Circular Dichroism of CdSe Nanocrystals Bound by Chiral Carboxylic

Nov 22, 2017 - Through this family of chiral carboxylic acid ligands, we performed a direct comparison between carboxylate-bound and thiolate-bound ch...
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Circular Dichroism of CdSe Nanocrystals Bound by Chiral Carboxylic Acids Mayank Puri and Vivian E. Ferry* Department of Chemical Engineering and Materials Science University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: Chiral semiconductor nanocrystals, or quantum dots (QDs), are promising materials for applications in biological sensing, photonics, and spin-polarized devices. Many of these applications rely on large dissymmetry, or g-factors, the difference in absorbance between left- and right-handed circularly polarized light compared to the unpolarized absorbance. The majority of chiral QDs, specifically CdSe, reported to date have used thiolated amino acid ligands to introduce chirality onto the nanoparticles, but these systems have ultimately reported small g-factors of ∼2 × 10−4. In an effort to realize chiral CdSe QDs with higher g-factors and to expand the set of designer chiral semiconductor nanocrystals, we have employed chiral carboxylic acids as a distinct class of ligands for chiral CdSe nanoparticles. Through this family of chiral carboxylic acid ligands, we performed a direct comparison between carboxylate-bound and thiolate-bound chiral CdSe QDs. Spectral analysis revealed that the resulting circular dichroism shifts originate from the splitting of the exciton by the ligand−nanocrystal interaction. Subsequent examination of a series of chiral carboxylic acid ligands revealed a 30fold range in g-factor through relatively small changes in the structure of the ligand. Finally, we showed that increasing the number of stereocenters on the ligand can further enhance the dissymmetry factors. This versatile and tunable combination of nanocrystals and ligands will inform future designs of chiral nanomaterials and their applications. KEYWORDS: quantum dots, chiral semiconductor nanocrystals, circular dichroism, carboxylic acid, ligand-induced optical activity

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pathways to tunable and switchable chiral optical properties in semiconductor nanocrystals. In designing an optimal chiral nanoparticle system for these applications, the anisotropy or dissymmetry factor, is a critical parameter. The g-factor is defined as g = Δε/ε = (AL − AR)/A, where AL and AR are the absorbance of circularly polarized lefthanded and right-handed light, respectively, and A is the absorbance of unpolarized light. Large g-factors lead to an increase in signal-to-noise for applications involving CD spectroscopy. For example, it has recently been shown that the electron-transfer kinetics of chiral QDs depends on the strength of the g-factors, which is important for spintronic devices.21 Applications of chiral materials in nanophotonics also require increased light−matter interactions. Unfortunately, current chiral QDs have revealed g-factors on the order of 10−5 to 10−4,31,34,35 which is low compared to that of larger assemblies containing QDs or metal plasmonic systems.40,41 One recent effort toward increased g-factors involved the synthesis of chiral CdSe nanorods, where an increase in g-factor

hiral semiconductor nanocrystals or quantum dots (QDs) are emerging materials with widespread potential applications,1−5 including as biological 6−12 in document security and anticounterfeiting sensors, materials, 13,14 in photonics,15−18 and in spin-polarized devices.19−21 Since their initial preparation by Moloney and co-workers in 2007, where chiral ligands were used as stabilizers in the microwave synthesis of CdS QDs,22 chirality has been shown to originate from the semiconductor nanocrystal core in the presence of dislocations and defects,23−26 from interactions between chiral ligands and the nanocrystal,27−37 and from the incorporation and arrangement of nanocrystals into chiral superstructures.38 Among these, the induction of chirality through postsynthetic ligand exchange, in which chiral ligands replace native achiral ligands on the surface, promises a large design space for tailored and tunable chiral semiconductor nanocrystals.34−36,39 The nanocrystals can be designed and synthesized independently from the ligands, while subtle differences in the binding of the chiral ligand to the semiconductor nanocrystal influence the circular dichroism (CD) spectra.35 Designer ligands that change their binding in response to a stimulus can also be envisioned, opening © 2017 American Chemical Society

