Thiolate-Protected Hollow Gold Nanospheres

XU Wenwu, GAO Yi

Division of Interfacial Water and Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics,Chinese Academy of Sciences, Shanghai 201800, P. R. China.

Abstract: We present the atomic structure of thiolate-protected hollow Au nanosphere (HAuNS), Au60(SR)20, with high symmetry and stability based on the grand unified model (GUM; Nat. Commun. 2016, 7, 13574) and density-functional theory (DFT) calculations. Using C20 fullerene (with Ih symmetry) as a template, 20 tetrahedral Au4 units were used to replace the C atoms of C20, and three Au atoms of each Au4 were fused with three neighboring Au4 units by sharing one Au atom to form an icosahedral Au50 fullerene cage as the inner core. Subsequently, the unfused Au atom in each Au4 was bonded with the [―RS―Au―SR―]staple to form the completely hollow Au60(SR)20 nanosphere. Therefore, the Au60(SR)20 is composed of an icosahedral Au50 fullerene hollow cage (constructed by fusing 20 tetrahedral Au4 units) with 10 [―RS―Au―SR―] staples, obeying the “divide and protect” rule. Each Au4 unit has 2e valence electrons, namely, the tetrahedral Au4(2e) elementary block in the grand unified model. The DFT calculations showed that this hollow Au60(SR)20 nanosphere had a large HOMO–LUMO(HOMO: the highest occupied molecular orbital; LUMO: the lowest unoccupied molecular orbital) gap (1.3 eV) and a negative nucleus-independent chemical shift (NICS) value (-5) at the center of the hollow cage, indicating its high chemical stability. Furthermore, the NICS values in the center of the tetrahedral Au4 units were much more negative than that in the center of the hollow cage, revealing that the overall stability of Au60(SR)20 likely stemmed from the local stability of each tetrahedral Au4 unit. The harmonic vibrational frequencies were all positive, suggesting that the HAuNS corresponded to the local minimum of the potential energy surface. In addition, the bilayer HAuNS was designed by fusing the tetrahedral Au4 layers, indicating the feasibility of tuning the thickness of the shell of HAuNS. In bilayer HAuNS, each tetrahedral Au4 unit in the first layer shared four Au atoms, while those in the second layer shared one Au atom. The other three Au atoms of each tetrahedral unit bonded with the SR groups, demonstrating that each tetrahedral Au4 unit has 2e valence electrons (namely the tetrahedral Au4(2e) elementary block in GUM). The HOMO-LUMO gap of the bilayer Au140(SH)60 nanosphere is 1.5 eV, indicating its chemical stability. The thicknesses of the shells in monolayer and bilayer HAuNS are about 0.2 and 0.4 nm, respectively. This process could be easily understood in terms of the local stabilities of the tetrahedral Au4(2e) elementary block in GUM. Finally, the design of larger HAuNS, Au180(SR)60, has also been presented. The HOMO-LUMO gap of Au180(SH)60 was 0.9 eV, which showed that it was also a stable HAuNS. This work provides a new strategy to controllably design the HAuNS.

Key Words: Thiolate-protected hollow Au nanosphere; Density-functional theory; Grand unified model; “Divide and protect” rule; Nucleus-independent chemical shift

1 Introduction

The gold nanospheres (AuNSs) have attracted tremendous research interest due to the strong and tunable surface plasmon resonance (SPR) absorption band in the near-infrared (NIR)region, which can arise strong photothermal coupling property suitable for photothermal ablation (PTA) therapy and light-activated drugs release1–6. Although much progress has been made in this area, the most attention has been paid on the AuNSs with large size (larger than 30 nm in diameter). In particular, the atomic structures of the AuNSs have not been disclosed and characterized, which highly hinders the synthesis of the desirable AuNSs precisely and controllably. On the other hand, several small-sized pure hollow gold cages were predicted(Au327, Au428, and etc.) or synthesized (Au189) though, the insolubility becomes the bottleneck for their real applications.

