Bioengineered Zn₂TiO₄ nanomaterial and its effective electrochemical, antibacterial and photocatalytic responses

Abstract

Bioengineering of bimetallic oxides is a significant requirement for both social and environmental applications. In this work, bimetal oxide nanomaterials, Zinc orthotitanate Zn₂TiO₄ was produced using a green process which utilizes natural extract of Hibiscus sabdariffa. More accurately, natural extract/phyto-compounds of Hibiscus sabdariffa was validated as an effective chelating agent at room temperature and atmospheric pressure. A variety of techniques were applied to analyse their physicochemical properties and verify the production of the Zn₂TiO₄ nanoparticles. The XRD analysis illustrated the crystalline structure of the Zn₂TiO₄ nanocomposites. The SEM analysis showed the morphology of the compounds Zn₂TiO₄ to be highly porous. The EDS analysis validated the presence of elements such as Ti, and Zn while FTIR analysis indicated the existence of multiple functional groups, including O–H, C–H, C=O, Ti–O, and Zn–O bonds in the compound. The PL peaks identified at 240 and 450 nm are attributed to the charge transfer characteristics present in the surface state of the resulting product. The single phase crystals of the bio-engineered Zn₂TiO₄ were found to be within the nanoscale while exhibiting a significantly elevated photocatalytic efficacy in decomposing Methylene Blue reaching a threshold of about 94.1% efficacy at about 120 min. From the electrochemistry viewpoint, the bioengineered Zn₂TiO₄ nanomaterial demonstrated enhanced redox activity and stability, exhibiting 94.2% current retention after 20 cycles. Cyclic voltammetry (CV) studies confirmed a diffusion-controlled process, while electrochemical impedance spectroscopy (EIS) revealed a 10-fold reduction in charge transfer resistance ®_ct), from 5.48 to 0.53 KΩ. This significant acceleration in electron transfer kinetics is corroborated by a nearly 2-fold increase in interfacial admittance (Y_o), indicating an expanded electroactive surface area. These results demonstrate the applicability and potential for Zn₂TiO₄ nanomaterial in advanced electrochemical applications. From the antibacterial perspective, Zn₂TiO₄ nanomaterial showed limited activity in the agar-disc diffusion assay, with no meaningful inhibition against most tested Gram-positive and Gram-negative strains and only a marginal response against K. pneumoniae. These results suggest that any antibacterial potential of the Zn₂TiO₄ nanomaterial may likely to be strongly dependent on assay format, nanoparticle dispersion, contact conditions, and possible light-driven ROS generation.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-54507-y.

Introduction

Nanoscaled metal oxides have been widely used in many technical fields, such as photonics, energy storage, gas sensing, environmental decontamination and water recovery, nano-medicine, micro-electronics, and catalysis^1–4. As nanoscale ZnO and TiO₂ have been identified as the most multifunctional oxides within this family⁵. Aside from the cosmetics industry, their primary applications are focused on the environment and energy because to their inherent opto-electronic characteristics and very similar band-gap values within the 3.2 eV region. They are employed in the Energy focus for oxidation/reduction, H₂O water splitting, and solar energy conversion. Among its applications in the environmental area are air purification, water decontamination, agriculture, and cosmetics^6,7.

ZnO is actually one of the most popular optically transparent conducting oxides in the photonics industry⁸. Smart windows with features like gas detection, solar cells, flat panel displays, and electrochromic mirrors are made possible by its special capacity to reflect infrared light⁹. Because of their low emissivity, optically transparent conductors are currently most frequently used in smart window applications^10,11. ZnO’s excitonic UV emission has sparked increased interest in opto-electronics due to its verified application in solid-state lighting systems, such as light-emitting diodes and laser diodes^10,11. ZnO is basically a semiconductor at room temperature, having a wide, straight band-gap of “~3.7 eV.”

Due to its wurtzite crystal structure and optical properties that are similar to those of GaN, it is presently being investigated and confirmed as a potential candidate for blue-UV light emitting diodes. In contrast, ZnO can tolerate high temperature treatments related to doping and the formation of ohmic connections since it is sufficiently stable with a melting point of 2248 K. In a similar vein, ZnO is a strong candidate to be the material of choice for short-wavelength optoelectronic and photonic devices due to its intrinsic properties. These features are most notable for their high excitonic binding energy of 60 meV, which allows devices to function effectively at room temperature and above¹².

One important multifunctional oxide, particularly in the photocatalysts family, is standard titania (TiO₂)¹. In fact, these later materials are a class of photoactive substances that, when exposed to light, produce excitonic pairs, which in turn produce extremely reactive superoxides and/or hydroxyl radicals. Among other phenomena, these quick chemical reactions are effective for air purification, H₂O splitting, and the breakdown of pollutants in waste H₂O^13,14.

Under typical pressure and temperature settings, TiO₂ exhibits three primary crystallographic phases: rutile, brookite, and anatase. These phases correspond to different bandgap values of 3.4 eV for brookite, 3.0 eV for rutile, and 3.2 eV for anatase. The bandgap further validates the oxygen excess characteristic of the samples¹⁵. According to the extensive literature, TiO₂ nano-scaled is without a doubt the preferred photocatalyst^13–16, particularly when it comes to multi-sectorial functioning as environment, and energy sectors. However, from a catalytic point of view, its usefulness is limited by the poor quantum yield caused by the rapid excitonic recombination of charge carriers (e_/h+) and the need for UV irradiation for its photo-activation. It has been demonstrated that nitrogen doping can get over this restriction^17,18.

