ON-CHIP DROPLET ENHANCED FLUORESCENCE EMISSION FOR LOW CONCENTRATION PROTEIN MEASUREMENT Y. F. Yu 1, 2 , T. Bourouina 2 , and A. Q. Liu 1 1 School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798 2 Ecole Supérieure d'Ingénieurs en Electronique et Electrotechnique, University of Paris Est, France, 93162 (Email: eaqliu@ntu.edu.sg , Tel: +65-6790-4336, URL: http://nocweba.ntu.edu.sg/laq_mems/ ) ABSTRACT On-chip droplet enhanced fluorescence emission for low concentration protein measurement is presented in this paper. The system includes T-junction of microchannel for micro-droplet generation and two integrated optical fibers for excitation laser input and fluorescence detection. The excitation light is confined in the droplet by total internal reflection to increase the optical path length, which enhances the fluorescence emission. The on-chip droplet features higher stability and easier manipulation. The size of the droplet can be varied by changing the flow conditions. The intensity of the droplet enhanced fluorescence emission is much higher than the intensity of the free space fluorescence, which promises wide applications in biological detection and measurement. KEYWORDS Droplet, Microfluidics, Fluorescence emission, Microphotonicfluidic systems and Optofluidics. INTRODUCTION Traditional methods for protein concentration measurement are mostly based on absorption techniques, such as ultraviolet absorption, alkaline copper, bicinchoninic acid, Coomassie blue staining or gold nano- particle labeling [1]. These methods have limitations on detection sensitivity, quantitative accuracy and compatibility with modern protein identification and characterization procedures as compared with the fluorescence based protein detection methods [2]. In on-chip droplet enhanced fluorescence emission, the intensity of fluorescence emission is increased using a spherical cavity [3]. The intensity enhancement has real significance in increasing the sensitivity of measurement. Fluorescence intensity is quantitatively depending on the molar extinction coefficient, the optical path length and the solute concentration, the excitation light intensity and the fluorescence collection efficiency of the system. But for the same sample solution, the molar extinction coefficient and the solute concentration are constants. The adjustable parameters are the optical path length and the fluorescence collection efficiency. They are directly proportional to the fluorescence intensity. In order to increase the optical path length, the excitation light is confined in a spherical cavity (here is a droplet) by total internal reflection. Due to the interfacial tension between two immiscible liquids, the multiphase liquid droplet has ultrahigh smooth interface, which insures low loss. The optical path length can be lengthened by thousand times through resonance. In this paper, a micro-optic-fluidic system is applied to replace the traditional electrophoresis detection method [4]. Optical fibers integrated in the chip for spectrum detection increases the fluorescence collection efficiency because the fibers are close to the droplets [5]. The micro-optic-fluidic chip is suitable for experiments with tiny volume of sample, which reduces the total cost and accelerates the detection time. Compared to the aerosol droplets for detection [6], the on-chip droplets have higher stability and simpler manipulation. DESIGN AND ANALYSIS The micro-optic-fluidic chip consists of two liquid inlets and a waste outlet, a two-inlet t-junction for multiphase plugs generation, an enlarged channel region and two integrated optical fibers as shown in fig. 1. The plugs are reformed into droplets due to the surface tension, and the length of the plugs determines the diameter of the droplets, which is determined by the flow conditions. For optical measurement, one of the methods is to input the excitation light from the top and use a CCD device to capture the images with a filter. Alternatively, the excitation light is injected by the optical fiber and the emission is collected by another optical fiber and detected by the Figure 1: Schematic of (a) optofluidic chip with optical detection (b) detection region. Excitation laser Fluorescence detection Oil inlet Glycerin inlet Outlet (a) (b) 978-1-4244-4193-8/09/$25.00 ©2009 IEEE Transducers 2009, Denver, CO, USA, June 21-25, 2009 1250 T3P.093