The Detection of Helicobacter hepaticus Using Whispering-Gallery Mode Microcavity Optical Sensors

Abstract

Current bacterial detection techniques are relatively slow, require bulky instrumentation, and usually require some form of specialized training. The gold standard for bacterial detection is culture testing, which can take several days to receive a viable result. Therefore, simpler detection techniques that are both fast and sensitive could greatly improve bacterial detection and identification. Here, we present a new method for the detection of the bacteria Helicobacter hepaticus using whispering-gallery mode (WGM) optical microcavity-based sensors. Due to minimal reflection losses and low material adsorption, WGM-based sensors have ultra-high quality factors, resulting in high-sensitivity sensor devices. In this study, we have shown that bacteria can be non-specifically detected using WGM optical microcavity-based sensors. The minimum detection for the device was 1 × 10(4) cells/mL, and the minimum time of detection was found to be 750 s. Given that a cell density as low as 1 × 10(3) cells/mL for Helicobacter hepaticus can cause infection, the limit of detection shown here would be useful for most levels where Helicobacter hepaticus is biologically relevant. This study suggests a new approach for H. hepaticus detection using label-free optical sensors that is faster than, and potentially as sensitive as, standard techniques.

Keywords: H. hepaticus; bacterial detection; microcavities; optical transducing mechanisms; sensors.

Figure 1

A model for WGM optical microcavity detection, based on [41] (adapted with permission). In the top image, light (thick black line around the device) enters a WGM optical microcavity, where it experiences total internal reflection (TIR) and generates an evanescent field. The evanescent field is an optical field extending to the surrounding environment and decreasing exponentially with the distance away from the resonator’s interface. When an analyte (red sphere), such as bacteria, binds or adsorbs onto the surface of the microsphere, it changes the effective refractive index of the circulating optical field resonator, and it pulls part of the evanescent field to the outside of the resonator. The expansion of the optical field’s boundary causes the round-trip wavelength of light to increase about 2πΔl. The increase in the optical field’s wavelength results in a corresponding frequency shift in the transmission spectrum (bottom image).

Figure 2

Microscopic images of the tip of a single mode optical fiber before and after gravimetric melting with a CO₂ laser.

Figure 3

A model of the open-flow flow cell.

Figure 4

A representative resonance peak of the silica microspheres used as the WGM optical microcavities in the sensing experiments, showing a high quality factor device (black line—data, red line—Lorentzian fit) during testing in air.

Figure 5

An overlay of the wavelength shift over time for a single representative silica microsphere (sphere 4, referenced in Table 1 and Table 2) used in the sensing experiments.