Disposable sensors for environmental monitoring of lead, cadmium and mercury

Accepted Manuscript Title: Disposable sensors for environmental monitoring of lead, cadmium and mercury Author: Kátia Duarte, Celine I.L. Justino, Ana C. Freitas, Ana M.P. Gomes, Armando C. Duarte, Teresa A.P. Rocha-Santos PII: DOI: Reference:

S0165-9936(14)00162-9 http://dx.doi.org/doi:10.1016/j.trac.2014.07.006 TRAC 14289

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Trends in Analytical Chemistry

Please cite this article as: Kátia Duarte, Celine I.L. Justino, Ana C. Freitas, Ana M.P. Gomes, Armando C. Duarte, Teresa A.P. Rocha-Santos, Disposable sensors for environmental monitoring of lead, cadmium and mercury, Trends in Analytical Chemistry (2014), http://dx.doi.org/doi:10.1016/j.trac.2014.07.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Disposable sensors for environmental monitoring of lead, cadmium and mercury Kátia Duarte a, *, Celine I.L. Justino a, Ana C. Freitas a, b, Ana M.P. Gomes c , Armando C. Duarte a, Teresa A.P. Rocha-Santos a, b a

Department of Chemistry & CESAM, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal b ISEIT/Viseu, Instituto Piaget, Estrada do Alto do Gaio, Galifonge, 3515-776 Lordosa, Viseu, Portugal c CBQF/Escola Superior de Biotecnologia, Catholic University, Rua Dr. António Bernardino de Almeida, P-4200-072 Porto, Portugal

HIGHLIGHTS  Disposable sensors for environmental monitoring of lead, cadmium and mercury  Disposable electrochemical sensors for detection of lead, cadmium and mercury  Operating principles and analytical parameters of disposable sensors  Advantages and limitations of disposable sensors in environmental monitoring ABSTRACT Miniaturization is an increasing trend in the field of analytical chemistry as a response to the need to develop new analytical techniques for food, clinical, and environmental applications. There is therefore also an increasing trend towards the use of miniaturized disposable sensors, which are inexpensive and designed to be one-shot and do not require pre-treatment prior to use or cleaning between measurements. This review describes disposable sensors for detection of lead, cadmium and mercury in the environment, taking into account their analytical performance. Further, we also discuss the role of certain factors, such as the immobilization procedure and surface modification affecting the analytical characteristics of sensors. Finally, we comment on future applications and potential research interest in this field. Keywords: Analytical performance Cadmium Detection Disposable sensor Electrochemical sensor Environmental monitoring Lead Mercury Potentially toxic element Screen-printed sensor *Corresponding author. Tel.: +351 232910100; Fax: +351 232910183. E-mail address: [email protected] (K. Duarte)

1. Introduction The presence of potentially toxic elements (PTEs) in the environment is of particular concern due to their adverse effects on ecosystems and human health. PTEs, namely 1 Page 1 of 13

lead, cadmium and mercury, were listed as priority substances in the field of water policy by the Directive 2008/105/EC of the European Union Parliament and of the European Council on environmental quality standards in the field of water policy [1]. In this list, cadmium and mercury, as well as being priority substances, were also identified as priority hazardous substances [1]. The standard and traditional techniques for analysis of traces of PTEs require some costly analytical techniques, such as atomic absorption spectroscopy and atomic fluorescence spectrometry [2], inductively-coupled plasma-mass spectrometry (ICP-MS) [3], optical emission spectroscopy (OES) [4], and X-ray fluorescence spectrometry [5], and specialized personnel to carry out the operational procedures [6]. In these methodologies, besides sample collection and transport to the laboratory, a sample pre-treatment step and pre-concentration of the target compounds in the sample are required and are labor intensive. Efforts are ongoing to develop rapid and inexpensive techniques, such as enzymatic biosensors [7], for insitu analysis of PTEs for the early detection of pollution in several environmental compartments. There has therefore been an increasing trend to develop miniaturized sensing strategies because of their advantages, such as disposability, simplicity, rapid response, and readiness for field application. The introduction in the past few years of nanomaterials, such as metal nanoparticles (NPs), quantum dots, magnetic NPs or nanotubes (NTs), which enhance selectivity, sensibility, and reproducibility, has been of paramount importance for improving limits of detection (LODs) and to allow adequate miniaturization of the sensing devices [8,9]. Furthermore the combination of the nanomaterials using the techniques and the tools of surface modification can give rise to highly selective, sensitive, cost-effective, disposable sensors for PTEs. In this review, we summarize the different environmental applications of disposable sensors in the determination of PTEs, namely lead, cadmium, and mercury. We discuss the role of various factors affecting the analytical characteristics of sensors, including the immobilization procedure used. We also consider the selectivity of sensors towards PTEs and their operational characteristics. We report trace-level LODs for lead, cadmium and mercury.

