A SELF-MAINTAINED SENSOR USING A LOW POWER ACTUATOR

BACKGROUND OF INVENTION

A signal drift over time arising from changes in the sensor surface, and its interface with the sample, are intrinsic problems for most chemical sensors and these issues are exacerbated in remote autonomous monitoring systems. A chemical sensor generally consists of a chemically-selective interface in or on a non-specific transducer. The interface can selectively interact with analyte at atomic, molecular or ionic levels and transfer one form of property into another so that the transducer can respond (Chemical Sensors; Blackie and Son Ltd: New York, 1988). Due to the variation between individual sensors and the material deterioration of the chemically-selective interface over time, a calibration process is required to convert the output signal to a measurable quantity. Additionally, the ability to regenerate the interface for long term use is preferable. Although calibration and regeneration of the chemically-selective interface is important, the main stream research has so far been devoted to understanding the interaction and/or recognition mechanism and the transducer design for better selectivity and sensitivity (Wallace, G. G. In Chemical Sensors; Edmonds, T. E., Ed.; Blackie and Son Ltd.: New York, 1988, pp 132). To a lesser extent, research on the above noted problems have focused on better measurement reproducibility. Unfortunately, human involvement is a default requirement of these methods. Recently, autonomous chemical sensors with minimal or no human input have become important due to security, terrorism and environmental issues. Additionally, decreased cost, size and power consumption also have become important factors in the design process for large scale deployments. Unfortunately, prior art approaches has produced only limited achievements in the above requirements. For application specific approaches, van der Schoot el al (See. van der Schoot, B.; Bergveld, P. Sensors and Actuators 1985, 8, 11-22) introduced an integrated self-calibrating sensor system to measure the concentration of acids and bases in situ by an absolute colorimetric titration method, in which the local pH change induced by the electrolysis of water on a gold electrode was used for a self-calibrating purpose. Following the van der Shoot study, oxygen and hydrogen bubbles generated electrochemically from two electrodes have been employed to do a two point calibration (100% and 0% oxygen) for an integrated oxygen sensor in microfluidic devices (Park, J.; Kim, C-S.; Kim, Y. Sensors and Actuators B 2005, 108, 633-638). For general

applications, several self-calibration methodologies have been reported and were based on flow injection analysis manifold to enabling automatic calibration and sample preconditioning possible (See. Bataillard, P.; Haemmerli, S.; Ludi, H.; Widmer, H. M. Sensors and Actuators B: Chemical 1991, 4, 309-313; Rapp, R.; Hoffmann, W.; Su[beta], W.; Ache, H. J.; GoIz, H. Electrochimica Acta 1997, 42, 3391-3398; DeGrandpre, M. D.; Baehr, M. M.; Hammar, T. R. Anal. Chem. 1999, 71, 1152-1159).

Among these above approaches, DeGrandpre describes a self-maintained sensor configuration by renewing the analyte-sensitive solution for absorbance and fluorescence- based chemical sensors. One of examples raised in DeGrandpre was for the autonomous mooring-based measurements of pCO ₂ (partial pressure of CO ₂ ) in sea water, where long- term laboratory and field studies showed that the response has no drift over extended periods (months).

Recently, Hahn (Hahn, F. Biosystems Engineering 2005, 92, 275-284) also reported an auto-maintained chloride-ion chemical sensor at low cost for remote river water monitoring. Hahn's method employs a two-point calibration methodology, in which two stepper motors and valves were utilized for the injection of a calibration reference liquid, distilled water aliquot and water sample to the sensor cavity for sensor calibration, cleaning, and sample water monitoring, respectively. Using this method, Hahn was able to achieve seven continuous days of operation. Unfortunately, approaches such as disclosed in Hahn's work use the mobility of analyte in the self-calibration process. Use of the mobility of sensitive chemical interfaces such as the moveable, bendable or rotary interchangeable electrode and/or sensitive surface does not appear in prior art approaches, presumably due to the lack of innovative and cost effective actuation materials.

Unfortunately, it is apparent that prior art approaches are not reliable or cost effective and that new innovative configurations are required to achieve desired performance levels, meaning that the potential of emerging technologies and materials need to be fully exploited. One class of emerging actuation materials that show promise are the inherently conducting polymers (ICPs). Their novel actuation mechanism is based on the reversible ion doping/dedoping process with the application of electrical stimulation in electrolyte solution (See. Baughman, R. H.; Shacklette, I. W.; Elsenbaumer, R. L. Microelectromechanical actuators based on conducting polymers; Kluwer, Dordrecht, 1991).

