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Green synthesis of silver nanoparticles for ammonia sensing
Available online at www.sciencedirect.com
Talanta 76 (2008) 29–33
Green synthesis of silver nanoparticles for ammonia sensing 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 Received 14 January 2008; received in revised form 28 January 2008; accepted 29 January 2008 Available online 13 February 2008
Abstract Silver nanoparticles synthesized by a reagent less method involving only UV radiation have been used in colorimetric assay for the detection of ammonia in solution. The silver nanoparticles were synthesized by the exposure of a silver nitrate solution to a low-power UV source in the presence of poly(methacrylic acid) (PMA), which acted both as reducing and capping agent. The synthesis of the silver nanoparticles was studied by monitoring the changes in position and amplitude of the localized plasmon resonance (LSPR) band using UV–vis spectroscopy. The morphology of the particles was studied using transmission electron microscopy which confirmed the formation of spherical particles with an average particle size around 8 nm. Interestingly, the silver nanoparticles solution was found to display a strong color shift from purple to yellow upon mixing with increasing concentration of ammonia ranging from 5 to 100 ppm. Hence, the nanoparticles prepared with this method could be used as colorimetric assay for sensing applications of ammonia in water. © 2008 Elsevier B.V. All rights reserved. Keywords: Ammonia; Silver nanoparticles; Sensor
1. Introduction Ammonia is produced in very large quantity (2.1–8.1 Tg/ year) by the chemical industry to be used as fertilizers and in refrigeration systems [1]. Because of its toxicity, ammonia presents serious health hazards, and the monitoring of its concentration in air and liquid is of major importance. Concentration control of ammonia not only interest environmental agencies but also, for example, the automotive industry as ammonia has been found in exhaust systems with concentrations up to 8 ppm [2,3]. There are also medical interests in measuring ammonia in the body, as it can be an indicator of disorder or disease. For instance, breath ammonia level can be a diagnostic for urea imbalance as a result of kidney disorder or stomach bacterial infection [4,5]. To this end, a wide range of sensors have been designed with detection limit lower than part per million (ppm). Sensors have been commercialized with detection mechanisms often based on metal-oxide or organic conducting films, whose electrical properties are perturbed upon the adsorption of ammonia molecules
∗
Corresponding author at: Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok, Thailand. Tel.: +66 2 218 4234. E-mail address: [email protected] (S.T. Dubas). 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.01.062
[6,7]. Films based on SnO2 or WO3 demonstrated good response to NH3 with a detection limit down to 1 ppm [8]. Films from conjugated polymer such as polypyrrole or polyaniline have also been used to detect ammonia by changes of their conduction properties [7]. All of these techniques display extremely good reproducibility and low detection limit but only work for ammonia gas and cannot be used to measure ammonia dissolved in water. For the detection of ammonia in solution, spectrophotometric methods based on the change in color of a reagent in the presence of ammonia have been developed. The best-known reaction is probably the Nessler reaction based on the color change of potassium tetraiodomercurate (II) in a dilute alkaline solution which has the inconvenience to produce a toxic precipitate [9]. Another coloration method is the Berthelot reaction that displays a blue color in the presence of ammonia with a detection limit of 90 ppb but has a very slow kinetic of reaction [10]. Recent report of low detection of ammonia in drinking water was made using carbon nanotube/copper composite paste coating for electrochemical electrode [11]. In their experiment, Valentini et al. reported sub ppm detection of ammonia with fast response time but the experimental setup is more complex and can be difficult to adapt for on-site measurements. Silver nanoparticles have already been extensively studied for their unique optical and sensing properties [12,13]. Solutions of
30
S.T. Dubas, V. Pimpan / Talanta 76 (2008) 29–33
