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Semiconductor Nanoparticle/Polystyrene Latex Composite Materials Robert L. Sherman, Jr. and Warren T. Ford* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received December 22, 2004. In Final Form: March 7, 2005 Cadmium sulfide and cadmium selenide/cadmium sulfide core/shell nanoparticles stabilized with poly(cysteine acrylamide) have been bound to polystyrene (PS) latexes by three methods. First, anionic 5 nm diameter CdS particles were electrostatically attached to 130 nm surfactant-free cationic PS latexes to form stable dispersions when the amount of CdS particles was less than 10% of the amount required to form a monolayer on the surface of the PS particles or when the amount of CdS particles exceeded the amount required to form a monolayer on the PS particles. Transmission electron microscopy (TEM) showed nanoparticles on the surface of the latex particles. Fluorescence spectra showed unchanged emission from the nanoparticles. Second, anionic, surfactant-free PS latexes were synthesized in the presence of CdS and CdSe/CdS nanoparticles. TEM showed monodisperse latex particles with trapped nanoparticles. Third, surfactant-stabilized latexes were synthesized by copolymerization of styrene with vinylbenzyl(trimethyl)ammonium chloride electrostatically bound to the CdSe/CdS nanoparticle surface. Brownian motion of the submicroscopic composite particles in water was detected by fluorescence microscopy.
Introduction Submicrometer-sized fluorescent spheres are valuable as imaging agents for confocal fluorescence microscopy and flow cytometry. Most fluorescent polymer spheres are either swollen with or copolymerized with organic dyes.1-5 The organic dyes have high fluorescence quantum yields but photobleach rapidly. The photobleaching problem can be overcome by use of semiconductor nanoparticles as the fluorophores. Semiconductor nanoparticles have been attached to polymer particles electrostatically,6-9 by ligand exchange,10 and by trapping during solvent swelling.11 Nanoparticles have been synthesized on the surface of polymer particles,12 and nanoparticles have been captured from solution during polymer particle synthesis.13 Aqueous latex dispersions are susceptible to aggregation in the presence of salts. This is a problem when working with aqueous nanoparticle dispersions, which usually contain an excess of ionic stabilizer and have a high pH.14 * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Bradley, M.; Ashokkumar, M.; Grieser, F. J. Am. Chem. Soc. 2003, 125, 525-529. (2) Bosma, G.; Pathmamanoharan, C.; de Hoog, E. H. A.; Kegel, W. K.; van Blaaderen, A.; Lekkerkerker, H. N. W. J. Colloid Interface Sci. 2002, 245, 292-300. (3) Jardine, R. S.; Bartlett, P. Collloids Surf., A 2002, 211, 127-132. (4) Hu, H.; Larson, R. G. Langmuir 2004, 20, 7436-7443. (5) Pham, H. H.; Gourevich, I.; Oh, J. K.; Jonkman, J. E. N.; Kumacheva, E. Adv. Mater. 2004, 16, 516-520. (6) Radtchenko, I. L.; Sukhorukov, G. B.; Gaponik, N.; Kornowski, A.; Rogach, A. L.; Mohwald, H. Adv. Mater. 2001, 13, 1684-1687. (7) Susha, A. S.; Caruso, F.; Rogach, A. L.; Sukhorukov, G. B.; Kornowski, A.; Mohwald, H.; Giersig, M.; Eychmuller, A.; Weller, H. Colloids Surf., A 2000, 163, 39-44. (8) Rogach, A.; Susha, A.; Caruso, F.; Sukhorukov, G.; Kornowski, A.; Kershaw, S.; Mo¨hwald, H.; Eychmuller, A.; Weller, H. Adv. Mater. 2000, 12, 333-337. (9) Caruso, F.; Lichtenfeld, H.; Giersig, M.; Moehwald, H. J. Am. Chem. Soc. 1998, 120, 8523-8524. (10) Hirai, T.; Saito, T.; Komasawa, I. J. Phys. Chem. B 2001, 105, 9711-9714. (11) Han, M.; Gao, X.; Su, J. Z.; Nie, S. Nat. Biotechnol. 2001, 19, 631-635. (12) Zhang, J.; Coombs, N.; Kumacheva, E. J. Am. Chem. Soc. 2002, 124, 14512-14513. (13) Yang, X.; Zhang, Y. Langmuir 2004, 20, 6071-6073.
