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R E V I E W Diverse biotechnological applications of multifunctional titanium dioxide nanoparticles: An up-to-date review

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Abstract

Titanium dioxide (TiO 2) nanoparticles (NPs) are one of the topmost widely used metallic oxide nanoparticles. Whether present in naked form or doped with metals or polymers, TiO 2 NPs perform immensely important functions. However, the alteration in size and shape by doping results in improving the physical, chemical, and biological behaviour of TiO 2 NPs. Hence, the differential effects of various TiO 2 nanostructures including nanoflakes, nanoflowers, and nanotubes in various domains of biotechnology have been elucidated by researchers. Recently, the exponential growth of research activities regarding TiO 2 NPs has been observed owing to their chemical stability, low toxicity, and multi-faceted properties. Because of their enormous abundance, plants, humans, and environment are inevitably exposed to TiO 2 NPs. These NPs play a significant role in improving agricultural attributes, removing environmental pollution, and upgrading the domain of nanomedicine. Therefore, the currently ongoing studies about the employment of TiO 2 NPs in enhancement of different aspects of agriculture, environment, and medicine have been extensively discussed in this review. K E Y W O R D S drug delivery, nanobiotechnology, nanofertilisers, nanosensors, phototherapy, TiO 2 NPs 1 | INTRODUCTION Nanotechnology is the branch of science that uses materials at the nanometre scale. Nanobiotechnology is the rapidly emerging field involving interface between nanotechnology and biology. This is interdisciplinary field having applications in almost all areas of biotechnology [1, 2]. Titanium dioxide (TiO 2) nanoparticles (NPs), often called titania, are the most exceptional NPs among all transition metal oxide nano-structures due to their distinctive behaviour. The four polymorphic forms in which the crystalline structure of TiO 2 NPs exists are rutile (tetragonal) of 2.96 eV bandgap, anatase (tetragonal) of 3.2 eV bandgap, brookite (orthorhombic) of 3.02 eV bandgap, and TiO 2-B (monoclinic) of 3.69 eV band gap [3]. All forms of titania are found in zero dimension (0D), one dimension (1D), two dimensions (2D), and three dimensions (3D) and have promising applications in biotechnology. However, the literature suggests that anatase and rutile have greater biotechnological applications than brookite and TiO 2-B owing to the highly stable nature of the former [3, 4]. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
Received: 17 December 2021
-
Revised: 13 February 2022
-
Accepted: 31 March 2022
-
IET Nanobiotechnology
DOI: 10.1049/nbt2.12085
REVIEW
Diverse biotechnological applications of multifunctional titanium
dioxide nanoparticles: An uptodate review
Rabia Javed
1,2
|Noor ul Ain
2
|Ayesha Gul
3
|Muhammad Arslan Ahmad
4
|
Weihong Guo
5
|Qiang Ao
1
|Shen Tian
6
1
NMPA Key Laboratory for Quality Research and Control of Tissue Regenerative Biomaterial, Institute of Regulatory Science for Medical Device, National Engineering Research Center for
Biomaterials, Sichuan University, Chengdu, China
2
Department of Biotechnology, QuaidiAzam University, Islamabad, Pakistan
3
NANOCAT Research Center, Institute for Advanced Studies, University of Malaya, Kuala Lumpur, Malaysia
4
Shenzhen Key Laboratory of Marine Bioresource and Ecoenvironmental Science, College of Life Sciences and Oceanography, Shenzhen University, Shenzhen, China
5
Fuwai Hospial, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China
6
Department of Neurology, The 4th Afliated Hospital of China Medical University, Shenyang, China
Correspondence
Rabia Javed, Department of Biotechnology, Quaidi
Azam University, Islamabad, Pakistan.
Email: rabia.javed@ymail.com
Shen Tian, Department of Neurology, The 4th
Afliated Hospital of China Medical University,
Shenyang, Liaoning 110032, China.
Email: cmu4h_ts1969@126.com
Funding information
Sichuan Science and Technology Program, Grant/
Award Number: 2020YFH0008; National Key R &
D Program of China, Grant/Award Number:
2020YFF0426289
Abstract
Titanium dioxide (TiO
2
) nanoparticles (NPs) are one of the topmost widely used metallic
oxide nanoparticles. Whether present in naked form or doped with metals or polymers,
TiO
2
NPs perform immensely important functions. However, the alteration in size and
shape by doping results in improving the physical, chemical, and biological behaviour of
TiO
2
NPs. Hence, the differential effects of various TiO
2
nanostructures including
nanoakes, nanoowers, and nanotubes in various domains of biotechnology have been
elucidated by researchers. Recently, the exponential growth of research activities regarding
TiO
2
NPs has been observed owing to their chemical stability, low toxicity, and multi-
faceted properties. Because of their enormous abundance, plants, humans, and envi-
ronment are inevitably exposed to TiO
2
NPs. These NPs play a signicant role in
improving agricultural attributes, removing environmental pollution, and upgrading the
domain of nanomedicine. Therefore, the currently ongoing studies about the employment
of TiO
2
NPs in enhancement of different aspects of agriculture, environment, and
medicine have been extensively discussed in this review.
KEYWORDS
drug delivery, nanobiotechnology, nanofertilisers, nanosensors, phototherapy, TiO
2
NPs
1
|
INTRODUCTION
Nanotechnology is the branch of science that uses materials at
the nanometre scale. Nanobiotechnology is the rapidly
emerging eld involving interface between nanotechnology
and biology. This is interdisciplinary eld having applications
in almost all areas of biotechnology [1, 2]. Titanium dioxide
(TiO
2
) nanoparticles (NPs), often called titania, are the most
exceptional NPs among all transition metal oxide nano-
structures due to their distinctive behaviour. The four
polymorphic forms in which the crystalline structure of TiO
2
NPs exists are rutile (tetragonal) of 2.96 eV bandgap, anatase
(tetragonal) of 3.2 eV bandgap, brookite (orthorhombic) of
3.02 eV bandgap, and TiO
2
B (monoclinic) of 3.69 eV band
gap [3]. All forms of titania are found in zero dimension (0D),
one dimension (1D), two dimensions (2D), and three di-
mensions (3D) and have promising applications in biotech-
nology. However, the literature suggests that anatase and rutile
have greater biotechnological applications than brookite and
TiO
2
B owing to the highly stable nature of the former [3, 4].
This is an open access article under the terms of the Creative Commons AttributionNonCommercialNoDerivs License, which permits use and distribution in any medium, provided the
original work is properly cited, the use is noncommercial and no modications or adaptations are made.
© 2022 The Authors. IET Nanobiotechnology published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology.
IET Nanobiotechnol. 2022;119. wileyonlinelibrary.com/journal/nbt2
-
1
TiO
2
NPs are very cost efcient and easy to synthesise on
laboratory and industrial scale. Among the various methods of
preparation, sol–gel, hydrothermal, and chemical vapour
deposition are the most common methods for TiO
2
NPs'
synthesis by which the NPs having desired characteristics are
obtained [5]. These NPs have been approved by the Food and
Drug Administration (FDA) as the safe, biocompatible, highly
reactive, and chemically stable substances. It is estimated that
the global annual production of TiO
2
NPs was 1,175,176 tons
in 2012 [6] and it will reach up to 2.5 million metric tons in
2050, which will convert nearly 100% of the total TiO
2
market
into nano [7]. It has been estimated that the concentration of
TiO
2
NPs in wastewater treatment plants is 5–20 μg/L and is
expected to increase gradually with the use of different NPs
[8–10]. TiO
2
NPs are strong oxidising agents capable of very
high photocatalysis. Other than that, nanostructures of TiO
2
have profound applications in the agriculture and food in-
dustry, food packaging, textile, energy, ceramics, cosmetics,
medical devices, pharmacy, and theranostics of various diseases
[11, 12]. These are excellent sanitisers and the remarkable
antibacterial, UV protection, and catalytic activity of TiO
2
NPs
makes them effective to be used in medicine, agriculture, and
environmental remediation. The signicance of TiO
2
NPs lies
in the fact that they can solve problems and challenges related
to the diverse scientic elds [13, 14].
The unique physicochemical and biological properties of
TiO
2
NPs make them potential candidates for scientic
investigation regarding their tremendous functionalities and
technological applications [3]. As the TiO
2
NPs have been
explored in different elds of science due to their versatility,
this review is a novel attempt to acknowledge the readers with
the recent advances in the inclusive usage of TiO
2
nano-
structures regarding the areas of medical biotechnology (bio-
imaging, drug delivery, phototherapy, antimicrobial activity, and
tissue regeneration), agricultural biotechnology (nano-
pesticides, nanofertilisers, and plant tolerance), and environ-
mental biotechnology (nanosensors and soil/water/air
remediation). According to our knowledge, this is the rst
comprehensive report outlining the applications of TiO
2
NPs
in the interrelated areas of biomedicine, agriculture, and envi-
ronmental remediation in a single document.
