Enhancement of Magnetic Surface-Enhanced Raman Scattering Detection by Tailoring Fe3O4@Au Nanorod Shell Thickness and Its Application in the On-site Detection of Antibiotics in Water

Abstract

Surface-enhanced Raman scattering (SERS) has become a promising method for the detection of contaminants or biomolecules in aqueous media. The low interference of water, the unique spectral fingerprint, and the development of portable and handheld equipment for in situ measurements underpin its predominance among other spectroscopic techniques. Among the SERS nanoparticle substrates, those composed of plasmonic and magnetic components are prominent examples of versatility and efficiency. These substrates harness the ability to capture the target analyte, concentrate it, and generate unique hotspots for superior enhancement. Here, we have evaluated the use of gold-coated magnetite nanorods as a novel multifunctional magnetic-plasmonic SERS substrate. The nanostructures were synthesized starting from core-satellite structures. A series of variants with different degrees of Au coatings were then prepared by seed-mediated growth of gold, from core-satellite structures to core-shell with partial and complete shells. All of them were tested, using a portable Raman instrument, with the model molecule 4-mercaptobenzoic acid in colloidal suspension and after magnetic separation. Experimental results were compared with the boundary element method to establish the mechanism of Raman enhancement. The results show a quick magnetic separation of the nanoparticles and excellent Raman enhancement for all the nanoparticles both in dispersion and magnetically concentrated with limits of detection up to the nM range (∼50 nM) and a quantitative calibration curve. The nanostructures were then tested for the sensing of the antibiotic ciprofloxacin, highly relevant in preventing antibiotic contaminants in water reservoirs and drug monitoring, showing that ciprofloxacin can be detected using a portable Raman instrument at a concentration as low as 100 nM in a few minutes, which makes it highly relevant in practical point-of-care devices and in situ use.

Figure 1

(A) Schematic representation of the synthesis procedure, starting with the solvothermal synthesis of magnetite NRs, which are modified with APTES, then by electrostatic binding of gold nanoclusters, and a final growth of a gold shell. (B) Schematic representation of the measurement procedure, by adding the NR@Au to an antibiotic-containing solution, magnetic separation and accumulation, and Raman measurement at the pellet spot.

Figure 2

a) TEM image of Fe₃O₄ NRs. (b) TEM image of the NR core-satellite structure. (c) TEM images of core@shell structures synthesized by adding different amounts of Fe₃O₄-NR satellites: NR@Au1 250 μL, NR@Au2 500 μL, NR@Au3 1000 μL, NR@Au4 3000 μL, NR@Au5 5000 μL, and NR@Au6 10,000 μL, from the thickest (NR@Au1) to the least complete (NR@Au6). The scale bar corresponds to 200 nm.

Figure 3

Representative UV–vis spectra of the synthesized NPs described in Figure 1.

Figure 4

BEM simulations of several morphology configurations: (a) Fe₃O₄ NRs functionalized with different amounts of Au nanospheres; (b) smooth (top) and rough (bottom) core@shell Fe₃O₄@Au NRs; (c) tip-to-tip assembled smooth (top) and rough (bottom) core@shell structures; (d) side-to-side assembled rough core@shell structures; (e) average local field enhancements of the different nanostructures (Fe₃O₄ NRs functionalized with different amounts of Au nanospheres randomly located, isolated Au nanospheres, and smooth and rough core@shell structures); and (f) average local field enhancement tip-to-tip assemblies of smooth and rough core@shell NRs compared with the individual ones.

Figure 5

(a) Raman spectra of MBA (0.1 mM) measured using a magnet and without it. (b) Intensity of the first characteristic peak of MBA (1072 cm^–1) at 0.1 mM with each core–shell structure using a magnet and without it. (c) Raman spectra of MBA (1 mM) performed with each of the core–shell structure. (d) Intensity of the first characteristic peak of MBA (1072 cm^–1) at different concentrations measured with each core@shell structure.

Figure 6

(a) Raman spectra of CIP at different concentrations using NR@Au1 as the substrate. (b) Intensity of one characteristic peak of CIP (1382 cm^–1) at different concentrations performed with the NR@Au1 structure.