Activatable Small‐Molecule Photoacoustic Probes that Cross the Blood–Brain Barrier for Visualization of Copper(II) in Mice with Alzheimer's Disease

Molecular imaging-guided diagnosis and treatment integration for brain diseases

In practical clinical scenarios, improved diagnostic methods have been developed for the precise visualization of molecular targets using molecular imaging in brain diseases. Recently, the introduction of innovative molecular imaging modalities across both macroscopic and mesoscopic dimensions, with remarkable specificity and spatial resolution, has expanded the scope of applications beyond diagnostic testing, with the potential to guide therapeutic interventions, offering real-time feedback in the context of brain therapy. The molecular imaging-guided integration of diagnosis and treatment holds the potential to revolutionize disease management by enabling the real-time monitoring of treatment responses and therapy adjustments. Given the vibrant and ever-evolving nature of this field, this review provides an integrated picture on molecular image-guided diagnosis and treatment integration for brain diseases involving the basic concepts, significant breakthroughs, and recent trends. In addition, based on the current achievements, some critical challenges are also discussed.

Nanophotonic-enhanced photoacoustic imaging for brain tumor detection

Photoacoustic brain imaging (PABI) has emerged as a promising biomedical imaging modality, combining high contrast of optical imaging with deep tissue penetration of ultrasound imaging. This review explores the application of photoacoustic imaging in brain tumor imaging, highlighting the synergy between nanomaterials and state of the art optical techniques to achieve high-resolution imaging of deeper brain tissues. PABI leverages the photoacoustic effect, where absorbed light energy causes thermoelastic expansion, generating ultrasound waves that are detected and converted into images. This technique enables precise diagnosis, therapy monitoring, and enhanced clinical screening, specifically in the management of complex diseases such as breast cancer, lymphatic disorder, and neurological conditions. Despite integration of photoacoustic agents and ultrasound radiation, providing a comprehensive overview of current methodologies, major obstacles in brain tumor treatment, and future directions for improving diagnostic and therapeutic outcomes. The review underscores the significance of PABI as a robust research tool and medical method, with the potential to revolutionize brain disease diagnosis and treatment.

Turn-On NIR-II Polymer Dots with Large Stokes Shift for In Vivo Visualizing Dynamical Brain Zinc in Alzheimer's Disease Mouse

It is a critical and broad prospect to evaluate ion levels and monitor their dynamic changes in the brain for early diagnosis, in-depth mechanism investigation, and accurate staging of neurodegenerative diseases including Alzheimer's disease (AD). It is still a great challenge to in vivo track Zn2+ levels in the brain by fluorescence imaging due to the drawbacks including short emission wavelength, poor selectivity and sensitivity, and unfavorable penetration across the blood-brain barrier (BBB) for currently developed fluorescent probes. We herein engineer a fluorescent probe with a large Stokes shift of 256 nm, NNDPTQ Pdots, which display substantial Zn2+-specific turn-on response in the NIR II region with the longest emission of 1064 nm up to now. The probe shows a fast response within seconds, high selectivity, low-nanomolar affinity of 6 nM, low detection limit of 3.4 nM, and efficient BBB-permeability efficacy of 37%. The results of brain imaging demonstrate that brain Zn2+ level in AD mice is substantially higher than normal mice and also is elevated with the prolonging of AD-bearing time. This study may provide a promising fluorescent indicator for in vivo tracing of brain Zn2+ levels to reveal AD pathogenesis.

The role of metal ions in stroke: Current evidence and future perspectives

Metal ions play a pivotal role in maintaining optimal brain function within the human body. Nevertheless, the accumulation of these ions can result in irregularities that lead to brain damage and dysfunction. Disruptions of metal ion homeostasis can result in various pathologies, including inflammation, redox dysregulation, and blood-brain barrier disruption. While research on metal ions has chiefly focused on neurodegenerative diseases, little attention has been given to their involvement in the onset and progression of stroke. Recent studies have identified cuproptosis and confirmed ferroptosis as significant factors in stroke pathology, underscoring the importance of metal ions in stroke pathology, including abnormal ion transport, neurotoxicity, blood-brain barrier damage, and cell death. Additionally, it provides an overview of contemporary metal ion chelators and detection techniques, which may offer novel approaches to stroke treatment.

Carbon dots-based dual-emission ratiometric fluorescent sensors for fluorescence and visual detection of hypochlorite and Cu2

Carbon dots (CDs) with blue emission were synthesized by solvothermal method using hydroquinone and 5-aminoisphthalic acid as precursors. The strong oxidation of ClO- caused the fluorescence quenching of CDs at 405 nm, and synchronously generated a new emission peak at 500 nm. Furthermore, upon the addition of Cu2+ to CDs-ClO- system, the green fluorescence at 500 nm was quenched, while the blue emission at 405 nm remained unchanged, due to the complexation between Cu2+ and the amino group on the CDs surface. Meanwhile, the fluorescence color of system changed from blue to bright green and then to dark blue by sequentially increasing the concentrations of ClO- and Cu2+. The fluorescence signal of F500/F405 exhibited a linear relationship with the concentration of ClO- and Cu2+ in a certain range, respectively. Thus, a ratiometric fluorescence sensor based on the obtained CDs were developed to sequentially detect ClO- and Cu2+ with detection limits of 0.40 μM and 0.31 μM, respectively. Additionally, the CDs were mixed with polyvinyl alcohol hydrogel to form test strips, which were successfully used for visual detection of ClO- and Cu2+. Satisfactory results were also obtained in the analysis of ClO- and Cu2+ in actual water samples.

