Mechanistic Insights into the Growth of Anisotropic Nanostructures

Article Cite This: J. Phys. Chem. B 2019, 123, 5324−5336

pubs.acs.org/JPCB

Mechanistic Insights into the Growth of Anisotropic Nanostructures Inside Reverse Micelles: A Solvation Perspective Nitin Yadav, Pramit K. Chowdhury, and Ashok K. Ganguli* Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India

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S Supporting Information *

ABSTRACT: Reverse micelles (RMs) as soft templates have been successfully used in tailoring the structural characteristics (size and morphology) of nanomaterials that in turn have been used in various applications. In this work, we have focused on the local perturbations in the different interior domains of the cetyltrimethylammonium bromide-reverse micelle-based soft template en route to nanorod formation by monitoring the solvation response of coumarin-based solvatochromic probes (C343 and C153). We have observed an appreciable retardation of the solvent coordinate during the initial phases of nanorod growth, which we have attributed to the reorientational motion of the water molecules lodged in the interfacial region. Moreover, these rigid nanostructures leave their imprints on the soft interfacial layer as was observed from the direct correlation in the solvation response of RMcontaining nanostructures and respective surfactant aggregates in supernatant solution. Supporting data from time-resolved anisotropy studies further reinforced our conclusions from the solvation experiments. Our study proves that the hydration dynamics can be a promising tool in tracking the heterogeneous growth evolution of nanostructure formation in RMs since solvent reorganization provides insights into the intrinsic, molecular-level features of the micellar assemblies.

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INTRODUCTION The field of nanoscience and nanotechnology has created a revolutionary impact and holds promising applications in the areas of medicine, environmental science, energy harvesting, imaging, and electronic devices.1−4 Anisotropic nanostructures (nanospheres, nanorods, nanowires, nanobelts, nanoplates, and nanohelices) have a wide array of potential applications.5−7 Fashioned with different functionalities, these nanomaterials have been used for targeted drug delivery, tumor targeting, multimodal MRI imaging, bioprobes, sensors, and so forth.8−10 Size and shape of nanostructures are the key characteristics for realizing the exotic properties of nanomaterials.11,12 Therefore, scientists have put in significant efforts on strategies for designing nanostructures with a variety of morphologies, in particular one-dimensional (1D) nanostructures that have wide applications.12−14 Among the well-established methods for fabricating nanostructures, reverse micelles (RMs) have proved to be ideal templates for precisely tailoring their structural characteristics (size/shape).14−16 A large body of scientific literature documenting the advantage of RMs in tuning the size and morphology of a variety of nanostructures with remarkable homogeneity and monodispersity already exists.17−19 This is possible by controlling the structure of RMs through several variables, namely, molar ratio of water to surfactant molecules, W0 (= [H2O]/[Surfactant]), surfactant structure, co-surfactant, solvent, and other factors such as reactant concentration and intermicellar exchange rate.17−21 The structural organization, i.e., surfactant layers with different types of water confined to © 2019 American Chemical Society

the polar core, the underlying molecular heterogeneity, and the rich structural phase diagram that lead to different micellar geometries are the very essence of RMs. These features of RMs facilitate a variety of applications other than nanomaterial synthesis that include drug delivery, use as biological membrane mimic, enzyme stabilization, extraction of biomolecules, and application in cosmetic and food industries.22−27 RMs, considered as reaction vessels for shaping nanomaterials, are thermodynamically stable supramolecular selfassemblies consisting of nanodimensional water droplets (polar liquid) dispersed in continuous oil phase stabilized by surfactants and co-surfactants. In these micellar templates, the chemical reaction occurs in the aqueous core, and the micellar interface, owing to its nonrigid nature (noncovalent intermolecular interactions), plays a decisive role in the evolution of a wide array of nanostructural architectures.17,20,21 Our group has been successful in synthesizing a variety of nanomaterials (metal nanoparticles, metal borates, rare-earth metal hexaboride anisotropic nanostructures, dielectric and magnetic oxides, and ternary and quaternary oxides) by using the CTAB-based quaternary RMs (surfactant/co-surfactant/ oil/water) as a synthetic route.28−36 CTAB being a cationic surfactant has a positively charged head group that aligns in a Received: March 15, 2019 Revised: June 3, 2019 Published: June 3, 2019 5324

DOI: 10.1021/acs.jpcb.9b02459 J. Phys. Chem. B 2019, 123, 5324−5336

Article

The Journal of Physical Chemistry B

consecutive days (synthesis completion period ∼ 8 days) at room temperature. Zinc Oxalate Nanostructures. The synthesis of zinc oxalate nanostructures by the reverse-micellar route gets completed in 15 h, this being much faster than that of the iron oxalate nanostructures. The composition of RM was kept the same as mentioned above. The equivalent volumes of the two reversemicellar solution-containing individual reactants (one with 0.1 M zinc nitrate solution and the other with 0.1 M ammonium oxalate solution) were slowly mixed for starting the reaction. The resultant reaction mixture was continuously agitated for 15 h at room temperature. Instrumentation. Steady-state fluorescence measurements were performed on an Edinburgh FLS900 fluorescence spectrometer. The emission spectra were recorded with fluorescence quartz cuvettes having a 1 cm path length, and all of the samples were excited at 405 nm. The excitation and emission slits were both fixed at 5 nm. Measurements of the excited singlet-state decay for both the molecular probes, C343 and C153, were accomplished using a time-correlated single photon counting (TCSPC) spectrometer (Pico Quant, Fluotime 300). A pulsed diode laser (FWHM ∼ 70 ps) was employed for the excitation of samples at 405 nm. Timeresolved emission decays of probe C343 were collected from 430 to 630 nm and for C153 from 450 to 650 nm at 10 nm intervals through a single monochromator with a 5 nm emission bandpass. For all of the samples, the fluorescence decays were collected at the magic angle of 54.7° to avoid complications arising from probe rotation. The instrument response function (IRF) was measured with the Ludox (scattering) solution in water with the detector set at the excitation wavelength (405 nm). The IRF for the mentioned setup was in the range of ∼200 ps (FWHM). TCSPC data sets were analyzed by least-squares iterative reconvolution of an Ncomponent multiexponential decay function with IRF. Acceptable χ2 and homogeneously distributed residuals were obtained by fitting data with a triexponential decay function. For the iron oxalate system, we have performed all fluorescence measurements on both the reaction mixture and its supernatant. An aliquot of the reaction mixture was taken out after every 24 h for measurements. Every day, a part of the obtained reaction mixture was centrifuged to settle down the evolved nanostructures and the resultant clear and colorless supernatant solution was carefully removed to carry out the fluorescence measurements. The fast reaction kinetics of formation of ZnOx nanostructures (∼15 h reaction completion time) led to the onset of turbidity in reaction mixture in a small period of time. Hence, we have confined our measurements to the supernatant solution only that was obtained from reaction mixture every hour. Anisotropy Measurements. For the rotational anisotropy measurements, r(t), the decays were collected with the aforementioned TCSPC setup at the emission peak. The time-dependent anisotropy function, r(t), was obtained using the following expression:

