Scalable and Precise Synthesis of Thiolated Au10−12, Au15, Au18, andAu25 Nanoclusters via pH Controlled CO ReductionYong Yu, Xi Chen, Qiaofeng Yao, Yue Yu, Ning Yan,* and Jianping Xie*

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117576, Singapore

*S Supporting Information

ABSTRACT: Large-scale production of thiolated Au nano-clusters (NCs) of tunable sizes is pivotal to realizing theirpractical applications. Here, we present a simple one-potsynthesis method for gram-scale production of various discrete-sized Au NCs (Au10−12, Au15, Au18, and Au25) protected bydifferent thiol ligands. The key design in our method is to usea gaseous reducing agent, carbon monoxide (CO), to supporta mild reaction environment for a slow and well-controlledgrowth of Au NCs. The pH of the reaction solution wasfurther used to fine-tune the reduction kinetics for the NCgrowth, leading to the formation of various sized Au NCs. Themonodispersity of our products was verified by a number ofcharacterization techniques (e.g., UV−vis, electrospray ioniza-tion (ESI)-MS, and X-ray photoelectron spectroscopy (XPS)). NMR spectroscopy was also used to investigate the structure ofas-synthesized thiolated Au NCs, which suggested that Au18(SG)14 NCs adopt a core−shell structure with two binding modes ofthe thiol ligands. The synthetic strategy developed in this study produced three small thiolated Au NCs (Au10−12, Au15, and Au18)in large quantities that are not available in the current NC synthesis due to the lack of direct and scalable synthetic protocols.

KEYWORDS: gold nanoclusters, CO reduction, pH-dependent redox potential, size tunable, large scale synthesis

1. INTRODUCTION

Thiol-protected gold nanoclusters (NCs) or thiolated Au NCsare ultrasmall particles below 2 nm in size typically containing<150 Au atoms.1−3 Thiolated Au NCs are currently emergingas a class of promising supermolecules owing to their uniquephysical and chemical properties, such as quantized chargingand luminescence.4−8 Such properties are not observed in bothsingle Au atoms and large Au nanocrystals (>2 nm).9−12 Due tothe strong quantum confinement effect, the optical andchemical properties of thiolated Au NCs are mainly determinedby their size. For example, thiolated Au NCs of varied sizes,such as Au10−12, Au15, Au18, Au25, Au38, and Au144, exhibitinteresting size-dependent molecular absorptions in the UV−vis-near infrared (NIR) region.13−15 Recent studies alsosuggested that the catalytic activities of Au NCs were alsosize dependent.16−18 Tailoring of the NC size is therefore a keyfocus in recent NC development. Moreover, large-scaleproduction of thiolated Au NCs of tunable sizes can furtherfacilitate the basic and applied research in NC chemistry, suchas identifying the NC structure, resolving the size-propertycorrelation, and exploiting the size-dependent optical andcatalytic properties.Recently, several kinetic control19−21 and thermodynamic

selection (or size focusing)22−25 methods have been developedfor the synthesis of atomically precise Au NCs. There are a fewreports on the large-scale production of thiolated Au NCs witha medium-to-large size (e.g., Au25,

26−28 Au38,22−24 Au102,

29 and

Au14415); however, the fabrication of smaller NC analogues

(<20 Au atoms) is attempted less. On the other hand, theeffects of protecting (thiol) ligands in thiolated Au NCs ontheir physicochemical properties are more pronounced ascompared to that in large Au nanocrystals. It is therefore ofparamount interest to synthesize Au NCs of varied sizes butstabilized by the same thiol ligand. To date, only high-resolution separation techniques [e.g., polyacrylamide gelelectrophoresis (PAGE)] are able to prepare a series ofdiscrete-sized Au NCs stabilized by the same thiolligand.13,30−32 Facile and scalable production of such thiolatedAu NCs still remains a grand challenge in the current NCdevelopment.More recently, we have developed a new synthetic strategy

for the large-scale production of thiolated Au25 NCs by using areactive gas, carbon monoxide (CO), as the reducing agent.33

The unique reaction environment created by gaseous COsupports a well-controlled growth of thiolated Au NCs, whichleads to the formation of high purity Au25 NCs in a one-potmanner. Distinct color changes were also observed during theformation of Au25 NCs, implying the discrete cluster sizegrowth for the NC formation. This observation inspired us tofurther exploit this unique reaction system for the tailoring of