2. Disposable sensors Disposable sensors are economical in nature and designed to be one-shot, so they do not experience so-called memory effects; they do not require further pre-treatment prior to use or cleaning between measurements and they are very versatile in different applications [10,11]. Disposable sensors and biosensors have many areas of application, such as environmental protection [12–14], food analysis [15–17], and medical diagnosis[11,18,19]. Various disposable sensors for environmental monitoring of Pb, Cd and Hg were fabricated using different technologies, such as screen-printing, toner transfer, and lithography, which we discuss in the following sub-sections. 2.1. Screen-printed Table 1 shows selected disposable screen-printed sensors [13,20–26] used for determination of Pb(II), Cd (II) and Hg(II) in environmental samples, taking into account their analytical figures of merit, such as LOD and linear range [27]. Screen-printed sensors (Fig. 1) are miniaturized devices fabricated by depositing metal or graphite-loaded inks on a support, and, due to their disposability, they avoid cross-contamination [28–30]. These sensors have several advantages, such as low cost 2 Page 2 of 13

of production, flexibility in design, and ease of mass production with consistent chemical performance [28–30]. There are many commercial sources of screen-printed sensors in different configurations, and they become very convenient to fabricate in small batches with screen-printing machines [30]. The formats of screen-printed sensors are changeable in line with the requirements of a specific analyte. Also, the surface of a screen-printed sensor can be easily modified to fit many different pollutants and to achieve a variety of improvements [30]. Screen-printed sensors combine ease of use and portability with simple, inexpensive analytical methods. Li et al. [30] reviewed the recent developments and applications of screen-printed electrodes (SPEs) in environmental assays, including the determination of Pb, Cd and Hg, but they did not cover disposable screen-printed devices. According to Arduini et al. [31], sensors for lead detection are usually modified with carbon, bismuth (Bi), gold or other materials in order to improve sensitivity. The modifiers can be transplanted onto the surface of SPEs [30,31]. Solid-contact Pb(II)-selective electrodes and solid-contact reference electrodes suitable for use as disposable sensing devices for environmental monitoring of lead in river waters were prepared on screen-printed substrates by Anastova et al. [20]. They used as immobilization procedures manual drop casting of poly(3-octylthiophene-2,5 diyl) (POT) onto carbon SPEs and the electropolymerization of poly(3,4ethylenedioxythiophene) (PEDOT) from its monomer onto carbon SPEs. By manual deposition of POT, the electrodes produced (n = 8) showed identical LODs of 1.20 ± 0.08 nM but the baseline values were more scattered, having an average of 46.7 mV and a standard deviation of 18.2 mV. The reason for this behavior most probably arises from the manual drop-casting of the polymeric layer, which may result in variable thickness and coverage, so leading to a variation of the potential at the carbon/POT interface [20]. Improving the precision of the conducting polymer-layer thickness using electropolymerization can reduce the inter-electrode offset. Anastova et al. [20] therefore electropolymerized PEDOT into carbon SPEs and obtained an increase of the LOD to 10.9 ± 1.1 nM, which was explained by the difference in lipophilicity between PEDOT and POT. While the inter-batch reproducibility of the PEDOT carbon SPEs is striking, Anastova et al. [20] stated that the variations in membrane thickness originating from manual drop-casting have virtually no influence on the entire response of the sensor, since a 20% change in membrane thickness causes a decrease in the zero-point of only ~1 mV. The sensors were successfully applied to the detection of Pb(II) in real samples of rivers, and the sensor performance was comparable to a method based on ICP-MS. A coefficient of determination R2 of 0.989 between the results obtained with the two methods was achieved. In summary, the highly reproducible nature of the sensors and the ability to use them as single-shot devices are their main advantages in the detection of Pb(II) in river water samples[20]. Bouden et al. [21] developed an SPE-based sensor to detect Pb(II) in spiked water samples in the nM range. The LOD of the sensor was 1.2×10−9 M. The SPE sensor was modified with 4-carboxy-phenyl functions by electrochemical reduction of diazonium salt, which was grafted onto the carbon electrodes. Diazonium electrochemical grafting is a method to obtain a covalently bound organic coating. This modification of the sensor allowed better definition of the analytical signal relative to Pb reoxidation. The influence of the adsorption time was evaluated in the 0–20 min range using CH3COONH4 buffer with Pb(II) (5x10-8 M). It was observed that the intensity of the Pboxidation peak increases proportionately with adsorption time in the first 5 min range, which has been attributed to chelation kinetics of the metallic cations by the carboxylate functions grafted on the electrode surface. It was demonstrated that the adsorption time 3 Page 3 of 13

