Preparation of SERS substrates by gel encapsulation of Ag nanoparticles prepared by laser ablation

Abstract

SERS effectiveness, nanoparticle stability and the overlapping from interference species eventually existing in the medium are the parameters to be considered in the application of metal nanoparticles for surface-enhanced spectroscopic techniques [1]. Silver nanoparticles (Ag NPs) are usually prepared by chemical reduction, using sodium citrate and hydroxylamine hydrochloride as reducing agents. In both cases, the by-products of the reduction process can affect negatively the adsorption of the molecule of interest [2]. Ag NPs prepared by laser ablation have the advantage of not having any interference in the aqueous media. However, their time stability is quite poor. For this reason, the encapsulation of Ag NPs in an agar gel can help to overcome that drawback with the addition of an increase of the SERS effectiveness [3]. In this work Ag NPs were fabricated by pulsed laser ablation of a silver target in milli-Q water. The laser irradiation was carried out with nanosecond laser pulses using the fundamental (1064 nm), 2nd (532 nm) and 4th harmonic (266 nm) output of a Q-switched Nd:YAG laser (pulse duration 15 ns, repetition rate 10 Hz). Two irradiation times (25 and 50 minutes) and two laser fluences (3 and 6 times the Ag ablation threshold fluence) were considered. Thus, 12 different Ag NPs preparations were tested in order to study their SERS effectiveness using Diamond Green as probe molecule. Characterization of Ag NPs was carried out by UV-Vis spectroscopy and TEM. After the study of the Ag NPs in aqueous media, encapsulation of the SERS substrates on Agar gels was carried out. Two different methods were used: (a) preparation of pieces of agar gel and their immersion it in the Ag NPs solution for 24 hours, and (b) preparation of pieces of agar gel dissolved in the Ag NPs solution. In both cases the Agar-Ag NPs were put in contact with a Diamond Green solution for 24 hours before the measurement of SERS spectra. The enhancement factor obtained by using each of the Ag NPs preparations, both in solution and encapsulated, were calculated in order to study their effectiveness as SERS substrates. Best results were obtained by using the Ag NPs prepared by irradiation for 50 min at 1046 nm and at 6 times the Ag ablation threshold fluence. Acknowledgments: This work was supported by CSIC (project 201560I029) and Comunidad de Madrid/EU (project GEOMATERIALES 2).