Molecular Level Understanding of Sodium Dodecyl Sulfate (SDS

Article Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

pubs.acs.org/JPCB

Molecular Level Understanding of Sodium Dodecyl Sulfate (SDS) Induced Sol−Gel Transition of Pluronic F127 Using Fisetin as a Fluorescent Molecular Probe Jhili Mishra, Jitendriya Swain, and Ashok Kumar Mishra* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India S Supporting Information *

ABSTRACT: The thermoreversible sol−gel transition of pluronic F127 is markedly altered even with addition of submicellar concentration of sodium dodecyl sulfate (SDS) surfactant. Multiple fluorescence parameters like fluorescence intensity, fluorescence anisotropy and fluorescence lifetime of both the prototropic forms (anion (A−*) and phototautomer FT*) of the photoprototropic fluorescent probe fisetin has been efficiently used to understand the molecular level properties like polarity and microviscosity of the PF127−SDS system as a function of temperature. The SDS-induced increase in the interfacial hydrophobicity level is seen to affect the sol− gel phase transition of PF127 (21−18 °C). The ET(30) polarity parameter value of anionic emission of fisetin suggests that there is a considerable decrease in the polarity of the PF127 medium with increase in temperature and with the addition of SDS. The microviscosity progressively increases from ∼5 mPa s (sol state, 10 °C) to ∼22.01 mPa s (gel state 35 °C) in aqueous solution of PF127. The variation in microviscosity with addition of SDS in PF127−SDS mixed system is significant in sol phase whereas in gel phase this variation is significantly less. Temperature dependent fluorescence lifetime of FT* indicates that there is heterogeneity in distribution of fisetin molecules at different domains of PF127. This work also show-cases the sensitivity of fisetin toward change in polarity and change in sol−gel transition temperature of copolymer PF127 with variation in temperature (both forward and reverse directions) and SDS.

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(ethylene oxide) (PEO)) groups.1 The PPO segments assemble together to form the core region whereas the PEO segments form the corona region of the polymeric micelle of PF127. Different regions of the PF127 micelle are represented in Figure 3. Because of the nontoxicity, biocompatibility and unique polymorphism character of PF127, it is used as a drug carrier as well as a component in skin cosmetics.2 Recently PF127 has drawn special attention toward dermal and trans dermal drug delivery system for poorly water-soluble drug and it is also helpful to delay the lipid digestion process.2−4 It is well-known that PF127 is both concentration and temperature sensitive toward the structural and microenvironmental changes of its organization.5,6 It is a lower critical solution temperature (LCST) type gel that means at lower temperature it is in a micellar less-viscous sol phase and at higher temperature the polymer chains aggregate together to form a gel network with a nonpolar and compact viscous environment.7,8 Molecular level organization of PF127 and its micropolarity has been studied recently by Swain et al.6 and Sarkar et al.9 using an ESPT molecule 1-naphthol and coumarin-cholesterol

INTRODUCTION

PF127 (Figure 1B) is an amphiphilic copolymer having both hydrophobic (polypropylene (PPO)) and hydrophilic (poly-

Received: October 13, 2017 Revised: December 3, 2017 Published: December 6, 2017

Figure 1. Molecular structure of SDS (A) and PF127 (B). © XXXX American Chemical Society

A

DOI: 10.1021/acs.jpcb.7b10170 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 2. Molecular structure of fisetin and its different prototropic forms of emission.

temperature of aqueous PF127 as a function of temperature and in the presence of additive like SDS. Pluoronic-surfactant interactions have also been a topic of interest. Bakshi et al.17 have reported that the mixed micelle of PF127 with both monomeric and dimeric alkylammonium bromides are formed due to synergistic interaction. Hecht et al.18 have reported that depending upon the concentration of SDS (Figure 1A) in PF127−SDS mixture, different types of aggregates are formed such as (i) aggregates with one poloxamer molecule and several surfactants, (ii) aggregates rich in poloxamer, and (iii) pure surfactant micelle etc. Formations of different types of aggregates indicate that there is a peel off mechanism behind the disintegration of copolymer micelle. For a fixed concentration of PF127, with increase in SDS concentration the phase transition temperature decreases this is because of the destruction of the polymeric micelle by adsorption of surfactant.19 Hecht et al.20 have reported that the binding of SDS to the hydrophobic PO block is saturated with surfactant which makes it hydrophilic so the micellization process of copolymer is suppressed. Chaibundit et al.21 have reported the effect of SDS on the hard gel boundaries of the copolymer PF127. Nambam et al. has reported that SDS destabilized the micellization process of pluronic.22 As the above literatures suggest the effect of SDS in altering the sol− gel transition property of PF127 is rather remarkable. The use of fluorescent probe in getting molecular level understanding of such interaction is known however no literature available for the sensitivity of different prototropic emission of fluorescent probe and estimation of polarity and microviscosity of the PF127−SDS mixture with varying concentration of SDS from sub pre micellar to pre micellar to post micellar. Therapeutically beneficial role of flavonoid was first recognized by Rusznyak and Szent Gyorgyi23 in 1936. Because of the presence of intrinsic fluorescence, medium dependent excited state intramolecular proton transfer (ESIPT) process, low toxicity, and higher potency nature, flavonoids draw special attention toward photophysical study and also pharmaceutical uses.4,23−26 Generally flavonoids are present in foods like tea, apple, onion, persimmons, grapes, kiwis, strawberries, and

Figure 3. Schematic representation of different regions of pluronicF127.

