The Art of Fluorescence Imaging with Chemical Sensors

Dual-/multi-organelle-targeted AIE probes associated with oxidative stress for biomedical applications

In situ monitoring of biological processes between different organelles upon oxidative stress is one of the most important research hotspots. Fluorescence imaging is especially suitable for biomedical applications due to its distinct advantages of high spatiotemporal resolution, high sensitivity, non-invasiveness, and in situ monitoring capabilities. However, most fluorescent probes can only achieve light-up imaging of single organelles, thus the combined use of two or more probes is usually required for monitoring biological processes between organelles, which can suffer from tedious staining and washing procedures, increased cytotoxicity and poor photostability. Exogenetic oxidants can affect broad-spectrum subcellular organelles, which are not conducive to in situ monitoring of biological processes between specific organelles. To tackle these challenges, a series of dual-/multi-organelle-targeted aggregation-induced emission (AIE) probes associated with oxidative stress have been designed and developed in the past few years. Herein, the recent progress of these AIE probes is summarized in biomedical applications, such as apoptosis monitoring, interplay between organelles, microenvironmental changes of organelles, organelle morphology tracking, precise cancer therapy, and so forth. Moreover, the further outlook for dual-/multi-organelle-targeted AIE probes is discussed, aiming to promote innovative research in biomedical applications.

Recent advancements in the sensors for food analysis to detect gluten: A mini-review [2019-2023]

People with celiac disease or gluten sensitivity may experience an immune reaction to the protein called gluten, which is present in wheat, barley, and rye. A strict gluten-free diet is the sole cure for these ailments. There are chances of food fraud about the claim of being gluten-free food items. As a result, there is a rising need for trustworthy and precise ways to identify gluten. There are many methods to detect gluten in food samples viz., enzyme-linked immunosorbent assay 1 Surface plasmon resonance (SPR), Electrochemical sensors, Fluorescence-based sensors, etc. The use of sensors is one of the most promising methods for gluten detection. For detecting gluten, a variety of sensors, including optical, electrochemical, and biosensors, have been developed with different limits of detection and sensitivity. The present review reports the recent advancements (2019-2023) in the development of sensors for gluten detection in food. We may conclude that sensitivity and limit of detection are not related to the type of sensor used (aptamer or antibody-based), however, there are advancements, with the year, on the simplicity of the material used like paper-based sensors and paradigm shift to reagent free sensors by the spectral analysis. Also, recent work shows the potential of IoT-based studies for gluten detection.

Biothiols-activated near-infrared frequency up-conversion luminescence probe for early evaluation of drug-induced hepatotoxicity

A novel biothiols-sensitive near-infrared (NIR) fluorescent probe RhDN based on a rhodamine skeleton was developed for early detection of drug-induced hepatotoxicity in living mice. RhDN can be used not only as a conventional large stokes shift fluorescent (FL) probe, but also as a kind of anti-Stokes frequency upconversion luminescence (FUCL) molecular probe, which represents a long wavelength excitation (808 nm) to short wavelength emission (760 nm), and response to Cys/Hcy/GSH with high sensitivity. Compared with traditional FL methods, the FUCL method exhibited a lower detection limit of Cys, Hcy, and GSH in 75.1 nM, 101.8 nM, and 84.9 nM, respectively. We exemplify RhDN for tracking endogenously biothiols distribution in living cells and further realize real-time in vivo bioimaging of biothiols activity in mice with dual-mode luminescence system. Moreover, RhDN has been successfully applied to visualize the detection of drug-induced hepatotoxicity in living mice. Overall, this report presents a unique approach to the development of large stokes shift NIR FUCL molecular probes for in vitro and in vivo biothiols biosensing.

Coumarin-Phosphazenes: Enhanced Photophysical Properties from Hybrid Materials

Phosphazenes have drawn a great deal of interest over the past 20 years as a potentially useful building block for the fabrication of fluorescent materials. The main objective of this work is to explore novel derivatives produced by coumarins, a class of chemicals well-known for their photophysical importance, and cyclophosphazenes. UV absorbance, fluorescence emission, quantum yield, and lifetime measurements were conducted to comprehend the optical properties. Furthermore, single-crystal X-ray analysis and theoretical calculations were carried out to confirm the structure of the molecule. The obtained findings collectively confirm the commendable optical properties exhibited by the studied compounds. Moreover, a detailed study of the crystal packing arrangement of DPP-Et-Kum-Et compound crystallized in the P21/n monoclinic space group revealed the presence of stacking interactions between the nonplanar conjugated benzene rings of the coumarins and the rigid diphenyl groups attached to the phosphazene ring. The crystal structure of the DPP-Kum-Me-Me compound is mainly based on classical C-H···O intermolecular hydrogen bonding interactions with an average distance of 2.52 Å. Importantly, the calculated absorption spectra of the compounds are in close agreement with the experimental data, further supporting their interesting electronic properties. Given that the DPP-Et-Kum-Et and DPP-Kum-Et compounds have the theoretically lowest band gaps (4.31 and 4.30 eV, respectively), the activation energies of these compounds were determined by an impedance analyzer using dc conductance values measured at different temperatures. The calculated activation energies for DPP-Et-Kum-Et and DPP-Kum-Et are 104.49 and 100.92 meV, respectively. The results demonstrate that both theoretical and experimental calculations are in agreement with each other and that the DPP-Kum-Et compound has the lowest conductivity.

