Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Method for the electrical characterisation at the nanoscale of biological samples by Atomic Force Microscopy

With the Atomic Force Microscope set up developed in the project we have developed a method to characterize the electrical properties of biological samples with nanoscale spatial resolution. The method implemented uses dynamic or jumping mode to image the sample topography and then contact mode approach to perform the electrical measurements.

We have performed a number of DC and AC measurements on a number of biological samples, and in particular on a single nanosomes containing olfactory receptors.

We have verified the following properties:
- The nanosome behaves as an insulator with respect to DC electronic transport, with a resistance far above hundreds of GigaOhms for the AFM tip-nanosome-gold substrate system. This result has been further confirmed with DC measurements performed on bacteriorhodopsin purple membrane only 5 nm thick. In this system only direct tunnel electric current has been observed with an AFM tip-purple membrane-gold substrate resistance of hundreds of GigaOhm.

- The nanosome displays an almost pure capacitive behaviour for AC electronic transport between 100Hz up to 300kHz. In the nanosomes studied no significant frequency dispersion on the electrical properties has been observed. However, on bigger nanosomes this result may need further investigation, since the water enclosed inside the nanosome could produce frequency dispersion phenomena.

-The relative dielectric constant of the whole nanosome has been estimated to be around 2. This value has been extracted from a capacitance distance curve performed on a single nanosome and comparing it with a capacitance distance curve performed on the gold substrate and with the help of a theoretical model involving realistic formulas for the capacitance of the AFM probe-substrate system.

The fact that the measurement has been performed on a single nanosome can be verified by a post measurement imaging of the nanosome, where the tip indentation can be clearly identified.

Finally, some attempts to perform an electrical map of the electrical properties of nanosomes were essayed in contact mode by using a softer cantilever (0.2 N/m). Even though capacitance images were efficiently recorded by the measuring set up and results are in qualitative agreement with capacitance-distance curve measurements, some concerns on the sample integrity remained after the images were performed.

Reported by

Marti i Franques, 1
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