Compositions, systems and methods for using a nanoparticle composite to act as a valve within a microfluidic conduit to regulate fluid flow therethrough are provided. The nanoparticle composite includes a core having magnetic particles and Au particles and includes a hydrogel coating surrounding the core. The size of the nanoparticle composite is controlled by causing the hydrogel coating to lose water or absorb water, thus decreasing or increasing the size of the nanoparticle composite within the microfluidic conduit.