Nanostructured and Selective Filter To Improve Detection of Arsenic ...

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Nanostructured and Selective Filter To Improve Detection of Arsenic on Surface Plasmon Nanosensors Yulieth C. Reyes,† Luis Emerson Coy,‡ Luis Yate,§ Stefan Jurga,‡,∥ and Edgar E. González*,† †

Pontificia Universidad Javeriana, Faculty of Engineering, Instituto Geofísico, Bogotá, 110231, Colombia NanoBioMedical Centre, Adam Mickiewicz University, 61614 Poznan, Poland § CIC biomaGUNE, Paseo Miramón 182, 20009, San Sebastián, Spain ∥ Department of Macromolecular Physics, Adam Mickiewicz University, 85 Umultowska str., 61-614 Poznan, Poland ‡

S Supporting Information *

ABSTRACT: The development of a pretreatment system to assist surface plasmon sensor-based measurement of arsenic in water is described. The system proposed addresses important issues, regarding the reliable in situ detection of arsenic in water. This system uses a primary filter made of nonactivated cotton fibers for particulate matter and chemical retention agents without modifying the arsenic concentration in the water sample. A secondary filter was designed for retention of mercury, lead, and other heavy metals without alteration of the arsenic concentration in the collected water samples to be sensed. This filter was made with amino-functionalized carbon nanotubes. The results of the operational assessment of this filter show a retention efficiency of 98% for suspended solids, 96% for mercury ions, and 2% for arsenic, a remarkable improvement toward the accurate detection and quantification of arsenic in contaminated waters. KEYWORDS: arsenic, nanofilter, nanosensor, surface plasmon resonance, carbon nanotubes

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mining, and so forth.5−7 Arsenic may remain in the natural ecosystems for a long time. It may also be present in the soil, groundwater and host lithologies.8 The above phenomenon represents a serious contamination problem in drinking water resources, which gives rise to severe consequences for public health.8−10 The World Health Organization (WHO), as well as other regulatory entities, has set 10 ppb as the allowable level of arsenic in drinking water. Detecting arsenic at such level is a challenge in the development of portable and economic highsensitivity detection systems. In this context, the science and the technology of nanomaterials may contribute to the measurement of arsenic given the measuring scales needed to quantify, measure, monitor, or even remediate.11−13

nvironmental pollution is one of the most relevant problems to deal with in the twenty-first century. The increase in concentration of heavy metals and other contaminants, in water for human consumption, have caused high environmental impact. Food security is, for example, dramatically affected by the effects of bioaccumulation on fauna and flora due to the presence of contaminants in water and sediments. Experimental processes and strategies to detect and measure contaminants in water for human consumption, especially heavy metals, have gained relevance in research. Their development has been propelled by the urgent necessity of carrying diagnosis tables out to devise plans aiming at mitigation and remediation.1−4 Arsenic is a naturally occurring element in the earth’s crust. It is found as cobalt ore as well as combined with other elements such as sulfur and metals (iron, manganese, silver, tin, and nickel) in the surface of rocks. It is introduced naturally into water bodies or anthropogenically, especially by coal combustion, insecticide and fertilizer use, industrial activity, © 2016 American Chemical Society

Received: March 29, 2016 Accepted: April 25, 2016 Published: April 25, 2016 725

DOI: 10.1021/acssensors.6b00211 ACS Sens. 2016, 1, 725−731

Article

ACS Sensors

Three collecting-sample places along the Bogota River Basin were selected: Point 1. (Puente) Puente La Virgen station, located in the SubaCota highway at latitude 4° 48′ 3.8″ N, longitude 74°5′ 57.8″ W and 2565 m altitude (this station is the first point in the Bogota River Medium Basin). Point 2. (Parque) La Florida Park, in Engativa district, located at latitude 4° 44′ 2″ N and longitude 74°8′ 36.8″ W Point 3. (Humedal) Puente Humedal Jaboque station, located in La Florida park in Engativa district, among El Dorado airport, the Juan Amarillo River, and the Medellin highway (this wetland is in the Salitre Basin). To select the sample collection places, the following aspects were taking into account: (i) the importance of the River Bogota for the country and its impact on agriculture given its pollution levels; (ii) the relevance that these places have in order to evaluate the work environments for the surface plasmon sensors; and (iii) the possibility of implementing the pretreatment system to produce maps showing the pollution caused by arsenic or other metals (mercury and lead) of the River Bogota, using SPR sensors. In order to confirm the quantity of suspended solids analysis, and arsenic concentration, three samples were taken at point 1, three at point 2, and one at point 3. Measurement of arsenic was crucial in order to carry out the necessary studies and for further design and construction of the pretreatment system. Suspended solids analysis and measurement of arsenic were carried out by ANTEK S.A. laboratories, with receipt No. 30184. Standard Methods for the Examination of Water and Wastewater APHA/AWWA/WEF were used for the measurement of parameters in the laboratory. Electrothermal atomic absorption spectrometry (ETAAS) was the analytical technique used for detection of arsenic. Gravimetric drying at 103−105 °C (Table 1) was the analytical

