Tumor Imaging Based on Photon Upconversion of ... - ACS Publications

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Letter

Tumor Imaging Based on Photon Upconversion of Pt(II) Porphyrin Rhodamine Co-modified NIR Excitable Cellulose Enhanced by Aggregation Atsushi Nagai, Jason B. Miller, Petra Kos, Sussana Elkassih, Hu Xiong, and Daniel John Siegwart ACS Biomater. Sci. Eng., Just Accepted Manuscript • DOI: 10.1021/acsbiomaterials.5b00389 • Publication Date (Web): 27 Oct 2015 Downloaded from http://pubs.acs.org on November 1, 2015

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ACS Biomaterials Science & Engineering

Tumor Imaging Based on Photon Upconversion of Pt(II) Porphyrin Rhodamine Co-modified NIR Excitable Cellulose Enhanced by Aggregation Atsushi Nagai,*1,2 Jason B. Miller,1 Petra Kos,1 Sussana Elkassih,1 Hu Xiong,1 and Daniel J. Siegwart*1 1

The University of Texas Southwestern Medical Center, Simmons Comprehensive Cancer

Center, Department of Biochemistry, Dallas, Texas 75390, USA. 2

Institute for Molecular Science, National Institutes of Natural Sciences, Department of

Materials Molecular Science, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan.

KEYWORDS Photon upconversion, cellulose, biomedical imaging, cancer.

ABSTRACT

We report a bio-inspired upconversion (UC) system using a cellulose template, in which an aggregated platinum (II)-tetraphenylporphyrin (PtTPP) sensitizer is able to excite Rhodamine B as an emitter, enabling near-infrared (NIR)-to-orange wavelength conversions. The comodified cellulose was observed to undergo J aggregation of PtTPP in DMSO solution, as

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indicated by broad, weak absorption bands in the NIR region of the absorption spectrum. Excitation of these NIR J aggregation peaks of PtTPP led to efficient UC emission in the orange wavelength region. These materials were shown to exhibit UC properties in biological settings both in vitro and in vivo, demonstrating utility of UC for tumor imaging.

TEXT (Letter format) Recently, there has been an exciting convergence of materials science and optics, where a variety of solid materials have been able to upconvert lower energy optical radiation into photons of higher energy via the triplet-triplet annihilation photon upconversion (TTA-UC) process.1-3 The excitation of lower energy photons (longer wavelength) followed by emission at higher energy (shorter wavelength) has been shown to proceed under various conditions.4 Materials capable of TTA-UC are attractive for many applications including solar cell devices5 and solar fuels.6 Recently, they have gained popularity in the biomedical imaging field7-11 because these materials can be excited in the so-called near-infrared (NIR) “imaging window” where there is minimal tissue absorbance and autofluorescence and emit light in regions more easily detected by existing equipment.12,13 Biomaterials with elegant properties have been the focus of recent research in the development of next-generation materials for various applications.14,15 Polymer templates are excellent candidates for the construction of ordered bio-inspired structures due to low cost, welldefined architecture, and robust and facile chemical processing.16,17 Cellulose is a highly abundant biopolymer that has been utilized in a variety of functional nanostructures including fibers, hydrogels, films, and microspheres.18-20 It is known that the identity and molecular


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orientation of the sensitizer / emitter pair is critical for successful TTA-UC. Using the UC mechanism, it was reported that cellulose nanofiber-based liquid core capsules containing sensitizer-emitter pairs showed high upconversion abilities when embedded in pristine cellulose nanofiber films, but was limited to air free conditions.11 Inspired by this work, we utilized copper-catalyzed click chemistry to quantitatively functionalize cellulose with a platinum (II)tetraphenylporphyrin (PtTPP) sensitizer and Rhodamine B emitter. Precise design and synthesis of the donor (sensitizer) and acceptor (emitter) pair is required for the efficient intensity of UC luminescence light. The energy levels of the acceptor triplet excited state (3T*) must be lower than the donor 3T* for the triplet-triplet energy transfer (TTET) to occur.1 In organic TTA-UC systems, efficient emission has been achieved using numerous fused polycyclic aromatic hydrocarbons and heterocyclic compounds as acceptors.3 These emitters have been successfully paired with sensitizers comprised of porphyrin and phthalocyanine metallo-complex derivatives in TTA-UC systems.21,22 Porphyrins are known to undergo pH dependent J aggregation in high polarity solvents.23 Furthermore, J aggregates of platinum (II) octaethyl porphyrin (PtOEP) in blends of inert polymer matrices have been proposed to be responsible for luminescence peaks at 800 nm.24 Hence, we explored aggregation of a biocompatible template, and excitation using NIR peaks, to enhance TTA-UC for biomedical imaging applications. These cellulose-based UC materials were formulated into nanoparticles using nanoprecipitation and demonstrated UC emission in biological settings both in vitro in cancer cells and in vivo in tumors in mice. In this Letter, we report a novel co-modified cellulose templated TTA-UC system employing PtTPP sensitizers and Rhodamine B (Rho) as emitters (Scheme 1). PtTPP was selected due to previously demonstrated utility as a sensitizer for photon upconversion materials and the ability of TPP to undergo J-aggregation.23 Surprisingly, NIR excitation of the J


