Chirality Inversion of CdSe and CdS Quantum Dots ... - ACS Publications

Mar 3, 2016 - (iv) chiral assemblies of achiral QDs.21−31 Since capping ligands can ... report the inversion of the CD spectra of CdSe and CdS QDs i...
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Chirality Inversion of CdSe and CdS Quantum Dots without Changing the Stereochemistry of the Capping Ligand Jung Kyu Choi,†,⊥ Benjamin E. Haynie,† Urice Tohgha,† Levente Pap,† K. Wade Elliott,† Brian M. Leonard,† Sergei V. Dzyuba,‡ Krisztina Varga,*,† Jan Kubelka,*,† and Milan Balaz*,§ †

Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071, United States Department of Chemistry and Biochemistry, Texas Christian University, Fort Worth, Texas 76129, United States § Underwood International College, Integrated Science & Engineering Division, Yonsei University, Seoul 03722, Republic of Korea ‡

S Supporting Information *

ABSTRACT: L-cysteine derivatives induce and modulate the optical activity of achiral cadmium selenide (CdSe) and cadmium sulfide (CdS) quantum dots (QDs). Remarkably, N-acetyl-L-cysteine-CdSe and L-homocysteine-CdSe as well as N-acetyl-L-cysteine-CdS and L-cysteine-CdS showed “mirrorimage” circular dichroism (CD) spectra regardless of the diameter of the QDs. This is an example of the inversion of the CD signal of QDs by alteration of the ligand’s structure, rather than inversion of the ligand’s absolute configuration. Non-empirical quantum chemical simulations of the CD spectra were able to reproduce the experimentally observed sign patterns and demonstrate that the inversion of chirality originated from different binding arrangements of N-acetyl-Lcysteine and L-homocysteine-CdSe to the QD surface. These efforts may allow the prediction of the ligand-induced chiroptical activity of QDs by calculating the specific binding modes of the chiral capping ligands. Combined with the large pool of available chiral ligands, our work opens a robust approach to the rational design of chiral semiconducting nanomaterials. KEYWORDS: chiral semiconductor nanocrystals, ligand-induced optical activity, circular dichroism, nanoparticles, quantum chemical simulations, density functional theory, quantum dots

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different chemical structure. Using non-empirical, fully quantum chemical simulations of the CD spectra for model structures, we rationalize the origins of the observed CD signals. Together, the experiments and simulations provide insights into the interactions and binding models of chiral ligands with QD surfaces to yet unprecedented levels of detail.

hiral, optically active nanoparticles (NPs) are promising candidates for biosensing, stereoselective reactions, and chiral memory applications as well as for the bottom-up fabrication of chiroptical nanomaterials.1−20 Chirality in semiconductor quantum dots (QDs) can concurrently originate from (i) intrinsic dissymmetry of a nanocrystal, (ii) the ligand-induced chiral surface of QDs, (iii) the electronic coupling between chiral capping ligands and achiral QDs, or (iv) chiral assemblies of achiral QDs.21−31 Since capping ligands can influence chemical and electronic properties of QDs,32,33 the postsynthetic functionalization of achiral QDs with chiral capping ligands is an ideal approach to induce and control chirality in semiconductor nanomaterials. Some of us reported the first example of chiral QDs prepared by postsynthetic phase transfer ligand exchange on achiral QDs: D- and L-cysteine-functionalized CdSe displayed mirror-image circular dichroism (CD) as well as circularly polarized luminescence (CPL) spectra.28,29 Here, for the first time, we report the inversion of the CD spectra of CdSe and CdS QDs induced by ligands of identical absolute configuration but © XXXX American Chemical Society

RESULTS AND DISCUSSION N-Acetyl-L-Cys-Functionalized QDs Show “MirrorImage” CD Compared to L-HomoCys. CdSe QDs (ØCdSe = 4.4 and 2.9 nm) functionalized with six different cysteine derivatives (Figure 1) were prepared from oleic-acid-capped CdSe QDs (OA-CdSe QDs) by postsynthetic phase transfer ligand exchange (see Supporting Information). CdSe QDs (ØCdSe = 4.4 nm) capped with cysteine derivatives displayed Received: January 24, 2016 Accepted: March 2, 2016

