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1Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA.2Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA.3Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA15261, USA.*These authors contributed equally to this work.†Present address: Department of Chemistry, Anhui University Hefei, Anhui230601, China.‡Corresponding author. Email: rongchao@andrew.cmu.edu

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Molecular “surgery” on a 23-gold-atom nanoparticleQi Li,1* Tian-Yi Luo,2* Michael G. Taylor,3* Shuxin Wang,1† Xiaofan Zhu,1 Yongbo Song,1

Giannis Mpourmpakis,3 Nathaniel L. Rosi,2 Rongchao Jin1‡

Compared to molecular chemistry, nanochemistry is still far from being capable of tailoring particle structureand functionality at an atomic level. Numerous effective methodologies that can precisely tailor specific groupsin organic molecules without altering the major carbon bones have been developed, but for nanoparticles, it isstill extremely difficult to realize the atomic-level tailoring of specific sites in a particle without changing thestructure of other parts (for example, replacing specific surface motifs and deleting one or two metal atoms).This issue severely limits nanochemists from knowing how different motifs in a nanoparticle contribute to itsoverall properties. We demonstrate a site-specific “surgery” on the surface motif of an atomically precise 23-gold-atom [Au23(SR)16]

− nanoparticle by a two-step metal-exchange method, which leads to the “resection” oftwo surface gold atoms and the formation of a new 21-gold-atom nanoparticle, [Au21(SR)12(Ph2PCH2PPh2)2]

+,without changing the other parts of the starting nanoparticle structure. This precise surgery of the nanoclusterreveals the different reactivity of the surface motifs and the inner core: the least effect of surface motifs onoptical absorption but a distinct effect on photoluminescence (that is, a 10-fold enhancement of luminescenceafter the tailoring). First-principles calculations further reveal the thermodynamically preferred reactionpathway for the formation of [Au21(SR)12(Ph2PCH2PPh2)2]

+. This work constitutes a major step toward the de-velopment of atomically precise, versatile nanochemistry for the precise tailoring of the nanocluster structure tocontrol its properties.

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INTRODUCTIONRecent years have witnessed significant research efforts on atomical-ly precise ultrasmall metal nanoparticles (often called nanoclusters)(1–4). Major advances have been achieved in the synthesis of gold,silver, and Au/Ag alloy nanoclusters and the identification of aestheticstructural patterns (5–22). Exploring effective methods to exquisitelytailor the size, structure, and composition of atomically precise nano-clusters constitutes an ultimate goal in this field, which will offer oppor-tunities to pursue fundamental understanding of the properties ofnanoclusters (23–26) and establish definitive structure-property rela-tionships (1, 5, 27). With precise formulas, molecular purity, and totalstructures solved by single-crystal x-ray analysis, it has become possibleto investigate the transformation chemistry of nanoclusters at the atomiclevel (28–31), akin to organic transformation chemistry. This atomic-levelnanochemistry may provide new opportunities to discover some un-expected chemical reactions, which so far remain a mystery.

Although different precise synthesis methods have been developedin the past decade, realizing the atomic-level tailoring of specific sitesin a nanoparticle without changing the structure of other parts (forexample, replacing specific surfacemotifs and deleting one or twometalatoms) remains extremely difficult. This “molecular surgery,” which ishighly desirable but is so far the least feasible in nanochemistry, hindersthe fundamental understanding of how different motifs in a nanopar-ticle contribute to its overall properties.

Here, we report the first case ofmolecular surgery on the nanoclustersurface via site-specific tailoring of the protectingmotif (Fig. 1A), whichleads to the “resection” of two gold atoms from the [Au23(SR)16]

−

(R: cyclo-C6H11) nanocluster and thus the formation of a new[Au21(SR)12(Ph2PCH2PPh2)2]

+ (abbreviated as P–C–P hereafter) na-nocluster. Direct transformation did not occur; thus, our methodconsists of a two-step metal exchange via the formation of a critical[Au23−xAgx(SR)16]

− (x ~ 1) intermediate. We determined the struc-tures of [Au21(SR)12(P–C–P)2]

+ and [Au23−xAgx(SR)16]− (x ~ 1) by

single-crystal x-ray analysis. First-principles calculations further re-veal the critical role of the first-step silver doping (step 1) in openingup the thermodynamically favorable pathway of surfacemotif exchangefromRS–Au–SR to P–C–Pmotifs in the second step.Meanwhile, a newmetal-exchange scenario in which an Ag dopant goes to a counterionis also presented.

