Real time monitoring of aminothiol level in blood using a near-infrared dye assisted deep tissue fluorescence and photoacoustic bimodal imaging

Tuning Tumor Targeting and Ratiometric Photoacoustic Imaging by Fine-Tuning Torsion Angle for Colorectal Liver Metastasis Diagnosis

Photoacoustic (PA) tomography is an emerging biomedical imaging technology for precision cancer medicine. Conventional small-molecule PA probes usually exhibit a single PA signal and poor tumor targeting that lack the imaging reliability. Here, we introduce a series of cyanine/hemicyanine interconversion dyes (denoted Cy-HCy) for PA/fluorescent dual-mode probe development that features optimized ratiometric PA imaging and tunable tumor-targeting ability for precise diagnosis and resection of colorectal cancer (CRC). Importantly, Cy-HCy can be presented in cyanine (inherent tumor targeting and long NIR PA wavelength) and hemicyanine (poor tumor targeting and short NIR PA wavelength) by fine-tuning torsion angle and the ingenious transformation between cyanine and hemicyanine through regulation optically tunable group endows the NIR ratiometric PA and tunable tumor-targeting properties. To demonstrate the applicability of Cy-HCy dyes, we designed the first small-molecule tumor-targeting and NIR ratiometric PA probe Cy-HCy-H2S for precise CRC liver metastasis diagnosis, activated by H2S (a CRC biomarker). Using this probe, we not only visualized the subcutaneous tumor and liver metastatic cancers in CRC mouse models but also realized PA and fluorescence image-guided tumor excision. We expect that Cy-HCy will be generalized for creating a wide variety of inherently tumor-targeting NIR ratiometric PA probes in oncological research and practice.

Activatable Probes for Ratiometric Imaging of Endogenous Biomarkers In Vivo

Dynamic variations in the concentration and abnormal distribution of endogenous biomarkers are strongly associated with multiple physiological and pathological states. Therefore, it is crucial to design imaging systems capable of real-time detection of dynamic changes in biomarkers for the accurate diagnosis and effective treatment of diseases. Recently, ratiometric imaging has emerged as a widely used technique for sensing and imaging of biomarkers due to its advantage of circumventing the limitations inherent to conventional intensity-dependent signal readout methods while also providing built-in self-calibration for signal correction. Here, the recent progress of ratiometric probes and their applications in sensing and imaging of biomarkers are outlined. Ratiometric probes are classified according to their imaging mechanisms, and ratiometric photoacoustic imaging, ratiometric optical imaging including photoluminescence imaging and self-luminescence imaging, ratiometric magnetic resonance imaging, and dual-modal ratiometric imaging are discussed. The applications of ratiometric probes in the sensing and imaging of biomarkers such as pH, reactive oxygen species (ROS), reactive nitrogen species (RNS), glutathione (GSH), gas molecules, enzymes, metal ions, and hypoxia are discussed in detail. Additionally, this Review presents an overview of challenges faced in this field along with future research directions.

A reductively convertible nickel phthalocyanine precursor as a biological thiol-responsive turn-on photoacoustic contrast agent

A nickel phthalocyanine precursor bearing poly(ethylene glycol) as a turn-on contrast agent for photoacoustic imaging was prepared. The water-soluble polymeric chains were smoothly eliminated through thiol-mediated reductive aromatization in cancer cells, enabling the detection of endogenous biological thiols in vitro and in vivo.

Full-Color Emissive D-D-A Carbazole Luminophores: Red-to-NIR Mechano-fluorochromism, Aggregation-Induced Near-Infrared Emission, and Application in Photodynamic Therapy

The preparation of multifunctionalized luminophores with full-color emission based on an identical core skeleton is a significative but challenging research topic. In this work, eight donor-donor-acceptor (D-D-A)-type luminogens based on a central carbazole core bearing a C₆ hydrocarbon chain were designed by using different kinds of donor and acceptor units on the left and right, and synthesized in good yields. These D-D-A carbazole derivatives display deep-blue, sky-blue, cyan, green, yellow-green, yellow, orange and red fluorescence in the solid state, achieving full-color emission covering the whole visible light range under UV light illumination. Notably, the dicyano-functionalized triphenylamine-containing carbazole derivative exhibits rare aggregation-induced near-infrared emission and red-to-near-infrared mechano-fluorochromism with high contrast beyond 100 nm. Furthermore, the red-emissive luminogen can serve as a potential candidate for cell imaging and photodynamic therapy (PDT). This work not only provides reference for the construction of full-color emissive systems but also opens a new avenue to the preparation of multifunctionalized luminophores capable of simultaneous application in near-Infrared mechanical-force sensors and PDT fields.

