Accessing Structurally Diverse Near-Infrared Cyanine Dyes for Folate Receptor-Targeted Cancer Cell Staining

Hemicyanine-Based Highly Water-Soluble Probe for Extracellular Nitroreductase

Nitroreductase (NTR) has long been a target of interest for its important role involved in the nitro compounds metabolism. Various probes have been reported for NTR analysis, but rarely able to distinguish the extracellular NTR from intracellular ones. Herein we reported a new NTR sensor, HCyS-NO2, which was a hemicyanine molecule with one nitro and two sulfo groups attached. The nitro group acted as the reporting group to respond NTR reduction. Direct linkage of nitro group into the hemicyanine π conjugate system facilitated the intramolecular electron transfer (IET) process and thus quenched the fluorescence of hemicyanine core. Upon reduction with NTR, the nitro group was rapidly converted into the hydroxylamino and then the amino group, eliminating IET process and thus restoring the fluorescence. The sulfo groups installed significantly increased the hydrophilicity of the molecule, and introduced negative charges at physiological pH, preventing the diffusion into bacteria. Both gram-negative and gram-positive bacteria were able to turn on the fluorescence of HCyS-NO2, without detectable diffusion into cells, providing a useful tool to probe the extracellular reduction process.

Activatable NIR Fluorescence Probe for Epinephrine Detection and Bioimaging Based on Anionic Heptamethine Cyanine

Epinephrine (EP) is an essential catecholamine in the human body. Currently, most EP detection methods are not suitable for in vivo detection due to material limitations. An organic small molecule fluorescent probe based on a chemical cascade reaction for the detection of EP was designed. Anionic heptamethine cyanine dye was selected as a fluorescent dye because of its NIR fluorescence emission with excellent biocompatibility. The secondary amine of EP nucleophilically attacks the carbonate of the probe with its stronger nucleophilicity and further undergoes intramolecular nucleophilic cyclization to release the fluorophore. Other substances containing only primary amines or no β-OH lack reaction competitiveness due to their weaker nucleophilicity or inability to undergo further cyclization. The fluorescence recovery of the probe was linearly related to the EP concentration of 2-75 μmol/L. The detection limit was 0.4 μmol/L. The recovery rate was 94.78-111.32%. Finally, we successfully achieved bioimaging of EP in living cells and EP analogue in nematodes.

Synthesis of Heptamethine Cyanines from Furfural Derivatives

Despite the widespread theranostic utilization of cyanine dyes (Cy7), their synthetic method is still limited with pyridine or cyclohexanone derivatives as starting materials. Herein, we report the synthesis of Cy7 from furfural derivatives. First, a one-pot reaction strategy is developed to solve the unstable problem of the Stenhouse salts. Second, a stepwise condensation strategy is exploited to regioselectively synthesize asymmetrical Cy7. The methodology possesses advantages, such as easy handling, high yield, wide substrate scopes, and good functional group tolerance.

Supramolecular Mitigation of the Cyanine Limit Problem

Currently, there is a substantial research effort to develop near-infrared fluorescent polymethine cyanine dyes for biological imaging and sensing. In water, cyanine dyes with extended conjugation are known to cross over the "cyanine limit" and undergo a symmetry breaking Peierls transition that favors an unsymmetric distribution of π-electron density and produces a broad absorption profile and low fluorescence brightness. This study shows how supramolecular encapsulation of a newly designed series of cationic, cyanine dyes by cucurbit[7]uril (CB7) can be used to alter the π-electron distribution within the cyanine chromophore. For two sets of dyes, supramolecular location of the surrounding CB7 over the center of the dye favors a nonpolar ground state, with a symmetric π-electron distribution that produces a sharpened absorption band with enhanced fluorescence brightness. The opposite supramolecular effect (i.e., broadened absorption and partially quenched fluorescence) is observed with a third set of dyes because the surrounding CB7 is located at one end of the encapsulated cyanine chromophore. From the perspective of enhanced near-infrared bioimaging and sensing in water, the results show how that the principles of host/guest chemistry can be employed to mitigate the "cyanine limit" problem.

