Frontiers in Probing Alzheimer's Disease Biomarkers with Fluorescent Small Molecules

Red emissive fluorescent carbon dots based on ternary carbon source for imaging α-synuclein fibrils

The misfolding and aggregation of α-synuclein monomers usually cause the occurrence and development of Parkinson's disease (PD). It is important to develop effective methods for detection of α-synuclein aggregates. Carbon dots (CDs) could be the potential fluorescence probe for this purpose owing to their appreciated optical properties. However, undefined structure of CDs and complicated three-dimensional structure of protein severely hindered the design of fluorescence probe towards protein aggregates. Herein, a red emissive fluorescent amphiphilic CD, named as CL-9, was designed with a high sensitivity to α-synuclein fibrils by a one-step heating process, using the ternary carbon source, including Congo red, l-tryptophan and urea. The CL-9 exhibited turn-on red emissive fluorescence towards α-synuclein fibril, but remained no change towards its monomer. Compared with the original Congo red dye, CL-9 exhibited stronger turn-on red fluorescence towards α-synuclein fibrils with better anti-photobleaching resistance, biocompatibility and signal-to-noise ratio. The CL-9 was successful as a fluorescent probe to image α-synuclein fibrils in NL-5901 C. elegans. The present study provided a feasible approach using the multiple carbon sources to construct the CDs based fluorescence probe targeting amyloid proteins.

Pyrene Functionalized Norbornadiene-Quadricyclane Fluorescent Photoswitches: Characterization of their Spectral Properties and Application in Imaging of Amyloid Beta Plaques

This study presents the synthesis and characterization of two fluorescent norbornadiene (NBD) photoswitches, each incorporating two conjugated pyrene units. Expanding on the limited repertoire of reported photoswitchable fluorescent NBDs, we explore their properties with a focus on applications in bioimaging of amyloid beta (Aβ) plaques. While the fluorescence emission of the NBD decreases upon photoisomerization, aligning with what has been previously reported, for the first time we observed luminescence after irradiation of the quadricyclane (QC) isomer. We deduce how the observed emission is induced by photoisomerization to the excited state of the parent isomer (NBD) which is then the emitting species. Thorough characterizations including NMR, UV-Vis, fluorescence, X-ray structural analysis and density functional theory (DFT) calculations provide a comprehensive understanding of these systems. Notably, one NBD-QC system exhibits exceptional durability. Additionally, these molecules serve as effective fluorescent stains targeting Aβ plaques in situ, with observed NBD/QC switching within the plaques. Molecular docking simulations explore NBD interactions with amyloid, unveiling novel binding modes. These insights mark a crucial advancement in the comprehension and design of future photochromic NBDs for bioimaging applications and beyond, emphasizing their potential in studying and addressing protein aggregates.

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.

Recent Advances in Fluorescent Theranostics for Alzheimer's Disease: A Comprehensive Survey on Design, Synthesis, and Properties

Alzheimer's disease (AD) is the most common form of neurodegenerative dementia that is rapidly becoming a major health problem, especially in developed countries because of their increasing life expectancy. Two main problems are often associated with the disease: (i) the absence of a widely accessible "gold-standard" for early diagnosis and (ii) lack of effective therapies with disease-modifying effects. The recent success of the monoclonal antibody lecanemab played an important role not only in clarifying a possible druggable pathway but also in spelling the revival of small molecule drug discovery. Unlike bulky biologics, small molecules are structurally less complex, generally cheaper, and compatible with at-home oral consumption, making it feasible for people to start their drug regimen early and stay on it longer. In this sense, small-molecule near-infrared fluorescent theranostics have been gaining more and more attention from the scientific community, as they have the potential to simultaneously provide diagnostic outputs and deliver therapeutic action, paving the way toward personalized medicine in AD patients. They also have the potential to shift the diagnostic "status-quo" from expensive and limited-access PET radiotracers toward inexpensive and handy imaging tools widely available for primary patient screening and preclinical animal studies. Herein, we review the most recent advances in the field of fluorescent theranostics for Alzheimer's disease, detailing their design strategies, synthetic approaches and imaging and therapeutic properties in vitro and in vivo. With this Review, we intend to provide a milestone in the acquired knowledge in the field of AD theranostics, encouraging the future development of properly designed theranostic compounds with improved chances to reach clinical applications.

Early onset diagnosis in Alzheimer's disease patients via amyloid-β oligomers-sensing probe in cerebrospinal fluid

Amyloid-β (Aβ) oligomers are implicated in the onset of Alzheimer's disease (AD). Herein, quinoline-derived half-curcumin-dioxaborine (Q-OB) fluorescent probe was designed for detecting Aβ oligomers by finely tailoring the hydrophobicity of the biannulate donor motifs in donor-π-acceptor structure. Q-OB shows a great sensing potency in dynamically monitoring oligomerization of Aβ during amyloid fibrillogenesis in vitro. In addition, we applied this strategy to fluorometrically analyze Aβ self-assembly kinetics in the cerebrospinal fluids (CSF) of AD patients. The fluorescence intensity of Q-OB in AD patients' CSF revealed a marked change of log (I/I0) value of 0.34 ± 0.13 (cognitive normal), 0.15 ± 0.12 (mild cognitive impairment), and 0.14 ± 0.10 (AD dementia), guiding to distinguish a state of AD continuum for early diagnosis of AD. These studies demonstrate the potential of our approach can expand the currently available preclinical diagnostic platform for the early stages of AD, aiding in the disruption of pathological progression and the development of appropriate treatment strategies.

Golgi-targeting viscosity probe for the diagnosis of Alzheimer's disease

Early diagnosis and intervention of Alzheimer's disease (AD) are particularly important to delay the pathological progression. Although fluorescent probes have been widely employed for investigating and diagnosing AD, their biological applications are significantly restricted due to the low penetration ability of the blood-brain barrier (BBB) in vivo. In this study, we reported the first Golgi-targeted two-photon (TP) fluorescent probe, DCM-DH, for detecting viscosity in the Golgi apparatus. The probe was rationally designed to exhibit superior analytical performance including high sensitivity, specific Golgi-targeting, efficient BBB penetration ability, and deep tissue penetration (247 μm) in the brains of AD model mice. Using the probe, we demonstrated that the fluorescence intensity in the human liver cancer cell (HepG2 cells) was higher than that of human normal liver cell (LO2 cells), and the brain viscosity of AD model mice increased significantly. We anticipate that this competent tool could be easily extended to other AD biomarkers for fundamental research on this detrimental disease.

