in PBS and 1:10 in MilliQ water lead to emission at 533 nm and
538 nm respectively, observed as a spectral shoulder in the first case
and as the main peak in the latter (Table 4). This wavelength range is
consistent with emission of curcumin in organic H-bonding solvents
(Nardo et al., 2008) and could indicate that the curcumin-NADES
network is weakened upon dilution in aqueous medium. There seems
to be a difference in emission spectrum and thereby the network structure
between MC3 samples diluted 1:10 and 1:50 in MilliQ water.
According to previous investigations on curcumin in organic
solvents instauration of intermolecular H-bonds results in decay mechanisms
involving delivery of excitation energy (or intermolecular
charge transfer) to the solvent molecules. A reduction in fluorescence
signal is then observed (Nardo et al., 2008, 2012). In the present work,
a linear increase (r N 0.99) in fluorescence signal was recorded upon dilution
of curcumin-MC3 in ethanol (Fig. 5). This increase was followed
by a linear (r N 0.98) blue-shift in the curcumin fluorescence emission
maximum up to 50% (v/v) ethanol (Table 4). These results indicate a
gradual weakening of the intermolecular interactions until the network
apparently breaks down around 50–60% (v/v) ethanol. Dilution of
curcumin-GS in ethanol resulted in an increase in curcumin fluorescence
intensity and a slight blue-shift in emission maximum up to
10–20% (v/v) ethanol followed by a nearly constant fluorescence up to
50% (v/v) ethanol (Fig. 5). This observation indicates that the intermolecular
network formed between curcumin and GS was ruptured by a
rather low concentration of ethanol (b20% (v/v)). The fact that
curcumin dissolved better in MC3 supports the theory about a stronger
curcumin-NADES interaction in MC3 than in GS.