Fluorimetric Determination of Carbamate Pesticides in Host-Guest Complexes

From the effect of β-cyclodextrin and hydroxypropylβ 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.


Introduction
The study of organized systems has proved to be very useful for analytical purposes.[1] These microheterogeneous systems can be classified into: molecular aggregates (micelles, vesicles, monolayers, etc.) and polymolecular species (polysilicates such as zeolites; polysugars such as cyclodextrins (CD); polyethers such as crown ethers ).The cavities or pores with defined sizes in these permanent molecular systems favour inclusion or complexing small molecules leading to high selectivity of these molecules.In these complexes, known as supramolecular species, are produced specific interactions that play an important role in the modification of physical and chemical properties of the substrates included.[2] Several photochemical and photophysical studies have been done with these receptors to investigate inclusion with substrates that have, for example, luminiscent properties.A remarkable increase of these properties was found.
The molecular luminiscent spectrometry and in particular molecular fluorescence has become a routine technique with many analytical applications [3] that offers a lower detection limit and with higher selectivity than the absorption spectroscopy.
Some pesticides from the carbamate group (-OCONH 2 ) derived from fluorescent nuclei such as naphthalene, for example carbaryl (1-naphthyl-N-methylcarbamate, CY), or such as benzofuran, for example carbofuran (2,2-dimethyl,2-3-dihidro-7-benzofuranyl-N-methylcarbamate, CF) are widely used for plant protection against insects.However, the high toxicity of these pesticides requires sensitive and reliable methods to detect their presence in fruits and seeds.Therefore, an improved determination of these pesticides in presence of certain receptors is here proposed.
Experimental [4] UV-Visible absorption spectra of CY and CF (analytical grade commercial reactives) were performed in neutral aqueous, acid or basic medium in absence and in presence of 10 mM β-CD (cyclic oligomer with 7 glucose units) or HP-β-CD (CD derivatized).The two carbamates CY and CF showed decomposition at alkaline pH so pH=7 was selected for the determinations.The presence of β-CD and HP-β-CD produced changes in the UV-Visible spectra.The excitation wavelengths (λ ex ) chosen for the fluorescent measurements were the λ where the substrate and the complexed substrate presented the same molar absorptivity (ε); it being 280 nm for CY and 273 nm for CF.Fluorescence emission spectra of the aqueous solutions were taken from each substrate at absorbance concentrations lower than 0.050 at the λ ex chosen in each case with 10 nm excitation and emission widthbands, at low gain for emision spectra.The spectrofluorimeter cell was thermostatized at 25.0±0.1 °C.The areas below the curves (F) and the fluorescent intensities at different λ (F λ ) were recorded.All measurements were performed using a substrate solution at pH=7 as reference, to check the response of the apparatus.

Results and Discussion
The association constants (K ass , M -1 ) could be determined by UV-visible and fluorimetry obtaining values of 190 and 123 for CF and 350 and 644 for CY with CD and HP-β-CD respectively.The values of the quantum yield ratios (Φ complexed /Φ free ) were 1.30 for CY but 7.02 and 9.48 for CF.The detection limits expressed as a limit concentration (C L = ng/mL) were determined employing the corresponding analytical and statistical methods.Notable enhancement of these limits was achieved for the compounds studied.