Article

# Shift of Whispering Gallery Modes in Microspheres by Protein Adsorption

The Rockefeller University, New York, New York, United States
(Impact Factor: 3.29). 03/2003; 28(4):272-4. DOI: 10.1364/OL.28.000272
Source: PubMed

ABSTRACT Biosensors based on the shift of whispering-gallery modes in microspheres accompanying protein adsorption are described by use of a perturbation theory. For random spatial adsorption, theory predicts that the shift should be inversely proportional to microsphere radius R and proportional to protein surface density and excess polarizability. Measurements are found to be consistent with the theory, and the correspondence enables the average surface area occupied by a single protein to be estimated. These results are consistent with crystallographic data for bovine serum albumin. The theoretical shift for adsorption of a single protein is found to be extremely sensitive to the target region, with adsorption in the most sensitive region varying as 1/R(5/2). Specific parameters for single protein or virus particle detection are predicted.

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Available from: Frank Vollmer, Sep 27, 2015
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• "While this approach has allowed for unprecedented sensing performance and Q factors [5] , it is limited in practice because any variation of the distance between the tapered fiber and resonator results not only in changes in the coupling efficiency but also in fluctuation of the resonance positions [6] [7] . Furthermore, using small optical resonators, which would enable improved performance, since the refractive index sensitivity is inversely proportional to the resonator diameter [8] , is challenging from a practical point of view using this approach. The alternative approach involves using resonators that contain a gain medium. "
##### Conference Paper: Whispering-Gallery Mode lasers for biosensing: a rationale for reducing the lasing threshold
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ABSTRACT: Whispering-gallery modes have been studied extensively for biosensing applications. Whilst the vast majority of work undertaken has focused on high Q factor resonators, with the main improvement being a reduction of the resonator size to improve sensitivity, we have chosen a different pathway by starting with resonators that exhibit extremely high refractive index sensitivity but low Q factor. A way forward to overcome this limitation is to introduce a gain medium and operate the resonator above its lasing threshold. This has been shown to result on average in a 5 fold increase in the Q factor. With the lasing threshold itself being dependent on the Q factor, amongst other parameters, the Q factor enhancement can be exploited to either reduce the lasing threshold or alternatively enable smaller resonators to be operated above their lasing threshold. As a demonstration we present a 10 μm diameter polystyrene microsphere lasing in aqueous solution for refractive index sensing applications, which to the best of our knowledge is the smallest polystyrene microsphere laser ever demonstrated in these conditions.
Laser Resonators, Microresonators, and Beam Control XVII, San Francisco, California, United States; 02/2015
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• "Any drift in the baseline data was corrected by subtracting a linear function from the data. The surface density (mass per unit area) was calculated from the resonance shift [34] [36] using the equation: "
##### Article: Combining an optical resonance biosensor with enzyme activity kinetics to understand protein adsorption and denaturation
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ABSTRACT: a b s t r a c t Understanding protein adsorption and resultant conformation changes on modified and unmodified silicon dioxide surfaces is a subject of keen interest in biosensors, microfluidic systems and for medical diagnostics. However, it has been proven difficult to investigate the kinetics of the adsorption process on these surfaces as well as understand the topic of the denaturation of proteins and its effect on enzyme activity. A highly sensitive optical whispering gallery mode (WGM) resonator was used to study a cat-alytic enzyme's adsorption processes on different silane modified glass substrates (plain glass control, DETA, 13F, and SiPEG). The WGM sensor was able to obtain high resolution kinetic data of glucose oxidase (GO) adsorption with sensitivity of adsorption better than that possible with SPR. The kinetic data, in combination with a functional assay of the enzyme activity, was used to test hypotheses on adsorption mechanisms. By fitting numerical models to the WGM sensograms for protein adsorption, and by con-firming numerical predictions of enzyme activity in a separate assay, we were able to identify mecha-nisms for GO adsorption on different alkylsilanes and infer information about the adsorption of protein on nanostructured surfaces.
Biomaterials 10/2014; 38. DOI:10.1016/j.biomaterials.2014.10.002 · 8.56 Impact Factor
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• "The most common types of WGM resonators used for sensing are fused silica microspheres [2], silica microtoroids [13] and polystyrene microbeads [18]. These devices share isotropic optical properties, high refractive indices compared to water and their amorphous nature. "
##### Article: High-Q MgF2 whispering gallery mode resonators for refractometric sensing in aqueous environment
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ABSTRACT: We present our experiments on refractometric sensing with ultrahigh-Q, crystalline, birefringent magnesium fluoride (MgF2) whispering gallery mode resonators. The difference to fused silica which is most commonly used for sensing experiments is the small refractive index of MgF2 which is very close to that of water. Compared to fused silica this leads to more than 50% longer evanescent fields and a 4.25 times larger sensitivity. Moreover the birefringence amplifies the sensitivity difference between TM and TE type modes which will enhance sensing experiments based on difference frequency measurements. We estimate the performance of our resonators and compare them with fused silica theoretically and present experimental data showing the interferometrically measured evanescent decay and the sensitivity of mm-sized MgF2 whispering gallery mode resonators immersed in water. They show reasonable agreement with the developed theory. Furthermore, we observe stable Q factors in water well above $1 \times 10^8$.
Optics Express 09/2014; DOI:10.1364/OE.22.030934 · 3.49 Impact Factor