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Solid-Phase Extraction Coupled to a Paper-Based Technique for Trace Copper in Drinking Water Casey W. Quinn, David Cate, Daniel Miller-Lionberg, Thomas Reilly, John Volckens, and Charles S. Henry Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05436 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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Environmental Science & Technology

Solid-Phase Extraction Coupled to a Paper-Based Technique for Trace Copper in Drinking Water Casey W. Quinn†, David M. Cate‡, Daniel D. Miller-Lionberg║, Thomas Reilly III║, John Volckens*†§, and Charles S. Henry*‡┴ # †Department of Environmental and Radiological Health Sciences, Colorado State University, Fort Collins, Colorado 80523, USA; ‡School of Biomedical Engineering, Colorado State University, Fort Collins, Colorado 80523, USA; §Department of Mechanical Engineering, Colorado State University, Fort Collins, Colorado 80523, USA; ║Access Sensor Technologies, LLC, Fort Collins, Colorado 80524, USA; ┴ Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA; #Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, Colorado 80523, USA 1

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ABSTRACT

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Metal contamination of natural and drinking water systems poses hazards to public and

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environmental health. Quantifying metal concentrations in water typically requires sample

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collection in the field followed by expensive laboratory analysis that can take days to weeks to

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obtain results. The objective of this work was to develop a low-cost, field-deployable method to

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quantify

trace

levels

of

copper

in

drinking

water

by

coupling

solid-phase

10

extraction/preconcentration with a microfluidic paper-based analytical device. This method has

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the advantages of being hand-powered (instrument-free) and using a simple ‘read by eye’

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quantification motif (based on color distance). Tap water samples collected across Fort Collins,

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CO were tested with this method and validated against ICP-MS. We demonstrate the ability to

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quantify the copper content of tap-water within 30% of a reference technique at levels ranging

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from 20 to 500,000 ppb. The application of this technology, which should be sufficient as a rapid

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screening tool, can lead to faster, more cost-effective detection of soluble metals in water

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systems.

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INTRODUCTION

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Recent municipal1 and environmental2 water contamination events have highlighted public

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health concerns regarding water quality. Events like those in Flint, MI and the Gold King Mine

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in Colorado have produced considerable outrage3,

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improved monitoring.5 Affected stakeholders have placed particular emphasis on increasing the

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spatial and temporal resolution of monitoring, in an effort to promote more effective risk

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communication (and control efforts) during a contamination event.6,

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methods for trace metals analysis, however, rely on expensive equipment and time-intensive

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procedures. Therefore, a need exists for water quality monitoring methods that are rapid, low-

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cost, and scalable to the needs not only of regulatory agencies but also the general public.

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and have increased public demand for

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Standard reference

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Microfluidic paper analytical devices (µPADs) represent an emerging technology platform that

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shows promise for scalable, low-cost monitoring of water quality. Most µPAD applications have

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focused on medical diagnostics,8-10 but this technology is also well poised to make advances in

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environmental monitoring.11 The present work focuses on µPAD detection of Cu in drinking

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water. Although low-levels of Cu intake are essential for human and aquatic health, high levels

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of intake can be toxic.12,

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monitoring of Cu in drinking water under the 1991 Copper and Lead rule14 and also under the

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National Recommended Aquatic Life Criteria.15 The current EPA Action Level (concentration

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that when exceeded warrants remedial action) for Cu in drinking water is 1.3 ppm.16 Although

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colorimetric intensity17-26 and distance-based27-28 µPADs have been developed for Cu, these

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existing designs have limitations. Intensity-based µPADs require the evaluation of the color hue

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and/or intensity, which can be difficult to access accurately without the aid of imaging

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equipment and software. Distance-based µPADs are easier to evaluate in the field as they can be

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The U.S. Environmental Protection Agency (EPA) requires


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read by eye; however, such methods, to date, cannot detect Cu concentrations at or below the

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EPA limit of 1.3 ppm.

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Solid-phase extraction of metal ions has been previously demonstrated as a sample preparation

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technique for Flame Atomic Absorption Spectrometry,29-31 X-Ray Fluorescence,32,

33

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Reflectance Spectroscopy,34-36 and for intensity-based µPADs.37 In this work, EmporeTM

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chelation disks were used to extract the metal ions from the sample to reduce the potential for

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interferences from sample matrix effects and to preconcentrate Cu from tap water samples for

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subsequent quantification using a distance-based µPAD. While we focused on Cu, the method

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has broad utility to other types of analytes.38 Once captured, the Cu was extracted from the disk

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with 100 µL of acid and eluted onto a distance-based µPAD for direct quantification. Using this

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approach, a detection limit of 20 ppb was achieved, approximately two orders of magnitude

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lower than methods without preconcentration. Finally, Cu levels were measured in drinking

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water from multiple sites and gave good agreement for paired samples analyzed by ICP-MS.

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EXPERIMENTAL METHODS

Diffuse

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Chemicals. Ultrapure water (18.2 MΩcm) from a Mill-Q system was used for the preparation

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of solutions and cleaning of supplies and equipment (Merck Millipore, Darmstadt, Germany).

