royalsocietypublishing.org/journal/rsos
Research
Cite this article: Ahmed F, Awada C, Ansari SA,
Aljaafari A, Alshoaibi A. 2019 Photocatalytic
inactivation of Escherischia coli under UV light
irradiation using large surface area anatase TiO2
quantum dots. R. Soc. open sci. 6: 191444.
http://dx.doi.org/10.1098/rsos.191444
Received: 24 August 2019
Accepted: 16 September 2019
Subject Category:
Chemistry
Subject Areas:
nanotechnology/materials science/
nanotechnology
Keywords:
TiO2, quantum dots, microwave–hydrothermal,
X-ray diffraction, photocatalysis
Authors for correspondence:
Faheem Ahmed
e-mail: fahmed@kfu.edu.sa
Chawki Awada
e-mail: cawada@kfu.edu.sa
This article has been edited by the Royal Society
of Chemistry, including the commissioning, peer
review process and editorial aspects up to the
point of acceptance.
Photocatalytic inactivation of
Escherischia coli under UV
light irradiation using large
surface area anatase TiO2
quantum dots
Faheem Ahmed, Chawki Awada, Sajid Ali Ansari,
Abdullah Aljaafari and Adil Alshoaibi
Physics Department, College of Science, King Faisal University, Hofuf, Al-Ahsa 31982,
Saudi Arabia
FA, 0000-0002-5436-1966
In this study, high specific surface areas (SSAs) of anatase
titanium dioxide (TiO2) quantum dots (QDs) were successfully
synthesized through a novel one-step microwave–hydrothermal
method in rapid synthesis time (20 min) without further heat
treatment. XRD analysis and HR-TEM images showed that the
as-prepared TiO2 QDs of approximately 2 nm size have high
crystallinity with anatase phase. Optical properties showed that
the energy band gap (Eg) of as-prepared TiO2 QDs was 3.60 eV,
which is higher than the standard TiO2 band gap, which might
be due to the quantum size effect. Raman studies showed
shifting and broadening of the peaks of TiO2 QDs due to the
reduction of the crystallite size. The obtained Brunauer–
Emmett–Teller specific surface area (381 m2 g−1) of TiO2 QDs is
greater than the surface area (181 m2 g−1) of commercial TiO2
nanoparticles. The photocatalytic activities of TiO2 QDs were
conducted by the inactivation of Escherischia coli under
ultraviolet light irradiation and compared with commercially
available anatase TiO2 nanoparticles. The photocatalytic
inactivation ability of E. coli was estimated to be 91% at
60 µg ml−1 for TiO2 QDs, which is superior to the commercial
TiO2 nanoparticles. Hence, the present study provides new
insight into the rapid synthesis of TiO2 QDs without any
annealing treatment to increase the absorbance of ultraviolet
light for superior photocatalytic inactivation ability of E. coli.
1. Introduction
Titanium dioxide (TiO2) is a significant nanomaterial which
has attracted a considerable attention because of its distinctive
© 2019 The Authors. Published by the Royal Society under the terms of the Creative
Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits
unrestricted use, provided the original author and source are credited.
Analytical grade precursors and reagents were used in the present experiments. The synthesis was performed
in a microwave–hydrothermal system (CEM-MARS 5). For the synthesis, to prepare aqueous solution, TiCl3
R. Soc. open sci. 6: 191444
2. Experimental details
2
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optoelectronic and photocatalytic properties. TiO2 has catalytic, dielectric and optical properties, which
leads to diverse industrial applications such as solar cell, pigments, fillers, catalyst supports and photocatalysts [1–5]. Specifically, the TiO2 nanoparticles-based photocatalysis technique is an important and
promising method for the complete removal of organic compounds [6,7] and microorganisms [8,9]. In
general, the organic compounds can be oxidized to carbon dioxide (CO2), water and simple mineral acids
at ambient temperature using TiO2 nanoparticles under the illumination of ultraviolet source [10,11].