An example utilizing a polypyrrole trilayer actuator combined with an oxygen sensor has been shown by Andrews, et al. (See. Andrews, M. K.; Jansen, M. L.; Spinks, G. M.; Zhou, D.; Wallace, G. G. Sensors and Actuators A: Physical 2004, 114, 65-72). Using the Andrews method, the galvanic cell, which is the oxygen sensor, is used to trigger the power required to move the polymer actuator. The flexibility of conducting polymer actuators has been demonstrated by Smela et al. (See. Smela, E.; Ingamas, O.; lundstrom, I. Science 1995, 268, 1735-1738; Smela, E.; Kallenbach, M.; Holdenried, J. IEEE Journal of Microelectromechanical Systems 1999, 8, 373-383).

It is clear that what is needed is a new approach to the calibration and regeneration of chemical sensors that are cost effective and have the ability to be deployed in a self- calibrating manner.

SUMMARY OF INVENTION

According to the inventive method that is disclosed is an innovative approach to the calibration and regeneration of chemical/biosensors. According to the invention, chemical/bio sensing is achieved which involves the use of low power polymer actuators to move and position the sensor. Using this inventive approach, the self calibrating chemical sensors are automatically placed to the calibration, regeneration or measurement environment at and for the required time. It is contemplated within the scope of the disclosure that sensors according to the invention can be used in remote environmental monitoring, where networks of such devices could be devised to operate collaboratively over large areas.

It is further contemplated within the scope of the disclosure that sensors according to the invention may be 'wearable sensors' for calibration within fabric and materials and sensing external to the fabric or material.

It is also an object of the disclosure that sensors according to the invention can be used in parallel or serial configurations were more than one moveable sensing unit could be used in tandem to allow for multiple or analytes differential measurements for the same analyte to be determined. It is a further objection of the disclosure that sensors according to the invention can use low power actuation to induce vibration for "stirring" of solution during cleaning or calibration.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages, objects and features of the invention will be apparent through the detailed description of the embodiments and the drawings attached hereto. It is also to be understood that both the foregoing general description and the following detailed description are exemplary and not restrictive of the scope of the invention.

Fig. IA depicts a configuration of side view of a polypyrrole actuator and colorimetric sensor;

Fig. IB depicts a configuration of top view of a polypyrrole actuator and colorimetric sensor; Fig. 2 depicts the experimental setup for the self-maintained acetic acid sensing according to the invention;

Fig. 3 graphically depicts the applied potential waveform and current response of the polypyrrole actuator used for the controlled positioning of sensor for analyte detection and calibration at different places according to the invention; Fig. 4 depicts a data processing diagram of the RGB analysis for the colorimetric response;

Fig. 5 is a schematic showing the principle of LED analysis;

Fig. 6A graphically depicts a RGB analysis data showing one measurement cycle of colorimetric response from sensor tip red, green, blue and overall response; Fig. 6B graphically depicts a RGB analysis data showing one measurement cycle of colorimetric response from the red response with standard derivation errors indicated; and

Fig. 7 shows continuous detection of analyte (acetic acid vapors) over 5 sampling cycles, where each measurement was performed after conditioning of sensor in ammonia vapors

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, a self-maintained chemical sensing system is disclosed. It has been found that a BCG-based colorimetric sensor could be reproducibly moved between a predefined site to accomplish a self-maintaining and colorimetric measurement. Colorimetric measurements are achieved by analyzing RGB changes using a video camera or by monitoring photocurrent using a LED emitter and a LED detector. Measurement cycles are reproducibly achieved with the use of PC as the solvent. The calculated energy consumption rate was about ~ 0.17 J / cm ² per cycle for the setup used according to the

invention. According to the invention, a self calibrating chemical sensor is disclosed herein that will allow self-maintained and/or autonomous chemical sensors.

The sensitive interface according to the invention is a BCG dye, which is a pH sensitive colorimetric reagent. It is contemplated within the scope of the invention that other colorimetric reagents known in the art can be used. At an equilibrium state, the color shifted between blue (λ _max - 610 nm in ammonia vapors) and yellow (λm _ax ~ 440 nm in acetic acid vapor), using the BCG dye. According to Maxwell diagram, blue is a primary color so that the dye would absorb other color of light except blue. When it turns yellow (a combination of green and red light), the dye absorbs other colors of light except the red and green light. By way of example, a red or other suitable color light emitting LED can be used for this study.

