Chemical Interface Damping as an Indicator for Hexadecyltrimethylammonium Bromide Replacement by Short-Chain Thiols on Gold Nanorods

Dániel P Szekrényes,Dávid Kovács,Zsolt Zolnai,András Deák

doi:10.1021/acs.jpcc.0c04629

Dániel P Szekrényes, Dávid Kovács + Show 2 more

Open Access

https://doi.org/10.1021/acs.jpcc.0c04629

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Abstract

The binding of thiols on hexadecyltrimethylammonium bromide (CTAB)-capped gold nanorods is investigated both at the ensemble and individual particle level using two similar sized but oppositely charged small molecules, namely, cysteamine and mercaptopropionic acid (MPA). Changes in the width and position of the localized surface plasmon resonance of individual gold nanorods are used to elucidate differences between the accumulation of these two short-chain thiols at the particles’ surface. It is shown that the interplay between the charged nature of the thiol molecules and the concentration of CTAB in the bulk phase determines to which extent the thiols bind to the particles. On the basis of the changes in the resonance widths, the binding of the negatively charged MPA is reduced compared to that of the positively charged cysteamine, especially at higher CTAB bulk concentrations. This is interpreted as the result of the interaction between the small molecule thiols and CTAB: while cysteamine is effectively replacing CTAB, the interaction of readily bound MPA and CTAB from the bulk results in a self-limiting process.

Highlights

Wet-chemical surface modification of nanoparticles is of central importance for practical applications, and for fundamental studies
The two molecules cysteamine and mercaptopropionic acid (MPA) investigated in the present study both clearly bind to the gold particles as shown by the increasing damping
There is a clear difference between the two ligands that can be interpreted by the different interaction of the thiol and the native CTAB layer of the rods

Summary

■ INTRODUCTION

Wet-chemical surface modification of nanoparticles is of central importance for practical applications, and for fundamental studies. A Lorentz-function fitted to the individual particles’ scattering spectrum provides the resonance energy and damping of the model dipole-oscillator These parameters are determined by factors that have an impact on the frequency or decay mechanism of the localized plasmon oscillation.[6] Recently, a new aspect of the thiol molecule-related gold nanoparticle surface modification came into focus by analyzing the scattering spectrum of individual gold nanorods. It is argued that it correlates with the chemical enhancement in the surface-enhanced Raman scattering process.[9,10] The increasing CID-associated peak broadening has been readily employed to indicate thiol molecule accumulation on gold nanoparticles, with the priority to clarify the origin and mechanism of the CID process.[7,8,11−13] These studies deal with nanoparticles where the original capping layer (typically CTAB) has been previously removed and the thiols are adsorbed at the cleaned particle interface. SEM images were obtained using a Zeiss LEO field-emission scanning electron microscope operated at 5 keV acceleration voltage

■ RESULTS AND DISCUSSION

Conclusions

■ REFERENCES