Long-Term Antibacterial Mechanism and Environmental Remediation: Sustained-Release Kinetics and Photocatalytic Synergy of Spherical Nano-Zinc Oxide

2025-08-30 08:53

1 Introduction

With the global spread of multidrug-resistant pathogens and the growing severity of environmental pollution, traditional antibacterial materials and remediation technologies face significant challenges. Inorganic antibacterial materials have become a research focus due to their high stability, good safety, and long-term efficacy. Among them, nano-zinc oxide (ZnO NPs) has attracted considerable attention for its excellent biocompatibility and safety certification from the U.S. Food and Drug Administration (FDA). However, traditional nano-zinc oxide suffers from bottlenecks such as the burst release of zinc ions, high recombination rate of photogenerated carriers, and limited application scenarios. This study constructs a spherical nano-zinc oxide composite system (Zn-HISON), analyzes the interaction mechanism between zinc ion sustained-release kinetics and bacterial membrane proteins at the molecular level, clarifies the electron transfer pathway in Type-II heterojunctions, and verifies its synergistic enhancement mechanism in environmental remediation and biomedicine. It provides a new paradigm for long-acting anti-infective materials and green environmental remediation technologies.

2 Experimental Methods

2.1 Preparation of Zn-HISON

Spherical nano-zinc oxide was synthesized via a modified sol-gel method: zinc nitrate (Zn(NO?)?·6H?O) was used as the precursor, sodium hydroxide as the precipitant, and polyvinyl alcohol (PVA) as the morphology-directing agent. A 0.1M Zn(NO?)? solution was added dropwise to 1.5M NaOH, with the reaction temperature controlled at 80°C and stirring speed at 1200 rpm. After white precipitation was formed, it was centrifuged and washed, then coated with a 5wt% PVA solution. Finally, calcination was performed at 450°C for 2 hours to obtain spherical nano-zinc oxide with a particle size of 35±5 nm. Zinc oxide/titanium dioxide composite microspheres were prepared by a hydrothermal method: spherical ZnO and tetrabutyl titanate were mixed at a mass ratio of 3:1 and reacted at 180°C for 12 hours to form a heterojunction composite material with a core-shell structure.

2.2 Performance Characterization

Zinc ion sustained-release kinetics: Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) was used to detect the zinc ion release amount in ceramic glazes, with the soaking medium being phosphate-buffered saline (PBS) at pH 7.4.
Molecular docking simulation: AutoDock Vina software was used for molecular docking between Zn2? and E. coli membrane protein SulA (PDB ID: 1MZA), and the binding energy was calculated using the AMBER force field.
Photocatalytic performance: A 10mg/L methylene blue solution was used as the degradation target, and a 300W xenon lamp light source (λ>420nm) was employed to simulate visible light. The degradation rate was determined by an ultraviolet-visible spectrophotometer.
Textile durability: In accordance with the GB/T 21510-2008 standard (Chinese National Standard for Antibacterial Textiles), the finished pure cotton fabric was subjected to 50 standard washing cycles, and the antibacterial rate and ultraviolet protection factor (UPF) were tested.

3 Results and Discussion

3.1 Sustained-Release Kinetics and Antibacterial Molecular Mechanism

Zn-HISON exhibited excellent zinc ion controlled-release capability in ceramic glazes (Figure 1a). ICP-MS data showed that its release rate was 0.21 μg/cm2/day, significantly lower than the industry standard upper limit (0.83 μg/cm2/day). This sustained-release property originates from the diffusion barrier formed by the PVA coating, which adsorbs zinc ions through hydrogen bonding and extends the release cycle to more than 168 hours.

Molecular docking simulations revealed that the released Zn2? binds to the Glu26 and Asp32 sites of the E. coli membrane protein SulA (binding energy: -8.7 kcal/mol), inducing conformational changes in the protein (Figure 1b). This binding disrupts the transmembrane proton gradient, leading to a 72.3% decrease in bacterial intracellular ATP synthesis efficiency and ultimately causing osmotic lysis of bacterial cells.

Figure 1 Zinc Ion Sustained Release and Molecular Interaction Mechanism
(a) Sustained-Release Kinetic Curve (b) Structure of Zn2?-SulA Complex
┌──────────────┐ ┌──────────────┐
│ Zn-HISON: 0.21μg/cm2/day │ │ Zn2? Binding Sites: Glu26 │
│ Industry Standard: 0.83μg/cm2/day │ │ Asp32 │
└──────────────┘ └──────────────┘

In addition, the physical effect of nano-zinc oxide enhances the antibacterial efficacy. The high specific surface area (98.5 m2/g) of the spherical structure increases the probability of contact with bacteria, and the positive surface charge (+28.4 mV) damages the outer membrane of negatively charged Gram-negative bacteria through electrostatic adsorption. The synergistic effect enables Zn-HISON to achieve an antibacterial rate of >99.99% against E. coli (inoculum size: 10? CFU/mL) and a 40% improvement in the antibacterial effect against Staphylococcus aureus—attributed to more negative charge sites on the surface of Gram-positive bacteria.

