Guest Commentary pubs.acs.org/JPCL
Plasmonics. Electron Oscillations and Beyond
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related to the more and more frequent use of electron energy loss spectroscopy (EELS) to characterize the optical response of metal nanoparticles at the single-particle level and even with nanometer-scale resolution. It is very interesting to see that electron-driven plasmon excitation allows one to not only identify the nature of the various plasmon modes that can be excited in different nanoparticles but also to explore in detail the formation of hot spots and the relevance to SERS. Additionally, comparison of the experimental data to numerical calculations based on the discrete dipole approximation (DDA) allows a detailed interpretation of the experimental results and prediction of the necessary conditions for single-molecule SERS. Knappenberger et al. (Knappenberger, K. L., Jr.; Dowgiallo, A.-M.; Chandra, M.; Jarrett, J. W. Probing the Structure− Property Interplay of Plasmonic Nanoparticle Transducers Using Femtosecond Laser Spectroscopy. J. Phys. Chem. Lett. 2013, 4, 1109−1119) focus on a different type of spectroscopic technique, which not only provides information regarding the effect of light on the nanoparticles but also correlates the structure of the material with the optical property. They describe the use of femtosecond time-resolved transient extinction measurements on gold nanoshells to probe the plasmon resonances upon photoexcitation to demonstrate the effect of particle size on environmental sensitivity. Additionally, nonlinear optical spectroscopy correlated with electron microscopy proves useful to determine plasmon modes within individual assemblies, for example, providing evidence of chiral electromagnetic surface plasmon fields. On the other hand, Scaiano and Stamplecoskie (Scaiano, J. C.; Stamplecoskie, K. Can Surface Plasmon Fields Provide a New Way to Photosensitize Organic Photoreactions? From Designer Nanoparticles to Custom Applications. J. Phys. Chem. Lett. 2013, 4, 1177−1187) get somehow away from the more physical aspects of plasmon resonances and discuss their potential use to influence chemical reactions. While this topic is closely related to the spectroscopic properties of plasmonic nanoparticles, the role of light during nanoparticle growth and its influence on chemical processes (such as catalysis) taking place at their surface has probably not been sufficiently considered, and thus, the discussion here may shed some light (never a more appropriate expression) and open the minds of many researchers working on other fields of chemistry that are not traditionally associated with plasmonics. Finally, the Perspective from Dickson’s group (Petty, J. T.; Story, S. P.; Hsiang, J.-C.; Dickson, R. M. DNA-Templated Molecular Silver Fluorophores. J. Phys. Chem. Lett. 2013, 4, 1148−1155) is perhaps farther away from plasmonic phenomena because it focuses on extremely small metallic nanostructures (few atoms), which therefore behave as molecules in the sense that they display discrete electronic levels, and therefore, absorbed light is efficiently re-emitted, rendering these clusters excellent fluorophores. It is very
lasmon resonances in small metal particles have been usually related to highly efficient interaction of light with the nanoparticles, inducing coherent resonant oscillations of conduction electrons, which is reflected in large extinction coefficients in the visible or the near-IR.1 This being true and leading by itself to a wide variety of phenomena and applications, the intense recent research in the newly quoted field of plasmonics has demonstrated that scientists must remain open-minded and be ready to observe and exploit new and unexpected phenomena in plasmonic nanoparticles and nanostructures. Additionally, while single particles display extremely interesting properties on their own, a plethora of new possibilities arise when they are used as building blocks and assembled into specific configurations, perhaps using molecular concepts as guidelines.2 Recent reports have stressed the concept of plasmonic nanomaterials as nanoantennas for light, indicating the ability to concentrate electromagnetic radiations within areas that are significantly smaller than their wavelengths. One immediate consequence of the nanoantenna effect is the ability of metal nanoparticles to largely enhance the incident electromagnetic field and thereby generate high electric fields that can be localized at extremely small areas, both within single particles (for example, at sharp edges or tips) and at small gaps between plasmonic materials.3 This is the main effect behind the so-called surface-enhanced spectroscopies, among which, surface-enhanced Raman scattering (SERS) has attracted the most attention because of its great potential to become a widely used ultrasensitive and ultraselective analytical tool. However, a number of other effects including plasmon chirality, surface-enhanced fluorescence, plasmon thermometry, or plasmon-induced chemical reactions have been recently reported. The fast dissipation of energy toward the environment upon plasmon excitation has been presented as an extremely efficient tool for localized heating, which has demonstrated its viability for novel therapeutic methods based on hyperthermia. Obviously, all of these developments and discoveries would have not been possible without the development of advanced fabrication methods (both top-down and bottom-up), as well as theoretical models and numerical methods. Fabrication has allowed the creation of nanoparticles within a wide range of sizes and shapes, while theory has been able not only to explain many experimental observations but also to predict phenomena that were subsequently demonstrated. Additionally, the development of characterization techniques has also been essential, in particular, for those related to single-particle spectroscopic analysis. This issue of J. Phys. Chem. Lett. features four Perspectives dealing with different aspects of plasmonics or closely related phenomena. Camden and colleagues (Iberi, V.; MirsalehKohan, N.; Camden, J. P. Understanding Plasmonic Properties in Metallic Nanostructures by Correlating Photonic and Electronic Excitations. J. Phys. Chem. Lett. 2013, 4, 1070− 1078) nicely show the similarities between the excitation of plasmon resonances with photons and with electrons, which is © 2013 American Chemical Society
Published: April 4, 2013 1197
dx.doi.org/10.1021/jz400495d | J. Phys. Chem. Lett. 2013, 4, 1197−1198
The Journal of Physical Chemistry Letters
Guest Commentary
interesting to see that the stabilization of silver clusters with DNA can lead to fine-tuning of the optical response through careful selection of the nucleotide sequence, again establishing a link between chemical interactions and physical properties. As a researcher who is interested in the field of plasmonics from different points of view, I have really enjoyed reading these Perspectives, and I am sure that many will be surprised by the whole range of possibilities that plasmon resonances offer toward basic research as well as applications in a variety of fields.
Luis M. Liz-Marzán*,†,‡ †
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BioNanoPlasmonics Laboratory, CIC biomaGUNE, Paseo de Miramón 182, 20009 Donostia - San Sebastián, Spain ‡ Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain
AUTHOR INFORMATION
Corresponding Author
* E-mail:
[email protected].
REFERENCES
(1) Liz-Marzán, L. M. Tailoring Surface Plasmon Resonance through the Morphology and Assembly of Metal Nanoparticles. Langmuir 2006, 22, 32−41. (2) Guerrero-Martínez, A.; Grzelczak, M.; Liz-Marzán, L. M. Molecular Thinking for Nanoplasmonic Design. ACS Nano 2012, 6, 3655−3662. (3) Alvarez-Puebla, R. A.; Liz-Marzán, L. M.; García de Abajo, F. J. Light Concentration at the Nanometer Scale. J. Phys. Chem. Lett. 2010, 1, 2428−2434.
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