RESULTSThe starting material, [Au23(SR)16]

− [counterion: tetraoctylammonium(TOA+)], was synthesized by a previously reportedmethod (32). Single-crystal x-ray analysis revealed the details of the two-step molecularsurgery process for the transformation of [Au23(SR)16]

− to the[Au21(SR)12(P–C–P)2]

+ nanocluster (Fig. 1B). In the first step, asmall amount of AgI(SR) at a molar ratio of Au/Ag = 1:0.07 was addedto target-dope the [Au23(SR)16]

−, and a silver-doped Au cluster,[Au23−xAgx(SR)16]

− (x ~ 1), was obtained. The dopant Ag was foundto be at two specific positions (centrosymmetric) in [Au23−xAgx(SR)16]

−

(Fig. 1B, middle). In the next step, [Au23−xAgx(SR)16]−was transformed

to [Au21(SR)12(P–C–P)2]+ by reacting with a gold(I)-diphosphine

complex,Au2Cl2(P–C–P). It was found that the dopantAgwas reverselyreplaced byAu, andmeanwhile, the twomonomeric RS–Au–SRmotifs,which protect the dopant Ag sites, are also exchanged by the P–C–Pmotif from theAu2Cl2(P–C–P) (33) reactant. This simultaneous exchangeofmetal and surface motif produced the new [Au21(SR)12(P–C–P)2]

+

cluster and a silver-containing counterion, AgCl2−, which was formed

after the dopant Ag was pulled out from the cluster.Details of the x-ray structures of [Au23(SR)16]

−, [Au23−xAgx(SR)16]−,

and [Au21(SR)12(P–C–P)2]+ are shown in Fig. 2. A previous study (32)

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reported on the structure of [Au23(SR)16]−, which has a 15-atom bipy-

ramidal Au core, that is, a 13-atom cuboctahedron plus two extra “hub”gold atoms (Fig. 2A, in blue) that links the surface-protecting staplemotifs together. The core is protected by two belt-like trimeric Au3(SR)4and two monomeric Au(SR)2 staple motifs, as well as four simplebridging SR ligands. The structure of [Au23−xAgx(SR)16]

− is shownin Fig. 2B. The 15-atom bipyramidal Au–Ag core is shown on the left.There are only two specific sites (centrosymmetric) where Ag can befound. This is different frompreviously reportedAu/Ag alloy nanoclus-ters inwhich theAg atoms are distributed inmany sites (31, 34, 35). Theoccupancy of Ag is determined to be 31.5 and 30.0% at position 1 inthe two crystallographically independent clusters, whereas position2 has a lower occupancy, determined to be 12.7 and 6.4%. The x-raycrystallography–averaged composition is [Au22.13Ag0.87(SR)16]

−, andthe fractional occupancy is caused by the compositional variation ofthe Au or Ag atom. One TOA+ counterion was found, although itshows heavy disorder, indicating that the −1 charge is retained in[Au23−xAgx(SR)16]

− during the silver doping process of [Au23(SR)16]−.

The structure of [Au21(SR)12(P–C–P)2]+ is shown in Fig. 2C.Although

the 15-atom bipyramidal core is retained in [Au21(SR)12(P–C–P)2]+, the

positions of the two hub gold atoms shift. In [Au23(SR)16]−, the two

hub gold atoms (labeled Au-2) are closer to Au-3, with distances of3.234 and 3.245 Å (Fig. 2A), whereas the distance between Au-1 andAu-2 is so large (3.462 Å) that no bond is formed. However, in[Au21(SR)12(P–C–P)2]

+, the Au-2 atom is closer to Au-1, with a muchshorter distance of 2.934 Å; therefore, a bond forms. The distancebetween Au-2 and Au-3 is 3.425 Å in [Au21(SR)12(P–C–P)2]

+, with

Li et al., Sci. Adv. 2017;3 : e1603193 19 May 2017

no Au2–Au3 bond formed. The average lengths of core Au–Au bonds in[Au21(SR)12(P–C–P)2]

+ and [Au23(SR)16]− are 2.95 and 2.98 Å, respectively.

