Optical switch from silver nanocomposite thin films

Materials Letters 62 (2008) 3361–3363

Contents lists available at ScienceDirect

Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Optical switch from silver nanocomposite thin films Stephan T. Dubas a,b, Vimolvan Pimpan b,⁎ a b

Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok, Thailand Department of Materials Science, Faculty of Science, Chulalongkorn University, Bangkok, Thailand

A R T I C L E

I N F O

Article history: Received 14 October 2007 Accepted 6 March 2008 Available online 20 March 2008 Keywords: Layer-by-layers Silver Nanoparticles Sensors

A B S T R A C T Silver nanoparticles capped with sodium alginate were assembled into thin films by using the layer-by-layer dipping technique. Composite films were built by sequential dipping of a glass slide in either anionic alginate capped nanoparticles or cationic Poly(diallyldimethylammonium chloride) (PDADMAC). The growth of the film was characterized using UV–Vis spectroscopy by monitoring the increase in absorbance at 420 nm which correspond to the silver nanoparticles plasmon band. The final films formed onto glass slides displayed and interesting color shift upon exposure to water or to a less polar solvent such as ethanol. In this research, changes in spectral absorbance of the nanoparticles film were monitored as a function of ethanol content (0, 20, 40, 60, 80 and 100%) in water. The color shift from yellow to red color was explained by the changes in the dielectric constant of the silver nanoparticles surrounding medium which induce a shift in their plasmon band absorbance. These composite thin films displayed fast color change and could therefore be used in sensing application as well as for optical switches. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The properties of silver nanoparticles have been extensively studied in the past few decades for their unique optical, sensing and anti-microbial properties [1,2]. While the surface enhance Raman scattering (SERS) is probably the most spectacular application of silver nanoparticles, other sensing methods have been developed simply based on shifts in the spectral transmission of nanoparticles solutions. Solutions of spherical silver nanoparticles are known to present a strong yellow color with an extinction UV–Vis spectrum featuring a sharp peak around 400 nm [3,4]. The absorbance is caused by the so-called localized surface plasmon absorption (LSPR) which arises from the coupled oscillations of the conduction electron in the metallic nanoparticles induced by the incident light electric field [5]. The position, shape and intensity of the LSPR is function of factors such as morphology (size and shape), dielectric constant of the environment (coating, surrounding medium and supporting substrate) as well as inter-particle coupling (state of aggregation) [6]. Changes in LSPR position and intensity due to variations in surrounding medium's dielectric occur when either a solvent of different polarity surrounds the particle or a molecules bind directly on the nanoparticles surface [8]. Changes in nanoparticles proximity

⁎ Corresponding author. Tel.: +66 02 218 5548. E-mail address: [email protected] (V. Pimpan). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.03.036

can be induced by DNA strands modified nanoparticles which leads to the aggregation of the particles upon multiple binding [7]. Changes in the shape of particles were also recently reported for the detection of mercury by gold nano-rods which reversed to a more thermodynamically stable spherical shape [8]. Although nanoparticles solutions can be made very sensitive, another interesting approach for the fabrication of sensor is to use nanoparticles immobilized as thin films. Among the different methods that can be used for the fabrication of thin films, the layer-by-layer (LbL) method developed by Decher in the early nineties is of great interest [9,10]. This simple technique based on the sequential deposition of oppositely charged species has received growing interest for its simplicity and versatility and has been the subject of several reviews [11–13]. It has been described as a very convenient method to incorporate nanoparticles, polymer, polyelectrolytes or bio-molecules in thin films with tunable thickness, and surface chemistry [14– 17]. Using this technique, silver nanoparticles have previously been assembled into thin films for anti-microbial or electrochemical sensors applications [18,19] but to the knowledge of the author have not yet been reported as optical sensors. In the present article the LbL assembly of silver nanoparticles into thin films and their response to increasing ethanol content in water is presented. UV–Vis spectroscopy was used to monitor the layer-bylayer deposition of silver nanoparticles as well as to observe the film's color shift when exposed to various ethanol concentrations. Finally a homemade device was used to evaluate the fast color switch of the film when exposed to alternating solution of water and ethanol. These

3362

S.T. Dubas, V. Pimpan / Materials Letters 62 (2008) 3361–3363

[20]. The alginate capped silver nanoparticles and PDADMAC were deposited by sequential dipping of a glass slide in either PDADMAC or silver nanoparticles solutions for 5 min followed by three rinses in distilled water for 1 min. These steps were repeated as many times as the number of layers desired. PDADMAC concentration was fixed to 10 mM and the ionic strength of the solution was adjusted with 0.1 M of sodium chloride (NaCl). The silver nanoparticles solution was used pure from the synthesis procedure. At the end of the deposition steps, the samples were gently dried in a stream of nitrogen before testing. 2.4. UV–Vis spectrophotometer The silver nanoparticles containing thin films were analyzed using a UV–Vis spectrophotometer (SPECORD S 100, Analytikjena). For the measurements, the coated film on a pre-cut glass slide was placed in the cuvette and measured. The absorbance of the film was measured in the 350 nm to 700 nm range, which includes the absorbance peak of the silver nanoparticles. 3. Results and discussion

