electronics Article
Chitosan-Based Quartz Crystal Microbalance for Alcohol Sensing Kuwat Triyana 1,2, * , Agustinus Sembiring 1 , Aditya Rianjanu 1 , Shidiq Nur Hidayat 1 , Riowirawan Riowirawan 1 , Trisna Julian 1 , Ahmad Kusumaatmaja 1,2 , Iman Santoso 1,2 and Roto Roto 2,3 1
2 3
*
Department of Physics, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia;
[email protected] (A.S.);
[email protected] (A.R.);
[email protected] (S.N.H.);
[email protected] (R.R.);
[email protected] (T.J.);
[email protected] (A.K.);
[email protected] (I.S.) Nanomaterial Research Group, Universitas Gadjah, Yogyakarta 55281, Indonesia;
[email protected] Department of Chemistry, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia Correspondence:
[email protected]; Tel.: +62-857-2815-2871
Received: 5 August 2018; Accepted: 7 September 2018; Published: 8 September 2018
Abstract: Short-chain alcohols are a group of volatile organic compounds (VOCs) that are often found in workplaces and laboratories, as well as medical, pharmaceutical, and food industries. Real-time monitoring of alcohol vapors is essential because exposure to alcohol vapors with concentrations of 0.15–0.30 mg·L−1 may be harmful to human health. This study aims to improve the detection capabilities of quartz crystal microbalance (QCM)-based sensors for the analysis of alcohol vapors. The active layer of chitosan was immobilized onto the QCM substrate through a self-assembled monolayer of L-cysteine using glutaraldehyde as a cross-linking agent. Before alcohol analysis, the QCM sensing chip was exposed to humidity because water vapor significantly interferes with QCM gas sensing. The prepared QCM sensor chip was tested for the detection of four different alcohols: n-propanol, ethanol, isoamyl alcohol, and n-amyl alcohol. For comparison, a non-alcohol of acetone was also tested. The prepared QCM sensing chip is selective to alcohols because of hydrogen bond formation between the hydroxyl groups of chitosan and the analyte. The highest response was achieved when the QCM sensing chip was exposed to n-amyl alcohol vapor, with a sensitivity of about 4.4 Hz·mg−1 ·L. Generally, the sensitivity of the QCM sensing chip is dependent on the molecular weight of alcohol. Moreover, the developed QCM sensing chips are stable after 10 days of repeated measurements, with a rapid response time of only 26 s. The QCM sensing chip provides an alternative method to established analytical methods such as gas chromatography for the detection of short-chain alcohol vapors. Keywords: Quartz crystal microbalances; chitosan; L-cysteine; self-assembled monolayer; alcohol compounds
1. Introduction Detection of volatile organic compounds (VOCs) such as alcohols, ethers, esters, haloalkanes, ammonia, and NO2 , has gained the interest of researchers across the globe since a few decades ago for environmental protection, human health, industrial processing, and quality control [1]. Volatile alcohols such as methanol, ethanol and propanol are organic compounds often found in workplaces and laboratories, as well as medical, pharmaceutical, and food industries. Long-term exposure to alcohol vapor with a concentration of up to 0.15–0.30 mg·L−1 can cause serious health problems, such as eyesight disturbance, nasal and mucous membrane inflammation, conjunctival inflammation,
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respiration disruption, nerve disease, lung irritation, and even death [2]. Thus, the real-time monitoring of the presence of alcohols in the air is required. Fourier transform infrared spectroscopy, gas chromatography, and mass spectrometry are often used to detect alcohol vapors [3]. Although these standard methods are accurate and reliable, they are not real-time, require expensive instrumentations, and are time-consuming, which make them difficult to apply in different conditions. The development of a low-cost sensing system with