3.1. Physicochemical Characterizations of Ti/RuO2-TiO2-SnO2
The structure of the synthesized electrosensing Ti/RuO
2-TiO
2-SnO
2 films were analyzed by XRD.
Figure 1 illustrates the crystal structure of the DSA
® film deposited on the titanium substrate. The characterization shows the presence of metallic titanium (PDF- 44-1294) associated with the titanium support, whose peaks were displaced to slightly greater values of 2θ due to the joint contribution of the three metals that introduce cell distortion. The diffractogram allows clearly identifying characteristic peaks associated with the tetragonal crystalline phase of RuO
2 (PDF- 40-1290), and the anatase structure of TiO
2 (PDF- 21-1272). Characteristic peaks associated with SnO
2 were not observed due to the low content of this metal in the mixed-metal oxide composition. The absence of peaks suggested the formation of a solid solution following the Hume-Rother rule as observed in other mixed metal oxide compositions [
16,
24]. This common behavior is explained by the small difference in the ionic radius of the elements Ru
4+ (0.062 nm), Ti
4+ (0.060 nm), and Sn
4+ (0.069 nm) that did not exceed 15%, which induces the substitution solid solution in the titanium structure [
19,
32,
33,
34].
From the XRD data, the apparent crystallite size was calculated and summarized in
Table 1. When comparing
Table 1 to the apparent size of the crystallite values for a solid solution RuO
2 and TiO
2 phase, the result implies that the ruthenium oxide might be incorporating titanium/tin atoms in their crystalline lattice and thus distorting the structure of TiO
2.
The surface morphology and composition of the formed films were analyzed using the SEM and EDS techniques.
Figure 2a depicts the characteristic electrode surface morphology of DSA
® electrodes with a mud cracked structure [
29,
34]. The EDS analyses of
Figure 2b allowed identifying signals for the three metals in the mixed metal oxide composition of Ti/RuO
2-TiO
2-SnO
2 (50:40:10 atom. %). The EDS demonstrates the presence of Sn in the electroactive film despite not having observed an associated crystalline structure in XRD (see
Figure 1), which allows inferring its solid solution in TiO
2 and RuO
2.
Table 2 collects the atomic composition determined through the EDS analyses, and indicates a good correlation between experimental and nominal compositions. Thus, the DSA preparation method effectively formed a mixed metal oxide film from the polymer precursor calcination.
3.2. Electrochemical Characterizations
The electroanalytical behavior of the DSA as a working electrode is shown in
Figure 3. The CV analysis of Ti/RuO
2-TiO
2-SnO
2 in the Na
2SO
4 supporting electrolyte at pH 7.0 shows an increase in current response at 1.1 V vs. Ag/AgCl that is associated with the oxygen evolution reaction (OER) from water oxidation. The onset potential of OER shows an overpotential (η) of 1.0 V which is commonly associated with active electrodes [
14]. When CV is conducted in KCl as a supporting electrolyte under an identical ionic strength of 0.10 two peaks were observed (see
Figure 4). The peak located in the region between 0.10–0.70 V vs. Ag/AgCl was attributed to Ru(III)/Ru(IV) redox transition. Meanwhile, the second peak in the region between 0.8–1.1 V vs. Ag/AgCl was attributed to Ru(IV)/Ru(VI) redox transition [
35,
36]. In the region from 1.0 V vs. Ag/AgCl the start of the chlorine evolution reaction (ClER) an increase is observed in the current response that is associated with the coexistence of chloride oxidation reaction (1) and water oxidation (i.e., OER) [
35,
36]. The most notorious difference is the clear reduction peak observed in the cathodic scan that is ascribed to the reduction of active chlorine species electrogenerated during the anodic scan. To demonstrate that the cathodic peak observed in the presence of chloride is indeed associated with the cathodic reduction of ClO
−, a blank experiment in Na
2SO
4 supporting electrolyte containing 2.68 × 10
−3 mol L
−1 of NaClO was carried out. Under these conditions, the reduction peak appeared at the same potential of 1.0 V vs. Ag/AgCl demonstrating that this charge transfer process is actually associated with the ClO
− cathodic reduction.
