Novel Thin-Film Polymeric Materials for the Detection of Heavy Metals

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Procedia Engineering 47 ( 2012 ) 322 – 325

Proc. Eurosensors XXVI, September 9-12, 2012, Kraków, Poland Novel Thin-Film Polymeric Materials for the Detection of Heavy Metals

H. Ikena, D. Kirsanovb,c, A. Leginb,c and M.J. Schöninga,d,*

a Institute of Nano- and Biotechnologies (INB), FH Aachen, Jülich Campus, Germany b Sensor Systems, LLC, St. Petersburg, Russia c Laboratory of Chemical Sensors, St. Petersburg University, St. Petersburg, Russia d Peter Grünberg Institute (PGI-8), Research Centre Jülich GmbH, Jülich, Germany

Abstract

A variety of transition metals, e.g., copper, zinc, cadmium, lead, etc. are widely used in industry as components for wires, coatings, alloys, batteries, paints and so on. The inevitable presence of transition metals in industrial processes implies the ambition of developing a proper analytical technique for their adequate monitoring. Most of these elements, especially lead and cadmium, are acutely toxic for biological organisms. Quantitative determination of these metals at low activity levels in different environmental and industrial samples is therefore a vital task. A promising approach to achieve an at-side or on-line monitoring on a miniaturized and cost efficient way is the combination of a common potentiometric sensor array with heavy metal-sensitive thin-film materials, like chalcogenide glasses and polymeric materials, respectively.

© 20122012 The Published Authors. by Published Elsevier by Ltd. Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o. Open access under CC BY-NC-ND license. Keywords: ion-selective microelectrodes, polymeric sensor membrane, sensor array, PLD

1. Motivation

The main methods currently used for the detection of heavy metals are atomic absorption (emission) and ICP-MS (inductivity coupled plasma mass spectrometry). These techniques ensure very low detection limits of transition metal detection but although require expensive and sophisticated equipment and skilled laboratory staff. Finally, they are hardly applicable for at- and on-line analysis, often demanded in common practice. Therefore, the determination of ions in solutions by electrochemical sensors represents a promising alternative since they allow performing analysis of heavy metals in real-world media, e.g., industrial waste-waters in a simple and inexpensive way. Another important feature of these sensors is that they determine not the total metal content as spectroscopic methods do, but the metal activity (content in free ionic form), which is usually responsible for toxicity of the metal. Thereto, a combination of organic and inorganic sensing materials on a single thin-film sensor array is envisaged (see Fig. 1). As inorganic compound chalcogenide glass materials are used. These are well known and have shown good sensor properties as thin-film sensor layers in former studies [1-7]. The drawback of the chalcogenide glass membranes is their lack of selectivity. The combination with an organic compound will deliver a higher information content, especially with regard to better selectivity coefficients of the polymeric materials compared to the chalcogenide glass materials. As organic material a PVC-based compound material is utilized, which offers the opportunity of “fine-tuning” the sensor properties by varying the receptor molecule in structure and concentration [8,9]. These issues are not only interesting for discrete sensors but for sensor arrays (“electronic tongue”), which may help to overcome many practical drawbacks in applications related to insufficient selectivity, detection limit and of simply unavailability of certain sensors for the demanded species in a complex analyte.

Corresponding author: Michael J. Schöning, FH Aachen – University of Applied Sciences, Heinrich-Mußmann-Str. 1, 52428 Jülich, phone: +49 (0)241-6009 53215, fax: +49 (0)241-6009 53235, [email protected]

1877-7058 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o. Open access under CC BY-NC-ND license. doi: 10.1016/j.proeng.2012.09.148 H. Iken et al. / Procedia Engineering 47 ( 2012 ) 322 – 325 323

Fig. 1: Scheme of the sensor set-up used for potentiometric measurements (left) and build-up of the sensor array (right), exemplarily shown with two electrodes.

2. Experimental

2.1. Sensor fabrication

For the fabrication of the transducer structure, a p-doped Si substrate was used (specific resistance >1000 Ωcm). Thereon, 500 nm SiO2 has been grown as insulator by means of a thermal wet oxidation process. After that, a photoresist has been spun on the wafer and was exposed with a mask aligner and developed. Since the electrodes (sensor spots) and the conducting lines are embedded in the insulating layer, an etching step has been performed before depositing the metal layers. This was done by using buffered hydrofluoric acid with an etching time of about 4 min. Then, the metal-layer sandwich, built-up of titanium, platinum and gold, has been deposited by means of electron-beam evaporation and was photolithographically patterned with lift-off process. In the next process step, the passivation consisting of a layer stack of SiO2, Si3N4 and SiO2 (ONO) has been deposited via plasma-enhanced chemical vapor deposition. The passivation layer had to be removed from the sensor spots, which has been carried out via a dry etching process. Therefore, the ONO layer was etched in reactive atmosphere with a gas mixture of fluoroform (CHF3) and tetrafluormethane (CF4). The chalcogenide glass thin films have been deposited with PLD technique on the electrode surface. Detailed information about the PLD process and used parameters, see e.g., in [10- 13]. The chalcogenide glass target materials were fabricated and delivered from St. Petersburg University [14]. The polymeric membrane was deposited via a drop-coating technique. Thereto, it was dissolved in tetrahydrofuran (THF) and droped on the electrodes surface. By means of this procedure single electrode sensors and hybrid sensor arrays have been fabricated (s. Fig. 2). Afterwards, the sensor chip was glued onto a printed circuit board, electrically contacted and encapsulated with a silicone epoxy, excepting the particular sensor spots to guarantee their necessary contact to the analyte.

