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Pd nanoparticles and their application in detection of biodeteriorating fungi
Materials Chemistry and Physics 240 (2020) 122172
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
L-glycine-assisted synthesis of SnO2/Pd nanoparticles and their application in detection of biodeteriorating fungi Leila Shokrzadeh a, Parisa Mohammadi a, *, Mohammad Reza Mahmoudian b, Wan Jeffrey Basirun c, Masoumeh Bahreini d a
Department of Microbiology, Faculty of Biological Sciences, Alzahra University, Tehran, Iran Department of Chemistry, University of Farhangian, Tehran, Iran Department of Chemistry, University of Malaya, 50603, Kuala Lumpur, Malaysia d Department of Biology, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran b c
H I G H L I G H T S
� The synthesis of SnO2/Pd NPs in the presence of L-glycine as a reducing agent. � SnO2/Pd nanoparticles was able to detect 1-octen-3-ol, a fungal VOC. � SnO2/Pd showed better response compared to the pure SnO2. � The SnO2 decorated with Pd showed acceptable selectivity towards 1-octen-3-ol. A R T I C L E I N F O
A B S T R A C T
Keywords: SnO2/Pd Gas sensor 1-octen-3-ol Fungi Biodeterioration
The presence of certain volatile organic compounds is used for the detection of fungi in the indoor environments. The synthesis of SnO2/Pd nanoparticles (NPs) was performed via the reduction of Pd2þ in the presence of Lglycine. The XRD and EDX results confirmed the presence of palladium on the surface of SnO2 NPs. The sensing properties of SnO2/Pd and SnO2 NPs towards the detection of 1-octen-3-ol were analyzed by a static process. The SnO2/Pd and SnO2 NPs fabricated sensors gave a response of 46.7% and 24.2%, respectively, for 700 ppm target gas at 250 � C. The estimated limit of detection and limit of quantification of the fabricated SnO2/Pd sensor at 250 � C in the linear segments are 20.94 ppm and 69.79 ppm, respectively. These results demonstrated that the synthesis of SnO2/Pd NPs in the presence of L-glycine as a reducing agent has potential applications in the detection of fungal biodeterioration in infected materials at the early stages of infection.
1. Introduction The role of microorganisms, especially fungi in the biodeterioration of old papers, manuscripts, parchments and artefacts of cultural heritage is well known. The damage of paper products and library materials is a great concern for libraries and historical archives all over the world. On the other hand, in spite of the stringent requirements of storage condi tions such as low temperature, low humidity, and adequate ventilation, the biodeterioration of library materials is still inevitable [1]. Therefore, effective mitigation strategies must be implemented to prevent the development and growth of fungi, the main cause of biodeterioration in indoor environments. Efficient prevention methods of deterioration or degradation of preserved materials should be also identified.
The classical method of detection of biodeteriogens is based on culturing and molecular techniques, but this is only applicable after visible fungi growth occurs in an advanced stage of biodeterioration. Therefore, this approach incurs a high cost for the conservation and preservation of historical archives and artefacts. Thus, it is very crucial to detect the presence of biodeteriorating fungi in the early stages of growth to prevent serious damage. At present, there are only few reports on the detection of volatile organic compounds (VOCs) released from the different fungi species in indoor environments. Anton et al. employed three micro-modules based on conducting polymers on interdigitated electrodes (IDE), for the early detection of VOCs from fungi in indoor environments [2]. Joblin et al. also used different types of polymer sensors to detect fungal contamination in different environments [3]. In
* Corresponding author. E-mail address: [email protected] (P. Mohammadi). https://doi.org/10.1016/j.matchemphys.2019.122172 Received 30 April 2019; Received in revised form 29 August 2019; Accepted 10 September 2019 Available online 12 September 2019 0254-0584/© 2019 Published by Elsevier B.V.
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both works, polymer sensors were employed for the detection of fungal VOCs. Some researchers have focused on the identification of microbial volatile organic compounds (MVOCs) released from different types of fungi found in indoor environments [4,5]. For example, Fiedler et al. identified around 150 VOCs released from fungal cultures using head space and solid-phase microextraction (HS-SPME) techniques [4]. While Moularat et al. determined various types of VOCs from fungal meta bolism such as ethylhexanoic acid methyl ester, 1-octen-3-ol, 3-hepta nol, 3-methyl-1-butanol, 2-methyl-1-butanol, 1,3-octadiene, 2-(5H)-furanone, 2-heptene, a-pinene, 2-methylisoborneol, 4-hepta none, 2-methylfuran, 3-methylfuran, dimethyldisulfide, methox ybenzene, as well as a terpenoid and three types of sesquiterpenes using gas chromatography [5]. In this context, our interest is mainly focused on the development of a simple and new method of nanoparticle syn thesis for the in situ electrochemical sensing of fungal VOCs. It was recently reported that fluorescence probes could be used to identify different types of biomolecules in buffer solutions and living cells. Sun et al. synthesized a sensitive and selective turn-on fluorescence probe for the detection of CO [6]. While Yu et al. successfully utilized fluorescent MUA-stabilized Au nanoclusters for the detection of peni cillamine in water samples [7]. In these reports, the fabrication of a biosensor based on specific molecules and probes was performed in buffer solutions and living cells. In contrast, our method of detection of VOCs released from living cells is based on a gas nanosensing material. The nanosensing material produces an electrical signal which is pro portional to the target gas concentration. Also, unlike other techniques of detection such as fluorescence probes, infrared and photoionization detection (PID) technologies, the fabricated electrochemical nanosensor is low cost and only requires simple operation. Although most of the VOCs released from buildings such as libraries, museums and historical archives are closely related to MVOCs, it was reported that among the different types of VOCs released from different fungi species, 1-octen-3-ol is released at the highest concentration (900 μg m 3 or 0.16 ppm) from the most destructive fungi species [8–10]. Furthermore, this type of fungal VOC is one of the main causes of sick building syndrome in indoor environments [10]. Therefore, 1-octe n-3-ol is a fingerprint molecule for the detection and identification of filamentous fungi, especially in the early stages of germination before serious deterioration occurs. Studies have shown that low-cost metal oxides such as WO3, SnO2, In2O2 and ZnO are the most sensitive materials for the detection of VOCs [11]. Nanostructured tin oxide (SnO2) is an n-type direct wide band gap semiconductor which has attracted great interest for the fabrication of gas sensor devices due to the superior chemical stability, electrical properties, sensitivity and wide reactivity [12,13]. It was reported that the size, structure, and shape of the SnO2 nanostructures, whether in composite with other inorganic/organic materials or in the pure form, greatly influence the sensing performance [13]. Among the different types of nanostructured tin oxides, the nanograin morphology is the most common and shows higher electrical conductivity at the grain boundary [11]. However, a higher operating temperature is required (e. g., 573–673 K) for a faster rate of reaction between the target VOC molecules with the nanostructured tin oxide surface [11]. In other re ports, noble metals such as Pd enhances the sensitivity of the sensing materials for the detection of VOCs [11,14]. Another research demon strated that the sensitivity of SnO2 towards VOC detection in indoor environments can be improved by the addition of TiO2 dopant [15]. Therefore, the synthesis of different types of nanostructured catalysts for the sensing process is crucial for the fabrication of gas sensors. At present, there are very few reports on the synthesis of SnO2 nanostructures via a simple biomolecule assisted hydrothermal approach [16–18]. In this work, SnO2 decorated with Pd nanoparticles in the presence of L-glycine was synthesized and utilized as an electro chemical gas sensor for the detection of 1-octen-3-ol. In this report, VOC such as 1-octen-3-ol is the fingerprint molecule for the presence of molds, especially destructive fungi species found in libraries, museums,
