The trace gas direction holds practical significance in human health, industrial production safety, national defense, and other key fields. The optical fiber whisper gallery mode (WGM) sensors can achieve high sensitivity and resolution sensing measurement due to their strong light-matter interaction. However, the common silica material of WGM sensors is not sensitive to gases, which limits their applications in gas sensing. As a kind of two-dimensional material, graphene oxide (GO) not only has sound physical properties such as high mechanical strength and flexibility, but also features a significant surface volume ratio, efficient surface adsorption, low noise level, and stable chemical properties. Based on optical WGM excitation, the GO film is coated on a hollow microsphere cavity inwall to achieve gas sensing. The gas molecule adsorption on the GO will affect the effective refractive index of the overall microcavity structure and be reflected by the WGM shift. It is worth noting that the unique hollow structure of the microbubble is a natural fluid channel, which is very suitable for gas transportation. It is unnecessary to design a separate fluid channel or external gas chamber.
The investigation is based on the WGM sensor theory. The changed refractive index induced by gas molecular adsorption is analyzed. The sensors are fabricated by melt pressured rheological method and injection of GO dispersion. First, the performance of the GO-coated WGM gas sensor is investigated, and the changes in WGM resonance wavelength are observed by injecting gases with different concentrations into the sensor. Next, the gas sensing performance below 40×10-6 is elaborately investigated. The sensitivity and resolution of the sensor are obtained. Finally, the real-time response to 10×10-6-40×10-6 NH3 is demonstrated to show the sound recoverability, response, and recovery time.
The designed GO-coated microbubble sensor exhibits deserved gas sensing performance. Fig. 4 shows the WGM spectrum of the structure with different gas concentrations. The resonance wavelength appears to be red-shifted as the gas concentration increases, and this trend is gradually slowing down. The optical quality factor Q is 3.7×105. Specifically, for the low concentrations from 0 to 40×10-6, the sensitivity is 0.73×106 pm with a fitting coefficient of 0.9994 (Fig. 5). According to the standard deviation of center wavelength fluctuations, detection resolution of the gas sensor is better than 1.9×10-6. The temperature response performance is shown in Fig. 6, and the response is 10.88 pm/℃. Finally, the time response of the gas sensor at low concentrations is shown in Fig. 7. At the concentration of 20×10-6, the response time and recovery time are 294 s and 329 s respectively.
We design a kind of gas sensor based on a GO-coated microbubble. The gas molecule adsorption affects the refractive index of GO and changes the overall effective refractive index of the microcavity sensor correspondingly. Gas sensing can be achieved by monitoring the WGM shifts via a power meter. The sensors are fabricated by melt pressurized stretching and injection of GO dispersion. The sensitivity is 0.73×106 pm within a gas concentration below 40×10-6. According to the wavelength drift standard deviation of the overall system, the resolution is 1.9×10-6. At the gas concentration of 20×10-6, the response time and recovery time of the sensor are 294 s and 329 s respectively. Meanwhile, the hollow sensor structure does not need additional gas channels or gas chamber packaging structures during gas sensing, thus providing convenience for practical applications.
