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Smart Fluorescent Nanoparticles in Water Showing TemperatureDependent Ratiometric Fluorescence Color Change Junjie Cui, Ji Eon Kwon, Hyeong-Ju Kim, Dong Ryeol Whang, and Soo Young Park* Center for Supramolecular Optoelectronic Materials, Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea S Supporting Information *

ABSTRACT: We synthesized two different amphiphilic small molecules 1 and 2 by attaching the same oligo(ethylene glycol) (OEG) unit to the same dicyanodistyrylbenzene (DCS) fluorophore but at different positions. These molecules self-assemble into nanoparticles in water and show lower critical solution temperature (LCST) at 26 and 58 °C, respectively. Upon heating, the transition of hydrophilic coils to hydrophobic globules of the OEG unit leads to the change in the stacking structure of the luminescent DCS cores. As a result, it shows significant ratiometric fluorescence color changes from excimeric yellow emission to monomerdominated green emission. Interestingly, the coassembly of 1 and 2 exhibits single transition temperature between the transition temperatures of the two components. Moreover, it is demonstrated that the transition temperature of the coassembly is delicately tuned over 26−58 °C by varying the molar mixing ratio of them. KEYWORDS: ratiometric fluorescence switching, LCST, amphiphilic, self-assembly, supramolecular chemistry

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nanostructures such as spherical or cylindrical micelles,29 helical fibers,30 vesicles,31 tubules,32,33 and ribbons34,35 in aqueous condition. Particularly, organic rod−coil molecules containing biocompatible oligo(ethylene glycol) (OEG) as a coil unit have been widely studied to fabricate smart and functional nanomaterials with finely tunable thermoresponsiveness.36−41 Similar to the PNIPAM derivatives, OEGylated small amphiphiles often show LCST behaviors attributed to the coil-to-globule transition of the ethylene glycol (EG) chains accompanying hydrophilic-to-hydrophobic change of the microenvironment inside their nanostructures upon heating.37,42,43 Recently, a few studies have been reported to incorporate π-conjugated fluorophores as rigid rod structures in the OEGylated amphiphiles, which have demonstrated enhancement of fluorescence emission with the LCST transition. Yin et al. reported that a series of compounds consisting of a tetraphenylethene (TPE) core and peripheral OEG chains showed 2-fold fluorescence turn-on above LCST in aqueous solution.44 Würthner group developed perlyene bisimide hydrogels bearing OEG units whose fluorescence intensity was greatly enhanced by a factor of ∼7 above LCST.26 In essence, such fluorescence intensity modulation is triggered by the increased microviscosity around the fluorophore through globularization of OEG chains to reduce nonradiative decay

INTRODUCTION Designing thermometers suitable to detect atmospheric and physiological temperature range is an important subject for environmental and biomedical studies.1−6 Fluorescence thermometers based on thermoresponsive organic materials have attracted much attention because they can provide both high spatial and temperature resolution with noninvasive detection.7−12 Typically, thermoresponsive polymers such as poly(Nisopropylacrylamide) (PNIPAM), which displays a lower critical solution temperature (LCST) of 32 °C in water,13 and its derivatives bearing various organic fluorescent dyes have been studied as the fluorescent thermometers, so far.14−22 These polymers undergo a phase transition from a soluble coiled-conformation state below LCST to an insoluble globular state above LCST upon heating due to dehydration of the polymer chains, which results in turbidity. Through incorporating polarity- or viscosity-sensitive organic fluorophores to the polymers, the temperature-induced coil-to-globule transition can be visualized in the fluorescence signal. However, rather difficult polymerization and complexation processes often limit their practical use significantly. In this context, nonpolymeric LCST materials based on supramolecular self-assembly of organic small molecules have only recently emerged as an elegant and simple alternative to fabricate nanothermometers.23−28 Organic small amphiphiles called rod−coil molecules, consisting of hydrophobic rigid rods and hydrophilic flexible coils, are excellent in forming various well-defined self-assembly © 2016 American Chemical Society

Received: October 28, 2016 Accepted: December 27, 2016 Published: December 27, 2016 2883

