Fluorescent Self-Threaded Peptide Probes for Biological Imaging

Steric protection of near-infrared fluorescent dyes for enhanced bioimaging

Near-fluorescent (NIR) dyes that absorb and emit light in the wavelength range of 650-1700 nm are well-suited for bioimaging due to the improved image contrast and increased penetration of the long-wavelength light through biological tissue. However, the imaging performance of NIR fluorescent dyes is limited by several inherent photophysical and physicochemical properties including, low fluorescence quantum yield, high chemical and photochemical reactivity, propensity to self-aggregate in water, non-specific association with off-target biological sites, and non-optimal pharmacokinetic profiles in living subjects. In principle, all these drawbacks can be alleviated by steric protection which is a structural process that surrounds the fluorophore with bulky groups that block undesired intermolecular interactions. The literature methods to sterically protect a long-wavelength dye can be separated into two general strategies, non-covalent dye encapsulation and covalent steric appendage. Illustrative examples of each method show how steric protection improves bioimaging performance by providing: (a) increased fluorescence brightness, (b) higher fluorophore ground state stability, (c) decreased photobleaching, and (d) superior pharmacokinetic profile. Some sterically protected dyes are commercially available and further success with future systems will require experts in chemistry, microscopy, cell biology, medical imaging, and clinical medicine to work closely as interdisciplinary teams.

Near-Infrared Fluorescent Probe for the Detection of Cysteine

Hemicyanine dyes are an ideal structure for building near-infrared fluorescent probes due to their excellent emission wavelength properties and biocompatibility in biological imaging field. Developing a near-infrared fluorescent probe capable of detecting cysteine (Cys) was the aim of this study. A novel developed fluorescent probe P showed high selectivity and sensitivity to Cys in the presence of various analytes. The detection limit of P was found to be 0.329 μM. The MTT assay showed that the probe was essentially non-cytotoxic. Furthermore, the probe was successfully used as cysteine imaging in living cells and mice.

Homo-Dyad with Outer Hydration Layer Approach for Developing NIR-II Chromophore of High Stability and Water-Solubility as Injectable and Sprayable Optical Probe

Dyes with extended conjugate structures are the focus of extensive design and synthesis efforts, aiming to confer unique and improved optical and electronic properties. Such advancements render these dyes applicable across a wide spectrum of uses, ranging from second-window near-infrared (NIR-II) bioimaging to organic photovoltaics. Nevertheless, the inherent benefits of long conjugation are often accompanied by persistent challenges like aggregation, fluorescence quenching, absorption blueshift, and low stability and poor water solubility. Herein, a unique structural design strategy termed "homo-dyad with outer hydration layer" is introduced to address these inherent problems, tailored for the development of imaging probes exhibiting long absorption/emission wavelengths. This approach involves bringing two heptamethine cyanines together through a flexible linker, forming a homo-dyad structure, while strategically attaching four polyethylene glycol (PEG9) chains to the terminal heterocycles. This approach imparts excellent water solubility, biocompatibility, and enhanced chemical, photo-, and spectral stability for the dyes. Utilizing this strategy, a biomarker-activatable probe (HD-FL-4PEG9-N) for NIR-II fluorescent and 3D multispectral optoacoustic tomography imaging is developed, and its effectiveness in disease visualization. It can not only serve as an injectable probe for acute kidney injury imaging due to its high water solubility, but also a sprayable probe for imaging bacterial-infected wounds.

Monofluorophore-based Two-Photon Ratiometric Fluorescent Probe for the Quantitative Imaging of Fatty Acid Amide Hydrolase in Live Neurons and Mouse Brain Tissues

Fatty acid amide hydrolase (FAAH) plays a crucial role in the metabolism of the endocannabinoid system by hydrolyzing a series of bioactive amides, whose abnormal levels are associated with neuronal disorders including Alzheimer's disease (AD). However, due to the lack of suitable quantitative sensing tools, real-time and accurate monitoring of the activity of FAAH in living systems remains unresolved. Herein, a novel enzyme-activated near-infrared two-photon ratiometric fluorescent probe (CANP) based on a naphthylvinylpyridine monofluorophore is successfully developed, in which the electron-withdrawing amide moiety is prone to be hydrolyzed to an electron-donating amine group under the catalysis of FAAH, leading to the activation of the intramolecular charge transfer process and the emergence of a new 80 nm red-shifted emission, thereby achieving a ratiometric luminescence response. Benefiting from the high selectivity, high sensitivity, and ratiometric response to FAAH, the probe CANP is successfully used to quantitatively monitor and image the FAAH levels in living neurons, by which an amyloid β (Aβ)-induced upregulation of endogenous FAAH activity is observed. Similar increases in FAAH activity are found in various brain regions of AD model mice, indicating a potential fatty acid amide metabolite-involved pathway for the pathological deterioration of AD. Moreover, our quantitative FAAH inhibition experiments further demonstrate the great value of CANP as an efficient visual probe for in situ and precise assessment of FAAH inhibitors in complex living systems, assisting the discovery of FAAH-related therapeutic agents.