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(ØCdSe = 4.4 nm) displayed a (−/+/−/+/−) CD pattern. The first excitonic CD band was thus negative for CdSe functionalized with N-Ac-L-Cys, whereas it was positive for all remaining cysteine derivatives. A comparison of the CD spectra of CdSe led to a remarkable observation: L-HomoCys-CdSe and N-Ac-L-Cys-CdSe exhibited mirror-image CD spectra: N-Ac-L-Cys-CdSe showed an alternating (−/+/−/+/−) CD profile, while the L-HomoCysCdSe displayed an inverse alternating (+/−/+/−/+) CD profile. To the best of our knowledge, this is the first example of mirror-image CD spectra of QDs induced by chiral ligands with the same central chirality. The cysteine derivatives also induced chiroptical activity in smaller CdSe QDs (Figure S2, ØCdSe = 2.9 nm) and CD spectra of N-Ac-L-Cys-CdSe (−/+/−/+/− profile) and L-HomoCys-CdSe (+/−/+/−/+ profile) showed again a clear mirror-image relationship (Figure S2f). Thus, the diameter of the CdSe had a negligible effect on the ligandinduced mirror-image CD signatures of the CdSe QDs. Chirality Inversion Is Independent of the Chemical Composition of the QDs. To further explore the scope of CD inversion in QDs by modification of the structure of the capping ligand, we studied the optical properties of CdS QDs (D = 4.3 nm) functionalized with L/D-Cys, N-Ac-L/D-Cys, and L/D-Pen (Figure 2). Mirror-image CD patterns (Figure 2e) were observed for L-Cys-CdS (+/−/+/−/+ CD profile, Figure 2a) and N-Ac-L-Cys-CdS (−/+/−/+/− CD profile, Figure 2c). Similarly, smaller CdS QDs (ØCdS = 3.7 nm) functionalized with L-Cys and N-Ac-L-Cys also displayed mirror-image CD spectra (Figure S3). Similarly to CdSe, CdS capped with N-Ac-

Figure 1. CD spectra of CdSe QDs (ØCdSe = 4.4 nm) functionalized with (a) L-Cys, (b) L-Cys-Me-ester, (c) L-Pen, (d) L-HomoCys, and (e) N-Ac-L-Cys and N-Ac-D-Cys capping ligands. (f) UV−vis absorption spectra. (g) Mirror-image CD spectra of L-HomoCysCdSe and N-Ac-L-Cys-CdSe. Conditions: [CdSe] = 2.8 μM (ØCdSe = 4.4 nm). (h) Structures and abbreviations of cysteine capping ligands.

band-edge absorption maxima at 595 nm.34 All ligands successfully induced chiroptical activity in CdSe QDs (Figure 1a−e). The structure of the capping ligand had a pronounced effect on the shape of the CD signature of the CdSe QDs. L-Cys induced the strongest CD with an alternating (+/−/+/−/+/−; from long to short wavelength) pattern at the excitonic wavelengths (Figure 1a), while L-Cys methyl ester gave rise to a similar CD spectrum, albeit with a weaker intensity (Figure 1b). In contrast, capping CdSe with L-Pen and L-HomoCys (introducing two methyl groups on the Cβ carbon or γmethylene group between the Cβ carbon and thiol group, compared to L-Cys, respectively) significantly changed the shape of the CD signal of the CdSe (Figure 1c,d): L-Pencapped CdSe exhibited a (+/−/+/+/−/+) CD pattern, while LHomoCys-CdSe showed an alternating (+/−/+/−/+) CD pattern. The presence of an acetyl moiety on cysteine (i.e., NAc-L-Cys) had the most dramatic effect on the induced CD signal (Figure 1e): N-Ac-L-Cys-functionalized CdSe QDs

