Environ Sci Pollut Res DOI 10.1007/s11356-016-6118-2
ENVIRONMENTAL ISSUES FACING CHEMICAL, BIOLOGICAL, RADIOLOGICAL AND NUCLEAR RISKS
Hg2+ detection using a disposable and miniaturized screen-printed electrode modified with nanocomposite carbon black and gold nanoparticles Stefano Cinti 1 & Francesco Santella 1 & Danila Moscone 1,2 & Fabiana Arduini 1,2
Received: 10 July 2015 / Accepted: 14 January 2016 # Springer-Verlag Berlin Heidelberg 2016
Abstract A miniaturized screen-printed electrode (SPE) modified with a carbon black-gold nanoparticle (CBNPAuNP) nanocomposite has been developed as an electrochemical sensor for the detection of inorganic mercury ions (Hg2+). The working electrode surface has been modified with nanocomposite constituted of CBNPs and AuNPs by an easy drop casting procedure that makes this approach extendible to an automatable mass production of modified SPEs. Square wave anodic stripping voltammetry (SWASV) was adopted to perform Hg2+ detection, revealing satisfactory sensitivity and detection limit, equal to 14 μA ppb−1 cm−2 and 3 ppb, respectively. The applicability of the CBNP-AuNP-SPE for the determination of inorganic mercury has been assessed in river water by a simple filtration and acidification of the sample as well as in soil by means of a facile acidic extraction procedure assisted by ultrasound. Keywords Mercury detection . Anodic stripping voltammetry . Screen-printed electrode . Carbon black . Gold nanoparticles . Soil Mercury (Hg), including inorganic and organic forms, represents one of the heavy metals of global concern. In the list of priority substances set by the United States Agency for Toxic Responsible editor: Philippe Garrigues * Fabiana Arduini [email protected]
1
Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma BTor Vergata^, Via della Ricerca Scientifica 1, 00133 Rome, Italy
2
Consorzio Interuniversitario Biostrutture e Biosistemi BINBB^, Viale Medaglie d’Oro, 305 Rome, Italy
Substances and Disease Registry (ATSDR) and the United States Environmental Protection Agency (EPA) in agreement with the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), Hg has occupied the third position (lead and arsenic are the first and the second ones), in 2013 and 2015 (Agency for Toxic Substances and Disease Registry 2015), demonstrating the relevant problem of Hg. In 2013, United Nations Environment Programme (UNEP) in a summary report claimed that Hg emissions in air arise from natural (10 %), anthropogenic (30 %), and re-emission (60 %) sources (UNEP 2013). Telmer and Veiga (2009) have estimated that approximately 1000 metric tons/year of mercury have been released from at least 70 countries. Approximately, 350 metric tons/year is directly emitted into the atmosphere, while 650 metric tons/year is released into the hydrosphere, e.g., rivers, lakes, and soils. For instance, Hg in urban soils of China has been found in the range of 0.16– 3.68 ppm (Liu et al. 2012). Obviously, aquatic and soil pollution are linked: indirect sources to the aquatic environment include rain runoff to water bodies or leaching of the upper soil layers by groundwater flows (Schroeder et al. 1989). Mercury is present in several forms; Hg can be easily oxidized to Hg2+ and, subsequently, atmospherically transported and accumulated in land, waterways, and oceans (Schroeder and Munthe 1998), aquatic microorganism can easily turn Hg2+ into methylated species, i.e., mono-methylmercury (MMHg) and di-methylmercury (DMHg), leading to an amplified bioaccumulation from plankton to fishes. However, as recently reviewed by Sonke et al. (2013), even if the relationship between inorganic and methylated Hg is not completely clear in terms of mechanism, seafood sources represent the greatest health risk to humans and wildlife. In an overall scenario, the development of simple, fast, efficient, and reliable methods to evaluate environmental
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levels of mercury is an important issue. Standardized methods are mainly based on cold vapor atomic absorption (AAS), atomic fluorescence spectrometry (AFS), and inductively coupled plasma mass spectrometry (ICP-MS) (Rio-Segade and Bendicho 1999; Huber et al. 2015; Cairns et al. 2008); yet, these approaches usually require expensive instrumentation, often hardly portable, and skilled personnel. An electroanalytical approach emerges as a valuable alternative for environmental monitoring of mercury (Wang 2006). In literature, the utilization of voltammetric stripping analysis is largely reported (Stozhko et al. 2008; Economou 2010). In particular, anodic stripping voltammetry (ASV) is known as the best electroanalytical technique to detect mercury: briefly, Hg is first reduced/pre-concentrated at the electrode surface and subsequently oxidized/re-dissolved in the solution, while the anodic current is recorded as it is proportional to the concentration of Hg. Gold electrodes (disk, film, arrays) or glassy carbon electrodes modified with gold nanoparticles, thanks to their high affinity for mercury, are widely reported as the best electrodes to perform ASV detection of Hg (Bonfil et al. 2000; Ordeig et al. 2006; Okçu et al. 2008; Laffont et al. 2015). Nevertheless, if the high Au-Hg affinity enhances the preconcentration step, this feature even demonstrates a relevant drawback of the gold electrode, as the structural surface changes after the formation of this amalgam and long cleaning treatments are required to achieve a satisfying reproducibility of measurements (Welch et al. 2004). Screen-printed electrodes (SPEs) represent an alternative substitute to conventional gold-based solid electrodes (Metters et al. 2011). Their versatility demonstrates a great benefit in analytical research areas; the ability to easily modify the electrodes with different and commercially available ink formulations onto a variety of substrates (plastic, ceramic, paper, skin) allows for highly specific and finely calibrated electrodes able to determine specific analytes. SPEs provide effective electroanalytical platforms, customizable and easy extendible for the detection of heavy metals. In the literature, several references are reported on the application of different SPEs for anodic stripping Hg2+ determination: Chiu et al. (2008) utilized a silver-based SPE to detect mercury in cosmetic samples; Mandil et al. (2010) detected mercury by electrodeposition of a gold film onto a carbon SPE; Meucci et al. (2009) fabricated screen-printed gold electrodes on a plastic substrate for determination of mercury in fish tissues; Bernalte et al. (2011) used a gold SPE for the determination of mercury in wastewaters by square wave anodic stripping voltammetry. However, using electroplating procedures as well as the use of noble metal inks (silver, gold) to produce sensing devices could not demonstrate to be the best solution. The utilization of nanomaterials to improve the analytical performance, and to reduce time/cost fabrication of SPEs, is being widely reported in last 5–10 years: many research groups have reported on the opportunity to modify SPEs with nanomaterials such as
gold nanoparticles (AuNPs), carbon nanotubes (CNTs), graphene, and hybrid nanocomposites (Bernalte et al. 2012; Niu et al. 2011; Martín-Yerga et al. 2012). In this work, we propose a disposable sensor based on a carbon screen-printed electrode modified with a nanocomposite constituted of AuNPs and carbon black nanoparticles (CBNPs) to detect Hg2+. CB is a cost-effective nanomaterial principally used as reinforcing filler in rubber compounds and was recently employed as an electrode modifier for successful electroanalytical analyses (Arduini et al. 2010; Arduini et al. 2012; Arduini et al. 2015; Vicentini et al. 2015; Talarico et al. 2015; Cinti et al. 2016). We have deposited a layer of AuNPs onto a CBNPs layer and, due to the high surface area of this latter, the CBNP layer allowed for a better dispersion of AuNPs onto the electrode surface (Cinti et al. 2014). In this work, we carried out the optimization with respect to both the modification (amount of modifiers) and the analytical parameters. CBNP-AuNP-SPEs resulted in a cost-effective, massproducible, and disposable sensing platform to detect Hg2+ in few milliliters of sample, without requiring tedious cleaning treatments and expensive instrumentations. Finally, the CBNP-AuNP-SPE has been successfully applied to Hg2+ detection in spiked river water sample and extracted soil.