affects the linearity, so an increase in adsorption time will result in a decrease of the lower limit of quantification. Moreover, an interference study was carried out with Cu(II), Hg(II), Al(III), Mn(II), Zn(II), Cd(II) and no major interference was observed from those cations on the quantification of Pb(II). Repeatability was evaluated after analysis of seven replicates of a 5x10-8 M solution of Pb(II) with a single sensor. Wellshaped voltammograms were obtained for all experiments and the calculated relative standard deviation (RSD) was 13%. In order to verify the sensor disposability, the reproducibility of the procedure was also evaluated by performing a series of analyses with six different electrodes (sensors). The RSD obtained was 8% [21]. SPEs modified with gold NPs have been employed for the determination of many analytes, such as lead [13]. Metal NPs have unique size-dependent properties, such as large effective surface area, enhancement of mass transport and catalysis. The changes in electronic structure and adsorption behavior at the nanoscale induce changes in the electrolytic reaction mechanism and kinetics. Also the different morphology of the NPmodified electrode surface changes the mass-transport characteristics at the interface [32]. Mandil et al. [22] developed a disposable SPE modified with gold film for the simultaneous detection of spikes of Pb(II)and Hg(II) in tap water. The sensor RSD (n = 6) was found to be 6.0% and 3.0% for lead and mercury, respectively, indicating good repeatability of the method. In addition, a pre-concentration step was performed using magnetic particles modified with thiols to enhance the LOD to concentrations below 1 μgL-1 of such compounds: the LODs for lead and mercury, after a deposition time of 120 s, were improved to 0.02 μgL-1 and 0.08 μgL-1, respectively. Bi is one of the most used modifiers in electrochemical sensing due to its good analytical performance and its environment-friendly characteristics [30,31]. The modification of an electrode/sensor with a Bi film for lead detection essentially involves electroplating a film of Bi onto the surface of the electrode/sensor [28, 31]. This process can be performed in three different ways (ex situ, in situ and bulk). Ex situ, the electroplating of the Bi film is carried out prior to the transfer of the electrode to the solution in which the analyte is present. In situ, Bi ions are added directly into the solution to be analyzed, and the analyte is incorporated as the Bi film is formed. In the bulk method, the modification with Bi takes place during the electrode production that, in SPEs, lies in preparing a mix of graphite ink and a certain amount of a Bi precursor, such as Bi citrate or Bi aluminate, before the printing procedure. Fu et al. [25] developed a multi-walled carbon-NT (MWCNT) and Nafion composite-modified SPE with in situ plated Bi film for detection of Pb(II) and Cd(II) in freshwater samples. Since the Bi film was plated on the electrode, the voltammetric analytical curve of the electrode depended on both the Bi film and the substrate at the time. The linear range was 0.05–100 μgL-1 for Pb(II) and 0.5–80 μgL-1 for Cd(II). The LOD was 0.01 μgL-1and 0.2 μgL-1for Pb(II) and Cd(II), respectively. The enhanced current response has been attributed to both Bi alloy, which causes Pb(II) and Cd(II) to be reduced more easily, and MWCNTs providing more absorbing sites for Pb(II) and Cd(II) binding, leading to an effective accumulation of Pb(II) and Cd(II). A series of determinations (n = 10) for 10 μgL-1 each of Pb(II) and Cd(II) were used to evaluate the reproducibility of the sensor. The RSDs were 1.0%, and 1.8%, respectively, and the results demonstrated that the sensor displayed a stable response. The experimental work confirmed that the proposed method was highly accurate, precise and reproducible for the analysis of Pb(II) and Cd(II) in real samples of tap, lake and drinking waters. CNTs are among the nanomaterials extensively studied for their physical properties, such as large surface area, chemical and thermal stability, and electronic and optical 4 Page 4 of 13