Since the first successful crystallization of Au102(SR)4410, a serial of thiolate-protected gold nanoclusters (RS-AuNCs),such as Au18(SR)1411,12, Au20(SR)1613, Au23(SR)-1614,Au24(SR)1615, Au24(SR)2016, Au25(SR)-181/017,18, Au28(SR)20,Au30S(SR)1819–21Au36(SR)2422, Au38(SR)2423,24, Au40(SR)2425,Au52(SR)3225, Au130(SR)5026, Au133(SR)5227,28, Au246(SR)8029have been determined from X-ray crystallography. Many new characteristics regarding the gold–sulfur bonding as well as the atomic packing structure in RS-AuNCs have been clearly uncovered, which benefits us to well understand the structures and properties of Au clusters30–33. In addition, the “divide and protect” (D&P) concept30combined with the densityfunctional theory (DFT) approach have been notably advanced as a viable theoretical tool to predict the structures of RS-AuNCs34–43. Considering both predicted and crystallized RS-AuNCs are compact44–48, to design the hollow-caged nanoclusters with high stability become a challenging and intriguing topic.

Recently, a grand unified model (GUM) has been identified to gain fundamental understanding of a plethora of complex structures of ligand-protected gold nanoclusters49,50. In GUM,there are three possible valence electrons for Au atoms, i.e.,Au(1e), Au(0.5e), and Au(0e). And the ligand-protected gold nanoclusters can be divided into a series of elementary blocks(i.e., triangular Au3(2e) and tetrahedral Au4(2e)) and the secondary block (icosahedral Au13(8e)). The proposition of GUM makes it possible to design the hollow-caged nanoclusters with high stability. In this paper, we report a highly stable atomic structure of thiolate-protected hollow Au60(SR)20nanosphere based on DFT calculations. Then the bilayer HAuNS is designed by fusing tetrahedral Au4layers.Finally, the design of larger HAuNS, Au180(SR)60, is introduced.

2 Computational method

The structure of Au60(SR)20as well as Au32and Au42fullerene cages were optimized by using two DFT methods implemented in Gaussian09 program package51, the generalized gradient approximation (GGA) functional of Perdew, Burke and Ernzerhof (PBE)52and Becke’s three parameter hybrid functional with the Lee-Yang-Parr correlation functional B3LYP53. The LANL2DZ basis set54–56for Au atom and 6-31G*basis set for S and H were adopted. The harmonic vibrational frequency calculations and the nucleus-independent chemical shift (NICS) analyses57were performed based on the optimized geometries. The R group is simplified by the hydrogen atom.

3 Results and discussions

Fig. 1 (a) The sketch map of the replacement of a tetrahedral Au4 unit with a C atom in C20 fullerene; (b) the icosahedral Au50 fullerene hollow cage constructed by 20 fused tetrahedral Au4; (c) the complete Au60(SR)20 formed by protecting 10 [―RS―Au―SR―] on the icosahedral Au50 fullerene hollow cage.Au, olive and yellow; S, red; C, black. The R groups are omitted for clarity.The spherical hollow characteristic are indicated by gray balls.

Using C20fullerene (with Ihsymmetry) as the template, 20 tetrahedral Au4units were used to replace the C atoms of C20(Fig. 1a). In GUM, the elementary blocks satisfy the duet rule.Therefore, in order to keep the 2e valence electrons of each tetrahedral Au4 units, the three Au atoms of each Au4 unit should be fused with three neighboring Au4 by sharing one Au atom to form an icosahedral Au50fullerene cage as the inner core (Fig. 1b). Then the unfused Au atom in each Au4bonded with [―RS―Au―SR―] staple to form the complete hollow Au60(SR)20nanosphere (Fig. 1c). Since three shared Au atoms and one Au atom bonded with the SR group contribute 0.5e valence electron to each tetrahedral Au4, each Au4 unit has 2e valence electrons (namely the tetrahedral Au4(2e) elementary block in GUM). In addition, it can be found that the porous Au50fullerene cage has 12 pentagons, which is different from the closed structures of Au32and Au42fullerene cages7,8. The Au60(SR)20 can be divided into Au50[Au(SR)2]10, obeying the“D&P” rule30. The symmetry point group of Au60S20 is D5d without considering the R group.

