We used a gel trapping technique to trap the particle at the water/1-octanol interface and analyzed the contact angle value of single nanoparticles with the help of SEM characterization and ImageJ analysis (39, 40). See Fig. 4A for a schematic of the gel trapping process. Specifically, gellan gum (2 wt %) was dispersed in water (10 ml) and heated to 60° to 70°C for 30 min to fully dissolve. The gelation temperature of 2 wt % gellan gum was 40° to 45°C. We preheated both 1-octanol (3 ml) and colloidal isopropanol suspension (SPs and M-SPs with diameter around ~400 nm, 0.5 mg/ml). Isopropanol was used as a spreading solvent to help particles disperse at the interface. Then, 1-ocatanol was added into gellan gum solution, followed by injection of the particle suspension into the water/1-octanol interface. Upon cooling to room temperature around 25°C, the gellan gum solution began to gel and was kept for another 30 min. 1-Octanol, existing on the top layer, was then carefully removed. PDMS (SYLGARD 184 Silicone Elastomer, Dow Corning) was mixed in a ratio of 10:1 and degassed before being layered over the gelled gellan gum. After PDMS was cured for 36 to 48 hours at room temperature, it was peeled off (with particles adhered to the PDMS surface) and rinsed with hot water (60° to 70°C) to wash away gellan gum residue. We coated particle-embedded PDMS with ~9 nm of osmium for SEM characterization. ROI (Region of Interest) manager in ImageJ was used to analyze the SEM images and measure how deeply particles were embedded in PDMS for the contact angle calculation. For each type of particle, we measured 10 particles to obtain an average value and standard deviation.

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