Both nanoscale ZnO and TiO₂ are valuable as opto-electronics compounds, despite their differences from a crystallographic standpoint. At ambient temperature and atmospheric pressure, ZnO crystallises in the hexagonal wurtzite phase. As previously established, TiO₂ exhibits three primary phases (anatase, rutile, and brookite).

Combining the inherent properties of ZnO and TiO₂ could result in an enhanced zinc titanate. ZnTiO₃ (Hexagonal), Zn₂TiO₄ (Cubic), and Zn₂Ti₃O₈ (Cubic) can be distinguished within this family. These binary type oxides are efficient electroceramics with a variety of technological uses, such as light-emitting materials, photocatalysts, sensors, dielectric resonator materials, filters, white pigment, etc.^19–24.

Zinc titanate have been synthesized through a variety of methods, including sol-gel, hydrothermal processes, magnetron reactive sputtering, pulsed laser ablation, and solid-state reactions^24–31. For the synthesis of single-phase Zn₂TiO₄, the predominant crystallographic structure and key challenges have been identified as amorphous with insufficient high crystallinity²⁰, multi-phase and biphase systems²⁴, significantly high crystallinity without complete epitaxial growth²⁶, the disparity in melting points between ZnO (975 °C) and TiO₂ (1830 °C) phases^{27–29,32,33}, and a high degree of crystallinity coupled with enhanced chemical homogeneity²⁹. In this research, green biosynthesis was utilized to produce Zn₂TiO₄ with a high degree of crystallinity, improved chemical homogeneity, and nanoscale dimensions.

The confirmation of the synthesis of single phase Zn₂TiO₄ nanoparticles utilising a totally green approach is presented for the first time in this contribution. More specifically, Hibiscus-sabdarifa natural extract was employed as an efficient chelating agent in place of harsh reducing agents. The process of creating nanoparticles through green synthesis entails the reduction of metal ions present in the precursor solution by means of biologically active compounds derived from Hibiscus extracts. The mechanisms involved vary according to the type of nanoparticles produced, the reaction conditions, and the composition of the plant extract³⁴. The biomolecules found in the Hibiscus extract, which include proteins, polysaccharides, and secondary metabolites such as phenols, flavonoids, and terpenoids, interact with metal ions, aiding in their stabilization and reduction. These biomolecules serve as stabilizing, reducing, capping, and reducing agents, thereby affecting the size, shape, and properties of the nanoparticles that are synthesized³⁵.

The benefits of this green procedure include:

• (i)The chelation/reduction process is held at room temperature and atmospheric pressure,
• (ii)Besides the Zn and Ti precursors, no hazard synthetic reducing agent is used,
• (iii)Only H₂O is used as a universal solvent,
• (iv)Nano-scaled particles are produced,
• (v)Initially Amorphous nanoscaled Zn₂TiO₄ are obtained; crystallizing to cubic phase upon annealing at about 500 °C in air,
• (vi)There is no need for additional compounds to control pH,
• (vii)No catalyst or vacuum is required.

Experimental section and methods

Materials

Zn(NO₃)₂·6H₂O and Titanium (IV) bis (ammonium lactato) dihydroxide ([CH₃CH(O-)CO₂NH₄]₂Ti(OH)₂) (both > 98% in purity/quality were obtained from Sigma Aldrich and used without further purification. The biomass Hibiscus sabdariffa was sourced from the local market in Dakar-Senegal. Deionized water with a conductivity lower than 10^− 6 S cm^− 1 was employed in the experimental procedures.

Synthesis

• (i)Preparation of the extract.

  Initially, to create the Hibiscus sabdariffa extract, 6 g of the leaves were added to 100 mL of deionized water at room temperature for a duration of one day. The following day, the extract was filtered to achieve the clearest possible solution. The extract was then stored in the refrigerator for future use in the production of Zn₂TiO₄.

• (ii)Production of the Zn₂TiO₄.

  For preparation of the Zn₂TiO₄ nanoparticles, 0.02 molar of [CH₃CH(O)CO₂NH₄]₂Ti (OH)₂ solution 50 wt%, and 5.95 g of Zn(NO₃)₂·6H₂O were dissolved in 10 mL deionized H₂O in separate 50 mL beakers. Both solutions were then added to 50 mL of the extract one by one while stirring at room temperature. This mixture was allowed to react under stirring for 24 h. The obtained precipitated was centrifuged out and washed 3 times with deionized H₂O. Subsequently, the precipitate was dried in the oven at 100 °C. The final powder was annealed at 500 °C for 2 h in view of inducing its crystallization. Thereafter, it was stored for further characterizations.

Characterization devices

The morphology of the material was explored via SEM analysis. The composites elements in the mixture were identified by using EDX and RBS techniques. Rutherford backscattering spectrometry (RBS) analysis was utilized to identify the elemental depth distribution. The crystalline form of the nanocompounds Zn₂TiO₄ was investigated by utilizing XRD analysis. FTIR spectroscopy was using to determinate the functional groups in the compounds and The Raman vibration analysis was utilized to identify the vibration modes. The absorbance and the diffuse reflectance were estimated by using UV-Vis spectroscopy. The luminescence was performed via PL spectroscopy measurement.

Photocatalytic investigations

Before performing the photodegradation analysis, 0.1 g of the nanocomposite was combined with 100 mL of methylene blue solution and agitated in the dark for half an hour. The solution was then exposed to a light source, and for a total of 120 min, we collected 2 mL every 15 min to analyse absorption using UV–visible spectroscopy.

