Highly Sensitive and Exceptionally Wide Dynamic Range Detection of

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Highly Sensitive and Exceptionally Wide Dynamic Range Detection of Ammonia Gas by Indium Hexacyanoferrate Nanoparticles Using FTIR Spectroscopy Supone Manakasettharn, Akira Takahashi, Tohru Kawamoto, Keiko Noda, Yutaka Sugiyama, and Tohru Nakamura Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00359 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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Analytical Chemistry

Highly Sensitive and Exceptionally Wide Dynamic Range Detection of Ammonia Gas by Indium Hexacyanoferrate Nanoparticles Using FTIR Spectroscopy Supone Manakasettharn†‡, Akira Takahashi†, Tohru Kawamoto†, Keiko Noda†, Yutaka Sugiyama†, and Tohru Nakamura*† †

Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8565, Japan. ‡ National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), 111 Thailand Science Park, Phahonyothin Road, Khlong Nueng, Khlong Luang, Pathum Thani 12120, Thailand. KEYWORDS. Nanoparticles, indium hexacyanoferrate, Prussian blue analogue, ammonia detection, infrared spectroscopy. ABSTRACT: Ammonia gas is useful but caustic thus its concentration is monitored depending on applications. We prepared indium hexacyanoferrate nanoparticles (InHCF-NPs, HCF = [FeII(CN)6]4−) with average diameter around 8 nm by simple reaction at room temperature between In cations and HCF anions, and found the unique functionality of InHCF-NPs which were capable of highly sensitive (16 ppb) and exceptionally wide range (190000, 16 ppb − 0.3 %) detection of ammonia gas within 6 min by IR measurements. Slope changes of the IR peak ratio of adsorbed ammonium over CN moieties in InHCF framework indicated a good log-log linear correlation along gas concentrations, in which wide dynamic range over 105 was realized for the first time in the field of ammonia gas detection.

There have been growing concerns about the impact of ammonia (NH3) pollution on public health issues. An internationally recommended exposure limit for NH3 has been 25 ppm.1 NH3 contaminant in hydrogen source for fuel cell must be less than 0.1 ppm.2 The International Technology Roadmap for Semiconductor 2.0 has required NH3 contaminant in lithography processing to be less than 50 ppb by volume (ppbV).3 In addition, leakage checks in wide range have been needed in ammonia production/processing industries, or food and live-stock fields. Therefore, monitoring of NH3 concentrations is crucial. Sensing principles for NH3 gas detection is able to be divided into two categories of electrical and optical measurements, including the present study, as shown in Table 1. In terms of the electrical measurements, most studies basically have used changes in electrical resistance of materials, i.e., chemiresistive sensing4–13. The chemiresistive sensing systems have a high potential of practical applications because of their compactness, moderate selectivity and response time, however these systems have not reached highly sensitive properties less than 100 ppb of NH3 gas and not indicated wide dynamic range extending over 104 either. The other electrical measurements for NH3 detection have used field-effect capacitors or electrochemical devices with detection limit down to 1 ppm and with narrow dynamic range.14,15 These electrical measurements have advantage of compact devices, but they have been suffering from two big problems. Their first problem has been maintainability because the electrical wirings of sensing devices inhibit disposability and long-term durability. Their second problem has been relatively low sensitivity because they cannot effectively condense the target gas molecules.

