AN INTELLIGENT DIGITAL VOLTAMMETRIC SYSTEM WITH

Vol. 14 No. 5 October 2002

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AN INTELLIGENT DIGITAL VOLTAMMETRIC SYSTEM WITH MULTIPLE FUNCTIONS EXECUTED THROUGH STAND-ALONE OPERATION OR PC-CONTROL CHING-CHING CHENG 1 , MING-SHING YOUNG1, SHANG-WEN YOUNG 1 , CHANG-LIN CHUANG2

department of Electrical Engineering, National Cheng Kung University, Tainan department of Electronic Engineering, Kun Shan University of Technology, Tainan, Taiwan

ABSTRACT This study presents an intelligent voltammetric system consisting of a personal computer and a digital voltammeter with VXIbus architecture of system control board, voltammetric measurement board and electrode evaluation board. System is designed to provide superior, comprehensive, versatile and convenient storage, analysis and display of electrochemical and voltammetric waveforms. Voltammeter is capable of stand-alone operation or direct PC control through a Labview program and serial communication interface. Stand-alone offers several general voltammetric functions such as electrochemical treatment and evaluation of electrodes and experimental voltammetry. PC connection gives additional functions such as automatic scanning of oxidation potential, expanded storage and processing of experimental data, arbitrary voltammetric waveform parameters, etc. Standalone uses microcontroller and three-bus structure, with EEPROM storing waveform parameters, experimental data and machine code program downloaded from PC. Electrode evaluation board tests electrode quality by measuring electrode equivalent resistance and capacitance, requiring only one button to perform the entire procedure. Minimum potential unit is 1 mV, at which setting the voltage range is -2.05 to +2.05 V. At a minimum unit of 4.9 mV, the voltage range is -10 to +10V. Experimental results are presented using carbon fiber electrode to measure the dopamine concentration in PBS solution, showing minimum oxidation current can be measured to less than 10 pA, with a minimum detectable bulk concentration of less than 10 ppb. The combination of PC with stand-alone voltammeter offers high-speed, precision, automation, versatility and portability, while the VXIbus architecture allows easy expansion capability. Biomed Eng Appl Basis Comm, 2002 (October); 14: 218-236. Keywords: voltammetry, Labview, VXIbus, electrochemical, carbon fiber electrode

1. INTRODUCTION Voltammetry for electrochemistry began with the simplest potential waveforms such as those used in chronoamperometry and chronocoulometry. Since then, a wide range of electrochemical voltammetric techReceived: Sept. 8, 2002; Accepted: Oct. 20, 2002 Correspondence: Ming-Shing Young, Ph. D., Professor Department of Electrical Engineering, National Cheng Kung University, No.l, University Rd., Tainan 701, Taiwan, E-mail : [email protected] -36-

niques has evolved. For example, there are several techniques for continuously varying potential scans of reaction potentials such as linear sweep voltammetry (LSV), cyclic voltammetry (CV), normal pulse volt­ ammetry (NPV), differential pulse voltammetry (DPV), differential normal pulse voltammetry (DNPV) and differential pulse amperommetry (DPA). Newer ap­ proaches include compound techniques such as triplepulse voltammetry and multiple square wave voltam­ metry (MSWV) [1-8]. Modern voltammetric instru­ ments have in many cases eliminated the charge cur­ rents caused by double layers, making it easier to get pure oxidation currents from total electrochemical re­ action currents. Unceasing development of voltammet­ ric sensitivity and techniques are improving the resolu-

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are usually designed as PC interface cards [15,18]. However, the mainboards and interface cards used in commercial PCs are of only moderate stability relative to high-level experimental requirements. Moreover, the useful life of PC products is notably short as PC component specifications change and products are up­ graded, producing rapid functional variance in CPU, system speed, interfaces and software. For example, PC interface card specifications have rapidly upgraded from XT-bus, to AT-bus to PCI-bus, with the XT-bus already weeded out and new PC mainboards no longer being equipped with AT-bus interface. Thus, volt­ ammetric systems using PC interface cards require fre­ quent replacement or else additional interface equip­ ment, and cannot be counted on for long and reliable service. Some voltammeters do not contain data acquisi­ tion functions and must use recorders to plot meas­ urement results. The values of current and time are thus denoted in the charts. Interpretation of these val­ ues depends on recorder paper grid and the naked eye. Sometimes chart values have to be modified by trans­ ferring the time axis to the voltage axis. For these rea­ sons, the results taken in this fashion suffer from high levels of inaccuracy. Digitization of the measurement

tion of oxidation potentials and concentrations, making voltammetry an increasingly important method of quantitative electrochemical analysis. Most commercial voltammeters and instruments are developed for specific applications in medicine, science, engineering, etc [1, 9-12]. They are often highly dedicated portable systems with only one or, at best, a few functions, and cannot be used to satisfy the various requirements and applications of sophisticated experimental voltammetry. Even the voltammeters called "versatile" can perform only a very limited number of functions. Exploration of new theoretical voltammetric waveforms or functions not provided in existing instruments requires combination of several instruments or design of an entirely new system [1316]. Additional problems are found with contemporary instruments such as the Biopulse (Tacussel, France) [ 17], in which the system clock is based on the French utility power frequency of 50Hz. In countries using other power frequencies such as 60Hz, experimenters must carefully recalculate time data and potentials, making research more complex and making it much easier to make mistakes. Alternately, there are PCbased voltammetric systems, whose measuring circuits v ▲ V

