sensors Article
A Functionalized Tetrakis(4-Nitrophenyl)Porphyrin Film Optical Waveguide Sensor for Detection of H2S and Ethanediamine Gases Gulimire Tuerdi, Nuerguli Kari, Yin Yan, Patima Nizamidin and Abliz Yimit * College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China;
[email protected] (G.T.);
[email protected] (N.K.);
[email protected] (Y.Y.);
[email protected] (P.N.) * Correspondence:
[email protected]; Tel./Fax: +86-099-185-835-75 Received: 8 October 2017; Accepted: 22 November 2017; Published: 24 November 2017
Abstract: The detection of hydrogen sulfide (H2 S) and ethanediamine, toxic gases that are emitted from industrial processes, is important for health and safety. An optical sensor, based on the absorption spectrum of tetrakis(4-nitrophenyl)porphyrin (TNPP) immobilized in a Nafion membrane (Nf) and deposited onto an optical waveguide glass slide, has been developed for the detection of these gases. Responses to analytes were compared for sensors modified with TNPP and Nf-TNPP composites. Among them, Nf-TNPP exhibited significant responses to H2 S and ethanediamine. The analytical performance characteristics of the Nf-TNPP-modified sensor were investigated and the response mechanism is discussed in detail. The sensor exhibited excellent reproducibilities, reversibilities, and selectivities, with detection limits for H2 S and ethanediamine of 1 and 10 ppb, respectively, and it is a promising candidate for use in industrial sensing applications. Keywords: nafion; tetrakis(4-nitrophenyl)porphyrin; ethanediamine; hydrogen sulfide; optical waveguide
1. Introduction Ethanediamine (EDA) is a common volatile organic compound (VOC) [1], and the National Institute for Occupational Safety and Health recommends an occupational exposure limit of under 10 ppm [2]. Potential symptoms of overexposure to EDA include nasal and respiratory system irritation, asthma, as well as liver and kidney damage. EDA is very soluble in water, ethanol, and methanol, and it is employed as a reaction intermediate for the industrial production of pharmaceuticals, polymers, and dyes [3–6]. Recently, several studies directed toward the development of EDA gas sensors have been reported; for example, electrochemical [7,8] and luminescence sensors [9] exhibit high sensitivities toward EDA. However, they have drawbacks, such as poor detection limits, lack of mobility, and large gas volumes that are required for detection, which limits their use in environmental-monitoring departments. Castillero and co-workers produced a protonated porphyrin/TiO2 composite thin film for the determination of amines (ammonia, EDA, and butylamine, etc.) by monitoring spectral changes resulting from protonation-deprotonation reactions of porphyrins before and after exposure to amines and HCl [10]. While this sensor exhibited better sensitivity and response-recovery to analytes, its response and recovery times were long. Therefore, lower detection limits and faster response-recovery times for the detection of EDA is a meaningful objective. On the other hand, hydrogen sulfide (H2 S), which has an unpleasant smell that similar to that of rotten eggs, is toxic and corrosive [11]. Natural emissions of H2 S into the atmosphere are predominantly related to the degradation of organic matter through the anaerobic reduction by bacteria [12]. Anthropogenic sources, such as fossil-fuel burning and petroleum extraction, result in substantial emissions of H2 S into the environment [13]. Common H2 S gas sensors are based on metal oxide semiconductors and various hybrid materials. For example, sensors based on porous CuO nanosheets exhibit detection limits of 0.7 to 10 ppb at room temperature, with response and recovery Sensors 2017, 17, 2717; doi:10.3390/s17122717
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times for the detection of 200 ppb H2 S of 336 s and 543 s, respectively [14]. Liu and co-workers prepared CuO-ZnO nanorods using a pulsed-laser-deposition method; these nanorods exhibited a 0.5 ppm detection limit for H2 S gas at room temperature [15]. A H2 S gas sensor, based on p-type CuO (111) nanocuboids, also exhibits a 1 ppm detection limit for H2 S at 200 ◦ C [16]. However, some drawbacks exist, including high operating temperatures, and the low sensitivities and selectivities of CuO, ZnO, Fe2 O3 , SnO2 , and In2 O3 /ZnO, which limits the wide application of these types of novel nanomaterials. Highly sensitive low-cost sensing materials that can rapidly identify analytes are in high demand for multiple applications, such as environmental monitoring, medical diagnostics, and food-quality control. Porphyrins and related macrocycles have been intensively exploited as materials for use in chemical sensors [17], since these devices can exploit most of their physicochemical functions [18], such as reversible binding [19], catalytic activation [20], and optical changes [21]. The optical properties of porphyrinoids have attracted significant attention because they are applicable to the development of gas chemosensors, in which detection is based on the differential absorption of left- and right-hand circularly polarized light in the ultraviolet, visible, or IR region. Toxic gases, including VOCs [22,23], NO2 [24], CO2 [25], H2 S [26], ammonia, and amines [27,28] modify the spectrum of the porphyrin, which is useful for optical gas sensing. However, intermolecular π-π interactions between porphyrin rings usually lead to aggregation that, in general, is detrimental for gas-sensing purposes. Porphyrins incorporated into Nafion (Nf), PVC (polyvinyl chloride), or nitrocellulose films have been found to be more sensitive to ammonia than other materials [29]. Among these, Nf has been used as a functional material due to notable features that include its superior chemical and photothermal stabilities, filming properties, and, more importantly, its hydrophilic -SO3 – ionic-cluster regions, hydrophobic fluorocarbon regions, and the interfacial regions that are formed between them that makes it more suitable than other polymers for sensing applications [30]. It has been reported that metalloporphyrins occupy the hydrophobic and interfacial regions of the Nf film due to their hydrophobic nature [31]. Moreover, the polyvalent ampholytic nature of the tetrakis(4-nitrophenyl)porphyrin (TNPP) inner core provides sensitivity toward bases and acids [32,33]. Thus, the advantages of the novel Nf-TNPP thin film include its high activity, low cost, and high selectivity for EDA and H2 S gases. While toxic gases can be examined using conventional analytical methods, such as GC/MS and AAS, leading to the highly sensitive and well resolved detection and quantification of analytes [34,35], these methods suffer from some disadvantages; they are generally costly, require experienced operators, and are inconvenient for real-time monitoring. Since the 1980s, researchers in the field of optical communications have paid close attention to the study and application of optical-waveguide (OWG) sensors [36]. OWG sensors have been developed for the measurement of trace amounts of a variety of gases [37–40]. To the best of our knowledge, EDA and H2 S gases have, to date, not been determined utilizing OWG methods, even though OWG sensors exhibit several desirable features, including portability, potential for high sensitivity, anti-electromagnetic interference properties (they suffer little or no interference in the waveguide element of the sensor), fast response-recovery times, and intrinsic safety for detection at room temperature when compared to other sensors. In this work, TNPP and Nf-TNPP were used as sensing elements embedded in an OWG sensor to determine the presence of analytes through changes in absorption properties. A highly sensitive and rapidly responding/recovering Nf-TNPP sensor was subsequently fabricated and applied to the detection of low concentrations of EDA and H2 S gases. Finally, porphyrin protonation and deprotonation in Nf-TNPP, and the mechanism that is responsible for the diversity of these structures, are discussed. 2. Methods 2.1. Synthesis and Characterization of Meso-tetrakis(4-Nitrophenyl) Porphyrin (TNPP) TNPP was synthesized using a procedure modified from that reported by Fasalu et al [41] Pyrrole (0.5 mL, 7.2 M) and 4-nitrobenzaldehyde (1.09 g, 7.2 M) were taken in a 1 L three-necked round-bottom flask and dissolved in 500 mL CHCl3 . The solution was purged with nitrogen gas for 10 min. Then, the acid catalyst BF3 -Et2 O (0.31 mL, 2.5 M) was dispensed into the mixture. After 2 h of stirring,
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the reaction was quenched by the addition of N,N-diethylethanamine (0.35 mL, 2.5 M), and then, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, 1.63 g, 7.2 M) was added. The reaction mixture was further mixed for another hour at 65 ◦ C. The solvent was evaporated by using a rotary evaporator, and then purified by column chromatography using neutral alumina and chloroform/methanol (95:5) as the eluent. The yield of TNPP was 219 mg (16%). 1 H NMR (Bruker AVANCE DMX400, 400 MHz, CDCl3 , Billerica, MA, USA) δ = 8.58–8.01 (m, 4H), 7.71 (d, J = 34.0 Hz, 2H), 6.90–6.12 (m, 2H), 5.19 (s, 4H), 1.25 (dd, J = 20.6, 13.3 Hz, 12H), 0.85 (s, 2H). 2.2. Preparation of Nafion Solution Nafion NR50 mesh beads were sonicated for 3 h in an ethanol solution, and then placed in a vacuum drying oven for 12 h at 60 ◦ C in order to further purify the beads from residues of small polymers and fragments that were left over from manufacture. To a 25 mL high-pressure reaction vessel, 0.1 g of Nafion was added after 5 mL ethanol and 5 mL distilled water were completely mixed. This was then taken into the vacuum drying oven again for 4 h at 230 ◦ C. The reaction vessel was removed from the oven and cooled to room temperature. The result was a clear and colourless solution, containing 0.01 wt % of Nafion. 2.3. Preparation and Analysis of Sensing Element Before fabrication of the sensing element, a potassium ion exchanged glass slide was washed with acetone and ethanol. After the drying process, the glass slide was coated with the previously prepared Nafion solution (0.01 wt %) at 2200 rpm for 30 s using a spin coater, followed by desiccation for 1 h at 70 ◦ C. Finally, a certain concentration of the TNPP solution (0.66 mg cm−3 ) was immobilized on the Nafion film OWG surface by spin coating at 2000 rpm for 30 s (Scheme 1), and was then used for gas detection. A Keysight 5500 Atomic force microscopy (AFM, Bruker, Billerica, MA, USA) was used to characterize the surface morphology and thickness of both TNPP and functionalized Nf-TNPP thin films. The acquired images were collected in the tapping mode with lateral dimensions ranging from submicrometre range to several micrometres. Further, the Nf-TNPP film was characterized by UV-Vis and emission (FTIR-650, Beijing Kaifeng Fengyuan Technology Co., Ltd., Beijing, China) spectroscopy before and after exposure to the analyte gases. The absorbance measurements were conducted using a UV-1780 spectrophotometer (Shimadzu Technology Co., Ltd., Beijing, China) on solutions deposited on an optical glass substrate. 3. Sensing Apparatus and Procedure Gas sensing experiments were performed using a homemade OWG detection system, in which the sensor was mounted with prism-coupler feedthroughs. This gas sensor system is similar to that described in previous reports [42–44]. The gas-testing apparatus [45] was composed of a compressed air source and an Nf-TNPP-film OWG gas-sensitive element, a laser source, a flow meter, a diffusion tube, a light detector, and a recorder. A gas-mixing manifold, into which VOCs and a stream of pure air were introduced, was used for mixing and introducing the mixture into the flow cell, which enclosed the waveguide sensor. The Nf-TNPP thin-film-sensor device inside the flow cell (2 × 1 × 1 cm) was mounted on a rotational stage possessing X-Y-Z translation capabilities. Two different semiconductor laser beams (532 nm for EDA, 650 nm for H2 S) were introduced into the Nf-TNPP thin-film-based OWG using a prism coupler; these beams exited through another prism coupler. The intensity of the output light was monitored using a photomultiplier detector, and the signal was recorded by a computer. For each measurement, a fresh syringe was used to inject 20 cm3 of the gas sample into a new flow cell, after which it was vented. All of the experiments were performed at room temperature during which pure N2 (99.99%) was introduced into the cell at a constant flow rate of 50 cm3 min–1 in order to transfer the sample gas to the sensor.