PESIN Conjugates for Multimodal Imaging: Can Multimerization Compensate Charge Influences on Cell Binding Properties? A Case Study

Recently, anionic charges were found to negatively influence the in vitro gastrin-releasing peptide receptor (GRPR) binding parameters of dually radioisotope and fluorescent dye labeled GRPR-specific peptide dimers. From this, the question arose if this adverse impact on in vitro GRP receptor affinities could be mitigated by a higher valency of peptide multimerization. For this purpose, we designed two different hybrid multimodal imaging units (MIUs), comprising either one or two click chemistry-compatible functional groups and reacted them with PESIN (PEG₃-BBN_7-14, PEG = polyethylene glycol) dimers to obtain a dually labeled peptide homodimer or homotetramer. Using this approach, other dually labeled peptide monomers, dimers, and tetramers can also be obtained, and the chelator and fluorescent dye can be adapted to specific requirements. The MIUs, as well as their peptidic conjugates, were evaluated in terms of their photophysical properties, radiolabeling efficiency with ⁶⁸Ga and ⁶⁴Cu, hydrophilicity, and achievable GRP receptor affinities. Here, the hydrophilicity and the GRP receptor binding affinities were found to be especially strongly influenced by the number of negative charges and peptide copies, showing log_D (1-octanol-water-distribution coefficient) and IC₅₀ (half maximal inhibitory concentration) values of -2.2 ± 0.1 and 59.1 ± 1.5 nM for the homodimer, and -1.9 ± 0.1 and 99.8 ± 3.2 nM for the homotetramer, respectively. From the obtained data, it can be concluded that the adverse influence of negatively charged building blocks on the in vitro GRP receptor binding properties of dually labeled PESIN multimers can, at least partly, be compensated for by the number of introduced peptide binding motives and the used molecular design.

Automatic medium exchange for micro-volume cell samples based on dielectrophoresis

Cell medium exchange is a crucial step for life science and medicine. However, conventional cell medium exchange methods, including centrifuging and filtering, show limited ability for micro-volume cell samples such as circulating tumor cell (CTC) and circulating fetal cell (CFC). In this paper, we proposed an automatic medium exchange method for micro-volume cell samples based on dielectrophoresis (DEP) in microfluidic chip. Fresh medium and cell suspension were introduced into the microfluidic channel as the laminar flow. Plane stair-shaped interdigital electrodes were employed to drive the cells from the cell suspension to fresh media directly by DEP force. Additionally, we characterized and optimized the cell medium exchange according to both the theory and experiments. In the end, we achieved a 96.9% harvest rate of medium exchange for 0.3 μL samples containing micro-volume cells. For implementing an automatic continuous cell medium exchange, the proposed method can be integrated into the automatic cell processing system conveniently. Furthermore, the proposed method is a great candidate in micro-volume cell analysis and processing, cell electroporation, single cell sequencing, and other scenarios.

A Cyanine-Bridged Somatostatin Hybrid Probe for Multimodal SSTR2 Imaging in Vitro and in Vivo: Synthesis and Evaluation

Multimodal imaging probes have attracted the interest of ongoing research, for example, for the surgical removal of tumors. Modular synthesis approaches allow the construction of hybrid probes consisting of a radiotracer, a fluorophore and a targeting unit. We present the synthesis of a new asymmetric bifunctional cyanine dye that can be used as a structural and functional linker for the construction of such hybrid probes. 68 Ga-DOTATATE, a well-characterized radiopeptide targeting the overexpressed somatostatin receptor subtype 2 (SSTR2) in neuroendocrine tumors, was labeled with our cyanine dye, thus providing additional information along with the data obtained from the radiotracer. We tested the SSTR2-targeting and imaging properties of the resulting probe 68 Ga-DOTA-ICC-TATE in vitro and in a tumor xenograft mouse model. Despite the close proximity between dye and pharmacophore, we observed a high binding affinity towards SSTR2 as well as elevated uptake in SSTR2-overexpressing tumors in the positron emission tomography (PET) scan and histological examination.

Explorations into the Effect of meso-Substituents in Tricarbocyanine Dyes: A Path to Diverse Biomolecular Probes and Materials

Polymethine cyanine dyes have been widely recognized as promising chemical tools for a range of life science and biomedical applications, such as fluorescent staining of DNA and proteins in gel electrophoresis, fluorescence guided surgery, or as ratiometric probes for probing biochemical pathways. The photophysical properties of such dyes can be tuned through the synthetic modification of the conjugated backbone, for example, by altering aromatic cores or by varying the length of the conjugated polymethine chain. Alternative routes to shaping the absorption, emission, and photostability of dyes of this family are centered around the chemical modifications on the polymethine chain. This Minireview aims to discuss strategies for the introduction of substituents in the meso-position, their effect on the photophysical properties of these dyes and some structure-activity correlations which could help overcome common limitations in the state of the art in the synthesis.

