This chapter describes the realization of a lab-on-a-chip optical sensor that is based on surface plasmon resonance (SPR) trapped microspheres acting as localized sensing elements for morphology-dependent resonance (MDR) sensing. The microfluidic device is fabricated by a combination of direct laser writing and hot embossing. This allows simple integration of SPR techniques by the evaporative coating of a metal layer on the surface of the microfluidic device. Trapping of 4, 10, and 15 µm polystyrene microspheres is demonstrated using SPR in static and dynamic fluidic environments. Patterning of the metal surface is demonstrated to increase the trapping potential of the SPR technique as well as provide a method of further localizing the position of the optical trap within the device. Comparison between the trapping of microspheres for both on- and off-resonance incident angles of the trapping beam shows strong difference in the strength of the optical trap allowing for an on/off switching of the trapping force within the device. The integrated SPR trapping technique provides a method for arbitrary trapping of a range of microspheres within a microfluidic environment. The MDR optical sensing technique was selected as a noninvasive, multivariable sensing technique that can be performed on a range of optically trapped microcavities. Coupling to the MDR of a spherical microcavity is achieved via evanescent wave coupling under total internal reflection within a static fluidic environment. Fluid refractive index detection is realized with a sensitivity of 9.66 × 10-2 refractive index units (RIU) by the characterization of the shift of the MDR positions. A quality (Q) factor of 1.1 × 104 is observed for a 90 µm glass microsphere with a stability of ΔΛ = ±0.04. The coupling of light to the MDR mode is realized for a 90 µm glass microsphere trapped in a dynamic microfluidic device via SPR-based optical trapping. The position of the trapped microsphere is defined by the location of the patterned region of the metal surface as well as the position of the location of the focal spot of the SPR incident light source. A Q-factor of 4 × 103 is observed under these coupling conditions. Detection of a change in the refractive index of the local fluidic environment is observed via change in the MDR of a microcavity held under SPR trapping conditions; a resolution of 7.75 × 10-2 RIU is observed under a flow rate of 20 µm/s. This research explores the integration of optical-based manipulation and localized sensing techniques into a microfluidic environment. From the work demonstrated, it is anticipated that this research will develop toward an opticalbased sensing system where localized sensing can be performed in an arbitrary location within a fluidic environment.
Handbook of Photonics for Biomedical Engineering / Aaron Ho-Pui Ho, Donghyun Kim, and Michael G. Somekh (eds.), Chapter 26, pp. 767-805
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