Activity-Based Sensing: A Synthetic Methods Approach for Selective Molecular Imaging and Beyond

Fluorescent Probes Visualize Phytohormone: Research Status and Opportunities

Phytohormones, as crucial regulatory factors in plant growth and development, have garnered increased interest in improving crop stress resistance. It is essential to comprehend the distribution of phytohormones in plants to assess their health status and investigate their functions. This knowledge also serves as a guide for developing and using plant growth regulators. The advancement of fluorescent probe technology, along with the wide range of fluorophores and improvements in imaging methods, has made it a successful approach for monitoring phytohormones in plants. This technique has been confirmed to be effective in plants, particularly in detecting the response of fluorescent probes to phytohormones. In this Perspective, we highlight the utility of fluorescent probes in measuring and visualizing the distribution of phytohormones in plants under external stress. However, the visualization of phytohormones with high spatial resolution and the achievement of high biocompatibility in living plants have posed significant challenges for researchers. Nonetheless, there are also many untapped opportunities in this field. This paper seeks to delve into the potential for further discussion on the subject.

An Activity-Based Sensing Approach to Multiplex Mapping of Labile Copper Pools by Stimulated Raman Scattering

Molecular imaging with analyte-responsive probes offers a powerful chemical approach to studying biological processes. Many reagents for bioimaging employ a fluorescence readout, but the relatively broad emission bands of this modality and the need to alter the chemical structure of the fluorophore for different signal colors can potentially limit multiplex imaging. Here, we report a generalizable approach to multiplex analyte imaging by leveraging the comparably narrow spectral signatures of stimulated Raman scattering (SRS) in activity-based sensing (ABS) mode. We illustrate this concept with two copper Raman probes (CRPs), CRP2181 and CRP2153.2, that react selectively with loosely bound Cu(I/II) and Cu(II) ions, respectively, termed the labile copper pool, through copper-directed acyl imidazole (CDAI) chemistry. These reagents label proximal proteins in a copper-dependent manner using a dye scaffold bearing a 13C≡N or 13C≡15N isotopic SRS tag with nearly identical physiochemical properties in terms of shape and size. SRS imaging with the CRP reagents enables duplex monitoring of changes in intracellular labile Cu(I) and Cu(II) pools upon exogenous copper supplementation or copper depletion or genetic perturbations to copper transport proteins. Moreover, CRP imaging reveals reciprocal increases in labile Cu(II) pools upon decreases in activity of the antioxidant response nuclear factor-erythroid 2-related factor 2 (NRF2) in cellular models of lung adenocarcinoma. By showcasing the use of narrow-bandwidth ABS probes for multiplex imaging of copper pools in different oxidation states and identifying alterations in labile metal nutrient pools in cancer, this work establishes a foundation for broader SRS applications in analyte-responsive imaging in biological systems.

Designing small-molecule and macromolecule sensors for imaging redox-active transition metal signaling

Transition metals play essential roles in biology, where these nutrients regulate protein activity as active site cofactors or via metalloallostery. In contrast, dysregulation of transition metal homeostasis can lead to unique metal-dependent signaling pathways connected to aging and disease, such as cuproptosis and ferroptosis for copper- and iron-dependent cell death or cuproplasia and ferroplasia for copper- and iron-dependent cell growth and proliferation, respectively. New methods that enable detection of bioavailable transition metal pools with both metal and oxidation state specificity can help decipher their contributions to health and disease. Here we summarize recent advances in designing sensors for imaging transition metals and their applications to uncover new metal biology.

Unlocking the electrochemical functions of biomolecular condensates

Biomolecular condensation is a key mechanism for organizing cellular processes in a spatiotemporal manner. The phase-transition nature of this process defines a density transition of the whole solution system. However, the physicochemical features and the electrochemical functions brought about by condensate formation are largely unexplored. We here illustrate the fundamental principles of how the formation of condensates generates distinct electrochemical features in the dilute phase, the dense phase and the interfacial region. We discuss the principles by which these distinct chemical and electrochemical environments can modulate biomolecular functions through the effects brought about by water, ions and electric fields. We delineate the potential impacts on cellular behaviors due to the modulation of chemical and electrochemical environments through condensate formation. This Perspective is intended to serve as a general road map to conceptualize condensates as electrochemically active entities and to assess their functions from a physical chemistry aspect.

Rational Design of an Activatable Near-Infrared Fluorogenic Platform for In Vivo Orthotopic Tumor Imaging and Resection

Rational and effective design of a universal near-infrared (NIR) light-absorbed platform employed to prepare diverse activatable NIR fluorogenic probes for in vivo imaging and the imaging-guided tumor resection remains less exploited but highly meaningful. Herein, mandelic acid with a core structure of 4-hydroxylbenzyl alcohol to link recognition unit, a fluorophore and a quencher was employed to prepare activatable probes. We exemplified ester as carboxylesterase (CE)-recognized unit, ferrocene as quencher and phenothiazinium as NIR fluorophore to afford fluorogenic probes termed NBS-Fe-CE and NBS-C-Fe-CE. These probes enabled the conversion toward CE with significant fluorescence increases and successfully discriminate CE activity in cells. NIR light enhances the tumor penetration and enable imaging-guided orthotopic tumor resection. This specific case demonstrated that this platform can be effectively used to construct diverse NIR probes for imaging analytes in biological systems.

Enhancing Molecular-Level Biological Monitoring with a Smart Self-Assembling 19F-Labeled Probe

Real-time monitoring of molecular transformations is crucial for advancements in biotechnology. In this study, we introduce a novel self-assembling 19F-labeled nuclear magnetic resonance (NMR) probe that disassembles upon interaction with various nucleotides. This interaction not only activates the 19F signals but also produces distinct signatures for each specific component, thereby enabling precise identification and quantification of molecules in evolving samples. We demonstrate the capability of this probe for real-time monitoring of adenosine triphosphate (ATP) hydrolysis and screening potential enzyme inhibitors. These applications highlight the probe's significant potential in enzyme analysis, drug development, and disease diagnostics.

