Trace detection of anthrax protective antigens via a competitive method based on surface-enhanced Raman scattering

Owing to the high mortality and infection rates of the Bacillus anthracis infection, commonly known as anthrax, the ability to detecting anthrax protective antigens (PA) is important in several domains, including food safety and bioterrorism. An ultrasensitive PA detection that employs a biosensor using a competitive method based on surface-enhanced Raman scattering (SERS) was developed herein. Au nanorod arrays (AuNRAs), conjugated with the PA-aptamer were prepared as substrates, whereas Au nanoparticles (AuNPs) were modified with 1,2-bis (4-pyridyl) ethylene (BPE) and PA as SERS tags. During detection, free PA and SERS tags bound competitively with active sites on the surface of AuNRAs. The Raman spectra of BPE were used to evaluate the limit of detection (LOD) for PA. Three enhanced Raman modes in this biosensor, namely AuNP self-enhancement, plasmonic resonance between single AuNP and AuNRAs, and nanogaps between two AuNP agglomerates and AuNRAs, were evaluated using the finite-difference time-domain (FDTD) method. Among AuNPs, AuNRAs and PA/BPE/AuNP/PA-apt/AuNRAs complex, the highest Raman signal of the Raman molecule BPE for the PA/BPE/AuNP/PA-apt/AuNRAs complex proved the existence of these three modes. Owing to the increased number of hotspots in biosensors, the ultra-sensitive detection of PA was achieved with an LOD of 1 pg/mL PA.