Modeling scattered intensity from microspheres in evanescent field

The technique of single particle Total Internal Reflection Microscopy (TIRM) has been used to study the scattering intensity from levitated microspheres. TIRM can be used to monitor the separation between microscopic spheres immersed in liquid (water in our case) and a surface with nm resolution. In the technique, microspheres scatter light when the evanescent waves are incident upon them. The intensity of the scattered light is directly related to the height above the surface and allows determination of the height. From the separation distance histograms, the interaction between the microsphere and interface may be characterized with a force resolution in the range of 0.01 picoNewtons. Such a system can be applied to the measurement of biomolecular interactions biomolecules attached to the microsphere and the surface. The intensity and scattering pattern of this light has been modeled using a modified Mie theory which accounts for the evanescent nature of the incident light. Diffusing Colloidal Probe Microscopy (DCPM) is an extension of the TIRM technique that simultaneously monitors multiple microsphere probes. The use of multiple probes introduces the issue of probe polydispersity. When measured at the surface, a variation in scattered light intensity of nearly one order of magnitude has been observed from a purchased microsphere sample. Thus the polydisperse collection of microspheres adds significant complexity to the scattered light signal. It is hypothesized that the dependence of the total scattered light intensity on microsphere size accounts for the scattered intensity distribution in a polydisperse microsphere sample. Understanding this variation in the scattered light with microsphere size will allow improved characterization of the microsphere/surface separation. Additionally, larger microspheres have the ability to resonantly confine light and produce spectrally narrow Whispering Gallery Modes (WGMs). It is hypothesized that WGMs may be excited in microspheres with the DCPM system. These modes may be used as a refractometric biosensor with high sensitivity to local refractive index changes on the surface of the microsphere. This research involves modeling scattered intensity distributions for polydispersed collections of microspheres based on modified Mie theory. The theoretical results are compared to experimentally obtained results and found to qualitatively explain the scattered light intensity distribution in a multiple probe DCPM system. This is an important result suggesting that microsphere size variation plays a major role in determining the distribution of scattered intensity in multiple microsphere probe systems. This work also suggests that it may be possible to excite such WGMs in a DCPM system. The introduction of WGMs would enable refractometric biosensing in such evanescent mode systems.