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Quick and Selective Dual Mode Detection of H2S Gas by Mobile App Employing Silver Nanorods Array Shashank Kumar Gahlaut, Kavita Yadav, Chandrashekhar Sharan, and Jitendra Pratap Singh Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04064 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 22, 2017

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Analytical Chemistry

Quick and Selective Dual Mode Detection of H2S Gas by Mobile App Employing Silver Nanorods Array S. K. Gahlaut, Kavita Yadav, C. Sharan and J. P. Singh* Department of Physics, Indian Institute of Technology Delhi, Hauz Khas New Delhi 110016, India. *Corresponding author’s email: [email protected]

Abstract Hydrogen sulfide (H2S) is a hazardous gas, which not only harms living beings but also poses a significant risk to damage materials placed in culture and art museums, due to its corrosive nature. We demonstrate a novel approach for selective rapid detection of H2S gas using silver nanorods (AgNRs) array on glass substrate at ambient conditions. The arrays were prepared by glancing angle deposition method. The colorimetric and water wetting properties of as-fabricated arrays were found to be highly sensitive towards the sulfurization, in the presence of H2S gas with a minimal concentration in ppm range. The performance of AgNRs as H2S gas sensor is investigated by its sensing ability of 5 ppm of gas with exposure time of only 30 seconds. We have developed an android based mobile app to monitor real time colorimetric detection of H2S. The wettability detection has been carried out by mobile camera. A comparative analysis for different gases reveals the highest sensitivity and selectivity of the array AgNRs towards H2S. The rapid detection has also been demonstrated for H2S emission from aged wool fabric. Thus, high sensing ability of AgNRs towards H2S gas may have potential applications in health monitoring and art conservation. Keywords: silver nanorods, glancing angle deposition, silver sulfide, hydrogen sulfide, gas sensor

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Gas sensing is a very striking field of research in the view of health and environmental safety. H2S is one of the most hazardous gases explored in the field of environmental pollution for health safety issues. It is a colorless and flammable gas with a smell of rotten eggs.1,2 This gas is as toxic as hydrogen cyanide and works with similar mechanism i.e. it inhibits the respiratory enzyme cytochrome oxidase, hence its excess inhalation can even cause death.3 It also affects the pulmonary and central nervous system. The spectrum of illness depends on the concentration and duration of exposure, the concentration being more important than duration. Higher concentrations (>700 ppm) have the potential to cause sudden death due to its effect on the brainstem respiratory center.1,4 It is a very threatening gas in the interest of human health as it quickly deadens the sense of smell therefore, the victim may breathe increasing quantities without noticing, until the appearance of severe symptoms. Apart from this, H2S is an endogenous gasotransmitter (H2S, CO and NO); a signaling molecules emitted from animal cells.5–7 Besides this it is known to be involved in many physiological and pathological functions. Therefore, its detection can be used for non-invasive diagnosis of many diseases.8 Thus, the rapid, sensitive and selective detection of H2S is of great importance for the well-being of living organisms. Researchers have widely explored various techniques over time for the sensing of H2S, such as gas chromatography,9 electrochemical,10,11 and florescence.12,13 Although some of the reported techniques show better detection limit but they are time consuming, require sophisticated instrumentation, and involve cumbersome procedures with demand of specialized skills. But, the important issue which most of the techniques failed to address is the selectivity of the sensor towards H2S gas.7 Recently, advancement in colorimetry14–17 based sensing has shown promising response towards a sensitive, cost effective and environment friendly technique for H2S sensing. Among the metal nanoparticles, the colorimetric sensing based on silver nanoparticles have attracted much attention because of the unique change in its optical properties upon exposure to H2S gas. Silver forms black layer due to sulfidation to form silver sulfide (Ag2S). The rate of this reaction depends 2 ACS Paragon Plus Environment

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on the relative humidity (RH) of the atmosphere as the gas dissolves in water and then readily reacts with the silver.18 Ag2S is a semiconductor having fairly enough conductivity, about 80% of which arises from the highly mobile silver ions.19 Therefore, it has a potential application in semiconductor industry. In the present research, we have demonstrated a dual mode detection based on colorimetric as well as wettability of AgNRs array. The detection is such user-friendly that can be used with any mobile phone having camera. Further, for the ease and widespread use, we have also developed an android mobile application software named "Colorimetric Detector” based on color change of AgNRs array. Thus a highly sensitive and selective method for H2S detection utilizing AgNRs array which is particularly attractive for their fast and sensitive response towards H2S under ambient condition. Along with colorimetric sensing, a new approach wettability detection based on contact angle measurements has been shown here for the first time. The water wetting properties of AgNRs were found to be even more sensitive than colorimetric detection.

