A Library of Quinoline-Labeled Water-Soluble Copolymers with pH

Apr 20, 2016 - A Library of Quinoline-Labeled Water-Soluble Copolymers with pH-Tunable Fluorescence Response in the Acidic pH Region. Ioannis Thivaios...
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A Library of Quinoline-Labeled Water-Soluble Copolymers with pHTunable Fluorescence Response in the Acidic pH Region Ioannis Thivaios,† Sofia Kakogianni,† and Georgios Bokias*,†,‡ †

Department of Chemistry, University of Patras, University Campus, Rio-Patras GR26504, Greece Foundation for Research and Technology Hellas/Institute of Chemical Engineering Sciences (FORTH/ICE-HT), Platani Str., Patras GR26504, Greece

‡

S Supporting Information *

ABSTRACT: A series of quinoline-labeled water-soluble copolymers, synthesized through free radical copolymerization of vinylmodified quinoline derivatives with water-soluble monomers such as the nonionic N,N-dimethylacrylamide (DMAM), the anionic sodium 2-acrylamidomethylpropanesulfonate (AMPSNa), and the cationic methacrylamidopropyltrimethylammonium chloride (MAPTAC), are investigated in the present work. As a result of the protonization/deprotonization equilibrium of the quinoline unit, the optical properties (absorption and, more important, emission of light) of these copolymers are controlled by the pH of the surrounding environment. In fact, the emitted light of the quinoline group changes from blue to green upon decreasing pH. It is shown that this pH-controlled color change can be tuned by the nature of the polymer chain (anionic, nonionic, or cationic) or the existence of substituents (such as cyanophenyl or perfluorophenyl groups), giving rise to a library of pH-responding quinoline-labeled materials that cover almost the entire acidic pH region.

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INTRODUCTION Labeling water-soluble polymers with chromophores, especially photoluminescent ones, has been proved an elegant, powerful tool to explore the properties of polymer chains in aqueous environment.1−13 With the exception of special cases, groups like pyrene and naphthalene are the most commonly used luminescent labels of (bio)macromolecules.14−18 Though such groups have been proved very useful to study the microenvironment, the conformational changes, and spatial proximity, taking advantage of properties like excimer formation or nonradiative energy transfer, they do not possess any stimuliresponsive properties. On the other hand, stimuli-responsive water-soluble conjugated polymers, such as fluorene and thiophene derivatives,19−37 have been widely applied, not only to probe the local microenvironment but also to impart novel optical features, potentially useful for applications as indicators or sensors. Optical sensing properties may be achieved through using adequate stimuli-responsive chromophores other than pyrene (or similar units). For example, the quinoline unit is a pHsensitive chromophore; as consequence of its weak basic character, it can be combined with other chromophores or complexing units for potential sensing applications, and it can be incorporated into polymeric chains through several synthetic methodologies. Thus, the quinoline unit has been proposed for several optical sensing applications, like in complexation reaction and detection of chemical species38 or in fluorescent pH sensors.39,40 For example, Jenekhe et al.41−43 have demonstrated that in the case of organo-soluble quinoline© XXXX American Chemical Society

containing polymers the protonation of quinoline unit, taking place upon dissolution in organic acids, leads to a strong red shift (∼70−100 nm) of the emission spectrum (green emitted color), as compared to the corresponding spectrum when the polymer is dissolved in a typical organic solvent like chloroform (blue emitted color). In fact, when homopolymers or quinoline-rich copolymers are used, the emission of the protonated form of quinoline unit is significantly quenched.44 Nevertheless, the green emission of the protonated quinoline unit can be readily observed if the design of the materials does not facilitate quenching. For example, quenching is greatly suppressed when quinoline-based polymers are fixed onto carbon nanotubes45−47 or low fractions of quinoline units are randomly fixed onto cross-linked polymeric nanoparticles.48 Concerning pH-sensing applications, we have recently developed a strategy to prepare water-soluble polymers labeled with a pH-sensitive fluorophore based on the quinoline unit, through free radical copolymerization of the vinyl-functionalized quinoline derivative 2,4-diphenyl-6-(4-vinylphenyl)quinoline (SDPQ) with water-soluble comonomers, like N,Ndimethylacrylamide (DMAM) or N-isopropylacrylamide (NIPAM). As a result of the low SDPQ content, excimer formation is not favored, and these quinoline-labeled watersoluble polymers exhibit sensitive pH-responsive photoluminescent properties, changing color from blue to green Received: February 3, 2016 Revised: April 11, 2016

