Fe-Doped ZnS Quantum Dot Photocatalysts for the Degradation of Cefalexin in Water

Abstract

This study reports the synthesis, structural characterization, adsorption studies, nanoscale interaction, and photocatalytic application of pure and Fe-doped ZnS quantum dots for the degradation of the antibiotic cefalexin in aqueous solution. Nanoparticles were synthesized via the microwave-assisted method, and Fe doping was introduced at a 1% molar ratio. HRTEM images confirmed quasi-spherical morphology and high crystallinity, with particle sizes averaging 2.4 nm (pure) and 3.5 nm (doped). XRD analysis showed a consistent cubic ZnS structure. UV-vis spectra showed strong absorption at 316 nm for both samples, and PL measurements revealed emission quenching upon Fe doping. Photocatalytic tests under UV light demonstrated significantly higher degradation rates of 10 ppm cefalexin with Fe-doped ZnS, reaching near-complete removal within 90 min. Adsorption experiments revealed higher affinity and adsorption capacity of Fe-doped ZnS toward cefalexin compared to pure ZnS, as demonstrated by the Freundlich isotherm analyses, contributing significantly to enhanced photocatalytic degradation performance. High-resolution QTOF LC-MS analysis confirmed the breakdown of the β-lactam and thiazolidine rings of cefalexin and the formation of low-mass degradation products, including fragments at m/z 122.0371, 116.0937, and 318.2241. These findings provide strong evidence for the structural destruction of the antibiotic and validate the enhanced photocatalytic performance of Fe-doped ZnS.

1. Introduction

Cefalexin, a first-generation cephalosporin antibiotic, is widely prescribed for treating skin and soft tissue infections, otitis media, respiratory tract infections, and uncomplicated urinary tract infections [1,2]. Its popularity stems from favorable oral bioavailability and a well-established safety profile. Cefalexin exerts bactericidal effects by irreversibly binding to penicillin-binding proteins (PBPs), inhibiting the transpeptidation step of peptidoglycan synthesis, and inducing bacterial cell lysis in both Gram-positive and select Gram-negative bacteria [3].

However, extensive use in human and veterinary medicine has led to frequent detection of cefalexin in wastewater effluents and surface waters [4,5], with concentrations ranging from tens to thousands of nanograms per liter. Approximately 90% of administered cefalexin is excreted unchanged into aquatic environments, where its accumulation contributes to the emergence of multidrug-resistant pathogens and poses significant ecological risks [6]. Residual antibiotics in the environment promote the selection of resistant bacterial strains and facilitate the spread of antibiotic resistance genes, raising serious public health concerns [7,8].

The persistence of cefalexin is exacerbated by its limited removal in conventional wastewater treatment plants, which often fail to eliminate cephalosporins and their metabolites. Moreover, cefalexin strongly interacts with dissolved organic matter in wastewater, particularly protein-like fluorophores under acidic conditions, through π-π interactions between aromatic groups. This binding reduces adsorption onto materials such as biochar and clay, influencing cefalexin’s environmental fate and persistence [9,10]. Ecological risks extend beyond direct toxicity. Chronic exposure to trace cefalexin disrupts microbial community structures, increases chemical oxygen demand, and promotes antibiotic-resistant bacteria and resistance gene proliferation [11]. Additionally, accumulation of cefalexin and its by-products in sludge and effluents can impair biological treatment efficiency and harm downstream aquatic ecosystems [12].

While chlorine-based disinfection effectively inactivates pathogens, it produces toxic by-products that pose health and environmental risks, prompting exploration of advanced treatment technologies [13,14]. Advanced photodegradation processes, especially semiconductor photocatalysis, have emerged as promising tertiary treatments for mineralizing persistent antibiotics. Under UV irradiation (~365 nm), semiconductors like TiO₂ generate highly reactive hydroxyl radicals (•OH) that non-selectively oxidize organic contaminants, including cefalexin, leading to mineralization [15]. Laboratory studies report removal efficiencies exceeding 90% under optimized conditions, highlighting the potential of UV-based photocatalytic systems for mitigating antibiotic pollution [16].