Therefore we focus on the optical measurements in which sensing materials can be disposable and condense gas molecules, resulting in good maintainability and high sensitivity. For instance, colorimetric change is a good method with highly sensitivity because it can condense NH3 gas by inherent chemical cassette tape.16 However its maximum detection is limited around 150 ppm, and its dynamic range is up to 1.5×104 because of its high sensitivity. On the other hand, Fourier transform infrared (FTIR) spectroscopy is quite a useful method, generally distinguishing each chemical species in the spectra. However, the conventional IR has been suffering from low sensitivity. To solve this problem, the detection of NH3 gas by IR has used a very long IR gas cell path,17 which leads to quite high sensitivity but relatively large size of a sensing instrument. This long path of the IR cell subsequently has caused the problem of its narrow dynamic range due to the extreme enhancement of sensitivity. With these in mind, our approach in this IR study focuses on challenges of dynamic range expanding to solve the drawbacks of the narrow dynamic range and the size in IR measurements by using reactions of nanoparticles (NPs) because of their potential as an adsorbent. We think that the wide dynamic range and the compact size of sensing parts are important because these would lead to a cost effective system. Especially, the wide dynamic range is able to enlarge field tests to check the unknown concentration of NH3 gas and also expand the application range of business by using the same system in diversified workplaces and factories. We simultaneously challenge to get highly sensitive materials for NH3 gas detection to outstrip properties in electrical measurements.

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Analytical Chemistry Table 1. Summary for comparison among NH3 gas sensing.

Electrical measurements

Sensing principle

Optical

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Change in electrical resistance

Change in voltage Change in current Change in color FTIR using a long path Change in IR peak ratio of InHCF-NPs (this work)

Sensing material CuBr4 Co3O45 SnO2 NW6 Cu3(HITP)27 SWCNTs8 WO3-Au-MoO39 Ag NPs/ PEDOT NTs10 3D RGOH11 N-doped activated C12 Polyaniline13 Ir/Pd14 n/a15 Flex CC XP ammonia16 n/a17

Dynamic range 10x (conc. range/ ppm) 10 (0.1-1) 5×102 (0.2-100) 4×102 (0.5-200) 2×10 (0.5-10) 104 (0.5-5000) 5×10 (1-50) 102 (1-100) 5×10 (20-1000) 1.1×10 (45-500) 104 (1-10000) 102 (1-100) 2.5×102 (20-5000) 1.5×104 (0.01-150) 5×103 (0.0001-0.5)

Sensitivity (ppb) 100 200 500 500 500 1000 1000 20000 45000 1000 1000 20000 10 0.1

InHCF-NPs

1.9×105 (0.0155-3000)

16

Nanoparticles consisting of porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) have become attractive due to their large specific surface area and capacity as adsorbents. This feature will realize the wide dynamic range and compact system because of the great adsorption capacity, as well as highly sensitivity if the interaction and the reactions with gaseous molecule are effective. Examples of PCPs are Prussian blue (PB) analogues or metal hexacyanoferrates (MHCFs) consisting of metal and cyano-ligand molecules and porous network. PB has been synthesized as a blue pigment for painting since the 18th century. By appropriate selection of the metal and ligand molecules, our research group can control the material properties of MHCFs such as color.18 Other applications of MHCFs have included electrochromic devices,19–21 electrodes for batteries22,23 and supercapacitors,24 biosensors,25,26 and radioactive cesium adsorbents.27 On the other hand, we found MHCFs exhibit good adsorption of gaseous NH3.28 Among various MHCFs, previous studies have mostly used indium hexacyanoferrate (InHCF) as an electrode material29,30 in batteries,23 electrochromic devices20 and biosensors31. Our research group has discovered PB-ammonia interaction. As shown in Figure 1(a), it is considered to be 2 mechanisms related to NH3 adsorption in MHCFs. An NH3 molecule is first adsorbed in MHCFs at the vacancy site, and then converted to NH4+ in the presence of H2O in MHCFs, and finally trapped at the interstitial site in nano-framework of NPs as shown in Figure 1(a). This conversion was observed in PB.28 On the other hand, our preliminary screening tests suggested that the NH3 adsorption properties of InHCF were relatively greater than those of other MHCFs, which will contribute to sensitive and wide-range detection (see S-1.4 Preliminary screening results of MHCFs in SI). We reported here timedependence changes in the IR absorbance of InHCF-NPs films in a very compact 3D-printed cell as illustrated in Figure 1(b) (see S-1.1 3D-printed cell in SI). The InHCF-NPs films showed effective adsorption rates which leaded to irreversible adsorption under ambient conditions, therefore each film was discarded in every experiment. These is a disadvantage that indium in InHCF-NPs is a kind of rare metal and expensive, however we believe that a small amount of InHCF-NPs (ca. 20

µmol·cm-2) which is used to prepare each film is acceptable in practical applications using commercially available systems such as disposable test paper-, tape-, and tube-procedures.

