Two-Photon Fluorescent Probes for Metal Ions in Live Tissues

Excellent ratiometric two-photon fluorescent probes for hydrogen sulfide detection based on the fluorescence resonance energy transfer mechanism

Fluorescence resonance energy transfer (FRET) is an important mechanism to design ratiometric fluorescent probes that are able to detect analytes quantitatively according to the ratio of two well-resolved emission signals. Two-photon (TP) fluorescent probes can realize the detection in living cells and tissues with deeper penetration depth, higher resolution, and lower photodamage in contrast to one-photon fluorescent probes. However, to date, fabricating TP-FRET ratiometric fluorescent probes possessing large two-photon absorption (TPA), high fluorescence quantum yield and perfect FRET efficiency is still challenging. Consequently, to develop excellent TP-FRET ratiometric probes and explore the relationship between their molecular structures and TP fluorescence properties, in this paper, we designed a series of H2S-detecting TP fluorescent probes employing the FRET mechanism based on an experimental probe BCD. Thereafter, we comprehensively evaluated the TP sensing performance of these probes by means of time-dependent density functional theory and quadratic response theory. Furthermore, we determined energy transfer efficiency and fluorescence quantum yield. Significantly, through regulating benzene-fused positions, we successfully improved fluorescence quantum yield and TPA cross-section simultaneously. Large spectral overlap between energy donor emission and acceptor absorption was achieved and near perfect energy transfer efficiency was acquired for all the studied probes. We revealed that these probes exhibit two well-resolved TPA bands, which are contributed by FRET donors and acceptors, respectively. Especially, both the wavelengths and the cross-sections of the two TPA bands agree well with those of energy donors and acceptors, which is the unique TPA spectral profile of FRET probes and has never been previously reported. Moreover, we proposed an excellent TP-FRET probe BCD3 and its product molecule BCD3-H2S, which exhibit large Stokes (141 nm and 88 nm) and emission shifts (5931 cm-1), as well as greatly increased TP action cross-sections (24-fold and 60-fold) in the near-infrared region with respect to BCD and BCD-H2S. Our detailed study can give an insight into the efficient design of novel TP-FRET fluorescent probes.

Tris(4'-Nitrobiphenyl)amine─An Octupolar Chromophore with High Two-Photon Absorption Cross-Section and Its Application for Uncaging of Calcium Ions in the Near-Infrared Region

Compounds with high two-photon absorption (2PA) performance in the near-infrared region have attracted great attention because of their application in the material and biological science. In this study, we have developed a simple and novel octupolar chromophore, tris(4'-nitrobiphenyl)amine 1, with three nitro peripheral groups attached to a triphenylamine core via biphenyl linkers. A mono-branched analogue 2 has also been prepared to investigate the effects of octupolar and dipolar systems on photophysical and 2PA behaviors. Compound 1, despite having a much simpler structure than the previous three-branched scaffolds, exhibits comparable σ₂ values, reaching 1330 GM at 730 nm and 900 GM at 820 nm in toluene. Combined with an outstanding σ₂/MW ratio (2.2 GM g^-1 mol) and a high fluorescence quantum yield (0.51), 1 displays potential as a promising two-photon (2P) probe for bioimaging. Subsequently, the ethylene glycol tetraacetic acid-substituted derivatives featuring octupolar (3 and 5) or dipolar (4 and 6) character have been synthesized and their one-photon (1P) and 2P photochemical reactions have been examined. Finally, 1P- and 2P-triggered uncaging of Ca²⁺ from these calcium chelators has been confirmed.

