In Situ Tracking of Colloidally Stable and Ordered Assemblies of Gold Nanorods

Dorota Grzelak,Petr Král,Sanoj Raj,Wiktor Lewandowski,Christopher Tollan,Piotr Szustakiewicz,Luis M Liz-Marzán

doi:10.1021/jacs.0c06446

Abstract

Solution-phase self-assembly of anisotropic nanoparticles into complex 2D and 3D assemblies is one of the most promising strategies toward obtaining nanoparticle-based materials and devices with unique optical properties at the macroscale. However, controlling this process with single-particle precision is highly demanding, mostly due to insufficient understanding of the self-assembly process at the nanoscale. We report the use of in situ environmental scanning transmission electron microscopy (WetSTEM), combined with UV/vis spectroscopy, small-angle X-ray diffraction (SAXRD) and multiscale modeling, to draw a detailed picture of the dynamics of vertically aligned assemblies of gold nanorods. Detailed understanding of the self-assembly/disassembly mechanisms is obtained from real-time observations, which provide direct evidence of the colloidal stability of side-to-side nanorod clusters. Structural details and the forces governing the disassembly process are revealed with single particle resolution as well as in bulk samples, by combined experimental and theoretical modeling. In particular, this study provides unique information on the evolution of the orientational order of nanorods within side-to-side 2D assemblies and shows that both electrostatic (at the nanoscale) and thermal (in bulk) stimuli can be used to drive the process. These results not only give insight into the interactions between nanorods and the stability of their assemblies, thereby assisting the design of ordered, anisotropic nanomaterials but also broaden the available toolbox for in situ tracking of nanoparticle behavior at the single-particle level.

Highlights

Ordered assemblies of gold nanorods (AuNRs) offer extraordinary properties with potential applications in various technologies, such as chemical and biological sensing,[1−3] in vivo medical studies,[4,5] catalysis,[6−8] data storage,[9] and optoelectronics.[10]
We demonstrate the use of environmental scanning transmission electron microscopy (WetSTEM28) to characterize in situ the dynamics of relatively large, 2D, vertical assemblies of AuNRs in the wet state
The modeled spectra reveal a gradual blue-shift and broadening of the localized surface plasmon resonance (LSPR) band for a growing number of particles in the cluster. This is in agreement with our experimental results (Figure 1d) and with previous examples in the literature,[36] so we can conclude that the observed changes in absorbance spectra for AuNR@MUDOL result from the gradual aggregation of nanorods into parallel clusters, in the aqueous phase

Summary

■ INTRODUCTION

Ordered assemblies of gold nanorods (AuNRs) offer extraordinary properties with potential applications in various technologies, such as chemical and biological sensing,[1−3] in vivo medical studies,[4,5] catalysis,[6−8] data storage,[9] and optoelectronics.[10]. We consistently observed fluctuations of the nanoparticles within the solvent−trembling, twisting, assembling, and disassembling− often similar to previous reports based on the in situ TEM method.[27] Third, we could monitor the displacement of individual nanorods (see schematic drawings, still images from WetSTEM recordings, and Movie S1 in the Supporting Information), in particular those which were located at the edges of vertical aggregates These observations correlate well with the bulk-scale observations of thermally driven disassembly for AuNR@MUDOL aggregates and confirm that we can analyze the process with single-particle precision. It is intriguing to think that, in the context of the experimentally confirmed reversibility of the disassembly process (Figure 2), reversing time in our recordings would enable us to get insight into the assembly process−assembly of single particles as well as clusters of ordered AuNRs (Supporting Information, Movies S11−S12), which is similar to the pre- and postattachment alignment pathways[27] previously reported in the literature for side-to-side assemblies of AuNRs

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES