Highly Selective and Responsive Visible to Near-IR Ytterbium Emissive Probe for Monitoring Mercury(II)

Lanthanide-dye hybrid luminophores for advanced NIR-II bioimaging

In vivo luminescence imaging in the second near-infrared window (NIR-II, 1000-2000 nm) is a potent technique for observing deep-tissue life activities, leveraging reduced light scattering, minimized autofluorescence, and moderate absorption attenuation to substantially enhance image contrast. Pushing the frontiers of NIR-II luminescence imaging forward, moving from static to dynamic event visualization, monochromatic to multicolor images, and fundamental research to clinical applications, necessitates the development of novel luminophores featuring bright emission, extendable wavelength, and optimal biocompatibility. Recently, lanthanide-dye hybrid luminophores (LDHLs) are gaining increasing attention for their wavelength extensibility, molecular size, narrowband emission, mega stokes shift, long lifetime, and high photostability. In this review, we will summarize the recent advances of NIR-II LDHLs and their applications in imaging and analysis of living mammals, and discuss future challenges in designing new LDHLs for deep-tissue imaging.

Lanthanide-tetrapyrrole complexes: synthesis, redox chemistry, photophysical properties, and photonic applications

Tetrapyrrole derivatives such as porphyrins, phthalocyanines, naphthalocyanines, and porpholactones, are highly stable macrocyclic compounds that play important roles in many phenomena linked to the development of life. Their complexes with lanthanides are known for more than 60 years and present breath-taking properties such as a range of easily accessible redox states leading to photo- and electro-chromism, paramagnetism, large non-linear optical parameters, and remarkable light emission in the visible and near-infrared (NIR) ranges. They are at the centre of many applications with an increasing focus on their ability to generate singlet oxygen for photodynamic therapy coupled with bioimaging and biosensing properties. This review first describes the synthetic paths leading to lanthanide-tetrapyrrole complexes together with their structures. The initial synthetic protocols were plagued by low yields and long reaction times; they have now been replaced with much more efficient and faster routes, thanks to the stunning advances in synthetic organic chemistry, so that quite complex multinuclear edifices are presently routinely obtained. Aspects such as redox properties, sensitization of NIR-emitting lanthanide ions, and non-linear optical properties are then presented. The spectacular improvements in the quantum yield and brightness of Yb^III-containing tetrapyrrole complexes achieved in the past five years are representative of the vitality of the field and open welcome opportunities for the bio-applications described in the last section. Perspectives for the field are vast and exciting as new derivatizations of the macrocycles may lead to sensitization of other Ln^III NIR-emitting ions with luminescence in the NIR-II and NIR-III biological windows, while conjugation with peptides and aptamers opens the way for lanthanide-tetrapyrrole theranostics.

A fluorescent probe for rapid detection of low concentration mercury ions and its application in biological cells

As a toxic substance, mercury can easily cause harm to organisms and humans. The development of methods that allow rapid detection of low concentrations of mercury ions has a positive effect on the natural environment and human health. The fluorescent probe RBSH reported in this paper has a detection limit as low as 5.9 nM, and a fast response time and allows naked eye detection. We characterized its structure by nuclear magnetic resonance and mass spectrometry, and explored the response mechanism of the probe using Job's plot, and ¹H NMR and mass spectrometry. UV-vis spectrophotometry and fluorescence spectroscopy show the excellent optical properties of the probe RBSH. The low toxicity and high cell penetration capacity demonstrated by the cellular assay open up the possibility of biological experiments. By selecting hosts (natural water samples, soybean plants and zebrafish) where mercury ions are likely to be present in the biological chain for low concentration Hg²⁺ detection, the results all demonstrated the excellent performance of the probe RBSH.

