Elsevier

Sensors and Actuators B: Chemical

Volumes 171–172, August–September 2012, Pages 181-189
Sensors and Actuators B: Chemical

Performance of an electronic tongue during monitoring 2-methylisoborneol and geosmin in water samples

https://doi.org/10.1016/j.snb.2012.02.092Get rights and content

Abstract

2-Methylisoborneol (MIB) and geosmin (GSM) are sub products from algae decomposition and, depending on their concentration, can be toxic; otherwise, they give unpleasant taste and odor to water. For water treatment companies it is important to constantly monitor their presence in the distributed water and avoid further costumer complaints. Lower-cost and easy-to-read instrumentation would be very promising in this regard. In this study, we evaluate the potentiality of an electronic tongue (ET) system based on non-specific polymeric sensors and impedance measurements in monitoring MIB and GSM in water samples. Principal component analysis (PCA) applied to the generated data matrix indicated that this ET was capable to perform with remarkable reproducibility the discrimination of these two contaminants in either distilled or tap water, in concentrations as low as 25 ng L−1. Nonetheless, this analysis methodology was rather qualitative and laborious, and the outputs it provided were greatly subjective. Also, data analysis based on PCA severely restricts automation of the measuring system or its use by non-specialized operators. To circumvent these drawbacks, a fuzzy controller was designed to quantitatively perform sample classification while providing outputs in simpler data charts. For instance, the ET along with the referred fuzzy controller performed with a 100% hit rate the quantification of MIB and GSM samples in distilled and tap water. The hit rate could be read directly from the plot. The lower cost of these polymeric sensors allied to the especial features of the fuzzy controller (easiness on programming and numerical outputs) provided initial requirements for developing an automated ET system to monitor odorant species in water production and distribution.

Introduction

Among several factors that can compromise water quality, it is of interest to this work, the eutrophication. The term eutrophication can be defined as a nutrient increase in water reservoirs, such as nitrogen and phosphorous, which favors algae bloom. Besides some toxic substances, off-flavor compounds are released from algae decomposition, which give to water unpleasant taste and odor. 2-Methylisoborneol (MIB) and geosmin (GSM) have been identified as responsible for the earthy-musty odor in water [1], [2]. The mutagenicity and hepatotoxicity by MIB and GSM have been pointed out [3], [4], but possible health hazards have not been reported. These compounds can be perceived by humans’ nose, even though they have an extremely low odor threshold, for example 0.6 ng L−1 for MIB and 1 ng L−1 for GSM [5], [6]. In fact, contamination of drinking water by MIB and GSM are the major source of complaints for water companies. Therefore, the presence of these compounds in drinking water has to be monitored and controlled by highly sensitive instrumentation. They also affects aquaculture, because they give an unpleasant taste to fish [7], [8] and beverages such as grape juice [9] and wines [10].

Determination of MIB and GSM has been usually performed by gas chromatography/mass spectrometry (GC/MS) provided that some extraction/enrichment technique is applied, for example solid-phase microextraction [11], [12]. Despite its high sensitivity and accuracy, GC/MS is costly and time consuming. Moreover, it can be hardly implemented at remote locations so that it cannot be used to perform real time monitoring. In many instances, the changes in water quality caused by a specific episode cannot be detected on time and, eventually, water is supplied without any preventive action could be done. Alternatively, non-specific chemical sensors and sensor arrays have also been developed and employed to monitor water quality [13], [14]. They offer some important advantages over more traditional instrumental analysis, including lower cost, faster response, and higher frequency sampling. In addition, chemical sensor arrays allow for in locu and real time monitoring, once they are based on miniaturized, less expensive and more robust components.

A few contributions have been made in the last decade regarding the development, application, and evaluation of the performance of individual gas sensors and arrays, or electronic nose, on the detection of MIB and GSM in water samples. For example, Ji et al. developed molecularly imprinted polymers in order to impart specificity to quartz crystal microbalance (QCM) electrodes toward MIB and GSM [15], [16]. At the best preparation conditions they succeeded in producing QCM electrodes with high sensitivity and specificity to MIB, with detection limits as low as 10 ppb [16]. Nonetheless, the same approach was unsuccessful to improve detection of GSM. They observed no difference between the responses to GSM of bare and molecularly imprinted electrodes. Stuetz et al. have evaluated the performance of commercially available electronic noses on the detection of different odor compounds, such as 2-chlorophenol, geosmin and diesel, in raw and treated potable waters [17]. According to their investigation, the EN could identify tainted samples and quantify the odor compounds at concentrations as low as 1 pg L−1. The main feature of this approach is the possibility to perform the signal integration to yield a unique response pattern for a complex media. However, some limitations have also to be considered, for example, the influence of environmental conditions on the sensors’ signals (humidity and temperature) and the difficulty to establish calibration procedures.