Experimental details Fabrication of AgNRs array The AgNRs arrays were grown over glass substrate by thermal evaporation of silver powder (99.9%) using glancing angle deposition (GLAD) technique. The substrates were rinsed with ethanol (70%), followed by piranha solution (4:1 sulfuric acid, hydrogen peroxide). Then rinsed with deionized water and dried with stream of nitrogen gas before loading into the deposition chamber. The substrates were mounted on a sample holder such that the angle between the substrate normal and the incident vapor flux was 85°.20–23 The chamber pressure was kept at 2×10−6 Torr during the deposition. During initial growth, the impinging atoms form isolated nucleation centers which cast shadows for the arriving vapor flux. The nucleated islands act as shadowing centers and hence, the larger nucleation centers will receive more impinging atoms as compared to the smaller 3 ACS Paragon Plus Environment


ones and only the larger islands will grow. The competition between limited adatom surface mobility and shadowing effect results in the evolution of the columnar structure with the growth of AgNRs in the direction of the incident vapor flux.24–27

Experimental setup These AgNRs array were employed for the H2S gas sensing in a homemade setup for controlled gas flow as shown in Figure 1. Sensing chamber of 2 inch diameter was made of stainless steel with inlet and outlet valves. All the measurements were carried out at ambient condition. At the time of experiment, the room temperature and relative humidity were 28 °C and 38% respectively.

Figure 1. Schematic of homemade setup for the sensing of H2S gas.

Mobile setup and app development We made an attachment for mobile phone in order to execute the dual mode detection of H2S gas. It can be attached in front of camera in such a way that the AgNRs array was placed perpendicular to the optical axis for the colorimetric detection. Whereas, for the wettability detection, the array was placed parallel to the optical axis, in such a way that the cross-sectional view of water droplet can be captured by the camera. To enhance the visibility of the image and constant illumination, we have used an LED light. 4 ACS Paragon Plus Environment

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Figure 2. The mobile setup for the dual mode detection of H2S gas. Port 1 is to place the glass slide vertically and port 2 is to place it horizontally. For a user-friendly portable module, we have also developed an android-based operating system app “Colorimetric Detector” in JAVA platform, for the colorimetric detection. This app “Colorimetric Detector” was successfully installed and tested in android version 6.0 smart-phones. A demonstration of the app is shown in Figure 2. This user-friendly portable device directly calculates and compares the ODR (optical darkness ratio) values. When it detects the presence of H2S gas, above the set point concentration, mobile vibrates with a red signal which otherwise is green.

Results and discussion For the rapid detection of H2S gas, array of AgNRs was fabricated on glass substrate by GLAD. The GLAD method is most suitable for the growth of uniform array of pure crystalline AgNRs. These nanorods has high surface area are the most suitable candidate for the detection, especially for low concentration of a gas. The photograph of as-fabricated AgNRs array is shown in Figure 3a. The scanning electron microscope (SEM) of ZEISS EVO 50 was used to confirm the morphology as well as measurement of dimension of the nanorods, is shown in Figure 3b. The nanorods are 5 ACS Paragon Plus Environment


found to be uniformly distributed and have an average diameter of 100 nm and length of about 1µm. The X-ray diffraction (XRD) spectra shows the pure crystalline phase of pristine AgNRs (JCPDS 04-0783) whereas, the H2S exposed array shows the presence of sulfur confirmed the formation of standard monoclinic phase of Ag2S (JCPDS # 14-0072) as shown in Figure 3c. The energy dispersive X-ray spectroscopy (EDX) as shown in Figure 3d, confirmed the presence of sulfur in the H2S treated AgNRs samples, due to sulfurization.

