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Tuneable, Pre-stored Paper-Based Electrochemical Cells (μPECs): an Adsorptive Stripping Voltammetric Approach to Metal Analysis

  • Keagan Pokpas
  • Nazeem JahedEmail author
  • Emmanuel Iwuoha
Open Access
Original Research
  • 79 Downloads

Abstract

The development of point-of-care (POC) devices for the detection of environmental contaminants and the early monitoring of disease outbreaks in the health and food sectors has steadily grown in recent times. The real-time analysis offered by POC devices is pivotal in developing areas where access to skilled labor is often lacking. Chelating agent–based signal amplification methods have previously been employed in conjunction with electroplated metallic films to improve electrode sensitivity in trace metal analysis. Here, we describe a method for the dry storage of electrochemical reagents: ammonia/ammonium chloride (NH3/NH4Cl) buffer as a supporting electrolyte, dimethylglyoxime (DMG) as a chelating agent, and mercury in paper-based electrochemical cells (PECs), fabricated from commercial filter paper. The stored PECs were applied to the trace, microliter analysis of nickel in water samples by adsorptive cathodic stripping voltammetry (AdCSV) in conjunction with screen-printed electrodes. The method relies on the single-step accumulation/adsorption of microvolumes of the metallic analyte onto stored DMG ligands to form [Ni(dmgH)2] complexes in the presence of mercury films. By integrating the AdCSV techniques with paper-based analytical devices, detection of Ni2+ in water samples with good resolution was achieved at 20 μL sample volumes in the low parts per billion range. The mechanism for pre-concentration and its subsequent reduction was demonstrated as it differs from bulk electroanalysis. Instrumental parameters of the PECs, accumulation time, and deposition potential were optimized along with reagent storage introduction and concentrations. Further, the PECs were investigated in the absence and presence of mercury and had shown good possibility for metal-free sensors in future work. The prepared PECs showed good reproducibility (4.36%, n = 4) and no intermetallic interferences in the presence of 100 μg L−1 Zn2+, Cd2+, Pb2+, Co2+, and In2+. Detection and quantitation limits were calculated and recorded as 6.27 μg L−1 and 18.8 μg L−1, respectively, for Ni2+ determination and 13.1 μg L−1 and 39.3 μg L−1 for the Hg-free derivative at the 90-s accumulation time. The pre-stored paper-based electrochemical cells were then applied to the detection of Ni2+ in real tap water samples and showed good recovery values of ± 94%. Further, the infusion of PECs with a range of chelating agents (dimethylglyoxime, nioxime, and morin hydrate) demonstrates the possibility for tuning PECs to specific applications and improving selectivity in metal analysis applications.

Graphical Abstract

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Keywords

Paper-based electroanalytical cells Adsorptive stripping voltammetry Nickel detection Dimethylglyoxime 

Introduction

Heavy metal pollution of the environment poses a dire threat to the ecological system and human health alike. Contamination via industrial, mining, processing, pollution, and food packaging processes remains prevalent in a modern society where hazardous exposure may exist. The toxic effects as a direct result of their non-biodegradable nature are of great concern to specialists in the health sector and form the basis of the metal analysis class of science. Exposure to nickel, one such heavy metal, occurs primarily through food and industrial sources where its use is ubiquitous in nickel-based alloys and nickel-rich foods including grains, cocoa, and beans. Nickel poisoning in humans is commonly presented as dermatitis, respiratory issues, and skin allergies. As a consequence, accurate, low-cost, and simplistic detection methods are required for the determination of its exposure to the environment and water sources particularly at the site of contamination. The World Health Organization (WHO) and South African drinking water standard have set a maximum contamination limit of 0.1 mg L−1 and 0.15 mg L−1 for Ni2+ in water samples, respectively.

On-site analysis of metal poisoning in the environment is particularly crucial in impoverished areas where access to skilled labor and suitable instrumentation is often limited. Portable instrumentation, low-cost substrates, ease of modification, integrated battery, electrode systems, and others are key areas in the development of reliable point-of-care (POC) devices capable of early detection and accurate monitoring of metal contamination in resource-limited settings. Here, cost, robustness, ease of use, and accuracy are determining factors in their applicability. The rise of microfluidic devices has gained significant traction in the last two decades due to their ability for accurate mixing, separation of species, and the possibility for low-volume analysis. However, high costs associated with the devices are based on the polydimethylsiloxane (PDMS) elastomer. To date, dipstick and lateral flow devices have dominated rapid diagnostic sensing, while paper-based microfluidic sensors have recently garnered tremendous support as praiseworthy alternatives.

