Molecular Mechanism of Viscosity Sensitivity in BODIPY Rotors and Application to Motion-Based Fluorescent Sensors

Precise photorelease in living cells by high-viscosity activatable coumarin-based photocages

Intracellular viscosity is a critical microenvironmental factor in various biological systems, and its abnormal increase is closely linked to the progression of many diseases. Therefore, precisely controlling the release of bioactive molecules in high-viscosity regions is vital for understanding disease mechanisms and advancing their diagnosis and treatment. However, viscosity alone cannot directly trigger chemical reactions. Inspired by molecular rotor fluorophores, we have developed a series of high-viscosity activated photocages by modifying the C3 position of the coumarin scaffold with electron-withdrawing groups. In low-viscosity environments, both fluorescence and photocleavage of the photocages are inhibited by nonradiative decay caused by intramolecular free rotation. In contrast, in high-viscosity environments, the restriction of this intramolecular rotation restores fluorescence and photocleavage. These unique photolysis properties enable the selective photorelease of these photocages in high-viscosity conditions. As a proof of concept, we have developed a drug delivery system that targets abnormal mitochondria with high viscosity. This system demonstrates enhanced photolysis efficiency in abnormal mitochondria compared to normal ones, allowing for precise drug release in diseased mitochondria while ensuring excellent biological safety in healthy mitochondria. We anticipate that these photocages will serve as convenient and efficient tools for the precise release of active molecules in high-viscosity environments.

Lipid droplet specific BODIPY based rotors with viscosity sensitivity to distinguish normal and cancer cells: impact of molecular conformation

Lipid droplets (LDs) are dynamic, multifunctional organelles critical for regulating energy balance, cell signaling, membrane formation, and trafficking. Recent studies have highlighted LDs as emerging cancer biomarkers, with cancer cells typically exhibiting a higher number and viscosity of LDs compared to normal cells. This discovery paves the way for developing molecular probes that can monitor intracellular viscosity changes within LDs, offering a powerful tool for early cancer diagnosis, recurrence monitoring, and therapeutic interventions. In this study, we designed and synthesized two series of donor-acceptor (D-A) conjugated BODIPY-cyanostilbene based fluorophores (5a-c and 6a-c) by fine-tuning the cyanostilbene unit with three distinct substituents (OMe, H, Cl) and modulating the molecular conformation via rigidifying the indacene core. While the terminal substituents had a minimal effect on the optical properties, changes in molecular conformation significantly impacted the photophysical behavior of the fluorophores. Compounds 5a-c function as molecular rotors, with the free rotation of the meso-biphenyl rings leading to non-radiative deactivation of the excited state, resulting in weak emission. Additionally, this structural feature makes them highly responsive to changes in viscosity. As the glycerol concentration increased from 0% to 99%, the fluorescence intensity of compounds 5a, 5b, and 5c increased dramatically by 17-fold, 78-fold, and 43-fold, respectively. In contrast, compounds 6a-c, with restricted phenyl ring rotation due to tetra-methyls on the indacene unit, showed only a modest 2-3-fold increment in fluorescence intensity under similar conditions. These fluorophores possess several key advantages, including high selectivity for LDs, good photostability, sensitivity to viscosity, and responsiveness to polarity and pH. Moreover, they effectively differentiate between normal and cancer cells, making them valuable tools for cancer diagnosis and potential therapeutic applications.

Photophysical behavior of meso-N-butylcarbazole-substituted BODIPY in different nano-scale organized media

The present work focuses on the photophysical behavior of meso-N-butylcarbazole-substituted BODIPY (CBZ-BDP) in different organized media towards exploring the possible use of the dye as a molecular sensor and imaging agent. The molecule shows an appreciable change in absorption and emission spectra at 75% water-acetonitrile mixture compared to pure acetonitrile. In water-acetonitrile mixture, it displays aggregate-induced emission (AIE) bands. New emission peaks are observed at 560 nm and 630 nm, corresponding to LE (locally excited) and ICT (intramolecular charge transfer) states of CBZ-BDP aggregates. The fluorescence anisotropy studies of CBZ-BDP in glycerol medium show its better sensitivity towards the microenvironment. CBZ-BDP was used to probe various microheterogeneous systems like bile salts, pluronics, and lipid bilayer systems in aqueous medium. The dye displays sensitive variation in emission intensity and fluorescence anisotropy in sodium cholate (NaC) bile salt in aqueous medium as a function of the bile salt concentration. The molecule detects the temperature-induced phase transitions in pluronic P123 and F127, as well as 1,2-dimyristoylphosphatidylcholine (DMPC) and 1,2-dipalmitoylphosphatidylcholine (DPPC) lipid bilayer systems in aqueous medium. These studies strongly suggest that CBZ-BDP can be used as an efficient fluorescent probe in sensing the micro-environmental changes in bile salts, pluronics, and lipid bilayers in aqueous medium. The imaging studies of CBZ-BDP-embedded Giant Unilamellar Vesicles (GUVs) were carried out. The molecule stains the lipid bilayers and displays bright-green fluorescent images, suggesting its potential in lipid bilayer imaging.

Unraveling Hydration Shell Dynamics and Viscosity Effects Around Cyanamide Probes via 2D IR Spectroscopy

Hydration dynamics and solvent viscosity play critical roles in the structure and function of biomolecules. An overwhelming body of evidence suggests that protein and membrane fluctuations are closely linked to solvent fluctuations. While extensive research exists on the use of vibrational probes to detect local interactions and solvent dynamics, fewer studies have explored how the behavior of these reporters changes in response to bulk viscosity. To address this gap, two-dimensional infrared spectroscopy (2D IR) was employed in this study to investigate the ultrafast hydration dynamics around a cyanamide (NCN) probe attached to a nucleoside, deoxycytidine, in aqueous solutions with varying glycerol content. The use of a small vibrational probe on a targeted nucleic acid offers the potential to capture more localized hydration dynamics than alternative methods. The time scales for the frequency correlation decays were found to increase linearly with bulk viscosity, ranging from 0.9 to 11.4 ps over viscosities of 0.96-49.1 cP. Additionally, molecular dynamics (MD) simulations were performed to model the local hydration dynamics around the NCN probe. Interestingly, increasing the glycerol content did not significantly alter the hydration of the deoxycytidine. The MD simulations further suggested that the NCN probe's frequency fluctuations were primarily influenced by the dynamics of water in the second solvation shell. Cage correlation functions, which measure the movement of water molecules in and out of the second solvation shell, exhibited decays that closely matched those of the frequency-fluctuation correlation function (FFCF). These findings offer new insights into hydration dynamics and the impact of viscosity on biological systems.

