Spectroscopic Characterization of Core-Based Hyperbranched Poly

Jun 22, 2010 - Katrina K. Kline and Sheryl A. Tucker*. Department of Chemistry, University of Missouri, 125 Chemistry Building, Columbia, Missouri 652...
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J. Phys. Chem. A 2010, 114, 7338–7344

Spectroscopic Characterization of Core-Based Hyperbranched Poly(ethyleneimine) and Dendritic Poly(propyleneimine) as Selective Uptake Devices Katrina K. Kline and Sheryl A. Tucker* Department of Chemistry, UniVersity of Missouri, 125 Chemistry Building, Columbia, Missouri 65211 ReceiVed: April 7, 2010; ReVised Manuscript ReceiVed: June 2, 2010

Dendritic polymers have a wide range of potential applications; however, the extensive synthesis and limited availability of bulk quantities of dendrimers restrict their use. Core-based hyperbranched polymers (CBHPs) are, therefore, an attractive alternative to dendrimers for many applications. The selectivity of core-based hyperbranched poly(ethyleneimine), as a host for phenol blue and 2-hydroxy Nile red guests, was investigated using absorption and fluorescence spectroscopies. Research results are compared to those for its dendritic counterpart, poly(propyleneimine). Phenol blue is known to associate near the core in both the CBHPs and dendrimers investigated. The interfering agent, 2-hydroxy Nile red, has also been shown to associate with these polymers; however, this interaction occurs in the outermost branches. In this work, it was found that phenol blue was sequestered in both the CBHPs and dendrimers in the presence of interfering agent, and this association appeared to be the same as that of the polymers with phenol blue alone. Although the presence of 2-hydroxy Nile red did affect the association of phenol blue, there was still considerable association even when 2-hydroxy Nile red was in 10-fold excess. The association of phenol blue with both the CBHPs and dendrimers was stable and robust; however, the association of 2-hydroxy Nile red was relatively weak and unstable. Introduction Traditional randomly hyperbranched polymers have limited applications because of their large molecular-weight distributions and wide range of polydispersities. Such hyperbranched polymers generally have a degree of branching between 50% and 75% and are nonsymmetrical.1-4 Recently, a new class of polymersscore-based, randomly hyperbranched polymers (CBHPs)shas been synthesized on an industrial scale.5 These new materials contain an initiator core molecule, similar to that of the dendritic polymers. Dendrimers are a class of polymers that have well-defined macromolecular structures. They are synthesized in a labor-intensive process of iterative additions or generations (G) of monomeric branching units that emanate from or are later attached to a core atom or molecule. In contrast to hyperbranched polymers, dendrimers have a degree of branching of 100%.6 There is a wide range of applications for dendrimers including drug transport,7 gene transport systems,8 high-loading supports for organic synthesis,9 water purification systems,10 and molecular nanocarriers.11 However, because of their labor-intensive synthesis and the resulting limited availability in bulk quantities, large-scale use of dendrimers is limited. Core-based, randomly hyperbranched polymers are a potentially attractive alternative to dendrimers. They can be prepared using a one-pot synthesis with a maximum of two steps, resulting in a polydispersity range of 1.3-2.5, where a higher polydispersity index indicates a wider range of molecular weights.12 The similar structural features of CBHPs and dendrimers make the former an interesting material. In addition to the fact that they both contain a core region, CBHPs also have a globular three-dimensional architecture, “interior cavities”, and the availability of a large number of surface functional groups.13-19 These architectural features are responsible for the * To whom correspondence should be addressed. Phone: (573) 882-8374. Fax: (573) 882-2754. E-mail: [email protected].

