A general design of pyridinium-based fluorescent probes for enhancing two-photon microscopy

Near-Infrared Light-Driven Condensation Using Branched Two-Photon-Absorbing Organic Photocatalysts with Viscosity-Dependent Properties

Most reported photocatalytic systems that primarily rely on the absorption of ultraviolet (UV) and short visible light irradiation face significant limitations, including shallow penetration in reaction media, competing absorption with substrates and catalysts, and incompatibility with light-sensitive molecules. These drawbacks can be largely avoided if the same reaction can be operated under near-infrared (NIR) light irradiation. Herein, we report a novel family of branched pyridinium-based photocatalysts designed with elegant donor-π-acceptor (D-π-A) architectures and exceptional two-photon absorption (TPA) capabilities in the deep-red and NIR regions. Among the three designed complexes, MPP-3arm demonstrates a remarkable TPA cross section and hence efficient NIR-driven photocatalytic condensation of aromatic diamines and aldehydes. Notably, it also exhibits a unique viscosity-dependent photocatalytic performance, attributed to restricted rotational mobility in a highly viscous environment, a feature rarely explored in organic photocatalysis. Overall, this study presents the design of multibranched organic TPA photocatalysts and their potential of overcoming the limitations encountered in conventional photocatalysis, thus unlocking new opportunities in NIR light-driven chemical synthesis.

Advances in fluorescent probes of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is the predominant chronic liver disease worldwide, with 20-30 % of individuals going on to develop non-alcoholic steatohepatitis (NASH), which could result in serious complications such as fibrosis, liver cirrhosis, and hepatocellular carcinoma. Since NAFLD is reversible in its early stages, early diagnosis is necessary. By using particular structural and functional designs, fluorescent probes can be made to detect NAFLD-related chemicals or biological processes with a high degree of sensitivity and selectivity. In this work, we summarize the existing fluorescent probes for identifying biomarkers in NAFLD, including microenvironment (viscosity, polarity), ROS, RNS, RSS, metal ions, enzymes, and RNA. Furthermore, future directions are envisioned to inform the creation of more accurate and reliable fluorescent probes for NAFLD diagnosis, emphasizing the benefits and challenges of fluorescence probes.

A wash-free red fluorescent probe for real-time monitoring of mitochondrial viscosity changes and tumor imaging

Mitochondria are important energy-producing organelles within cells, and abnormalities in the viscosity of their microenvironment are closely related to diseases such as cancer. Current methods for detecting viscosity still suffer from many limitations, whereas fluorescence imaging techniques can address these shortcomings. Therefore, there is an urgent need to develop fluorescence probes capable of detecting changes in mitochondrial viscosity. In this study, derivatives of triphenylamine were used as the core structure, and different triphenylphosphine derivatives were introduced through a condensation reaction to synthesize three red fluorescence probes with a D-π-A structure. Their photophysical properties have been systematically studied and it has been found that they are not only sensitive to viscosity but also exhibit strong anti-jamming capabilities. Among them, probe TPAP2 exhibits excellent optical properties, including large Stokes shifts and high sensitivity to viscosity. It was found that TPAP2 is mitochondria-targeted, washing-free, and fast (<10s), with the targeting process depending on the mitochondrial membrane potential. The probe has been successfully applied to tumor imaging in mice with subcutaneous tumors.

Unveiling the susceptibility of nanosecond pulsed electric field on intracellular function in breast cancerous and normal cells using fluorescence imaging

Modulation in cellular function and cell death through electrostimulation of intracellular organelles with the application of 50 ns pulsed electric field (nsPEF) have been investigated in breast cancerous MCF7 and normal MCF10A cells by developing a three-dimensional microelectrode device integrated with a fluorescence microscope. The findings revealed that nsPEF induced distinct effects on intracellular functions and dynamics in MCF7 and MCF10A cells. MCF10A cells exhibited significantly higher survivability than MCF7 cells, with different modes of cell death observed between them. MCF7 cells underwent caspase-dependent cell death, whereas non-survived MCF10A cells experienced a caspase-independent death mechanism. These multiple nsPEF-induced changes in cellular function and dynamics were characterized by monitoring several parameters in real-time, both before and after nsPEF application. These parameters included cell viability, phosphatidylserine externalization, changes in mitochondrial membrane potential, superoxide anion production, intracellular calcium ion level, NADH properties, and caspase-3/7 activity. Additionally, NADH lifetimes increased and decreased by ∼25% and ∼4% in MCF7 and MCF10A cells under nsPEF application, accompanied by changes in intracellular calcium levels. The present study, demonstrating the selective killing of breast cancerous cells using microelectrodes, paves the way for developing nsPEF-based breast cancer therapies.

