Electrical transport in haemoglobin Langmuir–Blodgett films

Abstract

Electrical-transport properties of Langmuir–Blodgett multilayers of haemoglobin with evaporated aluminium and silver electrodes have been investigated by means of frequency-response analysis, voltage-ramp measurements and thermally stimulated depolarization. Interpretation is given in terms of dielectric relaxation and field-assisted hopping with injection of carriers from the electrodes.

The frequency-response-analyser measurements on the hydrated haemoglobin display two field-dependent dielectric-relaxation processes whose characteristic frequencies approach each other with increasing field. Above a critical field, the protein is transformed into a long-lived metastable state of characteristically different electrical properties. Removal of the water of hydration resulted in the disappearance of the low-frequency relaxation process.

In voltage-ramp measurements a pre-breakdown exponential current–voltage dependence was recorded, from which is deduced an inter-charge-centre distance of 2.5 nm between which carrier hopping occurs.

Thermally stimulated depolarization experiments yielded thermograms from which two transport processes, probably involving carriers of opposite sign, are deduced with activation energies 0.38 and 0.5 eV.