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Fig. 9. Circuit diagram of oxidation current input and amplifier. waveforms, an output alternating current voltage from the standard waveform generator is sent to the DAC VREFIN input voltage by appropriate selection of analog multiplexer input channel. Digital data thus controls the output signal amplitude of the DAC, so that it be­ comes a programmable attenuator. (3) Oxidation Current Input and Amplifier Circuit The signals measured by the voltammetric meas­ -43-

urement board are 10 pA-level oxidation currents of the electrodes. Thus, the voltammetric measurement board preamplifiers must have ultralow input bias cur­ rent to obtain good accuracy. Consequently, the IV converter preamplifier uses an AD515A, a low-power, FET-input operational amplifier with a maximum in­ put bias current of 0.3 pA. The IV converter converts the oxidation current signal to a voltage signal. A general voltage amplifier amplifies the signal again,

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and then sends it to a peak voltage holder stage. The oxidation-current input circuit and voltage amplifier are shown in Fig. 9. This figure also shows that a relay controls the connection switches of the three electrode probes for performing oxidation current measurements.

voltage. If it is too small, the capacitor cannot keep the charge for a long enough time, so that the held voltage will droop. After testing, we selected R = R\ = R2 = 100 fi, C = Ci = C2 = 0.1 uF, giving a time constant r = RC= 10 jas. The positive and negative peak voltage holder circuits each has two operational amplifiers. The opamp in the holder circuit should have a high slew rate to reduce peak value distortion; the prototype uses a LF357 with a slew rate of 50 V/us. The second opamp in the holder circuit can be a general compo­ nent. Holder circuit diodes require small cut-in volt­ ages, hence a 1N60 diode is used, with a cut-in voltage of about 0.25 V. The 7407 buffer in the holder circuits is an open-collector driver which can drive a highcurrent load. In the positive hold circuit, the charge on capacitor Ci is drained by setting the input terminal of 7407 to low, thus discharging d via resistor Ri and the 7407. But in the negative holder circuit, the nega­ tive charge on capacitor C2 can't directly discharge via the 7407. Therefore, a photocoupler's transistor in se­ ries with resistor R2 is used, so that if the 7407 input terminal is set low, C2 then discharges via R2 and the photocoupler's transistor.

(4) Peak Voltage Holder Circuit In some voltammetric methods, for instance modified DNPV [15], peak current is an important pa­ rameter. The peak current waveform is a differential type of pulse, so the current width is very short, possi­ bly shorter than 1 us. After this current is converted to voltage, the peak voltage width is likewise short. Thus, the ADC circuit can't directly sample the volt­ age. We therefore use the peak voltage holder circuit shown in Fig. 10 to hold the peak voltage in a capaci­ tor. An analog multiplexer in this holder circuit stage is used to send either the held peak voltage or the original voltage signal to the following ADC stage shown in Fig. 11. The ADC circuit converts the analog voltage signal into digital data. After a peak voltage has been converted to digital data, the charged capaci­ tor holding the peak voltage is discharged. The peak voltage holder circuit has to hold positive or negative peaks, so two kinds of holder circuits are used, one for positive and the other for negative voltages. Both out­ put signals are simultaneously sent to the multiplexer input channels. The charging capacitance of the holder circuit has to be carefully selected. If it is too large, the capacitor doesn't have enough time to charge to peak

(5) Analog-to-Digital Converter Circuit The output signal from the analog multiplexer of the peak voltage holder is sent to an ADC, as shown in Fig. 11. The ADC circuit is a SPT754 12-bit ADC with a maximum conversion speed of 25 u,s. In order

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to obtain precision digital data, the ADC input voltage must be somewhere between half-scale and full-scale. The signal to be input to the ADC is thus sent first through two digitally programmable gain instrumenta­ tion amplifiers (PGA), functioning as software-driven AGCs (automatic gain control amplifiers), which dy­ namically adjust the gain to obtain a voltage that meets the optimal input range of the ADC. Each PGA has four possible gain values. The gains possible for PGA204 are 1, 10, 100 and 1000; for PGA205 the pos­ sible gains are 1, 2, 4 and 8. Cascading these two PGAs thus permits gains from 1 to 8000 in 16 incre­ mental steps, quite sufficient for the requirements of contemporary voltammetry. The digital data from the 12-bit DAC output are separated into one byte and one nibble. Hence the system CPU executes two reading cycles to read these data from the 74LS245 bus trans­ ceiver. (6) Analog Output Circuit for Measurement Re­ sults This DAC circuit converts the digital measure­ ment results obtained by the CPU into analog voltage signals fed to a voltammeter output, which is typically