particular array around the surface of metal oxalate nanostructures because of the negative ζ potential of the latter. This decelerates the growth along the lateral axis and thereby facilitates the formation of elongated nanorods.35,36 Evolution of nanostructures in RMs has mostly been studied using techniques like transmission electron microscopy (TEM), fluorescence correlation spectroscopy (FCS), smallangle X-ray scattering (SAXS), and dynamic light scattering (DLS) that rely more on the size and morphology of system, without revealing much details about the interior domains of these.37,38 In this study, we have used the tool of solvation dynamics to provide insights into the possible mechanistic pathways of nanorod formation in RMs. Only a few studies exist that have used this approach to address the templatebased synthesis of nanoparticles.39−42 In this study, we have performed extensive solvation dynamics experiments along with rotational relaxation studies on the growth of iron oxalate nanorods over 8 consecutive days and zinc oxalate nanorods over a period of 15 h inside the polar core of CTAB/water/nbutanol/isooctane RM system. In addition, we have also investigated the supernatant solution, which was obtained after the extraction of nanostructures from the aqueous core to investigate their correlation with the corresponding RMencapsulating nanostructures. We have used the solvatochromatic probes Coumarin 343 (C343) and Coumarin 153 (C153) to probe the different interior regions of RMs. The solvation data provide insights into not only the changes in the micelle interior during the nanostructure growth but also the manner in which different parts of the micellar assembly respond during the growth process. In addition, our study establishes a reasonable correlation between the perturbed micellar assembly encapsulating the nanostructures and respective bare aggregated structures in supernatant solution. This suggests that nanostructures leave their imprint on the soft templates even after their extraction.

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EXPERIMENTAL SECTION Materials and Methods. Materials: Cetyltrimethylammonium bromide (CTAB, Sigma, 99%), 1-butanol (UV-grade, Spectrochem, >99%), isooctane (UV-grade, Spectrochem, 99%), and HPLC water (Merck) were used as received. Ferric nitrate (Sigma, ≥98%) and ammonium oxalate (Sigma, ≥99.5%) were used as reactants to synthesize iron oxalate nanorods. Zinc nitrate (Sigma, ≥98%) and ammonium oxalate (Sigma, ≥99.5%) were used as reactants to synthesize zinc oxalate nanorods. Coumarin 343 (Sigma) and Coumarin 153 (Sigma) were used as received. Reaction Mixture. Iron Oxalate Nanostructures. For the iron oxalate nanostructures synthesis, RMs with the surfactant as CTAB, co-surfactant as 1-butanol, nonpolar phase as isooctane, and an aqueous solution of Fe3+ (0.1 M) and C2O42− (0.1 M) were prepared. The weight percentages of various constituents in a reverse-micellar solution were as follows: 16.76% of CTAB, 13.9% of n-butanol, 59.29% of isooctane, and 10.05% of aqueous phase. Each reverse-micellar solution, one with 0.1 M ferric nitrate solution and the other with 0.1 M ammonium oxalate solution, was stirred continuously at room temperature until the cloudy solution turned into transparent and colorless solution. The equivalent volumes of the two reverse-micellar solutions containing individual reactants were slowly mixed to initiate the reaction. The obtained reaction mixture was continuously stirred for 8

r=

I − GI⊥ I + 2GI⊥

(1)

where I|| and I⊥ refer to the vertical and horizontal polarized components of the emission intensity, respectively, and G is the correction factor for different sensitivities of the detection system toward the vertically and horizontally polarized light. 5325


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Figure 1. Emission spectra of the solvation probes C343 and C153 in iron oxalate (FeOx) RM system. (A) Emission spectra of C343 in pure RMs (H2O), RMs containing reactants (AmOx and FeNt), and RMs having FeOx nanostructures. (B) Emission spectra of C343 in pure RMs (H2O), RMs containing reactants (AmOx and FeNt), and RMs after extracting the FeOx nanostructures. (C) Emission spectra of C153 in pure RMs (H2O), RMs containing reactants (AmOx and FeNt), and RMs having FeOx nanostructures. (D) Emission spectra of C153 in pure RMs (H2O), RMs containing reactants (AmOx and FeNt), and RMs after extracting the FeOx nanostructures.

The G factor was determined for every individual measurement, and the anisotropy decays were subsequently fitted to obtain the rotational time constant using the following equation: r (t ) =

emission peaks (489−493 nm) of C343 in RMs containing the evolving FeOx nanostructures and in supernatant solution occur in between the emission peaks of C343 inside pure (479 nm) and reactant-containing RMs (FeNt, 497 nm). With C153 as the probe, we obtained a marginal red shift of 3 nm in the emission maximum in FeNt-containing RMs (540 nm) in comparison to that of AmOx-containing RMs (537 nm) and pure RMs (537 nm). In RMs encapsulating the evolving FeOx nanostructures (528−530 nm) and in surfactant aggregates present in supernatant (527−531 nm), the C153 probe molecules exhibited blue-shifted emission maxima with respect to those containing the reactants (FeNt, 540 nm and AmOx, 537 nm) only and that of pure RMs (537 nm). Interestingly, as evident by the different spectral shifts, the two solvation probes, C343 and C153, probe different region of RMs, as would be expected, based on the difference in overall charge, with C343 being anionic and C153 being neutral as well as hydrophobic. Similar experiments were also carried out with the ZnOx system having a much shorter reaction completion time of ∼15 h in comparison to that of the FeOx system that goes on for ∼8 days. The emission maximum of C343 in zinc nitrate (ZnNt)-containing RMs (491 nm) showed a red shift in comparison to that in pure RMs (479 nm) and in AmOxcontaining RMs (481 nm). This signifies that hydrated ZnNt ions increase the polarity in micellar interior similar to that of FeNt hydrated ions although the magnitude of shift observed is more for FeNt. The emission peak of C343 has a similar maximum inside the RMs containing the evolved ZnOx

