Highly Photostable Near-IR-Excitation Upconversion Nanocapsules

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Highly-photostable NIR-excitation upconversion nanocapsules based on triplet-triplet annihilation for in vivo bioimaging application Qian Liu, Ming Xu, Tianshe Yang, Bo Tian, Xinglin Zhang, and Fuyou Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17929 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 10, 2018

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ACS Applied Materials & Interfaces

Highly-photostable NIR-excitation upconversion nanocapsules based on triplet-triplet annihilation for in vivo bioimaging application Qian Liu,† Ming Xu,† Tianshe Yang,† Bo Tian,† Xinglin Zhang,‡ Fuyou Li*† ADDRESS: †Department of Chemistry, State Key Laboratory of Molecular Engineering of Polymers, Fudan University, 220 Handan Road, Shanghai 200433, P.R. China ‡ Institute of Advanced Materials (IAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing 211816, China

KEYWORDS: NIR, Triplet-triplet annihilation, upconversion luminescence, enhanced photostability, bioimaging

ABSTRACT: Triplet-triplet annihilation based upconversion (TTA-UC) imaging boasts a lowexcitation irradiance and an uncanny lack of auto-fluorescence interference. Owing to these promising features, this approach has been the subject of intensifying investigation. Despite the ideal features, the classical approach of TTA-UC imaging suffers from some crucial drawbacks. A major deficiency of the system lies within its poor photo-stability, especially for NIR excitation system. Here we report a reduction strategy to improve the TTA-UC photostability.

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The poor photostability of TTA-UC can be attributed singlet oxygen generation by the sensitizer under irradiation. We control the singlet oxygen by including a reductive solvent, which consumes the singlet oxygen, therby improving the TTA-UC photostability. We also prepared TTA-UC nanocapsules with reductive solvent soybean oil inside.In comparison to non-reductive solvents such as toluene and soybean oil, our system shows a significant enhancement to the TTA-UC photostability. The prepared TTA-UC nanocapsules was then used for whole-animal deep imaging with high signal to noise ratio.

Upconversion is an anti-Stokes’ process in which low-energy photons are converted into highenergy photons.1 Both lanthanide doped upconversion2-6 and triplet-triplet annihilation-based upconversion (TTA-UC) can effectively achieve upconversion luminescence.7-8 TTA-UC exhibits a high quantum efficiency (up to 76%)

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and an intense absorption coefficient (∼10-17

cm2)10. The approach, which involves the triplet state energy transfer from a sensitizer to an annihilator is the subject of intensifying investigation,11-13 with a special emphasis in the bioimaging field. TTA-UC bioimaging can eliminate the auto-fluorescence from biological tissue, which yields a high signal to noise ratio even under sub-optimal conditions such as those with low irradiance (mW/cm-2). Several TTA-UC emitting nanomaterials have been developed for low-irradiance excitation bioimaging of living cells and small animals.10, 14-17 The excitation and emission wavelengths in the reported TTA-UC systems for bioimaging are located in the visible region,

10, 14-17

which strongly limits their application for deeper whole-body imaging.

Therefore, it is certainly worth red-shifting both the excitation and emission wavelengths into a relatively longwavelength region (>600 nm) for improved penetration depth for in vivo observation.18


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A major stumbling block in long-wavelength TTA-UC systems is that they suffer from a poor photostability. In most long-wavelength TTA-UC systems, polycyclic aromatic hydrocarbons with a large π-conjugated degree are used as emissive annihilators8. In an aerated environment, the annihilators are easily bleached under the irradiation of the excitation light, which terminates the TTA-UC process. For example, Castellano and coworkers reported a 730-nm excitation and 560-nm emission TTA-UC system, in which palladium(II) octabutoxyphthalocyanine (PdPC(OBu)8) and rubrene were used as a sensitizer and an annihilator, respectively.19 In this TTA-UC system, the annihilator (rubrene) was rapidly bleached under the exposure of 730 nm laser in an aerated environment. The photobleaching was attributed to the singlet oxygen generated by sensitizer under light irradiation in an aerated environment.20 A typical energy transfer mechanism involved in TTAUC is that the sensitizer transfers its triplet energy to the annihilator, which then triggers the TTA between two triplet annihilators (Figure 1a) 8. In the presence of oxygen, there is an alternative energy transfer process in which the triplet energy of sensitizer or annihilator transfers to oxygen, generating the singlet oxygen.21 The large π-conjugated degree annihilators(eg. rubrene and 5,12-bis(phenylethynyl)tetracene (BPEN)), are oxidized by the generated singlet oxygen, terminating the TTA-UC. There is no way around the singlet oxygen as the presence of oxygen in most bio-systems. Thus, a major hurdle to the biological application of TTA-UC systems is the photostability under the presence of oxygen.