Received: August 10, 2017 Accepted: November 16, 2017 Published: November 22, 2017 12240

DOI: 10.1021/acsnano.7b05690 ACS Nano 2017, 11, 12240−12246

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Figure 1. (a) Histogram of CdSe nanoparticle sizes as determined by TEM experiments. (b) Transmission electron micrographs of CdSe nanoparticles prior to ligand exchange (c) Photograph demonstrating the dispersion of CdSe nanoparticles in the presence of 5, 10, 20, and 40 mg of L-(+)-tartaric acid (left to right in image) after ligand exchange in DMF.

was observed as the aspect ratio of the rod increased.37 However, the largest g-factor obtained was 4.8 × 10−4. Here, we adopt an alternative strategy in which we employ a family of chiral carboxylic acids to systematically tune the g-factor of CdSe QDs, reaching values of 7.0 × 10−4 through ligand modification alone. Thus far, the vast majority of chiral ligands used to synthesize chiral QDs have been based on thiolated amino acids, such as cysteine and its derivatives,34 where binding to the QD surface primarily occurs through an anionic thiolate functional group. We reasoned that a greater library of chiral QDs, specifically chiral CdSe nanoparticles, may be synthesized by utilizing commercially available chiral carboxylic acids where binding to the CdSe surface instead occurs through an anionic carboxylate group. This change in binding group also allows for direct comparison of the influence of thiolate versus carboxylate groups on the CD spectra, as the CD signals have been shown to be sensitive to the interaction between the surface ligand highest occupied molecular orbital (HOMO) and the generated hole of the QD.34,36

(Figure 1a,b and Figure S1). Complementary dynamic light scattering (DLS) experiments revealed an average hydrodynamic radius of 3.5−4.0 nm, which is consistent with an approximately 2.1 nm radius inorganic core bound by longchain carboxylate ligands, such as oleate and myristate (Figure S2). Established synthetic methodologies for chiral QD ligand exchange involve a phase-transfer reaction in which the chiral ligand, often cysteine or its derivatives, is dissolved in water and treated with a base, followed by rigorous stirring with QDs dispersed in a nonpolar solvent for 1−24 h.35,37 Efforts to employ this technique with some of the chiral carboxylic acids used in this paper failed, likely due to the nature of carboxylate ligand binding and exchange. Specifically, Hens and co-workers have previously demonstrated that a proton-transfer event must occur in order to displace a bound carboxylate ligand with a free carboxylic acid ligand.43,44 Therefore, the use of an external base may prevent the carboxylic acid ligand exchange from occurring. For this reason, we used an alternative synthetic route for chiral CdSe QDs, where the protic chiral ligand is directly added to a nonpolar solution of CdSe bound by oleate and myristate ligands. Rigorous stirring of the chiral carboxylic acid and carboxylate-bound CdSe QDs in toluene leads to ligand exchange, likely through a proton-transfer event, as made evident by the aggregation of the nanoparticles in the toluene solution after less than 5 min of stirring. The colorless supernatant is then decanted, and the remaining solid, a mixture of excess chiral carboxylic acid and CdSe nanoparticles, is redispersed in a polar organic solvent such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or methanol (MeOH). We have found that it is important to use a large excess of the chiral carboxylic acid, generally between 30 and 50 mg, in order to fully redisperse the CdSe QDs in the polar organic solvent (Figure 1c). In addition, chiral carboxylic acids with pKa values lower than those of myristic acid and oleic acid were used to enable proton transfer. It should also be noted that free protons must be available to participate in the ligand exchange process, thus preventing the use of zwitterionic chiral ligands such as cysteine. However, this limitation may easily be addressed by utilizing nonzwitterionic derivatives, such as Nacetylcysteine. Evidence for successful binding of the chiral carboxylic acid ligands comes from CD spectroscopy, which is a powerful tool for distinguishing between free and bound ligand states. The

