Iridium complex loaded polypyrrole nanoparticles for NIR laser induced photothermal effect and generation of singlet oxygen

Abstract

Iridium complex loaded polypyrrole nanoparticles (Ir-PPy NPs) have been prepared through the polymerization of pyrrole by Fe³⁺ in the presence of polyvinyl alcohol (PVA) and iridium complex. The Ir-PPy NPs exhibited a photothermal effect and the generation of singlet oxygen. In the Ir-PPy NPs, PPy could convert light energy to heat with a photothermal conversion efficiency of ∼35.5% when irradiated by a 730 nm continuous wave (CW) laser, and the iridium complex could generate singlet oxygen when irradiated by a 730 nm CW laser, as confirmed by monitoring the consumption of DPBF at 410 nm.

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1 Introduction

Polymer nanoparticles (PNPs) have been playing a pivotal role in bio-imaging and therapeutics in recent decades due to their good biocompatibility and drug delivery capacity.¹ They can be conveniently prepared by the direct polymerization of monomers.² PNPs have gained great attention as drug delivery systems for the sensitive detection and therapy of cancer because of their excellent biocompatibility and water solubility.³ To achieve multi-functional properties, fluorescent dye doped PNPs have recently been developed by the polymerization of monomers with the addition of an initiator and emitter.⁴ For example, Liu’s group has reported a series of fluorescent bio-imaging nanoparticles based on polyelectrolytes.^4a

Photothermal therapy utilizes semi-conductive polymers or nanocrystals with strong absorbance in the NIR region to transform NIR light energy into heat.⁵ The elevated temperature in the micro-environment of a tumor can ablate it. Among polymer nanoparticles, semi-conductive polymers have recently been paid great attention as therapeutic agents for photo-ablation of cancer cells due to their strong absorption in the NIR region, outstanding stability and high quantum efficiency.^5b For example, Prof. Dai and Prof. Liu have developed uniform polypyrrole nanoparticles (PPy NPs) as photothermal agents.⁶

Photodynamic therapy involves light, oxygen and a photosensitizer. When the nontoxic photosensitizer is exposed to light, oxygen in the tissue can be converted into singlet oxygen (¹O₂), which is toxic to tumor cells.⁷ Due to the heavy atom effect in iridium complexes, they can induce oxygen to generate ¹O₂, which can be applied in photodynamic therapy. We have synthesized a series of iridium complex based polymer nanoparticles and applied them for photodynamic therapy.⁸

In our previous work, we designed a nonlinear absorption iridium complex which could be induced by a 730 nm laser to convert oxygen into singlet oxygen.⁹ Herein, we report a one pot synthesis for fabricating iridium complex doped PPy NPs (Ir-PPy NPs). By using the absorption properties of PPy NPs and the nonlinear absorption properties of the iridium complex in the near-infrared region, the formed Ir-PPy NPs can combine photothermal and photodynamic therapy driven by a 730 nm continuous wave laser for potential biological applications (Scheme 1).

Scheme 1 Illustration of Ir-PPy NPs.

2 Experimental section

2.1 General

Iridium trichloride hydrate was purchased from Shanghai Jiuyue Chemical Company. 2-Phenylpyridine (ppy) was purchased from Shanghai Ruiyi Medical Tech. Co. Ltd. Polyvinyl alcohol (PVA), pyrrole, and 1,3-diphenylbenzo[c]furan (DPBF) were purchased from Energy Chemical Company. Ethylene glycol and other reagents were obtained from Sinopharm Chemical Reagent Co. Ltd. All solvents were dried using a conventional procedure prior to use.

Transmission electron microscope (TEM) images were collected at 200 kV with a JEOL JEM-2100 transmission electron microscope. Scanning electron microscope (SEM) images were obtained with a Hitachi S-4800 scanning electron microscope. The concentration of the iridium complex was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (VISTAMPXICP VARIAN, AMERICA). The hydration size was measured on a Malvern Zetasizer Nano ZS model ZEN3600 (Worcestershire, U.K.) equipped with a standard 633 nm laser.

