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monodispersed fine nanoparticles. Such work is being carried out in the laborato-
ries of the present authors. There are no reports in the literature on the synthesis of
anhydrous rare earth phosphate nanoparticles using the continuous flow reactor
under SCF conditions. This is an unexplored area at the moment for researchers.
10.6.13 Rare Earth Garnets
The preparation of rare earth garnets using SCF technology is popular. The alkaline
solvents are used in most of the works in the temperature range of 420
450
C.
Over the past few years, the active rare earth
doped garnet fine particles have
been prepared by several workers using SCF technology
[212,213]
. The nanopho-
sphor garnet is better than fine-powder phosphors with a larger particle size
because nanophosphors could reduce internal scattering when they were coated
onto a bare light-emitting diode surface. Using the continuous flow type of reactor
with SCF technology, only fine particles of rare earth garnets have been obtained.
Fine powders of YAG were prepared using glycothermal method with alkoxides
(aluminum isopropoxide) as the starting material. The whole problem in all these
experiments is the larger particle size and agglomeration. The results of the synthe-
sis of nanoparticles of YAG:Ce in the size range 60 nm using SCF technology with
batch reactors is perhaps the best in the literature
[212]
. The use of surface modi-
fier in the supercritical hydrothermal conditions would be the best solution to
synthesize high-quality small nanoparticles of YAG.
More recently, the nanoparticles of YAlO
3
(YAP) doped with active rare earth
ions like Nd, Er, and Eu belonging to the perovskite family with cubic, hexagonal,
and orthorhombic polymorphs are becoming popular materials for bioimaging pur-
poses. The orthorhombic YAP has several advantages over the conventional YAG
owing to the ideal distribution coefficients for active rare earth elements. The present
authors have successfully synthesized this orthorhombic garnet for the first time using
SCF technology. However, the particle size is still a problem. The use of surface
modifiers and capping agents would provide some solutions to these problems.
Rare earth oxides and hydroxides doped with active rare earth ions like Er and
Eu (Lu
2
O
3
:Eu
3
1
;Y
2
O
3
:Eu
3
1
;Y
2
O
3
:Er
3
1
;R
x
Lu
2
2
x
O
3
:Eu
3
1
; where R
Y,Gd; Y
(OH)
3
:Eu
3
1
) are some of the potential phosphors in recent years with a capability
in biological applications
[214
5
218]
. However, the use of SCF technology has
been seldom reported. Presently also, using conventional hydrothermal and
solvothermal syntheses routes, several bioimaging phosphors, upconverting nano-
phosphors for biological labels like LaPO
4
:Nd, NiP, RF
3
(where R
rare earths),
5
and Gd
2
O
3
:Eu have been reported in the literature.
Bioimaging using phosphor attracts keen interest among researchers. To recog-
nize how a drug delivers inside the body is essential to design a DDS. Since the
organic probes used in bioimaging cannot survive too long in the body, the replace-
ment was found in some semiconductor materials showing strong photolumines-
cence behavior, whose wavelength could be controlled with its particle size
(“quantum size effect”)
[219]
. With a same wavelength of excitation, wide range of
colors can be obtained by changing the particle size of the nanoparticles of CdSe,
