CHEF-Affected Fluorogenic Nanomolar Detection of Al3+ by an Anthranilic Acid-Naphthalene Hybrid: Cell Imaging and Crystal Structure

A Prompt Study on Recent Advances in the Development Of Colorimetric and Fluorescent Chemosensors for "Nanomolar Detection" of Biologically Important Analytes

Fluorescent and colorimetric chemosensors for selective detection of various biologically important analytes have been widely applied in different areas such as biology, physiology, pharmacology, and environmental sciences. The research area based on fluorescent chemosensors has been in existence for about 150 years with the development of large number of fluorescent chemosensors for selective detection of cations as metal ions, anions, reactive species, neutral molecules and different gases etc. Despite the progress made in this field, several problems and challenges still exist. The most important part of sensing is limit of detection (LOD) which is the lowest concentration that can be measured (detected) with statistical significance by means of a given analytical procedure. Although there are so many reports available for detection of millimolar to micromolar range but the development of chemosensors for the detection of analytes in nanomolar range is still a challenging task. Therefore, in our current review we have focused the history and a general overview of the development in the research of fluorescent sensors for selective detection of various analytes at nanomolar level only. The basic principles involved in the design of chemosensors for specific analytes, binding mode, photophysical properties and various directions are also covered here. Summary of physiochemical properties, mechanistic view and type of different chemosensors has been demonstrated concisely in the tabular forms.

Solvent-Selective Fluorescence Sensing of Mg2+ and Al3+ Ions by Pincer-Type NNO Schiff Base Ligand: An Experimental and DFT Optimized Approach

A newly developed dual-functional fluorescence sensing probe (phenylhydrazinyl pyridine) Schiff base (SB) has been designed with good selectivity for distinguishing Mg2+ and Al3+ metal ions in different solvent solutions. SB exhibits quick and visual turn-on fluorescence enhancement in response to Mg2+ and Al3+ detection. The addition of Mg2+ in ACN-HEPES buffer (1 : 1, v/v, pH 7.2) at (λmax=390 nm) and Al3+ in MeOH-HEPES buffer (1 : 1, v/v, pH 7.2) at (λmax=360 nm) resulted in significant enhancement of fluorescence, up to 7-9 times. These low detection limits of 7.1×10-6 M (7.1 μM) and 5.15×10-7 M (0.51 μM) for Mg2+ and Al3+, respectively, have been achieved by this solvent-controlled platform. Due to the sensing potential towards Mg2+, the probe was utilized as an imaging material for breast cancer cells. 1H-NMR studies were utilized to explore SB's sensing mechanism through turn-on fluorescence. Density functional theory (DFT) calculations were utilized to validate optimized SB and its intricate geometries, which govern the sensing mechanism in the solvent environment. Such a probe has extensive potential applications in bioimaging and the assessment of the quality of wastewater.

Robust Optical Detection of Ga3+ by a Rhodamine- and Coumarin-Based Proficient Probe: Theoretical Investigations and Biological Applications

The present investigation highlights a rhodamine-B- and coumarin-based efficient probe that selectively detects Ga3+ over other metal ions. The active pocket of the ligand for trapping the metal ions and the binding stoichiometry of its Ga3+ complex were discovered by single-crystal X-ray diffraction (SC-XRD) analysis. This binding stoichiometry was further confirmed in the solution state by mass spectrometry and Job's plot. The detection limit was found to be at the nanomolar level. Pyrophosphate being a well-known quencher could easily quench the fluorescence intensity of the RC in the presence of Ga3+ and reversibly recognize Ga3+ in the solution. The spiro ring opening of the ligand after Ga3+ insertion is proposed to be the principal mechanism for the turn-on fluorescence response. This ring opening was confirmed by SC-XRD data and nuclear magnetic resonance (NMR) titration experiments. Both ground- and excited-state calculations of the ligand and complex have been carried out to obtain information about their energy levels and to obtain the theoretical electronic spectra. Furthermore, the live-cell imaging of the probe only and the probe after the addition of Ga3+ have been carried out in HaCaT cells and satisfactory responses were observed. Interestingly, with the help of this probe, Ga3+ can be tracked inside the intracellular organelle such as lysosomes along with other regions of the cell. The article highlights a rhodamine-coumarin-based probe for the detection of Ga3+ over other metal ions with a nanomolar level detection limit. Structural characterization of the ligand and its Ga3+ complex was investigated by SC-XRD. Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies were carried out to explore the excited-state energies and electronic spectra. The application of the probe for the detection of Ga3+ in live cells has been explored, and positive responses were observed.