Figure 1. Molecular structure of the CBHP poly(ethyleneimine) (PEI).

extensive proposed applications of these advanced materials, which mirror those of dendritic polymers. Using spectroscopic investigations, this research compares the ability of the CBHP poly(ethyleneimine) (PEI, Figure 1) to selectively entrap a guest molecule in the presence of an interfering agent with that of its dendritic counterpart, poly(propyleneimine) (PPI, Figure 2). Although dendritic poly(ethyleneimine) has been synthesized,20 the PPI dendrimers were chosen for comparison with hyperbranched PEI because of commercial availability, as well as their structural similarities. The CBHPs investigated are PEI-5 and -25, which have average molecular weights of 5000 and 25000 g/mol, respectively. These polymers are compared to dendritic PPI G4 [core ) diaminobutane (DAB) (G ) 4); dendri- ) poly(propyleneimine)(NH2)32] and PPI G5 [core ) DAB (G ) 5); dendri- ) poly(propyleneimine)-(NH2)64]. Previous investigations of the host-guest properties of the CBHP PEI indicate that multiple phenol blue (PB; Figure 3, top) guest molecules are able to associate within the core region of these polymers. The resulting association is quite similar to

10.1021/jp103144c  2010 American Chemical Society Published on Web 06/22/2010

Characterization of Selective Uptake by PEI and PPI

Figure 2. Molecular structure of dendrimeric poly(propyleneimine) (PPI).

J. Phys. Chem. A, Vol. 114, No. 27, 2010 7339 at the bottom of Figure 3.21,23,24 Whereas previous work with PAMAM dendrimers utilized Nile red as an interfering agent, the low solubility of Nile red in aqueous solutions led to the use of a structural derivative, HONR. The probe molecules chosen are solvatochromic in both absorption and fluorescence emission spectra and are also structural analogues of one another. The solvatochromic range of the absorption of PB is quite broad and extends from 652 nm in water to 541 nm in n-heptane. The fluorescence emission of PB has a more limited solvatochromic range, from 620 nm in methanol to 602 nm in acetone, with no fluorescence emission observed at either of the polarity extremes.23-25 The solvatochromic range of HONR is extensive for both the absorption and fluorescence emission. The absorption of HONR shifts from 584 nm in water to 509 nm in n-heptane, and the fluorescence emission shifts from 648 nm in water to 531 nm in n-heptane.26 The observed absorption and fluorescence emission maxima of PB and HONR allow the dyes to be spectroscopically resolved. By utilizing these dyes in tandem, additional information about the CBHPs as host molecules and their selectivity for a particular guest from a heterogeneous population can be determined and compared to the corresponding properties of dendritic PPI. With PB acting as the desired guest and HONR acting as the potential interferent, the CBHPs might demonstrate the size selectivity observed in other materials such as PAMAM dendrimers and cyclodextrins.25,27-29 Experimental Section

Figure 3. Molecular structures of phenol blue (PB; top) and 9-diethylamino-2-hydroxy-5H-benz[a]phenoxazin-5-one (HONR; bottom).

that of PB with dendritic PPI.21 To date, there have been no reports concerning the ability of either traditional or CBHP PEI to selectively entrap guest molecules from a heterogeneous solution. It has been shown that dendrimers are able to selectively host molecular guests based on differences in the carbon skeleton.22-25 Previous work has shown that dendritic poly(amidoamine) (PAMAM) is able to effectively uptake a desired guest from a heterogeneous solution.25 These studies involved the fluorescent probes PB and Nile red as the guest mixture and examined three concentration ratios, namely, excess PB, equivalent concentrations of PB and Nile red, and excess Nile red, with PB acting as the desired guest. The results of these studies showed that PB is able to associate with the PAMAM dendrimers, even with a 10-fold excess of Nile red present in solution. The goal of the research presented here is to determine whether the CBHP PEI and dendritic PPI are able to selectively entrap PB as a guest molecule in the presence of an interfering agent and to compare the results obtained for the CBHP PEI and dendritic PPI with those of previously investigated PAMAM dendrimers. Such selectivity is relevant to many of the interesting polymeric applications that can be envisioned for CBHPs: environmental remediation, nanoreaction, and chemical sensing. In these studies, two guest molecules are used to determine the guest selectivity of the polymers, namely, PB and 9-diethylamino-2-hydroxy-5H-benz[a]phenoxazin-5-one (HONR), shown