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Figure 1. (a) Generalized energy-level diagram of the upconversion processes between the triplet state of the sensitizer molecule and the triplet annihilator molecule leading to singlet delayed luminescence, the interaction of sensitizer or annihilator with oxygen. (b) the absorption and fluorescence spectra of BPEN and PdPC(OBu)8, the molecular structure of PdPC(OBu)8 and BPEN. Here, we propos a strategy using reductive solvent to improve the photostability of TTA-UC. The reductive solvent consumes the generated singlet oxygen, protecting the annhilator from oxidation, and thus maintains the upconversion system stability in the presence of light and oxygen. In addition, the consumption of oxygen, a quencher of triplet state, can improve the upconversion quantum efficiency.22-24 We chose a NIR-excitation non-photostable TTA-UC system in which PdPC(OBu)8 is the sensitizer and 5,12-bis(phenylethynyl)tetracene (BPEN) (Figure 1a) as the annihilator at anexcitation of 730 nm and emission of 610 nm. In aerated toluene, the 610 nm upconversion emission disappears within 1 minute of irradiation with a 730 nm laser (1.592 W cm-2). We also investigate whether the reductive strategy can improve the


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photostability of the TTA-UC system. We then prepare TTA-UC nanoparticles (TTA-UCNCs) by encapsulating PdPC(OBu)8&BPEN into reductive nanocapsules, and investigate whether these TTA-UCNCs can be used for whole-body deep imaging of small animals. To assess the efficiency of reduction for enhancing the photostability of upconversion system, we first investigate the upconversion properties of PdPC(OBu)8&BPEN in different solvents. We use reductive solvents such as soybean oil (SO), oleylamine (OM), linoleic acid (LA), oleic acid (OA) and dimethylsulfoxide (DMSO), as well as non-reductive solvents, such as toluene (TOL), dimethyl formamide (DMF), chloroform(CF), octadecene (ODE), and paraffin oil (PAR). As show in Figure 2a, PdPC(OBu)8&BPEN in all the degassed solvents show a bright orange upconversion emission. In the aerated environment, it is difficult to capture the orange signal by camera in non-reductive TOL, DMF, CF, ODE, and PAR solvents. On the other hand, bright upconversion luminescences can be observed by the naked eye in reductive solvents of SO, OM, OA, LA, and DMSO.


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Figure 2. Upconversion properties of PdPC(OBu)8&BPEN in different solvents. (a) upconversion luminescence photographs in toluene (TOL), dimethyl formamide (DMF), chloroform(CF), octadecene (ODE), paraffin oil (PAR), soybean oil (SO), oleylamine (OM), linoleic acid (LA), oleic acid (OA) and dimethylsulfoxide (DMSO); emission spectra of PdPC(OBu)8&BPEN in toluene (b), and in soybean oil(c) under irradiation of 730 nm laser; the absorption spectra of PdPC(OBu)8&BPEN with different irradiating duration of 730-nm laser in


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toluene (d) and soybean oil (e); The concentration of PdPC(OBu)8 is 1.0×10-5 mol L-1 and BPEN is 2.0×10-3 mol L-1, the excitation power density is 1.592 W cm-2 .

The upconversion emission spectra and absorption spectra of PdPC(OBu)8&BPEN in toluene (non-reductive) and soybean oil (reductive) were obtained to quantify the effect of the reductive agent on upconversion luminescence. As shown in Figure 2b, the peak around 610 nm in the spectra is the upconversion emission, while the wide peak 740-850 nm belongs to the fluorescence of PdPC(OBu)8. Under an aerated environment, the upconversion emission in toluene is hard to be detected and the emission is only 0.1% of that observed in the degassed environment. In the aerated soybean oil, the upconversion emission showed an enhanced upconversion emission at 96% of that in degassed soybean oil. Figure S2 showed the upconversion luminescence spectra in different reductive solvents. The absorption spectra monitors the change of characteristic absorption peaks of BPEN (400-600 nm). The peaks of the 730 nm laser irradiated PdPC(OBu)8 &BPEN in toluene decreases quickly under irradiation, and fully disappears within a minute. We demonstrated the fast photo-bleaching should be due to the presence of sensitizer PdPC(OBu)8 (Figure S3). On the other hand, in soybean oil BPEN exhibits excellent stability under irradiation (Figure 2e). We also test the upconversion quantum efficiency (ΦUC) of PdPC(OBu)8&BPEN in different solvents (Table 1). In accord with the above mentioned results relatively high ΦUC was obtained in aerated reductive solvents. However, the ΦUC in aerated TOL, DMF, CF, ODE, or PAR, is close to zero. Based on the data, the reduction strategy has great potential to improve the upconversion photostability in aerated environments.


