| Optical chemosensors based on fluorescent or UV absorbance response have attracted enormous attention in molecular recognition, and play an important role in the modern biological medicine detection, benefiting from the advantages such as the high sensitivity, easy detection and so on. In the traditional optical chemosensors, a chromophore (or fluorophore) is covalently attached to a receptor commonly with precise design and complicate synthesis.In the present study, sensing ensembles built in a non-covalent way are used to identify target molecules with high selectivity and sensitivity. Sensor arrays comprised by 4d-transition metals and fluorescent dyes are able to detect various analytes from ions such as Li+, pyrophosphate (PPi) to organic molecules like peptides and deoxynucleotides. Competitive analyte is introduced into the system causing the displacement of the indicator from the host, which in turn modulates measurable changes in fluorescence or absorbance. A key advantage of the approach is its simplicity:all components of the sensor array are commercially available or can be easily synthesized, and the individual sensors are rapidly obtained by mixing stock solutions of the respective reagents.First, a simple but powerful method has been developed for the sensing of peptides in aqueous solution. The transition-metal complexes [{RuCl2 (p-cymene)}2], [(RhCl2Cp*)2], [PdCl2(en)] are combined with six different fluorescent dyes to build a cross-reactive sensor array. The fluorescence response can be observed after the addition of peptide analytes, resulting from complexation reactions between peptides and the dyes. The collective response of the sensor array in a time-resolved fashion is used as an input for multivariate analyses. A sensor array comprised of only six metal-dye combinations is able to differentiate ten different dipeptides in buffered aqueous solution. Furthermore, the cross-reactive sensor could be used to detect pharmacologically interesting dipeptides carnosine and homocamosine in a complex biological matrix such as human blood serum. The array is also able to differentiate mixtures of longer peptides nonapeptide bradykin and the decapeptide kallidin. Next, a (2 x 2) colorimetric sensor array composed of two Pd complexes ([PdCl2(en)] and [PdCl2(bipy)]) and two fluorescent dyes is created to identify different hexadeoxynucleotides. Upon addition of hexadeoxynucleotides, a colorimetric response can be detected due to the displacement of fluorescent dyes from the Pd-dye complexes. The assay can identify sequence-isomeric oligonucleotides and distinguish mixtures of them, providing the information of the concentration and ratio.Further study demonstrates that the Pd(â…¡) complex [Pd(NO3)2(bipy)] (bipy= 2,2'-bipyridine) can interact with pyrophosphate (PPi). A mixture of [Pd(NO3)2(bipy)] and the fluorescent dye Methylcalcein blue (MCB) constitutes a chemosensing ensemble, which can be used for the detection of PPi in the low micromolar concentration by fluorescence spectroscopy. The selectivity for PPi over inorganic anions such as F-,Cl-,Br-,AcO-,HCO3-,NO3-,H2PO4-,ClO4-,SO42-,Sal- is very good.Finally, two chemosensors for Li+have been designed by functionalizing Ru complex [12]metallacrown-3 with a covalently bound fluorophore 1,8-naphthalimide. Due to the low solubility in purely aqueous solution, time-consuming synthesis and low sensitivity of these two sensors, we further develop a sensing ensemble in which fluorescent dye (HPTS) is bound to the macrocyclic receptor in a non-covalent fashion. The receptor is able to act as a ditopic one for Li+and the fluorescent dye HPTS. The assay is easy-to-use and allows the sensing of sub-millimolar concentrations of Li+in buffered aqueous solution with high selectivity over Na+. Mg2+. It can also be used to detect Li+ion in a complex biological fluid such as human blood serum.
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