Advanced high resolution three-dimensional imaging to visualize the cerebral neurovascular network in stroke

As an important method to accurately and timely diagnose stroke and study physiological characteristics and pathological mechanism in it, imaging technology has gone through more than a century of iteration. The interaction of cells densely packed in the brain is three-dimensional (3D), but the flat images brought by traditional visualization methods show only a few cells and ignore connections outside the slices. The increased resolution allows for a more microscopic and underlying view. Today's intuitive 3D imagings of micron or even nanometer scale are showing its essentiality in stroke. In recent years, 3D imaging technology has gained rapid development. With the overhaul of imaging mediums and the innovation of imaging mode, the resolution has been significantly improved, endowing researchers with the capability of holistic observation of a large volume, real-time monitoring of tiny voxels, and quantitative measurement of spatial parameters. In this review, we will summarize the current methods of high-resolution 3D imaging applied in stroke.

Recent Advances in Two-Photon AIEgens and Their Application in Biological Systems

Two-photon fluorescence imaging technology has the advantages of high light stability, little light damage, and high spatiotemporal resolution, which make it a powerful biological analysis method. However, due to the high concentration or aggregation state of traditional organic light-emitting molecules, the fluorescence intensity is easily reduced or disappears completely, and is not conducive to optimal application. The concept of aggregation-induced emission (AIE) provides a solution to the problem of aggregation-induced luminescence quenching (ACQ), and realizes the high fluorescence quantum yield of luminescent molecules in the aggregation state. In addition, two-photon absorption properties can readily be improved just by increasing the loading content of AIE fluorogen (AIEgen). Therefore, the design and preparation of two-photon fluorescence probes based on AIEgen to achieve high-efficiency fluorescence imaging in vitro/in vivo has become a major research hotspot. This review aims to summarize representative two-photon AIEgens based on triphenylamine, tetraphenylethene, quinoline, naphthalene and other new structures from the past five years, and discuss their great potential in bioimaging applications.

Near-infrared dyes for two-photon absorption in the short-wavelength infrared: strategies towards optical power limiting

The recent advances in the field of two-photon absorbing chromophores in the short-wavelength infrared spectral range (SWIR 1100–2500 nm) are summarized, highlighting the development of optical power limiting devices in this spectral range.

Recent advances in two-photon absorbing probes based on a functionalized dipolar naphthalene platform

Two-photon microscopy (TPM) techniques have been highlighted over the past two decades throughout various fields, including physics, chemistry, biology, and medicine. In particular, the two-photon near-infrared excitation of fluorophores or molecular probes emitting fluorescence have ushered in a new biomedical era, specifically in the deep-tissue imaging of biologically relevant species. Non-linear two-photon optics enables the development of 3D fluorescence images via focal point excitation of biological samples with low photo-damage and photo-bleaching. Many studies have disclosed the relationship between the chemical structure of fluorophores and their two-photon absorbing properties. In this review, we have summarized the recent advances in two-photon absorbing probes based on a functionalized electron donor (D)-acceptor (A) type dipolar naphthalene platform (FDNP) that was previously reported between 2015 and 2019. Our systematic outline of the synthesis, photophysical properties, and examples of two-photon imaging applications will provide useful context for the future development of new naphthalene backbone-based two-photon probes.

Sensing Performance Investigations on Two-Photon Fluorescent Probes for Detecting β-Amyloid in Alzheimer's Disease

Alzheimer’s disease (AD) is one of the most common forms of senile disease. In recent years, the incidence of AD has been increasing significantly with the acceleration of the aging process of the global population. However, current clinical drugs can only alleviate the symptoms of AD patients without healing the disease fundamentally. Therefore, it is of great significance to develop an effective small molecule diagnostic reagent for the early diagnosis of AD. In this paper, we employ an integrated approach, including molecular docking simulation and quantum mechanics/molecular mechanics calculation, to investigate the sensing performance of a series of donor–acceptor structural probes for the marker protein of AD (β-amyloid). Results show that the probes display evident fluorescence enhancement when bound to the β-amyloid, suggesting the effect of the environment on the molecular properties. Especially, the two-photon absorption cross-section of the probes increase drastically in the β-amyloid compared to that in vacuum, which results from the larger electron delocalization and dipole moment in the fibrillary-like environment. Thus, one can propose that the studied probes are capable of application in two-photon fluorescent imaging, particularly those containing naphthalene rings as the donor or with a longer spacer group. Our calculations elucidate the experimental measurements reasonably, and further establish possible structure–property relationships that can be used to design novel biocompatible two-photon fluorescent probes for the diagnosis of Alzheimer’s.

