Researchers at Temple University have observed
and documented electron transfer reactions on an electrode surface at
the single molecule level for the first time, a discovery which could
have future relevance to areas such as molecular electronics,
electrochemistry, biology, catalysis, information storage, and solar
energy conversion.
The researchers have published their findings,
“Dynamics of Porphyrin Electron-Transfer Reactions at the
Electrode–Electrolyte Interface at the Molecular
Level,” in the international scientific journal, Angewandte
Chemie.
“The simplest chemical reactions are oxidation and
reduction,” says Eric Borguet, professor of chemistry at
Temple and the study’s main author. “Chemistry is
basically all about the transfer of electrons from one atom to another
or one molecule to another. Those reactions are called
‘redox’ reactions.”
According to Borguet, one important place where these
reactions occur is on an electrode surface. For example, metal
corrosion is essentially oxidation. Corrosion can sometimes be reversed
by reducing the oxides and reclaiming the metal.
“Most of our studies of oxidation and reduction
basically involve measuring the flow of electrons in and out of bulk
chemical systems,” he says. “We’ve never
really looked at this at the single molecule level, looking at it one
molecule at a time. And it wasn’t necessarily clear that we
could do that.”
As part of their research, Borguet and his collaborator were
looking on a metal electrode surface at porphyrins, an important class
of molecules that are involved in a number of biological processes, and
in fact, can act as a catalyst for these processes.
The Temple researchers used scanning tunneling microscopy, in
which a sharp metal tip scans the electrode surface and measures the
passage of electrons from the tip, through the molecules, to the metal
surface. They noted that the chemical state of the molecule changes the
ability of the electrons to pass from the metal tip to the electrode.
“We noticed that some of these molecules, under
certain conditions, appeared dark while others appeared
bright,” noted Borguet. “What we essentially
figured out was that the molecules change color and appear dark when we
apply a potential to the electrode that begins to oxidize, or
essentially pull out an electron from, the molecule. So now it seems
that we can see the difference between oxidized molecules - the dark
ones - and reduced molecules - the bright ones.”
Borguet says that by gaining a handle on the
molecules’ chemical state, researchers now have the ability
to identify oxidized and reduced molecules, and to track them
individually.
“As researchers, we can now ask questions such as
‘Do molecules oxidize one at a time or do entire domains or
areas on the surface oxidize together"’,” he says.
“Do they oxidize in pairs or in clusters" If one molecule
oxidizes, is it going to make the oxidation of a neighboring molecule
more or less likely" What is the timescale under which these processes
occur and what factors facilitate redox reactions"”
Borguet believes the Temple researchers are the first to
observe and understand this interfacial electron transfer process at
the single molecule level.
“We think if you look back in the literature and at
other peoples’ data there is some evidence for this, but I
don’t think they actually recognized that they were observing
this process,” he says.