Development of a Dual-Factor Activatable Covalent Targeted Photoacoustic Imaging Probe for Tumor Imaging

Photoacoustic imaging (PAI) is an emerging modality in biomedical imaging with superior imaging depth and specificity. However, PAI still has significant limitations, such as the background noise from endogenous chromophores. To overcome these limitations, we developed a covalent activity-based PAI probe, NOx-JS013, targeting NCEH1. NCEH1, a highly expressed and activated serine hydrolase in aggressive cancers, has the potential to be employed for the diagnosis of cancers. We show that NOx-JS013 labels active NCEH1 in live cells with high selectivity relative to other serine hydrolases. NOx-JS013 also presents its efficacy as a hypoxia-responsive imaging probe in live cells. Finally, NOx-JS013 successfully visualizes aggressive prostate cancer tumors in mouse models of PC3, while being negligibly detected in tumors of non-aggressive LNCaP mouse models. These findings show that NOx-JS013 has the potential to be used to develop precision PAI reagents for detecting metastatic progression in various cancers.

Biomedical applications of stimuli-responsive nanomaterials

Nanomaterials have aroused great interests in drug delivery due to their nanoscale structure, facile modifiability, and multifunctional physicochemical properties. Currently, stimuli-responsive nanomaterials that can respond to endogenous or exogenous stimulus display strong potentials in biomedical applications. In comparison with conventional nanomaterials, stimuli-responsive nanomaterials can improve therapeutic efficiency and reduce the toxicity of drugs toward normal tissues through specific targeting and on-demand drug release at pathological sites. In this review, we summarize the responsive mechanism of a variety of stimulus, including pH, redox, and enzymes within pathological microenvironment, as well as exogenous stimulus such as thermal effect, magnetic field, light, and ultrasound. After that, biomedical applications (e.g., drug delivery, imaging, and theranostics) of stimuli-responsive nanomaterials in a diverse array of common diseases, including cardiovascular diseases, cancer, neurological disorders, inflammation, and bacterial infection, are presented and discussed. Finally, the remaining challenges and outlooks of future research directions for the biomedical applications of stimuli-responsive nanomaterials are also discussed. We hope that this review can provide valuable guidance for developing stimuli-responsive nanomaterials and accelerate their biomedical applications in diseases diagnosis and treatment.

Benzorhodol derived far-red/near-infrared fluorescent probes for selective and sensitive detection of butyrylcholinesterase activity in living cells and the non-alcoholic fatty liver of zebrafish

Liver diseases are a growing public health concern and the development of non-alcoholic fatty liver disease (NAFLD) has a significant impact on human metabolism. Butyrylcholinesterase (BChE) is a vital biomarker for NAFLD, making it crucial to monitor BChE activity with high sensitivity and selectivity. In this study, we designed and synthesized a range of benzorhodol-derived far-red/near-infrared fluorescent probes, FRBN-B, NF-SB, and NF-B, for the quantitative detection and imaging of BChE. These probes differed in the size of their conjugated systems and in the number of incorporated cyclopropanecarboxylates, acting as the recognition site for BChE. Comprehensive characterization showed that FRBN-B and NF-SB fluorescence was triggered by BChE-mediated hydrolysis, while an additional cyclopropanecarboxylate in NF-B impeded the fluorescence release. High selectivity towards BChE was observed for FRBN-B and NF-SB, with a detection limit of 7.2 × 10-3 U mL-1 for FRBN-B and 1.9 × 10-3 U mL-1 for NF-SB. The probes were further employed in the evaluation of BChE inhibitor efficacy and imaging of intracellular BChE activity. Additionally, FRBN-B was utilized for imaging the BChE activity level in liver tissues in zebrafish, demonstrating its potential as a diagnostic tool for NAFLD.

A ratiometric fluorescent probe revealing the abnormality of acetylated tau by visualizing polarity in Alzheimer's disease

Abnormal neuronal polarity leads to early deficits in Alzheimer's disease (AD) by affecting the function of axons. Precise and rapid evaluation of polarity changes is very important for the early prevention and diagnosis of AD. However, due to the limitations of existing detection methods, the mechanism related to how neuronal polarity changes in AD is unclear. Herein, we reported a ratiometric fluorescent probe characterized by neutral molecule to disclose the polarity changes in nerve cells and the brain of APP/PS1 mice. Cy7-K showed a sensitive and selective ratiometric fluorescence response to polarity. Remarkably, unlike conventional intramolecular charge transfer fluorescent probes, the fluorescence quantum yield of Cy7-K in highly polar solvents is higher than that in low polar solvents due to the transition of neutral quinones to aromatic zwitterions. Using the ratiometric fluorescence imaging, we found that beta-amyloid protein (Aβ) inhibits the expression of histone deacetylase 6, thereby increasing the amount of acetylated Tau protein (AC-Tau) and ultimately enhancing cell polarity. There was a high correlation between polarity and AC-Tau. Furthermore, Cy7-K penetrated the blood-brain barrier to image the polarity of different brain regions and confirmed that APP/PS1 mice had higher polarity than Wild-type mice. The probe Cy7-K will be a promising tool for assessing the progression of AD development by monitoring polarity.

