Syntheses of podants and decoration of nanoparticles for chemosensor development

Abstract

Development of synthetic receptors is an important and active research field of chemistry. The construction of artificial chemosensors for cations/anions/biomolecules is of considerable importance due to the diverse applications of such receptors in chemistry, biology, medicine and environmental analysis. These chemosensors relay through change in magnetic, electronic or optical properties when it binds to any specific analyte. The fluorescent chemosensors have attracted considerable attention due to high sensitivity, convenient use and real application in biological systems. In order to achieve practical applications, the future of chemosensors is dependent upon various types of fluorescence mechanisms. A number of mechanisms direct the activities of fluorescent chemosensors. Earlier reports usually use photoinduced proton transfer (PET), modulation of charge transfer (CT) transitions, excited state proton transfer (ESPT), and eximer/exiplex formation. A PET sensor operates through “ON-OFF” / “OFF-ON” of the fluorescence intensity at a single wavelength. Measurements at a single wavelength may lead to an error in estimation due to phototransformation, receptor concentrations and environmental effects. Alternatively, sensors based on the ESIPT mechanism, sensors operating through modulations of the CT band, and excimer formation lead to a system in which the estimation is free from these drawbacks as they demonstrate dual channel fluorescence emissions. Nanotechnology is a rapidly evolving and promising research arena in which nanoparticles have been utilized in the development of imperative applications such as drug delivery, imaging, catalysis, chemical and biochemical. Nanostructured ZnO finds applications in spintronics due to its ferromagnetic properties which in turn depend upon the grain boundaries in doped ZnO. Also the adsorption ability of these nanostructures depends upon the grain size. Nanoparticles have been used extensively as sensors with tremendous selectivity and sensitivity. Mostly, nanoparticles provide a signaling sub-unit for a wide range of organic and bio-chemical ligands and therefore act as excellent sensors. However, only a few reports are available where nanoparticles afford a platform to organize the properties of sensors. The incorporation of a nonselective receptor on to the surface of CdSe/ZnS Quantum Dot (QD) or a polymeric framework dramatically changed the binding affinity of the receptor. The design offers some kind of steric organization to the pods of the receptor, making it selective towards a particular metal ion. Extending the same principle, we have decorated a range of Schiff base, urea and thiourea receptors on to the surface of ZnO nanoparticles and studied the recognition behavior of these receptors with the help of UV-vis and Fluorescence techniques. All the receptors were able to display dual channel emission due to Excited State Intramolecular (ESIPT) phenomenon. The design, synthesis and development of metal chemosensors with high sensitivity, selectivity, applicability in aqueous or semi-aqueous medium is highly desirable. Many metal ions like Al3+ , Zn2+, Cu2+ etc. are required for proper physiological functions, so it is crucial to monitor the level of metal ions in living cell and body fluids. Numerous scientific endeavours have been focused in the development of chemosensors for in vitro and in vivo detection of metal ions. However, for metal ion detection in body fluids and in cellular environment, the chemosensor must be able to operate in aqueous/semi-aqueous medium; while most of the reported metal sensors have poor water solubility and are susceptible to the interference of other metal ions. This poor solubility is due to the presence of hydrophobic moieties in the design of chemosensor such as anthracene, naphthalimide, pyrene etc. Contrary to these types of chemosensors, the binding units constituted by the polyamine scaffolds may offer good solubility in aqueous medium and the polyamine derivatives can form stable metal chelates. However, the selective binding and sensing of metal cation can be achieved by functionalization of polyamines with fluorescent/chromogenic moieties. For cellular application, the biocompatibility of sensor for living cell is another important concern. We have synthesized polyamine based receptors which were evaluated as fluorescent chemosensors. Polymer based Schiff base fluorescent sensor have been developed which act as “ON-OFF” fluorescent chemosensor for the biocompatible, selective, and sensitive detection of Zn2+ and can operative in real microbe cell. On binding with Zn2+, 9 fold enhancement in fluorescence intensity is observed due to the combinational effect of ESIPT, C=N isomerisation and the two-channel quenching of PET. The polymeric sensor is able to detect zinc in (1:99) DMF: H2O to a minimum concentration of 40 nΜ. Many receptors show high binding affinities for anions in organic solvents but the binding strength is often greatly diminished in an aqueous solution. This is due to solvent competition with the anion for the hydrogen bonding sites of the receptor in aqueous medium. So, a different strategy of metal displacement approach was used for recognition of anion owing to strong electrostatic force of attraction between the anion and metal complex of the receptor which is not influenced much by aqueous conditions. An Imine linked “ON-OFF”, multi responsive and selective chemosensor has been synthesized and checked for cation recognition properties.The dipodal receptor acts as a probe to monitor the Zn2+ concentration through changes in fluorescence intensity based on ESIPT and charge transfer (CT) mechanism. The receptor showed high sensitivity and selectivity for Zn2+ detection at concentrations ranging from 0 to 100 μM with detection limit of 10 μM. The changes in the fluorescence signature of zinc complex of receptor in the presence of phosphate anion are significantly promising. Therefore, the Zn2+ complex of the receptor can be used for phosphate quantification and for recognition of phosphorylated