Ammonium hydroxide

Dicyandiamide (DCN, 99%), hexadecyltrimethylammonium bromide (CTAB, 99%), polyvinylpyrrolidone K30 (PVP K30), hydrochloric acid (HCl, 37%), silicon dioxide (SBA-15, 99.6%), p-anisaldehyde (AAD, 98%), benzoquinone (BQ, 98%) and ethylenediaminetetraacetic acid (EDTA, ≥98%) were purchased from Sigma-Aldrich. Acetaldehyde (C₂H₄O, ≥99%), tetraethyl orthosilicate (TEOS, 99%) and formic acid (HCO₂H, 98%) were purchased from Fluka. Ammonium hydroxide (NH₄OH, 28%), tert-butanol (t-BuOH, 99%) and p-anisyl alcohol (AA, 98%) were purchased from Alfa Aesar. Sulfuric acid (H₂SO₄, 95–98%) was purchased from Labor and ethanol (CH₃CH₂OH, p. a.) was purchased from Panreac. Hydrofluoric acid (HF, 48%) and methanol (CH₃OH, 99.6%) were purchased by VWR™. Titanium dioxide (TiO₂) Aeroxide® P25 powder was supplied by Evonik and used as received. All the reagents were used without further purification. Ultra-pure water was obtained from a Direct-Q Millipore system.

A ZnO nanoparticle solution was obtained from a zinc amine precursor.^{16 (link)} The solution was prepared by mixing 0.0813 g ZnO powder (Sigma Aldrich, 99.999%) with 12 mL ammonium hydroxide (Alfa Aesar, 99.9%) and refrigerated for 5 h to increase the solubility of the ZnO powder.^{38 (link)}

Fluorescein isothiocyanate (FITC, 95%), 3-aminopropyltriethoxysilane (APTES, 98%), tetraethyl orthosilicate (TEOS, 98%), and ammonium hydroxide (NH₄OH, 28%−30% as NH₃) were purchased from Alfa Aesar Ltd (Shanghai, China). Absolute ethyl alcohol was acquired from the Beijing Chemical Reagent Company (Beijing, China). All materials were used as received and without further purification. Purified water was used throughout.

Sodium chloride, potassium chloride, potassium hydroxide, ammonium hydroxide, sulfuric acid, silver nitrate (AgNO₃), potassium tetrachloroplatinate (K₂PtCl₄), potassium hexachloroplatinate (K₂PtCl₆), and potassium chloroaurate (K₂AuCl₄) are from Alfa Aesar. Perchloric acid is from Fisher Scientific. Organic reagents including AR (analytical reagent) grade acetone, ethanol, and ethylenediamine tetraacetic acid disodium are from J&K Chemicals. The substrate Ni foam (purity, 99.9%; thickness, 1.5 mm; pore density, 100 ppi) is from Zhuoer Advanced Metallic Materials. The substrate was cleaned using ultrasonication with ethanol (>99.7%), hydrochloric acid (2%), and deionized (DI) water (18.3 megohms·cm, Merck Millipore).

The reactive Ag ink was prepared with modified Tollens’ ink (29 (link)): 1 g of Ag acetate (Sigma-Aldrich) was dissolved in 2.5 ml of ammonium hydroxide (28% NH₃, Alfa Aesar), and then 0.2 ml of formic acid (Thermo Fisher Scientific) was added, followed by 3-hour stirring at room temperature to obtain a transparent solution. PEDOT:PSS solution was prepared by mixing 95% (v/v) of PEDOT:PSS (Heraeus Clevios PH 1000) and 5% (v/v) of ethylene glycol (Sigma-Aldrich). The solution was sonicated for around 20 min and then filtered through a 1.2-μm PTFE (polytetrafluoroethylene) filter before using. To enhance the structure and conductivity stability of PEDOT:PSS fibers in the case of using them inside cell culture media, 1% (w/w) of DVS (divinylsulfone; Sigma-Aldrich) was then added to the solution as a cross-linker. DVS is able to cross-link PEDOT:PSS under room temperature, and the cross-linked PEDOT:PSS is proven to be biocompatible. After mixing with DVS, the solution was mixed carefully and filtered through a 1.2-μm PTFE filter. PEO (M_w = 4 million Da) was dissolved in DI water at 2% (w/w) and stirred at room temperature for 24 hours.