Stimulus-Responsive Supramolecular Aggregate ... - ACS Publications

Article pubs.acs.org/JPCB

Stimulus-Responsive Supramolecular Aggregate Assembly of Auramine O Templated by Sulfated Cyclodextrin Ankur A. Awasthi and Prabhat K. Singh* Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India S Supporting Information *

ABSTRACT: Self-aggregation of organic molecules is rarely seen with macrocyclic hosts like β-cyclodextrin, as they preferentially involve the formation of inclusion complexes with the guest molecule. In this contribution, we report the self-aggregation of a guest molecule induced by negatively charged sulfated β-cyclodextrin (SCD) to yield highly emissive aggregates of a recently projected amyloid marker dye, Auramine O (AuO). The SCD templated AuO aggregates display very different photophysics when compared to its reported behavior in a wide range of various chemical and biological environment but show a remarkable similarity with the recently reported photophysical behavior of AuO in human insulin fibrillar media, thus providing important insights into the molecular form of AuO responsible for its amyloid sensing ability. The self-assembled AuO aggregates formed in the presence of SCD display a significantly long excited-state lifetime, suggesting the retardation of the torsional relaxation of dye in the aggregated state, which otherwise leads to a very short excited-state lifetime for the monomeric form of the dye in the isolated form. Detailed time-resolved emission spectra (TRES) measurements show a dynamic Stokes shift suggesting excitonic migration within the AuO aggregates. The supramolecular aggregate assembly displays remarkable sensitivity to important external stimuli like temperature or ionic strength of the medium, pitching for its possible application in designing stimuli-responsive sensing schemes for important analytes.

1. INTRODUCTION Stimulus-responsive molecular self-assembly involving supramolecular host−guest interactions has emerged to be an exciting research field for generating attractive supramolecular architectures which is finding potential applications in various important areas such as biosensing and chemosensing or even as prospective models for targeted drug delivery.1−5 Among the various supramolecular host family such as cyclodextrins, cucurbiturils, caliaxarenes, etc., the p-sulfonatocalixarenes, in which the hydroxyl groups of upper rim of the calixarenes are substituted with sulfonato groups, have recently received significant attention from the perspective of supramolecular aggregation.6 The complexation of p-sulfonatocalixarene with various cationic aromatic and amphiphilic guest molecules has been reported to promote the self-assembly of guest molecules by enhancing the aggregate stability or by regulating the degree of order in aggregates.7−12 These self-assembled guest structures promoted by p-sulfonatocalixarene have found various exciting applications such as fluorescent sensing systems to monitor enzymatic reactions,13−18 pesticide detoxification,19,20 and in drug delivery by supramolecular binary vesicles,21−24 resulting from calixarene-induced aggregation. On the other hand, cyclodextrins (CDs) are a series of cyclic polysaccharides primarily consisting of 6−8 glucose units connected together by α-1,4 glycosidic linkages, and compared to calixarenes, CDs are more water-soluble, less toxic, and commercially available at low cost, which can all add together to significantly impact the practical prospect of the generated supramolecular architectures in real applications.25−27 Further, © 2017 American Chemical Society

the hydrophobic cavity of CDs can include a large variety of biological, organic, and inorganic molecules with a relatively higher shape or size selectivity, as compared to calixarenes, both in solution state and in solid state to generate functional supramolecular materials.28−32 More importantly, the sulfated derivative of β-cyclodextrin (SCD) carries a relatively higher negative charge density, as compared to p-sulfonatocalixarene, and thus SCD is expected to promote more efficient aggregation, especially for cationic guest molecules, when compared to p-sulfonatocalixarene. However, despite these inherent advantages associated with SCD, surprisingly, there are rare reports on the interaction of organic guest molecules with this potential polyanionic macrocyclic host molecule, i.e., SCD, from the perspective of supramolecular aggregation.33 Herein, we report the interaction of a cationic guest molecule Auramine O (AuO), a recently reported amyloid marker, with polyanionic macrocyclic host molecule, SCD. AuO belongs to the family of ultrafast molecular rotors, which are characterized by their ability to twist around a single bond in the excited state that constitutes a major nonradiative deexcitation channel for the molecular rotor. Molecular rotors are virtually nonemissive in the isolated free form in aqueous solution or in low-viscosity solvents but show large emission enhancement in viscous solvents or restricted media.34−37 Utilizing the extreme sensitivity of the photophysical features of molecular rotors Received: April 17, 2017 Revised: June 1, 2017 Published: June 2, 2017 6208

