Perturbations in the photophysical properties of isoxazole derivative of curcumin up on interaction with different anionic, cationic and non-ionic surfactants

Article abstract of DOI:10.1016/j.molliq.2021.116981

Surfactants are organized assemblies that are often used as models for biomembranes and they can also act as solubilizing agents for various hydrophobic drug molecules. Isoxazole derivative of curcumin (IOC) is a potential drug molecule whose biological e

Full text of DOI:10.1016/j.molliq.2021.116981

Journal of Molecular Liquids xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Perturbations in the photophysical properties of isoxazole derivative of
curcumin up on interaction with different anionic, cationic and non-
ionic surfactants
⇑
Manisha Sharma, Swati Rani, Subho Mozumdar
Department of Chemistry, University of Delhi, Delhi 110007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Surfactants are organized assemblies that are often used as models for biomembranes and they can also
act as solubilizing agents for various hydrophobic drug molecules. Isoxazole derivative of curcumin (IOC)
is a potential drug molecule whose biological efficacy is limited due to its low aqueous solubility. In the
present work, different supramolecular confined environments have been utilized to enhance the aque-
ous solubility of IOC. Besides this, they have also been used as models for studying the perturbations in
the photophysical properties of IOC upon encapsulation inside them. To serve this purpose, three differ-
ent surfactants have been chosen from each category of cationic (CTAB: Cetyltrimethylammonium
Bromide), anionic (SDS: Sodium Dodecylsulfate) and non-ionic (TX-100: Trition X-100) surfactants. In
order to study the mechanism of interaction of IOC with the three different types of surfactant molecules,
various steady-state and time-dependent spectroscopic techniques have been employed. The change in
the optical properties of IOC, such as; red or blue shifts in the absorption or emission maxima and
increase in the intensities has helped in gathering information about the successful encapsulation of
IOC inside the surfactant micelles as well as the effect of different confined environments on the photo-
physics of IOC. The time-dependent stability of all the three micellar systems divulged with the help of
UV–Visible spectroscopy has revealed that IOC-surfactant micellar systems are extremely stable in nature
even after 3 days. The large value of partition coefficient and increment in the quantum yield values as
well as average lifetime certainly indicates the partitioning of the IOC molecules inside the organized
assemblies of all the three surfactant micellar systems. Moreover, the effect of salt (NaCl) concentration
has also been investigated on the steady-state and time-dependent photophysical properties of IOC
encapsulated surfactant nano-micellar system. It could be observed that the salt has no effect on the pho-
tophysics of IOC even at very high concentration. Hence, the information obtained from the present work
opens door for a relatively new research area where novel drug formulations of IOC could be designed
and developed that would help the mankind in the near future.
Received 13 May 2021
Revised 6 July 2021
Accepted 9 July 2021
Available online xxxx
Keywords:
Isoxazole derivative of Curcumin
CTAB
SDS
TX-100
Spectroscopy
Ó 2021 Elsevier B.V. All rights reserved.
1. Introduction
is of prior importance in order to design and develop drug formu-
lations and delivery systems.[8] Surfactant molecules are amphi-
Surfactants are organized assemblies that play a pivotal role in
textile industry, chemical industry and pharmaceutical industry as
an emulsifying, disintegrating and solubilizing agent.[1–4] Besides
this, the micelles of surfactants are also used as models for
biomembranes.[5] Surfactants are enormously used in the drug
industry in order to increase the aqueous solubility of drugs,
improve bioavailability, maintain drug stability along with control
drug release and uptake.[6–7] Therefore, the knowledge to interac-
tion mechanism of feebly soluble drug molecules with surfactants
philic in nature as they contain a hydrophilic polar head group as
well as hydrophobic non-polar hydrocarbon chain. On the basis
of their head groups, the surfactants can be classified as either
ionic (anionic/ cationic) or neutral.[9] It is because of this amphi-
philic nature that surfactants are capable of self-assembling in
water so as to form micelles. The concentration at which the for-
mation of this self-assembly occurs is known as critical micelle
concentration (cmc).[10–11] At the cmc value, the surfactants are
observed to experience drastic alterations in various physico-
chemical properties like surface tension, electromotive force, con-
ductivity and so on.[12–13] Due to all these advantages, extensive
research has been carried out in the recent times where different
⇑
Corresponding author.
https://doi.org/10.1016/j.molliq.2021.116981
0167-7322/Ó 2021 Elsevier B.V. All rights reserved.
Please cite this article as: M. Sharma, S. Rani and S. Mozumdar, Perturbations in the photophysical properties of isoxazole derivative of curcumin up on
interaction with different anionic, cationic and non-ionic surfactants, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2021.116981