Materials. Amine-terminated dendrimers (PPI G4 and G5; tetrafunctional diaminobutane cores) and CBHPs (PEI-5 and -25; diaminoethane cores) were obtained as neat liquids from Aldrich (Milwaukee, WI) and HyperPolymers (North RhineWestphalia, Germany), respectively. Stock solutions were prepared by diluting known aliquots of the polymers in HPLCgrade water that had been purified in-house (Solution Consultants, Jasper, GA). Polymers and stock solutions were stored at 5 °C. Other materials were obtained from the following suppliers: 200-proof ethanol from AAPER Alcohol and Chemical (Shelbyville, KY) and HPLC-grade tetrahydrofuran (THF), methanol, and toluene from Fisher (Fair Lawn, NJ). All solvents were used as received. Phenol blue and HONR (Aldrich, 98% purity) were obtained and used as received. Stock solutions of PB and HONR were prepared by dissolving the dyes in THF and ethanol, respectively. All dye solutions were stored in the dark at ambient temperature. Sample Preparation. Samples of the polymers with a single dye were prepared by quantitatively transferring known aliquots of the dye stock solution into volumetric flasks and stripping off the solvent under ultra-high-purity nitrogen (Mid States Airgas, Sedalia, MO). Next, appropriate volumes of the polymers were transferred to the flasks. The samples were diluted to volume with HPLC-grade water. To facilitate solute solubilization, samples were stirred for ∼24 h in the dark at room temperature and then equilibrated for an additional ∼24 h in the dark at room temperature. The final concentrations for the polymers and dye (PB or HONR) were 1 × 10-4 and 1 × 10-6 M, respectively. Samples containing polymers and both PB and HONR dyes were prepared similarly to single-dye samples. The final concentrations of polymer, PB, and HONR (polymer/PB/HONR) were 1 × 10-4, 1 × 10-7, and 1 × 10-6 M, respectively (1000:1:10). All samples retained for temporal and postextraction studies were stored in the dark at ambient temperature.

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Methods UV-Vis Absorption Measurements. Absorption spectra were collected in 1 cm2 suprasil quartz cuvettes (Hellma, Forest Hills, NY) on a Hitachi U-3000 (Hitachi Instruments, Danbury, CT) double-beam spectrophotometer at a scan rate of 120 nm/ min, a slit width of 1 nm, and a thermostatted cell temperature of 25 °C. Spectra were blank corrected to account for the possible absorption of the solvent and/or polymers, although no signal due to the aqueous polymers was apparent in the wavelength region of interest. Fluorescence Emission Measurements. The fluorescence emission and steady-state anisotropy data were collected using the same cuvettes on an SLM 48000 DSCF/MHF spectrofluorometer (Jobin Yvon, Edison, NJ) at a thermostatted cell temperature of 25 °C. The excitation source was an Ion Laser Technology (Salt Lake City, UT) RPC-50-220 argon ion laser operated at 514 nm and 30 mW. For fluorescence emission spectra, the emission monochromator slit widths (entrance and exit) were set at 8 and 4 nm, respectively. The emission scan interval was 1 nm, and spectra were recorded from an internal average of five signal samplings per emission wavelength. The fluorescence emission was also passed through a Glan-Thompson polarizer set at 0° to correct for the Woods anomaly.30 Spectra were also absorption and solvent-blank corrected. The software program PeakFit 4.0 for Windows (AISN Software, Inc., SPSS Science, Chicago, IL) was used to deconvolute the overlapping bands present in the fluorescence emission spectra of samples containing both PB and HONR. All fluorescence emission spectra were deconvoluted using both a two-peak model (associated PB and associated HONR) and a three-peak model (associated PB, associated HONR, and aqueous HONR). The widths, amplitudes, and shapes of the bands were allowed to vary according to best fit. Baseline corrections were not made during deconvolution. Steady-State Anisotropy Measurements. Steady-state anisotropy measurements were collected in “L” format2 for both dyes using Glan-Thompson polarizers. For PB, a 550-nm longpass filter (KV-550 Schott Glass Technologies, Duryea, PA) and a 610-nm short-pass filter (03 SWP 619 Melles Griot, Irvine, CA) were used in combination to select the wavelength region of interest. For HONR, a 650-nm band-pass filter with a 40nm bandwidth (03 FIV 048 Melles Griot, Irvine, CA) was used. Means and standard deviations were calculated from five sample replicates, each containing an internal average of five samplings at each of the four polarizer orientations (0°, 0°; 0°, 90°; 90°, 90°; 90°, 0°). Extraction Techniques. Organic extractions were performed on all samples studied. Samples were extracted three times with a fresh aliquot of toluene, equal to one-half of the initial volume (1 mL) of the sample. The aqueous layers of all samples were retained for additional investigation performed under the same conditions as the initial studies, and the organic layers were examined by UV-vis absorption spectroscopy. Results Polymers with HONR. Absorption spectra of the polymers with HONR consist of a single broad band for all samples (Figure 4, top). However, the peak maxima are significantly different for HONR in CBHPs, compared to those in the dendritic polymers. HONR in dendritic PPI shows an absorbance maximum at around 540 nm, indicative of a nanoenvironmental polarity similar to that of polar N,N-dimethyl formamide. The absorbance maximum for HONR in CBHP PEI is closer to 575 nm, which is indicative of a more polar environment, such as

