Optical response of silver nanoparticles stabilized by

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Procedia Materials Science 1 (2012) 594 – 600

11th International Congress on Metallurgy & Materials SAM/CONAMET 2011.

Optical response of silver nanoparticles stabilized by amines to LSPR based sensors M. V. Roldán*, N. S. Pellegri , O. A. de Sanctis Instituto de Física de Rosario-CONICET-UNR, Pellegrini 250, Rosario (2000), Argentina.

Abstract Silver nanoparticles aminosilane capped were obtained as stable colloids. FTIR spectrum of these nanoparticles showed a strong interaction between amine groups and silver surface, being this property the source of the high stability of the colloidal particles. UV-visible spectrum showed the typical absorption owing the resonance of localized surface plasmon (LSPR). We show that the optical properties of the colloidal nanoparticles were influenced by medium conditions. A continuous shift of the maximum of absorption to longer wavelength was observed with the enhancement of Pb2+ and Cd2+concentrations. In this way, these nanoparticles can be used as LSPR sensors.

2012Published Publishedby byElsevier ElsevierLtd. Ltd.Selection Selectionand/or and/orpeer-review peer-review under responsibility 11th ©©2012 under responsibility ofof SAM/ International2011, Congress on Metallurgy CONAMET Rosario, Argentina. & Materials SAM/CONAMET 2011. Keywords: nanoparticles, LSPR, sensors

1. Introduction Silver nanoparticles (NPs) are quite interesting due their particular properties that produce special behavior different than the bulk material. One of those properties is the resonance of localized plasmon surface (LSPR) that is the effect of the collective oscillation of the excited conduction electrons by the incident electromagnetic radiation.

* Corresponding author. Tel.: +54-0341-4495467. E-mail address: [email protected]. Corresponding author. Tel.: +54-0341-4495467. E-mail address: [email protected].

2211-8128 © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SAM/CONAMET 2011, Rosario, Argentina. doi:10.1016/j.mspro.2012.06.080

M. V. Roldán et al. / Procedia Materials Science 1 (2012) 594 – 600

When the light interacted with spherical Ag nanoparticles of a few nanometers, the incident electromagnetic field radiation shifts the conduction electrons from their equilibrium position by forming a dipole, this at the same time induces a restoring force that results in a collective oscillation of the electrons at a characteristic frequency. This is the resonance of localized surface plasmon, near 400nm for Ag NPs. At the same time, the oscillation of the electrons induces a polarization in the opposite sense in the surrounding medium of the particle, and this polarization reduces the strength of the restoring force over the nanoparticle, shifting the LSPR depending of the surroundings characteristic (Chumanov, 2005). Then, the changes in the refractive index of the NPs surroundings can be detected from the maximum absorption shift. This relation between the absorption maximum and the characteristics of the medium constitutes the base of the functioning principle for the refractive index sensors based in LSPR. Moreover, when two metal nanoparticles are at a distance lower than the particle size, the resonance is shifted to red due to the near field electromagnetic coupling, and in some cases this phenomenon could be as big that a change in the color can be observed at glance. The latter case is the principle of so called sensors aggregation (Sepúlveda, 2009). In this work, we investigated the response of the resonance of localized plasmon surface of Silver nanoparticles stabilized by amines obtained in colloidal suspension regarding the changes in the surroundings conditions. Knowledge of this information is the first step toward the development of chemical and/or biologic sensors based on LSPR. 2. Experimental Synthesis of silver nanoparticles. Silver nanoparticles were synthesized by a colloidal method as described elsewhere (Frattini, 2006; Roldán, 2008). Briefly, a 4 mM AgNO3 (Merck) solution was prepared in presence of N-[3-(trimethoxysilyl)propyl]diethylenetriamine (Aldrich, hereafter ATS) with a final concentration of 0.0125 M. The solution was homogenized and placed in a temperature bath at 40 ºC with magnetic stirrer for 3 or 4 hours in a N2 atmosphere. So, a yellow homogeneous solution was obtained. The color was attributed to the formation of silver nanoparticles. Colloids were aged at room conditions of moisture to allow condensation of aminosilane molecules in excess over the particles surface. Characterization of silver nanoparticles. UV-visible spectra of reaction medium and colloidal nanoparticles were taken by a Jasco V-530 double beam spectrophotometer operating in transmittance mode in quartz cuvettes. AFM images of the silver nanoparticles were taken with a NanoTec ELECTRONICA microscopy working in tapping mode with a Si3N4 tip. To prepare the sample colloidal nanoparticles were dropped over a Si substrate evaporating the solvent at room conditions between each drop. Finally substrate was rinsed with absolute ethanol. Also, Transmition Electron Microscopy (TEM) studies were made with CM 200 Philips equipment over samples prepared similarly to those for AFM over a TEM grid. Sensing. Different Pb(NO3)2 solutions were prepared with concentrations between 0.1 and 0.9 mM. In turn 100 μl of each solution was added to different aliquots of colloidal silver nanoparticles. They were homogenized by magnetic stirring and then the UV-visible spectra were taken. Control experiments were made adding NaCl to colloidal nanoparticles to obtain a final concentration of 0.1 and 0.2 M respectively, so we hope know the response of colloidal nanoparticles because a strength ionic change. 3. Results and discussion AFM and TEM images of the obtained silver nanoparticles are shown in Figure 1. The mean size was approximately 5.2 nm as determined by the microscopic techniques. Nanoparticles were obtained as stable yellow colloids. UV-visible spectrum showed the absorption band typical of silver nanoparticles with

