Ratiometric Photoacoustic Nanoprobe for Bioimaging of Cu2

Small-Molecule Fluorescent Probes for Binding- and Activity-Based Sensing of Redox-Active Biological Metals

Although transition metals constitute less than 0.1% of the total mass within a human body, they have a substantial impact on fundamental biological processes across all kingdoms of life. Indeed, these nutrients play crucial roles in the physiological functions of enzymes, with the redox properties of many of these metals being essential to their activity. At the same time, imbalances in transition metal pools can be detrimental to health. Modern analytical techniques are helping to illuminate the workings of metal homeostasis at a molecular and atomic level, their spatial localization in real time, and the implications of metal dysregulation in disease pathogenesis. Fluorescence microscopy has proven to be one of the most promising non-invasive methods for studying metal pools in biological samples. The accuracy and sensitivity of bioimaging experiments are predominantly determined by the fluorescent metal-responsive sensor, highlighting the importance of rational probe design for such measurements. This review covers activity- and binding-based fluorescent metal sensors that have been applied to cellular studies. We focus on the essential redox-active metals: iron, copper, manganese, cobalt, chromium, and nickel. We aim to encourage further targeted efforts in developing innovative approaches to understanding the biological chemistry of redox-active metals.

Xanthene-Based Dyes for Photoacoustic Imaging and their Use as Analyte-Responsive Probes

Developing imaging tools that can report on the presence of disease-relevant analytes in multicellular organisms can provide insight into fundamental disease mechanisms as well as provide diagnostic tools for the clinic. Photoacoustic imaging (PAI) is a light-in, sound-out imaging technique that allows for high resolution, deep-tissue imaging with applications in pre-clinical and point-of-care settings. The continued development of near-infrared (NIR) absorbing small-molecule dyes promises to improve the capabilities of this emerging imaging modality. For example, new dye scaffolds bearing chemoselective functionalities are enabling the detection and quantification of disease-relevant analytes through activity-based sensing (ABS) approaches. Recently described strategies to engineer NIR absorbing xanthenes have enabled development of analyte-responsive PAI probes using this classic dye scaffold. Herein, we present current strategies for red-shifting the spectral properties of xanthenes via bridging heteroatom or auxochrome modifications. Additionally, we explore how these strategies, coupled with chemoselective spiroring-opening approaches, have been employed to create ABS probes for in vivo detection of hypochlorous acid, nitric oxide, copper (II), human NAD(P)H: quinone oxidoreductase isozyme 1, and carbon monoxide. Given the versatility of the xanthene scaffold, we anticipate continued growth and development of analyte-responsive PAI imaging probes based on this dye class.

A Small-Molecule Ratiometric Photoacoustic Probe for the High-Spatiotemporal-Resolution Imaging of Copper(II) Dynamics in the Mouse Brain

Copper dysmetabolism is associated with various neurodegenerative disorders, making high-spatiotemporal-resolution imaging of Cu2+ in the brain essential for understanding the underlying pathophysiological processes. Nevertheless, the current probes encounter obstacles in crossing the blood-brain barrier (BBB) and providing high-spatial-resolution in deep tissues. Herein, we present a photoacoustic probe capable of imaging Cu2+ dynamics in the mouse brain with high-spatiotemporal-resolution. The probe demonstrates selective ratiometric and reversible responses to Cu2+ , while also efficiently crossing the BBB. Using the probe as the imaging agent, we successfully visualized Cu2+ in the brain of Parkinson's disease (PD) model mouse with a remarkable micron-level resolution. The imaging results revealed a significant increase in Cu2+ levels in the cerebral cortex as PD progresses, highlighting the close association between Cu2+ alternations in the region and the disease. We also demonstrated that the probe can be used to monitor changes in Cu2+ distribution in the PD model mouse brain during L-dopa intervention. Mechanism studies suggest that the copper dyshomeostasis in the PD mouse brain was dominated by the expression levels of divalent metal transporter 1. The application of our probe in imaging Cu2+ dynamics in the mouse brain offers valuable insights into the copper-related molecular mechanisms underlying neurodegenerative diseases.

