Each new ability to visualize analyte concentrations in real-time portends the fabrication of new sensors. Potential applications include fiber optic-based remoting sensing of solution species and the monitoring of concentrations within living cells. A fluorescent chemosensor is a compound of abiotic origin that complexes to an analyte reversibly with concomitant fluorescence signal transduction transduction. Such chemosensors have been used for fluorimetric metal ion determination for over 100 years. The advent of ligand engineering has introduced a more systematic approach to the design of chemosensors with new selectivities and signal transduction schemes. The technological driving force to achieve useful chemosensors will stimulate investigation of new topics in molecule recognition, fluorescent signal transduction, and their intersection.
Oxidative stress occurs when the intracellular redox homeostasis is disrupted, which is considered to be one of the important factors leading to aging and disease. Antioxidants of non-enzymatic reactive chalcogenide species play an important role in redox homeostasis, among which sulfane sulfur species and reactive selenium species are particularly indispensable. As a class of antioxidants with potential clinical biomarker value, the intracellular levels and distribution of sulfane sulfur and reactive selenium species can directly prove the dynamic state of oxidative stress, which may reveal the difference between physiological and pathological processes. Fluorescence bioimaging technology has the advantages of high temporal and spatial resolution, low invasiveness and fast response, and has become a powerful tool for intracellular detection. Herein, we have summarized the design strategy and development of fluorescent probes for the detection of sulfane sulfur and reactive selenium species. We have also highlighted the important physiological and pathological roles and mechanisms of these reactive species. We expect to point out a path centred on fluorescence imaging for the detection of representative biological reactive species in living systems.
Two-photon microscopy (TPM) is an indispensable bioimaging tool. TPM, which uses photons of a near-infrared wavelength as an excitation source for fluorophores, can result in deeper tissue images, which can potentially be measured beyond a depth of 1000 µm with relative stability in biological systems. With continuous development in deep-tissue optical microscopy and improvements in imaging algorithms, TPM is gaining immense support in biomedical applications. Furthermore, it has propelled the continuous development of appropriate two-photon (TP) probes with improved sensitivities for measuring target analytes in biological specimens. In this chapter, the photo-physiological process, basic TP probe design strategies, and representative TP sensors of TPM that researchers have reported are presented to provide significant information regarding specific targets in biological systems.
Photodynamic therapy (PDT) is a clinically approved treatment modality used for a wide range of medical conditions, including malignant cancers. It employs cytotoxic reactive oxygen species (ROS), particularly singlet oxygen (1O2), to kill cells of interest and has attracted immense attention during the last decades. Molecular design of triplet photosensitizers is no doubt at the core of successful PDT action. Spatiotemporal control of ROS generation and consequent cancer cell selectivity is one of the highly sought characteristics of new-generation photosensitizers, to minimize severe adverse effects as well as to enhance the therapeutic outcome. Activatable photosensitizers have appeared to be a good candidate in this respect as they tend to stay in their “off” state prior to activation with various tumor-associated intracellular stimuli. In this chapter, we summarize the recent advances in the field of activatable photosensitizers by focusing on the design principles and biologically relevant activators.
Zinc homeostasis in cells is tightly regulated by zinc-binding metalloproteins and zinc transporters. Since disruption of zinc homeostasis is associated with various diseases, such as Alzheimer's disease and diabetes, investigating its functions and physiological roles is important. One of the powerful methods for visualizing Zn2+ dynamics in a cell is fluorescence imaging because of its high sensitivity and spatiotemporal resolution. Several organelle-targeting fluorescent Zn2+ probes have been developed for imaging intracellular Zn2+ distribution and dynamics. This chapter highlights organelle-targeting small-molecule probes and protein-tag–small-molecule hybrid probes, which are expected to have higher potential for accurately visualizing and quantifying labile Zn2+ at the organelle level.
Fluorescent nanoparticles are indispensable tools often utilised in analytical biology, fluorescence spectroscopy, bioimaging, biophysics, clinical diagnosis, and environmental sensing. Their specific photophysical properties, including brightness, emission wavelength, and analyte sensitivity, can be easily modulated. This affords fluorescent nanoparticles an expansive scope of applicability in imaging and sensing. This chapter gives an overview of various fluorescent carbon nanoparticles, supported by selected literature case examples, namely on carbon nano-onions, fluorescent carbon dots, and detonation nanodiamonds.