Enhancing Phototoxicity in BODIPY-Perylene Charge Transfer Dyads by Combined Iodination and Mesylation

The uptake and phototoxicity of a family of BODIPY-perylene charge transfer dyads are compared in live cancer and non-cancer cell lines to evaluate their performance in imaging and photodynamic therapy (PDT). The impact of iodination and mesylation of the meso position of the compounds on their optical properties, cell uptake and toxicity are compared. Notably, across all derivatives the probes were minimally dark toxic up to 50 μM, (the maximum concentration tested), but exhibited outstanding phototoxicity with nanomolar IC50 values and impressive phototoxic indices (PI, ratio of dark IC50 to light IC50), with best performance for the mesylated iodinated derivative MB2PI, which had a PI of >218 and >8.9 in MCF-7 cells and tumour spheroids respectively. This is significantly higher than non-iodinated analogue MB2P in MCF-7 cells with an observed PI of >109 and slightly higher than MB2PI in spheroids with a PI of >8. This compound also showed interesting emission spectral variation with localisation that responded to stimulation of inflammation. Additional studies confirmed efficient singlet oxygen generation by the BODIPYs, suggesting a Type II mechanism of phototoxicity. Overall, the data indicates that combining charge transfer and iodination is an effective strategy for enhancing phototherapeutic capacity of BODIPY PS.

Moderately Heavy Atom-Substituted BODIPY Photosensitizer with Mitochondrial Targeting Ability for Imaging-Guided Photodynamic Therapy

Advanced photodynamic therapy requires photosensitizers with targeting, diagnostic, and therapeutic properties. To fulfill this multifunctionality, we report the synthesis of two triphenylphosphonium (TPP)-functionalized boron-dipyrromethene (BODIPY) dyes, TPPB-H and TPPB-Br, which incorporate a hydrogen atom and dibrominated vinyl moiety at the 6-position of the BODIPY core, respectively. The heavy-atom effect of the moderately heavy bromine atoms allowed TPPB-Br to achieve a proper balance between the toxic singlet oxygen (1O2) production and fluorescence efficiencies. In this dye, the bromine atom-induced stimulation of the singlet-to-triplet intersystem crossing dynamics resulted in an approximately 45-fold increase in the 1O2 quantum yield with respect to that of the nonbrominated counterpart (0.0059 and 0.28 for TPPB-H and TPPB-Br, respectively). This increase was accompanied only a 2-fold reduction in the fluorescence quantum yield (0.54 and 0.22 for TPPB-H and TPPB-Br, respectively). During multicolor confocal laser scanning microscopy observations conducted using two carcinomas, MCF-7 and HeLa, both BODIPY dyes exhibited high targeting specificity toward cancer cell mitochondria owing to the TPP cation functionalization. The two dyes also showed the feasibility of fluorescence cell imaging; however, only the dibrominated BODIPY TPPB-Br manifested pronounced photocytotoxicity with half-maximal inhibitory concentrations of 0.12 and 0.77 μM obtained for MCF-7 and HeLa cells, respectively. These findings demonstrate the potential applicability of TPPB-Br as an imaging-guided photodynamic therapy agent with mitochondrial specificity.

Breaking the blue barrier of nucleobase fluorescence emission with dicyanovinyl-based uracil molecular rotor probes

Dicyanovinyl-modified uracil produces fluorescent molecular rotors (FMR) that display massively red-shifted emission and huge Stokes shifts. They are exemplified by DCVSU - an intrinsically fluorescent nucleobase analog (IFNA) with the longest emission wavelength of 592 nm (DMSO) reported thus far which also shows strong polarity sensitivity and large Stokes shift (λ = 181 nm). The IFNAs exhibited typical molecular rotor response to solvent viscosity with brightnesses (ε × φ) of up to 8700 cm-1 M-1. 1H NMR titration confirmed the expected association of the IFNA with the complementary nucleobase adenine-9-ethyl acetate.

A lysosomal-targeted and viscosity-ultrasensitive near-infrared fluorescent probe for sensing viscosity in cells and a diabetic mice model

Diabetes, as a metabolic disorder, has been implicated in organ dysfunction, often correlated with aberrant changes in viscosity. Lysosomal viscosity serves as an indicator of the lysosome's condition and activity, as it always varies synchronously with the change of lysosome's positioning, structure, and internal constituents. Diabetes, a condition within the metabolic disease category, has the potential to disrupt organ function due to irregular changes in viscosity. Therefore, early and precise diagnosis of diabetes is crucial for the prevention and management of diabetic conditions. Understanding the correlation between viscosity variations and lysosomal changes in vivo is vitally important for researching associated diseases. In this study, we developed Lyso-V, a near-infrared (NIR) fluorescent probe targeting lysosomes, with ultrasensitivity to viscosity changes. This probe, designed with a donor-π-bridge-acceptor (D-π-A) structure, exhibits a significant increase in NIR fluorescence intensity (approximately 690 times) when responding to viscosity, due to a twisted intramolecular charge transfer (TICT) mechanism. Furthermore, the probe designed specifically for lysosomes, enables the detection of changes in lysosomal viscosity as well as autophagy processes. Notably, through the application of this probe, we have detected an increased viscosity within the pathological model of the diabetic mouse. Moreover, Lyso-V was employed to measure the viscosity in diabetic mice. Owing to the multifaceted nature of the Lyso-V probe, it is anticipated to act as a practical and potent resource for deepening our understanding of the pathophysiological aspects of diabetes and aiding in its early detection.

Construction and evaluation of near-infrared fluorescent probes for imaging lipid droplet and lysosomal viscosity

Microenvironmental viscosity is a crucial parameter for biological systems, and its abnormal fluctuations are closely associated with various functional disorders and diseases. However, it is still important and urgent to develop improved near-infrared fluorescent probes for micro-viscosity with dual-organelle targeting properties, low background noise, and high sensitivity. Herein, two BODIPY-based small-molecule fluorescent probes were designed and synthesized, which were explored for their viscosity- and polarity-responsive properties, and were further applied to imaging sub-cellular viscosity in living cells. Interestingly, BSZ-Ph and BSZ-R displayed near-infrared fluorescence (more than 650 nm) and were sensitive to environmental viscosity and polarity due to the introduction of a benzothiazole at the 2-position and electron-rich aniline groups at the 5-position of the BODIPY core, respectively. The fluorescence intensity increased exponentially with the viscosity changes. Furthermore, the probe BSZ-Ph could successfully target lipid droplets and image cellular viscosity changes by treating lipopolysaccharides (LPS) and nystatin. Comparatively, the probe BSZ-R could successfully target the dual organelles of lipid droplets and lysosomes and image cellular viscosity changes by treating LPS and monensin. Therefore, in this work, we reported two new BODIPY-based near-infrared fluorescent probes, BSZ-Ph and BSZ-R, for cellular viscosity imaging, which could target lipid droplets and the dual organelles of lysosomes and lipid droplets, respectively. The study could provide a reference for the future development of fluorescent probes for viscosity in lipid droplets and lysosomes.