Paper, created from woven cellulose fibers, offers many unique advantages over other existing microfluidic substrate materials, with its low cost, biodegradable nature, and porous structure being the most beneficial features. As such, cost-effective and portable miniaturized sensing devices may be manufactured. Its ability to allow for liquid transport through simple wicking and capillary flow methods demonstrate sample introduction without the need for external pumps, replacing the reagent-loaded plug-in cartridges [1] and glass ampoules [2] methods previously investigated. Paper-based analytical devices (PADs) offer the most reliable reagent storage method for once-off use owing to its excellent sorption nature as a result of absorptive surface properties of the porous cellulose structure [3]. Further, patterning with hydrophobic materials including wax through a variety of fabrication approaches creates designated, spatially separated reaction zones for simultaneous analysis. To date, metal analysis at PADs, while still in their infancy stage, has been limited to calorimetric [4, 5], electrochemical [6, 7], fluorescent [8, 9, 10], nanoparticle [11, 12] complexes and hybrid detection methods. These techniques generally rely on qualitative analysis represented by color changes. Microfluidic paper-based electrochemical devices (μPEDs), introduced by the Whitesides Research Group in 2007, combine the advantageous properties of paper substrates with ultra-sensitive electrochemical detection methods to develop disposable, low-cost, and quantitative analytical techniques. Herein, the cellulose structure acts as a 3D scaffold to create the electrochemical cell, allowing for simple low-volume analysis. Owing to its interesting properties, four unique configurations relying on their flow, patterning, and electrode integration have been approached for μPED designs in recent history: strip, origami, stack, and disk in conjunction with integrated or conventional electrode systems. With only two studies conducted thus far, the disk approach remains the most sparsely studied of the four existing configurations. More interestingly, only a few studies have been investigated for the application of μPEDs to metal analysis. These works have been limited to the anodic stripping voltammetry (ASV) detection of heavy metals such as Pb2+, Cd2+, and Hg2+ in water samples. The application of μPEDs has yet to be expanded to include simple complexation-based accumulation techniques demonstrated by adsorptive stripping voltammetry (AdSV). Tan et al. [13] proposed for the first time a simple reagent storage system by employing paper disks for the one-step detection of Pb2+ in water with an internal calibration technique. Kong et al. [14] further investigated the use of paper disk for the determination of stored glucose analyte. To date, adsorptive cathodic stripping voltammetry (AdCSV) has remained the go-to electroanalytical technique for nickel and cobalt determination with much attention and interest geared towards its development. In order to enhance the adsorption of the formed metal complexes at the electrode surface, many metallic films have been investigated to date, including mercury [15, 16, 17], bismuth [18, 19, 20, 21], antimony [22, 23], and lead [24, 25]. While its use has been limited, mercury-based electrode systems remain the gold standard with works still being conducted [26, 27, 28, 29, 30, 31, 32, 33, 34, 35].

Herewith, we describe a novel microvolume analysis of Ni2+ in tap water samples at low-cost, disposable, and pre-stored paper-based electrochemical cells (PECs) relying on a simple paper disk approach. Herein, the cellulose structure of paper disks created from commercial filter paper is pre-stored with electrolyte, chelating agents, and metallic films and is coupled with unmodified screen-printed electrodes. An accurate, sensitive, and quantitative system for metal analysis by AdCSV was achieved. This work expands on previously reported reagent storage systems for electrochemical detection by demonstrating for the first time the incorporation of microscale quantities of dimethylglyoxime (DMG) ligands within the paper substrate in conjunction with electroplated mercury films. This storage allows for the one-step accumulation of the Ni2+ analyte within the paper-based cell pores by simple adsorption processes to form [Ni(dmgH)2] complexes prior to detection. Further, the results of a Hg-free approach were compared to the gold standard Hg sensors along with its influence on the pre-concentration mechanism.

Experimental Section

Apparatus and Reagents

All voltammetric experiments were performed using computer-controlled Nova 1.1 software on an Autolab PGSTAT101 potentiostat (Metrohm Autolab, The Netherlands). Screen-printed carbon electrodes (SPCEs) with carbon working (4 mm diameter), auxiliary electrodes, and Ag/AgCl reference electrode (DRP-C101), purchased from DropSens, were employed for all experiments. Conventional voltammetric cells were replaced with PPECs for all analyses unless stated otherwise. All experiments were carried out at room temperature.

Reagents were purchased from the chemical company Sigma-Aldrich (MO, USA) and were of analytical reagent grade. Ultra-pure water, collected from a Millipore Milli-Q system (Milford, MA, USA), was used to prepare all solutions. An ammonia buffer, used as a supporting electrolyte, was prepared from appropriate quantities of ammonia (NH3) and ammonium chloride (NH4Cl), respectively, and was obtained from Lasec. Hydrochloric acid (HCl) and nitric acid (HNO3) were purchased from Lasec. Standard stock solutions of Ni2+ and Hg2+ were prepared by diluting the appropriate quantities of standard 103 mg L−1 Ni2+ and Hg2+ solutions (Sigma-Aldrich). Dimethylglyoxime (2,3-butanedione dioxime, C4H8N2O2), nioxime (1,2-cyclohexanedione, C6H10N2O2), and morin hydrate (2,3,4,5,7-pentahydroxyflavone, C15H12O8), purchased from Sigma-Aldrich, were dissolved in ethanol, absolute 99.8% (CH3CH2OH) to the desired concentration to prepare stock solutions and used as is without further modification. All stock solutions used in the study were prepared fresh once per week. All chemicals purchased were used as received without further treatment or modification unless stated otherwise. Whatman no. 1 chromatography paper (200.0 mm × 200.0 mm, pure cellulose paper; GE Healthcare) was purchased from Sigma-Aldrich.

PPEC Preparation

PECs were prepared by cutting 8-mm-diameter disks (circles) out of Whatman no. 1 filter paper. Two successive additions of 10 μL portions of prepared solutions, containing the desired reagent concentrations (typically, 2 mM DMG and 10 mg L−1 Hg) in 0.1 M NH3/NH4Cl buffer (pH 9.4), were dropped onto the paper sheets and allowed to dry at room temperature (23 °C) for 30 min. Drop casting of the pre-prepared reagent samples provided superior sensitivity over successive additions of separate samples. Prior to analysis, the prepared PPECs were placed on the surface of commercially bought SPCEs so as to cover the entire three-electrode system.

