Time-of-flight studies of charge-carrier mobility in hydrated bovine serum albumin

Abstract

Charge-carrier mobilities are reported for compressed samples of bovine serum albumin as a function of hydration. A novel time-of-flight technique is employed in which positive or negative ions are instantaneously deposited on the free surface of the samples. Clear evidence for dispersive transport is obtained. The macroscopic mobility, µ, which may be deduced appears to be independent of sign of carrier, to vary inversely with field and to increase with hydration according to the law µ=µ_d exp(βm) where µ_d is the mobility in the dry state (ca. 10^–11 m² V^–1 S^–1), m(wt %) is the level of hydration and the coefficient β was ca. 1.3 at a field of 10⁴ V m^–1. At hydration levels of 18 wt %, the macroscopic mobility exceeds 10^–4 m² V^–1 S^–1 and it is concluded that electronic transport is being observed. A model based on transport in delocalised bands and localised bands tails is developed in which the effects of hydration can be introduced according to the polarization model of Rosenberg.