RESULTS AND DISCUSSION With the goal of placing chiral carboxylic acid ligands on the surface of CdSe QDs via postsynthetic ligand exchange, we started with carboxylate-bound CdSe QDs. Work by the Dempsey group has shown that certain X-type ligands, such as phosphonate or thiolate ligands, bound to CdSe QDs do not undergo exchange with carboxylic acids due to the greater binding strength of phosphonate and thiolate ligands, whereas carboxylate-bound CdSe QDs result in an equilibrium exchange with free carboxylic acid ligands.42 Similarly, Hens and coworkers have shown that addition of primary carboxylic acids to carboxylate-bound CdSe leads to a one-for-one exchange.43,44 For this reason, we employed myristic acid and oleic acid as native ligands in a noninjection synthetic route toward zinc blende CdSe QDs. Jasieniak, Cao, and Owen have demonstrated that this synthetic method is both reproducible and amendable to scale-up, allowing us to screen a variety of chiral carboxylic acids with the same large batch of achiral carboxylate-bound CdSe QDs.45−47 Zinc blende CdSe QDs were synthesized on a multigram scale, following the adapted workup method of Owen and coworkers.47 The resulting particles were monodisperse, with an average diameter of 4.2 nm and a standard deviation of 8%, based on transmission electron microscopy experiments 12241

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ACS Nano Table 1. Experimentally Measured g-Factors of Selected CdSe Nanoparticles Bound by Chiral Ligands ligand

solvent

CdSe diameter (nm)

N-acetyl-L-cysteine N-acetyl-L-cysteine L-cysteine

H2O H2O H2O

4.4 2.9 4.1

L-cysteine

H2O

4.4

D-cysteine

H2O

N-acetyl-L-cysteine

H2O

nanorod AR: 2.7 4.2

N-acetyl-L-cysteine

DMF

4.2

N-acetyl-L-aspartic acid

DMF

4.2

L-tartaric

acid

DMF

4.2

D-tartaric

acid

DMF

4.2

L-malic

acid

DMF

4.2

D-malic

acid

DMF

4.2

L-lactic

acid

DMF

4.2

(S)-(−)-2-chloropropionic acid

DMF

4.2

(R)-(−)-phenylsuccinic acid

DMF

4.2

free chiral ligands have absorption features and related CD signals in the UV region (Figures S3−S5), while the bound ligand−semiconductor system yields a CD response in association with the excitonic features of the CdSe QD throughout the visible region of the spectrum.36 Prior to ligand exchange, no CD response is observed for the oleate- and myristate-bound CdSe QDs in a toluene solution (Figure S6). However, upon exchange, a CD response is observed between 400 and 650 nm, supporting the binding of the chiral carboxylic ligand to the CdSe surface. It is important to highlight that CD spectroscopy is a selective method to study chiral ligand binding, even in the presence of large excesses of the free chiral ligand. This gives it advantages over other spectroscopic methods to gauge ligand binding, such as NMR and Fourier transform infrared (FTIR) spectroscopy, where the signals of the free and bound ligands may overlap. The chiral activity of CdSe QDs bound by N-acetyl-Lcysteine has previously been reported in aqueous solution, with a g-factor of 0.5 × 10−4 at 584.6 nm measured for CdSe particles with a 4.4 nm diameter (Table 1).35 In our attempts to reproduce this result using the phase-transfer ligand exchange method with similarly sized CdSe QDs (4.2 nm), we observed a 4-fold increase in the g-factor value at the λCD from 0.5 × 10−4 to 1.9 × 10−4 (Table 1 and Figure 2). This difference may originate from the coverage of the chiral ligand on the surface or from a difference between zinc blende and wurtzite CdSe nanoparticles. We compared these measurements to chiral CdSe nanocrystals bound by N-acetyl-L-cysteine, prepared via our alternate ligand exchange procedure, in which the protic ligand is directly added to the achiral CdSe QDs and the resulting chiral CdSe nanoparticles are redispersed in a polar

g-factors (×10−4)