Recently, the development of TiO2 and TiO2–Pt catalyst efficiently interacts with the microbial cells under
UV light source, showing the microbial cells were completely removed [12]. Moreover, various bacteria,
cancerous cells, viruses, algae and fungi were successfully deactivated under the irradiation of UV source
using TiO2 nanoparticles [13–17]. Furthermore, Sunada et al. [18] reported that the TiO2 nanoparticles
are not only killed by the bacteria through photocatalytic process, but also they are used for the
decomposition of toxic ingredient of bacteria. If the UV light illuminated for a reasonable time,
the bacteria are completely mineralized and converted into CO2, H2O and other mineral substances
[19,20]. On the other hand, the degradation efficiency of TiO2 nanoparticles depends on their morphology,
preparation methods and specially size of the particles. The small lateral sized TiO2 nanoparticles exhibit
higher specific surface areas (SSAs) [21]. In addition, when the size of TiO2 decreased to below 10 nm, its
energy band gap of TiO2 increased due to its quantum size effect [22,23].
TiO2 nanoparticles were prepared by many synthetic routes not limited to but including sol–gel method
[24], hydrothermal process [25], template routes [26] and reverse micelles [27]. The sol–gel synthesis process
is employed for the controlled synthesis of TiO2 nanoparticles; however, the synthesis of smaller TiO2
nanoparticles with homogeneous size distribution is still challenging. In general, TiO2 nanoparticles
prepared by a sol–gel method are amorphous in nature; therefore, a calcination process is required to
achieve the crystallinity. Another factor that plays a key role in increasing the photocatalytic activity is the
larger SSA of TiO2. Although calcination process could be improved by the crystallinity of TiO2
nanomaterials, it might induce the aggregation of small nanoparticles that leads to a decrease of the SSA.
Based on the above concern, we need to synthesize agglomeration-free photocatalytic active TiO2
nanoparticles without any further heat treatment.
Recently, Sofia et al. [28] used a novel sol–gel reflux condensation route to produce TiO2 QDs. In their
work, the process involved using titanium tetra-isopropoxide as the precursor that was hydrolysed and
then subjected to reflux condensation for 24 h. Spherical QD morphology with an average crystallite
size of 5–7 nm was obtained by subsequent drying and annealing (450°C for 1 h) treatments. In another
report, Xu et al. [29] prepared TiO2 quantum dots (QDs) by using an autoclave method, and the mixed
solution was heated at 150°C for 24 h in autoclave. They reported that the final product was in the form
of mixed structures of monodispersed QDs (3–6 nm) and islands (15–30 nm). Lalitha et al. [30]
synthesized TiO2 QDs by the sol–gel method. In their work, calcination at 350°C for 30 min was
required to obtain TiO2 QDs of 4.8 nm size. Deng et al. [31] synthesized anatase TiO2 QDs with surface
hydroxyl groups and particle size below 3 nm via a new synthetic route (sol–gel). They have reported
that the reaction was completed in a Teflon-lined autoclave, and kept in an oven at 90°C for 1 day.
These reports showed that the preparation methods used for TiO2 QDs are time- and energy-consuming
and do not fulfil the economic and industrial requirements of TiO2 QDs-based photo-catalysts. Thus, a
simple and fast route, for the synthesis of TiO2 QDs under ambient conditions without any annealing
treatment, is still required. Compared with the above-mentioned techniques, microwave–hydrothermal
method is much simpler and cheaper due to its unique features such as short reaction time, rapid and
homogeneous volumetric heating, enhanced reaction selectivity, energy saving, environment-friendliness
and high reaction rate [32].
In this work, we report the synthesis of the agglomeration-free anatase TiO2 quantum dot, for the first
time, by using TiCl3 and NaOH by microwave–hydrothermal method toward the photocatalytic
deactivation of Escherischia coli under UV light source. The microwave-assisted hydrothermal process
is adopted to synthesize with controlled size and shape of TiO2 QDs. Most importantly, there is no
requirement of further calcination steps to obtain final product as was required in earlier reports [28–
31]. The resulting QDs show remarkably high photocatalytic inactivation of E. coli as compared with
commercially available TiO2 nanoparticles.
