Functionalization of Ag Nanoparticles with Dithiocarbamate Calix[4]arene As an Effective Supramolecular Host for the Surface-Enhanced Raman Scattering Detection of Polycyclic Aromatic Hydrocarbons Luca Guerrini, Jose ´ V. Garcia-Ramos, Concepcio ´n Domingo, and Santiago Sanchez-Cortes* Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006-Madrid, Spain ReceiVed August 1, 2006. In Final Form: October 13, 2006 We report the use of 25,27-diethyl-dithiocarbamic-26,28-dihydroxy-p-tert-butylcalix[4]arene in the functionalization of Ag nanoparticles for pyrene detection by surface-enhanced Raman scattering (SERS). SERS spectra provided information about the calixarene orientation on the metal surface and the interaction mechanism with pyrene. Thus, in this work, we have combined a powerful spectroscopy technique such as SERS, the electronic plasmon-based properties of nanostructured metals, the molecular size-selective recognition of calixarene, and the strong chelating properties of the dithiocarbamate group toward the metal surface in the detection of polycyclic aromatic hydrocarbons. Calixarenes (CXs) are synthetic cyclooligomers with a “cup- like” shape, capable of size-selective molecular encapsulation. By changing the chemical groups of the upper and/or lower rim, it is possible to modify their affinity toward the guest and the metal surface. 1 Surface-enhanced Raman spectroscopy (SERS) is a extremely highly sensitive analytical technique based on the giant electromagnetic enhancement induced by nanostructured metal surfaces via plasmon resonance. This technique has been extensively used in the identification and orientation of adsorbates on a surface. 2 The combination of the physical properties of nanostructured metal and the advanced chemical properties of self-assembled CXs is a subject of great interest in supramolecular science, with promising applications in different fields such as nanotechnology, environment, chemical sensing, and so forth. Polycyclic aromatic hydrocarbons (PAHs) are a group of pollutants with condensed benzene rings formed during the incomplete combustion of coal, oil, and gas or other organic substances such as tobacco or charbroiled meat. Many of them have been reported to be strong carcinogens. 3 These molecules show very low affinity for adsorption on a metallic surface, thus limiting the use of surface-enhanced techniques in their detection. However, in recent works, 4,5 CXs with carboethoxy groups in the lower rim have been successfully applied in the detection of PAHs by using the SERS technique. 4,5 The strong interaction of dithiocarbamates (DTs) with metals has been known for quite some time, and it is related to the ability of the molecules to form chelate complexes. 6 DT is able to strongly interact with the surface of metals, as demonstrated in previous studies on the adsorption of DT containing fungicides on nanostructured metal surfaces. 7-9 Thus, the combination of the good host properties of CX and the high affinity of the DT group in the same molecule, self-assembled on a nanostructured metal, should be a good strategy to design new sensitive and selective surfaces for the detection of PAHs. In this work, we report the use of DTCX-functionalized Ag nanoparticles, prepared by the chemical reduction of AgNO 3 by hydroxylamine, in the pyrene detection by SERS spectroscopy. The synthesis of the 25,27-diethyl-dithiocarbamic-26,28-di- hidroxy-p-tert-butylcalix[4]arene (DTCX) was carried out from the corresponding amino calixarene (AmCX) following the method described by Zhao et al. 10 (Figure 1, inset) (see Supporting Information for experimental details). DTCX metal function- alization represents a remarkable improvement in the detection limit of pyrene in relation to carboethoxy CX. 4,5 In Figure 1, the Raman and IR spectra of AmCX and DTCX in the solid state are compared. The IR spectrum of DTCX (Figure 1d) shows new bands at 1522, 1034, 967, 678, and 485 cm -1 , corresponding to the DT group. The Raman spectrum of DTCX (Figure 1b) shows less differences with respect to AmCX, due to the polar character of DT. In fact, only an intensity decrease in the band at 1236 cm -1 (assigned to a C-N stretching mode or a twisting of the CH 2 group close to the NH 2 one 11 ), the new band at 963 cm -1 (assigned to the ν(C-S) mode), and the changes observed in the bands at 698 and 668 cm -1 are observed. The latter bands are actually due to the DT group and other groups already existing in AmCX, since two bands at approximately the same wavenumber appear in the solid of AmCX (Figure 1a). Thus they were not considered to evaluate the adsorption of DTCX on the Ag surface. The SERS spectrum of DTCX (Figure 2c) is much more intense than that of AmCX (spectrum not shown), thus indicating the existence of a stronger interaction of DTCX with the metal. This interaction takes place through the S atoms, as indicated by the band at 198 cm -1 (Figure 2, inset), attributed to the Ag-S stretching motion. 8 A consequence of the strong interaction of * Corresponding author. Address: Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain. Fax: + 34 91 5 64 55 57. Phone: + 34 91 5 61 68 00. E-mail: imts158@iem.cfmac.csic.es. (1) Houk, K. N.; Leach, G. L.; Kim, S. P.; Zhang, X. Angew. Chem., Int. Ed. 2003, 42, 4872. (2) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783. (3) Harvey, R. G. Polycyclic Aromatic Hydrocarbons; John Wiley & Sons: New York, 1997. (4) Leyton, P.; Domingo, C.; Sanchez-Cortes, S.; Campos-Vallette, M.; Garcı ´a- Ramos, J. V. Langmuir 2005, 21, 11814. (5) Leyton, P.; Sanchez-Cortes, S.; Campos-Vallette, M.; Domingo, C.; Garcı ´a- Ramos, J. V.; Saitz, C. Appl. Spectrosc. 2005, 59, 1009. (6) Thorn, G. D.; Ludwig, R. A. The Dithiocarbamates And Related Compounds; Elsevier Monographs: New York, 1962. (7) Sanchez-Cortes, S.; Vasina, M.; Francioso, O.; Garcı ´a-Ramos, J. V. Vib. Spectrosc. 1998, 17, 133. (8) Sanchez-Cortes, S.; Domingo, C.; Garcı ´a-Ramos, J. V.; Aznarez, J. A. Langmuir 2001, 17, 1157. (9) Morf, P.; Raimondi, F.; Nothofer, H. G.; Schnyder, B.; Yasuda, A.; Wessels, J. M.; Jung, T. A. Langmuir 2006, 22, 658. (10) Zhao, Y.; Perez-Segarra, W.; Shi, Q.; Wei, A. J. Am. Chem. Soc. 2005, 127, 20, 7328. (11) Socrates, G. Infrared and Raman Characteristic Group Frequencies; John Wiley&Sons, Ltd.: New York, 2004. 10924 Langmuir 2006, 22, 10924-10926 10.1021/la062266a CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2006