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Original Article
2025
:18;
1852025
doi:
10.25259/AJC_185_2025

Citrus maxima extract-mediated synthesis of pristine and Cu-doped TiO2: Impact of the structural and optical properties on the antimicrobial activity

, , , , , , ,
aDepartment of Physics, College of Science and Humanities in Al-Kharj, Prince Sattam bin Abdulaziz University, Al-Kharj, Saudi Arabia
bDepartment of Chemistry, College of Science and Humanities in Al-Kharj, Prince Sattam bin Abdulaziz University, Al-Kharj, Saudi Arabia
cRefining and Petrochemical Technologies Institute, King Abdulaziz City for Science and Technology, Riyadh, Saudi Arabia
dPrince Sattam bin Abdulaziz University, Alkharj, Saudi Arabia
eDepartment of Pharmaceutics, College of Pharmacy, Prince Sattam bin Abdulaziz University, Alkharj, Saudi Arabia
fDepartment of Biotechnology College of Commerce, Arts & science Patliputra University, KankarBagh, Patna-800020, Bihar, India
gDepartment of Pharmaceutical Chemistry, College of Pharmacy, Prince Sattam bin Abdulaziz University, Al-Kharj, Saudi Arabia

* Corresponding author: E-mail address: m.geesi@psau.edu.sa (M.H. Geesi)

Received: , Accepted: ,
© 2025 Arabian Journal of Chemistry - Published by Scientific Scholar
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This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

Anatase is the most active among all TiO2 polymorphs; however, its thermal instability and its absorption confined in the UV range constitute its main limitations. Alternatively, plant-based synthesis arose as a promising approach to address the thermal instability issue, while transition-metal doping is recognised to extend its response to the visible range. In this study, Citrus maxima extract was used as a reducing/capping agent for the preparation of both pristine and copper-doped TiO2 nanostructures. Several techniques, including X-ray diffraction (XRD), Fourier-transform infrared (FTIR), UV-visible spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), were used to explore the physico-chemical features of the prepared TiO2 samples. XRD results showed that TiO2 samples, sintered at 600°C for 2 hrs, exhibit predominantly an anatase phase with a crystallite size of 15.3 nm and 23.5 nm for pristine and Cu-doped TiO2, respectively. XPS analysis revealed that both samples are inherently doped with carbon originating from the organic phase of the extract. Combined analysis using XRD, XPS, and UV-vis shows that the copper element was successfully inserted in the TiO2 lattice, causing an optical absorption red shift, as well as a structural transformation consisting of a mixture of anatase-rutile phases. The inhibitory potentials of the prepared samples were evaluated in terms of minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and disc diffusion measurements over several microbial pathogens, namely Bacillus subtilis, Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, and Candida albicans. Remarkably, both pristine and Cu-doped have demonstrated variable inhibition power against all the tested microbes, with a zone of inhibition ranging from 10 to 20 mm, depending on the microbe type.

Keywords

Antimicrobial effect
Citrus maxima-based sol-gel method
Pristine and Cu-doped TiO2

1. Introduction

Recent years have seen a huge growth in the application of natural products as promising alternatives for green and sustainable synthesis of metal oxides nanostructures, notably TiO2 [1]. This approach is preferred for its advantage to overcome critical issues associated with traditional physical and chemical methods, primarily the use and/or generation of harmful chemicals. In this context, various biosystems, including bacteria [2], yeast [3,4], viruses [5], algae [6], fungi [7], and plants [8-10], have been reported in the effective synthesis of versatile metal oxide nanostructures. Generally, biomolecules and phytochemicals derived from microbes and plants function as both reducing and stabilizing agents for the green synthesis of nanoparticles [11]. However, a plant extract-based approach is considered simpler, safer, and faster than the microorganism-based one, as the latter involves additional culturing and isolation steps prior to the synthesis of nanoparticles.