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connected to an external recorder for plotting charts of the data. The configuration of this DAC circuit is the same as the DAC circuit of voltammetric waveform generator. It consists of one AD7541 DAC and two OP-07 operational amplifiers. The V^™ in this cir­ cuit is 10 V dc, so the output voltage can range from 10 to +10V. The proposed voltammeter can thus out­ put real-time analog results to external recorders and can simultaneously save experimental results into memory as digital data. The recorded digital data can, of course, be converted into analog signals and charted off-line or otherwise processed at later date. 3.2.3 Electrode Evaluation Circuit Board The goal of the electrode evaluation circuit board is the application of a voltage pulse to a test electrode and the measurement of a current from the silver plate electrode for computing equivalent resistance and equivalent capacitance, which are then used to evalu­ ate electrode quality. The block diagram of the elec­ trode evaluation circuit board is shown in Fig. 12. This figure illustrates the potential pulse of 1 KHz, 200 mV, which is produced by system control board. The poten­ tial pulse is sent to the input terminal of the voltage

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Fig. 12. Block diagram of electrode evaluation circuit board follower circuit so as to provide effective driver cur­ rent to the external electrode. After electrode electro­ chemical reaction, the charging current is sent back to circuit board via the silver plate. This charging current is amplified, digitized and stored. From peak current Ip and applied pulse voltage V, we can find the equiva­ lent resistance R^ = V / Ip, time ti/2 and finally the equivalent capacitance Ceq = (1 / ln2) (t^ / R«,). Charging current from the silver plate is first am­ plified and converted to voltage by the IV converter. The voltage adder adds 3 V dc to the signal, adjusting the voltage to the 3 to 5 V range required by the flash ADC. After the signal is converted to digital data, the data is stored in cache RAM. When the circuit has cached the entire current waveform, the CPU reads the data from RAM for subsequent processing and/or stor­ age. The output voltage of the IV converter must be between 0 V and 2 V to meet the input requirements of the MB40578 ADC (8-bit ultra-high speed video ADC, Fujitsu). In order to amplify different ranges of elec­ trode equivalent R and C values, the gain selector for the IV converter offers ten gain values ranging from 1 to 512. Consequently, the signal can be amplified to optimal voltage for better accuracy. The voltage adder adds 3 V to this signal to meet the 3 to 5 V range of the MB40578 ADC input terminal. The address multiplexer generates the cache memory address signals. The address multiplexer has two input channels, one channel being successive 12bit addresses produced by a counter circuit. The 20 MHz circuit board clock drives the counter so that it produces an address signal for writing data to memory every 50 ns. The input signals of the second multi­ plexer input channel come from the address bus of the system control board. The system control board can read any address of the cache memory at any time.

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Fig. 13 describes the detail circuits of the flash ADC, buffer, cache memory, counter and multiplexer. The MB40578 flash ADC performs A/D conversion every 50 ns, fast enough to reduce peak current meas­ urement errors to less than 1%. The digital data after converting is sent via the 74LS244 buffer to the data bus of the MS6264A-25 cache memory. This memory is an 8 K cache RAM with 25 ns access speed, fast enough to be written at 50 ns. Three 74LS161 ICs are configured to form a 12-bit counter that counts once every 50 ns. The multiplexer consists of three 74LS 157 ICs. It can pass either of two 12-bit input channels to the address bus of the cache memory, so that the circuit can address 4 K bytes of memory locations. During memory write, sequential memory addresses from the counter are sent via multiplexer to the mem­ ory address bus so flash ADC output data can be writ­ ten to cache memory in sequence. During memory read, multiplexer input is switched to the other channel, giving the system address bus access to the memory address bus, whereupon the system CPU can read di­ rectly cache memory data and save it to system mem­ ory for computational use. During memory read, the circuit disables the 74LS244 buffer to avoid conflict between read data and flash ADC output data. The data width of the multiplexer is only 12 bits, a mere 4 K bytes of cache memory, but nevertheless large enough to support the current requirements of the pro­ posed voltammeter in its current form. Obviously, a more sophisticated version with larger memory could be designed if the data width of the multiplexer and re­ lated circuits were expanded several bits. 3.3 Software We used C language to design the voltammeter's program and used an ICC8051 compiler to produce machine code for the 80C32 CPU. The machine code

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Fig. 13. Circuit diagram of acquisition components in electrode evaluation circuit board. was written to the Atmel AT28C256 EEPROM located on the system control board. C language is a general and popular programming language, thus allowing easy program modification. It further allows the pro­ gram to be used in other generic systems, and mini­ mizes problems caused by events such as system downsizing or replacement of the CPU with a more powerful CPU. The AT28C256 EEPROM needs only 5 V-dc for supplying, clearing and programming, pro­ viding EEPROM with on-line upgrade ability. Thus, voltammeter expansion and upgrade programs can be installed merely by downloading new machine code to the voltammeter. The main flow diagram of the voltammeter's pro­ gram is shown in Fig. 14, organized in terms of user operation flow. The progra is simple and menu-driven. The menu screen as displayed on the LCD module is shown in the upper left of Fig. 15(a). This figure also shows the 21 keys that control our current voltammeter. Fig. 15(b) shows the parameters screen for voltammet­ ric measurement, with an inset DPA waveform. LCD module screen size is 64 x 240 dots. It can display 5 rows of messages, with 30 characters in each row. Each character pattern consists of 8 x 13 dots. The ex­ tended ASCII code (28 = 256 characters) matrix pat­ terns are saved in the program memory data area. Be­ cause the LCD has only 64 dots in each vertical line, we assign 13 horizontal lines in first to fourth rows and -47-