spherical silver nanoparticles are known to have a strong yellow color and an extinction UV–vis spectrum featuring a sharp peak at 400 nm [14,15]. The color of silver nanoparticles solutions is due to 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 [16]. The position, shape and intensity of the LSPR are the function of factors such as morphology, dielectric constant of the environment as well as inter-particle coupling [17]. The properties of nanoparticles the most often used in sensing application are based on changes in dielectric constant of the surrounding medium by changing the solvent quality or by complex formation at the surface of the nanoparticles. It is well known that less polar surrounding medium induces a shift of the LSPR toward higher wavelength [16,17]. Silver nanoparticles present therefore great interest for sensing applications in both gas and liquid phase [18,19]. In this article a very simple method is presented for the synthesis of silver nanoparticles to be used in colorimetric sensors for ammonia. Silver nitrate salts are reduced using an 8-W UV lamp in the presence of poly-methacrylic acid which acts as capping and reducing agent. Commonly used reducing agents such as hydrazine or sodium borohydride are replaced by a more environmental friendly method based on mild UV-light illumination. The low power of the UV lamp provides a very slow kinetic of reaction which allows the preparation of purple solution of silver nanoparticles. UV–vis spectroscopy was used to monitor the kinetic of nanoparticles synthesis while their size and morphology were confirmed by transmission electron microscopy (TEM). Finally, the sensing properties of the nanoparticles solution were tested against increasing ammonia concentration in the range of 5–100 ppm by monitoring the changes in LSPR position and amplitude with a UV–vis spectrophotometer. 2. Experimental method 2.1. Chemicals Poly(methacrylic sodium salt) (PMA) and sodium borohydride were purchased from Aldrich and AR grade silver nitrate was purchased from Mallinckrodt, Thailand. The pH of the solution was adjusted in all experiments to a value of 4 using a 100 mM acetic-acetate buffer. Ammonia diluted from a 30% solution was used in the sensing experiments to prepare solutions with a final concentration ranging from 5 to 100 ppm. Double distilled water was used in all experiments. 2.2. Nanoparticles synthesis Silver nanoparticles were prepared by the reduction of silver nitrate solutions under exposure to UV light in the presence of poly(methacrylic). PMA which acts both as a reducing and capping agent can stabilize the silver nanoparticles in solution by electrostatic repulsion and steric hindrance. The synthesis steps of the silver nanoparticles can be summarized as follow: 25 ml of a 10 mM solution of PMA was mixed with 25 ml of a 10 mM silver nitrate solution and stirred for 5 min. All solution had a
pH of 4, which was fixed by a 100 mM acetic-acetate buffer. The solution was then exposed to the UV light and the solution slowly turned pink before finally acquiring a purple color within an hour. After 1 h exposure time, the solutions were stored in a dark bottle. Under storage conditions, the purple solution was stable for several months. 2.3. Characterization of the nanoparticles The position and amplitude of the LSPR of the silver nanoparticles solutions were analyzed using a UV–vis spectrophotometer (SPECORD S 100, Analytikjena) in the wavelength range of 350–700 nm. The morphology and particles size of freshly synthesized silver nanoparticles were evaluated using a transmission electron microscope model (Jeol JEM 100SX). For the sample preparation, a drop of diluted silver nanoparticles solution was dropped on the grid and left to dry overnight in a desiccator. The samples were measured without any further treatment the following day. 3. Results and discussion 3.1. Synthesis of the silver nanoparticles In contrast with conventional methods used in the synthesis of silver nanoparticles (e.g. sodium borohydride or hot sodium citrate), this experiment uses a low intensity (8 W) UV lamp. The choice of a low intensity UV lamp was dictated by the need of a reduction method which would have a very slow kinetic of reaction. UV–vis spectroscopy was used to monitor the changes in λmax absorbance and amplitude of the LSPR band during the reduction reaction. By recording the changes in absorbance as a function of time, the kinetic of formation of silver nanoparticles solution was monitored and the compilation of the absorbance spectra is shown in Fig. 1. The increase in absorbance at 515 nm as a function of time can be seen. The final absorbance peak of the nanoparticles solution contrasts strongly with the expected absorbance maximum at 400 nm, which is commonly presented as the characteristic of successful silver nanoparticles synthesis. For reference, are shown in Fig. 2, spectra and pictures of nanoparticles solutions prepared by two methods. The experiment A represents the typical silver nanoparticles synthesis from NaBH4 which leads to a characteristic yellow color solution with an absorbance maximum at 400 nm. In contrast, the solution prepared with the UV lamp had a deep purple color with a maximum absorbance at 515 nm. The author speculates that the difference in spectral absorbance is due to the incomplete reduction of the silver ions which perturb the dielectric surround of the nanoparticles. The unreacted silver ions can interact with the COO− carboxylic groups present at the surface of the nanoparticles and form an Ag+ /COO− complex. The color shift is then justified by the less hydrophilic environment due to the complex formation as it displaces the water molecules and modifies the dielectric constant of the medium in the vicinity of the nanoparticles surface. Less hydrated and less polar surrounding medium have been demonstrated to induce a red shifted absorbance spectrum [17].