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Macromolecules under UV irradiation upon decreasing pH.49 In addition, pHresponsive properties are combined with temperature-responsive properties in the case of P(NIPAM-co-SDPQ),50 as a consequence of the well-known thermosensitive character of poly(N-isopropylacrylamide). On the basis of the aforementioned initial findings, in the present work we focus on the systematic investigation of the optical properties of series of quinoline-labeled water-soluble polymers toward two objectives. The first objective is to enlarge the potential of quinoline-labeled polymers, using several substituted quinoline derivatives, and to evaluate the importance of the substituents on the optical properties of the final products. To this end, apart from SDPQ, we have also exploited cyano-modified (SDPQCN) and perfluorophenylmodified (5FSPQ) quinoline derivatives (Chart 1a). The

materials covering almost the whole acidic pH region, from pH ∼ 7 down to pH ∼ 2.

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RESULTS AND DISCUSSION Synthesis and Characterization of Quinoline-Labeled Water-Soluble Polymers. The primary objective of this study was to investigate the pH-dependent optical response of quinoline units, adequately copolymerized with suitable watersoluble monomers. In fact, the main goal of the study was to elucidate the effect of the nature of the water-soluble chain or the quinoline substituent on the protonation/deprotonation equilibrium of the quinoline unit, affecting subsequently the pH-controlled optical response of the respective quinolinelabeled polymers. Concerning the influence of quinoline substituents, three quinoline-containing monomers (SDPQ, SDPQCN, and 5FSPQ) were successfully synthesized, as verified through 1H NMR spectroscopy (Figures S1−S3 in Supporting Information). As a next step, the synthesis of random quinoline-labeled copolymers was performed through free radical copolymerization (FRP) in an organic solvent (DMF or THF) and using an organo-soluble initiator, since all three quinoline-containing monomers are practically insoluble in water. The feed composition for all copolymers was set at 0.2% (mol/mol) in quinoline units. Table 1 summarizes the

Chart 1. Chemical Structures of (a) the Three Quinoline Derivatives (SDPQ, SDPQCN, and 5FSPQ) Used as Labels and (b) the Three Water-Soluble Units (DMAM, AMPSNa, and MAPTAC) Used for the Polymeric Chain

Table 1. Characterization Results of All Quinoline-Labeled Copolymers Synthesized in the Present Study SDPQ content (mol %) (1H NMR)

SDPQ content (mol %) (UV−vis results)

polymer

Mw × 10

Mw/Mn

P(AMPSNaco-SDPQ) P(AMPSNacoSDPQCN) P(AMPSNaco-5FSPQ) P(DMAM-coSDPQ) P(MAPTACco-SDPQ)

161

3.5

0.4

0.22

60

2.6

0.1

0.20

0.15

0.30

0.2

0.18

not detectable

0.18

3

11

1.9

nomenclature used and the characterization results of the quinoline-labeled copolymers studied in the present work. Thus, the samples P(AMPSNa-co-SDPQ), P(AMPSNa-coSDPQCN), and P(AMPSNa-co-5FSPQ) are all based on the same anionic polymer chain (PAMPSNa) and are used to clarify the effect of the chemical structure of the quinoline derivative (SDPQ, SDPQCN, or 5FSPQ) on the pH-controlled optical response. Moreover, two additional samples were labeled with SDPQ, namely the nonionic P(DMAM-coSDPQ) and the cationic P(MAPTAC-co-SDPQ), offering the possibility to explore the influence of the ionic nature of the polymer on the pH-controlled optical response. The chemical characterization of the products was first attempted through 1H NMR spectroscopy. As a representative example, the 1H NMR spectrum of the quinoline-labeled copolymer P(AMPSNa-co-SDPQ) in D2O is shown in Figure 1. The 1H NMR spectra of the rest quinoline-labeled copolymers are given in Figures S4−S6. The characteristic peaks of the AMPSNa structural unit are clearly observable: the two methyl groups (5) are observed at 1.5 ppm, the methylene group (6) is observed at 3.3 ppm, while the methylene groups (1,2) and methine groups (3,4) of the backbone are seen at 1.8 and 2.2

