High‑Sensitivity Fe³⁺ Detection Using Ag‑Functionalized TiO₂ Nanotube Arrays via Anodic Stripping Voltammetry

Background

Iron is vital for oxygen transport but excess Fe³⁺ generates reactive oxygen species that contribute to diseases such as Alzheimer’s, Wilson’s, and Menkes disorders. Industrial and agricultural activities introduce Fe³⁺ into water bodies, posing global health risks. Conventional techniques (AAS, ICP‑MS, ICP‑ES) demand expensive equipment and extensive sample preparation.

Nanomaterials—graphene, quantum dots, noble metal nanoparticles—have emerged as promising Fe³⁺ sensors due to high selectivity, sensitivity, and operational simplicity. Recent advances include colorimetric detection of Fe³⁺ with Ag nanoparticles (detection limit 80 nM) and gold‑nanoparticle probes (limit 14.8 nM). However, these methods still rely on color changes or optical readouts that can be influenced by matrix effects.

TiO₂ nanotubes offer excellent chemical stability, nontoxicity, and directional charge transport, making them attractive for electrochemical sensing. When functionalized with Ag nanoparticles, TiO₂ nanotubes can further suppress recombination and increase conductivity, potentially enabling trace Fe³⁺ detection.

Methods

Sensing Principle

Photo‑excited electrons in TiO₂ migrate to the surface, transferring charge to adsorbed Fe³⁺ ions. The presence of Ag nanoparticles facilitates electron transfer and reduces recombination, leading to measurable current changes during anodic stripping voltammetry.

Experimental Procedure

Pure Ti sheets (99.9 %) were polished, etched with 40 % HF (0.5 % in deionized water) for 10 s, and ultrasonically cleaned in acetone, ethanol, and water. TiO₂ nanotube arrays were fabricated by anodic oxidation in a glycol‑based electrolyte (2 % H₂O, 0.3 % NH₄F) at 60 V for 2 h, followed by 500 °C calcination.

Ag nanoparticles (99.9999 %) were sputtered onto the nanotubes (60 W, 10⁻³ Torr) for 30–45 s. X‑ray photoelectron spectroscopy (Shimadzu AMICUS) confirmed Ag⁰ deposition and changes in Ti, O, and C binding energies.

An electrochemical workstation (CHI660E) performed anodic stripping voltammetry in a three‑electrode cell. Fe³⁺ standards (10–50 µg L⁻¹) were prepared in ammonium chloride. The potential was swept from –1 V to +1 V (0.005 V steps) at 10⁻⁵ A resolution over 120 s.

Results and Discussion

Nanotube Morphology and Ag Deposition

SEM images (Fig. 2) show uniform TiO₂ nanotubes (~50 nm diameter, 19.2 µm length) with a 30° tilt, providing a large contact area for Fe³⁺ adsorption.

After 30 s of Ag sputtering, SEM (Fig. 3) reveals ~10 nm Ag particles uniformly distributed on the nanotube walls. Longer deposition times (35–45 s) lead to particle clustering and larger diameters (20–25 nm).

XPS analysis (Fig. 4) demonstrates Ag⁰ presence and shifts in Ti, O, and C peaks, indicating strong TiO₂–Ag interaction, reduced lattice oxygen, and lowered surface carbon contamination.

Fe³⁺ Detection Performance

Stripping voltammograms (Fig. 5) exhibit a linear relationship between peak current and Fe³⁺ concentration (10–50 µg L⁻¹). The slope is 37.3 µA µg⁻¹ L, corresponding to a detection limit of 15 nM.

With Ag nanoparticles (30 s deposition), the current response increases by ~20 % (Fig. 7), achieving a maximum sensitivity of –1.38 × 10⁻⁴ A. This enhancement stems from improved electron transfer and reduced recombination.

Competitive tests with other metal ions confirm selective Fe³⁺ response, as evidenced by unchanged SPR and UV‑vis spectra when Fe³⁺ is absent.

Conclusions

Well‑aligned TiO₂ nanotube arrays functionalized with Ag nanoparticles constitute a low‑cost, highly selective, and sensitive platform for Fe³⁺ detection. The method delivers a 15 nM detection limit and operates under ambient conditions without sample pretreatment, offering a promising route for real‑time monitoring in biological and environmental samples.

Abbreviations

CB:

Conductance band

FWHM:

Full width at half maximum

NALC:

N‑acetyl‑L‑cysteine

NPs:

Nanoparticles

ROS:

Reactive oxygen species

SEM:

Scanning electron microscope

Ti:

Titanium

VB:

Valence band

XPS:

X‑ray photoelectron spectrometer