Enhancement of Circular Dichroism of a Chiral Material by Dielectric

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Enhancement of Circular Dichroism of a Chiral Material by Dielectric Nanospheres Daniel Vestler, Assaf Ben-Moshe, and Gil Markovich J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10975 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 10, 2019

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The Journal of Physical Chemistry

Enhancement of Circular Dichroism of a Chiral Material by Dielectric Nanospheres Daniel Vestler, Assaf Ben-Moshe# and Gil Markovich* Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University 6997801, Israel. AUTHOR INFORMATION Corresponding Author * Email address: [email protected] #

Present address: Department of Chemistry, University of California Berkeley, Berkeley, CA

94720-1460

ABSTRACT

Circular Dichroism (CD) spectroscopy is very useful for studies of biomolecular conformation but suffers from very weak signals. Several theoretical and experimental papers reported schemes for CD enhancement using enhanced local fields produced by plasmonic nanostructures. Here we report enhancement of visible wavelength CD of chiral mercury sulfide (HgS) nanocrystals by Mie

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resonances of amorphous selenium nanospheres. The spatially averaged CD enhancement factor was estimated to be 4.7±1.5 fold, while the peak enhancement at particular locations on the nanospheres is probably >10. This type of enhancement may be useful for increasing the sensitivity of CD measurements, and in particular towards achievement of single chiral nanostructure CD measurements.

1. INTRODUCTION Circular Dichroism (CD), the difference in absorption of left- and right-handed circularly polarized light, is an important spectroscopic technique for deducing (chiral) biomolecular conformations and their dynamics.1 However, typically, in biomolecules, the CD is orders of magnitude (~10-3-10-4) smaller than the absorbance itself, making the measurement of CD challenging, especially when studying small quantities of chiral molecules. In an attempt to improve the sensitivity of CD measurements, focus has been given to the study of enhancement of chiroptical effects (and CD specifically) using different schemes, mostly using plasmonic systems: chiral near fields2–5 and plasmon resonance enhancement,6-9 but also by strictly tailoring the illuminating light field (superchiral fields),10–13 and by inserting the chiral molecules in photonic crystals.14 Mie resonances in dielectric material particles embedded in a chiral medium were also suggested as a viable route to enhanced differential circular scattering cross-section.15 Studies of light absorption, scattering or emission enhancement by local fields were not restricted to plasmonic systems; modest local electric and magnetic field enhancements may occur also near the surface of dielectric materials.16,17 Enhanced Raman scattering18 and luminescence19 by


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dielectric nanostructures were already demonstrated. Recently, it has been shown theoretically by Dionne and co-workers that combinations of high-order electric and magnetic Mie resonances in nanoscale spheres (NSs) with high dielectric constant may significantly enhance CD of molecules located at certain domains of their surface.20 In fact, they emphasized that it is crucial to use overlapping (in frequency) electric and magnetic multipole resonances in phase, in order to achieve a spatially average enhancement of the electromagnetic density of chirality, i.e. the chirality of the local fields near the NS's surface. While the proposed material in Ref. 20 is silicon, colloidal synthesis of silicon NSs with controlled size and relatively small size distribution is highly challenging. On the other hand, amorphous Se NSs have been suggested recently as a high dielectric medium to exhibit electric and magnetic multipolar Mie resonances in the mid-visible to near-IR regime.21 Amorphous Se NSs are relatively easy to synthesize in colloidal dispersions with control of size, and consequent control of resonance wavelengths. Here we report a modified synthesis for the production of amorphous Se NSs, and consequent procedure for attachment of chiral -HgS nanocrystals (NCs) with high enantiomeric excess to their surface. We show that the main CD line of HgS at ~550 nm is moderately enhanced on average by the attached Se NSs. To effectively enhance the CD of a material this way, the chiral material's electronic transition must not just overlap the electric and magnetic Mie resonances of the dielectric NSs, but the material has to be held in the vicinity of the NS and ideally, at specific locations (angles) relative to the incident radiation propagation direction.20 While most chiral biomolecules are optically active in the UV region, several types of inorganic chiral crystals with strong CD bands in the visible regime were recently reported.22-25 Enantio-selectively synthesized -HgS NCs are ideally suited for CD enhancement demonstration in the visible range: they have a strong CD line around 550 nm which is characterized by a very weak absorption threshold (band-gap).23 We have recently used these