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maximum at 434 nm. Colloidal nanoparticles were stable for long times, without the appearance of precipitates or modification of their optical properties. 60

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The colloidal stability of metal NPs is mostly defined by the characteristics of the molecule used as stabilizer. For the system here described ATS is used as stabilizer, which is included in the group of ligands. As such, it is expected that the amino group of the molecule acts as electron donor group to the particle surface of Ag and thus surrounds the particle, exposing -Si(OCH3) 3 groups to the outside that together with the hydrocarbon chain constitute a steric hindrance for the growth of the metal particle. ATS interaction with the surface particle was studied by FTIR spectroscopy and in a previous work (Roldán, 2011) this interaction was also studied using XPS spectroscopies. Figure 2 shows comparatively the FTIR spectra of ATS and Ag aminosilane stabilized NPs. The spectrum of ATS shows a peak with maximum at 3291 cm -1 surrounded by a shoulder on each side. These absorptions are assigned to the stretching vibrations of N-H bonds of secondary amines (3291) and primary amines (~ 3345 and 3190 cm -1). The peaks between 2940 and 2840 are assigned to stretching vibrations of C-H bonds of CH2 and CH3 groups. Similar absorptions were observed in the spectrum of Ag NPs with practically no change in the bands attributed to C-H bonds but with a notable shift in the bands corresponding to N-H vibrations. These observations suggest a strong interaction between the ATS and the NPs surface, possible favored by the multiplicity of functional groups.

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Fig. 2. FTIR spectra of ATS and Ag-nanoparticles stabilized by ATS.

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Using XPS technique we study the spectra of ATS and ATS/Ag nanoparticles for various elements (Roldán, 2011). The anchoring of nitrogen to the NP surface was observed, suggested by the absence of shifts for the carbon and oxygen 1s levels, while the nitrogen 1s level presents a clear shift to higher binding energies, indicating electron donor behavior through the Ag surface. Finally, a bonding scheme is proposed that can be seen in the figure 3 for the ATS/Ag capped nanoparticle. Condensation of silane groups is expected -amine groups catalyze the formation of siloxane bonds enhancing binding reactivity of aminosilane molecules toward silica surfaces (Buining, P., 1997). We hope that condensation at mild conditions with atmospheric moisture favour the formation of a bilayer of ATS which expose amine groups outside. The functionalization of the nanoparticles surface with amine groups turns it a very versatile system due the reactivity of amine groups. Alkylamine are able to form covalent bonds with other organic compounds or coordinate with metals to form complexes. So, we hope take advantage of the affinity of amine group to metals to quantify the concentration of metallic cations at solutions. We studied the optical properties of these colloidal nanoparticles in presence of small quantities of Pb2+. Figure 4 shows UV-visible spectra of colloidal nanoparticles with different concentrations of Pb 2+. A shift of the maximum absorption to longer wavelength with creasing Pb 2+ concentration is clearly observed.