The Synergistic Mechanisms of AIE, ESIPT and ICT in the α-cyanostilbene-based Derivative: A Red-fluorescence Probe With a Large Stokes' Shift for Copper (II) Ion Determination and Reversible Response to Amine/acid Vapor

Herein, α-cyanostilbene-based luminogen with an electron donor-π-electron acceptor (D-π-A) architecture was formylated into the salicylaldehyde-analogue luminogen, followed by the Schiff base reaction with phenylamine, a red-emitting luminogen was elaborately designed and successfully synthesized in a high yield of 89%. Its well-defined structure was confirmed by FT-IR, MALDI-TOF-MS, HR-MS and 1H/13C NMR technologies. Based on the synergistic mechanisms of aggregation-induced emission (AIE), excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT), it enjoyed a red-fluorescence emission at 627 nm in THF/water mixtures (fw = 95%) and was used as a probe. Moreover, the TLC-based test strips loaded with the probe not only exhibited the reversible fluorescence response to amine/acid vapor but also showed sensitive and selective fluorescence response towards Cu2+. Furthermore, the fluorescence titration experiment between the probe and Cu2+ in THF/water mixtures (fw = 95%, pH = 7.4) revealed that the detection limit was 1.18 × 10-7 M and the binding constant was 1.59 × 105. Job's plot experiment and HR-MS analysis revealed the 2:1 binding stoichiometry of the probe with Cu2+. The method enabled real-time assessment for Cu2+ in real water samples. This study could offer insightful opinions on the development of long-wavelength emissive luminogens based on α-cyanostilbene.

Simultaneous photoacoustic and ultrasound imaging: A review

Photoacoustic imaging (PAI) is an emerging biomedical imaging technique that combines the advantages of optical and ultrasound imaging, enabling the generation of images with both optical resolution and acoustic penetration depth. By leveraging similar signal acquisition and processing methods, the integration of photoacoustic and ultrasound imaging has introduced a novel hybrid imaging modality suitable for clinical applications. Photoacoustic-ultrasound imaging allows for non-invasive, high-resolution, and deep-penetrating imaging, providing a wealth of image information. In recent years, with the deepening research and the expanding biomedical application scenarios of photoacoustic-ultrasound bimodal systems, the immense potential of photoacoustic-ultrasound bimodal imaging in basic research and clinical applications has been demonstrated, with some research achievements already commercialized. In this review, we introduce the principles, technical advantages, and biomedical applications of photoacoustic-ultrasound bimodal imaging techniques, specifically focusing on tomographic, microscopic, and endoscopic imaging modalities. Furthermore, we discuss the future directions of photoacoustic-ultrasound bimodal imaging technology.

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.

Functionalized Green Carbon dots for Specific Detection of Copper in Human Serum Samples and Living Cells

Intracellular copper ion (Cu2+) is irreplaceable and essential in regulation of physiological and biological processes, while excessive copper from bioaccumulation may cause potential hazards to human health. Hence, effective and sensitive recognition is urgently significant to prevent over-intake of copper. In this work, a novel highly sensitive and green carbon quantum dots (Green-CQDs) were synthesized by a low-cost and facile one-step microwave auxiliary method, which utilized gallic acid, carbamide and PEG400 as carbon source, nitrogen source and surface passivation agent, respectively. The decreased fluorescence illustrated excellent linear relationship with the increasing of Cu2+ concentration in a wide range. Substantial surface amino and hydroxyl group introduced by PEG400 significantly improved selectivity and sensitivity of Green-CQDs. The surface amino chelation mechanism and fluorescence internal filtration effect were demonstrated by the restored fluorescence after addition of EDTA. Crucially, the nanosensor illustrated good cell permeability, high biocompatibility and recovery rate, significantly practical application in fluorescent imaging and biosensing of intracellular Cu2+ in HepG-2 cells, which revealed a potential and promising biological applications in early diagnosis and treatment of copper ion related disease.

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.

All-in-One Zinc-Doped Prussian Blue Nanozyme for Efficient Capture, Separation, and Detection of Copper Ion (Cu2+ ) in Complicated Matrixes

Copper is a vital micronutrient for lives and an important ingredient for bactericides and fungicides. Given its indispensable biological and agricultural roles, there is an urgent need to develop simple, affordable, and reliable methods for detecting copper in complicated matrixes, particularly in underdeveloped regions where costly standardized instruments and sample dilution procedures hinder progress. The findings that zinc-doped Prussian blue nanoparticle (ZnPB NP) exhibits exceptional efficiency in capturing and isolating copper ions, and accelerates the generation of dissolved oxygen in a solution of H2 O2 with remarkable sensitivity and selectivity, the signal of which displays a positive correlation with the copper level due to the copper-enhanced catalase-like activity of ZnPB NP, are presented. Consequently, the ZnPB NP serves as an all-in-one sensor for copper ion. The credibility of the method for copper assays in human urine and farmland soil is shown by comparing it to the standard instrumentation, yielding a coefficient of correlation (R2 = 0.9890), but the cost is dramatically reduced. This ZnPB nanozyme represents a first-generation probe for copper ion in complicated matrixes, laying the groundwork for the future development of a practical copper sensor that can be applied in resource-constrained environments.

