Govind B. Kolekar

Highly Selective and Sensitive Recognition of Cobalt(II) Ions Directly in Aqueous Solution Using Carboxyl-Functionalized CdS Quantum Dots as a Naked Eye Colorimetric Probe: Applications to Environmental Analysis

Quantum dots (QDs) are usually used as fluorescent probe, and they are difficult to use in colorimetric detection. However, in this report carboxyl-functionalized CdS (COF-CdS) QDs were synthesized in aqueous solution for colorimetric detection following a classic method. On the basis of inducing the aggregation of COF-CdS QDs, a simple naked eye colorimetric method with high sensitivity and selectivity was developed for the sensing of Co(2+) ions in aqueous solutions. The Co(2+) ions induced COF-CdS QDs results in a marked enhancement of the UV-vis absorption spectra at 360 nm, and the process was accompanied by a visible color change from colorless to yellowish brown within 5 min, which proves a sensitive detection of Co(2+) ions. The sensing of Co(2+) ions can therefore be easily achieved by a UV-vis spectrophotometer or even by the naked eye. Under the optimized circumstances, this method yields excellent sensitivity (LOD = 0.23 μg mL(-1)) and selectivity toward Co(2+) ions. The calibration plot of (A - A(0)) at 360 nm against concentration of Co(2+) ions was linear over the range from 0.5 to 14 μg mL(-1) with a correlation coefficient of 0.9996. The accuracy and reliability of the method were further ascertained by recovery studies via standard addition method with percent recoveries in the range of 99.63-102.46%. The plausible mechanism for the color change reaction has also been discussed. Our attempt may provide a cost-effective, rapid, and simple solution for the inspection of Co(2+) ions in the presence of a complex matrix from environmental aqueous samples.

A spectral deciphering the perturbation of model transporter protein (HSA) by antibacterial pyrimidine derivative: Pharmacokinetic and biophysical insights

Steady state fluorescence and UV-vis absorption spectroscopic techniques have been exploited to explore the binding interaction of a antibacterial pyrimidine derivative 2-amino-6-hydroxy-4-(4-hydroxyphenyl)-pyrimidine-5-carbonitrile (AHHPPC) with the model transporter protein, human serum albumin (HSA) under the physiological conditions. It exhibits antibacterial activity against Escherichia coli and Staphylococcus aureus. Analysis of fluorescence quenching data of HSA at different temperatures using Stern-Volmer methods revealed the formation of AHHPPC-HSA complex with binding affinities of the order 10(4) M(-1). The binding site number (n≈1) and corresponding thermodynamic parameters (ΔG), (ΔH) and (ΔS) were calculated, indicated that binding reaction was endothermic and the hydrophobic interactions plays a major role in stabilizing the complex. The binding distance (r=3.13 nm) between donor (HSA) and acceptor (AHHPPC) was obtained according to FRET. Changes in the albumin secondary structure imparted by the compound was confirmed using synchronous fluorescence, electronic absorption, circular dichroism (CD) and three-dimensional (3D) fluorescence spectroscopy. All these experimental results clarified that AHHPPC could bind to HSA and be effectively transported and eliminated in body, which could be a useful guideline for further drug design.

Fluorescence resonance energy transfer from tryptophan to folic acid in micellar media and deionised water.

The fluorescence resonance energy transfer (FRET) from tryptophan (Trp) to folic acid (FA) in aqueous sodiumdodecyl sulphate, cetyltrimethyl ammonium bromide, and Brij-35 as well as deionised water was investigated using steady state and time resolved fluorescence techniques. The data obtained from steady state fluorescence spectral studies and time resolved measurement indicated that the FRET from Trp to FA occurred most effectively in aqueous sodium dodecyl sulphate micellar solutions. The distance between Trp and FA were evaluated. Binding constant, number of binding sites and thermodynamic parameters were determined for Trp-FA interactions in deionised water. The values of the thermodynamic parameters suggest that the hydrophobic forces and hydrogen bonding are the key interacting forces between Trp-FA interaction.