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room temperature during which pure N2 (99.99%) was introduced into the cell at a constant flow rate of 50 cm3 min–1 in order to transfer the sample gas to the sensor. Different concentrations concentrations of of analytes analytes were were prepared prepared in two steps. First, First, standard standard H H22SS gas gas was was Different obtained by reacting a given amount of FeS with HCl at room temperature at atmospheric pressure. obtained by reacting a given amount of FeS with HCl at room temperature at atmospheric pressure. 3 ). The gas that was allowed to flow intointo a sealed standard vessel (600 cm The that was wasproduced producedininthis thismanner manner was allowed to flow a sealed standard vessel (600 3). Standard Standard EDAEDA gas was produced by by vaporizing a given amount inside cm gas was produced vaporizing a given amountofofEDA EDAsolution solution (99.5%) inside 3 ). The concentrations of the H S and EDA gases, calibrated using 3 a sealed standard vessel (600 cm a sealed standard vessel (600 cm ). The concentrations of the H2S2 and EDA gases, calibrated using a a commercial S gas detection tube(working (workingrange: range:2–200 2–200ppm, ppm,Gastec, Gastec,Beijing BeijingMunicipal MunicipalInstitute, Institute, commercial HH 2S2gas detection tube Beijing, China) and an EDA gas detection tube (Gastec, GV-100, Kobe, Japan), were approximately Beijing, China) and an EDA gas detection tube (Gastec, GV-100, Kobe, Japan), were approximately equal to to the the calculated calculated values. values. Second, Second, different different amounts amounts of of the the standard standard H H22SS and and EDA EDA gases gases were were equal 3 ) to obtain the desired concentrations. dilutedwith withpure pureairair a second standard vessel diluted inin a second standard vessel (600(600 cm3) cm to obtain the desired concentrations. Using Using this standard-vessel very low concentrations of HEDA (in the ppb this standard-vessel dilutiondilution method,method, very low concentrations of H2S and (inEDA the ppb range) 2 S and range)be could be produced. could produced. 4. Result Result and and Discussion Discussion 4. Previous studies studies investigated investigated the the interactions interactions of of porphyrins porphyrins bearing bearing free free bases bases with with analytes analytes and and Previous quantitatively determined determined acidic acidic and and alkaline alkaline gases gases on on the the basis basis of of UV-vis UV-vis spectral spectral shifts shifts [46]. [46]. In In the the quantitatively present study, we determined the concentrations of EDA and H S gases from Soret-band absorbances present study, we determined the concentrations of EDA and H22S gases from Soret-band absorbances using the the OWG OWG technique. technique. The Thegas-sensing gas-sensingresponses responsesof ofNf-TNPP Nf-TNPPand andTNPP TNPP films films to to 100 100 ppm ppm of of using VOCs and inorganic gases are shown in Figure 1a,b, respectively. The response factor of the sensor VOCs and inorganic gases are shown in Figure 1a,b, respectively. The response factor of the sensor is defined defined by: by: ΔI ∆I == IIgas Iair , where Igas and Iairthe aresignal the signal intensities of gas the and gas and ambient gas is -Iair−, where Igas and Iair are intensities of the ambient air, air, respectively. Figure 1 reveals thatNf-TNPP-film-based the Nf-TNPP-film-based is sensitive more sensitive and respectively. Figure 1 reveals that the OWGOWG sensorsensor is more and more more stable toward TNPP-film-based sensor. This ascribabletotoelectrostatic electrostatic 2 S than stable toward EDA EDA and and H2S H than the the TNPP-film-based sensor. This is isascribable – groups of the interactions between the four -NO functional groups on the porphyrin and the -SO interactions between the four -NO22 functional groups on the porphyrin and the -SO33– groups of the Nafion film film [46]. [46]. Consequently, Consequently, the the Nf-TNPP Nf-TNPP film film was Nafion was further further investigated. investigated. Selectivity is a contributing factor to gas-sensing performance, since good good selectivity selectivity determines determines Selectivity is a contributing factor to gas-sensing performance, since whether or areare detectable in ainmulti-component gas environment [47]. Here, the sensor whether or not nottarget targetanalytes analytes detectable a multi-component gas environment [47]. Here, the responses of the Nf-TNPP film to 100 ppm VOCs and inorganic gases, including HCl, H S, SO , NO 2 2 sensor responses of the Nf-TNPP film to 100 ppm VOCs and inorganic gases, including HCl, H2S,2 , and CO , were measured at room temperature. Clearly, as shown in Figure 1, Nf-TNPP exhibited SO 2, NO22, and CO2, were measured at room temperature. Clearly, as shown in Figure 1, Nf-TNPP a higher response to EDA (at H2 S and (at 650 among The typical exhibited a higher response to 532 EDAnm) (at and 532 nm) H2Snm) (at 650 nm) similar among analytes. similar analytes. The response of this sensor to EDA was approximately six-times greater than its response to methylamine, typical response of this sensor to EDA was approximately six-times greater than its response to and its response to its H2 Sresponse was about greater2.7-times than its response to SOits results reveal that 2 . These methylamine, and to 2.7-times H2S was about greater than response to SO 2. These the Nf-TNPP film the exhibits goodfilm selectivity togood EDA selectivity and H2 S gases. results reveal that Nf-TNPP exhibits to EDA and H2S gases. 1050
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Figure 1. Responses of tetrakis(4-nitrophenyl)porphyrin (TNPP) and Nafion membrane (Nf)-TNPP Figure 1. Responses of tetrakis(4-nitrophenyl)porphyrin (TNPP) and Nafion membrane (Nf)-TNPP thin film/K+-exchanged OWG sesnors upon exposure to (a) VOCs and (b) inorganic gases at room thin film/K+ -exchanged OWG sesnors upon exposure to (a) VOCs and (b) inorganic gases at room temperature (gas concentrations: 100 ppm). temperature (gas concentrations: 100 ppm).
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AFM, performed using a Multimode 8 system, was used to characterize the surface AFM, performed using a Multimode 8 system, was used to characterize the surface morphologies morphologies of the TNPP and Nf-TNPP thin films (Figure 2). The surface of the Nf-TNPP thin film of the TNPP and Nf-TNPP thin films (Figure 2). The surface of the Nf-TNPP thin film exhibits higher exhibits higher molecular ordering when compared to that of the TNPP thin film; the average surface molecular ordering when compared to that of the TNPP thin film; the average surface roughnesses roughnesses of the Nf-TNPP and TNPP sensing films were determined to be 9.97 nm and 20.4 nm, of the Nf-TNPP and TNPP sensing films were determined to be 9.97 nm and 20.4 nm, respectively. respectively. In addition, the peaks of the Nf-TNPP film are less high and the valleys less deep (i.e., In addition, the peaks of the Nf-TNPP film are less high and the valleys less deep (i.e., the film is flatter, theFigure film 2a) is flatter, Figure 2a)TNPP thanfilm those of the 2b), owing to groups the presence than those of the (Figure 2b),TNPP owingfilm to the(Figure presence of functional on the of functional groups on the film surface [48–51]. The strong interactions between the Nf and TNPP film surface [48–51]. The strong interactions between the Nf and TNPP are recognizable by a noticeable are recognizable by achange noticeable change in the morphology film layer, which conformational in the conformational morphology of the film layer, which highlightsof thethe inclusion of TNPP highlights the inclusion of TNPP molecules into the Nf structure; this also indicates that the TNPP molecules into the Nf structure; this also indicates that the TNPP molecules are well distributed across molecules areofwell distributed across the surface ofsurface the Nfwith film.TNPP Functionalization of the Nf surface the surface the Nf film. Functionalization of the Nf molecules significantly affects with affects theTNPP OWG molecules output-lightsignificantly intensity (Figure 1).the OWG output-light intensity (Figure 1).
