Papers in Physics, vol. 2, art. 020011 (2010) Received: 13 July 2010, Accepted: 10 January 2011 Edited by: V. Lakshminarayanan Licence: Creative Commons Attribution 3.0 DOI: 10.4279/PIP.020011 www.papersinphysics.org ISSN 1852-4249 Commentary on “Experimental determination of distance and orientation of metallic nanodimers by polarization dependent plasmon coupling” Sukhdev Roy1∗ The paper by H. E. Grecco and O. E. Martinez [1] describes an experimental technique to deter- mine (a) sub-diffraction distances based on near- field coupling of metallic nanoparticles, and (b) rel- ative orientations due to symmetry breaking in the scattering cross-section. The novelty is in sequen- tial illumination by two wavelengths to separate out the background from scattering from the nanopar- ticles, and in the illumination scheme to facilitate rotation of the polarization. The experimental re- sults are shown to be in good agreement with the- oretical predictions made earlier by the authors. The authors had earlier theoretically shown that the interparticle separation dependent polarization anisotropy of discrete nanoparticle dimers enables nanoscale distance measurements [2]. Their theo- retical approach has also been recently experimen- tally implemented to simultaneously measure dis- tance and orientation changes in discrete dimers of DNA linked nanoparticles [3]. In this Commentary, I briefly discuss some points to provide a better perspective of the contribution and make suggestions to improve the presentation. These suggestions have been taken on board by the authors and the published version of the paper is substantially improved. Noble metal nanoparticles have been intensively ∗E-mail: sukhdevroy@dei.ac.in 1 Department of Physics and Computer Science, Dayal- bagh Educational Institute, Dayalbagh, Agra 282110, In- dia studied, both theoretically and experimentally, for their unique optical properties. For a review of ap- plications in biosystems, readers can refer for ex- ample to Ref. [4]. The excitation of the localized surface plasmon (LSP) resonance by incident elec- tromagnetic radiation is responsible for a very large field enhancement at their surface. This singular and spatially localized optical response has proven to be of main interest for specific applications such as high sensitivity detection and spectroscopy of molecules suitably attached to the nanoparticle sur- face (fluorescence, Raman scattering and biochemi- cal sensing). For a single nanoparticle, the spectral features of the LSP are known to closely depend on its size, shape and dielectric environment. More re- cent studies have shown that interparticle coupling effects can be used for tailoring LSP resonances with even more flexibility. This has been clearly demonstrated for nanoparticle pairs which are the most basic systems of such interacting objects. Gold nanoparticles have stimulated tremendous research interest due to their unique optical proper- ties. It is well established that the optical response of an individual gold nanoparticle can not only be varied by changing the dielectric environment, but more dramatically by changing the nanoparticle ge- ometry itself. This has prompted extensive interest in the synthesis and characterization of the opti- cal response of a wide variety of gold nanoparticle structures such as shells [5], rods [6,7], stars [8] and dumbbells [9], with the prime objective of gener- ating a controlled plasmon resonance at a desired 020011-1 Papers in Physics, vol. 2, art. 020011 (2010) / S. Roy wavelength. Dimers [1–3], chains [10] and other arrays of nanoparticles are alternative methods of controlling the position or shape of the plasmon res- onance. The optical properties of these arrays can be analyzed approximately in two regimes: far-field where the interparticle gap is large and the near field where the gap is sufficiently small so that the near fields of the particles are coupled. In the near- field regime the plasmon resonance can be tuned deep into the infrared by decreasing the interparti- cle gap, resulting in a very strong enhancement of the electric field between the particles. The authors address the important problem of experimental determination of distance and orien- tation of gold nanoparticles by measuring scatter- ing by illumination with polarized light. They show that scattering polarization microscopy of coupled nanoparticles (with radius in the range of 4-20 nm) can provide an alternative method to fluorescence resonance energy transfer (FRET) and standard super-resolution techniques. The technique helps in filling the gap for distance and orientation mea- surements by these techniques, as in this range, the particles are closer than the resolution limit of the microscope and will appear as a single spot on the detector. The distance in which the technique is sensitive scales with the radii of the particles. The authors claim that the proposed technique can be an alternative to fluorescence based techniques when photostability, frame rate or coupling range are insufficient. It is interesting to note that