2
|
TiO
2
NANOPARTICLES IN
BIOMEDICINE
In recent times, there has been a steadily growing interest in
using metallic oxide NPs in various biomedical domains [15].
TiO
2
NPs are extensively studied due to their fascinating
properties such as low toxicity, biological and chemical alert-
ness, thin lm transparency, and high chemical stability. Among
other applications, TiO
2
NPs' medical applications are
remarkable, which may play an important role in the
improvement of healthcare sector. For example, the excellent
photocatalytic ability of photoexcited TiO
2
NPs depicts the
ability to kill cancer cells signicantly, and it can also be utilised
in genetic engineering as a nucleic acid endonuclease.
Moreover, TiO
2
NPbased nanostructures and nano-
composites may act as candidate tools for various biomedical
applications [16]. However, the toxicological prole of TiO
2
NPs should also be considered under both in vitro and in vivo
conditions [17]. So far, ample research work focussing on the
TiO
2
NPs' biomedical applications has been reported. The
following subsection describes the potential role of TiO
2
NPs
in drug delivery, bioimaging, phototherapy, tissue engineering,
and as antibacterial agents.
2.1
|
Bioimaging
There are a wide variety of imaging techniques that can be used
in scientic research and for the biomedical purposes,
including spectroscopy techniques such as (near) infrared (IR)
spectroscopy and Raman spectroscopy, nuclear magnetic
resonance (MRI), radioimaging using specic nuclides,
computed tomography (CT) scanning, and more advanced
scanning techniques that include laser ablation, ICPMS,
MALDIMS, and so forth [18]. Improvement in diagnostic
techniques leads to early treatment and recovery of patient.
NPs in general and TiO
2
NPs in particular are extensively
studied and used in the diagnostic techniques including MRI
and CT as contrast agents. TiO
2
NPs are activated by irradi-
ation, therefore, act as both diagnostic and therapeutic agents
at the same time (Figure 1). In a study, image contrast prop-
erties of TiO
2
NPs were assessed by means of MRI and CT
scanner. A clear change in imaging was detected between the
control and TiO
2
NP samples on T
2
weighted images, showing
that TiO
2
NPs can possibly be expended as an innovative
theranostic drug with radiosensitising capacity and radiological
diagnostic ability because of chemical group modication on
their surface [19, 20]. Also, TiO
2
NPs have numerous bene-
cial photophysical characteristics, particularly a photocatalytic
feature. The photocatalytic activity of TiO
2
NPs has been
investigated for use in diagnostic assays.
As per reports, the in situ labelling approaches for uo-
rescence microscopy to stain the TiO
2
NPs taken up by the
cells were studied in detail. Fluorescent biotin and uorescent
streptavidin were used to label the NPs before and after
cellular uptake in the rst approach. Whereas, in second
approach, coppercatalysed azidealkyne cycloaddition was
used for labelling and detection of azideconjugated TiO
2
NPs.
Synchrotron Xray uorescence microscopy (XFM) was used
to detect TiO
2
NPs. The results exhibiting TiO
2
NPs by XFM
displayed outstanding overlay with the site of optical uores-
cence as detected by the confocal microscopy [21]. TiO
2
NPs
are prepared easily and modied, such as with europium (III),
and TiO
2
hollow nanoshells are feasible twophoton nanop-
robes. They cling to HeLa cervical cancer cells when coated
with poly(ethylene imine) and hence detected [22]. According
to the literature, mesoporous titania was coated on silver–silica
core–shell NPs, resulting in an Ag@SiO
2
@mTiO
2
nano-
architecture. The metal core worked as a uorescence
enhancer. The particles were loaded with uorescent avin
mononucleotide and the uorescent cancer medication
2
-
JAVED ET AL.
doxorubicin, and they were employed for simultaneous
bimodal (uorescence and surfaceenhanced Raman spectros-
copy [SERS]) drug delivery imaging [23].
2.2
|
Drug delivery
Conventionally, routes of drug administration are oral, nasal,
and parenteral, by which drug is distributed to the whole body.
This limits the effectiveness of drug as well as causes the po-
tential side effects. Thus, more efcient drug delivery systems
are needed to overcome any drawbacks. Advancement in
biotechnology, material science, and engineering approaches
has incorporated the nanotechnology in biomedicine for tar-
geted drug delivery. TiO
2
NPs, owing to their unique elec-
trochemical properties as well as controllable structures and
biocompatibility, have been utilised in drug delivery processes
[24]. Figure 2represents the generalised process of drug de-
livery in living cells with and without TiO
2
NPs. More spe-
cically, the targeted drug delivery mechanism of TiO
2
NPs
involves the photocatalytic degradation of the organic com-
pounds upon UV light excitation. When the energy absorbed
from UV exceeds the band gap of TiO
2
NPs, valence electrons
are excited to the conduction band, forming an electron (e) and
hole (h+) pair as well as active free radicals (OH and O
2
) that
effectively decompose the organic compounds on the TiO
2
NPs' surface, providing a theoretical foundation for light
triggered drug release at a targeted site. It's worth noting
that TiO
2
NPs have been claimed to possess anticancer
properties due to the generation of active free radicals on UV
light exposure [25, 26].
To make TiO
2
NPs more biocompatible and to enhance
their property of controlled drug release, these are coated with
polyethylene glycol (PEG). TiO
2
NPs were used as nano-
carriers for the targeted drug delivery of an anticancer drug,
paclitaxel. Although TiO
2
NPs are stable and nontoxic, their
biocompatibility is enhanced by an attachment of PEG to their
surface and folic acid (FA) grafted as a ligand for the targeted
delivery of the drug. The characterisation studies revealed
successful PEGylation and grafting of ligand to these NPs.
Also, the drugloaded NPs held a signicantly greater
adsorption capability. Moreover, the in vitro release studies of
paclitaxel from FA–PEG–TiO
2
NPs showed an initial fast
release followed by a sustained release phase [27].
Furthermore, an antitumour potential of TiO
2
NPs loaded
with paclitaxel was enhanced due to the incorporation of
chitosan owing to its antiinammatory and antioxidant
properties against osteosarcoma [28]. In another report, hol-
low TiO
2
nanotubes were used along with iron oxide NPs as a
nanocarrier through thermal annealing for magnetic guidance
and drug delivery. Cytotoxic studies revealed that this nano-
carrier was nontoxic to HeLa cells at therapeutic concentra-
tions (200 μg/ml). Under magnetic eld gradient, adhesion
and endocytosis to a layer of HeLa cells of nanotubes was
observed. Two drugs, topoisomerase inhibitor camptothecin
and oligonucleotides, for cell transfection were loaded in
nanotubes exhibiting 90% and 100% killing and cellular up-
take, respectively, hence suggesting the use of TiO
2
NPs in
therapeutics [29].
2.3
|
Phototherapy
Cancer is still one of the leading causes of death around the
world and its treatment is also challenging. Primary treatment
involves the surgery in combination with chemotherapy or
radiotherapy. Chemotherapy faces challenges such as drug
resistance or drug is inaccessible to the tumour cells, thus
compromising its effectiveness [30]. Phototherapy, such as
photodynamic therapy (PDT) and photothermal therapy
(PTT), has become an emerging therapeutic technique for the
treatment of different types of cancer because of its limited
FIGURE 1 TiO
2
nanoparticlebased detection and killing of tumour cells
JAVED ET AL.
-
3
severity, high efcacy, and minimal side effects [31]. PDT is a
comparatively recent therapeutic approach that has garnered
attention in the last 3 decades. Its therapeutic principle needs
the existence of a photosensitiser, proper light wavelength, and
molecular oxygen. Photosensitiser and light are two adjustable
elements among these three components, and the development
of tumourspecic photosensitiser is of interest to many
chemists and pharmaceutical researchers. The photochemical,
photobiological, and pharmacokinetic features of the photo-
sensitiser inuence the success of PDT therapy. Most strong
photosensitisers contain a prolonged delocalised aromatic
electron system, which permits them to absorb light effectively.
In aqueous conditions, they quickly combine due to stacking
and hydrophobic interactions. This leads to the aggregation,
consequently leading to bioavailability and hampering of
reactive oxygen species (ROS) generation [32].
