Nanomaterials application in electrochemical detection of heavy metals

Electrochimica Acta 84 (2012) 49–61

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Nanomaterials application in electrochemical detection of heavy metals Gemma Aragay a , Arben Merkoc¸i a,b,∗ a b

Nanobioelectronics & Biosensors Group, Institut Català de Nanotecnologia, 08193, Bellaterra, Barcelona, Spain ICREA, Barcelona, Spain

a r t i c l e

i n f o

Article history: Received 7 November 2011 Received in revised form 8 April 2012 Accepted 11 April 2012 Available online 20 April 2012 Keywords: Nanoparticles Nanotubes Electrochemical stripping analysis Ion-selective electrodes Heavy metals

a b s t r a c t Recent trends in the application of nanomaterials for electrochemical detection of heavy metals are shown. Various nanomaterials such as nanoparticles, nanowires, nanotubes, nanochannels, graphene, etc. have been explored either as modifiers of electrodes or as new electrode materials with interest to be applied in electrochemical stripping analysis, ion-selective detection, field-effect transistors or other indirect heavy metals (bio)detection alternatives. The developed devices have shown increased sensitivity and decreased detection limits between other improvements of analytical performance data. The phenomena behind nanomaterials responses are also discussed and some typical responses data of the developed systems either in standard solutions or in real samples are given. The developed nanomaterials based electrochemical systems are giving new inputs to the existing devices or leading to the development of novel heavy metal detection tools with interest for applications in field such as diagnostics, environmental and safety and security controls or other industries. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Heavy metal ions such as cobalt, copper, iron, manganese and zinc among others, play an important role in living organisms being essential in small amounts for the metabolism maintenance. However, excessive levels or even small doses of very toxic metals (i.e. lead, cadmium, mercury, arsenic, stibium, etc.) can cause serious problems on the environment and human health. Accumulation of such metals on the human body can cause diseases in the central nervous system, liver and kidneys, or skin, bones, and teeth [1–3]. Main sources of heavy metals are often coming from human activities such as industrial processes, agriculture and mining industry. Such activities may cause the release of heavy metals to the aquatic and terrestrial systems which are further transferred to the living systems including plants, animals and humans. Therefore, the development of selective and sensitive methods for the early warning pollution of trace heavy metals in different chemical systems including living systems and the whole environment is of great concern. Typical analyses of heavy metals have been based in standard spectroscopic techniques, such as atomic absorption spectrometry (AAS) [4,5] and inductively coupled plasma optical mass spectrometry (ICP-MS) [6,7]. These time-consuming techniques require complex and expensive instruments and specialized personnel to

∗ Corresponding author at: Nanobioelectronics & Biosensors Group, Institut Català de Nanotecnologia, 08193, Bellaterra, Barcelona, Spain. E-mail address: [email protected] (A. Merkoc¸i). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.04.044

carry out the operational procedures. For this reason, there is an ongoing research on the development of rapid, low-cost and friendly-use techniques for trace heavy metals detection suitable for in situ monitoring assays. Electrochemical methods compared to optical techniques offer several advantages related to their cost, simplicity and the possibility of in-field application. Among the different electrochemical techniques voltammetric and potentiometric techniques are the most reported for heavy metals detection [8,9]. Voltammetry involves the perturbation of the initial zerocurrent condition of an electrochemical cell by applying either time-constant or time-varying potential (ramp) to an electrode surface and the further measurement of the resulting current. Among the different known voltammetric techniques, stripping techniques (i.e. anodic stripping voltammetry, ASV) are the most used for trace heavy metal detection due to the high selectivity and sensitivity they present coming from the combination of the separation, pre-concentration and determination steps in one single process [10,11]. In potentiometry, information on the sample composition is obtained through the potential change/s appearing and indicated at the two electrodes. Two main devices fall within the category of potentiometric sensors: ion selective electrodes (ISEs), the chemical sensors with longest history, and field-effect transistors (FETs). They combine the fundamental membrane science with fundamental host–guest chemistry. Several ISEs have been developed for the detection of ultra-low activities of heavy metal ions [12,13]. The key point to obtain a good and reliable electrochemical sensor lies on the kind of material that constitutes the detection

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platform. In this field, nanomaterials have brought many advantages on the development of new electrochemical transducing platforms beside their use as electrochemical labels or tags for signal enhancement with interest for sensing technologies [14,15]. The unique electronic, chemical and mechanical properties of nanomaterials (i.e. carbon nanotubes, metal nanoparticles, graphene) make them extremely attractive for electrochemical sensors in comparison to conventional materials [16–18]. Sensing using nanostructured materials takes advantage of the increased electrode surface area, increased mass-transport rate, and fast electron transfer compared to electrodes based on bulk materials [19] between other factors. The synergy between electrochemical sensors technology and nanomaterials is expecting to bring interesting advantages in the field of heavy metals detection and is therefore a promising area of research and development. In this review, the aim is to give an overview on the latest trends in the development of electrochemical heavy metals sensing strategies using nanomaterials during the last 5 years although their relatively longer history. 2. Voltammetric techniques Heavy metals detection using voltammetric techniques have been deeply described in the literature as very sensitive techniques achieving very low detection limits in the nanomolar and even picomolar range of concentrations [20,21]. The electrochemical transduction material is a key point in the electrochemical enhancement sensing strategies. In this context, nanostructured platforms ranging from carbon nanomaterials to metallic nanoparticles and other natural nanostructured adsorbents are being deeply investigated [22–24]. In Table 1 are gathered different works reported during the last 5 years involving voltammetric techniques and the use of nanostructured platforms. 2.1. Carbon nanomaterials based platforms Carbon nanostructured materials such as carbon nanoparticles, carbon nanotubes, fullerenes or graphene among others have attracted considerable attention to the researchers working in the field of heavy metals detection for their use as electrodes (or electrodes modifiers) due to their excellent properties and the fact that carbon material can act simultaneously as adsorbent/preconcentrator agent and transducer platform. A proof of this, is the great number of publications referred to this field [25–27]. A review, involving the use of nanoscale carbon-based materials in heavy metal sensing and detection, has been recently published by Wanekaya et al. [28]. The most commonly used carbon material for heavy metals detection happens to be carbon nanotubes (CNTs) (single or multiwalled carbon nanotubes, SWCNT or MWCNT) owing to their large apparent surface areas and their electrocatalytic nature attributed to the activity of the edge-plane-like graphite sites at their ends [29,30]. Functionalization of CNTs with molecules with affinity toward heavy metals is a good strategy to accumulate higher amounts of metal ions on the surface, achieving lower detection limits while using lower accumulation times. Cysteine modified CNTs have been reported by different authors [31,32]. Morton and co-workers focused on the sensitive voltammetric Pb2+ and Cu2+ detection, achieving detection limits of 8.9 and 23 nM, respectively. Fig. 1a shows the scheme and the proposed accumulation and voltammetric mechanisms involving the complex formation between the cysteine and the metal ion (A), the reduction of the accumulated M2+ (B), and the final stripping step (C). The typical voltammograms of 3.9 ␮M of Cu2+ and 1.4 ␮M of Pb2+ on cysteine modified

