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Graphene and its derivative graphene oxide (GO) have been the focus of attention in the field of chemical and biological sensing. In this paper, we report a fiber-optic sensor for chemical gas sensing by using graphene oxide coated microfiber knot resonator (GMKR). The refractive index of GO was changed when the gas molecules were adsorbed to the surface of GO, and the gas concentration varying induced refractive index change can be detected by measuring the interference fringes shift of the GMKR. The experimental results show the sensitivities of ~0.35pm/ppm for NH3 and ~0.17pm/ppm for CO detection, due to the different adsorption energy and charge transfer ability between the gas molecules and GO. Experimental results show GO is a promising candidate for gas sensing and can be combined with various fiber-optic devices due to the easy transfer process.
© 2016 Optical Society of America
1. Introduction
Graphene have attracted worldwide attention because it possesses one or few layers thickness of carbon atoms, which has superior photonic and optoelectronic properties [1]. Graphene oxide, is a kind of important derivative of graphene, which shows potential applications like energy storage, solar cell, electrochemical devices [2–5]. Its properties are very similar to graphene such as large interaction surface, high surface activity, excellent thermal conductivity and electrical conductivity [2, 6–8]. GO is a layered material consisting of hydrophilic oxygen functional groups on their basal planes and edges such as hydroxyl and carboxyl. Because of the existence of these oxygen groups, it has better water solubility than graphene and can be applied widely in the bio-chemical systems [9–12]. In addition, GO is also an ideal material for biomolecules and gas molecules adsorption with large specific surface area, high adsorption capacity and strong affinity. The complex refractive index of the GO is determined by its permittivity and conductivity which could be changed via laser irradiation, voltage modulation, molecules adsorption [13–15]. Hence, GO is a kind of promising material to be applied in biological sensors and chemical sensors combined with optical fiber.
There are many methods used for gas sensing based on graphene [16–21] or reduced graphene oxide (RGO) [22–24] have been reported. GO as the ideal material for gas sensing is also attracting many attentions [25, 26]. In this paper, by coating the GO on the surface of a microfiber knot resonator, we demonstrate that the refractive index of GO is sensitive to the change of the surrounding gas concentration. Comparing to the complicated transfer process of graphene, GO can be easy to transfer to the substrate by drying GO solution. The complex reduced process of RGO is also omitted. In this work, NH3 and CO gas were used as examples for the sensing experiments. A significant interference fringe shift is obtained due to the refractive index change of the GO by the adsorption of gas molecules on GO film coated microfiber surface. The experimental results demonstrated that this sensing structure shows the different sensitivity for the NH3 and CO gas sensing because of the different adsorption energy and charge transfer ability between the gas molecules and GO [27–29].
2. Sensing principles and fabrication of the GMKR
According to the microfiber based ring resonator interference theory [30], the dip shift (Δλ) of the interference fringe can be evaluated as
Here L is the loop length, n is the refractive index of the microfiber. ΔL and Δn are the variation of the loop length and refractive index of the transmission light, respectively. In our experiment, the loop length can be approximately considered to be invariable, resulted in the ΔL to be negligible. Δn is determined by the refractive index of GO which is in the evanescence field of the microfiber. So Eq. (1) is simplified asDue to the close relationship between the carbon nanotube and the graphene, the sensing mechanism is based on the charge transfer due to gas molecules adsorbed on the GO sheet that act as donors. When GO adsorbed the gas molecules, the “free electrons” were transfer from gas molecules to carbon atoms of GO. The hydroxyl, carboxyl functionalities decorating the basal planes and edges of GO may form oxo- or hydroxo-bridges with gas