A new family of imine and amine-based racemic ligands containing furan or pyridine as an aromatic donating ring [N,N′-bis(2-pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine, L1; N,N′-bis(2-furanylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine, L2; N,N′-bis(2-pyridylmethyl)-1,2-(R,R + S,S)-cyclohexanediamine, L3; and N,N′-bis(2-furanylmethyl)-1,2-(R,R + S,S)-cyclohexanediamine, L4] and their trifluoromethanesulphonate (CF3SO3(−), OTf(−)) and nitrate Eu(III) complexes is studied in acetonitrile (AN) solution. The stoichiometry and stabilities of the formed complexes are obtained by means of spectrophotometric titrations: when Eu(III) triflate is used as a starting salt, two mononuclear species (1:1 and 1:2) are detected, while only the 1:1 complex is observed when the nitrate salt is employed. The stability of these complexes, as well as the geometry of their Eu(III) environment, is significantly dependent on the nature of the ligand employed (imine or amine, furan or pyridine-based). DFT calculations show that all donor atoms are coordinated to the metal ion in the 1:1 EuL(L = L1–L4) species and suggest that the higher stability of the complexes with L1 and L2 with respect to L3 and L4 is mostly due to the higher degree of preorganization of the former species. The optical response of the imine-based L1 and L2 Eu complexes, produced by NO3(−) coordination, has been studied in order to assess their application as sensing devices. With both ligands, an increase of the emission intensity on the addition of the nitrate ion is observed. This is higher for the EuL2 complex and underlines the important role of the nature of the heteroaromatic ring. Finally, it is worth noting that an efficient energy transfer process from the ligand to the metal is present in the case of the 1:1 triflate Eu(III) complex with the ligand L1.

Deep Eutectic Solvents (DESs) are a new class of solvents characterized by a remarkable decrease in melting point compared to those of the starting components. The eutectic mixtures can be simply prepared by mixing a Hydrogen Bond Acceptor (HBA) with a Hydrogen Bond Donor (HBD) at a temperature of about 80 °C. They have found applications in different research fields; for instance, they have been employed in organic synthesis, electrochemistry, and bio-catalysis, showing improved biodegradability and lower toxicity compared to other solvents. Herein, we review the use of DESs in biosensor development. We consider the emerging interest in different fields of this green class of solvents and the possibility of their use for the improvement of biosensor performance. We point out some promising examples of approaches for the assembly of biosensors exploiting their compelling characteristics. Furthermore, the extensive ability of DESs to solubilize a wide range of molecules provides the possibility to set up new devices, even for analytes that are usually insoluble and difficult to quantify.

Paper has been widely employed as cheap material for the development of a great number of sensors such as pregnancy tests, strips to measure blood sugar, and COVID-19 rapid tests. The need for new low-cost analytical devices is growing, and consequently the use of these platforms will be extended to different assays, both for the final consumer and within laboratories. This work describes a paper-based electrochemical sensing platform that uses a paper disc conveniently modified with recognition molecules and a screen-printed carbon electrode (SPCE) to achieve the detection of gluten in a deep eutectic solvent (DES). This is the first method coupling a paper biosensor based on aptamers and antibodies with the DES ethaline. Ethaline proved to be an excellent extraction medium allowing the determination of very low gluten concentrations. The biosensor is appropriate for the determination of gluten with a limit of detection (LOD) of 0.2 mg L−1 of sample; it can detect gluten extracted in DES with a dynamic range between 0.2 and 20 mg L−1 and an intra-assay coefficient of 10.69%. This approach can be of great interest for highly gluten-sensitive people, who suffer from ingestion of gluten quantities well below the legal limit, which is 20 parts per million in foods labeled gluten-free and for which highly sensitive devices are essential. Graphical abstract

A new family of imine-based ligands containing pyridine or furan as an aromatic donating ring [N,N'-bis(2-pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine, L1; N,N'-bis(2-furanylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine, L2 and N,N'-bis(2-thienylmethylidene)-1,2-(R,S)-cyclohexanediamine, L3] has been prepared in high yield by means of an easy synthetic protocol. Their trifluoromethansulphonate (CF3SO3(-), OTf(-)) Eu(iii) complexes have been employed for luminescence sensing of the NO3(-) anion in an anhydrous acetonitrile solution. Spectrophotometric titrations have been carried out to define the speciation in the solution and study the formation of ternary species occurring with the addition of NO3(-) anions. The sensing response towards this anion is strongly dependent on the nature of the ligand, the stoichiometry of the complexes and their concentration.