For the surfaceAu–Aubonds, the average lengths in [Au21(SR)12(P–C–P)2]+

and [Au23(SR)16]− are 3.08 and 3.16 Å, respectively. The shorter

bond distances in Au21 might enhance the radiative decay of thephotoexcited particles. More significant changes occur on the surface.Surprisingly, the two monomeric S–Au–S motifs, which originally pro-tect the two dopants Ag-1 and Ag-2 (partial occupancy), are ex-changed by the P–C–P motifs from the Au2Cl2(P–C–P) complexes;in addition, the two doping sites become homogold. The two P atomsare bondedwith theAu-1 andAu-2with distances of 2.288 and 2.293Å,respectively. The motif exchange leads to the loss of two staple goldatoms and thus gives rise to [Au21(SR)12(P–C–P)2]

+.As shown in Fig. 3A, two phenyl rings are arranged in parallel

through p-p stacking in each P–C–P motif. A silver-containing coun-terion, AgCl2

−, is also identified. The AgCl2− anion has been previously

reported in the Cs complex (36), but in metal nanoclusters, before thiscurrent work the Ag-containing counterion has never been identified.TheAg–Cl bond length is 2.348Å, and theAgCl2

− shows a nearly linearconfiguration (the angle of Cl–Ag–Cl is about 175°), which is similarto the previously reported metal complex (36). The packing of[Au21(SR)12(P–C–P)2]

+[AgCl2]− in the single crystal is shown in Fig. 3B,

and each unit cell comprises two [Au21(SR)12(P–C–P)2]+ and

[AgCl2]−. The presence of the AgCl2

− counterion in a one-to-one ratiowith the cluster indicates a +1 charge of [Au21(SR)12(P–C–P)2]

+. Inview of the monovalent thiolate ligand and the neutral phosphineligand, the nominal count of gold core free valence electrons (6s1) is

Fig. 1. Molecular surgery on the atomicallyprecise 23-gold-atomnanoclusterby a two-stepmetal-exchangemethod: peeling off twoparts of the clusterwrapper andclosing the gaps with two P–C–P plasters. (A) Schematic of the molecular surgery on [Au23(SR)16]

−; all carbon tails are omitted for clarity. (B) Site-specific surface motiftailoring with a two-step metal-exchange method. The transformation from [Au23(SR)16]

− through [Au23−xAgx(SR)16]− (x ~ 1) to [Au21(SR)12(P–C–P)2]

+ is revealed bysingle-crystal x-ray analysis. Magenta and blue, Au; gray, Ag; yellow, S; orange, P; green, C; light green, Cl. Other C and all H atoms are omitted for clarity.

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8e (that is, 21 − 12 − 1 = 8e), which is isoelectronic as [Au23(SR)16]−

and [Au23−xAgx(SR)16]−.

It can be concluded from the above single-crystal analysis that thenew [Au21(SR)12(P–C–P)2]

+ cluster holds a very similar atomic struc-ture to the starting [Au23(SR)16]

−, except for the site-specific replace-ment of the surface motif. It will be interesting to study how thistargeted surgery influences the nanocluster’s overall property. The op-tical absorption spectrum of [Au21(SR)12(P–C–P)2]

+ is shown in Fig. 3C,and is similar to that of [Au23(SR)16]

− as well as [Au23−xAgx(SR)16]−

(fig. S1), all with a distinct peak at ~570 nm and a less prominentone at 460 nm. Surprisingly, the photoluminescence (PL) efficiency of[Au21(SR)12(P–C–P)2]

+ is found to be enhanced~10 times compared tothat of [Au23(SR)16]

− (Fig. 3D). This 10-fold improvement may arisefrom the motif exchange–induced change of electronic interaction be-tween the surfacemotifs and the Au core, leading to enhanced radiativerecombination of the electron-hole pair, as reported in gold and othernanoparticles (37, 38). These results unambiguously reveal the surfacemotifs have little effect on optical absorption but a distinct effect on PL.