For detecting heavy metals, principally arsenic, spectrometric, electroanalytical, and chromatographic methods stand out among the conventional ones used in chemical analysis of water samples. These methods have some limitations in terms of their cost, portability, measurement time, sample pretreatment, and so forth. Besides, they imply to run some contamination and alteration risks due to the necessity of implementing protocols of packaging, conservation, and transport of the sample from the place where it is collected to the laboratory. These operational and cost constraints trigger the development of economic, portable, and sensitive systems to be used in situ. Surface plasmon resonance (SPR)-based sensors have shown the highest efficiency, sensitivity, and precision in detecting and quantifying chemical and biological agents.14−21 In nanoengineering, they are a widely accepted tool for characterization of interfaces; thin films and kinetic surface processes (especially hybridization reactions).22 They have also been successfully used for detection of contaminants, micotoxins, pesticides, and allergenics in food; measurement of physical quantities; and detection and biodetection of chemical products.22−24 The use of surface-plasmon-based sensors has, however, some constraints for detection of heavy metals ions in water from in situ collected samples. The sensor operability depends on the changes of the refractive index; hence, it could be altered by microorganisms, organic compounds, inorganic colloidal compounds, and chemical agents in the water sample to be sensed. These agents modify the dielectric function at the interface of the metallic substrate, and produce changes in the output signal so that the measurement of the analyte is prevented. In addition, the presence of other heavy metals, such as mercury and lead, in the collected water samples causes interference problems in the measurement. This is because the type of functionalization used in the sensors surfaces enables them to capture elements such as mercury and lead, and thus, measurements not linked to the analyte itself, arsenic, are erroneously detected. As a solution to this problem, it is suggested to perform a selective pretreatment in the collected sample to eliminate micro- and nanoparticulated material along with the heavy metal ions (mercury and lead), without affecting the arsenic concentration. A few papers on the detection and quantification of elements by SPR from in situ collected water refer to a pretreatment of samples.18−25 These studies have reported problems in the detection of heavy metal ions, as a result of the impurities in the nanostructured surface at the moment of contact with the sample. It was then concluded that sample pretreatment is essential to minimize the interference in the accurate measurement of the analyte concentration by SPR. There are no literature reports on the implementation or performance of a pretreatment system for SPR sensors. In this Article, a system capable of filtering heavily contaminated waters for SPR based sensors is presented. The nanostructures filter proposed is tested in heavily polluted waters and its performance described.

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Table 1. Analytical Technique Employed by ANTEK SA and Method for Measuring Parameters description

analytical technique

method

suspended solids arsenic

drying at 103−105 °C ETAAS

SM2540 SM3113B, SM3030

technique used for measurement of the suspended solids parameter; it was carried out with the collaboration of the IDEAM accreditation by the resolution No. 0463 of April 09, 2013. Evaluation of the Effect of the Water Samples on the Sensing Surface. The characterization of the sensing surface (which is part of surface plasmon resonance sensor), to determine the effects caused by particulate matter and other chemical agents on it, was made with spectrophotometric techniques. For this characterization, it was used a Shimadzu UV−visible spectrophotometer-UV 2600 with integrating sphere attachment for transmittance measurement in thin films. To carry out the experimental study of the effect an in situ collected water sample has on the sensing surface, a flow cell was designed and constructed under the following specifications: two square cells (2.2 cm each side and 1 mm in depth) were carved in an acrylic plate (9 cm in length, 4.5 cm in width and 2 mm in depth) to work with two samples simultaneously. Each cell was covered with a glass foil and provided with access holes. A gold film was placed in the corresponding cell and exposed to a flow of collected water. Discharge and duration of the flow were the parameters controlled by setting a syringe pump, coupled to the system, as shown in Figure 1. There was no difference in placing the sensing surface in the gravitational field direction, downward, or in the opposite direction, upward. Filter Configuration. The filter for retention of particulate matter (primary filter) is, in its first stage, a hollow polypropylene cylinder with holes on its surface for water flow. The filter was covered with a 3.5 cm in length and 0.8 cm in width stainless steel mesh for retention of bigger particulate matter. In a second stage, the filter is a hollow

