Design and Development of Enzymatically Activatable Photosensitizer Based on Unique Characteristics of Thiazole Orange

β-Galactosidase-Triggered Photodynamic Elimination of Senescent Cells with a Boron Dipyrromethene-Based Photosensitizer

Senescence is a cellular response having physiological and reparative functions to preserve tissue homeostasis and suppress tumor growth. However, the accumulation of senescent cells would cause deleterious effects that lead to age-related dysfunctions and cancer progression. Hence, selective detection and elimination of senescent cells are crucial yet remain a challenge. A β-galactosidase (β-gal)-activated boron dipyrromethene (BODIPY)-based photosensitizer (compound 1) is reported here that can selectively detect and eradicate senescent cells. It contains a galactose moiety connected to a pyridinium BODIPY via a self-immolative nitrophenylene linker, of which the photoactivity is effectively quenched. Upon interactions with the senescence-associated β-gal, it undergoes enzymatic hydrolysis followed by self-immolation, leading to the release of an activated BODIPY moiety by which the fluorescence emission and singlet oxygen generation are restored. The ability of 1 to detect and eliminate senescent cells is demonstrated in vitro and in vivo, using SK-Mel-103 tumor-bearing mice treated with senescence-inducing therapy. The results demonstrate that 1 can be selectively activated in senescent cells to trigger a robust senolytic effect upon irradiation. This study breaks new ground in the design and application of new senolytic agents based on photodynamic therapy.

Design strategies and biological applications of β-galactosidase fluorescent sensor in ovarian cancer research and beyond

Beta-galactosidase (β-galactosidase), a lysosomal hydrolytic enzyme, plays a critical role in the catalytic hydrolysis of glycosidic bonds, leading to the conversion of lactose into galactose. This hydrolytic enzyme is used as a biomarker in various applications, including enzyme-linked immunosorbent assays (ELISAs), gene expression studies, tuberculosis classification, and in situ hybridization. β-Galactosidase abnormalities are linked to various diseases, such as ganglioside deposition, primary ovarian cancer, and cell senescence. Thus, effective detection of β-galactosidase activity may aid disease diagnoses and treatment. Activatable optical probes with high sensitivity, specificity, and spatiotemporal resolution imaging capabilities have become powerful tools for visualization and real time tracking in vivo in the past decade. This manuscript reviews the sensing mechanism, molecular design strategies, and advances of fluorescence probes in the biological application of β-galactosidase, particularly in the field of ovarian cancer research. Current challenges in tracking β-galactosidase and future directions are also discussed.

Detection and Elimination of Senescent Cells with a Self-Assembled Senescence-Associated β-Galactosidase-Activatable Nanophotosensitizer

Senescent cells have become an important therapeutic target for many age-related dysfunctions and diseases. We report herein a novel nanophotosensitizing system that is responsive to the senescence-associated β-galactosidase (β-gal) for selective detection and elimination of these cells. It involves a dimeric zinc(II) phthalocyanine linked to a β-galactose unit via a self-immolative linker. This compound can self-assemble in aqueous media, forming stable nanoscale particles in which the phthalocyanine units are stacked and self-quenched for fluorescence emission and singlet oxygen production. Upon internalization into senescent HeLa cells, these nanoparticles interact with the overproduced senescence-associated β-gal inside the cells to trigger the disassembly process through enzymatic cleavage of the glycosidic bonds, followed by self-immolation to release the photoactive monomeric phthalocyanine units. These senescent cells can then be lit up with fluorescence and eliminated through the photodynamic action upon light irradiation with a half-maximal inhibitory concentration of 0.06 μM.

Lysosome-targeting phenalenones as efficient type I/II photosensitizers for anticancer photodynamic therapy