Photocatalytic degradation begins when photons with energy equal to or greater than the semiconductor’s bandgap excite electrons from the valence band to the conduction band, creating electron–hole pairs. These charge carriers migrate to the surface, where holes oxidize water or hydroxide ions to generate hydroxyl radicals, and electrons reduce oxygen to form superoxide anions [17]. These reactive oxygen species synergistically degrade complex organic molecules into smaller intermediates and eventually mineralize them. Despite its potential in environmental remediation, photocatalysis is often limited by the efficient recombination of photogenerated charge carriers (electrons and holes), which reduces their availability for redox reactions and consequently diminishes the formation of reactive oxygen species essential for pollutant degradation. Strategies such as doping with transition metals or non-metal anions, constructing heterojunctions, and depositing noble-metal nanoparticles are employed to overcome this. These modifications prolong charge-carrier lifetimes and often extend light absorption into the visible range, enhancing degradation rates and broadening applicability.

Zinc sulfide quantum dots (ZnS QDs) offer distinct advantages among emerging photocatalysts due to their high surface-to-volume ratio, quantum confinement effects, and tunable optical properties. Their ~3.35 eV bandgap restricts activity to UV light; however, Fe²⁺ doping can introduce mid-gap trap states that quench UV fluorescence while promoting visible trap-state emissions. This modification has been associated with enhanced charge separation and improved generation of reactive oxygen species, thereby increasing the photocatalytic degradation efficiency of organic contaminants [18,19].

A critical initial step in photocatalytic degradation is the adsorption of cefalexin onto the ZnS QDs surface, which concentrates the antibiotic and facilitates interaction with reactive species [20]. The large specific surface area and exposed Zn²⁺/S²⁻ sites enable electrostatic interactions with cefalexin’s functional groups, while surface hydroxyl groups can form hydrogen bonds with polar moieties. Fe²⁺ doping alters surface properties, potentially affecting adsorption capacity and photocatalytic activity.

Accordingly, the objectives of this study are to: (i) synthesize pure and Fe-doped ZnS QDs via a one-step microwave-assisted method to achieve uniform particle size and dopant distribution; (ii) characterize the structural and optical properties of the QDs using X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–Vis absorption, and photoluminescence spectroscopy; (iii) evaluate and compare the photocatalytic performance of both QD systems for cefalexin degradation under UV irradiation; (iv) investigate adsorption interactions between QDs and cefalexin through adsorption isotherms; and (v) identify photodegradation by-products using high-performance liquid chromatography (HPLC) and mass spectrometry to elucidate the degradation pathway.

By integrating advanced materials synthesis with comprehensive mechanistic studies, this work aims to develop efficient photocatalytic systems for removing persistent antibiotics from aquatic environments, thereby contributing to mitigating antibiotic resistance risks.

2. Materials and Methods

2.1. Materials

Pure and iron-doped zinc sulfide (ZnS) nanoparticles were synthesized using analytical-grade reagents to ensure high chemical purity and reproducibility. Zinc sulfate heptahydrate (ZnSO₄·7H₂O, ReagentPlus^®, ≥99.0%, Sigma-Aldrich, St. Louis, MO, USA) and sodium sulfide (Na₂S, 3% w/w, Ricca Chemical, Arlington, TX, USA) were used as the primary precursors. Iron doping was achieved through the incorporation of iron sulfate heptahydrate (FeSO₄·7H₂O, ReagentPlus^®, ≥99.0%, Sigma-Aldrich). No additional stabilizing agents, surfactants, or pH modifiers were used in the process. All glassware employed was thoroughly cleaned by soaking in a 10% (v/v) nitric acid solution for at least 24 h, followed by extensive rinsing with high-purity deionized water to eliminate potential contaminants. Photocatalytic and sorption experiments were conducted using aqueous solutions of cefalexin, prepared from cefalexin hydrate (≥99%, Sigma-Aldrich). Ultrapure water with a resistivity of 18.2 MΩ·cm was used in all steps to minimize ionic interference and maintain the integrity of the materials.

2.2. Synthesis of Pure and Doped Quantum Dots

Zinc sulfide (ZnS) quantum dots were synthesized using a microwave-assisted hydrothermal approach, which offers rapid and uniform heating throughout the reaction medium, promoting controlled nucleation and crystal growth (Figure 1). In a typical synthesis, 5.0 mL of a 0.4 M aqueous solution of zinc sulfate heptahydrate (ZnSO₄·7H₂O) was combined with 2.5 mL of a 3% (w/w) sodium sulfide (Na₂S) solution, which served as the sulfur precursor. The solutions were freshly prepared using ultrapure water to prevent contamination and ensure reproducibility. The reaction mixture was thoroughly stirred to ensure homogeneity and then transferred into a Teflon-lined microwave digestion vessel (MARS 6, CEM Corporation, Matthews, NC, USA). The vessel was sealed and subjected to microwave irradiation at 140 °C for 15 min under a constant frequency of 2.45 GHz. The microwave system ensured efficient volumetric heating, eliminating thermal gradients typically observed in conventional hydrothermal methods and enhancing the uniformity of the resulting nanoparticles.