Figure 1. Schematic diagrams of (a) mechanisms of NH3 adsorption in MHCFs and (b) in situ detection of NH3 adsorption in InHCF-NPs dropped onto a membrane inside a 3D-printed cell by FTIR spectroscopy.

EXPERIMENTAL SECTION Synthesis of InHCF-NPs. Indium hexacyanoferrate nanoparticles (InHCF-NPs) were synthesized by mixing 0.334 mol·L-1 aqueous solution of indium (III) chloride (practical grade) and 0.25 mol·L-1 aqueous solution of potassium hexacyanoferrate (II) trihydrate (JIS special grade). Both raw materials were purchased from Wako Pure Chemical Industries Ltd. The obtained InHCF-NPs were washed with pure water for six times using centrifugation at 4000 rpm for 5 min. The InHCF-NPs were kept as a suspension in pure water to prevent from adsorption of NH3 in ambient air (Figure S-1(a)).


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Analytical Chemistry Fabrication of InHCF-NPs Thin Film. For NH3 adsorption experiments, InHCF-NPs thin films were prepared from the InHCF-NPs suspension and were subsequently coated. The 2 mL of 51.75 mg·mL-1 InHCF-NPs suspension was shook and sonicated for 5 min by using the ultrasonic cleaner (Honda electronics W-113MK-II) and was then shook again. After that, 10 µL of the InHCF-NPs suspension was dropped on a hydrophilic PTFE membrane filter (JCWP04700, Merckmillipore) with pore size of 10 µm and thickness of 85 µm. The InHCF-NPs sample was then dried at 60 ̊C for 2 min in the oven. To take images of InHCF-NPs with a field emission scanning electron microscope, 10 µL of the InHCFNPs suspension was dropped on a silicon substrate and then was dried at 60 ̊C for 2 min in the oven. To obtain powder diffraction data using an X-ray diffractometer (XRD), 10 µL of the InHCF-NPs suspension was dropped on an XRD low background holder and was dried similarly in the oven. Methods of Characterization. The chemical compositions of indium (In), iron (Fe), and potassium (K) in InHCFNPs were analyzed by using a microwave plasma-atomic emission spectrometer (MP-AES, Agilent 4100, Agilent Technologies Japan Ltd.) and using a microwave sample preparation system (Multi-wave 3000, PerkinElmer Corp.) for prior decomposition. The hydration number of InHCF-NPs was analyzed by using a thermogravimetry-differential thermal analysis (TG-DTA, Thermo plus evo II, Rigaku Corp.). The amount of Cl in the sample was analyzed by ion chromatography (Integrion, Thermo Fisher Scientific) in the Japanese analysis company named MC Evolve Technologies. Sample images were obtained by using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi High Technologies Corp.). The crystal structure of InHCF-NPs was investigated at room temperature by using an X-ray diffractometer (XRD, D2 Phaser, Bruker Corp.) with Cu Kα radiation (λ=0.154 nm) at 30 kV and 10 mA. Water vapor adsorption/desorption isotherms for InHCF-NPs were obtained by using a gas-adsorption/desorption system (BELSORP-maxII, Microtrac BEL Inc.). Measurement of NH3 Concentration. Three methods were used to measure NH3 concentration. The first method was using a sensor gas chromatograph (ODNA-P2-A, FIS Inc.) which the measurement time was 4 min per measurement and the measurement range was from 30 ppb to 10 ppm with the accuracy of ±15% at the calibration concentration, immediately after calibration. The second method was using shortterm measurement detector tubes (3L, 3La, 3M, 3HM, GASTEC Corp.) which their measurement range depended on the tube number with relative standard deviation of 5% and sampling time within a few minutes. The third method was using ion chromatography (IC, 833 basic plus, Metrohm AG). For the first method and the second method, NH3contaminated air was sampled and measured directly. For the third method, NH3-contaminated air was passed into two impingers in series filled with 20 mL of 5 g·L-1 boric acid solution with subsequent measurement of the NH4+ ions in the impingers by using IC. We used the first method and the second method to roughly check NH3 concentration below 10 ppm and confirmed by the third method. We used the second method to measure NH3 concentration above 10 ppm. We measured NH3 concentration in NH3-contaminated air before and after NH3 adsorption experiments.

