Iron oxide nanoparticles embedded in porous films for tannic acid detection

A novel method was developed for the detection of tannic acid (TA) by iron oxide (Fe2O3) nanoparticles (NPs) immobilized on the porous films prepared by the modified breath figure (mBF) method. For the mBF method, ferric chloride (FeCl3) was included in the polystyrene polymer solution and sodium borohydride (NaBH4) was added in the humidity to induce an interfacial reaction between the two materials during the BF process. FeCl3 present in the polymer solution is driven towards the water droplets containing NaBH4 due to the coffee stain effect. This phenomenon induces the formation of Fe2O3 NPs at the pore surface as FeCl3 reacts with NaBH4 at the air/polymer solution interface. The morphology of the hybrid films was evaluated by scanning electron microscopy (SEM). The doped Fe2O3 NP on the porous film was analyzed by energy dispersive X-ray (EDS), Fourier transform infrared spectroscopy (FT-IR), UV–vis spectrophotometer, thermogravimetric analyses (TGA), and X-ray diffraction (XRD). The wettability and conductivity of the films were also measured to further support the incorporation of Fe2O3 NP into the porous film. Detection of TA by Fe2O3 NPs is due to the binding of Fe(III) with galloyl groups of TA forming a coordination compound (Fe2O3-TA complex) which induces the color change of the TA solution from yellow to bluish-black for quick and easy naked-eye detection. The developed sensor system yields a limit of detection (LOD) and limit of quantitation (LOQ) of 370 nM and 1230 nM, respectively. The films exhibited excellent stability and selectivity towards TA and showed no sensitivity towards other compounds possessing a similar structure such as gallic acid, phenol, and glucose.