1. Introduction

Gas detection is particularly important in both environmental protection and human health. In recent decades, polyaniline (PANI) has been widely used for electrical or optical detection of a variety of gases such as HCl, NH₃, H₂S and NO₂, due to its reversible optical and electrical response to certain gas species, as well as easy processing and environmental stability [1–9]. Recently, there has been a move towards making nanoscale sensors using single PANI-based nanowires due to the high surface-to-volume ratios that facilitate the diffusion of gas molecules into the structures [4,6]. To date, most of these single-nanowire devices rely on the electrical conductance change when exposed to the gas species. However, in electrical response, it is difficult to determine individual responses in gas mixtures with cross-sensitivity. So far selective multicomponent detection on a single-nanowire scale has not been realized, although integrated nanowire arrays have been reported for selective multicomponent detection based on multiple-nanowire electrical responses [10–13]. In contrast to electrical schemes, optical sensing offers more options for signal retrieval from optical intensity, spectrum, phase, polarization, and fluorescence lifetime, and may offer possibility for selective detection of gas mixtures using a single sensing element [14–16].

Most recently, using the evanescent coupling method, the waveguiding polymer single nanowires drawn from solvated polymers have been demonstrated for gas sensing with extraordinary fast response and high sensitivity [17]. Owing to its high efficiency and compactness, the evanescent coupling between a nanowire and a nanoscale fiber taper enables broadband light launching and collection from the nanowire with wavelengths selected for spectral analysis in the visible/near-infrared (VIS/NIR) region [17–20]. The spectral selectivity could provide identity information of an analyte at multiple wavelengths; thus the possibility to determine a multicomponent mixture using a waveguiding single nanowire could be realized [14]. In this work, we demonstrate a facile approach to highly selective optical detection of gas mixtures of NH₃ and humidity using a single waveguiding polyaniline/polystyrene (PANI/PS) nanowire in the VIS/NIR region, in which optical absorption bands change of PANI is used for NH₃ sensing, and the coupling efficiency change between the PANI/PS nanowire and the nanoscale fiber tapers is used for relative humidity (RH) sensing. Our results show that a polymer single nanowire is very promising for developing highly sensitive optical sensors to detect individual responses in complex chemical and biological mixtures.

2. Sensor configuration

Due to the poor mechanical property of PANI [21], it is essential to blend it with soluble matrix polymers to fabricate waveguiding nanowires with good mechanical property and low optical loss. Here, polystyrene (PS) is used to blend with PANI due to its compatibility with PANI and excellent optical properties, such as good transparency in VIS/NIR region and high refractive index (about 1.59). PANI/PS nanowires are fabricated by direct drawing of a polymer-blend solution of 2 wt % PANI (M _w = 50 000; Fluka) doped with 10-camphorsulfonic acid (Alfa Aesar) and 5 wt % PS (M _w = 100 000; Alfa Aesar) in chloroform [4,17]. As-fabricated PANI/PS nanowire is placed across a 250-μm-width MgF₂ microchannel and two ends of the nanowire are coupled to fiber tapers with an overlap of a few micrometers as shown in Fig. 1(a) , where the close contact between the nanowire and the fiber taper can be maintained by Van der Waals or electrostatic attraction [18,19]. For robust gas sensing, the tapering region on MgF₂ substrate are firmly bound at both sides using low-index UV-cured fluoropolymer (EFIRON PC-373; Luvantix Co. Ltd.). For reference, Fig. 1(b) shows an optical microscope image of a 370-nm-diameter PANI/PS nanowire guiding a 633-nm light while being supported and coupled with two fiber tapers. The total coupling loss between the nanowire and the nanotapers is typically around 10 dB.

 

Fig. 1 (a) Schematic diagram of a nanowire sensing device. (b) Optical micrographs of a 370-nm-diameter nanowire guiding a 633-nm light. Scale bar, 50 μm. The white arrow indicates the direction of light propagation. (c) Optical micrograph of a nanowire device. Scale bar, 1 cm.

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The sensor elements are sealed in a poly(methyl methacrylate) chamber (Fig. 1(c)) with a vapor inlet/outlet and a hygrometer monitoring the humidity. The analyte gas diluted with air is introduced into the chamber at a flow rate of about 100 mL/min and the output of the nanowire is measured simultaneously. The broadband light is from a fiber-coupled supercontinuum source (SC450, Fianium Ltd.), and the 633-nm and 808-nm light sources are from continuous wave lasers.