MATERIALS AND METHODS

Sample Collection and Characterization. It is important to characterize the type of environments to which a typical SPR sensor is exposed during measurements. Therefore, it requires identifying recollection places that are representative of those places where the SPR sensor will be used for detecting contaminants in water. 726

DOI: 10.1021/acssensors.6b00211 ACS Sens. 2016, 1, 725−731

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ACS Sensors

Figure 1. Left: Experimental setup to evaluate the effects of exposure to the sensor gold surface to water samples taken in situ. Right: Structure of primary filter. polypropylene cylinder filled with cotton fibers as absorbent of microand nanoparticles and chemical agents (Figure 1). This material offers the following advantages: (i) high rate retention of solids in collected water samples; (ii) capacity of being functionalized for selective retention of heavy metals;26 (iii) no retention of arsenic (when the cotton fibers are not activated); (iv) low cost; (v) easy operation; and (vi) degradability. Activated carbon is widely used in water purification and filter design, but its use does not accomplish the purposes of this work. Despite the high specific surface area of the activated carbon, given its microporous morphology, obstructions of the micropores during absorption is a limitation. Because of this constraint, the pretreatment system lifetime can be drastically reduced under the operation conditions of this work. Besides, activated carbon has some limitations in its selectivity to retain certain ions.21,27 The use of carbon nanotubes has been a step forward in the processes of retention and adsorption of heavy metals and other chemical agents. Carbon nanotubes allow the design of filters given their capacity to eliminate numerous chemical agents (heavy metals, bacterial contaminants, etc.), their high specific surface areas, and their physical and chemical stability for extended periods of time use makes them ideal for sensing applications. When inserted in matrices that prevent them from moving in the environment, nanotubes are a remarkable alternative for configuring and implementing water pretreatment and water purification systems. Nevertheless, carbon nanotubes are considered, currently, high-risk material for the living beings and the environment; this consideration limits its use. It is vital to use them carefully and responsibly, following the manipulation safety and operability protocols of the containing system. As well as the activated carbon, carbon nanotubes have high specific surface area for adsorption processes. As long as these processes occur on the surface, no obstructions limit the access to the available places for pollutants capture; therefore, activated carbon and carbon nanotubes are appropriate in lifetime terms. Multiwalled carbon nanotubes were the chosen material to configure the barrier in the secondary f ilter, aimed at retaining the agents above-mentioned. Despite the small quantities of carbon nanotubes synthesized for the secondary filter construction, they were manipulated and stored under optimal security standards, and following the ASTM E2535-07 standards. Carbon nanotubes were synthesized by the microwave-based selective heating method. In the synthesis, pure graphite was used as precursor material, silver nitrate as catalyst, and a microwave oven (1500 W in power and 2.4 MHz in operating frequency) as source of energy for carbon volatilization.27 Multiwalled carbon nanotubes (MWCNTs) from 50 to 100 nm in diameter were obtained. The nanotubes used in the configuring of the filter were 60 ± 9 nm in diameter (Figure 2a). When using catalyst in the synthesis, open nanotubes are obtained, making the access to the interior easier and, therefore, increasing the number of available places for adsorption of heavy metal ions. To assess the efficiency of the filter in terms of its selectivity of heavy metal ions, it is crucial to have a carbon nanotubes sample pure enough to correlate the observed performances with the carbon

