Structure-conserving spontaneous transformations between nanoparticles

Ambient, structure- and topology-preserving chemical reactions between two archetypal nanoparticles, Ag25(SR)18 and Au25(SR)18, are presented. Despite their geometric robustness and electronic stability, reactions between them in solution produce alloys, AgmAun(SR)18 (m+n=25), keeping their M25(SR)18 composition, structure and topology intact. We demonstrate that a mixture of Ag25(SR)18 and Au25(SR)18 can be transformed to any arbitrary alloy composition, AgmAun(SR)18 (n=1–24), merely by controlling the reactant compositions. We capture one of the earliest events of the process, namely the formation of the dianionic adduct, (Ag25Au25(SR)36)2−, by electrospray ionization mass spectrometry. Molecular docking simulations and density functional theory (DFT) calculations also suggest that metal atom exchanges occur through the formation of an adduct between the two clusters. DFT calculations further confirm that metal atom exchanges are thermodynamically feasible. Such isomorphous transformations between nanoparticles imply that microscopic pieces of matter can be transformed completely to chemically different entities, preserving their structures, at least in the nanometric regime.

force-field global minimum of adduct from a molecular docking simulation with II on the left and I on the right. Dashed lines show the shortest distances found between atoms in the staples of the two clusters which are marked with letters A to E on II and F to I on I. Distances between the pairs of metal and sulfur atoms labelled are AF (Sb(Au25)-Ag)=3.90 Å, BG (Au-Sb(Ag25))=4.05 Å, CG (Snb(Au25)-Sb(Ag25))=4.74 Å, DH (Snb(Au25)-Snb(Ag25))=3.45 Å, DI (Snb(Au25)-Ag)=4.28 Å and EI (Au-Ag)=4.95 Å. The brackets after the pair of letters gives details of the element types and the bridging and non-bridging sulfur positions on staples that are denoted by the subscripts b and nb, respectively. The hydrogen atoms are omitted from the ligands for clarity. Color code for the atoms: Au (red), Ag (green), S (yellow), C (blue). PET is 2-phenylethanethiol and DMBT is 2,4dimethylbenzenethiol.      Fig. 13

Supplementary Note 1: Possible reason for the abundance of Ag13Au12(SR)18
Our DFT calculations show that substitution energy for an Au atom to occupy the I and the S positions of Ag25(SR)18 are almost the same which indicates that Au12 can be located in positions I or the S of Ag25(SR)18 with equal probability. Thus substitution of the twelve staple (S)/ icosahedral (I) Ag atoms in Ag25(SR)18 by twelve Au atoms produce Ag13Au12(SR)18. Hence, more of Ag13Au12(SR)18 could be formed as a result of Au substitution into Ag25(SR)18 on either at the I or at the S positions. Therefore, the probability of formation of Ag13Au12(SR)18 is higher due to the availability of two types of (I and S) twelve-atom sites for Au atoms. We do not think that the abundance of Ag13Au12(SR)18 is due to any shell closing effects as this abundance is observed only when the concentrations of the reacting clusters are comparable. Though this species was observed with higher abundance immediately after mixing (Figure 1c), and it existed for about 5 min (Supplementary Figure 6), no such species was observed after 1h (panel i of Supplementary Figure 20). Further, Supplementary Figure 20 shows that Ag13Au12 was not observed with any significantly higher abundance (even at higher concentrations of Au25), in contrast to what is seen in Figure 1c and Supplementary Figure 6. These observations show that significantly higher abundance of Ag13Au12 is observed only for a few minutes after mixing the clusters. As the reaction proceeds, this species also undergoes further doping. If the observed abundance of Ag13Au12(SR)18 is due to its higher stability due to any shell closing effects, this species is expected to remain at higher abundance for longer time intervals of the reaction.
In summary, the Ag13Au12(SR)18 detected can be due to a number of isomers depending on (i) the cluster from which it is derived and (ii) the exact locations of the Ag12/Au12 and the thirteenth Ag/Au atom. However, standard mass spectrometry cannot distinguish all the isomers of the formula, Ag13Au12(SR)18. We think that the abundance of Ag13Au12(SR)18 could be due to the larger number of ways by which Ag13Au12(SR)18 can be formed. was taken as the "ligand" i.e. the movable molecule whose degrees of freedom would be varied and Ag25(DMBT)18 as the "receptor" which was the fixed and completely rigid central molecule.

