Virtual Issue: Plasmon Resonances - A Physical Chemistry Perspective

EDITORIAL pubs.acs.org/JPCC

Virtual Issue: Plasmon Resonances - A Physical Chemistry Perspective

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lasmons are collective oscillations of conduction electrons in metals. For metal nanostructures, surface plasmons give rise to resonances at distinct wavelengths and are responsible for the brilliant colors of solutions of Ag and Au colloids. This was first recognized by Faraday in 1856, and studies of how these resonances are affected by the size, shape, and composition of metal nanoparticles have been an active area of research ever since. Surface plasmons also give rise to very large electro-magnetic fields at the surface of the particles, which is important for surface-enhanced spectroscopies, such as surface-enhanced Raman scattering (SERS). This general area of study is known as “Plasmonics”, and it is an extremely active topic in Physical Chemistry research, for both experiment and theory. Experimental studies in this area include the development of new synthetic techniques for making particles, spectroscopic studies of the properties of the particles and of molecules on the particles, correlated spectroscopic and structural studies, and the development of applications, such as detection of single molecules and enhancing optical absorption in solar energy conversion devices. Some of the challenges in theory are how to perform accurate and efficient electro-magnetic calculations over the wide range of length scales present in plasmonic materials and the inclusion of quantum effects for treating molecules at the surface of the particles. This Virtual Issue (http://pubs.acs.org/page/vi/2011/ plasmon_resonances.html) brings together a few representative papers published in the Journals of Physical Chemistry A/B/C and the Journal of Physical Chemistry Letters in recent years on Plasmonics. The articles cover the areas of the synthesis and spectroscopy of metal nanostructures, theory and modeling of plasmonic structures, and the more applied topics of surfaceenhanced spectroscopies and biomedical applications. These articles represent contributions from scientists around the world, who are actively involved in this dynamic and diverse field of research. Space restrictions are such that we have only been able to highlight a small fraction of the many interesting papers that have been published.

Representative images from the papers collected in this virtual issue. Top: correlated electron microscopy and optical scattering measurements from (a) gold nanorods [10] and (b) and gold decahedra [12]. Middle: r 2011 American Chemical Society

calculations of the optical response of (c) metal nanoparticles near surfaces [23], (d) gold clusters passivated by chiral ligands [27], and (e) chiral dimers of nanorods [28]. Bottom: SERS studies of (f) dimers [33] and (g) aggregates [5], and (h) mapping SERS hot spots in arrays [35].

’ SPECTROSCOPY OF METAL NANOSTRUCTURES The size and shape of the nanostructure dictate the frequency of the plasmon resonance of the particle.1 This has led to considerable research in synthetic techniques for making particles of different sizes and shapes2 and understanding how this affects the spectra of the particles and the fields at the particle surface.3 Among the synthetic advances that have occurred recently are the development of techniques to make samples with very narrow size distributions4,5 and the creation of materials with unusual shapes.6 In particular, hollow structures7 and voids in metal films (inverted particles) have attracted much recent attention.8 A particularly powerful way to study plasmonic materials involves single-particle absorption and scattering measurements.9 Single-particle experiments provide meaningful information about the width of the plasmon resonance as well as the frequency (in ensemble measurements the width is usually dominated by the size distribution of the sample). These experiments give insight into how effects such as radiation damping, electron-surface scattering, and retardation affect the plasmon resonance.10 Single-particle measurements can also be done in combination with electron-microscopy analysis, which yields precise structural information about the particle being interrogated.11 13 This is very important for studying coupled nanoparticles14 because the couplings depend sensitively on the distance between the particles.15 This sensitivity has led to the use of metal nanoparticle dimers as “rulers” for optically measuring distances at the nanoscale.16 The coupling of the plasmon resonance to transitions in molecules,17 or exciton resonances of quantum dots,18 is also an important area of research for plasmonic materials. Applications include optical nanolithography19 and enhancing the performance of solar cells20 and photocatalysts.21 Hand in hand with advances in experimental techniques have been advances in theories to model how metal nanoparticles interact with molecules22 and with each other3 and the substrate.23 A particular challenge is to accurately describe the size dependence of the dielectric function of small metal particles.24 The creation of high-quality metal nanoparticle samples capped with chiral ligands has also led to the discovery of intense circular dichroism in the metal-based optical transitions of the particles.25 This observation has spurred a number of theoretical studies aimed at understanding the origin of the effect.26,27 Researchers have also devised ways to make assemblies of nanoparticles that can enhance the circular dichroism signal at specific frequencies.28

