Electrophoresis 2012, 33, 751–757 751 Chia-Lin Chen Ruey-Jen Yang Department of Engineering Science, National Cheng Kung University, Tainan, Taiwan Received September 14, 2011 Revised October 15, 2011 Accepted October 17, 2011 Research Article Effects of microchannel geometry on preconcentration intensity in microfluidic chips with straight or convergent–divergent microchannels Preconcentration microfluidic devices are fabricated incorporating straight or convergent– divergent microchannels and hydrogel or Nafion membranes. Sample preconcentration is achieved utilizing concentration–polarization effects. The effects of the microchannel geometry on the preconcentration intensity are systematically examined. It is shown that for the preconcentrator with the straight microchannel, the time required to achieve a satisfactory preconcentration intensity increases with an increasing channel depth. For the convergent–divergent microchannel, the preconcentration intensity increases with a reducing convergent channel width. Comparing the preconcentration performance of the two different microchannel configurations, it is found that for an equivalent width of the main microchannel, the concentration effect in the convergent–divergent microchannel is faster than that in the straight microchannel. Keywords: Concentration polarization / Electrokinetics / Microfluidics / Nanofluidics / Preconcentration DOI 10.1002/elps.201100493 1 Introduction Advancesin microfabrication techniques in recent decades have led to the development of micro total analysis systems (TAS) and lab-on-a-chip (LOC) devices for a wide variety of chemical and biological applications. Microfluidic chips have many advantages compared to their macroscale counter- parts, including a greater throughput, an improved precision, and a reduced sample and reagent consumption. However, a smaller sample volume with dilute concentration inevitably degrades the sensitivity of the optical detection systems used with many microfluidic devices, and therefore reduces the reliability of the measurement results. As a result, it is neces- sary to increase the sample concentration in order to improve the efficiency of the detection process. Various strategies have been proposed for achieving sample preconcentration in liquids, including field-amplified sample stacking (FASS) [1], isotachophoresis [2], electroki- netic trapping [3–5], and temperature gradient focusing (TGF) [6]. Amongst these techniques, electrokinetic trap- ping is particularly advantageous since it can be used for any charged molecule species. The electrokinetic trapping method exploits the concentration polarization (CP) effect Correspondence: Professor Ruey-Jen Yang, Department of Engi- neering Science, National Cheng Kung University, Tainan, Taiwan E-mail: rjyang@mail.ncku.edu.tw Fax: +886-6-2766549 Abbreviations: DP, depletion; EDL, electric double layer that occurs at a micro/nano interface in the presence of an applied external electric field [7, 8]. CP prompts the devel- opment of a depletion layer near the interface which hin- ders the migration of charged analytes, and therefore creates a sample preconcentration effect. Khandurina et al. [9] and Culbertson et al. [10] designed injection valves incorporating a porous membrane structure in which sample concentra- tion was achieved by using an electric driving field to trap the larger molecules in the solution on the upstream side of the membrane. Wang et al. [3] presented a microfluidic con- centrator comprising two microchannels for sample trans- portation purposes connected by a nanochannel to create an ionic depletion/enrichment effect. The results showed that the device was capable of concentrating green fluorescent protein by a factor of 10 million times in 40 min. Lee et al. [5] proposed a simple method for the fabrication of microflu- idic preconcentrators, in which a Nafion ion-selective mem- brane was patterned on a glass substrate and the substrate was then plasma-bonded to a polydimethylsiloxane (PDMS) microfluidic device. It was shown that a concentration fac- tor as high as 10 000 could be obtained in around 5 min. In addition to Nafion, cross-linked polymer plugs [11], silica gel [12], and polycarbonate nanopore membranes [13] have all been successfully used to perform CP in microfluidic de- vices. Kim et al. [14] showed that stable electrokinetic trapping can also be achieved using heterogeneous ionic hydrogel. Chun et al. [15] showed that a negatively-charged hydrogel Colour Online: See the article online to view Figs. 1–4 and 6 in colour. C  2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com