A standard synthesis procedure consists of blending HDA
(180 mg) into an aqueous solution (10 ml) of anhydrous CuCl2
(30 mg) and glucose (50 mg) using an ultrasonic probe, giving
a light blue emulsion (Fig. 1A). The emulsion would then be
heated at 102 °C for 6 h without stirring. The Cu NWs formed
would settle to the bottom of the vial, while the solution
turned dark brown. Average yields of 40–50% were obtained.
Fig. 1B shows an SEM image of tangled Cu NWs having an
average diameter of 40 nm. Realistically, the NWs were too
long and too entangled to be reliably determined.
To better understand the roles of HDA and glucose, variable
amounts of both reagents were employed while keeping all
other parameters unchanged. Vials containing different
amounts of HDA (0–0.3 g) and glucose (0–0.1 g) are indicated
by the 2-by-2 matrix in Fig. 1C. Those containing Cu NWs after
growth are boxed in red. We first noticed that when either
glucose or HDA was absent, the solution remained blue and
no metallic Cu precipitates (Cu0) were formed. Furthermore,
Cu NWs were only produced when both glucose and HDA were
added in sufficient amounts. While it is known that glucose
itself is able to reduce Cu2+ ions to Cu+ in Benedict’s and Fehling’s
tests, it does not appear to be able to reduce Cu2+ to Cu0.
Evidently, glucose alone was not functioning as the reducing
agent as was originally proposed. Indeed, previous reports of
Cu0 formation with HDA took place at significantly higher
temperatures (>180 °C).13
In order to explain the above observations, we postulate
that the Maillard reaction is responsible for the reduction of
Cu2+. This reaction, brought about by the reaction of an amine
with a reducing sugar, is responsible for non-enzymatic browning
and the characteristic aromas in cooked meats and baked
bread. A general reaction scheme is shown in Fig. 2A. Briefly,
the Maillard reaction starts with a nucleophilic interaction of
the amine nitrogen with the carbonyl in glucose. A general
amine is denoted by RNH2, where R represents [–(CH2)15CH3]
in HDA. Condensation subsequently occurs, and after Amadori
rearrangement, the glucosamine is converted to the Amadori