Effect of both protective and reducing agents in the synthesis of multicolor silver nanoparticles
© Rivero et al.; licensee Springer. 2013
Received: 3 January 2013
Accepted: 14 February 2013
Published: 22 February 2013
In this paper, the influence of variable molar ratios between reducing and loading agents (1:100, 1:50, 1:20, 1:10, 1:5, 1:2, 1:1, 2:1) and between protective and loading agents (0.3:1, 0.75:1, 1.5:1, 3:1, 7.5:1, 30:1, 75:1) in the synthesis of silver nanoparticles by chemical reduction has been evaluated to obtain multicolor nanoparticles with a high stability in time. The protective agent poly(acrylic acid, sodium salt) (PAA) and reducing agent dimethylaminoborane (DMAB) play a key role in the formation of the resultant color. Evolution of the optical absorption bands of the silver nanoparticles as a function of PAA and DMAB molar ratios made it possible to confirm the presence of silver nanoparticles or clusters with a specific shape. The results reveal that a wide range of colors (violet, blue, green, brown, yellow, red, orange), sizes (from nanometer to micrometer), and shapes (cubic, rod, triangle, hexagonal, spherical) can be perfectly tuned by means of a fine control of the PAA and DMAB molar concentrations.
KeywordsSilver nanoparticles Multicolor Morphology Localized surface plasmon resonance
Metal nanoparticles (NPs) (e.g., Ag, Au, Cu NPs) have attracted great interest in a number of disciplines because of their potential applications in optical, medical, or electronic devices. The control of their size and shape is a challenging goal, and a large number of reports have been published for the preparation of metal nanoparticles of various morphologies [1–5], mainly for plasmonic and sensing applications . Very recently, our group has incorporated silver nanoparticles (AgNPs) in polymeric films for detecting fast changes of humidity (human breathing) [7, 8] and, at the same time, preventing the growth of bacteria very likely in high-humidity atmosphere [9–11].
One of the most frequently used methods is the production of AgNPs from aqueous solutions of Ag+ salts by exposure to radiation (ambient light, UV–vis, gamma) [12–15] or via chemical reduction [16, 17]. A wide number of solvents and encapsulating agents have been used to produce AgNPs and prevent their agglomeration [18–21]. However, the addition of water-soluble polymers such as poly(acrylic acid, sodium salt) (PAA) made possible a better control of the particle growth. This polymer in aqueous solution produces polyacrylate anions (PA−) with uncoordinated carboxylate groups which can bind metallic cations such as silver (Ag+ salts), forming intermediate charged clusters [22, 23]. Due to this, PAA is of special interest because it can control and stabilize both silver nanoparticles and clusters along the polymeric chains with a high stability in time. Several groups of investigation have carried out experiments to report the composition and evolution of these positively charged clusters [24–26].
One of the most relevant aspects of the synthesis of AgNPs is that their optical properties (the resultant color) present high dependence on their crystal morphology (specially size and shape) [27, 28]. These AgNPs exhibit localized surface plasmon resonance (LSPR) spectra (colors), enabling the monitoring of their evolution and color formation by UV–vis measurements.
In this work, the aim is the development of an easy chemical method to synthesize both clusters and silver nanoparticles of different colors in aqueous polymeric solution at room temperature and in a short period of time with a well-defined shape, using PAA as protecting agent. With this goal, an experimental matrix of results is generated by changing two parameters: the concentration of the protecting agent PAA (from 1 to 250 mM); and the different molar ratio between the reducing agent, dimethylaminoborane (DMAB) (concentration from 0.033 to 6.66 mM), and the loading agent, silver nitrate (AgNO3) (at a fixed concentration of 3.33 mM). The experimental matrix is formed by 56 different combinations of protective agent concentration and ‘reducing agent/loading agent’ ratio. From these 56 combinations, a wide range of AgNPs can be obtained with different colors (yellow, orange, red, violet, blue, green, brown) and tunable shape and size. Henceforward, for the sake of simplicity, this experimental matrix will be named the multicolor silver map. To our knowledge, this is the first time that an experimental study based on the influence of both PAA and DMAB molar concentrations to obtain colored silver nanoparticles and clusters has been reported in the literature.
The materials used were as follows: poly(acrylic acid, sodium salt) 35 wt.% solution in water (Mw 15.000), silver nitrate (>99% titration), and dimethylaminoborane complex. All chemicals were purchased from Sigma-Aldrich Corporation (St. Louis, MO, USA) and used without any further purification. All aqueous solutions were prepared using ultrapure water with a resistivity of 18.2 MΩ·cm.
Preparation of the multicolor silver map
A chemical reduction method at room temperature was performed using AgNO3 as loading agent, DMAB as reducing agent, and PAA as protective agent. In order to investigate the influence of both PAA and DMAB on color formation, several concentrations of this water-soluble polymer (from 1 to 250 mM PAA) and reducing agent (from 0.033 to 6.66 mM DMAB) were prepared. The samples of the multicolor silver map have been synthesized several times under the same experimental conditions (room conditions), and no significant difference in the optical absorption spectra of the AgNPs was observed.