Fluorescent Probes for Disease Diagnosis

The identification and detection of disease-related biomarkers is essential for early clinical diagnosis, evaluating disease progression, and for the development of therapeutics. Possessing the advantages of high sensitivity and selectivity, fluorescent probes have become effective tools for monitoring disease-related active molecules at the cellular level and in vivo. In this review, we describe current fluorescent probes designed for the detection and quantification of key bioactive molecules associated with common diseases, such as organ damage, inflammation, cancers, cardiovascular diseases, and brain disorders. We emphasize the strategies behind the design of fluorescent probes capable of disease biomarker detection and diagnosis and cover some aspects of combined diagnostic/therapeutic strategies based on regulating disease-related molecules. This review concludes with a discussion of the challenges and outlook for fluorescent probes, highlighting future avenues of research that should enable these probes to achieve accurate detection and identification of disease-related biomarkers for biomedical research and clinical applications.

N-Aryl-DABCO Salts as an Unprecedented Sensing Platform for the Detection of Thiols and Selenols

Quaternary N-aryl-DABCO salts were introduced for the first time as a highly selective sensing platform for thiols and selenols. By employing this platform, a highly sensitive coumarin based "off-on" fluorescent probe was designed and synthesized. The probe possesses a good solubility in water, low background fluorescence, and, most importantly, demonstrates high selectivity to aryl thiols and selenols over their aliphatic counterparts and other common nucleophiles. A dramatic increase in fluorescence intensity is achieved through the selective cleavage of the quaternized DABCO-ring, yielding a piperazine derivatives with a high fluorescence quantum yield (~72 %). Moreover, stability of the probe to the most used reducing agents DTT and TCEP was demonstrated. The limits of detection for p-thiocresol and phenyl selenide were evaluated to be 22 nM and 6 nM, respectively.

Nanocrystals for Deep-Tissue In Vivo Luminescence Imaging in the Near-Infrared Region

In vivo imaging technologies have emerged as a powerful tool for both fundamental research and clinical practice. In particular, luminescence imaging in the tissue-transparent near-infrared (NIR, 700-1700 nm) region offers tremendous potential for visualizing biological architectures and pathophysiological events in living subjects with deep tissue penetration and high imaging contrast owing to the reduced light-tissue interactions of absorption, scattering, and autofluorescence. The distinctive quantum effects of nanocrystals have been harnessed to achieve exceptional photophysical properties, establishing them as a promising category of luminescent probes. In this comprehensive review, the interactions between light and biological tissues, as well as the advantages of NIR light for in vivo luminescence imaging, are initially elaborated. Subsequently, we focus on achieving deep tissue penetration and improved imaging contrast by optimizing the performance of nanocrystal fluorophores. The ingenious design strategies of NIR nanocrystal probes are discussed, along with their respective biomedical applications in versatile in vivo luminescence imaging modalities. Finally, thought-provoking reflections on the challenges and prospects for future clinical translation of nanocrystal-based in vivo luminescence imaging in the NIR region are wisely provided.

Optical sensors (optodes) for multiparameter chemical imaging: classification, challenges, and prospects

Chemical gradients and uneven distribution of analytes are common in natural and artificial systems. As a result, the ability to visualize chemical distributions in two or more dimensions has gained significant importance in recent years. This has led to the integration of chemical imaging techniques into all domains of analytical chemistry. In this review, we focus on the use of optical sensors, so-called optodes, to obtain real-time and multidimensional images of two or more parameters simultaneously. It is important to emphasize that multiparameter imaging in this context is not confined solely to multiple chemical parameters (analytes) but also encompasses physical (e.g., temperature or flow) or biological (e.g., metabolic activity) parameters. First, we discuss the technological milestones that have paved the way for chemical imaging using optodes. Later, we delve into various strategies that can be taken to enable multiparameter imaging. The latter spans from developing novel receptors that enable the recognition of multiple parameters to chemometrics and machine learning-based techniques for data analysis. We also explore ongoing trends, challenges, and prospects for future developments in this field. Optode-based multiparameter imaging is a rapidly expanding field that is being fueled by cutting-edge technologies. Chemical imaging possesses the potential to provide novel insights into complex samples, bridging not only across various scientific disciplines but also between research and society.

Near-Infrared Rechargeable Persistent Luminescence Nanoparticles for Biomedical Implants In Vivo Noninvasive Bioimaging

Luminescent imaging has garnered significant attention for in vivo tracking of biomedical implants during and after surgery due to its human friendliness, affordability, and high sensitivity. However, conventional fluorescent probes are susceptible to background autofluorescence interference from living tissues, often resulting in poor signal-to-noise ratios. Herein, we report a background interference-free persistent luminescent implant (PLI) with excellent persistent luminescence (PL) performance, which can be clearly and long-term detected by an optical imaging system after implantation. Rechargeable near-infrared persistent luminescence nanoparticles (PLNPs) were prepared first via a simple hydrothermal approach and then modified by PEGylation to improve their hydrophilicity, biocompatibility, and compatibility with polymer substrates. The PEGylated PLNPs were facilely complexed into a polymer matrix to fabricate the PLI. The obtained PLIs can well inherit the PL properties of PLNPs, exhibiting good PL optical imaging performance without tissue autofluorescence interference. Furthermore, both PLNPs and PLIs possess good biocompatibility, and the addition of PLNPs has no negative impact on the biocompatibility of the polymer matrix. This work fully utilizes the luminescent properties of PLNPs and adapts this PL to the field of biomedical implant imaging, which provides new insight for designing biomedical imaging systems.

Nanoscatterer-Assisted Fluorescence Amplification Technique

Weak fluorescence signals, which are important in research and applications, are often masked by the background. Different amplification techniques are actively investigated. Here, a broadband, geometry-independent and flexible feedback scheme based on the random scattering of dielectric nanoparticles allows the amplification of a fluorescence signal by partial trapping of the radiation within the sample volume. Amplification of up to a factor of 40 is experimentally demonstrated in ultrapure water with dispersed TiO2 nanoparticles (30 to 50 nm in diameter) and fluorescein dye at 200 μmol concentration (pumped with 5 ns long, 3 mJ laser pulses at 490 nm). The measurements show a measurable reduction in linewidth at the emission peak, indicating that feedback-induced stimulated emission contributes to the large gain observed.