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aggregation peak enabled strong emission in orange wavelength range, which was red-shifted due to aggregation induced emission enhancement (AIEE). This is, to the best of our knowledge, the first example of a Rhodamine upconversion TTA system that utilizes the excitation of J aggregated NIR band of PtTPP. The ability to excite in NIR region and detect emission in the orange region makes it a promising system for applications including biomedical imaging.

Scheme 1. A three component-click reaction of azide-cellulose with sensitizer and emitter alkyne compounds was used to prepare the co-modified cellulose templated TTA-UC system.

We utilized copper-catalyzed click chemistry to quantitatively functionalize cellulose with optimal content of emitter and sensitizer molecules. First, cellulose was modified to contain azide groups by tosylation of cellulose and subsequent nucleophilic displacement. In a typical experiment, 6-O-tosylcellulose with a degree of substitution of 0.97 could be prepared25 and


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transferred to 6-azide-6-deoxy cellulose (N3-cellulose). The IR spectrum of N3-cellulose shows a significant signal at 2110 cm-1 typical for the azide moiety and complete disappearance of the aromatic bands of the tosyl functionality at 1580 and 1453 cm-1 (Figure S1). Thus, cellulose with 99% azide modification was obtained and could be further utilized in click reactions to attach the two alkyne-functionalized UC components. Alkyne-functionalized Rhodamine B emitters were synthesized by simple condensation with propargyl alcohol, while alkynyl-PtTPP was prepared by esterification of the parent porphyrin acid with propargyl bromide (Scheme S1). N3-cellulose was reacted with Rhodamine B alkyne ester (Rho corresponding to total amounts of azide and sensitizer contents), and a small amount of sensitizer PtTPP alkyne (0.06, 0.12, and 0.75%) as shown in Scheme 1. It was found that increasing the amount of sensitizer led to quenching of UC-TTA. Singlet Förester resonance energy transfer from emitter to sensitizer was also observed at higher doping ratios. N3-cellulose modification reactions were performed in a mixed solvent of DMSO and water at 100 °C for 2 days. Cellulose-Rho99.94%PtTPP0.06%, Cellulose-Rho99.88%PtTPP0.12%, and Cellulose-Rho99.25%PtTPP0.75% were obtained as powdery pale pink solids in quantitative yields. Evidence for the successful click reaction of the alkyne groups in the emitters with the azide groups on cellulose was obtained from IR spectroscopy (Figure S2). The azide peaks of Cellulose-Rho99.25%PtTPP0.75% disappear and signals of the ester carbonyl groups for respective emitter or sensitizer are observed at 1740 and 1590 cm-1, confirming the covalent binding of the emitters and sensitizer moieties. Figure 1a shows the normalized absorption of monomeric PtTPP and Rho in DMSO at 25 °C. The PtTPP sensitizer showed a strong Soret band at 400 nm and Q-bands at 515 nm. Although the maximum absorbance of Cellulose-Rho99.98%PtTPP0.12% possesses the strong Soret


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band at 405 nm, a clear Q-band of cellulose-bearing sensitizer PtTPP was not observed around at 500 nm. The absorption maximum of Rhodamine B was at 565 nm and the weak J-aggregation broad peak around at 750-900 nm, as compared to free PtTPP (405 nm as Soret band) and monomeric Rho (560 nm as Rhodamine B absorption maximum), whereas the Q-band of PtTPP (515 nm) in the Cellulose-Rho99.88%PtTPP0.12% overlapped with the Rhodamine B moiety (565 nm) on the surface of cellulose (Figure 1b). As a result, cellulose equipped with Rhodamine B emitter and PtTPP sensitizer would have the J-aggregated construction due to aggregation induced self-assembly containing dimer-like edge-to-edge association from platinum (II) porphyrin arrays on the surface of the cellulose template.

