Croconaine rotaxane for acid activated photothermal heating and ratiometric photoacoustic imaging of acidic pH

Croconaine-based NIR-II fluorescence imaging-guided tumor photothermal therapy induces long-term antitumor immune memory

Photothermal therapy (PTT) for cancers guided by optical imaging has recently shown great potential for precise diagnosis and efficient therapy. The second near-infrared window (NIR-II, 1000-1700 nm) fluorescence imaging (FLI) is highly desirable owing to its good spatial and temporal resolution, deep tissue penetration, and negligible tissue toxicity. Organic small molecules are attractive as imaging and treatment agents in biomedical research because of their low toxicity, fast clearance rate, diverse structures, ease of modification, and excellent biocompatibility. Various organic small molecules have been investigated for biomedical applications. However, there are few reports on the use of croconaine dyes (CRs), especially NIR-II emission CRs. To our knowledge, there have been no prior reports of NIR-II emissive small organic photothermal agents (SOPTAs) based on CRs. Herein, we report a croconaine dye (CR-TPE-T)-based nanoparticle (CR NP) with absorption and fluorescence emission in the NIR-I and NIR-II windows, respectively. The CR NPs exhibited intense NIR absorption, outstanding photothermal properties, and good biological compatibility. In vivo studies showed that CR NPs not only achieved real-time, noninvasive NIR-II FLI of tumors, but also induced significant tumor ablation with laser irradiation guided by imaging, without apparent side effects, and promoted the formation of antitumor immune memory in a colorectal cancer model. In addition, the CR NPs displayed efficient inhibition of breast tumor growth, improved longevity of mice and triggered efficient systemic immune responses, which further inhibited tumor metastasis to the lungs. Our study demonstrates the great potential of CRs as therapeutic agents in the NIR-II region for cancer diagnosis.

Oxocarbon Acids and their Derivatives in Biological and Medicinal Chemistry

The biological and medicinal chemistry of the oxocarbon acids 2,3- dihydroxycycloprop-2-en-1-one (deltic acid), 3,4-dihydroxycyclobut-3-ene-1,2-dione (squaric acid), 4,5-dihydroxy-4-cyclopentene-1,2,3-trione (croconic acid), 5,6-dihydroxycyclohex- 5-ene-1,2,3,4-tetrone (rhodizonic acid) and their derivatives is reviewed and their key chemical properties and reactions are discussed. Applications of these compounds as potential bioisosteres in biological and medicinal chemistry are examined. Reviewed areas include cell imaging, bioconjugation reactions, antiviral, antibacterial, anticancer, enzyme inhibition, and receptor pharmacology.

Targeted delivery of organic small-molecule photothermal materials with engineered extracellular vesicles for imaging-guided tumor photothermal therapy

Imaging-guided photothermal therapy (PTT) for cancers recently gathered increasing focus thanks to its precise diagnosis and potent therapeutic effectiveness. Croconaine (CR) dyes demonstrate potential in expanding utility for near infrared (NIR) dyes in bio-imaging/theranostics. However, reports on CR dyes for PTT are scarce most likely due to the short of the efficacious delivery strategies to achieve specific accumulation in diseased tissues to induce PTT. Extracellular vesicles (EVs) are multifunctional nanoparticle systems that function as safe platform for disease theragnostics, which provide potential benefits in extensive biomedical applications. Here, we developed a novel delivery system for photothermal molecules based on a CR dye that exerts photothermal activity through CDH17 nanobody-engineered EVs. The formed CR@E8-EVs showed strong NIR absorption, excellent photothermal performance, good biological compatibility and superb active tumor-targeting capability. The CR@E8-EVs can not only visualize and feature the tumors through CR intrinsic property as a photoacoustic imaging (PAI) agent, but also effectively retard the tumor growth under laser irradiation to perform PTT. It is expected that the engineered EVs will become a novel delivery vehicle of small organic photothermal agents (SOPTAs) in future clinical PTT applications.

Water-solubility croconic acid-bisindole dye with morpholine ring for tumor NIRF/PA imaging and photothermal therapy activated by lysosome pH-response

A novel croconic acid-bisindole dye CR-630 with a morpholine ring showed good water-solubility and obvious lysosome-targeting. The protonation of the nitrogen atom in the indole and lysosome-targeting of morpholine ring let it exhibit stronger pH-responsive NIR/PA imaging and photothermal effect in the lysosome acidic microenvironment (pH 4.0-5.5) than in the tumor acidic microenvironment. In the animal study, compound CR-630 could NIRF/PA image in the tumor tissues in 1.5-2.0 h, effectively inhibit the growth of the tumor, and even ablate the tumor at the drug dose of 1 mg/kg. It also demonstrated good biosafety. This study gives a new idea to develop water-solubility organic dyes with lysosome targeting, stronger pH-responsive NIRF/PA imaging and PTT for breast cancer.

