Electrochemical devices for ultrasensitive detection and renewable energy applications

                                                
1. Ultrasensitive biosensing – the enzymatic field-effect transistor

The present thrust for ultrasensitive detection in areas such as early detection of diseases, environment protection and homeland security demands techniques that provide unprecedented detection sensitivity. Recently, we have invented a bioelectrochemical detection technique, which allowed us to push the detection limit of biosensors from the micro-molar (10^-6 M) range into the pico-molar (10^-12 M) range with pico-molar detection resolution. By applying a gating voltage to the enzyme-immobilized working electrode of a conventional electrochemical cell, the biocatalytic output current of the detector was increased significantly, resulting in voltage-controlled amplification of the output current. The current amplification could be reversibly controlled by the applied voltage. We have applied this technique to the bio-catalyzed oxidation of glucose and ethanol using immobilized glucose oxidase and alcohol dehydrogenase, respectively. The enzymes’ bio-specificities were preserved in the presence of the voltage. The voltage-controlled amplification of the output current was also observed in the reduction of hydrogen peroxide using immobilized microperixidase, a biomolecule that reduces H₂O₂ to water.

The detector, with its output current controlled by a voltage applied at a third electrode, behaves as a field-effect transistor, whose current-generating mechanism is the conversion of a substance (the analyte) to a product using an enzyme as catalyst. The realization of the enzymatic transistor has significant technological implication. Our demonstration of the operation of the transistor shows that the kinetics of the bio-catalyzed reactions can be controlled by a voltage. Controlled reaction kinetics of biological catalysis has been achieved using an electrostatic technique. The technique allows independent controls of the thermodynamics of an electrochemical system and the quantum mechanical tunneling at the interface between electroactive molecules and the working electrode by applying a voltage to the electrode.

 

2. Nanoparticle-based (enzymeless) sensors

The newly-discovered ultrasmall (1 nm) silicon nanoparticle was used as the sensing element for non-enzyme sensors for substances that are important in food industry, biomedicine and environmental applications. With this sensor, we have demonstrated direct electrochemical amperometric detection of different forms of sugar (glucose, fructose and lactose), dopamine, hydrogen peroxide and phenol. In the sensing of glucose, the sensor showed exclusive detection of glucose in the presence of interfering species within their physiological concentration ranges. The sensor also showed negligible electrode poisoning and detection stability over a 14-week period of repeated use. A comparison between the glucose detection characteristics of the nanoparticle-based sensor and an enzyme-based showed an enhanced amperometric response of the particle sensor. The nanoparticle appears as a suitable active material for the making of implantable devices and nanoscale devices.

 

3. Supercapacitors

Supercapacitors are electrochemical devices. They can be used to store electrical energy generated by intermittent renewable energy sources such as wind or solar radiation to ensure that energy is available at all times. Supercapacitors can also be used as rechargeable stand-alone power sources for portable electronic equipment with moderate energy demands. We have developed a composite material consisting of polyaniline, a conducting polymer, and ultrasmall silicon nanoparticles for making supercapacitors. Because of the hybrid nature of its capacitance, the composite shows a significantly enhanced specific capacitance of 409.27F/g. The enhanced capacitance results in high power (220 kW /kg) and energy-storage (30 Wh/kg) capabilities of the composite material. The specific capacitance of the composite electrode was observed to be stable during 2000 charging-discharging cycles. We have produced flexible sheets of supercapacitors using a simple “brushing-on” technique with conducting polymers, ultrasmall silicon nanoparticles and carbon nanotubes. A stack of capacitor sheets was used to drive a system of LEDs.