Abstract
Fluorescent nanodiamonds (FNDs) are a new class of carbon nanomaterials that offer great promise for biological applications such as cell labeling, imaging, and sensing due to their exceptional optical properties and biocompatibility. Implementation of these applications requires reliable and precise surface functionalization. Although diamonds are generally considered inert, they typically possess diverse surface groups that permit a range of different functionalization strategies. This review provides an overview of nanodiamond surface functionalization methods including homogeneous surface termination approaches (hydrogenation, halogenation, amination, oxidation, and reduction), in addition to covalent and non-covalent surface modification with different functional moieties. Furthermore, the subsequent coupling of biomolecules onto functionalized nanodiamonds is reviewed. Finally, biomedical applications of nanodiamonds are discussed in the context of functionalization.
Highlights
Among carbon nanomaterials, nanosized diamond particles or nanodiamonds (NDs), including detonation nanodiamonds (DNDs) and nanodiamonds containing fluorescent color centers, termed fluorescent nanodiamonds (FNDs), have received increasing interest.In particular, due to their high biocompatibility and unique optical properties including extraordinary photostability, near infrared (NIR)fluorescence emission (≈650–900 nm), long fluorescence lifetime (≈20 ns) and high fluorescence quantum yield, FNDs containing nitrogen-vacancy (NV) centers are being used in an expanding number of biomedical applications [1,2,3,4,5,6]
Biomedical applications of nanodiamonds are discussed in the context of functionalization
In addition to imaging and drug delivery or theranostic nanoparticles, the properties of NV− centers within FNDs are promising for sensing applications in biological environments due to their biocompatibility, exceptional optical properties, and high magnetic sensitivity with nanoscale spatial resolution under ambient conditions (Figure 5A) [140]
Summary
Among carbon nanomaterials, nanosized diamond particles or nanodiamonds (NDs), including detonation nanodiamonds (DNDs) and nanodiamonds containing fluorescent color centers, termed fluorescent nanodiamonds (FNDs), have received increasing interest. The charge state of an NV center is not fixed but rather can transition between the NV0 and NV− states in response to functional groups and impurities on the diamond surface [11]. Functionalization schemes that eliminate surface impurities such as graphitic carbon, and that leave the FND surface negatively charged will favor the formation and stabilization of the more useful NV− charge state. The HPHT method produces large diamonds that are subsequently irradiated with high energy electrons, protons, or helium ion to create NV centers, followed by mechanical size reduction and selection to generate FNDs [29,32,33,34]. Approaches that have been applied to FNDs are indicated, though we anticipate that the vast majority, if not all, of the functionalization schemes would be suitable and applicable for FNDs
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