Materials and methods Equipment Cyclic voltammetry and anodic stripping voltammetry measurements were performed using a portable potentiostat PalmSens (Palm Instruments, The Netherlands, http://www.palmsens. com/en/) (Fig. 1). UV–vis measurements were carried out using a spectrophotometer UV 1800 (Shimadzu, Japan). Screen-printed electrodes As previously reported (Arduini et al. 2007), screen-printed electrodes were home produced with a 245 DEK (Weymouth, England) screen-printed machine using graphite-based conductive ink (Electrodag 421) for working and counter electrode and silver-silver chloride conductive ink (Electrodag 477 SS) for pseudo-reference electrode, obtained from Acheson (Milan, Italy). The insulating layer was Vinilflat 38.101E from Argon (Italy). The substrate was a flexible polyester film (Autostat HT5) obtained from Autotype (Milan, Italy). The diameter of the working electrode was 0.3 cm, resulting in an apparent geometric area of 0.07 cm2. Chemicals All chemicals from commercial sources were of analytical grade. All solutions were prepared using ultrapure water
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from Millipore. All Hg2+ solutions were prepared using ICP-MS standard solution 1000 mg L −1 (Inorganic Ventures, USA); subsequent dilutions were obtained using HCl (Carlo Erba, Italy).
Hg2+ measurement in river water sample River water was sampled in Aniene Valley (Rome, Italy). The sample was filtered using 0.45-μm-pore-size disposable filter. For 10 mL of river water, concentrated HCl (final concentration 0.05 M) was added and the sample was analyzed. For the accuracy measurement, the river water was spiked with Hg2+, and concentrated HCl (final concentration 0.05 M) was added in order to analyze the sample.
Extracted Hg2+ measurements Supernatant containing the extracted mercury was diluted 10 times with ultrapure water and analyzed as described above. CBNP dispersion The dispersion of CBNPs was prepared by adding 10 mg of CB-N220 powder (Cabot Corporation, Italy, ) to 10 mL of solvent (a mixture dimethylformamide (DMF):water (1:1)) sonicated for 60 min at 59 kHz. CB-N220 is characterized by carbon nanoparticles with diameters comprised between 17.95 and 32.5 nm (Arduini et al. 2010). AuNP synthesis
Soil spiking Soil was sampled in Villa Borghese (Rome, Italy). One gram of soil was spiked to 5 mg/kg by adding 50 μL of a 100 mg/L Hg2+ solution. Spiked soil was homogenized by vortex mixing for 1 min.
Extraction of Hg2+ from soil Mercury was extracted from a soil sample by taking the procedure developed by Han et al. (2003) as a model, with slight modifications. Briefly, 1 g of soil was weighed and added to a 10-mL centrifuge tube with 5 mL of 0.5 M HCl. The sample and the extract solution were well mixed by vortex mixing for 1 min, then sonicated at 25 °C for 60 min. A centrifuge (3200 rpm, 6 min) was used to separate supernatant and soil.
In this work, two kinds of AuNPs were evaluated in the development of the sensor. Firstly, we synthesized citrate- and chloride-capped AuNPs, respectively, following Turkevich et al. (1951) and Zanardi et al. (2010) as models. In the latter case, AuNPs were synthesized by following Zanardi et al. (2010) with slight modifications, as 314 μL of 43 mM NaBH4 aqueous solution was added by two equal aliquots to 3 mL of 1 mM HAuCl4 aqueous solution under vigorous stirring every 15 min. The synthesis proceeded at room temperature for 20 min. We found that all the glassware and magnetic stir bar used in this synthesis should be thoroughly cleaned in aqua regia (HCl/HNO3 3:1, v/v), rinsed in ultrapure water and then cleaned with piranha solution (H2SO4/H2O2 7:3, v/v) and rinsed again with ultrapure water before the use. By UV–vis spectroscopy, we observed the characteristic plasmonic surface resonance (SPR) band of the AuNPs located at 514 nm. Citrate-capped AuNPs were prepared by adding 5 mL of 34 mM sodium citrate to 95 mL of boiling 0.1 mM
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HAuCl4 solution. The reaction was allowed to proceed for 30 min. The SPR band result centered at 525 nm. CBNP-AuNP-SPE preparation SPEs were modified with CBNPs by drop casting: 2 μL of the dispersion was drop cast onto the working electrode surface, with the solvent left to evaporate. The CBNP-AuNP-SPEs were prepared by depositing different drops (2 μL each, from 2 up to 20 μL) of the AuNPs dispersion on the CBNP-modified SPE. Hg2+ measurements Measurements of Hg2+ were performed using square wave anodic stripping voltammetry (SWASV) technique. The portable potentiostat is accessorized by a switch box that automatically controls the magnetic stirrer during the experiment (Calvo Quintana et al. 2012). In detail, 0.05 M HCl was used as working solution, a deposition potential of 0.2 V was applied for 400 s, followed by a stripping range potential from 0 to 0.6 V with a frequency = 60 Hz, E amplitude = 0.1 V and E step = 0.01 V. Prior to Hg2+ deposition, the working electrode was treated with 0.6 V for 10 s to clean the surface to avoid any memory effect.