Ligand-displacement-based two-photon fluorogenic probe for visualizing mercapto biomolecules in live cells, Drosophila brains and zebrafish

Investigating the change in expression level of mercapto biomolecules (GSH/Cys/Hcy) necessitates a rapid detection method for a series of physiological and pathological processes. Herein, we present a ligand-displacement-based two-photon fluorogenic probe based on an Fe(iii) complex, TPFeS, which is a GSH/Cys/Hcy rapid detection fluorogenic probe for in vitro analysis and live cell/tissue/in vivo imaging. The "in situ" probe is non-fluorescent and was prepared from a 1 : 2 ratio of Fe(iii) and TPS, a novel two-photon (TP) fluorophore with excellent one-photon (OP) and TP properties under physiological conditions, as a fluorescent ligand. This probe shows a rapid and remarkable fluorescence restoration (OFF-ON) property due to the ligand-displacement reaction of mercapto biomolecules in a recyclable manner in vitro. A significant two-photon action cross-section, good selectivity for biothiols, low cytotoxicity, and insensitivity to pH over the biologically relevant pH range allowed the direct visualization of mercapto biomolecules at different levels between normal/drug-treated live cells, as well as in Drosophila brain tissues/zebrafish based on the use of two-photon fluorescence microscopy.

Phosphorescent iridium(iii) complexes capable of imaging and distinguishing between exogenous and endogenous analytes in living cells

Many luminescent probes have been developed for intracellular imaging and sensing. During cellular luminescence sensing, it is difficult to distinguish species generated inside cells from those internalized from extracellular environments since they are chemically the same and lead to the same luminescence response of the probes. Considering that endogenous species usually give more information about the physiological and pathological parameters of the cells while internalized species often reflect the extracellular environmental conditions, we herein reported a series of cyclometalated iridium(iii) complexes as phosphorescent probes that are partially retained in the cell membrane during their cellular uptake. The utilization of the probes for sensing and distinguishing between exogenous and endogenous analytes has been demonstrated using hypoxia and hypochlorite as two examples of target analytes. The endogenous analytes lead to the luminescence response of the intracellular probes while the exogenous analytes are reported by the probes retained in the cell membrane during their internalization.

Chromis-1, a Ratiometric Fluorescent Probe Optimized for Two-Photon Microscopy Reveals Dynamic Changes in Labile Zn(II) in Differentiating Oligodendrocytes

Despite the significant advantages of two-photon excitation microscopy (TPEM) over traditional confocal fluorescence microscopy in live-cell imaging applications, including reduced phototoxicity and photobleaching, increased depth penetration, and minimized autofluorescence, only a few metal ion-selective fluorescent probes have been designed and optimized specifically for this technique. Building upon a donor-acceptor fluorophore architecture, we developed a membrane-permeant, Zn(II)-selective fluorescent probe, chromis-1, that exhibits a balanced two-photon cross section between its free and Zn(II)-bound form and responds with a large spectral shift suitable for emission-ratiometric imaging. With a Kd of 1.5 nM and wide dynamic range, the probe is well suited for visualizing temporal changes in buffered Zn(II) levels in live cells as demonstrated with mouse fibroblast cell cultures. Moreover, given the importance of zinc in the physiology and pathophysiology of the brain, we employed chromis-1 to monitor cytoplasmic concentrations of labile Zn(II) in oligodendrocytes, an important cellular constituent of the brain, at different stages of development in cell culture. These studies revealed a decrease in probe saturation upon differentiation to mature oligodendrocytes, implying significant changes to cellular zinc homeostasis during maturation with an overall reduction in cellular zinc availability. Optimized for TPEM, chromis-1 is especially well-suited for exploring the role of labile zinc pools in live cells under a broad range of physiological and pathological conditions.