Electrochemical investigations

Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to examine the electrochemical characteristics of the Zn₂TiO₄ nanocomposite material. All electrochemical measurements were carried out using Metrohm Autolab PGSTA302N potentiostat controlled by NOVA version 2.1 electrochemical software (Metohm Autolab NOVA Software). A standard three-electrode setup was utilized, comprising bare and modified glassy carbon electrode (GCE) as the working electrode, a platinum wire as the counter electrode, and a silver/silver chloride (Ag/AgCl in saturated KCl as the reference electrode.

Electrode modification: Prior to modification, the GCE surface was gently polished to a mirror-like finish using alumina slurries (1.0, 0.5, and 0.3 μm) on a polishing pad. To ensure the removal of residual alumina, the electrode was sequentially sonicated in ethanol and deionized water for 5 min each. To prepare the modified electrode, 0.04 g of Zn₂TiO₄ nanocomposite was mixed with 0.006 g of carbon black, and 0.006 g of polyvinylidene fluoride (PVDF). This mixture was homogenized using a mortar and pestle with the addition of N-methly-2-pyrrolidone (NMP) as a solvent. The resulting mixture was sonicated for 2 h to obtain a uniform slurry. Finally, a few microlitres of the resulting nanocomposite paste were drop-cast onto the surface and dried in an oven at 35 °C for 2 min to yielding the modified electrode.

Antibacterial investigations

The antibacterial activity of Zn₂TiO₄ nanoparticles was assessed using the Kirby–Bauer disk diffusion susceptibility test. This evaluation targeted Gram-positive bacteria, including Methicillin-resistant Staphylococcus aureus (MRSA) and Staphylococcus aureus (S. aureus), as well as Gram-negative bacteria, namely Escherichia coli (E. coli), Acinetobacter baumannii (A. baumannii), Klebsiella pneumoniae (K. pneumoniae), and Pseudomonas aeruginosa (P. aeruginosa), in accordance with a previously established protocol (BAUER, 1996). The bacterial strains, obtained from the American Type Culture Collection (ATCC) in Manassas, USA, were cultured on Mueller-Hinton agar (MHA) from Merck, Germany, as the growth medium. A fresh 24-hour stock culture was used to create a bacterial suspension in sterile saline, which was then adjusted to a 0.5 McFarland turbidity standard. After that, 50 µl of the suspension was applied to the MH agar plates, which were there after allowed to air dry at room temperature while being kept sterile. Fifty microlitres of the test solutions and controls were placed on sterile 6 mm discs. Zn₂TiO₄ nanoparticles were present in discs with the labels C1 through C4 at a concentration of 1000 µg/mL. As positive controls, discs C5 and C6 contained 100 µg/ml and 10 µg/mL of ciprofloxacin, respectively. The negative control was sterile saline. After placing the discs on agar plates, they were incubated for twenty-four hours at 37 °C.

Results and discussion

Morphology investigations

The Scanning Electron Microscopy of the targeted Zinc Titanate samples is shown in Fig. 1. More specifically, it shows the morphology of two samples that were annealed at 500 °C and 100 °C, respectively. The samples show a porous structure in both situations. On the sample that was annealed at 500 °C, this later appears to be more noticeable. Furthermore, the related porosity is comparatively poly-disperse between 381 nm and 5 nm. As re ported by Katarzyna, the SEM microphotographs of the analyzed samples validate the existence of particles with accurately specified diameters, which align with those presented in the particle size distributions³⁶. As shown and explained in the sections that follow, such a porosity may be advantageous for photocatalytic and/or antibacterial activities as well as light scattering.

Fig. 1.

Scanning Electron Microscopy images of the targeted Zinc Titanate samples annealed at a 100 °C and b 500 °C.

Elemental analysis and investigations

The elemental analysis profiles of the bio-engineered zinc titanate that was annealed at 500 °C are shown in Fig. 2. More specifically, the related Energy Dispersive X-rays Spectroscopy (EDS) spectrum is reported in Fig. 2a. The different energy pathways are associated with oxygen, titanium, and zinc, as can be shown in Fig. 2a. The Rutherford Backscattering (RBS) spectrum is reported by Fig. 2b. Although not flawless, the modelling profile and associated experimental data confirm the elemental composition of O, Ti, and Zn once more. As a preliminary conclusion, it is safe to assume that the bio-engineered substance is Zn_xTi_yO_z combination.

Fig. 2.

Elemental analysis profiles of the bio-engineered nano-scaled Zinc Titanate annealed at 500 °C: a energy dispersive X-rays spectroscopy EDS spectrum and b Rutherford backscattering (RBS) spectrum.

Structural and crystallographic investigations

The Zn_xTi_yO_z sample’s X-ray diffraction pattern after being annealed at 500 °C is shown in Fig. 3. Its high degree of crystallographic symmetry is shown by the related diffraction pattern, which shows a limit number of diffraction Bragg peaks; these are probably cubic according to the JCPDS card Pattern: 00–025-1164. The different diffraction peaks of the single phase crystalline Zn₂TiO₄ were determined to be the (220), (311), (400), (511), and (400). Notably, Zn₂TiO₄ is found in two main phases: a high-temperature cubic inverse-spinel structure (space group Fd3m) and a low-temperature tetragonal phase (space group P4₁22). As shown schematically in the inset of Fig. 5, the cubic inverse-spinel phase is a more ordered structure where Zn²⁺ and Ti²⁺ are distributed between tetrahedral and octahedral sites within the spinel framework, whereas the tetragonal phase has a more complex atomic arrangement with both TiO₄ octahedra and ZnO₄ tetrahedra. The average size of Zn₂TiO₄ crystallites is determined to be ~Ø~58.8 nm using the Scherrer approximation. The average strain(ε), average dislocation density(δ), and average number of crystallites(N) are among the key metrics that are summarised in Table 1.