Results and discussion Choice of the sensor Prior to developing the sensor, two different AuNPs were investigated to select the best typology in order to obtain maximum sensitivity towards mercury detection. Chloride- and citrate-capped AuNPs were employed. Bare SPEs were modified with 10 μL of the two AuNP dispersions. Cyclic voltammetry studies were carried out in order to qualitatively determine the difference regarding the amount of AuNPs deposited onto the working electrode. As displayed in Fig. 2, chloride-capped AuNPs gave the highest reduction peak, observed at around 0.6 V (vs. Ag/AgCl). The significance of the obtained peaks close to 0.6 and 1.2 V is related to the formation (1.2 V) and removal (0.6 V) of oxide layers onto gold sites, as well described in the literature (El-Deab et al. 2003). Figure 2 displays the obvious rise of anodic (close to 1.1 V) and cathodic (close to 0.6 V) currents, due to the formation and removal of a monolayer of oxygen species at the gold surface, respectively. Peak intensities depend on the exposed Au surface which can be firstly oxidized and then reduced, as described by the equations written in Fig. 2. Due to the diverse capping agent, chloride-capped AuNPs allowed for a better exposure of Au atoms with respect to the citrate-capped ones, giving higher voltammetric responses in acidic media.
Fig. 2 Cyclic voltammetry in 0.1 M H2SO4 using SPE modified with 10 μL of citrate-capped AuNPs (a) and chloride-capped AuNPs (b). Scan rate 0.1 V/s
Because of this, chloride-capped AuNPs were chosen as the favorable SPE modifier. Then, the amount of deposited AuNPs was investigated, ranging from 2 to 20 μL. SWASV measurements were conducted in the presence of a fixed concentration of Hg2+ (10 ppb), evaluating the best sensitivity of each system (Fig. 3a). Figure 3A displays a bell-shaped behavior with the maximum centered at 10 μL of deposited AuNPs. As shown, doubling the amount of AuNPs (from 10 to 20 μL) dramatically decreased the recorded signal. This behavior can be ascribed to the excessive thick layer of AuNPs, representing an obstacle for Hg oxidation. Subsequently, after choosing 10 μL of AuNPs as the optimal amount for the SPE modification, CBNPs were investigated to help the deposition of AuNPs. Due to their high surface area, CBNPs allow a more homogenous distribution than bare SPE (Cinti et al., 2014). As showed in Fig. 3B, it is evident how the presence of 2 μL of CBNPs, deposited under AuNP layer, enhances the spatial distribution of AuNPs, promoting a higher surface availability for the gold-mercury interaction. These experiments highlighted the suitability to modify SPEs with a double layer of CBNPs (2 μL) and AuNPs (10 μL) in order to achieve the best sensitivity towards Hg2+ detection. Parameter optimization To achieve the best sensitivity of the sensor, several parameters were examined and optimized by using SWASV analyses. As widely reported in the literature (Munoz et al. 2006; Mandil et al. 2010), hydrochloric acid (HCl) is usually used as supporting electrolyte to electrochemically detect Hg2+. Furthermore, the presence of a high concentration of chloride ions in solution helps to maintain the stability of the screen-printed Ag pseudo-reference electrode potential (Güell et al. 2008). Figure 4 shows the influence of HCl concentration on the sensitivity of a 10-ppb Hg2+ solution, ranging from 0.03 to 0.1 M.
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Fig. 3 A Optimization of the amount of AuNPs deposited onto SPE, from 2 to 20 μL. SWASV measurements were carried out using a 50-ppb Hg2+ standard solution in HCl 0.1 M, E dep = 0.3 V, E cond = 0.7 V, E ampl = 0.04 V, freq = 20 Hz, t cond = 10 s, t dep = 300 s, t eq = 10 s, E step = 0.006 V. B SWASV measurements in presence of a 50-ppb Hg2+ standard solution in the same condition of A using a SPE modified with 10 μL of AuNPs (a) and 2 μL of CBNPs + 10 μL of AuNPs (b)
As it is possible to observe in Fig. 4, a slight variation of sensitivity at varying acid concentrations was observed (Fig. 4). A low repeatability was obtained at 0.03 M, probably due to stabilizing effects of Hg ions by chloride in solution (Louie et al. 2012). Although chloride-rich solution helps to stabilize Hg2+, 0.03 M HCl might be lower than the minimum requirement for mercury stability. Even if the mean values are comparable within the experimental errors, it was decided to work with a concentration of hydrochloric acid equal to 0.05 M as the best compromise of sensitivity and repeatability (RSD < 8 %) of measurements. After having optimized the concentration of supporting electrolyte, electrochemical parameters were taken into account. The required potential to reduce Hg2+ to Hg0, known as deposition potential, was studied from −0.1 to 0.3 V, as reported in Fig. 5. The best sensitivities are shown at 0.1 and 0.2 V towards the detection of a 50-ppb Hg2+ solution. These two potentials
Fig. 5 Optimization of deposition potential in the presence of 50-ppb Hg2+ standard solution in 0.05 M HCl, using CBNP-AuNP-SPE. E cond = 0.7 V, E ampl = 0.04 V, freq = 20 Hz, t cond = 10 s, t dep = 300 s, t eq = 10 s, E step = 0.006 V
were further analyzed in triplicate to choose the best operative conditions to carry out the detection (data not shown). Thus, 0.2 V was chosen as the optimal deposition potential because of its higher sensitivity in respect to 0.1 V with satisfactory repeatability (RSD = 10 %). Moreover, the deposition time was evaluated from 20 to 500 s (data not shown) and it was decided to apply the deposition potential for 400 s, as compromise between sensitivity and analysis time. Finally, since square wave was chosen as dissolution technique (stripping), parameters of frequency and amplitude were optimized by examining the mercury response at a CBNP-AuNP-SPE in 0.05 M HCl (Fig. 6). Frequency of the square wave was studied from 20 to 100 Hz. As shown in Fig. 6A, the obtained trend is bellshaped with the maximum centered at 60 Hz. This frequency was used in the following experiments because it gave the optimal signal-to-noise ratio and the lowest background current (inset of Fig. 6A). The pulse amplitude, from 0.02 to 0.15 mV, was also tested. As clearly visible in Fig. 6B, 0.1 V amplitude gave the best sensitivity and a RSD lower than 9 %. Other optimized electrochemical parameters were the square wave step potential (step height of the staircase), the equilibration time (time between deposition and stripping), and the cleaning potential applied for 10 s (useful to reduce memory effect), equal to 0.01 V, 10 s and 0.6 V (data not shown), respectively. Analytical performance of CBNP-AuNP-SPE