A novel method for ranitidine hydrochloride determination in aqueous solution based on fluorescence quenching of functionalised CdS QDs through photoinduced charge transfer process: Spectroscopic approach

A novel method for the quantitative determination of ranitidine hydrochloride (RNH) based on the fluorescence quenching of functionalised CdS quantum dots (QDs) by RNH in aqueous solution was proposed. The method is simple, rapid, specific and highly sensitive with good precision. The thioglycolic acid (TGA)-capped CdS QDs were synthesized from cadmium nitrate and sodium sulfide in alkaline solution. Under the optimal conditions, the Stern-Volmer calibration plot of F(0)/F against concentration of RNH was linear in the range of 0.50-15.0 μg mL(-1) with a correlation coefficient of 0.996. The limit of detection (LOD) was 0.38 μg mL(-1). The method was satisfactorily applied to the direct determination of RNH in pharmaceutical formulations with no significant interference from excipients. The results were found to be in good agreement with those obtained by the reference method and the claimed value. The accuracy and reliability of the method were further ascertained by recovery studies via the standard-addition method, with percentage recoveries in the range of 98.47 to 102.30%. The possible fluorescence quenching mechanism for the reaction was also discussed.

Cetyltrimethylammonium Bromide Capped 9-Anthraldehyde Nanoparticles for Selective Recognition of Phosphate Anion in Aqueous Solution Based on Fluorescence Quenching and Application for Analysis of Chloroquine

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 PO43− 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 (Ksv) 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.

Fluorescence quenching studies of CTAB stabilized perylene nanoparticles for the determination of Cr(VI) from environmental samples: spectroscopic approach

Cetyl trimethyl ammonium bromide (CTAB) stabilized perylene nanoparticles (PNPs) were prepared by a modified reprecipitation method in aqueous solution under ultrasonic treatment. A spectrofluorimetric method for the quantitative determination of hexavalent chromium (Cr(VI), dichromate species) based on the fluorescence (FL) quenching of CTAB-stabilized PNPs (CTAB-PNPs) in aqueous solution was proposed. Under the most favourable conditions, the FL intensity of PNPs monitored at an excitation wavelength of λex = 382 nm was quenched by the successive addition of increasing concentrations of dichromate ions. The FL quenching results were found to fit the Stern–Volmer (S–V) relationship in the range of 0.5–50 μg mL−1 with a correlation coefficient of 0.9997. The limit of detection (LOD) was 0.008 μg mL−1. The method based on FL quenching was successfully applied for the quantitative analysis of Cr(VI) in water samples collected from different environments.

A novel pyrimidine derivative as a fluorescent chemosensor for highly selective detection of Aluminum (III) in aqueous media

An efficient fluorescent chemosensor Al(3+) receptor based on pyrimidine derivative,2-amino-6-hydroxy-4-(4-N,N-dimethylaminophenyl)-pyrimidine-5-carbonitrile (DMAB), has been synthesized by three-component condensation of aromatic aldehyde, ethyl cyanoacetate and guanidine hydrochloride in ethanol under alkaline medium. High selectivity and sensitivity of DMAB towards Aluminum ion (Al(3+)) in water: ethanol and acetate buffer at pH 4.0 makes it suitable to detect Al(3+) with steady-state UV-vis and fluorescence spectroscopy. Method shows good selectivity towards Al(3+) over other coexisting metal ions tested, viz. Fe(2+), Ni(2+), Cu(2+), Co(2+), Pb(2+), Sb(3+), Na(+), Ca(2+), Mg(2+), Zn(2+), Hg(2+), Ba(2+), Cd(2+) and K(+). A good linearity between the Stern-Volmer plots of F0/F versus concentration of Al(3+) was observed over the range from 10 to 60 μg mL(-1) with correlation coefficient of 0.991. The accuracy and reliability of the method were further confirmed by recovery studies via standard addition method with percent recoveries in the range of 101.03-103.44% and lowest detection limit (LOD=7.35 μg mL(-1)) for Al(3+) was established. This method may offer a new cost-effective, rapid, and simple key to the inspection of Al(3+) ions in water samples in the presence of a complex matrix and can be capable of evaluating the exceeding standard of Al(3+) in environmental water samples. The probable mechanism for fluorescence quenching was also discussed.

