Specific heat keeps constant only within a temperature interval, that is for definition, therefore it doesn't seem strange the increasing behaviour you are observing when temperature rises.

 Thanks for your answer but my question focused on the relation between the specific heat of a nanofluid and the nanoparticles concentration dispersed in the base fluid.

the increase of the nanoparticles concentration increases thermal conductivity, while the specific heat decrease as u proposed, u need to understand the thermal diffusivity of the nanofluid,  

α = k/ρCp

Specific heat of nanoparticles is less that the common working fluids. This increasing the solid concentration of nanoparticles results in decreasing the specific heat of the base fluid.