Comment Cite This: ACS Photonics XXXX, XXX, XXX−XXX
Response to “Comment on ‘Enantioselective Optical Trapping of Chiral Nanoparticles with Plasmonic Tweezers’” Yang Zhao* and Jennifer Dionne* Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States
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However, based on ensemble measurements, such a chirality value is achievable with synthetic chiral structures, including the above-mentioned chiral nanoparticles.4 To our knowledge, single particle chirality values have not been determined. Lastly, we appreciate M. Schaferling and H. Giessen’s suggestion of fine-tuning the incident power to better balance the dielectric gradient force and the chiral gradient force. This approach is indeed promising for enabling enantioselective trapping of structures smaller than those investigated in our manuscript. Indeed, this scheme should complement our paper’s proposal to embed chiral nanoparticles within an index-matching solvent.5 In particular, our index-matching solvent scheme aims to work for synthetic chiral nanoparticles. For example, carbon nanotubes have a scalar effective refractive index in the range of 1.1 to 2.6 at visible frequencies;6 this wide range of refractive indexes is measured or modeled by either taking into account the porosity of the nanotubes or employing the refractive index of graphite. If a carbon nanotube nanoparticle is immersed in water, the ratio between the gradient force and chiral force can be lowered, which will improve the differentiation in the enantioselective trapping. We note, however, that embedding small chiral molecules in various solvents would require modeling methods that are distinct from the nanoparticle model used in our report.
e thank Martin Schaferling and Harald Giessen for their interest in our work. Their main critique is that our proposed technique for enantioselective optical trapping would be challenging to apply to chiral molecules, given their size and polarizability. They also indicate that our analytic methods used may not be suitable for larger chiral nanostructures, like chiral meta-molecules. First, we note that our manuscript is focused on chiral nanoparticles, not on small chiral molecules or large chiral metamaterials. The ability to trap dielectric nanoparticles (as opposed to micron-sized particles) has only emerged in the past decade, thanks to advances in plasmonic optical traps. As highlighted in references 14−22 of our paper, direct optical trapping of dielectric nanoparticles and proteins down to about 10 nm in size can now be achieved. To date, direct optical trapping of small molecules remains an outstanding challenge and one that would be worthy of its own publication, even without the enantiospecificity. Our theoretical study builds upon the concept of plasmonic optical trapping, showing that selective trapping of nanoparticles based on their shape (i.e., chirality) is possible. As outlined in our paper, we considered, as a model system, a nanoparticle with a diameter of 20 nm. Numerous synthetic chiral nanoparticles fall into this size regime, such as carbon nanotubes,1 chiral nanocrystals,2 chiral quantum dots,3 and DNA-assembled nanoparticles.4 These synthetic chiral nanoparticles can also exhibit tunable absorption peaks in the ultraviolet and visible spectral range. With these conditions in mind, we designed our plasmonic coaxial aperture with a resonance in the near-infrared region, away from the resonance of the chiral nanoparticles, therefore satisfying the quasi-static limit that we assumed in the paper. Accordingly, these nanoparticles could serve as chiral nanomaterials to experimentally validate our theory. Chiral nanoparticles with absorption peaks close to or matching the resonance of the plasmonic coaxial aperture would not satisfy the assumed quasi-static limit. In these cases, the chiral nanoparticles often take on bigger dimensions. To ensure that the quasi-static limit is satisfied in these cases, the resonance of the plasmonic coaxial aperture needs to be redesigned to be tuned away from the absorption peaks of the nanoparticle. Second, we note that the chirality (κ) investigated in our manuscript spans from −1.45 to 1.45, or a relative chirality (κ/ n) from −1 to 1 (with n being the refractive index of the nanoparticle). Our theory suggests that our plasmonic trap can be used to selectively trap chiral nanoparticles with a chirality ranging from 0.2 to 1 (i.e., a relative chirality from 0.14 to 0.69). We recognize that a relative chirality of 0.14 is indeed larger than many chiral molecules, as described in our paper. © XXXX American Chemical Society
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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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REFERENCES
(1) Wei, X.; Tanaka, T.; Yomogida, Y.; Sato, N.; Saito, R.; Kataura, H. Experimental determination of excitonic band structures of singlewalled carbon nanotubes using circular dichroism spectra. Nat. Commun. 2016, 7, 12899. (2) Ben-Moshe, A.; Wolf, S. G.; Sadan, M. B.; Houben, L.; Fan, Z.; Govorov, A. O.; Markovich, G. Enantioselective control of lattice and shape chirality in inorganic nanostructures using chiral biomolecules. Nat. Commun. 2014, 5, 5302. (3) Varga, K.; Tannir, S.; Haynie, B. E.; Leonard, B. M.; Dzyuba, S. V.; Kubelka, J.; Balaz, M. CdSe quantum dots functionalized with chiral, thiol-free carboxylic acids: unraveling structural requirements for ligand-induced chirality. ACS Nano 2017, 11, 9846. (4) Mastroianni, A. J.; Claridge, S. A.; Alivisatos, A. P. J. Am. Chem. Soc. 2009, 131, 8455−8459.
Received: December 27, 2017
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DOI: 10.1021/acsphotonics.7b01610 ACS Photonics XXXX, XXX, XXX−XXX
Comment
ACS Photonics (5) Gharbavi, K.; Badehian, H. Optical properties of armchair (7, 7) single walled carbon nanotubes. AIP Adv. 2015, 5, 077155. (6) de los Arcos, T.; Oelhafen, P.; Mathys, D. Optical characterization of alignment and effective refractive index in carbon nanotube films. Nanotechnology 2007, 18, 26.
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DOI: 10.1021/acsphotonics.7b01610 ACS Photonics XXXX, XXX, XXX−XXX