Matrix-assisted laser desorption ionization (MALDI) spectra of thethree clusters are also shown in fig. S2, and confirm the compositionof the three clusters. The positive-mode electrospray ionization (ESI)spectrum of [Au21(SR)12(P–C–P)2]

+ is shown in fig. S3, and the 1-Daspacing of the isotropic peaks confirms the +1 charge of the cluster. Theexperimental isotope pattern is consistent with the simulated pattern.No signals of silver-doped [AgxAu21−x(SR)12(P–C–P)2]

+ can be ob-served in the ESI mass spectrum. Energy-dispersive x-ray spectroscopic(EDS) measurement under scanning electron microscopy (SEM) wasalso conducted on several [Au21(SR)12(P–C–P)2]

+[AgCl2]− crystals to

further confirm the elemental ratio of Au/Ag. The average elemental

Li et al., Sci. Adv. 2017;3 : e1603193 19 May 2017

ratio of Au/Ag from a number of crystals is calculated to be 21:1.04(table S1), which is very close to 21:1 determined by single-crystal x-rayanalysis. In addition, a 31P nuclear magnetic resonance (31P-NMR)experiment was also carried out, which showed a doublet peak at ~24/26parts permillion (fig. S4), corresponding to theAu2(P–C–P) environmentin theAu21 cluster. To gain insight into theAu23 transformation toAu21via the (Au–Ag)23 intermediate, we performed a control experimentin which the Au2Cl2(P–C–P) complex was directly reacted with[Au23(SR)16]

− under the same conditions, but no [Au21(SR)12(P–C–P)2]+

was obtained. This result suggests the importance of the intermediateproduct of [Au23−xAgx(SR)16]

− (x ~ 1), which is deemed to serve as aprerequisite for [Au21(SR)12(P–C–P)2]

+. More Au complex/salt [forexample, AuI(SR) and AuCl] have been tested to investigate how thecomposition or structure of Au complex/salt influences structural trans-formation. The control experimental results show that [Au23−xAgx(SR)16]

−

(x ~ 1) was retained when reacting with AuI(SR), whereas the reactionwith AuCl led to the conversion of [Au23−xAgx(SR)16]

− (x ~ 1) to otherclusters. These results indicate that the Cl− may act as the driving forceto extract the dopant Ag out of the cluster. Meanwhile, the structure ofP–C–P in theAu complex, which perfectlymatches the original S–Au–Smotif, is also critical to stabilize the whole structure of the cluster.

On the other hand, structure/size transformation of [Au23(SR)16]− to

[Au25−xAgx(SR)18]− upon heavy silver doping (x ~ 19) was reported in

our previous work (31). Compared to the present work, the differentresults (Au23−xAgxwith light doping versusAu25−xAgxwithheavydoping)arise from the different amounts of AgI(SR) added in the reaction. Theevolution of ultraviolet-visible (UV-Vis) absorption spectrum from[Au23(SR)16]

− that reacted with increasing amounts of AgI(SR) is shownin fig. S5, which suggests the transformation from [Au23(SR)16]

− through

Fig. 2. Comparison of the [Au23(SR)16]−, [Au23−xAgx(SR)16]

−, and [Au21(SR)12(P–C–P)2]+ structures. (A) Crystal structure of [Au23(SR)16]

−. Left: 15-atom Au bipyramidalcore. Right: Au23S16 framework. (B) Crystal structure of [Au23−xAgx(SR)16]

−. Left: 15-atom Au–Ag bipyramidal core. Right: Au23−xAgxS16 framework. (C) Crystal structure of[Au21(SR)12(P–C–P)2]

+. Left: 15-atom bipyramidal core. Right: Au21S12(P–C–P)2 framework. Magenta and blue, Au; gray, Ag; yellow, S; orange, P; green, C. Other C and allH atoms are omitted for clarity. The counterions TOA+ and AgCl2

− are also omitted.

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[Au23−xAgx(SR)16]− to [Au25−xAgx(SR)18]

−. The x-ray structure ofAg-doped [Au25−xAgx(SR)18]

− (x ~ 4) is also solved (fig. S6), and theentire transformation pathway from [Au23(SR)16]

− to the heavilydoped [Au25−xAgx(SR)18]

− is thus unveiled. As shown in fig. S7, the[Au23(SR)16]

− is first transformed to [Au23−xAgx(SR)16]− (x = 1 to 2),

and then it undergoes a size/structural change to [Au25−xAgx(SR)18]−

(x ~ 4), with the dopant Ag located exclusively on the sites of the12-atom icosahedral inner shell. Upon heavy doping, the dopantAg will also go onto the surface motifs, and the heavily doped[Au25−xAgx(SR)18]