A Red-Emission Fluorescent Probe with Large Stokes Shift for Detection of Viscosity in Living Cells and Tumor-Bearing Mice

Abnormal viscosity is closely related to the occurrence of many diseases, such as cancer. Therefore, real-time detection of changes in viscosity in living cells is of great importance. Fluorescent molecular rotors play a critical role in detecting changes in cellular viscosity. Developing red emission viscosity probes with large Stokes shifts and high sensitivity and specificity remains an urgent and important topic. Herein, a novel viscosity-sensitive fluorescent probe (TCF-VIS1) with a large stokes shift and red emission was prepared based on the 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) skeleton. Due to intramolecular rotation, the probe itself does not fluorescence at low viscosity. With the increase in viscosity, the rotation of TCF-VIS1 is limited, and its fluorescence is obviously enhanced. The probe has the advantages of simple preparation, large Stokes shift, good sensitivity and selectivity, and low cytotoxicity, which make it successfully used for viscosity detection in living cells. Moreover, TCF-VIS1 showed its potential for cancer diagnosis at the cell level and in tumor-bearing mice by detecting viscosity. Therefore, the probe is expected to enrich strategies for the detection of viscosity in biological systems and offer a potential tool for cancer diagnosis.

Excited State Rotational Freedom Impacts Viscosity Sensitivity in Arylcyanoamide Fluorescent Molecular Rotor Dyes

The microviscosity of intracellular environments plays an important role in monitoring cellular function. Thus, the capability of detecting changes in viscosity can be utilized for the detection of different disease states. Viscosity-sensitive fluorescent molecular rotors are potentially excellent probes for these applications; however, the predictable relationships between chemical structural features and viscosity sensitivity are poorly understood. Here, we investigate a set of arylcyanoamide-based fluorescent probes and the effect of small aliphatic substituents on their viscosity sensitivity. We found that the location of the substituents and the type of π-network of the fluorophore can significantly affect the viscosity sensitivity of these fluorophores. Computational analysis supported the notion that the excited state rotational energy barrier plays a dominant role in the relative viscosity sensitivity of these fluorophores. These findings provide valuable insight into the design of molecular rotor-based fluorophores for viscosity measurement.

Design and Application of Fluorescent Probes to Detect Cellular Physical Microenvironments

The microenvironment is indispensable for functionality of various biomacromolecules, subcellular compartments, living cells, and organisms. In particular, physical properties within the biological microenvironment could exert profound effects on both the cellular physiology and pathology, with parameters including the polarity, viscosity, pH, and other relevant factors. There is a significant demand to directly visualize and quantitatively measure the fluctuation in the cellular microenvironment with spatiotemporal resolution. To satisfy this need, analytical methods based on fluorescence probes offer great opportunities due to the facile, sensitive, and dynamic detection that these molecules could enable in varying biological settings from in vitro samples to live animal models. Herein, we focus on various types of small molecule fluorescent probes for the detection and measurement of physical parameters of the microenvironment, including pH, polarity, viscosity, mechanical force, temperature, and electron potential. For each parameter, we primarily describe the chemical mechanisms underlying how physical properties are correlated with changes of various fluorescent signals. This review provides both an overview and a perspective for the development of small molecule fluorescent probes to visualize the dynamic changes in the cellular environment, to expand the knowledge for biological process, and to enrich diagnostic tools for human diseases.

Molecular Rotors: Fluorescent Sensors for Microviscosity and Conformation of Biomolecules

The viscosity and crowding of biological environment are considered vital for the correct cellular function, and alterations in these parameters are known to underly a number of pathologies including diabetes, malaria, cancer and neurodegenerative diseases, to name a few. Over the last decades, fluorescent molecular probes termed molecular rotors proved extremely useful for exploring viscosity, crowding, and underlying molecular interactions in biologically relevant settings. In this review, we will discuss the basic principles underpinning the functionality of these probes and will review advances in their use as sensors for lipid order, protein crowding and conformation, temperature and non-canonical nucleic acid structures in live cells and other relevant biological settings.

No-wash fluorogenic labeling of proteins for reversible photoswitching in live cells

Photoswitchable fluorescent molecules (PSFMs) are positioned as valuable tools for biomolecule localization tracking and super-resolution imaging technologies due to their unique ability to reversibly control fluorescence intensity upon light irradiation. Despite the high demand for PSFMs that are suitable for live-cell imaging, no general method has been reported that enables reversible fluorescence control on proteins of interest in living cells. Herein, we have established a platform to realize reversible fluorescence switching in living cells by adapting a protein labeling system. We have developed a new PSFM, named HTL-Trp-BODIPY-FF, which exhibits strong fluorogenicity upon recognition of Halo-tag protein and reversible fluorescence photoswitching in living cells. This is the first example of a PSFM that can be applicable to a general-purpose Halo-tag protein labeling system for no-wash live-cell imaging.

Dual role far red fluorescent molecular rotor for decoding the plasma membrane and mitochondrial viscosity

The dysfunctions in the mitochondria are associated with various pathological conditions like neurodegeneration, metabolic disorder, and cancer, leading to dysregulated cell death. Here, we have designed and synthesized a julolidine-based molecular rotor (JMT) to target mitochondria with far-red emission accounting for mitochondrial dysfunction. JMT showed viscosity sensitivity with 160-fold enhancement in fluorescence intensity. The origin of the dark state in a lower viscous environment was investigated through density functional calculations. We have employed JMT to monitor mitochondrial dysfunction induced by nystatin using confocal and fluorescence lifetime imaging microscopy. Further, we investigated mitochondrial abnormalities under inflammatory conditions triggered by lipopolysaccharide in live HeLa cells. The cellular uptake mechanisms of JMT were studied using various endocytosis inhibitors. Moreover, we reported tracking small fluorescent molecule switching from mitochondria to the plasma membrane upon introducing mitochondrial depolarizer in cells. On treating the mitochondria potential uncoupler, JMT relocates to the cell membrane and can be utilized for understanding the interplay between mitochondria and cell membranes. Moreover, JMT was applied to stain the RBC plasma membrane isolated from human blood.

