380
N. Ashwin Kumar et al.
(hMSCs) in an in-vivo mouse model in the second NIR window region [294]. The
results also indicate an increased chemical- and photostability, making Ag 2 S QDs a
potential candidate for long-term cell tracking.
To have clear insights into the physiological and pathological processes connected
with the circulatory system Ag 2 S QDs have been implemented in dynamic imaging
of the circulatory system. NIR II imaging with Ag 2 S QDs significantly enhanced the
clarity and the penetration depth for envisaging lymph nodes and lymphatic vessels
[295]. Subcutaneous injections of PEGylated Ag 2 S QDs exhibited a narrowing of
the lymphatic vessel cross-sectional intensity profiles due to very low scattering and
decreased absorbance in the NIR III region. A comparison with ICG (used in clinical
practice for the detection and resection of sentinel lymph nodes) demonstrated that
ICG was unable to resolve the deeper lymph nodes. The studies presented here
provided agreeable results contrastingly there are some limitations associated with
QDs in imaging applications. First, heavy metals are incorporated in the core and
the materials used for capping, which could result in toxicity [296]. Second, the
luminescence emitted from quantum dots is demonstrated by variations in intensity,
leading to the appearance of ON and OFF states [297]. This downtime between
the states could hinder the establishment between two successive frames in singlemolecule tracking applications.
Upconverting NPs (UCNP) is a new generation of imaging compounds doped
with lanthanide ions capable of absorbing light in the NIR region (usually 980 nm)
and emits within the visible region [298, 299]. The UCNP usually consists of number
lattices of a ceramic material such as LaF 3 , YF 3 , Y 2 O 3 , NaYF 4 embedded with a trivalent transition metal, actinide, or lanthanide ions such as Yb
3+ , Er
3+ and Tm
3+ [300–
302]. The properties associated with UCNP include non-photo blinking emission,
biocompatibility, long anti-stokes shifts (up to 500 nm), excellent photochemicaland thermal- stability [303–306]. For long-term non-invasive in vivo optical imaging
PEG-lipid functionalized and biocompatible NaYF 4 : Tm
3+ , Yb
3+ UCNP was implemented in real-time to track excretion pathways, retention, and clearance [307]. This
was evaluated using a high-quality NIR-to-NIR upconversion luminescence (UCL)
imaging. The study revealed that UCNPs remained in the injected site for 14 days and
were significantly dropped after 30 days. The evaluation of lymphatic tissue status
is an imperative aspect to be considered in the diagnosis of people with tumors. A
new kind of imaging probes based on PEGylated NaGdF 4 : Yb, Er@NaGdF 4 UCNPs
was investigated for the detection of lymphatic metastasis of gastric cancer [308].
Additionally, due to a very low background at 980 nm laser excitation, lymphatic
metastasis lesser than 1 mm were successfully identified. Recently, Tian et al. [309]
evaluated novel carboxy-terminated silica-coated UCNPs with different surface functional groups –COOH, -PEG, D-SP5, and -UEA-I. Their study revealed that the functional group—UEA-I presented a bright UCL at the tumor sites and has an efficient
tumor targeting capacity in the mouse model. The limitations related to UCNPs are
lower upconversion quantum yield, and conventional fluorescence probes, including
QDs, have a very short lifetime [298].
Persistent luminescent nanoparticles (PLNPs) could serve as an alternate candidate to overcome the shortcomings associated with QDs and UCNPs. Persistent
N. Ashwin Kumar et al.
(hMSCs) in an in-vivo mouse model in the second NIR window region [294]. The
results also indicate an increased chemical- and photostability, making Ag 2 S QDs a
potential candidate for long-term cell tracking.
To have clear insights into the physiological and pathological processes connected
with the circulatory system Ag 2 S QDs have been implemented in dynamic imaging
of the circulatory system. NIR II imaging with Ag 2 S QDs significantly enhanced the
clarity and the penetration depth for envisaging lymph nodes and lymphatic vessels
[295]. Subcutaneous injections of PEGylated Ag 2 S QDs exhibited a narrowing of
the lymphatic vessel cross-sectional intensity profiles due to very low scattering and
decreased absorbance in the NIR III region. A comparison with ICG (used in clinical
practice for the detection and resection of sentinel lymph nodes) demonstrated that
ICG was unable to resolve the deeper lymph nodes. The studies presented here
provided agreeable results contrastingly there are some limitations associated with
QDs in imaging applications. First, heavy metals are incorporated in the core and
the materials used for capping, which could result in toxicity [296]. Second, the
luminescence emitted from quantum dots is demonstrated by variations in intensity,
leading to the appearance of ON and OFF states [297]. This downtime between
the states could hinder the establishment between two successive frames in singlemolecule tracking applications.
Upconverting NPs (UCNP) is a new generation of imaging compounds doped
with lanthanide ions capable of absorbing light in the NIR region (usually 980 nm)
and emits within the visible region [298, 299]. The UCNP usually consists of number
lattices of a ceramic material such as LaF 3 , YF 3 , Y 2 O 3 , NaYF 4 embedded with a trivalent transition metal, actinide, or lanthanide ions such as Yb
3+ , Er
3+ and Tm
3+ [300–
302]. The properties associated with UCNP include non-photo blinking emission,
biocompatibility, long anti-stokes shifts (up to 500 nm), excellent photochemicaland thermal- stability [303–306]. For long-term non-invasive in vivo optical imaging
PEG-lipid functionalized and biocompatible NaYF 4 : Tm
3+ , Yb
3+ UCNP was implemented in real-time to track excretion pathways, retention, and clearance [307]. This
was evaluated using a high-quality NIR-to-NIR upconversion luminescence (UCL)
imaging. The study revealed that UCNPs remained in the injected site for 14 days and
were significantly dropped after 30 days. The evaluation of lymphatic tissue status
is an imperative aspect to be considered in the diagnosis of people with tumors. A
new kind of imaging probes based on PEGylated NaGdF 4 : Yb, Er@NaGdF 4 UCNPs
was investigated for the detection of lymphatic metastasis of gastric cancer [308].
Additionally, due to a very low background at 980 nm laser excitation, lymphatic
metastasis lesser than 1 mm were successfully identified. Recently, Tian et al. [309]
evaluated novel carboxy-terminated silica-coated UCNPs with different surface functional groups –COOH, -PEG, D-SP5, and -UEA-I. Their study revealed that the functional group—UEA-I presented a bright UCL at the tumor sites and has an efficient
tumor targeting capacity in the mouse model. The limitations related to UCNPs are
lower upconversion quantum yield, and conventional fluorescence probes, including
QDs, have a very short lifetime [298].
Persistent luminescent nanoparticles (PLNPs) could serve as an alternate candidate to overcome the shortcomings associated with QDs and UCNPs. Persistent