Figure 3. (a) The photograph, (b) SEM image of pristine AgNRs array. (c) XRD and (d) EDX spectra of pristine and 1 % H2S gas exposed AgNRs array for 5 minutes. The colorimetric method, relies on the change in color of sensing array from silver-white to blackish, upon exposure to H2S gas. The grayscale intensity depends upon the concentration as well as duration of exposure. Further, the simple yet very promising way of detection of H2S using wettability method is proposed. In this method, the drastic change in static contact angle of water droplet upon the sensing array determines the presence of H2S. The nanorods were initially exposed to 1% H2S concentration with exposure time of 5 minutes at room temperature. The change in the 6 ACS Paragon Plus Environment

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color of the array and respective color intensity plots as shown in Figure 4a. The sample before and after H2S exposure, were captured by mobile camera at a constant illumination. For clear visualization of the change in intensity of array, the average pixel intensity of image was calculated using MATLAB software. The wettability test was performed by using 5 µl water droplets. The contact angle measurements were repeated five times at different positions of each sample. The images of droplet were captured by CMOS camera equipped with magnifying lens. Captured images of droplets were analyzed by Image J (National Institute of Health, USA) software to calculate the value of contact angle. After the exposure of H2S gas, a sharp decrease in contact angle from 126° to 57° was also observed, as shown in Figure 4b.

Figure 4. Demonstration of dual mode detection (a) Colorimetric mode: photographs of AgNRs array showing the change in color and average intensity plot (b) Wettability mode: change in contact angle of water droplet on pristine and 1 % H2S exposed AgNRs array for 5 minutes. 7 ACS Paragon Plus Environment


The contact angle variation with exposure time for 1% H2S gas is shown in Figure 5a. The contact angle results reveal that the maximum change occurs within 1 min of H2S exposure time and after that, the contact angle value almost unchanged. Figure 5b indicates the change in optical darkness ratio (ODR), inset shows respective photographs of the AgNRs array after 1% H2S exposure for different exposure time. Formula used for ODR value calculation as given below.28

Where, Ib and Is are greyscale intensity of pristine (background) and exposed AgNRs array respectively. The color of array significantly changes with increase in the exposure time up to 5 minutes, thereafter it remains almost constant. In support of this significant change in color, the reflectance spectra of H2S exposed arrays for varying exposure time were measured using Perkin Elmer Lambda 1050 UV-Vis spectrometer and the results shown in Figure 5c.

Figure 5. Variation of (a) contact angle, (b) optical darkness ratio (ODR), inset shows respective photographs and (c) surface reflectance of AgNRs array samples after 1% H2S exposure time of 1, 3 and 5 minutes. The results shown here clearly represent that the initial exposure for 1 min is most effective and after that a little change in reflectance has been observed. Thus, it can be inferred that in silver 8 ACS Paragon Plus Environment

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sulfurization is initially fast and gradually decreases afterwards. This can be attributed to the decrease in the rate of sulfurization with the increased tarnish layer thickness.29 The sulfide layer formed on the silver surface acts as a physical barrier between the metal surface and the H2S environment, preventing further sulfurization. In order to check the minimum detection limit of the system, gas concentrations were taken 10 ppm and 5 ppm for the same exposure time. The change in contact angle and ODR values for AgNRs samples with 10 ppm and 5 ppm H2S gas for different exposure time as shown in Figure S1 and S2. To determine sensitivity and lower detection limit of the system, we have further reduced the exposure time. The experiments were also performed at 5 ppm concentration of H2S gas with an exposure time of 30 seconds. We found, the average intensity changes from 140 to 115 shown in Figure S3. The results clearly indicate that the colorimetric method is highly sensitive and rapid. In addition to this, the wettability method is found to be faster than the previous one. The contact angle based sensing methods would be promising and can successfully detect concentrations as low as 5 ppm within 5 seconds. The drastic change (126° to 60°) at the same parameters makes it superior over the former method. Therefore, the dual mode detection has advantage of having both higher sensitivity and selectivity.

