Luminescent porous silicon decorated with iron oxide nanoparticles synthesized by pulsed laser ablation

Abstract

Nanomaterials are playing an increasingly prominent role in recent biomedical applications, particularly due to their promising potential to combine diagnostic and therapeutic functions within a single multifunctional carrier. In this context, intrinsically luminescent silicon nanostructures offer a compelling alternative to conventional fluorophores. Their integration with magnetic nanoparticles could pave the way for the development of a traceable, multimodal platform in the field of nanomedicine. With this objective, we investigated the decoration/infiltration of light-emitting porous silicon (pSi) with iron oxide nanoparticles (FeO _x NPs) synthesized by pulsed laser ablation at two different liquid-gas interfaces: water-air (FeO _x NPs-Air), and water-argon (FeO _x NPs-Ar). This kind of polydispersed NPs are well-suited to filling the wide pore size range of the porous network. Moreover, their intrinsic positive surface charge enables straightforward and direct interaction with negatively charged carboxyl-functionalized porous silicon, without requiring additional surface modifications, chemical agents, or time-consuming intermediate processing steps such as the thermal oxidation or dehydration procedures reported in previous studies. The effectiveness of this simple infiltration/decoration approach-achieved through basic chemical mixing in a standard container-was successfully demonstrated by electron microscopies, Z-potential, optical, and magnetization experiments, which indicate a ferromagnetic behavior of the porous Si FeO _x nanocomposites (pSi + FeO _x NCs). The optical emission properties of the pSi + FeO _x NCs were maintained with respect to the bare ones, although slightly less intense and blue-shifted (about 15 nm), in agreement with the change of radiative lifetime from about 30 μs to 20 μs. Magnetic measurements reveal that pSi + FeO _x NCs obtained using FeO _x NPs synthesized at the air-water interface exhibit a weaker, noisier signal with ∼80 Oe coercivity and lower remanence. Conversely, those produced at the argon-water interface show a stronger magnetic response, with ∼170 Oe coercivity and higher remanence. Notably, the magnetic properties of the Ar-synthesized sample remained stable for months without affecting its intrinsic photoluminescence, offering a stable micro-nano optical and magnetic system for theranostics applications.

Fig. 1. TEM Images and size distribution of (A and B) FeO_xNPs–Air and (C and D) FeO_xNPs–Ar.

Fig. 2. Monitoring of the UV-Vis extinction spectra of the FeO_xNPs–Air (A) and FeO_xNPs–Ar (B) along 48 hours.

Fig. 3. Effect of a magnet on the pSi microparticles before (A) and after (B) decoration/infiltration with FeO_xNPs–Ar. Effect of a magnet and UV lamp (365 nm, 6 W) irradiation on the pSi microparticles after decoration with FeO_xNPs–Ar (C). The arrow in yellow indicate the presence of the pSi.

Fig. 4. TEM images. Left panel: pSi portion (500 nm scale). Middle (500 m scale) and right (200 m scale) panel: pSi after infiltration with FeO_xNPs–Ar nanoparticles. Circles and arrows point out the presence of iron oxide nanoparticles.

Fig. 5. Morphological characterization of pSi microparticles decorated with iron oxide NPs. TEM image of (A) pSi + FeO_xNPs–Air (100 nm scale) and (B) pSi + FeO_xNPs–Ar (200 nm scale). (C) Size distributions and (D) surface charge modifications of pSi microparticles before and after decoration with FeO_xNPs (DLS measurements).

Fig. 6. Elemental analyses of FeO_xNPs–Ar sample. Gray: STEM-HAADF image; green: iron map; red: oxygen map; blue: silicon map.

Fig. 7. Photoluminescence properties of pSi microparticles before and after decoration with iron oxide NPs: PL spectra (A) and decay curves (B) at λ_ex 325 nm of bare pSi, pSi + FeO_xNPs–Air and pSi + FeO_xNPs–Ar. In panel C the 3 cuvettes with the samples under UV lamp excitation.

Fig. 8. pSi + FeO_xNPs–Ar_2 characterization. (A) Magnetic pSi in absence and presence of a magnet. (B) and (C) TEM images of pSi + FeO_xNPs–Ar_2 (500 nm scale and 100 nm scale, respectively).

Fig. 9. Magnetic response curves comparing similar masses of pSi decorated with FeO_xNPs–Air (black curve) and pSi decorated with FeO_xNPs–Ar (red curve). These curves display the magnetic moment as a function of the applied magnetic field. The inset provides a zoomed-in view of the low-field region, emphasizing the differences in coercivity and remanence between the two samples.