pubs.acs.org/Langmuir © 2009 American Chemical Society
Phase Transfer of Large Anisotropic Plasmon Resonant Silver Nanoparticles from Aqueous to Organic Solution Abhishek P. Kulkarni, Keiko Munechika, Kevin M. Noone, Jessica M. Smith, and David S. Ginger* Department of Chemistry, University of Washington, Seattle, Washington 98195 Received February 17, 2009. Revised Manuscript Received April 24, 2009 We describe the phase transfer of large, anisotropic, silver nanoparticles (∼50-100 nm edge length) from water to polar organics such as alcohols, acetone, dimethylformamide and to nonpolar hexanes. We transferred the silver nanoparticles to the polar organic solvents via their precipitation in water by centrifugation and redispersion in organics. Using scanning electron microscopy (SEM) imaging and UV-vis extinction spectra, we confirmed that there was little to no shape change in the nanoparticles upon transfer to the polar solvents. The nanoparticles were stable for months in the polar organics. We also transferred the nanoparticles to hexanes with up to 75% phase transfer efficiency by using sodium oleate as a surfactant. We found the extinction spectra and transmission electron microscopy (TEM) images of the nanoparticles were similar in water and hexanes, indicating that exchange into hexanes resulted in an only slight change in shape. The nanoparticles were stable for at least 10 days in hexanes under appropriate conditions. The phase transfer efficiency decreased with an increase in the size of the nanoparticles. These results open the possibility for the conjugation of large, anisotropic plasmon resonant silver nanoparticles with organic dyes or their blends with conjugated polyelectrolytes for fundamental optical studies and applications.
Introduction Metal nanoparticles continue to attract great research interest because of the unique size- and shape-dependent optical properties that result from their localized surface plasmon resonances (LSPRs).1-6 For instance, the local electric field enhancements due to LSPR excitation of nanoparticles can enhance the absorption and emission of light from nearby fluorophores.4,7-10 Plasmon resonant silver nanoparticles are also being currently explored as optical antennas in a variety of applications, including bioassays, surface-enhanced Raman spectroscopy (SERS),1-5 and thin-film organic optoelectronics.11-13 In the context of local field enhancements, large anisotropic nanoparticles such as silver nanoprisms offer many advantages over their smaller spherical counterparts, including larger scattering cross sections and LSPRs that can be tuned across the visible spectrum into the near infrared.14-18 Existing colloidal syntheses of silver nanoparticles
with large scattering cross sections, such as nanocubes,19,20 photoconverted prisms (PNP),14,15 or “thermally synthesized” prisms (TNP),18,21-23 are performed almost exclusively in aqueous solution. On the other hand, many fluorophores such as the conjugated polymers being explored in organic light-emitting diodes (OLEDs) and solar cells are soluble primarily in organic solvents which are generally incompatible with aqueous solutions. This limits the use of the more promising large anisotropic silver nanoparticles in organic devices. As a result, most reports to date of plasmon-enhanced organic solar cells have been with evaporated silver films comprising irregular-shaped islands that have LSPR peaks below 450 nm.13 To facilitate solution processing for the fabrication of the organic optoelectronic devices, colloidal silver nanoprisms need to be dispersed in organic solvents. The need to disperse metal colloids in different physicochemical environments has led to a great deal of work on phase transfer of metal nanoparticles from aqueous to organic solution24-37 and
*To whom correspondence should be addressed. E-mail: ginger@ chem.washington.edu.