A conformationally-locked p-hydroxybenzylidene imidazolinone derivative for detecting Aβ42 aggregation

Alzheimer's disease (AD) is a common type of neurodegenerative disease, which can only be symptomatically relieved but does not yet have a cure. Among the different Aβ species, amyloid-β 42 (Aβ42) aggregates are proposed to be more neurotoxic than that of Aβ40, and oligomeric Aβ42 is thought to play a harmful role in the pathophysiology of AD. Therefore, the detection of Aβ42 aggregation is very meaningful in the AD field. We herein report a conformationally-locked p- hydroxybenzylidene imidazolinone derivative, BDI, which exhibits selectivity and specificity towards Aβ42 aggregation and remarkable fluorescent enhancement with a large Stokes shift (more than 100 nm). In the fluorescent co-localization study, BDI can sensitively detect a large population of Aβ42 aggregation over that of Aβ40 in the brain tissues of AD transgenic mouse models. Therefore, this new probe could provide a useful tool for the rapid detection of important Aβ species in AD.

Synthesis and biological evaluation of indane-based fluorescent probes for detection of amyloid-β aggregates in Alzheimer's disease

In this article, the development of fluorescent imaging probes for the detection of Alzheimer's disease (AD)-associated protein aggregates is described. Indane derivatives with a donor-π-acceptor (D-π-A) structure were designed and synthesized. The probes were evaluated for their ability to bind to β-amyloid (Aβ) protein aggregates, which are a key pathological hallmark of AD. The results showed that several probes exhibited significant changes in fluorescence intensity at wavelengths greater than 600 nm when they were bound to Aβ aggregates compared to the Aβ monomeric form. Among the tested probes, four D-π-A type indane derivatives showed promising binding selectivity to Aβ aggregates over non-specific proteins such as bovine serum albumin (BSA). The molecular docking study showed that our compounds were appropriately located along the Aβ fibril axis through the hydrophobic tunnel structure. Further analysis revealed that the most active compound having dimethylaminopyridyl group as an election donor and dicyano group as an electron acceptor could effectively stain Aβ plaques in brain tissue samples from AD transgenic mice. These findings suggest that our indane-based compounds have the potential to serve as fluorescent probes for the detection and monitoring of Aβ aggregation in AD.

Rational design synthesis and evaluation of a novel near-infrared fluorescent probe for selective imaging of amyloid-β aggregates in Alzheimer's disease

Alzheimer's disease (AD) is a degenerative neurological disorder that remains incurable to date, seriously affecting the quality of life and health of those affected. One of the key neuropathological hallmarks of AD is the formation of amyloid-β (Aβ) plaques. Near-infrared (NIR) probes that possess a large Stokes shift show great potential for imaging of Aβ plaques in vivo and in vitro. Herein, we proposed a rational strategy for design and synthesis of a series of NIR fluorescent probes that incorporate a tricarbonitrile group as a strong electron-withdrawing group (EWG) to enable NIR emission and large Stokes shift for optimal imaging of Aβ plaques. The probe TCM-UM exhibited remarkable in vitro performance, including strong NIR emission (λem = 670 nm), large Stokes shift (120-245 nm), and its affinity for Aβ42 aggregates (Kd = 43.78 ± 4.09 nM) was superior to the commercially available probe Thioflavin T (ThT, Kd = 896.04 ± 33.43 nM). Further, TCM-UM was selected for imaging Aβ plaques in brain tissue slices and APP/PS1 transgenic (AD) mice, the results indicated that TCM-UM had an excellent ability to penetrate the blood-brain barrier (BBB) compared with ThT, and it could effectively distinguish wild-type (Wt) mice and APP/PS1 transgenic (AD) mice.

RNA-Selective Small-Molecule Ligands: Recent Advances in Live-Cell Imaging and Drug Discovery

RNA structures, including those formed from coding and noncoding RNAs, alternative to protein-based drug targets, could be a promising target of small molecules for drug discovery against various human diseases, particularly in anticancer, antibacterial and antivirus development. The normal cellular activity of cells is critically dependent on the function of various RNA molecules generated from DNA transcription. Moreover, many studies support that mRNA-targeting small molecules may regulate the synthesis of disease-related proteins via the non-covalent mRNA-ligand interactions that do not involve gene modification. RNA-ligand interaction is thus an attractive approach to address the challenge of "undruggable" proteins in drug discovery because the intracellular activity of these proteins is hard to be suppressed with small molecule ligands. We selectively surveyed a specific area of RNA structure-selective small molecule ligands in fluorescence live cell imaging and drug discovery because the area was currently underexplored. This state-of-the-art review thus mainly focuses on the research published within the past three years and aims to provide the most recent information on this research area; hopefully, it could be complementary to the previously reported reviews and give new insights into the future development on RNA-specific small molecule ligands for live cell imaging and drug discovery.

Multifunctional theranostic carbazole-based cyanine for real-time imaging of amyloid-β and therapeutic treatment of multiple pathologies in Alzheimer's disease

Alzheimer's disease (AD) is a progressive and irreversible neurodegenerative disorder characterized by the synaptic and neuronal loss, which results in cognitive impairment in particular learning and memory. Currently, AD is incurable and no single confirmative test can clinically be used to diagnose AD. In light of the complex and multifactorial nature of AD etiology, the development of multifunctional/multi-target drugs that act on multiple pathological pathways and mechanisms shows great therapeutic potential for intervention of this devastating disease. We report herein a multifunctional theranostic cyanine, SLCOOH, which serves not only as a highly sensitive fluorescent probe for real-time imaging of amyloid-β (Aβ) contents in different age groups of transgenic (Tg) AD mice but also as an effective therapeutic agent for early AD intervention via multiple pathological targets in the AD mouse model. Remarkably, treatment with SLCOOH gives rise to multiple therapeutic benefits, including the amelioration of cognitive decline, a reduction in Aβ levels, a decrease in hyperphosphorylated tau proteins and tau depositions, and the alleviation of synaptic loss and dysfunctions in young triple Tg AD mice. Our results have demonstrated that in addition to superior Aβ imaging capability, SLCOOH exhibits versatile and effective multiple modes of drug action, signifying outstanding therapeutic potential to treat early onset AD. Our work also paves the way for the development of effective Aβ-targeted theranostic agents for AD.