Comparative Observation of Distinct Dynamic Stokes Shifts in Diaryl BODIPY Triads with Broadband Two-Photon Absorption

Time-resolved evolution of excited states in the twist-conjugated chromophores is of great fundamental interest for photoluminescent applications. The four diaryl BODIPY triads modified with diverse end-cappers at 2,6-positions were investigated properly, and considerable two-photon absorption capabilities in the first biological spectral window were obtained. Fast relaxations from the initially twisted conformation to the planarized conformation in the excited state were resolved spectrally and kinetically, accompanied by the discernible phenomenon of the fluorescence dynamic Stokes shift (DSS). Along with increasing electron donating capabilities and solvent polarities, the characteristics of structural rearrangement and intramolecular charge transfer have been estimated by enhanced DSS behaviors. Especially, the blue-shifted DSS was rationalized as the sequence conversion between the planarized state and the twisted charge transfer state. A molecular-level picture for relaxation pathways in different polarities was depicted and supported by the theoretical simulations. Significant and fast structural motions in this work contribute to the excited-state dynamics and rational development of versatile BODIPY chromophores.

A review on solution- and vapor-responsive sensors for the detection of phthalates

Phthalic acid esters, largely referred to as phthalates, are today acknowledged as important pollutants used in the manufacture of polyvinyl chloride (PVC)-based plastics, whose use extends to almost every aspect of modern life. The risk of exposure to phthalates is particularly relevant as high concentrations are regularly found in drinking water, food-contact materials and medical devices, motivating an immense body of research devoted to methods for their detection in liquid samples. Conversely, phthalate vapors have only recently been acknowledged as potentially important atmospheric pollutants and as early fire indicators; additionally, deposition of these vapors can pose significant problems to the proper functioning of spacecraft and diverse on-board devices, leading to major space agencies recognizing the need of developing vapor-responsive phthalate sensors. In this manuscript we present a literature survey on solution- and vapor-responsive sensors and analytical assays for the detection of phthalates, providing a detailed analysis of a vast array of analytical data to offer a clear idea on the analytical performance (limits of detection and quantification, linear range) and advantages provided by each class of sensor covered in this review (electrochemical, optical and vapor-responsive) in the context of their potential real-life applications; the manuscript also gives detailed fundamental information on the various physicochemical responses exploited by these sensors and assays that could potentially be harnessed by new researchers entering the field.

Viscosity-Sensitive Membrane Dyes as Tools To Estimate the Crystalline Structure of Lipid Bilayers

Lipid membranes are crucial for cellular integrity and regulation, and tight control of their structural and mechanical properties is vital to ensure that they function properly. Fluorescent probes sensitive to the membrane's microenvironment are useful for investigating lipid membrane properties; however, there is currently a lack of quantitative correlation between the exact parameters of lipid organization and a readout from these dyes. Here, we investigate this relationship for "molecular rotors", or microviscosity sensors, by simultaneously measuring their fluorescence lifetime to determine the membrane viscosity, while using X-ray diffraction to determine the membrane's structural properties. Our results reveal a phase-dependent correlation between the membrane's structural parameters and mechanical properties measured by a BODIPY-based molecular rotor, giving excellent predictive power for the structural descriptors of the lipid bilayer. We also demonstrate that differences in membrane thickness between different lipid phases are not a prerequisite for the formation of lipid microdomains and that this requirement can be disrupted by the presence of line-active molecules. Our results underpin the use of membrane-sensitive dyes as reporters of the structure of lipid membranes.

Bodipy Dimer for Enhancing Triplet-Triplet Annihilation Upconversion Performance

Triplet-triplet annihilation upconversion (TTA-UC) has considerable potential for emerging applications in bioimaging, optogenetics, photoredox catalysis, solar energy harvesting, etc. Fluoroboron dipyrrole (Bodipy) dyes are an essential type of annihilator in TTA-UC. However, conventional Bodipy dyes generally have large molar extinction coefficients and small Stokes shifts (<20 nm), subjecting them to severe internal filtration effects at high concentrations, and resulting in low upconversion quantum efficiency of TTA-UC systems using Bodipy dyes as annihilators. In this study, a Bodipy dimer (B-2) with large Stokes shifts was synthesized using the strategy of dimerization of an already reported Bodipy annihilator (B-1). Photophysical characterization and theoretical chemical analysis showed that both B-1 and B-2 can couple with the red light-activated photosensitizer PdTPBP to fulfill TTA-UC; however, the higher fluorescence quantum yield of B-2 resulted in a higher upconversion efficiency (η_UC) for PdTPBP/B-2 (10.7%) than for PdTPBP/B-1 (4.0%). This study proposes a new strategy to expand Bodipy Stokes shifts and improve TTA-UC performance, which can facilitate the application of TTA-UC in photonics and biophotonics.

Acid-Resistant BODIPY Amino Acids for Peptide-Based Fluorescence Imaging of GPR54 Receptors in Pancreatic Islets

The G protein-coupled kisspeptin receptor (GPR54 or KISS1R) is an important mediator in reproduction, metabolism and cancer biology; however, there are limited fluorescent probes or antibodies for direct imaging of these receptors in cells and intact tissues, which can help to interrogate their multiple biological roles. Herein, we describe the rational design and characterization of a new acid-resistant BODIPY-based amino acid (Trp-BODIPY PLUS), and its implementation for solid-phase synthesis of fluorescent bioactive peptides. Trp-BODIPY PLUS retains the binding capabilities of both short linear and cyclic peptides and displays notable turn-on fluorescence emission upon target binding for wash-free imaging. Finally, we employed Trp-BODIPY PLUS to prepare some of the first fluorogenic kisspeptin-based probes and visualized the expression and localization of GPR54 receptors in human cells and in whole mouse pancreatic islets by fluorescence imaging.

Acid-Resistant BODIPY Amino Acids for Peptide-Based Fluorescence Imaging of GPR54 Receptors in Pancreatic Islets

The G protein-coupled kisspeptin receptor (GPR54 or KISS1R) is an important mediator in reproduction, metabolism and cancer biology; however, there are limited fluorescent probes or antibodies for direct imaging of these receptors in cells and intact tissues, which can help to interrogate their multiple biological roles. Herein, we describe the rational design and characterization of a new acid-resistant BODIPY-based amino acid (Trp-BODIPY PLUS), and its implementation for solid-phase synthesis of fluorescent bioactive peptides. Trp-BODIPY PLUS retains the binding capabilities of both short linear and cyclic peptides and displays notable turn-on fluorescence emission upon target binding for wash-free imaging. Finally, we employed Trp-BODIPY PLUS to prepare some of the first fluorogenic kisspeptin-based probes and visualized the expression and localization of GPR54 receptors in human cells and in whole mouse pancreatic islets by fluorescence imaging.