DOI: 10.1021/acsami.6b13818 ACS Appl. Mater. Interfaces 2017, 9, 2883−2890

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ACS Applied Materials & Interfaces rates of the excited-state of the fluorophore, which in turn gives the simple turn-on switching. However, such fluorescence nanothermometers based on the intensity changes of single emission center have significant shortcoming that the signal is easily affected by some other factors, such as concentration of the fluorophores, excitation power of the instruments, and sensitivity of detectors.45 In contrast, ratiometric fluorescence thermometers can provide more accurate and sensitive signals via the self-calibration effect.46−52 Recently, Wang et al.53 reported a BODIPY-based ratiometric fluorescent thermometer showing three-fold emission enhancement simultaneous with a bathochromic shift upon heating; but the spectral shift was rather limited (∼15 nm shift in λem,max). Thus, to develop a rationally designed rod−coil small molecule showing temperaturedependent ratiometric fluorescence color change is highly desirable and still remains a big challenge. Herein we present the smart fluorescent nanoparticles obtained by the aqueous self-assembly of amphiphilic small molecules based on a dicyanodistyrylbenzene (DCS) rod segment which is grafted by OEG chains on its center or terminal phenyl group (Figure 1). The self-assembled nano-

Scheme 1. Schematic Representation of the Reversible Transformation of 1 from Self-Assembled Core (T < LCST) to Disassembled Core Structure (T > LCST)

Compound 1 is clearly dissolved in water (100 μM) at ambient temperature (∼20 °C). As depicted in Figure 2a, the clear water solution changes to turbid one upon heating which can be reverted to clear solution again upon cooling, indicating that 1 exhibits temperature-responsive LCST behavior. Concomitantly, the aqueous system shows reversible fluorescence color switching upon heating and cooling. In order to illustrate the origin of the fluorescent color change we performed temperature-dependent photoluminescence (PL) experiment. Figure 2b and Figure S1 show the PL spectra of 1 in 100 μM water solution at different temperatures. Compound 1 emits yellow fluorescence (λem,max = 600 nm, ΦF = 0.18) at low temperature (15 °C), while the emission color changes distinctly to green (λem,max = 540 nm, ΦF = 0.17) upon elevating the temperature to 80 °C (Table 1). Figure 2c shows that the ratio of the emission intensities at 540 and 600 nm (I540/I600) is invariant at low temperature but shows linear increase from around 26 °C (I540/I600 = 0.0142T/°C + 0.315, where R2 = 0.985; see Figure S2). Moreover, the temperaturedependent fluorescence ratio changes (I540/I600) of 1 in 100 μM aqueous solution show excellent reproducibility during subsequent 20 heating and cooling cycles (Figure S3). Such a unique temperature-dependent PL emission shift indicates that the packing structure of DCS cores changes upon heating beyond 26 °C. The rod−coil amphiphilic molecules containing hydrophobic aromatic rings and hydrophilic OEG chains usually selfassemble into various supramolecular nanostructures in aqueous solution and have been widely investigated.64−67 Our molecules 1 and 2 also belong to this category but with specially designed DCS cores for thermally induced ratiometric fluorescence modulation. First, the self-assembly behavior of 1 in aqueous solution at 15 °C was investigated by varying its concentrations in water. Upon increasing the concentration from 1 μM to 30 μM in water, 1 showed its molecularly dissolved monomeric emission with its PL maximum unchanged (λem,max = 522 nm). Upon further increase in the concentration, however, the PL maximum was gradually redshifted up to 600 nm at 100 μM with notable emission peak broadening, which can be assigned to an excimeric emission of the self-assembled DCS cores of 1 (Figure 3a). The transition of monomeric to excimeric emission is in accordance with that of β-MODCS molecule which shares the same DCS core as reported earlier.68 This result thus indicates the formation of self-assembled nanostructure due to the amphiphilic nature of 1 augmented by the strong self-assembling tendency of DCS cores when the concentration is above 30 μM which is considered as critical aggregation concentration (CAC). Then, formation of the nanoparticles was directly evidenced by dynamic light scattering (DLS) and energy-filtering transmission electron microscopy (EFTEM) measurements. The