properties advantageous in the design of electrochemical sensors. Martín-Yerga et al. [13] developed gold nanostructured screen-printed transducers modified with CNTs for the detection of Hg(II). For this sensor, a linear range of 0.5–50 gL-1 was obtained in acidic solutions of Hg(II) with an intra-sensor reproducibility of 11.4% (eight repeated measurements for 10 gL-1 of mercury). The LOD was 0.2 gL-1 of mercury. The performance of the sensor was evaluated using real samples of tap water and river water, and was found to be comparable with ICP-MS. To study the stability of the sensors after fabrication, several sensors were prepared and stored at room temperature. A solution of 10 gL-1 of mercury was measured for different days of fabrication. The sensor lost 15% of the analytical signal measured on the day of its fabrication, but then remained constant during the first 30 days [13]. Furthermore, the ability of the sensor in the simultaneous detection of lead and mercury was also evaluated [13], with the following results:  linear range of 2–100 gL-1 and 2–60 gL-1 with an LOD of 2.0 gL-1 and 1.9 gL-1 for lead and mercury, respectively;  inter-sensor reproducibility of 3.4% for lead and 4.1% for mercury. The competition between lead and mercury for gold sites may be the main cause of obtaining an LOD for Hg that was not as low as in the separate determination of lead and mercury. These results demonstrated the possibility of fabricating a cheap, fast, low-cost sensor for the simultaneous determination of two potentially toxic ions at the gL-1 range of concentration. Graphene is a nanomaterial with two-dimensional layers of sp2 hybridization carbon. Graphene has several properties that make it interesting for use as a modifier that increases sensor sensitivity, including high conductivity, large surface area, fluorescence quenching; it also offers the possibility of functionalization for increasing detection capability and efficiency [33]. Graphene can be employed as a support material for dispersion and stabilization of other materials, and the incorporated material can also prevent the problem of the aggregation of graphene. Martín-Yerga et al. [13] developed gold-nanostructured screen-printed transducers modified with graphene for the detection of Hg(II). Moreover the performance of the graphene-modified sensor was compared with the previously described CNT-modified sensor and a gold-nanostructured screen-printed sensor without graphene/CNT modifications. It obtained an intra-sensor reproducibility of 9.4% (eight repeated measurements for 20 gL-1 of mercury) and an LOD of 1.9 gL-1. Although the reproducibility was comparable, the LOD was almost 10 times higher than the LOD obtained for the sensor modified with CNTs. Nevertheless, the LODs and the linear ranges of both graphene- and CNT-modified sensors were considerably enhanced compared to the LOD and the linear range obtained for the non-modified sensor. This improvement may be because the electrode surface in these sensors entirely comprises nanomaterials with the advantages that entails, since both CNTs and graphene oxide completely covered the working electrode and the non-modified carbon electrode only worked as an electrical contact. 2.2. Fabricated by other technologies or processes

