Alicia V. Veglia

Cyclodextrin enhanced fluorimetric method for the determination of tryptamine

The effect of native cyclodextrins (alpha-, beta- or gamma- with six, seven or eight glucose units, respectively), hydroxypropyl-beta-cyclodextrin, beta-cyclodextrin solubilized in urea, soluble starch and glucose in water solution on the fluorescence behaviour of tryptamine [3-(2-aminoethyl)indole] (T) was determined. In addition, the effect of methanol and propanol with and without beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin was ascertained. From the fluorescence changes with pH and with beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin, the values of the pKa of the ground and excited states and the association constants of T and TH [3-(2-ammoniumethyl)indole] with the two hosts were determined. The values are pKa = 9.5 +/- 0.2 and pKa* = 8.4 +/- 0.2; KAssocTH = (1.6 +/- 0.3) x 10(2) mol-1 dm3 and KAssocT = (2.8 +/- 0.3) x 10(2) mol-1 dm3 with beta-cyclodextrin, KAssocTH = (1.8 +/- 0.5) x 10(2) mol-1 dm3 and KAssocT = (4.9 +/- 0.9) x 10(2) mol-1 dm3 with hydroxypropyl-beta-cyclodextrin. The ratio of the fluorescence quantum yields for the bound and free substrate (phi b/phi f) were in the range 1.25-1.33. The detection limit for the better conditions where the host-guest interactions produce fluorescence enhancement was 0.454 +/- 0.002 ng ml-1 for the complex T-hydroxypropyl-beta-cyclodextrin in water. The method is simpler than others reported previously.

Spectrofluorimetric determination of benzoimidazolic pesticides: Effect of p-sulfonatocalix[6]arene and cyclodextrins

The effect of the addition of a macrocyclic host (H) such as p-sulfonatocalix[6]arene (C6S), native and modified cyclodextrins (CDs), on the fluorescence of benzoimidazolic fungicides (P), like Benomyl (BY) and Carbendazim (CZ), has been studied. The fluorescence of BY in water at pH 1.000 and 25.0 degrees C was increased in the presence of C6S, alphaCD and hydroxypropyl-beta-CD (HPCD). The association constants determined by fluorescence enhancement showed weak interactions (K(A) approximately 10(1) to 10(2) M(-1)) between the fungicide with both CDs, whereas they were stronger with C6S (K(A) approximately 10(5) M(-1)). Molecular recognition of BY for C6S was mainly attributed to electrostatic interactions, and for CDs to the hydrophobic effect and hydrogen bond formation. On the other hand, the fluorescent behaviour of CZ in the presence of C6S at pH 6.994 was interpreted as the formation of two complexes with 1:1 (P:H) and 1:2 (P:H(2)) stoichiometry, the latter being less fluorescent than the free analyte. Relative fluorescence quantum yield ratios between the complexed and free BY (phi(P:H)/phi(P)) were 2.00+/-0.05, 1.40+/-0.03 and 2.8+/-0.4 for C6S, alphaCD and HPCD, respectively. The analytical parameters improved in the presence of C6S and CDs. The best limit of detection (L(D), ng mL(-1)) was 17.4+/-0.8 with HPCD. The proposed method with C6S and HPCD was successfully applied to fortified samples of tap water and orange flesh extract with good recoveries (91-106%) and R.S.D. (< or = 2%) by triplicate analysis. The method is rapid, direct and simple and needs no previous degradation or derivatization reaction.