Square-Wave Adsorptive Cathodic Stripping Voltammetric Detection of Ni

A 20-μL aliquot of the desired metal analyte concentration, prepared in supporting electrolyte, was added to the PPEC prior to analysis. Twenty microliters was the desired sample volume to prevent floating of the PPEC and still allow for excellent contact with the working electrode surface. The sample, in aqueous media, through wicking wets the PPEC and allows for the dissolution of the dried, stored reagents. Stripping voltammetry was performed by in situ accumulation and deposition of the formed [Ni(dmgH)2] complex onto an electroplated Hg film. Stripping voltammetric measurements were then performed by square-wave adsorptive cathodic stripping voltammetry (SW-AdCSV) between − 0.7 and − 1.4 V, unless stated otherwise. A fixed accumulation potential of − 0.7 V was employed for 90 s during analysis, followed by a 10-s equilibration time. Square-wave instrumental parameters of 35 mV amplitude, 20 Hz frequency, and 5 mV potential step were employed for all analyses.

Results and Discussion

Water Sorption in the Porous PEC

Figure 1 shows an HRSEM image of the unmodified PEC at × 100 magnification. The porous nature of the chromatography paper within the entangled/woven cellulose fiber structure is observed. Slight artifacts are further seen between pores, which may arise from the carbon coating process required for conductive imaging in HRSEM analysis. These are attractive features in our application as it allows liquid to penetrate within the hydrophilic fiber matrix without the need for an external pump source [36]. Pore sizes of 11 μm are characteristic of the Whatman no. 1 chromatography paper and indicate the retention pore size of the PEC. The fluid flow in the porous media is crucial in its ability to be used as an electrochemical cell and also in dry reagent storage techniques. Fluid penetration in the PEC is governed by capillary-driven flow via simple wicking processes.
Fig. 1

HRSEM image of the entangled cellulose fiber structure of unmodified PEC at × 100 magnification

Characteristic Quantitative Detection of Ni2+

The AdCSV technique relies on the accumulation of Ni2+ in a suitable, complexed [Ni(dmgH)2] form by adsorption processes at the electrode surface prior to its electrolytic reduction in basic electrolyte solutions. This method has proven to be particularly useful in the detection of nickel and cobalt owing to its ability to readily complex with oxime-based chelating agents. While the exact nature of AdCSV detection of Ni2+ is not clear, two distinct approaches have been proposed, differing in their complex formation process and subsequent accumulation mechanism. The two pre-concentration routes are detailed in Scheme 1. Route I describes the simplest case where Ni2+ cations form the absorbable [Ni(dmgH)2] complex with the DMG ligand in solution prior to its adsorption on the SPCE surface. In the second route (route II), the DMG complexing agent is adsorbed on the electrode surface, so that [Ni(dmgH)2] complex formation takes place on the SPCE surface. This method is limited to materials present at or very close to the Nernst diffusion layer or solution interface of the electrode system and, therefore, usually only accounts for small amounts of the detected species. Hg-free platforms follow a similar complexation mechanism; however, accumulation and adsorption of the analyte species is hindered in the absence of a metallic film.
Scheme 1

Schematic illustration, along with accompanying mechanisms, detailing two possible pre-concentration routes in the AdCSV determination of Ni2+ in the presence of DMG, chelating agents in bulk electrolyte solutions

The subsequent AdCSV reduction of Ni2+ from [Ni(dmgH)2] complexes involves an overall ten-electron transfer reduction process of Ni2+ metal cation from the [Ni(dmgH)2] complex to Ni0 along with the reduction of the DMG ligand to 2,3-bishydroxylaminebutane (DHAB). The overall reduction is summarized in Eq. (1), according to the work described by Baxter et al. [37].
$$ {\left[\mathrm{Ni}{\left(\mathrm{dmgH}\right)}_2\right]}_{\mathrm{ads}}\mathrm{Hg}+10\ {e}^{-}+10\ {\mathrm{H}}^{+}\to \mathrm{Ni}\left(\mathrm{Hg}\right)+2\ \mathrm{DHAB} $$
(1)
A typical square-wave voltammogram for Ni2+ detection at a bare SPCE in the presence of a DMG ligand and electroplated Hg film in a bulk solution of 0.1 M NH3/NH4Cl buffer (pH 9.4) as a supporting electrolyte is shown in Fig. 2. Two distinct cathodic stripping peaks are observed at − 1.032 V and − 1.113 V, respectively. Both may be attributed to the characteristic cathodic two-electron electrochemical reduction of Ni2+ to Ni0 via routes I and II, respectively. The faradaic current of the first peak at − 1.032 V is significantly larger than that of the second (− 1.113 V) due to the migration of a large number of formed complex molecules from the bulk solution to the electrode surface. The more negative potential associated with the peak at − 1.113 V is largely due the quicker migration of adsorbed species to the SPCE surface. An increase in background current is seen with progressive scanning to increasingly negative potentials (− 0.7 V to − 1.3 V).
Fig. 2

Square-wave adsorptive cathodic stripping voltammogram (SW-AdCSV) of 50 μg L−1 Ni2+ in 0.1 M NH3/NH4Cl buffer (pH 9.4) as the supporting electrolyte containing 2 mM DMG and 10 mg L−1 Hg at an SPCE. SWV parameters: Eacc = − 0.7 V, tacc = 90 s, amplitude = 35 mV, and f = 20 Hz

