Two birds with one stone: A highly sensitive near-infrared BODIPY-based fluorescent probe for the simultaneous detection of Fe2+ and H+ in vivo

Ferrous ion (Fe²⁺) plays an essential role in many physiological and pathological processes, and its cellular metabolism is closely related to acidic pH. However, the lack of multifunctional Fe²⁺ probes has hindered the further study of Fe²⁺ in vivo. Herein, we report a dual-responsive near-infrared (NIR) fluorescent probe BODIPY-Fe for the simultaneous of Fe²⁺ and H⁺ in vivo by harnessing the N-oxide strategy and photoinduced electron transfer (PeT) mechanism. BODIPY-Fe exhibited NIR fluorescence at 671 nm, rapid response to Fe²⁺ within 90 s, and high sensitivity of low LOD of 292 nM towards Fe²⁺. Moreover, BODIPY-Fe could sensitively and selectively detect Fe²⁺ and H⁺ in the lysosomes of living cells simultaneously. Notably, BODIPY-Fe was able to noninvasively visualize Fe²⁺ and H⁺ in vivo, showing "ON-OFF-ON" NIR fluorescence signal changes. This work demonstrates that BODIPY-Fe has great potential to promote the simultaneous imaging of Fe²⁺ and H⁺ in biological systems.

High-Contrast Fluorescence Detection of Metastatic Breast Cancer Including Bone and Liver Micrometastases via Size-Controlled pH-Activatable Water-Soluble Probes

Breast cancer metastasis is the major cause of cancer death in women worldwide. Early detection would save many lives, but current fluorescence imaging probes are limited in their detection ability, particularly of bone and liver micrometastases. Herein, probes that are capable of imaging tiny (<1 mm) micrometastases in the liver, lung, pancreas, kidneys, and bone, that have disseminated from the primary site, are reported. The influence of the poly(ethylene glycol) (PEG) chain length on the performance of water-soluble, pH-responsive, near-infrared 4,4'-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) probes is systematically investigated to demonstrate that PEG tuning can provide control over micrometastasis tracking with high tumor-to-background contrast (up to 12/1). Optimized probes can effectively visualize tumor boundaries and successfully detect micrometastases with diameters <1 mm. The bone-metastasis-targeting ability of these probes is further enhanced by covalent functionalization with bisphosphonate. This improved detection of both bone and liver micrometastases (<2 mm) with excellent tumor-to-normal contrast (5.2/1). A versatile method is thus introduced to directly synthesize modular water-soluble probes with broad potential utility. Through a single intravenous injection, these materials can image micrometastases in multiple organs with spatiotemporal resolution. They thus hold promise for metastasis diagnosis, image-guided surgery, and theranostic PEGylated drug therapies.

Dual-stimuli responsive and reversibly activatable theranostic nanoprobe for precision tumor-targeting and fluorescence-guided photothermal therapy

The integrated functions of diagnostics and therapeutics make theranostics great potential for personalized medicine. Stimulus-responsive therapy allows spatial control of therapeutic effect only in the site of interest, and offers promising opportunities for imaging-guided precision therapy. However, the imaging strategies in previous stimulus-responsive therapies are ‘always on' or irreversible ‘turn on' modality, resulting in poor signal-to-noise ratios or even ‘false positive' results. Here we show the design of dual-stimuli-responsive and reversibly activatable nanoprobe for precision tumour-targeting and fluorescence-guided photothermal therapy. We fabricate the nanoprobe from asymmetric cyanine and glycosyl-functionalized gold nanorods (AuNRs) with matrix metalloproteinases (MMPs)-specific peptide as a linker to achieve MMPs/pH synergistic and pH reversible activation. The unique activation and glycosyl targetibility makes the nanoprobe bright only in tumour sites with negligible background, while AuNRs and asymmetric cyanine give synergistic photothermal effect. This work paves the way to designing efficient nanoprobes for precision theranostics.

A number of nanomaterials for dual diagnostic and therapeutic application have a number of limitations including poor signal-to-noise ratio. Here, the authors developed dual stimuli-responsive and reversibly activatable nanoprobes for tumour targeting and fluorescence-guided photothermal therapy.

Activatable Water-Soluble Probes Enhance Tumor Imaging by Responding to Dysregulated pH and Exhibiting High Tumor-to-Liver Fluorescence Emission Contrast

Dysregulated pH has been recognized as a universal tumor microenvironment signature that can delineate tumors from normal tissues. Existing fluorescent probes that activate in response to pH are hindered by either fast clearance (in the case of small molecules) or high liver background emission (in the case of large particles). There remains a need to design water-soluble, long circulating, pH-responsive nanoprobes with high tumor-to-liver contrast. Herein, we report a modular chemical strategy to create acidic pH-sensitive and water-soluble fluorescent probes for high in vivo tumor detection and minimal liver activation. A combination of a modified Knoevenagel reaction and PEGylation yielded a series of NIR BODIPY fluorophores with tunable pKas, high quantum yield, and optimal orbital energies to enable photoinduced electron transfer (PeT) activation in response to pH. After intravenous administration, Probe 5c localized to tumors and provided excellent tumor-to-liver contrast (apparent T/L = 3) because it minimally activates in the liver. This phenomenon was further confirmed by direct ex vivo imaging experiments on harvested organs. Because no targeting ligands were required, we believe that this report introduces a versatile strategy to directly synthesize soluble probes with broad potential utility including fluorescence-based image-guided surgery, cancer diagnosis, and theranostic nanomedicine.

Near-infrared asymmetrical heptamethine cyanines specifically imaging cancer cells by sensing their acidic lysosomal lumen

Asymmetrical heptamethine cyanine based near-infrared fluorophores specifically imaging cancer cells by sensing their acidic lysosomal lumen.

Long-wavelength analyte-sensitive luminescent probes and optical (bio)sensors

Long-wavelength luminescent probes and sensors become increasingly popular. They offer the advantage of lower levels of autofluorescence in most biological probes. Due to high penetration depth and low scattering of red and NIR light such probes potentially enable in vivo measurements in tissues and some of them have already reached a high level of reliability required for such applications. This review focuses on the recent progress in development and application of long-wavelength analyte-sensitive probes which can operate both reversibly and irreversibly. Photophysical properties, sensing mechanisms, advantages and limitations of individual probes are discussed.

Multi-parametric imaging of the invasiveness-permissive acidic microenvironment in human glioma xenografts

Non-invasive multi-parametric imaging demonstrated the positive correlation between the invasiveness and extracellular acidity in glioma xenografts.