This chapter outlines the development of small-molecule fluorescent chemosensors for phosphate species. Phosphate species are ubiquitous in nature, play diverse roles in biological systems, and display extensive variation in the functionality attached to the phosphate group. These molecules have a diffuse negative charge, are highly solvated, and are all linked by a common phosphate group. Together, these features present a significant challenge for the development of species-selective chemosensors that function in aqueous media with appropriate binding affinities. This challenge has been tackled via a variety of approaches, including chemosensors that bind the phosphate group via charge–charge, hydrogen-bonding, and metal–cation interactions. Key examples of each of these interaction types, varied approaches to chemosensor design, and fluorescence response mechanisms are highlighted.
Fluorescent techniques have attracted significant attention in bioimaging, analyte sensing, and disease diagnosis in recent years. Conventional fluorescent chemosensors provide significant advantages in monitoring/detecting different analytes; however, physiological or experimental factors may influence the single-targeted fluorophore absolute intensity-dependent signal acquisition, which can be cause misleading and strong non-specific background signals in molecular sensing and imaging applications. The simple alternative to minimize these non-specific effects is a ratiometric measurement strategy. This is a self-calibration method for recording two or more analyte-induced signals, in which one signal is a reference factor to normalize other signals. Due to its self-calibrating internal standard system obtained from the ratio between two or more emission bands, ratiometric approaches have become the most effective fluorescence method for quantitative analysis measurements, compensating for a number of analyte-independent parameters and eliminating most ambiguities that may affect the fluorescence signal. In particular, by taking advantage of various photophysical/chemical sensing theories, ratiometric fluorophores successfully endow structural design for detection of biologically/environmentally important analytes. This chapter will highlight the basic principles and design strategies of ratiometric fluorescent chemosensors, including photophysical/chemical sensing mechanisms based on different molecular types (i.e., small molecules and nanoparticles) with appropriate examples.
Chemical sensors for detection of biological analytes in their native settings with spatial and temporal resolution can enable the study of their physiological and pathological contributions by molecular imaging. An emerging area of sensor research is activity-based sensing (ABS), which leverages the unique chemical reactivity of a given analyte of interest, rather than traditional binding-based approaches that rely on lock-and-key molecular recognition, to achieve selectivity in the complex biological environments. This chapter summarizes foundational design principles of ABS and provides a survey of three representative examples of activity-based imaging probes using oxidative, reductive, or redox-neutral reactivity for selective analyte detection, highlighting the broad applicability of this synthetic reaction chemistry approach to identify and characterize new biology.
In chemical biology the concept of chemosensors and chemical probes implicates the transduction of a chemical signal upon binding with the chemical substance. There are enormous challenges for developing superior chemical probes using the target-oriented synthesis approach with limited knowledge of the recognition site. The emerging field of the diversity-oriented fluorescent library approach (DOFLA) enables a conceptually novel and new strategy for designing chemosensors/probes. Vast chemical diversity in optically active fluorophores provides limitless opportunity for probe development, likewise drug discovery from diversity-oriented synthesis approaches. Synergistic effects of diverse chemical structures, high-throughput screening methodology, and improved microscopic automation have led to several chemical probes in the last two decades. This chapter summarizes the recent developments of chemical probes/sensors and chemosensors using DOFLA and their biological and environmental applications. The chapter consists of six sections, including the concept of the diversity-oriented fluorescent library, mechanisms of signal transduction, screening strategies, and reviews of recent examples from in vitro spectra-based screening, cell-based screening, and in vivo whole organelle-based screening.
Metal ions are involved in various biological processes, and their essential roles and pathological involvement have facilitated the development of new synthetic chemical tools for the detection and monitoring of metal ions in cells and living organisms. For example, fluorescent and other optical probes have been used to study metal homeostasis and alterations with spatial and temporal resolution. They are categorized into chelation-based and activity-based probes based on their detection mechanism. Chelation-based probes have been used since the 1980s and operate through the complexation of a metal ion with a chelator tethered to a fluorophore, which is detected as a change in fluorescence intensity and/or wavelength. On the other hand, activity-based sensing has only been developed in the last decade for the selective detection of metal ions that are difficult to detect using chelation-based probes. These activity-based probes provide new possibilities in the detection of biological metals because they can avoid the disadvantages of chelation-based probes, including the fluorescence-quenching effect of d-unsaturated metal ions and metal-ion depletions. In this chapter, specific examples of activity-based probes for metal-ion detection are introduced, and their sensing mechanisms and features are discussed.
