Determination of Trace Levels of Nickel(II) by Adsorptive Stripping Voltammetry Using a Disposable and Low-Cost Carbon Screen-Printed Electrode

A commercial and disposable screen-printed carbon electrode (SPCE) has been proposed for a fast, simple and low-cost determination of Ni(II) at very low concentration levels by differential pulse adsorptive stripping voltammetry (DPAdSV) in the presence of dimethylglyoxime (DMG) as complexing agent. In contrast with previously proposed methods, the Ni(II)-DMG complex adsorbs directly on the screen-printed carbon surface, with no need of mercury, bismuth or antimony coatings. Well-defined stripping peaks and a linear dependence of the peak area on the concentration of Ni(II) was achieved in the range from 1.7 to 150 μg L−1, with a limit of detection of 0.5 μg L−1 using a deposition time of 120 s. An excellent reproducibility and repeatability with 0.3% (n = 3) and 1.5% (n = 15) relative standard deviation, respectively, were obtained. In addition, the suitability of the SPCE as sensing unit has been successfully assessed in a wastewater certificated reference material with remarkable trueness and very good reproducibility.


Introduction
Stripping voltametric methods are especially suitable for trace metal analysis in environmental samples, due to their exceptional features such as good detection limits, reproducibility and selectivity [1]. It is well known that the performance of a voltametric method is closely linked to the material of the working electrode. Trace metal ions determination was traditionally associated with the use of mercury-based electrodes, due to both their high reproducibility and their wide cathodic window [2]. However, the toxicity and the poor versatility as sensing devices of mercury electrodes have been argued as relevant drawbacks; as a result, in the last few years, their use has been progressively decreasing.
Accordingly, in the last two decades, the number of works devoted to the development of sensors with similar electrochemical performance than that exhibited by mercury electrodes has increased rapidly. In addition to the essential requirement of being less toxic, the new electrodes had to be more versatile as sensors than mercury and with a large quantity of supports and modifiers available [3][4][5]. In this sense, the boom of the screen-printed electrodes (SPEs) as a versatile and low-cost option to traditional solid electrodes has contributed in a very positive way to the development of sensors that are able to work either submerged into a solution, in a unique drop or coupled to a flow system. SPEs can be used either in its bare form or as a support for a large variety of modifications. In addition, their disposability, low-cost and reproducible character allow the substitution of every device after some measurements by a new unit with comparable performance, thus avoiding the periodic and tedious polishing and cleaning protocols of traditional solid electrodes. In this way, taking advantage of the above-mentioned specific features, sensors based on SPEs are very appropriate for continuous monitoring, sample screening and on-site analysis [6][7][8].
On the other hand, it is well reported the potential effects that heavy metals may cause on both human health and environment. Heavy metals enter the surroundings by natural ways and through anthropogenic activities. Indeed, it should be noted that heavy metals are closely linked to water since they can be easily collected in surface or groundwater. While it is true that some of these metals may remain in the water, it is also known that some others can come out and gather on accessible surfaces like plants. Therefore, heavy metals can enter the body via food or drinking water, being particularly hazardous because they tend to bioaccumulate. The build-up of heavy metals can produce malfunctions in the kidneys, liver, the circulatory system and the movement of nerve signals. Some heavy metals may be also involved in the development of several cancers [9,10].
Heavy metals most frequently found in wastewater include arsenic, nickel, chromium, cadmium, copper, lead and zinc, representing all of them an important risk for human health and natural ecosystems [11]. Particularly, nickel is a frequent industrial pollutant that can be found in wastewater around industrial parks with concentrations at times achieving the low millimolar range [12]. Nickel is usually found in the Ni(0) or Ni(II) state because of the stability of these species in water [13]. Various sources of Ni(II) include industry, electroplating, tobacco, primarily refining and welding since nickel is frequently used to produce industrial machines components, electronics, jewellery, stainless steel kitchen utensils, dental or orthopedic implants, coins and magnets. High levels of nickel can also be found in spinach, legumes, nuts and lettuce, as well as in cocoa powder and baking powder [14]. Nuts and cocoa may have nickel levels as high as 3 and 10 ppm, respectively. In a market basket survey completed in the United States, the highest average levels of nickel in µg/100 g were found in nuts (128.2), legumes (55), sweeteners (31.6), grains and grain products (26.2) and mixed dishes and soups (25.3) [15]. Nickel is a potential immunotoxic and immunomodulatory agent apart from its action as an allergen in humans. Moreover, nickel toxicity to humans has obtained serious attention due to its carcinogenic action [16][17][18]. In this regard, the United States Environmental Protection Agency (EPA) recommends that the nickel levels in drinking water do not exceed 0.1 mg L −1 . In view of the statements above, it is critical to develop suitable methods that make possible the on-site determination and monitoring of nickel at low concentrations.