2.2 Synthesis of Ir-PPy NPs

The cyclometalated iridium complex was synthesized according to our previous report.⁹ 0.5 g PVA (M_w: 9000) and 0.63 g iron(III) chloride hexahydrate were dissolved in 10 mL deionized water, and then a 2 mL ethanol solution of 0.02 g iridium complex was added. After stirring for 30 min, 69 μL pyrrole was added dropwise to the reaction mixture. After polymerization for 24 h, the black dispersion was washed three times by centrifugation at 15 000 rpm for 10 min.

2.3 Photothermal experiments with Ir-PPy NPs

An aqueous suspension (1 mL) containing Ir-PPy NPs at different concentrations was put in a quartz cuvette with an optical path length of 1.0 cm. The cuvette was illuminated by a 730 nm laser (GG-730-1500 MW) with a power density of 0.5 W cm⁻² for 600 seconds. The increase in temperature was monitored by a digital thermocouple device.

2.4 The generation of singlet oxygen by Ir-PPy NPs

To measure the absorption spectra, a solution of Ir-PPy NPs (3 mL) in ethanol/water (v/v = 4 : 1), DPBF (5.0 × 10⁻⁵ M) and NaN₃ (2.5 × 10⁻⁴ M) were put in a quartz cuvette with an optical path length of 1.0 cm. The cuvette was illuminated by a 730 nm CW laser with a power density of 0.5 W cm⁻² to observe the decrease in the absorbance of DPBF centred at 410 nm every 10 min.

3 Results and discussion

3.1 Synthesis and characterization of Ir-PPy NPs

As shown in Scheme 1, uniform Ir-PPy NPs were prepared by a one-step micro-emulsion polymerization using Fe³⁺ as the oxidizing agent and introducing polyvinyl alcohol (PVA) to enhance the water solubility and biocompatibility of the nanoparticles.^6b During the formation of nanoparticles, the iridium complex was enwrapped in the PPy NPs by π–π interactions. After the removal of excess free PVA by centrifugation, the Ir-PPy NPs showed good dispersity and stability in physiological solutions. The content of iridium complex in the PPy NPs was ∼0.25%, which was determined by ICP-MS. The presence of iridium complex in the nanoparticles was further confirmed by the UV-Vis spectra (Fig. 1c). The absorption peak at ∼400 nm for PPy nanoparticles was assigned to the π–π* transition of the PPy ring and the absorption peak at ∼455 nm for the Ir complex was due to the ³MLCT transition. Compared with PPy nanoparticles, the red shift of the absorption edge for Ir-PPy nanoparticles confirmed that Ir complex was doped into the PPy nanoparticles.¹⁰ SEM and TEM (Fig. 1) images showed that the Ir-PPy NPs were monodispersed nanospheres with an average size of ∼60 nm. The hydrodynamic diameter of the Ir-PPy NPs measured by dynamic light scattering (DLS) was ∼100 nm and the zeta potential was ∼16 mV in aqueous solution.

Fig. 1 SEM (a) and TEM (b) images of Ir-PPy NPs. Inset of (b): the distribution of Ir-PPy NPs. (c) Normalized UV-Vis-NIR spectra of Ir-PPy NPs in aqueous solution (black), PPy NPs in aqueous solution (red) and iridium complex in ethanol/water solution (v/v 4/1; blue). (d) Curves of the hydrodynamic diameter as a function of time. Ir-PPy NPs were dispersed in water or 1640 plus 10% FBS.

To further confirm the good stability of the Ir-PPy NPs, their hydrodynamic diameter tracking was measured by DLS for about one month in aqueous solution and in a simulated cell growth environment. After 25 days, the hydrodynamic diameter showed no obvious change either in aqueous solution or in the simulated cell growth environment. The hydrodynamic diameter was ∼120 nm in aqueous solution and ∼160 nm in the simulated cell growth environment, as shown in Fig. 1d. The results indicated that the Ir-PPy NPs could be applied in biological systems.

