Gold nanoparticle surface functionalization

One milliliter of the 50-nm gold nanoparticle stock solution (Nanopartz) was mixed with 100 μl of 20 mM 11-MUA (Sigma-Aldrich) in ethanol. The mixture was sonicated at 55°C for 90 min and kept at room temperature overnight. These nanoparticles were centrifuged at 5500 rcf (relative centrifugal force) for 10 min and redispersed in 1 ml deionized water to remove excess 11-MUA. Then, 100 μl of 10 μM XPA (Novus Biologicals) was mixed with 10 μl of 1 mM EDC, 1 mM NHS, and 100 μl of the above functionalized nanoparticle solution at room temperature for 2 hours. To trap a single nanoparticle in the photonic crystal cavity, nanoparticles were diluted from 3.9 × 10⁷ particles/μl (stock solution) in heavy water to about 500 particles/μl. The nanoparticles were flowed into the microfluidic channel with a flow rate of 1 μl/min. When a single gold nanoparticle was trapped, we observed a discrete resonance shift of 440 pm, and the Q-factor dropped from 1.0 × 10⁵ to 8.2 × 10³ (Fig. 1F).