Fig. 3. Proposed bonding scheme for the ATS/Ag capped nanoparticles

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Fig. 6. UV-Vis spectra of colloidal Ag NPs with different concentrations of Na +.

In other experiment, we tested the behavior of the same colloidal nanoparticles in presence of small quantities of Cd2+. Figure 5 shows UV-visible spectra with a shift of the maximum absorption to longer wavelength with increasing Cd2+ concentration. In this case, the sensibility to the [Cd2+] is not as high as with [Pb2+] and more studies must be done to achieve a good understanding of these results.


Li (2009) showed that colloidal nanoparticles could aggregate changing the ionic force of the medium by salt addition, but, it could be avoided using a good stabilizer. To rule out that this shift was associated to a change of ionic strength, a control experiment was made, by adding to the colloidal nanoparticles sodium salt, where the cation is not able to form complexes with amines. Figure 6 shows UV-visible spectra of colloidal nanoparticles with different concentration of sodium cation. No shift was observed, so it is not the ionic force change the reason of the change of optical properties. We propose a mechanism where the surface nanoparticles are functionalized with an aminosilane bilayer which exposes amine groups outside. Cations Pb2+ added to colloidal NPs are placed around the particles due the interaction with amine groups. Then, the conditions around the particles are changed and the characteristic LSPR is modified, shifting the maximum absorption to longer wavelength proportionally to Pb2+ concentration. A complete scheme about the proposed sensing mechanism is showed in Figure 7.

Fig. 7. Schematic graph of the proposed sensing mechanism.

4. Conclusion Silver nanoparticles aminosilane capped were obtained as stable colloids. We show that aminosilane acts as a good stabilizing due the strong interaction between amine groups and silver surface. Colloidal nanoparticles showed the typical absorption due the localized surface plasmon resonance. Optical properties of the colloidal silver were modified by Pb2+ concentration in the solution, showing a direct relationship between the heavy metal cation concentration and the maximum position of the LSPR band. So these nanoparticles seem to be able to design LSPR based sensors.

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Acknowledgements Authors acknowledge the financial support of the following entities: Agencia Nacional de Promoción Científica y Tecnológica (ANPCT), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Universidad Nacional del Litoral (UNL) and Universidad Nacional de Rosario (UNR). Authors deeply thank H. Troiani (CAB) for TEM images. References Buining, P, Humbel, B., Philipse, A. Verkleij, A. 1997. Preparation of functional silane-stabilized gold colloids in the (sub)nanometer size range. Langmuir 13, p. 3921. Chumanov, G., Evanoff, D. 2005. Synthesis and optical properties of silver nanoparticles and arrays. ChemPhysChem 6, p. 1221. Frattini A., Pellegri N., Nicastro D., de Sanctis O.. 2005. Effect of amine groups in the synthesis of Ag nanoparticles using aminosilanes Materials Chemistry and Physics, 94,p 148. Li, X., Jiang, L., Zhan, Q., Qian, J., He, S. 2009. Localized surface plasmon resonance (LSPR) of polyelectrolyte-functionalized goldnanoparticles for bio-sensing. Colloids and Surfaces A: Physicochem. and Eng. Aspects 332, p. 172. Roldán,M., Scaffardi, L., de Sanctis, O., Pellegri, N. 2008. Optical properties and extinction spectroscopy to characterize the synthesis of amine capped silver nanoparticles. Materials Chemistry and Physics 112, p. 984. Roldán M., Pellegri N., de Sanctis O., 2011 SiO2-K2O-MgO vitreous films doped with erbium and silver nanoparticles for optical applications, Optical Materials 33, p. 1921. Sepúlveda, B., Angelomé, P., Lechuga, L., Liz-Marzán, L. 2009. LSPR-based nanobiosensors. Nano Today 4, p. 244.