Investigating New Applications of a Photoswitchable Fluorescent Norbornadiene as a Multifunctional Probe for Delineation of Amyloid Plaque Polymorphism

Amyloid beta (Aβ) plaques are a major pathological hallmark of Alzheimer's disease (AD) and constitute of structurally heterogenic entities (polymorphs) that have been implicated in the phenotypic heterogeneity of AD pathology and pathogenesis. Understanding amyloid aggregation has been a critical limiting factor to gain understanding of AD pathogenesis, ultimately reflected in that the underlying mechanism remains elusive. We identified a fluorescent probe in the form of a turn-off photoswitchable norbornadiene derivative (NBD1) with several microenvironment-sensitive properties that make it relevant for applications within advanced fluorescence imaging, for example, multifunctional imaging. We explored the application of NBD1 for in situ delineation of structurally heterogenic Aβ plaques in transgenic AD mouse models. NBD1 plaque imaging shows characteristic broader emission bands in the periphery and more narrow emission bands in the dense cores of mature cored plaques. Further, we demonstrate in situ photoisomerization of NBD1 to quadricyclane and thermal recovery in single plaques, which is relevant for applications within both functional and super-resolution imaging. This is the first time a norbornadiene photoswitch has been used as a probe for fluorescence imaging of Aβ plaque pathology in situ and that its spectroscopic and switching properties have been studied within the specific environment of senile Aβ plaques. These findings open the way toward new applications of NBD-based photoswitchable fluorescent probes for super-resolution or dual-color imaging and multifunctional microscopy of amyloid plaque heterogeneity. This could allow to visualize Aβ plaques with resolution beyond the diffraction limit, label different plaque types, and gain insights into their physicochemical composition.

An ESIPT-Based Fluorescent Probe for Aqueous Cu+ Detection through Strip, Nanofiber and Living Cells

Constructed on the benzothiazole-oxanthracene structure, a fluorescent probe RBg for Cu⁺ was designed under the ESIPT mechanism and synthesized by incorporating amide bonds as the connecting group and glyoxal as the identifying group. Optical properties revealed a good sensitivity and a good linear relationship of the probe RBg with Cu⁺ in the concentration range of [Cu⁺] = 0-5.0 μmol L^-1. Ion competition and fluorescence-pH/time stability experiments offered further possibilities for dynamic Cu⁺ detection in an aqueous environment. HRMS analysis revealed a possible 1:1 combination of RBg and Cu⁺. In addition, colorimetric Cu⁺ detection and lysosome-targeted properties of the probe RBg were analyzed through RBg-doped PVDF nanofiber/test strips and RBg-Mito/Lyso trackers that were co-stained in living HeLa cells, enabling the probe's future applications as real-time detection methods for dynamic Cu⁺ tracking in the lysosomes and Cu⁺ detection under diversified conditions.

Preparation of near-infrared AIEgen-active fluorescent probes for mapping amyloid-β plaques in brain tissues and living mice

Fibrillar aggregates of the amyloid-β protein (Aβ) are the main component of the senile plaques found in brains of patients with Alzheimer's disease (AD). Development of probes allowing the noninvasive and high-fidelity mapping of Aβ plaques in vivo is critical for AD early detection, drug screening and biomedical research. QM-FN-SO3 (quinoline-malononitrile-thiophene-(dimethylamino)phenylsulfonate) is a near-infrared aggregation-induced-emission-active fluorescent probe capable of crossing the blood-brain barrier (BBB) and ultrasensitively lighting up Aβ plaques in living mice. Herein, we describe detailed procedures for the two-stage synthesis of QM-FN-SO3 and its applications for mapping Aβ plaques in brain tissues and living mice. Compared with commercial thioflavin (Th) derivatives ThT and ThS (the gold standard for detection of Aβ aggregates) and other reported Aβ plaque fluorescent probes, QM-FN-SO3 confers several advantages, such as long emission wavelength, large Stokes shift, ultrahigh sensitivity, good BBB penetrability and miscibility in aqueous biological media. The preparation of QM-FN-SO3 takes ~2 d, and the confocal imaging experiments for Aβ plaque visualization, including the preparation for mouse brain sections, take ~7 d. Notably, acquisition and analyses for in vivo visualization of Aβ plaques in mice can be completed within 1 h and require only a basic knowledge of spectroscopy and chemistry.

A PET Fluorescent Probe for Dynamic Pd2+ Tracking with Imaging Applications in the Nanofiber and Living Cells

Constructed on the moiety of a lactam screw ring, a near-infrared fluorescent probe RCya for Pd²⁺ was designed under the PET mechanism and synthesized by incorporating 2,4-dihydroxybenzaldehyde as the recognition group. Dynamic detection of aqueous Pd²⁺ by the probe RCya could be accomplished through ion competition, linear response, fluorescence-pH/time stabilities, and other optical tests. Moreover, the high selectivity, low cytotoxicity, cell permeability, and lysosome accumulation properties of RCya enabled the imaging applications on solid-state RCya-PAN composite nanofibers and in living cells. The recognition mechanism of probe RCya toward Pd²⁺ was further studied through simulation calculation and MS analysis.

Trace Elements Open a New Direction for the Diagnosis of Atherosclerosis

Abnormal or excessive accumulation of adipose tissue leads to a condition called obesity. Long-term positive energy balance arises when energy intake surpasses energy expenditure, which increases the risk of metabolic and other chronic diseases, such as atherosclerosis. In industrialized countries, the prevalence of coronary heart disease is positively correlated with the human development index. Atherosclerotic cardiovascular disease (ACD) is among the primary causes of death on a global scale. There is evidence to support the notion that individuals from varied socioeconomic origins may experience varying mortality effects as a result of high blood pressure, high blood sugar, raised cholesterol levels, and high body mass index (BMI). However, it is believed that changes in the concentration of trace elements in the human body are the main contributors to the development of some diseases and the transition from a healthy to a diseased state. Metal trace elements, non-metal trace elements, and the sampling site will be examined to determine whether trace elements can aid in the diagnosis of atherosclerosis. This article will discuss whether trace elements, discussed under three sections of metal trace elements, non-metal trace elements, and the sampling site, can participate in the diagnosis of atherosclerosis.