From the optimized Au60(SR)20structure, the Au―Au bond lengths are 0.278 and 0.280 nm with PBE and 0.282 and 0.286 nm with B3LYP calculation, which are in consistence with the results from density functional studies for clusters such as Au32and Au42. Table 1 presents the calculated properties of hollow Au60(SR)20 nanosphere and its Au50 core. The properties of Au32and Au42fullerene cages are also calculated for comparison, which are in agreement with previous results7,8.The diameter of the icosahedral Au50core in Au60(SR)20is 0.8 nm, slightly smaller than that of icosahedral Au32and Au42fullerene cages. Due to the protected staples at the surface of Au50 core, the diameter of Au60(SR)20 reaches about 1.7 nm(the Au–Au distance between two opposite staples). The hollow Au60(SR)20nanosphere has a HOMO–LUMO gap of 1.3 eV with PBE and 2.4 eV with B3LYP calculations, which is close to the gap of Au32(PBE: 1.6 eV, B3LYP: 2.3 eV) but much larger than that of Au42(PBE: 0.4 eV, B3LYP: 0.9 eV),indicating the Au60(SR)20 is chemically stable. The harmonic vibrational frequencies of Au60(SR)20are all positive,demonstrating the nanosphere is at least the local minimum of the potential energy surface.

Table 1 The calculated properties of Au60(SH)20 and its Au50 core versus Au32 and Au42 fullerene cages using the PBE/B3LYP method implemented in Gaussian09 program package.

The NICS analyses have been performed to evaluate the stability of Au60(SR)20(Table 1). There is a negative NICS value (-5) at the center of the hollow cage, which can also illustrate the stability of Au60(SR)20. Recently, NICS analyses on the two tetrahedral Au4units of Au20(SR)16cluster36have been performed to support the concept of superatom-network(SAN)58, a theory to explain the stability of thiolate-protected gold nanoclusters. Very recently, the NICS values are also successfully applied to understand the stabilities of Au20(SR)1636, Au28(SR)2019,20, Au36(SR)2422, Au44(SR)2843, and Au52(SR)3225composed of fused tetrahedral Au4units with double-helix59. As the Au50core of Au60(SR)20is composed of twenty tetrahedral Au4fusing-units, it is necessary to examine the NICS value at the center of the tetrahedral Au4unit. It can be seen that the absolute NICS values (see Table 1) in the center of the tetrahedral Au4units exhibit a large number (-25),much larger than that in the center of hollow cage (-5),demonstrating that overall stability of Au60(SR)20stems likely from the local stability of each tetrahedral Au4unit. The local orbitals across the tetrahedral Au4units can be observed in Fig.S1 (Supporting Information), which further confirms their local stabilities.

The optical absorption spectra of hollow Au60(SH)20nanosphere are shown in Fig. 2. Three prominent absorption peaks, α (1.40 eV), β (1.59 eV), and γ (1.92 eV), corresponding to the transition from HOMO, HOME-7, and HOMO-24 to LUMO, respectively, can be observed.

Fig. 2 Theoretical optical absorption spectra of hollow Au60(SH)20 nanosphere using the PBE method implemented in Gaussian09 program package.The olive curve denotes TDDFT-computed spectra obtained from the individual optical transitions (wine vertical lines).

Fig. 3 The optimized monolayer Au60(SH)20 (a) and its Au50 core (b), bilayer Au140(SH)60 (c) and its Au110 core (d) using the PBE method implemented in Dmol3 code.The first and second layers are in olive and blue, respectively. Au, olive, blue, and yellow; S, red. The H atoms are omitted for clarity. The spherical hollow characteristic are indicated by gray balls.

