Simultaneous Voltammetric Determination of Zn(II), Pb(II), Cu(II), and Hg(II) in Ethanol Fuel Using an Organofunctionalized Modified Graphite-Polyurethane Composite Disposable Screen-Printed Device

Double-Emission Ratiometric Fluorescent Sensors Composed of Rare-Earth-Doped ZnS Quantum Dots for Hg2+ Detection

Quantum dots (QDs) are a class of zero-dimensional nanocrystal materials, whose lengths are limited to 2-10 nm. Their unique advantages such as wide excitation spectra, narrow emission spectra, and high quantum yield make their application possible in fluorescence sensing, wherein QDs such as CdSe, CdTe, and CdS are used. Indeed, QDs have a wide range of applications in fluorescence sensing, and there have been many reports of applications based on QDs as ion probes. The emission spectra of QDs can be adjusted by changing the size of the QDs or doping them with other ions/elements. However, the high toxicity of Cd and the poor anti-interference ability of single-emission fluorescent probes greatly limit the applications of QDs in many fields. In this paper, ZnS QDs are doped with the rare-earth element Ce to form a low-toxicity double-emission ratiometric fluorescent sensor, ZnS:Ce, for Hg²⁺ detection. The results of transmission electron microscopy (TEM), X-ray diffractometry, X-ray photoelectron spectroscopy, and optical spectroscopy show that ZnS:Ce QDs were successfully synthesized. Under the optimal conditions, the concentration of Hg²⁺ was in the range of 10-100 μM, which had a linear relationship with the fluorescence intensity of the ZnS:Ce QDs: the linear correlation coefficient was 0.998, and the detection limit was 0.82 μM L^-1. In addition, the fluorescent sensor had good selectivity for Hg²⁺, and it was successfully applied to the detection of Hg²⁺ in laboratory water samples.

Developing a screen-printed graphite-polyurethane composite electrode modified with gold nanoparticles for the voltammetric determination of dopamine

A screen-printed electrode (SPGPUE) was prepared with graphite-polyurethane composite ink containing gold nanoparticles (AuNPs), resulting in a screen-printed graphite-polyurethane composite electrode modified with gold nanoparticles (SPGPUE-AuNPs). Gold nanoparticles were prepared by the citrate method and extracted from the water medium since polyurethane is not compatible with humidity. After extraction to chloroform, they were characterized via transmission electron microscopy (TEM). The presence of gold on the SPGPUE-AuNP surface was confirmed via SEM and EDX analyses, while thermogravimetry revealed the presence of approximately 3.0% (m/m) gold in the composite. An electrochemical pretreatment in 0.10 mol L-1 phosphate buffer (pH 7.0) with successive cycling between -1.0 V and 1.0 V (vs. pseudo-Ag/AgCl) under a scan rate of 200 mV s-1 and 150 cycles was required in order to provide a suitable electrochemical response for the voltammetric determination of dopamine. After the optimization of the parameters of differential pulse voltammetry (DPV), an analytical curve was obtained within a linear dynamic range of 0.40-60.0 μmol L-1 and detection limit (LOD) of 1.55 ×10-8 mol L-1 for dopamine at the SPGPUE-AuNP. A non-modified SPGPUE was used for comparison and a linear range was obtained between 2.0 and 10 μmol L-1 with an LOD of 2.94 × 10-7 mol L-1. During the dopamine determination in cerebrospinal synthetic fluid (CSF), recoveries between 89.3 and 103% were achieved. There were no significant interferences from ascorbic acid and uric acid, but some from epinephrine due to the structural similarity.

Direct identification and analysis of heavy metals in solution (Hg, Cu, Pb, Zn, Ni) by use of in situ electrochemical X-ray fluorescence

The development and application of a new methodology, in situ electrochemical X-ray fluorescence (EC-XRF), is described that enables direct identification and quantification of heavy metals in solution. A freestanding film of boron-doped diamond serves as both an X-ray window and the electrode material. The electrode is biased at a suitable driving potential to electroplate metals from solution onto the electrode surface. Simultaneously, X-rays that pass through the back side of the electrode interrogate the time-dependent electrodeposition process by virtue of the XRF signals, which are unique to each metal. In this way it is possible to unambiguously identify which metals are in solution and relate the XRF signal intensity to a concentration of metal species in solution. To increase detection sensitivity and reduce detection times, solution is flown over the electrode surface by use of a wall-jet configuration. Initial studies focused on the in situ detection of Pb(2+), where concentration detection limits of 99 nM were established in this proof-of-concept study (although significantly lower values are anticipated with system refinement). This is more than 3 orders of magnitude lower than that achievable by XRF alone in a flowing solution (0.68 mM). In situ EC-XRF measurements were also carried out on a multimetal solution containing Hg(2+), Pb(2+), Cu(2+), Ni(2+), Zn(2+), and Fe(3+) (all at 10 μM concentration). Identification of five of these metals was possible in one simple measurement. In contrast, while anodic stripping voltammetry (ASV) also revealed five peaks, peak identification was not straightforward, requiring further experiments and prior knowledge of the metals in solution. Time-dependent EC-XRF nucleation data for the five metals, recorded simultaneously, demonstrated similar deposition rates. Studies are now underway to lower detection limits and provide a quantitative understanding of EC-XRF responses in real, multimetal solutions. Finally, the production of custom-designed portable in situ EC-XRF instrumentation will make heavy metal analysis at the source a very realistic possibility.