Reductive Amination Reaction for the Functionalization of Cellulose Nanocrystals

Abstract

Cellulose nanocrystals (CNCs) represent intriguing biopolymeric nanocrystalline materials, that are biocompatible, sustainable and renewable, can be chemically functionalized and are endowed with exceptional mechanical properties. Recently, studies have been performed to prepare CNCs with extraordinary photophysical properties, also by means of their functionalization with organic light-emitting fluorophores. In this paper, we used the reductive amination reaction to chemically bind 4-(1-pyrenyl)butanamine selectively to the reducing termini of sulfated or neutral CNCs (S_CNC and N_CNC) obtained from sulfuric acid or hydrochloric acid hydrolysis. The functionalization reaction is simple and straightforward, and it induces the appearance of the typical pyrene emission profile in the functionalized materials. After a characterization of the new materials performed by ATR-FTIR and fluorescence spectroscopies, we demonstrate luminescence quenching of the decorated N_CNC by copper (II) sulfate, hypothesizing for these new functionalized materials an application in water purification technologies.

Tailoring the Chemical Structure of Cellulose Nanocrystals by Amine Functionalization

“…The choice of DMSO as solvent not only supported the effective dispersion of cellulose nanocrystals, but also the formation of pyrene excimers, necessary to appreciate emission from this system in the visible region (∼450 nm, blue emission). In our previous work, [24] we had used a pyrenyl derivative with a pending aliphatic linker, that displayed a very weak capacity to generate excimers. In this work, we use pyrene‐1‐amine, which displays a strong tendency to form π‐stacked aggregates.…”

“…This procedure allows to isolate nanocrystals mainly consisting of cellulose I β crystalline phase with an average length of ~150 nm. [12,24] These nanocrystals were used as substrates for the following functionalization steps. N_CNCs surface modification with aliphatic amine functionalities was achieved by applying the synthetic sequence summarized in the Scheme 1.…”

“…The new chemical functionalities are introduced both on the surface of the nanocrystals, applying a synthetic sequence interesting the transformation of the C 6 carbon of the glucopyranose ring, and on the reducing termini of cellulose nanocrystals, by the reductive amination reaction applied in the presence of sodium cyanoborohydride. [24] Indeed, we consider surface amination a good approach to access cellulose nanocrystals with deacidifying properties as these would mimic the action of ethanolamines (used in the Book Preservation Associates, BPA, conservation process). [25] At the same time, the decoration of N_ CNCs with aromatic amines can give access to nanocrystals with photophysical properties or with potential applications in electrochemistry.…”

“…This frequently limits the true possibilities of a fine molecular characterization since there is often only access to indirect evidences of the presence of the new chemical functionality. [24] In this paper, using dimethylsulfoxide as the dispersion medium, we obtain stable colloidal suspensions of N_CNCs and of their functionalized derivatives. Then, we elucidate their surface structure applying 1 H NMR experiments.…”

“…For this reason, in this work we first focus on the functionalization of N_CNCs, proposing the decoration of nanocrystals structure with aliphatic or aromatic amines. The new chemical functionalities are introduced both on the surface of the nanocrystals, applying a synthetic sequence interesting the transformation of the C 6 carbon of the glucopyranose ring, and on the reducing termini of cellulose nanocrystals, by the reductive amination reaction applied in the presence of sodium cyanoborohydride [24] . Indeed, we consider surface amination a good approach to access cellulose nanocrystals with deacidifying properties as these would mimic the action of ethanolamines (used in the Book Preservation Associates, BPA, conservation process) [25] .…”