Synthesis of Near‐Infrared‐Emitting Benzorhodamines and Their Applications to Bioimaging and Photothermal Therapy

Recent advances and design strategies for organic afterglow agents to enhance autofluorescence-free imaging performance

Long-lasting afterglow luminescence imaging that detects photons slowly being released from chemical defects has emerged, eliminating the need for real-time photoexcitation and enabling autofluorescence-free in vivo imaging with high signal-to-background ratios (SBRs). Organic afterglow nano-systems are notable for their tunability and design versatility. However, challenges such as unsatisfactory afterglow intensity, short emission wavelengths, limited activatable strategies, and shallow tissue penetration depth hinder their widespread biomedical applications and clinical translation. Such contradiction between promising prospects and insufficient properties has spurred researchers' efforts to improve afterglow performance. In this review, we briefly outline the general composition and mechanisms of organic afterglow luminescence, with a focus on design strategies and an in-depth understanding of the structure-property relationship to advance afterglow luminescence imaging. Furthermore, pending issues and future perspectives are discussed.

Semiconducting polymer dots for multifunctional integrated nanomedicine carriers

The expansion applications of semiconducting polymer dots (Pdots) among optical nanomaterial field have long posed a challenge for researchers, promoting their intelligent application in multifunctional nano-imaging systems and integrated nanomedicine carriers for diagnosis and treatment. Despite notable progress, several inadequacies still persist in the field of Pdots, including the development of simplified near-infrared (NIR) optical nanoprobes, elucidation of their inherent biological behavior, and integration of information processing and nanotechnology into biomedical applications. This review aims to comprehensively elucidate the current status of Pdots as a classical nanophotonic material by discussing its advantages and limitations in terms of biocompatibility, adaptability to microenvironments in vivo, etc. Multifunctional integration and surface chemistry play crucial roles in realizing the intelligent application of Pdots. Information visualization based on their optical and physicochemical properties is pivotal for achieving detection, sensing, and labeling probes. Therefore, we have refined the underlying mechanisms and constructed multiple comprehensive original mechanism summaries to establish a benchmark. Additionally, we have explored the cross-linking interactions between Pdots and nanomedicine, potential yet complete biological metabolic pathways, future research directions, and innovative solutions for integrating diagnosis and treatment strategies. This review presents the possible expectations and valuable insights for advancing Pdots, specifically from chemical, medical, and photophysical practitioners' standpoints.

Nucleophile-Triggered π-Topological Transformation: A New Synthetic Approach to Near-Infrared-Emissive Rhodamines

We describe a π-topological transformation-based synthetic method for the preparation of a new type of near-infrared (NIR)-emissive rhodamine dye called Polymethine-embedded Rhodamine Fluorophore (PeR Fluor). In contrast to conventional NIR-emissive dyes that require tedious synthetic steps and/or a high cost, linear fully π-conjugated PeR Fluor can be regioselectively prepared in one step by mixing different nucleophiles with ABPXs, a family of rhodamines with a cross-conjugated structure. PeR Fluor exhibits bright NIR fluorescence emission and high photostability owing to the cooperative π-electron system of rhodamines and polymethine scaffolds. Large bathochromic shifts of the absorption and fluorescence emission maxima can be achieved by modifying the N-substituted group to obtain NIR-absorbing/emitting PeR Fluor. We also demonstrate the stimulus-responsive functionality of PeR Fluor through the addition of chemicals (acid/base), which shows switchable NIR and visible fluorescence response. Our π-topological transformation-based synthetic method is a promising approach to produce new functionalized rhodamine dyes.

Strategies to convert organic fluorophores into red/near-infrared emitting analogues and their utilization in bioimaging probes

Organic fluorophores aided by current microscopy imaging modalities are essential for studying biological systems. Recently, red/near-infrared emitting fluorophores have attracted great research efforts, as they enable bioimaging applications with reduced autofluorescence interference and light scattering, two significant obstacles for deep-tissue imaging, as well as reduced photodamage and photobleaching. Herein, we analyzed the current strategies to convert key organic fluorophores bearing xanthene, coumarin, and naphthalene cores into longer wavelength-emitting derivatives by focussing on their effectiveness and limitations. Together, we introduced typical examples of how such fluorophores can be used to develop molecular probes for biological analytes, along with key sensing features. Finally, we listed several critical issues to be considered in developing new fluorophores.

