Sapphire microspheres: possible applications to biosensing

Abstract

Circular cavities [1] such as microspheres [2], microdisks [3], and microtoroids [4] with high quality factor whispering gallery modes (WGMs) continue to open up new perspectives in sensing [5-8]. Here, we report an optical setup for a DNA optical biosensor based on sapphire microsphere. The transmitted and elastic scattering intensity at 1510 nm were analyzed from a sapphire microsphere on an optical fiber half coupler. The 0.43 nm angular mode spacing of the WGMs correlated well with the optical size of the sapphire microsphere. The spectral linewidths of the WGMs were on the order of 0.01 nm, which corresponded to quality factors on the order of 104. Afterwards, the sapphire microsphere was modified with DNA and an optical biosensor is demonstrated using an insulating implant material such as sapphire. A probe DNA, consisting of a 36-mer fragment was covalently immobilized on sapphire microsphere and hybridized with a 29-mer target DNA. WGMs were monitored prior to the sapphire’s functionalization with DNA and after the sapphire was functionalized with single stranded DNA (ssDNA) and double stranded DNA (dsDNA).