# A Low-Cost and Portable Smart Instrumentation for Detecting Colorectal Cancer Cells
## 2 Materials and Methods  
Most substances absorb light. The substances which absorb light at a particular wavelength and energy but emit light at a longer wavelength but lower energy are called fluorescence substances and the principle is called fluorescence. Fluorescent dyes or quantum dots are most commonly used as fluorescence substance. Hence, when a light of sufficient intensity is incident upon samples which are conjugated with a fluorophore dye, these samples absorb light and emit at a longer wavelength. However, these are of low intensity. The proposed device exploits this principle of fluorescence to detect colorectal cancer cells. Figure 1a shows the working principle of a typical fluorescence microscope [7], whereas Figure 1b shows the working principle of the proposed fluorimeter. In this paper, IRFP702 is used as a dye which is conjugated with colorectal cancer cells (human colorectal carcinoma (HCT116)). The peak absorption and emission wavelength of infrared protein (IRFP702) are 673 nm and 702 nm respectively. As can be seen in Figure 1b, the excitation light (in the visible range) from the flashlight is focused on the top of the sample. The emitted light at a longer wavelength from the sample is then passed through a hard-coated longpass filter (Edmund Optics, Stock# 62-980) whose longpass wavelength is 700 nm. The fluorescent light is detected by the photodiode. The photodiode is placed inside a custom-built sample chamber which is black in color to minimize the interference from the ambient light. The microcontroller reads the data from the photodiode and displays it on an liquid crystal display (LCD). The proposed device is comprised of a flashlight (Ultrafire C8) [18], a photodiode (OSRAM Opto Semiconductors SFH 203) [19; 20], a microcontroller (Arduino UNO REV3 (surface-mount device) SMD version) [21] and a LCD (Basic \(16\times 2\) Character LCD--White on Black 3.3 V version) screen [22] as major components. The photodiode is used to detect fluorescent signal while the microcontroller is used to read and transmit data detected from the photodiode to the LCD display. The chosen photodiode has a short switching time of 5 ns while its responsivity lies in the range of 400-1100 nm and is available in a 5 mm LED plastic packaging. The relative spectral sensitivity of this photodiode at 702 nm is 0.82 and it has a radiant sensitive area of 1 mm\({}^{2}\). This wide spectral characteristic of the photodiode makes it versatile and an ideal candidate to be used in the designed fluorimeter. The overall working principle is shown in Figure 1b where it is clear that the flashlight is placed in a straight line on top of the sample. The sample is placed in between the longpass filter and flashlight. The optical longpass filter/high pass filter is designed to transmit wavelengths greater than the cut-on wavelength of the filter and block the lower wavelengths. The detector circuitry is placed inside the sample chamber. The received signal from the photodiode is processed with the help of an Arduino Uno microcontroller and displayed on an LCD display. The major components that are required to assemble the fluorimeter are shown in Figure 2. The chosen flashlight has a dimension of 14.5 cm \(\times\) 4.5 cm \(\times\) 4.5 cm and can operate in three modes: strobe, low and high. It is operated with a rechargeable 3.7 li-ion battery and has a 3000 mAh current rating. For this experiment, the flashlight was operated in high mode. It uses a Cree XR-E Q5 type emitter, which has a luminous flux of 200 lm [23]. The sample chamber is designed with _SolidworksTM_ and printed with the help of a 3D printer (CR-10), which is black in color in order to reduce unwanted noise. The complete dimension of the custom-built sample chamber is shown in Figure 3. To further minimize the interference, a longpass filter is used to pass only the fluorescence wavelengths while filtering out the unwanted spectrum from the flashlight. This longpass filter is placed between the sample and the photodiode.