Recent Advances in Biological Applications of Aptamer-Based Fluorescent Biosensors

Aptamers have been spotlighted as promising bio-recognition elements because they can be tailored to specific target molecules, bind to targets with a high affinity and specificity, and are easy to chemically synthesize and introduce functional groups to. In particular, fluorescent aptasensors are widely used in biological applications to diagnose diseases as well as prevent diseases by detecting cancer cells, viruses, and various biomarkers including nucleic acids and proteins as well as biotoxins and bacteria from food because they have the advantages of a high sensitivity, selectivity, rapidity, a simple detection process, and a low price. We introduce screening methods for isolating aptamers with q high specificity and summarize the sequences and affinities of the aptamers in a table. This review focuses on aptamer-based fluorescence detection sensors for biological applications, from fluorescent probes to mechanisms of action and signal amplification strategies.

Polymerization Induced Microphase Separation for the Fabrication of Nanostructured Materials

Polymerization induced microphase separation (PIMS) is a strategy used to develop unique nanostructures with highly useful morphologies through the microphase separation of emergent block copolymers during polymerization. In this process, nanostructures are formed with at least two chemically independent domains, where at least one domain is composed of a robust crosslinked polymer. Crucially, this synthetically simple method is readily used to develop nanostructured materials with the highly coveted co-continuous morphology, which can also be converted into mesoporous materials by selective etching of one domain. As PIMS exploits a block copolymer microphase separation mechanism, the size of each domain can be tightly controlled by modifying the size of block copolymer precursors, thus providing unparalleled control over nanostructure and resultant mesopore sizes. Since its inception 11 years ago, PIMS has been used to develop a vast inventory of advanced materials for an extensive range of applications including biomedical devices, ion exchange membranes, lithium-ion batteries, catalysis, 3D printing, and fluorescence-based sensors, among many others. In this review, we provide a comprehensive overview of the PIMS process, summarize latest developments in PIMS chemistry, and discuss its utility in a wide variety of relevant applications.

Julolidine-based small molecular probes for fluorescence imaging of RNA in live cells

Intracellular RNA imaging with organic small molecular probes has been an intense topic, although the number of such reported dyes, particularly dyes with high quantum yields and long wavelength excitation/emission, is quite limited. The present work reports the design and synthesis of three cationic julolidine-azolium conjugates (OX-JLD, BTZ-JLD and SEZ-JLD) as turn-on fluorescent probes with appreciably high quantum yields and brightness upon interaction with RNA. A structure-efficiency relationship has been established for their potential for the interaction and imaging of intracellular RNA. Given their chemical structure, the free rotation between the donor and the acceptor gets restricted when the probes bind with RNA resulting in strong fluorescence emission towards a higher wavelength upon photoexcitation. A detailed investigation revealed that the photophysical properties and the optical responses of two probes, viz. BTZ-JLD and SEZ-JLD, towards RNA are very promising and qualify them to be suitable candidates for biological studies, particularly for cellular imaging applications. The probes allow imaging of intracellular RNA with prominent staining of nucleoli in live cells under a range of physiological conditions. The results of the cellular digest test established the appreciable RNA selectivity of BTZ-JLD and SEZ-JLD inside the cellular environment. Moreover, a comparison between the relative intensity profile of SEZ-JLD before and after the RNA-digestion test inside the cellular environment indicated that the interference of cellular viscosity in fluorescence enhancement is insignificant, and hence, SEZ-JLD can be used as a cell membrane permeable cationic molecular probe for deep-red imaging of intracellular RNA with a good degree of selectivity.

Bright and Stable Nanomaterials for Imaging and Sensing

This review covers strategies to prepare high-performance emissive polymer nanomaterials, combining very high brightness and photostability, to respond to the drive for better imaging quality and lower detection limits in fluorescence imaging and sensing applications. The more common approaches to obtaining high-brightness nanomaterials consist of designing polymer nanomaterials carrying a large number of fluorescent dyes, either by attaching the dyes to individual polymer chains or by encapsulating the dyes in nanoparticles. In both cases, the dyes can be covalently linked to the polymer during polymerization (by using monomers functionalized with fluorescent groups), or they can be incorporated post-synthesis, using polymers with reactive groups, or encapsulating the unmodified dyes. Silica nanoparticles in particular, obtained by the condensation polymerization of silicon alcoxides, provide highly crosslinked environments that protect the dyes from photodegradation and offer excellent chemical modification flexibility. An alternative and less explored strategy is to increase the brightness of each individual dye. This can be achieved by using nanostructures that couple dyes to plasmonic nanoparticles so that the plasmon resonance can act as an electromagnetic field concentrator to increase the dye excitation efficiency and/or interact with the dye to increase its emission quantum yield.

