NANO LETTERS
Reply to “Comment on ‘Gold Nanoshells Improve Single Nanoparticle Molecular Sensors’”
2005 Vol. 5, No. 4 811-812
G. Raschke,* S. Brogl, A. S. Susha, A. L. Rogach, T. A. Klar, and J. Feldmann Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Amalienstr. 54, D-80799 Munich, Germany
B. Fieres, N. Petkov, and T. Bein Department of Chemistry and CeNS, Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Butenandtstr. 5-13, D-81377 Munich, Germany
A. Nichtl and K. Ku1 rzinger Roche Diagnostics GmbH, Nonnenwald 2, D-82372 Penzberg, Germany Received January 21, 2005
In their comment, Zhang et al. point out that in principle both gold shells and gold nanoparticle aggregates could give rise to the near-IR absorption and scattering bands that are observed after the HAuCl4-Na2S reaction. In fact, we do observe solitary and randomly clustered small (5-10 nm) gold nanoparticles. However, in TEM studies we also observe a substantial amount of spherical particles of approximately 40 nm in diameter, which we and the groups of Zhou and Halas claim to be Au2S/Au core/shell structures.1-5 We believe that our work,6 which was initially intended to report on improved single nanoparticle molecular sensors using nanoparticles prepared with the HAuCl4-Na2S reaction, can add an important tessera to the dispute of the products of this reaction: In general, if there are many different products coming out of a reaction like the HAuCl4-Na2S reaction, ensemble measurements may not provide reliable information. Single nanoparticle measurements remove the ensemble averaging and provide an insight into the properties of individual particles.7-10 In our experiments we used a polarizer in front of the detector. All spectra reported in our letter6 are taken from single nanoparticles that show a scattering spectrum that is essentially independent of the orientation of the polarizer. Therefore, the single nanoparticles under investigation must be spherically symmetric. Additionally, the spectra show a narrow line width of only 200 meV. Both arguments together make it very unlikely that our reported spectra could stem from aggregates. In contrary, they can easily be reproduced by Mie theory calculations, assuming a dielectric core and a gold shell. Testing different reaction conditions during the synthesis revealed that the relative amount of large spherical particles compared to small nanoparticles depends very critically on the route of synthesis. In particular, we want to stress that many ions such as S2-, HS-, and SO42- can play a decisive 10.1021/nl050130b CCC: $30.25 Published on Web 02/18/2005
© 2005 American Chemical Society
role in the synthesis, not only H2S as is claimed by Zhang et al., especially when an aged solution of Na2S is used, which is a prerequisite for successful synthesis. Therefore, there are certainly several alternatively possible reaction pathways in addition to the pathway 2HAuCl4 + 3H2S ) 3S + 2Au + 8H+ + 8Cl-. If the latter would be the only possible route, only 1/3 of the used gold salt would be reduced in the synthesis by Zhang et al.11 This is inconsistent with our ensemble extinction measurements showing that the absorption peak of gold salt at approximately 300 nm disappears completely during synthesis. Instead, our ensemble spectra taken after the first addition of H2S as well as those taken by Zhou and Halas are consistent with the formation of dielectric nanocrystals but not with the formation of metallic nanoparticles. Surprisingly, Zhang and coworkers never show an optical ensemble spectrum at this stage of the synthesis. We also want to note that our sample solutions are stable for a few months without any addition of stabilizers. Contrary, Zhang et al. report that they observe a reddish precipitate 48 h after the reaction.11 A further strong evidence of the formation of Au2S is provided by electron diffraction performed by Zhou and Halas.1,5 Nonetheless we agree with Zhang et al. that supplementary experiments must be performed for further clarification of the possible routes of synthesis and the products of the HAuCl4-Na2S reaction. For example a 1:1 comparison of gold nanoparticles in the electron microscope and the dark field microscope has recently been performed on gold nanoshells that were prepared using the SiO2/Au seed route.12 It has been shown that the narrow scattering spectra indeed stem from spherical nanoparticles and not from aggregates. In summary, our single nanoparticle measurements strongly support the claim of Zhou and Halas that gold nanoshells are a product of the HAuCl4-Na2S reaction.
Note Added after ASAP Publication. Reference 12 was corrected. This paper was published ASAP on 2/18/05. The corrected version was posted on 3/16/05. References (1) Zhou, H. S.; Honma, I.; Komiyama, H.; Haus, J. W. Phys. ReV. B 1994, 50, 12052. (2) Averitt, R. D.; Sarkar, D.; Halas, N. J. Phys. ReV. Lett. 1997, 78, 4217. (3) Averitt, R. D.; Westcott, S. L.; Halas, N. J. Phys. ReV. B 1998, 58, R10203. (4) Averitt, R. D.; Westcott, S. L.; Halas, N. J. J. Opt. Soc. Am. B-Opt. Phys. 1999, 16, 1814. (5) Averitt, R. D.; Westcott, S. L.; Halas, N. J. J. Opt. Soc. Am. B-Opt. Phys. 1999, 16, 1824. (6) Raschke, G.; Brogl, S.; Susha, A. S.; Rogach, A. L.; Klar, T. A.; Feldmann, J.; Fieres, B.; Petkov, N.; Bein, T.; Nichtl, A.; Ku¨rzinger, K. Nano Lett. 2004, 4, 1853.
812
(7) Klar, T.; Perner, M.; Grosse, S.; von Plessen, G.; Spirkl, W.; Feldmann, J. Phys. ReV. Lett. 1998, 80, 4249. (8) So¨nnichsen, C.; Geier, S.; Hecker, N. E.; von Plessen, G.; Feldmann, J.; Ditlbacher, H.; Lamprecht, B.; Krenn, J. R.; Aussenegg, F. R.; Chan, V. Z. H.; Spatz, J. P.; Mo¨ller, M. Appl. Phys. Lett. 2000, 77, 2949. (9) So¨nnichsen, C.; Franzl, T.; Wilk, T.; von Plessen, G.; Feldmann, J.; Wilson, O.; Mulvaney, P. Phys. ReV. Lett. 2002, 88, 077402. (10) Mu¨ller, J.; So¨nnichsen, C.; von Poschinger, H.; von Plessen, G.; Klar, T. A.; Feldmann, J. Appl. Phys. Lett. 2002, 81, 171. (11) Norman, T. J.; Grant, C. D.; Magana, D.; Zhang, J. Z.; Liu, J.; Cao, D. L.; Bridges, F.; Van Buuren, A. J. Phys. Chem. B 2002, 106, 7005. (12) Nehl, C. L.; Grady, N. K.; Goodrich, G. P.; Tam, F.; Halas, N. J.; Hafner, J. H. Nano Lett. 2004, 4, 2355.
NL050130B
Nano Lett., Vol. 5, No. 4, 2005