S.T. Dubas, V. Pimpan / Talanta 76 (2008) 29–33
31
Fig. 3. TEM image of silver nanoparticles prepared by exposure of a 10 mM silver nitrate and 10 mM PMA solution at pH 4 under an 8-W UV lamp for 1 h. Fig. 1. Changes in absorbance as a function of reaction time of a 10 mM silver nitrate solution mixed with a 10 mM PMA solution exposed to an 8-W UV lamp. The spectra were recorded every 5 min.
The morphology of produced silver nanoparticles using this method was monitored by TEM. The nanoparticles seen in Fig. 3 appear spherical and have a narrow particles size distribution with the largest having a diameter of 10 nm. TEM analysis confirms thus the formation of spherical nanoparticles and not nanorods as it might have been though since silver nanorods solution would also posses a purple/red color. Although the polymer coating cannot be seen on the TEM image, the carboxylic groups from the PMA can interact with the silver metals surface as a protective coating and prevent Ostwall ripening of the nanoparticles by electrostatic repulsion and stearic hindrance. 3.2. Ammonia sensing
Fig. 2. UV–vis spectrums and pictures of silver nanoparticles solution produced by reduction of AgNO3 with sodium borohydride (A) and by exposure to an 8-W UV lamp (B). In both experiments PMA was used as capping agent.
The spectral absorbance of the nanoparticles in solution was found to display a strong shift upon adjunction of ammonia and the color of solution changed from purple to yellow. Using UV–vis spectroscopy the changes in absorbance can be monitored as a function of the ammonia content. Shown in Fig. 4 is the plot of changes in spectral absorbance as a function of ammonia concentration increasing from 0, 5, 15, 40, 60, 75, 85 and 100 ppm. It can be seen that the initial absorbance peak intensity at 515 nm decreases and is replaced by another peak appearing at 460 nm while the ammonia content is increased. The presence of an isosbectic point at 490 nm indicates the presence of two species in equilibrium. Considering the already reported association of ammonia with Ag+ ions to form Ag(NH3 )2 + coordination complex, the two species in our case are the silver nanoparticles with and without excess silver ions. Spectral shifts of LSPR band are usually explained by a combination of changes in interparticles distance and by a shift of the dielectric constant of the surrounding medium. Upon the addition of ammonia in solution,
32
S.T. Dubas, V. Pimpan / Talanta 76 (2008) 29–33
Fig. 5. Ratio of the absorbance peak at 460 and 515 nm for a silver nanoparticles solution exposed to increasing ammonia content (5–100 ppm). In the cartouche the detailed changes in absorbance for each wavelength are given (square = absorbance at 515 nm; diamonds = absorbance at 460 nm).
Fig. 4. UV–vis spectrum of a silver nanoparticles solution exposed to various concentration of ammonia (0, 5, 15, 40, 60, 75, 85, and 100 ppm).