Fig. 4 Optimization of the HCl concentration. SWASV measurements were performed using CBNP-AuNP-SPE in presence of a 50-ppb Hg2+ standard solution. E dep = 0.3 V, E cond = 0.7 V, E ampl = 0.04 V, freq = 20 Hz, t cond = 10 s, t dep = 300 s, t eq = 10 s, E step = 0.006 V
Once the parameters were optimized, the CBNP-AuNP-SPE was tested using standard solutions of Hg2+ as shown in Fig. 7. Two linear ranges were obtained as relationship between the peak current of Hg2+ and its concentration: from 0 to 60 ppb described by y = −6.2 + 0.96x with a R2 = 0.997 and a sensitivity of 14 μA ppb−1 cm−2 and from 60 to 100 ppb described by y = 22 + 0.48x with a R2 = 0.993 and a sensitivity
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than the 1.1 ppb obtained with gold SPE (Bernalte et al. 2011), the use of CBNPs-AuNPs allowed to reach a higher sensitivity (0.98 vs. 0.036 μA ppb−1) and a wider linear range with the respect to the gold-based ink (up to 100 ppb using SPE modified with CBNPs-AuNPs and up to 30 ppb using gold SPE). The presence of CB, as recently demonstrated by our group (Cinti et al. 2014), guaranteed a high dispersion of AuNPs and, consequently, a large number of sites available for Hg2+. Determination of Hg2+ in river water and soil samples
Fig. 6 A Optimization of square wave frequency in presence of 50-ppb Hg2+ standard solution in 0.05 M HCl, using a CBNP-AuNP-SPE. Inset: voltammograms recorded at 60 Hz (a) and 100 Hz (b) are reported. E dep = 0.2 V, E cond = 0.7 V, E ampl = 0.04 V, t cond = 10 s, t dep = 400 s, t eq = 10 s, E step = 0.006 V. B Optimization of square wave amplitude potential in the same operative conditions of A using freq = 60 Hz
of 6.8 μA ppb−1 cm−2. Limit of detection (LOD), determined according to a signal-to-noise ratio (S/N) = 3, was equal to 3 ppb, and limit of quantification (LOQ), calculated as S/N = 10, was equal to 10 ppb. The reproducibility was evaluated by measuring a fixed concentration of Hg2+ (50 ppb) with different CBNP-AuNP-SPEs of the same batch, and the RSD resulted lower than 10 %. Even if the LOD was higher
The analytical efficacy of the developed electrochemical sensor for the determination of mercury ions was demonstrated by applying it to the determination of Hg2+ in river water and soil samples. For river water, the sample was only acidified with HCl as described in the BMaterials and methods^ section. The river water sample was analyzed, and Hg2+ was not measured at the detection limit of the sensor, since no voltammetric peak around 0.4 V (vs. Ag/AgCl) was observed. To verify the accuracy of the method, the sample was spiked with 10 ppb and was analyzed after, obtaining a recovery value of 107 ± 14 %. For the soil sample, 1 g of soil was treated and extracted as described in the BMaterials and methods^ section. The extract was diluted tenfold with ultrapure water and analyzed. Furthermore, 1 g of soil previously spiked with Hg2+ at the 5-ppm level, in agreement with the allowed limit for industrial soils (D.M.121 1992), was extracted and analyzed as previously reported. The recovery percentage was evaluated by standard addition method. Inorganic mercury ions belong to the Breactive Hg species^ (Lindqvist et al. 1984), the most complicated group to extract. The extraction step is the critical one, and percentage of extracted inorganic mercury is characterized by high scattered values (Hammerschmidt and Fitzgerald 2001; Issaro et al. 2009). Extraction was not optimized, nor deeply investigated, as it was not the aim of the present work. Firstly, soil sample was analyzed and Hg2+ was not measured at the detection limit of the sensor, since no voltammetric peak around 0.4 V (vs. Ag/AgCl) was observed. To verify the accuracy of the method, the spiked sample was later analyzed, obtaining a recovery value of 56 ± 5 %. We were only interested in highlighting the effectiveness of CBAuNPs-SPE to detect Hg2+ in the extracted matrix. Therefore, no attempt was made to develop a more effective extraction procedure.
Conclusion Fig. 7 Voltammograms recorded performing SWASV from 0 to 100 ppb of Hg2+ in 0.05 M HCl using CB-AuNP-SPE. Inset: calibration curve with two linear ranges, 0–60 and 60–100 ppb. E dep = 0.2 V, E cond = 0.6 V, E ampl = 0.1 V, freq = 60 Hz, t cond = 10 s, t dep = 400 s, t eq = 10 s, E step = 0.01 V
This work demonstrates the development of a facile method to detect inorganic mercury ions. Screen-printed electrodes were easily modified with carbon black nanoparticles (CBNPs) and
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gold nanoparticles (AuNPs) by an automatable (www.biodot. com) drop casting procedure. The developed sensor (CBNPAuNP-SPE) was challenged at parts per billion level range, achieving a limit of detection equal to 3 ppb and a satisfying linearity. Furthermore, the proposed sensor was applied to Hg2+ determination in real river water and soil sample by means of quick acidification in the case of river water sample and a mild extraction protocol (HCl + ultrasound) in the case of soil sample. We demonstrated for the first time the applicability of a miniaturized nano-modified sensor for Hg2+ detection in soil sample. Screen-printed technology represents a helpful approach for the production of electrochemical platforms for decentralized pollutant sensing; even more, the use of portable instruments (www.palmsens.com) could allow for field measurements with no need of specialized employees. The suitability of this sensor in river water and soil paves the way for a green analysis, bearing in mind that one of the 12 principles of the green chemistry concerns real-time analyses. Acknowledgments F.A. likes to acknowledge the Minister of Defense, Aptamer BW project for financial support. The authors thank Julian Ramirez for revising the English manuscript.