Figure 2. (a) Image showing the morphology of the purified MWCNTs obtained by scanning electron microscopy and their average diameter. (b) General scheme of pretreatment system with the location of the first and secondary filters, cylindrical support, and nanotube film. nanotubes without the noise figures caused by carbonaceous products. The obtained MWCNTs contained carbonaceous products, such as amorphous carbon, fullerenes, and graphitic nanoparticles. Thus, the carpets were purified after synthesis. The purification of the carbon nanotubes was carried out by oxidative processes, specifically by a process using nitric acid and hydrochloric acid in conjunction with mechanical processes of centrifugation. The use of acid nitric provides the surface of the nanotubes with carboxyl groups (COOH); these contribute to improve the adsorption of mercury and other heavy metals. Carboxyl groups also increase the ion-exchange capacity and make the carbon nanotubes acquire a hydrophilic character. To have a completely dispersed sample that allows the construction of the filter, it was used the nonionic surfactant Triton X-100 in a ratio of 0.4% volume of nanotubes dissolved in 10 mL of deionized water. After sonicating the sample, a stable suspension was obtained, which was usable in the construction of the filter. In order remove the liquid out of the suspension, a vacuum filtration method was used.28 In this procedure, a PES (polyether sulfone) membrane, 0.20 μm in pore size and 150 μm in thickness (PES 029025, Sterlitech Corporation), was used in a Buchner funnel. After the vacuum filtration, the solute was rinsed with isopropyl alcohol to remove the surfactant, and the circular film of nanotubes used to configure the secondary filter (386 μm in thickness) was carefully 727

DOI: 10.1021/acssensors.6b00211 ACS Sens. 2016, 1, 725−731

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ACS Sensors removed. The resulting material was cut in a rectangular shape (4 cm × 1.88 cm) and placed into the detection system). A film of nanotubes, with mean size of 60 nm in diameter, was obtained and used in the filter construction. This film was cohesive enough to have a high mechanical stability when installed and used in the pretreatment system. In constructing the secondary filter, the film of carbon nanotubes was set between two layers of high-density propylene filter paper to avoid the movement of the carbon nanotubes out of the filter, and strengthen the barrier aimed at impeding the flow of waste material in the water into the flow cell. The secondary filter was rolled around a hollow cylinder with perforations on its surface, similar to the one used in the filter using cotton Filter Architecture. Figure 2b shows the architecture of the pretreatment system. It is a primary filter followed by a secondary filter, a micropump that produces the flow of the in situ collected water, and the connection caps. Surface Plasmon Resonance Measurements. The montage used to assess the feasibility of the filter, followed the Kretschmann configuration,13−15 shown in Figure 3. The setup used an EK2000

Figure 4. (a) Concentration of suspended solids and (b) arsenic in water samples taken at selected collection points. Figure 3. Surface plasmon resonance setup used to test the quality of the pretreatment system.

Transmittance measurements of the gold sensing surface allowed establishing the deposition of particulate matter and the adhesion of other agents in the in situ collected water samples. Figure S1 shows the transmittance curves for the glass substrate coated with the gold film that contained the sensing surface for samples from point 2. Curves were recordedfor a constant flow of 0.2 mL/s and at times (t) of 0, 5, 15, 25, and 50 min. At t = 0, which corresponds to the gold surface before being exposed to the water flow, the highest transmittance value (lowest absorbance) was recorded at a wavelength of 508 nm. At wavelengths near the UV and NIR spectrum, the transmittance of the gold foil film dropped dramatically. This means that the gold foil film absorbs radiation efficiently at those ranges of the electromagnetic spectrum. At t = 5, the highest peak for transmittance values was also recorded at a wavelength of 508 nm; however, it slumped from 1363 to 652 nm. This change represents a 48% transmittance loss (considerable absorbance increment) and points out the effects of untreated water samples on the sensing surface due to the chemical agents and particulate matter in it. The inset in Figure S1a shows an exponential decay of the transmittance as the time at which the sensing surface is exposed to the water sample increases. As time increases, the sorption of chemical agents and deposition of particulate matter decreases drastically. A water sample of 500 mL collected from point 2 was used to assess the filter of cotton fibers. Arsenic was added to the sample in a ratio of 1 ppm to determine if retention is performed once the sample has gone through it; results are shown in Figure S2. These results support the selection of