Functional Hybrid Molecules for the Visualization of Cancer: PESIN-Homodimers Combined with Multimodal Molecular Imaging Probes for Positron Emission Tomography and Optical Imaging: Suited for Tracking of GRPR-Positive Malignant Tissue*

We describe multimodal imaging probes for gastrin-releasing peptide receptor (GRPR)-specific targeting suited for positron emission tomography and optical imaging (PET/OI), consisting of PESIN (PEG3 -BBN7-14 ) dimers connected to multimodal imaging subunits. These multimodal agents comprise a fluorescent dye for OI and the chelator ((1,4,7-triazacyclononane-4,7-diyl)diacetic acid-1-glutaric acid) (NODA-GA) for PET radiometal isotope labelling. Special focus was put on the influence of the used dyes on the properties of the whole bioconjugates. For this, several compounds with different fluorescent dyes and non-dye carrying subunits were synthesized and investigated. As fluorescent dyes, dansyl, NBD, derivatives of fluorescein, coumarin and rhodamine as well as three pyrilium-based dyes were employed. Considerable influence of the charge of the colored unit on hydrophilicity as well as in vitro target receptor binding was observed and classified. High radiochemical yields and purities were found during radiolabeling of the multimodal imaging subunits as well as their GRPR-specific bioconjugates with 68 Ga. Examinations of the photophysical properties of both molecule species displayed no loss or alteration of fluorescence characteristics.

Synthesis, characterization and optimization of in vitro properties of NIR-fluorescent cyclic α-MSH peptides for melanoma imaging

Melanoma are malignant tumors derived from melanocytes being responsible for the majority of skin cancer deaths with an increasing rate of incidence. The Melanocortin-1 receptor (MC1R) has been recognized as a molecular target for melanoma detection. Here, we report on the development and optimization of molecular probes which are based on novel conjugates of near-infrared (NIR) fluorescent indocyanine dyes and an MC1R-targeting peptide intended for optical fluorescence imaging enabling an early, specific, accurate and sensitive diagnosis of malignant melanomas. The introduction of anionic groups into the aromatic ring of the indolenine substructure of the conjugated dyes has shown to result in a strong fluorescence in aqueous solution and a concomitant increase of binding affinities of the peptide conjugates to the target receptor. The length and flexibility of the PEG chain introduced as a linker, as well as the nature of its attachment to the dye also affect the binding affinities, albeit to a lower extent. The conjugates have been successfully applied in the MC1R-specific staining of B16F10 melanoma cells, both in cell cultures and in microtome sections of solid tumors.

Cyanine Conjugate-Based Biomedical Imaging Probes

Cyanine is a class of fluorescent dye with meritorious fluorescence properties and has motivated numerous researchers to explore its imaging capabilities by miscellaneous structural modification and functionalization strategies. The covalent conjugation with other functional molecules represents a distinctive design strategy and has shown immense potential in both basic and clinical research. This review article summarizes recent achievements in cyanine conjugate-based probes for biomedical imaging. Particular attention is paid to the conjugation with targeting warheads and other contrast agents for targeted fluorescence imaging and multimodal imaging, respectively. Additionally, their clinical potential in cancer diagnostics is highlighted and some concurrent impediments for clinical translation are discussed.

Cyanine-Like Boronic Acid-Derived Salicylidenehydrazone Complexes (Cy-BASHY) for Bioimaging Applications

Boronic acid-derived salicylidenehydrazone complex (BASHY) dyes with a polymethine backbone were designed to yield efficient red-emitting and two-photon absorbing fluorophores that can be used as markers for astrocytes. The dyes are chemically stable in aqueous solution and do not undergo photodecomposition. Their photophysical properties can be electronically fine-tuned and thereby adapted to potentially different imaging situations and requirements.