Dual-Mode Tumor Imaging Using Probes That Are Responsive to Hypoxia-Induced Pathological Conditions

Hypoxia in solid tumors is associated with poor prognosis, increased aggressiveness, and strong resistance to therapeutics, making accurate monitoring of hypoxia important. Several imaging modalities have been used to study hypoxia, but each modality has inherent limitations. The use of a second modality can compensate for the limitations and validate the results of any single imaging modality. In this review, we describe dual-mode imaging systems for the detection of hypoxia that have been reported since the start of the 21st century. First, we provide a brief overview of the hallmarks of hypoxia used for imaging and the imaging modalities used to detect hypoxia, including optical imaging, ultrasound imaging, photoacoustic imaging, single-photon emission tomography, X-ray computed tomography, positron emission tomography, Cerenkov radiation energy transfer imaging, magnetic resonance imaging, electron paramagnetic resonance imaging, magnetic particle imaging, and surface-enhanced Raman spectroscopy, and mass spectrometric imaging. These overviews are followed by examples of hypoxia-relevant imaging using a mixture of probes for complementary single-mode imaging techniques. Then, we describe dual-mode molecular switches that are responsive in multiple imaging modalities to at least one hypoxia-induced pathological change. Finally, we offer future perspectives toward dual-mode imaging of hypoxia and hypoxia-induced pathophysiological changes in tumor microenvironments.

PDA modified NIR-II NaEr0.8Yb0.2F4nanoparticles with high photothermal effect

Polydopamine (PDA)-modified NaEr_0.8Yb_{0.2 F4}nanoparticles were synthesized, with strong NIR-II emission, quantum yield of 29.63%, and excellent photothermal performance. Crystal phases and microstructures are characterized. Optical properties such as absorption, NIR-II emission, and light stability are studied, and the luminescence mechanism is discussed in detail. Key factors in NIR-II imaging were evaluated in fresh pork tissue, including penetration depth, spatial resolution, and signal-to-noise ratio (SNR). A high penetration depth of 5 mm and a high spatial resolution of 1 mm were detected. Mice are imaged in vivo afterintravenousinjection. Due to the accumulation of nanoparticles in the liver, high image quality with an SNR of 5.2 was detected in the abdomen of KM mice with hair. The photothermal conversion effect of PDA-modified NPs was twice that of the reported material. These NIR-II nanoparticles have superior optical properties, high photothermal efficiency and low cytotoxicity, and are potential fluorescent probes for further disease diagnosis and treatment.

Rational design of stable heptamethine cyanines and development of a biomarker-activatable probe for detecting acute lung/kidney injuries via NIR-II fluorescence imaging

Heptamethine cyanines exhibiting high photo- and chemostability have been developed. And an activatable probe was developed for H2O2 to visualize acute lung and kidney injuries via NIR-II fluorescence imaging.

NIR-II cell endocytosis-activated fluorescent probes for in vivo high-contrast bioimaging diagnostics

Fluorescence probes have great potential to empower bioimaging, precision clinical diagnostics and surgery. However, current probes are limited to in vivo high-contrast diagnostics, due to the substantial background interference from tissue scattering and nonspecific activation in blood and normal tissues. Here, we developed a kind of cell endocytosis-activated fluorescence (CEAF) probe, which consists of a hydrophilic polymer unit and an acid pH-sensitive small-molecule fluorescent moiety that operates in the "tissue-transparent" second near-infrared (NIR-II) window. The CEAF probe stably presents in the form of quenched nanoaggregates in water and blood, and can be selectively activated and retained in lysosomes through cell endocytosis, driven by a synergetic mechanism of disaggregation and protonation. In vivo imaging of tumor and inflammation with a passive-targeting and affinity-tagged CEAF probe, respectively, yields highly specific signals with target-to-background ratios over 15 and prolonged observation time up to 35 hours, enabling positive implications for surgical, diagnostic and fundamental biomedical studies.

A General Approach to Design Dual Ratiometric Fluorescent and Photoacoustic Probes for Quantitatively Visualizing Tumor Hypoxia Levels In Vivo

Herein, we describe an energy balance strategy between fluorescence and photoacoustic effects by sulfur substitution to transform existing hemicyanine dyes (Cy) into optimized NIRF/PA dual ratiometric scaffolds. Based on this optimized scaffold, we reported the first dual-ratio response of nitroreductase probe AS-Cy-NO₂ , which allows quantitative visualization of tumor hypoxia in vivo. AS-Cy-NO₂ , composed of a new NIRF/PA scaffold thioxanthene-hemicyanine (AS-Cy-1) and a 4-nitrobenzene moiety, showed a 10-fold ratiometric NIRF enhancement (I₇₇₃ /I₇₃₃ ) and 2.4-fold ratiometric PA enhancement (PA₇₃₀ /PA₆₇₀ ) upon activation by a biomarker (nitroreductase, NTR) associated with tumor hypoxia. Moreover, the dual ratiometric NIRF/PA imaging accurately quantified the hypoxia extent with high sensitivity and high imaging depth in xenograft breast cancer models. More importantly, the 3D maximal intensity projection (MIP) PA images of the probe can precisely differentiate the highly heterogeneous oxygen distribution in solid tumor. Thus, this study provides a promising NIRF/PA scaffold that may be generalized for the dual ratiometric imaging of other disease-relevant biomarkers.

