt Erickson
Microfabrication without a cleanroom
trench." CSuCP is best suited for patterns requiring narrow (down to 1 um) but Standard methods of designing and generat- widely separated (several microns) ing microscale patterns on surfaces are slow features. and expensive. Chrome masks are typically Masters were prepared by spin coating made by commercial suppliers, and the pho- a silicon wafer with a mixture of poly (methtolithography must be carried out in a clean- yl methacrylate) and rhodamine B in chloroom. For researchers whose needs aren't roform. The desired patterns were ablated particularly complex, such hurdles to trying into the film with a laser operating at new simple patterns may be disappearing. .n 532 nm. Rhodamine B was selected bethe November 15 issue of Analytical Chemis- cause its maximum absorption is near the try (p. 4645)) George M. Whitesides and co- laser wavelength. They tried other polyworkers at Harvard University describe a mers, including polystyrene and polyuremethod for rapid generation of elastomeric thane, and organic solvents, such as dichlomasters for microcontact printing (uCP) and romethane and ethyl acetate, but found the a new variant of uCP called controlled sagcombination of PMMA and chloroform to ging microcontact printing (CSuCP). Neibe the best. Whitesides says, "The ther of these methods needs to be carried PMMA/CHC1 system used because out in a cleanroom. both the dye and the polymer dissolve well in chloroform and the resultant spincoated surface of the dye/polymer matrix is flat This choice was largely empirical" Stamps are made by casting PDMS on the master Devices were created by wetting the stamps with a solution of hexadecanethiol in ethanol and pressing them to the gold substrate. As examples, they fabricated a minielectrode array, which was used for cyclic voltammetry of a hexamineruthenium (III) chloride and lithium perchlorate redox/electrolyte solution. They also created a stamp with an array of microwells that were used as microreactors for the crystallization of KN03. The quality of the devices depends on three factors—how fresh the gold surface is, the concentration of the thiol solution, and the etching proceSteps involved in using CSyCP for dure. The stamps are hardy—some have qeneratinq 1 -pm features. survived more man 200 uses over a period of four months. Both techniques use an elastomeric They have been able to make features master to transfer a pattern to a substrate. as small as 5 um with conventional uCP In conventional uCP, the pattern is in the and 1 um with CSuCP. Whiiesides says, raised portion of the poly(dimethylsilox'We believe it might be possible to go to ane) (PDMS) stamp; the pattern is trans1 um [with uCP] and 0.5 um [for CSuCP] ferred to the surface when the stamp is with more elaborate optical instrumentabrought into contact with the substrate. In tion [that provides] better focusing of the contrast, the pattern for CSuCP is a series laser beam and beam profile correction." of trenches on the stamp. Most of the surThese techniques hold the promise of face of the stamp touches the substrate— bringing micropatterning within reach of only the trenches don't touch the substrate, the average chemistry laboratory. "For According to Whitesides, the major differchemical laboratories that do not need ence between the two patterning techcomplex patterns or multilayer fabrication, niques is that the latter results in features and that do not have access to a cleanroom, on the surface that are smaller than the this method provides an alternative," says features on the stamp. "These trenches Whitesides. "All that is really needed for tend to 'shrink'" says Whitesides "so that simple patterns is an x-y stage and a low the unprinted portion of the substrate is power laser." narrower than the original width of the Celia Henry Analytical Chemistry News & Features, December 1, 1998 773 A
