Development of Small-Molecule Fluorescent Probes Targeting Enzymes

Design, Synthesis, and Evaluation of Boron Dipyrromethene-Based Fluorescent Probes Targeting BRAF for Melanoma Diagnosis

Fluorescent dyes are widely applied in clinical diagnosis, detection, and treatment of diseases. Several image probes such as ICG, MB, and 5-ALA have been approved by FDA. However, the limited tumor-targeting capability of these dyes hinders their effectiveness in oncological imaging. Currently, various ligand-based targeting probes have been developed to minimize nonspecific background emission. BRAF, especially BRAF V600E, is a common cancer gene and undergoes frequent mutation in melanoma. Small molecular BRAF kinase inhibitors have been approved for the treatment of melanoma patients carrying the BRAF V600E mutation, including Vemurafenib, Dabrafenib and so on. Boron dipyrromethene (BODIPY) as an important fluorescent class has been investigated extensively. Vemurafenib-BODIPY has been reported to visualize BRAF V600E mutated cancer cells. Herein, the designed BODIPY-based Vemurafenib derivatives targeting BRAF for cancer cell imaging are reported. The fluorescent probes are characterized and evaluated of photophysical properties, targeted binding and live cell imaging. Compound 1a exhibited promising fluorescence imaging ability. To improve fluorescence quantum yield, structural optimization is performed by incorporating meso N,N'-dialkyl-substituted amides to BODIPY core. Compound 1d shows excellent fluorescence properties and nice binding affinity. It allows visualization of BRAF V600E mutated cancer cells at low concentrations.

Fluorescent probes in autoimmune disease research: current status and future prospects

Autoimmune diseases (AD) present substantial challenges for early diagnosis and precise treatment due to their intricate pathogenesis and varied clinical manifestations. While existing diagnostic methods and treatment strategies have advanced, their sensitivity, specificity, and real-time applicability in clinical settings continue to exhibit significant limitations. In recent years, fluorescent probes have emerged as highly sensitive and specific biological imaging tools, demonstrating substantial potential in AD research.This review examines the response mechanisms and historical evolution of various types of fluorescent probes, systematically summarizing the latest research advancements in their application to autoimmune diseases. It highlights key applications in biomarker detection, dynamic monitoring of immune cell functions, and assessment of drug treatment efficacy. Furthermore, this article analyzes the technical challenges currently encountered in probe development and proposes potential directions for future research. With ongoing advancements in materials science, nanotechnology, and bioengineering, fluorescent probes are anticipated to achieve higher sensitivity and enhanced functional integration, thereby facilitating early detection, dynamic monitoring, and innovative treatment strategies for autoimmune diseases. Overall, fluorescent probes possess substantial scientific significance and application value in both research and clinical settings related to autoimmune diseases, signaling a new era of personalized and precision medicine.

Sensing and Imaging Agents for Cyclooxygenase Enzyme

In this concept, we present a comprehensive study on the development and application of COX-2-specific fluorescent probes for cancer imaging and diagnosis. To target cancer cells and measuring cancer-related activities in specific organelles quickly and accurately are crucial factors for early diagnosis and research on cancer pathology and treatment. This concept explores a variety of probes based on indomethacin (IMC), celecoxib, rofecoxib as well as CoxFluor and each one demonstrates unique mechanisms and high selectivity towards COX-2 enzymes. These probes were designed to enhance fluorescence upon binding to COX-2 which enable precise visualization of tumor and inflamed tissues. The research emphasizes the importance of COX-2 as a biomarker in cancer diagnostics, particularly in identifying cancer stem cells and inflamed tissues. This concept highlights the potentiality of these probes in non-invasive imaging techniques which offering significant advancements in cancer diagnosis and monitoring. The in vivo and in vitro experiments, including applications in mouse models and human tissue samples, confirm the efficacy of these probes in providing detailed imaging for clinical and research applications.

Advancements in phasor-based FLIM: multi-component analysis and lifetime probes in biological imaging

Fluorescence lifetime imaging microscopy (FLIM) is a reliable method that achieves imaging by detecting fluorescence lifetimes within samples. Owing to its unique temporal characteristic, it can complement fluorescence intensity measurement. Technological and methodological advancements in FLIM have broadened its applications across various domains. The processing of fluorescence lifetime data is crucial for enhancing the speed and accuracy of imaging. Thus, various lifetime fitting algorithms have been developed to improve the imaging speed. The phasor analysis (PA) method is an approach for processing fluorescence lifetime data, capable of directly converting lifetime signals into visual graphics without fitting, which outperforms traditional approaches in speed. Furthermore, lifetime probes with distinct lifetimes are readily implemented for visualization and cluster analysis combined with PA, facilitating the prediction of specific biological states or functions. This review examines various lifetime probes employed in phasor-based FLIM and discusses their roles in the PA method. The methods for multi-component PA within complex biological environments were also described. Additionally, we focused on the advantages of the phasor vector rule and the unmixing of multi-component analysis based on PA. The integration of lifetime probes with phasor-based FLIM facilitates rapid and intuitive detection methods for analyzing complex biological environments.

