Determination of iron in biodiesel based on fluorescence quenching of CdTe quantum dots

Abstract

The determination of trace elements in biodiesel is essential to assess oil qualities, identify adulterations and prevent undesirable effects in environment and in humans. However, due to the low sample concentration, sensitive instrumental techniques are required.In this work an effective and simple quantification method using water-soluble mercaptopropionic acid (MPA)-capped CdTe quantum dots (QDs) was for the first time implemented for the fluorescence quantification of iron in biodiesel obtained from different vegetable oils and fat. The developed methodology was based on the iron capacity to establish surface interactions with the nanocrystals that result in a quenching of the fluorescence intensity (F0−F), proportional to the iron concentration. Size and concentration of QDs, concentration and pH of the buffer solution showed a strong effect on the quenching efficiency influencing linear working range and sensitivity of the methodology.An ultrasonic bath was used for the extraction of iron from oil samples with a mixture of 1:1 (v/v) concentrated HCl and H2O2. The extraction efficiency was approximately 100% after 50min of ultrasound.Under the optimized experimental conditions, a linear working range was obtained for iron concentrations from 4.16 to 100.0μgL−1 (R=0.9996, n=6). The determined detection limit (LOD) was about 1.25μgL−1. Six biodiesel samples were analyzed by the proposed methodology, and the results revealed good agreement with those obtained through a high-resolution continuum source graphite furnace atomic absorption spectrometry (HR-CS GFAAS) comparison procedure. Relative deviations lower than 4.5% were achieved, and the application of a paired Student’s t-test, confirmed the absence of any statistical difference for a confidence level of 95% (n=5). Recoveries varied from 90.2% to 105.2%. The slopes of the analytical curves obtained with different matrices showed that the quantification of the analytes was not influenced by matrix effects. The quenching mechanism of CdTe QDs is discussed. The accuracy, precision and operational simplicity of the method, without additional electronic devices, demonstrated an excellent alternative for determination of iron in biodiesel samples.