Expanding the Hydrophobic Cavity Surface of Azocalix[4]arene to Enable Biotin/Avidin Affinity with Controlled Release

The development of artificial receptors that combine ultrahigh-affinity binding and controllable release for active guests holds significant importance in biomedical applications. On one hand, a complex with an exceedingly high binding affinity can resist unwanted dissociation induced by dilution effect and complex interferents within physiological environments. On the other hand, stimulus-responsive release of the guest is essential for precisely activating its function. In this context, we expanded hydrophobic cavity surface of a hypoxia-responsive azocalix[4]arene, affording Naph-SAC4A. This modification significantly enhanced its aqueous binding affinity to 1013 M-1, akin to the naturally occurring strongest recognition pair, biotin/(strept-)avidin. Consequently, Naph-SAC4A emerges as the first artificial receptor to simultaneously integrate ultrahigh recognition affinity and actively controllable release. The markedly enhanced affinity not only improved Naph-SAC4A's sensitivity in detecting rocuronium bromide in serum, but also refined the precision of hypoxia-responsive doxorubicin delivery at the cellular level, demonstrating its immense potential for diverse practical applications.

De Novo Screening and Mirror Image Isomerization of Linear Peptides Targeting α7 Nicotinic Acetylcholine Receptor

As ligand-gated ion channels, nicotinic acetylcholine receptors (nAChRs) are widely distributed in the central and peripheral nervous systems and are associated with the pathogenesis of various degenerative neurological diseases. Here, we report the results of phage display-based de novo screening of an 11-residue linear peptide (named LKP1794) that targets the α7 nAChR, which is among the most abundant nAChR subtypes in the brain. Moreover, two d-peptides were generated through mirror image and/or primary sequence inverso isomerization (termed DRKP1794 and DKP1794) and displayed improved inhibitory effects (IC50 = 0.86 and 0.35 μM, respectively) on α7 nAChR compared with the parent l-peptide LKP1794 (IC50 = 2.48 μM), which markedly enhanced serum stability. A peptide-based fluorescence probe was developed using proteolytically resistant DKP1794 to specifically image the α7 nAChR in living cells. This work provides a new peptide tool to achieve inhibitory modulation and specifically image the α7 nAChR.

Sub-100 ms Level Ultrafast Detection and Near-Infrared Ratiometric Fluorescence Imaging of Norepinephrine in Live Neurons and Brains

Norepinephrine (NE) is a key neurotransmitter in the central and sympathetic nervous systems, whose content fluctuates dynamically and rapidly in various brain regions during different physiological and pathophysiological processes. However, it remains a great challenge to directly visualize and precisely quantify the transient NE dynamics in living systems with high accuracy, specificity, sensitivity, and, in particular, high temporal resolution. Herein, we developed a series of small-molecular probes that can specifically detect NE through a sequential nucleophilic substitution-cyclization reaction, accompanied by a ratiometric near-infrared fluorescence response, within an impressively short time down to 60 ms, which is 3 orders of magnitude faster than that of present small-molecular probes. A unique water-promoted intermolecular proton transfer mechanism is disclosed, which dramatically boosted the recognition kinetics by ∼680 times. Benefiting from these excellent features, we quantitatively imaged the transient endogenous NE dynamics under external stimuli at the single living neuron level and further revealed the close correlations between NE fluctuations and Parkinson's disease pathology at the level of acute brain slices and live mouse brains in vivo.

An adhesive peptide specifically induces microtubule condensation

Cell function-associated biomolecular condensation has great potential in modulation of molecular activities. We develop a microtubule-trapping peptide that first self-assembles into nanoparticles and then in situ transforms into nanofibers via ligand-receptor interactions when targeted to tubulin. The nanofibers support the increased exposed targets for further adhering to microtubules and induce the self-assembly of microtubules into networks due to multivalent effects. Microtubule condensation with prolonged retention in cells for up to 24 h, which is 6 times longer than that of the non-transformable nanoparticle group, efficiently induces in vitro cell apoptosis and inhibits in vivo tumour growth. These smart transformable peptide materials for targeted protein condensation have the potential for improving retention and inducing cell apoptosis in tumour therapy.

An Interlocked Figure-of-Eight Molecular Shuttle

Here is reported the synthesis and characterization of an interlocked figure-of-eight rotaxane molecular shuttle from a dibenzo-24-crown-8 (DB24C8) derivative. This latter bears two molecular chains, whose extremities are able to react together by click chemistry. One of the two substituting chain holds an ammonium function aimed at driving the self-entanglement through the complexation of the DB24C8 moiety. In the targeted figure-of-eight rotaxane, shuttling of the DB24C8 along the threaded axle from the best ammonium station to the weaker binding site triazolium was performed through deprotonation or deprotonation-then-carbamoylation of the ammonium. This way, two discrete co-conformational states were obtained, in which the folding and size of the two loops could be changed.

