Chirality, Optical Activity, and Enantiospecific Adsorption in Gold Nanoparticles

Ignacio L. Garz?n

Instituto de F?sica, Universidad Nacional Aut?noma de México,

Apartado Postal 20-364, 01000 México, D. F., México

Chirality is a fundamental property of many bio-organic molecules and organometallic compounds, with great relevance in several fields like the research on the origin of life, or in the pharmacology industry. Chirality in metal nanoparticles was not expected, however recent experimental results on the structural, electronic and optical properties of bare and organically-modified gold nanoclusters suggested the existence of chiral structures for these systems [1,2]. Theoretical studies had also provided support on the energetic stability and optical activity of chiral isomers of bare and passivated gold nanoclusters with different size [3-5].

In this work, we will describe a theoretical study on the structural, vibrational, electronic, and optical properties of chiral gold nanoclusters. Firstly, we consider the case of the Au₃₄^- cluster for which extensive experimental studies on its structural and electronic behavior had been published recently [1,6,7]. Our results show that the lowest-energy isomers of the Au₃₄^- cluster correspond to two chiral structures with C₁ and C₃ point symmetry groups [7]. The two isomers are nearly degenerate in energy, being the C₁ isomer more stable than the C₃ one by only 0.031 eV. Zero point energies and vibrational entropy at finite temperatures do not produce any change in the energy ordering between these isomers. Moreover, the calculated structure factors, which have been measured using trapped ion electron diffraction [1], indicate that both isomers are almost indistinguishable [7]. On the other hand, the electronic DOS of these chiral isomers shows different features around the HOMO-LUMO energy gap, which may be detected through optical spectroscopies. In fact, our calculated absorption and circular dichroism spectra show clear differences in the optical behavior of these chiral clusters [7]. Another important property that distinguishes the C₁ and C₃ isomers is the different spatial distribution of the atomic coordination on the cluster surface, which would generate distinct enantiospecific adsorption patterns with chiral molecules [7].

Enantiospecific adsorption of a chiral amino acid (cysteine) on a C₁ (chiral) isomer of the Au₅₅ nanocluster had also been theoretically predicted using DFT calculations [8]. The physical origin of this effect is related with the different bond strength and location of the carboxyl group, forming the L- and D- enantiomers of cysteine, when it is adsorbed on one of edges of the Au₅₅ cluster [8]. In this work, the most active sites on the Au₅₅ chiral cluster surface for the adsorption of the thiolate, amino and carboxyl groups of the cysteine will be described. The relation of these theoretical results with the optical activity measured on chiral monolayer-protected gold nanoclusters will also be discussed.

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[2] C. Noguez, I.L. Garz?n Chem. Soc. Rev.  DOI: 10.1039/b800404h (2009).             

[3] I. L. Garz?n et al. Phys. Rev B 66, 073403 (2002).

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[5] C.E. Rom?n-Velazquez, C. Noguez, and I.L. Garz?n J. Phys. Chem. B 107, 12035 (2003).

[6] X. Gu et al. J. Phys. Chem. C 111, 8228 (2007).

[7] I. E. Santizo, F. Hidalgo, L. A. Pérez, C. Noguez, I.L. Garz?n J. Phys. Chem. C 112, 17533 (2008).

[8] X. L?pez-Lozano, L. A. Pérez, and  I. L. Garz?n Phys. Rev. Lett. 97, 233401 (2006)