Microwave Debye relaxation analysis of dissolved proteins: Towards free-solution biosensing

Aqueous solutions of a variety of proteins at different concentrations are examined through microwave spectroscopy and compared to sodium chloride and polystyrene nanospheres. The complex permittivity is analysed in terms of the Debye model and the Stokes-Einstein-Debye relation in conjunction with the Maxwell-Garnett equation. According to Einstein's classical theory of viscosity with Brenner's adaptation [H. Brenner, Chem. Eng. Sci. 27, 1069 (1972)] for arbitrary solute shapes, the ratio of the alterations of static permittivity and relaxation time of low concentration solutions is found to be independent of concentration and determined by the molecular shape. Our results represent a route towards free-solution identification through molecular finger-printing. (C) 2011 American Institute of Physics. [doi:10.1063/1.3665413]