Design of Water-Soluble Rotaxane-Capped Superparamagnetic, Ultrasmall Fe3O4 Nanoparticles for Targeted NIR Fluorescence Imaging in Combination with Magnetic Resonance Imaging

Integrating an NIR fluorescent probe with a magnetic resonance imaging (MRI) agent to harvest complementary imaging information is challenging. Here, we have designed water-soluble, biocompatible, noncytotoxic, bright-NIR-emitting, sugar-functionalized, mechanically interlocked molecules (MIMs)-capped superparamagnetic ultrasmall Fe3O4 NPs for targeted multimodal imaging. Dual-functional stoppers containing an unsymmetrical NIR squaraine dye interlocked within a macrocycle to construct multifunctional MIMs are developed with enhanced NIR fluorescence efficiency and durability. One of the stoppers of the axle is composed of a lipophilic cationic TPP+ functionality to target mitochondria, and the other stopper comprises a dopamine-containing catechol group to anchor at the surface of the synthesized Fe3O4 NPs. Fe3O4 NPs surface-coated with targeted NIR rotaxanes help to deliver ultrasmall magnetic NPs specifically inside the mitochondria. Two carbohydrate moieties are conjugated with the macrocycle of the rotaxane via click chemistry to improve the water solubility of MitoSQRot-(Carb-OH)2-DOPA-Fe3O4 NPs. Water-soluble, rotaxane-capped Fe3O4 NPs are used for live-cell mitochondria-targeted NIR fluorescence confocal imaging, 3D and multicolor imaging in combination with T2-weighted MRI on a 9.4 T MR scanner with a high relaxation rate (r2) of 180.7 mM-1 s-1. Biocompatible, noncytotoxic, ultrabright NIR rotaxane-capped superparamagnetic ultrasmall monodisperse Fe3O4 NPs could be a promising agent for targeted multimodal imaging applications.

New "Destruction Seek to Survive" Strategy Based on a Serum Albumin Assembly with a Squaraine Molecule for the Detection of Peroxynitrite

Peroxynitrite (ONOO^-), a kind of active nitrogen species, plays an important role in biological systems. Overproduction of ONOO^- is closely related to the pathogenesis of many diseases. Therefore, it is necessary to quantify intracellular ONOO^- for differentiating health and disease states. Fluorescent probes with near-infrared (NIR) fluorescence can detect ONOO^- with high sensitivity and selectivity. However, there is an inevitable problem that many NIR fluorophores are easily oxidized by ONOO^- to give a false-negative result. To avoid this problem, herein, we ingeniously propose a "destruction to seek to survive" strategy to detect ONOO^-. Two NIR squaraine (SQ) dyes were connected together to form a fluorescent probe (SQDC). This method utilizes the destructive effect of peroxynitrite on one of the SQ moieties of SQDC to eliminate the steric hindrance, enabling the other "survived" SQ segment to enter the hydrophobic cavity of bovine serum albumin (BSA) via the well-known host-guest interactions. The encapsulation of albumin protects the "survived" SQ from further attack of ONOO^-. As a result, a NIR fluorescence turn-on response coming from the host-guest interaction between BSA and the "survived" SQ escaped from SQDC was found, which can be used for the detection of ONOO^-. The assembly of SQDC mixed with BSA can be located in mitochondria to detect endogenous and exogenous ONOO^- sensitively in living cells. As a proof-of-concept method, it is envisioned that this novel detection strategy with a simple assembly would become a powerful means for the detection of ONOO^- when employing NIR fluorophores.

Formation of Direction-Controllable Pseudorotaxane and Catenane Using Chemically Cyclized Oligodeoxynucleotides and Their Noncovalent RNA Labeling

The formation of interlocked structures, such as rotaxane and catenane, enables noncovalent conjugations. We previously confirmed that the chemically cyclized pseudorotaxane-forming oligodeoxynucleotides (prfODNs) with double-tailed parts formed a pseudorotaxane structure with the target DNA and RNA via the slipping process. Here, we report the one-step synthesis of cyclized prfODNs from alkyne-modified ODNs, after which we investigated the properties and mechanism of the slipping process and performed noncovalent RNA labeling with prfODNs. Additionally, the catenane structure was formed by the combination of pseudorotaxane formation with a 5'-end-phosphorylated RNA and enzymatic ligation. The newly synthesized prfODN represents a new tool for achieving the noncovalent conjugation of various functional moieties to RNAs.

Two-photon fluorescence imaging and specifically biosensing of norepinephrine on a 100-ms timescale

Norepinephrine (NE) is a key neurotransmitter in the central nervous system of organisms; however, specifically tracking the transient NE dynamics with high spatiotemporal resolution in living systems remains a great challenge. Herein, we develop a small molecular fluorescent probe that can precisely anchor on neuronal cytomembranes and specifically respond to NE on a 100-ms timescale. A unique dual acceleration mechanism of molecular-folding and water-bridging is disclosed, which boosts the reaction kinetics by ˃10⁵ and ˃10³ times, respectively. Benefiting from its excellent spatiotemporal resolution, the probe is applied to monitor NE dynamics at the single-neuron level, thereby, successfully snapshotting the fast fluctuation of NE levels at neuronal cytomembranes within 2 s. Moreover, two-photon fluorescence imaging of acute brain tissue slices reveals a close correlation between downregulated NE levels and Alzheimer's disease pathology as well as antioxidant therapy.

