A physical approach for the estimation of the SERS enhancement factor through the enrichment and separation of target molecules using magnetic adsorbents

Abstract

The controllable synthesis of nanosized Fe₃O₄ (10-20 nm) encapsulated in different numbers of graphene layers (1-5 layers) (Fe₃O₄@DGL NPs) was realized through a facile and green hydrothermal reaction at a temperature as low as 200 °C. The competitive reduction-oxidation between reducing ethylene glycol (EG) and oxidizing H₂O under hydrothermal conditions resulted in the emergence of a magnetic Fe₃O₄ core. Then, the pyrolytic reaction of the polyvinyl alcohol (PVA) molecules attached to the surface of the Fe₃O₄ core with different surface densities led to the formation of graphene with a controlled number of layers. These Fe₃O₄@DGL NPs exhibited fast adsorption and sensitive SERS detection for rhodamine B (RhB). A physical and mathematical model was proposed for the estimation of the enhancement factor (EF) by combining the adsorption efficiency and SERS of RhB. This approach and model are applicable for the adsorption, sensitive SERS detection and determination of SERS EF when using functional magnetic nanoparticles as the adsorbent. The Fe₃O₄@1G NPs were also used as a novel nano-adsorbent for the fast removal of Escherichia coli (E. coli) from an aqueous solution. The Fe₃O₄@1G NPs regenerated after 3 cycles also showed high efficiency in the adsorption and separation of RhB and E. coli.

Scheme 1. Schematic illustration for the synthesis of Fe₃O₄@DGL products.

Fig. 1. (a) Low magnification and (b) corresponding high-resolution TEM images of the Fe₃O₄@1G NPs; (c) and (d) Fe₃O₄@3G NPs; (e) and (f) Fe₃O₄@5G NPs.

Fig. 2. (a) XRD patterns and (b) Raman spectra of the Fe₃O₄@DGL products.

Fig. 3. Magnetization curves of the three Fe₃O₄@DGL products obtained at 300 K.

Fig. 4. (a) SERS spectra of 10⁻³, 10⁻⁴ and 10⁻⁵ mol L⁻¹ RhB adsorbed on Fe₃O₄@1G NPs. (b) SERS spectra of 10⁻³ mol L⁻¹ RhB adsorbed on Fe₃O₄@1G, Fe₃O₄@3G, and Fe₃O₄@5G, respectively.

Fig. 5. Schematic illustration of (a) in situ SERS detection of RhB adsorbed on the Fe₃O₄@1G NPs, and (b) ideally illuminated area and penetration depth profile of the laser spot for the estimation of the number of RhB molecules in the bulk sample.