Controlled by a programmed potential waveform as shown in Fig. 3, the colorimeter sensor moved repeatedly between two beakers to demonstrate the concept of self- maintained chemical sensor. Fig. 6A shows that the red line data gives the most pronounced shift due to the relative large amount of red light being absorbed by BCG dye of the sensor when exposed to acetic acid vapors. The sensor was held in acetic acid vapor for 15 seconds to allow for signal stabilization before being brought back to the ammonia vapors for the interface re-generation. Fig. 6B shows the color response of the red fraction alone with standard derivation errors for one measurement cycle. The initial color in the ammonia vapors was stable and the color values deviated only slightly. As the sensor was brought into the acetic acid vapor, there was a large and rapid color shift that resulted in large deviations across the film surface. According to the invention, a polypyrrole trilayer actuator can be employed for use in the self-maintained chemical sensor.

To minimize the large deviations across the film surface and simplify data processing, the idea of self-maintained colorimetric sensor according to the invention was further exploited using a red LED light emitter and a detector. A period of about 15 seconds was allowed to stabilize the absorption signal. Fig. 7 shows the light absorption data from the red LED detector. Initially, at the calibration site, the photo current generated from the LED detector was ~ 15000 The LED detector output is a function of the time to discharge the reverse biased junction; units are milliseconds, but are essentially dimensionless, related to the light density striking the junction

While the sensor moved to the detection site, the dye changed color from blue to yellow in about a few seconds, allowing red light to pass through, generating more

photocurrent through the LED detector, therefore, decreasing the sensor reading from ~ 15000 to - 7800.

Reproducible results were obtained when the process was repeated between the calibration site and the detection site. Although some instability in measurement in a preliminary setup was observed, including the inappropriate position of sensor tip in between the LEDs and some deterioration of the dye layer for prolonged measurements, this result illustrates that a remote self-maintained chemical sensing system according to the invention is achievable using low cost polypyrrole actuators. Advantageously, this system utilizes low power consumption and advantageously has durability of the polypyrrole actuator system.

Energy efficiency was examined for the polypyrrole actuator used according to the invention. Since the operation of polypyrrole trilayer actuator is analogue to the charging and discharging of capacitors, energy efficiency is dependant on an applied electric stimulation waveform. Under the conditions used according to the invention (near zero loading), - 0.17 J/cm ² per cycle was consumed for the triangular potential waveform method. At this rate of energy consumption, it was shown that the polypyrrole actuator was able to reproducibly operate for at least 2 hours and complete at least 120 cycles, however, the propylene carbonate gradually evaporated and the actuator gradually decreased its bending amplitude over time. Without being bound to any particular theory, it is thought that the use of an ionic liquid as the electrolyte for polypyrrole actuators would significantly extend cycle life since ionic liquids have extremely low vapor pressure and excellent electrochemical properties for the operation of inherently conducting polymer based devices. The lifetime is also significantly enhanced in situations where evaporation is prevented, or where the sensor is maintained in contact with an aqueous electrolyte.

EXAMPLES The following examples are provided to illustrate the methods and products of the present invention with particular choices for the several components described above. As described above, many variations on these particular examples are possible. These examples are merely illustrative and not limiting of the present invention.

Example I Reagents and Materials

Polyvinylindene fluoride (PVDF) hydrophobic porous membrane with ~ 110 Dm thickness and average pore size ~ 0.45 Dm (Millipore) was used as received without additional treatment. It was utilized as the backing material for the construction of laminated polypyrrole actuators. Pyrrole (Merck) was distilled and stored under nitrogen at — 20 ⁰ C before use. Propylene carbonate (PC) (Aldrich), tetrabutylammonium hexafluoro- phosphate (TBA.PF ₆ ) (Aldrich), lithium trifluoromethanesulfonimide (LiTFSI) (3M),

Acetic acid (Aldrich) and aqueous ammonia solution (30%) (Aldrich) were used without further purification. Bromocreosol green (BCG) / TiO ₂ colorimetric dye composite, a red

LED emitter, a detector and a data logging system were obtained from the National Centre for Sensor Research, Dublin City University, Ireland.

