Monitoring the lead-and-copper rule with a water-gated field effect transistor

We use the natural zeolite clinoptilolite as the sensitive element in a plasticised PVC membrane. Separating a sample pool and a reference pool with such a membrane in water-gated SnO2 thin-film transistor (SnO2 WGTFT) leads to membrane potential, and thus transistor threshold shift in response to the common drinking water pollutants Pb2þ or Cu2þ in the sample pool. Threshold shift with ion concentration, c, follows a Langmuir–Freundlich (LF) characteristic. As the LF characteristic shows the steepest slope in the limit c→ 0, this opens a window to limits-of-detection (LoDs) far below the ‘action levels’ of the ‘lead-and-copper rule’ for drinking water: Pb2þ: LoD 0.9 nM vs 72 nM action level, Cu2þ: LoD 14 nM vs 20.5 μM action level. LoDs are far lower than for membranes using organic macrocycles as their sensitive elements. Threshold shifts at the lead and copper action levels are more significant than shifts in response to variations in the concentration of non-toxic co-cations, and we discuss in detail how to moderate interference. The selective response to lead and copper qualifies clinoptilolitesensitised WGTFTs as a low footprint sensor technology for monitoring the lead-and-copper rule, and to confirm the effectiveness of attempts to extract lead and copper from water. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wh.2020.186 ://iwaponline.com/jwh/article-pdf/18/2/159/709109/jwh0180159.pdf Zahrah Alqahtani (corresponding author) Nawal Alghamdi Martin Grell Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Rd, Sheffield S3 7RH, UK E-mail: zjalqahtani1@sheffield.ac.uk Zahrah Alqahtani Department of Physics, University of Taif, Taif-Al-Haweiah 21974, Saudi Arabia Nawal Alghamdi Department of Physics, University of Tabuk, King Fahad Road, Tabuk 47731, Saudi Arabia This article has been made Open Access thanks to the generous support of a global network of libraries as part of the Knowledge Unlatched Select initiative.


INTRODUCTION
The report by Kergoat et al. () that thin-film transistors can be gated across water as electrolytic gate medium (water-gated thin-film transistors, WGTFTs) has paved the way for new potentiometric sensors: when a WGTFT is sen- where z is the valency of the cation, and c ref > >c st is the ion concentration in a reference solution. c st depends on ion and ionophore, but typically is in the range 100 nM to 1 μM (Choi et al. ; Althagafi et al. b; Al Baroot & Grell ). This LoD is sufficient for common waterborne cations (e.g., Na þ , K þ , Ca 2þ , Mg 2þ ) as these occur naturally in concentrations far higher than c st . However, the 'potability' limit (highest acceptable concentration) of radioisotopes (e.g., Cs þ , Sr 2þ ) or heavy metals (e.g., Pb 2þ , Cd 2þ ) is often significantly lower, e.g., 7.5 nM for Cs þ ( (1): where K is the stability constant for the analyte/sensitiser binding and ΔV th (sat) the saturated value of threshold shift in the limit c > >c 1/2 ¼ 1/K, with c 1/2 defined as ΔV th (c 1/2 ) ¼ ½ ΔV th (sat). We found a very large K ¼ 3.9 × 10 9 L/mole and very low LoD of 33 pM, well below the 'potability' limit of 7.5 nM for Cs þ .
Two common low-level toxic pollutants in drinking water are the heavy metal cations lead (Pb 2þ ) and copper (Cu 2þ ), e.g., lead leaches from historic water pipes, copper from 'low tech' water sterilisation (Sudha et al. ; Masindi & Muedi ). Lead and copper are subject to regulation, e.g., the US Environmental Protection Agency's (EPA) 'lead-and-copper rule' (EPA ) sets 'action levels' of 0.015 mg/L ¼ 72 nM for lead and 1.3 mg/L ¼ 20.5 μM for copper in the domestic water supply.
In drinking water treatment, another zeolite, 'clinoptilolite', is used to extract Pb 2þ and Cu 2þ from water (Perić et al. ). Clinoptilolite forms naturally by volcanic ash alternation in water (Mumpton ) and is mined from natural deposits (Erdem et al. ). Here, we show that WGTFTs sensitised with a clinoptilolite-filled membrane provide a simple potentiometric sensor with very low limit-of-detection, suitable for monitoring the lead-and-copper rule.
Response characteristic is described by a generalisation of Equation (2), known as the 'Langmuir-Freundlich' (LF) isotherm, Equation (3): The additional parameter β < 1 describes inhomogeneity in the analyte/ionophore binding sites (Turiel et al.  conditioned for 4 hours in tap water which did not contain any deliberately added ions. Finally, the membrane was glued in between two plastic pools with epoxy (see Figure 1).