Near-Infrared Bioimaging Using Two-photon Fluorescent Probes

Near-infrared (NIR) bioimaging has emerged as a transformative technology in biomedical research. Among many fluorescent probes that are suitable for NIR imaging studies, two-photon absorption (TPA) ones represent a particularly promising category, because TPA fluorescent probes can overcome the inherent limitations of one-photon absorption (OPA) counterparts. By leveraging the unique properties of two-photon absorption, TPA fluorescent probes achieve superior tissue penetration, significantly reduced photodamage, and enhanced spatial resolution. This perspective article delves into the fundamental principles, design strategies, and representative TPA probes for various imaging applications. In particular, a number of molecular fluorescent probes, ranging from organic, inorganic, and COF/MOF-based systems are highlighted to showcase the vast scope of possible TPA probe design and application scenarios. In addition, the employment of stimulated TPA probes that are responsive to different external factors, including pH, redox species, enzymes, and hypoxia, is also discussed. In the end, the future perspectives for the continuous advancement of TPA fluorescent probes in the NIR bioimaging field are presented. For instance, it is essential to transition from cellular to in vivo imaging studies to obtain more physiologically relevant insights. Additionally, the development of "dual-function" TPA probes for both disease diagnosis and therapeutic treatment is particularly promising.

Covalent assembly-based two-photon fluorescent probes for in situ visualizing nitroreductase activities: From cancer cells to human cancer tissues

Nitroreductase (NTR) is widely regarded as a biomarker whose enzymatic activity correlates with the degree of hypoxia in solid malignant tumors. Herein, we utilized 2-dimethylamino-7-hydroxynaphthalene as fluorophore linked diverse nitroaromatic groups to obtain four NTR-activatable two-photon fluorescent probes based on covalent assembly strategy. With the help of computer docking simulation and in vitro assay, the sulfonate-based probe XN3 was proved to be able to identify NTR activity with best performances in rapid response, outstanding specificity, and sensitivity in comparison with the other three probes. Furthermore, XN3 could detect the degree of hypoxia by monitoring NTR activity in kinds of cancer cells with remarkable signal-to-noise ratios. In cancer tissue sections of the breast and liver in mice, XN3 had the ability to differentiate between healthy and tumorous tissues, and possessed excellent fluorescence stability, high tissue penetration and low tissue autofluorescence. Finally, XN3 was successfully utilized for in situ visualizing NTR activities in human transverse colon and rectal cancer tissues, respectively. The findings suggested that XN3 could directly identify the boundary between cancer and normal tissues by monitoring NTR activities, which provides a new method for imaging diagnosis and intraoperative navigation of tumor tissue.

Modulating Aggregation and Deaggregation Based on Assembling Strategy to Switch on NIR-II Light-Excited Fluorescence for Self-Reporting Viability of Eliminating Cancer Cell

The fabrication of self-reporting photosensitizers (PSs), enabling real-time evaluation of the extent of elimination of cancer cells, holds significant scientific importance in the photodynamic therapy (PDT) process. To address the intrinsic challenge of the short-wavelength light source, this work proposed an innovative approach of rational design second near-infrared (NIR-II, 1000-1700 nm) light-excited fluorescent PS systems (named HOEt-PI, Me-PI, and Et-PI, respectively) through modulating aggregation and deaggregation based on assembling strategy. Therein, the suitable interplanar distance of adjacent Et-PI linked with C-H···π interactions was an idea for relieving compact π···π packing for fluorescent imaging as well as elevating the spin-orbit coupling for reactive oxygen species (ROS) generation. With ROS continuously increasing, Et-PI underwent cell membrane-to-mitochondria migration, ultimately accumulated in nucleoli, symbolizing programmed cell death, thus distinguishing dead/live cells via three-photon fluorescence imaging (excited on 1250 nm) under photogeneration ROS. Meaningfully, the three-photon fluorescence of Et-PI was triggered by RNA of nucleoli, for which the higher signal-to-noise ratio and in-depth fluorescence imaging observed cancer cellular viability. Collectively, the proposed findings presented a constructing strategy for NIR-II light-mediated self-reporting PS for guiding the PDT of deep cancerous tissue in the future.