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Table 2 shows the disposable sensors fabricated by other technologies or processes (i.e., non-screen-printed) [10,34,35,37–40] used for determination of Pb(II), Cd(II) and Hg(II) in environmental samples taking into account their analytical figures of merit. Figueiredo-Filho et al. [10] developed a voltammetric disposable copper mini-sensor ex-situ modified Bi film for sensing Pb(II) and Cd(II). The electrodes were fabricated using the toner-transfer method and polyester paper. First, the design of the electrode was transferred to a copper plate using a press with heating; second, the copper not protected by the ink was removed, third, the ink was cleaned and nail varnish was applied as an insulator; and, finally, the electrodeposition of Bi was performed ex situ. Under optimal experimental conditions, the voltammetric response linearly depended on the analyte concentrations over the range 1.3x10-6–1.3x10-5 molL-1 and 9.9x10-7– 1.2x10-5 molL-1 with LODs of 8.3x10-7 molL-1and 5.3x10-7 molL-1 for Pb(II) and Cd(II), respectively. Using the sensor, the determinations of both Pb(II) and Cd(II) were carried out in natural waters of rivers and the results obtained were comparable with those obtained using flame atomic absorption spectrometry. Repeatability studies were carried out by successive measurements (n = 10) using the sensor in the presence of concentrations of 1.1x10-5 molL-1of Pb(II) and 5.7x10-5 molL-1 of Cd(II). After each measurement, the working sensor was rinsed thoroughly with water, and transferred into the blank electrolyte to remove any adsorbed species. The continuous use of the electrode, without surface renewal, yielded RSDs of 1.8% and 2.4% for Pb(II) and Cd(II), respectively. Also, the response of the multiple mini-sensors (n = 5) in the presence of Pb(II) and Cd(II) ions, under the same conditions, was also evaluated. In this case, an RSD below 5% was obtained for both appraised PTEs, indicating that the performance of the fabricated sensor was good. Kokkinos et al. [35] compared the performance of a disposable lithographicallyfabricated Bi-microelectrode array with an in-situ electroplated Bi-film electrode in the determination of Pb(II) and Cd(II). Fig. 2 shows the process of fabricating the microelectrode array. The Bi-microelectrode array produced well-defined peaks in the linear range 0–60 μgL-1. The LODs were 3.4 μgL-1 for Pb(II) and 6.7 μgL-1 for Cd(II). The electroplated Bi-film electrode produced a clear Cd peak only at Cd(II) concentrations greater than 20 μgL-1; this behavior was attributed to the low coverage of the substrate surface with Bi in static solution. The higher LODs for the electroplated electrodes of 8.2 μgL-1 for Pb(II) and 28.2 μgL-1 for Cd(II) were attributed to convective effects during pre-concentration in static solution compared to the Bi-microelectrode array [35]. It was verified that the Bi-microelectrode array could perform 8–10 consecutive determinations (equivalent to 15–20 min of operation) without significant change in sensitivity. After this time of operation, loss or fracturing of the Bi film was observed. Despite the promising results, the Bi-microelectrode array was not applied to the determination of Pb(II) and Cd(II) in real environmental samples. The functionalization of a CNT surface by incorporating a large variety of functional groups improves the CNT properties [33,36], so Bagheri et al. [39] assembled a triphenyl-phosphine-modified CNT composite with a room temperature ionic liquid as pasting binder for simultaneous determination of Pb(II), Cd(II) and Hg(II). For the electrode fabrication, a carbon-paste mixture was prepared by thoroughly hand mixing graphite powder with paraffin oil. The composite mixture was packed firmly into a piston-driven carbon-paste electrode holder. Finally, a CNT-paste electrode was prepared by mixing MWCNTs with graphite powder. Fig. 3 shows the working principles of the fabricated sensor. The suggested mechanism for the working principles of the fabricated electrode arose from the metal ions in solution being capable of adsorption or incorporation on the surface of the 6 Page 6 of 13