conjugate. Swain et al. have quantified the micropolarity and microviscosity of the aqueous PF127 solution using nile red as a fluorescent molecular probe.10 Roy et al. have studied the efficiency of fluorescence resonance energy transfer between coumarin 153 and rhodamine 6G in pluronic f127 medium.11 Sarkar has monitored the thermo-reversible dehydration of pluronic microenvironment using 4-chloro-1-naphthol as a fluorescent molecular probe12 Khateb et al. have reported the ocular drug delivery application of pluronic F127 along with pluronic F68.13 Devi et al. have developed the drug delivery and gene therapy utility of different poloxamers.14 Gu et al. have studied the pluronic F127/chitosan blend microspheres for mucoadhesive drug delivery.15 Recently Meng et al. have prepared a polymeric mixed micelle comprised of pluronic F127 and D-α-tocopheryl poly(ethylene glycol) succinate to improve the delivery of fluorescent dyes and protein across the blood brain barrier.16 Several studies have done to explore the biological utility of pluronic F127 and also to understand the molecular organization, sol−gel phase transition of pluronic F127 but there is no literature indicates the sensitivity of fisetin toward microenvironmental changes of PF127. In this work, we have tried to explore the sensitivity of fisetin for the quantification of polarity, microviscosity and sol−gel transition B


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Figure 4. (A) Emission spectra of fisetin in aqueous PF127 with variation of temperature (8−35 °C) (B) Plot for emission maximum of anionic and tautomeric emission of fisetin in PF127 with variation in temperature. (C) Fluorescence intensity ratio of tautomeric and anionic emission (535 nm/ 473 nm) of fisetin in PF127 with variation in temperature. (D) ET(30) plot for anionic emission maximum with variation in temperature. [fisetin] = 5 μM, [PF127] = 10% (w/v). λex = 370 nm, slit width = 5 nm (error = ±2%).

cucumbers and beverages like red wine.27Among flavonoids, fisetin (Figure 2)28 is an important candidate for photophysical study because of its environment dependent prototropism.26 Several studies have been done to understand the photophysics of fisetin in different organized media: Sengupta et al.27,28 have studied the binding and photophysical study of fisetin in DNA and beta-CD medium, Selvam et al.29 have shown the multiple prototropism of fisetin in alkaline bile salt media, Mohapatra et al.26 have studied the photophysics of fisetin in lipid bilayer membrane. All these studies suggest that fisetin is multiemissive in nature, depending on its surrounding environment. Fisetin fluorescence is remarkably sensitive toward the phase transition temperature of DMPC lipid bilayer membrane and also in mixed lipid system.26 Some special properties are seen like (i) the large difference in anionic (470 nm) and tautomeric (535 nm) fluorescence band maxima, (ii) the intense emission of the phototautomer form in the green region, (iii) a reasonably high extinction coefficient (2.6 × 104 M−1 cm−1) at absorption maximum (370 nm), (iv) the sensitivity of anionic emission toward the local polarity, and (v) the high sensitivity of fluorescence anisotropy to the local viscosity changes. In this work, a molecular level understanding of the SDS induced changes in PF127 in its sol and gel state is studied using multiple fluorescence parameters of fisetin.

and spectroscopic grade solvents were used for the sample preparation for all experiments. Fluorescence intensity and fluorescence anisotropy measurements were performed using a Fluoromax-4 fluorescence spectrophotometer. A series of temperature range was maintained by circulating water through a jacketed cuvette holder from a refrigerated bath (Julabo, Germany). The fluorescence lifetime measurement was carried out using a Horiba Jobin Yvon TCSPC lifetime instrument. A 370 nm nano-LED was used as the light source for the experiment. The pulse repetition rate was set to 1 MHz, and the pulse width was ∼1.1 ns for 370 nm LED. The detector response time is less than 1 ns. Instrument response function was collected by using Ludox AS40 colloidal silica which acts as a scatterer. The decay data were analyzed using IBH software. A value of χ2, in between 0.99 and 1.22 with a symmetrical distribution of the residual was considered as a good fit. The average fluorescence lifetime (τavg) values were calculated by the following equation, eq 1a30 ⎛ n ⎞ ⎛ n ⎞ τavg = ⎜⎜∑ αiτi⎟⎟ /⎜⎜∑ αi⎟⎟ ⎝ i=1 ⎠ ⎝ i=1 ⎠

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(1a)

Here τi is the individual lifetime with corresponding amplitude αi. Lifetime distribution fitting was also done by using IBH software having a χ2 value in between 0.99 and 1.3, by using the eq 1b, which is extracted from the DAS 6 manual.

MATERIALS AND METHODS Pluronic F127 and fisetin were purchased from Sigma Chemical Co. (Bangalore, India). Sodium dodecyl sulfate (SDS) was purchased from SD Fine Chemicals Ltd. Triple distilled water C


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The Journal of Physical Chemistry B 5

