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# Amino-acid- and peptide-directed synthesis of chiral plasmonic gold nanoparticles

## Abstract

Understanding chirality, or handedness, in molecules is important because of the enantioselectivity that is observed in many biochemical reactions1, and because of the recent development of chiral metamaterials with exceptional light-manipulating capabilities, such as polarization control2,3,4, a negative refractive index5 and chiral sensing6. Chiral nanostructures have been produced using nanofabrication techniques such as lithography7 and molecular self-assembly8,9,10,11, but large-scale and simple fabrication methods for three-dimensional chiral structures remain a challenge. In this regard, chirality transfer represents a simpler and more efficient method for controlling chiral morphology12,13,14,15,16,17,18. Although a few studies18,19 have described the transfer of molecular chirality into micrometre-sized helical ceramic crystals, this technique has yet to be implemented for metal nanoparticles with sizes of hundreds of nanometres. Here we develop a strategy for synthesizing chiral gold nanoparticles that involves using amino acids and peptides to control the optical activity, handedness and chiral plasmonic resonance of the nanoparticles. The key requirement for achieving such chiral structures is the formation of high-Miller-index surfaces ({hkl}, hkl ≠ 0) that are intrinsically chiral, owing to the presence of ‘kink’ sites20,21,22 in the nanoparticles during growth. The presence of chiral components at the inorganic surface of the nanoparticles and in the amino acids and peptides results in enantioselective interactions at the interface between these elements; these interactions lead to asymmetric evolution of the nanoparticles and the formation of helicoid morphologies that consist of highly twisted chiral elements. The gold nanoparticles that we grow display strong chiral plasmonic optical activity (a dis-symmetry factor of 0.2), even when dispersed randomly in solution; this observation is supported by theoretical calculations and direct visualizations of macroscopic colour transformations. We anticipate that our strategy will aid in the rational design and fabrication of three-dimensional chiral nanostructures for use in plasmonic metamaterial applications.

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## Acknowledgements

This research was supported by a Seoul National University research grant in 2015, the LG Display under LGD-SNU Incubation Program, the Creative Materials Discovery Program (2017M3D1A1039377), which is funded by the National Research Foundation (NRF) under the Ministry of Science, ICT and Future Planning (MSIP), South Korea, and the KIST-SNU Joint Research Program (0543-20180021). K.T.N. acknowledges financial supports from the Global Frontier R&D Program of the Center for Multiscale Energy System (2012M3A6A7054855), which is funded by the NRF-MSIP, South Korea. J.R. acknowledges financial support from the Engineering Research Center Program of the Center for Optically-assisted Mechanical Systems (2015R1A5A1037668) and the Global Frontier R&D Program of the Center for Advanced Meta-Materials (2014M3A6B3063708), which is funded by the NRF-MSIP, South Korea. H.-Y.A. and M.K. are grateful to the Global PhD Fellowship Program (2014H1A2A1020809 and 2017H1A2A1043204, respectively), which is funded by the NRF-MSIP, South Korea. J.R. thanks T. Liedl (Ludwig Maximilian University of Munich) and N. Liu (University of Heidelberg) for discussions.

### Reviewer information

Nature thanks L. M. Liz-Marzán and the other anonymous reviewer(s) for their contribution to the peer review of this work.

## Author information

### Author notes

1. These authors contributed equally: Hye-Eun Lee, Hyo-Yong Ahn.

### Affiliations

1. #### Department of Materials Science and Engineering, Seoul National University, Seoul, South Korea

• Hye-Eun Lee
• , Hyo-Yong Ahn
• , Yoon Young Lee
• , Nam Heon Cho
•  & Ki Tae Nam
2. #### Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, South Korea

• Jungho Mun
•  & Junsuk Rho
3. #### Department of Mechanical Engineering, POSTECH, Pohang, South Korea

• Minkyung Kim
•  & Junsuk Rho
4. #### R&D Center, LG Display, LG Science Park, Seoul, South Korea