Hydrogen peroxide responsive theranostics for cancer-selective activation of DNA alkylators and real-time fluorescence monitoring in living cells

Triple negative breast cancer (TNBC) is a notoriously difficult disease to treat, and many of the existing TNBC chemotherapeutics lack tumor selectivity and the capability for simultaneously visualizing and monitoring their own activity in the biological context. However, TNBC cells have been known to generate high levels of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2). To this end, three novel small molecule theranostics 1a, 1c, and 2 consisting of both H2O2-responsive nitrogen mustard prodrug and profluorophore character have been designed, synthesized, and evaluated as targeted cancer therapeutics and bioimaging agents. The three theranostics comprise of boronate esters that deactivate nitrogen mustard functional groups and fluorophores but allow their selective activation through H2O2-specific oxidative deboronation for the release of the active drug and fluorophore. The three theranostics demonstrated H2O2-inducible DNA-alkylating capability and fluorescence turn-on properties in addition to selective anticancer activity. They are particularly effective in killing TNBC MDA-MB-468 cells with high H2O2 level while safe to normal epithelial MCF-10A cell. The conjugated boron-masked fluorophores in 1c and 2 are highly responsive towards H2O2, which enabled tracking of the theranostics in living cellular mitochondria and nucleus organelles. The three theranostics 1a, 1c, and 2 are capable of both selective release of the active drug to take effect in H2O2-rich cancer sites and simultaneously monitoring its activity. This single molecule system is of utmost importance to understand the function, efficacy, and mechanism of the H2O2-activated prodrugs and theranostics within the living recipient.

Dual-stimuli responsive theranostic agents based on small molecules

Stimuli-responsive theranostic agents represent a class of molecules that integrate therapeutic and diagnostic functions, offering the capability to respond to disease-associated biomarkers. Dual-stimuli responsive agents, particularly those based on small molecules, have shown considerable promise for precise imaging-guided therapeutic applications. In this Highlight, we summarize the progress of dual-stimuli responsive theranostic agents based on small molecules, for diagnostic and therapeutic studies in biological systems. The Highlight focuses on comparing different responsive groups and chemical structures of these dual-stimuli responsive theranostic agents towards different biomarkers. The potential future directions of the agents for further applications in biological systems are also discussed.

Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

A tandem activity-based sensing and labeling strategy reveals antioxidant response element regulation of labile iron pools

Iron is an essential element for life owing to its ability to participate in a diverse array of oxidation-reduction reactions. However, misregulation of iron-dependent redox cycling can also produce oxidative stress, contributing to cell growth, proliferation, and death pathways underlying aging, cancer, neurodegeneration, and metabolic diseases. Fluorescent probes that selectively monitor loosely bound Fe(II) ions, termed the labile iron pool, are potentially powerful tools for studies of this metal nutrient; however, the dynamic spatiotemporal nature and potent fluorescence quenching capacity of these bioavailable metal stores pose challenges for their detection. Here, we report a tandem activity-based sensing and labeling strategy that enables imaging of labile iron pools in live cells through enhancement in cellular retention. Iron green-1 fluoromethyl (IG1-FM) reacts selectively with Fe(II) using an endoperoxide trigger to release a quinone methide dye for subsequent attachment to proximal biological nucleophiles, providing a permanent fluorescent stain at sites of elevated labile iron. IG1-FM imaging reveals that degradation of the major iron storage protein ferritin through ferritinophagy expands the labile iron pool, while activation of nuclear factor-erythroid 2-related factor 2 (NRF2) antioxidant response elements (AREs) depletes it. We further show that lung cancer cells with heightened NRF2 activation, and thus lower basal labile iron, have reduced viability when treated with an iron chelator. By connecting labile iron pools and NRF2-ARE activity to a druggable metal-dependent vulnerability in cancer, this work provides a starting point for broader investigations into the roles of transition metal and antioxidant signaling pathways in health and disease.

Xanthene-Based Dyes for Photoacoustic Imaging and their Use as Analyte-Responsive Probes

Developing imaging tools that can report on the presence of disease-relevant analytes in multicellular organisms can provide insight into fundamental disease mechanisms as well as provide diagnostic tools for the clinic. Photoacoustic imaging (PAI) is a light-in, sound-out imaging technique that allows for high resolution, deep-tissue imaging with applications in pre-clinical and point-of-care settings. The continued development of near-infrared (NIR) absorbing small-molecule dyes promises to improve the capabilities of this emerging imaging modality. For example, new dye scaffolds bearing chemoselective functionalities are enabling the detection and quantification of disease-relevant analytes through activity-based sensing (ABS) approaches. Recently described strategies to engineer NIR absorbing xanthenes have enabled development of analyte-responsive PAI probes using this classic dye scaffold. Herein, we present current strategies for red-shifting the spectral properties of xanthenes via bridging heteroatom or auxochrome modifications. Additionally, we explore how these strategies, coupled with chemoselective spiroring-opening approaches, have been employed to create ABS probes for in vivo detection of hypochlorous acid, nitric oxide, copper (II), human NAD(P)H: quinone oxidoreductase isozyme 1, and carbon monoxide. Given the versatility of the xanthene scaffold, we anticipate continued growth and development of analyte-responsive PAI imaging probes based on this dye class.

Nanomedicine Targeting Cuproplasia in Cancer: Labile Copper Sequestration Using Polydopamine Particles Blocks Tumor Growth In Vivo through Altering Metabolism and Redox Homeostasis

Copper plays critical roles as a metal active site cofactor and metalloallosteric signal for enzymes involved in cell proliferation and metabolism, making it an attractive target for cancer therapy. In this study, we investigated the efficacy of polydopamine nanoparticles (PDA NPs), classically applied for metal removal from water, as a therapeutic strategy for depleting intracellular labile copper pools in triple-negative breast cancer models through the metal-chelating groups present on the PDA surface. By using the activity-based sensing probe FCP-1, we could track the PDA-induced labile copper depletion while leaving total copper levels unchanged and link it to the selective MDA-MB-231 cell death. Further mechanistic investigations revealed that PDA NPs increased reactive oxygen species (ROS) levels, potentially through the inactivation of superoxide dismutase 1 (SOD1), a copper-dependent antioxidant enzyme. Additionally, PDA NPs were found to interact with the mitochondrial membrane, resulting in an increase in the mitochondrial membrane potential, which may contribute to enhanced ROS production. We employed an in vivo tumor model to validate the therapeutic efficacy of PDA NPs. Remarkably, in the absence of any additional treatment, the presence of PDA NPs alone led to a significant reduction in tumor volume by a factor of 1.66 after 22 days of tumor growth. Our findings highlight the potential of PDA NPs as a promising therapeutic approach for selectively targeting cancer by modulating copper levels and inducing oxidative stress, leading to tumor growth inhibition as shown in these triple-negative breast cancer models.

