Integrated Refractive Index Optical Ring Resonator Detector for Capillary Electrophoresis

University of Cambridge, Cambridge, England, United Kingdom
Analytical Chemistry (Impact Factor: 5.64). 03/2007; 79(3):930-7. DOI: 10.1021/ac061279q
Source: PubMed


We developed a novel miniaturized and multiplexed, on-capillary, refractive index (RI) detector using liquid core optical ring resonators (LCORRs) for future development of capillary electrophoresis (CE) devices. The LCORR employs a glass capillary with a diameter of approximately 100 mum and a wall thickness of a few micrometers. The circular cross section of the capillary forms a ring resonator along which the light circulates in the form of the whispering gallery modes (WGMs). The WGM has an evanescent field extending into the capillary core and responds to the RI change due to the analyte conducted in the capillary, thus permitting label-free measurement. The resonating nature of the WGM enables repetitive light-analyte interaction, significantly enhancing the LCORR sensitivity. This LCORR architecture achieves dual use of the capillary as a sensor head and a CE fluidic channel, allowing for integrated, multiplexed, and noninvasive on-capillary detection at any location along the capillary. In this work, we used electro-osmotic flow and glycerol as a model system to demonstrate the fluid transport capability of the LCORRs. In addition, we performed flow speed measurement on the LCORR to demonstrate its flow analysis capability. Finally, using the LCORR's label-free sensing mechanism, we accurately deduced the analyte concentration in real time at a given point on the capillary. A sensitivity of 20 nm/RIU (refractive index units) was observed, leading to an RI detection limit of 10-6 RIU. The LCORR marries photonic technology with microfluidics and enables rapid on-capillary sample analysis and flow profile monitoring. The investigation in this regard will open a door to novel high-throughput CE devices and lab-on-a-chip sensors in the future.

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    • "As a sensor, the HBR takes advantage of intracavity enhancement (because it is a microresonator), and in addition it combines the advantages of capillary-based optical ring resonators (because it is hollow) and whispering-gallery bottle resonators (because of the bottle shape). Capillary-based optical ring resonators [1] [2] [3] [4] [5] [6] are advantageous because they permit internal sensing, which means that much smaller volumes of analyte are required, and it is easy to incorporate the sensor into microfluidic and/or chromatographic systems. Bottle resonators [7] [8] [9] [10] [11] [12] provide the benefits of high quality factor (Q), tunability (by stretching), axial mode confinement, and mode selectivity (e.g., by positioning the coupling fiber). "
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    • "In the last few years, several methods have been proposed to measure the RI of liquid by using a capillary. A typical example is the capillary interferometry123456789101112, where the RI of the liquid can be obtained by measuring the interference fringes. However, the capillary interferometry is still a little complicated, in both experiment and theoretical analysis . "
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    • "In a ring resonator, light propagates in the whispering gallery mode (WGM), which allows for a great miniaturization of the sensors while maintaining a longer effective interaction period. The capillary nature of the ring resonator also enables the convenient sample delivery [4,5]. The 2-dimensional array arrangement enables high efficiency detection and multiple-analyte detection, as shown in Figure 1(a). "
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