Small Two-Photon Organic Fluorogenic Probes: Sensing and Bioimaging of Cancer Relevant Biomarkers

A novel isophorone-based NIR fluorescent and colormetric probe for Al3+ sensing and its application for living cells and plants imaging

In this paper, an isophorone-based NIR fluorescent and colormetric probe BDDH for Al3+ was synthesized and characterized, it showed highly selectivity and sensitivity through significant fluorescence enhancement and visible color change towards Al3+. The job plot confirmed that the binding ratio of BDDH with Al3+ was 1:1. Furthermore, the limit of detection (LOD) of Al3+ was determined to be 4.01 × 10-8 M. Moreover, BDDH was successfully applicated in identification of Al3+ in the different water samples, cell imaging in alive MCF-7 cells and plant imaging in soybean roots.

Mitochondria-targetable small molecule fluorescent probes for the detection of cancer-associated biomarkers: A review

Cancer represents a global threat to human health, and effective strategies for improved cancer early diagnosis and treatment are urgently needed. The detection of tumor biomarkers has been one of the important auxiliary means for tumor screening and diagnosis. Mitochondria are crucial subcellular organelles that produce most chemical energy used by cells, control metabolic processes, and maintain cell function. Evidence suggests the close involvement of mitochondria with cancer development. As a consequence, the identification of cancer-associated biomarker expression levels in mitochondria holds significant importance in the diagnosis of early-stage diseases and the monitoring of therapy efficacy. Small-molecule fluorescent probes are effective for the identification and visualization of bioactive entities within biological systems, owing to their heightened sensitivity, expeditious non-invasive analysis and real-time detection capacities. The design principles and sensing mechanisms of mitochondrial targeted fluorescent probes are summarized in this review. Additionally, the biomedical applications of these probes for detecting cancer-associated biomarkers are highlighted. The limitations and challenges of fluorescent probes in vivo are also considered and some future perspectives are provided. This review is expected to provide valuable insights for the future development of novel fluorescent probes for clinical imaging, thereby contributing to the advancement of cancer diagnosis and treatment.

Chemical-chemical redox cycling for improving the sensitivity of the fluorescent assay: A proof-of-concept towards DNA methylation detection

Ultrasensitive analytical methods are still urgent for the discovery of trace level biomarkers and the early clinical diagnosis of disease. In this work, an ultrasensitive universal sensing platform was constructed by integrating fluorescent assay with chemical-chemical redox cycling signal amplification strategy. Using Ru@SiO2 nanoparticles wrapped by MnO2 nanosheets (Ru@SiO2@MnO2) as fluorescent probe, the chemical-chemical redox cycling system was conducted upon ascorbic acid (AA) and tris(2-carboxyethyl)phosphine (TCEP) as reductants and MnO2 nanosheets as oxidant. The MnO2 nanosheets not only could quench the fluorescence of Ru@SiO2 nanoparticles to reduce the background, but also could serve as oxidants to react with AA, generating dehydroascorbic acid (DHA). The DHA was reduced by TCEP in turn to form AA that participated in the next cycling of chemical-chemical redox reaction. Thus, the constantly released AA from the chemical-chemical redox cycling system could massively etch MnO2 nanosheets on Ru@SiO2 surface, making the fluorescence of Ru@SiO2 nanoparticles greatly recovered. It was shown that the sensitivity of the fluorescent assay was improved almost 52 times by utilizing the chemical-chemical redox cycling signal amplification strategy. This strategy was further employed to detect DNA methylation with the aid of AA-encapsulated liposomes that were modified with 5 mC antibodies to bind with the methylated DNA captured in 96-well plate. A detection of limit down to 16.2 fM was achieved for the detection of methylated DNA. It's believed that the incorporation of chemical-chemical redox cycling signal amplification strategy into fluorescent sensing paves a new way for ultrasensitive detection of biomarkers.

