Preparation Of Receptormodified Metal Nanoparticles

There has been a great advance[3] in the development of synthetic methodology for the preparation of MPNPs after a seminar report from Brust et al.[4] In their approach, metal nanoparticles were prepared by chemical reduction of corresponding salts under a mild condition using NaBH4 as the reducing agent. The presence of thiol ligands with various stoichiometries to metal salt in the reaction resulted in metal nanopar-ticles ranging in size between 2 and 8nm. Molecular receptors on metal nanoparticles can be introduced by directly using the thiolated receptor as the capping agent during the particle formation. An alternative approach uses the ligand-exchange[3] method to further elaborate the preformed MPNPs with thiolated receptors. In the first case, a crucial consideration is the solubility of the receptor in the reaction media because metal salts are not soluble in nonpolar organic solvents. For example, Kaifer et al. developed a special method to prepare the water- soluble Au,[5] Pt,[6] and Pd[6,7] nanoparticles modified with perthiolated cyclo-dextrins (PSH-CDs, Chart 1). In this approach, the special solvent, dimethyl sulfoxide (DMSO) or dimethylformamide (DMF), was used as the reaction media to bring all reacting components, the PSH-CD, the corresponding metal salt, and NaBH4 in the same phase. This one-phase, one-step (1P-1S) approach eliminates the use of phase transfer reagents in a typical two-phase, one-step (2P-1S) method.[4] In the 1P-1S method, the final particle size is influenced not only by the concentration of thiols[5] but also by the nature of these molecules. For instance, a "macro-cyclic effect'' was observed during the preparation of Ag nanoparticles in DMF.[8] Perthiolated b-CD (PSH-b-CD) was found to be more efficient as a capping ligand than monothiolated b-CD (MSH-b-CD, Chart 1). Similarly, a tetrathiol cavitand derivative (Chart 1) was also more efficient than a simple mono-thiol model compound. This sort of "macrocyclic effect'' probably results from an increased probability of surface attachment for the multidentate thiols, as they possess more functional groups capable of initiating their chemisorption process. In addition, once the surface attachment starts, their multidentate character may lead to a more robust, multipoint anchoring of the ligand. Therefore, this macrocyclic effect in metal nanoparticle capping reactions may have both a kinetic and a thermodynamic origin.

PSH-CD-modified metal nanoparticles prepared by 1P-1S approach have a diameter less than 10 nm. To obtain PSH-b-CD-modified gold nanoparticles (PSH-b-CD-Au) larger than 10 nm, a 2S method was applied. Preprepared, citrate-stabilized gold nanoparti-cles (13 nm in diameter)[9] were further modified with PSH-b-CDs by ligand exchange with citrate on the particle surface in the aqueous solution.[10] A similar strategy was used by Lin et al.[11] to prepare Au nano-particles (18 nm in diameter) modified with thiolated crown ether derivatives (Chart 1). Interestingly, surface-modified gold nanoparticles with even larger size (16-87 nm in diameter) were prepared by extracting citrate-stabilized particles in aqueous solutions into toluene or chloroform with thiolated resorcinarene derivatives (Chart 1),[12] the compounds relevant to some macrocyclic receptors. These tetrathiolated resor-cinarenes improved the dispersion and robustness of midnanometer-sized gold nanoparticles in organic solvents. However, the authors clearly demonstrated that

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