by Professor Paolo Facci
Biomolecular Electronics is a branch of nano-science and technology dealing
with the investigation and the technological exploitation of electron transport
properties in special classes of biomolecules. Albeit it deals with molecules
that can donate to or receive electrons, biomolecular electronics has nothing to
do with the molecular bases ruling the generation and propagation of electrical
signals in neural cells, i.e. the action potential. This very important example
of electrical activity pertaining biological matter, depends in fact on ionic
currents and involves a well known interplay between protein channels varying
their permeability to ions in response to certain stimuli and the dielectric
properties of the axon membrane they are embedded in.
Biomolecular electronics, instead, deals with biomolecules which are able to
transfer electrons between molecular partners as a result of redox
reactions1. These molecules can be redox
metalloproteins, proteins bearing redox moieties (e.g. disulfide bonds) or redox
cofactors (e.g quinone-based molecules).
It is believed that about 25-30% of the entire proteome is composed by
metalloproteins; therefore understanding their behavior, possibly at the single
molecule level, represents an extremely relevant enterprise. Furthermore, the
physiological functional activity of redox metalloproteins of shuttling
electrons between redox partners has been optimized by more than 4 billions
years natural evolution and, as such, it results to be extremely effective and
appealing for applications.
The scientific activity on biomolecular electronics dates back to the early
nineties and has been triggered by the advent of scanning probe microscopes,
especially the scanning tunnelling microscope (STM).
Nowadays, the experimental tool of choice for the investigation of electron
transport in redox metalloproteins at the single molecule level is an evolution
of STM that can be operated in a four-electrode electrochemical cell: the
electrochemical scanning tunnelling microscope (ECSTM)2. It features the possibility of measuring tunnelling
currents in physiologic-like, salty aqueous solution through molecular
adsorbates on atomically flat, conductive substrates. Therefore, at variance
with the well known STM, it makes use of insulated tips and of a bipotentiostat
that can drive independently the potential of the metal substrate and the tip,
thus preventing faradic currents to take place at both working electrodes (tip
and substrate). The result is a microscope which provides spectroscopic-like
images of molecular adsorbates. A generic ECSTM setup is depicted in figure
Figure 1. Scheme of an
ECSTM. The inset shows an insulated ECSTM
A prototypical redox metalloprotein that has been widely investigated by
ECSTM is azurin from Pseudomonas aeruginosa, a molecule belonging to the
family of the blue copper proteins, as its intense blue colour reveals. This
redox protein shuttles electrons between proteinaceous partners by changing
reversibly the oxidation state of a copper atom in its active site
(Cu2+⇔Cu1+). Furthermore, its structure is characterized
by an exposed disulfide bridge (Cys3-Cys26) which turns out to be extremely
useful for anchoring the molecule on an atomically flat gold substrate, figure
Figure 2. The 3d
structure of azurin from Pseudomonas aeruginosa. Structural information
from PDB file 1E5Y.
ECSTM investigation of azurin reveals firstly a substrate potential dependent
contrast in constant current images3; the appearance
of molecular features in the images results from the proper alignment of the
Fermi levels of substrate and tip, as determined by the bipotentiostatic
control, with respect to the molecular redox levels (the "density of oxidized or
reduced levels"). Furthermore, ECSTM studies show the possibility of
distinguishing between molecules, identical in their structure but bearing
different metal ions in their active site (eg. Cu vs Zn)4. This possibility is enabled by the radically different
redox potential of the two ions. From an applied standpoint, the reported
behavior qualifies azurin as a molecular electronic switch and enables solid
state electronic applications5.
Direct access to tunnelling current enables also a detailed analysis of the
electron transport mechanisms involved in the phenomenon. Implementing a "change
of perspective" one can immobilize azurin on a gold ECSTM tip, achieving the
advantages of i) avoiding to track the molecules adsorbed on the substrate; ii)
measuring directly the tunnelling current by switching the feedback system off,
once the current set point has been established, while sweeping tip voltage.
Under these conditions, it is possible to extract data that allow one to
elucidate the mechanism underlying electron transport as a two step electron
transfer with partial molecular relaxation6. A this
point it is worth noting that the realized setup configures a single protein
transistor with an electrochemical gate7.
