Colorimetric Sensor Array Reader System Using Smartphone Camera
Authors: Yoon Sumi , Lee Kook-Nyung , Hong Dong-ki , Kang Hye-Lim , Song Sunga , Kim Won-Hyo , Seong Woo Kyeong , Shin Dong-Sik , Cho Hana , Kim Hyewon 
Published: May 2020

Colorimetric sensor arrays were developed to detect gas molecules by changing colors a decade ago.[1] Since the colors of the sensors change while specific gas molecules interact with an array of the colorimetric chemical sensor, the technique for measuring and analyzing the changes of the colors upon gas interactions is very important. Most of the color reading method for the sensor array is to obtain images using color scanners or digital cameras. If the spatiotemporal condition for obtaining images is inconstant, reading and interpreting the color change information upon the interaction of the gas sensors with the gas molecules must be inaccurate. In addition, most systems for analyzing color changes are laborious and time-consuming to find locations for accurate analyses using a desktop computer.[2] Here we have developed a smartphone-based colorimetric sensor array reader system that simply analyzes color changes. This system consists of a housing unit that maintains a constant position of the sensor array and provides illumination with the same brightness, a smartphone camera and an image analysis Android OS-based application. After acquiring the sensor array images before and after injection of the target gas by a smartphone camera, the application program automatically assigns positions of the color sensor array for the color analysis and obtains the color difference of the sensor array. The result of the color change analysis visualizes the RGB difference with color display images and numerical values created as excel files.
The colorimetric sensor array was fabricated as a 6x6 array guided by an array template using PEEK polymer which is not reactive to organic solvents. The dimension of the sensor array substrate was 30 mm(l)x 30 mm(w)x 4 mm(h) and the diameter of each hole for an individual sensor is 4 mm. The each spots of sensors was created by hydrogel matrices that can provide uniform color signals. The white LED was illuminated from the bottom of the template with a constant light source. The housing unit was equipped within the system to place the sensor array and the smartphone camera in a fixed position and to prevent the light from outside disturbing the intrinsic signals from the sensor array. This housing unit enables acquiring the sensor array images under the same conditions as the camera settings. Figure 1 shows the structure of the colorimetric sensor array reader system. The housing unit is a closed gas chamber and has a 50 mL of room to enable gas reaction experiments. The gas chamber has inlet and outlet so that gas can be injected and color change image can be obtained at each reaction time. The smartphone camera (Samsung Galaxy 7) was perpendicularly positioned over the sensor array at the fixed distance of 6 cm. An image of the color sensor array is acquired before injecting the target gas into gas chamber and the images of the color sensor array are temporally obtained while the target gas is injected at a constant volumetric rate. The structure of the smartphone application for image analysis is divided into the procedure of selecting the color sensor array, finding the array position, acquiring RGB color information and analyzing difference of color information as shown in Figure 2. After selecting images with a certain time lapse experiment to be analyzed, the application will automatically find the position of the sensor array. An image thresholding algorithm in openCV library was used to locate the sensor array and shape 36 sensor spots. The spots can be detected individually and 111 pixel region of the center of each spot can be calculated by HoughCircle algorithm in openCV library as shown in Figure 3. The 36 analysis areas obtained by the algorithm overlap the images obtained before and after the target gas was injected to take the RGB value in those areas. After obtaining the RGB median values of each sensor array images, the application can analyze the RGB deviation value of the image before the gas injection and the time-lapse image after the gas injection. This RGB difference shows that the degree of color change of the sensor array, when the color array sensor responds to the molecules of target gas. The analyzed results of the time-lapse response of the color sensor array exposed to 50 ppm of propionic acid are shown in Figure 4. This program could extract the median, mean and standard deviation of the RGB colors of all spots in the senor array images into an excel file and save the virtual array images of the RGB differences.
In conclusion, we implemented the application program that analyzes color difference of colorimetric sensor array. The smartphone camera generates image files by measuring the color array sensor at each elapsed time points. The android application analyzes the acquired images and simply calculates the color change of the color array. We envision that the useful color sensor array reader system allows the color sensor array to be easily recorded reaction of gas molecules through the analyzed RGB difference values.
This work was supported by the Technology Innovation Program (Grant No. 10077599) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea)
[1] Anal. Chem. 2006, 78, 3591-3600
[2] L. Shen, J.A. Hagen, I. Papautsky, Point-of-care colorimetric detection with a smartphone, Lab Chip 12 (2012) 4240–4243.

Figure 1