fast and it only took a few seconds for the color change to occur. 4. Conclusion
a Ag(NH3 )2 + complex is formed between ammonia and silver ion which increases the surface charge of the particles and allows the repulsion between the particles as well as increase the water content in the surrounding of the particles. This results in more hydrophilic and isolated particles with a blue shifted spectrum. Similarly, the addition of sodium chloride was found to also induce the color shift between purple to yellow but in this case the AgCl complex is non-soluble and leads to the appearance of a turbid yellow solution. These results confirm that the excess of unreacted silver ions which are responsible for the purple color can be removed by any complexing agent. For the design of optical sensors, the monitoring of changes in absorbance at specific wavelengths instead of the entire UV–vis spectrum is preferred. The choice of the wavelengths is usually dictated by the area of the spectrum which displays the largest variation in absorbance during the sensing experiment. The two wavelengths of interest here are 460 and 515 nm as they display the increase and decrease in absorbance respectively upon mixing of the solution with dilute ammonia solutions as it can be seen from Fig. 4. Shown in the cartouche Fig. 5 is a plot of the absorbance for these wavelengths across the isosbectic point as a function of the ammonia content which displays a linear relationship. Further improvement is possible by data processing and taking the ration of these two values (abs460 /abs515 ). This has for effect to provide a less scattered plot as it can be seen in Fig. 5 which displays a linear relationship as a function of ammonia concentration with a correlation factor R2 equal to 0.988. The kinetic of color shift was found to be extremely
We have presented a new method for the synthesis of silver nanoparticles based on the incomplete reduction of silver ions when exposed to a weak UV light and capped by PMA. The nanoparticles solution displays an absorbance maximum at 515 nm instead of the 400 nm usually expected for spherical silver nanoparticles. The unreacted silver is thought to be responsible for the color shift by forming an Ag+ /COO− complex with the PMA stabilizing polyelectrolyte. The resulting solution displays a purple color which can be reversed to yellow upon adjunction of ammonia. Based on this mechanism, an ammonia sensor has been proposed and shows a linear response in the range of 5–100 ppm. Acknowledgement This research was funded by the Chulalongkorn University through the Center for Innovative Nanotechnology (CIN). References [1] P. Warneck, Chemistry of the Natural Atmosphere, Academic Press Inc., 1998. [2] C. Pijolat, C. Pupier, M. Sauvan, G. Tournier, R. Lalauze, Sens. Actuators B 59 (1999) 195. [3] T.D. Durbin, R.D. Wilson, J.M. Norbeck, J.W. Miller, T. Huai, S.H. Rhee, Atmos. Environ. 36 (2002) 1475. [4] W. Ament, J.R. Huizenga, E. Kort, T.W. Van Der Mark, R.G. Grevink, G.J. Verkerke, Int. J. Sports Med. 20 (1999) 71.
S.T. Dubas, V. Pimpan / Talanta 76 (2008) 29–33 [5] L.R. Narasimhan, W. Goodman, N. Patel, Proc. Natl. Acad. Sci. U.S.A. 98 (2001) 4617. [6] K. Zakrzewska, Thin Solid Films 391 (2001) 229. [7] M. Sahm, A. Oprea, N. Barsan, U. Weimar, Sens. Actuators B 127 (2007) 204. [8] X. Wang, N. Miura, N. Yamazoe, Sens. Actuators B 66 (2000) 74. [9] A.I. Vogel, Vogel’s Qualitative Inorganic Analysis, Longman Scientific & Technical, 1987. [10] P.L. Saerle, Analyst 109 (1984) 549. [11] F. Valentini, V. Biagiotti, C. Lete, G. Palleschi, J. Wang, Sens. Actuators B 28 (2007) 326.
33
[12] Y. Shen, P.N. Prasad, Appl. Phys. B: Lasers Opt. 74 (2002) 641. [13] H. Guo, S. Tao, Sens. Actuators B 123 (2007) 578. [14] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107 (2003) 668. [15] J. Perez-juste, I. Pastoriza-Santos, L. Liz-Marzan, P. Mulvaney, Coordin. Chem. Rev. 249 (2005) 1870. [16] U. Kreibig, M. Vollmer, J.P. Toennies, Optical Properties of Metal Clusters, Springer-Verlag, Berlin, 1995. [17] S. Link, M.A. El-Sayed, Int. Rev. Phys. Chem. 19 (2000) 409. [18] H.Y. Wang, Y.F. Li, C.Z. Huang, Talanta 72 (2007) 1698. [19] C.S. Cheng, Y.Q. Chen, C.J. Lu, Talanta 73 (2007) 358.