Tracking the binding of multi-functional fluorescent tags for Alzheimer's disease using quantitative multiphoton microscopy

A recent theranostic approach to address Alzheimer's disease (AD) utilizes multifunctional targets that both tag and negate the toxicity of AD biomarkers. These compounds, which emit fluorescence with both an activation and a spectral shift in the presence of Aβ, were previously characterized with traditional fluorescence imaging for binary characterization. However, these multifunctional compounds have broad and dynamic emission spectra that are dependent on factors such as the local environment, presence of Aβ deposits, etc. Since quantitative multiphoton microscopy is sensitive to the binding dynamics of molecules, we characterized the performance of two such compounds, LS-4 and ZY-12-OMe, using Simultaneous Label-free Autofluorescence Multi-harmonic (SLAM) microscopy and Fast Optical Coherence, Autofluorescence Lifetime imaging and Second harmonic generation (FOCALS) microscopy. This study shows that the combination of quantitative multiphoton imaging with multifunctional tags for AD offers new insights into the interaction of these tags with AD biomarkers and the theranostic mechanisms.

Versatile Dicyanomethylene-Based Fluorescent Probes for the Detection of β-Amyloid in Alzheimer’s Disease: A Theoretical Perspective

Motivated by the growing demand for target chemosensors designed with diagnostic or therapeutic capability for fibrils related to amyloidosis diseases, we investigated in the present work the response mechanism of dicyanomethylene-based fluorescent probes for amyloid fibril using a combined approach, including molecular docking, quantum mechanics/molecular mechanics (QM/MM), and the quantum chemical method. Various binding modes for the probes in β-amyloid (Aβ) are discussed, and the fibril environment-induced molecular optical changes at the most stable site are compared to the fibril-free situation in aqueous environments. The results reveal that the fluorescence enhancement for the probes in Aβ observed experimentally is an average consequence over multiple binding sites. In particular, the conformational difference, including conjugation length and donor effect, significantly contributes to the optical property of the studied probes both in water and fibril. To further estimate the transition nature of the molecular photoabsorption and photoemission processes, the hole-electron distribution and the structural variation on the first excited state of the probes are investigated in detail. On the basis of the calculations, structure–property relationships for the studied chemosensors are established. Our computational approach with the ability to elucidate the available experimental results can be used for designing novel molecular probes with applications to Aβ imaging and the early diagnosis of Alzheimer’s disease.

Image-guided/improved diseases management: From immune-strategies and beyond

Timely and accurate assessment and diagnosis are extremely important and beneficial for all diseases, especially for some of the major human disease, such as cancers, cardiovascular diseases, infectious diseases, and neurodegenerative diseases. Limited by the variable disease microenvironment, early imperceptible symptoms, complex immune system interactions, and delayed clinical phenotypes, disease diagnosis and treatment are difficult in most cases. Molecular imaging (MI) techniques can track therapeutic drugs and disease sites in vivo and in vitro in a non-invasive, real-time and visible strategies. Comprehensive visual imaging and quantitative analysis based on different levels can help to clarify the disease process, pathogenesis, drug pharmacokinetics, and further evaluate the therapeutic effects. This review summarizes the application of different MI techniques in the diagnosis and treatment of these major human diseases. It is hoped to shed a light on the development of related technologies and fields.

Picomolar-sensitive β-amyloid fibril fluorophores by tailoring the hydrophobicity of biannulated π-elongated dioxaborine-dyes

The pathological origin of Alzheimer's disease (AD) is still shrouded in mystery, despite intensive worldwide research efforts. The selective visualization of β-amyloid (Aβ), the most abundant proteinaceous deposit in AD, is pivotal to reveal AD pathology. To date, several small-molecule fluorophores for Aβ species have been developed, with increasing binding affinities. In the current work, two organic small-molecule dioxaborine-derived fluorophores were rationally designed through tailoring the hydrophobicity with the aim to enhance the binding affinity for Aβ1-42 fibrils -while concurrently preventing poor aqueous solubility-via biannulate donor motifs in D-π-A dyes. An unprecedented sub-nanomolar affinity was found (K d = 0.62 ± 0.33 nM) and applied to super-sensitive and red-emissive fluorescent staining of amyloid plaques in cortical brain tissue ex vivo. These fluorophores expand the dioxaborine-curcumin-based family of Aβ-sensitive fluorophores with a promising new imaging agent.

Endoplasmic Reticulum-Targeting Near-Infrared Fluorescent Probe for CYP2J2 Activity and Its Imaging Application in Endoplasmic Reticulum Stress and Tumor

CYP2J2 as an endoplasmic reticulum (ER)-expressed vital cytochrome P450 isoform participates in the metabolism of endogenous polyunsaturated fatty acids. Its abnormal expression and function are closely related to the progress of cancer and cardiovascular diseases. Herein, an ER-targeting near-infrared (NIR) fluorescent probe ER-BnXPI was developed for monitoring CYP2J2 activity, which possessed a high selectivity and sensitivity toward CYP2J2 among various CYP450 isoforms and exhibited excellent subcellular localization for ER. Then, the CYP2J2 variation behavior under the ER stress model was imaged by ER-BnXPI in living cells and successfully used for the in vivo imaging in different tumors that well distinguished tumor tissues from para-cancerous tissues. All these findings fully demonstrated that ER-BnXPI could be used as a promising tool for exploring the physiological function of CYP2J2 and provided some novel approach for the diagnosis and therapy of CYP2J2-related vascular inflammation and cancer.

Analyses on intracellular Fe3+ with a rhodamine probe: "turn-on" response, specific recognition and bioimaging

To overcome the existing difficulty in distinguishing ferric from ferrous ions, a rhodamine-containing probe was designed to combine with Fe³⁺ based on an opening-closing transformation of the spirolactam ring in the rhodamine moiety. Through a specific and stoichiometric fluorescence response towards Fe³⁺ by 1 : 1 binding accompanied by an obvious color change in the recognition process of Fe³⁺, a "naked-eye" detection method of Fe³⁺ in an aqueous environment is possible. Theoretical calculations gave a possible recognition mechanism of the probe-Fe³⁺ system. Further cytotoxicity and bioimaging in living L929 cells suggested the probe's future applications as a real-time analytical method for intracellular Fe³⁺ in clinical diagnosis. Besides, bioimaging applications enable the dynamic labelling and tracking of Fe³⁺ in biological systems.

