Due to an immense popularity of HPHTND
in bioimaging, this topic has been covered in many reviews. Fluorescence
associated with ND NVN and NV defects is most widely used in bioimaging. Both
defects are produced by high-energy particles irradiation of NDs followed by
annealing. The advantages of NVN and NV defects for bioimaging are related to
their stable photoemission in the 500–800 nm spectral range and were used
in many in vivo and in vitro
studies.Other defects in nanodiamond also demonstrate interesting optical properties. For example, europium-vacancy (EuV), and silicon-vacancy (SiV)
defects can be potentially exploited as bright biomarkers withexcellent
photostability, which is not compromised by tissue autofluorescence.
Given the low probability of existence of NV centres
in NDs smaller than 40 nm, the possibility to create SiV defects in the ND
crystals less than 10 nm in diameter seems to be a great step forward. SiV
defects can be created in NDs for example byplasma CVD growth on a silicon
substrate. Shorter excited state lifetime, weak and narrow vibronic sideband, and zero phonon (ZP)
emission maximum at 739 nm with little overlap with broad photo-luminescence of
ND (450–650 nm),
are the advantages of SiV over NV defects.In addition to widely studied ‘intrinsic’fluorescence
associated with diamond structure defects, the so-called ‘extrinsic fluorescence’ associated
with ND surface functionalgroups has also been analyzed recently. The intensity
of the ‘extrinsic fluorescence’ depends
on the type of ND functionalgroups and varies from very high (e.g.,
for octadecyl amine terminated ND) to low (many
other types of surface termination).
Octadecyl amine , hydroxyl, ketone, and ester groups have a significant
effect on the fluorescent properties of DND.
The ‘extrinsic fluorescence’
of DND with simultaneously present hydroxyl, ester, and ketone
groups is wavelength-dependent. However, when the surface is covered by mainly
one type of functional groups, for example, hydroxyl, the fluorescence
becomes independent on the excitation. Importantly, each type of functional
groups results in slightly different emission wavelength potentially allowing
multicolor imaging using DNDs with well-defined surface chemistry.NV defects of
bulk diamond have been exploited in the areas spanning beyond conventional fluorescence

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