Nano Focus: Plasmonic gold nanotubes serve as sensitive refractive-index reporters when suspended in solution

Of the many potential applications of plasmonic metallic nanostructures, their use as refractive-index reporters has proven particularly successful. This behavior can be optimized when resonances occur in the near-infrared region, and through the use of hollow nanostructures, which experience reduced plasmonic damping and exhibit stronger surface plasmon resonance (SPR) signals than their solid counterparts. Many applications also demand that such structures can be homogeneously suspended in solution.

As reported in the March 27 issue of Chemistry of Materials (DOI: 10.1021/cm203184d; p. 963), C. Bridges, P. DiCarmine, and D. Seferos of the University of Toronto in Canada describe the first-known successful synthesis of solution-suspendable gold nanotubes. The synthesis was accomplished by deposition of metals at the base of the template followed by electro-polymerization of a sacrificial hydrophobic polymer rod. Following collapse of the polymer core due to hydrophobic effects, a gold shell was deposited, and all sacrificial materials were then removed to yield hollow gold nanotubes. The synthesized nanotubes were ≈ 260 nm in length, ≈ 55 nm wide, and ≈ 15 nm thick, where the dimensions could be tailored according to the pore size and deposition time.

Extinction spectra of gold nanotubes that are 55 ± 7 nm in diameter and aligned in an AAO template (a). Cartoon depiction (b) and SEM image of aligned nanotubes (c). Extinction spectra of gold nanotubes suspended in D₂O (length = 258 ± 42 nm; width = 55 ± 7 nm) (d), accompanied by their TEM (e, f) images. Scale bars are 100 nm. Reproduced with permission from Chem. Mater. 24 (6) (2012), DOI: 10.1021/cm203184d; p. 963. ©2012 American Chemical Society.

A series of spectroscopic experiments were performed to determine the SPR behavior of the nanotubes. Investigation of the nanotubes in water, in situ in the template showed a single peak corresponding to the transverse SPR at 553 nm, as compared with 574 nm for a similarly sized gold nanorod. Alternatively, suspension of the nanotubes in D₂O resulted in transverse and longitudinal SPR modes at 527 nm and 1210 nm, respectively, together with a weak quadrupolar mode at ~700 nm. In a final experiment, glycerol was added incrementally to a colloidal suspension of the nanotubes, and it was observed that the sensitivities of the longitudinal and transverse SPRs were approximately double those of solid nanorods.

The Seferos group said this discovery may open the door to further development and study of hollow nanoparticles, particularly in applications that require a high sensitivity to subtle changes in refractive index, such as sensing and imaging. Also of potential interest are the photothermal heating properties of these hollow nanostructures.