We have solved this problem by using nanoparticles stabilized with polymers of cysteine acrylamide (1) that
are dialyzed to remove salts.15 The free carboxylate ions of poly-1 stabilize the surface of the particle. The nanopaticles are colloidally stable in aqueous solution for at least 2 years. We now have used these exceptionally stable nanoparticles to produce water-dispersable composite polystyrene latexes that are 100-300 nm in diameter, are easily detected by fluorescence microscopy, do not photobleach readily, and are colloidally stable with the nanoparticles irreversibly bound to the latex particle. Results and Discussion Electrostatic Attachment of Nanoparticles to Oppositely Charged Latexes. CdS and CdSe/CdS core/ shell semiconductor nanoparticles stabilized with poly-1 were prepared as described elsewhere.15 Their properties are reported in Table 1. Cationic polystyrene latexes were prepared as reported earlier by shot growth emulsion polymerization of styrene, varied amounts of vinylbenzyl chloride, and 1% of vinylbenzyl(trimethyl)ammonium chloride as described in the Experimental Section.18,19 Particle diameters were mea(14) Rogach, A. L.; Nagesha, D.; Ostrander, J. W.; Giersig, M.; Kotov, N. A. Chem. Mater. 2000, 12, 2676-2685. (15) Sherman, R. L. J.; Chen, Y.; Ford, W. T. J. Nanosci. Nanotechnol. 2004, 4, 1032-1038. (16) Rockenberger, J.; Troeger, L.; Kornowski, A.; Vossmeyer, T.; Eychmueller, A.; Feldhaus, J.; Weller, H. J. Phys. Chem. B 1997, 101, 2691-2701. (17) Weller, H.; Schmidt, H. M.; Koch, U.; Fojtik, A.; Baral, S.; Henglein, A.; Kunath, W.; Weiss, K.; Dieman, E. Chem. Phys. Lett. 1986, 124, 557-60. (18) Ford, W. T.; Yu, H.; Lee, J. J.; El-Hamshary, H. Langmuir 1993, 9, 1698-1703. (19) Tan, S.; Sherman, R. L., Jr.; Ford, W. T. Langmuir 2004, 20, 7015-7020.
10.1021/la0468139 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/15/2005
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Table 1. Semiconductor Nanoparticles sample (ID)
absorbance cutoff (nm)a
emission maximum (nm)
diameter (nm)b
CdS (236) CdS (288) CdS (496) CdSe/CdS (239) CdSe/CdS (333) CdSe/CdS (359)
430 435 415 600 597 605
585 580 600
5.2 5.3 4.9 5 5 5
a Wavelength of the tangent to band edge absorption extrapolated to zero absorbance. b Based on the band gap of CdS16,17 or CdSe,14 taken as the absorbance cutoff.
Figure 1. TEM of composite 262 having more than a monolayer of CdS on a polystyrene latex.
Table 2. Cationic Polystyrene Latexes sample
mol% N+Cl-
diameter, TEM (nm)
PDIa
diameter, DLS (nm)
230b 345
5.1 11.3
117 271
1.01 1.01
167 250
a PDI ) polydispersity index ) 1 + (standard deviation/mean diameter) from TEM measurements. b Cross-linked with 1% divinylbenzene.