Sources of NH3-contaminated Air. Three sources of NH3-contaminated air were used. The first source was ambient room air with 15.5 ± 0.6 ppbV of NH3, relative humidity of 75.0 ± 5.0% and temperature of 24.6 ± 0.2 °C. The base air of the second and third sources originated from a compressed ambient air which was filtered and dried at the beginning of the gas-line in our institute. The second source was provided by the mixing of NH3 generated from a calibration gas generation system (PD-1B-2, GASTEC Corp.) with humidified air generated by MilliQ-water bubbling of the base air. The NH3 concentrations of the second source were 45.0 ± 16.2 ppbV, 198.4 ± 24.9 ppbV, 250.6 ± 9.7 ppbV with a relative humidity of 92.0 ± 5.0% and a temperature of 25.0 ± 0.6 °C, and 666.0 ± 29.7 ppbV, 1.6 ± 0.5 ppmV, 12.0 ± 0.6 ppmV with a relative humidity of 54.0 ± 5.0% and a temperature of 25.0 ± 0.6 °C. The third source was from the dilution of 99.999% NH3 with humidified air in an aluminum foil-reinforced polyethylenebased plastic bag. The NH3 concentrations of the third source were 45 ± 2 ppmV, 120 ± 14 ppmV, 455 ± 7 ppmV, 750 ± 38 ppmV and 3000 ± 150 ppmV with a relative humidity of 54.0 ± 5.0% and a temperature of 25.0 ± 0.6 °C. The relative humidity and the temperature were monitored by a temperature and humidity data logger (DL171, As One Corp) and a digital thermo-hygrometer (TH-321, As One Corp). For the blank tests, the base air was passed and bubbled into a series of 2 impingers filled with 95% concentrated sulfuric acid solution (Wako Pure Chemical Industries Ltd) cooled by ice bath with sodium chloride and subsequently passed through a trapping Schlenk tube in liquid nitrogen to remove NH3 as much as possible, and the outcome was passed through a bottle added Milli-Q ultrapure water to produce humid air, and then slightly heated with a ribbon heater in the gas-line to adjust temperature. The NH3 concentrations of the output blank air was checked by IC to be at very low, around 1 ppb, and the relative humidity before passed into the gas cell was 92.0 ± 5.0% and the temperature was 25.4 ± 0.4 °C. In-situ Detection of NH3 Adsorption. InHCF-NPs on the membrane filter inside the 3D-printed cell was set up on a conventional FTIR spectrometer (Nicolet iS5, Thermo Fisher Scientific Inc.) as an IR source and an IR detector (Figure S1(d)). NH3-contaminated air was supplied to the cell inlet by 3 methods depending on the sources. For the second source, the NH3-contaminated air was firstly generated from the calibration gas generation system equipped with check valves to prevent backflow, and was subsequently mixed with humidified air, and the resulting gas was flowed to the 3D-printed cell with the internal pressure of the pressurized air. When the internal pressure balance of the pressurized air was not appropriate, a DC diaphragm pump (DSA-2F-12, Denso Sangyo Co. Ltd.) was used to control the pressure of the NH3contaminated air, resulting in the reach of an expected flow rate. For the third source, the NH3-contaminated air in the aluminum bag coated with polymers was flowed to the 3Dprinted cell by the other small DC diaphragm pump (DSA-2F12, Denso Sangyo Co. Ltd.) which was connected between the aluminum bag and the 3D-printed cell. Two flow meters (RK1250, KOFLOC Co., Ltd.) were connected before the cell inlet and after the cell outlet to measure the flow rate of the NH3contaminated air. The inlet flow rate and the outlet flow rate were 155 ± 5 mL·min-1 and 150 ± 5 mL·min-1, respectively. IR spectra of InHCF-NPs were automatically collected by using the FTIR spectrometer (Nicolet iS5, Thermo Fisher Sci-