high accuracy and sensitivity for the detection of alcohol has drawn significant attention recently. Several sensing systems have been developed based on the sensing principle of electrical resistance, photoelectricity, optics, and acoustic waves [4]. The quartz crystal microbalance (QCM) is a type of acoustic wave sensor. It provides a highly sensitive, real-time, and low-cost method for vapor detection and is often used for gas/vapor [5–7] and humidity [8,9] detection. The sensitivity and selectivity of the QCM system can be improved by coating the electrode with specific materials, including polymers [9,10], metal oxides [11], and other functional materials [12]. Chitosan is one of the natural polysaccharide polymers containing the hydroxyl (-OH) and amino (-NH2 ) groups. Chitosan has many promising properties, such as excellent film-forming capability, high mechanical strength, hydrophilicity, and even antibacterial properties. Therefore, chitosan has been widely used as a gas sensor to detect alcohol vapors and other VOCs [13]. Several methods have been developed to coat the QCM substrate with polymer thin film, including drop casting [14], electrospinning [15,16], and self-assembled monolayer (SAM) methods [17]. A vapor-sensing system fabricated via the SAM method is physically more robust than those fabricated by other methods. Moreover, the SAM method is easy to implement and economical [18]. We used L-cysteine and glutaraldehyde to fabricate the chitosan-based QCM sensors. This study aims to develop an alcohol vapor detector based on the QCM coated with chitosan polymer attached to the surface through a covalent bond. The capability of the QCM-based chitosan sensing system will be illustrated by the frequency shift due to variation in alcohol concentrations. The chitosan-based QCM sensing systems are tested for the detection of four different alcohols that are n-amyl alcohol, isoamyl alcohol, n-propanol, and ethanol. For comparison, a non-alcohol of acetone was also tested. A calibrated measurement system, including the frequency counter of QCM used in this study, was build using a standard Pierce-gate oscillator and microcontroller. 2. Materials and Method Chitosan with medium molecular weight (MW = 190,000 to 310,000 g·L−1 ), L-cysteine (HSCH2 CH(NH2 ) CO2 H, CAS#52-90-4), glutaraldehyde (GA) (50%), n-amyl alcohol, isoamyl alcohol, n-propanol, and ethanol were purchased from Sigma-Aldrich, St. Louis, MO, USA. Acetic acid and doubly distilled water were purchased from Merck, Darmstadt, Germany. The chemicals were used as received without further purification. The 10-MHz resonant frequency of AT-cut QCM chips with Au electrode diameter of 6 mm was purchased from Novaetech, Turin, Italy. L-cysteine solution of 1% (w/w) was obtained by dissolving 0.1-g L-cysteine powder in distilled water using a magnetic stirrer at 25 ◦ C and diluted using a 10-mL volumetric flask. Glutaraldehyde solution of 5% (w/w) was obtained by dissolving 1 mL of concentrated glutaraldehyde (50% w/w) in distilled water and diluted to the mark with a 10-mL volumetric flask. Chitosan solution was prepared by mixing 0.05-g chitosan powder with 5.05 mL of 1% (w/w) acetic acid and was mechanically stirred at 600 rpm for 3 h at 60 ◦ C. The QCM sensor chip was prepared by the following steps. The QCM substrate was first immersed in a 1% L-cysteine solution for 24 h. It was rinsed with distilled water and acetone. After air drying, the QCM chip was immersed in a 5% glutaraldehyde solution for 30 min, followed by rinsing with distilled water. In addition, after air drying, the QCM chip was immersed in a 1% chitosan solution for 2 h. The chip was air dried for 24 h in a dry box. Figure 1 displays the layout of the QCM substrate.
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Figure 1. Schematic chemical structure and active layer deposition of the chitosan-based QCM chip for alcohol sensing.
