1342
`
`Chem. Mater. 1997, 9, 1342-1347
`
`The Enzymatic Synthesis of Thiol-Containing Polymers
`to Prepare Polymer-CdS Nanocomposites
`R. Premachandran,† S. Banerjee,† V. T. John,*,† and G. L. McPherson*,‡
`Department of Chemical Engineering and Department of Chemistry, Tulane University,
`New Orleans, Louisiana 70118
`
`J. A. Akkara and D. L. Kaplan
`U.S. Army Soldier Systems Command, Natick, Massachusetts 01760
`
`Received August 5, 1996. Revised Manuscript Received March 18, 1997X
`
`Thiol-containing polyphenolics are enzymatically synthesized in the microstructured
`environment of reversed micelles. CdS semiconductor nanocrystallites, also synthesized in
`the water pools of reversed micelles, are then attached to these polymers through binding
`to the sulfhydryl groups. The polymer-CdS nanocomposite thus prepared exhibits
`luminescence characteristic of such quantum dot particles. Passivation of the CdS by the
`polymer suppresses low-energy emissions associated with surface recombinations, while
`slightly enhancing higher energy emissions resulting from the recombinations in the excitonic
`state of the crystallite interior. The polymer-CdS complex is stable in solution, and the
`solid form can be obtained in the morphology of microspheres.
`
`Introduction
`
`Nanometer-size semiconductor particles are of par-
`ticular interest because of their size-dependent photo-
`physical, photochemical, and nonlinear optical proper-
`ties.,1-3 In contrast to the bulk solid, such quantum dot
`particles exhibit quantum effects that arise from the
`spatial confinement of photogenerated charge carriers.
`Quantum dot particles exhibit structured absorption
`and emission with energies characteristic of particle
`size.4-6 Typically, such semiconductor colloids also have
`a high density of surface defect sites.7 The sites cover
`a broad range of energies and structures with many
`defect states existing at midbandgap energies. The
`surface defects are involved in trapping initially pro-
`duced electron-hole pairs, as shown by the fact that
`emission is significantly red-shifted from the absorption
`edge. The existence of different trap states provides
`multiple pathways for radiative and nonradiative re-
`combination.
`Quantum state particles have a tendency to associate
`because of their large surface-to-volume ratio. To
`overcome the problem, various strategies of nanoparticle
`preparation and size control have been investigated.
`These include encapsulation in sol-gels8 and in polymer
`
`† Department of Chemical Engineering.
`‡ Department of Chemistry.
`X Abstract published in Advance ACS Abstracts, May 1, 1997.
`(1) Brus, L. E. J. Chem. Phys. 1983, 79, 5566.
`(2) Rosetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys.
`1985, 82, 552.
`(3) (a) Alivisatos, A. P.; Harris, A. L.; Levinos, N. J.; Steigerwald,
`M. L.; and Brus, L. E. J. Chem. Phys. 1988, 89, 4000. (b) Alivisatos,
`A. P. Science 1996, 271, 933.
`(4) Weller, H.; Koch, U.; Gutierrez, M.; Henglein, A. Ber. Bunsen-
`Ges. Phys. Chem. 1984, 88, 649.
`(5) Ramdsen, J. J.; Gratzel, M. J. Chem. Soc., Faraday. Trans. 1
`1984, 80, 919.
`(6) Service, R. F. Science 1996, 271, 920.
`(7) (a) Brus, L. E. J. Phys. Chem. 1986, 90, 2555. (b) Petit, C.; Lixon,
`P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598.