Figure 4. Representative absorption (top) and absorption-corrected fluorescence emission (middle) spectra for dendritic PPI and CBHP PEI (10-4 M) with HONR (10-6 M). A magnification of the lowerintensity emission spectra is also provided (bottom).

that of the dye in pure 1,2-ethanediol.26 These results indicate a more polar nanoenvironment for HONR in CBHPs compared to PB in CBHPs, with an absorption wavelength maximum previously reported as λmax ≈ 550 nm. There is signal enhancement for HONR with the dendritic PPI samples, which is consistent with previous studies of the polymers with PB.21,23,24 Corresponding fluorescence emission spectra (Figure 4, middle and bottom) of HONR in the PPI dendrimers consist of a single band centered around 620 nm for both generations, which indicates a nanoenvironmental polarity similar to that of N,N-dimethyl formamide, in agreement with the absorption results. Fluorescence emission of the CBHP PEI-25 sample also consists of a single band centered on 612 nm, indicative of a nanoenvironmental polarity similar to that of the polar acetonitrile. The fluorescence emission spectrum of CBHP PEI-5 also contains another band centered around 650 nm that is the result of the contribution from unassociated HONR in solution. Unlike aqueous, polymer-free PB samples, the fluorescence emission of HONR dissolved in water is easily detected under these experimental conditions. The intensity of HONR emission decreases when HONR is associated with the polymers, which is most likely due to the polymer solution pH of 8-10.24 The pH dependence of HONR emission has been previously reported,31 and in-house pH studies verified the decrease in emission intensity in basic solutions.

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TABLE 1: Anisotropy Values of PB and HONR in Single-Dye Studies polymer

HONR anisotropy ((σ)

PB anisotropy ((σ)

water dendritic PPI G4 hyperbranched PEI-5 dendritic PPI G5 hyperbranched PEI-25

0.071 ((0.002) 0.167 ((0.004) 0.084 ((0.003) 0.237 ((0.008) 0.142 ((0.006)

0.087 ((0.003) 0.235 ((0.007) 0.208 ((0.005) 0.275 ((0.008) 0.246 ((0.004)

