Carboxylate-Containing Two-Photon Probe for the Simultaneous Detection of Extra- and Intracellular pH Values in Colon Cancer Tissue

Rational Synthesis of Imine-Linked Fluorescent Covalent Organic Frameworks with Different pKa for pH Sensing In Vitro and In Vivo

Precise modulation of pH in living cells plays a vital role in the study of many diseases, such as cancer and rheumatoid arthritis. Here, a series of imine-linked covalent organic frameworks (COFs) were rationally designed and developed for pH sensing in tumor cells and zebrafish. Four monomers were chosen to synthesize COFs (COF₁-COF₄) with different pK_a by a simple orthogonal combination through condensation reaction. The as-obtained COFs exhibited a sensitive pH-dependent fluorescence response compared to their building blocks. Among them, COF₂ possessed a high crystallinity, excellent fluorescence, and suitable pK_a for biosensing. For bioimaging applications, COF₂ was modified with poly-d-lysine (PDL) to improve its biocompatibility and endocytosis efficiency. After that, PDL-modified COF₂ (PDL@COF₂) was used as a novel fluorescence probe with a superior linear pH response over the range from 5.0 to 8.0 due to its fully reversible protonation and deprotonation. The fluorescent PDL@COF₂ was further employed as a good candidate for pH imaging in tumor cells and zebrafish. The as-constructed environment-sensitive fluorescent COFs have greatly expanded the applications of COFs in the biological area.

A MOF-Shell-Confined I-Motif-Based pH Probe (MOFC-i) Strategy for Sensitive and Dynamic Imaging of Cell Surface pH

Dynamic imaging of cell surface pH is extremely challenging due to the slight changes in pH and the fast diffusion of secreted acid to the extracellular environment. In this work, we construct a novel metal-organic framework (MOF)-shell-confined i-motif-based pH probe (MOFC-i) strategy that enables sensitive and dynamic imaging of cell surface pH. The CY3- and CY5-labeled i-motif, which is hybridized via its short complementary chain with two-base mismatches, is optimized for sensing at physiological pH. After efficiently anchoring the optimized pH probes onto the cell membrane with the aid of cholesterol groups, a biocompatible microporous MOF shell is then formed around the cell by cross-linking ZIF-8 nanoparticles via tannic acid. The microporous MOF shell can confine secreted acid without inhibiting the normal physiological activities of cells; thus, the MOFC-i strategy can be used to monitor dynamic changes in the cell surface pH of living cells. Furthermore, this method can not only clearly distinguish the different metabolic behaviors of cancer cells and normal cells but also reveal drug effects on the cell surface pH or metabolism, providing promising prospects in pH-related diagnostics and drug screening.

A Diagnostic Method for Gastric Cancer Using Two-Photon Microscopy With Enzyme-Selective Fluorescent Probes: A Pilot Study

BACKGROUND

Endoscopy is the most important tool for gastric cancer diagnosis. However, it relies on naked-eye evaluation by endoscopists, and the histopathologic confirmation is time-consuming. We aimed to visualize and measure the activity of various enzymes through two-photon microscopy (TPM) using fluorescent probes and assess its diagnostic potential in gastric cancer.

METHODS

β-Galactosidase (β-gal), carboxylesterase (CES), and human NAD(P)H: quinone oxidoreductase (hNQO1) enzyme activities in the normal mucosa, ulcer, adenoma, and gastric cancer biopsy samples were measured using two-photon enzyme probes. The fluorescence emission ratio at long and short wavelengths (Ch2/Ch1) for each probe was comparatively analyzed. Approximately 8,000 - 9,000 sectional images in each group were obtained by measuring the Ch2/Ch1 ratio according to the tissue depth. Each probe was cross-validated by measuring enzymatic activity from a solution containing lysed tissue.

RESULTS

Total of 76 subjects were enrolled in this pilot study (normal 21, ulcer 18, adenoma 17, and cancer 20 patients, respectively). There were significant differences in the mean ratio values of β-gal (0.656 ± 0.142 vs. 1.127 ± 0.109, P < 0.001) and CES (0.876 ± 0.049 vs. 0.579 ± 0.089, P < 0.001) between the normal and cancer, respectively. The mean ratio value of cancer tissues was different compared to ulcer and adenoma (P < 0.001). The hNQO1 activity showed no significant difference between cancer and other conditions. Normal mucosa and cancer were visually and quantitatively distinguished through β-gal and CES analyses using TPM images, and enzymatic activity according to depth, was determined using sectional TPM ratiometric images. The results obtained from lysis buffer-treated tissue were consistent with TPM results.

CONCLUSIONS

TPM imaging using ratiometric fluorescent probes enabled the discrimination of gastric cancer from normal, ulcer, and adenoma. This novel method can help in a visual differentiation and provide quantitative depth profiling in gastric cancer diagnosis.

Recent advances in two-photon absorbing probes based on a functionalized dipolar naphthalene platform

Two-photon microscopy (TPM) techniques have been highlighted over the past two decades throughout various fields, including physics, chemistry, biology, and medicine. In particular, the two-photon near-infrared excitation of fluorophores or molecular probes emitting fluorescence have ushered in a new biomedical era, specifically in the deep-tissue imaging of biologically relevant species. Non-linear two-photon optics enables the development of 3D fluorescence images via focal point excitation of biological samples with low photo-damage and photo-bleaching. Many studies have disclosed the relationship between the chemical structure of fluorophores and their two-photon absorbing properties. In this review, we have summarized the recent advances in two-photon absorbing probes based on a functionalized electron donor (D)-acceptor (A) type dipolar naphthalene platform (FDNP) that was previously reported between 2015 and 2019. Our systematic outline of the synthesis, photophysical properties, and examples of two-photon imaging applications will provide useful context for the future development of new naphthalene backbone-based two-photon probes.

Rational Design of a Long-Wavelength Fluorescent Probe for Highly Selective Sensing of Carboxylesterase 1 in Living Systems

Rational design of practical probes with excellent specificity and improved optical properties for a particular enzyme is always a big challenge. Herein, a practical and highly specific fluorescent probe for carboxylesterase 1 (CES1) was rationally designed using meso-carboxyl-BODIPY as the basic fluorophore based on the substrate preference and catalytic properties of CES1. Following molecular docking-based virtual screening combined with reaction phenotyping-based experimental screening, we found that MMB (probe 7) exhibited the optimal combination of sensitivity and specificity toward human CES1 in contrast to other ester derivatives. Under physiological conditions, MMB could be readily hydrolyzed by CES1 and release MCB; such biotransformation brought great changes in the electronic properties at the meso position of the fluorophore and triggered a dramatic increase in fluorescence emission around 595 nm. Moreover, MMB was cell membrane permeable and was successfully applied to monitor the real activities of CES1 in various biological samples including living cells, tissue slices, organs, and zebrafish. In summary, this study showed a good example for constructing specific fluorescent probe(s) for a target enzyme and also provided a practical and sensitive tool for real-time sensing of CES1 activities in complicated biological samples. All these findings would strongly facilitate high-throughput screening of CES1 modulators and the studies on CES1-associated physiological and pathological processes.