Synthesis of Two-Photon Active Tricomponent Fluorescent Probe for Distinguishment of Biotin Receptor Positive and Negative Cells and Imaging 3D-Spheroid

Chronological development of functional fluorophores for bio-imaging

Functional fluorophores represent an emerging research field, distinguished by their diverse applications, especially in sensing and cellular imaging. After the discovery of quinine sulfate and subsequent elucidation of the fluorescence mechanism by Sir George Stokes, research in the field of fluorescence gained momentum. Over the past few decades, advancements in sophisticated instruments, including super-resolution microscopy, have further promoted cellular imaging using traditional fluorophores. These advancements include deciphering sensing mechanisms via photochemical reactions and scrutinizing the applications of fluorescent probes that specifically target organelles. This approach elucidates molecular interactions with biomolecules. Despite the abundance of literature illustrating different classes of probe development, a concise summary of newly developed fluorophores remains inadequate. In this review, we systematically summarize the chronological discovery of traditional fluorophores along with new fluorophores. We briefly discuss traditional fluorophores ranging from visible to near-infrared (NIR) in the context of cellular imaging and in vivo imaging. Furthermore, we explore ten new core fluorophores developed between 2007 and 2022, which exhibit advanced optical properties, providing new insights into bioimaging. We illustrate the utilization of new fluorophores in cellular imaging of biomolecules, such as reactive oxygen species (ROS), reactive nitrogen species (RNS), and proteins and microenvironments, especially pH and viscosity. Few of the fluorescent probes provided new insights into disease progression. Furthermore, we speculate on the potential prospects and significant challenges of existing fluorophores and their potential biomedical research applications. By addressing these aspects, we intend to illuminate the compelling advancements in fluorescent probe development and their potential influence across various fields.

Photoinduced Ring-Opening and Phototoxicity of an Indolin-3-one Derivative

The study of a fluorescent indolin-3-one derivative is reported that, as opposed to its previously described congeners, selectively undergoes photoactivated ring-opening in apolar solvents. The excited state involved in this photoisomerization was partially deactivated by the formation of singlet oxygen. Cell studies revealed lipid droplet accumulation and efficient light-induced cytotoxicity.

A two-photon fluorescence probe for cell membrane imaging under temporal-focusing multiphoton excitation microscopy (TFMPEM)

A small-sized chromophore, BTTA-2OH, manifesting favorable solubility, large two-photon excitation efficiency, and good fluorescence photostability was synthesized to label the membrane of living cells for visualizing the dynamic movement of membrane-related vesicles via a two-photon fluorescence imaging technique based on wavelength-tunable temporal-focusing multiphoton excitation microscopy.

Strategic engineering of alkyl spacer length for a pH-tolerant lysosome marker and dual organelle localization

Long-term visualization of lysosomal properties is extremely crucial to evaluate diseases related to their dysfunction. However, many of the reported lysotrackers are less conducive to imaging lysosomes precisely because they suffer from fluorescence quenching and other inherent drawbacks such as pH-sensitivity, polarity insensitivity, water insolubility, slow diffusibility, and poor photostability. To overcome these limitations, we have utilized an alkyl chain length engineering strategy and synthesized a series of lysosome targeting fluorescent derivatives namely NIMCs by attaching a morpholine moiety at the peri position of the 1,8-naphthalimide (NI) ring through varying alkyl spacers between morpholine and 1,8-naphthalimide. The structural and optical properties of the synthesized NIMCs were explored by 1H-NMR, single-crystal X-ray diffraction, UV-Vis, and fluorescence spectroscopy. Afterward, optical spectroscopic measurements were carefully performed to identify a pH-tolerant, polarity sensitive, and highly photostable fluoroprobes for further live-cell imaging applications. NIMC6 displayed excellent pH-tolerant and polarity-sensitive properties. Consequently, all NIMCs were employed in kidney fibroblast cells (BHK-21) to investigate their applicability for lysosome targeting and probing lysosomal micropolarity. Interestingly, a switching of localization from lysosomes to the endoplasmic reticulum (ER) was also achieved by controlling the linker length and this phenomenon was subsequently applied in determining ER micropolarity. Additionally, the selected probe NIMC6 was also employed in BHK-21 cells for 3-D spheroid imaging and in Caenorhabditis elegans (C. elegans) for in vivo imaging, to evaluate its efficacy for imaging animal models.

Probing biotin receptors in cancer cells with rationally designed fluorogenic squaraine dimers

Fluorogenic probes enable imaging biomolecular targets with high sensitivity and maximal signal-to-background ratio under non-wash conditions. Here, we focus on the molecular design of biotinylated dimeric squaraines that undergo aggregation-caused quenching in aqueous media through intramolecular H-type dimerization, but turn on their fluorescence in apolar environment due to target-mediated disaggregation. Our structure-property study revealed that depending on the linkers used to connect the squaraine dyes, different aggregation patterns could be obtained (intramolecular dimerization versus intermolecular aggregation) leading to different probing efficiencies. Using a relatively short l-lysine linker we developed a bright fluorogenic probe, Sq2B, displaying only intramolecular dimerization-caused quenching properties in aqueous media. The latter was successfully applied for imaging biotin receptors, in particular sodium-dependent multivitamin transporter (SMVT), which are overexpressed at the surface of cancer cells. Competitive displacement with SMVT-targets, such as biotin, lipoic acid or sodium pantothenate, showed Sq2B targeting ability to SMVT. This fluorogenic probe for biotin receptors could distinguish cancer cells (HeLa and KB) from model non-cancer cell lines (NIH/3T3 and HEK293T). The obtained results provide guidelines for development of new dimerization-based fluorogenic probes and propose bright tools for imaging biotin receptors, which is particularly important for specific detection of cancer cells.