A detection system using sensing motif-tethered oligodeoxynucleotides for multiplex biomolecular analysis

We developed a system to detect multiple target biomolecules through sensing motif-tethered oligodeoxynucleotides. DNA-based molecular probes gave the primary amine motif upon reaction with the target biomolecules, glutathione (GSH) and H2O2. After labelling with biotin, the product DNAs were selectively collected to be quantified by qPCR.

A mitochondrial-targeted activity-based sensing probe for ratiometric imaging of formaldehyde reveals key regulators of the mitochondrial one-carbon pool

Formaldehyde (FA) is both a highly reactive environmental genotoxin and an endogenously produced metabolite that functions as a signaling molecule and one-carbon (1C) store to regulate 1C metabolism and epigenetics in the cell. Owing to its signal-stress duality, cells have evolved multiple clearance mechanisms to maintain FA homeostasis, acting to avoid the established genotoxicity of FA while also redirecting FA-derived carbon units into the biosynthesis of essential nucleobases and amino acids. The highly compartmentalized nature of FA exposure, production, and regulation motivates the development of chemical tools that enable monitoring of transient FA fluxes with subcellular resolution. Here we report a mitochondrial-targeted, activity-based sensing probe for ratiometric FA detection, MitoRFAP-2, and apply this reagent to monitor endogenous mitochondrial sources and sinks of this 1C unit. We establish the utility of subcellular localization by showing that MitoRFAP-2 is sensitive enough to detect changes in mitochondrial FA pools with genetic and pharmacological modulation of enzymes involved in 1C and amino acid metabolism, including the pervasive, less active genetic mutant aldehyde dehydrogenase 2*2 (ALDH2*2), where previous, non-targeted versions of FA sensors are not. Finally, we used MitoRFAP-2 to comparatively profile basal levels of FA across a panel of breast cancer cell lines, finding that FA-dependent fluorescence correlates with expression levels of enzymes involved in 1C metabolism. By showcasing the ability of MitoRFAP-2 to identify new information on mitochondrial FA homeostasis, this work provides a starting point for the design of a broader range of chemical probes for detecting physiologically important aldehydes with subcellular resolution and a useful reagent for further studies of 1C biology.

Accelerated Activity-Based Sensing by Fluorogenic Reporter Engineering Enables to Rapidly Determine Unstable Analyte

Accurate detection of labile analytes through activity based fluorogenic sensing is meaningful but remains a challenge because of nonrapid reaction kinetic. Herein, we present a signaling reporter engineering strategy to accelerate azoreduction reaction by positively charged fluorophore promoted unstable anion recognition for rapidly sensing sodium dithionite (Na2S2O4), a kind of widespread used but harmful inorganic reducing agent. Its quick decomposition often impedes application reliability of traditional fluorogenic probes in real samples because of their slow responses. In this work, four azo-based probes with different charged fluorophores (positive, zwitterionic, neutral, and negative) were synthesized and compared. Among of them, with sequestration effect of positively charged anthocyanin fluorophore for dithionite anion via electrostatic attraction, the cationic probe Azo-Pos displayed ultrafast fluorogenic response (∼2 s) with the fastest response kinetic (kpos' = 0.373 s-1) that is better than other charged ones (kzwi' = 0.031 s-1, kneu' = 0.013 s-1, kneg' = 0.003 s-1). Azo-Pos was demonstrated to be capable to directly detect labile Na2S2O4 in food samples and visualize the presence of Na2S2O4 in living systems in a timely fashion. This new probe has potential as a robust tool to fluorescently monitor excessive food additives and biological invasion of harmful Na2S2O4. Moreover, our proposed accelerating strategy would be versatile to develop more activity-based sensing probes for quickly detecting other unstable analytes of interest.

Small-Molecule Fluorescent Probes for Binding- and Activity-Based Sensing of Redox-Active Biological Metals

Although transition metals constitute less than 0.1% of the total mass within a human body, they have a substantial impact on fundamental biological processes across all kingdoms of life. Indeed, these nutrients play crucial roles in the physiological functions of enzymes, with the redox properties of many of these metals being essential to their activity. At the same time, imbalances in transition metal pools can be detrimental to health. Modern analytical techniques are helping to illuminate the workings of metal homeostasis at a molecular and atomic level, their spatial localization in real time, and the implications of metal dysregulation in disease pathogenesis. Fluorescence microscopy has proven to be one of the most promising non-invasive methods for studying metal pools in biological samples. The accuracy and sensitivity of bioimaging experiments are predominantly determined by the fluorescent metal-responsive sensor, highlighting the importance of rational probe design for such measurements. This review covers activity- and binding-based fluorescent metal sensors that have been applied to cellular studies. We focus on the essential redox-active metals: iron, copper, manganese, cobalt, chromium, and nickel. We aim to encourage further targeted efforts in developing innovative approaches to understanding the biological chemistry of redox-active metals.

Friedel-Crafts reactions for biomolecular chemistry

Chemical tools and principles have become central to biological and medical research/applications by leveraging a range of classical organic chemistry reactions. Friedel-Crafts alkylation and acylation are arguably some of the most well-known and used synthetic methods for the preparation of small molecules but their use in biological and medical fields is relatively less frequent than the other reactions, possibly owing to the notion of their plausible incompatibility with biological systems. This review demonstrates advances in Friedel-Crafts alkylation and acylation reactions in a variety of biomolecular chemistry fields. With the discoveries and applications of numerous biomolecule-catalyzed or -assisted processes, these reactions have garnered considerable interest in biochemistry, enzymology, and biocatalysis. Despite the challenges of reactivity and selectivity of biomolecular reactions, the alkylation and acylation reactions demonstrated their utility for the construction and functionalization of all the four major biomolecules (i.e., nucleosides, carbohydrates/saccharides, lipids/fatty acids, and amino acids/peptides/proteins), and their diverse applications in biological, medical, and material fields are discussed. As the alkylation and acylation reactions are often fundamental educational components of organic chemistry courses, this review is intended for both experts and nonexperts by discussing their basic reaction patterns (with the depiction of each reaction mechanism in the ESI) and relevant real-world impacts in order to enrich chemical research and education. The significant growth of biomolecular Friedel-Crafts reactions described here is a testament to their broad importance and utility, and further development and investigations of the reactions will surely be the focus in the organic biomolecular chemistry fields.

