Singlet Oxygen Kinetics in Polymeric Photosensitizers - The Journal of

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C: Physical Processes in Nanomaterials and Nanostructures

Singlet Oxygen Kinetics in Polymeric Photosensitizers Steffen Hackbarth, Sebastian Pfitzner, Liang Guo, Jiechao Ge, Pengfei Wang, and Beate Roeder J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02052 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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The Journal of Physical Chemistry

Singlet Oxygen Kinetics in Polymeric Photosensitizers

Steffen Hackbarth1*, Sebastian Pfitzner1, Liang Guo2, Jiechao Ge2, Pengfei Wang2, Beate Röder1

1 Photobiophysics, Physics Department, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany 2 Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China *

Corresponding author: [email protected]; Tel.: +49 30 2093 7648

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Abstract Nanoparticles built up from polythiophenes act as photosensitizers without embedding any additional chromophore. The self-organized water-soluble nanoparticles in this work are made from polythiophene precursors with alkyl side chains which was combined with DSPCPEG in different mass ratios. The fluorescence and singlet oxygen quantum yields of such nanoparticles can be tuned by varying the mass ratio of the two components. Two unique properties of these polymeric photosensitizers result in special singlet oxygen kinetics. First, the backbone orientation of the polythiophene influences the probability of triplet excitons, which have a high mobility across all the nanoparticle and second, oxygen can diffuse in and out of the polymeric photosensitizer. Therefore, most of the singlet oxygen is generated inside the nanoparticle, close to the surface, soon after oxygen diffuses in. After generation, the majority of the singlet oxygen diffuses out of the nanoparticle. Using highly sensitive time- and spectrally resolved singlet oxygen phosphorescence detection, the oxygen diffusion can be confirmed, the observed kinetics and quantum yield variations can be explained based on the polymer-semiconductor model. Whenever singlet oxygen kinetics in polymeric nanostructures are investigated, such oxygen diffusion effects have to be taken into account.

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Introduction Recently, a new class of water-soluble polymeric photosensitizing nanoparticles (NP) has been reported, which can efficiently generate singlet oxygen without any additional chromophors1. Such nanoparticles can be of great interest for various applications, since they are comparably easy to produce, exhibit low reactivity, good photostability, and have the potential to exploit the EPR effect (Enhanced Permeability and Retention)2 for targeted delivery due to their size. Polymeric photosensitizers (PS) - in contrast to conventional PS - have the unique feature that they can take up oxygen molecules. The scope of this paper is to evaluate the singlet oxygen kinetics of a NP series consisting of polythiophene (PT) and Di phospatidylcholinepolyethylene glycol (DSPC-PEG), to better understand the process of singlet oxygen generation in such NPs. Singlet oxygen (1O2), the lowest electronically excited state of molecular oxygen, is known as the main mediator in photodynamic therapy

3,4

or photodynamic inhibition of

microorganisms 5. 1O2 can be directly detected via its characteristic, yet very weak nearinfrared (NIR) phosphorecence. The intensity of this phosphorescence as well as its decay time strongly depend on its environment 6. In homogeneous environments, the kinetics of 1

O2 phosphorescence can be described by a double exponential function:

c∆(t) = (cPS(0)·Φ∆·τT-1/(τT-1-τ∆-1)) ·(exp(-t/τ∆)-exp(-t/τT))

(1)

where τT is the PS triplet decay time and τ∆ is the 1O2 decay time, Φ∆ is the quantum yield and cPS(0) is the amount of excited PS molecules at time zero. When fitting the above mentioned formula to experimentally obtained intensities I(t), the constant factors are often merged to one parameter A1 = (cPS(0)·Φ ∆·τT-1/(τT-1-τ∆-1)). Keep in 3 ACS Paragon Plus Environment


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mind that A1 is positive just as long as τ∆ > τT. To account for spectrally overlapping emissions like PS phosphorescence, another exponentially decaying signal component has to be added: I(t) = A1 ·(exp(-t/τ∆)-exp(-t/τT))+ A2 ·(exp(-t/τΤ)

(2)

Whenever a PS phosphorescence is observable at the detection wavelength, Eq. 2 is a much better choice than Eq. 1. Fitting a 1O2 kinetic with additional PS phosphorescence using Eq. 1 would yield wrong results (too short PS phosphorescence decay time and too long 1O2 decay time). Any heterogeneity causes further deviations from these simple models, since the different radiative rate constants of 1O2 in different materials become of importance 7. Diffusion of 1