Rational Design of Biomaterials to Potentiate Cancer Thermal Therapy

Cancer thermal therapy, also known as hyperthermia therapy, has long been exploited to eradicate mass lesions that are now defined as cancer. With the development of corresponding technologies and equipment, local hyperthermia therapies such as radiofrequency ablation, microwave ablation, and high-intensity focused ultrasound, have has been validated to effectively ablate tumors in modern clinical practice. However, they still face many shortcomings, including nonspecific damages to adjacent normal tissues and incomplete ablation particularly for large tumors, restricting their wide clinical usage. Attributed to their versatile physiochemical properties, biomaterials have been specially designed to potentiate local hyperthermia treatments according to their unique working principles. Meanwhile, biomaterial-based delivery systems are able to bridge hyperthermia therapies with other types of treatment strategies such as chemotherapy, radiotherapy and immunotherapy. Therefore, in this review, we discuss recent progress in the development of functional biomaterials to reinforce local hyperthermia by functioning as thermal sensitizers to endow more efficient tumor-localized thermal ablation and/or as delivery vehicles to synergize with other therapeutic modalities for combined cancer treatments. Thereafter, we provide a critical perspective on the further development of biomaterial-assisted local hyperthermia toward clinical applications.

A water-soluble thiophene-croconaine dye with a high molar extinction coefficient for NIR fluorescence imaging-guided synergistic photothermal/photodynamic therapy of cancer

Phototherapeutic agents with near-infrared (NIR) fluorescence, strong reactive oxygen species generation and photothermal conversion capabilities are highly desirable for use in cancer therapy. Herein, a water-soluble NIR croconaine dye (TCR) with a thiophene-croconaine rigid core and two symmetric alkyl chains was designed and synthesized. TCR exhibits intense NIR absorption and fluorescence that peaked at 780 and 815 nm, respectively, with a high molar extinction coefficient of 1.19 × 10⁵ M^-1 cm^-1. Moreover, TCR has a high photothermal conversion efficiency of 77% and is capable of generating hydroxyl radicals (OH˙) under 735 nm laser irradiation. Based on these outstanding properties, TCR has proven its application in NIR fluorescence imaging-guided synergistic photothermal/photodynamic therapy of cancer.

Chemical Design of Activatable Photoacoustic Probes for Precise Biomedical Applications

Photoacoustic (PA) imaging technology, a three-dimensional hybrid imaging modality that integrates the advantage of optical and acoustic imaging, has great application prospects in molecular imaging due to its high imaging depth and resolution. To endow PA imaging with the ability for real-time molecular visualization and precise biomedical diagnosis, numerous activatable molecular PA probes which can specifically alter their PA intensities upon reacting with the targets or biological events of interest have been developed. This review highlights the recent developments of activatable PA probes for precise biomedical applications including molecular detection of the biotargets and imaging of the biological events. First, the generation mechanism of PA signals will be given, followed by a brief introduction to contrast agents used for PA probe design. Then we will particularly summarize the general design principles for the alteration of PA signals and activatable strategies for developing precise PA probes. Furthermore, we will give a detailed discussion of activatable PA probes in molecular detection and biomedical imaging applications in living systems. At last, the current challenges and outlooks of future PA probes will be discussed. We hope that this review will stimulate new ideas to explore the potentials of activatable PA probes for precise biomedical applications in the future.

Phototherapy and optical waveguides for the treatment of infection

With rapid emergence of multi-drug resistant microbes, it is imperative to seek alternative means for infection control. Optical waveguides are an auspicious delivery method for precise administration of phototherapy. Studies have shown that phototherapy is promising in fighting against a myriad of infectious pathogens (i.e. viruses, bacteria, fungi, and protozoa) including biofilm-forming species and drug-resistant strains while evading treatment resistance. When administered via optical waveguides, phototherapy can treat both superficial and deep-tissue infections while minimizing off-site effects that afflict conventional phototherapy and pharmacotherapy. Despite great therapeutic potential, exact mechanisms, materials, and fabrication designs to optimize this promising treatment option are underexplored. This review outlines principles and applications of phototherapy and optical waveguides for infection control. Research advances, challenges, and outlook regarding this delivery system are rigorously discussed in a hope to inspire future developments of optical waveguide-mediated phototherapy for the management of infection and beyond.

Stimuli-activated molecular photothermal agents for cancer therapy

Taking advantage of activatable and imaging-guided properties, stimuli-activated molecular photothermal agents (MPTAs) have drawn great attention in photothermal therapy (PTT) over the past decades. In this review, the recent progress in the study of stimuli-activated MPTAs is summarized from different stimuli, including pH, bioactive small molecules, and enzymes. The features and challenges of stimuli-activated MPTAs are also discussed. This review aims to motivate readers to design and synthesise more efficient MPTAs.

