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Unraveling the origin of chirality from plasmonic nanoparticle-protein complexes

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Science  27 Sep 2019:
Vol. 365, Issue 6460, pp. 1475-1478
DOI: 10.1126/science.aax5415

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Probing protein-nanorod aggregates

The interaction of proteins with nanoparticles can enhance circular dichroism signals and provide a route for sensitive biodetection. However, ensemble measurements are insufficient for resolving the origin of plasmon-coupled circular dichroism (PCCD). Zhang et al. used single-particle circular differential scattering spectroscopy along with correlated tomographic reconstruction and electromagnetic simulations to study individual aggregates of bovine serum albumin and gold nanorods (see the Perspective by Kim and Kotov). Aggregates contribute to PCCD activity through formation of plasmonic hotspots, but single nanoparticles do not contribute. The protein does not function simply as a chiral chromophore but enables assembly of the chiral nanorod-protein complexes.

Science, this issue p. 1475; see also p. 1378

Abstract

Plasmon-coupled circular dichroism has emerged as a promising approach for ultrasensitive detection of biomolecular conformations through coupling between molecular chirality and surface plasmons. Chiral nanoparticle assemblies without chiral molecules present also have large optical activities. We apply single-particle circular differential scattering spectroscopy coupled with electron imaging and simulations to identify both structural chirality of plasmonic aggregates and plasmon-coupled circular dichroism induced by chiral proteins. We establish that both chiral aggregates and just a few proteins in interparticle gaps of achiral assemblies are responsible for the ensemble signal, but single nanoparticles do not contribute. We furthermore find that the protein plays two roles: It transfers chirality to both chiral and achiral plasmonic substrates, and it is also responsible for the chiral three-dimensional assembly of nanorods. Understanding these underlying factors paves the way toward sensing the chirality of single biomolecules.

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