Small-Molecule Fluorescent Probes for Binding- and Activity-Based Sensing of Redox-Active Biological Metals

Although transition metals constitute less than 0.1% of the total mass within a human body, they have a substantial impact on fundamental biological processes across all kingdoms of life. Indeed, these nutrients play crucial roles in the physiological functions of enzymes, with the redox properties of many of these metals being essential to their activity. At the same time, imbalances in transition metal pools can be detrimental to health. Modern analytical techniques are helping to illuminate the workings of metal homeostasis at a molecular and atomic level, their spatial localization in real time, and the implications of metal dysregulation in disease pathogenesis. Fluorescence microscopy has proven to be one of the most promising non-invasive methods for studying metal pools in biological samples. The accuracy and sensitivity of bioimaging experiments are predominantly determined by the fluorescent metal-responsive sensor, highlighting the importance of rational probe design for such measurements. This review covers activity- and binding-based fluorescent metal sensors that have been applied to cellular studies. We focus on the essential redox-active metals: iron, copper, manganese, cobalt, chromium, and nickel. We aim to encourage further targeted efforts in developing innovative approaches to understanding the biological chemistry of redox-active metals.

Potentially toxic elements and environmentally-related pollutants recognition using colorimetric and ratiometric fluorescent probes

A safer detection or sensing of toxic pollutants is one among several environmental contamination issues, across the globe. The ever-increasing industrial practices and controlled or uncontrolled release of toxic pollutants from various industrial sectors is a key source of this environmental problem. Significant research efforts have been or being made to tackle this problematic issue to fulfill the growing needs of the modern world. Despite many useful aspects, heavy metals are posing noteworthy toxicological concerns and human-health related issues at various levels of the ecosystem. In this context, notable efforts from various regulatory authorities, the increase in the concentration of these toxic heavy metals in the environment is of serious concern, so real-time monitoring is urgently required. Herein, we reviewed fluorescent sensor based models and their potentialities to address the detection fate of hazardous pollutants including chromium, manganese, cobalt, nickel, copper, and zinc as model elements. The novel aspects of turn-on/off fluorescent sensors have also been discussed from a state of the art viewpoint. In summary, comprehensive literature regarding fluorescent sensor based models and their potentialities to detect various types of toxic pollutants is reviewed.

Design and Synthesis of Nanosensor Based on CdSe Quantum Dots Functionalized with 8-Hydroxyquinoline: a Fluorescent Sensor for Detection of Al3+ in Aqueous Solution

A novel nanosensor based on CdSe quantum dots (QDs) capped with 8-hydroxyqunoline (HQ) was developed for Al³⁺ ions determination in aqueous solutions. The method is based on the fluorescence enhancement of the HQ functionalized QDs in the presence of Al³⁺ ions, due to the strong interaction between Al³⁺ and HQ. Prepared nanosensor exhibited an acceptable selectivity and sensitivity for Al³⁺ ions in the presence of other metal ions. Plot of Log(I/I₀) against Log[Al³⁺] shows a good linearity in the range of 0.02-3.0 mM, and the method could be used for detection of Al³⁺ ions concentration in aqueous solutions.

Coplanarity driven fluorescence turn-on sensor for chromium(iii) and its application for bio-imaging

The sensing behaviour of benzimidazole and thiozole derivatives from heteroaromatic aldehyde was studied towards various cations and anions. S1 showed selectivity towards Cr³⁺ with fluorescence enhancement and live cell imaging of Hela cells was successfully demonstrated.

A new turn-on benzimidazole-based greenish-yellow fluorescent sensor for Zn2+ions at biological pH applicable in cell imaging

A newly designed and structurally characterized non-cytotoxic benzimidazole containing quinazoline derivative (HL) acts as a ‘turn-on’ greenish-yellow fluorescent sensor selective for Zn²⁺ions at as low as 39.91 nM in 5 mM HEPES buffer (DMSO/water: 1/5, v/v) at biological pH.

A supramolecular self-assembly host–guest system from cyclodextrin as an absolute water-soluble fluorescence sensor for aluminium ions: synthesis, characterization and sensing activity

In the study, an organic molecule derived from paeonol and anthraniloyl hydrazine (L) was synthesized via a Schiff-base reaction.

A single chemosensor for bimetal Cu(ii) and Zn(ii) in aqueous medium

A new quinazoline-based bimetal chemosensor for Cu(ii) and Zn(ii) in aqueous medium was synthesized. It can act as a “turn-off” sensor for Cu(ii) with the excitation wavelength of 356 nm, and a “turn-on” sensor for Zn(ii) when excited at 416 nm.

Effect of metal oxidation state on FRET: a Cu(i) silent but selectively Cu(ii) responsive fluorescent reporter and its bioimaging applications

A new structurally characterized cell permeable rhodamine-cinnamaldehyde hybrid (HL) behaves as a Cu(ii) ions selective chemosensor through FRET process which depends on +2 state of copper ion exclusively.

Selective and sensitive turn-on chemosensor for Al(iii) ions applicable in living organisms: nanomolar detection in aqueous medium

A new crystallographically characterized non-cytotoxic CHEF based highly sensitive Al(iii) ion selective chemosensor (L) is useful to detect the Al(iii) ions' distribution in A549 cell lines and also able to sense Al(iii) ions in tea extract.