Anisotropy values measure the overall rotational freedom of a probe during the fluorescence lifetime and, therefore, represent an overall, weighted average of the rotational motions in solution. Anisotropy values for HONR in Table 1 range from 0.07 (freely rotating) in aqueous solution to 0.24 (hindered) when associated with dendrimeric PPI G5. The trends in the anisotropy values for PB and HONR samples are in agreement with polymer type and size. In previous PB studies,21,23-25 the increasing anisotropy values primarily reflected the increasing molecular volume of the polymers. In addition, the dendrimercontaining solutions exhibited higher anisotropy values than the CBHPs with PB,21 as seen here. Polymers with PB. Absorption and emission spectra of PB in both polymers consist of a single band, characteristic of polymer-associated dye (Figure 5).21,23,24 The maximum absorption wavelength for associated PB is centered around 555 nm for both the dendrimers and the CBHPs. This absorption wavelength is easily differentiated from the absorption wavelength obtained for the association of HONR with both the dendrimers (∼540 nm) and the CBHPs (∼575 nm). The fluorescence emission wavelength is ca. 580 nm for dendritic PPI, G4 and G5, and ca. 600 nm in CBHP PEI-5 and -25. The ∼20-nm difference in emission wavelength between the dendrimers and CBHPs indicates that PB is in a slightly more polar nanoenvironment when associated within the CBHPs. Even with this difference, the fluorescence emission spectra of polymer-

Figure 5. Representative absorption (top) and absorption-corrected fluorescence emission (bottom) spectra of dendrimeric PPI and CBHP PEI (10-4 M) with PB (10-6 M).

Figure 6. Representative initial absorption (top) and absorptioncorrected fluorescence emission (bottom) spectra of dendritic PPI and hyperbranched PEI (10-4 M) with PB (10-7 M) and HONR (10-6 M).

associated PB are still differentiable from the fluorescence emission spectra of polymer-associated HONR. The emission of aqueous PB is not detected under the steady-state conditions used in these studies; therefore, the intensity enhancements observed for the polymer-containing samples are further evidence of dye-polymer association. Previous studies have shown that fluorescence intensity increases with increasing dendrimer generation;23,24 these results were also observed in this study, as the larger of each polymer type (dendritic or CBHP) exhibited an enhanced intensity. An increase in anisotropy values for polymer-containing samples is additional evidence that PB has associated with the polymers. Anisotropy values in Table 1 range from approximately 0.09 for aqueous phenol blue to 0.28 for dendritic PPI G5. As was observed for the polymers with HONR, the dendrimer-containing solutions exhibited higher anisotropy values than the CBHPs with PB. Polymers, PB, and HONR in a 1000:1:10 Ratio. Initial absorption spectra for the 1000:1:10 polymer/PB/HONR sample show a single broad peak with a shoulder for both polymer types and both sizes of each polymer, indicating the presence of both associated PB and HONR (Figure 6, top). Based on deconvolution, there is no spectral evidence of aqueous dye present. The corresponding fluorescence emission data (Figure 6, bottom) show a single broad band present for both dendritic samples and the larger CBHP PEI-25. Two peaks are present in the emission spectrum for the dyes in the smaller CBHP PEI-5, indicative of associated PB and HONR, as well as aqueous HONR. Deconvolution of the emission spectra reveals two overlapping bands for all samples, except PEI-5, which contains three, indicating that there is still a significant contribution from associated PB, even at the lower concentration. Initial anisotropy values in Table 2 for dendrimer-associated PB are greater for multiple dyes than was observed with a single

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TABLE 2: Representative Anisotropy Values of PB for Dendritic PPI and Hyperbranched PEI with PB and HONR (1000:1:10) r ((σ) polymer

initial study

temporal study

postextraction

water dendritic PPI G4 hyperbranched PEI-5 dendritic PPI G5 hyperbranched PEI-25

0.054 ((0.006) 0.270 ((0.004)

0.052 ((0.005) 0.192 ((0.006)

0.052 ((0.004) 0.153 ((0.005)

0.172 ((0.006)

0.170 ((0.006)

0.184 ((0.004)

0.286 ((0.009)

0.202 ((0.004)

0.184 ((0.002)

0.247 ((0.001)

0.235 ((0.004)

