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spheroid
optical fiber
100µm
(b)
tunable DFBlaser diode1340 nm
photodetector
sample cell
thermocouple optical fiber
spheroid
4 mm
input coupler
(a)
(c)
L = 30
here:
a
light orbit in the geo-metrical optics limit
light orbit as a waveoptics illustration
Whispering Gallery Modein a Dielectric Microsphere
Protein Detection by Optical Shift of a Resonant MicrosphereFrank Vollmer1, Maziar Khoshsima2, Dieter Braun1, Iwao Teraoka2, Albert Libchaber1, Stephen Arnold2
1Center for Studies in Physics and Biology, The Rockefeller University, New York, NY 10021email: [email protected]
2Microparticle Photophysics Lab, Polytechnic University, Brooklyn, NY 11201www.poly.edu/microparticle
We present a novel optical biosensor with unprecedented sensitivity for detectionof unlabeled molecules. Our device uses optical resonances in a dielectric micro-particle (Whispering Gallery Modes) as the physical transducing mechanism.The resonances are excited by evanescent coupling to an eroded optical fiber anddetected as dips in the light intensity transmitted through the fiber at differentwavelengths. Binding of proteins on the microparticle surface is measured from ashift in resonance wavelength. We demonstrate the sensitivity of our device bymeasuring adsorption of bovine serum albumin and we show its use as a biosen-sor by detecting streptavidin binding to biotin.
An optical resonance (or WhisperingGallery Mode, WGM) occurs whencircumnavigating light, trapped bytotal internal reflection, constructivelyinterfers with itself. Each resonancecan be characterized by the number ofwavelength L within an orbit.
(a) A spheroidal microparticle fabri-cated by melting the tip of a fiber isplaced in contact with the erodedpart of a single mode optical fiber.
The output wavelength of a distrib-uted feedback laser diode is tunedby its laser current. Resonances aredetected as dips in the transmittedintensity at different wavelength.
Buffer solution is retained in thesample cell due to surface tension, athermocouple measures the solutiontemperature.
(b) Picture of the spheroid coupledto the optical fiber.
(c) Resonances (WGMs) versuswavelength.
Proteins binding to the micro-sphere surface increase itseffective radius.
A given optical resonance willshift its resonance wavelengthto accomodate the larger cir-cumverence.
BSA solution is injected into thesample cell to a final concentrationof 1.5 µM (PBS, pH 7.4).
A Labview program traces thewavelength shift of one given opti-cal resonance.
The initial negative shift is due tothermal contraction of the spheroid.The overall positive shift is entirelydue to BSA adsorption onto the ami-nosilanized microparticle.
The inset shows the binding iso-therm.
Streptavidin binding to previously surface immobilized BSA-biotin. The initialshift is due to adsorption of BSA-biotin. After injecting Streptavidin, a secondshift is observed.
λ ... wavelengthδλ ... wavelength shiftαex ... excess polarizability of the bound proteinσs ... surface density of the bound proteinn1, n2 ... refractive indices of the sphere and the buffer solution, respectivelyR ... orbital radiusε0 ... vacuum permittivity
FV is supported by a fellowship of the Boehringer Ingelheim Fonds, DB by aFellowship of the Deutsche Forschungsgemeinschaft, research at the Polytech-nic is supported by a National Science Foundation grant BES-0119273.
In the asymptotic limit for the number of wavelength within an orbit L >> 1,the resonance wavelength shift δλ is caused by the energy needed to polarize aprotein molecule. The protein needs only be characterized by its excess polariz-ability. The resulting shift is
WGMs of a microsphere can be excited by evanescent cou-pling to the core of an optical fiber. Resonances are detectedin the scattered light from the microsphere or by dips in thetransmitted intensity through the fiber end.