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 stable ratiometric fluorescent probe for hypochlorous acid detection and rheumatoid arthritis evaluation

A ratiometric fluorescent probe (MeO-CNPPV Pdots) based on the principle of fluorescence resonance energy transfer (FRET) was designed for hypochlorous acid (HOCl) and rheumatoid arthritis (RA) detection. The presence of HOCl can block the energy transfer from CNPPV to MeOTPATBT, resulting in a ratio change in the fluorescence of Pdots (I600 nm/I680 nm). This strategy provides a valuable paradigm in early RA evaluation.

3D-printed colorimetric copper ion detection kit and portable fluorescent sensing device using smartphone based on ratiometric fluorescent probes

Copper ion (Cu2+) is not only a transition metal ion but also a significant environmental pollutant. The imbalance of Cu2+ content will threaten the safety of the environment and even life. The portable detection devices based on ratiometric fluorescent probes have garnered increasing attention and acclaim because of their reliable analysis parameters. Therefore, two Cu2+ ratiometric fluorescent probes (RH-1 and RH-2) were developed, which exhibit pronounced fluorescence changes, high sensitivity, excellent selectivity, and large Stokes shift. Both probes are capable of detecting Cu2+ in water and milk samples. It is worth noting that a 3D-printed fluorescence sensing device was constructed using RH-1, and a new 3D-printed copper ion detection kit was developed based on RH-2, enabling on-the-spot estimation of Cu2+ concentration. These devices significantly facilitate Cu2+ detection in daily life. RH-2 has been successfully employed for imaging Cu2+ in living cells and zebrafish. In conclusion, this work provides, for the first time, the 3D-printed ideal tools for detecting Cu2+. It also provides valuable insights for the establishment of on-site portable detection methods for other important substances.

Biobased Agents for Single-Particle Detection with Optoacoustics

Optoacoustic (OA, photoacoustic) imaging synergistically combines rich optical contrast with the resolution of ultrasound within light-scattering biological tissues. Contrast agents have become essential to boost deep-tissue OA sensitivity and fully exploit the capabilities of state-of-the-art OA imaging systems, thus facilitating the clinical translation of this modality. Inorganic particles with sizes of several microns can also be individually localized and tracked, thus enabling new applications in drug delivery, microrobotics, or super-resolution imaging. However, significant concerns have been raised regarding the low bio-degradability and potential toxic effects of inorganic particles. Bio-based, biodegradable nano- and microcapsules consisting of an aqueous core with clinically-approved indocyanine green (ICG) and a cross-linked casein shell obtained in an inverse emulsion approach are introduced. The feasibility to provide contrast-enhanced in vivo OA imaging with nanocapsules as well as localizing and tracking individual larger microcapsules of 4-5 µm is demonstrated. All components of the developed capsules are safe for human use and the inverse emulsion approach is known to be compatible with a variety of shell materials and payloads. Hence, the enhanced OA imaging performance can be exploited in multiple biomedical studies and can open a route to clinical approval of agents detectable at a single-particle level.

An oral ratiometric NIR-II fluorescent probe for reliable monitoring of gastrointestinal diseases in vivo

Early monitoring of gastrointestinal diseases via orally delivered NIR-II ratiometric fluorescent probes represents a promising noninvasive diagnostic modality, but is challenging due to the limitation of harsh digestive environment. Here, we report a single-component NIR-II ratiometric molecular nanoprobe (LC-1250 NP) to monitor gastrointestinal disease with high specificity to its biomarker H₂O₂ via oral administration. LC-1250 NP displays stable fluorescence in the channel of 1250 long-pass (F_1250LP) before and after the gastrointestinal disease detection as the reference, while it presents significantly enhanced fluorescence signal in the response channel of 1150 nm short-pass (F_1150SP) in diseased gastrointestinal environment due to the intramolecular cyclization of LC-1250 molecules activated by H₂O₂. The fluorescence ratio (F_1150SP/F_1250LP) increases linearly with the concentration of H₂O₂ with a low detection limit of 20 nM. Therefore, when delivered orally, LC-1250 NP can accurately map the diseased areas and surmount the false-positive interference from biological heterogeneity by NIR-II ratiometric fluorescence imaging, providing sensitive and reliable evaluation for the progress of gastroenteritis.

