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.