molecules. The sensing mechanism of GO interaction with gas molecule can be written as [31]Where Δσgo is the change of the conductivity of GO; Δσmax,go is the maximum change of conductivity with sufficient gas exposure; c is the gas concentration; K is the binding equilibrium constant; k is the surface reaction rate; t is the time for gas exposure. Here, considering the ngo (equivalent refractive index of GO) with different conductivity via εgo = -σgo,i/ωΔ + iσgo,r/ωΔ and εgo = (ngo)2 in Eq. (4) [19,20]. εgo and σgo are the permittivity and the conductivity of GO, ω is the angular frequency, Δ is the thickness of GO.Here Eqs. (3) and (4) show that the refractive index of GO is related to gas concentration. The higher the gas concentration (c), the more gas is absorbed by GO. The change of refractive index of the GO is bigger. The relationship between the gas concentration and resonance dip shift can be Δλ∝Δn∝Δc.Thus the external gas concentration change can be detected by measuring the dip shift of the interference fringe.The schematic GMKR structure is shown in Fig. 1(a). A microfiber knot resonator is attached tightly onto the MgF2 substrate, which are further covered by the transferred GO sheet. Figure 1(b) shows the microscopic picture of the GMKR. The diameter of the knot is about 1.85 mm. Figure 1(c) illustrates the scanning electron microscope (SEM) image of the microfiber coated with GO sheet. The microfiber with diameter of 5.1 μm is fabricated from stretching the fused single mode fiber (SMF) which heated by a hydrogen flame. The GO was further characterized by the Raman spectroscopy. The spectrum of GO shows two features in the 800-3000 cm−1 region, a D band at ~1330 cm−1, a G band at ~1577 cm−1 as shown in Fig. 1(d). A strong D band, a signature for disorder, indicates a high density of defects and vacancies in the GO sheet because of the oxidation [32].
Fig. 1 (a) The schematic diagram of the GMKR. (b) The optical microscope picture of the GMKR and (c) The SEM image of the microfiber coated with GO layers.
Figure 2 shows the depositing process of the GMKR. In this work, we fabricated a microfiber by the heating-and-drawing method and then made a knot. The microfiber knot was attached on the MgF2 by surface electrostatic and van der Waals force. Both pigtails of the microfiber are fixed onto the MgF2 substrate via UV glues. GO was first dissolved in the deionized (DI) water. Then, the GO solution was sonicated in an ultrasonic bath for 30 min. The concentration of GO is 100mg/L. The GO was dropped onto the knot. Then the whole structure was put on a heating stage with a temperature of 40°C until the GO solution was dried.
3. Gas sensing experiment
The GO is an ideal material for gas sensing because it has dangling bonds on its surface, which are essential for detecting gas molecules. GO has many polar functional groups such as hydroxyl and carboxyl. They make GO sheet to have higher adsorption efficiency of gas molecules [33]. To investigate the sensitivity of GO to different gases, we use NH4 and CO as examples for demonstration. The GO sheet receives an electron from the “activated” surface functional groups and promotes the conductivity of the GO result in the refractive index changed as shown in the inset of Fig. 3. In our experiment, the gas sensing responses of the sensor were tested in a constant-temperature environment. The experimental setup is shown in Fig. 3. The GMKR sensor was placed in a gas chamber. In this experiment, the pure NH3 and CO gases with the purity come up to 99.99% are injected into the sample chamber through the flow meter. Firstly, the dry N2 gas is used to exhaust the small amount of air and water vapor in the air chamber to eliminate the interference of water and oxigen for the sensing experiments. The concentration of NH3 or CO gas can be controlled by adjusting the flow meter, respectively. The outlet of the gas chamber is connected with the external environment. So the pressure inside the gas chamber is equal to the atmospheric pressure. A tunable laser with a wide wavelength range was used as the light source (81960A, Agilent, USA). The transmission light from the sensor was collected and analyzed by a power detected-based optical spectrum analyzer (OSA, N744A, Agilent, USA). A polarizer controller is adopted to optimize the polarization of the transmission light.
