Tunable NIR Absorption Property of a Dithiolene Nickel Complex: A Promising NIR-II Absorption Material for Photothermal Therapy

A dipyridophenazine Ni(II) dithiolene complex as a dual-acting cancer phototherapy agent activatable within the phototherapeutic window

A combination of photodynamic therapy (PDT) and photothermal therapy (PTT) within the phototherapeutic window (600-900 nm) can lead to significantly enhanced therapeutic outcomes, surpassing the efficacy observed with PDT or PTT alone in cancer phototherapy. Herein, we report a novel small-molecule mixed-ligand Ni(II)-dithiolene complex (Ni-TDD) with a dipyridophenazine ligand, demonstrating potent red-light PDT and significant near-infrared (NIR) light mild-temperature PTT activity against cancer cells and 3D multicellular tumour spheroids (MCTSs). The four-coordinate square planar complex exhibited a moderately intense absorption band (ε ∼ 3700 M-1cm-1) centered around 900 nm and demonstrated excellent dark and photostability in an aqueous phase. Ni-TDD induced a potent red-light (600-720 nm) PDT effect on HeLa cancer cells (IC50 = 1.8 μM, photo irritation factor = 44), triggering apoptotic cell death through efficient singlet oxygen generation. Ni-TDD showed a significant intercalative binding affinity towards double-helical calf thymus DNA, resulting in a binding constant (Kb) ∼ 106 M-1. The complex induced mild hyperthermia and exerted a significant mild-temperature PTT effect on MDA-MB-231 cancer cells upon irradiation with 808 nm NIR light. Simultaneous irradiation of Ni-TDD-treated HeLa MCTSs with red and NIR light led to a remarkable synergistic inhibition of growth, exceeding the effects of individual irradiation, through the generation of singlet oxygen and mild hyperthermia. Ni-TDD displayed minimal toxicity towards non-cancerous HPL1D and L929 cells, even at high micromolar concentrations. This is the first report of a Ni(II) complex demonstrating red-light PDT activity and the first example of a first-row transition metal complex exhibiting combined PDT and PTT effects within the clinically relevant phototherapeutic window. Our findings pave the way for designing and developing metal-dithiolene complexes as dual-acting cancer phototherapy agents using long wavelength light for treating solid tumors.

Fluorescence-free bis(dithiolene)nickel dyes for surface-enhanced resonance Raman imaging in the second near-infrared window

Second near-infrared window (NIR-II, 1000-1700 nm) imaging is one of the foremost optical imaging techniques. However, surface-enhanced Raman scattering (SERS)-based research in this optical region remains in its infancy, mainly because of a lack of suitable NIR-II Raman reporters. Herein, we report the first example of a nickel dithiolene complex as a NIR-II resonance Raman reporter with intense long wavelength absorption (ε = 9.58 × 104 m-1 cm-1 at 1007 nm), fluorescence-free features and ultrahigh affinity to noble metal surfaces with its eight sulfur atoms. Surface-enhanced resonance Raman scattering nanoprobes constructed with such reporters enable high contrast and highly photostable lymph node imaging far superior to that possible with existing NIR-I and NIR-II SERS nanoprobes. The developed NIR-II nanoprobes allow deep optical penetration (8 mm) as well as in vivo SERS detection of deep-seated microtumors in mice.

Near-infrared metal agents assisting precision medicine: from strategic design to bioimaging and therapeutic applications

Metal agents have made incredible strides in preclinical research and clinical applications in recent years, but their short emission/absorption wavelengths continue to be a barrier to their distribution, therapeutic action, visual tracking, and efficacy evaluation. Nowadays, the near-infrared window (NIR, 650-1700 nm) provides a more accurate imaging and treatment option. Thus, there has been ongoing research focusing on developing multifunctional NIR metal agents for imaging and therapy that have deeper tissue penetration. The design, characteristics, bioimaging, and therapy of NIR metal agents are covered in this overview of papers and reports published to date. To start with, we focus on describing the structure, design strategies, and photophysical properties of metal agents from the NIR-I (650-1000 nm) to NIR-II (1000-1700 nm) region, in order of molecular metal complexes (MMCs), metal-organic complexes (MOCs), and metal-organic frameworks (MOFs). Next, the biomedical applications brought by these superior photophysical and chemical properties for more accurate imaging and therapy are discussed. Finally, we explore the challenges and prospects of each type of NIR metal agent for future biomedical research and clinical translation.

Near-IR Absorbers Based on Pt(II)-Dithiolene Donor–Acceptor Charge-Transfer (CT) Systems: A Structural Analysis to Highlight DA Interactions

The packing interactions of a series of electron donor (D) and electron acceptor (A) charge transfer (CT) near-IR absorbers based on platinum-dithiolene complexes are reinvestigated here as a case study also by using the Hirshfeld surface analysis. This analysis on systems, which exhibit the 1:1, 2:1 and 2:2 columnar stacking patterns between D and A, allows us to point out that several interactions of atoms and fragments are involved in the stacking interactions but also that only a limited fraction of these interactions, limited to the 1:1 D/A columnar stacking case, can be relatable to the absorption features of this class of compounds.

Nickel Complexes Bearing ONS Chelating Ligands: A Promising Contender for In Vitro Cytotoxicity Effects on Human Pancreatic Cancer MIA-PaCa-2 Cells

The highly chronic human pancreatic cancer cell is one of the major reasons for cancerous death. Nickel complexes are recently gaining interest in anticancer activities on different types of cancer cells. Hence, in this study, we synthesized and characterized a series of ONS donor ligands [2-HO-C₆H₄-CH═N-(C₆H₄)-SH] (L1), [2-OH-3-OMe-C₆H₃-CH═N-(C₆H₄)-SH] (L2), [2-OH-3,5-(C(Me)₃)₂-C₆H₂-CH═N-(C₆H₄)-SH] (L3), [2-OH-C₆H₄-CH═N-(C₆H₄)-SMe] (L4), [2-OH-3-OMe-C₆H₃-CH═N-(C₆H₄)-SMe] (L5), [2-OH-3,5-(C(Me)₃)₂-C₆H₂-CH═N-(C₆H₄)-SMe] (L6) and their Ni(II) metal complexes [(MeOH)Ni(L1-L1-4H)] (1), [(MeOH)Ni(L2-L2-4H)] (2), [(MeOH)Ni(L3-L3-4H)] (3), [(L4-H)₂Ni] (4), [(L5-H)₂Ni] (5), and [(L6-H)₂Ni] (6). The single-crystal X-ray diffraction data of complexes 1 and 4 were collected to elucidate the geometry around the metal center. The anticancer activity of complexes 1-6 was investigated on human pancreatic cancer cell line MIA-PaCa-2, which revealed that complexes 4 and 6 were the most significantly effective in decreasing the cell viability of cancer cells at the lowest dose. The structure parameters obtained from single-crystal X-ray diffraction data are found to be in good agreement with the data from density functional theory and Hirshfeld surface analysis for complex 1.

Revealing the synergetic interaction between amino and carbonyl functional groups and their effect on the electronic and optical properties of carbon dots

Nitrogen and oxygen-based functionalized carbon dots (CDs) surfaces have attracted significant attention due to their ability to tailor the optical and electronic properties of CDs.