DOI: 10.1021/acs.jpcb.7b03592 J. Phys. Chem. B 2017, 121, 6208−6219

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The Journal of Physical Chemistry B Scheme 1. Molecular Structures of Sulfated β-Cyclodextrin (A) and Auramine O (B)

experiment was performed by dropwise addition of SCD to an aqueous solution containing AuO, while keeping the AuO concentration fixed during titration. An incubation time of 20− 25 min was used for each SCD addition to ensure equilibration, and data points were collected only after solution equilibration was achieved. A Hitachi spectrofluorometer (model F-4500) was used to perform steady-state fluorescence measurement. The spectrometers were equipped with a Peltier-based arrangement for temperature-dependent measurement with an accuracy of ±1 °C. Rayleigh scattering measurements were performed on a Hitachi F-4500 spectrofluorometer in a 1 cm path length cell at 25 °C. The samples were excited at 300 nm, and the emission wavelengths were scanned for 250−350 nm. Circular dichroism spectra were measured over a wavelength range of 300−600 nm, using a Biologic MOS 450 spectropolarimeter, at room temperature, having continuous flow of nitrogen, and using a cell of 1 cm path length. Scan rate was set at 50 nm/min while recording the spectra, with an average of three scans for each spectrum. Baseline was subtracted from all the spectra with water being used as baseline. CD spectra were recorded as ellipticity (θ) in mdeg. 1 H NMR spectroscopic data were recorded with a 500 MHz Varian spectrometer. 1H chemical shifts are given in ppm (δ scale) and are measured relative to D2O (4.65 ppm), as internal standards. A diode laser based time-correlated single-photon counting (TCSPC) spectrometer (IBH, U.K.) was used to obtain the time-resolved fluorescence measurements and has been described in details elsewhere.55−57 Excitation of Auramine O was done using a 406 nm diode laser (1 MHz repetition rate). The fluorescence transients were collected at magic angle (54.7°) configuration which ensures that the observed transient decays are not influenced by the rotational relaxation of the probe molecule. To obtain instrument response function (IRF), scattered excitation light from the suspended TiO2 particles in water was monitored. The IRF obtained was ∼160 ps. To construct the time-resolved emission spectra (TRES), fluorescence transients were collected at an interval of 10 nm, covering the entire range of the fluorescence spectrum of the dye. Time-resolved emission spectra were constructed using the method proposed by Maroncelli and Fleming58 and are also detailed in our earlier publications.55−57 All the measured fluorescence transients were fitted with a triexponential function using an iterative convolution method. The best fitting parameters of the fluorescence decays, for different wavelengths, were used to construct the time-resolved emission spectra. The reconstructed spectrum was fitted using a lognormal function. The decay traces are fitted with a multiexponential function of the following form:59

toward restricted environment, they have been rapidly growing as sensitive reporters for microviscosity measurement in complex biological media and in the field of bioanalytical applications.35,36,38−40 In line with the behavior of molecular rotors, AuO also displays very weak emission in free isolated form and in lowviscosity solvents, whereas it shows large emission enhancement in high-viscosity solvents or restricted media.41−49 Very recently, AuO has been reported to be a fluorescent marker for important toxic biological assemblies such as G-quadruplexes50 and insulin amyloid fibrils.46,51 Interestingly, only in the case of human insulin fibrils, AuO displays distinct photophysical features, which are remarkably different from its conventional photophysical features reported earlier in a variety of chemical systems including protein and DNA. For example, AuO displays a large red-shifted intense emission band at ∼560 nm in the presence of human insulin fibrillar media as compared to its conventional emission band at ∼500 nm, reported in a variety of other media ranging from conventional solvents to proteins.45,47,52−54 It has been assumed that this red-shifted emission band for AuO in human insulin fibrils might be associated with the aggregated form of AuO.46 In this contribution, we report that the interaction of SCD with AuO induces aggregation of guest molecule rather than the conventional host−guest inclusion complexation of AuO with SCD. In the presence of highly charged environment of polyanionic SCD, AuO displays intense emission with a large red-shifted emission maximum at 560 nm, a band that was also observed for AuO in human insulin fibril media. The emission features have been assigned to the J-type aggregates of AuO. The photophysical features of AuO in the presence of SCD have been investigated in detail by using ground-state absorption as well as steady-state and time-resolved emission measurements. The present AuO-SCD assembly has been also shown to be remarkably responsive to important external stimuli like temperature and ionic strength of the medium.

2. EXPERIMENTAL SECTION Auramine O (AuO) was received from Sigma-Aldrich and was purified by multiple sublimation steps. β-Cyclodextrin sulfated sodium salt (degree of substitution ∼12−14), dextran sulfate sodium salt (mol wt ∼ 40 000), and sodium chloride were also obtained from the Sigma-Aldrich and were used as received. Nanopure water (conductivity less than 0.1 μS cm−1), used for all sample preparations, was obtained from a Millipore Milli-Q system. Fresh solutions of the AuO in SCD were prepared before measurements. The concentration of AuO used in all the experiments is 40 μM unless otherwise stated. A JASCO spectrophotometer (model V650) was used to carry out the ground-state absorption measurements. The 6209