• Kiseok Chang
•  & Wook Sung Kim
5. #### Department of Electrical Engineering, POSTECH, Pohang, South Korea

• Wook Sung Kim

### Contributions

K.T.N. and J.R. conceived the idea. H.-E.L., H.-Y.A., N.H.C. and Y.Y.L. synthesized and characterized the materials. J.M., M.K. and J.R. performed the numerical simulations and analysed the data. H.-E.L., H.-Y.A., K.C. and W.S.K. conducted the optical characterization of the materials. All authors discussed the experiments and contributed to writing the manuscript. J.R. guided the numerical simulations and optical measurements. K.T.N. guided all aspects of the work.

### Competing interests

The authors declare no competing interests.

### Corresponding authors

Correspondence to Junsuk Rho or Ki Tae Nam.

## Extended data figures and tables

1. ### Extended Data Fig. 1 Chiral morphology and characterization of 432 helicoids I and II.

a, Large-area SEM image of 432 helicoid I. l-Cys was used as an additive. Inset, extinction spectra of 432 helicoid I synthesized using l-Cys and d-Cys. b, Large-area SEM image of 432 helicoid II, synthesized with l-GSH.

2. ### Extended Data Fig. 2 Chiral morphology development of 432 helicoids I and II.

a, Schematic illustration of stellated octahedron with differentiated {321} facets ({321} nanoparticle). Each triangular facet of a stellated octahedron is divided into two convex {321} facets with R and S surface conformation. b, SEM images showing the detailed geometry of a {321} nanoparticle. c, Bright-field TEM image along the [110] direction showing angles (α, β, γ) between the eight outermost edges. d, Calculated angles between the outermost edges of an {hkl}-enclosed nanoparticle. The exposed facets of the nanoparticle in c were indexed as {321}. e, Schematic illustration of the time-dependent evolution of 432 helicoids I and II. All models are viewed along the [110] direction. Starting from a {321}-indexed nanoparticle with an equal ratio of R and S regions, different RS boundaries are split, thickened and distorted. f, SEM images of 432 helicoids I and II at different growth times. The chiral components that developed in 432 helicoids I and II are highlighted in red and blue, respectively.

3. ### Extended Data Fig. 3 Interaction of l-Cys with high-index planes.

a, Atomic structure of a chiral nanoparticle at the initial stage. SEM image (i) and TEM images (ii and iii) of a chiral nanoparticle after 20 min of growth. Because the nanoparticle was oriented along the 〈110〉 direction, the projected boundaries in the TEM image consist of chirally distorted edges. The high-resolution TEM image of distorted edges corresponds to the red dotted box in ii. The atoms of the microfacets are marked with coloured circles, and different colours are assigned to the Miller index of each microfacet. Using microfacet nomenclature, the microstructure of (551) can be divided into three units of (111) and two units of $$(11\bar{1})$$. Inset, corresponding fast Fourier transform (FFT) showing typical patterns along the $$[1\bar{1}0]$$ zone. b, Temperature-programmed desorption spectra of l-Cys of 432 helicoid I and a low-index cubic nanoparticle, with monitoring of CO2 (m/q = 44 amu). As the temperature was raised at a rate of 3 K min−1, helium carrier gas flowed over the dried nanoparticle sample. The distinguishable temperature-programmed desorption peak at 635 K for 432 helicoid I indicates a specific interaction of l-Cys with a kink atom on the gold surface. Cys on the cube (100) surface shows no observable peak at high temperatures. c, Cyclic voltammograms for a cube, a high-index stellated octahedron (with differentiated {321} facets) and 432 helicoid I, with l-Cys measured in 0.1 M KOH-ethanol solution at a scan rate of 0.1 V s−1. Negative peaks between about −1.8 V and −1.1 V originate from the reductive desorption of l-Cys by the cleavage of an Au–S bond, Au–SR + e → Au + RS. Desorption peaks at more negative potentials indicate the higher adsorption energy of l-Cys on high-index gold surfaces.

