Development of a Label-Free LSPR-Apta Sensor for Staphylococcus aureus Detection

The risk of foodborne diseases has increased over the last years. We have developed a simple, portable, and label-free optical sensor via aptamer recognition of Staphylococcus aureus at nanostructured plasmonic elements. The developed aptamers conjugated to a localized surface plasmon resonance (LSPR) sensing device were applied in both pure culture and artificially contaminated milk samples enabling a limit of detection of 10³ CFU/mL for S. aureus in milk. There was no need for a pre-enrichment step, and the total analysis time decreased from 30 min to 120 s. Finite-difference time-domain was used to simulate the experimentally measured optical responses for a range of different sensor designs (100 and 200 nm disks), addressing the role of the near field and intrinsic refractive index sensitivity. A comparison of the aptamer to antibody-based recognition approaches showed that the thickness of the sensing layer was critical with a significantly larger response for the thinner aptamer layer. Comparison of differently sized metal nanostructures showed a significantly higher sensitivity for 200 nm diameter compared to 100 nm diameter disk structures resulting from both increases in bulk refractive index sensitivity and the extent to which the local field extends out from the metal surface. These findings confirmed that the developed gold nanodisk-based LSPR sensing chips could facilitate sensitive detection of S. aureus in food samples.