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particles for demonstration of CD enhancement by a metal nanorod metamaterial.24 Both HgS and Se form fairly strong bonds with thiol groups. We thus tested 3 different thiol bearing molecules as cross-linkers between the Se NSs and HgS NCs, where two were alkanedithiols: 1,4butanedithiol (BDT) and 1,9-nonanedithiol (NDT), and the third, acetylthiocholine (ATC), has one thiol group (expected to attach to the HgS) while the other end had a positively charged amine group, expected to be electrostatically attracted to the negatively charged carboxylate groups which exist in the partially hydrolyzed polyvinylpirrolidone (PVP) coating the Se NSs' surface.

2. EXPERIMENTAL SECTION Synthesis of Se NSs. All water used was ultrapure (18.2 MΩ), obtained from a Millipore DirectQ3 UV. Se NSs were prepared using a modified version of a synthesis reported by Cho et-al.21 3.8 mL ethylene glycol were placed in a vial under mild stirring at room temperature. 0.45 mL of hydrazine 80% (in water) was added to the vial and allowed to stir for several minutes. We then added 0.3 mL of PVP (40000 g/mol) 1 mM aqueous solution, followed by 0.1 mL of H2SeO3 0.07 M. The solution gradually changed color to pinkish within about 10 minutes past the addition of the selenous acid. The reaction was allowed 3-4 hours to reach completion and the resulting size was preliminarily determined using extinction measurements (S2000 Ocean Optics spectrophotometer). Variation of the added volume of ethylene glycol and hydrazine was used to prepare different sphere sizes, where increasing the hydrazine concentration generally results in smaller spheres. Synthesis of HgS NCs. HgS NCs were prepared as reported by Ben-Moshe et al.23 To 3 mL of vigorously stirred water we added 0.9 mL Mercury Nitrate 100 mM, 0.9 mL D-penicillamine (or L-penicillamine) 100 mM, 0.3 mL NaOH 1M and 0.9 mL thioacetamide 100 mM, in this order.


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The solution color immediately changed to black (β-HgS) with the addition of thioacetamide and gradually changed to orange (α-HgS) as the reaction progresses. The reaction was allowed 20 hours to complete followed by product purification: addition of 1:1 volume of isopropyl alcohol and centrifugation at 600 RCF for 5 min, and redissolution of the precipitate in 6 mL of water. Cross-linking Se NSs and HgS NCs. In order to bind the HgS NCs to the Se NSs, 3 linker molecules were used: acetylthiocholine iodide (ATC), 1,4-butanedithiole (BDT) and 1,9nonanedithiol (NDT). To link the particles, 0.5 mL of the Se NS solution was added to 2 mL of water in a vial under stirring, followed by addition of 0.1 mL NaOH 0.1 M, 0.05 mL potassium iodide 30 mM and the linker: 45 µL of 89 µM ATC aqueous solution or 45 µL of 49.5 µM of BDT or NDT solutions. The Se-linker solution was left to stir for 10 minutes and then 0.1 mL of the HgS NC solution was added to the mix and the solution was left to stir further for 15 minutes. This amount of HgS was calculated to be roughly the required amount to completely cover the NSs’ total area in the solution by a monolayer of HgS particles (the calculation is detailed in the Supporting Information). For a reference sample, a solution of 2.695 mL water and 0.1 mL HgS NC solution was prepared, to obtain the exact same HgS NC concentration present in the crossed-linked particle mixture before centrifugation. Characterization. In order to separate bound Se-linker-HgS from free HgS NCs, 1:1 volume ratio of water was added to the cross-linking solution mix followed by centrifugation at 1070 RCF for 10 minutes, and the precipitate was redissolved in water. Centrifugation was then repeated at 600 RCF for 5 minutes and the precipitate was redissoved to form a final solution volume of 3 mL. Extinction measurements of the redissolved precipitate produced the same NS spectral shape, as the original one.


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The Se-linker-HgS precipitate was also characterized using transmission electron microscope (TEM, FEI Tecnai F20 FEG-TEM), where high resolution measurements were performed at small spot size and objective to minimize deformation of the spheres due to increasing temperature beyond the glass transition temperature (~31°C).21 CD measurements were performed using a commercial CD spectrometer (Applied Photophysics Chirascan) on all samples as are without further dilution. We present raw CD data without any smoothing or other manipulations.

