I have prepared various metal nanoparticles but I observed that Nickel nanoparticles's black colour solution becomes transparent after few hours,while others nanoparticles such as Cr, Co,Ag solutions don't . What happened with Nickel nanoparticles the solution? Why colour disappears? Even all above discussed nanoparticles are capped by various capping agents in the solutions.Kindly share your valuable knowledge,thanks in advance.