Among the large spectrum of metal oxides, TiO2 has aroused extensive interest because of its low cost, non-toxicity, and mainly its outstanding optical and electrical properties. TiO2 can exist in two predominant crystal polymorphs: rutile and anatase. Among them, rutile is the most thermally stable, while anatase is typically the most active [12]. Although, the reasons behind such difference in activity are still under debate, multiple hypotheses have been reported including mainly: i) a larger band gap of anatase (3.2 eV) with respect to rutile polymorph (3 eV) believed to increase the oxidation strength of photoexcited electrons, which promotes its activity [13]; ii) a larger charge carrier lifetime in indirect bandgap semiconductors (anatase; > 1µs) than those with direct bandgap (rutile; ∼20-50 ns) [14]; iii) the average effective mass of charge carriers was reported to be smaller in anatase (∼m0, where m0 is the electron mass) compared to rutile (∼7-8 m0), which makes their migration easy, inhibits their recombination, and therefore increases the photocatalytic activity [15]; and iv) different surfaces display different activities and bandgaps [16].

However, despite its higher apparent activity with respect to other TiO2 polymorphs, the optical absorption of anatase is limited to the UV light spectrum due to its wide bandgap, stated to be about 3.2 eV [17]. Therefore, extending its optical absorption to the visible spectrum region is of paramount importance for versatile applications. This can be achieved using numerous approaches, namely by sensitization with organic dyes [18,19], by coating with noble metals serving as trapping sites for the photoexcited electrons which impede the charge carrier’s recombination [20,21], by forming heterostructure with other semiconductors [22,23], and particularly by doping with metal or non-metal elements [24-27].

On the other hand, the high temperature transition of anatase to rutile represents a critical problem that denies the apparent activity potential of the anatase polymorph. Alternatively, plant-based synthesis promotes a highly thermal-stable anatase phase even when the calcination temperature is rather high (up to 600°C) (Table 1). From them, citrus fruits including Citrus maxima are deliberated as a potential biosource, offering the reduction and the stabilization of row metal precursors into metal oxide nanostructures [28-31], thanks to the abundance of valuable phytoelements and bioactive molecules in their composition such as flavonoids, carotenoids, ascorbic acid [32,33], etc.

Previously, only few papers have been reported on the use of Citrus maxima for the synthesis of metal or metal oxide nanoparticles, including ZnO [34], CuO [35], Fe [36], Ag [37], and Au [38], whereas no study has been reported on its application for the synthesis of TiO2.

Drawing on our previous work on the biosynthesis of TiO2 using plant extracts [39,40], in this study, both pristine and Cu-doped TiO2 samples were prepared by a biogenic-based sol-gel method using Citrus maxima juice extract as a source of reducing/capping agents. The prepared samples were subjected to structural, morphological, and optical investigations. In addition, their antimicrobial inhibitory potentials were assessed against selected microbial pathogens.

Table 1. Reported plant-based synthesis of TiO2 nanoparticles calcined at different temperatures and providing anatase polymorph.
Precursor Plant Calcination temperature phase Ref.
Titanium (IV) isopropoxide Calotropis Gigantea 500°C Anatase [41]
Titanium sulfoxide Gum kondagogu 500-900°C Anatase up to 700°C [42]
Titanium tetrachloride Jatropha curcas L. leaves extract 450°C Anatase [43]
Titanium(IV) butoxide Citrus limon juice extract 450°C Anatase [44]
Titanium(IV) butoxide Citrus limon juice extract 600°C Anatase [45]
Titanium isopropoxide Diospyros ebenum leaves extract 600°C Anatase [46]
Titanium tetraisopropoxide Mango leaf extract 400°C Anatase [47]
Titanium isopropoxide Green tea extract 500°C Anatase [48]
Titanium tetraisopropoxide Mangosteen pericarp extract 500°C Anatase [49]
Tetrabutyl orthotitanate Acacia nilotica 400°C Anatase [50]
Titanium(IV) butoxide Citrus limetta extract 550°C Anatase [51]
Titanium tetraisopropoxide Black pepper, Coriande, r and Clove 400°C Anatase [52]
Titanium tetraisopropoxide Mentha arvensis leaves extract 500°C Anatase [53]
Titanium sulfoxide Trigonella foenum-graecum leaves extract 700°C Anatase [54]
Titanium isopropoxide Adansonia digitata leaves extract 700-900°C Anatase [55]
Titanium (IV) bis (ammonium lactate) dihydroxide Citrus sinensis Up to 800°C Anatase [56]
Titanium tetraisopropoxide Leaf extract of Citrus aurantifolia 150-450°C Anatase [57]
Titanium butoxide Citrus limetta extract 550°C Anatase [58]

