Andrea R. da Silva

Determination of enrofloxacin by room-temperature phosphorimetry after solid phase extraction on an acrylic polymer sorbent.

A phosphorimetric method was developed to enable the determination of enrofloxacin using photochemical derivatization which was used to both improve detection limits and to minimize the uncertainty of measurements. Phosphorescence was induced on cellulose containing TlNO(3). Absolute limit of detection at the ng range and linear analytical response over three orders of magnitude were achieved. A metrological study was made to obtain the combined uncertainty value and to identify that the precision was mainly affected by the changing of substrates when measuring the signal from each replicate. Pharmaceutical formulations containing enrofloxacin were successfully analyzed by the method and the results were similar to the ones achieved using a HPLC method. A solid phase extraction on an acrylic polymer was optimized to separate enrofloxacin from interferents such as diclofenac and other components from biological matrices, which allowed the successful use of the method in urine analysis.

Determination of lapachol in the presence of other naphthoquinones using 3MPA-CdTe quantum dots fluorescent probe

3MPA-CdTe QDs in aqueous dispersion was used as a fluorescent probe for the determination of lapachol, a natural naphthoquinone found in plants of the Bignoniaceae family genus Tabebuia. Working QDs dispersions (4.5×10(-8) mol L(-1) of QDs) was prepared in aqueous media containing Tris-HCl buffer pH 7.4 and methanol (10% in volume). The excitation was made at 380 nm with signal measurement at 540 nm. To establish a relationship between fluorescence (corrected to inner filter effect) and concentration of lapachol, a Stern-Volmer model was used. The linear range obtained was from 1.0×10(-5) to 1.0×10(-4) mol L(-1). The limit of detection (x(b)-3s(b)) was 8.0×10(-6) mol L(-1). The 3MPA-CdTe QDs probe was tested in the determination of lapachol in urine, previously cleansed in an acrylic polymer. The average recovery was satisfactory.

Voltammetric determination of sibutramine in beverages and in pharmaceutical formulations

A simple and sensitive method has been proposed for the determination of sibutramine-HCl in energy drinks, green tea and pharmaceutical formulations using differential pulse voltammetry performed on a hanging mercury drop electrode. In the chosen experimental condition (Mcllvaine pH 4.0 buffer, 50 mV pulse amplitude and 40 mV s-1 scan velocity), sibutramine-HCl presented a reversible behavior and a peak maximum at -80 mV. Detection limit was 0.4 mg L-1 and the working linear range extended up to 33.3 mg L-1 (r = 0.99). Analysis of real and fortified samples enabled recoveries between 91 and 102%. The electroanalytical method was compared with a HPLC method which indicated it accuracy.

Spherical gold nanoparticles and gold nanorods for the determination of gentamicin.

Gentamicin is an antibiotic indicated to treat mastitis in dairy cattle and for the treatment of bacterial resistance in the context of hospital infections. The effect caused by gentamicin on the optical properties of gold nanoparticles aqueous dispersions were used to develop quantitative methods to determine this antibiotic. Two different aqueous dispersions, one containing spherical Au nanoparticles (AuNPs) and the other containing Au nanorods (AuNRs), had their conditions adjusted to enable a stable and sensitive response towards gentamicin. The use of AuNPs, with measurement at 681nm of the rising coupling plasmon band, enabled a limit of detection (LOD) of 0.4ngmL-1 (0.02ng absolute LOD), ten times lower than the one achieved by measuring the decreasing of the longitudinal surface plasmon resonance band (at 662nm). The linear analytical response of AuNPs measured at 681nm did not require rationing of signal values to correct for linearity. Stability of the analytical response resulted in intermediary precision below 2%. No significant interference was imposed by excipients traditionally present in injectable solutions for veterinary use. Percent recoveries obtained in such formulations were between 94.5 and 98.2% regardless the existence of any difference in the proportion of the compounds known as gentamicin (C1, C1a and C2) in standard and in the samples. The method requires no derivatization with toxic reagents as usually is required in other spectroscopic approaches.

Study of the interaction of flavonoids with 3-mercaptopropionic acid modified CdTe quantum dots mediated by cetyltrimethyl ammonium bromide in aqueous medium

