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Biomedical Optics & Medical Imaging

Pure silicon dioxide waveguides allow UV detection in microfluidic separation systems

Increasing sensitivity by one to two orders of magnitude promotes effective use of UV light in microanalytic devices.
8 January 2008, SPIE Newsroom. DOI: 10.1117/2.1200801.0995

Miniaturized lab-on-a-chip systems have the advantage of lowering sample volume, decreasing analysis time, and in some cases enabling portability. By the same token, a complication of the reduced channel volume is that, especially in detection of absorbed light, sensitivity is compromised since it is proportional to the optical path length. The use of external bulk optics typically allows adequate response only through a depth of 10–100μm. For this reason researchers generally use fluorescence detection in lab-on-a-chip applications because it is very sensitive over small volumes. The problem with this approach is that labeling (as with fluorescent dyes) alters the properties of the analytes, which makes the results difficult to interpret. Hence, fluorescence detection is used in only around 1% of all high-performance liquid chromatography analyses, while UV absorbance detection is used in around 94% of applications (see the extensive survey by Rao et al.1).

Optical waveguides are structures that combine components of different refractive indices to steer light along their length. Our approach for making UV absorbance available for microchip chromatography is to use integrated waveguides for detection in the plane of the device and thereby along the length of a channel segment.

Here we present a chip for electrochromatography that uses a waveguide with absorbance detection in the UV range (see Figure 1). The path length is 1000μm, which increases sensitivity one to two orders of magnitude compared with free-space optics. The waveguides are made of pure silicon dioxide with air as the side and top cladding, and were measured to be transparent down to at least 200nm.2

Figure 1. Sketch of the chip layout. The lines indicate fluidic channels, optical waveguides, and fiber couplers. The separation column is 33mm long, and the U-shaped detection cell, at the end of the column, has a length of 1.0mm. The waveguides are used to excite and collect the light and have fiber couplers for easy, alignment-free connection to an external light source and detector.2

The chip consists of fluidic channels comprising an injection cross, a separation column, and a detection cell, and optical parts that include planar waveguides and fiber couplers. All elements of the chip are made simultaneously in a single silicon etching process, so that only one mask step is necessary. This ensures self-alignment between all components and easy fabrication. After etching into silicon, the structures are oxidized to make the waveguides transparent and to insulate the fluidic channel electrically to support pumping by electro-osmotic flow.2 Finally, a lid is bonded to the top of the device. The separation column is filled with an array of 16μm-wide hexagonal pillars spaced 4μm apart (see Figure 1). This was done to increase the surface-to-volume area for chromatography, which relies on fast interaction between the analyte molecules and those immobilized on the channel surface. The pillars are relatively large in this first implementation of the chip to make sure that they can withstand the deep etching process. Hydrophobic octylsilane chains immobilized onto the pillar sidewalls were used as the stationary phase. The chip was tested by using three uncharged molecules, each with a different degree of hydrophobicity: acetophenone, valerophenone, and hexanophenone. The molecules were injected simultaneously and migrated downstream with different velocities due to different degrees of interaction with the stationary phase. Figure 2 shows the arrival time at the end of the column, where the waveguides are placed for detection.

Figure 2. On-chip chromatographic separations of (1) acetophenone (100μg/mL), (2) valerophenone (200μg/mL), and (3) hexanophenone (300μg/mL). The three chromatograms are for different percentages of acetonitrile, which modifies the interaction between the analytes and the stationary phase. In (A) the concentration is so high that the interaction is decreased to a level where all peaks co-elute, whereas in (C) there is a clear baseline separation of the analytes. The last peak is dispersed because it is highly retained, which results in slower longitudinal diffusion in the column. Conditions: 10mM tetraborate buffer at pH 9.2, E=306V/cm, 1s gated injection, sampling frequency 50Hz, and UV absorbance detection at 254nm with an optical path length of 1.0mm. A.U.: Arbitrary units.

This is the first time capillary electrochromatography has been shown on a microchip with integrated waveguides for detection. In addition, to our knowledge the waveguides are the most transparent in the UV reported to date. In the future we plan to optimize the separation performance of the chip by reducing the size of the pillars in the column, and to lower the detection limits by replacing the bulky mercury arc lamp with a small UV laser.

Klaus B. Mogensen, Omar Gustafsson, Pedro Nunes, and Jörg P. Kutter 
Department of Micro- and Nanotechnology
Technical University of Denmark
Kgs. Lyngby, Denmark

Klaus Mogensen obtained his PhD from the Department of Micro- and Nanotechnology at the Technical University of Denmark in 2002 on the topic of integration of planar waveguides for optical detection in microfabricated analytical devices. He is now working as an associate professor in the same department.