Macroliter vs. Microliter Detection of Ni2+

The reported AdCSV technique demonstrated in the literature has shown to provide a simple, sensitive analytical technique for the detection of Ni2+ in water samples. Large volumes of the sample required for accurate detection make it a disadvantageous method for on-site analysis of real samples. The feasibility of the fabricated PECs was investigated by comparing its electrochemical detection in both bulk and microliter sample volumes. Figure 3 shows a schematic of the comparative systems of bulk and microliter sample volumes used and the appropriate pre-concentration routes: (a) immersed in the bulk solution, (b) with 100 μL droplet dropped directly onto the three-electrode system, and (c) with 20 μL sample volume dropped onto PEC covering the three-electrode area.
Fig. 3

Schematic illustration of complexation routes at the a SPCE immersed in 10 mL of 50 μg L−1 Ni2+ [solution], b SPCE with a 100-μL sample of 50 μg L−1 Ni2+ dropped directly onto the three-electrode system [droplet], and c SPCE with PEC covering the three-electrode system

The recorded SW-AdCSVs are shown in Fig. 4. Bulk solution (Fig. 3a) analysis of a square-wave cathodic sweep in 0.1 M NH3/NH4Cl buffer (pH 9.4) shows three separate peaks: a large, well-defined peak at − 1.032 V; a significantly smaller peak at − 1.13 V due to the reduction of Ni2+ from the [Ni(dmgH)2] complex according to routes I and II (as previously discussed); and a broad peak at more negative potentials (− 1.23 V) arising from reduction of dissolved oxygen. Replacing bulk electrolyte solution with microliter volumes of metal analyte samples (Fig. 3b) was investigated and recorded in Fig. 4. Accurate wetting and spreading of the sample droplet across the three-electrode system was achieved when 100 μL of Ni2+ sample volume was used. Similar to bulk analysis, two well-resolved reduction peaks at − 1.09 V and − 1.18 V are observed. The calculated peak currents at 100 μL sample volumes (droplet) were significantly smaller than those observed in bulk metal samples as well as a negative shift in reduction peak potentials. This is attributed to the fixed amount of metal cations available in the microliter sample volume during electrode pre-concentration. The size of the stripping peaks resulting from routes I and II was found to be more similar than that in in scenario A, with an almost 1:1 ratio as a result of a closer proximity of Ni2+ and DMG to the electrode surface. Utilizing the excellent sorption properties of the filter paper used in the fabrication of the PECs, microliter sample volumes were significantly reduced when using PECs in conjunction with the SPCE, as discussed in the introduction. The maximum allowed volume to allow for a good contact on the electrode surface so as to prevent floating of the PEC in the sample droplet was found to be 20 μL volumes of the sample. The porous structure of the PECs acts as a 3D scaffold which holds a low volume of samples. PECs were found to only exhibit a single cathodic reduction peak (− 1.18 V) due to the reduction of Ni2+ according to Eq. (1) via pre-concentration route II, where complexation occurs at the Nernst diffusion layer interface. The absence of bulk electrolyte volumes limits the formation of [Ni(dmgH)2] complexes in solution prior to adsorption. The results highlight the use of PECs to utilize low microliter volumes of sample while still offering good detection.
Fig. 4

Square-wave adsorptive cathodic stripping voltammograms (SW-AdCSVs) of 50 μg L−1 Ni2+ at the SPCE immersed in 10 mL of 50 μg L−1 Ni2+ [solution], SPCE with a 100-μL sample of 50 μg L−1 Ni2+ dropped directly onto the three-electrode system [droplet], and SPCE with PEC covering the three-electrode system, in which a 20 μL solution of 50 μg L−1 Ni2+ [paper disk] and 0.1 M NH3/NH4Cl buffer as the supporting electrolyte containing 2 mM DMG and 10 mg L−1 Hg were used. SWV parameters: Eacc = − 0.7 V, tacc = 90 s, amplitude = 35 mV, and f = 20 Hz

Development of PPECs

Reagent storage is crucial in developing effective POC devices. The excellent sorption properties of the cellulose fiber structure make PECs capable of acting as 3D electrochemical cells to not only significantly reduce sample volume but also to allow for sorption of reagents prior to analysis. The dry reagent storage method was investigated in fabricating PPECs. Typically, 20-μL samples containing appropriate quantities of reagents in preferred buffer solutions were drop cast onto PECs and allowed to dry at room temperature for 30 min. The reagents are absorbed within the porous cellulose structure. Prior to analysis, a fixed volume of liquid analyte was dropped onto the PPEC, allowing for the dissolution of the stored reagents in aqueous media.

Figure 5 shows comparative voltammograms of paper-based electrochemical cells on a three-electrode SPCE with (a) 20 μL of 50 μg L−1 Ni2+ solution containing 10 mg L−1 Hg and 2 mM DMG in 0.1 M NH3/NH4Cl buffer (pH 9.4) and (b) 20 μL of 50 μg L−1 Ni2+ solution. The PEC was previously impregnated or absorbed with 20 μL of a solution containing 10 mg L−1 Hg and 2 mM DMG in 0.1 M NH3/NH4Cl buffer (pH 9.4). The square-wave voltammogram of PECs in conjunction with SPCEs and samples containing reagents and target metal ions demonstrates a single, symmetrical reduction peak at − 1.20 V. Pre-mixing of analyte ions and reagents in suitable solvents prior to analysis resulted in improved sensitivities of the PEC. While offering accurate and sensitive detection, pre-mixing and pre-treatment of samples are tedious and time-consuming. Similarly, a single stripping peak (− 1.198 V), attributed to the cathodic reduction of Ni2+, is observed for the PPECs with absorbed reagents (electrolyte, DMG, and Hg). A steady increase in the background current at more negative potentials can be seen due to increased charge build-up at the electrode surface resulting in asymmetry due to the irreversible nature of the [Ni(dmgH)2] reduction reaction [38]. Comparison of the stripping reduction peaks of the un-stored and pre-stored PECs indicates similar peak heights (1.75 μA vs. 1.51 μA, respectively) for both PEC derivatives with minimum shifts in peak potential (± 2 mV). This result is in agreement with the hypothesis and shows the applicability of PPECs towards the detection of a metal analyte by AdCSV.
Fig. 5