Rapid Biomarker Screening of Alzheimer's Disease by Interpretable Machine Learning and Graphene-Assisted Raman Spectroscopy

The study of Alzheimer's disease (AD), the most common cause of dementia, faces challenges in terms of understanding the cause, monitoring the pathogenesis, and developing early diagnoses and effective treatments. Rapid and accurate identification of AD biomarkers in the brain is critical to providing key insights into AD and facilitating the development of early diagnosis methods. In this work, we developed a platform that enables a rapid screening of AD biomarkers by employing graphene-assisted Raman spectroscopy and machine learning interpretation in AD transgenic animal brains. Specifically, we collected Raman spectra on slices of mouse brains with and without AD and used machine learning to classify AD and non-AD spectra. By contacting monolayer graphene with the brain slices, the accuracy was increased from 77% to 98% in machine learning classification. Further, using a linear support vector machine (SVM), we identified a spectral feature importance map that reveals the importance of each Raman wavenumber in classifying AD and non-AD spectra. Based on this spectral feature importance map, we identified AD biomarkers including Aβ and tau proteins and other potential biomarkers, such as triolein, phosphatidylcholine, and actin, which have been confirmed by other biochemical studies. Our Raman-machine learning integrated method with interpretability will facilitate the study of AD and can be extended to other tissues and biofluids and for various other diseases.

Animal Models of Neurodegenerative Disease: Recent Advances in Fly Highlight Innovative Approaches to Drug Discovery

Neurodegenerative diseases represent a formidable challenge to global health. As advances in other areas of medicine grant healthy living into later decades of life, aging diseases such as Alzheimer's disease (AD) and other neurodegenerative disorders can diminish the quality of these additional years, owed largely to the lack of efficacious treatments and the absence of durable cures. Alzheimer's disease prevalence is predicted to more than double in the next 30 years, affecting nearly 15 million Americans, with AD-associated costs exceeding $1 billion by 2050. Delaying onset of AD and other neurodegenerative diseases is critical to improving the quality of life for patients and reducing the burden of disease on caregivers and healthcare systems. Significant progress has been made to model disease pathogenesis and identify points of therapeutic intervention. While some researchers have contributed to our understanding of the proteins and pathways that drive biological dysfunction in disease using in vitro and in vivo models, others have provided mathematical, biophysical, and computational technologies to identify potential therapeutic compounds using in silico modeling. The most exciting phase of the drug discovery process is now: by applying a target-directed approach that leverages the strengths of multiple techniques and validates lead hits using Drosophila as an animal model of disease, we are on the fast-track to identifying novel therapeutics to restore health to those impacted by neurodegenerative disease.

Rational design and synthesis of a novel BODIPY-based probe for selective imaging of tau tangles in human iPSC-derived cortical neurons

Numerous studies have shown a strong correlation between the number of neurofibrillary tangles of the tau protein and Alzheimer's disease progression, making the quantitative detection of tau very promising from a clinical point of view. However, the lack of highly reliable fluorescent probes for selective imaging of tau neurofibrillary tangles is a major challenge due to sharing similar β–sheet motifs with homologous Amyloid-β fibrils. In the current work, we describe the rational design and the in silico evaluation of a small-size focused library of fluorescent probes, consisting of a BODIPY core (electron acceptor) featuring highly conjugated systems (electron donor) with a length in the range 13–19 Å at C3. Among the most promising probes in terms of binding mode, theoretical affinity and polarity, BT1 has been synthesized and tested in vitro onto human induced pluripotent stem cells derived neuronal cell cultures. The probe showed excellent photophysical properties and high selectivity allowing in vitro imaging of hyperphosphorylated tau protein filaments with minimal background noise. Our findings offer new insight into the structure-activity relationship of this class of tau selective fluorophores, paving the way for boosting tau tangle detection in patients possibly through retinal spectral scans.

Strategic Design of Amyloid-β Species Fluorescent Probes for Alzheimer's Disease

Alzheimer's disease (AD) is a high mortality and high disability rates neurodegenerative disease characterized by irreversible progression and poses a significant social and economic burden throughout the world. However, currently approved AD therapeutic agents only alleviate symptoms and there is still a lack of practical therapeutic regimens to stop or slow the progression of this disease. Thus, there is urgently needed novel diagnosis tools and drugs for early diagnosis and treatment of AD. Among several AD pathological hallmarks, amyloid-β (Aβ) peptide deposition is considered a critical initiating factor in AD. In recent years, with the advantages of excellent sensitivity and high resolution, near-infrared fluorescence (NIRF) imaging has attracted the attention of many researchers to develop Aβ plaque probes. This review mainly focused on different NIRF probe design strategies for imaging Aβ species to pave the way for the future design of novel NIRF probes for early diagnosis AD.

Development of Planar Illumination Strategies for Solving Mysteries in the Sub-Cellular Realm

Optical microscopy has vastly expanded the frontiers of structural and functional biology, due to the non-invasive probing of dynamic volumes in vivo. However, traditional widefield microscopy illuminating the entire field of view (FOV) is adversely affected by out-of-focus light scatter. Consequently, standard upright or inverted microscopes are inept in sampling diffraction-limited volumes smaller than the optical system's point spread function (PSF). Over the last few decades, several planar and structured (sinusoidal) illumination modalities have offered unprecedented access to sub-cellular organelles and 4D (3D + time) image acquisition. Furthermore, these optical sectioning systems remain unaffected by the size of biological samples, providing high signal-to-noise (SNR) ratios for objective lenses (OLs) with long working distances (WDs). This review aims to guide biologists regarding planar illumination strategies, capable of harnessing sub-micron spatial resolution with a millimeter depth of penetration.