3. RESULTS AND DISCUSSION Colloidal Se NSs were prepared by a simple procedure in a mixture of water and ethylene glycol, where selenic acid was reduced by hydrazine in the presence of PVP, which serves as a surfactant. By tuning the synthesis conditions (amounts of ethylene glycol and hydrazine) we obtained Se NSs of 225±25 nm average diameter which had two broad extinction peaks at wavelengths of 550 and 620 nm (Figure 1). A comparison to the work of ref. 21 led to the conclusion that the 550 nm extinction peak corresponds to a combination of dipolar and quadrupolar magnetic Mie resonance peaks which overlap a broader dipolar electric resonance of the NSs, and the first CD peak of the HgS nanoparticles.23 -HgS, also called Cinnabar, is a chiral material, namely crystallizes in a chiral space group (P3121 and P3221). The -HgS NCs produced here were previously shown to have a large enantiomeric excess and strong visible CD activity due to their synthesis in the presence of the strongly binding D-penicillamine chiral ligand, as well as other chiral ligands.23-25 As detailed in the introduction, the specific relative location of the HgS adsorbed to the Se NSs is crucial for enhancement. Since control over spatial attachment of the HgS to Se NSs in solution is hardly possible, and is not useful as long as the NSs are freely rotating in colloidal solution, we


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use isotropic coverage of the dielectric NSs by the chiral material and expect an enhancement factor which will be a weighted average of the enhancement of CD in HgS NCs attached to the regions of high enhancement and regions of basically no enhancement. 0.8

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Figure 1. (a) Extinction plots for Se NSs and HgS NCs. The peak at 625 nm and shoulder at 560 nm correspond to magnetic Mie resonances in the NSs. (b) TEM image of the Se NSs. Average


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size of 225±25 nm. (c) TEM image of a Se NS coated with HgS NCs, cross-linked by ATC molecules.

Obtaining high coverage of the Se NSs with HgS NCs without significant aggregation of the Se NSs (which would cause precipitation), or of the HgS NCs, is not a trivial task. Both HgS and Se form fairly strong bonds with thiol groups, making dithiol molecules natural candidates for linking the HgS to the Se. However, getting preferential Se-HgS linking, over Se-Se and HgS-HgS linking, is challenging. The strategy to minimize Se-Se and HgS-HgS cross-linking was to separate the bonding process in two consecutive steps, where first, the linker molecules were added at very low concentration to the Se NSs solution, and only then the HgS NCs were added to the linker coated NS solution. In order to avoid an excess of HgS NCs and to minimize cross-linking between the HgS NCs themselves, we roughly estimated the number of HgS NCs needed to form a tightly packed monolayer on all the NSs present in the sample. This concentration of HgS NCs was first validated separately to be sufficient to have a measureable CD signal on its own, prior to any potential enhancement. Specifically, the ATC linker, has a positively charged end (trimethylamine end) while the thiol group on its other end is protected by an acetyl group. This allows the ATC to electrostatically attach to the Se NSs first, followed by addition of a base which removes the acetyl protection group and exposes the thiol end to bind the HgS NCs. Figure 2 shows the CD spectrum of the raw Se-BDT-HgS cross-linked sample, compared to a reference sample of only HgS particles, both having the exact same concentration of HgS NCs. We repeated the experiment for the two types of HgS NC enantiomers, i.e. HgS prepared either with D- and L-penicillamine. The 550 nm CD peak of the Se-BDT-D-HgS sample was higher than


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the reference sample's peak by ~7±2%, while the smaller and broader peak at ~460 nm did not exhibit significant enhancement. Comparable results were obtained for the Se-BDT-L-HgS sample. Similar experiments with the other linker molecules resulted in 6±2% increase for the SeATC-HgS sample and 9±2% for the NDT linked sample (see Supporting Information, Figures S1, S2i). It should be stressed that the true CD enhancement level of the HgS NCs is much higher than the small difference observed in Fig. 2, as shown in the following.