Square-wave adsorptive cathodic stripping voltammograms (SW-AdCSVs) of 50 μg L−1 Ni2+ at the SPCE with PEC covering the three-electrode system, in which a 20 μL solution comprised of 0.1 M NH3/NH4Cl buffer as the supporting electrolyte containing 2 mM DMG and 10 mg L−1 Hg was used, and at the SPCE with PEC pre-stored with a 20 μL solution containing 2 mM DMG and 10 mg L−1 Hg in 0.1 M NH3/NH4Cl buffer as the supporting electrolyte. SWV parameters: Eacc = − 0.7 V, tacc = 90 s, amplitude = 35 mV, and f = 20 Hz

Effect of Reagent Storage on the Electrochemical Detection at PPECs

Reagent storage in PPECs is largely responsible for the simple sample introduction without a need for sample pre-treatment prior to analysis. Interrogation of a variety reagents (namely, (a) NH3/NH4Cl buffer as a supporting electrolyte, (b) Hg films for adsorption of target species in NH3/NH4Cl buffer, (c) DMG as a chelating agent in NH3/NH4Cl buffer, and (d) DMG and Hg in NH3/NH4Cl buffer on the stripping reduction peak of Ni2+) was performed and recorded in Fig. 6. The square-wave voltammogram of the PPEC, pre-stored with only electrolyte, indicates the absence of any stripping peaks due to the reduction of Ni2+. Pre-concentration of the electrode surface with Ni2+ metallic cations was not possible by simple adsorption, and the deposition/accumulation potentials were unsuitable for Ni2+ reduction and electrode pre-concentration. The low solubility of Ni2+ within an electroplated Hg film, reported in the literature, is evident upon sorption of the PEC with Hg cations prior to analysis. Low sensitivity of the PEC towards Ni2+ detection is shown by an extremely small reduction peak for Ni2+ conversion to Ni0 at − 1.18 V. This supports literature findings of the AdCSV technique at a variety of carbon-based electrode substrates. Dry storage of the DMG ligand and 0.1 M NH3/NH4Cl buffer within the cellulose fiber structure produces a well-defined reduction peak at − 1.14 V upon dissolution in an aqueous solution containing 50 μg L−1 Ni2+. Pre-concentration of the electrode surface results from [Ni(dmgH)2] complex formation within the PPEC by adsorption according to route II, in Scheme 1. Ease of reduction is demonstrated over the Hg-PPEC by a 40 mV peak shift to a more positive reduction potential. A slight enhancement in stripping peak current is seen upon the impregnation of DMG along with Hg and a suitable electrolyte solution. Pre-storage of chelating agent and metallic film in suitable electrolyte solution shows accurate and sensitive detection of Ni2+ cations and confirms the incorporation of the hypothesized reagents within the PEC devices. Here, [Ni(dmgH)2] complexes adsorb onto the present Hg ions, improving electrode sensitivity.
Fig. 6

Square-wave adsorptive cathodic stripping voltammograms (SW-AdCSVs) of the 20-μL sample of 50 μg L−1 Ni2+ at the SPCE with PEC pre-stored with ammonia buffer, SPCE with PEC pre-stored with a 20 μL solution containing 10 mg L−1 Hg, SPCE with PEC pre-stored with a 20 μL solution containing 2 mM DMG, and SPCE with PEC pre-stored with a 20 μL solution containing 2 mM DMG and 10 mg L−1 Hg; 0.1 M NH3/NH4Cl buffer was used as the supporting electrolyte. SWV parameters: Eacc = − 0.7 V, tacc = 90 s, amplitude = 35 mV, and f = 20 Hz

Influence of DMG Ligand Concentration and Storage

The presence of dimethylglyoxime within the PPEC is important towards the detection of Ni2+, resulting in the metal-ligand complex formation and electrode pre-concentration by adsorption. The dependence of [Ni(dmgH)2] stripping peak current as a function of complexing agent concentration was investigated between 0 and 5 mM DMG-ligand concentrations (Figure S1). This was achieved by drop casting fixed microliter volumes of a sample containing the appropriate DMG-ligand concentrations onto developed PECs. The DMG ligand fills the pores in the cellulose structure by sorption methods creating a 3D electrochemical cell in which metal complex formation is possible. A constant, linear dependence and an increase in stripping peak current are observed with increasing DMG concentration in the range under investigation. A further increase to 0.1 M DMG concentration (not shown here) results in saturation of the electrode surface with non-conductive DMG ligands. Electron transfer kinetics through the adsorbed ligand species is significantly hindered, and stripping peak currents are adversely affected. Further, a slight shift to more negative potentials is observed with increasing DMG concentration owing to ease of reduction. A 2 mM DMG concentration was selected for all further experiments to lower reagent storage concentrations, providing a more environmentally friendly device.