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Stozhko NY, Malakhova NA, Fyodorov MV, Brainina KZ (2008) Modified carbon containing electrodes in stripping voltammetry of metals. J Solid State Electrochem 12:1185–1204 Talarico D, Cinti S, Arduini F, Amine A, Moscone D, Palleschi G (2015) Phosphate detection through cost-effective carbon black nanoparticle-modified screen-printed electrode embedded in a continuous flow system. Environ Sci Technol DOI. doi:10.1021/acs.est. 5b00218 Telmer KH, Veiga MM (2009) World emissions of mercury from artisanal and small scale gold mining. In: Mason R, Pirrone N (eds) Mercury Fate and Transport in the Global Atmosphere: Emissions, Measurements and Models. Springer, New York, pp 131–172 Turkevich J, Stevenson PC, Hillier J (1951) A study of the nucleation and growth processes in the synthesis of colloidal gold. Discuss Faraday Soc 11:55–75 UNEP (2013) Global Mercury Assessment 2013: Sources, Emissions, Releases and Environmental Transport. UNEP Chemicals Branch, Geneva, Switzerland Vicentini FC, Ravanini AE, Figueiredo-Filho LC, Iniesta J, Banks CE, Fatibello-Filho O (2015) Imparting improvements in electrochemical sensors: evaluation of different carbon blacks that give rise to significant improvement in the performance of electroanalytical sensing platforms. Electrochim Acta 157:125–133 Wang J (2006) Analytical Electrochemistry. Wiley-VCH, New Jersey Welch CM, Nekrassova O, Dai X, Hyde ME, Compton RG (2004) Fabrication, characterisation and voltammetric studies of gold amalgam nanoparticle modified electrodes. Chem Phys Chem 5:1405– 1410 Zanardi C, Terzi F, Zanfrognini B, Pigani L, Seeber R, Lukkari J, Ääritalo T (2010) Effective catalytic electrode system based on polyviologen and Au nanoparticles multilayer. Sens Actuators B 144:92–98
ENVIRONMENTAL ISSUES FACING CHEMICAL, BIOLOGICAL, RADIOLOGICAL AND NUCLEAR RISKS
Hg2+ detection using a disposable and miniaturized screen-printed electrode modified with nanocomposite carbon black and gold nanoparticles Stefano Cinti 1 & Francesco Santella 1 & Danila Moscone 1,2 & Fabiana Arduini 1,2
Received: 10 July 2015 / Accepted: 14 January 2016 # Springer-Verlag Berlin Heidelberg 2016
Abstract A miniaturized screen-printed electrode (SPE) modified with a carbon black-gold nanoparticle (CBNPAuNP) nanocomposite has been developed as an electrochemical sensor for the detection of inorganic mercury ions (Hg2+). The working electrode surface has been modified with nanocomposite constituted of CBNPs and AuNPs by an easy drop casting procedure that makes this approach extendible to an automatable mass production of modified SPEs. Square wave anodic stripping voltammetry (SWASV) was adopted to perform Hg2+ detection, revealing satisfactory sensitivity and detection limit, equal to 14 μA ppb−1 cm−2 and 3 ppb, respectively. The applicability of the CBNP-AuNP-SPE for the determination of inorganic mercury has been assessed in river water by a simple filtration and acidification of the sample as well as in soil by means of a facile acidic extraction procedure assisted by ultrasound. Keywords Mercury detection . Anodic stripping voltammetry . Screen-printed electrode . Carbon black . Gold nanoparticles . Soil Mercury (Hg), including inorganic and organic forms, represents one of the heavy metals of global concern. In the list of priority substances set by the United States Agency for Toxic Responsible editor: Philippe Garrigues * Fabiana Arduini [email protected]
1
Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma BTor Vergata^, Via della Ricerca Scientifica 1, 00133 Rome, Italy
2
Consorzio Interuniversitario Biostrutture e Biosistemi BINBB^, Viale Medaglie d’Oro, 305 Rome, Italy
Substances and Disease Registry (ATSDR) and the United States Environmental Protection Agency (EPA) in agreement with the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), Hg has occupied the third position (lead and arsenic are the first and the second ones), in 2013 and 2015 (Agency for Toxic Substances and Disease Registry 2015), demonstrating the relevant problem of Hg. In 2013, United Nations Environment Programme (UNEP) in a summary report claimed that Hg emissions in air arise from natural (10 %), anthropogenic (30 %), and re-emission (60 %) sources (UNEP 2013). Telmer and Veiga (2009) have estimated that approximately 1000 metric tons/year of mercury have been released from at least 70 countries. Approximately, 350 metric tons/year is directly emitted into the atmosphere, while 650 metric tons/year is released into the hydrosphere, e.g., rivers, lakes, and soils. For instance, Hg in urban soils of China has been found in the range of 0.16– 3.68 ppm (Liu et al. 2012). Obviously, aquatic and soil pollution are linked: indirect sources to the aquatic environment include rain runoff to water bodies or leaching of the upper soil layers by groundwater flows (Schroeder et al. 1989). Mercury is present in several forms; Hg can be easily oxidized to Hg2+ and, subsequently, atmospherically transported and accumulated in land, waterways, and oceans (Schroeder and Munthe 1998), aquatic microorganism can easily turn Hg2+ into methylated species, i.e., mono-methylmercury (MMHg) and di-methylmercury (DMHg), leading to an amplified bioaccumulation from plankton to fishes. However, as recently reviewed by Sonke et al. (2013), even if the relationship between inorganic and methylated Hg is not completely clear in terms of mechanism, seafood sources represent the greatest health risk to humans and wildlife. In an overall scenario, the development of simple, fast, efficient, and reliable methods to evaluate environmental
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levels of mercury is an important issue. Standardized methods are mainly based on cold vapor atomic absorption (AAS), atomic fluorescence spectrometry (AFS), and inductively coupled plasma mass spectrometry (ICP-MS) (Rio-Segade and Bendicho 1999; Huber et al. 2015; Cairns et al. 2008); yet, these approaches usually require expensive instrumentation, often hardly portable, and skilled personnel. An electroanalytical approach emerges as a valuable alternative for environmental monitoring of mercury (Wang 2006). In literature, the utilization of voltammetric stripping analysis is largely reported (Stozhko et al. 2008; Economou 2010). In particular, anodic stripping voltammetry (ASV) is known as the best electroanalytical technique to detect mercury: briefly, Hg is first reduced/pre-concentrated at the electrode surface and subsequently oxidized/re-dissolved in the solution, while the anodic current is recorded as it is proportional to the concentration of Hg. Gold electrodes (disk, film, arrays) or glassy carbon electrodes modified with gold nanoparticles, thanks to their high affinity for mercury, are widely reported as the best electrodes to perform ASV detection of Hg (Bonfil et al. 2000; Ordeig et al. 2006; Okçu et al. 2008; Laffont et al. 2015). Nevertheless, if the high Au-Hg affinity enhances the preconcentration step, this feature even demonstrates a relevant drawback of the gold electrode, as the structural surface changes after the formation of this amalgam and long cleaning treatments are required to achieve a satisfying reproducibility of measurements (Welch et al. 2004). Screen-printed electrodes (SPEs) represent an alternative substitute to conventional gold-based solid electrodes (Metters et al. 2011). Their versatility demonstrates a great benefit in analytical research areas; the ability to easily modify the electrodes with different and commercially available ink formulations onto a variety of substrates (plastic, ceramic, paper, skin) allows for highly specific and finely calibrated electrodes able to determine specific analytes. SPEs provide effective electroanalytical platforms, customizable and easy extendible for the detection of heavy metals. In the literature, several references are reported on the application of different SPEs for anodic stripping Hg2+ determination: Chiu et al. (2008) utilized a silver-based SPE to detect mercury in cosmetic samples; Mandil et al. (2010) detected mercury by electrodeposition of a gold film onto a carbon SPE; Meucci et al. (2009) fabricated screen-printed gold electrodes on a plastic substrate for determination of mercury in fish tissues; Bernalte et al. (2011) used a gold SPE for the determination of mercury in wastewaters by square wave anodic stripping voltammetry. However, using electroplating procedures as well as the use of noble metal inks (silver, gold) to produce sensing devices could not demonstrate to be the best solution. The utilization of nanomaterials to improve the analytical performance, and to reduce time/cost fabrication of SPEs, is being widely reported in last 5–10 years: many research groups have reported on the opportunity to modify SPEs with nanomaterials such as
gold nanoparticles (AuNPs), carbon nanotubes (CNTs), graphene, and hybrid nanocomposites (Bernalte et al. 2012; Niu et al. 2011; Martín-Yerga et al. 2012). In this work, we propose a disposable sensor based on a carbon screen-printed electrode modified with a nanocomposite constituted of AuNPs and carbon black nanoparticles (CBNPs) to detect Hg2+. CB is a cost-effective nanomaterial principally used as reinforcing filler in rubber compounds and was recently employed as an electrode modifier for successful electroanalytical analyses (Arduini et al. 2010; Arduini et al. 2012; Arduini et al. 2015; Vicentini et al. 2015; Talarico et al. 2015; Cinti et al. 2016). We have deposited a layer of AuNPs onto a CBNPs layer and, due to the high surface area of this latter, the CBNP layer allowed for a better dispersion of AuNPs onto the electrode surface (Cinti et al. 2014). In this work, we carried out the optimization with respect to both the modification (amount of modifiers) and the analytical parameters. CBNP-AuNP-SPEs resulted in a cost-effective, massproducible, and disposable sensing platform to detect Hg2+ in few milliliters of sample, without requiring tedious cleaning treatments and expensive instrumentations. Finally, the CBNP-AuNP-SPE has been successfully applied to Hg2+ detection in spiked river water sample and extracted soil.
Materials and methods Equipment Cyclic voltammetry and anodic stripping voltammetry measurements were performed using a portable potentiostat PalmSens (Palm Instruments, The Netherlands, http://www.palmsens. com/en/) (Fig. 1). UV–vis measurements were carried out using a spectrophotometer UV 1800 (Shimadzu, Japan). Screen-printed electrodes As previously reported (Arduini et al. 2007), screen-printed electrodes were home produced with a 245 DEK (Weymouth, England) screen-printed machine using graphite-based conductive ink (Electrodag 421) for working and counter electrode and silver-silver chloride conductive ink (Electrodag 477 SS) for pseudo-reference electrode, obtained from Acheson (Milan, Italy). The insulating layer was Vinilflat 38.101E from Argon (Italy). The substrate was a flexible polyester film (Autostat HT5) obtained from Autotype (Milan, Italy). The diameter of the working electrode was 0.3 cm, resulting in an apparent geometric area of 0.07 cm2. Chemicals All chemicals from commercial sources were of analytical grade. All solutions were prepared using ultrapure water
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from Millipore. All Hg2+ solutions were prepared using ICP-MS standard solution 1000 mg L −1 (Inorganic Ventures, USA); subsequent dilutions were obtained using HCl (Carlo Erba, Italy).
Hg2+ measurement in river water sample River water was sampled in Aniene Valley (Rome, Italy). The sample was filtered using 0.45-μm-pore-size disposable filter. For 10 mL of river water, concentrated HCl (final concentration 0.05 M) was added and the sample was analyzed. For the accuracy measurement, the river water was spiked with Hg2+, and concentrated HCl (final concentration 0.05 M) was added in order to analyze the sample.
Extracted Hg2+ measurements Supernatant containing the extracted mercury was diluted 10 times with ultrapure water and analyzed as described above. CBNP dispersion The dispersion of CBNPs was prepared by adding 10 mg of CB-N220 powder (Cabot Corporation, Italy, ) to 10 mL of solvent (a mixture dimethylformamide (DMF):water (1:1)) sonicated for 60 min at 59 kHz. CB-N220 is characterized by carbon nanoparticles with diameters comprised between 17.95 and 32.5 nm (Arduini et al. 2010). AuNP synthesis
Soil spiking Soil was sampled in Villa Borghese (Rome, Italy). One gram of soil was spiked to 5 mg/kg by adding 50 μL of a 100 mg/L Hg2+ solution. Spiked soil was homogenized by vortex mixing for 1 min.
Extraction of Hg2+ from soil Mercury was extracted from a soil sample by taking the procedure developed by Han et al. (2003) as a model, with slight modifications. Briefly, 1 g of soil was weighed and added to a 10-mL centrifuge tube with 5 mL of 0.5 M HCl. The sample and the extract solution were well mixed by vortex mixing for 1 min, then sonicated at 25 °C for 60 min. A centrifuge (3200 rpm, 6 min) was used to separate supernatant and soil.
In this work, two kinds of AuNPs were evaluated in the development of the sensor. Firstly, we synthesized citrate- and chloride-capped AuNPs, respectively, following Turkevich et al. (1951) and Zanardi et al. (2010) as models. In the latter case, AuNPs were synthesized by following Zanardi et al. (2010) with slight modifications, as 314 μL of 43 mM NaBH4 aqueous solution was added by two equal aliquots to 3 mL of 1 mM HAuCl4 aqueous solution under vigorous stirring every 15 min. The synthesis proceeded at room temperature for 20 min. We found that all the glassware and magnetic stir bar used in this synthesis should be thoroughly cleaned in aqua regia (HCl/HNO3 3:1, v/v), rinsed in ultrapure water and then cleaned with piranha solution (H2SO4/H2O2 7:3, v/v) and rinsed again with ultrapure water before the use. By UV–vis spectroscopy, we observed the characteristic plasmonic surface resonance (SPR) band of the AuNPs located at 514 nm. Citrate-capped AuNPs were prepared by adding 5 mL of 34 mM sodium citrate to 95 mL of boiling 0.1 mM
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HAuCl4 solution. The reaction was allowed to proceed for 30 min. The SPR band result centered at 525 nm. CBNP-AuNP-SPE preparation SPEs were modified with CBNPs by drop casting: 2 μL of the dispersion was drop cast onto the working electrode surface, with the solvent left to evaporate. The CBNP-AuNP-SPEs were prepared by depositing different drops (2 μL each, from 2 up to 20 μL) of the AuNPs dispersion on the CBNP-modified SPE. Hg2+ measurements Measurements of Hg2+ were performed using square wave anodic stripping voltammetry (SWASV) technique. The portable potentiostat is accessorized by a switch box that automatically controls the magnetic stirrer during the experiment (Calvo Quintana et al. 2012). In detail, 0.05 M HCl was used as working solution, a deposition potential of 0.2 V was applied for 400 s, followed by a stripping range potential from 0 to 0.6 V with a frequency = 60 Hz, E amplitude = 0.1 V and E step = 0.01 V. Prior to Hg2+ deposition, the working electrode was treated with 0.6 V for 10 s to clean the surface to avoid any memory effect.