ij

t yzzz z τ zz k rj {

∑ r0j expjjjjj− j=2

(2)

where correspond to the fractional anisotropies, and τr1 and τr2 present the local and global (with the micellar assembly) motions of fluorescent probe (C343 or C153), respectively. Results. We have monitored the emission maxima of two fluorescent probes, C343 and C153, during the synthesis of iron oxalate (FeOx ∼ 8 days) and zinc oxalate (ZnOx ∼ 15 h) nanostructures and post extraction of the evolved nanostructures inside the CTAB/butanol/water/isooctane quaternary RMs (surfactant/co-surfactant/water/oil). The spectral features of C343 and C153 in pure RMs, in reactant-containing RMs, in micellar assemblies with evolving nanostructures (FeOx ∼ 8 days, ZnOx ∼ 15 h) and in respective supernatant solutions have been shown in Figure 1 (and Supporting Information Figure S1). The emission maximum of C343 was 497 nm in ferric nitrate (FeNt)-containing RMs, while the same in ammonium oxalate (AmOx)-containing RMs and pure RMs was obtained at 481 and 479 nm, respectively, signifying a red shift for the probe in the FeNt-containing reverse micelles. This implies that the polarity-sensitive dye C343 experiences a region of higher micropolarity in the presence of FeNt. The rj0

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Figure 2. Decay of solvent response function, C(t), of C343 (A) in pure RMs, reactants (AmOx and FeNt) containing RMs, and RMs encapsulating FeOx nanostructures, (B) in pure RMs, reactants (AmOx and FeNt) containing RMs, and in surfactant aggregates obtained after extracting evolved FeOx nanostructures.

Figure 3. Decay of solvent response function, C(t), of C153 (A) in pure RMs, reactants (AmOx and FeNt) containing RMs and RMs encapsulating FeOx nanostructures, (B) in pure RMs, reactants (AmOx and FeNt) containing RMs and in surfactant aggregates obtained after extracting evolved FeOx nanostructures.

spectral reconstruction protocol of Maroncelli and Fleming,43 as follows

nanostructures and micellar aggregates in supernatant solution (490−492 nm), although it showed a red shift with respect to pure RMs (479 nm) and AmOx-containing RMs (481 nm). On the contrary, C153 exhibited similar emission maxima in pure RMs (537 nm) and in other perturbed RMs including RMs containing ZnNt (538 nm), RMs encapsulating evolved ZnOx nanostructures (539−540 nm) and in surfactant aggregates in supernatant (536−539 nm). Dynamic Fluorescence Stokes Shift. We have investigated the solvation dynamics of the molecular probes C343 and C153 in RMs during the evolution of the anisotropic nanostructures of iron oxalate (∼8 days), zinc oxalate (∼15 h), as well as the impression left by these nanostructures on the micellar assembly after extracting the evolved nanostructures. The fluorescence decays of C343 at different wavelengths ranging from 430 to 630 nm and those of C153 at wavelengths ranging from 450 to 650 nm (at 10 nm intervals) were recorded. The transients showed remarkable wavelength dependency characterized by a significant growth toward the red side (log-linear representation in Supporting Information Figures S2−S7), indicating that the solvation probes, C343 and C153, are gradually solvated as a function of time. The time-resolved emission spectra (TRES), S(λ, t) have been obtained using the fitted decay parameters, D(λ, t), and the normalized steady-state intensities S0(λ), by following the

S (λ , t ) =

D(λ , t )S0 (λ) ∞

∫0 D(λ , t )

(3)

Each obtained TRES was adequately fit by a log-normal line shape function, defined as ÄÅ É 2Ñ ÅÅ ij ln(1 + 2b(v − vp)/Δ yz ÑÑÑÑ ÅÅ zz ÑÑ g (v) = g0 expÅÅÅ−ln 2jjjj zz ÑÑ ÅÅ b ÅÅÇ k { ÑÑÑÖ (4) where g0, Δ, vp, and b are expressed as peak height, width parameter, peak frequency, and asymmetry parameter, respectively. Some representative normalized TRES have been shown in the Supporting Information (Supporting Information Figures S8−S13). The solvent relaxation dynamics were obtained by fitting the solvent correlation function, C(t), defined as C(t ) =

ν(t ) − ν(∞) ν(0) − ν(∞)

(5)

where ν(t), ν(0), and ν(∞) describe the corresponding peak frequencies at times t, 0, and ∞, respectively, with ∞ referring to the time at which system had arrived at equilibrium. The 5327


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Figure 4. Decays of solvent correlation function, C(t), of C343 (A) and C153 (B) in pure (H2O) RMs, reactants (AmOx and ZnNt) containing reverse micelles, and in micellar aggregates in supernatant solution after extracting ZnOx nanostructures.

decays of solvent correlation function, C(t), are shown in Figures 2−4. The results presented in Table 1 show the decay

characteristics of the solvent correlation function, C(t), of the probe C343 in RMs containing FeOx nanostructures over 8 consecutive days. In pure RMs and AmOx-containing RMs, the average solvation times were similar, with the decay of C(t) being well described by a biexponential function. The faster component can be attributed to the presence of relatively free water molecules in the middle of the polar core and the slower component can be ascribed to those water molecules residing more toward the interfacial region where motion of the water molecules remains constrained.44,45 Interestingly, the C(t) decays in FeNt-containing RMs and during the course of FeOx nanostructure formation had to be modeled using a triexponential function, thereby reflecting on the distinct alteration in dynamics of water and the added heterogeneity (Table 1). Along with two relatively faster components of solvation, the third component was quite slow, varying from ∼11 ns (day 0) to ∼6.6 ns (day 8), thereby signaling a significant slowdown of a certain and significant fraction of the water molecules. The magnitude of the third component maximizes at ∼17 ns on day 4, subsequent to which there was a gradual drop (Table 1, Figure 5A). As a result, the average solvation time ⟨τs⟩ also shows a similar trend by peaking on day 4 with a value of ∼4.1 ns (Table 1, Figure 5B). We have also carried out a detailed solvation study on the micellar aggregates in the supernatant solution obtained after

Table 1. Decay Parameters of Solvent Correlation Function, C(t), of C343 during the Synthesis of the FeOx Nanostructures RM system Wo12

a1

τ1 (ns)

a2

τ2 (ns)

H2Oa AmOxb FeNtb day 0c day 0.5 day 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8

0.88 0.87 0.43 0.67 0.50 0.49 0.61 0.63 0.60 0.65 0.45 0.53 0.52

0.24 0.30 0.20 0.16 0.12 0.16 0.14 0.19 0.15 0.10 0.09 0.09 0.09

0.12 0.13 0.24 0.16 0.28 0.29 0.15 0.11 0.17 0.20 0.40 0.32 0.33

1.92 2.13 0.62 0.95 0.43 0.34 0.60 0.78 0.43 0.40 0.35 0.33 0.33

a3

0.33 0.17 0.22 0.22 0.24 0.26 0.23 0.15 0.15 0.15 0.15

τ3 (ns)

τavg (ns)

8.30 10.97 10.80 12.19 12.38 11.66 17.02 8.05 8.25 7.18 6.62

0.443 0.537 2.97 2.12 2.56 2.85 3.15 3.23 4.08 1.35 1.41 1.23 1.15

a In bare RMs. bIn reactant-containing RMs. cDay refers to the progress of reaction happening inside the RMs.