Croconaine-based nanoparticles enable efficient optoacoustic imaging of murine brain tumors

Contrast enhancement in optoacoustic (photoacoustic) imaging can be achieved with agents that exhibit high absorption cross-sections, high photostability, low quantum yield, low toxicity, and preferential bio-distribution and clearance profiles. Based on advantageous photophysical properties of croconaine dyes, we explored croconaine-based nanoparticles (CR780RGD-NPs) as highly efficient contrast agents for targeted optoacoustic imaging of challenging preclinical tumor targets. Initial characterization of the CR780 dye was followed by modifications using polyethylene glycol and the cancer-targeting c(RGDyC) peptide, resulting in self-assembled ultrasmall particles with long circulation time and active tumor targeting. Preferential bio-distribution was demonstrated in orthotopic mouse brain tumor models by multispectral optoacoustic tomography (MSOT) imaging and histological analysis. Our findings showcase particle accumulation in brain tumors with sustainable strong optoacoustic signals and minimal toxic side effects. This work points to CR780RGD-NPs as a promising optoacoustic contrast agent for potential use in the diagnosis and image-guided resection of brain tumors.

Facile Synthesis of a Croconaine-Based Nanoformulation for Optoacoustic Imaging and Photothermal Therapy

Near-infrared (NIR) light absorbing theranostic agents can integrate optoacoustic imaging and photothermal therapy for effective personalized precision medicine. However, most of these agents face the challenges of unstable optical properties, material-associated toxicity, and nonbiodegradability, all of which limit their biomedical application. Several croconaine-based organic agents able to overcome some of these limitations have been recently reported, but these suffer from complicated multistep synthesis protocols. Herein, the use of CR760, a croconaine dye with excellent optical properties, is reported for nanoparticle formulation and subsequent optoacoustic imaging and photothermal therapy. Importantly, CR760 can be conveniently prepared in a single step from commercially available materials. Furthermore, CR760 can be covalently attached, via a polyethylene glycol linker, to the α_v β₃ integrin ligand c(RGDyC), resulting in self-assembled nanoparticles (NPs) with cancer-targeting capability. Such CR760RGD-NPs exhibit strong NIR absorption, high photostability, high optoacoustic generation efficiency, and active tumor-targeting, making them ideal candidates for optoacoustic imaging. Due to favorable electron transfer, CR760RGD-NPs display a 45.37% photothermal conversion efficiency thereby rendering them additionally useful for photothermal therapy. Targeted tumor elimination, biosafety, and biocompatibility are demonstrated in a 4T1 murine breast tumor model. This work points to the use of CR760RGD-NPs as a promising nanoagent for NIR-based cancer phototheranostics.

Hydrazone exchange: a viable route for the solid-tethered synthesis of [2]rotaxanes

Using a hydrazone exchange methodology, resin beads were functionalised with [2]rotaxanes at up to 80% efficiency—higher than using other dynamic or irreversible synthetic approaches to form self-assembled structures on solid supports.

Highly photostable croconium dye-anchored cell membrane vesicle for tumor pH-responsive duplex imaging-guided photothermal therapy

The development of tumor acidic microenvironment-responsive theranostic agents is a research hotspot. Herein, we developed highly photostable amphiphilic croconium dye-anchored red blood cell membrane vesicle (denoted as LET-5) for tumor pH-responsive near-infrared fluorescence (NIRF) and photoacoustic (PA) duplex imaging-guided photothermal therapy. In tumor acidic microenvironment, both NIRF and PA signals of LET-5 were significantly enhanced and the photothermal effect of LET-5 was activated. Notably, cell membrane-based vesicle with enhanced stability and long blood circulation significantly improved the tumor accumulation of croconium dye, thus achieving better therapeutic effect than free croconium dye. These findings provide a promising approach to construct amphiphilic dye-anchored cell membrane vesicle for cancer theranostics.

Design Strategies of Photoacoustic Molecular Probes

Photoacoustic (PA) probes have been developed very quickly and applied in broad areas in recent years. Most of them are constructed based on organic dyes with intrinsic near-infrared (NIR) absorption properties. To increase PA contrast and improve imaging resolution and the sensitivity of detection, various methods for the design of PA probes have been developed. This minireview mainly focuses on the development and design strategies of activatable small-molecule PA probes in four aspects: reaction-cleavage, metal ion chelation, photoswitch, and protonation-deprotonation. It highlights some key points of designing PA probes corresponding to their properties and applications. The challenges and perspectives for small-molecule PA probes are also discussed.

Biradical-Featured Stable Organic-Small-Molecule Photothermal Materials for Highly Efficient Solar-Driven Water Evaporation

With recent progress in photothermal materials, organic small molecules featured with flexibility, diverse structures, and tunable properties exhibit unique advantages but have been rarely applied in solar-driven water evaporation owing to limited sunlight absorption resulting in low solar-thermal conversion. Herein, a stable croconium derivative, named CR-TPE-T, is designed to exhibit the unique biradical property and strong π-π stacking in the solid state, which facilitate not only a broad absorption spectrum from 300 to 1600 nm for effective sunlight harvesting, but also highly efficient photothermal conversion by boosting nonradiative decay. The photothermal efficiency is evaluated to be 72.7% under 808 nm laser irradiation. Based on this, an interfacial-heating evaporation system based on CR-TPE-T is established successfully, using which a high solar-energy-to-vapor efficiency of 87.2% and water evaporation rate of 1.272 kg m^-2 h^-1 under 1 sun irradiation are obtained, thus making an important step toward the application of organic-small-molecule photothermal materials in solar energy utilization.