Figure 1. Molecular structures of 1 and 2.

particles made of V-EO7DCS (1) or H-EO7DCS (2) show thermoresponsive, ratiometric, and reversible fluorescence color switching in water. DCS based small molecules, consisting of oligo(p-phenylenevinylene) (OPV) with two electron-withdrawing cyano-groups, are one of the most excellent luminescent materials which have been widely investigated due to their versatility in tuning optical and electronic properties.54−60 These highly luminescent DCS based molecules exhibit strong tendencies toward excimer formation and show pronounced PL emission shift between dilute solution (monomer) and crystalline state (excimer).61−63 By tethering the DCS core to the thermoresponsive OEG chains, which exhibit hydrophilic to hydrophobic transition with LCST behavior upon heating, we expected LCST-tuned packing structure change of DCS core to give ratiometric fluorescence color switching in water (Scheme 1). We have synthesized compounds 1 and 2 differing in a substitution position of OEG unit in DCS to obtain the same fluorescent behavior but with different LCST temperatures.

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RESULTS AND DISCUSSION The target rod−coil molecules 1 and 2 were synthesized according to the Scheme S1 in the Supporting Information (SI). The resulting molecules were characterized by 1H NMR spectroscopy, elemental analysis, and MALDI-TOF mass spectroscopy to be in full agreement with its structures (see the experimental section in the SI). 2884


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Figure 2. (a) Photoimages illustrating the luminescent color and transmittance changes of 1 upon heating in water (100 μM) under 365 nm handheld UV lamp (middle) and room light (side). (b) Temperature-dependent normalized PL spectra of 100 μM 1 in water (The bold solid lines represent 15 and 80 °C, respectively, λex = 353 nm). (c) Plot of fluorescence intensity ratio between 540 and 600 nm (I540/I600) as a function of temperature.

Table 1. Photophysical Properties of 1 in Water λem,max [nm]a ΦF τav [ns]b kr/107 [s−1]c knr/107 [s−1]c

T < LCST, c > CAC (15 °C, 100 μM)

T > LCST, c > CAC (80 °C, 100 μM)

T < LCST, c < CAC (15 °C, 8 μM)

600 0.18 12.4 1.45 6.61

540 0.17 2.8 6.07 29.6

522 0.10 0.7 14.3 128.6

a Maximum emission wavelengths. bIntensity weighted average fluorescence lifetimes. cCalculated using the following equations: kr = ΦF/τav, knr = 1−ΦF/τav.

DLS size distribution curve for 100 μM (c > CAC) of 1 in water reveals formation of the nanoparticles of 108 ± 27 nm size (Figure 3b). An EFTEM image obtained from drop-cast sample of 100 μM aqueous solution of 1 (Figure 3c) also shows uniform spherical nanoparticles with average size of 105 nm, which is consistent with the DLS result. From the DLS and EFTEM results, we can undoubtedly conclude that 1 forms well-defined spherical nanoparticles in aqueous solution above the CAC. We have further investigated the self-assembly states of rod segment DCS cores in 1 with increasing concentration by time-correlated single-photon counting (TCSPC) measurement. Figure 3d shows fluorescence decay profiles of 1 at c < CAC and c > CAC in water, of which the fitting results are shown in Figure S4 and Table S1. It clearly shown that the fluorescence lifetime is greatly extended when the concentration is increased above CAC; the average fluorescence lifetime (τav) is 0.7 ns at c < CAC (8 and 10 μM) but is increased up to 10.3 and 12.4 ns at 80 and 100 μM (c > CAC), respectively. Much longer fluorescence decay lifetimes at the concentrations above CAC strongly indicate excimeric emission features of the DCS cores in their self-assembled nanoparticles.68 The mechanism of LCST behavior of rod−coil molecules containing OEG chains in water has been well established.66,69 At lower temperature below LCST, the OEG chains of the