Utilization of photo-luminescent technique toward viscosity detection in the liquid food system with triphenylamine-michaelitic acid molecular sensor

A noninvasive and effective viscosity inspection method is expected to ease the burden of continued increased health problems caused by liquid food safety. In this study, we proposed the viscosity of the liquid food micro-environment as a marker and further developed a versatile optical sensor, DPTMDD, for monitoring liquid food micro-environmental viscosity alterations. This sensor was strategically constructed by the triphenylamine-thiophene derivate and michaelitic acid, rotatable conjugate structure was utilized as the recognition site. The molecular sensor was synthesized in a one-step facile way, and DPTMDD displayed a longer emission wavelength (592 nm), low detection limit (1.419 cP), and larger Stokes shift (193.7 nm in glycerol and 177.8 nm in water) with narrower energy band, endowing the sensor with the capacity of achieving high signal-to-noise ratio imaging. Meanwhile, DPTMDD exhibits high adaptability, selectivity, sensitivity, and good photo-stability in various liquid foods, bright fluorescent signal (37.5-fold) of DPTMDD is specifically activated in the high viscosity media. Thickening efficiencies can be identified as well. More importantly, the viscosity fluctuations during the metamorphic stages of liquid foods are also screened through in situ monitoring. We expected that this unique strategy will reinvigorate the continued perfection of liquid food safety investigation systems.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05699-y.

FRET-based fluorescent probe with favorable water solubility for simultaneous detection of SO2 derivatives and viscosity

The intracellular viscosity is an important parameter of the microenvironment and SO₂ is a vital gas signal molecule. At present, some dual-response fluorescence probes for simultaneous measurements of viscosity and SO₂ derivatives (HSO₃^-/SO₃^2-) possessed poor water solubility. In this work, we developed a water-soluble fluorescence probe CIJ (0.0864 g/100 mL of water at 20 °C) for simultaneous measurements of viscosity and SO₂ derivatives. CIJ exhibited a sensitive fluorescence enhancement to environmental viscosity from 0.97 to 28.04 cP based on a twisted intramolecular charge transfer mechanism and was applied to effective measurement of viscosity in vitro and in vivo. CIJ could also respond to SO₂ derivatives with a low detection limit (44 nM) and a fast response time (5 min) based on the nucleophilic addition reaction. Furthermore, CIJ was applied to monitor SO₂ derivatives in ratiometric response manner in living cells.

Nuclear bodies protect phase separated proteins from degradation in stressed proteome

RNA-binding proteins (RBPs) containing intrinsically disordered domains undergo liquid-liquid phase separation to form nuclear bodies under stress conditions. This process is also connected to the misfolding and aggregation of RBPs, which are associated with a series of neurodegenerative diseases. However, it remains elusive how folding states of RBPs changes upon the formation and maturation of nuclear bodies. Here, we describe SNAP-tag based imaging methods to visualize the folding states of RBPs in live cells via time-resolved quantitative microscopic analyses of their micropolarity and microviscosity. Using these imaging methods in conjunction with immunofluorescence imaging, we demonstrate that RBPs, represented by TDP-43, initially enters the PML nuclear bodies in its native state upon transient proteostasis stress, albeit it begins to misfolded during prolonged stress. Furthermore, we show that heat shock protein 70 co-enters the PML nuclear bodies to prevent the degradation of TDP-43 from the proteotoxic stress, thus revealing a previously unappreciated protective role of the PML nuclear bodies in the prevention of stress-induced degradation of TDP-43. In summary, our imaging methods described in the manuscript, for the first time, reveal the folding states of RBPs, which were previously challenging to study with conventional methods in nuclear bodies of live cells. This study uncovers the mechanistic correlations between the folding states of a protein and functions of nuclear bodies, in particular PML bodies. We envision that the imaging methods can be generally applied to elucidating the structural aspects of other proteins that exhibit granular structures under biological stimulus.

Green extract rosemary acid as a viscosity-sensitive molecular sensor in liquid systems

The liquid micro-environment plays a momentous role in the regulation of various activities, and the abnormal changes are often closely related to the deterioration phenomena in multiple beverages. The local viscosity fluctuation has long been regarded as a key indicator to reflect the micro-environmental status changes. Herein, we proposed a versatile optical sensor, rosmarinic acid (RA), one kind of green natural product extracted from rosemary, for monitoring liquid micro-environmental viscosity alterations. RA displays a larger Stokes shift (123.8 nm) with narrow-band energy and exhibits wide adaptability, high selectivity, good sensitivity, and excellent photostability in various commercial liquids. When in high viscous media, a bright fluorescent signal of RA is specifically activated, and a high signal-to-noise ratio signal was released (58-fold). With the assistance of the fluorescence analytical technique, we have successfully achieved tracking the viscosity fluctuations during the deterioration stage of liquids via an in situ and visualization method. Our study will spur additional research on the molecular tools extracted from natural products for liquid safety inspection, and a convenient and sustainable application pathway has been established.

Triphenylamine-Modified Cinnamaldehyde Derivate as a Molecular Sensor for Viscosity Detection in Liquids

Liquid safety is considered a serious public health problem; a convenient and effective viscosity determination method has been regarded as one of the powerful means to detect liquid safety. Herein, one kind of triphenylamine-modified cinnamaldehyde-based fluorescent sensor (3-(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl)acrylaldehyde (DPABA)) has been developed for sensing viscosity fluctuations in a liquid system, where a cinnamaldehyde derivative was extracted from one kind of natural plant cinnamon and acted as an acceptor, which has been combined with a triphenylamine derivate via the Suzuki coupling reaction within one facile step. Twisted intramolecular charge transfer (TICT) was observed, and the rotation could be restricted in the high-viscosity microenvironment; thus, the fluorescent signal was released at 548 nm. Featured with a larger Stokes shift (223.8 nm in water, 145.0 nm in glycerol), high adaptability, sensitivity, selectivity, and good photostability, the capability of high signal-to-noise ratio sensing was achieved. Importantly, this sensor DPABA has achieved noninvasively identifying thickening efficiency investigation, and viscosity fluctuations during the liquid deterioration program have been screened as well. We believed that this unique strategy can accelerate intelligent molecular platforms toward liquid quality and safety inspection.

