A composition is disclosed which is capable of being used for detection, comprising a noble metal nanoparticle. The inventive compositions exhibit little interaction with serum proteins while exhibiting pH-dependent adsorption onto live cell membranes. The nanoparticles of the claimed invention are capable of interacting with cell membranes, which in turn permits the advantages of nanoparticle bio-imaging to be extended to many pH dependent biological processes such as targeting acidic tumor microenvironment.