12 lines to the fifth row, giving the fifth row characters only 8 x 12 dots. This is a feasible economy since there are only a couple of special ASCII symbols that have data on the thirteenth line in our character set, and these characters are not used in our work. Thus, the inability of the final row to display these characters does not affect the display quality. The proposed voltammeter as a portable stand­ alone system cannot provide all the functions of a good PC. It thus is provided with a RS232 serial interface for direct connection with a PC, permitting the PC to directly operate all voltammeter functions and perform additional functions such as parameter setting, results analysis, statistical operations, file processing, etc. Our PC voltammetric program was designed with Labview graphic programming language [26], a language which includes a wide range of drivers for control of instru­ ment screens and peripheral interfaces. The prototype program provides PC-based menu-driven control of the voltammetric operational functions, with userfriendly screens with characteristics. Further, the pro­ gram is readily upgradable. 3.3.1 EEPROM Program The EEPROM program initializes default control signal values and status of all I/O interfaces as follows: (a) Cut off connections between internal circuits and external electrodes, and set voltages of output termi-

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Fig. 15. (a) Front panel of voltammeter; (b) submenu screen for voltammetric parameters. nals in these circuits to zero; (b) Start timer and inter­ rupt subprograms, (c) Set interface circuits of periph­ erals to idle or low voltage status; (d) Set the parame­ ters of basic waveforms; (e) Display home menu on LCD module. The program then performs a polling to check if buttons have been pushed and whether the serial port is receiving external control signals. If true, the program decodes the instruction and directs program flow to an appropriate submenu which lists items such as parame­ ter setting, electrochemical treatment, voltammetric measurement, electrode evaluation, data transfer and control status. They are described in greater detail as follows:

The user can select one of a variety of standard triangle, sine and square waveforms to output for elec­ trochemical treatment. The system reads the corre­ sponding parameter set from memory and displays the parameters and waveform on the screen. After the user pushes the start button, the system uses the parameter set for circuit control. First, it sets the output waveform frequency of the ICL8038 waveform generator. Sec­ ond, it selects the input channel of the analog multi­ plexer for outputting the assigned waveform to the output terminal. Third, it adds an offset voltage to the output waveform. Finally, it sets the waveform ampli­ tude by setting the digital input of the AD7541 DAC circuit shown in Fig. 8. Because the electrochemical treatment process doesn't use the DAC circuits shown in Fig. 6 for plotting, the idle DAC is used to generate offset voltage for addition to the output waveform. In this circuit, the voltage range of the output waveform can be set from -10 V to +10 V.

(1) Settable Parameter Items User can set voltage, duration and waveform of the output waveform. These parameters are stored in EEPROM and can be read out for future use. The space for each electrochemical parameter is 16 bytes, each voltammetric parameter is 32 bytes, and each electrode evaluation parameter is 16 bytes, permitting several hundreds of parameter sets for each different parameter type.

(3) Voltammetric Measurement Items The user selects a general DPV, DNPV, DPA or other waveform to output for voltammetric measure­ ment. The system reads a corresponding parameter set from memory and displays the parameters and wave­ form on the screen. After pushing the start button, the

(2) Electrochemical Treatment Items 48-

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parameter set is used for circuit control. It sets the digi­ tal input data of the AD7541 DAC shown in Fig. 8 so as to output the selected waveform. Second, it sets a dc voltage to the DAC VREFIN input. Finally, the system continuously updates the digital data input to the DAC according to the parameter set. To improve output voltage precision for all waveform points, the program looks at the maximum value of the waveform and then, with reference to the 12-bit word, assigns the mini­ mum integer voltage value required to map the mini­ mum digital data unit of the output waveform. This voltage unit is also called voltage resolution and is typically either 1 mV or 2 mV. Both positive and nega­ tive voltage ranges are available, each range having 211 possible voltage values. If the voltage unit is 1 mV, the maximum positive voltage is +2.048 V. However, the circuit accuracy is computed only for three significant digits. Thus, the maximum positive voltage is assumed to be approximately +2.05 V. Similarly, the minimum negative voltage is approximately -2.05 V. Therefore, we can set voltages for all points of a waveform in the range of ±2.05 V. Likewise, if the minimum voltage unit is 2 mV, we can set voltages for all points of a waveform in the range of ±4.10 V. If we want to set a waveform with a voltage range over ± 4.10 V, the pro­ gram also can provide a maximum voltage range of ±10 V. However, the minimum unit of voltage value is 10000/211 mV and it isn't an integer. Thus, under these circumstances, some of the lower voltage values of the waveform may have significant error. When we want to output a voltammetric waveform not contained in the voltammeter operating system, the voltammeter can provide a maximum of 8 K voltage points for each period for the generation of an arbitrary waveform. For each voltammetric cycle, the system obtains a set of measured digital data including charging cur­ rents or oxidation currents from the ADC. These data are stored in memory. These data are then read, proc­ essed, converted to analog voltage by the other DAC circuit, Fig. 6, and sent to external equipment, usually a recorder to plot a response current chart. These digi­ tal data can be also transferred to a PC through the RS232 serial interface for displaying the response curve on the PC's screen and for disk storage. During amplification of the voltammetric response current, the PGA circuit dynamically adjusts its own gain for more precise measurement results. (4) Electrode Evaluation Item The voltammeter outputs a potential pulse, for example a 1 KHz, 200 mV square wave, to the electrode being tested and, after electrochemical reaction, obtains a response charging current from the other electrode, typically a silver plate electrode. The current is sent to the electrode evaluation circuit of the voltammeter to be processed and stored. The potential pulse is generated by the CPU timer and interrupt