− is accordingly obtained. These results suggestthat the number of dopant Ag atoms is very important for the finalstructure of the alloy nanocluster product in this case. The doping-induced transformation from [Au23(SR)16]

− to [Au21(SR)12(P–C–P)2]+

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and [Au25−xAgx(SR)18]− is summarized in Fig. 4. Here, a small amount

of AgI(SR) is critical to place the necessary dopant Ag atoms in specificpositions (targeted doping) and retain the structure of the original[Au23(SR)16]

−, which provides the way for the subsequent motifexchange to obtain [Au21(SR)12(P–C–P)2]

+.To further understand the driving forces in the synthesis of

[Au21(SR)12(P–C–P)2]+ and [Au25−xAgx(SR)18]

−,weperformeda thermo-dynamic analysis of elementary growth steps using density functionaltheory (DFT) calculations as shown in Fig. 5 (see the SupplementaryMaterials for computational details). The growth and doping reactionsteps involving the addition ofM1(SR) species (M =Au or Ag) have beenreferenced to the energies of the thermodynamically very stable tetra-mer species [M4(SR)4] that have been both computationally (39) and

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Fig. 3. Single-crystal structure and optical properties of [Au21(SR)12(P–C–P)2]+[AgCl2]

−. (A) The counteranion [AgCl2]− and coordination of PPh2CH2PPh2 motifs.

Other carbon tails and all H atoms are removed for clarity. (B) Total structure and arrangement of [Au21(SR)12(P–C–P)2]+[AgCl2]

− in a single-crystal unit cell. Magenta, Au;gray, Ag; yellow, S; orange, P; green, C; light green, Cl; white, H. (C) UV-Vis absorption spectrum of [Au21(SR)12(P–C–P)2]

+. (D) PL spectrum of the Au21 (solid line); the PLefficiency is enhanced ~10 times compared to Au23 (dashed line). Inset shows a photograph of the Au21 sample under 365-nm UV light.

Fig. 4. Metal-exchange transformation from [Au23(SR)16]− to [Au21(SR)12(P–C–P)2]

+ and [Au25−xAgx(SR)18]−.

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experimentally (40) observed in thiolated group XI metal complexes(this reference selection does not affect the relative thermodynamicstability of the species and results in accurate reaction energy calcula-tions; see table S4). First, we observe that for the [Au23(SR)16]

− and[Au22Ag(SR)16]

− clusters, Ag doping reactions are exothermic andslightly preferred over growth reactions to form [Au25−xAgx(SR)18]

−

nanoclusters. Additionally, we see that the growth of [Au23(SR)16]− to

[Au25(SR)18]− is unfavorable.However, for the [Au21Ag2(SR)16]

− cluster,growth to [Au21Ag4(SR)18]

− becomes energetically more preferredthan the doping step to [Au20Ag3(SR)16]

−, rationalizing the lack ofobserved [Au20Ag3(SR)16]

−. This preference in the Ag growth step overAg doping is further enhanced in the reaction of [Au20Ag3(SR)16]

− toform [Au20Ag5(SR)18]

− over [Au19Ag4(SR)16]−. This demonstrates an

increasing energetic preference for growth to [Au25−xAgx(SR)18]− nano-

clusters, in agreement with the observation that [Au23−xAgx(SR)16]−,

with xmore than 2, is not formed. Next, we observe a significantly uphillthermodynamic reaction betweenAu2Cl2(P–C–P) and [Au23(SR)16]

− toform the [Au21(SR)12(P–C–P)2(AuCl2)2]

− cluster (representing motif ex-change reactions). However, when Ag atoms are doped into [Au23(SR)16]

−

to form [Au21Ag2(SR)16]−, the presence of the higher-energy (2,2) iso-

mer of [Au21Ag2(SR)16]− creates an almost thermoneutral path, enabling

the formation of [Au21(SR)12(P–C–P)2(AgCl2)2]−. Note that all these

theoretical findings are in agreement with the experimental observa-tions, demonstrating that a thermodynamic (free energy) analysis cancapture the growth behavior of these nanoclusters (at least for thesystems of interest).

