The SPR phenomenon can be
described as a resonance that is a coherent oscillation of
the surface conduction electrons, which are excited by electromagnetic
radiation [33]. When the conducting electrons
on the NP are exposed to certain frequencies of the electromagnetic
spectrum, they undergo a collective oscillation/
excitation that is induced by the electric field of the incident
light [34]. When the frequency of the incident light satisfies
the resonance conditions of the noble-metal NP, SPR
occurs, together with absorption of light [35]. The characteristic
SPR absorption corresponds to the visible range
in the case of gold, silver and copper NP [36]. Moreover,
when the light–matter interaction involves particles that
are much smaller than the incident wavelength, the plasmon
oscillates locally around the NP and, in this case,
the frequency is known as localized surface plasmon resonance
(LSPR) [37]. Additionally, when noble-metal NP
are embedded in a titania matrix, the UV-photo-excited
electrons arising from that TiO2 matrix are able to reduce
such NP to their metallic state, hence giving them intense
colours as a result of the afore-mentioned LSPR. This leads
to a phenomenon known as photochromism, of which the
process was described on a Ag-loaded TiO2 film by Ohko
et al. [38–40].