Golgi-targeting viscosity probe for the diagnosis of Alzheimer's disease

Early diagnosis and intervention of Alzheimer's disease (AD) are particularly important to delay the pathological progression. Although fluorescent probes have been widely employed for investigating and diagnosing AD, their biological applications are significantly restricted due to the low penetration ability of the blood-brain barrier (BBB) in vivo. In this study, we reported the first Golgi-targeted two-photon (TP) fluorescent probe, DCM-DH, for detecting viscosity in the Golgi apparatus. The probe was rationally designed to exhibit superior analytical performance including high sensitivity, specific Golgi-targeting, efficient BBB penetration ability, and deep tissue penetration (247 μm) in the brains of AD model mice. Using the probe, we demonstrated that the fluorescence intensity in the human liver cancer cell (HepG2 cells) was higher than that of human normal liver cell (LO2 cells), and the brain viscosity of AD model mice increased significantly. We anticipate that this competent tool could be easily extended to other AD biomarkers for fundamental research on this detrimental disease.

Design of 2-Pyridone Fluorophores for Brighter Emissions at Longer Wavelengths

The recent discovery of blue fluorophores with high quantum yields based on pyridone structures inspired the development of new low-molecular-weight fluorophores with bright emissions at tunable wavelengths, which are highly attractive for various applications. In this study, we propose a rational design strategy for 2-pyridone-based fluorophores with bright emissions at long wavelengths. With a detailed understanding of the positional substitution effects on each carbon atom of the 2-pyridone core, we developed a bright blue fluorophore (λabs =377 nm; λem =433 nm; ϵ=13,200 M-1  cm-1 ; ϕF =88 %) through C3 -aryl and C4 -ester substitutions followed by cyclization. Furthermore, by applying the intramolecular charge transfer (ICT) principle, we invented a bright green fluorophore through C3 - and C4 -diester and C6 -aryl substitutions. The ICT fluorophore based on the pyridone structure shows large molar absorptivity (ϵ=20,100 M-1  cm-1 ), longer emission wavelength (λem =539 nm), high emission quantum yield (ϕF =74 %), and large Stokes shift (Δv=5720 cm-1 ), which are comparable to those of practical fluorescent probes.

Fluorogen-Functionalized Mesoporous Silica Hybrid Sensing Materials: Applications in Cu2+ Detection

Since Cu2+ ions play a pivotal role in both ecosystems and human health, the development of a rapid and sensitive method for Cu2+ detection holds significant importance. Fluorescent mesoporous silica materials (FMSMs) have garnered considerable attention in the realm of chemical sensing, biosensing, and bioimaging due to their distinctive structure and easily functionalized surfaces. As a result, numerous Cu2+ sensors based on FMSMs have been devised and extensively applied in environmental and biological Cu2+ detection over the past few decades. This review centers on the recent advancements in the methodologies for preparing FMSMs, the mechanisms underlying sensing, and the applications of FMSMs-based sensors for Cu2+ detection. Lastly, we present and elucidate pertinent perspectives concerning FMSMs-based Cu2+ sensors.

Fidelity-oriented fluorescence imaging probes for beta-galactosidase: From accurate diagnosis to precise treatment

Beta-galactosidase (β-gal), a typical glycosidase catalyzing the hydrolysis of glycosidic bonds, is regarded as a vital biomarker for cell senescence and cancer occurrence. Given the advantages of high spatiotemporal resolution, high sensitivity, non-invasiveness, and being free of ionizing radiations, fluorescent imaging technology provides an excellent choice for in vivo imaging of β-gal. In this review, we detail the representative biotech advances of fluorescence imaging probes for β-gal bearing diverse fidelity-oriented improvements to elucidate their future potential in preclinical research and clinical application. Next, we propose the comprehensive design strategies of imaging probes for β-gal with respect of high fidelity. Considering the systematic implementation approaches, a range of high-fidelity imaging-guided theragnostic are adopted for the individual β-gal-associated biological scenarios. Finally, current challenges and future trends are proposed to promote the next development of imaging agents for individual and specific application scenarios.

