Capillary-Type Microfluidic Sensors Based on Optical Whispering Gallery Mode Resonances

Abstract

Microfluidic capillary-type sensors could potentially find applications in a range of medical and environmental diagnostic technologies. Refractive-index sensors-as well as biosensors-based on the whispering gallery mode resonances of a thin-walled capillary offer several modes of operation and the capacity to detect a wide range of analytes through standard silica surface functionalization methods. This article reviews the basic optical physics associated with the resonances and how they respond to changes in the local refractive index. The sensitivity and detection limits of these devices are explored, and at least one old assumption concerning the inverse relationship between the quality factor of the resonance and the detection limit is shown to be valid only under a restricted set of circumstances. Problems and pitfalls associated with whispering-gallery-mode capillary sensors are examined, as well as the experimental methods needed for analysis. Finally, relevant capillary sensor examples from the literature are summarized.

Protein biosensing with fluorescent microcapillaries

“…This blueshift occurs because the effective path length decreases as the high-index material is gradually removed. The spectral shift data could be related directly to an etching rate by using standard methods to calculate the resonance wavelength for a layered cylinder [52], as adapted from earlier theoretical work on TM-polarized layered spheres [53]. For the sake of brevity we don't repeat the equations here but merely state the results: for conditions matching the experimental ones the resonance is calculated to blueshift by approximately 25 pm per nm etched, yielding an etching rate for the QD film of ~1.4 nm/h in continuously pumped 10 M NaOH solution.…”

“…Also, our relatively low-resolution spectrograph (which has a pitch of slightly over 0.1 nm/pixel) and the moderate sensitivity for the QD film thicknesses used here both offer room for improvement. The latter two methods could, in principle, improve the detection limits from the range of nM to tens of pm [52].…”

Whispering Gallery Mode Devices for Sensing and Biosensing

“…The equations are based on the solutions to the spherical or cylindrical Bessel functions under the appropriate boundary conditions, which depend on the polarization and the number of layers (generally two or three, but more are possible) in the sensor structure. Here, we briefly review the equations that define the electric field profiles, noting that the detailed derivation of these expressions can be found elsewhere [47][48][49][50].…”

“…The methods used to calculate dk/k have, however, been shown both for spheres [52] and for cylinders [50]. The mathematical procedure is too lengthy to repeat here; instead, we simply report the results for bulk changes in the analyte medium.…”

Refractometric sensing of Li salt with visible-light Si3N4 microdisk resonators

“…[1][2][3] Sensors based on whispering-gallery mode resonators have been used for the label-free detection of single viruses, 4,5 nanoparticles, 6-9 single proteins 10 , nucleotides, 11,12 and are even used commercially. 13 Many geometries have been used for bulk refractometric sensing.…”