2. Materials and Methods

2.1. Biogenic synthesis of pristine and copper-doped TiO2

Citrus maxima fruit was purchased from a local market and thoroughly cleaned with deionized water, cut into four pieces, and then squeezed to extract the juice. The collected juice was filtered twice using a Whatman filter paper. The harvested clean juice was used as a stock solution for the biogenic synthesis of TiO2 samples. The preparation of the pristine TiO2 nanoparticles was achieved using the sol-gel method in a similar way as previously reported for the synthesis of TiO2 using Citrus limon and grapefruit juice extracts [39,40]. Specifically, 23 mL of titanium butoxide precursor was mixed with 23 mL of ethanol under stirring for 30 mins. Then, 23 mL from the Citrus maxima stock solution was added to the above-prepared precursor/ethanol solution under stirring for 30 mins till the mixture transformed rapidly into a gel. The obtained gel was then transferred to a pre-heated oven at 80°C and kept to dry overnight. The collected dry powder was ground in a mortar, poured into a ceramic crucible, and finally, sintered in a furnace at 600°C for 2 h. A similar process was performed for the preparation of the Cu-doped TiO2. Thus, copper chloride, 10% w/w with respect to the titanium butoxide precursor, was dissolved in 23 mL of the Citrus maxima stock solution under stirring, then added to a pre-prepared solution consisting of 23 mL of titanium butoxide in 23 mL of ethanol under stirring. The formed gel was dried in an oven at 80°C and sintered at 600°C for 2 h. Figure 1 represents a schematic diagram of the above-described synthesis protocol.

Schematic diagram of Citrus maxima-based sol-gel synthesis of TiO2.
Figure 1.
Schematic diagram of Citrus maxima-based sol-gel synthesis of TiO2.

2.2. Analytical techniques

X-ray diffraction (XRD) powder analysis was performed using a Rigaku Ultima IV diffractometer from Bruker equipped with a Cu-Kα source providing a wavelength λ= 0.15406 nm. Diffraction data were performed in the 2θ angle range 10-80° with a step of 0.02°. UV-visible absorption spectra were recorded using a Cary 5000 UV–Vis-NIR spectrophotometer from Agilent by scanning the wavelength between 200 nm and 800 nm. Fourier transform infrared (FTIR) spectroscopy investigations were carried out on a Thermo Scientific Nicolet iS50 FTIR spectrometer in the wavenumber range 400 - 4000 cm-1. An Inspect F50 scanning electron microscope (SEM) from FEI Company was used for the morphological studies of the biogenic synthesized TiO2 samples.

2.3. Antimicrobial methodology

The agar-cup diffusion method was used to assess antibacterial activity. This experiment was conducted for both gram-positive (Bacillus subtilis, Staphylococcus aureus) and gram-negative bacteria (Escherichia coli, Klebsiella pneumonia), as well as for Candida albicans (fungus).

All of these strains were cultivated in nutrient agar at 37°C for 24 hrs and grown on Mueller-Hinton agar Oxoid from England. After solidification, 5-mm holes were drilled with a sterile borer, and 0.25 mL of the test strains was separately injected into the culture media. Next, 100 μL of the antimicrobial agents (pristine and Cu-doped TiO2), taken at various concentrations, as well as negative and positive control solutions, were added into each hole. Next, plates were incubated at 37°C for 24 hrs to measure their ability to kill bacteria. The zone of inhibition (mm) surrounding the well was measured for each examined samples.