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entific Inc.), every ca. 16 seconds for the first 2 min to monitor the fast adsorption of NH3 gas. After that, the collection period was longer (e.g., typically every 2 min after first 2 min, and next every 10 min, and then every 2 h) to monitor the additional adsorption. Three experiments were repeated for each NH3 concentration and therefore three data sets of IR spectra were obtained for each NH3 concentration. After each NH3 adsorption experiment was finished, the gas flow system was washed with isopropanol and ethyl acetate, and flushed with ambient air at the flow rate of 150 mL·min-1 for at least 5 min. Before exhaust in a fume hood, NH3-contaminated air was impregnated in a boric acid solution to trap NH3 out of the exhaust air. RESULTS AND DISCUSSION The chemical composition of InHCF-NPs was analyzed to be In[Fe(CN)6]0.749·3.83H2O·K0.0390·Cl0.0321 including a small amount of K+ and Cl- which originated from raw materials. X-ray diffraction (XRD) patterns of InHCF-NPs with the Fm3m space group (Figure 2(a), upper) are in good agreement with a calculated curve (see Figure S-4 in SI) and revealed its lattice constant (the distance between M-to-M in Figure 1(a)) to be 1.039 nm obtained by the Pawley method and its crystallite size to be around 8.0 nm estimated by the Double-Voigt approach (see S-1.3 Powder diffraction data in SI). The SEM image in Figure 2(b) indicates InHCF-NPs with sizes approximately ranging from 8 to 35 nm, in which some InHCF-NPs are single-crystalline and the other InHCF-NPs are polycrystalline. Interestingly, it is confirmed that the crystal structures of InHCF-NPs are maintained after pure NH3 adsorption (Figure 2(a), bottom). Compared to PB with its lattice constant of 1.019 nm28, InHCF-NPs show relatively the larger lattice constant, which will be conductive to the greater adsorption ability.

tetrafluoroethylene (PTFE) membrane filters. The area density of InHCF-NPs on the membrane filter was approximately 20 µmol·cm-2. The dried edge of the InHCF-NPs sample formed wrinkled ellipsoidal shape on the membrane filter with the lengths of the semi-major and semi-minor axes of approximately 2 and 1.5 mm, respectively (Figure S-1(b)). The films were set inside the 3D-printed gas cell by the FTIR spectrometer at room temperature. InHCF-NPs were exposed to NH3-contaminated air with relatively high humidity of 54-92% considering the actual conditions of practical sensing technology. Figures 3(a) and 3(b) show two examples of the time variation of the transmission IR spectrum of InHCF-NPs exposed to 16 (15.5±0.6) ppbV and 12 (12±0.6) ppmV of NH3, respectively, at an outlet flow rate of 150±5 mL·min-1. These figures indicate that measurable signals are observed at ~1416 cm-1,32 corresponding to the symmetric deformation28 of NH4+ in InHCF-NPs.

Figure 3. Transmission FTIR spectra changes of InHCF-NPs exposed to (a) 15.5±0.6 ppbV and (b) 12±0.6 ppmV NH3 at 150 mL·min-1.