CNTs electrode are shown in Fig. 1b. The main drawback of this system is the long accumulation times (up to 30 min) used for the detection. Electrodes based on CNTs arrays have shown promising results for heavy metals detection [33]. Guo et al. recently reported a CNT tower electrode for simultaneous trace heavy metal ions detection: Zn2+ , Cd2+ , Cu2+ and Pb2+ [34]. CNT tower is one kind of CNT array where each CNT tower contains approximately 25 million of ordered super long carbon nanotubes [35]. They report calculated detection limits (DL) of 12, 25, 44 and 67 nM for Pb2+ , Cd2+ , Cu2+ and Zn2+ , respectively for a deposition time of 120 s. A DL of 0.5 nM for lead detection using longer deposition times (10 min) was also achieved. However, the reproducibility at this concentration level was not as good as at higher concentrations. Arduini et al. worked on the development of a screen-printed electrode (SPE) modified by a carbon black film, a nanostructured material characterized by a high number of defect sites [36]. With the aim of mercury detection, they describe an indirect method involving a first step of thiols detection. In the absence of mercury, thiols are oxidized and can be easily detected by CV. When mercury is present, the thiol-Hg2+ complex is formed and it cannot be oxidized at the electrode surface resulting in a decrease of the oxidation current. A detection limit of 5 ␮M of Hg2+ is achieved. The detection of heavy metals using a carbon nanoparticles (CNP) based screen-printed electrode (SPE) has recently been explored by our group [37]. We demonstrated the effect of temperature upon the electrochemical stripping of heavy metal ions and compared the results obtained with a commercial carbonmicroparticle based SPE. While both sensors were prone to an improved heavy metals deposition yield upon a temperature increase, the CNP-based electrode showed an increased effect related to the higher available surface area and the higher number of edge-planes which allow a higher electron transfer. In Fig. 2(a.1)–(a.4) are shown the SEM images of the working electrode surface after the deposition of different heavy metals (Cd2+ , Pb2+ , Cu2+ and Hg2+ ) at two different temperatures (room temperature and 40 ◦ C) using two different surfaces (CNP and carbon-microparticles based SPE). At room temperature different deposition profiles are observed for each heavy metal deposited using both SPE (Fig. 2(a.1) and (a.3)). However, when temperature was increased the deposition of metals was found to be more homogeneous giving rise to a uniform layer (see Fig. 2(a.2) and (a.4)). The increase on the intensity of the metal ions voltammetric peaks upon the temperature increase is shown in Fig. 2b. Beside the use of CNTs and CNP, graphene nanosheets (GNS) based electrodes are gaining interest the last few years due to the excellent electronic, thermal and mechanical properties they possess [38,39]. Wang et al. recently described a simple and economical approach to produce GNS and their further use for voltammetric detection of Cd2+ , Pb2+ and Cu2+ [40]. Although reporting low detection limits for the described metals (between 1 × 10−7 and 1 × 10−11 mol/L), the paper lacks of important analytical performance parameters such as reproducibility, sensitivity, linearity and a deep interferences studies that make not easy the comparison with the results obtained with other nanomaterials. A Nafion-graphene nanocomposite electrode has been used by Willemse et al. for the electrochemical detection of Zn2+ , Cd2+ and Pb2+ [41]. However, the main drawback of this work is the use of mercury (highly toxic although in situ plated) in order to preconcentrate the metal ions. Contrary to some works reported in the literature [42,43], Browson and Banks demonstrated that commercially available graphene (NanoIntegris, IL, USA) enhanced nucleation of heavy metals but inhibited the stripping step of cadmium ions due to the presence of surfactants leading to a poor electrochemical detection of the metals [44].

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Table 1 Nanomaterials applications in relation to voltammetric strategies used for heavy metals detection. Electrochemical platform Carbon-based material MWCNT based GCE MWCNT tower based GCE NanoCB SPE Carbon NPs SPE Graphene NS MNPs-based material BiNPs SPE Bi nanostructures GNEE electrode AuNPs-graphene NS Bimetallic Au–Pt NPs DNA-Au bio-bar codes amplified gold electrode AuNPs amplified DNA-gold electrode AuNPs amplified DNA-gold electrode AuNPs amplified DNA-gold electrode Others NHAP CNT-HAP Cucumber-like HAP Nanoarray membrane Nanostructured MIP

Technique

Analyte

LOD

Sample matrix

[Ref]

DPASV SWASV Amperometry SWASV SWASV

Pb(II) and Cu(II) Pb(II), Cd(II), Cu(II) and Zn(II) Hg(II) Pb(II), Cd(II), Cu(II) and Hg(II) Pb(II), Cd(II) and Cu(II)

8.9 and 23 nM 12, 25, 44 and 67 nM 5 nM 4.8, 4.4, 7.9, 5 nM 1 × 10−11 , 1 × 10−7 and 1 × 10−8 M

Spiked lake water Acetate buffer Spiked drinking water Spiked sea water Acetate buffer

[32] [34] [36] [37] [40]

SWASV SWASV SWASV SWASV SWASV DPV SWASV DPV CV

Pb(II), Cd(II) and Zn(II) Pb(II) As(III), Hg(II), Cu(II) Hg(II) Hg(II) Pb(II) Hg(II) Hg(II) Hg(II)

0.9–4.9 ng/mL 2.5 ␮g/L 0.27, 0.1 and 0.31 nM 0.03 nM 0.04 nM 1 nM 0.5 nM 0.5 nM 10 nM