and historical archives. 2. Materials and methods 2.1. Synthesis of SnO2/Pd in the presence of the L-glycine as a reducing agent Tin oxide nanoparticles were prepared by a simple sol-gel process according to the method of Feng Gu et al. with minor modifications [19]. First, 10 g tin chloride (hydrous SnCl4⋅5H2O, Sigma Aldrich) was dis solved in distilled water. Then, 2 M aqueous ammonia solution was added drop-wise into the tin chloride solution until pH 4 and kept stirred for 10 min. The final product was filtered and washed several times with distilled water to eliminate the chloride ions from the mixture. The gel particles were dried at 80 � C, followed by calcination at 500 � C for 2 h. The synthesized SnO2 NPs were used as a substrate in the next section, and Pd was decorated on the surface of synthesized SnO2 NPs as the following process. The SnO2 NPs decorated with palladium (SnO2/Pd NPs) were syn thesized in the presence of L-glycine (Fluka, Buchs, Switzerland) as a reducing agent by the hydrothermal method. One of the objectives of this work is to use the minimum amount of palladium for improving SnO2 because of its cost. Therefore, 4 μl, 8 μl, and 16 μl of prepared so lution of 0.5 M palladium chloride (Sigma Aldrich) were added to 1% (w/v) SnO2 NPs dispersion in the distilled water using a homogenizer. This was followed by the addition of 10 mL aqueous (1%) glycine into the mixture. The mixture was heated at 100 � C for 1 h until precipita tion, followed by filtration and washing with distilled water several times to remove the excess reagents. Finally, the SnO2/Pd NPs were dried at room temperature and stored in the dark. The 16 μl prepared solution of 0.5 M palladium chloride was selected based on the weight percentage of deposited Pd on the surface of SnO2 NPs become close to 1% w/w. Moreover, the results showed that the prepared sample by 4 and 8 μl of prepared solution of 0.5 M palladium chloride did not show a significant response to the detection of 1-octen3-ol in comparison to pure SnO2 NPs. 2.2. Material characterization The phase structure of the SnO2/Pd NPs and SnO2 NPs were char acterized by X-ray powder diffraction (Siemens D5000) with Cu Kα ra diation (wavelength ¼ 1.541874 Å) from 20� to 80� . The morphology and structural investigations were carried out using a transmission electron microscope [(TEM); Philips CM120, Netherlands] while the elemental analysis was performed by EDS integrated with the FESEM system. 2.3. Fabrication of gas sensing device Conductive fluorine-doped tin oxide (FTO; Sharif Solar, Iran) glass was used as the substrate (cheap, simple and available) for the sensing electrode. Firstly, the FTO-coated glass was etched with zinc powder (Fluka, Buchs, Switzerland) and 2 M HCl (Fluka, Buchs, Switzerland), to obtain the required electrode pattern according to the following steps. The etch-resistant stickers were cut into a comb-shaped pattern and pasted onto the FTO (2 cm � 2 cm) glass substrate. Next, the substrate was coated with a layer of zinc powder and immersed in a 2 M HCl so lution to complete the reaction. Finally, the etched FTO was washed and rinsed with distilled water. Two silver electrodes, each on top and at the bottom of the comb-shaped pattern, were used as the electrical contacts to the sensing material. The resistance across the etched area was measured by a digital multimeter (VICTOR VC97 Digital Multimeter 4000 Count). In the second step, ethyl cellulose (99%), alfa-terpineol (99%) and ethanol (all from Sigma-Aldrich) were mixed by successive sonication and stirring, was used as the paste for the SnO2 and SnO2/Pd NPs. 2
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Fig. 1. The XRD patterns of (a) SnO2 NPs and (b) SnO2/Pd NPs in present of L-glycine.
Precisely, 0.05 g ethyl cellulose (binder) was mixed with 2 g ethanol and 0.35 g α-terpinol (solvents) and stirred for 15 min. Then, 0.1 g nano particle was added into the solution and dispersed, for an hour by stir ring for 30 min with an ultrasonicator and stirred again for an hour, until the grains dissolved into a homogeneous solution [20]. A thin layer of the slurry was drop-casted onto the etched FTO substrate and annealed at 500 � C for an hour to establish good electrical contacts.
Fig. 3. Schematic interaction between L-Glycine and Pd2þ.
2.4. Gas sensing experiments
C¼
The gas sensing properties of SnO2 and SnO2/Pd nanostructures were investigated in a homemade gas chamber according to the following procedures. The sensor was placed onto a small heater in a sealed glass and PTFE (polytetrafluoroethylene) chamber at the desired temperature with precision control. When the resistance of the sensor was stabilized, it was recorded as the electrical resistance of dry air (Ra). Then, a known amount of 1-octen-3-ol (Merck), as the target VOC released from fungal biodeterioration, was injected into the chamber using a syringe and mixed with air. The resistance of the sensor began to change upon the injection of the gas. The resistance value stabilizes after a few minutes and was recorded as the electrical resistance of the target gas at the specified temperature (Rg). The response of the gas sensor toward various VOCs (S) is defined as (Ra-Rg)/Ra � 100 [21]. The concentra tion of the injected target gas in the chamber was calculated (ppm) and described by the formula:
22:4ρTVs � 1000 273MV
(1)
Where C and ρ is the concentration of the target gas (ppm) and the density of liquid gas (g/mL), respectively. In equation (1), T and Vs replace the temperature (K) and volume of liquid gas (μL), respectively. While M and V are the molecular weight of the target gas (g/mol) and volume of the chamber (L), respectively [22]. The time taken for the sensor to reach 90% of the total resistance is defined as the response time for gas adsorption or the recovery time for gas desorption [21]. 3. Results and discussions 3.1. Material characterization The crystal structures of SnO2 and SnO2/Pd were investigated by XRD. Fig. 1(a) illustrates the XRD patterns of the synthesized SnO2 NPs (reference 01-077-044). The diffraction peaks at 26.56� , 33.83� , 37.91� ,
Fig. 2. EDS spectra of SnO2/Pd NPs preparing in present of L-glycine. 3
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Fig. 4. TEM images and particle size histograms of SnO2 NPs (a, b), SnO2/Pd NPs (c, d).
38.93� , 51.71� , 54.70� , 57.74� , 62.53� , 65.89� , 69.16� , 71.18� and 78.60� are attributed to the crystal planes of (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 1), (2 2 0), (0 0 2), (221), (3 1 1), (2 0 2) and (3 2 1) of tetragonal SnO2 NPs, respectively. The diffraction peaks of Sn and SnO are absent in the SnO2 diffractogram. The XRD pattern of the synthesized SnO2/Pd in the presence of L-glycine (Fig. 1b) confirms the tetragonal structure of SnO2 while the peaks at 39.67� (111) and 66.76� (220) are attributed to the Pd metal (reference code 00-001-1310). The EDX result of SnO2/Pd NPs confirms the presence of Pd (Fig. 2). As shown in Figs. 1 and 2, the XRD and EDX results not only confirm the presence of Pd, but also shows the peaks of Pd metal from the reduction of Pd (II) to Pd (0) in the presence of L-glycine. Recent reports also suggest that amino acids can be used as the reducing agent. Most researchers believe that the carbonyl group forms coordination bonds with the metal cations and reduces them to metallic elements [16,17]. Fig. 3 shows the schematic interac tion between L-glycine and Pd2þ, with the first binding step and second reducing step, respectively. The TEM images of SnO2 and synthesized SnO2/Pd in the presence of L-glycine at different magnifications are shown in Fig. 4(a–b) and (c–d), respectively. The morphology of the particles is predominantly spherical and granular. The histogram shows that the diameter of SnO2 NPs and SnO2/Pd NPs are approximately 20.66 and 20.97 nm, respectively. As shown in Fig. 4, Pd incorporation into the SnO2 NPs had no significant effect on the grain size and morphology of tin oxide.