In addition, the minimum inhibitory concentration (MIC) of a given antimicrobial substance, referred to as the concentration at which the growth of microbes is inhibited, was also determined. MIC values were determined via the Kirby disc diffusion method. As a result, MIC values were determined using 96-well microdilution plates, as described in references.

3. Results and Discussion

3.1. XRD analysis

Crystal structure and phase composition of the synthesized samples were investigated by powder XRD. As shown in Figure 2, the pattern of the pristine TiO2 sample exhibits, predominantly, typical peaks of the anatase phase of TiO2, confirmed by the Retvield refinement of the raw diffractogram and identified at 2θ values of 25.34°, 36.92°, 37.86°, 38.6°, 48.06°, 53.74°, 55.04°, 62.12°, 62.81°, 68.75°, 70.3°, and 75.11° assigned to the (hkl) crystallographic planes (101), (013), (004), (112), (020), (015), (121), (123), (024), (116), (220), and (125), respectively. Besides, the pattern shows a small shoulder located at 27.48° corresponding to the plane (110) of the rutile phase. In contrast, the XRD pattern of the Cu-doped TiO2 displays a clear biphasic anatase-rutile structure where the characteristic peaks of the anatase phase appear at 25.14°, 36.58°, 37,68°, 47.92°, 53.79°, 54.93°, 62.59°, 68.59°, 70.01°, 74.97° and attributed to the (hkl) crystallographic planes (101), (013), (004), (020), (015), (121), (024), (116), (220) and (125) while the peaks of the rutile phase are located at 27.26°, 35.96° and 41.24° and attributed to the following (110), (101) and (111) (hkl) planes. Overall, the XRD pattern of the Cu-doped TiO2 shows an almost anatase phase without any diffraction peak, corresponding to metallic copper or any form of copper oxides. Qualitatively, the sharpness of the diffraction peaks, as well as the absence of any unidentified peaks, confirmed the efficacy of the synthesis approach to achieve high-crystalline and pure samples.

XRD patterns of pristine and Cu-doped TiO2 samples calcined at 600°C for 2 h.
Figure 2.
XRD patterns of pristine and Cu-doped TiO2 samples calcined at 600°C for 2 h.

Quantitatively, the percentages of the anatase and the rutile phases are estimated using Spurr formula (Eq. 1) [59]:

(1)
  A = ( 1 1 + 1.256   I R I A ) × 100

Where IR and IA represent the intensities of the most prominent peaks of the (101) and (110) corresponding to the anatase and rutile polymorphs, respectively. Table 2 summarizes the percentages of the rutile and anatase phases in the pristine and the Cu-doped TiO2 samples. Remarkably, the percentage of rutile increased after Cu-doping, which indicates that the Cu-doping promoted the transformation from anatase to rutile as previously reported [60].

Table 2. Crystallite size and percentage of anatase and rutile phases in the synthesized pristine and Cu-doped TiO2 samples.
Sample Anatase phase (%) Rutile phase (%) Crystallite size (nm)
Pristine TiO2 95.2 4.8 15.3
Cu-doped TiO2 83.8 16.2 23.5

Moreover, the average crystallite size was calculated using the following Sherrer’s Eq. (2):

(2)
d h k l = k β h k l c o s θ h k l

Where d h k l represent the crystallite size, λ = 0.15406   n m is the wavelength of the XRD source used, k is a shape factor regularly taken as 0.95, β h k l is the full width at half maxima of the diffraction peak in radians, and θ h k l is the Bragg’s diffraction angle. The average crystallite sizes were 15.3 nm and 23.5 nm for pristine and Cu-doped TiO2, respectively. The copper doping caused an enlargement of the average size of the TiO2 particles. This increase in crystallite’s size after doping was likely due to the insertion of larger Cu2+ ions (ionic radius of Cu2+ is 0.73 Å and that of Ti4+ is 0.64 Å) of different charge than Ti4+ in the anatase phase [61].