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
cationic, anionic, non-ionic and gemini surfactants have been uti-
lized in enhancing the aqueous solubility and stability of various
drug molecules and ionic liquids. Some of them are ceftriaxone
sodium trihydrate, various antidiabetic drugs like: gliclazide, gly-
buride, glimepiride, glipizide, repaglinide, pioglitazone and rosigli-
tazone, and in
a series of antibiotics including cloxacillin,
cephalothin, cefotaxime, meropenem, and gentamicin.[5,14] Lee
et al. have reported the utilization of surfactants (Tris and
Trition-X 100) in enhancing the anti-bacterial activity of shikonin
which is a highly liposoluble naphthoquinone pigment isolated
from the roots of L. erythororhizon.[15] Additionally, there are
numerous reports where surfactants have been employed to
enhance the aqueous solubility of an extremely efficient drug
named curcumin.[7,16–18]
Curcumin, a natural yellow medicinal pigment found in the
roots of Curcuma longa has various beneficial pharmacological
and biological applications like anti-inflammatory, antioxidant,
anti-Alzheimer, antitumor, anticarcinogenic and free radical scav-
enger properties.[19–20] Despite all these applications, the biolog-
ical efficacy of curcumin is hindered due to its extremely low
aqueous solubility, rapid degradation at physiological conditions
and low bioavailability.[21–22] To overcome these limitations,
the scientific community has made several attempts to enhance
the aqueous solubility of curcumin. One of the many approaches
adopted by researchers is to entrap curcumin inside the core of
micelles formed by surfactants. Surfactants from almost all the cat-
egories either independently or in combinations have been
employed for this purpose such as; sodium dodecylsulfate (SDS,
anionic surfactant),[18,23–24] Sodium dodecylbenzenesulfonate
(SDBS, anionic surfactant),[25] Cetyltrimethylammonium bromide
(CTAB, cationic surfactant),[16] cetyltrimethylammonium tosylate
(CTAT, cationic surfactant),[26] Dodecyltrimethylammonium bro-
mide (DTAB, cationic surfactant),[7,24] Triton X-100 (TX-100,
non-ionic surfactant),[24] Tween-20 (non-ionic surfactant)[20,27]
and many more. Kee et al. have reported that the alkaline hydrol-
ysis of curcumin could be suppressed after encapsulation inside
cationic micelles.[16] Despite all these efforts, the desired stability
and bioavailability of curcumin could not be achieved by the scien-
tific community. In the recent years, another approach that has
been exploited by the researchers to extract the effectiveness of
curcumin is to explore the structurally modified curcumin deriva-
tives.[28]
One of the commonly investigated curcumin analogues is the
isoxazole derivative of curcumin (IOC). IOC is the structurally mod-
ified curcumin derivative where the b-diketo group of curcumin
molecule has been transformed into the five membered isoxazole
ring (Scheme 1).[29] It has been reported that IOC possesses wide-
spread biological applications like antitumor, anticancer, antioxi-
dant, antimalarial, anti-proliferative, anti-Parkinson activities and
many more.[28,30–34] IOC has also been reported to have modest
inhibitory activity against COX-2 enzymes. [35] Although the
aqueous solubility of IOC is greater than curcumin, the effective-
ness of IOC is still largely compromised due to its low aqueous sol-
ubility. Therefore, keeping in mind the influence of surfactants in
solubilizing curcumin, efforts have been made in the directions
of enhancing the solubility of IOC using different types of surfac-
tants. The information about the mechanism of interaction of IOC
with different kinds of supramolecular confined environments
would also help in understanding its mode of action inside biolog-
ical entities and would also further help in designing and develop-
ing new formulations as well as drug delivery vehicles for IOC.
The present work aims in utilizing three different surfactants,
one from each category of cationic (CTAB), anionic (SDS), and
non-ionic (TX-100) surfactants in solubilizing poorly water-
soluble drug, IOC. These surfactants have been selected in order
to understand the effect of different supramolecular confined envi-
Scheme 1. Chemical structures of Isoxazole derivative of Curcumin (IOC) along
with the surfactants used in this study: Cetyltrimethylammonium bromide (CTAB),
Sodium dodecylsulfate (SDS) and Triton X-100 (TX-100).
ronments on the photophysics of IOC. The interaction mechanism
of IOC with all the three surfactants have been studied using both
steady-state and time-dependent spectroscopic techniques. The
stability of the micelles formed in each case has been scrutinized
by investigating the degradation profile which is achieved via
recording of the absorption spectra at regular intervals of time.
The thermodynamic parameters like change in Gibbs free energy,
enthalpy and entropy have also been calculated for all the three
kinds of nanomicellar systems. Besides this, the effect of salt (NaCl)
on the photophysics of IOC encapsulated inside all the three types
of nano-micellar systems, has also been scrutinized in depth. Fur-
ther, the TCSPC studies have been deployed to study the excited
state photophysical properties of IOC encapsulated inside different
kinds of supramolecular confined environments.
2. Experimental section
2.1. Materials
Curcumin, ethanol and hydroxylamine hydrochloride (NH2OH.
HCl) were purchased from Sigma Aldrich. Moreover, cetyltrimethy-
lammonium bromide (CTAB), sodium dodecylsulfate (SDS) and Tri-
ton X-100 (TX-100) were also purchased from Sigma Aldrich. All
the chemicals obtained were of analytical grade and were used
without any further purification. The synthesis of isoxazole deriva-
tive of curcumin (IOC) was done by method reported in our previ-
ous studies.[29] In brief, 1 mmol of curcumin and 10 mL of ethanol
were taken in a round bottom (RB) flask and the contents were
allowed to stir for 15 mins. Subsequently, catalytic amount of
acetic acid was added to the RB, followed by addition of 1.5 mmol
of NH2OH.HCl. Then, under N2 atmosphere the reaction mixture
was allowed to stir at 80ꢀC for 24 h and the thin layer chromatog-
raphy (TLC) was used to study the progress of the reaction. Finally,
2

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
after the completion of the reaction, the contents of the RB were
transferred to a beaker containing ice cold water which led to
the formation of brown color precipitates. The crude formed was
extracted via centrifugation and was recrystallized in methanol
to obtain the pure form of IOC which was used for further studies.
The stock solution of IOC was prepared in methanol whereas the
stock solution of all the four surfactant solutions were prepared
in potassium phosphate buffer solution of pH 7.4. HPLC grade
water was used to prepare all the samples. In all the experiments
terns were obtained at the emission wavelength of 500 nm of the
corresponding sample under investigation. During measurements,
the emission polarizer angle was fixed at 55° while the excitation
was vertically polarized. A long pass filter with cut-off at 400 nm
was added in the emission channel to eliminate all the probability
of the scattered excitation light. The obtained decay patterns were
fitted using tri-exponential functions. The average lifetime of
excited state of IOC molecule in the presence of different surfactant
environments could be calculated using Eq. (2).[36]
the concentration of IOC was fixed at 10 lM.
sav ¼ a1s1 þ a2s2 þ a3s3
ð2Þ
g:
Here, s1, s2 and s3 are the three components of the decay time of
IOC in presence of different surfactant solutions and a1, a2 and a3
represents their corresponding relative weightage.
The values of radiative and non-radiative decay rate constants
could be determined using the Eq. (3).[36]
2.2. Preparation of samples of IOC in different surfactant solutions
Firstly, stock solution of IOC was prepared in methanol. Then,
the stock solutions of 0.1 M CTAB, 0.1 M SDS and 0.01 M TX-100
were prepared in 100 mM phosphate buffer solution of pH 7.4. In
order to formulate the samples of IOC with surfactant, required
amount of methanolic stock solution of IOC was added to vessels
and the solvent was allowed to evaporate at room temperature.
Then, after the formation of thin film, it was scratched from the
vessel and requisite amount of surfactant solution was added to
it. The resultant solution thus formed was introduced to ultrason-
ication for about 3 h. Following this, the samples were filtered
0EF
sav
1
sav
F
kr ¼
and
¼ kr þ knr
:
g
ð3Þ
g:
Here, kr corresponds to the rate constant for radiative process
whereas knr represents non-radiative decay rate constant. The sym-
bols savg and ɸF corresponds to the average lifetime of the excited
state of IOC molecule as determined by Eq. (2) and fluorescence
quantum yield as calculated from Eq. (1); respectively.
through a 0.45
l filter to get rid of any unbounded surfactant
and IOC molecules. Finally, these samples were instantly used for
further analyses.
All the measurements were carried out at 25 °C.
3. Results and discussion
2.3. Characterization techniques
The structure of isoxazole derivative of curcumin (IOC) along
with all the surfactants used in the present study i.e., CTAB, SDS
and TX-100 have been indicated in Scheme 1. In the present study,
initially the characterization for the successful formation of
micelles of IOC in the presence of different surfactants have been
accomplished by using DLS and TEM analysis and the results
obtained have been presented in the supplementary information
(SI: Figure S1 and S2). Then after, the study of interaction mecha-
nism has been achieved by using different steady-state and time-
dependent spectroscopic techniques. The effect of temperature
and salt (NaCl) on the photophysics of IOC has been evaluated in
depth. The details of the results obtained along with the elaborated
discussion and analysis follows below.
The absorption spectra of IOC in the presence of different sur-
factant solutions were recorded using Shimadzu spectrophotome-
ter (UV-2600) in the wavelength range of 200 nm ꢀ 800 nm. The
quartz cuvettes having 1 cm path length were used for the mea-
surements. The steady state emission and excitation spectra were
recorded using Varian Carry Eclipse Fluorescence spectrophotome-
ter in the wavelength range of 355 nm ꢀ 800 nm and from 200 nm
to 450 nm; respectively. The samples of CTAB, SDS and TX-100
were excited at a wavelength of 338 nm, 336 nm and 339 nm;
respectively. Similarly, for recording the excitation spectra, the
emission wavelength was fixed at 408 nm, 430 nm and 408 nm;
respectively. During all the measurements, the temperature was
maintained at 25 °C by circulating the water through the ther-
mostated cuvette holders and both the excitation and emission
slits were kept at 5 nm throughout the measurements.
3.1. Steady state absorption and emission studies
The fluorescence quantum yield of IOC in presence of CTAB, SDS
and TX-100 solutions was calculated using the below written
equation.[36]
In order to understand the effect of different microenvironment
provided by different cationic, anionic and non-ionic surfactants,
initially the absorption spectra of IOC (10 lM) in presence of CTAB
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
(5 mM), SDS (20 mM) and TX-100 (1.5 mM) have been recorded.
The overlay of all the three absorption spectra have been repre-
sented in supplementary information (SI: Figure S3a). In our previ-
ous studies, it has been reported that IOC in pure aqueous medium
shows a main peak at 333 nm accompanied by a shoulder at
295 nm.[29] In general, the absorption spectra depicted in Fig-
ure S3a displays one main peak and one shoulder at around
295 nm. However, in case of TX-100 as surfactant, the peak at
around 270 nm solely corresponds to that of TX-100. Moreover,
it could be noticed that in presence of surfactants the main peak
is red shifted. The IOC entrapped inside the micelles of CTAB, SDS
and TX-100 manifests an absorption maxima at 338 nm, 336 nm
and 339 nm; respectively. Thus, a red shift of 5 nm, 3 nm and
6 nm could be observed for CTAB, SDS and TX-100 micelles;
respectively. This red shifted absorption spectra clearly demon-
strates the presence of some specific interactions of IOC with all
the three surfactant molecules. Further, to get fundamental under-
standing about the excited state photophysical properties of IOC
AS
AR
AbsR gS
AbsS gR
0EF ¼ 0EF
ð1Þ
s
R
In this equation, the subscripts ‘‘S” and ‘‘R” corresponds to sample
solutions and reference sample; respectively. The symbols ɸ, , A,
g
and Abs. have been designated to the fluorescence quantum yield,
refractive index, integrated area under the curve of emission spectra
and absorption intensity at maxima; respectively. In this experi-
ment, quinine sulfate in 0.1 N H2SO4 (which has quantum yield
value of 0.546) has been used as reference.[37]
In order to conduct the experiments related to the picosecond
resolved excited state lifetime measurements, the commercially
available time-correlated single- photon counting (TCSPC) set up
was used. This instrument has been provided by Edinburgh instru-
ments, U.K. and it possesses MCP-PMT detectors. The picosecond
resolved fluorescence transients could be recorded by employing
a 375 nm picosecond pulsed laser diode as an excitation source
with an instrument response function (IRF) of 75 ps. The decay pat-
3

