  The manipulation of microparticles using optical forces has led to many
applications in the life and physical sciences. To extend optical trapping
towards the nano-regime, in this work we demonstrate trapping of single
nanoparticles in arrays of plasmonic coaxial nano-apertures with various inner
disk configurations and theoretically estimate the associated forces. A high
normalised experimental trap stiffness of 3.50fN/nm/mW for 20nm polystyrene
particles is observed for an optimum design of 149nm for the nanodisk diameter
at a trapping wavelength of 980nm. Theoretical simulations are used to
interpret the enhancement of the observed trap stiffness. A quick particle
trapping time of less than 8sec is obtained at a concentration of
14$\times$10$^{11}$ particles/ml with low incident laser intensity of
0.59mW/$\mu$m$^{2}$. This good trapping performance with fast delivery of
nanoparticles to multiple trapping sites emerges from a combination of the
enhanced electromagnetic near-field and spatial temperature increase. This work
has applications in nanoparticle delivery and trapping with high accuracy, and
bridges the gap between optical manipulation and nanofluidics.