Figure 1 depicts the XRD patterns of the as-prepared TiO2 QDs and the commercial TiO2 nanoparticles.
All the diffractions peaks in TiO2 QDs and commercial nanoparticles are well matched and indexed to
anatase phase, and are in good agreement with standard JCPDS card no. 89-4921. From this pattern,
the as-synthesized TiO2 QDs exhibit well crystalline peaks with pure anatase, indicating the complete
crystallization of the stable anatase phase without any further heat treatment. Mainly, the localized
high temperatures through microwaves caused the rapid crystallization of TiO2 QDs [33]. The major
diffraction peaks of TiO2 QDs indicate at the same peak position (2θ) as commercial TiO2
nanoparticles. Moreover, the major peak of TiO2 QDs shows broader and the relative peak intensity
decreases, which indicates very smaller crystallite size. The average crystallite size (D) of TiO2 QDs
and commercial nanoparticles estimated using Debye–Scherrer formula [34] using most intense (101)
plane diffraction peaks were found to be approximately 2 nm and approximately 20 nm, respectively.
To identify the morphology and dimension of the TiO2 QDs and commercial TiO2 nanoparticles,
FESEM and TEM were used. FESEM images (low magnification) of the TiO2 QDs showed
nanoparticles ranging from 2 to 5 nm (figure 2b), which can be seen in the high-magnification images
as shown in the inset of figure 2b. On the other hand, commercial nanoparticles of TiO2 are larger
ranging 20–30 nm (figure 2a).
Moreover, TEM and HRTEM studies were performed to get the information about morphologies and
the structural features of TiO2 QDs. Figure 3 displays the TEM image (low magnification) of the TiO2
QDs of approximately 2 nm size (upper inset of figure 3) which is well matched with the XRD
analysis and uniformly distributed (lower inset of figure 3). The HRTEM image (figure 4) displays
clear lattice fringes of as-prepared QDs, and it was completely crystalline and entirely consists of an
anatase phase. The lattice spacing d is 0.34 nm corresponding to the (101) crystallographic planes of
anatase TiO2.
R. Soc. open sci. 6: 191444
3. Results and discussion
3
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(Sigma Aldrich) and NaOH (99.99%; Sigma Aldrich) in 1 : 10 molar ratio were dissolved in 50 ml deionized
water (Milli-Q Gradient A-10 system (Millipore)). The solution was stirred for 20 min at room temperature
and transferred into a 100 ml Teflon-lined digestion vessel at 160°C for 20 min with a pressure in the range
of 150 psi and 500 W power in a microwave–hydrothermal. After completing the reaction, the solution
was cooled down to room temperature. The precipitate was collected and washed several times with water
and ethanol. The final samples were dried in an oven at 80°C for 24 h. For the comparison purpose,
commercial TiO2 nanoparticles (Anatase, nanopowder, 99.7%; Sigma Aldrich) were used.
X-ray diffraction (XRD) analysis of the samples was carried out using a Phillips X’pert (MPD-3040) X-ray
diffractometer with Cu Kα radiation (λ = 1.5406 Å) operated at a current of 30 mA and a voltage of 40 kV. The
morphological studies of QDs were explored through high-resolution transmission electron microscopy (HRTEM; JEOL/JEM-2100F) operated at 200 kV and a field emission scanning electron microscope (FESEM;
MIRA II LMH). UV–Vis spectrophotometer (Agilent-8453) was used to obtain the optical behaviour of the
samples ranging from 200 to 800 nm. The optical band gap of the QDs was determined from the UV–Vis
diffuse reflectance spectra recorded at room temperature. Raman spectrometer (NRS-3100, λ = 532 nm)
was used to study the structural properties of TiO2 QDs. The SSA of the samples was estimated using
Brunauer–Emmett–Teller (BET; Autosorb-1, Quantachrome) analysis.