3.2 Photothermal properties of Ir-PPy NPs

Due to the strong absorption of the Ir-PPy NPs in the NIR region (as seen in the UV-Vis-NIR spectra), the photothermal effect was investigated by monitoring the temperature of a 3 mL water solution containing Ir-PPy NPs at different concentrations (0, 50, 100, 200, and 400 μg mL⁻¹) irradiated by a 730 nm laser (0.5 W cm⁻²). As shown in Fig. 2a, when Ir-PPy NPs were dispersed in aqueous solution (400 μg mL⁻¹) and exposed to a 730 nm laser for 10 min, the temperature increased by 12 °C. In comparison, the temperature of the solution in the absence of Ir-PPy NPs increased by only 0.7 °C. Moreover, six cycles of laser on/off with a 730 nm laser were used to examine the photostability of the Ir-PPy NPs. The solution of Ir-PPy NPs was exposed to a 730 nm laser for 10 min, then naturally cooled to room temperature. After six cycles, the solution could still rise to the same temperature, which further confirmed the photostability of the Ir-PPy NPs.

Fig. 2 (a) Heating curves of water and different concentrations of Ir-PPy NPs in aqueous solution under the irradiation of a 730 nm laser with a power density of 0.5 W cm⁻². (b and c) The photothermal conversion efficiency of Ir-PPy nanoparticles and PPy nanoparticles. (d) Six cycles of laser on/off with a 730 nm laser.

In order to verify the efficiency of Ir-PPy NPs as a photothermal agent, a 200 μg mL⁻¹ solution of Ir-PPy NPs was exposed to a 730 nm laser with a power density of 0.5 W cm⁻². An obvious temperature increase was observed from 25.6 °C to 31.7 °C. The photothermal conversion efficiency (η) of Ir-PPy and pure PPy NPs was calculated to be ∼33.5% and ∼35.5%, respectively, using the equation by Roper’s method (Fig. 2c and d). Therefore, the photothermal properties of the Ir-PPy NPs resulted from the PPy component.

3.3 The generation of singlet oxygen by Ir-PPy NPs

The iridium complex has been reported as a novel reagent for nonlinear absorption photodynamic therapy by our group.⁹ DPBF, a well-known highly efficient ¹O₂ scavenger, was introduced to react with singlet oxygen to obtain 1,2-dibenzoylbenzene.¹¹ When exposed to a 730 nm continuous wave laser in the presence of the iridium complex, DPBF was consumed directly, and this was monitored by the decrease in intensity of the UV-Vis spectra at 410 nm (Fig. 3a). However, in the presence of PPy NPs and NaN₃, there was no obvious consumption of DPBF, which confirmed that singlet oxygen was generated by the iridium complex in Ir-PPy NPs (Fig. 3c). As shown in Fig. 3b, the value of ln(A_t/A₀) exhibited a good linear relationship with the irradiation time. The photo-oxidation rate expressed by the slope of the linear plot was calculated to be ∼0.0086 min⁻¹.

Fig. 3 (a) The production of singlet oxygen by Ir-PPy NPs with a concentration of 50 μg mL⁻¹ irradiated by a 730 nm laser (0.5 W cm⁻²). (b) Comparative plots of ln(A_t/A₀) as a function of time. A₀ is the initial absorbance and A_t is the absorbance at different irradiation times. (C) Decay profiles of the absorbance of DPBF centred at 410 nm in the presence of Ir-PPy (blue), PPy (red), and Ir-PPy NPs with NaN₃ (black) in air-saturated ethanol/water solution (v/v 4/1) irradiated by a 730 nm laser. (d) The reaction of ¹O₂ with DPBF.

4 Conclusions

We have reported a one pot method for the synthesis of multi-functional nanoparticles. The synthesized Ir-PPy NPs showed photothermal properties and the ability to generate singlet oxygen when irradiated by a 730 nm CW laser. This strategy will be extended to similar systems to design multi-functional nanoparticles for photothermal and photodynamic agents.

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (No. 21271130 and 21371122), Shanghai Science and Technology Fund Program (No. 12ZR1421800 and 13520502800), Shanghai Normal University (No. SK201339), and International Joint Laboratory on Resource Chemistry (IJLRC).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22092k

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