`
`matrixes9 and the use of organic capping agents.10
`Among them, it is believed that chemical modification
`with organic reagents such as thiols is a useful prepara-
`tion technique that inhibits particle aggregation, im-
`proves suspension stability, and results in new surface
`functionalities.11 On capping with thiols, the weak
`sulfhydryl bond is replaced with a bond between sulfur
`and surface cadmium ions. The capping can lead to
`surface passivation by the annealing of surface defects.12
`In this paper, we report the synthesis and properties
`of sulfhydryl group containing polymer that can be used
`to bind CdS thus resulting in a polymer-semiconductor
`nanocomposite. The monomer used is 4-hydroxythiophe-
`nol and we report the synthesis of copolymers of
`4-hydroxythiophenol and an alkyl-substituted phenol,
`4-ethylphenol. The polymer synthesis is carried out
`enzymatically using an oxidative enzyme, horseradish
`peroxidase. Enzyme-catalyzed phenol polymerizations
`have been previously shown to be feasible in monophasic
`organic solvents.13,14 The novelty of the present work
`is the synthesis of thiol-substituted polyphenols and the
`synthesis medium which is the microstructured envi-
`ronment of water-in-oil microemulsions, or reversed
`micelles as they are often called. This micellar environ-
`ment is ideally suited for the enzymatic synthesis of
`water-insoluble polymers. The organic phase helps
`sustain solubility of the monomer (and growing chain),
`while the microaqueous phase solubilizes the enzyme
`
`(8) Breitscheidel, B.; Zeider, J.; Schubert, U. Chem. Mater. 1991,
`3, 559.
`(9) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. J. Chem. Phys.
`1987, 87, 7315.
`(10) Steigerwald, M. I.; Alivisatos, A. P.; Gibson, J. M.; Haris, T.
`D.; Kortan, R.; Muller, A. J.; Thayer, A. M.; Duncan, T. M.; Douglas,
`D. C.; Brus, L. E. J. Am. Chem. Soc. 1988, 110, 3047.
`(11) Yoneyama, H.; Torimoto, T. Adv. Mater. 1995, 7, 492.
`(12) Majetich, S. A.; Carter, A. C. J. Phys. Chem. 1993, 97, 8727.
`(13) Dordick, J. S.; Marletta, M. A.; Klibanov, A. M. Biotechnol.
`Bioeng. 1987, 30, 31.
`(14) Akkara, J. A.; Senecal, K. J.; Kaplan, D. L. J. Polym. Sci., Part
`A: Polym. Chem. 1991, 29, 1561.
`
`S0897-4756(96)00418-8 CCC: $14.00 © 1997 American Chemical Society
`
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`
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`Polymer-CdS Nanocomposites
`
`with full retention of catalytic activity. In this paper,
`we therefore report the feasibility of preparing polymers
`in reversed micelles, the morphology of such polymers,
`and the attachment of CdS nanoparticles to the polymer.
`The micellar environment can be used to synthesize
`both the polymer and the CdS nanoparticles. We report
`the steady-state luminescence characteristics of these
`interesting polymer-semiconductor nanocomposites.
`
`Materials and Methods
`
`The enzyme peroxidase (type II, from horseradish), the
`buffer HEPES [N-(2-hydroxyethyl)piperazine-N¢-(2-ethane-
`sulfonic acid)], and hydrogen peroxide, were purchased from
`Sigma Chemicals, St. Louis, MO. The monomers, 4-ethylphe-
`nol (EP), 4-hydroxythiophenol (HTP), the anionic surfactant
`bis(2-ethylhexyl)sodium sulfosuccinate (AOT), were all ob-
`tained from Aldrich Chemical Co. (Milwaukee, WI). The
`solvent isooctane (ACS reagent grade) was also obtained from
`Aldrich.
`Polymer-CdS Synthesis. While both the polymer and
`CdS are synthesized in reversed micelles, the fact that Cd2+
`deactivates horseradish peroxidase necessitates synthesis in
`separate micellar systems. The procedure used in polymer
`synthesis was a simple variation of our earlier procedure.15
`Essentially, the monomers were dissolved in a water-free
`reversed micellar solution containing 0.5 M of the anionic
`surfactant AOT (bis(2-ethylhexyl)sodium sulfosuccinate) in
`isooctane. The total monomer concentration was fixed at 0.15
`M. To this solution, the required volume of enzyme-containing
`HEPES buffer was added to bring the water content to a w0
`value of 15 (w0 is the water-to-surfactant molar ratio). The
`enzyme level in the buffer was adjusted so that the final
`enzyme concentration in the overall solution was 0.5 mg/mL.