Synergetic spin singlet-quintet switching and luminescence in mononuclear Fe(II) 1,3,4-oxadiazole tetradentate chelates with NCBH3 co-ligand

We report the multi-step synthesis of the tetradentate 2-(naphthalen-2-yl)-5-[N,N-bis(2-pyridylmethyl)aminomethyl]-1,3,4-oxadiazole ligand (LTetra-ODA) along with its corresponding [FeII(LTetra-ODA)(NCBH3)2]·1.5CH3OH (C1) complex, which is the first mononuclear 1,3,4-oxadiazole based Fe(II) spin crossover (SCO) complex, and its zinc analogue [ZnII(LTetra-ODA)(NCBH3)2]·0.5H2O (C2). The spin transition is followed by variable temperature (VT-) X-ray crystallography of [Fe(LTetra-ODA)(NCBH3)2]·1.5CH3OH (C1) at 120 and 220 K. The magnetic susceptibility measurements on the bulk sample recorded from 2 to 300 K show that the complex exhibits a complete abrupt reversible spin transition with a T1/2 of 207 K. The loss of the lattice solvent methanol shifts the T1/2 slightly to around 210 K. The spin transition in solution for [Fe(LTetra-ODA)(NCBH3)2]·1.5CH3OH (C1) was followed using the VT-1H-NMR Evans method in CD3CN, with a T1/2 of 357 K. Solid state VT luminescence studies provide some preliminary evidence of interplay of luminescence and spin transition in the [Fe(LTetra-ODA)(NCBH3)2]·1.5CH3OH (C1) complex.

Connecting Ruthenium Photocatalysis to 1,2-Dioxetane-Mediated Chemiluminescence: a Versatile Combination for Optical Detection and Read-Out

Optical storage and photon quantification systems based on sensitive photoreactions have numerous applications. Herein, we report a highly efficient photocatalytic reaction, in which ruthenium photoredox catalysis is combined with a 1,2-dioxetane from which chemiluminescence can be triggered. In this system, blue light irradiation as optical input enables a defined inverse correlation with base-triggered, blue light emission as optical output. Comparison of readout by 1 H NMR and chemiluminescence, relative to previous optical input, underlines the reliability and usefulness of the ruthenium-dioxetane system for optical storage, sensing and ruthenium detection.

ON-OFF Fluorescent Cyanine Dye Based on a Benzothiophenyl Rotor Enables Selective Illumination of G-Quadruplexes in Mitochondria

Conventional cyanine dyes exist as "always-on" fluorescent probes leading to inevitable background signals which often limit their performance and scope of applications. To develop specific fluorescent probes with high sensitivity and robust OFF/ON switching for targeting G4s, we introduced aromatic heterocycles through conjugation with polymethine chains to construct a rotor-π system. Here, a universal strategy is presented to synthesize pentamethine cyanines with different aromatic heterocycle substituents on the meso-polymethine chain. In these probes, SN-Cy5-S is self-quenched in aqueous solution due to H-aggregation. The structure indicates that SN-Cy5-S with a flexible meso-benzothiophenyl rotor conjugated to the cyanine backbone matches adaptively with G-tetrad planes, enhancing π-π stacking and resulting in triggered fluorescence. This allows recognition of G-quadruplexes due to the synergy of disaggregation-induced emission (DIE) and inhibited twisted intramolecular charge-transfer effects. This combination leads to a robust lighting-up fluorescence response for c-myc G4 with superior fluorescence enhancement (98-fold), allowing for a low detection limit of 1.51 nM, which is much more sensitive than the previously reported DIE-based G4 probes (22-83.5 nM). In addition, the superior imaging properties and rapid internalization time (5 min) in mitochondria allow SN-Cy5-S to also have a high potential for mitochondrially targeting anti-cancer therapy.

Optode-based chemical imaging of laboratory burned soil reveals millimeter-scale heterogeneous biogeochemical responses

Soil spatial responses to fire are unclear. Using optical chemical sensing with planar 'optodes', pH and dissolved O₂ concentration were tracked spatially with a resolution of 360 μm per pixel for 72 h after burning soil in the laboratory with a butane torch (∼1300 °C) and then sprinkling water to simulate a postfire moisture event. Imaging data from planar optodes correlated with microbial activity (quantified via RNA transcripts). Post-fire and post-wetting, soil pH increased throughout the entire ∼13 cm × 17 cm × 20 cm rectangular cuboid of sandy loam soil. Dissolved O₂ concentrations were not impacted until the application of water postfire. pH and dissolved O₂ both negatively correlated (p < 0.05) with relative transcript expression for galactose metabolism, the degradation of aromatic compounds, sulfur metabolism, and narH. Additionally, dissolved O₂ negatively correlated (p < 0.05) with the relative activity of carbon fixation pathways in Bacteria and Archaea, amoA/amoB, narG, nirK, and nosZ. nifH was not detected in any samples. Only amoB and amoC correlated with depth in soil (p < 0.05). Results demonstrate that postfire soils are spatially complex on a mm scale and that using optode-based chemical imaging as a chemical navigator for RNA transcript sampling is effective.

Quinolinium-Based Fluorescent Probes for Dynamic pH Monitoring in Aqueous Media at High pH Using Fluorescence Lifetime Imaging

Spatiotemporal pH imaging using fluorescence lifetime imaging microscopy (FLIM) is an excellent technique for investigating dynamic (electro)chemical processes. However, probes that are responsive at high pH values are not available. Here, we describe the development and application of dedicated pH probes based on the 1-methyl-7-amino-quinolinium fluorophore. The high fluorescence lifetime and quantum yield, the high (photo)stability, and the inherent water solubility make the quinolinium fluorophore well suited for the development of FLIM probes. Due to the flexible fluorophore-spacer-receptor architecture, probe lifetimes are tunable in the pH range between 5.5 and 11. An additional fluorescence lifetime response, at tunable pH values between 11 and 13, is achieved by deprotonation of the aromatic amine at the quinolinium core. Probe lifetimes are hardly affected by temperature and the presence of most inorganic ions, thus making FLIM imaging highly reliable and convenient. At 0.1 mM probe concentrations, imaging at rates of 3 images per second, at a resolution of 4 μm, while measuring pH values up to 12 is achieved. This enables the pH imaging of dynamic electrochemical processes involving chemical reactions and mass transport.