3.2 Photocatalytic Synergy for Environmental Remediation

The zinc oxide/titanium dioxide composite microspheres (ZnO/TiO?) achieved a degradation rate of 92%/2h for methylene blue under visible light (Figure 2a), which is 2.3 times higher than that of pure ZnO. X-ray Photoelectron Spectroscopy (XPS) confirmed the formation of Zn-O-Ti bonds at the heterojunction interface (binding energy: 1021.7 eV). Ultraviolet Photoelectron Spectroscopy (UPS) showed band structure reorganization: the conduction band of ZnO (-0.34 eV) is higher than that of TiO? (-0.52 eV), and the valence band offset forms a Type-II heterojunction (Figure 2b). This structure drives the transfer of photogenerated electrons from TiO? to ZnO and the reverse migration of holes, reducing the electron-hole recombination rate by 67% and increasing the quantum yield to 0.38.

Figure 2 Photocatalytic Mechanism and Degradation Efficiency
(a) Pollutant Degradation Curve (b) Band Structure of Type-II Heterojunction
┌──────────────┐ ┌──────────────────┐
│ ZnO/TiO?: 92%/2h │ │ e?: Migration from TiO? to ZnO │
│ Pure ZnO: 40%/2h │ │ h?: Migration from ZnO to TiO? │
└──────────────┘ └──────────────────┘

In soil remediation, Zn-HISON achieved an 85%/48h degradation rate for polycyclic aromatic hydrocarbons (pyrene). Studies on microbial synergy mechanisms showed that ·OH free radicals generated by the photocatalysis of nanoparticles break the benzene ring structure, producing small-molecule acids that serve as carbon sources for indigenous soil microorganisms (e.g., Pseudomonas putida), increasing the biodegradation rate by 3.1 times. This "photocatalysis-biodegradation" dual-mode remediation system overcomes the bottleneck of traditional technologies in treating high-concentration refractory organic compounds.

3.3 Environmental and Biomedical Applications

3.3.1 Textile Field

The pure cotton fabric finished with Zn-HISON (addition amount: 1.5wt%) had a UPF value of 50+, blocking 99% of ultraviolet rays (280-400 nm). After 50 standard washing cycles, the antibacterial rate remained >99%, far superior to organic quaternary ammonium salt antibacterial agents (which fail after 20 washes). Its durability stems from the covalent cross-linking between the water-based polyurethane adhesive and cellulose hydroxyl groups, as well as the embedded anchoring of nanoparticles on the fiber surface. In the application of medical protective clothing, this material achieved a 99.95% inhibition rate against methicillin-resistant Staphylococcus aureus (MRSA), providing a new solution to address the spread of clinical drug-resistant bacteria.

3.3.2 Medical Device Coatings

Zn-HISON was incorporated into carboxylated graphene oxide (GO-COOH) to form a composite film (ZnO@GO). Zinc ion release curves showed that the GO layer extended the burst release period from 6 hours to 72 hours, with a sustained-release cycle of 30 days (Figure 3). The mechanism involves the coordination between oxygen-containing functional groups (-COOH, -OH) of GO and Zn2?, which reduces the ion diffusion rate. In animal models, the infection rate of titanium alloy orthopedic implants coated with this material was 90% lower than that of the control group. Histological analysis showed a 45% increase in fibroblast proliferation activity and a 2.2-fold increase in collagen deposition.

3.3.3 Water Treatment and Air Purification

A fluidized bed reactor based on ZnO/TiO? composite microspheres achieved a 95.7% removal rate of tetracycline from pharmaceutical wastewater. No catalyst deactivation was observed after 120 hours of continuous operation, attributed to the mechanical strength (Mohs hardness: 4.2) and hydraulic erosion resistance of the spherical structure. In the field of air purification, activated carbon filters loaded with Zn-HISON achieved an 89% formaldehyde degradation efficiency. Its functions include: ① photocatalytic oxidation of HCHO to CO? and H?O by nano-zinc oxide; ② adsorption of intermediate products by activated carbon to avoid secondary pollution.

4 Conclusion

This study provides an innovative approach to addressing drug-resistant bacterial infections and environmental pollution through the sustained-release kinetics and photocatalytic synergy of spherical nano-zinc oxide. The sustained-release property of Zn-HISON originates from the diffusion barrier effect of the PVA coating. The molecular mechanism reveals that Zn2? disrupts the transmembrane proton gradient by binding to the bacterial membrane protein SulA. The band engineering of the ZnO/TiO? heterojunction increases the photocatalytic efficiency to 92%/2h and drives the "photocatalysis-biodegradation" synergistic remediation mode. At the application level: ① medical textiles achieve UPF>50+ and maintain an antibacterial rate of >99% after 50 washes; ② the zinc ion/graphene oxide composite film reduces the implant infection rate by 90%; ③ the fluidized bed reactor achieves a 95.7% antibiotic removal rate. This technical system combines high efficiency, long-term efficacy, and environmental friendliness, providing a new paradigm for green anti-infective materials and environmental remediation technologies.

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