Example II Preparation of Polypyrrole Trilayer Actuator

Au was sputter coated on each side of porous PVDF membranes at a sputtering current of about 30 mA for about 30 minutes with an argon pressure of 2 * 10 ^"3 mBar. The as-coated membrane was used as the anode for the electrochemical deposition of polypyrrole, carried out galvanostatically from PC solution containing 0.06 M pyrrole monomer, 0.06 M TBA ₁ PF ₆ and 0.5 % (w/w) water on both sides of the membrane at 20

⁰ C in a freezer. A current density of 0.10 mA/cm ² was applied for 12 hours. Following electropolymerisation, films were cut to rectangular strips of dimension lcm * 5 cm and thoroughly rinsed with PC to remove any residue of pyrrole. After pad-drying with tissue, actuator strips were stored in 0.25 M TBA.PF ₆ / PC electrolyte until used to construct the sensor system.

Example III Sensor Setup and Operation The sensor compromised of two components: a polypyrrole trilayer actuator

(bender) and a BCG coated colorimetric sensor tip jointed together by a light polyethylene clamp as shown in Fig IA and IB. BCG dye solution was coated manually and used to provide the colorimetric signal.

The sensor was clamped and placed between two beakers, one containing ammonia and the other acetic acid as shown in Fig. 2. Sensor re-generation was achieved by moving the sensor above the two beakers, thereby exposing it to basic or acidic vapors. Exposure to ammonia vapor was regarded as "sensor regeneration", while exposure to acetic acid vapor was considered as measurement.

A programmed potential waveform depicted in Fig. 3 was used to operate the polypyrrole actuator, thereby moving the attached colorimetric sensor between two beakers for the acetic acid monitoring. In this study, the polypyrrole trilayer actuator was clamped and the electrical contact to each side of polypyrrole was made by platinum wire. A Solartron Potentiostat was used to control the applied potential, in which the working lead was connected to one side of polypyrrole actuator and the auxiliary and reference electrodes were shorted and connected to the other side of the polypyrrole actuator. CorrWare v.2.2 software (Scribner Associates, Inc.) was used for data acquisition and CView2 v. 2.2 (Scribner Associates, Inc.) was used for data manipulation.

Example IV Measurement of Colorimetric Response

Two methods of data analysis were employed for the bending sensor. The first method involved the extrapolation of "red-green-blue" (RGB) values for the color changes in the sensing tip across one measurement cycle, for which, digital images were captured using a commercial digital camera and Paint Shop Professional software was used for RGB analysis. In this method, images were taken at 2-second intervals and the evaluation of color change was based on the averaged value over the surface of the BCG coated sensor tip, where five segments were taken from across the film. The details of data process are illustrated in Fig. 4. A second method, according to the invention, involved automatic optical measurements using a light emitting diode (LED) as shown in Fig. 5. In this measurement, the setup was modified by placing an LED and a detector in the beaker containing acetic acid facing each other a short distance apart ~ 3 mm. The LED acted as an emitter, sending out five flashes per second, while another LED was set up in reverse bias to act as a photon detector. The sensing tip on the bender was brought between the two LEDs so that the intensity of light reaching the detector is dependent on the light absorption property of dye, whereby, the dye color change in response to analyte becomes detectable. To minimize the effect of ambient light, LEDs were thoroughly coated with black ink using a permanent marker, except for the surfaces used for emitting and detection.

The principles, preferred embodiments and modes of operation of the presently disclosed have been described in the foregoing specification. The presently disclosed however, is not to be construed as limited to the particular embodiments shown, as these

embodiments are regarded as illustrious rather than restrictive. Moreover, variations and changes may be made by those skilled in the art without departing from the spirit and scope of the instant disclosure and disclosed herein and recited in the appended claims.

Although detection in the illustrated examples is through RGB image analysis or duel LED optical detection, it will be appreciated by those skilled in the art that other colorimetric methods could be used for detection employing a variety of photo detectors, or simple visual analysis. Likewise, it will be appreciated that fluorescence detection is also contemplated within the scope of the invention. Additionally, it is further contemplated within the scope of the invention that electrochemical methods can be used including but not limited to po tensometric, amperometric and conductimetric methods of detection. It will be further appreciated that a combination of detection methods could be used.

Although the illustrated examples show the use of optical sensors, it will be appreciated by those skilled in the art that the inventive method could also be applied to chemical sensors based on various transducers such as acoustic wave chemi-sensors, chemo-thermistors.