Abstract
It is highly desirable, while still challenging, to obtain noble metal nanocrystals with custom capping ligands, because their colloidal synthesis relies on specific capping ligands for the shape control while conventional ligand exchange processes suffer from “the strong replaces the weak” limitation, which greatly hinders their applications. Herein, we report a general and effective ligand exchange approach that can replace the native capping ligands of noble metal nanocrystals with virtually any type of ligands, producing flexibly tailored surface properties. The key is to use diethylamine with conveniently switchable binding affinity to the metal surface as an intermediate ligand. As a strong ligand, it in its original form can effectively remove the native ligands; while protonated, it loses its binding affinity and facilitates the adsorption of new ligands, especially weak ones, onto the metal surface. By this means, the irreversible order in the conventional ligand exchange processes could be overcome. The efficacy of the strategy is demonstrated by mutual exchange of the capping ligands among cetyltrimethylammonium, citrate, polyvinylpyrrolidone, and oleylamine. This novel strategy significantly expands our ability to manipulate the surface property of noble metal nanocrystals and extends their applicability to a wide range of fields, particularly biomedical applications.
Highlights
Colloidal noble metal nanocrystals have received extensive interest due to their broad applications in catalysis [1,2,3,4], electronics [5, 6], biosensing [7,8,9], imaging [10, 11], and medicine [12, 13]
We report the use of diethylamine (DEA), whose binding affinity to the metal surface can be conveniently switched by controlling the pH, to serve as an intermediate ligand to enable the replacement of strong native capping ligands with weak ones
We carried out density functional theory (DFT) calculations to evaluate the binding energies (Ebinding) of DEA in its original and protonated forms on the Au surface, in comparison with common capping ligands such as trisodium citrate (TSC), cetyltrimethylammonium bromide (CTAB), and PVP
Summary
Colloidal noble metal nanocrystals have received extensive interest due to their broad applications in catalysis [1,2,3,4], electronics [5, 6], biosensing [7,8,9], imaging [10, 11], and medicine [12, 13]. Oleylamine (OAm) and polyvinylpyrrolidone (PVP) are widely used capping ligands due to their strong adhesion to the metal surface, but they block the catalytically active sites or electromagnetic hotspots of the nanocrystals and significantly reduce their activities in catalysis and surfaceenhanced Raman scattering [19, 20]. Many capping ligands such as cetyltrimethylammonium bromide (CTAB) are biotoxic, which is an obvious obstacle for the use of these noble metal nanocrystals in biological applications [21, 22]. In some investigations, the surface property of the nanocrystals should be kept consistent for an unambiguous
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