In photocatalytic experiments, 20, 40 and 60 µg ml−1 of aqueous TiO2 solution was prepared through the
normal saline water under dark condition. Afterwards, 10 ml of TiO2 solution and 10% fresh standard
inoculums of E. coli (≈108 cfu ml−1) were added into 80 ml sterilized normal saline. For the
standardization of the overnight grown culture, where standard inoculum is prepared by diluting and
making a 10% inoculum in fresh broth. This fresh 10% inoculum is equivalent to approximately
108 cfu ml−1. Before the light exposure, the suspension was stirred with a magnetic stirrer for 30 min in
the dark condition. During the dark experiment and irradiation, the beaker was wrapped with an
aluminium foil to shield it from the ambient light and to increase reflection. The complete suspension was
stirred through the magnetic stirrer, while UV light (Spectronics ENF-240C, (λ = 365 nm) 4 W tubes) at
15 cm distance from the surface of the medium was illuminated and the suspension was collected every
30 min interval for 4 h. The viable concentration of E. coli was estimated with dispersion plate method on
nutrient agar. For control, experiment was conducted without the addition of TiO2 into E. coli suspension
under UV light irradiation. The collected plates were incubated at 37°C for 24 h and the colony counter
was used for counting the colonies. For a comparative study, similar concentrations of 20, 40 and
60 µg ml−1 of commercial TiO2 nanoparticles solution were used.
50
2q (°)
60
70
(215)
(116)
(220)
(204)
(105)
(211)
(200)
(004)
intensity (arb. units)
(101)
40
80
Figure 1. XRD patterns of as-synthesized TiO2 QDs, commercial TiO2 nanoparticles and standard JCPDS 89-4921 of TiO2.
(a)
(b)
200 nm
200 nm
500 nm
500 nm
Figure 2. Low magnification FESEM images of (a) commercial TiO2 nanoparticles, (b) TiO2 QDs. Insets of (a) and (b) show highmagnification FESEM images.
To study the quantum confinement effect of as-prepared TiO2 QDs on the band gap, UV–Vis
spectroscopy was employed. Figure 5a shows the UV–Vis diffuse reflectance spectra of TiO2 QDs and
commercial TiO2 nanoparticles. The band gap energies of the TiO2 QDs and commercial TiO2
nanoparticles were evaluated using Kubelka–Munk function [35,36]. The plot of (F(R)hν)2 versus
photon energy (hν) for TiO2 QDs and commercial TiO2 nanoparticles is shown in figure 5b. The
energy band gap of TiO2 QDs was found to be 3.60 eV which is larger than the value of commercial
TiO2 nanoparticles as well as the reported value for anatase (3.2 eV) [37]. This increase in Eg might be
due to the quantum size effect [38].
Raman spectrum carried out at room temperature further supported the formation of a tetragonal
anatase structure of TiO2 QDs confirmed in the XRD. An earlier report [39] showed that for anatase
TiO2, six Raman active modes, i.e. A1g, two B1g and three Eg, were obtained, and could be detected
at 144 cm−1 (Eg), 197 cm−1 (Eg), 399 cm−1 (B1g), 513 cm−1 (A1g), 519 cm−1 (B1g) and 639 cm−1 (Eg).
Figure 6 illustrates the Raman spectra of both samples, which indicate the presence of anatase phases
TiO2 for both the QDs and commercial nanoparticles. Moreover, the peak corresponding to the B1g
mode, A1g and Eg modes of TiO2 QDs shows significant broadening and a small shift toward the
higher frequencies than that of the commercial TiO2 (figure 6). It is well known that this shift is
attributed to the phonon confinement size effect [40]. In the present work, TiO2 QDs are of
approximately 2 nm size; thus, the shift of Raman peaks is due to the quantum size effect.
