Syntheses of organic based nanomaterials receptors and their analytical studies

Abstract

The development of chemosensors for the identification and quantification of important physiological and environmental analytes are of considerable importance. The search is focused for the high sensitivity, selectivity and multifunctionality of these chemosensors for their effective operational usage. For example, transition metal ions are a paradox to life and trace amount of several transition metal ions are required in cellular processes and many enzymatic reactions are under the direct control of these metal ions. However, excess intake may lead to adverse health effects, including allergic and autoimmune diseases. Therefore, detection of these metal ions in a water sample is of great interest. Anions are ubiquitous in Nature playing significant roles in both biological processes and large scale industrial processes. DNA, responsible for carrying genetic information is anionic as are most enzyme substrates. Anions are significant in medicine with new treatments for cystic fibrosis focusing on the regulation of anion transport. While the field of cation detection is well developed, anion detection is not yet a mature field as the design of anion detection systems capable of sensing more than one analyte is particularly challenging. In part this is due to the large size of anions compared to cations leading to lower charge to radius ratios and the wide range of geometries anions commonly adopt. In recent years, considerable efforts have been devoted to develop fluorescent sensors because of their high sensitivity, the low detection limits obtained and their convenient use. An efficient strategy for the design of a fluorescent sensor includes the combination of a receptor moiety with a signaling subunit, which responds through the change in the fluorescent profile upon binding with particular analyte. This change in signal was raised from various phenomena depending upon the size of analyte, solvent system and fluorophore moiety. Photoinduced electron transfer (PET), modulation of charge transfer (CT) transitions, excited state proton transfer (ESPT) and eximer/exiplex formation are mainly reported in literature. Based upon these mechanisms, several reports existed in literature for detection of metal ions and anions. However, major challenge is solubility in aqueous medium and most of physiological and environmental processes are occurred in water. Therefore, it is highly essential to develop the strategy for detection of metal ions in aqueous medium. To achieve the same, fluorescent organic nanoparticles were synthesized using reprecipitation method. This strategy gives a solution of organic receptor in 99% water. The dipodal and tripodal receptors are synthesized for the fabrication of fluorescent organic nanoparticles (FONPs). These receptors attached with rhodamine/naphthalimide based fluorophores. To counter the poor solubility of sensor in pure water, fluorescent organic nanoparticles (FONPs) are developed using reprecipitation method. This involves an injection of particular sensor (1 ml of stock solution of particular sensor in pure DMF/THF) to 100 ml of double distilled pure water. There was great disparity between the solubility of organic receptor in DMF (good solubility) and water (highly insoluble). However, the mutual solubility compatibility of the two solvents is the governing features of the method i.e. mixing the both solutions under sonication ensured the rapid mixing of both the solutions. As the molecules of organic receptor are exposed to water surroundings in a very short time and the water is expected to change the microenvironment; inducing the nucleation and growth of the molecules to nanoparticles. The formation of FONPs was confirmed with the help of TEM, UV-Visible spectrophotometer, fluorescence and DLS. The rhodamine-based dipodal and tripodal framework results in the enhancement of fluorescence intensity due to spirolactam ring-opening. To the best of our knowledge, this is the first report where mesitylene anchored rhodamine containing dipodal–tripodal scaffolds have been used for the detection of Hg(II). Among the various toxic metal ions, mercury contamination is extensive with highly toxic impact on the environment and human health. The rhodamine-based dipodal is highly sensitive to Hg(II) ion and coordinate in 1:1 stoichiometry. The addition of Hg(II) results into opening of spirolactam ring-opening and significant enhancement was observed in emission intensity of FONPs (7). The opening of spirolactam ring was confirmed through 1H NMR titration. This strategy give nano-molar detection of Hg(II) ion in aqueous medium with a detection limit of 0.1 nM. For real application, strips coated with rhodamine-based dipodal are prepared and can be utilized even after months. Furthermore, dipodal rhodamine-based mercury complex was utilized for the selective detection of 3-mercaptopropionicacid (MPA). To avoid the poor solubility of rhodamine-based ligand in pure water, the Hg(II) complex of fluorescent organic nanoparticles (FONPs) of ligand