Visualization of Endogenous Hypochlorite in Drug-Induced Liver Injury Mice via a Bioluminescent Probe Combined with Firefly Luciferase mRNA-Loaded Lipid Nanoparticles

Drug-induced liver injury (DILI) is a common liver disease with a high rate of morbidity, and its pathogenesis is closely associated with the overproduction of highly reactive hypochlorite (ClO-) in the liver. However, bioluminescence imaging of endogenous hypochlorite in nontransgenic natural mice remains challenging. Herein, to address this issue, we report a strategy for imaging ClO- in living cells and DILI mice by harnessing a bioluminescent probe formylhydrazine luciferin (ClO-Luc) combined with firefly luciferase (fLuc) mRNA-loaded lipid nanoparticles (LNPs). LNPs could efficiently deliver fLuc mRNA into living cells and in vivo, expressing abundant luciferase in the cytoplasm in situ. In the presence of ClO-, probe ClO-Luc locked by formylhydrazine could release cage-free d-luciferin through oxidation and follow-up hydrolysis reactions, further allowing for bioluminescence imaging. Moreover, based on the luciferase-luciferin system, it was able to sensitively and selectively detect ClO- in vitro with a limit of detection of 0.59 μM and successfully monitor the endogenous hypochlorite generation in the DILI mouse model for the first time. We postulate that this work provides a new method to elucidate the roles of ClO- in related diseases via bioluminescence imaging.

Cd2+-Specific Fluorescence Response of Methoxy-Substituted N,N-Bis(2-quinolylmethyl)-2-methoxyaniline Derivatives

The N3O1 tetradentate ligand, TriMeOBQMOA (N,N-bis(5,6,7-trimethoxy-2-quinolylmethyl)-2-methoxyaniline), was developed as a Cd2+-specific fluorescent sensor. The structure of TriMeOBQMOA is half of TriMeOBAPTQ (N,N,N',N'-tetrakis(5,6,7-trimethoxy-2-quinolylmethyl)-1,2-bis(2-aminophenoxy)ethane), which is a tetrakisquinoline derivative of the well-known calcium chelator BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid). The fluorescent Cd2+ selectivity of TriMeOBAPTQ (IZn/ICd = 5.3% in the presence of 3 equiv of metal ions in MeOH-HEPES buffer (9:1)) comes from the formation of fluorescent dinuclear cadmium (M2L) and nonfluorescent OH-bridged dizinc ((μ-OH)M2L) complexes. TriMeOBQMOA also exhibits excellent Cd2+ specificity in fluorescence enhancement (IZn/ICd = 2.3% in the presence of 5 equiv of metal ions in DMF-HEPES buffer (1:1, HEPES 50 mM, KCl 0.1 M, pH = 7.5)) via substantial formation of a highly fluorescent bis(μ-chloro)dinuclear cadmium complex ([Cd2(μ-Cl)2L2]2+), which is in equilibrium with the mononuclear Cd2+ complex ([CdLCl]+), and extremely poor stability of the TriMeOBQMOA-Zn2+ complex. The all-nitrogen derivatives of BQMOA and BAPTQ, namely, N,N-BQDMPHEN (N,N-bis(2-quinolylmethyl)-N',N'-dimethyl-1,2-phenylenediamine) and BPDTQ (N,N,N',N'-tetrakis(2-quinolylmethyl)-2,2'-(N,N'-dimethylethylenediamino)dianiline), respectively, and their methoxy-substituted derivatives were also prepared, and the fluorescent metal ion sensing properties are discussed.

Fluorescent Protein-Based Sensors for Detecting Essential Metal Ions across the Tree of Life

Genetically encoded fluorescent metal ion sensors are powerful tools for elucidating metal dynamics in living systems. Over the last 25 years since the first examples of genetically encoded fluorescent protein-based calcium indicators, this toolbox of probes has expanded to include other essential and non-essential metal ions. Collectively, these tools have illuminated fundamental aspects of metal homeostasis and trafficking that are crucial to fields ranging from neurobiology to human nutrition. Despite these advances, much of the application of metal ion sensors remains limited to mammalian cells and tissues and a limited number of essential metals. Applications beyond mammalian systems and in vivo applications in living organisms have primarily used genetically encoded calcium ion sensors. The aim of this Perspective is to provide, with the support of historical and recent literature, an updated and critical view of the design and use of fluorescent protein-based sensors for detecting essential metal ions in various organisms. We highlight the historical progress and achievements with calcium sensors and discuss more recent advances and opportunities for the detection of other essential metal ions. We also discuss outstanding challenges in the field and directions for future studies, including detecting a wider variety of metal ions, developing and implementing a broader spectral range of sensors for multiplexing experiments, and applying sensors to a wider range of single- and multi-species biological systems.

One Carbon to Rule Them All: Formaldehyde is a One-Carbon Signal Connecting One-Carbon Metabolism and Epigenetic Methylation

Formaldehyde is commonly thought of as an environmental toxin or laboratory fixation reagent, but there is a growing appreciation for its broader physiological contributions as a naturally generated one-carbon metabolite across all kingdoms of life. In this In Focus article, we summarize emerging advances in the field that show how formaldehyde plays diverse roles as a one-carbon signal in DNA damage, one-carbon metabolism, and epigenetic regulation.

"Covalent-Disassembly"-Based Approaches For Sensing Applications

The detection of analytes with small molecular probes is crucial for the analysis and understanding of chemical, medicinal, environmental and biological situations as well as processes. Classic detection approaches rely on the concept of molecular recognition and bond formation reactions. Bond breakage reactions have been less explored in similar contexts. This concept article introduces metal-salen and metal-imine complexes as "covalent-disassembly"-based (DB)-probes for detecting polyoxophosphates, thiols, amino acids, HCN and changes in pH. It discusses the roles, importance and combinations of structurally functionalized molecular building blocks in the construction of DB-probes. Applications of optimized DB-probes for analyte detection in live cells and foodstuff are also discussed. Furthermore, the mechanism of the disassembly of a Fe(III)-salen probe upon pyrophosphate binding is presented. Extraordinary selectivity for this analyte was achieved by a multistep disassembly sequence including an unprecedented structural change of the metal complex (i. e. "induced-fit" principle). Design principles of probes for sensing applications following the "covalent-disassembly" approach are summarized, which will help improving current systems, but will also facilitate the development of new DB-probes for challenging analytic targets.

