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Mechanically interlocked gold nanocatenanes

Abstract

The design of nanostructures is often inspired by the geometries of molecular systems. Inspired by mechanically interlocked catenanes, here we report the synthesis of interlocked plasmonic nanochains, starting from triangular gold nanoplates. The gold nanocatenanes (AuNCats) are prepared in a strategy analogous to the metal-templated synthesis of [2]catenane. We present an analysis of the behaviour of the localized surface plasmon resonance of AuNCats comprising two gold nanorings in terms of D2d spatial symmetry. Moreover, the rocking motion of one nanoring relative to the other causes desymmetrization into D2 symmetry, and induces mechano-helical chirality of an AuNCat as a structural analogy of the [2]catenane, which results in the occurrence of chiroptical responses and remarkably high g factors. As a proof of concept, we constructed a nanoactuator using an AuNCat in tandem with a thermoresponsive polymer. The light-induced thermal actuation of the plasmonic nanomachine transforms rectilinear force into rotational mechanical motion, which enables control of the circular dichroism of the plasmonic nanostructure. We anticipate that mechanically interlocked plasmonic platforms will introduce new classes of nanostructures with unprecedented capabilities.

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Fig. 1: Synthetic scheme for a mechanically interlocked AuNCat.
Fig. 2: Characterization of mechanically interlocked AuNCats.
Fig. 3: Mechano-helical chirality of AuNCats.
Fig. 4: Light-induced thermal actuation of a mechanically interlocked plasmonic nanomachine.

Data availability

The data reported in this paper are available in the main text, Extended Data figures or Supplementary Information. Source data are provided with this paper.

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Acknowledgements

We thank Y. Jung, J. Son, J.-H. Kim, S. Cha, Y. Lee, S. Kim and D. H. Kim for helpful discussions. This work was supported by National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIT; nos. NRF-2021R1A2C3010083 and NRF-2017R1A5A1015365.)

Author information

Authors and Affiliations

Authors

Contributions

Y.K. and J.-M.N. conceived the initial ideas and designed the experiments. Y.K. performed the synthesis, single-particle measurements, numerical simulations and data analysis under the guidance of J.-M.N. Y.K. and J.-M.N. wrote the manuscript. J.-M.N. supervised the entire study.

Corresponding author

Correspondence to Jwa-Min Nam.

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The authors declare no competing interests.

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Nature Synthesis thanks the anonymous reviewers for their contribution to the peer review of this work. Primary Handling Editor: Alison Stoddart, in collaboration with the Nature Synthesis team.

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Integrated supplementary information

Extended Data Fig. 1 Rational design for the length of PEG linkers to provide an appropriate space in the subsequent Au overgrowth step.

a, AFM image (top) and AFM height scan (bottom) of a PtSNR. The measured height of PtSNR is similar to its thickness as ca. 18 nm, which enables it to be considered as a torus structure. b,c, TEM image of a PtSNR (b) and imaginary position of the other PtSNR that will form pre-interlocked entwined conformation after EDC/sulfo-NHS coupling (c). The cross section of PtSNR is expressed as a circle (dotted line) in the way that the PtSNR can be considered as a split torus structure. The diameter of 15 nm is the average value of the thickness and height. The scale bar indicates 20 nm. d, Cross section of AuNCat and the amide linker between two constituent gold nanorings. 17 nm of carboxylic PEG and amine-PEG should be used to acquire roughly 35 nm of linker that guarantees a proper interparticle distance. e,f, Possible mechanisms of the PtSNR assembly (e) and Au overgrowth-based ring-closing step (f).

Source data

Extended Data Fig. 2 Structural configuration dependency on optical responses of AuNCat.

Desymmetrization angle (a–d) and interparticle distance (e–h) between two constituent Au nanorings affect LSPR of AuNCat. a, Scattering cross-sections calculated by BEM simulation when the desymmetrization angle changes from φ = 0° to φ = 30°. The AuNCat structures used in the simulation have 100 nm of major diameter and 40 nm of minor diameter where the interparticle distance, defined as a distance between two centres of the gold nanorings, is fixed at 50 nm. b,c, The SEM images of a AuNCat where d ~ 50 nm and φ ~ −28° (b) and the corresponding structure constructed for BEM simulation (c). d, Experimental scattering spectrum of the AuNCat shown in b and calculated scattering cross-section for the structure in c. e, Scattering cross-sections calculated by BEM simulation when the interparticle distance changes from 42 nm to 58 nm. The AuNCat structures used in the simulation have 100 nm of major diameter and 40 nm of minor diameter where the desymmetrization angle is fixed to 0°. f,g, The SEM images of a AuNCat where d ~ 58 nm and φ ~ +10° (f) and the corresponding structure constructed for BEM simulation (g). h, Experimental scattering spectrum of the AuNCat shown in f and calculated scattering cross-section for the structure in g. The prevalent redshifts over the measured spectra are attributed to slightly triangular shape of each nanoring. Field-emission SEM (MERLIN Compact, Carl Zeiss, Germany) with different tilt angles was used to obtain all the SEM images, and scale bars in them indicate 100 nm.