Fig. 5. Dependence of response on operating temperature of pure SnO2 NPs and SnO2/Pd NPs to 100 ppm.
was utilized as a biomarker in the gas sensor. The gas sensing device was fabricated from pure SnO2 and SnO2/Pd synthesized in the presence of Lglycine as the reducing agent. The electrical resistance of pure SnO2 and SnO2/Pd samples in the air as a function of temperature is presented in Fig. 5. In both samples, the sensitivity of the sample increases with the operating temperature. This is attributed to the adsorbed oxygen mol ecules on the semiconductor metal oxide surface in the form of O and O2 , in addition to the increase of thermal energy with operating tem perature [23]. On the other hand, as shown in Fig. 5, the presence of Pd on the SnO2 substrate enhances the sensor response at low and high temperatures (100–300 � C). However, the sensor based on pure tin oxide did not show any response between 100 and 150 � C. In the other words,
3.2. 1-octen-3-ol sensing properties The common fungal VOC, 1-octen-3-ol, released from the growth of biodeteriorative fungi on ancient artefacts and contaminated materials 4
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Fig. 6. Transient response curves of the pure SnO2/Pd NPs (a) and SnO2 NPs (c) upon exposure to different concentrations of 1-octen-3-ol. Resistance of the SnO2/Pd NPs (b) and pure SnO2 NPs (d) samples at 50 ppm 1-octen-3-ol in 250 � C. The magnified response–recovery characteristic curves for 50 ppm SnO2 NPs and SnO2/Pd NPs in 250 � C (e).
the reduction of Pd (II) on SnO2 NPs gave better response even at lower temperatures between 100 and 150 � C, which is one of the advantages of Pd deposition onto the surface of SnO2 NPs. The sensing measurements at temperatures above 300 � C could not be performed due to the FTO substrate which degrades at higher temperatures and the limitation of the sensing apparatus. Fig. 6 presents the transient response curve of the two functionalized samples, i.e. pure SnO2 NPs and SnO2/Pd NPs towards 50–200 ppm 1octen-3-ol at 250 � C. As shown in Fig. 6a, the response of SnO2/Pd NPs increases upon the exposure to 1-octen-3-ol. However, the resis tance of the sensor declines to the baseline when exposed to air. This observation confirms the reversibility of the fabricated SnO2/Pd NPs sensor. The change in the resistance of pure SnO2 NPs upon the exposure to 1-octen-3-ol is similar to SnO2/Pd, however, the SnO2/Pd shows a significant response compared to the pure SnO2 NPs (Fig. 6e). As can be seen from Fig. 6 (b, d), the electrical resistance of SnO2 rose after the Pd reducing process. The Ra changed from 1.93 kΩ (SnO2) to 53 kΩ (SnO2/ Pd), furthermore in 50 ppm 1-octen-3-ol, Rg increased from 1.72 kΩ (SnO2) to 45 kΩ (SnO2/Pd). As it was expected, the sensor response improved by 1.39 times from 10.9 (SnO2) to 15.1 (SnO2/Pd). It has been reported that in the air, Pd forms a stable oxide. In the oxygen phase, Pd acts as an electron acceptor and traps the charge from SnO2. Therefore, the depletion of electrons from the metal oxide surface, causing a rise in resistance [30]. On the other hand, the SnO2/Pd sensor exhibited a higher sensitivity compared to the pure SnO2 NPs. These results are encouraging and in accordance with previous reports [24–26]. The addition of catalytically active metals such as Pd and Pt can promote chemical and electrical
sensitization, and besides the sensitivity and selectivity of sensing de vices can be improved [27–30]. Hence, in present findings the reducing Pd on SnO2 NPs improve the electrical and chemical sensitization and, these effects are responsible to accelerate sensing reaction and improve sensing performance. The enhanced response towards the target gas is attributed to the presence of Pd metal which provides effective catalytic adsorption sites for oxygen and other reducing gases [30]. On the other hand, the Pd atoms entrap the electrons from the metal oxide. The entrapped charges provide stronger depletion of electrons from the metal oxide surface which increases the sensitivity [31]. From previous reports [23–25], the trapped oxygen species can be adsorbed onto the surface of SnO2 NPs even in an air-conditioned atmosphere. In the next step, the adsorbed oxygen is ionized to O , O2 and O2 by capturing free electrons from the n-type metal oxide. The presence of Pd increases the electron mobility and the available surface area of the semiconductor which enhances the oxygen adsorption. On the other hand, 1-octen-3-ol is a reducing gas, therefore it reacts with the adsorbed oxygen ions which release the adsorbed electrons when exposed to the sensor sur face. The following reaction mechanism shows the interaction between the oxygen ions and 1-octen-3-ol on the sensor surface. O2 (g)→O2 (ads)
(2)
O2 (ads) þ1e→O2 (ads)
(3)
O2 (ads) þ1e→2O (ads)
(4)
C8H15OH (g) þ2O (ads)→CO2 (g) þ H2O (g) þ2e
(5)
In the above equations (2)–(5), “ads” represents “adsorbed” ions. 5
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Table 1 The LOD and LOQ value of the fabricated sensor with SnO2/Pd at the 100, 150 and 250 � C temperature (a). The stability results of the SnO2/Pd-based sensor for 250 ppm of 1-octen-3-ol at 250 � C (b). a
b
Temperature LOD (ppm) LOQ (ppm)
100 � C 82.33 274.42
150 � C 33.51 111.70
250 � C 20.94 69.79
Stability Response%
First experiment 25.10
After ten days 25.01
After seven months 24.20
Fig. 7. Calibration plot for effect of reduced Pd on the SnO2 in comparing to pure SnO2 in different concentration (50–350 ppm) of 1-octen-3-ol in 250 � C.
Fig. 9. The selectivity of the SnO2 and SnO2/Pd, toward the 700 ppm of 1octen-3-ol, acetone, isopropanol, methanol, ethanol, formaldehyde, and n-hexane.
coefficient (R2) of SnO2 NPs and the curve fitting of SnO2/Pd NPs values of 0.88 and 0.97, respectively. Therefore, the linear rate increased by reduced Pb on SnO2 NPs. Fig. 8 shows the variation of Ra-Rg with different concentrations of 1-octen-3-ol at 100 � C (a), 150 � C (b) and 250 � C (c), respectively. These results clearly show that the sensitivity increases from 0.0003 kOhm ppm 1 to 0.0435 kOhm ppm 1 with the increase of temperature. These temperatures were chosen based on the results in Fig. 5. From the results in Fig. 8, equations (6) and (7), the limit of detection (LOD) and limit of quantification (LOQ) of the SnO2/Pd sensor at the above temperatures are given in Table 1(a) [32]. LOD ¼ 3 SB / b
(6)
LOQ ¼ 10 SB / b
(7)
The estimated LOD and LOQ values improved with the increase of temperature from 100 � C to 250 � C. The stability of the SnO2/Pd NPs sensor towards the detection of 1-octen-3-ol at 250 � C was investigated and shown in Table 1(b). The experiments were performed at 250 ppm 1-octen-3-ol, after keeping the sensor in ambient air at room tempera ture for 10 days and seven months interval. The determination of 1octen-3-ol at 250 � C was performed after a storage interval of 10 days and seven months, where the response percentage decreased to only 0.34% and 3.56%, respectively. This confirms that the SnO2/Pd NPs has acceptable stability towards the detection of 1-octen-3-ol at 250 � C. According to WHO, volatile organic compounds are the most important pollutants of indoor and outdoor environments. As mentioned in the literature review, the main VOCs including isopropanol, meth anol, ethanol, formaldehyde, and n-hexane may be found in the indoor air and are the environmental pollutants that can constitute a health hazard [33,34]. In the present study, the selectivity of fabricated sensors was tested towards 1-octer-3-ol, which represents the fungal growth, and the other VOCs, which can be usually present in libraries. Prior studies that have noted the selectivity characteristic is related to the discrimination capacity of a sensor in front of a different of gases. This factor is closely associated with many factors such as morphology, the
Fig. 8. The variation of Ra-Rg vs different concentrations of 1-octen-3-ol at 100 � C (a), 150 � C (b) and 250 � C (c) of the SnO2/Pd NPs.