A Transfer Hydrogenation Approach to Activity-Based Sensing of Formate in Living Cells

Formate is a major reactive carbon species in one-carbon metabolism, where it serves as an endogenous precursor for amino acid and nucleic acid biosynthesis and a cellular source of NAD(P)H. On the other hand, aberrant elevations in cellular formate are connected to progression of serious diseases, including cancer and Alzheimer's disease. Traditional methods for formate detection in biological environments often rely on sample destruction or extensive processing, resulting in a loss of spatiotemporal information. To help address these limitations, here we present the design, synthesis, and biological evaluation of a first-generation activity-based sensing system for live-cell formate imaging that relies on iridium-mediated transfer hydrogenation chemistry. Formate facilitates an aldehyde-to-alcohol conversion on various fluorophore scaffolds to enable fluorescence detection of this one-carbon unit, including through a two-color ratiometric response with internal calibration. The resulting two-component probe system can detect changes in formate levels in living cells with a high selectivity over potentially competing biological analytes. Moreover, this activity-based sensing system can visualize changes in endogenous formate fluxes through alterations of one-carbon pathways in cell-based models of human colon cancer, presaging the potential utility of this chemical approach to probe the continuum between one-carbon metabolism and signaling in cancer and other diseases.

Ratiometric near-infrared fluorescent probe for nitroreductase activity enables 3D imaging of hypoxic cells within intact tumor spheroids

Fluorescent molecular probes that report nitroreductase activity have promise as imaging tools to elucidate the biology of hypoxic cells and report the past hypoxic history of biomedical tissue. This study describes the synthesis and validation of a "first-in-class" ratiometric, hydrophilic near-infrared fluorescent molecular probe for imaging hypoxia-induced nitroreductase activity in 2D cell culture monolayers and 3D multicellular tumor spheroids. The probe's molecular structure is charge-balanced and the change in ratiometric signal is based on Förster Resonance Energy Transfer (FRET) from a deep-red, pentamethine cyanine donor dye (Cy5, emits ∼660 nm) to a linked near-infrared, heptamethine cyanine acceptor dye (Cy7, emits ∼780 nm). Enzymatic reduction of a 4-nitrobenzyl group on the Cy7 component induces a large increase in Cy7/Cy5 fluorescence ratio. The deep penetration of near-infrared light enables 3D optical sectioning of intact tumor spheroids, and visualization of individual hypoxic cells (i.e., cells with raised Cy7/Cy5 ratio) as a new way to study tumor spheroids. Beyond preclinical imaging, the near-infrared fluorescent molecular probe has high potential for ratiometric imaging of hypoxic tissue in living subjects.

Activity-Based Nitric Oxide-Responsive Porphyrin for Site-Selective and Nascent Cancer Ablation

Nitric oxide (NO) generated within the tumor microenvironment is an established driver of cancer progression and metastasis. Recent efforts have focused on leveraging this feature to target cancer through the development of diagnostic imaging agents and activatable chemotherapeutics. In this context, porphyrins represent an extraordinarily promising class of molecules, owing to their demonstrated use within both modalities. However, the remodeling of a standard porphyrin to afford a responsive chemical that can distinguish elevated NO from physiological levels has remained a significant research challenge. In this study, we employed a photoinduced electron transfer strategy to develop a panel of NO-activatable porphyrin photosensitizers (NOxPorfins) augmented with real-time fluorescence monitoring capabilities. The lead compound, NOxPorfin-1, features an o-phenylenediamine trigger that can effectively capture NO (via N2O3) to yield a triazole product that exhibits a 7.5-fold enhancement and a 70-fold turn-on response in the singlet oxygen quantum yield and fluorescence signal, respectively. Beyond demonstrating excellent in vitro responsiveness and selectivity toward NO, we showcase the potent photodynamic therapy (PDT) effect of NOxPorfin-1 in murine breast cancer and human non-small cellular lung cancer cells. Further, to highlight the in vivo efficacy, two key studies were executed. First, we utilized NOxPorfin-1 to ablate murine breast tumors in a site-selective manner without causing substantial collateral damage to healthy tissue. Second, we established a nascent human lung cancer model to demonstrate the unprecedented ability of NOxPorfin-1 to halt tumor growth and progression completely. The results of the latter study have tremendous implications for applying PDT to target metastatic lesions.

Advances and Perspectives of Responsive Probes for Measuring γ-Glutamyl Transpeptidase

Gamma-glutamyltransferase (GGT) is a plasma-membrane-bound enzyme that is involved in the γ-glutamyl cycle, like metabolism of glutathione (GSH). This enzyme plays an important role in protecting cells from oxidative stress, thus being tested as a key biomarker for several medical conditions, such as liver injury, carcinogenesis, and tumor progression. For measuring GGT activity, a number of bioanalytical methods have emerged, such as chromatography, colorimetric, electrochemical, and luminescence analyses. Among these approaches, probes that can specifically respond to GGT are contributing significantly to measuring its activity in vitro and in vivo. This review thus aims to highlight the recent advances in the development of responsive probes for GGT measurement and their practical applications. Responsive probes for fluorescence analysis, including "off-on", near-infrared (NIR), two-photon, and ratiometric fluorescence response probes, are initially summarized, followed by discussing the advances in the development of other probes, such as bioluminescence, chemiluminescence, photoacoustic, Raman, magnetic resonance imaging (MRI), and positron emission tomography (PET). The practical applications of the responsive probes in cancer diagnosis and treatment monitoring and GGT inhibitor screening are then highlighted. Based on this information, the advantages, challenges, and prospects of responsive probe technology for GGT measurement are analyzed.

Electrochemical Diselenation of BODIPY Fluorophores for Bioimaging Applications and Sensitization of 1 O2

We report a rapid, efficient, and scope-extensive approach for the late-stage electrochemical diselenation of BODIPYs. Photophysical analyses reveal red-shifted absorption - corroborated by TD-DFT and DLPNO-STEOM-CCSD computations - and color-tunable emission with large Stokes shifts in the selenium-containing derivatives compared to their precursors. In addition, due to the presence of the heavy Se atoms, competitive ISC generates triplet states which sensitize 1 O2 and display phosphorescence in PMMA films at RT and in a frozen glass matrix at 77 K. Importantly, the selenium-containing BODIPYs demonstrate the ability to selectively stain lipid droplets, exhibiting distinct fluorescence in both green and red channels. This work highlights the potential of electrochemistry as an efficient method for synthesizing unique emission-tunable fluorophores with broad-ranging applications in bioimaging and related fields.

