Multifunctional Gas and pH Fluorescent Sensors Based on Cellulose Acetate Electrospun Fibers Decorated with Rhodamine B-Functionalised Core-Shell Ferrous Nanoparticles

Abstract

Ferrous core-shell nanoparticles consisting of a magnetic γ-Fe₂O₃ multi-nanoparticle core and an outer silica shell have been synthesized and covalently functionalized with Rhodamine B (RhB) fluorescent molecules (γ-Fe₂O₃/SiO₂/RhB NPs). The resulting γ-Fe₂O₃/SiO₂/RhB NPs were integrated with a renewable and naturally-abundant cellulose derivative (i.e. cellulose acetate, CA) that was processed in the form of electrospun fibers to yield multifunctional fluorescent fibrous nanocomposites. The encapsulation of the nanoparticles within the fibers and the covalent anchoring of the RhB fluorophore onto the nanoparticle surfaces prevented the fluorophore's leakage from the fibrous mat, enabling thus stable fluorescence-based operation of the developed materials. These materials were further evaluated as dual fluorescent sensors (i.e. ammonia gas and pH sensors), demonstrating consistent response for very high ammonia concentrations (up to 12000 ppm) and fast and linear response in both alkaline and acidic environments. The superparamagnetic nature of embedded nanoparticles provides means of electrospun fibers morphology control by magnetic field-assisted processes and additional means of electromagnetic-based manipulation making possible their use in a wide range of sensing applications.

Figure 1

Schematic presentation of the experimental setup employed in ammonia gas sensing and pH sensing experiments (A) and of the ammonia gas sensing set-up (B).

Figure 2

TEM images of γ-Fe₂O₃/SiO₂/RhB NPs at low (A) and high (B) magnification, room-temperature measurement of the magnetization as a function of magnetic field strength (C), and intensity-weighted distribution of the γ-Fe₂O₃/SiO₂/RhB NPs hydrodynamic diameters obtained from the DLS measurements in ethanol-based suspension at concentration 1 mgmL⁻¹ (D). Emission spectra of γ-Fe₂O₃/SiO₂/RhB NPs with high (9.33 µmol of RhB) and low (3.11 µmol of RhB) RhB loading (E). Emission spectra of the γ-Fe₂O₃/SiO₂/RhB NPs and of the aqueous solution containing free RhB molecules (F).

Figure 3

Left: Schematic of the electrospinning set-up used in the fabrication of electrospun CA fibers. Right: Indicative photographs of the γ-Fe₂O₃/SiO₂/RhB NPs-functionalized CA fibers obtained via spray deposition of the γ-Fe₂O₃/SiO₂/RhB NPs onto the fibers’ surfaces – sprayed magnetic fibers – (up) and by mixing of the γ-Fe₂O₃/SiO₂/RhB NPs dispersion with the polymer solution prepared in acetone, followed by electrospinning – electrospun magnetic fibers – (down).

Figure 4

SEM images of the as prepared CA fibers (A), the sprayed magnetic fibers (B) and the electrospun magnetic fibers (C).

Figure 5

TEM images of the electrospun magnetic fibers (A–C) and the sprayed magnetic fibers (E–G). Fibers diameter distributions were determined by analysis of the TEM images corresponding to the electrospun magnetic fibers (D) and the sprayed magnetic fibers (H).

Figure 6

Fluorescence microscopy image of the sprayed magnetic fibers (A) and the electrospun magnetic fibers (B).

Figure 7

Photoluminescence spectra of the γ-Fe₂O₃/SiO₂/RhB NPs-functionalized CA fibers obtained via spray deposition (via spray deposition and electrospinning) (excitation wavelength: 520 nm).

Figure 8

A room-temperature measurement of the magnetization as a function of magnetic field strength of the γ-Fe₂O₃/SiO₂/RhB NPs-functionalized CA fibers obtained via spray deposition (A) and electrospinning (B).

Figure 9

Sensing (fluorescence quenching) mechanism of RhB molecules undergoing structural changes when exposed to NH₃ vapours, resulting to the generation of the non-fluorescent lactone form.

Figure 10

(A) Fluorescence spectra of the electrospun magnetic fibers for different concentrations of ammonia gas. (B) Response of the electrospun magnetic fibers at 577 nm for different ammonia concentrations. (C) Fluorescence spectra of the sprayed magnetic fibers for different concentrations of ammonia gas. (D) Response of the sprayed magnetic fibers at 577 nm for different ammonia concentrations.

Figure 11

(A) Fluorescence spectra of the electrospun magnetic fibers for acidic aqueous solutions with different pH values. (B) Fluorescence intensity at 580 nm versus pH. (C) Fluorescence spectra of the electrospun magnetic fibers for basic aqueous solutions with different pH values. (D) Fluorescence intensity at 580 nm versus pH.

Figure 12

Reversibility of the pH-dependent on-off-on fluorescence intensity profile of electrospun magnetic fibers. The intensity was normalised to the intensity measured at pH = 7. The response time of the pH sensor is 30 s as estimated by the fluorescence measurements instrumentation.