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
encapsulated inside different micelles, the emission spectra have
been recorded and is presented in Figure S3b. The inset of the Fig-
ure S3a and S3b corresponds to the intensity magnified normalized
emission spectra of the same samples. The emission spectra of pure
IOC in an aqueous medium exhibits a maximum at 431 nm.[29]
Whereas, in case of CTAB, SDS and TX-100, the emission maxima
appears at 408 nm, 430 nm and 408 nm; respectively. An expected
blue shift could be analyzed for all the three types of surfactants.
The blue shift in the emission spectra illustrates the existence of
IOC in comparatively more hydrophobic environment inside the
surfactant aggregates. It has been reported by Sarkar et al. that
upon increase in the solvent polarity, the absorption energy
increases and the emission energy experiences a decrement. In
other words, the absorption maxima exhibit a red shift whereas
the emission spectra show a blue shift when the microenviron-
ment around the probe becomes more and more non-polar in nat-
ure.[38] Depending on the extent of the blue shift, it could be easily
said that the IOC molecules experience minimum hydrophobicity
in the case of SDS micelles. Similar results have been obtained
when curcumin was used as a fluorescence probe and was ana-
lyzed in the presence of CTAB, SDS and TX-100.[24,39]
before and after micellization stages are quite similar in case of SDS
and TX-100. Whereas it could be stated that the interaction mech-
anism of IOC is completely different for CTAB surfactant below and
after its cmc value.
Moreover, the trend of augmentation in the absorption inten-
sity upon increase in the surfactants’ concentration could be easily
understood with the help of the plot of absorbance versus concen-
tration of surfactant. These graphs have been represented in the
inset of their corresponding absorption spectra overlay (Fig. 1a,
1c and 1e). After the cmc value of CTAB, SDS and TX-100, it could
be observed that the absorption intensity could not increase much,
in fact a plateau could be declared at higher concentrations. This
could imply that maximum solubilization of IOC by the surfactant
molecules could be achieved with concentrations just above the
cmc value and addition of the surfactant in the excess quantity
has no significant impact on the solubility of hydrophobic drug
IOC. The solubility of IOC in the presence of CTAB, SDS and TX-
100 surfactant molecules has been calculated using the traditional
Lambert-Beer’s law. It has already been reported in the previously
that the molar extinction coefficient of IOC in presence of methanol
is around 4.521 ꢁ 104 dm3mol-1cmꢀ1.[29] Substituting this value
in the formula, the maximum concentration of IOC inside the
Now, in order to investigate the interaction mechanism of IOC
with different micellar environments, the absorption spectra of
micellar aggregates has been found to be around 9.7 0.2
0.2 M and 9.3 0.2 M for CTAB, SDS and TX-100; respec-
tively. Now, considering that the amount of IOC in all the formula-
tions was initially fixed at 10 M, these solubility values could
lM, 9.
IOC at fixed concentration (10
lM) were recorded in the presence
5
l
l
of increasing concentrations of CTAB, SDS and TX-100. The concen-
tration of CTAB and SDS has been varied from 0 mM to 100 mM,
whereas the concentration of TX-100 has been increased from
0 mM to 10 mM. The overlay of the absorption spectra in all the
three type of surfactants have been presented in Fig. 1. The results
indicate that in the absence of surfactants, the absorption peak is at
333 nm escorted by a shoulder at around 295 nm. But when the
CTAB surfactant is added even in the minute quantity i.e.,
0.4 mM, the absorption maxima shifts from 333 nm in pure aque-
ous medium to around 346 nm, in other words, a red shift of about
13 nm could be observed together with a new additional peak at
around 400 nm. This new peak in case of cationic surfactant has
also been reported for curcumin by Adhikary et al. where they have
described that this peak is due to the presence of a small popula-
tion of deprotonated curcumin at pH 7.4.[24] Further, when the
concentration of CTAB is gradually increased up to its cmc (i.e.,
0.92 mM to 1.0 mM), the absorption intensity is also found to
increase which could be seen to get accompanied by a prominent
peak at around 400 nm. But when the concentration of CTAB was
further raised above the cmc value i.e., from 1 mM up to
100 mM, the shoulder at around 400 nm completely disappeared
and this could be clearly manifested in the Fig. 1a. The absorption
spectra after the cmc of CTAB depicted a blue shift in maxima from
346 nm to around 338 nm followed by a shoulder at around
295 nm. This could imply that after the cmc value, the IOC mole-
cules are travelling from the bulk water phase towards more and
more hydrophobic environment i.e., towards the core of the
micelle. Additionally, in the case of SDS surfactant, after the addi-
tion of SDS in extremely low quantity i.e., 4 mM, the absorption
maxima got shifted to 336 nm (Fig. 1c). Meanwhile, when the con-
centration of SDS has been further increased, only the absorption
intensity increased and no effect could be observed on the absorp-
tion maxima. Moving forward, the absorption spectra of IOC in
presence of non-ionic surfactant (TX-100) demonstrates a red shift
from 333 nm to 339 nm (Fig. 1e). This information could assist us
to deduce that after micellization in each one of the surfactants,
TX-100 is capable of providing maximum hydrophobicity to the
ground state of IOC molecules. Also, on increasing the concentra-
tion of TX-100 after cmc value, no alterations in the nature of
absorption spectra could be observed and the only parameter that
is getting effected is the absorption intensity. No shift in the
absorption maxima could elucidate that the nature of interactions
l
account for a remarkable solubilization of around 97%, 95% and
93% for CTAB, SDS and TX-100; respectively Besides this, after com-
paring the solubilization tendencies of all the three types of surfac-
tants, it could be conjectured that TX-100 provides minimum
solubilization and CTAB provides maximum solubilization to the
IOC molecules. For the sake of effortless understanding, all these
values have been tabulated in the Table 1.
In order to understand the excited state photophysical proper-
ties of IOC encapsulated nano-micellar systems, one must record
the steady-state emission spectra of the aforementioned systems.
Therefore, the emission spectra of the IOC-surfactant micellar for-
mulations were recorded in the range of 355 nm to 800 nm. The
overlay of the emission spectra is represented in Fig. 1. In the
absence of surfactants, the emission maxima of pure IOC could
be obtained at around 431 nm.[29] In case of CTAB, even when
minimal amount of CTAB is added (i.e., 0.4 mM), the emission max-
ima got shifted to 480 nm. A huge red shift of approximately 49 nm
could be observed. This comprehensive red shift could possibly be
due to the drastic enhancement in the micropolarity experienced
by the IOC molecules which could probably be due to some sort
of specific interactions amongst the two molecules in the pre-
micellar state. Further, when the concentration of CTAB is around
its cmc value, a broad emission maxima could be obtained ranging
from 408 nm to 480 nm. Moreover, when the concentration of
CTAB was substantially higher than its cmc value (i.e., from
1 mM to 100 mM), a comparatively sharper emission spectra could
be obtained with the emission maxima at around 408 nm. This
could imply that the IOC molecules are getting sifted from the
stern layer towards the core of the micelles formed by the CTAB
surfactant molecules. Based on the findings of absorption and
emission spectra, it could simply be stated that below cmc the
microenvironment encountered by the IOC molecules is polar in
nature and beyond cmc it is hydrophobic in nature. The inset graph
of the Fig. 1b depicts a plot between the fluorescence intensity of
IOC at 408 nm and the concentration of CTAB in the formulations.
The graph reveals that below the cmc of CTAB, the increase in the
fluorescence intensity is quite rapid and after the cmc value the
increase is gradual. When similar studies were performed with
SDS surfactant, after addition of minimal amount of SDS solution,
the emission maxima got blue shifted from 431 nm in pure aque-
4

