978-1-4673-8156-7/15/$31.00 ©2015 IEEE Protected Hollow Metal Nanoresonators for Raman Analysis of Surfaces Heman Burhnalden Abdulrahman, Jan Krajczewski, Karol Kołątaj, Andrzej Kudelski Department of Chemistry, Warsaw University, Pasteur 1, PL-02-093 Warsaw, Poland e-mail (AK): akudel@chem.uw.edu.pl Abstract: In this contribution we report the first example of Raman analysis of various surfaces using hollow-silver and hollow-gold nanoparticles protected by the silica layer. In contrast to solid spherical gold nanoparticles typically used for construction of nanoresonators for shell-isolated nanoparticle- enhanced Raman spectroscopy (SHINERS) measurements, for which a change of the diameter of the nanoparticle causes only a small change of the position of its plasmon band, the plasmon band for hollow spherical gold nanoparticles can be changed within broad range of the visible electromagnetic radiation. It suggests that silica-covered hollow-gold nanoresonators can be used in a broader frequency range than previously used "solid" nanoparticles. We also found that the samples of hollow Au or Ag nanoparticles induce, on average, stronger enhancement of the Raman signal than the respective samples of solid nanoparticles synthesized from the same amount of metal. Keywords: surface-enhanced Raman scattering; SERS; hollow- nanoparticles; core-shell nanoparticles; shell-isolated nanoparticle- enhanced Raman spectroscopy I. INTRODUCTION Analysis of surfaces of various materials is very important from the economic and scientific point of view. Such studies are especially difficult for so-called buried interfaces (e.g., the surface of the solid sample in the liquid or the high pressure gas), a situation which occurs in very important from the practical point of view interfaces of various biological samples in their “natural” environment. One of the analytical methods of chemical analysis of buried interfaces is so-called shell- isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). SHINERS was developed in 2010 by Tian et al. [1]. In this approach the analyzed surface is covered with the layer of gold or silver nanoparticles covered by the few nanometers thick protective layer or silica or alumina, and then the Raman spectrum of the investigated sample is recorded [1-5]. Metal nanoparticles act as electromagnetic resonators, significantly enhancing the electric field of the incident electromagnetic radiation, and hence leading to very large increase of the Raman signal from the surface on which nanoparticles have been spread. The protecting coating separates metal cores from direct contact with the probed material and keeps them from agglomerating. In this contribution we report on modification of SHINERS method by using hollow silver (h-Ag) and hollow gold (h-Au) nanoparticles as electromagnetic nanoresonators; in other words we show the first example of using of h-Ag@SiO 2 and h-Au@SiO 2 nanoparticles for the SHINERS measurements. Hollow nanostructures exhibit surface plasmonic properties different (and, in some cases, superior to) their solid counterparts [6-9]. For example, the position of the surface plasmon resonance (SPR) band could be changed in the significantly broader wavelength range than for solid spherical nanoparticles [6-9]. Therefore, combination of the SHINERS technique with the resonance Raman effect should be significantly easier when using h-Ag@SiO 2 and h-Au@SiO 2 clusters. II. EXPERIMENTAL SECTION Materials. CoCl 2 , AgNO 3 , NaOH, tri-sodium citrate dihydrate, and 37% hydrochloric acid (all pure p.a.) were acquired from POCH S.A. NaBH4 ≥99% and methyl parathion (O,O-dimethyl-O-(4-nitrophenyl) phosphorothioate) were purchased from Fluka. (3-aminopropyl)trimethoxysilane (>97%), aqueous solution of Na 2 SiO 3 (ca. 26.5% of SiO 2 , reagent grade), L-glutathione (>98%) and 4-mercaptobenzoic acid (>99%) were acquired from Sigma-Aldrich. HAuCl 4 (30% solution in dilute HCl, 99.99% trace metals basis) was purchased from Mennica Państwowa. All of the reagents were used as received. Water (with resistivity of 18.2 MW cm) was purified with Millipore Milli-Q system. Argon (≥99.999%) was purchased from Air Products. Preparation of metal nanoparticles. Hollow silver nanoparicles were prepared according to a modified Moshe and Markovich method [10]. The preparation of the hollow Ag nanoparticles starts with the preparation of Ag 2 O nanoparticles. Briefly speaking, to 2.6 ml of water 0.15 ml of 10 mM aqueous solution of AgNO 3 and 0.018 ml of 10 mM aqueous solutions of glutathione were added. Then, 0.5 ml of 0.1 M aqueous solutions of NaOH was added while vigorously stirring. The solution had turned pale yellowish due to the formation of Ag 2 O nanocrystals. Then, 0.18 ml of 10 mM aqueous solution of NaBH 4 was introduced, silver oxide nanoparticles were reduced, and the reaction mixture had turned orange. Figure 1 shows the TEM image of synthesized hollow-Ag nanoparticles. Hollow gold nanospheres with tunable interior-cavity sizes were synthesized by using cobalt nanoparticles as sacrificial templates and varying the molar ratio of used reagents (it Sponsor: National Science Centre (Poland).