Scheme 1. Electrostatic Attachment of Nanoparticles to Latexes
Table 3. Stable Latex/CdS Nanoparticle Composite Mixtures sample
latex
nanoparticles
color
coverage
242 262 337B 337A
230 230 345 230
CdS 236 CdS 236 CdS 236 CdSe/CdS 239
yellow yellow yellow orange
0.1 monolayer >monolayer >monolayer all coverages
sured by TEM and by dynamic light scattering (DLS). The properties are reported in Table 2. To form latex/semiconductor nanoparticle composite materials, anionic CdS nanoparticles were attached to cationic latex particles as shown in Scheme 1. Properties of the composite particles are listed in Table 3. The composite latexes were colloidally stable only when the amount of CdS particles was less than 10% of the amount required to form a monolayer on the surface of the PS particles or when the amount of CdS particles exceeded the amount required to form a monolayer on the PS particles. Calculation of a monolayer was based on square packing of nanoparticle spheres on a planar latex surface. A PS latex coated with more than a monolayer of CdS is shown in the TEM image of Figure 1. Our TEM was not capable of resolving individual CdS particles, but the fuzzy surface of the PS particles appeared only after treatment with the CdS nanoparticles. Attempts at forming composites with 50% of the amount of CdS particles required for a monolayer on the PS surface resulted in aggregation and precipitation of the polymer particles within seconds. Further addition of CdS particles with agitation failed to redisperse the yellow precipitate, even when the amount of CdS particles exceeded the amount required for one monolayer. To minimize aggregation of PS particles, all reported samples having at least one monolayer of CdS particles on the PS surface were prepared by rapid addition of the nanoparticle solution to the latex with vigorous stirring.
Figure 2. Emission spectra of CdSe/CdS nanoparticles (239) and CdSe/CdS/polystyrene composite (337A) from 400 nm excitation.
A TEM image of a larger cationic latex with more than a monolayer of CdS particles (337B, Table 3) showed no aggregates. However, as the amount of CdS approached two layers, nanoparticles appeared in the background on the TEM grid. To determine if the CdS was attached to the latex, composites such as samples 242 and 262 were filtered using a 100 nm cutoff syringe filter. This resulted in a colorless filtrate containing neither CdS nor latex and retention of a yellow solid. In a control experiment, a CdS nanoparticle solution (sample 236, Table 1) filtered by the same method gave a yellow filtrate containing CdS particles and no retained solid. Thus, all of the CdS particles were bound to the latex. CdSe/CdS nanoparticles (sample 239, Table 1) as 18 nm diameter aggregates20,21 also were attached to cationic latex 230. The composite 337A did not aggregate at any ratio of nanoparticles to latex. TEM analysis showed that most, but not all, of the nanoparticles were bound to the latex. The CdSe/CdS-coated latexes fluoresce with no shift in emission wavelength, as shown in Figure 2. Emission studies from CdS-coated latexes were more difficult because CdS nanoparticles fluoresce best when the excitation wavelength is below 400 nm, where the latex dispersions strongly scatter light. Since the CdS nanoparticles had absorption cutoffs of 415-435 nm, an excitation wavelength of 400 nm was chosen. Further experiments with the stable composites of anionic CdS bound to cationic latexes showed that the CdS easily separated from the latex in the presence of (20) Ji, T.; Fang, J. Abstracts of Papers, 225th ACS National Meeting, New Orleans, LA, United States, March 23-27, 2003 2003, INOR-241. (21) Yan, Y.-l.; Li, Y.; Qian, X.-f.; Yin, J.; Zhu, Z.-k. Mater. Sci. Eng., B 2003, B103, 202-206.
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Table 4. Trapping Nanoparticles during Two Step Shot Growth Emulsion Polymerization samplea 292 316 317 374 497
water (mL)
nanoparticlesb
styrene (mL)
NaSS (mg)
K2S2O8 (mg)
T (°C)
diameter (nm)
PDI
30 6 20 10 20 10 30 60 12
CdS 288b,c 6 CdS 288b,c 10 CdS 288b,f 10 CdSe/CdS 355b,c CdS 496b,c 12
5.0 1.0 2.0 0.5e 2.0 0.5e 2.5 10 1.0
26 17 16 10 16 10 15 50 30
33 7 15 5 15 5 15 66 14
75
177d
1.00
60
166d
1.01
60
140d
1.21
60 75
121d 195g
1.01 -
a First and second row data for each sample are materials used for the first and second steps of the polymerization. b 0.8 mg of nanoparticles/ mL. c CdS coated with monomer 1. d TEM. e Methyl methacrylate used in place of styrene. f CdS coated with poly-1. g DLS.
