Diagnostic Biosensor Polymers - ACS Publications - American

Chapter 11

Flow-Injection Analysis of Some Anionic Species Automated, Miniaturized, Dual Working Electrode Potentiostat Using a Poly(3-methylthiophene)-Modified Electrode 1

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Downloaded by UNIV LAVAL on May 6, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/bk-1994-0556.ch011

Elmo A. Blubaugh, George Russell , Merril Racke , Dwight Blubaugh, T. H. Ridgway, and H. B. Mark Jr. Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221-0172 The work presented in this manuscript is composed of two parts. The first part is concerned with the design and fabrication of a computer controlled dual working electrode potentiostat (DWEP). This automated andminiaturized DWEP was designed and built with utilization of a new technology in the field of electronics, specifically, surface mount electronic components (passive and active). This potentiostat design, also allows for the automated zeroing of current at each working electrodes, with the same nanoampere to microampere range, and a range of sensitivities from nanoampere to microampere. The second part describes the application of this automated potentiostat as an electrochemical detector for various electro-inactive anionic species; nitrate (NO -), nitrite (NO -), sulfate (SO 2-), chloride (Cl ), etc. (1,2). The electrochemical detection involves the use of a conducting poly(3methylthiophene) (P3MT) polymer modified platinum electrode in a flow injection analysis (FIA) mode. The analytically important criterion of detection limits were determined to be 10 to 10 molar in anion concentration. The dynamic range was five to six orders in magnitude. -

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A new area within the field of organic electrochemistry was initiated by the first successful electropolymerization by Kern etal. (3 ) in the 1950's. However, it was not until 1973 with the discovery of the metallic properties of (SN) by Green etal., that the area of conducting polymers started to evolve. Interest in the area of conducting polymers dramatically increased in the late seventies with publications by MacDiarmid (4 ) and Diaz (5). These publications centered on the ability of electrochemically synthesized polymers to exhibit relatively high electronic conductivity and undergo electrochemical redox transitions. Since the seventies, the publications using conducting polymers as electrodes hasflourished.Also, conducting polymers is one of the more diverse and interdisciplinary fields found within science at the present time. Some of the possible applications for conducting polymers include electrocatalysis (6 ), electrochemical and electrochromic displays ( 7 ), thin layers for solar cells (5), rechargeable battery electrodes (0), corrosion protection and the extraction of X

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Current address: Sabine River Laboratory, DuPont Chemical Company, P.O. Box 1089 (FM-1006), Orange, TX 77631-1089 Current address: National Forensic Chemistry Center, U.S. Food and Drug Administration, 1141 Central Parkway, Cincinnati, OH 45202

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0097-6156/94/0556-0137$08.00/0 © 1994 American Chemical Society

Usmani and Akmal; Diagnostic Biosensor Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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DIAGNOSTIC BIOSENSOR POLYMERS

