Caged-Sphere Optofluidic Sensors: Whispering Gallery Resonators in Wicking Microfluidics

Abstract

The rapid development of optofluidic technologies in recent years has seen the need for sensing platforms with ease-of-use, simple sample manipulation, and high performance and sensitivity. Herein, an integrated optofluidic sensor consisting of a pillar array-based open microfluidic chip and caged dye-doped whispering gallery mode microspheres is demonstrated and shown to have potential for simple real-time monitoring of liquids. The open microfluidic chip allows for the wicking of a thin film of liquid across an open surface with subsequent evaporation-driven flow enabling continuous passive flow for sampling. The active dye-doped whispering gallery mode microspheres placed between pillars, avoid the use of cumbersome fibre tapers to couple light to the resonators as is required for passive microspheres. The performance of this integrated sensor is demonstrated using glucose solutions (0.05–0.3 g/mL) and the sensor response is shown to be dynamic and reversible. The sensor achieves a refractive index sensitivity of ~40 nm/RIU, with Q-factors of ~5 × 103 indicating a detection limit of ~3 × 10−3 RIU (~20 mg/mL glucose). Further enhancement of the detection limit is expected by increasing the microsphere Q-factor using high-index materials for the resonators, or alternatively, inducing lasing. The integrated sensors are expected to have significant potential for a host of downstream applications, particularly relating to point-of-care diagnostics.

Microsphere-Enabled Micropillar Array for Whispering Gallery Mode Virus Detection

“…The Q factor is of the same order of magnitude as the unfunctionalized WGM microresonators reported previously. 14 This suggests a homogeneous antibody coating on the surface of the resonator, as the Q factor is affected by the resonator curvature, material, surface inhomogeneities, and roughness. 24 The optical shift (Δλ) caused by molecular binding onto the microresonator is related to the refractive index change on the surface and the radius increase, as shown in eq 1.…”

“…Micropillar arrays have been demonstrated to be an outstanding microfluidic component for handling small volumes of liquid . The WGM-based optofluidic sensor was shown to be practical and robust for continuously monitoring refractive index changes in liquid samples . In addition, the micropillar array can act as a filter , to remove impurities and even separate particles for sample pretreatment.…”

“…Previously, we reported the proof-of-concept integration of active WGM microsphere resonators in a micropillar array that achieve sensing in autonomous flow. 14 Taking advantage of capillarity in the microfluidic pillar array to create a supported thin liquid film, 15 flow is then driven by evaporation. 16 Micropillar arrays have been demonstrated to be an outstanding microfluidic component for handling small volumes of liquid.…”

“…17 The WGMbased optofluidic sensor was shown to be practical and robust for continuously monitoring refractive index changes in liquid samples. 14 In addition, the micropillar array can act as a filter 16,18 to remove impurities and even separate particles for sample pretreatment. Other microfluidic techniques, such as inertial and centrifugal microfluidics, 19 may also be incorporated to deliver a high-purity or concentrated sample to the sensor.…”

Feature Papers in Optical Sensors 2022

“…In the ninth paper, by Riesen et al [ 9 ], the authors have demonstrated an integrated optofluidic sensor consisting of a pillar array-based open microfluidic chip and caged dye-doped whispering gallery mode microspheres, which shows potential for the simple real-time monitoring of liquids. The open microfluidic chip allows for the wicking of a thin film of liquid across an open surface with subsequent evaporation-driven flow enabling a continuous passive flow for sampling.…”