A novel near-infrared ratiometric fluorescent probe targeting lysosomes for imaging HOCl in vitro and in vivo

Hypochlorous acid (HOCl), as an important biological reactive oxygen species (ROS), plays an important role in microbial immune defense and inflammatory response. Abnormal levels of HOCl in lysosomes can cause lysosomal membrane rupture and release of various hydrolases, leading to a variety of diseases, including cancer. In order to better monitor the level of HOCl in lysosomes, phenothiazine was chosen as fluorophore to construct a NIR fluorescent probe PMM with intramolecular change transfer process (ICT). PMM is a colorimetric and ratiometric fluorescent probe, which has high sensitivity with a low detection limit (20 nM), high selectivity and anti-interference. PMM has good stability in the weakly acidic environment of pH 4.0-5.5. PPM has good localization ability for lysosomes and has been successfully used for fluorescence imaging of exogenous and endogenous HOCl in HepG2 cells. Moreover, nude mouse imaging also demonstrated that PMM could be used to detect HOCl in vivo.

Single-molecule characterization of a bright and photostable deep-red fluorescent squaraine-figure-eight (SF8) dye

Squaraine Figure Eight (SF8) dyes are a unique class of deep-red fluorescent dyes with self-threaded molecular architecture that provides structural rigidity while simultaneously encapsulating and protecting the emissive fluorochrome. Previous cell microscopy and bulk phase studies of SF8 dyes indicated order of magnitude enhancements in photostability over conventional pentamethine cyanine dyes such as Cy5. Studies conducted at the single molecule level now reveal that these ensemble level enhancements carry over to the single molecule level in terms of enhanced emission quantum yields, longer times to photobleaching, and enhanced total photon yields. When compared to Cy5, the SF8-based dye SF8(D4)2 possesses a three-fold larger single molecule emission quantum yield, exhibits order of magnitude longer average times before photobleaching, and exhibits twenty times larger photon yields. Additional features such as water solubility, fluorochrome encapsulation to protect it against nucleophilic attack, and selective biomarker targeting capability make SF8-based dyes promising candidates for biological labeling and microscopy applications and single molecule tracking.

Silica nanoparticle remodeling under mild conditions: versatile one step conversion of mesoporous to hollow nanoparticles with simultaneous payload loading

A binary mixture of mesoporous silica nanoparticles plus organic polyammonium additive (dye or drug) is cleanly converted upon mild heating into hollow nanoparticles. The remodeled nanoparticle shell is an organized nanoscale assembly of globular additive/silica subunits and cancer cell assays show that a loaded drug additive is bioavailable.

A rotaxane-based platform for tailoring the pharmacokinetics of cancer-targeted radiotracers

Radiolabelled monoclonal antibodies (mAbs) are a cornerstone of molecular diagnostic imaging and targeted radioimmunotherapy in nuclear medicine, but one of the major challenges in the field is to identify ways of reducing the radiation burden to patients. We reasoned that a rotaxane-based platform featuring a non-covalent mechanical bond between the radionuclide complex and the biologically active mAb could offer new ways of controlling the biophysical properties of cancer-specific radiotracers for positron emission tomography (PET). Herein, we present the photoradiosynthesis and characterisation of [⁸⁹Zr]ZrFe-[4]rotaxane-azepin-onartuzumab ([⁸⁹Zr]ZrFe-2), a unique rotaxane-antibody conjugate for PET imaging and quantification of the human hepatocyte growth factor receptor (c-MET). Multiple component self-assembly reactions were combined with simultaneous ⁸⁹Zr-radiolabelling and light-induced bioconjugation methods to give [⁸⁹Zr]ZrFe-2 in 15 ± 1% (n = 3) decay-corrected radiochemical yield, with >90% radiochemical purity, and molar activities suitable for PET imaging studies (>6.1 MBq mg^-1 of protein). Cellular assays confirmed the specificity of [⁸⁹Zr]ZrFe-2 binding to the c-MET receptor. Temporal PET imaging in athymic nude mice bearing subcutaneous MKN-45 gastric adenocarcinoma xenografts demonstrated specific binding of [⁸⁹Zr]ZrFe-2 toward c-MET in vivo, where tumour uptake reached 9.8 ± 1.3 %ID g^-1 (72 h, n = 5) in a normal group and was reduced by ∼56% in a control (blocking) group. Head-to-head comparison of the biodistribution and excretion profile of [⁸⁹Zr]ZrFe-2versus two control compounds, alongside characterisation of two potential metabolites, showed that the rotaxane-radiotracer has an improved clearance profile with higher tumour-to-tissue contrast ratios and reduced radiation exposure to critical (dose-limiting) organs including liver, spleen, and kidneys. Collectively, the experimental results suggested that non-covalent mechanical bonds between the radionuclide and mAb can be used to fine-tune the pharmacokinetic profile of supramolecular radiopharmaceuticals in ways that are simply not accessible when using traditional covalent design.

