Cadmium-free quantum dots as time-gated bioimaging probes in highly-autofluorescent human breast cancer cells.

We report cadmium-free, biocompatible (Zn)CuInS(2) quantum dots with long fluorescence lifetimes as superior bioimaging probes using time-gated detection to suppress cell autofluorescence and improve the signal : background ratio by an order of magnitude. These results will be important for developing non-toxic fluorescence imaging probes for ultrasensitive biomedical diagnostics.

Synthesis, characterization, photophysical properties of a novel organic photoswitchable dyad in its pristine and hybrid nanocomposite forms.

In the present paper the method of synthesis and characterization of a novel organic dyad, 3-(1-Methoxy-3,4-dihydro-naphthalyn-2-yl-)-1-p-chlorophenyl propenone, have been reported. In this paper our main thrust is to fabricate new hybrid nanocomposites by combining the organic dyad with different noble metals, semiconductor nanoparticle and noble metal-semiconductor core/shell nanocomposites. In this organic dyad, donor part is 1-Methoxy-3, 4-dihydro-naphthalen-2-carboxaldehyde with the acceptor p-chloroacetophenone. We have carried out steady state and time-resolved spectroscopic measurements on the dyad and its hybrid nanocomposite systems. Some quantum chemical calculations have also been done using Gaussian 03 software to support the experimental findings by theoretical point of view. Both from the theoretical predictions and NMR studies it reveals that in the ground state only extended (E-type or trans-type) conformation of the dyad exists whereas on photoexcitation these elongated conformers are converted into folded forms (Z- or cis-type) of the dyad, showing its photoswitchable character. Time resolved fluorescence spectroscopic (fluorescence lifetime by TCSPC method) measurements demonstrate that in chloroform medium all the organic-inorganic hybrid nanocomposites, studied in the present investigation, possess larger amount of extended conformers relative to folded ones, even in the excited singlet state. This indicates the possibility of slower energy destructive charge recombination rates relative to the rate processes associate with charge-separation within the dyad. It was found that in CHCl3 medium, the computed charge separation rate was found to be approximately 10(8) s(-1) for the dyad alone and other hybrid nanocomposite systems. The rate is found to be faster than the energy wasting charge recombination rate approximately 10(2)-10(1) s(-1), as observed from the transient absorption measurements for the corresponding hybrid systems. It indicates the conformational geometry has a great effect on the charge-separation and recombination rate processes. The suitability for the construction of efficient light energy conversion devices especially with Ag-Dyad nanocomposite of all the systems studied here is hinted from the observed long ion-pair lifetime.

The rates of charge separation and energy destructive charge recombination processes within an organic dyad in presence of metal-semiconductor core shell nanocomposites.

Steady state and time resolved spectroscopic measurements were made at the ambient temperature on an organic dyad, 1-(4-Chloro-phenyl)-3-(4-methoxy-naphthalen-1-yl)-propenone (MNCA), where the donor 1-methoxynaphthalene (1 MNT) is connected with the acceptor p-chloroacetophenone (PCA) by an unsaturated olefinic bond, in presence of Ag@TiO2 nanoparticles. Time resolved fluorescence and absorption measurements reveal that the rate parameters associated with charge separation, k(CS), within the dyad increases whereas charge recombination rate k(CR) reduces significantly when the surrounding medium is changed from only chloroform to mixture of chloroform and Ag@TiO2 (noble metal-semiconductor) nanocomposites. The observed results indicate that the dyad being combined with core-shell nanocomposites may form organic-inorganic nanocomposite system useful for developing light energy conversion devices. Use of metal-semiconductor nanoparticles may provide thus new ways to modulate charge recombination processes in light energy conversion devices. From comparison with the results obtained in our earlier investigations with only TiO2 nanoparticles, it is inferred that much improved version of light energy conversion device, where charge-separated species could be protected for longer period of time of the order of millisecond, could be designed by using metal-semiconductor core-shell nanocomposites rather than semiconductor nanoparticles only.

Is dye mixture more suitable rather than single dye to fabricate dye sensitized solar cell?