Pyrrolopyrrole aza boron dipyrromethene based two-photon fluorescent probes for subcellular imaging

A series of two-photon-absorbing pyrrolopyrrole aza boron dipyrromethenes have been prepared which can serve as fluorescent probes for subcellular imaging.

Design of a two-photon fluorescent probe for selective recognition of Au(III) over Au(I) and its application of imaging in vitro and in vivo

A highly selective two-photon fluorescent probe (PyCM-1) for Au³⁺ was developed with distinct "turn on" fluorescence response, low detection limit (22nM) and large two-photon absorption cross-sections (696 GM at 860nm). Its high selectivity for Au³⁺ over Au⁺ was achieved via the modification on the type of coordination atoms in the Schiff base receptor. Co-staining experiments showed that the probe PyCM-1 could co-localize specifically with mitochondria. Moreover, the two-photon confocal fluorescence imaging results demonstrated the probe's capability for visualizing Au³⁺in vitro and in vivo.

Theoretical Studies on Two-Photon Fluorescent Hg2+ Probes Based on the Coumarin-Rhodamine System

The development of fluorescent sensors for Hg²⁺ has attracted much attention due to the well-known adverse effects of mercury on biological health. In the present work, the optical properties of two newly-synthesized Hg²⁺ chemosensors based on the coumarin-rhodamine system (named Pro1 and Pro2) were systematically investigated using time-dependent density functional theory. It is shown that Pro1 and Pro2 are effective ratiometric fluorescent Hg²⁺ probes, which recognize Hg²⁺ by Förster resonance energy transfer and through bond energy transfer mechanisms, respectively. To further understand the mechanisms of the two probes, we have developed an approach to predict the energy transfer rate between the donor and acceptor. Using this approach, it can be inferred that Pro1 has a six times higher energy transfer rate than Pro2. Thus the influence of spacer group between the donor and acceptor on the sensing performance of the probe is demonstrated. Specifically, two-photon absorption properties of these two probes are calculated. We have found that both probes show significant two-photon responses in the near-infrared light region. However, only the maximum two-photon absorption cross section of Pro1 is greatly enhanced with the presence of Hg²⁺, indicating that Pro1 can act as a potential two-photon excited fluorescent probe for Hg²⁺. The theoretical investigations would be helpful to build a relationship between the structure and the optical properties of the probes, providing information on the design of efficient two-photon fluorescent sensors that can be used for biological imaging of Hg²⁺ in vivo.

An N-nitrosation reactivity-based two-photon fluorescent probe for the specific in situ detection of nitric oxide

In situ fluorescence imaging of nitric oxide (NO) is a powerful tool for studying the critical roles of NO in biological events. However, the selective imaging of NO is still a challenge because most currently available fluorescent probes rely on the o-phenylenediamine (OPD) recognition site, which reacts with both NO and some abundant reactive carbonyl species (RCS) (such as dehydroascorbic acid and methylglyoxal) and some reactive oxygen/nitrogen species (ROS/RNS). To address this problem, a new fluorescent probe, NCNO, based on the N-nitrosation of aromatic secondary amine was designed to bypass the RCS, ROS, and RNS interference. As was expected, the probe NCNO could recognize NO with pronounced selectivity and sensitivity among ROS, RNS, and RCS. The probe was validated by detecting NO in live cells and deep tissues owing to its two-photon excitation and red-light emission. It was, hence, applied to monitor NO in ischemia reperfusion injury (IRI) in mice kidneys by two-photon microscopy for the first time, and the results vividly revealed the profile of NO generation in situ during the renal IRI process.

