Diabetes is a kind of non-infectious human metabolic disease caused by the inability to secrete insulin or secrete insulin normally. In recent years, the prevalence of diabetes has become much higher than before. In order to improve the life of patients and save the medical resources, it is necessary to further optimize the detection methods of diabetes. Human breath contains numerous information related to health. The breath of healthy people contains hundreds of volatile organic compounds (VOCs), whose composition and concentration can reflect the metabolism of a human body. The concentration of glucose in the blood of diabetic patients can be reflected by detecting the acetone concentration in breath. At present, the acetone detection methods mainly include gas chromatography, mass spectrometry, and laser spectroscopy. The required equipment is usually expensive and complex, which limits their application as a daily breath detection method for diabetic patients. As an ultra-high-precision 3D printing technology, femtosecond-laser induced two-photon polymerization microfabrication has been widely used in the fields of photonics, micromachines, microfluidics, and biomedicine, due to its high manufacturing precision and flexibility. Combining the two-photon polymerization technology with fiber optical sensing, the developed integrated device has the characteristics of high precision and flexibility, which provides a new solution for gas sensing. In this study, we report an acetone gas sensing method based on the polymer micropillar structure fabricated on the end face of a fiber. The absorption of acetone molecules by the photoresist micropillars polymerized on the end face of the fiber leads to the change of refractive index of the polymer-acetone mixed material, and subsequently makes the reflection interference spectrum from the Fabry-Perot interferometer (composed of the photoresist micropillars and the fiber end face) shift.

Two-photon polymerization is used to prepare the polymerized micropillars on the end face of a single-mode fiber. First, the single-mode fiber is cut flatly and placed on a glass slide. Then the cut end face is immersed in the negative photoresist. The sample is fixed on a three-dimensional air-floating displacement platform and polymerized by a femtosecond laser. After the laser polymerization process is complete, the sample is developed in a mixed solution of acetone and isopropanol. After the development, the micropillar structure is firmly adhered on the end face of the optical fiber. The morphology of the fabricated device is characterized by scanning electron microscope. The reflection spectrum of the polymerized micropillar sensor is measured by a broadband light source and a spectrum analyzer. The reflection spectra and free spectral range (FSR) values of the micropillar at different heights are compared. The spectral response of the sensor to acetone concentration is also tested. The temperature response of the sensor and the influence of temperature change on acetone concentration in the daily breath detection are studied.

In this study, the micropillar with a designed height of 20 μm is first characterized by scanning electron microscopy. The measured height of the micropillar is about 19.6 μm after polymerization (Fig. 3). The reflection spectra of micropillars with different heights and their FSR values at ~1550 nm are tested, showing that the heights of the micropillar devices are inversely proportional to their FSR values (Fig. 4). The spectral response of the sensor with a micropillar height of 30 μm to acetone concentration is measured. In the concentration range of 1×10^-9 to 1×10^-3, the reflection spectrum shifts by 5 nm. The mechanism is that after the polymerized micropillar absorbs acetone gas molecules, the material refractive index increases (Fig. 5). The trough wavelength decreases with the increase of acetone concentration in the environment. When the acetone gas concentration is less than 1×10^-7, the decreasing trend of trough wavelength becomes slow. This is because when the acetone gas concentration is very low, the acetone molecules absorbed by the polymerized material are limited, which is close to the lower detection limit of the sensor. Thus, the detection limit of the sensor is determined to be 1×10^-9. The sensor is repeatedly tested using acetone gas with the concentration range of 1×10^-9 to 1×10^-3. The blue shift of the sensor near 1550 nm in the reflection spectrum and the red shift of the sensor near 1550 nm are very similar, and both are with a close standard deviation of wavelength shift (less than 5%) (Fig. 6). The temperature response of the sensor from 25 ℃ to 55 ℃ is also studied. The reflection spectrum of the sensor shows a red shift with the increase of temperature. The reason is that with the increase of ambient temperature, the thermal expansion of polymerized micropillars leads to the increase of the FPI cavity length. During the detection, the temperature change introduces a large influence on the detection results of acetone concentration. It is necessary to strictly control the temperature of the gas to be tested and ensure the detection stability of acetone concentration (Fig. 7).

In this paper, the method based on two-photon polymerization for polymerizing micropillars on end face of a fiber and constructing an FPI acetone gas sensor is proposed. The sensor has a detection range of 1×10^-9 to 1×10^-3 for acetone concentration and a detection limit of 1×10^-9. This work provides a new, high-sensitivity, high-integration, and simple-to-use method for the detection of acetone in breath. If a new type of photoresist material with specific absorption capacity for acetone is further applied in sensor preparation, the sensor proposed in this paper will be more useful in practical clinical applications. Through the non-invasive breath detection, it will become a new method for the detection of blood glucose concentration in diabetic patients.