We reported a well-defined amphiphilic fluorescent polymer bearing indole repeating units with naphthalimide pendants obtained by ATRP followed by chemical modification. The obtained polymer poly (N-allyl indole-2-(4-hydroxybutyl)-1, 8-naphthalimide) (PAIHN) was characterized by 1H NMR, FTIR, and GPC. The polymer showed a specific fluorescence emission maximum at 538 nm excited at 435 nm in aqueous solution. The amphiphilic nature of the polymer was investigated in aqueous solution using two spectroscopic methods, namely, absorption and emission spectroscopy. The Critical Miceller Concentration values obtained by UV-visible and fluorescence are in good agreement with each other. The micelles were characterized by TEM at concentration of 0.014 mg/mL. The polymer PAIHN prepared after tweaking exhibits green fluorescence at 538 nm due to the significant effect of naphthalimide moieties, whereas before tweaking the polymer poly (1-allyl indole-3-carbaldehyde) (PAIC) exhibits blue fluorescence at 425 nm due to indole repeating units of the polymer.
Amphiphilic property in polymers is a consequence of presence of segments of opposite philicity, that is, hydrophobic and hydrophilic segments, which are covalently bonded. This gives rise to unique properties at surfaces as well as in bulk due to the differential solvation of the hydrophilic and hydrophobic segments where the hydrophobic part forms the inner core and hydrophilic part forms the outer shell to form micellar structure in aqueous media [
The amphiphilic polymer architecture generally consists of block copolymers and several research groups have reported the synthesis and applications thereof where one block is hydrophilic while the other is hydrophobic [
Therefore, we intended to transform hydrophobic indole-based polymer into amphiphilic polymeric architecture by using naphthalimide-based hydrophilic fluorophore. Naphthalimide moiety was introduced in polymer, since amphiphilic polymer with intrinsic fluorescence is generally labeled with naphthalene or pyrene moiety [
Indole-3-carboxaldehyde (97%, Aldrich), 2,2′-bipyridyl (99%, Himedia), 4-bromo-1, 8-naphthalic anhydride (95%, Aldrich), 4-amino-1-butanol (98%, Aldrich), ethylenediamine (97%, Spectrochem), dimethylacetamide (DMA) (AR grade, Merck, India), absolute ethanol (99%, s. d. fine), acetonitrile (HPLC grade, Merck, India), methanol (99%, s. d. fine), and 2-bromoisobutyrylbromide (98%, Aldrich) were used as received. CuBr (98%, Aldrich) was purified by consecutive washing with glacial acetic acid in order to remove residual CuBr2 [
1H NMR spectra were recorded at 300 MHz resonance frequency in a JEOL AL300 FT NMR at 25°C in DMSO-d6 and CDCl3 using trimethylsilane (TMS) as an internal reference. FTIR spectra were recorded on a Varian Excalibur 3000 (Palo Alto, CA) spectrophotometer in the region 4000–400 cm−1 on KBr disc. The absorption spectra were recorded on Perkin Elmer-Lambda 35 UV-VIS spectrophotometer connected with PTP-1 Peltier system at 25°C in the range 200–700 nm. The optical path length of measurement cell was 10 mm. Fluorescence measurements were taken on Flurolog3-11 spectrophotometer from Horiba Jobin-Yvon, USA, with excitation at 435 nm using a 10 mm path length quartz cuvette in the wavelength range 430–700 nm. All fluorescence experiments were carried out at 25°C. Average molecular weights (
The fluorophore AAHN was prepared by small modification in procedures [ 4-Bromo-1, 8-naphthalic anhydride (1 mmol, 277 mg) was taken in absolute ethanol (15 mL) followed by the addition of 4-amino-1-butanol (1.5 mmol, 134 mg). The reaction was refluxed for 8 h, cooled, and poured in crushed ice. Product was precipitated, filtered, and washed with cold water. The precipitate was then dried under vacuum at 20°C for 4 h to give light yellow-colored solid
Synthesis of fluorophore AAHN (
1H NMR spectrum of the fluorophore AAHN in CDCl3.
13C NMR spectrum of AAHN in CDCl3.
FTIR spectra of AAHN and PAIHN in KBr.
Polymer of substituted indole PAIC was prepared by the ATRP protocol as reported elsewhere [
Schematic representation of chemical modification of PAIC.
1H NMR spectrum of modified fluorescent polymer PAIHN in CDCl3.
CMC of the polymer PAIHN was determined by absorption as well as emission phenomenon. It may be noted that due to the high composition of hydrophobic moieties in the polymer PAIHN, the polymer could not be dispersed directly into water. Therefore, the polymer was first dissolved in minimum amount of acetone (0.1 mL) and water. The mixture was sonicated to yield a clear solution followed by evaporation of acetone in nitrogen atmosphere and addition of distilled water to obtain a final stock solution of PAIHN of 1 mg/mL and the concentration was varied from 3.5 × 10−6 to 1 × 10−1 mg/mL by diluting the stock solution with distilled water. All the solutions were equilibrated at 25°C for 24 h, well shielded from light. The CMC determinations were carried out at 25°C and the values were calculated from the interception point of two tangent straight lines passing through the plotted points.