Fig. 1. Plot of the increase in absorbance at 420 nm as a function of the number PDADMAC and silver nanoparticles layers. The corresponding absorbance spectra are given in the cartouche.

thin films show reproducible, very stable, and fast-switching optical characteristics, which could be of interest in the field of photonic or to fabricate colorimetric sensors. 2. Experimental method and chemicals

Silver nitrate salts were reduced by sodium borohydride and stabilized with polyanionic sodium alginate. Polyelectrolytes such as sodium alginate have already been reported for their use as stabilizing agent in the synthesis of silver nanoparticles due to the presence of carboxylic and hydroxyl groups on the polymer backbone [21]. The electrostatic repulsion between particles as a result of the alginate capping insures the formation of only small nanoparticles by preventing their aggregation and growth through Ostwald ripening [22]. The alginate capping around the particles also provide a negative charge which allow later their assembly into thin films using the layer-by-layer deposition method [21]. Using the layer-by-layer technique, up to 10 layers of silver nanoparticles and polycationic PDADMAC were sequentially deposited. The electrostatic interactions between oppositely charged polyelectrolytes allow the fabrication of uniform films. The growth of the film was confirmed by monitoring the changes in absorbance of the coated glass slide as a function of the number of deposited layers (Fig. 1). The linear increase of the absorbance at 420 nm as a function of the number of deposited layers (0 to 10 layers) confirms the formation of a multilayer film which is characteristic of this process [23]. Nevertheless it is interesting to note that a change in growth rate after the deposition of the 10th layer appears. This change in slope seems to correspond to the appearance of a second absorbance peak at 545 nm as in can be seen on the UV–Vis spectrum added in the cartouche of the Fig. 1. The second peak at 545 nm is possibly due to the inter-

2.1. Chemicals Poly(diallyldimethylammonium chloride) (PDADMAC), 20 wt.% in water, typical MW 200,000–350,000, sodium borohydride and alginic acid were purchased from Aldrich. AR grade silver nitrate was purchased from Mallinckrodt, Thailand. Analytical grade sodium chloride was purchased from Carlo Erba. All chemicals and solvents were used as received without any further purification or treatment. Double distilled water was used in all experiments. 2.2. Nanoparticles synthesis Silver nanoparticles were prepared by reduction of silver nitrate salts with sodium borohydride and stabilized using sodium alginate. The silver nanoparticles synthesis can be summarized as follow: 10 ml of a 0.1 mM solution of sodium alginate was mixed with 10 ml of a 1 mM silver nitrate solution and stirred for 5 min. A 10 ml solution of a freshly prepared sodium borohydride solution having a concentration of 10 mM was added quickly. The solution immediately turned yellow which is characteristic of the formation of silver nanoparticles. The solution was stirred overnight and then stored in a dark bottle. The solutions of nanoparticles were found to be stables for several months without appearance of any precipitates. 2.3. Layer-by-layer assembly Prior to the layer-by-layer deposition, the glass slide substrates were cleaned from organic contaminants by a 15 min dipping in an oxidizing “piranha solution” following procedure reported elsewhere

Fig. 2. Absorbance spectra from a 20 layers PADAMAC/Silver nanoparticles as a function of the ethanol content in the cuvette. The corresponding absorbance value at 550 nm was shown in the cartouche.

S.T. Dubas, V. Pimpan / Materials Letters 62 (2008) 3361–3363

3363

second since no intermediate point could be recorded with a sampling rate of 1 s. Shown on the graph is a section of our cycling test but up to 100 cycles were performed with similar efficiency and switching speed which suggest that the coating is stable.

4. Conclusion In conclusion sensing properties of silver nano-thin films were presented. The layer-by-layer method used for the assembly allows for a good control over the total thickness and optical properties of the film. Films composed of 20 layers of silver nanoparticles and PDADMAC showed a linear dependence between absorbance at 550 nm and the ethanol content of the surrounding solution. The color switching was found to be extremely fast as it required less than a second to switch from one color to the other. The film color shift also displayed a good reproducibility without any loss of efficiency in color intensity and switching time. Further researches are being pursued to improve the resolution and selectivity of the sensor film. Acknowledgements This work was financially supported by the Chulalongkorn University through the center for innovative research in nanotechnology. Appendix A. Supplementary data Fig. 3. Changes in transmittance of the silver nanoparticles film when exposed to either water or 100% ethanol solution. The sampling rate used in this experiment is 1 s per data point.

