Overview
At present, there is no effective testing method for detecting bacteria and monitoring bacterial contamination of refrigerator. If bacterial contamination occurs, it is often a threat to people’s physical health. To monitor the pathogenic bacteria in refrigerators in a timely manner, we developed a hydrogel biosensor for detecting the contamination of Shigella, E. coli and Salmonella which are reported as pathogenic bacteria. In this page, we describe the development of hardware (detector) for detecting the bacteria on the hydrogel biosensor. Due to the convenience and inexpensive, it is easily to be applied in refrigerator at home, for reminding people to clean or disinfect the refrigerator in case of bacterial contamination occurrence.
1. Principle of RGB sensor used in our hardware
The color of any object can be obtained by fitting the three primary colors of red, green, and blue in a certain proportion using the principle of color matching constant law for color quantitative analysis.
An RGB color sensor helps us accurately detect an object’s color. RGB stands for Red, Green, and Blue. An RGB sensor can independently detect the color intensity of red, green, and blue. It can also measure brightness. It is realized by using an RGB color filter at its input. For example, the red color filter allows only the red light to pass through. The light falls on the photodiode, whose current will vary depending on the amount of incident light. The current variation will be converted to a voltage variation using a signal conditioner which can be read using an Analog-to-Digital Converter (ADC) (Fig. 1).
Fig.1 The block diagram of RGB sensor used in our bacteria detector.
Since the three colors can be used to create many colors with different proportions, RGB sensors can also detect other colors. With an 8-bit RGB color sensor, the red, green, and blue colors can take any values from 0 to 255, which means that it can detect 256 * 256 * 256 = ~16 million colors in total. So, the RGB color sensor was used in our detector to detect a vivid color variation of the hydrogel, monitoring the bacterial contamination in refrigerator.
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2. Detector construction
Based on these principles mentioned above, we used multiple components to construct the detector, including the hydrogel biosensor made from cell free system with X-gal, main board, RGB color sensor, Dupont wire, Pin header, USB cable, Breadboard, Resistance, LED light, and the shell of the detector which was printed using 3D printer, all of which were showed in Fig. 2A.
Fig. 2B shows the assembled detector: main board put in the board cell, LED light placed on the breadboard, RGB color sensor connected to the main board.
Fig.2 The construction of detector.
(A) The components to construct the detector. (B) The assembled detector.
When we completed the preliminary construction of the detector, we conducted color detection experiment to test if the detector is working properly, and found that our detector could work well (Fig. 3A). The analysis software used is Arduino IDE 2.3.2 (Fig. 3B), which was download on the website. Please refer to the web page (https://www.arduino.cc/en/software) for detailed information.
Fig.3 Test of the Detector.
(A) Color detection experiment to test the detector. (B) The software used in detector was Arduino IDE 2.3.2.
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3. Usage of Arduino IDE 2.3.2
Arduino IDE resource library has the TCS34725 code example adapted for UNO R3 main board. After downloading, select the correct main board and port, connect the UNO R3 main board and TCS34725 RGB sensor correctly, check and upload the code, wait for the computer to compile, and then open the serial monitor to see the output results. Some screenshots are shown in the following Figures (Fig.4 and Fig.5).
Fig.4 Some screenshots shows how to use Arduino IDE 2.3.2.
Fig.5 Some screenshots shows the usage of Arduino IDE 2.3.2.
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4. Experiment on the cell free system hydrogel using complementary fragment of aptamer
The hydrogel was made using the extracted cell free system containing aptamer toehold switch, LacZ serving as the reporter gene, as well as X-gal. Then the hydrogel was placed in the hydrogel cell of the detector. Add a drop of complementary fragment of aptamer with the concentration of 2 uM onto the hydrogel, and keep it at room temperature for 1 h. After the hydrogel appeared blue color, connected the detector to computer using USB cable. The RGB sensor scanned the color on the hydrogel and transferred the information to the processor. If the detected value exceeds a certain range, the red LED light on the breadboard would turn on, otherwise, the green light will turn on.
Fig.6 Experiment on the cell free system hydrogel using complementary fragment of aptamer, showing how to detect the color variation on the hydrogel.
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5. Advantage
(1) The detector with cell free system hydrogel may be applied in refrigerator at home, suitable for monitoring pathogenic bacteria in refrigerators, reminding people to clean or disinfect the refrigerator to protect people's health.
(2) The detector converts the detected color signal which is amplified by enzyme into an electrical signal, and displays it on the screen of the refrigerator door. If the detected value exceeds a certain range, the red LED light on the screen of refrigerator door will turn on, making it convenient for people to observe.
(3) The detector is small, the volume is approximately 10 cm * 6 cm * 1 cm, which is easy to put in the refrigerator. And the hydrogel is replaceable, which is very convenient and practical.
(4) The production cost of this detector is relatively low, around $5, which is economically feasible for both manufacturers and users. From the survey, we know that all the interviewers can accept the 10% price increase of a refrigerator with a bacteria detector. $5 is lower than 10% price of a refrigerator, means that all the interviewers can accept our detector.
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6. Disadvantage
For safety reasons, we designed and only demonstrated a detector using the complementary fragment of bacterial aptamer. However, it can be predicted that it is feasible to use this method for detecting pathogenic bacteria in refrigerators in the future.