Recently, NPs have been utilised to deliver photosensitisers
for PDT, which is a good way to improve the targeting of
photosensitisers [33]. In PDT, ROS generation takes place via
stimulation of light absorbing photosensitisers causing cell
damage. PDT is expended in the treatment of various malig-
nancies and abnormal vasculatures [34]. Several studies are
now underway to investigate the use of molybdenum oxide,
TiO
2
, ZnO, and tungsten oxide NPs as photosensitisers in
PDT [35]. Various reports have shown the promising results of
using TiO
2
NPs as photosensitisers or as conjugates to deliver
photosensitisers in PDT and PTT as TiO
2
NPs have unique
catalytic activity. Moosavi and colleagues reported the photo-
dynamic nitrogendoped titanium dioxide (NTiO
2
) NPs in
conjugation with visible light. This novel PDT system showed
not only the generation of ROS but also autophagy in
leukaemia K562 cells. It implies that PDT with NTiO
2
NPs is
an efcient approach of priming autophagy via ROS genera-
tion. The potential of photoactivated NTiO
2
NPs to achieve
desirable cellular outcomes constitutes a novel cancer cell
treatment method [36]. In another study, scientists reported the
combined effect of 2,2,6,6, tetramethylpiperidineNoxyl
(TEMPO)coated TiO
2
nanorods (NRs) for PDT. The sol–
gel technique was employed for the synthesis of TiO
2
NRs
followed by TEMPO grafting via oxoammonium salt. It was
reported that TEMPOgrafted TiO
2
NRs produced signicant
therapeutic response against human breast cancer cell line
(MCF7) in combination with PDT under UV light as
compared to TiO
2
NRs in dark. Also, this nanocomposite had
overcome the hindrance of multidrug resistance (MDR)
alongside the PDT treatment [37].
Another phototherapy that employs NPs is PTT, which is a
novel procedure to competently treat malignancies with no
major limitation or side effect. In PTT, NPs convert the energy
of photon into heat owing to their explicit physicochemical
characteristics and generating hyperthermia in tumour tissues.
A promising candidate with denite features for application in
PTT of tumours is TiO
2
NPs [38, 39]. TiO
2
NPs along with a
magnetic core make nanohybrid for imbedding an extensive
range of theranostics qualities including magneticguided and
triggered therapeutic delivery systems. One such system has
recently been developed, which is mesoporous TiO
2
coated
Fe
3
O
4
NPs. By employing combined production approach, the
solvent thermal method was used to generate an amino
functional magnetic core and porous shell formation via
FIGURE 2 Diagrammatic representation of drug delivery with and without TiO
2
nanoparticles
4
-
JAVED ET AL.
homogeneous precipitation of TiOSO
4
. To produce hollow
TiO
2
NPs, manufacture of iron oxide TiO
2
core–shell nano-
composite is a benet because of the easy magnetic core
removal within the process [40].
2.4
|
Antimicrobial activity
Antibiotic overuse has resulted in the emergence of MDR
bacterial strains, which is currently a source of concern for
food safety and human health [41]. Metal oxide NPs have
attracted the attention of researchers while looking for novel
antibacterial compounds. One of the main contributing factors
in antibiotic resistance is the formation of biolms [42].
Therefore, research is shifted to the exploration of antimi-
crobial potential of metal and metal oxide NPs [43]. Because of
their photocatalytic nature and the fact that they are chemically
stable, nontoxic, affordable, and Generally Recognised as Safe,
TiO
2
NPs have been regarded as an appealing antimicrobial
agents [44]. Moreover, TiO
2
NPs are one of the increasingly
being used metal oxide NPs in biomedicine because of their
antimicrobial activity [45].
Photocatalytic antimicrobial activity is exhibited by TiO
2
NPs when irradiated with UV light (<385 nm). The effec-
tiveness of TiO
2
NPs' antimicrobial activity depends on the
microbial cell surface thickness and the order is virus >bacte-
rial wall >bacterial spore. In case of antibacterial activity, the
generation of hydroxyl radicals as a response to photocatalysis
by TiO
2
NPs causes the oxidative stress on the cell membrane
of bacteria. The photocatalytic activity of TiO
2
NPs augments
peroxidation of unsaturated phospholipids present in the
plasma membrane, consequently damaging the bacterial
membrane. It also interferes with vital biological mechanisms
including respiration, oxidative phosphorylation reaction, and
semipermeability [46]. The simplied mechanism of action of
TiO
2
NPs against bacteria has been illustrated in Figure 3.
Among the major nosocomial infections, methicillin
resistant Staphylococcus aureus (MRSA) is involved in broad
spectrum infections and the emergence of bacterial resis-
tance. According to the literature, the antibacterial activity of
TiO
2
NPs against MRSA was explored. A study was designed
to evaluate the effect of TiO
2
NPs against the biolm for-
mation by MRSA using tissue culture plate method. Total 30
isolates were taken and out of them, 22 were involved in strong
biolm formation and 2 were weak in the formation of bio-
lm. The TiO
2
NPs (500 μg/ml) inhibited the growth of both
strong and weak MRSA, thus showing that the TiO
2
NPs are
promising antibacterial candidates [47]. In another study, TiO
2
NPs were used in combination with antibiotics including cef-
tazidime and cefotaxime against MDR Pseudomonas aerugi-
nosa. The samples were isolated from sputum, pus,
bronchoalveolar lavage, and endotracheal tract. On exposure to
UV light for 1 h, a bactericidal effect was observed at
>350 μg/ml. The minimum inhibitory concentration values of
TiO
2
NPs were observed six fold higher than the antibiotics.
Hence, the combination of antibiotics and TiO
2
NPs improved
the antimicrobial activity [48].
The inclusion of antibacterial agents into synthetic
polymerbased nanocomposites has resulted in the creation of
adaptable antibacterial materials suitable for a wide range of
biomedical, packaging, and generalpurpose applications.
However, the production of such materials using TiO
2
NPs is
difcult due to thermodynamic and kinetic hurdles that pre-
vent inorganic and generally hydrophilic nanoparticles from
dispersing in hydrophobic polymer matrices. Nevertheless,
antimicrobial nanocomposites based on titania have received a
lot of attention in recent years [49]. For example, Petica and
coworkers reported the synthesis of silver–titania nano-
composites by the electrochemical method for enhancement of
photocatalytic characteristics and antifungal and antibacterial
activity [50]. In another study, researchers developed the
parafn and silvercoated titania NPs in polyethylene nano-
composite for food packaging. Addition of 3% and 5% of
TiO
2
/Ag NPs into polyethylene with low density through
meltblending nanocomposite lms was performed and results
revealed that 5% addition of TiO
2
/Ag NPs caused signicant
reduction of bacterial growth [51]. Thus, adding the TiO
2
alone
as well as in combination with the polymers reduces the mi-
crobial growth, therefore, preventing food spoilage and
increasing the shelflife of food.
2.5
|
Tissue regeneration
Organ and tissue transplantation has been limited due to the
nonavailability or lack of donor, tissue rejection, and use of
immunosuppressants. This has increased the need and demand
for tissue engineering and regenerative medicine [52]. Efforts
have been shifted to the development of miniaturised articial
organs resembling the in vivo environment of the body.
Nanoscale entities have shown better results as compared to
macroscopic ones by providing good mechanical strength,
delivery of bioactive agents, and monitoring cell activities [53].
Moreover, disadvantages and restraining issues, for instance,
short halflife, low solubility, and instability of bioactive mol-
ecules and contrast agents, have made the NPs as promising
carriers for bioactive agents' delivery and monitoring for
biomedical applications [54]. Table 1shows the regeneration of
different tissues using TiO
2
NPs.
Burn wounds are one of the major causes of morbidity and
mortality around the world and regeneration of skin tissue is
the most challenging issue. In a recent study, the dispersion of
TiO
2
NPs has been used to improve the burn wound healing
process and regeneration of the tissue via interaction with
blood serum proteins. In this study, it was proposed that TiO
2
NPs are capable of adsorbing proteins by means of protective
coating and then render them a robust capacity for the stim-
ulation of coagulation of body uid. In vitro and in vivo studies
elucidated the initiation of cascade response and adherence of
nanocomposite covering that prevented infection and inam-
mation with rapid diminution of wound area in comparison to
control. Thus, the suspension of TiO
2
NPs evidently improved
damaged tissue regeneration along with considerable reduction
in the formation of scar as well as skin colour differences [62].
JAVED ET AL.
-
5
TiO
2
NPs have also been applied to enhance bone tissue
regeneration in combination with chitosan. According to report,
the chitosan hybrid with TiO
2
NPs making a nanosponge
scaffold for improved bone regeneration was investigated.
Although chitosan is frequently used in tissue regeneration, its
use is limited due to poor mechanical strength and non
osteogenic inductivity. Thus, chitosan sponges were imbedded
with TiO
2
NPs, and morphological and crystallographic inves-
tigation displayed even distribution of NPs. Integrity of the
nanocomposite remained intact upon addition of TiO
2
NPs,
which was conrmed by degradation studies. Biomineralisation,
molecular, and cytotoxicity assays revealed that the addition of
TiO
2
NPs enhanced apatite formation, upregulation of regen-
eration genes, and biocompatibility. It clearly suggested chitosan
and 50% TiO
2
nanohybrid sponge as a potentially unique
scaffold in the tissue engineering of bones [63].