Xanthene-based functional dyes: towards new molecules operating in the near-infrared region

Xanthene-based functional dyes have diverse applications in life science and materials science. A current challenge is to develop new dyes with suitable physicochemical properties, including near-infrared (NIR) operation, for advanced biological applications such as medical diagnostics and molecular imaging. In this review, we first present an overview of xanthene-based functional dyes and then focus on synthetic strategies for modulating the absorption and fluorescence of dyes that operate in the NIR wavelength region with bright emission and good photostability. We also introduce our work on aminobenzopyranoxanthenes (ABPXs) and bridged tetra-aryl-p-quinodimethanes (BTAQs) as new xanthene-based far-red (FR)/NIR absorbing/emitting molecules whose absorption/fluorescence wavelengths change in response to external stimuli.

Two Hawks with One Arrow: A Review on Bifunctional Scaffolds for Photothermal Therapy and Bone Regeneration

Despite the significant improvement in the survival rate of cancer patients, the total cure of bone cancer is still a knotty clinical challenge. Traditional surgical resectionof bone tumors is less than satisfactory, which inevitably results in bone defects and the inevitable residual tumor cells. For the purpose of realizing minimal invasiveness and local curative effects, photothermal therapy (PTT) under the irradiation of near-infrared light has made extensive progress in ablating tumors, and various photothermal therapeutic agents (PTAs) for the treatment of bone tumors have thus been reported in the past few years, has and have tended to focus on osteogenic bio-scaffolds modified with PTAs in order to break through the limitation that PTT lacks, osteogenic capacity. These so-called bifunctional scaffolds simultaneously ablate bone tumors and generate new tissues at the bone defects. This review summarizes the recent application progress of various bifunctional scaffolds and puts forward some practical constraints and future perspectives on bifunctional scaffolds for tumor therapy and bone regeneration: two hawks with one arrow.

Recent advances in nanoparticles mediated photothermal therapy induced tumor regression

Photothermal therapy (PTT) is a minimally invasive procedure for treating cancer. The two significant prerequisites of PTT are the photothermal therapeutic agent (PTA) and near-infrared radiation (NIR). The PTA absorbs NIR, causing hyperthermia in the malignant cells. This increased temperature at the tumor microenvironment finally results in tumor cell damage. Nanoparticles play a crucial role in PTT, aiding in the passive and active targeting of the PTA to the tumor microenvironment. Through enhanced permeation and retention effect and surface-engineering, specific targeting could be achieved. This novel delivery tool provides the advantages of changing the shape, size, and surface attributes of the carriers containing PTAs, which might facilitate tumor regression significantly. Further, inclusion of surface engineering of nanoparticles is facilitated through ligating ligands specific to overexpressed receptors on the cancer cell surface. Thus, transforming nanoparticles grants the ability to combine different treatment strategies with PTT to enhance cancer treatment. This review emphasizes properties of PTAs, conjugated biomolecules of PTAs, and the combinatorial techniques for a better therapeutic effect of PTT using the nanoparticle platform.

NAD(P)H Quinone Oxidoreductase-1 in Organ and Tumor Tissues: Distinct Activity Levels Observed with a Benzo-rosol-Based Dual-Excitation and Dual-Emission Probe

NAD(P)H quinone oxidoreductase-1 (NQO1), a protective enzyme against cellular oxidative stress, is expressed abnormally high in solid tumors and thus recognized as a cancer biomarker. To develop a fluorescent NQO1 probe with practicality, we investigated benzo-rosol fluorophores linked with a known self-immolative quinone substrate. Four probe candidates exhibited ratiometric sensing behavior toward the enzyme, satisfying our orbital mismatch stratagem proposed before, under dual-excitation and dual-emission conditions that alleviate the spectral overlap issue commonly observed with the ratiometric probes based on intramolecular charge-transfer change. Among the candidates, two ester-linked compounds exhibited hydrolytic instability to water or an esterase, discouraging us to develop such ester-linked probes. One ether-linked, hydrolytically stable probe provided brighter cellular fluorescence than the other and thus was applied to ratiometric imaging of NQO1 in cells and tissues. We found that the enzyme activity levels are much different in organ tissues: stomach (56), kidney (22), colon (9.8), testis (7.8), bladder (5.6), lung (1.2), and muscle (1.0). Furthermore, a markedly high enzyme level (14.6-fold) was observed in a xenograft tumor tissue compared with that in a normal tissue, which suggests that such an NQO1 probe is promising for cancer diagnosis and for studying the enzyme-associated biology.