`Polymerization was initiated with the addition (in aliquots)
`of the stoichiometric amount of hydrogen peroxide required
`for complete monomer conversion (0.15 M). The initially clear
`solution becomes dark almost immediately and polymer begins
`to precipitate. Although most of the monomer conversion is
`completed within 10-15 min of reaction initiation, the solution
`was stirred for 24 h. The polymer was recovered, washed
`repeatedly with isooctane to remove adsorbed surfactant and
`air-dried.
`CdS nanoparticles were synthesized in reversed micelles
`following standard procedures, using an 0.5 M AOT concentra-
`tion level.16 A 2:1 ratio of Cd2+ containing micelles (from CdCl2
`dissolved in the water pools) mixed with S2- containing
`micelles (from Na2S dissolved in the water pools) yields CdS
`with surface-enriched Cd2+. The w0 for synthesis was adjusted
`to 5 to yield CdS particles of diameter 2-3 nm.17 The CdS
`containing reversed micellar solution was then dried to a
`residue containing CdS and AOT.
`To prepare the composite, the polymer was dissolved in
`dimethyl sulfoxide (DMSO). The CdS + AOT was also
`dispersed into this solution resulting in CdS attachment to
`the polymer thiol groups. The solvent was then removed by
`evaporation in a vacuum oven at room temperature, and the
`residue washed with isooctane. The CdS that is not covalently
`linked to the isooctane-insoluble polymer becomes resuspended
`with AOT in the isooctane wash. After repeated washing with
`isooctane, much of the CdS not covalently attached to the
`polymer is removed. The isooctane wash also removes the
`AOT that is adsorbed to the polymer. The residue, which is
`now the polymer-CdS complex, with CdS covalently attached
`to the polymer, was recovered, dried, and stored for further
`characterization.
`Characterization. The morphologies of the polymer and
`the polymer-CdS nanocomposite were characterized by scan-
`
`(15) Karayigitogulu, C. F.; Kommareddi, N.; Gonzalez, R. D.; John,
`V. T; McPherson, G. L.; Akkara, J. A.; Kaplan, D. L. Mater. Sci. Eng.
`1995, C2, 165.
`(16) Karayitogulu, C. F.; Murthy T.; John, V. T.; McPherson, G. L.
`Colloids Surf. A 1994, 82, 151.
`(17) Henglein, A. Chem. Rev. 1989, 89, 1861.
`
`Chem. Mater., Vol. 9, No. 6, 1997 1343
`
`ning electron microscopy (SEM) and transmission electron
`microscopy (TEM). For SEM analysis, a small amount of the
`solid polymer or the composite was dispersed in isooctane. A
`drop of this suspension was placed on an aluminum stub and
`allowed to dry at room temperature. The stub was then
`coated, first with 20 nm thickness carbon black and then with
`gold. The micrograph was taken at an accleration voltage of
`15-30 kV in a JEOL JSM-820 scanning electron . For TEM
`analysis, a Philips 410 transmission electron microscope with
`a LaB6 (lanthanum hexaboride) crystal electron source was
`used. The polymer containing CdS was dissolved in DMSO,
`and a drop of the solution was placed on a carbon coated copper
`grid and the grid was left to dry for a few minutes before
`transferring into the TEM sample chamber. Micrographs were
`taken at an acceleration voltage of 80 kV.
`Molecular weight distributions of the polymers were mea-
`sured using gel permeation chromatography (GPC). The setup
`consisted of a 25 cm Jordi Gel DVB mixed bed columnm a
`Perkin-Elmer biocompatible binary pump (Model 250) and a
`Perkin-Elmer diode array detector (model 235) interfaced with
`a personal computer. The eluent was THF, and the column
`operated under the eluent head pressure adjusted to maintain
`a flow rate of 0.75 mL/min. The dry polymer was dissolved in
`THF at a concentration of 0.1% (w/v) and 20 (cid:237)L of this solution
`was used for the measurements. Polystyrene samples in the
`molecular weight range of 600-200 000 were used as calibra-
`tion standards.
`An ATI-Mattson Galaxy 6021 FTIR spectrometer was used
`for FTIR measurements. A liquid sample cell (SpectraTech)
`with CaF2 windows and path length 0.3 mm was used to
`measure liquid samples. KBr pellets made from a 1% (by
`weight) polymer/KBr mixture was used to record polymer
`spectra. The UV-vis spectra of the polymers and copolymers
`were recored using a Shimadzu UV-160 spectrophotometer.
