Biomedical Engineering Reference
In-Depth Information
and its biocompatibility has long been established in dentistry [84, 85]. The low toxicity of
gold nanoparticles for labeled cells has been confirmed
in vitro
[86]. That was followed by
effective imaging with micro-CT of cells tagged with gold nanoparticles [87]. Despite the
favorable characteristics of gold nanoparticles, the cost of gold is still prohibitive for wide-
spread application. Thus, there is a search for an inexpensive replacement for gold. Tantalum
oxide nanoparticles have been considered as an alternative attractive candidate, since they are
also characterized by a high X-ray absorption, biocompatibility, bioinertia, and low toxicity
[88, 89], all of which comes at a much lower cost.
Polymer Dots
While quantum dots have been used with increased frequency for cell labeling, a new
organic alternative has arisen, in the form of π-conjugated polymer-based semiconductor
nanoparticles, characterized by high fluorescence [90]. There are many advantages of these
nanoparticles, such as a great emission rate, minimal “blinking,” and outstanding photo-
stability. The diversity of emitted photon wavelengths, depending on polymer type, opens
up the possibility of multicolor imaging. The progress in the assembly of near-infrared
π-conjugated polymers heralds a wide utility in stem-cell imaging
in vivo
[91]. The synthesis
of inorganic-organic/nanocrystals-polymer hybrids with these exceptional characteristics
will enable a better fit for specific experimental conditions [92].
Nanodiamonds
Fluorescent nanodiamonds are another group of optical labels. They are highly photosta-
ble, chemically nonreactive, biocompatible, and, most importantly, can emit photons of
far-red bandwidth, which makes them advantageous for
in vivo
stem-cell imaging [93]. The
atomic composition of nanodiamonds enables enhancement by fluorescence resonance
energy transfer (FRET), but at the cost of the introduction of another label into the cells
[94, 95]. Nanodiamonds have been shown to be relatively safe for the cells [96-98], and are
characterized by a low exocytosis rate [99]; however, a slight increase in DNA repair proteins
in nanodiamond-labeled embryonic stem cells has been reported [100].
Upconverting Luminescence Nanoparticles
Nanoparticles characterized by near-infrared fluorescence have been shown to be suitable
for
in vivo
stem-cell tracking; however, the tissue autofluorescence interferes with the process
of detection of specific fluorescence, thus decreasing sensitivity. In fluorescence imaging,
the absorption of light is followed by the emission of photons of longer wavelength, which
must be detected against a usually high autofluorescence signal. The phenomenon of upcon-
verting luminescence (UCL) may address this constraint. However, there are specific sub-
stances characterized by the emission of shorter wavelengths, called the anti-Stokes process,
which is practically never encountered in living organisms; thus, any signal for UCL materials
is devoid of background [101]. The anti-Stokes emission has been recognized for some time,
but only after suitable techniques for the manufacture of UCL nanoparticles for use
in vivo
were developed did this field develop further. In addition, near-infrared upconversion observed
in rare-earth nanophosphors resulted in the production of photoluminescent probes suitable
for
in vivo
imaging in small animals [102]. Proof-of-principle experiments performed on tissue
