N. Vasimalai

Picomolar melamine enhanced the fluorescence of gold nanoparticles: Spectrofluorimetric determination of melamine in milk and infant formulas using functionalized triazole capped goldnanoparticles

We wish to report a simple and sensitive method to determine the melamine in milk and infant formulas using 3-amino-5-mercapto-1,2,4-triazole capped gold nanoparticles (AMTr-AuNPs) as fluorophore. The AMTr-AuNPs were synthesized by a wet chemical method and were characterized by high-resolution transmission electron microscopy (HR-TEM), and X-ray diffraction, UV-visible and fluorescence spectroscopic techniques. The AMTr-AuNPs show the absorption maximum at 520 nm and emission maximum at 759 nm (λ(ex)=520 nm). While adding 10 μM melamine, the wine red color of AMTr-AuNPs was changed into purple and the absorption band at 520 nm was decreased. The observed changes were ascribed to the hydrogen bonding interaction between melamine and AMTr-AuNPs, which led to the aggregation of the nanoparticles. This was confirmed by dynamic light scattering and HR-TEM measurements. No appreciable absorption change was observed for AMTr-AuNPs in the presence of less than micromolar concentrations of melamine. But, the emission intensity of AMTr-AuNPs was enhanced even in the presence of picomolar concentration of melamine. Based on the enhancement of emission intensity, the concentration of melamine was determined. The present fluorophore showed an extreme selectivity towards the determination of 100 nM melamine in the presence of 500-fold common interferents. The good linearly was observed from 1×10⁻⁹ to 100×10⁻¹² M melamine and a detection limit was found to be 10 fM/L (S/N=3). The proposed method was successfully applied to determine melamine in cow milk and infant formulas. The obtained results were validated with HPLC.

Ultrasensitive fluorescence-quenched chemosensor for Hg(II) in aqueous solution based on mercaptothiadiazole capped silver nanoparticles

This manuscript describes a highly selective and ultra sensitive determination of Hg(II) in aqueous solution using functionalized mercaptothiadiazole capped silver nanoparticles (AgNPs) by spectrofluorimetry. We have synthesized 2,5-dimercapto-1,3,4-thiadiazole (DMT), 2-mercapto-5-methyl-1,3,4-thiadiazole (MMT) and 2-mercapto-5-amino-1,3,4-thiadiazole (AMT) capped AgNPs by wet chemical method. Among these AgNPs, DMT capped AgNPs (DMT-AgNPs) were more stable and highly fluorescent than the other two AgNPs. DMT-AgNPs show the emission maximum at 677 nm while exciting at 400 nm. After the addition of Hg(II), the emission intensity was decreased at 677 nm. The observed decreased emission intensity was ascribed to the aggregation of AgNPs and it was confirmed by TEM. Based on the decrease in emission intensity, the concentration of Hg(II) was determined. The lowest detection limit (LOD=3S/m) of 1.0 pg L(-1) was achieved for the first time using DMT-AgNPs by spectrofluorimetry. The quantum yield (φ(F)), Stern-Volmer constant (K(SV)), Gibbs free energy changes (ΔG°), association constant (K(f)) were calculated and the quenching mechanism also was discussed. Finally, the proposed method was successfully utilized for the determination of Hg(II) in river water, industrial effluent water and tap water samples. The obtained results were fairly matches with the ICP-AES method.

Mercaptothiadiazole capped gold nanoparticles as fluorophore for the determination of nanomolar mercury(II) in aqueous solution in the presence of 50 000-fold major interferents