Recent Development of Photothermal Agents (PTAs) Based on Small Organic Molecular Dyes

Photothermal therapy (PTT) has attracted great attention due to its noninvasive and effective use against cancer. Various photothermal agents (PTAs) including organic and inorganic PTAs have been developed in the last decades. Organic PTAs based on small-molecule dyes exhibit great potential for future clinical applications considering their good biocompatibility and easy chemical modification or functionalization. In this review, we discuss the recent progress of organic PTAs based on small-molecule dyes for enhanced PTT. We summarize the strategies to improve the light penetration of PTAs, methods to enhance their photothermal conversion efficiency, how to optimize PTAs' delivery into deep tumors, and how to resist photobleaching under repeated laser irradiation. We hope that this review can rouse the interest of researchers in the field of PTAs based on small-molecule dyes and help them to fabricate next-generation PTAs for noninvasive cancer therapy.

Activatable Phototheranostic Materials for Imaging-Guided Cancer Therapy

Cancer theranostics, which combines diagnostic and therapeutic effects into one entity, holds promise in precision medicine. Conventional theranostic agents possess always-on imaging signals and cytotoxic effects and thus often encounter poor selectivity or specificity in cancer treatment. To tackle this issue, activatable phototheranostic materials (PMs) have been developed to simultaneously and specifically turn on their diagnostic signals (fluorescence/self-luminescence/photoacoustic signals) and photothermal/photodynamic effects in response to cancer hallmarks. This Review summarizes the recent progress in the design, synthesis and proof-of-concept applications of activatable PMs. The molecular engineering strategy to increase tumor accumulation and enhance treatment efficacy are highlighted. Current challenges and future perspectives in this emerging field are also discussed.

Tumor pH-Responsive Albumin/Polyaniline Assemblies for Amplified Photoacoustic Imaging and Augmented Photothermal Therapy

Tumor-microenvironment-responsive theranostics have great potential for precision diagnosis and effective treatment of cancer. Polyaniline (PANI) is the first reported pH-responsive organic photothermal agent and is widely used as a theranostic agent. However, tumor pH-responsive PANI-based theranostic agents are not explored, mainly because the conversion from the emeraldine base (EB) to emeraldine salt (ES) state of PANI requires pH < 4, which is lower than tumor acidic microenvironment. Herein, a tumor pH-responsive PANI-based theranostic agent is designed and prepared for amplified photoacoustic imaging guided augmented photothermal therapy (PTT), through intermolecular acid-base reactions between carboxyl groups of bovine serum albumin (BSA) and imine moieties of PANI. The albumin/PANI assemblies (BSA-PANI) can convert from the EB to ES state at pH < 7, accompanied by the absorbance redshift from visible to near-infrared region. Both in vitro and in vivo results demonstrate that tumor acidic microenvironment can trigger both the photoacoustic imaging (PAI) signal amplification and the PTT efficacy enhancement of BSA-PANI assemblies. This work not only highlights that BSA-PANI assemblies overcome the limitation of low-pH protonation, but also provides a facile assembly strategy for a tumor pH-responsive PANI-based nanoplatform for cancer theranostics.

In Vivo Photoacoustic Tracking of Mesenchymal Stem Cell Viability

Adult stem cell therapy has demonstrated improved outcomes for treating cardiovascular diseases in preclinical trials. The development of imaging tools may increase our understanding of the mechanisms of stem cell therapy, and a variety of imaging tools have been developed to image transplanted stem cells in vivo; however, they lack the ability to interrogate stem cell function longitudinally. Here, we report the use of a nanoparticle-based contrast agent that can track stem cell viability using photoacoustic imaging. The contrast agent consists of inert gold nanorods coated with IR775c, a reactive oxygen species (ROS) sensitive near-infrared dye. Upon cell death, stem cells produce ROS to degrade the cell. Using this feature of stem cells, the viability can be measured by comparing the IR775c signal to the ROS insensitive gold nanorod signal, which can also be used to track stem cell location. The nanoprobe was successfully loaded into mesenchymal stem cells (MSCs), and then, MSCs were transplanted into the lower limb of a mouse and imaged using combined ultrasound and photoacoustic imaging. MSC viability was assessed using the nanoprobe and displayed significant cell death within 24 h and an estimated 5% viability after 10 days. This nanoparticle system allows for longitudinal tracking of MSC viability in vivo with high spatial and temporal resolution which other imaging modalities currently cannot achieve.