Intense red emissive organic crystals with elastic bending ability and optical waveguiding behaviour

Orange (Cry-1O) and red (Cry-1R) emissive crystals were obtained based on a green emissive molecule 1. The light transducing capability of Cry-1R in both the straight and bent states can be applied as an active optical waveguiding medium.

The Aggregation Regularity Effect of Multiarylpyrroles on Their Near-Infrared Aggregation-Enhanced Emission Property

Increasing the quantum yield of near-infrared (NIR) emissive dyes is critical for biological applications because these fluorescent dyes generally show decreased emission efficiency under aqueous conditions. In this work, we designed and synthesized several multiarylpyrrole (MAP) derivatives, in which a furanylidene (FE) group at the 3-position of the pyrrole forms donor-π-acceptor molecules, MAP-FE, with a NIR emissive wavelength and aggregation-enhanced emission (AEE) features. Different alkyl chains of MAP-FEs linked to phenyl groups at the 2,5-position of the pyrrole ring resulted in different emissive wavelengths and quantum yields in aggregated states, such as powders or single crystals. Powder XRD data and single crystal analysis elucidated that the different lengths of alkyl chains had a significant impact on the regularity of MAP-FEs when they were forced to aggregate or precipitate, which affected the intermolecular interaction and the restriction degree of the rotating parts, which are essential components. Therefore, an increasing number of NIR dyes could be developed by this design strategy to produce efficient NIR dyes with AEE. Moreover, this method can provide general guidance for other related fields, such as organic solar cells and organic light-emitting materials, because they are all applied in the aggregated state.

Cell-Membrane-Localizing, Two-Photon Probe for Ratiometric Imaging of γ-Glutamyl Transpeptidase in Cancerous Cells and Tissues

γ-Glutamyl transpeptidase (GGT), a cell surface-bound protease, is associated with various diseases including cancer. The detection of the enzyme activity is an important subject, leading to about 40 activatable fluorescent probes so far. All of them, however, lack the membrane-localizing ability, raising a reliability issue in the quantitative analysis. Disclosed is the first fluorescent probe that senses the cell surface-bound enzyme, which, furthermore, is capable of ratiometric as well as two-photon imaging with desirable features. Ratiometric imaging of cancer cell lines reveals a 6.4-8.4-fold higher GGT levels than those in normal cell lines. A comparison of the enzyme activity in organ tissues of normal and tumor xenograft mice reveals notably different levels of enzyme activity depending on the kind of tissue. Normal tissues exhibited comparable levels of enzyme activity, except the kidney that has significantly higher GGT activity (2.7-4.0-fold) than the other organs. Compared with the normal tissues, considerably higher enzyme activity was observed in the tumor tissues of the thigh (4.0-fold), colon (2.5-fold), lung (3.6-fold), and liver (2.1-fold), but essentially no enhanced activity in the tumor tissues of the spleen, stomach, and pancreas and a comparable level in both the tumor and normal kidney tissues were observed. The probe offers practical means for studying GGT-associated biology in cells and tissues by one- as well as two-photon ratiometric imaging.

Development of photo- and chemo-stable near-infrared-emitting dyes: linear-shape benzo-rosol and its derivatives as unique ratiometric bioimaging platforms

Microscopic imaging aided with fluorescent probes has revolutionized our understanding of biological systems. Organic fluorophores and probes thus continue to evolve for bioimaging applications. Fluorophores such as cyanines and hemicyanines emit in the near-infrared (NIR) region and thus allow deeper imaging with minimal autofluorescence; however, they show limited photo- and chemo-stability, demanding new robust NIR fluorophores. Such photo- and chemo-stable NIR fluorophores, linear-shape π-extended rosol and rosamine analogues, are disclosed here which provide bright fluorescence images in cells as well as in tissues by confocal laser-scanning microscopy. Furthermore, they offer unique ratiometric imaging platforms for activatable probes with dual excitation and dual emission capability, as demonstrated with a 2,4-dinitrophenyl ether derivative of benzo-rosol.

A systematic study on the discrepancy of fluorescence properties between in solutions and in cells: super-bright, environment-insensitive benzocoumarin dyes

The benzocoumarins show distinctive emission behaviour from some commonly-used dyes in organic, aqueous buffer, and cellular media, which compels us to recognize the cellular environment as the third space for fluorophores.