control unit (Thorlabs) with a constant current control of a laser diode of 0−100 mA and a tolerance factor of 0.1%. The photodetector was set between 20 and 125 with a bandwidth of 3 dB and 10 kHz. The tension range was of 8−12 V DC with a source limit of 130 mA. The laser diode used is a L808P10, wavelength λ = 808 ± 7 nm (Thorlabs), with a power of 10 mW at 25 °C. Additionally, the prism used was made of BK7 glass with a refraction index η = 1.51 ± 0.01. The prism is optically coupled to the system by immersion oil, with a refraction index η = 1.51 ± 0.01 and 1.25 mm of thickness. The gold thin film resides on top of the prism. Gold thin film Au(111) textured of 50 nm in thickness and 0.4 nm of roughness was deposited on commercial corning glass by sputtering method at room temperature using an adhesive Ti layer with 5 nm thickness. The gold film was functionalized with a monolayer of dithiothreitol (DDT), following a reported protocol.31 Finally, the photodetector used consisted in four cells with a signal range of 0−0.3 V, bandwidth of 250 kHz and amplification constant of k = 10.000.14,15

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RESULTS AND DISCUSSION The quantity of arsenic present in the samples taken at the collection points was below the recommended limit set by environmental authorities. Figure 4 shows the quantity of suspended solid particles (SSP) and arsenic (As). We chose the sample collection place 3 (point 3, Humedal Jaboque) for the assessment of the pretreatment system because, according to the results, it presented the highest quantity of suspended solids, which were even higher than the recommended limit (600 mg/L). 728

DOI: 10.1021/acssensors.6b00211 ACS Sens. 2016, 1, 725−731

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ACS Sensors cotton fibbers for the construction of the primary filter with the 2% arsenic retention and the 98.8% solids retention. For mercury, about 22% retention is obtained by using the primary filter. With the secondary filter manufactured with carbon nanotubes functionalized with COOH groups only from the purification treatment as noted above, concentration measurements of arsenic, mercury, and lead with and without the filter are shown in Figure 5. These tests are performed with a flow of

Figure 6. Oxidation and amino-functionalization of carbon nanotubes for sorption Hg2+.

Filters designed with amino-functionalized nanotubes exhibit effective mercury sorption without retention of arsenic.Mercury retention obtained with this type of functionalization reaches values close to 96%. In the secondary filter, carbon nanotubes-made, total arsenic retention remains below 1.26%; mercury retention increases to 55%, and lead retention to 48%. An amino-functionalized nanotubes filter was used to improve mercury retention and allow only arsenic ions flow to the sensor cell. To verify the efficiency of this type of functionalization for mercury retention, an absorbance measurement of the gold surface was made with dithiothreitol under conditions of exposure and no exposure to filtered water (1000 ppb mercury before entering the filter). This method for detection of mercury using absorbance has been used to measure arsenic concentrations near 1 ppb.31 It may also be implemented to measure mercury with the same levels of sensitivity. The curve in Figure S3 shows no change in the absorbance. This means the concentration of mercury in the filtered water sample is lower than 1 ppb. Besides, it is verified that the sample did not absorb any particulate matter or other agents that may interfere in the absorbance change. In the same way as that for mercury, selectivity for lead may be achieved as a water sample pretreated enough is obtained to make measurements using surface plasmon sensors. Finally, SPR measurements were performed using the proposed nanostructured filter coupled with the architecture described before in this Article. It is important to determine the accuracy of the detection system used. Therefore, a mixture of 10 mL of distilled water (19 MΩ·cm) with 5 ppb As(III) was prepared. Figure 7a shows the angular dependency of both the pristine surface, resonance angle 57°, and the exposed to the mixture with an angular shift of 0.5° after 5 min of recirculating flow of 2 mL/min. This displacement is well within the sensibility limits of our previously reported sensor14 and shows the accuracy of the system. Having determined the optimal resolution of the system, water from point 3 with an arsenic concentration 0.66 ppb was used; the SPM response was evaluated after the first filter and after both filters. Figure 7b and c shows the results from such measurements, respectively. The results clearly show that, by only using the primary filter on a 0.6 pbb contaminated water sample, the SPR signal has a shift 6 times bigger than that observed for a 5 pbb controlled detection. Remarkably enough the use of both filters results in much more accurate detection of the As(III) concentration. Figure 7c shows a displacement of 0.2° on the resonant angle and a change of voltage equivalent to 1:0.08. These results, conclusively show that the nanostructured filter provides a clean enough water sample to successfully detect As(III) in water with a total error of 8% from the nominal concentration.

Figure 5. Concentrations of As, Hg, and Pb before and after using the secondary filter. Measurements were made with EAAE.

