  We show that the dispersive force between a spherical nanoparticle (with a
radius $\le$ 100 nm) and a substrate is enhanced by several orders of magnitude
when the sphere is near to the substrate. We calculate exactly the dispersive
force in the non-retarded limit by incorporating the contributions to the
interaction from of all the multipolar electromagnetic modes. We show that as
the sphere approaches the substrate, the fluctuations of the electromagnetic
field, induced by the vacuum and the presence of the substrate, the dispersive
force is enhanced by orders of magnitude. We discuss this effect as a function
of the size of the sphere.