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

Two-photon absorbing fluorescent probes have emerged as powerful imaging tools for subcellular-level monitoring of biological substances and processes, offering advantages such as deep light penetration, minimal photodamage, low autofluorescence, and high spatial resolution. However, existing two-photon absorbing probes still face several limitations, such as small two-photon absorption cross-section, poor water solubility, low membrane permeability, and potentially high toxicity. Herein, we report three small-molecule probes, namely MSP-1arm, Lyso-2arm, and Mito-3arm, composed of a pyridinium center (electron-acceptor) and various methoxystyrene "arms" (electron-donor). These probes exhibit excellent fluorescence quantum yield and decent aqueous solubility. Leveraging the inherent intramolecular charge transfer and excitonic coupling effect, these complexes demonstrate excellent two-photon absorption in the near-infrared region. Notably, Lyso-2arm and Mito-3arm exhibit distinct targeting abilities for lysosomes and mitochondria, respectively. In two-photon microscopy experiments, Mito-3arm outperforms a commercial two-photon absorbing dye in 2D monolayer HeLa cells, delivering enhanced resolution, broader NIR light excitation window, and higher signal-to-noise ratio. Moreover, the two-photon bioimaging of 3D human forebrain organoids confirms the successful deep tissue imaging capabilities of both Lyso-2arm and Mito-3arm. Overall, this work presents a rational design strategy in developing competent two-photon-absorbing probes by varying the number of conjugated "arms" for bioimaging applications.

Photoactivable AIEgen-based Lipid-Droplet-Specific Drug Delivery Model for Live Cell Imaging and Two-Photon Light-Triggered Anticancer Drug Delivery

Lipid droplets (LDs) are dynamic complex organelles involved in various physiological processes, and their number and activity are linked to multiple diseases, including cancer. In this study, we have developed LD-specific near-infrared (NIR) light-responsive nano-drug delivery systems (DDSs) based on chalcone derivatives for cancer treatment. The reported nano-DDSs localized inside the cancer microenvironment of LDs, and upon exposure to light, they delivered the anticancer drug valproic acid in a spatiotemporally controlled manner. The developed systems, namely, 2'-hydroxyacetophenone-dimethylaminobenzaldehyde-valproic (HA-DAB-VPA) and 2'-hydroxyacetophenone-diphenylaminobenzaldehyde-valproic (HA-DPB-VPA) ester conjugates, required only two simple synthetic steps. Our reported DDSs exhibited interesting properties such as excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) phenomena, which provided advantages such as AIE-initiated photorelease and ESIPT-enhanced rate of photorelease upon exposure to one- or two-photon light. Further, colocalization studies of the nano-DDSs by employing two cancerous cell lines (MCF-7 cell line and CT-26 cell line) and one normal cell line (HEK cell line) revealed LD concentration-dependent enhanced fluorescence intensity. Furthermore, systematic investigations of both the nano-DDSs in the presence and absence of oleic acid inside the cells revealed that nano-DDS HA-DPB-VPA accumulated more selectively in the LDs. This unique selectivity by the nano-DDS HA-DPB-VPA toward the LDs is due to the hydrophobic nature of the diphenylaminobenzaldehyde (mimicking the LD core), which significantly leads to the aggregation and ESIPT (at 90% volume of fw, ΦF = 20.4% and in oleic acid ΦF = 24.6%), respectively. Significantly, we used this as a light-triggered anticancer drug delivery model to take advantage of the high selectivity and accumulation of the nano-DDS HA-DPB-VPA inside the LDs. Hence, these findings give a prototype for designing drug delivery models for monitoring LD-related intracellular activities and significantly triggering the release of LD-specific drugs in the biological field.

Photophysical Exploration of Alectinib and Rilpivirine: Insights from Theory and Experiment

Due to the excellent characteristics of fluorescence-based imaging, such as non-invasive detection of biomarkers in vitro and in vivo with high sensitivity, good spatio-temporal resolution and fast response times, it has shown significant prospects in various applications. Compounds with both biological activities and fluorescent properties have the potential for integrated diagnosis and treatment application. Alectinib and Rilpivirine are two excellent drugs on sale that represent a clinically approved targeted therapy for ALK-rearranged NSCLC and have exhibited more favorable safety and tolerance profiles in Phase III clinical trials, ECHO and THRIVE, respectively. The optical properties of these two drugs, Alectinib and Rilpivirine, were deeply explored, firstly through the simulation of molecular structures, electrostatic potential, OPA/TPA and emission spectral properties and experiments on UV-vis spectra, fluorescence and cell imaging. It was found that Alectinib exhibited 7.8% of fluorescence quantum yield at the 450 nm excited wavelength, due to a larger electronic transition dipole moment (8.41 Debye), bigger charge transition quantity (0.682 e) and smaller reorganization energy (2821.6 cm-1). The stronger UV-vis spectra of Rilpivirine were due to a larger electron-hole overlap index (Sr: 0.733) and were also seen in CDD plots. Furthermore, Alectinib possessed obvious active two-photon absorption properties (δmaxTPA* ϕ = 201.75 GM), which have potential TPA imaging applications in bio-systems. Lastly, Alectinib and Rilpivirine displayed green fluorescence in HeLa cells, suggesting the potential ability for biological imaging. Investigation using theoretical and experimental methods is certainly encouraged, given the particular significance of developing integrated diagnosis and treatment.