Chemical modification leads to many changes in chemical as well as physical properties of polymer. Thus, designing and incorporating suitable fluorophore result in tunable fluorescent polymer. Attempts were carried out to chemically modify PAIC such that the emitting color of the polymer solution could be tuned from initial blue to green. The attachment of many polar groups onto one hydrophobic polymer chain affords the polymer high amphiphilicity. Thus, in an effort to gain amphiphilicity naphthalimide unit containing hydroxyl group was attached to the polymer PAIC. PAIHN was self-assembled into micellar aggregates with hydrophobic polymer backbone segments as the core and hydrophilic hydroxyl group segments as the corona. A detailed discussion regarding the characterization of PAIHN along with amphiphilicity and fluorescence properties is given below.
The homopolymer of N-allyl indole-3-carboxaldehyde was synthesized by atom transfer radical polymerization in the presence of 2,2′-bipyridyl, CuBr, and 2-bromoisobutyryl bromide as ligand, catalyst, and initiator, respectively, in the ratio 200 : 4 : 2 : 4. In the case of fluorophore AAHN, all the aromatic protons of the naphthalimide unit appeared in the range
The analysis of the FTIR spectrum of the fluorophore AAHN (Figure
The number average molecular weight (
GPC chromatogram of PAIC and PAIHN.
Both the polymers PAIC and PAIHN have narrow molar mass dispersion (Đ
Characterization of indole-based polymer PAIC and chemically modified fluorescent polymer PAIHN by GPC.
Sample |
|
Đ |
---|---|---|
PAIC | 7200 | 1.19 |
PAIHN | 11500 | 1.21 |
The emission spectra of PAIHN in dimethyl sulfoxide, tetrahydrofuran, acetone, methanol, acetonitrile, and 1,4-dioxane are illustrated in Figure
Fluorescence spectra of PAIHN in different solvents.
PAIC and PAIHN dissolved in acetonitrile were investigated in normal as well as UV light for their change in color. Naked eye observation of PAIC and PAIHN exhibited brown and green coloration, whereas when exposed to UV light (354 and 365 nm) PAIC shows strong blue fluorescence and PAIHN emits bright green fluorescence as can be seen from Figures
Photographs exhibiting the color of PAIC and PAIHN in (a) normal light, (b) UV light (354 nm), and (c) UV light (365 nm).
Since the polymer PAIHN itself exhibits intrinsic fluorescent properties, hence, there was no need of adding any external fluoroprobe for the determination of CMC [
UV-visible absorption spectra of PAIHN at various concentrations (3.5 × 10−6 to 1 × 10−1 mg/mL) in aqueous solution.
Fluorescence emission spectra at various concentrations of PAIHN (3.5 × 10−6 to 1 × 10−1 mg/mL) in aqueous solutions, excited at 435 nm.
Plot of absorbance versus concentration of PAIHN for the determination of CMC.
Plot of emission intensity versus concentration of PAIHN for the determination of CMC.
The results obtained by the two spectroscopic methods are summarized in Table
Critical micelle concentration of PAIHN obtained by UV-visible and fluorescence methods.
Sample |
|
|
---|---|---|
PAIHN | 0.015 | 0.013 |
The successful formation of micelles was confirmed by TEM measurement where Figure
TEM micrograph showing micelles of the modified fluorescent polymer PAIHN (
In this paper, fluorescent amphiphilic polymer bearing indole repeating units with naphthalimide pendants was obtained by ATRP followed by chemical modification. The polymer showed a specific fluorescence emission maximum at 538 nm excited at 435 nm in aqueous solution. The amphiphilic nature of the polymer was investigated using two spectroscopic methods, namely, absorption and emission spectroscopy. The two methods revealed critical micelle concentrations of PAIHN solution to be 0.015 and 0.013 mg/mL, respectively; thus, the results of these two methods have evidenced fair agreement with each other. TEM studies also supported self-assembled micelles of PAIHN in water. These results demonstrate the possibility of preparing modified amphiphilic polymers with tunable fluorescence.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors are thankful to the Head of Department of Chemistry for providing laboratory facilities as well as NMR, fluorometer, and UV-visible spectrometer for the research work. The authors extend their thanks to SAIF, IIT Bombay, for providing TEM micrographs. Financial assistance from CSIR, New Delhi, in the form of project (01(2362)/10/EMR-II) and senior research fellowships to Ambika Srivastava and Pooja Singh are gratefully acknowledged.