Hydroxypropyl-β-cyclodextrin enhanced fluorimetric method for the determination of melatonin and 5-methoxytryptamine

The effects of native cyclodextrins (alpha, beta or gamma), hydroxypropyl-beta-cyclodextrin, beta-cyclodextrin solubilized in urea, soluble starch and glucose in water solution on the fluorescence behaviour of melatonin (N-acetyl-5-methoxytryptamine) (M) and 5-methoxytryptamine [5-methoxy-3-(2-aminoethyl)indole] (5M) were determined. In addition, the effect of methanol and propanol with and without beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin was assessed. From the fluorescence changes with pH, the values of the pKa for the ground (9.9 +/- 0.2) and the excited state (7.7 +/- 0.2) for 5M were determined. From the fluorescence changes with beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin, the association constants of M, 5MH [5-methoxy-3-(2-ammoniumethyl)indole] and 5M with the two hosts were determined. The values with beta-cyclodextrin were KAssoc5MH = (1.4 +/- 0.4) x 10(2) mol-1 dm3, KAssoc5M = (1.6 +/- 0.1) x 10(2) mol-1 dm3 and KAssocM = (1.1 +/- 0.2) x 10(2) mol-1 dm3, and with hydroxypropyl-beta-cyclodextrin KAssoc5MH = (1.1 +/- 0.3) x 10(2) mol-1 dm3, KAssoc5M = (2.5 +/- 0.1) x 10(2) mol-1 dm3 and KAssocM = (1.51 +/- 0.07) x 10(2) mol-1 dm3. The ratios of the fluorescence quantum yields for the bound and free substrate (phi b/phi f) were in the range 1.15-1.48. The detection limits under the optimum conditions were 0.381 +/- 0.001 ng cm-3 for the complex 5MH-hydroxypropyl-beta-cyclodextrin in water and 0.290 +/- 0.001 ng cm-3 for the complex M-hydroxypropyl-beta-cyclodextrin in water with 5% of methanol. The recovery of melatonin from pharmaceutical preparations was 98-103% with an RSD of 2%. The recovery from rat pineals was also good. The method is direct, simple and accurate.

Synthetic Routes for the Preparation of Silver Nanoparticles

In this chapter, we revise some of the most relevant and widely used synthetic routes available for the preparation of metallic silver nanoparticles. Particular emphasis has been focused in the rationale involved in the formation of the nanostructures, from the early metallic silver atoms formation, passing by atoms nucleation and concluding in the growth of silver nanostructures. We hope the reader will find in this chapter a valuable tool to better understand the relevance of the experimental conditions in the resulting silver nanoparticle size, shape and overall properties.

Cucurbit[6]uril nanocavity as an enhanced spectrofluorimetric method for the determination of pyrene

The effect of the addition of macrocyclic host cucurbit[6]uril (CB6) on the photophysical properties of polycyclic aromatic hydrocarbon pyrene (PYR) was analyzed. The fluorescence emission spectra of the aromatic compound were determined at 25.0°C in different acidic media (HCl 18%, w/v, or HCOOH 55%, w/v) with and without CB6. A significant enhancement in the fluorescence signals in the presence of CB6 was observed. The average values of the association constant (K(A)) for the 1:1 stoichiometry complex and the relative fluorescence quantum yield ratio between the complexed and free PYR (φ(PYR-CB6)/φ(PYR)) in acidic media were (4.0 ± 0.5) × 10(2) M(-1) and (5.7 ± 0.2), respectively. The analytical parameters improved in the presence of CB6. The relative decrease in the limit of detection was 92%. The matrix effect was evaluated in fortified samples of tap water and tea extracts. Apparent recoveries obtained by the proposed method in tap water and tea extracts were (82-103)% and (89-99)%, respectively. Selectivity studies with inorganic and organic species were performed. The method is rapid, direct, selective and simple.

Spectrofluorimetric determination of serotonin and 5-hydroxyindoleacetic acid in urine with different cyclodextrin media.