3. Experimental results and discussions

It is well known that camphorsulfonic acid doped PANI shows the emeraldine salt (ES) form and can be transferred to the emeraldine base (EB) form when exposed to NH₃ [1–3]. The ES and EB forms absorb light at different spectral regions, which can be used for optical sensing. The output spectral intensity of a 370-nm-diameter PANI/PS nanowire in dry air is shown in Fig. 2(a) (gray line). Upon exposure to 20 ppm NH₃ diluted with dry air, a decrease in intensity of around 600-nm wavelength and an increase of around 830-nm wavelength are observed (black line), suggesting that ammonia gas molecules diffuse into the nanowire and deprotonate PANI from ES form to EB form. It should be noticed that a pure PS nanowire is non-dissipative within the spectral range and shows no detectable optical response to NH₃ in dry air.

 

Fig. 2 (a) Output intensity of a 370-nm-diameter PANI/PS nanowire versus wavelength in dry air without NH₃ (gray line) and with 20 ppm NH₃ (black line). (b) Absorption spectra of the 370-nm-diameter PANI/PS nanowire exposed to dry air with NH₃ concentrations from 0.5 to 32 ppm. Inset shows the NH₃-concentration dependence of ΔA _600-835.

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The absorbance of light guided along the nanowire (defined as A _r = log [I ₀/I], where I ₀ and I are the output intensity in the absence and presence of the analyte gas, respectively) at a single wavelength λ can be expressed as [14,15]

where κ_λ, ε_λ, and η_λ is the coupling efficiency between the nanowire and the nanotapers, the nanowire absorptivity, and the fractional energy confined in the nanowire at the wavelength λ, respectively; and A_n is the attenuation induced by optical scattering and/or background absorbance. The output intensity spectrum in dry air is used as a reference; thus the absorption spectra of the 370-nm-diameter PANI/PS nanowire exposed to NH₃ diluted with dry air with concentrations from 0.5 to 32 ppm can be obtained (see Fig. 2(b)). The absorption spectra show a peak around 600 nm (λ _max) that increases monotonously with the increasing NH₃ concentrations, and a valley (λ _min) around 835 nm that decreases monotonously with the increasing NH₃ concentrations. Because of the opposite response of the nanowire to NH₃ at λ _max and λ _min, the differential absorbance (defined as ΔA _λ1-λ2 = A _r(λ ₁) - A _r(λ ₂)) between the λ _max and λ _min can be used to quantify the NH₃ concentrations with enhanced sensitivity [14]. The NH₃-concentration dependence of the ΔA _600-835 is plotted in the inset of Fig. 2(b). The near linear dependence of the ΔA _600-835 with the NH₃ concentration (< 10 ppm) indicates that the PANI/PS nanowire could function as a NH₃ optical sensor in dry air. With the increase in NH₃ concentration (e.g., > 15 ppm), the A _r shows a nonlinear NH₃ dependence, which can be contributed to the saturation at high NH₃ concentrations due to the limited number of imine-nitrogen sites that can be protonated in the nanowire [4,5].

Water molecules are ubiquitous in the environment and humidity is a parameter that must be considered in a PANI-based device for reliable NH₃ sensing [3,22,23]. While it is difficult to determine individual responses in gas mixtures of NH₃ and humidity with a single PANI-based nanowire by electrical detection [22], here we employ a single PANI/PS nanowire for RH sensing based on a RH-dependent evanescent coupling efficiency between the PANI/PS nanowire and the nanoscale fiber tapers. Figure 3(a) gives the absorption spectra of a 350-nm-diameter PANI/PS nanowire with RH ranging from 37 to 84%. It shows that the valleys in the absorption spectra around the wavelengths of 617-, 770- and 860 nm decrease with increasing RH level. The distinct response spectra for humidity and NH₃ (in Fig. 2) are apparent. It can be used to identify the response to humidity and NH₃ using multiple-wavelength measurement. The behavior can be explained as follows: as the RH increases, the refractive index of the PANI/PS nanowire decreases as a result of diffusion of water molecules and subsequently changes the coupling efficiency, resulting in RH-dependent intensity measured at the output end [20,23]. Figure 3(b) shows the RH-dependent A_r of the nanowire with RH ranging from 37 to 84% at the wavelengths of 617-, 770- and 860 nm, respectively. It suggests that the PANI/PS nanowire could operate as a RH sensor within the range of 37% and 84%.

 

Fig. 3 (a) Absorption spectra of a 350-nm-diameter PANI/PS nanowire exposed to air with RH ranging from 37% to 84%. (b) RH-concentration dependence of A _r at wavelengths of 617-, 770-, and 860 nm, respectively.