Development of safe and effective photosensitizers is important for enhancing the efficacy of photodynamic cancer therapy. Phenalenone is a type II photosensitizer with a high singlet oxygen quantum yield; however, its short UV absorption wavelength hinders its application in cancer imaging and in vivo photodynamic therapy. In this study, we report a new redshift phenalenone derivative, 6-amino-5-iodo-1H-phenalen-1-one (SDU Red [SR]), as a lysosome-targeting photosensitizer for triple-negative breast cancer therapy. SDU Red produced singlet oxygen (Type II reactive oxygen species [ROS]) and superoxide anion radicals (Type I ROS) upon light irradiation. It also exhibited good photostability and a remarkable phototherapeutic index (PI > 76) against triple-negative breast cancer MDA-MB-231 cancer cells. Additionally, we designed two amide derivatives, SRE-I and SRE-II, with decreased fluorescence and photosensitizing capabilities based on SDU Red as activatable photosensitizers for photodynamic cancer therapy. SRE-I and SRE-II could be further converted into the active photosensitizer SDU Red via carboxylesterase-catalyzed amide bond cleavage. Moreover, SDU Red and SRE-II induced DNA damage and cell apoptosis in the presence of light. Therefore, SRE-II can act as a promising theranostic agent for triple-negative breast cancer.

Selective photodynamic eradication of senescent cells with a β-galactosidase-activated photosensitiser

A β-galactosidase-responsive photosensitiser has been designed and synthesised. It contains a galactosyl substrate, a boron dipyrromethene-based photosensitising unit and a black hole quencher 2 connected via an AB₂-type self-immolative linker. This novel photosensitiser can be selectively activated by the senescence-associated β-galactosidase in senescent cells, leading to restoration in fluorescence emission and effective killing of the cells via photodynamic action.

Locked and Loaded: β-Galactosidase Activated Photodynamic Therapy Agent Enables Selective Imaging and Targeted Treatment of Glioblastoma Multiforme Cancer Cells

Selective detection and effective therapy of brain cancer, specifically, the very aggressive glioblastoma multiforme (GBM), remains one of the paramount challenges in clinical settings. While radiotherapy combined surgery is proposed as the main treatment course, it has several drawbacks such as complexity of the operation and common development of recurrent tumors in this course of patient care. Unique opportunities presented by photodynamic therapy (PDT) offer promising, effective, and precise therapy against GBM cells along with simultaneous imaging opportunities. However, activatable, theranostic molecular systems in PDT modality for GBM remained scarce. Specifically, even though elevated β-galactosidase (β-gal) activity in glioblastoma cells is well-documented, targeted, activatable therapeutic PDT agents have not been realized. Herein, we report a β-galactosidase (β-gal) activatable phototheranostic agent based on an iodinated resorufin core (RB-1) which was realized in only three steps with commercial reagents in 29% overall yield. RB-1 showed very high singlet oxygen (1O2) quantum yield (54%) accompanied by a remarkable turn-on response in fluorescence upon enzymatic activation. RB-1 was tested in different cell lines and revealed selective photocytotoxicity in U-87MG glioblastoma cells. Additionally, thanks to almost 7% fluorescence quantum yield (ΦF) despite extremely high 1O2 generation yield, RB-1 was also demonstrated as a successful agent for fluorescence imaging of U-87MG cells. Due to significantly lower (β-gal) activity in healthy cells (NIH/3T3), RB-1 stayed in a passive state and showed minimal photo and dark toxicity. RB-1 marks the first example of a β-gal activatable phototheranostic agent toward effective treatment of glioblastoma.

Multicomponent synthesis of fully substituted thiazoles using glycine-based dithiocarbamates, acetic anhydride and nitroalkenes

Reaction of glycine-based dithiocarbamates with nitroalkenes in the presence of acetic anhydride was utilized for the synthesis of fully substituted 2-(alkylsulfanyl)-4-(nitroalkyl)-5-acyloxy-1,3-thiazoles. The reaction proceeds via the in situ formation of thiazol-5(4H)-one from glycine-based dithiocarbamates, followed by the Michael addition of this intermediate to nitroalkenes, aromatization, and esterification reaction cascade. This new one-pot three-component reaction afforded a diverse library of fully substituted thiazoles in high to excellent yields under solvent-free conditions.

Developing a far-red fluorogenic beta-galactosidase probe for senescent cell imaging and photoablation

A methylene blue (MB)-based beta-galactosidase (β-gal) activatable molecule, Gal-MB, was developed for senescence imaging and light-triggered senolysis. When in contact with LacZ β-gal or senescence-associated β-gal (SA-β-gal), the photoinsensitive Gal-MB becomes fluorescent. Gal-MB also offered selective phototoxicity toward LacZ β-gal expressing cells and drug-induced senescent cells, which express SA-β-gal, after light illumination at 665 nm.