Long-lived lanthanide emission via a pH-sensitive and switchable LRET complex

Lanthanide-based luminescence resonance energy transfer (LRET) can be used as a tool to enhance lanthanide emission for time-resolved cellular imaging applications. By shortening lanthanide emission lifetimes whilst providing an alternative radiative pathway to the formally forbidden, weak lanthanide-only emission, the photon flux of such systems is increased. With this aim in mind, we investigated energy transfer in differently spaced donor–acceptor terbium–rhodamine pairs with the LRET “on” (low pH) and LRET “off” (high pH). Results informed the design, preparation and characterisation of a compound containing terbium, a spectrally-matched pH-responsive fluorophore and a receptor-targeting group. By combining these elements, we observed switchable LRET, where the targeting group sensitises lanthanide emission, resulting in an energy transfer to the rhodamine dye with an efficiency of E = 0.53. This strategy can be used to increase lanthanide emission rates for brighter optical probes.

A pH-sensitive luminescence resonance energy transfer (LRET) was explored as a method to increase photon flux in a terbium-rhodamine-receptor targeting group construct. At low pH, long-lived dye emission and shorter terbium lifetimes were observed.

A nanomolar detection of mercury(II) ion by a chemodosimetric rhodamine-based sensor in an aqueous medium: Potential applications in real water samples and as paper strips

A new promising rhodamine based colorimetric and fluorometric chemosensor, RDV has been designed and synthesized for specific detection of Hg²⁺ ion. It acts as highly selective "turn-on" fluorescent chemosensor for Hg²⁺ ion without interference from other competitive metal ions in aqueous acetonitrile medium. The drastic color change with addition of Hg²⁺, from colorless to pink, indicates RDV can acts as "naked-eye" indicator for Hg²⁺. The Hg²⁺ promoted selective hydrolysis of appended vinyl ether group in RDV followed by Hg²⁺ chelated complex formation with concomitant opening of spirolactam ring is the plausible sensing mechanism. The detailed absorption, fluorescence, ¹H NMR, ¹³C NMR and mass spectrometry confirms the proposed sensing mechanism. The limit of detection (LOD) of Hg²⁺ by RDV is 136 nM indicating the high sensitivity towards Hg²⁺. The RDV shows consistent spectroscopic response in biological pH range 4-10. In addition to explore practical applicability of RDV, its paper strips have been made and used to detect Hg²⁺ in pure water solution up to 10 ppm level. Furthermore, the potential application of RDV for the sensing of Hg²⁺ in real water samples (tap water and drinking waters from different sources) were also monitored and demonstrated.

Highly luminescent, biocompatible ytterbium(iii) complexes as near-infrared fluorophores for living cell imaging

We report three synthetic methods to prepare biocompatible Yb³⁺ complexes, which displayed high NIR luminescence with quantum yields up to 13% in aqueous media. This renders β-fluorinated Yb³⁺ porphyrinoids a new class of NIR probes for living cell imaging including time-resolved fluorescence lifetime imaging.

Herein, we report the design and synthesis of biocompatible Yb³⁺ complexes for near-infrared (NIR) living cell imaging. Upon excitation at either the visible (Soret band) or red region (Q band), these β-fluorinated Yb³⁺ complexes display high NIR luminescence (quantum yields up to 23% and 13% in dimethyl sulfoxide and water, respectively) and have higher stabilities and prolonged decay lifetimes (up to 249 μs) compared to the β-non-fluorinated counterparts. This renders the β-fluorinated Yb³⁺ complexes as a new class of biological optical probes in both steady-state imaging and time-resolved fluorescence lifetime imaging (FLIM). NIR confocal fluorescence images showed strong and specific intracellular Yb³⁺ luminescence signals when the biocompatible Yb³⁺ complexes were uptaken into the living cells. Importantly, FLIM measurements showed an intracellular lifetime distribution between 100 and 200 μs, allowing an effective discrimination from cell autofluorescence, and afforded high signal-to-noise ratios as firstly demonstrated in the NIR region. These results demonstrated the prospects of NIR lanthanide complexes as biological probes for NIR steady-state fluorescence and time-resolved fluorescence lifetime imaging.