0.231 ((0.008)

TABLE 3: Representative Anisotropy Values of HONR for Dendritic PPI and Hyperbranched PEI with PB and HONR (1000:1:10) r ((σ) polymer

initial study

temporal study

postextraction

water dendrimeric PPI G4 hyperbranched PEI-5 dendrimeric PPI G5 hyperbranched PEI-25

0.0713 ((0.0006) 0.249 ((0.008)

0.0733 ((0.0005) 0.176 ((0.007)

0.0716 ((0.0006) 0.169 ((0.009)

0.104 ((0.007)

0.106 ((0.006)

0.159 ((0.008)

0.277 ((0.009)

0.215 ((0.001)

0.209 ((0.007)

0.235 ((0.003)

0.215 ((0.009)

0.229 ((0.009)

PB guest. For the CBHP-associated PB, anisotropy values were slightly lowered for PEI-5 compared to those observed with PB as the only guest, but the larger reported anisotropy values for PEI-25 are very similar to those observed with PB as the only guest. Initial anisotropy values for HONR in Table 3 are higher for all polymer-containing samples than those observed with HONR as the only guest. To determine whether the association of the guests changed over time, samples were kept in the dark at room temperature, and temporal measurements were collected one week after the initial measurements. Temporal absorption spectra are nearly identical to the initial absorption data for the dendritic polymers; however, the absorption intensities for the CBHP samples are slightly lowered than in the initial study (Figure 7, top). The corresponding fluorescence emission (Figure 7, bottom) is nearly identical to that of the initial study, with a slight increase in emission for the dendritic polymer samples, indicating an increase in dye association. Deconvolution of the fluorescence emission spectra again reveals two overlapping bands, indicating that both PB and HONR are contributing to the overall observed fluorescence emission spectra. Temporal anisotropy values measured in the PB emission region are listed in Table 2 and are almost identical for the CBHPs but lowered for the dendritic polymers. The temporal anisotropy values measured in the HONR emission range are listed in Table 3 and are lower for both the dendritic polymers, PPI G4 and PPI G5, as well as the larger CBHP PEI-25. The temporal anisotropy value for the smaller hyperbranched PEI-5 is nearly identical to the initial measurement. After the temporal studies, samples were then extracted as described above. For the polymer-containing samples, postextraction absorption spectra show a decrease in absorption for all samples (Figure 8, top). Fluorescence emission spectra are similar to those of the temporal study, with one main difference (Figure 8, bottom): The additional band present in the CBHP PEI-5 indicative of aqueous HONR has disappeared. Deconvolution of the emission spectra reveals two overlapping bands, as was seen in both of the previous studies. Postextraction

Figure 7. Representative temporal absorption (top) and absorptioncorrected fluorescence emission (bottom) spectra of dendritic PPI and hyperbranched PEI (10-4 M) with PB (10-7 M) and HONR (10-6 M).

Figure 8. Representative postextraction absorption (top) and absorption-corrected fluorescence emission (bottom) spectra of dendrimeric PPI and CBHP PEI (10-4 M) with PB (10-7 M) and HONR (10-6 M).

anisotropy values of PB in the dendritic polymers are lower than in the temporal study. The anisotropy values in the CBHP PEI-25 samples remain unchanged, and PEI-5 exhibits a slight increase in anisotropy (Table 2). The postextraction HONR anisotropy values listed in Table 3 are higher than those of the temporal study for the CBHPs, whereas those of the dendritic polymers exhibit a slight decrease from the temporal measurements.

