Click-Chemistry-Enabled Functionalization of Cellulose Nanocrystals with Single-Stranded DNA for Directed Assembly

Abstract

Controlling the self-assembly of cellulose nanocrystals (CNCs) requires precise control over their surface chemistry for the directed assembly of advanced nanocomposites with tailored mechanical, thermal, and optical properties. In this work, in contrast to traditional chemistries, we conducted highly selective click-chemistry functionalization of cellulose nanocrystals with complementary DNA strands via a three-step hybridization-guided process. By grafting terminally functionalized oligonucleotides through copper-free click chemistry, we successfully facilitated the assembly of brushlike DNA-modified CNCs into bundled nanostructures with distinct chiral optical dichroism in thin films. The complexation behavior of grafted DNA chains during the evaporation-driven formation of ultrathin films demonstrates the potential for mediating chiral interactions between the DNA-branched nanocrystals and their assembly into chiral bundles. Furthermore, we discuss the future directions and challenges that include new avenues for the development of functional, responsive, and bioderived nanostructures capable of dynamic reconfiguration via selective complexation, further surface modification strategies, mitigating diverse CNC aggregation, and exploring environmental conditions for the CNC-DNA assembly.

Keywords: CNC surface functionalization; DNA-mediated self-assembly; cellulose nanocrystal chiral complexation; cellulose nanocrystal chiral grafting.

Figure 1

Schematic of (a) the surface modification steps implemented in this work and (b) DNA complexation of the complementary DNA strands grafted on the CNC surface and the final CNC assembly morphology.

Figure 2

(a) FTIR transmittance spectra of CNCs in DMF and modified CNC-Br and CNC-N₃. UV–vis absorbance spectra of (b) unmodified CNCs in water and modified CNC-Br and CNC-N₃. Note that unmodified CNCs were measured in water because of the high absorbance of DMF below 250 nm. (c) UV-vis absorbance spectra of azide-modified CNCs (CNC-N₃) and CNCs modified with complementary oligonucleotides (CNC-O₁ and CNC-O₂), as well as spectra for O₁ and O₂ in water. XPS narrow high-resolution scan of the C 1s and N 1s regions for (d, g) CNC-N₃, (e, h) CNC-O₁, and (f, i) CNC-O₂.

Figure 3

AFM topography images of (a) CNC-OH in DMF, (b) CNC-Br, (c) CNC-N₃, and the corresponding phase images (d–f). All scale bars are 400 nm. Samples were drop cast onto a Si wafer. Red circles visualize local bundles of nanocrystals.

Figure 4

High-resolution AFM topography and phase scans of (a) CNC in DMF, (d) CNC-O₁, and (g) CNC-O₂. (b) CNC in DMF, (e) CNC-O₁, and (h) CNC-O₂ topography profiles obtained from the AFM topography images, with the average diameter (Avg d) of the CNCs corresponding to max Z height. (c, f, i) Apparent diameter from the phase images.

Figure 5

(a) Schematics of the changes in CNC dimensions according to the suggested grafting density. From left to right are the pristine CNC, initial modest grafting, and the final nonuniform “shell” of overlapped oligonucleotides in the mushroom regime. (b) 3D AFM topography image of oligonucleotide-modified nanocrystals demonstrating aggregated oligonucleotide “shell” in the dry state along the CNC visualizing nonuniform 3D surface topography.

Figure 6

(a, b) CD spectra of the CNC-O₁ + CNC-O₂ assembly. (c) CB of CNC-O₁ + CNC-O₂ obtained from the Mueller matrix analysis of ellipsometry data, with positions 1–3 corresponding to the sample’s different rotations. Unpolarized (d) bright-field and (e) dark-field optical microscopy images of the self-assembled CNCs drop cast on the quartz slide from the mixture of CNC-O₁ + CNC-O₂. (f, g) AFM topography images showing CNC organization within this assembly in a solid film and of isolated nanostructures, respectively. Red circles are added to help guide the attention to the ordered nanostructures. (h) FTIR spectra of the CNCs modified with oligonucleotides before their complexation (CNC-O₁ and CNC-O₂) and after they form a double helix (CNC-O₁ + CNC-O₂). (i) Cartoon of the CNC nanostructures formed via chiral complexation of the ssDNA on their surface.