For the synthesis of iron-doped ZnS quantum dots, the same procedure was followed with the addition of 1.0 mL of a 0.15 M solution of iron (II) sulfate heptahydrate (FeSO₄·7H₂O), yielding a final Fe²⁺ doping level of approximately 1% relative to zinc. The iron-containing reaction mixture was subjected to identical microwave conditions (140 °C, 15 min) to ensure consistency across samples. After completion of the microwave treatment, the reaction vessels were cooled to room temperature. The colloidal suspensions containing the synthesized nanoparticles, either pure or iron-doped, were then purified via centrifugation at 5000 rpm for 15 min. The supernatant was discarded, and the precipitate was washed with 2-propanol to remove unreacted precursors and any loosely bound impurities. This washing-centrifugation cycle was repeated three times to ensure purity. Finally, the purified nanoparticles were resuspended in deionized water and stored at 4 °C for further physicochemical and optical characterization.

2.3. Characterization of Pure and Doped Quantum Dots

The optical properties of pure and iron-doped quantum dots were evaluated using UV-visible absorption and photoluminescence spectroscopy. UV-vis absorption spectra were recorded in the 200–900 nm range using a UV-2700i spectrophotometer (Shimadzu, Columbia, MD, USA), while photoluminescence measurements were performed with an RF-6000 spectrofluorometer (Shimadzu, Columbia, MD, USA). Morphological characterization was conducted by high-resolution transmission electron microscopy (HRTEM) using a JEM-ARM200cF instrument (JEOL) operated at 200 kV, equipped with an Oxford Aztec energy-dispersive X-ray spectroscopy (EDS) system. Crystallographic structure was assessed using electron diffraction (ED) and X-ray diffraction (XRD); the latter was carried out with a Siemens Powder Diffractometer D5000 using Cu-K radiation to determine the crystal phase of the nanoparticles. Elemental composition was quantified via energy-dispersive X-ray spectroscopy (EDX) using the Cliff-Lorimer method (k-factor approach), calibrating against a pure copper standard.

2.4. Photocatalytic Experiments of Cefalexin

Photocatalytic degradation studies were conducted to assess the efficacy of pure and iron-doped ZnS quantum dots in degrading cefalexin in aqueous solution. Cefalexin was dissolved in deionized water to prepare a working solution with a final concentration of 10 ppm. The photocatalytic activity of the quantum dots was tested by introducing them into the antibiotic solution at two different concentrations: 250 ppm and 500 ppm. The nanoparticles were added to the cefalexin solution immediately before light exposure to ensure homogenous dispersion and maximize interaction. To ensure the validity of the degradation process, two sets of control groups were prepared using cefalexin (10 ppm) without any nanomaterial: (1) a UV control, exposed to light in the absence of photocatalyst, and (2) a dark control, maintained under identical mixing conditions but fully protected from light to rule out photolysis or hydrolysis in the absence of irradiation. All experiments were conducted at room temperature (approximately 25 °C) and kept under continuous agitation using a Rotamix (20–25 rpm) to prevent nanoparticle sedimentation and ensure uniform exposure.

UV irradiation was applied using a handheld 8-watt UV lamp (302 nm) emitting a total irradiance of 10 mW cm⁻². Each sample, including control and experimental groups, was exposed to the UV source for predetermined time intervals ranging from 0 to 180 min. Dark control samples were stored under identical physical conditions but wrapped in aluminum foil to block light exposure entirely. At each time point, aliquots (typically 1.0 mL) were analyzed for the residual cefalexin concentration by high-performance liquid chromatography (HPLC) using a Shimadzu LC-2030 system (Columbia, MD, USA). Calibration curves were generated using standard cefalexin solutions ranging from 0.3 ppm to 25 ppm, prepared from a 100 ppm stock solution. Chromatographic separation was achieved using an Agilent Zorbax Eclipse XDB-C8 column (Santa Clara, CA, USA), operating under isocratic conditions with a mobile phase composed of 68% methanol and 32% aqueous formic acid (0.1% v/v). The injection volume was 15 μL, and detection was carried out by UV absorbance at 260 nm. The column oven was maintained at a constant temperature of 35 °C throughout the analysis. All degradation experiments were conducted in triplicate to ensure reproducibility and statistical robustness.