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The Journal of Physical Chemistry B I(t ) = I(0) ∑ αi exp( −t /τi)

band for AuO. Recently, a similar red-shifted emission band has been reported for AuO in human insulin fibril media46,51 and has been implicated to arise from the aggregated form of the dye.46 Please note that the selection of an excitation wavelength at 410 nm for steady-state emission measurements was motivated by the intention to capture the whole range of emission spectra consisting of both monomer (λem ∼ 500 nm) and aggregated species (λem ∼ 560 nm). However, as will be discussed later, even when the excitation wavelength (410 nm) is reasonably apart from the excitation maximum of the aggregated species (470 nm, Figure S3), the emission spectrum is still dominated by the aggregate emission. This is largely because of the very weakly emissive nature of the AuO monomer as compared to their aggregates. To know more about the AuO species existing in the presence of SCD, we have carried out ground-state absorption spectral measurements. Ground-State Absorption Measurement. Figure 2 presents the ground-state absorption spectra of AuO in water

(1)

The mean fluorescence lifetime is calculated according to the equation59 τavg =

∑ Aiτi

where Ai = αiτi /∑ αiτi

(2)

3. RESULTS AND DISCUSSION Steady-State Emission Measurements. The emission spectra of AuO in aqueous solution and with varying concentrations of SCD have been recorded and are presented in Figure 1. As evident from Figure 1, the addition of SCD to

Figure 1. Steady-state fluorescence spectrum (λexc = 410 nm) of AuO at varying concentration of SCD: (1) 0.3, (2) 0.6, (3) 0.9, (4) 1.8, (5) 2.1, (6) 2.4, (7) 3.0, (8) 4.0, and (9) 6.5 μM. The red dashed line represents the emission spectra of AuO in water. Inset: variation of emission intensity at 560 nm with increasing concentration of SCD.

the aqueous solution of AuO leads to a very large enhancement in emission intensity of AuO as compared to that in water. However, interestingly, the emission spectrum of AuO in the presence of SCD displays a remarkably different emission maximum (∼560 nm) which is largely red-shifted as compared to that in water (∼500 nm, Figure S1, see Supporting Information). The gradual addition of SCD to the aqueous solution of AuO leads to further enhancement of intensity, at the red-shifted emission band, reaching an enhancement factor of ∼60 times at the saturation condition (6.5 μM). AuO has been reported to display emission enhancement in a variety of media such as viscous solvent, 47 confined environments,42,43,48,49 and more recently in important biological species such as insulin amyloid fibrils45,46,51 and G-quadruplexes.50 Such emission enhancement of AuO has been attributed to the restriction of its intramolecular torsional relaxation in the excited state, which otherwise leads to very low emission yield in the free form in aqueous solution or low-viscosity solvents.43,48,49 However, in all these previous reports except for human insulin fibril, the emission maximum is centered at ∼500 nm presenting an interesting and different case of new AuO emission in SCD. This indicates that AuO is present in a different molecular form in the presence of SCD as compared to the most frequently encountered monomeric form of AuO that emits at ∼500 nm. Note that the unsubstituted β-CD does not lead to any observable changes for AuO when compared at same concentration (6 μM) or even at 100 times higher concentration as that of SCD (Figure S2). This suggests that highly anionic charged environment of sulfated β-cyclodextrin is responsible for the intense and largely red-shifted emission

Figure 2. Ground-state absorption spectra of AuO at different concentrations of SCD: (1) 0, (2) 0.3, (3) 0.6, (4) 0.9, (5) 1.2, (6) 1.5, (7) 2.1, (8) 2.4, (9) 3.0, and (10) 6.5 μM. Inset: variation of absorbance at 435 nm (red, triangle) and 525 nm (blue, circles) with increasing concentration of SCD.

and at different SCD concentrations. In aqueous solution, AuO shows its characteristic absorption maxima at 368 and 431 nm, corresponding to the transitions from ground state to second and first excited state, respectively.47 Upon addition of SCD, a gradual decrease in the absorbance along with a nominal redshift of ∼3 nm was observed. These changes in the absorption spectral features may be attributed to the electrostatic interaction between the cationic AuO and anionic sulfated groups of SCD. A similar red-shift in the absorption spectra has been observed for AuO in the presence of anionic surfactants60,61 and DNA molecules52 which offers anionic phosphate backbone as interaction sites. However, interestingly, the increasing concentration of SCD also leads to slight turbidity in the solution in a concentration dependent manner, which appears as an offset at the red side of the absorption spectra. The increase in tubidity of the solution is concomitant with the decrease in absorbance of the main absorption band (at 430 nm) of AuO. To quantify this feature, absorbance at 525 nm was plotted as a function of SCD concentration, presented in the inset (left) of Figure 2. Abosrbance at 525 nm increases with increasing concentration of SCD, reaching a maximum at 6.5 μM which correlates very well with the decrease in absorbance at 435 nm with increasing concentration of SCD (Figure 2, right inset). This further 6210


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Figure 3. Proton NMR spectra of AuO (120 μM) in D2O (upper panel, blue) and in 2 μM SCD (lower panel, red).