The SSA of TiO2 plays a key role in photocatalysis [41]. Thus, the primary objective was to prepare
larger SSA TiO2 QDs. Figures 7a and 8a show the nitrogen adsorption–desorption isotherms, and the
R. Soc. open sci. 6: 191444
30
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20
4
JCPDS-89-4921
(commercial TiO2)
(TiO2 quantum dots)
5
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5 nm
average diameter = 2.5 nm
counts
15
10
5
0
10 nm
2
3
particle diameter (nm)
4
Figure 3. TEM image of TiO2 QDs (low magnification), the upper inset shows high-magnification TEM images and the lower inset
shows corresponding particle size distribution plot.
0.34 nm (101)
2 nm
2 nm
Figure 4. HRTEM image of TiO2 QDs, inset shows high-magnification image of the zoomed area.
SSA plot of the as-synthesized TiO2 QDs and commercial TiO2 nanoparticles are shown in figures 7b and 8b,
respectively. The isotherm shows that the nitrogen adsorption volume gradually increases with the relative
pressure and then decreases with the decrease of relative pressure (figure 7a). The SSA of the TiO2 QDs
was calculated to be approximately 381 m2 g−1 (figure 7b) higher than that of commercial TiO2 particles
of approximately 181 m2 g−1 (figure 8b). Also, the TiO2 QDs synthesized by this method showed higher
SSA than already reported TiO2 nanoparticles. Yan et al. [42] reported preparation of TiO2 nanoparticles
with diameter ranging 4–12 nm having an SSA of approximately 64 m2 g−1, on the other hand,
Suttiponparnit et al. [43] showed an SSA of 254 m2 g−1 of TiO2 nanoparticles. In addition, Lee et al. [44]
reported TiO2 nanoparticles produced from the sludge of TiCl4 flocculation of wastewater and seawater
with average crystallite sizes of 6, 15 and 40 nm with the surface area of 76, 103 and 168 m2 g−1,
respectively from artificial wastewater (AW), biologically treated sewage effluent (BTSE) and seawater
(SW), respectively. By comparing our results with these reports, the synthesized QDs in this study have
a smaller particle size of approximately 2 nm and very large SSA of 381 m2 g−1, thus the presented
R. Soc. open sci. 6: 191444
20
(a)
(b)
6
(F(R) hn)2
% reflectance
200
300
600
2.0
700
2.5
3.0
3.5 4.0
hn (eV)
4.5
5.0
5.5
Figure 5. (a) UV–Vis diffuse reflectance spectra, and (b) Kubelka–Munk plots for TiO2 QDs and commercial TiO2 nanoparticles.
Raman intensity (arb. units)
(commercial TiO2)
(TiO2 quantum dots)
A1g
Eg
B1g
300
600
500
400
wavenumber (cm–1)
700
800
Figure 6. Raman spectra of TiO2 QDs and commercial TiO2 nanoparticles.
(a)
(b)
220
(adsorption)
(desorption)
3.0
180
1/[W((Po/P)–1)]
volume (cc g–1)
200
160
140
120
100
80
TiO2 quantum dots
60
0
0.2
0.4
0.6
0.8
relative pressure, P/P0
1.0
TiO2 quantum dots
2.5
2.0
1.5
1.0
slope = 9.061
intercept = 0.0774
correlation coefficient, r = 0.99996
C constant = 118.005
0.5
surface area = 381.088 m2 g−1
0.05 0.10 0.15 0.20 0.25 0.30 0.35
relative pressure, P/P0
Figure 7. (a) Nitrogen adsorption–desorption isotherm of as-synthesized TiO2 QDs, (b) corresponding BET surface area plot.
method is more efficient to produce QDs with large SSA. The higher SSA of TiO2 could enhance the
surface reactivity [45].
