Citric acid: An ecofriendly cross-linker for the production of functional biopolymeric materials

Biopolymeric materials have been the focus of research recently owing to their wide range of applications in a variety of sectors such as pharmaceutical, biomedical, cosmetics and food industries. Cross-linking of carbohydrates such as starch, pectin, chitosan, agar, cellulose, hemicellulose etc. is one of the most accepted approaches for fabrication of biopolymeric materials. Citric acid (CA), a tricarboxylic acid, being affordable, easily available and nature-originated, has emerged as an eco-friendly and sustainable cross-linker for the development of biopolymeric materials. CA has been used to synthesize a variety of materials such as films, hydrogels, bioplastics, composites, and membranes. Cross-linking with CA is accomplished through covalent intermolecular di-ester interactions between –COO of CA and the –OH groups of the biopolymer. This article discusses the mechanism of cross-linking with CA and its impact on the physical, functional, structural, morphological, mechanical, thermal, and antimicrobial properties of biopolymeric materials. Recent applications of CA cross-linked biopolymeric materials in diverse fields have also been reviewed.