Development of a small molecule-based two-photon photosensitizer for targeting cancer cells

Photodynamic therapy (PDT) employing two-photon (TP) excitation is increasingly recognized to induce cell damage selectively in targeted areas, underscoring the importance of developing TP photosensitizers (TP-PSs). In this study, we developed BSe-B, a novel PS that combines a selenium containing dye with biotin, a cancer-selective ligand, and is optimized for TP excitation. BSe-B demonstrated enhanced cancer selectivity, efficient generation of type-I based reactive oxygen species (ROS), low dark toxicity, and excellent cell-staining capability. Evaluation across diverse cell lines (HeLa, A549, OVCAR-3, WI-38, and L-929) demonstrated that BSe-B differentiated and targeted cancer cells while sparing normal cells. BSe-B displayed excellent in vivo biocompatibility. In cancer models such as three-dimensional spheroids and actual colon cancer tissues, BSe-B selectively induced ROS production and cell death under TP irradiation, demonstrating precise spatial control. These findings highlight the potential of BSe-B for imaging-guided PDT and its capability for micro treatment within tissues. Thus, BSe-B demonstrates robust TP-PDT capabilities, making it a promising dual-purpose tool for cancer diagnosis and treatment.

Advances in small-molecule fluorescent probes for the study of apoptosis

Apoptosis, as type I cell death, is an active death process strictly controlled by multiple genes, and plays a significant role in regulating various activities. Mounting research indicates that the unique modality of cell apoptosis is directly or indirectly related to different diseases including cancer, autoimmune diseases, viral diseases, neurodegenerative diseases, etc. However, the underlying mechanisms of cell apoptosis are complicated and not fully clarified yet, possibly due to the lack of effective chemical tools for the nondestructive and real-time visualization of apoptosis in complex biological systems. In the past 15 years, various small-molecule fluorescent probes (SMFPs) for imaging apoptosis in vitro and in vivo have attracted broad interest in related disease diagnostics and therapeutics. In this review, we aim to highlight the recent developments of SMFPs based on enzyme activity, plasma membranes, reactive oxygen species, reactive sulfur species, microenvironments and others during cell apoptosis. In particular, we generalize the mechanisms commonly used to design SMFPs for studying apoptosis. In addition, we discuss the limitations of reported probes, and emphasize the potential challenges and prospects in the future. We believe that this review will provide a comprehensive summary and challenging direction for the development of SMFPs in apoptosis related fields.

A naphthalimide-based portable fluorescent sensor integrated with a photoelectric converter for rapid and on-site detection of type II pyrethroids in celery

The on-site detection of pyrethroids, particularly type II pyrethroids, remains a challenging task in complex vegetable samples. Herein, a novel method based on naphthalimide was developed to realize the specific detection of type II pyrethroids by hydrolyzing and utilizing the compound m-phenoxybenzaldehyde (3-PBD). Hydrazine group, used as the appropriate moiety, was introduced into the fluorescent dye 1,8-naphthalimide to construct the fluoroprobe NAP. In the presence of 3-PBD, NAP displayed the prominently enhanced fluorescence and also exhibited high selectivity. This proposed method exhibited high anti-inference effects in complex media, realizing sensitive detection of 3-PBD with linear range of 2.15-800 μM and a low detection limit (LOD) of 0.64 μM. The underlying fluorescence-responsive mechanisms were in-depth elucidated by combining spectral analyses with TD-DFT theoretical calculations. Additionally, a direct and rapid hydrolysis method for deltamethrin in celery was established, achieving a high hydrolysis efficiency of >90% within 15 min. Furthermore, a portable fluorescence sensor (PFS) was developed based on high-power LEDs and photodetectors. PFS supplied a LOD of 2.23 μM for 3-PBD and exhibited comparable stability by a fluorescence spectrometer when detecting celery hydrolysate. Moreover, external power source is not required for PFS operations, thereby enabling rapid and on-site detection by transmitting data to a smartphone via bluetooth. These findings extend the academic knowledge in the field of specific pyrethroids detection and contribute to the development of on-site methods for pesticide residual analyses in food matrices.