An indanone-based fluorescent probe for detection and imaging of Cys/Hcy in living cells

Effective detection of Cys and Hcy plays an important role in the diagnosis of diseases. In this work, a novel indanone-based fluorescent probe INIAc-CN for sensitively and effectively detecting Cys and Hcy was developed. The probe exhibited weak fluorescence, but obvious fluorescent enhancement after reacted with Cys/Hcy. Moreover, the good anti-interference and low cytotoxicity of the probe made it successfully applied for monitoring Cys and Hcy of in living cells.

P800SO3-PEG: a renal clearable bone-targeted fluorophore for theranostic imaging

BACKGROUND

Due to the deep tissue penetration and reduced scattering, NIR-II fluorescence imaging is advantageous over conventional visible and NIR-I fluorescence imaging for the detection of bone growth, metabolism, metastasis, and other bone-related diseases.

METHODS

Bone-targeted heptamethine cyanine fluorophores were synthesized by substituting the meso-carbon with a sulfur atom, resulting in a bathochromic shift and increased fluorescence intensity. The physicochemical, optical, and thermal stability of newly synthesized bone-targeted NIR fluorophores was performed in aqueous solvents. Calcium binding, bone-specific targeting, biodistribution, pharmacokinetics, and 2D and 3D NIR imaging were performed in animal models.

RESULTS

The newly synthesized S-substituted heptamethine fluorophores demonstrated a high affinity for hydroxyapatite and calcium phosphate, which improved bone-specific targeting with signal-background ratios > 3.5. Particularly, P800SO3-PEG showed minimum nonspecific uptake, and most unbound molecules were excreted into the urinary bladder. Histological analyses demonstrated that P800SO3-PEG remained stable in the bone for over two weeks and was incorporated into bone matrices. Interestingly, the flexible thiol ethylene glycol linker on P800SO3-PEG induced a promising photothermal effect upon NIR laser irradiation, demonstrating potential theranostic imaging.

CONCLUSIONS

P800SO3-PEG shows a high affinity for bone tissues, deeper tissue imaging capabilities, minimum nonspecific uptake in the major organs, and photothermal effect upon laser irradiation, making it optimal for bone-targeted theranostic imaging.

Diagnosis of Alzheimer's Disease and In Situ Biological Imaging via an Activatable Near-Infrared Fluorescence Probe

Alzheimer's disease (AD) is a common neurodegenerative disease that makes the brain nervous system degenerate rapidly and is accompanied by some special cognitive and behavioral dysfunction. Recently, butyrylcholinesterase (BChE) was reported as an important enzyme, whose activity can provide predictive value for timely discovery and diagnosis of AD. Therefore, it is indispensable to design a detection tool for selective and rapid response toward BChE. In this study, we developed a novel near-infrared fluorescent probe (Chy-1) for the detection of BChE activity. An excellent sensitivity, good biocompatibility, and lower limit of detection (LOD) of 0.12 ng/mL made the probe extremely specific for BChE, which was successfully used in biological imaging. What is more, Chy-1 can not only clearly distinguish tumor from normal cells but also forms a clear boundary between the normal and cancer tissues due to the obvious difference in fluorescence intensity produced via in situ spraying. Most important of all, Chy-1 was also successfully applied to track the BChE activity in AD mouse models. Based on this research, the novel probe may be a powerful tool for clinical diagnosis and therapy of tumor and neurodegenerative diseases.

Construction of Red Fluorescent Dual Targeting Mechanically Interlocked Molecules for Live Cancer Cell Specific Lysosomal Staining and Multicolor Cellular Imaging

We have designed and synthesized red fluorescent mechanically interlocked molecules with dual targeting functionality for live cancer cell specific active targeting followed by selective internalization and imaging of malignant lysosomes along with real-time tracking, 3D, and multicolor cellular imaging applications.

Supramolecular Rotaxane‐Based Multi‐Modal Probes for Cancer Biomarker Imaging**

Mechanically interlocked molecules present opportunities to construct therapeutic drugs and diagnostic imaging agents but harnessing supramolecular chemistry to make biologically active probes in water is a challenge. Here, we describe a rotaxane‐based approach to synthesise radiolabelled proteins and peptides for molecular imaging of cancer biomarkers in vivo. Host–guest chemistry using β‐cyclodextrin‐ and cucurbit[6]uril‐catalysed cooperative capture synthesis produced gallium‐68 or zirconium‐89 radiolabelled metallo[4]rotaxanes. Photochemical conjugation to trastuzumab led to a viable positron emission tomography (PET) radiotracer. The rotaxane architecture can be tuned to accommodate different radiometal ion complexes, other protein‐ or peptide‐based drugs, and fluorophores for optical detection. This technology provides a platform to explore how mechanical bonding can improve drug delivery, enhance tumour specificity, control radiotracer pharmacokinetics, and reduce dosimetry.

Cell-permeable fluorescent indicator for imaging formaldehyde activity in living systems

Formaldehyde (FA), as a reactive signaling molecule, plays an important role in living systems through a diverse array of cellular pathways. However, no systematic investigation for detection and imaging of FA by rendering cells transiently permeable has been reported yet. Specifically, we developed a new cell-permeable fluorescence probe functionality that was enhanced cellular entry efficiency and well retained intracellularly after activation for visualizing endogenous FA changes. Moreover, a smart "multi-lock system -key-and-lock" strategy，which have provoked a starting point for the use of probe and related biochemical tools to monitor FA in lysosomes. The versatile "latent" fluorophore that can undergo a subsequent self-immolative spacer for interrogating the roles and functions of FA in living systems as well as related biomedical applications.