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Table 1. Upconversion quantum efficiency (ΦUC), for the system of PdPC(OBu)8&BPEN in different solvents. PdPC(OBu)8 in benzene was chosen as reference. Toluene (TOL), dimethyl formamide (DMF), chloroform (CF), octadecene (ODE), paraffin oil (PAR), soybean oil (SO), oleylamine (OM), linoleic acid (LA), oleic acid (OA) and dimethylsulfoxide (DMSO).

Solvent

TOL DMF

CF

ODE

PAR

SO

OM

OA

LA

DMSO

a

0.01

~0

0.01

0.02

0.01

1.14

2.13

0.75

0.53

1.47

2.82

1.91

1.62

1.39

0.66

1.23

2.20

0.93

0.55

1.69

ΦUC(%)

b a)

ΦUC(%)

indicated under the aerated environment, and b) indicated under degassed environment.

The continuous kinetic scan of upconversion emission was measured to confirm the effectiveness of the reduction process to photostability. As shown in Figure 3, the photostability of the system in reductive solvents such as SO (T90% =1658 s), OM (T90% =2056 s), OA(T90% =1481 s), LA(T90% =1213 s), and DMSO(T90% =294 s) were greatly enhanced compared to the stability in non-reductive solvents (T90%, the scanning duration when the upconversion emission intensity dropped down to the 90% of the initial, is 3 s in toluene). Furthermore, a volume % as low as 5% of soybean oil in toluene can improve the upconversion photostability (T90% = 78 s) a lot. We demonstrate that the presence of a reductive agent can enhance the upconversion photostability. In addition, previously reported photo-unstable sensitizer and annihilator couples in air (PdPC(OBu)8&rubrene), were adopted19 to demonstrate the generality of this reductive strategy (Figure S4).


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Figure 3. Continuous kinetic scan of TTA-UC emission of PdPC(OBu)8&BPEN in different solvent, including SO, OM, LA, OA, DMSO, TOL, PAR, ODE, DMF and the mixture of TOL and SO, in aerated environment under continuous irradiation of 730 nm laser. The concentration of PdPC(OBu)8 is 1.0×10-5 mol L-1 and BPEN is 2.0×10-3 mol L-1, the excitation power density is 1.592 W cm-2.

To investigate the mechanism of the photostabilization by the reductive agents, we dissolved the sensitizer of PdPC(OBu)8 (1.0×10-5 mol L-1) and reductive agent (LA, OA or DMSO) in dtoluene, and compared the 1H NMR before and after the irradiation with 730 nm laser. As we expected, the peak assigned to -HC=CH- of LA (Figure S5) and OA (Figure S6) disappeared after irradiation of 730 nm laser, indicating cleavage of the carbon-carbon double bond. In the sample containing DMSO, a new peak attributed to dimethyl sulfone (-CH3, Figure S7) was observed. The above described 1H NMR results indicated that reductive solvent can react with the singlet oxygen generated by sensitizer of PdPC(OBu)8 under irradiation, thus protecting the annihilator from oxidation by singlet oxygen. Soybean oil was used as the solvent for next studies, given it has been approved by FDA as an injectable component. In soybean oil, the quadratic dependence of the upconversion emission


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intensity on laser power density was demonstrated in the range of lower power (Figure 4), in higher power, the upconversion emission intensity exhibited a linear dependence, indicating the nature of TTA-UC photochemistry. The lifetime of TTA-UC was measured to be as long as 89 µs due to the presence of long lifetime triplet state (Figure S8). This result also supports the hypothesis that the TTA-UC emission is based on triplet-triplet annihilation.

Figure 4. Integrated TTA-UC emission intensity data plotted as a function of incident power of PdPC(OBu)8&BPEN in soybean oil.