F (t ) = A +

∑ Bk [1 − (1 − qk)t /τk]1/1 − q

polarity Study. Figure 4A shows the emission behavior of fisetin (λex = 370 nm) in aqueous solution of PF127(10% w/v) with variation in temperature (8−35 °C).The presence of an iso-emissive point at ∼506 nm indicates that two prototropic emissions of fisetin are in equilibrium with each other. In the aqueous solution of PF127 the ESIPT process of fisetin results in intense tautomer (FT* = 535 nm) emission which is comparable to the emission of fisetin from nonpolar solvent28 and less intense anion emission (excited state anion A−* = 473−443 nm).28 The most noticeable feature in the emission spectra is the enhanced intensity of the FT* emission at 535 nm and the blue shift in emission maximum of the A−* from 473 to 443 nm with increase in temperature (8−35 °C) (Figure 4A). Figure 4B shows that, the variation in emission maximum of FT* (535 nm) and A−* (473 nm) with temperature. The fluorescence intensity of FT* increases with increase in temperature reaches a maximum at 21 °C and after that, it decreases slightly. Similarly, A−* intensity decreases with a significant drop at 21 °C (Figure 4B. A clear understanding of this slight decrease in fluorescence intensity at gel phase requires close studies of fluorescence anisotropy and fluorescence lifetime parameters, which will be discussed subsequently. This variation in emission intensity of two prototropic forms indicate that fisetin acts as a sensor toward sol−gel transition of PF127 as this transition temperature (20− 22 °C) also matches with the literatures report.6,9 Figure 4C represents the fluorescence intensity ratio plot for two prototropic forms of fisetin FT*/A−* with the increase in temperature. The intensity ratio plot increases with increase in temperature having a maximum at 21 °C and after that, it almost remains constant. This increase value indicates that, the surrounding environment of FT* is aprotic in nature where the ESIPT process is significant as proposed by Banerjee et al.27 The hydrophobicity of the microheterogeneous organization of aqueous PF127 increases with increase in temperature as reported by Swain et al.6 From these two reports, it can be concluded that the prototropic emissions of fisetin are efficiently sensing the hydrophobic environment of PF127.The process of dehydration is continuing up to phase transition temperature and after sol−gel transition it is approaching toward uniformity as expected from fisetin spectral feature. The polarity of the aqueous solution of PF127 with increase in temperature was estimated by using A−* emission maximum (frequency cm−1) and polarity ET(30) parameter value (Figure 4D). A calibration plot was done by recording the emission spectra of fisetin in different solvents (Figure S1).The comparison of the emission maximum of fisetin anionic species in PF127 medium with variation in temperature with the anionic emission maximum in different solvents concludes that A−* emission experience methanol−ethanol (ET(30) = 55.4, 51.9) like polarity at sol phase whereas acetonitrile (ET(30) = 46.7) like polarity at gel phase of PF127. This is supported by the literature report also.32 It is well-known that, in the micellar phase of PF127 the corona region is sufficiently hydrated, whereas in the gel phase the decrease in aqueous solubility of the PEO block (corona region) leads to the compact hydrophobic surface.33 The blue shift of A−* fluorescence emission is maximum in the gel state as compared to sol state, which is a result of the increase in hydrophobicity of the microheterogenious organization of PF127 in water. The increase in fluorescence intensity of

k

k=1

(1b)

Here τk = the mean value of the lifetime distribution and q = parameter of heterogeneity. The simplified form is q=1+

⟨(γ − ⟨γ ⟩2 )⟩ 2 =1+ N ⟨γ ⟩2

Here q describes the fluctuation according to the mean value of the decay rate ⟨γ ⟩ =

1 τ

, which is also indicative of the width

of the distribution and the number of decay channels (N). The components of F(t) become exponentials as qk tends toward one 1/(1 − qk ) ⎡ q(k → 1) t⎤ ⎯⎯⎯⎯⎯→ e−1/ τk ⎢1 − (1 − qk ) ⎥ τk ⎦ ⎣

The analysis software places limits on the value of q (1.01 ≤ q ≤ 1.3), with the lower limit representing q → 1 and the upper limit allowing the mean value of F(t) to be well-defined.When q = 1.01, then the lifetime is tending toward a simple exponential term. When q = 1.3, then the distribution of lifetimes is significantly large. Sample Preparation. PF127 solutions were prepared by dissolving 10% (w/v) of the copolymer in 5 μM probe solution. The samples were left in the refrigerator for 1day for the complete dissolution as pluronics are soluble in cold water. Pluronic−SDS mixture solutions were prepared by fixing the PF127 (10% w/v) concentration with varying SDS concentration (0−20 mM) in 5 μM probe solution. Microviscosity Calculation. The microviscosity of the medium can be calculated by using the Perrin equation, eq 2, which includes fluorescence anisotropy and fluorescence lifetime data.31 ro Tτ = 1 + C(r ) rss η (2) Here ro = Limiting anisotropy value of the probe in the absence of any depolarizing process. T = temperature of the medium in kelvin. rss = measured steady state fluorescence anisotropy. τ = lifetime value of the fluorophore. C(r) = k/v (k = Boltzmann constant, v = molecular volume parameter, which relates to the molecular shape and the location of the transition dipoles of the rotating fluorophores). A simplified form of eq 2 can be written as

η=

C(r )Tτrss δr

(3)

where δr = ro − rss Here η represents the microviscosity of the surrounding environment of the fluorophore. The C(r) value is calculated by using Boltzmann constant (k = 1.3806 × 10−23 m2 kg s−2 k−1) and molecular volume of the probe. The value of η was calculated at different temperatures by using the average lifetime value of fisetin tautomer emission in the aqueous solution of PF127.

