M. R. Molla and S. Ghosh
a current of 40 kV and 30 mA, respectively. In a typical XRD experi-
ment, a solution of NDI-1 in water (1 mm) was repeatedly drop-cast onto
a glass slide to make a thick film. The film was then dried under a high
vacuum and the data were recorded from 1–308 with a sampling interval
of 0.028/step.
Conclusion
In summary, we have reported the vesicular assembly of
a bis-hydrazide-functionalized NDI-based bolaamphiphile in
aqueous medium by using the synergistic effect of hydro-
gen-bonding and p-stacking interactions. Control experi-
ments clearly showed the necessity of proper placement of
the hydrazide functionality to ensure shielding from the
bulk water so that they could remain engaged amongst
themselves in intermolecular hydrogen-bonding. Further-
more, the electron-deficient NDI chromophores in the vesic-
ular membrane quite-remarkably provided space for the in-
tercalation of completely water-insoluble electron-rich
pyrene guest molecules in a stoichiometric ratio to gain ad-
ditional stabilization through CT interactions without dis-
rupting the hydrogen-bonding framework. Interestingly,
such intercalation-induced 2D-to-1D vesicle-to-fiber mor-
phological transitions through intervesicular fusion eventual-
ly resulted in gelation at higher concentrations. This process
could be further extended to achieve a tunable, in situ, non-
covalent functionalization of the membrane by using
a water-soluble-functionalized intercalator, such as pyrene
butyrate. These results could inspire the exploration of vari-
ous other directional non-covalent interactions to enrich the
field of self-assembly of amphiphilic molecules and macro-
molecules to generate complex self-assembled functional
materials. In particular, it will be interesting to examine
such NDI-based vesicles as model systems to understand the
fusion of cell membranes and also for selective ion-trans-
port.[17] Intercalation-based non-covalent surface-functional-
Fluorescence microscopy: A solution of NDI-1 (40 mL) and various dye-
encapsulated solutions were placed between two clean glass slips and
images were taken on a fluorescence microscope (OLIMPUS BX-61) at
ꢂ40 magnification.
Determination of the critical aggregation concentration (CAC): The
CAC of NDI-1 was determined by using two independent methods: In
the first method, a stock solution of NDI-1 in water (10 mm) was pre-
pared and from this a series of solutions of various concentrations were
produced (1–0.05 mm) that were equilibrated for 1 h at RT before UV/
Vis spectroscopic analysis. Then, the absorbance of NDI-1 at 380 nm was
plotted against concentration and the CAC value was estimated from the
inflection point. In the second method (fluorescence method by Nile-red
encapsulation), a measured amount of a solution of Nile red in THF
(100 mL, 0.1 mm) was placed in various screw-capped vials and the sol-
vent was evaporated. Solutions of various concentrations of NDI-1 were
added to vials that contained Nile red and the mixture was sonicated and
allowed to stand for 2 h before fluorescence spectroscopic analysis (lex
=
530 nm). The final concentration of Nile red was 10ꢀ5 m. The emission in-
tensity of the encapsulated Nile red at 606 nm was plotted versus the
concentration of NDI-1 and the inflection point of such a plot was taken
as the CAC of NDI-1.
Dynamic light scattering (DLS): DLS experiments were carried out on
a Malvern instrument. A solution of (2 mL, 1 mm) the sample was pre-
pared and equilibrated at RT for 2 h before the DLS analysis.
Calcein encapsulation: An aqueous solution of Calcein (0.2 mL, 10ꢀ3 m)
was mixed with a solution of NDI-1 in water (1.8 mL, final concentration
of NDI-1=1 mm) and the mixture was sonicated for 15 min and then
stirred overnight at RT. The mixture was further dialyzed against water
by using 3000 Da MWCO membrane for 48 h (outside water was re-
placed with fresh water every 2 h) to remove any unencapsulated dye.
Then, the fluorescence spectra of the dialyzed solution was recorded and
compared with an absorption-normalized aqueous solution of the free
dye.
ACHTUNGTRENNUNGization approaches may also find great relevance in areas
such as targeted delivery and sensing; currently, such efforts
are underway in our laboratory.
Calcein-release experiment: A solution of NDI-1 vesicles (2 mL, 1 mm)
was treated with Calcein and extensive dialysis was performed to remove
any unencapsulated dye. When no more dye was coming out (confirmed
by the lack of an emission band from the water outside the dialysis bag),
excess urea (about 200 mg) was added to the solution inside the dialysis
bag and the bag was dipped into a container of fresh water (20 mL),
from which an aliquot (0.5 mL) was taken and emission spectra were re-
corded at different time intervals.
Experimental Section
UV/Vis spectroscopy: In a typical UV/Vis experiment, a stock solution of
NDI-1 (20 mm) was made in THF, from which an aliquot (0.1 mL) was
transferred into a glass vial and the solvent was evaporated to produce
a thin film. To this film was added a measured amount of THF/water to
adjust the desired final concentration (1 mm) of the solution and the vial
was shaken vigorously to produce an optically clear solution. The solu-
tion was allowed to equilibrate at RT for 2 h before spectroscopic meas-
urements were recorded.
Gelation test and the determination of Tgel: Solutions of NDI-1 and
pyrene (0.2 mL, 10 mm each) in THF were mixed together and the sol-
vent was evaporated to produce a deep-red-colored thin film; 0.4 mL
water was added and the mixture was sonicated for 3 min to produce an
optically clear red solution. The solution was allowed to stand for 3 days
at RT, after which time gelation was noted by the “stable to inversion of
a vial” method. The Tgel value was determined by using the “dropping-
ball” method: In a typical experiment, a glass ball (weight: 85.0 mg) was
slowly placed on top of the gel (10 mm) in a screw-capped vial and then
the vial was placed in a water bath. The temperature of the water bath
was gradually increased and the temperature at which the ball touched
the bottom of the vial was taken as Tgel (the gel-to-sol transition tempera-
ture).
Atomic force microscopy (AFM): An aqueous solution of NDI-1 (20 mL,
1 mm) was placed on a silicon wafer and allowed to dry overnight in air
before the images were taken. In the case of the hydrogel, the gel
(0.1 mL, 5 mm) was diluted with water (0.1 mL) and the samples were
prepared by following a similar procedure to that described above.
Transmission electron microscopy (TEM): A solution of the sample in
water (20 mL, 1 mm) was placed on a TEM grid (300 mesh carbon-coated
Cu grid). The samples were allowed to dry under vacuum for a few hours
before the measurements were recorded.
Determination of the association constant (Ka) for the CT complex: A
deep-red-colored solution of NDI-1+pyrene (10 mm, 1:1) was gradually
diluted with water and spectra were recorded at each concentration. The
intensity of the CT-absorption band at 529 nm was monitored at each
concentration and, from this data, the association constant (Ka) was de-
termined according to Equation (1), in which c, A, e, and l are the con-
centration, absorbance, extinction coefficient, and optical path length of
the cuvette, respectively (l=0.1 cm). To obtain the association constant,
FTIR spectroscopy: A solution of the sample in either THF or water
(50 mL, 2 mm) was placed in the spectroscopic cell, which was sandwiched
between two CaF2 windows, and the spectra of the solutions were record-
ed.
X-ray diffraction: XRD data were recorded on a Seifert XRD3000P dif-
fractometer with Cu Ka radiation (a=0.15406 nm) and a voltage and
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