Figure 2. CD spectra of CdS QDs functionalized with (a) L-Cys and D-Cys, (b) L-Pen and D-Pen, and (c) N-Ac-L-Cys and N-Ac-D-Cys capping ligands. (d) UV−vis absorption spectra. (e) Mirror-image CD spectra of L-Cys-CdS and N-Ac-L-Cys-CdS. Conditions: [CdS] = 1.4 μM (ØCdS = 4.3 nm). B

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ACS Nano exhibited a negative first excitonic CD band, while all other cysteine derivatives induced a positive CD band. The fluorescence-detected circular dichroism (FDCD) and CD spectra of N-Ac-L-Cys-CdS and L-Cys-CdS exhibited matching polarities, thus confirming the presence of optically active band gap states in CdS QDs (Figure S3c).35 Notably, CdSe and CdS QDs capped with D-enantiomers of cysteine derivatives induced true mirror-image CD spectra in comparison to L-ligands (Figures 1 and 2). To quantify the influence of the capping ligand on the dissymmetry of the chiral CdSe and CdS QDs, we calculated their CD anisotropy g factors (Table 1). The g factor is defined L-Cys

Table 1. CD Anisotropy g Factors for Chiral CdSe and CdS QDs

Øa

CdSe

4.4

CdSe

2.9

CdS

4.3

CdS

3.7

ligand L-HomoCys

N-Ac-L-Cys L-HomoCys N-Ac-L-Cys L-Cys N-Ac-L-Cys L-Cys N-Ac-L-Cys

λmaxb 595.0 595.2 548.0 546.4 426.8 426.2 410.1 407.5

λCDd

gCDc +1.3 +0.5 +0.2 +0.4 +0.4 +0.4 +0.8 +0.4

× × × × × × × ×

−4

10 10−4 10−4 10−4 10−4 10−4 10−4 10−4

550.4 584.6 548.6 531.6 430.0 415.5 379.5 401.0

Figure 3. (a) CD spectra of L-Cys-CdS and N-Ac-L-Cys-CdS (ØCdS = 4.3 nm) recorded at [CdS] = 3.2 μM (top) and 0.81 μM (bottom). (b) UV−vis (426 nm) and CDmax (∼435 nm) signals of L-Cys-CdS as functions of absorbance. (c) UV−vis (426 nm) and CDmax (∼422 nm) signals of N-Ac-L-Cys-CdS as functions of absorbance. HRTEM images of (d) N-Ac-L-Cys-CdSe and (e) L-HomoCysCdSe (ØCdSe = 4.4 nm). Insets: Images of QDs showing a 111 lattice spacing of ∼3.48 Å (box size: 5 × 5 nm).

a

QD diameter (in nm) determined by Peng’s equation34 from the absorption spectra. bBand-edge absorption maximum (in nm). cCD anisotropy g factors of positive CD bands within the CdSe band-edge absorption region. dPositive CD band wavelengths (in nm).

the CdSe−ligand systems and their corresponding CD spectra theoretically. To our knowledge, with the exception of our own preliminary study on chiral semiconductor QDs28 and the very recent semiempirical (ZINDO) simulations for the chiral graphene QDs by Kotov et al.,20 calculations of CD spectra for model QD−ligand structures that allow direct comparison to the experimental data have not been attempted. Such simulations are crucial because the correspondence of the predicted and experimental CD patterns (or lack thereof) is the strongest, decisive indicator of the validity of the models. To allow for CD spectra simulations over a sufficient wavelength range in order to make the comparison with experiment meaningful, the size of the model system had to be kept to a minimum. To approximate the QD, we have used a small Cd9Se7 cluster (Figure 4a,b), constructed from the cubic (zinc blende) crystalline form cut along the (111) surface. The (111) plane represents the dominant type of surface of truncated octahedrons, a common type of structure adopted by semiconductor QDs36,37 and consistent with the HRTEM images (Figures 3d,e and S5) of the chiral CdSe QDs under investigation. The Cd9Se7 model consists of the central hexagon formed by three Cd and three Se atoms on the (111) surface, each bonded to its nearest-neighbor Cd or Se both on and below the surface, which were terminated by hydrogen atoms to satisfy the unsaturated valences. The structure of the model Cd9Se7 fragment was kept constant at the experimental geometry38 to allow properly modeling the binding of ligands to the QD surface, and only the capping hydrogen atoms were initially optimized (for computational details, see the Methods section and the Supporting Information). Despite its simplicity, the Cd9Se7 cluster captures very well the electronic structure of the CdSe QDs, as evidenced by the excellent agreement of the predicted shape of the UV−vis absorption (Figure S18). The blue shift in the computed spectra compared to the experimental data is expected due to the much smaller size of the model Cd9Se7 cluster. However,