Fig. 2: Photograph of a single-electrode sensor with heavy metal-sensitive PVC membrane (a) and a hybrid sensor array with both polymeric- and chalcogenide glass-type membranes (b). 324 H. Iken et al. / Procedia Engineering 47 ( 2012 ) 322 – 325

2.2. Sensor characterization

Ion-selective potentiometry has been applied to investigate the electrochemical behavior of the sensors in different heavy metal solutions. The measurement setup is shown in Fig. 1 (left). The setup consists of the sensor array and an external, conventional double-liquid junction Ag/AgCl reference electrode, which are both submerged in the analyte solution and connected via a highly ohmic multimeter (10 GΩ) to close the electrical circuit. The reference electrode was filled with 0.1 mol/l KNO3 (inner electrolyte) and 0.1 mol/l -3 KCl (outer electrolyte), respectively. The analyte consists of 0.1 mol/l KNO3 and 10 mol/l HNO3 as background solution (BGS). The BGS was mixed with the desired heavy metal salts (Cd(NO3)2, Pb(NO3)2) to achieve the particular heavy metal solution. All measurements were carried out in a dark Faraday cage at room temperature to avoid outer influences. The measurements have been done in the concentration range from 10-9 mol/l to 10-3 mol/l of the respective heavy metal ion. Figure 3 shows a potentiometric measurement of a Cd2+-sensitive polymeric membrane in Cd solution. The concentration-depending potential change is clearly visible (Fig. 3a) and the calibration curve (Fig. 3b) results in a nearly-Nernstian sensitivity.

Figure 3: Potentiometric measurement of a Cd2+-sensitive polymeric membrane in Cd solution (a) and resulting calibration curve (b); S: sensitivity, LDL: lower detection limit, BGS: background solution.

Table 1 summarizes the results of the electrochemical characterization of the four different thin-film membranes. One can see that the chalcogenide glass membranes and the polymeric membranes have a comparable electrochemical behavior regarding their sensitivity towards the primary ion and the lower detection limit.

Table 1. Analytical characteristics of the different heavy metal-sensitive materials as thin-film membrane.

Membrane material Primary ion Sensitivity Lower detection limit

2+ -7 CdSAgIAs2S3 Cd 25.5 ± 0.9 mV/pCd 4 x 10 mol/l 2+ -8 PbSAgIAs2S3 Pb 23.9 ± 2.1 mV/pPb 5 x 10 mol/l Cd-sensitive PVC Cd2+ 30.3 ± 1.1 mV/pCd 5 x 10-8 mol/l Pb-sensitive PVC Pb2+ 24.3 ± 1.8 mV/pPb 8 x 10-7 mol/l

3. Conclusion

An ISE-based sensor array on the basis of chalcogenide glass and polymeric thin-film materials has been applied for the detection of heavy metals. Thereby, the chalcogenide glass systems CdSAgIAs2S3 and PbSAgIAs2S3, and Pb- and Cd-sensitive PVC membranes have been successfully deposited as thin films onto an array arrangement of potentiometric, semiconductor-type electrodes. The sensor properties of the individual membranes demonstrate a low detection limit towards the respective primary ion (up to 4 x 10-8 mol/l in case of Pb2+), a response time of less than 1 min and a nearly-Nernstian sensitivity. H. Iken et al. / Procedia Engineering 47 ( 2012 ) 322 – 325 325

Moreover, an organic/inorganic hybrid sensor array, consisting of a Pb2+- and Cd2+-sensitive polymer and chalcogenide glass thin-film membrane, respectively, has been fabricated via silicon planar technology. The organic/inorganic sensor hybrid might serve as an electronic tongue for future applications in environmental monitoring.

Acknowledgements

The authors thank the joint program between the Federal Ministry for Education and Research (BMBF, Germany) and FASIE (Russia) within the project “Multimodal sensors for control of ecological safety of aqueous media in industrial environment” for financial support of these studies.

References

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