Our biosensor is composed of three major components: the biological recognition unit, the circuit, and the user interface. The probe also has a structural unit that holds the whole shebang together.
How Does it Work?
Op amp 1 is a voltage follower; the voltage output of this op amp is equal to the voltage of the reference electrode. Op amp 2 is a summing amplifier; its output is the inverse of the sum of the user applied voltage and the output of op amp 1. Op amp 3 inverts the signal from op amp 2, setting the voltage of the working electrode with respect to the reference electrode at a defined voltage. Op amp 4 converts the current running through the counter electrode into a voltage.
While testing circuit function, our circuit was wired on a protoboard.
The finalized potentiostat circuit design was soldered to a circuit board.
Op amp 1 is a voltage follower; the voltage output of this op amp is equal to the voltage of the reference electrode. Op amp 2 is a summing amplifier; its output is the inverse of the sum of the user applied voltage and the output of op amp 1. Op amp 3 inverts the signal from op amp 2, setting the voltage of the working electrode with respect to the reference electrode at a defined voltage. Op amp 4 converts the current running through the counter electrode into a voltage.
Biorecognition
Our recognition unit relies on a biochemical process called oxidation, which releases electrons from molecules. Electrons are small, negatively charged particles. These electrons are passed to a metal probe to generate an electrical current proportional to the number of electrons released. We use an enzyme known as glucose oxidase (which is specific to glucose molecules) fixed to a carbon paste and stainless steel probe to accomplish this task.
This is the structure of the glucose dehydrogenase enzyme used in the probe. The ball and stick structures pictured are covalently bound electron mediators known as PQQ.
A glucose molecule is composed of six carbons (black), twelve hydrogens (white), and six oxygen molecules (red).
The two possible reaction mechanisms for s-GDH. (A) The addition-elimination mechanism comprises general base-catalyzed proton abstraction followed by covalent addition of the substrate and subsequent elimination of the product. (B) Mechanism comprising general base-catalyzed proton abstraction in concert with direct hydride transfer from substrate to PQQ, and tautomerization to PQQH2. Taken from "The EMBO Journal".
This is the structure of the glucose dehydrogenase enzyme used in the probe. The ball and stick structures pictured are covalently bound electron mediators known as PQQ.
Circuit
The circuit used in this design is what is known as a potentiostat. The circuit has three electrodes: a working electrode, a reference electrode, and a counter electrode. The working electrode is the site of our biological recognition unit. By holding voltages constant between the working and reference electrodes, the circuit forces the same current that flows into the working electrode to flow out the counter electrode. This current is converted to a voltage and is sent to the user interface and is proportional to the glucose concentration in the solution.
Designed in LabVEIW, the user interface imports, displays, and converts the raw voltage data. A calibration function allows the user to see their voltage data directly as glucose concentration. Other capabilities, such as file saving and controls for the circuit, make the interface more user friendly and the whole system more versatile.
Interface
This is what the user sees as they interact with the program.
This is the program behind the user interface that allows it to do what it does.
This is what the user sees as they interact with the program.
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The probe structure is what holds the sensing elements of our instrument in place. The body of the structure is a plastic disk with holes drilled to the size of the electrodes. The electrodes are short, thin rods of stainless steel hypotubing. These electrodes are equally spaced. The disk has a steel handle fixed in its center to aid in the use of the device.
Structure
This is the portion of the probe tip that contains the biological recognition unit and interacts with the test solution.
This is the portion of the probe tip that contains the biological recognition unit and interacts with the test solution.
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