A Ratiometric Two-Photon Fluorescent Probe for Tracking Lysosomal ATP: Direct In Cellulo Observation of Lysosomal Membrane Fusion Processes

A Small-Molecule Fluorescence Probe for Nuclear ATP

Fluorescence monitoring of ATP in different organelles is now feasible with a few biosensors developed, which, however, show low sensitivity, limited biocompatibility, and accessibility. Small-molecule ATP probes that alleviate those limitations thus have received much attention recently, leading to a few ATP probes that target several organelles except for the nucleus. We disclose the first small-molecule probe that selectively detects nuclear ATP through reversible binding, with 25-fold fluorescence enhancement at pH 7.4 and excellent selectivity against various biologically relevant species. Using the probe, we observed 2.1-3.3-fold and 3.9-7.8-fold higher nuclear ATP levels in cancerous cell lines and tumor tissues compared with normal cell lines and tissues, respectively, which are explained by the higher nuclear ATP level in the mitosis phase. The probe has great potential for studying nuclear ATP-associated biology.

In Situ Observation of Lysosomal Hypobromous Acid Fluctuations in the Brain of Mice with Depression Phenotypes by Two-Photon Fluorescence Imaging

Excessive oxidative stress is the main cause of neurotransmitter metabolism disorder in the brain with depression. Lysosomal hypobromic acid (HOBr) is an important reactive oxygen species produced in oxidative stress. Its abnormal content can lead to macromolecular damage and neurodegenerative diseases. However, due to the high reactivity and low concentration of HOBr and the lack of in situ imaging methods, the role of HOBr in depression is not clear. Herein, based on the HOBr-initiated aromatic substitution of a tertiary amine, we developed a novel two-photon (TP) fluorescence probe (NH-HOBr) for real-time visual monitoring of trace HOBr in living systems. NH-HOBr introduces N-(2-aminoethyl)-morpholine as a new recognition receptor for HOBr and a targeting group for lysosomes. It not only has excellent selectivity compared with other biomolecules (including hypochlorous acid), fast response (≤5 s) and high sensitivity (LOD = 15 nM) but also realizes sensitive detection of HOBr in cells, zebrafish, and mice tissues. It is worth noting that the in situ TP fluorescence imaging of mouse brain reveals the positive correlation between HOBr content and depression phenotype for the first time, providing strong direct evidence for the relationship between oxidative stress and depression. This work can provide reference to further study depression and the pathological mechanism of HOBr. In addition, HOBr-initiated aromatic substitution of a tertiary amine provides a new idea for the construction of specific and sensitive HOBr probes.

Rational Design, Synthesis of Fluorescence Probes for Quantitative Detection of Amyloid-β in Alzheimer's Disease Based on Rhodamine-Metal Complex

The efficient detection and monitoring of amyloid-β plaques (Aβ42) can greatly promote the diagnosis and therapy of Alzheimer's disease (AD). Fluorescence imaging is a promising method for this, but the accurate determination of Aβ42 still remains a challenge. The development of a reliable fluorescent probe to detect Aβ42 is essential. Herein, we report a rational design strategy for Aβ42 fluorescence probes based on rhodamine-copper complexes, Rho1-Cu-Rho4-Cu, among them Rho4-Cu exhibits the best performance including high sensitivity (detection limit = 24 nM), high affinity (K_d = 23.4 nM), and high selectivity; hence, Rho4-Cu is selected for imaging Aβ42 in AD mice, and the results showed that this probe can differentiate normal mice and AD mice effectively.

Rhodamine-thiourea Linked Naphthalimide Derivative to Image ATP in Mitochondria using Two Channels

Adenosine 5'-triphosphate (ATP), synthesized in mitochondria, is an energy molecule in all living things. ATP not only serves as an energy source for protein synthesis and muscle contraction, but also as an important indicator for various diseases, such as Parkinson's disease, cardiovascular disease, and others. Accordingly, detection and sensing of ATP, especially in mitochondria, are important. In this study, a unique ring-opening process of rhodamine was coupled to recognition of ATP via introduction of a thiourea moiety, which was further linked to a naphthalimide group. A strong fluorescent emission at ∼580 nm was accompanied by a color change from colorless to pink upon addition of ATP at pH 7.4. Fluorescent probe 1 successfully imaged mitochondrial ATP with a Pearson's coefficient of 0.8. In addition, green emission from the naphthalimide moiety at ∼530 nm was observed without any change upon addition of ATP. This emission can be considered equivalent to an internal standard to utilize probe 1 as a dual-channel probe for ATP. Furthermore, probe 1 showed negligible cytotoxicity based on MTT assays.