3
|
TiO
2
NANOPARTICLES IN
AGRICULTURE
Agriculture is the core reason for human growth and the na-
tion's economy; thus, it is crucial to create and innovate green
technologies for sustainable crop sector development [64]. As
per reports, in 2050, the world population is expected to rise to
10 billion, which will lead towards the rise in demand for food
products. To meet the rising global food demand, the Food
and Agriculture Organization (FAO) has predicted that be-
tween 2020 and 2056 the food production will be needed to get
enhanced by 70% [65]. The existing modern technologies such
as aquaculture system, photovoltaic green houses, synthetic
biology, vertical farming, and fungicides etc., have brought
signicant advantages to agriculture by producing products
with maximum yield and cultivation of outofseason products
FIGURE 3 Antibacterial action mechanism of TiO
2
nanoparticles
TABLE 1TiO
2
nanocomposites used in tissue engineering
Tissue
Hierarchical structure
of TiO
2
NPs
Concentration of
TiO
2
NPs Polymer/hybrid Characteristics/effects Reference
Bone Irregular 1.0% Chitosan Improved density, osteogenesis, and vascularisation [52]
Skin Spherical 0.5% Gelatin Angiogenesis, granulation, proliferation,
antibacterial wound dressing
[55]
Bone Nanowires 1%–2% (w/w) Poly(vinylidene uoride
triuoroethylene) (P[VDF
TrFE])
Good mechanical strength, improved cell
adherence and proliferation
[56]
Skin Spherical 1.0% Gelatin composite Stability of wound area, rearrangement of
granulation tissue and collagen bres
[57]
Tooth Spherical clusters 0.5% Allyltriethoxysilane (ATES) Microhardness, exural strength [58]
Tooth Spherical 1%–2% Poly methyl methacrylate Enhanced mechanical properties [59]
Bone Needlelike 0.2% Poly(D,Llactidecoglycolide) Improved mechanical properties and wettability [60]
Bone 1%–3% Poly(vinyl alcohol) Increase in the hydrophilicity and mechanical
strength
[61]
Abbreviation: NPs, nanoparticles.
6
-
JAVED ET AL.
but also caused considerable health and eco problems due to
the inappropriate use of fertilisers and pesticides. Nanotech-
nology is an interdisciplinary fast emerging eld in developing
novel technical tools for maximum crop yield and protection
with the strategy of improving plants' capacity to absorb
maximum nutrients. Amongst various nanomaterials, extensive
research has been directed towards the agricultural applications
of TiO
2
based nanomaterials because of their unique structural
conguration, chemical stability, hydrophilicity, and eco
friendliness [66]. The following subsection demonstrates the
applications of TiO
2
NPbased nanopesticides and nano-
fertilisers as well as the TiO
2
NPs' interaction with crops
leading to plant tolerance.
3.1
|
Nanopesticides
Crop protection from different diseases and pests presents an
enduring challenge, which directs the development of inno-
vative approaches and agents. Chemically designed pesticides
are utilised to control or kill microbes, weeds, insects, and
fungi. However, the excessive use of pesticides can cause
serious health effects [67]. The use of NPs or nano-
formulations of pesticides is more efcacious in contrast to the
commercial formulations of pesticides. This might be the result
of enhanced uptake of active ingredients and greater
bioavailability with NPs, consequently leading to killing of in-
fectious organisms [68].
TiO
2
NPs are used to kill Spodoptera littoralis, the
Egyptian cotton leaf worm. It is basically a pest having wide
host range in plants that attacks certain eld crops such as
cotton and vegetables including tomato etc. In this context, an
experiment was performed with six different concentrations
(1000, 500, 250, 125, 62.5, and 31.25 ppm) of TiO
2
NPs. In
this study, the larvae were fed on TiO
2
treated leaves of cotton
and 2 weeks post application, the mortality was detected. The
results clearly showed that TiO
2
NPs exhibited toxic action at
all the concentrations applied against the larvae of S. littoralis
[69]. Figure 4shows the positive effects produced by TiO
2
based nanopesticides on plants.
Recently, TiO
2
NPs were used alone and in combination
with ZnO NPs to study their insecticidal effect against Bac-
tericera cockerelli nymphs. The laboratory study and a
greenhouse study were separately executed in tomato plant.
The leaf immersion bioassay method was performed in labo-
ratory experiment while direct plant spraying method was
carried out under greenhouse conditions. Results indicated that
the TiO
2
NPs caused 99% and 100% mortality alone (at 100
ppm concentration) and in combination with ZnO NPs (at 250
ppm concentration), respectively, after 96h of treatment in
laboratory. Whereas, 32% and 23% mortality was observed at
500 and 250 ppm concentration of TiO
2
NPs and
TiO
2
+ZnO NPs, respectively, in the greenhouse experiment
[70]. In another study, TiO
2
NPs were prepared by green
synthesis route using aqueous leaf extract of Pouteria cam-
pechiana and were analysed for insecticidal potential. It was
demonstrated that at 900 μg/ml of TiO
2
NPs, remarkable
lethal activity was obtained against the larvae and pupa stages
of Aedes aegypti [71].
TiO
2
NPs when codoped with nitrogen (N) and uoride
(F) were utilised to inhibit fungal growth of Fusarium oxy-
sporum in tomato. These NPs acted as antifungal agents in
visible light. The colloidal form of NPs provided competent
interactions of NPs with the cell wall of fungi owing to their
appropriate surface chemistry. In fact, the synergistic effects
were produced by the TiO
2
NPs and N & F attached on their
surface, producing stronger toxic inuence on fungal strain,
eventually killing it. The generation of ROS under visible light
actually caused the disinfection of fungus. Thus, these NPs
could be utilised for visible lightinduced bacterial and fungal
disinfection [72].
In another study, the antibacterial aptitude of TiO
2
NPs
and silver (Ag)and zinc (Zn)doped TiO
2
NPs was evaluated
for the underlying agent of bacterial spot disease in tomato,
Xanthomonas perforans. The dose dependency and the pho-
tocatalytic activity were performed in vitro on glass cover slips
that were coated with NPs. The known population of X.
perforans was added on the cover slip, which were then illu-
minated with visible light. The in vitro and greenhouse studies
showed that the doped TiO
2
NPs at a concentration of 500–
800 ppm had higher photocatalytic activity against X. perfo-
rans under visible light, which was obviously due to the
combinatorial inuence of TiO
2
NPs with their doping agents,
that is, Ag and Zn [73].
3.2
|
Nanofertilisers
Agricultural production can be enhanced with the introduction
of fertilisers. To overcome the challenges such as pollution
problems and use of nutrients with high efciency, nano-
fertilisers serve as a best choice and might be the best alternative
and more efcient tool than conventional fertilisers [74]. Also,
they improve the condition of soil by reducing the toxic effects
resulting from the overuse of conventional fertilisers [75].
FIGURE 4 Plants grown (a) without TiO
2
nanopesticides and (b) with
TiO
2
nanopesticides
JAVED ET AL.
-
7
The impact of nano TiO
2
(a nanofertiliser) on spinach
seeds was observed during its growth and development. The
seed vigour and rate of germination showed enhanced effects
with TiO
2
NPs' treatment. Moreover, at 2.5% concentration of
nano TiO
2
, the dry weight of plant, Rubisco activity, the for-
mation of chlorophyll and the rate of photosynthesis were
increased during growth stage [76]. To analyse the effects of
TiO
2
NPs on the development and growth of canola, a study
was designed in which canola seeds were independently treated
with various concentrations of TiO
2
NPs and the impact of
these treatments was focussed on seed and seedling vigour. It
was inferred from the results that the higher concentration
(2000 mg/L) of TiO
2
NPs demonstrated enormous growth of
seedling plumule and radicle [77].
In another study, the impact of TiO
2
NPs' spray treatment
on Zea mays was investigated. The TiO
2
NPs' spray was
applied and the two factors were mainly considered for treat-
ment; in the rst step, the growth of the plant was observed
(growth of vegetative parts and the appearance of male and
female owers). Then, the second factor including chlorophyll,
anthocyanins and carotenoids content etc., at various concen-
trations of TiO
2
NPs was observed. The results clearly showed
that the impact of TiO
2
NP concentration 0.03% was note-
worthy on chlorophyll (a & b), carotenoids, total chlorophyll (a
+b), and anthocyanins. Besides, the nano TiO
2
spray at the
male and female owers (reproductive stage) resulted in the
greater amount of pigmentation as compared to control.
Hence, it was concluded that the utilisation of TiO
2
NPs can
expedite the corn yield and several other parameters. Most
importantly, TiO
2
NPs produced a signicant effect on
photoreduction capabilities of photosystem II and electron
transport chain (ETC), thus promoted photosynthesis by
triggering the photochemical reaction of plant chloroplasts and
subsequently enhanced the production of pigments [78].