Fig. 7 presents the linear fitting, together with the correlation 6
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Fig. 10. The transient response curves of the pure SnO2/Pd NPs upon exposure of the 3 contaminated (Aspergillus sp. on YGC media) atmospheres, separately.
presence of a catalyst, and operating temperature [35]. Fig. 9 shows the response of SnO2 NPs and SnO2/Pd NPs toward 700 ppm 1-octen-3-ol, as the biomarker for fungal VOCs and 1400 ppm of acetone, isopropanol, methanol, ethanol, formaldehyde, and n-hex ane as the common environmental VOC pollutants. The current study found that the SnO2 NPs and SnO2/Pd NPs displayed a higher response to 1-octen-3-ol in comparison with other pollutants. The higher response of the sensors to 1-octen-3-ol can be related to the polarity of the detected molecules. The response of the SnO2/Pd NPs and SnO2 NPs fabricated sensor towards 700 ppm 1-octen-3-ol is 46.70% and 24.16%, respectively. Therefore, as it was expected, the response of the sample based on SnO2/Pd NPs showed better selectivity to 1-octen-3-ol than SnO2 NPs. In the other words, the selectivity was enhanced to analyte by the reduction of Pd2þ on the SnO2 NPs used as a substrate via the Lglycine-assisted synthesis. Hence, as mentioned in the literature review, the presence of catalytic metallic additives such as Pd can often improve the selectivity of SnO2 NPs based gas sensors.
placed. Upon the injection of the fungal VOCs, the resistance of the sensor began to decrease. The value stabilizes after a few minutes and was recorded as the electrical resistance of the VOCs at the specified temperature. After switching from fungal VOCs to air, the resistance of the sensor changed to the baseline, and the headspace above the second Aspergillus sp. culture was introduced into the test chamber. After the resistance was stabilized, a third test was repeated for the third real sample. The response was calculated similarly to the response of the 1-octen-3-ol. Fig. 10 illustrates the transient response curves of SnO2/Pd NPs upon exposure to the 3 contaminated (Aspergillus sp. on YGC media) atmosphere, separately. With respect to Figs. 7 and 10, the estimated concentration of fungal VOCs in 17.74%, 18.17%, and 19.25% sensor responses are 97.56, 99.03 and 101.62 ppm, respectively. These tests confirm the feasibility of the fabricated sensor for the detection of the biodeteriorating fungi. As mentioned before, albeit SnO2/Pd showed the highest sensitivity and good selectivity towards the target gas released from real samples, it is still sensitive to temperature changes, therefore the temperature stability is an important requirement for highly sensitive and selective detection of the fungal VOC. To comprise, a summary of reviewed gas sensors that reported in previous researches and this work is displayed in Table 2.
3.3. Real sample analysis According to Fidler et al. (2001) and other researches (4, 5, 8) most of Aspergillus species such as Aspergillus fumigatus, Aspergillus versicolor, Aspergillus niger, Aspergillus ochraceus, and other species can produce 1octen-3-ol. Therefore, in the present study Aspergillus sp. was selected. This genus was isolated from an old biodeteriorated manuscript located in one of the repositories in Iran, with high-frequency. To investigate the detection of the target fungi by the presence of the VOCs, the same concentration of Aspergillus sp. spores isolated from an old infected manuscript was inoculated in 3 sealed YGC (Yeast Extract. Glucose Chloramphenicol. Agar FIL-IDF, Merck) media. After 72 h of incubation at room temperature, the headspace above the 3 real samples was introduced into the test chamber separately where the sensor is
4. Conclusions To conclude, the synthesis of SnO2 nanoparticles decorated with Pd in the presence of L-glycine and its sensing performance towards 1octen-3-ol, a biomarker for the presence of destructive fungi species in indoor environments, was successfully fabricated and tested. The nanoparticles were spherical and granular in shape. The SnO2/Pd sensor showed better response compared to the pure SnO2 NPs at the lower selected temperature (100–150 � C). Furthermore, the SnO2/Pd sample gave significant enhancement in the sensitivity towards the detection of
Table 2 Summary of gas sensors review in previous reports and present work. Target gas
Sensor structure
Operation
LOD
References
Octane Toluene 1-octanol 1-octen-3-ol
SAW sensors based on PIB/MWNTs SAW sensors based on PIB/MWNTs SWCNTs in FET structures SnO2-SP (Electrochemical)
1-octen-3-ol
TiO2-SP (Electrochemical)
1-octanol 1-octen-3-ol
Polymer-based sensor SnO2/Pd
Resistance response vs. concentration change Resistance response vs. concentration change Response vs. saturated vapor Cyclic voltammetry Differential pulse voltammetry Cyclic voltammetry Differential pulse voltammetry Response vs. resistivity change Resistance response vs. concentration change
Not detected Not detected Not detected 82 nM 62 nM 128 nM 35 nM Not detected 20.94 ppm
[36] [36] [37] [38] [38] [38] [38] [39] This work
SAM: Surface acoustic wave. PIB: Polyisobutylene. MWCNTs: Multi-walled carbon nanotube. SWCNTs: Single-walled semiconducting carbon nanotubes. FET: Fieldeffect transistor. 7
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1-octen-3-ol, compared to the pure SnO2. Besides, the sensor based on SnO2/Pd gave the highest response towards 1-octen-3-ol compared to other gases present in the same environment but not from the fungal origin. It can be concluded that the reduction of Pd2þ on the SnO2 NPs surface via the L-glycine-assisted synthesis showed acceptable selectivity towards certain fungal VOCs. From the real sample tests, the SnO2/Pdbased sensor is feasible for the detection of deteriorating fungi, although further tests must be conducted before it can be used for in situ applications.