To complement the quantitative analysis, the structural identification of degradation products was performed using high-resolution mass spectrometry. A 6530 accurate-mass quadrupole time-of-flight liquid chromatography mass spectrometry (LC/MS) system (Agilent Technologies, Santa Clara, CA, USA) was employed to analyze selected samples. Both total ion chromatograms (TIC) and extracted ion chromatograms (EIC) were generated to detect and identify cefalexin and its photodegradation by-products. The high mass accuracy and resolution of the QTOF-LC/MS system allowed for precise characterization of transformation products, providing mechanistic insight into the degradation pathway.

2.5. Adsorption Experiments

Stock solutions of cefalexin were prepared by dissolving cefalexin hydrate (purity ≤ 100%, Sigma-Aldrich) in ultrapure deionized water under constant stirring to ensure complete dissolution. From the stock solution, a series of working solutions with concentrations ranging from 1 to 130 ppm were prepared via serial dilution. These solutions were freshly prepared prior to each experiment to ensure stability and accuracy.

Adsorption experiments were conducted using 50 mL polypropylene centrifuge tubes, each containing 5.0 mL of a given cefalexin working solution and ZnS quantum dots added at a fixed concentration of 1000 ppm. The ZnS nanoparticles used in these assays were previously synthesized via microwave-assisted methods and suspended in deionized water. The pH of the solution remained unchanged in this phase, thereby maintaining the native aqueous conditions of the system. The prepared suspensions were then placed on an orbital shaker and agitated at 350 rpm for a total contact time of 3 h, at a constant temperature of 20 ± 2 °C. This contact time was selected based on prior kinetic studies, which indicated that equilibrium conditions for cefalexin adsorption onto ZnS surfaces were achieved within this period. All experiments were conducted in duplicate or triplicate to ensure reproducibility and reduce variability. Upon completion of the agitation period, the suspensions were immediately filtered to separate the nanoparticles from the solution. Filtration was performed using sterile polyethersulfone (PES) syringe filters (Millex^®, 0.22 μm pore size, Cork, Ireland), ensuring that no nanoparticles remained in the filtrate. The resulting clear supernatants were collected for subsequent quantification of residual cefalexin.

Quantitative analysis of cefalexin concentration in the filtrates was carried out using high-performance liquid chromatography (HPLC) on a Shimadzu LC-2030 platform (Columbia, MD, USA). Chromatographic separation was achieved using an Agilent Zorbax Eclipse XDB-C8 column (150 mm × 4.6 mm, 5 μm particle size; Santa Clara, CA, USA). The mobile phase consisted of 85% aqueous formic acid solution (0.1% v/v) and 15% acetonitrile, delivered at a flow rate of 0.8 mL/min under isocratic conditions. The column oven temperature was maintained at 35 °C, and UV detection was performed at 260 nm. A 100 μL injection volume was used for all analyses to ensure optimal sensitivity and precision. Calibration was achieved using cefalexin standards prepared in the same mobile phase composition, covering the range of 1 to 120 ppm. The calibration curve exhibited excellent linearity with a correlation coefficient (R²) greater than 0.999, validating the method for quantitative analysis of residual antibiotic concentrations in adsorption studies. The equilibrium adsorption capacity, Q_e (mg g⁻¹), was calculated as:

Q_e = [(C₀ − C_e) × V]/m

(1)

where C₀ and C_e (ppm) are the initial and equilibrium cefalexin concentrations, V is the solution volume (L), and m is the adsorbent mass (g).

3. Results and Discussion

3.1. Morphology and Crystalline Structure

Microwave irradiation has emerged as a powerful tool in synthesizing nanomaterials, primarily due to its capacity for rapid and direct energy delivery through electromagnetic waves. Unlike conventional heating methods based on thermal conduction or convection, microwave systems enable immediate and uniform heating throughout the reaction medium, especially in systems containing polar solvents like water. This approach significantly shortens reaction times, improves product yield and purity, and lowers energy and reagent demands. Moreover, the application of microwave pulses allows for further reductions in energy consumption and enhances the efficiency of the process.

A critical aspect of this heating mechanism involves dielectric polarization, in which polar molecules absorb microwave energy and convert it into thermal energy. This effect is particularly advantageous in aqueous media, facilitating the dissociation of metal precursors such as zinc sulfate, sodium sulfide, and iron sulfate. The resulting conditions favor the rapid nucleation and controlled growth of ZnS nanoparticles, enabling the formation of well-defined structures with tunable size and crystallinity in pure and doped systems.