Fig. 3.

X-Rays Diffraction pattern of the bio-engineered nano-scaled Zn₂TiO₄ annealed at 500 °C.

Fig. 5.

a UV–VIS–NIR diffuse reflectivity and b photoluminescence spectrum of the bio-engineered nanoscaled Zn₂TiO₄ annealed at 500 °C.

Table 1.

Derived parameters from the XRD pattern of the Zn₂TiO₄ sample annealed at 500 °C.

Miller index (hkl)	2 θExp rad	FWHM 10−2 rad	D nm	ε 10−4	δ (1015) line/m2	N (1022) m−3
(220)	0.521	0.033	48.314	58,10	0.428	0.887
(311)	0.614	0.025	64.645	71.80	0.240	0.370
(400)	0.746	0.027	61.271	54.50	0.270	0.435
(511)	0.986	0.042	41.640	104.10	0.580	1.385
(440)	1.082	0.022	78.140	201.24	0.164	0.210

Vibrational spectroscopy investigations: ATR-FTIR and Raman studies

Attenuated total reflection (ATR) Fourier Transform Infrared spectroscopy of the Zn_xTi_yO_z sample annealed at 500 °C is shown in Fig. 4a. Different functional groups can be distinguished. The O–H functional group is responsible for the vibrational mode near 3390 cm⁻¹. The C–H functional group is associated with the strong mode at 2885 cm⁻¹, whereas the C–H bending vibrations in di-alkyl or tri-alkyl alkenes are probably responsible for the mode near 1410 cm⁻¹. In addition to the modes mentioned above, the stretching vibrations of the Ti–O groups are responsible for the mode at 585 cm⁻¹²⁸, and Zn–O vibrations are responsible for the mode at 1435 cm^−132–37. As a preliminary conclusion, the measured vibrational modes, particularly those of 585 cm⁻¹ and 1635 cm⁻¹, support Zn₂TiO₄’s single phase nature, as reported by Sharma et al.³¹. Zn₂TiO₄ is an inverse spinel with a space group Fd-3 m and typically described by a 56-ato m unit cell. In its unit cell, 32 oxygen atoms comprise an fcc lattice, with associated interstitial tetrahedral (T) and octahedral (O) sites. The T sites are occupied by half of the divalent Zn cations and the O sites are filled by a stoichiometric mixing of the divalent Zn and tetravalent Ti cations randomly.

Fig. 4.

a Attenuated total reflection Fourier transform infrared and b Raman spectra of the bio-engineered nano-scaled Zn₂TiO₄ annealed at 500 °C.

Figure 4.b displays the Raman spectrum of the Zn_xTi_yO_z sample annealed at 500 °C. The spectrum exhibits several Raman active modes within the spectral range up to 3000 cm⁻¹. It consists a priori of 2 separate regions; the Zn₂TiO₄ modes and that governed by TiO₂or/and ZnO mides. More precisely, the modes are centered at the following spectral positions 139,272(F2g), 371(E-2 H),744 cm⁻¹ (A1g) for the Zn₂ TiO₄ and 1994, 2482 and 2806 cm⁻¹ for the ZnO/TiO₂ labelled region. In the case of the labelled Zn₂TiO₄ region, the corresponding modes are likely to be attributed to the A1g, F2g and E-2Hg modes of the spinel structure of the Zn₂TiO₄ perovskite as observed by Li et al.²⁹ and Matsumuto et al.³⁸ in bulk crystalline Zn₂TiO₄ perovskite. However, there is a significant spectral differences. This later difference is likely to be attributed to size/shape effect of the Zn₂TiO₄ nano-crystals^39,40.

Optical investigations: absorbance and photoluminescence studies

Figure 5a reports the Diffuse reflectivity of the Zn₂TiO₄ sample annealed at 500 °C. The UV-Vis diffuse reflectance spectrum shows a sharp absorption edge at λ_cut-off ≈ 271 nm, which lies in the UV region. The spectrum shows reflectance of approximately 80% across the visible range (400–1100 nm), indicating that the material is highly reflective. The decrease in the wavelength range of 374–413 nm for the sample is due to the intrinsic absorption of Zn₂TiO₄ as a direct semiconductor³¹. This shows that this material is a wide bandgap oxide semiconductor. The absorption edge wavelength was used to calculate energy band gap to be 4.58 eV using Planck equation. This shows a blue shift when compared to the TiO₂ bulk energy band gap (3.0–3.2 eV)⁴¹. However, this agrees with the values published within the literature⁴² in line with band structure of the Zn₂TiO₄ perovskite structure, which confirms the results obtained from the XRD showing the formation of Zn₂TiO₄.

Figure 5b reports the room temperature photoluminescence of the Zn_xTi_yO_z sample annealed at 500 °C. The spectrum consists of 2 emissions centered at 428.2 and 648.5 nm. The near band-edge emission is responsible for the comparatively narrow peak in the UV range that is centred at 384 nm^41–44.

The dominant emission at 428.2 nm reflects radiative recombination through shallow trap states, likely associated with slightly oxygen-deficient TiO₂ units or Zn–O surface defects⁴³. The sharpness of this peak suggests a relatively ordered defect structure, which is favourable for reproducible photocatalytic behaviour⁴⁴. This also explains the 94.1% degradation of methylene blue by this material with 4.58 eV using white light is due to some surface defects^45,46.