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matlet.2008.03.036. References

particles interaction due to increased packing as the film grows. The LSPR of the silver nanoparticles is also known to be highly sensitive to changes in dielectric constant of the surrounding medium and tends to be red shifted when exposed to less polar medium such as organic solvent or dehydrated layers [24]. The resulting film formed by the successive deposition of 20 layers of PDADMAC and silver nanoparticles capped with alginate displayed and interesting color switch depending on the surrounding solvent. The film appeared yellow when dipped in water but changed to a strong orange-red color upon drying or dipping in more polar solvent such as ethanol. The color shift between yellow and orange was found to be reversible and very fast. This color switch suggested the potential development of a sensor or optical device which could be trigger by the changing surrounding solvent such as an ethanol/water mixture. In order to characterize the sensing properties of the film, the changes in absorbance when exposed to increasing ethanol content (0, 20, 40, 60, 80 and 100% ethanol in water) were monitored and plotted in Fig. 2. It can be seen that the characteristic yellow peak centered at 400 nm gradually shift to a peak centered at 545 nm as ethanol content is increased from 0 to 100%. The color change is due to the variation in dielectric constant of the surrounding of the silver nanoparticles upon exposure to less polar solvent. Reported calculations from Mie theory confirm such behavior of a red shifted plasmon absorbance band of the nanoparticles upon exposure to less polar solvent. The cartouche in Fig. 2 is a plot of the relationship between absorbance at 550 nm versus ethanol content in the solution which appears to be linear. The 550 nm wavelength was used because it present the widest range change in absorbance as it can be seen from the spectra shown in the cartouche of Fig. 2. The kinetic of the nanoparticles' color change was found to be extremely fast which suggest that these films could be used as optical switches. To characterize the fast switching of these silver nanoparticles film, was used a simple homemade light sensor similar to that already reported in the literature [25]. The film was placed in a flow through cuvette and exposed to solutions of either 100% ethanol or pure water. As seen on Fig. 3, the color switch is fast and occurs in less than a

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]

Y. Shen, P.N. Prasad, Appl. Phys. B-Lasers Opt. 74 (2002) 641–645. H. Guo, S. Tao, Sens. Actuators B-Chem. 123 (2007) 578–582. K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107 (2003) 668–677. J. Perez-juste, I. Pastoriza-Santos, L. Liz-Marzan, P. Mulvaney, Coord. Chem. Rev. 249 (2005) 1870–1901. U. Kreibig, M. Vollmer, J.P. Toennies, optical properties of metal clusters, SpringerVerlag, Berlin, 1995. S. Link, M.A. El-Sayed, Int. Rev. Phys. Chem. 19 (2000) 409–453. J.J. Storhoff, R. Elghanian, R.C. Mucic, C.A. Mirkin, R.L. Letsinger, J. Am. Chem. Soc. 120 (1998) 1959–1964. M. Rex, F.E. Hernandez, A.D. Campiglia, Anal. Chem. 78 (2006) 445–451. G. Decher, Science 277 (1997) 1232. G. Decher, J.B. Schlenoff (Eds.), Wiley-VCH, Weinheim, 2003. X. Shia, M. shen, H. Mohwald, Prog. Polym. Sci. 29 (2004) 987. P.T. Hammond, Curr. Opin. Colloid Interface Sci. 4 (1999) 430. N.A. Kotov, I. Dekany, J.H. Fendler, J. Phys. Chem. 99 (1995) 13065. J.A. Jaber, J.B. Schlenoff, Langmuir 23 (2007) 896–901. P. Podsiadlo, L. Sui, Y. Elkasabi, P. Burgardt, J. Lee, A. Miryala, et al., Langmuir 23 (2007) 7901–7906. T.R. Hendricks, E.E. Dams, S.T. Wensing, I. Lee, Langmuir 23 (2007) 7404–7410. J. Bravo, Zhai, Z.Z. Wu, R.E. Cohen, M.F. Rubner, Langmuir 23 (2007) 7293–7298. B. Abu-sharkh, Langmuir 22 (2006) 3028–3034. Z. Li, D. Lee, X.X. Sheng, R.E. Cohen, M.F. Rubner, Langmuir 22 (2006) 9820–9823. S.T. Dubas, J.B. Schlenoff, Macromolecules 32 (1999) 8153–8160. L. Limsavarn, V. Sritaveesinsub, S.T. Dubas, Mater. Lett. 61 (2007) 3048–3051. A. Imre, D.L. Beke, E. Gontier-Moya, I.A. Szabo, E. Gillet, Appl. Phys. A-Mater. 71 (2000) 19–22. C. Jiang, S. Markutsya, V. Tsukruk, Langmuir 20 (2004) 882–890. H. Xu, M. Kall, Sens. Actuators B-Chem. 87 (2002) 244–249. K.T. Lau, R. Shepherd, D. Diamond, Sensors 6 (2006) 848–859.