Emerging contrast agents for multispectral optoacoustic imaging and their biomedical applications

Optoacoustic imaging is a hybrid biomedical imaging modality which collects ultrasound waves generated via photoexciting contrast agents in tissues and produces images of high resolution and penetration depth. As a functional optoacoustic imaging technique, multispectral optoacoustic imaging, which can discriminate optoacoustic signals from different contrast agents by illuminating samples with multi-wavelength lasers and then processing the collected data with specific algorithms, assists in the identification of a specific contrast agent in target tissues and enables simultaneous molecular and physiological imaging. Moreover, multispectral optoacoustic imaging can also generate three-dimensional images for biological tissues/samples with high resolution and thus holds great potential in biomedical applications. Contrast agents play essential roles in optoacoustic imaging, and they have been widely explored and applied as probes and sensors in recent years, leading to the emergence of a variety of new contrast agents. In this review, we aim to summarize the latest advances in emerging contrast agents, especially the activatable ones which can respond to specific biological stimuli, as well as their preclinical and clinical applications. We highlight their design strategies, discuss the challenges and prospects in multispectral optoacoustic imaging, and outline the possibility of applying it in clinical translation and public health services using synthetic contrast agents.

Indocyanine green dye based bimodal contrast agent tested by photoacoustic/fluorescence tomography setup

Multimodal imaging systems are in high demand for preclinical research, experimental medicine, and clinical practice. Combinations of photoacoustic technology with other modalities including fluorescence, ultrasound, MRI, OCT have been already applied in feasibility studies. Nevertheless, only the combination of photoacoustics with ultrasound in a single setup is commercially available now. A combination of photoacoustics and fluorescence is another compelling approach because those two modalities naturally complement each other. Here, we presented a bimodal contrast agent based on the indocyanine green dye (ICG) as a single signalling compound embedded in the biocompatible and biodegradable polymer shell. We demonstrate its remarkable characteristics by imaging using a commercial photoacoustic/fluorescence tomography system (TriTom, PhotoSound Technologies). It was shown that photoacoustic signal of the particles depends on the amount of dye loaded into the shell, while fluorescence signal depends on the total amount of dye per particle. For the first time to our knowledge, a commercial bimodal photoacoustic/fluorescence setup was used for characterization of ICG doped polymer particles. Additionally, we conducted cell toxicity studies for these particles as well as studied biodistribution over time in vivo and ex vivo using fluorescent imaging. The obtained results suggest a potential for the application of biocompatible and biodegradable bimodal contrast agents as well as the integrated photoacoustic/fluorescence imaging system for preclinical and clinical studies.

A novel fluorescence probe for the selective detection of cysteine in aqueous solutions and imaging in living cells and mice

A novel fluorescent probe based on hydroxyquinoline conjugated with a charged trimethylindolenine unit Toc-Ac was developed, which exhibited long wavelength emission (560 nm) and a large Stokes shift (∼140 nm) due to the intrinsic mechanism of the intramolecular charge transfer process. The probe Toc-Ac showed a highly sensitive response to Cys (the detection limit was 3.86 × 10^-8 M) in aqueous solution and was successfully applied for detecting endogenous Cys in living cells and mice.

A new pentacyclic pyrylium fluorescent probe that responds to pH imbalance during apoptosis

Efficient fluorophores with easy synthetic routes and fast responses are of great importance in clinical diagnostics. Herein, we report a new, rigid pentacyclic pyrylium fluorophore, PS-OMe, synthesised in a single step by a modified Vilsmeier-Haack reaction. Insights into the reaction mechanism facilitated a new reaction protocol for the efficient synthesis of PS-OMe which upon demethylation resulted in a "turn-on" pH sensor, PS-OH. This new fluorescent probe has been successfully used to monitor intracellular acidification at physiological pH. From the fluorescence image analysis, we were able to quantify the intracellular dynamic pH change during apoptosis. This new pH probe is a potential chemical tool for screening, drug discovery and dose determination in cancer therapy.

Grafted semiconducting polymer amphiphiles for multimodal optical imaging and combination phototherapy

Semiconducting polymer nanoparticles (SPNs) have gained growing attention in biomedical applications. However, the preparation of SPNs is usually limited to nanoprecipitation in the presence of amphiphilic copolymers, which encounters the issue of dissociation. As an alternative to SPNs, grafted semiconducting polymer amphiphiles (SPAs) composed of a semiconducting polymer (SP) backbone and hydrophilic side chains show increased physiological stability and improved optical properties. This review summarizes recent advances in SPAs for cancer imaging and combination phototherapy. The applications of SPAs in optical imaging including fluorescence, photoacoustic, multimodal and activatable imaging are first described, followed by the discussion of applications in imaging-guided phototherapy and combination therapy, light-triggered drug delivery and gene regulation. At last, the conclusion and future prospects in this field are discussed.