ionic species (10). The ability of these thin polymeric layers to modify the behavior and response of the electrode from that behavior associated with the substrate electrode to the chemical and physical behavior associated with the modifying polymer layer is the focus of intense study. In this particular study the ability of conducting polymers to extract or become doped by ionic materials is the basis for this study. Polythiophene and its derivatives can be polymerized by chemical or electrochemical techniques. In this study, the electrochemical method was utilized.The mechanism is a cationic radical polymerization (11 ). The polymerization pathway can be summarized in the following steps: 1) oxidation of the monomer to form a radical cation, 2) dimerization of the radical cations, 3) loss of proton to yield a neutral dimer, 4) oxidation of dimer to form a radical cation, 5) reaction of dimer radical cation with another radical cation, 6) repeat of the this study, are 3-methylthiophene, tetrabutylammonium tetrafluoroborate (TBATFB), as the supporting electrolyte. The organic solvent was acetonitrile. The resulting polymer was the first conducting polymer family found to be stable in air and water in both their doped or undoped state. The doping/undoping mechanism for poly(3-methylthiophene) is shown in Figure 1. The oxidation or doping of the P3MT involves the transfer of an electron with the gain of an anion to form the charge compensated, or doped P3MT. Since Q is approximately equal to QRED' t doping/undoping process is reversible (12). The changes from doped to undoped polymer are accompanied by color changes from dark blue to red.There is also a difference in the conductivity for thesefilmsbetween the doped or undoped state. The conductivity of a doped film approaches that of the metallic state, while the conductivity found for the undoped polymer approaches an insulator. One of the first reported uses of a conducting polymer modified electrode for the detection of anions by flow injection analysis (FIA) utilized poly (pyrrole) (13). Low detection limits were achieved. However, poly(pyrrole) is irreversibly oxidized in the presence of oxygen at potentials of around +1.0 volts. Poly(3-methylthiophene) does not exhibit this behavior. Therefore, poly(3-methylthiophene appeared to be a good candidate as an electrochemical detector for anionic species by FIA with electrochemical detection at an poly(3-methylthiophene) modified electrode. Also P3MT does not need to be undoped (cycled between the negative and positive detection potentials) between successive injections as many other polymer systems must. One of the objectives for this study was the design and fabrication of a physically small and highly automated DWEP. The eventual role that this potentiostat will play involves the remote sampling and monitoring of hazardous waste streams. Both the potentiostat hardware and the FIA sampling hardware lend themselves readily to computer control and multiple sample collection and evaluation, (14,15). The approach taken in the design and manufacture for the potentiostat, which must have a small physical size, involved the utilization of a relatively new technology in the field of electronics, namely Surface Mount Technology (SMT). Not only does this technology allow for a smaller physical size, but also greater reliability. Both of these advantages are of a prime importance for a remote monitoring situation. Surface mounted electronic components do not have leads for connection to the printed circuit board (and hence no need for plated-thru holes), but rather are attached to the printed circuit board by soldering pads on only one side of the printed circuit board. The smaller physical size, which results from utilizing surface mounted components is the result of two factors: 1) surface mounted components are approximately one-third to one-fourth the size of analogous conventional thru-board components, 2) fewer plated thru-board holes, hence a smaller required space for placement of the component. There is a possible disadvantage associated with using SMT components. This disadvantage involves the very small size for the SMT components and the resulting difficulty in; 1) the assembly of these printed circuit boards and 2) the difficulty in the reworking or repair of these printed circuit boards without the aid of specialized and expensive equipment. o x

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Experimental Chemicals and Solvents. HPLC grade acetonitrile (Fisher Scientific Co.) was the solvent Usmani and Akmal; Diagnostic Biosensor Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

11. BLUBAUGH ET AL.

Flow-Injection Analysis of Some Anionic Species

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used for the electropolymerization. The monomer, 3-methylthiophene (3MT) (Aldrich Chemical Co., Milwaukee, WI), was distilled prior to use. The sodium salts of fluoride, chloride, bromide, iodide, nitrate and nitrite were purchased from Aldrich Chemical Company. Tetrabutyl ammonium tetrafluoroborate (TBATFB) was also purchased from Aldrich Chemical Company and used as received. Aqueous solutions were prepared by dilution from stock solution or by dissolution of a weighed amount into distilled water. Distilled water was obtained from a NanoPure II system purchased from Barnstead (Newton, PA). The specific conductivity of the distilled water was approximately 10" S/cm.


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Figure 1:

Doping/Undoping Mechanism for Poly (3-methylthiophene)