Preparation of test solutions
To simulate realistic conditions for practical use of our sensor, we did not work with deionised water but drew water samples from drinking water taps at Sheffield

Twin-pool gating setup
To test the response of membrane-sensitised WGTFTs to The SnO 2 transistor substrate was in contact with tap water held in an inner (reference) pool that is separated from an outer (sample) pool by the sensitised PVC membrane. The water in the reference pool was tap water as drawn, with no deliberately added ions. For sensor calibration, the outer pool is initially also filled with tap water, but this is then subsequently replaced with solutions of known and increasing concentrations of lead or copper, prepared as described in the section 'Preparation of test solutions', while the inner pool remains filled with tap water as a reference. For practical use of the WGTFT as lead and copper sensor, rather than calibration, the sample pool will be filled with the potentially contaminated water. The transistor is gated by a tungsten (W) contact needle that is submerged in the outer pool. As with all electrolyte-gated transistors, the potential applied to the gate contact will be communicated to the semiconductor surface via interfacial electric double layers (EDLs). However, the potential at the semiconductor surface will be different from the potential applied to the gate needle by any membrane potential, V M (c) in response to different ion concentrations, c, in the outer (sample) vs inner (reference) pool. The setup is illustrated in Figure 1.

WGTFT characterisation and analysis
As V M (c) adds to the applied gate potential, it can be measured as a shift in the WGTFT's threshold voltage, ΔV th . We, therefore, recorded linear transfer characteristics by connecting all tungsten contact needles/probeheads shown in Figure 1 (3)) using the nonlinear fitting routine in Origin 2,018 software.

RESULTS AND DISCUSSION
Lead and copper sensing results In Figure 2     Quantitative analysis of Pb 2þ and Cu 2þ sensing Figure 4 shows ΔV th (c Pb ) and ΔV th (c Cu ) as evaluated from the shift of transfer characteristics along the V G axis to construct 'master' characteristics.
We find that the threshold shifts observed in WGTFTs with increasing concentration of Pb 2þ and Cu 2þ increase rapidly for low concentrations and approach saturation ΔV th (sat) of several 100 mV at high concentration. This response is different from the Nikolsky-Eisenman law (Equation (1)) but similar to our previous results with zeolite mordenite (Alghamdi et al. ), albeit we required the LF isotherm (Equation (3)) rather than the simpler Equation (2), for the fits shown in Figure 4. We find a satisfactory match for Pb 2þ and excellent match for Cu 2þ . The values for the fit parameters K, β and ΔV th (sat) from Equation (3) for both Pb 2þ and Cu 2þ sensing are summarised in Table 1.
The three-orders-of-magnitude larger K for lead vs copper indicates the stronger extraction of lead rather than copper by clinoptilolite, which is already evident from the concentration scales used in Figure 2  and Cu 2þ , respectively, from Table 1. We then fit straight lines of the form y ¼ mx þ b; resulting parameters m (slope) and b (intercept) with their respective errors are listed in Table 2.
As expected from Equation (3), b overlaps with zero within its error Δb. The concentration corresponding to LoD can be determined with the standard '3 errors' criterion, Equation (4): We here find LoD(Pb 2þ ) ¼ 0.9 nM and LoD(Cu 2þ ) ¼ 14 nM, which are already included in Table 2. To make sure Cu 2þ LoD is realistic rather than an artefact of   LoD for lead is relatively larger than for copper when compared to 1/K, which reflects the larger scatter (poorer fit to the model, Equation (3)) in the original data, particularly at higher concentrations. Visually, the lead LoD formally evaluated by Equation (4)  LoDs for both lead and copper are significantly smaller than the action levels of the lead-and-copper rule, which qualifies our sensors for its monitoring.

Sensor performance in acidic conditions
While the tap water drawn in our laboratory has nearneutral pH (pH ¼ 7.2, measured with pH meter (CyberScan PH 300)), drinking water generally may vary in pH, with the permitted range for drinking water (in the EU) being pH 6.5-9.5 (Council of the European Union ). Practically, water samples can be tested for pH with a pH meter and adjusted to pH 7 by adding small amounts of a strong base (or acid) before lead and copper testing. Contamination with, e.g., Na þ from NaOH will in itself not lead to significant threshold shift, as we show below in the section 'Interference from common co-cations'. However, we here show that the impact of pH on sensing of lead and copper is small. We added a drop of acetic acid to our tap water to deliberately make it mildly acidic, pH 5.2 as measured with the same pH meter. We then tested clinoptilolitebased WGTFTs to sense lead and copper in acidified tap water. Threshold shifts at one representative heavy metal concentration for as-drawn (pH 7.2) vs acidified (pH 5.2)  Table 1.  Table 3. Concentrations were chosen to lead to near-saturated threshold shift according to Figure 4.
Heavy metal-induced threshold shifts under acidic conditions are slightly smaller than under near-neutral pH.
However, shifts are still significant at pH 5.2, which is more than one pH unit below the permitted pH range for drinking water. Clinoptilolite membranes are therefore suitable to detect lead and copper within the permitted pH range of drinking water. For accurate quantitative determination at significantly non-neutral pH, we advise calibration (as in Figures 2 and 3) at several pHs, or prior neutralisation of acidic samples with small amounts of a strong base, e.g., NaOH.