Ultrathin Zinc Selenide Nanoplatelets Boosting Photoacoustic Imaging of In Situ Copper Exchange in Alzheimer's Disease Mice

The critical role of transition metal dyshomeostasis in Alzheimer's disease (AD) pathology poses demands of in vivo imaging for brain copper levels. Nanostructured probes afford prolonged retention time, increased accumulation, and enhanced photostability; however, their development for activatable photoacoustic (PA) imaging remains largely unexplored. We develop a principle of concept for activable PA imaging using in situ cation exchange of ultrathin zinc selenide (ZnSe) nanoplatelets for monitoring brain copper levels in AD mice. We start from quantitative modeling of optical absorption, time-resolved temperature field, and thermal expansion of copper selenide (CuSe) nanocrystals of different morphologies and reveal that ultrathin nanoplatelets afford substantial enhancement of near-infrared (NIR) absorption and PA pressures as compared to nanodots and nanoparticles. By tethering with a blood-brain barrier (BBB)-targeting peptide ligand, the ultrathin ZnSe nanoplatelet probe efficiently transports across the BBB and rapidly exchanges with endogenous copper ions, boosting activatable PA imaging of brain copper levels. We also demonstrate that the efficient exchange of ZnSe nanoplatelets with copper ions can reduce oxidative stress of neurons and protect neuronal cells from apoptosis. The nanoplatelet probe provides a paradigm for activatable PA imaging of brain copper levels, highlighting its potential for pathophysiologic study of AD.

Biomedical Photoacoustic Imaging for Molecular Detection and Disease Diagnosis: "Always-On" and "Turn-On" Probes

Photoacoustic (PA) imaging is a nonionizing, noninvasive imaging technique that combines optical and ultrasonic imaging modalities to provide images with excellent contrast, spatial resolution, and penetration depth. Exogenous PA contrast agents are created to increase the sensitivity and specificity of PA imaging and to offer diagnostic information for illnesses. The existing PA contrast agents are categorized into two groups in this review: "always-on" and "turn-on," based on their ability to be triggered by target molecules. The present state of these probes, their merits and limitations, and their future development, is explored.

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.

A novel dual-emission fluorescent probe for ratiometric and visual detection of Cu2+ ions and Ag+ ions

In this work, the biomolecule glutathione was used to prepare cyan fluorescent carbon dots (GSH@CDs) by a hydrothermal method. The GSH@CDs were adopted as the scaffolds to synthesize fluorescent gold nanoclusters (GSH@CDs-Au NCs) with two independent emission peaks at 430 nm and 700 nm. A fluorescent method for the Cu²⁺ and Ag⁺ ion assay was established based on the fluorescence quenching or enhancement at 700 nm of GSH@CDs-Au NCs. The fluorescent test strips were successfully prepared for visual detection of Cu²⁺ ions and Ag⁺ ions based on GSH@CDs-Au NCs. In addition, GSH@CDs-Au NCs were found to possess well peroxidase-like activity.

A Unique Multifunctional Nanoenzyme Tailored for Triggering Tumor Microenvironment Activated NIR-II Photoacoustic Imaging and Chemodynamic/Photothermal Combined Therapy

The accurate diagnosis and targeted therapy of malignant tumors face significant challenges. To address these, an oxidized molybdenum polyoxometalate-copper nanocomposite (Ox-POM@Cu) is designed and synthesized here. The doping with Cu determines the formation of oxygen vacancies, which can increase the carrier concentration in Ox-POM@Cu, accelerate electron transfer, and enhance the redox activity, thus playing an efficient catalytic role. The nanocomposite presents unique enzymatic functions characterized by a multielement catalytic activity in the tumor microenvironment (TME). In addition, it can be employed as an NIR-II photoacoustic imaging (PAI) probe and cancer therapy agent. First, it participates in a redox reaction with glutathione (GSH) in tumor tissues, activates the PAI and photothermal therapy functions via NIR-II irradiation, and depletes the GSH supply in cancerous cells. Subsequently, it catalyzes a Fenton-like reaction with H₂ O₂ in tumor tissues to form hydroxyl radicals, thereby performing a chemodynamic therapy function. The findings show that the developed nanoenzyme is very efficient in the diagnosis and treatment of malignant tumors. This work not only provides a new strategy for the design of TME-induced NIR-II PAI but also presents new insights into enhanced cancer therapy.