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Acknowledgments This study was financed by Vice-Chancellor of Alzahra and carried out at Shayesteh Sepehr Laboratories of Alzahra University. The authors would like to thank Mohsen Cheraghizade, a Ph.D. student in Electronic Engineering from Islamic Azad University (IAU), Mahshahr branch, for his helpful advice on the technical issues in the paper. References [1] A. Micheluz, S. Manente, V. Tigini, V. Prigione, F. Pinzari, G. Ravagnan, G. C. Varese, The extreme environment of a library: xerophilic fungi inhabiting indoor niches, Int. Biodeterior. Biodegrad. 99 (2015) 1–7. https://doi.org/10.1016/j. ibiod.2014.12.012. [2] R. Anton, S. Moularat, E. Robine, A new approach to detect early or hidden fungal development in indoor environments, Chemosphere 143 (2016) 41–49. https://do i.org/10.1016/j.chemosphere.2015.06.072. [3] Y. Joblin, S. Moularat, R. Anton, F. Bousta, G. Orial, E. Robine, O. Picon, T. Bourouina, Detection of moulds by volatile organic compounds: application to heritage conservation, Int. Biodeterior. Biodegrad. 64 (2010) 210–217. https://doi. org/10.1016/j.ibiod.2010.01.006. [4] K. Fiedler, E. Schütz, S. Geh, Detection of microbial volatile organic compounds (MVOCs) produced by moulds on various materials, Int. J. Hyg Environ. Health 204 (2001) 111–121. https://doi.org/10.1078/1438-4639-00094. [5] S. Moularat, E. Robine, O. Ramalho, M.A. Oturan, Detection of fungal development in closed spaces through the determination of specific chemical targets, Chemosphere 72 (2008) 224–232. https://doi.org/10.1016/j.chemosphere.200 8.01.057. [6] M. Sun, H. Yu, K. Zhang, S. Wang, T. Hayat, A. Alsaedi, D. Huang, M. Sun, H. Yu, K. Zhang, S. Wang, T. Hayat, A. Alsaedi, A palladacycle based fluorescence turn-on probe for sensitive detection of carbon monoxide, ACS Sens. 3 (2) (2018) 285–289. https://doi.org/10.1021/acssensors.7b00835. [7] H. Yu, X. Chen, L. Yu, M. Sun, K.A. Alamry, A.M. Asiri, K. Zhang, Fluorescent MUAstabilized Au nanoclusters for sensitive and selective detection of penicillamine, Anal. Bioanal. Chem. 410 (2018), 2629–36, https://doi.org/10.1007/s00216-0 18-0936-7. [8] S. Moularat, E. Robine, O. Ramalho, M.A. Oturan, Detection of fungal development in a closed environment through the identification of specific VOC: demonstration of a specific VOC fingerprint for fungal development, Sci. Total Environ. 407 (2008) 139–146. https://doi.org/10.1016/j.scitotenv.2008.08.023. [9] A. Sunesson, W. Vaes, C. Nilsson, G. Blomquist, B. Andersson, A. Sunesson, W.H. J. Vaes, C. Nilsson, Identification of volatile metabolites from five fungal species cultivated on two media. These include: identification of volatile metabolites from five fungal species cultivated on two media, Appl. Environ. Microbiol. 61 (1995) 2911–2918. [10] S.U. Morath, R. Hung, J.W. Bennett, Fungal volatile organic compounds: a review with emphasis on their biotechnological potential, Fungal Biol. Rev. 26 (2012) 73–83. https://doi.org/10.1016/j.fbr.2012.07.001. [11] L. Spinelle, M. Gerboles, G. Kok, S. Persijn, T. Sauerwald, Review of portable and low-cost sensors for the volatile organic compounds, Sensors 17 (2017) 1520. http s://doi.org/10.3390/s17071520. [12] G. Jim�enez-Cadena, J. Riu, F.X. Rius, Gas sensors based on nanostructured materials, Analyst 132 (2007) 1083–1099. https://doi.org/10.1039/b704562j. [13] C. Wang, L. Yin, L. Zhang, D. Xiang, R. Gao, Metal oxide gas sensors: sensitivity and influencing factors, Sensors 10 (2010) 2088–2106. https://doi.org/10.3390/s 100302088. [14] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles, Nano Lett. 5 (2005) 667–673. https://doi.org/10.1021/ nl050082v.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
L-glycine-assisted synthesis of SnO2/Pd nanoparticles and their application in detection of biodeteriorating fungi Leila Shokrzadeh a, Parisa Mohammadi a, *, Mohammad Reza Mahmoudian b, Wan Jeffrey Basirun c, Masoumeh Bahreini d a
Department of Microbiology, Faculty of Biological Sciences, Alzahra University, Tehran, Iran Department of Chemistry, University of Farhangian, Tehran, Iran Department of Chemistry, University of Malaya, 50603, Kuala Lumpur, Malaysia d Department of Biology, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran b c
H I G H L I G H T S
� The synthesis of SnO2/Pd NPs in the presence of L-glycine as a reducing agent. � SnO2/Pd nanoparticles was able to detect 1-octen-3-ol, a fungal VOC. � SnO2/Pd showed better response compared to the pure SnO2. � The SnO2 decorated with Pd showed acceptable selectivity towards 1-octen-3-ol. A R T I C L E I N F O
A B S T R A C T
Keywords: SnO2/Pd Gas sensor 1-octen-3-ol Fungi Biodeterioration
The presence of certain volatile organic compounds is used for the detection of fungi in the indoor environments. The synthesis of SnO2/Pd nanoparticles (NPs) was performed via the reduction of Pd2þ in the presence of Lglycine. The XRD and EDX results confirmed the presence of palladium on the surface of SnO2 NPs. The sensing properties of SnO2/Pd and SnO2 NPs towards the detection of 1-octen-3-ol were analyzed by a static process. The SnO2/Pd and SnO2 NPs fabricated sensors gave a response of 46.7% and 24.2%, respectively, for 700 ppm target gas at 250 � C. The estimated limit of detection and limit of quantification of the fabricated SnO2/Pd sensor at 250 � C in the linear segments are 20.94 ppm and 69.79 ppm, respectively. These results demonstrated that the synthesis of SnO2/Pd NPs in the presence of L-glycine as a reducing agent has potential applications in the detection of fungal biodeterioration in infected materials at the early stages of infection.
1. Introduction The role of microorganisms, especially fungi in the biodeterioration of old papers, manuscripts, parchments and artefacts of cultural heritage is well known. The damage of paper products and library materials is a great concern for libraries and historical archives all over the world. On the other hand, in spite of the stringent requirements of storage condi tions such as low temperature, low humidity, and adequate ventilation, the biodeterioration of library materials is still inevitable [1]. Therefore, effective mitigation strategies must be implemented to prevent the development and growth of fungi, the main cause of biodeterioration in indoor environments. Efficient prevention methods of deterioration or degradation of preserved materials should be also identified.
The classical method of detection of biodeteriogens is based on culturing and molecular techniques, but this is only applicable after visible fungi growth occurs in an advanced stage of biodeterioration. Therefore, this approach incurs a high cost for the conservation and preservation of historical archives and artefacts. Thus, it is very crucial to detect the presence of biodeteriorating fungi in the early stages of growth to prevent serious damage. At present, there are only few reports on the detection of volatile organic compounds (VOCs) released from the different fungi species in indoor environments. Anton et al. employed three micro-modules based on conducting polymers on interdigitated electrodes (IDE), for the early detection of VOCs from fungi in indoor environments [2]. Joblin et al. also used different types of polymer sensors to detect fungal contamination in different environments [3]. In
* Corresponding author. E-mail address: [email protected] (P. Mohammadi). https://doi.org/10.1016/j.matchemphys.2019.122172 Received 30 April 2019; Received in revised form 29 August 2019; Accepted 10 September 2019 Available online 12 September 2019 0254-0584/© 2019 Published by Elsevier B.V.
