Two-step FRET mediated metal ion induced signalling responses in a probe appended with three fluorophores.

A tri-fluorophoric molecular probe (1) with three different derivatized fluorophores, i.e. anthracene (An), 7-nitrobenz-2-oxa-1,3-diazole (NBD) and rhodamine-B (Rh) appended on to a Tren [tris-(2-aminoethyl)amine] receptor was demonstrated to exhibit metal ion induced ratiometric fluorescence signalling through the initiation of a two-step fluorescence resonance energy transfer (FRET) process owing to a compatible and substantial spectral overlap of electronic absorption and fluorescence of initial donor-intermediate donor/acceptor-final acceptor pairs. The steady-state and time-resolved fluorescence spectral investigation of probes (1a-1f) with individual excitation energy donor (D)/acceptor (A) fluorophores as well as different bi-fluorophoric D-A pairs appended to the same receptor, and comparison with those of 1 revealed a higher energy transfer efficiency between the initial donor (FAn) and the final acceptor (FRh) in the presence of an intermediate (FNBD) fluorophore. The determination of FRET efficiency involving rhodamine-B derivatized systems is complicated as the dynamics and equilibria of their complexation induced spirolactam to ring-opened transformations are driven by various influential parameters; despite this, the FRET efficiencies in 1 and other intermediates were determined and compared under similar conditions for a better comprehension of individual fluorophoric contribution in two-step FRET-based sensing. The selectivity in chelation induced two-step FRET mediated fluorescence signalling in 1 is controlled by a coordination environment and solvent medium; it exhibited Hg2+ ion selective signalling responses in aqueous-MeCN medium while rendering discriminatory responses towards Fe3+/Fe2+ redox couples in MeCN medium.

Dual mode signaling responses of a rhodamine based probe and its immobilization onto a silica gel surface for specific mercury ion detection.

A 3-aminomethyl-(2-amino-1-pyridyl) coupled amino-ethyl-rhodamine-B based probe (2) exhibited simultaneous chromogenic and fluorogenic dual mode signaling responses in the presence of Hg(II) ions only among all the metal ions investigated in an organic aqueous medium. The spiro-cyclic rhodamine signaling subunit undergoes complexation induced structural transformation to result in absorption and fluorescence modulation. Its complexation induced signaling exhibited reversibility with various contrasting reagents having higher affinity towards Hg(II) ions, such as anions (AcO(-)) and competing chelating agents (En). It also exhibited Hg(II)-specific photophysical signaling responses when immobilized onto a silica gel surface attached through its amino-ethyl-receptor end, owing to its structure-conformational advantages for effective coordination. The surface modified silica appended with 2 (SiR-1), as evaluated through the FTIR spectral pattern, thermogravimetric analysis, FESEM images, elemental analysis, X-ray diffraction, surface area determination and particle size analysis, also exhibited reversible Hg(II)-specific signaling in its suspension state in an aqueous medium, enhancing the probe's utility for practical applications such as the detection, isolation and extraction of Hg(II) ions in the presence of other competitive metal ions.

Water disinfection through photoactive modified titania.

TiO(2), N-TiO(2) and S-TiO(2) samples have been prepared by various chemical methods. These samples were characterized by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), Laser Raman spectrometer, UV-Visible spectrophotometer, field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). X-ray powder diffraction study reveals that all three samples are single anatase phase of titania and the crystallinity of titania decreases with sulphur doping whereas nitrogen doping does not affect it. UV-Visible (diffuse) reflectance spectra shows that doping of titania with nitrogen and sulphur shift the absorption edge of titania from ultraviolet to visible region. XPS study confirms that both nitrogen and sulphur are well doped in the titania lattice. It is observed that nitrogen occupies at both substitutional and interstitial position in the lattice of titania. FE-SEM and TEM studies demonstrate that the particles are below 50nm range. It is found that S and N doping of titania increased its water disinfection property in the order TiO(2)

Rhodamine-based probes for metal ion-induced chromo-/fluorogenic dual signaling and their selectivity towards Hg(II) ion.

The new signaling probes 2-6, rhodamine-B derivatives of various receptors which contain different donor atoms for effective metal ion coordination, were synthesized and their absorption as well as fluorescence spectral responses were evaluated in the presence of various metal ions. All these probes along with the reference probe 1 have exhibited optimal metal ion-induced absorption and fluorescence enhancement with Hg(II) ion in the longer wavelength region (>500 nm) in MeCN, exploiting the spectral characteristics of metal ion-induced structural transformation of rhodamine. The selectivity and sensitivity towards Hg(II) ion were better pronounced in MeCN-H(2)O (1 : 1 v/v) medium, implying the role of the solvent molecules, water in particular, in the preferential Hg(II) coordination environment. Complexation of Hg(II) to 1-6 not only enhanced the absorption at ~560 nm, which turned the colourless solution into pink to facilitate a naked eye detection, but also amplified the fluorescence intensity simultaneously to offer high sensitivity of detection at lower concentration. The Hg(II)-induced photo-physical spectral responses of 1-6 in presence of other competitive metal ions rendered their high selectivity towards Hg(II). Further, their reversible dual channel signaling pattern under the action of counter anions, exploiting coordination tendency of anions towards Hg(II), which compete with probe-metal interaction, implied the reversibility in their Hg(II) coordination. The selectivity, sensitivity and reversibility, in principle, establishes the potential of these probes as chemosensors for Hg(II) ion detection.

Water induced chromogenic and fluorogenic signal modulation in a bi-fluorophore appended acyclic amino-receptor system.

An acyclic amino-receptor based bi-fluorophoric signaling system 3 exhibits water-induced simultaneous dual channel chromogenic and fluorogenic signal modulation. Its micromolar solution in various organic solvents exhibits an enhancement in absorption in the presence of water in trace amounts through the water-induced delactonization of rhodamine dye, rendering a visual perception as a function of colour change. The presence of water molecules also facilitates a fluorescence resonance energy transfer (FRET) from the excited nitro-benz-oxa-diazole fluorophore to rhodamine dye of 3 and leads to an enhancement of emission up to a second order of magnitude. The failure of the reference compounds 2 and 4 to respond under similar conditions reveals the role of structural variation of the receptor on the perturbation of photophysical processes.