Different agronomic species of plants have been exposed
to different concentrations of nano TiO
2
to examine the po-
tential outcomes on germination, early seedling development,
and other parameters such as root length etc. The seeds treated
with TiO
2
NPs have shown an enhanced rate of germination
and an increase in the growth of seedlings and root length in
different crop plants as explained in Table 2. In all of these
studies, TiO
2
NPs are either applied via foliar spray or given to
seeds soaked in them and by soil medium. In the rst case, the
increased absorption of NPs by increasing the surface area of
stomata of leaves might occur and later on, dissolution in the
shoots leads to increase in growth parameters. While in the
second case, enhancement of seed germination, plant growth,
and crop yield might be due to the increased surface area of
roots for absorption of NPs, which take part in the nutrients'
uptake by acting as nutrient carriers. These effects are observed
at optimum concentrations of TiO
2
NPs and are dosage
dependent. The NPs have been observed to produce nega-
tive effects mostly under high dosages while are benecial at
lower concentrations.
Recently, TiO
2
NPs synthesised by the green synthesis
route using fruit peel extract of Citrus medica L. were used as
nanofertilisers to enhance the yield of Capsicum annuum [86].
In another study, TiO
2
NPs were applied via foliar spray to
sunower at a concentration of 2.6 mg/L. The results showed
improved physiology and enhanced nutritional parameters
such as oil content [87]. In an investigation, the coriander
plants were sprayed with TiO
2
NPs (2, 4, 6 ppm) that led to the
increase in height of plant and other such physiological pa-
rameters. Moreover, signicant rise of carotenoids, total sugars,
phenols, and amino acids was observed [88].
3.3
|
Plant tolerance
The NPs of TiO
2
are utilised broadly in different commercial
items and plants. Despite their lavished utilisation, the inves-
tigation of the uptake of these NPs and their translocation in
plants is restricted. Asli and Neumann examined the uptake of
nano TiO
2
and its translocation in Z. mays. The roots were
excised having apices that were intact. The results showed that
the NPs were not taken up through the cells of root, pre-
sumably because of their enormous size contrasted with the
size of the diameter of root pore in the maize cell wall. This
was appeared to diminish the water passage through the roots,
thus causing diminished transpiration and growth of leaf [89].
A combination of SiO
2
and TiO
2
NPs in soybean augmented
the uptake of fertiliser and water and incited the enzymatic
antioxidant activity including catalase (CAT), superoxide dis-
mutase (SOD), and peroxidase (POD). The synergism of
SiO
2
–TiO
2
nanocomposite increased the oxidative stress
against which the enzymatic antioxidants were produced [90].
The high utilisation of TiO
2
NPs has prompted their
augmented delivery into the environment, where they might
interact or associate with various plants and inuence their
physiological capacities [91, 92]. A study was designed to
examine the response of different physiological parameters of
Triticum aestivum L. with respect to the escalating amount of
TiO
2
NPs. The TiO
2
NPs were applied to soil at a concen-
tration of 0 (control), up to 100 mg/kg. The physiological
parameters such as length of root and shoot, phosphorus (P)
availability, chlorophyll content, biomass, and H
2
O
2
produc-
tion were then recorded. All investigations were repeated twice.
After 2 months of the NPs' exposure, the length of root and
shoot as well as P uptake by the plants was notably greater with
increasing nano TiO
2
concentration contrasted with the con-
trol; however, it was decreased at higher doses. The utilisation
of NPs prompted chlorophyll content being greater than in the
control; however, lesser content was seen at higher concen-
trations. The outcomes proposed that wheat could not tolerate
much greater concentration of TiO
2
due to the excessive
production of H
2
O
2.
Actually, H
2
O
2
acts as a regulator of
growth and chlorophyll content. The rising concentration of
TiO
2
NPs increase the formation of H
2
O
2
because it is an
important stresssignalling molecule, and hence the more
oxidative stress means more H
2
O
2
production while lesser
growth and chlorophyll content [93].
TiO
2
NPs play an important role in the growth of plants,
particularly under exposure to abiotic stresses. In an interesting
study, the impact of TiO
2
NPs at different concentrations
8
-
JAVED ET AL.
(0, 50, 100 and 200 mg/L) on the agronomic characteristics of
Dracocephalum moldavica L. plants was explored that were
grown under various dosages of salinity. Outcomes showed
that all agronomical attributes were adversely inuenced by all
saltiness amounts; however, utilisation of TiO
2
NPs alleviated
those adverse effects. The treatments of TiO
2
NPs on Mol-
davian, which was nurtured in the stress of salt settings,
enhanced every single agronomical quality and increased the
activity of antioxidant enzymes contrasted with the plants
cultivated in soil that was devoid of TiO
2
treatments under
saltiness. The utilisation of TiO
2
NPs brought down the
quantity of H
2
O
2
by reducing the oxidative stress and the
quantity of essential oil was attained highest in plants treated
with TiO
2
NPs. In conclusion, the use of TiO
2
NPs was found
to fundamentally improve the impact of salinity in Dracoce-
phalum moldavica L. plant [94].
Numerous studies have reported that the use of rutile TiO
2
NPs can moderate the oxidative stress of plants. Addition of
TiO
2
NPs (50 mg/L) to the growth media reduced the growth
of roots and shoots while enhanced enzymatic antioxidant
activities (SOD, POD, and CAT) in maize tissues [95]. Another
study reported that the application of TiO
2
NPs (100–300 mg/
kg) to Cdenriched soil effectively reduced the plant biomass
and growth while increased the proline and malondialdehyde
(MDA) content [96]. Therefore, TiO
2
NPs have great appli-
cation prospective in reducing oxidative stress in plants caused
by the heavy metals. According to reports, the four types of
TiO
2
NPs (anatase, rutile with hydrophilic surface, rutile with
hydrophobic surface, and pristine rutile) at 10–1000 mg/L
concentration in rice plants effectively decreased the Pb con-
tent in plant roots and shoots [97].
Exposure medium plays a signicant role in determining
the plants' tolerance potential. Recently, in a study, TiO
2
NPs
(100 and 250 mg/L) were applied both through soil and foliar
application in maize grown in Cdcontaminated soil. Foliar
application of TiO
2
NPs inhibited Cd accumulation in maize
and enhanced biomass production, while TiO
2
NPs'
application in soil upheld the accumulation of Cd in maize and
substantially reduced the biomass. Foliar spraying of TiO
2
NPs
also improved SOD and glutathione Stransferase (GST)
enzyme activities, and galactose, aspartame acid, and other
metabolic pathways were activated to alleviate abiotic stress in
maize [98]. Figure 5explains the interaction of TiO
2
NPs with
crop plants and associated mechanism.
4
|
TiO
2
NPS IN ENVIRONMENTAL
REMEDIATION
Technological advancement brought by rapid growth in world
population and urbanisation and industrial evolution have led
towards the drastic exploitation of natural resources such as
water, air, and soil. Major causes of water and soil pollution are
untreated industrial efuents, improper sewage water disposal,
and haphazard application of pesticides and fertilisers. At
present, the water and soil are polluted with lethal heavy
metals, chlorinated compounds, and dyes. Air is loaded with
abundant contaminants such as nitrogen oxides (NO), carbon
monoxide (CO), volatile organic compounds, chlorouoro-
carbons (CFCs), hydrocarbons etc. There is a substantial need
for efcient technologies capable of tracking, identifying, and
handling such kinds of contaminants in water, air, and soil [99].
In recent years, environmental nanotechnology has made
tremendous developments in terms of environmental protec-
tion. Amongst the most promising contributions, environ-
mental applications in an area of water/air remediation are
signicant. The unique properties of NPs such as their nano-
scale size, large surface area to volume ratio, high exibility for
in situ and exsitu practices, and reluctance to ecofactors make
them suitable candidates for different environmental ap-
proaches. Different kinds of available nanomaterials and
nanotools are used to remediate ecocontaminants [100].
Among various materials, use of TiO
2
NPs is increasing day by
day as a remediating agent to clean water, purify air, and to
TABLE 2Inuence of TiO
2
nanofertilisers on different crops
Plant Concentration Application method Effects Reference
Maize 0, 100, 300, and
500 mg/L
Foliar spray Increase in plant height, dry weight, and yield [79]
Lettuce 0, 25, 50, 75, and
100 mg/L
NPs in sandy soil Fivefold increase in phosphorus (P) uptake and plant growth (shoot and root
length)
[80]
Onion 0, 100, 200, and
400 mg/L
Seeds soaked in NPs Promote seed germination, maximum germination rate achieved at 100 mg/L [81]
Cabbage 0, 25, 50, and 100 mg/L Seeds soaked in NPs Promote seed germination and root growth [82]
Wheat 0, 30, 50, and 100 mg/
kg
NPs in soil with
phosphorus (P)
Promote plant growth and nutrient uptake [83]
Cucumber 0–4000 mg/L Seeds soaked in NPs Promote seed germination and root growth, >300% increase in root length at all
concentrations
[84]
Barley 0, 100, 200, and
300 mg/L
Foliar spray Increase in crop yield [85]
Abbreviation: NP, nanoparticle.
