HEAVY METALS AND LACTATE MONITORING SYSTEM

Transcription

1 II WORKSHOP ON CHEMICAL SENSORS AND BIOSENSORS HEAVY METALS AND LACTATE MONITORING SYSTEM Manuela Adami 1, Giorgio Martinazzo 1, Simona Villari 1, Mario Panza 1, Marco Sartore 1, Claudio Nicolini 2 1 Parco Scientifico e Tecnologico dell Elba, Marciana (LI), Italy 2 Istituto di Biofisica, C.so Europa 32, Genova, Italy ABSTRACT: a recent research program at the PST Elba was focalized on the development of low-cost, portable electrochemical apparatus for the detection of analytes both in environmental and health care field. The instrument is capable to perform the detection in several matrices such as water (clear, sea, drinking, etc.), beverages and biological fluids. The operations are simple and completely computerassisted by means of user-friendly menus. The operator is only requested to add the sample into the analytical cell, along with a small quantity of supporting electrolyte, and to set up a few measuring parameters. The system is completely automated and performs different electrochemical techniques, i.e. Potentiometric Stripping Analysis, Chronoamperometry and Linear, Cyclic, Differential Pulse and Square Wave Voltammetry. The working electrodes are graphite-based, eventually including the catalytic elements when required. Applications of the device in the detection of the presence of metal ions in solution, as well as of metabolites, are presented. Keywords: metal ion detection, lactate detection, electrochemical instrument, electrochemical techniques. INTRODUCTION There is an increasing demand for analytical devices suitable for the quantitative detection of analytes in field operating conditions. This need often stems from the impossibility to perform on-field analysis with portable systems, even for a rough, first screening. Our project has demontrated the possibility to realize low-cost apparatus for this purpose. The system can be connected to any portable computer and 207

2 M. ADAMI ET AL. HEAVY METALS AND LACTATE MONITORING SYSTEMS utilizes graphite-based working electrodes, being based on a high performance potentiostat and galvanostat. One of the main features of the developed devices is the utilization of untreated samples: no mineralization process, nor deareation steps are required. Actual lactate measurements are satisfactory in terms of detection limit, whereas heavy metal ions are currently detected, using plain graphite based electrodes, with a resolution not yet satisfactory for some applications, but the system can use a mercury doped graphite sensor as well, achieving much higher performances. The choice of lessperforming electrodes is justified by the portability and easyness of use of the system, when intended for a first screening field application. THE SYSTEM The core of the analysis system consists of the combination of the measuring sensor and an electronic board, which includes the interface towards a portable (or desktop) personal computer. The electronics comprises a base circuitry which implements the electrochemical technique suitable for the desired experiments [1]. It consists of a potentiostat capable to switch to a galvanostat, and of a precision signal adder for the superimposition of driving signal waves. Therefore the core electronics is reduced to a minimal critical set of circuits, driven by different software routines depending on the analysis to be performed. This solution yields to the maximum system flexibility, and allows to reduce the overall cost of the instrument, being the software an integral portion of it. Outside the sensor control circuit, an interface electronics allows direct connection to a personal computer by means of its parallel port. This block, hence, implements both A/D and D/A signal convertions, in addition to the generation od digital control signals for the control of the core components. A proprietary protocol allows direct data exchange to/from the PC. The block diagram of the system is depicted in Fig

3 II WORKSHOP ON CHEMICAL SENSORS AND BIOSENSORS Portable Computer Interface electronics (AD, DA, Digital Output) Sensors driver/switch (Potentiostat/Galvanostat) Electrodes Figure 1: block diagram of the electrochemical system For environmental applications the working electrode is a glassy carbon transducer. In the case of lactate detection it is a composite transducer containing graphite powder, an electronic mediator (tetrathiafulvalene), a stabilizer (polyethylenimine), a low melting solid binding matrix (2-Hexadecanone), preservative and the specific enzymes (lactate oxidase and horse-radish peroxidase) [2-5]. The electrochemical cell is equipped also with a Ag/AgCl or calomel reference electrode and a platinum counter electrode. RESULTS AND DISCUSSION In this section we present some examples of the analytical possibilities offered by our system, showing results of interest for environment and health care. About heavy metal detection, the investigation was performed using the Potentiometric Stripping Analysis (PSA) [6]. This technique presents a preliminary pre-concentration step in which the metal ions are plated on the working electrode. The electronics is automatically set in potentiostatic operational mode. Then the metal stripping phase follows, when no biasing control is active and the electroncs records the potential drop variation in time between reference and working 209

4 M. ADAMI ET AL. HEAVY METALS AND LACTATE MONITORING SYSTEMS electrodes. A constant and fixed oxidizing current is superimposed to the working electrode during this phase, and the electronics is essentially a galvanostat. A typical example of a calibration curve obtained by this acquisition method is shown in Fig. 2, where a starting solution of 1000 ppm of Cd ++ in KCl 0.1 M was diluted in successive steps. Cadmium calibration I.R. (sec.) y = x R 2 = Cd (ppm) Figure 2: Calibration curve relative to Cadmium detection. The detection limit in this case is 10 ppb. A typical example related to health care application is lactate determination. In this case the system is used as a Chronoamperometer: the electrochemical cell is biased at 50 mv and the working electrode current is measured as a function of time, by means of the electronic circuitry, set up in potentiostatic mode. A typical result of such determination is shown in Fig. 3, which condenses four series of experiments in a single plot (standard solutions of lactic acid were prepared in Dulbecco s buffer, ph 7.4 and fed into the electrochemical cell, which contained the already mentioned working electrode, modified with the proper enzymes). Table 1 reports the final experimental results. 210

5 II WORKSHOP ON CHEMICAL SENSORS AND BIOSENSORS I (na) y = 0,4214x + 8,261 R 2 = 0, Lactate (um) Figure 3: calibration curve relative to Lactate detection. Each curve point is an average over four distinct results. Table 1: Lactate determination characteristics. Sensitivity Detection limit Rensponse time Base noise Stability (na/µm) (µm) (sec) (na) (monthes) ±1 6 ACKNOWLEDGEMENT The present work has been supported by a grant within the Italian Minister Contract: Sviluppo di sistemi di biosensori per l ambiente e la salute given to Polo Nazionale Bioelettronica Parco Scientifico e Tecnologico dell Elba. REFERENCES 1. A.J. Bard and L.R. Faulkner, Electrochemical methods: Fundamentals and Applications, Wiley, New York (1980) 2. G.M. Varga et al, Analyst, 117 (1992) 211

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