Copper binding and protein aggregation: a journey from the brain to the human lens

Metal ions have been implicated in several proteinopathies associated to degenerative and neurodegenerative diseases. While the molecular mechanisms for protein aggregation are still under investigation, recent findings from Cryo-EM point out to polymorphisms in aggregates obtained from patients, as compared to those formed in vitro, suggesting that several factors may impact aggregation in vivo. One of these factors could be the direct binding of metal ions to the proteins engaged in aggregate formation. In this opinion article, three case studies are discussed to address the question of how metal ion binding to a peptide or protein may impact its conformation, folding, and aggregation, and how this may be relevant in understanding the polymorphic nature of the aggregates related to disease. Specifically, the impact of Cu2+ ions in the amyloid aggregation of amyloid-β and amylin (or IAPP- islet amyloid polypeptide) are discussed and then contrasted to the case of Cu2+-induced non-amyloid aggregation of human lens γ-crystallin proteins. For the intrinsically disordered peptides amyloid-β and IAPP, the impact of Cu2+ ion binding is highly dependent on the relative location of the metal binding site and the hydrophobic regions involved in β-sheet folding and amyloid formation. Further structural studies of how Cu2+ binding impacts amyloid aggregation pathways and the molecular structure of the final amyloid fibril, both, in vitro and in vivo, will certainly shed light into the molecular origins of the polymorphisms observed in diseased tissue. Finally, contrasting these cases to that of Cu2+-induced non-amyloid aggregation of γ-crystallins, it is evident that, although the impact in aggregation - and the nature of the aggregate - may differ in each system, at the molecular level there is a competition between metal ion coordination and the stability of β-sheet structures. Considering the importance of the β-sheet fold in biology, it is fundamental to understand the energetics and molecular details behind such competition. This opinion article aims to highlight future research directions in the field that can help tackle the important question of how metal ion binding may impact protein folding and aggregation and how this relates to disease.

Design of electron-donating group substituted 2-PAM analogs as antidotes for organophosphate insecticide poisoning

The use of organophosphate (OPs) pesticides is widespread in agriculture and horticulture, but these chemicals can be lethal to humans, causing fatalities and deaths each year. The inhibition of acetylcholinesterase (AChE) by OPs leads to the overstimulation of cholinergic receptors, ultimately resulting in respiratory arrest, seizures, and death. Although 2-pralidoxime (2-PAM) is the FDA-approved drug for treating OP poisoning, there is difficulty in blood-brain barrier permeation. To address this issue, we designed and evaluated a series of 2-PAM analogs by substituting electron-donating groups on the para and/or ortho positions of the pyridinium core using in silico techniques. Our PCM-ONIOM2 (MP2/6-31G*:PM7//B3LYP/6-31G*:UFF) binding energy results demonstrated that 13 compounds exhibited higher binding energy than 2-PAM. The analog with phenyl and methyl groups substituted on the para and ortho positions, respectively, showed the most favorable binding characteristics, with aromatic residues in the active site (Y124, W286, F297, W338, and Y341) and the catalytic residue S203 covalently bonding with paraoxon. The results of DS-MD simulation revealed a highly favorable apical conformation of the potent analog, which has the potential to enhance reactivation of AChE. Importantly, newly designed compound demonstrated appropriate drug-likeness properties and blood-brain barrier penetration. These results provide a rational guide for developing new antidotes to treat organophosphate insecticide toxicity.

Ratiometric and visual determination of copper ions with fluorescent nanohybrids of semiconducting polymer nanoparticles and carbon dots

Developing nanohybrid composition based fluorescent carbon dots (CDs) for ratiometric detection of copper ions is highly appealing. Herein, a ratiometric sensing platform (GCDs@RSPN) for copper ions detection has been developed by loaded green fluorescence carbon dots (GCDs) on the surface of red emission semiconducting polymer nanoparticles (RSPN) through electrostatic adsorption. The GCDs, featuring abundant amino groups, can selectively bind copper ions to induce the photoinduced electron transfer, leading to fluorescence quenching. A good linearity within the range of 0-100 μM is obtained, and the limit of detection (LOD) is 0.577 μM by using obtained GCDs@RSPN as ratiometric probe to detect copper ion. Moreover, the paper-based sensor derived from GCDs@RSPN was successfully applied for the visual detection of Cu²⁺.

Activity-Based Photoacoustic Probes for Detection of Disease Biomarkers beyond Oncology

The earliest activity-based photoacoustic (PA) probes were developed as diagnostic agents for cancer. Since this seminal work over a decade ago that specifically targeted matrix metalloproteinase-2, PA instrumentation, dye platforms, and probe designs have advanced considerably, allowing for the detection of an impressive list of cancer types. However, beyond imaging for oncology purposes, the ability to selectively visualize a given disease biomarker, which can range from aberrant enzymatic activity to the overproduction of reactive small molecules, is also being exploited to study a myriad of noncancerous disease states. In this review, we have assembled a collection of recent papers to highlight the design principles that enable activity-based sensing via PA imaging with respect to biomarker identification and strategies to trigger probe activation under specific conditions.

