Molecular Tuning of Quantum Plasmon Resonances Peter Nordlander Science 343, 1444 (2014); DOI: 10.1126/science.1252245

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PERSPECTIVES Concurrently with the new cameras, powerful maximum likelihood image processing routines became available. These routines define reliable and objective criteria for averaging tens or hundreds of thousands of single-particle images, as is necessary to achieve high resolution (11). This combination of advanced detectors and software now produces cryo-EM structures that look, in terms of clarity and map definition, considerably better than x-ray structures at the same nominal resolution, owing to the high quality of the phase information contained in cryoEM images. Does the resolution revolution in cryo-EM mean that the era of x-ray protein crystallography (12) is coming to an end? Definitely not. For the foreseeable future, small proteins—in

cryo-EM, anything below 100 kD counts as small—and resolutions of 2 Å or better will remain the domain of x-rays. But for large, fragile, or flexible structures (such as membrane protein complexes) that are difficult to prepare yet hold the key to central biomedical questions, the new technology is a major breakthrough. In the future, it may no longer be necessary to crystallize large, well-defined complexes such as ribosomes. Instead, their structures can be determined elegantly and quickly by cryo-EM. These are exciting times. References and Notes 1. A. Amunts et al., Science 343, 1485 (2014). 2. M. Liao, E. Cao, D. Julius, Y. Cheng, Nature 504, 107 (2013). 3. M. Allegretti et al., eLife 3, e01963 (2014). 4. X. Li et al., Nat. Methods 10, 584 (2013). 5. N. Ban et al., Science 289, 905 (2000).

6. B. T. Wimberly et al., Nature 407, 327 (2000). 7. A. R. Faruqi, R. Henderson, Curr. Opin. Struct. Biol. 17, 549 (2007). 8. Henderson (MRC Laboratory of Molecular Biology, Cambridge, UK) formed a consortium with engineers at the Rutherford Appleton Laboratory and scientists at the Max Planck Society to fund and develop a first prototype. The consortium then joined forces with the electron microscope manufacturer FEI to roll out and market the new design. At about the same time, Gatan Inc. of Pleasanton, California came out with a similar detector designed by Peter Denes (Lawrence Berkeley National Laboratory) and David Agard (University of California, San Francisco). A third type of camera was developed by Nguyen-Huu Xuong at the Direct Electron company (San Diego, California). 9. B. Daum, A. Walter, A. Horst, H. D. Osiewacz, W. Kühlbrandt, Proc. Natl. Acad. Sci. U.S.A. 110, 15301 (2013). 10. F. K. Schur et al., J. Struct. Biol. 184, 394 (2013). 11. S. H. Scheres, J. Struct. Biol. 180, 519 (2012). 12. Special section on Crystallography at 100, Science (7 March 2014). 10.1126/science.1251652

APPLIED PHYSICS

Molecular Tuning of Quantum Plasmon Resonances

The tuning of nanostructure plasmon resonances with bridging molecules offers opportunities for both plasmonics and molecular electronics.

Peter Nordlander A C etallic nanoparticles D exhibit plasmon resoD nances, which are colEgap = Eext dg lective, coherent oscillations of their conduction electrons that can dg couple very efficiently to light. Originally a subfield of condensedB matter physics, the past decade has ε FL (t) ε RF(t’) e– e– ε RF (t) ε FL(t’) seen tremendous growth of plasmonics as an interdisciplinary field spanning chemistry, materials science, and biology. On page 1496 of this issue, Tan et al. (1) discuss an experiment that will almost certainly further fuel this Plasmon-induced electron tunneling. (A) The electric potential associated with the incident radiation field (black dashed growth—the coupling of plasmon line) is screened by the nanoparticles and creates an enhanced local field in the gap (solid red line) determined by the strucexcitations to molecular conduc- tural parameters (blue) D and dg. (B) The field in the junction shifts the relative positions of the Fermi levels εF of the left and tion. The merging of plasmonics right particles as a function of time t and enables electrons to transfer between nanoparticles in each cycle of the incident with molecular electronics prom- radiation. (C) A schematic of the molecular junction used in the present experiment shows the planar junction geometry and ises both novel fundamental dis- interrogation by a light beam (green) that allows a large number of molecules to serve as conduction channels for electrons. coveries and new applications. The energy of the plasmon resonances ties of plasmonic nanoparticles, perhaps the Nanostructures with sharp protrusions depends strongly on nanostructure shape and most mature is the use of the large plasmon- naturally induce large local field enhancecomposition. Given the emergence of highly induced enhancements in local electromag- ments, but the optimal structures for local precise nanofabrication methods, it is possi- netic fields near the surfaces of the nanoparti- field enhancements consist of a pair of metalble to design nanostructures that have plas- cles for surface-enhanced spectroscopies (2). lic nanoparticles separated by a nanometermon resonances ranging from the ultraviolet Amplified local fields can enhance the prob- scale gap (a “plasmonic dimer”). Several into the infrared. Among the many important abilities for molecular transitions by many studies have demonstrated that the local field applications of the light-harvesting proper- orders of magnitude. In nonlinear spectros- enhancements in the “hot spots” in the gaps copies such as Raman scattering, the signal of dimers can be sufficiently strong that the scales as the fourth power of the local electric Raman scattering from an individual molDepartment of Physics and Astronomy, Rice University, field across the molecule. ecule can be detected (3). Classical electroHouston, TX 77005, USA. E-mail: [email protected]