Characterization of Selective Uptake by PEI and PPI Discussion Previous investigations of both the dendritic PPI and CBHP PEI with PB have shown that the fluorophore associates within the core region of these polymers, as seen here.21,23-25 However, the results obtained for polymer-associated HONR indicate that this larger dye does not associate with the polymers in the same manner. For the dendritic PPI samples, absorption and fluorescence emission polarity measurements of polymer-associated HONR agree. Although the polarity results of PB in the CBHP PEI samples exhibit some variance between absorption and emission data, experimental results indicate a moderately polar nanoenvironment in both polymer families. Previous studies indicated that the polymeric core regions are significantly more nonpolar than the environment within the branching groups.23,24 The HONR association, in a moderately polar region of both polymer types, is indicative of the fluorophore intercalating in the outermost branches of the polymer, not completely shielded from the bulk solution. The lower anisotropy values of polymer-associated HONR are further evidence of a loose association within the PPI and PEI polymer branches. Initial fluorescence emission spectra of both dyes with the polymers contain significant spectral evidence of associated PB (∼580 nm for dendrimers and ∼600 nm for CBHPs), as well as associated HONR (∼620 nm for dendrimers and ∼612 nm for CBHPs). There is also indication of some unassociated HONR (∼650 nm) for the smaller CBHP, PEI-5. Association of both dyes within the polymers is also demonstrated by the anisotropy values obtained. In general, the anisotropy values for all polymer-containing samples with both dyes present is greater than those for either dye alone in the polymers. For the CBHP samples, the anisotropy value of associated PB with the larger PEI-25 was very similar to that obtained with PB as the only guest, whereas the smaller PEI-5 exhibited a decrease in anisotropy for PB, possibly because of the contribution of both polymer-associated and unassociated HONR in solution. In the temporal investigation, evidence of association of both dyes remains for all samples. There is a ∼10% increase in the fluorescence emission intensity of PB for both dendritic and CBHP samples. This increase indicates that the presence of HONR does have some inhibiting effect on the association of PB with the polymers. Temporal anisotropy values of both PB and HONR decrease for all samples except the smaller CBHP PEI-5. It is likely that some of the loosely associated HONR is released from the polymers over time, and the contribution from the freely rotating dye in solution lowers the recorded anisotropy. Because the filters chosen to isolate the dyes have similar wavelengths (640 vs 610 nm), it is possible to have some contribution from both dyes for each wavelength region selected. After extraction of any unassociated or loosely associated dye from solution, the fluorescence emission of all samples is lowered slightly. However, there is a greater decrease in emission intensity for the smaller dendritic PPI G4 from the temporal investigations. Deconvolution revealed a significant decrease in the contribution from HONR for all samples, indicating that the majority of the HONR present is removed during the extraction process. There is also a slight decrease in PB fluorescence emission postextraction, indicating that, although some of the dye is removed, the association of the guest with the polymers is strong enough to withstand extraction.21 The extraction process removed more HONR from the dendritic samples than the CBHPs; this is most likely due to the structural differences of the two polymer families. Variation in both core and branching units and the less symmetrical structure of the