Received: December 22, 2012Revised: February 21, 2013Published: February 28, 2013

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NC sizes. It is well-demonstrated in the synthesis of large Aunanocrystals that kinetics control offers a convenient way forthe tailoring of the particle size.34−36 The reduction kineticscould be readily varied by redox potentials of the reaction pairs.In our reaction system, the reactants are CO (reducing agent)and Au(I)-thiolate complexes (Au precursors). The redoxpotentials of both CO and Au(I)-thiolate complexes are highlysensitive to the reaction pH, which could be readily used toadjust the reduction kinetics for the NC formation. Theincrease of the pH of the reaction solution would result in theformation of larger-sized thiolated Au NCs, because a higherreaction pH could lead to a stronger reducing capability of COand therefore a faster reduction kinetics.Herein, we present a systematic study of the pH effects on

the formation of thiolated Au NCs in the CO-reductionmethod. We found that pH is a key factor to adjust thereduction kinetics and therefore to tailor the size of as-synthesized Au NCs. By simply varying the pH of the reactionsolution, discrete sized Au NCs (Au10−12, Au15, Au18, and Au25),protected by various thiol ligands [e.g., glutathione (GSH) and3-mercaptopropionic acid (MPA)] can be easily synthesized inlarge quantities (∼0.1 g) in the CO-reduction method. Anadditional attractive feature of our synthetic method is thelarge-scale production of three small thiolated Au NCs(Au10−12, Au15, and Au18). This advancement provides directand scalable synthetic protocols for NCs in the missing size linkbetween single Au atoms and large Au NCs in the currentdevelopment.

2. EXPERIMENTAL SECTION2.1. Materials. Hydrogen tetrachloroaurate (III) hydrate

(HAuCl4·3H2O) was from Alfa Aesar; L-glutathione (GSH) and 3-mercaptopropionic acid (MPA) were from Sigma-Aldrich. Carbonmonoxide (CO, 99.9%) was from Singapore Oxygen Air Liquide PteLtd. (SOXAL), and all other chemicals were used as received. Allglassware was washed with Aqua Regia (HCl/HNO3 volume ratio =3:1) and rinsed with ethanol and copious ultrapure water. Ultrapurewater with a specific resistance of 18.2 MΩ was used throughout theexperiment.2.2. Synthesis of Thiolated Au NCs of Different Sizes. In a

typical synthesis of GSH-protected Au NCs, aqueous solutions ofHAuCl4 (12.5 mL, 20 mM) and GSH (10.0 mL, 50 mM) were addedto a 500-mL glass flask containing 227.5 mL of ultrapure water. Thereaction solution was stirred vigorously, and the greenish-yellowsolution (the original color of HAuCl4 solution) quickly turned tocolorless, implying the formation of Au(I)-thiolate complexes.31,37

After 2 min, a certain amount of 1 M NaOH was introduced to thereaction solution to bring the pH to a particular value (7, 9, 10, and 11;the NC samples are therefore denoted as NCs-pH7, NCs-pH9, NCs-pH10, and NCs-pH11, respectively). One bar of CO gas was bubbledthrough the reaction solution for 2 min, after which the solution wassealed airtight and stirred at 500 rpm at room temperature. After 24 h,the products were collected. Au15(MPA)13, Au18(MPA)14, andAu25(MPA)18 were synthesized by the same method, except the pHof the reaction solutions were adjusted to 10, 11, and 12, respectively.See Supporting Information for full synthetic details.2.3. Materials Characterizations. UV−vis spectra were recorded

on a Shimadzu UV-1800 spectrometer. The electrospray ionizationtime-of-flight (ESI-TOF) mass spectra were obtained on a BrukerMicroTOF-Q system. The samples were injected directly into thechamber at 120 μL·min−1. Typical instrument parameters were asfollows: capillary voltage, 4 kV; nebulizer, 0.4 bar; dry gas, 2 L·min−1 at120 °C; m/z range, 100−4000. Transmission electron microscopy(TEM) images were taken on a JEOL JEM-2010 microscope operatingat 200 kV. X-ray photoelectron spectroscopy (XPS) measurementswere performed on a VG ESCALAB MKII spectrometer. Nuclear

magnetic resonance (NMR) analysis was conducted on a BrukerAvance DMX 500 spectrometer operating at 500.13 MHz for 1H and125.77 MHz for 13C, using standard Bruker software. The data werecollected with samples dissolved in D2O.

