2 research outputs found
Measuring Proton Currents of Bioinspired Materials with Metallic Contacts
Charge
transfer at the interface between the active layer and the contact
is essential in any device. Transfer of electronic charges across
the contact/active layer interface with metal contacts is well-understood.
To this end, noble metals, such as gold or platinum, are widely used.
With these contacts, ionic currents (especially protonic) are often
neglected because ions and protons do not transfer across the interface
between the contact and the active layer. Palladium hydride contacts
have emerged as good contacts to measure proton currents because of
a reversible redox reaction at the interface and subsequent absorption/desorption
of H into palladium, translating the proton flow reaching the interface
into an electron flow at the outer circuit. Here, we demonstrate that
gold and palladium contacts also collect proton currents, especially
under high relative humidity conditions because of electrochemical
reactions at the interface. A marked kinetic isotope effect, which
is a signature of proton currents, is observed with gold and palladium
contacts, indicating both bulk and contact processes involving proton
transfer. These phenomena are attributed to electrochemical processes
involving water splitting at the interface. In addition to promoting
charge transfer at the interface, these interfacial electrochemical
processes inject charge carriers into the active layer and hence can
also modulate the bulk resistivity of the materials, as was found
for the studied peptide fibril films. We conclude that proton currents
may not be neglected a priori when performing electronic measurements
on biological and bioinspired materials with gold and palladium contacts
under high humidity conditions
Protonic and Electronic Transport in Hydrated Thin Films of the Pigment Eumelanin
The
electrical properties of eumelanin, a ubiquitous natural pigment,
have fascinated scientists since the late 1960s. For several decades,
the hydration-dependent electrical properties of eumelanin have mainly
been interpreted within the amorphous semiconductor model. Recent
works undermined this paradigm. Here we study protonic and electronic
charge carrier transport in hydrated eumelanin in thin film form.
Thin films are ideal candidates for these studies since they are readily
accessible to chemical and morphological characterization and potentially
amenable to device applications. Current–voltage (<i>I</i>-<i>V</i>) measurements, transient current measurements
with proton-transparent electrodes, and electrochemical impedance
spectroscopy (EIS) measurements are reported and correlated with the
results of the chemical characterization of the films, performed by
X-ray photoelectron spectroscopy. We show that the electrical response
of hydrated eumelanin films is dominated by ionic conduction (10<sup>–4</sup>–10<sup>–3</sup> S cm<sup>–1</sup>), largely attributable to protons, and electrochemical processes.
To propose an explanation for the electrical response of hydrated
eumelanin films as observed by EIS and <i>I</i>-<i>V</i>, we considered the interplay of proton migration, redox
processes, and electronic transport. These new insights improve the
current understanding of the charge carrier transport properties of
eumelanin opening the possibility to assess the potential of eumelanin
for organic bioelectronic applications, e.g. protonic devices and
implantable electrodes, and to advance the knowledge on the functions
of eumelanin in biological systems