2D and 3D metal-organic frameworks constructed with a mechanically rigidified [3]rotaxane ligand

A mechanically interlocked [3]rotaxane was newly designed, synthesized, and employed as a ligand for constructing metal-organic frameworks (MOFs). The nano-confinement by macrocycles forces the soft bis-isophthalate axle into a pseudo-rigid conformation and coordinates to zinc(II) ions, affording a two- or three-dimensional MOF under controlled conditions. The 2D MOF exhibits a neutral framework with a periodic puckering sheet structure, while an anionic framework with a pts topology was observed for the 3D MOF. The phase purity of both bulk materials was confirmed by powder X-ray diffraction. Thermogravimetric analysis reveals that both materials are stable up to 250 °C. The success of applying mechanical bonds to rigidify flexible ligands provides new insights for the design of metal-organic frameworks.

Pillar[5]arene-Based Fluorescent Sensor Array for Biosensing of Intracellular Multi-neurotransmitters through Host-Guest Recognitions

Neurotransmitters are very important for neuron events and brain diseases. However, effective probes for analyzing specific neurotransmitters are currently lacking. Herein, we design and create a supramolecular fluorescent probe (CN-DFP5) by synthesizing a dual-functionalized fluorescent pillar[5]arene derivative with borate naphthalene and aldehyde coumarin recognition groups to identify large-scale neurotransmitters. The developed probe can detect seven model neurotransmitters by generating different fluorescence patterns through three types of host-guest interactions. The obtained signals are statistically processed by principal component analysis, thus the high-throughput analysis of neurotransmitters is realized under dual-channel fluorescence responses. The present probe combines the advantages of small-molecule-based probes to easily enter into living neurons and cross-reactive sensor arrays. Thus, the selective binding enables this probe to identify specific neurotransmitters in biofluids, living neurons, and tissues. High selectivity and sensitivity further demonstrate that the molecular device could extend to more applications to detect and image neurotransmitters.

Achieving Adjustable Multifunction Based on Host-Guest Interaction-Manipulated Reversible Molecular Conformational Switching

Small molecules that are capable of toggling between multiple and definite conformational states under external stimuli have great potential for use in molecular switches or sensors. However, currently developed regulation approaches for these switchable molecules mostly involve covalent bond-breaking/reforming processes, thereby inevitably producing byproducts or causing fatigue accumulation. Herein, we report a simple but successful model whose molecular conformation can be precisely manipulated between stretched and folded forms by employing host-guest interactions with rigid macrocycles, thus avoiding possible side reactions and fatigue accumulation and possessing excellent reversibility. Moreover, the conformation states of this molecule can be visualized and identified by luminous readout, endowing it with real-time self-reporting features. Furthermore, this controllable and reversible conformational conversion is accompanied by various valuable functions, including controllable multicolor emission; ratiometric fluorescent thermosensing with high temperature resolution, excellent reversibility, lock/unlock switching, and especially linear detection range tunability; and in addition real-time intracellular temperature sensing and imaging, disclosing the intriguing microscopic "conformation-function" relationship based on a single molecule.

A deep-red lysosome-targetable fluorescent probe for detection of hypochlorous acid in pure water and its imaging application in living cells and zebrafish

Hypochlorite plays a significant role in physiological processes, particularly regulation of lysosomal functions, and is involved in various diseases. Thus, it is crucial to develop highly sensitive and selective molecule tools to detect HClO in lysosomes. Herein, a novel 2H-benzo[h]chromene-pyridine derivative (1) was synthesized through condensation reaction, which exhibited a notable deep-red emission at 640 nm in pure water. This deep-red emission was specifically quenched by adding ClO^-. The response of probe 1 toward ClO^- was rapid (within 10 s), sensitive (detection limit of 0.012 μM), and effective over a wide range of pH (1.0-12.0). Due to the existence of morpholine as the lysosome-targeting unit, the probe was successfully utilized to monitor lysosomal ClO^-. Moreover, the probe 1 was also applied to detecting ClO^- in zebrafish.

Rational design of stable heptamethine cyanines and development of a biomarker-activatable probe for detecting acute lung/kidney injuries via NIR-II fluorescence imaging

Heptamethine cyanines exhibiting high photo- and chemostability have been developed. And an activatable probe was developed for H2O2 to visualize acute lung and kidney injuries via NIR-II fluorescence imaging.