Organic Nanotheranostics for Photoacoustic Imaging-Guided Phototherapy

Phototherapies including photothermal therapy (PTT) and photodynamic therapy (PDT) have emerged as one of the avant-garde strategies for cancer treatment. Photoacoustic (PA) imaging is a new hybrid imaging modality that shows great promise for real-time in vivo monitoring of biological processes with deep tissue penetration and high spatial resolution. To enhance therapeutic efficacy, reduce side effects and minimize the probability of over-medication, it is necessary to use imaging and diagnostic methods to identify the ideal therapeutic window and track the therapeutic outcome. With this regard, nanotheranostics with the ability to conduct PA imaging and PTT/PDT are emerging. This review summarizes the recent progress of organic nanomaterials including nearinfrared (NIR) dyes and semiconducting polymer nanoparticles (SPNs) in PA imaging guided cancer phototherapy, and also addresses their present challenges and potential in clinical applications.

Croconaine Rotaxane Dye with 984 nm Absorption: Wavelength-Selective Photothermal Heating

Croconaines are an emerging class of near-infrared dyes that are useful for various sensing, photothermal, optoelectronic, and photoacoustic applications. Previous work encapsulated a dumbbell-shaped croconaine dye whose structure contains two thiophene flanking units inside a tetralactam macrocycle and produced a croconaine rotaxane 1 with a narrow 824 nm absorption band. Herein, a new rotaxane 2 is reported that encapsulates a croconaine dye with two thienothiophene flanking units. The new croconaine rotaxane 2 exhibits a narrow 984 nm absorption band that is distinct from the 824 nm absorption of rotaxane 1. Photothermal heating experiments showed that an 830 nm diode laser selectively heats a solution containing rotaxane 1, with no heating of a solution containing rotaxane 2. Conversely, a 980 nm diode laser selectively heats a solution containing rotaxane 2, with no heating of a solution containing rotaxane 1. The new croconaine rotaxane 2 shows no fatigue after four cycles of laser heating and cooling.

Integrating Stimuli-Responsive Properties in Host-Guest Supramolecular Drug Delivery Systems

Host-guest motifs are likely the most recognizable manifestation of supramolecular chemistry. These complexes are characterized by the organization of small molecules on the basis of preferential association of a guest within the portal of a host. In the context of their therapeutic use, the primary application of these complexes has been as excipients which enhance the solubility or improve the stability of drug formulations, primarily in a vial. However, there may be opportunities to go significantly beyond such a role and leverage key features of the affinity, specificity, and dynamics of the interaction itself toward "smarter" therapeutic designs. One approach in this regard would seek stimuli-responsive host-guest recognition, wherein a complex forms in a manner that is sensitive to, or can be governed by, externally applied triggers, disease-specific proteins and analytes, or the presence of a competing guest. This review will highlight the general and phenomenological design considerations governing host-guest recognition and the specific types of chemistry which have been used and are available for different applications. Finally, a discussion of the molecular engineering and design approaches which enable sensitivity to a variety of different stimuli are highlighted. Ultimately, these molecular-scale approaches offer an assortment of new chemistry and material design tools toward improving precision in drug delivery.

Pre-blocked molecular shuttle as an in-situ real-time theranostics

Real-time monitor of drug-release from drug formulations in a noninvasive way can provide spatio-temporal information for drug activation and guide further clinical rational administration. In this work, a molecular shuttle, as a typical nanosized artificial molecular machine, was managed to act as a conceptually-new nanotheranostics for oxaliplatin. A post-recognition strategy was utilized, where a default supramolecular-dye couple was pre-blocked. The rational design, synthesis, characterization and proof-of-concept of this strategy were described in detail. The drug-release upon reducing environment can be translated into near-infrared (NIR) fluorescence signal (OFF-to-ON), allowing to track the drug-release procedure by multi-modal images including IVIS, FLECT and photoacoustic imaging. The versatile nanotheranostics system can target to triple negative breast tumor via conjugated F3 peptide, and show an improved anti-tumor efficacy with much lower side effect. The intelligent nanotheranostics system based on molecular shuttle provides new reference for precision medicine in preclinical trial and postclinical evaluation.

Photoacoustic Imaging: Contrast Agents and Their Biomedical Applications

Photoacoustic (PA) imaging as a fast-developing imaging technique has great potential in biomedical and clinical applications. It is a noninvasive imaging modality that depends on the light-absorption coefficient of the imaged tissue and the injected PA-imaging contrast agents. Furthermore, PA imaging provides superb contrast, super spatial resolution, and high penetrability and sensitivity to tissue functional characteristics by detecting the acoustic wave to construct PA images. In recent years, a series of PA-imaging contrast agents are developed to improve the PA-imaging performance in biomedical applications. Here, recent progress of PA contrast agents and their biomedical applications are outlined. PA contrast agents are classified according to their components and function, and gold nanocrystals, gold-nanocrystal assembly, transition-metal chalcogenides/MXene-based nanomaterials, carbon-based nanomaterials, other inorganic imaging agents, small organic molecules, semiconducting polymer nanoparticles, and nonlinear PA-imaging contrast agents are discussed. The applications of PA contrast agents as biosensors (in the sensing of metal ions, pH, enzymes, temperature, hypoxia, reactive oxygen species, and reactive nitrogen species) and in bioimaging (lymph nodes, vasculature, tumors, and brain tissue) are discussed in detail. Finally, an outlook on the future research and investigation of PA-imaging contrast agents and their significance in biomedical research is presented.

