Thiophene-based electrochemically active probes for selective calcium detection

A Comprehensive Review on Thiophene Based Chemosensors

The recognition and sensing of various analytes in aqueous and biological systems by using fluorometric or colorimetric chemosensors possessing high selectivity and sensitivity, low cost has gained enormous attention. Furthermore, thiophene derivatives possess exceptional photophysical properties compared to other heterocycles, and therefore they can be employed in chemosensors for analyte detection. In this review, we have tried to explore the design and detection mechanism of various thiophene-based probes, practical applicability, and their advanced models (design guides), which could be thoughtful for the synthesis of new thiophene-based probes. This review provides an insight into the reported chemosensors (2008-2020) for thiophene scaffold as effective emission and absorption-based chemosensors.

A Transparency Sheet-Based Colorimetric Device for Simple Determination of Calcium Ions Using Induced Aggregation of Modified Gold Nanoparticles

A simple and novel transparency sheet-based colorimetric detection device using gold nanoparticles (AuNPs) modified by 4-Amino-6-hydroxy-2-mercaptopyrimidine monohydrate (AHMP) was fabricated and developed for the determination of calcium ions (Ca²⁺). The detection was based on a colorimetric reaction as a result of the aggregation of modified AuNPs induced by Ca²⁺ due to the ability to form strong electrostatic interactions between positively charged Ca²⁺ and negatively charged modified AuNPs. Probe solution changes color from red to blue in the presence of Ca²⁺ and can be observed by the naked eyes. To verify the complete self-assembly of the AHMP onto the AuNP surface, the modified AuNPs were characterized using ultraviolet–visible spectroscopy and zeta potential measurements. Under optimal conditions, a quantitative linearity was 10 to 100 ppm (R² = 0.9877) with a detection limit of 3.05 ppm. The results obtained by the developed method were in good agreement with standard atomic absorption spectrometry (AAS) results and demonstrated that this method could reliably measure Ca²⁺. Overall, this novel alternative approach presents a low-cost, simple, sensitive, rapid, and promising device for the detection of Ca²⁺.

A highly selective fluorescence "turn-on" sensor for Ca2+ based on diarylethene with a triazozoyl hydrazine unit

A new photochromic diarylethene derivative with a triazozoyl hydrazine unit has been designed and synthesized. Its photochromism and photoswitchable fluorescence behaviors were studied systematically by the stimuli of lights and chemical substances in acetonitrile solution. With the addition of Ca2+, the emission intensity enhanced 6.7 fold, accompanied by an obvious fluorescent color change from dark to light blue. The complexation between the derivative and Ca2+ is reversible with the 1 : 1 stoichiometry, which was verified by Job's plot and MS. The limit of detection (LOD) for Ca2+ was determined to be 2.49 × 10-8 mol L-1. Based on this unimolecular platform, a logic circuit was designed with fluorescence intensity at 482 nm as the output and the combined stimuli of UV/vis and Ca2+/EDTA as four inputs.

Redox-electrodes for selective electrochemical separations

Redox-active materials hold great promise as platforms for selective liquid-phase separations. In contrast to capacitive electrodes that rely purely on double-layer charge for deionization, redox-modified electrodes can be used to control Faradaic reactions at the interface to selectively bind various charged and uncharged molecules, thus modulating surface interactions through electrochemical potential solely. These electrodes can be composed of a range of functional materials, from organic and organometallic polymers to inorganic crystalline compounds, each relying on its own distinct ion-exchange process. Often, redox electrochemical systems can serve as pseudocapacitors or batteries, thus offering an advantageous combination of adsorption selectivity and energy storage/recovery. This review summarizes redox-interfaces for electrosorption and release, outlines methods for preparation and synthesis, discusses the diverse mechanisms for interaction, and gives a perspective on the future of redox-mediated separations.

Solvent Effects on the Electrochemical Behavior of TAPD-Based Redox-Responsive Probes for Cadmium(II)

Two tetralkylated phenylenediamines (TAPD)1and2have been prepared by reductive alkylation ofpara-dimethylaminoaniline with furfural or thiophene 2-carboxaldehyde, respectively. Their chelation ability has been evaluated as electrochemical guest-responsive chemosensors for Cd(II) in acetonitrile (ACN), dimethylformamide (DMF), propylene carbonate (PC), and nitromethane (NM). The voltamperometric studies showed that these compounds are able to bind the Cd(II) cation with strong affinities except in DMF. The redox features of the chemosensors changed drastically when they are bounded to Cd(II) to undergo important anodic potential peak shifts comprised between ca. 500 and ca. 900 mV depending on the solvent. The addition of ∼4–10% molar triflic acid (TfOH) was found to be necessary to achieve rapidly the cation chelation which is slow without the acid. The electrochemical investigations suggested the formation of 1 : 2 stoichiometry complexes [Cd(L)2]2+. The results are discussed in terms of solvent effects as a competitive electron donating ligand to the cation. The reaction coupling efficiency (RCE) values were determined and were also found to be solvent-dependent.

Water-soluble, redox-active organometallic calcium chelators

This paper describes a new series of organometallic water-soluble chelators combining a redox moiety (ferrocene) and a selective Ca2+ chelator (BAPTA) separated by an ethynyl bridge. We report the synthesis and characterization of organometallic derivatives of the BAPTA chelator featuring one (2a) and two ferrocenyl (2b) moieties. Single crystal X-ray structural analysis on these chelators revealed unexpected conformations for the ferrocenyl substituent with respect to the phenyl ring of the BAPTA unit. DFT calculations on a model system of the ferrocenyl-ethynyl-BAPTA molecule were carried out to evaluate the energy separation between the two limiting conformations observed experimentally in the solid state, and to check the effective electronic communication between the binding pocket and the redox probe. The binding affinity of 2a–b for Ca2+, as probed by UV-Vis and cyclic voltammetry, revealed distinct behaviors in the presence of a metal ion depending on whether BAPTA is substituted by one or two ferrocenyl groups.

Redox-responsive probes for selective chelation of bivalent cations

N,N-disubstituted bis(furanyl-2-methyl)aminoanilines are new electrochemically-active probes for cations relying on the phenylenediamine moiety as an electroactive transducer and the difuranylamino group as an ionophore site. The electrochemical investigations, by means of cyclic and Osteryoung square wave voltammetries (CV and SWV, respectively), showed that these compounds are able to bind Mg2+, Ca2+, Ni2+ and Zn2+ cations with strong affinities. The addition of catalytic amounts of trifluoromethanesulfonic acid (TfOH) was found necessary to achieve rapid cation complexation. The electroactive redox features of the probes were drastically modified when the ionophore site was bonded to the cations. The anodic potential shifts of the oxidation peaks were between 905 and 1030 mV depending on the cations. The electrochemical investigations suggested the formation of a 1:2 stoichiometric complex: [M(L) 2]2+, M=Mg, Ca, Ni and Zn. These probes were found to be selective of Ca2+ and chelates, with strong preference for Ca2+ even in presence of others cations (Ca2+> Mg2+, Ca2+> Ni2+ and Ni2+> Zn2+). UV-visible spectrophotometric studies also showed blue shifts of the absorption bands comprising between 5 and 29 nm ligands when the metal ions were added to the solution, which confirmed the complexes formation.