Silver nanoparticles synthesized by direct ... - Wiley Online Library

Research Article Received: 25 February 2008

Accepted: 23 August 2008

Published online in Wiley Interscience: 20 October 2008

(www.interscience.wiley.com) DOI 10.1002/jrs.2135

Silver nanoparticles synthesized by direct photoreduction of metal salts. Application in surface-enhanced Raman spectroscopy ´ America ´ ´ Roberto Sato-Berru, Vazquez-Olmos and ´ ∗ Rocío Redon, Jose´ M. Saniger A simple synthesis method of silver nanoparticles and its application as an active surface-enhanced Raman spectroscopy (SERS) colloid are presented in this work. The photoreduction of AgNO3 in presence of sodium citrate (NaCit) was carried out by irradiation with different light sources (UV, white, blue, cyan, green, and orange) at room temperature. The evaluation of silver nanoparticles obtained as a function of irradiation time (1–24 h) and light source was followed by UV-visible absorption spectroscopy. This light-modification process results in a colloid with distinctive optical properties that can be related to the size and shape of the particles. The Ag colloids, as prepared, were employed as active colloids in SERS. Pyridine and caffeine c 2008 John Wiley & Sons, Ltd. were used as test molecules. Copyright Keywords: silver nanoparticles; photoreduction; surface-enhanced Raman spectroscopy; pyridine; caffeine

Introduction

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Metal nanoparticles are the most promising systems for applications in different areas, like optics or electronics. Ag, Au, and Cu nanoparticles are especially interesting due to their characteristic surface plasmons arising from the resonance of the conduction electrons of individual nanoparticles upon interaction with an incoming electromagnetic field. The surface plasmon resonance results in a strong local enhancement of the electromagnetic field in the vicinity of the nanoparticles.[1 – 4] Silver nanoparticles have been intensively studied in the past few years since their properties depend on particle size and shape.[5 – 7] Among the different shapes (such as cubes, prisms, plates, disks, rods and wires),[8 – 14] spherical particles play a unique role as the only type for which the classical Mie’s theory provides an analytical solution of Maxwell equations. Furthermore, spherical metallic particles are used in many applications such as surfaceenhanced Raman spectroscopy (SERS),[15 – 17] sensors,[18,19] and plasmonic devices.[20] A great amount of synthetic routes for the preparation of metallic nanoparticles have been reported, however, two methods predominate: chemical reduction of metal salts[21 – 24] and photoinduced aggregation of small nanoparticle seeds.[25 – 30] The latter has recently become very popular, especially for silver. Jin et al.[25] induced the aggregation of seeds using a laser beam of certain wavelength and proposed that the aggregation process is controlled mainly by the charge distribution on the seeds. Callegari et al.[26] demonstrated that the use of filters during illumination directly influences the final geometry of silver nanoprisms. Bastys et al.[29] used light-emitting diodes of different emission wavelengths, or white light combined with different color filters for illumination, which allows the preparation of Ag nanoprisms with high-aspect ratios. These methods are based on the photoinduced aggregation of metallic seeds with the development of other morphologies, specifically, Ag prisms.

J. Raman Spectrosc. 2009, 40, 376–380

In this work, the direct photoreduction process of the AgNO3 NaCit solutions was carried out without the previous addition of silver seeds. Besides, we studied the wavelength influence on the synthesis of Ag nanoparticles. Finally, we used these nanoparticles as SERS colloids. SERS is of great interest because it constitutes an important tool for the high-sensitivity detection of a broad kind of compounds. For this purpose, pyridine and caffeine were used as test molecules. Pyridine (C5 H5 N) is an intermediate in the synthesis of insecticides, herbicides, pharmaceuticals, etc. On the other hand, caffeine (C8 H10 N4 O2 ) is used as a central nervous system stimulant, having the effect of temporarily warding off drowsiness and restoring alertness.

Experimental Materials All reagents were analytical grade, purchased from Sigma and Merck and used without further purification. Pyridine, 99.9%; caffeine, 99%; AgNO3 , 99.9999%; and sodium citrate, 99% were used in this work. Aqueous solutions were prepared using tridistilled water. Pyridine and caffeine aqueous solutions with a concentration of 2000 ppm were used for SERS tests.