Photocatalytic response investigations

The absorbance variation and the time photo-degradation efficiency of a typical water solution polluted with Methylene Blue under xenon lamp UV light at different times, ranging from 0 to 120 min, are shown in Fig. 6. Considerable focus has been placed on the use of bimetal oxides for the remediation of polluted water, with significant treatment efficiencies reported through the photocatalysis process⁴⁷. Typically, the photocatalytic process involving semiconductors includes several critical reactions, such as the generation of charge carriers, their relaxation, separation, transfer, capture, transport, and recombination. The catalytic characteristics of these bimetal oxides are affected by various factors, including structure, size, shape, composition, and surface properties, all of which are influenced by the different methodologies employed⁴⁸. The techniques commonly used for synthesizing bimetal oxides encompass both chemical and physical methods⁴⁹. The efficacy of the Zn₂TiO₄ photocatalyst in degrading MB dye was assessed in relation to existing studies on photocatalysts. Figure 6.a depicts the absorption spectrum of the MB dye solution combined with the Zn₂TiO₄ nanocomposite. As illustrated in the graph, it shows the variation in absorbance of the standard methylene blue contaminated water solution and Zn₂TiO₄ under xenon lamp UV radiation over various time intervals, specifically within the range of while the spectrum of the treated solution was recorded at intervals of 15 min from 0 to 120 min. More specifically, it presents the absorbance profiles recorded at 0.0, 30.0, and 90.0 min. One can distinctly observe the steady decline in the intensity of the intrinsic methylene blue absorbance peak within the wavelength range of 500–700 nm. The extent of degradation of the composite was estimated using the equation provided below D = ⁵⁰. In this context, A₀ represents the initial absorbance of the dye, while A denotes the absorbance of the dye at time t. In this work, the subsequently seems to suggest an efficacy of 94.1% at around 90 min, indicating that the synthesized bimetallic oxide exhibits remarkable catalytic performance and can greatly enhance catalytic efficiency. As reported by Praveena Panner et al.⁵¹, the redox potential of the photoexcited electron–hole pairs rises due to a reduction in band gap values, which significantly enhances the activity of the visible light photocatalyst. In a research study carried out by Khatua et al., the UV photocatalytic dye decomposition capabilities of Zn₂TiO₄ nanoparticles were compared with those of the individual metal oxides ZnO and TiO₂ nanoparticles, demonstrating that Zn₂TiO₄, ZnO, and TiO₂ nanoparticles can decompose 90%, 63%, and 32% of MB dye, respectively⁵². According to Fulvia Pinzari⁵³, the photocatalytic activity is enhanced when the material is in a nanocrystalline state. It has been observed that these characteristics are also found in the nanocomposite material, with the degree of enhancement depending on several factors, including crystallinity, structural composition, crystallite size, and morphology. These structural attributes can be modified by manipulating the synthesis of the material, leading to a diverse range of composites. When an interface is established between the nanocrystallites of the two oxides, the composite system exhibits photocatalytic activity that surpasses that of the individual oxides^54,55. Furthermore, Fig. 6.b illustrates the experimental results of the logarithm of the normalized concentrations as a function of irradiation time for the MB solution in the presence of the Zn₂TiO₄ nanocomposite. Furthermore, the Langmuir-Hinshelwood model serves as a framework for comprehending the kinetics associated with the photocatalytic degradation efficiencies of MB, as illustrated by the formula below⁵⁶.

Fig. 6.

UV photocatalytic activity of the bioengineered nanoscaled Zn₂TiO₄; a absorbance profiles taken at 0.0, 30.0 and 90.0 min, b its corresponding time photo-degradation efficiency.

where signifies the initial reaction rate, A denotes the dye concentration, and t indicates the reaction time. Additionally, k and represent the Langmuir-Hinshelwood reaction rate constant and the Langmuir adsorption equilibrium constant, respectively. In scenarios involving minimal pollutant concentrations (KA < < 1), pseudo-first-order kinetics is applicable, as expressed by the following equations:

where the apparent constant serves as the kinetics parameter, and A represent the initial and residual concentrations of MB dyes in the aqueous phase prior to and following light irradiation, respectively. To determine the degradation rate of MB dyes in the presence of Zn₂TiO₄, pseudo-first-order kinetics can be utilized, as represented by the equation = Kt^57,58, where indicates the initial concentration before irradiation, A signifies the concentration post-irradiation, t represents the duration of irradiation, and K is a constant that can be identified as the slope after graphing the photodegradation efficiencies, as demonstrated in figure-6(b). According to Kamo et al.^59,60, the photocatalytic mechanism of the nanocomposite can be elucidated through the following reactions: in summary, when photons possessing sufficient energy are absorbed by the nanocomposite catalyst, electron–hole pairs are generated. These pairs arise through a series of reductive or oxidative redox pathways. Initially, the photogenerated holes in the valence band react with water to produce hydroxyl radicals.

Electrochemical response investigations

The electrochemical properties of the Zn₂TiO₄ nanocomposite were investigated using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). CV is a widely utilized electroanalytical technique known to provide intuitive insights into electrochemical reactions, by rapidly determining the redox potentials of electroactive species. It offers valuable information on the thermodynamics of redox reactions, kinetics of heterogeneous electron transfer, as well as the examination of linked electrochemical reactions or adsorption phenomena⁴⁷.

