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. 2023 May 26;14(22):5095–5101. doi: 10.1021/acs.jpclett.3c00794

Transforming Silver Nanoclusters from Racemic to Homochiral via Seeded Crystallization

Along Ma †, Wenjun Du §, Jiawei Wang †, Kefan Jiang †, Cheng Zhang †, Wenhan Sheng †, Haiyan Zheng †, Rongchao Jin ‡,*, Shuxin Wang †,*
PMCID: PMC10258841  PMID: 37234017

Abstract

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Chirality has risen as an attractive topic in materials research in recent years, but the attainment of enantiopure materials remains a major challenge. Herein, we obtained homochiral nanoclusters by a recrystallization strategy, without any chiral factors (i.e., chiral ligands, counterions, etc.). Through the rapid flipping of configuration of silver nanoclusters in solution, the initial racemic Ag40 (triclinic) nanoclusters are converted to homochiral (orthorhombic) as revealed by X-ray crystallography. In the seeded crystallization, one homochiral Ag40 crystal is used as a seed to direct the growth of crystals with specific chirality. Furthermore, enantiopure Ag40 nanoclusters can be used as amplifiers for the detection of chiral carboxylic drugs. This work not only provides chiral conversion and amplification strategies to obtain homochiral nanoclusters but also explains the chirality origin of nanoclusters at the molecular level.

In nature, chirality is a common feature in numerous systems, ranging from small molecules (e.g., tartaric acid and amino acids) to atomic aggregates (e.g., nanoparticles and nanoclusters) and macroscopic objects (e.g., human hands and atmospheric cyclones). In recent years, great progress has been made in the application of chiral nanomaterials in asymmetric catalysis, biorecognition, chiral medicine, and chiroptics.1−5 However, the origin of chirality generation is still unclear. Further, achieving symmetry breaking and amplification of chiral products are challenging. On the one hand, absolute asymmetric synthesis (AAS) was attained; that is, in the absence of chiral ligands or chiral environments, an enantiomer is spontaneously obtained.6,7 It is worth noting that external forces are usually required to achieve symmetry breaking, such as stirring, ultrasound, temperature gradients, and so on.8−11 Without the presence of such factors, the enantiomeric excess (ee) value of the obtained product will be significantly lower, and even racemic results will be obtained. This motivated us to ponder whether the molecule itself can complete the symmetry breaking and give rise to a product with homochirality in the absence of external forces. On the other hand, the seeded crystallization method has been widely used in the synthesis of nanocrystals.12,13 Furthermore, seed crystals can affect the morphology and properties of crystal products.12,13 For example, NaClO3 is achiral in solution, but it can crystallize into chiral crystals, and chiral seed crystals of NaClO3 can induce the formation of chiral crystal products.13

Ligand-protected nanoparticles with atomically precise nature, often called nanoclusters (NCs), have been of tremendous interest due to their precise structures and extraordinary properties.14−32 By introducing chiral factors, such as chiral ligands or chiral counterions, optically active nanoclusters have been synthesized.33−40 In addition, racemic nanoclusters such as Au38 were separated into enantiomers through chiral resolution.41−45 Further studies have shown that it is difficult to make homogold nanoclusters undergo a configuration reversal at room temperature. However, it is interesting that, after doping silver atoms into the kernel of gold nanoclusters, the temperature for configuration reversal can be significantly reduced.46,47 Theoretically, thiolated silver or heavily silver-doped nanoclusters can only exist in the form of racemates in solution.47−49 The rapid flip of chirality is achieved by changing the arrangement of the outer metal complex shell. Since the nanocluster itself has a metal core, its chiral construction does not depend on primary nucleation as in the AAS process. In theory, external factors (such as stirring, ultrasound, etc.) are not necessary to achieve symmetry breaking. For the cluster protected by the achiral coligands and crystallized in a noncentrosymmetric space group, if one can build a stable system and obtain only one crystal, then nearly 100% ee value could be obtained.50 In the seeded crystallization method, the chirality of seed crystals can affect the chirality of final products. The racemic nanoclusters can rapidly flip between chiral structures in solution. If a chiral crystal is used as a seed, it may be feasible for racemic clusters to grow into homochiral crystals with the initial seed; then amplification of chiral crystals will be realized.