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The Journal of Physical Chemistry C

’ SURFACE-ENHANCED SPECTROSCOPIES AND BIOMEDICAL APPLICATIONS The high fields associated with the plasmon resonances of metal nanostructures enhance the optical response of molecules attached to their surface. This has led to a variety of surfaceenhanced spectroscopies, the most well-known of which is surface-enhanced Raman spectroscopy (SERS). Research in SERS has examined the effect of the size of the particles on the SERS signal,29 their shape,30 the presence of sharp features,31,32 and junctions between particles.33 Junctions can create large-field enhancements at specific locations, the so-called SERS “hot spots”. Understanding the nature of these hot spots34,35 and engineering films to reproducibly create them is a longstanding challenge in this area of research.36 The field enhancements at the hot spots are clearly important in surface-enhanced spectroscopies, but the contribution from chemical enhancement effects on the SERS signal is still a significant issue from both theory and experimental perspectives.37,38 The enhancements that occur in SERS in principle allow detection of single molecules; understanding the conditions where this can happen is an active area of research.39,40 Recently developed femtosecond time-resolved SERS measurements provide important new capabilities in this area.41 The fields at the surface of metal nanoparticles can also enhance other photophysical processes, such as emission.42,43 These enhancements also occur in nanohole arrays.44 Another important application of plasmonic structures is in biomolecule sensing. The frequency of the plasmon resonance is sensitive to the environment of the particle,45 and this can be used to detect binding of target molecules to the surface of the nanostructure.46 An important issue in this field of research is the length scale for sensing, which depends on the size and geometry of the particle and determines the ultimate sensitivity of the technique.47 The intense optical absorption and scattering associated with the plasmon resonance of metal nanoparticles has also made these materials useful for imaging applications.48,49 In these experiments, the particles are used to label specific parts of cells, or specific cells, and questions about how the particles are taken up by the cells and cytotoxicity are important to resolve.50 In some cases (for example, cancer cells) it is desirable to kill the cells. This can be achieved by exciting the surface plasmon resonance of metal nanoparticles attached to the cell, which causes rapid local heating, disrupting functions important for the cell’s viability.7,51 The papers outlined above represent the recent physical chemistry advances in the field of metal nanoparticle research and the future challenges in surface plasmon research. Over the next few years, we expect that there will be continued progress in developing techniques to produce particles with well-defined sizes and shapes, controlling their positions in arrays, and modeling their response to optical fields. This will lead to improved performance of sensors based on these materials,46 as well as new applications, such as metamaterials.3 The importance of plasmons in physical science is also highlighted by the recent virtual issue on Plasmonics in ACS Nano52 and the special issue on Plasmonics in Chemical Reviews.53 Gregory V. Hartland Senior Editor University of Notre Dame