J. Phys. Chem. A, Vol. 114, No. 27, 2010 7343 CBHPs potentially allows a better fit for the HONR within the CBHP branches. Postextraction anisotropy values for both dyes show a decrease from the temporal study for both dendritic PPI G4 and G5, whereas the CBHPs exhibit an increase in anisotropy from the temporal investigations. This further indicates that any unassociated or loosely associated dye was removed through the extraction process. Although the absorption spectra for the samples of dendritic PPI and hyperbranched PEI with PB and HONR are complex, with overlapping bands and multiple possible deconvolution models, they do offer some insight into the nature of these samples. There is evidence of associated PB in all investigations, even with a 10-fold concentration excess of HONR. The fluorescence emission data agree with these findings, indicating that there is contribution of both associated PB and HONR in all samples investigated. Moreover, all samples contain the characteristic emission profile of associated PB.21 There is also obvious evidence of associated HONR with both the dendrimers and CBHPs, indicating that, although HONR does associate with the polymers, it does not inhibit the association of PB. As expected based on the larger size of the dye, the association of HONR with the polymers is not as strong an association as that of PB with the polymers. Conclusions Previous studies of dendritic PPI have employed a single dye to gain information about this dendrimer family. Studies with PB have revealed much about the PPI dendrimers including the nature of association, interior cavity size, and interior polarity.24 Similar information was obtained on the CBHP PEI by examining the association of PB, and the results were compared to those for the dendritic PPI.21 The use of both PB and HONR simultaneously provides more information on these important, but relatively unstudied new materials, namely, CBHP molecules. Investigations utilizing these two dyes in tandem were undertaken to investigate the host-guest properties of both the dendritic PPI and the CBHP PEI. Several interesting aspects of the polymer-dye association were revealed. The presence of excess HONR (1000:1:10 polymer/PB/HONR) confirms that the ability of PB to associate with the polymers does depend on the concentration of HONR present. However, even at a 10fold concentration excess of HONR, the association of PB with the polymers is only inhibited, not prevented entirely. The association of PB does appear to increase after one week in the presence of HONR, whereas the association of HONR decreases, indicating HONR does induce some inhibition of the PBpolymer association. This is not surprising considering that PB has been shown to associate near the core of the dendrimeric PPI and is more or less completely protected from the bulk solution, whereas HONR is unable to reach the core region of the polymers and associates in the outer branches. The presence of HONR does not change the nature of the association of PB with the polymers. All datasabsorption, emission, and anisotropysindicate that the PB remains located at the core region of the polymers.21 Results are similar regardless of the presence of HONR, revealing that the association of PB with the polymers must be similar to that of PB without the presence of HONR. Previous investigations of the structurally different PAMAM dendrimer family utilized the fluorescent probes PB and the structurally similar Nile red as the guest mixture, and the results of those studies showed that PB was also able to associate with the PAMAM dendrimers, even with an excess of Nile red present in solution.25 The results obtained in these studies reveal that both the dendritic PPI and

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the CBHP PEI exhibit the same ability to effectively uptake a desired guest from a heterogeneous population. The similarities in the selective uptake of the CBHPs and the dendrimers further support the use of CBHPs as a replacement to dendrimers in potential applications, such as environmental remediation, separations, and water purification. References and Notes (1) Seiler, M. Chem. Eng. Technol. 2002, 25, 237–253. (2) Sunder, A.; Heinemann, J.; Frey, H. Chem.sEur. J. 2000, 6, 2499– 2506. (3) Yates, C. R.; Hayes, W. Eur. Polym. J. 2004, 40, 1257–1281. (4) Frey, H.; Haag, R. ReV. Mol. Biotechnol. 2002, 90, 257–267. (5) Product Information Polyglycerol. HyperPolymers, North RhineWestphalia, Germany. http://www.hyperpolymers.com/prodinf.html (accessed Aug 8, 2007). (6) Hanselmann, R.; Ho¨lter, D.; Frey, H. Macromolecules 1998, 31, 3790–3801. (7) Suttiruengwong, S.; Rolker, J.; Smirnova, I.; Arlt, W.; Seiler, M.; Lu¨deritz, L.; Pe´rez de Diego, Y.; Jansens, P. Pharm. DeV. Technol. 2006, 11, 55–70. (8) Paleos, C. M.; Tsiourvas, D.; Sideratou, Z. Mol. Pharm. 2007, 4, 169–188. (9) Haag, R. Chem.sEur. J. 2001, 7, 327–335. (10) Arkas, M.; Tsiourvas, D.; Paleos, C. M. Chem. Mater. 2005, 17, 3439–3444. (11) Kra¨mer, M.; Stumbe´, J.; Tu¨rk, H.; Krause, S.; Komp, A.; Prokhorova, S.; Kautz, H.; Haag, R. Angew. Chem., Int. Ed. 2002, 41, 4252– 4256. (12) Kra¨mer, M.; Stumbe´, J.; Grimm, G.; Kaufmann, B.; Kru¨ger, U.; Weber, M.; Haag, R. ChemBioChem. 2004, 5, 1081–1087. (13) Mo¨ck, A.; Burgath, A.; Hanselmann, R.; Frey, H. Macromolecules 2001, 34, 7692–7698.

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