λCD (nm)

+0.5 +0.4 +1.7 −1.9 +1.9 −2.1 −3.3 +4.8 −1.8 +1.9 −1.7 +2.0 −3.2 +6.0 +3.8 −7.0 −3.4 +5.8 −2.2 +3.6 +2.3 −3.6 −1.2 +1.0 −0.25 +0.26 +0.48 −0.87

584.6 531.6 560.8 580.0 569.0 587.8 553 571 572 538 572 538 560 528 557 528 557 528 557 528 557 528 572 538 572 538 557 528

ref 35 35 34 34 37 this work this work this work this work this work this work this work this work this work this work

Figure 2. Overlaid CD spectra (a) and UV−vis spectra (b) of Nacetyl-L-cysteine bound to CdSe nanoparticles, dispersed in H2O (pink) and DMF (blue).

organic solvent such as DMF instead of H2O. Notably, these nanocrystals had a nearly identical g-factor of 2.0 × 10−4 in DMF at the CD maximum of 538 nm, suggesting that a change in ligand exchange method or solvent does not drastically affect the CD response (Figure 2). These chiral CdSe QDs have been both synthesized and examined in an organic solvent, allowing 12242

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HOMO and the QD valence band states. This hybridization effectively splits the exciton into two sublevels with opposite angular momentum, which are each preferentially absorbed by opposite circularly polarized light and give rise to the derivativeshaped peaks in the CD spectrum. This analysis suggests that the observed changes in the CD signals between N-acetyl-Lcysteine and N-acetyl-L-aspartic acid may be due to differences in the ligand binding functional groups or to the binding modes, potentially leading to differences in hybridization with the CdSe valence bands. Specifically, N-acetyl-L-aspartic acid has the potential to bind to the surface of CdSe through two carboxylate groups, as well as the acetyl oxygen group, whereas previous computational evidence suggests that N-acetyl-Lcysteine likely binds to the CdSe surface through the thiolate and acetyl groups.35 To clarify these differences, the ligand framework was simplified by comparing the CD response of CdSe QDs bound by L-(+)-lactic acid and D-(+)-malic acid (Figure 4). The L-

for potential applications that are not compatible with aqueous environments. The influence of ligand binding on the CD response was then probed by comparing the CD response of CdSe QDs bound by N-acetyl-L-cysteine to those bound via a carboxylate functional group. We chose to employ N-acetyl-L-aspartic acid, as it is a close structural analogue of N-acetyl-L-cysteine with the thiol group replaced with a carboxylic acid group. Figure 3

Figure 3. Overlaid CD spectra (a) and UV−vis spectra (b) of CdSe nanoparticles (5 μM, DMF) bound by N-acetyl-L-cysteine (green points) and N-acetyl-L-aspartic acid (blue points), where the CD spectral fits are shown by solid lines.

shows that the CD response for CdSe bound by N-acetyl-Laspartic acid exhibits a higher g-factor of 6.0 × 10−4 at 528 nm compared to 1.9 × 10−4 at 538 nm for N-acetyl-L-cysteine (Table 1). These measurements were also taken with solutions containing N-acetyl-L-aspartic acid and various concentrations of CdSe: 2.5 μM, 5 μM, and 10 μm. The CD signals increase in intensity with an increase in CdSe absorbance, yielding comparable g-factors at all three concentrations, as expected (Figure S7 and Table S1). Interestingly, the CD signals for CdSe bound by N-acetyl-Laspartic acid appear to show a ∼10 nm blue shift compared to the CD signals for CdSe bound by N-acetyl-L-cysteine (Figure 3a), despite no significant blue shift observed in the UV−vis absorption spectrum (Figure 3b). Further analysis of the spectra indicates that this shift can be explained by interactions of the chiral ligand with the excitonic features of the QD.36 The absorption spectrum of the QDs was fit to a sum of Gaussians corresponding to the excitonic levels, and the corresponding CD spectra fit well to a linear sum of the derivatives of the Gaussians without significant changes to the central wavelength (Figure 3a, solid lines). The CD spectra are thus well explained by splitting of the exciton by the chiral ligands, where the observed blue shift may be rationalized as a difference in energy splitting terms, manifested as linear coefficients of the sum of Gaussian derivatives used to reconstruct the CD spectra. These energy splitting terms are summarized in Table S2. Markovich and co-workers have demonstrated that splitting of the exciton likely originates from coupling, or hybridization, of the ligand

