  Diffusion of tracer dye molecules in water confined to nanoscale is an
important subject with a direct bearing on many technological applications. It
is not yet clear however, if the dynamics of water in hydrophilic as well as
hydrophobic nanochannels remains bulk-like. Here, we present diffusion
measurement of a fluorescent dye molecule in water confined to nanoscale
between two hydrophilic surfaces whose separation can be controlled with a
precision of less than a nm. We observe that the fluorescence intensities
correlate over a fast($\sim$ 30 $\mu$s) and slow ($\sim$ 1000 $\mu$s) time
components. The slow timescale is due to adsorption of fluorophores to the
confining walls and it disappears in presence of 1 M salt. The fast component
is attributed to diffusion of dye molecules in the gap and is found to be
bulk-like for sub-10 nm separations and indicates that viscosity of water under
confinement remains unaltered up to confinement gap as small as $\sim$ 5 nm.
Our findings contradict some of the recent measurements of diffusion under
nanoconfinement, however they are consistent with many estimates of
self-diffusion using molecular dynamics simulations and measurements using
neutron scattering experiments.