Energy Donor Effect on the Sensing Performance for a Series of FRET-Based Two-Photon Fluorescent Hg2+ Probes

Nonlinear optical properties of a series of newly-synthesized molecular fluorescent probes for Hg2+ containing the same acceptor (rhodamine group) are analyzed by using time-dependent density functional theory in combination with analytical response theory. Special emphasis is placed on evolution of the probes' optical properties in the absence and presence of Hg2+. These compounds show drastic changes in their photoabsorption and photoemission properties when they react with Hg2+, indicating that they are excellent candidates for ratiometric and colorimetric fluorescent chemosensors. Most importantly, the energy donor moiety is found to play a dominant role in sensing performance of these probes. Two-photon absorption cross sections of the compounds are increased with the presence of Hg2+, which theoretically suggests the possibility of the probes to be two-photon fluorescent Hg2+ sensors. Moreover, analysis of molecular orbitals is presented to explore responsive mechanism of the probes, where the fluorescence resonant energy transfer process is theoretically demonstrated. Our results elucidate the available experimental measurements. This work provides guidance for designing efficient two-photon fluorescent probes that are geared towards biological and chemical applications.

A New Two-Photon Ratiometric Fluorescent Probe for Detecting Alkaline Phosphatase in Living Cells

Alkaline phosphatase (ALP) is an important diagnostic indicator of many human diseases. To quantitatively track ALP in biosystems, herein, for the first time, we report an efficient two-photon ratiometric fluorescent probe, termed probe 1 and based on classic naphthalene derivatives with a donor-π-acceptor (D-π-A) structure and deprotection of the phosphoric acid moiety by ALP. The presence of ALP causes the cleave of the phosphate group from naphthalene derivatives and the phosphate group changes the ability of the intramolecular charge transfer (ICT) and remarkably alters the probe's photophysical properties, thus an obvious ratiometric signal with an isoemissive point is observed. The fluorescence intensity ratio displayed a linear relationship against the concentration of ALP in the concentration range from 20 to 180 U/L with the limit of detection of 2.3 U/L. Additionally, the probe 1 is further used for fluorescence imaging of ALP in living cells under one-photon excitation (405 nm) or two-photon excitation (720 nm), which showed a high resolution imaging, thus demonstrating its practical application in biological systems.

Elucidating the Mechanism of Zn(2+) Sensing by a Bipyridine Probe Based on Two-Photon Absorption

In this work, we examine, by means of computational methods, the mechanism of Zn(2+) sensing by a bipyridine-centered, D-π-A-π-D-type ratiometric molecular probe. According to recently published experimental data [Divya, K. P.; Sreejith, S.; Ashokkumar, P.; Yuzhan, K.; Peng, Q.; Maji, S. K.; Tong, Y.; Yu, H.; Zhao, Y.; Ramamurthy, P.; Ajayaghosh, A. A ratiometric fluorescent molecular probe with enhanced two-photon response upon Zn(2+) binding for in vitro and in vivo bioimaging. Chem. Sci. 2014, 5, 3469-3474], after coordination to zinc ions the probe exhibits a large enhancement of the two-photon absorption cross section. The goal of our investigation was to elucidate the mechanism behind this phenomenon. For this purpose, linear and nonlinear optical properties of the unbound (cation-free) and bound probe were calculated, including the influence of solute-solvent interactions, implicitly using a polarizable continuum model and explicitely employing the QM/MM approach. Because the results of the calculations indicate that many conformers of the probe are energetically accessible at room temperature in solution and hence contribute to the signal, structure-property relationships were also taken into account. Results of our simulations demonstrate that the one-photon absorption bands for both the unbound and bound forms correspond to the bright π → π* transition to the first excited state, which, on the other hand, exhibits negligible two-photon activity. On the basis of the results of the quadratic response calculations, we put forward a notion that it is the second excited state that gives the strong signal in the experimental nonlinear spectrum. To explain the differences in the two-photon absorption activity for the two lowest-lying excited states and nonlinear response enhancement upon binding, we employed the generalized few-state model including the ground, first, and second excited states. The analysis of the optical channel suggests that the large two-photon response is due to the coordination-induced increase of the transition moment from the first to the second excited state.