Dense and Acidic Organelle-Targeted Visualization in Living Cells: Application of Viscosity-Responsive Fluorescence Utilizing Restricted Access to Minimum Energy Conical Intersection

Cell-imaging methods with functional fluorescent probes are an indispensable technique to evaluate physical parameters in cellular microenvironments. In particular, molecular rotors, which take advantage of the twisted intramolecular charge transfer (TICT) process, have helped evaluate microviscosity. However, the involvement of charge-separated species in the fluorescence process potentially limits the quantitative evaluation of viscosity. Herein, we developed viscosity-responsive fluorescent probes for cell imaging that are not dependent on the TICT process. We synthesized AnP₂-H and AnP₂-OEG, both of which contain 9,10-di(piperazinyl)anthracene, based on 9,10-bis(N,N-dialkylamino)anthracene that adopts a nonflat geometry at minimum energy conical intersection. AnP₂-H and AnP₂-OEG exhibited enhanced fluorescence as the viscosity increased, with sensitivities comparable to those of conventional molecular rotors. In living cell systems, AnP₂-OEG showed low cytotoxicity and, reflecting its viscosity-responsive property, allowed specific visualization of dense and acidic organelles such as lysosomes, secretory granules, and melanosomes under washout-free conditions. These results provide a new direction for developing functional fluorescent probes targeting dense organelles.

Thiophene-Extended Fluorescent Nucleosides as Molecular Rotor-Type Fluorogenic Sensors for Biomolecular Interactions

Fluorescent molecular rotors are versatile tools for the investigation of biomolecular interactions and the monitoring of microenvironmental changes in biological systems. They can transform invisible information into a fluorescence signal as a straightforward response. Their utility is synergistically amplified when they are merged with biomolecules. Despite the tremendous significance and superior programmability of nucleic acids, there are very few reports on the development of molecular rotor-type isomorphic nucleosides. Here, we report the synthesis and characterization of a highly emissive molecular rotor-containing thymine nucleoside (^ThexT) and its 2'-O-methyluridine analogue (2'-OMe-^ThexU) as fluorogenic microenvironment-sensitive sensors that emit vivid fluorescence via an interaction with the target proteins. ^ThexT and 2'-OMe-^ThexU may potentially serve as robust probes for a broad range of applications, such as fluorescence mapping, to monitor viscosity changes and specific protein-binding interactions in biological systems.

Solvatochromic Sensitivity of BODIPY Probes: A New Tool for Selecting Fluorophores and Polarity Mapping

This research work is devoted to collecting a high-quality dataset of BODIPYs in a series of 10-30 solvents. In total, 115 individual compounds in 71 solvents are represented by 1698 arrays of the spectral and photophysical properties of the fluorophore. Each dye for a series of solvents is characterized by a calculated value of solvatochromic sensitivity according to a semiempirical approach applied to a series of solvents. The whole dataset is classified into 6 and 24 clusters of solvatochromic sensitivity, from high negative to high positive solvatochromism. The results of the analysis are visualized by the polarity mapping plots depicting, in terms of wavenumbers, the absorption versus emission, stokes shift versus - (absorption maxima + emission maxima), and quantum yield versus stokes shift. An analysis of the clusters combining several dyes in an individual series of solvents shows that dyes of a high solvatochromic sensitivity demonstrate regular behaviour of the corresponding plots suitable for polarity and viscosity mapping. The fluorophores collected in this study represent a high quality dataset of pattern dyes for analytical and bioanalytical applications. The developed tools could be applied for the analysis of the applicability domain of the fluorescent sensors.

BODIPY-Ethynylestradiol molecular rotors as fluorescent viscosity probes in endoplasmic reticulum

Due to their capability for sensing changes in viscosity, fluorescent molecular rotors (FMRs) have emerged as potential tools to develop several promising viscosity probes; most of them, however, localize non-selectively within cells, precluding changes in the viscosity of specific cellular microdomains to be studied by these means. Following previous reports on enhanced fluorophore uptake efficiency and selectivity by incorporation of biological submolecular fragments, here we report two potential BODIPY FMRs based on an ethynylestradiol spindle, a non-cytotoxic semisynthetic estrogen well recognized by human cells. A critical evaluation of the potential of these fluorophores for being employed as FMRs is presented, including the photophysical characterization of the probes, SXRD studies and TD-DFT computations, as well as confocal microscopy imaging in MCF-7 (breast cancer) cells.

A palette of site-specific organelle fluorescent thermometers

Intracellular micro-temperature is closely related to cellular processes. Such local temperature inside cells can be measured by fluorescent thermometers, which are a series of fluorescent materials that convert the temperature information to detectable fluorescence signals. To investigate the intracellular temperature fluctuation in various organelles, it is essential to develop site-specific organelle thermometers. In this study, we develop a new series of fluorescent thermometers, Thermo Greens (TGs), to visualize the temperature change in almost all typical organelles. Through fluorescence lifetime-based cell imaging, it was proven that TGs allow the organelle-specific monitoring of temperature gradients created by external heating. The fluorescence lifetime-based thermometry shows that each organelle experiences a distinct temperature increment which depends on the distance away from the heat source. TGs are further demonstrated in the quantitative imaging of heat production at different organelles such as mitochondria and endoplasmic reticulum in brown adipocytes. To date, TGs are the first palette batch of small molecular fluorescent thermometers that can cover almost all typical organelles. These findings can inspire the development of new fluorescent thermometers and enhance the understanding of thermal biology in the future.

Synergistic interplay between photoisomerization and photoluminescence in a light-driven rotary molecular motor

Photoactuators and photoluminescent dyes utilize light to perform mechanical motion and undergo spontaneous radiation emission, respectively. Combining these two functionalities in a single molecule would benefit the construction of advanced molecular machines. Due to the possible detrimental interaction between the two light-dependent functional parts, the design of hybrid systems featuring both functions in parallel remains highly challenging. Here, we develop a light-driven rotary molecular motor with an efficient photoluminescent dye chemically attached to the motor, not compromising its motor function. This molecular system shows efficient rotary motion and bright photoluminescence, and these functions can be addressed by a proper choice of excitation wavelengths and solvents. The moderate interaction between the two parts generates synergistic effects, which are beneficial for lower-energy excitation and chirality transfer from the motor to the photoluminescent dye. Our results provide prospects towards photoactive multifunctional systems capable of carrying out molecular rotary motion and tracking its location in a complex environment.

Combining photofunctionalities in a single molecule is challenging due to inherent detrimental interactions. Here, the authors construct a molecular motor that exhibits photoinduced rotary motion together with bright photoluminescence.