3. RESULTS AND DISCUSSIONGlutathione (GSH or H-SG), a natural thiol-containingtripeptide (γ-Glu-Cys-Gly), was chosen as our model thiolligand. As shown in Scheme 1, thiolated Au NCs of four sizes,

Au10−12, Au15, Au18, and Au25, were successfully synthesized inour method by simply adjusting the reaction pH from 7 to 11.In a typical Au NC synthesis, aqueous solutions of HAuCl4(12.5 mL, 20 mM) and GSH (10 mL, 50 mM) were mixedtogether to form Au(I)-SG complexes. The pH of the reactionsolution was then brought to a certain value (7, 9, 10, and 11)by the addition of a certain amount of 1 M NaOH. GaseousCO was then introduced to the reaction solution for about 2min. The solution was sealed airtight, and the reaction wasallowed to proceed at room temperature under mild stirring(500 rpm) for 24 h. Dispersions of Au NCs with differentcolors, colorless, yellow, brown, and red-brown (Figure 1a−d,inset), were then collected for the reaction pH of 7, 9, 10, and11, which are denoted as NCs-pH7, NCs-pH9, NCs-pH10, andNCs-pH11, respectively.The crude products synthesized under different pH display

different colors, implying that they have different sizes. The sizeinformation of the crude products was first suggested by theirUV−vis spectra. It has been well-documented that atomicallyprecise thiolated Au NCs have characteristic optical absorptionspectrum, and different sized Au NCs display differentcharacteristic absorption peaks.38−40 As shown in Figure 1a−d, NCs-pH7, NCs-pH9, NCs-pH10, and NCs-pH11 show well-defined absorption spectra with distinct peaks at 330/370, 375/415, 560/620, and 675/805 nm, which match nicely with thereported Au10−12(SG)10−12, Au15(SG)13, Au18(SG)14, andAu25(SG)18, respectively.

13,40 The well-defined spectra, espe-cially the respective fingerprint absorption peaks, also imply thehigh purity of our crude products. To the best of ourknowledge, this is the first successful attempt on the direct andlarge-scale production (∼0.1 g) of Au10−12, Au15, and Au18 NCsin a single method. Our protocol can be further scaled-up togram-scale (see Figure S1, Supporting Information, for oneexample) considering its simple and one-pot feature.The size information was further confirmed by the

electrospray ionization mass spectrometry (ESI-MS). ESI-MS

Scheme 1. Schematic Illustration of Producing Au10−12, Au15,Au18, and Au25 NCs by Adjusting the pH of the ReactionSolution in the CO-Reduction Method

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is a powerful technique to determine the NC formula byanalyzing the intact NC mass.41,42 The ESI mass spectra(Figure 1e−h) of NCs-pH7, NCs-pH9, NCs-pH10, and NCs-pH11 unambiguously confirmed their formula to beAu10−12(SG)10−12, Au15(SG)13, Au18(SG)14, and Au25(SG)18,respectively. For example, the ESI mass spectrum of the NCs-pH7 (Figure 1e) shows four sets of intense peaks in the m/zrange of 1500−3000. The first set of peaks at m/z ∼1680consists of a series of peaks spaced m/z 7.35 apart. Thecorresponding isotope pattern (Figure 1e, inset) suggests thatthese ionized species carry three negative charges. Therefore,the spacing between two adjacent peaks is 22 Da (7.35 × 3),attributing to a common [−H + Na].33 The first set of peaksare therefore assigned to be [Au10(SG)10 − (3 + n)H + nNa]3−