Supplementary Note 2: General instrumental parameters used for ESI measurements
We assigned charges by following the procedure to Guberman-Pfeffer et al. 7 Supplementary Fig. 8).
The 10 lowest minima structures that were obtained with binding free energies ranging from -4.62 to -17.53 kcal/mol with Au25(PET)18 in various orientations and at varying distances on various different sides the Ag25(DMBT)18. The free energies of binding were calculated by summing the intermolecular and internal and torsional terms and subtracting the unbound energy which is a calculation that is performed within the Autodock program. The relative orientation of the clusters in the minimum energy configuration is shown in Supplementary Fig. 11 and is such that the overlap between the molecular surfaces, or van der Waals envelope, of the ligands of the two clusters has maximal area of contact and their protrusions and pocket fit closely together in a lock and key fashion, as can be seen in Supplementary Fig 10. Two of the C5 axes are nearly aligned with each other as seen in Supplementary Fig 11. Strength of interactions between atoms in the staples of the two clusters in the minimum energy configuration can be partially gauged by comparing their interatomic distances to the sum of their van der Waals radii, since the attractive van der Waals interaction (-A/r -6 ) is stronger than the repulsive term (B/r -12 ) at distances close to and greater their van der Waals radii. The shortest distances between the pairs of different types of metal and sulfur atoms (S-Au, S-Ag, Au-Ag) in the staples, shown in Supplementary Fig. 9, of the two clusters in the adduct show that all these interatomic distances are greater than those needed for covalent bonding.
Thus in our force-field minimum geometry, only weak non-covalent interactions between (i) metal atoms of one cluster and sulfur atoms in the staples of the neighbouring cluster and (ii) the alkyl/aryl groups of the ligands of the clusters are only expected in this force-field global minimum geometry of adduct at this separation between I and II, unlike in the case of onedimensional chains of clusters 11 .
However, DFT optimization of the force-field global minimum of adduct ( Supplementary Fig. 12 and 13) revealed that the clusters I and II undergo significant structural distortions in the adduct geometry in terms of the bond lengths and bond angles. Moreover, the DFT-optimized geometry shows the formation of a weak bonding between Ag atom and the sulfur atoms in the staples of I and II. However, the local minimum in DFT-PES, shown in these figures, cannot be confirmed as the actual global minimum of the DFT PES of the dimer, without a complete search of PES but it is reasonable to assume that this would at least resemble in essential aspects such as the overall separation and orientation of the two clusters to one of the lower lying minima in the DFT PES.
The energy difference (binding energy) between the DFT-optimized adduct geometry and the sum of the DFT-optimized energies of isolated clusters was -90 eV. A quite significant lowering of the total energy of the cluster is observed. However, the per atom binding energy is 90/698 eV = 0.12 eV (698 is the total number of atoms in the adduct, [Ag25Au25(DMBT)18(PET)18] 2-). The large magnitude of the binding energy can be attributed to a number of factors such as the presence of large number of atoms and large common interfacial area, which facilitates attractive binding interactions, such Ag-S(Au25) chemical bonding in the adduct, pi-pi interactions, ligand orbital overlaps, dipole-dipole interactions. Secondly, there is additional structural relaxation taking place through the structures resulting in distortions such as bond strain, angular and dihedral distortions in the core and staples, which lower the total energy below that of the sum of the DFT-optimized geometry of the two isolated clusters. However, we are unable to decompose the total energy into these different contributions.
Such a substantial energy reduction for an intermediate indicates that the overall reaction is also thermodynamically favorable.