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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    • "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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    ABSTRACT: The opto-fluidic ring resonator (OFRR) biosensor is numerically characterized in whispering gallery mode (WGM). The ring resonator includes a ring, a waveguide and a gap separating the ring and the waveguide. Dependence of the resonance characteristics on the resonator size parameters such as the ring diameter, the ring thickness, the waveguide width, and the gap width between the ring and the waveguide are investigated. For this purpose, we use the finite element method with COMSOL Multiphysics software to solve the Maxwell's equations. The resonance frequencies, the free spectral ranges (FSR), the full width at half-maximum (FWHM), finesse (F), and quality factor of the resonances (Q) are examined. The resonant frequencies are dominantly affected by the resonator diameter while the gap width, the ring thickness and the waveguide width have negligible effects on the resonant frequencies. FWHM, the quality factor Q and the finesse F are most strongly affected by the gap width and moderately influenced by the ring diameter, the waveguide width and the ring thickness. In addition, our simulation demonstrates that there is an optimum range of the waveguide width for a given ring resonator and this value is between ~2.25 μm and ~2.75 μm in our case.
    Sensors 12/2012; 12(10):14144-57. DOI:10.3390/s121014144 · 2.25 Impact Factor
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    • "Many optical properties, such as refractive index (RI), fluorescence, Raman scattering, absorption and polarization, can be exploited individually or in combination to generate the sensing signal. Moreover, traditional analytical chemistry technologies such as chromatography and electrophoresis can be adopted to optofluidic devices further increases their functionality in biological/chemical analysis [5] [6]. Furthermore, optofluidic microsystems can also employ optical forces in tandem with microfluidics to trap and manipulate targets [7], thus further enhancing the system's analytical capabilities. "
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    ABSTRACT: Surface-enhanced Raman scattering (SERS) has emerged as a powerful analytical technique for direct detection of chemical and biological analytes because of high sensitivity, selectivity, and rapid response. Here we propose and develop a novel optofluidic SERS structure, i.e., nanoparticle-functionalized flow-through multihole capillary. This unique platform provides many advantages. First, its 3-dimensional (3-D) structure, similar to nanoporous aluminum membranes, nanoporous polymer monoliths, and photonic crystal fibers (PCFs), provides large surface area for the deposition of noble nanoparticles or nanoclusters to achieve high SERS intensity. Second, it has well-defined flow-through channels. It provides robust and controllable nanoparticle immobilization like PCFs, but much higher nanoparticle density thus large SERS-active sites due to large surface within the detection volume, and also enables fast and convenient analyte delivery for real-time, online detection. Third, the well-defined multihole capillary can also confine and transmit light along the longitudinal direction, accumulating large SERS signal like PCFs. Fourth, it is easy to integrate with other sensing platforms, such as label-free biosensors, to provide comprehensive information on molecular interaction. Moreover, the multihole capillary can be mass-produced easily and cost effectively using the fiber drawing method. In this report, using a capillary consisting of thousands of micrometer-sized holes adsorbed with gold nanoparticles, we investigated the proposed optofluidic SERS system using the transverse and longitudinal detection methods, where the SERS excitation and collection were perpendicular to and along the capillary, respectively. A detection limit better than 100 fM for rhodamine 6G was achieved with an enhancement factor exceeding 108.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2012; 8212:8-. DOI:10.1117/12.909527 · 0.20 Impact Factor
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    ABSTRACT: The liquid core optical ring resonator (LCORR) integrates an array of optical ring resonators into a microfluidics channel. The LCORR is made of a micro-sized glass capillary; the circular cross-section of the capillary acts as an optical ring resonator while the resonating light interacts with the fluid sample passing through the core. Q-factors larger than 107 have been achieved in LCORRs on the order of 100 micrometers in diameter. This implies an effective interaction length between the evanescent field of the resonator and the fluidic core of over 10 cm. The novel integrated architecture and excellent photonic performance lead to a number of applications in sensing, analytical chemistry, and photonics. For the last decade, optical ring resonators have been explored for label-free bio/chemical detection. The LCORR architecture possesses the same capabilities as other optical ring resonator bio/chemical sensors while also integrating micro-capillary-based fluidics with the sensor head. The integrated fluidics design in combination with the micro-sized sensor head and pico-liter sample volume lead to a lab-on-a-chip sensor for biomolecules, such as biomarkers and specific DNA sequences. Also, because the ring resonator creates a high-intensity field inside the microfluidic channel, the LCORR is an excellent microfluidic platform for surface-enhanced Raman scattering (SERS) detection in silver colloids. Finally, the high quality factor of the capillary-based resonator enables novel opto-fluidic devices, such as dye lasers. We will discuss the details of these concepts and present our research results in each of these applications.
    Proceedings of SPIE - The International Society for Optical Engineering 01/2007; 6757. DOI:10.1117/12.732949 · 0.20 Impact Factor
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