Comparison of Fe₂O₃ and Fe₂CoO₄ core-shell plasmonic nanoparticles for aptamer mediated SERS assays

Conjugation of oligonucleotides or aptamers and their corresponding analytes onto plasmonic nanoparticles mediates the formation of nanoparticle assemblies: molecularly bound bundles of nanoparticles which cause a measurable change in the colloid's optical properties. Here, we present further optimization of a "SERS off" competitive binding assay utilizing plasmonic and magnetic nanoparticles for the detection of the toxin bisphenol A (BPA). The assay involves 1) a 'target' silver nanoparticle functionalized with a Raman reporter dye and PEGylated BPA-binding DNA aptamers, and 2) a version of the toxin BPA, bisphenol A diglycidyl ether (BADGE), PEGylated and immobilized onto a silver coated magnetic 'probe' nanoparticle. When mixed, these target and probe nanoparticles cluster into magnetic dimers and trimers and an enhancement in their SERS spectra is observed. Upon introduction of free BPA in its native form, target AgNPs are competitively freed; reversing the nanoparticle assembly and causing the SERS signal to "turn-off" and decrease in response to the competitive binding event. The assay particles were housed inside two types of optofluidic chips containing magnetically active nickel pads, in either a straight or spotted pattern, and both Fe₂O₃ and Fe₂CoO₄ were compared as magnetic cores for the silver coated probe nanoparticle. We found that the Ag@ Fe₂O₃ particles were, on average, more uniform in size and more stable than Ag@ Fe₂CoO₄, while the addition of cobalt significantly improved the collection time of particles within the magnetic chips. Using 3D Raman mapping, we found that the straight channel design with the Ag@ Fe₂O₃ particles provided the most uniform nanoparticle organization, while the spotted channel design with Ag@ Fe₂CoO₄ demonstrated a larger SERS enhancement, and thus a lower limit of detection.