function. Frequency and amplitude of the pulse can be set and modified for different electrode materials and applications. At the start of each pulse cycle, the timer automatically generates an interrupt request signal to trigger CPU execution of an interrupt service program. The program first checks the status of the start button. If the button is pushed, the program starts the 12-bit counter counting from zero. The flash A/D converter also starts to execute its conversion, and the output re­ sult is written to cache RAM. After starting operation of the above circuits, the program sets the potential of the pulse signal from zero volts to the user-defined voltage and outputs the pulse to the external electrode to start the electrochemical reaction. Because operation timing is controlled by program, the voltammeter can certainly record the entire response waveform. After cache RAM has recorded data about 1000 data sam­ ples (about 50 us), the control circuits automatically stops operations of counter, cache RAM and other cir­ cuits. The program then sequentially reads the digital charging current data from cache RAM and computes these data for electrode equivalent resistance and ca­ pacitance. Finally, the measurement results are dis­ played on LCD module or PC screen. (5) Data Transfer Item This function uses the RS232 serial interface con­ necting the voltammeter to the PC, allowing the volt­ ammeter to transfer data to and download data from the PC, and can perform following functions: (a) Transferring waveform parameters stored in the voltammeter's EEPROM to the PC. These waveform parameters are used in electrochemical, voltammetry and electrode evaluation, (b) Loading waveform parameters stored in the PC to the voltammeter. (c) Transferring the response current data from voltammetric experiments or electrode evaluation to the PC. (d) Loading machine code from the PC to the voltammeter's program memory for upgrading the voltammeter's functions. (6) Remote Control Item This function uses the voltammeter/PC RS232 se­ rial interface to form an intelligent voltammetric sys­ tem, giving the PC direct control of voltammeter func­ tions. The voltammeter functions then as a PC periph­ eral device. Because a PC's user interface is typically more convenient and a PC's computational ability and speed are typically better than the voltammeter's, this function makes performance of many of the voltamme­ ter's functions more flexible, faster and easier. 3.3.2. Program of Personal Computer The PC used in these tests is a Pentium II proces­ sor operating in Windows 98 environment. The soft­ ware development tool used for designing the voltam­ metric application program is Labview 5.01 (National

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Instruments) graphic programming language. Labview is a general-purpose programming system like C and BASIC, but while C and BASIC are text-based lan­ guages, Labview is a graphic programming language. In addition, Labview provides a library of various functions and subroutines, including applicationspecific modules for data acquisition, communication interface, data analysis, data presentation, data storage and others. Because the voltammeter microprocessor functions well for control of its internal systems but is weak in computational and data handling abilities, us­ ing the voltammeter as a PC peripheral supplements the relative weaknesses of the stand-alone voltamme­ ter's microprocessor. The library of Labview functions for instrument I/O, signal processing, file I/O, report generation and others allowed us to design a voltammetric program with operating functions and screens for our various requirements, with a user-friendly in­ terface. Fig. 16 shows a PC displaying a voltammetric op­ eration screen. The graph displayed in the upper right corner of the screen is a simulated oscilloscope display. It is used to display the voltammogram. Because the program has filing function, we don't need a recorder to plot the voltammogram. The oscilloscope display in the lower right of the screen displays the electrode ex­ citation waveforms. If the waveform parameters on left side of screen are set or modified by the user, the waveform displayed on the lower screen is immedi­ ately and automatically modified. Main selector dis­ played in the lower right corner of the screen is mapped to the main menu displayed on LCD of the

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voltammeter. If the electrochemical item is selected from the main selector, the sub-selector on the lower screen displays a menu of standard waveforms for the experimenter to use. If the voltammetric item is se­ lected from main selector, the sub-selector displays a menu of general voltammetric waveforms for the ex­ perimenter to use. The waveform parameters on the left side of the screen can load default values from memory. New parameters set by user can be saved to memory as default values for future downloading. The above parameters and test results may be stored on PC disk, or in the voltammeter's memory. Because our Labview voltammetric program can be installed in portable notebook computers, and because the volt­ ammeter is portable, the combination forms a mobile and versatile voltammetric measurement system.

