Monodispersed Nanodiamonds and their Applications

Introduction

Among various nanomaterials, nanoparticles of less than 10 nm size remain the most sought-after products. Their production, however, is a challenging task since the final structure and structural stability of a particle is largely dictated by the identity of surface atoms. Once produced, these “single-digit” nanoparticles open new horizons. For example, 5 nm particles offer 10-fold higher specific surface area and the absorbing capacity compared to 50 nm particles. Also, the inter-particle distance between 5 nm particles uniformly dispersed at 1 vol% in a polymer matrix is around 20 nm, which is of the order of radii of gyration for polymer chains. For 100 nm particles at the same loading, the inter-particle distance increases to a few hundred nanometers and becomes significantly higher than the radii of gyration of polymer chains and thus provide less effect on the composite mechanical properties. In fact, the “single-digit” nanoparticles are thought to be capable of penetrating the blood brain barrier thereby playing a special role in nanomedicine.¹

Though macroscopic diamonds fascinate due to their brilliance and prettiness, they possess excellent technical properties such as the highest hardness and thermal conductivity, and the widest optical transparency window. Similarly, diamond nanoparticles (nanodiamonds (NDs)) remain arguably the highest impact nanomaterial as they demonstrate a distinct combination of outstanding mechanical performance, chemical resistance, biocompatibility, magneto-optical and electronic properties induced by doping. While there are a variety of synthesis methods for the production of NDs, the first method which was discovered more than 50 years ago,^2,3 produces NDs with diameters of 4-6 nm. The method involves the detonation of carbon-containing explosives in an oxygen-deficit environment to avoid carbon oxidation. High temperature and pressure created during the detonation are favorable for diamond formation. Since the explosion lasts only for a fraction of a microsecond, it highly restricts the ND growth time and thus the sizes of the resultant particles to a few nanometers. However, since NDs collide and fuse during synthesis, the as-produced detonation nanodiamonds (DNDs) form tight aggregates of primary particles which are challenging to separate. Quite often, the commercially available DNDs are listed as “5 nm nanodiamond” based on the size of the primary particles, while in reality the materials contain larger aggregates. This general misconception hampered the field for quite a long time and the same problem is still encountered by newcomers to the field.

The past 50 years have seen significant advancements in the production of detonation nanodiamonds; however, one of the greatest challenges has been the isolation of the primary particles (~5 nm in size) from the 200-300 nm aggregates produced during synthesis. It was not until 2005 that the isolation of these primary particles was implemented through media-assisted milling of the tight aggregates.⁴ These “single digit” ND particles opened a wide plethora of new opportunities in materials science, electronics, optics and life science applications.⁵

Properties, Structural Features and Applications of Monodispersed NDs

The most essential features of monodispersed single-digit ND particles and related properties are summarized in figure 1. Each of these distinguishing structural features contributes to the unique identity of nanodiamonds, and each characteristic renders the material for diverse applications.⁶ Currently, a major source of “single-digit” ND particles that can be produced at a large scale is detonation ND (DND). Therefore applications discussed below are related to “single-digit” DNDs.

The strong covalent bonding between carbon atoms in the nanodiamond crystallographic lattice results in the material with the highest known atomic density. This, in turn results in the most hard and chemically inert material with stabilities over a wide range of environments. DNDs are stable in acidic and basic environments, can be heated in a vacuum without extensive graphitization up to ~800 ^oC, and can withstand combustion when heated in air at up to ~450 ^oC. Diamond micro- and nanoparticles have long been used as a polishing agent due to their high-wear resistance and hardness. The high chemical stability of NDs makes it useful in applications involving harsh conditions, such as microelectronic processing, where NDs are currently used for seeding for growth of diamond films by chemical vapor deposition (CVD). NDs, when used as an additive in lubricant, provide fine polishing of surfaces resulting in decreased friction and promote an increase in fuel efficiency for both diesel and gasoline based vehicles.^7,8 Thus, the core is the primary structural feature that distinguishes NDs from other nanomaterials. The structure also lays the foundation for excellent optical properties such as a large bandgap and transparency from the ultraviolet to infrared spectral regions. The crystallographic core is also responsible for the high refractive index of diamonds (~2.4) providing strong light scattering in NDs and making them useful as non-toxic UV shielding nanoadditives in sunscreens and polymer coatings.⁹

