The zwitterion, N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic
acid (HEPES), facilitates the
formation and stability of gold nanostars; however, little is known
about how this molecule interacts with the metal postsynthesis. Herein,
restructuring of gold nanostar morphology is induced upon acidification,
an effect that depends on both pH and acid composition as well as
on the protonation state of HEPES. Changes in molecular protonation
are measured using zeta potential and modeled using DFT. The surface-sensitive
technique, surface-enhanced Raman scattering (SERS), reveals that
pH variations induce reversible activation of the amine and sulfonate
groups in HEPES and that electron redistribution weakens its affinity
to the metal thus promoting the adsorption and SERS detection of benzene.
By selecting a molecule that does not induce significant desorption
of the stabilizing agent, binding energies of benzene to gold are
measured even though only weak London dispersion and π–π
interactions promote adsorption. All in all, this molecular-level
insight is expected to facilitate new applications of these nanostructures
in ways that have not been possible to date.
Unwanted nanoparticle aggregation and/or agglomeration may occur when anisotropic nanoparticles are dispersed in various solvents and matrices. While extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory has been successfully applied to predict nanoparticle stability in solution, this model fails to accurately predict the physical stability of anisotropic nanostructures; thus limiting its applicability in practice. Herein, DLVO theory was used to accurately predict gold nanostar stability in solution by investigating how the choice of the nanostar dimension considered in calculations influences the calculated attractive and repulsive interactions between nanostructures. The use of the average radius of curvature of the nanostar tips instead of the average radius as the nanostar dimension of interest increases the accuracy with which experimentally observed nanoparticle behavior can be modeled theoretically. This prediction was validated by measuring time-dependent localized surface plasmon resonance (LSPR) spectra of gold nanostars suspended in solutions with different ionic strengths. Minimum energy barriers calculated from collision theory as a function of nanoparticle concentration were utilized to make kinetic predictions. All in all, these studies suggest that choosing the appropriate gold nanostar dimension is crucial to fully understanding and accurately predicting the stability of anisotropic nanostructures such as gold nanostars; i.e., whether the nanostructures remain stable and can be used reproducibly, or whether they aggregate and exhibit inconsistent results. Thus, the present work provides a deeper understanding of internanoparticle interactions in solution and is expected to lead to more consistent and efficient analytical and bioanalytical applications of these important materials in the future. Graphical abstract ᅟ.
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