A novel photostable near-infrared-to-near-infrared fluorescent nanoparticle for in vivo imaging

Water-soluble K₅ HoLi₂ F₁₀ (KHLF) nanoprobes with the excitation and emission both in the near-infrared (NIR) region were developed and first demonstrated for in vivo imaging of living mice. The PEG₄₀₀ coating endows the nanoprobes with good water solubility and biocompatibility. Doping with Ho³⁺ ions is capable of emitting NIR fluorescence with two peaks centered, respectively, at 887 and 1,180 nm once excited by a 808 nm laser; meanwhile, it also possess good photothermal conversion performance. The KHLF matrix with specifically structure of large ion-distance and low photon energy imparts the nanoprobes low quenching effect and excellent photostability (fluorescence decrease <5% upon 120 min illumination of 808 nm continuous laser with a power density of 1 W/cm² ). The nanoparticles (NPs) were tested for in vitro bioimaging with living mice. The results show the NPs have low biotoxicity, rapid metabolism, normal biodistribution, together with the photothermal imaging performance and a high-contrast fluorescence images (signal-to-background ratio of 14:1). The superior performances of these nanoprobes in vivo imaging of mice proclaim the great potential of this type of probe for high-contrast imaging and photothermal treatment in practical applications.

Development of a fast-responsive and turn on fluorescent probe with large Stokes shift for specific detection of cysteine in vivo

Cysteine has a great effect on the physiological and pathological processes, which could bring out various diseases such as skin lesions, edema, hair depigmentation, Alzheimer's, Parkinson's, and liver damage due to the abnormal concentrations of cysteine. Therefore, it is of great impoatance to develop a method for imaging Cys. Herein, a novel fluorescent probe was developed for imaging Cys in vivo specially. This turn-on probe exhibited favorable advantages including large Stokes shift (90 nm), fast response (10 min), good selectivity, low cytotoxicity and so on. Furthermore, the probe could be applied to monitoring cysteine in living HeLa cells, which indicates that this turn-on probe could penetrate viable cell membranes and image Cys over other analystes especially HCy and GSH.

An Unsymmetrical Squaraine-Based Activatable Probe for Imaging Lymphatic Metastasis by Responding to Tumor Hypoxia with MSOT and Aggregation-Enhanced Fluorescent Imaging

Optoacoustic imaging has great potential for preclinical research and clinical practice, and designing robust activatable optoacoustic probes for specific diseases is beneficial for its further development. Herein, an activatable probe has been developed for tumor hypoxia imaging. For this probe, indole and quinoline were linked on each side of an oxocyclobutenolate core to form an unsymmetrical squaraine. A triarylamine group was incorporated to endow the molecule with the aggregation enhanced emission (AEE) properties. In aqueous media, the squaraine chromophore aggregates into the nanoprobe, which specifically responds to nitroreductase and produces strong optoacoustic signals due to its high extinction coefficient, as well as prominent fluorescence emission as a result of its AEE feature. The nanoprobe was used to image tumor metastasis via the lymphatic system both optoacoustically and fluorescently. Moreover, both the fluorescence signals and three-dimensional multispectral optoacoustic tomography signals from the activated nanoprobe allow us to locate the tumor site and to map the metastatic route.

An Activatable Near-Infrared Chromophore for Multispectral Optoacoustic Imaging of Tumor Hypoxia and for Tumor Inhibition

Hypoxia is a key hallmark of solid tumors and tumor hypoxia usually contributes to cancer progression, therapeutic resistance and poor outcome. Accurately detecting and imaging tumor hypoxia with high spatial resolution would be conducive to formulating optimized treatment plan and thus achieving better patient outcome. Methods: Tumor hypoxia can cleave the azo linker and release a NIR fluorophore (NR-NH2) and release the active drug as well. NR-NH2 shows a strong absorption band at around 680 nm and a strong fluorescence band at 710 nm, allowing for both multispectral optoacoustic tomography imaging (MSOT) and fluorescent imaging of tumor hypoxia in a tumor-bearing mouse model. Results: Liposome encapsulated with the activatable chromophore (NR-azo) for detecting/imaging tumor hypoxia and for tumor inhibition was demonstrated. For this chromophore, a xanthene-based NIR fluorophore acts as the optoacoustic and fluorescent reporter, an azo linker serves as the hypoxia-responsive moiety and a nitrogen mustard as the therapeutic drug. NR-azo shows an absorption at around 575 nm but exhibits negligible fluorescence due to the existence of the strong electron-withdrawing azo linker. Conclusion: We demonstrated an optoacoustic and fluorescent system for not only imaging tumor hypoxia in vivo but also achieving tumor inhibition.

In situ photoacoustic imaging of cysteine to reveal the mechanism of limited GSH synthesis in pulmonary fibrosis

We developed a photoacoustic and fluorescent dual-mode imaging probe, CCYS, for the detection of cysteine with high selectivity and sensitivity in a living system for the first time. By using CCYS, we found that the limited synthesis of glutathione in pulmonary fibrosis was not caused by cysteine deficiency.

