Purpose: The objective of this study was to develop a rapid, sensitive method to detect food contaminants by using highly specific aptamers coupled with AuNPs to perform colorimetric and SERS measurements.
Methods: Aptamers (ssDNA) specific to ampicillin and phorate were mixed with their target at varying concentration ratios (up to 1:100) and incubated for 10 min (n = 3). Next, 13 nm citrate-capped AuNPs was added and incubated for 5 min. Finally, NaCl was added. If no target was present, the aptamers would physically adsorb on AuNPs and prevent aggregation. However, if target was present, the aptamer would bind to target instead, causing aggregation of AuNPs (red to purple/blue). One µl of sample was deposited and dried on gold slide for Raman analysis. The absorbance measurements of the samples were analyzed between 450 - 750 nm. Total detection time was less than 40 min.
Results: SERS was more sensitive than colorimetric measurements and could detect as low as 100 nM. Sample spectra showed new peaks and target peak shifts, owing to molecular interactions between aptamer and target, which was confirmed using principal component analysis. Our results also suggest colorimetric measurements were not very sensitive possibly due to co-interaction between aptamer-AuNPs and aptamer-target. Method optimization may help to further increase sensitivity of the colorimetric assay.
Significance: This method can potentially be used for rapid screening of small molecules such as antibiotics and pesticides. We can also validate colorimetric measurements with SERS.