1,5-Diaminonaphtalene is a Highly Performing Electron-Transfer Secondary-Reaction Matrix for Laser Desorption Ionization Mass Spectrometry of Indolenine-Based Croconaines

Croconaine dyes are appealing molecules synthesized via the condensation of croconic acid and reactive electron-donating aromatic or heterocyclic systems. Here, matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) investigation of indolenine-based croconaines is presented for the first time. Archetype proton-transfer matrices, such as 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid (CHCA), 9-aminoacridine (9AA) as the protonating/deprotonating matrix, and electron-transfer (ET) secondary-reaction matrices, such as 1,5-diaminonapthalene (DAN) and trans-2-[3-(4-t-butyl-phenyl)-2-methyl-2-propenylidene]malononitrile (DCTB), were investigated. DHB, CHCA, and 9AA generate a mix of odd-electron molecular ions and protonated, sodiated, and potassiated adducts. Among the ET matrices, DAN was found to be capable of directing the ionization process toward the exclusive formation of odd-electron molecular ions M^+• without fragmentation. MALDI tandem MS provides useful structural characterization of croconaine dyes, thus making identification very straightforward for all investigated compounds. Interestingly the fragmentation of bromo-containing croconaines revealed, for the first time, the gas-phase formation of a bromime cation [Br]⁺.

Synthesis and Computational Study of Semicroconaines and Nonsymmetric Croconaines

The synthesis of two new dye families of croconic acid derivatives, semicroconaine and nonsymmetric croconaine dyes, is reported for the first time. These compounds show strong absorption in the UV-visible and NIR, respectively. Semicroconaine dyes were obtained by a scalable and efficient condensation of croconic acid with aromatic heterocyclic methylene-active compounds. The subsequent reaction of the semicroconaine dyes with aromatic heterocyclic compounds affords nonsymmetric croconaines. The structure and electronic properties of the synthesized compounds have been investigated by preliminary theoretical calculations at DFT level of approximation.

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.

Recent progress in the development of near-infrared organic photothermal and photodynamic nanotherapeutics

Phototherapies including photothermal therapy (PTT) and photodynamic therapy (PDT) have gained considerable attention due to their high tumor ablation efficiency, excellent spatial resolution and minimal side effects on normal tissue. In contrast to inorganic nanoparticles, near-infrared (NIR) absorbing organic nanoparticles bypass the issue of metal-ion induced toxicity and thus are generally considered to be more biocompatible. Moreover, with the guidance of different kinds of imaging methods, the efficacy of cancer phototherapy based on organic nanoparticles has shown to be optimizable. In this review, we summarize the synthesis and application of NIR-absorbing organic nanoparticles as phototherapeutic nanoagents for cancer phototherapy. The chemistry, optical properties and therapeutic efficacies of organic nanoparticles are firstly described. Their phototherapy applications are then surveyed in terms of therapeutic modalities, which include PTT, PDT and PTT/PDT combined therapy. Finally, the present challenges and potential of imaging guided PTT/PDT are discussed.

Photoacoustic Probes for Molecular Detection: Recent Advances and Perspectives

Photoacoustic (PA) imaging (PAI) is a noninvasive and nonionizing biomedical imaging modality that combines the advantages of optical imaging and ultrasound imaging. Based on PAI, photoacoustic detection (PAD) is an emerging approach that is involved with the interaction between PA probes and analytes resulting in the changes of photoacoustic signals for molecular detection with rich contrast, high resolution, and deep tissue penetration. This Review focuses on the recent development of PA probes in PAD. The following contents will be discussed in detail: 1) the construction of PA probes; 2) the applications and mechanisms of PAD to different types of analytes, including microenvironments, small biomolecules, or metal ions; 3) the challenges and perspectives of PA probes in PAD.