Talanta 76 (2008) 29–33
Green synthesis of silver nanoparticles for ammonia sensing 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 Received 14 January 2008; received in revised form 28 January 2008; accepted 29 January 2008 Available online 13 February 2008
Abstract Silver nanoparticles synthesized by a reagent less method involving only UV radiation have been used in colorimetric assay for the detection of ammonia in solution. The silver nanoparticles were synthesized by the exposure of a silver nitrate solution to a low-power UV source in the presence of poly(methacrylic acid) (PMA), which acted both as reducing and capping agent. The synthesis of the silver nanoparticles was studied by monitoring the changes in position and amplitude of the localized plasmon resonance (LSPR) band using UV–vis spectroscopy. The morphology of the particles was studied using transmission electron microscopy which confirmed the formation of spherical particles with an average particle size around 8 nm. Interestingly, the silver nanoparticles solution was found to display a strong color shift from purple to yellow upon mixing with increasing concentration of ammonia ranging from 5 to 100 ppm. Hence, the nanoparticles prepared with this method could be used as colorimetric assay for sensing applications of ammonia in water. © 2008 Elsevier B.V. All rights reserved. Keywords: Ammonia; Silver nanoparticles; Sensor
1. Introduction Ammonia is produced in very large quantity (2.1–8.1 Tg/ year) by the chemical industry to be used as fertilizers and in refrigeration systems [1]. Because of its toxicity, ammonia presents serious health hazards, and the monitoring of its concentration in air and liquid is of major importance. Concentration control of ammonia not only interest environmental agencies but also, for example, the automotive industry as ammonia has been found in exhaust systems with concentrations up to 8 ppm [2,3]. There are also medical interests in measuring ammonia in the body, as it can be an indicator of disorder or disease. For instance, breath ammonia level can be a diagnostic for urea imbalance as a result of kidney disorder or stomach bacterial infection [4,5]. To this end, a wide range of sensors have been designed with detection limit lower than part per million (ppm). Sensors have been commercialized with detection mechanisms often based on metal-oxide or organic conducting films, whose electrical properties are perturbed upon the adsorption of ammonia molecules
∗
Corresponding author at: Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok, Thailand. Tel.: +66 2 218 4234. E-mail address: [email protected] (S.T. Dubas). 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.01.062
[6,7]. Films based on SnO2 or WO3 demonstrated good response to NH3 with a detection limit down to 1 ppm [8]. Films from conjugated polymer such as polypyrrole or polyaniline have also been used to detect ammonia by changes of their conduction properties [7]. All of these techniques display extremely good reproducibility and low detection limit but only work for ammonia gas and cannot be used to measure ammonia dissolved in water. For the detection of ammonia in solution, spectrophotometric methods based on the change in color of a reagent in the presence of ammonia have been developed. The best-known reaction is probably the Nessler reaction based on the color change of potassium tetraiodomercurate (II) in a dilute alkaline solution which has the inconvenience to produce a toxic precipitate [9]. Another coloration method is the Berthelot reaction that displays a blue color in the presence of ammonia with a detection limit of 90 ppb but has a very slow kinetic of reaction [10]. Recent report of low detection of ammonia in drinking water was made using carbon nanotube/copper composite paste coating for electrochemical electrode [11]. In their experiment, Valentini et al. reported sub ppm detection of ammonia with fast response time but the experimental setup is more complex and can be difficult to adapt for on-site measurements. Silver nanoparticles have already been extensively studied for their unique optical and sensing properties [12,13]. Solutions of
30
S.T. Dubas, V. Pimpan / Talanta 76 (2008) 29–33
spherical silver nanoparticles are known to have a strong yellow color and an extinction UV–vis spectrum featuring a sharp peak at 400 nm [14,15]. The color of silver nanoparticles solutions is due to 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 [16]. The position, shape and intensity of the LSPR are the function of factors such as morphology, dielectric constant of the environment as well as inter-particle coupling [17]. The properties of nanoparticles the most often used in sensing application are based on changes in dielectric constant of the surrounding medium by changing the solvent quality or by complex formation at the surface of the nanoparticles. It is well known that less polar surrounding medium induces a shift of the LSPR toward higher wavelength [16,17]. Silver nanoparticles present therefore great interest for sensing