3.2. UV–vis analysis

UV-vis spectroscopy is a common and useful technique used to investigate the optical properties of semiconductor materials, notably the determination of the band gap energy and follow-up of its change associated with any electronic structure disruptions. The UV-vis absorption spectra of experiments performed on the synthesized TiO2 samples over the spectral range 200-800 nm have been shown in Figure 3, subpart Figure 3(a). Qualitatively, the absorption spectrum of the pristine TiO2 presents a single peak around 335 nm, followed by a sharp decrease (cut off) around 403 nm characteristic of the optical inter-band absorption [62]. In contrast, in comparison with the pristine TiO2, the spectrum of Cu-doped TiO2 shows a further visible-light absorption shoulder around 403-575 nm, consistent with the change of the sample’s color from white to yellow [63]. Such absorption shoulder referred to as the Urbach tail, appearing often in semiconductor materials and associated, typically, with structural disorder generating a below-gap optical absorption phenomenon near the band edge [64]. The mechanism behind such behavior is described by an exponential absorption edge as follows (Eq. 3):

(3)
α ( h ν ) =   α 0   e h ν − E g E U ( T ) [65]

(a) UV-visible absorption spectra of biogenic synthesized pristine and Cu-doped TiO2. The inset shows optical images of pristine and Cu-doped TiO2, respectively, (b) represents the corresponding Tauc plots, where bandgap energy was determined using the baseline approach [68], (c) Plot of ln (α) versus energy for the determination of Urbach energy of Cu-doped TiO2.
Figure 3.
(a) UV-visible absorption spectra of biogenic synthesized pristine and Cu-doped TiO2. The inset shows optical images of pristine and Cu-doped TiO2, respectively, (b) represents the corresponding Tauc plots, where bandgap energy was determined using the baseline approach [68], (c) Plot of ln (α) versus energy for the determination of Urbach energy of Cu-doped TiO2.

where E U is called the Urbach energy that describes the total energy disorder in the material including both the temperature-dependent dynamical disorder term related to the thermal occupation of phonon states [66], and the static term associated with the compositional disorder in the material assumed as the width of the exponential distribution of sub-gap states induced by such static disorder [67]. By application of the logarithm to both sides of the above equation, we find that (Eq. 4):

(4)
  ln ⁡ α =   1 E U h   + ln ⁡ α 0 − E g E U .

Thus, the plot of l n α versus the energy; h is a linear curve with a slope equal to the inverse of the Urbach energy. Herein, the Urbach energy for Cu-doped TiO2 was found to be about 735 meV (Figure 3c).

The Tauc plot derived from the Kubelka-Munk function, considering TiO2 as an indirect semiconductor, achieved further insight into the bandgap energy. As illustrated in Figure 3b, the bandgap energy was determined using the baseline approach detailed in reference [68] and was found to be about 3.08 eV for both pristine and Cu-doped TiO2 samples, less than the typical 3.2 eV reported for pure anatase polymorph. This indicates that the bandgap is not altered by doping, but localized states associated with the copper doping are inserted in the bandgap of TiO2, which agree well with previous reported results on TiO2 doped with either metal or nonmetal ions [69-71].

Overall, UV analysis revealed that copper is inserted into the TiO2 lattice causing the alteration of the crystal and the electronic properties of the TiO2 sample.

3.3. XPS elemental analysis of TiO2 samples

Elemental analysis of the prepared biogenic TiO2 samples was performed by X-ray photoelectron spectroscopy (XPS). As can be seen in Figure 4(a), both spectra are quite similar, indicating the presence of Ti and O elements, except for the appearance of an additional small peak located at 935 eV in the spectrum of the Cu-doped sample associated with the Cu 2p spin-orbit splitting, confirming the successfulness of the doping process (Figure 4b).