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
Fig. 1. The absorption spectra of IOC in the presence of increasing concentration of (a) CTAB (0 to 0.1 M), (c) SDS (0 to 0.1 M) and (e) TX-100 (0 to 0.01 M); respectively. The
inset in each figure represents the absorbance (at absorption maxima) versus the concentration of surfactants. The emission spectra of IOC in the presence of increasing
concentration of (b) CTAB (0 to 0.1 M), (d) SDS (0 to 0.1 M) and (f) TX-100 (0 to 0.01 M); respectively. The inset in each figure represents the variation of emission intensity
with the increasing concentration of the surfactant.
ous medium to 425 nm in the presence of SDS surfactant. Whereas,
when the concentration of SDS was further enhanced, the emission
maxima got shifted from 425 nm to 430 nm. Thus, it could be sta-
ted that IOC molecules experience almost similar microenviron-
ment upon increase in the SDS concentration. Another
discrepancy in the emission spectra in the presence of SDS with
Table 1
Values of critical micellar concentration of different surfactants along with solubility, absorption maxima, emission maxima, Stokes’ shift and ET(30) values of IOC in the presence
of different surfactants. (Error within 5%).
Surfactant
cmc
Solubility (mM)
kmax (abs.) (nm)
kmax (emsn.) (nm)
Stokes’Shift (cmꢀ1
)
ET(30) (kcal/mol)
Water
CTAB
SDS
–
3.5 0.2
9.7 0.2
9.5 0.2
9.3 0.2
333
338
336
339
431
408
430
408
6828
5076
6506
4989
63.1
0.92 mM ꢀ 1 mM
40.80
62.91
40.80
8.2 mM
TX-100
0.22 mM ꢀ 0.24 mM
5