A novel and simple imidazo[1,2-a]pyridin fluorescent probe for the sensitive and selective imaging of cysteine in living cells and zebrafish

Cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) play many crucial physiological roles in organisms. Their abnormal levels can cause and indicate various diseases. In the present study, a small-molecule fluorescent probe 2-(imidazo[1,2-a]pyridin-2-yl)phenyl acrylate (IPPA) was designed, synthesized and characterized by NMR, FT-IR and HRMS. IPPA can selectively detect Cys over other analytes because of an approximately 76 times enhancement in fluorescence intensity. The limit of detection of IPPA for Cys was 0.33 μM. The pseudo-first-order rate constant of the reaction between IPPA and Cys was approximately 10 times that of the reaction between IPPA and Hcy (KCys 3.18 × 10-3 S-1vs KHcy 4.92 × 10-4 S-1), indicating that Cys can be distinguished from Hcy. In addition, IPPA exhibits strong anti-interference ability, small molecular weight, high efficiency, low toxicity and good cell permeability. It was successfully used in imaging HepG2 cells and zebrafish. The fluorescence response of IPPA for calf serum are powerful proofs for practical application. Therefore, IPPA has high potential for bioassay applications.

Oxygenated theranostic nanoplatforms with intracellular agglomeration behavior for improving the treatment efficacy of hypoxic tumors

Hypoxia plays vital roles in the development of tumor resistance against typical anticancer therapies and local reoxygenation has proved effective to overcome the hypoxia-induced chemoresistance. Perfluorocarbon (PFC) is an FDA approved oxygen carrier and currently vigorously investigated for oxygen delivery to tumors. This study reports a perfluorocarbon and etoposide (EP) loaded porous hollow Fe₃O₄-based theranostic nanoplatform capable of delivering oxygen to solid tumors to enhance their susceptibility against EP. Results show that oxygen could be released at a moderate rate from the porous hollow magnetic Fe₃O₄ nanoparticles (PHMNPs) over an extended period of time, therefore effectively reducing the hypoxia-induced EP resistance of tumor cells. Moreover, the surface of PHMNPs was modified with lactobionic acid (LA)-containing amphiphilic polymers via hydrophobic interaction, which could provide targeting effect against certain types of tumors. The hydrophilic moiety would be subsequently shed by the intratumoral GSH after cellular internalization and result in the agglomeration of nanocarriers inside tumor cells, consequently impeding the nanoparticle exocytosis to enhance their intracellular retention. The enhanced retention could elevate the intracellular EP level and effectively boost the tumor cell killing effect. In addition to the therapeutic benefits, the Fe₃O₄ nanocage could also be used for the magnetic resonance imaging of the tumor area. The assorted benefits of the composite nanosystem are anticipated to be advantageous for the treatment of drug-resistant hypoxic tumors.

Multispectral optoacoustic tomography (MSOT) for imaging the particle size-dependent intratumoral distribution of polymeric micelles

PURPOSE

This study proposes the utilization of multispectral optoacoustic tomography (MSOT) to investigate the intratumoral distribution of polymeric micelles and effect of size on the biodistribution and antitumor efficacy (ATE).

MATERIALS AND METHODS

Docetaxel and/or optoacoustic agent-loaded polymeric micelles (with diameters of 22, 48, and 124 nm) were prepared using amphiphilic block copolymers poly (ethylene glycol) methyl ether-block-poly (D,L lactide) (PEG2000-PDLLAx). Subcutaneous 4T1 tumor-bearing mice were monitored with MSOT imaging and IVIS® Spectrum in vivo live imaging after tail vein injection of micelles. The in vivo results and ex vivo confocal imaging results were then compared. Next, ATE of the three micelles was found and compared.

RESULTS

We found that MSOT imaging offers spatiotemporal and quantitative information on intratumoral distribution of micelles in living animals. All the polymeric micelles rapidly extravasated into tumor site after intravenous injection, but only the 22-nm micelle preferred to distribute into the inner tumor tissues, leading to a superior ATE than that of 48- and 124-nm micelles.

CONCLUSION

This study demonstrated that MSOT is theranostically a powerful imaging modality, offering quantitative information on size-dependent spatiotemporal distribution patterns after the extravasation of nanomedicine from tumor blood vessels.

Molecular Photoacoustic Contrast Agents: Design Principles & Applications

Photoacoustic imaging (PAI) is a rapidly growing field which offers high spatial resolution and high contrast for deep-tissue imaging in vivo. PAI is nonionizing and noninvasive and combines the optical resolution of fluorescence imaging with the spatial resolution of ultrasound imaging. In particular, the development of exogenous PA contrast agents has gained significant momentum of late with a vastly expanding complexity of dye materials under investigation ranging from small molecules to macromolecular proteins, polymeric and inorganic nanoparticles. The goal of this review is to survey the current state of the art in molecular photoacoustic contrast agents (MPACs) for applications in biomedical imaging. The fundamental design principles of MPACs are presented and a review of prior reports spanning from early-to-current literature is put forth.

Azacalixphyrins as NIR photoacoustic contrast agents

Near-infrared (NIR) azacalixphyrins bearing aryl substituents strongly impacting the physico-chemical properties of the macrocycles were designed, enabling hyperchromic and bathochromic shifts of the absorption compared to their N-alkylated analogues. This engineering enhances the photoacoustic response under NIR excitation, making azacalixphyrins promising organic contrast agents that reach the 800-1000 nm range.

