Biomedical Engineering Reference
In-Depth Information
If an artifi cial molecular motor has to work by inputs of chemical energy, it will
need addition of fresh reactants (“fuel”) at any step of its working cycle, with the
concomitant formation of waste products. Accumulation of waste products, however,
compromises the operation of the device unless they are removed from the system,
as it happens in our body as well as in macroscopic internal combustion engines. The
need to remove waste products introduces noticeable limitations in the design and
construction of artifi cial molecular motors based on chemical fuel inputs.
Chemists have since long known that photochemical and electrochemical energy
inputs can cause the occurrence of
endergonic
and
reversible
reactions. In recent
years, the outstanding progress made by supramolecular photochemistry (Balzani
and Scandola
1991
; Balzani
2003
) and electrochemistry (Kaifer and Gómez-Kaifer
1999
; Marcaccio et al.
2004
) has, thus, led to the design and construction of molec-
ular machines powered by light or electrical energy, which work without the for-
mation of waste products. In the case of photoexcitation, the commonly used
endergonic and reversible reactions are isomerization and redox processes. In the
case of electrochemical energy inputs, the induced endergonic and reversible reac-
tions are, of course, heterogeneous electron transfer processes. Photochemical and
electrochemical techniques offer further advantages, since lasers provide the
opportunity of working in very small space and very short time domains, and elec-
trodes represent one of the best ways to interface molecular-level systems with the
macroscopic world.
A very important feature of molecular motors, related to points (1) and (4), is
their capability to exhibit an
autonomous
behavior; that is, to keep operating, in a
constant environment, as long as the energy source is available. Natural motors are
autonomous, but the vast majority of the artifi cial molecular motors reported so far
are
not autonomous
since, after the mechanical movement induced by a given input,
they need another opposite input to reset.
Needless to say, the operation of a molecular machine is accompanied by partial
conversion of free energy into heat, regardless of the chemical, photochemical, and
electrochemical nature of the energy input.
1.2.2
Other Features
The motions performed by the component parts of a molecular motor [point (2)]
may imply rotations around covalent bonds or the making and breaking of inter-
component noncovalent bonds, as we shall see later on.
In order to control and monitor the device operation [point (3)], the electronic
and/or nuclear rearrangements of the component parts should cause readable
changes in some chemical or physical property of the system. In this regard, photo-
chemical and electrochemical techniques are very useful since both photons and
electrons can play the dual role of
writing
(i.e., causing a change in the system) and
reading
(i.e., reporting the state of the system). Luminescence spectroscopy, in par-
ticular, is a valuable technique since it is easily accessible and offers good sensitiv-
ity and selectivity, along with the possibility of time-resolved studies.
