End-to-end and side-by-side assemblies of gold nanorods induced by dithiol poly(ethylene glycol)

Abstract

The assemblies of gold nanorods (Au NRs) exhibit unique properties distinct from the isolated Au NR. We report an effective and simple method for the end-to-end (E-E) and side-by-side (S-S) assemblies of Au NRs with a molecularly defined nanogap (1–2 nm) only in the presence of dithiol poly(ethylene glycol) (HS-PEG-SH). The assembled methods need neither the pH value adjustments nor the addition of other organic solvent. With increasing amount of dithiol molecules, assembled modes of Au NRs experience an interesting procedure, changing from E-E to S-S orientation. The experimental results indicate that when the concentration of HS-PEG-SH is less than 0.25 μM, electrostatic repulsion of positive-charged CTA+ is stronger than the affinity of the Au-S binding, resulting in the E-E oriented assembly. Otherwise, the S-S oriented mode is predominated. The current assembled method will be potentially useful for the optoelectronics and biomedical engineering.

Interactions and Attachment Pathways between Functionalized Gold Nanorods

“…Bonding between the thiol moiety and Au is more prominent because of the strong Au−S covalent bond formation (∼209 kJ/mol or 84.5 k B T), 61,62 which has been widely used in NR functionalization. 15,36,42 Au-bonded cysteamine linkers can connect two adjacent Au NRs via hydrogen bonding (N−H•••N), as illustrated in Figure 1A. We found that depending on the linker concentration in solution, NRs assemble through two different attachment modes: (1) endto-end attachment (Figure 1B) at low (150 μM) and (2) sideto-side (Figure 1C) attachment at high (500 μM) cysteamine concentrations.…”

“…At low linker concentrations, the CTA + ions that have less affinity for the {111} facets at the NR ends 63,64 are easily displaced by thiol. 42 This results in NRs with cysteamine-coated ends. The formation of hydrogen bonds between the cysteamine molecules of two NRs then leads to their end-to-end attachment.…”

“…41 Therefore, the surfactant packing density on the sides of NRs is much higher than at its ends. 36,42 The assembly of NRs can be induced by introducing linker molecules with an affinity for Au into the system. 20,43,44 These linker molecules preferentially absorb onto the ends of the NRs because the ends are sparsely covered by surfactants.…”

“…15,36,38,45 End-to-end and side-to-side NR assemblies can be induced by tuning the concentration of the linker molecules. 42 However, the exact mechanism through which linkers mediate these end-to-end and side-to-side NR assemblies is unclear.…”

Directional self-assembly of gold nanorods into 1D and 2D arrays by quadruple hydrogen bonding

“…We speculate that these two assembly modes were closely related to the affinity of the Au-S binding and the repulsion between the cationic CTAB molecules on the side. 33 Crosslinker 1 has two functional groups, a thiol group and a 2-ureido-4(1H)-pyrimidinone moiety, and both can form bonds with Au. Bonding between the thiol moiety and Au is more favored because of the strong Au-S covalent bond formation.…”