The voltametric determination of Ni(II) at the very low concentrations found in natural water samples involves the use of preconcentration methods. In the years dominated by mercury electrodes, it was clear that, unlike Pb(II)-or Cd(II)-ions, the preconcentration of Ni(II) could not be done by anodic stripping voltammetry (ASV) since reduced Ni atoms are poorly soluble in mercury. A solution to this problem was found with a "classical" method of adsorptive stripping voltammetry (AdSV) with dimethylglyoxime (DMG) [19,20]. In this method, Ni(II) is complexed by DMG in solution and the resulting Ni(DMG) 2 complex adsorbs and accumulates onto the surface of the mercury electrode, where Ni(II) can be further reduced by means of a cathodic scan. In this way, an enhanced reduction signal is obtained. When mercury electrodes started to become obsolete, alternative approaches were required with new electrode materials. In this regard, two basic strategies are found in the literature: (i) using AdSV with DMG again but replacing the mercury surface with bismuth or antimony coatings which can reasonably reproduce its most relevant characteristics [21][22][23][24][25][26][27][28][29][30][31][32][33][34]; and (ii) using stripping techniques with modified electrodes based on DMG which incorporate the ligand to the electrode and not to the solution [35][36][37][38][39][40][41][42]. In both cases, either the classical glassy carbon electrodes or the screen-printed electrodes were used as a support for the corresponding modifications.
Thus, to the best of our knowledge, on the search for an alternative to mercury, the determination of Ni(II) has mainly been studied using modified electrodes, neglecting the possibilities of bare electrodes. While it is true that conventional solid electrodes, usually based on carbon (carbon paste or glassy carbon), but also based on gold, silver or platinum, are less toxic and more versatile as sensing units than mercury-based electrodes, they have the disadvantage of having a lower reproducibility and need to be properly cleaned and polished before each determination. However, most of the drawbacks of non-modified solid electrodes can be overcome using SPEs as sensing devices because of their abovementioned specific characteristics. Moreover, today, many disposable SPEs from different materials are available on the market at very affordable prices. There is no doubt that the possibility of using an electrode that does not require any modification procedure before being used would certainly simplify and make faster the determination of Ni(II), since the stage related to the preparation of the electrode, which in some cases is quite laborious and time consuming, would be completely avoided.
Hence, in this work, screen-printed carbon electrode (SPCE) and screen-printed gold electrode (SPAuE) have been selected as two of the most economical, simple, reproducible and popular commercial screen-printed electrodes, and have been applied to the determination of Ni(II) at trace levels by AdSV using DMG as complexing agent. While SPAuE has not proved to be a valid sensor for the determination of Ni(II), excellent preliminary results were achieved with SPCE. Thus, SPCE was analytically characterized and applied for the determination of Ni(II) in a certified wastewater sample, as an example of its suitability for a fast, simple and low-cost analysis of Ni(II) in samples of environmental interest.

Chemicals
All chemicals were of analytical grade. Stock standard solutions of Ni(II) at 10 mg L −1 , 1 mg L −1 and 100 µg L −1 were prepared in ultrapure water from 1000 mg L −1 atomic absorption standard purchased from Alfa Aesar (Ward Hill, MA, USA). Atomic absorption standard solutions of Fe(III), Pd(II), Pt(II) and Co(II), 1000 mg L −1 , were supplied by Merck (Darmstadt, Germany).
A 0.1 mol L −1 stock solution of dimethylglyoxime (DMG) was prepared in ethanol 96% and 0.1 mol L −1 ammonia/ammonium solution at pH 9.2 served as a buffer.
The water used through the experiments was purified via Milli-Q plus 185 system (Millipore, Burlington, MA, USA).