biomolecules through cation displacement approach. Similarly, another Imine linked chemosensor has been synthesized and examined for cation recognition properties. The sensor shows strong affinity for Al3+ over other cations such as Na+ , K+ , Mg2+, Ca2+, Ba2+ , Sr2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+ , Hg2+ and Pb2+. The resultant Al3+ complex have been synthesized and characterized on the basis of elemental analysis, mass spectra, and IR spectra. The recognition behavior of this complex was tested towards various anions and complex has offered an interesting opportunity for the ratiometric determination of HSO4 - in DMF/H2O (7:3, v/v) solvent system. This is the first Al3+ complex used for the ratiometric determination of HSO4 - in semi-aqueous medium. A new series of dipodal receptors has been synthesized containing disulphide linkages and hydroxyl group in the near proximity of imine linkage. The receptors are the ideal candidates to exhibit the phenomenon of keto-enol tautomerism. The recognition properties of the receptors have been studied by dual techniques: fluorescence as well as UV-Vis absorption spectroscopy. The disulphide receptors acted as ideal candidates for quantification of transition metal ions like Ni2+ and Cu2+ . Receptors have been developed for estimation of Ni2+ and Al3+ using two different approaches. It acted as a fluorescent receptor for Al3+ recognition with high selectivity and without any interference from other competitive metal ions. The enhancement in fluorescence intensity is the combined result of “PET” channel “OFF”, ESIPT, ICT and molecular rigidification. The receptor acted as a ratiometric chromogenic probe for Ni2+ through changes in the absorption profile. The color change from brownish to yellow allowed naked eye detection of Ni2+ with excellent sensitivity and a low detection limit of 0.8 µM. Mesitylene anchored dipodal receptors have been synthesized and checked for recognition affinity towards metal ions. The mesitylene anchored dipodal receptor acted as a ratiometric chemosensor for the simultaneous estimation of Cu 2+ and Ni2+ ions with excellent selectivity and sensitivity. The color change of the receptor upon the addition of Cu2+ and Ni2+ ions allowed the naked eye detection of both of these ions. The receptor was able to detect Cu2+ up to a detection limit of 1 µM and has a low detection limit of 1.5 µM for Ni2+ ions. The tripodal podants contains symmetrical pods attached to mesitylene anchor. Soft S donar sites have been incorporated in the upper portion and the lower part offers relatively harder sp2 hybridised -N donar sites. Synthesis and study of these tripodal moieties is very fascinating as the tripodal ligands can form complexes with metal ions by sandwiching the metal ion between the flexible pods and thereby offering excellent thermodynamic and kinetic stability to the complexes. By changing the substitution in the tripodal arms, a variety of receptors can be prepared. A tripodal receptor acted as a ratiometric chemosensor for the simultaneous estimation of Cu2+ and Al3+ ions with high selectivity and sensitivity. The receptor showed a color change from pale brown to greenish with the addition of Cu2+ and pale brown to colorless upon the addition of Al3+ and thus can be employed for the naked eye detection of both these ions. Fluorescent organic nanoparticles (FONPs) are prepared by simply injecting the non-aqueous solution of organic compound in aqueous phase under vigorous stirring at constant temperature. Aqueous phase insolubility of the organic compound compels the molecules to get aggregated in roughly spherical particles whose size can very well be controlled by the concentration, temperature, and stirring rate. Several pharmaceutical and consumer products develop a range of organic compounds for their diverse use. To improve the performance of organic products especially in aqueous phase while keeping the environmental concerns in consideration, researchers use formulations with surfactants, other composite mixtures, and surface decoration. Conversion of organic compounds into organic nanoparticles (ONPs) dramatically increases their applications in diverse environments including biological systems due to their interesting medicinal, electronic, and optical properties. Fluorescent organic nanoparticles (FONPs) are known to have marked improved optical and photo-physical properties than their parent organic molecules, and hence can be used as chemosensors for the detection of biologically important ions, biomolecules, molecular imaging probes etcWe demonstrate a simple method for the synthesis of fine spherical FONPs by using a OTL in aqueous phase. OTL itself is not a fluorescent molecule but shows radiative emission when it aggregates in a specific order in amorphous nanoparticle whose emission increases with the increase in the size and decreases with the rise in temperature. Because of its specific Ag+ ions binding ability, FONPs also show the same property and possess an ability to reduce Ag(I) into Ag(0) resulting in the growth of Ag NPs on the surface of FONPs to produce organic – inorganic hybrid nanoparticles. Ag@FONPs are not fluorescent due to the quenching of fluorescence emission of FONPs when Ag NPs grow on their surface. However, Ag@FONPs are the fine chromogenic agents for the detection of Cu2+ even in the presence of blood serum and hence can be used in the biological systems. Apart from this, NPs of ~200 nm show little hemolysis up to 200 µg/ml, thus making them suitable vehicles for the drug release in systemic circulation. The ZnO coated receptors exhibited very interesting properties and helped in fine tuning the recognition properties of free organic receptors. We have demonstrated the modulation of receptor properties by decorating them on the surface of ZnO nanoparticles. Thus, we have developed the ZnO nanoparticle surface decorated with otherwise non–selective receptor for the simultaneous estimation of Cr3+ and Al3+ in semi-aqueous medium through changes in fluorescence profile. This puts forwards the ability to monitor these ions in competitive media by fluorescent technique.