(n = 0−9). The comparison of the experimental and simulatedisotope distributions (Figure S2, Supporting Information)further confirms our assignment. Similarly, other sets of peaksin Figure 1e are assigned to be [Au11(SG)11 − (3 + n)H +nNa]3− (n = 0−12), [Au12(SG)12 − (3 + n)H + nNa]3− (n =0−12), and [Au10(SG)10 − (2 + n)H + nNa]2− (n = 0−6).These assignments are also confirmed by their experimentaland simulated isotope distributions (Figures S3−S5, Support-ing Information). Using the same method, the molecularformulas of NCs-pH9, NCs-pH10, and NCs-pH11 (Figure 1f−h) are determined to be [Au15(SG)13 − (3 + n)H + nNa]3− (n= 0−3), [Au18(SG)14 − (3 + n)H + nNa]3− (n = 0−2), and[Au25(SG)18]

z− (z = 6 and 7), respectively (see the detailedassignment in Figures S6 and S7, Supporting Information; itshould be noted that the isotope analysis of Au25(SG)18 was notsuccessful due to the severe charging of the NCs during theionization process). Both UV−vis and ESI-MS analysesunambiguously determined the composition of our products:Au10−12(SG)10−12 (NCs-pH7), Au15(SG)13 (NCs-pH9),Au18(SG)14 (NCs-pH10), and Au25(SG)18 (NCs-pH11).The representative transmission electron microscopy (TEM)

images of our products [Au10−12(SG)10−12, Au15(SG)13,Au18(SG)14, and Au25(SG)18] suggest that they were indeedultrasmall and below 1.5 nm in size (Figure S8, SupportingInformation). The oxidation states of Au10−12(SG)10−12,Au15(SG)13, Au18(SG)14, and Au25(SG)18 were examined by

X-ray photoelectron spectroscopy (XPS) (Figure S9, Support-ing Information). The Au 4f7/2 binding energies of our Au NCswere slightly higher than that of large Au nanocrystals (>3 nm,prepared by the reduction of HAuCl4 with NaBH4 without anyprotecting agent). The slight shift of Au 4f7/2 binding energiesin thiolated Au NCs could be attributed to the initial-stateeffect, where the electron donation from Au atoms to thiolatesoccurs on the NC surface.13 A size-dependent shift of Au 4f7/2binding energies (Figures S9, Supporting Information) was alsoobserved because the initial-state effect is more pronouncedwhen the size of the thiolated Au NCs decreases, similar to theobservations in other studies.13

The size-tailoring feature of our synthetic strategy is fairlygeneric. For example, the protocol can be easily adapted tosynthesize Au15, Au18, and Au25 NCs protected by other thiolligands [e.g., 3-mercaptopropionic acid (MPA)]. By simplysubstituting GSH with MPA and varying the reaction pH to 10,11, and 12, high purity Au15, Au18, and Au25 NCs protected byMPA can be synthesized. As shown in Figure 2a−c (inset), theas-synthesized Au NCs protected by MPA show identical coloras the respective Au NCs synthesized in the GSH system(yellow, Au15; brown, Au18; and red-brown, Au25). They alsohave similar absorption spectra (Figure 1b−d for Au NCsprotected by GSH and Figure 2a−c for Au NCs protected byMPA): 375/410 (Au15), 560/620 (Au18), and 670/785 nm(Au25). ESI-MS analyses further confirmed their molecularformulas to be Au15(MPA)13, Au18(MPA)14, and Au25(MPA)18,respectively (Figure 2d−f) [see the detailed assignment(experimental and simulated isotope distributions) in FiguresS10−S12, Supporting Information]. It should be noted that thereaction pH in the MPA system is relatively higher than that inthe GSH system. For example, for the synthesis of Au15 NCs,the optimal pH in the MPA system is 10, while 9 is the optimalpH in the GSH system. The pH differences in other systemsare 11 (MPA) vs 10 (GSH) for Au18 and 12 (MPA) vs 11(GSH) for Au25. This observation coincides with the pKa of thethiol group in GSH and MPA, which is 9.12 and 10.27,43,44

respectively (see their molecule structures in Figures S13 andS14, Supporting Information). The pKa of the thiol groupreflects the readiness of the thiolates to donate electrons to the

Figure 1. UV−vis (top) and ESI mass (down) spectra of (a, e) Au10−12, (b, f) Au15, (c, g) Au18, and (d, h) Au25 NCs protected by GSH. The insets in(a−d) show digital photos of the respective products and the insets in (e−g) are the corresponding isotope patterns.