Fӧrster resonance energy transfer analysis of amyloid state of proteins

The Förster resonance energy transfer (FRET) is a well-established and versatile spectroscopic technique extensively used for exploring a variety of biomolecular interactions and processes. The present review is intended to cover the main results of our FRET studies focused on amyloid fibrils, a particular type of disease-associated protein aggregates. Based on the examples of several fibril-forming proteins including insulin, lysozyme and amyloidogenic variants of N-terminal fragment of apolipoprotein A-I, it was demonstrated that: (i) the two- and three-step FRET with the classical amyloid marker Thioflavin T as an input donor has a high amyloid-sensing potential and can be used to refine the amyloid detection assays; (ii) the intermolecular time-resolved and single-molecule pulse interleaved excitation FRET can give quantitative information on the nucleation of amyloid fibrils; (iii) FRET between the membrane fluorescent probes and protein-associated intrinsic or extrinsic fluorophores is suitable for monitoring the membrane binding of fibrillar proteins, exploring their location relative to lipid-water interface and restructuring on a lipid matrix; (iv) the FRET-based distance estimation between fibril-bound donor and acceptor fluorophores can serve as one of the verification criteria upon structural modeling of amyloid fibrils.

Emergence of distinct and heterogeneous strains of amyloid beta with advanced Alzheimer's disease pathology in Down syndrome

Amyloid beta (Aβ) is thought to play a critical role in the pathogenesis of Alzheimer's disease (AD). Prion-like Aβ polymorphs, or "strains", can have varying pathogenicity and may underlie the phenotypic heterogeneity of the disease. In order to develop effective AD therapies, it is critical to identify the strains of Aβ that might arise prior to the onset of clinical symptoms and understand how they may change with progressing disease. Down syndrome (DS), as the most common genetic cause of AD, presents promising opportunities to compare such features between early and advanced AD. In this work, we evaluate the neuropathology and Aβ strain profile in the post-mortem brain tissues of 210 DS, AD, and control individuals. We assayed the levels of various Aβ and tau species and used conformation-sensitive fluorescent probes to detect differences in Aβ strains among individuals and populations. We found that these cohorts have some common but also some distinct strains from one another, with the most heterogeneous populations of Aβ emerging in subjects with high levels of AD pathology. The emergence of distinct strains in DS at these later stages of disease suggests that the confluence of aging, pathology, and other DS-linked factors may favor conditions that generate strains that are unique from sporadic AD.

Recent Advances in the Application Peptide and Peptoid in Diagnosis Biomarkers of Alzheimer's Disease in Blood

Alzheimer's disease (AD) is one of the most common neurodegenerative diseases with irreversible damage of the brain and a continuous pathophysiological process. Early detection and accurate diagnosis are essential for the early intervention of AD. Precise detection of blood biomarkers related to AD could provide a shortcut to identifying early-stage patients before symptoms. In recent years, targeting peptides or peptoids have been chosen as recognition elements in nano-sensors or fluorescence detection to increase the targeting specificity, while peptide-based probes were also developed considering their specific advantages. Peptide-based sensors and probes have been developed according to different strategies, such as natural receptors, high-throughput screening, or artificial design for AD detection. This review will briefly summarize the recent developments and trends of AD diagnosis platforms based on peptide and peptoid as recognition elements and provide insights into the application of peptide and peptoid with different sources and characteristics in the diagnosis of AD biomarkers.

Laser Emission of Thioflavin T Uncovers Protein Aggregation in Amyloid Nucleation Phase

There is currently no definitive test for early detection of neurodegeneration which is linked with protein aggregation. Finding methods capable of detecting intermediate states of protein aggregates, named oligomers, is critical for the early stage diagnosis of over 30 neurodegenerative diseases including Alzheimer's or Parkinson's. Currently, fluorescence-based imaging using Thioflavin T (ThT) dye is the gold standard for detecting protein aggregation. It is used to detect aggregation in vitro and in various tissues, including the cerebrospinal fluid (CSF), whereby the disease-related protein recombinant is seeded with the patient's fluid. The major drawback of ThT is its lack of sensitivity to oligomeric forms of protein aggregates. Here, we overcome this limitation by transferring a ThT-oligomer mixture into solid state thin films and detecting fluorescence of ThT amplified in the process of stimulated emission. By monitoring the amplified spontaneous emission (ASE) we achieved a remarkable recognition sensitivity to prefibrillar oligomeric forms of insulin and lysozyme aggregates in vitro, to Aβ42 oligomers in the human protein recombinants seeded with CSF and to Aβ42 oligomers doped into brain tissue. Seeding with Alzheimer patient's CSF containing Aβ42 and Tau aggregates revealed that only Aβ42 oligomers allowed generating ASE. Thus, we demonstrated that, in contrast to the current state-of-the-art, ASE of ThT, a commonly used histological dye, can be used to detect and differentiate amyloid oligomers and evaluate the risk levels of neurodegenerative diseases to potential patients before the clinical symptoms occur.

Three-step Förster resonance energy transfer on an amyloid fibril scaffold

The present study provides evidence that the energy transfer chain consisting of the benzothiazole dye Thioflavin T as an input donor, a phosphonium dye TDV and a squaraine dye SQ4 as mediators, and one of the three squaraines SQ1/2/3 as an output acceptor displays an excellent amyloid-sensing ability when applied to differentiating between the amyloid and non-fibrillized states of insulin. The ensemble of fluorophores offers the advantages of a large effective Stokes shift (∼240 nm), well-resolved 3D fluorescence patterns and strong enhancement of the terminal fluorescence (up to two orders of magnitude). The occurrence of multistep energy transfer on an amyloid fibril scaffold opens new possibilities for the more sensitive detection of fibrillar protein assemblies and their applications in nanophotonics.