Development of a Pyrone-Fused Tricyclic Scaffold-based Ratiometric Fluorescent Probe for Al3+ Detection

Aluminum (Al3+) is environmentally abundant and can harm living organisms in various ways, such as by inhibiting root growth, damaging faunal nervous systems, and promoting tumor cell proliferation. However, the dynamics of Al3+ in living organisms are largely unknown; thus, detecting Al3+ in the environment and organisms is crucial. Fluorescent probes are useful tools for the selective detection of metal ions. In particular, ratiometric fluorescent probes exhibit a detection response at two different maximum fluorescence emission wavelengths; which is advantageous for avoiding the influence of background fluorescence. A novel pyrone-fused tricyclic scaffold-based ratiometric fluorescent probe for detecting Al3+, ethyl 11-imino-1-oxo-3-phenyl-1H,11H-pyrano[4,3-b] quinolizine-5-carboxylate (PQ), was developed in this study. The PQ fluorescence blue shifted from 505 to 457 nm upon the addition of Al3+. The blue shift was accompanied by a change in the fluorescence color of the PQ solution from green to blue. Fluorescence titration experiments demonstrated that the fluorescence intensity ratio at the two peaks of interest (457/505 nm) increased in a concentration-dependent manner upon the addition of Al3+. Moreover, this study demonstrated that a PQ-soaked paper displays a visible color change under ultraviolet light upon exposure to Al3+. The above results suggest that PQ is an effective ratiometric probe for the detection of Al3+ in the environment. Future studies will be conducted to introduce various substituents and develop fluorescent probes by leveraging the fluorescence property of a pyrone-fused tricyclic scaffolds.

A comparative study of two thienopyrimidine Schiff base probes for sequential monitoring of Ga3+ and Pd2

Two Schiff base probes (S1 and S2) were prepared and synthesized by incorporating thienopyrimidine into salicylaldehyde or 3-ethoxysalicylaldehyde individually, with the aim of detecting Ga3+ and Pd2+ sequentially. Upon chelation with Ga3+, S1 and S2 exhibited fluorescence enhancement in DMSO/H2O buffer. Both S1-Ga3+ and S2-Ga3+ were quenched by Pd2+. The limit of detection for S1 in response to Ga3+ and Pd2+ was 2.86 × 10-7 and 4.4 × 10-9 M, respectively. For S2, the limit of detection for Ga3+ and Pd2+ was 4.15 × 10-8 and 3.0 × 10-9 M, respectively. Furthermore, the complexation ratios of both S1 and S2 with Ga3+ and Pd2+ were determined to be 1:2 through Job's plots, ESI-MS analysis, and theoretical calculations. Two molecular logic gates were constructed, leveraging the response behaviors of S1 and S2. Moreover, the potential utility of S1 and S2 for monitoring Ga3+ and Pd2+ in domestic water was verified.

A novel fluorescent probe based on carbazole-thiophene for the recognition of hypochlorite and its applications

A carbazole thiophene-aldehyde and 4-methylbenzenesulfonhydrazide conjugate CSH was synthesized by introducing 5-thiophene aldehyde at the 3-position of the carbazole group as the precursor and then condensing it with 4-methylbenzenesulfonhydrazide. CSH has high selectivity and sensitivity towards ClO-, which can specifically identify ClO- by UV-Vis and fluorescence spectroscopy. CSH can rapidly respond to ClO- in the physiological pH range through a fluorescence quenching pattern, accompanied by the color of CSH changing markedly from turquoise to yellowish green under the 365 nm UV light. Probe CSH exhibits a quantitative response to ClO- (0-11 μM) with a low detection limit (1.16 × 10-6 M). Cell imaging experiments have shown that CSH can capture fluorescent signals in the cyan and yellow channels of HeLa cells through fluorescence confocal microscopy, and can successfully identify exogenous ClO- in HeLa cells. In addition, probe CSH can also be used to detect ClO- in environmental water samples. These results indicate that CSH has potential application prospects in the environmental analysis and biological aspects.

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.

Recent advances in small-molecule fluorescent probes with the function of targeting cancer receptors

Cancer is "the sword of Damocles" that threatens human life and health. Therefore, the diagnosis and treatment of cancer have been receiving much attention. Many overexpressed receptors on the surface of cancer cells provide us with an effective way to specifically identify the cancer cells, and receptor targeting strategies are becoming one of the hot ideas to enhance the ability of fluorescent probes to target tumors. Fluorescent probes connected to ligands are targeted at cancer cell surfaces through receptor-mediated endocytosis. Receptor-targeting probes can image and track cancer cells, determine tumor boundaries, monitor deep lesions, and play a role in clinical medicine, such as fluorescent imaging-guided surgery. In this review, based on the perspective of small molecule fluorescent probes, we reviewed the design ideas, photophysical properties, and applications of receptor-targeting probes for detecting biomarkers in imaging and tracing cancer cells and prospected the future developmental direction of such probes. We hope that this review will provide more ideas for the design and development of active targeting probes for receptors and lead to more applications in the medical field.