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both works, polymer sensors were employed for the detection of fungal VOCs. Some researchers have focused on the identification of microbial volatile organic compounds (MVOCs) released from different types of fungi found in indoor environments [4,5]. For example, Fiedler et al. identified around 150 VOCs released from fungal cultures using head space and solid-phase microextraction (HS-SPME) techniques [4]. While Moularat et al. determined various types of VOCs from fungal meta bolism such as ethylhexanoic acid methyl ester, 1-octen-3-ol, 3-hepta nol, 3-methyl-1-butanol, 2-methyl-1-butanol, 1,3-octadiene, 2-(5H)-furanone, 2-heptene, a-pinene, 2-methylisoborneol, 4-hepta none, 2-methylfuran, 3-methylfuran, dimethyldisulfide, methox ybenzene, as well as a terpenoid and three types of sesquiterpenes using gas chromatography [5]. In this context, our interest is mainly focused on the development of a simple and new method of nanoparticle syn thesis for the in situ electrochemical sensing of fungal VOCs. It was recently reported that fluorescence probes could be used to identify different types of biomolecules in buffer solutions and living cells. Sun et al. synthesized a sensitive and selective turn-on fluorescence probe for the detection of CO [6]. While Yu et al. successfully utilized fluorescent MUA-stabilized Au nanoclusters for the detection of peni cillamine in water samples [7]. In these reports, the fabrication of a biosensor based on specific molecules and probes was performed in buffer solutions and living cells. In contrast, our method of detection of VOCs released from living cells is based on a gas nanosensing material. The nanosensing material produces an electrical signal which is pro portional to the target gas concentration. Also, unlike other techniques of detection such as fluorescence probes, infrared and photoionization detection (PID) technologies, the fabricated electrochemical nanosensor is low cost and only requires simple operation. Although most of the VOCs released from buildings such as libraries, museums and historical archives are closely related to MVOCs, it was reported that among the different types of VOCs released from different fungi species, 1-octen-3-ol is released at the highest concentration (900 μg m 3 or 0.16 ppm) from the most destructive fungi species [8–10]. Furthermore, this type of fungal VOC is one of the main causes of sick building syndrome in indoor environments [10]. Therefore, 1-octe n-3-ol is a fingerprint molecule for the detection and identification of filamentous fungi, especially in the early stages of germination before serious deterioration occurs. Studies have shown that low-cost metal oxides such as WO3, SnO2, In2O2 and ZnO are the most sensitive materials for the detection of VOCs [11]. Nanostructured tin oxide (SnO2) is an n-type direct wide band gap semiconductor which has attracted great interest for the fabrication of gas sensor devices due to the superior chemical stability, electrical properties, sensitivity and wide reactivity [12,13]. It was reported that the size, structure, and shape of the SnO2 nanostructures, whether in composite with other inorganic/organic materials or in the pure form, greatly influence the sensing performance [13]. Among the different types of nanostructured tin oxides, the nanograin morphology is the most common and shows higher electrical conductivity at the grain boundary [11]. However, a higher operating temperature is required (e. g., 573–673 K) for a faster rate of reaction between the target VOC molecules with the nanostructured tin oxide surface [11]. In other re ports, noble metals such as Pd enhances the sensitivity of the sensing materials for the detection of VOCs [11,14]. Another research demon strated that the sensitivity of SnO2 towards VOC detection in indoor environments can be improved by the addition of TiO2 dopant [15]. Therefore, the synthesis of different types of nanostructured catalysts for the sensing process is crucial for the fabrication of gas sensors. At present, there are very few reports on the synthesis of SnO2 nanostructures via a simple biomolecule assisted hydrothermal approach [16–18]. In this work, SnO2 decorated with Pd nanoparticles in the presence of L-glycine was synthesized and utilized as an electro chemical gas sensor for the detection of 1-octen-3-ol. In this report, VOC such as 1-octen-3-ol is the fingerprint molecule for the presence of molds, especially destructive fungi species found in libraries, museums,
and historical archives. 2. Materials and methods 2.1. Synthesis of SnO2/Pd in the presence of the L-glycine as a reducing agent Tin oxide nanoparticles were prepared by a simple sol-gel process according to the method of Feng Gu et al. with minor modifications [19]. First, 10 g tin chloride (hydrous SnCl4⋅5H2O, Sigma Aldrich) was dis solved in distilled water. Then, 2 M aqueous ammonia solution was added drop-wise into the tin chloride solution until pH 4 and kept stirred for 10 min. The final product was filtered and washed several times with distilled water to eliminate the chloride ions from the mixture. The gel particles were dried at 80 � C, followed by calcination at 500 � C for 2 h. The synthesized SnO2 NPs were used as a substrate in the next section, and Pd was decorated on the surface of synthesized SnO2 NPs as the following process. The SnO2 NPs decorated with palladium (SnO2/Pd NPs) were syn thesized in the presence of L-glycine (Fluka, Buchs, Switzerland) as a reducing agent by the hydrothermal method. One of the objectives of this work is to use the minimum amount of palladium for improving SnO2 because of its cost. Therefore, 4 μl, 8 μl, and 16 μl of prepared so lution of 0.5 M palladium chloride (Sigma Aldrich) were added to 1% (w/v) SnO2 NPs dispersion in the distilled water using a homogenizer. This was followed by the addition of 10 mL aqueous (1%) glycine into the mixture. The mixture was heated at 100 � C for 1 h until precipita tion, followed by filtration and washing with distilled water several times to remove the excess reagents. Finally, the SnO2/Pd NPs were dried at room temperature and stored in the dark. The 16 μl prepared solution of 0.5 M palladium chloride was selected based on the weight percentage of deposited Pd on the surface of SnO2 NPs become close to 1% w/w. Moreover, the results showed that the prepared sample by 4 and 8 μl of prepared solution of 0.5 M palladium chloride did not show a significant response to the detection of 1-octen3-ol in comparison to pure SnO2 NPs. 2.2. Material characterization The phase structure of the SnO2/Pd NPs and SnO2 NPs were char acterized by X-ray powder diffraction (Siemens D5000) with Cu Kα ra diation (wavelength ¼ 1.541874 Å) from 20� to 80� . The morphology and structural investigations were carried out using a transmission electron microscope [(TEM); Philips CM120, Netherlands] while the elemental analysis was performed by EDS integrated with the FESEM system. 2.3. Fabrication of gas sensing device Conductive fluorine-doped tin oxide (FTO; Sharif Solar, Iran) glass was used as the substrate (cheap, simple and available) for the sensing electrode. Firstly, the FTO-coated glass was etched with zinc powder (Fluka, Buchs, Switzerland) and 2 M HCl (Fluka, Buchs, Switzerland), to obtain the required electrode pattern according to the following steps. The etch-resistant stickers were cut into a comb-shaped pattern and pasted onto the FTO (2 cm � 2 cm) glass substrate. Next, the substrate was coated with a layer of zinc powder and immersed in a 2 M HCl so lution to complete the reaction. Finally, the etched FTO was washed and rinsed with distilled water. Two silver electrodes, each on top and at the bottom of the comb-shaped pattern, were used as the electrical contacts to the sensing material. The resistance across the etched area was measured by a digital multimeter (VICTOR VC97 Digital Multimeter 4000 Count). In the second step, ethyl cellulose (99%), alfa-terpineol (99%) and ethanol (all from Sigma-Aldrich) were mixed by successive sonication and stirring, was used as the paste for the SnO2 and SnO2/Pd NPs. 2
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Fig. 1. The XRD patterns of (a) SnO2 NPs and (b) SnO2/Pd NPs in present of L-glycine.
Precisely, 0.05 g ethyl cellulose (binder) was mixed with 2 g ethanol and 0.35 g α-terpinol (solvents) and stirred for 15 min. Then, 0.1 g nano particle was added into the solution and dispersed, for an hour by stir ring for 30 min with an ultrasonicator and stirred again for an hour, until the grains dissolved into a homogeneous solution [20]. A thin layer of the slurry was drop-casted onto the etched FTO substrate and annealed at 500 � C for an hour to establish good electrical contacts.
Fig. 3. Schematic interaction between L-Glycine and Pd2þ.
2.4. Gas sensing experiments
C¼
The gas sensing properties of SnO2 and SnO2/Pd nanostructures were investigated in a homemade gas chamber according to the following procedures. The sensor was placed onto a small heater in a sealed glass and PTFE (polytetrafluoroethylene) chamber at the desired temperature with precision control. When the resistance of the sensor was stabilized, it was recorded as the electrical resistance of dry air (Ra). Then, a known amount of 1-octen-3-ol (Merck), as the target VOC released from fungal biodeterioration, was injected into the chamber using a syringe and mixed with air. The resistance of the sensor began to change upon the injection of the gas. The resistance value stabilizes after a few minutes and was recorded as the electrical resistance of the target gas at the specified temperature (Rg). The response of the gas sensor toward various VOCs (S) is defined as (Ra-Rg)/Ra � 100 [21]. The concentra tion of the injected target gas in the chamber was calculated (ppm) and described by the formula:
22:4ρTVs � 1000 273MV
(1)
Where C and ρ is the concentration of the target gas (ppm) and the density of liquid gas (g/mL), respectively. In equation (1), T and Vs replace the temperature (K) and volume of liquid gas (μL), respectively. While M and V are the molecular weight of the target gas (g/mol) and volume of the chamber (L), respectively [22]. The time taken for the sensor to reach 90% of the total resistance is defined as the response time for gas adsorption or the recovery time for gas desorption [21]. 3. Results and discussions 3.1. Material characterization The crystal structures of SnO2 and SnO2/Pd were investigated by XRD. Fig. 1(a) illustrates the XRD patterns of the synthesized SnO2 NPs (reference 01-077-044). The diffraction peaks at 26.56� , 33.83� , 37.91� ,
Fig. 2. EDS spectra of SnO2/Pd NPs preparing in present of L-glycine. 3
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Fig. 4. TEM images and particle size histograms of SnO2 NPs (a, b), SnO2/Pd NPs (c, d).
