Letter pubs.acs.org/NanoLett
Intrinsic Chirality of CdSe/ZnS Quantum Dots and Quantum Rods Maria V. Mukhina,*,† Vladimir G. Maslov,† Alexander V. Baranov,† Anatoly V. Fedorov,† Anna O. Orlova,† Finn Purcell-Milton,‡ Joseph Govan,‡ and Yurii K. Gun’ko*,†,‡ †
ITMO University, St. Petersburg, 197101, Russia School of Chemistry and CRANN, University of Dublin, Trinity College, Dublin 2, Ireland
‡
S Supporting Information *
ABSTRACT: A new class of chiral nanoparticles is of great interest not only for nanotechnology, but also for many other fields of scientific endeavor. Normally the chirality in semiconductor nanocrystals is induced by the initial presence of chiral ligands/stabilizer molecules. Here we report intrinsic chirality of ZnS coated CdSe quantum dots (QDs) and quantum rods (QRs) stabilized by achiral ligands. As-prepared ensembles of these nanocrystals have been found to be a racemic mixture of D- and L-nanocrystals which also includes a portion of nonchiral nanocrystals and so in total the solution does not show a circular dichroism (CD) signal. We have developed a new enantioselective phase transfer technique to separate chiral nanocrystals using an appropriate chiral ligand and obtain optically active ensembles of CdSe/ZnS QDs and QRs. After enantioselective phase transfer, the nanocrystals isolated in organic phase, still capped with achiral ligands, now display circular dichroism (CD). We propose that the intrinsic chirality of CdSe/ZnS nanocrystals is caused by the presence of naturally occurring chiral defects. KEYWORDS: Intrinsic chirality of nanocrystals, separation of nanocrystal enantiomers, chiral defects, screw dislocation, circular dichroism
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nanoparticles has received a great deal of attention due to the range of potential applications offered by these materials in chiral sensing, catalysis, and as metamaterials in advanced optical devices.7,9 A recent example of this was demonstrated using DNA-origami-scaffolded plasmonic gold nanoparticle helices attached to a substrate, which can be used to reversibly switch the optical response between two distinct CD spectra corresponding to either a perpendicular or parallel helix orientation with respect to the light beam.13 These switchable chiral plasmonic nanostructures can be used as sensors for the detection of molecules, their chirality, and their orientation and could find applications in data storage systems or as materials with switchable refractive indices. The use of stereospecific chiral stabilizer molecules/ligands has also opened another avenue of interest in the area of optically active quantum dot research.14−17 Initially optically active chiral QDs (penicillamine stabilized CdS) with surface defect luminescence have been prepared by using microwave induced heating with the racemic (Rac), D- and L-enantiomeric forms of penicillamine as stabilizers.14 Density functional calculations of the electronic states demonstrated that circular
hirality is one of the most fascinating occurrences in the natural world.1,2 Chiral compounds play an extremely important role in the fields of chemistry, pharmacology, biology, and medicine. Chirality is also one of the key factors in molecular recognition,3,4 which has many applications in both chemistry and biology. Discovering efficient methods to produce and identify enantiopure molecules5 is critical for the development of pharmaceuticals, agrochemicals, fragrances, and food additives.6 Chirality has also been envisaged to play an important role in nanotechnology. Potentially, any nanocrystal can be chiral since they frequently have low symmetry due to the presence of chiral defects in bulk and at the surface.7 However, nanocrystals in a macroscopic ensemble in solution typically show no optical activity (circular dichroism) since nanocrystal chirality is random. In overall the development of new chiral nanoparticles is of great interest not only for nanotechnology, but also for many other fields of science including chemistry, biochemistry, pharmacology, and medicine. In addition, the understanding of the fundamental concepts relevant to chirality in nanosystems is very important for the advancement of nanoscience and nanotechnology in general.7−12 It has been demonstrated that various optically active inorganic nanocrystals can be designed and produced. In particular over the last number of years, the area of chiral metal © XXXX American Chemical Society
Received: November 18, 2014 Revised: April 22, 2015
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Figure 1. Chiroptical properties of the CdSe/ZnS QD and QR solutions after chiral resolution. The absorption (dotted lines) and CD (solid lines) spectra of the chloroform and aqueous phases of the QDs (A and C) and QRs (B and D) after enantioselective phase transfer with L-cysteine (A and B) and D-cysteine (C and D).
tellurium, and gold nanostructures with enantioselectively controlled lattice and shape chirality using chiral ligands.26 In this work, colloidal nanostructures of Se and Te with various morphologies have been prepared by applying strongly binding chiral ligands (glutathione, cysteine, and penicillamine), with all samples demonstrating a strong CD response. Moreover it was found that the chiral Te nanostructures of a size in the order of 100 nm can act as chiral optical resonators that may be useful for the optical sensing of chiral molecules. In all publications discussed, the chirality in these quantum dot structures was determined to have been induced by the initial presence of chiral ligands/stabilizer molecules. Here we report the observation of intrinsic chirality for CdSe/ZnS based quantum dots and quantum rods which were originally prepared without use of chiral capping ligands or stabilizers. In this case the intrinsic chirality of the CdSe/ZnS based nanocrystals is caused by the presence of naturally occurring chiral defects which are formed in an equal D- to L-ratio (50:50 racemic mixture) along with achiral nanocrystals, and as a result there is no overall CD response of the original nanocrystals after the hot injection synthesis. We demonstrate that these intrinsically chiral semiconducting nanocrystals can be selectively separated using an appropriate chiral ligand and a standard phase transfer approach. This chiral resolution enabled us to isolate L- and D- chiral CdSe/ZnS based nanocrystals into different organic and aqueous phases and investigate their CD behavior. Results. Separation of Left-Handed and Right-Handed Nanocrystals. To separate enantiomers of CdSe/ZnS nanocrystals, the samples were first suspended in chloroform. Absorption and CD spectra of the QD and QR suspensions before the enantioselective phase transfer are shown in the Supporting Information (Figure 1A). These show that an ensemble of the nanocrystals does not possess optical activity before enantiomeric separation. The solutions were cooled to
dichroism at longer wavelengths is associated with near-surface cadmium atoms that are enantiomerically distorted by chiral penicillamine ligands, which translate their enantiomeric structure to the surface layers and associated electronic states, while the QD core is found to remain undistorted and achiral.15 Following that work, the preparation of chiral CdSe,18 CdTe,19,20 and chiral CdS nanotetrapods (NTP) were also reported.16 All of these chiral nanostructures showed characteristic CD responses within the band-edge region of their spectrum as well as very broad photoluminescence band which both originate from defect states. The concept of a chiral ligand induced optical activity was also reported for CdTe nanocrystals bearing various chiral ligands.21−23 Interestingly, it was shown that the chirality of the QDs surface was retained even after ligand exchange with an achiral thiol and subsequent transfer of the CdTe QDs into a different (organic) phase. In this case, chiral QDs demonstrated a very interesting chiral memory effect.22 More recently, chiral ligand induced circular dichroism in CdSe QDs was reported by M. Balaz et al.24,25 The researchers have found that chiral thiol capping ligands such as L- and Dcysteines can induce chiroptical properties in originally achiral CdSe QDs. The process involved a simple phase transfer of achiral trioctylphosphine oxide or oleic acid capped CdSe QDs from toluene into aqueous phase using L- or D-cysteines by stirring the mixture at room temperature in the absence of light for 24 h. It was found that L- or D-cysteine stabilized QDs in aqueous phase demonstrated size-dependent electronic circular dichroism (CD) and circularly polarized luminescence (CPL). As expected, mirror image CD and CPL signals have been shown by CdSe QDs capped with D- and L-cysteine. In this case, the origin of the induced CD in QDs was explained by the hybridization of chiral ligand molecular orbitals with QD valence band states. In another recent publication, Markovich et al. reported the synthesis of a range of colloidal selenium, B
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Figure 2. Spectra of induced (A) and intrinsic (B) CD (solid lines) of the L- and D-CdSe/ZnS QDs. Dotted lines show absorption spectra.