For biological application, we prepared NIR-excitation TTA-UC nanocapsules (TTAUCNCs).14 These nanocapsules were fabricated by nanometer oil droplets with stable bovine serum albumin (BSA) film at the oil-water interface and hydrophilic dextran on the droplet surface. TEM image of TTA-UCNCs (Figure S9) showed the polygonal shapes due to the soft and deformable nature of these nanocapsules. Table S1 presented the size of TTA-UCNCs obtained by DLS. The z-average hydrodynamic diameter (Dh) is 156 nm in pH 7.4 solution. No


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significant change in the nanocapsules size is observed in the pH range of 2−8, which is attributed to the stable BSA film at the oil-water interface and the hydrophilic dextran on the nanocapsules surface. Table S2 shows the ζ-potential of TTA-UCNCs as a function of pH. The isoelectric point of TTA-UCNCs is pH = 5. At physiological pH 7.4, the ζ-potential value is 19.4 mV, suitable for biological application.25 The prepared TTA-UCNCs

keeps excellent

photostability in water (Figure 5), as there is no obvious color-change after the exposure of 730nm laser. The TTA-UC can be also be observed by the naked eye. On the other hand, the TTAUC of nanocapsules made from the toluene dissolving the PdPC(OBu)8&BPEN couple was almost fully bleached within 1 minute.

Figure 5. Continuous kinetic scan of TTA-UCNCs emission prepared from soybean oil (TTAUCNCs) and toluene, respectively, the concentration of PdPC(OBu)8 and BPEN is 5.0×10-7 mol L-1 and 1.0×10-4 mol L-1, respectively. The insert shows the bright-filed images of TTA-UCNCs with soybean oil as core before and after exposure under 730 nm laser, the power density of 730nm laser is 1.592 W cm-2. Due to the NIR excitation, which is favorable for deep imaging, TTA-UCNCs were used for whole-body deep imaging of nude mouse. The excitation wavelength is 730 nm and collected


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wavelength is from 575 to 700 nm. As shown in Figure 6, obvious TTA-UC signals can be observed in vivo with signal-to-noise ratio (SNR) of 15 after 10 min injection of TTA-UCNCs. Imaging in situ and ex vivo confirmed the TTA-UC signals were from the liver.

Figure 6. Upconversion imaging of TTA-UCNCs in vivo, in vitro, and ex vivo, the concentration of PdPC(OBu)8 and BPEN is 5.0×10-7 mol L-1 and 1.0×10-4 mol L-1, respectively. Signal-to-noise ratio (SNR)= [(mean luminescence intensity of signal region 1) - (mean luminescence intensity of background region 3)] / [(mean luminescence intensity of the noise region 2) - (mean luminescence intensity of background region 3)]. 1, liver; 2, spleen; 3, kidney; 4, heart; 5, lung; 6, stomach; 7, intestines.

In summary, we have developed a reductive agent strategy to minimize the photobleaching of annihilator and improve the TTA-UC photostability. The poor photostability of TTA-UC in aerated environments is attributed to the singlet oxygen, which is generated under the irradiation


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of excitation light, leading to energy transfer from the triplet-state sensitizer or annihilator to oxygen. Reduction molecules can consume the singlet oxygen, thus protecting TTA-UC system from oxidation. This reductive agent strategy is utilized to prepare highly-photostable and highly-efficient TTA-UCNCs, which is used for deep imaging in vivo of small animal with high signal-to-noise ratio of 15. This study opens up new perspectives for preparing highphotostability and high-efficiency TTA-based upconversion nanometerials. ADDITIONAL INFORMATION Supporting Information Experimental section; Upconversion luminescence of PdPC(OBu)8&BPEN in vacuum degassed toluene;

Continuous

PdPC(OBu)8&rubrene;

kinetic 1

scan

of

the

upconversion

luminescence

intensity

of

HNMR of Linoleic acid, oleic acid, and DMSO in d-toluene;

Hydrodynamic diameter, zeta-pontential and TEM imange of TTA-UCNCs; TTA-UC life-time of PdPC(OBu)8&BPEN in soybean oil. AUTHOR INFORMATION Author Contributions Q.L., M.X., T.Y., and B.T. were responsible for the experimental work. Q.L., and F.L. conceived the project. F.L. supervised the research. All authors discussed the results. The manuscript was written by Q.L., and F.L. Corresponding Author *[email protected] Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank National Science Foundation of China (21401102) and State Key Basic Research Program of China (2015CB931800) for financial support.

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SYNOPSIS


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Highly Photostable Near-IR-Excitation Upconversion Nanocapsules

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