Recent advances in dual-target-activated fluorescent probes for biosensing and bioimaging

Fluorescent probes have been powerful tools for visualizing and quantifying multiple dynamic processes in living cells. However, the currently developed probes are often constructed by conjugation a fluorophore with a recognition moiety and given signal-output after triggering with one singly target interest. Compared with the single-target-activated fluorescent probes mentioned above, the dual-target-activated ones, triggering with one target under stimulus (such as photoirradiation, microenvironment) or another targets, have the advantages of advoiding nonspecific activation and "false positive" results in complicated environments. In recent years, many dual-target-activated fluorescent probes have been developed to detect various biologically relevant species. In view of the importance of a comprehensive understanding of dual-target- activated fluorescent probes, a thorough summary of this topic is urgently needed. However, no comprehensive and critical review on dual target activated fluorescent probes has been published recently. In this review, we focus on the dual-target-activated fluorescent probes and briefly outline their types and current state of development. In each type, the chemical structure, proposed responsive mechanism and application of probes are highlighted. At last, the challenges and prospective opportunities of every type were proposed.

A novel dual-capability naphthalimide-based fluorescent probe for Fe3+ ion detection and lysosomal tracking in living cells

We design and synthesize a novel 1,8-naphthalimide-based fluorescent probe MNP that features the dual capabilities of tracking lysosomes in living HeLa cells and sensitively detecting Fe³⁺ ions in aqueous solution. The MNP is obtained by modifying the morpholine group with a lysosomal targeting function and the piperazine group with an Fe³⁺ ion recognition function on the 1,8-naphthalimide matrix. In the presence of Fe³⁺ ions, the MNP acts as a recognition ligand to coordinate with the central Fe³⁺ ion, and the protonated [MNPH]⁺ is eventually generated, in which significant fluorescence enhancements are observed due to the intramolecular photo-induced electron transfer (PET) process being blocked. The limit of detection of Fe³⁺ ions is as low as 65.2 nM. A cell imaging experiment shows that the MNP has low cytotoxicity and excellent lysosomal targeting ability. Therefore, the MNP offers a promising tool for lysosomal tracking and relevant life process research.

A newly prepared 1,8-naphthalimide-based fluorescent probe, MNP, allows the detection of Fe³⁺ ions in aqueous medium and lysosomal tracking in living cells. MNP was used in situ for the imaging of lysosomes in HeLa cells, a new strategy for lysosome-related medical diagnosis.

Current status and future prospects of nanoscale metal–organic frameworks in bioimaging

The importance of diagnosis and in situ monitoring of lesion regions and transportation of bioactive molecules has a pivotal effect on successful treatment, reducing side effects, and increasing the chances of survival in the case of diseases.

Recent Advances in Nucleic Acid Modulation for Functional Nanozyme

Nanozymes have the potential to replace natural enzymes, so they are widely used in energy conversion technologies such as biosensors and signal transduction (converting biological signals of a target into optical, electrical, or metabolic signals). The participation of nucleic acids leads nanozymes to produce richer interface effects and gives energy conversion events more attractive characteristics, creating what are called “functional nanozymes”. Since different nanozymes have different internal structures and external morphological characteristics, functional modulation needs to be compatible with these properties, and attention needs to be paid to the influence of nucleic acids on nanozyme activity. In this review, “functional nanozymes” are divided into three categories, (nanozyme precursor ion)/ (nucleic acid) self-assembly, nanozyme-nucleic acid irreversible binding, and nanozyme-nucleic acid reversible binding, and the effects of nucleic acids on modulation principles are summarized. Then, the latest developments of nucleic acid-modulated nanozymes are reviewed in terms of their use in energy conversion technology, and their conversion mechanisms are critically discussed. Finally, we outline the advantages and limitations of “functional nanozymes” and discuss the future development prospects and challenges in this field.

AIEgen-based nanoprobe for the ATP sensing and imaging in cancer cells and embryonic stem cells

A turn-on fluorescent nanoprobe (named AAP-1), based on an aggregation-induced emission luminogen (AIEgen), is disclosed for the detection of adenosine triphosphate (ATP), which is an essential element in the biological system. Organic fluorophore (named TPE-TA) consists of tetraphenylethylene (TPE, sensing and signaling moiety) and mono-triamine (TA, sensing moiety), and it forms an aggregated form in aqueous media as a nanoprobe AAP-1. The nanoprobe AAP-1 has multiple electrostatic interactions as well as hydrophobic interactions with ATP, and it displays superior selectivity toward ATP, reliable sensitivity, with a detection limit around 0.275 ppb, and fast responsive (signal within 10 s). Such a fluorescent probe to monitor ATP has been actively pursued throughout fundamental and translational research areas. In vitro assay and a successful cellular ATP imaging application was demonstrated in cancer cells and embryonic stem cells. We expect that our work warrants further ATP-related studies throughout a variety of fields.

