Reversing the Thermodynamics of Galvanic Replacement Reactions by Decreasing the Size of Gold Nanoparticles

Abstract

Here, we describe the surprising reactivity between surface-attached (a) 0.9, 1.6, and 4.1 nm diameter weakly stabilized Au nanoparticles (NPs) and aqueous 1.0 × 10–4 M Ag+ solution, and (b) 1.6 and 4.1 nm diameter weakly stabilized Au NPs and aqueous 1.0 × 10–5 M PtCl4 2–, which are considered to be antigalvanic replacement (AGR) reactions because they are not thermodynamically favorable for bulk-sized Au under these conditions. Anodic Stripping Voltammetry (ASV) and Scanning Transmission Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (STEM-EDS) mapping provide quantitation of the extent of Ag and Pt replacement as a function of Au NP diameter. The extent of the reaction increases as the Au NP size decreases. The percentage of Ag in the AuAg alloy following AGR based on ASV is 17.8 ± 0.6% for 4.1 nm diameter Au NPs, 87.2 ± 2.9% for 1.6 nm Au NPs, and an unprecedented full 100% Ag for 0.9 nm diameter Au NPs. STEM-EDS mapping shows very close agreement with the ASV-determined compositions. In the case of PtCl4 2–, STEM-EDS mapping shows AuPt alloy NPs with 3.9 ± 1.3% and 41.1 ± 8.7% Pt following replacement with 4.1 and 1.6 nm diameter Au NPs, respectively, consistent with qualitative changes to the ASV. The size-dependent AGR correlates well with the negative shift in the standard potential (E0) for Au oxidation with decreasing NP size.

Programmed ripening of nanoparticles using a DNA template

“…It has been reported that the oxidation potential of metal NPs decreases with their size. 43,44 We confirmed this trend for our core-shell Au@AgNPs, by measuring their oxidation potential as a function of size, using linear sweep voltammetry (Supporting information S-VII). While DNA-bound Au@AgNPs grow homogeneously at the beginning of the silver deposition (Scheme 1b), the subtle differences in their size and surface conditions result in a small divergence in their electrochemical potentials during growth.…”

“…We systematically positioned two AuNP seeds on the DNA origami with controlled interparticle distances of 16, 32 or 48 nm (Figure 1a-i, 1b-i and 1c-i). Increasing amounts of AgNO 3 solution were then added to grow Ag shells on the Au seeds, followed by stabilization with a small molecule ligand (4,7,10,13,16,19,22,25,32,35,38,41,44,47,50, 53-hexadecaoxa-28, 29-dithiahexapentacontanedioic acid, OEG) and H 2 O wash, before being imaged by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). In all cases, we observed the formation of asymmetric dimers (Figure 1, magenta circles)with one large and one small Au@AgNPpast a critical concentration of Ag growth.…”

Site-Selective Photosynthesis of Ag–AgCl@Au Nanomushrooms for NIR-II Light-Driven O2- and O2•–-Evolving Synergistic Photothermal Therapy against Deep Hypoxic Tumors

“…The Ag–AgCl@Au NMs were obtained by irradiating the mixed dispersion of Ag–AgCl NCs, PVP, and HAuCl 4 in water, where HAuCl 4 was reduced to Au and deposited on the surface of Ag–AgCl NCs under photoreduction. Generally, when the standard reduction potential of the oxidized metal is greater than that of the reduced metal, a galvanic replacement reaction can occur between the two. , The standard reduction potential of Au (0.99 V) is larger than that of Ag (0.80 V), so HAuCl 4 and the Ag component of Ag–AgCl NCs can undergo the following galvanic replacement reaction to obtain Ag–AgCl@Au NMs: Panels b and c in Figure show that the as-synthesized Ag–AgCl@Au nanostructures are mushroom-like and their surfaces are very rough. The hydrodynamic sizes of Ag–AgCl NCs and Ag–AgCl@Au NMs are 123.12 (PDI = 0.148) and 199.04 nm (PDI = 0.193), respectively, based on the dynamic light scattering analysis (Figure S7).…”

“…Generally, when the standard reduction potential of the oxidized metal is greater than that of the reduced metal, a galvanic replacement reaction can occur between the two. 44,45 The standard reduction potential of Au (0.99 V) is larger than that of Ag (0.80 V), so HAuCl 4 and the Ag component of Ag−AgCl NCs can undergo the following galvanic replacement reaction to obtain Ag−AgCl@Au NMs: 46…”

Effect of Metal Nanoparticle Aggregate Structure on the Thermodynamics of Oxidative Dissolution

“…All electrochemical measurements were made with a CH Instruments model CHI 660E or 620 electrochemical workstation (Austin, TX). The working electrode was indium-tin-oxide-coated glass (glass/ITO) functionalized with (3-aminopropyl)­triethoxysilane (APTES) as described by our group previously. , The isolated and aggregated 15 nm Au NPs were attached to glass/ITO/APTES electrodes by soaking the electrodes in the appropriate solution for 3–5 min. A Pt wire counter electrode and an Ag/AgCl (3 M KCl) reference electrode completed the three-electrode cell set up.…”