– – – River water samples – Buffer Buffer Tap and river water –

[53] [54] [59] [60] [61] [65] [66] [67] [68]

DPASV DPASV SWASV DPV DPV

Pb(II) Cd(II) Pb(II) and Cd(II) Cu(II) Pb(II)

1.0 nM 4 nM 0.00423 and 0.027 nM 0.4 ␮M 0.6 nM

Lake and tap water Lake and tap water Buffer Freshwater Waste, tap and river water

[73] [74] [75] [76] [77]

MNPs: metal nanoparticles; MWCNT: muti-walled carbon nanotubes; GCE: glassy carbon electrode; NPs: nanoparticles; SPE: screen-printed electrodes; nano CB: nano carbon black; NS: nanosheets; MIP: molecular imprinted polymer; NHAP: nano hydroxyapatite; CNT-HAP: carbon nanotubes-hydroxyapatite; SWASV: square wave anodic stripping voltammetry; DPV: differential pulse voltammetry; CV: cyclic voltammetry; DPASV: differential pulse anodic stripping voltammetry.

Fig. 1. Carbon nanotubes based electrode. (a) Proposed accumulation and voltammetric mechanism; (b) typical differential pulse anodic stripping voltammograms of 3.9 ␮M Cu2+ and 1.4 ␮M Pb2+ accumulated on GCE-MWNT-CO-Cys electrode. Ref. [32].

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Fig. 2. Carbon nanoparticle based screen-printed electrodes. (a.1–a.4) SEM images of the working electrode surface of a carbon microparticle-based SPE after the deposition of heavy metals (Cd2+ , Pb2+ , Cu2+ and Hg2+ ) at room temperature (a.1) and at 40 ◦ C (a.2) and for the working electrode surface of a carbon nanoparticles based SPE at room temperature (a.3) and at 40 ◦ C (a.4). Insets show the schematic mechanism of heavy metal deposition at the micro (a.1 and a.2) and nano (a.3 and a.4) particle based surfaces; (b) multidetection SW voltammograms for Cd2+ , Pb2+ , Cu2+ and Hg2+ (0.9, 0.5, 1.6, and 0.5 nM, respectively) at different temperatures. Inset: comparison of the voltammetric peak height of each metal at different temperatures. Ref. [37].

2.2. Metal nanostructures based platforms Platforms based on metal nanostructures have also been extensively studied and remain of great interest for electrochemical applications. Mercury or bismuth have the ability to preconcentrate heavy metals through an alloy formation and have extensively been used in their electrochemical stripping analysis [45]. Being mercury very toxic, bismuth and antimony have become alternative materials [46–50]. Nanomodified electrodes with the mentioned materials have shown distinct advantages over the conventional macroelectrodes due to the attractive and unique behavior of bismuth and antimony combined with the special properties of metal nanoparticles based electrodes which may act as random arrays of microelectrodes [51,52]. Several works have been reported for heavy metal ions voltammetric detection using bismuth or antimony nanoparticles based electrodes [53–57]. and collaborators recently described a Saturno micro/nanoparticle bismuth film for lead, cadmium and chromium

determination [55]. The bismuth film was electrodeposited onto a glassy carbon electrode (GCE) with the aid of a hydrated aluminum oxide template. The authors reported detection limits of 87 and 98 nM for lead and cadmium, respectively. Such levels are higher than the ones reported by EPA and WHO as safe limits for drinking waters [58]. On the other hand, the chromium analysis reported have much lower detection limits (2.3 × 10−15 M) which could make the system useful for real sample analysis. Zhang et al. described a self-organized morphology of Bi (called bunch-like bismuth, see Fig. 3(a.1)) grown in a bare GCE [54]. Although this system could achieve the detection of lower concentration of lead than the ones previously mentioned (lower than 12 nM) some important electrochemical parameters (i.e. reproducibility, LOD, LOQ, or selectivity) are either not mentioned or not sufficiently studied. Fig. 3(a.2) shows typical voltammetric peaks and the corresponding background for the multidetection of heavy metals with bunch-like bismuth electrode. In the context of antimony nanoparticles based electrodes, Urvanova et al. reported a porous antimony film electrode electrochemically deposited into the interstitial space of a colloidal crystal

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Fig. 3. Bismuth nanostructured electrode. (a) Scanning electron microscopy images of a bunch-like bismuth electrode; (a.2.) square wave anodic stripping voltammograms recorded (blank and 0.2, 0.4, 0.6 and 0.2 nM of Pb2+ , Cd2+ , Cu2+ and Hg2+ , respectively). Ref. [54]. (b) Gold nanoelectrode ensemble (GNEE) electrode. (b.1.) Schematic illustration of the GNEE electrode; (b.2.) SWASV response of the GNEE electrode at different concentration of As(III) in the presence of 0.2 nM Cu(II). Ref. [59].

template [57]. Such electrodes were tested and evaluated for heavy metals stripping showing reproducible and well-defined peaks for cadmium and lead with low detection limits (6.2 nM for Cd2+ and 2.4 nM for Pb2+ ). The main advantages of such electrodes are the fact of being less toxic than mercury and their partial nonsensivity to dissolved oxygen, being well suited for electrochemical analysis. Beside bismuth and antimony, gold and silver nanostructured electrodes play also an important role in the field of heavy metals electrochemical detection [59–63]. Fig. 3b shows a summary of the work reported by Jena and Raj [59]. The authors of the work have explored the possible application of gold nanoelectrode ensemble (GNEE) (Fig. 3(b.1)) for the simultaneous detection of arsenic, mercury and copper. They take advantage of SWASV for the As(III) detection without any interference of Cu(II) as shown in Fig. 3(b.2). Detection limits (0.26 nM) below the guideline value set by WHO were achieved for mercury and arsenic. Gold nanoparticles have also been combined with carbon materials such as graphene with the aim to facilitate the electron-transfer processes [60]. Gong et al. investigated on an electrochemical platform for mercury voltammetric detection based on monodispersed AuNPs decorated graphene. The detection limit was found to be 0.03 nM, much below the guideline established by WHO [64]. Real samples were also evaluated exhibiting fine applicability for river water samples. In the same context of mercury voltammetric detection, the same research group reported on bimetallic Au–Pt nanoparticles/organic nanofibers (Au–Pt NPs/NF) modified GCE [61]. The detection limit was found to be in the same range as the one previously described (0.04 nM of Hg2+ ). The analytical performance of the system was carefully evaluated including the study of the interferences effect and its application for real tap and river water samples. Authors point out the excellent properties of bimetallic nanoparticles as the high conductivity, and better catalytic properties than their monometallic counterparts.

