Team:BSC United/Sensor
DESIGN
SENSOR
Electrochemical Biosensor
We use electrochemical biosensors to detect the glucose concentration.
The electrochemical biosensor is a device that can transform chemical quantities (contained by organic matters including human body) into electric quantities (sensible and “understandable” by electronic devices) for detection of a certain determinand.
We chose this kind of sensor because of its fast measurements, sensitivity and ability to perform measurements on complicated solutions (turbid/opaque)[1]. A Typical blood sugar electrochemical sensor contained three electrodes: working electrode (the electrode in an electrochemical system on which the reaction of interest is occurring), counter electrode (an electrode which is used to close the current circuit in the electrochemical cell) and reference electrode (an electrode which has a stable and well-known electrode potential and it is used as a point of reference in the electrochemical cell for the potential control and measurement).
Of many applications of electrochemical sensors, the most common one would be blood sugar monitoring. In order to monitor blood glucose concentration, Enzyme electrochemistry is applied. In an enzyme-based electrochemical sensor, enzymes serves as catalysts in the reduction reaction caused by change in glucose concentration and finally produce an electrical current. The redox reaction is as followed[3]:
We choose enzyme-based electrochemical sensors to evaluate the change of glucose concentration in our project so as to obtain the effect of engineering bacteria constructed by the project.
However, we cannot reach this purpose using market electrochemical sensors. Market electrochemical sensors have short life, and their enzymes on the working electrodes are easy to fall off. To improve the sensor’s life of usage, we applied the technique of immobilized enzyme. Typically, the crosslinking method is applied in the design of the electrochemical biosensors, but in our project, we use entrapping method to fix our enzyme -- glucose oxidase -- on the electrode. The entrapping method includes a polymer to entrap the enzyme inside, which is Chitosan in our project. The entrapping method has a comparably high rate of enzyme recovery than crosslinking method, which makes it the best method for our project.
Design
Specialized electrochemical sensors need specialized design. First, we designed the abstract structure of our electrochemical sensor with the help of Solidworks. After that, we simulate the model using COSMOL Multiphysics theoretically. We chose carbon paste to make the working electrode and the counter electrode; reference electrode was made of silver and silver chloride paste in the mixture proportion of 1:1. We chose these two materials because of their inertness so they would not participate in the electrochemical reactions.
Our sensor generally contains three layers. The PET layer at the bottom serves as carrier of the components; the paste of silver chloride layer in the middle serves as conductor of electricity; the carbon paste layer at the top serves as both the working electrode and the counter electrode. The design of the electrochemical sensor is shown in Fig.1.
Fig.1 The design of the electrochemical sensor
Firstly, we had the PET layer (27cm×36cm) as the basis. Secondly, we used silk-screen printing (printing plate 32cm×44cm) to prepare the first layer of the electrodes (Working electrode, Counter electrode) with carbon paste (material). After sintering (90℃ in sinter box for 10 minutes), the second layer of the electrodes were printed with Ag (material) and further sintered. On the top of the second layer of WE, we prepared the layer of enzymes (which was glucose oxidase (GOx) of 10 mg/mL in 1x PBS solution) and applied the protection (the entrapping method) on the layer. The Gox solution would be the mixture of 0.5 mL of 1 M acetic acid and 4 ug of Chitosan. The actual image of the sensors were shown in Fig.2.
Fig.2 The actual image of the sensors
Testing
After the designing, we constructed the graph of CV (Cyclic Voltammetry) curve in electrochemical workstation and tested the performance and practicability of our sensors. As a type of electrochenmical measurement, it measured the peak current (and the corresponding bias voltage) and the reversibility of the redox reaction by manipulating the electrode potential. A CV curve would be the graph of the current versus potential. According to the CV graph shown in Fig.3, the bias voltage should be approximately 0.15 V.
Fig.3 CV curve
Sensors in Organ-on-a-chip
Finally, in order to detect the change in glucose concentration after proinsulin is released, we set two electrochemical sensors, one on the top and the other at the bottom of the organ chip. The design is shown in Fig.4.
The top channel of the design is used to mimic the “blood” of human. The sensor in the top part of the organ-on-a-chip contacts with the solution in the top channel to detect the concentration of the glucose in “blood”. Moreover, the bacterial solution was added into the top channel to mimic the injection of proinsulin.
The sensor in the bottom part of the organ-on-a-chip contacted with the solution in the bottom channel. During this process, the liver cell line was cultured and used to mimic the liver side of the body for monitoring the concentration of the glucose after the effects of liver with proinsulin.
Fig.4 Three-electrode sensors on organ-on-a-chip
Reference
[1] “Types of Biosensors.” Types of Biosensors, www.idc-online.com/technical_references/pdfs/chemical_engineering/Type s_of_biosensors.pdf.
[2] Grieshaber, Dorothee, et al. “Electrochemical Biosensors - Sensor Principles and Architectures.” Sensors (Basel, Switzerland), Molecular Diversity Preservation International (MDPI), 7 Mar. 2008, www.ncbi.nlm.nih.gov/pmc/articles/PMC3663003/.
[3] Zhang, John X.J. “Glucose Sensor.” Glucose Sensor - an Overview | ScienceDirect Topics, ScienceDirect, 2014, www.sciencedirect.com/topics/engineering/glucose-sensor.