A water soluble copper(ii) complex as a HSO4− ion selective turn-on fluorescent sensor applicable in living cell imaging

A water soluble non-fluorescent biofriendly cell permeable structurally characterised copper(ii) complex (1) selectively senses HSO₄⁻ ions as low as 3.18 × 10⁻⁷ M in water : DMSO (9 : 1, v/v) HEPES buffer at biological pH.

A rhodamine-based ‘turn-on’ Al3+ ion-selective reporter and the resultant complex as a secondary sensor for F− ion are applicable to living cell staining

A new cell permeable rhodamine based Schiff base (L) senses nanomolar level of Al³⁺ ions through CHEF process and its Al(iii) complex (2) behaves as a highly F⁻ ions selective biomarker through fluorescence quenching in HEPES buffer.

Metal ion-assisted ring-opening of a quinazoline-based chemosensor: detection of copper(ii) in aqueous media

A fluorescence chemosensor for Cu(ii) in aqueous media was synthesized. The experiments and theoretical calculations confirmed that the Cu(ii) ion assisted the ring-opening of the quinazoline derivative, forming a Cu(ii) Schiff base during the detection.

A Highly Selective and Sensitive Turn-On Fluorescent Chemosensor Based on Rhodamine 6G for Iron(III)

Recently, more and more rhodamine derivatives have been used as fluorophores to construct sensors due to their excellent spectroscopic properties. A rhodamine-based fluorescent and colorimetric Fe(3+) chemosensor 3',6'-bis(ethylamino)-2-acetoxyl-2',7'-dimethyl-spiro[1H-isoindole-1,9'-[9H]xanthen]-3(2H)-one (RAE) was designed and synthesized. Upon the addition of Fe(3+), the dramatic enhancement of both fluorescence and absorbance intensity, as well as the color change of the solution, could be observed. The detection limit of RAE for Fe(3+) was around 7.98 ppb. Common coexistent metal ions showed little or no interference in the detection of Fe(3+). Moreover, the addition of CN(-) could quench the fluorescence of the acetonitrile solution of RAE and Fe(3+), indicating the regeneration of the chemosensor RAE. The robust nature of the sensor was shown by the detection of Fe(3+) even after repeated rounds of quenching. As iron is a ubiquitous metal in cells and plays vital roles in many biological processes, this chemosensor could be developed to have applications in biological studies.

Fluorescent, MRI, and colorimetric chemical sensors for the first-row d-block metal ions

Transition metals (d-blocks) are recognized as playing critical roles in biology, and they most often act as cofactors in diverse enzymes; however, improper regulation of transition metal stores is also connected to serious disorders. Therefore, the monitoring and imaging of transition metals are significant for biological research as well as clinical diagnosis. In this article, efforts have been made to review the chemical sensors that have been developed for the detection of the first-row d-block metals (except Cu and Zn): Cr, Mn, Fe, Co, and Ni. We focus on the development of fluorescent sensors (fall into three classes: "turn-off", "turn-on", and ratiometric), colorimetric sensors, and responsive MRI contrast agents for these transition metals (242 references). Future work will be likely to fill in the blanks: (1) sensors for Sc, Ti, and V; (2) MRI sensors for Cr, Mn, Co, Ni; (3) ratiometric fluorescent sensors for Cr(6+), Mn(2+), and Ni(2+), explore new ways of sensing Fe(3+) or Cr(3+) without the proton interference, as well as extend applications of MRI sensors to living systems.

A napthelene–pyrazol conjugate: Al(iii) ion-selective blue shifting chemosensor applicable as biomarker in aqueous solution

A new crystallographically characterized napthelene–pyrazol conjugate acts as an Al(III) ion selective chemosensor in 100 mM HEPES buffer (water–DMSO 5 : 1, v/v) at biological pH. It is an efficient biomarker in detecting Al(III) ions in living cells.

A simple donor–acceptor probe for the detection of Cr3+ cations

A probe exhibits fluorescence enhancement upon exposure to Cr(iii) in a THF–water (v/v = 9/1) Tris–HCl buffer solution. Fluorescence imaging experiments of the exciplex in human glioma cells evidenced its potential application in biological systems.

A FRET-based ‘off–on’ molecular switch: an effective design strategy for the selective detection of nanomolar Al3+ ions in aqueous media

A FRET-based ratiometric chemosensor (L1) is highly selective for Al³⁺ ions (as low as 6.19 × 10⁻⁹ M) and could be used to acquire images of Al³⁺ ions in living cells.

A quinazoline derivative as quick-response red-shifted reporter for nanomolar Al3+and applicable to living cell staining

A newly designed and structurally characterized non-cytotoxic quinazoline based ratiometric chemosensor (L) selectively detects Al³⁺ions upto 1.48 nM through ICT and CHEF processes in water–DMSO (9 : 1, v/v) and it is also applicable in living cell staining.

Development of a rhodamine–benzimidazol hybrid derivative as a novel FRET based chemosensor selective for trace level water

A newly designed FRET-based rhodamine–benzimidazol hybrid molecule has been developed as a dual channel chemosensor for the detection of trace level water in both protic and aprotic organic solvents.