Nanoplasmonic biodetection based on bright-field imaging of resonantly coupled gold-silver nanoparticles

This study presents an alternative experimental strategy for the colorimetric biodetection based on modulation of the bright-field color of nano-engineered Au-chips through plasmonic coupling instead of conventional detection based on dark-field imaging in colloidal solutions. In this approach, not only the optical detection system is simplified such that it does not require a dark-field condenser lens and precise optical alignment but also the formation of plasmonic colored letters using dip-pen nanolithography makes the proposed strategy a promising candidate for smartphone-based detection and facilitates the detection process by untrained users. For this purpose, e-beam lithography was employed for precise fabrication of the nanoplasmonic chips as solid binding sites comprising gold nano-disks with 120 nm diameter and 25 nm thickness. Dip-pen nanolithography enabled the microscale patterning of probes onto its surface. A color change is induced by plasmonic coupling between silver nanoparticle labels and the gold nano-disk-binding sites, which can be verified by surface-enhanced Raman spectroscopy. Sensitivity investigation shows that at least 150 Ag-nanoparticles are required to observe the bright-field colorimetric change. Dip-pen printing of a noncomplementary probe alongside a complementary probe demonstrates the selectivity of the detection. Patterning of the probes onto the plasmonic surface in the form of a letter in addition to the compatibility of this technology with smartphone platforms for imaging makes this technique desirable for future biomedical applications.