20 15

D-HgS

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Se-BDT-HgS HgS Reference

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Figure 2. Circular Dichroism (CD) spectrum of the raw Se-BDT-HgS samples, containing also a majority of unbound HgS NCs, compared to a reference HgS sample of the same exact concentration. Experiments with the two enantiomers of the HgS NCs are presented. D-HgS and L-HgS were prepared with D-penicillamine and L-penicillamine, respectively. The Se-BDT-D-


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HgS sample’s CD peak at 550 nm is 1.1 mdeg larger compared to the HgS reference, corresponding to ~7±2% increase in CD peak. Very similar effect is observed for the L-HgS enantiomer. Above 600 nm both samples showed zero CD signal and below 400 nm the extinction was too large for reliable CD measurements.

To estimate the fraction of HgS NCs attached to the Se NSs, all linked samples were centrifuged at 600 RCF for 5 minutes to precipitate the bound Se-linker-HgS particles, while the unbound HgS NCs remained in solution (tested by TEM images of the two fractions). The precipitate was separated from the unbound HgS nanoparticles supernatant, and redissolved in the same original volume of water. CD measurements were then performed on the separated samples. Surprisingly, the supernatant CD spectrum showed no visible decline relative to the control HgS sample shown in Figure 3a, as the difference between the curves (before and after separation of the bound particles) lies within the noise level of about ±2% of the peak CD value. This indicates that only a small fraction (≤2%) of the HgS NCs in the solution were bound to the Se NSs. TEM imaging of the precipitate samples of bound Se-linker-HgS with the three different linker types revealed bound HgS particles in their respective separated samples, though an incomplete, loosely packed film, mostly a monolayer, formed in all three cases (see Figure 1c and Supporting Information, Figure S3). In the absence of the linker molecules negligible amounts of HgS NCs were bound to the Se NSs. The Se-BDT-HgS precipitate (redissolved) showed a measureable CD peak at 550 nm, of about 1.5 mdeg (Figure 3b). To quantify the 550 nm CD peak enhancement level, we evaluated the expected CD signal of the HgS particles bound to the Se NSs by a rough estimate of the number of HgS particles bound to Se NSs from the TEM images (see detailed calculation in the Supporting Information). We estimate that 2.2±0.1% of the HgS NCs were bound


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to the Se NSs in the BDT-linked sample. The ATC linked sample had similar coverage of HgS NCs (2.3%, see Supporting Information). In Figure 3b we display the reference unbound HgS NC CD spectrum scaled to 2.2% of its original magnitude. By dividing the enhanced HgS peak by the rescaled reference peak magnitude we were able to roughly estimate the average enhancement factor to be 4.7±1.5 for the BDT-linked sample, and also 4.7±1.5 for the ATC linked sample. We did not calculate the exact enhancement factor in the NDT sample, but it seems to be a little higher than the other two samples. 20

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Figure 3. CD spectra of the two separated fractions after binding part of the HgS NCs to the Se NSs using BDT: (a) CD spectra of the HgS reference sample and the separated unbound HgS NCs. No measureable difference is observed in the resonance peak within the ±2% noise level of the


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CD peak. (b) CD spectra of the separated bound Se-BDT-D-HgS of the same sample presented in Fig. 2, compared to estimated CD signal of the HgS NCs of the concentration attached to the Se NSs (reference HgS NC CD spectrum shown in Fig. 2 scaled to 2.2% of its original intensity). The measured CD of the bound Se-BDT-HgS sample is larger by ~4.7.

In order to further confirm that the enhancement of the CD is due to the Mie resonances of the Se NSs, we performed a control experiment in which smaller Se NSs were prepared (diameter of 130±10 nm), with a relatively broad magnetic dipolar Mie resonance appearing at wavelengths below 500 nm and quadrupolar resonance below 400 nm (Supporting Information, Figure S5), and coated with HgS NCs in the same manner. The measurements followed the exact same procedure and the spheres were imaged with TEM to confirm HgS attachment as in the case of the 225 nm NSs (Supporting Information, Figure S4). The resulting CD measurement (Figure S4) showed no indication of enhancement, within the measurement noise, resulting in the same CD intensity at 550 nm measured for both Se-linker-HgS sample and the HgS only reference sample for all three linkers. Dionne and coworkers have calculated that for 75 nm Si NSs the overlapping magnetic and electric dipole resonances would enhance the CD of a chiral medium spread at a spherical shell at 1 nm distance from the surface by an average factor of ~8.27 No similar data is available for a quadrupolar resonance. In ref. 20, only the peak values of quadrupolar magnetic mode CD enhancement for the Si NSs is estimated (~15). Since the high order resonances (quadrupolar and up) have both positive and negative enhancement regions, then part of the enhanced CD signal might cancel out for this enhancement mode. On the other hand, Yoo and Park have shown that the differential circular scattering magnitude of similar Si NSs embedded in a chiral medium is