aggregates are excited, possibly due to broader distribution of aggregate species (indicated by broader excitation spectra, Figure S3 and Job’s plot, Figure 4 discussed later), the aggregate species predominates the emission spectra. According to exciton theory, the dipole−dipole interaction between the neighboring molecules, in the aggregate, leads to splitting of the excited-state energy levels which are known as exciton levels and are shared by all the molecules in the aggregated state.62 Among these exciton energy levels, whether the higher or lower exciton state is allowed is determined by the arrangement of the molecules in the aggregated state. When the transition dipole of the molecule adopts a cofacial arrangement (H-aggregates), the electronic transition between ground state and the higher exciton state is allowed leading to a blue-shift in the absorption spectra.62 On the other hand, if the transition dipole of the molecules are offset along the long axis (Jaggregate), the lower energy exciton state is allowed, leading to an overall red-shifted absorption spectra.62 Thus, the large redshift observed in the excitation spectra of AuO in the presence of SCD, as compared to the monomeric form, indicates the Jtype nature of the aggregates in the presence of SCD. Although pure J-aggregates display sharp red-shifted absorption band with respect to the monomer absorption band, the broadening observed in the red-side of the absorption spectra of AuO in the presence of SCD may be attributed to the less optimal or disordered J-type aggregates, as has been observed for some other organic dye molecules.64 The formation of AuO aggregates has been further confirmed by Rayleigh scattering measurements (Figure S5). The light scattering intensity monitored at 300 nm was found to gradually increase with increasing concentraion of SCD, reaching a maximum at ∼6.5 μM. It should be noted that the same concentration of SCD in the absence of AuO does not lead to any change in the scattering intensity. The excitonic coupling between the AuO molecules in the aggregated state is further investigated by electronic circular dichroism (CD) spectra which ideally leads to a bisignate feature, in the CD spectra, in the case of excitonic interaction.65 The induced chirality in the present case of AuO in chiral SCD molecule also displays a bisignate feature ranging the whole S0− S1 electronic transition of AuO chromophore between 350 and 500 nm (Figure S6). The bisignate signal is attributed to the

correlates quite well with the SCD concentration-dependent increase in emission intensity of AuO (Figure 1). Thus, the SCD concentration-dependent behaviors of absorbance of AuO at these two wavelengths correlate with each other very well, suggesting that the two species, one absorbing at 430 nm, corresponding to monomeric AuO, and the other AuO species, possibly absorbing on the red side, are at equilibrium with each other. Thus, to gain more insight into these possibly red absorbing species, we have collected the excitation spectrum (Figure S3) of AuO at the maximum complexation concentration (∼6.5 μM) by monitoring emission peak at 560 nm. Interestingly, the excitation spectrum reveals a peak maxima at 470 nm, which is quite red-shifted as compared to the absorption maximum of monomeric form of AuO (∼430 nm) and establishes that the emission at 560 nm, in the presence of SCD, arises predominantly from the AuO species absorbing at 470 nm. Such a large red-shift in the excitation spectra of AuO species, as compared to monomeric form, indicates the J-type nature of the AuO aggregates in the presence of SCD. This presumably leads to the broadening and tailing in the red-side of the absorption spectra in the presence of SCD. The red-shift in the absorption spectra of AuO aggregates can be understood in terms of exciton theory associated with molecular aggregates which has been discussed in the next paragraph.62,63 Since the monomer and aggregtaes of AuO absorb at different wavelength, so to check how the excitation wavelength affects the emission spectra of AuO-SCD system, we have measured excitation wavelength-dependent emission spectra (Figure S4). The emission intensity was found to be dependent on the excitation wavelength which has been attributed to varying extent of excitation of the aggregated species. For example, at a longer excitation wavelength (λexc = 460 nm), the emission intensity is comparatively high due to increased extent of excitation of aggregated species, whereas at a shorter excitation wavelength (λexc = 390 nm), the emission intensity is comparatively less due to reduced extent of excitation of the aggregated species. The dominance of aggregate emission, even when the excitation wavelength (390 nm) is kept reasonably apart from the aggregate excitation maximum (470 nm, Figure S3), is attributed to the fact that monomers of AuO are very weakly emissive as compared to their aggregates, so even when a small fraction of AuO 6211


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Figure 4. Job’s plot for AuO−SCD system obtained from the (A) absorption changes (ΔAbs = AbsAuO/SCD − AbsAuO only) at 430 nm (inset: 525 nm) as a function of the mole fraction of AuO (XAuO) and (B) fluorescence changes (ΔIf = IAuO/SCD − IAuO only) at 560 nm (λex = 410 nm) as a function of the mole fraction of AuO (XAuO). The sum of the concentrations of the AuO and the SCD was kept as 50 μM.

It should be noted that the formation of AuO aggregates, in the presence of SCD, presents a very different perspective on the interaction of organic dyes with β-CD and its derivatives. For example, unsubstituted β-CD usually leads to deaggregation of different organic dye molecules by forming an inclusion complex with the monomeric form of the dyes.72−74 Contrastingly, in the present case, we are dealing with a βCD derivative with a very high negative charge density (degree of sulfation = 12−14) in comparison to unsubstituted β-CD which is neutral. It is the high negative charge density of SCD which induces the aggregation of the cationic AuO, by virtue of charge neutralization of the AuO molecules, because of which the electrostatic repulsion between the AuO molecules is significantly reduced, thereby facilitating the formation of aggregates on the surface of SCD. Time-Resolved Emission Measurements. As briefly mentioned in the Introduction that AuO belongs to the family of ultrafast molecular rotors which are characterized by their ability to twist around a single bond in the excited state, leading to very fast dissipation of excitation energy in the excited state. Thus, to know how the excited-state relaxation is affected in the AuO aggregates, formed in the presence of SCD, we have performed time-resolved emission measurements for AuO in water and in the presence of SCD. Figure 5 displays the transient decay traces for AuO in water and in SCD. In water, AuO decays very rapidly, and its excited-state lifetime in water cannot be measured by our current TCSPC setup (IRF ∼ 160 ps). The lifetime of AuO in water is reported