The present work describes the determination of picogram Hg(II) using 2,5-dimercapto-1,3,4-thiadiazole stabilized gold nanoparticles (DMT-AuNPs) by a spectrofluorimetry method. DMT-AuNPs show emission maximum at 773 nm with excitation at 514 nm. They show a large stock shift (259 nm), narrow emission profile and good photostability. While adding 10 μM Hg(II) the red color solution of DMT-AuNPs changes to purple and the UV-visible spectrum of DMT-AuNPs band at 514 nm was decreased. This is due to aggregation of DMT-AuNPs and it was confirmed by high resolution transmission electron microscopy (HR-TEM). UV-visible spectra of DMT-AuNPs in the presence of nanomolar concentrations of Hg(II) do not show any significant changes at 514 nm. However, the emission intensity of DMT-AuNPs was enhanced during adding even at picomolar concentration of Hg(II) due to photoinduced electron transfer and metal binding-induced conformational restriction upon complexation. Based on the enhancement of emission intensity the concentration of Hg(II) was determined. The binding constant (K(A) = 2.6514 × 10(4) mol(-1) L) value suggested that there is a strong binding force between Hg(II) and DMT-AuNPs. The present fluorophore showed an extreme selectivity towards Hg(II). The emission intensity was increased linearly against a wide range of Hg(II) concentration from 1 × 10(-12) to 1 × 10(-7) M and a detection limit of 0.64 pg L(-1) Hg(II) (S/N = 3) was achieved for the first time using DMT-AuNPs by spectrofluorimetry method. The proposed method was successfully applied for the determination of Hg(II) in environmental samples. The obtained results were validated by ICP-AES.

Micromolar Hg(II) induced the morphology of gold nanoparticles: a novel luminescent sensor for femtomolar Hg(II) using triazole capped gold nanoparticles as a fluorophore

In this paper, we report the morphological changes of gold nanoparticles induced by micromolar Hg(II) and the determination of femtomolar Hg(II) by a luminescent method. 3,5-Diamino-1,2,4-triazole capped gold nanoparticles (DAT-AuNPs) were synthesized by a wet chemical method. The HR-TEM images show that the spherical structure of DAT-AuNPs was changed into a chain-like structure after the addition of micromolar Hg(II) due to the strong coordination of DAT-AuNPs with Hg(II). The binding of Hg(II) with DAT-AuNPs was confirmed by XPS. The XPS of Hg5p shows two peaks at 69.3 eV for 5p1 and 74.35 eV for 5p3, suggesting that mercury was present as Hg(II), coordinated with DAT-AuNPs. The DAT-AuNPs show the emission maximum at 776 nm while exciting at 520 nm and the emission intensity was enhanced after the addition of even nanomolar Hg(II). The quantum yield estimated for DAT-AuNPs in the presence of Hg(II) was 1.5-fold higher than that of free DAT-AuNPs. This suggests that Hg(II) induced the fluorescence properties of DAT-AuNPs due to photoinduced electron transfer and metal binding-induced conformational restriction upon complexation. Based on the enhancement of emission intensity, the concentration of Hg(II) was determined. Further, the DAT-AuNPs showed an extreme selectivity towards the determination of 10 nM Hg(II) in the presence of a 50 000-fold higher concentration of common interferents. The emission intensity increases linearly in the concentration range of 1 × 10−7 to 5 × 10−13 M Hg(II) and the detection limit was found to be 0.75 fM L−1 Hg(II) (S/N = 3). This method was successfully utilized for the determination of Hg(II) in environmental samples.

Biopolymer capped silver nanoparticles as fluorophore for ultrasensitive and selective determination of malathion.

This paper describes a novel luminescent sensor for malathion using chitosan capped silver nanoparticles (Chi-AgNPs) as fluorophore. The Chi-AgNPs were synthesized by the wet-chemical method and were characterized by absorption, fluorescence, HR-TEM, XRD and DLS techniques. The Chi-AgNPs show the absorption maximum at 394 nm and emission maximum at 536 nm. While adding 10 µM malathion, yellow color Chi-AgNPs was changed to brown and the absorbance was decreased along with a redshift. The observed spectral and color changes were mainly due to the aggregation of Chi-AgNPs. This was confirmed by zeta potential, DLS and HR-TEM studies. No significant absorption spectral change was observed for Chi-AgNPs in the presence of less than micromolar concentrations of malathion. However, the emission intensity of Chi-AgNPs was decreased and the emission maximum was shifted toward higher wavelength in the presence of picomolar concentration of malathion. Based on the decrease in emission intensity, the concentration of malathion was determined. The Stern-Volmer constant, Gibbs free energy change, association constant, quantum yield and binding constant were calculated and the quenching mechanism was proposed. The Chi-AgNPs show good selectivity toward the determination of 10nM malathion in the presence of 1000-fold higher concentrations of common interferents. A good linearity was observed for the emission intensity against 1 × 10(-9)-10 × 10(-12)M malathion and the detection limit was found to be 94 fM L(-1) (S/N=3). The proposed method was successfully applied to determine malathion in fruits and water samples and the obtained results were validated with HPLC.

