Josefa A. García Calzón

Fluorescent carbon nanodots for sensitive and selective detection of tannic acid in wines

Herein we describe an easy one step synthesis of carbon nanodots (C-dots) by thermal carbonization of 6-bromohexylboronic acid using two different amine compounds, polyethyleneglycol bis(3-aminopropyl (PEGA) and 1,2-aminopropane (DPA), at 180 °C in atmospheric oxygen. The as-synthesized C-dots were characterized by FTIR, HRTEM, NMR and fluorescence. The C-dots prepared using PEGA showed a strong emission at 440 nm with excitation at 362 nm. These C-dots exhibited analytical potential as sensing probes for tannic acid (TA) determination. pH effect, interferences, and analytical performance of the method were investigated. The method was found effective in the linear concentration range from 0.1 to 10 mg L(-1) TA achieving a limit of detection equal 0.018 mg L(-1) TA. The applicability of the method was demonstrated by direct measurements of TA in red and white wine samples. Validation of the method was achieved by spiking the wine samples with different standard TA concentrations obtaining recoveries in the range (90-112.5%). A probable mechanism by which TA quenched the C-dots fluorescence was proposed.

Electrocatalytic effects of deoxyribonucleic acids, adenine and guanine on the reduction of Ni(II) at a mercury electrode

Adenine, guanine, and native and denatured DNA lower the overpotential for the reduction of Ni(II) at a mercury electrode in a sodium acetate medium. A mechanism for the electrode reaction underlying the electrocatalytic effects observed is proposed. The similarity of the electrocatalytic effects of native and thermally denatured DNA adsorbed on the electrode at full coverage reveals that Ni(II) species interact with basic residues within the double-helical structure, thus reflecting the high diffusivity of these species throughout the native DNA molecule. A simple differential pulse voltammetric method for the simultaneous determination of adenine and guanine in mixtures based on the peak for the reduction of protonated adenine and the single prepeak produced by the reduction of Ni(II) catalysed by unprotonated adenine and guanine was developed. Finally, a sensitive adsorptive stripping differential pulse voltammetric method for the determination of native and denatured DNA is proposed.

Square-wave voltammetry of the o-catechol-Ge(IV) catalytic system after adsorptive preconcentration at a hanging mercury drop electrode.

Square-wave (SW) voltammetry in connection with a hanging mercury drop electrode has been applied for studying the reduction of Ge(IV) catalyzed by o-catechol after adsorptive preconcentration. Acetate buffer solutions with low and high analytical concentrations of o-catechol [with respect to that of Ge(IV)] have been used. Dependences of SW catalytic peaks on accumulation conditions have been attributed to the formation of the catalytic complexes via a direct-adsorption mechanism. The nature of the electrocatalytic process (involving an apparently quasi-reversible electrode reaction with an adsorbed reactant and a diffusing product) allows us to explain complex dependences of catalytic peaks on SW-parameters. Adequate conditions for determination of o-catechol and Ge(IV) by means of SW catalytic adsorptive voltammetry have been established.

Catalytic reduction of metal ions by adsorbed ligands in differential pulse voltammetry : evaluation of catalytic effects from the non-catalysed reduction peaks

A new method for estimating catalytic effects in differential pulse polarography and differential pulse voltammetry is proposed. Based on an approximate equation relating the catalytic peak current with that of the uncatalysed reduction of metal ions, the new method is applicable when the ligand concentration is much smaller than the analytical concentration of the metal ion. The method has been tested with previously studied Ni(II)-(native or denatured) DNA electrocatalytic systems. Its main advantage appears when the electrocatalytic peaks are poorly resolved.

Facile synthesis of water-soluble carbon nano-onions under alkaline conditions

Summary Carbonization of tomatoes at 240 °C using 30% (w/v) NaOH as catalyst produced carbon onions (C-onions), while solely carbon dots (C-dots) were obtained at the same temperature in the absence of the catalyst. Other natural materials, such as carrots and tree leaves (acer saccharum), under the same temperature and alkaline conditions did not produce carbon onions. XRD, FTIR, HRTEM, UV–vis spectroscopy, and photoluminescence analyses were performed to characterize the as-synthesized carbon nanomaterials. Preliminary tests demonstrate a capability of the versatile materials for chemical sensing of metal ions. The high content of lycopene in tomatoes may explain the formation of C-onions in alkaline media and a possible formation mechanism for such structures was outlined.