Nanodiamonds have attracted remarkable scientific attention for bioimaging and therapeutic applications owing to their low toxicity with many cell lines, convenient surface properties and stable fluorescence without photobleaching. Newer techniques are being applied to enhance fluorescence. Interest is also growing in exploring the possibilities for modifying the nanodiamond surface and functionalities by attaching various biomolecules of interest for interaction with the targets. The potential of Raman spectroscopy and fluorescence properties of nanodiamonds has been explored for bioimaging and drug delivery tracing. The interest in nanodiamonds' biological/medical application appears to be continuing with enhanced focus. In this review an attempt is made to capture the scope, spirit and recent developments in the field of nanodiamonds for biomedical applications.
We investigate the antibacterial effect of ultrafine nanodiamond particles with an average size of 5 nm against the gram-negative bacteria Escherichia coli (E. coli). UV-visible, Raman spectroscopy, and scanning electron microscopy (SEM) have been employed to elucidate the nature of the interaction. The influence on bacterial growth was monitored by measuring optical densities of E. coli at 600 nm as a function of time in the presence of carboxylated nanodiamond (cND) particles (100 μg/ml ) in highly nutritious liquid Luria-Bertani medium. The SEM images prove that cND particles are attached to the bacterial cell wall surface and some portion of the bacterial cell wall undergoes destruction. Due to the change of the protein structure on the bacterial wall, a small Raman shift in the region of 1400 to 1700 cm⁻¹ was observed when E. coli interacted with cNDs. Raman mapping images show strong evidence of cND attachment at the bacterial cell wall surface. Electrotransformation of E. coli with a fluorescent protein markers experiment demonstrated that the interaction mechanisms are different for E. coli treated with cND particles, E. coli by lysozyme treatment, and E. coli that suffer lysis.
Fluorescent nanodiamonds with nitrogen‐vacancy (NV) centers respond to local changes in electric and magnetic fields. These responses can be read optically as changes in fluorescence. NV centers do not suffer from photoblinking or photobleaching, making nanodiamonds a viable platform for long‐term imaging of processes inside living cells. However, for any bioapplication, the surface of these particles must be modified to prevent aggregation and nonspecific protein adsorption and to effectively transduce the changes in local environment to NV center. Modular biomimetic interface has been developed on nanodiamonds with remarkable sensitivity for relaxometric readout that takes advantage of self‐assembled phospholipid bilayers supported by the nanoparticle surface. This rapid and robust approach provides synthetic pathway to tunable composition, demonstrated by tuning surface charge and content of spin labels on nanodiamond. The supported phospholipid bilayer interface increases the detection sensitivity about one‐order‐of‐magnitude. Also, a theoretical model of the system is provided, which shows excellent agreement with experimental results. Merging biocompatibility, modularity, and outstanding spin sensitivity in one nanomaterial provides a foundation for development of multifunctional nanoparticles suitable for highly sensitive monitoring of local magnetic field fluctuations and paramagnetic species under physiological conditions.
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