However, it is no secret that there are advantages and disadvantages associated with very small cars. Likewise, nanotechnology has good and bad features as well. It is therefore
pertinent to state that nanoparticles, which are very small
versions of their precursors, may have desirable or undesirable properties. To earn the title of nanoparticle, it must be
small enough that at least one dimension, but preferably two,
should be 100 nm or less. Nanoparticles of titanium dioxide
are present in many products which are used in common
places. Nanotitanium dioxides (nano-TiO 2 ) are being used in
several products (Bis and Wu 2005) such as toothpaste,
personal care products, sunscreen, pigments and outdoor
building materials like paving stones for decoration. The
roles of nanotitanium dioxide in the above and other applications will become clear after the properties of titanium
dioxide are described and discussed.
1.1 Background Information on the Source
of Titanium Dioxide (TiO 2 )
Titanium dioxide (TiO 2 ) is classified as a mineral, which is
commonly known as titania. Just for clarity, we should note
the difference between a mineral and an ore. A mineral
occurs naturally in the earth’s crust and has a definite range
of formulae whereas an ore refers to a rock which is rich in
minerals making extraction of the metal an economically
viable process. The naturally occurring mineral forms of
titanium dioxides are rutile, ilmenite and anatase. There are
other crystal phases which are formed under specific conditions, especially in high pressures. The purpose in dwelling on the different crystalline forms of titanium dioxide is
that the literature (Riaz and Naseem 2015; Tan et al. 2017)
abounds with reports of titanium dioxide in a specific crystalline form. Also, titanium dioxide appears to be ubiquitous;
so much, so that, it is even present in significant amounts, in
beach sea sand (Shalini et al. 2020), in the rutile modification. The nanoforms of titanium dioxide were estimated
(Robichaud et al. 2009), to amount 4 million metric tons per
annum (MT year
−1 ). As a guesstimate, the present production of nano-TiO 2 could be double or more.
1.2 The Motivation for Focusing on Nano-TiO 2
Several personal care products, especially in most toothpastes contain significant amounts of nano-TiO 2 . The
end-of-use destiny of these items is wastewater treatment
works (WWTW) where varying amounts of nano-TiO 2 are
removed as sludge which is disposed on land. The water
from WWTWs containing residual amounts nano-TiO 2 is
invariably passed into rivers from the water may be utilized
for agricultural purposes. If this scenario is currently in
vogue, it implies that increasing amounts of nano-TiO 2 will
be found in soil and water. Thus, there is a significant chance
that these nanoparticles may be accumulated by edible
plants. Therefore, the presence of nano-TiO 2 or its changed
form may be an integral part of foods consumed by humans.
Furthermore, nano-TiO 2 present in such soil and water may
enter into the plants and affect them positively or negatively
with regard to growth. If there is a need to deliberately
reduce the amounts of nano-TiO 2 in the soils, the phenomenon of nanophytoremediation may be employed. Several reports covering both these cases involving interactions
of nano-TiO 2 with plant life will be used to get an understanding of how these interactions occur and the results
thereof in respect of the health of humans. The impact of
factors such as crystal type, size, shape and possible pathways will be considered. This will be preceded by a brief
report of the methods of generating nano-TiO 2 , characterization of the products and very importantly the properties of
nano-TiO 2 . To accentuate the importance of the interactions
of nano-TiO 2 with plants, brief notes on other applications of
nano-TiO 2 will also be included.
2 Importance and Classification
of the Methods of Synthesis of Nano-TiO 2
For describing the methods of synthesis, it has to be stated
that the properties of nanoparticles such as size, shape and
the reactivity of the faces of the crystal would be extremely
useful. It is thus necessary to provide information on the
various methods of synthesis of nanoparticles of TiO 2 with
special reference to properties pertinent to the interaction of
nano-TiO 2 . To get a good perspective of the various methods
used already, a general classification such as the following
may be useful. Firstly, they could be physical or
non-physical methods. Another classification may be ‘green’
and ‘non-green’ methods. The various methods may be
driven by heat, electricity, sound, microwaves, solvents,
chemicals and biochemicals. Green methods are taken to be
driven by chemicals in plants; hence, biochemical for classification purposes. There may be methods which do not fall
into these categories but the broad classification, given
above, may have the benefits of highlighting the concepts
involved. Synopses of some of the array of methods available to synthesize nanoparticles of nano-TiO 2 (Fig. 1) are
given below.