CV measurements were performed in 1 M NaOH electrolyte over a potential range − 2.0 to 0.5 V (vs. Ag/AgCl) at a scan rate of 50 mV/s. As shown in Fig. 7a, the bare GCE exhibits a relatively low current response, whereas the Zn₂TiO₄-modified GCE displays enhanced current response with distinct cathodic peaks at −1.6 V, −0.46 V, and − 0.12 V (vs. Ag/AgCl), alongside corresponding anodic features. This increase in current response suggests improved electrochemical activity and enhanced electron transfer at the modified electrode surface. The stability of the modified electrode was assessed over 20 consecutive CV cycles (Fig. 7b). The anodic peak current exhibited excellent retention (94.2%), with only ∼5.8% current loss between the 1 st and 20th scans, demonstrating outstanding stability to electrode fouling.

Fig. 7.

a CVs of bare and Zn₂TiO₄ modified GCE in 1 M NaOH over a potential range of − 2.0 to 0.5 V (vs. Ag/AgCl) at a scan rate of 50 mV/s, b stability study of Zn₂TiO₄ modified GCE over 20 repeated CV scans under the same conditions, c Scan rate study (50–1000 mV/s) of Zn₂TiO₄ modified GCE in 1 M NaOH, d Plots of anodic (I_p.a.) and cathodic (I_pc) peak currents versus the square root of the scan rate (v^1/2), e Nyquist plots of bare and Zn₂TiO₄ modified GCE in 5mM [Fe(CN)₆]^3−/4− solution prepared in 0.1 M KCl over a frequency range of 0.1 Hz to 10 kHz, and f the corresponding Randles equivalent circuit used for fitting the EIS data.

The effect of scan rate (50–1000 mV/s) was investigated by CV in 1 M NaOH at the Zn₂TiO₄ modified GCE. Figure 7c depicts the results obtained, and it was observed that both the anodic and cathodic peak currents increased with increasing scan rate. Furthermore, a linear correlation was observed between peak currents (I_p.a. and I_pc) and the square root of the scan rate (v^1/2) (Fig. 7d), suggesting that the redox process is primarily diffusion-controlled, in accordance with the Randles-Sevcik model⁶¹.

The interfacial electron transfer kinetics were further evaluated by EIS in a 5 mM [Fe(CN)₆]^3−/4− solution prepared in 0.1 M KCl over a frequency range of 0.1 Hz to 10 kHz. The equivalent circuit for the Nyquist plots of the asymmetric supercapacitor device typically exhibits a semicircular shape in the mid-frequency range, a spike in the low-frequency range, and a characteristic equivalent series resistance in the high-frequency range⁶². The resulting Nyquist plots (Fig. 7e) display a semicircular region at high frequencies corresponding to charge transfer resistance (R_ct), followed by a linear tail at low frequencies associated with diffusion-controlled processes. The experimental data were fitted using a modified Randles equivalent circuit (Fig. 7f) comprising solution resistance (R_s), R_ct, a constant phase element (Q), and a Warburg impedance element (W). As summarised in Table 2, the bare GCE exhibited an R_ct value of 5.48 kΩ, which decreased significantly to 0.53 kΩ upon modification with Zn₂TiO₄. This nearly 10-fold reduction in R_ct indicates that the bioengineered Zn₂TiO₄ nanomaterial facilitates faster electron transfer kinetics. The constant phase element (CPE) exponent (n ≈ 0.73) suggests a porous and heterogeneous surface morphology, typical of bioengineered nanomaterials. Furthermore, the Zn₂TiO₄ modified GCE displayed a higher admittance (Y_o = 3.94 µΩ⁻¹sⁿ) compared to bare GCE (2.02 µΩ⁻¹sⁿ), suggesting an increase in electroactive surface area. The decrease in the Warburg parameter (from 351.1 to 250.42 µMho) reflects changes in diffusion-related impedance at the electrode interface. These findings demonstrate the superior conductivity and charge-transfer properties of Zn₂TiO₄ nanomaterial, highlighting their potential in advanced electrochemical applications.

Table 2.

Summary of the EIS fitting parameters for bare and Zn₂TiO₄ modified GCE in 5 mM [Fe(CN)₆]^3−/4− solution prepared in 0.1 M KCl. Values in parentheses correspond to the percentage errors associated with the fitting procedure.

Electrode	Rs (kΩ)	Rct (kΩ)	Q (CPE)		W (µMho)	Chi-square
			Yo (µΩ−1sn)	n
GCE	1.19 (2.1)	5.48 (2.2)	2.02 (1.4)	0.74 (2.2)	351.1 (6.7)	0.094
GCE/Zn2TiO4	1.43 (1.1)	0.53 (5.2)	3.94 (3.4)	0.73 (5.4)	250.42 (0.7)	0.008

Antibacterial response investigations

The inhibition zone measurements are summarized in Fig. 8 and Table 3. After 24 h of incubation, the Zn₂TiO₄ nanocomposite did not produce measurable inhibition zones against the Gram-positive strains tested, namely S. aureus and MRSA, nor against most of the Gram-negative strains, including A. baumannii, E. coli, and P. aeruginosa. A very small zone was observed only for K. pneumoniae, with mean inhibition values of 6.85 ± 0.07, 6.95 ± 0.21, 7.08 ± 0.28, and 7.10 ± 0.22 mm for samples C1–C4, respectively. Since the sterile disc diameter was approximately 6 mm, these values indicate only marginal inhibition beyond the immediate disc area and should therefore be interpreted cautiously. By contrast, ciprofloxacin produced clear and concentration-dependent inhibition zones across all bacterial strains, confirming the validity of the assay conditions and the responsiveness of the bacterial panel.