Recent progress of TP/NIR fluorescent probes for metal ions

Metal ions are of significance in various pathological and physiological processes. As such, it is crucial to monitor their levels in organisms. Two-photon (TP) and near-infrared (NIR) fluorescence imaging has been utilized to monitor metal ions because of minimal background interference, deeper tissue depth penetration, lower tissue self-absorption, and reduced photodamage. In this review, we briefly summarize recent progress from 2020 to 2022 of TP/NIR organic fluorescent probes and inorganic sensors in the detection of metal ions. Additionally, we present an outlook for the development of TP/NIR probes for bio-imaging, diagnosis of diseases, imaging-guided therapy, and activatable phototherapy.

Using time-shared scanning optical tweezers assisted two-photon fluorescence imaging to establish a versatile CRISPR/Cas12a-mediated biosensor

Based on the admirable precision to identify target nucleic acids and the particular trans-cleavage feature, CRISPR/Cas12a system is a useful means to further improve the sensing accuracy and the design flexibility of fluorescence biosensors. However, the current construction concepts still suffer from insufficient sensitivity, unsuitable for complicated real samples and limited detection species. In this work, much efforts are achieved to address these obstacles. At first, we adopt a microsphere sustained signal enrichment, under which a home-made time-shared scanning optical tweezers assisted fluorescence imaging is employed to guarantee a stable excitation and also realize multiflux measurement. Furthermore, by taking advantage of the low background merit of the near-infrared light excited two-photon fluorescence, a commendable anti-interference capability is endowed to operate in complex media. After utilizing a functional DNA (e.g. aptamer and DNAzyme) regulated mediation pathway to respond non-nucleic acid analytes (alpha fetal protein and Pb²⁺), the newly-established CRISPR/Cas12a-mediated fluorescence biosensor is found to display favorable assay performance. More importantly, our analytical methodology can act as a versatile and reliable toolbox in various applications such as disease diagnosis and environmental analysis, propelling the development of CRISPR system in biosensing field.

ESIPT-, AIE-, and AIE + ESIPT-Based Light-Activated Drug Delivery Systems and Bioactive Donors for Targeted Disease Treatment

Targeted release of bioactive molecules for therapeutic purposes is a key area in the biomedical field that is growing quickly, where bioactive molecules are released passively or actively from drug delivery systems (DDSs) or bioactive donors. In the past decade, researchers have identified light as one of the prime stimuli that can implement the efficient spatiotemporally targeted delivery of drugs or gaseous molecules with minimal cytotoxicity and a real-time monitoring ability. This perspective emphasizes recent advances in the photophysical properties of ESIPT- (excited-state intramolecular proton transfer), AIE- (aggregation-induced emission), and AIE + ESIPT-attributed light-activated delivery systems or donors. The three major sections of this perspective describe the distinctive features of DDSs and donors concerning their design, synthesis, photophysical and photochemical properties, and in vitro and in vivo studies demonstrating their relevance as carrier molecules for releasing cancer drugs and gaseous molecules in the biological system.

An alkaline phosphatase-induced immunosensor for SARS-CoV-2 N protein and cardiac troponin I based on the in situ fluorogenic self-assembly between N-heterocyclic boronic acids and alizarin red S

