Plasma & Microwaves as Greener Options for Nanodiamond Purification: Insight Into Cytocompatibility

Abstract

The potential biomedical applications of nanodiamond have been considered over the last few decades. However, there is still uncertainty regarding the extent to which the surface characteristics of this material can influence potential applications. The present study investigated the effects of surface characteristics alongside the prospective of improving nanodiamond production using cold plasma and microwave technologies for the surface tailoring of the nanocarbons. Numerous approaches were applied to purify, refine and modify a group of nanosized diamonds at each step of their production cycle: from the detonation soot as the initial raw material to already certified samples. The degree of surface changes were deliberately performed slowly and kept at different non-diamond carbon presence stages, non-carbon elemental content, and amount converted superficial moieties. In total, 21 treatment procedures and 35 types of nanosize diamond products were investigated. In addition cultures of human fibroblast cells showed enhanced viability in the presence of many of the processed nanodiamonds, indicating the potential for dermal applications of these remarkable nanomaterials.

Keywords: cytotoxicity; microwave; modification; nanocarbon; nanodiamond; nanomaterial; plasma treatment; purification.

FIGURE 1

(A) Comparative distribution of 18 non-carbon impurity elements in the initial DSoot, vs. microwave-purified DND of types NASA, PANA and NSPA (ppm, logarithmic scale 0.1–4,000; check also Supp. Information 4); (B) Elemental distribution of acid-dichromate purified DND type MyD, compared with that of the parent DS, MW-purified NSPA and MW-refined MyD (MyD ac.NSPA); scale 0.1–5,000; (C) Elemental content of initial PlCh, PlCh ac.NSPA and PlCh UR; scale 0.1–5,000.

FIGURE 2

Effects of the complexant-driven refinement (Na₂EDTA and 2,6-Pyridinedicarboxylic acid (here indexed “EDTA” and “DPA”; see also Supplementary Table B2): (A) treatment of MyD; scale 0.1–4,000; (B) treatment of PlCh; scale 0.1–5,000. Effects of the fluorides-driven refinement (NaF and NH₄F; see also Supplementary Table B2): (C) treatment of MyD; scale 0.1–4,000; (D) treatment of PlCh; scale 0.1–5,000.

FIGURE 3

(A) Comparative elemental distribution of O₂-plasma treated DS; (B) Air- and H₂-plasma treated DS; scales 0.1–4,000.

FIGURE 4

ATR-FTIR (A) and Raman spectra of PL- and MW-treated DND (B), as compared with the parent DS and HPHT micronsized diamond as a standard (Raman; the top). (C) B & W colour ratios for all plasma-purified DND, parent DS and several others (bottom; the percentage shows the relative darkness of the powder; far right: B & W standards, the black square is with the used 101 × 101 pixels sampling size).

FIGURE 5

ATR-FTIR spectra of PL-modified NSPA type of DND.

FIGURE 6

SEM-EDX elemental content of PL-sulphhydrylated and fluorinated samples, as compared with parent NSPA (“CNO-only” comparison: the inset plot at top right).

FIGURE 7

Electrokinetic (zeta-) potential and intrinsic pH ranges (strips close to base) of 0.05% ND suspensions in DI water.

FIGURE 8

Cell cultures of HDFs in presence of cytotoxic “H₂S-treated” ND (above) vs. beneficial plasma-aminated ND of the same types (below). Scale bar: 100 μm.

FIGURE 9

Metabolic activity of HDFs cultured with DND for 72 h under three deposition methods, using the alamarBlue^TM assay (normalised to the non-treated cells; n = 4; ^∗∗∗p < 0.001; ^∗∗p < 0.01; ^∗p < 0.05; ± SD).

FIGURE 10

Metabolic activity of HDFs cultured with completely purified (“end-of-cycle”) nano- and micronsized diamonds for 72 h, using the alamarBlue^TM assay (normalised to the non-treated cells; n = 3; ^∗∗∗p < 0.001; ^∗∗p < 0.01; ^∗p < 0.05; ± SD).

FIGURE 11

Metabolic activity of HDFs cultured with incompletely sp²-C-purified DND, using the alamarBlue^TM assay (normalised to the non-treated cells; n = 3; ^∗∗∗p < 0.001; ^∗∗p < 0.01; ^∗p < 0.05; ± SD).

FIGURE 12

Metabolic activity of HDFs cultured with MW-refined DND samples, using the alamarBlue^TM assay (normalised to the non-treated cells; n = 3; ^∗∗∗p < 0.001; ^∗∗p < 0.01; ^∗p < 0.05; ± SD).

FIGURE 13

Metabolic activity of HDFs cultured with plasma-modified DND, using the alamarBlue^TM assay (normalised to the non-treated cells; n = 3; ^∗∗∗p < 0.001; ^∗∗p < 0.01; ^∗ p < 0.05; ± SD).