Figure 5. Plot of the average solvation time, (⟨τ⟩), and the slowest solvation time, (τs), as a function of increasing reaction time (day) in reverse micelles encapsulating FeOx nanostructures and in surfactant-aggregated structures in supernatant solution obtained after extracting evolved FeOx nanostructures during the synthesis. 5328


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micellar assemblies encapsulating evolving ZnOx nanostructures) itself. However, based on the observed similarity in dynamics between RMs encapsulating FeOx nanostructures and the respective micellar aggregates in supernatant solution obtained after the extraction of nanostructures, we constructed TRES for the ZnOx system supernatant solution that is, after the nanoparticles were extracted, for 15 h (Supporting Information Figures S12 and S13). Similar to the FeOx system, the decays of C(t) of C343 and C153 in ZnOx system were satisfactorily fit to triexponential and biexponential functions, respectively (Supporting Information Tables S3 and S4). With C343, the magnitude of the slowest component and therefore the average solvation time maximized at ∼13.6 and ∼3.72 ns, respectively, at 6 h, and subsequently showed a gradual decrease (Figure 6A). With C153, the long component and average solvation time rise to maxima of ∼7.96 and ∼1.30 ns, respectively, at the 3 h mark. Thereafter, the magnitude decreased until the 8 h mark and stayed constant (Figure 6B). The obtained trends in the solvent relaxation times of C343 and C153 probes collectively allude to the induced structural perturbations in soft micellar templates as lasting imprints made by the rigid nanostructures as the reaction progressed. Time-Resolved Anisotropy. The rotational time of probe molecules have a strong dependence on the rigidity (or viscosity) of the surroundings. Therefore, to further probe into the evolution of nanostructures, we have also performed timeresolved anisotropy measurements of the systems as a function of the progress of the reaction(s). For both the probes, C343 and C153, the r(t) fits well to a biexponential function for all of the studied RM systems (Tables 3 and 4, and Supporting Information Tables S5−S8). The biexponential nature of r(t) can be attributed to two different kinds of rotational dynamics. The faster component may be ascribed to the rotational dynamics of the probe in the bulklike water.46 On the other hand, the slower component represents the global rotational dynamics, which in turn is representative of the motion of the whole micellar assembly.46 The rotational time, τr1, of the probe C343 in RMs containing FeOx nanoparticles is slower than that in the pure RMs and AmOx-RMs, but faster than that in the FeNt-RMs (Table 3). This indicates a distinct change in the surrounding rigidity of the probe in FeNt-RMs and RMs containing FeOx nanoparticles. The slower rotational time, τr2, of probe C343 progressively increases and maximizes on day 4 and then decreases, similar to that observed for the C(t) variation (Table 3). The profile of τr2 in the case of C153 for RMs containing FeOx nanoparticles was found to be similar to that of C343 (Table 4). In the case of surfactant aggregates present in supernatant solution, the trend of τr2 was found to be similar as it was with the nanoparticles for both probes, C343 and C153, with the magnitude of the rotational time constant being a bit lower because of the absence of the nanostructures (Supporting Information Tables S5 and S6). The τr2 values of C343 (Supporting Information Table S8) and C153 (Supporting Information Table S8) in RMs containing ZnNt and in micellar aggregates were nearly invariant irrespective of the reaction time.

extracting the FeOx nanostructures from the hydrophilic core of RMs over the course of the reaction extending over a period of 8 days. The above study had two explicit aims: (1) to unveil the state of perturbed micellar aggregates in the supernatant solution and (2) to explore whether the rigid nanoparticles have been able to imprint their morphology on the soft micellar templates. The decay parameters (Supporting Information Table S1) of the solvent correlation function, C(t), were also characterized by triexponential fits that reflected the existing complexity of the dynamics of solvation inside the assembly even after the extraction of nanoparticles. The profile of the solvation dynamics of CTAB aggregates in supernatant is virtually the mirror image of the profile obtained in the presence of the encapsulated nanoparticles (Figure 5). This implies that the nanoparticles left an impression on the soft micellar templates, with the latter seemingly frozen in the likely perturbed state of the micellar core prior to the extraction of the evolved nanostructures. Since C343 is a charged species, it is expected that it would be sensitive to changes in the more polar regions of the RM interior. Hence, we have selected the hydrophobic probe C153 to provide information on how the relatively nonpolar regions of the RM would respond to the changes in structure en route nanoparticle formation. The decays of C(t) of C153 exhibited biexponential features as shown in Table 2 (and Supporting Table 2. Decay Parameters of Solvent Correlation Function, C(t), of C153 during the Synthesis of the FeOx Nanostructures RM system Wo12

a1

τ1 (ns)

a2

τ2 (ns)

τavg (ns)


0.94 0.89 0.94 0.83 0.52 0.88 0.89 0.93 0.71 0.92 0.95 0.79 0.71

0.27 0.21 0.18 0.22 0.18 0.22 0.22 0.26 0.18 0.29 0.29 0.21 0.14

0.06 0.11 0.06 0.17 0.48 0.12 0.11 0.07 0.29 0.08 0.05 0.21 0.29

2.88 2.81 1.91 1.02 0.46 1.41 1.14 1.34 0.55 2.39 2.02 0.59 0.48

0.43 0.50 0.29 0.35 0.31 0.36 0.32 0.34 0.29 0.46 0.37 0.29 0.24

a

In bare RMs. bIn reactant-containing RMs. cDay refers to the progress of reaction happening inside the RMs.