Effect of Hg Concentration on Ni2+ Stripping Peak Current

It follows that a dependence of the [Ni(dmgH)2] stripping peak current on the Hg ion concentration in the fabricated PPEC exists. Due to the high sensitivity of Hg film electrodes, the validity of the proposed method was studied in the presence of Hg ions. As such, optimal Hg concentrations for pre-storage were investigated. Figure S2 shows the effect of Hg concentration on the recorded stripping peak currents of Ni2+ reduction. An increase in peak currents up to 10 mg L−1 was observed. At concentrations greater than 10 mg L−1, electrode saturation occurs and a steady decrease in peak currents is recorded. Relatively low changes in stripping peak currents with increasing Hg concentrations show a weak dependence of the stripping peak currents on Hg ion concentration. As a result, Hg storage partially affects the AdCSV reduction of [Ni(dmgH)2]. The shift to more negative reduction potentials with increasing Hg concentration, however, suggests preferential complex formation and its subsequent adsorption at the electroplated Hg film due to increased solubility of the [Ni(dmgH)2] at a metallic film. Consequently, 10 mg L−1 Hg ion concentration was selected for further analysis.

Instrumental Parameter Optimization

Square-wave waveforms were selected for analysis of the PPECs over other commonly used waveforms including linear sweep voltammetry (LSV), differential pulse voltammetry (DPV), and cyclic voltammetry (CV) due to its high sensitivity towards metal characterization. Optimization of the instrumental parameters associated with the square-wave waveform was performed and recorded in Figure S3.

The effect of varying accumulation potentials in the range from 0.0 to − 1.0 V was determined in a basic solution containing 30 μg L−1 Ni2+ cations, 2 mM DMG, and 10 mg L−1 Hg (Figure S3A). The peak current associated with the reduction of [Ni(dmgH)2] showed an increase in peak sensitivity and Eacc up to − 0.75 V. At accumulation potentials more negative than − 0.75 V, the efficiency of Ni2+ pre-concentration significantly declined. The observed phenomena indicated a distinct dependence of accumulation potential on stripping peak currents. An Eacc value of − 0.75 V indicated the most effective accumulation potential towards detection of [Ni(dmgH)2] and was selected for future experiments.

Figure S3B demonstrates the dependence of [Ni(dmgH)2] stripping peak current on the applied accumulation time. Accumulation times were investigated from 0 to 300 s. Pre-concentration of the electrode surface results in improved sensitivities. Prolonged accumulation times up to 120 s in 0.1 M NH3/NH4Cl buffer resulted in an increase in peak currents. Saturation of the electrode surface due to adsorption of [Ni(dmgH)2] occurred for voltammograms run between 150 and 300 s as seen by constant stripping peaks. An accumulation time of 120 s was chosen for further analysis.

The analysis of stripping peak currents recorded with instrumental parameters such as amplitude ranging between 5 and 50 mV is described in Figure S3C. A constant increase in stripping reduction peak heights is shown between 5 and 35 mV, before a significant decrease at 50 mV. An amplitude of 35 mV provided the optimum conditions for Ni2+ detection by AdCSV.

Frequency changes between 5 and 65 Hz were investigated as a means to improve the stripping peak current associated with [Ni(dmgH)2] reduction. The obtained results are summarized in Figure S3D. Low-frequency (5–20 Hz) changes showed the best electrode sensitivity. Increasing the frequency above 20 Hz indicated a decrease in [Ni(dmgH)2] stripping. A frequency of 20 Hz was selected for all further testing.

PPEC Reproducibility and Stability

The stability and reproducibility of the developed PPECs towards [Ni(dmgH)2] detection were performed at SPCEs in conjunction with pre-stored PECs for 30 μg L−1 Ni2+ concentrations. Square-wave voltammograms, recorded on four identically fabricated PPECs on a single SPCE, are illustrated in Figure S4. Single, well-defined, and reproducible peaks were recorded at an average reduction potential of − 1.16 V with little deviation for [Ni(dmgH)2] reduction. An average stripping peak current of 3.93 × 10−6 A ± 1.71 × 10−7 A was reported for the four replications (n = 4). A relative standard deviation (RSD, %) of 4.36% was calculated. The low calculated percentage error demonstrates excellent reproducibility in the fabrication of the PPEC system. Regeneration of the SPCE surface is observed without any further electrochemical cleaning steps.

Interference Studies of the PPECs

Accurate quantitative analysis is pivotal in the fabrication of POC devices for real sample analysis. Understanding the effects of common interferences present in tap water samples on the stripping peak current obtained for [Ni(dmgH)2] complex detection is vital in the development of the PPECs. Commonly found metallic cations such as Zn2+, Cd2+, Pb2+, Co2+, and In2+, present in tap water samples and with redox potentials found in the potential window under investigation, were interrogated as possible metallic interferences (Figure S5). Under optimized conditions, the effects of the metallic cations on the stripping reduction of 30 μg L−1 [Ni(dmgH)2] were investigated by square-wave voltammetry (SWV). (a) The recorded SW-AdCSV of 20 μL of 100 μg L−1 Zn2+, Cd2+, Pb2+, Co2+, and In2+ and the (b) repeated runs of 20 μL of 30 μg L−1 Ni2+ in the presence of 100 μg L−1 Zn2+, Cd2+, Pb2+, Co2+, and In2+ are shown in Figure S5A. The voltammograms show no distinct stripping reduction peaks for Zn2+, Cd2+, Pb2+, Co2+, and In2+ up to 100 μg L−1 between − 0.7 and − 1.4 V (Eacc = − 0.7 V, tacc = 90 s). The stripping peak currents recorded for six replications of Ni2+ in the presence of metallic cations are demonstrated in Figure S5B. The RSD (%) was calculated for each metal cation. Any metal cation with calculated RSD % over 10 was deemed to be an interference. A recorded RSD % of 7.58 was calculated in the presence of all metal cations (n = 6). The results show no interference of Zn2+, Cd2+, Pb2+, Co2+, and In2+ up to 100 μg L−1 towards the AdCSV detection with [Ni(dmgH)2] and indicate the satisfactory analytical performance of the sensor.