excitonic coupling of the rotationally displaced AuO transition dipole moment. A similar bisignate feature for the aggregates of other dye molecules has been observed earlier in the presence of chiral templates.33,65 The formation of AuO aggregates, in the presence of SCD, was further investigated by 1H NMR measurements. AuO in D2O shows 1H NMR peaks previously reported for this molecule in literature.66 However, when SCD was added to the same solution, a drastic reduction in the 1H NMR signal by ∼50 times was observed (around 3000 scans were collected to get a reasonable spectra for the AuO−SCD system). Only AuO yields NMR signals, in the aromatic region, for two sets of protons (δ = 6.75−6.77 and 7.47−7.49); however, upon addition of SCD, these aromatic protons appear as a broad single peak at δ = 6.755 and 7.45, respectively (Figure 3). Apart from aromatic protons, the only other signal that could be detected in the 1H NMR spectra belongs to the N−CH3 protons (δ = 2.957) of AuO, which also display a drastic reduction in signal intensity (Figure S7). This result implies that AuO exists as aggregates in the presence of SCD which leads to the broadening of the 1H NMR signals that could be due to the magnetic dipolar interactions generated among the nuclei of the individual dye molecule in the aggregate.67,68 Further, the drastic reduction in the NMR signal can be assigned to the semicolloidal nature of the AuO aggregates (also suggested by Rayleigh light scattering measurements, Figure S5) that has been induced by SCD. A similar observation of the reduction of the signal intensity69 as well as the broadening 1H NMR signal has been reported earlier for the semicolloidal aggregates of other probe molecules.67,70,71 To gain an idea about how many units of AuO are approximately present in this aggregate assembly, we carried out Job’s plot measurements, using both emission and absorption studies, as a function of mole fraction of AuO (XAuO) by keeping the sum of the concentration of AuO and SCD constant. At the maximum concentration of the complex, the molar ratio equals the stoichiometry of the complexation. In the present case, the Job’s plot, by both the absorption and emission method (Figure 4A,B, respectively), displays a maximum at XAuO = 0.9, indicating a stoichiometry of 1:9 for SCD:AuO. However, owing to the broadening observed in the Job’s plot, the possibility of aggregates with other stoichiometry cannot be ruled out. Such a high value of AuO units, involved in the complexation, strongly suggests the formation of aggregates of AuO in the presence of SCD.

Figure 5. Transient decay trace for AuO (λex = 406 nm, λem = 560 nm) in (1) water and (2) 6.5 μM SCD. The solid black line represents instrument response function (IRF). 6212


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The Journal of Physical Chemistry B to be ∼1 ps.51 However, it is quite evident that AuO displays very slow excited-state decay in the presence of SCD as compared to that of water. Further, the decay trace was found to follow a non-single-exponential kinetics. Such non-singleexponential decay kinetics has been well documented in the literature for AuO in a variety of medium including both homogeneous and heterogeneous medium.42,48,75,76 This nonsingle-exponential decay kinetics has been attributed to the fact that during the relaxation on the potential energy surface, where the radiative decay constant becomes dependent on the phenyl bond twisting angle, the emission from different positions on the excited state potential energy surface is associated with different time constant and leads to the decay of the excited state becoming non-single exponential.42,75 This non-single-exponential decay kinetics is not specific for AuO but has been also observed for several other molecules which similarly undergo conformational relaxation in the excited state.77−79 Because of non-single-exponential decay kinetics, assigning a specific decay constant to a specific process in the excited state of the dye is very difficult. Under such circumstances, it is a common practice to represent such dynamics by average lifetime.42,79,80 The average lifetime of AuO in the presence of SCD (at λem = 560 nm) was found to be ∼1.8 ns. The large increase in the excited-state lifetime is consistent with the large increase observed in steady-state emission intensity for AuO in the presence of SCD and indicates that the nonradiative torsional relaxation process in AuO is largely suppressed in the aggregated form as compared to the free isolated form in aqueous solution. However, apart from displaying a long excited-state lifetime, interestingly, transient decay traces are found to be strongly dependent on the monitoring emission wavelength. It is obvious from Figure 6 that the transient decay traces display

for other molecular rotors in the aggregated state.46,81 Thus, in the present system of AuO in SCD, the observed emission wavelength-dependent decay features can be associated with the AuO aggregates. However, to gain more insights into the nature of emissive species, we utilized the wavelength-dependent fluorescence decay traces to construct time-resolved emission spectra (TRES) following the procedures proposed by Maroncelli and Fleming.58 The resulting time-dependent spectral profile was fitted using a log-normal function and is shown in Figure 7.