A methylene blue (MB)-based beta-galactosidase (β-gal) activatable molecule, Gal-MB, was developed for senescence imaging and light-triggered senolysis.

Recent Strategies to Develop Innovative Photosensitizers for Enhanced Photodynamic Therapy

This review presents a robust strategy to design photosensitizers (PSs) for various species. Photodynamic therapy (PDT) is a photochemical-based treatment approach that involves the use of light combined with a light-activated chemical, referred to as a PS. Attractively, PDT is one of the alternatives to conventional cancer treatment due to its noninvasive nature, high cure rates, and low side effects. PSs play an important factor in photoinduced reactive oxygen species (ROS) generation. Although the concept of photosensitizer-based photodynamic therapy has been widely adopted for clinical trials and bioimaging, until now, to our surprise, there has been no relevant review article on rational designs of organic PSs for PDT. Furthermore, most of published review articles in PDT focused on nanomaterials and nanotechnology based on traditional PSs. Therefore, this review aimed at reporting recent strategies to develop innovative organic photosensitizers for enhanced photodynamic therapy, with each example described in detail instead of providing only a general overview, as is typically done in previous reviews of PDT, to provide intuitive, vivid, and specific insights to the readers.

In Situ Generated Novel 1H MRI Reporter for β-Galactosidase Activity Detection and Visualization in Living Tumor Cells

For wide applications of the lacZ gene in cellular/molecular biology, small animal investigations, and clinical assessments, the improvement of noninvasive imaging approaches to precisely assay gene expression has garnered much attention. In this study, we investigate a novel molecular platform in which alizarin 2-O-β-d-galactopyranoside AZ-1 acts as a lacZ gene/β-gal responsive 1H-MRI probe to induce significant 1H-MRI contrast changes in relaxation times T 1 and T 2 in situ as a concerted effect for the discovery of β-gal activity with the exposure of Fe3+. We also demonstrate the capability of this strategy for detecting β-gal activity with lacZ-transfected human MCF7 breast and PC3 prostate cancer cells by reaction-enhanced 1H-MRI T 1 and T 2 relaxation mapping.

Enhancing the tumor cell selectivity of a rhodamine-decorated iridium(III) complex by conjugating with indomethacin for COX-2 targeted photodynamic therapy

A rhodamine-iridium (III) complex bearing indomethacin moiety, named IM-rho-Ir, was synthesized and evaluated for COX-2 targetable photodynamic therapy. By integrating COX-2 directing group, IM-rho-Ir exhibited enhanced cellular uptake in cancer cells than in normal cells compared to rhodamine-iridium (III) complex without indomethacin moiety.

Recent Advances in Activatable Organic Photosensitizers for Specific Photodynamic Therapy

Photodynamic therapy is an alternative modality for the therapy of diseases such as cancer in a minimally invasive manner. The essential photosensitizer, which acts as a catalyst when absorbing light, converts oxygen into cytotoxic reactive oxygen species that ablate malignant cells through apoptosis and/or necrosis, destroy tumor microvasculature, and stimulate immunity. An activatable photosensitizer whose photoactivity could be turned on by a specific disease biomarker is capable of distinguishing healthy cells from diseased cells, thereby reducing off-target photodamage. In this Minireview, we highlight progress in activatable organic photosensitizers over the past five years, including: (i) biorthogonal activatable BODIPYs; (ii) activatable Se-rhodamine with single-cell resolution; (iii) silicon phthalocyanine targeting oxygen tension; (iv) general D-π-A scaffolds; and (v) AIEgens. The potential challenges and opportunities for developing new types of activatable organic photosensitizers to overcome the hypoxia dilemmas of photodynamic therapy are discussed.

Chromo-fluorogenic probes for β-galactosidase detection

β-Galactosidase (β-Gal) is a widely used enzyme as a reporter gene in the field of molecular biology which hydrolyzes the β-galactosides into monosaccharides. β-Gal is an essential enzyme in humans and its deficiency or its overexpression results in several rare diseases. Cellular senescence is probably one of the most relevant physiological disorders that involve β-Gal enzyme. In this review, we assess the progress made to date in the design of molecular-based probes for the detection of β-Gal both in vitro and in vivo. Most of the reported molecular probes for the detection of β-Gal consist of a galactopyranoside residue attached to a signalling unit through glycosidic bonds. The β-Gal-induced hydrolysis of the glycosidic bonds released the signalling unit with remarkable changes in color and/or emission. Additional examples based on other approaches are also described. The wide applicability of these probes for the rapid and in situ detection of de-regulation β-Gal-related diseases has boosted the research in this fertile field.