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
that to CTAB is that; in case of SDS, the emission intensity attains a
plateau just above its cmc value. The inset of Fig. 1d certainly helps
us to clearly analyze the trend of emission intensity with the con-
centration of SDS with the saturation concentration of SDS for IOC
reaching at 10 mM. Furthermore, the Fig. 1f shows the emission
spectra of IOC in the presence of increasing concentration of TX-
100 surfactant. Here, even with a slight addition of TX-100 solu-
tion, the emission maxima could display a blue shift and at the sat-
urated concentration of TX-100 (i.e., 100 mM), the emission
maxima got shifted to 408 nm. This blue shift of 23 nm suggests
that the IOC molecules are traveling from the bulk water phase
to the hydrophobic core of the TX-100 micelle. Moreover, it could
also be noticed that in the presence of low concentrations of TX-
100, the emission spectra are quite broad and it keeps on becoming
sharper as the concentration of TX-100 increases. This sharpness
could indicate that the rotational and vibrational modes of the
IOC molecules are getting restricted and the IOC molecules are
residing in more compact hydrophobic microenvironment. For
the sake of clarity, a graph has been plotted between emission
intensity @408 nm on y-axis and concentration of TX-100 on the
x-axis and it has been displayed in the inset of Fig. 1f. Furthermore,
the trend in the emission maxima for all the three types of surfac-
tants follows the order: CTAB (408 nm) ~ TX-100 (408 nm) < SDS
(430 nm). This information aids us to infer that the IOC molecules
experiences almost similar hydrophobic microenvironment in case
of CTAB and TX-100 micelles whereas the SDS surfactant aggre-
gates provides the most hydrophilic environments. Based on these
findings, it could also be inferred that in case of the CTAB and TX-
100 micelles, the IOC molecules are present in the palisade layer
whereas in the case of SDS micelles, the IOC molecules are present
in the stern layer of the micelles which felicitates strong interac-
tion with the water molecules. Similar results have also been
reported when curcumin has been investigated as the fluorescent
probe inside these supramolecular confined environments.[24] In
addition, a comparison between the absorption and emission max-
ima of IOC in the presence of these surfactants with those inside
some organic solvents and water could help us to get an insight
about the resemblance of the microenvironments. It could be ana-
lyzed that the microenvironment experienced by the IOC mole-
cules in the presence of CTAB and TX-100 is similar to that of
aprotic solvent like Tetrahydrofuran whereas that in the presence
of SDS molecules is analogous to that of polar protic solvents like
methanol and water.[29]
excitation spectra is almost similar in nature where both possesses
two peaks i.e., one main peak around 338 nm accompanied by a
shoulder at around 295 nm. Thus, it could be easily interpreted
that the there is only one emitting specie in the different IOC-
surfactant nano-micellar formulations and it responds distinctively
in presence of different microenvironments.
Furthermore, the effect of excitation wavelength on the emis-
sion spectra and vice versa have also been investigated on all the
three IOC-surfactant formulations. The overlay of both red and blue
end wavelengths on both the fluorescence excitation as well as
emission spectra have been represented in the Supplementary
Information (SI: Figure S4). From the figure, it could be easily seen
that the effect of excitation wavelength on emission spectra and
emission wavelength on the excitation spectra is almost negligible.
This could further confirm that there is only one specie i.e., IOC,
responsible for the fluorescence spectra.
3.2. Determination of ET(30) values
A more precise information about the micropolarity of the envi-
ronment experienced by the IOC molecules in presence of different
cationic, anionic and non-ionic surfactant could be obtained by
determining the solvent polarity parameter (ET(30)). This could
be executed by studying the changes occurring in the spectral
properties of the fluorescent probe molecule in the presence of dif-
ferent environments of known polarity. Thus, the emission spectra
of IOC have been recorded in the presence of varying ratios of dif-
ferent 1,4-Dioxane/ water mixtures. The overlay of the emission
spectra of IOC in presence of different mixtures has been illustrated
in the supplementary information (SI: Figure S5). The ET(30) values
obtained for CTAB, SDS and TX-100 micellar systems are around
40.80 kcal/ mol, 62.91 kcal/ mol and 40.80 kcal/ mol; respectively
(Table 1). From the values of Table 1, it could also be analyzed that
the increment in the values of Stokes shift is in accordance with the
increase in the micropolarity parameter. These results echo with
the results obtained in our previous study where the Stokes shift
of IOC in presence of various solvents increases with the polarity
of the solvent.[29]
3.3. Determination of partition coefficient
In order to check the efficiency of a newly studied system as a
potential drug delivery vehicle, one must have some quantitative
knowledge about the penetration of the drug inside the micelle
and this could be efficiently done by calculating the partition coef-
ficient. In this, the fluorescence spectra of different samples con-
Now, in order to accumulate more details about the extent of
reorganization of the local environment of the IOC molecules, the
fluorescence Stokes shift has been calculated for all the three sur-
factants. The obtained values in case of pure aqueous medium as
well as for the three surfactants have been tabulated in Table 1.
The value of Stokes shift of IOC in the absence of surfactants is
around 6828 cmꢀ1 whereas the values obtained in case of CTAB
and TX-100 were 5076 cmꢀ1 and 4989 cmꢀ1; respectively and that
acquired for SDS micellar system is 6506 cmꢀ1. Therefore, it is
clearly evident that the local microenvironment experienced by
the IOC molecules in the presence of all the three surfactants is def-
initely less polar as the values of Stokes shift have decreased and it
could be stated that the SDS surfactant could provide a rather polar
microenvironment to the IOC molecules as compared to the other
two surfactant assemblies, further confirming that in case of SDS,
IOC molecules are present in the stern layer of the micelle.
taining
a
fixed amount of IOC (10 lM) and variable
concentration of surfactants (definitely above cmc) were recorded
and the values of fluorescence intensities of different samples were
substituted in the equation mentioned below.
I1 ꢀ I0
It ꢀ I0
½Waterꢂ
1
¼ 1 þ
ꢁ
kp
½Surfactantꢂ
Here, Io, It and I are the fluorescence intensities of IOC in the pres-
1
ence of bulk water, at various concentrations of surfactant and at
concentration where the fluorescence intensity becomes saturated.
The value of partition coefficient could be obtained by plotting the
graph between (I - Io)/(It and I0) and [Surfactant]-1 (Fig. 2). Based
1
Now, in order to gather information about the nature of the
emitting species, the fluorescence excitation spectra have been
recorded in the presence of all the three surfactants. The overlay
has been represented in the supporting information (SI: Figure S3c).
The maxima in case of CTAB, SDS and TX-100 nano-micellar sys-
tems could be obtained at 338 nm, 339 nm and 341 nm; respec-
tively. Also, the nature of the absorption and fluorescence
on the slope of these graphs, the value of partition coefficients at
25 °C could be calculated to be around 1.63 ꢁ 104, 1.55 ꢁ 104 and
2.133 ꢁ 105 for CTAB, SDS and TX-100 nano-micellar systems;
respectively. These values of partition coefficient are considered
to be quite high for such newly studied systems. Also, it could be
observed that the value of Kp in case of TX-100 micelles is almost
10 times higher than those obtained in the case of CTAB or SDS. This
6

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
information benefits us to presume that maximum extent of pene-
tration of the IOC molecules from the bulk water phase towards the
core of the micelle could be achieved in case of the non-ionic surfac-
tant i.e., TX-100.
3.4. Stability of IOC-Surfactant nano-micellar system
The prolonged stability of a newly designed nano-formulation is
utterly important criteria which must be fulfilled so that it could be
utilized as a drug delivery vehicle. The stability of different IOC-
surfactant nano-micellar formulations has been evaluated by
studying the degradation profile via UV–Visible spectroscopy by
recording the absorption spectra of all the three formulations at
regular intervals of time up to 3 days. The overlay of the absorption
spectra for all the three formulations have been represented in
Fig. 3. It can be easily identified from the figure that the stability
of the IOC-surfactant nano-micellar system is maximum in case
of non-ionic surfactant i.e., TX-100. The rate of degradation of the
formulations has been determined for all the samples by plotting
a graph between % change in the absorption intensity at maxima
versus time and it has been depicted in the inset of Fig. 3a, 3b
and 3c for CTAB, SDS and TX-100; respectively. The overall rate
of degradation of the micellar system of CTAB, SDS and TX-100
could be calculated to be around 6.8%, 3.06% and 3.28%; respec-
tively. Such low values of the rate of degradation for nano-
micellar formulations correspond to the extraordinary stability of
IOC encapsulated surfactants. However, the time-dependent
absorption spectra of IOC inside CTAB and SDS micellar solutions
shows some variation with time. In the case of CTAB surfactant,
after 24 h an additional peak could be seen to appear at around
400 nm. This peak at around 400 nm in case of CTAB surfactant
has also been obtained when CTAB was added to IOC in minute
quantities (below its cmc value). Similarly, a shift in the shoulder
peak from 295 nm to 280 nm along with slight increase in the
absorption intensity could be noticed for the IOC-SDS micellar sys-
tem. These observations indicate that with prolonged duration of
time, some additional specific interactions might be occurring
between IOC and CTAB/ SDS surfactants. However, no signs of such
interactions could be observed in case of non-ionic surfactant (TX-
100).
3.5. Effect of temperature on micellar systems
The study of effect of temperature on a newly pioneered system
is highly necessary because it gives the information about the
types of interactions occurring amongst the two constituents at
different temperatures. The temperature dependent studies also
help in analyzing the thermal stability of the system and changes
occurring in the interaction mechanism at elevated temperatures.
The substantial interaction forces that could occur between the
small organic molecules and macromolecules are electrostatic
interactions, hydrogen bonding interactions, hydrophobic forces,
and van der Waals’ forces.[40] Ross et. al. have reported the ther-
modynamic rules to estimate the type of interactions between
the small organic molecule and macromolecules.[41] In the pre-
sent work, the effect of temperature on the IOC-surfactant nano-
formulations has been studied using fluorescence spectroscopy.
The temperature has been varied from 5 °C to 80 °C for CTAB and
TX-100 and in the range of 5 °C to 95 °C in case of SDS. The overlay
of temperature effect in the entire above mentioned temperature
range for all the three micellar systems have been shown in
Fig. 4. It can be clearly seen from this figure that the fluorescence
intensity is definitely decreasing with increase in the temperature
for the three surfactant micellar systems. This implies that the
forces of interactions that were responsible for holding the IOC
molecules inside the micelles are definitely been torn down.
Increase in temperature could only weaken the forces of interac-
tions that were responsible for keeping the micellar system intact.
Due to this, the number of IOC molecules bounded with surfactant
molecules could be seen to be decreasing as the IOC molecules
were moving out of the micelles as the micelle gets ruptured at
Fig. 2. Plot of (I - Io)/(It - Io) vs. 1/[surfactant] for the determination of partition
coefficient of IOC in the presence of (a) CTAB, (c) SDS and (e) TX-100 micelles;
respectively.
1
7

