Gold nanocages of <40 nm in dimension have been synthesized using the galvanic replacement reaction between Ag nanocubes and HAuCl4 in an aqueous solution. By controlling the molar ratio between Ag and HAuCl4, the gold nanocages could be tuned to display surface plasmon resonance peaks around 800 nm, a wavelength commonly used in optical coherence tomography (OCT) imaging. OCT measurements on phantom samples indicate that these gold nanocages have a moderate scattering cross-section of approximately 8.10 x 10(-16) m2 but a very large absorption cross-section of approximately 7.26 x 10(-15) m2, suggesting their potential use as a new class of contrast agents for optical imaging. When bioconjugated with antibodies, the gold nanocages have also been demonstrated for specific targeting of breast cancer cells.
The galvanic replacement reaction between a Ag template and HAuCl4 in an aqueous solution transforms 30–200 nm Ag nanocubes into Au nanoboxes and nanocages (nanoboxes with porous walls). By controlling the molar ratio of Ag to HAuCl4, the extinction peak of resultant structures can be continuously tuned from the blue (400 nm) to the near‐infrared (1200 nm) region of the electromagnetic spectrum. These hollow Au nanostructures are characterized by extraordinarily large cross‐sections for both absorption and scattering. Optical coherence tomography measurements indicate that the 36 nm nanocage has a scattering cross‐section of ∼ 0.8 × 10–15 m2 and an absorption cross‐section of ∼ 7.3 × 10–15 m2. The absorption cross‐section is more than five orders of magnitude larger than those of conventional organic dyes. Exposure of Au nanocages to a camera flash resulted in the melting and conversion of Au nanocages into spherical particles due to photothermal heating. Discrete‐dipole‐approximation calculations suggest that the magnitudes of both scattering and absorption cross‐sections of Au nanocages can be tailored by controlling their dimensions, as well as the thickness and porosity of their walls. This novel class of hollow nanostructures is expected to find use as both a contrast agent for optical imaging in early stage tumor detection and as a therapeutic agent for photothermal cancer treatment.
A New Class of Self-Assembled Monolayers: closo-B 12 H 11 S 3− on Gold.-closo-B 12 H 11 S 3− Monolayers form spontaneously when Au substrates are exposed to solutions of (I). The adsorption process is similar to that of organothiol self-assembly on Au; absorption occurs via the S atom and is accompanied by cleavage of the S-H bond and the formation of a strong polar covalent S-Au bond. The monolayers are characterized by surface-enhanced Raman spectroscopy using 735 nm excitation. -(YEAGER, L. J.; SAEKI, F.; SHELLY, K.; HAWTHORNE, M. F.; GARRELL, R. L.; J. Am. Chem. Soc. 120 (1998) 38, 9961-9962;
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