Designing Stimuli-Responsive Upconversion Nanoparticles that Exploit the Tumor Microenvironment

Tailoring personalized cancer nanomedicines demands detailed understanding of the tumor microenvironment. In recent years, smart upconversion nanoparticles with the ability to exploit the unique characteristics of the tumor microenvironment for precise targeting have been designed. To activate upconversion nanoparticles, various bio-physicochemical characteristics of the tumor microenvironment, namely, acidic pH, redox reactants, and hypoxia, are exploited. Stimuli-responsive upconversion nanoparticles also utilize the excessive presence of adenosine triphosphate (ATP), riboflavin, and Zn²⁺ in tumors. An overview of the design of stimulus-responsive upconversion nanoparticles that precisely target and respond to tumors via targeting the tumor microenvironment and intracellular signals is provided. Detailed understanding of the tumor microenvironment and the personalized design of upconversion nanoparticles will result in more effective clinical translation.

Visualization of endogenous β-galactosidase activity in living cells and zebrafish with a turn-on near-infrared fluorescent probe

β-Galactosidase (β-gal) is an important biomarker for primary ovarian cancers. Developing noninvasive bioimaging probes for studying the activity of β-gal is highly desirable for cancer diagnosis. Herein, a turn-on near-infrared (NIR) fluorescent probe， 2-((6-(((2S, 3R, 4S, 5R, 6R)-3,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran -2-yl)oxy)-2,3-dihydro-1H-xanthen-4-yl)methylene)malononitrile named DXM-βgal, was rationally designed based on enzymatic reaction for the detection of β-gal activity both in vitro and in vivo. Upon incubating with β-gal, DXM-βgal displayed a significant fluorescence enhancement at 640 nm, accompanying by a color change of solution color from red to purple. DXM-βgal exhibited high selectivity and sensitively to β-gal with low limit of detection (2.92 × 10^-4 U mL^-1). Besides, based on its advantages of long-wavelength emission and excellent biocompatibility, DXM-βgal was successfully applied to imaging β-gal in living cells and zebrafish. Given these prominent properties, we believe that DXM-βgal will be a potential tool for investigating β-gal activity in biomedical research.

Activatable Photosensitizer for Targeted Ablation of lacZ-Positive Cells with Single-Cell Resolution

To achieve highly selective ablation of lacZ-positive cells in a biological milieu in vivo, we developed an activatable photosensitizer, SPiDER-killer-βGal, targeted to β-galactosidase encoded by the lacZ reporter gene. Hydrolysis of SPiDER-killer-βGal by β-galactosidase simultaneously activates both its photosensitizing ability and its reactivity to nucleophiles, so that the phototoxic products generated by light irradiation are trapped inside the lacZ-positive cells. The combination of SPiDER-killer-βGal and light irradiation specifically killed lacZ-positive cells in coculture with cells without lacZ expression. Furthermore, β-galactosidase-expressing cells in the posterior region of cultured Drosophila wing discs and in pupal notum of live Drosophila pupae were selectively killed with single-cell resolution. This photosensitizer should be useful for specific ablation of targeted cells in living organisms, for example, to investigate cellular functions in complex networks.

Recent Advances of Molecular Optical Probes in Imaging of β-Galactosidase

β-Galactosidase (β-Gal), as a lysosomal hydrolytic enzyme, plays an important physiological role in catalyzing the hydrolysis of glycosidic bonds which convert lactose into galactose. Moreover, upregulation of β-Gal is often correlated with the occurrence of primary ovarian cancers and cell senescence. Thereby, detection of β-Gal activity is relevant to cancer diagnosis. Optical imaging possesses high spatial and temporal resolution, high sensitivity, and real-time imaging capability. These properties are beneficial for the detection of β-Gal in living systems. This Review summarizes the recent progress in development of molecular optical probes for near-infrared fluorescence (NIRF), bioluminescence (BL), chemiluminescence (CL), or photoacoustic (PA) imaging of β-Gal in biological systems. The challenges and opportunities in the probe design for detection of β-Gal are also discussed.