Figure 3. (A) Comparison of the CD signals induced by the full and enantioselective phase transfer of QDs. Inset shows time dependence of CD normalized to absorbance during full phase transfer. (B) Comparison of the CD spectra TOPO-capped CdSe/ZnS QDs, D-cysteine capped QDs, and QDs after substitution of the chiral ligand D-cysteine for achiral dodecanethiol (DDT).
aqueous phase we assume that the induced contribution prevails over the intrinsic one in the CD signal. As shown in Figure 1A−C, there are some noteworthy differences between the spectra of the induced and intrinsic CD. In particular, the induced CD signal intensity is three times stronger for the QDs and seven times stronger for the QRs when compared to the intrinsic CD signal. Also, a comparison of the CD spectra clearly shows the appearance of a new CD band at longer wavelengths of the band gap in the case of the intrinsic CD. The differences observed may be caused by a number of reasons. (1) It seems very likely that the chloroform and aqueous phases are enriched unequally. (2) According to the literature,15,24 adsorption of the chiral ligands on the nanocrystal surface induces the appearance of the CD signal. In our experiments, an increase in the intensity of the CD signal is observed in the aqueous phase compared with the chloroform phase. We assume that this increase can be partly induced by adsorption of chiral ligands. And, importantly, this effect is more distinct for the QRs, probably, because of their elongated shape and therefore larger surface area. (3) The chloroform phase solution is slightly unstable after extraction of the aqueous phase and is believed to contain charged species that can lead to aggregation of the nanocrystals. This is combated by the addition of a diluted solution of HCl, which partly stabilizes the chloroform phase (see Methods for details). However, the solution in chloroform does not become optically clear which may be due to the retention of a small amount of charged species. The influence of these three factors likely leads to the observed differences in the values of the CD signal in water and chloroform, producing the shift and broadening of the CD bands, as well as to the appearance of an additional CD band. An absence of an increase in the CD signal in the aqueous
slow the process of phase transfer. Then a concentrated methanol solution of D- or L-cysteine was added to the nanocrystal solution followed by stirring and the addition of distilled water. To initiate the phase transfer, the mixture was vigorously shaken and left for 1−2 min. After complete separation of the chloroform and aqueous phases, the optically enriched fraction of nanocrystals capped by cysteine transferred to water. The aqueous phase was enriched with levorotatory (L) or dextrorotatory (D) nanocrystals in the case of using L- or Dcysteine, respectively. In contrast, the chloroform phase was enriched with nanocrystals of the opposite chirality that were still capped by achiral molecules of trioctylphosphine oxide (TOPO). Schematic representation of the enantioselective phase transfer and the image of the QD sample are shown in the Supporting Information (Figure 1B). Chiroptical Properties. The CD spectra of the cysteine- and TOPO-capped CdSe/ZnS QDs as well as QRs are shown in Figure 1 and were measured after the enantioselective phase transfer assisted by L- and D-cysteine. There was a possibility that CD spectra in the aqueous and organic phases could be both induced by the interaction with chiral stabilizer, because, as FTIR data in Supporting Information Figure 2 indicated, some amount of cysteine remained in the organic phase after chiral resolution of nanocrystal enantiomers. In this case, an opposite sign of the CD bands could be caused by different molecular geometry of cysteine in the aqueous and organic phases. However, analysis of the CD spectra of D-cysteine in aqueous and chloroform solutions clearly shows that it remains dextrorotatory (see Supporting Information Figure 3). Therefore, CD spectra of TOPO-capped nanocrystals in chloroform were ascribed by us as intrinsic, and CD spectra of the nanocrystals, whose chiroptical properties were modified by capping with cysteine, were ascribed as induced. For the C
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Figure 4. CD spectra of CdS nanotetrapods stabilized by L- and D-penicillamine after substitution of the chiral ligands for achiral dodecanethiol (A) and following chiral resolution of racemic mixture of nanotetrapods in different phases with L-cysteine (B). The absorption and CD spectra are shown with dotted and solid lines, respectively.
out with dodecanthiol for the sample of D-cysteine capped CdSe/ZnS QDs. The degree of substitution of cysteine for DDT was monitored by FTIR spectroscopy (see Supporting Information Figure 2). As one can see in Figure 3B, the QDs capped with DDT display a CD signal with the same sign as observed in the aqueous phase but with a decrease in intensity equal to a maximum of two-thirds of the original. Chiral Resolution of Racemic Mixture of Initially Chiral Nanotetrapods. To check whether the method of enantioselective phase transfer works for initially chiral nanocrystals, CdS nanotetrapods (NTP) were investigated. The CdS NTPs were synthesized with chiral ligands penicillamine (for details see Methods), and, as shown in Supporting Information Figure 4, displayed chiroptical activity before chiral resolution. The chiral ligands were removed from nanotetrapods surface via reverse phase transfer procedure analogous to the method described for the QDs above. Figure 4A shows the CD spectra of the NTP surface capped with achiral DDT after the reverse phase transfer. As shown, L- and D-CdS NTPs still exhibit near mirrorimage CD spectra and, importantly, a mixture of L- and Dnanotetrapods, as shown in Figure 4A, is almost chiroptically inactive. Chiral resolution of this optically inactive mixture of Land D-nanotetrapods was then carried out via standard enantioselective phase transfer procedure previously demonstrated with CdSe/ZnS QD and QR samples. CD spectra for the aqueous and chloroform phases are shown in Figure 4B. As L-cysteine was used for the phase transfer, L-CdS NTPs transferred to water, and the chloroform phase was enriched with D-CdS NTPs. As it was in the case of the QDs and QRs, the chloroform phase was not optically clear and may have contained nanocrystal aggregates. Most likely, this led to the shift of the CD bands of D-CdS NTPs compared with L-CdS NTPs that was observed in our experiments. Discussion. There is great scientific and technological interest in the development of new approaches for the production of chiral nanoparticles. Over the last 7 years, there have been a number of reports on optically active chiral quantum dots.14−16,22−24,28,29 There are also several different theories explaining the chirality in various types of QDs. Using density functional theory (DFT) calculations Gun’ko et al. have proposed that the circular dichroism in QDs with surface defect luminescence is associated with near-surface cadmium atoms that are enantiomerically distorted by chiral penicillamine ligands, which translate their enantiomeric structure to the surface layers and associated electronic states.15 However, chiral surface defects are not the only factor for the origin of chirality in chiral QDs. For example, Kotov et al. postulated that the
phase of the D-cysteine capped QRs shown in Figure 1D also can be associated with aggregation of the nanocrystals. The influence of aggregation on the spectral position of the CD bands is more obvious if, as it is shown in Figure 2, the spectra of the induced (Figure 2A) and intrinsic (Figure 2B) CD of the D- and L-CdSe/ZnS QDs are compared. In this case, the CD band’s positions coincide, because the L- and Dnanocrystals are compared in the same state. In all cases shown in Figure 1, the CD spectra have a few features with alternating sign. The position of the first CD bands clearly corresponds to the absorption band gap wavelength. Also, the first two intrinsic CD bands of both the QDs and QRs have the same signs: ∓ and ± for L- and Dnanocrystals, respectively. According to data published in the previous work,27 the first and second groups of the QR excitonic transitions involve components with different polarization, whose absorbance positions correspond to the observed alterations in CD sign very well. Time Dependence Study. Increasing the induced CD signal depends on the selectivity of the process of nanocrystal extraction from chloroform to the aqueous phase. If the solution is not cooled before extraction, the volume of cysteine addition is increased 2-fold, and the solubilization time is increased 3-fold, all the nanocrystals, regardless of their handedness, will transfer to the aqueous phase. As shown in Figure 3A, the CD signal after the enantioselective phase transfer is approximately 2.5 times higher than those after the full phase transfer. If L-cysteine (as in the case shown in Figure 3A) is used for extraction, the smaller CD signal after the full phase transfer can be attributed to the superposition of the induced CD signal of achiral and L-nanocrystals with the intrinsic CD signal of D-nanocrystals modified after adsorption of L-cysteine. As shown in the inset to Figure 3A, the CD signal depends on the time of the full phase transfer reaction. The signal increases for the first 8 min of the reaction until equal to the CD signal measured for the enantioselective phase transfer. After that, the CD signal intensity is found to decrease by approximately 2.6 times. The explanation for this time dependence behavior is that, in the case of using L-cysteine, L-nanocrystals transfer to water faster than D-nanocrystals, producing the greater signal as the intrinsic L-nanocrystals transfer, which then reduces as more D-nanocrystals transfer to the aqueous phase. Chiroptical Activity Test after Removal of the Chiral Ligands. To check whether the nanocrystals display chiroptical activity after removal of the chiral ligands, a reverse phase transfer from water to chloroform (see Methods) was carried D