Thienylpiperidine Donor NIR Xanthene-Based Dye for Photoacoustic Imaging

Few xanthene-based near-infrared (NIR) photoacoustic (PA) dyes with absorbance >800 nm exist. As accessibility to these dyes requires long and tedious synthetic steps, we designed a NIR dye (XanthCR-880) with thienylpiperidine donors and a xanthene acceptor that is accessible in 3-4 synthetic steps. The dye boasts a strong PA signal at 880 nm with good biological compatibility and photostability, yields multiplexed imaging with an aza-BODIPY reference dye, and is detected at a depth of 4 cm.

A General Approach to Convert Hemicyanine Dyes into Highly Optimized Photoacoustic Scaffolds for Analyte Sensing*

Most photoacoustic (PA) imaging agents are based on the repurposing of existing fluorescent dye platforms that exhibit non-optimal properties for PA applications. Herein, we introduce PA-HD, a new dye scaffold optimized for PA probe development that features a 4.8-fold increase in sensitivity and a red-shift of the λabs from 690 nm to 745 nm to enable ratiometric imaging. Computational modeling was used to elucidate the origin of these enhanced properties. To demonstrate the generalizability of our remodeling efforts, we developed three probes for β-galactosidase activity (PA-HD-Gal), nitroreductase activity (PA-HD-NTR), and H2 O2 (PA-HD-H2 O2 ). We generated two cancer models to evaluate PA-HD-Gal and PA-HD-NTR. We employed a murine model of Alzheimer's disease to test PA-HD-H2 O2 . There, we observed a PA signal increase at 735 nm of 1.79±0.20-fold relative to background, indicating the presence of oxidative stress. These results were confirmed via ratiometric calibration, which was not possible using the parent HD platform.

NIR-II Photoacoustic Reporter for Biopsy-Free and Real-Time Assessment of Wilson's Disease

Wilson's disease (WD) is a rare inherited disorder of copper metabolism with pathological copper hyperaccumulation in some vital organs. However, the clinical diagnosis technique of WD is complicated, aggressive, and time-consuming. In this work, a novel ratiometric photoacoustic (PA) imaging nanoprobe in the NIR-II window is developed to achieve noninvasive, rapid, and accurate Cu²⁺ quantitative detection in vitro and in vivo. The nanoprobe consists of Cu²⁺ -responsive IR970 dye and a nonresponsive palladium-coated gold nanorod (AuNR-Pd), achieving a concentration-dependent ratiometric PA₉₇₀ /PA₁₂₆₀ signal change. The urinary Cu²⁺ content is detectable within minutes down to a detection limit of 76 × 10^-9 m. This report acquisition time is several orders of magnitude shorter than those of existing detection approaches requiring complex procedure. Moreover, utilizing the ratiometric PA nanoprobe, PA imaging enables biopsy-free measurement of the liver Cu²⁺ content and visualization of the liver Cu²⁺ biodistribution of WD patient, which avoid the body injury during the clinical Cu²⁺ test using liver biopsy method. The NIR-II ratiometric PA detection method is simple and noninvasive with super precision, celerity, and simplification, which holds great promise as an alternative to liver biopsy for clinical diagnosis of WD.