Hybrid Multimodal Imaging Synthons for Chemoselective and Efficient Biomolecule Modification with Chelator and Near-Infrared Fluorescent Cyanine Dye

The development of hybrid multimodal imaging synthons (MIS), carrying in addition to a chelator for radiometal labeling also a near-infrared (NIR) fluorescent cyanine dye was the aim of this work. The MIS should be introducible into biomolecules of choice via an efficient and chemoselective click chemistry reaction. After chemical optimization, a successful synthetic strategy towards such hybrid MIS was developed, based on solid phase-based synthesis techniques and applying different near-infrared fluorescent cyanine dyes. The developed hybrid agents were shown to be easily introducible into a model homobivalent peptidic gastrin-releasing peptide receptor- (GRPR)-specific carrier without forming any side products and the MIS as well as their bioconjugates were radiolabeled with the positron-emitter ⁶⁸Ga³⁺. The hybrid multimodal agents were characterized with regard to their log_Ds, GRPR target affinities and photophysical characteristics. It could be shown that the properties of the bioconjugates were not per se affected by the introduction of the MIS but that the cyanine dye used and specifically the number of comprised negative charges per dye molecule can have a considerable influence on target receptor binding. Thus, the molecular toolbox described here enables the synthesis of tailored hybrid multimodal imaging synthons for biomolecule modification, meeting the specific need and envisioned application of the combined imaging agent.

Cyanine-Flavonol Hybrids for Near-Infrared Light-Activated Delivery of Carbon Monoxide

Carbon monoxide (CO) is an endogenous signaling molecule that controls a number of physiological processes. To circumvent the inherent toxicity of CO, light-activated CO-releasing molecules (photoCORMs) have emerged as an alternative for its administration. However, their wider application requires photoactivation using biologically benign visible and near-infrared (NIR) light. In this work, a strategy to access such photoCORMs by fusing two CO-releasing flavonol moieties with a NIR-absorbing cyanine dye is presented. These hybrids liberate two molecules of CO in high chemical yields upon activation with NIR light up to 820 nm and exhibit excellent uncaging cross-sections, which surpass the state-of-the-art by two orders of magnitude. Furthermore, the biocompatibility and applicability of the system in vitro and in vivo are demonstrated, and a mechanism of CO release is proposed. It is hoped that this strategy will stimulate the discovery of new classes of photoCORMs and accelerate the translation of CO-based phototherapy into practice.

Sterically Shielded Heptamethine Cyanine Dyes for Bioconjugation and High Performance Near-Infrared Fluorescence Imaging

The near-infrared window of fluorescent heptamethine cyanine dyes greatly facilitates biological imaging because there is deep penetration of the light and negligible background fluorescence. However, dye instability, aggregation, and poor pharmacokinetics are current drawbacks that limit performance and the scope of possible applications. All these limitations are simultaneously overcome with a new molecular design strategy that produces a charge balanced and sterically shielded fluorochrome. The key design feature is a meso-aryl group that simultaneously projects two shielding arms directly over each face of a linear heptamethine polyene. Cell and mouse imaging experiments compared a shielded heptamethine cyanine dye (and several peptide and antibody bioconjugates) to benchmark heptamethine dyes and found that the shielded systems possess an unsurpassed combination of photophysical, physiochemical, and biodistribution properties that greatly enhance bioimaging performance.

Stable and Well-Organized Near-Infrared Platinum(II)-Acetylide-Based Metallacycles-Mediated Cancer Phototherapy

The development of metallacycles with high stability and intense near-infrared (NIR) absorption is important for biomedical applications. However, very few molecular design strategies have been developed on such metallacycles. Herein, we report a new series of stable and well-defined NIR-absorbing metallacycles (M1-M3) through the Pt-acetylide coordination with highly efficient photoconversion performance for cancer phototherapy. The metallacycles showed high stability and strong NIR absorption, and the absorption peaks were red shifted approximately 30 nm in comparison with their corresponding precursors. The introduction of Pt into metallacycles promotes significant photoconversions, including the singlet-to-triplet and nonradiative transitions. Moreover, the fabricated M3 nanoparticles (M3-NPs) showed favorable photoconversions into both thermal effect and singlet oxygen generation upon NIR irradiation, achieving tumor ablation. This novel design of Pt-acetylide metallacycles possesses not only complex topological architectures but also a valuable paradigm for precise cancer phototherapy, which is important for grafting stimuli-responsive functional groups into metallacycles for the development of high-performance biomedical supramolecular materials.