∗

Correspondence to: Roberto Sato-Berrú, Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, Apdo. Postal 70-186, México, D.F. C.P. 04510, México. E-mail: [email protected] Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, Apdo. Postal 70-186, México, D.F. C.P. 04510, México

c 2008 John Wiley & Sons, Ltd. Copyright

Silver nanoparticles by photoreduction and SERS Synthesis of Ag nanoparticles

Results and Discussion

this wavelength more strongly on a rotationally averaged basis in solution than the transverse mode which shifts to a shorter wavelength. This explanation is normally accepted but, in our case, some other effect was observed because of the lack of initial metallic seeds and the use of a specific wavelength. The reaction evolution with white light was monitored by electronic absorption UV-visible spectroscopy. The colloids show a plasmonic resonance band which is more intensive and becomes wider as the irradiation time increases, and the maximum is shifted from 416 to 439 nm (Fig. 2). The wavelength of the maximum, as well as the shape of this band, had been associated with the formation of silver nanospheres.[30] After 18 h of irradiation, the plasmonic band remains practically unchanged. The band observed below 320 nm is assigned to the intraband electronic transitions of silver.[4] The band centered at 226 nm is observed in the initial nonirradiated sample which can be assigned to citrate and nitrate ions. In order to study the behavior of these colloids as active supports for SERS, 0.25 ml of Ag colloid, 0.05 ml of NaCl (8.5 mM), and 0.25 ml of the test molecule (pyridine or caffeine diluted in water at 2000 ppm), in that order, were mixed in a quartz cell and then analyzed by Raman spectroscopy. The SERS in solution is usually induced by addition of NaCl.[34] The role of such salts is to induce some kind of aggregation of colloidal nanoparticles through the interaction of chloride anions with the surface of the Ag nanoparticles. Raman and SERS spectra of pyridine diluted to 2000 ppm used as reference, and pyridine diluted around 1000 ppm in the silver colloids are shown in Fig. 3(a). Although the increase of the signal Raman intensity is evident, the enhancement

At the beginning, the AgNO3 -NaCit solutions are colorless. After irradiation with white light for 4 h, the solution turns light yellow, and a plasmon band at 416 nm is observed. When the solutions were irradiated with white light for 1–24 h, its color changed from light to dark yellow, Fig. 1. The role of the citrate in the synthesis can be explained as follows: the citrate has three carboxylic groups and it has been shown by Munro et al.[31] that mainly two of them would bind to the silver, leaving the third on the Ag nanoparticles surface to stabilize the system by electrostatic repulsion. Moreover, the citrate is a reducing agent that can be photo-oxidized to acetone1,3-dicarboxylate, as well as carbon dioxide, according to Ahern and Garrell[32] and Sato et al.[33] and their adsorption onto the nanoparticle surface induces an electron transfer.[28] According to Maillard et al.[28] after being formed, the seeds absorb light isotropically (equally for all orientations) due to their spherical shape. Now, inhomogeneity is assumed in the deposit of the metallic layer; then the particle starts to become ellipsoidlike and the degenerate plasmon resonance splits into transverse and longitudinal modes, and the longitudinal plasmon (along the longer dimension) shifts to a longer wavelength and absorbs

Figure 2. UV-visible spectra of Ag colloids in aqueous solutions at different irradiation times with white light.

In a typical synthesis, 42.5 mg of AgNO3 and 73.5 mg of sodium citrate (NaCit) were dissolved in 250 ml of tri-distilled water at room temperature. This solution was separated in 15 aliquots, the first was used as reference and the other 14 were exposed to white light (conventional fluorescent tube, Sanalec 26W, Coolwhite; 350 nm < λ < 700 nm) from 1–24 h. The same procedure was carried out in order to explore the colloid behavior at different wavelengths; colored glass filters in the blue 350 < λ < 482 nm; cyan 370 < λ < 563 nm; green 396 < λ < 600 nm; and orange 563 < λ < 700 nm were interposed between the fluorescent tube and the samples. On the other hand, a UV lamp (λ = 365 nm) was used as another light source to evaluate the reduction process. Instrumentation Raman spectra were recorded with a Raman dispersive spectrometer, model Almega XR. Macro compartment was used both for focusing the laser on the liquid samples and for collecting the scattered light in a 180◦ backscattering configuration. The Raman spectra were accumulated over 25 s with a resolution of ∼4 cm−1 and the excitation source was 532 nm radiation from a Nd : YVO4 laser (frequency-doubled). UV-visible absorption spectra were measured by a fiber-optic Ocean Optics USB-2000 spectrometer.

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Figure 1. Silver colloids at different irradiation time of white light (1, 4, 8, 12, 16, and 24 h).