O2 from one material to another may change the observed phosphorescence signal

intensity7,8. The emission of singlet oxygen molecules in a volume with higher than average radiative rate constant is over-pronounced in the detected signal9 and may alter the detected phosphorescence kinetics. Accurate analysis of these kinetics can give additional information about the place of 1O2 generation 10, but such analysis often requires numerical simulations9,10. In case of the polymeric NPs in this work, the NP itself has to be taken into account. Oxygen can diffuse into the polymer, which means that most of the 1O2 will be generated near the surface or inside of the NP. In most polymers the radiative rate constant of 1O2 is higher compared to water which results in over-pronounced 1O2 phosphorescence shortly after generation. If such results are fitted with Eq. 1, too short PS triplet decay times and too long 1O2 decay times will be the result. This paper aims to find a reliable method to determine 1O2 quantum yields for such NPs by analyzing the 1O2 kinetics, taking the influence of excitonic movement in the polymer semiconductor and 1O2 diffusion into account. 4 ACS Paragon Plus Environment

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Materials and Methods All the nanoparticles (NPs) were made from Polythiophene (PT) precursors with molecular weight of 64.000 Da and Phospholipid – Polyethylene glycol (DSPC-PEG) (Fig.1). All substances are named after the mass percentage of the PT used to build them. Hence, the investigated NPs are PT10, PT25, PT50, PT75 and PT90.

Figure 1: Structural formula of the Polythiophen (PT) and DSPC-PEG The synthesis of the NPs has been described before 1. In short, the DSPC-PEG is solved in 9 ml water and PT, dissolved and sonicated for 3 minutes in a mixture of THF and water (1ml, 1:1), is added under intensive stirring. Subsequently, THF was removed by rotary evaporation, and the PNPs were purified through a 0.22 μm filter membrane. Variation of NPs was done by varying the mass ratio of PT and DSPC-PEG. Absorption spectra were recorded using the commercial UV-Vis spectrophotometer UV-1800 (Shimadzu). 1

O2 luminescence kinetics were measured using a compact table-top 1O2 luminescence

detection system TCMPC1270 (SHB Analytics) with the included LED excitation module at 402 nm/12 kHz - 160 ns (~2mW) and the module for using external lasers. In the latter case we used a frequency doubled Nd3+-YAG Laser Vector from Coherent/Germany (532 nm, 12 5 ACS Paragon Plus Environment


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kHz - 10 ns) attenuated to 5 mW as excitation source. The quality of each fit was judged using reduced χ2 (χ 2red) in the time region 0.5-30 µs. The lower time limit was given by the pronounced short time signal of the NPs. Time and spectrally resolved 1O2 luminescence kinetics were measured in a whole batch of NPs directly after synthesis to have a constant signal over a long time. A multifurcated quartz fiber was submerged into the solution with fixed positioning and intensive stirring to ensure constant conditions throughout the time of this measurement. The fiber can simultaneously be used to accomplish excitation (532 nm, 12kHz, 5 mW) and 1O2 phosphorescence detection. Spectral resolution was realized using a tiltable interference filter. Technical details and proof of concept are about to be published separately. Data were collected for 10 seconds at each filter angle (0° to 40° in steps of 1°). Tilting the filter from 0° to 40°, the central wavelength of the detection window is shifted from 1325 nm to 1190 nm, where an angle of 23° results in a central transmission wavelength of 1270 nm. Fluorescence lifetimes and anisotropy were determined using time-correlated single photon counting (TCSPC), comprising of a thermoelectrically cooled microchannel plate (R3809-01, Hamamatsu), a monochromator (77200, LOT-Oriel) and a PCI plug-in card (SPC630, Becker & Hickl): Samples were excited at 532 nm, fluorescence was recorded at 590 nm. TEM images were taken on a JEOL JEM-2100F transmission electron microscope at an acceleration voltage of 150 kV. Numerical modelling of the 1O2 diffusion in- and outside of the nanoparticles uses a system of concentric spherical shells of identical thickness (0.25 nm) where the innermost shells represent the NP. The behavior of 1O2 in each of these shells is defined by several parameters: the diffusion constants in- and outside were chosen as 2·10-7 cm²/s 11 for PT and 6 ACS Paragon Plus Environment

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2·10-5cm²/s

12

for water. The 1O2 decay time inside the NP has very little influence on the

simulation unless it is very short (

Singlet Oxygen Kinetics in Polymeric Photosensitizers - The Journal of

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