(a) XPS surveys and high-resolution spectra of: (b) Cu2p, (c), (d) C1s, (e) (f) O1s, and (g), (h) Ti2p of pristine TiO2 and Cu-doped TiO2.
Figure 4.
(a) XPS surveys and high-resolution spectra of: (b) Cu2p, (c), (d) C1s, (e) (f) O1s, and (g), (h) Ti2p of pristine TiO2 and Cu-doped TiO2.

In addition, to gather more information about the electronic states of the different elements present in the samples, high-resolution scans were carried out. Interestingly, the high-resolution spectra of C1s for both the pristine and the Cu-doped TiO2 samples demonstrated a peak associated with the Ti-O-C bond, indicating their doping with carbon (Figure 4c,d). Such a result is expected in virtue of the abundance of organic compounds in the synthetic medium originating from the plant extract. The high-resolution spectra of O1s of the two samples can be resolved into three peaks, assigned to the lattice oxygen for peaks around 529.3 eV, to the oxygen vacancy in peaks 530.8 eV and 530.37 eV, and to the adsorbed oxygen 531.91 eV and 532.95 eV peaks (Figure 4e,f). The peak associated with oxygen vacancy in the Cu-doped TiO2 samples seems to be more intense than that in pristine TiO2, indicating that the Cu doping promoted more oxygen vacancies in the sample. The fitting of the high resolution peak of Cu2p (Figure 4b) revealed two peaks lactated at 932.45 eV and 952.23 eV ascribed to the indicates that copper Cu2p3/2 and Cu2p1/2 spin-orbit splitting with a splitting energy Δ E = 19.78   e V consistent with the reported values for Cu metal [72]. In addition, the absence of satellite features near 943 eV in the Cu2p spectrum characterizing the presence of CuO indicated that the Cu was doped in the TiO2 lattice and did not form CuO structures [73]. Further, as Cu2+ has a lower valency than Ti4+, to conserve the lattice charge after the insertion of Cu ions in TiO2, oxygen vacancies are promoted as revealed in analysis of the O1s spectrum of the Cu-doped TiO2 sample (Figure 4f). Ti2p spectrum of pristine TiO2 reveals two peaks located at 458.67 eV and 464.39 assigned to Ti2p3/2 and Ti2p1/2 spin-orbit splitting states with a splitting energy Δ E = 5.72   e V   indicating that Ti ions are tetravalent Ti4+ (Figure 4g) In contrast, the Ti2p spectrum of the Cu-doped TiO2 (Figure 4h) shows two additional shoulders peaks located at 459.2 eV and 562.09 eV that can be ascribed to the Ti2p of Ti3+ ions [74].

3.4. FTIR analysis

To gain further insights into the features of the biogenic synthesized TiO2 nanoparticles, FTIR analysis of the prepared samples was carried out. Spectra of the pristine and the Cu-doped TiO2 samples recorded over the wavenumber range 400- 4000 cm-1 are depicted in Figure 5. The wide band observed in the range of 400–800 cm-1 is typically ascribed to Ti–O and Ti–O–O bonds and considered as the fundamental characteristic of the FTIR spectrum of TiO2. However, as the Ti–O bond is shorter than the Cu–O bond, the doping of copper in TiO2 may lead to a shift of the wavenumber of Ti–O lattice vibration [75-77]. In addition, the absence of any vibration bands except those associated with TiO2 indicates the elimination of any organic residues after the calcination step. This reveals that the prepared TiO2 samples are pure, which corroborates with earlier XRD and XPS results.

FTIR spectra of pristine and Cu-doped TiO2 samples.
Figure 5.
FTIR spectra of pristine and Cu-doped TiO2 samples.