Figure 4. Overlaid CD spectra (a) and UV−vis spectra (b) of CdSe nanoparticles (5 μM, DMF) bound by D-(+)-malic acid (black points) and L-(+)-lactic acid (orange points), where the CD spectral fits are shown in solid lines.

(+)-lactic acid has one available carboxylic acid functional group, whereas D-(+)-malic acid has two available carboxylic acid functional groups. Measurement of the g-factors reveals a 3-fold increase in g-factor for CdSe bound by D-(+)-malic acid over L-(+)-lactic acid (Table 1). The intensity of the CD signals associated with CdSe bound by D-(+)-malic acid are also sharper and more intense than those associated with CdSe bound by L-(+)-lactic acid. Interestingly, while the sign of the CD spectrum is the same at the first spectral feature, at higher energies, the sign between these two ligands is reversed. A similar spectral analysis reveals that all of the features in the CD spectra can be explained by splitting of the exciton via interactions with the ligand, as verified by the same fitting procedure as above (Figure 4a, solid lines). The central wavelengths of the Gaussians obtained from the absorption spectra for both ligands are within 3 nm of each other. However, since the CD spectra are the sum of the derivatives of the Gaussians associated with each exciton, different 12243

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DLS measurements confirm that no aggregates are apparent in the solution (Figure S12). We also compared the CD response of CdSe QDs bound by two closely related chiral dicarboxylic acids, L-(−)-malic acid and (R)-(−)-phenylsuccinic acid (Figure S13), where the only structural difference is replacement of the chiral substituent from a hydroxide group to a phenyl group. Interestingly, this single substitution causes a dramatic change in the CD signals, where the CD signals associated with CdSe QDs bound by (R)(−)-phenylsuccinic acid are not only broader but also inverted, despite having the same absolute configuration. Notably, we still observe a good fit of the experimental CD spectrum of CdSe QDs bound by (R)-(−)-phenylsuccinic acid (Figure S14). This phenomenon of chirality inversion has recently been highlighted by Balaz and co-workers for cysteine derivatives and was attributed to differences in surface binding modes.35 In this case, replacement of a coordinating hydroxide group for a noncoordinating phenyl group may be responsible for the inversion. However, additional studies are necessary to support this hypothesis. Thus far, CdSe nanocrystals bound by L-(+)- and D(−)-tartaric acid demonstrate the largest g-factors of the systems we have measured.31,37 Interestingly, the CD response of CdSe nanocrystals bound by an additional monocarboxylic acid ligand, (S)-(−)-2-chloropropionic acid (Figure S15), resulted in the smallest g-factors in this study, with values of ±0.25 × 10−4, 30-fold smaller than that of L-(+)- and D(−)-tartaric acid (Table 1). This significant range of g-factors suggests that the chemical structure of the chiral ligand is critical for optimizing performance of chiral QDs.

interactions between the ligand and the nanocrystal can result in very different CD spectra. This indicates the importance of careful tailoring of the ligand to achieve the desired sign of the CD spectrum at target wavelengths: small structural variations can produce oppositely signed CD spectra. To further explore the effects of ligand structure, additional dicarboxylic acid ligands were examined, including L-(−)-malic acid and the closely related derivatives L-(+) and D-(−)-tartaric acid in which the chiral ligand now possesses two stereocenters. The resulting CD spectra are overlaid in Figure 5, where CdSe

Figure 5. Overlaid CD spectra of CdSe nanoparticles (5 μM, DMF) bound by L-(+)-tartaric acid (dotted orange trace), D-(−)-tartaric acid (solid orange trace), D-(+)-malic acid (dotted green trace), L(−)-malic acid (solid green trace), and meso-tartaric acid (black trace).