George Schatz Editor-in-Chief Northwestern University

EDITORIAL

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The Journal of Physical Chemistry C (20) Nishijima, Y.; Ueno, K.; Yokota, Y.; Murakoshi, K.; Misawa, H. Plasmon-Assisted Photocurrent Generation from Visible to Near-Infrared Wavelength Using a Au-Nanorods/TiO2 Electrode. J. Phys. Chem. Lett. 2010, 1, 2031–2036. (21) Yu, J. G.; Dai, G. P.; Huang, B. B. Fabrication and Characterization of Visible-Light-Driven Plasmonic Photocatalyst Ag/AgCl/TiO2 Nanotube Arrays. J. Phys. Chem. C 2009, 113, 16394–16401. (22) Ruan, Z. C.; Fan, S. H. Temporal Coupled-Mode Theory for Fano Resonance in Light Scattering by a Single Obstacle. J. Phys. Chem. C 2010, 114, 7324–7329. (23) Wu, Y. P.; Nordlander, P. Finite-Difference Time-Domain Modeling of the Optical Properties of Nanoparticles near Dielectric Substrates. J. Phys. Chem. C 2010, 114, 7302–7307. (24) Jensen, L. L.; Jensen, L. Atomistic Electrodynamics Model for Optical Properties of Silver Nanoclusters. J. Phys. Chem. C 2009, 113, 15182–15190. (25) Jiang, D. E.; Whetten, R. L.; Luo, W. D.; Dai, S. The Smallest Thiolated Gold Superatom Complexes. J. Phys. Chem. C 2009, 113, 17291–17295. (26) Aikens, C. M. Electronic Structure of Ligand-Passivated Gold and Silver Nanoclusters. J. Phys. Chem. Lett. 2011, 2, 99–104. (27) Noguez, C.; Sanchez-Castillo, A.; Hidalgo, F. Role of Morphology in the Enhanced Optical Activity of Ligand-Protected Metal Nanoparticles. J. Phys. Chem. Lett. 2011, 2, 1038–1044. (28) Auguie, B.; Alonso-Gomez, J. L.; Guerrero-Martinez, A.; LizMarzan, L. M. Fingers Crossed: Optical Activity of a Chiral Dimer of Plasmonic Nanorods. J. Phys. Chem. Lett. 2011, 2, 846–851. (29) Seney, C. S.; Gutzman, B. M.; Goddard, R. H. Correlation of Size and Surface-Enhanced Raman Scattering Activity of Optical and Spectroscopic Properties for Silver Nanoparticles. J. Phys. Chem. C 2009, 113, 74–80. (30) Rodriguez-Lorenzo, L.; Alvarez-Puebla, R. A.; Garcia de Abajo, F. J.; Liz-Marzan, L. M. Surface Enhanced Raman Scattering Using StarShaped Gold Colloidal Nanoparticles. J. Phys. Chem. C 2010, 114, 7336– 7340. (31) Pazos-Perez, N.; Barbosa, S.; Rodriguez-Lorenzo, L.; AldeanuevaPotel, P.; Perez-Juste, J.; Pastoriza-Santos, I.; Alvarez-Puebla, R. A.; LizMarzan, L. M. Growth of Sharp Tips on Gold Nanowires Leads to Increased Surface-Enhanced Raman Scattering Activity. J. Phys. Chem. Lett. 2010, 1, 24–27. (32) Kumar, J.; Thomas, K. G. Surface-Enhanced Raman Spectroscopy: Investigations at the Nanorod Edges and Dimer Junctions. J. Phys. Chem. Lett. 2011, 2, 610–615. (33) Rycenga, M.; Camargo, P. H. C.; Li, W. Y.; Moran, C. H.; Xia, Y. Understanding the SERS Effects of Single Silver Nanoparticles and Their Dimers, One at a Time. J. Phys. Chem. Lett. 2010, 1, 696–703. (34) Mahajan, S.; Cole, R. M.; Soares, B. F.; Pelfrey, S. H.; Russell, A. E.; Baumberg, J. J.; Bartlett, P. N. Relating SERS Intensity to Specific Plasmon Modes on Sphere Segment Void Surfaces. J. Phys. Chem. C 2009, 113, 9284–9289. (35) Farcau, C.; Astilean, S. Mapping the SERS Efficiency and HotSpots Localization on Gold Film over Nanospheres Substrates. J. Phys. Chem. C 2010, 114, 11717–11722. (36) Mahmoud, M. A.; Tabor, C. E.; El-Sayed, M. A. SurfaceEnhanced Raman Scattering Enhancement by Aggregated Silver Nanocube Monolayers Assembled by the Langmuir-Blodgett Technique at Different Surface Pressures. J. Phys. Chem. C 2009, 113, 5493–5501. (37) Mahajan, S.; Cole, R. M.; Speed, J. D.; Pelfrey, S. H.; Russell, A. E.; Bartlett, P. N.; Barnett, S. M.; Baumberg, J. J. Understanding the Surface-Enhanced Raman Spectroscopy “Background”. J. Phys. Chem. C 2010, 114, 7242–7250. (38) Saikin, S. K.; Chu, Y. Z.; Rappoport, D.; Crozier, K. B.; AspuruGuzik, A. Separation of Electromagnetic and Chemical Contributions to Surface-Enhanced Raman Spectra on Nanoengineered Plasmonic Substrates. J. Phys. Chem. Lett. 2010, 1, 2740–2746. (39) Bohn, J. E.; Le Ru, E. C.; Etchegoin, P. G. A Statistical Criterion for Evaluating the Single-Molecule Character of SERS Signals. J. Phys. Chem. C 2010, 114, 7330–7335.

