Optical microspherical resonators for biomedical sensing

Abstract

Optical resonators play an ubiquitous role in modern optics. A particular class of optical resonators is constituted by spherical dielectric structures, where optical rays are total internal reflected. Due to minimal reflection losses and to potentially very low material absorption, these guided modes, known as whispering gallery modes, can confer the resonator an exceptionally high quality factor Q, leading to high energy density, narrow resonant-wavelength lines and a lengthy cavity ringdown. These attractive characteristics make these miniaturized optical resonators especially suited as laser cavities and resonant filters, but also as very sensitive sensors. First, a brief analysis is presented of the characteristics of microspherical resonators, of their fabrication methods, and of the light coupling techniques. Then, we attempt to overview some of the recent advances in the development of microspherical biosensors, underlining a number of important applications in the biomedical field.

Keywords: biomedical sensors; microspheres; optical resonators; whispering gallery modes.

Figure 1.

(a) Total internal reflection for the light rays in correspondence of the surface of the microsphere; (b) Spherical coordinate system and mode propagation along the equatorial plane of the sphere.

Figure 2.

Spherical mode fields for the fundamental (n = 1) WGM.

Figure 3.

WGM resonator detection system (bottom); resonance shift after analyte binding to the surface of the microsphere and sensorgram showing the resonance signal change with time (top-left).

Figure 4.

Functionalization of a WGM sensor: (a) a silane agent is used. In this case, the microsphere surface is previously functionalized with primary amine groups. In a second step, the receptors can be covalently bound to these groups; (b) after functionalization with Eudragit® the carboxyl groups (COOH) are activated with EDC/NHS chemistry.

Figure 5.

Top: 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: main analytes (antigens, biotin(ylated) proteins, aminoacids).

Figure 6.

A schematic diagram of the experimental arrangement employing a bi-conical tapered fiber coupler.

Figure 7.

(a) Size of the microspheres produced at the tip of a standard 125 μm telecom fiber, as a function of the arc shots in a commercial fiber fusion splicer. (b) Optical image of a microsphere with a diameter of about 250 μm. In the background (out of focus) one can see the coupling bi-conical tapered fiber.

Figure 8.

Experimental set-up based on a disposable cuvette of 1 cm³ volume, temperature controlled and magnetically stirred. Reprinted with permission from [57] © 2005, American Institute of Physics.

Figure 9.

(a) Schematic of the microsphere and fluidic cell (side view); (b) top view of the fiber prism and of (c) the fused silica microsphere placed in contact with the fiber prism. Reprinted with permission from [38] © 2005, American Institute of Physics.

Figure 10.

Integrated WGMR and microfluidic system for biosensing. Reprinted with permission from [59] © 2007, American Institute of Physics.

Figure 11.

Top: Optical image of two microspheres coupled to a taper, showing two resonances orbiting inside the spheres. Bottom: Single nucleotide mismatch detection; left: time traces of the two microspheres, the perfect match sequence gives a signal 10 times larger; right: difference signal, allowing to achieve SNR = 54. Reprinted with permission from [47] © 2003, Elsevier.

Figure 12.

Sensorgrams for BSA cleavage at various concentrations of trypsin. Inset: exponential fit decay for the 0.1 mg/mL curve in the time range of 400–2,000 s; the decay constant is 950 s, and the dashed line indicates the baseline (−32.6 pm). Reproduced with permission from [48] ©2005 American Scientific Publishers.

Figure 13.

WGM spectral shift vs. thrombin concentration. Solid line: theoretical fit; dashed line: WGM shift for maximal thrombin binding. Reproduced with permission from [22] © 2006 MDPI.

Figure 14.

Comparison between Phix174 (blue line) and MS2 (red line) detection experiments on the same microspherical WGMR [21]—Reproduced by permission of The Royal Society of Chemistry.

Figure 15.

(A) Sketch of a WGM confined at the equator of the microsphere. N_cav, n_sol, n_w, n_int are the refractive indexes of cavity, surrounding aqueous solution, the bacteria cell wall and interior (protoplasm) of the bacteria; (B) Fluorescent image of green fluorescent protein (GFP)-labeled bacteria bound to the surface. Only few bacteria (arrows) are bound at their tip; Right: Shift and broadening of the linewidth of the resonance due to adsorption of E.coli. Reproduced with permission from [20] © (2007), Optical Society of America.

Figure 16.

Fluorescent spectrum of the TMR-dyed microsphere. Right: WGM emission spectra of the same microsphere acquired pre- and post-complementary strand treatment. The peaks get red shifted following 90 s exposure to cDNA probe (dashed line). Reproduced with permission from [49]. ©Wiley-VCH Verlag GmbH & Co. KGaA (2007).