Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Simple and Versatile Platform for Air-Tolerant Photon Upconverting Hydrogels by Biopolymer−Surfactant−Chromophore Co-assembly Pankaj Bharmoria,† Shota Hisamitsu,† Hisanori Nagatomi,† Taku Ogawa,† Masa-aki Morikawa,† Nobuhiro Yanai,*,† and Nobuo Kimizuka*,† †
Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan
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S Supporting Information *
ABSTRACT: Exploration of triplet−triplet annihilation based photon upconversion (TTA-UC) in aqueous environments faces difficulty such as chromophores insolubility and deactivation of excited triplets by dissolved oxygen molecules. We propose a new strategy of biopolymer−surfactant−chromophore coassembly to overcome these issues. Air-stable TTA-UC with a high upconversion efficiency of 13.5% was achieved in hydrogel coassembled from gelatin, Triton X-100 and upconverting chromophores (triplet sensitizer and emitter). This is comparable to the highest UC efficiency observed to date for air-saturated aqueous UC systems. Moreover, this is the first example of airstable TTA-UC in the form of hydrogels, widening the applicability of TTA-UC in biological applications. The keys are two-fold. First, gelatin and the surfactant self-assemble in water to give a developed hierarchical structure with hydrophobic domains which accommodate chromophores up to high concentrations. Second, thick hydrogen-bonding networks of gelatin backbone prevent O2 inflow to the hydrophobic interior, as evidenced by long acceptor triplet lifetime of 4.9 ms. Air-stable TTA-UC was also achieved for gelatin with other nonionic surfactants (Tween 80 and Pluronic f127) and Triton X-100 with other gelling biopolymers (sodium alginate and agarose), demonstrating the versatility of current strategy.
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INTRODUCTION Triplet−triplet annihilation based photon upconversion (TTAUC) is at the forefront of developing technologies for nextgeneration renewable energy devices, mainly on account of its ability to function at low excitation intensity (mW cm−2) and with noncoherent excitation sources.1 TTA-UC typically occurs in multichromophore systems (Figure S1 of the Supporting Information, SI) wherein excited triplet energy of sensitizer is transferred to an acceptor via Dexter energy transfer. When two sensitized acceptor triplets collide, the annihilation produces a high energy singlet state (S1) which radiates the delayed anti-Stokes fluorescence.1−13 Besides its potential to enhance the efficiency of sunlightpowered devices, TTA-UC has also become the focus of attention in various biological applications including bioimaging,14,15 photodynamic therapy,16 and drug delivery.17 Although such in vivo operation of TTA-UC requires efficient upconversion in aerated aqueous environments, it is confronted by two major problems: (1) limited solubility of hydrophobic sensitizer and emitter dyes in water and (2) quenching of triplet state by dissolved molecular oxygen.18 Hydrophobic interiors of aqueous micelles19 and lipid bilayers20,21 have been employed as matrixes to dissolve hydrophobic TTA-UC dyes, however, they require deaerated conditions or addition of oxygen scavengers in high © XXXX American Chemical Society
concentration. Several research groups have tried to overcome these issues by using water-dispersible upconverting micro/ nanoparticles coated with silica or polymer shell, in which pairs of sensitizer/emitter were dissolved in viscous organic solvents with or without oxygen scavengers.13,22−27 The triplet energy transfer and annihilation operating via diffusion mechanism are inevitably suppressed in such viscous environments, and consequently, maximum UC efficiency (100% theoretical maximum) had been limited to 4.5%.13 Simon and Weder et al. has recently prepared TTA-UC nanoparticles consisting of poly(ε-caprolactone)-tagged emitter and sensitizer by oil-inwater microemulsion technique, whose aqueous suspension showed thermoresponsive UC emission with a high UC efficiency of 15% under nondeoxygenated conditions.28 Alternatively, our group developed photon harvesting TTAUC systems based on triplet energy migration in acceptor molecular assemblies29,30 and extended this concept to aqueous systems.31 The amphiphilically designed cationic acceptors self-assemble in water to give molecular membranes in which emitter chromophores are organized in the hydrophobic interior.31 In these systems, the emitter chromophores are well kept away from the aqueous interface Received: June 3, 2018 Published: July 27, 2018 A
DOI: 10.1021/jacs.8b05821 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society
Figure 1. A schematic representation of G-TX-DPAS-PtOEP coassembly in photon upconverting hydrogel. Donor (PtOEP) and acceptor (DPAS) molecules are accumulated in the nonpolar domains of gelatin-TX100 hydrogel. The photoexcitation of donor is followed by a sequence of donorto-acceptor triplet−triplet energy transfer (TTET), triplet energy migration (TEM), and triplet−triplet annihilation (TTA) among the acceptor molecules, and upconverted fluorescence from the acceptor excited singlet state. The hydrogen-bond networks of gelatin work as the oxygen barrier layers to protect the excited triplet state from the quenching by molecular oxygen.