Information Table S2). The results obtained with C153 were in contrast to what was observed with C343. The ⟨τs⟩ for FeNt-containing RMs was nearly reduced to half of that of pure RMs and AmOx-containing RMs, which were nearly comparable (Table 2 and Supporting Information Table S2). During the course of nanoparticle formation, both τs2 and ⟨τs⟩ do not seem to follow a trend. In addition, there was a much smaller change in hydration dynamics as the nanoparticle formation progressed. These results suggest that C343 and C153 are probing quite different local environments of the interior regions of RMs. An onset of turbidity in the reaction mixture was found to occur within a small period of time indicating fast reaction kinetics for the formation of ZnOx nanostructures (∼15 h reaction completion time). This prevented us from investigating solvation dynamics in the reaction medium (reverse-

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DISCUSSION The potential utility of quaternary reverse micelles (surfactant/ co-surfactant/oil/water) as nanoreactors in synthesizing novel anisotropic nanostructures gives them an advantage over the conventional ternary reverse micelles (surfactant/oil/ water).17−21,28−38 The CTAB/1-butanol/water/isooctane qua5329


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Figure 6. Plot of the average solvation time, (τ), of C343 (A) and C153 (B) as a function of increasing reaction time (h) in surfactant-aggregated structures in supernatant solution obtained after extracting the ZnOx nanostructures during synthesis.

like intermicellar exchange.17,36,47−50 Consequently, the presence of co-surfactants at the interface significantly influences the solvation dynamics in RMs, this arising from considerable alteration in the extent of solvent reorganization in quaternary RMs compared to ternary RMs.51−54 In the present study, we have investigated two different systems in CTAB quaternary RMs (W0 = 12) having significant dissimilarity with regard to the time of completion of reaction, namely, (1) synthesis of FeOx nanorods monitored over a period of 8 days, (2) ZnOx nanorod synthesis having a much faster reaction completion time of ∼15 h. The reason for choosing the RM system of W0 = 12 value as a case study was that this system has been shown to be composed of a fair population of both types of ensembles of water molecules, one having the characteristics of bulklike water and the other one showing substantially slow orientational relaxation, widely known as bound water.55 Additionally, in our earlier reports, this RM system of W0 = 12 has been optimized for the synthesis of FeOx and ZnOx nanorods that were well characterized with a variety of techniques.56,57 Insights into the evolution of anisotropic nanostructures inside the RMs have been obtained with complementary techniques as reported earlier, such as TEM, FCS, DLS, and SAXS, which however have only addressed the morphological aspects of the micelles.37,38 Since it is well known that the nature of water inside the RMs is quite altered with respect to that of bulk water, our primary goal in this study has been to probe the modulation in solvent response as a function of the evolving nanostructures. Solvation is quite sensitive to small perturbations even under conditions wherein major structural changes might not take place. Hence, the latter stands to provide us with valuable information on alterations in the RM interior, thereby allowing one to put forward a mechanistic viewpoint with regard to the nanostructure synthesis. Previous studies have shown that the evolution of nanorods inside the micellar assembly is heterogeneous in nature and have different stages in their growth.37,38 In general, nanoparticles must acquire a critical dimension before breaking of asymmetry occurs and finally grow into nanorods.37,38,58 In pure CTAB RMs, we have observed a bimodal nature of solvation dynamics attributed to “free” and “bound” water reorganization with the solvation response being much slower than the bulk water response.59 Prior to the study of RMs containing nanoparticles, we monitored the solvation response with the reactants only inside the water pool of the RMs. Using C343 as the probe, the solvation dynamics of the FeNt-RMs

Table 3. Anisotropy Decay Parameters of C343 during the Synthesis of the FeOx Nanostructures RM system Wo12 a

H2O AmOxb FeNtb day 0c day 0.5 day 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8

r10

τr1 (ns)

r10

τr2 (ns)

χ2

0.11 0.16 0.26 0.24 0.26 0.33 0.26 0.28 0.28 0.31 0.29 0.19 0.20

0.29 0.24 0.52 0.30 0.35 0.36 0.37 0.36 0.37 0.30 0.26 0.22 0.26

0.11 0.09 0.02 0.04 0.03 0.02 0.01 0.02 0.02 0.02 0.05 0.05 0.03

1.03 1.06 1.04 1.01 1.38 1.64 1.76 1.70 2.18 1.43 0.88 0.90 0.91

1.1 1.2 1.2 1.1 1.2 1.1 1.2 1.2 1.0 1.2 1.0 1.1 1.2

a

In bare RMs. bIn reactant-containing RMs. cDay refers to the progress of reaction happening inside the RMs.

Table 4. Anisotropy Decay Parameters of C153 during the Synthesis of the FeOx Nanostructures RM system Wo12

r10

τr1 (ns)

r20

τr2 (ns)

χ2


0.12 0.16 0.23 0.26 0.18 0.27 0.32 0.23 0.20 0.22 0.29 0.30 0.30

0.19 0.24 0.38 0.31 0.24 0.29 0.29 0.29 0.28 0.25 0.22 0.24 0.24

0.02 0.03 0.03 0.02 0.04 0.02 0.02 0.01 0.03 0.04 0.03 0.03 0.03

1.12 1.22 1.19 1.26 1.24 1.35 1.39 1.71 1.09 0.97 1.10 1.22 1.22

1.0 1.1 1.1 1.1 1.2 1.1 1.2 1.1 1.1 1.1 1.1 1.1 1.06

a In bare RMs. bIn reactant-containing RMs. cDay refers to the progress of reaction happening inside the RMs.

ternary RMs system has proven to be an ideal template for the synthesis of homogeneous and monodispersed nanorods.17,19,28−38 The addition of co-surfactants in RMs is known to influence the curvature of interface, interfacial microviscosity, and interfacial fluidity, thereby affecting the size, shape, and other important parameters of reverse micelles 5330


