NANO LETTERS
Comment on “Gold Nanoshells Improve Single Nanoparticle Molecular Sensors”
2005 Vol. 5, No. 4 809-810
Jin Z. Zhang,*,† Adam M. Schwartzberg,† Thaddeus Norman, Jr.,‡ Christian D. Grant,§ Jun Liu,| Frank Bridges,⊥ and Tony van Buuren# Department of Chemistry, UniVersity of California, Santa Cruz, California 95064, Lawrence LiVermore National Laboratories, P.O. BOX 808, L-370, LiVermore, California 94551, Department of Chemistry and Chemical Biology, Rutgers, The State UniVersity of New Jersey, Piscataway, New Jersey 08854, Sandia National Laboratories, Biomolecular Materials and Interfaces Department 1140, Mail Stop 1413, P.O. Box 5800, Albuquerque, New Mexico 87185, Department of Physics, UniVersity of California, Santa Cruz, California 95064, and Lawrence LiVermore National Laboratories, 7000 East AVe., LiVermore, California 94550 Received December 13, 2004
In a recently published article in Nano Letters by Raschke et al. entitled “Gold Nanoshells Improve Single Nanoparticle Molecular Sensors,”1 the authors present data from nanostructure samples generated from the HAuCl4-Na2S reaction. While experimentally the authors present some interesting light scattering data, their interpretation of this system as Au2S/Au “core/shell” in structure is most likely not accurate. The authors used the “shell” model, but there is no convincing evidence (for example, microscopy or analytical data) to fully justify this model. In addition, the paper did not refer to several papers we have published on the same reaction. This reaction has been studied systematically and extensively by us in the past few years, and we believe that we have clearly and consistently shown that this reaction does not produce Au2S/Au “core/shell” structures, but instead generates aggregates of gold nanoparticles. In 1994, Zhou and co-workers in Japan published the first paper on the named reaction above and proposed a model of Au2S/Au “core/shell” structure to explain the new nearIR absorption band based on mainly UV-visible-near-IR spectroscopy and some TEM and electron-diffraction data.2 This work was followed by several publications by other groups, all suggesting a Au2S/Au “core/shell” structure;3-5 however, no direct structural evidence was presented. About five years ago, we found out that the suggested Au2S/Au “core/shell” structures could not be verified, even though the UV-visible-near-IR spectroscopy data could be reproduced. Over the course of two years of research that included different measurements (EELS, HRTEM, XAFS, * Corresponding author. E-mail:
[email protected]. † Dept. of Chemistry, U.C., Santa Cruz. ‡ LLNL, P.O. Box 808. § Rutgers University. | Sandia National Laboratories. ⊥ Dept. of Physics, U.C., Santa Cruz. # LLNL, 7000 East Ave. 10.1021/nl0479379 CCC: $30.25 Published on Web 02/18/2005
© 2005 American Chemical Society
dynamic light scattering, AFM, optical spectroscopy) of many samples, we concluded that the only consistent explanation for the near-IR absorption band is an aggregate structure.6 We found no evidence of Au2S/Au “core/shell” structure whatsoever. Later, the aggregate model was further supported by ultrafast dynamics and hole burning studies.7 Furthermore, recently the structure of the aggregates has been studied by small-angle X-ray scattering.8 We have more recently demonstrated that these aggregates are excellent SERS substrates.9,10 Our data clearly show that there are no discrete Au2S nanoparticles formed from the reaction of HAuCl4 with Na2S. The data also indicate that virtually all of the gold atoms present in the system exist in a gold lattice with only a few gold atoms possessing an additional sulfur coordination shell on the gold nanoparticle surface. In other words, a very thin sulfur layer covers the gold nanoparticle aggregates. Furthermore, it is well-known that the reaction of aqueous Au3+ at room temperature with sulfides (e.g., Na2S or H2S) never produces Au2S, but rather Au0.11 We have also independently studied the ultrafast relaxation processes and structure of Au2S nanoparticles.12 The ultrafast dynamics data on the reaction product of HAuCl4 with Na2S were consistent with that of gold nanoparticles but not that of Au2S nanoparticles or of Au2S nanoparticles with a Au shell. The model of aggregates of gold nanoparticles is the most consistent with all the observed spectroscopic and structural data from our and other labs. After our publication of the aggregate model in 2002,6 Prodon et al. published a theoretical study of gold nanoshell structures supporting the Au2S/Au “core/shell” structure in the HAuCl4 + Na2S reaction in 2003.13 This paper did not reference our published work even though one co-author is fully aware of our published model of aggregates. We should point out that in principle both gold shells and gold
nanoparticle aggregates could give rise to the near-IR absorption band. However, our structural and optical results clearly support the aggregate model for the product of the HAuCl4 + Na2S reaction. In summary, our structural studies strongly disagree with the proposed Au2S/Au “core/shell” model from the HAuCl4+ Na2S reaction. All the data we obtained support the aggregate model. Nanoshells and nanoparticle aggregates are fundamentally different, and the differences have significant implications in their properties and functionalities. Gold nanoshells may be synthesized in some other cases, but not in the HAuCl4+Na2S reaction. We hope that this communication can stimulate further discussion on this topic and help us arrive at an accurate model for this important reaction. References (1) Raschke, G.; Brogl, S.; Susha, A. S.; Rogach, A. L.; Klar, T. A.; Feldmann, J.; Fieres, B.; Petkiv, N.; Bein, T.; Nichtl, A.; Kurzinger, K. Nano Lett. 2004, 4, 1853.
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(2) Zhou, H. S.; Honma, I.; Komiyama, H.; Haus, J. W. Phys. ReV. B 1994, 50, 12052. (3) Averitt, R. D.; Sarkar, D.; Halas, N. J. Phys. ReV. Lett. 1997, 78, 4217. (4) Averitt, R. D.; Westcott, S. L.; Halas, N. J. Phys. ReV. B-Condens. Matter 1998, 58, 10203. (5) Averitt, R. D.; Westcott, S. L.; Halas, N. J. J. Opt. Soc. Am. B-Opt. Phys. 1999, 16, 1814. (6) Norman, T. J., Jr.; Grant, C. D.; Magana, D.; Zhang, J.; Liu, J.; Cao, D.; Bridges, F.; van Buuren, A. J. Phys. Chem. B 2002, 106, 70057012. (7) Grant, C. D.; Schwartzberg, A. M.; Norman, T. J., Jr.; Zhang, J. Z. J. Am. Chem. Soc. 2003, 125, 549. (8) Norman, T. J., Jr.; Grant, C. D.; Zhang, J. Z. Opt. Mater., in press. (9) Schwartzberg, A. M.; Grant, C. D.; Wolcott, A.; Talley, C.; Huser, T.; Bogomolni, R.; Zhang, J. Z. J. Phys. Chem. B 2004, 108, 19191. (10) Schwartzberg, A. M.; Wolcott, A.; Willey, T.; van Buuren, A.; Zhang, J. Z. SPIE Proc. 2004, 5513, 213. (11) Puddephatt, R. J. The Chemistry of Gold; Elsevier: Amsterdam, 1978, Chapter 2. (12) Grant, C. D.; Norman, T. J., Jr.; Morris, T.; Szulczewski, G.; Zhang, J. Z. SPIE Proc. 2002, 4807, 216-222. (13) Prodan, E.; Nordlander, P.; Halas, N. J. Nano Lett. 2003, 3, 1411.
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Nano Lett., Vol. 5, No. 4, 2005