this is the first report of air-stable photon upconverting hydrogels which are designed based on the concept of synergistic biopolymer−surfactant interactions. The current bioinspired strategy is simple and versatile, and opens up a new potential of biopolymers for their applications in advanced light-harvesting systems, which would also have spillover effects on pharmaceutical, cosmetic, and detergent industries.35−37
and the formation of hydrogen bond networks around the chromophores lead to efficient triplet energy migration and high UC efficiency of 7% in air-saturated water.31 While these approaches are useful, the synthesis of such amphiphilic or polymeric acceptors involve an immense amount of time and effort. It is desired to develop a more straightforward methodology that allows accumulation of TTA-UC chromophores in hydrophobic microenvironments in water without the use of organic solvent and to secure protection from the quenching by dissolved oxygen. In this work, we show a simple and versatile methodology to achieve air-stable photon upconversion in supramolecular hydrogels via protein−surfactant−chromophore coassembly. We took this inspiration from the naturally occurring spectrinbased membrane cytoskeleton that serves as a scaffold of plasma membranes,32 and the pigment−protein complexes in bacterial light-harvesting antennas wherein chlorophylls and bacteriochlorophylls are arranged in exquisite geometries.33,34 As a proof of concept, photon upconverting hydrogels were self-assembled from protein gelatin, nonionic surfactant Triton-X 100 (TX100), and a donor−acceptor UC pair; Pt(II) octaethylporphyrin (PtOEP) and sodium 9,10-diphenylanthracene-2-sulfonate (DPAS) (Figure 1). The donor and acceptor chromophores are accumulated in the nonpolar surfactant domains covered with thick gelatin hydrogen-bond networks that exerted remarkable O2 blocking ability. Consequently, the synergic protein−surfactant interactions drive efficient green-to-blue TTA-UC in air-saturated hydrogels. We found that the modification of acceptor with sulfonate group is crucial to the high molecular dispersibility in the hydrogels by effective interactions of polar sulfonate group with gelatin and TX100, which was reflected in the high transparency of the hydrogels. This is in stark contrast with neutral 9,10-diphenylanthracene (DPA) that formed turbid hydrogels with lower UC efficiency due to the aggregation of DPA. In addition to the combination of cationic gelatin and neutral TX100, we further generalize the current design concept with other nonionic surfactants such as Tween 80 and Pluronic f127, and with other biopolymers, anionic sodium alginate, and neutral agarose. To the best of our knowledge,
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EXPERIMENTAL SECTION
Materials. All the solvents and reagents were used as-received. Platinum(II) octaethylporphyrin (PtOEP), 9,10-diphenylanthracene (DPA) and gelatin type A from porcine skin were purchased from Sigma-Aldrich. Sodium alginate was purchased from Wako, agaroseLE was purchased from Nacalai Tesque, Triton X-100, and Tween-80 were purchased from Chameleon, and Pluronic f127 was purchased from AnaSpec. Inc. All the chemicals were used as obtained otherwise noted. Sodium 9,10-diphenylanthracene-2-sulfonate (DPAS) was synthesized according to our previous work.38 Optical Measurements. UV−vis absorption spectra were recorded on JASCO V-670 and V-770 spectrophotometers. Luminescence spectra were measured by using a PerkinElmer LS 55 fluorescence spectrometer. Circular dichroism spectra were recorded on a JASCO J-820 spectrophotometer. Due to the high concentration of DPAS and gelatin, the spectra were recorded in a quartz cell having 0.025 mm path length adjusted with a Teflon spacer (FLAB50-UV-01, GL Sciences Inc.). The hot solution of gelatin, TX100, and chromophores mixture was poured into the cell and allowed to undergo gelation for overnight at room temperature. Time-resolved photoluminescence lifetime measurements were carried out by using a time-correlated single photon counting lifetime spectroscopy system, HAMAMATSU Quantaurus-Tau C11367−02 (for fluorescence lifetime)/C11567−01(for delayed luminescence lifetime). The quality of the fit has been judged by the fitting parameters such as χ2 (