A carbon screen-printed electrode (SPCE, ref. 110) or a gold screen-printed electrode (SPAuE, ref. 220AT) purchased from Metrohm DropSens (Oviedo, Spain) were served as working electrode (4 mm diameter), which was attached by means of a flexible cable (ref. CAC, Metrohm DropSens) to the Autolab System. These electrodes were used as acquired since no previous treatment is needed. Ag/AgCl/KCl (3 mol L −1 ), to which all potentials are referred, supplied by Metrohm (Switzerland) was employed as a reference electrode and as auxiliary electrode it was used the one included in the screen-printed unit. Considering that a reasonable volume of sample was analyzed, the included reference electrode was not used to ensure a better accuracy and for a longer durability of the devices.
For pH measurements a Crison micro pH 2000 was employed.

Voltammetric Measurements
Ni(II) calibration curves at SPCE were performed by means of AdSV by adding small volumes of the Ni(II) stock standard solutions into the electrochemical cell containing 20 mL of 0.1 mol L −1 ammonia/ammonium buffer solution at pH 9.2 and 5 × 10 −5 mol L −1 DMG. AdSV measurements were carried out by applying a deposition potential (E d ) of −0.7 V during a deposition time (t d ) of 120 s with stirring and after a rest period (t r ) of 5 s without stirringthe AdSV curves were recorded in the potential range from −0.7 to −1.3 V. A pulse times of 50 ms, a pulse amplitude of 100 mV and a step potential of 5 mV were used.
The analysis of the certified wastewater sample using a SCPE was performed by the standard addition calibration method. The AdSV measurements started by putting in the cell a volume of the wastewater sample in 0.1 mol L −1 ammonia/ammonium buffer solution (dilution factor 1:500) at pH 9.2 and 5 × 10 −5 mol L −1 DMG and the voltammogram was recorded. Then, five aliquots of Ni(II) stock standard solution were added and the respective AdSV curves were recorded.
Three scans are made for each new unit in buffer solution before taking reliable measurements to equilibrate de SPEs. Moreover, for the purpose of ensuring the elimination of the remaining bound Ni(II)-DMG complex from the working SPCE surface, in both Ni(II) calibration curves and analysis of the certified wastewater sample, a conditioning potential (E cond ) of −1.3 V for 30 s was applied before each measurement. All experiments were carried out without any oxygen elimination and at room temperature (20 • C).

Differential Pulse Adsorptive Stripping Voltammetry of Ni(II)
Looking for the best analytical response of both SPCE and SPAuE in connection with Ni(II) determination, two key electrochemical parameters such as the deposition potential (E d ) and the deposition time (t d ) were initially assessed by AdSV.
DPAdSV measurements of a solution containing 10 µg L −1 Ni(II) at pH 9.2 and 5 × 10 −5 mol L −1 DMG were done at several E d and t d values varying from −0.6 V to −1.0 V and from 30 s to 240 s, respectively, searching for a compromise between both the analytical response and the time of analysis. A well-defined Ni(II) peak close to −1.1 V that proportionally increased with t d until 120 s was obtained using a SPCE ( Figure S1). For longer t d a diminution of peak area was progressively observed. Therefore, the highest analytical response was achieved applying a E d of −0.7 V and a t d of 120 s as a good agreement between the analytical response and the analysis time. However, when a SPAuE was used, the characteristic Ni(II) peak was not observed either in any of the E d values tested or t d applied. Instead, a background signal was found whose current dramatically increased as the potential was getting more negative (Figure 1), surely due to the H 2 discharge. Indeed, gas bubbles were detected on the electrode surface at the most negative potentials. In view of these results, the use of a SPAuE was discarded as a sensing unit for the DPAdSV determination of Ni(II) and, from here on out, SPCE was used.