Highly Sensitive Fluorescence and Photoacoustic Detection of Metastatic Breast Cancer in Mice Using Dual-Modal Nanoprobes

The biomedical imaging of metastatic breast cancer, especially in lymphatic and lung metastasis, is highly significant in cancer staging as it helps assess disease prognosis and treatment. Using an albumin-indocyanine green dual-modal nanoprobe developed in our laboratory, in vivo fluorescence imaging and photoacoustic imaging of metastatic breast cancer tumors were performed separately. Fluorescence imaging at the near-infrared window features high imaging sensitivity but is generally limited by a low imaging depth. Thus, tumors can only be observed in situ whereas tumor cells in the lymph nodes and lung cannot be imaged in a precise manner. In contrast, photoacoustic imaging often helps overcome the limitations of imaging depth with high acoustic spatial resolution, which could provide complementary information for imaging cancer metastases. Ex vivo fluorescence and photoacoustic imaging were also performed to verify the tumor metastatic route. This study may not only provide insights into the design of dual-modal nanoprobes for breast cancer diagnosis but may also demonstrate the superiority of combined fluorescence imaging and photoacoustic imaging for guiding, monitoring, and evaluating lymphatic and lung metastatic stages of breast cancer with a high imaging specificity as well as sensitivity.

A low background D-A-D type fluorescent probe for imaging of biothiols in living cells

Two probes, structurally symmetric CBFB and asymmetric CBFM, constructed by a D-A-D (donor-acceptor-donor) type curcuminoid as the fluorophore and the DNBS (2,4-dinitrobenzenesulfonyl) group as the biothiol recognition site were designed and synthesized here. The DNBS group can quench the emission of the fluorophore by the PET (photoinduced electron transfer) process, and in the presence of biothiols, the emission of the probe was switched on as a result of the cleavage of the quencher by a nucleophilic aromatic substitution reaction. Experimental analyses and theoretical calculations revealed that two recognition moieties in the molecule can quench the fluorescence more efficiently, therefore, CBFB showed a much higher SNR (signal to noise ratio) than CBFM in biothiol detection with an emission maximum at 610 nm. This "low background" and "turn-on" fluorescent probe, CBFB, was successfully utilized to map endogenous biothiols in living cells.

Recent developments in multimodality fluorescence imaging probes

Multimodality optical imaging probes have emerged as powerful tools that improve detection sensitivity and accuracy, important in disease diagnosis and treatment. In this review, we focus on recent developments of optical fluorescence imaging (OFI) probe integration with other imaging modalities such as X-ray computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), single-photon emission computed tomography (SPECT), and photoacoustic imaging (PAI). The imaging technologies are briefly described in order to introduce the strengths and limitations of each techniques and the need for further multimodality optical imaging probe development. The emphasis of this account is placed on how design strategies are currently implemented to afford physicochemically and biologically compatible multimodality optical fluorescence imaging probes. We also present studies that overcame intrinsic disadvantages of each imaging technique by multimodality approach with improved detection sensitivity and accuracy.

Cancer Cell Membrane-Biomimetic Nanoprobes with Two-Photon Excitation and Near-Infrared Emission for Intravital Tumor Fluorescence Imaging

Biomimetic fluorescent nanoprobes capable of emitting near-infrared (NIR) fluorescence (λ_max ≈ 720 nm) upon excitation of 800 nm light were developed. The key conjugated polymer enabled two-photon absorption and Förster resonance energy transfer (FRET) processes within the nanoprobes, which imparted the nanoprobes with ideal NIR-incoming-NIR-outgoing fluorescence features. The cancer cell membrane (CM) coating endowed these nanoprobes with perfect biocompatibility and highly specific targeting ability to homologous tumors. It was believed that CM encapsulation provided an additional protecting layer for the photoactive components residing in the core of nanoprobes for retaining their intrinsic fluorescing ability in the physiological milieu. The long-term structural integrity, excellent photostability (fluorescence decrease <10% upon 30 min illumination of 800 nm pulse laser), high NIR fluorescence quantum yield (∼20%), and long in vivo circulation time of the target nanoprobes were also confirmed. The ability of these feature-packed nanoprobes for circumventing the challenges of absorption and light scattering caused by cellular structures and tissues was definitely confirmed via in vivo and in vitro experiments. The superior performances of these nanoprobes in terms of fluorescence signaling as well as targeting specificity were verified in intravital fluorescence imaging on tumor-bearing model mice. Specifically, these nanoprobes unequivocally enabled high-resolution visualization of the fine heterogeneous architectures of intravital tumor tissue, which proclaims the great potential of this type of probe for high-contrast fluorescence detection of thick biological samples in practical applications.

Disintegrable NIR Light Triggered Gold Nanorods Supported Liposomal Nanohybrids for Cancer Theranostics

In this work, facile synthesis and application of targeted, dual therapeutic gold nanorods-liposome (GNR-Lipos) nanohybrid for imaging guided photothermal therapy and chemotherapy is investigated. The dual therapeutic GNR-Lipos nanohybrid consists of GNR supported, and doxorubicin (DOX) loaded liposome. GNRs not only serve as a photothermal agent and increase the drug release in intracellular environment of cancer cells, but also provide mechanical strength to liposomes by being decorated both inside and outside of bilayer surfaces. The designed nanohybrid shows a remarkable response for synergistic chemophotothermal therapy compared to only chemotherapy or photothermal therapy. The NIR response, efficient uptake by the cells, disintegration of GNR-Lipos nanohybrid, and synergistic therapeutic effect of photothermal and chemotherapy over breast cancer cells MDA-MB-231 are studied for the better development of a biocompatible nanomaterial based multifunctional cancer theranostic agent.