4. ### Extended Data Fig. 4 Comparison of Cys and GSH by time-dependent concentration quantification and adsorption assay.

a, Schematic experimental procedure for thiol quantification on the gold surface. The reduction of thiolate by NaBH4 cleaved the Au–S bond, and the thiol group of the released molecule spontaneously reacted with the thiol-specific dye, producing a fluorescent derivative. The excitation and emission wavelengths were 405 nm and 535 nm, respectively. b, Concentration curve from 0 μM to 5 μM for a fluorometric assay of l-Cys. The linear fitting and corresponding R2 value show good linearity within the measured range. c, Measured surface density of l-Cys and l-GSH for 432 helicoids I and II, respectively. Surface coverage is calculated using the previously reported surface densities of l-Cys and l-GSH at the fully saturated monolayer condition. Mean ± s.d. (n = 3) is shown. d, Increase in g-factor of 432 helicoids I (Cys) and II (GSH) with time. The CD signal was measured and the normalized g-factor is displayed every 5 min during growth. The maximum g-factors (gmax) of 432 helicoids I and II at 120 min were 0.02 and 0.04, respectively. e, Amount of GSH adsorbed on 432 helicoid II at different growth times. For a detailed quantification of the amount of GSH on a nanoparticle, see Methods. f, Adsorption study of Cys and GSH on {321} nanoparticles. Different concentrations of Cys and GSH were added and aged for 2 h, and the amount of adsorbate was measured by subtracting the Cys and GSH concentrations in the supernatant from the initial concentration. See Methods for a detailed Cys and GSH quantification study. g, h, Effect of Cys and GSH concentrations on chiral morphology. SEM images of chiral nanoparticles synthesized with different concentrations of Cys (g) and GSH (h). The highest g-factor was observed at the optimum amino acid and peptide concentration (red text). At low concentrations, only achiral nanoparticles formed, but with incremental additions, chiral edges started to appear. An excess of molecule results in the overgrowth of edges and a greatly decreased CD signal, indicating that an optimal concentration exists for chirality formation. Scale bars, 100 nm.

5. ### Extended Data Fig. 5 Effect of molecular structure on chirality evolution.

a, Effect of functional-group change in l-Cys. Comparison of g-factor and SEM images of synthesized nanoparticles with C-terminal blocked l-Cys (l-cysteine ethyl ester) (top), N-terminal blocked l-Cys (N-acetyl-l-cysteine) (middle) and l-Cys (bottom). C-blocked l-Cys changed the chiral morphology and decreased the CD intensity of the resulting nanoparticles. Furthermore, nanoparticles produced with N-blocked l-Cys showed achiral morphology without an observable CD signal. b, Schematic illustration of chirality formation on a {321} nanoparticle. Boundary shifts of 432 helicoids I (l-Cys) and II (l-GSH) are indicated in red and blue, respectively. c, Schematic (111) cross-section of (312)S–(321)R–(231)S facets. Original and newly shifted RS boundaries are indicated with dashed lines. d, Atomic arrangement of (312)S–(321)R–(231)S facets in a (111) cross-section view. The {321} surface consists of a (111) terrace and alternating {100} and {110} microfacets. The $$\bar{{\rm{A}}{\rm{C}}}$$ boundary in 432 helicoid I shifts in the $$[\bar{1}01]$$ direction and the $$\bar{{\rm{A}}{\rm{B}}}$$ boundary in 432 helicoid II shifts in the $$[01\bar{1}]$$ direction. The differentiated growth directions at (312)S and (231)S, indicated with thick arrows, resulted in contrasting morphology for the different chiral nanoparticles. e, Effect of functional-group change in l-GSH. SEM images are shown of the synthesized nanoparticles prepared with l-glutathione ethyl ester (C-blocking), γ-E-C-A, E-C-G and γ-E-C sequences. f, SEM images of nanoparticles synthesized with different dipeptide sequences. Alanine (A), proline (P), cysteine (C) or tyrosine (Y) was added to the N terminus of l-Cys, which modified the morphology of the resulting nanoparticle substantially.