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R. Sato-Berru´ et al. factor is only ∼102 , which is very small in comparison with the average factors obtained for SERS that can be as high as 1014 .[35] This small value can be explained by the mismatch between the wavelengths of the laser used as excitation source (532 nm) and the plasmonic band (439 nm). The apparent enhancement factor was experimentally measured by direct comparison using the following relation: EF = (RSENH /RSREF ) × (C REF /C ENH )

(1)

where RSENH and RSREF are the measured Raman intensities, and C REF and C ENH are the concentrations in reference and enhanced samples.[36] Figure 3(b) shows the Raman spectrum of caffeine diluted to 2000 ppm and the SERS spectra of caffeine which were diluted to 1000 ppm in silver colloids. It can be applied the same consideration on the small value of the factor enhancement. SERS spectra of pyridine show two typical bands located at 1006 and 1035 cm−1 corresponding to the ring breathing mode and symmetric deformation, respectively. Moreover, other peaks can be observed, which can be associated to the modes of pyridine absorbed on Ag nanoparticles.[37,38] SERS spectra of caffeine show the usual Raman peaks corresponding to the interaction of caffeine on Ag nanoparticles.[39] The band at 234 cm−1 is assigned to the Ag-Cl vibration. On the other hand, in order to study the wavelength effect on the photoreduction process, we used a UV lamp or filtered fluorescent light as other irradiation source. These colloids were analyzed by UV-visible electronic absorption spectroscopy. Figure 4 shows the UV-visible spectra of Ag colloids prepared at different times of irradiation with UV light. At low irradiation times of 4 h or less, a plasmonic resonance band similar to that obtained with white light appeared. On the contrary, for irradiation times more than 8 h, a broad plasmon complex band is observed with a clear red shift as a function of irradiation time. Under these conditions, silver nanoparticles of multi-dispersed size and/or shape are obtained (top spectrum). In the literature, there are numerous references related to complex plasmon bands, which have been associated with different nanoparticle shapes.[25,26,29,30] The UV-visible spectra of Ag colloids prepared with different colored filters interposed between the fluorescent bulb and the samples are shown in Fig. 5. Going from blue to orange, the decrease of the relative intensity of the plasmon band (428 nm)

can be seen, indicating that the efficiency of the photoreduction process decreases when the red irradiation wavelength shifts. In fact, when the orange filter was employed, only a band at 226 nm is observed (Fig. 5(d)). Finally, the Raman spectrum of pyridine diluted to 2000 ppm in water together with SERS spectra of pyridine diluted to 1000 ppm in the Ag colloids, prepared by UV irradiation or using filtered fluorescent light during 24 h are shown in Fig. 6. In the Ag colloid obtained after 24 h of irradiation, the pyridine Raman signal increases close to one hundred times, whereas in the other colloids (obtained after 4, 8, 12, 16, and 20 h of irradiation) the pyridine signal increases from 60 up to 100 times.

Conclusion A simple AgNO3 -NaCit photoreduction synthesis method results in Ag nanoparticles, using UV and visible light at room temperature. The wavelength involved in the photoreduction process, as well as the irradiation time, influences the production and the yield of the silver nanoparticles; higher wavelength values correspond to lower yields. The irradiation with orange light (563 < λ < 700 nm)

Figure 4. UV-visible spectra of Ag colloids in aqueous solutions at different irradiation times with UV light (365 nm).

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Figure 3. Raman and SERS spectra of pyridine (a) and caffeine (b). Ag colloids prepared under white light at different times.




Silver nanoparticles by photoreduction and SERS

Figure 5. UV-visible spectra of Ag colloids in aqueous solutions at different irradiation times with blue (a), cyan (b), green (c) and orange (d) light.

Figure 6. Raman and SERS spectra of pyridine obtained with Ag colloids irradiated with UV, blue, cyan and green light (24 h).

does not produce Ag colloids or any SERS activity, whereas in the other Ag colloids it was observed that the pyridine and caffeine Raman signals increased by a factor of ∼102 times.

´ de la Investigacion ´ Científica Sato RY acknowledges Coordinacion ´ and Centro de Ciencias Aplicadas y Desarrollo Tecnologico (UNAM) for the Postdoctoral fellowship.

Acknowledgements


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The authors acknowledge PAPIIT IN120508 and IN106405-3 ´ (UNAM) projects, and the Fondo de Fomento a la Investigacion ´ Científica y Tecnologica del Gobierno del D.F. for their support.

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