as g = Δε/ε = (AL − AR)/A, where A is the conventional absorbance of nonpolarized light and AL and AR are the absorptions of left and right circularly polarized light, respectively. Table 1 shows that the structure of the capping ligand, chemical composition, and diameter of QDs had small effects on the ligand-induced CD anisotropy of studied CdSe and CdS QDs. Aggregation Is Not the Source of QD Chirality and CD Inversion. The mirror-image CD patterns of QDs capped with L-Cys and N-Ac-L-Cys could result either from electronic and structural interactions of chiral ligands with individual QDs or from chiral ligand-induced stereoselective aggregation of achiral QDs into chiral nanostructures. Based on UV−vis spectra, aggregation was not observed for any QD samples as no scattering was detected (Figures 1f, 2d, S2, and S3). In addition, variable concentration CD and UV−vis experiments showed that the mirror-image relationship of N-Ac-L-Cys-CdS and LCys-CdS spectra was not affected (ØCdS = 4.3 nm, Figures 3a and S4) and the CD intensity decreased nearly linearly with concentration and UV−vis absorption signal (Figure 3b,c). Finally, high-resolution transmission electron microscopy (HRTEM) data did not reveal the presence of any chiral selfassembled nanostructures2 or chiral defects on the surface of QDs23 (Figures 3d,e, S5, and S6). Therefore, chiral aggregation and intrinsic chirality of QDs were excluded as a source of the QDs’ chiroptical activity and CD signal inversion. Since thiolanchored cysteine ligands could interact with the QDs’ surface via the carboxylate groups and, in the case of N-Ac-L-Cys, also the acetyl group, different structural and electronic interactions between chiral capping ligands and achiral CdSe and CdS QDs must be the source of chiral induction and CD signal inversion. Theoretical Modeling of the Chiral Inversion. In order to demonstrate that different binding modes are responsible for the major sign changes in the induced CD signal, we modeled C

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Figure 4. Model structures for theoretical investigations of ligandinduced chirality of CdSe QDs. (a) Top view and (b) side view of the Cd9Se7H22 quantum dot fragment. The Cd atoms are colored beige, Se bronze, and capping hydrogens white. (c) Two possible bidentate binding arrangements of L-HomoCys ligand to the surface of the model CdSe fragment, termed diastereomer #1 and diastereomer #2. Black dots denote Cd atoms. (d) Optimized structures (at PBEPBE/3-21G/sbkjc-VDZ* level) of L-HomoCys corresponding to diastereomers #1 and #2.

Figure 5. Simulated UV and CD spectra for model CdSe complexes with single L-HomoCys or N-Ac-L-Cys ligand. The spectra were simulated at the TD-DFT CAM-B3LYP/3-21G/sbkjcVDZ* level. The Ac and COO for N-Ac-L-Cys denotes binding to the model CdSe surface by the acetyl or carboxylate groups, respectively, in addition to the thiol. The numbers correspond to the chelation arrangements depicted in Figures 4 and S19. (a) UV spectra for the L-HomoCys and two N-Ac-L-Cys complexes with different modes of binding. (b,c) CD spectra for the more energetically stable LHomoCys/CdSe complex compared to the CD of more energetically stable N-Ac-L-Cys/CdSe complex with the ligand bound via the acetyl group (b) and both complexes of N-Ac-L-Cys bound via carboxylate groups (c).