38.93� , 51.71� , 54.70� , 57.74� , 62.53� , 65.89� , 69.16� , 71.18� and 78.60� are attributed to the crystal planes of (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 1), (2 2 0), (0 0 2), (221), (3 1 1), (2 0 2) and (3 2 1) of tetragonal SnO2 NPs, respectively. The diffraction peaks of Sn and SnO are absent in the SnO2 diffractogram. The XRD pattern of the synthesized SnO2/Pd in the presence of L-glycine (Fig. 1b) confirms the tetragonal structure of SnO2 while the peaks at 39.67� (111) and 66.76� (220) are attributed to the Pd metal (reference code 00-001-1310). The EDX result of SnO2/Pd NPs confirms the presence of Pd (Fig. 2). As shown in Figs. 1 and 2, the XRD and EDX results not only confirm the presence of Pd, but also shows the peaks of Pd metal from the reduction of Pd (II) to Pd (0) in the presence of L-glycine. Recent reports also suggest that amino acids can be used as the reducing agent. Most researchers believe that the carbonyl group forms coordination bonds with the metal cations and reduces them to metallic elements [16,17]. Fig. 3 shows the schematic interac tion between L-glycine and Pd2þ, with the first binding step and second reducing step, respectively. The TEM images of SnO2 and synthesized SnO2/Pd in the presence of L-glycine at different magnifications are shown in Fig. 4(a–b) and (c–d), respectively. The morphology of the particles is predominantly spherical and granular. The histogram shows that the diameter of SnO2 NPs and SnO2/Pd NPs are approximately 20.66 and 20.97 nm, respectively. As shown in Fig. 4, Pd incorporation into the SnO2 NPs had no significant effect on the grain size and morphology of tin oxide.
Fig. 5. Dependence of response on operating temperature of pure SnO2 NPs and SnO2/Pd NPs to 100 ppm.
was utilized as a biomarker in the gas sensor. The gas sensing device was fabricated from pure SnO2 and SnO2/Pd synthesized in the presence of Lglycine as the reducing agent. The electrical resistance of pure SnO2 and SnO2/Pd samples in the air as a function of temperature is presented in Fig. 5. In both samples, the sensitivity of the sample increases with the operating temperature. This is attributed to the adsorbed oxygen mol ecules on the semiconductor metal oxide surface in the form of O and O2 , in addition to the increase of thermal energy with operating tem perature [23]. On the other hand, as shown in Fig. 5, the presence of Pd on the SnO2 substrate enhances the sensor response at low and high temperatures (100–300 � C). However, the sensor based on pure tin oxide did not show any response between 100 and 150 � C. In the other words,
3.2. 1-octen-3-ol sensing properties The common fungal VOC, 1-octen-3-ol, released from the growth of biodeteriorative fungi on ancient artefacts and contaminated materials 4
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Fig. 6. Transient response curves of the pure SnO2/Pd NPs (a) and SnO2 NPs (c) upon exposure to different concentrations of 1-octen-3-ol. Resistance of the SnO2/Pd NPs (b) and pure SnO2 NPs (d) samples at 50 ppm 1-octen-3-ol in 250 � C. The magnified response–recovery characteristic curves for 50 ppm SnO2 NPs and SnO2/Pd NPs in 250 � C (e).
the reduction of Pd (II) on SnO2 NPs gave better response even at lower temperatures between 100 and 150 � C, which is one of the advantages of Pd deposition onto the surface of SnO2 NPs. The sensing measurements at temperatures above 300 � C could not be performed due to the FTO substrate which degrades at higher temperatures and the limitation of the sensing apparatus. Fig. 6 presents the transient response curve of the two functionalized samples, i.e. pure SnO2 NPs and SnO2/Pd NPs towards 50–200 ppm 1octen-3-ol at 250 � C. As shown in Fig. 6a, the response of SnO2/Pd NPs increases upon the exposure to 1-octen-3-ol. However, the resis tance of the sensor declines to the baseline when exposed to air. This observation confirms the reversibility of the fabricated SnO2/Pd NPs sensor. The change in the resistance of pure SnO2 NPs upon the exposure to 1-octen-3-ol is similar to SnO2/Pd, however, the SnO2/Pd shows a significant response compared to the pure SnO2 NPs (Fig. 6e). As can be seen from Fig. 6 (b, d), the electrical resistance of SnO2 rose after the Pd reducing process. The Ra changed from 1.93 kΩ (SnO2) to 53 kΩ (SnO2/ Pd), furthermore in 50 ppm 1-octen-3-ol, Rg increased from 1.72 kΩ (SnO2) to 45 kΩ (SnO2/Pd). As it was expected, the sensor response improved by 1.39 times from 10.9 (SnO2) to 15.1 (SnO2/Pd). It has been reported that in the air, Pd forms a stable oxide. In the oxygen phase, Pd acts as an electron acceptor and traps the charge from SnO2. Therefore, the depletion of electrons from the metal oxide surface, causing a rise in resistance [30]. On the other hand, the SnO2/Pd sensor exhibited a higher sensitivity compared to the pure SnO2 NPs. These results are encouraging and in accordance with previous reports [24–26]. The addition of catalytically active metals such as Pd and Pt can promote chemical and electrical
sensitization, and besides the sensitivity and selectivity of sensing de vices can be improved [27–30]. Hence, in present findings the reducing Pd on SnO2 NPs improve the electrical and chemical sensitization and, these effects are responsible to accelerate sensing reaction and improve sensing performance. The enhanced response towards the target gas is attributed to the presence of Pd metal which provides effective catalytic adsorption sites for oxygen and other reducing gases [30]. On the other hand, the Pd atoms entrap the electrons from the metal oxide. The entrapped charges provide stronger depletion of electrons from the metal oxide surface which increases the sensitivity [31]. From previous reports [23–25], the trapped oxygen species can be adsorbed onto the surface of SnO2 NPs even in an air-conditioned atmosphere. In the next step, the adsorbed oxygen is ionized to O , O2 and O2 by capturing free electrons from the n-type metal oxide. The presence of Pd increases the electron mobility and the available surface area of the semiconductor which enhances the oxygen adsorption. On the other hand, 1-octen-3-ol is a reducing gas, therefore it reacts with the adsorbed oxygen ions which release the adsorbed electrons when exposed to the sensor sur face. The following reaction mechanism shows the interaction between the oxygen ions and 1-octen-3-ol on the sensor surface. O2 (g)→O2 (ads)
(2)
O2 (ads) þ1e→O2 (ads)
(3)
O2 (ads) þ1e→2O (ads)
(4)
C8H15OH (g) þ2O (ads)→CO2 (g) þ H2O (g) þ2e
(5)
In the above equations (2)–(5), “ads” represents “adsorbed” ions. 5
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Table 1 The LOD and LOQ value of the fabricated sensor with SnO2/Pd at the 100, 150 and 250 � C temperature (a). The stability results of the SnO2/Pd-based sensor for 250 ppm of 1-octen-3-ol at 250 � C (b). a
b
Temperature LOD (ppm) LOQ (ppm)
100 � C 82.33 274.42
150 � C 33.51 111.70
250 � C 20.94 69.79
Stability Response%
First experiment 25.10
After ten days 25.01
After seven months 24.20
Fig. 7. Calibration plot for effect of reduced Pd on the SnO2 in comparing to pure SnO2 in different concentration (50–350 ppm) of 1-octen-3-ol in 250 � C.