The scheme of the static-type alcohol vapor analytical system was described in a previous study [19]. Figure 1. Schematic chemicalwas structure and active a layer depositionPierce-gate of the chitosan-based QCM and chip for The frequency QCM built using standard oscillator microcontroller Figure 1. counter Schematicofchemical structure and active layer deposition of the chitosan-based QCM chip alcohol sensing. with 32-bit timer/counter resolution and 42 MHz frequency clock. The configuration of the frequency for alcohol sensing. Thedouble schemeedge of themode static-type vapor analytical system was described in aHz. previous counter uses to getalcohol a measurement resolution of less than 0.5 The frequency study [19]. The frequency counter of QCM was built using a standard Pierce-gate oscillator and [19]. of directly the static-type alcohol vapor analytical system described in a previous study counterThe wasscheme placed on top of the oscillator circuit to was avoid interference. The QCM chip was microcontroller with of 32-bit timer/counter resolution and 42 MHz frequency clock. The configuration The frequency counter QCM was built using a standard Pierce-gate oscillator and microcontroller installed in a 1.25-L testing chamber. The temperature and humidity inside the chamber were the frequency counterresolution uses double edge get a measurement of lessof than Hz. withof32-bit timer/counter and 42 mode MHz to frequency clock. Theresolution configuration the0.5 frequency adjusted to 33 ± 2 °C and 45–55 %, respectively. It was monitored with a calibrated Sensirion SHT31. The uses frequency counter was placed on top of resolution the oscillator circuit to 0.5 avoid counter double edge mode to getdirectly a measurement of less than Hz.interference. The frequency The measurement system wasin connected to chamber. a PC equipped with and LabVIEW This The QCM chip was installed testing temperature humidity software. counter was placed directly on topaof1.25-L the oscillator circuit toThe avoid interference. The QCMinside chip was ◦ the chamber were was adjusted to 33 ± 2 using C and AFG-2125 45–55%, respectively. It was monitored withINSTRUMENT a calibrated measurement system calibrated Function Generator (GW Co., installed in a 1.25-L testing chamber. The temperature and humidity inside the chamber were Sensirion SHT31. The measurement system was connected to a PC equipped with LabVIEW software. Ltd., Taiwan). µL syringe 701 RN SYR, Hamilton Company, Switzerland) was used adjusted to 33A±1–10 2 °C and 45–55 %,(Model respectively. It was monitored with a calibrated Sensirion SHT31. This measurement system was calibrated using AFG-2125 Function Generator (GW INSTRUMENT forThe analyte injection. The concentration of the injected analyte in the chamber was expressed in mg·L−1 measurement connected PCSYR, equipped LabVIEW software. Co., Ltd., Taiwan).system A 1–10 was µL syringe (Modelto701a RN Hamiltonwith Company, Switzerland) was This andmeasurement calculated using density, percent volume ofthe analytes. Figure 2 shows system was calibrated usingpurity, AFG-2125 Function Generator (GW INSTRUMENT Co., the used for analyte injection. The concentration of the and injected analyte in chamber was expressed − 1 experimental of the QCMpercent system. Ltd.,inTaiwan). A 1–10 µLmodified syringe (Model 701 RNpurity, SYR, and Hamilton was mg·L setup and calculated using density, volumeCompany, of analytes.Switzerland) Figure 2 shows the used for analyte injection. concentration of the injected analyte in the chamber was expressed in mg·L−1 experimental setupThe of the modified QCM system. and calculated using density, percent purity, and volume of analytes. Figure 2 shows the experimental setup of the modified QCM system.
Figure 2. Experimental setup of the QCM system for gas or vapor sensing.
Figure 2. Experimental setup of the QCM system for gas or vapor sensing. Figure 2. Experimental setup of the QCM system for gas or vapor sensing.