DOI: 10.1021/nl504439w Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters atomic origin of chiral sites in cysteine stabilized CdTe QDs is geometrically similar to that in organic compounds. Using theoretical calculations and experimental data, the researchers demonstrated that atoms in chiral cysteine stabilized CdTe nanocrystals are arranged as tetrahedrons and therefore chirality occurs when all tetrahedral apexes have different chemical substitutions.23 More recently, Balaz et al. have explained the origin of induced CD in QDs by the hybridization and coupling of the chiral ligand HOMO with the QD valence band states.24,25 In all cases discussed the presence of chiral ligand during the formation of QDs or at the surface of produced QDs is a crucial factor for inducing circular dichroism and therefore the specific peaks in CD spectra of chiral QDs. By contrast, according to our presented results the formation of chiral QDs takes place even in the absence of chiral ligands, resulting in intrinsically chiral semiconducting nanocrystals which can be selectively separated using an appropriate chiral ligand and an organicaqueous phase transfer approach. This enables us to suggest that CdSe/ZnS QDs and QRs initially contain some chiral defects which are produced at the surface of nanocrystals during their formation. In this case there is an equal probability of the formation of L- and D-chiral defects in nanocrystals such as for example screw and edge dislocations and potentially other chiral defects (e.g., interstitial point defects or atom vacancies) that results in the formation of racemic mixtures of D- and L-nanocrystals along with nonchiral nanocrystals. Indeed if we look at TEM images of CdSe/ZnS nanocrystals we can identify the possible presence of QDs with left- and righthanded screw dislocations (Figure 5A and B). If the resulting mixture contains equal amounts of nanocrystals with right- and left-handed dislocations (racemate) it does not show any
optical activity and has no visible signals in its CD spectra. However, our chiral-ligand assisted phase separation allowed us to separate D- and L-chiral nanostructures into different phases. Chiral resolution of the nanocrystals requires energy difference in binding of chiral molecules of cysteine to the oppositely distorted surfaces of the nanocrystal. To verify enantioselectivity of the surface of CdSe/ZnS nanocrystal distorted with screw dislocation, we performed DFT calculations (for details see Methods). Quantum dots with right-handed and left-handed dislocations were modeled as shown in Figure 5C and D. For the calculation we choose the distorted (+)Zn13S13 and (−)Zn13S13 clusters and attached to their end-surfaces molecule of L-cysteine (where (+)/(−) is left/right handedness of the ZnS clusters). The calculation of the binding energy of L-cysteine and Zn13S13 cluster is described in Methods. The difference between binding energies of the Lcys-(−)Zn13S13 complex and the L-cys-(+)Zn13S13 complex was found to be equal to 0.2 eV, exceeding room temperature kT nearly eight times, and therefore indicating the possibility of chiral molecular recognition in the system of CdSe/ZnS QD and cysteine. This result is in agreement with the previous reports for cysteine on the surface of gold.30,31 It has been shown in previous publications that screw dislocations strongly influence the growth process and that the morphology of nanocrystals, frequently resulting in asymmetric nanostructures.32−34 It is also well-known that wurtzite CdSe nanostructures grow layer by layer in direction of the crystallographic axis c.35 Therefore, it is expected that the formation of screw dislocations resulting in asymmetric and chiral nanostructures can occur during the hot injection synthesis of CdSe/ZnS quantum dots and rods. The influence of both crystal structure and size on the chiroptical activity in colloidal QDs has been previously reported.24,29,36 Chiral twisting also strongly affects optical activity of ensembles of nanocrystals37,38 and macroscopic crystals.39 In addition, the formation of gold clusters of different sizes stabilized by achiral thiolates, and their enatioseparation by using different techniques have been reported recently.40−42 In these cases, the chirality of the nanoclusters was caused by the chiral arrangement of the thiolates on their surfaces, forming staple motifs. We believe that chiral defects such as screw dislocations can play an important role in the appearance of chirality in the case of semiconductor nanocrystals, at the same time interactions with the chiral ligands can affect CD signal of nanocrystals as it is confirmed by our observation. In addition we have demonstrated that premade chiral nanostructures such as previously reported chiral D- and Lpenicilamine stabilized CdS nanotetrapods16 can be transferred into organic phase using nonchiral dodecanethiol ligand and that this racemic mixture of nanotetrapods can be easily separated using the phase transfer method reported, with Lcysteine ligands resulting in the separation and concentration of D- and L-CdS nanostructures in organic and aqueous phases. In this case, the chiral defects at the surface of CdS nanotetrapods and optical activity are retained in these nanostructures even after their transfer into the organic phase using the nonchiral dodecanethiol ligand (chiral memory effect). However, if we prepare the racemic mixture composed of D- and L-CdS tetrapods, the use of chiral L-cysteine ligands enables the recognition of chiral defects on the surface of these nanotetrapods followed by the enantioselective separation of these chiral nanostructures into different phases (Figure 4).
Figure 5. (A, B) TEM images of the CdSe/ZnS QDs. The arrows indicate possible screw dislocations. (C, D) Atomistic models of CdSe/ZnS QDs with right (C) and left (D) screw dislocations. Dislocations are set in the (010) plane of CdSe core of nanocrystals with (−) Burgers vector (C) and (+) Burgers vector (D). Atomistic models were prepared using Vesta software.43 Red dotted lines indicate the direction of the dislocations. E
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Methods. Synthesis of CdSe/ZnS Quantum Dots and Quantum Rods. Semiconductor core/shell CdSe/ZnS quantum dots and rods were synthesized by a method described previously45 without using any chiral chemicals. The dots had a diameter of ∼2.5 nm and exhibit a photoluminescence band with a peak emission at 520 nm. The rods had a diameter of ∼5 nm with an average aspect ratio of around 7 and exhibit a photoluminescence band with a peak emission at 630 nm. Asprepared CdSe/ZnS nanocrystals were surface capped by achiral TOPO. Synthesis of Penicilamine Stabilized CdS Tetrapods. Aqueous solutions of D- or L-penicillamine (0.01 M, 10 mL) and CdCl2 (0.01 M, 8 mL) were mixed with 40 mL of Millipore water in a round bottomed flask. Then 2 M NaOH solution was added dropwise to the mixture to adjust the pH to 11. After that thioacetamide solution (0.01 M, 2 mL) was added to the mixture, and it was heated under reflux for 2 h. Then the flask was wrapped in aluminum foil and was allowed to cool to ambient temperature. The solution was concentrated by rotary evaporation of the water under reduced pressure followed by filtration via ultracentrifugation with a 30 kDa MWCO filter. The precipitate was washed with water and then redispersed in Millipore water and stored at 4 °C. Enantioselective Phase Transfer. For the optically enriched samples, 15 mg of nanocrystals were dissolved in 750 μL of chloroform. The solutions obtained were cooled for 15 min at 4 °C to slow the process. Then 5 vol % of methanol solution of D- or L-cysteine (8 mg of cysteine dissolved in 300 μL of methanol) were added to the nanocrystals solution followed by stirring. After 1−2 min of stereospecific solubilization, 750 μL of distilled water with pH 10−11 were added to the solution. pH was varied by the addition of aqueous solution of KOH. To initiate the phase transfer, the mixture was vigorously shaken and left for 1−2 min. After complete separation, the chloroform phase and the aqueous phase were collected in separate vials. The chloroform phase contained a small amount of the nanocrystals partially capped by charged molecules of cysteine. It could destabilize the solution and lead to aggregation of the nanocrystals. To avoid aggregation, a few drops of 3% solution of hydrochloric acid and 50−100 μL of chloroform solution of TOPO were added to the chloroform phase. Full Phase Transfer. To suppress selectivity of interaction between the chiral ligands and the nanocrystals, a number of changes were made to the method described above. In particular, the solution was not cooled; the addition of cysteine was increased up to 10 vol % with the same concentration. After the cysteine solution was added, the solution was stirred for 5 min. Under such conditions, nearly 100% of the nanocrystals transferred to the aqueous phase after addition of water and extraction. Reverse Phase Transfer. For phase transfer from water to chloroform,46 1 mL of dodecanthiol (DDT) and 2 mL of acetone were added to 1 mL of aqueous solution of cysteinecapped nanocrystals obtained after the enantioselective phase transfer. To initiate the phase transfer, the mixture was vigorously shaken and heated to 56 °C. After a few minutes the mixture were centrifuged, and the nanocrystals capped with DDT were redissolved in chloroform. CD Measurements. The spectra of circular dichroism and absorption were studied using a Jasco J-1500 spectrometer. The spectra of the chloroform and aqueous phases were measured separately at 20 °C. Quartz cuvettes with a 1 cm path length were used for all experiments.