Fig. 9. The selectivity of the SnO2 and SnO2/Pd, toward the 700 ppm of 1octen-3-ol, acetone, isopropanol, methanol, ethanol, formaldehyde, and n-hexane.
coefficient (R2) of SnO2 NPs and the curve fitting of SnO2/Pd NPs values of 0.88 and 0.97, respectively. Therefore, the linear rate increased by reduced Pb on SnO2 NPs. Fig. 8 shows the variation of Ra-Rg with different concentrations of 1-octen-3-ol at 100 � C (a), 150 � C (b) and 250 � C (c), respectively. These results clearly show that the sensitivity increases from 0.0003 kOhm ppm 1 to 0.0435 kOhm ppm 1 with the increase of temperature. These temperatures were chosen based on the results in Fig. 5. From the results in Fig. 8, equations (6) and (7), the limit of detection (LOD) and limit of quantification (LOQ) of the SnO2/Pd sensor at the above temperatures are given in Table 1(a) [32]. LOD ¼ 3 SB / b
(6)
LOQ ¼ 10 SB / b
(7)
The estimated LOD and LOQ values improved with the increase of temperature from 100 � C to 250 � C. The stability of the SnO2/Pd NPs sensor towards the detection of 1-octen-3-ol at 250 � C was investigated and shown in Table 1(b). The experiments were performed at 250 ppm 1-octen-3-ol, after keeping the sensor in ambient air at room tempera ture for 10 days and seven months interval. The determination of 1octen-3-ol at 250 � C was performed after a storage interval of 10 days and seven months, where the response percentage decreased to only 0.34% and 3.56%, respectively. This confirms that the SnO2/Pd NPs has acceptable stability towards the detection of 1-octen-3-ol at 250 � C. According to WHO, volatile organic compounds are the most important pollutants of indoor and outdoor environments. As mentioned in the literature review, the main VOCs including isopropanol, meth anol, ethanol, formaldehyde, and n-hexane may be found in the indoor air and are the environmental pollutants that can constitute a health hazard [33,34]. In the present study, the selectivity of fabricated sensors was tested towards 1-octer-3-ol, which represents the fungal growth, and the other VOCs, which can be usually present in libraries. Prior studies that have noted the selectivity characteristic is related to the discrimination capacity of a sensor in front of a different of gases. This factor is closely associated with many factors such as morphology, the
Fig. 8. The variation of Ra-Rg vs different concentrations of 1-octen-3-ol at 100 � C (a), 150 � C (b) and 250 � C (c) of the SnO2/Pd NPs.
Fig. 7 presents the linear fitting, together with the correlation 6
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Fig. 10. The transient response curves of the pure SnO2/Pd NPs upon exposure of the 3 contaminated (Aspergillus sp. on YGC media) atmospheres, separately.
presence of a catalyst, and operating temperature [35]. Fig. 9 shows the response of SnO2 NPs and SnO2/Pd NPs toward 700 ppm 1-octen-3-ol, as the biomarker for fungal VOCs and 1400 ppm of acetone, isopropanol, methanol, ethanol, formaldehyde, and n-hex ane as the common environmental VOC pollutants. The current study found that the SnO2 NPs and SnO2/Pd NPs displayed a higher response to 1-octen-3-ol in comparison with other pollutants. The higher response of the sensors to 1-octen-3-ol can be related to the polarity of the detected molecules. The response of the SnO2/Pd NPs and SnO2 NPs fabricated sensor towards 700 ppm 1-octen-3-ol is 46.70% and 24.16%, respectively. Therefore, as it was expected, the response of the sample based on SnO2/Pd NPs showed better selectivity to 1-octen-3-ol than SnO2 NPs. In the other words, the selectivity was enhanced to analyte by the reduction of Pd2þ on the SnO2 NPs used as a substrate via the Lglycine-assisted synthesis. Hence, as mentioned in the literature review, the presence of catalytic metallic additives such as Pd can often improve the selectivity of SnO2 NPs based gas sensors.
placed. Upon the injection of the fungal VOCs, the resistance of the sensor began to decrease. The value stabilizes after a few minutes and was recorded as the electrical resistance of the VOCs at the specified temperature. After switching from fungal VOCs to air, the resistance of the sensor changed to the baseline, and the headspace above the second Aspergillus sp. culture was introduced into the test chamber. After the resistance was stabilized, a third test was repeated for the third real sample. The response was calculated similarly to the response of the 1-octen-3-ol. Fig. 10 illustrates the transient response curves of SnO2/Pd NPs upon exposure to the 3 contaminated (Aspergillus sp. on YGC media) atmosphere, separately. With respect to Figs. 7 and 10, the estimated concentration of fungal VOCs in 17.74%, 18.17%, and 19.25% sensor responses are 97.56, 99.03 and 101.62 ppm, respectively. These tests confirm the feasibility of the fabricated sensor for the detection of the biodeteriorating fungi. As mentioned before, albeit SnO2/Pd showed the highest sensitivity and good selectivity towards the target gas released from real samples, it is still sensitive to temperature changes, therefore the temperature stability is an important requirement for highly sensitive and selective detection of the fungal VOC. To comprise, a summary of reviewed gas sensors that reported in previous researches and this work is displayed in Table 2.
3.3. Real sample analysis According to Fidler et al. (2001) and other researches (4, 5, 8) most of Aspergillus species such as Aspergillus fumigatus, Aspergillus versicolor, Aspergillus niger, Aspergillus ochraceus, and other species can produce 1octen-3-ol. Therefore, in the present study Aspergillus sp. was selected. This genus was isolated from an old biodeteriorated manuscript located in one of the repositories in Iran, with high-frequency. To investigate the detection of the target fungi by the presence of the VOCs, the same concentration of Aspergillus sp. spores isolated from an old infected manuscript was inoculated in 3 sealed YGC (Yeast Extract. Glucose Chloramphenicol. Agar FIL-IDF, Merck) media. After 72 h of incubation at room temperature, the headspace above the 3 real samples was introduced into the test chamber separately where the sensor is
4. Conclusions To conclude, the synthesis of SnO2 nanoparticles decorated with Pd in the presence of L-glycine and its sensing performance towards 1octen-3-ol, a biomarker for the presence of destructive fungi species in indoor environments, was successfully fabricated and tested. The nanoparticles were spherical and granular in shape. The SnO2/Pd sensor showed better response compared to the pure SnO2 NPs at the lower selected temperature (100–150 � C). Furthermore, the SnO2/Pd sample gave significant enhancement in the sensitivity towards the detection of
Table 2 Summary of gas sensors review in previous reports and present work. Target gas
Sensor structure
Operation
LOD
References
Octane Toluene 1-octanol 1-octen-3-ol
SAW sensors based on PIB/MWNTs SAW sensors based on PIB/MWNTs SWCNTs in FET structures SnO2-SP (Electrochemical)
1-octen-3-ol
TiO2-SP (Electrochemical)
1-octanol 1-octen-3-ol
Polymer-based sensor SnO2/Pd
Resistance response vs. concentration change Resistance response vs. concentration change Response vs. saturated vapor Cyclic voltammetry Differential pulse voltammetry Cyclic voltammetry Differential pulse voltammetry Response vs. resistivity change Resistance response vs. concentration change
Not detected Not detected Not detected 82 nM 62 nM 128 nM 35 nM Not detected 20.94 ppm
[36] [36] [37] [38] [38] [38] [38] [39] This work
SAM: Surface acoustic wave. PIB: Polyisobutylene. MWCNTs: Multi-walled carbon nanotube. SWCNTs: Single-walled semiconducting carbon nanotubes. FET: Fieldeffect transistor. 7
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1-octen-3-ol, compared to the pure SnO2. Besides, the sensor based on SnO2/Pd gave the highest response towards 1-octen-3-ol compared to other gases present in the same environment but not from the fungal origin. It can be concluded that the reduction of Pd2þ on the SnO2 NPs surface via the L-glycine-assisted synthesis showed acceptable selectivity towards certain fungal VOCs. From the real sample tests, the SnO2/Pdbased sensor is feasible for the detection of deteriorating fungi, although further tests must be conducted before it can be used for in situ applications.
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