Figure 7. Time-resolved emission spectroscopy (TRES) of AuO in SCD at different times: 0.1 ns (red), 0.15 ns (yellow), 0.2 ns (green), 0.3 ns (blue), 0.45 ns (pink), 0.7 ns (cyan), 1.0 ns (purple), and 2.0 ns (orange). The circles are the experimental data points, and the solid lines are the log-normal fits to the data points. Inset: variation in the area under the emission spectra for AuO in SCD with time.

From the inspection of the time-resolved emission spectra, it is observed that the emission intensity gradually decreases (Figure 7, inset) with time along with a gradual red-shift, which becomes even more evident from the normalized TRES (Figure 8A). The peak frequency of the spectra, calculated using lognormal function, gradually decreases with time (Figure 8B). A dynamic red-shift of 800 cm−1 is observed within 3 ns. A similar dynamic red-shift in TRES has been observed for AuO in human insulin fibrillar media and in the presence of premicellar concentration of surfactant, and this dynamic red-shift has been attributed to the excitonic migration in the aggregated state, from aggregates with higher HOMO−LUMO energy gap to aggregates with lower HOMO−LUMO energy gap.46 This kind of dynamic red-shift for other dye aggregates has also been well reported.82−85 Thus, based on previous literature reports, the dynamic red-shift observed with the TRES in the present system of AuO in SCD can be assigned to the excitonic migration in AuO aggregates. Effect of Stimulus (Ionic Strength). Self-assembled systems involving supramolecular host−guest interactions are driven by multiple weak and dynamic noncovalent interactions which provide easy and facile approaches to manipulate such self-assembled systems leading to fascinating stimulus responsive systems. Thus, the self-assembled fluorescent J-type aggregate of AuO, observed in the presence of SCD, may provide tunable and reversible response to external stimuli like temperature, salt, etc. Since in the present case we have a polyanionic host due to presence of multiple sulfate groups (SCD), and a cationic guest (AuO), a predominant contribution of electrostatic interaction toward the present self-assembly is expected. Thus, to verify this, and to understand the response of the present supramolecular

Figure 6. Transient decay trace for AuO in SCD at various emission wavelengths (λex = 406 nm): (1) 490, (2) 520, (3) 550, (4) 560, and (5) 650 nm. The solid black line represents instrument response function (IRF).

rapid dynamics on the blue side of the steady-state fluorescence spectra and gradually slows down when monitoring emission wavelength is scanned from shorter to longer wavelength. A similar emission wavelength-dependent decay kinetics has been recently observed for AuO in human insulin fibril media as well as in the presence of premicellar concentration of anionic surfactants, which also display a red-shifted emission band at ∼560 nm. Such emission wavelength-dependent decay features in these previously reported systems have been attributed to the excitonic migration in AuO aggregates.46 Further, similar wavelength-dependent decay dynamics have been observed 6213


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To understand the effect of ionic strength better and to complement the steady-state emission measurements, the ground-state absorption measurements were performed for the AuO−SCD system in the presence of various concentrations of sodium chloride. Figure 10 shows the ground-state

Figure 10. Ground-state absorption spectra of AuO in SCD (6.5 μM) at various concentration of sodium chloride: (1) 0, (2) 0.06, (3) 0.12, (4) 0.18, (5) 0.24, (6) 0.30, (7) 0.36, and (8) 0.42 mM. Inset: variation of absorbance at 430 nm (red, triangle) and 525 nm (blue, circle) with varying concentration of sodium chloride.

absorption spectra of the AuO−SCD system at various concentrations of sodium chloride. As discussed earlier, upon formation of aggregates of AuO with polyanionic SCD, the absorbance at its peak maximum (430 nm) is significantly reduced along with a broadening and tailing on the red side of the absorption spectra. Now, upon addition of salt, the absorbance at the peak position starts recovering (increasing) with increase in salt concentration (inset of Figure 10). Also, the extent of tailing on the red side of the absorption spectra which is a manifestation of aggregate assembly gradually decreases with increase in salt concentration (represented as absorbance at 525 nm, inset of Figure 10). These observations together indicate the disruption of SCD templated aggregate assembly upon addition of salt and correlate quite well with the steady-state emission measurements. The steady-state emission and ground-state absorption measurements of the effect of salt on the present supramolecular aggregate assembly are further supported by transient emission measurements, where the transient decay traces were found to gradually decay faster with increasing salt concentration in the solution, thus suggesting disassembly of the supramolecular aggregate (Figure 11). This disassembly of supramolecular aggregates leads to the gradual release of AuO molecules in the bulk aqueous phase, where the rapid torsional relaxation in the free AuO molecule dominates. This causes gradual rapidity in the transient decay traces of the AuO−SCD system as a function of salt concentration. Thus, the timeresolved measurements addressing the salt effect on the AuO− SCD system are in concordance with the ground-state absorption and steady-state emission measurements. The large effect of ionic strength on the interaction between AuO and SCD suggests the predominance of electrostatic interaction in forming the aggregate assembly which involves interaction of the AuO with the highly charged surface of SCD rather than forming a conventional host−guest inclusion complexation. Effect of Temperature. Apart from strong electrostatic interaction, these supramolecular aggregates, in general, also involve multiple, relatively weak, and dynamic noncovalent