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similar both for the dipolar and quandrupolar magnetic resonances.15 This may indicate that also the extinction CD, which is also largely due to differential circular scattering, would exhibit comparable dipolar and quadrupolar contributions. Those contributions to CD enhancement should be somewhat smaller in the case of our Se NSs around 550 nm due to the lower refractive index of Se compared with Si and the increasing loss in Se at this wavelength range compared with the IR range for which the calculations for the Si NSs were done.20,27 Consequently, an average enhancement factor of ~5 in our Se NSs compared with the estimated 8 (depending on the quadrupolar contribution) for Si would seem reasonable. The effect observed here should be distinguished from the dipolar interaction model developed by Govorov and coworkers for small plasmonic nanoparticles,7 where the chiral medium is described as combined molecular electric and magnetic transition moment dipoles and the interaction with the electric component (only) of the local plasmonic field is calculated within the quasi-electrostatic approximation as the metal particles in the model are much smaller the wavelength of the illuminating light. In the present work the orientation of the HgS NCs with respect to the NS surface is random, hence this type of interaction should average out to zero CD enhancement. This further strengthens the notion that it is the combined magnetic and electric Mie resonance modes which cause the CD enhancement. 4. CONCLUSION In conclusion, we report measurements of a modest factor of ~3-6 average enhancement of the CD signal measured for HgS NCs, by magnetic dipole and quadrupole Mie resonances (and electric dipole) of the Se NSs, both occurring at a wavelength of ~550 nm. Improvement of HgS NC coating density over the Se NSs would help to better quantitate the average enhancement factor. This would probably require more elaborate Se-HgS coupling


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schemes, possibly using an ultra-thin coating of another material on the Se surface, such a silica, to enable more selective bifunctional cross-linking between the two materials. Since part of the HgS NCs should experience CD enhancement factors >10, it should make the measurement of single HgS particle CD spectra feasible. Previously it was possible to measure individual, ~100 nm long HgS NCs’ CD spectra,28 and with proper tuning of the Se NS’s sizes, and in the absence of size dispersion of the Se NSs (single particle measurement), it is conceivable that certain 10 nm HgS NCs would even have ~100 enhanced CD, which should be relatively easy to measure as single particle CD. It should also be interesting to try detecting the attachment of the HgS NCs to the Se NSs through Se NS scattering CD, as done with plasmonic nanoparticles.29

ASSOCIATED CONTENT Supporting Information. CD data for Se-linker-HgS systems based on NDT and ATC linker molecules. Information about the estimates of number of HgS NCs attached to a NS. Data on the experiments with the smaller, 130 nm NSs.

AUTHOR INFORMATION The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by Israel Science Foundation grant no. 507/14.

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(26) Vestler, D.; Shishkin, I.; Gurvitz, E. A.; Nasir, M. E.; Ben-Moshe, A.; Slobozhanyuk, A. P.; Krasavin, A. V.; Levi-Belenkova, T.; Shalin, A. S.; Ginzburg, P.; et al. Circular Dichroism Enhancement in Plasmonic Nanorod Metamaterials. Opt. Express 2018, 26, 17841. (27) García-Etxarri, A.; Dionne, J. A. Surface-enhanced circular dichroism spectroscopy mediated by nonchiral nanoantennas. Phys. Rev. B 2013, 87, 235409. (28) Vinegrad, E.; Vestler, D.; Ben-Moshe, A.; Barnea, A. R.; Markovich, G.; Cheshnovsky, O. Circular Dichroism of Single Particles. ACS Photonics 2018, 5, 2151-2159. (29) Schnoering, G.; Poulikakos, L. V.; Rosales-Cabara, Y.; Canaguier-Durand, A.; Norris, D. J.; Genet, C. Three-Dimensional Enantiomeric Recognition of Optically Trapped Single Chiral Nanoparticles. Phys. Rev. Lett. 2018, 121, 1–8.


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Table of Contents Graphic

Ci rcul ar Dichroism

CPL



Se chiral HgS .


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Enhancement of Circular Dichroism of a Chiral Material by Dielectric

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