These results confirm our hypothesis and indicate both the generic and universal nature of our approach for the separation of chiral nanostructures in different phases. Our method of enantioselective phase transfer of quantum nanostructures demonstrates that chiral recognition and discrimination which is very well-known for enantiomeric molecules3,4 can be extended to nanosized inorganic particles. The detailed mechanism of this chiral recognition between chiral nanocrystals and chiral ligand is not quite clear at this stage. However, most likely the process is similar to the three point recognition44 which is described for cysteine molecules at gold metal surface.4,30,31 More precise studies of the mechanism as well as computational modeling will be the subject of future work. In conclusion, we have developed a new enantioselective phase transfer technique for the separation of chiral nanostructures using an appropriate chiral ligand. We have shown that this method can be used for chiral resolution of various quantum nanostructures such as CdSe/ZnS QDs and QRs and CdS nanotetrapods. We expect that this approach will open up new exciting possibilities in the separation and investigation of various chiral nanosized objects and may find a number of technologically important applications in chiral resolution, asymmetric catalysis, and chiral recognition and sensing. In addition we have found that in some nanocrystals (CdSe/ ZnS QDs and QRs) the chirality and corresponding optical activity are intrinsic features that can occur even without the presence of chiral ligands. We propose that the intrinsic chirality of the CdSe based nanocrystals is caused by the presence of intrinsically occurring chiral defects such as dislocations or point defects in these nanostructures. The use of chiral ligands enabled us to separate these nanostructures into different phases and enhance their chiroptical activity. We believe that not only CdSe based nanocrystals but also many other colloidal inorganic nanoparticulate systems have similar intrinsic chiral properties, in particular twinned nanocrystals similar to the ones described in ref 34. Also it is very likely that the chiral defects can exist not only in wurtzite but can be intrinsic to the other types of crystal lattice.39 Therefore, some of the well-known existing inorganic nanostructures could have D- and L-chiral species which coexist alongside nonchiral nanoparticles. Since the probability of formation of left- and right-handed chiral features is equal and therefore will form racemic mixtures, it is normally impossible to detect these nanostructures using CD spectroscopy. The possibility to separate these chiral nanostructures into different phases using our approach opens up new horizons in nanoscience and nanotechnology. We consider this research will be of particular importance for future development of nanobiotechnology due to biomolecules well-known chirality and therefore sensitivity to D- or L-chiral nanoparticles. Therefore, the possibility of the existence of intrinsically chiral nanoparticles (without chiral ligands) forming racemic mixtures must be taken into account, and as result this would require a significant revision of existing views and approaches in very important areas such as nanotoxicology and the use of nanoparticles in both medical diagnostics and drug delivery systems. In addition, this research may have an impact on the applications of nanoparticulate systems in biopharmaceutical industry. However, further intensive research will be necessary to understand the detailed mechanisms of formation and behavior of chiral optically active colloidal nanoparticles, as well as to develop their applications. F
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(8) Ben-Moshe, A.; Maoz, B. M.; Govorov, A. O.; Markovich, G. Chem. Soc. Rev. 2013, 42, 7028−7041. (9) Guerrero-Martínez, A.; Alonso-Gómez, J. L.; Auguié, B.; Cid, M. M.; Liz-Marzán, L. M. Nano Today 2011, 6, 381−400. (10) Wang, Y.; Xu, J.; Wang, Y.; Chen, H. Chem. Soc. Rev. 2013, 42, 2930−2962. (11) Jing, W.; Shuang, L.; Chun, Z.; Huibi, X.; Xiangliang, Y. Prog. Chem. 2011, 23, 669−678. (12) Liu, H.; Shen, X.; Wang, Z.-G.; Kuzyk, A.; Ding, B. Nanoscale 2014, 6, 9331−9338. (13) Schreiber, R.; Luong, N.; Fan, Z.; Kuzyk, A.; Nickels, P. C.; Zhang, T.; Smith, D. M.; Yurke, B.; Kuang, W.; Govorov, A. O.; Liedl, T. Nat. Commun. 2013, 4, 2948. (14) Moloney, M. P.; Gun’ko, Y. K.; Kelly, J. M. Chem. Commun. 2007, 38, 3900−3902. (15) Elliott, S. D.; Moloney, M. P.; Gun’ko, Y. K. Nano Lett. 2008, 8, 2452−2457. (16) Govan, J. E.; Jan, E.; Querejeta, A.; Kotov, N. A.; Gun’ko, Y. K. Chem. Commun. 2010, 46, 6072−6074. (17) Moloney, M. P.; Govan, J.; Loudon, A.; Mukhina, M.; Gun’ko, Y. K. Nat. Protoc. 2015, 10, 558−573. (18) Gallagher, S. A.; Moloney, M. P.; Wojdyla, M.; Quinn, S. J.; Kelly, J. M.; Gun’ko, Y. K. J. Mater. Chem. 2010, 20, 8350−8355. (19) Moloney, M. P., Gallagher, S. A., Gun’ko, Y. K. Chiral CdTe quantum dots; MRS Proc.: Cambridge, 2009; pp 124-XX02-10. (20) Gérard, V. A.; Freeley, M.; Defrancq, E.; Fedorov, A. V.; Gun’ko, Y. K. J. Nanomater. 2013, 2013, 3. (21) Xia, Y.; Zhou, Y.; Tang, Z. Nanoscale 2011, 3, 1374−1382. (22) Nakashima, T.; Kobayashi, Y.; Kawai, T. J. Am. Chem. Soc. 2009, 131, 10342−10343. (23) Zhou, Y.; Yang, M.; Sun, K.; Tang, Z.; Kotov, N. A. J. Am. Chem. Soc. 2010, 132, 6006−6013. (24) Tohgha, U.; Deol, K. K.; Porter, A. G.; Bartko, S. G.; Choi, J. K.; Leonard, B. M.; Varga, K.; Kubelka, J.; Muller, G.; Balaz, M. ACS Nano 2013, 7, 11094−11102. (25) Tohgha, U.; Varga, K.; Balaz, M. Chem. Commun. 2013, 49, 1844−1846. (26) Ben-Moshe, A.; Wolf, S. G.; Sadan, M. B.; Houben, L.; Fan, Z.; Govorov, A. O.; Markovich, G. Nat. Commun. 2014, 5, 4302. (27) Mukhina, M. V.; Maslov, V. G.; Baranov, A. V.; Artemyev, M. V.; Orlova, A. O.; Fedorov, A. V. Opt. Lett. 2013, 38, 3426−3428. (28) Naito, M.; Iwahori, K.; Miura, A.; Yamane, M.; Yamashita, I. Angew. Chem., Int. Ed. 2010, 49, 7006−7009. (29) Ben Moshe, A.; Szwarcman, D.; Markovich, G. ACS Nano 2011, 5, 9034−9043. (30) Greber, T.; Šljivančanin, Ž ; Schillinger, R.; Wider, J.; Hammer, B. Phys. Rev. Lett. 2006, 96, 056103. (31) Kühnle, A.; Molina, L.; Linderoth, T. R.; Hammer, B.; Besenbacher, F. Phys. Rev. Lett. 2004, 93, 086101. (32) Yu, Z.; Hahn, M. A.; Maccagnano-Zacher, S. E.; Calcines, J.; Krauss, T. D.; Alldredge, E. S.; Silcox, J. ACS Nano 2008, 2, 1179− 1188. (33) Meng, F.; Morin, S. A.; Forticaux, A.; Jin, S. Acc. Chem. Res. 2013, 46, 1616−1626. (34) Chen, C.-C.; Zhu, C.; White, E. R.; Chiu, C.-Y.; Scott, M.; Regan, B.; Marks, L. D.; Huang, Y.; Miao, J. Nature 2013, 496, 74−77. (35) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. Nature 2000, 404, 59−61. (36) Baimuratov, A. S.; Rukhlenko, I. D.; Gun’ko, Y. K.; Baranov, A. V.; Fedorov, A. V. Nano Lett. 2015, 15, 1710−1715. (37) Singh, G.; Chan, H.; Baskin, A.; Gelman, E.; Repnin, N.; Král, P.; Klajn, R. Science 2014, 345, 1149−1153. (38) Yeom, J.; et al. Nat. Mater. 2015, 14, 66−72. (39) Shtukenberg, A. G.; Punin, Y. O.; Gujral, A.; Kahr, B. Angew. Chem., Int. Ed. 2014, 53, 672−699. (40) Dolamic, I.; Knoppe, S.; Dass, A.; Bürgi, T. Nat. Commun. 2012, 3, 798. (41) Knoppe, S.; Wong, O. A.; Malola, S.; Häkkinen, H.; Bürgi, T.; Verbiest, T.; Ackerson, C. J. J. Am. Chem. Soc. 2014, 136, 4129−4132.
