Surface functionalization of Nanodiamonds using the Biomacromolecule Chitosan (cs)

Abstract

NDs are highly promising drug delivery vehicles (DDVs) due to the materials low cytotoxicity and biocompatibility. The advantage of these materials lies within their tunable surface chemistry and inherent optical properties. Despite the numerous benefits, the use of NDs in biological systems is impeded by their high aggregation propensity in polar liquid medium, a caveat from its rich chemistry. ND particles typically aggregate into much larger groups >200 nm, which are too large for drug delivery applications. In this work, salt assisted attrition milling was utilized to decrease ND aggregates. After 5 hours of milling, dynamic light scattering (DLS) measurements revealed a maximum particle distribution at 127 nm, with significant reduction in the mass fraction of larger particle size distribution from 300-1000nm. The average size of the nanodiamond particles was also shown to increase with an increasing concentration of chitosan (0.25 w/v%; 142nm and 1 w/v%; 825nm). In the modification of nanodiamonds, FTIR was used to examine the chemical composition of the nanodiamonds, both before and after functionalization. FTIR results of the pristine nanodiamonds showed characteristic bands at 3400cm-1 (-OH) 1630 cm-1 (C=O) and 1080 (-C-O). Chitosan exhibited spectral regions of interest at 3350 cm-1 (-OH), 1665 cm-1 (NH2) and 1030 cm-1 (C-O). Successful functionalization of the nanodiamaond surface was confirmed by the appearance of the amide band I at 1664 cm-1 and presence of strong C-O stretching at 1197cm-1 from the chitosan backbone. This is indicative of the covalent binding of NH2 on chitosan with COOH on the ND surface. X-ray diffraction was performed to observe the structure and crystallinity of the NDs and showed crystalline characteristic peaks at 2ϴ 44º (111), 75º (220), and 91º (113) for the pristine ND. The appearance of an amorphous band at 2ϴ 22º in the analysis of ND-COOH-CS is indicative of the surface modification of the nanodiamond with the biopolymer, CS. Zeta potential analysis show an increase in the zeta potential with an increase in concentration of CS (pristine ND, -40mV; 0.25 w/v%, -26.7mV; and 1 w/v%; 12.9mV). This increase in zeta potential is indicative that a more stable dispersion of nanodiamonds was created upon functionalization with the biopolymer, CS. TGA was also used to confirm the composition of the pristine and modified NDs. Loss of water was observed initially for all samples at 100C followed by onsets of degradation for CS and ND-COOH-CS occurred at 212°C and 232.16 °C for CS and ND-COOH-CS, respectively. The presence of weight loss profiles characteristic of CS in the ND-COOH-CS thermogram confirmed modification of the ND surface when compared to the pristine ND. Morphological investigations of the nanodiamonds using TEM showed a particle size of ~150-300nm supporting PSA results. TEM images also showed characteristic aggregation of the NDs due to the presence of attractive surface functional groups. This work confirms the capability of surface functionalization of nanodiamonds using biomacromolecules and reveals the potential application regarding the use of these materials as DDVs with enhanced permeability and compatibility properties.