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metallic core. The relatively lower pH in GSH than in MPA forthe synthesis of Au NCs of the same size suggests that GSH is abetter protecting agent for Au NCs as compared to MPA.The size tailorability of our synthetic method is realized by

creating a unique reaction environment for the NC growth. Areactive gas CO is a mild reducing agent and provides a mildand controllable reduction kinetics for the formation of NCs.The slow reduction kinetics in the CO-reduction method wasevidenced by the slow and gradual color changes in the reactionsolution. The distinct color changes also implied the formationof small NC intermediates during the NC growth. However,this phenomenon (distinct color changes during the formationof the thiolated Au NCs) was not observed when a strongreducing agent (e.g., NaBH4) was used because of the rapidreduction and a subsequent burst formation of Au NCs.32

Similar to the synthesis of large Au nanocrystals (>2 nm),45 thesize of the Au NCs was also determined by the delicate controlof the reduction kinetics of the Au(I)-thiolate complexes andthe passivation (protection) of Au NCs by the thiol ligands. Aslower reduction kinetics allows a better passivation orstabilization of smaller-sized NCs at the early stage of theNC growth because it provides more time for the thiol ligandsto cover and protect the newly formed Au NC surface. Therecent study on the synthesis of Au19(SR)13 NCs alsohighlighted the importance of slow reduction kinetics for theformation of small Au NCs.20,46

In addition, the reduction kinetics in the CO-reductionmethod can be further fine-tuned by adjusting the reaction pH.The reducing capability of CO is largely pH dependent. Forexample, the reduction of Au(I)-thiolate complexes by CO canbe expressed by the following equation: 2Au(I)−SR + CO +2H2O → 2Au0 + 2RSH + CO3

2− + 2H+. The kinetics of thisreaction is pH dependent, where the reduction kineticsincreases as the reaction pH increases. A faster reductionkinetics at a higher reaction pH therefore produced Au NCs ofa larger size. This corresponds well with our experimentalobservations that the size of as-synthesized thiolated Au NCs

increases as the increase of the reaction pH (e.g., Au15 at pH 9and Au25 at pH 11 in the GSH system).The mild reduction kinetics made possible by CO can also be

readily used to monitor the growth process of thiolated AuNCs. For example, we have observed distinct color changes(from colorless, light-yellow, yellow, to finally brown; Figure3a) in the reaction solution during the formation of Au18(SR)14

NCs. The time-course UV−vis spectra (Figure 3b) were thenmeasured to reconstruct the formation process of Au18(SR)14NCs. A key NC intermediate, Au15(SR)13, was first identified inthe formation of Au18(SR)14 NCs. As shown in Figure 3a, uponthe introduction of CO to the reaction solution, the originalcolorless solution (0 min) rapidly changed to light-yellow at 5min. The UV−vis spectrum of the resulting light-yellowsolution shows a distinct peak at ∼370 nm and a shoulderpeak at ∼420 nm (Figure 3b, red line). These two peaks (370/420 nm) are fingerprint absorption of Au15(SR)13 NCs (Figure1b).13 As the reaction proceeds from 10 min to 1 h, the light-yellow solution deepened in color. This color change matcheswell with the intensity increase at 370 nm in the UV−visspectra (Figure 3b). At ∼1 h, the reaction solution changed tobrown. Meanwhile, a prominent peak at ∼560 nm and ashoulder peak at ∼610 nm were observed. These two peaks arethe characteristic absorptions of Au18(SR)14 NCs (Figure 1c).

13

From 1 to 8 h, the reaction solution deepened in brown, andthe absorption intensity at 560 nm increased with time. Inaddition, the absorption spectra for Au18(SR)14 NCs becamebetter-defined as the time increased from 1 to 8 h. Theabsorption spectrum of the reaction solution at 8 and 12 h wasidentical, which suggested that the formation of Au18(SR)14NCs was completed at ∼8 h. On the basis of the analyses of thetime-course optical absorption, the formation of Au18(SR)14NCs was most likely from Au(I)-thiolate complexes, to Au15NC intermediates, and finally to Au18 NCs. The discrete sizegrowth from Au15 to Au18 is currently unclear which needs toawait the resolving of their cluster structures.

Figure 2. UV−vis and ESI mass spectra of (a, d) Au15, (b, e) Au18, and(c, f) Au25 NCs protected by MPA. The insets in (a−c) show digitalphotos of the respective products and the insets in (d−f) are thecorresponding isotope patterns.

