Nanomaterials for Medical Imaging …
381
luminescence is an optical phenomenon in some materials which continue to emit
light even after ceasing excitation [310]. In persistent luminescence materials, the
energy is stored in the intrinsic traps or defects, which are intentionally introduced
upon excitation [311]. To release the stored energy, the thermal de-excitation process
is required after which the delayed emission of light occurs over minutes or hours after
the exciting radiation has been removed [312]. Chermont et al. reported a methodology to synthesize Ca 0.2 Zn 0.9 Mg 0.9 Si 2 O 6 nanoparticles doped using NIR persistent
luminescence. The NPs used in this investigation were excited at UV or visible light,
and over a while, they release the stored energy by the emission of photons with
lower energy. The NPs were excited before the injection to mouse, and the emission
can be monitored in real-time for more than one hour without a need for an excitation source. The NPs with an emission wavelength around 690 nm were able to
image 3LL tumors in an in a vivo mouse model.
Abdukayum and co-workers [313] developed a fabrication strategy utilizing the
citrate sol-gel method of functional PLNPs through the minimal composition of
Zn 2 . 94 Ga 1 . 96 Ge 2 O 10 : Cr
3+ , Pr
3+ . The co-doping of Pr
3+ /Cr
3+ and regulating the Zn
deficiency in the host material enhanced persistent luminescence intensity and afterglow time of PLNPs. Experiments on animal models presented an increased signal-tonoise ratio and provided in vivo bio-imaging for more than 15 h without an excitation
source. NPs used in persistent luminescence imaging are usually based on rare earth
metals Europium, Praseodymium is poorly biocompatible due to its associated toxicity [314]. Alternatively, semiconductor-based conjugated polymer poly[2-methoxy5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) was used as PLNP with
the NIR dye NIR775 encapsulation for enabling emission in the NIR range. This
investigation demonstrated that this semiconductor-based PLNP emitted persistent
luminescence for nearly one hour after a single exposure to white light and presented
an alternative contender for in vivo imaging applications.
Photoacoustic imaging (PAI), also referred to as optoacoustic imaging (OAI) has
evolved as an in vivo and non-ionizing modality with the relative ability for deep
tissue imaging [315]. Here, a nanosecond laser with a pulse duration of < 10 ns is
used to illuminate a biological sample of interest [315]. The light absorbed by the
object results in a rise in temperature followed by thermal expansion and production
of ultrasonic waves [316]. These waves are detected by an ultrasonic transducer to
form images. Certain endogenous contrast agents used in PAI include hemoglobin,
melanin, lipid, and water in the visible and NIR regions [317].
In some cases, endogenous contrast agents are inadequate to provide useful
information for clinical diagnostics. Since the strong light scattering in the shorter
wavelength region, the light intensity and henceforth the photon acoustic amplitude
reduces exponentially concerning tissue depth [318]. To overcome these limitations,
different types of NP based contrast agents such as plasmonic NPs (e.g., gold and
silver), carbon nanotubes, UCNP, semiconducting polymer NP, and quantum dots
have been explored [319, 320].
UCNP provides narrow excitation and emission profiles for PAI. Lanthanide
doped ions in UCNP could lead to quenching of luminescence due to the solvent
relaxation process in aqueous conditions. These luminescence quenching phenomena
381
luminescence is an optical phenomenon in some materials which continue to emit
light even after ceasing excitation [310]. In persistent luminescence materials, the
energy is stored in the intrinsic traps or defects, which are intentionally introduced
upon excitation [311]. To release the stored energy, the thermal de-excitation process
is required after which the delayed emission of light occurs over minutes or hours after
the exciting radiation has been removed [312]. Chermont et al. reported a methodology to synthesize Ca 0.2 Zn 0.9 Mg 0.9 Si 2 O 6 nanoparticles doped using NIR persistent
luminescence. The NPs used in this investigation were excited at UV or visible light,
and over a while, they release the stored energy by the emission of photons with
lower energy. The NPs were excited before the injection to mouse, and the emission
can be monitored in real-time for more than one hour without a need for an excitation source. The NPs with an emission wavelength around 690 nm were able to
image 3LL tumors in an in a vivo mouse model.
Abdukayum and co-workers [313] developed a fabrication strategy utilizing the
citrate sol-gel method of functional PLNPs through the minimal composition of
Zn 2 . 94 Ga 1 . 96 Ge 2 O 10 : Cr
3+ , Pr
3+ . The co-doping of Pr
3+ /Cr
3+ and regulating the Zn
deficiency in the host material enhanced persistent luminescence intensity and afterglow time of PLNPs. Experiments on animal models presented an increased signal-tonoise ratio and provided in vivo bio-imaging for more than 15 h without an excitation
source. NPs used in persistent luminescence imaging are usually based on rare earth
metals Europium, Praseodymium is poorly biocompatible due to its associated toxicity [314]. Alternatively, semiconductor-based conjugated polymer poly[2-methoxy5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) was used as PLNP with
the NIR dye NIR775 encapsulation for enabling emission in the NIR range. This
investigation demonstrated that this semiconductor-based PLNP emitted persistent
luminescence for nearly one hour after a single exposure to white light and presented
an alternative contender for in vivo imaging applications.
Photoacoustic imaging (PAI), also referred to as optoacoustic imaging (OAI) has
evolved as an in vivo and non-ionizing modality with the relative ability for deep
tissue imaging [315]. Here, a nanosecond laser with a pulse duration of < 10 ns is
used to illuminate a biological sample of interest [315]. The light absorbed by the
object results in a rise in temperature followed by thermal expansion and production
of ultrasonic waves [316]. These waves are detected by an ultrasonic transducer to
form images. Certain endogenous contrast agents used in PAI include hemoglobin,
melanin, lipid, and water in the visible and NIR regions [317].
In some cases, endogenous contrast agents are inadequate to provide useful
information for clinical diagnostics. Since the strong light scattering in the shorter
wavelength region, the light intensity and henceforth the photon acoustic amplitude
reduces exponentially concerning tissue depth [318]. To overcome these limitations,
different types of NP based contrast agents such as plasmonic NPs (e.g., gold and
silver), carbon nanotubes, UCNP, semiconducting polymer NP, and quantum dots
have been explored [319, 320].
UCNP provides narrow excitation and emission profiles for PAI. Lanthanide
doped ions in UCNP could lead to quenching of luminescence due to the solvent
relaxation process in aqueous conditions. These luminescence quenching phenomena