Name: synthesis and operation of fluorescent-core microcavities for refractometric sensing
Uploaded: 2013-03-13T13:00:00
Description: Watch this Scientific Journal Video about Synthesis and Operation of Fluorescent-core Microcavities for Refractometric Sensing at JoVE.com

This research was funded by NSERC, Canada.

1. Preparation of Materials
<ol>
 <li>Microcapillaries Obtain silica capillaries from a commercial supplier. We purchase our capillaries from Polymicro Technologies . Choose a small inner diameter (~25 - 30 &mu;m) for more widely separated spectral resonances (i.e. a larger free spectral range) or a larger inner diameter (~100 &mu;m) for more closely spaced resonances with higher quality factors. A large outer diameter ensures the FCMs are durable and easily manipulated.
 <ol>
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<ol>
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Fluorescent-core microcavities can be used as refractometric sensors. While there are isolated examples of "rolled up" microtubes that could act as microfluidic sensors,22 compared to microtubes, capillaries will be easier to integrate into microfluidic setups and have considerable practical advantages, since they are easily handled and simple to interface with an analysis setup. Using conventional Fourier analysis methods, wavelength shifts that are at least an order of magnitude smaller than the pitc...

Chemical sensing systems that require only small sample volumes and that can be incorporated into hand-held or field-operable devices could lead to the development of a wide range of new technologies. Such technologies could include field diagnostics for diseases and pathogens,1 environmental contaminants,2 and food safety.3 Several technologies are being actively explored for microfluidic chemical sensors, with devices based on the physics of surface plasmon resonances (SPR) among the most advanced.4 These sensors are now capable of detecting many specific biomolecules and have achieved commercial success, although mainly as larger-scale lab equipment.5
In recent years, optical microcavities have risen to compete with SPR-based systems. Microcavities can be amazingly sensitive, with demonstrated ability to detect single viruses6 and perhaps even single biomolecules7 (the latter remains the subject of some debate,8 however there is no doubt that the mass detection limits are small9 ). In microcavities, the detection mechanism relies upon changes in the optical resonances caused by the presence of an analyte within the electric field profile of the resonance. Typically, a given analyte will cause the resonance to change in in central frequency, visibility, or linewidth. As with SPR systems, microcavities can act as non-specific refractometric sensors, or as biosensors functionalized for a specific analysis.
Dielectric microstructures with a circular cross section (e.g. microspheres, disks, or cylinders) are characterized by electromagnetic resonances known as the whispering gallery modes, or WGMs, a term dating back to Lord Rayleigh's investigations of analogous acoustic effects.10 Essentially, an optical WGM occurs when a wave circumnavigates the circular cross section by total internal reflection, and returns to its starting point in phase. An example of an electromagnetic resonance for a silica microsphere is illustrated in Figure 1a. This resonance is characterized by one maximum in the radial direction (n = 1), while a total of 53 wavelengths fit around the equator (l = 53), only some of which are shown. The evanescent part of the field intensity extends into the medium outside the sphere boundary; thus the microsphere WGM can sense the external medium.
Capillaries are an especially interesting example of a WGM-based sensor. In a capillary, cylindrical WGMs can form around the circular cross section, similar to the case for a sphere. If the capillary wall is very thin, part of the electromagnetic field extends into the capillary channel (Figure 1b). Thus, a capillary can be a microfluidic sensor for analytes injected into the channel. This is the basis of operation of the liquid core optical ring resonator (LCORR).11 LCORRs rely on the evanescent coupling of light from a precision tuneable laser source to probe the WGMs. An important aspect of the LCORR is that the capillary walls must be thin (~1 &mu;m) to ensure that the mode samples the channel medium. This places some difficulties on their fabrication and causes them to be mechanically fragile.
In our work, we have developed an alternative structure we call a fluorescent core microcavity (FCM).12,13 To form an FCM, we coat the channel walls of a capillary with a high-refractive-index fluorophore (specifically, a layer of oxide-embedded silicon quantum dots). The high index of the film is required to confine the emitted radiation, thereby building up the WGMs (Figure 1c). In contrast to the LCORR, in an FCM the modes appear as sharp maxima in an emitted fluorescence spectrum. The thickness of the film is critically important; if it is too thick the WGM does not sample the medium in the capillary channel, and if it is too thin the optical confinement is lost and the WGMs become weak. Thus, the fabrication of an FCM is a difficult process, requiring careful preparation. This is the main topic of the current paper.

<table border="1">
<tbody>
 <tr>
 <td>Table of Materials</td>
 <td>Company</td>
 <td>Catalog #</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>silica microcapillaries</td>
 <td></td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>flexible microbore tubing</td>
 <td></td>
 <td></td>
 <td>polyethylene, tygon, etc</td>
 </tr>
 <tr>
 <td>adhesive</td>
 <td>Mascot, Norland NOA</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>HSQ dissolved in MIBK</td>
 <td></td>
 <td></td>
 <td>e.g., FOx-15</td>
 </tr>
 <tr>
 <td>methanol</td>
 <td></td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>ethanol</td>
 <td></td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>distilled water</td>
 <td></td>
 <td></td>
 <td></td>
 </tr>
 </tbody>
</table>
Table 1. List of materials used.

synthesis and operation of fluorescent-core microcavities for refractometric sensing

This paper discusses fluorescent core microcavity-based sensors that can operate in a microfluidic analysis setup. These structures are based on the formation of a fluorescent quantum-dot (QD) coating on the channel surface of a conventional microcapillary. Silicon QDs are especially attractive for this application, owing in part to their negligible toxicity compared to the II-VI and II-VI compound QDs, which are legislatively controlled substances in many countries. While the ensemble emission spectrum is broad and featureless, an Si-QD film on the channel wall of a capillary features a set of sharp, narrow peaks in the fluorescence spectrum, corresponding to the electromagnetic resonances for light trapped within the film. The peak wavelength of these resonances is sensitive to the external medium, thus permitting the device to function as a refractometric sensor in which the QDs never come into physical contact with the analyte. The experimental methods associated with the fabrication of the fluorescent-core microcapillaries are discussed in detail, as well as the analysis methods. Finally, a comparison is made between these structures and the more widely investigated liquid-core optical ring resonators, in terms of microfluidic sensing capabilities.

Small deviations in the capillary fabrication procedure can lead to significant changes in the sample success rate. In Figure 5(a-d), we show representative examples of failed capillaries as well as a successful one. Generally, the visual indication of a successful sample is a red fluorescence combined with a high intensity at the capillary walls and a featureless interior. The fluorescence spectrum also clearly indicates the difference between success and failure (Figure 5e). A ...

Watch this Scientific Journal Video about Synthesis and Operation of Fluorescent-core Microcavities for Refractometric Sensing at JoVE.com

Synthesis and Operation of Fluorescent-core Microcavities for Refractometric Sensing

Fluorescent-core microcavity sensors employ a high-index quantum-dot coating in the channel of silica microcapillaries. Changes in the refractive index of fluids pumped into the capillary channel cause shifts in the microcavity fluorescence spectrum that can be used to analyze the channel medium.

This paper discusses  fluorescent core microcavity-based sensors that can operate in a microfluidic  analysis setup. These structures are based on the ...

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