(a)
(b)
Figure 2. 2.Atomic of the thespin-coated spin-coated(a)(a)Nf-TNPP Nf-TNPP thin-film surface, Figure Atomicforce forcemicroscopy microscopy (AFM) (AFM) images images of thin-film surface, 22). and (b) TNPP thin-film surface (area: 10 × 10 μm and (b) TNPP thin-film surface (area: × 10 µm ).
After the Nf-TNPP film, which was initially black in color, turned green, Afterexposure exposuretotoHH2S, 2 S, the Nf-TNPP film, which was initially black in color, turned green, whereas itsitscolor toEDA EDAgas; gas;the theassociated associated UV-vis spectral changes whereas colorchanged changedto tored redupon upon exposure exposure to UV-vis spectral changes areare shown in Figure 3. When the Nf-TNPP film was exposed to H 2 S gas, remarkable changes were shown in Figure 3. When the Nf-TNPP film was exposed to H2 S gas, remarkable changes were observed inin the spectrum.The TheSoret Soret(B) (B)band band Nf-TNPP film observed thevisible visibleregion regionof ofits itsabsorption absorption spectrum. ofof thethe Nf-TNPP film waswas observed toto (red) while the thefour fourQQbands, bands,atat519, 519, 557, 595, and observed (red)shift shiftfrom from431 431nm nmto to 463 463 nm, while 557, 595, and 642642 nm,nm, merged into a single Q band at 667 (Figure No noticeable shifts in positions the positions ofBthe merged into a single Q band at 667 nmnm (Figure 3a).3a). No noticeable shifts in the of the band B band at 436 nm and the Q-bands at 480–750 nm were observed when the film was exposed to EDA at 436 nm and the Q-bands at 480–750 nm were observed when the film was exposed to EDA gas; gas; however, absorption intensities strengthenednoticeably noticeably (Figure (Figure 3b). are are however, their their absorption intensities strengthened 3b). These Thesechanges changes attributable to the protonation of TNPP by H S gas. On the other hand, when the same film is exposed 2 attributable to the protonation of TNPP by H2S gas. On the other hand, when the same film is exposed EDAgas, gas,the theporphyrin porphyrin becomes (see Scheme 1 for1 possible interactions between the to to EDA becomesdeprotonated deprotonated (see Scheme for possible interactions between – thethe analytes) andand coordinated to the –SO3–SO groups to enhance the intensities of the thefilm filmand and analytes) coordinated to residual the residual 3– groups to enhance the intensities of absorption peaks of the film. The absorption changes that are observed for the Nf-TNPP film upon the absorption peaks of the film. The absorption changes that are observed for the Nf-TNPP film exposure to analytes also reveal that readily available semiconductor laser beams should be effective upon exposure to analytes also reveal that readily available semiconductor laser beams should be light sources for the Nf-TNPP film OWG sensor. The refractive index and thickness of the OWG effective light sources for the Nf-TNPP film OWG sensor. The refractive index and thickness of the device, as measured by an SGC-10 ellipsometer, were 1.68 and 138 nm, respectively. A theoretical OWG device, as measured by an SGC-10 ellipsometer, were 1.68 and 138 nm, respectively. A calculation [52] reveals that a 138 nm thick Nf-TNPP film should support the TE0 mode of the optical theoretical calculation [52] reveals that a 138 nm thick Nf-TNPP film should support the TE0 mode of waveguide [53]. Propagation of the TE0 mode in the Nf-TNPP-film-based OWG sensor by analyte thegases optical waveguide [53]. Propagation the TE0 mode in the Nf-TNPP-film-based OWG sensor by was examined by laser-beam prismofcoupling. analyte gases was examined by laser-beam prism coupling.
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Figure 3.3.Absorption spectra of Nf-TNPP film before after exposure to (a) hydrogen sulfide Figure Absorption spectra ofthe the Nf-TNPP film hydrogen sulfide Figure 3. Absorption spectra of the Nf-TNPP film before andand after exposure to to (a)(a) hydrogen sulfide and(b) (b)Ethanediamine Ethanediamine (EDA) gas vapor. 2S) (EDA) vapor. 2 S)and (H2(H S)(H and (b) Ethanediamine (EDA) gasgas vapor.
Scheme1.1. Stepwise Stepwisefabrication fabricationof of the the functionalized functionalized Nf-TNPP-composite Nf-TNPP-composite thin Scheme thin film film and and possible possible Scheme 1. Stepwise fabrication of the functionalized Nf-TNPP-composite thin film and possible interactions with analytes. interactions with analytes. interactions with analytes.