the pronounced exciton transitions on the sloping absorption continuum to the blue reflect the characteristic experimental spectra (Figure 1f), as well as similar computations on larger semiconductor clusters.28,39,40 We considered complexes of the model CdSe cluster with a single ligand as the simplest system for studying of the QD−ligand interactions and with two ligands in order to include any potential effects of ligand− ligand interactions (for details of the simulations, see the Methods section and the Supporting Information). Two Possible Bidentate Ligand Binding Modes of a Single Ligand Yield Oppositely Signed CD Spectra. The ligand geometry optimizations were carried out with the ionic states of carboxylic and amino groups corresponding to the experimental pH (NH3+ and COO−) and with aqueous solvent approximated by a polarized continuum model.38 The entire CdSe fragment, including the capping hydrogens, was kept frozen. The energy minimizations clearly demonstrated the bidentate nature of L-HomoCys (Figure 4), which binds to the Cd atoms by the thiol and carboxylate (CO2−) groups. A key property of such bidentate binding is that there are two distinct arrangements depending on where the thiol and carboxylate groups attach (termed #1 and #2, Figure 4c,d). These two diastereomers gave rise to oppositely signed simulated CD spectra (Figure S20) with the +/−/+ pattern computed for the more energetically favored #1 (by ∼1.4 kcal mol−1, Table S3), qualitatively consistent with the experimentally observed CD (Figure 5). N-Ac-L-Cys is, by contrast, a possible tridentate ligand (Figure S19). However, it is not difficult to rationalize why two-site binding may occur or even be preferred: the most obvious reason is the presence of other bound ligand molecules that restrict the number of available surface interaction sites (Cd atoms). Tridentate binding would also impair the electrostatic stabilization of the colloidal CdSe QDs by

decreasing the net negative charge on the outer surface of QDs, potentially causing their aggregation. Importantly, to the best of our knowledge, a tridentate binding mode has never been experimentally observed for QDs with N-acetyl cysteine derivatives.15,41 Finally, the UV−vis spectra simulations (Figure S20a) suggest that tridentate binding of N-Ac-L-Cys causes a significant hypochromic shift with respect to the L-HomoCys, while experimentally the spectra are superimposable (Figure 1). Monodentate binding is also unlikely because experimental data have shown that decreasing the ratio of the available QD binding sites to ligands below 1:1 does not affect the CD, as shown in Supporting Information Figure S9 and Table S1. Two-site binding via thiol and either Ac or CO2− groups therefore appears to be the most likely scenario, giving rise to four possible binding arrangements (Figure S19). Although full structure optimization of the single doubly bound N-Ac-L-Cys is not possible as it always converges to the triply bound state (see Supporting Information), a single torsional angle constraint was sufficient to find local energy minima. Interestingly, in contrast to L-HomoCys, binding via thiol and Ac groups favors diastereomer #2 over #1 by ∼3.1 kcal/mol (Table S3). The simulated CD spectrum for this more stable N-Ac-L-Cys binding has a −/+/− sign pattern, again in agreement with the experimental data (Figures 1 and 2) and, compared to the L-HomoCys simulation, roughly reproducing the mirror-image CD (Figure 5b). The other possible binding of N-Ac-L-Cys by thiol and CO2− groups favors the chelation arrangement #1 (Figure S19c) but only by D