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RESULTS AND DISCUSSION Thermotropic Microenvironmental Changes of Aqueous Pluronic F127. Fluorescence Intensity and MicroD


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Fluorescence Lifetime Study. FT* emission of fisetin (λem = 535 nm) shows a biexponential decay (Figure 6) with two

FT* can also be understood by similar model proposed by Swain et al.6 Since the number of proton receiving water molecules decrease with increase in temperature from sol state to gel state, the enhancement of FT* intensity directly relates to the absence of proton receiving water in the microenvironments of the fisetin. So all the above explanations reflect that both the prototropic emissions of fisetin are sensitive toward temperature induced changes of PF127. Fluorescence Anisotropy and Microviscosity Study. Microfluidity obtained from fluorescence anisotropy value and microviscosity calculated by using fluorescence anisotropy and fluorescence lifetime value (using eq 2) gives information about the rigidity of the surrounding environments of the probe in an organized media.31 Figure 5 shows the variation in fluorescence

Figure 6. Plot for the decay profile of FT* emission in PF127 (10% w/v) medium with variation of temperature. λex = 370 nm, λem = 535 nm, [fisetin] = 5 μM, and [PF127] = 10% (w/v).

Table 1. Temperature Dependent Fluorescence Lifetime Data of FT* Emission in Aqueous PF127 (10% w/v)a

Figure 5. Plot of fluorescence anisotropy and calculated microviscosity (mPa s) of fisetin in PF127 with variation in temperature (λex = 370 nm, λem = 535 nm): [fisetin] = 5 μM, [PF127] = 10% (w/v) (error = ±2%).

anisotropy and microviscosity values of fisetin in PF127 (10% w/v) medium with temperature. Here we have used the fluorescence anisotropy and fluorescence lifetime value of phototautomic emission band34 to quantify microviscosity of PF127 (10% w/v) with variation in temperature. From a reasonably low value of fluorescence anisotropy rss (0.07) and the corresponding microviscosity η (5 mPa s) at sol state (below 18 °C), these values are keep on increasing with increase in temperature and remains constant at higher value of rss (0.18) and η (22−20 mPa s) above 21 °C in gel phase. There is a good correlation between fluorescence anisotropy and corresponding microviscosity exist as explained by Mohanty et al.35 In the gel state (above 25 °C), the small decrease in the fluorescence anisotropy and microviscosity value may be due to the approaching of cloud point of the PF127 medium where the change in refractive index and phase separation of the network is more resulting in increased light scattering. This may be the possible reason for the decrease in fluorescence intensity of FT* emission in gel phase (Figure 4A) also. The larger value of both fluorescence anisotropy and microviscosity suggest a significant resistance to the motional dynamics of fisetin molecules at the gel surface of PF127. It is remarkable to observe that both polarity (Figure 4D) and microviscosity studies (Figure 5) indicate an inverse correspondence in their variation patterns with temperature. The two variation patterns observed from different fluorescence parameters of fisetin closely correspond to two different states of aggregation of PF127 medium like sol state and gel state.

a

temp (°C)

τs (ns) (αs)

10 12 14 16 18 20 22 24 26 28 30

1.37 (40) 1.40 (42) 1.31 (43) 1.32 (45) 1.20 (46) 1.32 (47) 1.65 (48) 1.71 (50) 1.64 (52) 1.58{55) 1.53 (59)

τ| (ns) (α|) 3.69 3.63 3.55 3.51 3.41 3.37 3.23 3.01 2.89 2.76 2.66

(60) (58) (57) (55) (54) (53) (52) (50) (48) (45) (41)

χ2 1.23 1.22 1.11 1.24 1.08 1.25 1.24 1.23 1.07 1.24 1.09

λex = 370 nm, λem = 535 nm, [fisetin] = 5 μM. Error = ±5%.

different lifetime components (Table 1). The variation in both shorter and longer lifetime values with temperature is represented in Figure 8. The sol−gel phase transition temperature of PF127 is observed from the Figure 8. The noticeable observations in the lifetime study is the variation in both relative amplitude (α) and lifetime of both the components. The relative amplitude of shorter component (αs) increases with a concomitant decrease in the relative amplitude of longer component (αl) with the increase in temperature from sol state to gel state (Figure 7). The presence of two different lifetime components of FT* suggest that there is heterogeneity in the distribution of fisetin molecules in the different domains of PF127. In order to understand the local environmental effect leading to the two decay times, an experiment involving the variation of FT* decay times with varying acetonitrile (polar aprotic)−ethanol (polar protic) composition was made. It is observed that, in acetonitrile the decay is monoexponential with a lifetime value of ∼0.7 ns (Figure S2 and Table S1). Introduction of ethanol as the second solvent component results in the appearance of a second longer lifetime component at ∼3.0 ns, and the relative amplitude of this longer component increases with increasing ethanol fraction (v/v) in acetonitrile−ethanol mixture (Table S1). FT* being zwitter ionic in nature, it appears that a E


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aqueous corona interfacial domain decreases with temperature due to the expected increase of nonradiative rates with temperature. Figure 9 presents the lifetime distribution plot for FT* emission in sol and gel phases of PF127.The raw data

Figure 7. Plot for the relative amplitudes of shorter lifetime component (α1) and longer lifetime component (α2) with variation of temperature, λex = 370 nm, λem = 535 nm, [fisetin] = 5 μM, [PF127] = 10% w/v, and error = ±2%.

Figure 9. Fluorescence lifetime distribution plot for both longer and shorter components of FT* emission in sol and gel phases of PF127. [fisetin] = 5 μM, λex = 370 nm, and λem = 535 nm.