Development of a two-photon fluorescent probe for the selective detection of β-galactosidase in living cells and tissues

We have developed a two-photon fluorescent β-gal probe (G-GAL), which was demonstrated to be an efficient two-photon biosensor for β-gal in living cells and tissues.

Engineering of Electrochromic Materials as Activatable Probes for Molecular Imaging and Photodynamic Therapy

Electrochromic materials (EMs) are widely used color-switchable materials, but their applications as stimuli-responsive biomaterials to monitor and control biological processes remain unexplored. This study reports the engineering of an organic π-electron structure-based EM (dicationic 1,1,4,4-tetraarylbutadiene, 1²⁺) as a unique hydrogen sulfide (H₂S)-responsive chromophore amenable to build H₂S-activatable fluorescent probes (1²⁺-semiconducting polymer nanoparticles, 1²⁺-SNPs) for in vivo H₂S detection. We demonstrate that EM 1²⁺, with a strong absorption (500-850 nm), efficiently quenches the fluorescence (580, 700, or 830 nm) of different fluorophores within 1²⁺-SNPs, while the selective conversion into colorless diene 2 via H₂S-mediated two-electron reduction significantly recovers fluorescence, allowing for non-invasive imaging of hepatic and tumor H₂S in mice in real time. Strikingly, EM 1²⁺ is further applied to design a near-infrared photosensitizer with tumor-targeting and H₂S-activatable ability for effective photodynamic therapy (PDT) of H₂S-related tumors in mice. This study demonstrates promise for applying EMs to build activatable probes for molecular imaging of H₂S and selective PDT of tumors, which may lead to the development of new EMs capable of detecting and regulating essential biological processes in vivo.

The good, the bad, and the ugly - controlling singlet oxygen through design of photosensitizers and delivery systems for photodynamic therapy

Singlet oxygen, although integral to photodynamic therapy, is notoriously uncontrollable, suffers from poor selectivity and has fast decomposition rates in biological media. Across the scientific community, there is a conscious effort to refine singlet oxygen interactions and initiate selective and controlled release to produce a consistent and reproducible therapeutic effect in target tissue. This perspective aims to provide an insight into the contemporary design principles behind photosensitizers and drug delivery systems that depend on a singlet oxygen response or controlled release. The discussion will be accompanied by in vitro and in vivo examples, in an attempt to highlight advancements in the field and future prospects for the more widespread application of photodynamic therapy.

Photosensitizers with Aggregation-Induced Emission: Materials and Biomedical Applications

Photodynamic therapy is arising as a noninvasive treatment modality for cancer and other diseases. One of the key factors to determine the therapeutic function is the efficiency of photosensitizers (PSs). Opposed to traditional PSs, which show quenched fluorescence and reduced singlet oxygen production in the aggregate state, PSs with aggregation-induced emission (AIE) exhibit enhanced fluorescence and strong photosensitization ability in nanoparticles. Here, the design principles of AIE PSs and their biomedical applications are discussed in detail, starting with a summary of traditional PSs, followed by a comparison between traditional and AIE PSs to highlight the various design strategies and unique features of the latter. Subsequently, the applications of AIE PSs in photodynamic cancer cell ablation, bacteria killing, and image-guided therapy are discussed using charged AIE PSs, AIE PS molecular probes, and AIE PS nanoparticles as examples. These studies have demonstrated the great potential of AIE PSs as effective theranostic agents to treat tumor or bacterial infection. This review hopefully will spur more research interest in AIE PSs for future translational research.

Controllable supramolecular self-assemblies (rods–wires–spheres) and ICT/PET based perylene probes for palladium detection in solution and the solid state

AC-PDIshows solvent dependent self-assembly into nanowires, rods and spheres. It could be used for detection of Pd⁰in 50% HEPES buffer–DMSO (39 nM, UV-Vis; 45 nM, fluorescence) and the solid state (0.58 pg cm⁻²).

Recent progresses in small-molecule enzymatic fluorescent probes for cancer imaging

An overview of recent advances in small-molecule enzymatic fluorescent probes for cancer imaging, including design strategies and cancer imaging applications.

