Di water

All the materials, including dopamine hydrochloride (DA, 98%), graphite, tris(hydroxymethyl)-aminomethane (Tris, ≥99%), and polyacrylonitrile (PAN, Mw 150,000) were used as-received from Aladdin Chemical Co., Ltd. (Shanghai, China). All aqueous solutions were prepared with doubly distilled water (DI water) from a Millipore system (Chengdu, China) (>18 MΩ cm).

Uncoated AgNPs were produced by the solution-phase method (Polte et al. 2012 (link); Van Hyning and Zukoski 1998 (link)). Silver nitrate (AgNO₃, Sigma-Aldrich) was used as the precursor and reduced by sodium borohydride (NaBH₄, Sigma-Aldrich) to form AgNPs at room temperature (19 ± 4 °C). All the glassware for AgNPs synthesis was soaked in 10 % HNO₃ overnight and rinsed with copious amounts of deionized (DI) water (18.2 mΩ, Millipore), followed by drying in an ambient environment. The AgNO₃ solution (100 mL, 0.12 mM) was poured into the NaBH₄ solution (100 mL, 3 mM) in a 500-mL beaker. The NaBH₄ concentration was in 25-fold excess. The mixture was homogenized by magnetic stirring (1200 rpm). The NaBH₄ solution was freshly prepared to reduce the degradation resulting from its reaction with water to produce H₂ and BO₄⁻. The solution turned to grey within a few seconds after mixing and changed to light yellow after a few minutes. As the reaction continued, the colour slowly changed to dark yellow at ~25 min and back to yellow. After 1 h, the stirring was stopped, and the solution was stored for 24 h in the dark at room temperature. Finally, the stirring bar was removed and the AgNP suspension was transferred into glass bottles (250 mL, Duran) and stored at 4 °C in the dark.

Vegetable oil and petroleum oils with different carbon numbers, namely n-hexane, 2,2,4-trimethylpentane (iso-octane), isopar-G, n-hexadecane and mineral oil were tested (see further information of these oils in Supplementary Experimental Details 2.3). The oil concentrations of emulsions were ranged from 2.5 to 100 g/L (g oil/L water). Triton X-100 was used as surfactant with the surfactant/oil ratios varying from 0.0 to 0.2. In order to simulate shale gas wastewater, hexadecane-in-water emulsions of different salinity (0 ~ 256 g/L total dissolved salts) were prepared according to the following procedure. Firstly, NaCl, MgSO₄ and Al₂(SO₄)₃ were dissolved in DI water (Millipore ultrapure water, 18 MΩ cm) with molar concentration 1:1:1. Secondly, surfactant and oil were added into the salt solution sequentially under mechanical stirring. Thirdly, the mixture was sonicated under 100 W at 20 °C for 3 hours to obtain a homogenous milky emulsion. Fresh emulsions were immediately used in the subsequent fouling tests.

Tetrachloroauric(iii) acid trihydrate (HAuCl₄·3H₂O, 99.5%), trisodium citrate dihydrate (TSC, 99.0%), silver nitrate (AgNO₃, 99.8%), sodium borohydride (NaBH₄, 96.0%), l(+)-ascorbic acid (AA, 99.0%), n-hexane (C₆H₁₄, anhydrous, >99%), and DI water (>18 MW, Millipore, conductivity <4.3 μS cm⁻¹) were purchased from Sigma-Aldrich (Darmstadt, Germany). Cetyltrimethylammonium bromide (CTAB, 99.0%) was purchased from HIMEDIA (Mumbai, India). Crystal violet (C₂₅N₃H₃₀Cl, 99.0%) was purchased from AK Scientific, Inc. (Union City, USA). The above-mentioned chemicals were analytical grade and used without further purification. All experiment solutions were prepared using deionized water (DI). Before synthesizing the nanoparticles, glassware and stirring bars were washed with aqua regia water (HCl : HNO₃, optimal in a molar ratio of 3 : 1), and then thoroughly washed with deionized water.

Chloroauric acid (HAuCl₄·4H₂O) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Silver nitrate (AgNO₃, 99.0%), L (+)-ascorbic acid (C₆H₈O₆, 99.0%), sodium borohydride (NaBH₄, 96.0%), trisodium citrate (TSC, 99.0%), and DI water (>18 MW, Millipore, conductivity <4.3 μS cm⁻¹) were obtained from Sigma-Aldrich (Darmstadt, Germany). Hydrogen peroxide (H₂O₂, 30.0%) and sodium hydroxide (NaOH, 96.0%) were obtained from Xilong Scientific Chemical Reagent Co., Ltd. (Shantou, China). Cetyltrimethylammonium bromide (CTAB, 99.0%) was purchased from HIMEDIA (Mumbai, India). Thiram (C₆H₁₂N₂S₄, 97.0%) was purchased from AK Scientific, Inc. (Union City, USA). All reagents were used without further purification, and the aqueous solution experiments were prepared using DI water. All glassware and magnetic stirring bars were cleaned with aqua regia (HCl/HNO₃, volume ratio 3 : 1) and thoroughly rinsed with Millipore water (conductivity <4.3 μS cm⁻¹) several times prior to use in all experiments.