Alkaline phosphatase (ALP)-induced in situ fluorescent immunosensor is less investigated and reported. Herein, a high-performance ALP-labeled in situ fluorescent immunoassay platform was constructed. The developed platform was based on a fluorogenic self-assembly reaction between pyridineboronic acid (PyB(OH)₂) and alizarin red S (ARS). We first used density functional theory (DFT) to theoretically calculate the changes of Gibbs free energy of the used chemicals before and after the combination and simulated the electrostatic potential on its' surfaces. The free ARS and PyB(OH)₂ exist alone, neither emits no fluorescence. However, the ARS/PyB(OH)₂ complex emits strong fluorescence, which could be effectively quenched by PPi based on the stronger affinity between PPi and PyB(OH)₂ than that of ARS and PyB(OH)₂. PyB(OH)₂ coordinated with ARS again in the presence of ALP due to the ALP-catalyzed hydrolysis of PPi, and correspondingly, the fluorescence was restored. We chose cTnI and SARS-CoV-2 N protein as the model antigen to construct ALP-induced immunosensor, which exhibited a wide dynamic range of 0-175 ng/mL for cTnI and SARS-CoV-2 N protein with a low limit of detection (LOD) of 0.03 ng/mL and 0.17 ng/mL, respectively. Moreover, the proposed immunosensor was used to evaluate cTnI and SARS-CoV-2 N protein level in serum with satisfactory results. Consequently, the method laid the foundation for developing novel fluorescence-based ALP-labeled ELISA technologies in the early diagnosis of diseases.

Characterization of Chlorogenic Acid as a Two-Photon Fluorogenic Probe that Regulates Glycolysis in Tumor Cells under Hypoxia

High levels of steady-state mitochondrial reactive oxygen species (ROS) and glycolysis are hallmarks of cancer. An improved understanding of interactions between tumor energetics and mitochondrial ROS modulation is useful for the development of new anticancer strategies. Here, we show that the natural product chlorogenic acid (CGA) specifically scavenged abnormally elevated mitochondrial O₂^•- and exhibited a two-photon fluorescence turn-on response to tumor cells under hypoxia and tumor tissues in vivo. Furthermore, we illustrated that CGA treatment reduced O₂^•- levels in cells, hampered activation of AMP-activated protein kinase (AMPK), and shifted metabolism from glycolysis to oxidative phosphorylation (OXPHOS), resulting in inhibition of tumor growth under hypoxia. This study demonstrates an efficient two-photon fluorescent tool for real-time assessment of mitochondrial O₂^•- and a clear link between reducing intracellular ROS levels by CGA treatments and regulating metabolism, as well as undeniably helpful insights for the development of new anticancer strategies.

Small-Molecule Fluorescent Probes for Detecting Several Abnormally Expressed Substances in Tumors

Malignant tumors have always been the biggest problem facing human survival, and a huge number of people die from cancer every year. Therefore, the identification and detection of malignant tumors have far-reaching significance for human survival and development. Some substances are abnormally expressed in tumors, such as cyclooxygenase-2 (COX-2), nitroreductase (NTR), pH, biothiols (GSH, Cys, Hcy), hydrogen sulfide (H₂S), hydrogen sulfide (H₂O₂), hypochlorous acid (HOCl) and NADH. Consequently, it is of great value to diagnose and treat malignant tumors due to the identification and detection of these substances. Compared with traditional tumor detection methods, fluorescence imaging technology has the advantages of an inexpensive cost, fast detection and high sensitivity. Herein, we mainly introduce the research progress of fluorescent probes for identifying and detecting abnormally expressed substances in several tumors.

Green Light-Activated Single-Component Organic Fluorescence-Based Nano-Drug Delivery System for Dual Uncaging of Anticancer Drugs

Developing green or red light-activated drug delivery systems (DDSs) for cancer treatment is highly desirable. Herein, we have reported a green light-responsive single component-based organic fluorescence nano-DDS by simply anchoring 2-hydroxy-6-naphthacyl (phototrigger) on both sides of the 1,5-diaminonaphthalene (DAN) chromophore. This green light (λ ≥ 500 nm)-activated DDS released two equivalents of the anticancer drug (valproic acid) in a spatio-temporally controlled manner. Our photoresponsive DDS [DAN-bis(HO-Naph-VPA)] exhibited interesting properties such as excited-state intramolecular proton transfer (ESIPT) accompanied with aggregation-induced emission (AIE) phenomena. AIE initiated the photorelease, and ESIPT enhanced the rate of the photorelease. Further, in vitro studies revealed that our green light-activated nano-DDS exhibited good cytocompatibility, excellent cellular internalization, and effective cancer cell killing ability.

A tracer-type fluorescent probe for imaging adenosine triphosphate under the stresses of hydrogen sulfide and hydrogen peroxide in living cells

Adenosine triphosphate (ATP) is a direct energy source in cells and the core of the biochemical system, and is closely related to various metabolic activities in living organisms.