Electrochemical Techniques. Cyclic Voltammetry (CV) was performed on most of the poly(3-methylthiophene) anion sensor electrodes. Cyclic voltammetry provided the electrochemical characterization and evaluation of the post treatment of the modified electrodes. The potential of the platinum electrodes (Model MF-1012, BAS Inc.), with or without modifying film of poly(3-methylthiophene) was controlled relative to an Ag/AgCl reference electrode (Bioanalytical Systems Inc., West Lafayette, IN). The auxiliary electrode was a platinum flag electrode or in the case of the FIA experiments a stainless steel block electrode (BAS, West Lafayette, IN). Several different instruments were utilized to perform the cyclic voltammetry experiments. The model CV-IB (or CV-IA), Cyclic Voltammetry Unit, BAS-100 or BAS-100A Electrochemical Analyzer (BAS, West Lafayette, IN) were used as the instruments to provide potential control. The voltammograms were recorded using the HIPLOT DMP-40 Digital Series Plotter (Houston Instruments, Houston, TX). Cyclic voltammograms were also obtained with the automated DWEP, which is described elsewhere in this manuscript. Data storage and manipulation were accomplished via a PC-286 clone, in conjunction with an inhouse developed data acquisition and software package (16). Polymerfilmswere formed by applying a potential of +1.600 volts to a platinum working electrode for 10 to 160 seconds. This variation of time gave polymer films, which had a thickness that varied in a direct proportion. Post-treatment of thesefilmswas performed by scanning the potential of the modified working electrode, which was immersed in a monomer free supporting electrolyte solution, between the potential limits of -200 to +1000 millivolts. The electrosynthesis of the poly(3-methylthiophene) and any non-cyclic post-treatments were performed using a PAR Model-173 potentiostat/galvanostat equipped with a PAR Model-176 Current to Voltage Converter, which was connected to a PAR Model-179 Digital Coulometer (Princeton Applied Research, Princeton, NJ). Electrodes and Electrochemical Cells. The majority of the work for this study used a dual platinum working electrode available from BAS, Inc. (Model MF-1012). All platinum electrodes were polished with metallurgical papers, followed by polishing with fine grade polishing paper, using a water and alumina suspension. After polishing, the electrode was thenrinsedwith distilled water and sonicated with methanol. The electrode wasrinsedwith the same solvent that was used in the electropolymerization experiment and used immediately. The electrochemical cell used for the electropolymerization is of a unique design and has been described elsewhere (17) The electrochemical cell used in the FIA/anion analysis was a commercial flow-thru electrochemical cell available from BAS.


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Automated Dual Working Electrode Potentiostat Design Our design philosophy was based on several criteria. These criteria included, small physical size for the printed circuit board, computer control of both working electrodes independently of one another, a range of current sensitivities, which included nanoamperes through microamperes and automated compensation of background current in the same nanoampere to microampere range. We were also interested in the evaluation of a relatively new technology in thefieldof electronics, Surface Mount Technology (SMT). The hardware design began with the capture of the hardware circuit schematic with the aid of a CAD software package called Schema Π (Omation, Richardson, TX). The schematic capture software allows for identification, of each component and the interconnections between components. The schematic for the DWEP is presented in Figure 2. After the successful capture and verification for the DWEP schematic, the net list file containing the components and the interconnections used in the design was transferred to a printed circuit board artwork CAD program called Tango Π (ACCEL Technology, San Diego, CA) For the DWEP printed circuit board, afinalphysical size of 3 by 4 inches was achieved. In addition, to the small physical size, the routed printed circuit board was also composed of four layers of conductive copper. After post router verification of the board artwork design, five four-layered prototype printed circuit boards were manufactured and electrically and thermally tested by the fabrication shop (Sovereign Electronics, Youngstown, OH) prior to shipping. The total cost for these five prototype printed circuit boards was approximately $1800. Surface mount components, such as resistors, capacitors and electrolytic capacitors were purchased from a commercial source (Digi-Key, Thief River Falls, MN). The operational amplifiers, OPA121 and INA105 were purchased from Burr-Brown, Tucson, AZ. Conventional soldering and desoldering techniques are applicable in the final step of populating and testing the printed circuit board with both active and passive surface mount components. Flow Injection Analysis (FIA) with Poly (3-methylthiophene) Modified Electrodes Figure 3 shows a schematic of the FIA system and details the thin-layer transducer cell, which contains the poly (3-methylthiophene) anion sensor electrode(s). Electrochemical detection in the amperometric mode was employed for the FIA/EC (flow injection analysis with electrochemical detection) studies. A typical potential used was +1.00 volts vs. Ag/AgCl reference electrode. The FIA-EC system consisted of an Altex 100A double reciprocation pump and a Rheodyne injector (Cotai, CA) with either a 20 or 50 microliter sample loop. Constant potentials were applied using the herein described DWEP, which is software controlled. The electronically transduced signals are converted from analog to digital by use of A/D and D/A converter circuits. The option exists to automatically save the data on the computer or print the results as the experiment progresses. Results and Discussion Hardware Design. The block diagram for the dual working electrode potentiostat is shown in Figure 4. We will use this diagram to explain in detail features associated with this hardware design. The two independently controlled working electrode voltages, Ε and E , signals are DAC voltages which have a full scale voltage range of + /-10 volts. Each voltage signal is reduced by a factor of minus one-third (x-1/3). This attenuation is needed, in order to, bring these input voltages into a range of values (-/ + 3.33 volts), which are more commonly needed for solution electrochemistry. The attenuated voltage signal, E is then sent to the control (or auxiliary) amplifier and is summed with the voltage signal from the reference electrode follower. This control amplifier (or feedback control loop) arrangement is of conventional design (18). However, the voltage signal, E , is a voltage that is amplified by a factor of three times the voltage level found at the output of the voltage follower amplifier. χ