JAVED ET AL.
-
9
decontaminate soil due to properties such as appropriate
electronic band structure, high quantum efciency, stability,
and chemical inertness [101].
4.1
|
Nanosensors
Nanosensors are sensitive detecting devices having at least
100 nm as one of their sensing dimensions. These are instru-
mental for detecting and evaluating physical and chemical
changes, observing biochemical and biomolecular alterations
inside the cells, and measuring toxic environmental contami-
nants. Sensitive detection of pollutants is accredited to NPs'
small size together with high surface to volume ratio for moni-
toring via nanosensor devices. Various kinds of nanostructured
materials are utilised in the production of nanosensors, namely,
nanoscale wires, carbon nanotubes (CNTs), graphene (G),
polymers, biomaterials, thin lms, metal and metal oxide NPs.
Nanosensors function in the scale down, effective, accurate, and
sensitive recognition of contaminants [102]. Analyte (sample, i.e.
pollutants), bioreceptor (molecule that recognises the analyte,
i.e. NPs), transducer (converts one form of energy to another, i.e.
signaliser), electronics (processes/amplies the transduced
signal), and display (interpretation system, i.e. quantier) are the
basic components of biosensor. Various factors inuencing the
performance of nanosensors include selectivity and sensitivity of
bioreceptor and linearity and reproducibility of response [103].
The process of nanosensing along with its various biological
applications has been well explained in Figure 6.
TiO
2
nanostructures possess unique physical and chemical
characteristics such as nontoxicity, strong oxidation ability,
and high chemical inertness obtained at a relatively low cost.
Based on this, TiO
2
NPbased nanosensors have been pro-
foundly examined and employed for the detection of ions and
substances [104, 105]. Henceforth, nowadays tremendous in-
terest has been observed in TiO
2
nanomaterial structures as
well as their transduction principles and simulations for
nanosensor applications [106–110]. In recent times, several
new TiO
2
based nanomaterials with advanced compositions
and structures have been employed for a variety of sensors.
Based on different sensing targets or measurement principles,
the TiO
2
nanosensors can be stated as (1) TiO
2
based gas
sensors, (2) TiO
2
based electrochemical sensors, and (3) TiO
2
based biosensors [111–113].
According to the report, a highly selective nanosensor, that
is, thiazolylazopyrimidinedoped titanium dioxide (TiO
2
TAP),
FIGURE 5 Interaction of TiO
2
nanoparticles (NPs) with crop plants: (a) exposure of TiO
2
NPs to the crops using soil medium and foliar spray; (b) TiO
2
NPs entering the plant cells through apoplast and symplast pathways; (c) TiO
2
NPs controlling the production of oxidative stress and its possible mechanism
10
-
JAVED ET AL.
was synthesised via diazo coupling reaction and surface
modication reaction for colourimetric detection of Cu
2+
in
water samples. The novel nanosensor exhibited maximum
sensitivity, high afnity, and selectivity for copper ions in
aqueous media at pH 5.0. Moreover, the TiO
2
TAP revealed
wide linear detection range for Cu
2+
(0.01–12.5 μM) [114]. In a
recent study, a novel approach was used for the detection of
toxic metal (Pb) using carbon quantum dot (CQD) nanosensor
with 0.070 μM sensitivity and selectivity, and its doping with
TiO
2
nanostructures to make Pb–CQDs–TiO
2
(PCT) com-
posite was later applied for the photocatalysis of industrial
dyes. The adsorption ability of TiO
2
NPs, light absorption
capacity of Pb–CQDs, and photocatalytic ability of PCT were
utilised. Using wet impregnation method, the Pb–CQDs so-
lution was immersed in TiO
2
to achieve Pb–CQDs–TiO
2
(PCT) showing photocatalytic ability of 3.2–2.8 eV energy gap.
Results illustrated 100% degradation efciency for RBX dye
and 1.8 μmols of CO
2
release was noticed in 1 h [115]. As per
the literature, the removal of Bi (III) ions using efcient,
sensitive, and highly selective mesoporous TiO
2
NPbased
sensing system was examined. The formation of TiO
2
NPs
and [Bi (DZ)
3
] complex led towards the successful removal of
Bi (III) ions. TiO
2
NPs of 174 m
2
/g surface area and 10 nm
particle size played important role in [Bi (DZ)
3
] complex for-
mation. Bi (III) ions detection limit was observed to be
1 ppb. Moreover, the colourimetric nanosensor depicted
selective sensing performance for up to 5000 times in the
presence of other competent anions and cations [116]. In
another study, the sensing properties of CdTe quantum dot
(QD)doped TiO
2
nanotubes (TiO
2
NTs) for polycyclic aro-
matic hydrocarbon (PAH) detection using uorescence
resonance energy transfer (FRET) were evaluated. FRET
occurred between the CdTe QDs and PAHs, with CDTe QDs
as donors and PAHs as receptors. The sensors' maximum
sensitivity was found to be dependent upon the number of
PAH rings having the highest sensitivity observed against
benzopyrene (BaP). Results demonstrated that the proposed
sensor could be used for efcient scanning of PAHs because
the sensor (i) described a linear response to the log of BaP
concentration having range of 400 nM–40 pM and (ii)
exhibited 15 mP detection limit which is set by USEPA [117].
In another report, sensing potential of graphenedoped
anatase TiO
2
carbon paste (GTC) for electrochemical detec-
tion of phenol was elucidated. Investigation showed the ef-
cient detection of phenol down to 3.6620 10
5
μM. GTC
electrode demonstrated better selectivity and stability and
predicted the corresponding reproducibility standard deviation
to be 1.33% and 2.83%. Also, the electrode performance was
found to be concentrationdependent with the optimum
electrode using G and TiO
2
samples as lower as 0.015 and
0.3 g, respectively. A very low detection limit was obtained in
this study, which is attributed to the graphene and TiO
2
that
were involved in acceleration of the process of charge transfer
and surface reaction [118]. As per ndings, sensing character-
istics of ZIF8@ZnO/TiO
2
1DTDPC (zeoliticimidazolate
framework8, 1D) (topdefect photonic crystalline structures)
against carbon tetrachloride (CCl
4
) synthesised using spin
coating technique were studied. ZIF8@ZnO/TiO
2
1DTDPC represented excellent quality in terms of sensitivity
(0.008 nm/ppm towards water vapours). Besides, the ZIF
8@ZnO/TiO
2
1DTDPC showed higher selectivity and
sensitivity (0.05 nm/ppm) for CCl
4
vapours. Moreover, at
FIGURE 6 Process of nanosensing applied in biotechnology
JAVED ET AL.
-
11
320 ppm concentration, 300 ms was found to be the optical
response time [119]. According to another report, a successful
development of high performance nanosensor based on
ZnO@TiO
2
nanorods and their applications for nbutanol
detection was observed. Results revealed higher sensitivity,
better selectivity, low detection limit of 133 ppb, and fast
response recovery against nbutanol investigation. The
enhanced sensing capability of the developed nanosensor was
attributed to the coreshell nanostructures' heterojunction,
maximum oxygen sorption owing to TiO
2
nanoshells, and
complete electron depletion having a Debye length comparable
thickness [120].
As per data available, the role of anatase TiO
2
–CPE (carbon
paste electrode) nanocomposite electrochemical sensor for
cypermethrin detection was explored. Results showed
maximum sensing performance with increased anatase TiO
2
NPs' concentration of 2 w/w (CPE 3 w/w). The synthesised
nanosensor exhibited 0.1 ppm detection limit, which is less than
the cypermethrin residual limits in the environment (0.5 ppm)
and in the food items (0.05–0.2 ppm). Also, the nanoelectrode
exhibited remarkable stability with only 0.37% performance
reduction (%RSD) for 11 times repeated measurement. The
maximum sensing performance was ascribed to excellent
physicochemical properties and surface chemistry of anatase
TiO
2
due to which greater electron transfer rate occurred in the
CPE matrix [121]. According to the literature, sensing potential
of CuO–TiO
2
hybrid nanocomposite decorated on glass carbon
electrode as electrochemical sensor for methyl parathion
(pesticide) detection was investigated. Results illustrated lower
detection limit of 1.21 ppb. Also, the as prepared nanosensor
exhibited efcient sensing capability and maximum selectivity
for methyl parathion. Additionally, the novel electrochemical
sensor also showed effective sensing performance for the
pesticide detection in actual ground water samples [122].