Guo et al. presented another alternative to reduce the toxicity of mercury electrodes by using a Nafion film nano Ag–Hg amalgam [63]. The weak toxicity coming from this material is due to the amalgam formation and the little amount of mercury used. However, given the efforts of the scientific community on the use of environmentally friendly materials in order to achieve similar or even better analytical performances than the best ones obtained with mercury electrodes a careful consideration of this work is necessary prior real/in-field future applications.

2.3. Nanoparticles as tags for signal enhancement Metal nanoparticles have not only been used as modifiers of electrochemical platforms but also as amplifying tags for novel biosensing systems mainly based on DNA for heavy metals detection [65–68]. The use of DNA probes for heavy metals detection (commonly Hg2+ and Pb2+ ) is based in two well-known phenomena: DNA hybridization and the affinity for heavy metal cations that thymine–thymine mismatches of complimentary sequences have [69]. In addition, other biomolecules such as DNAzymes have also been extensively used due to the fact that their cleavage can be specifically catalyzed by metal ions [70]. In this context, Shen and co-workers reported the use of the so-called DNA-Au bio-bar codes for signal enhancement upon Pb2+ detection [65]. The system is based on the use of a lead-dependent DNAzyme as the target recognition element. The DNA-Au bio-bar is based on AuNPs functionalized with a large number or oligonucleotides strands (the bar codes) and a corresponding recognition agent. The bar code strands are used as means of amplification. The electrochemical response comes from the electroactive hexaamineruthenium (III) chloride complex (RuHA) interaction with DNA which can be investigated by DPV technique. The authors reported detection limits of 1 nM of Pb2+ within an extreme selectivity for this metal cation.

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Fig. 4. AuNPs as tags for signal enhancement. (a.1) Schematic illustration of the AuNPs signal enhancement procedure for Hg2+ detection; (a.2) cyclic voltammograms of 50 ␮M of RuHA obtained at a gold electrode modified (A) probe 1 and (B) probe 1 and probe 2 loaded on AuNPs with the presence of 10 nM of Hg2+ . Ref. [68].

Another system based on the use of AuNPs as tags for signal amplification was reported by Zhu et al. [66]. In this work, the authors take advantage of the easy surface modification of AuNPs to immobilize mercury-specific oligonucleotide (MSO) probes. They demonstrated that the use of AuNPs brings an amplification factor of more than 3 orders of magnitude leading to a detection limit of 0.5 nM of Hg2+ ions. RuHA complex is again used for the final electrochemical SWV measurements and the responses were further correlated to the Hg2+ concentration. A quite similar work was described by Kong et al. achieving the same detection limit as the previous one by Zhu and co-workers [67]. Methylene blue as electrochemical label instead of RuHA complex and DPV technique as detection technique instead of SWV were used. In addition, the authors studied real tap and river water samples showing good recoveries values. Fig. 4a shows another example of the use of AuNPs as signal amplification tags [68]. As depicted in Fig. 4a DNA strand (probe 1) on a gold electrode surface was firstly immobilized. A complementary DNA strand (probe 2) containing six strategically placed T-T mismatches was immobilized onto the AuNPs surface. When Hg2+ is present T–Hg2+ –T complexes can be formed and AuNPs (with high amount of DNA strands) can be immobilized onto the gold electrode. The final measurement is obtained by performing CV of the RuHA complex present which could be related to the amount of DNA on the surface which in turn could be further related to the amount of Hg2+ ions. Fig. 4b shows the cyclic voltammograms of

50 ␮M of RuHA obtained at a gold electrode modified with probe 1 (A) and with probe 1 and probe 2 (B) loaded on AuNPs with the presence of 10 nM of Hg2+ . Beside the use of AuNPs, vertically aligned TiO2 nanotube arrays have been used to immobilize DNA for Pb2+ detection (Fig. 5a) [71]. The authors attributed the remarkable characteristics of the system to the abundant immobilization of target biomolecules, and the enhanced bioelectrochemical activity. SEM images of as-grown TiO2 NTs are shown in Fig. 5b (top view at Fig. 5(b.1) and cross section at Fig. 5(b.2)). The results showed that the system possesses a wide linear calibration (Fig. 5c) ranging from 0.01 to 160 nM with the detection limit at picomolar range (3.3 pM) using DPV technique. The strategies described above have the advantages of high selectivity, low cost and low detection limits typical for biosensing devices. However, working with biological receptors such as DNA and enzymes, has also possible drawbacks related to their short lifetime and the need of working in soft mediums which limits their applications in some real samples. 2.4. Other nanostructured materials In recent years, researchers have focused also their interests in natural adsorbents, either inorganics materials or minerals, as electrochemical surfaces for heavy metal ions detection. Nanosized hydroxyapatite (NHAP) Ca10 (PO4 )6 (OH)2 , a bioceramic analogous

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Fig. 5. TiO2 nanotubes (NTs) based electrode. (a) Schematic illustration for DNA/C-TiO2 NTs construction and its Pb2+ ion monitoring; (b) SEM images of as-grown TiO2 NTs ((b.1): top view; (b.2): cross sections); (c) DPASV for Pb2+ with various concentrations on DNA/C-TiO2 NTs. Inset: the calibration plot of stripping peak current density with Pb2+ concentration. Ref. [71].

Fig. 6. (a) Sea cucumber-like hydroxyapatite (HAp) based electrode. (a.1) SEM images of sea cucumber-like HAp; (a.2) square-wave stripping voltammograms and calibration plots of the HAp based electrode containing different amounts of Pb2+ and Cd2+ . Ref. [75]. (b) Nanoarray electrode-nafion-electrode (NENE). (b.1) Triple layer design of the NENE sensor and the SEM showing Nafion deposits within 200 nm wide membrane nanochannels; (b.2) calibration plot of DPAV peak current response of the sensor vs Cu2+ concentration. Ref. [76].

to the mineral component of bones with great biocompatibility and particular multiadsorbing sites, has been used to provide unique three-dimensional network structures in heavy metals electrochemical sensors [72]. It is very interesting to take into account that Ca2+ ions of HAP can be replaced by bivalent cations including heavy metals such as Cd2+ and Pb2+ due to its apatite “open structure”.