Scheme 2. Capture of Nanoparticles during Latex Formation
cationic surfactants, cationic initiators, or cationic monomers. Under these conditions, the nanoparticles rapidly aggregated into a yellow oil leaving behind a white latex dispersion. This led to a search for a stronger more permanent method of attachment of nanoparticles. Trapping CdS Nanoparticles during Emulsion Polymerization. Latexes were produced by surfactantfree shot growth emulsion polymerization of styrene and sodium 4-styrenesulfonate22,23 in the presence of semiconductor nanoparticles, as shown in Scheme 2. The majority of the monomer was polymerized in the presence of CdS nanoparticles stabilized with monomer 1 to form a latex. Before the polymerization was complete, the remainder of the monomer (the second shot) was added to increase the size and introduce more ionic units to stabilize the surface of the latex. This two-step process allows for capture of the nanoparticles in the first step of the polymerization and then allows for further latex growth from the second shot of monomer. Conditions for the syntheses are shown in Table 4, and TEM images are shown in Figure 3. Sample 292 was analyzed by TEM and DLS to have a diameter of 177 and 226 nm, respectively, as seen in Figure 3. Although the TEM image in Figure 3 does not show the individual nanoparticles, the presence of CdS is certain because of the color and the fluorescence of the polymer particles. This sample was stable for more than 2 years with no precipitate and no loss of nanoparticle color. The method was also found to be reproducible: Duplicate sample 497 was identical in appearance and almost the same size as sample 292. Attempts to integrate methyl methacrylate into polystyrene latexes having CdS nanoparticles stabilized with either monomer 1 or poly-1 during the second step of emulsion polymerization left some nanoparticles in solution and some nanoparticles in the latex (samples 316 and 317 in Table 4). An attempt at integrating CdSe/CdS nanoparticles into a polystyrene latex by batch emulsion polymerization (sample 374 in Table 4) also left some particles in solution. (22) Kim, J. H.; Chainey, M.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 3187-99. (23) Sunkara, H. B.; Jethmalani, J. M.; Ford, W. T. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1431-5.
Figure 3. TEM image of polystyrene/CdS sample 292.
The CdS and CdSe/CdS particles not trapped in the latexes were removed from the water easily by ultrafiltration. The remaining latex containing CdSe/CdS had strong fluorescence almost the same as that of the nanoparticles alone. Brownian motion of sample 497 was observed by fluorescence microscopy, as shown in Supporting Information. During the microscopy experiment, the particles did not photobleach over 45 min of analysis. Formation of Polystyrene Latexes using CdSe/ CdS Nanoparticles with Vinylbenzyl(trimethyl)ammonium Counterions. Since anionic nanoparticles attract quaternary ammonium ions, we prepared an anionic PS latex with a cationic monomer prebound to CdSe/CdS nanoparticles. CdSe/CdS was titrated with vinylbenzyl(trimethyl)ammonium chloride to one tenth of the concentration that was determined in a separate experiment to precipitate the nanoparticles. The cationic monomer provided polymerizable groups on the nanoparticle surface. The nanoparticles were then dispersed into a styrene/water mixture with sodium dodecyl sulfate, as illustrated in Scheme 3. Polymerization gave the latexes reported in Table 5 and pictured in Figure 4. The particles were colloidally stable and showed strong fluorescence from CdSe. During the polymerization, the orange nanoparticles turned to purple before latex particles were formed. The mixture became milky pink when the particles formed and gave the blue-shifted fluorescence spectrum in Figure 5. In control experiments, mixing and heating CdSe/CdS nanoparticles with each individual reactant and with combinations of the reactants did not result in a change in fluorescence. Only when all of the reactants were present did the fluorescence shift occur. We have no explanation for the blue shift of the fluorescence. The latexes reported in Table 5 all showed good emission and colloidal stability. Because sample 336 was 80 nm in diameter, less than wanted, samples 365 and 367 were made using smaller amounts of surfactant. As expected, a decrease in the amount of surfactant increased the size
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Scheme 3. Nanoparticle Capture via Monomer Coordination
Table 5. Latex Synthesis Using Precoordinated Nanoparticles CdSe/CdSa sample
(mg)
H 2O (mL)
VBTMAC (mg)b
styrene (mL)
SDS (mg)
AIBN (mg)
T (°C)
diameter (nm)
PDI
336 365 367
333 (32) 342 (16) 342 (16)
45 20 20
100 50 50
4.3 3.0 3.0
250 120 90
49 20 20
70 60 60
80c 94d 119c
1.2 1.1
a
See Table 1. b Vinylbenzyl(trimethyl)ammonium chloride. c TEM.