4.2
|
Soil remediation
The existence of hazardous compounds causes contamination
of the naturally occurring soil. Heavy metals are the most
common pollutants of the soil, especially when present at toxic
level. There are different sources on which soil contamination
depends including manufacturing, mining, and landll sites
especially those which are built to receive the wastes of
different industries such as paint residues, electrical wastes,
batteries etc., as well as industrial or municipal sludge. Thus,
heavy metals are one of the challenging pollutants of the soil
because they are considered as the nondegradable substances
that remain there once been introduced [123, 124]. Various
studies have reported an interesting information related to
nanoTiO
2
, considering the soil remediation by the UV
mediated degradation of the organic pollutants of soil [125].
Photodegradation of diphenyl arsenic acid (DPAA) by
employing TiO
2
NPs has enabled us to investigate that either
nanoTiO
2
or the derivatives of TiO
2
can wipe out the organic
pollutants via the process of photocatalytic oxidation. The
formation of DPAA usually results from the leakage of arsenic
weapons and it also poses various adverse effects on human
health. To optimise the removal, all the operational parameters
required for removal of DPAA, which include dosage of TiO
2
NPs, radiation time, light intensity, and also soil–water ratio,
are studied well. It has also been observed that all the pa-
rameters mentioned above result in the removal efcacy of
82.7% of the DPAA. It has been reported that the DPAA is
not completely converted by TiO
2
NPs, rather its inorganic
arsenic products are adsorbed by these NPs, hence playing
crucial role in eradicating DPAA pollution [126].
In a report, the photocatalysis of nanoTiO
2
, when used in
combination with the pulse discharge plasma, required for the
purpose to remove it from the contaminated soil was studied.
The mechanism involved removal of pnitrophenol and then its
degradation by the enhancement of pulse discharge voltage. The
nitrophenols are termed as the biorefractory and the noxious
organic compounds that are signicantly used as both the in-
termediate as well as the raw materials to produce pharmaceu-
ticals, dyes, rubber chemicals, pigments, wood preservatives,
pesticides, and explosives. Wang et al. predicted that the pulsed
discharge plasma could be the force used to drive the photo-
catalysis of nanoTiO
2
. The report also revealed that with the use
of this route, the pnitrophenol could be removed up to 88% in a
succession of just 10 min [127]. Figure 7depicts the mechanism
of soil remediation by the TiO
2
based photocatalyst.
A study related to the application of plant growth pro-
moting rhizobacteria (PGPR) and TiO
2
NPs, when combined,
suggests the promotion of the phytoremediation of Cd
contaminated soil. The percent of clay, sand, and silt in soil
was 28%, 37%, and 35%, respectively. Soil had 0.47% N,
7.1 mg/kg of phosphorus, and its pH was 7.8. Different doses
of the TiO
2
NPs and the PGPR were used for the seedlings of
Trifolium repens, both separately as well as in combination,
with the purpose to analyse their effects on the uptake of Cd,
growth of plants, and the content of chlorophyll present in
plants. Thus, it was revealed that their combined application
resulted in the enhanced plant growth with high chlorophyll
content. It also reduced the amount of TiO
2
NPs required for
the phytoremediation of soils particularly polluted with heavy
metals. It also promoted the growth of T. repens with increased
Cd uptake in the Cdcontaminated soil by the plants [128].
In a study, the association of biochar and TiO
2
NPs was
investigated extensively for the phytoremediation of soil
contaminated with antimony. Soil pH was 7.7 and it had 1.12%
N and 8.7 mg/kg of phosphorus. The percent of clay, sand,
and silt was 28%, 37%, and 35%, respectively. Different con-
centrations of TiO
2
and biochar were applied to the Sorghum
bicolour seedlings individually and in combinations. The
treatments were applied to study their impact on the growth of
plants, the absorption of antimony and its accumulation, and
the physiological response of the plants in antimony
contaminated soil. The results demonstrated that the combi-
nation of biochar and TiO
2
NPs had positive effects on the
growth of plant. The accumulation of antimony was increased
signicantly with TiO
2
and biochar combination. The results
clearly revealed this technique to be favourable for the phy-
toremediation of heavy metal contaminated soil [129].
12
-
JAVED ET AL.
4.3
|
Water remediation
Clean and fresh water is essential for daily life and life cycle of
living organisms. Due to different mankind activities, water is
contaminated by physical, chemical (heavy metals, inorganic/
organic etc.), and biological pollutants. Recently, nano-
technological approach for wastewater treatment using
adsorption and photocatalytic technology has attained
tremendous attention owing to its ecofriendliness, sustain-
ability, costeffectiveness, and other effective properties [130].
Among nanomaterials, TiO
2
based nanomaterials have ach-
ieved the core importance because of their unique physical and
chemical features, remarkable biocompatibility, strong oxida-
tion, and maximal photocatalytic properties [131, 132].
According to the report [133], a novel porous magnetic Ag/
TiO
2
/Fe
3
O
4
@GO nanocomposite was synthesised and its
photocatalytic ability for As (III) and As (V) was examined.
Results showed optimum adsorption capacity of 91% at pH 5
and 20 mg sorbent concentration for 24 ppm A (III), 90 min
duration, and at room temperature. pH plays a pivotal role in the
adsorption technology for wastewater treatment application. At
pH <3, H
3
AsO
3
are primary nonanionic species and the As
(III) adsorption on the adsorbent surface is lowered. While at
pH >8, the surface of the adsorbent becomes less positive and
As (III) removal efciency is reduced. Also, for As (V), 87%
adsorption capacity was achieved at pH 3, 17 ppm A (V), 11 mg
sorbent concentration, 30 min duration, and at room tempera-
ture. As (V) adsorption was also affected by varying the pH of
the medium solution. Since As (V) existed in negative form
H
2
AsO
4
, at acidic pH, H
+
ions easily interacted with the
negative charges of the adsorbent (Ag NPs) surface, TiO
2
and
Fe
3
O
4
oxygen groups, and GO functional groups to make sur-
face complexes. Overall, at lower pH, the electrostatic interac-
tion of H
2
AsO
4
on the positive sites of the adsorbent resulted
in maximum adsorption. Moreover, the nature of adsorption for
As (III) and As (V) was best interpreted by Langmuir isotherms.
Table 3describes various examples of the application of TiO
2
based nanomaterials in wastewater treatment.
In a study, TiO
2
NPs along with the ZnO NPs were
immobilised on a natural clay for the removal of ammonium
ions from wastewater. The parameters adjusted were initial
concentration, solution pH, agitation time, and adsorbent
dosage. The adsorption process followed the pseudosecond
order kinetics and Langmuir model. The structure and sur-
face properties of NPs were involved in regulating the
adsorption capacity [140]. Another report showed the immo-
bilisation of TiO
2
NPs on bentonite and kaolin adsorbent for
the removal of cationic polymer. The experimental conditions
involved alteration in pH, ionic strength, and dosage. The
rising pH was observed to improve the removal process from
wastewater [141].
TiO
2
NPs have shown very high photocatalytic activity as
compared to other NPs [142]. Recently, Indira and colleagues
prepared Ni–TiO
2
nanoakes using the leaf extract of Mukia
madrasapatna and their photocatalytic potential in wastewater
was analysed. Results indicated that the Congo red dye was
degraded by UV illumination [143]. In another study, niobium
doped TiO
2
nanotubes (Nb–TNT) were developed via dip
coating route for the catalytic degradation of organic pollutants
and the results demonstrated excellent charge separation ef-
ciency [144].
In a most recent study, TiO
2
NPs were combined with a
microalgae, Chlorella vulgaris, forming bionano hybrid
catalyst for downstream wastewater treatment. The photo-
catalytic behaviour of TiO
2
NPs and sorption potential of C.
vulgaris were combined. The synergistic effects of NPs and
algal biomass enhanced the biosorption of copper ions (Cu
2+
)
from 103 to 4000 mg/g and photodegradation of rhodamine B
(RhB) in 1 h, increasing the kinetic constant from 8.7 to
10.7 10
2
min
1
[145].
4.4
|
Air remediation
With the growing world population, the industrial pollution
and transportation will continue to increase causing environ-
mental burden. Air pollution also has bad impact on the
ecosystem. Today, the most important challenge is global
warming, which is caused by different gases present in the
atmosphere [146]. Various technologies have been developed
to monitor, control, and eliminate the harmful gases from
environment. Taking advantage of the properties of nano-
materials, the eld of nanotechnology has been utilised as an
effective treatment to control or eliminate air pollutants by
using them as adsorbents and catalysts [147].
Nanocrystalline TiO
2
photocatalysts formed by the
process of sol–gel, which were then calcined at 100°C–
800°C, were assessed for the deterioration of trichloro-
ethylene (TCE) in indoor air. Catalysts were made up of
pure rutile, pure anatase, and a combination of anatase/
rutile that were immobilised on borosilicate glass as thick
lms and then introduced in a photocatalytic reactor.