6. ### Extended Data Fig. 6 Characterization of surfaces inside the gaps of 432 helicoid III.

a, Large-area SEM image of 432 helicoid III nanoparticles, synthesized using an octahedral seed and l-GSH. b, Helium-ion microscopy secondary electron images of 432 helicoid III during the He+-ion milling process. The original pinwheel-like structure of 432 helicoid III is highlighted in yellow. Exposure to a He+-ion beam with an acceleration voltage of 30 keV and a beam current of 0.733 pA allows visualization of the interior parts of the curved surfaces, as indicated by red arrows. c, Modelling of the 432 helicoid III surface. A magnified SEM image of 432 helicoid III (i), the corresponding three-dimensional model (ii) and the interpolated curved surface of 432 helicoid III (iii) are shown. The curved outlines of the chiral arm at the front and side face are indicated by green and red lines, respectively, and the internal boundary is indicated by the blue dotted lines. The three-dimensional curved-surface model of 432 helicoid III was constructed by using the interpolation of surface outlines. d, Distribution of Miller indices on the modelled surface. The Miller indices were calculated from a normal vector at each point on the surface.

7. ### Extended Data Fig. 7 FDTD simulation results for 432 helicoid III.

ac, Calculated absorbance (dashed lines) and CD (solid lines) for 432 helicoid III with triangular (a), rectangular (b) and curved (c) gap shapes. The corresponding three-dimensional models are displayed on the left. All three models derived from SEM images of the particles successfully reproduce the experimentally observed characteristic spectrum patterns: two main absorbance peaks, a sharp absorbance feature overlapped on the fundamental absorbance peak, a CD peak overlapped on the sharp feature, and the ‘bisignate’ CD signals, with negative peaks near 700 nm and a positive peak around 800–850 nm. This reproduction of the general features of 432 helicoid III suggests that the models can be used to study this helicoid theoretically and that they do not have to be perfect, but only need to resemble the helicoid shape sufficiently. All of the results are averaged over 756 discrete orientations and were estimated using a particle number density of N = 1015 m−3 and a cell path length of l = 10−3 m. d, Three-dimensional model and orientation of 432 helicoid III. e, Orientation-averaged CD spectrum (〈CD〉Ω, black solid line) and CD spectra calculated at selected orientations (dots). 〈CD〉Ω is averaged over 756 discrete orientations. The CD spectrum at a single orientation resembles 〈CD〉Ω with some deviations. f, CD and absorbance spectra calculated with a normal incidence. g, Scattering cross-section decomposed by multipole analysis. The total scattering is contributed by a broad and large electric dipole mode (E1) around 1,200 nm, and a magnetic dipole (M1) and electric quadrupole (E2) around 650 nm and 950 nm near the chiro-optical peaks. A strong chiro-optical signal was observed from two other high-order modes (650 nm and 950 nm). h, i, Electric- and magnetic-field intensities on an illuminated helicoid surface upon normal incidence of LCP and RCP light at three different wavelengths (650 nm, 950 nm and 1,200 nm).

8. ### Extended Data Fig. 8 FDTD simulations of differently modified 432 helicoid III nanoparticles to identify design guidelines.

a, Calculated g-factors of chiral nanoparticles corresponding to models using parameterized chiral nanoparticles (samples 1–19). The different samples represent chiral nanoparticles with: 1–3, edge lengths L of 100–200 nm; 4–7, gap widths w of 10–40 nm; 8–15, gap depths d of 30–100 nm; or 16–19, gap angles t of 30°–75°. The default parameters are L = 150 nm, w = 20 nm, d = 70 nm and t = 60°. b, Calculated absorbance and CD of chiral nanoparticle samples 7 (L = 150 nm, w = 40 nm, d = 70 nm, t = 60°), 12 (L = 150, w = 20, d = 70, t = 60) and 14 (L = 150, w = 20, d = 90, t = 60), using N = 1015 m−3 and l = 10−3 m (i and ii). The calculated electric-field intensity of each of these samples on the illuminated face (z = −75 nm) at RCP illumination at the first CD peak—of 600 nm, 670 nm and 720 nm, respectively—is also shown (iii–v). c, Calculated g-factors of chiral nanoparticles corresponding to models 20–32, using chiral nanoparticles with various geometry changes: 20–22, chiral nanoparticles with increasing curvature; 23–26, chiral nanoparticles with aspect ratios of 1–3; 27–31, chiral nanoparticles with hollow structures constructed by removing cubic domains with side lengths of 70–130 nm; and 32, planar-triangle-based chiral nanoparticle with an edge length of 150 nm. The default size of chiral nanoparticles 20–31 is 150 nm.