CONCLUSIONS Here, we highlight a different ligand exchange method for the synthesis of chiral QDs using commercially available carboxylic acids. These chiral carboxylate-bound CdSe QDs yield sharper and more intense signals than their thiolate-bound CdSe counterparts, with g-factors up to 7.0 × 10−4. This expanded family of chiral carboxylate-bound CdSe QDs has also allowed for systematic tuning of the number of stereocenters on the chiral ligand, revealing an increase in the g-factor as the number of stereocenters increases. In addition, we have observed a 30fold range in g-factors depending on the exact chemical structure of the chiral carboxylic acid used, highlighting the importance of optimizing the chiral ligands used in future applications. This work is complemented by a recent publication from Balaz and co-workers in which chiral carboxylic acids are bound to CdSe QDs via the phase-transfer ligand exchange method.48

nanocrystals bound by opposite enantiomers yield mirrorimage CD spectra, as has been shown previously for cysteine and its derivatives.35,39 Using the previously described method, we also observed good fitting of the experimental CD spectrum of CdSe QDs bound by L-(+)-tartaric acid (Figure S8). Interestingly, CdSe bound by L-(+) and D-(−)-tartaric acid demonstrates even more intense CD signals than L-(−) and D(+)-malic acid, with a 2-fold increase in the corresponding gfactors (Table 1). A full understanding for the increase in gfactor with increasing number of stereocenters would likely require theoretical density functional theory support. As control experiments, the CD spectrum of CdSe bound by meso-tartaric acid, a racemic mixture of L-(+) and D-(−)-tartaric acid, and succinic acid were collected and revealed no CD activity (Figure S9). This confirms that the CdSe surface has no preferential binding of one enantiomeric form of tartaric acid over the other. FTIR experiments support loss of the native ligands and binding of L-(+)-tartaric acid to the CdSe surface (Figures S10 and S11). Specifically, a disappearance of the νasymm COO stretch associated with the native ligands at 1530 cm−1 is observed, along with the appearance of a vibrational feature at 1567 cm−1, tentatively assigned to a νCOO stretch for bound L-(+)-tartaric acid. Notably, this vibrational feature is distinct from the νCOO stretch associated with free L(+)-tartaric acid at 1720 cm−1. DLS experiments on CdSe QDs bound by L-(+)-tartaric acid revealed a hydrodynamic radius of 4.5 nm (Figure S12). This is a small increase over the corresponding radius of the CdSe QDs prior to ligand exchange. Despite this increase in hydrodynamic radius, the

METHODS Cadmium nitrate tetrahydrate (98%, Sigma-Aldrich), myristic acid (98%, Acros Organics), sodium hydroxide pellets (98.9%, Fisher Chemicals), selenium dioxide (99.8%, Acros Organics), oleic acid (tech. grade 90%, Alfa Aesar), and 1-octadecene (90%, Acros Organics) were all purchased from the stated vendor and used without further purification. Cadmium myristate was prepared on a multigram scale according to the method reported by Cao and coworkers.46 Toluene (anhydrous, Alfa Aesar) and methyl acetate (anhydrous, Sigma-Aldrich) were further treated with 3 Å molecular sieves for at least 24 h prior to use. The chiral ligands, including Nacetyl-L-cysteine (>99%, Sigma-Aldrich), N-acetyl-L-aspartic acid (>99%, Sigma-Aldrich), L-(+)-lactic acid (90% in H2O, Acros Organics), L-(−)-malic acid (99%, Acros Organics), D-(+)-malic acid (98%, Acros Organics), L-(+)-tartaric acid (99%, Alfa Aesar), D12244