Moreover, the combination of such materials with specific ionophores can provide excellent electrochemical platforms for heavy metals analysis due to the combination of the enlarged active surface area and strong adsorptive capability of the nanomaterial and the specific complexing ability of the ionophore [73]. The combination of HAP with other nanostructured material such as carbon nanotubes has also been evaluated [74]. The authors

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Table 2 Nanomaterials applications in relation to potentiometric techniques used for heavy metals detection. Electrochemical platform Carbon-based material MWCNT MWCNT MWCNT MWCNT-NS MWCNT-NS SWCNT Others Nanoporous silica PANI NP-silicon rubber solid contact electrode SGAuNP-CE SGAuNP-CE SiNWs

Technique

Analyte

LOD

Sample matrix

[Ref]

ISE ISE ISE ISE ISE FET

Hg2+ Pb2+ Pb2+ Pb2+ Cu2+ Hg2+

2.5 × 10−9 M >10−6 M 3.2 × 10−10 M >10−7 M >10−5 M 10 × 10−9 M

Dental amalgam and water samples Buffer Waste water and black tea Waste water and black tea Waste water Buffer

[79] [80] [81] [84] [85] [95]

ISE ISE ISE ISE FET

Hg2+ Ag+ Al3+ Al3+ and Cu2+ Cu2+

7.0 × 10−8 M 2 × 10−8 M 2.0 × 10−10 M 1.6 × 10−7 and 4.0 × 10−7 M 1 × 10−9 M

Waste water and fish samples Buffer River, tap and mineral waters Buffer Buffer

[82] [83] [86] [87] [93]

MWCNT: multi-walled carbon nanotubes; MWCNT-NS: multi-walled carbon nanotubes-nanosilica; SWCNT: single wall carbon nanotubes; PANI NP: polyaniline nanoparticles; SGAuNP-CE: sol–gel gold nanoparticles based carbón electrode; SiNWs: silica nanowires; ISE: ion-selective electrode; FET: field effect transistors.

attribute the low detection limit (4 nM of Cd2+ ) to the combination of the strong absorption ability of HAP and the excellent electroanalytical properties of CNTs. One of the main advantages of these systems is the wide potential window, the excellent chemical stability and unique three dimensional networks giving rise to an increase on the active surface area. In addition, the morphology of the HAP has great influence on its chemical and biological properties. For example, sea cucumber-like HAP (Fig. 6(a.1)) can exhibit large surface areas with adsorbing sites which will allow the enhancement of the sensitivity toward heavy metals [75]. Zhang et al. recently described a sea cucumber-like HAP modified carbon paste electrode for Pb2+ and Cd2+ ions detection by means of SWAV [75]. The metals were detected over the range of 10−11 –10−7 M when using a deposition potential of −1.2 V (vs. SCE) with a deposition time of 540 s (Fig. 6(a.2)). Detection limits of 0.00423 nM and 0.027 nM for Pb2+ and Cd2+ , respectively were achieved. Beside NHAP based sensors, other strategies have been developed in the same field. Zhuo et al. developed a nanoarray electrode-Nafion-electrode (NENE) electrochemical sensor based on a triple layer design (Fig. 6(b.1)) comprising a polyelectrolyte (perfluorinated ionmer Nafion) entrapped within micrometerlength nanochannels (nanoporous alumina) and sandwiched between two nanometer-thick electrode layers [76]. The NENE sensor was tested for Cu2+ ions detection using DPV technique. Fig. 6(b.2) shows the calibration plot (with two linear ranges) of anodic differential peak current responses of the sensor versus Cu2+ concentration. The authors attribute the two different linear responses to the partial removal of Cu2+ through an ion-exchange process in the Nafion layer of the sensor causing the gentle slope response, followed by a steeper sensitive slope at high Cu2+ concentrations when the limited amount of Nafion entrapped in the nanochannels becomes saturated with Cu2+ ions. An 80% of confidence level was achieved for Cu2+ determination in real reservoir water samples using this system. A well-known technique for selective and high affinity sensors development is the use of molecular-imprinted polymers (MIPs). In the same context, ion-imprinted polymers (IIPs) are similar to MIPs but, they recognize specific metal ions via specific nanocavities created after imprinting polymeric membranes [77]. Although some works related to this field have been described, IIPs have the drawbacks related to template leakage, incompatibility in aqueous media, low binding capacity and slow mass transfer. 3. Potentiometric techniques Potentiometric techniques have been widely used in the field of heavy metals detection in complex environmental matrices.

Potentiometry represents a very attractive option for numerous analyses due to the low cost, short response time, high selectivity and broad range of response. However, potentiometric techniques still suffer from problems related sometimes to the lack of sensitivity, high detection limits and difficulties in electrode miniaturization beside others. In this context, the combination of nanomaterials with potentiometric sensing devices is a promising area of research in order to overcome the limitations of the technique [15,78]. Beside the exceptional electrical properties and the extraordinary electrical capacities generated at the nanomaterials interface, the extremely high surface-to-volume ratio of nanomaterials such as carbon nanotubes (CNTs) or metal nanoparticles (MNPs) promotes a greater interaction with targets when nanostructures are present in the recognition layer. Two main devices fall within the category of potentiometric sensors: ion-selective electrodes (ISEs) and field-effect transitors (FETs). Both have been successfully used for many years in the heavy metals detection field. Table 2 gathers different works related to heavy metals detection using potentiometric techniques.