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ACS Nano about 1 kcal/mol. While predicted to be more stable (∼1.5 kcal/mol) than the thiol and Ac binding, the computed CD did not exhibit clear opposite sign patterns (Figure 5c) to that of LHomoCys in either chelation arrangement. In both cases, couplet-like CD corresponding to the excitonic UV band was predicted, while the experimental spectra exhibited a single sign. These results suggest that the observed sign patterns arise from the preferred bidentate binding mode, which can change depending on the ligand. This further implies that the N-Ac-LCys may preferably bind through the thiol and Ac moieties. Although the latter is somewhat at odds with the computed energetic stability, the differences are relatively small, and due to the obvious simplifications, the relative energies for binding through different groups may not be entirely reliable. Relative energies for the two diastereomers of the same binding, on the other hand, should be more trustworthy. Most importantly, the agreement of the simulated CD, which is highly sensitive to the binding geometry, with experiment lends strong support for these conclusions. Binding Arrangements of Two Ligands Result in Mirror-Image CD Spectra. As already pointed out, the presence of other ligands is likely to influence the QD−ligand interactions. To have more realistic models, we therefore considered binding of two ligands to the model CdSe cluster. Assuming again bidentate interaction, there are six distinct arrangements of sulfur and oxygen binding (Figures 6e and S21−S24). For N-Ac-L-Cys, since the oxygen could be either the carboxylic or the acetyl oxygen, this leads to 24 possibilities (considering that the binding to the minimal CdSe model involves the H-capped “outer” Cd atoms, which are not entirely equivalent to the “inner” ones bound to Se). Geometry optimizations and UV and CD spectra simulations were carried out for all of these structures (Figures S25−S29 and Table S4) with no geometry constraints on the cysteine ligands (the Cd9Se7H22 cluster was again held fixed). Although the relative energies again favored carboxylate binding (Table S4), the energetics here is further complicated by the chemical non-equivalence of the Cd atoms within the model QD fragment and potentially by the capping H atoms. The computed CD (and UV) spectra should be a much more reliable guide for assessing the actual arrangements of the ligands on real QDs than the relative energies. As shown in Figure 6, simulated CD spectra for some of the acetyl and mixed binding arrangements were in excellent qualitative agreement with experiment, exhibiting very near mirror-image symmetry between L-HomoCys and N-Ac-L-Cys. For carboxylate binding of both ligands, the sign pattern was more complex, and the couplet-like character of the excitonic band CD was also evident paralleling the single ligand results (Figure 5). With both carboxylate-bound N-Ac-L-Cys ligands, the simulated UV maximum was also blue-shifted (304 nm) with respect to that for the L-HomoCys complex (308 nm), whereas the mixed carboxylate and acetyl groups binding of N-Ac-L-Cys gave the same wavelength, and bis-acetyl binding produced a slight blue shift (307 nm). Both single ligand simulations and two ligand simulations are therefore consistent with bidentate binding of L-HomoCys and N-Ac-L-Cys to the QD surface, with the N-Ac-L-Cys attached predominantly via the acetyl group. Distinct binding arrangements of the two ligands, despite their same stereochemistry, explain the observed induced mirror-image CD of L-HomoCysCdSe and N-Ac-L-Cys-CdSe. These finding show that the

Figure 6. Simulated UV and CD spectra for selected model CdSe complexes with two L-HomoCys and N-Ac-L-Cys ligands. The spectra were simulated at the TD-DFT CAM-B3LYP/3-21G/ sbkjcVDZ* level. Ac/Ac, Ac/COO, and COO/COO for N-Ac-LCys denote the mode of ligand binging to the QD surface in addition to the thiols; numbers correspond to the chelation arrangements depicted in (e). (a) UV spectra of L-HomoCys and three N-Ac-L-Cys complexes with different modes of binding, whose CD spectra are most consistent with the experimental data. (b−d) CD spectra of the L-HomoCys/CdSe complex compared to the CD spectra of the N-Ac-L-Cys complex with (b) both ligands bound via the acetyl groups, (c) one ligand bound through carboxylate and one through the acetyl group, and (d) both ligands bound via carboxylate groups. (e) Schematic representation of the possible binding arrangements for two bidentate ligands, with S (thiol) and O (carboxyl or acetyl) binding sites, on the model QD fragment. Black dots denote Cd atoms.

induced chirality of QDs is not the result of a simple orbital mixing between the chiral ligand electronic states and the achiral ones of the QDs,20,28,35 but in addition to the ligand’s absolute configuration, the mixing also critically depends on the binding geometry and conformation of the chiral ligand. This advances our understanding of the origins of ligand-induced chirality in the QDs and stresses the importance of the QD itself and its influence on the conformations of the anchored ligands. We anticipate that these findings may have important implications in rational design of chiral ligands as a means to control optical properties of chiral semiconductor nanoparticles.