Figure 2. (a) XRD patterns of InHCF-NPs before (upper, blue line) and after pure NH3 adsorption (bottom, red line). (b) SEM image of InHCF-NPs. Scale bar, 20 nm.

We investigated in-situ NH3 adsorption at various NH3 concentrations in InHCF-NPs dropped onto hydrophilic poly-

We evaluated NH3 adsorption in InHCF-NPs on the hydrophilic PTFE membrane filter by the IR peak ratio of NH4+ (~1413 cm-1) to CN− peak (~2100 cm-1) because it is reasonable that concentrations of NH4+ are proportional to the ratio based on cyanide (CN−) as the InHCF backbone. It is found that CN− peaks remain unchanged for each InHCF-NPs sample (one sample shown in Figure 3(a) and the other shown in Figure 3(b)), which can be used as a reference to evaluate changes of the other IR peaks in the same sample. The IR peak height of NH4+ and CN− depended on the film thickness or the amount of InHCF-NPs, however, the IR peak ratio of NH4+ to CN− was independent of the film thickness or the amount of InHCF-NPs. Therefore, we could use each InHCF-NPs sample one by one, prepared by the simple method which does not


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Analytical Chemistry strictly maintain the film thickness or the amount of InHCFNPs. Although the hydrophilic PTFE membrane filters as a substrate highly absorb IR in the region from ~1100 to ~1300 cm-1, it did not affect our evaluation of NH4+ peaks provided by NH3 adsorption. We found that the IR peak ratio of NH4+ to CN− increased with the retention time at the initial stage by averaging IR signals over a fixed range of wavenumber (see S1.2 IR data in SI). As shown in Figures 4(a) and 4(b), even for very thin concentration (16 ppbV), the NH3 detection is realized as it gives a sufficiently large signal as compared with the blank data (dotted line). We plotted the IR peak ratio of NH4+ to CN− vs. the retention time for about 6 min, giving the linear slope of 0.0065 (R2 = 0.9122) as shown in Figure 4(a). For 12 ppmV, the NH4+ peak increased more quickly; therefore, we plotted the IR peak ratio of NH4+ to CN− vs. the retention time for about one minute, giving the linear slope of 0.0754 (R2 = 0.9916) as shown in Figure 4(b).

(within 6 min) at initial IR measurements of 30 sec (750 3000 ppm), 1 min (12 - 455 ppm), 2 min (0.666 - 1.6 ppm), and 6 min (16 - 251 ppb) (see S-1.2 IR data in SI). In Figure 4(a), the origin of large error bars at initial measurements of about 2 min is presumably from the fluctuating of an automatic air sampling pump during ramp-up at the beginning and the noise of IR signals. The fluctuation basically decreases after about 4 min under steady moving of the pump and increase of IR signals in the case of ammonia concentration of 16 ppb. Therefore, the response time of about 6 min is used to analyze the slope of IR peak ratio of NH4+ to CN− at 16 ppb in Figures 4(a) and 5(a). Some analyses of the other slope of IR peak ratio showed the same trend. There should be no large error in the case of the InHCF-NPs films exchanged in every ammonia monitoring test since the same experimental setup was carefully repeated. From these experimental results, it was clear that the error bars in adsorption slopes were not critical when we used the slope values obtained by the measurements as shown in Figures 4(a), 5(a), and S-5(b) to S-17(b) in SI. The minimum limit of determination (min-LOD)33 of the NH3 gas concentration was estimated under the concept of 10 times standard deviation of data based on blank data, resulting in the minLOD around 3 ppb (see S-1.2 IR data in SI). The result explicitly indicates the high sensitivity and previously unknown dynamic range over 105 (ca. 190000) including ppb-level less than 50 ppb for ammonia gas detection using the simple film system of InHCF-NPs.