Just as the arrangement of carbon gives diamond its robust mechanical properties, defects in the carbon lattice network are equally unique. Nitrogen-based color centers impart fluorescent properties to diamonds.¹⁰ Detonation NDs containing nitrogen impurities are derived from explosives. While an excessive amount of nitrogen in typical DNDs prevents formation of optically active color centers, opportunities exist to produce DNDs containing color centers.² In addition to optically active fluorescent defect centers, doping with boron to produce electrically conductive nanodiamonds is of high technological importance. Doping of the DND core with tritium is another recent exciting opportunity that provides highly stable radiolabeled nanodiamonds for bioimaging applications while leaving the surface available for further functionalization with targeted proteins and for drug uploading.¹¹

The size and shape of ND particles render themselves to a number of applications. Primary particles of detonation nanodiamonds are 4-6 nm in size and typically have an approximate spherical shape (figure 2).² This well-defined shape and size makes them ideal for applications involving the uploading of sorbent molecules,¹² or the formation of a dense network of bonds between ND nanofillers and a surrounding polymer matrix,¹³ where a high specific surface area (~300-400 m²/g) is required. Rounded particles are inherently more effective in lubrication and polishing applications,^7,8 where NDs can, in principle, act as nanoscale ball bearings. In biological applications, well-controlled sizes and shapes likely enhance the loading capacity for drug delivery or protein adsorption.^12,14 DND primary particles also have a distinct advantage over other comparably-sized materials in applications where true sub-10 nm particles with low cytotoxicity are required. DND primary particles are inherently non-toxic, however their toxicity can be dependent on sufficient purification from sp² carbon and metals, and also their toxicity depend on the type of cell used.¹⁵

As produced detonation NDs are hydrophilic, covered with oxygen-containing groups resulting from the purification by oxidation from sp² carbons². Acid treatment of deagglomerated primary particles results in carboxylated NDs (4-6 nm) with a zeta potential of about -45 mV that can be dispersed in a variety of polar solvents.¹² Chemical reduction of the particles provides 4-6 nm NDs with a prevalence of hydroxylic surface groups and zeta potential around +30 mV.¹⁶ The value of the zeta potential is important for controlled electrostatic interactions with target sorbent molecules and for interaction with cells (surfaces of cell membranes are negatively charged).¹² A number of interesting functionalization schemes have been developed for tailoring the surface groups of NDs. This includes NDs with surface amino functionalities for bio-applications and attachment of biotin, streptavidin and nucleic acids, etc.^2,3,15 These surface functionalities enable the functionalization of NDs with a wide variety of groups and thus make them suitable for potential applications involving in vitro and/or in vivo targeted delivery and bio-imaging.^14,15

Monodispersed Nanodiamond Particles in Solvents

Single digit NDs with well-defined surface functionality in application-appropriate solvents are of utter importance from the users’ point of view. Therefore, the development of colloidally stable dispersions of primary DND particles in a wide range of solvents is of paramount importance. Control of the surface functionality (with carboxylic, hydroxylic or other specific groups) ensures that interactions between NDs and other media, such as a solvent or polymer, can be more easily predicted. Although there are issues related to aggregation upon mixing with polymers, tailored surface functionalization schemes can presumably promote dispersion. As shown in figure 3, DND primary particles of ~4-6 nm can be stabilized in a number of solvents, and each of these solvents can be used in a number of applications (see Table 1 for details).

Summary

Applications of nanodiamonds in lubricants & polishing materials, fillers in electroplated films, polymer coatings with enhanced mechanical properties and thermal resistance, and UV and radiation-resistant materials and formulations are based on the exceptional bulk properties. However, applications in immobilization of biomolecules, drug delivery, and composites are based on the surface properties. The combination of the unique bulk and surface properties of nanodiamonds make them extremely versatile materials with a number of applications. Some of the important applications of NDs include their use in drug delivery,^14,15 polymer strengthening,^2,13 high density nucleation and growth of CVD diamond films,² and as additives for oils, lubricants and fuels,^7,8 catalysts support,² antibacterial and antifungal coating material.² The outstanding chemical and mechanical properties, along with their small size and approximate spherical shape, render NDs ideal candidates for the aforementioned applications.