3.1. Ion selective electrodes Ion-selective electrodes (ISEs) are commonly known as potentiometric sensors that include a selective polymeric membrane which minimize matrix interferences. The development and application of ISEs for sensing of heavy metal cations have been an interesting area of analytical research due to their accuracy, fast response, non-destructive and low cost of analysis [13]. The introduction of nanostructured material as transducers in ISEs enables the development of new types of potentiometric sensors in which polymeric membrane is replaced by receptors directly linked to the nanostructured transducer (i.e. CNTs, MNPs, fullerene, graphene, NWs). In this context, carbon nanostructured materials have been proved to be excellent ion-to-electron transducers in ISEs owing their great physicochemical properties. Specially, many works have been reported dealing with the use of multi-walled carbon nanotubes (MWCNT) as transducers materials between the source and the drain electrodes as can be seen in Table 2. Khani et al. reported the use of a carbon paste electrode based on a Hg2+ selective ionophore (1-(2-ethoxyphenyl)-3-(3nitrophenol)triazene) coupled with MWCNT [79]. The authors report a detection limit of 2.5 × 10−9 M for Hg2+ with a short response time (ca. 5 s) with the main advantage that the device can be used for long-term analysis (at least 55 days). Real samples show good correlations between mercury concentrations found by

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Fig. 7. Carbon nanotubes based ion-selective electrode (ISE). (a) Scheme of the synthesis of hybrid MWCNT/Pb2+ ISE transducer. (b) Time response for the MWCNT/Pb2+ -ISE. Ref. [80].

the developed ISE and by ICP techniques for a dental amalgam used as sample. Fig. 7a shows the immobilization procedure of the Pb2+ ionophore (benzo-18-crown-6)onto MWCNT described by Parra et al. [80]. The MWCNT are first oxidized to introduce carboxylic groups which are further activated as acid chloride to react with the corresponding ionophore. After this first immobilization, hybrid MWCNT-Pb2+ ionophore was captured into a polymeric membrane to ensure a good dispersion of the recognition moieties. Fig. 7b shows the characteristic potentiometric response of the ISEs developed as function of time at different increasing concentrations of Pb2+ ions for MWCNT-Pb2+ ionophore membrane (black line), for dispersed MWCNT and Pb2+ ionophore membrane (red line) and MWCNT with no ionophore membrane (blue line). The authors point out that by using the hybrid system with the Pb2+ ionophore immobilized on the top of the MWCNT, both molecular recognition and ion-to-electron transduction have been successfully carried out on a single material in potentiometric solid-contact ISEs with the main advantage of high stability and reproducibility. The immobililization of the ionophore on the top of MWCNT avoids the leaching of the recognition molecules from the electrode surface allowing longer life-times. However, in this work the lifetime of the sensor is not reported. Recently, Guo et al. worked on the development of another potentiometric sensor based on MWCNT for the determination of Pb2+ ions [81]. The system is very similar to the ones previously described but using a different ionophore and achieving lower detection limits (down to 3.2 × 10−1 M of Pb2+ ions). Important analytical parameters have been well-determined including the lifetime of the sensor (at least 3 months). In addition, real water samples have been successfully evaluated with the described sensor. Beside the use of carbon nanostructured electrodes, other nanomaterials have been applied for ISEs development. For example, silica based nanostructures offer great advantages to the ISEs development in terms of robustness, easy functionalization and high surface area which allows a large number of accessible binding sites for the analytes. Some works have been described involving the use of nanoporous silica materials and heavy metals selective ionophores [82]. Lindfors et al. showed an example of the use of nanostructured silicon rubber based solid contact ISEs with polyaniline (PANI) nanodispersion as conducting polymer for Ag+ ions sensing [83]. The analytical performance including selectivity toward other metal ions have been studied in detail showing the best selectivity for Ag+ ions reported so far. Combination of both carbon nanotubes and nanoporous silica has been also investigated [84,85]. Although the idea of such combination is quite logical the reported detection limits are not lower

than the ones reported for the single materials (see Table 2). For example, Ganjali et al. reported an electrode based on graphite powder, MWCNT, nanosilica, Pb2+ ionophore and paraffin oil for lead determination [84]. The detection limit achieved was not determined by the authors, but the response range was found to be from 1 × 10−7 to 1 × 10−2 M. Compared to the system based on MWCNT described by Guo et al. [81] the range of response is wider and the lowest detected concentration is lower with the system using only MWCNT than with the one using both MWCNT and nanoporous silica. Mashhadizadeh and collaborators have reported different works involving the use of AuNPs for the development of sol–gel–AuNPs modified carbon paste ISEs [86,87]. The research group has worked on the potentiometric detection of different heavy metals such as Al3+ [86,87] and Cu2+ [87] among others [88,89], using different ionophores. Detection limits of 2.0 × 10−10 M for Al3+ [86] and4.0 × 10−7 M for Cu2+ were achieved [87]. Sol–gel method and AuNPs are used in these cases to prepare three-dimensional networks for the encapsulation of the ionophore molecules in order to achieve higher sensibility and stability and a longer lifetime of the electrode. 3.2. Field-effect transistors Field-effect transistors (FETs) often measure the flow of the current across a transistor that links the source and drain electrodes (see Fig. 8a). FETs use to contain a semiconducting channel whose conductivity is affected by external fields which in this case corresponds to a potential variation. This variation in the potential, or field effect, is the reason why FETs are classified as potentiometric sensors. Incorporation of nanostructured materials into FET designs allows to overcome drawbacks such as the unstable response. The most used nanostructured materials for FETs construction are carbon nanotubes and semiconductor nanowires owing to their capability to form channels for the source and drain connection without being buried underneath the dielectric layer [90,91]. The main advantages coming from the nano-FETs can be attributed to their ultra-low detection limits, possibility of direct functionalization with the nanostructured material and easy miniaturization. Some nano-FETs have been applied in the field of heavy metals sensing achieving very low detection limits [92]. SiNWs have been used by Bi et al. to construct a FET for copper ions detection [93]. SiNWs were modified with glycyl-glycylhistidine as a copper ion recognition element. The measurement of Cu2+ concentration was performed by measuring the change on the conductance of the SiNW upon the copper ions concentration.

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Fig. 8. (a) FETs general scheme. (b) Single walled carbon nanotubes based FET. (b.1) Plausible mechanism for Hg2+ detection (upper) and Pb2+ detection (lower). (b.2) A real-time current measurement obtained from the SWCNT-FET after the introduction of Hg2+ at various concentrations. Arrows indicate the points of Hg2+ injections. Ref. [95].