DISCUSSIONHere, a two-step metal-exchange method has been developed forsite-specific surface motif exchange on [Au23(SR)16]

− to form a new[Au21(SR)12(P–C–P)2]

+ nanocluster. Both experimental andDFTcalcu-lations indicate that the formation of the [Au23−xAgx(SR)16]

− (x = 1 to 2)intermediate is critical, and the dopant Ag atoms are target-doped attwo specific positions (partial occupancy). Doping lowers the

Li et al., Sci. Adv. 2017;3 : e1603193 19 May 2017

transformation barrier and thus enables transformation from Au23 to Au21when[Au23−xAgx(SR)16]

− reactswithAu2Cl2(P–C–P),whereas the reactionwithmoreAgI(SR) leads to size/structure changes and the formation ofAu25−xAgx. This work offers a promising strategy for molecular sur-gery on nanoclusters to tailor their structure and functionality.

MATERIALS AND METHODSChemicalsTetrachloroauric(III) acid (HAuCl4·3H2O, 99.99%; Sigma-Aldrich),cyclohexanethiol (C6H11SH, 99%; Sigma-Aldrich), bis[chlorogold(I)]bis(diphenylphosphino)methane [Au2Cl2(P–C–P), 97%; Sigma-Aldrich],sodiumborohydride (NaBH4, 99.99%; Sigma-Aldrich), tetraoctylammo-nium bromide (TOAB, 98%; Fluka), pentane [high-performance liquidchromatography (HPLC) grade, 99.9%; Sigma-Aldrich], ethanol (HPLCgrade; Sigma-Aldrich), methanol (HPLC grade, 99%; Sigma-Aldrich),and dichloromethane (DCM) (HPLC grade, 99.9%; Sigma-Aldrich)were used as received.

Synthesis and crystallizationTo synthesize [Au23−xAgx(SR)16]

− (x = 1 to 2), 20 mg of molecularlypure [Au23(SR)16]

− [made using a previously reported method (32)]was first dissolved in 3 ml of DCM, and 1.2 mg of AgI(SR) (powder)(29) was added, with amolar ratio of elemental Au/Ag in reactants of1:0.07. The reaction was allowed to proceed for ~1 hour and was moni-tored by MALDI mass and UV-Vis spectroscopy. After [Au23(SR)16]

−

was converted to [Au23−xAgx(SR)16]− (judging from MALDI spectra),

the DCM solution containing the [Au23−xAgx(SR)16]− product was

directly transferred to a glass tube, and pentane was diffused into thesolution at room temperature for crystallization of clusters. After~2 days, crystals of [Au23−xAgx(SR)16]

−were obtained on the inner walland at the bottom of the glass tube. Notably, this crystallization step alsoserves as isolation/purification of the [Au23−xAgx(SR)16]

− product, and toobtain high-quality single crystals, the crystals were redissolved and thenrecrystallized for several times. The product yield of [Au23−xAgx(SR)16]

−

was ~70% (crystals after the final step).To synthesize [Au21(SR)12(P–C–P)2]

+, 30 mg of accumulated[Au23−xAgx(SR)16]

− crystals was first redissolved in 6 ml of DCM,and 8.0 mg of the Au2Cl2(P–C–P) complex was added (mole ratio ofreactants = 1:2). The reaction was allowed to proceed at room tem-perature for ~3 hours, which was also monitored by MALDI massand UV-Vis spectroscopy. After [Au23−xAgx(SR)16]

− was converted to[Au21(SR)12(P–C–P)2]

+, theDCMsolution containing [Au21(SR)12(P–C–P)2]+

clusters was transferred to a glass tube, to which pentane was diffused atroom temperature for crystallization. After 3 to 5 days, black crystals of[Au21(SR)12(P–C–P)2]

+ were observed at the bottom and side wall ofthe glass tube and then collected. The redissolving and recrystallizationprocesses were performed to obtain high-quality [Au21(SR)12(P–C–P)2]

+

single crystals. The product yield was ~80% at the final step. Notably,if [Au23(SR)16]

− was used as the reference, then the product yield of[Au21(SR)12(P–C–P)2]

+ would be ~56%.For the synthesis of [Au25−xAgx(SR)18]

− (x ~ 4), 20 mg of molecular-pure [Au23(SR)16]

− was first dissolved in 3 ml of DCM, and 4 mg ofAgI(SR) (powder) was then added (molar ratio of elemental Au/Agin reactants = 1:0.23). The reaction was allowed to proceed overnight at0°C, andwasmonitored byMALDImass andUV-Vis spectroscopy un-til the [Au23(SR)16]

− reactant was converted to [Au25−xAgx(SR)18]−.

