  The optical properties of nanofluids are peculiar and interesting for a
variety of applications. Among them, the high light extinction coefficient of
nanofluids can be useful in linear parabolic concentrating solar systems, while
their properties under high light irradiation intensities can be exploited for
direct solar steam generation. The optical characterization of colloids,
including the study of non-linear optical properties, is thus a needed step to
design the use of such novel materials for solar energy exploitation. In this
work, we analysed two different types of nanofluids, consisting of
polycarboxylate chemically modified graphene nanoplatelets (P-GnP) and sulfonic
acid-functionalized graphene nanoplatelets (S-GnP) dispersed in water, at three
concentrations from 0.005 wt% to 0.05 wt%. Moderately stable nanofluids were
achieved with favourable light extinction properties, as well as a non-linear
optical behaviour under high input solar intensities.