After selecting the optimal electrochemical conditions, the repeatability and reproducibility of the SPCE were evaluated. Table 1 summarizes the reproducibility and repeatability values found for the determination of Ni(II) on SPCE. Repeatability was computed using the same SPCE device for five and fifteen repetitive DPAdSV measurements, respectively, performed in a solution containing 20 µg L −1 of Ni(II) in ammonia/ammonium buffer solution at pH 9.2 and 5 × 10 −5 mol L −1 DMG following the above-established conditions. A highly stable response along 15 consecutive measurements with a relative standard deviation (RSD) of 1.5% was obtained, proving the excellent response repeatability of SPCE. Reproducibility was calculated from the slope corresponding to the linear range of three independent calibration curves carried out from 0.5 to 150 µg L −1 using three different SPCE units. The reproducibility among SPCE units was also outstanding, with RSD values of 0.3%. These repeatability and reproducibility values are better than those achieved for ex-situ antimony film screen-printed carbon electrode and ex-situ bismuth film screen-printed carbon electrode associated to the Ni(II) voltametric stripping peak [34]. After selecting the optimal electrochemical conditions, the repeatability and reproducibility of the SPCE were evaluated. Table 1 summarizes the reproducibility and repeatability values found for the determination of Ni(II) on SPCE. Repeatability was computed using the same SPCE device for five and fifteen repetitive DPAdSV measurements, respectively, performed in a solution containing 20 µg L −1 of Ni(II) in ammonia/ammonium buffer solution at pH 9.2 and 5 × 10 −5 mol L −1 DMG following the above-established conditions. A highly stable response along 15 consecutive measurements with a relative standard deviation (RSD) of 1.5% was obtained, proving the excellent response repeatability of SPCE. Reproducibility was calculated from the slope corresponding to the linear range of three independent calibration curves carried out from 0.5 to 150 µg L −1 using three different SPCE units. The reproducibility among SPCE units was also outstanding, with RSD values of 0.3%. These repeatability and reproducibility values are better than those achieved for exsitu antimony film screen-printed carbon electrode and ex-situ bismuth film screen-printed carbon electrode associated to the Ni(II) voltametric stripping peak [34]. The analytical performance evaluation was conducted by means of DPAdSV calibration of Ni(II) metal ion performed in triplicate (a new SPCE unit was used in each replicate). Linear calibration curves were carried out at the optimized conditions by analyzing thirteen Ni(II) concentrations increasing from 0.5 to 150 µg L −1 . As it can be seen in Figure  2a, a well-shaped Ni(II) voltametric stripping peak close to −1.1 V that increases with the   The analytical performance evaluation was conducted by means of DPAdSV calibration of Ni(II) metal ion performed in triplicate (a new SPCE unit was used in each replicate). Linear calibration curves were carried out at the optimized conditions by analyzing thirteen Ni(II) concentrations increasing from 0.5 to 150 µg L −1 . As it can be seen in Figure 2a, a well-shaped Ni(II) voltametric stripping peak close to −1.1 V that increases with the Ni(II) concentration was obtained. As shown in Table 1 and Figure 2b, good linear responses of peak area vs. Ni(II) concentration was attained.
On the other hand, sensitivity was established from the slope of the calibration curve and the limit of detection (LOD) and the limit of quantification (LOQ) were computed as three and ten times, respectively, the standard deviation of the intercept over the slope of the calibration curves. Table 1 reports the calibration data obtained using a SPCE. LOD and LOQ were at the level of µg L −1 and a linear response up to 150 µg L −1 was achieved.
Typical concentrations of nickel in sediments and soils range from 4 to 80 mg Kg −1 , nickel concentrations in surface water and groundwater range between 3 and 10 µg L −1 and in drinking water the average concentration of nickel is between 2 and 4.3 µg L −1 [15]. Moreover, as it is stated in the introduction section, the EPA recommends that the nickel levels in drinking water do not exceed 0.1 mg L −1 . Therefore, the reported calibration data suggest that SPCE could be a fully appropriate sensor for the determination of Ni(II) at trace levels in environmental samples. Ni(II) concentration was obtained. As shown in Table 1 and Figure 2b, good linear responses of peak area vs. Ni(II) concentration was attained. On the other hand, sensitivity was established from the slope of the calibration curve and the limit of detection (LOD) and the limit of quantification (LOQ) were computed as three and ten times, respectively, the standard deviation of the intercept over the slope of the calibration curves. Table 1 reports the calibration data obtained using a SPCE. LOD and LOQ were at the level of µg L −1 and a linear response up to 150 µg L −1 was achieved.
Typical concentrations of nickel in sediments and soils range from 4 to 80 mg Kg −1 , nickel concentrations in surface water and groundwater range between 3 and 10 µg L −1 and in drinking water the average concentration of nickel is between 2 and 4.3 µg L −1 [15]. Moreover, as it is stated in the introduction section, the EPA recommends that the nickel levels in drinking water do not exceed 0.1 mg L −1 . Therefore, the reported calibration data suggest that SPCE could be a fully appropriate sensor for the determination of Ni(II) at trace levels in environmental samples.