`The fluorescence spectra were recorded using a Perkin-Elmer
`luminescence spectrophotometer (LB-50) equipped with a Xe
`lamp as the excitation source. The emission slit was kept at
`10 nm resolution in all measurements.
`A Perkin-Elmer atomic absorption spectrometer, fitted with
`a Cd lamp was used to determine cadmium loading in the
`polymer-CdS complex. the complex (0.01 g) was dissolved in
`1 mL of DMSO. To this solution, 25 (cid:237)L of concentrated HCl
`was added to ensure complete solubility of CdS. The above
`solution was diluted with water to a concentration of 0.1 mg
`composite/mL solution. The absorption of this solution at 322
`nm was used to determine the Cd concentration from a
`standard calibration curve. The instrument absorbance was
`autozeroed using the DMSO/water blank.
`
`Results and Discussion
`
`General Characteristics of Polymerization. The
`monomers used and the structure of the anionic sur-
`factant AOT are shown in Figure 1. Peroxidase-
`catalyzed polyphenol synthesis follows reaction mech-
`anisms similar to those involved in biological lignin
`synthesis.18 Phenoxy radical centers delocalize onto the
`ortho positions from which coupling occurs. The heme
`group of the enzyme (HRP) undergoes 2-electron redox
`reactions during monomer coupling.19 The overall
`condensation reaction can be written as
`
`(R1)H + (R2)H + H2O2 f R1-R2 + 2H2O
`
`where (R1)H and (R2)H are the phenolic monomers or
`oligomers. The direct ring-to-ring attachment generates
`a conjugated polymer in contrast to the more conven-
`tional phenol-formaldehyde resins. Thus, enzymati-
`
`(18) Freudenberg, K. Science 1965, 148, 595.
`(19) Halliwell, B.; Gutteridge, J. M. C. Free Radicals in Biology and
`Medicine; Clarendon Press: Oxford, 1989.
`
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`1344 Chem. Mater., Vol. 9, No. 6, 1997
`
`Premachandran et al.
`
`Figure 1.
`(a) Structure of the anionic surfactant AOT, bis-
`(2-ethylhexyl)sodium sulfosuccinate. (b) A simplified schematic
`of the coupling between 4-ethylphenol and 4-hydroxythiophe-
`nol.
`
`Figure 2. FTIR of AOT CdO vibrations in reversed micelles
`as perturbed by the monomers 4-ethylphenol and 4-hydroxy-
`thiophenol. (a) 0.5 M AOT/isooctane micelles with w0 ) 5. (b)
`Solution (a) with addition of 0.15 M 4-ethylphenol. (c) Solution
`(a) with addition of 0.15 M 4-hydroxythiophenol.
`
`cally synthesized polyphenols are of interest for their
`electrooptical properties.20
`An interesting characteristic of enzymatic polyphenol
`synthesis in reversed micelles is the observation of
`monomer-surfactant interactions and the possible par-
`titioning of the monomer to the oil-water interface.
`Evidence of monomer access to the micellar interface
`is usually seen through IR evidence of perturbations to
`AOT headgroup vibrational frequencies as a result of
`AOT-phenol hydrogen bonding.21 Figure 2 illustrates
`the shift of AOT CdO vibrations to lower frequency,
`upon the addition of the two monomers 4-ethylphenol
`and 4-hydroxythiophenol to AOT reversed micelles. We
`also note in this case, that the monomer, 4-ethylphenol
`
`(20) Akkara, J. A.; Ayyagari, M.; Bruno, F.; Samuelson, L.; John,
`V. T.; Karayigitoglu, C.; Tripathy, S.; Marx, K. A.; Rao, D. V. G. L. N.;
`Kaplan, D. L. Biomimetics 1994, 2, 331.
`(21) Rao, A. M.; Gonzalez, R. D.; John, V. T.; Akkara, J. A.; Kaplan,
`D. L. Biotechnol. Bioeng. 1993, 140, 912.