AND-Logic Strategy for Accurate Analysis of Alzheimer's Disease via Fluorescent Probe Lighted Up by Two Specific Biomarkers

Alzheimer's disease (AD) has become a global threat to the elderly health with a short survival time after diagnosis. Due to the asymptomatic stage during the early development, patients are usually diagnosed at the middle or late stage. Therefore, an efficient tool for AD early diagnosis deserves considerable attention, which could make a significant contribution to the treatment intervention. A fluorescent probe has been widely applied for detecting and visualizing species of interest in vitro and in vivo, and the proper reaction between the probe and analytes is responsible for the fluorescence change to provide a lighting-on or ratiometric responsive pattern with satisfactory sensing behavior. In this work, we report the first attempt to build up an AND-logic probe P2 for AD accuracy diagnosis taking butyrylcholinesterase (BChE) and reactive oxygen species (ROSs) as dual targets. Upon the co-stimulation by these two factors through enzymatic hydrolysis and redox reaction, the NIR emission could be readily turned on. This AND sensing pattern avoided the false-positive response effectively, and other diseases sharing one biomarker could hardly induce a NIR fluorescence response. The sensing assay has also been confirmed to be feasible in vitro and in vivo with good sensibility and selectivity. It is worth mentioning that the probe structure has been optimized in terms of the linkage length. This study shows that probe P2 with a connecting arm of medium length (one methylene, n = 1) has superior sensing performance, promising to provide a reference for the relative structure design.

Synthesis and evaluation of curcumin-based near-infrared fluorescent probes for the in vivo optical imaging of amyloid-β plaques

The abnormal self-assembly of amyloid-beta (Aβ) peptides into oligomers, as well as insoluble fibrils, has been identified as a key factor for monitoring the progression of Alzheimer's disease (AD). The noninvasive imaging of Aβ aggregates utilizing chemical probes can be a powerful and practical technique for accurately diagnosing and monitoring the progress of AD, as well as evaluating the effectiveness of therapeutic drug candidates in treating or managing it. Particularly, the near-infrared (NIR) fluorescence imaging of Aβ plaques is a potentially promising approach toward the efficient detection of the biomarker. In this study, we describe a new NIR fluorophore, which was based on curcumin derivatives. The fluorophore is equipped with desirable optical properties for in vivo brain imaging. The emission wavelength of the probe, 8b, is 667 nm, and its fluorescent intensity is significantly increased through binding with the Aβ aggregates. The probe allows the clear visualization of the Aβ plaques 10 min post administration, and the intensity of the fluorescent signal in the brain of a 5XFAD transgenic mouse model is more than three times higher than that of the normal control group. These results demonstrate that the designed probe can be an effective tool for visualizing Aβ plaques, as well as investigating the pathological progress of AD.

Review on the recent progress in the development of fluorescent probes targeting enzymes

Enzymes are very important for biological processes in a living being, performing similar or multiple tasks in and out of cells, tissues and other organisms at a particular location. The abnormal activity of particular enzyme usually caused serious diseases such as Alzheimer's disease, Parkinson's disease, cancers, diabetes, cardiovascular diseases, arthritis etc. Hence, nondestructive and real-time visualization for certain enzyme is very important for understanding the biological issues, as well as the drug administration and drug metabolism. Fluorescent cellular probe-based enzyme detectionin vitroandin vivohas become broad interest for human disease diagnostics and therapeutics. This review highlights the recent findings and designs of highly sensitive and selective fluorescent cellular probes targeting enzymes for quantitative analysis and bioimaging.

Evaluation of Fluorescent Cu2+ Probes: Instant Sensing, Cell Permeable Recognition and Quantitative Detection

By incorporating a rhodamine spirolactam structure as the recognition site for Cu2+, two novel probes were synthesized through a connection of rhodamine 6G acylhydrazine and 5-formyl-6-hydroxyl-4-methylcoumarin/2,4-dihydroxybenzaldehyde. In the recognition process of probes towards Cu2+, the spirolactam ring exhibited opening and closing, accompanying an instant and specific change in fluorescence and in color, which could also achieve a naked-eye and semiquantitative recognition of aqueous Cu2+ besides the fluorescent Cu2+ detection method. Fluorescent analyses and ECV304 cell imaging further revealed the probes' good optical stability, instant response, low toxicity, and membrane permeability, which offers future possibilities for the probes' instant detection and the real-time tracking of Cu2+ in biological systems.

Fluorescent probes for bioimaging of potential biomarkers in Parkinson's disease

This review comprehensively summarizes various types of fluorescent probes for PD and their applications for detection of various PD biomarkers.

Fluorescent Diagnostic Probes in Neurodegenerative Diseases

Neurodegenerative diseases are debilitating disorders that feature progressive and selective loss of function or structure of anatomically or physiologically associated neuronal systems. Both chronic and acute neurodegenerative diseases are associated with high morbidity and mortality along with the death of neurons in different areas of the brain; moreover, there are few or no effective curative therapy options for treating these disorders. There is an urgent need to diagnose neurodegenerative disease as early as possible, and to distinguish between different disorders with overlapping symptoms that will help to decide the best clinical treatment. Recently, in neurodegenerative disease research, fluorescent-probe-mediated biomarker visualization techniques have been gaining increasing attention for the early diagnosis of neurodegenerative diseases. A survey of fluorescent probes for sensing and imaging biomarkers of neurodegenerative diseases is provided. These imaging probes are categorized based on the different potential biomarkers of various neurodegenerative diseases, and their advantages and disadvantages are discussed. Guides to develop new sensing strategies, recognition mechanisms, as well as the ideal features to further improve neurodegenerative disease fluorescence imaging are also explored.

BODIPY Dyes as Probes and Sensors to Study Amyloid-β-Related Processes

Amyloid formation plays a major role in a number of neurodegenerative diseases, including Alzheimer's disease. Amyloid-β peptides (Aβ) are one of the primary markers associated with this pathology. Aβ aggregates exhibit a diverse range of morphologies with distinct pathological activities. Recognition of the Aβ aggregates by using small molecule-based probes and sensors should not only enhance understanding of the underlying mechanisms of amyloid formation, but also facilitate the development of therapeutic strategies to interfere with amyloid neurotoxicity. BODIPY (boron dipyrrin) dyes are among the most versatile small molecule fluorophores. BODIPY scaffolds could be functionalized to tune their photophysical properties to the desired ranges as well as to adapt these dyes to various types of conditions and environments. Thus, BODIPY dyes could be viewed as unique platforms for the design of probes and sensors that are capable of detecting and tracking structural changes of various Aβ aggregates. This review summarizes currently available examples of BODIPY dyes that have been used to investigate conformational changes of Aβ peptides, self-assembly processes of Aβ, as well as Aβ interactions with various molecules.