Isoindolium-Based Allenes: Reactivity Studies and Applications in Fluorescence Temperature Sensing and Cysteine Bioconjugation

The reaction of a series of electron-deficient isoindolium-based allenes with sulfhydryl compounds has been studied, leading to the formation of isoindolium-based vinyl sulfides. The vinyl sulfides generated could be readily converted into the corresponding indanones and amines upon heating at 30-70 °C with good yields up to 61 %. The thermal cleavage reaction of vinyl sulfides was further studied for developing temperature-sensitive systems. Notably, a novel FRET-based fluorescent temperature sensor was designed and synthesized for temperature sensing at 50 °C, giving a 6.5-fold blue fluorescence enhancement. Moreover, chemoselective bioconjugation of cysteine-containing peptides with the isoindolium-based allenes for the construction of multifunctional peptide bioconjugates was investigated. Thermal cleavage of isoindoliums on the modified peptides at 35-70 °C gave indanone bioconjugates with up to >99 % conversion. These results indicated the biocompatibility of this novel temperature-sensitive reaction.

Emerging ultrasmall luminescent nanoprobes for in vivo bioimaging

Photoluminescence (PL) imaging has become a fundamental tool in disease diagnosis, therapeutic evaluation, and surgical navigation applications. However, it remains a big challenge to engineer nanoprobes for high-efficiency in vivo imaging and clinical translation. Recent years have witnessed increasing research efforts devoted into engineering sub-10 nm ultrasmall nanoprobes for in vivo PL imaging, which offer the advantages of efficient body clearance, desired clinical translation potential, and high imaging signal-to-noise ratio. In this review, we present a comprehensive summary and contrastive discussion of emerging ultrasmall luminescent nanoprobes towards in vivo PL bioimaging of diseases. We first summarize size-dependent nano-bio interactions and imaging features, illustrating the unique attributes and advantages/disadvantages of ultrasmall nanoprobes differentiating them from molecular and large-sized probes. We also discuss general design methodologies and PL properties of emerging ultrasmall luminescent nanoprobes, which are established based on quantum dots, metal nanoclusters, lanthanide-doped nanoparticles, and silicon nanoparticles. Then, recent advances of ultrasmall luminescent nanoprobes are highlighted by surveying their latest in vivo PL imaging applications. Finally, we discuss existing challenges in this exciting field and propose some strategies to improve in vivo PL bioimaging and further propel their clinical applications.

Bioelectronic devices for light-based diagnostics and therapies

Light has broad applications in medicine as a tool for diagnosis and therapy. Recent advances in optical technology and bioelectronics have opened opportunities for wearable, ingestible, and implantable devices that use light to continuously monitor health and precisely treat diseases. In this review, we discuss recent progress in the development and application of light-based bioelectronic devices. We summarize the key features of the technologies underlying these devices, including light sources, light detectors, energy storage and harvesting, and wireless power and communications. We investigate the current state of bioelectronic devices for the continuous measurement of health and on-demand delivery of therapy. Finally, we highlight major challenges and opportunities associated with light-based bioelectronic devices and discuss their promise for enabling digital forms of health care.

Molecular Engineering of Near-Infrared Fluorescent Probes for Cell Membrane Imaging

Cell membrane (CM) is a phospholipid bilayer that maintains integrity of a whole cell and relates to many physiological and pathological processes. Developing CM imaging tools is a feasible method for visualizing membrane-related events. In recent decades, small-molecular fluorescent probes in the near-infrared (NIR) region have been pursued extensively for CM staining to investigate its functions and related events. In this review, we summarize development of such probes from the aspect of design principles, CM-targeting mechanisms and biological applications. Moreover, at the end of this review, the challenges and future research directions in designing NIR CM-targeting probes are discussed. This review indicates that more efforts are required to design activatable NIR CM-targeting probes, easily prepared and biocompatible probes with long retention time regarding CM, super-resolution imaging probes for monitoring CM nanoscale organization and multifunctional probes with imaging and phototherapy effects.

Luminescent Lanthanide Complexes for Effective Oxygen-Sensing and Singlet Oxygen Generation

Oxygen quantification using luminescence has attracted considerable attention in various fields, including environmental monitoring and clinical analysis. Among the reported luminophores, trivalent lanthanide complexes have displayed characteristic narrow emission bands with high brightness. This bright emission is based on photo-sensitized energy transfer via organic triplet states. The organic triplet states in lanthanide complexes effectively react with the triplet oxygen, enabling oxygen quantification by lanthanide luminescence. Some Tb^III and Eu^III complexes with slow deactivation processes have also formed the excited state equilibrium, thus resulting in the emission-lifetime based oxygen sensing property. The combination of Tb^III /Eu^III emission, Eu^III /Sm^III emission, Eu^III /ligand phosphorescence, and ligand fluorescence/ligand phosphorescence provide the ratiometric oxygen-sensing properties. Moreover, the reaction generates singlet oxygen species which exhibit numerous applications in the photo-medical field. The ligands with large π-conjugated aromatic systems, such as porphyrin, phthalocyanine, and polyaromatic compounds, induces highly efficient oxygen generation. The combination of effective luminescence with singlet-oxygen generation by the lanthanide complexes render them suitable for photo-driven theranostics. This review summarizes the research progress of lanthanide complexes with efficient oxygen-sensing and singlet-oxygen generation properties.

