## 2 Experimental Section
### The Principle of Measuring Vc Using the Iodine-Turbidimetric Method  
The schematic presentation of the detection of VC is shown in Figure 4.  
When a VC solution is added to iodine ethanol solution, the solubility of iodine will decrease significantly due to the decrease of alcohol resulting from the VC solution, and an iodine suspension is formed [21]. Iodine is reduced to iodide ions and then iodide ions react with iodine to form triiodide, which is soluble. This reaction process is shown in Equations (6) and (7). The turbidity of the iodine suspension is relative to the VC content:  
\[C_{6}H_{8}O_{6}+I_{2}=C_{6}H_{6}O_{6}+2\,H^{+}+2\,I^{-} \tag{6}\] \[I_{2}+I^{-}\Longleftrightarrow I_{3}^{-}. \tag{7}\]  
The 850 nm light source used in this study belongs to near-infrared (NIR, 780-1000 nm). NIR is widely used in turbidity measurements, and commercial turbidity instruments are based on 850 nm/860 nm light [22]. NIR light is less influenced by the color of the sample and is not sensitive to soluble particles, which essentially do not absorb and scatter infrared light, and the light band of the 850 nm light source is narrow (\(\Delta\lambda=50\) nm when I = 20 mA); thus, it is not absorbed by most organic substances [23]. Additionally, [PERSON] et al. proved that \(I_{3}^{-}\) has no sensitive light absorption in the NIR band [24], and [PERSON] et al. reported that the absorption of 850 nm light by water was small, and did not affect turbidity measurements [25]. Therefore, in the suspension, only suspended particles reduce the intensity of the transmitted light at 850 nm. With the exception of substances used to precipitate \(I^{-}\) or with strong oxidation or reducibility, the soluble chemicals (such as pigments, inorganic salts, and sugars) do not interfere with the measurement [23], which is in keeping with our Interference Experiments. These results demonstrated the selectivity and feasibility of the method.  
When light passes through a suspension, the suspended particles block the propagation of light in the suspension. To what degree the propagation of light is affected by the suspended substances depends on the size, shape, and composition of particles, and the wavelength of the incident light. In addition to the scattering effect, the transmitted light is absorbed by the particles and the light intensity reduces [23], and the change in the light intensity follows the Lambert-Beer law Equation (8) or Equation (9).  
Figure 4: Schematic presentation of the detection of vitamin C (VC) with our proposed method.  
\[I=I_{0}e^{-[\alpha a+\alpha b]xc} \tag{8}\] \[\ln I=\ln I_{0}-(\alpha a+\alpha b)xc \tag{9}\]  
where \(I\) is the intensity of the transmitted light, \(I_{0}\) is the intensity of the incident light, \(X\) is the length of the solution the light passes through, \(c\) is the particle concentration, \(\alpha a\) is the absorption coefficient, and \(\alpha b\) is the scattering coefficient.  
The suspensions with different turbidity were obtained by adding the VC solution to a certain amount of iodine ethanol solution. When light passes through the suspension, light is absorbed and scattered. The absorption and scattering affects the image, and this effect is mainly related to the turbidity of the suspension. After using the infrared camera to obtain the images of the incident light, the RGB values of the suspension were obtained by image processing. The grayscale and log-grayscale were calculated and the Lab values of the images were obtained by the color space conversion from RGB to Lab. By analyzing the relationship between the VC and RGB, and Lab and grayscale values, the relation expressions between the VC concentration and L, and grayscale and log-grayscale values were obtained, which were used to determine the VC in real samples.