Source data

Extended Data Fig. 3 Gap distance dependency on LSPR of AuNCat.

a, Structures of AuNCat constructed for BEM simulations. The propagating direction and electric field polarization of the incident light are shown. BEM simulation was performed while increasing the minor diameter of the AuNCat. The interparticle distance is fixed at 50 nm and the gap distance is determined by the minor radius as gap distance = 50 – 2×minor radius. b, Calculated scattering cross-sections of the structures shown in a. c, Gap distance dependency of two peak wavelengths in b. Lower energy LSPR is the symmetric bonding mode (SBM, top panel, solid line) and higher energy LSPR is the gap dominating mode (GDM, bottom panel, solid line). If the gap distance is large enough, decrease in the gap distance generally causes blueshift of SBM. This is concomitant with the simultaneous increase of minor radius and resulting blueshift of symmetric dipolar mode (SDM, top panel, dotted line) of Au nanoring. Blue box indicates the regime where the gap mode significantly affects LSPR features. In this regime, SBM begins to redshift and GDM emerges due to the gap mode participation.

Source data

Extended Data Fig. 4 Basis decomposition for LSPRs of a AuNCat by group representation analysis.

a, Symmetry operations for the D2d symmetric structure of a AuNCat. D2d point group has one C2 main axis and two perpendicular C2 axes with two mirror planes. b, Structure of a AuNCat used in the group representation analysis. The direction of light propagation is perpendicular to the C2 main axis and parallel to one of mirror planes while the polarization direction of electric field is parallel to C2 main axis. The major and minor diameter are 100 nm and 47 nm, respectively, and the gap distance is 3 nm. c, Theoretical extinction (black line), scattering (red line), and absorption (blue line) cross-section of the AuNCat shown in b. Two LSPRs, SBM and GDM, are marked by green and violet triangles, and their resonance wavelengths are 761 nm and 603 nm, respectively. d, Decomposed basis modes of the LSPRs of the AuNCat corresponding to their own irreducible representations. The contribution of the B2 symmetric basis dominates inter alia in both resonances, which implies that the two LSPs can interfere according to their respective phases.

Extended Data Fig. 5 Plasmon hybridization model for thin (34 nm thick) and thick (47 nm thick) AuNCats.

a, Decomposed B2 symmetric basis mode of a LSP mode of a thin AuNCat. The mode can be expressed as a simple addition of dipole modes of each constituent Au nanoring. b, Decomposed B2 symmetric basis mode of each LSP mode of a thick AuNCat, that is, SBM and GDM. The modes can be expressed as a linear combination among two dipolar modes of each Au nanoring and a gap mode. The LSP modes of Au nanorings indicate the charge density inside the surface. The gap mode is calculated outside the surface by subtracting the two dipolar modes of Au nanorings from the SBM mode of the AuNCat. c,d, Theoretical scattering cross-section of a AuNCat (middle panel) and Au nanorings that compose the AuNCat (right and left panels) with parallel polarization of the electric field along the C2 main axis. The inset images indicate respective surface charge distributions at each LSPR peak. Small difference in the resonance frequency of the thin AuNCat (c) means the two dipolar plasmon modes of nanorings merely couple with each other (d). e,f, Schematic illustration of the hybridization model to construct LSPRs of a AuNCat. As per plasmon hybridization between two symmetric dipole modes of each Au nanoring, simple subtraction and addition of these modes are generated. The subtracted mode has zero dipole moment and is optically dark, so that the only one LSP, the addition of ring dipolar modes, can be excited by planewaves as in c (e). In the case of the thick AuNCat, the simple subtraction and addition of these modes are intermediately formed; the modes are hybridized further with a B2 symmetric gap mode to generate the LSP modes of the AuNCat. Created SBM and GDM are optically bright with their own dipole moment, whereas the symmetric anti-bonding mode (SAM) is optically dark, which cannot be excited by planewaves (f).

