Ligand exchange is an important reaction pathway in synthetic chemistry, enabling – for example – the post-production modification of nanoparticles. However, characterization of the degree of “exchange” is challenging, especially for non-soluble particles such as cellulose nanocrystals (CNCs). For example, elemental analysis is accurate but is costly, requires careful sample preparation and cannot be performed in-flow. Similarly, spectroscopies like FTIR typically require at least 5 wt.% of the moiety of interest to be present in the sample in order to acquire a useful signal and are often not quantitative. Here, we investigate whether cluster-triggered emission (CTE) luminescence – previously used to characterize CNCs (Johns et al., Nanoscale, in press) and cetyltrimethylammonium (CTA) surfactants (Tang et al., Angew. Chem. Int. Ed., 2022, 61, e202117368) – can be used as a new spectroscopic method to rapidly determine the degree of ligand exchange without the need for laborious additional sample preparation steps. Modifying CNCs with surfactants through counterion exchange is a straightforward method to increase the hydrophobicity of nanocelluloses (Salajková et al., J. Mater. Chem., 2012, 22:19798; Abitbol et al., Nord. Pulp. Paper Res., 2014, 29:46; Nagalakshmaiah et al., ACS Appl. Mater. Interfaces, 2016, 8:8755). Herein, we have exchanged the counterion (H+) of the sulfate-half ester group of CNCs for the amphiphilic CTA cation as a model ligand exchange reaction. The nitrogen group present in CTA results in new fluorescent spectral bands due to intermolecular interactions involving the free-electron pairs, which increase in intensity as the degree of counterion exchange increases (Figure 1). Ongoing research is investigating whether other luminescent techniques – phosphorescence, luminescent lifetime – can be utilised to track the modification reaction, and whether the choice of solvent in which CNCs are suspended has an impact. This will enable us to confirm whether CTE luminescence is suitable for characterizing ligand exchange reactions with nanocelluloses.

Tracking ligand exchange in surfactant-modified cellulose nanocrystals via autoluminescence

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