Compact optical characterization platform for detection of bio-molecules in fluidic and aerosol samples
There is an urgent need for fast, sensitive, and specific detection methods to identify nanoparticles, micro-organisms, bio agents, and toxins in water, blood, food, aerosols and other specimens. This generally involves a laboratory which is expensive, time consuming, and requires skilled personnel. Yet many applications actually require a compact, fast and automated detection system for point-of-care detection. It is generally a necessity to detect the analyte of interest "on-the-fly," that is, as it is moving in order to allow for continuous and real time detection. Optical detection methods offer high sensitivity, but current approaches capture only a "snapshot" of the moving particle which yields limited information. Furthermore, most detection schemes have limited performance due to weak interaction between the excitation light and analyte, as well as the high cost and large size of the typical optical instruments (e.g., commercial spectrometers). This presentation will describe an optical subsystem that can provide both an "event trigger" and "identification into classes" of bio-molecules in water. The detection platform is designed to record the auto-fluorescence properties of an unknown analyte in water "on the flow" and can be assembled into an extremely compact subsystem. Our approach integrates a method for enhanced light-target interaction with a chip-size wavelength detector for spectral characterization of a moving analyte. Unlike most other approaches for detecting and identifying an analyte, ours does not require concentrating or immobilizing suspect particles for interrogation. Rather, it takes advantage of the general necessity to detect such particles in real time as they are moving. Our concept permits warning (triggering) of suspicious particles and class identification simultaneously. This information can be used to initiate further processing in a second stage, such as identification, removal, disinfection, capturing or sorting of agents. Our compact optical characterization platform is schematically illustrated in Fig. 1. The building blocks of the platform are (1) an anti-resonant waveguide to enhance light-target interaction and (2) a chip-size spectrometer. Each of these components and their integration are discussed below. In optical biosensors the interaction between light and target molecules is typically very weak. In order to improve this interaction we are using an anti-resonant waveguide, in which the core region has a lower refractive index than the cladding layers. With this concept the light can be guided within the target-containing medium, thereby enabling an extended interaction length. An anti-resonant waveguide is especially compatible with a fluidic biosensor because the fluidic channel itself can be used as the core of the anti-resonant waveguide. (Of course, this approach can be used in various other configurations, e.g., aerosols in glass capillary or liquid films between glass slides). Various optical characterization techniques can be performed along the length of the fluidic channel by using different intensity and/or wavelength selective detectors that are sequentially addressed. The anti-resonant waveguide concept is particularly well suited for a multi-signal analysis approach, since it is relatively unaffected by changes in both wavelength and film thickness. Novel compact spectrometers are integrated along the fluidic channel. Most presently used spectrometers are bulky and expensive systems in which the incident wavelength spectrum is split into its components with gratings or prisms in order to determine the intensity of the various wavelength fractions. Many approaches for miniaturization exist, based on various technologies, e.g., MEMS, but they are still very expensive, not compact enough and most importantly not suited for integration into small systems as required in our case. We have identified concepts that enable the realization of a compact and low-cost spectrometer that is especially well suited for the spectral characterization of moving particles. The size of the spectrometer can be only slightly larger than that of the detector array, and the cost of fabrication requires only conventional, readily available processing technology which should enable a cost effective manufacturing process. System applications requiring either very high wavelength resolution (e.g., for absorption spectroscopy or Raman spectroscopy) or a wide spectral range (e.g., fluorescence spectroscopy) can be accommodated with our design. Significant advantages of our spectrometer are realized at the systems level with the enablement of interactive detection schemes. With fast data acquisition, the result of a prior characterization can trigger or influence a subsequent measurement or action. For example, a light scattering and fluorescence measurement may indicate the presence of a certain agent. This can trigger a more specific investigation for this species using, for example, multi-wavelength fluorescence.
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Kiesel, P. ; Schmidt, O. ; Bassler, M. ; Johnson, N. M. Compact optical characterization platform for detection of bio-molecules in fluidic and aerosol samples. ISSSR 2006 (International Symposium on Spectral Sensing Research); 2006 May 29 - Jun 2; Bar Harbor; ME; USA.
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