Quantitative Analytical Performance of the Fabricated PPECs

The favorable adsorptive stripping voltammetric performance of the pre-stored PEC in the presence and absence of toxic Hg is demonstrated in Fig. 7. Due to limitations set on Hg use in electrochemical sensing, the study was performed to determine the validity of the proposed method without the use of Hg in the pre-storage step. A set of square-wave voltammograms and calculated calibration plots recorded over 15–120 μg L−1 Ni2+ ions at optimum conditions were performed in deaerated samples. The voltammograms were recorded between − 0.7 and − 1.3 V. PPECs were used as disposable electrochemical cells for each Ni2+ cation concentration. To verify a linear relationship of the [Ni(dmgH)2] peak current on Ni2+ concentration, calibration plots were analyzed. Both PPECs showed an increase in faradaic stripping peak current with increasing Ni2+ concentration. The PPEC with pre-stored Hg, however, showed improved sensitivity (three times) over the Hg-free substrate. This could be due to favorable adsorption of DMG ligands at the Hg surface. The operating or dynamic linear range of the sensors was however influenced by the use of Hg films. A linear relationship was established between 15 and 90 μg L−1 [Ni(dmgH)2] concentration for the PPEC with Hg use (Fig. 7b). Electrode saturation occurs at concentrations greater than 90 μg L−1 [Ni(dmgH)2], and a plateau is observed in the obtained calibration curve due to pre-concentration of the electrode surface with [Ni(dmgH)2] at accumulation times of 90 s, blocking the electron transfer rate and diminishing the increase in peak current as a result of increased Ni2+ cations. This feature was not observed in the Hg-free precursor, with a dynamic linear range of 15–120 μg L−1 Ni2+ ions observed. The findings show that the analysis is possible in the absence of toxic Hg films; however, the superior performance due to Hg inclusion is still noted.
Fig. 7

Square-wave adsorptive cathodic stripping voltammograms of a, b Hg-PPECs and c, d Hg-free PPECs in conjunction with SPCEs and corresponding calibration plots recorded with 20 μL volumes of 15–120 μg L−1 [Ni(dmgH)2] in 0.1 M NH3/NH4Cl buffer. SWV parameters: Eacc = − 0.7 V, tacc = 90 s, amplitude = 35 mV, and f = 20 Hz (n = 3)

The detection limit of the PPEC in the presence and absence of Hg at a SPCE was estimated from three times the standard deviation of five replications of square-wave voltammograms recorded in the absence of [Ni(dmgH)2] (3σ), divided by the slope of the calibration curve between 15 and 90 μg L−1 (3σ/slope). The calculated limit of detection (LOD), limit of quantitation (LOQ), sensitivity, and the recorded correlation coefficient obtained from the calibration curve are recorded in Table 1. The calculated detection limits for [Ni(dmgH)2] detection were 6.27 μg L−1 and 13.1 μg L−1, respectively, with and without Hg use. A linear regression of R2 = 0.997 and 0.996 was indicated with a sensitivity of 7.08 μA L−1 μg−1 and 1.89 μA L−1 μg−1 determined. Three replications were performed for all calibration curves. The high sensitivity and good linearity of the dimethylglyoxime-PPEC in the concentration range between 15 and 90 μg L−1 indicate the possibility for high accuracy quantitation of the developed system.
Table 1

Analytical performance of the Hg and Hg-free PPECs (n = 3) in the 15–120 μg L−1 Ni2+ concentration

Analytical parameter

Individual analysis of Ni2+

Hg-DMG-PPEC

  Sensitivity (μA L−1 μg−1)

7.08 × 10−8

  Correlation coefficient (R2)

0.998

  Limit of detection (μg L−1)

6.27 ± 1.32

  Limit of quantitation (μg L−1)

18.8 ± 4.2

Hg-free DMG-PPEC

  Sensitivity (μA L−1 μg−1)

1.89 × 10−8

  Correlation coefficient (R2)

0.996

  Limit of detection (μg L−1)

13.1 ± 4.02

  Limit of quantitation (μg L−1)

39.3 ± 7.51

The effectiveness of the developed DMG-Hg-PPEC, to quantitatively detect Ni2+ metal cations in water samples, was investigated by comparing the determined LODs with recently reported literature values. Table 2 illustrates a summary of previously reported Ni2+ sensing techniques and its analytical performance. The developed sensor shows comparable results to other reported sensor technologies based on the adsorptive stripping voltammetric technique in the low micrograms per liter range under short evaluation times. Slightly higher limits of detection were achieved for the paper-based sensor over other solid electrode materials with low analysis times. This is attributed to lower sensitivity and lower sample volumes utilized in the DMG-Hg-PPEC. Larger concentration additions were required for an adequate change in stripping peak current attributed to lower electrode sensitivity.
Table 2

A summary of the previously reported detection limits for in situ Ni2+ detection in water samples

Metal ions

Substrate

Technique

Accumulation time (s)

Dynamic linear range (μg L−1)

Limit of detection (μg L−1)

Reference

Ni2+

mpBiF-SPCE

AdCSV

180

1–10

0.027

[39]