applications in both gas and liquid phase [18,19]. In this article a very simple method is presented for the synthesis of silver nanoparticles to be used in colorimetric sensors for ammonia. Silver nitrate salts are reduced using an 8-W UV lamp in the presence of poly-methacrylic acid which acts as capping and reducing agent. Commonly used reducing agents such as hydrazine or sodium borohydride are replaced by a more environmental friendly method based on mild UV-light illumination. The low power of the UV lamp provides a very slow kinetic of reaction which allows the preparation of purple solution of silver nanoparticles. UV–vis spectroscopy was used to monitor the kinetic of nanoparticles synthesis while their size and morphology were confirmed by transmission electron microscopy (TEM). Finally, the sensing properties of the nanoparticles solution were tested against increasing ammonia concentration in the range of 5–100 ppm by monitoring the changes in LSPR position and amplitude with a UV–vis spectrophotometer. 2. Experimental method 2.1. Chemicals Poly(methacrylic sodium salt) (PMA) and sodium borohydride were purchased from Aldrich and AR grade silver nitrate was purchased from Mallinckrodt, Thailand. The pH of the solution was adjusted in all experiments to a value of 4 using a 100 mM acetic-acetate buffer. Ammonia diluted from a 30% solution was used in the sensing experiments to prepare solutions with a final concentration ranging from 5 to 100 ppm. Double distilled water was used in all experiments. 2.2. Nanoparticles synthesis Silver nanoparticles were prepared by the reduction of silver nitrate solutions under exposure to UV light in the presence of poly(methacrylic). PMA which acts both as a reducing and capping agent can stabilize the silver nanoparticles in solution by electrostatic repulsion and steric hindrance. The synthesis steps of the silver nanoparticles can be summarized as follow: 25 ml of a 10 mM solution of PMA was mixed with 25 ml of a 10 mM silver nitrate solution and stirred for 5 min. All solution had a
pH of 4, which was fixed by a 100 mM acetic-acetate buffer. The solution was then exposed to the UV light and the solution slowly turned pink before finally acquiring a purple color within an hour. After 1 h exposure time, the solutions were stored in a dark bottle. Under storage conditions, the purple solution was stable for several months. 2.3. Characterization of the nanoparticles The position and amplitude of the LSPR of the silver nanoparticles solutions were analyzed using a UV–vis spectrophotometer (SPECORD S 100, Analytikjena) in the wavelength range of 350–700 nm. The morphology and particles size of freshly synthesized silver nanoparticles were evaluated using a transmission electron microscope model (Jeol JEM 100SX). For the sample preparation, a drop of diluted silver nanoparticles solution was dropped on the grid and left to dry overnight in a desiccator. The samples were measured without any further treatment the following day. 3. Results and discussion 3.1. Synthesis of the silver nanoparticles In contrast with conventional methods used in the synthesis of silver nanoparticles (e.g. sodium borohydride or hot sodium citrate), this experiment uses a low intensity (8 W) UV lamp. The choice of a low intensity UV lamp was dictated by the need of a reduction method which would have a very slow kinetic of reaction. UV–vis spectroscopy was used to monitor the changes in λmax absorbance and amplitude of the LSPR band during the reduction reaction. By recording the changes in absorbance as a function of time, the kinetic of formation of silver nanoparticles solution was monitored and the compilation of the absorbance spectra is shown in Fig. 1. The increase in absorbance at 515 nm as a function of time can be seen. The final absorbance peak of the nanoparticles solution contrasts strongly with the expected absorbance maximum at 400 nm, which is commonly presented as the characteristic of successful silver nanoparticles synthesis. For reference, are shown in Fig. 2, spectra and pictures of nanoparticles solutions prepared by two methods. The experiment A represents the typical silver nanoparticles synthesis from NaBH4 which leads to a characteristic yellow color solution with an absorbance maximum at 400 nm. In contrast, the solution prepared with the UV lamp had a deep purple color with a maximum absorbance at 515 nm. The author speculates that the difference in spectral absorbance is due to the incomplete reduction of the silver ions which perturb the dielectric surround of the nanoparticles. The unreacted silver ions can interact with the COO− carboxylic groups present at the surface of the nanoparticles and form an Ag+ /COO− complex. The color shift is then justified by the less hydrophilic environment due to the complex formation as it displaces the water molecules and modifies the dielectric constant of the medium in the vicinity of the nanoparticles surface. Less hydrated and less polar surrounding medium have been demonstrated to induce a red shifted absorbance spectrum [17].