ATP-Activatable Photosensitizer Enables Dual Fluorescence Imaging and Targeted Photodynamic Therapy of Tumor

Targeted delivery of intracellular stimuli-activatable photosensitizers (PSs) into tumor cells to achieve selective imaging and on-demand photodynamic therapy (PDT) of tumors has provided a vital opportunity for precise cancer diagnosis and therapy. In this paper, we report a tumor targeting and adenosine triphosphate (ATP)-activatable nanophotosensitizer Apt-HyNP/BHQ₂ by modifying hybrid micellar nanoparticles with both nucleolin-targeting aptamer AS1411 and quencher BHQ₂-labeled ATP-binding aptamer BHQ₂-ATP-apt. We demonstrated that both of the fluorescence emissions at 555 and 627 nm were quenched by BHQ₂ in Apt-HyNP/BHQ₂, resulting in low PDT capacity. After selective entry into tumor cells through nucleolin-mediated endocytosis, the high concentration of intracellular ATP could bind to BHQ₂-ATP-apt and trigger Apt-HyNP/BHQ₂ dissociation, leading to turning "on" both fluorescence and PDT. The "off-on" fluorescence emissions at both 555 and 627 nm were successfully applied for dual color fluorescence imaging of endogenous ATP levels and real-time monitoring of intracellular activation of Apt-HyNP/BHQ₂ in tumor cells. Moreover, imaging-guided precise PDT of tumors in living mice was also demonstrated, allowing for selective ablation of tumors without obvious side effects. This study highlights the potential of using a combination of tumor-targeting and ATP-binding aptamers to design ATP-activatable PSs for both fluorescence imaging and imaging-guided PDT of tumors in vivo.

Differences in the sensitivity of ovarian cancer to photodynamic therapy and the mechanisms for those differences

Protoporphyrin IX (PpIX) levels are crucial to the antitumor action of photodynamic therapy (PDT). In the present study, the underling molecular mechanisms for the variation in PpIX levels in ovarian cancer cells were investigated. Five ovarian cancer cell lines were subcutaneously grafted onto the backs of nude mice. Once tumors had developed, 5-aminolevulinic acid methyl ester hydrochloride (methyl-ALA) was administered intraperitoneally and the tumor was irradiated twice/week. PpIX levels in the tumor were assayed using high-performance liquid chromatography. Enzymes involved in heme synthesis and degradation were screened using a microarray technique. Expression of the glutathione transferase Omega-1 (GSTO1) gene involved in the conversion of PpIX into heme in cells was quantified using the reverse transcription-quantitative polymerase chain reaction. In HTOA, HRA and DISS cells, PDT resulted in significant tumor shrinkage in comparison with the controls. In MCAS and TOV21G cells, no significant alterations in tumor growth were identified compared with the untreated cells. PpIX levels increased significantly in HTOA, DISS and HRA cells compared with in MCAS and TOV21G cells. A comparison of genetic profiles using PDT-sensitive DISS cells and PDT-resistant MCAS cells indicated that MCAS cells exhibited significantly increased levels of δ-aminolevulinate synthase (a rate-limiting enzyme in heme synthesis), heme oxygenase 2 (an enzyme that degrades heme into biliverdin), and biliverdin reductase B (an enzyme that reduces biliverdin into bilirubin) in comparison with DISS cells. The level of GSTO1 expression in HTOA, HRA and DISS cells was ~2.5-fold that in MCAS and TOV21G cells. Sensitivity to PDT is related to PpIX levels in cells. The results of the present study suggested that PpIX tends not to accumulate in PDT-resistant cells despite active heme synthesis and degradation, and that high levels of GSTO1 expression are associated with increased sensitivity to PDT.

A novel near-infrared fluorescent probe for sensitive detection of β-galactosidase in living cells

A novel near-infrared fluorescent probe for β-galactosidase has been developed based on a hemicyanine skeleton, which is conjugated with a d-galactose residue via a glycosidic bond. The probe serves as a substrate of β-galactosidase and displays rapid and sensitive turn-on fluorescent responses to β-galactosidase in aqueous solution. A 12.8-fold enhancement of fluorescence intensity at 703 nm was observed after incubation of 10 nM of β-galactosidase with 5 μM probe for 10 min. The probe can sensitively detect as little as 0.1 nM of β-galactosidase and shows linear responses to the enzyme concentration below 1.4 nM. The kinetic study showed that the probe has high binding affinity to β-galactosidase with K_m = 3.6 μM. The probe was used to detect β-galactosidase in living cells by employing the premature cell senescence model. The probe exhibited strong fluorescent signals in senescent cells but not in normal cells, which demonstrates that the probe is able to detect the endogenous senescence-associated β-galactosidase in living cells.