modified electrode followed by reduction and oxidation steps (stripping). The LODs were 6.0x10−5 M, 7.4x10−5 M and 9.2x10−5 M for Pb(II), Cd(II) and Hg(II), respectively. The method was applied to the determination of cadmium, lead and mercury in soil, gasoline, fish, tap water and wastewater samples, and its performance was comparable with a method based on ICP-OES. The repeatability of the sensor was investigated by repetitive measurements for Pb(II), Cd(II) and Hg(II) concentrations of 1.0 and 15.0 nM. The RSD of the peak currents for five replicate determinations of 1.0 nM and15.0 nM of cadmium were 1.6% and 1.5%, respectively, and the corresponding values for lead and mercury were 2.5% and 2.3%, 1.9% and 1.7%, respectively. The preparation of five sensors and the application for the simultaneous determination of Pb(II), Cd(II) and Hg(II) at 1.0 nM and 15.0 nM concentrations allowed estimation of the reproducibility of the sensor fabrication. The corresponding RSD values were 2.9% and 2.4% for Cd(II), and 3.3% and 4.2%, and 3.1% and 2.8% for Pb(II) and Hg(II), respectively. The results indicate that the modified electrode has high reproducibility and excellent repeatability using this fabrication procedure. Furthermore, this sensor constitutes an alternative to ICP-OES, which in the determination of ions in complex matrices requires very expensive, complex equipment.

3. Conclusions and future trends Research, development, design and fabrication of disposable sensors for PTE detection in environmental samples has become of great interest to the scientific community. Several disposable sensors based in microtechnologies and nanotechnologies are under development, and these technologies and materials combined with electrochemical techniques are offering new disposable sensors for determination of PTEs with advantages, such as high sensitivity, speed, cost efficiency, and ease of use. The disposable sensors developed for determination of lead, cadmium and mercury in environmental applications are based on electrochemical transducers. This review shows that the materials used as contact layer or as modifiers can greatly affect the analytical performance of the disposable electrochemical sensors. The disposable sensors used for one-shot measurements of Pb(II), Cd(II) and Hg(II) in environmental samples showed adequate inter-sensor reproducibility, sensitivity and selectivity, achieving very low LODs. These disposable sensors detect Pb(II), Cd(II), and Hg(II) and are best suited to speciation analysis and monitoring of those cations, rather than the determination of total elemental composition, as done with standard methods based on atomic absorption spectroscopy, atomic fluorescence spectrometry, ICP-MS, and ICPOES. In the coming years, this field should focus research efforts on unique nanomaterials and transducer techniques other than electrochemical in order adequately to measure the concentration of PTEs. Further work is needed on the development of simple, costeffective, rapid, sensitive and selective disposable sensors for detection of trace concentrations of Pb(II), Cd(II) and Hg(II) under field conditions, ensuring that in-situ results are comparable with those of laboratory instruments. Due to the great interest and high performance of disposable sensors in environmental applications, their commercialization is likely to come in the near future. Research efforts should therefore focus on recycling the materials used in fabricating these sensors, since we found no studies on this subject. Acknowledgements 7 Page 7 of 13