Next, the solution containing [Au25−xAgx(SR)18]− was mixed with ace-

tonitrile (DCM/acetonitrile ratio = 3:2) and was allowed to evaporate

Fig. 5. DFT-calculated free energies (DGrxn) of elementary reaction steps of theexperimentally synthesized pure and Ag-doped Au nanoclusters. Detailed re-action network energetics analysis can be found in table S4. Inset shows the dif-ferent (thermodynamically stable) doping positions of Ag in the Au15 core of the[Au23−xAgx(SR)16]

− clusters. The different energy levels of the [Au23−xAgx(SR)16]− clus-

ters represent the lowest-energy isomers (based on doping positions of the inset),which are also analyzed in fig. S8.

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slowly for ~4 days at 4°C. Black crystals of [Au25−xAgx(SR)18]− were

observed at the bottomand sidewall of the glass tube and then collected.The product yield of [Au25−xAgx(SR)18]

− was ~30%.

CharacterizationUV-Vis spectra of the clusters (dissolved in CH2Cl2) were acquired on aHewlett-Packard Agilent 8453 diode array spectrophotometer atroom temperature. MALDI mass spectrometry was performed on aPerSeptive Biosystems Voyager DE super-STR time-of-flight (TOF)mass spectrometer. ESI mass spectra were recorded using a WatersQ-Tof mass spectrometer equipped with Z-Spray Source. 31P-NMRspectrum was obtained using Bruker Avance III 400 MHz spectrom-eters. EDS-SEM was conducted with ZEISS Sigma 500 VP SEM withOxford AZtec X-EDS.

DFT simulationDFT calculations were performed using the BP86 (41, 42) functionalcombined with the def2-SV(P) basis set (43), accelerated with the RI(resolution of identities) approximation (44, 45), as implemented inthe TURBOMOLE 6.6 package (46). Geometry optimizations werecarried out using the quasi–Newton-Raphson method without anysymmetry constraints. Gibbs free energies were calculated using theharmonic oscillator approach applied to the vibrational modes cal-culated for the entire nanoparticles at 298.15 K. See equations belowfor reference in the calculation of free energy

Etot ¼ Eelectronic þ ZPEþ RT � RT � lnðqrot � qvib � qtransÞ

whereEtot represents the totalmolar free energy,Eelectronic represents theelectronic energy, ZPE represents the zero-point vibrational energy, R isthe ideal gas constant, T is the temperature, qrot is the rotationalpartition function, qvib is the vibrational partition function, and qtransis the translational vibrational motion (set to 1 atomic unit to neglect inthis case).

qvib ¼ ∏i

1

1� exp �w� eðiÞ=kBT� �

where e(i) represents the vibrational energy of the ith vibrational stateand kB is Boltzmann’s constant. The factor of w = 0.9914 was used as acorrection to the vibrational energies (47).

qrot ¼ ½ð2� p� kB � TÞ2 � A� B� C�0:5p

whereA,B, andC represent themoments of inertia of themolecules ands represents the symmetry number.

The [Au23(SR)16]− and [Au25(SR)18]

− structures were taken from previ-ously published crystallographic information, the R groups of the thiolateswere substituted by methyl groups, and the positive counterions were re-moved (15, 32). The structure of [Au21(SR)12(P–C–P)4(AgCl2)z]

− wastaken from table S3, and the R groups (–C6H11) of the thiolates weresubstituted for (–CH3). Note that the BP86 functional has been suc-cessfully used on thiolated metal nanocluster systems (48), and R =methyl group substitution has had little impact on RS–Au bondstrength, as has been previously applied in computational nanoparticlestructural determinations (48). From the optimized [Au23(SCH3)16]

−

Li et al., Sci. Adv. 2017;3 : e1603193 19 May 2017

particle, Au atoms were substituted for Ag atoms in each position,and the optimized isomers and relative energies are presented infig. S8. From the [Au25(SCH3)18]