4. RESULTS AND DISCUSSION 4.1 System Performance An 11.059 MHz oscillator is used as clock input for the 80C32 CPU in the voltammeter system control board. The frequency of one 80C32 machine cycle is one twelfth the input clock frequency so that a single instruction cycle is about 1.085 jos. Because electro­ chemical reaction speed is slow, the program devel­ oped in the CPU instruction speed is quick enough to control all system peripheral and circuit operations. Although the CPU speed is slow for software compu­ tation, this does not affect the correctness and com­ pleteness of data. Higher online computation speeds

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are achieved by data transfer to a higher speed PC. The 32 K bytes of data memory space in the voltammeter is for storing electrochemical waveform pa­ rameters, voltammetric parameters and the data of voltammetric response currents. Of this storage space, 2 K bytes are allocated to electrochemical waveform parameters, 10 K bytes to voltammetric waveform pa­ rameters, 4 K bytes to electrode equivalent RC values and 16 K bytes to response current data. Because a set of electrochemical waveform parameter needs 16 bytes of space and a set of voltammetric waveform parame­ ter needs 32 bytes of space, the memory can store 128 sets of electrochemical waveform parameters and 320 sets of voltammetric waveform parameters. In addition, the memory can also store 256 sets of electrode evaluation RC values and 8192 points of dopamine re­ sponse current. If we assign a DPA cycle of 2 s, the storage time for continuous experiment is at least 4 hours. Because the proposed voltammetric circuit uses 12-bit DAC to generate voltammetric potentials, in each scanning voltammetric experiment the total num­ ber of different pulse potential values is not larger than 4096. If we use DNPV and either the 1 mV or 2 mV minimum voltage unit value, the total of applied twopulse waveforms in each experiment will not be more than 4096 sets. In other words, the number of resulting current data in one experiment is not more than 4096 points. Cyclic voltammetry measures the redox poten­ tial, while DNPV measures only the reduction or oxi­ dation potential. Thus, the maximum memory space for one cyclic experiment will not be more than 2l2x2 = 8192 points. From the above, it can be seen that when this voltammeter is operating stand-alone, the 16K bytes of memory space allocated for data results is enough to store at least one set of voltammetric response current data. In fact, most common experiments rarely use the largest data space. Consequently, the memory space is adequate for storage of ten or more sets of typical ex­ perimental data. For situations requiring greater amounts of memory space, the communication inter­ face can transfer and store these data to PC. Electrochemical standard waveform parameters stored in memory include waveform type W, pulse amplitude AE, initial potential Pj, frequency F and duration D. Standard waveforms include dc and ac of sine, triangle and square waves. The amplitude range for ac waves is from 10 mV to 10 V, with a frequency range from 0.1 Hz to 100 Hz. The initial potential of dc and ac waves may range from -5 to 5 V. Wave du­ ration is counted by program, so it is almost without limit. However, the voltammeter LCD screen is limited by field length, to which we have assigned a duration range from 1 s to 999 s. For requirements over this range, PC control may be used. Voltammetric waveforms parameters stored in -51

memory include voltammetric type W, initial potential Pi, final potential Pf, pulse amplitude A E, pulse height A V, prepulse duration Ti, measuring pulse du­ ration T2, pulse period T and measuring duration D. The commonly used voltammetry types include CV, SV, SWV, NPV, DPV, DNPV, DPA and others. In this voltammeter, the minimum voltage unit for output voltammetric waveforms can be set to 1 mV, giving a waveform potential range of ±2.05 V, with Pi, Pf, AE and AV ranges also of ±2.05 V. If the waveform po­ tential range is expanded to a ±10 V range, the mini­ mum voltage unit must be set to 20/212 = 4.9 mV. The minimum time unit used for all output waves is 1ms. The maximum output measuring pulse duration T2 is 999 ms. Thus, the maximum pulse period T is 99 s, and the maximum output duration D is 9,999 s. For arbitrary waveforms, the maximum number of potential points for one waveform cycle is 8 K points. The minimum potential unit is lmV. The minimum potential is -10 V. The maximum potential is 10 V. The minimum time unit between two potential points is 100 us. Because the arbitrary waveform func­ tion has a higher maximum frequency, it may be used to generate electrochemical waveforms whose desired frequency is higher than that available via the standard waveform function generator. The experimenter can edit waveform patterns in the PC and download the patterns to voltammeter. More than ten points of hold­ ing time in each cycle may be used to catch response currents. The voltage amplitude and time duration of the waveforms applied to the external electrodes in this system have high accuracy. The voltage accuracy can reach better than a thousandth of full scale by sup­ pressing noise and using a precise voltage regulator. Because the duration is directly computed by the pro­ gram, there is no accuracy problem concerning time, which is computed to a milli-second scale. The accu­ racy of the input oxidation current via the electrode, however, is a different matter. The input oxidation cur­ rent is only a few pA, and is easily affected by noise. Therefore, even if the amplifier is very precise, it is difficult to obtain an accuracy greater than a hundredth of full scale. Nevertheless, this is high enough for the requirements of medical experimentation. In the pro­ posed system, noise suppression has been a primary concern. We have paid great attention to wires shield­ ing, system grounding, noise filtering of circuits, selec­ tion of low-noise components, etc, but further im­ provement of noise performance remains possible and is being pursued. When we write voltammetric waveform parame­ ters and response currents into EEPROM, 1 K bytes of data can be finished within 0.25 seconds, and 32 K bytes of data can be finished within about 8 seconds. The baud rate of the RS232 serial interface is 9600 bps, allowing an upload of 32 K bytes of data from volt-