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This arrangement gives, E voltage values, which are in the + /-10 volt range. The voltage range for E and the input voltages, E and E , use the full resolution available from the +/-10 volt bipolar voltage range, which was configured for the 12 bit DAC and ADC converter circuits. Working electrode, W is connected to a "virtual ground", current to voltage amplifier. The voltage signal out, E , is proportional to the current flowing to or from electrode W multiplied with the feedback resistance, R used. There are eight ranges of feedback resistances, R , therefore eight ranges of current sensitivities, from nanoamperes to microamperes. The voltage signal,E , is a 12 bit DAC voltage which can be applied to one of eight resistances, R ^ ^ . The resulting current, I p p is the background compensating current which is summed at the summing point node for this current to voltage converter amplifier. The current measuring and potential control circuitry for working electrode, W , is somewhat different than the circuitry previously described for working electrode, W The attenuated input voltages, Ε and E are subtracted with a differential amplifier, to give an output voltage which is proportional to (Ej-Eg). This voltage signal is directed to three separate amplifiers. One signal path involves subtracting a voltage, E , from the voltage (Ej-Eg). The resulting output signal is applied across one of eight compensation resistances, R , which provides a background compensation current at the summing point node for the second working electrode, W . The second signal path involves directing the voltage signal (Ej-Eg) to the non-inverting inputs for two operational amplifier. This circuitry arrangement allows us to subtract the voltage (Ej-Eg) from the composite voltage output from the current to voltage converter for the second working electrode, W . The voltage, E , at the subtracter circuit is a voltage, that is proportional to the current flowing from (or to) the working electrode, W . The schematic for the DWEP is shown in Figure 2. There are several interesting design features which were incorporated into our hardware design. The inclusion of these features was dictated by the dual requirements of small physical size and superb reliability. The first feature was the exclusive use of the operational amplifier INA105, for each subtracter circuit. In addition to this amplifier being available as an SMT component, the four resistors required for a subtracter circuit are integrated into the op-amp package. These resistors are pre-trimmed at the factory, thereby assuring a total error of less than + /-0.015%. With the resistors integrated into the op-amp package, there is also an inherent space savings. Another feature, which was included in the design to enhance space savings and reliability was the use of matched resistor contained in a network package (e.g. R and R ). Using the resistors in the resistor network package, for both the input and feedback resistors, gives a circuit whose temperature sensitivity is nearly zero, due to nearly equivalent temperature coefficient values for each resistor in the package. The resistance values for the compensation resistances and the feedback resistances, for each current to voltage converter range in magnitude from 10 to 10 ohms. These resistance values are quite large and as a result problems with stability may become evident due to inductive coupling (19). We decided to avoid this potential problem by utilizing a Thevenin equivalent circuit. The Thevenin equivalent circuit requires three resistors of much lower resistance values. This approach mitigates the inductive coupling problem, thereby improving the stability. There are two disadvantages associated with the Thevenin circuit. Thefirstdisadvantage is the additional cost, in terms, of space required for three resistors versus one resistor. The second disadvantage is that due to the inherent limitation in the range of resistance values, which can be achieved using the Thevenin circuit. This range of resistance values is finite, and has a value of about 1000/1 (19). For a lower resistance value of 10 ohms, the equivalent circuit will allow an upper resistance value of about 10 ohms. Electrical testing of an assembled prototype board involved the integration of this hardware, with the data acquisition hardware and the software driver package. A dual electrode dummy cell "testfixture"was constructed to aid in the electrical testing of this integrated system. This dummy cell was designed to make available eight discrete levels of resistance and capacitance values. The DWEP package could be tested for gain-frequency stability, signal to noise ratio as a function of the amount of current (10' to 10" amperes) Q

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Diagnostic Biosensor Polymers - ACS Publications - American

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