Near-Infrared-Activated Upconversion Nanoprobes for Sensitive Endogenous Zn2+ Detection and Selective On-Demand Photodynamic Therapy

As a light-activated noninvasive cancer treatment paradigm, photodynamic therapy (PDT) has attracted extensive attention because of its high treatment efficacy and low side effects. Especially, spatiotemporal control of singlet oxygen (¹O₂) release is highly desirable for realizing on-demand PDT, which, however, still remains a huge challenge. To address this issue, a novel switchable near-infrared (NIR)-responsive upconversion nanoprobe has been designed and successfully applied for controlled PDT that can be optically activated by tumor-associated disruption of labile Zn²⁺ (denoted as Zn²⁺ hereafter) homeostasis stimuli. Upon NIR irradiation, this theranostic probe can not only quantitatively detect the intracellular endogenous Zn²⁺ in situ but also selectively generate a great deal of cytotoxic reactive oxygen species (ROS) for efficiently killing breast cancer cells under the activation of excessive endogenous Zn²⁺, so as to maximally avoid adverse damage to normal cells. This study aims to propose a new tumor-specific PDT paradigm and, more importantly, provide a new avenue of thought for efficient cancer theranostics based on our designed highly sensitive upconversion nanoprobes.

A bacteria-activated photodynamic nanosystem based on polyelectrolyte-coated silica nanoparticles

A novel bacteria-activated photodynamic nanosystem (SiO₂/PAH–Ce6) has been reported for selective fluorescence sensing and photodynamic elimination of pathogenic bacteria.

Key Structural Elements of Unsymmetrical Cyanine Dyes for Highly Sensitive Fluorescence Turn-On DNA Probes

Unsymmetrical cyanine dyes, such as thiazole orange, are useful for the detection of nucleic acids with fluorescence because they dramatically enhance the fluorescence upon binding to nucleic acids. Herein, we synthesized a series of unsymmetrical cyanine dyes and evaluated their fluorescence properties. A systematic structure-property relationship study has revealed that the dialkylamino group at the 2-position of quinoline in a series of unsymmetrical cyanine dyes plays a critical role in the fluorescence enhancement. Four newly designed unsymmetrical cyanine dyes showed negligible intrinsic fluorescence in the free state and strong fluorescence upon binding to double-stranded DNA (dsDNA) with a quantum yield of 0.53 to 0.90, which is 2 to 3 times higher than previous unsymmetrical cyanine dyes. A detailed analysis of the fluorescence lifetime revealed that the dialkylamino group at the 2-position of quinoline suppressed nonradiative decay in favor of increased fluorescence quantum yield. Moreover, these newly developed dyes were able to stain the nucleus specifically in fixed HeLa cells examined by using a confocal laser-scanning microscope.

Intermolecular Structural Change for Thermoswitchable Polymeric Photosensitizer

We developed a thermoswitchable polymeric photosensitizer (T-PPS) by conjugating PS (Pheophorbide-a, PPb-a) to a temperature-responsive polymer backbone of biocompatible hydroxypropyl cellulose. Self-quenched PS molecules linked in close proximity by π-π stacking in T-PPS were easily transited to an active monomeric state by the temperature-induced phase transition of polymer backbones. The temperature-responsive intermolecular interaction changes of PS molecules in T-PPS were demonstrated in synchrotron small-angle X-ray scattering and UV-vis spectrophotometer analysis. The T-PPS allowed switchable activation and synergistically enhanced cancer cell killing effect at the hyperthermia temperature (45 °C). Our developed T-PPS has the considerable potential not only as a new class of photomedicine in clinics but also as a biosensor based on temperature responsiveness.

pH-Response Optimization of Amino-Substituted Tetraphenylporphyrin Derivatives as pH-Activatable Photosensitizers