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[30]M. Li, Y-T Li, D-W Li, Y-T Long, Recent developments and applications of screen-printed electrodes in environmental assays- A review, Anal. Chim. Acta 734 (2012) 31-44. [31] F. Arduini, J.Q. Calvo, A. Amine, G. Palleschi, D. Moscone, Bismuthmodified electrodes for lead detection, Trends Anal. Chem.29 (2010)1295-1304. [32]C. Gao, X.-J. Huang, Voltammetric determination of mercury(II), Trends Anal. Chem. 51 (2013) 1-12. [33] K.V. Ragavan, N. K. Rastogi, M. S. Thakur, Sensors and biosensors for analysis of bisphenol-A, Trends Anal. Chem. 52 (2013) 248-260. [34] H. Li, J. Li, Z. Yanga, Q. Xua, C. Houa, J. Penga, X. Hua, Simultaneous determination of ultratrace lead and cadmium by square wave stripping voltammetry with in situ depositing bismuth at Nafion-medical stone doped disposable electrode, J Hazard. Mat. 191 (2011) U26–31. [35] C. Kokkinos, A. Economou, I. Raptis, T. Speliotis, Disposable lithographically fabricated bismuth microelectrode arrays for stripping voltammetric detection of trace metals, Electrochem. Commun. 13 (2011) 391-395. [36]C. I. L. Justino, T. A. P Rocha-Santos, A. C. Duarte, Advances in point-ofcare technologies with biosensors based on carbon nanotube, Trends Anal. Chem. 45 (2013) 24-36. [37] M.-P. N.Bui, C. A. Li, K. N. Han, X.-H. Pham, G. H. Seong, Electrochemical Determination of Cadmium and Lead on Pristine Single-walled Carbon Nanotube Electrodes, Anal. Sci.28 ( 2012) 699-704. [38] J. Guo, Y. Chai, R. Yuan, Z. Song, Z. Zou, Lead (II) carbon paste electrode based on derivatized multi-walled carbon nanotubes: Application to lead content determination in environmental samples, Sens. Actuators, B 155 (2011) 639–645. [39] H. Bagheri, A. Afkhami, H. Khoshsafar, M. Rezaei, A. Shirzadmehr, Simultaneous electrochemical determination of heavy metals using a triphenylphosphine/MWCNTs composite carbon ionic liquid electrode, Sens. Actuators, B 186 (2013) 451–460. [40] M. Chen, M. Chao, X. Ma, Poly(crystal violet)/graphene-modified electrodefor the simultaneous determination of trace lead and cadmium ions in water samples, J Appl. Electrochem. in press DOI 10.1007/s10800-013-0641.

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Captions Fig. 1. Screen-printed device with three-electrode configuration. {Reprinted from [28], © 2013 with permission from Elsevier}. Fig. 2. (A) The fabrication process of the bismuth-microelectrode arrays. (B) Scanning electron microscopy image showing the two-dimensional surface geometry of the bismuth-microelectrode arrays. {Reprinted from [35], ©2011, with permission from Elsevier). Fig. 3. The suggested mechanism for the working principles of the fabricated sensor in 0.1 M KNO3/0.1 M HCl solution containing Cd(II), Pb(II) and Hg(II). (A) Cd(II), Pb(II) and Hg(II) in solution can be adsorbed or incorporated on the surface of modified electrode; (B) reduction step; and, (C) oxidation step (stripping). {Reprinted from [39], ©2013, with the permission from Elsevier}. Table 1. Selected examples of screen-printed disposable sensors used for detection of Pb(II), Cd(II)and Hg(II) in environmental samples

Immobilization procedure/sensor fabrication

Transducer

Analytes

LOD

Dropping AuCl4 on the screen-printed electrode surface at constant current intensity

Electrochemical

Hg(II)

0.2 µgL-1

Electrochemical

Hg(II)

1.9 µgL-1

Electrochemical

Pb(II)

1.20 nM

Electrochemical

Pb(II)

10.9 nM

Electrochemical

Pb(II)

1.2×10−9 M

Pb(II)