DFT Calculations. DFT calculations were performed using the GAMESS program47 with the Hay−Wadt valence basis with pseudopotential48 and the B3LYP functional.49 The binding energy of L-cysteine and Zn13S13 cluster was calculated as Ebinding = Ecomplex − Ecluster − Ecys, where Ecomplex is the total energy of the L-cys-(+)Zn13S13 complex or L-cys-(−)Zn13S13 complex, Ecluster is the total energy of the Zn13S13 cluster with the same geometry as in the complexes, and Ecys is the total energy of L-cysteine molecule. Total energies were calculated for optimized geometry with the exception of the atoms belonging to the Zn13S13 cluster, which were assumed to be fixed. No solvation models were used in the calculations.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 1A shows absorption and CD spectra of initial chloroform solutions of the QR and QD; Figure 1B shows schematic representation of the enantioselective phase transfer and photography of the sample after the enantioselective phase transfer; Figure 2 shows FTIR spectra of the QD samples in initial solution, the chloroform phase after the enantioselective phase transfer, the aqueous phase after the enantioselective phase transfer and the chloroform phase after the back phase transfer of the aqueous phase from water to the chloroform with achiral ligand; Figure 3 shows absorption and CD spectra of D-cysteine and complexes of D-cysteine with QDs in aqueous and chloroform solutions; Figure 4 shows absorption and CD spectra of initial water solutions of the nanotetrapods synthesized with chiral ligand penicillamine. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/nl504439w.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Government of the Russian Federation (Grant 074-U01), and the Ministry of Education and Science of the Russian Federation (Grant No. 14.B25.31.0002). We also acknowledge the financial support from Science Foundation Ireland (Grant SFI 12/IA/1300) and EU FP7 FutureNanoNeeds grant. M.V.M. also thanks the Ministry of Education and Science of the Russian Federation for support via the Scholarships of the President of the Russian Federation for Young Scientists and Graduate Students (2015−2017).
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REFERENCES
(1) Jorissen, A.; Cerf, C. Orig. Life Evol. Biosph. 2002, 32, 129−142. (2) Wagnière, G. H. On chirality and the universal asymmetry: reflections on image and mirror image; Wiley: New York, 2008. (3) McKendry, R.; Theoclitou, M.-E.; Rayment, T.; Abell, C. Nature 1998, 391, 566−568. (4) Kühnle, A.; Linderoth, T. R.; Hammer, B.; Besenbacher, F. Nature 2002, 415, 891−893. (5) Carreño, M. C. Chem. Rev. 1995, 95, 1717−1760. (6) Williams, K.; Lee, E. Drugs 1985, 30, 333−354. (7) Govorov, A. O.; Gun’ko, Y. K.; Slocik, J. M.; Gérard, V. A.; Fan, Z.; Naik, R. R. J. Mater. Chem. 2011, 21, 16806−16818. G
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Nano Letters (42) Knoppe, S.; Burgi, T. Acc. Chem. Res. 2014, 47, 1318−1326. (43) Momma, K.; Izumi, F. J. Appl. Crystallogr. 2011, 44, 1272−1276. (44) Booth, T. D.; Wahnon, D.; Wainer, I. W. Chirality 1997, 9, 96− 98. (45) Artemyev, M.; M?ller, B.; Woggon, U. Nano Lett. 2003, 3, 509− 512. (46) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Eychmüller, A.; Weller, H. Nano Lett. 2002, 2, 803−806. (47) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347−1363. (48) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310. (49) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.
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Intrinsic Chirality of CdSe/ZnS Quantum Dots and Quantum Rods Maria V. Mukhina,*,† Vladimir G. Maslov,† Alexander V. Baranov,† Anatoly V. Fedorov,† Anna O. Orlova,† Finn Purcell-Milton,‡ Joseph Govan,‡ and Yurii K. Gun’ko*,†,‡ †
ITMO University, St. Petersburg, 197101, Russia School of Chemistry and CRANN, University of Dublin, Trinity College, Dublin 2, Ireland
‡
S Supporting Information *
ABSTRACT: A new class of chiral nanoparticles is of great interest not only for nanotechnology, but also for many other fields of scientific endeavor. Normally the chirality in semiconductor nanocrystals is induced by the initial presence of chiral ligands/stabilizer molecules. Here we report intrinsic chirality of ZnS coated CdSe quantum dots (QDs) and quantum rods (QRs) stabilized by achiral ligands. As-prepared ensembles of these nanocrystals have been found to be a racemic mixture of D- and L-nanocrystals which also includes a portion of nonchiral nanocrystals and so in total the solution does not show a circular dichroism (CD) signal. We have developed a new enantioselective phase transfer technique to separate chiral nanocrystals using an appropriate chiral ligand and obtain optically active ensembles of CdSe/ZnS QDs and QRs. After enantioselective phase transfer, the nanocrystals isolated in organic phase, still capped with achiral ligands, now display circular dichroism (CD). We propose that the intrinsic chirality of CdSe/ZnS nanocrystals is caused by the presence of naturally occurring chiral defects. KEYWORDS: Intrinsic chirality of nanocrystals, separation of nanocrystal enantiomers, chiral defects, screw dislocation, circular dichroism
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nanoparticles has received a great deal of attention due to the range of potential applications offered by these materials in chiral sensing, catalysis, and as metamaterials in advanced optical devices.7,9 A recent example of this was demonstrated using DNA-origami-scaffolded plasmonic gold nanoparticle helices attached to a substrate, which can be used to reversibly switch the optical response between two distinct CD spectra corresponding to either a perpendicular or parallel helix orientation with respect to the light beam.13 These switchable chiral plasmonic nanostructures can be used as sensors for the detection of molecules, their chirality, and their orientation and could find applications in data storage systems or as materials with switchable refractive indices. The use of stereospecific chiral stabilizer molecules/ligands has also opened another avenue of interest in the area of optically active quantum dot research.14−17 Initially optically active chiral QDs (penicillamine stabilized CdS) with surface defect luminescence have been prepared by using microwave induced heating with the racemic (Rac), D- and L-enantiomeric forms of penicillamine as stabilizers.14 Density functional calculations of the electronic states demonstrated that circular
hirality is one of the most fascinating occurrences in the natural world.1,2 Chiral compounds play an extremely important role in the fields of chemistry, pharmacology, biology, and medicine. Chirality is also one of the key factors in molecular recognition,3,4 which has many applications in both chemistry and biology. Discovering efficient methods to produce and identify enantiopure molecules5 is critical for the development of pharmaceuticals, agrochemicals, fragrances, and food additives.6 Chirality has also been envisaged to play an important role in nanotechnology. Potentially, any nanocrystal can be chiral since they frequently have low symmetry due to the presence of chiral defects in bulk and at the surface.7 However, nanocrystals in a macroscopic ensemble in solution typically show no optical activity (circular dichroism) since nanocrystal chirality is random. In overall the development of new chiral nanoparticles is of great interest not only for nanotechnology, but also for many other fields of science including chemistry, biochemistry, pharmacology, and medicine. In addition, the understanding of the fundamental concepts relevant to chirality in nanosystems is very important for the advancement of nanoscience and nanotechnology in general.7−12 It has been demonstrated that various optically active inorganic nanocrystals can be designed and produced. In particular over the last number of years, the area of chiral metal © XXXX American Chemical Society
Received: November 18, 2014 Revised: April 22, 2015
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Figure 1. Chiroptical properties of the CdSe/ZnS QD and QR solutions after chiral resolution. The absorption (dotted lines) and CD (solid lines) spectra of the chloroform and aqueous phases of the QDs (A and C) and QRs (B and D) after enantioselective phase transfer with L-cysteine (A and B) and D-cysteine (C and D).