74
K. G. Moodley and V. Arumugam
pertinent to state that nanoparticles, which are very small
versions of their precursors, may have desirable or undesirable properties. To earn the title of nanoparticle, it must be
small enough that at least one dimension, but preferably two,
should be 100 nm or less. Nanoparticles of titanium dioxide
are present in many products which are used in common
places. Nanotitanium dioxides (nano-TiO 2 ) are being used in
several products (Bis and Wu 2005) such as toothpaste,
personal care products, sunscreen, pigments and outdoor
building materials like paving stones for decoration. The
roles of nanotitanium dioxide in the above and other applications will become clear after the properties of titanium
dioxide are described and discussed.
1.1 Background Information on the Source
of Titanium Dioxide (TiO 2 )
Titanium dioxide (TiO 2 ) is classified as a mineral, which is
commonly known as titania. Just for clarity, we should note
the difference between a mineral and an ore. A mineral
occurs naturally in the earth’s crust and has a definite range
of formulae whereas an ore refers to a rock which is rich in
minerals making extraction of the metal an economically
viable process. The naturally occurring mineral forms of
titanium dioxides are rutile, ilmenite and anatase. There are
other crystal phases which are formed under specific conditions, especially in high pressures. The purpose in dwelling on the different crystalline forms of titanium dioxide is
that the literature (Riaz and Naseem 2015; Tan et al. 2017)
abounds with reports of titanium dioxide in a specific crystalline form. Also, titanium dioxide appears to be ubiquitous;
so much, so that, it is even present in significant amounts, in
beach sea sand (Shalini et al. 2020), in the rutile modification. The nanoforms of titanium dioxide were estimated
(Robichaud et al. 2009), to amount 4 million metric tons per
annum (MT year
−1 ). As a guesstimate, the present production of nano-TiO 2 could be double or more.
1.2 The Motivation for Focusing on Nano-TiO 2
Several personal care products, especially in most toothpastes contain significant amounts of nano-TiO 2 . The
end-of-use destiny of these items is wastewater treatment
works (WWTW) where varying amounts of nano-TiO 2 are
removed as sludge which is disposed on land. The water
from WWTWs containing residual amounts nano-TiO 2 is
invariably passed into rivers from the water may be utilized
for agricultural purposes. If this scenario is currently in
vogue, it implies that increasing amounts of nano-TiO 2 will
be found in soil and water. Thus, there is a significant chance
that these nanoparticles may be accumulated by edible
plants. Therefore, the presence of nano-TiO 2 or its changed
form may be an integral part of foods consumed by humans.
Furthermore, nano-TiO 2 present in such soil and water may
enter into the plants and affect them positively or negatively
with regard to growth. If there is a need to deliberately
reduce the amounts of nano-TiO 2 in the soils, the phenomenon of nanophytoremediation may be employed. Several reports covering both these cases involving interactions
of nano-TiO 2 with plant life will be used to get an understanding of how these interactions occur and the results
thereof in respect of the health of humans. The impact of
factors such as crystal type, size, shape and possible pathways will be considered. This will be preceded by a brief
report of the methods of generating nano-TiO 2 , characterization of the products and very importantly the properties of
nano-TiO 2 . To accentuate the importance of the interactions
of nano-TiO 2 with plants, brief notes on other applications of
nano-TiO 2 will also be included.
2 Importance and Classification
of the Methods of Synthesis of Nano-TiO 2
For describing the methods of synthesis, it has to be stated
that the properties of nanoparticles such as size, shape and
the reactivity of the faces of the crystal would be extremely
useful. It is thus necessary to provide information on the
various methods of synthesis of nanoparticles of TiO 2 with
special reference to properties pertinent to the interaction of
nano-TiO 2 . To get a good perspective of the various methods
used already, a general classification such as the following
may be useful. Firstly, they could be physical or
non-physical methods. Another classification may be ‘green’
and ‘non-green’ methods. The various methods may be
driven by heat, electricity, sound, microwaves, solvents,
chemicals and biochemicals. Green methods are taken to be
driven by chemicals in plants; hence, biochemical for classification purposes. There may be methods which do not fall
into these categories but the broad classification, given
above, may have the benefits of highlighting the concepts
involved. Synopses of some of the array of methods available to synthesize nanoparticles of nano-TiO 2 (Fig. 1) are
given below.
74
K. G. Moodley and V. Arumugam