CTAB-PNPs are bound to dichromate ion by electrostatic interaction to form stable non-fluorescent micellar complex which is responsible for the ‘FL quenching’ of CTAB-PNPs.

The nanoparticles of highly fluorescent Rubrene prepared by reprecipitation method using Sodium Dodecyl Sulphate (SDS) exhibited narrower particle size distribution when examined by Dynamic Light Scattering (DLS) technique. The average particle size obtained is 87.2 nm and zeta potential -13.8 mV given by zeta sizer indicated that the Rubrene Nanoparticles (RUBNPs) entrapped in SDS surfactant has charged negatively by which it can change the photo absorption and emission properties of aqueous suspension of nanoparticles. The red shifted UV -photo absorption band of RUBNPs in comparison with absorption band of rubrene in Tetrahydrofuran (THF) solution is because of J-type aggregates in aqueous suspension of nanoparticles, which also results into Aggregation Induced Enhanced Emission (AIEE) at λmax = 564 nm. The presence of Fe 2+ in the aqueous suspension of RUBNPs showed quenching of fluorescence at 564 nm and quenching results fits into conventional Stern-Volmer relation in the concentration range of 0-80 μg/mL of Fe 2+ ion solution with good linear relationship. The possible mechanism of fluorescence quenching of RUBNPs is explained by considering adsorption of Fe 2+ cation electrostatistically on the negatively charged surface of the nanoparticle generated by SDS capping. The proposed sensing method ofRUBNPsto selective detection of Fe 2+ ion is successfully applied for quantification of Fe 2+ from pharmaceutical tablet.

Cetyltrimethylammonium bromide (CTAB) capped 9-Anthraldehyde nanoparticles (9-AANPs) in aqueous suspension prepared by reprecipitation method are seen brick shaped in Scanning Electron Microscope image. The Dynamic Light Scattering histogram of nanoparticle suspension reveals narrow particle size distribution and average particle size is 89 nm. The positive zeta potential 20.8 mV measured on zeta sizer indicates high level stability of nanoparticle suspension. The blue shift of 65359.47 cm(-1) observed in the UV-Visible absorption spectrum of CTAB capped 9-AANPs from the absorption maximum of dilute solution of 9-Anthraldehyde (9-AA) in acetone is an indication of formation of H-bonded aggregates by π stacking effect. The strong Aggregation Induced Enhanced Emission (AIEE) of CTAB capped 9-AANPs at 537 nm is selectively quenched with addition of phosphate anion solution. The fluorescence quenching results of the nanoparticle in aqueous solution fit into conventional Stern-Volmer relation in the range of phosphate ion concentration of 0-40 μM. The possible mechanism of fluorescence quenching of nanoparticle is explained by considering adsorption of phosphate anion electrostatistically on positively charged surface of nanoparticle generated by CTAB cap. The Langmuir adsorption plot constructed for PO(4)(3-) adsorption on the basis of fluorescence quenching results of CTAB capped 9-AANPs is linear. The estimated value of Langmuir constant (K) and Stern - Volmer constant (K(sv)) are in close agreement within experimental limits. The sensing method of phosphate ion based on fluorescence quenching of 9-AANPs is applied successfully for quantification of phosphate from pharmaceutical tablet chloroquine phosphate and hence to determine the amount of chloroquine.

Fluorescent Tetracene Nanoparticles (TNPs) have been prepared by a reprecipitation method using cetyl trimethyl ammonium bromide (CTAB) as a stabilizer. These TNPs are more photostable against photobleaching and high solubility in water minimize the utilization of hazardous organic solvents compared with single organic fluorophore in sensing applications. The method based on fluorescence quenching TNPs as a novel fluorescent sensor for selective recognition of fast green FCF dye in aqueous solution. The fluorescence intensity of TNPs was quenched by the successive addition on increasing concentrations of fast green FCF dye. The fluorescence quenching results were found to fit the Stern-Volmer (S-V) relationship in the range of 0.5-7.0 µg L -1 with a correlation coefficient of 0.999. The limit of detection (LOD) was 0.136 µg L -1 . Moreover, the excited state lifetime of TNPs remains unchanged even after increasing concentration fast green FCF dye suggest that fast green FCF dye adsorbed over the surface of the nanoparticles to form non-fluorescent ground state complex i.e. nature of quenching process is static. The proposed method was successfully applied for the quantitative analysis of fast green FCF dye in commercial samples with no necessity of prior separation of analyte molecules form the interfering constituents.

Static quenching process of AIEE active SDS-2-NNPs by CV.