Evanescent wave cavity ring-down spectroscopy based interfacial sensing of prostate-specific antigen

Evanescent wave cavity ring-down spectroscopy (EW-CRDS) is implemented as a sensitive probe for the detection of prostate-specific antigen (PSA) with a sub-femtomolar limit of detection (LOD). PSA, a glycoprotein, is an essential biomarker for scrutinizing prostate cancer. Gold nanoparticles (Au NPs) were synthesized to serve as a sensitive reporter which are strongly bound to the thiol-modified target DNA. While Au NPs/target DNA flowed via a microfluidic channel, the target DNA may hybridize with aptamer DNA immobilized on the silica surface. Thus, the adsorption kinetics of hybridization between the Au NPs/target and aptamer was measured with interfacial optical absorbance using Langmuir fit of adsorption equilibrium. The LOD of Au NPs/PSA sensor reaches 0.54 fM signifying tremendous detecting proficiency of EW-CRDS. The evanescent wave is highly sensitive for small changes in absorption of material leading to a significant variation of the ring-down time constant. Moreover, Au NPs possess inherent excellent surface plasmonic properties that made them a promising probe material for the interfacial sensing process. To assess the significance of probe material, 5-carboxytetramethylrhodamine was substituted as a reporter to determine sensing kinetics, showing LOD of 99.1 fM. EW-CRDS approach could be used as an efficient sensing technique for the examination of malignant diseases and many biological reactions.