Clinically oriented Alzheimer's biosensors: expanding the horizons towards point-of-care diagnostics and beyond

The development of minimally invasive and easy-to-use sensor devices is of current interest for ultrasensitive detection and signal recognition of Alzheimer's disease (AD) biomarkers. Over the years, tremendous effort has been made on diagnostic platforms specifically targeting neurological markers for AD in order to replace the conventional, laborious, and invasive sampling-based approaches. However, the sophistication of analytical outcomes, marker inaccessibility, and material validity strongly limit the current strategies towards effectively predicting AD. Recently, with the promising progress in biosensor technology, the realization of a clinically applicable sensing platform has become a potential option to enable early diagnosis of AD and other neurodegenerative diseases. In this review, various types of biosensors, which include electrochemical, fluorescent, plasmonic, photoelectrochemical, and field-effect transistor (FET)-based sensor configurations, with better clinical applicability and analytical performance towards AD are highlighted. Moreover, the feasibility of these sensors to achieve point-of-care (POC) diagnosis is also discussed. Furthermore, by grafting nanoscale materials into biosensor architecture, the remarkable enhancement in durability, functionality, and analytical outcome of sensor devices is presented. Finally, future perspectives on further translational and commercialization pathways of clinically driven biosensor devices for AD are discussed and summarized.

Recent development of near-infrared photoacoustic probes based on small-molecule organic dye

Photoacoustic imaging (PAI), which integrates the higher spatial resolution of optical imaging and the deeper penetration depth of ultrasound imaging, has attracted great attention. Various photoacoustic probes including inorganic and organic agents have been well fabricated in last decades. Among them, small-molecule based agents are most promising candidates for preclinical/clinical applications due to their favorite in vivo features and facile functionalization. In recent years, PAI, in the near-infrared region (NIR, 700-1700 nm) has developed rapidly and has made remarkable achievements in the biomedical field. Compared with the visible light region (400-700 nm), it can significantly reduce light scattering and meanwhile provide deeper tissue penetration. In this review, we discuss the recent developments of near-infrared photoacoustic probes based on small molecule dyes, which focus on their "always on" and "activatable" form in biomedicine. Further, we also suggest current challenges and perspectives.

Dual Ratiometric SERS and Photoacoustic Core-Satellite Nanoprobe for Quantitatively Visualizing Hydrogen Peroxide in Inflammation and Cancer

Excessive production of oxidative species alters the normal redox balance and leads to diseases, such as chronic inflammation and cancer. Oxidative species are short-lived species, which makes direct, precise, and real-time measurements difficult. Herein, we report a novel core-satellite gold nanostructure for dual, ratiometric surface-enhanced Raman scattering (SERS) and photoacoustic (PA) imaging to enable the precise detection of inflammation/cancer-related H₂ O₂ . The combination of H₂ O₂ -activated second near-infrared (NIR-II) PA imaging and SERS imaging enables the differentiation between the inflamed region and normal tissue with high accuracy. The mesoporous silica shell of the nanoprobe could be used to deliver drugs to the target area to precisely treat disease. Therefore, this core-satellite nanostructure can not only quantitatively and precisely monitor H₂ O₂ produced in inflammation, tumor, and osteoarthritis in rabbits in real-time, but can also be used to track the progress of the anti-inflammatory treatment in real-time.

Stimuli-Responsive Plasmonic Assemblies and Their Biomedical Applications

Among the diverse development of stimuli-responsive assemblies, plasmonic nanoparticle (NP) assemblies functionalized with responsive molecules are of a major interest. In this review, we outline a comprehensive and up-to-date overview of recently reported studies on in vitro and in vivo assembly/disassembly and biomedical applications of plasmonic NPs, wherein stimuli such as enzymes, light, pH, redox potential, temperature, metal ions, magnetic or electric field, and/or multi-stimuli were involved. Stimuli-responsive assemblies have been applied in various biomedical fields including biosensors, surfaced-enhanced Raman scattering (SERS), photoacoustic (PA) imaging, multimodal imaging, photo-activated therapy, enhanced X-ray therapy, drug release, stimuli-responsive aggregation-induced cancer therapy, and so on. The perspectives on the use of stimuli-responsive plasmonic assemblies are discussed by addressing future scientific challenges involving assembly/disassembly strategies and applications.

A novel copper ion sensing fluorescent probe for fast detection of pyrophosphate and alkaline phosphatase

A Cu²⁺ sensing fluorescent probe is synthesized via a Mannich reaction and is applied in the fluorescence detection of pyrophosphate and alkaline phosphatase.