# Supramolecular Effects of Alkyl Sulfonates in Silver Nanocrystal Synthesis

## TL;DR

This study shows how adding long-chain alkyl sulfonates affects the color and structure of silver nanocrystals during and after synthesis.

## Contribution

The paper reveals that long-chain alkyl sulfonates influence nanocrystal absorbance through surface interactions, not micelle formation.

## Key findings

- Short-chain sulfonates have minimal impact on nanocrystal synthesis.
- Long-chain sulfonates cause a blue-shift in absorbance at low concentrations and a red-shift at higher concentrations.
- Surface interactions, not micelles, drive the observed changes in nanocrystal properties.

## Abstract

While cationic surfactants such as hexadecyltrimethylammonium
bromide
(CTAB) are ubiquitous in the synthesis of noble metal nanocrystals,
anionic surfactants are rarely used. This work explores the addition
of sodium alkyl sulfonates with chain lengths ranging from one to
eight carbons to a silver nanoplatelet reaction. Short-chain sulfonates
comprised of one to four carbons show little effect on the nanocrystal
synthesis, but alkyl sulfonates comprised of five or more carbons
at concentrations above 1 mM have a pronounced effect on the absorbance
of the nanocrystals, causing a blue-shift in the wavelength of maximum
absorbance (λmax) from approximately 800 to 400 nm
as the sulfonate concentration is increased to 7 mM. Higher concentrations
of sulfonate result in a subsequent red-shift of the peak. Investigation
into the possible formation mechanisms responsible for this synthetic
control revealed the absence of sulfonate micelles under the reaction
conditions. Instead, we hypothesize that sulfonate bilayers are nucleated
around the silver nanocrystals at concentrations below the critical
micelle concentration and interact with either the citrate ligands
or the silver surface to influence nanocrystal morphology, and thus
absorbance. Strikingly, the addition of long-chain alkyl sulfonates
to already-synthesized nanocrystals results in similar changes to
the nanocrystal absorbance that occur within seconds, providing further
support for the proposal that these effects are related to the surface
chemistry of the nanocrystals, which appear to be highly dynamic.