Radiolabeled Angiogenesis-Targeting Croconaine Nanoparticles for Trimodality Imaging Guided Photothermal Therapy of Glioma

To meet the criteria of effective theranostics, biocompatible nanomedicine endowing intrinsic therapeutic and imaging properties have gained extraordinary momentum. In this study, an ultra-stable near-infrared (NIR) dye croconaine (CR780) was engineered with arginine-glycine-aspartic acid (RGD) peptide and polyethylene glycol (PEG), which was then self-assembled into uniform nanoparticles (NPs). These RGD-CR780-PEG5K assemblies were radiolabeled with ¹²⁵I through a facile standard Iodo-Gen method. The resulting [¹²⁵I]RGD-CR780-PEG5K NPs showed effective accumulation in α_vβ₃ integrin expressing glioblastoma, as evidenced by single photon emission computed tomography (SPECT)/CT and NIR fluorescence imaging. More importantly, high-resolution photoacoustic imaging revealed that these NPs selectively targeted to angiogenic tumor vessels. With the favorable tumor selective accumulation and high photothermal conversion efficiency, the [¹²⁵I]RGD-CR780-PEG5K NPs allowed thorough tumor ablation and inhibition of tumor relapse at a relatively low laser energy (0.5 W/cm²). Overall, this work offers a proper methodology to fabricate tumor-targeted multi-modal nanotheranostic agents, providing great opportunity for precision imaging and cancer therapy.

Ratiometric optical nanoprobes enable accurate molecular detection and imaging

Exploring and understanding biological and pathological changes are of great significance for early diagnosis and therapy of diseases. Optical sensing and imaging approaches have experienced major progress in this field. Particularly, an emergence of various functional optical nanoprobes has provided enhanced sensitivity, specificity, targeting ability, as well as multiplexing and multimodal capabilities due to improvements in their intrinsic physicochemical and optical properties. However, one of the biggest challenges of conventional optical nanoprobes is their absolute intensity-dependent signal readout, which causes inaccurate sensing and imaging results due to the presence of various analyte-independent factors that can cause fluctuations in their absolute signal intensity. Ratiometric measurements provide built-in self-calibration for signal correction, enabling more sensitive and reliable detection. Optimizing nanoprobe designs with ratiometric strategies can surmount many of the limitations encountered by traditional optical nanoprobes. This review first elaborates upon existing optical nanoprobes that exploit ratiometric measurements for improved sensing and imaging, including fluorescence, surface enhanced Raman scattering (SERS), and photoacoustic nanoprobes. Next, a thorough discussion is provided on design strategies for these nanoprobes, and their potential biomedical applications for targeting specific biomolecule populations (e.g. cancer biomarkers and small molecules with physiological relevance), for imaging the tumor microenvironment (e.g. pH, reactive oxygen species, hypoxia, enzyme and metal ions), as well as for intraoperative image guidance of tumor-resection procedures.

Organic molecule-based photothermal agents: an expanding photothermal therapy universe

Over the last decade, organic photothermal therapy (PTT) agents have attracted increasing attention as a potential complement for, or alternative to, classical drugs and sensitizers involving inorganic nanomaterials. In this tutorial review, we provide a structured description of the main classes of organic photothermal agents and their characteristics. Representative agents that have been studied in the context of photothermal therapy since 2000 are summarized and recent advances in using PTT agents to address various cancers indications are highlighted.

Liquid Exfoliation of Colloidal Rhenium Disulfide Nanosheets as a Multifunctional Theranostic Agent for In Vivo Photoacoustic/CT Imaging and Photothermal Therapy

Near-infrared light-mediated theranostic agents with superior tissue penetration and minimal invasion have captivated researchers in cancer research in the past decade. Herein, a probe sonication-assisted liquid exfoliation approach for scalable and continual synthesis of colloidal rhenium disulfide nanosheets, which is further explored as theranostic agents for cancer diagnosis and therapy, is reported. Due to high-Z element of Re (Z = 75) and significant photoacoustic effect, the obtained PVP-capped ReS₂ nanosheets are evaluated as bimodality contrast agents for computed tomography and photoacoustic imaging. In addition, utilizing the strong near-infrared absorption and ultrahigh photothermal conversion efficiency (79.2%), ReS₂ nanosheets could also serve as therapeutic agents for photothermal ablation of tumors with a tumor elimination rate up to 100%. Importantly, ReS₂ nanosheets show no obvious toxicity based on the cytotoxicity assay, serum biochemistry, and histological analysis. This work highlights the potentials of ReS₂ nanosheets as a single-component theranostic nanoplatform for bioimaging and antitumor therapy.

In vivo quantitative imaging of tumor pH by nanosonophore assisted multispectral photoacoustic imaging

Changes of physiological pH are correlated with several pathologies, therefore the development of more effective medical pH imaging methods is of paramount importance. Here, we report on an in vivo pH mapping nanotechnology. This subsurface chemical imaging is based on tumor-targeted, pH sensing nanoprobes and multi-wavelength photoacoustic imaging (PAI). The nanotechnology consists of an optical pH indicator, SNARF-5F, 5-(and-6)-Carboxylic Acid, encapsulated into polyacrylamide nanoparticles with surface modification for tumor targeting. Facilitated by multi-wavelength PAI plus a spectral unmixing technique, the accuracy of pH measurement inside the biological environment is not susceptible to the background optical absorption of biomolecules, i.e., hemoglobins. As a result, both the pH levels and the hemodynamic properties across the entire tumor can be quantitatively evaluated with high sensitivity and high spatial resolution in in vivo cancer models. The imaging technology reported here holds the potential for both research on and clinical management of a variety of cancers.

