Fabrication and characterization of electrospun semiconductor nanoparticle-polyelectrolyte ultra-fine fiber composites for sensing applications

Abstract

Fluorescent composite fibrous assembles of nanoparticle-polyelectrolyte fibers are useful multifunctional materials, utilized in filtration, sensing and tissue engineering applications, with the added benefits of improved mechanical, electrical or structural characteristics over the individual components. Composite fibrous mats were prepared by electrospinning aqueous solutions of 6 wt% poly(acrylic acid) (PAA) loaded with 0.15 and 0.20% v/v, carboxyl functionalized CdSe/ZnS nanoparticles (SNPs). The resulting fluorescent composite fibrous mats exhibits recoverable quenching when exposed to high humidity. The sensor response is sensitive to water concentration and is attributed to the change in the local charges around the SNPs due to deprotonation of the carboxylic acids on the SNPs and the surrounding polymer matrix.

Keywords: composite-nanofibers; electrospinning; nano-effects; polyelectrolytes.

Figure 1.

(a) Chemical structure of poly acrylic acid; (b) Schematic diagram of the electrospinning apparatus.

Figure 2.

Composite fiber characterization: The SEM micrographs display smooth uniform diameter fibers for the composite fibers (a,b) and pristine fibers (c). The mean fiber diameters for both composite populations (d,e) are within one standard deviation. The pristine fibers (f) are nanoscale but have spindle like defects. The composite fibers (g,h) are fluorescent as shown in the false colored fluorescence micrographs. (SEM scale bar = 500 nm and fluorescence micrograph scale bar = 50 μm).

Figure 3.

TEM micrograph of a composite fiber. Arrows are pointing to single and small clusters of SNPs in the fiber.

Figure 4.

(a) Absorption (Photoluminescence Excitation); and (b) Band edge PL spectra (λ_ex = 366 nm) of 0.20% v/v Qdot 525 in aqueous PAA solution and composite fibers; (c) PL spectra (λ_ex = 405 nm) of composite fibers with new peaks due to confinement; (d) Fluorescence decay curves of 0.20% v/v Qdot 525 in aqueous PAA. The extracted excited lifetimes are as follows: 20 ns ± 2 ns, 19 ns ± 4 ns, and 14 ns ± 2 ns for the Qdot 525 in H₂O, Qdot 525 in aqueous PAA and the composite fibers respectively.

Figure 5.

Photograph of the flourometer configured for sensing experiments. The cuvette (a) was modified with a Teflon adapter to secure the fibrous composite mat. For the humidity experiments, a 10 mL syringe was used to introduce the humid (b) air at the inlet and the second syringe was used to withdraw the air through the exhaust.

Figure 6.

The normalized integrated intensity of the PL peaks to assess the response of the composite mat to changes in pH. Each blue bar represent the median area under the curve and the upper error bar represents the largest area and the lower error bar represents the lowest area in of the three spectra taken. The bars labeled Air represent the spectra taken after the mat was left to dry. Inset (a) displays the PL spectra curves. Inset (b) display the normalized integrated intensities in order of increasing pH. The toluene and pyridine spectra in this graph are adjusted by 38% to account for the overall decrease in intensity possibly caused by movement of the mat during the experiment.

Figure 7.

Plot of the magnitude change of the intensity of the band edge emission at λ = 523 when humid air is injected into the cuvette. (a) The plot on the right (b) illustrates the repeatability and volume sensitivity of the sensor’s response. In plot b, curve 1 represents two mL of moist air and curve 2 represents one mL of moist air.

Figure 8.

SEM micrograph (a) of the composite fibers imaged right after they were electrospun. Images (b–e) are ESEM micrographs of the composite fibers experiencing different ambient moisture contents.

Figure 9.

SEM micrographs of composite mats (a) as spun and (b) after soaking in toluene.

Figure 10.

FT-IR spectra of the composite mats. In the spectra on the right, the presence of the shoulder indicated the carboxylic acids are deprotonated.