Regarding previous results connected with the voltametric determination of Ni(II) and, as can be extracted from Table 2, it should be highlighted that (i) the linear range Regarding previous results connected with the voltametric determination of Ni(II) and, as can be extracted from Table 2, it should be highlighted that (i) the linear range obtained using a SPCE is much wider and less restricted to lower concentrations than most of the values obtained using bismuth-based, antimony-based, lead-based or mercurybased electrodes; (ii) the obtained LODs and LOQs are considerably lower than most of those provided by modified electrodes based on DMG. Compared to those reported using bismuth-based, antimony-based and lead-based electrodes, the LOD and LOQ obtained using a SPCE are of the order or even slightly better depending on the considered electrode. However, slightly lower LODs were reported when a mercury based-electrode was used as sensing unit; and (iii) the deposition time (120 s) applied using a SPCE is in the lower range of the deposition times reported for the voltametric determination of Ni(II), which range from 30 to 1500 s, depending on the considered electrode. Therefore, the good analytical performance, together with the fact that SPCEs are non-toxic, low-cost and reproducible devices that do not require any polishing, cleaning or modification procedure, allows us to assert that SPCE can be applied for a fast, simple and low-cost stripping voltametric determination of Ni(II) at very low concentration levels. In-situ plated lead film on carbon fiber working microelectrode SWAdSV 0.1-6 0.05 240 [47] Screen-printed carbon electrode DPAdSV 1.7-150 0.5 120 This work a The quantification of nickel takes place after 12 min of accumulation and 5 min of deposition. b Dimethylglyoxime modified screen-printed electrode was prepared from nine additions of 25 µL of DMG solution with a rest period of 15 min between additions. c Dimethylglyoxime modified screen-printed electrode was prepared from three additions of 75 µL of DMG solution with a rest period of 30 min between additions. AAdSV: anodic adsorptive stripping voltammetry, AdSV: adsorptive stripping voltammetry, AdCSV: Adsorptive cathodic stripping voltammetry; CCAdSCP: adsorptive stripping constant current chronopotentiometry; CSWV: cathodic square wave voltammetry, DPAdSV: differential pulse adsorptive stripping voltammetry, DPV: differential pulse voltammetry, SWAdSV: square wave adsorptive stripping voltammetry; SWV: square wave voltammetry.

Interference Study
The potential interference of some frequently occurring metal ions such us Co(II), Pd(II), Pt(II) and Fe(III) in the determination of Ni(II) by DPAdSV using a SPCE was assessed. These metal ions could competitively complex with DMG or generate reduction peaks that overlap with or almost entirely suppress the Ni(II) peak.
Ni(II):metal ion ratios of 4:1, 2:1, 1:1, 1:2, 1:5, 1:10 and 1:50 were investigated, starting with a concentration of Ni(II) of 10 µg L −1 (Figure 3). As an interference criterion, a difference between the peak area of the Ni(II) without and with the presence of the possible interfering ion higher than 15% was considered. Stripping voltametric measurements achieved until a 1:5 Ni(II):metal ion ratio indicate that Co(II) and Fe(III) do not interfere with the Ni(II) peak, whereas Ni(II):metal ion ratios of 1:10 and 1:50 result in a significant increase of the Ni(II) peak area with respect to that without Co(II) or Fe(III). Pd(II) also does not display interference until a 1:2 Ni(II):Pd(II) ratio, whereas a considerable increase of the Ni(II) peak area is observed for ratios of 1:5 and 1:10 Ni(II):Pd(II). However, at a 1:50 Ni(II):Pd(II) ratio, the Ni(II) is practically suppressed. In the case of Pt(II), at a 1:50 Ni(II):Pt(II) ratio, an increase of the Ni(II) peak area with respect to that without Pt(II) of 19% was achieved, whereas lower Ni(II):Pt(II) ratios did not result in interference. Unlike other previous results achieved using bismuth and antimony-based electrodes [27,34], it can be noted that the use of SPCE as sensing unit does not result in overlapping AdSV signals between Ni(II) and Co(II), at least until a 1:5 Ni(II):Co(II) ratio, which ensures the successful determination of Ni(II). This is especially important because both metals ions usually present very close reduction peaks that hinder the determination of one of the metals in the presence of the other.