Figure 8. (A) Normalized time-resolved emission spectroscopy (TRES) of AuO in SCD at different times: 0.1 ns (red), 0.15 ns (green), 0.3 ns (pink), 1.0 ns (yellow), 1.5 ns (cyan), and 5.0 ns (purple). The circles are the experimental data points, and the solid lines are the log-normal fits to the data points. (B) Variation of peak frequency for AuO in SCD with time.

fluorescent assembly toward the ionic strength of the medium, we investigated the response of SCD templated J-aggregates of AuO as a function of salt concentration. Figure 9 shows the variation of emission intensity of the AuO−SCD supramolecular aggregates at varying concentra-

Figure 9. Steady-state fluorescence spectra (λexc = 410 nm) of AuO in SCD (6.5 μM) at varying concentrations of sodium chloride: (1) 0, (2) 0.06, (3) 0.12, (4) 0.18, (5) 0.24, (6) 0.30, (7) 0.36, (8) 0.42, (9) 0.47, (10) 0.53, and (11) 0.59 mM. Inset: variation of emission intensity at 560 nm with increasing concentration of sodium chloride.

tions of sodium chloride. It can be seen that with increasing concentration of sodium chloride, the emission intensity gradually decreases and nearly reaches to bulk water like state before leveling off at a concentration of 0.55 mM. 6214


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Figure 13. Ground-state absorption spectra of AuO in SCD (6.5 μM) at varying temperatures: (1) 20, (2) 25, (3) 30, (4) 35, (5) 40, (6) 45, (7) 50, (8) 55, (9) 60, and (10) 75 °C. The black dashed line represents the absorption spectrum of AuO in water. Inset: variation of absorbance at 430 nm (red, triangle) and 525 nm (blue, circle) with temperature.

Figure 11. Transient decay trace for AuO in SCD (6.5 μM) (λex = 406 nm, λem = 560 nm) at different concentrations of NaCl: (1) 0, (2) 0.65, (3) 0.88, and (4) 0.99 mM. The solid black line represents the instrument response function (IRF).

interactions, such as van der Waals interaction, aromatic− aromatic interaction, London dispersion forces, and hydrophobic interaction. Thus, these self-assembled aggregates are expected to be sensitive to temperature. Figure 12 shows the

gradually decreasing with increase in temperature (represented by absorbance at 525 nm, inset of Figure 13). This temperature-induced disassembly of AuO−SCD aggregates was further supported by transient emission measurements. Figure 14 summarizes the results of these

Figure 12. Steady-state fluorescence spectra (λexc = 410 nm) of AuO in SCD (6.5 μM) at varying temperatures: (1) 20, (2) 25, (3) 30, (4) 35, (5) 40, (6) 45, (7) 50, (8) 55, (9) 60, and (10) 70 °C. The black dotted line represents the fluorescence spectrum of AuO in water. Inset: variation of emission intensity at 560 nm with temperature.

Figure 14. Transient decay trace for AuO in SCD (6.5 μM) (λex = 406 nm, λem = 560 nm) at different temperatures: (1) 15, (2) 35, (3) 45, and (4) 55 °C. The solid black line represents instrument response function (IRF). Inset: variation of average excited-state lifetime (τavg) of AuO in SCD with temperature.

changes in emission spectra of SCD templated J-aggregate assembly of AuO as a function of temperature. As evident from Figure 12, the emission intensity gradually decreases with increase in temperature. Also, cooling the solution back to room temperature completely recovers the emission intensity. This temperature responsiveness of the present system may be perceived in terms of weakening of noncovalent interactions involved in the formation of these supramolecular aggregates and which, upon disassembly, activates the intramolecular torsional relaxation process in AuO molecules that subsequently weakens the light emission from the system. To complement the steady-state emission measurement, the effect of temperature on the AuO−SCD assembly was also evaluated by ground-state absorption measurements. As previously stated, the absorbance of AuO at its peak maximum (430 nm) significantly decreases in the presence of SCD. Now, upon increase in temperature, the absorbance at 430 nm starts gradually increasing with gradual increase in temperature (inset, Figure 13). At the same time, absorbance on the red side of the spectra, which is a representation of aggregate assembly, starts

experiments. From Figure 14, it is evident that the transient decay traces become gradually faster with increase in temperature, suggesting the temperature-induced disassembly of AuO−SCD aggregates. This disassembly activates the torsional relaxation in the AuO molecules and leads to gradually faster excited-state decay dynamics. The average excited-state lifetime shows a linear decrease with temperature (inset of Figure 14). Thus, the remarkable temperature sensitivity of the AuO−SCD system is promising for its usage in temperature sensing applications. Effect of Preorganized Structure of SCD on the Aggregation Process. To understand the effect of preorganized structure of SCD, on the aggregation process of AuO, we have further investigated the effect of dextran sulfate (DeXS), which is an open chain analogue of sulfated cyclodextrin. Figure S8 represents the emission spectra of AuO in the presence of dextran sulfate. Similar to the case of SCD, dextran sulfate also induces a red-shifted emission for the AuO, which has been assigned to the AuO aggregates. The 6215


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Figure 15. Changes in (A) steady-state emission intensity at 560 nm and (B) absorbance at 525 nm of AuO with increasing charge ratio in for SCD (●, red) and (2) dextran sulfate (DeXS) (▲, blue).

