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Figure 28. Frequency shift of a WGM versus internal pressure for a microbubble having a wall thickness of 3.7 lm.
Figure 27.( a) Sketch of the microbottle,( b) Sketch of the GNR adsorbed into the inner surface of the microbottle,( c) Wavelength shift of the wall mode( black), the interface mode( orange) and the core mode( brown). Adapted from [ 107 ].
estimate of the applied force. In 2011, Henze et al. [ 111 ] demonstrated the possibility of tuning the resonances by changing the internal aerostatic pressure. Yang et al. [ 112 ] showed that the quasi-droplet regime for MBR gives the best sensing resolution. Lu et al. [ 113 ] proposed to use the internal aerostatic pressure sensing for measuring the thickness of the MBR wall. The authors obtained a sensitivity of 6.21 GHz / bar with standard error of 0.10 GHz / bar for a MBR wall thickness of 2.73 lm. Aymerich et al. have measured a similar sensitivity 9.99 GHz / bar for a wall thickness of 3.7 lm( see Fig. 28)[ 114 ].
WGMRs are very sensitive to temperature and their sensitivity has been extensively studied over the years [ 115 ], mostly because it can be a serious hindrance especially in biosensing [ 116 ]. Two temperature related papers were published almost simultaneously by Guan et al. [ 117 ] and Brenci et al. [ 118 ]. Guan et al. measured the temperature changes in air and in water using a silica microsphere [ 117 ], whereas Brenci et al. [ 118 ] measured the optical shifts at room temperature and in a hot environment( around 54 ° C). The authors measured a sensitivity of about 14.2 pm /° C, which is very close to the calculated one( 13.5 pm /° C). Cosci et al. [ 119 ] proposed crystalline microdisks as THz detectors using the thermo-optical features of LN to design a bolometer with a NEP value of 9.66 nW Hz �1 / 2. Gagliardi et al. [ 120 ] reported on a silica microsphere based bolometer with a NEP value of 1pWHz �1 / 2. Hybrid WGMR [ 121 ] have been proposed to compensate the thermal shifts and also as humidity sensors [ 122, 123 ]. New interesting materials such as silk fi- broin have been proposed as thermal sensors [ 124 ]. In 2019, Gu et al. [ 125 ] proposed a MBR filled with ethanol for temperature sensing. The authors measured a shift of almost 397 pm /° C and postulated that such a device could be used for measuring ocean temperature. Obviously, the device needs a strongly insulating packaging to work in a harsh environment and prevent water infiltrations and / or
environmental pressure changes. Packaging the resonator and the coupler is also good option for thermally insulating the sensor [ 126 – 128 ].
Another way to reduce the thermal drift is resorting to self-referencing techniques such as mode splitting. Yang’ s group reported on a nanoparticle-size spectrometry scheme for label-free real-time continuous detection and sizing of single Influenza A virions, polystyrene and GNRs using the mode splitting technique in a microtoroid [ 129 ]. However, back-scattered mode splitting is less effective when the goal is to detect multiple particles. Some authors have proposed the use of coupled cavities. In coupled cavities architectures, mode splitting is achieved by having a coresonance [ 130 ]. Figure 29 shows the self-referencing optofluidic ring resonator( SR-OFRR) or coupled microbubbles. The authors demonstrated that their device lowered the noise by two orders of magnitude and detected BSA at 1 pg / mL. We remark that the device cannot be classified as a biosensor, since it does not have a BRE: the inner wall of OFRR 2 is silanized, but no specific element is used to recognize BSA.
5.4 Active sensing
Arnold et al. [ 69 ] reported on nanoparticle detection using the so-called WGM carousel trap. This light-induced trapping force provides a sensitive means for sizing individual particles and detecting their interactions close to the surface. The low power threshold of the effect(< 200 lW) is due to the resonant build up in the cavity. Lopez et al. [ 131 ] proposed the WGMR carousel trap in coulometry. The carousel trap is used to first measure the cavity charge density and then the nanoparticle charge. Wang et al. [ 132 ] proposed an active sensor based on FRET to detect cell derived exosomes. The functionalization procedure is different from the most standard ones, since they relied on electrostatic interactions using poly-L-lysine( PLL), a layer of streptavidin( SA), and a biotinylated antibody to detect Nile Red( NR) labeled exosomes in a complex liquid matrix. The WGMR here is a Liquid Crystal( LC) microdroplet doped with C6 dye( green emission). Microdroplets, which act as microlasers, can flow in the liquid matrix due to intrinsic convection and Marangoni effect. Figure 30 shows