Surface Plasmon Resonance (SPR)
on Microfluidic Chips

We have demonstrated the concept of CE-SPR on-a-chip using a chip fabricated from PDMS. Figure 1A is a typical electropherogram on this instrument showing the separation of BSA and fluorescein and the dynamic range (1B) for the data. As an extension to this work we developed a miniaturized CE system coupled to a SPR sensor by incorporating a previously described split-flow injection technique to first manipulate sample into the microfluidic 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 relatively 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.

Figure 1. (Left) Representative electropherogram showing the separation of bovine serum albumin (BSA)
and fluorescein in 10 mM HEPES, (pH 7.4). The total analysis time was 3.0 min at 2.5 kV (5 micro-A).
(Right) Slope response for bovine serum albumin (BSA) and fluorescein.

We have used SPR to study, in real-time and, by label-free means, the reversible and irreversible adsorption of small molecules, pharmaceuticals, detergents and proteins on PDMS surfaces (Figure 2). The SPR sensor is first covered with 0.2 % (w/v) PDMS in octane. During the timescale of a typical lab-on-a-chip analysis or an electrophoretic separation, it was found that small neutral components containing a hydrophobic part do not adsorb/absorb onto PDMS, while larger, water-soluble polymer-like materials (dextran and proteins) generally irreversibly adsorb to PDMS. The technique can be used to monitor the kinetics of adsorption and desorption of the molecules.

Figure 2. (A) Water droplets (10 ?L placed on the SPR sensor (gold coated glass slide) uncoated and coated with PDMS. (B) Sensorgram showing multiple injections of solutions of SDS onto a SPR sensor coated
with non- crosslinked PDMS. As a comparison, an ethanolic solution is initially injected, which does
not modify the baseline after the injection (flow rate: 60 ?L/min, sample volume: 200 ?L, carrier; water.)
    References
  1. "Facile Fabrication of an Interface for On-Line Coupling of Microchip Capillary Electrophoresis to Surface Plasmon Resonance", Liu, X.; Du, M.; Zhou, F.; Gomez, F. A. Bioanalysis 2012, 4, 373-379.
  2. "Development of an Ultra-Low Volume Flow-Cell for Surface Plasmon Resonance Detection in a Miniaturized Capillary Electrophoresis System", Gaspar, A.; Gomez, F. A. Electrophoresis, 2012, 33, 1723-1728.
  3. "Use of Surface Plasmon Resonance to Study the Adsorption of Detergents on Poly(dimethysiloxane) Surfaces", Gaspar, A.; Kecskemeti, A.; Gomez, F. A. Electrophoresis, 2012, submitted.
  4. "Application of Surface Plasmon Resonance Spectroscopy for Adsorption Studies of Different Types of Components on Poly(dimethylsiloxane)", Gaspar, A.; Gomez, F. A. Anal. Chim. Acta, 2012, submitted