`
`Figure 3. Scanning electron micrograph of copoly(4-ethylphe-
`nol/4-hydroxythiophenol) with a 1:1 monomer ratio. The
`spherical morphology is obtained with up to 60% 4-hydroxy-
`thiophenol monomer content. At higher levels of 4-hydroxy-
`thiophenol, the morphology is lost and the polymer becomes
`insoluble.
`
`is virtually insoluble in water but highly soluble in
`isooctane. On the other hand 4-hydroxythiophenol is
`very sparingly soluble in isooctane, but its solubility is
`considerably increased by the addition of AOT and
`water. The micellar system therefore offers a method
`to bring mutually incompatible monomers together to
`the vicinity of the enzyme located in the microaqueous
`pools. The IR data also indicate that 4-hydroxythiophe-
`nol perturbs AOT CdO vibrations significantly more
`than 4-ethylphenol, perhaps implying stronger hydrogen
`bonding to AOT.
`Polymerization is very rapid and within a few min-
`utes, most of the conversion is complete. The polymer
`formed precipitates out of the solution and is collected,
`dried, and analyzed. As stated in the earlier section,
`the pure polymer from 4-hydroxythiophenol is an in-
`soluble, intractable material and was not studied fur-
`ther. Copolymers from 4-ethylphenol and 4-hydroxy-
`thiophenol have been synthesized and characterized.
`Figure 3 illustrates the microspherical morphology
`obtained through synthesis in reversed micelles. The
`microsphere morphology is obtained reproduceably with
`a 3/1 AOT/monomer ratio in agreement with our earlier
`work with pure 4-ethylphenol polymers.15 With the
`copolymers studied here, the microsphere morphology
`is obtained as long as the 4-hydroxythiophenol content
`is less than about 60% of the total monomer content.
`At these compositions, the copolymer and its composites
`with CdS are fully soluble in a variety of polar organic
`solvents. The implication here is that the incorporation
`of 4-ethylphenol into the system separates 4-hydroxy-
`thiophenol monomers during synthesis and thereby
`minimizes disulfide bridging.
`While the microsphere morphology is observed with
`the synthesized copolymer, it is noted that the method
`of CdS attachment involves dissolving the copolymer in
`DMSO containing dispersed CdS, as described earlier.
`This procedure destroys the microsphere morphology of
`the nanocomposite. The microspheres can be regener-
`ated by dissolving the nanocomposite in a polar solvent
`(e.g., acetone), followed by reprecipitation using an
`excess of an AOT-isooctane reversed micellar solution
`
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`Polymer-CdS Nanocomposites
`
`Chem. Mater., Vol. 9, No. 6, 1997 1345
`
`Table 1. Copolymer Molecular Weight and CdS Content
`as a Function of Monomer Compositiona
`CdS content
`monomer composition
`polydispersity
`in polymer
`(% 4-hydroxythiophenol) Mn
`(Mn/Mw)
`(wt%)
`Mw
`2300
`0%
`1650
`1.4
`0
`2550
`10%
`1750
`1.5
`3.2
`3490
`30%
`2500
`1.4
`5.4
`50%
`4390 10775
`2.2
`7.2
`a The copolymer molecular weights were measured prior to
`attachment of CdS.
`
`Figure 5. Transmission electron micrograph of CdS in
`polymer-CdS complex. The copolymer was prepared with a
`1:1 concentration of 4-hydroxythiophenol and 4-ethylphenol.
`
`copolymer prior to CdS attachment. The loss of this
`absorbance in curve (c) for the polymer-CdS complex
`is indirect evidence of CdS attachment at the copolymer
`thiol groups. The loss of the S-H stretch upon func-
`tionalization was first reported by Torimoto et al.23 in
`their study of PbS attachment to 4-hydroxythiophenol.
`The present study implies that there is retention of thiol
`functionality upon enzymatic copolymer synthesis, and
`that CdS nanoparticles bind to these thiol groups.
`Figure 4 also illustrates the essential phenolic nature
`of the polymer through retention of the hydroxyl stretch
`band between 3100 and 3500 cm-1. We also examined
`the 1H NMR of the polymer-CdS complex, but the
`structural information is limited because of the broad
`and unresolved peaks in the aromatic region, typical of
`macromolecular species.
