Nathan Clack

	Nathan Clack
	Class of 2001 Graduated in 2007
	Undergraduate Institution: University of Texas at Austin Major: B.S. Biochemistry and B.S. Math Origin: Houston, TX
	Lab: Jay Groves  Location: 109 Lewis

We have imaged surface electrostatic charge patterns by compiling the three dimensional positions of a population of passively diffusing colloidal probe particles. The sedimented particles adopt an equilibrium height distribution above the substrate based on a balance between gravitational and local electrostatic forces. Using dual wavelength reflection interference contrast microscopy, particle heights can be measured with ~1nm precision, based on the phase of the interferogram, while lateral positions can be determined to ~10 nm. Hundreds of particles can be tracked simultaneously as they passively sample the substrate (Langmuir 2005). In the present implementation, surface charge densities are measured to an accuracy of 200 e/nm2 over an absolute range of 500 - 2000 e/nm², with a spatial resolution of 2 um. Although the readout is entirely optical, the resolution of this imaging technique is not diffraction-limited. We have also applied this to the development of low-power surface-based analytical methods (manuscript Clack, Salaita and Groves 2007).