Environment-Sensitive Probes for Illuminating Amyloid Aggregation In Vitro and in Zebrafish

The aberrant aggregation of certain peptides and proteins, forming extracellular plaques of fibrillar material, is one of the hallmarks of amyloid diseases, such as Alzheimer's and Parkinson's. Herein, we have designed a new family of solvatochromic dyes based on the 9-amino-quinolimide moiety capable of reporting during the early stages of amyloid fibrillization. We have rationally improved the photophysical properties of quinolimides by placing diverse amino groups at the 9-position of the quinolimide core, leading to higher solvatochromic and fluorogenic character and higher lifetime dependence on the hydrophobicity of the environment, which represent excellent properties for the sensitive detection of prefibrillar aggregates. Among the different probes prepared, the 9-azetidinyl-quinolimide derivative showed striking performance in the following β-amyloid peptide (Aβ) aggregation in solution in real time and identifying the formation of different types of early oligomers of Aβ, the most important species linked to cytotoxicity, using novel, multidimensional fluorescence microscopy, with one- or two-photon excitation. Interestingly, the new dye allowed the visualization of proteinaceous inclusion bodies in a zebrafish model with neuronal damage induced by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Our results support the potential of the novel fluorophores as powerful tools to follow amyloid aggregation using fluorescence microscopy in vivo, revealing heterogeneous populations of different types of aggregates and, more broadly, to study protein interactions.

A mitochondria-specific fluorescent probe for rapidly assessing cell viability

Rapid detection of cell viability is of importance for various aspects of basic and applied research. However, the most commonly used MTT assay suffers from some disadvantages of time-consuming, poor solubility of formazan crystals, and interference of cell extracts, resulting in either delayed results or inconsistent cell viability. Accumulating evidences show that mitochondrial activity is closely associated with cell proliferation and apoptosis, making it valuable for real-time monitoring of cell fates. Herein, we present a novel type of cell-permeant mitochondrial dyes, composed of coumarin-quinoline conjugates (CQCs), for assessing cell viability through monitoring mitochondrial dynamic changes. CQCs exhibited superior preference for mitochondria, and could sensitively monitor their fissions under pathological conditions. Particularly, CQCs could accurately assess the cell viability within 10 min, allowing for rapidly evaluating therapeutic effects with high sensitivity and accuracy, in comparison of MTT assay. Therefore, CQCs provide a proof-of-concept assay that is appropriately practical either independently or in complementation with other assays for the analysis of cellular viability in broad range of biomedical applications.

Differentiating Aβ40 and Aβ42 in amyloid plaques with a small molecule fluorescence probe

Differentiating amyloid beta (Aβ) subspecies Aβ40 and Aβ42 has long been considered an impossible mission with small-molecule probes. In this report, based on recently published structures of Aβ fibrils, we designed iminocoumarin–thiazole (ICT) fluorescence probes to differentiate Aβ40 and Aβ42, among which Aβ42 has much higher neurotoxicity. We demonstrated that ICTAD-1 robustly responds to Aβ fibrils, evidenced by turn-on fluorescence intensity and red-shifting of emission peaks. Remarkably, ICTAD-1 showed different spectra towards Aβ40 and Aβ42 fibrils. In vitro results demonstrated that ICTAD-1 could be used to differentiate Aβ40/42 in solutions. Moreover, our data revealed that ICTAD-1 could be used to separate Aβ40/42 components in plaques of AD mouse brain slides. In addition, two-photon imaging suggested that ICTAD-1 was able to cross the BBB and label plaques in vivo. Interestingly, we observed that ICTAD-1 was specific toward plaques, but not cerebral amyloid angiopathy (CAA) on brain blood vessels. Given Aβ40 and Aβ42 species have significant differences of neurotoxicity, we believe that ICTAD-1 can be used as an important tool for basic studies and has the potential to provide a better diagnosis in the future.

A small molecule fluorescence probe ICTAD-1 was rationally designed for differentiating Aβ40 and Aβ42 in solutions and in Aβ plaques.

Recent advances in two-photon absorbing probes based on a functionalized dipolar naphthalene platform

Two-photon microscopy (TPM) techniques have been highlighted over the past two decades throughout various fields, including physics, chemistry, biology, and medicine. In particular, the two-photon near-infrared excitation of fluorophores or molecular probes emitting fluorescence have ushered in a new biomedical era, specifically in the deep-tissue imaging of biologically relevant species. Non-linear two-photon optics enables the development of 3D fluorescence images via focal point excitation of biological samples with low photo-damage and photo-bleaching. Many studies have disclosed the relationship between the chemical structure of fluorophores and their two-photon absorbing properties. In this review, we have summarized the recent advances in two-photon absorbing probes based on a functionalized electron donor (D)-acceptor (A) type dipolar naphthalene platform (FDNP) that was previously reported between 2015 and 2019. Our systematic outline of the synthesis, photophysical properties, and examples of two-photon imaging applications will provide useful context for the future development of new naphthalene backbone-based two-photon probes.

Sensing Performance Investigations on Two-Photon Fluorescent Probes for Detecting β-Amyloid in Alzheimer's Disease

Alzheimer’s disease (AD) is one of the most common forms of senile disease. In recent years, the incidence of AD has been increasing significantly with the acceleration of the aging process of the global population. However, current clinical drugs can only alleviate the symptoms of AD patients without healing the disease fundamentally. Therefore, it is of great significance to develop an effective small molecule diagnostic reagent for the early diagnosis of AD. In this paper, we employ an integrated approach, including molecular docking simulation and quantum mechanics/molecular mechanics calculation, to investigate the sensing performance of a series of donor–acceptor structural probes for the marker protein of AD (β-amyloid). Results show that the probes display evident fluorescence enhancement when bound to the β-amyloid, suggesting the effect of the environment on the molecular properties. Especially, the two-photon absorption cross-section of the probes increase drastically in the β-amyloid compared to that in vacuum, which results from the larger electron delocalization and dipole moment in the fibrillary-like environment. Thus, one can propose that the studied probes are capable of application in two-photon fluorescent imaging, particularly those containing naphthalene rings as the donor or with a longer spacer group. Our calculations elucidate the experimental measurements reasonably, and further establish possible structure–property relationships that can be used to design novel biocompatible two-photon fluorescent probes for the diagnosis of Alzheimer’s.