Spanning BODIPY fluorescence with self-assembled micellar clusters

BODIPY dyes possess favorable optical properties for a variety of applications including in vivo and in vitro diagnostics. However, their utilization might be limited by their water insolubility and incompatibility with chemical modifications, resulting in low aggregation stability. Here, we outline the route for addressing this issue. We have demonstrated two approaches, based on dye entrapment in micellar coordination clusters (MCCs); this provides a general solution for water solubility as well as aggregation stability of the seven BODIPY derivatives. These derivatives have various bulky aromatic substituents in the 2,3,5,6- and meso-positions and can rotate relative to a dipyrrin core, which also provides molecular rotor properties. The molecular structural features and the presence of aromatic groups allows BODIPY dyes to be used as "supporting molecules", thus promoting micelle-micelle interaction and micellar network stabilization. In the second approach, self-micellization, following BODIPY use, leads to MCC formation without the use of any mediators, including chelators and/or metal ions. In both approaches, BODIPY exhibits an excellent optical response, at a concentration beyond its solubilization limit in aqueous media and without undesired crystallization. The suggested approaches represent systems used to encapsulate BODIPY in a capsule-based surfactant environment, enabling one to track the aggregation of BODIPY; these approaches represent an alternative system to study and apply BODIPY's molecular rotor properties. The stabilized compounds, i.e., the BODIPY-loaded MCCs, provide a unique feature of permeability to hydrophilic ligand-switching proteins such as BSA; they exhibit a bright "turn-on" fluorescence signal within the clusters via macromolecular complexation, thus expanding the possibilities of water-soluble BODIPY-loaded MCCs utilization for functional indicators.

Red fluorescent BODIPY molecular rotor for high microviscosity environments

Microviscosity has a strong impact for diffusion-controlled processes in biological environments. BODIPY molecular rotors are viscosity-sensitive fluorophores that provide a simple and non-invasive way to visualise microviscosity. Although green fluorescent probes are already well developed for imaging, thick biological samples require longer wavelengths for investigation. This work focuses on the examination of novelβ-substitutedmeso-phenyl-BODIPYs possessing a red emission. We report a new red fluorescent BODIPY-based probe BP-Vinyl-NO₂suitable for sensing microviscosity in rigid environments of over 100 000 cP viscosities. Furthermore, we demonstrate that changing the methyl position fromorthotometaon theβ-phenyl-substituted conjugate BP-PH-m2M-NO₂redshifts absorbance and fluorescence spectra while maintaining viscosity sensitivity. Finally, we show that nitro-substitution ofmeso-phenyl is a versatile approach to improve the sensitivity to viscosity while suppressing sensitivity to polarity and temperature of such derivatives. In summary, we present two nitro-substituted red fluorescent probes that could be used as lifetime-based microviscosity sensors.

Exploring BODIPY-Based Sensor for Imaging of Intracellular Microviscosity in Human Breast Cancer Cells

BODIPY-based molecular rotors are highly attractive imaging tools for imaging intracellular microviscosity in living cells. In our study, we investigated the ability to detect the microviscosity of biological objects by using BDP-NO₂ and BDP-H molecular rotors. We describe in detail the optical properties of BDP-NO₂ and BDP-H molecular rotors in aqueous media with and without proteins, together with their accumulation dynamics and localization in live and fixed human breast cancer cells. Furthermore, we investigate the applicability of these molecules to monitor microviscosity in the organelles of human breast cancer cells by fluorescence lifetime imaging microscopy (FLIM). We demonstrate that the BDP-NO₂ molecular rotor aggregates in aqueous media and is incompatible with live cell imaging. The opposite effect is observed with BDP-H which preserves its stability in aqueous media, diffuses through the plasma membrane and accumulates in lipid droplets (LDs) and the cytosol of both live and fixed MCF-7 and MDA-MB-231 cancer cells. Finally, by utilizing BDP-H we demonstrate that LD microviscosity is significantly elevated in more malignant MDA-MB-231 human breast cancer cells, as compared to MCF-7 breast cancer cells. Our findings demonstrate that BDP-H is a water-compatible probe that can be successfully applied to measure microviscosity in the LDs of living cells.

Rationales Design von Phe-BODIPY-Aminosäuren als fluorogene Bausteine für den peptidbasierten Nachweis von Candida-Infektionen im Harntrakt

Pilzinfektionen, die durch Candida‐Arten verursacht werden, gehören zu den häufigsten Infektionen bei Krankenhauspatienten. Die derzeitigen Methoden zum Nachweis von Candida‐Pilzzellen in klinischen Proben beruhen jedoch auf zeitaufwändigen Analysen, die eine schnelle und zuverlässige Diagnose erschweren. In diesem Beitrag beschreiben wir die rationale Entwicklung neuer Phe‐BODIPY‐Aminosäuren als kleine fluorogene Bausteine und ihre Anwendung zur Erzeugung fluoreszierender antimikrobieller Peptide für die schnelle Markierung von Candida‐Zellen im Urin. Mit Hilfe von computergestützten Berechnungen haben wir das fluorogene Verhalten von BODIPY‐substituierten aromatischen Aminosäuren analysiert und Bioaktivitäts‐ und konfokale Mikroskopieexperimente bei verschiedenen Stämmen durchgeführt, um den Nutzen und die Vielseitigkeit von Peptiden mit Phe‐BODIPYs zu bestätigen. Schließlich haben wir einen einfachen und sensitiven fluoreszensbasierten Test zum Nachweis von Candida albicans in menschlichen Urinproben entwickelt.

Rational Design of Phe-BODIPY Amino Acids as Fluorogenic Building Blocks for Peptide-Based Detection of Urinary Tract Candida Infections

Fungal infections caused by Candida species are among the most prevalent in hospitalized patients. However, current methods for the detection of Candida fungal cells in clinical samples rely on time-consuming assays that hamper rapid and reliable diagnosis. Herein, we describe the rational development of new Phe-BODIPY amino acids as small fluorogenic building blocks and their application to generate fluorescent antimicrobial peptides for rapid labelling of Candida cells in urine. We have used computational methods to analyse the fluorogenic behaviour of BODIPY-substituted aromatic amino acids and performed bioactivity and confocal microscopy experiments in different strains to confirm the utility and versatility of peptides incorporating Phe-BODIPYs. Finally, we have designed a simple and sensitive fluorescence-based assay for the detection of Candida albicans in human urine samples.