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of the aromatic moiety and ammonium cationic head group.51,80,81 It has been suggested that the C343 molecules lodge themselves in between the surfactant head groups and therefore small changes occurring in the interfacial region can be selectively monitored with this probe molecule.42,51,52,82,83 Earlier reports have demonstrated that the different regions of complex systems (RMs, micelles) can be site-selectively monitored with the appropriate choices of solvatochromic probes.84,85 In PEO−PPO−PEO triblock copolymer micelles, the hydrophobic PPO core, PPO core and PEO regions, and bulk water region outside the micelles were studied by employing a very hydrophobic probe (C153), a hydrophobic probe (C102), and a relatively hydrophilic probe (C343), respectively.84 Similarly, the coumarin-based solvatochromic probes, C153, C480, and C343, have also been utilized to specifically monitor the different interior regions of ionic liquid-containing RMs in the absence and presence of palladium nanoparticles.85 In both pure and perturbed RMs, the trend in solvent relaxation time has been observed as C153 < C480 < C343. This has been attributed to the well partitioning of C343 inside the polar core of RMs because of its higher solubility and thereby sensing more restricted milieu imposed by the nanoparticles. The interfacial layer plays a pivotal role in the formation of nanoparticles as after the intermicellar exchange of reactants via nanochannels between the RMs, the reaction and nucleation start at the micellar edges with the subsequent growth occurring at this nucleation point.17,86,87 The selfassembly of surfactant molecules, i.e., interfacial layer around the aqueous core, incorporates mostly noncovalent intermolecular interactions. As a consequence, the growth of nanoparticles can easily be modified and thereby be able to induce the desired structural changes (size and shape) in such a soft template. Thus, one would envision that such changes will also be reflected on the dynamics of water molecules, the latter therefore providing an added dimension to the manner in which the progress in nanoparticle formation can be monitored. As a result, we have focused on the slowest relaxation component which represents the dynamics of the most impeded water molecules associated with the interfacial layer. Furthermore, from the trend of the day-to-day variation of this long component, it was observed to be the most sensitive to the induced structural alterations in RMs. Thereby, this slow component is indeed a reliable marker of the progress of reaction. On day 0, fusion of RMs containing equimolar reactants (AmOx and FeNt) led to the origin of new RMs encapsulating FeOx nanoparticles. The decay parameters of solvent response function C(t) (Table 1 for C343) revealed that after the start of reaction (i.e., after 15 min on day 0), the slowest solvent relaxation time in the case of FeOx-RMs (∼11 ns) increased with respect to that of FeNt-RMs (∼8 ns). However, at the same time, the amplitude of the slowest component of FeOx-RMs (day 0) almost reduced to half with respect to FeNt-RMs resulting in a concomitant decrease in ⟨τs⟩. The reduction of amplitude of the slowest component inside RMs after the coalescence of RMs containing reactants signifies the consumption of Fe3+ ions in the formation of nanoparticles. Furthermore, the increase in solvation time of the third component (∼11 ns) with respect to RMsencapsulated FeNt (8.3 ns) also reflects the formation of nanoparticles in the micellar core. This result thus suggests that owing to the initiation of reaction and nucleation at the micellar edges,17,86,87 the orientational motion of water

had to be described using three characteristic solvation times as opposed to a biexponential fit for the C(t) of the bare and AmOx-containing RMs. It was observed that while the solvation dynamics in the presence of bare RMs and AmOxcontaining RMs were nearly similar, that in the presence of FeNt was distinctly different, with the latter exhibiting an appreciable slowdown of the solvation coordinate. This difference was further manifested in the shift in the emission maximum of C343. For the FeNt-RMs, the emission maximum was substantially red-shifted (by 18 nm) compared to the bare RMs. RMs have proved to be an elegant model for probing the structural and dynamical properties of confined water molecules on a nanometer length scale.55,60−63 Therefore, water dynamics in the RMs have been the subject of extensive interest. Ultrafast IR and FTIR studies have explicitly shown the presence of the distinct water relaxation inside the RMs and have attributed these to the dynamics of “free water, “trapped water”, and “bound water”.64,65 Simulation studies, also in agreement with the experimental measurements, have identified the three structural regions of water in RMs.66 From our results, the fast, slower, and slowest relaxation times may reflect the dynamics of free, trapped, and bound water, respectively. These three different water types have been described as follows: (i) free water, which constitutes the least perturbed water molecules that are far away from the interfacial region (more toward the center of the polar core) and thereby appear to have bulklike water characteristics; (ii) the trapped water that includes those perturbed water molecules that are trapped between the counterions and surfactant polar head groups; and (iii) the bound water region, this being composed of those water molecules that reside in close proximity to the interface and are thereby strongly affected and have much retarded orientational relaxation. In addition, it is well established from both experimental67−70 and theoretical studies71−73 that water molecules in the close vicinity of ions in solutions exhibit substantially slower orientational dynamics in comparison to that of bulk water. The solvation response in RMs containing AmOx differed only modestly from that of pure RMs because of the considerably weaker interaction of water with NH4+ ions.74 However, the Fe3+ hydrated ions inside the micellar interior induced substantial perturbation, the same being well reflected in the significantly slowed down water reorientational dynamics. It has been shown that small multivalent ions (e.g., Mg2+, Fe3+, Cr3+) possess larger charge density with respect to larger monovalent ions and thereby slow down the reorientational motions of water molecules substantially, with their influence persisting beyond the first hydration shell.67,75−78 Furthermore, the slow component of water dynamics could also originate from solvated ion pairs because of the rigid, locked hydrogen bond structures existing in several hydration layers between the ions.67,79 Another point worth underlining is the remarkable difference in the solvent response of both probes C343 and C153. C343 exhibits an appreciable retardation in the solvent response, whereas C153 shows faster solvent reorganization dynamics. This implies that the C343 responds more to the “bound” water immobilized via the hydration of chloride counterions and ionized ammonium head group moieties of CTAB surfactants self-assembly. This is most likely owing to the electrostatic interaction between the anionic C343 dye molecules and cationic ammonium head group moiety of surfactant molecules (ion pairing) and a cation−π interaction 5331