Part of these difficulties can be minimized by application of chemical sensor arrays in liquid phase, also known as electronic tongue (ET) [18], [19], [20], [21]. In this configuration, the humidity plays a secondary role. The sensors are soaked in the test solution so that the sample/sensor interface is well developed and sensitivity is thus enhanced. Furthermore, the calibration set can be as ample as possible because the preparation of single and multi-composition solutions is more easily attained. This approach has been applied to water analysis, with focus in different factors, including quantification of ions [22], [23], discrimination between brands of mineral water [19], [20], detection of pollutants [24], drinking water quality [25] and production [21] and environmental friendly applications [26]. However, none of these studies was devoted to detection of either MIB or GSM.

In previous contributions we have employed an ET system based on non-specific conjugated polymer sensors and capacitance measurements to detect MIB at levels as low as 10 ng L−1 [27]. Herein, we present the results from a more detailed investigation on the performance of this ET system employed on the analysis of water samples tainted with different concentrations of MIB and GSM. One of the main features of the system described herein is the implementation of a fuzzy logic program which enables the recognition of test samples at less laborious procedures than those inherent to other pattern recognition tools, such as PCA or artificial neural networks. The fuzzy logic provides numerical outputs which can be associated with linguistic variables (proper or improper, contaminated or clean, etc.) and can thus be readily implemented in information panels, addressing at the same time requirements for system automation and friendly interface to be used by non-specialized operators. This investigation was meant to give information on the capabilities and limitations of this ET in detecting and quantifying different levels of MIB and GSM, and the possibility to provide such information by means of linguistic outputs via the fuzzy program.

Section snippets

Materials

Polyaniline (PANI, Mw 10,000 g mol−1), polypyrrole (PPy, Mw 67.089 g mol−1), polyallylamine hydrochloride (PAH, Mw 56,000 g mol−1), sodium sulfonated polystyrene (PSS, Mw 70,000 g mol−1), and camphorsulfonic acid (CSA) were all purchased from Aldrich Co (USA) and used without additional purification. Sulfonated-lignin (SL, Mw 2000 g mol−1) was supplied by Melbar Produtos de Lignina SA (Brazil). 2-Methylisoborneol (MIB) and geosmin (GSM) from Supelco were provided as individual methanolic solutions (100 

Sensors response

The frequency response of polymeric sensors in contact with water samples contaminated with MIB and GSM were analyzed in terms of the equivalent circuit proposed by Taylor and Macdonald [35] and later applied by Riul et al. [36] in impedimetric electronic tongues. The electrical impedance of such a circuit depends on the electrical processes involving the electrode coating (the polymeric film), the coating/electrode interface and the bulk solution. Each of them is physically represented by

Conclusion

An electronic tongue system based on polymeric sensors, impedance measurements and multivariate data analysis (PCA and fuzzy logic) was employed to perform discrimination and quantification of water samples tainted with 2-methylisoborneol and geosmin. The electronic tongue easily discriminated and quantified (as low as 25 ng L−1) these contaminants with remarkable reproducibility, regardless the samples were prepared with either distilled or tap water. The electronic tongue also exhibited

Acknowledgements

The authors wish to thank CNPq, CAPES, EMBRAPA and FAPESP for their financial support.

G.S. Braga received his Ph.D. degree in electrical engineering in 2011 from the Escola Politécnica/University of São Paulo, Brazil. During his Ph.D. studies, he had an internship period (April 2010–November 2010) at the Autonoma University of Barcelona, Spain. His interests are mainly nanotechnology related subjects, including layer-bu-layer films, conducting polymers, chemical sensors and electronic tongue.

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    G.S. Braga received his Ph.D. degree in electrical engineering in 2011 from the Escola Politécnica/University of São Paulo, Brazil. During his Ph.D. studies, he had an internship period (April 2010–November 2010) at the Autonoma University of Barcelona, Spain. His interests are mainly nanotechnology related subjects, including layer-bu-layer films, conducting polymers, chemical sensors and electronic tongue.

    L.G. Paterno has a Bachelor degree in Chemistry from University of Sao Paulo (1998) and Ph.D. degree in Materials Science and Engineering from Federal University of Sao Carlos (2003), both in Brazil. He is currently an assistant professor of the Chemistry Institute of the University of Brasilia since 2011. His research activities are mainly focused on nanomaterials (inorganic particles, carbon-related materials and conjugated polymers), their processing into thin films and application in chemical sensors, organic light emitting diodes and dye-sensitized solar cells.

    F.J. Fonseca received his Ph.D. in electrical engineering in 1994 from the Escola Politécnica/University of São Paulo, Brazil where he is currently an associate professor. His field of interests includes organic electronics, chemical sensors, OLEDS, organic solar cells, and microfabrication.

    1

    Permanent address: Universidade de Brasília, Instituto de Química, Campus Universitário Darcy Ribeiro, CEP 70904-970 Asa Norte, Brasília, DF, Brazil.

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