In Vivo Chemoselective Photoacoustic Imaging of Copper(II) in Plant and Animal Subjects

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.

Different Valence States of Copper Ion Delivery against Triple-Negative Breast Cancer

Different valence states of copper (Cu) ions are involved in complicated redox reactions in vivo, which are closely related to tumor proliferation and death pathways, such as cuproptosis and chemodynamic therapy (CDT). Cu ion mediated Fenton-like reagents induced tumor cell death which presents compelling attention for the CDT of tumors. However, the superiority of different valence states of Cu ions in the antitumor effect is unknown. In this study, we investigated different valence states of Cu ions in modulating tumor cell death by Cu-chelated cyanine dye against triple-negative breast cancer. The cuprous ion (Cu+) and copper ion (Cu2+) were chelated with four nitrogen atoms of dipicolylethylenediamine-modified cyanine for the construction of Cu+ and Cu2+ chelated cyanine dyes (denoted as CC1 and CC2, respectively). Upon 660 nm laser irradiation, the CC1 or CC2 can generate reactive oxygen species, which could disrupt the cyanine structure, achieving the rapid release of Cu ions and initiating the Fenton-like reaction for CDT. Compared with Cu2+-based Fenton-like reagent, the CC1 with Cu+ exhibited a better therapeutic outcome for the tumor due to there being no need for a reduction by glutathione and a shorter route to generate more hydroxyl radicals. Our findings suggest the precision delivery of Cu+ could achieve highly efficient antitumor therapy.

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.

Leveraging coordination chemistry to visualize metal ions via photoacoustic imaging

Metal ions are indispensable to all living systems owing to their diverse roles. Perturbation of metal homeostasis have been linked to many pathological conditions. As such, visualizing metal ions in these complex environments are of utmost importance. Photoacoustic imaging is a promising modality that combines the sensitivity of fluorescence to the superior resolution of ultrasound, through a light-in sound-out process, making it an appealing modality for metal ion detection in vivo. In this review, we highlight recent advances in the development of photoacoustic imaging probes for in vivo detection of metal ions, such as potassium, copper, zinc, and palladium. In addition, we provide our perspective and outlook on the exciting field.

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.

Monitoring of the copper persistence on plant leaves using pulsed thermography

Copper-based fungicides are largely used in agriculture in the control of a wide range of plant diseases. Applied on plants, they remain deposited on leaf surfaces and are not absorbed into plant tissues. Because of accumulation problems and their ecotoxicological profiles in the soil, their use needs to be monitored and controlled, also by using modern technologies to better optimize the efficacy rendering minimum the amount of copper per season used. In this work, we test a novel approach based on pulsed thermography to evaluate the persistence of the copper on plant leaves so that the time between two applications should be the minimum needs. We monitored the thermal response observed on different treatments of both grapevine and tobacco plants over a 3-week period. Our experimental results demonstrate that the new methodological approach based on pulsed thermography can be an effective tool to evaluate in real time the presence of copper on differently treated plants allowing a tentative quantification and, therefore, to optimize its use in the agricultural practices, according also to the European Regulation n. 1107/2009.

Engineering Molecular Probes for In Vivo Near-Infrared Fluorescence/Photoacoustic Duplex Imaging of Human Neutrophil Elastase

Early detection of human neutrophil elastase (HNE), the potential biomarker of lung cancer, is crucial for the accurate diagnosis and evaluation of lung cancer. Currently, little progress of HNE-activated probes has been made for in vivo imaging. Herein, assisted by probe-active pocket match engineering, we synthesized a series of near-infrared fluorescence (NIRF) and photoacoustic (PA) duplex imaging probes by conjugating diverse fluorinated amide chains onto hemi-cyanine. Finally, we identified that probe 2 (denoted as LET-8), with the pentafluoroethyl group, is a superior probe to detect HNE with the best selectivity as well as good response ability and thus successfully realized NIRF/PA duplex imaging of HNE activity both in vitro and in vivo.

Targeted contrast agents and activatable probes for photoacoustic imaging of cancer

Photoacoustic (PA) imaging has emerged as a powerful technique for the high resolution visualization of biological processes within deep tissue. Through the development and application of exogenous targeted contrast agents and activatable probes that can respond to a given cancer biomarker, researchers can image molecular events in vivo during cancer progression. This information can provide valuable details that can facilitate cancer diagnosis and therapy monitoring. In this tutorial review, we provide a step-by-step guide to select a cancer biomarker and subsequent approaches to design imaging agents for in vivo use. We envision this information will be a useful summary to those in the field, new members to the community, and graduate students taking advanced imaging coursework. We also highlight notable examples from the recent literature, with emphasis on the molecular designs and their in vivo PA imaging performance. To conclude, we provide our outlook and future perspective in this exciting field.

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.