Furthermore, the photocatalytic deactivation of E. coli was conducted by TiO2 QDs and commercial
TiO2 nanoparticles powder concentration ranging from 20 to 60 µg ml−1 under UV light irradiation, as
shown in figure 9. The nanomaterial will show stronger antibacterial activity if the change occurs in
R. Soc. open sci. 6: 191444
400
500
wavelength (nm)
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(commercial TiO2 nanoparticles)
(TiO2 quantum dots)
(a)
(b)
1/[W((Po/P)–1)]
volume (cc g–1)
250
200
7
6
(adsorption)
(desorption)
commercial TiO2
150
100
commercial TiO2
5
4
3
2
slope = 19.0474
intercept = 0.2367
correlation coefficient, r = 0.99994
C constant = 81.472
1
surface area = 180.589 m2 g−1
50
0
0
0.2
0.4
0.6
0.8
relative pressure, P/P0
1.0
0.05
0.10 0.15 0.20 0.25
relative pressure, P/P0
0.30
control (E. coli, UV light)
survival ratio (N/N0)
1.0
0.8
commercial TiO2
(60 mg ml–1)
0.6
0.4
(20 mg ml–1)
(40 mg ml–1)
0.2
(b) 100
survival percentage (%)
(a)
80
60
40
20
(60 mg ml–1)
0
0
0
1
2
time (h)
3
4
rol
cont
iO 2
dots
al T
um
t
n
a
qu
TiO 2
ci
mer
com
(c) 1.8 × 10–4
K (s–1)
1.6 × 10–4
1.4 × 10–4
1.2 × 10–4
1.0 × 10–4
8.0 × 10–5
slope = 0.01
20
40
50
60
30
TiO2 concentration (µg ml–1)
Figure 9. (a) Plot of change in the survival ratio of E. coli, (b) survival percentage of E. coli, (c) the relation between death rate
constant and TiO2 QDs concentration towards E. coli.
the survival ratio (N/N0; N = number of cells at time t, N0 = number of cell at time t = 0) with the specified
time. It is clear from figure 9a that with the increase of UV irradiation time, the survival ratio decreased,
which is illustrating the inactivation of E. coli. At a specified time, with the increase in powder
concentration, the values became smaller, which showed that the higher the powder concentration, the
higher the antibacterial activity. In particular, the survival ratio of TiO2 QDs decreased more steeply
in short time as compared with commercial TiO2 nanoparticles. Figure 9b shows the survival
percentage of E. coli for control, commercial TiO2 nanoparticles and TiO2 QDs. It was observed that
the highest inactivation of 91% of E. coli was achieved in the presence of 60 µg ml−1 of TiO2 QDs,
while only 45 and 3% of E. coli were inactivated in the presence of commercial TiO2 nanoparticles and
control, respectively. It is clear that the TiO2 QDs show higher photocatalytic inactivation of E. coli
R. Soc. open sci. 6: 191444
Figure 8. (a) Nitrogen adsorption–desorption isotherm of commercial TiO2 nanoparticles, (b) corresponding BET surface area plot.
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300
dN
¼
dt
KN,
ð3:1Þ
4. Conclusion
In summary, rapid and cost-effective one-pot microwave–hydrothermal route was used to prepare
anatase TiO2 QDs within 20 min without any additional heat treatment, and the photocatalytic
inactivation of E. coli was investigated. XRD, Raman and HRTEM investigations confirmed the
tetragonal anatase structure with well crystalline and single-phase nature. TEM results revealed TiO2
QDs with the size of approximately 2 nm. The BET surface area analysis showed that the anatase
TiO2 QDs exhibited a much higher SSA (381 m2 g−1) than commercial nanoparticles (181 m2 g−1) as
well as earlier reported TiO2. The band gap energy for QDs was found to be 3.60 eV higher than that
of the commercial nanoparticles. As-synthesized TiO2 QDs exhibit higher photocatalytic inactivation
of E. coli than commercially available nanoparticles under UV light irradiation. TiO2 concentration of
60 µg ml−1 is sufficient to inactivate about 91% of E. coli. The higher photocatalytic inactivation
properties of the TiO2 QDs are believed to be due to smaller particle size and higher band gap
resulted in the higher SSA and prevent electron–hole recombination rate compared with the
commercial nanoparticles. These QDs effectively inactivate E. coli by photocatalysis and offer an
improved charge separation and promote the photoactivity significantly. This work suggests that to
obtain excellent photocatalytic properties, tuning of particle size might be a key parameter for
promising future biomedical applications.