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To test the capability of selective detection of NH₃ and humidity in gas mixtures, the PANI/PS nanowire is first exposed to 70% RH air, and then to 5 ppm NH₃ diluted with 84% RH air (2-minute exposure). Figure 4 shows the output intensity spectra of the 350-nm-diameter PANI/PS nanowire in 70% RH air (gray line) and 5 ppm NH₃/84% RH air (black line). In this case, the spectrum of 70% RH air is used as a reference. As can be seen from Fig. 4, an increase in intensity in the long wavelengths range (> 650 nm) is observed when 5 ppm NH₃/84% RH air is introduced. Inset of Fig. 4 shows the absorption spectrum of the PANI/PS nanowire to 5 ppm NH₃/84% RH air with a λ _min around 780 nm. The reason for this result is that for detection of NH₃ and RH in a gas mixture, the absorbance of the nanowire at a given wavelength is caused by both NH₃ and RH: at long wavelengths the sensitivity of the nanowire to NH₃ is enhanced by RH and results in a blue shift of the λ _min in absorption spectrum compared with that in Fig. 2; on the contrary, at short wavelengths the sensitivity to NH₃ is counteracted by RH.

 

Fig. 4 Output intensity spectra of a 350-nm-diameter PANI/PS nanowire in 70% RH (gray line) and 5 ppm NH₃ diluted with 84% RH air (black line). Inset, absorption spectrum of the PANI/PS nanowire to 5 ppm NH₃ diluted with 84% RH air.

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Using the two independent sensing mechanisms above, gas mixtures of NH₃ and humidity can be determined using a single nanowire by dual-wavelength measurements. Here 633-nm and 808-nm-wavelength lights are used to simultaneously monitor the A_r of the 350-nm-diameter PANI/PS nanowire. The nanowire is first exposed to 70% RH air as a reference, and then to the analyte gases. As shown in Fig. 5(a) , upon exposure to 84% RH air, decrease in A_r at both 633- and 808-nm are observed; yet upon exposure to 5 ppm NH₃/dry air, an increase in A_r at 633 nm and a decrease in A_r at 808 nm are observed. The distinguishable response between humidity and NH₃ is clearly seen. Upon addition of 5 ppm NH₃/79% RH air, an increase in A_r at 633 nm and a decrease in A _r at 808 nm are observed; however, the A_r at the two wavelengths show little decrease. This effect is much more obvious in the response to 5 ppm NH₃/84% RH air: only decreases in A_r at 633 nm and 808 nm are observed, which is in accordance with that in Fig. 4. These results make it possible in principle to determine the individual responses in a gas mixture of NH₃ and humidity by dual-wavelength measurement.

 

Fig. 5 (a) Time-dependent response of the nanowire exposed to (1) 84% RH air, (2) 5 ppm NH₃, (3) 5 ppm NH₃ with 79% RH air, and (4) 5 ppm NH₃ with 84% RH air respectively, which are simultaneously monitored using two lasers with wavelengths of 633 and 808 nm. (b) Bar graph summarizing the optical response of the nanowire to the analyte gases at wavelengths of 633- and 808 nm.

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The bar graph in Fig. 5(b) summarizes the response of the nanowire to the analyte gases at 633-nm and 808-nm wavelengths. The height of each bar represents the maximum A_r of the responses to analyte gases at each wavelength. The distinction between each analyte gas is immediately apparent. The concentrations of individual gas components may be calculated by solving the simultaneous equations associated with the nanowire absorptivity ε_λ, which can be determined from the individual response to NH₃ and humidity respectively, the sensing length b, and the fractional energy η _λ confined in the nanowire at wavelength λ which can be calculated by solving the Maxwell’s Equations [24]. Moreover, the measurement precision can be further increased by including more sensing results or by choosing more suitable wavelengths [14].

4. Conclusions

In conclusion, by using spectral analysis in the VIS/NIR region, we have demonstrated optical waveguiding PANI/PS single-nanowire devices for highly selective detection of gas mixtures of NH₃ and humidity, based on the absorption band change of PANI and the coupling efficiency change between the PANI/PS nanowire and the nanoscale fiber taper. In addition, compared with existing techniques for gas sensing relying on electrical response of nanowires such as metal oxide nanowire gas sensors, the use of functionalized polymer nanowire makes it simpler and easier to operate: the high temperature or UV exposure that is required for refreshing the sensing elements in most of metal oxide nanowire gas sensors [11,25–28], is not needed here. Furthermore, the nanoscale fiber taper is directly connected to a standard optical fiber through the tapering region and has a simple optical power readout, which offer several advantages: possibility of remote sensing, continuous monitoring, and feasibility of multiplexing the information from different sensors in one optical fiber, which may open interesting perspectives for highly selective detection in complex chemical and biological environments with a single nanowire.