second objective is to elucidate the influence of the nature of the polymer chain on the pH-responsive optical properties of the labeled polymers. To achieve this goal, we prepared quinoline-labeled nonionic polymers based on DMAM as well as quinoline-labeled polyelectrolytes, either anionic, based on sodium 2-acrylamido-2-methylpropanesulfonate (AMPSNa), or cationic ones, based on methacrylamidopropyltrimethylammonium chloride (MAPTAC) (Chart 1b). The UV−vis absorption and photoluminescence properties of these materials reveal that such water-soluble quinoline-labeled polymeric derivatives consist a library of pH-responsive B

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an example, the absorption spectra of the copolymer P(AMPSNa-co-SDPQ) are shown in Figure 2 for pH values

Figure 2. Absorption spectra of P(AMPSNa-co-SDPQ) in buffer solutions, covering the range 2 < pH < 7. CQ = 2 × 10−5 M.

Figure 1. 1H NMR spectrum of the quinoline-labeled copolymer P(AMPSNa-co-SDPQ) in D2O.

covering the range 2 < pH < 7. At low pH, the characteristic absorption peaks at ∼280 and ∼350 nm (attributed to the benzene rings and the quinoline group, respectively), as well as a shoulder at 390 nm (characteristic of the protonated form of the quinoline ring), are observed. The two peaks appear to shift to shorter wavelengths (blue shift), while the shoulder seems to disappear as the pH increases. This is the result of the gradual deprotonation of the quinoline ring by decreasing the concentration of H+ in the aqueous solution. Similar characteristics of the absorption spectra were observed for all copolymers investigated in the present study (Figure 3). For reasons of clarity, the absorption spectra at two extreme pH values, specifically at pH ∼ 2 and at pH ∼ 7, are shown in this figure. This comparison is based both on the charge of the water-soluble chain (anionic/nonionic/cationic, Figure 3a) or the electronegativity of the substituent of the quinoline ring (−H/−CN/−perfluorophenyl, Figure 3b). The position of the absorption peaks is not affected at all by the presence or the absence of anionic or cationic charge on the polymeric backbone (Figure 3a), whereas all absorption peaks are clearly blue-shifted, by a few nanometers, when the quinoline derivative changes from SDPQCN to SDPQ and finally to 5FSPQ (Figure 3b). Moreover, the position λmax of the peaks for each copolymer is affected by the pH of the solution. In fact, the peaks are shifted about ∼10−15 nm to higher wavelength as pH decreases (Figure S10). The absorbance at low pH appears to depend significantly both on the nature of the polymer backbone and on the quinoline’s substituent. This is clearly illustrated in Figure S11, where the pH dependence of the absorbance of all quinolinelabeled copolymers at the three characteristic peaks (i.e., at ∼280, ∼350, and 390 nm) is shown. The pH dependence of the absorbance observed at 390 nm is of special interest (Figure 4), since it can be directly related to the protonation/ deprotonation equilibrium of the quinoline derivatives as detected by UV−vis absorption spectroscopy. At high pH the absorbance at 390 nm is low, suggesting that the quinoline unit is practically under the basic unprotonated form. However, as pH decreases, the absorbance starts to increase at a characteristic for each copolymer pH value, corresponding to the first detectable through UV−vis spectroscopy quantity of protonated quinoline species.