A novel fluorescent probe with red emission and a large Stokes shift for selective imaging of endogenous cysteine in living cells

A new probe ANT selectively mapped endogenous Cys in living cells with bright red-emission and a large Stokes shift.

Ratiometric photoacoustic imaging of endoplasmic reticulum polarity in injured liver tissues of diabetic mice

As one of the complications of diabetes, liver injury results in significant hazards. Therefore, accurately diagnosing diabetes-induced liver injury beforehand is crucial for the warning and treatment of hepatic diseases. Diabetes-induced liver injury can cause changes in the microstructure and morphology of liver tissue, leading to changes in the hydrophilic and hydrophobic domains in the endoplasmic reticulum (ER), which is closely associated with changes in cellular ER polarity. So, differences in the ER polarity can indicate the degree of diabetes-induced liver injury. Herein, we develop a new fluorescent and photoacoustic dual-mode probe, ER-P, for detection of the ER polarity of liver tissue in normal and diabetic mice. Upon excitation with a 633 nm laser, ER-P showed increasing fluorescence intensity at 800 nm accompanying a decline in the polarity. Due to its polarity-sensitivity, ER-P was utilized for confocal fluorescence imaging in live cells, and the results demonstrate that ER-P can exclusively accumulate in the ER and indicate an increase in the polarity during ER stress. Importantly, ER-P displayed different absorbance intensities at 700 nm and 800 nm in different polarity environments because of intramolecular charge transfer. The photoacoustic intensity ratios between 700 nm and 800 nm will enable quantification of polarity to be achieved. The ratiometric photoacoustic imaging data demonstrate that the polarity of the liver tissue of diabetic mice is higher than that of the liver tissue of normal mice. Meanwhile, after treatment with the antidiabetic drug metformin, diabetic mice exhibit a reduced polarity environment in their liver tissue. The proposed study may serve as a new approach for the early diagnosis and therapeutic evaluation of diabetes-induced liver injury.

Quinone-fused porphyrins as contrast agents for photoacoustic imaging

Photoacoustic (PA) imaging is an emerging non-invasive diagnostic modality with many potential clinical applications in oncology, rheumatology and the cardiovascular field. For this purpose, there is a high demand for exogenous contrast agents with high absorption coefficients in the optical window for tissue imaging, i.e. the near infrared (NIR) range between 680 and 950 nm. We herein report the photoacoustic properties of quinone-fused porphyrins inserted with different transition metals as new highly promising candidates. These dyes exhibit intense NIR absorption, a lack of fluorescence emission, and PA sensitivity in concentrations below 3 nmol mL-1. In this context, the highest PA signal was obtained with a Zn(ii) inserted dye. Furthermore, this dye was stable in blood serum and free thiol solution and exhibited negligible cell toxicity. Additionally, the Zn(ii) probe could be detected with an up to 3.2 fold higher PA intensity compared to the clinically most commonly used PA agent, ICG. Thus, further exploration of the 'quinone-fusing' approach to other chromophores may be an efficient way to generate highly potent PA agents that do not fluoresce and shift their absorption into the NIR range.

An unprecedented amplification of near-infrared emission in a Bodipy derived π-system by stress or gelation

A meso-substituted Bodipy derived π-gelator exhibits amplified near-infrared (NIR) emission upon shearing of its film from n-decane or drying of its gel from DMSO.

We report an unprecedented strategy to generate and amplify near-infrared (NIR) emission in an organic chromophore by mechanical stress or gelation pathways. A greenish-yellow emitting film of π-extended Bodipy-1, obtained from n-decane, became orange-red upon mechanical shearing, with a 15-fold enhancement in NIR emission at 738 nm. Alternatively, a DMSO gel of Bodipy-1 exhibited a 7-fold enhancement in NIR emission at 748 nm with a change in emission color from yellow to orange-red upon drying. The reason for the amplified NIR emission in both cases is established from the difference in chromophore packing, by single crystal analysis of a model compound (Bodipy-2), which also exhibited a near identical emission spectrum with red to NIR emission (742 nm). Comparison of the emission features and WAXS and FT-IR data of the sheared n-decane film and the DMSO xerogel with the single crystal data supports a head-to-tail slipped arrangement driven by the N–H···F–B bonding in the sheared or xerogel states, which facilitates strong exciton coupling and the resultant NIR emission.