Amino-substituted tetraphenylporphyrin derivatives have been designed as pH-activatable photosensitizers for photodynamic cancer therapy. The photophysical processes of the monoamino-substituted derivative N1 and nonsubstituted derivative N0 have been studied. The quantum yields of the fluorescence and photosensitization of singlet oxygen by N1 were very low in the neutral condition (OFF state), but these quantum yields were recovered by adding acid (ON state). These changes were not observed for N0; therefore, N1 is expected to be applicable as a pH-activatable photosensitizer. The ON/OFF switching mechanism of N1 has also been clarified. To optimize the pH response, tri- and tetraamino-substituted derivatives (N3 and N4) have also been explored. The pH response intensified as the number of amino groups increased. Furthermore, the ON/OFF switching ratio of N3 was 100, which is quite high.

Protein sensing in living cells by molecular rotor-based fluorescence-switchable chemical probes

In this paper, we introduce a general design to construct fluorescence-switching probes by using conjugates of a fluorescent molecular rotor and protein specific ligands for the selective protein detection and real-time tracking of protein degradation in living cells. Upon the interaction of the ligand with the protein ligand-binding domain, the crowded surroundings restrict the bond rotation of the fluorescent molecular rotor to trigger the emission of a strong fluorescence signal, which is reduced upon the addition of a competitive ligand or after protein degradation. With this probe design, two fluorescent probes for MGMT and hCAII proteins were constructed and applied for detecting the endogenous proteins in living cells. In addition, real-time degradation kinetics of the alkylated-MGMT at the single living cell level were revealed for the first time. We believe that this fluorescence-switching probe design can possibly be extended for the analysis of other proteins, for which there are still no effective tools to visualize them in living cells.

Recent advances in the development of synthetic chemical probes for glycosidase enzymes

The emergence of synthetic glycoconjugates as chemical probes for the detection of glycosidase enzymes has resulted in the development of a range of useful chemical tools with applications in glycobiology, biotechnology, medical and industrial research. Critical to the function of these probes is the preparation of substrates containing a glycosidic linkage that when activated by a specific enzyme or group of enzymes, irreversibly releases a reporter molecule that can be detected. Starting from the earliest examples of colourimetric probes, increasingly sensitive and sophisticated substrates have been reported. In this review we present an overview of the recent advances in this field, covering an array of strategies including chromogenic and fluorogenic substrates, lanthanide complexes, gels and nanoparticles. The applications of these substrates for the detection of various glycosidases and the scope and limitations for each approach are discussed.

Development of spiropyran-based electrochemical sensor via simultaneous photochemical and target-activatable electron transfer

In traditional electrochemical sensors, the electrochemical signal transduction of the redox-active material is usually controlled by the analytical target. Due to non-specific interaction between the redox mediator and the target, false signal by single stimulus may not be avoided. To address this issue, we have developed a new electrochemical sensor that uses a functional spiropyran, an important class of photo and thermochromic compounds, as both recognition receptor and latent redox mediator, to realize simultaneous photochemical and target-modulated electron transfer. As a proof of principle, β-galactosidase was chosen as a model target. The new synthesized spiropyran probe, SP-β-gal, undergoes reversibly structural isomerization to form merocyanine under UV light irradiation. After the glycosidic bond being cleaved by β-galactosidase, the opened merocyanine of SP-β-gal forms redox-active 2-(2.5-dihydroxystyryl)-1.3.3-trimethyl-3H-indolium, and thus produces a pair of reversible redox current peaks under the electrochemical scanning. To amplify the detection signal, SP-β-gal was self-assembled with single-walled carbon nanotubes (SWCNTs) on the surface of glass carbon electrode. Kinetics experiments confirm that the probe is an ideal candidate for the determination of different concentrations of β-galactosidase digestion kinetics. Further, the SP-β-gal/SWCNTs-modified electrode is chemically stable in complex biological fluids. It was successfully applied to monitor β-galactosidase activity in the 10% calf thymus. This work represents not only a significant step forward in the further development of low-dimensional carbon nanomaterials/small organic molecular probes-based electrochemical biosensors, but also a new platform which may be extended to the assay of other enzyme such as β-D-glycosidase and so on by translating the biorecognition into electrochemical signal responses.