Page 101 - Organic Electronics in Sensors and Biotechnology

78 Cha pte r T w o

Because of their size- and shape-dependent properties 178–182 NPs in
the size range of 1 to 100 nm may show an intermediate behavior
between the atomic and the bulk material, although frequently they
are demonstrated to possess brand-new properties. A deep under-
standing of how the electron energy distribution changes from dis-
crete levels (for atoms and very small particles) to continuous bands
(for bulk metals) is at the basis of many useful applications. The high
relative abundance of surface atoms (and electronic states) that is
typical of nano-sized materials dramatically increases the impor-
tance of surface chemistry in tuning the macroscopic properties of
the nanomaterial. Au-NPs show surprising properties: while in the
bulk form, gold is commonly considered an inert material; when it is
finely dispersed, at the nanometer scale, it shows promising catalytic
158
properties that can be tuned by controlling the particle size and
structure. 183–185
Tens of different preparation strategies of Au-NPs have been
proposed, and their detailed discussion is beyond the scope of this
chapter. Many reviews on this topic and useful information can be
found in Ref. 158 and Refs. 185 to 197.
In the work described here, the synthesis of gold nanoparticles
was carried out according to an electrochemical process called the
sacrificial anode electrolysis (SAE), which was reported for the first time
by Reetz and Helbig in 1994. 198
The SAE synthesis was carried out in a three-electrode cell, filled
with an electrolytic solution composed of a quaternary ammonium
salt (tetra-octyl-ammonium chloride, TOAC) dissolved in tetrahy-
drofuran/acetonitrile mixed solution (see reference for experimental
198
details). In SAE processes, the ammonium salt acts both as support-
ing electrolyte and as NP stabilizer, thus leading to a stabilized col-
loidal suspension of core-shell NPs. The shell of such nanostructures
has been demonstrated to be composed of a monolayer of quaternary
ammonium moieties, and its thickness approximately corresponds to
the length of the alkyl chains. 199
SAE electrochemical route offers several advantages in terms of
reduction of the overall cost, high morphological and chemical stabil-
ity, as well as the possibility of easily tuning the nanoparticle size, the
NP diameter being correlated to the process parameters and particu-
larly to the applied current density.

2.4.3 Key Features of the Nanostructured Active Layers
Since particle size is expected to have a substantial impact on sensor
performance, a great deal of work was devoted to investigate the Au
core modulation by means of the Reetz and Helbig’s SAE approach.
An inverse correlation between the applied current density and the
198
NP core diameter is generally expected in galvanostatic SAE. In the