Optical-waveguide sensors rely two principles. Firstly, absorbance changes exhibited Optical-waveguide gas sensors rely on two principles. Firstly, absorbance changes exhibited Optical-waveguide gasgas sensors rely on on two principles. Firstly, thethe absorbance changes exhibited by film are aredirectly directly related to interactions with the detected gas, and, secondly, the sensor bythe the film related to thethe detected gas, and, the sensor by the film are directly related to interactions interactionswith with detected gas, secondly, and, secondly, the responds sensor responds to in changes in the absorbance ofthat the film that are a consequence of interactions with analytes. to changes the absorbance of the film are a consequence of interactions with analytes. In order responds to changes in the absorbance of the film that are a consequence of interactions with analytes. Intoorder to elucidate the response of the sensor toEDA, H2S and and the suitabilities oflaser-beam the chosen elucidate the response of the sensor to H and EDA, the suitabilities of the chosen 2 S and In order to elucidate the response of the sensor to H2S and EDA, and the suitabilities of the chosen wavelengths for OWG detection, responsethe of the sensor of was 532 and 650 at nm using laser-beam wavelengths for OWGthe detection, response theinvestigated sensor wasatinvestigated 532 and laser-beam wavelengths for OWG detection, the response of the sensor was investigated at 532 and thenm same concentrations of VOCs and of inorganic gases (Figuresgases S1–S4). The absorbance changes and 650 using the same concentrations VOCs and inorganic (Figures S1–S4). The absorbance 650 nm using the same concentrations of VOCs and inorganic gases (Figures S1–S4). The absorbance responses of responses the sensor to and EDA andEDA 650 nm are listed in Table 1, which that changes and of H the to at H2532 S and at 532 and 650 nm are listed reveals in Table 1, larger which 2 S sensor changes and responses of the sensor to H2S and EDA at 532 and 650 nm are listed in Table 1, which absorbance are observed in response to each analyte at its preferred 532 nm, reveals that changes larger absorbance changes are observed in response to each wavelength. analyte at itsAtpreferred reveals that larger absorbance changes are observed in response to each analyte at its preferred the responseAt of 532 the sensor EDA is 9.7 greater is 9.7 to H at 650 nm, of wavelength. nm, thetoresponse oftimes the sensor tothan EDAit is times greater than it the is toresponse H2S, while 2 S, while wavelength. At 532 nm, the response of the sensor to EDA is 9.7 times greater than it is to H2S, while to H 2.5 sensor times greater than to EDA. Therefore, semiconductor atthe 650sensor nm, the response of the to H2S is about 2.5 times greaterthese thantwo to EDA. Therefore, laser these 2 S is about at 650 nm, the response of the sensor to H2S is about 2.5 times greater than to EDA. Therefore, these wavelengths were used detect the target 532 nmthe fortarget EDA and 650 nm fornm H2for S. EDA and two semiconductor lasertowavelengths wereanalytes: used to detect analytes: 532 two semiconductor laser wavelengths were used to detect the target analytes: 532 nm for EDA and 650 nm for H2S. 650 nm for H2S.
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Table 1. Absorbance changes (∆Abs = |Absgas − Absfilm |) and responses (∆I) of the Nf-TNPP sensor upon exposure to H2 S and EDA at different laser-beam wavelengths. Sensors 2017, 17, 2717
Gas Sample
∆Abs (532 nm)
∆Abs (650 nm)
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∆I (650 nm)
Table (ΔAbs = |Absgas − Abs film|) and responses100 (ΔI) of the Nf-TNPP530 sensor H1. 0.001 0.005 2 SAbsorbance changes upon exposure to H2S and 0.005 EDA at different laser-beam EDA 0.002wavelengths. 970 212 Gas Sample
ΔAbs (532 nm)
ΔAbs (650 nm)
ΔI (532 nm)
ΔI (650 nm)
H2S 0.001 0.005 530 Figure 4 displays FT-IR spectra of the TNPP powder (see100 in Figure 4a) and thin film (see in EDA 0.005 0.002 970 212 Figure 4b). In the powder, the νN-H stretching band of TNPP appears at 3416 cm–1 , while CH2 Figure 4 displays FT-IR spectra of the TNPP powder (see in Figure 4a) and thin–1film (see in asymmetric and symmetric stretching bands of the alkyl chain appear at 2922 cm and at 2851 cm–1 , Figure 4b). In the powder, the νN-H stretching band of TNPP appears at 3416 cm–1, while –1 CH2 respectively. The spectrum of the powder also exhibits a middle strong band at 1594 cm , which can asymmetric and symmetric stretching bands of the alkyl chain appear at 2922 cm–1 and at 2851 cm–1, –1, which be attributed to theThe νC spectrum band of pyrrole, twostrong strong bands andcan1107 cm–1 = C vibration respectively. of the powder also exhibits aand middle band at 1594at cm1145 be attributed to the νC = Cνvibration band and two strong macrocycle, bands at 1145 and 1107 cm–1 that that can be attributed to the νCof= pyrrole, porphyrin respectively. Moreover, C = C and N of the –1 can be attributed to the ν C = C and νC = N of the porphyrin macrocycle, respectively. Moreover, the the spectrum of the powder also shows two weaker bands at 843 and 798 cm , which can be attributed spectrum of the powder also shows two weaker bands at 843 and 798 cm–1, which can be attributed to νC-C of the two substitutes of the phenyl benzene ring. When compared with the peaks obtained to νC-C of the two substitutes of the phenyl benzene ring. When compared with the peaks obtained from the from powder, the absorption peaks ofofthe thinfilm film very similar the between region between the powder, the absorption peaks theTNPP TNPP thin areare very similar in the in region –1 –1 –1 –1 4000 and4000 600and cm600. cm However, the bands and in theatmacrocycle at . However, theabsorption absorption bands of νof N-H ν atN-H 3416at cm3416 andcm in the macrocycle 1145– –1 –1ννC = C (or 1107 ν C= N) are visibly different in the spin-coated film. This indicates that the molecules 1145–1107 cmcm (or ν ) are visibly different in the spin-coated film. This indicates that the C=C C= N the film are arrayed more closely [54]. moleculesinin the film are arrayed more closely [54].