have been developed using reprecipitation method and the formation of 1:1 complex has been confirmed with various spectroscopic techniques. The resultant chemosensor can detect MPA in a concentration range of 60 nM–1 μM (in buffered aqueous medium) with detection limit of 60 nM. However, naphthalimide-based dipodal receptor showed ratiometric response for Ag(I) ion on emission spectroscopy. In design of naphthalimide-based dipodal receptor, the sp2 nitrogen binding sites from imine linkages are provided in such a way that the receptor should offer five-membered chelate ring. The meta substituted derivative of naphthalimidebased dipodal receptor was utilized for the selective and ratiometric sensing of Ag(I) in a concentration range of 15–65 nM with a detection limit of 15.5 nM. The work was extended to monitor the Ag(I) concentration in samples of environmental importance. Moreover, naphthalimide-based monopodal receptors were synthesized and their FONPs were used for detection of metal ions like Cu(II), Cr(III), and Al(III) in aqueous medium. The receptor and its metal complexes are evaluated for the cytotoxic activities and studies have led to construction of the molecular logic gate. Due to the broad biological activities of benzothiazole/benzimidazole derivatives, much effort has been devoted to the development of synthetic methods for the synthesis of the benzothiazole/benzimidazole framework. However, these methods require harsh reaction conditions, the use of expensive, air-sensitive and toxic reagents, which potentially introduce serious hazardous materials into the ecosystem. A convenient solvent-free method for the synthesis of 2-substituted benzothiazole/benzimidazole and 1,2-disubstituted benzimidazole derivatives has been developed using recyclable ZnO-NPs via a ball-milling strategy. The method affords environmentally friendly reaction conditions that score high on the ecoscale with the low E-factor. The process is also highly efficient even on a multi-gram scale and provides easy product isolation. The as synthesized 2-substituted benzothiazole/benzimidazole and 1,2-disubstituted benzimidazole derivatives have used for the detection of metal ions in aqueous medium. For aqueous medium, FONPs were prepared using reprecipitation method. These small molecules showed high affinity for various toxic metals like Ag(I), Al(III), Ni(II), Cr(III) and Pb(II). The pico-molar detection of Al(III) was achieved using this strategy. Developing a receptor to detect anions in aqueous media is challenging because of competition between anions and water molecules for the receptor binding sites. Thus, most hydrogen bonding-based receptors for anions are not compatible with an aqueous system. Metal complexes have been used to overcome this problem. Metal displacement from a complex or electrostatic interactions between the metal center of a complex and an anion can cause changes in its photophysical properties. UV–Visible and fluorescence spectroscopic analysis techniques are frequently employed to detect these changes. Electrochemical techniques offer several benefits over other detection techniques, including simplicity, relative low cost, portability, high performance with lower background, sensitivity and applicability to turbid samples. The benzimidazole/benzothiazole-based Co(III) complexes have been employed for the direct sensing of biomolecules without modification of the surface of electrodes. Using this approach, nano-molar detection of various biomolecules like guanine, adenine, NAD and phosphate have been achieved. These complexes also showed good selectivity to anions like iodide and hydrogen sulfate. ZnO-NPs were employed as a platform to organize non-selective receptor binding to an assembly, which lead to highly selective sensor for particular analyte. Although, similar to CdSe/ZnS (QDs), the relatively cheaper ZnO decorated with non-selective receptor may results into a nanocrystal hybrid, with unique selectivity for particular metal ion. The theme of this strategy is based upon the idea that a free organic receptor is expected to be highly flexible and may adopt any geometry according to the steric requirement of any metal ion. However, if flexibility of this receptor is retarded, then this may improve the selectivity of the receptor. Under this strategy, the decoration of organic receptor on the surface of nanoparticle may restrict some of the coordination modes; this will result into a relatively more selective sensor. Through this strategy, imine-linked ZnO coated receptors were synthesized and compared their recognition behavior with receptor alone. Interestingly, ZnO coated receptors showed high selectivity for metal ions [Mg(II), Al(III), Zn(II), Mn(II) and CO(II)] in semi-aqueous medium. This strategy was further explored through decoration of two different ligands in 1:1 ratio on the surface of ZnO. The 1:1 ratio was confirmed through number of proton in 1H NMR. Some of them have detection in nano-molar range. Due to non-toxic in nature, ZnO coated receptors were successfully employed for detection of metal ions in biological conditions.