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.

A Photoacoustic Probe with Blood-Brain Barrier Crossing Ability for Imaging Oxidative Stress Dynamics in the Mouse Brain

Spatiotemporal assessment of the oxidative stress dynamics in the brain is crucial for understanding the molecular mechanism underlying neurodegenerative diseases. However, existing oxidative stress probes have poor blood-brain barrier permeability or poor penetration depth, making them unsuitable for brain imaging. Herein, we developed a photoacoustic probe that enables real-time imaging of oxidative stress dynamics in the mouse brain. The probe not only responds to oxidative stress in a reversible and ratiometric manner, but it can also cross the blood-brain barrier of the mouse brain. Notably, the probe displayed excellent photoacoustic imaging of oxidative stress dynamics in the brains of Parkinson's disease mouse models. In addition, we investigated the antioxidant properties of natural polyphenols in the brain of a Parkinson's disease mouse model using the probe as an imaging agent and suggested the potential of the probe for screening anti-oxidative stress agents.

Photostable Small-Molecule NIR-II Fluorescent Scaffolds that Cross the Blood-Brain Barrier for Noninvasive Brain Imaging

The second near-infrared (NIR-II, 1000-1700 nm) fluorescent probes have significant advantages over visible or NIR-I (600-900 nm) imaging for both depth of penetration and level of resolution. Since the blood-brain barrier (BBB) prevents most molecules from entering the central nervous system, NIR-II dyes with large molecular frameworks have limited applications for brain imaging. In this work, we developed a series of boron difluoride (BF₂) formazanate NIR-II dyes, which had tunable photophysical properties, ultrahigh photostability, excellent biological stability, and strong brightness. Modulation of the aniline moiety of BF₂ formazanate dyes significantly enhances their abilities to cross the BBB for noninvasive brain imaging. Furthermore, the intact mouse brain imaging and dynamic dye diffusion across the BBB were monitored using these BF₂ formazanate dyes in the NIR-II region. In murine glioblastoma models, these dyes can differentiate tumors from normal brain tissues. We anticipate that this new type of small molecule will find potential applications in creating probes and drugs relevant to theranostic for brain pathologies.

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.

Potential of fluorescent nanoprobe in diagnosis and treatment of Alzheimer's disease

Alzheimer's disease (AD) is well known for its insidious nature, slow progression and high incidence as a neurodegenerative disease. In the past, diagnosis of AD mainly depended on analysis of a patient's cognitive ability and behavior. Without a unified standard for analysis methods, this is prone to produce incorrect diagnoses. Currently, definitive diagnosis mainly relies on histopathological examination. Because of the advantages of precision, noninvasiveness, low toxicity and high spatiotemporal resolution, fluorescent nanoprobes are suitable for the early diagnosis of AD. This review summarizes the research progress of different kinds of fluorescent nanoprobes for AD diagnosis and therapy in recent years and provides an outlook on the development prospects of fluorescent nanoprobes.

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.

Simultaneous Sensing of H2S and ATP with a Two-Photon Fluorescent Probe in Alzheimer's Disease: toward Understanding Why H2S Regulates Glutamate-Induced ATP Dysregulation

Energy deprivation and reduced levels of hydrogen sulfide (H₂S) in the brain is closely associated with Alzheimer's disease (AD). However, there is currently no fluorescent probe for precise exploration of both H₂S and adenosine triphosphate (ATP) to directly demonstrate their relationship and their dynamic pattern changes. Herein, we developed a two-photon fluorescent probe, named AD-3, to simultaneously image endogenous H₂S and ATP from two emission channels of fluorescent signals in live rat brains with AD. The probe achieved excellent selectivity and good detection linearity for H₂S in the 0-100 μM concentration range and ATP in the 2-5 mM concentration range, respectively, with a detection limit of 0.19 μM for H₂S and 0.01 mM for ATP. Fluorescence imaging in live cells reveals that such probe could successfully apply for simultaneous imaging and accurate quantification of H₂S and ATP in neuronal cells. Further using real-time quantitative polymerase chain reaction and Western blots, we confirmed that H₂S regulates ATP synthesis by acting on cytochrome C, cytochrome oxidase subunit 3 of complex IV, and protein 6 of complex I in the mitochondrial respiratory chain. Subsequently, we constructed a high-throughput screening platform based on AD-3 probe to rapidly screen the potential anti-AD drugs to control glutamate-stimulated oxidative stress associated with abnormal H₂S and ATP levels. Significantly, AD-3 probe was found capable of imaging of H₂S and ATP in APP/PS1 mice, and the concentration of H₂S and ATP in the AD mouse brain was found to be lower than that in wild-type mice.

Looking deep inside tissue with photoacoustic molecular probes: a review

Significance

Deep tissue noninvasive high-resolution imaging with light is challenging due to the high degree of light absorption and scattering in biological tissue. Photoacoustic imaging (PAI) can overcome some of the challenges of pure optical or ultrasound imaging to provide high-resolution deep tissue imaging. However, label-free PAI signals from light absorbing chromophores within the tissue are nonspecific. The use of exogeneous contrast agents (probes) not only enhances the imaging contrast (and imaging depth) but also increases the specificity of PAI by binding only to targeted molecules and often providing signals distinct from the background.

