Simulation of Whispering-gallery Mode Microsensing in a Microfluidic System

Label-free detection using a whispering-gallery model biosensor in a micro fluidic channel is simulated. The analyte transport in the solution is controlled by applied electric potentials and diffusion. The finite element method is employed for solving the charged species transportation equations, the Poisson equation, the equations of conservation of momentum and energy, and the Helmholtz equations for electromagnetic waves. The adsorption process of analyte on the micro resonator surface is monitored by the resonance wavelength shift in the sensor. Shift caused by temperature variation due to Joule heating is found to be negligible compared to that induced by analyte deposition. The deposition induced shifts behave in a manner similar to Langmuir-like adsorption kinetics. A linear correlation between the frequency shift and the analyte concentration in the solution is obtained. The applied voltage is found to affect the adsorption capability; and thus, the sensor sensitivity. Detection of very low concentration to the sub-ppm level using the sensor is demonstrated. INTRODUCTION Thanks to the recent advance in optical techniques, many different detection methods have been widely applied in biomedical analyses. These methods can be broadly classified into two main categories: labeling methods and label-free methods. Labels can structurally and functionally interfere with an assay. As a result, scientists and biotechnologists have been exploring the area of label-free detection in order to overcome these disadvantages with label methods. Without the need for labels or agents, detection can be done in situ and in real time, which is an important advantage in emerging point-of-care detecting applications. Several label-free optical techniques have been developed, like autofluorescence [1], confocal Raman spectroscopy [2], optical scattering [3], and surface plasmon resonance [4]. However, biophysical studies are hindered by the bulky and expensive equipment needed, and limited cell and biomolecule manipulation and techniques. Consequently, the micro optical fluidic system (MOFS) [5, 6], which employs optics and micro fluidics in a micro system environment to perform novel functionality and in-depth analysis in the biophysical area, has been highly driven by recent development in biophysical studies. In this work, we briefly present the primary mechanism of a micro optical fluidic system, consisting of a microfluidic channel to manipulate samples and a whispering-gallery mode (WGM) biosensor. We believe that the miniature optical WGM device can be used in MOFS to provide an advantage in labelfree detection with high sensitivity in the analysis of biomolecules or single living cells [7]. It is well known that light can be confined in a microresonator which can be a sphere [8], a disc [9], a ring [10], or a torus [11]. At resonance, electromagnetic (EM) waves of a specific wavelength are trapped and internally reflected in an orbit within the resonator surface of circular shape. The socalled WGM resonance induces an evanescent wave field in the surrounding medium and extends up to one wavelength outward. This wave can be explored to detect polarizable molecules binding to the surface of the micro-resonator, which can slightly change the resonance condition through interaction with the evanescent waves [7, 12]. WGM resonance frequencies depend on the size and refractive index of the resonator. As the photon guided by total internal reflection (TIR) in a resonator circulates many times, it interacts repeatedly with the adsorbate on the surface of the resonator, leading to change of effective size or refractive index. This feature is broadly explored for use in WGM-based sensors [13, 14]. In general, the resonant modes are approximately predicted by 0 2 / ( m r mc f n π ) = ⋅ , where m is an integer representing the possible mode, r is the resonator radius, is 0 c Copyright © 2009 by ASME 1 the light speed in vacuum and n is the refractive index of micro resonator. The wavelength shift of a given resonant mode assuming constant refractive index is estimated as