(corresponding main graph and residual graph) of the lifetime distribution fitting of FT* emission in both sol and gel phase is represented in Figure S4. The “full width at half-maximum” (fwhm) value of shorter component almost remains constant whereas the fwhm value of longer component decreases from sol to gel phase (Table 2). The modal lifetime value also Table 2. Fluorescence Lifetime Distribution Data of FT*in Sol and Gel Phases of PF127a Figure 8. Plot for the both shorter and longer lifetime components of phototautomer emission (λex = 370 nm, λem = 535 nm) with temperature in PF127. [PF127] = 10%(w/v), [fisetin] = 5 μM. Error = ±2%.

phases of PF127

fwhms

fwhm1

modal lifetimes

modal lifetime1

χ2

sol phase gel phase

1.02 0.9

1.24 0.074

1.11 1.17

3.18 2.46

1.22 1.09

a [fisetin] = 5 μM. λex = 370 nm, and λem = 535 nm (error = ± 5%). FWHMS = full width at half maximum of shorter component; FWHML = full width at half maximum of longer component. Both FWHM and modal lifetime in nano second (ns).

hydrogen bonding solvent stabilizes the FT* state resulting in the appearance of the long lifetime component (Table S1). The result of temperature dependence experiment of fisetin lifetime in pluronic medium can thus be explained as follows. The longer lifetime component (∼3.6 ns) originates from FT* emission distributed in aqueous-corona domain whereas the shorter component (∼1.5) originates from the core−corona domain of PF127. Banerjee et al.27 had made a similar assignment for assigning both the shorter and longer lifetime components of FT* emission in β-cyclodextrin medium. The changes in relative amplitudes of both the lifetime components of the phototautomer form at above the gelation temperature suggest that larger fraction of fisetin molecules experience the water deficient nonpolar environment of the gel domain. The shorter lifetime values increase with increase in temperature and the increase is marked at the sol−gel transition temperature. Since the gel phase is characterized by a much higher local viscosity, the nonradiative decay rates are expected to be lower in the gel domain which explains the behavior. In contrast, the longer lifetime component originating from the

follows the same trend like the fwhm value (Table 2). As explained, the shorter lifetime component originates from the core−corona domain whereas longer component originates from the interface of corona-aqueous region of PF127. The changes in the distribution profile of the longer lifetime component from wider distribution to narrower distribution indicates that at sol phase the local environmental heterogeneity sensed by fisetin tautomeric species in the interface of corona-aqueous domain is more whereas fisetin senses almost the same nonpolar environment in the gel phase. On the other hand the environmental heterogeneity sensed by fisetin tautomeric species in the core−corona domain remains almost constant from sol to gel phase as observed from Table 2. Thermoreversibility of PF127 Aqueous Solution. Fluorescence Intensity Study. The emission behavior of fisetin in PF127 with variation in temperature is represented in Figure 10A (10−35 °C) and Figure 10B (35−10 °C) respectively. It is F


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Figure 10. Emission behavior of fisetin in PF127 aqueous solution with variation in temperature (A) 10−35 °C (forward direction), (B) 35−10 °C (reverse direction). (C) Response of FT* intensity in PF127 solution both in forward (black line) and reverse (red line) with variation in temperature. (D) Wavenumber (cm−1) plot of the A−* with variation in temperature both in forward (black line) and reverse (red line) direction. Error = ±2%.

observed that (Figure 10C), the tautomeric emission of fisetin is so sensitive toward the sol−gel transition temperature of PF127 both in forward and reverse directions. The change in polarity of the PF127 medium with variation in temperature is reflected in the shifting of anionic emission maximum toward shorter wavelength region in both directions. The change in rigidity of the medium with variation in temperature is reflected in the fluorescence anisotropy of FT* emission (Figure 11). The fluorescence lifetime value and decay profile of fisetin in PF127 also follows the reversibility behavior (Figure S4, Table S2, and Table S3). All the above results indicate that fisetin is an efficient fluorescent molecular probe for proofing the thermoreversibility behavior of PF127. Differential Scanning Calorimetric (DSC) Analysis of Fisetin−SDS Mixture. Differential scanning calorimetry (DSC) is an efficient analytical tool for the analysis of thermotropic phase behavior in organized media like polymer system,37 lipid bilayer membrane38,39 niosomes membrane36,40etc. Figure 12 shows the DSC thermogram plot for PF127 (10% w/v) in water and also in PF127−SDS mixture. Table 3 shows the onset and sol−gel phase transition temperature of PF127 (10% w/v) in presence and absence of surfactant as obtained from DSC analysis. The phase transition temperature and onset temperature of PF127 decreases with increase in SDS concentration from premicellar to postmicellar region. A clearer understanding of the effect of premicellar postmicellar concentration of SDS on PF127 copolymer

Figure 11. Plot of fluorescence anisotropy of fisetin in PF127 with variation in temperature. (λex = 370 nm, λem = 535 nm). [fisetin] = 5 μM, [PF127] = 10% (w/v). Error = ±2%.

requires a detailed analysis of the fluorescence intensity and lifetime data of both the prototropic emissions of fisetin which will be discussed subsequently. Thermotropic Microenvironmental Changes of the PF127−SDS Mixture. Fluorescence Intensity Study. The emission spectra of fisetin in PF127−SDS mixture with variation in both SDS concentration and temperature (8−35 G