Modular Design of High-Brightness pH-Activatable Near-Infrared BODIPY Probes for Noninvasive Fluorescence Detection of Deep-Seated Early Breast Cancer Bone Metastasis: Remarkable Axial Substituent Effect on Performance

We herein report a series of high-brightness pH-activatable near-infrared (NIR) BODIPY probes for high-contrast intravital imaging of deep-seated early breast cancer bone metastasis by harnessing the axial substituent effect. These probes exhibit tunable pK _a, higher brightness, and antiquenching capabilities in aqueous solution, which can be simultaneously adjusted by axial steric substituents. The optimized probe BODO-3 bearing axial dimethyl substituents exhibited a higher pK _a value of 5.6 and a brighter NIR fluorescence under tumor acidic pH, showing 10.3-fold and 6.5-fold enhanced brightness (εΦ) at pH 5.5 and 6.5, respectively. Due to the higher brightness, BODO-3 with a brilliant NIR emission at 700 nm allows for deep optical penetrations of 5 and 8 mm at pH 6.5 and 4.5, respectively. Meanwhile, covalent functionalization with glucose (BODO-3-Glu) could further enhance breast cancer and its soft tissue metastasis imaging in vivo. Notably, covalent functionalization with bisphosphonate (BODO-3-PO ₃ H ₂ ) allowed the successful targeting and visualization of deep-seated bone metastases of breast cancer with a high tumor to normal contrast of 8/1, outperforming X-rays in early detection. This strategy may provide insights for designing high-brightness activatable NIR probes for detecting deep-seated tumors and metastases.

Post-Synthetic Macrocyclization of Rotaxane Building Blocks

Although not often encountered, cyclic interlocked molecules are appealing molecular targets because of their restrained tridimensional structure which is related to both the cyclic and interlocked shapes. Interlocked molecules such as rotaxane building blocks may be good candidates for post-synthetic intramolecular cyclization if the preservation of the mechanical bond ensures the interlocked architecture throughout the reaction. This is obviously the case if the modification does not involve the cleavage of either the macrocycle's main chain or the encircled part of the axle. However, among the post-synthetic reactions, the chemical linkage between two reactive sites belonging to embedded elements of rotaxanes still consists of an underexploited route to interlocked cyclic molecules. This Review lists the rare examples of macrocyclization through chemical connection between reactive sites belonging to a surrounding macrocycle and/or an encircled axle of interlocked rotaxanes.

Terminal protection of peptides by interactions with proteins: A "signal-on" peptide-templated gold nanocluster beacon for label-free protein detection

Characterization of the protein-peptide interactions are a critical for understanding the functions and signal pathways of proteins. Herein, a new finding of universal terminal protection that protein bind specifically with peptide and provide a protective coating to prevent peptide hydrolysis in the presence of peptidase. On the basis of this mechanism, we first reported a novel label-free fluorescence biosensor strategy that utilizes the protection of specific terminal protein on peptide-templated gold nanocluster (AuNCs) beacon for the detection of proteins. The fluorescence quenching of peptide-templated AuNCs can be effectively inhibited with increasing concentration of the specific protein, exhibiting a satisfactory sensitivity and selectivity toward protein with the detection limit of MDM2 and gp120 are 0.0019 U/mL and 0.0012 U/mL, respectively. The developed label-free fluorescence biosensor strategy provides new ideas to detect and screen protein for analyzing protein-peptide interaction in biomedical applications.

Dynamic covalent self-assembly of mechanically interlocked molecules solely made from peptides

Mechanically interlocked molecules (MIMs), such as rotaxanes and catenanes, have captured the attention of chemists both from a synthetic perspective and because of their role as simple prototypes of molecular machines. Although examples exist in nature, most synthetic MIMs are made from artificial building blocks and assembled in organic solvents. Synthesis of MIMs from natural biomolecules remains highly challenging. Here we report on a synthesis strategy for interlocked molecules solely made from peptides—mechanically interlocked peptides (MIPs). Fully peptidic, cysteine-decorated building blocks were self-assembled in water to generate disulfide-bonded dynamic combinatorial libraries consisting of multiple different rotaxanes, catenanes and daisy chains as well as more exotic structures. Detailed NMR spectroscopy and mass spectrometry characterization of a [2]catenane comprised of two peptide macrocycles revealed that this structure has rich conformational dynamics reminiscent of protein folding. Thus, MIPs can serve as a bridge between fully synthetic MIMs and those found in nature.

Ultrabright Fluorescent Polymeric Nanofibers and Coatings Based on Ionic Dye Insulation with Bulky Counterions

Preparation of bright fluorescent materials based on polymers is hampered by a fundamental problem of aggregation-caused quenching (ACQ) of encapsulated dyes. Here, ultrabright fluorescent polymeric nanofibers and coatings are prepared based on a concept of ionic dye insulation with bulky hydrophobic counterions that overcomes the ACQ problem. It is found that bulky hydrophobic counterion perfluorinated tetraphenylborate can boost >100-fold the fluorescence quantum yields of cationic dye octadecyl rhodamine B at high loading (30 wt %) in biocompatible poly(methyl methacrylate) (PMMA). The concept is applicable to both rhodamine and cyanine dyes, which results in bright fluorescent polymeric materials of four different colors spanning from blue to near-infrared. It allows for preparation of electrospun polymeric nanofibers with >50-fold higher dye loading by mass (30 wt %, >20-fold higher molarity for rhodamine dyes) while preserving good fluorescence quantum yields (31%), which implies drastic improvement in their fluorescence brightness. The counterion-based polymeric materials are also validated as coatings of model medical devices, such as stainless steel fiducials and 3D-printed stents of complex geometry. Spin-coated fluorescent polymeric films loaded with a dye paired with bulky counterions exhibit excellent biocompatibility and low toxicity. Moreover, counterion-modified materials show much better stability against dye leakage in the presence of living cells and a serum-containing medium, compared to materials based on the dye with a small inorganic anion. Overall, by pushing the barriers of ACQ, our counterion approach emerges as a powerful tool to develop ultrabright fluorescent polymeric materials ranging from nano- and macroscale.