Fig. 8.

Disc diffusion results showing (1–4) nanoscaled Zn₂TiO₄ (1000 µg/mL), (5) standard/positive control ciprofloxacin 100 µg/mL, (6) standard/positive control ciprofloxacin 10 µg/mL, and (X) negative control (sterile saline) for a A. baumannii, b E. coli, c K. pneumoniae, d P. aeruginosa, e MRSA, and f S. aureus.

Table 3.

The zone of inhibition values for the Zn₂TiO₄ nanoparticles and ciprofloxacin (n = 3).

Sample	Zone of inhibition (mm)
	Gram-positive strains		Gram-negative strains
	S. aureus MRSA		A. baumannii	E. coli	K. pneumoniae	P. aeruginosa
C1	–	–	–	–	6.85 ± 0.07	–
C2	–	–	–	–	6.95 ± 0.21	–
C3	–	–	–	–	7.08 ± 0.28	–
C4	–	–	–	–	7.1 ± 0.22	–
C5 (ciprofloxacin 100 µg/mL)	25.8 ± 0.42	26.75 ± 0.78	30.35 ± 0.21	30.25 ± 0.64	30.4 ± 0.14	30 ± 1.70
C6 (Ciprofloxacin 10 µg/mL)	18.5 ± 0.71	19.3 ± 0.28	14.55 ± 0.92	26.4 ± 0.99	24.8 ± 1.84	20.3 ± 2.55

The antibacterial action of metal-oxide nanomaterials is commonly associated with three interrelated processes: reactive oxygen species generation, disruption of the bacterial membrane envelope, and subsequent loss of cellular viability. In semiconductor oxides such as ZnO-, TiO₂-, and titanate-based systems, photon absorption or surface redox reactions can promote electron–hole separation. The photogenerated holes may react with adsorbed water or hydroxide ions to form hydroxyl radicals, while electrons can reduce dissolved oxygen to superoxide radicals. These reactive oxygen species, including ·OH, O₂^·⁻, and H₂O₂, can oxidize membrane lipids, proteins, and nucleic acids. At the bacterial surface, this oxidative stress weakens the structural integrity of the cell envelope, increases membrane permeability, disturbs ion homeostasis, and may lead to leakage of intracellular contents. Once membrane damage exceeds the bacterium’s repair capacity, respiratory activity, nutrient transport, and metabolic regulation are compromised, resulting in growth inhibition or cell death.

For Zn₂TiO₄ specifically, such ROS-mediated pathways are mechanistically plausible because the material showed strong photocatalytic activity toward methylene blue degradation, indicating that the nanocomposite can participate in light-driven oxidative reactions. However, the antibacterial experiment was performed using a disc-diffusion format under standard incubation conditions. ROS-driven antibacterial effects usually require close contact between the nanoparticle surface and bacterial cells, sufficient oxygen and water at the interface, and, in many titanate or Ti-containing oxides, suitable light activation. In the present agar-diffusion assay, Zn₂TiO₄ particles are unlikely to diffuse efficiently through agar in the same way as a small-molecule antibiotic. Therefore, even if ROS are generated locally at the nanoparticle surface, the oxidative species may remain confined near the application site, decay rapidly, or fail to reach bacteria located at the expanding growth front⁵⁹. This may explain why the nanocomposite produced little to no measurable inhibition zone despite its demonstrated photocatalytic functionality.

The relationship between ROS production, membrane damage, and bacterial inactivation may therefore be understood as a sequential process: Zn₂TiO₄ surface activation may generate ROS; ROS can oxidatively attack bacterial envelopes; membrane oxidation can increase permeability and cause intracellular leakage; and these events can culminate in bacterial inactivation. Nevertheless, under the present experimental conditions, this sequence was not sufficiently expressed as a clear diffusion-based inhibition zone. The negligible activity against S. aureus, MRSA, A. baumannii, E. coli, and P. aeruginosa suggests that ROS generation, ion release, or direct particle–cell contact was insufficient in the agar environment to produce observable bactericidal effects. The marginal response against K. pneumoniae may reflect limited local contact-dependent stress at the disc boundary rather than strong diffusible antibacterial activity.

Several factors may have limited the expression of antibacterial activity. First, particle agglomeration can reduce the available reactive surface area and decrease the effective contact frequency between Zn₂TiO₄ and bacterial cells. Second, limited release of Zn²⁺ species from the titanate matrix may reduce metal-ion-mediated stress^63,64. Third, incubation in the absence of controlled illumination may suppress photocatalytic ROS pathways associated with Ti-containing oxides⁶⁵. Fourth, Gram-negative and Gram-positive bacteria differ in cell-envelope architecture, which influences nanoparticle attachment, ROS susceptibility, and permeability responses. The outer membrane of Gram-negative bacteria may restrict nanoparticle access, while the thick peptidoglycan layer of Gram-positive bacteria can also act as a physical barrier to oxidative or contact-mediated damage.