tellurium, and gold nanostructures with enantioselectively controlled lattice and shape chirality using chiral ligands.26 In this work, colloidal nanostructures of Se and Te with various morphologies have been prepared by applying strongly binding chiral ligands (glutathione, cysteine, and penicillamine), with all samples demonstrating a strong CD response. Moreover it was found that the chiral Te nanostructures of a size in the order of 100 nm can act as chiral optical resonators that may be useful for the optical sensing of chiral molecules. In all publications discussed, the chirality in these quantum dot structures was determined to have been induced by the initial presence of chiral ligands/stabilizer molecules. Here we report the observation of intrinsic chirality for CdSe/ZnS based quantum dots and quantum rods which were originally prepared without use of chiral capping ligands or stabilizers. In this case the intrinsic chirality of the CdSe/ZnS based nanocrystals is caused by the presence of naturally occurring chiral defects which are formed in an equal D- to L-ratio (50:50 racemic mixture) along with achiral nanocrystals, and as a result there is no overall CD response of the original nanocrystals after the hot injection synthesis. We demonstrate that these intrinsically chiral semiconducting nanocrystals can be selectively separated using an appropriate chiral ligand and a standard phase transfer approach. This chiral resolution enabled us to isolate L- and D- chiral CdSe/ZnS based nanocrystals into different organic and aqueous phases and investigate their CD behavior. Results. Separation of Left-Handed and Right-Handed Nanocrystals. To separate enantiomers of CdSe/ZnS nanocrystals, the samples were first suspended in chloroform. Absorption and CD spectra of the QD and QR suspensions before the enantioselective phase transfer are shown in the Supporting Information (Figure 1A). These show that an ensemble of the nanocrystals does not possess optical activity before enantiomeric separation. The solutions were cooled to
dichroism at longer wavelengths is associated with near-surface cadmium atoms that are enantiomerically distorted by chiral penicillamine ligands, which translate their enantiomeric structure to the surface layers and associated electronic states, while the QD core is found to remain undistorted and achiral.15 Following that work, the preparation of chiral CdSe,18 CdTe,19,20 and chiral CdS nanotetrapods (NTP) were also reported.16 All of these chiral nanostructures showed characteristic CD responses within the band-edge region of their spectrum as well as very broad photoluminescence band which both originate from defect states. The concept of a chiral ligand induced optical activity was also reported for CdTe nanocrystals bearing various chiral ligands.21−23 Interestingly, it was shown that the chirality of the QDs surface was retained even after ligand exchange with an achiral thiol and subsequent transfer of the CdTe QDs into a different (organic) phase. In this case, chiral QDs demonstrated a very interesting chiral memory effect.22 More recently, chiral ligand induced circular dichroism in CdSe QDs was reported by M. Balaz et al.24,25 The researchers have found that chiral thiol capping ligands such as L- and Dcysteines can induce chiroptical properties in originally achiral CdSe QDs. The process involved a simple phase transfer of achiral trioctylphosphine oxide or oleic acid capped CdSe QDs from toluene into aqueous phase using L- or D-cysteines by stirring the mixture at room temperature in the absence of light for 24 h. It was found that L- or D-cysteine stabilized QDs in aqueous phase demonstrated size-dependent electronic circular dichroism (CD) and circularly polarized luminescence (CPL). As expected, mirror image CD and CPL signals have been shown by CdSe QDs capped with D- and L-cysteine. In this case, the origin of the induced CD in QDs was explained by the hybridization of chiral ligand molecular orbitals with QD valence band states. In another recent publication, Markovich et al. reported the synthesis of a range of colloidal selenium, B
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Figure 2. Spectra of induced (A) and intrinsic (B) CD (solid lines) of the L- and D-CdSe/ZnS QDs. Dotted lines show absorption spectra.
Figure 3. (A) Comparison of the CD signals induced by the full and enantioselective phase transfer of QDs. Inset shows time dependence of CD normalized to absorbance during full phase transfer. (B) Comparison of the CD spectra TOPO-capped CdSe/ZnS QDs, D-cysteine capped QDs, and QDs after substitution of the chiral ligand D-cysteine for achiral dodecanethiol (DDT).
aqueous phase we assume that the induced contribution prevails over the intrinsic one in the CD signal. As shown in Figure 1A−C, there are some noteworthy differences between the spectra of the induced and intrinsic CD. In particular, the induced CD signal intensity is three times stronger for the QDs and seven times stronger for the QRs when compared to the intrinsic CD signal. Also, a comparison of the CD spectra clearly shows the appearance of a new CD band at longer wavelengths of the band gap in the case of the intrinsic CD. The differences observed may be caused by a number of reasons. (1) It seems very likely that the chloroform and aqueous phases are enriched unequally. (2) According to the literature,15,24 adsorption of the chiral ligands on the nanocrystal surface induces the appearance of the CD signal. In our experiments, an increase in the intensity of the CD signal is observed in the aqueous phase compared with the chloroform phase. We assume that this increase can be partly induced by adsorption of chiral ligands. And, importantly, this effect is more distinct for the QRs, probably, because of their elongated shape and therefore larger surface area. (3) The chloroform phase solution is slightly unstable after extraction of the aqueous phase and is believed to contain charged species that can lead to aggregation of the nanocrystals. This is combated by the addition of a diluted solution of HCl, which partly stabilizes the chloroform phase (see Methods for details). However, the solution in chloroform does not become optically clear which may be due to the retention of a small amount of charged species. The influence of these three factors likely leads to the observed differences in the values of the CD signal in water and chloroform, producing the shift and broadening of the CD bands, as well as to the appearance of an additional CD band. An absence of an increase in the CD signal in the aqueous
slow the process of phase transfer. Then a concentrated methanol solution of D- or L-cysteine was added to the nanocrystal solution followed by stirring and the addition of distilled water. To initiate the phase transfer, the mixture was vigorously shaken and left for 1−2 min. After complete separation of the chloroform and aqueous phases, the optically enriched fraction of nanocrystals capped by cysteine transferred to water. The aqueous phase was enriched with levorotatory (L) or dextrorotatory (D) nanocrystals in the case of using L- or Dcysteine, respectively. In contrast, the chloroform phase was enriched with nanocrystals of the opposite chirality that were still capped by achiral molecules of trioctylphosphine oxide (TOPO). Schematic representation of the enantioselective phase transfer and the image of the QD sample are shown in the Supporting Information (Figure 1B). Chiroptical Properties. The CD spectra of the cysteine- and TOPO-capped CdSe/ZnS QDs as well as QRs are shown in Figure 1 and were measured after the enantioselective phase transfer assisted by L- and D-cysteine. There was a possibility that CD spectra in the aqueous and organic phases could be both induced by the interaction with chiral stabilizer, because, as FTIR data in Supporting Information Figure 2 indicated, some amount of cysteine remained in the organic phase after chiral resolution of nanocrystal enantiomers. In this case, an opposite sign of the CD bands could be caused by different molecular geometry of cysteine in the aqueous and organic phases. However, analysis of the CD spectra of D-cysteine in aqueous and chloroform solutions clearly shows that it remains dextrorotatory (see Supporting Information Figure 3). Therefore, CD spectra of TOPO-capped nanocrystals in chloroform were ascribed by us as intrinsic, and CD spectra of the nanocrystals, whose chiroptical properties were modified by capping with cysteine, were ascribed as induced. For the C
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Figure 4. CD spectra of CdS nanotetrapods stabilized by L- and D-penicillamine after substitution of the chiral ligands for achiral dodecanethiol (A) and following chiral resolution of racemic mixture of nanotetrapods in different phases with L-cysteine (B). The absorption and CD spectra are shown with dotted and solid lines, respectively.