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ammeter data memory to PC or visa versa within 40 seconds. Similarly, we can download 32 K bytes of machine codes upgrade to the voltammeter program memory within 40 seconds and finish the writing func­ tion within 8 seconds. The response time of the stand­ alone voltammeter can be seen from the fact that the time from press of keypad to result on LCD is typi­ cally less than 2 seconds. Response time for remote control operation can be seen from the fact that the time from push of PC keyboard until result on volt­ ammeter LCD is typically less than 3 seconds. 4.2 Circuit Testing and Dopamine Concentra­ tion Measuring Experiment The system was calibrated by measuring a known concentration of dopamine (DA). The voltammogram and resulting current from this test are shown in Fig. 17. The waveforms in the figure are obtained by using this voltammetric system and a three-electrode A90mV

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(b) Fig. 17. Voltammeter with three electrodes measur­ ing dopamine concentration in PBS solution; work­ ing electrode is carbon fiber, reference electrode is Ag/AgCI and auxiliary electrode is platinum, (a) voltammogram obtained using DNPV technique (Pi = - 220 mV, P, = 420 mV, AE = 50 mV, AV = 4 mV, T, = 90 ms, T2 = 40 ms and T = 2 s); (b) re­ sponse oxidation current waveform, obtained using DPA technique (Pj = - 220 mV, Pf = 90 mV, A E = 50 mV, T, = 90 ms, T2 = 40 ms and T = 2 s).

cell (carbon fiber working electrode, Ag/AgCI refer­ ence electrode and platinum auxiliary electrode) to measure dopamine in PBS solution. Fig. 17(a) is a voltammogram obtained using DNPV to measure do­ pamine oxidation potentials at P, = -220 mV, Pf = 420 mV, AE = 50mV, AV = 4 mV, T, =90 ms, T2 = 40 ms and T = 2 s. Fig. 17(b) shows the oxidation cur­ rent obtained using DPA to measure several different concentrations of dopamine in PBS with Pj = -220 mV, Pf = 90 mV, AE = 50 mV, T, = 90 ms, T2 = 40 ms and T = 2 s. During DPA measurement, 100 nM of dopamine solution was dropped into 80 ml of blank PBS solution at fixed time intervals, indicated in the figure by arrows. This is equivalent to adding 0.237 ppm of dopamine into PBS at each time interval. After each addition, the response oxidation current increased slowly. The solution was stirred constantly to speed dopamine diffusion into PBS and thus stabilize the current. After several dopamine additions, the wave­ form of Fig. 17(b) was obtained. Comparison with the output waveforms of our previously-used voltammetry equipment (Biopulse, TACUSSEL) showed that the sensitivity of our system is higher. Also, because the output waveforms of our system are processed by digi­ tal technique, they are more stable and accurate. Do­ pamine tests of our system demonstrate that the mini­ mum oxidation current can be measured to about 10 pA and the minimum dopamine concentration can be measured to about 10 ppb. It was found that the pro­ posed system outperforms the Biopulse system in all parameters. Consequently, this voltammetric system certainly meets the general requirements of contempo­ rary voltammetric experimentation. 4.3 Electrode Evaluation Experiment The electrode evaluation circuit is tested and cali­ brated using different values of 1 %-error resistors and general capacitors. For each test, the test probe is con­ nected to the circuit board by a series resistor/capacitor pair. For each resistor/capacitor pair, circuit compo­ nents and programs are adjusted so that the data and waveforms displayed on the LCD screen correspond to actual values. The electrode evaluation acquisition cir­ cuit operates with a 50 ns sample speed to capture the charging current, which is not fast enough to avoid current data error. However, comparing the captured digital waveform with the actual analog waveform of oscilloscope measurement, it is found that the error is very small. The worst case is observed when capturing peak currents, but the difference between captured values and actual peak values is less than 1%. In addi­ tion, the resistors error is also 1%, and capacitor error does not affect the peak current value. Therefore, the actual peak current error is less than 2%, and the total error of equivalent resistance is likewise less than 2%. The computation of equivalent capacitance is based only on equivalent resistance and test time. Conse-