Dual color pH probes made from silica and polystyrene nanoparticles and their performance in cell studies

Ratiometric green-red fluorescent nanosensors for fluorometrically monitoring pH in the acidic range were designed from 80 nm-sized polystyrene (PS) and silica (SiO₂) nanoparticles (NPs), red emissive reference dyes, and a green emissive naphthalimide pH probe, analytically and spectroscopically characterized, and compared regarding their sensing performance in aqueous dispersion and in cellular uptake studies. Preparation of these optical probes, which are excitable by 405 nm laser or LED light sources, involved the encapsulation of the pH-inert red-fluorescent dye Nile Red (NR) in the core of self-made carboxylated PSNPs by a simple swelling procedure and the fabrication of rhodamine B (RhB)-stained SiO₂-NPs from a silane derivative of pH-insensitive RhB. Subsequently, the custom-made naphthalimide pH probe, that utilizes a protonation-controlled photoinduced electron transfer process, was covalently attached to the carboxylic acid groups at the surface of both types of NPs. Fluorescence microscopy studies with the molecular and nanoscale optical probes and A549 lung cancer cells confirmed the cellular uptake of all probes and their penetration into acidic cell compartments, i.e., the lysosomes, indicated by the switching ON of the green naphthalimide fluorescence. This underlines their suitability for intracellular pH sensing, with the SiO₂-based nanosensor revealing the best performance regarding uptake speed and stability.

Elucidation of complex triplet excited state dynamics in Pd(II) biladiene tetrapyrroles

Pd(II) biladienes have been developed over the last five years as non-aromatic oligotetrapyrrole complexes that support a rich triplet photochemistry. In this work, we have undertaken the first detailed photophysical interrogation of three homologous Pd(II) biladienes bearing different combinations of methyl- and phenyl-substituents on the frameworks' sp³-hybridized meso-carbon (i.e., the 10-position of the biladiene framework). These experiments have revealed unexpected excited-state dynamics that are dependent on the wavelength of light used to excite the biladiene. More specifically, transient absorption spectroscopy revealed that higher-energy excitation (λ_exc ∼ 350-500 nm) led to an additional lifetime (i.e., an extra photophysical process) compared to experiments carried out following excitation into the lowest-energy excited states (λ_exc = 550 nm). Each Pd(II) biladiene complex displayed an intersystem crossing lifetime on the order of tens of ps and a triplet lifetime of ∼20 μs, regardless of the excitation wavelength. However, when higher-energy light is used to excite the complexes, a new lifetime on the order of hundreds of ps is observed. The origin of the 'extra' lifetime observed upon higher energy excitation was revealed using density functional theory (DFT) and time-dependent DFT (TDDFT). These efforts demonstrated that excitation into higher-energy metal-mixed-charge-transfer excited states with high spin-orbit coupling to higher energy metal-mixed-charge-transfer triplet states leads to the additional excitation deactivation pathway. The results of this work demonstrate that Pd(II) biladienes support a unique triplet photochemistry that may be exploited for development of new photochemical schemes and applications.

Microdroplet-Facilitated Assembly of Thermally Activated Delayed Fluorescence-Encoded Microparticles with Non-interfering Color Signals

Encoded microparticles (EMPs) have shown demonstrative value for multiplexed high-throughput bioassays such as drug discovery and diagnostics. Herein, we propose for the first time the incorporation of thermally activated delayed fluorescence (TADF) dyes with low-cost, heavy metal-free, and long-lived luminescence properties into polymer matrices via a microfluidic droplet-facilitated assembly technique. Benefiting from the uniform droplet template sizes and polymer-encapsulated structures, the resulting composite EMPs are highly monodispersed, efficiently shield TADF dyes from singlet oxygen, well preserve TADF emission, and greatly increase the delayed fluorescence lifetime. Furthermore, by combining with phase separation of polymer blends in the drying droplets, TADF dyes with distinct luminescent colors can be spatially separated within each EMP. It eliminates optical signal interference and generates multiple fluorescence colors in a compact system. Additionally, in vitro studies reveal that the resulting EMPs show good biocompatibility and allow cells to adhere and grow on the surface, thereby making them promising optically EMPs for biolabeling.

Design Concepts for Solution and Solid-State Emitters - A Modern Viewpoint on Classical and Non-Classical Approaches

For a long time, luminescence phenomena were strictly distinguished between the emission of isolated molecules in dilute solutions or close-packed structures such as in powders or aggregates. This changed with the breakthrough observation of dual-state efficient materials, which led to a rapid boost of publications examining the influence of structural features to achieve balanced emission with disregarded molecular surroundings. Some first general structural design concepts have already been proposed based on reoccurring patterns and pivotal motifs. However, we have found another way to classify these solution and solid-state emitters (SSSEs). Hence, this minireview aims to present an overview of published structural features of SSSEs while shining light on design concepts from a more generalized perspective. Since SSSEs are believed to bridge the gap of hitherto known aggregation-sensitive compound classes, we hope to give future scientists a versatile tool in hand to efficiently design novel luminescent materials.

Unexpected luminescence of non-conjugated biomass-based polymers: new approach in photothermal imaging

Population growth, depletion of world resources and persistent toxic chemical production underline the need to seek new smart materials from inexpensive, biodegradable, and renewable feedstocks. Hence, "metal-free" ring-opening copolymerization to convert biomass carvone-based monomers into non-conventional luminescent biopolymers is considered a sustainable approach to achieve these goals. The non-conventional emission was studied in terms of steady-state and time-resolved spectroscopy in order to unravel the structure-properties for different carvone-based copolymers. The results highlighted the importance of the final copolymer folding structure as well as its environment in luminescent behavior (cluster-triggered emission). In all cases, their luminescent behavior is sensitive to small temperature fluctuations (where the minimum detected temperature is T_m ∼ 2 °C and relative sensitivity is S_r ∼ 6% °C) even at the microscopic scale, which endows these materials a great potential as thermosensitive smart polymers for photothermal imaging.