Extended Data Fig. 6 Chiroptical responses of desymmetrized AuNCats under circularly polarized light.

a–e and f–j show the chiroptical responses of a (P)- and an (M)-AuNCat shown in Fig. 3b, respectively. a,f, SEM images of AuNCats at different tilt angles. b,g, Corresponding structures to a and f used in FEM calculations. c,h, Experimental spectra (top panel) and calculated scattering cross-sections (bottom panel) under LCP (red line) and RCP (blue line). The simulations agree well with the experiments for the desymmetrized AuNCats. The small difference in peak wavelengths and intensity of the resonances could have been resulted from the structural inexactitude from the SEM images and imparity in the portion between vertically and parallelly propagating light. d,e,i,j, Measured circular dichroism (d,i, top panels) and g-factor of each AuNCat (e,j, top panels), originated from the different optical responses to LCP and RCP light in c and h. The theoretical circular dichroisms and g-factors are shown in the bottom panels, and they accord closely with the experiments. All SEM images were taken by field-emission SEM (MERLIN Compact, Carl Zeiss, Germany) at 10 kV and the scale bars indicate 100 nm.

Source data

Extended Data Fig. 7 Difference in circular dichroic behaviours of (M)-AuNCats and triangular AuNCats with respect to light propagating directions.

a, The structure of an (M)-AuNCat used in FEM simulation for LCP and RCP light. The light that propagates parallel to the C2 main axis (x axis) is marked by a black arrow. For vertically travelling light, the optical responses for two orthogonal axis, y and z axis (red arrows), are individually calculated and averaged out. b–g, The circular dichroic behaviours under parallelly (black line) and vertically (red line) propagating light are presented in regard to several desymmetrization angles. h–j, The circular dichroic behaviours of triangular AuNCats under light propagating along x (black line), y (red line), and z (blue line) axes with respect to several rotational angles of a triangular nanoring.

Source data

Extended Data Fig. 8 Circular dichroism in extinction, scattering, and absorption cross-section of (M)-AuNCats.

a–c, Calculated circular dichroisms in extinction (black line), scattering (red line), and absorption (blue line) are shown with respect to different desymmetrization angles. d, The ratio of scattering (red line) and absorption (blue line) CD to extinction CD at the wavelength of bonding modes. As the desymmetrization angle increases, the dipole moment of the bonding mode decreases due to the opposite oscillating direction between dipolar modes of the nanorings, which causes the decrease in scattering CD. e, Change in the peak wavelength of bonding (violet line) and anti-bonding mode (green line) according to the degree of desymmetrization. As the desymmetrization angle increases, the lateral distance between two nanorings gets closer. As a result, the coupling between each dipolar mode becomes stronger and the peak splitting extent increases, which is expressed as the redshift of bonding mode and blueshift of anti-bonding mode.

Source data

Extended Data Fig. 9 Origin of circular dichroism for vertically propagating light.

a, Chiroptical response of an (M)-AuNCat where φ = −25° under LCP (red line) and RCP (blue line). b, Corresponding CD spectrum to a. The two main peaks are marked with green and violet triangles. c, Surface charge distribution under LCP at the peak wavelength of positive sign in b. The resonance is mainly comprised of the vertically oscillating charges of rings, rather than normal lateral oscillation over the whole ring structure. As the charges are localized near the gap region where the lateral proximity of rings forms, the oscillating direction is tilted (black arrows), which induces circular dichroic responses. d, Surface charge distribution under RCP at the peak wavelength of negative sign in b. Opposite to c, two lateral charge oscillations get involved in this resonance; but similarly, the charge accumulation near the gap region causes the rotation of the oscillating direction of the dipolar modes (black arrows).

Source data

Extended Data Fig. 10 Light-induced thermal actuation of a PNIPAM-modified (M)-AuNCat.

a, TEM image of a PNIPAM-modified Au nanorings. b, UV-vis spectra of Au nanorings (black), PNIPAM-modified Au nanorings (red), and heated PNIPAM-modified Au nanorings (blue). c, Scattering spectra of an (M)-AuNCat at weak irradiation powers. d, Irreversible spectral changes at 970 μW of the irradiation power observed before the PNIPAM reaches an equilibrium state. The numbering in the spectra is chronological sequence. e, The SEM image (left) and corresponding structure (right) of the (M)-AuNCat used in the measurement of Fig. 4. The SEM image was obtained by Hitachi S-4300 at 15 kV with Pt sputtering after all optical measurements. f,g, Contour plots of scattering spectra (f) and circular dichroisms (g) throughout the whole light-induced thermal actuating processes. The peak wavelengths are marked by light black lines. In both cases, the intensity of the peak decreases and the peak redshifts under irradiation, which hypothetically indicates the increment in the desymmetrization angle.

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Kim, Y., Nam, JM. Mechanically interlocked gold nanocatenanes. Nat. Synth 1, 649–657 (2022). https://doi.org/10.1038/s44160-022-00116-2

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