Synthesis and functionalization of plasmonic nanoparticles
This protocol is extracted from research article:
Nano-bio-computing lipid nanotablet
Sci Adv, Feb 22, 2019; DOI: 10.1126/sciadv.aau2124

Gold nanorods with silver shells, gold nanospheres, and silver nanospheres on gold seeds that exhibit red, green, and blue scattering signals were synthesized and referred to as red, green, and blue nanoparticles, respectively. For the preparation of red nanoparticles, gold nanorods with an aspect ratio of 4 were first synthesized according to previous methods based on a seed-mediated growth mechanism (36). Silver shells (5 nm thick) were formed around the gold nanorods by incubating the gold nanorod solution (1 ml, 100 nM) with cetyltrimethylammonium chloride solution (1 ml, 10 mM), AgNO3 (1 ml, 0.2 mM), and l-ascorbic acid (1 ml, 50 mM) for 4 hours. The resulting red nanoparticles (diameter, ~22 nm; length, ~56 nm) were washed three times by centrifugation, supernatant removal, and redispersion in DI water. Spherical gold nanoparticles (diameter, ~50 nm) were purchased from BBI Solutions (Cardiff, UK) and used as green nanoparticles. Blue nanoparticles (diameter, ~54 nm) were prepared by growing 17-nm-thick silver shells on 20-nm spherical gold nanoparticle seeds. To form silver shells on the seeds, sodium ascorbate solution (100 μl, 50 mM) was rapidly injected into the mixture containing 150 pM of 20-nm gold seeds, 0.2% polyvinylpyrrolidone (PVP), and 0.24 mM AgNO3. The nanoparticles were characterized by transmission electron microscopy (TEM; JEM-2100, JEOL Ltd., Japan), UV-Vis spectrophotometry (Agilent 8453, Agilent Technologies, USA), DFM (Axiovert 200 M, Carl Zeiss, Göttingen, Germany), and field-emission scanning electron microscopy (FE-SEM; JSM-7600F, JEOL Ltd., Japan). Correlative DFM-SEM imaging was performed to analyze single-particle scattering signals from the three nanoparticles: Nanoparticles loaded on a Cr-patterned glass substrate were first imaged by DFM, treated with Pt coating (Cressington 108auto, Cressington Scientific Instruments Ltd., UK), and imaged in the same position by FE-SEM. The characterization data are summarized in fig. S2. TEM and FE-SEM imaging were carried out at the National Center for Inter-University Research Facilities and at the Research Institute of Advanced Materials (both at Seoul National University, Seoul, South Korea), respectively.

Thiolated DNA oligonucleotides (Bioneer, Daejeon, Korea) were treated with 100 mM dithiothreitol in 100 mM phosphate buffer (PB) (pH 8.0) for 1 hour. Afterward, the oligonucleotides were purified via size-exclusion chromatography with a NAP-5 column (GE Healthcare, Buckinghamshire, UK). Nanoparticles (final concentration of 15 pM) were incubated with 216 nM (for blue nanoparticles) or 288 nM (for red and green nanoparticles) thiolated oligonucleotides for 1 hour at 25°C. The ratios of biotinylated DNA linkers to total surface DNA ligands were 0.5, 0.5, 0.5, 35, 35, and 50% for R-NF, G-NF, B-NF, R-NR, G-NR, and B-NR, respectively. The solution was then adjusted to 0.1% (w/v) PVP in 10 mM PB for red nanoparticles, to 0.1% (w/v) SDS in 10 mM PB for green nanoparticles, and to 10 mM PB for blue nanoparticles. Three aliquots of 1 M NaCl, 0.1% SDS, and 10 mM PB salt solution were added with a 1-hour interval to achieve a final concentration of 0.3 M NaCl. After each salt addition, the mixture was heated at 50°C for 10 min and incubated at 25°C for 50 min. Two hours after reaching the final concentration, the red nanoparticle solution was centrifuge-washed and redispersed in 1× PBS. Other nanoparticle solutions were incubated for 12 hours, centrifuge-washed, and redispersed in DI water (green nanoparticles) or in 1× PBS (blue nanoparticles).

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