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of the interfacial surfactant layer. This structural rearrangement might be rationalized based on the following aspects: (1) finite bending modulus of surfactant arrays and (2) adsorption of surfactant molecules on nanoparticles surface. First, the finite bending modulus of surfactant arrays restricts the monotonic increase in size of isotropic RMs assembly and stimulates the structural rearrangement of assembly.19 Second, once the size of RMs and encapsulated nanoparticles become comparable, the surfactant molecules start adsorbing on nanoparticle surface, which lead to decrease in the coagulation rate.88−90 In addition, earlier studies have reported that the cationic CTAB surfactants, owing to the inherent positive surface charges on the head groups, organize themselves into arrays along the surface of metal oxalate nanostructures because of the negative ζ potential of the latter.17,35 As a result, these enhance the directional characteristics, i.e., retard the further growth of nanoparticles along the lateral axis and increase the growth rate along the longitudinal axis and finally evolve in the anisotropic nanorods.17,35 Therefore, in the period from day 5 to day 8, the RMs encapsulating the anisotropic nanostructures undergo structural rearrangement in such a manner that coalescence rate increased along the longer axis and remained much reduced along the lateral axis leading to the formation of nanorods. These modulated micellar structures were no longer having their normal RMs structure morphology because of the complete disappearance of free water regions.91 It has been suggested that in such case, the water molecules inside the RMs get bound to surfactant molecules or to the encapsulated particle surface and thereby lose the typical characteristics of free water.91 Consequently, the probe molecules might not get partitioned into these modified micellar structures. Hence, no solvation could be observed for the modified RMs encapsulating the large nanorods. Support for this hypothesis is obtained from a previous report wherein for FCS-based studies (on FeOx nanorods in CTAB RMs), no signal could be obtained from RMs encapsulating the large nanorods in the same reaction period of day 5 to day 8.37 During this period, through FCS, only the presence of RMs containing small nanorods/ nanoparticles was detected.37 In the same report, TEM images revealed a significant increase in the aspect ratio of FeOx nanorods from day 5 (3:1) to day 8 (14:1).37 Taken together, our modulation in solvation data can therefore be analyzed based on the aforementioned discussion. In other words, the solvation observed from day 5 to day 8 comes predominantly from the smaller reverse micelles harboring smaller nanoparticles and hence having faster solvation times. We have also examined the supernatant liquid obtained after each day of reaction to reveal how the solvation dynamics were modulated in the micellar aggregates after the extraction of the nanoparticles and their correlation with respective RMs system encapsulating nanostructures. Surprisingly, the results showed appreciable similarity in the types of water population, i.e., a triexponential function well described the reorientational dynamics profile of surfactant aggregates in supernatant as of RMs encapsulating the FeOx nanoparticles (Supporting Information Table S1). Owing to the marked similarity in solvation profile between the FeOx-RMs and the respective surfactant aggregates in supernatant solution (Figure 5), the above-mentioned explanations for FeOx-RMs system remain equally applicable for surfactant aggregates in supernatant. This implies that the RM assemblies have been imprinted upon by the rigid nanoparticles such that even after the extraction of

molecules around the interfacial region slowed down further and thereby the magnitude of slowest solvent response time increased. Interestingly, from day 0.5 (after 12 h of reaction) to the end of day 2, both the slowest and average relaxation time showed a monotonic increase, i.e., hydration dynamics became continuously slower during this period of reaction. This increased perturbation in the RM interior can be explained based on a more constrained environment brought around by the growth of nanoparticles populating the core of the micellar assembly that in turn renders reduced reorientational mobility to surrounding water molecules. Previous studies37 on the same RM system showed a monotonous increase in the size distribution of RMs containing the FeOx nanostructures during the reaction, implying that larger dimensions under the present conditions lead to reduction in solvent orientation. At the end of day 3 of reaction, the slowest relaxation time decreased moderately with respect to the end of day 2 although the increased population of bound water led to a modest increase in the average solvent relaxation time. It is known that the nanoparticle formation incorporates an expansion of the size of the RM core.41 In other words, the growth of rigid nanoparticles induces structural modifications in interfacial layer of RMs for their accommodation inside the small hydrophilic core.41 The indication of the release in constraint inside the RMs core can be thus rationalized with respect to an enlarged size of the micellar core (swollen micellar core) to accommodate the growing nanoparticles. Intriguingly, at the end of day 4 of the reaction, the solvation dynamics strikingly slowed down indicating the presence of strong perturbation within the micellar core. The slowest relaxation time and hence the average relaxation time maximized after the end of day 4 (Figure 5). This might be due to the most constrained interfacial layer of micellar assembly, which in turn gives the impression of marked structural modulation. This morphological alteration might be related to the switching of isotropic assembly to anisotropic micellar assembly. In other words, the phenomenon of switching of geometry of micellar assembly from isotropic to anisotropic structures as signaled by the appreciable retardation of interfacial dynamics of FeOx-RMs assembly is sensed by the probe molecules lodged in the interfacial layer. The above-proposed idea of breaking of isotropic symmetry has been found to be consistent with earlier findings. Sharma et al.37 monitored the FeOx nanostructure growth in CTAB RMs with FCS, DLS, and TEM. They have shown with TEM images that after day 4, isotropic nanoparticles start aligning to form anisotropic nanostructures, and at the end of day 5, formation of nanorods takes place. Thus, the alignment of isotropic nanoparticles toward the anisotropic shape concurrently leads to the switching of isotropic assembly to anisotropic micellar assembly. Therefore, after the fourth day of reaction, the grown isotropic nanoparticles start aligning in the direction of anisotropic nanostructures inside the micellar aqueous core, the same bringing about an appreciable retardation of the solvation coordinate. However surprisingly, the end of day 5 was marked by a 3fold decrease in ⟨τs⟩ suggesting a relatively less restricted milieu of micellar core, perceived by probe molecules C343. From day 5 to day 8 of reaction, the reduction of the constrained milieu inside the hydrophilic core encapsulating the elongated large nanostructures (possibly nanorods) provides evidence of another major structural reorganization 5332


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axis. Therefore, we propose that the C153 molecules and hence the part of the micellar assembly, these are located in, are able to respond to the initiation of the nanorod formation followed by that region wherein the C343 molecules were located. To further investigate the alterations in the micellar structure during the synthesis of nanorods, we performed time-resolved anisotropy measurements and have mostly focused on the longer rotational constant, which reflects the tumbling of the whole micellar assemblies harboring the nanoparticles inside them. The progressive increase of the slower rotational time, τr2, of C343 till day 4 indicates that the rotation of the micellar assembly encapsulating FeOx nanostructures gets slowed down until this period of reaction (Table 3). This might be seen as the increase in size and the mass loading of the nanoparticles because of the growth of nanoparticles as the reaction progressed and is in good agreement with the observed trends from the solvation data. During the end of reaction, the rotation of RMs gets faster, this being similar to solvation results and thereby further confirming our proposition of the presence of smaller micelles with nanoparticles in the reaction solution. The profile of τr2 of C153 during the reaction was similar to that of C343 but with smaller changes, which we attribute to the different locales of C153 compared to that of C343 (Table 4). For ZnOx system, the obtained rotational times were nearly invariant for both C343 and C153, although a closer inspection of the tables does reveal a minor trend similar to the solvation data, in particular for C343 (Supporting Information Tables S7 and S8). We attributed the relative invariance of the rotation for the ZnOx supernatant to the difference in the micellar dynamics and size because of the difference in the charge of the ions involved as mentioned above. The dynamics of solvation has always been an area of burgeoning interest among the scientific community. Solvation measures the dynamical response of solvent molecules and is very sensitive to even minor structural and dynamical perturbations in the surroundings. Solvent relaxation appreciably affects the energetics of the solute and thus the dynamics of reaction.93 The dynamics of biological macromolecules (proteins, RNA, and DNA) and their biomolecular functions has been explored at the molecular level through molecular simulations and experimental measurements of solvation dynamics.60,61,93 It has also been utilized in thoroughly investigating the nature of confined water in complex systems like micelles and reverse micelles, sol−gel mixtures, and polymers.60−62,93 These studies are of utmost significance for unraveling the role of confined water in biological functions. Ion- and charge-transfer phenomena at the interfaces have also been studied using solvation dynamics, which have appreciable relevance.55,63 Presently, we have taken recourse to solvation dynamics to gain insights into the growth mechanism of nanorod formation in RMs. We have monitored the different interior domains of perturbed RMs during the course of reaction using site-selective solvation of fluorescent probes. Complementary techniques like FCS, SAXS, TEM, and DLS have helped track the growth process by monitoring the changes in the particle size distribution and in the particle morphology as the reaction progresses.37,38 However, changes in the internal dynamics of the micelles during such a reaction have rarely been studied.39−42 Extracting the in-detail intrinsic information of the temporal evolution of RMs en route nanorod formation is of utmost importance not only for