The Detection and Imaging of pH Values in Living Cells with Hemicyanine Based Colorimetric Mitochondria-Targeted Fluorescent Probe

pH plays a crucial role in cells, especially mitochondria, an important organelle. Developing probes with well-performance for pH detection is still in great demand. Therefore, we first synthesized an indole-based probe (MC-ID-OL) to detect mitochondrial pH changes. The emission wavelength of MC-ID-OL is 649 nm, which does not reach the near-infrared region (650-900 nm). To further enlarge the emission wavelength, probe MC-BI-OL was developed by replacing indolenine with benzoindole. As expected, the emission wavelength changed from 649 to 656 nm. MC-BI-OL probes could also detect pH changes and mitochondria's highly reversible proportional fluorescence localization. In addition, the fluorescence imaging of the MC-BI-OL in HeLa cells demonstrated that this probe could sense changes in the pH of mitochondria in cells.

Recent advances in NIR-II fluorescence based theranostic approaches for glioma

Gliomas are among the most common malignant tumors in the central nervous system and lead to poor life expectancy. However, the effective treatment of gliomas remains a considerable challenge. The recent development of near infrared (NIR) II (1000-1700 nm) theranostic agents has led to powerful strategies in diagnosis, targeted delivery of drugs, and accurate therapy. Because of the high capacity of NIR-II light in deep tissue penetration, improved spatiotemporal resolution can be achieved to facilitate the in vivo detection of gliomas via fluorescence imaging, and high contrast fluorescence imaging guided surgery can be realized. In addition to the precise imaging of tumors, drug delivery nano-platforms with NIR-II agents also allow the delivery process to be monitored in real-time. In addition, the combination of targeted drug delivery, photodynamic therapy, and photothermal therapy in the NIR region significantly improves the therapeutic effect against gliomas. Thus, this mini-review summarizes the recent developments in NIR-II fluorescence-based theranostic agents for glioma treatment.

Investigation of the Spatial Generation of Stimulated Raman Scattering Using Computer Simulation and Experimentation

Stimulated Raman scattering is a phenomenon with potential use in providing real-time molecular information in three-dimensions (3D) of a sample using imaging. For precise imaging, the knowledge about the spatial generation of stimulated Raman scattering is essential. To investigate the spatial behavior in an idealized case, computer simulations and experiments were performed. For the computer simulations, diffraction theory was used for the beam propagation complemented with nonlinear phase modulation describing the interaction between the light and matter. For the experiments, a volume of ethanol was illuminated by an expanded light beam and a plane inside the volume was imaged in transmission. For generating stimulated Raman scattering, a pump beam was focused into this volume and led to a beam dump after passing the volume. The pulse duration of the two beams were 6 ns and the pump beam energy ranged from 1 to 27 mJ. The effect of increasing pump power on the spatial distribution of the Raman gain and the spatial growth of the signal at different interaction lengths between the beam and the sample was investigated. The spatial width of the region where the stimulated Raman scattering signal was generated for experiments and simulation was 0.21 and 0.09 mm, respectively. The experimental and simulation results showed that most of the stimulated Raman scattering is generated close to the pump beam focus and the maximum peak of the Stokes intensity spatially comes shortly after the peak of the pump intensity.

Selective detection of carboxylesterase 2 activity in cancer cells using an activity-based chemiluminescent probe

Carboxylesterase 2 (CES2) has crucial roles in both xenobiotic metabolism and formation of pathogenic states including cancer. Thus, it is highly critical to monitor intracellular CES2 activity in living cancer cells. Here, we report a CES2 activatable phenoxy 1,2-dioxetane based chemiluminescent agent (CL-CES2). The probe exhibited a selective turn-on response in the presence of CES2 enzyme and enabled detection of CES2 activity in three different cancer cells that possess varying enzyme concentrations with high signal to noise ratios. In contrast no signal was obtained with CES1, an isoform of CES2 enzyme. CL-CES2 marks the first ever example of a CES2-responsive chemiluminescent luminophore and holds a great potential in further understanding the roles of CES2 activity in tumorogenesis.

Schiff Base Aggregation-Induced Emission Luminogens for Sensing Applications: A Review

Fluorescence sensing can not only identify a target substrate qualitatively but also achieve the purpose of quantitative detection through the change of the fluorescence signal. It has the advantages of immense sensitivity, rapid response, and excellent selectivity. The proposed aggregation-induced emission (AIE) concept solves the problem of the fluorescence of traditional fluorescent molecules becoming weak or quenched in high concentration or aggregated state conditions. Schiff base fluorescent probes have the advantages of simple synthesis, low toxicity, and easy design. They are often used for the detection of various substances. In this review we cover late developments in Schiff base compounds with AIE characteristics working as fluorescence sensors.

Lighting up Micro-/Nanorobots with Fluorescence

Micro-/nanorobots (MNRs) can be autonomously propelled on demand in complex biological environments and thus may bring revolutionary changes to biomedicines. Fluorescence has been widely used in real-time imaging, chemo-/biosensing, and photo-(chemo-) therapy. The integration of MNRs with fluorescence generates fluorescent MNRs with unique advantages of optical trackability, on-the-fly environmental sensitivity, and targeting chemo-/photon-induced cytotoxicity. This review provides an up-to-date overview of fluorescent MNRs. After the highlighted elucidation about MNRs of various propulsion mechanisms and the introductory information on fluorescence with emphasis on the fluorescent mechanisms and materials, we systematically illustrate the design and preparation strategies to integrate MNRs with fluorescent substances and their biomedical applications in imaging-guided drug delivery, intelligent on-the-fly sensing and photo-(chemo-) therapy. In the end, we summarize the main challenges and provide an outlook on the future directions of fluorescent MNRs. This work is expected to attract and inspire researchers from different communities to advance the creation and practical application of fluorescent MNRs on a broad horizon.