S.T. Dubas, V. Pimpan / Talanta 76 (2008) 29–33
31
Fig. 3. TEM image of silver nanoparticles prepared by exposure of a 10 mM silver nitrate and 10 mM PMA solution at pH 4 under an 8-W UV lamp for 1 h. Fig. 1. Changes in absorbance as a function of reaction time of a 10 mM silver nitrate solution mixed with a 10 mM PMA solution exposed to an 8-W UV lamp. The spectra were recorded every 5 min.
The morphology of produced silver nanoparticles using this method was monitored by TEM. The nanoparticles seen in Fig. 3 appear spherical and have a narrow particles size distribution with the largest having a diameter of 10 nm. TEM analysis confirms thus the formation of spherical nanoparticles and not nanorods as it might have been though since silver nanorods solution would also posses a purple/red color. Although the polymer coating cannot be seen on the TEM image, the carboxylic groups from the PMA can interact with the silver metals surface as a protective coating and prevent Ostwall ripening of the nanoparticles by electrostatic repulsion and stearic hindrance. 3.2. Ammonia sensing
Fig. 2. UV–vis spectrums and pictures of silver nanoparticles solution produced by reduction of AgNO3 with sodium borohydride (A) and by exposure to an 8-W UV lamp (B). In both experiments PMA was used as capping agent.
The spectral absorbance of the nanoparticles in solution was found to display a strong shift upon adjunction of ammonia and the color of solution changed from purple to yellow. Using UV–vis spectroscopy the changes in absorbance can be monitored as a function of the ammonia content. Shown in Fig. 4 is the plot of changes in spectral absorbance as a function of ammonia concentration increasing from 0, 5, 15, 40, 60, 75, 85 and 100 ppm. It can be seen that the initial absorbance peak intensity at 515 nm decreases and is replaced by another peak appearing at 460 nm while the ammonia content is increased. The presence of an isosbectic point at 490 nm indicates the presence of two species in equilibrium. Considering the already reported association of ammonia with Ag+ ions to form Ag(NH3 )2 + coordination complex, the two species in our case are the silver nanoparticles with and without excess silver ions. Spectral shifts of LSPR band are usually explained by a combination of changes in interparticles distance and by a shift of the dielectric constant of the surrounding medium. Upon the addition of ammonia in solution,
32
S.T. Dubas, V. Pimpan / Talanta 76 (2008) 29–33
Fig. 5. Ratio of the absorbance peak at 460 and 515 nm for a silver nanoparticles solution exposed to increasing ammonia content (5–100 ppm). In the cartouche the detailed changes in absorbance for each wavelength are given (square = absorbance at 515 nm; diamonds = absorbance at 460 nm).
Fig. 4. UV–vis spectrum of a silver nanoparticles solution exposed to various concentration of ammonia (0, 5, 15, 40, 60, 75, 85, and 100 ppm).