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PERSPECTIVES magnetic theory predicts that the local field enhancement in the junction of a plasmonic dimer increases monotonically with decreasing gap width. This property can be understood in a very simplified fashion as a “lightning rod” effect (see the figure, panel A). In the long wavelength regime, the nanoparticles are essentially equipotential; the potential drop from the electric component of the incident field across the dimer occurs only in the gap (4). For a dimer of length D with a gap of width dg, the local field enhancement will scale as D/dg and can be made very large by fabricating structures with large D and small dg. The large fields in the gap also result in a voltage drop between the nanoparticles that shifts the relative position of the Fermi energies εF of the nanoparticles (see the figure, panel B). In principle, these shifts allow electrons to transfer back and forth between the nanoparticles. This plasmon mode, referred to as the charge-transfer plasmon (CTP), coexists with the standard nonconductively coupled dimer plasmons but is distinct and appears at different frequencies because the local fields in the junctions are altered by the charge transport across the junction (5). For a vacuum junction, the tunneling matrix elements, which determine the transition rates of electrons between the two nanoparticles, decrease exponentially with increasing dg, and the CTP can only be observed for gaps a few angstroms in width. The creation of dimers with such narrow gaps is experimentally challenging but has recently been accomplished and has enabled the experimental observation of well-defined CTPs (6, 7). Theoretical calculations have predicted that if the nanoscale gap is filled by a conductive medium such as molecules, the tunneling transition matrix element will no longer exhibit an exponential dependence on dg and the CTP may be observed at larger dg (8, 9). The experimental verification of these predictions is a challenging task, but Tan et al. were able to nanofabricate dimers with extremely small gap widths, as well as assemble molecules that span the gap. By fabricating a silver nanocube dimer with very flat surfaces (see the figure, panel C), the junction area became large and uniform and could accommodate a sufficiently large number of conducting molecules. The CTP was observed for junction widths greater than 1 nm. These results not only allow quantum plasmonics to be studied with structures that have substantially larger gap widths than in previous experiments but also bridge two vibrant subfields of nanoscience, plasmonics, and

molecular electronics. Tan et al. show that the CTP depends on the type of molecule used, thus firmly establishing that its electronic properties are a key parameter in determining the optical frequency transport response of the nanoscale interparticle junction. Molecular tunnel junctions are normally characterizing with direct current (dc) or lowfrequency alternating current conductivity measurements. In the present experiment, the molecular conductance was probed instead at optical frequencies determined by the collective resonance of the plasmonic dimer. At high frequencies, additional intramolecular processes not present in dc transport, such as electron-electron and electron-vibrational scattering, could strongly influence molecular conductance. By exploiting the facile geometric tunability of plasmonic antennas, it should be possible to measure molecular

conductances at much higher frequencies. This experiment by Tan et al. also paves the way for novel device and molecular sensing applications. The conductance of a molecule can be switched or modulated, for example, by the application of an external dc field or by chemical reaction with another molecule. References 1. S. F. Tan et al., Science 343, 1496 (2014). 2. S. Lal et al., Chem. Soc. Rev. 37, 898 (2008). 3. J. A. Dieringer, R. B. Lettan 2nd, K. A. Scheidt, R. P. Van Duyne, J. Am. Chem. Soc. 129, 16249 (2007). 4. F. Le et al., ACS Nano 2, 707 (2008). 5. J. Zuloaga et al., Nano Lett. 9, 887 (2009). 6. K. J. Savage et al., Nature 491, 574 (2012). 7. J. A. Scholl, A. García-Etxarri, A. L. Koh, J. A. Dionne, Nano Lett. 13, 564 (2013). 8. O. Pérez-González et al., Nano Lett. 10, 3090 (2010). 9. P. Song, P. Nordlander, S. Gao, J. Chem. Phys. 134, 074701 (2011). 10.1126/science.1252245

CANCER

Cholesterol and Cancer, in the Balance Sandrine Silvente-Poirot and Marc Poirot Cholesterol metabolites can promote or suppress breast cancer, raising questions about how therapies might disrupt this balance.

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ammalian cells synthesize cholesterol through a series of 21 enzymatic steps, generating numerous metabolites that are involved in the control of physiological and developmental processes. Cholesterol itself is the precursor of steroid hormones and sterols, the latter of which can be further modified into molecules that induce specific biological responses. Epidemiological studies have investigated the role of cholesterol in breast cancer risk, with contradictory findings. Recent studies, however, linking cholesterol metabolism to breast cancer may provide some insights. Certain cholesterol metabolites can promote (1, 2) or suppress (3) breast cancer. This raises the important question of how to regulate or inhibit the cholesterol metabolic pathway, and at which steps, in a therapeutic approach to cancer. Cholesterol is a unique lipid, essential for membrane biogenesis, cell proliferation, and cell differentiation (4). It is provided by the diet but is also mainly synthesized by the liver in humans and distributed throughout the UMR 1037 INSERM–University Toulouse III, Cancer Research Center of Toulouse, and Institut Claudius Regaud, 31052 Toulouse, France. E-mail: [email protected]; [email protected]

body via low-density lipoprotein (LDL) and high-density lipoprotein (HDL) transporters. Cancer has been associated with cholesterol, as it is the obligatory precursor of steroid hormones that are involved in tumor promotion (estrogens, androgens) as well as tumor death (glucocorticoids). Oncogenic processes enable cancer cells to synthesize their own cholesterol, which can be further metabolized to support their rapid proliferation. But contradictory results of epidemiologic studies make conclusions difficult regarding breast cancer. Thus, it is still not clear whether total cholesterol, HDL, or LDL can predict the occurrence of breast cancer. Many studies have investigated whether breast cancer risk is affected by blocking cholesterol synthesis with anticholesterol drugs such as statins, which inhibit the enzyme 3-hydroxy-3-methylglutaryl– coenzyme A reductase (HMG-CoA reductase or HMGCR). Here too, the findings are inconsistent. Statin use is associated with both an increased and decreased risk of breast cancer, and other studies report no association at all. A recent study has found that long-term (10 years) treatment with statins doubled the risk of invasive ductal

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