*2.2. Sample Preparation*

Zn3Li0.4Ga1.6GeO8:0.25% Mn phosphor was synthesized by using a hydrothermal method [8,11]. The synthesis procedure is described below. GeO2 powders were dissolved in a sodium hydroxide solution to achieve 0.5 M Na2GeO3 solution. Then, 3.0 mL 0.5 M Zn(NO3)2, 0.0375 mL 0.1 M Mn(NO3)2 and 0.3 mL concentrated HNO3 were together added slowly into 10 mL deionized water and then violently stirred. Subsequently, 0.4 mL 0.5 M LiNO3, 1.6 mL 0.5 M Ga(NO3)3 and 1.1 mL 0.5 M Na2GeO3 were added to the above solution. After the solution was stirred vigorously for 1 h, concentrated ammonium hydroxide was added into the mixture of precursor solution to tune the pH of the mixed solution to 7.5, and then the mixed solution was stirred vigorously for 1 h at ambient temperature. The final colloid was transferred into a 50 mL polytetrafluoroethylene reactor, which was then placed in an oven and heated at 220 ◦C for 10 h. Finally, the resulting suspension was centrifuged and washed 3 times using deionized water. The collected products were dried at 70 ◦C, and were then annealed in a chamber-type electric resistance furnace in air at 1100 ◦C for 2 h.

The synthesis procedure of Zn2.4Li0.6Ga2GeO8:Mn phosphors by hydrothermal method was similar to that of Zn3Li0.4Ga1.6GeO8:0.25% Mn only via 0.6 mL 0.5 M LiNO3 substituting for 20% Zn(NO3)2. According to our previous study [8,11], 20% Li substitution is an optimal doping concentration.

Zn3Ga2GeO8:0.25% Mn phosphors were prepared via a solid-state reaction approach using ZnO, Ga2O3, GeO2, and MnO as raw materials [12]. Raw materials were weighted on the basis of the formula of Zn3Ga2GeO8:0.25%Mn and finely mixed in an agate. All the ground powders were then pre-calcined at 900 ◦C for 2 h. Subsequently, the pre-calcined products were sintered at 1100 ◦C for 2 h and then cooled down to ambient temperature.

The synthesis procedure of Zn2.4Li0.6Ga2GeO8:Mn phosphor synthesized by a solid state reaction approach was similar to that of Zn3Ga2GeO8:0.25% Mn only via Li substituting for 20% ZnO.

#### *2.3. Inks Preparation and Anti-Counterfeiting Patterns*

Luminescence inks were fabricated based on a modified approach [13]. Typically, the prepared phosphors were dispersed into a mixed solution of 1.0 mL hydrochloric acid (0.2 mol/L) and 1.0 mL anhydrous C2H5OH, and then the mixed solution was centrifuged and washed with deionized water and anhydrous C2H5OH several times. Finally, the powders were mixed with PVA aqueous solution in a ratio of 1:1 to fabricate luminescent anti-counterfeiting inks. The luminescent patterns printed on the paper were achieved using screen printing method.