Niosome-Assisted Delivery of DNA Fluorescent Probe with Optimized Strand Displacement for Intracellular MicroRNA21 Imaging

MicroRNAs play a vital role in cancer development and are considered as potential biomarkers for early prognostic assessment. Here, we propose a novel biosensing system to achieve fluorescence imaging of miRNA21 (miR21) in cancer cells. This system consists of two components: an optimized "off-on" double-stranded DNA (dsDNA) fluorescent for miR21 sensing by efficient strand-displacement reaction and a potent carrier vesicle, termed niosome (SPN), to facilitate the efficient intracellular delivery of the dsDNA probe. A series of dsDNA probes based on fluorescence energy resonance transfer (FRET) was assembled to target miR21. By optimizing the appropriate length of the reporter strand in the dsDNA probe, high accuracy and sensitivity for miR21 recognition are ensured. To overcome the cellular barrier, we synthesized SPN with the main components of a nonionic surfactant Span 80 and a cationic lipid DOTAP, which could efficiently load dsDNA probes via electrostatic interactions and potently deliver the dsDNA probes into cells with good biosafety. The SPN/dsDNA achieved efficient miR21 fluorescent imaging in living cells, and could discriminate cancer cells (MCF-7) from normal cells (L-02). Therefore, the proposed SPN/dsDNA system provides a powerful tool for intracellular miRNA biosensing, which holds great promise for early cancer diagnosis.

Dual-Mode Tumor Imaging Using Probes That Are Responsive to Hypoxia-Induced Pathological Conditions

Hypoxia in solid tumors is associated with poor prognosis, increased aggressiveness, and strong resistance to therapeutics, making accurate monitoring of hypoxia important. Several imaging modalities have been used to study hypoxia, but each modality has inherent limitations. The use of a second modality can compensate for the limitations and validate the results of any single imaging modality. In this review, we describe dual-mode imaging systems for the detection of hypoxia that have been reported since the start of the 21st century. First, we provide a brief overview of the hallmarks of hypoxia used for imaging and the imaging modalities used to detect hypoxia, including optical imaging, ultrasound imaging, photoacoustic imaging, single-photon emission tomography, X-ray computed tomography, positron emission tomography, Cerenkov radiation energy transfer imaging, magnetic resonance imaging, electron paramagnetic resonance imaging, magnetic particle imaging, and surface-enhanced Raman spectroscopy, and mass spectrometric imaging. These overviews are followed by examples of hypoxia-relevant imaging using a mixture of probes for complementary single-mode imaging techniques. Then, we describe dual-mode molecular switches that are responsive in multiple imaging modalities to at least one hypoxia-induced pathological change. Finally, we offer future perspectives toward dual-mode imaging of hypoxia and hypoxia-induced pathophysiological changes in tumor microenvironments.

Mechanism assay of interaction between blood vessels-near infrared probe and cell surface marker proteins of endothelial cells

In vivo blood vessels imaging is crucial to study blood vessels related diseases in real-time. For this purpose, fluorescent based imaging is one of the utmost techniques for imaging a living system. The discovery of a new near-infrared probe (CyA-B2) by screening chemical probe library in our previous report which showed the most specific binding on the blood capillaries of the 3D-tissue models give us interest to study more about the binding site of this probe to the surface of endothelial cells main component cell of blood capillaries. By studying the competition assays of CyA-B2 using several potential surface markers of endothelial cells found through the chemical database (ChEMBL) and manually selected, CD133 gave the lowest IC₅₀ (half maximal inhibitory concentration) value. Hence, CD133 protein which is expressed on the endothelial cell membrane was postulated to be the binding site due to the suppression of CyA-B2 on the blood capillaries by the competition assays. Since, CD133 is also expressed on many types of cancer cells, it would be useful to use CyA-B2 as a bioprobe to monitor or diagnostic tumor growth.

A pH-sensor scaffold for mapping spatiotemporal gradients in three-dimensional in vitro tumour models

The detection of extracellular pH at single cell resolution is challenging and requires advanced sensibility. Sensing pH at high spatial and temporal resolution might provide crucial information in understanding the role of pH and its fluctuations in a wide range of physio-pathological cellular processes, including cancer. Here, a method to embed silica-based fluorescent pH sensors into alginate-based three-dimensional (3D) microgels tumour models, coupled with a computational method for fine data analysis, is presented. By means of confocal laser scanning microscopy, live-cell time-lapse imaging of 3D alginate microgels was performed and the extracellular pH metabolic variations were monitored in both in vitro 3D mono- and 3D co-cultures of tumour and stromal pancreatic cells. The results show that the extracellular pH is cell line-specific and time-dependent. Moreover, differences in pH were also detected between 3D monocultures versus 3D co-cultures, thus suggesting the existence of a metabolic crosstalk between tumour and stromal cells. In conclusion, the system has the potential to image multiple live cell types in a 3D environment and to decipher in real-time their pH metabolic interplay under controlled experimental conditions, thus being also a suitable platform for drug screening and personalized medicine.

A New Phenylazo-Based Fluorescent Probe for Sensitive Detection of Hypochlorous Acid in Aqueous Solution

In this paper, we designed and synthesized a novel phenylazo-based fluorescent probe (RHN) for the sensing and imaging of hypochlorous acid (HClO) in mitochondria in living cells. In this process, HClO promoted the oxidation of the phenylazo group to generate a free Rhodol fluorophore moiety, which in turn restored strong fluorescence and realized the detection of HClO. As expected, RHN exhibited high selectivity, high sensitivity and rapid response, with detection limits as low as 22 nM (1.155 ng/mL). Importantly, the results of the cell imaging experiments indicated that RHN has the ability to image and sense HClO in mitochondria, which is of great significance for exploration of the specific role of HClO in both the immune system and diseases.