Interrogating the nature of aggregates formed in a model azine based ESIPT coupled AIE active probe: stark differences in photodynamics in the solid state and aggregates in water

A novel Schiff base 4-bromo-2-((E)-((E)-(1-(naphthalen-2-yl)ethylidene)hydrazono)methyl)phenol (BNHMP) was synthesized and characterized by NMR, ESI-MS, FTIR and single crystal X-ray diffraction studies. In the solution phase, BNHMP shows prominent emission from the keto-form, a consequence of excited state intramolecular proton transfer (ESIPT). The quantum yield and excited state lifetime decrease in polar solvent THF compared to relatively non-polar solvent DCM. Interestingly, in aqueous solution (pH 7.0), the quantum yield along with the excited state lifetime undergoes tremendous increment. Dynamic light scattering experiments and FESEM reveal the formation of aggregates in water as reflected by the increased hydrodynamic radius of BNHMP in water. Hence, aqueous phase studies revealed BNHMP to be an AIE active probe. On the other hand, BNHMP shows huge emission intensity in the solid state. Interestingly, the emission decay behavior of BNHMP changes upon excitation, as BNHMP shows very broad absorption in the solid state. Upon excitation at 360 nm, a triexponential decay pattern is found, which changes to a biexponential one upon excitation at 450 nm. Meticulous analysis of the fluorescence lifetimes led to the assignment of J and H aggregates coexisting in the solid state with the former dominating the photodynamics. A judicious comparison of the lifetime behavior in the solid state to that in water leads to the conclusion that BNHMP undergoes AIE by the formation of J and H aggregates to an equal extent, a phenomenon starkly different from the solid-state scenario. The current results hold significance as this is among a few reports where such comprehensive spectrodynamic dissection has been performed for an ESIPT-AIE active Schiff base in solution as well as in the solid phase, thereby giving a holistic vision of the nature and fate of aggregation occurring in such azine based systems and subsequently advancing the understanding of such systems in terms of their photo behavior.

Fluorescence detection of Al3+ ion in aqueous medium and live cell imaging by ESIPT probe (E)-N'-(5-bromo-2-hydroxybenzylidene)-4-hydroxybenzohydrazide

The molecule (E)-N'-(5-bromo-2-hydroxybenzylidene)-4-hydroxybenzohydrazide (BHHB) has been synthesized and its photophysical properties have been investigated by using steady state absorption, emission and time resolved emission spectroscopy. The molecule shows excited state intramolecular proton transfer (ESIPT) process with characteristics large Stoke shifted emission. Fluorescence enhancement of BHHB only in presence of Al³⁺ ion is used as selective aluminium ion sensor in the sub-nano molar scale in aqueous solution. BHHB-Al³⁺ ion complex can penetrate through live Hepatocellular Carcinoma (HepG2) cell membranes and is capable for imaging of nucleus of live cells by fluorescence confocal microscopy.

Fluorescent Sensors for Detecting and Imaging Metal Ions in Biological Systems: Recent Advances and Future Perspectives

Metal ions play a crucial role in many biochemical processes, and when in a state of scarcity or surplus, they can lead to various diseases. Therefore, the development of a selective, sensitive, cost-effective, and fast-responding sensor to detect metal ions is critical for in vitro medical diagnostics. In recent years, fluorescent sensors have been extensively investigated as potent kits for the effective assessment of metal ions in living systems due to their high sensitivity, selectivity, ability to perform real-time, non-invasive monitoring, and versatility. This review is an overview of recent advances in fluorescent sensors for the detection and imaging of metal ions in biosystems from 2018 to date. Specifically, we discuss their application in detecting essential metal ions and non-essential metal ions for in vitro diagnostics, living cell imaging, and in vivo imaging. Finally, we summarize remaining challenges and offer a future outlook on the above topics.

Real time sensor for Fe3+, Al3+, Cu2+ & PPi through quadruple mechanistic pathways using a novel dipodal quinoline-based molecular probe

A quinoline-based small molecular probe, H₂L was designed, synthesized and characterized by different spectroscopic methods. It was utilized as a multi-responsive probe for the detection of Fe³⁺, Al³⁺, Cu²⁺ and PPi. It showed very selective instant turn-on fluorimetric response towards Fe³⁺and Al³⁺ with a detection limit in nanomolar range. Solutions of H₂L containing Fe³⁺ or Al³⁺ could sequentially sense PPi by a turn-off mechanism. Also, H₂L could determine the presence of Cu²⁺ very selectively among a series of other metal ions by a sharp change in colour. Detection of Cu²⁺ through colorimetry was further investigated by systematic UV-Vis studies and the potential of H₂L to act as a potential colorimetric sensor for Cu²⁺ was suitably established. Filter-paper strip experiments were conducted to demonstrate the practical utility of the proposed sensor. Potential applications of H₂L as a sensor for pH in the acidic range has also been explored.