Activatable NIR-II Fluorescence Probe for Highly Sensitive and Selective Visualization of Glutathione In Vivo

Visualization of glutathione (GSH) enables us to understand GSH-related pathophysiological processes in living subjects. Currently, in vivo visualization methods of GSH are based on visible or first near-infrared (NIR-I) window fluorescence (FL) probes, which possess limitations due to their low tissue penetration depth and strong tissue autofluorescence. Herein, we developed a GSH-activatable second near-infrared (NIR-II) window FL probe (denoted as LET-7) for highly sensitive and selective visualization of GSH in vivo. LET-7, composed of an anionic polymethylcyanide skeleton linked with a FL quenching group of 3,5-bis(trifluoromethyl)benzenethiol, can be specifically activated by GSH, thus triggering a significant NIR-II FL emission enhancement with excellent photostability, which enables us to efficiently distinguish GSH from closely related low-molecular-weight biothiols. The limit of detection of LET-7 for GSH was determined to be as low as 85 nM. Most intriguingly, the in vivo studies demonstrated that LET-7 showed high sensitivity and good selectivity toward GSH. Therefore, our study provides a solution to design activatable NIR-II FL probes for in vivo imaging of GSH and other disease-related biomarkers.

Responsive fluorescence enhancement for in vivo Cu(II) monitoring in zebrafish larvae

Several neurodegenerative diseases are ascribed to disorders caused by the secretion of Cu ions. However, a majority of the current techniques for copper ion detection are restricted to in vivo monitoring and nonspecific interactions. Their methods are limited to the systematic analysis of Cu ions in living organisms. Thus, a synthetic molecular fluorophore, 5-amino 2,3-dihydroquinolinimine (NDQI), has been developed and successfully utilized in in vivo monitoring of the distribution of Cu(II) in zebrafish larvae. The reversible formation of the NDQI-Cu complex allows its use with high metal concentrations and in oxidative stress conditions. The NDQI-directed strategy developed here can quantitatively differentiate cells with different Cu(II) concentrations. Remarkably, dynamic distribution of Cu(II) in the intestine and liver can be observed.

Plasmonic Oxygen Defects in MO3- x (M = W or Mo) Nanomaterials: Synthesis, Modifications, and Biomedical Applications

Nanomedicine is a promising technology with many advantages and provides exciting opportunities for cancer diagnosis and therapy. During recent years, the newly developed oxygen-deficiency transition metal oxides MO_{3- x} (M = W or Mo) have received significant attention due to the unique optical properties, such as strong localized surface plasmon resonance (LSPR) , tunable and broad near-IR absorption, high photothermal conversion efficiency, and large X-ray attenuation coefficient. This review presents an overview of recent advances in the development of MO_{3- x} nanomaterials for biomedical applications. First, the fundamentals of the LSPR effect are introduced. Then, the preparation and modification methods of MO_{3- x} nanomaterials are summarized. In addition, the biological effects of MO_{3- x} nanomaterials are highlighted and their applications in the biomedical field are outlined. This includes imaging modalities, cancer treatment, and antibacterial capability. Finally, the prospects and challenges of MO_{3- x} and MO_{3- x} -based nanomaterial for fundamental studies and clinical applications are also discussed.

Illuminating the hidden world of calcium ions in plants with a universe of indicators

The tools available to carry out in vivo analysis of Ca²⁺ dynamics in plants are powerful and mature technologies that still require the proper controls.

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.

Activatable Photoacoustic Probe for In Situ Imaging of Endogenous Carbon Monoxide in the Murine Inflammation Model

Photoacoustic (PA) imaging is an emerging biomedical imaging modality that combines the advantages of optical and ultrasound imaging. Carbon monoxide (CO), which is a vital endogenous cell-signaling molecule in the human body, exerts critical physiological functions such as anti-inflammatory, antiapoptotic, and antiproliferative. The imbalance of CO homeostasis is also associated with numerous diseases. Therefore, it is critically important to noninvasively monitor the steady-state changes of CO in vivo. However, the activatable photoacoustic (PA) probes for detecting CO-associated complicated diseases have not yet developed. In this work, we developed the first turn-on PA probe (MTR-CO) to visualize the CO level in the lipopolysaccharide (LPS)-induced acute inflammation murine model through PA imaging technology. MTR-CO is composed of a near-infrared absorption cyanine-like dye (MTR-OH) and allyl formate, showing a 10.2-fold PA signal enhancement at 690 nm upon activation by CO. Furthermore, the results revealed that MTR-CO has high sensitivity, excellent specificity, and good biocompatibility for CO in vivo. MTR-CO was then applied for PA imaging of CO in cells and for monitoring the development of acute inflammation in the murine model by tracking the changes of the CO level. These findings provide a promising strategy for accurately detecting the steady-state changes of CO in living organisms.