molecules are stretched freely in water by forming ether−water hydrogen bonds. Raising the temperature above LCST, however, ether−water hydrogen bonds are broken to form globular aggregates and precipitates. The LCST characteristics of 100 μM aqueous solution of 1 (note that CAC is 30 μM) was examined by turbidity measurements in water using 700 nm light source (both 1 and 2 in water have no absorption at 700 nm, see Figure S5 in the SI). As shown in Figure 4a, the transmittance at 700 nm shows sudden drop above ∼26 °C. To the naked eye, the transparent solution gradually changes to turbid one with increasing temperature. This phenomenon is caused by the temperature dependent dehydration of the OEG chains with phase separation of the globular OEG aggregates in water. The onset temperature of this phase transition is named as LCST (26 °C for 1),26 which is completely identical to the onset temperature of fluorescence color change shown in Figure 2c. To investigate the size change of smart fluorescent nanoparticles of 1 as a function of temperature, temperaturecontrolled DLS experiment was performed. As shown in Figure S6, unimodal 108 ± 27 nm sized yellow fluorescent nanoparticles below LCST change to the gradually largersized bimodal globular aggregates (∼400 nm and 6.7 μm at 50 °C) showing green fluorescence, which concomitantly increases the light scattering to give turbidity. The somewhat decrease of the particle size at the highest temperature of 80 °C is 2885


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Figure 3. (a) Normalized fluorescence spectra of 1 aqueous solution at different concentrations in water. Inset shows the un-normalized original fluorescence spectra of 1 in water. (b) DLS result of 100 μM aqueous solution of 1 at 15 °C. (c) EFTEM image obtained from drop-cast sample of 100 μM aqueous solution of 1, scale bar = 0.5 μm. (d) Fluorescence decay profiles of aqueous systems of 1 at different concentrations below (c = 8 and 10 μM) and above (c = 80 and 100 μM) the CAC.

Figure 4. (a) Transmittance changes of 100 μM 1 in water upon increasing temperature measured at 700 nm. (b) Fluorescence decay profiles of 100 μM 1 in water at 15, 30, 50, and 80 °C.

monomer-dominated emission due to the dehydration-induced globularization of OEG chains tethered to them. We have further investigated the assembly states of rod segment DCS cores in 1 with increasing temperature by TCSPC measurement. Figure 4b shows fluorescence decay profiles of 1 (100 μM in water) at various temperatures, of which fitting results are shown in Figure S8 and Table S2. It is clearly noted that the fluorescence lifetime is gradually decreased with increasing temperature; τav of 12.4 ns at 15 °C gradually changes to 10.4, 6.4, and 2.8 ns at 30, 50, and 80 °C, respectively. Such a monotonic decrease of fluorescence decay time suggests that the self-assembled DCS cores showing excimeric emission is broken up to disassembled state showing monomer-dominated emission above the LCST. Interestingly, both the radiative rate constant (kr) and nonradiative rate constant (knr) increase upon heating (15 to 80 °C) to give the decent value of fluorescent quantum yield (0.18 and 0.17, respectively) almost unchanged upon heating. Relatively

presumably due to a severe densification and fast Brownian motion of the globular aggregates. The EFTEM image shown in Figure S7b consistently supports the results of this temperature-varied DLS measurement. To get an insight into the assembling status of DCS cores in these larger size globular aggregates above LCST, we compared their fluorescence behavior with those at the temperature below LCST and also with monomeric emission (8 μM solution) as shown in Table 1. As summarized in Table 1, the PL emission maxima of the T > LCST (80 °C, λex,max = 540 nm, c > CAC) are intermediate between those of T < LCST (c > CAC) and monomer emission (c < CAC, 8 μM, λem,max = 522 nm) but much more close to the latter. From these results, we can deduce that the fluorescent DCS cores of 1 self-assemble together and emit excimeric emission at the temperature below LCST. With increasing temperature above LCST, however, the selfassembled DCS cores are gradually disassembled to emit 2886


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Figure 5. (a) Temperature-dependent normalized PL spectra of 100 μM 2 in water (bold solid lines represent 15 and 80 °C, λex = 374 nm). (b) Plot of fluorescence intensity ratio between 539 and 585 nm (I539/I585) as a function of temperature. (c) EFTEM image obtained from drop-cast sample of 100 μM aqueous solution of 2, scale bar = 0.5 μm. (d) Transmittance changes of 100 μM 1 and 2 in water measured at 700 nm wavelength.