A novel full symmetric diarylethene-based ratiometric fluorescent sensor for lysine and the application for a logic circuit

A novel diarylethene-based ratiometric fluorescent sensor with full symmetric structure, 1o, was designed and synthesized successfully. 1o could identify lysine (Lys) with high selectivity and sensitivity and the fluorescence emission peak was red shifted 85 nm upon addition of Lys, which could realize ratio recognition. It exhibited excellent anti-interference performance in the presence of various amino acids in CH₃ CN/H₂ O (7/3, v/v) solution. Moreover, the limit of detection of 1o to Lys could reach 0.019 μM based on a good linear range of 0-40 μM. In addition, the fluorescence emission intensity of 1o could be turned off/on by ultraviolet/visible light due to the special structure of diarylethene. A logic circuit was designed with three inputs. The ratiometric fluorescent sensor 1o could be as a new tool and provide a new method for detection of Lys.

A dual fluorescence probe for Zn2+ and Al3+ through differentially response and bioimaging in living cells

A novel Schiff base fluorescent probe 7-Hydroxy-8-(((2-(hydroxymethyl)quinolin-8-yl)imino)methyl)-coumarin (XL) consist of formylcoumarin and aminoquinoline moieties was synthesized for dual detection of Zn²⁺ and Al³⁺ ions. Probe XL exhibited high selective and sensitive response towards Zn²⁺ and Al³⁺ ions through different color changes and significant fluorescence turn-on response (270 fold higher for Zn²⁺ and 230 fold higher for Al³⁺) in MeOH-H₂O (4/1, v/v) over other cations, with detection limits (LOD) as low as 3.75 × 10^-8 and 1.14 × 10^-8 M, respectively. Moreover, probe XL exhibited preferential selectivity for Al³⁺ through displacing Zn²⁺ from the XL-Zn²⁺ complex by ligand-to-ligand transfer process. The binding mechanism of intramolecular charge transfer (ICT) were proposed from fluorescence and UV-vis titrations, Job's plot, ¹H NMR titration, HRMS and DFT calculations. The probe was proven to be suitable for actual samples detection of Zn²⁺ and Al³⁺ ions. The complex XL-Zn²⁺ and XL-Al³⁺ exhibited dramatic fluorescent "turn-off" properties for PPi and PPi/F^- respectively through snatching metal ions and released free XL. Moreover, probe XL showed low biotoxicity and sequentially "off-on-off" fluorescent bio-imaging of Zn²⁺/Al³⁺ and PPi/F^- in PC12 cells.

Turn-On Fluorescent Sensors for the Selective Detection of Al3+ (and Ga3+) and PPi Ions

Rationally designed multiple hydroxyl-group-based chemosensors L1-L4 containing arene-based fluorophores are presented for the selective detection of Al³⁺ and Ga³⁺ ions. Changes in the absorption and emission spectra of L1-L4 in ethanol were easily observable upon the addition of Al³⁺ and Ga³⁺ ions. Competitive binding studies, detection limits, and binding constants illustrate significant sensing abilities of these chemosensors with L4, showing the best results. The interaction of Al³⁺/Ga³⁺ ions with chemosensor L4 was investigated by fluorescence lifetime measurements, whereas Job's plot, high-resolution mass spectrometry, and ¹H NMR spectral titrations substantiated the stoichiometry between L4 and Al³⁺/Ga³⁺ ions. The solution-generated [L-M³⁺] species further detected pyrophosphate ion (PPi) by exhibiting emission enhancement and a visible color change. The binding of Al³⁺/Ga³⁺ ions with chemosensor L4 was further supported by density functional theory studies. Reversibility for the detection of Al³⁺/Ga³⁺ ions was achieved by utilizing a suitable proton source. The multiionic response, reversibility, and optical visualization of the present chemosensors make them ideal for practical applications for real samples, which have been illustrated by paper-strip as well as polystyrene film-based detection.

Water-soluble aluminium fluorescent sensor based on aggregation-induced emission enhancement

Development of a portable miniature system for Al(iii) detection in pure aqueous solutions using a novel AIEE compound.