Detection of H2S from aged wool fabric The faster detection in dual would be highly useful especially for low concentration of H2S, for example emission from aged wool fabric. It is widely recognized that many materials produce volatile compounds as they deteriorate over time. The artifacts placed in heritage museums release H2S gas. Such emissions from storage and display materials can put the preservation of artifacts at risk. The material’s stability evaluation known as ‘Oddy test’ are often done to identify such objects which is at risk.30 Aged wool fabric textile and rubber are major sources of sulfur compound in the museum. Wool is a natural protein fiber having sulfur containing amino acids (methionine and 9 ACS Paragon Plus Environment


cysteine). On the exposure of UV light, disulfide bonds in these amino-acids gets broken and produce H2S gas. Conventional method for material testing (for example Oddy test) requires keeping samples for 28 days at 65 oC temperature with high humidity.30 The high sensitivity of AgNRs towards H2S presents an opportunity to use them as indicators of material degradation/ageing under ambient condition. We have used AgNRs array sensor for the detection of H2S emission from aged wool fabric and surprisingly, a significant response was found in ambient condition within three days. The measurements were conducted by placing the array with aged wool fabric in a closed glass vessel and monitored intermittently as shown in Figure 6a. A significant change in color and water contact angle was found in this small period. The variations in contact angle as well as ODR plots with time are shown in Figure 6b. The color of array was significantly changed after three days. It was found that the contact angle value changes from 126° to 87°, 70° and 56° after one, two and three days of exposure, respectively of AgNRs sample to aged wool fabric whereas the color of the AgNRs array does not changes significantly in one day exposure. It is important to note that during these measurements we have not used any UV light irradiation on the wool fabrics. The UV light irradiation on wool fabric is known to increases the degradation rate and H2S production. Earlier report on Ag nanoparticles for detection of H2S from wool fabric was based on the radiation ageing where UV light was used to degrade the wool fabric.31 In contrast to earlier existing literature, we report a novel approach to sense H2S emission from the aged wool fabric having similar response time as mentioned in the earlier reports without using any UV exposure. The presence of sulfur in EDX measurements (not shown here) Ag2S validates the feasibility of this method. This dual mode detection method could be promising in order to test such materials.


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Figure 6. (a) Schematic showing the setup for detection of H2S from the aged wool (b) Plots showing variation of contact angle (CA) and optical darkness ratio (ODR) with exposure time, when the AgNRs array was placed with aged wool fabric for 3 days. We have also explored the crucial issue of selectivity that limits practical usage of most of the currently available H2S sensors. In order to verify the selectivity of the present approach for H2S gas the AgNRs array samples were exposed to different gases e.g. O2, H2, CO, He and N2 .The measurements were carried out under similar conditions with the relative humidity and temperature values of 38% and 28°C, respectively and the exposure time was chosen to be 1 minute. Figure 7 shows the change in contact angle and ODR values of AgNRs after the exposure of different gases. A surprisingly higher change in both contact angle and ODR values were observed in the case of H2S gas exposure in comparison to the all other gases, which clearly reveals the selectivity of AgNRs array towards H2S gas sensing. Thus, the proposed AgNRs based sensing presents an assured, low-cost and facile method for the highly sensitive and selective detection of H2S at low concentrations.



Figure 7. Showing selectivity of AgNRs array towards H2S as depicted by the change in contact angle (∆θ) and optical darkness ratio (ODR) after exposure of AgNRs arrays to different gases for 1 minute.

The feasibility of AgNRs over Ag thin films was also tested by performing the similar experiments on conventional Ag thin film. It was found that the AgNRs are highly sensitive in comparison to the silver thin film due to their high surface area, which results into more absorption of H2S gas molecules on the surface of sensor array. After exposure to 5 ppm H2S gas for 5 minutes it gets decreased by approximately 15% only whereas in the case of AgNRs, the contact angle decreases by 50%. The comparative results are shown in Figure S5. Raman spectroscopy was done by Horiba LabRam HR Evolution on AgNRs samples before and after the exposure of H2S gas. The Raman spectra of H2S exposed and pristine AgNRs array are shown in Figure S6. The pristine AgNRs samples represent low intensity Raman bands at 115 and 230 cm-1 which are attributed to Ag lattice vibrations and Ag-O vibrational modes respectively. While in case of H2S exposed AgNRs sample one extra Raman peak at 182 cm-1 was observed which is assigned to Ag-S vibrational modes,32 which reveals the covalent bond between Ag and S. The XPS spectra of AgNRs arrays, pristine and exposed to 5 ppm H2S for 30 s as shown in Figure 8. The data were taken at ultra-high vacuum in multi-probe surface analysis system of


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Omicron, with Mg Kα X-ray source (hν = 1253.6 eV). The peak of adventitious carbon at 284.8 eV,33 were used as charge reference standard to calibrate the whole spectrum for both the samples. The high-resolution (0.5 eV) narrow range core-level spectra of Ag and S were plotted separately. The minimum numbers of peaks were fitted to Lorentzian–Gaussian functions using XPSPEAK41

C 1s

O 1s

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Ag 3d5/2

Ag 3d5/2

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Ag MNN

software, after background correction based on Shirley algorithms.