out with dodecanthiol for the sample of D-cysteine capped CdSe/ZnS QDs. The degree of substitution of cysteine for DDT was monitored by FTIR spectroscopy (see Supporting Information Figure 2). As one can see in Figure 3B, the QDs capped with DDT display a CD signal with the same sign as observed in the aqueous phase but with a decrease in intensity equal to a maximum of two-thirds of the original. Chiral Resolution of Racemic Mixture of Initially Chiral Nanotetrapods. To check whether the method of enantioselective phase transfer works for initially chiral nanocrystals, CdS nanotetrapods (NTP) were investigated. The CdS NTPs were synthesized with chiral ligands penicillamine (for details see Methods), and, as shown in Supporting Information Figure 4, displayed chiroptical activity before chiral resolution. The chiral ligands were removed from nanotetrapods surface via reverse phase transfer procedure analogous to the method described for the QDs above. Figure 4A shows the CD spectra of the NTP surface capped with achiral DDT after the reverse phase transfer. As shown, L- and D-CdS NTPs still exhibit near mirrorimage CD spectra and, importantly, a mixture of L- and Dnanotetrapods, as shown in Figure 4A, is almost chiroptically inactive. Chiral resolution of this optically inactive mixture of Land D-nanotetrapods was then carried out via standard enantioselective phase transfer procedure previously demonstrated with CdSe/ZnS QD and QR samples. CD spectra for the aqueous and chloroform phases are shown in Figure 4B. As L-cysteine was used for the phase transfer, L-CdS NTPs transferred to water, and the chloroform phase was enriched with D-CdS NTPs. As it was in the case of the QDs and QRs, the chloroform phase was not optically clear and may have contained nanocrystal aggregates. Most likely, this led to the shift of the CD bands of D-CdS NTPs compared with L-CdS NTPs that was observed in our experiments. Discussion. There is great scientific and technological interest in the development of new approaches for the production of chiral nanoparticles. Over the last 7 years, there have been a number of reports on optically active chiral quantum dots.14−16,22−24,28,29 There are also several different theories explaining the chirality in various types of QDs. Using density functional theory (DFT) calculations Gun’ko et al. have proposed that the circular dichroism in QDs with surface defect luminescence is associated with near-surface cadmium atoms that are enantiomerically distorted by chiral penicillamine ligands, which translate their enantiomeric structure to the surface layers and associated electronic states.15 However, chiral surface defects are not the only factor for the origin of chirality in chiral QDs. For example, Kotov et al. postulated that the
phase of the D-cysteine capped QRs shown in Figure 1D also can be associated with aggregation of the nanocrystals. The influence of aggregation on the spectral position of the CD bands is more obvious if, as it is shown in Figure 2, the spectra of the induced (Figure 2A) and intrinsic (Figure 2B) CD of the D- and L-CdSe/ZnS QDs are compared. In this case, the CD band’s positions coincide, because the L- and Dnanocrystals are compared in the same state. In all cases shown in Figure 1, the CD spectra have a few features with alternating sign. The position of the first CD bands clearly corresponds to the absorption band gap wavelength. Also, the first two intrinsic CD bands of both the QDs and QRs have the same signs: ∓ and ± for L- and Dnanocrystals, respectively. According to data published in the previous work,27 the first and second groups of the QR excitonic transitions involve components with different polarization, whose absorbance positions correspond to the observed alterations in CD sign very well. Time Dependence Study. Increasing the induced CD signal depends on the selectivity of the process of nanocrystal extraction from chloroform to the aqueous phase. If the solution is not cooled before extraction, the volume of cysteine addition is increased 2-fold, and the solubilization time is increased 3-fold, all the nanocrystals, regardless of their handedness, will transfer to the aqueous phase. As shown in Figure 3A, the CD signal after the enantioselective phase transfer is approximately 2.5 times higher than those after the full phase transfer. If L-cysteine (as in the case shown in Figure 3A) is used for extraction, the smaller CD signal after the full phase transfer can be attributed to the superposition of the induced CD signal of achiral and L-nanocrystals with the intrinsic CD signal of D-nanocrystals modified after adsorption of L-cysteine. As shown in the inset to Figure 3A, the CD signal depends on the time of the full phase transfer reaction. The signal increases for the first 8 min of the reaction until equal to the CD signal measured for the enantioselective phase transfer. After that, the CD signal intensity is found to decrease by approximately 2.6 times. The explanation for this time dependence behavior is that, in the case of using L-cysteine, L-nanocrystals transfer to water faster than D-nanocrystals, producing the greater signal as the intrinsic L-nanocrystals transfer, which then reduces as more D-nanocrystals transfer to the aqueous phase. Chiroptical Activity Test after Removal of the Chiral Ligands. To check whether the nanocrystals display chiroptical activity after removal of the chiral ligands, a reverse phase transfer from water to chloroform (see Methods) was carried D
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Letter
Nano Letters atomic origin of chiral sites in cysteine stabilized CdTe QDs is geometrically similar to that in organic compounds. Using theoretical calculations and experimental data, the researchers demonstrated that atoms in chiral cysteine stabilized CdTe nanocrystals are arranged as tetrahedrons and therefore chirality occurs when all tetrahedral apexes have different chemical substitutions.23 More recently, Balaz et al. have explained the origin of induced CD in QDs by the hybridization and coupling of the chiral ligand HOMO with the QD valence band states.24,25 In all cases discussed the presence of chiral ligand during the formation of QDs or at the surface of produced QDs is a crucial factor for inducing circular dichroism and therefore the specific peaks in CD spectra of chiral QDs. By contrast, according to our presented results the formation of chiral QDs takes place even in the absence of chiral ligands, resulting in intrinsically chiral semiconducting nanocrystals which can be selectively separated using an appropriate chiral ligand and an organicaqueous phase transfer approach. This enables us to suggest that CdSe/ZnS QDs and QRs initially contain some chiral defects which are produced at the surface of nanocrystals during their formation. In this case there is an equal probability of the formation of L- and D-chiral defects in nanocrystals such as for example screw and edge dislocations and potentially other chiral defects (e.g., interstitial point defects or atom vacancies) that results in the formation of racemic mixtures of D- and L-nanocrystals along with nonchiral nanocrystals. Indeed if we look at TEM images of CdSe/ZnS nanocrystals we can identify the possible presence of QDs with left- and righthanded screw dislocations (Figure 5A and B). If the resulting mixture contains equal amounts of nanocrystals with right- and left-handed dislocations (racemate) it does not show any
optical activity and has no visible signals in its CD spectra. However, our chiral-ligand assisted phase separation allowed us to separate D- and L-chiral nanostructures into different phases. Chiral resolution of the nanocrystals requires energy difference in binding of chiral molecules of cysteine to the oppositely distorted surfaces of the nanocrystal. To verify enantioselectivity of the surface of CdSe/ZnS nanocrystal distorted with screw dislocation, we performed DFT calculations (for details see Methods). Quantum dots with right-handed and left-handed dislocations were modeled as shown in Figure 5C and D. For the calculation we choose the distorted (+)Zn13S13 and (−)Zn13S13 clusters and attached to their end-surfaces molecule of L-cysteine (where (+)/(−) is left/right handedness of the ZnS clusters). The calculation of the binding energy of L-cysteine and Zn13S13 cluster is described in Methods. The difference between binding energies of the Lcys-(−)Zn13S13 complex and the L-cys-(+)Zn13S13 complex was found to be equal to 0.2 eV, exceeding room temperature kT nearly eight times, and therefore indicating the possibility of chiral molecular recognition in the system of CdSe/ZnS QD and cysteine. This result is in agreement with the previous reports for cysteine on the surface of gold.30,31 It has been shown in previous publications that screw dislocations strongly influence the growth process and that the morphology of nanocrystals, frequently resulting in asymmetric nanostructures.32−34 It is also well-known that wurtzite CdSe nanostructures grow layer by layer in direction of the crystallographic axis c.35 Therefore, it is expected that the formation of screw dislocations resulting in asymmetric and chiral nanostructures can occur during the hot injection synthesis of CdSe/ZnS quantum dots and rods. The influence of both crystal structure and size on the chiroptical activity in colloidal QDs has been previously reported.24,29,36 Chiral twisting also strongly affects optical activity of ensembles of nanocrystals37,38 and macroscopic crystals.39 In addition, the formation of gold clusters of different sizes stabilized by achiral thiolates, and their enatioseparation by using different techniques have been reported recently.40−42 In these cases, the chirality of the nanoclusters was caused by the chiral arrangement of the thiolates on their surfaces, forming staple motifs. We believe that chiral defects such as screw dislocations can play an important role in the appearance of chirality in the case of semiconductor nanocrystals, at the same time interactions with the chiral ligands can affect CD signal of nanocrystals as it is confirmed by our observation. In addition we have demonstrated that premade chiral nanostructures such as previously reported chiral D- and Lpenicilamine stabilized CdS nanotetrapods16 can be transferred into organic phase using nonchiral dodecanethiol ligand and that this racemic mixture of nanotetrapods can be easily separated using the phase transfer method reported, with Lcysteine ligands resulting in the separation and concentration of D- and L-CdS nanostructures in organic and aqueous phases. In this case, the chiral defects at the surface of CdS nanotetrapods and optical activity are retained in these nanostructures even after their transfer into the organic phase using the nonchiral dodecanethiol ligand (chiral memory effect). However, if we prepare the racemic mixture composed of D- and L-CdS tetrapods, the use of chiral L-cysteine ligands enables the recognition of chiral defects on the surface of these nanotetrapods followed by the enantioselective separation of these chiral nanostructures into different phases (Figure 4).