Fluorescent probe strategy for live cell distinction

Live cell discrimination is the first and essential step to understand complex biosystems. Conventional cell discrimination involving various antibodies relies on selective surface biomarkers. Compared to antibodies, the fluorescent probe strategy allows the utilisation of intracellular biomarkers, providing broader options with unique chemical principles to achieve the live cell distinction. In general, fluorescent probes can be retained in cells by interacting with biomolecules, accumulating via transporters, and participating in metabolism. Based on the target difference, fluorescent probe strategy can be divided into several categories: protein-oriented live cell distinction (POLD), carbohydrate-oriented live cell distinction (COLD), DNA-oriented live cell distinction (DOLD), gating-oriented live cell distinction (GOLD), metabolism-oriented live cell distinction (MOLD) and lipid-oriented live cell distinction (LOLD). In this review, we will outline the concepts and mechanisms of different strategies, introduce their applications in cell-type discrimination, and discuss their advantages and challenges in this area. We expect this tutorial will provide a new perspective on the mechanisms of fluorescent probe strategy and facilitate the development of cell-type-specific probes.

Triphenylamine indanedione as an AIE-based molecular sensor with one-step facile synthesis toward viscosity detection of liquids

AIE-based triphenylamine indanedione molecular sensors were synthesized in a one-step facile manner and designed for viscosity detection in liquids.

Two-photon fluorescence imaging of mitochondrial viscosity with water-soluble pyridinium inner salts

Viscosity-induced emission of fluorogenic probes was used to detect intracellular mitochondrial viscosity, even in different tissues and/or zebrafish via TPFM.

Molecular rotor based on an oxidized resorcinarene

Rate of rotation of substituents in a molecular single stator-double rotor based on an oxidized resorcinarene with unsaturated hemiquinonoid groups at its meso positions (i.e., a fuchsonarene) has been controlled according to solvent polarity and acidity.

Give or Take: Effects of Electron-Accepting/-Withdrawing Groups in Red-Fluorescent BODIPY Molecular Rotors

Mapping microviscosity, temperature, and polarity in biosystems is an important capability that can aid in disease detection. This can be achieved using fluorescent sensors based on a green-emitting BODIPY group. However, red fluorescent sensors are desired for convenient imaging of biological samples. It is known that phenyl substituents in the β position of the BODIPY core can shift the fluorescence spectra to longer wavelengths. In this research, we report how electron-withdrawing (EWG) and -donating (EDG) groups can change the spectral and sensory properties of β-phenyl-substituted BODIPYs. We present a trifluoromethyl-substituted (EWG) conjugate with moderate temperature sensing properties and a methoxy-substituted (EDG) molecule that could be used as a lifetime-based polarity probe. In this study, we utilise experimental results of steady-state and time-resolved fluorescence, as well as quantum chemical calculations using density functional theory (DFT). We also explain how the energy barrier height (Ea) for non-radiative relaxation affects the probe’s sensitivity to temperature and viscosity and provide appropriate Ea ranges for the best possible sensitivity to viscosity and temperature.

Fluorescence-based sensing of the bioenergetic and physicochemical status of the cell

Fluorescence-based sensors play a fundamental role in biological research. These sensors can be based on fluorescent proteins, fluorescent probes or they can be hybrid systems. The availability of a very large dataset of fluorescent molecules, both genetically encoded and synthetically produced, together with the structural insights on many sensing domains, allowed to rationally design a high variety of sensors, capable of monitoring both molecular and global changes in living cells or in in vitro systems. The advancements in the fluorescence-imaging field helped researchers to obtain a deeper understanding of how and where specific changes occur in a cell or in vitro by combining the readout of the fluorescent sensors with the spatial information provided by fluorescent microscopy techniques. In this review we give an overview of the state of the art in the field of fluorescent biosensors and fluorescence imaging techniques, and eventually guide the reader through the choice of the best combination of fluorescent tools and techniques to answer specific biological questions. We particularly focus on sensors for probing the bioenergetics and physicochemical status of the cell.

Photophysics of faecal pigments stercobilin and urobilin in aliphatic alcohols: introduction of a sensitive method for their detection using solvent phase extraction and fluorometry

Faecal pigments (FPs) are ubiquitous in the environment and are a primary contaminant in groundwater and surface water. This article presents a new analytical paradigm by a fluorescence coupled extraction-based method involving FP fluorescence enhancement and minimization of background fluorescence for high sensitivity detection. FPs show higher fluorescence intensity in aliphatic alcohols due to the breaking down of higher-order H-aggregates into lower-order H-aggregates (dimers). DFT studies using the B3LYP functional and LANL2DZ basis set show π-π stacking and hydrogen-bonding contributions towards forming H-aggregated dimers of FPs in the implicit and explicit solvent environments of 1-hexanol. This study is the first report on the extractability of FPs using 1-hexanol as an efficient extraction medium in comparison to higher-order aliphatic alcohols (1-butanol, 1-hexanol and 1-octanol). Furthermore, FP-Zn(II) complexes in 1-hexanol medium significantly enhance the fluorescence emission intensity (∼14-17 times), and the emission intensity remains stable over time. This further helps to increase the detection limit of FPs in the picomolar to sub-picomolar concentration range. This study proposes a protocol involving extraction of FPs by 1-hexanol followed by the complexation of FPs with Zn(II) in the alcohol media and subsequent fluorimetric detection of the FP-Zn(II) complex with a high level of sensitivity, enabled by reduced interference from the background fluorescence of humic acid. The complexation behaviour of FPs with various metal salts was also examined, which provided an understanding of the fluorescence behaviour of FPs with various other metal ions commonly present in natural environmental water. The proposed analytical method has been further validated using real water samples.

Visual detection of viscosity through activatable molecular rotor with aggregation-induced emission

Food safety has become one of the urgent affairs in the global public health studies, and irregular viscosity is closely associated with the food spoilage extent. In this study, one kind of activatable molecular rotor (TPA-PBZ) based on triphenylamine derivates has been synthesized via the Schiff base condensation reaction. This rotor is comprised by donor-accepter conjugated structure, with aggregation induced-emission feature and a large Stokes shift of 160 nm in water. The rotation of aromatic rings in TPA-PBZ is restricted in high-viscosity microenvironment, with the gradually increasing fluorescence emission signal at 568 nm. Significantly, this rotor TPA-PBZ has successfully been applied not only in the determination of thickening effects of food gum, but also in the detection of viscosity enhancement during the liquid food spoilage process. This molecular rotor can be utilized as an intelligent monitor platform for food quality and safety inspection in viscosity-related conditions.

Molecular rotors as reporters for viscosity of solutions of collagen like peptides

We have studied the suitability of using a molecular rotor-based steady-state fluorometric assay for evaluating changes in both the conformation and the viscosity of collagen-like peptide solutions. Our results indicate that a positive charge incorporated on the hydrophobic tail of the BODIPY molecular rotor favours the dye specificity as a reporter for viscosity of these solutions.