Electrochemical microsensors for the detection of cadmium(II) and lead(II) ions in plants

Abstract

Routine determination of trace metals in complex media is still a difficult task for many analytical instruments. The aim of this work was to compare three electro-chemical instruments [a standard potentiostat (Autolab), a commercially available miniaturized potentiostat (PalmSens) and a homemade micropotentiostat] for easy-to-use and sensitive determination of cadmium(II) and lead(II) ions. The lowest detection limits (hundreds of pM) for both metals was achieved by using of the standard potentiostat, followed by the miniaturized potentiostat (tens of nM) and the homemade instrument (hundreds of nM). Nevertheless, all potentiostats were sensitive enough to evaluate contamination of the environment, because the environmental limits for both metals are higher than detection limits of the instruments. Further, we tested all used potentiostats and working electrodes on analysis of environmental samples (rainwater, flour and plant extract) with artificially added cadmium(II) and lead(II). Based on the similar results obtained for all potentiostats we choose a homemade instrument with a carbon tip working electrode for our subsequent environmental experiments, in which we analyzed maize and sunflower seedlings and rainwater obtained from various sites in the Czech Republic.

Keywords: amperometry; cadmium; heavy metals; lead; maize; miniaturization; plant; screen printed electrode; sunflower; voltammetry; water.

Figure 1.

(A) DP voltammograms of lead(II) and cadmium(II) ions: a (Pb²⁺ 10.0 μM, Cd²⁺ 10.0 μM); b (Pb²⁺ 15.6 μM, Cd²⁺ 25.0 μM); c (Pb²⁺ 32.3 μM, Cd²⁺ 100.0 μM); d (Pb²⁺ 62.5 μM, Cd²⁺ 175.0 μM); e (Pb²⁺ 125.0 μM, Cd²⁺ 250.0 μM). (B) The dependence of peak height on concentration of the metals as follows for cadmium (0.75–100 μM) and for lead (0.5–1,000 μM); in insets: for cadmium (0.75–12.5 μM) and for lead (0.5–15.6 μM). Potentiostat: Autolab.

Figure 2.

Photos of screen printed carbon electrode and/or carbon tips electrode connected to PalmSens potentiostat.

Figure 3.

Differential pulse voltammetric detection of cadmium(II) ions at SPE and/or CTE connected to PalmSens potentiostat. (A) The dependence of cadmium(II) ions peak height on accumulation time (concentration of Cd(II) is 20 μM), in inset: typical DP voltammograms of cadmium(II) ions measured at various times of accumulation with CTE. (B, C) Calibration curves. (D) DP voltammograms of various concentrations of cadmium(II) ions measured with SPE. Experimental parameters were as follows: initial potential 0 V, end potential −0.8 V, potential step 5 mV.

Figure 4.

Microdetection of lead(II) ions at carbon tip. (A) The dependence of lead(II) ions peak height on time of accumulation, in inset: typical DP voltammograms of lead(II) ions under various times of accumulation. (B) Calibration curve. (C) DP voltammograms of various concentrations of lead(II) ions. (D) Detection of lead(II) ions in contaminated flour.

Figure 5.

Photo of homemade potentiostat with chip and controlling circuits.

Figure 6.

Cadmium(II) ions. (A) DP voltammogram of cadmium(II) ions detected at SPE. Calibration curves measured with (B) SPE and/or (C) CTE. Lead(II) ions. (A) DP voltammogram of lead(II) ions detected at SPE. Calibration curves measured with (B) SPE and/or (C) CTE. Poteniostat: homemade. Experimental parameters were as follows: initial potential 0 V, end potential −0.8 V, potential step 5 mV.

Figure 7.

Sunflower seedlings. Changes in fresh weight, growth, AST and ALT activities in sunflower seedlings treated with (A) cadmium(II) ions and/or (B) lead(II) ions. Content of (C) cadmium(II) ions and /or (D) lead(II) ions in shoots and roots of the treated seedlings.

Figure 8.

Maize seedlings. Changes in fresh weight, growth, AST and ALT activities in maize seedlings treated with (A) cadmium(II) ions and/or (B) lead(II) ions. Content of (C) cadmium(II) ions and /or (D) lead(II) ions in shoots and roots of the treated seedlings.

Figure 9.

(A) Concentration of zinc(II), cadmium(II) and lead(II) ions in rainwater sampled from 14th November 2007 to 31st July 2008 in Boritov, Czech Republic. Concentration of (B) cadmium(II) ions and/or (C) lead(II) ions in water samples obtained from the Zidlochovice area (southern Moravian region, Czech Republic). The metal ions were determined by using homemade potentiostat and CTE as working electrode.