Aggregation and de-aggregation of gold nanoparticles induced by polyionic drugs: spectrofluorimetric determination of picogram amounts of protamine and heparin drugs in the presence of 1000-fold concentration of major interferences

Spectrofluorimetric determination of protamine and heparin was reported using folic acid capped gold nanoparticles (FA-AuNPs) as fluorophore. The FA-AuNPs were synthesized by a wet chemical method and were characterized by UV-visible, photoluminescence, HR-TEM and XRD techniques. They show an absorption maximum at 510 nm and an emission maximum at 780 nm (λex: 510 nm). On addition of 0.05 μg mL-1 protamine, the wine red color of FA-AuNPs turned purple and the absorption maximum attained a red shift. The observed spectral and color changes were attributed to the aggregation of FA-AuNPs and this was confirmed by HR-TEM. Interestingly, on addition of 0.5 μg mL-1 heparin into aggregated FA-AuNPs, the absorption maximum attained a blue shift and the wine red color reverted back. The observed spectral and color changes were due to the strong coordination of protamine with heparin which leads to de-aggregation of AuNPs. Intriguingly, addition of 25 pg mL-1 protamine decreased the emission intensity of FA-AuNPs at 780 nm even in the presence of 1000-fold higher concentrations of Na+, K+, Ca2+, Mg2+, Fe2+, SO4 2-, Cl-, PO4 3- NO3 -, ascorbic acid, glucose interferences and bovine serum albumin interferences. In contrast, addition of 65 pg mL-1 heparin into aggregated FA-AuNPs enhanced their emission intensity at 780 nm in the presence of 1040-fold higher concentrations of the above-mentioned interferences. Based on the increase and decrease in emission intensities, the concentrations of protamine and heparin, respectively, were determined. The lowest detection limits were found to be 4.8 × 10-15 g mL-1 for protamine and 12.6 × 10-15 g mL-1 for heparin (S/N = 3). The present method was successfully applied to determine protamine and heparin in human blood serum samples.

Spectrofluorimetric determination of picogram level Pb(II) using a dimercaptothiadiazole fluorophore

This paper describes the spectrofluorimetric determination of picogram level Pb(II) using 2,5-dimercapto-1,3,4-thiadiazole (DMT) as a fluorophore. Excitation of DMT at 330 nm shows an emission maximum at 435 nm. The colorless solution of DMT changes into highly emittive yellow color immediately after the addition of 0.5 μM Pb(II) and nearly 245-fold increase in emission intensity at 435 nm was observed. These changes were attributed to the complex formation between Pb(II). The emission intensity linearly increases in the concentration range of 10-100 nM Pb(II) and DMT. Based on the fluorescence enhancement, the concentration of Pb(II) was determined. Interestingly, the emission intensity was increased even in the presence of 0.1 pM Pb(II). The fluorophore showed an extreme selectivity towards 100 nM Pb(II) even in the presence of 50,000-fold higher concentrations of common metal ions interferences such as Na(+), K(+), Ca(2+), Mg(2+), Fe(2+), Cd(2+), Cr(3+), Mn(2+), Zn(2+), Co(2+), Ni(2+) and 5000-, 100- and 40-fold of Cu(2+), Hg(2+) and Ag(+) ions, respectively. The lowest detection of 20 pg L(-1) Pb(II) was achieved for the first time using DMT. The proposed method was successfully utilized for the determination of Pb(II) in tap water, polluted river water and industrial waste water samples. The results obtained in the present study were validated with both AAS and ICP-AES methods.