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values. This phase transition temperature also matches with the DSC data (Figure 12). Parts A and B of Figure 13 show the emission spectra of fisetin in PF127−SDS mixture ([SDS] = 0−20 mM and [PF127] = 10% w/v) at 8 °C (sol phase) and 35 °C (gel phase). Similarly, parts C (sol phase) and D (gel phase) of Figure 13 represent the emission behavior of fisetin in the same mixture having an SDS concentration of 0−2 mM. At sol state, the enhancement in FT* (535 nm) emission reflects the rate of excited state intramolecular proton transfer (ESIPT) process of fisetin in the PF127 copolymer solution with the addition of SDS. The increase in hydrophobicity at the corona region leads to increase the FT* intensity. There is not much variation in A−* emission. In the gel state (35 °C) both A−* and FT* emission remains same irrespective of the SDS concentration. In the presence of anionic (SDS) surfactant, the FT* emission intensity increases as compared to aqueous PF127. The increase in FT* emission with variation in SDS concentration indicates the increase in hydrophobicity of the medium. Temperature Dependent Emission Behavior of FT* Emission. The emission behavior of the phototautomer form of fisetin in PF127−SDS mixture is represented in Figure 14 with variation in temperature and SDS concentration (from subpremicellar (Figure 14A) to premicellar to postmicellar (Figure 14B). In the PF127−SDS mixture, FT* intensity increases with increase in temperature reaches the maximum value at phase transition temperature and then slightly decreases. In sol phase there is a significant increase in FT* intensity with increase in SDS concentration this is because of the increase in hydrophobicity of the corona domain of PF127 as explained in the intensity study. The enhancement in FT* intensity is more significant in the subpremicellar concentration of SDS (0−2 mM) as compare to premicellar-postmicellar concentration (2−20 mM). This can be explained as, at lower concentrations (≪CMC, i.e., 0−2 mM), SDS locates preferably in the corona domain. With increase in SDS concentration, the process of SDS micellization (>CMC, i.e., 8−20 mM) starts and the effect of SDS appears to saturate. So the presence of SDS molecules in the corona domain increase the hydrophobicity which reflects in the enhancement of FT* emission as it is reported in literature that FT* intensity increases with increase in hydrophobicity of the medium. On the other hand, in post gelation, however the phototautomer emission intensity do not vary much with SDS concentration as almost all fisetin molecules are expected to emit from phototautomer form. The slight decrease in FT* intensity at post gelation temperature is due to increased light scattering where phase separation of the network is more. A temperature dependent study is also carried in pure SDS micelle by varying both SDS concentration (0−20 mM) and temperature using fisetin as a fluorescent molecular probe. There is no such interesting observation like PF127− SDS mixture observed in pure SDS micelle where there is only normal temperature effect is observed on FT* intensity with variation in temperature (Figure S7). Figure 14C represents the plot of the phase transition temperature of PF127 with SDS concentration as obtained from DSC and fluorescence data. The phase transition temperature obtained from DSC matches with the fluorescence data which shows the sensitivity behavior of fisetin toward phase transition of PF127 in aqueous solution and also in the presence of additive like SDS. Estimation of Polarity. It is observed that the anionic emission of fisetin is so sensitive toward the polarity of its surrounding environment both in the PF127 and PF127−SDS

Figure 12. Differential scanning calorimetric plot for PF127 with and without SDS [PF127] = 10% (w/v), [SDS] = 0 mM (black line), 1 mM (red line), 2 mM (green line), 6 mM (blue line), 8 mM (cyan line), 10 mM (pink line), and 15 mM (navy line).

Table 3. DSC data for Sol−Gel Phase Transition Temperature and Onset Temperature of PF127 in Presence and Absence of Surfactant (SDS), [PF127] = 10% (w/v) and [SDS] = 0−15 mM (Premicellar−Postmicellar Concentration) [SDS] (mM)

sol−gel phase transition temperature (°C)

Tonset (°C)

0 1 2 6 8 10 15

20.89 20.03 18.56 18.07 17.94 17.84 17.41

17.15 16.59 15.31 14.82 14.14 14.02 13.96

°C) are represented in Figure S5 (A−I, 0−2 mM) and Figure S6 (A−E, 4−20 mM). The inset of the Figures S5 and S6 shows the point plot for both anionic (470 nm) and tautomeric (535 nm) emissions of fisetin in PF127−SDS mixture with temperature. From the inset plot, it is observed that, with the increase in SDS concentration, the phase transition temperature of PF127 shift toward lower temperature region. This shift in phase transition temperature can be rationalized as follows: in the sol phase in which pluronic exists in micellar form, SDS molecules are expected to be found predominantly in the corona region of the PF127 micelle due to their amphiphilicity. SDS molecule has a long hydrophobic tail of 12 carbon atoms. Therefore, with increase in SDS concentration, the hydrophobicity of the corona domain tends to increase due to the replacement of some water molecules by SDS molecules, which helps the copolymer to reach the gel network at lower temperature in the presence of SDS. This increase in hydrophobicity in the corona region is responsible for the shift of sol−gel transition temperature of PF127 to lower H


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Figure 13. Emission spectra of fisetin in PF127−SDS mixture with varying concentration of SDS at sol(8 °C) and gel(35 °C) phase of PF127. (A and B) [SDS] = 0−20 mM, (C and D) [SDS] = 0−2 mM, [PF127] = 10%(w/v), slit width = 5 nm, λex = 370 nm, and error = ±2%.