Scheme 2. Sulfated β-Cyclodextrin Induces a StimulusResponsive J-Aggregate Assembly of Auramine O

formation of AuO aggregate, in the presence of dextran sulfate, is further complemented by ground-state absorption measurements (Figure S9), which displays slight turbidity in the solution and leads to an offset in the red side of the absorption spectra, in a concentration-dependent manner, as has been observed in the case of SCD. This induced aggregation of AuO by dextran sulfate is assigned to the polyelectrolyte nature of dextran sulfate with high negative charge density, which leads to charge compensation of the guest molecule followed by possible π−π stacking interaction between the guest molecules. However, dextran sulfate lacks a preorganized structure as compared to SCD which bears a cyclic cavity and is expected to bring a difference in the aggregation behavior of AuO. Thus, a definite difference with respect to the behavior in SCD is observed when excess of dextran sulfate is added to the AuO aggregate solution. Figure 15 shows the comparison of the behavior of SCD and dextran sulfate. It is quite obvious from Figure 15A that the addition of excess SCD drastically diminishes the aggregate emission, whereas excess of dextran sulfate causes minimal changes in the aggregate emission at the same charge ratio. A similar trend has been observed in the ground-state absorption measurements (Figure 15B). Dextran sulfate is a flexible polymer without any preorganized structure. Thus, besides electrostatic interaction, there is no special “host−guest interaction” that contributes to the induced aggregation process in this case. On the contrary, in the case of SCD, the aggregates are dispersed by excess SCD, where host−guest interaction predominates the other interactions. Before we close, it is also important to remind that the present photophysical features for SCD templated AuO aggregates are quite similar to that of AuO in human insulin fibrillar medium, and the similarity in the spectroscopic features can be listed as follows: (1) Both the SCD templated AuO aggregates as well as self-assembled AuO in human insulin fibrillar media display a large red-shifted emission band at 560 nm. (2) A long excited state lifetime of 1.8 ns has been observed for both the SCD template AuO aggregates as well as self-assembled AuO in human insulin fibrillar media. (3) In both the cases, the transient emission decay traces displays an extreme dependence on the monitoring emission wavelength. (4) The constructed time-resolved emission spectra (TRES), in both the cases, display a dynamic red-shift, which is a manifestation of an excitonic migration within AuO aggregates formed in both the cases. Thus, the results of the present system, obtained in this study, also provide important insights into the nature of emissive species that might be responsible for its amyloid sensing ability.

4. CONCLUSIONS To summarize, we have reported here an interesting assembly between AuO and SCD, switching on the fluorescence in solution. Interestingly, in contrast to our expectations, the emission enhancement is associated with J-aggregate of the dye rather than inclusion complexation between SCD and monomers of AuO. Here polyanionic SCD serves as a template for the formation of fluorescent aggregates of noncovalently attached AuO molecules. The fluorescence enhancement shown by the J-aggregates of AuO, over its isolated form, is an outcome of suppression of the nonradiative torsional relaxation of AuO in the aggregated form. The present supramolecular aggregate assembly also displays extreme sensitivity to important external stimulus like temperature and salt and may find application in designing of stimulusresponsive systems. The unique advantage of emission of the present supramolecular assembly in the biologically advantageous red region, coupled with the large contrast between the photophysical features of the aggregated form of AuO and the momeric AuO, may allow design of differential and ratiometric sensors for important external analytes, including bioanalytes, which is currently being undertaken in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b03592. Normalized emission spectra of AuO in water and SCD, emission spectra of AuO in β-cyclodextrin, excitation spectra of AuO in SCD, Rayleigh scattering, excitation wavelength dependent emission spectra, NMR spectra, circular dichroism spectra of AuO in the presence of SCD, steady-state emission and ground-state absorption spectra of AuO in the presence of dextran sulfate (PDF) 6216


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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], [email protected]; Tel 91-22-25590296; Fax 91-22-5505151 (P.K.S.). ORCID

Prabhat K. Singh: 0000-0002-4612-547X Present Address

A.A.A.: On M.Sc. research project from Department of Chemistry, Jai Hind College, Mumbai 400020, India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Department of Atomic Energy, India, for the financial support. The authors acknowledge Dr. H. Pal for his constant support and encouragement during the course of this work. The authors also thank Dr. D. Goswami, Bio-organic Division, for help with NMR measurements. A.A.A. thanks the Human Resource Development Division, Bhabha Atomic Research Centre, for approving the project internship.

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