In this work, we devise a crystallization approach for achieving homochiral recrystallization and seeded crystallization. We first synthesized racemic Ag40 nanoclusters. Then, we developed an automatic crystallization system by using a robot in order to quickly screen the crystallization conditions. After optimizing the conditions, the probability of obtaining one crystal in the crystallization vial is close to 10%. After testing more than 40 crystals, we found that 2 of the crystals are in the orthorhombic system (P212121, flack = 0.006), which means that an enantiomeric excess close to 100% has been achieved, and the chirality of the resulting crystal is random in our current work without introducing any chiral factors and external forces. Further, based on the principle of rapid structure flipping of silver nanoclusters in solution, using one homochiral crystal obtained by recrystallization as the seed allows rapid construction of crystals with conspecific chirality in a racemic solution. The chirality of the final product crystals is consistent with the chirality of the initial seed. It should be noted that the state of the resulting products is many crystals and not a single one. In addition, Ag40 nanoclusters can be used as chiral amplifiers to determine the ee values of chiral carboxylic acids. This work not only provides new insight into the chiral origin of nanomaterials but also provides new insight for chiral separation and chiral crystal amplification.

Racemic Ag40 was first synthesized, and crystals were obtained (Figure S1a). Details of the synthesis are provided in the Supporting Information. X-ray crystallography revealed that racemic Ag40 NCs were crystallized in a triclinic P1̅ space group, which is achiral. The crystal structure of Ag40 can be viewed as a kernel-shell structure, which contains an icosahedral Ag13 kernel and an Ag27 complex shell (Figure S2). Notably, the Ag27 complex shell consists of three Ag7 chains, every two Ag7 chains are connected by an Ag2(CH3COO)(TBBM)4 “button”, and the chiral structure is determined by the direction of the chain rotation (Figures 1 and S3). The structure of Ag40 obtained in this work is not the same as the Ag40 previously reported;51 however, it is similar to the doped AuAg39 nanocluster.51 The UV–vis absorption spectrum of Ag40 in CH2Cl2 (DCM) exhibits three characteristic peaks at 460, 500, and 610 nm (Figure S4a), the corresponding circular dichroism (CD) spectrum shows that Ag40 is racemic in DCM solution (Figure S4b). ESI-MS was performed; however, no meaningful signal of Ag40 was obtained in either the positive or negative ion mode (Figure S5). After the addition of cesium ions, the situation did not improve (Figure S5c). This may be due to the Ag40 nanocluster being neutral.

Figure 1.

Figure 1

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Chiral structure of the Ag40 nanoclusters. (a) X-ray structure of the two mirror-symmetric enantiomers of chiral Ag40. (b) Ag5(TBBM) units on top and bottom of Ag40. (c) Ag21(TBBM)2(CH3COO)9 spiral structure formed by three Ag7 chains in Ag40. (d) Spiral arrangement of Ag2(CH3COO)(TBBM)4 units at the waist of Ag40. Color codes: green or silver, Ag; yellow, S; red, O; gray, C. All H atoms and C atoms in TBBM are omitted for clarity.

The ratio between TBBM and acetic ligands were confirmed by 1H NMR in Figure S6. The 1H signals at 5.97–8.25 ppm (80 H = 20 × 4), 2.57–5.12 ppm (40 H = 20 × 2), and 0.58–1.48 ppm (180 H = 20 × 9) correspond to phenyl groups, benzyl groups, and tertiary butyl groups in TBBM. The 1H signals at 1.66–2.35 ppm (36 H = 12 × 3) correspond to the CH3COO–. These results are consistent with 20 TBBM and 12 CH3COO– ligands as well as single-crystal X-ray diffraction (SC-XRD) in the Ag40 nanocluster. Additionally, X-ray photoelectron spectroscopy (XPS) revealed the elemental composition of Ag40, and semiquantitative analysis indicates the Ag/S/O ratio of 1:0.47:0.49, which is close to the 1:0.5:0.6 of the SC-XRD result (Figure S7). The EDS further proved the presence of Ag, S, and O elements in the Ag40 nanocluster (Figure S8), which was consistent with the cluster compositions obtained by SC-XRD. TGA was also performed to verify the ratio of metals to ligands in Ag40, and the experimental value of 49.49% was in good agreement with the theoretical value of 49.88% (Figure S9).

As shown in Figure 1, the origin of the chirality in the Ag40 lies in the asymmetric arrangement of one Ag21 framework (Figure 1c) and three Ag2(CH3COO)(TBBM)4 buttons (Figure 1d), whereas the icosahedral Ag13 kernel is achiral. Three chains are bonded to the top and the bottom of the Ag21(CH3COO)9(TBBM)8 framework by TBBM ligands, forming two orthogonal Ag5(TBBM) units. For the enantiomers, three Ag2(CH3COO)(TBBM)4 buttons form a left- and right-handed helical arrangement at the waist of Ag40 and are related by a C3 axis. From the top view, the helical arrangement of the Ag2(CH3COO)(TBBM)4 buttons resembles a triblade fan.