− particle, Ag atoms were substitutedinto the icosahedral core, where Ag is determined to preferentially sit, toform [Au25−xAgx(SCH3)18]

− (x = 1 to 5) and were then relaxed (35).Gibbs free energies for Ag doping and growth reactions are presentedin table S4. Tetramers [M4(SR4)] were used as a common reference forthe growth and doping reactions for the M1(SR) complexes (M = Auor Ag) because these tetramers have been previously shown to be highlythermodynamically stable (39) and similar in structure to the surfacestaplemotifs of the clusters.Moreover, these tetramers have been exper-imentally observed as prenucleation species in thiolated group XI bi-metallic solutions (40).

X-ray analysisDetails of the x-ray crystallographic analysis are provided in theSupplementary Materials.

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/5/e1603193/DC1X-ray Experimentalsfig. S1. UV-Vis absorption spectra of [Au23(SR)16]

− and [Au23−xAgx(SR)16]−.

fig. S2. MALDImass spectraof [Au23(SR)16]− (black), [Au23−xAgx(SR)16]

− (green), and [Au21(SR)12(P–C–P)2]+

(gray) sample.fig. S3. ESI mass spectrum of [Au21(SR)12(P–C–P)2]

+.fig. S4. 31P-NMR spectrum of [Au21(SR)12(P–C–P)2]

+.fig. S5. UV-Vis absorption spectra of samples with increasing mass ratio of AgI(SR) that reactedwith [Au23(SR)16]

−.fig. S6. X-ray crystal structure of [Au25−xAgx(SR)18]

− (x ~ 4).fig. S7. AgI(SR) complex–induced transformation from [Au23(SR)16]

− to heavily Ag-doped[Au25−xAgx(SR)18]

−.fig. S8. DFT-relaxed [Au23−xAgx(SR)18]

− (x = 1 to 3) and [Au25−yAgy(SR)18]− (y = 2, 3) nanoclusters

and associated relative electronic energies.table S1. AtomicpercentagesofAuandAg in the [Au21(SR)12(P–C–P)2]

+[AgCl2]−obtainedbyEDS-SEM.

table S2. DFT free energies of reactions of intermediates and possible reaction pathways.table S3. Crystal data and structure refinement for Au22.13Ag0.87(SR)16

− TOA+.table S4. Crystal data and structure refinement for Au20.51Ag4.49(SR)18

− TOA+.table S5. Crystal data and structure refinement for [Au21(SR)12(P–C–P)2]

+[AgCl2]−.

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AcknowledgmentsFunding: This work was financially supported by the Air Force Office of Scientific Researchunder award no. FA9550-15-1-9999 (FA9550-15-1-0154). M.G.T. acknowledges support fromthe NSF Graduate Research Fellowship under grant no. 1247842. Author contributions: Q.L.and R.J. designed the research; Q.L. performed synthesis and crystallization; M.G.T. and G.M.performed DFT calculations; and T.-Y.L. and N.L.R conducted x-ray crystallographic analysis.All authors analyzed data and contributed to the writing of the manuscript. Competinginterests: The authors declare that they have no competing interests. Data and materialsavailability: All data needed to evaluate the conclusions in the paper are present in the paperand/or the Supplementary Materials. Additional data related to this paper may be requestedfrom the authors.

Submitted 15 December 2016Accepted 20 March 2017Published 19 May 201710.1126/sciadv.1603193

Citation: Q. Li, T.-Y. Luo, M. G. Taylor, S. Wang, X. Zhu, Y. Song, G. Mpourmpakis, N. L. Rosi,R. Jin, Molecular “surgery” on a 23-gold-atom nanoparticle. Sci. Adv. 3, e1603193 (2017).

7 of 7

Molecular ''surgery'' on a 23-gold-atom nanoparticle

Rongchao JinQi Li, Tian-Yi Luo, Michael G. Taylor, Shuxin Wang, Xiaofan Zhu, Yongbo Song, Giannis Mpourmpakis, Nathaniel L. Rosi and

DOI: 10.1126/sciadv.1603193 (5), e1603193.3Sci Adv 

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MATERIALSSUPPLEMENTARY http://advances.sciencemag.org/content/suppl/2017/05/15/3.5.e1603193.DC1

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