Lead and copper extraction with clinoptilolite
As the usual application of clinoptilolite is to extract lead and copper pollution from the drinking water supply  Figure 6, which for comparison also includes the transfer characteristic for asdrawn tap water that has not been spiked/extracted.
We find that characteristics for both spiked/extracted samples do display a small threshold shift compared to a tap water sample that has never been spiked. However, the shift is significantly lower than what we found in the section 'Lead and copper sensing results'. This response suggests that the extraction procedure has significantly reduced the initial 1 μM heavy metal concentration, albeit a small amount of pollution remains. Results are summarised in Table 4, which also shows the heavy metal concentration remaining after extraction. These are calculated with Equation (3) from the measured threshold shifts after extraction, using the parameters listed in Table 1. Table 4 shows that clinoptilolite is, indeed, effective in extracting lead and copper from drinking water. The remaining heavy metal pollution after extraction is far below the action level. The larger K for lead vs copper established previously is reflected again in the lower residual concentration after extraction.  (1 μM heavy metal spiked/extracted) tap water sample vs tap water as drawn.
Triangles: Pb 2þ spiked/extracted; circles: Cu 2þ spiked/extracted; squares: tap water as drawn. Interference from common co-cations These concentrations are significantly greater than the 'action levels' for heavy metals under the lead-and-copper rule, but alkaline and alkaline earth metal co-cations at these levels are not harmful and should not lead to 'false positives'. As described in 'Preparation of test solutions', we account for the common tap water interference 'cocktail' by preparing calibration solutions and testing our WGTFTs, using tap water rather than DI water. We have, nevertheless, studied the interference from co-cations on our WGTFT heavy metal sensor. Figure 7 shows the transfer characteristics of a SnO 2 WGTFT transistor sensitised with a clinoptilolite membrane when using tap water with deliberately added sodium (Na þ ) ions (from NaCl) or calcium (Ca 2þ ) ions (from CaCl 2 ) in the sample pool vs tap water as drawn in the reference pool.
There are measurable threshold shifts under co-cations, as summarised in Table 5.
We find that the highest threshold shifts due to Na þ and Ca 2þ are significantly smaller than ΔV th (sat) under Pb 2þ or Cu 2þ . At 100 μM, we find a shift of 85 mV for Na þ and 120 mV for Ca 2þ while ΔV th (100 μM) >400 mV for Cu 2þ .
According to Equation (3) with the parameters listed in Table 1, the action levels of 72 nM for lead and 20.5 μM for copper would lead to threshold shifts of 289 mV (lead) or 356 mV (copper), both significantly larger than 100 mV.
Hence, at least qualitatively, we can still decide potability with respect to lead and copper despite interference. To quantify selectivity, we observe from Figure 7(a) that c 1/2 ≈ 30 μM for Na þ , hence, selectivity S for lead over sodium is S(Pb 2þ vs Na þ ) ¼ K(Pb)/K(Na) ¼ c 1/2 (Na)/c 1/2 (Pb) ≈ 13,000; log S ≈ 4.5.   Figures 2 and 3. Then, we replace the extracted solution in the sample pool with non-extracted (i.e., actual) sample and test for threshold shift. We tested this procedure by applying it to a control, and a sample contaminated with lead at potability limit (72 nM) ( Figure 8).  In Figure 8(

SUMMARY AND CONCLUSIONS
The cheap and naturally abundant zeolite clinoptilolite is not only useful for the extraction of the toxic heavy metals copper and lead from contaminated water but also their sensing and monitoring of the lead-and-copper rule. When we embed powdered clinoptilolite into a plasticised PVC membrane that we use to separate a sample pool and a reference pool in water-gated SnO 2 thin-film transistor, we find a membrane potential that leads to transistor threshold shift in response to the presence of either Pb 2þ or Cu 2þ in the sample pool. Threshold shift follows a Langmuir-Freundlich (LF) characteristic (Equation (3). This is in contrast to Nikolsky-Eisenman (NE) characteristics (Equation (1) WGTFTs as a low footprint sensor technology for monitoring the lead-and-copper rule, and to confirm the effectiveness of attempts to extract lead and copper from water. For the practical use of such sensors, potential interference from common co-cations such as Na þ , Ca 2þ and Mg 2þ is a more serious challenge than LoD. However, we provide and verify a routine for generating interferant-matched reference solutions by using clinoptilolite as extractant as well as a sensitiser, closing the interference 'loophole'.
The reason for the unusual but useful LF response characteristic warrants further study. We note an important difference between conventional macrocycle-sensitised potentiometric sensors and zeolite-based sensors; namely, macrocycles capture the target ion and hence charge the membrane. Zeolites are ion exchangers, so acquire no net charge under target ion exposure, but may well build up superficial dipoles.