Analysis of a Wastewater Certified Reference Material by Using a SPCE
The applicability of the DPAdSV method using SPCE was assessed by analyzing Ni(II) in a wastewater certified reference material. The wastewater sample was not subjected to any treatment such as digestion, filtration or preconcentration. The determination of Ni(II)-ions was carried out by means of the standard addition method. Then,  Unlike other previous results achieved using bismuth and antimony-based electrodes [27,34], it can be noted that the use of SPCE as sensing unit does not result in overlapping AdSV signals between Ni(II) and Co(II), at least until a 1:5 Ni(II):Co(II) ratio, which ensures the successful determination of Ni(II). This is especially important because both metals ions usually present very close reduction peaks that hinder the determination of one of the metals in the presence of the other.

Analysis of a Wastewater Certified Reference Material by Using a SPCE
The applicability of the DPAdSV method using SPCE was assessed by analyzing Ni(II) in a wastewater certified reference material. The wastewater sample was not subjected to any treatment such as digestion, filtration or preconcentration. The determination of Ni(II)-ions was carried out by means of the standard addition method. Then, DPAdSV measurements were performed at the above optimized conditions, including five additions of Ni(II). The DPAdSV determination of Ni(II), in the wastewater sample, was performed in triplicate using a new SPCE unit for each replicate. Figure 4a shows a series of DPAdS voltammograms after standard additions for the determination of Ni(II) in the wastewater sample. As in the calibration data, a well-shaped Ni(II) stripping voltametric peak was obtained. A good correlation of the DPAdSV measurement carried out using SPCE was achieved as it is shown in the Ni(II) standard addition calibration plot (Figure 4b). Table 3 reports the Ni(II) concentration data obtained from the DPAdSV determination of the three replicates of the certified wastewater sample made using the SPCE. An excellent agreement was achieved between all the three replicates, as well as with the certified value of Ni(II) in the wastewater. Moreover, it should be noted that the certified wastewater solution contains 12 more elements, Al, As, Cd, Co, Cr, Cu, Fe, Mn, P, Pb, V and Zn, at concentration ratios Ni:element (calculated in grams) of 1:2, 10:1, 50:1, 17:1, 5:1, 2.5:1, 1:1, 2.5:1, 1:1, 10:1; 10:1 and 1.7:1, respectively, without affecting the determination of Ni(II) in the sample.     Looking at the excellent results obtained in the determination of Ni(II) in the wastewater sample, we can assert that a disposable and commercial screen-printed carbon electrode can be successfully applied for the AdSV determination of Ni(II) in the presence of dimethylglyoxime as complexing agent in real water samples at very low concentration levels, even in the presence of other elements at comparable concentrations as Ni(II). Thus, SPCE is an authentic alternative not only to the most classical mercury and bismuth or antimony-based electrodes, but also to the modified electrodes based on DMG for the determination of Ni(II) in natural samples.

Conclusions
In this work, two commercial screen-printed electrodes based on carbon (SPCE) and gold (SPAuE) were tested as sensing devices for the determination of Ni(II) by DPAdSV in the presence of DMG as a complexing agent.
The use of SPAuE did not give rise to the typical Ni(II) peak, being ruled out as sensing device for the stripping determination of Ni(II). However, when a SPCE is used, a well-defined Ni(II) stripping peak appears that increases proportionally with both t d and Ni(II) concentration. After the optimization of the electrochemical parameters, calibration curves applying an E d of −0.7 V during a t d of 120 s were conducted. LOD and LOQ at levels of µg L −1 and a linearity of up to 150 µg L −1 were achieved. Moreover, as can be inferred from the obtained repeatability values, a SPCE unit can be applied for many measurements without degradation signals. Thus, it can be concluded that all the analytical parameters evaluated were similar or even better than those reported in previous studies for bismuth-based, antimony-based, lead-based or modified electrode-based on DMG.
In addition, SPCE was successfully applied for Ni(II) determination in a wastewater sample as a demonstration of its applicability for natural samples analysis, being able to quantify Ni(II) concentration with very high trueness and excellent reproducibility, inferred by the relative error = 0.9% and the RSD = 0.4%, respectively.
Thus, the good stability, selectivity, reproducibility, and LOD and LOQ at µg L −1 levels added to the inherent strengths of using screen-printed electrodes, i.e., low-cost, disposable character, reproducible units . . . , and the lack of modification procedures involved make the SPCE particularly well suited to being implemented in the monitoring of Ni(II) at low µg L −1 levels in samples of environmental interest.