Influence of Donor on the Sensing Performance of a Series of Through-Bond Energy Transfer-Based Two-photon Fluorescent Cu(2+) Probes

Optical properties of a series of molecular two-photon fluorescent Cu(2+) probes containing the same acceptor (rhodamine group) are analyzed using time-dependent density functional theory in combination with analytical response theory. Special emphasis is placed on evolution of the probes' optical properties in the presence of Cu(2+) . In this study, the compound with naphthalene as the donor is shown to be excellent ratiometric fluorescent chemosensor, whereas the compound with quinoline derivative as the donor shows off/on-typed colorimetric fluorescent response. For the compound with naphthalimide derivative as the donor, changing the connection between the donor and acceptor can efficiently prevent the fluorescent quenching of the probe both in the absence and presence of Cu(2+) . The donor moiety and the connection between donor and acceptor are thus found to play dominant roles on sensing performance of these probes. Moreover, distributions of molecular orbitals involved in the excitation and emission of the probes are analyzed to explore responsive mechanism of the probes. The through-bond energy transfer process is theoretically demonstrated. Our results are used to elucidate the available experimental measurements. This work is helpful to understand the relationships of structure with optical properties for the studied probes.

A mitochondria-targeted ratiometric two-photon fluorescent probe for biological zinc ions detection

A mitochondria-targeted ratiometric two-photon fluorescent probe (Mito-MPVQ) for biological zinc ions detection was developed based on quinolone platform. Mito-MPVQ showed large red shifts (68 nm) and selective ratiometric signal upon Zn(2+) binding. The ratio of emission intensity (I488 nm/I420 nm) increases dramatically from 0.45 to 3.79 (ca. 8-fold). NMR titration and theoretical calculation confirmed the binding of Mito-MPVQ and Zn(2+). Mito-MPVQ also exhibited large two-photon absorption cross sections (150 GM) at nearly 720 nm and insensitivity to pH within the biologically relevant pH range. Cell imaging indicated that Mito-MPVQ could efficiently located in mitochondria and monitor mitochondrial Zn(2+) under two-photon excitation with low cytotoxicity.

Rational Design of a Water-Soluble, Lipid-Compatible Fluorescent Probe for Cu(I) with Sub-Part-Per-Trillion Sensitivity

Knowledge-driven optimization of the ligand and fluorophore architectures yielded an ultrasensitive Cu(i)-selective fluorescent probe featuring a 180-fold fluorescence contrast and 41% quantum yield.

Fluorescence probes represent an attractive solution for the detection of the biologically important Cu(i) cation; however, achieving a bright, high-contrast response has been a challenging goal. Concluding from previous studies on pyrazoline-based fluorescent Cu(i) probes, the maximum attainable fluorescence contrast and quantum yield were limited due to several non-radiative deactivation mechanisms, including ternary complex formation, excited state protonation, and colloidal aggregation in aqueous solution. Through knowledge-driven optimization of the ligand and fluorophore architectures, we overcame these limitations in the design of CTAP-3, a Cu(i)-selective fluorescent probe offering a 180-fold fluorescence enhancement, 41% quantum yield, and a limit of detection in the sub-part-per-trillion concentration range. In contrast to lipophilic Cu(i)-probes, CTAP-3 does not aggregate and interacts only weakly with lipid bilayers, thus maintaining a high contrast ratio even in the presence of liposomes.