Figure 1. UV-vis absorption of (a) monomeric PtTPP and Rho, and (b) CelluloseRho99.88%PtTPP0.12% in DMSO (c = 0.25 g/L). The inset red curve and red arrow show the PtTPP J aggregate peaks used for UC excitation.


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Next, we investigated the UC efficiency of Cellulose-Rho99.94%PtTPP0.06%, CelluloseRho99.88%PtTPP0.12%, and Cellulose-Rho99.25%PtTPP0.75% upon excitation of the aggregation absorption area (850 nm) in DMSO solution (c = 0.5 g/L) (Figure 2). A longpass cut-in filter (550 nm) was used to eliminate low wavelength excitation beams and prevent two-photon excitation of Rhodamine B emission. By exciting at 850 nm and removing all excitation wavelengths below 550 nm, we could ensure proper measurement of higher energy emission coming from TTA-UC and not from Rhodamine B excitation alone. Cellulose-Rho-PtTPP shows weak orange emissions around 595 nm, meaning that each Rhodamine molecule was located on efficient triplet energy migration between the emitters in spite of aggregation state from cellulose templating. In other words, the chemical structure and morphology of cellulose may position the UC components for effective UC via aggregation and induction of self-assembly and Jaggregation. Although higher concentration of the sensitizer is desirable to generate excited triplet states with higher density, the donor/acceptor molecular ratios employed were optimized in terms of UC intensity. Increasing the sensitizer content resulted in the quenching of UC under higher aggregation states of emitter/sensitizer-appending celluloses due to poor solubility. As a result, the sensitizer content for efficient TTA was fixed to be Cellulose-Rho99.88%PtTPP0.12%. The optimization of excited wavelengths of Cellulose-Rho99.88%PtTPP0.12% in DMSO (c = 0.5 g/L) is shown in Figure S3. Different excitation wavelengths of CelluloseRho99.88%PtTPP0.12% from the range of 700 nm to 850 result in larger increase of the UCPL intensity. On the other hand, the aggregation state loaded on the NIR wavelength led to efficient triplet energy transfer from donor to acceptor, i.e., the aggregated donors have higher internal conversion efficiency than monomeric acceptors. Intriguingly, this UC-TTA system enabled large anti-Stokes shifts from NIR to orange wavelengths (>250 nm).


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Figure 2. Photoluminescence of Rho monomer upon the excitation at 415 nm in DMSO (c = 1.0 × 10-5 g/L) (a) and UC photoluminescence of Cellulose-Rhox%PtTPPy% upon the excitation at 850 nm in DMSO (c = 5.0 × 10-1 g/L) (b).

The concentration dependence of Cellulose-Rho99.88%PtTPP0.12% in DMSO (0.10 – 0.50 g/L) was examined relative to UC intensity (Figure 3). The concentration change of the Rhoappended cellulose allows gradual increase of the UC-TTA emission, suggesting that aggregation-enhanced emission (AEE) nature of the Rho skeleton led to the UC-TTA emission enhancement in an aggregated state.26 The observed red shift is likely due to aggregation-induced emission enhancement (AIEE). As a result, this UC-TTA strategy can lead to next-generation systems by utilizing AIEE molecules as emitters. A possible mechanism for this process is illustrated in Figure S4. PtTPP molecules can form J aggregates enabling excitation at nearinfrared wavelengths.27,28 We hypothesize that the formation of J aggregation of PtTPP


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molecules is capable of producing an S0 → S2 transition,27,28 enabling PtTPP aggregates to be excited at near-infrared wavelengths. At the same time, Rhodamine B as an emitter undergoes AIEE aggregation-enhanced environment between Rho molecules. Then, singlet population from S2 to S1 generates internal conversion (IC). The photon energy is absorbed by the PtTPP sensitizer and stored in its triplet state via a process of inter-system crossing (ISC). This energy is then transferred to the Rho emitter by triplet-triplet energy transfer (TTET). Next, the excited triplet states of two emitter molecules undergo triplet-triplet annihilation (TTA), in which one emitter molecule returns to its singlet ground state and the other molecule gains the energy of both triplet states and is excited to the higher singlet state. As the singlet state emitter molecule decays back to the ground state, a fluorescence photon is emitted. The red shift is due to aggregation-induced emission enhancement (AIEE). Overall, templated organization effects and aggregation of the cellulose template enables transfer of the UC photon from the PtTPP sensitizers to the Rho emitters, allowing detection of the UCPL emission.