Figure 4(a) illustrates the transmission spectra of microfiber knot resonator with and without GO coating, respectively. In this microfiber knot resonator, the single mode formed multi-beam interference is dominant, which can be confirmed by the narrow FSR of the interference spectrum. GO can cause very significant loss of light because it can absorb and scatter light. The quality factory (Q) is 7.8 × 104 and 4.9 × 103 without GO coated and with GO coated, respectively. We directly observed that the GMKR has larger transmission loss and lower quality factory than the pure microfiber knot resonator. It could be explicated that the GO sheet cladding introduced additional loss on the transmitted light. In order to prove that the dip shift was caused by the refractive index change of GO, a pure microfiber knot resonator without GO was used as a reference. For pure microfiber knot resonator, no wavelength shift was observed with NH3 gas concentrations of 0 ppm,150 ppm, and 300 ppm, as shown in the Fig. 4(b). The results indicated that MKR without GO is insensitive to the ambient gas.
Fig. 4 (a) The transmission spectra of microfiber knot resonator without GO coating and with GO coating. (b) The experiment results of microfiber knot resonator without GO coating for gas sensing.
Two sets of experiments have done in order to investigate the relationship among refractive index of GO and the different gases. For the NH3 and CO gases with concentrations of 0ppm, 75ppm, 150ppm, 225ppm, 300ppm, the resonant dip shifts of the GKNR in the gases sensing experiments are shown in the Figs. 5(a) and 5(b), respectively. The resonant dips of the GKNR red shift as the gas concentration increases.
Fig. 5 The transmission spectra changes of the GMKR with different concentrations: (a) for NH3 gas and (b) for CO gas
By comparing the experimental results of the NH3 gas sensing and CO gas sensing, the linear relationship between dip shift and gas concentration is shown in Fig. 6(a). For NH3 and CO gas sensing, the obtained sensitivities are about 0.35pm/ppm and 0.17pm/ppm respectively, with the concentration lower than 150ppm. We can get the conclusion that the sensitivity of the GKNR for NH3 sensing is much higherthan that of CO because the NH3 gas molecule has larger adsorption energy and stronger charge transfer ability to GO, compared with CO molecules [27–29]. The blue dashed circles and the red dashed squares show the microfiber knot resonator without GO coated for NH3 gas and CO gas sensing, respectively. The dip wavelengths were almost kept constant. It proves that the resonant dip shifts are induced by the refractive index changes of GO to adsorb the gas molecules. As shown in Fig. 6(a), it is approximate to a linear response to the gas concentration in the low concentration under the 150ppm. The molecular adsorptions on graphene oxide tend to be saturated with the gas concentration increased. So the resonant shifts tend to be saturated with the gas concentration up to 300 ppm. It can be observed that the resonant shifts tend to unchanged when the gas concentration over 300 ppm. It indicates this sensor can be applied to high precision measurement in low concentration with the range from 0 ppm to 150 ppm. Figure 6(b) presents the temporal recoverability of the GMKR sensor. Experimental results indicate that the good reversibility of GKNR sensor is obtained by cyclically exposing the sensor in NH3 and CO gases with concentrations of 150ppm and 300ppm, respectively.
Fig. 6 (a) Dip shift of GMKR as a function of the gas concentration change. (b) The recoverability of the GMKR.
4. Conclusion
A GO-deposited microfiber knot resonator has been fabricated by a simple and efficient fabrication process and used for gas sensing. The experimental results indicated that the refractive index of GO is very sensitive to the external changes of gas concentration, and GO also shows different sensitivity for NH3 and CO gas detection due to the different adsorption energy and charge transfer ability. The sensitivities of ~0.35 pm/ppm for NH3 and ~0.17 pm/ppm for CO have been achieved with concentration range lower than 150 ppm. Here, the different sensitivities due to the different adsorption energy and charge transfer ability of NH3 and CO, so it shows not a real selectivity for differentiate gas sensing. In the future work, the sensitivity of this sensor could be also improved by dropped some special metal nano-particles into the GO such as Pd or Pt. The characteristics of high sensitivity, real-time, easy fabrication and low cost make this structure a big promising for various gas sensing applications.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) under Grants 61290312, 61475032, and 61575039. It was also supported by the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT1218), and the 111 Project (B14039).
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