Fluorescence spectroscopy and microscopy as tools for monitoring redox transformations of uranium in biological systems

We report a study of redox reactions of uranium in model conditions using luminescence spectroscopy, which with its ease and wide availability has the potential to offer new insights into a bioremediation strategy of particular interest - the enzymatic reduction of U^VIO₂²⁺ by bacteria such as Geobacter sulfurreducens. The inherent luminescent properties of U^VIO₂²⁺ have been combined with confocal fluorescence microscopy techniques and lifetime image mapping to report directly on uranium concentration, localisation and oxidation state in cellular systems during uranium bioreduction, suggesting that localisation of uranyl species on the cell membrane surface plays an important role and that extracellular biogenic features form alongside uranyl sorbed cellular species during early stages of the bioreduction. The use of confocal microscopy in tandem with lifetime image mapping offers both improved temporal and spatial resolution (nanoseconds to microseconds and sub-micron respectively) than more conventional X-ray based techniques and offers the potential to image redox reactions occurring in situ. Together, these techniques provide an excellent and sensitive probe to assess the coordination environment of uranium during bioreduction processes that are currently being considered for remediation strategies of redox active radionuclides present in contaminated land.

Two-photon fluorescent probe derived from naphthalimide for cysteine detection and imaging in living cells

A maleimide coupling naphthalimide was reported as new two-photon fluorescent (TPF) probe for cysteine (Cys). The probe was weakly fluorescent itself due to the donor-excited photoinduced electron transfer (d-PET). After reaction with Cys, d-PET process was blocked and fluorescence enhancement of the probe was observed at 470 nm. The d-PET principle was rationalized by theoretical calculations with density functional theory and time-dependent density functional theory. Thiol-maleimide addition between the probe and Cys was confirmed by (1)H NMR and mass spectrum measurements. TPF analysis demonstrated a 24.7-fold emission increase of the probe induced by Cys upon excitation at 760 nm. The two-photon action cross-section of probe-Cys adduct at 760 nm reached 42 GM compared to 1.7 GM for free probe. The probe showed high sensitivity and selectivity to Cys over other potential interferences; especially it had the capability to discriminate Cys from glutathione and homocysteine. Through TPF imaging, the probe was successfully applied in the detection of Cys in living cells.

1,8-Naphthyridine-based molecular clips for off–on fluorescence sensing of Zn2+ in living cells

New 1,8-naphthyridine-based clip-like receptor as “off–on” fluorescent probe was designed and synthesised for selectively sensing Zn²⁺ in living cells.

The role of copper ions in pathophysiology and fluorescent sensors for the detection thereof

Copper ions are crucial to life, and some fundamental roles of copper in pathophysiology have been elucidated using fluorescent sensors.

Current developments in fluorescent PET (photoinduced electron transfer) sensors and switches

A fluorophore can be combined with a receptor according to a molecular engineering design in order to yield fluorescent sensing and switching devices.

Two-photon multiplexing bio-imaging using a combination of Eu- and Tb-bioprobes

Two europium and terbium luminescent bio-probes whose luminescence can be sensitized by a two-photon sensitisation process have been designed and the proof-of-concept of biphotonic multiplexing experiment is described.

Photoluminescence imaging of Zn2+in living systems

Advances in PL imaging techniques, such as confocal microscopy, two photon microscopy, lifetime and optical imaging techniques, have made remarkable contributions in Zn²⁺tracking.

Recent development of two-photon fluorescent probes for bioimaging

Fluorescent probes are essential tools for studying biological systems.

Zn(II)-coordination modulated ligand photophysical processes - the development of fluorescent indicators for imaging biological Zn(II) ions

Molecular photophysics and metal coordination chemistry are the two fundamental pillars that support the development of fluorescent cation indicators. In this article, we describe how Zn(II)-coordination alters various ligand-centered photophysical processes that are pertinent to developing Zn(II) indicators. The main aim is to show how small organic Zn(II) indicators work under the constraints of specific requirements, including Zn(II) detection range, photophysical requirements such as excitation energy and emission color, temporal and spatial resolutions in a heterogeneous intracellular environment, and fluorescence response selectivity between similar cations such as Zn(II) and Cd(II). In the last section, the biological questions that fluorescent Zn(II) indicators help to answer are described, which have been motivating and challenging this field of research.