compared to 2−20 mM SDS. This significant variation can be explained by using the same model (location of different concentration of SDS molecules in PF127 micelle) which already incorporated in the fluorescence intensity study. This observation indicates that even at sub premicellar concentration of SDS also the rigidity of the medium increases. The anisotropy value of fisetin in pure SDS micelle with variation in temperature at different concentration of SDS is represented in Figure S8. It is observed that irrespective of the SDS concentration the anisotropy value decreases with increase in temperature whereas in PF127−SDS mixture it senses the phase transition temperature of PF127. Fluorescence Lifetime Study. The fluorescence lifetime value and decay profile of fisetin in PF127−SDS mixture with variation in SDS concentration at sol and gel phases are represented in Table 4, Table 5 and Figure S9. The FT* emission shows biexponential decay fitting both in sol (8 °C) and gel state (30 °C) of PF127 (10% w/v) with and without SDS. It is interesting to note that the shorter lifetime value increases with increase in SDS concentration until about the micellization concentration of SDS. After micellization it shows saturating behavior (Figure S10), but on the other hand the variation in longer component is also follows the same trend like the shorter component but not as significant. The viscosity of the medium increases with increase in SDS concentration as observed from fluorescence anisotropy study (Figure 16). Here, the interesting observation is that a 1:1 mapping exists in the variation pattern of both anisotropy and shorter lifetime value

mixtures (Figures 4D and 15). There is a blue shift of anionic maxima (470−459 nm for 0−2 mM and 458−451 nm for 4−20 mM) in the PF127−SDS mixture at sol phase (Figure 13A,C). The polarity of the PF127−SDS mixture with variation in both SDS concentration and temperature was estimated by using the A−* emission maximum (frequency cm−1) and ET(30) polarity parameter value. It is observed that at the lower temperature A−* experiences methanolic like environment and with increase in temperature it feels acetonitrile like environment. At sol phase, with variation in SDS concentration, the polarity of the medium changes from polar to nonpolar this is because of the increase in hydrophobicity of the PF127 medium in the presence of SDS whereas in gel phase irrespective of the SDS concentration the medium is nonpolar in nature. Fluorescence Anisotropy Study. Figure 16 shows the fluorescence anisotropy data of the tautomeric emission of fisetin in PF127−SDS mixture (10% w/v) with variation in SDS concentration and temperature. Anisotropy value gives information about the rigidity of the surrounding environment.9At subpremicellar concentration of SDS (0−2 mM) the anisotropy value increases with increase in SDS concentration at sol phase (8 °C) whereas in gel phase (35 °C) the anisotropy value remains same which indicates that the rigidity of the medium is same irrespective of the concentration of SDS. Figure 16B represents the anisotropy of the FT* emission in PF127−SDS mixture with variation in SDS concentration from premicellar to postmicellar range (2−20 mM). The variation in anisotropy from 0 to 2 mM SDS is more prominent as I


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Figure 14. Response of FT* emission in PF127−SDS mixture with variation in both temperature and SDS concentration. [SDS] = 0−2 mM (A) and [SDS] = 0−20 mM (B). (C) Plot for the phase transition temperature of PF127 obtained from fluorescence data and DSC data with variation in SDS concentration. [PF127] = 10%(w/v), [SDS] = 0−20 mM, and error = ±2%.

Figure 15. Wave number (cm−1) plot of the anionic emission of fisetin with variation in temperature and SDS concentration. (A) [SDS] = 0−2 mM, (B) 2−20 mM, and error = ±2%.

of FT* emission at the sol phase, i.e., both the anisotropy and shorter lifetime value of FT* increase until about the micellization concentration of SDS and after that it almost remains constant whereas in gel phase the variation is not very significant. The variation in longer lifetime value also follows the same trend like shorter lifetime value both at sol and gel phase but the variation is not very significant. This negligible variation of longer lifetime value at both the phases of PF127 in the presence of SDS molecules conclude that the aqueous− corona interfacial domain of PF127 is not as affected by the

SDS molecule as is the corona domain. From the above observation, it can be concluded that at the sol phase, the premicellar concentration of SDS is more effective for the tunability of the microdomain of PF127 as compared to postmicellar concentration. In the sol phase, the relative amplitude of shorter lifetime component (αs) increases whereas the relative amplitude of longer component decreases (with increase in SDS concentration) (Table 4). At the gel phase, the relative amplitude of both shorter and longer components are almost constant. With increase in SDS concentration, a larger J


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Figure 16. Fluorescence anisotropy of FT* emission (535 nm) in PF127−SDS mixture with variation in temperature and SDS concentration. (A) subpremicellar concentration of SDS (0−2 mM), (B) premicellar to post micellar concentration of SDS (0−20 mM), and error = ±2%.

fraction of fisetin molecules experience the water deficient nonpolar environment, which results in the variation in relative amplitude at the sol phase. Microviscosity Study. The microviscosity of PF127−SDS mixture was calculated by using the Perrin equation, which involves two important spectroscopic parameters like fluorescence anisotropy and fluorescence lifetime value.35 From the fluorescence anisotropy and fluorescence lifetime study, it is observed that at the sol phase, the viscosity of the medium increases with an increase in SDS concentration. This increase is more prominent up to premicellar concentration (8 mM) of SDS. Here Figure 17 represents the variation in

Table 4. Fluorescence Life Time Data for the FT* Form of Fisetin in PF127 Medium with SDS at the Sol (8 °C) State of PF127a SDS (mM) 0 0.1 0.3 0.5 0.7 1 1.4 1.8 2 4 6 10 15 20