Silver or silver-doped nanoclusters undergo rapid racemization in solution.47 We rationalize that it should be possible for a rapidly racemizing nanocluster to produce a monochirality crystal during the crystallization. There are two processes involved: (i) synthesizing racemic silver nanoclusters; here, the newly obtained Ag40 nanocluster is chosen as a model in testing our chiral crystallization approach; (ii) screening the crystallization conditions to obtain one crystal in the solution.

In order to achieve rapid screening, we designed a set of high-throughput automated crystallization system (Figures 2 and S10). As such, a grain of homochiral Ag40 crystal was acquired and divided into five pieces (Figure 3). All pieces were homochiral and right-handed unraveled by SC-XRD. Furthermore, the homochiral Ag40 was found to crystallize in an orthorhombic P212121 space group. There are three important variables to be concerned with in this model: (1) Concentration of the DCM solution. To obtain Ag40 nanoclusters in one- and fine-crystal form, the concentration of Ag40 in the DCM solution is very crucial. When the solution is highly concentrated, it gives rise to the primary form of racemic crystals. Here, we control the concentrations to 3, 4, 5, 6, 7, 8, 9, and 10 mg/mL, respectively. (2) Ratio and volume of the diffusion layer. In this experiment, the ratio of the two-solvent diffusion layer is chosen as VDCM:VCH3CN = 1:1, 1:2, and 1:3, and the volume of the diffusion layer is 0.5 or an equal multiple of the Ag40 DCM solution that is injected. Too much diffusion layer can make the crystal growth difficult. (3) Recrystallization temperature. To explore the temperature gradient influence, temperature gradients of 10, 20, and 30 °C are controlled to recrystallize Ag40 crystals. With the help of the automatic crystallization system, 144 vials of crystallization can be completed uniformly each time (Figures 2g,h and S10). All the crystallization results are shown in Table S1.

Figure 2.

Figure 2

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Illustration of crystallization. (a) Adding DCM solution of racemic Ag40. (b) Spread the diffusion layer above the DCM solution of Ag40. (c) Inject CH3CN to the top of the diffusion layer. (d) One triclinic Ag40 crystal resulted. (e) One triclinic Ag40 crystal is recrystallized and converted into one triclinic (Case 1) or homochiral Ag40 (Case 2) crystal by dissolution and recombination. (f) One homochiral crystal (Case 2) was used as seed to yield homochiral crystals. (g) Diagram of an automatic crystallization system. (h) Photograph of Ag40 crystals under different recrystallization conditions. (i) Photograph of Ag40 before and after recrystallization. (j) One homochiral Ag40 crystal after recrystallization.

Figure 3.

Figure 3

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Photographs of the five parts from one homochiral Ag40 crystal. One homochiral Ag40 crystal is divided into five pieces (Parts 1–5), and their structures are all of the right-handed form (solved by SC-XRD). Color codes: silver, Ag; yellow, S; red, O; gray, C. All H atoms are omitted for clarity.

Interestingly, in the crystallization conditions of 30 °C, concentration of 4 mg/mL, and volume of the diffusion layer (VDCM:VCH3CN = 1:1) was half of the DCM solution, homochiral Ag40 crystal was successfully acquired (Figure 2i,j). X-ray diffraction identified orthorhombic Ag40 crystals. This remarkable discovery indicates that the crystal system of Ag40 crystals is converted from the originally triclinic system (racemic) to the orthorhombic system (homochiral) after recrystallization. Further, Ag40 nanoclusters show excellent stability in the solution and solid state, which indicates that the clusters are not decomposed during the recrystallization (Figure S11). Under this set of crystallization conditions, more than 40 crystals from different vials were tested, and two of them were found to be homochiral (one crystal is left-handed and the other is right-handed) and orthorhombic (P212121), therefore the probability of racemic Ag40 nanoclusters being converted to one homochiral Ag40 crystal is 5% (Figure 2d,e).

