Photonic detection and characterization of DNA using sapphire microspheres

Abstract

Abstract. A microcavity-based deoxyribonucleic acid (DNA) optical biosensor is demonstrated for the first time using synthetic sapphire for the optical cavity. Transmitted and elastic scattering intensity at 1510 nm are analyzed from a sapphire microsphere (radius 500 μm, refractive index 1.77) on an optical fiber half coupler. The 0.43 nm angular mode spacing of the resonances correlates well with the optical size of the sapphire sphere. Probe DNA consisting of a 36-mer fragment was covalently immobilized on a sapphire microsphere and hybridized with a 29-mer target DNA. Whispering gallery modes (WGMs) were monitored before the sapphire was functionalized with DNA and after it was functionalized with single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA). The shift in WGMs from the surface modification with DNA was measured and correlated well with the estimated thickness of the add-on DNA layer. It is shown that ssDNA is more uniformly oriented on the sapphire surface than dsDNA. In addition, it is shown that functionalization of the sapphire spherical surface with DNA does not affect the quality factor (Q ≈ 10 4 ) of the sapphire microspheres. The use of sapphire is especially interesting because this material is chemically resilient, biocompatible, and widely used for medical implants.

Whispering gallery mode sensors

“…The choice of resonator material is hence also a crucial factor in sensor design, motivating the search for novel materials as a means to improve sensor performance and cost. Recent developments in this context for sensing purposes include the use of titanium dioxide (TiO 2 ) [58], silicon nitride (Si 2 N 3 ) [59], silicon carbide (SiC) [60], hydrogenated amorphous silicon [61], poly(methyl methacrylate) (PMMA) [47], polydimethylsiloxane (PDMS) [62], magnesium fluoride (MgF 2 ) [63,64], sapphire [65,66], and liquid (paraffin oil) droplets [67,68]. Composite resonators, such as coated spheres, and hybrid metallo-dielectric resonators, have also recently been investigated as a route to improving sensor robustness and signal enhancement, e.g., [69–72].…”

“…The ability to detect small changes in refractive index has led researchers to measure low concentrations of analyte materials in solution or as an adsorbed layer, e.g., [66,147,148]. Naturally the question arises as to whether single particle or molecule sensitivity is achievable.…”

Photonic studies on polymer-coated sapphire-spheres: A model system for biological ligands

“…We also verified that PDA layers are homogeneous which resulted in high Q factors, comparable with the values achieved with silica spheres coated with polylactic-acid (PLLA, a crystalline biodegradable polymer) [13], showing that PDA coating is a valid alternative method to silanization for coupling bioreceptors such as DNA, proteins and even cells to the sphere. The experimental feasibility of the photonic biosensing concept with sapphire microspheres was recently proven within DNA-characterization studies using a silane crosslinker between DNA and the sapphire spheres [48]. The PDA coatings described in this article are expected to offer even more experimental flexibility due to the numerous available coupling protocols between organic-and biological compounds.…”

Label-Free Biological and Chemical Sensing Using Whispering Gallery Mode Optical Resonators: Past, Present, and Future

“…In terms of cavity material, sapphire resonator biosensors have been used to detect DNA [64]. Liquid droplet resonators have been used to measure concentrations of oil.…”