1.6
a Nf-TNPP powder
Transmittance (%)
1.4 1.2 1.0
b Nf-TNPP film
0.8 0.6 4000
3500
3000
2500
2000
1500
1000
Wavenumber(cm-1) 4. FT-IR spectra TNPP: (a) andand (b) thin film film. FigureFigure 4. FT-IR spectra ofofTNPP: (a)powder powder (b) thin
Generally, the sensing mechanism that is used by gas sensors involves the absorption/desorption
Generally, the at sensing mechanism that isonused by gas sensors thematerials absorption/desorption of analytes their surfaces. Depending the chemical nature ofinvolves the sensing and the analytes, absorption/binding can occur through weak and/or strong chemical interactions, such as and the of analytes at their surfaces. Depending on the chemical nature of the sensing materials van der Waals, dipole-dipole, hydrogen bonding, and - interactions [21]. In this paper, in order to analytes, absorption/binding can occur through weak and/or strong chemical interactions, such as examine possible interactions between the composite thin film and detected gases, the gas response van der Waals, dipole-dipole, hydrogen bonding, and π-π interactions [21]. Transform In this paper, in order to of the thin film to saturated H2S or EDA vapor was examined by FT-IR (Fourier Infrared examine Spectoscopy) possible interactions between theofcomposite thin film detected the spectroscopy, the results which are displayed in and Figure 5. Upon gases, exposure to gas each response analyte, strong hydrogen may form between –NH– or –N– groups(Fourier of the pyrrole ring and Infrared of the thin film to saturated H2bonds S or EDA vapor wasthe examined by FT-IR Transform the hydrogen atoms of the H2S results or the –NH groups are of EDA, which may result in5. proton transfer. The to each Spectoscopy) spectroscopy, of 2which displayed in Figure Upon exposure spectra of the thin film before (Figure 5a) and after exposure to H2S gas (Figure 5b), reveal that the analyte, strong hydrogen bonds may form between the –NH– or –N– groups of the pyrrole ring peak positions and the absorption intensities are essentially the same, indicating that the functional and the hydrogen atoms ofeffectively H2 S or the –NH2 asgroups EDA, which may proton transfer. groups of the film are unchanged a result of of their interactions with result H2S gas.in However, The spectra of exposure the thin to film before 5a) strong and after exposure to H2band S gasappeared (Figureat5b), upon EDA vapor,(Figure a relatively and broad stretching 3313reveal cm–1 that the (Figure 5c), which is partly due to primary amines that are formed through interactions between peak positions and the absorption intensities are essentially the same, indicating that thethe functional group of the pyrrole ring and the secondary amines of the EDA as a result of the acidity of groups of–NH– the film are effectively unchanged as a result of their interactions with H2 S gas. However, TNPP, and partly due to the νN-H stretching band of the EDA vapor. upon exposure to EDA vapor, a relatively strong and broad stretching band appeared at 3313 cm–1 (Figure 5c), which is partly due to primary amines that are formed through interactions between the –NH– group of the pyrrole ring and the secondary amines of the EDA as a result of the acidity of TNPP, and partly due to the νN-H stretching band of the EDA vapor.
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3108 3108 3108 3108 3108 3108 2923 2923 2923 2923 2923 2923 2852 2852 2852 2852 2852 2852
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Nf-TNPP film+EDA
Nf-TNPP film+H2S Nf-TNPP film+H2S Nf-TNPP film Nf-TNPP film
3000
2500 2000 1500 Wavenumber (cm-1) 3000 2500 2000 1500 Wavenumber (cm-1)
1000 1000
Figure spectra of the Nf-TNPP thinthin film film (a) before, and after to (b) Hto 2S and (c) EDA Figure 5.5.FT-IR FT-IR spectra of the Nf-TNPP (a) before, andexposure after exposure (b) H 2 S and vapor. (c) EDA5.vapor. Figure FT-IR spectra of the Nf-TNPP thin film (a) before, and after exposure to (b) H2S and (c) EDA vapor.
The TNPP molecule is square in shape, with two imine-type nitrogen atoms in the center of the The TNPP molecule is square in shape, with two imine-type nitrogen atoms in the center of the ring. The Protonation occurs at two nitrogen atoms certain conditions. Inin order to elucidate TNPP molecule is these square in shape, with twounder imine-type nitrogen atoms the center of the ring. Protonation occurs at these two nitrogen atoms under certain conditions. In order to elucidate this phenomenon, sample solutions of TNPP were characterized by UV-vis spectroscopy in ring. Protonation occurs at these two nitrogen atoms under certain conditions. In order to elucidate this phenomenon, sample solutions of TNPP were characterized by UV-vis spectroscopy in chloroform, chloroform, before and after exposure to H 2 S and EDA gases, the results of which are displayed in this phenomenon, sampletosolutions TNPP were by are UV-vis spectroscopy before and after exposure H2 S and of EDA gases, the characterized results of which displayed in Figurein 6. Figure 6. Thebefore TNPPand solution exhibits aB band at 424 nmgases, and Qthe bands fromof500 to 650 nm in its UVchloroform, after exposure to H 2S and EDA results which are displayed in The TNPP solution exhibits a B band at 424 nm and Q bands from 500 to 650 nm in its UV-vis vis spectrum. These solution bands are attributed to transitions the highest and lowest Figure 6. The TNPP exhibits a Btransitions band at 424 nmbetween and bands from 500 to molecular 650 nmmolecular in orbitals, its UVspectrum. These bands are attributed to between theQhighest and lowest orbitals, and correspond to π → π transitions. The intensity of the B-band absorption peak of TNPP vis These bands are attributed to transitions the highestpeak and of lowest andspectrum. correspond to π → π transitions. The intensity of thebetween B-band absorption TNPPmolecular increases increases upon exposure to EDA gas, leading to an increase in molar absorptivity (Figure 6c). On the orbitals, and correspond to leading π → π transitions. The of the B-band absorption peak of TNPP upon exposure to EDA gas, to an increase inintensity molar absorptivity (Figure 6c). On the other hand, other hand, theexposure B band becomes red leading shifted, to from 424 to 455 nm (Figure 6b), when the 6c). solutionthe is increases upon to EDAfrom gas, an increase in molar the B band becomes red shifted, 424 to 455 nm (Figure 6b), whenabsorptivity the solution(Figure is exposed On to H2 S, exposed to H 2 S, and the four Q bands strengthen and merge into a single band. These spectral changes other hand, B band becomes and red merge shifted,into from 424 toband. 455 nm (Figure 6b), changes when the is and the fourthe Q bands strengthen a single These spectral aresolution observed are observed in the solution, and in the solid film, and are in agreement with reports in the literature exposed to H 2both S, and four Q bands strengthen and merge into a single band. These spectral changes both in solution, and in the solid film, and are in agreement with reports in the literature [55]. The red [55]. The red both shift in of solution, the Soretand band the enhancement the Q bandwith at 659 nm indicate that the are inand the solid are of in the literature shiftobserved of the Soret band and the enhancement offilm, the and Q band at agreement 659 nm indicatereports that theininner nitrogen inner nitrogen atoms of the TNPP ring become protonated by H 2 S, leading to enhanced porphyrin [55]. The red TNPP shift ofring thebecome Soret band and the by enhancement oftothe Q band porphyrin at 659 nm indicate that and the atoms of the protonated H2 S, leading enhanced conjugation conjugation andatoms an overall reduction in the energy of the porphyrin inner nitrogen of the TNPP ring become protonated by H 2S, system. leading to enhanced porphyrin an overall reduction in the energy of the porphyrin system. conjugation and an overall reduction in the energy of the porphyrin system. 4.0
Absorbance Absorbance
3.5 3.0 3.0 2.5 2.5 2.0
a 424 ba 424 cb
0.6
516
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552
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591
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Absorbance Absorbance
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659 646
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552 591 550
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Figure 6. UV-vis absorption spectra of TNPP solutions (a) before and after exposure to (b) H2 S and (c) EDA6.vapors. Figure UV-vis absorption spectra of TNPP solutions (a) before and after exposure to (b) H2S and (c) EDA vapors. Figure 6. UV-vis absorption spectra of TNPP solutions (a) before and after exposure to (b) H2S and (c)
EDA vapors.
