<title>High-Q whispering-gallery mode sensor in liquids</title>

Abstract

Optical sensing of biomolecules on microfabricated glass surfaces requires surface coatings that minimize nonspecific binding while preserving the optical properties of the sensor. Microspheres with whispering-gallery (WG) modes can achieve quality factor (Q) levels many orders of magnitude greater than those of other WG-based microsensors: greater than 10<SUP>10</SUP> in air, and greater than 10<SUP>9</SUP> in a variety of solvents, including methanol, H<SUB>2</SUB>O and phosphate buffered saline (PBS). The presence of dyes that absorb in the wavelength of the WG excitation in the evanescent zone can cause this Q value to drop by almost 3 orders of magnitude. Silanization of the surface with mercapto-terminal silanes is compatible with high Q (&gt;10<SUP>9</SUP>), but chemical cross-linking of streptavidin reduces the Q to 10<SUP>5</SUP>-10<SUP>6</SUP> due to build-up of a thick, irregular layer of protein. However, linkage of biotin to the silane terminus preserves the Q at a ~2x10<SUP>7</SUP> and yields a reactive surface sensitive to avidin-containing ligands in a concentration-dependent manner. Improvements in the reliability of the surface chemistry show promise for construction of an ultrasensitive biosensor.