An Excimer Clamp for Measuring Damaged-Base Excision by the DNA Repair Enzyme NTH1

Direct measurement of DNA repair enzyme activities is important both for the basic study of cellular repair pathways as well as for potential new translational applications in their associated diseases. NTH1, a major glycosylase targeting oxidized pyrimidines, prevents mutations arising from this damage, and the regulation of NTH1 activity is important in resisting oxidative stress and in suppressing tumor formation. Herein, we describe a novel molecular strategy for the direct detection of damaged DNA base excision activity by a ratiometric fluorescence change. This strategy utilizes glycosylase-induced excimer formation of pyrenes, and modified DNA probes, incorporating two pyrene deoxynucleotides and a damaged base, enable the direct, real-time detection of NTH1 activity in vitro and in cellular lysates. The probe design was also applied in screening for potential NTH1 inhibitors, leading to the identification of a new small-molecule inhibitor with sub-micromolar potency.

Design of a Fluorescence Turn-on and Label-free Aptasensor Using the Intrinsic Quenching Power of G-Quadruplex to AMT

A novel fluorescent aptasensor based on the G-quadruplex induced fluorescent quenching of psoralen and the competitive interactions between 4'-aminomethyl-4,5',8-trimethylpsoralen (AMT), adenosine triphosphate (ATP) and G-rich DNA functionalized split ATP aptamer was proposed. The binding of ATP to the G-rich DNA functionalized split aptamer induced a significant enhancement in fluorescence emission intensity while undergoing excitation at 340 nm. Under the optimal conditions, the developed aptasensor showed high selectivity and good accuracy for detecting ATP. The practicality of the proposed aptasensor has been confirmed by successfully analyzing ATP in spiked human blood serum samples with satisfactory results. As far as we know, this is the first time that the intrinsic quenching ability of G-quadruplex was applied to simply construct a fluorescence turn-on and label-free aptasensor. On account of the superiority of the simplicity of the design strategy, more work is expected in the future to develop a variety of novel sensors for other important analytes using the quenching capability of G-quadruplex through reasonable designs.

A Self-Assembled ATP Probe for Melanoma Cell Imaging

In this investigation, a new terpyridine metal complex was developed as a probe for selective detection of ATP and imaging of melanoma cells. The probe takes advantage of the ability of the metal complex to be transformed to its imaging competent turn-on state through assembly with ATP.

Observation of Acetylcholinesterase in Stress-Induced Depression Phenotypes by Two-Photon Fluorescence Imaging in the Mouse Brain

Oxidative stress in depression is a prime cause of neurotransmitter metabolism dysfunction in the brain. Acetylcholinesterase (AChE), a key hydrolase in the cholinergic system, directly determines the degradation of neurotransmitters. However, due to the complexity of the brain and lack of appropriate in situ imaging tools, the mechanism underlying the changes in AChE activity in depression remains unclear. Hence, we generated a two-photon fluorescence probe (MCYN) for real-time visualization of AChE with excellent sensitivity and selectivity. AChE can specifically recognize and cleave the carbamic acid ester bond in MCYN, and MCYN emits bright fluorescence at 560 nm by two-photon excitation at 800 nm. By utilizing MCYN to monitor AChE, we discovered a significant increase in AChE activity in the brains of mice with depression phenotypes. Notably, with the assistance of a two-photon fluorescence imaging probe of the superoxide anion radical (O₂^•-), in vivo visualization for the first time revealed the positive correlation between AChE and O₂^•- levels associated with depressive behaviors. This finding suggests that oxidative stress may induce AChE overactivation, leading to depression-related behaviors. This work provides a new and rewarding perspective to elucidate the role of oxidative stress regulating AChE in the pathology of depression.

Ultrasmall Au nanocatalysts supported on nitrided carbon for electrocatalytic CO2 reduction: the role of the carbon support in high selectivity

Au is one of the most promising electrocatalysts to convert CO2 into CO in an aqueous-phase electrochemical reduction. However, ultrasmall Au nanocatalysts (AuNCs, <2 nm) have proven to be favorable for water reduction over CO2, although they possess a large surface-to-volume ratio and potentially are ideal for CO2 reduction. We herein report that ultrasmall AuNCs (1.9 ± 0.3 nm) supported on nitrided carbon are remarkably active and selective for CO2 reduction. The mass activity for CO of AuNCs reaches 967 A g-1 with a faradaic efficiency for CO of ∼83% at -0.73 V (vs. reversible hydrogen electrode) that is an order of magnitude more active than the state-of-the-art results. The high activity is endowed by the large surface area per unit weight and the high selectivity of ultrasmall AuNCs for CO2 reduction originates from the cooperative effect of Au and the nitrided carbon support where the surface N sites act as Lewis bases to increase the surface charge density of AuNCs and enhance the localized concentration of CO2 nearby catalytically active Au sites. We show that our results can be applied to other pre-synthesized Au catalysts to largely improve their selectivity for CO2 reduction by 50%. Our method is expected to illustrate a general guideline to effectively lower the cost of Au catalysts per unit weight of the product while maintaining its high selectivity for CO2 reduction.