A Single Organic Fluorescent Probe for the Discrimination of Dual Spontaneous ROS in Living Organisms: Theoretical Approach

Single-organic-molecule fluorescent probes with double-lock or even multi-lock response modes have attracted the attention of a wide range of researchers. The number of corresponding reports has rapidly increased in recent years. The effective application of the multi-lock response mode single-molecule fluorescent probe has improved the comprehensive understanding of the related targets' functions or influences in pathologic processes. Building a highly efficient functional single-molecule fluorescent probe would benefit the diagnosis and treatment of corresponding diseases. Here, we conducted a theoretical analysis of the synthesizing and sensing mechanism of this kind of functional single-molecule fluorescent probe, thereby guiding the design and building of new efficient probes. In this work, we discuss in detail the electronic structure, electron excitation, and fluorescent character of a recently developed single-molecule fluorescent probe, which could achieve the discrimination and profiling of spontaneous reactive oxygen species (ROS, •OH, and HClO) simultaneously. The theoretical results provide insights that will help develop new tools for fluorescent diagnosis in biological and medical fields.

A Turn-On Lipid Droplet-Targeted Near-Infrared Fluorescent Probe with a Large Stokes Shift for Detection of Intracellular Carboxylesterases and Cell Viability Imaging

Carboxylesterases (CEs) play important physiological roles in the human body and are involved in numerous cellular processes. Monitoring CEs activity has great potential for the rapid diagnosis of malignant tumors and multiple diseases. Herein, we developed a new phenazine-based "turn-on" fluorescent probe DBPpys by introducing 4-bromomethyl-phenyl acetate to DBPpy, which can selectively detect CEs with a low detection limit (9.38 × 10^-5 U/mL) and a large Stokes shift (more than 250 nm) in vitro. In addition, DBPpys can also be converted into DBPpy by carboxylesterase in HeLa cells and localized in lipid droplets (LDs), emitting bright near-infrared fluorescence under the irradiation of white light. Moreover, we achieved the detection of cell health status by measuring the intensity of NIR fluorescence after co-incubation of DBPpys with H₂O₂-pretreated HeLa cells, indicating that DBPpys has great potential applications for assessing CEs activity and cellular health.

Fluorescent "AND" logic gates for simultaneous detection of thiols and protons: photophysical properties, mechanism and bioimaging of living cells

Five fluorescent probes TP1-5 were demonstrated as two-input "AND" molecular logic gates for the detection of thiols and protons. The molecules were designed based on "thiol receptor-spacer₁-fluorophore-spacer₂-proton receptor" mode. The logic gates were constructed by employing maleimide, naphthalimide and morpholine (TP1-3)/N-methyl piperazine (TP4-5) as the thiol receptor, fluorophore and proton receptor, respectively. All probes show significant fluorescence enhancements upon addition of both protons and thiols. However, much weaker spectral responses were observed with the addition of only one single analyte. The fluorescence outputs, based on photoinduced electron transfer (PET) and (twisted) intramolecular charge transfer (TICT/ICT), were modulated by the proton receptor and linker. The length of spacer₁ affects the responses toward thiols, whereas spacer₂ influences the sensing performance toward protons. The difference between the pK_a values of morpholine (∼5.80) and N-methyl piperazine (∼7.10) enables us to detect thiols in divergent pH circumstances. TP1-3 exhibit an excellent "AND" logic function for simultaneous detection of protons and thiols as well as bioimaging thiols in weakly acidic living cells. However, TP4 and TP5 are not good candidates for executing "AND" logic operation possibly due to the stronger electron donating properties and steric effect of N-methyl piperazine.

Advances in small molecule two-photon fluorescent trackers for lipid droplets in live sample imaging

Two-photon fluorescent trackers for monitoring of lipid droplets (LDs) would be highly effective for illustrating the critical roles of LDs in live cells or tissues. Although a number of one-photon fluorescent trackers for labeling LDs have been developed, their usability remains constrained in live sample imaging due to photo damage, shallow imaging depth, and auto-fluorescence. Recently, some two-photon fluorescent trackers for LDs have been developed to overcome these limitations. In this mini-review article, the advances in two-photon fluorescent trackers for monitoring of LDs are summarized. We summarize the chemical structures, two-photon properties, live sample imaging, and biomedical applications of the most recent representative two-photon fluorescent trackers for LDs. Additionally, the current challenges and future research trends for the two-photon fluorescent trackers of LDs are discussed.