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
to calculate the thermodynamic parameters like change in Gibbs
free energy (DG), enthalpy change (DH) and entropy change
(D
S); initially the values of partition coefficient were calculated
at different temperatures. The temperature values chosen for this
experiment were in the range of 20 °C–60 °C at an interval of
5 °C. The overlay of the graph for calculation of partition coefficient
at each temperature in case of all the three surfactants has been
represented in the supplementary information (SI: Figure S6). Fol-
lowing this, the values of change in Gibbs free energy at each tem-
perature were calculated using the equation represented below.
D
G ¼ ꢀRTlnðkpÞ
In this equation, ‘‘R” represents the universal gas constant, ‘‘T” is the
absolute temperature (in Kelvin) and Kp is the value of partition
coefficient at temperature T. The value of change in Gibbs free
energy at 25 °C for CTAB, SDS and TX-100 were found to be around
ꢀ24.035 kJ molꢀ1, ꢀ24 kJ molꢀ1 and ꢀ21.33 kJ molꢀ1; respectively.
Based on these values, it could be easily stated that the process of
micelle formation in all the three cases is highly spontaneous in
nature. The values of other two mentioned thermodynamic param-
eters could be calculated using the van’t Hoff equation described
below.
ꢀ
ꢁ
D
R
H
1
T
DS
ln Kp ¼ ꢀ
þ
R
In the above-mentioned equation, the terms have their usual mean-
ings as discussed previously. Using this equation, a graph could be
plotted between ln Kp on the y-axis versus 1/T on the x-axis as
shown in Fig. 4b, 4d and 4f for CTAB, SDS and TX-100; respectively.
The values of
DH and DS could be obtained from slope and intercept
of this straight-line graph; respectively. The value of change in
enthalpy (DH) for CTAB, SDS and TX-100 were found to be around
ꢀ14.973 kJ molꢀ1, ꢀ0.65 kJ molꢀ1 and ꢀ10.5 kJ molꢀ1; respectively,
whereas the values for change in entropy (DS) could be calculated
to be around 30.57 J Kꢀ1 molꢀ1, 78.024 J K-1molꢀ1 and 65.51 J K-1
-
molꢀ1 for CTAB, SDS and TX-100; respectively. All the corresponding
thermodynamic parameters at their respective temperatures are
tabulated in Table 2. Based on these calculated values, the following
findings could easily be postulated.
ꢃ Such high negative values of change in Gibbs free energy sug-
gests that the process of micellization is highly spontaneous
in nature.
ꢃ As indicated by the negative value of change in enthalpy, this
process is exothermic in nature.
ꢃ Such high positive values of change in entropy could reveal that
the process of micellization for all the three surfactants is both
entropy-driven as well as enthalpy driven.
3.6. Fluorescence quantum yield and time-resolved fluorescence
studies
Fig. 3. The overlay of the absorption spectra of IOC recorded at regular intervals of
time, in the presence of (a) CTAB, (b) SDS and (c) TX-100; respectively. The inset in
each figure represents the percentage change in the absorbance value (at
absorption maxima) versus time.
The fluorescence quantum yield for IOC has been calculated in
the presence of all the three surfactants at varying concentrations
and the values have been arranged in Table 3. This table clearly sig-
nifies that the value of fluorescence quantum yield (/x) increases
with increase in the concentration of CTAB and TX-100 but the
concentration of SDS has almost negligible effect. Further, it could
be seen that the values of fluorescence quantum yield are relatively
higher in case of non-ionic surfactant i.e., TX-100 with almost 60-
fold increment as compared to that obtained in the pure aqueous
medium. These observations are in agreement with our previous
solvent dependent studies where it has been inferred that the flu-
orescence quantum yield of IOC is maximum in case of non-polar
and polar aprotic solvents.[29] In addition, with the intentions of
accumulating knowledge about the impact of different kinds of
higher temperatures and this could account for the decrease in the
emission intensity. The inset of Fig. 4 which is a plot between emis-
sion intensity versus temperature, provides a clear picture of the
effect of temperature on the IOC molecules encapsulated in the
surfactant micelles.
Moreover, to get an insight about the nature of interactions
between the IOC molecules and various surfactant molecules and
also about the feasibility of formation of the micellar systems,
the calculations for thermodynamic parameter is essential. In order
8

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
Fig. 4. The overlay of the temperature-dependent emission spectra of IOC in the presence of (a) CTAB, (c) SDS and (d) TX-100; respectively. The inset in each figure represents
the variation of emission intensity with the increasing temperature. The van’t Hoff plot of IOC at different temperatures in the presence of (b) CTAB, (d) SDS and (f) TX-100
micelles; respectively.
surfactant micellar systems on the excited state photophysics of
IOC, picosecond resolved fluorescence decay transients of IOC have
been recorded at increasing concentration of CTAB, SDS and TX-
100. The overlay of the concentration dependent excited state
decay patterns of IOC encapsulated micellar systems have been
illustrated in Fig. 5. The decay patterns have been fitted using
tri-exponential function and the values obtained for the three
decay components along with their relative contributions have
been tabulated in Table 3. It could be noted that the value of first
component is in the range of 60 ps to100 ps, the second component
ranges from 500 ps to 750 ps whereas the third component varies
from 1.4 ns to 2.5 ns. The average lifetime of IOC in aqueous solu-
tion has been found to be around 100 ps. However, when the con-
centration of CTAB was increased from 0.5 mM to 50 mM, initially
the average lifetime of IOC did not change much but got elevated to
163 ps when the concentration of CTAB was 50 mM. In a similar
manner, when the concentration of SDS and TX-100 were varied
in the range of 5 mM–50 mM and 0.1 mM–50 mM; respectively,
it was found that while the SDS micelles could only escalate the
average lifetime up to 115 ps, the TX-100 micelles could enhance
the savg. to around 160 ps even at the lowest concentration of
0.1 mM. Moreover, the contribution of second and third compo-
nent was found to be comparatively higher in the case of TX-100
than in the cases of CTAB or SDS. Thus, it could be inferred that
the aliphatic anionic surfactant (SDS) is less efficient in elevating
the average lifetime of IOC than the cationic (CTAB) and non-
ionic (TX-100) surfactants. Furthermore, from the values obtained
for the fluorescence quantum yield and average lifetime, the values
9

