Silicon quantum dot coated microspheres for microfluidic refractive index sensing

Abstract

The electromagnetic resonances of optical microspheres-the so-called whispering gallery modes (WGMs)-can be used for refractometric sensing of surrounding fluids. Microspheres are attractive because they offer high sensitivity and can be utilized with fluorescent dyes or quantum dots. One issue with microspheres, however, is that they are difficult to integrate into microfluidic systems. Here, we develop a microfluidic structure that permits sensing applications using a single microsphere in a capillary. To achieve this, a microsphere formed on the end of a tapered fiber was first coated with fluorescent silicon quantum dots (QDs). The sphere was then inserted into a microcapillary and the fluorescence WGMs were monitored as different fluids were pumped through the channel. The sensitivity and detection limits for this sphere-in-a-capillary device were measured for several different QD film thicknesses and for two different microsphere sizes. Because of the relatively high-visibility mode structure, the sensitivity and detection limit can be defined by Fourier analysis of the free spectral range and WGM spectral shifts.

Single Nanoparticle Detection Using Optical Microcavities

“…[134] The statistical analysis of the resonance shift due to MS2 virus binding events is shown in Figure 11e. [138][139][140][141][142] Placing the nanoparticle closer to the peak of the mode field by fabricating a notched microcavity could also lead to an enhancement of the sensitivity. Although plasmonicphotonic PhCs have not been used for sensing applications, hybrid PhCs fabrication techniques have been demonstrated.…”

Protein biosensing with fluorescent microcapillaries

“…The most common types of WGM-based resonator include microscale spheres [9][10][11] toroids [12], disks [13], fibers [14], and thin-walled capillaries [15]. The basic idea behind each of these is the same: In the first four examples, light is confined by total internal reflection as it propagates around the circumference of the structure.…”

Whispering Gallery Mode Devices for Sensing and Biosensing

“…As discussed in Sects. 9.2 and 9.3, further enhancements of the sensitivity can be obtained by coating the sphere with a high-index layer [53], as has been demonstrated experimentally [132,136].…”

“…Fluorescent microspheres have also been fabricated by melting the tip of an optical fiber containing rare earth ions such as erbium [39,134,135]. Alternatively, bulk fluorescent coatings can be deposited onto the microsphere, such as a thin film of silicon nanocrystals [136]. Finally the intrinsic microsphere fluorescence can be exploited to achieve similar results.…”