Figure 4. TEM image of composite 336.
Figure 5. Fluorescence spectra of CdSe/CdS nanoparticles 333 and composite latex 336.
of the latex. These 80-120 nm diameter latexes should also be useful for fluorescence microscopy. Discussion Nanoparticle/polystyrene latex composite materials have been synthesized by three methods. (1) Electrostatic attachment of CdS nanoparticles to the surfaces of preformed cationic latexes gave colloidally stable com-
d
DLS.
posites at less than 10% of a calculated monolayer and at greater than a monolayer of CdS particles. Concentrations between 10% and a full monolayer caused aggregation and precipitation. However, the attachment of the nanoparticles to the latex surface was weak, and organic cations such as surfactants could remove the nanoparticles from the latex. These materials were not suitable for particle tracking by microscopy due to the low fluorescence intensity at 100% coverage. (2) Attachment of the nonpolymerized CdS nanoparticles to latexes by entrapment during anionic latex formation was simple and effective. The size of the latexes is easily tunable. In some cases, not all of the nanoparticles were integrated into the latexes. Brownian motion of 195 nm diameter polystyrene-CdS composite particles was observed by fluorescence microscopy. (3) Polymerization of styrene in the presence of CdSe/CdS nanoparticles having vinylbenzyl(trimethyl)ammonium chloride electrostatically attached to the surface resulted in efficient uptake of nanoparticles and strong emission suitable for optical tracking experiments. Similar monodisperse spheres containing luminescent semiconductor particles have been reported by others: CdS particles on the surface of 150-600 nm diameter poly(methyl methacrylate) spheres,12 CdSe/ZnS and CdS/ZnS particles in the outer shell of 154-954 nm diameter silica spheres,24 and CdSe dots and rods in 250-1000 nm diameter spheres of composites composed of silica with minor amounts of polystyrene.25 The fluorescence quantum yields of the nanoparticle/silica composites were higher than ours owing to preparation of the nanoparticles by a high-temperature method using trioctylphosphine oxide as the surface stabilizer in nonpolar solvent followed by transfer of the nanoparticles to water/ethanol or water/ toluene for synthesis of the silica spheres. All of these semiconductor/polymer and semiconductor/silica composite particles conveniently are dispersed in water and have (24) Chan, Y.; Zimmer, J. P.; Stroh, M.; Steckel, J. S.; Jain, R. K.; Bawendi, M. G. Adv. Mater. 2004, 16, 2092-2097. (25) Mokari, T.; Sertchook, H.; Aharoni, A.; Ebenstein, Y.; Avnir, D.; Banin, R. Chem. Mater. 2005, 17, 258-263.
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potential uses as colloidal crystals for photonic band gap materials, biological labeling, and optical tracking. Despite the lower fluorescence, our poly(cysteine acrylamide)stabilized nanoparticles have the advantage of synthesis of all components under mild conditions.15 Experimental Section General Methods. Except as noted, reagents from Aldrich Chemical Co. and Fisher Scientific Co. were used without purification. VA-044 (2,2′-azobis(2-(2-imidazolin-2-yl)propane dihydrochloride) was from Wako Specialty Chemicals. Styrene and methyl methacrylate were purified by passing the monomers down a column of basic alumina to remove inhibitors. Water was purified via a three-column Barnstead e-pure water filtration system to a conductivity of