`A transmission electron micrograph (TEM) of the
`polymer-CdS nanocomposite is shown in Figure 5 with
`the dark patches representing CdS particles (or particle
`agglomerates). The TEM shows sizes ranging from less
`than 5 nm to about 30 nm, with the vast majority of
`the particles below 20 nm. There appears to be a region
`with a high density of particles, but this region is not
`representative of CdS attached to a single polymer
`chain. Copolymer chains with 10-30 units represent
`end-to-end chain lengths of 5-12 nm. Thus, the CdS
`particle size is comparable to the length of the polymer
`chain. Let us consider a simple calculation with a
`copolymer chain 20 units long assuming a 1/1 distribu-
`tion of the two monomers alternating on the chain. In
`a polar solvent, the chain adopts an open structure since
`segment-solvent interactions dominate over the in-
`tramolecular hydrogen bonding responsible for chain
`folding and curvature.22 In such an open structure, a
`20-unit copolymer chain has an extended length of 7.4
`nm with the thiol groups separated by 0.74 nm (calcu-
`lated using the HyperChem Molecular Modeling Soft-
`ware). Assuming a CdS particle size of 3 nm, the
`picture of the nanocomposite is one where there are just
`two or three CdS nanoparticles attached to a chain at
`
`Figure 4. FTIR spectra of copoly(4-ethylphenol/4-hydroxy-
`thiophenol) prepared with an equimolar concentration of
`monomers. The S-H stretch is the small feature at 2560 cm-1.
`The inset shows the expanded region. In the inset (a) il-
`lustrates the S-H stretch of the monomer 4-hydroxythiophe-
`nol, (b) the S-H stretch of the copolymer and (c) the lack of
`an S-H stretch when CdS is attached to the copolymer. These
`are solid-state IR samples prepared in a KBr pellet.
`
`which is a nonsolvent for the composite.22 It is also
`possible to attach CdS to the copolymer by reprecipi-
`tating the copolymer using CdS-containing micellar
`solutions. The use of the reversed micellar medium as
`a nonsolvent for presynthesized polyphenolics, the
`generation of microspheres, and the encapsulation of
`intramicellar solutes in the polymer matrix are de-
`scribed in detail in another paper.22
`Table 1 lists the molecular weight data for the
`copolymers, and it is seen that the peak molecular
`weight shows a consistent increase with the 4-hydroxy-
`thiophenol monomer content. From oligomers with
`approximately 13 units (pure 4-ethylphenol monomer),
`the molecular weight increases to about 35 units when
`the 4-hydroxythiophenol content is increased to 50%
`total monomer. Additionally, the polydispersity index
`increases at 50% 4-hydroxythiophenol content. The
`table also lists saturation CdS contents of the compos-
`ites synthesized from these copolymers indicating the
`increase in loading with an increase in the 4-hydroxy-
`thiophenol
`level. Evidence of CdS binding to the
`polymer is further shown by the inset to the FTIR
`spectra of Figure 4, where the weak band at 2560 cm-1
`is attributed to the vibrations of the S-H bond. Curve
`(a) of the inset shows the S-H stretch for the monomer,
`4-hydroxythiophenol, while curve (b) represents the
`
`(22) Banerjee, S.; Premchandran, R.; John, V. T.; McPherson, G.;
`Akkara, J.; Kaplan, D. L. Ind. Eng. Chem. Res. 1996, 35, 3100.
`
`(23) Torimoto, T.; Uchida, H.; Sakata, T.; Mori, H.; Yoneyama, H.
`J. Am. Chem. Soc. 1993, 115, 1874.
`
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`1346 Chem. Mater., Vol. 9, No. 6, 1997
`
`Premachandran et al.
`
`Figure 6. The emission spectra of copoly(50% 4-ethylphenol/
`50% 4-hydroxythiophenol)-CdS complex with excitation at 400
`nm. The inset shows the corresponding absorbance spectra.
`The CdS particles here were first synthesized in w0 5 micelles.
`Emission spectra obtained in chloroform.
`
`most. It is also possible that chains could be bridged
`by being bound to the same CdS particle, and a
`multiplicity of such bridged chains and particles could
`lead to the particle clustering. GPC measurements of
`the polymer molecular weight after CdS attachment
`show a small (about 100 Da) but consistent increase in
`the peak molecular weight. This may simply be a result
`of changing GPC elution characteristics upon CdS
`attachment and is not necessarily indicative of chain
`bridging.
