Intestrumentation for Electrochemical Time of Flight Experiments

Current methods of calibration of biosensors take place before or after use.  In situ calibration methods are either nonexistent or ineffective at dealing with the changes in sensitivity that occur while the sensor is in situ. Electrode-based sensors are usually covered with a protective membrane that helps prevent fouling and adds some degree of selectivity. External calibration calculates a sensitivity based on an external calibration curve that does not reflect the in situ conditions. As soon as the sensor is placed in situ, fouling occurs at the electrode’s surface/membrane, which changes its permeability, a product of the solubility of the analyte in the membrane and the diffusion coefficient of the analyte through the membrane.  The sensitivity of the electrode will change when the permeability of the membrane is affected by the foulants that adsorb to the surface of the electrode/membrane. In order to achieve accurate results, one needs to have a method for tracking the changes in the permeability of the membrane as fouling occurs.

Currently the electrochemical methods for determining the diffusion coefficient of an analyte through a membrane are either to use electrical impedance spectroscopy (EIS) or to use a rotating disk electrode (RDE) experiment.  Neither of these methods are particularly fast, nor do they lend themselves to being performed in an in situ environment. Electrochemical time of flight (ETOF) is an experiment performed using an electrode array where a small amount of analyte is generated at an electrode called the generator while monitoring the current at another electrode in the array, called the collector, which are a known distance apart (Figure 1). The relationship between the distance travelled, d, and the time of maximum collection, t_ms, is as follows: d=K√(Dt_ms), where D is the diffusion coefficient and K is a constant based on geometric parameters of the electrodes. ETOF has been used to test diffusional models for analytes, such as ferricyanide or other model compounds, in bulk solution through gels and through solid polymer electrolytes, or to enhance signals of an analyte by redox cycling. ETOF has not yet been used to test models of diffusion through membranes, the effects of foulants on that membrane, or to re-determine the sensitivity of a sensor that has been fouled.

The instrumental limitations of most bi- and multipotentiostats, including the popular CHI 750a or CHI 760b potentiostats, have limited the development and applications of ETOF.  None of the most popular multistats  are able to apply a potential pulse to the generating electrode.  Here we report on the construction of a device that applies and controls the generator electrode by connecting and disconnecting the lead to the electrode.  Figure 2 illustrates the connections to the CHI potentiostat, an in-house constructed generation-electrode-controller (GEC), and the National Instruments Compact DAQ through the NI 9403 Digital I/O module.  Circuitry in the GEC uses a flip-flop to catch an outgoing trigger pulse from the potentiostat at the start of an experiment, this starts a timer that times the placement of the potential pulse to the generating electrode. Figure 3 shows an ETOF experiment gathered from a CHI 750 potentiostat using the National Instruments controlled GEC.  The red curve is the generation pulse, then the green curve is the transient of a collector 6 microns away, blue at 14 microns away, and purple at 22 microns away.  The time of maximum collection is measured from the center of the generation pulse to the peak of the collector current.  From this data a line is constructed plotting the separation distance, d, as a function of the square root of t_ms. The slope contains the geometric constant, K, and the square root of the diffusion coefficient, D. Using an analyte of known diffusion coefficient one can determine the geometry constant of the electrode. In a second experiment and knowing the geometry constant, one can determine the diffusion coefficient of an analyte. Currently ETOF-determined diffusion coefficients in our lab have been measured within 10% of diffusion coefficients determined for ferricyanide in bulk solution using RDE experiments, reporting diffusion coefficients of 2.0x10^-5 cm²/s and 1.8x10^-5 cm²/s respectively.  Using the newly developed instrumentation, our plan is to use electrode arrays and ETOF to determine diffusion coefficients through a membrane and therefore monitor sensitivity changes in membrane coated sensors.