The morphology and crystallinity of the synthesized ZnS nanoparticles were investigated using high-resolution transmission electron microscopy (HRTEM), as shown in Figure 2 and Figure 3. Low-magnification images reveal that the pure and Fe-doped ZnS samples exhibit reasonable particle dispersion, although with some aggregation. The individual nanocrystals are distinguishable and uniformly distributed throughout the examined regions. Energy-dispersive X-ray spectroscopy (EDS) confirmed the elemental composition of the samples, showing that the undoped ZnS nanoparticles consisted of approximately 47% zinc and 53% sulfur, in close agreement with the expected stoichiometry. Fe was detected in trace amounts ranging from 0.5 to 1.0 atomic percent in the doped samples (Figure 4). At the same time, Zn and S remained the dominant elements, indicating successful low-level incorporation of the dopant without altering the primary composition. High-resolution images confirm that the nanoparticles in both samples possess a well-defined crystalline structure. Resolved lattice fringes are observed in both cases, with an interplanar distance of approximately 0.32 nm, attributed to ZnS’s (111) planes. These periodic lattice features demonstrate high crystallinity in the as-synthesized nanomaterials.

Quantitative particle size analysis was performed using ImageJ software version 1.54p, measuring 20 individual particles per sample. The average particle diameter was estimated to be 2.4 nm for the undoped ZnS and 3.5 nm for the Fe-doped ZnS nanoparticles. This increase in average particle size upon Fe incorporation suggests that Fe²⁺ ions may influence the nucleation and growth processes during synthesis, potentially promoting more extensive crystal development without compromising lattice integrity.

Overall, the TEM and HRTEM observations demonstrate that pure and Fe-doped ZnS nanoparticles are highly crystalline, nanometric in size, and morphologically uniform. The slight increase in particle size observed in the doped sample, together with its preserved crystallinity, highlights the role of Fe as a structural modulator during synthesis, an effect that may be advantageous for applications where particle size and crystal order impact performance, such as in photocatalysis.

Figure 5 shows the X-ray diffraction (XRD) patterns of pure ZnS and ZnS nanoparticles doped with 1% Fe. Both samples display three characteristic diffraction peaks centered at approximately 2θ = 28.6°, 48.0°, and 56.6°, which are indexed to the (111), (220), and (311) crystallographic planes, respectively. These reflections are consistent with the face-centered cubic (FCC) structure of ZnS, corresponding to the zinc blende phase (JCPDS Card No. 65-0309) [21]. Notably, a slight shift in the (111) peak toward lower 2θ values was observed in the Fe-doped sample, indicating a minor expansion of the unit cell due to the substitutional incorporation of Fe²⁺ ions (ionic radius = 0.077 nm) in place of Zn²⁺ (ionic radius = 0.074 nm). This shift is expected from Bragg’s law, as the incorporation of slightly larger dopant ions increases the interplanar spacing (d), leading to a decrease in the diffraction angle [22]. The absence of additional peaks confirms the phase purity of the samples and indicates that Fe²⁺ was successfully incorporated into the ZnS lattice without forming detectable secondary phases, such as iron oxides or sulfides, within the sensitivity range of the instrument [21,23,24,25,26].

Crystallite size calculations based on the Debye–Scherrer equation revealed that the average crystallite size of pure ZnS was approximately 2.5 nm. In comparison, that of Fe-doped ZnS increased slightly to 3.2 nm. This result is consistent with the HRTEM observations, where the Fe-doped nanoparticles also appeared slightly larger than the undoped ones. The agreement between both techniques supports the conclusion that Fe incorporation induces a modest increase in particle size, likely associated with changes in the crystal growth mechanism during synthesis [21,25,26].

The observed increase in crystallite size upon Fe doping is attributed to the effect of Fe²⁺ ions on the crystal growth dynamics. Although the ionic radius of Fe²⁺ (0.077 nm) is slightly larger than that of Zn²⁺ (0.074 nm), its incorporation into the ZnS lattice can influence the strain energy and reduce the formation of crystal defects, thereby facilitating grain growth during synthesis. Notably, the retention of the FCC structure in both samples suggests that low-level Fe doping does not disrupt the crystal phase but rather modifies the microstructural properties of the material [27].