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Analytical-grade chemical reagents were used. Copper(II) sulfate pentahydrate, dithiooxamide

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(98.5%), and hydroxylamine hydrochloride were purchased from Sigma-Aldrich (St. Louis, MO,

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USA). Sodium acetate, nitric acid, sodium hydroxide, glacial acetic acid, and isopropanol were

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obtained from Fisher Scientific (Pittsburgh, PA, USA).

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µPAD Fabrication. The distance-based µPAD design39 and dithiooxamide Cu detection ink

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reported by Cate et. al.27 were leveraged in this work. The µPAD was created on Whatman


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(grade 4) filter paper (GE Healthcare Bio-Sciences, Pittsburgh, PA, USA) using a red wax

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barrier to enhance visibility of the colorimetric product. The distance-based µPAD design is

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shown in the supporting materials (Figure S1). In brief, red wax barriers (46 devices per 8”x11”

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sheet) were printed on filter paper using a wax printer (Xerox ColorQube 8860). Next, a

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piezoelectric ink-jet printer (Epson R280) deposited the dithiooxamide reagent along the flow

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channel according to a pre-determined gradient (five printing cycles in all) with more reagent

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deposited near the sample addition zone and decreasing amounts of reagent along the detection

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zone. The reagent gradient improves the linear response by offsetting the Lucas-Washburn40

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capillary flow as found by Cate et. al.27 For this work, dithiooxamide was printed as two

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gradients (Figure S1) in series. At the midpoint of the detection zone a second identical gradient

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was started; this second gradient provided a visible warning indication for the presence of high

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Cu concentrations (Figure S1). After printing, a 350 nL aliquot of 10%(w/w) hydroxylamine was

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pipetted onto the pretreatment zone. Next, the Whatman paper was placed inside a lamination

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sheet and passed through a laminator (Apache AL13P) six times at 340°F to melt the wax

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through the paper and to seal the device. The wax barrier and the plastic laminate sheets provide

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hydrophobic barriers to confine the sample flow within the channel and to prevent evaporation

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during analysis. A 3 mm hole was then punched into each device to form a sample addition zone;

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clear packing tape was applied to the underside of each hole creating a small reservoir for sample

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addition.

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Preconcentration and µPAD Analysis. The sample workflow for analysis of Cu is depicted

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in Figure 1. Standard solutions prepared from analytic reagents were used to calibrate the

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method.


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1 Sample Draw

2 Sample Preconcentration

3 Acid Elution

4 pH Adjustment

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6 Quantification

5 Sample Addition

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Figure 1: Schematic for sample collection, preconcentration, and µPAD analysis in six steps: 1)

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Sample water is drawn into a syringe, 2) Sample water is passed through an EmporeTM chelation

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disk for metal capture, 3) A drop of acid is passed through the EmporeTM chelation disk,

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releasing the metal ions back into solution and depositing them into a micro-centrifuge tube

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containing pre-mixed amounts of base and buffer, 4) A pH adjusted aliquot is pulled from the

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centrifuge tube 5) The aliquot is added to the distance-based µPAD, 6) After 30 min (once the

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sample has fully been wicked along the length of the µPAD), the color formation distance is read

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to determine the sample concentration.

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Sample Preconcentration. Initial development of the preconcentration technique leveraged

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work by Gazda et. al.,35 which used a polystyrene-divinylbenzene EmporeTM solid-phase

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extraction Chelation disk (3M) housed in a 13 mm Millipore Swinnex filter holder; however,

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modifications were made to optimize this technique. We used computer-aided design software

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(SolidWorks® ANSYS, Inc., Canonsburg, PA, USA) and rapid-prototype machining to develop

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a holder that is chemically resistant (made from Sabic Ultem 1000 thermoplastic) and thus,

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reusable with 10 mm chelation disks. Head space for the chelation disk was integrated into the

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upper portion of the filter holder to accommodate the swelling that occurs when the disk is


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wetted; this feature reduces the amount of backpressure during preconcentration (Figure 1 step

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2). Drain manifold tracks were added to the lower portion of the filter holder and a polyether

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ether ketone (PEEK) mesh was used to eliminate blockage of the outlet of the filter cartridge. As

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a result, the holder allows for a high acid extraction efficiency (Figure 1, step 3) with a minimal

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acid wash volume. The final preconcentration design consisted of a 10 mm punch of solid-phase

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extraction chelation disk (3M) paired with a 10 mm punch of fine woven PEEK mesh which

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were placed inside the custom holder (Figure 2).

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Copper was adsorbed to the disk by passing a 30-mL aqueous sample through the EmporeTM

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chelation disk by hand with a disposable syringe. Each 30-mL sample required approximately

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five minutes to complete the preconcentration (Figure 1, steps 1-3). Most of that time was

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attributed to the sample preconcentration (Figure 1, step 2) due to the limited flow rate of the

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chelation disk; estimated flow rates were comparable to the 3M documentation (400 ppb, n=5) were found in


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some of the oldest buildings on campus are shown in Figure 6 (orange/red dots), and are likely

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the result of old infrastructure and/or pipe corrosion.

