Metal nanoparticles coated with SAMs of carboxylic acid thiols have been used widely for the immobilization of biomolecules. In these applications, the pH of the NP suspension needs to be adjusted slightly above the pKa of the COOH groups in the SAM (ca. 6-8; see J. Am. Chem. Soc. 2006, 128, 15046; 2007, 129, 6664). The negatively charged NPs thus prepared interact attractively with the positively-charged amino acid side chains of proteins or with heme-containing redox enzymes (Biochemistry, 1993, 32, 13771). Toxicity studies (summarized in Small 2008, 1, 26) show that these NPs have very low toxicity against multiple cell types. Recently, Au/COOH nanoparticles have also been used as building blocks for nanoparticle supracrystals (Science 2006, 312, 420; Nano Lett. 2007, 7, 1018) and as components of “patchy” nanoparticle coatings formed on different types of materials by electrostatic interactions (J. Am. Chem. Soc. 2007, 129, 15623.