Transmission electron microscopy (TEM) was used to determine the morphology of both silver nanoparticles and clusters. TEM analysis was carried out with a Carl Zeiss Libra 120 (Carl Zeiss, AG, Oberkochen, Germany). Samples for TEM were prepared by dropping and evaporating the solutions onto a collodion-coated copper grid.
UV-visible (vis) spectroscopy was used to characterize the optical properties of the multicolor silver map. Measurements were carried out with a Jasco V-630 spectrophotometer (Jasco Analytical Instruments, Easton, MD, USA).
Results and discussion
Multicolor silver map
Effect of the protective agent
One of the major findings of the present study was the significant influence of the PAA concentration on the final color of each sample. Due to its molecular structure with PA− in water solution, the binding of PA− with metal cations (silver) was made possible, forming Ag+PA− complexes wherein a posterior reduction of the silver cations to silver nanoparticles takes place [24–26]. Moreover, PAA concentration plays a key role for the stabilization of silver nanoparticles and metal clusters along the polymeric chains, controlling their size and shape. In fact, the multicolor silver map of Figure 1 demonstrates that with a lower PAA concentration (1 or 2.5 mM), stable silver nanoparticles are generated, showing only yellow, orange, and red colors. These AgNPs showed no changes in the position of their optical absorption bands even after 6 months. Our study demonstrates that by increasing the PAA concentration from 5 to 250 mM, a wider range of colors (violet, blue, green, brown, orange) is obtained with a high stability in time. In fact, a higher range of blue colors is obtained for higher PAA concentrations (25, 100, or 250 mM; see Figure 1). This blue color has been reported in previous works using photochemical or chemical reduction [14, 15, 17], but not using DMAB as reducing agent in the presence of various PAA concentrations.
The results reveal that varying the PAA concentration induces a change in the shape and size of the particles from 100 to 300 nm (nanoparticles) with lower PAA concentration (orange color) to 0.5 to 1 μm (clusters) with higher PAA concentration (brown, green, or blue color).
Effect of the reducing agent
The spectra reveal that the evolution of the absorption bands as a function of the DMAB added to the solution shows just the opposite behavior to the phenomenon observed when PAA was added. The position of the maximum absorption bands shifted to shorter wavelengths when DMAB concentration was increased, and the resulting colors are formed in a different order (from violet to orange) during the synthesis process.
According to the results shown in Figure 5, the evolution of both regions demonstrated that an absorption band at long wavelengths (region 2) is obtained in the first steps of color formation (violet or blue) with lower DMAB molar in the solution. However, when the DMAB molar was increased, the maximum absorption band shifted to short wavelengths (region 1) with a corresponding change of color (brown or green). Furthermore, when higher DMAB molar was added to the solution (with orange color only), a new intense absorption band appeared at 410 nm which was indicative of the formation of nanoparticles with spherical shape. These same spectral absorption variations in both regions have been observed with higher PAA concentrations (100 or 250 mM).
In this study, we have successfully synthesized a multicolor silver map as a function of variable PAA and DMAB concentrations with a constant concentration of silver cations using a chemical reduction method. It has been demonstrated that a fine control of both PAA and DMAB concentrations made it possible to obtain a wide range of colors with specific shapes. Initially, only yellow, orange, or red color is obtained with lower PAA concentrations (1.0 or 2.5 mM PAA), whereas violet, blue, green, brown, or orange color is obtained with higher PAA concentrations (from 5 to 250 mM).
Samples have been characterized using TEM and UV–vis spectroscopy in order to verify the shape and evolution of their maximum absorption bands in two spectral regions (region 1, 400 to 500 nm; region 2, 600 to 700 nm). Firstly, when PAA concentration varies (from 1 to 250 mM) for a constant DMAB concentration (0.33 mM) and, secondly, when DMAB concentration varies (from 0.033 to 6.66 mM) for a constant PAA concentration (10 or 25 mM), the results indicate that for higher PAA or lower DMAB molar concentrations, an absorption band at longer wavelengths (region 2) appears, which implies violet, blue, or green solutions of AgNPs with hexagonal, triangle, and rod shapes. On the other hand, for lower PAA or higher DMAB concentrations, an intense absorption band at shorter wavelengths around 410 nm (region 1) appears, which implies orange red solutions of AgNPs of spherical shape. In summary, the fine control of PAA and DMAB concentrations in the AgNPs synthesis makes possible the color selection of the AgNPs solutions, from violet to red, as well as the shape (spherical, rod, triangle, hexagonal, cube), and size (from nanometer to micrometer) of the nanoparticles. To our knowledge, this is the first time that an experimental matrix showing multicolor silver nanoparticle solutions with well-defined shape and size using both protective agent (PAA) and reducing agent (DMAB) has been reported in the bibliography.
Localized surface plasmon resonance
Poly(acrylic acid sodium salt)
Transmission electron microscopy
The authors express their gratitude to David García-Ros (Universidad de Navarra) for his help with the TEM images. This work was supported in part by the Spanish Ministry of Education and Science CICYT FEDER TEC2010-17805 research grant.
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