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Figure 6: Cu concentrations by sample location. Inset shows all samples collected across Fort

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Collins (n=33) in August of 2015. The Colorado State University main campus is enclosed with

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the red dotted lines on the inset, and the samples (n=19) collected there are displayed on the main

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map. Maps were generated with the ggmap44 library using R 3.4.1 (R Core Team, Vienna,

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Austria).

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Implications for Metal Detection in Water Sources. A preconcentration technique coupled

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with a distance-based µPAD with minimal equipment requirements has been developed. This

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new method can be used for quantitative detection of Cu at parts-per-billion levels in water and

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has been validated against ICP-MS. Although both qualitative and quantitative colorimetric kits


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for Cu detection are commercially available (in addition to aforementioned laboratory methods),

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the distance-based measurement described here offers several key advantages. For example,

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distance-based detection is more precise than existing low-cost methods that rely on color chart

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comparisons.48 Although a single measurement is limited to a smaller range of concentrations

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(e.g. 20-375 ppb when using a 188× preconcentration factor), one can adapt the method to

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measure a wide range of Cu concentrations (from approximately 20 to 500,000 ppb) by adjusting

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the volume of sample that is preconcentrated and/or adjusting the aliquot volume that is added to

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the µPAD.

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The implementation of a second gradient along the µPAD detection region provides a warning

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indicator that the optimal detection range has been exceeded (Figure 3); this line also may be

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used to alert a user that Cu concentrations have reached or exceeded action limits. Finally, the

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output from the µPAD can be easily captured using a smartphone for long-term data storage, the

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advantages of which have been described in a number of reports.49-57 A phone application similar

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to these could be developed to measure the length of the color formation and automatically

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report that data back to a central data server. Adding this functionality would allow the

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image/results to be quickly relayed back to a central location.

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There are several limitations worth noting. 1) The color formation produced by the Cu-

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dithiooxamide complex is relatively faint especially when Cu concentrations are low (Figure 3).

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The use of another complexing reagent with higher molar absorptivity would improve

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readability. Bathocuproine has been successfully used in µPADs for the detection of Cu;22

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however, the development of a bathocuproine ink compatible with an ink-jet printer has yet to be

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developed. Silver nanoparticles (AgNP) have been used with traditional µPADs to detect ppb

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levels of Cu.23 The use of AgNP could be a potential candidate for future work and improve the


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sensitivity of the method; however, deposition of the AgNP on the µPAD and immobilization of

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the Cu-AgNP complex would need to be considered. Recent work has successfully demonstrated

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the use of Meso-tetrakis(1,2-dimethylpyrazolium-4-yl)porphyrin sulfonate (TDMPzP), a water-

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soluble porphyrin derivative for Cu detection using a distance-based µPAD.28 The TDMPzP

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ligand provides a very visible response to Cu and future work will likely try to couple this µPAD

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motif with the preconcentration technique to quantify ppb Cu in water. 2) The distance-based

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µPAD demonstrated in this work required 30 minutes to complete sample wicking and color

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formation. The Cu-TDMPzP color formation will help reduce the quantification time by

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providing a distinct color formation. Work is also underway to reduce the time to transport the

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sample aliquot along the length of the µPAD measurement zone. 3) Currently Cu is the only

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analyte that has been evaluated with this method; however, there is potential to extend this

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method to other metals. For example, colorimetric reagents have been described for arsenic,58

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chromium,17 nitrate/nitrite,59 and lead,37 all of which are regulated in drinking water by the U.S.

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Environmental Protection Agency.

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The preconcentration-µPAD system has the potential to provide water quality experts,

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technicians, and other stakeholders with a tool that can provide quick and inexpensive analysis of

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a large number of water samples. This method will not eliminate the need for standard laboratory

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sample analysis; however, this method can be used for the identification of a pollutant source in

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a river section, provide spatial-temporal feedback during accidental events (similar to the Gold

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King Mine incident), or provide municipalities with a cost-effective means to implement a

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monitoring network without a substantial investment of resources.

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ASSOCIATED CONTENT


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Supporting Information.

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The following file is available free of charge.

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One 16-page PDF document containing 4 figures, 1 equation, 8 tables, and 1 method.

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AUTHOR INFORMATION

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Corresponding Authors

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*Phone: 1-970-491-2852. Fax: 1-970-491-1801. E-mail: [email protected] (C.S.H.).

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Phone: 1-970-491-6341. E-mail:[email protected] (J.V.).

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ORCID

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Charles S. Henry: 0000-0002-8671-7728

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Casey W. Quinn: 0000-0002-6802-5250

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENT

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This work was supported by grant 1415655 from the National Science Foundation and a State of

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Colorado Advanced Industries Grant. The authors wish to thank all of the volunteers whom

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provided water samples from their homes.

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REFERENCES

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