Evaluation of interparticle interaction between colloidal Ag nanoparticles coated with trisodium citrate and safranine by using FRET: Spectroscopic and mechanistic approach

Current study employs fluorescence spectroscopy, UV-Vis absorbance spectroscopy, dynamic light scattering (DLS) and cyclic voltammetry (CV) to investigate the interaction of safranine dye with spherical shaped silver nanoparticles (AgNPs) coated with trisodium citrate. In fluorescence spectroscopic study we used the AgNPs and safranine dye as component molecules for the construction of FRET, whereas AgNPs serve as donor fluorophore and safranine as acceptor. The fluorescence quenching of AgNPs followed by sensitization of safranine occurs almost simultaneously by addition of safranine dye with different concentrations, indicating fluorescence energy transfer observed between them. Interaction between safranine and AgNPs is also confirmed by using UV-Vis absorption spectroscopy. Addition of safranine results in the significant decrease in the absorbance of AgNPs at 423 nm and simultaneous increase in the absorbance of safranine at 518, 276 and 248 nm which is indication of rapid binding of safranine molecules with AgNPs. However CV measurements reveals that the safranine molecule does not alter the redox properties of the AgNPs but the safranine molecule lose their redox properties upon getting bonded with AgNPs. This clearly confirms that the safranine molecules get attached on the surface of AgNPs which was also supported by the DLS as well as zeta potential measurement.

A novel FRET probe for selective and sensitive determination of vitamin B12 by functionalized CdS QDs in aqueous media: applications to pharmaceutical and biomedical analysis

Vitamin B12 (VB12) is one of the essential B complex vitamins, it is a water soluble nutrient and a medicine. Herein we developed, a novel but straightforward method for the selective and sensitive quantitative determination of VB12 based upon measuring the fluorescence quenching of functionalized CdS quantum dots (QDs) in aqueous media. The mercaptopropionic acid (MPA) functionalized CdS QDs were directly synthesized from cadmium chloride and sodium sulfide in aqueous media by a chemical method. The detection mechanism involves photoinduced charge transfer from CdS QDs towards VB12 at the surface of the CdS QDs resulting in the fluorescence quenching of CdS QDs followed by nonradiative fluorescence resonance energy transfer. Under the optimized conditions, the relationship between the fluorescence intensity of the CdS QDs and VB12 concentration was linear in the range of 5.0–100 μg mL−1 with a limit of detection of 6.91 μg mL−1. The selectivity experiment indicated excellent selectivity for VB12 over a number of interfering species. Moreover the method was successfully applied in the determination of VB12 in complex matrices, namely blood serum, urine and pharmaceutical formulations (multivitamin injection), without sample pretreatment. The accuracy and reliability of the method was further ascertained by recovery studies via the standard addition method with percentage recoveries in the range of 97.15%–99.49% being obtained. In addition, the probable fluorescence quenching mechanism of the CdS QDs induced by VB12 is also discussed.

Off–on fluorescent polyanthracene for recognition of ferric and fluoride ions in aqueous acidic media: application in pharmaceutical and environmental analysis

Fluorescent polyanthracene prepared by a chemical oxidative route was shown to sense ferric ions (Fe3+) in aqueous acidic solutions. The fluorescence titration indicates that the intensity of fluorescence of the polyanthracene (PAT) was gradually quenched by the successive addition of an Fe3+ ion solution (‘Off-fluorescence’). The fluorescence quenching data fit well into the conventional Stern–Volmer relationship. A mechanism of Photoinduced Electron Transfer (PET) is proposed to explain the observed ‘Off fluorescence’ state of the PAT. Based on the strong affinity of Fe3+ to F− ions, the mixture of polyanthracene–F− ions exhibiting ‘On fluorescence’, could behave as a fluorescent ‘On type probe’. Thus a method using a single probe for the detection of both Fe3+ and F− ions is developed using a fluorescence ‘Off–On’ approach, and is successfully applied for the determination of these ions in commercial samples.