`Absorbance and Fluorescence Properties. The
`fluorescence spectrum for a copolymer-CdS complex is
`shown in Figure 6, with excitation at 400 nm. The
`corresponding absorbance spectrum is shown in the
`inset to the figure. The CdS nanoparticles were syn-
`thesized in reversed micelles of w0 5 prior to capping
`with the copolymer. The CdS absorption edge of 400
`nm is virtually unchanged upon capping. The particle
`size of the nanocrystals was calculated from the absorp-
`tion edge to be 3 nm.24 An interesting observation
`regarding the stability of CdS in the complex is the
`retention of the absorption spectrum upon extended
`storage in the absence of light. In contrast to uncapped
`CdS where Ostwald ripening effects are seen upon
`extended storage,25 there is no red-shift in the absorp-
`tion edge that would indicate particle growth. Such
`stabilization to agglomeration is a general phenomena
`of CdS and PbS capping by thiol-based compounds.23
`The emission spectrum of Figure 6 is rather interest-
`ing as it exclusively shows a peak at 470 nm. CdS
`nanoparticles typically have two characteristic emis-
`sions, one at 540 and the other at 470 nm.25,26 The
`emission at 540 nm is assigned to hole-electron recom-
`binations at surface traps, while the higher energy
`emission at 470 nm near the band edge is attributed to
`recombination from the excitonic state in the crystallite
`interior.27,28 To understand whether there is indeed a
`quenching of the low-energy fluorescence upon nano-
`
`(24) Moffit, M.; Eisenberg, A. Chem. Mater. 1995, 7, 178.
`(25) Robinson, B. H.; Towey, T. F.; Zourab, S.; Visser, A. J. W. G.;
`van Hoek, A. Colloids Surf. 1991, 61, 175.
`(26) Noglik, H.; Pietro, W. J. Chem. Mater. 1994, 6, 1593.
`
`Figure 7. Emission spectra of (a) 2 (cid:2) 10-4 M CdS (Cd2+/S2-
`) 2) prepared in reversed micelles dried, and redispersed in
`chloroform (b) sample (a) with addition of 0.012 mg/mL copoly-
`(50% 4-ethylphenol/50% 4-hydroxythiophenol) (c) sample (a)
`with addition of 0.012 mg/mL poly(4-ethylphenol). Excitation
`at 400 nm.
`
`particle capping with the copolymer, the following
`experiment was carried out, the results of which are
`shown in Figure 7. Here, CdS nanoparticles were
`prepared in reversed micelles, following which the
`micellar system was completely dried leaving a CdS +
`AOT residue. This residue was then redispersed in
`chloroform and the fluorescence spectrum taken (spec-
`trum a). The fluorescence shows typical CdS emission
`characteristics with a broad emission maximum extend-
`ing from 470 to 540 nm. Copoly(4-ethylphenol, 50%/4-
`hydroxythiophenol, 50%) was then dissolved into the
`solution. An immediate quenching of the low-energy
`fluorescence is observed (spectrum b) with a small
`increase in the high-energy emission. The observed
`overall decrease in quantum yield is indicative of
`nonradiative pathways introduced through such surface
`passivation and capping. The selective quenching of the
`540 nm emission has been observed by others upon
`addition of mercaptopyridines26 and methylviologen.29,30
`These earlier studies attributed such selective quench-
`ing to the destruction of surface trap states through
`electron capture by the capping organic compound
`which acts as an electron acceptor.26 Similar mecha-
`nisms may be operational here. Verification that the
`blue-shift is due to the 4-hydroxythiophenol content of
`the copolymer is obtained by adding pure poly(4-
`ethylphenol) rather than the copolymer (spectrum c).
`While there is some quenching of fluorescence, there is
`little change in the fluorescence maximum. The appar-
`ent quenching may be simply due to polymer self-
`absorbance at the excitation wavelength (400 nm).
