Monolithic integration of optical waveguides for absorbance detection in microfabricated electrophoresis devices

Abstract

The fabrication and performance of an electrophoretic separation chip with integrated optical waveguides for absorption detection is presented. The device was fabricated on a silicon substrate by standard microfabrication techniques with the use of two photolithographic mask steps. The waveguides on the device were connected to optical fibers, which enabled alignment free operation due to the absence of free-space optics. A 750 microm long U-shaped detection cell was used to facilitate longitudinal absorption detection. To minimize geometrically induced band broadening at the turn in the U-cell, tapering of the separation channel from a width of 120 down to 30 microm was employed. Electrical insulation was achieved by a 13 microm thermally grown silicon dioxide between the silicon substrate and the channels. The breakdown voltage during operation of the chip was measured to 10.6 kV. A separation of 3.2 microM rhodamine 110, 8 microM 2,7-dichlorofluorescein, 10 microM fluorescein and 18 microM 5-carboxyfluorescein was demonstrated on the device using the detection cell for absorption measurements at 488 nm.

Optical waveguide devices for bioanalysis

“…However, in applications where the volume of a liquid or of a micron-scale object such as a biological cell is to be interrogated, in-plane optofluidic approaches must be adopted [10,11]. Optical waveguide devices must be integrated with microfluidic channels to excite a defined volume in the fluid and to collect the scattered spectrum, in terms of angle or wavelength.…”

Performance of an in-plane detection cell with integrated waveguides for UV/Vis absorbance measurements on microfluidic separation devices

“…The last step was packaging of the individual devices, which involved gluing of optical fibers to the end-faces of the integrated waveguides and attachment of reservoirs on top of the device, also by gluing. The fabrication of the waveguides and the electrically insulated fluidic channels is described in more detail elsewhere [6,17].…”

“…On the microchip device presented here stray light could bypass the detection cell by propagating in the oxide layer insulating the channels from the conducting silicon wafer [6]. The UV transparent fibers used for coupling light into the waveguides had a diameter of 50 mm and were therefore oversized compared to the waveguides, which only had a cross section of 2466 mm.…”

“…In a previous paper [6] describing absorption measurements in the visible wavelength range no indication of stray light was observed. In that device a single mode fiber with a core diameter of 3.3 mm was used to couple in light at l = 488 nm and a multimode fiber was used for the collection of the transmitted light.…”

On-chip absorption measurements using an integrated waveguide

“…Several groups have reported hybrid integration that include detector and source either fabricated on the separations chip or bonded to the chip [1,2]. Other groups have used waveguides fabricated on the separations chip to carry light to and from the detector [3]. A drawback to such schemes is the cost associated with production of these specialty devices.…”