R. Soc. open sci. 6: 191444
where N corresponds to survival ratio (N/N0) and t is the time. Figure 9c shows the relationship between
K value and TiO2 QDs concentration. The slope value in death rate constant plot of E. coli was found to be
approximately 0.01. In comparison with photocatalysis-based antibacterial activity of TiO2 QDs and
commercial TiO2 nanoparticles towards E. coli, it was found that the antibacterial activity of TiO2 QDs
were much stronger than the commercial TiO2 nanoparticles.
Different activity of the prepared samples is associated with their SSA and the size of the particles.
It has been reported that ultra-small particles (i.e. quantum-sized particles) showed better
photochemical characteristics than Degussa P25 [47], and have the characteristics between molecular
and bulk semiconductor. Thus, there is improvement in the surface-limited reactions due to high
surface area-to-volume ratios [45], since TiO2 QDs are purely anatase phase and have extremely large
SSA as compared with commercial nanoparticles, which provides better reactivity with the
microorganisms and resulted in higher photocatalytic inactivation.
Sunada et al. [48] reported that the photocatalytic mechanism using TiO2 on E. coli is a three-stage
process where the decomposition of the dead cell occurred. Fujishima et al. [49] showed that E. coli
will be totally mineralized with the illumination time.
In a photocatalytic process, the light with a wavelength greater than or equal to the band gap (Eg) of
the semiconductor irradiates onto a semiconductor such as TiO2. When the QDs absorb the light, the
electrons in the valance band excited to the conduction band, resulting in the generation of
photoexcited electron–hole pairs. These photoexcited electron–holes might diffuse to the surface of the
semiconductor resulted in the interfacial electron transfer. The oxidation reactions in the solution are
caused by holes which resulted in the mineralization of organic substances [50]. In the photocatalytic
process, OH• radicals formed which are governed by OH groups and or physisorbed H2O. The
production of highly reactive hydroxyl radicals (OH•) occurred due to the reaction of holes with
water, and caused the oxidation of organic materials and biomolecules [51]. To achieve a high
efficiency by the adsorption of higher OH groups on the surface of QDs, the large SSA of the TiO2
QDs is a key factor. In another factor the wider band gap of TiO2 QDs prevents the recombination
effects of charge carriers, resulting in higher photocatalytic activity against E. coli. Moreover, when the
crystallite size of the particle decreases to below or approximately 10 nm, the charge carriers acted
quantum mechanically [45]. Due to the confinement, the band gap increased with the decrease of
particle size. Thus, with the increase in band gap, the potential of oxidation of the photon-generated
holes and the reducing potential of the electrons might increase. Consequently, TiO2 QDs exhibit
excellent photocatalytic properties, and this property was used for the inactivation of E. coli.
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than the commercial nanoparticles. By converting the survival ratio of vertical axis into logarithmic value
as depicted in figure 9a, a linear decrease for time resulted to the ratio. Thus, death rate constant, k can be
determined by first-order kinetics [46];
Data accessibility. Data available from the Dryad Digital Repository: https://dx.doi.org/10.5061/dryad.9pc7mj1 [52].
Authors’ contributions. F.A. designed the study and prepared all the samples for analysis. F.A., S.A.A. and C.A. collected
9
and analysed the data. F.A., A.A., and A.A.S. contributed in interpreting the results and writing the manuscript. All
authors read the manuscript and gave final approval for publication.
Competing interests. The authors declare no competing interest.
Funding. This work is funded by Deanship of Scientific Research at King Faisal University through NASHER track
(grant no. 186101).
Acknowledgements. The authors would like to thank the Deanship of Scientific Research at King Faisal University for
supporting this research through NASHER track (grant no. 186101).
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