ppm, respectively. The absence of any peak in the 5−6 ppm region, corresponding to vinylic protons of any unreacted monomers, verifies the successful copolymerization reaction and the purification of the final materials. Given that the quinoline content of the copolymer is low and approaches the sensitivity limits of the instrument, the peaks attributed to the aromatic protons of SDPQ are not easily detected. However, these peaks appear in the magnification of the 7−8 ppm region, shown in the inset of Figure 1. The 1H NMR spectra permit a first rough estimation of the quinoline content of the final polymers. These results, shown in Table 1, are in the order of 0.1−0.4 mol %, whereas the feed composition is 0.2 mol %. The discrepancy observed arises from the extremely low and broadened NMR signals. For this reason, we proceeded to the characterization of the copolymers through UV−vis spectroscopy. Thus, the calibration curves of the monomers SDPQ, SDPQCN, and 5FSPQ in THF were established (Figure S7) and used for the determination of the content of the respective quinoline derivative of the organosoluble P(DMAM-co-SDPQ), P(DMAM-co-SDPQCN), and P(DMAM-co-5FSPQ) copolymers. This methodology in not applicable to the anionic or cationic copolymers, since these polyelectrolytes are hardly organo-soluble. Therefore, the calibration curves of P(DMAM-co-SDPQ), P(DMAM-coSDPQCN), and P(DMAM-co-5FSPQ) copolymers in water were established (Figure S8) and used for the determination of the quinoline content in the case of quinoline-labeled polyelectrolytes. A more precise determination of the quinoline content of the copolymers is now achieved. The molar extinction coefficients of all products used to establish the calibration curves are summarized in Table S1 and are well comparable with the respective values reported for similar quinoline derivatives.51 Finally, the molar mass characterization through size exclusion chromatography (SEC) was possible for some of the copolymers with a considerable molar mass (Figure S9). UV−Vis Absorption Spectroscopy. Though the three quinoline-containing monomers are practically insoluble in water, the quinoline-labaled polymers are readily water-soluble, making feasible the investigation of the optical properties in aqueous solution as a function of pH, using citrate buffers. As C

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Figure 3. Absorption spectra at pH ∼ 2 (solid lines) and at pH ∼ 7 (dashed lines) for copolymers with different (a) polymer charge P(AMPSNa-coSDPQ) (red), P(DMAM-co-SDPQ) (blue), and P(MAPTAC-co-SDPQ) (green) and (b) quinoline’s substituent P(AMPSNa-co-SDPQ) (red), P(AMSPNa-co-SDPQCN) (magenta), and P(AMPSNa-co-5FSPQ) (orange). CQ = 2 × 10−5 M.

Figure 4. pH dependence of the absorbance at 390 nm of the copolymers with different (a) polymer charge P(AMPSNa-co-SDPQ) (■), P(DMAMco-SDPQ) (●), and P(MAPTAC-co-SDPQ) (▲) and (b) quinoline’s substituent P(AMPSNa-co-SDPQ) (■), P(AMSPNa-co-SDPQCN) (●), and P(AMPSNa-co-5FSPQ) (▲). CQ = 2 × 10−5 M.

electron-withdrawing substituents in SDPQ does not favor protonation and the pHUV onset values are found ∼6 and ∼4 for P(AMSPNa-co-SDPQCN) and P(AMPSNa-co-5FSPQ), respectively. pH-Responsive Photoluminescence. After these initial steps, the second part of the present work was devoted to the investigation of the luminescence of the quinoline-labeled water-soluble copolymers and its dependence on the pH of the aqueous solution. Moreover, the effect of the ionic strength of the solution on this visual response was explored. Upon excitation, all quinoline-labeled copolymers exhibited the characteristic color change from blue to green as pH decreases. As an example, the emission spectra of P(AMPSNa-co-SDPQ) upon excitation at 350 nm are presented in Figure 5. All spectra are normalized with respect to the emission band observed at the highest pH value studied for this copolymer. As it can be seen, the emission band at ∼410 nm gradually decreases while a new emission band at ∼490 nm appears, as pH decreases from pH= 6.8 down to pH= 2.1. For solutions with an intermediate acidity, both emission bands are observed. The study of all quinoline-labeled water-soluble copolymers has shown considerable variations in both the maximum emission intensities and the peak positions. In fact, in line with the UV−vis observations, the position of the peaks seems to be considerably affected by the existence of substituents on the chromophore, while no significant shifts are observed when the polymer chain changes from anionic to nonionic and, finally, to cationic (Figure S12). We attempted to determine the quantum