Protein protected gold nanoparticles as a fluorophore for the highly selective and ultrasensitive determination of bisphenol A in plastic samples

We wish to report a highly selective and ultrasensitive chemosensor for bisphenol A in plastic samples using bovine serum albumin capped gold nanoparticles (BSA–AuNPs) as the fluorophore. The BSA–AuNPs were synthesized by a wet chemical method and were characterized by UV-visible, fluorescence, high resolution transmission electron microscopy (HR-TEM) and X-ray diffraction techniques. The BSA–AuNPs show the absorption maximum at 523 nm and the emission maximum at 765 nm (λex: 523 nm). Interestingly, after the addition of 10 μM bisphenol A, the wine red color of BSA–AuNPs was changed to purple and the surface plasmon resonance (SPR) band at 523 nm was red shifted with decreased absorbance. The observed spectral and color changes were ascribed to the aggregation of AuNPs and this was confirmed by HR-TEM. However, no spectral changes were observed for BSA–AuNPs in the presence of nanomolar range concentrations. Interestingly, the emission intensity was enhanced at 765 nm for the addition of 10 pM bisphenol A. Based on the enhancement of emission intensity, the concentration of bisphenol A was determined. The present fluorophore showed preferential selectivity towards the determination of 1 nM bisphenol A in the presence of 1000-fold higher concentration of common interferences. A good linearity was observed from 1 × 10−9 to 100 × 10−12 M bisphenol A and the detection limit was found to be 2.3 fM L−1 (S/N = 3). The proposed method was successfully utilized to determine bisphenol A in a PVC drinking cup, babys dummy and food packaging, tap water and river water samples. The obtained results were validated by HPLC.

Economically viable sensitive and selective luminescent sensor for the determination of Au(III) in environmental samples

This paper describes the ultrasensitive and selective determination of Au(III) in an aqueous solution using a functionalized mercapto thiadiazole ligand. UV-visible and spectrofluorimetry studies of functionalized mercapto thiadiazole ligands such as 2,5-dimercapto-1,3,4-thiadiazole (DMT), 2-amino-5-mercapto-1,3,4-thiadiazole (AMT) and 2-mercapto-5-methyl-1,3,4-thiadiazole (MMT) were carried out in the presence of Au(III) ions in solution. DMT, AMT and MMT exhibit an absorption maximum at 330, 310 and 299 nm, respectively. The emission intensities of the respective compounds were enhanced at 435, 428 and 442 nm after the addition of 1 nM Au(III); furthermore, the color of the solutions also changed to yellow. The observed color changes and emission intensity enhancement are ascribed to the effective complex formation of Au(III) with DMT, AMT and MMT ligands. When 8 nM Au(III) was added into the aqueous solutions of DMT, AMT and MMT, the emission intensity was enhanced to 102, 8 and 5-fold, respectively. The binding constants for DMT, AMT and MMT–Au(III) complexes were found to be 1.52, 1.05 and 1.04 × 105 mol−1 L, respectively. The higher emission intensity and binding constant value obtained for DMT reveals that the DMT–Au(III) complex is highly fluorescent compared to the other two complexes. Thus, DMT was chosen as a fluorophore for the determination of Au(III). Interestingly, even after the addition of 1 pM Au(III) into DMT solution, the emission intensity was enhanced at 435 nm. Based on the enhancement of emission intensity, we have determined the concentration of Au(III) and the detection limit was found to be 1 pg L−1 (S/N = 3). Furthermore, 60 000-fold higher concentrations of common interferences and 500-fold higher concentration of Cu(II), Pb(II), Ag(I), Ag(II) and Ag(III) did not interfere for the determination of 8 nM Au(III). The proposed method was successfully applied to determine Au(III) in different water samples and the obtained results were validated with ICP-AES. The present method of determination has several advantages, including low cost and environmental friendly nature.