Citric acid

PBNPs were synthesized using a one-pot scheme as previously described [9 (link),35 (link),41 (link),44 (link)]. Briefly, 20 mL of 10 mM aqueous FeCl₃•6H₂O (54.06 mg) (Millipore Sigma, Darmstadt, Germany) containing 5 mmol of citric acid (961 mg) (Millipore Sigma, Darmstadt, Germany) was added to an equal volume of 10 mM aqueous K₄[Fe(CN)₆]•3H₂O (84.5 mg) (Millipore Sigma, Darmstadt, Germany) containing 5 mmol of citric acid, under vigorous stirring at 60 °C using a magnetic stirring hot plate. After heating and stirring for 60 s, the solution was allowed to cool to room temperature (RT) for 5 min under constant stirring. The 40 mL solution was then divided equally between two 50 mL tubes and PBNPs were then collected by adding equal volume of acetone (Millipore Sigma, Darmstadt, Germany) and 5 mL 5 M NaCl (Millipore Sigma, Darmstadt, Germany). The solutions were then centrifuged at 10,000 rpm for 15 min at RT. The centrifugation step with 20 mL MilliQ water, 20 mL acetone, and 5 mL 5M NaCl was repeated twice. The final PBNP pellet was sonicated (at 40% amplitude for 30 s using a microtip probe) in 10 mL MilliQ water to achieve colloidal resuspension. The PBNPs were stored under ambient conditions in DI water prior to further bioconjugation.

Cells were fixed for 15 min (room temperature) in 1% formaldehyde, washed with PBS and incubated at 37°C (no CO2) with fresh staining solution: 0.3 mg/mL of 5-bromo4-chloro-3-indolyl β-D-galactoside (X-Gal, Fermentas), 40 mM citric acid (Sigma), 40 mM sodium phosphate (Sigma) (stock solution (400 mM citric acid, 400 mM sodium phosphate) must be at pH6), 5 mM potassium ferrocyanide (Sigma), 5 mM potassium ferricyanide (Sigma), 150 mM NaCl (Sigma), 150 mM MgCl2 (Sigma). SA-β-GAL-positive cells were quantified after 16-20 hrs as compared to unstained cells.

Calcium chloride dihydrate, sodium carbonate (anhydrous), iron(III) chloride hexahydrateiron(II), chloride tetrahydrate, sodium hydroxide, tannic acid (TA), bovine serum albumin (BSA, lyophilized powder) and TRITC-conjugated BSA, phosphate-buffered saline (PBS), citric acid, glycyl alcohol, ethanol, mitoxantrone (Mit), Dulbecco’s modified Eagle’s medium (DMEM) (high glucose), Bradford reagent, phosphate buffer solution (PBS), citric acid, glycyl alcohol, trypsin (from porcine pancreas, ~1500 U/mg), pepsin (from porcine gastric mucosa, ≥2400 U/mg), tris(hydroxymethyl)aminomethane (≥99.8%), and sodium citrate were purchased from SigmaAldrich (Germany, Taufkirchen). Fetal bovine serum (FBS) was purchased from Gibco (Waltham, MA, USA). AlamarBlue was purchased from Thermo Fisher Scientific (Waltham, MA, USA).
Millipore Milli-Q deionized (DI) water (18.2 MΩ·cm) was used in all sets of experiments.

The VO₂ (a) was prepared by hydrothermal method. 1 mmol V₂O₅ (99.6%; Sigma-Aldrich) and 3.3 mmol citric acid (99.0%; Sigma-Aldrich) were added to 10 ml distilled water, followed by stirring at 80 °C for 30 min. About 2.5 mL H₂O₂ (30 wt%; Aladdin) was added dropwise to the previous solution and stirred, then added 25 ml alcohol and a piece of CF (CeTech) and transferred to a 50 ml Teflon-lined stainless steel autoclave, which was maintained at 180 °C for 3 h. Lastly, the product was washed thoroughly with deionized water and ethanol and dried under vacuum at 60 °C for 12 h. VO₂ (M) was obtained using the same process as VO₂ (a), except that the citric acid was replaced by oxalic acid (99.0%; Sigma-Aldrich) and annealed at 500 °C for 4 h under Ar atmosphere. VO₂ (B) was also made from oxalic acid, but compared with VO₂ (a), the amount of all raw materials was increased threefold^{20 ,55} . Since the synthesis method mentioned above could not obtain the individual amorphous VO₂ powder sample, 3 mmol VOSO₄·xH₂O (99.9%; Aladdin) and 6 mmol NaOH (97.0%; Aladdin) were stirred into deionized water for 1 h. The precipitates were centrifuged with water and ethanol and dried at 80 °C for 12 h. Next, the product was calcined at 300 °C for 8 h to obtain the amorphous VO₂ powder⁸ . The B phase and M phase VO₂ powder were prepared in the same way as VO₂ (B) and VO₂ (M), but without adding the CF.