Due to the fact that copper ions are positively charged, they can act as positive gate potential and increase the conductance of the n-type SiNWs which will be the final measurement. Other works involving the use of modified SiNWs for Hg2+ and 2+ Cd ions detection have been reported [94]. Fig. 6b shows details on the design and the obtained response of a SWCNT based FET developed by Kim et al. for the Hg2+ ions detection [95]. SWCNT are very sensitive to the chemical environment because of the high sensitivity of their band gap energies to the local dielectric or redox environment. The authors of the work point out that a strong response of SWCNT conductance upon Hg2+ exposure is caused by the strong redox reaction between SWCNT and Hg2+ in which mercury is reduced to Hgo by SWCNT and deposited on the walls of the nanotubes. The selectivity of the sensor can be explained by looking at the different standard potentials of the metal ions which indicate that Hg2+ ions are the only ones that can be thermodynamically favorable reduced by SWCNT. Fig. 8(b.1) (upper part) shows the plausible mechanism for Hg2+ reduction on the top of SWCNT. In contrast, Pb2+ ions cannot be reduced by SWCNT and remain as Pb2+ ions in solution (Fig. 8(b.1)). Real-time current measurement obtained from the SWCNT-FET after the introduction of Hg2+ at various concentrations is presented in Fig. 8(b.2). The addition of Hg2+ from 1 pM to 1 nM showed no significance effect on the source–drain current, while drastic current increase with fast response was observed after the addition of 10 nM of Hg2+ . The same strategy involving the reduction of Hg2+ through CNTs was recently used by Lee et al. with the additional theoretical study for the prediction of the sensor response [96]. 4. Other electrochemical strategies using nanostructured materials 4.1. Electrochemiluminescence Electrochemiluminescence (ECL) is known as a process in which electrochemically generated species combine to undergo highly energetic electron transfer (redox or enzymatic) reactions that emit

light from excited states. It has attracted a lot of attention due to its versatility, simple setup and high sensitivity [97]. In this context, ECL detection methods for heavy metal ions, and more specifically nanostructured-based ones, are growing importance in the last years [98]. Different metal nanostructures such as silica nanoparticles (SiNPs), AuNPs, or even quantum dots (QDs) have been used for this purpose. SiNPs have been demonstrated to be a good platform for ECLbased detection due to their surface chemistry that allows easy modification and functionalization. Ru(bpy)3 2+ -doped SiNP have been used as DNA labels for Hg2+ determination [99]. In this case, the immobilization of the ruthenium complex onto the surface of SiNPs acts as signal enhancement due to the higher amount of complex attached to the DNA. Fig. 9a shows the principle of Hg2+ detection using the Ru–SiNPsDNA ECL sensor. The principle of detection is based on the blocking of the electron transfer by thymine–thymine mismatches of two complementary DNA strands. When mercury is present the T–Hg2+ –T interaction takes places allowing the electron transfer between the electrode and the Ru–SiNPs and resulting in enhancing of the ECL intensity. Fig. 9b shows the ECL intensity of the sensor under different Hg2+ concentrations by using cyclic voltammetry as scan mode. The detection limit found for mercury is 2.3 × 10−9 M. The same research group has recently reported an ECL sensor for Pb2+ ions using gold nanoclusters [100]. They reported for the first time the ECL emission from Au nanoclusters at 1.45 V vs Ag/AgCl (3 M KCl) using triethylamine as the coreactant for Pb2+ ions. The system is presented as selective to Pb2+ ions and justified for the higher affinity of Pb2+ to sulphur atoms coming from the BSA protected nanoclusters. In addition, cysteine was added as masking agent to avoid interferences from other metals such as Cd2+ . The work presented can be just considered a proof-of-concept and more studies involving the analytical performance of the system including some more studies about interferences are necessary for real sample applications. Other interesting works recently reported by Chen et al. research group involve the detection of Cu2+ ions using click chemistry onto

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Fig. 10. Peptide nanotube based electrode. (a) Strategy for the ultralow detection of Pb2+ ions with peptide nanotube detection platform; (b) conductance of the peptide nanotube between electrodes after incubation with different heavy metals. Ref. [103].

Fig. 9. Electrochemiluminescent (ECL) detection method. (a) Principle of ECL Hg2+ detection; (b) ECL intensity curves from different Hg2+ concentrations under CV scanning. Inset: calibration curve of ECL intensity vs the logarithm of Hg2+ concentration. Ref. [99].

a gold electrode and Ru–SiNPs [101]. Firstly, 1-azidoundecan-11thiol was assembled on the Au electrode surface. Then Ru–SiNPs functionalized with diethylenetriamine were covalently coupled onto the electrode via click chemistry forming 1,2,3-triazoles using Cu+ (derived from Cu2+ reduction) as catalyst. Using this interesting strategy ultra low detection limits (ca. 1.0 × 10−16 M Cu2+ ) are achieved. Such a low detection limit is not necessary for environmental samples but could be useful for clinical applications. Another strategy involving ECL techniques for Cu2+ detection was proposed by Zhang et al. [102]. The authors reported the Cu2+ detection by using electrochemiluminiscence quenching of mercaptosuccinic acid capped CdTe QDs (MSA-QDs). The system is based on the removal of the MSA from the surface of QDs by Cu2+ ions followed by the further ECL quenching of the QDs. The obtained detection limit of 30 nM of Cu2+ is much higher than the one mentioned above. 4.2. Other strategies Other electrical/electrochemical strategies related for example to conductimetry or biofuel cells have also been reported for heavy metal detection. For example De la Rica et al. used a bioinspired crystal growth concept with peptide nanotube detection platform for lead ions (Fig. 10) [103]. In this work, peptide nanotubes can bind Pb2+ ions selectively and template the growth of Pb crystals via molecular recognition (see Fig. 10a). After an electrochemical reduction step, the metallization of the nanotubes induce changes in the electrical properties increasing the conductivity between electrodes which could be used as electrochemical signal.

