Guest Commentary pubs.acs.org/JPCL
Surface Plasmons as Versatile Analytical Tools
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The Perspective by Khatua and Orrit focuses on a simple plasmonic nanostructure for sensing, single gold nanorods. They argue that although larger electric fields can be generated by more complex nanostructures and in particular in nanosized gaps between nanoparticles, nanorods present a versatile sensing platform because of: (1) their ease of chemical preparation9,10 and surface functionalization for targeting specific analyte molecules, (2) narrow and tunable plasmon resonance with minimized Ohmic dissipation in the nearinfrared region, and (3) consistent, well-understood, and aspect ratio controllable electric field enhancement. They show that adsorption of single protein molecules can be detected through shifts in the plasmon absorption of a single nanorod measured with high sensitivity by photothermal contrast. Individual, weakly fluorescent molecules are amplified by up to 1000 times in the vicinity of single gold nanorods, making it possible to detect fluorescence from a single molecule that otherwise is too weak of an emitter to be imaged. Similar to the plasmonic optical trap, detection volumes can therefore be drastically reduced as the fluorescence enhancement is confined to the local electric field. Alternatively, the authors show how the nonblinking and nonbleaching luminescence of the nanorods themselves can be used as a local orientational probe. One drawback of plasmonics in the visible wavelength range is that the metals typically used such as silver and gold are not perfect conductors and nonradiative losses lead to ohmic heating of the nanostructures and their surroundings. Both Perspective articles address this aspect and point out that excessive heating can indeed be detrimental for applications: nanorods reshape into spheres; and in a plasmonic optical trap thermophoresis acts against the trapping force, whereas thermal convection assists the supply of molecules toward the trap. However, it takes significant heating to completely turn rods into spheres and thermal effects in plasmonic optical trapping can be mitigated by adding a heat sink or choosing appropriate densities of plasmonic nanostructures per illumination area. Furthermore, as clearly demonstrated by the achievements of both groups, effects due to photothermal heating, though important to consider, can certainly be mostly overcome. The outlook that both sensing approaches offer, namely the possibility of detection and spectroscopy of individual molecules free in solution either passing by a gold nanorod as in fluorescence enhanced correlation spectroscopy or held in a plasmonic optical trap without the presence of an interfering nearby substrate, is extremely exciting for the future of Physical Chemistry.
nderstanding and exploiting the interaction between electromagnetic radiation and matter represents a major theme in Physical Chemistry. The wavelength of the radiation typically determines the smallest spatial dimensions that can be sampled and resolved. For visible light, this criterion corresponds to hundreds of nanometers, much larger than molecules, polymers, and most nanoparticles. Surface plasmons, the collective excitation of conduction band electrons in metallic nanostructures,1 are often exploited to overcome this limitation because the electric fields created by plasmon excitation are strongly localized within nanometer dimensions at the nanostructure surface. In addition to strong spatial localization, these electric fields amplify both incoming and outgoing radiation, making plasmonic nanostructures ideal antennas for the nanoscale manipulation of light as plasmon resonances and local fields can be straightforwardly tuned through nanostructure material, size, and shape2 as well through near-field coupling in nanoparticle arrays.3 Among the different applications of plasmonic nanostructures, sensing of analyte molecules takes particular advantage of these local electric fields in two complementary ways: (1) spectroscopic signals of analyte molecules are strongly enhanced such as in surface-enhanced Raman spectroscopy;4 (2) the presence of molecules near the nanostructure surface modify the plasmon resonance through their dielectric properties. In the latter case, detection is achieved through shifts in usually the plasmon scattering wavelength and is well known as plasmon resonance sensing.5,6 In this current issue of The Journal of Physical Chemistry Letters, two excellent Perspective articles by Shoji and Tsuboi7 and Khatua and Orrit8 explore, in quite different ways, how the electric fields of plasmonic nanostructures can be exploited to study single molecules, the ultimate detection limit in sensing. The Perspective by Shoji and Tsuboi advocates the use of patterned plasmonic substrates with strong local electric fields in an optical trap. Optical trapping of small molecules is difficult because of their small polarizability. Although the trapping strength can be increased by using larger laser intensities, a plasmonic optical trap achieves not only deeper but also steeper potentials as the object is also subjected to the gradient forces created through the excitation of the more strongly confined and enhanced local electromagnetic hot spot compared to only the laser focus in a conventional optical trap. The authors discuss chiral nanostructures as substrates for plasmonic optical trapping, a comparison between femtosecond and continuous wave trapping lasers, and resonant plasmonic optical trapping. In addition, plasmonic optical trapping combined with fluorescence microspectroscopy enabled them to follow conformational changes of flexible polymer chains. Proteins, polymers, and DNA can already be trapped, and it is expected that even the manipulation of much smaller molecules will be possible. An interesting possibility is to expand on the function of the plasmonic substrate and use it also as a sensor while molecules are trapped. © 2014 American Chemical Society
Stephan Link
Department of Chemistry, Department of Electrical and Computer Engineering, Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States Published: September 4, 2014 3007
dx.doi.org/10.1021/jz501665j | J. Phys. Chem. Lett. 2014, 5, 3007−3008
The Journal of Physical Chemistry Letters
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Guest Commentary
AUTHOR INFORMATION
Notes
Views expressed in this Editorial are those of the author and not necessarily the views of the ACS.
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REFERENCES
(1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (2) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668−677. (3) Slaughter, L.; Chang, W.-S.; Link, S. Characterizing Plasmons in Nanoparticles and Their Assemblies with Single Particle Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 2015−2023. (4) Stiles, P. L.; Dieringer, J. A.; Shah, N. C.; Van Duyne, R. R. Surface-Enhanced Raman Spectroscopy. Annu. Rev. Anal. Chem. 2008, 1, 601−626. (5) Ament, I.; Prasad, J.; Henkel, A.; Schmachtel, S.; Sönnichsen, C. Single Unlabeled Protein Detection on Individual Plasmonic Nanoparticles. Nano Lett. 2012, 12, 1092−1095. (6) Mayer, K. M.; Hafner, J. H. Localized Surface Plasmon Resonance Sensors. Chem. Rev. 2011, 111, 3828−3857. (7) Shoji, T.; Tsuboi, Y. Plasmonic Optical Tweezers toward Molecular Manipulation: Tailoring Plasmonic Nanostructure, Light Source, and Resonant Trapping. J. Phys. Chem. Lett. 2014, 5, 2957− 2967. (8) Khatua, S.; Orrit, M. Probing, Sensing and Fluorescence Enhancement with Single Gold Nanorods. J. Phys. Chem. Lett. 2014, 5, 3000−3006. (9) Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method. Chem. Mater. 2003, 15, 1957−1962. (10) Jana, N. R.; Gearheart, L.; Murphy, C. J. Wet Chemical Synthesis of High Aspect Ratio Cylindrical Gold Nanorods. J. Phys. Chem. B 2001, 105, 4065−4067.
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dx.doi.org/10.1021/jz501665j | J. Phys. Chem. Lett. 2014, 5, 3007−3008
