Re-crystallization of silver nanoparticles in a highly concentrated NaCl environment—a new substrate for surface enhanced IR-visible Raman spectroscopy† Robert Prucek, * a Ale  s Pan a cek, a Ariana Farga  sov a, a V aclav Ranc, b Vlastimil Ma  sek, c Libor Kv  ıtek a and Radek Zbor ˇil * a Received 26th October 2010, Accepted 24th November 2010 DOI: 10.1039/c0ce00776e The common approach of silver nanoparticles activation for surface enhanced Raman spectroscopy often exploits an addition of chloride ions, generally at low concentrations of about 0.1–10 mM in the final dispersion. For the first time, we report the applicability of a highly concentrated NaCl solution (final concentration of 400 mM) for the SERS activation of silver nanoparticles (30 nm). Microscopic, optical and particle size distribution measurements reveal the rapid and reproducible re-crystallization of the primary silver nanoparticles to one-order larger crystallites (400 nm) already after 15 min after NaCl addition. The crystal growth mechanism is discussed with respect to the proved essential role of oxygen in the reaction system. The specific action of chloride ions is demonstrated through a comparison with NaBr and NaI solutions of the identical concentrations, which do not induce the analogous crystallization process. The recrystallized silver particles are efficient in an enhancement of the Raman signal not only for visible (488 nm) but also for near infrared laser excitation (1064 nm) as illustrated with the representative spectra of adenine. Introduction Fleischmann’s 1974 discovery of Surface Enhanced Raman Scattering (SERS) on a silver electrode, 1 and especially its re- discovery on colloidal silver particles in 1977 by Creighton, 2 started the extensive development of a new and very sensitive analytical method 3–7 enabling to detect molecules in the concentration range from pico- to femtomols. 8 High enhance- ments of SERS even allowed the detection of individual mole- cules adsorbed on a single silver particle. 9–12 Some studies have shown that the highest value of enhancement is achieved only on the silver particles of a certain size which are referred to as ‘hot particles’. The optimal size of these hot particles depends on the wavelength of the laser used for excitation and ranges from approximately 70 nm to 200 nm for excitation wavelengths between 488 and 647 nm. 13 For the commonly used argon laser with a wavelength of 514.5 nm, the ‘hot particle size’ is reported from 80 to 100 nm. 8,14 For a use of lasers with an excitation wavelength in the red region (l ¼ 785 nm), and especially in the NIR region (l ¼ 1064 nm), we can assume that the highest enhancement of the Raman signal would be achieved with particles of around 400 nm in diameter. Nevertheless, the prep- aration of such large particles represents a hard task from the synthetic viewpoint. Moreover, particles of these dimensions are unstable and usually settle within a few hours. Silver nano- particles (Ag NPs) of 20–30 nm in sizes can be stable for several months or years, even without any extra stabilization. However, these small particles themselves usually do not provide enhancement of the Raman signal. For this purpose, they must be activated for example by the addition of some inorganic ions. 7,15–18 The most frequently used activation agents of the silver nanoparticles prepared by the common reduction proce- dures 2,19,20 include halide ions, particularly chlorides. 21–24 However, the mechanism of the activation has not yet been fully explained. 25–29 One possible explanation is based on the forma- tion of silver particle aggregates. 29 Recently, it has been shown that a very strong increase in the Raman signal is achieved in nanocrystal junction sites between two nanoparticles. 30–32 In this work, we report a simple and reproducible method for the activation of monodispersed Ag NPs by introducing the concentrated NaCl solution (resulting concentration of 400 mM) into the Ag NP dispersion. It is worth mentioning that the effect of as-high concentrations of chloride ions, which is one to three orders higher than commonly mentioned in the literature, have a Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University, 17 Listopadu 1192/12, 771 46 Olomouc, Czech Republic. E-mail: zboril@prfnw.upol.cz; robert.prucek@upol.cz; Fax: +420 585 634 761; Tel: +420 585 634 427 b Department of Analytical Chemistry, Faculty of Science, Palacky University, 17 Listopadu 12, Olomouc, 771 46, Czech Republic c Department of Pharmacology, Faculty of Medicine and Dentistry, Palacky University, Hneˇvot  ınsk a 3, Olomouc, 775 15, Czech Republic † Electronic supplementary information (ESI) available: Additional UV-vis absorption spectra, DLS measurement data, TEM images, XRD and SAED patterns. See DOI: 10.1039/c0ce00776e 2242 | CrystEngComm, 2011, 13, 2242–2248 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < CrystEngComm Cite this: CrystEngComm, 2011, 13, 2242 www.rsc.org/crystengcomm PAPER Downloaded by Universite De Fribourg Suisse on 17 August 2012 Published on 21 January 2011 on http://pubs.rsc.org | doi:10.1039/C0CE00776E View Online / Journal Homepage / Table of Contents for this issue