These structural modifications, particularly the increase in crystallite size and the possible introduction of mid-gap energy states, can enhance the photocatalytic activity of the material. Doping with transition metals such as Fe²⁺ improves charge carrier separation by acting as electron or hole traps, reducing recombination rates and increasing the lifetime of photogenerated charge carriers [28]. Additionally, incorporating Fe can promote the generation of reactive oxygen species (ROS) under light irradiation, which are critical for the oxidative degradation of organic pollutants [28]. Therefore, the Fe-doped ZnS nanoparticles are expected to exhibit superior photocatalytic performance compared to their undoped counterparts, particularly in applications targeting the degradation of persistent contaminants in aqueous environments.

3.2. Optical Characterization

The optical absorption properties of pure and 1% Fe-doped ZnS nanoparticles were analyzed using UV–Visible spectroscopy, as shown in Figure 6. Both samples exhibit a well-defined absorption edge centered around 316 nm, corresponding to the fundamental electronic transition from the valence band to the conduction band. This value represents a blue shift relative to the absorption edge of bulk ZnS (∼336 nm), confirming the influence of quantum confinement effects in the nanoscale regime [29,30].

A similar absorption onset was observed for pure and Fe-doped ZnS, suggesting that low-level Fe incorporation (1%) does not significantly alter the band gap energy of the material. This is consistent with the structural characterization by XRD, which showed only a modest increase in crystallite size from 2.5 nm (pure) to 3.2 nm (doped). Such a slight change in size remains within the range where quantum confinement dominates, preserving the high-energy optical transition near 316 nm. The optical band gap energies of the pure ZnS and the Fe-doped (1%) ZnS nanoparticles were estimated using the Tauc method. The calculated band gap for the undoped ZnS is 3.52 eV, while the Fe-doped sample exhibits a slightly lower band gap of 3.50 eV. This minor redshift in the band gap is consistent with the introduction of Fe²⁺ ions into the ZnS lattice, which leads to sp-d exchange interactions between the conduction band electrons and the localized d electrons of the dopant. These interactions slightly modify the electronic structure of the material, supporting the successful incorporation of Fe [29,30].

The absorption behavior of both samples in the UV region indicates strong photon–matter interaction in this range, which is particularly relevant for photocatalytic applications. The high absorption coefficient in the 300–350 nm range enhances the generation of electron–hole pairs under UV light, a critical step in driving redox reactions for contaminant degradation [31].

Photoluminescence (PL) spectra collected under 302 nm excitation (Figure 7) revealed a strong near-band-edge emission at approximately 353 nm in the undoped ZnS sample, which is attributed to radiative exciton recombination within the pristine crystal lattice. In contrast, this characteristic emission was significantly quenched in the Fe-doped ZnS nanoparticles, indicating the introduction of non-radiative recombination pathways likely associated with Fe²⁺ incorporation [32,33]. Additionally, both materials exhibited broader defect-related emissions in the visible region, centered at 450 nm for pure ZnS and red-shifted to 468 nm for the Fe-doped sample. These emissions are associated with intrinsic point defects, such as sulfur vacancies and zinc interstitials.

Interestingly, the observed red shift and attenuation of the trap-related emission in the doped sample occur despite the slight increase in crystallite size detected by XRD (from 2.5 nm in the pure sample to 3.2 nm in the doped one). This suggests that while doping promoted crystal growth, it also altered the local electronic structure of the material, likely due to the substitution of Zn²⁺ (ionic radius 0.074 nm) by Fe²⁺ (0.077 nm). This substitution can introduce localized energy states within the band gap, acting as recombination centers that suppress UV emission and modify the trap-state landscape. Elemental analysis confirming Fe concentrations below 1.6% supports the idea of controlled doping, where the dopant subtly influences the optical properties without disrupting the overall crystal phase. These changes in PL behavior reinforce the structural evidence from XRD and highlight the role of Fe²⁺ in tailoring the defect chemistry and recombination dynamics of ZnS nanocrystals.

3.3. Photodegradation of Cefalexin

Figure 8 and Figure 9 present the photocatalytic performance of pure ZnS and 1% Fe-doped ZnS nanoparticles, evaluated at two different catalyst concentrations (250 ppm and 500 ppm). The degradation of cefalexin was monitored over time by calculating the relative concentration (C_f/C_i), where C_i represents the initial antibiotic concentration and C_f the residual concentration at each sampling point. This ratio served as a reliable metric to assess photocatalytic efficiency, with decreasing C_f/C_i values indicating progressive degradation of the target molecule.

