Importance of Electrostatic Forces in Supracolloidal Self-Assembly of Polymer-Functionalized Gold Nanorods

Abstract

Devices and applications that exploit nanoparticle (NP) coupling interactions require the controlled arrangement of primary NPs into defined supracolloidal assemblies. Solution self-assembly of polymer-coated NPs is a scalable approach; however, imparting assembly directionality is still challenging. Current strategies for achieving such directionality include surface encoding of distinct NP sites with polymer ligands, which adds significantly to the complexity of the approach. Moreover, at present, a mechanistic understanding of the self-assembly behavior of polymer-coated NPs is limited due to the hurdle of locally resolved, quantitative characterization under the assembly condition. Herein, we reveal that polystyrene-coated gold nanorods can undergo directional tip-to-tip self-assembly, even when they are uniformly coated with a polymer ligand layer. Our results suggest that directionality in the self-assembly arises from the intrinsic surface charge of the nanorods. These findings open up a straightforward and generic way for achieving directional assembly of anisotropic NPs for future materials concepts.

Nanoparticle Size Influences the Self-Assembly of Gold Nanorods Using Flexible Streptavidin–Biotin Linkages

“…79 These stacks mimic a top-down approach to selfassembly 13,18,19,21,80−83 using the bottom layer of NRs as the adherent surface to build up the more linkages as gravitational forces bring them down to the bottom 78 and electrostatic forces generate an amplified electric field for in-solution streptavidin to localize and bind to the ends of the NRs for self-assembly of longer chains to occur. 11,84,85 Kinetics of the self-assembly process at short times was monitored by extinction spectroscopy (Figure 8a). Intuitively, one may expect that the larger the monomer, the slower the assembly kinetics, just based on diffusion coefficients, per what one may expect for a linear condensation polymer.…”

“…Eckert et al showed the effect of gravitational fields on bulk phase behavior of NRs to cause NRs to layer on top of each other as sedimentation occurs to form sedimentation stacks with the ends most likely vertically . These stacks mimic a top-down approach to self-assembly ,,,,− using the bottom layer of NRs as the adherent surface to build up the more linkages as gravitational forces bring them down to the bottom and electrostatic forces generate an amplified electric field for in-solution streptavidin to localize and bind to the ends of the NRs for self-assembly of longer chains to occur. ,, …”

“…Gold nanorods (AuNRs) are appealing materials for optical sensors due to their plasmonic properties, which can be tuned by their size, shape, and surface functionalization . AuNRs are well-known to exhibit both transverse and longitudinal plasmon bands, where the difference in the extinction cross sections along the two axes leads to substantially stronger electromagnetic field enhancements at the ends of the NRs upon appropriate illumination. − When two or more NRs are brought into close proximity and their ends align, they create interparticle junctions with amplified electric field strengths between the two ends, known as second-generation hot spots. , These hot spots result from the coupling of the individual NRs’ surface plasmons and combine distinct spatial and angular photoexcitation distributions − that intensify electromagnetic fields. ,, The resultant amplification at the second-generation hot spots has been utilized for signal enhancement of analytes as the analytes interact with these nanogaps. ,,, This amplification process holds great efficacy in nanoengineering of devices for sensing, imaging, waveguides, and catalysis, among others . The increased intensity of the electromagnetic fields allows for lower limits of detection for fluorescence, luminescence, or other techniques that utilize plasmonic enhancement …”

“…1,3 These hot spots result from the coupling of the individual NRs' surface plasmons 6 and combine distinct spatial and angular photoexcitation distributions 7−9 that intensify electromagnetic fields. 5,10,11 The resultant amplification at the secondgeneration hot spots has been utilized for signal enhancement of analytes as the analytes interact with these nanogaps. 1,5,12,13 This amplification process holds great efficacy in nanoengineering of devices for sensing, imaging, waveguides, and catalysis, among others.…”

Kinetically Controlled Site-Specific Self-assembly of Hairy Colloids

“…This resulted in a grafting density of 0.11 chains nm −2 as quantified for PS50@AuNR using energy filtered transmission electron microscopy (EFTEM) measurements (as shown in Figure 2 and Figure S4). 23 These three distinct hybrid nanoparticles were used as building blocks to obtain tipto-tip assembled supracolloidal structures in solution by reducing the solvent quality for PS brushes in a mixture of 80% DMF-20% water (see Methods). TEM measurements were performed after drop-casting from DMF (a good solvent for polystyrene) 37 and from the DMF/water mixture (Figure 3).…”