## 2 Results and Discussion
### Performance of Anion-\(\pi\) Type PNPs in Water-Based Inkjet Inks  
CMYK water-based inkjet inks were prepared by ball milling based on the optimized mixing ratios of PNPs-5 with four pigments (Figure 6a). The viscosities of CMYK inks were measured as 3.74 \(\pm\) 0.03, 3.43 \(\pm\) 0.02, 3.33 \(\pm\) 0.04, and 2.92 \(\pm\) 0.04 mPa\(\cdot\)s (Figure 6b), respectively. This remarkably demonstrated the low-viscosity advantage of PNPs-5 and thus met the viscosity requirements for inkjet printing. Surface tension is a key parameter affecting the spreading performance of inkjet inks on the surface of substrates [50]. The surface tension values of CMYK inks were 31.85 \(\pm\) 0.16, 30.82 \(\pm\) 0.22, 30.04 \(\pm\) 0.12, and 28.82 \(\pm\) 0.09 mN/m (Figure 6c), respectively, indicating their capability to wet various substrates. The average particle sizes of CMYK inks were 125, 225, 213, and 106 nm, respectively (Figure 6d), which suggested that the pigments were dispersed in the form of primary particles. In a planetary ball mill, with a minimum particle size of 100 nm, the pigments in K ink had already been dispersed to an ideal particle size. The viscosities, surface tension values, and particle size distributions of the inks satisfied the performance requirements necessary for effective operation in inkjet printers. Next, the inks were stored at room temperature for three months, and the particle size distribution curves remained essentially unchanged (Figures 6e and S19). This stability was beneficial for the long-term storage and use of the CMYK inks. At a centrifugation speed of 3000 rpm/min for 20 min, the average particle size changes of CMYK inks were only 1.95, 0.25, 0.06, and 4.90%, respectively (Figure S20). The results from UV-Vis spectra showed no decrease in absorbance values (Figures 6f and S21), indicating that CMYK inks exhibited  
Figure 5: Interaction mechanism of anion-\(\pi\) type PNPs with different organic pigments and the effect of mixing ratios. (**a**) Schematic illustrations of the interactions between PNPs and organic pigments. Taking PNPs-5 for example, it formed electrostatic and \(\pi\)-\(\pi\) interactions with CuPc, while it formed classical and salt-bridge H-bonds with R-254 or Y-180. \(\zeta\) values of PNPs-5 upon mixing with pigments (**b**) CuPc, (**c**) CB, (**d**) R-254, and (**e**) Y-180 in water at different mixing ratios. UV-Vis DR spectra of PNPs-5 with pigments (**f**) CuPc, (**g**) R-254, and (**h**) Y-180 in preferred mixing ratios. (**i**) Transmittance of water-based dispersion of PNPs-5 at 4% content after drying. excellent stability during centrifugation. Furthermore, the temperature stability of CMYK inks was investigated (Figure 6g). The average particle size changes of CMYK inks were 0.70, 0.46, 2.05, and 2.14% after four cycles of temperature tests (the conditions for each cycle were 4 \({}^{\circ}\)C for 24 h, and then 50 \({}^{\circ}\)C for 48 h). This demonstrated that CMYK inks exhibited superior temperature resistance and could be applied to inkjet printing under different ambient temperatures. These temperature stability results, combined with the previous findings, proved the high quality and reliability of the CMYK inks for various printing applications. The contact angles of CMYK inks on the surfaces of different substrates were further investigated. The substrates were polystyrene board, glass board, and coated paper, respectively. Among them, the polystyrene board exhibited the lowest surface energy, while the coated paper showed the highest surface energy. The measured contact angles of water on the three substrates were 64.2 \(\pm\) 0.3, 36.4 \(\pm\) 1.3, and 26.1 \(\pm\) 0.3\({}^{\circ}\), respectively (Figure S22). The contact angles of CMYK inks on polystyrene board, glass board, and coated paper decreased in that order. Taking C ink as an example, its contact angles on the three substrates were 27.7 \(\pm\) 0.7, 15.7 \(\pm\) 0.9, and 8.9 \(\pm\) 0.2\({}^{\circ}\), respectively (Figure 6h).