Quantitative analysis of cefalexin concentration was performed using a calibration curve (Figure 10), enabling precise determination of removal efficiency over time. The data show that photocatalytic degradation depends on catalyst concentration, irradiation time, and doping composition. In contrast, the control groups, those exposed to UV radiation without catalysts and those kept in complete darkness, exhibited negligible reduction in cefalexin concentration, reinforcing the molecule’s high photostability and low natural degradation rate in aqueous media. This is consistent with the well-documented persistence of cefalexin in the environment, due to its stable β-lactam structure and poor biodegradability [34].

Complete degradation of cefalexin was achieved in both experimental conditions, using pure and Fe-doped ZnS nanoparticles. However, a significantly faster degradation rate was observed when using 1% Fe-doped ZnS. In particular, after 90 min of UV exposure, cefalexin was nearly eliminated in the presence of the doped nanoparticles, regardless of whether the concentration was 250 ppm or 500 ppm. This acceleration in degradation highlights the role of Fe²⁺ ions in enhancing the photocatalytic activity of the semiconductor.

The improved performance of the doped system is likely due to enhanced generation of reactive oxygen species (ROS) and more efficient separation of photogenerated charge carriers. Upon exposure to UV light (302 nm), electrons are excited from the valence band (VB) to the conduction band (CB) of the ZnS semiconductor. In Fe-doped ZnS, Fe²⁺ ions could act as intermediate electron traps within the crystal lattice. These traps suppress recombination of electron–hole pairs by temporarily capturing conduction band electrons, which increases their availability for redox reactions at the nanoparticle surface.

This charge separation mechanism facilitates the formation of highly reactive species such as hydroxyl radicals (•OH) and superoxide anions (O₂•⁻) through reactions with water and dissolved oxygen, respectively [34,35,36]. These species are known to oxidize a broad range of organic pollutants, including β-lactam antibiotics like cefalexin, by attacking the aromatic rings and cleaving functional groups critical for biological activity.

Overall, the superior photocatalytic behavior observed in Fe-doped ZnS systems underscores the beneficial effect of low-level doping in tuning the electronic and catalytic properties of semiconductor nanomaterials. These findings support the potential of Fe-doped ZnS nanoparticles as efficient photocatalysts for removing persistent pharmaceutical contaminants from aquatic environments.

3.4. Adsorption Studies and Nanoscale Interaction

Adsorption experiments were conducted to comprehensively understand the nanoscale interactions between cefalexin and pure and Fe-doped ZnS quantum dots. The adsorption data for both pure and Fe-doped ZnS quantum dots exhibited the best fit to the Freundlich isotherm model, indicating that the adsorption occurred on heterogeneous surfaces with multilayer coverage, which aligns well with the observed surface properties and adsorption behaviors of the quantum dots. The Freundlich model yielded correlation coefficients (R²) of 0.8741 for pure ZnS (Figure 11a) and 0.9531 for Fe-doped ZnS (Figure 11b), clearly indicating enhanced adsorption capacity and surface heterogeneity for the Fe-doped system.

The superior adsorption behavior of Fe-doped ZnS quantum dots is attributed to the structural and chemical modifications induced by Fe²⁺ ions doping. The incorporation of Fe²⁺ significantly modifies the electronic structure of ZnS, leading to localized charge density fluctuations on the nanoparticle surface. These fluctuations intensify electrostatic attraction between the cefalexin’s negatively charged functional groups (notably the carboxylate group) and positively charged metal sites (Zn²⁺ and Fe²⁺) on the quantum dot surface. Additionally, Fe²⁺ doping generates supplementary coordination sites, enhancing the availability and efficacy of surface-active sites that facilitate adsorption.

Moreover, abundant hydroxyl groups on the surface of Fe-doped ZnS quantum dots play a critical role in strengthening adsorption interactions through robust hydrogen bonding with cefalexin molecules. These hydroxyl-mediated interactions further stabilize the antibiotic molecules on the nanoparticle surface, leading to a higher localized concentration of cefalexin near catalytic sites.

This enhanced adsorption capacity significantly influences subsequent photocatalytic degradation processes. By concentrating cefalexin molecules in proximity to the reactive sites, Fe-doped ZnS quantum dots improve interactions with photogenerated reactive oxygen species (ROS), such as hydroxyl radicals (•OH) and superoxide anions (O₂•⁻). This proximity and enhanced concentration expedite the oxidative degradation of cefalexin, leading to more efficient disruption of critical structural moieties such as the β-lactam and thiazolidine rings.