A Ratiometric Near-Infrared Fluorogen for the Real Time Visualization of Intracellular Redox Status during Apoptosis

Direct monitoring of apoptotic progression is a major step forward for the early assessment of therapeutic efficacy of certain treatments and the accurate evaluation of the spread of a disease. Here, the regulatory role of glutathione (GSH) is explored as a potential biomarker for tracking apoptosis. For this purpose, a near- infrared (NIR) squaraine dye is introduced that is capable of sensing GSH in a ratiometric manner by switching its emission from NIR (690 nm) to visible region (560 nm). The favorable biocompatible attributes of the probe facilitated the real-time monitoring of apoptotic process in line with the conventional apoptotic assay. Furthermore, the robust nature of the probe was utilized for the quantitative estimation of GSH during different stages of apoptosis. Through this study, an easy and reliable method of assaying apoptosis is demonstrated, which can provide valuable insights in translational clinical research.

Highly Sensitive Squaraine-Based Water-Soluble Far-Red/Near-Infrared Chromofluorogenic Thiophenol Probe

A squaraine-based far-red/near-infrared fluorescent probe (SQ-DNBS) was exploited for thiophenol detection. SQ-DNBS is a colorimetric and "off-on" fluorometric dual-channel "naked-eye" chemosensor showing high selectivity, high sensitivity (detection limit: 9.9 nM), and rapid response to thiophenol in aqueous solution. SQ-DNBS also can be used in practical applications for the detection of thiophenol in water samples. Photophysical and spectral characterization results revealed that the probing mechanism of SQ-DNBS toward thiophenol lies in the thiolate-mediated cleavage reaction. Our discovery demonstrates the potential of the arylmethylene-squaraine skeleton as a promising fluorophore unit to construct high-performance far-red/near-infrared chemosensors.

Highly specific noninvasive photoacoustic and positron emission tomography of brain plaque with functionalized croconium dye labeled by a radiotracer

Highly-efficient targeting probes are desirable for disease diagnosis and functional imaging. However, most of the current near-infrared (NIR) probes suffer from low signal conversion, insufficient photostability, poor probe specificity, and limited functions. Herein, an NIR ultrahigh absorbing croconium dye for amyloid (CDA) was designed and synthesized to specifically bind to cerebrovascular amyloid without antibody linkage. This unique CDA is able to strongly bind the hydrophobic channels of amyloid beta (Aβ) fiber with a very strong binding energy of -9.3 kcal mol-1. Our experimental results demonstrate that the amphipathic dye with an intense absorption peak at 800 nm generated a significant local temperature surge under low-power laser irradiation. Compared with representative prominent indocyanine green, Prussian blue, and gold nanorods, this probe can produce the strongest photoacoustic signal based on the same mass concentration. Labeled with radioactive 18F, this multifunctional probe allowed for the ultrasensitive photoacoustic tomography (PAT)/positron emission tomography (PET)/fluorescence imaging of Aβ plaques in the brain cortex. Featured with high spatial resolution and optical specificity, PAT was intrinsically suitable for imaging pathological sites on cortical vessels, whereas PET revealed whole-body anatomy with quantitative biodistribution information. Our study shows that a CDA-based functionalized dye aided with PAT and PET is capable of plaque diagnosis and localization.

Combinatorial Strategy to Identify Fluorescent Probes for Biothiol and Thiophenol Based on Diversified Pyrimidine Moieties and Their Biological Applications

We present a feasible paradigm of developing original fluorescent probes for target biomolecules via combinatorial chemistry. In this developmental program, pyrimidine moieties were investigated and optimized as unique recognition units for thiols for the first time through a parallel synthesis in combination with a rapid screening process. This time-efficient and cost-saving process effectively facilitated the developmental progress and provided detailed structure-reactivity relationships. As a result, Res-Biot and Flu-Pht were identified as optimal fluorescent probes for biothiol and thiophenol, respectively. Their favorable characteristics and superior applicability have been well demonstrated in both chemical and biological contexts. In particular, Res-Biot enables the direct visualization of biothiol fluctuations during oxidative stress and cell apoptosis, indicating its suitability in elucidation of a specific pathophysiological process in both living cells and living animals. Meanwhile, Flu-Pht is competent to visualize thiophenols without the interference from endogenous biothiols in living cells.

Zwitterionic near infrared fluorescent agents for noninvasive real-time transcutaneous assessment of kidney function

Zwitterionic near infrared fluorescent agents were developed for non-invasive real-time transcutaneous assessment of kidney function.

We developed novel zwitterionic near infrared (NIR) fluorescent agents (ABZWCY-HPβCD and AAZWCY-HPβCD), which exhibit favorable hydrophilicity, low plasma protein binding, high stability and non-toxicity. These attractive characteristics ensure that they are excreted rapidly, without any skin accumulation or metabolism in vivo. More importantly, zwitterionic HPβCD based agents can be efficiently filtrated by the glomerulus and completely excreted through the kidneys into urine without reabsorption or secretion in the kidney proximal tubule. Relying on these novel zwitterionic NIR agents and a transcutaneous device, we demonstrate a rapid, robust and biocompatible approach for assessing kidney function in rat models of both healthy rats and those with kidney disease, without the need for time-consuming blood/urine sample preparation. Our work provides a promising tool for in vivo real-time non-invasive kidney function assessment in preclinical applications.

Evaluation of electron or charge transfer processes between chromenylium-based fluorophores and protonated–deprotonated aniline

PET and ICT processes in chromenylium hybrid fluorescent dyes.