The pH values where the absorbance onset is observed, pHUV onset, for the copolymers studied here are reported in Table 2. Though these values are expected to be affected by the Table 2. pH Values Where the Onset of the Absorbance at 390 nm and the Emission at 490 nm (upon Excitation at 390 nm) Is Observed for the Quinoline-Labeled Copolymers (Quantum Yields at pH ∼ 2 and pH ∼ 7 Are Also Shown) quantum yield polymer

pHUV onset

pHEm onset

pH ∼ 2

pH ∼ 7

P(AMPSNa-co-SDPQ) P(AMPSNa-co-SDPQCN) P(AMPSNa-co-5FSPQ) P(DMAM-co-SDPQ) P(MAPTAC-co-SDPQ)

>6 5.8 4 5.5 4

>6 ∼6 4−4.5 5−5.5 5−5.5

0.55 0.31 0.32 0.38 0.46

0.84 0.73 0.77 0.85 0.90

extinction coefficients at 390 nm, they can be considered as a reliable first criterion of the protonation ability of the quinoline unit, closely related to the respective protonation/deprotonation equilibrium. Thus, quinoline units of the anionic P(AMPSNa-co-SDPQ) copolymer can be easily protonated, since the first protonated quinoline species are observed at a pH value higher than 6. Protonation of the nonionic P(DMAM-co-SDPQ) copolymer is less easy, since a pHUV onset value around 5.5 is observed, while the protonation of the cationic P(MAPTAC-co-SDPQ) copolymer seems to be more difficult (pHUV onset value around 4). Moreover, the introduction of D

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quantity of protonated quinoline species. As it is seen in Table 2, with the exception of P(MAPTAC-co-SDPQ), the pHEm onset values are in a rather good agreement with the respective pHUV onset for all copolymers studied. Though the quantum yields of the two peaks at 410 and 490 nm are different, these observations are also related to changes of the protonation/deprotonation equilibrium of the quinoline groups of the copolymers. In fact, this equilibrium maybe is shifted as a consequence of the electrostatic interactions developed in the aqueous solution between the protonated quinoline groups and the charged water-soluble polymer chains (Figure 7a) or of the electronegativity of the substituent of quinoline (Figure 7b). The introduction of a more electronegative substituent leads to an electron deficiency in the quinoline ring, making harder the protonation by the cationic H+ species. Concerning the importance of the nature of the macromolecular chain, when the quinoline group is covalently attached to an anionic backbone, it can be readily protonated (leading to a pH-controlled polyampholyte chain), in comparison with the nonionic copolymer, as a result of the electrostatic attraction between the negatively charged chain and the protonated quinoline units. In contrast, when the quinoline group is attached onto the cationic copolymer, protonation is much harder due to the electrostatic repulsion between the positively charged chain and protonated qunoline units. Since the acid−base equilibrium depends on the charge of the polymer chain, it would be expected that the ionic strength of the solution is of great importance, as it controls the strength of electrostatic interactions. In fact, this is clearly demonstrated in Figure 8, where the intensity ratio r is plotted against pH at different ionic strengths (from 0.02 up to 2 M). The ratio r is defined as

Figure 5. Normalized emission spectra of P(AMPSNa-co-SDPQ) in buffer solutions, covering the range 2 < pH < 7, upon excitation at 350 nm. CQ = 2 × 10−6 M.

yields of these two emission bands, blue and green, at pH ∼ 7 and pH ∼ 2, respectively, using quinine as a standard (Figures S13 and S14). The results are reported in Table 2 and are in agreement with previous observations.52 Quantum yields at pH ∼ 7 are high, 0.7−0.9, comparable with the values reported for similar quinoline-based molecules. In fact, the SDPQ-labeled polymers exhibit higher quantum yields as compared to the ones labeled with the substituted chromophores. At pH ∼ 2 the quantum yields remain significant, indicating that the usually observed quenching under acidic conditions41−43 is avoided to a large extent, as a consequence of the high “dilution” of the chromophores along the polymer chain. Probably, the same reason is the origin of the higher quantum yields observed in the cases of labeled polyelectrolytes (either anionic or cationic) as compared to the respective nonionic homologue. From Figure 3 it is clear that the labeled copolymers do not absorb at 390 nm unless the pH of the solution is sufficiently decreased. Therefore, upon excitation at this wavelength just the green emission peak at ∼490 nm is observed as pH decreases (Figure S12). The pH dependence of the normalized emission intensity for these studies is shown in Figure 6. At high pH the emission at 490 nm, upon excitation at 390 nm, is around zero for all copolymers, suggesting that the quinoline unit is fully under the basic unprotonated form. The pH values where the emission onset is observed, pHEm onset, correspond to the first detectable, through photoluminescence spectroscopy,