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For an OWG sensor, any variation in the optical properties of the thin film alters the intensity of For an OWG sensor, any variation in the optical properties of the thin film alters the intensity of absorbed light, and, consequently, the amount of transmitted light. To explore sensor reversibility, absorbed light, and, consequently, the amount of transmitted light. To explore sensor reversibility, we prepared a sensor using a film that was composed of the dipotassium salt of Nf-TNPP, in which we prepared a sensor using a film that was composed of the dipotassium salt of Nf-TNPP, in which both acidic protons were exchanged for K++; typical responses of this sensor to H2S and EDA vapor both acidic protons were exchanged for K ; typical responses of this sensor to H2 S and EDA vapor are are shown in Figure 7. As can be seen in Figure 7, when continuous air flows during the entire shown in Figure 7. As can be seen in Figure 7, when continuous air flows during the entire detection detection process, the intensity of the transmitted light decreases in response to the presence of H2S process, the intensity of the transmitted light decreases in response to the presence of H2 S and EDA and EDA gases in the 10 ppb to 100 ppm range. The as-prepared OWG device exhibited an excellent gases in the 10 ppb to 100 ppm range. The as-prepared OWG device exhibited an excellent response response (ΔI = 77) to 1 ppb H2S, with a signal-to-noise ratio (SNR) of 7.8 dB, and an excellent response (∆I = 77) to 1 ppb H2 S, with a signal-to-noise ratio (SNR) of 7.8 dB, and an excellent response (∆I = 79) (ΔI = 79) to 10 ppb EDA, with an SNR of 31 dB. In addition, the response/recovery times of each gas to 10 ppb EDA, with an SNR of 31 dB. In addition, the response/recovery times of each gas were were calculated. These times are two fundamental and important factors that, together with response, calculated. These times are two fundamental and important factors that, together with response, are considered to define gas-sensing performance. The response time is defined as the time required are considered to define gas-sensing performance. The response time is defined as the time required to establish 90% of the equilibrium value following exposure to the analyte, and recovery time is the to establish 90% of the equilibrium value following exposure to the analyte, and recovery time is the time required for the sensor to return to within 10% of the initial response in air upon release of the time required for the sensor to return to within 10% of the initial response in air upon release of the analyte gas [56]. Figures 7 and 8 reveal that, at commencement, the gas sensor exhibits a constant and analyte gas [56]. Figures 7 and 8 reveal that, at commencement, the gas sensor exhibits a constant stable baseline. When each analyte gas was introduced into the flow cell, the intensity of the and stable baseline. When each analyte gas was introduced into the flow cell, the intensity of the corresponding transmitted light decreased. However, when the flow cell was refilled with pure N2, corresponding transmitted light decreased. However, when the flow cell was refilled with pure N2 , the signal returned to the original baseline value. The response and recovery times were 1 s and 20 s, the signal returned to the original baseline value. The response and recovery times were 1 s and 20 s, and 3 s and 57 s, to 100 ppb H2S (Figure 7b) and EDA (Figure 8b), respectively. Good Nf-TNPP-film and 3 s and 57 s, to 100 ppb H2 S (Figure 7b) and EDA (Figure 8b), respectively. Good Nf-TNPP-film OWG sensor reproducibilities were demonstrated by successively exposing it to 100 ppb H2S or EDA OWG sensor reproducibilities were demonstrated by successively exposing it to 100 ppb H2 S or EDA gases over five cycles at room temperature (Figures 7b and 8b). In addition, the response-recovery gases over five cycles at room temperature (Figures 7b and 8b). In addition, the response-recovery times of EDA were about three times longer than those of H2S at the same concentrations, which may times of EDA were about three times longer than those of H2 S at the same concentrations, which may due to the viscosity of EDA gas [57]. At this concentration, the relative standard deviations of the due to the viscosity of EDA gas [57]. At this concentration, the relative standard deviations of the transmitted light intensity are ±4.29 for H2S and ±5.10 for EDA. transmitted light intensity are ±4.29 for H2 S and ±5.10 for EDA. H2S ON OFF 1ppb 2000
10ppb 100ppb 1ppm
1800
10ppm
1600
1400
gas in
1750
1775
gas in
gas in
gas in
H2S
2080
2040
1s
2000
1800
1825
1850
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Time (s) (a)
20 s
1960
1920
100ppm
1725
gas in
2120
Output Light Intensity (a.u)
Output Light Intensity (a.u)
2200
gas out
2660
2680
gas out gas out gas out 2700
2720
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2760
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gas out 2800
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Time(s) (b)
(a)
(b)
Figure Nf-TNPP-film-based/K++-rxchanges -rxchangesoptical-waveguide optical-waveguide (OWG)-sensor performance. (a) Figure7.7. Nf-TNPP-film-based/K (OWG)-sensor performance. (a) Response Response reproducibility and (b) response-recovery curve five for H 100 ppb H 2 S gas. reproducibility and (b) response-recovery curve over fiveover cycles forcycles 100 ppb S gas. 2
When Whenthe thesensor sensorwas wasexposed exposedtotoanalyte analytegases, gases,the theoptical opticalproperties propertiesofofthe thesensitive sensitiveelement element were changed. As shown in the following equation, the intensity of transmitted light is related were changed. As shown in the following equation, the intensity of transmitted light is relatedtotothe the absorbance absorbancecoefficient, coefficient,thickness, thickness,and andrefractive refractiveindex indexofofthe thesensitive sensitivethin thinlayer: layer: (1)
I I 0 1 - aNde
I = I0 (1 − aNde ) (1) In which I denotes the intensity of the output light, I0 is the intensity of input light, a is the absorbance In which Iofdenotes the intensity of the light, I0 isnumbers the intensity input light, is the the surface absorbance coefficient the sensitive thin layer, N output is the refractive of theofguided light aon of coefficient of the sensitive thin layer, N is the refractive numbers of the guided light on the surface the OWG with length L, and de is the real propagated distance of light, which is related to the thickness thesensitive OWG with L, andondethe is the realformula, propagated distance of light, which is relatedortothe the ofofthe thin length layer. Based above it is apparent that when the thickness absorbance coefficient of the sensitive thin layer decreases, the intensity of the output light will increase.