Probing two PESIN-indocyanine-dye-conjugates: significance of the used fluorophore

Peptide-dye-conjugates hold a great promise in application for biological and medical imaging of cellular processes and in delineation and characterization of human tumors. In particular, indocyanine dyes are of great interest due to their reported superior properties such as absorption and emission in the near-infrared (NIR) spectral range, favorable Stokes shifts and their well-studied safety profile in humans. In this study, we investigated and describe the influence of indocyanine dyes on different properties of the final peptide-dye-conjugates. As a target peptide, PESIN, a bombesin derivative, was used as a model peptide which addresses GRP receptors overexpressed on different malignancies. Here, we map similarities and differences of the fluorescent conjugates and by this elucidate the influence of the dyes on different properties of the formed conjugates. We performed the dye syntheses, subsequent bioconjugation reactions and in the following investigated the optical properties, water/octanol distribution coefficients and target receptor affinities by in vitro competitive binding studies on PC-3 cells. The obtained results give a handrail to medical and biological researchers planning studies involving indocyanine dye biomolecule conjugates.

Glutathione-responsive disassembly of disulfide dicyanine for tumor imaging with reduction in background signal intensity

Near-infrared (NIR) fluorescence imaging has been proved as an effective modality in identifying the tumor border and distinguishing the tumor cells from healthy tissue during the oncological surgery. Developing NIR fluorescent probes with high tumor to background (T/B) signal is essential for the complete debulking of the tumor, which will prolong the survival rate of tumor patients. However, the nonspecific binding and "always-on" properties of the conventional fluorescent probes leads to high background signals and poor specificity. Method: To address this problem, glutathione (GSH)-responsive, two disulfide-bonded dicyanine dyes (ss-diCy5 and ss-diNH800CW) were synthesized. As synthesized dyes are quenched under normal physiological conditions, however, once reached to the tumor site, these dyes are capable of emitting strong fluorescence signals primarily because of the cleavage of the disulfide bond in the tumor microenvironment with high GSH concentration. Besides, the GSH-responsive behavior of these dyes was monitored using the UV-vis and fluorescence spectroscopy. The diagnostic accuracy of the aforementioned dyes was also tested both in tumor cells and 4T1-bearing mice. Results: The fluorescence signal intensity of disulfide dicyanine dyes was quenched up to 89% compared to the mono cyanine dyes, thus providing a very low fluorescence background. However, when the disulfide dicyanine dye reaches the tumor site, the dicyanine is cleaved by GSH into two mono-dyes with high fluorescence strength, thus producing strong fluorescent signals upon excitation. The fluorescent signal of the dicyanine was enhanced by up to 27-fold after interacting with the GSH solution. In vivo xenografts tumor studies further revealed that the fluorescence signals of aforementioned dyes can be quickly recovered in the solid tumor. Conclusion: In summary, the disulfide dicyanines dyes can provide a promising platform for specific tumor-activatable fluorescence imaging with improved T/B value.

Near-infrared heptamethine cyanines (Cy7): from structure, property to application

Heptamethine cyanine dyes (Cy7) have attracted much attention in the field of biological application due to their unique structure and attractive near infrared (NIR) photophysical properties.

Rational design of near-infrared platinum(ii)-acetylide conjugated polymers for photoacoustic imaging-guided synergistic phototherapy under 808 nm irradiation

We have developed a novel near-infrared Pt-acetylide conjugated polymer CP3 with highly efficient photoconversion behaviors for synergistic cancer phototherapy.

Toward a Molecular Reorganization Energy-Based Analysis of Third-Order Nonlinear Optical Properties of Polymethine Dyes and J-Aggregates

This work demonstrates the feasibility and applicability of the theory of extended multiphonon electron transitions for the description of nonlinear optical properties of polymethine dyes using quantum chemistry and model calculations. The transformation of a strong one-photon absorption band in dye monomers to a weak two-photon absorption band is rationalized from the electron-nuclear resonance condition. The power law fitting of the results of quantum chemical computations of nonlinear optical properties allows the predicting of the shift of the corresponding Egorov-like resonance curve to the shortest dye in the vinylogous series of dye monomers. The results presented provide an insight and guide for the rational molecular design and application of polymethine dyes.

New Polyfluorinated Cyanine Dyes for Selective NIR Staining of Mitochondria

In this communication, the synthesis of three unknown polyfluorinated cyanine dyes and their application as selective markers for mitochondria are presented. By incorporating fluorous side chains into cyanine dyes, their remarkable photophysical properties were enhanced. To investigate their biological application, several different cell lines were incubated with the synthesized cyanine dyes. It was discovered that the presented dyes can be utilized for selective near-infrared-light (NIR) staining of mitochondria, with very low cytotoxicity determined by MTT assay. This is the first time that polyfluorinated cyanine fluorophores are presented as selective markers for mitochondria. Due to the versatile applications of polyfluorinated fluorophores in bioimaging and materials science, it is expected that the presented fluorophores will be stimulating for the scientific community.

