Whispering Gallery Mode Biosensors Research Articles

Resonance fluctuations of a whispering gallery mode biosensor by particles undergoing Brownian motion

Nanoparticles suspended in the vicinity of a whispering gallery mode (WGM) biosensor are detected from fluctuations in the driving light-guide transmission. These fluctuations are described by Brownian particles perturbing the resonance wavelength in reaction to being polarized by the WGM’s evanescent field. Comparison between the autocorrelation of the measured fluctuations and theory provides a first order approximation for the nanoparticle size and lays the basis for future studies of interfacial dynamics. With this advance, the WGM biosensor goes beyond low-frequency measurements of adsorption and desorption and into a world which has been dominated by fluorescence correlation spectroscopy, but without labels.

Enhancing sensitivity of a whispering gallery mode biosensor by subwavelength confinement

The authors demonstrate enhanced sensitivity of a spherical whispering gallery mode biosensor (WGMB) by confining orbiting light near the surface using a subwavelength (sub-λ) high refractive index layer on a fluorine doped silica microsphere (radius R∼200μm). Their experiments at a free space wavelength λ∼1310nm show that the frequency shift sensitivity by changing the external refractive index is increased by more than 700% by adding a 340nm thick polystyrene layer. This advance is expected to move the WGMB well into the lead as the most sensitive method for unlabeled biosensing.

Simulation of whispering-gallery-mode resonance shifts for optical miniature biosensors

Finite element analyses are made of the shifts of resonance frequencies of whispering-gallery-mode (WGM) for a fiber–microsphere coupling miniature sensor. The time-domain Maxwell's equations were adopted to describe the near-field radiation transport and solved by the in-plane TE waves application mode of the FEMLAB. The electromagnetic fields as well as the radiation energy distributions can be easily obtained by the finite element analysis. The resonance intensity spectrum curves in the frequency range from 213 to 220 THz were studied under different biosensing conditions. Emphasis was put on the analyses of resonance shift sensitivity influenced by changes of the effective size of the sensor resonator (i.e., microsphere) and/or the refractive index of the medium surrounding the resonator. It is estimated that the WGM biosensor can distinguish molecular size change to the level of 0.1 nm and refractive index change in the magnitude of ∼ 10 - 3 even with the use of a general optical spectrum analyzer of one GHz linewidth. Finally, the potential of the WGM miniature biosensor for monitoring peptide growth is investigated and a linear sensor curve is obtained.

Multiplexed DNA Quantification by Spectroscopic Shift of Two Microsphere Cavities

We have developed a novel, spectroscopic technique for high-sensitivity, label-free DNA quantification. We demonstrate that an optical resonance (whispering gallery mode) excited in a micron-sized silica sphere can be used to detect and measure nucleic acids. The surface of the silica sphere is chemically modified with oligonucleotides. We show that hybridization to the target DNA leads to a red shift of the optical resonance wavelength. The sensitivity of this resonant technique is measured as 6pg/mm2 mass loading, higher as compared to most optical single-pass devices such as surface plasmon resonance biosensors. Furthermore, we show that each microsphere can be identified by its unique resonance wavelength. Specific, multiplexed DNA detection is demonstrated by using two microspheres. The multiplexed signal from two microspheres allows us to discriminate a single nucleotide mismatch in an 11-mer oligonucleotide with a high signal-to-noise ratio of 54. This all-photonic whispering gallery mode biosensor can be integrated on a semiconductor chip that makes it an easy to manufacture, analytic component for a portable, robust lab-on-a-chip device.