J. Eur. Opt. Society-Rapid Publ. 21, 26( 2025) 263
Figure 21. Top row: schematic representation of a WGM biosensor, resulting from the union of a WGM resonator and a sensing layer. Middle row: main ligands or receptors( antibodies, streptavidin, aptamers, enzymes). Bottom row: main analytes( antigens, biotin( ylated) proteins, aminoacids). Reproduced from [ 88 ].
Monifi et al. [ 78 ] reported a proof-of-concept by measuring a PA signal using an encapsulated microtoroidal WGMR and a taper using an UV-curable low index polymer, whereas Chistiakova et al. [ 79 ] used a microsphere: both groups immersed the resonators in water to match the acoustic signal impedance. In 2017, Kim et al. [ 80 ] used a microbottle to measure an external PA signal in air. In 2019, Frigenti et al. [ 19 ] used a MBR to sense the PA signal generated by a gold nanorod solution( GNR) inside the MBR, with the whole system placed in air.
5 Sensing applications
5.1 Chemical and biochemical sensing
The first straightforward sensing application of WGMR is a refractometer. The change on refractive index( RI) is usually the proof of concept when authors report on a new architecture. Microspheres were the first WGMR studied and proposed as RI sensors [ 81 – 83 ], followed by microtoroids [ 84 ], thin capillaries or liquid core optical resonators( LCOR) [ 85, 86 ], microbubbles [ 44, 48 ], and crystalline microtoroids [ 87 ].
To have a selective sensor, especially a biochemical sensor, the surface of the WGMR needs to be functionalized. The surface functionalization is a chemical modification of the transducer surface, usually through covalent reactions. There are several chemical procedures or protocols to minimize the unwanted non-specific effects and coat the surface with very thin( maximum thickness below the penetration depth of the evanescent field) and homogeneous layers to preserve the high quality of the transducer. The choice of the chemical protocol must be based on the specificity, the affinity and the stability of the bio-receptor. Silanization is the most common approach and it is based on covalent binding of the silane groups with the glass surface and enables a further functionalization with ligands or receptors. Another technique is the use of biotin and / or streptavidin layers to bind biotinylated molecules. Figure 21 shows a render of four different biosensing schemes that are based on the following biological recognition elements( BRE): antibodies, streptavidins, aptamers and enzymes. The
Figure
22. WGM resonance recorded at different steps of the functionalisation process:( a) before the process started,( b) after silanization and( c) after an antibody is covalently bound.
BRE will bind to the corresponding analytes: antigen, biotin / biotinylated proteins, proteins, and amino acids. Immunosensors are antibody-based sensors, while aptasensors are aptamer-based sensors. Antibodies and aptamers show high affinity and specificity towards the analytes, due to their molecular complementarity. The beauty of aptamers relies on the possibility of being selected in vitro for any given target. Enzymes are specific for both the reaction they catalyze and the substrate they recognize, and are subject to regulation of their activity by other molecules, but their activity depends on physical and chemical environments. Detailed descriptions of the different chemical protocols can be found in other extensive reviews such as Foreman et al. [ 89 ] or Cai et al. [ 90 ].
Biochemical sensors are highly specific due to the molecular affinity we just mentioned, but the surface functionalization can degrade the quality factor of the resonators. Figure 22b shows the slight change in Q-factor and contrast of the resonance after silanization, while Figure 22c shows their drop by an order of magnitude once the antibody is covalently bound to the silane and the free binding sites blocked with a short peptide [ 63, 91 ].
Stephen Arnold’ s group pioneered the sensing applications of WGMR. The group demonstrated the feasibility of detecting very large objects like bacteria [ 92 ] as well as very small ones like virus [ 93, 94 ]. In [ 93 ] themodelsystem is a RNA virus known as MS2, which kills E. coli, but is harmless to humans, while the negative control is Phix174, another E. coli virus having different epitopes. Both viruses are icosahedral viruses like HIV. In the case of influenza A [ 94 ], the authors were able to detect single virions, even though this result was obtained for non-specific sensing. Arnold’ s group also studied protein orientation by measuring the shift ratio between the fields polarized parallel and perpendicular to the surface [ 95 ]. Following this paper, Vollmer et al. studied conformational changes using bacteriorhodopsin as a model system [ 96 ], showing potentiality for studying protein misfolding diseases or anomalous aggregations like Creutzfeldt-Jakob’ s disease.