A Single-Shot Technique for Measuring Broadband Two-Photon Absorption Spectra in Solution

Applications involving two-photon activation, including two-photon fluorescence imaging, photodynamic therapy, and 3D data storage, require precise knowledge of the two-photon absorption (2PA) spectra of target chromophores. Broadband pump-probe spectroscopy using femtosecond laser pulses provides wavelength-dependent 2PA spectra with absolute cross sections, but the measurements are sometimes complicated by cross-phase modulation effects and dispersion of the broadband probe. Here, we introduce a single-shot approach that eliminates artifacts from cross-phase modulation and enables more rapid measurements by avoiding the need to scan the time delay between the pump and the probe pulses. The approach uses counterpropagating beams to automatically integrate over the full interaction between the two pulses as they cross. We demonstrate this single-shot approach for a common 2PA reference, coumarin 153 (C153), in three different solvents using the output from a Yb:KGW laser. This approach provides accurate 2PA cross sections that are more reliable and easier to obtain compared with scanning pump-probe methods using copropagating laser beams. The single-shot method for broadband two-photon absorption (BB-2PA) spectroscopy also has significant advantages compared with single-wavelength measurements, such as z-scan and two-photon fluorescence.

A Mitochondria-Targeted Nitric Oxide Probe for Multimodality Imaging of Macrophage Immune Responses

Nitric oxide (NO) is a crucial signal molecule closely linked to the biological immune response, especially in macrophage polarization. When activated, macrophages enter a pro-inflammatory state and produce NO, a marker for the M1 phenotype. In contrast, the anti-inflammatory M2 phenotype does not produce NO. We developed a mitochondria-targeted two-photon iridium-based complex (Ir-ImNO) probe that can detect endogenous NO and monitor macrophages' different immune response states using various imaging techniques, such as one- and two-photon phosphorescence imaging and phosphorescence lifetime imaging. Ir-ImNO was used to monitor the immune activation of macrophages in mice. This technology aims to provide a clear and comprehensive visualization of macrophage immune responses.

Rational construction and evaluation of a dual-functional near-infrared fluorescent probe for the imaging of Amyloid-β and mitochondrial viscosity

Alzheimer's disease is a fatal, incurable, chronic neurodegenerative disease. Diagnosis in its early and even preclinical stages will be beneficial for its prevention and treatment. In the accepted pathological theory, abnormal accumulation of Aβ protein and abnormal mitochondrial function, including changes in mitochondrial viscosity, is closely related to Alzheimer's disease. To date, rare fluorescent probes have been reported that can simultaneously image Aβ plaques and mitochondrial viscosity. Therefore, the development of a dual-functional fluorescent probe for real-time fluorescence imaging of Aβ plaques and mitochondrial viscosity is crucial to discover a novel approach and strategy for the treatment of Alzheimer's disease, and to understand the pathological process and crosstalk between different biomarkers of Alzheimer's disease. Herein, we rationally designed and synthesized a series of fluorescent probes QM-SF-1∼5 with dimethylamino-quinolinium as the skeleton and thiophene as the π bridge to connect the groups with different electron-push/pull capacities. Among them, QM-SF-2 exhibited excellent properties such as large Stokes shift (168 nm), near-infrared emission (689 nm), and high selectivity and sensitivity (limit of detection was 1.07 μM) to Aβ aggregate and mitochondrial viscosity changes, indicating its promising prospects as a dual-functional imaging tool in the pathological study of Alzheimer's disease.

Quantifying cell viability through organelle ratiometric probing

Detecting cell viability is crucial in research involving the precancerous discovery of abnormal cells, the evaluation of treatments, and drug toxicity testing. Although conventional methods afford cumulative results regarding cell viability based on a great number of cells, they do not permit investigating cell viability at the single-cell level. In response, we rationally designed and synthesized a fluorescent probe, PCV-1, to visualize cell viability under the super-resolution technology of structured illumination microscopy. Given its sensitivity to mitochondrial membrane potential and affinity to DNA, PCV-1's ability to stain mitochondria and nucleoli was observed in live and dead cells, respectively. During cell injury induced by drug treatment, PCV-1's migration from mitochondria to the nucleolus was dynamically visualized at the single-cell level. By extension, harnessing PCV-1's excellent photostability and signal-to-noise ratio and by comparing the fluorescence intensity of the two organelles, mitochondria and nucleoli, we developed a powerful analytical assay named organelle ratiometric probing (ORP) that we applied to quantitatively analyze and efficiently assess the viability of individual cells, thereby enabling deeper insights into the potential mechanisms of cell death. In ORP analysis with PCV-1, we identified 0.3 as the cutoff point for assessing whether adding a given drug will cause apparent cytotoxicity, which greatly expands the probe's applicability. To the best of our knowledge, PCV-1 is the first probe to allow visualizing cell death and cell injury under super-resolution imaging, and our proposed analytical assay using it paves the way for quantifying cell viability at the single-cell level.