Figure 5. (A, B) TEM images of the CdSe/ZnS QDs. The arrows indicate possible screw dislocations. (C, D) Atomistic models of CdSe/ZnS QDs with right (C) and left (D) screw dislocations. Dislocations are set in the (010) plane of CdSe core of nanocrystals with (−) Burgers vector (C) and (+) Burgers vector (D). Atomistic models were prepared using Vesta software.43 Red dotted lines indicate the direction of the dislocations. E
DOI: 10.1021/nl504439w Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Methods. Synthesis of CdSe/ZnS Quantum Dots and Quantum Rods. Semiconductor core/shell CdSe/ZnS quantum dots and rods were synthesized by a method described previously45 without using any chiral chemicals. The dots had a diameter of ∼2.5 nm and exhibit a photoluminescence band with a peak emission at 520 nm. The rods had a diameter of ∼5 nm with an average aspect ratio of around 7 and exhibit a photoluminescence band with a peak emission at 630 nm. Asprepared CdSe/ZnS nanocrystals were surface capped by achiral TOPO. Synthesis of Penicilamine Stabilized CdS Tetrapods. Aqueous solutions of D- or L-penicillamine (0.01 M, 10 mL) and CdCl2 (0.01 M, 8 mL) were mixed with 40 mL of Millipore water in a round bottomed flask. Then 2 M NaOH solution was added dropwise to the mixture to adjust the pH to 11. After that thioacetamide solution (0.01 M, 2 mL) was added to the mixture, and it was heated under reflux for 2 h. Then the flask was wrapped in aluminum foil and was allowed to cool to ambient temperature. The solution was concentrated by rotary evaporation of the water under reduced pressure followed by filtration via ultracentrifugation with a 30 kDa MWCO filter. The precipitate was washed with water and then redispersed in Millipore water and stored at 4 °C. Enantioselective Phase Transfer. For the optically enriched samples, 15 mg of nanocrystals were dissolved in 750 μL of chloroform. The solutions obtained were cooled for 15 min at 4 °C to slow the process. Then 5 vol % of methanol solution of D- or L-cysteine (8 mg of cysteine dissolved in 300 μL of methanol) were added to the nanocrystals solution followed by stirring. After 1−2 min of stereospecific solubilization, 750 μL of distilled water with pH 10−11 were added to the solution. pH was varied by the addition of aqueous solution of KOH. To initiate the phase transfer, the mixture was vigorously shaken and left for 1−2 min. After complete separation, the chloroform phase and the aqueous phase were collected in separate vials. The chloroform phase contained a small amount of the nanocrystals partially capped by charged molecules of cysteine. It could destabilize the solution and lead to aggregation of the nanocrystals. To avoid aggregation, a few drops of 3% solution of hydrochloric acid and 50−100 μL of chloroform solution of TOPO were added to the chloroform phase. Full Phase Transfer. To suppress selectivity of interaction between the chiral ligands and the nanocrystals, a number of changes were made to the method described above. In particular, the solution was not cooled; the addition of cysteine was increased up to 10 vol % with the same concentration. After the cysteine solution was added, the solution was stirred for 5 min. Under such conditions, nearly 100% of the nanocrystals transferred to the aqueous phase after addition of water and extraction. Reverse Phase Transfer. For phase transfer from water to chloroform,46 1 mL of dodecanthiol (DDT) and 2 mL of acetone were added to 1 mL of aqueous solution of cysteinecapped nanocrystals obtained after the enantioselective phase transfer. To initiate the phase transfer, the mixture was vigorously shaken and heated to 56 °C. After a few minutes the mixture were centrifuged, and the nanocrystals capped with DDT were redissolved in chloroform. CD Measurements. The spectra of circular dichroism and absorption were studied using a Jasco J-1500 spectrometer. The spectra of the chloroform and aqueous phases were measured separately at 20 °C. Quartz cuvettes with a 1 cm path length were used for all experiments.
These results confirm our hypothesis and indicate both the generic and universal nature of our approach for the separation of chiral nanostructures in different phases. Our method of enantioselective phase transfer of quantum nanostructures demonstrates that chiral recognition and discrimination which is very well-known for enantiomeric molecules3,4 can be extended to nanosized inorganic particles. The detailed mechanism of this chiral recognition between chiral nanocrystals and chiral ligand is not quite clear at this stage. However, most likely the process is similar to the three point recognition44 which is described for cysteine molecules at gold metal surface.4,30,31 More precise studies of the mechanism as well as computational modeling will be the subject of future work. In conclusion, we have developed a new enantioselective phase transfer technique for the separation of chiral nanostructures using an appropriate chiral ligand. We have shown that this method can be used for chiral resolution of various quantum nanostructures such as CdSe/ZnS QDs and QRs and CdS nanotetrapods. We expect that this approach will open up new exciting possibilities in the separation and investigation of various chiral nanosized objects and may find a number of technologically important applications in chiral resolution, asymmetric catalysis, and chiral recognition and sensing. In addition we have found that in some nanocrystals (CdSe/ ZnS QDs and QRs) the chirality and corresponding optical activity are intrinsic features that can occur even without the presence of chiral ligands. We propose that the intrinsic chirality of the CdSe based nanocrystals is caused by the presence of intrinsically occurring chiral defects such as dislocations or point defects in these nanostructures. The use of chiral ligands enabled us to separate these nanostructures into different phases and enhance their chiroptical activity. We believe that not only CdSe based nanocrystals but also many other colloidal inorganic nanoparticulate systems have similar intrinsic chiral properties, in particular twinned nanocrystals similar to the ones described in ref 34. Also it is very likely that the chiral defects can exist not only in wurtzite but can be intrinsic to the other types of crystal lattice.39 Therefore, some of the well-known existing inorganic nanostructures could have D- and L-chiral species which coexist alongside nonchiral nanoparticles. Since the probability of formation of left- and right-handed chiral features is equal and therefore will form racemic mixtures, it is normally impossible to detect these nanostructures using CD spectroscopy. The possibility to separate these chiral nanostructures into different phases using our approach opens up new horizons in nanoscience and nanotechnology. We consider this research will be of particular importance for future development of nanobiotechnology due to biomolecules well-known chirality and therefore sensitivity to D- or L-chiral nanoparticles. Therefore, the possibility of the existence of intrinsically chiral nanoparticles (without chiral ligands) forming racemic mixtures must be taken into account, and as result this would require a significant revision of existing views and approaches in very important areas such as nanotoxicology and the use of nanoparticles in both medical diagnostics and drug delivery systems. In addition, this research may have an impact on the applications of nanoparticulate systems in biopharmaceutical industry. However, further intensive research will be necessary to understand the detailed mechanisms of formation and behavior of chiral optically active colloidal nanoparticles, as well as to develop their applications. F
DOI: 10.1021/nl504439w Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
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DFT Calculations. DFT calculations were performed using the GAMESS program47 with the Hay−Wadt valence basis with pseudopotential48 and the B3LYP functional.49 The binding energy of L-cysteine and Zn13S13 cluster was calculated as Ebinding = Ecomplex − Ecluster − Ecys, where Ecomplex is the total energy of the L-cys-(+)Zn13S13 complex or L-cys-(−)Zn13S13 complex, Ecluster is the total energy of the Zn13S13 cluster with the same geometry as in the complexes, and Ecys is the total energy of L-cysteine molecule. Total energies were calculated for optimized geometry with the exception of the atoms belonging to the Zn13S13 cluster, which were assumed to be fixed. No solvation models were used in the calculations.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 1A shows absorption and CD spectra of initial chloroform solutions of the QR and QD; Figure 1B shows schematic representation of the enantioselective phase transfer and photography of the sample after the enantioselective phase transfer; Figure 2 shows FTIR spectra of the QD samples in initial solution, the chloroform phase after the enantioselective phase transfer, the aqueous phase after the enantioselective phase transfer and the chloroform phase after the back phase transfer of the aqueous phase from water to the chloroform with achiral ligand; Figure 3 shows absorption and CD spectra of D-cysteine and complexes of D-cysteine with QDs in aqueous and chloroform solutions; Figure 4 shows absorption and CD spectra of initial water solutions of the nanotetrapods synthesized with chiral ligand penicillamine. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/nl504439w.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Government of the Russian Federation (Grant 074-U01), and the Ministry of Education and Science of the Russian Federation (Grant No. 14.B25.31.0002). We also acknowledge the financial support from Science Foundation Ireland (Grant SFI 12/IA/1300) and EU FP7 FutureNanoNeeds grant. M.V.M. also thanks the Ministry of Education and Science of the Russian Federation for support via the Scholarships of the President of the Russian Federation for Young Scientists and Graduate Students (2015−2017).
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