Highly selective generation of singlet oxygen from dioxygen with atomically dispersed catalysts

Abstract

Singlet oxygen (¹O₂) as an excited electronic state of O₂ plays a significant role in ubiquitous oxidative processes from enzymatic oxidative metabolism to industrial catalytic oxidation. Generally, ¹O₂ can be produced through thermal reactions or the photosensitization process; however, highly selective generation of ¹O₂ from O₂ without photosensitization has never been reported. Here, we find that single-atom catalysts (SACs) with atomically dispersed MN₄ sites on hollow N-doped carbon (M₁/HNC SACs, M = Fe, Co, Cu, Ni) can selectively activate O₂ into ¹O₂ without photosensitization, of which the Fe₁/HNC SAC shows an ultrahigh single-site kinetic value of 3.30 × 10¹⁰ min^-1 mol^-1, representing top-level catalytic activity among known catalysts. Theoretical calculations suggest that different charge transfer from MN₄ sites to chemisorbed O₂ leads to the spin-flip process and spin reduction of O₂ with different degrees. The superior capacity for highly selective ¹O₂ generation enables the Fe₁/HNC SAC as an efficient non-radiative therapeutic agent for in vivo inhibition of tumor cell proliferation.

Fig. 1. Synthesis of M₁/HNC SACs and structural characterization of the Fe₁/HNC SAC. (a) Schematic depiction of the route to the fabrication of M₁/HNC SACs. (b) TEM image of Fe₁/HNC. (c) HAADF-STEM image and the corresponding elemental mapping images of Fe₁/HNC (C, red; Fe, green; N, orange). (d) Aberration-corrected HAADF-STEM image of Fe₁/HNC showing atomically dispersed Fe atoms as bright dots highlighted with red circles.

Fig. 2. XAFS analysis of the Fe₁/HNC SAC. (a) Normalized XANES spectra and (b) corresponding Fourier-transform EXAFS spectra at the Fe K-edge of Fe₁/HNC (red line), Fe foil (purple line), and Fe₂O₃ (blue line). (c) Wavelet transforms of the Fe K-edge EXAFS spectra for Fe₁/HNC, Fe foil, and Fe₂O₃. (d) Fourier-transform EXAFS spectrum and the corresponding fitting curve of Fe₁/HNC in R-space. (e) Schematic structure of Fe₁/HNC.

Fig. 3. Selective generation of ¹O₂ with M₁/HNC SACs. Time-dependent absorbance changes of ox-TMB monitored at 652 nm (a) with 5 μg mL⁻¹ catalysts synthesized at different temperatures or (b) with 5 μg mL⁻¹ M₁/HNC and HNC in the Britton–Robinson (B–R) buffer (pH 4.0). (c) UV-vis absorption spectra of ox-TMB catalyzed by 5 μg mL⁻¹ Fe₁/HNC at 2 min in the N₂-saturated (blue line) and air-saturated (red line) B–R buffer (pH 4.0). (d) Time-dependent absorbance changes of ox-TMB monitored at 652 nm catalyzed by 5 μg mL⁻¹ Fe₁/HNC in the B–R buffer (pH 4.0) without scavengers (brown line) or with SOD (blue line), mannite (purple line), and NaN₃ (red line). (e) UV-vis absorption spectra of ox-TMB catalyzed by 5 μg mL⁻¹ Fe₁/HNC in 2 min without (red line) or with (blue line) light in the B–R buffer (pH 4.0). (f) Oxidation rate of ABDA with M₁/HNC and HNC in the B–R buffer (pH 4.0). (g) k value of the ABDA oxidation reaction with 5 μg mL⁻¹ M₁/HNC and HNC in the B–R buffer (pH 4.0). (h) Decrease in fluorescence intensity (λ_ex/λ_em = 380/433 nm) of ABDA with 5 μg mL⁻¹ Fe₁/HNC in 5 min in the B–R buffer at different pH values. (i) TEMPO ESR signals in the absence (blue line) and presence of 20 μg mL⁻¹ Fe₁/HNC in the B–R buffer (pH 4.0).

Fig. 4. DFT calculations. PDOS of the M 3d (red line) and O 2p (blue line) orbitals of (a) FeN₄, (b) CoN₄, (c) CuN₄ and (d) NiN₄ sites adsorbed with O₂. Calculated charge density differences of (e) FeN₄, (f) CoN₄, (g) CuN₄ and (h) NiN₄ centers adsorbed with O₂. (i) Comparison of the DFT results for constructed MN₄ structures.

Fig. 5. In vitro and in vivo inhibition of tumor growth with the Fe₁/HNC SAC. (a) CLSM fluorescence image of HeLa cells treated with FITC-loaded P-Fe₁/HNC for 6 h. (b) Cell viability of HeLa cells after 12 h-treatment of P-HNC or P-Fe/HNC at the concentrations of 0.06–0.3 mg mL⁻¹. CLSM fluorescence images of HeLa cells stained with Calcein-AM and (c) PI or (d) DCFH-DA after the 12 h-treatment with PBS, P-HNC and P-Fe₁/HNC. (e) Cell viability of HeLa cells pre-incubated with AA (0.25 mM, 0.5 mM, and 1 mM) after 12 h-treatment with 0.1 mg mL⁻¹ P-Fe/HNC. In vivo (f) tumor proliferation and (g) body weight curves of mice treated with the intravenous injection of PBS (blue line), P-HNC (purple line), or P-Fe₁/HNC (red line).