Aim

We aim to review the current development and future progression of photoacoustic molecular probes/contrast agents.

Approach

First, PAI and the need for using contrast agents are briefly introduced. Then, the recent development of contrast agents in terms of materials used to construct them is discussed. Then, various probes are discussed based on targeting mechanisms, in vivo molecular imaging applications, multimodal uses, and use in theranostic applications.

Results

Material combinations are being used to develop highly specific contrast agents. In addition to passive accumulation, probes utilizing activation mechanisms show promise for greater controllability. Several probes also enable concurrent multimodal use with fluorescence, ultrasound, Raman, magnetic resonance imaging, and computed tomography. Finally, targeted probes are also shown to aid localized and molecularly specific photo-induced therapy.

Conclusions

The development of contrast agents provides a promising prospect for increased contrast, higher imaging depth, and molecularly specific information. Of note are agents that allow for controlled activation, explore other optical windows, and enable multimodal use to overcome some of the shortcomings of label-free PAI.

Advances of functional nanomaterials for magnetic resonance imaging and biomedical engineering applications

Functional nanomaterials have been widely used in biomedical fields due to their good biocompatibility, excellent physicochemical properties, easy surface modification, and easy regulation of size and morphology. Functional nanomaterials for magnetic resonance imaging (MRI) can target specific sites in vivo and more easily detect disease-related specific biomarkers at the molecular and cellular levels than traditional contrast agents, achieving a broad application prospect in MRI. This review focuses on the basic principles of MRI, the classification, synthesis and surface modification methods of contrast agents, and their clinical applications to provide guidance for designing novel contrast agents and optimizing the contrast effect. Furthermore, the latest biomedical advances of functional nanomaterials in medical diagnosis and disease detection, disease treatment, the combination of diagnosis and treatment (theranostics), multi-model imaging and nanozyme are also summarized and discussed. Finally, the bright application prospects of functional nanomaterials in biomedicine are emphasized and the urgent need to achieve significant breakthroughs in the industrial transformation and the clinical translation is proposed. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Diagnostic Tools > Diagnostic Nanodevices Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

A multifunctional metal regulator as the potential theranostic agent: Design, synthesis, anti-AD activities and metallic ion sensing properties

Although there is no cure for Alzheimer's disease (AD) due to its complex pathogenesis, early detection and treatment can help delay the development of the disease. So, it is necessary to develop multifunctional metal regulators that can integrate the therapeutics and diagnostics effect against AD. In this work, N-(anthracene-9-ylmethylene)benzohydrazide (probe 1), a fluorescent probe with imine and carbonyl as chelating sites was designed and synthesized. Results showed that 1 had good activities related to AD, such as regulation of metal homeostasis, inhibition of β-amyloid (Aβ) aggregation and scavenging of reactive oxygen species. The selectivity experiment showed that probe 1 had a good recognition effect on Cu²⁺. Fluorescence imaging assay also indicated that probe 1 had a good fluorescence imaging effect on Cu²⁺ in living cells. Furthermore, probe 1 had showed no cytotoxicity and good BBB permeability. These results indicated that probe 1 had potential diagnostic and therapeutic capabilities, and can be used as the multifunctional theranostic agent for AD.

The Schiff Base Probe With J-aggregation Induced Emission for Selective Detection of Cu2

Here, three Schiff bases 3a-c, differing by the substitutions (-H, -Cl, and -N(CH₃)₂) on the phenyl ring, have been designed and synthesized via the reaction of ortho-aminophenol with benzaldehyde, 2,4-dichlorobenzaldehyde and para-dimethylamine benzaldehyde in 1:1 molar ratio with favourable yields of 89-92%, respectively. Their structural characterizations were studied by FT-IR, NMR, MALDI-MS and elemental analysis. The fluorescence behaviours of compounds 3a and 3b exhibited a severe aggregation caused quenching (ACQ) effect in EtOH/water system. On the contrary, compound 3c had an obvious J-aggregation induced emission (AIE) feature in EtOH/water mixture (v/v = 1:1), and exhibited excellent sensitivity and anti-interference towards Cu²⁺ with the limit of detection (LOD) of 1.35 × 10^-8 M. Job's plot analysis and MS spectroscopic study revealed the 2:1 complexation of probe 3c and Cu²⁺. In addition, probe 3c was successfully applied to the determination of Cu²⁺ in real aqueous samples.

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.

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.

Peptide-based semiconducting polymer nanoparticles for osteosarcoma-targeted NIR-II fluorescence/NIR-I photoacoustic dual-model imaging and photothermal/photodynamic therapies

Background

The overall survival rate of osteosarcoma (OS) patients has not been improved for 30 years, and the diagnosis and treatment of OS is still a critical issue. To improve OS treatment and prognosis, novel kinds of theranostic modalities are required. Molecular optical imaging and phototherapy, including photothermal therapy (PTT) and photodynamic therapy (PDT), are promising strategies for cancer theranostics that exhibit high imaging sensitivity as well as favorable therapeutic efficacy with minimal side effect. In this study, semiconducting polymer nanoparticles (SPN-PT) for OS-targeted PTT/PDT are designed and prepared, using a semiconducting polymer (PCPDTBT), providing fluorescent emission in the second near-infrared window (NIR-II, 1000 - 1700 nm) and photoacoustic (PA) signal in the first near-infrared window (NIR-I, 650 - 900 nm), served as the photosensitizer, and a polyethylene glycolylated (PEGylated) peptide PT, providing targeting ability to OS.