Fe doping amplifies surface heterogeneity and increases adsorption capacity, directly augmenting photocatalytic efficiency. This robust nanoscale interaction underscores the strategic importance of doping in optimizing nanomaterials for environmental remediation applications, particularly in the targeted removal of persistent pharmaceutical contaminants from aqueous media. This enhanced adsorption behavior is directly reflected in the subsequent photocatalytic degradation performance, as the higher concentration of cefalexin at the surface facilitates faster and more complete breakdown under UV irradiation.

3.5. Mass Spectrometric Analysis

The mass spectrometric analysis depicted in Figure 12 reveals the degradation pathway of cefalexin (C₁₆H₁₇N₃O₄S; [M − H]⁺, m/z 348.2410) after treatment with UV-activated Fe-doped ZnS nanoparticles at a concentration of 500 ppm. The structures were elaborated using the m/z ratio values and the ChemDraw software version 22.2.0 (ChemOffice program, Perkin Elmer, Waltham, MA, USA). Based on the identified degradation products, a mechanistic pathway for cefalexin degradation is proposed. The process is initiated by the photocatalytic activation of Fe-doped ZnS under UV irradiation, which generates reactive oxygen species (ROS), especially hydroxyl radicals (•OH), on the surface of the nanoparticles. These radicals play a crucial role in attacking the functional groups of the antibiotic molecule.

The primary step involves the cleavage of the β-lactam ring, a key structural component responsible for the antibiotic activity of cefalexin. This ring opening is indicated by the appearance of fragments with m/z values of 336.2388 and 364.2391. The 364.2391 m/z fragment is attributed to hydroxylation at the β-lactam ring, likely due to direct attack by •OH radicals, as previously described in the literature [17]. This oxidative process weakens the amide bond, facilitating ring opening. The 336.2388 m/z fragment may result from subsequent deamination and loss of a hydroxyl group from the 364.2391 m/z species.

Further degradation involves secondary transformations such as demethylation, evidenced by the 324.2300 m/z fragment, consistent with the loss of a methyl group from the molecule, a process also reported by other authors [17,37]. The cleavage of the thiazolidine ring, which is part of the cephem nucleus, leads to the formation of the 318.2241 m/z fragment. This fragmentation is particularly important as it marks the destabilization of the core structure, enabling further breakdown into smaller, less complex molecules.

Finally, the detection of low-mass fragments, such as those at 116.0937 and 122.0371 m/z, suggests complete molecular disintegration through decarboxylation and further oxidation steps. The cumulative evidence supports a degradation mechanism initiated by radical-induced β-lactam ring opening, followed by sequential oxidative and hydrolytic transformations, including hydroxylation, deamination, demethylation, ring scission, and decarboxylation [17,37,38].

4. Conclusions

In this work, pure and Fe-doped ZnS quantum dots were successfully synthesized using a microwave-assisted method and evaluated for their photocatalytic efficiency in degrading the antibiotic cefalexin in aqueous media. Structural and morphological characterizations confirmed that both materials possessed nanometric dimensions, crystalline structures, and quasi-spherical morphology, with Fe doping inducing a slight increase in particle size without disrupting crystallinity. Optical analyses revealed strong UV absorption around 316 nm and photoluminescence quenching in the doped sample, suggesting improved charge carrier separation. Adsorption tests demonstrated that Fe-doped ZnS quantum dots exhibited superior affinity and adsorption capacity toward cefalexin compared to pure ZnS, as evidenced by the Freundlich isotherm analysis. This enhanced adsorption was attributed to surface modifications induced by Fe doping, specifically the introduction of additional active sites and enhanced electrostatic and hydrogen bonding interactions with cefalexin molecules. Consequently, this increased adsorption significantly contributed to concentrating the antibiotic near active catalytic sites, thus improving photocatalytic degradation efficiency. Photocatalytic degradation experiments showed that Fe-doped ZnS exhibited enhanced performance, achieving nearly complete degradation of cefalexin after 90 min of UV irradiation at both 250 and 500 ppm catalyst concentrations. QTOF LC–MS analysis confirmed the formation of multiple degradation products, consistent with the breakdown of the β-lactam and thiazolidine rings, and the generation of smaller, more polar fragments. These findings demonstrate that Fe doping modulates the structural and optical properties of ZnS nanoparticles and significantly improves their photocatalytic activity. Therefore, Fe-doped ZnS quantum dots represent a promising platform for the efficient degradation of pharmaceutical contaminants in water, contributing to the development of advanced nanomaterials for environmental remediation.