3. Results and Discussion
3. The Results andgold Discussion QCM electrode was coated with chitosan via a self-assembly monolayer of L-cysteine.
Chitosan was attached to L-cysteine through coupling of glutaraldehyde. After consecutive The QCM gold electrode was coated with achitosan viaagent a self-assembly monolayer of L-cysteine. coating withwas L-cysteine, and chitosan, obtained frequency shift ofconsecutive QCM was 176.5 Chitosan attached glutaraldehyde, to L-cysteine through a couplingthe agent of glutaraldehyde. After glutaraldehyde, and chitosan, the obtained frequency shift of QCM was 176.5 Hz,coating 571.5 with Hz, L-cysteine, and 12750.5 Hz, respectively. The decrease in resonant frequency indicates that the
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3. Results and Discussion The QCM gold electrode was coated with chitosan via a self-assembly monolayer of L-cysteine. Chitosan was attached to L-cysteine through a coupling agent of glutaraldehyde. After consecutive 4 of 11 Electronics 2018, 7, 181 coating with L-cysteine, glutaraldehyde, and chitosan, the obtained frequency shift of QCM was 176.5 Hz, 571.5 Hz, and 12750.5 Hz, respectively. The decrease in resonant frequency indicates that the on the chitosan-based QCM sensor could be attributed to successful surface modification by selfactive layer has been deposited on top of the QCM electrodes. The progression of the frequency shift on assembly monolayer and polymer attachment. The thickness and hydrophilicity of the film deposited the chitosan-based QCM sensor could be attributed to successful surface modification by self-assembly on the QCM substrate affect the frequency shift upon contact with different humidities [20,21]. monolayer and polymer attachment. The thickness and hydrophilicity of the film deposited on the Knowing that the the response of shift the modified QCM is humidities highly dependent QCM substrate affect frequency upon contact withsensor different [20,21]. on the ambient relative Knowing humiditythat (RH), also investigated theQCM response ofisour chitosan-modified QCM sensor to the we response of the modified sensor highly dependent on the ambient the relative RH value. By injecting a small amount of distilled water intochitosan-modified the chamber, theQCM RH increased humidity (RH), we also investigated the response of our sensor to from 48%the to RH 54%.value. The linear correlation between shift andinto frequency response QCM sensor By injecting a small amounthumidity of distilled water the chamber, the of RHthe increased from 48% to 54%.3.The linearnote correlation shift Hz/%RH, and frequency of the QCM is shown in Figure Please that thebetween slope ishumidity about −6.58 withresponse a correlation coefficient 2 sensor is shown in Figure 3. Please note that the slope is about − 6.58 Hz/%RH, with a correlation (R ) of 0.995. The results show the significant effect of the RH on the frequency change. This 2 ) of 0.995. The results show the significant effect of the RH on the frequency change. coefficient (R phenomenon could be attributed to the hydrophilicity of the chitosan. This phenomenon could be attributed to the hydrophilicity of the chitosan.
Figure 3. The resonantfrequency frequency shift shift of QCM alcohol sensor under the influence of Figure 3. The resonant ofchitosan-based chitosan-based QCM alcohol sensor under the influence of different RH values. different RH values.
After investigating the effect of the relative humidity (RH) on the QCM response, we used it to
After investigating the effect of the relative humidity (RH) on the QCM response, we used it to investigate four different analytes that are n-amyl alcohol, isoamyl alcohol, n-propanol, and ethanol. investigate different analytesofthat n-amyl QCM alcohol, isoamyl n-propanol, andatethanol. Figure 4afour shows the response the are developed sensor after alcohol, contacting with alcohol a − 1 Figure 4a shows the response of the developed QCM sensor after contacting with alcohol concentration of 7 mg·L . The analyte was injected into the chamber after 50 s when the resonant at a concentration 7 QCM mg·L−1sensor . The was analyte was injected intothe the chamber after 50decreased s when the resonant frequency ofofthe stable. After injection, resonant frequency rapidly for a few andsensor reached a steady 100 s. the The resonant frequencyfrequency shift after decreased injection was frequency ofseconds the QCM was stable.value Afterafter injection, rapidly to beand about 39, 24, a5, steady and 3 Hz for n-amyl alcohol, alcohol, n-propanol, and was for determined a few seconds reached value after 100 s. Theisoamyl frequency shift after injection ethanol, respectively. Figure 4b shows the RH and temperature readings in the gas sensing chamber determined to be about 39, 24, 5, and 3 Hz for n-amyl alcohol, isoamyl alcohol, n-propanol, and duringrespectively. measurement. The RH temperature values changed slightly during measurement, about ethanol, Figure 4band shows the RH and temperature readings in the gas sensing chamber ◦ 0.25% and 0.4 C, respectively. As mentioned previously, the QCM vapor sensor was profoundly during measurement. The RH and temperature values changed slightly during measurement, about influenced by the humidity and temperature changes. Therefore, the RH and temperature were 0.25% and 0.4°C, respectively. As mentioned previously, the QCM vapor sensor was profoundly controlled during measurement to minimize their effect on the frequency reading. influenced by the humidity and temperature changes. Therefore, the RH and temperature were controlled during measurement to minimize their effect on the frequency reading. Figure 5a shows the response of the developed QCM sensor to different analytes. The analyte with the concentrations of 1–37 mg·L−1 was consecutively injected into the chamber after the frequency response reached a steady state. The frequency shift was recorded after 150 s of analyte injection. The slope of the linear fit of frequency shift versus analyte concentration, shown in Figure 5a, shows the sensitivity of the QCM alcohol sensor. The sensitivity is the highest for n-amyl alcohol and the lowest for ethanol. The correlation coefficient (R2) of the proposed QCM sensor for all analytes is
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Figure 5a shows the response of the developed QCM sensor to different analytes. The analyte with the concentrations of 1–37 mg·L−1 was consecutively injected into the chamber after the frequency response reached a steady state. The frequency shift was recorded after 150 s of analyte injection. The slope of the linear fit of frequency shift versus analyte concentration, shown in Figure 5a, shows the sensitivity of the QCM alcohol sensor. The sensitivity is the highest for n-amyl alcohol and the lowest for ethanol. The correlation coefficient (R2 ) of the proposed QCM sensor for all analytes is 0.999, which indicates good linearity. The QCM sensor also shows good reproducibility, as indicated by its Electronics 2018, 7, 181 5 of 11 low standard deviation.