Pristine Exposed

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Ag 3d 5/2

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Ag 3d 3/2

Exposed Exposed Pristine

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166 366

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Figure 8. The XPS spectra showing (a) broad view survey scan and high-resolution core level spectra with their various deconvolution components of (b) S 2p and (c) Ag 3d. No impurity signal was observed in the XPS spectra, shows the high purity of the samples. The presence of sulfur (S 2p at 162.3 and S 2s at 226.3) and shift (0.2 eV) of Ag 3d peaks confirms the conversion of Ag to Ag2S. Since recovery is one of the prominent issues in gas sensing, we have shown the recovery of H2S exposed AgNRs array by dipping it into tarnish remover solution for 1 min. The solution removes sulfide layer from the exposed array, which can be reused. The recovery of the array is shown in Figure S4.



Mechanism of H2S sensing The change in color and wettability of the AgNRs array can be attributed to the conversion of Ag to Ag2S on the surface of array. This conversion can be explained by following reactions.

→ + (1) 8 + 4 → 4 + 2 + 4 (2) + 2 + 4 ⟶ 4 (3) The first reaction is believed to occur at the adsorbed aqueous layer present on the surface of array in the humid atmosphere. It is believed that the gaseous H2S molecules dissociate in the presence of water molecules and make it more readily available for reaction with silver19 as shown in eq 2. Oxygen present in the air atmosphere acts as a cathodic species and consumes electrons as shown in the eq 3 to form hydroxyl ion species on the surface of silver. The literature also suggests that the presence of oxygen and moisture is important for the reaction between H2S and silver to take place at ambient temperature and pressure.34–36 The rate of reaction increases with the increase in RH. In low humidity (< 50 %), the amount of absorbed water on the silver surface is approximately constant and the reaction rate is steady. However, at high RH (70 – 80 %), surface moisture increases and accelerates the reaction rapidly.29 Also, the chemical reaction took place during silver sulfurization form hydroxyl ions on the surface of AgNRs as shown by equation 3. The presence of hydroxyl ions and higher water adsorption on the surface accounts for the low contact angle.37,38 The enhanced hydroxyl ions on the surface, therefore, lead to lowering down the contact angle of water on the surface of array. This mechanism fully supports our wettability detection method.

Conclusions In summary, we have demonstrated simple, fast and novel approach using colorimetric and contact angle measurements for H2S gas detection by AgNRs array at room temperature under air ambient. In order to make it easy and portable, an android mobile app has been developed for 14 ACS Paragon Plus Environment

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detection of the gas. The surface color and water wetting properties of nanorods are found to be highly sensitive towards the H2S gas environment. H2S gas with different concentrations viz. 1%, 10 ppm and 5 ppm have been utilized for the measurements. Different gases were tested and this array was found highly selective for H2S. Along with high sensitivity and selectivity, the response time of the sensor was found to be significantly low (within 5 s). We have also demonstrated the significance of present approach in detecting H2S gas emission from aged wool fabric within oneday exposure in ambient atmosphere. The present approach has promising applications in art conservation and future studies of H2S emission from bio-systems (live cells).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Measurements with 10 ppm (Figure S-1), 5 ppm (Figure S-2), and for exposure time 30 sec (Figure S-3), Recovery measurements (Figure S-4), comparative data for Ag thin film (Figure S-5), Raman spectra of pristine and exposed AgNRs (Figure S-6).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements The author (SKG) kindly acknowledges Department of Science and Technology (DST), India for providing junior research fellowship. This research is supported by DST, India (grant number EMR/2015/ 001477) and Nanoscale Research Facility, IIT Delhi, India. The authors are grateful to Mr. Samir Kumar for sample preparation and fruitful discussion during this work. The author 15 ACS Paragon Plus Environment


(SKG) is thankful to Ms. Anisha Pathak for consistent support in manuscript preparation. We are also thankful to Dr. Govind Gupta, NPL Delhi for XPS measurements.

Conflicts of interest There are no conflicts to declare.

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