Nanohoop Rotaxane Design to Enhance the Selectivity of Reaction-Based Probes: A Proof-of-Principle Study

Mechanical interlocking of a nanohoop fluorophore and a reactive thread couples the benefits of a reaction-based probe with a sterically congested active site for enhanced selectivity. Advantageously, the thread design uses dual function stoppers that act as both a quencher and a trigger for sensing. In progress toward expanding this approach to biologically relevant analytes, this system is used to demonstrate steric differentiation and provide a selective turn-on fluorescent response with size selectivity for HS^- rather than larger thiolates.

Structural Engineering of Fluorescent Self-Threaded Peptide Probes for Targeted Cell Imaging†

Squaraine figure-eight (SF8) molecules are a new class of deep-red fluorescent probes that are well suited for fluorescence cell microscopy due to their very high fluorescence brightness and excellent stability. Three homologous SF8 probes, with peptidyl loops that differ by very minor changes in the peptide sequence, were synthesized and assessed for probe uptake by cancer cells. One of probes included the RGD motif that is recognized by many classes of integrin receptors that reside on the surface of the cancer cells, and it permeated the cells by receptor-mediated endocytosis. In contrast, cell microscopy showed that there was negligible cell uptake of the two homologous SF8 probes indicating differences in probe targeting capability. The synthetic method allows for easy alteration of the peptide sequence; thus, it is straightforward to develop new classes of peptidyl SF8 probes with loop sequences that target other cancer biomarkers.

Specific intracellular binding peptide as sPD-L1 antibody mimic: Robust binding capacity and intracellular region specific modulation upon applied to sensing research

Programmed death ligand 1 (PD-L1) immune checkpoint has been regarded as a new target for predicting cancer immunotherapy. As a transmembrane protein, PD-L1 has very low blood concentration and is likely to deplete their native activity when separated from the membrane environment due to significant hydrophobic domains, which make it difficult to measure sensitively. The reported PD-L1 aptamers and antibodies are both extracellular region binding molecules with the overlapping binding sites, which seriously limit with the construction of biosensor. Specific intracellular binding peptide (SIBP) as a unique PD-L1 intracellular region homing probe molecule is utilized for specifically capture targets. A simple and sensitive surface plasmon resonance (SPR) sandwich assay was constructed to detect serum soluble PD-L1 (sPD-L1) based on the unique and strong binding ability of SIBP to the intracellular region of sPD-L1. The designed SPR sensor showed great selectivity and wide dynamic response range of sPD-L1 concentration from 10 ng/mL to 2000 ng/mL. The limit of detection was calculated to be 1.749 ng/mL (S/N = 3). Owing to the SIBP's strong and specific binding ability with sPD-L1, the sensitive sensor can successfully detect sPD-L1 in serum samples, paving the way for the development of efficient test tools for clinical diagnosis and analysis.

Chiral figure-eight molecular scaffold for fluorescent probe development

Targeted fluorescent molecular probes are useful for cell microscopy, diagnostics, and biological imaging. An emerging discovery paradigm is to screen libraries of fluorescent molecules and identify hit compounds with interesting targeting properties. However, a current limitation with this approach is the lack of fluorescent molecular scaffolds that can produce libraries of probe candidates with three dimensional globular shape, chiral centers, and constrained conformation. This study evaluated a new probe scaffold called squaraine figure-eight (SF8), a self-threaded molecular architecture that is comprised of an encapsulated deep-red fluorescent squaraine dye, surrounding tetralactam macrocycle, and peripheral loops. Easy synthetic variation of the loops produced four chiral isomeric SF8 probes, with the same log P values. Cell microscopy showed that subtle changes in the loop structure led to significant differences in intracellular targeting. Most notably, a comparison of enantiomeric probes revealed a large difference in mitochondrial accumulation, very likely due to differences in affinity for a chiral biomarker within the organelle. A tangible outcome of the research is a probe candidate that can be: (a) developed further as a bright and photostable, deep-red fluorescent probe for mitochondrial imaging, and (b) used as a molecular tool to identify the mitochondrial biomarker for selective targeting. It will be straightforward to expand the SF8 probe chemical space and produce structurally diverse probe libraries with high potential for selective targeting of a wide range of biomarkers.

Hydrazone exchange: a viable route for the solid-tethered synthesis of [2]rotaxanes

Using a hydrazone exchange methodology, resin beads were functionalised with [2]rotaxanes at up to 80% efficiency—higher than using other dynamic or irreversible synthetic approaches to form self-assembled structures on solid supports.