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ACS Nano (−)-tartaric acid (99%, Alfa Aesar), meso-tartaric acid monohydrate (90%, Sigma-Aldrich), DL-tartaric acid (99.5%, Acros Organics), succinic acid (99%, Acros Organics), (S)-(−)-2-chloropropionic acid (99%, Sigma-Aldrich), and (R)-(−)-phenylsuccinic acid (>96%, Sigma-Aldrich) were used without further purification from the vendor. Dimethylformamide (ACS grade, Fisher Chemical) was used without further drying or purification. Synthesis of Zinc Blende CdSe QDs. Zinc blende CdSe QDs were synthesized on a multigram scale following the procedure reported by Owen and co-workers,47,49 which was based off the original synthesis by Cao and co-workers.46 Large-scale purification of the QDs was carried out following the method of Owen and coworkers,49 except the resulting CdSe nanoparticles were only precipitated with methyl acetate once prior to undergoing ligand exchange with the desired chiral ligand. Ligand Exchange Procedure. Ligand exchange was carried out simply by mixing an excess of the solid ligand (50 mg) with 0.5 mL of a 20 μM toluene solution of CdSe nanoparticles. Rapid mixing for 5 min causes aggregation of the CdSe nanoparticles, and centrifugation of the mixture results in a colorless supernatant with aggregated quantum dots mixed with undissolved excess ligand as a solid. The supernatant was then decanted, and 2 mL of a polar solvent, DMF, MeOH, or DMSO, was added. Centrifugation of the resulting chiral CdSe nanoparticles in polar organic solvent allowed any undissolved ligand to be removed from the solution. UV−Visible Spectroscopy Procedure. UV−vis spectroscopy experiments were carried out using a 1.0 cm quartz cuvette containing either a 5 μM DMF solution of chiral CdSe nanoparticles or a 200 μM H2O solution of chiral ligands. The UV−vis spectroscopy measurements were carried out with either an Agilent Cary 5000 UV−vis−NIR spectrometer or a Spectronic Genesys 5 spectrometer at 25 °C. Circular Dichroism Spectroscopy Procedure. CD spectroscopy experiments were carried out using a 1.0 cm quartz cuvette containing either a 5 μM DMF solution of chiral CdSe nanoparticles or a 200 μM H2O solution of chiral ligands. The CD experiments were carried out with a Jasco J-815 circular dichroism spectrometer at 25 °C. The scan rate was set to 50 nm/min for the chiral CdSe QDs in DMF and 100 nm/min for the chiral ligands in H2O. The data pitch was set to 0.5 nm, the digital integration time to 1 s, and the bandwidth to 4 nm. All g-factors were calculated from absorbance data measured simultaneously with the Jasco J-815 circular dichroism spectrometer. Transmission Electron Microscopy Procedure. TEM samples were prepared by drop-casting a dilute solution of CdSe QDs dispersed in toluene onto a 300 mesh copper TEM grid supplied by Ted Pella, Inc. TEM experiments were run on a Tecnai T12 microscope at an operating voltage of 120 kV. Sizing of the nanoparticles was carried out using the PEBBLES software,50 and images were produced using ImageJ software.