Co2+

1–10

0.094

Ni2+

RBiABE

DP-AdSV

30

0.6–41

0.18

[40]

Co2+

0.06–4.1

0.018

Ni2+

PbF-SPE

SWV

60

0.6–2.9

0.2

[24]

Co2+

0.6–5.9

0.3

Ni2+

SBVE

SW-AdCSV

30

0–10

0.6

[41]

Ni2+

DMG-CPE

DP-AdSV

120

80–600

27

[42]

Ni2+

DMG-N-SPE

DP-AdSV

120

60–500

30

[43]

Ni2+

ERGO-PG-MFE

SW-AdCSV

210

2–16

0.12

[44]

Ni2+

NGr-DMG-GCE

SW-AdCSV

120

2–20

1.5

[45]

Ni2+

DMG-Hg-μPPEC

SW-AdCSV

90

15–90

6.27

This Work

Ni2+

DMG-Hg-free μPPEC

SW-AdCSV

90

15–90

13.1

This Work

DP-AdSV differential pulse adsorptive stripping voltammetry

Recovery Studies of PPECs

Recovery studies in (a) test solutions in the presence of possible metallic interferences, (b) test solutions in the presence of dirt, and (c) real tap water samples were performed by simple calibration techniques since the standard addition methods could not be employed. No Ni2+ was detected in any samples prior to spiking with known concentrations. As previously stated, this may be attributed to low concentration present below the LOD. Accurate recovery data were obtained for the test sample and real water sample within a 10% error. Contaminated dirt samples, however, reduced the ability to accurately detect metal cations. The results are summarized in Table 3.
Table 3

Recovery studies for in situ Ni2+ detection in the test sample, contaminated sample, and real water sample

Ni2+ samples

Original (μg L−1)

Added (μg L−1)

Found (μg L−1)

Recovery (%)

Test sample

ND

45

45.61

101.34

Test sample with added dirt

ND

45

53.45

118.77

Real water sample

ND

45

41.46

93.12

ND not determined

Effect of Chelating Agent on Ni2+ Detection at PPECs

A variety of chelating agents have previously been reported for the AdSV detection of Ni2+. Ni-chelate complexes formed with dimethylglyoxime, nioxime, and morin hydrate have been studied. The probable structures of (a) dimethylglyoxime, (b) nioxime, and (c) morin hydrate with Ni2+ are illustrated in Figure S6.

Figure 8 represents the SW-AdCSVs obtained from reduction of 30 μg L−1 Ni2+ at PPEC, pre-stored with 10 mg L−1 Hg and 2 mM (a) dimethylglyoxime, (b) nioxime, and (c) morin hydrate. Single, well-defined stripping peaks are obtained for all three chelating agents. The stripping reduction peak attribute to the Ni2+-dmg, Ni2+-nioxime, and Ni2+-MR complexes showed cathodic stripping reduction peaks at − 1.13 V, − 1.16 V, and − 1.08 V, respectively. The nioxime complex showed the largest peak current response over all three Ni-ligand complexes, a minimum of 3 and 2.5 times response of Ni2+-dmg and Ni2+-MR complexes. This increased stripping peak current may be attributed to the affinity of Ni2+ to form the Ni-nioxime complex and its affinity towards the electroplated Hg film. The results demonstrate future applications of the chelating agents towards Ni2+ detection.
Fig. 8

Square-wave adsorptive cathodic stripping voltammograms (SW-AdCSVs) obtained from the 20-μL sample of 50 μg L−1 Ni2+ at the SPCE with PEC pre-stored with a 20 μL solution containing 2 mM DMG and 10 mg L−1 Hg, SPCE with PEC pre-stored with a 20 μL solution containing 2 mM nioxime and 10 mg L−1 Hg, and SPCE with PEC pre-stored with a 20 μL solution containing 2 mM morin hydrate and 10 mg L−1 Hg; 0.1 M NH3/NH4Cl buffer was used as the supporting electrolyte. SWV parameters: Eacc = − 0.7 V, tacc = 90 s, amplitude = 35 mV, and f = 20 Hz

Conclusions and Future Work

A paper-based electrochemical device was for the first time applied to the detection of metal cations in water samples, making use of an adsorptive cathodic stripping voltammetry technique. To the best of our knowledge, this was the first work reported on the use of AdCSV for metal detection in μPECs, building on the work performed by Tan et al. [13] for the detection of Pb2+ by ASV using pre-stored paper disks. The μPECs make use of microliter sample volumes along with dry storage of reagents within the paper structure. The cellulose structure acts as a 3D scaffold and cell for the reaction of dimethylglyoxime, Hg, and electrolyte solution with Ni2+. Since it is the first work of its kind for AdCSV, the study was performed in the absence and presence of Hg and a comparison was drawn. While toxic, it was found that Hg still exhibited superior detection performance over the Hg-free platform; however, promising results were determined to suggest that Hg use is not required. The PPECs could therefore be applied along with alternative metallic films in future work. The μPECs showed good sensitivity and reproducibility towards Ni2+ detection in water samples with excellent selectivity in the presence of Zn2+, Cd2+, Pb2+, Co2+, and In2+. In addition, morin hydrate and nioxime were compared to its DMG counterpart and showed the possibility for future studies in the field, particularly due to the excellent response of Ni2+-nioxime complexes.

Notes

Supplementary material

12678_2019_516_MOESM1_ESM.docx (1001 kb)
ESM 1 (DOCX 1000 kb)

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Authors and Affiliations

  1. 1.SensorLab, Department of ChemistryUniversity of the Western CapeBellvilleSouth Africa

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