these nanoparticles, the aggregates in supernatant solution retained the impression of nanostructures. On the contrary, in the case of C153, much faster hydration dynamics were observed. As mentioned before, the C343 molecules selectively probed the interfacial region, where reorientational mobility of water molecules gets severely retarded. Also it is the most perturbed region of RMs during the formation of rigid nanoparticles. This indicates that the C153 molecules were probing those regions, which were relatively less affected during the synthesis of nanoparticles. Moreover, it is interesting to note that C153 also exhibited faster solvent response in surfactant aggregates in supernatant solution. This finding of similar kind of solvent response in both RMs encapsulating the nanostructures and surfactant aggregates in supernatant solution underlined the lasting effect that these nanostructures can have on the micellar assembly. Since the time resolution of our setup is ∼200 ps, we are not being able to detect a part of the early solvation. We have calculated the missing component using the method described earlier42,92 and have tabulated the same (Supporting Information Tables S9−S14). However, both these probes (C343 and C153) show structured absorption spectra in the nonpolar solvent, which can lead to nonaccurate estimation of the extent of missed solvation. The synthesis of ZnOx nanorods in CTAB RMs is a fast reaction having duration of only ∼15 h until completion. The motivation of this study was to ascertain that the solvation dynamics approach can be equally promising in providing the mechanistic aspects of relatively fast nanostructure synthesis. Monitoring the progress of ZnOx reaction was complicated by the appearance of turbidity. Hence, based on our aforementioned results that the supernatant retained all of the properties of the solution with the nanostructures during their synthesis, we performed our measurements with the supernatant solution only for the ZnOx system. A distinctive feature of the ZnOx nanorod synthesis is that the longest solvent relaxation time of C343 was significantly less compared to that observed for the FeOx system while that of C153 was notably more than the FeOx system. This implies that the extent of perturbation to the solvent environment is quite different for the two systems (FeOx and ZnOx), which can be rationalized on the basis of substantial difference in the nature of Fe3+ ions and Zn2+ ions in terms of charge density and thus their ability to modulate the solvent properties. A similar effect is also manifested in the shifts in emission maxima of the solvation probes, the same being far less for the ZnOx nanorods compared to the FeOx system. In spite of these differences, the overall trend in the solvation profile remains similar to that of FeOx with the solvation time peaking at an intermediate period of the reaction, revealing that the general solvation response and hence the mechanism of nanorod formation remains the same in both the cases. It is interesting to note that the C343 and C153 probes showed the maximum solvent relaxation time at two different time intervals of 6 and 3 h, respectively. This signifies that the C153 probe experienced the maximum perturbation in milieu at the end of 3 h while the C343 probe perceived the same at the end of 6 h. Sharma et al.38 demonstrated with the complementary techniques SAXS, DLS, and TEM that the nucleation-dominant growth period of ZnOx reaction exists up to 2 h of the reaction, beyond which critical-sized nanoparticles start transitioning into nanorods and subsequently from the end of 4 h, the growth of nanorod occurs more along the long 5333



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understanding the growth process but also for the development of new potential synthetic methodologies for the anisotropic nanostructures. In other words, such an approach allows us to envision possible control of the reaction times and/or size and shapes of nanostructures by systematic variation of the water content in the reverse micelle-based templates. Moreover, selecting probes that show preferential partitioning into different microregions of the soft templates can provide much needed insights into how the core and periphery of such reverse micelles respond to the changing structure during the nanostructure growth.

CONCLUSIONS A detailed and comprehensive solvation dynamics and rotational relaxation studies have been performed for the synthesis of iron oxalate and zinc oxalate nanostructures in CTAB-based RMs. The obtained solvent reorientational dynamics profiles for both iron oxalate and zinc oxalate suggest three distinct stages in the heterogeneous evolution of the anisotropic nanostructures: (1) nucleation and growth of isotropic nanoparticles; (2) the transition phase, this being the most perturbed state of RM wherein switching of geometry from isotropic to anisotropic nanostructures takes place; and (3) the directional growth of aligned nanostructures into the nanorods. Our data show that the solvation dynamics are a reliable signature of the structural perturbations in RMs encapsulating the nanostructures en route to their formation. Interestingly, our study also revealed an appreciable correlation between the RMs containing nanostructures and respective surfactant aggregates in supernatant solution, which shows that encapsulated rigid nanostructures imprint their marks on the soft templates. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.9b02459.

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Emission spectra of the solvation probes C343 and C153 in the respective solvent systems; representative normalized time-resolved fluorescence decays of C343 and C153 and TRES plots of these probes; representative decay parameters of C(t) of C343 and C153; anisotropy decay parameters of the coumarin-based probes; and percentage of solvation missed (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Phone: +91 011 2659 1511. Fax 91-11-26854715. ORCID

Pramit K. Chowdhury: 0000-0002-9593-2577 Ashok K. Ganguli: 0000-0003-4375-6353 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.K.G. and P.K.C. acknowlegde the Department of Science and Technology (DST), New Delhi, India, for financial support. N.Y. acknowledges Indian Institute of Technology (IIT) Delhi for financial support. 5334


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