0.5 µgL-1

Hg(II)

1.5 µgL-1

Deposition of the MWCNT–COOH dispersion on the electrode surface Dropping AuCl4 on the electrode surface at constant current intensity Deposition of the graphene oxide dispersion on the working electrode surface Manual drop-casting of poly(3octylthiophene–2,5 diyl) (POT) onto carbon-screen printed electrodes Electropolymerization of poly(3,4ethylenedioxythiophene) (PEDOT) from its monomer onto carbon-screen printed electrodes Grafting by reduction of 4-CPD in H2SO4 by chronoamperometry onto carbon-based screen-printed electrodes Deposition of gold onto screenprinted electrode surface

Electrochemical

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Deposition of gold onto screenprinted electrode surface

Screen-printed gold-film electrode

Electrochemical

Hg(II)

1.1 ngmL−1

Electrochemical

Hg(II)

0.8 μgL-1

Bi film plated in-situ on screenprinted electrodes coated with multiwalled carbon nanotubes (MWCNTs)

Pb(II)

Bi film plated in-situ onto screen-

graphene-poly(sodium 4-

--

Cd(II)

dispersed into a Nafion solution

printed electrode coated with

0.01 µgL-1

Electrochemical

Pb(II)

0.089 µgL−1

Cd(II)

0.042 µgL−1

Electrochemical

styrenesulfonate) composite film

Table 2. Selected examples of disposable sensors fabricated by technologies or processes, other than screen-printing, used for detection of Pb(II), Cd(II) and Hg(II) in environmental samples Immobilization procedure/sensor fabrication

Transducer

Ex-situ Bi electrodeposition onto plated copper electrodes prepared by tonertransfer method

Analytes

LOD

Pb(II)

8.3x10-7

Linear range

1.3x10-6–1.3x10-5 m

molL-1

Electrochemical

9.9x10-7–1.2x10-5 m

Cd(II)

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5.3x10-7 molL-1 In-situ Bi electrodeposition onto the medical stone-doped electrode coated with Nafion

0.07 gL-1

2.0-12.0 gL-1

0.47 gL-1

2.0-12.0 gL-1

3.4 gL-1

0-60 gL-1

Cd(II)

6.7 gL-1

0-60 gL-1

Pb(II)

0.6 ppb

0.033-0.280 pp

Cd(II)

2.2 ppb

0.033-0.280 pp

Pb(II) Electrochemical

Electrodes fabricated by automatic vacuum

Cd(II)

laminating machine method

Lithography followed by sputtering of Bi to fabricate Bi-microelectrode array

Pb(II) Electrochemical

Single-walled carbon nanotube (SWCNT) film electrode prepared by vacuum filtering methods. Photolithographic patterning of a

Electrochemical

photoresist polymer on the SWCNT surface Oxidation of MWCNTs (MWCNTCOOH) Preparation of 2-aminothiophenol grafted MWCNTs (L-g-MWCNTS) L-g-MWCNTS along with graphite

Electrochemical

Pb(II)

5.9x10-10 M -

3.2x10-10 M

10.0x10-2 M

powder and paraffin oil were thoroughly mixed to prepare the derivatized MWCNT-based carbon-paste electrode Carbon paste mixture prepared by 6.0x10−5 M

thoroughly hand mixing of graphite

Pb(II)

powder with paraffin oil Composite mixture packed firmly a piston-driven carbon-paste electrode

Electrochemical

holder. CNT-paste electrode prepared mixing

Cd(II)

7.4x10−5 M

Hg(II)

9.2x10−5 M

0.1-150.0 nM

MWCNTs with graphite powder Graphene suspension was cast on the surface of the pretreated bare glassycarbon electrodes

Pb(II)

1.0×10−8 molL−1

Cd(II)

1.0×10−8 molL−1

Electrochemical

2.00×10

1.95×10−5 m

9.00×10

5.58×10−5 m

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