Carbonic Anhydrase IX-Targeted Near-Infrared Dye for Fluorescence Imaging of Hypoxic Tumors

Use of tumor-targeted fluorescence dyes to help surgeons identify otherwise undetected tumor nodules, decrease the incidence of cancer-positive margins, and facilitate localization of malignant lymph nodes has demonstrated considerable promise for improving cancer debulking surgery. Unfortunately, the repertoire of available tumor-targeted fluorescent dyes does not permit identification of all cancer types, raising the need to develop additional tumor-specific fluorescent dyes to ensure localization of all malignant lesions during cancer surgeries. By comparing the mRNA levels of the hypoxia-induced plasma membrane protein carbonic anhydrase IX (CA IX) in 13 major human cancers with the same mRNA levels in corresponding normal tissues, we document that CA IX constitutes a nearly universal marker for the design of tumor-targeted fluorescent dyes. Motivated by this expression profile, we synthesize two new CA IX-targeted near-infrared (NIR) fluorescent imaging agents and characterize their physical and biological properties both in vitro and in vivo. We report that conjugation of either acetazolamide or 6-aminosaccharin (i.e., two CA-IX-specific ligands) to the NIR fluorescent dye, S0456, via an extended phenolic spacer creates a brightly fluorescent dye that binds CA IX with high affinity and allows rapid visualization of hypoxic regions of solid tumors at depths >1 cm beneath a tissue surface. Taken together, these data suggest that a CA IX-targeted NIR dye can constitute a useful addition to a cocktail of tumor-targeted NIR dyes designed to image all human cancers.

Evaluation of a Centyrin-Based Near-Infrared Probe for Fluorescence-Guided Surgery of Epidermal Growth Factor Receptor Positive Tumors

Tumor-targeted near-infrared fluorescent dyes have the potential to improve cancer surgery by enabling surgeons to locate and resect more malignant lesions where good visualization tools are required to ensure complete removal of malignant tissue. Although the tumor-targeted fluorescent dyes used in humans to date have been either small organic molecules or high molecular weight antibodies, low molecular weight protein scaffolds have attracted significant attention because they penetrate solid tumors almost as efficiently as small molecules, but can be infinitely mutated to bind almost any antigen. Here we describe the use of a 10 kDa protein scaffold, a Centyrin, to target a near-infrared fluorescent dye to tumors that overexpress the epidermal growth factor receptor (EGFR) for fluorescence-guided surgery (FGS). We have developed and optimized the dose and time required for imaging small tumor burdens with minimal background fluorescence in real-time fluorescence-guided surgery of EGFR-expressing tumor xenografts in murine models. We demonstrate that the Centyrin-near-infrared dye conjugate (CNDC) binds selectively to human EGFR+ cancer cells with an EC50 of 2 nM, localizes to EGFR+ tumor xenografts in athymic nude mice and that uptake of the dye in xenografts is significantly reduced when EGFR are blocked by preinjection of excess unlabeled Centyrin. Taken together, these data suggest that CNDCs can be used for intraoperative identification and surgical removal of EGFR-expressing lesions and that Centyrins targeted to other tumor-specific antigens should prove similarly useful in fluorescence guided surgery of cancer. In addition, we demonstrate that the CNDC is detected in the NIR region of the spectrum and can be utilized for fluorescence-guided surgery (FGS). In addition, we propose that with its eventual complete clearance from EGFR-negative tissues and its quantitative retention in the tumor mass for >24 h, a Centyrin-targeted NIR dye should provide excellent tumor contrast when injected at least 6-8 h before initiation of cancer surgery in human patients.

Near-Infrared Fluorescent Probe for Sensitive Detection of Pb(II) Ions in Living Cells

A new near-infrared fluorescent probe (NIR-PbP) for sensitive detection of Pb(II) ions in solution and living cells has been rationally designed and synthesized. The NIR-PbP is inherently non-fluorescent and gains fluorescence in the presence Pb(II) ions. The ion detection is based on Pb(II)-induced unmasking the fluorophore through the opening of the spyrocycle, with more than 500-fold fluorescence for sub-micromolar Pb(II) concentration. The NIR-PbP has high sensitivity, good photo-stability, low detection limit, and reversible response to Pb(II) ions.