Alternative and sensitive spectrofluorimetric methods for the determination of hydroxyindoles, such as serotonin (5HT) and 5-hydroxyindoleacetic acid (5HIA), were developed on the basis of supramolecular interaction with cyclodextrin (CD) nanocavities (βCD and hydroxypropyl-βCD, HPCD) at different pH values. Both substrates and receptors have acidic protons, therefore the interactions produced in different systems were considered. The effects of neutral CD at pH 2.00 and 6.994, and of anionic CD at pH 13.00 on the specific acid-base species of the compounds at each pH were determined. In all the conditions studied, the fluorescence of the substrates in the presence of CD increased. The association constants (K(A), mol(-1)L) between the substrates and CD were determined (30-300) and interpreted. A zero-crossing first-derivative spectrofluorimetric method with and without HPCD was developed for the simultaneous determination of 5HT and 5HIA. The limits of detection (L(D), ng mL(-1)) for the best conditions were 0.37 for 5HT and 0.50 for 5HIA at pH 2.00 with HPCD. These L(D) proved to be better than others reported. The applicability of the direct and derivative spectrofluorimetric methods to urine samples was demonstrated with good recoveries 92-110% and R.S.D. 1-10%.

Structural characterization of N-methylcarbamate: β-Cyclodextrin complexes by experimental methods and molecular dynamics simulations

Detailed insights regarding the inclusion process between β-cyclodextrin and the N-methylcarbamates insecticides like Bendiocarb, Carbaryl, Carbofuran and Promecarb, are proposed in bases of experimental and computational methods. The results from Fourier transform infrared spectroscopy, differential scanning calorimetry, induced circular dichroism and molecular dynamics indicate that only in the case of Promecarb the interaction with the macrocycle is produced by the alkyl rest of the molecule. In all other cases the aromatic moiety is the part of the insecticide that is partially included in the cavity of β-cyclodextrin.

On the product distribution in the iodination of phenol

Abstract The product distribution in the iodination of phenol with I 2 , in water solution is buffer and pH dependent with the ortho product increasing at low pH and low buffer concentration. This effect is attributed to different rates of proton transfer from the 4-iodo-2,5-; and 2-iodo-3,5-cyclohexadienone intermediate.

Synthesis of ultraluminescent gold core–shell nanoparticles as nanoimaging platforms for biosensing applications based on metal-enhanced fluorescence

Core–shell nanoparticles are versatile nanostructures that can be used as luminescent biosensing platforms in many nanotechnological developments. Ultraluminescent fluorescent gold core–shell nanoparticles based on Metal-Enhanced Fluorescence (MEF) were synthesized. The nanoparticles obtained were formed by 40.0 nm cores and variable silica spacer lengths. Silica spacer lengths from 6.0 to 25.0 nm were obtained. The plasmon maximal wavelength of the core–shell nanoparticles was shifted to a longer wavelength from a gold nanoparticle plasmon centered at 537.0 nm to 545.0 nm and 548 nm from 6.0 nm to 20.0 nm spacer length, respectively. The effect of the gold core on emission was evaluated by determination of Metal Enhanced Fluorescence enhancement factors (MEFEF), applying the sodium cyanide method for core leaching. We observed maximal MEFEF = 8.1 and 7.2 for 6.0 and 14.0 nm, respectively, and a significant decrease at longer silica spacer lengths. From nanoimaging by confocal fluorescence microscopy it was possible to detect ultraluminescent gold core–shell nanoparticle aggregates and obtain an MEFEF that can rise to 40. These parameters and properties were discussed from the point of view of fluorescent platform applications. Moreover in order to show the potential application of these nanoparticles in biodetection and nanomedicine, Escherichia coli bacteria were labelled with ultraluminescent nanoparticles. Bright and clear bacteria images were obtained by laser fluorescence microscopy. Based on these results, future applications for individual bacterial detection will be developed.

Fluorimetric Determination of Carbamate Pesticides in Host-Guest Complexes

From the effect of β-cyclodextrin and hydroxypropyl-β-cyclodextrin on the UVvisible and fluorescence spectra of carbaryl and carbofuran, the values of association constants were determined. The ratio of the fluorescence quantum yields for the bound and free substrates indicated an enhanced fluorimetric method of detection.