The reduction peak observed is a key aspect for the indirect electrochemical quantification of urea since the concentration of the target analytes can be indirectly estimated from the HClO/ClO
− consumed by the chemical reactions (2) and (3).
Figure 4 illustrates how the presence of urea decreases the intensity of the cathodic peak associated with the reduction of HClO/ClO
−. This trend is associated with the lower accumulation of active chlorine species in the solution due to their consumption by fast chlorine breaking point chemical reactions.
The cathodic charge densities (
qc) determined in the different solutions tested are collected in
Table 3. It can be seen that the
qc-values obtained for the solutions of urea showed a
qc reduction of 88% in relation to the (
qc) KCl solution value. This electrochemical response is related to the amount of non-consumed HClO remaining in the solution.
The LSV analyses were conducted to determine the relationship between the cathodic peak intensity and the concentration of urea in the solution. The initial potential of 1.2 V vs. Ag/AgCl was held for 60 s to ensure the electrogeneration of active chlorine species required for the analyses. Thereafter, the current response was registered during the negative-going scan from 1.20 to 0.20 V vs. Ag/AgCl. The LSV readings registered for urea concentrations ranging between 6.66 × 10
−6 to 3.33 × 10
−4 mol L
−1 of urea are depicted in
Figure 5a. Interestingly, it can be observed that the cathodic peak intensity decreases while increasing the concentration of urea in the solution. The cathodic peak intensity (
Ipeak), which is related to the urea concentration, presented a linear relationship with
R2 = 0.997. From the slope of the analytical curve, the limits of detection and quantification were calculated according to the formulas: LOD = (3 × SD
blank)/Slope and LOQ = (10 × SD
blank)/Slope, where the SD blank is the standard deviation of 10 voltametric measurements of blank and slope of the analytical curve [
37], which are summarized in
Table 4. A low LOQ of 7.66 × 10
−6 mol L
−1 encourages the possible application of this indirect method for the quantification of urea.
If we compare these highly promising results (
Table 4) with the previous literature reports shown in
Table 5, the proposed electrochemical quantification of urea by Ti/RuO
2-TiO
2-SnO
2 (50:40:10 atom. %) outperforms other electroanalytical approaches in terms of LOD, linearity, stability, and reproducibility. The repeatability and reproducibility tests showed low standard deviations, which indicate a good agreement between the analyses performed by these materials. Thus, the DSA produced in the present work demonstrates an excellent efficiency linking the qualities of being an easily produced electrode and the ability to detect and quantify urea in a simple and fast way.
The presence of several metal ions that can interfere in the urea analysis was analyzed [
43,
44]. The interferents were analyzed in the proportions of 1:1 (interferent: urea) and it showed a loss (−) and current gain (+) as a percentage. The results obtained are shown in
Table 6.
Table 6 shows that nickel(II) and zinc(II) ions did not significantly interfere with the analytical urea signal, considering the tolerable limit of ±10% for interference [
45]. The results obtained from sulfur(II) and iron(II) ions showed decreased analytical signals in the current. According to Wilson et al., 2019, this may be due to the fact that the metallic species suffer oxidation in the presence of electrogenerated active chlorine in situ [
46]. In addition, cadmium(II), lead(II), and copper(II) ions showed decreased analytical signals in the current as similarly reported elsewhere [
47,
48]. This can be related to the lower generation of active chlorine species due to their ability to complex chloride. Note that any aspect of the system that conditions the electrogeneration of active chlorine species (our indirect measure) can decrease the overall peak signal registered during the cathodic scan.
In order to discuss a real scenario, we conducted the analyses with synthetic urine samples. After obtaining the analytical parameters of the sensor Ti/RuO2-TiO2-SnO2 (50:40:10 atom. %), the method proposed was applied for the analysis in a complex sample of synthetic urine. The analyses of synthetic urine samples were prepared containing 2.77 × 10−4 mol L−1 of urea in 0.10 mol L−1 KCl. The final concentration of urea found using the DSA electrode following the indirect electroanalytical method was 3.31 × 10−4 mol L−1 with an error estimated at +16%, which is acceptable for an online measurement that provides a continuous evaluation of urine in real effluents. These results can be related to the presence of interferents in synthetic urine as discussed during the study of the effect of coexisting species.