CONCLUSIONS We have presented the first demonstration that the chirality of QDs could be induced, tuned, and inverted by modifying the structure of the chiral capping ligands, without changing the absolute configuration of the ligand. The CD spectra of the CdSe QDs capped with L-HomoCys and N-Ac-L-Cys as well as E

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AUTHOR INFORMATION

those of the CdS QDs capped with L-Cys and N-Ac-L-Cys displayed mirror-image profiles. The CD spectra and CD anisotropy data suggested that the interactions between ligands and QDs and the resulting chiroptical properties were dictated by the structure of the capping ligands rather than the chemical composition and diameter of the QDs. Matching polarities of the FDCD and CD spectra showed the presence of chiral bandedge states in CdS QDs capped with L-Cys and N-Ac-L-Cys. The origin of the inverted CD for ligands with the same stereochemistry was shown to arise from different ligand binding modes to the QD surfaces using non-empirical, fully quantum chemical simulations of the CD spectra for the first time. The induced chirality of the QDs is thus the result of hybridization of the (achiral) QD and (chiral) ligand electronic states, where the sign of the resulting induced CD of QDs is determined by both the ligand’s geometrical arrangement on the QD surface and the ligand’s absolute configuration. The results of these first principle calculations underscore the utility of the high-level CD spectra simulations in chiral ligand functionalized QDs. A combination of experimental and theoretical approaches presented here opens up many exciting possibilities in the rational design of chiroptical semiconducting nanomaterials, including chiral recognition and sensing of biomolecules.

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Present Address ⊥

POSTECH, Pohang, South Korea.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported in part by the U.S. Department of Energy, Office of Basic Energy Sciences (Award DE-FG0210ER46728 to M.B.) and National Science Foundation (Awards CBET-1403947 to M.B., J.K., S.V.D., and K.V. and DGE-0948027 to M.B.). Graduate students were supported by U.S. Department of Energy Award DE-FG02-10ER46728 (J.K.C., L.P., B.E.H.) and National Science Foundation Awards CBET-1403947 (L.P.) and DGE-0948027 (B.E.H., U.T., K.W.E.). REFERENCES (1) Gautier, C.; Bürgi, T. Chiral Nanoparticles. Chirality at the Nanoscale; Wiley-VCH Verlag GmbH & Co. KGaA, 2009; pp 67−91. (2) Yeom, J.; Yeom, B.; Chan, H.; Smith, K. W.; Dominguez-Medina, S.; Bahng, J. H.; Zhao, G.; Chang, W.-S.; Chang, S.-J.; Chuvilin, A.; Melnikau, D.; Rogach, A. L.; Zhang, P.; Link, S.; Král, P.; Kotov, N. A. Chiral Templating of Self-Assembling Nanostructures by Circularly Polarized Light. Nat. Mater. 2015, 14, 66−72. (3) Ma, W.; Kuang, H.; Xu, L.; Ding, L.; Xu, C.; Wang, L.; Kotov, N. A. Attomolar DNA Detection with Chiral Nanorod Assemblies. Nat. Commun. 2013, 4, 2689. (4) Ben-Moshe, A.; Maoz, B. M.; Govorov, A. O.; Markovich, G. Chirality and Chiroptical Effects in Inorganic Nanocrystal Systems with Plasmon and Exciton Resonances. Chem. Soc. Rev. 2013, 42, 7028−7041. (5) Nakashima, T.; Kobayashi, Y.; Kawai, T. Optical Activity and Chiral Memory of Thiol-Capped CdTe Nanocrystals. J. Am. Chem. Soc. 2009, 131, 10342−10343. (6) Xia, Y.; Zhou, Y.; Tang, Z. Chiral Inorganic Nanoparticles: Origin, Optical Properties and Bioapplications. Nanoscale 2011, 3, 1374−1382. (7) Zhu, Z.; Guo, J.; Liu, W.; Li, Z.; Han, B.; Zhang, W.; Tang, Z. Controllable Optical Activity of Gold Nanorod and Chiral Quantum Dot Assemblies. Angew. Chem., Int. Ed. 2013, 52, 13571−13575. (8) Hu, T.; Isaacoff, B. P.; Bahng, J. H.; Hao, C.; Zhou, Y.; Zhu, J.; Li, X.; Wang, Z.; Liu, S.; Xu, C.; Biteen, J. S.; Kotov, N. A. SelfOrganization of Plasmonic and Excitonic Nanoparticles into Resonant Chiral Supraparticle Assemblies. Nano Lett. 2014, 14, 6799−6810. (9) Sánchez-Castillo, A.; Noguez, C.; Garzón, I. L. On the Origin of the Optical Activity Displayed by Chiral-Ligand-Protected Metallic Nanoclusters. J. Am. Chem. Soc. 2010, 132, 1504−1505. (10) Shukla, N.; Bartel, M. A.; Gellman, A. J. Enantioselective Separation on Chiral Au Nanoparticles. J. Am. Chem. Soc. 2010, 132, 8575−8580. (11) Gautier, C.; Bürgi, T. Chiral N-Isobutyryl-Cysteine Protected Gold Nanoparticles: Preparation, Size Selection, and Optical Activity in the Uv−Vis and Infrared. J. Am. Chem. Soc. 2006, 128, 11079− 11087. (12) Gellman, A. J.; Huang, Y.; Feng, X.; Pushkarev, V. V.; Holsclaw, B.; Mhatre, B. S. Superenantioselective Chiral Surface Explosions. J. Am. Chem. Soc. 2013, 135, 19208−19214. (13) Hidalgo, F.; Noguez, C. Optical Activity of Achiral Ligand SCH3 Adsorbed on Achiral Ag55 Clusters: Relationship between Adsorption Site and Circular Dichroism. ACS Nano 2013, 7, 513−521.