the use of two-color imaging can provide an efficient, faster and reliable way to detect scattering centers that have plasmon resonances. The authors should highlight the importance of the proposed contribution in the Introduction section. It should include the importance of (i) spherical metal nanoparticles and plasmon reso- nances, which are an indispensable tool for exam- ining optical near-fields, imaging and sensing and (ii) distance and orientation measurements, which are very important to understand many biologi- cal systems, such as molecular structural dynamics and conformational transitions in proteins. Gold nanoparticles, in addition to their enhanced ab- sorption and scattering, and usefulness as contrast agents in cellular and biological imaging, offer good biocompatibility, facile synthesis and conjugation to a variety of biomolecular ligands, antibodies and other targeting moieties, making them suitable for use in biochemical sensing and detection, medical diagnostics and therapeutic applications. Since the proposed technique is based on scat- tering polarization microscopy, it would be in the interest of readers to also mention scanning parti- cle enhanced Raman microscopy. Since the spec- tral properties of the overall plasmon resonance of two coupled spherical metal nanoparticles are sub- ject to their material, shape, size, orientation, dis- tance and surrounding medium, modulation of the spectral position and the spectral line width can be used to estimate the distance between two cou- pled metallic nanoparticles. Important related ref- erences, such as, Reinhard et al., Nano Lett. 5, 2246 (2005) and Olk et al., Nano Lett. 8, 1174 (2008) can be included to make the paper more comprehensive. The details of the experimental setup should in- clude parameters such as the powers or intensities of the laser beams used, the spot size of the beams at the object plane, accuracy in the angular mea- surements, beam splitting ratio, bandwidth of the filters used, etc. to facilitate better understanding of the experiment. It should be clarified whether black and white or color recordings were made. A color CCD camera can improve the identification of particles by efficient measurement of scattered in- tensities and their color. A discussion on the photo- stability of the proposed technique, which has been mentioned as the main advantage over fluorescence and super-resolution based techniques in the Re- sults and Discussion section, would provide justi- fication and highlight the importance of scattering polarization microscopy. Since it is an experimental study, a discussion on the various factors that can lead to uncertainty in the distance and orientation measurement, such as non-uniformity of the particles (both size and shape), noise, accuracy of the measurement of an- gles etc., is required to clearly assess the limitations and help in future efforts to improve the technique. Since relative error depends on distance, it would be good to mention where the best spatial resolu- tion for inter-particle separation occurs for 20 nm Au plasmon ruler. The efficient use of two color imaging to detect scattering centers demonstrated in the illumina- tion setup by Grecco and Mart́ınez in combination with other techniques, provides a robust anisotropy based microscopic tool for further studies. 020011-2 Papers in Physics, vol. 2, art. 020011 (2010) / S. Roy [1] H E Grecco, O E Mart́ınez, Experimental determination of distance and orientation of metallic nanodimers by polarization depen- dent plasmon coupling, Pap. Phys. 2, 020010 (2010). [2] H E Grecco, O E Mart́ınez, Distance and ori- entation measurement in the nanometric scale based on polarization anisotropy of metallic dimers, Opt. Exp. 14, 8716 (2006). [3] H Wang, B M Reinhard, Monitoring simulta- neous distance and orientation changes in dis- crete dimers of DNA linked gold nanoparticles, J. Phys. Chem. C 113, 11215 (2009). [4] P K Jain, X Huang, I H El-Sayed, M A El- Sayed, Review of some interesting surface plas- mon resonance-enhanced properties of noble metal nanoparticles and their applications to biosystems, Plasmonics 2, 107 (2007). [5] S J Oldenburg, R D Averitt, S L Westcott, N J Halas, Nanoengineering of optical resonances, Chem. Phys. Lett. 288, 243 (1998). [6] C J Murphy, T K Sau, A M Gole, C J Oren- dorff, J Gao, L Gou, S E Hunyadi, T Li, Anisotropic metal nanoparticles: Synthesis, assembly, and optical applications, J. Phys. Chem. B 109, 13857 (2005). [7] L S Slaughter, Y Wu, B A Willingham, P Nordlander, S Link, Effects of asymmetry breaking and conductive contact on the plas- mon coupling in gold nanorod dimers, ACS Nano 4, 4657 (2010). [8] C L Nehl, H Liao, J H Hafner, Optical prop- erties of star-shaped gold nanoparticles, Nano Lett. 6, 683 (2006). [9] D K Lim, K S Jeon, H M Kim, J M Nam, Y D Suh, Nanogap-engineerable Raman-active nanodumbbells for single-molecule detection, Nat. Mater. 9, 60 (2010). [10] N Harris, M D Arnold, M G Blaber, M J Ford, Plasmonic resonances of closely coupled gold nanosphere chains, J. Phys. Chem. C 113, 2784 (2009). 020011-3