2 mL/min for a time of 5 min (this is the minimum time has been identified for the sensor can measure). According to these results, arsenic maintains the concentration while mercury and lead reduce their concentration by approximately 50%, indicating that carbon nanotubes functionalized with COOH are not enough active to fully retain the mercury that enters the system. Thus, a further functionalization of the MWCNTs was performed. As it has been reported, the carbon nanotubes have a high affinity for adsorption of mercury Hg(II) ions.29 The adsorption capacity of carbon nanotubes is 440% greater than that of activated carbon. The Hg(II) can be found as mercury cation Hg2+ in aqueous solutions with pH below 2. By increasing the pH of the solvent, it is hydrolyzed to form HgOH+ until it precipitates as mercury hydroxide Hg(OH)2 and finally spontaneously evolves into HgO (s). The adsorption capacity of Hg (II) dramatically depends on the pH of collected water as it affects the cleavage sites of nanotubes and precipitation and complexation of metal ions. At acidic pH, the surface can have net positive charge restricting the adsorption of positive ions of mercury. For alkaline pH, the surface is expected to have a net negative charge, which facilitates adsorption of positive ions of mercury. Thus, adsorption of Hg(II) increases with increasing pH to a maximum value near neutral and decreases as the pH is increased to a basic value. As it has been reported,30 the optimal pH for removal of Hg with double-wall carbon nanotubes is close to the neutral value, i.e. between 6.5 and 7.5. The pH measurements of the water samples used are close at 6.8, which favors the adsorption of mercury with carbon nanotubes. To improve the rate of adsorption of mercury in the secondary filter maintaining selectivity (unabsorbed As(III)), carbon nanotubes were functionalized with ethylenediamine. Ethylenediamine is an organic compound that dissolves in water to form a solution with basic pH. The functionalization was carried out following the protocol reported in ref 29 (Figure 6). 729

DOI: 10.1021/acssensors.6b00211 ACS Sens. 2016, 1, 725−731

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metals in water and opens an important field of research and development in remediation of water with sustainable systems, efficiency, low cost, and absence of environmental impact.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.6b00211. Transmittance spectra (S1), concentration of suspended solids and arsenide before and after cotton filter (S2), and absorbance spectra (S3) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the Vice-Rectory of Research of the Pontificia Universidad Javeriana for funding the Project No. 006288 whose results are in this work. S.J. acknowledges the partial financial support from the National Centre for Research and Development under research grant “Nanomaterials and their application to biomedicine”, Contract Number PBS1/A9/ 13/2012.

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Figure 7. Plots of photodetector voltage vs angle of incidence for (a) pure surface, straight line, and after exposure of 5 pbb (As) control water, dashed line. (b) Pure surface, straight line, and exposed to 0.6 pbb (As) contaminated water after first filter only. (c) Pure surface, straight line, and after treatment with 0.6 pbb (As) contaminated water using both filters, dashed.

REFERENCES

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CONCLUSIONS The development of the pretreatment system required two filters that fulfill specific functions to allow the operation of the surface plasmon sensors in on-field measurements. The primary filter retained particulate matter and chemical agents; however, the quality of this retention was not enough to avoid the effect of interference factors on the sensing surface. The use of cotton fibers to construct the primary filter showed satisfactory results in terms of retention of particulate matter: 98% for suspended solids and chemical agents, such as lead and mercury ions. Nonetheless, the nonactivated cotton fibers allow the flow of arsenic ions; it is not altering the arsenic concentration in the collected water samples, which is one of the most important specifications for the system to accomplish. The secondary filter, constructed with amino-functionalized carbon nanotubes, retains mercury up to 96%, and 2% of arsenic, thus ensuring that the water sample is completely free of interferences caused by mercury ions at the moment of accessing the sensing surface. The SPR results showed that the pretreatment system allows measurements without a noise factor until values nearly to 5 ppb. These results lead us to conclude that the pretreatment system is an efficient method of reducing contaminants and measuring heavily polluted waters without affecting the performance of the sensor and the accuracy of arsenic detection. Finally, with appropriate modifications, the pretreatment system developed can be used to sense other types of heavy 730

DOI: 10.1021/acssensors.6b00211 ACS Sens. 2016, 1, 725−731

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DOI: 10.1021/acssensors.6b00211 ACS Sens. 2016, 1, 725−731