pH‐Controlled Reversible Assembly and Disassembly of Gold Nanorods

Abstract

Gold nanorods exhibit rich surface-plasmon-resonance (SPR)derived properties, which have made discrete nanorods useful for many interesting applications such as optical data storage, [1] submicrometer metallic barcodes, [2] sensing, [3] biological imaging, [4] and controlled gene delivery. [5] Future scientific and technological applications of Au nanorods require the capability to assemble into complex one-, two-, or even three-dimensional (3D) functional architectures. The assembly of Au nanorods also allows for the utilization of their collective properties that result from the coupling of the optical and electronic properties between neighboring individual nanorods. Several approaches have been developed for the assembly of Au nanorods in either end-to-end (EE) or side-by-side (SS) orientations. They include i) assembly through electrostatic interactions, hydrogen bonding, or covalent bonding, [6] ii) antibody/antigen and streptavidin/biotin biorecognitions, [7] iii) use of carbon nanotubes and silica nanofibers as templates, [8] and iv) interactions between functionalized polymers in selective solvents. [9] Au nanorods assembled by these approaches are generally difficult to disassemble. Even though significant progress has been made in the organization of nanomaterials, reversible assembly and disassembly of Au nanorods in either EE or SS orientations has remained a big challenge. So far, reversible aggregation of spherical Au nanoparticles has been demonstrated by functionalizing them with thiol-modified DNA oligomers. [10] Here, we report on a robust strategy for the reversible assembly and disassembly of Au nanorods in both EE and SS fashion. Thiol-containing bifunctional molecules are selectively bound to the end or side surface of individual Au nanorods. The bound molecules induce the assembly of Au nanorods if the pH of the nanorod solution is adjusted within an optimal range. Outside the optimal pH range, Au nanorods are disassembled. This pH-controlled assembly and disassembly is reversible and can be repeated many times. Moreover, the distances between assembled nanorods are estimated to vary from 0.080 to 1.8 nm for different assembling molecules and assembly orientations.As-prepared Au nanorods are stabilized in 0.1 M aqueous cetyltrimethylammonium bromide (CTAB) solutions with pH ¼ 3.5. Their ensemble transverse and longitudinal plasmon wavelengths are 515 and 780 nm, respectively. The ensemble extinction values at the two plasmon peaks are 1.0 and 3.7, respectively, suggesting a high yield of Au nanorods. The nanorod concentration is estimated to be %0.8 nM according to previously determined molar extinction coefficients. [11] The average length, diameter, and aspect ratio, determined from %500 nanorods on transmission electron microscopy (TEM) images, are 38 AE 4 nm, 11 AE 1 nm, and 3.6 AE 0.5 nm, respectively. The used bifunctional molecules include 3-mercaptopropionic acid (MPA), 11-mercaptoundecanoic acid (MUA), glutathione (GSH), and cysteine (CYS). Their concentrations in nanorod so...

Colorimetric assay of lead using unmodified gold nanorods

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