Tuning the Surface Plasmon Resonance of Gold Dumbbell Nanorods

Abstract

Gold has always fascinated humans, occupying an important functional and symbolic role in civilization. In earlier times, gold was predominantly used in jewelry; today, this noble metal's surface properties are taken advantage of in catalysis and plasmonics. In this article, the plasmon resonance of gold dumbbell nanorods is investigated. This unusual morphology was obtained by a seed-mediated growth method. The concentration of chemical precursors such as cetyltrimethylammonium bromide and silver nitrate plays a significant role in controlling the shape of the nanorods. Indeed, the aspect ratio of dumbbell nanostructures was varied from 2.6 to 4. UV-visible absorption spectra revealed a shift of the longitudinal surface plasmon resonance peak from 669 to 789 nm. Having the plasmon resonance in the near infrared region helps to use those nanostructures as photothermal agents.

Figure 1

Effect of the precursors on the dumbbell growth. (a) Normalized absorption spectra of GNRs at different concentrations of CTAB (the volume concentration of CTAB at 3.0, 3.5, 4.0, and 4.5 mL corresponds to the molar concentration of 0.08902, 0.09044, 0.0915, and 0.0924 M, respectively). (b) LSPR peak position as a function of CTAB. (c) Normalized UV–vis absorption spectra with the variation of AgNO₃ (the volume concentration of AgNO₃ at 10, 20, 40, and 60 μL corresponds to the molar concentration of 1.9 × 10^–5, 3.7 × 10^–5, 7.4 × 10^–5, and 1.1 × 10^–4 M, respectively). (d) Shift of the LSPR peak to longer wavelengths as an effect of AgNO_3. The dashed line in (a,c) corresponds to the gold plasmon resonance at 525 nm. The first biological window (I-BW) is also marked for reference.

Figure 2

Effect of concentration of (a) CTAB and (b) AgNO₃ on the aspect ratio of dumbbell-shaped particles. Aspect ratio increases linearly with CTAB and AgNO₃.

Figure 3

(a) TEM image showing the population of nanorods and nanospheres within the colloidal solution. (b) UV–visible spectra of the nanorods and nanospheres obtained after centrifugation. The black curve displays a majority of nanospheres within the colloidal solution, while the blue curve shows more nanorods within the colloidal solution. The colloidal solution was centrifuged in an attempt to separate the two populations present in the colloidal solution.

Figure 4

(a) TEM image of nanorods showing the dumbbell-shaped morphology having a length of approximately 40 nm and a width of ∼15 nm. (b) Diffraction pattern of dumbbell nanorods showing different planes.

Figure 5

Effect of surfactants on the growth of GNDs. (1) In the absence of surfactants, the growth favors the isotropic direction {111} resulting in nanospheres. (2) In the presence of CTAB, anisotropic growth occurs through the energetic facet {100}. (3) In the presence of AgNO₃, it favors {110} in the longitudinal direction.

Figure 6

Typical DLS spectra showing two peaks corresponding to the diameter of the rod and the length of the rod.

Figure 7

Effect of variation of (a) CTAB and (b) AgNO₃ over the length of nanorods. Schematic showing the consequence of change in the (c) length and (d) diameter of dumbbells on the surface-to-volume ratio.

Figure 8

Schematic of a six-step synthesis leading to the growth of GNDs. In steps (1–3), we prepared tiny gold seeds (light brown color) by reducing gold salts by CTAB in the presence of NaBH₄. In step 4a, we added CTAB in the stock solution of gold salts and AgNO₃ to study the effect of CTAB concentration on the formation of rods. In step 4b, we studied the effect of silver nitrate, keeping the gold salts and CTAB constant using seeds from the first stage. In step 5, ascorbic acid was dropped into the solution obtained from steps 4a and 4b to obtain the final product as dumbbell-shaped nanoparticles (purple/violet) (step 6).