Calixarenes and cyclodextrins as off- and on-fluorescence probes for carbazole.

Absorption and fluorescence spectra of the nitrogen polycyclic aromatic hydrocarbon carbazole (CZL) were analyzed with native cyclodextrins (CD;α,ß,γ); derivatizedCD(hydroxypropyl-ß-cyclodextrin,HPCD; methyl-ß-cyclodextrin,MeCD) and p-sulfonated calixarenes (SCAn, with n = 6 and 8) macrocycles. The results showed a slight increase in the absorbance ofCZLwithCD, but the mixture ofCZLwithSCAshowed lower absorption than the sum of the individual spectra. Also, changes in fluorescence were observed by adding the macrocycles, quenching withSCA, and significant increases withCD. The higher fluorescence enhancement was withHPCDrationalized as a complex formation with 1:1 stoichiometry, with an average value for the association constant (KA) of (12 ± 1) x 102M-1, and a quantum yield ratio between the complexedCZLand freeCZL(ΦCZL-HPCD/ΦCZL) of (1.56 ± 0.02) at neutral pH and 25.0 °C. These increases in fluorescence were used as an on-fluorescence switch to develop a supramolecular analytical method forCZLin aqueous samples. The best analytical parameters were inHPCD(LOD = 1.41 ± 0.01 ng mL-1). The method was validated in aqueous samples of river and tap water with recoveries between 96%-104%. The proposed supramolecular method is quick, direct, selective and represents an alternative and low-cost analysis method.

Fluorescence quenching of p-tert-buthylthiacalix[4]arene by 1-ethyl-3-methylimidazolium bromide.

The interaction between the ionic liquid 1-ethyl-3-methylimidazolium bromide (EMIMBr) and the macrocyclic compound p-tert-buthylthiacalix[4]arene (TC4) in solution ethanol/water 3/1 V/V was investigated by spectrophotometry and spectrofluorimetry. The UV-Vis spectra of the system (EMIMBr + TC4) were completely additive. The addition of EMIMBr decreases the fluorescence of the TC4 significantly, excited at 325.0 nm. The Stern-Volmer quenching constants were determined at different temperatures (15.0, 25.0, and 35.0 °C), and the obtained values were the same, with an average value of KQav = (3.8 ± 0.9) 103 M-1 for the quenching constant. No changes in the emission spectra of TC4 were obtained by the addition of inorganic salts, such as NaCl, KBr, NH4Cl, and NH4PF6 or the organic compound N-methylimidazole (NMI). More evidence of EMIMBr-TC4 interaction was found with the determination of the fluorescence lifetime and the anisotropy of the mixture. Also, the slope of the quenching plot was the analytical sensitivity of calibration for EMIMBr using TC4 as an indirect sensor, and a LOD = (26 ± 2) µM was obtained.

In flow metal-enhanced fluorescence for biolabelling and biodetection.

Escherichia coli bacteria were determined by in flow cytometry with laser excitation and fluorescence detection applying ultraluminescent core-shell nanoparticles based on Metal Enhanced Fluorescence (MEF). Core-shell nanoparticles consisted of a 40 nm core modified with a silica spacer grafted with Rhodamine B (RhB). The electromagnetic field in the near field of the core surface enhanced the fluorescence of RhB by plasmonic and fluorophore coupling. The hydrophilic silica spacer allowed the non-covalent interaction with the polar E. coli surface and thus ultraluminescent bacteria biolabelling was developed. Clearly, well defined and bright bacteria imaging was recorded by Laser Fluorescence Microscopy based on the non-covalent deposition of the ultraluminescent nano-emitters. Using these nano-labellers, it was possible to detect labelled E. coli by in flow cytometry. Higher values of Side-scattered light (SSC) and Forward-scattered light (FSC), and number of fluorescent event detections, were observed for labelled bacteria compared to those non-labelled. The sensitivity of the methodology was evaluated by varying bacteria concentration and acceptable analytical figures of merit were determined. Applying this methodology we could quantify E. coli from a synthetic real sample of fortified water. Similar results were obtained by bacteria counting with Laser Fluorescence Microscopy and with a cell-bacteria counter.

Structural characterization of N-methylcarbamate: ß-cyclodextrin complexes by experimental methods and molecular dynamics simulations.

Detailed insights regarding the inclusion process between ß-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 ß-cyclodextrin.

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 (ßCD and hydroxypropyl-ßCD, 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%.

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.

Determination of poorly fluorescent carbamate pesticides in water, bendiocarb and promecarb, using cyclodextrin nanocavities and related media.

The effect of native cyclodextrins (alpha, beta, or gammaCD with six, seven and eight glucose units, respectively), hydroxypropyl-beta-cyclodextrin (HPCD), chitosan (CHT) and glucose in water solution or water with n-propylamine (PA) as co-solvent upon the UV-vis and fluorescence properties of poorly fluorescent N-methyl carbamates pesticides (C) as bendiocarb (2,2-dimethyl-1,3-benzodioxol-4-ol methylcarbamate, BC) and promecarb (3-methyl-5-(1-methylethyl)phenol methylcarbame, PC) was examined. Fluorescent enhancement was found for both substrates with all CDs in water or PA-water except from PC with alphaCD. The addition of CHT increases the fluorescence of BC but decreases the fluorescence of PC, and glucose addition gives in both cases no spectral changes. Host-guest interaction was clearly determined by fluorescence enhancement with betaCD and HPCD with a 1:1 stoichiometry for the complexes (C:CD). The values obtained for the association constants (K(A), M(-1)) were (6+/-2)x10(2) and (2.3+/-0.3)x10(2) for BC:betaCD and BC:HPCD complexes, respectively. For PC:betaCD and PC:HPCD the values of K(A) were (19+/-2)x10(2) and (21+/-2)x10(2), respectively. The ratio of the fluorescence quantum yields for the bound and free substrates (phi(CCD)/phi(C)) was in the range 1.74-3.8. The limits of detection (L(D), microg mL(-1)) for the best conditions were (0.57+/-0.02) for BC with HPCD and (0.091+/-0.002) for PC with betaCD in water. Application to the analysis in pesticide spiked samples of tap water and fruit yields satisfactory apparent recoveries (84-114%), and for the extraction procedure in fruits and a commercial formulation, recoveries were of 81-98% and 104%, respectively. The method is rapid, simple, direct, sensitive and useful for pesticide analysis.