Whole-Body and Whole-Organ 3D Imaging of Hypoxia Using an Activatable Covalent Fluorescent Probe Compatible with Tissue Clearing

Elucidation of biological phenomena requires imaging of microenvironments in vivo. Although the seamless visualization of in vivo hypoxia from the level of whole-body to single-cell has great potential to discover unknown phenomena in biological and medical fields, no methodology for achieving it has been established thus far. Here, we report the whole-body and whole-organ imaging of hypoxia, an important microenvironment, at single-cell resolution using activatable covalent fluorescent probes compatible with tissue clearing. We initially focused on overcoming the incompatibility of fluorescent dyes and refractive index matching solutions (RIMSs), which has greatly hindered the development of fluorescent molecular probes in the field of tissue clearing. The fluorescent dyes compatible with RIMS were then incorporated into the development of activatable covalent fluorescent probes for hypoxia. We combined the probes with tissue clearing, achieving comprehensive single-cell-resolution imaging of hypoxia in a whole mouse body and whole organs.

Genetically Encoded Photocatalysis Enables Spatially Restricted Optochemical Modulation of Neurons in Live Mice

Light provides high temporal precision for neuronal modulations. Small molecules are advantageous for neuronal modulation due to their structural diversity, allowing them to suit versatile targets. However, current optochemical methods release uncaged small molecules with uniform concentrations in the irradiation area, which lack spatial specificity as counterpart optogenetic methods from genetic encoding for photosensitive proteins. Photocatalysis provides spatial specificity by generating reactive species in the proximity of photocatalysts. However, current photocatalytic methods use antibody-tagged heavy-metal photocatalysts for spatial specificity, which are unsuitable for neuronal applications. Here, we report a genetically encoded metal-free photocatalysis method for the optochemical modulation of neurons via deboronative hydroxylation. The genetically encoded photocatalysts generate doxorubicin, a mitochondrial uncoupler, and baclofen by uncaging stable organoboronate precursors. The mitochondria, nucleus, membrane, cytosol, and ER-targeted drug delivery are achieved by this method. The distinct signaling pathway dissection in a single projection is enabled by the dual optogenetic and optochemical control of synaptic transmission. The itching signaling pathway is investigated by photocatalytic uncaging under live-mice skin for the first time by visible light irradiation. The cell-type-specific release of baclofen reveals the GABABR activation on NaV1.8-expressing nociceptor terminals instead of pan peripheral sensory neurons for itch alleviation in live mice.

Recent Advances in the Development of Non-Invasive Imaging Probes for Cancer Immunotherapy

The last two decades have witnessed a major revolution in the field of tumor immunology including clinical progress using various immunotherapy strategies. These advances have highlighted the potential for approaches that harness the power of the immune system to fight against cancer. While cancer immunotherapies have shown significant clinical successes, patient responses vary widely due to the complex and heterogeneous nature of tumors and immune responses, calling for reliable biomarkers and therapeutic strategies to maximize the benefits of immunotherapy. Especially, stratifying responding individuals from non-responders during the early stages of treatment could help avoid long-term damage and tailor personalized treatments. In efforts to develop non-invasive means for accurately evaluating and predicting tumor response to immunotherapy, multiple affinity-based agents targeting immune cell markers and checkpoint molecules have been developed and advanced to clinical trials. In addition, researchers have recently turned their attention to substrate and activity-based imaging probes that can provide real-time, functional assessment of immune response to treatment. Here, we highlight some of those recently designed probes that image functional proteases as biomarkers of cancer immunotherapy with a focus on their chemical design and detection modalities and discuss challenges and opportunities for the development of imaging tools utilized in cancer immunotherapy.

Single Atom Stabilization of Phosphinate Ester-Containing Rhodamines Yields Cell Permeable Probes for Turn-On Photoacoustic Imaging

Photoacoustic imaging (PAI) is an emerging imaging technique that uses pulsed laser excitation with near-infrared (NIR) light to elicit local temperature increases through non-radiative relaxation events, ultimately leading to the production of ultrasound waves. The classical xanthene dye scaffold has found numerous applications in fluorescence imaging, however, xanthenes are rarely utilized for PAI since they do not typically display NIR absorbance. Herein, we report the ability of Nebraska Red (NR) xanthene dyes to produce photoacoustic (PA) signal and provide a rational design approach to reduce the hydrolysis rate of ester containing dyes, affording cell permeable probes. To demonstrate the utility of this approach, we construct the first cell permeable rhodamine-based, turn-on PAI imaging probe for hypochlorous acid (HOCl) with maximal absorbance within the range of commercial PA instrumentation. This probe, termed SNR700 -HOCl, is capable of detecting exogenous HOCl in mice. This work provides a new set of rhodamine-based PAI agents as well as a rational design approach to stabilize esterified versions of NR dyes with desirable properties for PAI. In the long term, the reagents described herein could be utilized to enable non-invasive imaging of HOCl in disease-relevant model systems.

A FRET-Based Ratiometric H2S Sensor for Sensitive Optical Molecular Imaging in Second Near-Infrared Window

Second near-infrared (NIR-II) window optical molecular imaging kicks off a new revolution in high-quality imaging in vivo, but always suffers from the hurdles of inevitable tissue autofluorescence background and NIR-II probe development. Here, we prepare a Förster resonance energy transfer-based ratiometric NIR-II window hydrogen sulfide (H2S) sensor through the combination of an H2S-responsive NIR-II cyanine dye (acceptor, LET-1055) and an H2S-inert rhodamine hybrid polymethine dye (donor, Rh930). This sensor not only exhibits high sensitivity and selectivity, but also shows rapid reaction kinetics (~20 min) and relatively low limit of detection (~96 nM) toward H2S, allowing in vivo ratiometric NIR-II fluorescence imaging of orthotopic liver and colon tumors and visualization of the drug-induced hepatic H2S fluctuations. Our findings provide the potential for advancing the feasibility of NIR-II activity-based sensing for in vivo clinical diagnosis.

Quantum mechanisms for selective detection in complex gas mixtures using conductive sensors

In this paper, we consider new quantum mechanisms for selective detection in complex gaseous media which provide the highest possible efficiency of quantum sensors and for the first time analyze their nature. On the basis of these quantum mechanisms, the concepts of quantum detection and innovative methods of analysis are developed, which are virtually impossible to implement in the conventional conductive sensors and nanosensors. Examples of original solutions to problems in the field of detection and analysis of human breath using point-contact sensors are considered. A new method of analysis based on detection of metastable quantum states of the "point-contact sensor-breath" system in dynamic mode is proposed. The conductance histogram of dendritic Yanson point contacts recorded for this system is a unique energy signature of breath which allows differentiation between the states of human body. We demonstrate that nanosized Yanson point contacts, which, thanks to their quantum properties, can replace a massive spectrometer, open up wide opportunities for solving complex problems in the field of breath analysis using a new generation of portable high-tech quantum sensor devices.

