A comprehensive study of biocompatibility of detonation nanodiamonds

Highlights

• DFT and molecular dynamics approaches for DND–water binary system were applied.

• Size distribution and ζ-potentials of DND nanoparticles were obtained.

• Complex biocompatibility investigation of DND was performed.

Abstract

The article describes a complex study of detonation nanodiamonds (DND) aqueous dispersions. In this research, DND sample was characterised by means of IR, NMR spectroscopy, TEM, thermogravimetric analysis, size distribution, and ζ-potentials. It was shown that DND sample includes several surface groups, mainly hydroxylic, carboxylic, and carbonyl ones. Dynamic light scattering results revealed that in the concentration range C = 0.002–0.3 wt%, DND nanoparticles size is equal to 55 ± 5 nm. It was demonstrated that DND possessed weak antiradical activity, had an inhibitory effect on F₁F₀-ATPase activity, almost did not affect platelet aggregation, formed a stronger complex with human serum albumin (HSA) in subdomain IB (digitoxin, K_b = 20.0 ± 2.4 l·g⁻¹) and a less strong complex in subdomain IIA (warfarin, K_b = 3.7 ± 0.1 l·g⁻¹), inhibited the esterase activity of HSA, DND dispersions (C = 0.0012–0.15 wt%) revealed genotoxic effect towards PBMCs, did not affect cellular proliferation in the experiment with HEK293 cell line, did not reveal cytotoxic effect up to 0.01 wt%. Using DFT and MD approaches allowed us to perform a simulation of interaction between DND nanoparticle and water molecules.

Introduction

Being, along with fullerenes, carbon nanotubes and graphene, one of the representatives of carbon nanostructures, detonation nanodiamonds (DND) that are produced by detonation of explosives have become a commercially available material in recent years [[1], [2], [3]]. DND is isolated by oxidative treatment from diamond-containing condensed carbon formed during the detonation of explosives with a negative oxygen balance in a non-oxidising environment [4]. The conditions of detonation synthesis determine the spherical shape and small size (2–10 nm) of the primary diamond nanoparticles [[5], [6], [7]].

DND, however, cannot be applied for biomedical applications in the unmodified form and therefore require additional surface modification [8]. Usually DND surface modification is divided into two groups, namely initial surface termination and covalent immobilisation of functional groups onto previously homogenised DND [9].

The possibilities of using DND in biology and medicine [[10], [11], [12], [13], [14]] are determined by the following unique properties:

• highly developed surface. In DND particles with a diameter of 4 nm, up to 15% of all atoms are located on the surface [6], and the total surface area can reach 300–400 m²·g⁻¹ [15]. As a result, DND have a high sorption capacity;

• hydrophilic surface of DND, which distinguishes them from other hydrophobic carbon nanostructures (fullerenes, nanotubes, graphene). Various oxygen-containing functional groups are attached to the DND surface (carboxyl, ether and ester, hydroxyl, keto, anhydrides and lactone rings) [[15], [16], [17], [18]];

• possibility of carrying out redox reactions on electrodes coated with DND, attractive for use in biosensors [19];

• chemical inertness of the “diamond” core, which results in its metabolic stability [[19], [20], [21], [22], [23]];

• the possibility of obtaining nanodiamonds capable of fluorescence [24,25];

• the ability of nanodiamonds to penetrate cells [[26], [27], [28]];

• commercial availability [19].

The surface of diamonds can be regarded as an example of the transition from inorganic solid crystals to organic molecules [29]. The DND surface is predominantly formed by carbon atoms in sp³ hybridisation, which are bonded to hydrogen atoms or oxygen-containing groups [[30], [31], [32]]. Unpaired electrons were also found in nanodiamond particles (about 40 electrons per particle of 4.8 nm in size). It is assumed that they are not “dangling bonds” on the surface, but are located inside the particle at a distance of 0.4 to 1 nm from the surface [33].

A highly developed surface, the presence of various functional groups, as well as the relative simplicity of modification reveals wide possibilities both for direct interaction of DND nanoparticles with biological molecules and for targeted drug delivery [34,35].

Data on the cytotoxicity of DND are ambiguous. Horie et al. showed that DND has a less negative effect on the cell viability than fullerenes and nanotubes [36]. To do this, the authors studied the cytotoxic effect of DND associates of various sizes (from 40 to 100 nm) on the HaCaT keratinocyte cell line. Only at comparatively high DND concentrations (1 mg∙ml⁻¹), a slight suppression of the growth of HaCaT cells was observed. In general, the effect of DND on cell viability, the integrity of their membranes was not noticed. In addition, intracellular oxidative stress characteristic of fullerenes and nanotubes was not observed. However, Solarska et al. noted that DND has significant cytotoxicity towards human umbilical vein endothelial cells [37].

This article focuses on the biocompatibility study of DND aqueous dispersions with negative ζ-potential values. For characterisation of DND, the following physicochemical methods were applied: NMR, IR, UV/Vis spectroscopy, thermogravimetric analysis, TEM; the particle size and ζ-potentials of aqueous DND dispersions were studied. The biocompatibility study of DND included investigation of cyto- and genotoxicity, antiradical and photodynamic activity, binding to HSA and DNA, platelet aggregation, F₁F₀-ATPase activity. The DFT and MD methods were used to model the interaction of DND nanoparticles with water molecules.