Figure 1. Structure of a fluorescent-core microcav

Figure 1. Structure of a fluorescent-core microcavity sensor. Whispering gallery modes (WGMs) are illustrated as red ‘oscillations’ mainly in the quantum-dot layer. A fraction of the mode field extends into the channel, sampling any fluids pumped inside.

We demonstrated biosensing with a fluorescent microcapillary of the type shown in Figure 1. We used the well-known specific interaction between biotin and neutravidin as a model system, mainly because the strong binding interaction between these biomolecules makes this an excellent test system. The silicon quantum dots produced a bright red fluorescence in the channel region and showed clear WGM oscillations in their emission spectrum (see Figure 3). The thickness of the coating could be controlled via etching using strong sodium hydroxide solutions, alleviating the need to use hydrofluoric-acid etching.9

In summary, we have developed a sensitive, laser-free biosensor using fluorescent WGMs of a quantum-dot-coated capillary structure that is durable and easy to use. These devices could represent a microfluidic alternative to some types of currently available bioassay technologies. The protein detection limits were in the nanomolar range for these quite simple and durable sensor devices, approaching the desired detection limits for many biosensing applications. There are, however, a number of ways we can improve the sensing performance of this platform in future work. These include achieving a better signal-to-noise ratio by projecting the spectrum onto full vertical CCD arrays, sensitivity enhancement via further thinning the quantum-dot film, the use of a higher-resolution spectroscopic system (although this comes at a cost), or even by plasmon enhancement using methods similar to those demonstrated for microsphere sensors.

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http://spie.org/x113539.xml?highlight=x2406&a...ID=x113539