Results

The results showed that SPN-PT nanoparticles significantly accelerated OS-specific cellular uptake and enhanced therapeutic efficiency of PTT and PDT effects in OS cell lines and xenograft mouse models. SPN-PT carried out significant anti-tumor activities against OS both in vitro and in vivo.

Conclusions

Peptide-based semiconducting polymer nanoparticles permit efficient NIR-II fluorescence/NIR-I PA dual-modal imaging and targeted PTT/PDT for OS.

Graphic Abstract

Photoacoustic imaging of elevated glutathione in models of lung cancer for companion diagnostic applications

Companion diagnostics (CDx) are powerful tests that can provide physicians with crucial biomarker information that can improve treatment outcomes by matching therapies to patients. Here, we report a photoacoustic imaging-based CDx (PACDx) for the selective detection of elevated glutathione (GSH) in a lung cancer model. GSH is abundant in most cells, so we adopted a physical organic chemistry approach to precisely tune the reactivity to distinguish between normal and pathological states. To evaluate the efficacy of PACDx in vivo, we designed a blind study where photoacoustic imaging was used to identify mice bearing lung xenografts. We also employed PACDx in orthotopic lung cancer and liver metastasis models to image GSH. In addition, we designed a matching prodrug, PARx, that uses the same SNAr chemistry to release a chemotherapeutic with an integrated PA readout. Studies demonstrate that PARx can inhibit tumour growth without off-target toxicity in a lung cancer xenograft model.

Recent Technical Progression in Photoacoustic Imaging—Towards Using Contrast Agents and Multimodal Techniques

For combining optical and ultrasonic imaging methodologies, photoacoustic imaging (PAI) is the most important and successful hybrid technique, which has greatly contributed to biomedical research and applications. Its theoretical background is based on the photoacoustic effect, whereby a modulated or pulsed light is emitted into tissue, which selectively absorbs the optical energy of the light at optical wavelengths. This energy produces a fast thermal expansion in the illuminated tissue, generating pressure waves (or photoacoustic waves) that can be detected by ultrasonic transducers. Research has shown that optical absorption spectroscopy offers high optical sensitivity and contrast for ingredient determination, for example, while ultrasound has demonstrated good spatial resolution in biomedical imaging. Photoacoustic imaging combines these advantages, i.e., high contrast through optical absorption and high spatial resolution due to the low scattering of ultrasound in tissue. In this review, we focus on advances made in PAI in the last five years and present categories and key devices used in PAI techniques. In particular, we highlight the continuously increasing imaging depth achieved by PAI, particularly when using exogenous reagents. Finally, we discuss the potential of combining PAI with other imaging techniques.

Activity-based photoacoustic probe for biopsy-free assessment of copper in murine models of Wilson's disease and liver metastasis

The development of high-performance photoacoustic (PA) probes that can monitor disease biomarkers in deep tissue has the potential to replace invasive medical procedures such as a biopsy. However, such probes must be optimized for in vivo performance and exhibit an exceptional safety profile. In this study, we have developed PACu-1, a PA probe designed for biopsy-free assessment (BFA) of hepatic Cu via photoacoustic imaging. PACu-1 features a Cu(I)-responsive trigger appended to an aza-BODIPY dye platform that has been optimized for ratiometric sensing. Owing to its excellent performance, we were able to detect basal levels of Cu in healthy wild-type mice as well as elevated Cu in a Wilson's disease model and in a liver metastasis model. To showcase the potential impact of PACu-1 for BFA, we conducted two blind studies in which we were able to successfully identify Wilson's disease animals from healthy control mice in each instance.

Photoacoustic probe of targeting intracellular Staphylococcus aureus infection with signal-enhanced by self-assembly

In this chapter, we introduce the photoacoustic imaging probe in detecting the infection of bacterial in host cells. We give detailed protocols for the synthesis and confirmation of a PA probe which response to caspase-1 and then self-assembly to enhance the signal. With this PA contrast agent, we may provide a new approach for an intracellular bacterial infection with the selective and sensitive diagnosis.

A TME-activated in situ nanogenerator for magnetic resonance/fluorescence/photoacoustic imaging

With the rapid development of biomedical imaging, non-invasive imaging method particularly has been widely used in clinical diagnosis. Different imaging methods have their own advantages, such as higher resolution of optical imaging, deeper penetration of acoustic imaging, high resolution photoacoustic imaging (PAI), and multi-parameter guidance of magnetic resonance imaging (MRI). Recent years, multimodal MRI, fluorescence imaging, PAI and others have been verified to play an important role in the field of molecular imaging and provided detail information for accurate in vivo diagnosis. Therefore, the design of multimodal probe that can integrate the above advantages to carry out combined imaging will be more accurate to assist diagnosis and treatment. While tumor microenvironment (TME) is highly critical in validating and optimizing current therapeutic strategies. Herein, we highlight the TME-triggered supramolecular system as an in situ nano-generator for multimodal imaging-guided treatment. The experimental protocols on the PAI/fluorescence imaging/MRI-based diagnosis are described in this chapter.

