Fluorescent probes for the detection of disease-associated biomarkers

Fluorescent probes have emerged as indispensable chemical tools to the field of chemical biology and medicine. The ability to detect intracellular species and monitor physiological processes has not only advanced our knowledge in biology but has provided new approaches towards disease diagnosis. In this review, we detail the design criteria and strategies for some recently reported fluorescent probes that can detect a wide range of biologically important species in cells and in vivo. In doing so, we highlight the importance of each biological species and their role in biological systems and for disease progression. We then discuss the current problems and challenges of existing technologies and provide our perspective on the future directions of the research area. Overall, we hope this review will provide inspiration for researchers and prove as useful guide for the development of the next generation of fluorescent probes.

Recent Advances in Luminescent Recognition and Chemosensing of Iodide in Water

This Minireview covers the latest developments of chemosensors based on transition-metal receptors and organic fluorophores with specific binding sites for the luminescent detection and recognition of iodide in aqueous media and real samples. In all selected examples within the last decade (made-post 2010), the iodide sensing and recognition is probed by monitoring real-time changes of the fluorescence or phosphorescence properties of the chemosensors. This review highlights effective strategies to iodide sensing from a structural approach where the iodide recognition/sensing process, through supramolecular interactions as coordination bonds, hydrogen bonds, halogen bonds and electrostatic interactions, is transduced into an optical change easily measurable. The selective iodide sensing is an active field of research with global interest due to the importance of iodide in biological, medicinal, industrial, environmental and chemical processes.

New colorimetric and fluorometric chemosensor for selective Hg2+ sensing in a near-perfect aqueous solution and bio-imaging

We report a new 7-nitrobenzo-2-oxa-1, 3-diazolyl (NBD)-based chemosensor containing a piperazine derivative, NBDP, for detection of mercury ions in almost 100% aqueous medium. The chemosensor shows sensing exclusively toward Hg²⁺ with a switch-on fluorescence response at 543 nm, which could be attributed to the blocking of PET (photo-induced electron transfer) process upon complexation with mercury ions. The molar ratio of Hg(Ⅱ) to NBDP in the complex is 1:1 based on the Job's plot and HRMS studies. Optimized configurations of NBDP and NBDP-Hg²⁺ complexes were simulated by means of DFT calculations. The reversible fluorescence response with low detection limit (19.2 nM) in the pH range of 6.0-7.5 renders NBDP a promising candidate for Hg²⁺ detection in neutral aqueous environments. For the practical application of the chemosensor, test strips were successfully fabricated for rapid detection of Hg²⁺ ions. Moreover, the utility of NBDP showing the mercury recognition in Human liver cancer cells (SMMC-7721) and zebrafish as well as in live tissues of Arabidopsis thaliana has been demonstrated as monitored by fluorescence imaging.

A fluorescent "ON-OFF-ON" switch for the selective and sequential detection of Hg2+ and I- with applications in imaging using human AGS gastric cancer cells

A new fluorescent "on-off-on" probe (BIPQ) is designed and developed which selectively binds with Hg²⁺; its emission intensity is quenched almost 40-fold at 455 nm without interference from other metal cations. On gradual addition of I^- to the solution of BIPQ-Hg²⁺, the emission reverts to its original intensity. The limits of detection of BIPQ for Hg²⁺ and I^- are found to be on the order of 3.12 × 10^-9 and 5.48 × 10^-8 M, respectively, which shows clearly that BIPQ can sense Hg²⁺ at a very minute level. DFT and TDDFT studies are conducted with the probe to establish similarity between theoretical and experimental outcomes. Finally, to demonstrate its practical benefit in biological fields, live cell imaging experiments with BIPQ are carried out to detect Hg²⁺ in human AGS gastric cancer cell lines.

Facile method for iodide ion detection via the fluorescence decrease of dihydrolipoic acid/beta-cyclodextrin protected Ag nanoclusters

In this work, novel photoluminescent Ag nanoclusters (Ag NCs) with red emission are synthesized and successfully used for detecting iodide ion (I^-). The dihydrolipoic acid (DHLA) is used as the stabilizing agent and beta-cyclodextrin (β-CD) is used as the auxiliary stabilizing agent. DHLA and β-CD are combined with Ag atoms by the formation of AgS bonds and hydrophobic interaction, respectively. Functionalization of β-CD endows good photoluminescent properties and solubility in water to the Ag NCs. The obtained DHLA and β-CD-protected Ag NCs (DHLA/β-CD-Ag NCs) are spherical and display a dispersed state. However, the DHLA/β-CD-Ag NCs are aggregated in the presence of I^-, accompanied by the decrease in their fluorescence intensity. Because the integrity of β-CD cavities is retained on the surface of DHLA/β-CD-Ag NCs, which preserves their capability for I^- host-guest recognition, the DHLA/β-CD-Ag NCs combine with I^- through the formation of inclusion complexes. Based on this phenomenon, the prepared DHLA/β-CD-Ag NCs can be designed as a novel fluorescent probe for I^- detection. The limit of detection (LOD) is calculated as 0.06 μM, indicating that it is an ideal probe for I^- detection in practical applications.