Background optical absorption of several biomolecules impedes an effective in vivo pH imaging in tumors. Here, the authors developed a visible light-based in vivo pH mapping method by coupling photoacoustic imaging and pH-responsive modified nanoparticles that selectively target tumor cells.

Advanced Photoacoustic Imaging Applications of Near-Infrared Absorbing Organic Nanoparticles

Progress of nanotechnology in recent years has stimulated fast development of nanoparticles in biomedical research. Photoacoustic (PA) imaging as an emerging non-invasive technique in molecular imaging has improved imaging depth relative to conventional optical imaging, demonstrating great potential in clinical applications. The convergence of nanotechnology and PA imaging has enabled a broad spectrum of new opportunities in fundamental biology and translation medicine. This review focuses on the recent advances of organic nanoparticles in PA imaging applications. Near-infrared absorbing organic nanoparticles are classified and discussed according to their different imaging applications, which include tumor imaging, gastrointestinal imaging, sentinel lymph node imaging, disease microenvironment imaging and real-time drug imaging. The chemistry and PA properties of organic nanoparticles are discussed in details to highlight their own merits, and their challenges and perspectives in PA imaging are also discussed.

pH and Urea Estimation in Urine Samples using Single Fluorophore and Ratiometric Fluorescent Biosensors

Kidney diseases remain often undiagnosed due to inefficient screening methods available at patient's disposal. Early diagnosis and effective management of kidney problems can best be addressed by the development of biosensors for commonly occurring clinical biomarkers. Here we report the development of single fluorophore and dual fluorophore ratiometric biosensors based on alginate microspheres for pH and urea analysis in urine samples. A facile method of air driven atomization was used for developing these polymeric fluorophore and enzyme based biosensors. Ratiometric biosensors were developed using layer-by-layer coating of polyelectrolyte conjugated to reference fluorophores. Biosensing studies using these biosensors showed that samples in pathophysiological range can be measured having pH range of 4-8 and urea levels between 0-50 mM. Testing of urine samples using these biosensors showed that both pH and urea detection can be accurately performed without interference. Thus, we believe that FITC-Dextran and FITC-Dextran/RuBpy based pH and urea biosensors show a great potential to be translated as a point of care device for pH and urea biosensing in early detection and continuous monitoring of kidney diseases.

Cancer-Associated, Stimuli-Driven, Turn on Theranostics for Multimodality Imaging and Therapy

Advances in bioinformatics, genomics, proteomics, and metabolomics have facilitated the development of novel anticancer agents that have decreased side effects and increased safety. Theranostics, systems that have combined therapeutic effects and diagnostic capabilities, have garnered increasing attention recently because of their potential use in personalized medicine, including cancer-targeting treatments for patients. One interesting approach to achieving this potential involves the development of cancer-associated, stimuli-driven, turn on theranostics. Multicomponent constructs of this type would have the capability of selectively delivering therapeutic reagents into cancer cells or tumor tissues while simultaneously generating unique signals that can be readily monitored under both in vitro and in vivo conditions. Specifically, their combined anticancer activities and selective visual signal respond to cancer-associated stimuli, would make these theranostic agents more highly efficient and specific for cancer treatment and diagnosis. This article focuses on the progress of stimuli-responsive turn on theranostics that activate diagnostic signals and release therapeutic reagents in response to the cancer-associated stimuli. The present article not only provides the fundamental backgrounds of diagnostic and therapeutic tools that have been widely utilized for developing theranostic agents, but also discusses the current approaches for developing stimuli-responsive turn on theranostics.

Advanced optoacoustic methods for multiscale imaging of in vivo dynamics

Visualization of dynamic functional and molecular events in an unperturbed in vivo environment is essential for understanding the complex biology of living organisms and of disease state and progression. To this end, optoacoustic (photoacoustic) sensing and imaging have demonstrated the exclusive capacity to maintain excellent optical contrast and high resolution in deep-tissue observations, far beyond the penetration limits of modern microscopy. Yet, the time domain is paramount for the observation and study of complex biological interactions that may be invisible in single snapshots of living systems. This review focuses on the recent advances in optoacoustic imaging assisted by smart molecular labeling and dynamic contrast enhancement approaches that enable new types of multiscale dynamic observations not attainable with other bio-imaging modalities. A wealth of investigated new research topics and clinical applications is further discussed, including imaging of large-scale brain activity patterns, volumetric visualization of moving organs and contrast agent kinetics, molecular imaging using targeted and genetically expressed labels, as well as three-dimensional handheld diagnostics of human subjects.

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.

Molecular imaging probes for multi-spectral optoacoustic tomography

In this review, we discuss recent progress in emerging optoacoustic probes, their mechanisms, applications and challenges for biological imaging using MSOT.

Selective photothermal inactivation of cells labeled with near-infrared croconaine dye

Laser irradiation of a mixed population of two cell lines produced cell death only in the cells that were labeled with croconaine dye.