Vacancy-enhanced generation of singlet oxygen for photodynamic therapy

Oxygen vacancy introduced defects in the band gap of BiOBr–H allow facile electron transfer from a photo-excited ruthenium complex to the semiconductor, thereby increasing ROS yields and PDT efficiency.


Synthesis of Ru(bpy) 2 C-pyCl 2
Ruthenium chloride (0.05 g, 0.24 mmol) and 2,2'-bipyridine (0.075 g, 0.48 mmol) were dissolved in 25 mL of anhydrous N, N-dimethylformamide (DMF), and refluxed at 120 °C for 12 h under a nitrogen atmosphere. The reaction liquid was filtered and the filtrate evaporated to dryness under vacuum. The obtained Ru(bpy)2Cl2 was washed three times with diethyl ether, collected by centrifugation at 8000 r.p.m and then allowed to dry naturally at room temperature. The product (0.02 g, 0.04 mmol) was then dissolved in anhydrous DMF, to which 2,2'-biisonicotinic acid (0.010 g, 0.04 mmol) was added and the resulting mixture stirred at 120 °C for 12 h in the dark under a nitrogen atmosphere. The Ru(bpy)2C-pyCl2 product was collected by centrifugation, washed several times with diethyl ether, then air dried. For simplicity, Ru(bpy)2C-pyCl2 is denoted as Rub2d in this manuscript.

Synthesis of BiOBr with and without oxygen vacancies (OVs)
For the preparation of BiOBr without oxygen vacancies, the following procedure was used. Bi(NO3)·5H2O (3 mmol) and 2 mg of polyvinylpyrrolidone (PVP) were added slowly to 32 mL of an ethylene glycol solution containing 3 mmol of KBr. The resulting mixture was then stirred for 1 h at room temperature in air, then poured into a 100 mL Teflon-lined stainless autoclave. The autoclave was then heated at 160 °C for 12 h under autogenous pressure, and then allowed to cool naturally to room temperature. The solid precipitate was collected and washed several times with deionized water and then ethanol. The product was then dried at 60 °C in vacuum. BiOBr with OVs, denoted herein as BiOBr-H, was obtained by heating BiOBr at 300 °C in an O2 atmosphere for 4 h.

Synthesis of BiOBr-H/Rub 2 d
A solution of Rub2d in DI water (1 mg/mL) was added to a suspension of BiOBr-H in DI water (BiOBr-H concentration ranging from 0.25-4 mg mL -1 ) under magnetic stirring, and the stirring continued for 20 min at RT. The BiOBr-H/Rub2du products were then collected by centrifugation and re-dispersed in water for later use.

Synthesis of BiOBr-H/PS
A solution of photosensitizers (denoted as PS, e.g. zinc phthalocyanine and idocyanine green) in DI water (1 mg/mL) was added to a suspension of BiOBr-H in DI water (1 mg /mL) under magnetic stirring, and the stirring continued for 20 min at RT. The BiOBr-H/PS products were then collected by centrifugation and re-dispersed in water for later use.

Model construction
The model of bulk BiOBr was constructed in the space group of P4/nmm with the following lattice parameters: a = b = 3.92 Å, c = 8.11 Å, ɑ = β = γ = 90°. 1  In order to determine the most preferentially exposed facets of BiOBr, the surface energy (γ) of BiOBr was derived from equation 1: 5 where Eslab is the total energy of the optimized slab possessing the same formula unit as bulk BiOBr, Ebulk is the energy of bulk BiOBr, and A is the surface area for one side of the slab.
The electronic band gap energy, Eg, of BiOBr was calculated using equation 2: 6 where ECBM and EVBM represent the energy of the conduction band minimum (CBM) and valence band maximum (VBM), respectively. By calculating the band structure of BiOBr or BiOBr-H, the energy difference (x) between the Fermi level (EF) and the CBM can be obtained by equation 3: The work function of the (011) facet of BiOBr can be calculated using equation 4: where e is the charge of an electron, ϕ is the electrostatic potential in the vacuum close to the surface. The values of ϕ and EF can be directly obtained from the DMol 3 code.
The band edge positions of BiOBr and BiOBr-H were then calculated using equations 5 and 6: 7 The positions of the HOMO and LUMO for Rub2d were directly obtained from the DMol 3 code by analyzing the molecular orbitals.
The binding energy, , between Rub2d and BiOBr was calculated with Where / , , and is the energy of BiOBr/Rub2d, BiOBr, and Rub2d, respectively. Ebind between Rub2d and BiOBr-H was calculated via a similar method.

In vitro experiments
The in vitro cytotoxicity of various samples were evaluated on human various cells (FT-IR) spectra were obtained on a Varian Excalibur 3100 FTIR spectrometer over the range 4000-500 cm -1 at 2 cm -1 resolution. X-ray photoelectron spectra (XPS) were collected on a PHIQ2000 X-ray photoelectron spectrometer equipped with an Al Kα Xray source. Electron spin resonance (ESR) spectra were collected on a Bruker 500 spectrometer. A Thermo Multiskan FC was used to investigate cell viability. The visible light source was obtained by a 300 W Xenon lamp (EOSun, Au-Light, America) with cutoff filter (520 ± 15 nm). The irradiance was measured to be 100 mW/cm 2 (CEL-NP2000).

Statistical Analysis
Statistical significance was assessed using one-way ANOVA analyses on SPSS 16.0 software. The difference was considered to be statistically significant if the probability value was less than 0.05 (i.e. p < 0.05). Mean values and standard deviations (SD) were calculated from replicate experiments. Data were presented as mean ± SD. Figure S1. The chemical structure of Rub2d.