Indeed, it is physically equivalent to a single particle transistor typical of
nanoelectronics: in the latter the gating is provided by capacitive coupling
between a (back) gate and the electronic level of the dot, whereas, in the
former, there is a sort of "diffused gating" brought about by the
electrochemical control of tip and substrate Fermi levels.
The demonstration of a single metalloprotein wet biotransistor, as well as
similar findings on other redox molecules8, can in
principle pave the way to the exploitation of the switching behaviour of
suitable biomolecules for implementing nanoelectronic devices operating in a wet
In spite this configures a suggestive scenario, it is questionable whether
such kind of approach will be ever competitive with solid state nanoelectronics.
Indeed, we believe its relevance in applications should be sought in a different
context. The novelty of the described findings stands in that they establish the
concept of "electrically controlled biological reactions". This concept
encompasses not only redox reactions and is not limited to redox proteins;
rather, it includes also electrically-induced conformational changes in charged
biomolecules and extends to other kinds of proteins such as enzymes, antibodies,
redox cofactors as they are involved in many diverse biological phenomena. This
quite suggestive perspective aims to gather the most advanced technology that
Mankind have ever developed (Electronics) with the most sophisticated
organization level of the matter that Nature has so far produced: the biological
Exemplifications of the aforementioned concept span from modulation of the
binding affinity of antibodies for the corresponding antigens by electrical
induced conformational changes, to modulation of gene expression profile via
tuning the conformation of redox enzymes involved in the control of gene
expression. All of these examples are currently object of intense investigation
and represent the most proming future trends in Biomolecular Electronics.
- A. Alessandrini, P. Facci "Metalloprotein Electronics" in CRC Handbook in
Nano- and Molecular Electronics Ed. S. Lyshevsky, Boca Raton, 14, 1-47,
- Andrea Alessandrini, & Paolo Facci "Electrochemically Assisted Scanning
Probe Microscopy: a Powerful Tool in Nano(Bio)Science" in Biophysical Aspects of
Nanotechnology, V. Erokhin, M.K. Ram, O. Yavuz Eds., Elsevier, 2007.
- P. Facci, D. Alliata, S. Cannistraro "Potential-Induced Resonant Tunneling
through a Redox Metalloprotein Probed by Electrochemical Scanning Probe
Microscopy", Ultramicroscopy, 89(4), 291-298, (2001).
- A. Alessandrini, M. Gerunda, G. Canters, M. Ph. Verbeet, P. Facci "Electron
Tunneling through Azurin is Mediated by the Active Site Cu Ion", Chem. Phys.
Lett., 376/5-6 pp. 625-630, (2003).
- R. Rinaldi, A. Biasco, G. Maruccio, R. Cingolani, D. Alliata, L. Andolfi, P.
Facci, F. De Rienzo, R. Di Felice, E. Molinari "Solid-State Molecular Rectifier
Based on Self-Organized Metalloproteins", Adv. Mater., 14, 1449-1453, (2002); R.
Rinaldi, A Biasco, G. Maruccio, R. Cingolani, D. Alliata, L. Andolfi, P. Facci,
F. De Rienzo, R. Di Felice, E. Molinari, M. Verbeet and G. Canters, "Electronic
rectification in protein devices", Appl. Phys. Lett., 82, 472 (2003).
- A. Alessandrini, S. Corni, P. Facci "Unraveling single metalloprotein
electron transfer by scanning probe techniques" Phys. Chem. Chem. Phys., 8,
- A. Alessandrini, M. Salerno, S. Frabboni, P. Facci "Single-metalloprotein
wet biotransistor" Appl. Phys. Lett., 86, 133902, (2005).
- P. Petrangolini, A. Alessandrini, L. Berti, P. Facci "An Electrochemical
Scanning Tunneling Microscopy study of 2-(6-mercaptoalkyl)hydroquinone molecules
on Au (111)", J. Am. Chem. Soc., 2010, 132, 7445–7453.
Copyright AZoNanoo.com, Professor Paolo Facci