Near Infrared Light-Triggered Photocatalytic Decaging for Remote-Controlled Spatiotemporal Activation in Living Mice

Spatiotemporal manipulation of biological processes in living animals using noninvasive, remote-controlled stimuli is a captivating but challenging endeavor. Herein, we present the development of a biocompatible photocatalytic technology termed CAT-NIR, which uses external near infrared light (NIR, 740 nm) to trigger decaging reactions in living mice. The Os(II) terpyridine complex was identified as an efficient NIR photocatalyst for promoting deboronative hydroxylation reactions via superoxide generation in the presence of NIR light, resulting in the deprotection of phenol groups and the release of bioactive molecules under living conditions. The validation of the CAT-NIR system was demonstrated through the NIR-triggered rescue of fluorophores, prodrugs as well as biomolecules ranging from amino acids, peptides to proteins. Furthermore, by combining genetic code expansion and computer-aided screening, CAT-NIR could regulate affibody binding to the cell surface receptor HER2, providing a selective cell tagging technology through external NIR light. In particular, the tissue-penetrating ability of NIR light allowed for facile prodrug activation in living mice, enabling noninvasive, remote-controlled rescue of drug molecules. Given its broad adaptability, this CAT-NIR system may open new opportunities for manipulating the functions of bioactive molecules in living animals using external NIR light with spatiotemporal resolution.

In vivo bioluminescence imaging of labile iron in xenograft models and liver using FeAL-1, an iron-activatable form of D-luciferin

Dysregulated iron homeostasis underlies diverse pathologies, from ischemia-reperfusion injury to epithelial-mesenchymal transition and drug-tolerant "persister" cancer cell states. Here, we introduce ferrous iron-activatable luciferin-1 (FeAL-1), a small-molecule probe for bioluminescent imaging of the labile iron pool (LIP) in luciferase-expressing cells and animals. We find that FeAL-1 detects LIP fluctuations in cells after iron supplementation, depletion, or treatment with hepcidin, the master regulator of systemic iron in mammalian physiology. Utilizing FeAL-1 and a dual-luciferase reporter system, we quantify LIP in mouse liver and three different orthotopic pancreatic ductal adenocarcinoma tumors. We observed up to a 10-fold increase in FeAL-1 bioluminescent signal in xenograft tumors as compared to healthy liver, the major organ of iron storage in mammals. Treating mice with hepcidin further elevated hepatic LIP, as predicted. These studies reveal a therapeutic index between tumoral and hepatic LIP and suggest an approach to sensitize tumors toward LIP-activated therapeutics.

Red-shifted activity-based sensors for ethylene via direct conjugation of fluorophore to metal-carbene

A number of Activity-Based Sensors (ABS) for relatively unreactive small molecules, such as ethylene, necessitates a transition metal for reaction under ambient conditions. Olefin metathesis has emerged as one of the primary strategies to achieve ethylene detection, and other transition metals are used for similarly challenging-to-detect analytes. However, limited studies exist investigating how fluorophore-metal attachment impacts photophysical properties of such ABS. Two new probes were prepared with the chelating benzlidene Ru-ligand directly conjugated to a BODIPY fluorophore and the photophysical properties of the new conjugated ABS were evaluated.

Copper trafficking systems in cells: insights into coordination chemistry and toxicity

Transition metal ions, such as copper, are indispensable components in the biological system. Copper ions which primarily exist in two major oxidation states Cu(I) and Cu(II) play crucial roles in various cellular processes including antioxidant defense, biosynthesis of neurotransmitters, and energy metabolism, owing to their inherent redox activity. The disturbance in copper homeostasis can contribute to the development of copper metabolism disorders, cancer, and neurodegenerative diseases, highlighting the significance of understanding the copper trafficking system in cellular environments. This review aims to offer a comprehensive overview of copper homeostatic machinery, with an emphasis on the coordination chemistry of copper transporters and trafficking proteins. While copper chaperones and the corresponding metalloenzymes are thoroughly discussed, we also explore the potential existence of low-molecular-mass metal complexes within cellular systems. Furthermore, we summarize the toxicity mechanisms originating from copper deficiency or accumulation, which include the dysregulation of oxidative stress, signaling pathways, signal transduction, and amyloidosis. This perspective review delves into the current knowledge regarding the intricate aspects of the copper trafficking system, providing valuable insights into potential treatment strategies from the standpoint of bioinorganic chemistry.

Screening and detection of multivalent human papillomavirus antibodies using a high-throughput liquid chip fluoroimmunoassay system

Immunoassays are commonly used in disease diagnosis and vaccine evaluation but can be costly and time-consuming when confronted with multivalent targets, such as antisera containing antibodies to human papillomavirus (HPV), because of their limited ability to discriminate between multiple analytes in a single reaction well. This study describes the development of a high-throughput liquid chip system that combines immunoassay techniques and magnetic beads to allow the simultaneous screening and quantitative detection of antibodies to four types of HPV using the Luminex fluoroimmunoassay system. Groups of beads embedded with fluorescent dyes at various ratios were coated with optimized HPV capture antigens and demonstrated excellent dose-dependent response to four monoclonal antibodies used as reference standards. This assay is sensitive, accurate, repeatable, and simple to perform, enabling multiplex antibody detection with a high degree of orthogonality. The performance of the Luminex system was compared with conventional immunoassays for quantitative detection of quadrivalent HPV antibodies in antisera of mice immunized with five lots of HPV vaccines, verifying the accuracy and detection efficiency of the assay. This strategy is a promising approach to characterizing antibodies present in polyclonal antisera and has promising applications in research, clinical, and industrial settings, for example, streamlining vaccine efficacy trials and vaccine lot inspection and release procedures.

Hydrogen Sulfide Responsive Phototherapy Agents: Design Strategies and Biological Applications

Hydrogen sulfide (H2S) is one of the critical gasotransmitters, which play important roles in regular physiological processes, especially in vital signaling pathways. However, fluctuations in endogenous H2S concentration can be linked to serious health problems, such as neurodegenerative diseases, cancer, diabetes, inflammation, cardiovascular diseases, and hypertension. Thus, it has attracted a great deal of attention in therapeutic applications, specifically in the field of phototherapy. Photodynamic therapy (PDT) and photothermal therapy (PTT) are two subclasses of phototherapy, which utilize either reactive oxygen species (ROS) or local temperature increase upon irradiation of a photosensitizer (PS) to realize the therapeutic action. Phototherapies offer unique advantages compared to conventional methods; thus, they are highly promising and popular. One of the design principles followed in new generation PSs is to build activity-based PSs, which stay inactive before getting activated by disease-associated stimuli. These activatable PSs dramatically improve the selectivity and efficacy of the therapy. In this review, we summarize small molecule and nanomaterial-based PDT and PTT agents that are activated selectively by H2S to initiate their cytotoxic effect. We incorporate single mode PDT and PTT agents along with synergistic and/or multimodal photosensitizers that can combine more than one therapeutic approach. Additionally, H2S-responsive theranostic agents, which offer therapy and imaging at the same time, are highlighted. Design approaches, working principles, and biological applications for each example are discussed in detail.