τs (αs) 1.10 1.05 1.08 1.11 1.27 1.32 1.37 1.43 1.40 1.72 1.93 2.17 2.16 2.20

(23) (24) (25) (28) (32) (32) (33) (33) (33) (38) (42) (56) (57) (58)

τ| (α|) 3.12 3.28 3.30 3.36 3.39 3.30 3.29 3.25 3.22 3.19 3.22 3.44 3.33 3.42

(77) (76) (75) (72) (68) (68) (67) (67) (67) (62) (58) (44) (43) (42)

χ2 1.28 1.15 1.33 1.25 1.27 1.27 1.23 1.10 1.27 1.11 1.13 1.00 1.07 1.06

λex = 370 nm, λem = 535 nm, [fisetin] = 5 μM, [PF127] = 10% (w/v), [SDS] = 0−20 mM, error = ± 5%, and lifetime in nanoseconds. a

Table 5. Fluorescence Lifetime Data for the FT* Form of Fisetin in PF127 Medium with SDS at the Gel (30 °C) State of PF127a SDS(mM) 0 0.1 0.3 0.5 0.7 1 1.4 1.8 2 4 6 10 15 20

τs (αs) 1.13 1.12 1.15 1.07 1.12 1.06 1.09 1.07 1.12 1.13 l.23 1.23 1.15 1.21

(56) (55) (56) (54) (56) (51) (53) (52) (54) (50) (59) (57) (53) (54)

τ| (α|)

χ2

2.20 (44) 2.21 (45) 2.22 (44) 2.19 (46) 2.22 (44) 2.14 (48) 2.16 (47) 2.14 (48) 2.16 (47) 2.05 (50) 2.13 (41) 2.10 (43) 2.04 (47) 2.06 (46)

1.28 1.15 1.33 1.25 1.27 1.27 1.23 1.10 1.27 1.11 1.13 1.33 0.99 1.21

Figure 17. Plot of calculated microviscosity (mPa s) of PF127−SDS mixture with variation in SDS concentration at 8 and 35 °C. (λex = 370 nm, λem = 535 nm). [fisetin] = 5 μM, [PF127] = 10% (w/v), and [SDS] = 0−20 mM. Error = ±2%.

microviscosity of the PF127−SDS mixture at sol and gel phases with variation in SDS concentration. It is observed that, with the increase in SDS concentration, the microviscosity of the environment increases and at higher concentration it remains constant. Whereas in the gel phase the microviscosity of the medium remains almost the same up to premicellar concentration of SDS, at post micellar concentration there is a slight variation in microviscosity value. So it can be concluded that premicellar concentration of SDS is also effective for the increase in microviscosity at sol phase of PF127.

λex = 370 nm, λem = 535 nm, [fisetin] = 5 μM, [PF127] = 10% (w/v), [SDS] = 0−20 mM, error = ± 5%, and lifetime in nanoseconds. a

K


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Notes

CONCLUSIONS This work uses the sensitivity of multiple prototropism of fisetin fluorescence in garnering a molecular level understanding of PF127−SDS interaction in terms of local heterogeneities of micropolarity and microviscosity in the sol and gel phase of PF127. The use of multiple fluorescence parameters like fluorescence intensity, fluorescence anisotropy and fluorescence lifetime value of two prototropic emission of fisetin enables the quantification of micropolarity, microviscosity and sol−gel transition temperature of PF127 medium with variation in temperature and SDS concentration.Some important observations of this study: (i) SDS molecules preferentially locate at the corona domain of the PF127 micellar system, (ii) The presence of SDS molecules shifted the phase transition temperature of PF127 to lower region by increasing the hydrophobicity of the medium. (iii) The premicellar concentration of SDS is most effective for the tunabilitity of the different parameters (polarity, anisotropy microviscosity etc.) in the corona region of PF127. Fluorescence anisotropy and microviscosity of the PF127 medium increases whereas polarity decreases in the presence of SDS as estimated by using the fluorescence properties (emission maximum, anisotropy and lifetime value) of the different prototropic emission of fisetin. (iv) It was interesting to note that the fluorescence lifetimes of FT* emission and emission maximum of anionic emission signify polarity of the different regions of pluronic medium which sensitively reflects the changes in hydrophobicity of PF127 with and without surfactant. (v) Fisetin is also sensitive toward the theroreversible behavior of PF127. Thus, fisetin is an unusual molecular probe that effectively quantifies polarity, microviscosity, and sol−gel transition temperature with variation of temperature and in the presence of additive.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a research project funded by the Department of Science and Technology, Government of India. J.M. thanks IIT Madras and J.S thanks DST for research fellowships.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b10170. Emission behavior of fisetin in different solvents, fluorescence lifetime decay and corresponding lifetime value of fisetin in different solvents, main graph and residual graph for the phototautomer (FT*) emission of fisetin in sol and gel phase of PF127 for the nonextensive distribution fitting, fluorescence lifetime decay and corresponding lifetime values of FT* emission of fisetin in PF127 both in forward (10−35 °C) and reverse (35− 10 °C) directions, emission behavior of fisetin in PF127 with variation in SDS concentration, emission behavior of fisetin in pure SDS micelle, fluorescence anisotropy value of FT* in SDS micelle, fluorescence lifetime decay profile of FT* in PF127 with variation in SDS concentration at sol and gel phases, and variation in shorter and longer lifetime components of FT* emission in PF127 with variation in SDS concentration at sol and gel phases (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(A.K.M.) [email protected]. ORCID

Ashok Kumar Mishra: 0000-0002-0309-3332 L


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