3.5. Field emission scanning electron microscopy micrograph

The morphology of the biogenic prepared pristine and Cu-doped TiO2 samples was revealed by field emission scanning electron microscopy (FESEM). The micrographs of the pristine and the Cu-doped TiO2 have been displayed in Figures 6(a) and (b), respectively. The two images share quite similar features, consisting of clusters of agglomerated nanoparticles. However, the Cu-doped TiO2 samples appear to be rather aggregated than the pristine TiO2. Such high morphological similarity supported the inclusion of copper in the TiO2 matrix as confirmed by the above techniques.

SEM micrographs of the synthesized TiO2 samples (a) pristine TiO2, (b) Cu-doped TiO2.
Figure 6.
SEM micrographs of the synthesized TiO2 samples (a) pristine TiO2, (b) Cu-doped TiO2.

3.6. Antimicrobial performance

Both pristine and Cu-doped TiO2 were assessed for their antimicrobial activity against five microbial pathogens, namely, E. Coli, K. pneumoniae, S. aureus, B. subtilis, and C. albicans, by performing MIC, minimum bactericidal concentration (MBC), and disc diffusion measurements. Dimethyl sulfoxide (DMSO) and a conventional antibiotic, ciprofloxacin, were used as negative and positive controls, respectively. Antimicrobial investigations data have been summarized in Table 3.

Table 3. Antimicrobial effects of the pristine and Cu-doped TiO2 against different pathogens, evaluated in terms of MIC and MBC values expressed in µg/mL, as well as in terms of disc diffusion (diameter of the inhibition zone: ZOI in mm).
Microbial strains
E. Coli K. pneumoniae S. aureus B. subtilis C. albicans
Pristine TiO2 MIC 12.5 9.37 12.5 9.37 9.37
MBC 18.75 12.5 18.75 12.5 12.5
ZOI ±SD 16.66 ± 1.15 19.66 ± 0.29 18.66 ± 1.52 17.66 ± 0.58 20.5 ± 0.5
Cu-doped TiO2 MIC 18.75 18.75 25 12.5 12.5
MBC 25 25 37.5 18.75 18.75
ZOI ±SD 14.33 ± 1.15 14.16 ± 1.04 10.83 ± 0.29 16.33 ± 0.58 17.33 ± 0.26
Ciprofloxacin MIC 12.5 18.75 12.5 12.5 -

By using pristine TiO2 as an antimicrobial agent, the MIC was found to be 12.5 µg/mL for E. Coli (gram-negative) and S. aureus (gram-positive) against a value of 9.75 µg/mL for K. pneumoniae (gram-negative), B. subtilis (gram-positive), and C. albicans (fungi). It is worth noting that the above results are quite similar to the obtained data using standard antibiotic ciprofloxacin, which suggested an MIC value of 12.5 µg/mL for E. Coli, S. aureus, and B. subtilis and 18.75 µg/mL against K. pneumoniae. In contrast, higher MIC values were registered while using the Cu-doped TiO2 agent, specifically 18.75 µg/mL for both E. Coli and K. pneumoniae, 12.5 µg/mL for both B. subtilis and C. albicans, and 25 µg/mL for S. aureus. Advantageously, both biogenic synthesized TiO2 samples demonstrated a clear inhibition potency against all the screened pathogens. Furthermore, a qualitative study of the inhibitory potential of the biogenic synthesized TiO2 samples was evaluated by performing agar-well disc diffusion experiments. The inhibitory zones for target microorganisms, E. Coli, K. pneumoniae, S. aureus, B. subtilis, and C. albicans using a 25 µg/mL concentration of pristine TiO2 were 16.66±1.15, 19.66±0.29, 18.66±1.52, 17.66±0.58. and 20.5±0.5, respectively, significantly higher than previous reported values obtained using grapefruit-synthesized TiO2 [40]. On the other hand, the zone of inhibition decreased when using Cu-TiO2 at a concentration of 25 µg/mL and found to be 14.33±1.15, 14.16±1.04, 10.83±0.29, 16.33±0.58, and 17.33±0.26 for E. Coli, K. pneumonia, S. aureus, B. subtilis, and C. albicans, respectively, which agreed well with the MIC results.

