Electrophoresis 2012, 33, 1723–1728 1723 Attila Gaspar * Frank A. Gomez Department of Chemistry and Biochemistry, California State University, Los Angeles, CA, USA Received December 7, 2011 Revised February 27, 2012 Accepted February 27, 2012 Research Article Development of an ultra-low volume flow cell for surface plasmon resonance detection in a miniaturized capillary electrophoresis system A miniaturized capillary electrophoresis system coupled to a surface plasmon resonance (SPR) sensor on a microfluidic platform fabricated from PDMS is detailed. A previously described split-flow injection technique is first utilized to manipulate sample into the mi- crofluidic chip, followed by separation within the fused-silica capillary and final off-capillary detection of analytes via SPR. Instead of using commercial SPR flow cells requiring rela- tively large detection volumes, samples of less than 1 nL volume are utilized. The interface between the CE system and SPR sensor made it possible to detect minute volumes of sample with minimal dispersion. The flow cell has the potential to be applicable to minia- turized flow-injection (FI) systems where submicroliter volumes of sample are frequently only available for analysis. The components present in solution, but not bound to the sensor surface, were also investigated. The sensitivity of the CE-SPR system was similar to that found in UV-spectrometric instruments and nonchromophoric components could also be measured. Keywords: Capillary electrophoresis / Microchip / Poly(dimethylsiloxane) / Surface plasmon resonance DOI 10.1002/elps.201100673 1 Introduction Surface plasmon resonance (SPR) is a surface-sensitive op- tical technique that is used to study interaction/adsorption processes occurring on the surface of a thin layer on a metal surface [1]. SPR enables real-time, label-free measurements of biomolecular binding kinetics and affinity and plays an im- portant role for drug discovery and proteomics research [2]. Surface plasmons (SPs) are surface waves that propagate at the interface between a metallic film and a dielectric medium and are coupled to the free-electron plasma in the metal (usu- ally gold). The SPs energy is tightly confined in a decaying evanescent field 100–200 nm above the surface making SPs more sensitive to local changes in the refractive index (RI) than bulk measurement techniques [2]. In other words, the change of RI comes from the change of the composition of the bulk solution and the adsorption/desorption of the molecules onto/from the sensor surface [3]. Although a myriad of interactions have been studied in the last two decades using SPR [3–5], only a few have detailed its coupling to separation techniques. Namely, separation of Correspondence: Dr. Frank A. Gomez, Department of Chemistry and Biochemistry, California State University, Los Angeles, 5151 State University Drive, Los Angeles, California 90032-8202, USA E-mail: fgomez2@calstatela.edu Abbreviations: FI, flow injection; RI, refractive index; SPR, surface plasmon resonance carbohydrates that are difficult to analyze by common de- tection techniques [6, 7] and weakly associating antigens to antibodies [8]. In addition, Du and Zhou [9] used a postcol- umn sensor surface regeneration injecting 0.1 M NaOH or 0.1 M HCl through a six-port valve to continuously detect proteins strongly bonded to the SPR sensor. The advantage of coupling CE instrumentation to SPR is evident since sample volumes in CE are in the subnanoliter order requiring a sensitive detection system. Whelan and Zare have demonstrated a postcolumn detector in a CE-SPR separation [10]. Here, the separation capillary was cracked and grounded above the outlet to protect the detection electronics from the separation voltage. In their homemade CE system, a three-component mixture of high RI materials is injected into the capillary by gravity, and the solution leaving the capillary entered a 0.4 or 3.5 L flow cell of the SPR. Protein samples were electrophoretically delivered to a functionalized surface. PDMS is a widely used material to create microfluidic devices and several PDMS-based flow cells for SPR studies have been described [11,12]. For example, microfabrication of a flow cell improved the efficiency of the interaction between the solid-state sensing surface and the sample [13, 14]. In addition, multiplexed analysis was shown by combining SPR imaging (iSPR) with multichannel PDMS chips [15, 16]. ∗ Current address: Attila Gaspar, Department of Inorganic and Analytical Chemistry, University of Debrecen, Hungary Colour Online: See the article online to view Fig. 1 in colour. C  2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com