Fig. 10b shows the conductance of the peptide nanotube between the electrodes after the incubation with different heavy metals. It can be observed than only Pb2+ ions induce a change in the conductance due to the peptide specificity. The response range of the system stays between 0.01 nM and 1 nM. Concentrations higher than 1 nM Pb2+ cannot be quantified due to a saturation of the binding sites on the nanotube sensor. The work represents an original idea for selective ions detection. However, a large analytical study must be performed before application of this system in real samples. The reusability of the peptide nanotube detection platform by recirculation of fresh peptide through the platform can probably be with interest for flow through and long term applications. Wen et al. recently described a self-powered sensor for trace Hg2+ ions based on an enzymatic biofuel cell (BFC) with single-walled carbon nanohorns (SWNHs) modified carbon fiber microelectrodes (CFMEs) [104]. Authors point out that the presence of Hg2+ in the BFC solution inhibited the catalytic activities of the biocatalysts (bilirubin oxidase (BO) and alcohol deshydrogenase (ADH)), and thus, led to a remarkable decrease in the open circuit potential of the BFC. Detection limits of 1 nM of Hg2+ have been found. 5. Conclusions and outlook Nanomaterials are showing to be interesting alternative materials to be integrated into various electrochemical devices with interest in heavy metals detections. Among the various electrochemical techniques, voltammetric and potentiometric based ones are taking several advantages by such incorporations. Various materials such as carbon nanoparticles, carbon nanotubes, nanowires, graphene nanosheets, metal nanoparticles, etc. have been extensively explored for their use as modifiers of conventional electrodes (i.e. graphite, glassy carbon, metallic electrodes, etc.) or even as electrode itself (nanomaterials deposited onto non-conducting platforms). Nanomaterials modified electrodes have shown improved performance in electrochemical stripping

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analysis or ion-selective detection of heavy metals due probably to the improved electronic/catalytic properties. Such improvements related also to the increased active electrode area due to the size of nanomaterials still need to be better clarified so as to understand the obtained responses and design better detection strategies. The advantage of the use of nanomaterials is also related to their size and consequently their flexibility/easy application in various biosensing systems (i.e. DNA sensing) that in turn are shown to be useful for indirect detection of heavy metals. Such combination is bringing new advantages in the heavy metal detection selectivity and sensitivity due to the combination of biorecognition specificity with the sensitivity and easy integration of nanomaterials. Given the size of nanomaterials and the various electrical/electrochemical detection technologies the possibility exists to further miniaturize the heavy metal detection sensors so as to obtain in situ devices that can easily integrated into complex monitoring systems that use to incorporate a variety of devices and where the compactness of the systems is a challenging issue. Such a system would be with great interest for environmental monitoring but also for biological/toxicological studies. Nevertheless, issues related to the reproducibility and stability of these systems as well as aspects associated to the toxicity of the used materials need great consideration prior the application of the nanomaterial based devices in real samples. Acknowledgements The authors would like to acknowledge MICINN for PIB2010JP00278 project. G.A. thanks Generalitat de Catalunya for the predoctoral fellowship (FI 2009). References [1] M. Bodo, S. Balloni, E. Lumare, M. Bacci, M. Calvitti, M. Dell’Omo, N. Murgia, L. Marinucci, Toxicology In Vitro 24 (2010) 1670. [2] N. Johri, G. Jacquillet, R. Unwin, Biometals 23 (2010) 783. [3] P. Triunfante, M.E. Soares, A. Santos, S. Tavares, H. Carmo, M. Bastos, Forensic Science International 184 (2009) e1. [4] I.M.M. Kenawy, M.A.H. Hafez, M.A. Akl, R.R. Lashein, Analytical Sciences 16 (2000) 493. [5] P. PohL, Trends in Analytical Chemistry 28 (2009) 117. [6] E.L. Silva, P.S. Roldan, M.F. Giné, Journal of Hazardous Materials 171 (2009) 1133. [7] S. Caroli, G. Forte, A.L. Iamiceli, B. Galoppi, Talanta 50 (1999) 327. [8] E. Bakker, Y. Qin, Analytical Chemistry 78 (2006) 3965. [9] B.J. Privett, J.H. Shin, M.H. Schoenfisch, Analytical Chemistry 80 (2008) 4499. [10] J. Wang, Stripping Analysis. Encyclopedia of Electrochemistry, Willey VCH, 2007. [11] A. Merkoc¸i, S. Alegret (Eds.), Comprehensive Analytical Chemistry, vol. 49, 2009, p. 143. [12] J. Bobacka, A. Ivaska, A. Lewenstam, Chemical Reviews 108 (2008) 329. [13] A. Radu, D. Diamond, in: S. Alegret, A. Merkoc¸i (Eds.), Comprehensive Analytical Chemistry, vol. 49, 2007, p. 25. [14] G. Aragay, J. Pons, A. Merkoc¸i, Chemical Reviews 111 (2011) 3433. [15] A. Düzgün, G.A. Zelada-Guillén, G.A. Crespo, S. Macho, J. Riu, F.X. Rius, Analytical and Bioanalytical Chemistry 399 (2011) 171. [16] L. Wang, W. Ma, L. Xu, W. Chen, Y. Zhu, C. Xu, N.A. Kotov, Materials Science and Engineering Reports 70 (2010) 265. [17] L. Zhang, M. Fang, Nano Today 5 (2010) 128. [18] T. Pradeep, Thin Solid Films 517 (2009) 6441. [19] A. Merkoc¸i (Ed.), Electroanalysis, vol. 19, no. 7–8, Wiley InterScience, 2007. [20] W. Yantasee, Y. Lin, K. Hongsirikarn, G.E. Fryxell, R. Addleman, C. Timchalk, Environmental Health Perspectives 115 (2007) 1683. [21] K.C. Honeychurch, J.P. Hart, Trends in Analytical Chemistry 22 (2003) 456. [22] M. Pumera, A. Ambrosi, A. Bonanni, E.L. KhimChng, H. Ling Poh, Trends in Analytical Chemistry 29 (2010) 954. [23] D.T. Pierce, J.X. Zhao, Trace Analysis with Nanomaterials, Wiley-VCN Verlag, Germany, 2010. ˜ A. Ambrosi, A. Merkoc¸i, Trends in Analytical Chem[24] A. de la Escosura-Muniz, istry 27 (2008) 568. [25] (a) M. Pumera, Chemistry- A European Journal 15 (2009) 4970. [26] M. Trojanowicz, Trends in Analytical Chemistry 25 (2006) 480. [27] M.M. Musameh, M. Hickey, I.L. Kyratzis, Research on Chemical Intermediates 37 (2011) 675. [28] A.K. Wanekaya, Analyst 136 (2011) 4383. [29] A. Merkoc¸i, Microchimica Acta 152 (2006) 157.

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