`Finally, Figure 8 illustrates the emission character-
`istics of copolymer-CdS complexes containing various
`levels of 4-hydroxythiophenol. As intuitively expected,
`the fluorescence intensity increases with the 4-hydroxy-
`thiophenol content of the copolymer. The data are
`
`(27) Fojtik, A.; Weller, H.; Koch, U.; Henglein, A. Ber. Bunsen-Ges.
`Phys. Chem. 1984, 88, 969.
`(28) Chen, W.; McLendon, G.; Marchetti, A.; Rehm, J. M.; Freedhoff,
`M. I.; Myers, C. J. Am. Chem. Soc. 1994, 116, 1585.
`(29) Weller, H.; Koch, U.; Gutierrez, M.; Henglein, A. Ber. Bunsen-
`Ges. Phys. Chem. 1984, 88, 649.
`(30) Gratzel, M. In Electrochemistry in Colloids and Dispersions;
`Mackay, R. A., Texter, J., Eds.; VCH Publishers: New York, 1992.
`
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`Polymer-CdS Nanocomposites
`
`Figure 8. Fluorescence emission spectra of CdS from com-
`plexes containing (a) copoly (50% 4-ethylphenol/50% 4-hy-
`droxythiophenol) (b) copoly(70% 4-ethylphenol/30% 4-hydroxy-
`thiophenol) (c) copoly (90% 4-ethylphenol/10% 4-hydroxy-
`thiophenol). (d) copoly(70% 4-ethylphenol/30% 4-hydroxy-
`thiophenol) but without CdS. Emission spectra obtained in
`chloroform with excitation at 400 nm. Saturation loadings of
`CdS used in all samples.
`
`consistent with the observation that the saturation CdS
`loadings on the polymer increase with the 4-hydroxy-
`thiophenol content (Table 1). There is a small red-shift
`in the emission with increasing CdS loading and at the
`highest loading (spectrum a) a distinct shoulder is
`observed at 540 nm indicative of surface recombinations.
`Spectrum (d) is the polymer of spectrum (a) but without
`CdS, which verifies that the polymer has no intrinsic
`fluorescence when excited at 400 nm.
`
`Conclusions
`
`This work demonstrates a biocatalytic approach to
`synthesize polymers that can be formulated into pho-
`toluminescent composites by the attachment of semi-
`conductor nanoparticles. We note that the micellar
`environment, while convenient for the synthesis of CdS
`
`Chem. Mater., Vol. 9, No. 6, 1997 1347
`
`nanoparticles, is not the best environment to synthesize
`monodispersed nanoclusters, and there are much su-
`perior synthetic routes to monodisperse II-VI semicon-
`ductor nanoparticles.31 Regardless of how the semicon-
`ductor component is prepared, the task addressed here
`is the synthesis of the thiol-containing polymer, and this
`seems to be well-suited to the micellar environment.
`Once the polymer is synthesized, attachment of the
`semiconductor component is straightforward. The re-
`sulting nanocomposite is highly processible and can be
`fabricated into films, coatings, etc. An interesting
`aspect of polymer synthesis in reversed micelles is the
`observation that polymer microspheres can be generated
`implying an ease of dispersion in coatings applications.
`The complex displays the fluorescence characteristics
`of CdS but without the low-energy emissions associated
`with surface recombinations. The polymer thus caps
`and passivates the CdS nanoparticles. The complex is
`easily soluble in polar organic solvents. It is indefinitely
`stable in solution both in its solubility characteristics
`and in its luminescent properties.
`Continuing studies seek to expand the application
`potential of these polymer-CdS nanocomposites. Due
`to conjugation in the polymer, these materials may be
`electroluminescent with consequent applications in sen-
`sor and display technologies. The conjugated polymer-
`CdS complex may also have applications in nonlinear
`optics as both components should have high third-order
`nonlinear susceptibilities. From another perspective,
`the hydroxyl groups present on the polymer imply ease
`in functionalization. For example, it is possible to
`attach affinity ligands to the polymer leading to further
`application in biosensor technologies. These materials
`may therefore represent a new class of polymer-
`inorganic composites with useful fluorescent properties.
`
`Acknowledgment. Financial support from National
`Science Foundation and the US Army is gratefully
`acknowledged.
`CM960418P
`
`(31) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem.
`Soc. 1993, 115, 8706.
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