Figure 3. (a) Digital photos and (b) corresponding UV−vis spectra ofthe reaction solution for the synthesis of Au18(SG)14 NCs at differenttimes.

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Structure information is the key to understand the importantproperties (e.g., stability) of thiolated Au NCs. Although thegrowth of single crystals of small-sized NCs (<20 Au atoms)still remains a challenge, the scalable and precise synthesis ofthe three small thiolated Au NCs (Au10−12, Au15, and Au18)enables us to reveal their structural information by nuclearmagnetic resonance (NMR) spectroscopy. NMR has beenproved to be a powerful analysis tool to investigate thestructure of thiolated Au NCs.47−49 The 1H NMR spectrum ofAu18(SG)14 NCs is shown in Figure 4, and the inset image is

the molecular structure representing the Au18(SG)14 NCs withcarbon atoms bearing protons being labeled. Note that thestrong peak at 4.7 ppm is from D2O NMR solvent, and the J-coupling peaks at 3.6 and 1.1 ppm is from the residual ethanol,which was used to precipitate the Au18(SG)14 NCs fromaqueous solution. All the other peaks could be categorized intotwo groups, that is, the broad, major set of peaks from theAu18(SG)14 NCs and the shaper, less abundant peaks fromresidual disulfide GS-SG. By comparison with a previous NMRstudy on Au25(SG)18 NCs,

47 we assign the merged broad peaksat 3.6−3.8 ppm to the CH at C-1 and CH2 at C-6 and thebroad peaks at ∼2.1 and 2.5 ppm to the CH2 at C-2 and C-3,respectively.For further information, 2D correlation NMR spectroscopy

including homonuclear correlation spectroscopy (COSY) andheteronuclear single quantum correlation (HSQC) wereconducted. The COSY and HSQC spectra are shown in Figure5a,b. In the 2D correlation NMR spectroscopy, the signals ofthe Au18(SG)14 NCs and the residual disulfide GS-SG areobserved. While both sets of signals have been unambiguouslyassigned, only the assignment of the Au18(SG)14 NCs wasdescribed below. First of all, the assignments for C1, C2, C3,and C6 were further confirmed by their 2D patterns. Then, wefocus on the assignments of C-4 and C-5, which could not beassigned by 1H NMR but are more informative as they are themost adjacent carbons to Au atom. In the HSQC, the signals at3.2 and 3.4 ppm are CH2 signals connected to the same carbon(∼51 ppm), and this pair of signals is J-coupled as shown in theCOSY. In a similar way, the pair of peaks at 2.5 and 2.7 ppm isassigned to be a J-coupled CH2 signal connected to anothersingle carbon (∼60 ppm). The two pairs are assigned to theprotons at C-5, labeled as 5 and 5′, respectively (Figure 4 andFigure 5a). Note that the splitting of the proton signals, which

is also observed in the disulfide GS-SG spectrum, is induced bythe chiral carbon (C-4). Meanwhile, Figure 5a,b reveals thatthere’s a pair of peaks hidden under the D2O solvent peak(denoted as 4 and 4′). The signals are assigned to be the CH atC-4 because their J-coupling behavior with protons at C-5 isobserved in Figure 5a (highlighted by the two squares). InFigure 5b, signals for these two CH groups appear at 4.6 and4.7 ppm, and the chemical shifts of the corresponding carbonsare also distinguishable. As such, we have unequivocallyassigned all the NMR peaks of the Au18(SG)14 NCs.The most important finding in NMR analysis is the

demonstration of two distinct chemical environments of surfaceligands in the Au18(SG)14 NCs, indicated by the two sets of C-4and C-5 proton/carbon signals. A similar circumstance wasfound previously in the Au25(SG)18 NCs where two bindingmodes of the SG ligand were identified by NMR study.47 Inone type, each ligand connects the centered icosahedra Au13core and the exterior Au12 shell, whereas in the other type eachligand connects two exterior Au atoms. Our NMR resultssuggest the Au18(SG)14 NCs plausibly adopt a core−shellstructure and consequently two distinct ligand binding modes.Interestingly, the chemical shifts of C4 and C5 and relatedprotons induced by Au are different in these two clusters. Theproton signal on C5 in Au25(SG)18 NCs experienced adownfield shift, irrespective of ligand binding modes. For thesame protons on the Au18(SG)14 NCs, however, one set of

Figure 4. 1H NMR spectrum of Au18(SG)14 NCs.

