Label-Free, Single-Molecule Detection with Optical Microcavities
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It is possible to perform single-molecule detection without first sensitizing the surface. This type of detection is shown in Fig. 4 where single-molecule detection experiments of Protein G and QSY-21 were successfully demonstrated without first sensitizing the surface. However this type of detection has limited applications in biology because specificity is often as important as sensitivity. This surface sensitization is distinct from a label because the antibody is attached to the surface of the microtoroid and not to the molecule of interest (e.g. antigen or streptavidin) whereas a label is attached to the molecule of interest.
Materials and methods are available as supporting material on Science Online.
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The majority of the optical field intensity (over 90%) resides within the toroidal boundary. This fact and the similar magnitudes of the thermal conductivity of water and silica (0.6 and 1.38 W/mK) make it possible to attribute all thermal tuning in ( 1 ) (and indeed the shape of the thermal plume) to the silica. The resulting error in the calculated tuning shift is estimated to be about 5%. We also note that the indicated shift is in steady state and the response time of the system is set by the thermal response (on the order of microseconds).
The actual form of the temperature plume in the vicinity of the molecule is likely complex and has been combined into a single empirical parameter ϵ. In contrasting a perfect point source of heat with a molecule this parameter captures the essential fact that the temperature profile is not singular at the source and instead rises steadily until reaching some radius on the order of the molecular size. This approximation is justified first because the thermal transport process itself rapidly smoothes nanoscale spatial variations created by molecular shape and second because the ensuing temperature field created by the molecular hot spot is long-range (i.e. l/r dependence). For this reason the tuning shift is only a weak function of the parameter ϵ. Along these lines it is the optical cross section σ as opposed to the physical radius ϵ that is of far greater relevance to the thermal-induced tuning. However the size of ϵ strongly suggests a maximum temperature in the vicinity of the molecule; therefore on physical grounds (i.e. molecules are not denaturing in the present work) we expect ϵ to be many times the actual physical size of the molecule.
A cavity linewidth measurement is the minimum sensitivity measurement that can be performed without requiring additional equipment or employing more complex techniques (such as locking onto the resonant wavelength). The primary assumption in this measurement is that the resonant wavelength shifts an entire linewidth upon the binding of a molecule. For example a cavity with a Q of 100 million operating at 680 nm would need to shift 6.8 × 10 –6 nm. As mentioned this limit is not fundamental but can be viewed as what is detectable by methods that do not enable detection of sub-linewidth shifts. Thus this limit is the most desirable one to consider when balancing detection limits and experimental complexity.
The absorption cross sections of the IL-2 protein G streptavidin and the two different IL-2 antibodies were determined with an ultraviolet-visible spectrophotometer (Shimadzu BioSpec-1601 San Diego CA USA) whereas the absorption cross section of the Cy5-labeled antibody and the QSY-21 quencher were determined from absorption spectra available from the manufacturer (Invitrogen Carlsbad CA USA). The absorption spectra of the Cy5-labeled antibody and of the bleached Cy5-labeled antibody were verified with the spectrophotometer.
We thank O. Painter for useful discussions B. Min for FEMLAB simulations N. Pierce for spectrophotometer measurements and D. Armani for microtoroid fabrication. A.M.A. is supported by the Clare Boothe Luce Postdoctoral Fellowship. This work was supported by the Defense Advanced Research Projects Agency's Center for OptoFluidic Integration and the Biological Imaging Center of the Beckman Institute at the California Institute of Technology.