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7932 DOI: 10.1021/la900600z
Published on Web 05/14/2009
Langmuir 2009, 25(14), 7932–7939
Kulkarni et al.
vice versa.38,39 Many authors have reported transferring small, spherical (e20 nm diameter) metal nanoparticles into an organic phase by hydrophobization of the particle surface using various ligands and surfactants such as alkanethiols,24-26 alkylamines,27,28 long-chain unsaturated carboxylates,29-33 polymeric ligands,34,35 and others.36,37 On the other hand, there are very few reports of transferring larger spherical nanoparticles (>20 nm diameter) to organic solvents25,26 because the stronger van der Waals forces between the larger nanoparticles, and the weaker electrostatic stabilization in organics, usually lead to aggregation. The stability of the nanoparticles in the organic medium depends on various factors, including the shape of the nanoparticles and the chemical nature, surface coverage, and binding strength of the surfactants. Specially synthesized multidentate or bulky thiolbased ligands have been used to enable stable dispersions of large spherical gold nanoparticles (20-100 nm diameter) in organic solvents such as toluene and chloroform.25,26 In addition to the aqueous-to-organic phase transfer approach, direct synthesis of metal colloids, especially for larger particles, in organic solvents has also proven to be more challenging because of the poor dielectric stabilization in organics.40-43 For example, nearly all previous reports of the direct synthesis of silver nanoparticles in organic solvents are for small spheres (98.5%), acetone, methanol, and 2-propanol (HPLC grade) were from EMD Chemicals. DMF and DMSO were reagent grade (J. T. Baker), and ethanol (200 proof) was from Decon Laboratories, Inc. All the reagents and solvents were used as received without any further purification. Synthesis of Silver Nanoparticles. Silver nanoprisms were synthesized following two different literature protocols, the photochemical conversion route14 and the thermal route.18 In this paper, these nanoprisms are denoted as PNP-XYZ and TNP-XYZ, respectively, where XYZ indicates the position of the strongest LSPR scattering peak (typically the in-plane dipole) (in nanometers) of the nanoparticles in water. For instance, TNP-500 denotes a thermal route batch with an extinction maximum at 500 nm in water. Photochemical Route. Small silver nanoparticles were synthesized by borohydride reduction of AgClO4 at 0 °C by rapidly injecting 1 mL of 0.01 M AgClO4 into 99 mL of an ice-cold solution of 1 mM NaBH4 and 0.30 mM sodium citrate in water. The solution immediately turned light yellow and deepened to a bright yellow after being stirred for 20 min. The presence of a strong peak at 400 nm in the extinction spectrum confirmed the presence of small silver particles. These spherical nanoparticles were then photochemically converted into flat triangular/hexagonal shapes by placing the colloidal solution ∼10 cm from a white fluorescent tube light for ∼100-120 h. The solution color changed from bright yellow to green, with the new extinction spectrum showing two LSPR peaks at ∼470 and ∼630-640 nm. Thermal Route. Typically, aqueous solutions of silver nitrate (0.1 mM, 25 mL), sodium citrate (30 mM, 1.5 mL), hydrogen peroxide (H2O2, 30 wt %, 60 μL), and polyvinylpyrrolidone (PVP, 0.7 mM, 1.5 mL) were mixed with different amounts of NaBH4 (100 mM, 100-250 μL) under vigorous stirring. The solution was light yellow initially, indicating the presence of small silver nanoparticles, but turned dark yellow after ∼30 min. Suddenly, the color changed from light pink to pale blue depending upon the amount of NaBH4 added. Silver nanoparticles with scattering peaks ranging from ∼500 to ∼850 nm in water were made using this method. Phase Transfer Protocol.
Highly Polar Organics.
A small amount (0.5-1.0 mL) of the as-synthesized aqueous silver nanoparticle solution was put into a 1.5 mL Eppendorf tube and the tube spun in a centrifuge at 14500 rpm for 5-10 min. The aqueous supernatant was removed, and an equal amount of the polar organic solvent was added to the tube. When the sample was vortex mixed for 10-15 s, the nanoparticles were fully dispersed in the organic solvent. The aqueous colloidal solution could also be concentrated by factors of up to 20 in the organic solvent using this method. Nonpolar Organics. The large, anisotropic silver nanoprisms were transferred from water to nonpolar hexanes using a previously reported method for transfer of small, spherical (