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quently, the total error of equivalent capacitance is also small. We tested the electrode evaluation circuit using a carbon fiber electrode, though other types could be used. First, we measure the equivalent resistance R and the equivalent capacitance C in PBS solution. Then we measure the oxidation current in 80 ml PBS mixed with 200 nM dopamine. According to the analy­ sis of the electrical characteristics of carbon fiber elec­ trodes by professor Liao [25], a higher equivalent re­ sistance causes a lower oxidation response current, and a higher equivalent capacitance causes a higher oxida­ tion response current. Our test results found that the equivalent resistances of good carbon fiber electrodes ranged from 0.8 to 2 Kfl, while the equivalent capaci­ tances ranged 2 to 6 nF. These electric characteristics correspond to the analysis of Liao. If R is too large or C is too small, electrode sensitivity will be too low and the inner connection between carbon fiber and wire could be opened. If R is too small or C is too large, the inner materials of the electrode are in danger of elec­ trical short. Therefore, the use of this electrode evalua­ tion circuit can help select effective electrodes, thereby avoiding wasted time, materials, and avoiding nonpredictive results. The functions provided by the elec­ trode evaluation circuit and software are very simple to operate, requiring only the push of a single key. After pushing the start key, the circuit immediately measures the values of R and C, the charging current and the time. At the same time, the waveform of the response current is displayed on the LCD screen and simultane­ ously displays the R and C values, helping the experi­ menter ensure the results are correct and free of sig­ nificant noise interference. The accuracy of this circuit is high so that electrodes selected by this test are reli­ able.

5. CONCLUSION This stand-alone and versatile digital voltammeter is highly compatible with common voltammetric ap­ plications. A full range of standard waveforms for standard testing procedures are preset and provided by this voltammeter. Special waveforms not provided as defaults in the voltammeter can be set by the arbitrary waveform function. The arbitrary waveform generators in contemporary general-purpose voltammetric ex­ periments cannot simultaneously catch response sig­ nals, and the experimenter is forced to rely on graphic recorders to draw response curves. The coordinated functions provided by the proposed voltammeter al­ lows both the setting of arbitrary waveform parameters and the accurate setting of suitable time-point parame­ ters to catch response current signals. The proposed in­ strument includes preset standard electrode fabrication functions for electrochemical treatment, electrocoating

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and electrode evaluation, and also allows the pro­ gramming of arbitrary waveforms for electrochemical treatment, electrocoating and testing of electrodes. Electrode testing and quality evaluation of carbon fiber electrodes is accomplished by measuring the equiva­ lent resistance and equivalent capacitance, a conven­ ient and accurate method. The programmable abilities of this testing system allow easy modification or up­ grade for alternative or future electrodes or test proce­ dures. Test functions of the proposed system have automatic circuits to catch and handle response wave­ forms, giving a speed, simplicity and accuracy to the electrode evaluation process superior to previous evaluation systems. The built-in RS232 serial port can connect the proposed voltammeter with a standard PCs, allowing PC remote control of the voltammeter and its various functions. This gives the proposed voltammeter the advantages of the PC's greater data handling speed and memory capacity. Waveform parameters and test re­ sults can be stored in PC, processed into a variety of forms, transferred to other systems around the world, and are readily available for a variety of analytical, sta­ tistical and other processing by software utilities. The EEPROM in the voltammeter which stores the wave­ form parameters, operating commands and test results can be upgraded by downloading machine code from the PC, providing the instrument with the ability to add new voltammetric functions. Because the voltammeter's architecture is de­ signed around a low-cost 80C32 CPU with standard logic circuits, it can easily be transplanted to smaller computer architectures or designed as ASIC or cus­ tomer IC. Thus, our current voltammetric system can readily and simply be downsized for more convenient and portable experimental tools. Our PC system uses Labview language to expand the voltammetric programs and capacities. Because Labview provides a library of modules and drivers for a wide range of standard needs, it was easy to design a PC program to suit the voltammeters needs for instru­ ment screens, controls operating functions and userfriendliness. Modification and upgrade of this program are likewise simple. The minimum unit of the waveform potential in this voltammeter is 1 mV, and the minimum unit of the time duration is 1 ms, enough to cover the different set­ ting requirements of the waveforms in contemporary voltammetric experiments. Response current ampli­ tudes can be measured to about 10 pA, and a minimum bulk concentration of dopamine can be detected to an accuracy of 10 ppb, good enough to meet the require­ ments of contemporary voltammetric experiments. Noise elimination is a main consideration when designing voltammetric circuits for maximizing re­ sponse current resolution. In the proposed system, noise influences have been reduced to acceptable con-

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temporary resolution levels, but further reductions in noise are possible and will be part of our further efforts. Clearly, improved noise performance will allow more precise test results, lower minimum response currents, and lower minimum bulk concentrations of dopamine. An embodiment of the proposed voltammeter us­ ing VXIbus as the system architecture has been de­ signed as a low cost stand-alone system. The reserved expansion slots in VXIbus mechanism provide space for addition of other circuit boards, allowing hardware extension and expansion of this device and functions for the foreseeable future. Using data link with high performance PCs, the proposed system is then capable of software upgrade, both for onboard EEPROM op­ eration and for PC remote controlled operation. These summarized characteristics clearly recommend this system to researchers as a powerful and versatile tool.

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