r = Ibasic/Iacidic

(1)

where Ibasic and Iacidic is the normalized intensity of the emission at ∼410 and ∼490 nm, respectively. Having always as a comparison the nonionic water-soluble copolymer P(DMAM-co-SDPQ), it is clear that upon increasing the ionic strength of the solution, the quinolinelabeled polyelecrolytes, P(AMPSNa-co-SDPQ) and P(MAPTAC-co-SDPQ), tend to approach the photophysical behavior of the nonionic copolymer, which remains practically unaffected by the ionic strength of the aqueous solution. Thus,

Figure 6. pH dependence of the normalized emission at 490 nm, upon excitation at 390 nm, of copolymers with different (a) polymer charge P(AMPSNa-co-SDPQ) (■), P(DMAM-co-SDPQ) (●), and P(MAPTAC-co-SDPQ) (▲) and (b) quinoline substituent P(AMPSNa-co-SDPQ) (■), P(AMSPNa-co-SDPQCN) (●), and P(AMPSNa-co-5FSPQ) (▲). CQ = 2 × 10−6 M. E

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Figure 7. Intensity dependence of the emission peaks at 410 nm (closed symbols) and 490 nm (open symbols), upon excitation at 350 nm, as a function of the pH and the dependence (a) of the backbone charge P(AMPSNa-co-SDPQ) (■, □), P(DMAM-co-SDPQ) (●, ○), and P(MAPTACco-SDPQ) (▲, △) and (b) of the quinoline’s substituent P(AMPSNa-co-SDPQ) (■, □), P(AMSPNa-co-SDPQCN) (●, ○), and P(AMPSNa-co5FSPQ) (▲, △). CQ = 2 × 10−6 M.

Figure 8. Ratio r as a function of pH at ionic strength 0.02 M solution (■), 0.1 M (●), and 2 M (▲) for P(AMPSNa-co-SDPQ) (red), P(DMAM-co-SDPQ) (blue), and P(MAPTAC-co-SDPQ) (green).

Figure 9. Color change upon exposure of the P(AMPSNa-co-SDPQ), P(DMAM-co-SDPQ), and P(MAPTAC-co-SDPQ) solutions in UV light. The ionic strength of the solutions is 0.02 M.

the protonation/deprotonation equilibrium of the polycation P(MAPTAC-co-SDPQ) is shifted toward higher pH (i.e., it is protonated more easily upon increasing the ionic strength), while the protonation/deprotonation equilibrium of the polyanion P(AMPSNa-co-SDPQ) is shifted toward lower pH (i.e., it is protonated less easily upon increasing the ionic strength). The gradual color shift from blue to green is more conceivable upon exposure of the polymeric solutions in UV light. Representative photos showing the appearance of the lower ionic strength solutions are shown in Figure 9. As seen for the P(AMPSNa-co-SDPQ) solution, this happens when the pH decreases below pH = 6.0. On the other hand, for the P(DMAM-co-SDPQ) solution this color change takes place at pH = 3.4 and for the P(MAPTAC-co-SDPQ) solution at pH = 3. In agreement with the results of Figure 8, the green color at low pH becomes less clear and is mixed with blue color, when the three quinoline-labeled copolymers are compared, since the emission peak of the blue color is not negligible or remains significant at low pH for P(DMAM-co-SDPQ) and P(MAPTAC-co-SDPQ), respectively. However, Figure 9 is a visual demonstration of the potential of such quinoline-labeled materials as luminescent pH indicators covering the whole useful acidic pH region. In fact, to construct a library of such indicators and to fine-tune the color change at the desired pH, apart from the selection of the water-soluble monomer, we can

also adequately design the chemical structure of the quinolinebased monomer.