A halogen ion-selective phosphorescence turn-on probe based on induction of Pt–Pt interactions

A halogen ion induced self-assembly of square-planar platinum complexes has been, for the first time, observed and applied as a turn-on phosphorescent probe for Cl⁻.

A highly selective chromogenic sensor for Mn2+, turn-off fluorometric for Hg2+ ion, and turn-on fluorogenic sensor for F- ion with the practical application

A colorimetric and fluorometric probe (E)-2-((8-hydroxy-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9-yl)methylene)hydrazinecarbothioamide based on thiosemicarbazide and julolidine moieties has been synthesised in pure crystalline form and characterized by ¹H NMR, UV-vis, elemental analysis and single crystal XRD. The probe functioned as multitarget ion sensor, detect biologically important metal ions Hg²⁺ and Mn²⁺ in dual channel mode. Meanwhile, in mixed solvent media DMF/H₂O [8:2], probe displayed selectivity for Hg²⁺ over other cations by the emission spectrum. Interestingly probe has been explored to recognize F^- anion in DMF through ESIPT mechanism. The 1:1 binding stoichiometry of probe with Hg²⁺ and Mn²⁺ is confirmed by Job's plot through emission titration and UV-vis titration respectively. Probe is selective and sensitive to Hg²⁺ and Mn²⁺ with detection limit as low as 15μM and 0.2μM respectively. The sensing mechanism for selective ions was also scrutinized using ¹H NMR experiments and computational studies.

A G-quadruplex based fluorescent oligonucleotide turn-on probe towards iodides detection in real samples

A basket-type G-quadruplex (GQ) fluorescent oligonucleotide (OND) probe is designed to detect iodides dependent on thymine-Hg(II)-thymine (T-Hg(II)-T) base pairs and the intrinsic fluorescence quenching capacity of GQ. In the presence of Hg(II) ions (Hg²⁺), the two hexachloro-fluorescein-labeled ONDs form a hairpin structure and the fluorophores are dragged close to the GQ, leading to fluorescence quenching of the probe due to photoinduced electron transfer. Upon addition of iodide anions, Hg²⁺ are extracted from T-Hg(II)-T complexes which attributes to the stronger binding with iodide anions, resulting in the fluorescence recovery. Through performing the fluorescence quenching and recovery processes, this probe developed a fluorescence turn-on sensor for iodide anions determination over a linear range of 20-200nmol/L with a limit of detection of 5nmol/L. The practical use of the turn-on technology was demonstrated by its application in determination of iodides in water, food, pharmaceutical products and biological samples.

Fostering protein–calixarene interactions: from molecular recognition to sensing

An highly selective direct sensing of cytochrome c by a bis-calix[4]arene-carbazole conjugate (CCC-1) in aqueous-based medium at nanomolar levels is disclosed. An electron transfer (ET) between complexed partners mediates the sensory event.

Syntheses, crystal structures and steady state and time-resolved fluorescence properties of a PET based macrocycle and its dinuclear ZnII/CdII/HgII complexes

The present report deals with solution spectroscopic (absorption and emission) and colorimetric studies of a Robson type tetraaminodiphenol macrocyclic ligand and its ZnII2, CdII2 and HgII2 compounds.

Ratiometric fluorophore for quantification of iodide under physiological conditions: applications in urine analysis and live cell imaging

The ratio of fluorescence intensity of fluorophoreI_{395 nm}/I_{475 nm}vs.log [I⁻] undergoes linear change over a broad iodide concentration range of 10⁻⁹to 10⁻⁵M and finds application in urine analysis and live cell imaging.

Porphyrinic MOFs for reversible fluorescent and colorimetric sensing of mercury(ii) ions in aqueous phase

A simple, rapid and visual assay for Hg²⁺ detection is designed based on assembling Zr–O clusters and porphyrin ligands.