Figure 4. Time dependence of the FTIR absorption peak ratio of NH4+ to CN− of InHCF-NPs exposed to (a) 15.5±0.6 ppbV and (b) 12±0.6 ppmV NH3. The dotted lines represent the result of the blank test.

Figure 5(a) shows the time variation of the IR peak ratio of NH4+ to CN− for InHCF-NPs exposed to NH3 concentrations ranging from 16 ppbV to 3000 (±150) ppmV. The results demonstrate that the slope increases with the concentration of NH3. Figure 5(b) is a log-log plot of the slope of the IR peak ratio of NH4+ to CN- vs. NH3 concentration, which indicates a good linearity over an exceptionally wide dynamic range (16 ppb to 3000 ppm (0.3%)) even under relatively high humid conditions. The time unit and the concentration of NH3 are converted into second and mol·dm-3 (mol·L-1) to estimate the reaction order and rate. Each slope was determined depending on NH3 concentrations in reasonable response time

Figure 5. (a) Time dependence of FTIR absorption peak ratio of NH4+ to CN− at various NH3 concentrations (inset: expansion of 16 ppb and blank data). (b) Log-log plot of the slope of the IR peak ratio of NH4+ to CN− vs. NH3 concentrations.

We performed adsorption experiments using three materials of InHCF, NH3, and H2O under controlled conditions on the basis of a method of initial rates.34 In this method, at the



beginning of reaction and/or adsorption, the initial rate v0 can be generally expressed by the following equation, v0 = kq[Y0]q (eq.1) where kq is a rate constant under a condition in which the values caused from InHCF and H2O could be included in kq as a constant, q is the reaction order for NH3, and Y0 is the NH3 concentration. Our experiments fixed the concentration of ammonia gas, Y0, as one dot in Figure 5(b). Therefore, taking logarithms of (eq.1) gives: log v0 = log kq + q log[Y0] (eq.2) which is in good agreement with the result in Figure 5(b), depending on several different concentrations of ammonia gas, Y 0. It is quite common that q is a fractional order, e.g., 0.5, not integral orders such as first- or second-order,34 especially in the case of gas-phase reactions or complex reactions including consecutive reactions. We think that our results in Figure 5 are encompassed in these reaction categories. According to the slope in Figure 5(b), the reaction order, q, is ca. 0.33, and the rate constant, kq, is around 0.9 s-1. We inferred from the obtained data that the result of reaction order was associated with consecutive complex reactions at the interface between gas phase containing ammonia and the surface of InHCF crystals, and also diffusion of ammonia gas into the crystals of several units of InHCF to react with water molecule inside. On the other hand, we assumed that the reaction rate of adsorption around 0.9 s-1 by InHCF-NPs is relatively faster than that by iron catalyst in the ammonia decomposition reactions.35 Finally we examined the selectivity of InHCF-NPs to some extent. Although infrared spectroscopy can principally distinguish each chemical species in the spectra based on the different vibration or the other modes, resulting in high selectivity, a practical response of InHCF-NPs in FTIR spectra was tested by using n-butanethiol and propionic acid as impurity gases, which are encompassed in the category of odorous and volatile compounds, similar to ammonia. Interestingly, InHCF-NPs showed no significant response in FTIR measurements even under mostly saturated gas of n-butanethiol and propionic acid in room air. On the other hand, ammonia gas of ca. 500 ppm together with mostly saturated gas of nbutanethiol and propionic acid indicated apparent changes in FTIR charts at around 1415 cm-1 which was consistent with ammonium cation species described above in this study (see Figure S-19, insets: expansion (a) and (b) in SI). The obtained FTIR charts showed no peaks corresponding to thiol SH moiety (S-H, ca. 2600 cm-1) and alkyl group (C-H, ca. 29003000 cm-1). In the case of propionic acid, small peaks were observed, especially at around 1540 cm-1 which presumably corresponded to carbonyl group (C=O) of carboxylate salt form, however no peak of alkyl chain (C-H) was observed. We think that these results basically suggest the good selectivity of InHCF-NPs among ammonia, organic thiol, carboxylic acid gaseous molecules, though carboxylic acid such as propionic acid might have an interaction and reactions with InHCF. We infer that nano-space of InHCF crystal framework and different reactivity of InHCF toward the different type of gas molecules render the intrigued selectivity possibly. It is still unclear about the intrigued NH3-adsorption properties of InHCF-NPs. At the present stage, we speculate the following reasons: (i) The very small particle size of InHCF-