Design of Near-Infrared Luminescent Lanthanide Complexes Sensitive to Environmental Stimulus through Rationally Tuning the Secondary Coordination Sphere

The design of near-infrared (NIR) emissive lanthanide (Ln) complexes sensitive to external stimulus is fundamentally important for the practical application of Ln materials. Because NIR emission from Ln is extremely sensitive to X-H (X = C, N and O) bond vibration, we herein report to harness the secondary coordination sphere to design NIR luminescent lanthanide sensors. Toward this goal, we designed and synthesized two isomeric [(η⁵-C₅H₅)Co{(D₃CO)₂P = O}₃]-Yb(III)-7,8,12,13,17,18-hexafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porpholactol NIR emitters, Yb-up and Yb-down, based on the stereoisomerism of porphyrin peripheral β-hydroxyl group. Yb-up, in which β-OH is at the same side of Yb(III) center, can form an intramolecular hydrogen bond with the axial Kläui ligand, whereas Yb-down cannot because its β-OH is opposite to Yb(III) center. X-ray crystal structures and photophysical studies suggested that the intramolecular hydrogen bond plays important roles on the NIR luminescence of ytterbium(III), which shortens the distance between β-OH and Yb(III) and facilitates the nonradiative deactivation of Ln excited state. Importantly, Yb-up/down were demonstrated to be highly sensitive toward temperature and viscosity. The PMMA polymer using Yb-up as the dopant NIR emitter showed thermosensitivity up to 6.0% °C^-1 in the wide temperature range of 77-400 K, higher than that of Yb-down (3.8% °C^-1). These complexes were also explored as the first NIR viscosity sensor, revealing their potential applications as optical sensors without visible light interference. This work demonstrates the importance of secondary coordination sphere on designing NIR Ln luminescent functional materials.

Long-wavelength analyte-sensitive luminescent probes and optical (bio)sensors

Long-wavelength luminescent probes and sensors become increasingly popular. They offer the advantage of lower levels of autofluorescence in most biological probes. Due to high penetration depth and low scattering of red and NIR light such probes potentially enable in vivo measurements in tissues and some of them have already reached a high level of reliability required for such applications. This review focuses on the recent progress in development and application of long-wavelength analyte-sensitive probes which can operate both reversibly and irreversibly. Photophysical properties, sensing mechanisms, advantages and limitations of individual probes are discussed.

A Indole-Trizole-Rhodamine Triad as Ratiometric Fluorescent Probe for Nanomolar-Concentration Level Hg(2+) Sensing with High Selectivity

A new type of ratiometric fluorescent probe capable of detecting Hg(2+) ions at nanomolar-concentration level with high selectivity was developed based on an indole-trizole-rhodamine triad and its practicability for intracellular Hg(2+) sensing was verified. The as-prepared fluorescent probe is capable of detecting Hg(2+) over other competing metal ions including Ag(+) with high selectivity. The synergistic effect of Hg(2+)-assisted conversion of the nonfluorescent ring-closed rhodamine moiety to the highly fluorescent ring-open form as well as the fluorescence signal amplification originating from the Förster resonance energy transfer (FRET) from indole-trizole conjugate to rhodamine moiety contributed to a detection limit of 11 nM of the probe for Hg(2+) sensing.

A bifunctional colorimetric fluorescent probe for Hg2+and Cu2+based on a carbazole–pyrimidine conjugate: chromogenic and fluorogenic recognition on TLC, silica-gel and filter paper

A bifunctional fluorescent probe based on a carbazole–pyrimidine conjugate exhibited a colorimetric and ratiometric turn-on response towards Hg²⁺/Cu²⁺in the nanomolar range.

Fluorescent carbon nanoparticles for the fluorescent detection of metal ions

Fluorescent carbon nanoparticles (F-CNPs) as a new kind of fluorescent nanoparticles, have recently attracted considerable research interest in a wide range of applications due to their low-cost and good biocompatibility. The fluorescent detection of metal ions is one of the most important applications. In this review, we first present the general detection mechanism of F-CNPs for the fluorescent detection of metal ions, including fluorescence turn-off, fluorescence turn-on, fluorescence resonance energy transfer (FRET) and ratiometric response. We then focus on the recent advances of F-CNPs in the fluorescent detection of metal ions, including Hg(2+), Cu(2+), Fe(3+), and other metal ions. Further, we discuss the research trends and future prospects of F-CNPs. We envision that more novel F-CNPs-based nanosensors with more accuracy and robustness will be widely used to assay and remove various metal ions, and there will be more practical applications in coming years.