Phenolic thio- and selenosemicarbazones as multi-target drugs

A series of isosteric phenolic thio- and selenosemicarbazones have been obtained by condensation of naturally-occurring phenolic aldehydes and thio(seleno)semicarbazides. Title compounds were designed as potential multi-target drugs, and a series of structure-activity relationships could be established upon their in vitro assays: antioxidant activity, α-glucosidase inhibition and antiproliferative activity against six human tumor cell lines: A549 (non-small cell lung), HBL-100 (breast), HeLa (cervix), SW1573 (non-small cell lung), T-47D (breast) and WiDr (colon). For the antiradical activity, selenium atom and 2 or 3 phenolic hydroxyl groups proved to be essential motifs; remarkably, the compound with the most potent activity, with a trihydroxyphenyl scaffold (EC50 = 4.87 ± 1.57 μM) was found to be stronger than natural hydroxytyrosol, a potent antioxidant present in olive oil (EC50 = 13.80 ± 1.41 μM). Furthermore, one of the thiosemicarbazones was found to be a strong non-competitive inhibitor of α-glucosidase (Ki = 9.6 ± 1.6 μM), with an 8-fold increase in activity compared to acarbose (Ki = 77.9 ± 11.4 μM), marketed for the treatment of type-2 diabetes. Most of the synthesized compounds also exhibited relevant antiproliferative activities; in particular, seleno derivatives showed GI50 values lower than 6.0 μM for all the tested cell lines; N-naphthyl mono- and dihydroxylated derivatives behaved as more potent antiproliferative agents than 5-fluorouracil or cisplatin.

A new selective chromogenic and turn-on fluorogenic probe for copper(ii) in solution and vero cells: recognition of sulphide by [CuL]

A new coumarin-appended thioimidazole-linked imine conjugate,viz.Lhas been synthesized and characterized for a selectiveturn-onfluorescence chemosensor for Cu²⁺.

An azodye–rhodamine-based fluorescent and colorimetric probe specific for the detection of Pd2+ in aqueous ethanolic solution: synthesis, XRD characterization, computational studies and imaging in live cells

An azodye–rhodamine hybrid colorimetric fluorescent probe (L) has been designed and synthesized.

Phenothiazine based sensor for naked-eye detection and bioimaging of Hg(ii) and F− ions

The design and development of new phenothiazine-based fluorescent probes, which displays selective fluorescence response to Hg²⁺ and F⁻ ions in a reversible manner. The probe is the first example that facilitates the detection of Hg²⁺ at nanomolar concentration.

Luminescent probes for the bioimaging of small anionic species in vitro and in vivo

This comprehensive review examines recent developments in the use of fluorescent/luminescent probes for the bioimaging of anionic species. Images in cover art reproduced with permission from ref. 290 and 306.

A mechanosynthesized, sequential, cyclic fluorescent probe for mercury and iodide ions in aqueous solutions

A fluorescent Hg(2+)-selective chemosensor, 2,5-dimethoxybenzaldehyde thiosemicarbazone (1), was quantitatively prepared by grinding 2,5-dimethoxybenzaldehyde and thiosemicarbazide together in a ball mill for 15min. The excitation and emission maxima of compound 1 are 347 and 450nm, respectively. The reaction of this ligand with Hg(2+) was investigated by FT-IR, (1)H NMR, and fluorescence titration. Results show that the composition of the resulting Hg complex 1-Hg is 2:1 1:Hg, and that the S and imino N atoms serve as the binding sites of the ligand to the Hg(2+) ions. Coordination-assisted fluorescence quenching results show that compound 1 exhibits a highly selective fluorescence response to trace amounts of Hg(2+) in water. More importantly, the resulting complex 1-Hg can be used as a turn-on fluorescence probe for I(-) at a detection limit of 8.4×10(-8)M. Thus, compound 1 is a relatively stable, sequential, cyclic fluorescent probe for Hg(2+) and I(-).

Colorimetric detection of in situ metal acetates and fluorides by a bipyridyl-linked Schiff base

Here, we present a new bipyridyl moiety linked Schiff base (bipy-1) that is well characterized using spectroscopic techniques. Colorimetric and UV-vis titrations were used to study the photophysical properties of bipy-1 in the presence of various tetrabutyl ammonium salt of anions and metal salts containing different counter cations. bipy-1 showed selective recognition of dimethyl sulphoxide solution of tetrabutyl ammonium salt of F(-) ion accompanied with a UV-vis band at 529 nm and interesting binding of aqueous Co, Ni, and Cu acetates/fluorides, as confirmed by distinct color changes from fluorescent green to pink or orange and a strong band around 480-510 nm in the UV-vis spectrum. However, in the presence of Co, Ni, and Cu countercations, any form of metal acetate/fluorides was found to be able to respond to similar color changes from fluorescent green to pink or orange, showing a band around 480-510 nm. This type of output clearly indicates that the in situ formation of Co, Ni, and Cu acetates/fluorides also coordinates with bipyridyl nitrogen atoms.

A pyrene thiazole conjugate as a ratiometric chemosensor with high selectivity and sensitivity for tin (Sn4+) and its application in imaging live cells

A new pyrene thiazole-conjugate amine based fluoroionophore, PTC was developed for ratiometric detection of Sn⁴⁺ ion in organo-aqueous medium.