METHODS General Procedure for Phase Transfer Ligand Exchange Synthesis of Chiral QDs and Their Characterization. An aqueous solution of cysteine derivative was added to a solution of achiral OAcapped QDs in toluene. The heterogeneous mixture was deoxygenated and stirred at room temperature under N2 in the absence of light for 24 h. The aqueous layer was separated, and the QDs capped with cysteine derivatives were precipitated with acetone and separated by centrifugation. Isolated chiral QDs were dissolved in DI water, and CD and UV−vis spectra were recorded using Jasco J-815 and V-650 spectrometers, respectively. TEM samples were prepared by dropcasting of the chiral QD solution onto carbon-coated copper grids and drying in air. Imaging was performed on an FEI Tecnai G2 F20 scanning transmission electron microscope operating at 200 kV. Theoretical Modeling. All quantum chemical calculations were done at density functional theory (DFT) and time-dependent DFT (TD-DFT) levels using Gaussian 09 software.42 All geometries were optimized with PBEPBE functional,43 while TD-DFT calculations were carried out using CAM-B3LYP. The same basis set, sbkjcVDZ*28,40 (see Supporting Information), for Cd, Se, and S and 3-21G for all light atoms was employed. Aqueous solvent was approximated by the conductor-like polarized continuum model with default parameters for water.38 The UV and CD spectral contours were simulated from the computed wavelengths and intensities of the individual transitions by assuming Gaussian line shapes with a constant width of 25 nm.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b00567. Experimental conditions; XRD patterns, absorption and emission spectra, and HRTEM images of CdSe and CdS; variable concentration CD and UV−vis spectra; TGA curves; CD anisotropy g factors; MAS ssNMR spectra, FTIR spectra; schematic representations of QD−ligand binding modes; relative energies of QD−ligand optimized structures and simulations of CD spectra for all QD−ligand geometries; QD cluster coordinate files (PDF) F

DOI: 10.1021/acsnano.6b00567 ACS Nano XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acsnano.6b00567 ACS Nano XXXX, XXX, XXX−XXX