## 3 Results and Discussion
### Analysis of Savitzky-Golay Second-Order-Derivative Spectra  
Second-order-derivative treatment of spectra was performed on PAW samples treated with CAP for 5, 10 and 15 min, respectively. Subsequently, the spectra were compared with those of the blank control, revealing a clear and regular change in the spectra. Studies in aquaphotomics have shown that specific absorption bands can be identified in the first overtone region of the OH stretching band [20]. In addition, significant spectral variations in the OH bending and stretching band region (1800-1860 nm) of the NIR spectra were also identified [29], as depicted in Figure 4. Figure 4 shows a partial enlargement of the second-order-derivative spectrum ranging from 1300 to 1380 nm and 1800 to 1850 nm, respectively. The two regions could be assigned to different aspects of the structure of water molecules. In Figure 4A, the wavelengths 1360 nm, 1361 nm, 1363 nm, 1365 nm, 1366 nm and 1368 nm could be assigned to the solvation shell absorption of water, i.e., OH-(H\({}_{2}\)O)\({}_{1,2,4}\)[27], and 1373 nm could be identified as the symmetric and asymmetric telescoping vibrations of the water molecular structure, i.e., \(\upsilon_{1}+\upsilon_{3}\)[27]. In Figure 4B, all six wavelengths correspond to the stretching and bending vibrations of the OH bond [29]. By making full use of the  
Figure 2: Raw NIR spectra of PAW samples: (**A**). Raw NIR spectra of PAW samples obtained by treating with CAP for 5 min, 10 min, and 15 min, respectively. (**B**). Raw NIR spectra were acquired by continuous measurement of a 15 min PAW sample for 8 h.  
potential information contained in these wavelengths, we could use water information as a mirror to reflect the active ingredients in PAW. The variation in the spectra demonstrates an increase with longer treatment time, owing to the impact of reactive substances on the water molecules' structure. This increase in reactive substances is a result of the production of high-energy electrons by the CAP emitter during the discharge process. These electrons bombard the feed gas argon, Ar, producing argon plasma, which then reacts with O\({}_{2}\), N\({}_{2}\), and water vapor in the air, creating active substances. Alternatively, energetic electrons may come into direct contact with air, producing active substances that eventually enter the water. As the treatment time increases, the number of energetic electrons produced rises, leading to an increase in the amount of active substances in the water [19; 30]. The ratio of fluorescence intensities shown in Figure 5 indicates a rise in ROS concentration with treatment time, consistent with the change in the spectra. Figure 3: Local magnification of the original spectra at 1300–1600 nm and 1750–1860 nm: (**A**). Magnified view of the spectra of the samples treated for 5, 10 and 15 min at 1300–1600 nm. (**B**). Magnified view of the spectra of the samples treated for 5, 10 and 15 min at 1750–1860 nm. (**C**). Magnified view of the spectra obtained from the samples treated for 15 min with continuous collection for 8 h at 1300–1600 nm. (**D**). Magnified view of the spectra at 1750–1860 nm obtained from the samples treated for 15 min with 8 h of continuous collection. Figure 4: Local magnification of the second-order-derivative spectral changes after processing the 5, 10, and 15 min PAW samples demonstrates a trend of increasing spectral changes with longer processing times from 5 to 15 min: (**A**). Local magnification of the spectrum in the range 1359–1375 nm. (**B**). Local magnification of the spectrum in the range 1821–1847 nm. Figure 5: The ratio of fluorescence intensity of PAW samples with different treatment times to the blank control. Black, light grey and grey in the graphs indicate the ratio of the fluorescence intensity measured by the 5, 10 and 15 min samples to that measured by the blank control, respectively. As mentioned above, the results obtained using the chemical and NIR spectroscopy methods are the same.