Aggregation-Induced Emission (AIE), Life and Health

Light has profoundly impacted modern medicine and healthcare, with numerous luminescent agents and imaging techniques currently being used to assess health and treat diseases. As an emerging concept in luminescence, aggregation-induced emission (AIE) has shown great potential in biological applications due to its advantages in terms of brightness, biocompatibility, photostability, and positive correlation with concentration. This review provides a comprehensive summary of AIE luminogens applied in imaging of biological structure and dynamic physiological processes, disease diagnosis and treatment, and detection and monitoring of specific analytes, followed by representative works. Discussions on critical issues and perspectives on future directions are also included. This review aims to stimulate the interest of researchers from different fields, including chemistry, biology, materials science, medicine, etc., thus promoting the development of AIE in the fields of life and health.

A selective fluorescent turn-on probe for imaging and sensing of hydrogen peroxide in living cells

Fluorescent turn-on probes have been extensively used in disease diagnosis and research on pathological disease mechanisms because of their low background interference. Hydrogen peroxide (H2O2) plays a vital role in regulating various cellular functions. In the current study, a fluorescent probe, HCyB, based on hemicyanine and arylboronate structures, was designed to detect H2O2. HCyB reacted with H2O2 and exhibited a good linear relationship for H2O2 concentrations ranging from 15 to 50 μM and good selectivity over other species. The fluorescent detection limit was 76 nM. Moreover, HCyB exhibited less toxicity and mitochondrial-targeting abilities. HCyB was successfully used to monitor exogenous or endogenous H2O2 in mouse macrophage RAW 264.7, human skin fibroblast WS1, breast cancer cell MDA-MB-231, and human leukemia monocytic THP1 cells.

An activity-based fluorescent sensor with a penta-coordinate N-donor binding site detects Cu ions in living systems

An activity-based sensor afforded a 63 times fluorescence-enhancement with Cu2+/Cu+ ions and could image Cu2+/Cu+ in living cells and in a multicellular organism. The sensor functioned only in the presence of ambient dioxygen and glutathione, and the characterization of intermediates and products hinted toward a sensing mechanism involving a CuII hydroperoxo species.

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

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

A Novel Fluorescent Probe for the Detection of Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) is one of the important reactive oxygen species (ROS), which is closely related to many pathological and physiological processes in living organisms. Excessive H₂O₂ can lead to cancer, diabetes, cardiovascular diseases, and other diseases, so it is necessary to detect H₂O₂ in living cells. Since this work designed a novel fluorescent probe to detect the concentration of H₂O₂, the H₂O₂ reaction group arylboric acid was attached to the fluorescein 3-Acetyl-7-hydroxycoumarin as a specific recognition group for the selective detection of hydrogen peroxide. The experimental results show that the probe can effectively detect H₂O₂ with high selectivity and measure cellular ROS levels. Therefore, this novel fluorescent probe provides a potential monitoring tool for a variety of diseases caused by H₂O₂ excess.

Molecular Thin Films Enable the Synthesis and Screening of Nanoparticle Megalibraries Containing Millions of Catalysts

Megalibraries are centimeter-scale chips containing millions of materials synthesized in parallel using scanning probe lithography. As such, they stand to accelerate how materials are discovered for applications spanning catalysis, optics, and more. However, a long-standing challenge is the availability of substrates compatible with megalibrary synthesis, which limits the structural and functional design space that can be explored. To address this challenge, thermally removable polystyrene films were developed as universal substrate coatings that decouple lithography-enabled nanoparticle synthesis from the underlying substrate chemistry, thus providing consistent lithography parameters on diverse substrates. Multi-spray inking of the scanning probe arrays with polymer solutions containing metal salts allows patterning of >56 million nanoreactors designed to vary in composition and size. These are subsequently converted to inorganic nanoparticles via reductive thermal annealing, which also removes the polystyrene to deposit the megalibrary. Megalibraries with mono-, bi-, and trimetallic materials were synthesized, and nanoparticle size was controlled between 5 and 35 nm by modulating the lithography speed. Importantly, the polystyrene coating can be used on conventional substrates like Si/SiO_x, as well as substrates typically more difficult to pattern on, such as glassy carbon, diamond, TiO_2, BN, W, or SiC. Finally, high-throughput materials discovery is performed in the context of photocatalytic degradation of organic pollutants using Au-Pd-Cu nanoparticle megalibraries on TiO₂ substrates with 2,250,000 unique composition/size combinations. The megalibrary was screened within 1 h by developing fluorescent thin-film coatings on top of the megalibrary as proxies for catalytic turnover, revealing Au_0.53Pd_0.38Cu_0.09-TiO₂ as the most active photocatalyst composition.

Interface of biomolecular condensates modulates redox reactions

Biomolecular condensates mediate diverse cellular processes. The density transition process of condensate formation results in selective partitioning of molecules, which define a distinct chemical environment within the condensates. However, the fundamental features of the chemical environment and the mechanisms by which such environment can contribute to condensate functions have not been revealed. Here, we report that an electric potential gradient, thereby an electric field, is established at the liquid-liquid interface between the condensate and the bulk environment due to the density transition of ions and molecules brought about by phase separation. We find that the interface of condensates can drive spontaneous redox reactions in vitro and in living cells. Our results uncover a fundamental physicochemical property of the interface of condensates and the mechanism by which the interface can modulate biochemical activities.

Genetically encoded fluorescent sensors for metals in biology

Metal ions intersect a wide range of biological processes. Some metal ions are essential and hence absolutely required for the growth and health of an organism, others are toxic and there is great interest in understanding mechanisms of toxicity. Genetically encoded fluorescent sensors are powerful tools that enable the visualization, quantification, and tracking of dynamics of metal ions in biological systems. Here, we review recent advances in the development of genetically encoded fluorescent sensors for metal ions. We broadly focus on 5 classes of sensors: single fluorescent protein, FRET-based, chemigenetic, DNAzymes, and RNA-based. We highlight recent developments in the past few years and where these developments stand concerning the rest of the field.