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.

Photoacoustic Neuroimaging - Perspectives on a Maturing Imaging Technique and its Applications in Neuroscience

A prominent goal of neuroscience is to improve our understanding of how brain structure and activity interact to produce perception, emotion, behavior, and cognition. The brain's network activity is inherently organized in distinct spatiotemporal patterns that span scales from nanometer-sized synapses to meter-long nerve fibers and millisecond intervals between electrical signals to decades of memory storage. There is currently no single imaging method that alone can provide all the relevant information, but intelligent combinations of complementary techniques can be effective. Here, we thus present the latest advances in biomedical and biological engineering on photoacoustic neuroimaging in the context of complementary imaging techniques. A particular focus is placed on recent advances in whole-brain photoacoustic imaging in rodent models and its influential role in bridging the gap between fluorescence microscopy and more non-invasive techniques such as magnetic resonance imaging (MRI). We consider current strategies to address persistent challenges, particularly in developing molecular contrast agents, and conclude with an overview of potential future directions for photoacoustic neuroimaging to provide deeper insights into healthy and pathological brain processes.

Tapered Mach-Zehnder interferometer based on PbS quantum dots modified by polymers for copper ion sensing

An optical fiber interferometer coated with PbS quantum dots (QDs) was developed for copper ion (${{\rm{Cu}}^{2 +}}$) detection. The QDs were modified by a multifunctional copolymer that enabled QD surface ligation, dispersion, and coordination with ${{\rm{Cu}}^{2 +}}$. ${{\rm{Cu}}^{2 +}}$ coordination with the polymer induced changes in the surrounding refractive index of the interferometer. The sensor was highly selective for ${{\rm{Cu}}^{2 +}}$ and showed a linear detection range of 0-1000 µM with a limit of detection of 2.20 µM in both aqueous and biological solutions.

Responsive optical probes for deep-tissue imaging: Photoacoustics and second near-infrared fluorescence

Optical imaging has played a vital role in development of biomedicine and image-guided theragnostic. Nevertheless, the clinical translation of optical molecular imaging for deep-tissue visualization is still limited by poor signal-to-background ratio and low penetration depth owing to light scattering and tissue autofluorescence. Hence, to facilitate precise diagnosis and accurate surgery excision in clinical practices, the responsive optical probes (ROPs) are broadly designed for specific reaction with biological analytes or disease biomarkers via chemical/physical interactions for photoacoustic and second near-infrared fluorescence (NIR-II, 900-1700 nm) fluorescence imaging. Herein, the recent advances in the development of ROPs including molecular design principles, activated mechanisms and treatment responses for photoacoustic and NIR-II fluorescence imaging are reviewed. Furthermore, the present challenges and future perspectives of ROPs for deep-tissue imaging are also discussed.

Development of NIR-II Photoacoustic Probes Tailored for Deep-Tissue Sensing of Nitric Oxide

Photoacoustic (PA) imaging has emerged as a reliable in vivo technique for diverse biomedical applications ranging from disease screening to analyte sensing. Most contemporary PA imaging agents employ NIR-I light (650-900 nm) to generate an ultrasound signal; however, there is significant interference from endogenous biomolecules such as hemoglobin that are PA active in this window. Transitioning to longer excitation wavelengths (i.e., NIR-II) reduces the background and facilitates the detection of low abundance targets (e.g., nitric oxide, NO). In this study, we employed a two-phase tuning approach to develop APNO-1080, a NIR-II NO-responsive probe for deep-tissue PA imaging. First, we performed Hammett and Brønsted analyses to identify a highly reactive and selective aniline-based trigger that reacts with NO via N-nitrosation chemistry. Next, we screened a panel of NIR-II platforms to identify chemical structures that have a low propensity to aggregate since this can diminish the PA signal. In a head-to-head comparison with a NIR-I analogue, APNO-1080 was 17.7-fold more sensitive in an in vitro tissue phantom assay. To evaluate the deep-tissue imaging capabilities of APNO-1080 in vivo, we performed PA imaging in an orthotopic breast cancer model and a heterotopic lung cancer model. Relative to control mice not bearing tumors, the normalized turn-on response was 1.3 ± 0.12 and 1.65 ± 0.07, respectively.

Brain-Targeted Dual Site-Selective Functionalized Poly(β-Amino Esters) Delivery Platform for Nerve Regeneration

Brain injuries are devastating central nervous system diseases, resulting in cognitive, motor, and sensory dysfunctions. However, clinical therapeutic options are still limited for brain injuries, indicating an urgent need to investigate new therapies. Furthermore, the efficient delivery of therapeutics across the blood-brain barrier (BBB) to the brain is a serious problem. In this study, a facile strategy of dual site-selective functionalized (DSSF) poly(β-amino esters) was developed using bio-orthogonal chemistry for promoting brain nerve regeneration. Fluorescence colocalization studies demonstrated that these proton-sponge DSSF poly(β-amino esters) targeted mitochondria through electrostatic interactions. More importantly, this delivery system could effectively accumulate in the injured brain sites and accelerate the recovery of the injured brain. Finally, this DSSF poly(β-amino esters) platform may provide a new methodology for the construction of dual regioselective carriers in protein/peptide delivery and tissue engineering.