Figure 5. Two-dimensional NMR of Au18(SG)14 NCs. (a) 1H−1HCOSY; (b) 1H−13C HSQC. Note that (1) only proton signalsderived from the Au18(SG)14 NCs are labeled in the 1H NMR on thetop of the figure, and (2) only signals from 4, 4′, 5, 5′, indicating thepresence of two sets of ligands on the cluster, are labeled in the 2DNMR spectra.

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peaks shifted downfield to ∼3.3 ppm whereas the other shiftedupfield to ∼2.7 ppm. In the 13C NMR spectrum of Au25(SG)18,the signal of C-5 comprises two peaks close to each other (34and 38 ppm), and C-4 displays a single peak at ∼56 ppm. ForAu18(SG)14, the two C-5 signals are further apart (51 and 62ppm) and C-4 splits into two peaks (50 and 51 ppm). Thesedifferences highlight that the Au25 and Au18 are significantlydifferent in structure and/or electronics, despite both adoptinga core−shell structure. The finding of two binding modes of SGligand in the Au18(SG)14 NCs also agrees well with recenttheoretical studies. For example, Jiang et al. predicted thestructure of Au18(SG)14 NCs as an Au8 core protected by twodimer (−S−Au−S−Au−S−) and two trimer (−S−Au−S−Au−S−Au−S−) motifs based on the “divide and protected”scheme.50 More recently, Garzon’s group presented theoreticalcalculation on the geometrical structure of the Au18(SR)14 NCsand confirmed that the proposed structure of the Au8 core (twofused tetrahedral Au4 units) covered by two dimer and twotrimer motifs was indeed most stable.51

4. CONCLUSIONIn summary, we presented here a versatile method to tailor thesize of thiolated Au NCs by simply adjusting the reaction pH inthe CO-reduction method. Our one-pot method is very simpleand can produce four different sized Au NCs (Au10−12, Au15,Au18, and Au25) protected by different thiol ligands (GSH andMPA) in large quantities (∼0.1 g). The mild reductionenvironment made possible by the reactive gaseous CO alsohelped one to monitor the cluster growth for thiolated Au18NCs, where an intermediate species of Au15 NCs was firstidentified during the formation of Au18 NCs. We also carriedout unprecedented 1D and 2D NMR analysis on theAu18(SG)14 NCs, which reveals two ligand binding modessupporting a core−shell structure of the NCs. The productsand the synthetic strategies demonstrated in this study are ofinterest not only because they provide a versatile and scalableprotocol for small Au NCs (Au10−12, Au15, and Au18) whichbridge the synthetic missing link between the single Au atomand large Au NCs (>20 Au atoms), but also because theyexemplify the usefulness of the reaction pH in tailoring thecluster size, which could also be a key factor in other reactionsystems for the synthesis of atomically precise Au NCs.Detailed studies on the structures and properties of these AuNCs are currently underway.

■ ASSOCIATED CONTENT*S Supporting InformationDetailed synthetic procedures for Au NCs protected by GSHand MPA; UV−vis spectrum and digital photos of a large-scalesynthesis of Au18(SG)14 NCs (Figure S1); comparison ofexperimental and simulated isotope patterns of different sizedAu NCs (Figures S2−S7 and S10−S12); TEM images ofAu10−12, Au15, Au18, and Au25 protected by GSH (Figure S8);XPS spectra of Au4f of as-synthesized GSH-protected Au NCsand large Au nanocrystals (Figure S9); schemes of pH-dependent protonation and deprotonation of the thiol ligandsused in this study (Figure S13 and S14) (PDF). This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: chexiej@nus.edu.sg (J.X.); cheyann@nus.edu.sg(N.Y.). Fax: +65 6779 1936. Tel: +65 6516 1067.

Author ContributionsThe manuscript was written through contributions of allauthors.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work is financially supported by the Ministry of Education,Singapore, under Grants R-279-000-295-133 and R-279-000-327-112. Y. Yu and X. Chen acknowledge the NationalUniversity of Singapore for their research scholarships.

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