smaller nonradiative rate constant of the smart fluorescent nanoparticles compared to that of monomeric emission is due to the rigidity effect and the smaller radiative rate is due to the exciton coupling effect. On the other hand, at a concentration lower than CAC (c = 8 μM in water), 1 showed no thermoresponsive changes in monomeric fluorescence (Figure S9), indicating that fluorescence color switching occurs only when the concentration is above the CAC. Taken all together, the mechanism of fluorescent color switching upon heating is as follows. At the low temperature aqueous solution below LCST, OEG chains are fully dissolved in aqueous solution and the long wavelength excimer emission are secured due to the strong self-assembly of densely π−π stacked hydrophobic DCS rod segments. With increasing temperature above LCST, however, short wavelength monomer-dominated emission is obtained because hydrophilic OEG coils are dehydrated to form large hydrophobic globules, which concomitantly disassembles the covalently tethered DCS cores. To further evidence this mechanism of smart fluorescent nanoparticle of 1, we also synthesized its structural analog 2 with different tethering position of OEG units but otherwise the same chemical structure. 2 in aqueous solution also shows monomeric emission at the concentration below the CAC (0.8 μM, λem,max = 509 nm, τav = 0.5 ns) and excimer emission above the CAC (100 μM, λem, max = 585 nm, τav = 30.9 ns) (Figures S10 and S11a). Formation of self-assembled 142 ± 35 nm spherical nanoparticles of 2 was measured both by EFTEM and DLS (Figures 5c and S11b). 2 also shows the thermoresponsive fluorescent switching in aqueous solution in exactly the same way as 1 but with different LCST temperature of 58 °C (Figure 5d). As shown in Figure 5a and b, 2 exhibits ratiometric fluorescent color changes from yellow emission (λem,max = 585 nm, ΦF = 0.37) at low temperature (15 °C) to green emission (λem,max = 539 nm, ΦF = 0.29) upon elevating the temperature to 80 °C. It is found that the ratio I539/I585 increases linearly above the LCST temperature (I539/I585 = 0.0159T/°C +

0.0109, where R2 = 0.998; see Figure S12). The significant decrease in the average lifetime (τav from 30.9 to 6.3 ns; see Figure S13) accompanying this fluorescence color change suggests that it is the transition from excimer emission to the monomer-dominated emission alike the case of 1. The globularization with size increase of smart fluorescent nanoparticle of 2 was also observed with increasing temperature by DLS and EFTEM measurement (Figures S14 and S15). It should be noted that LCST of 2 (58 °C) is much higher than that of 1 (26 °C). This implies that the substituted position of OEG chains in the DCS core plays the key role in controlling the LCST. It is most likely that 2 with OEG chains substituted in horizontal direction along molecular long axis has stronger stacking tendency than 1 which has the substitution in the vertical direction with hindering effect of OEG chains against the intermolecular DCS stacking. Therefore, 2 demands higher energy than 1 to break the hydrogen bonding between OEG chains and water. The fluorescence switching is also reversible and reproducible upon heating and cooling cycles (Figure S16). Inspired by the previous studies on tuning the LCST by controlling the ratio of two different polymer building block,70,71 we supposed that 1 and 2 can make coassembly together in water because of their structural similarity. And if so, it is expected that LCST-tunable smart fluorescence nanoparticles can be prepared by varying the mixing ratio of the two components in the conanoparticles. To prove our hypothesis, the 1:1 aqueous mixture solution of 1 and 2 was prepared (the total concentration of 1 and 2 was 100 μM). The EFTEM and DLS measurements (Figure S17) demonstrate that around 100 nm-sized nanoparticles are well formed in the mixture solution. It is noteworthy that the 1:1 mixture nanoparticles exhibits a single LCST transition temperature at 45 °C which is between the transition temperature of 1 (26 °C) and 2 (58 °C). It suggests that 1 and 2 are coassembled together into the nanoparticles. Interestingly, the LCST transition temperature of mixed nanoparticles is easily tuned 2887