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
Table 2
Change in partition coefficient and Gibbs free energy of IOC in the presence of different surfactants at varying temperature. (Error within 5%).
Surfactant
Temp. (K)
CTABa
SDSb
TX-100c
Kp
D
G (kJ/mol)
Kp
D
G (kJ/mol)
Kp
DG (kJ/mol)
(ꢁ104)
(ꢁ104)
(ꢁ104)
293.15
298.15
303.15
308.15
313.15
318.15
323.15
328.15
333.15
1.93
1.63
1.46
1.31
1.26
1.16
1.06
0.96
0.87
ꢀ24.05
ꢀ24.04
ꢀ24.17
ꢀ24.30
ꢀ24.58
ꢀ24.75
ꢀ24.89
ꢀ25.02
ꢀ25.14
1.554
1.548
1.541
1.537
1.531
1.525
1.518
1.511
1.505
–23.52
–23.91
ꢀ24.30
ꢀ24.70
ꢀ25.09
ꢀ25.48
ꢀ25.87
ꢀ26.25
ꢀ26.64
24.76
21.33
19.63
18.23
16.94
15.96
15.55
14.61
14.03
ꢀ30.27
ꢀ30.42
ꢀ30.72
ꢀ31.03
ꢀ31.35
ꢀ31.69
–32.12
–32.45
–32.83
a
b
c
In the above-mentioned temperature range for CTAB surfactant;
In the above-mentioned temperature range for SDS surfactant;
In the above-mentioned temperature range for TX-100 surfactant;
D
H = -14.97 kJ/mol and
D
S = 30.57 JK-1molꢀ1
.
D
H = -0.65 kJ/mol and
H = -10.9 kJ/mol and
D .
S = 78.02 JK-1molꢀ1
D
D .
S = 65.51 JK-1molꢀ1
of decay rate constants for the radiative and non-radiative pro-
cesses could be calculated using Eq. (3) and the values have been
tabulated in Table 3. It is evident that an increase in the concentra-
tion of any of the surfactant does not have much effect on the value
of kr but the values of knr definitely get decreased. Also, the non-
radiative process predominates in the process of deactivation of
the excited state of IOC than the radiative ones.
state photophysics of the IOC molecules encapsulated inside the
surfactant micelles. But in order to check its effect on the excited
state properties on the IOC molecule, the steady-state emission
spectra was recorded in the wavelength range of 355 nm to
800 nm. The overlay of the emission spectra depicting the effect
of NaCl on the IOC-surfactant nano-micellar system has been rep-
resented in Fig. 6. It could be easily noticed from this figure that the
fluorescence intensity monotonically increased on increasing the
concentration of salt in all the three micellar systems. This increase
in the emission intensity could be ascribed to the salting-in effect.
Further, in order to get a clear picture of the effect of NaCl on the
excited state photophysical properties of IOC-surfactant micellar
system, the time-dependent excited state lifetime measurement
experiments were performed. In the case of CTAB and TX-100,
the concentration of NaCl was maintained at 1 M, whereas in the
case of SDS it was fixed at 0.1 M (due to insolubility at higher con-
centration of NaCl). The overlay of the decay transients in the pres-
ence and absence of salt have been represented in Fig. 6b, 6d and 6f
for CTAB, SDS and TX-100; respectively. The values of the fitting
parameters for all the decay parameters have been arranged in
Table 3. It could be observed that while the addition of 1 M NaCl
to 5 mM CTAB solution increased the average lifetime of IOC from
96 ps to 138 ps and the addition of 0.1 M NaCl to 20 mM SDS solu-
tion had very minimal effect on the average lifetime (108 ps to
127 ps). Similarly, the addition of salt has negligible effect on the
savg. value of IOC encapsulated inside TX-100 micelles. Thus, it
could be concluded that salt has no effect on the excited state pho-
tophysics of IOC in presence of non-ionic surfactants (TX-100). In
contrast, the introduction of salt in the presence of cationic surfac-
3.7. Effect of salt concentration
In order to study the effect of salt on the photophysical proper-
ties of IOC-surfactant nano-micellar system, the concentration of
IOC was fixed at 10
lM and the concentrations of CTAB, SDS and
TX-100 were maintained at 5 mM, 20 mM and 1.5 mM; respec-
tively. Meanwhile, the concentration of salt; i.e., sodium chloride,
was varied up to 2 M. A total of six solutions were prepared in
which the concentration of NaCl was altered as 0 mM, 0.05 M,
0.1 M, 0.5 M, 1 M and 2 M; respectively. While preparing the sam-
ples, something unusual was sighted happening in case of SDS. In
the IOC-SDS micellar system, at higher concentrations of NaCl
(greater than 0.5 M), thick white precipitates could be observed.
Kratsey et al. have reported that NaCl and SDS are immiscible in
the micellar state and they have called it as the salting out effect
of SDS in the presence of NaCl.[42] It has been observed that the
concentration of NaCl has an insignificant and unnoticeable change
in the absorption spectra for all the three nano-micellar systems in
terms of both nature of the spectra and the absorbance value.
Based on these observations, it could easily be said that in the stud-
ied concentration range, salt (NaCl) has no effect on the ground
Table 3
Picosecond resolved decay parameters, fluorescence quantum yield, and radiative and non-radiative decay rate constants of IOC in the presence of aqueous solution with
increasing concentrations of surfactants. (Experimental error 6%).
Surfactants
Concentration
s1 (ps) (%a1)
s 2 (ps) (%a2)
s 3 (ps) (%a3)
s avg. (ps)
v2
/
kr (sꢀ1) (ꢁ108)
knr (sꢀ1) (ꢁ109)
x
Water
CTAB
–
64.01 (96.5)
68.9 (96.91)
74.7 (97.38)
76.1 (97.82)
90.7 (95.12)
90.1 (95.12)
75.8 (96.84)
76.9 (96.7)
78 (96.55)
81.1 (96.62)
83 (95.35)
94.6 (91.25)
95.4 (92.5)
96 (92.66)
690.2 (2.46)
535.8 (2.01)
539.9 (1.57)
629.7 (1.08)
723.4 (2.44)
750.6 (3.66)
578.1 (2.11)
584.2 (2.20)
592.2 (2.33)
612.1 (2.25)
661.8 (3.49)
447.6 (6.25)
463.5 (5.01)
480.3 (5.15)
475.7 (5.06)
503.7 (6.71)
2123.1 (0.98)
1455.7 (1.04)
1405.9 (1.04)
1517.5 (1.09)
2424.7 (2.44)
2028.3 (1.22)
1821.9 (1.05)
1878.4 (1.10)
1995.8 (1.11)
1998.3 (1.12)
2101.2 (1.16)
1824.1 (2.5)
1912.1 (2.5)
2124.1(2.19)
1857.7 (2.66)
1929.1 (2.01)
99.72
92.82
95.94
1.095
1.045
1.065
1.026
1.039
1.067
1.001
1.023
0.998
1.025
1.007
1.005
1.083
1.081
1.068
1.092
0.0066
0.0207
0.0216
0.0229
0.0241
0.0355
0.0173
0.0172
0.0172
0.0172
0.0172
0.0380
0.0392
0.0396
0.0401
0.0462
0.66
2.42
2.25
2.34
1.48
2.57
1.65
1.60
1.54
1.50
1.53
2.37
2.46
2.47
2.47
2.86
9.96
10.55
10.20
9.99
5.98
6.99
9.38
9.11
8.83
8.58
8.81
6.02
6.03
5.99
5.93
5.91
0.5 mM
5 mM
10 mM
50 mM
5 mM + 1 M NaCl
5 mM
20 mM
35 mM
50 mM
20 mM + 0.1 M NaCl
0.1 mM
1.5 mM
10 mM
50 mM
97.78
163.09
137.90
104.73
107.87
111.27
114.52
126.67
159.87
159.21
160.16
161.89
161.40
SDS
TX-100
95.9 (92.28)
97.2 (91.27)
1.5 mM + 1 M NaCl
10

