*2.3. Method*

#### 2.3.1. Synthesis of NaGdF4:49%Yb,1%Tm Core Nanocrystals

NaGdF4 doped with 49 mol % of Yb and 1 mol % of Tm (NaGdF4:49%Yb,1%Tm) was synthesized via a modified literature procedure [28,35,36]. A water solution of Gd(CH3CO2)3 (0.067 g; 0.2 mmol), Yb(CH3CO2)3 (0.069 g; 0.196 mmol), and Tm(CH3CO2)3 (0.001 g; 0.004 mmol) was combined with OA (5 mL) and ODE (5 mL) in a 50 mL two-neck round-bottom flask. The mixture was heated to 150 ◦C and maintained at this temperature for 1.5 h to form the lanthanide–oleate precursor. After cooling to 50 ◦C, a methanol solution consisting of NH4F (0.05 g; 1.36 mmol) and NaOH (0.04 g; 1 mmol) was added to the mixture and stirred for 30 min. The solution was heated to 100 ◦C for 20 min in vacuo to remove methanol. The resulting solution was quickly heated to 300 ◦C and maintained at this temperature for 1.5 h with nitrogen before cooling to room temperature. The obtained nanocrystals were precipitated by centrifugation at 8000 rpm for 5 min and then washed with cyclohexane and ethanol three times. The core nanoparticles were dispersed in cyclohexane (4 mL) for further shell coating.

#### 2.3.2. Synthesis of NaGdF4:49%Yb,1%Tm@NaYF4:20%Yb Core–Shell Nanocrystals

The synthesis procedure for core–shell nanoparticles was similar to that in our previous paper [36]. We use the obtained NaGdF4:49%Yb,1%Tm nanocrystals as seeds for subsequent shell coating. NaYF4 with 20 mol % of Yb (NaYF4:20%Yb) precursor was prepared via the same procedure as mentioned above, except that different amounts of OA (3 mL) and ODE (7 mL) were used. After cooling to 80 ◦C, the cyclohexane solution of NaGdF4:Yb/Tm nanoparticles (4 mL) was added and kept at 80 ◦C for 30 min to remove cyclohexane. Then, a methanol solution of NH4F (0.05 g; 1.36 mmol) and NaOH (0.04 g; 1 mmol) was added to the mixture and stirred at 50 ◦C for 30 min. Subsequently, the mixture was heated to 100 ◦C for 20 min in vacuo to remove methanol. The solution was then heated to 300 ◦C for 1.5 h under a nitrogen atmosphere. After cooling to room temperature, the core–shell nanoparticles were collected and washed using the same post-treatment approach as for core nanocrystals. NaGdF4@NaGdF4:49%Yb,1%Tm and NaYF4@NaGdF4:49%Yb,1%Tm were synthesized using a similar method to core–shell nanocrystals except for the use of NaGdF4 and NaYF4 as seeds.

2.3.3. Synthesis of NaGdF4:49%Yb,1%Tm@NaYF4:20%Yb@NaGdF4:50%Nd,10%Yb and NaGdF4:49%Yb,1%Tm@NaYF4:20%Yb@NaGdF4:50%Nd,10%Yb@NaGdF4 Core–Multishell Nanocrystals

The following multishelled core–shell nanoparticles were prepared using a procedure similar to the NaGdF4:49%Yb,1%Tm@NaYF4:20%Yb core–shell nanoparticles: NaGd F4@NaGdF4:49%Yb,1%Tm@ NaYF4:20%Yb; NaYF4@NaGdF4:49%Yb,1%Tm@NaYF4: 20%Yb; NaGdF4@NaGdF4:49%Yb, 1%Tm@NaYF4:20%Yb@NaGdF4:50%Nd,10%Yb; NaYF4 @NaGdF4: 49%Yb,1%Tm@NaYF4:20%Yb@NaGdF4:50%Nd,10%Yb;NaGdF4@NaGdF4:49%Yb,1%Tm@

NaYF4:20%Yb@NaGdF4:50%Nd,10%Yb@NaGdF4; NaYF4@NaGdF4:49%Yb,1% Tm@NaYF4: 20%Yb@ NaGdF4:50%Nd,10%Yb@NaGdF4.

#### 2.3.4. Preparation of Dye-Sensitized Upconversion Nanoparticles

The synthesis of IR-806 followed a well-established method [32]. Then, the IR-806 was dissolved in CHCl3 (0.01 mg/mL). The as-prepared core–multishell nanocrystals were centrifuged and dissolved in CHCl3 to a final concentration of 0.375 mg/mL. The samples were prepared by adding different amounts of IR-806 to Gd-CSYS2S3 CHCl3 solution (4 mL) and stirring for 2 h at a speed of 700 rpm at room temperature before UV– vis–NIR absorption and standard fluorescence measurements. All samples were prepared and measured in a dark environment.