NPs would be helpful to increase the adsorption speed compared to particles with larger sizes. (ii) The relatively larger lattice constant will possibly render the enhancement of adsorption ability possible. (iii) An interaction of the lone-pair electron orbital of NH3 with In cations in the InHCF framework would be conducive to coordination and trapping of NH3 molecules. (iv) Water molecules adsorbed in the InHCF framework would also promote trapping of NH3 molecules by the protonation of water molecules. According to a preliminary result of water vapor isotherm (Figure S-3 in SI), InHCF-NPs showed significant adsorption of water molecules which facilitated the effective conversion of NH3 to NH4+. Along this line, acidity inside the InHCF framework after hydration of water molecules is very intrigued, however the scenario of the mechanism is still ambiguous. We think that the clue to understand it exists in the phenomena of aqua-ions consisting of a simple metal cation, in which different pKa constants of aqua-ions hydrolysis are summarized at room temperature including indium (III) hexahydrate.36 The information lets us to be considered that 2+ cations such as Cu2+, Co2+, Mn2+ show relatively large pKa over 7.6, which is in acceptable agreement with relatively weak acidity and adsorption trend. On the other hand, 3+ cations such as In3+ and Fe3+ show relatively low pKa values (e.g., 3.2 (In3+) and 2.0 (Fe3+)), which lead to the strong acidity, however, the order is not in agreement with our results due to the other factors. The further investigations and studies are needed to clarify the mechanism since it is still unclear at this moment. (v) Maintaining the crystal structure of InHCF-NPs before and after adsorption of pure NH3 would contribute to wide range detection, specifically no decomposition under thick concentration of NH3, compared to the degradation of materials such as CuHCF28. Studies to clarify these aspects are under way. CONCLUSION We demonstrated the facile synthesis of indium hexacyanoferrate nanoparticles (InHCF-NPs) with the size around 8 nm, which can be used as an effective NH3 adsorption material, with selectivity, under ambient-like conditions, utilizing the changes in IR absorbance. The dynamic range is exceptionally large (190000, 0.016 to 3000 ppmV (0.3%)) with the minimum limit of determination around 3 ppbV and response time is within 6 min. Future work will include investigations of NH3 adsorption behavior depending on metal-cation in hexacyanoferrate, improved IR optical sensing technology by using the other type of membrane filters, and an effect of inhibitor gases and humidity/temperature in real work places or factories. Studies to scrutinize their properties such as sensitivity, selectivity and durability, and further to develop practical applications are under way.

ASSOCIATED CONTENT Supporting Information. The Supporting Information (SI) is available free of charge via the Internet at http://pubs.acs.org. Method details and supplementary results, i.e., 3Dprinted cell, IR data and analyses, power diffraction data, preliminary screening results of MHCFs, and selectivity examination.


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Analytical Chemistry

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank K. Minami, K. Sakurai, H. Watanabe, C. Takamura, M. Asai, and Y. Jan for their technical supports and discussions. This work has been financially supported by NMRI, AIST, Japan and NANOTEC, NSTDA, Thailand (Grant no. P1750113 and F216004667).

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Indium hexacyanoferrate nanoparticles are studied as an effective NH3 gas sensing material with an IR system in the dynamic range of 190000, i.e., 0.0155-3000 ppm.


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Highly Sensitive and Exceptionally Wide Dynamic Range Detection of

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