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586 IEEE SENSORS JOURNAL, VOL. 9, NO. 5, MAY 2009

An On-Site Heavy Metal Analyzer

With Polymer Lab-on-a-Chips for Continuous

Sampling and Monitoring

Zhiwei Zou, Student Member, IEEE, Am Jang, Eric T. MacKnight, Pei-Ming Wu, Jaephil Do, Joon Sub Shim, Paul L. Bishop, and Chong H. Ahn, Member, IEEE

Abstract—An on-site analyzer system for monitoring of heavy metals has been presented. This analyzer can automatically perform long-term continuous water sampling and on-site heavy metals measurement using an array of disposable polymer lab-on-a-chips (lab chip) and a continuous flow sensing method. The system consists of a plastic fluidic motherboard with a mi- crochannels network, microvalves and pump, control circuits, a wireless communication module, a potentiostat, LabVIEW control, and seven disposable heavy metal lab chips. Square wave anodic stripping voltammetry was performed using a microfab- ricated planar bismuth electrode on the chip for detecting heavy metal (e.g., cadmium, Cd) concentrations. Sensing performance sensitivity was improved with by the continuous flow sensing method propelled by the analyzer. On-site measurement of the Cd concentration change of the soil pore and ground water samples from a lab-scale reactor was automatically performed to evaluate the performance of the analyzer with lab chips.

Index Terms—Bismuth electrode, environmental monitoring, heavy metal sensor, lab-on-a-chip.

I. INTRODUCTION

SOIL pore and ground water pollution by heavy metals is a major environmental concern, especially around heavy metal waste disposal sites, such as mining, pigment, and battery factories. Unlike organic pollutants, most of the important envi- ronmental heavy metals tend to accumulate in soils because of their chemical stability for long periods and their nonbiodegrad- able nature [1]–[4]. Typical environmental monitoring methods for these contaminants are realized by collecting samples in the field in person and transporting them to centralized labo- ratories for analysis. Unfortunately, long time delays associated with centralized analysis of samples retrieved from the field may result in alteration of the sample’s constitution as a result of

Manuscript received October 21, 2008; accepted November 14, 2008. Cur- rent version published April 10, 2009. This work was supported in part by the National Institute of Environmental Health Sciences (NIEHS, R01 ES015446) in the National Institute of Health (NIH). The associate editor coordinating the review of this paper and approving it for publication was Prof. Evgeny Katz.

Z. Zou, E. T. MacKnight, P.-M. Wu, J. Do, J. S. Shim, and C. H. Ahn are with the MicroSystems and BioMEMS Laboratory, Department of Electrical and Computer Engineering, University of Cincinnati, Cincinnati, OH 45221 USA (e-mail: zouz@ececs.uc.edu).

A. Jang and P. L. Bishop are with the Department of Civil and Environment Engineering, University of Cincinnati, Cincinnati, OH 45221 USA.

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2009.2018348

chemical, biological, and physical reactions. Furthermore, cen- tralized sampling and detection methods can be very costly and time consuming. Therefore, one of the best preventive measures is to rapidly and continuously monitor heavy metals in the field automatically [5].

Various sensing methods have been developed for on-site measurement of heavy metals, such as optical [6], piezoelectric [7], and ion selective electrodes [8], [9]. One of the most promising methods is anodic stripping voltammetry (ASV), which satisfies almost all requirements for on-site measurement of heavy metals. ASV is a voltammetric method for quantita- tive determination of specific metal ionic species. The analyte of interest is electroplated on a working electrode during a deposition step, and oxidized from the electrode during a subsequent stripping step. The oxidation current is measured during the stripping step. The oxidation of species is registered as a peak in the current signal associated with the potential at which the species is known to be oxidized [10]. Very sensitive sensors that employ mercury or mercury precursors as the working electrode have been developed for ASV to measure heavy metals [11]–[16], however, the toxicity of mercury and its precursors make them undesirable for disposable on-site environmental sensing applications.

An environmentally friendly heavy metal sensor using a microfabricated bismuth (Bi) working electrode has been de- veloped in our group [17]. Bi was introduced as an alternative working electrode material to replace mercury in ASV for heavy metal sensing in different applications [18]. Compared with mercury, Bi is an environmentally friendly element, with a very low toxicity, and has widespread pharmaceutical use [19]–[25]. The Bi film electrodes are most commonly prepared by plating Bi ions onto a carbon substrate. Bi (III) ions (0.25–1 ppm) are directly added into the sample solution, and simultaneously deposited on the carbon substrate with target heavy metals [18]. However, in some cases, this method is not applicable for on-site heavy metal measurement in natural environment because it requires the presence of Bi in the sample solution. By using the MEMS and polymer micromachining techniques [26], [27], our newly developed sensors are ready-to-use, have a high yield, and are low cost, which is extremely suitable as a disposable sensor for on-site environmental monitoring applications.

To achieve a better sensing performance and comprehensive functions, variety of integrated sensing systems and analyzers built on single or array of sensor have been developed and reported in different application areas [5], [28]–[37]. Based

1530-437X/$25.00 © 2009 IEEE

Authorized licensed use limited to: University of Cincinnati. Downloaded on April 14, 2009 at 19:19 from IEEE Xplore. Restrictions apply.

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