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
radiative (kr) decay rate constants were also determined and the
values have been tabulated in Table 3. It could be analyzed that
when CTAB and TX-100 were used as surfactants, the value of kr
increased and that of knr decreased whereas in case of SDS, there
was no significant alterations in the values of both the rate con-
stants. It has been already been established in literature that salts
are good candidate for the hydrogen bond making as well as break-
ing substances.[19] Also, the major contribution in the non-
radiative processes is from the H-bonding between the IOC and
the surfactant molecules. After the addition of salt, there is a pos-
sibility of formation of new H-bond network which perturbs the
already existing H-bonding. Hence, as a result, the value of non-
radiative decay rate constant decreases.
In order to understand the stability of the three nano-micellar
formulations in the presence of salt, the time-dependent absorp-
tion spectra of the three above-mentioned samples have been
recorded up to 3 days. The overlay of the degradation profiles
has been represented in the Fig. 7. In the case of CTAB surfactant,
no variation in the intensity of the main peak could be observed,
indicating the magnificent stability in the presence of cationic sur-
factant. In addition to this, the peak at around 400 nm (which
could be noticed in the absence of salt) was completely absent fol-
lowing the addition of NaCl. Moreover, in the case of IOC-SDS-NaCl
micellar system, the absorbance value of the main peak remained
unaltered, whereas the absorption intensity of the peaks in the
range of 270 nm ꢀ 285 nm kept getting decreased. The peaks in
the later mentioned range is due to the interaction of the micellar
system with NaCl only and the decrement might be due to the
salting-out effect of SDS in the presence of NaCl.[42] Besides this,
as represented in the Fig. 7e, the addition of NaCl to the IOC-TX-
100 surfactant micellar solution seemed further enhanced the
aqueous stability of the nano-formulation. Thus, it could be
inferred that the addition of NaCl (even at higher concentration)
had absolutely no effect on the stability of the nano-micellar sys-
tems and therefore, these formulations could be further investi-
gated as potential drug delivery vehicles.
The binding interactions in the IOC-surfactant micellar system
in the presence of NaCl could be further explored by studying
the effect of temperature on the micelles formed with the samples
at the aforementioned concentrations. The overlay of emission
spectra of IOC encapsulated surfactant micelles in the presence
of 1 M/ 0.1 M NaCl at different temperatures is shown in Fig. 7b,
7d and 7f, for CTAB, SDS and TX-100; respectively with the insets
describing the plot between the fluorescence intensity versus tem-
perature. Finally, in the case of all the three surfactants in the pres-
ence of NaCl, the only change in the temperature-dependent
emission spectra that could be observed was the decrease in the
fluorescence intensity on increasing the temperature. Hence, it
could be stated that salt had almost negligible effect on the ther-
mal stability of the IOC encapsulated surfactant micelles.
4. Conclusion
Fig. 5. The overlay of the picosecond resolved decay patterns of IOC in the presence
of increasing concentrations of (a) CTAB, (b) SDS and (c) TX-100 micelles;
respectively.
In the present work, the aqueous solubility of the potential
hydrophobic drug molecule termed as Isoxazole derivative of cur-
cumin (IOC) has been successfully enhanced by using surfactants
of various surface charges. The surfactants chosen for this purpose
are CTAB, SDS and TX-100 picking up one from each category of
cationic, anionic and non-ionic surfactants. The steady state spec-
troscopic techniques have revealed that the mechanism of interac-
tion is entirely different for different kinds of surfactants. The
absorption and emission maxima as well as the intensities are
completely dependent on the microenvironment provided by var-
ious surfactants. The maximum partition of the IOC molecules from
the bulk water phase to palisade layer of the micelles could be
tants could slow down the process of decay of the excited state of
the IOC molecules.
In addition, the fluorescence quantum yield of all the three
micellar system were calculated in the presence of salt and the val-
ues have been arranged in Table 3. It could be seen that although
the addition of salt increased the fluorescence quantum yield of
IOC in case of CTAB and TX-100, it had almost negligible effect in
the case of SDS. Based on this data, the radiative (kr) and non-
11

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
Fig. 6. The effect of increasing salt (NaCl) concentration on the emission spectra of IOC in the presence of (a) CTAB, (c) SDS and (e) TX-100 micelles; respectively. The effect of
addition of salt (NaCl) on the picosecond resolved decay patterns of IOC in the presence of (b) CTAB, (d) SDS and (f) TX-100 micelles; respectively.
found in case of non-ionic surfactant i.e., TX-100. The low degrada-
tion rate constants and high negative values of change in Gibbs free
energy for all the three surfactants certainly reveals that the pro-
cess of micellization for all the surfactants is highly spontaneous
in nature and once formed they are extremely stable. The effect
of salt concentration has also been examined on the photophysical
properties of IOC inside all the three nano-micellar systems. The
time-dependent studies have revealed that the addition of salt
increases the fluorescence quantum yield as well as average life-
time of the IOC molecules encapsulated inside surfactant micelles.
These outcomes have helped us concluding that the surfactants are
very well efficient in enhancing the aqueous solubility of IOC and
also form stable formulations (in terms of time-dependent and
temperature-dependent stability in the presence as well as
absence of salt). Thus, this work opens a wide area of research
where the design and development of new drug formulations
and delivery vehicles could be done using different kinds of surfac-
tants, polymers, liposomes and many more.
CRediT authorship contribution statement
Manisha Sharma: Writing - original draft, Formal analysis,
Methodology, Conceptualization, Investigation, Writing - review
& editing. Swati Rani: Validation, Formal analysis. Subho Mozum-
dar: Conceptualization, Resources, Validation, Writing - review &
editing, Supervision.
12

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
Fig. 7. The overlay of the absorption spectra of IOC recorded at regular intervals of time, in the presence of (a) CTAB, (c) SDS and (e) TX-100 micelles; respectively with the
addition of NaCl. The overlay of the temperature-dependent emission spectra of IOC in the presence of (b) CTAB, (d) SDS and (f) TX-100 micelles; respectively with the
addition of NaCl. The inset in each figure represents the variation in the emission intensity of IOC encapsulated surfactant micelles in the presence of NaCl with the increasing
temperature.
Chemistry and University Science Instrumentation Center (U.S.I.
C.), University of Delhi, New Delhi for providing the sophisti-
cated analytical instruments. The authors also acknowledge the
Advanced Instrumentation Research Facility (A.I.R.F.) at Jawahar-
lal Nehru University, Delhi and Sophisticated Analytical Instru-
mentation Facility (SAIF) at All Indian Institute of Medical
Sciences (AIIMS), Delhi for providing the instrumentation facility.
Thanks are due to DST-SERB EMR/2016/006678 for funding this
work.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgement
Manisha Sharma acknowledges the C.S.I.R., India for the
financial support. The authors also thank the Department of
13

M. Sharma, S. Rani and S. Mozumdar
Journal of Molecular Liquids xxx (xxxx) xxx
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Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molliq.2021.116981.
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