Results and discussion Choice of the sensor Prior to developing the sensor, two different AuNPs were investigated to select the best typology in order to obtain maximum sensitivity towards mercury detection. Chloride- and citrate-capped AuNPs were employed. Bare SPEs were modified with 10 μL of the two AuNP dispersions. Cyclic voltammetry studies were carried out in order to qualitatively determine the difference regarding the amount of AuNPs deposited onto the working electrode. As displayed in Fig. 2, chloride-capped AuNPs gave the highest reduction peak, observed at around 0.6 V (vs. Ag/AgCl). The significance of the obtained peaks close to 0.6 and 1.2 V is related to the formation (1.2 V) and removal (0.6 V) of oxide layers onto gold sites, as well described in the literature (El-Deab et al. 2003). Figure 2 displays the obvious rise of anodic (close to 1.1 V) and cathodic (close to 0.6 V) currents, due to the formation and removal of a monolayer of oxygen species at the gold surface, respectively. Peak intensities depend on the exposed Au surface which can be firstly oxidized and then reduced, as described by the equations written in Fig. 2. Due to the diverse capping agent, chloride-capped AuNPs allowed for a better exposure of Au atoms with respect to the citrate-capped ones, giving higher voltammetric responses in acidic media.
Fig. 2 Cyclic voltammetry in 0.1 M H2SO4 using SPE modified with 10 μL of citrate-capped AuNPs (a) and chloride-capped AuNPs (b). Scan rate 0.1 V/s
Because of this, chloride-capped AuNPs were chosen as the favorable SPE modifier. Then, the amount of deposited AuNPs was investigated, ranging from 2 to 20 μL. SWASV measurements were conducted in the presence of a fixed concentration of Hg2+ (10 ppb), evaluating the best sensitivity of each system (Fig. 3a). Figure 3A displays a bell-shaped behavior with the maximum centered at 10 μL of deposited AuNPs. As shown, doubling the amount of AuNPs (from 10 to 20 μL) dramatically decreased the recorded signal. This behavior can be ascribed to the excessive thick layer of AuNPs, representing an obstacle for Hg oxidation. Subsequently, after choosing 10 μL of AuNPs as the optimal amount for the SPE modification, CBNPs were investigated to help the deposition of AuNPs. Due to their high surface area, CBNPs allow a more homogenous distribution than bare SPE (Cinti et al., 2014). As showed in Fig. 3B, it is evident how the presence of 2 μL of CBNPs, deposited under AuNP layer, enhances the spatial distribution of AuNPs, promoting a higher surface availability for the gold-mercury interaction. These experiments highlighted the suitability to modify SPEs with a double layer of CBNPs (2 μL) and AuNPs (10 μL) in order to achieve the best sensitivity towards Hg2+ detection. Parameter optimization To achieve the best sensitivity of the sensor, several parameters were examined and optimized by using SWASV analyses. As widely reported in the literature (Munoz et al. 2006; Mandil et al. 2010), hydrochloric acid (HCl) is usually used as supporting electrolyte to electrochemically detect Hg2+. Furthermore, the presence of a high concentration of chloride ions in solution helps to maintain the stability of the screen-printed Ag pseudo-reference electrode potential (Güell et al. 2008). Figure 4 shows the influence of HCl concentration on the sensitivity of a 10-ppb Hg2+ solution, ranging from 0.03 to 0.1 M.
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Fig. 3 A Optimization of the amount of AuNPs deposited onto SPE, from 2 to 20 μL. SWASV measurements were carried out using a 50-ppb Hg2+ standard solution in HCl 0.1 M, E dep = 0.3 V, E cond = 0.7 V, E ampl = 0.04 V, freq = 20 Hz, t cond = 10 s, t dep = 300 s, t eq = 10 s, E step = 0.006 V. B SWASV measurements in presence of a 50-ppb Hg2+ standard solution in the same condition of A using a SPE modified with 10 μL of AuNPs (a) and 2 μL of CBNPs + 10 μL of AuNPs (b)
As it is possible to observe in Fig. 4, a slight variation of sensitivity at varying acid concentrations was observed (Fig. 4). A low repeatability was obtained at 0.03 M, probably due to stabilizing effects of Hg ions by chloride in solution (Louie et al. 2012). Although chloride-rich solution helps to stabilize Hg2+, 0.03 M HCl might be lower than the minimum requirement for mercury stability. Even if the mean values are comparable within the experimental errors, it was decided to work with a concentration of hydrochloric acid equal to 0.05 M as the best compromise of sensitivity and repeatability (RSD < 8 %) of measurements. After having optimized the concentration of supporting electrolyte, electrochemical parameters were taken into account. The required potential to reduce Hg2+ to Hg0, known as deposition potential, was studied from −0.1 to 0.3 V, as reported in Fig. 5. The best sensitivities are shown at 0.1 and 0.2 V towards the detection of a 50-ppb Hg2+ solution. These two potentials
Fig. 5 Optimization of deposition potential in the presence of 50-ppb Hg2+ standard solution in 0.05 M HCl, using CBNP-AuNP-SPE. E cond = 0.7 V, E ampl = 0.04 V, freq = 20 Hz, t cond = 10 s, t dep = 300 s, t eq = 10 s, E step = 0.006 V
were further analyzed in triplicate to choose the best operative conditions to carry out the detection (data not shown). Thus, 0.2 V was chosen as the optimal deposition potential because of its higher sensitivity in respect to 0.1 V with satisfactory repeatability (RSD = 10 %). Moreover, the deposition time was evaluated from 20 to 500 s (data not shown) and it was decided to apply the deposition potential for 400 s, as compromise between sensitivity and analysis time. Finally, since square wave was chosen as dissolution technique (stripping), parameters of frequency and amplitude were optimized by examining the mercury response at a CBNP-AuNP-SPE in 0.05 M HCl (Fig. 6). Frequency of the square wave was studied from 20 to 100 Hz. As shown in Fig. 6A, the obtained trend is bellshaped with the maximum centered at 60 Hz. This frequency was used in the following experiments because it gave the optimal signal-to-noise ratio and the lowest background current (inset of Fig. 6A). The pulse amplitude, from 0.02 to 0.15 mV, was also tested. As clearly visible in Fig. 6B, 0.1 V amplitude gave the best sensitivity and a RSD lower than 9 %. Other optimized electrochemical parameters were the square wave step potential (step height of the staircase), the equilibration time (time between deposition and stripping), and the cleaning potential applied for 10 s (useful to reduce memory effect), equal to 0.01 V, 10 s and 0.6 V (data not shown), respectively. Analytical performance of CBNP-AuNP-SPE