The steady state and time resolved spectroscopic studies reveal that two xanthene dyes Rhodamine 6G (R6G) and Rhodamine B (RB), used in the present investigations, form ground state hydrogen -bonded complexes with meso-tetrakis(4-carboxyphenyl) porphyrin (TCPP). However, it is apparent that upon photoexcitation the H-bonding complexes formed in the ground state decompose into the individual reacting components. This presumption was confirmed from the observation of the presence of only static quenching mode in the steady state fluorescence of the dyes in presence of porphyrin. The photoelectrochemical properties of the free dyes and the mixtures of each dye with porphyrin are investigated by measuring incident photon-to-current conversion efficiency (IPCE) using ZnO electrode and also with TiO2 electrode. It is seen that Rhodamine B-porphyrin mixture has attained maximum IPCE among the four samples studied at approximately 550 nm using ZnO electrode. Using TiO2 electrode, slight improvement in the value of IPCE was found for the same mixture. Therefore Rhodamine B-porphyrin mixture may act as a good sensitizer for converting solar energy to electrical energy.

Interaction and photodegradation characteristics of fluorescein dye in presence of ZnO nanoparticles.

The interaction between xanthene dye Fluorescein (Fl) and zinc oxide (ZnO) nanoparticles is investigated under physiological conditions. From the analysis of the steady state and time resolved spectroscopic studies in aqueous solution static mode is found to be responsible in the mechanism of fluorescence quenching of the dye Fl in presence of ZnO. ZnO nanoparticles are used as photocatalyst in order to degrade Fl dye. At pH 7, a maximum degradation efficiency of 44.4% of the dye has been achieved in presence of ZnO as a nanophotocatalyst and the photodegradation follows second-order kinetics.

Detailed spectroscopic investigations to reveal the nature of interaction of anionic porphyrin with calf thymus DNA.

The interaction between anionic form of meso-tetrakis(4-carboxyphenyl) porphyrin (TCPP) and calf thymus deoxyribonucleic acid (CT DNA) is investigated by measuring UV-vis absorption, steady-state fluorescence, steady-state fluorescence anisotropy, time-resolved fluorescence, resonance light scattering (RLS), FT-IR and circular dichroism (CD) spectra along with the help of atomic force microscopy (AFM) under Tris-Borate-EDTA (TBE) buffer solution of pH 8.3. The static mode of fluorescence quenching of porphyrin by calf thymus deoxyribonucleic acid indicates the formation of a ground-state complex. The formation of ground-state complex is a spontaneous molecular interaction procedure in which outside groove binding through hydrogen bond or van der Waals force plays a major role. For biomedical application this investigation is very important as here TCPP, i.e. the anionic porphyrin does not bring any changes in the original structure of the CT DNA to selectively cleaving the nucleic acid to destroy the cancer or tumor cells whereas cationic porphyrin makes change in the protein structure significantly during the same process.

Interaction of bovine serum albumin and albumin-gold nanoconjugates with l-aspartic acid. A spectroscopic approach.

The interaction of an essential transport protein bovine serum albumin (BSA) and albumin-gold nanoconjugates (BSA-GNPs) with amino acid l-aspartic (ASP) are investigated by steady state and time resolved spectroscopic techniques. In both the cases, static fluorescence quenching is observed indicating that a ground state complex is formed between the donor BSA/BSA-GNP with the acceptor ASP. High values of quenching constant suggest that energy transfer also occurred from BSA and BSA-GNPs to ASP. Distance between the fluorophore in the protein and the amino acid (ASP) is evaluated. Binding constants and the number of binding sites were determined in both the cases. The observed thermodynamic parameters suggest that the key interacting forces involved in both cases are hydrophobic interactions. Circular dichroism (CD) spectrum of BSA molecule suffers marginal change in the presence of ASP both in its pure as well as bio-nanoconjugate forms. As no structural deformation is occurred, the biological activity along with the activity of immune response of protein and the biocompatibility of protein-nanoconjugate remain as such.

Development of a nanocomposite system by combining an organic dyad 1-(4-chloro-phenyl)-3-(4-methoxy-naphthalen-1-yl)-Propenone with semiconductor TiO2 nanoparticles.

Steady state and time resolved spectroscopic measurements on an organic dyad, 1-(4-chlorophenyl)-3-(4-methoxy-naphthalen-1-yl)-propenone, where the donor 1-methoxynaphthalene is connected with the acceptor p-chloroacetophenone by an unsaturated olefinic bond, in presence of TiO2 nanoparticles were made at the ambient temperature. Time resolved fluorescence measurements reveal that the rate parameters associated with charge separation, kCS, within the dyad increases whereas charge recombination rate reduces when the surrounding medium is changed from chloroform to TiO2 nanoparticles. The observed results indicate that the dyad being combined with TiO2 nanoparticles may form organic-inorganic nanocomposite systems useful for developing light energy conversion devices.