9. ### Extended Data Fig. 9 Effect of defects in the low-index-plane-exposed seeds on the morphology of 432 helicoid III.

a, Characterization of twin boundary defects in seed nanoparticles. Twin boundaries were observed as bright lines in a single nanoparticle by scanning TEM imaging. Nanoparticles with a single twin and fivefold twins are indicated in red and yellow, respectively. b, Defect-induced morphology deformation of 432 helicoid III. SEM (left), TEM (middle) and selected-area electron diffraction (SAED; right) images are shown for an ideal 432 helicoid III (i), an irregular achiral nanoparticle (ii) and an irregular nanoparticle with broken 432 symmetry (iii). In the case of the irregular achiral particle (ii), several diffraction spots that deviate from the regular diffraction pattern of the 〈100〉 zone (red) show polycrystalline character. In case of the particle with partially broken symmetry (iii), dark-field TEM images originating from diffraction spots 1 and 2 are also shown on the right, and demonstrate different crystallographic orientations in a single nanoparticle. We believe that the irregular, non-homogeneous shapes represented by ii and iii may originate from the twin boundary defects in seeds. By decreasing the population of twinned seeds, we expect that the g-factor can be further increased.

10. ### Extended Data Fig. 10 Transmitted colour modulation by a dispersed solution of 432 helicoid III nanoparticles.

a, b, Lorentz reciprocity of 432 helicoid III nanoparticles. The CD spectra of 432 helicoid III nanoparticles were measured from dispersion in aqueous solution (a) and deposition on a glass substrate (b). In both cases, CD measurements in the forwards and backwards directions produced identical responses. c, SEM images of 432 helicoid III nanoparticles with different sizes controlled by seed concentrations. Increasing the nanoparticle size resulted in a redshift in the plasmon resonance. The wavelengths at maximum CD intensity (λmax) are indicated in the images. d, Polarization-resolved transmittance spectra at different analyser angles. As the angle increased from −10° to 10°, transmittance at 550 nm gradually decreased, whereas that at 620 nm increased, resulting in a distinct asymmetric transition pattern. e, Colour transition patterns of 432 helicoid III nanoparticles traced on CIE xy 1931 colour space (CIE, International Commission on Illumination). The white triangle indicates the RGB boundary. Each pattern shows elliptical traces with a clockwise rotational direction that reflects the asymmetric colour transition. f, CD spectra of a 432 helicoid III mixture. The spectral features of the broad and split CD peaks show linear superposition of the original components. g, Colour transition traces of the mixture on a colour space. The trace of the mixture was distinct from that of each original component and displays tailored colour transformation. h, Polarization-resolved transmission image of a 432 helicoid III mixture. Compared to the original components, the mixture shows different colour-transition patterns depending on the polarization angle.

## Supplementary information

### DOI

https://doi.org/10.1038/s41586-018-0034-1

• ### Seeded growth of gold nanoparticles in aqueous solution of cationic gemini surfactants with different spacer length: influences of molecular and aggregate structures

• Wentao Wang
• , Yuchun Han
• , Jiang Zhu
• , Yaxun Fan
•  & Yilin Wang

Journal of Nanoparticle Research (2019)

• ### Peptides used to make light-twisting nanoparticles

• Guillermo González-Rubio
•  & Luis M. Liz-Marzán

Nature (2018)