Abstract
Native plant cellulose has an intrinsic supramolecular structure. Consequently, it can be isolated as nanocellulose species, which can be utilized as building blocks for renewable nanomaterials. The structure of cellulose also permits its end‐wise modification, i.e., chemical reactions exclusively on one end of a cellulose chain or a nanocellulose particle. The premises for end‐wise modification have been known for decades. Nevertheless, different approaches for the reactions have emerged only recently, because of formidable synthetic and analytical challenges associated with the issue, including the adverse reactivity of the cellulose reducing end and the low abundance of newly introduced functionalities. This Review gives a full account of the scientific underpinnings and challenges related to end‐wise modification of cellulose nanocrystals. Furthermore, we present how the chemical modification of cellulose nanocrystal ends may be applied to directed assembly, resulting in numerous possibilities for the construction of new materials, such as responsive liquid crystal templates and composites with tailored interactions.
Highlights
A direct comparison of the methods clearly shows that a combination of complimentary analysis should always be employed when confirming, quantifying, and assessing the applicability of any reducing end-groups (REGs) modification method (Table 2)
The only direct method with sufficient resolution to discriminate between REG chemical species is solution-state NMR spectroscopy in an ionic liquid electrolyte.[75,177]
While the workflow required to validate this approach for each new functionality can be tedious, the value in applying this method in the long run is likely considerable
Summary
Synthetic tailoring of natural compounds such as proteins, DNA/RNA, and lipids has unleashed a vast area of new research and technological development in materials science.[1] Chemical modifications to better control the nanoscale assembly of native units has proved a viable approach in many instances.[2] For naturally derived carbohydrate polymers, like starch and cellulose, synthetic tools and aims have traditionally functioned within a somewhat cruder framework. By contrast to the mass scale production, the present century has seen the rise of nanosized objects derived from naturally occurring polysaccharides.[7,8] The development has been markedly strong in the case of cellulose, which has been touted as a source for a new family of nanomaterials, i.e., nanocellulose.[9,10] the top-down isolation of cellulose nanofibers (CNFs)[11] and nanocrystals (CNCs)[14] from plantbased fibers has advanced enormously during the past decade with several semi-industrial production sites emerging worldwide (see Supporting Information (SI), Table S1). End-wise modification exploits the reducing end unit of cellulose and the parallel alignment[20] of the chains in the native cellulose I crystal, which dictates that all reducing end-groups (REGs) are at the same terminal end of a CNC or a CNF (Figure 1)
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