Accordingly, the present findings indicate that Zn₂TiO₄, as formulated and tested here, does not behave as an effective diffusible antibacterial agent in an agar-disc diffusion assay. However, the absence of large inhibition zones should not be taken as definitive evidence that the material lacks antibacterial potential under all conditions. Rather, it suggests that the antibacterial mechanism, if present, is more likely to be contact-, surface-, dose-, dispersion-, or illumination-dependent. Future work should therefore quantify ROS generation directly under antibacterial assay conditions, for example using ROS-sensitive probes, and should correlate ROS levels with membrane damage assays such as propidium iodide uptake, protein/nucleic acid leakage, lipid peroxidation, or electron microscopy of bacterial envelope disruption. In addition, broth microdilution, time-kill kinetics, illuminated versus dark incubation, nanoparticle-coated surface assays, and Zn²⁺ release measurements would help determine whether Zn₂TiO₄ can induce ROS-mediated membrane damage and bacterial inactivation under conditions better aligned with nanoparticle mechanisms. Importantly, the negligible activity observed in the agar-diffusion assay also explains why additional antibacterial assays, such as broth microdilution, MIC/MBC determination, and time-kill studies, were not pursued within the present work. The initial disc-diffusion test was used as a first-line screening method to establish whether Zn₂TiO₄ produced a measurable antibacterial response under standard conditions. Since the material showed no meaningful inhibition zones for most organisms, and only a near-baseline response against K. pneumoniae, subsequent antibacterial assays were considered beyond the immediate scope of this study and are now identified as essential follow-up investigations rather than confirmatory tests. At the same time, we acknowledge that the absence of activity cannot be fully interpreted without quantitative physicochemical information under biological test conditions. In particular, nanoparticle dispersion state, hydrodynamic size, ζ-potential, agglomeration behaviour in Mueller–Hinton medium, and Zn²⁺ ion release could strongly influence bacterial contact, ROS generation, and membrane interaction⁶⁶. These parameters were not measured during the antibacterial assay and therefore could not be quantitatively correlated with the observed lack of inhibition. Similarly, because the antibacterial experiments were performed under standard incubation conditions rather than controlled illumination, the possible photocatalytic bactericidal effect of Zn₂TiO₄ was not directly assessed. This is a relevant limitation, especially given the strong photocatalytic activity observed for methylene blue degradation in this study. In sum, future antibacterial work should therefore be specifically designed to compare dark and illuminated conditions, quantify ROS and Zn²⁺ release in the assay medium, evaluate colloidal stability and ζ-potential, and determine MIC/MBC and time-kill profiles under conditions that better reflect the likely surface-contact and light-activated mechanisms of Zn₂TiO₄.

Overall, the antibacterial results should be interpreted conservatively (Table 3). The Zn₂TiO₄ nanocomposite showed no meaningful inhibition against most tested organisms in the disc-diffusion format, with only a minimal response against K. pneumoniae. Mechanistically, ROS-mediated oxidative membrane damage remains a plausible pathway for Zn₂TiO₄-based antibacterial action, particularly under light-activated or contact-rich conditions, but the present assay did not provide direct evidence of sufficient ROS generation, membrane disruption, or bacterial inactivation to support a strong antibacterial claim.

As a follow up of this study, the bio-synthesis of other potential Titanates would be explored. Thin includes but not limited to nano-scaled Ca, Sn, Cu, Cr, Ni Titanate among others^67–73.In addition, we would compare the physical-chemical properties relatively to their simple oxides in view of potential technological applications^74–79.

Conclusion

The bio-inspired synthesis of single phase Zinc Orthotitanate (Zn₂TiO₄) is validated in this work. More precisely, Hibiscus sabdariffa’s natural extract and phytocompounds were shown to be an efficient chelating agent at room temperature and atmospheric pressure.

• (i)It was discovered that the bio-engineered Zn₂TiO₄ single phase crystals were nanoscale and exhibited an effective photocatalytic efficacy in breaking down Methylene Blue, reaching a threshold of roughly 94.1% efficacy upon exposure for 120 min with 4.58 eV as band gap.
• (ii)Likewise, the bio-engineered Zn₂TiO₄ nanomaterial demonstrated superior electrochemical perfomance, characterized by a 10-fold reduction in charge transfer resistance (R_ct) from 5.48 kΩ to 0.53 KΩ and a nearly 2-fold increase in interfacial admittance (Y_o). These results, coupled with a 94.2% current retention over 20 cycles, confirm that the material provides a stable, highly conductive interface, with significantly increased electroactive surface are suitable for advance electrochemical applications.
• (iii)In the antibacterial assays, the bio-inspired Zn₂TiO₄ nanomaterial did not produce meaningful zones of inhibition against most tested bacterial strains, with only a minimal response observed for K. pneumoniae. The results therefore suggest that Zn₂TiO₄ does not function as an effective diffusible antibacterial agent under the present agar-disc diffusion conditions, although ROS-mediated and contact-dependent antibacterial mechanisms may warrant further investigation under optimized illumination, dispersion, and surface-contact conditions.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

A.F. was responsible for synthesizing the nanocomposites and drafting the initial version of the manuscript. A. A. F performed the XRD analysis. N.L. (B) undertook the evaluations of UV-visible, Diffuse Reflectance, and PL. KJ. C executed the SEM and EDS analyses. A. F. played a role in interpreting the FTIR and Raman results. KJ. C. and R. A. carried out the antibacterial tests. A. F. finalized the assessments of photocatalytic activity. K. K. M. conducted the electrochemical analysis. M. M. was responsible for the final revisions and data analysis of the manuscript.

Funding

No funding for this research.

Data availability

The information that underpins the findings presented can be obtained upon reasonable request from the corresponding author.

Declarations

Competing interests

The authors declare no competing interests.

Ethics approval and consent to participate

In this research, we utilized a natural extract of Hibiscus obtained from waste materials. All procedures were conducted in compliance with applicable guidelines and regulations. No consent, approval, or licenses were necessary.