Quantitative Assessment of Copper(II) in Wilson's Disease Based on Photoacoustic Imaging and Ratiometric Surface-Enhanced Raman Scattering

Cu²⁺ is closely related to the occurrence and development of Wilson's disease (WD), and quantitative detection of various copper indicators (especially liver Cu² and urinary Cu²⁺) is the key step for the early diagnosis of WD in the clinic. However, the clinic Cu²⁺ detection approach was mainly based on testing the liver tissue through combined invasive liver biopsy and the ICP-MS method, which is painful for the patient and limited in determining WD status in real-time. Herein, we rationally designed a type of Cu²⁺-activated nanoprobe based on nanogapped gold nanoparticles (AuNNP) and poly(N-isopropylacrylamide) (PNIPAM) to simultaneously quantify the liver Cu²⁺ content and urinary Cu²⁺ in WD by photoacoustic (PA) imaging and ratiometric surface-enhanced Raman scattering (SERS), respectively. In the nanoprobe, one Raman molecule of 2-naphthylthiol (NAT) was placed in the nanogap of AuNNP. PNIPAM and the other Raman molecule mercaptobenzonitrile (MBN) were coated on the AuNNP surface, named AuNNP-NAT@MBN/PNIPAM. Cu²⁺ can efficiently coordinate with the chelator PNIPAM and lead to aggregation of the nanoprobe, resulting in the absorption red-shift and increased PA performance of the nanoprobe in the NIR-II window. Meanwhile, the SERS signal at 2223 cm^-1 of MBN is amplified, while the SERS signal at 1378 cm^-1 of NAT remains stable, generating a ratiometric SERS I₂₂₂₃/I₁₃₇₈ signal. Both NIR-II PA_1250 nm and SERS I₂₂₂₃/I₁₃₇₈ signals of the nanoprobe show a linear relationship with the concentration of Cu²⁺. The nanoprobe was successfully applied for in vivo quantitative detection of liver Cu²⁺ of WD mice through NIR-II PA imaging and accurate quantification of urinary Cu²⁺ of WD patients by ratiometric SERS. We anticipate that the activatable nanoprobe might be applied for assisting an early, precise diagnosis of WD in the clinic in the future.

Light-Triggered Transformable Ferrous Ion Delivery System for Photothermal Primed Chemodynamic Therapy

Chemodynamic therapy (CDT) involves the catalytic generation of highly toxic hydroxyl radicals (^. OH) from hydrogen peroxide (H₂ O₂ ) through metal-ion-mediated Fenton or Fenton-like reactions. Fe²⁺ is a classical catalyst ion, however, it suffers easy oxidation and systemic side-effects. Therefore, the development of a controllable Fe²⁺ delivery system is a challenge to maintain its valence state, reduce toxicity, and improve therapeutic efficacy. Reported here is a near-infrared (NIR) light-triggered Fe²⁺ delivery agent (LET-6) for fluorescence (FL) and photoacoustic (PA) dual-modality imaging guided, photothermal primed CDT. Thermal expansion caused by 808 nm laser irradiation triggers the transformation of LET-6 to expose Fe²⁺ from its hydrophobic layer, which primes the catalytic breakdown of endogenous H₂ O₂ within the tumor microenvironment, thus generating ^. OH for enhanced CDT. LET-6 shows remarkable therapeutic effects, both in vitro and in vivo, achieving 100 % tumor elimination after just one treatment. This high-performance Fe²⁺ delivery system provides a sound basis for future synergistic metal-ion-mediated cancer therapy.

A dual-round signal amplification strategy for colorimetric/photoacoustic/fluorescence triple read-out detection of prostate specific antigen

The detection of prostate specific antigen (PSA) is extremely important for the early diagnosis of prostate cancer. Herein, we report a dual-round signal amplification strategy for colorimetric/fluorescence/photoacoustic triple read-out detection of PSA using a silica coated Au@Ag core-shell nanorod (denoted Au@Ag@SiO2) based enzyme-linked immunosorbent assay (ELISA) system. In the presence of PSA, monoclonal primary antihuman PSA antibody (Ab1) captured PSA and was subsequently recognized by the secondary antihuman PSA detection antibody (Ab2) which was conjugated with glucose oxidase (GOx) functionalized magnetic beads (MBs) for signal amplification, then GOx catalyses the addition of glucose to generate hydrogen peroxide that etches the silver layer in Au@Ag@SiO2, thus producing abundant Ag+ to realize the second signal amplification. With the degradation of the silver layer, an obvious color change (green-to-pink) of the Au@Ag@SiO2 solution could be observed by the naked eye and its surface plasmon resonance (SPR) absorption had a red-shift, enhancing photoacoustic signal read-out at 780 nm. Additionally, the released Ag+ was caught by a Ag+-fluorescent probe (Ag+-FP) for enhanced fluorescence signal read-out. These results suggested that this ELISA system achieves a triple read-out detection of PSA. This work provides a promising strategy for multiple read-out detection of biomarkers, which has great potential in clinical diagnosis.

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.

Co-delivery of Cu(I) chelator and chemotherapeutics as a new strategy for tumor theranostic

Chelating Cu from tumors has been verified as an effective and promising strategy for cancer therapy through antiangiogenesis. However, systematic removal Cu by injecting with Cu chelators will result unavoidable side effects, since Cu is indispensable to the body. In this work, a micelle targeting to tumors' newborn vessels based on a polypeptide was developed to co-load DOX and Probe X, which can go through an "OFF-to-ON" procedure to report the Cu⁺-capture events in vivo in a real-time way by giving near infrared (NIR) fluorescence and photoacoustic signal. By co-delivering antiangiogenesis and chemotherapeutic reagents, the tumor can be significantly suppressed, meanwhile with a low systematic toxicity. Hopefully, this work can offer new insights in designing sophisticated antitumor strategy.