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Vivian E. Ferry: 0000-0002-9676-6056 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Air Force Office of Scientific Research under Contract Number FA9550-16-1-0282. Circular dichroism experiments reported in this paper were performed at the Biophysical Technology Center, University of Minnesota Department of Biochemistry, Molecular Biology, and Biophysics. Transmission electron microscopy and infrared spectroscopy experiments were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. The authors would like to thank Whitney Wenger for assistance with DLS measurements and analysis, as well as Pavlos Pachidis and Professor Mahesh Mahanthappa for valuable discussions. REFERENCES (1) 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. (2) Milton, F. P.; Govan, J.; Mukhina, M. V.; Gun’ko, Y. K. The Chiral Nano-World: Chiroptically Active Quantum Nanostructures. Nanoscale Horiz 2016, 1, 14−26. (3) Govan, J.; Gun’ko, Y. K. Recent Progress in Chiral Inorganic Nanostructures. In Nanoscience; O’Brien, P., Thomas, P. J., Eds.; Royal Society of Chemistry: Cambridge, 2016; Vol. 3, pp 1−30. (4) Kumar, J.; Thomas, K. G.; Liz-Marzán, L. M. Nanoscale Chirality in Metal and Semiconductor Nanoparticles. Chem. Commun. 2016, 52, 12555−12569. (5) Ma, W.; Xu, L.; de Moura, A. F.; Wu, X.; Kuang, H.; Xu, C.; Kotov, N. A. Chiral Inorganic Nanostructures. Chem. Rev. 2017, 117, 8041−8093. (6) Han, C.; Li, H. Chiral Recognition of Amino Acids Based on Cyclodextrin-Capped Quantum Dots. Small 2008, 4, 1344−1350. (7) Carrillo-Carrión, C.; Cárdenas, S.; Simonet, B. M.; Valcárcel, M. Selective Quantification of Carnitine Enantiomers Using Chiral Cysteine-Capped CdSe(ZnS) Quantum Dots. Anal. Chem. 2009, 81, 4730−4733. (8) Hendry, E.; Carpy, T.; Johnston, J.; Popland, M.; Mikhaylovskiy, R. V.; Lapthorn, A. J.; Kelly, S. M.; Barron, L. D.; Gadegaard, N.; Kadodwala, M. Ultrasensitive Detection and Characterization of Biomolecules Using Superchiral Fields. Nat. Nanotechnol. 2010, 5, 783−787. (9) Xia, Y.; Zhou, Y.; Tang, Z. Chiral Inorganic Nanoparticles: Origin, Optical Properties and Bioapplications. Nanoscale 2011, 3, 1374. (10) Li, Y.; Zhou, Y.; Wang, H.-Y.; Perrett, S.; Zhao, Y.; Tang, Z.; Nie, G. Chirality of Glutathione Surface Coating Affects the Cytotoxicity of Quantum Dots. Angew. Chem., Int. Ed. 2011, 50, 5860−5864. (11) Delgado-Pérez, T.; Bouchet, L. M.; de la Guardia, M.; Galian, R. E.; Pérez-Prieto, J. Sensing Chiral Drugs by Using CdSe/ZnS Nanoparticles Capped with N -Acetyl- L -Cysteine Methyl Ester. Chem. - Eur. J. 2013, 19, 11068−11076. (12) Ghasemi, F.; Hormozi-Nezhad, M. R.; Mahmoudi, M. TimeResolved Visual Chiral Discrimination of Cysteine Using Unmodified CdTe Quantum Dots. Sci. Rep. 2017, 7, 890. (13) Hou, X.; Ke, C.; Bruns, C. J.; McGonigal, P. R.; Pettman, R. B.; Stoddart, J. F. Tunable Solid-State Fluorescent Materials for Supramolecular Encryption. Nat. Commun. 2015, 6, 6884.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05690. Procedures for DLS experiments, FTIR experiments, CD spectral fitting and analysis; data from supporting experiments, including additional TEM micrographs, DLS spectra, FTIR spectra of CdSe QDs before and after exchange with L-(+)-tartaric acid, as well as UV−vis and CD spectra of chiral carboxylic acids dissolved in H2O and of CdSe bound by DL-tartaric acid, succinic acid, meso-tartaric acid, (R)-(−)-phenylsuccinic acid, (S)(−)-2-chloropropionic acid, and N-acetyl-L-aspartic acid with varying CdSe concentrations, all dispersed in DMF (PDF) 12245

DOI: 10.1021/acsnano.7b05690 ACS Nano 2017, 11, 12240−12246

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DOI: 10.1021/acsnano.7b05690 ACS Nano 2017, 11, 12240−12246