Based on the principle that silver nanoclusters flip rapidly in solution, we have obtained homochiral Ag40 crystals. Meanwhile, this principle also provides the possibility for the amplification of chiral crystals. We used one homochiral Ag40 crystal as seed to make the racemic Ag40 clusters in solution convert into homochiral crystals by the seeded crystallization (Figure 2e,f). Simply, we first identified one homochiral Ag40 crystal (i.e., left-handed Ag40, abbrev. L-Ag40) by SC-XRD. Then, it was placed in a saturated DCM solution of Ag40 (racemic) and CH3CN was added; this L-Ag40 crystal remained in the crystalline state. After about 1 week, many L-Ag40 crystals were produced (Figure S1b). The morphology of all L-Ag40 crystals via seeded growth is similar to the homochiral Ag40 crystal and racemic Ag40 crystals, which is of black block (Figures 2j and S1). Expectedly, during the seeded crystallization process, racemic Ag40 clusters in solution rapidly flip, nucleate, and grow into homochiral crystals in the presence of one chiral crystal. We also note that the chirality of the crystals via seeded growth is consistent with the chirality of the initial seed.

It is interesting that the Ag40 nanoclusters which are crystallized in P212121 space group show optical activity in solid state but no activity in solution. This is due to the rapid racemization of the crystal upon redissolution. The CD spectra and corresponding UV–vis spectra of Racemic-Ag40 (Rac-Ag40), L-Ag40, and right-handed Ag40 (R-Ag40) crystals in solid state were collected at 250–800 nm (Figures 4a,b and S12a). The CD spectra of L-Ag40 and R-Ag40 are in mirror image of each other, confirming that they are pure enantiomers. As shown in Figure 4b, the L-Ag40 curve (blue) exhibits seven prominent peaks from 250 to 800 nm, including 261 (+), 283 (−), 326 (+), 386 (−), 432 (+), 469 (−), and 535 nm (+). The corresponding anisotropy factors g = ΔA/A = θ[mdeg]/(32980 × A) were calculated over the spectral range; gmax is about 5.9 × 10–4 at 261 nm (Figure 4c). It is worth noting that, using homochiral crystals (i.e., L-Ag40) as seeds, the obtained crystals have CD spectra similar to that of the original crystal (Figure 4d). Notably, the CD value of L-Ag40 seeded crystals decreased by approximately 10%, compared to the initial seed. Additionally, in order to better analyze the chirality of the samples, the corresponding UV–vis spectra and g-factors of L-Ag40 crystal and L-Ag40 seeded crystals in solid state were compared (Figures 4a, S12b, and S13). Notably, the g-factors of L-Ag40 seeded crystals decreased by about 16%. The gmax of L-Ag40 seeded crystals is about 4.5 × 10–3 at 260 nm. The possible reason might be that not all crystals were nucleated and grown with the added chiral crystal.

Figure 4.

Figure 4

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(a) UV–vis spectra of R-Ag40 crystals (red) and L-Ag40 crystals (blue) in solid state. (b) Experimental CD spectra of Rac-Ag40 crystals (black), R-Ag40 crystals (red), and L-Ag40 crystals (blue) in solid state. (c) Corresponding anisotropy factors of R-Ag40 and L-Ag40 enantiomers. (d) Experimental CD spectra of R-Ag40 crystals (red), L-Ag40 crystals (blue), and L-Ag40 seeded crystals (cyan).

There are 12 Ag40 molecules around one central Ag40 molecule in racemic or homochiral Ag40 crystals (Figures S14–S27). The density of the two crystals is calculated. After deducting the influence of the solvent, the density of the racemic Ag40 crystals is 1.6 g/cm3 and the homochiral is 1.9 g/cm3, explaining that homochiral Ag40 crystals has a higher packing density. For the intermolecular weak interactions, the C–H···π and π–π interactions play a vital role in the packing of Ag40 crystals.52 In racemic Ag40 crystals, only Ag40 molecules that are located in the b-axis direction have the C–H···π interaction with the central Ag40 molecule, as shown in Figure S28. The average distance for the C–H···π interaction is 3.073 Å for the left or right Ag40 molecule. In plane 1 of the b-axis direction of homochiral Ag40, the average distance of the eight C–H···π interactions is 3.466 Å (Figure S29). As shown in Figure S30, the average distance for the C–H···π interaction is 3.560 Å in plane 2 of the c-axis direction of homochiral Ag40. The π···π interactions are also discovered, and the average spacing is 5.023 Å. In homochiral Ag40 crystals, the 12 C–H···π interactions and four π···π interactions allow a central molecule to form a closed-ring structure with eight surrounding molecules in planes 1 and 2, which results in restrictions of the motions of the whole nanocluster. The connectivity via C–H···π and π···π interactions continues all the way along the a-axis of the unit cell, leading to the formation of the needle-like single crystal along the [100] direction. By contrast, C–H···π interactions exist only in a single plane along the [010] direction, which makes a less compact packing in racemic Ag40 crystals. Under the recrystallization, it is highly likely to obtain the racemic Ag40 crystal of the P1̅ space group. With a difference in the kinetic barrier (Ea, high for homochiral crystals), such crystals are not easy to form. This also shows that the acquisition of chiral crystals is a thermodynamic process in this case: higher temperatures favor the denser phase, which happens to be the chiral phase.