Fig. 4 Optimization of the HCl concentration. SWASV measurements were performed using CBNP-AuNP-SPE in presence of a 50-ppb Hg2+ standard solution. E dep = 0.3 V, E cond = 0.7 V, E ampl = 0.04 V, freq = 20 Hz, t cond = 10 s, t dep = 300 s, t eq = 10 s, E step = 0.006 V
Once the parameters were optimized, the CBNP-AuNP-SPE was tested using standard solutions of Hg2+ as shown in Fig. 7. Two linear ranges were obtained as relationship between the peak current of Hg2+ and its concentration: from 0 to 60 ppb described by y = −6.2 + 0.96x with a R2 = 0.997 and a sensitivity of 14 μA ppb−1 cm−2 and from 60 to 100 ppb described by y = 22 + 0.48x with a R2 = 0.993 and a sensitivity
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than the 1.1 ppb obtained with gold SPE (Bernalte et al. 2011), the use of CBNPs-AuNPs allowed to reach a higher sensitivity (0.98 vs. 0.036 μA ppb−1) and a wider linear range with the respect to the gold-based ink (up to 100 ppb using SPE modified with CBNPs-AuNPs and up to 30 ppb using gold SPE). The presence of CB, as recently demonstrated by our group (Cinti et al. 2014), guaranteed a high dispersion of AuNPs and, consequently, a large number of sites available for Hg2+. Determination of Hg2+ in river water and soil samples
Fig. 6 A Optimization of square wave frequency in presence of 50-ppb Hg2+ standard solution in 0.05 M HCl, using a CBNP-AuNP-SPE. Inset: voltammograms recorded at 60 Hz (a) and 100 Hz (b) are reported. E dep = 0.2 V, E cond = 0.7 V, E ampl = 0.04 V, t cond = 10 s, t dep = 400 s, t eq = 10 s, E step = 0.006 V. B Optimization of square wave amplitude potential in the same operative conditions of A using freq = 60 Hz
of 6.8 μA ppb−1 cm−2. Limit of detection (LOD), determined according to a signal-to-noise ratio (S/N) = 3, was equal to 3 ppb, and limit of quantification (LOQ), calculated as S/N = 10, was equal to 10 ppb. The reproducibility was evaluated by measuring a fixed concentration of Hg2+ (50 ppb) with different CBNP-AuNP-SPEs of the same batch, and the RSD resulted lower than 10 %. Even if the LOD was higher
The analytical efficacy of the developed electrochemical sensor for the determination of mercury ions was demonstrated by applying it to the determination of Hg2+ in river water and soil samples. For river water, the sample was only acidified with HCl as described in the BMaterials and methods^ section. The river water sample was analyzed, and Hg2+ was not measured at the detection limit of the sensor, since no voltammetric peak around 0.4 V (vs. Ag/AgCl) was observed. To verify the accuracy of the method, the sample was spiked with 10 ppb and was analyzed after, obtaining a recovery value of 107 ± 14 %. For the soil sample, 1 g of soil was treated and extracted as described in the BMaterials and methods^ section. The extract was diluted tenfold with ultrapure water and analyzed. Furthermore, 1 g of soil previously spiked with Hg2+ at the 5-ppm level, in agreement with the allowed limit for industrial soils (D.M.121 1992), was extracted and analyzed as previously reported. The recovery percentage was evaluated by standard addition method. Inorganic mercury ions belong to the Breactive Hg species^ (Lindqvist et al. 1984), the most complicated group to extract. The extraction step is the critical one, and percentage of extracted inorganic mercury is characterized by high scattered values (Hammerschmidt and Fitzgerald 2001; Issaro et al. 2009). Extraction was not optimized, nor deeply investigated, as it was not the aim of the present work. Firstly, soil sample was analyzed and Hg2+ was not measured at the detection limit of the sensor, since no voltammetric peak around 0.4 V (vs. Ag/AgCl) was observed. To verify the accuracy of the method, the spiked sample was later analyzed, obtaining a recovery value of 56 ± 5 %. We were only interested in highlighting the effectiveness of CBAuNPs-SPE to detect Hg2+ in the extracted matrix. Therefore, no attempt was made to develop a more effective extraction procedure.
Conclusion Fig. 7 Voltammograms recorded performing SWASV from 0 to 100 ppb of Hg2+ in 0.05 M HCl using CB-AuNP-SPE. Inset: calibration curve with two linear ranges, 0–60 and 60–100 ppb. E dep = 0.2 V, E cond = 0.6 V, E ampl = 0.1 V, freq = 60 Hz, t cond = 10 s, t dep = 400 s, t eq = 10 s, E step = 0.01 V
This work demonstrates the development of a facile method to detect inorganic mercury ions. Screen-printed electrodes were easily modified with carbon black nanoparticles (CBNPs) and
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gold nanoparticles (AuNPs) by an automatable (www.biodot. com) drop casting procedure. The developed sensor (CBNPAuNP-SPE) was challenged at parts per billion level range, achieving a limit of detection equal to 3 ppb and a satisfying linearity. Furthermore, the proposed sensor was applied to Hg2+ determination in real river water and soil sample by means of quick acidification in the case of river water sample and a mild extraction protocol (HCl + ultrasound) in the case of soil sample. We demonstrated for the first time the applicability of a miniaturized nano-modified sensor for Hg2+ detection in soil sample. Screen-printed technology represents a helpful approach for the production of electrochemical platforms for decentralized pollutant sensing; even more, the use of portable instruments (www.palmsens.com) could allow for field measurements with no need of specialized employees. The suitability of this sensor in river water and soil paves the way for a green analysis, bearing in mind that one of the 12 principles of the green chemistry concerns real-time analyses. Acknowledgments F.A. likes to acknowledge the Minister of Defense, Aptamer BW project for financial support. The authors thank Julian Ramirez for revising the English manuscript.
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