The development of lanthanide-based signalling systems that can monitor the concentration, nature and presence of certain analytes has grown rapidly over the last 25 years or so. Given their unique photophysical properties, which includes line-like emission bands/transitions, long-wavelength emission range (spanning from the visible to the near-infrared) and long-lived excited states, it comes as no surprise that today luminescent lanthanide sensors, probes and materials (e.g. films, polymers, particles, etc.) have been developed for the recognition, sensing and imaging of ions, small molecules as well as large biomolecules (nucleic acids, peptides and proteins, etc.), systems and processes (such as enzymes, etc.). This chapter gives some insight into this rapidly developing field of research. A selected number of samples will be featured and their properties and function discussed. The object herein is to demonstrate the structural versatility and the various applications that such lanthanide luminescence systems have to offer, along with expanding on how the lanthanide ions act as sensing platforms, which is seldomly seen for their organic counterparts.
Fluorescent sensors have proved to be useful for environmental monitoring of chemical pollutants and other substances that can cause harm to society and the natural world. Industrial advances since the 19th century have led to a significant increase in polluting substances in the air, water and land. Currently, the most common methods for detecting environmental pollutants include inductively coupled plasma–mass spectrometry (ICP–MS), gas chromatography–mass spectrometry (GC–MS) and atomic absorption spectroscopy (AAS). Although sensitive, these techniques involve the use of large and expensive instruments operated by specially trained personnel, limiting their availability in places other than major cities. Fluorescent sensing platforms provide an attractive alternative to these techniques, as high sensitivity can be achieved while using much cheaper and smaller instrumentation, such as benchtop spectrometers, or even portable devices. In this chapter, we present highlights from the vast array of fluorescent platforms developed for environmental sensing, with a particular focus on examples that have been successfully applied to studies in the field.
Chirality-sensing techniques utilizing UV-visible, fluorescence, and circular dichroism spectroscopy are discussed in detail. The design strategies, applications, and limitations associated with these techniques are considered, as well as relevant historical details. For fluorescence and circular dichroism, brief introductions to the photophysics enabling sensing using these techniques are provided. Finally, we give a broad overview of the impact of these methods in the context of high-throughput experiments.
Fluorescent materials have gained extensive attention owing to their highly photophysical properties and potential applications in diverse areas. Aggregation-induced emission (AIE) is a new type of photophysical phenomenon discovered in 2001, in which molecules in condensed solution or solid state exhibit enhanced fluorescence emission. In this chapter, we aim to cover the developments in the exciting field of AIE over the past decade, including organic luminescent molecules and clusteroluminogens, of which clusteroluminogens are highlighted. The fluorescence mechanism is one of the most important concepts within the scientific community. Therefore, we systematically summarize the latest research achievements on the luminescence mechanism of AIE-based fluorescent systems.
As the chemical by-products of cell metabolism, reactive oxygen species (ROS) are widely involved in signal transduction of physiological processes. However, when redox homeostasis is out of balance, excessive production of ROS may lead to or promote multiple pathologies. Currently, the diverse chemical properties of ROS and their biological mechanism are still poorly understood. Therefore, powerful tools need to be exploited to elucidate the chemical biology of ROS. Non-invasive fluorescence imaging techniques have gradually matured for tracking of active molecules. This chapter summarizes the fluorescent sensors for ROS in living cells and in vivo.
Porphyrins, phthalocyanines, and their metallo-compounds are representative macrocycles with 18-π structures and a diversity of properties. Construction of well-defined architectures of porphyrins, phthalocyanines, and their metallo-compounds is highly desirable and important as they offer functional materials with advanced properties and functions. In this chapter, we summarize the chemistry for the designed synthesis of crystalline covalent organic frameworks (COFs) with long-range orderings of porphyrins, phthalocyanines, and their metallo-compounds, with the aim of showing the capabilities of precise structural design and diversity of structural scope. We analyze various properties of these frameworks by emphasizing their interactions with photons, electrons, holes, ions, and molecules to elucidate their structure–function correlations, and to disclose the way to develop unique properties and functions that are specific to their framework structures. We predict key issues to be addressed and show future directions from the viewpoints of design, synthesis, and properties.