Higher photocatalytic activity was exhibited by the treated
pure anatase TiO
2
as compared to the commercial TiO
2
(P25) in the presence of UVA radiation [148]. The ac-
tion mechanism of TiO
2
NPbased photocatalyst is pre-
sented in Figure 8.
FIGURE 7 Schematic illustration of TiO
2
nanoparticlebased
photocatalyst for soil remediation
JAVED ET AL.
-
13
TiO
2
NPs codoped with NF (NFT) powders, which
were prepared through the process of spray pyrolysis (SP),
were described by photoluminescence (PL) spectra and
ultravioletvisible (UVVis) absorption spectroscopy. For the
purpose of evaluation of the photocatalytic properties of
NFT powder, the decomposition of acetaldehyde was
utilised as probe response and the results showed that un-
der the inuence of UV or visible irradiations, the NFT
powder exhibited greater photocatalytic activity when
compared with the commercial P25. These ndings also
showed the effectiveness of TiO
2
nanocomposite against
toluene and TCE [149].
TABLE 3Different TiO
2
based nanomaterials applied in wastewater treatment
TiO
2
based adsorbent Sorbate
Optimum
conditions
Removal
(%)
Adsorption capacity
(mg/g)
Isotherm
model
Kinetics
model Reference
Ag/TiO
2
/Fe
3
O
4
@GO As (III) pH: 5 91 Langmuir [133]
Ag/TiO
2
/Fe
3
O
4
@GO As (V) pH: 3 87 Langmuir
PP@TiO
2
As (III) 76.92 [134]
[EMIMBF
4
] assisted GO/TiO
2
nanocomposite
Cd (II) pH: 7.5 69.36 [135]
Sorbent: 0.5 mg/L
Sorbate: 0.5 mg/L
Temp: 20 2°C
Time: 40 min
[EMIMBF
4
] assisted GO/TiO
2
nanocomposite
Pb (II) pH: 3 89 ‐ ‐
Sorbent: 0.5 mg/L
Sorbate: 0.5 mg/L
Temp: 20°C 2°C
Time: 40 min
Sodium modied TiO
2
Zn (II) pH: 1–7.4 93 Freundlich PSO [136]
Sorbent: 50 mg
Time: 90 min
Sr (II) pH: 8–10 2084
Sorbent: 50 mg
Time: 90 min
Ba (II) Sorbent: 50 mg 2746
Time: 90 min
Pristine TiO
2
Cr (VI) pH: 5.36 100 [137]
Sorbent: 50 mg/L
Sorbate: 10 mg/L
Time: 60 min
Chitosan/gC
3
N
4
/TiO
2
(CS/CNT)
nanobres
Cr (VI) pH: 1–7 165.3 Langmuir PSO [138]
Sorbent: 10 mg/L
Sorbate: 20–
800 mg/L
Temp: 24 h
Time: 0–1440 min
Ti
3
C
2
/TiO
2
Cr (VI) pH: 2 99.34 [139]
Sorbent: 0.05 g/L
Sorbate: 50 mg/L
Time: 72 h
14
-
JAVED ET AL.
As per ndings, the TiO
2
NPcoated nano CaCO
3
became able to prevent the sintering of nano CaCO
3
and
capture the CO
2
effectively with the help of adsorption
phase technique. It was a nano CaObased CO
2
adsorbent.
The durability and compactness of CO
2
adsorption was
enhanced by TiO
2
NP coating [150]. Moreover, the pho-
tocatalytic degradation of greenhouse gases, viz., methane
(CH
4
) and CO
2
was done by the TiO
2
NPs coated on
stainless steel webnet. In a gasphase batch reactor, UV was
irradiated on precursors and formate and acetate derivatives
were formed as conversion products along with the degra-
dation of CH
4
and CO
2
[151]. In another study, a hybrid
nanomaterial PtrGOTiO
2
having a wideranging light ab-
sorption wavelength (800–2500 nm) was developed. This
well active and responsive photothermal catalyst could
decompose the volatile organic compounds (VOCs) under
the inuence of infrared (IR) radiations. The intensity of
light could affect the rate and efciency of PtrGOTiO
2
mixtures on toluene conversion with the production of
CO
2
. When the intensity of IR radiations was 116 mW/
cm
2
, the 14.1% of the photothermal conversion efciency
of substantial toluene was achieved at a conversion rate of
95% with the production of 72% CO
2
. Moreover, the
duration of stability was almost 50 h [152].
TiO
2
NPs are being employed as photochemical deodor-
isers in Japan as they convert oxides and air pollutants to less
lethal products like calcium nitrate (CaNO
3
) and carbon di-
oxide (CO
2
) [153]. Furthermore, titanium mesh lters have
been developed to remove pollutants from cigarette smoke
[154]. Toluene, an important volatile organic compound, can
be treated with carbon © coated TiO
2
NPs and the oxidation
of indoor contaminants present in air by TiO
2
NPs, helps in
the cleansing of air, which is vital for smooth breathing of all
living organisms [155].
5
|
CONCLUDING REMARKS AND
FUTURE PERSPECTIVES
The biotechnological approaches of TiO
2
NPs can be broadly
categorised into medical nanobiotechnology, agricultural
nanobiotechnology, and environmental nanobiotechnology.
The employment of TiO
2
NPs in medical nanobiotechnology
can be divided into different areas, that is, bioimaging, drug
delivery, phototherapy, antimicrobial potential, and tissue
regeneration. The applications of TiO
2
NPs in agricultural
nanobiotechnology discussed in this review can be narrowed
down to nanopesticides, nanofertilisers, and plant tolerance.
TiO
2
NPs' administration in environmental nanobiotechnology
can be sectioned into nanosensors and the soil/water/air
remediation. All of the sections are interrelated, for instance,
nanosensors could be used for the monitoring and detection of
plant or human pathogens as well as ecological toxicants.
Although several studies have reported the improvement in
crop yield and quality on administration of TiO
2
NPs, the toxic
or negative impact of these NPs is also achieved at relatively
higher dosages. Therefore, further investigations on the po-
tential interactions of TiO
2
NPs with toxic heavy metals are
needed to improve our understanding of NPs–crop plants
interactions for the safer applications of nanofertilisers in
future. Moreover, the benecial effects of TiO
2
NPs as plant
disease suppressing agents are dependent upon multiple fac-
tors, including physicochemical properties (size/morphology/
charge/coating), concentration, duration of application, expo-
sure dosage, plant species, and type of pathogen. Thus, the
selection of appropriate concentration and application regime
are critical for ensuring benecial outcomes, also in case of
biomedicine. Largescale application of TiO
2
NPs could
potentially lead to collateral damage in the terrestrial environ-
ment. Besides, the remediation potential of TiO
2
NPs also
FIGURE 8 Schematic illustration of TiO
2
nanoparticlebased photocatalyst for air remediation
JAVED ET AL.
-
15
depends on its surface properties and exposure medium.
Regarding the domain of biomedicine, the detailed studies
depicting photocatalytic performance, cytotoxicity, biodegrad-
ability, and biocompatibility of TiO
2
NPs should be carried out
to tackle challenges of translation of advanced nanomedical
approaches from preclinical to clinical settings. In addition,
TiO
2
NPs should be designed keeping in view of the biological
microenvironment in order to get maximal biological response
and negligible side effects in the human body.
In future, the planning, development, and implementation
of nanoenabled antiviral strategies should occur that would
draw attention of a broad group of plant/human biologists,
pathologists, and agricultural/biomedical engineers to collab-
oratively address the mounting challenges in the eld of agri-
culture and biomedicine. The efforts should focus on exploring
and establishing sustainable, environment friendly, longlasting,
and orthogonal approaches that would optimise crop/human
protection and forward efforts to achieve global food security
and longevity of human beings. Furthermore, the evaluation of
toxicity and toxicological pathways of TiO
2
nanostructures is
essential for the broad spectrum use of these NPs in different
elds of biotechnology. In a nutshell, the inherent character-
istics of TiO
2
NPs must be improved to ll existing knowledge
gaps and overcome future challenges in different functional-
ities of these NPs in biotechnology.
ACKNOWLEDGEMENT
Authors are obliged to the Sichuan University for providing all
funding. This work was supported by the Sichuan Science and
Technology Program (No. 2020YFH0008) and the National
Key R & D Program of China (No. 2020YFF0426289).
CONFLICT OF INTEREST
The authors declare that they have no competing interest.
PERMISSION TO REPRODUCE MATERIALS
FROM OTHER SOURCES
None.
DATA AVAILABILITY STATEMENT
The data that support the ndings of this study are available
from the corresponding author upon reasonable request.
ORCID
Rabia Javed
https://orcid.org/0000-0002-3599-8144
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How to cite this article: Javed, R., et al.: Diverse
biotechnological applications of multifunctional
titanium dioxide nanoparticles: An uptodate review.
IET Nanobiotechnol. 1–19 (2022). https://doi.org/10.
1049/nbt2.12085
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