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CONCLUSIONS The physicochemical behavior of quinoline-labeled watersoluble copolymers is investigated in the present work through the techniques of absorption and photoluminescence spectroscopies. The optical properties, especially luminescence, of these new materials are found to be pH-responsive. This pHcontrolled optical response is tuned by the charge of both the polymeric chain and the substituents of the quinoline unit. Moreover, in the case of charged copolymers, the ionic strength of the aqueous solution provides an additional factor to tune the pH of the color change, since ionic strength controls the strength of electrostatic interactions. The aforementioned findings show that these new quinolinelabeled water-soluble copolymers allow the construction of a library of luminescent polymeric materials to fine-tune the pHresponsive color change within the entire acidic pH range. This is important for the construction of optical pH-sensors/ indicators for a variety of applications. Such a library can readily grow through several approaches, including use of alternative vinyl-modified quinoline derivatives, use of alternative water-soluble monomers, and copolymerization of several monomers. In addition, the complexation of adequately F

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and the initiator was set at 1 mol % in all cases. The reaction mixtures were heated with an oil bath set at 110 °C for DMF, under inert atmosphere for 24 h. When THF was used as solvent, the reactions were carried out under reflux. After cooling down to room temperature, the resulting copolymers were precipitated in ethyl acetate (diethyl ether in the cases of the reactions in THF), filtrated, and dried under vacuum. Then, the materials were dissolved in ultrapure water and further purified through dialysis (membrane cutoff: 12 kDa) and freeze-dried. Prior to dialysis, the AMPSA-based copolymers were changed into the sodium salt form through neutralization with an excess of NaOH (Supporting Information). Characterization of Quinoline-Labeled Polymers. 1H NMR spectra were obtained on a Bruker Avance-DPX 400 MHz, using CDCl3 (containing TMS as internal standard) or deuterium oxide (D2O) as solvents for the characterization of the quinoline derivatives and the characterization of the copolymeric materials, respectively. For the organosoluble nonionic copolymers, gel permeation chromatography (GPC) was performed on a Polymer Lab chromatographer with two Ultra Styragel linear columns (104 Å × 500 Å), UV detector (254 nm) polystyrene standards, and CHCl3 as eluent, at 25 °C with a flow rate of 1 mL/min. For the ionic water-soluble copolymers a Millipore Waters 501 HPLC chromatograph with two Shodex B-804, B-805 linear columns (8 mm × 500 mm), a differential refractometer (R401) detector, poly(ethylene oxide) standards, and 0.1 M LiNO3 as eluent, at 25 °C with a flow rate of 1 mL/min, was used. The absorbance at 350 nm was used to establish the calibration curves of the three quinoline derivatives (SDPQ, SDPQCN, and 5FSPQ) in THF solutions, allowing thus the determination of the quinoline content of the soluble in THF DMAM-based quinoline-labeled copolymers (P(DMAM-co-SDPQ), P(DMAM-co-SDPQCN), and P(DMAM-co5FSPQ)). These copolymers were then used to construct the respective calibration curves in water, permitting the determination of the quinoline content of the quinoline-labeled polyelectrolytes in water, since these materials are hardly soluble in THF. The quantum yield, Q, of the final water-soluble copolymers was determined using quinine as standard molecule, dissolved in 0.1 M H2SO4 at a concentration of 1 × 10−4 M. The quantum yield of this standard upon excitation at 350 nm, Q = 0.557,51,56 was used for calculations. The methodology is presented in detail in the Supporting Information. Photophysical Properties in Aqueous Buffer Solutions. The UV−vis absorption and emission spectra of the copolymers in buffer solutions, covering the acidic pH region, were recorded on a Hitachi spectrophotometer, model U 1800, and a PerkinElmer LS45 luminescence spectrometer, respectively. All spectroscopic measurements were performed in quartz glass cuvettes (optical path = 1 cm). Both instruments were equipped with a circulating water bath. For the photoluminescence study, the excitation and emission slits were set at 5 and