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(40) Lombardi, J. R.; Birke, R. L.; Haran, G. Single Molecule SERS Spectral Blinking and Vibronic Coupling. J. Phys. Chem. C 2011, 115, 4540–4545. (41) Frontiera, R. R.; Henry, A.-I.; Gruenke, N. L.; Van Duyne, R. P. Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 1199–1203. (42) Touahir, L.; Jenkins, A. T. A.; Boukherroub, R.; GougetLaemmel, A. C.; Chazalviel, J.; Peretti, J.; Ozanam, F.; Szunerits, S. Surface Plasmon-Enhanced Fluorescence Spectroscopy on Silver Based SPR Substrates. J. Phys. Chem. C 2010, 114, 22582–22589. (43) Cameron, P. J.; Zhong, X.; Knoll, W. Electrochemically Controlled Surface Plasmon Enhanced Fluorescence Response of Surface Immobilized CdZnSe Quantum Dots. J. Phys. Chem. C 2009, 113, 6003– 6008. (44) Guo, P.-F.; Wu, S.; Ren, Q. J.; Lu, J.; Chen, Z.; Xiao, S.-J.; Zhu, Y.-Y. Fluorescence Enhancement by Surface Plasmon Polaritons on Metallic Nanohole Arrays. J. Phys. Chem. Lett. 2010, 1, 315–318. (45) Zheng, Y. B.; Jensen, L.; Yan, W.; Walker, T. R.; Juluri, B. K.; Jensen, L.; Huang, T. J. Chemically Tuning the Localized Surface Plasmon Resonances of Gold Nanostructure Arrays. J. Phys. Chem. C 2009, 113, 7019–7024. (46) Murray, W. A.; Auguie, B.; Barnes, W. L. Sensitivity of Localized Surface Plasmon Resonances to Bulk and Local Changes in the Optical Environment. J. Phys. Chem. C 2009, 113, 5120–5125. (47) Galopin, E.; Noual, A.; Niedziolka-Jonsson, J.; JonssonNedziolka, M.; Akjouj, A.; Pennec, Y.; Djafari-Rouhani, B.; Boukherroub, R.; Szunerits, S. Short- and Long-Range Sensing Using Plasmonic Nanostructures: Experimental and Theoretical Studies. J. Phys. Chem. C 2009, 113, 15921–15927. (48) Saha, A.; Basiruddin, S. K.; Sarkar, R.; Pradhan, N.; Jana, N. R. Functionalized Plasmonic-Fluorescent Nanoparticles for Imaging and Detection. J. Phys. Chem. C 2009, 113, 18492–18498. (49) Hu, R.; Yong, K. T.; Roy, I.; Ding, H.; He, S.; Prasad, P. N. Metallic Nanostructures as Localized Plasmon Resonance Enhanced Scattering Probes for Multiplex Dark-Field Targeted Imaging of Cancer Cells. J. Phys. Chem. C 2009, 113, 2676–2684. (50) Parab, H. J.; Chen, H. M.; Lai, T. C.; Huang, J. H.; Chen, P. H.; Liu, R. S.; Hsiao, M.; Chen, C. H.; Tsai, D. P.; Hwu, Y. K. Biosensing, Cytotoxicity, and Cellular Uptake Studies of Surface-Modified Gold Nanorods. J. Phys. Chem. C 2009, 113, 7574–7578. (51) Cole, J. R.; Mirin, N. A.; Knight, M. W.; Goodrich, G. P.; Halas, N. J. Photothermal Efficiencies of Nanoshells and Nanorods for Clinical Therapeutic Applications. J. Phys. Chem. C 2009, 113, 12090–12094. (52) Hafner, J. H.; Nordlander, P.; Weiss, P. S. Virtual Issue on Plasmonics. ACS Nano 2011, 5, 4245–4248. (53) Odom, T. W.; Schatz, G. C. Introduction to Plasmonics. Chem. Rev. 2011, 111, 3667–3668.

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