Furthermore, the Ag40 nanocluster could react with chiral 2-chloropropionic acid/ibuprofen/naproxen to result in an optically pure enantiomer. 1H NMR and CD spectra showed that the CH3COO– ligand on the Ag40 surface could be rapidly replaced by these chiral acids (Figures S31–S50). The UV–vis spectra of Ag40 did not change with ligand exchange, indicating that the structure of Ag40 was retained (Figures S46a and S50b). The CD spectra of Ag40 (R-2-chloropropionic acid) and Ag40 (S-2-chloropropionic acid) showed symmetrical peaks at 241, 275, 337, 423, 462, 507, and 623 nm, respectively (Figure S46b). The anisotropy factors were calculated, and gmax is about 1.1 × 10–3 at 343 nm (Figure S46c), which is comparable to that of chiral silver nanoclusters.53,54 When using Ag40 to detect chiral compounds such as ibuprofen or naproxen, the maximum CD signal can be obtained when the molar ratio of the chiral compound is around 7 times higher than the cluster (Figure S47c,d). The 1H NMR results showed an average of ∼7 ibuprofen or naproxen per cluster (Figures S39, S40, S43, and S44). For chiral 2-chloropropionic acid, the CD signals approached the maximum at 1:12 (Figure S47a). The 1H NMR results showed an average of ∼6 “2-chloropropionic acid” per cluster (Figures S34 and S36). These results indicate that chiral inversion can be achieved in the Ag40 clusters if approximately half of the carboxyl groups are replaced by chiral acids. In order to prove that excessive chiral acids can cause the chiral inversion of Ag40 nanoclusters, the ligand exchange between the homochiral Ag40 (chiral acids) and the chiral acids with the opposite chirality and the corresponding CD spectra confirm that the opposite chirality can be induced (Figure S48).48 Meanwhile, the enantiomer-dependent CD intensity in the R/S-2-chloropropionic acid ligand-exchange process also linearly correlates with the ee values, and the detection range was from 0 to 100% ee (Figure 5a,b). Notably, R/S-2-chloropropionic acid in CH2Cl2 exhibited only one CD signal at 232 nm (Figure S49), which is close to the far-ultraviolet region and requires high sensitivity for the instrument. For R/S-2-chloropropionic acid, the Ag40 cluster can effectively extend the detection range from deep ultraviolet to visible light (Figure S47a). Meanwhile, the Ag40 nanocluster can be used as chiral amplifiers for chiral carboxyl drugs such as R/S-ibuprofen and R/S-naproxen (Figures S50 and S51).

Figure 5.

Figure 5

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(a) CD spectra of Ag40 and R/S-2-chloropropionic acid of different ee% (0, 20, 40, 60, 80, 100%) in CH2Cl2. (b) The fitted curve by plotting CD readings at 337 nm in panel a against ee values of chiral R/S-2-chloropropionic acid.

In summary, the Ag40 nanoclusters are analyzed by SC-XRD, UV–vis, 1H NMR, XPS, TGA, ESI-MS, and CD. Chiral symmetry breaking, absolute asymmetric synthesis, and the amplification of chiral crystals in metal nanoclusters from racemic Ag40 nanoclusters to homochiral ones are accomplished with the help of the crystallization model. In addition, Ag40 nanoclusters can be used as chiral amplifiers and provide guidance for cluster-based chiral sensors. Our work demonstrates the possibility of homochiral construction without the influence of chiral factors (including ligands, environment, etc.), nor the external forces. Further, the seeded crystallization method proved that the effect of chiral crystal seed on the chirality construction of final products and amplification of chiral crystals were achieved. The results provide important implications for the transformation of chirality between nanoclusters, chiral separation, the amplification of chiral crystals, chiral detection, and the origin of chirality generation.

Acknowledgments

The authors thank Prof. Di Sun (Shandong University, Jinan, China) for assistance in single-crystal testing. S.W. acknowledges the support from the National Natural Science Foundation of China (22171156 and 21803001) and Startup Funds from Qingdao University of Science and Technology.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.3c00794.

  • Additional experimental details, materials, and methods, including supporting Figures S1–S51 and Tables S1–S8 (PDF)

The authors declare no competing financial interest.

Supplementary Material

jz3c00794_si_001.pdf (5.9MB, pdf)

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