ACS Applied Materials & Interfaces by controlling the mixing ratio of 1 and 2 as shown in Figure 6. By increasing the portion of 1 in the mixed nanoparticles from

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Materials. Compound 1 and 2 was synthesized according to the procedure in the Supporting Information. All chemicals were purchased commercially, and used without further purification. Instrumentation. Chemical structures were identified by 1H NMR (Bruker AVANCE-300, 300 MHz) spectroscopy and elemental analysis (Flash 1112 elemental analyzer). The molecular weight of final product was measured with matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra (Voyager DE, STR biospectrometry workstation). Transmission electron microscopy observation was carried out with an energy-filtering transmission electron microscope (EFTEM) (Carl Zeiss, LIBRA 120) operated at 120 kV. For the TEM sample preparation, a drop of aqueous solution was dried on a carbon-coated copper grid. Fluorescence spectra were obtained by a Varian Cary Eclipse fluorescence spectrometer. Single cell Peltier accessory was used to control the sample’s temperature. The absorption and transmittance measurements were performed by UV−visible spectrophotometer (Shimadzu UV 2550). The transmittance was measured at 700 nm wavelength. Time-resolved fluorescence lifetime experiment were measured by the time-correlated single-photon counting (TCSPC) technique with a FluoTime200 spectrometer (PicoQuant) equipped with a PicoHarp300 TCSPC board (PicoQuant) and a PMA182 photomultiplier (PicoQuant). Dynamic light scattering (DLS) experiment was performed by using a DLS-7000 instrument at different temperatures.

Figure 6. Transmittance changes of 100 μM 1 (■), 2 (◀), and their mixed nanoparticles (the molar mixing ratios of 1 and 2 were (●) 2:1, (▲) 1.5:1, and (▼) 1:1) in water measured at 700 nm wavelength.

1:1 to 2:1, the transition temperature gradually decreased to 36 °C, which are very close to the human body temperature. Finally, to test the application potential of these thermoresponsive nanoparticles in the physiological environment, we measured the PL and LCST transition behaviors of 1 and 2 in phosphate buffer solution (0.05 M KH2PO4, pH 7.2) which is widely used in biological research. In the buffer solution, both the nanoparticles of 1 and 2 show almost identical thermoresponsive fluorescence switching behaviors compared with those in water. The emission color of the nanoparticles changes from yellow (λem,max = 603 nm for 1 and 587 nm for 2 at 15 °C) to green (λem,max = 535 nm for 1 and 539 nm for 2 at 80 °C) upon increasing temperature (Figures S18a and S19a). It should also be noted that, even though the salt concentration is relatively high in the buffer solution, the LCST transition temperatures of the nanoparticles are almost unchanged. (Figures S18b and S19b).

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13818. Experimental details for synthesis procedures of compounds 1 and 2, absorption and fluorescence spectrum, DLS, TEM and TCSPC of compounds 1 and 2 in water (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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CONCLUSION

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) through a grant funded by the Korean Government (MSIP; No. 2009-0081571[RIAM041720150013]).

In conclusion, we have developed novel small molecule-based thermoresponsive amphiphilic nanoparticles showing ratiometric fluorescence color changes from excimeric yellow to monomeric green emission upon heating. It is unambiguously found that the reversible fluorescence color change is attributed to the LCST transition of the peripheral OEG units of 1 and 2 which concomitantly alters the stacking structure of the fluorescent DCS cores. Interestingly, although 1 and 2 have same core structure, they exhibit different LCST transition temperature depending on the substitution position of the OEG units. But, because of their structural similarity, 1 and 2 are able to make coassembly in the water and show single LCST transition temperature between the transition temperatures of the two. It should also be noted that the LCST transition temperature of the coassembled nanoparticles is conveniently tuned from 26 to 58 °C by changing their mixing ratios. It is expected that this LCST temperature-tunable smart nanoparticles showing ratiometric fluorescence color change would have a significant impact on the various applications such as fluorescent thermometers and bioimaging.

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