Although there is still considerable controversy surrounding the mechanistic nontoxicity and the activity of nanoparticles depends, in general, on both the core nanoparticle chemistry and physics, as well as the target microbe properties. This explains why it is not surprising that different microbes response differently to the same nanoparticle type. In addition, the interaction between nanoparticles and microbes is short–range; hence, the diffusion of nanoparticles to the proximity of the microbe membrane surface is of pivotal importance for its activity. This contact-based mechanism of inhibition includes the mechanical damage of the cell membrane [78], the generation of free radicals (ROS) by direct electron transfer [79], the release of metal ions [80], and the internalization of nanoparticles [81]. In pristine TiO2, the electronic transition occurs directly from the valence band (VB) to the conduction band (CB). Conversely, in Cu-doped TiO2, both Cu2+ sub-band states and oxygen defect band states are formed underneath the CB. Thus, electrons are not directly excited to the CB, and both the unoccupied Cu2+ states and oxygen vacancies can capture the electrons, causing an increase in photogenerated charge carriers’ lifetimes, which results in a high photocatalytic activity. In addition, these defects create energy states between the VB and CB, causing a redshift of optical absorbance of the Cu-doped TiO2 material, extending its response to the visible range.

As such, the unexpected lower antimicrobial activity of the Cu-doped TiO2 with respect to the pristine TiO2 is likely due to its more aggregated morphology as highlighted by both SEM and XRD analysis, which hinders its diffusion, lowers its specific surface area, and hence diminishes its activity. On the other hand, standard antibiotic acts on bacteria by targeting specific membrane sites. However, due to the misuse and the overuse of antibiotics, microbes have developed more abilities to resist the mechanism of action of antibiotics including the alteration of the target sites of antibiotics [82,83], the overexpression of a specific enzymes that destroy or inactive the antibiotics [84] and by overexpression of efflux pump, which enables bacteria to escape different antibiotics simultaneously [84]. Alternatively, nanoparticles via their non-specific interactions with bacteria could alleviate the problem of drug resistance. Nanoparticles can act either alone or in association with antibiotics or natural products for more efficient antibacterial effect [85,86].

4. Conclusions

Pristine and Cu-doped TiO2 samples were successfully prepared by a Citrus maxima-based sol-gel method. The impact of the synthetic approach on the antimicrobial effect of the derived nanoparticles in relation to their structural and optical features was discussed. Structural investigations reveal dominantly an anatase phase structure for both samples without any diffraction peak associated with copper doping. Although the Cu-doped TiO2 demonstrated additional visible light absorption in the band range 403-575 nm, unexpectedly, its antimicrobial activity was typically less than that of the pristine TiO2, likely because Cu-doped TiO2 was morphologically more aggregated than the pristine TiO2 as revealed by SEM microscopy, which impedes its diffusion, lowers its specific surface area, and thus diminishes its activity.

Acknowledgement

This study is supported via funding from Prince sattam bin Abdulaziz University project number (PSAU/2024/01/29697).

CRediT authorship contribution statement

Oussama Ouerghi: Writing – original draft, Writing – review & editing, Methodology, Data curation, Investigation, Visualization, Formal Analysis; Abdulaziz Alanazi: Writing – original draft, Data curation, Resources, Supervision, Validation; Talal A. Aljohani: Conceptualization, Methodology, Writing – review & editing, Formal Analysis, Resources, Supervision, Validation, Project administration. Elmutasim O. Ibnouf: Writing – original draft, Formal Analysis, Validation; Mohammad Azhar Kamal: Writing – original draft, Formal Analysis, Validation; Mahjabeen Rahmani: Writing – original draft, Data curation; Yassine Riadi: Conceptualization, Methodology, Writing – original draft, Data curation, Formal Analysis, Visualization; Mohammed H. Geesi: Conceptualization, Resources, Writing – original draft, Formal Analysis, Visualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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