Two-photon luminescence imaging of Bacillus spores using peptide-functionalized gold nanorods

Bacillus subtilis spores (a simulant of Bacillus anthracis) have been imaged by two-photon luminescence (TPL) microscopy, using gold nanorods (GNRs) functionalized with a cysteine-terminated homing peptide. Control experiments using a peptide with a scrambled amino acid sequence confirmed that the GNR targeting was highly selective for the spore surfaces. The high sensitivity of TPL combined with the high affinity of the peptide labels enables spores to be detected with high fidelity using GNRs at femtomolar concentrations. It was also determined that GNRs are capable of significant TPL output even when irradiated at near infrared (NIR) wavelengths far from their longitudinal plasmon resonance (LPR), permitting considerable flexibility in the choice of GNR aspect ratio or excitation wavelength for TPL imaging.