Grafting Nature‐Inspired and Bio‐Based Phenolic Esters onto Cellulose Nanocrystals Gives Biomaterials with Photostable Anti‐UV Properties

Abstract

New nature‐inspired and plant‐derived p‐hydroxycinnamate esters and p‐hydroxycinnamate diesters provide excellent protection against UV radiation when incorporated into a matrix. Herein, an efficient and sustainable pathway is reported to graft these phenolic compounds onto cellulose nanocrystals (CNCs) via click‐type copper‐catalyzed azide/alkyne cycloaddition (CuAAC) reaction. The successful grafting of the phenolic esters on CNC surface was evidenced by a range of chemical analyses, and the degrees of substitution (DS) of the CNC were found to depend on the structure of the phenolic ester grafted. Moreover, aqueous suspensions of the phenolic ester‐grafted CNCs not only strongly absorb in both the UVA and UVB regions, but they also exhibit average to very high photostability. Their wide spectrum UV‐absorbing properties and their stability upon exposure to UV are highly influenced by the structure of the phenolic ester, particularly by the extra ester group in p‐hydroxycinnamate diesters. These findings demonstrate that cellulose nanocrystals decorated with such plant‐derived and nature‐inspired phenolic esters are promising sustainable nanomaterials for anti‐UV applications.

Diethyl sinapate-grafted cellulose nanocrystals as nature-inspired UV filters in cosmetic formulations

“…Cream N (Neutrogena® Oil-Free Moisture) and Cream S (Neutrogena® Oil-Free Moisture Sun Protection Factor (SPF) 15,13.5% active ingredients) are commercially available facial creams purchased in a local pharmacy. DES was synthesized via proline-mediated Knoevenagel condensation of syringaldehyde and diethyl malonate ( Figure S1 ), as we have reported previously [ 14 ].…”

“…Alkyne-modified DES was synthesized via Williamson etherification with propargyl bromide ( Figure S2 ) as reported previously [ 14 , 21 ]. Basically, DES (1 eq, 4.41 g) was initially dissolved in 90 mL anhydrous DMF and cooled to 0 °C.…”

“…Azide-grafted CNCs (CNC–N 3 ) were initially synthesized by tosylation of CNC with p -toluenesulfonyl chloride at room temperature for 48 h followed by azidation with NaN 3 at 100 °C for 24 h ( Figure S5 ). DES was then grafted onto CNCs via a click-type Cu(I)-catalyzed azide-alkyne cycloaddition reaction between CNC–N 3 and alkyne-modified DES (DES-PB) as previously described [ 14 , 17 ] ( Figure S6 ). CNC–N 3 (1 eq, 1 g) and DES-PB (1 eq, 1 g) were suspended in 100 mL DMF.…”

“…Recently, we reported a new and innovative synthetic route to graft phenolic esters on cellulose nanocrystals (CNCs) [ 14 ]. We grafted a family of p -hydroxycinnamate esters (including DES) on CNCs via a click-type Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) [ 15 , 16 ] reaction and reported that the resulting click products display strong anti-UV activity and photostability.…”

“…The novel phenolic esters and their CNC derivatives are obtained from naturally occurring precursors and prepared via green chemistry methods. Our previous study also showed the important role of the CNC component as an effective dispersing medium for these non-polar phenolic ester derivatives in water and other hydrophilic systems [ 14 , 17 ]. For instance, we showed that grafting diethyl ferulate (DEF) on CNCs allows the efficient dispersion of the phenolic ester in polyvinyl alcohol (PVA) films [ 17 ].…”