fast and it only took a few seconds for the color change to occur. 4. Conclusion
a Ag(NH3 )2 + complex is formed between ammonia and silver ion which increases the surface charge of the particles and allows the repulsion between the particles as well as increase the water content in the surrounding of the particles. This results in more hydrophilic and isolated particles with a blue shifted spectrum. Similarly, the addition of sodium chloride was found to also induce the color shift between purple to yellow but in this case the AgCl complex is non-soluble and leads to the appearance of a turbid yellow solution. These results confirm that the excess of unreacted silver ions which are responsible for the purple color can be removed by any complexing agent. For the design of optical sensors, the monitoring of changes in absorbance at specific wavelengths instead of the entire UV–vis spectrum is preferred. The choice of the wavelengths is usually dictated by the area of the spectrum which displays the largest variation in absorbance during the sensing experiment. The two wavelengths of interest here are 460 and 515 nm as they display the increase and decrease in absorbance respectively upon mixing of the solution with dilute ammonia solutions as it can be seen from Fig. 4. Shown in the cartouche Fig. 5 is a plot of the absorbance for these wavelengths across the isosbectic point as a function of the ammonia content which displays a linear relationship. Further improvement is possible by data processing and taking the ration of these two values (abs460 /abs515 ). This has for effect to provide a less scattered plot as it can be seen in Fig. 5 which displays a linear relationship as a function of ammonia concentration with a correlation factor R2 equal to 0.988. The kinetic of color shift was found to be extremely
We have presented a new method for the synthesis of silver nanoparticles based on the incomplete reduction of silver ions when exposed to a weak UV light and capped by PMA. The nanoparticles solution displays an absorbance maximum at 515 nm instead of the 400 nm usually expected for spherical silver nanoparticles. The unreacted silver is thought to be responsible for the color shift by forming an Ag+ /COO− complex with the PMA stabilizing polyelectrolyte. The resulting solution displays a purple color which can be reversed to yellow upon adjunction of ammonia. Based on this mechanism, an ammonia sensor has been proposed and shows a linear response in the range of 5–100 ppm. Acknowledgement This research was funded by the Chulalongkorn University through the Center for Innovative Nanotechnology (CIN). References [1] P. Warneck, Chemistry of the Natural Atmosphere, Academic Press Inc., 1998. [2] C. Pijolat, C. Pupier, M. Sauvan, G. Tournier, R. Lalauze, Sens. Actuators B 59 (1999) 195. [3] T.D. Durbin, R.D. Wilson, J.M. Norbeck, J.W. Miller, T. Huai, S.H. Rhee, Atmos. Environ. 36 (2002) 1475. [4] W. Ament, J.R. Huizenga, E. Kort, T.W. Van Der Mark, R.G. Grevink, G.J. Verkerke, Int. J. Sports Med. 20 (1999) 71.
S.T. Dubas, V. Pimpan / Talanta 76 (2008) 29–33 [5] L.R. Narasimhan, W. Goodman, N. Patel, Proc. Natl. Acad. Sci. U.S.A. 98 (2001) 4617. [6] K. Zakrzewska, Thin Solid Films 391 (2001) 229. [7] M. Sahm, A. Oprea, N. Barsan, U. Weimar, Sens. Actuators B 127 (2007) 204. [8] X. Wang, N. Miura, N. Yamazoe, Sens. Actuators B 66 (2000) 74. [9] A.I. Vogel, Vogel’s Qualitative Inorganic Analysis, Longman Scientific & Technical, 1987. [10] P.L. Saerle, Analyst 109 (1984) 549. [11] F. Valentini, V. Biagiotti, C. Lete, G. Palleschi, J. Wang, Sens. Actuators B 28 (2007) 326.
33
[12] Y. Shen, P.N. Prasad, Appl. Phys. B: Lasers Opt. 74 (2002) 641. [13] H. Guo, S. Tao, Sens. Actuators B 123 (2007) 578. [14] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107 (2003) 668. [15] J. Perez-juste, I. Pastoriza-Santos, L. Liz-Marzan, P. Mulvaney, Coordin. Chem. Rev. 249 (2005) 1870. [16] U. Kreibig, M. Vollmer, J.P. Toennies, Optical Properties of Metal Clusters, Springer-Verlag, Berlin, 1995. [17] S. Link, M.A. El-Sayed, Int. Rev. Phys. Chem. 19 (2000) 409. [18] H.Y. Wang, Y.F. Li, C.Z. Huang, Talanta 72 (2007) 1698. [19] C.S. Cheng, Y.Q. Chen, C.J. Lu, Talanta 73 (2007) 358.