Flavonoids are polyphenols that help the maintenance of health, aiding the prevention of diseases. In this work, CdTe QDs coated with 3-mercaptopropionic acid (3MPA), with an average size of 2.7nm, were used as photoluminescence probe for flavonoids in different conditions. The interaction between 14 flavonoids and QDs was evaluated in aqueous dispersions in the absence and in the presence of cetyltrimethylammonium bromide (CTAB). To establish a relationship between photoluminescence quenching and the concentration of flavonoids, the Stern-Volmer model was used. In the absence of CTAB, the linear ranges for quercetin, morin and rutin were from 5.0×10-6molL-1 to 6.0×10-5molL-1 and from 1.0×10-5molL-1 to 6.0×10-4molL-1 for kaempferol. The sensibility of the Stern-Volmer curves (Ks) indicated that quercetin interacts more strongly with the probe: Ks quercetin>Ks kaempferol>Ks rutin>Ks morin. The conjugation extension in the 3 rings, and the acidic hydroxyl groups (positions 3and 4) in the B-ring enhanced the interaction with 3MPA-CdTe QDs. The other flavonoids do not interact with the probe at 10-5molL-1 level. In CTAB organized dispersions, Ks 3-hydroxyflavone>Ks 7-hydroxyflavone>Ks flavona>Ks rutin in the range from 1.0×10-6molL-1 to 1.2×10-5molL-1 for flavones and of 1.0×10-6molL-1 to 1.0×10-5molL-1 for rutin. Dynamic light scattering, conductometric measurements and microenvironment polarity studies were employed to elucidate the QDs-flavonoids interaction in systems containing CTAB. The quenching can be attributed to the preferential solubility of hydrophobic flavonoid in the palisade layer of the CTAB aggregates adsorbed on the surface of the 3MPA CdTe QDs.

Electrooxidation of trifloxystrobin at the boron-doped diamond electrode: electrochemical mechanism, quantitative determination and degradation studies

ABSTRACT The boron-doped diamond (BDD) presents attractive electrochemical sensing characteristics that are useful in analytical applications based on voltammetry and amperometry. It has a wide potential window in aqueous solutions enabling the quantification of the fungicide trifloxystrobin, measured at +1744 mV (versus Ag/AgCl), by square-wave anodic voltammetry in a Britton–Robinson (BR) buffer (0.04 mol L−1; pH 4.00)/acetonitrile 70/30% v/v. The activation of the electrode was made using galvanostatic chronopotentiometry and cyclic voltammetry (CV). The linear analyte addition curve, Ip (µA) = (1.0 × 10–1 ± 4.8 × 10–6) C (mol L−1) + (8.8 × 10–2 ± 1.1 × 10–3); R2 = 0.997, was obtained using amplitude of 40 mV, frequency of 30 Hz, step potential of 20 mV. The instrumental limit of detection (LOD) was 1.4 × 10–7 mol L−1 (0.058 mg L−1) and the dynamic linear range covered three decades (up to 1 × 10–5 mol L−1 or 4.1 mg L−1). The samples were analysed with recoveries about 80% in orange juice samples and from 92.4% to 104.0% in water samples. A study to evaluate potential interferences was made in the presence of other fungicides. Diagnostic studies indicated that oxidation of trifloxystrobin in aqueous medium at the surface of the BDD is irreversible, involving two steps, each one with two electrons. The UV degradation of trifloxystrobin was evaluated using the proposed electrochemical method and the kinetics of degradation established with half-life of 1.07 min.

Gold nanoparticles coupled with graphene quantum dots in organized medium to quantify aminoglycoside anti-biotics in yellow fever vaccine after solid phase extraction using a selective imprinted polymer

HIGHLIGHTSAuNPs were synthesized on a dispersion of GQDs‐amino using NaBH4 as reducing agent.Reaction was made in organized solution containing the cationic surfactant CTAB.Kanamycin amplify photoluminescence from the AuNPs‐GQDs‐amino‐CTAB probe.The instrumental detection limit was 30nmolL−1.Solid phase extraction using an aminoglycoside selective MIP ensured selectivity.Kanamycin was detected in pharmaceutical formulation and in yellow‐fever vaccine. ABSTRACT The determination of kanamycin sulfate was made indirectly by measuring its effect on photoluminescent amino functionalized graphene quantum dots (GQDs‐amino) associated with gold nanoparticles (AuNPs) that were produced by the reduction of AuCl4 with NaBH4 in an aqueous dispersion of GQDs‐amino (obtained by the pyrolysis of citric acid and glutathione) also containing the cationic surfactant CTAB. The AuNPs‐GQDs‐amino‐CTAB system presents a suppressed photoluminescence that is amplified in the presence of kanamycin. Under optimized experimental conditions, the photoluminescence amplification of the nanomaterial system showed a linear response as a function of kanamycin concentration, covering three orders of magnitude (10−7 to 10−5molL−1). The use of solid phase extraction with a cartridge packed with aminoglycoside selective molecularly imprinted polymer ensured selectivity in determinations made on yellow‐fever vaccine and veterinary pharmaceutical formulations. The analytical results were statistically similar to those obtained with an HPLC‐based fluorescence method (after chemical derivatization). The proposed method is a simple, sensitive and selective approach that does not involve the use of toxic reagents employed for chemical derivatization of aminoglycoside antibiotics.

Electroanalytical-based Approaches for the Determination of Pesticides from the Strobilurin Class

Pesticides of the strobilurin class have been recently introduced against fungi and have become widely used in various crops. Electroanalytical-based methods for these substances have been recently emerged to either detect them or previously separate them before detection. In the present study these approaches are critically revised in order to show the potential of voltammetry, amperometry and capillary electrophoresis to determine the strobilurin in various samples.