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thickness of the sensitive thin layer. Based on the above formula, it is apparent that when the thickness or the absorbance coefficient of the sensitive thin layer decreases, the intensity of the output light Sensors 2017, 17, 2717 10 of 14 will increase. 2700
Output Light Intensity (a.u)
Output Light Intensity (a.u)
100ppb 1ppm
1800
gas in
2320
2400 10ppb 2100
EDA
gas in
EDA
ON OFF
1500
10ppm
1200 900
100ppm
gas in
gas in
gas in
2240
2160
3s
57 s
2080
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gas out
gas out
gas out
gas out
gas out
1920
600 220
330
440
550
660
770
880
1800
990
1850
1900
Time (s) (a) (a)
1950
(b)
2000
2050
2100
2150
2200
Time (s) (b)
++ + Figure Responsereproducibility reproducibility Figure8.8.Nf-TNPP-film-based/K Nf-TNPP-film-based/K-rxchanges -rxchangesOWG-sensor OWG-sensor performance. performance. (a) (a) Response and (b) response-recovery curve over 5 cycles for 100 ppb EDA gas. and (b) response-recovery curve over 5 cycles for 100 ppb EDA gas.
The response intensity of the sensor, defined LogA = Log(I0 − It), where I0 is the initial output The response intensity of the sensor, defined LogA = Log(I0 − It ), where I0 is the initial output light intensity and It is the lowest point of the output light intensity, corresponding to before and after light intensity and It is the lowest point of the output light intensity, corresponding to before and after the injection of analytes into the flow cell, respectively. The calibration curve of the Nf-TNPP film the injection of analytes into the flow cell, respectively. The calibration curve of the Nf-TNPP film OWG intensity LogA LogA against againstthe theconcentration concentrationofofthe the OWGsensor sensorwas wasobtained obtainedby byplotting plotting the the response response intensity analyte are given given in in Figure Figure9a, 9a,and andthe therelationships relationships analytegases, gases,and andthe thelinear linearrelationships relationships between between them them are between increasing concentrations of H 2S (see in Figure 9a) and EDA (see in Figure 9b) gases and between increasing concentrations of H2 S (see in Figure 9a) and EDA (see in Figure 9b) gases and signals werefound found be Welinear. We therefore acquire the following signals were to beto linear. therefore acquire the following relations: LogAhydrogenrelations: sulfide = LogA hydrogen sulfide = (2.38 ± 0.004) + (0.23 ± 0.002) C(H2S), R = 0.99, and LogAEDA = (2.63 ± 0.008) + (0.25 ± (2.38 ± 0.004) + (0.23 ± 0.002) C(H2 S), R = 0.99, and LogAEDA = (2.63 ± 0.008) + (0.25 ± 0.004) C(EDA), 0.004) C(EDA), R = 0.99. R = 0.99. 3.0
3.2 2.8
3.0 2.8
2.4
LogA
LogA
2.6
2.2 y = a + b ×x R22 = 0.9996 Intercept = 2.379 Slope = 0.228
2.0 1.8
2.6 y = a + b ×x R22 = 0.9986 Intercept = 2.633 Slope = 0.248
2.4 2.2
1.6
2.0 -3
-2
-1
0
LogC (ppm) (a)
(a)
1
2
-2
-1
0
LogC (ppm) (b)(b)
1
2
Figure Relationship between response intensity the concentration H22S(b) and (b) vapour. EDA Figure 9. 9. Relationship between thethe response intensity and and the concentration of (a)of H2(a) S and EDA vapour
5. Conclusions In summary, TNPP was synthesized and dissolved in a solution of Nafion, after which it was used to prepare Nf-TNPP thin films by spin coating. Absorption and 1H NMR spectra of TNPP were acquired. The surface morphologies of the TNPP and Nf-TNPP thin films were investigated by AFM, which revealed that the surface of the Nf-TNPP film was smoother than that of TNPP. Planar OWG
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5. Conclusions In summary, TNPP was synthesized and dissolved in a solution of Nafion, after which it was used to prepare Nf-TNPP thin films by spin coating. Absorption and 1 H NMR spectra of TNPP were acquired. The surface morphologies of the TNPP and Nf-TNPP thin films were investigated by AFM, which revealed that the surface of the Nf-TNPP film was smoother than that of TNPP. Planar OWG sensors of K+ -exchanged TNPP and Nf-TNPP thin films on glass were prepared and were used to detect VOCs. Among them, the Nf-TNPP thin film exhibited preferable optical responses to H2 S and EDA at room temperature. The sensor also exhibited noticeable responses to EDA and H2 S in the presence of amines and some inorganic gases (Figure S5). We developed a sensitive, fast, and simple optical sensor for the determination of H2 S and EDA based on the absorption spectrum of Nf-TNPP. In addition, possible interactions between the sensor and the analytes are discussed in conjunction with FT-IR and UV-vis spectroscopy. The sensor film exhibited high sensitivity and selectivity toward H2 S and EDA gases, and detected ppb concentration levels of these gases through the use of two different semiconductor laser-beam wavelengths. Supplementary Materials: The Supplementary Materials are available online at http://www.mdpi.com/14248220/17/12/2717/s1. Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grant No. 21765021). Author Contributions: Abliz Yimit and Patima Nizamidin conceived and designed the experiments; Gulimire Tuerdi performed the experiments; Gulimire Tuerdi analyzed the data; Nuerguli Kari, Yin Yan contributed reagents/materials/analysis tools; Gulimire Tuerdi wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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