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Figure 3. UC photoluminescence of Cellulose-Rho99.88%PtTPP0.12% upon the excitation of 850 nm in DMSO (c = 0.1 – 0.5 g/L) In order to demonstrate the utility of these TTA-UC probes in biomedical applications, imaging studies were performed both in vitro and in vivo, which confirmed the UC phenomenon in biological settings (Figure 4). To help promote the sensitizer aggregation and enhance UCPL emission for imaging applications, Cellulose-Rho99.88%PtTPP0.12% nanoparticles (d = 350 nm) were prepared by nanoprecipitation (Figure S6). A Tween 20 coating was used to improve stability in PBS (Figure S5-S6). In vitro studies in HeLa cells by confocal laser scanning microscopy confirmed the UC properties of the co-modified cellulose. Although aggregation was observed, the nanoparticles exhibited strong emission at 600 nm with both direct excitation of the rhodamine moiety (λEx 561 nm) and excitation of the PtTPP J aggregates (λEx 850 nm), which were also shown to co-localize. This phenomenon was also preserved in vivo after intratumoral injection in mice bearing Huh7 xenograft tumors, and demonstrated tumor retention of these materials. Importantly, excitation of monomeric Rhodamine B showed fluorescence on direct excitation, but not using the upconversion parameters (Figure S6). Future studies will focus on optimization of nanoparticle stability and the introduction of stimuli responsive aggregation, which will greatly broaden the utility of these materials as biomedical imaging materials.


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Figure 4. Confocal imaging of Cellulose-Rho99.88%PtTPP0.12% in vitro (top panels) in cell culture and in vivo (bottom panels) in xenograft tumor sections. Rhodamine B (red) was directly excited at 561 nm, while the TTA-UC material (green) was excited at 800 nm.

Nuclei were

counterstained with DAPI and the channels merged to show co-localization (yellow, indicated by arrowheads for in vivo images). Scale bars for in vitro = 20 µm, and for in vivo = 50 µm.

In conclusion, we have succeeded in efficient upconversion photoluminescence (UCPL) of Rhodamine B via a bio-inspired cellulose template TTA system upon the excitation of J aggregation absorbance of PtTPP. A small amount of conjugated PtTPP into the cellulose results in J aggregation of PtTPP and aggregation-induced emission enhancement of Rhodamine moieties. Orange emission peaks of Rhodamine B moiety in the cellulose are red-shifted, depending on the concentration. Increasing the concentration of cellulose with Rhodamine B and


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PtTPP moieties leads to higher aggregation state and enhancement of UCPL intensity. Our cellulose templated UC system employs an aggregated sensitizer of PtTPP and should enable UC to other emitters, allowing for tunable TTA-UC. Therefore, in principle, future systems can likely excite RGB emitters, enabling near-infrared (NIR)-to-red, NIR-to-green and NIR-to-blue wavelength conversions. Importantly, the cellulose-Rho-PtTPP UC material showed utility as an imagining probe in a proof-of-principle biomedical imaging experiment both in vitro and in vivo. We are currently working on expanding the scope of AIE potential applications in the field of biomedical imaging, organic EL, and solar cells, utilizing broader ranges of emissions.

ASSOCIATED CONTENT Supporting Information. Equipment, Materials, Methods and additional Figures appear in the SI. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Correspondence should be addressed to A.N. ([email protected]) and D.J.S. ([email protected])

ACKNOWLEDGMENT A.N. gratefully acknowledges support from Professor Ohmine Iwao, the Director-General of the Institute for Molecular Science (IMS) in Japan. D.J.S. acknowledges grant support from the


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Cancer Prevention and Research Institute of Texas (CPRIT) (R1212) and the Welch Foundation (I-1855). J.B.M. acknowledges fellowship support from CPRIT (RP140110). We acknowledge assistance from the UT Southwestern Live Cell Imaging Facility, a Shared Cancer Center Resource supported by an NCI Cancer Center Support Grant (1P30 CA142543-05).

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