Furthermore, the thickness of the shell of HAuNS can be tuned based GUM. As shown in Fig. 3d, a bilayer shell (Au110core) can be formed by fusing 20 tetrahedral Au4units on monolayer shell (Fig. 3b). Then the unfused Au atoms in the outer layer bonded with 30 [―RS―Au―SR―] staples to form the complete hollow Au140(SR)60 nanosphere (Fig. 3c). In the bilayer HAuNS (Fig. 3c), each tetrahedral Au4unit in the first layer have four shared Au atoms, and in the second layer it has one shared Au atom and three Au atoms bonded with SR groups, demonstrating each tetrahedral Au4unit has 2e valence electrons (namely the tetrahedral Au4(2e) elementary block in GUM). The optimized structure of bilayer Au140(SH)60 HAuNS has been presented using the PBE method52together with the double numeric polarized (DNP) basis set and the semi-core pseudopotential implemented in the Dmol3code60,61. The HOMO-LUMO gap of hollow Au140(SH)60nanosphere is 1.5 eV, indicating its chemical stability. The thicknesses of the shells in two HAuNSs (Fig. 3) are about 0.2, 0.4 nm,respectively. This process could be easily understood by the local stabilities of the tetrahedral Au4units discussed above.

Fig. 4 The optimized Au180(SH)60 (a) using the PBE method implemented in Dmol3 code and its Au150 core (b).Au, olive and yellow; S, red. The H atoms are omitted for clarity.The spherical hollow characteristic are indicated by gray balls.

Following the same building procedure of Au60(SR)20, the hollow Au nanospheres with larger diameters could be easily built based on the large-sized carbon fullerene. Fig. 4a gives the DFT (PBE method implemented in Dmol3code) optimized structure of Au180(SH)60by using C60fullerene as a template.The icosahedral Au150fullerene cage in Au180(SH)60is composed of 60 tetrahedral Au4fusing-units (Fig. 4b). The HOMO-LUMO gap of Au180(SH)60 is 0.9 eV, which shows it is a stable HAuNS. In addition, the bilayer nanosphere Au420(SH)180by fusing one layer tetrahedral Au4on the Au150fullerene cage of Au180(SR)60has also been presented schematically (Fig. S2).

4 Conclusions

In conclusion, we have presented the atomic structure of thiolate-protected hollow Au nanosphere, Au60(SR)20, with high symmetry and stability based on the grand unified model and density-functional theory (DFT) calculations. The icosahedral Au50fullerene cage in Au60(SR)20is composed of twenty fused tetrahedral Au4by replacing each carbon atom of C20fullerene with tetrahedral Au4unit. Each tetrahedral Au4unit in the nanosphere has 2e valence electrons (namely the tetrahedral Au4(2e) elementary block in GUM). DFT calculations show that this hollow Au60(SR)20nanosphere has a large HOMO-LUMO gap (1.3 eV) and a negative nucleusindependent chemical shift (NICS) value (-5) at the center of hollow cage, indicating its chemical stability. Furthermore, the absolute NICS values in the center of the tetrahedron Au4 units exhibit a larger number than that in the center of hollow cage,demonstrating that overall stability of Au60(SR)20 stems likely from the local stability of each tetrahedron Au4(2e) elementary block. The harmonic vibrational frequencies analyses illustrate Au60(SR)20nanosphere is at least the local minimum of the potential energy surface. More importantly, the shell thickness and the cage size of hollow gold nanospheres (HAuNSs) can be tuned. This work is not only beneficial for us to understand the growth of HAuNSs, but enriches the tuning possibilities for their future applications.

Acknowledgment: The computational resources utilized in this research were provided by Shanghai Supercomputer Center, National Supercomputing Center in Tianjin and Shenzhen, and Special Program for Applied Research on Super Computation of the Natural Science Foundation of China(NSFC)-Guangdong Joint Fund (the second phase) under Grant No.U1501501.

Supporting Information:available free of charge via the internet at http://www.whxb.pku.edu.cn.