Figure 4. (a) Frequency shift of the chitosan-based QCM sensing system after contact with alcohols at
Figure 4. ·(a) ofRH theand chitosan-based sensing system after contact with alcohols at 7 mg L−1Frequency and (b) the shift drift of temperature ofQCM the sensing chamber throughout the measurement. −1 7 mg·L and (b) the drift of RH and temperature of the sensing chamber throughout the measurement.
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Figure Frequency shift thethe chitosan-based QCM sensing system after being after exposed to alcohols Figure 5. 5.(a)(a)Frequency shiftof of chitosan-based QCM sensing system being exposed to with different concentrations (error bar was determined based on standard deviation of 3-times repeated alcohols with different concentrations (error bar was determined based on standard deviation of 3measurement) and (b) calculated times repeated measurement) andsensitivity. (b) calculated sensitivity.
The sensitivity for different alcohols is shown in Figure 5b. The sensitivity increases in the
The sensitivity for different alcohols is shown in Figure 5b. The sensitivity increases in the following order: ethanol, n-propanol, isoamyl alcohol, and n-amyl alcohol. This seems to be directly following ethanol, n-propanol, and n-amyl alcohol. seemsweight to be directly related order: to the molecular weight of theisoamyl alcohols.alcohol, N-amyl alcohol has the highestThis molecular of −1 , giving aweight 1 ·L. Meanwhile, related of the alcohols. alcohol ethanol has the has highest molecular weight of 88.15tog·the molmolecular sensitivity of 4.4 Hz·mg−N-amyl the lowest molecular −1·L. Meanwhile, 1 , with a sensitivity , giving a −sensitivity of 4.4 Hz mgHz ethanol has theoflowest molecular 88.15 g·mol weight of−146.07 g·mol of ·0.4 ·mg−1 ·L. The physical properties the analytes are listed in Table 1.−1, with a sensitivity of 0.4 Hz·mg−1·L. The physical properties of the analytes are weight of 46.07 g·mol listed in Table 1. Table 1. Sensitivity and response time of chitosan-based QCM sensor for different types of alcohol.
Molecular Boiling QCMSensitivity LOD types of Time Table 1. Sensitivity and responseVapor time Pressure of chitosan-based sensor for different alcohol. Analytes Weight (g·mol−1 )
(mmHg)
Point (◦ C)
(Hz·mg−1 ·L)
(mg·L−1 )
Constant/t90 (s)
n-Amyl alcohol Isoamyl alcohol Analytes n-Propanol Ethanol Acetone
88.15 Molecular 88.14 weight 60.09 46.07 (g·mol−1)
2.20 Vapor 2.37 pressure 21 55 (mmHg)
138 Boiling 131 point97(°C)
231
78 56
n-Amyl alcohol
88.15
2.20
138
4.4
0.25
42/91
Isoamyl alcohol
88.14
2.37
131
3.1
0.50
28/62
n-Propanol
60.09
21
97
0.9
1.22
7.2/17
Ethanol
46.07
55
78
0.4
2.86
6.7/15
58.08
4.4 Sensitivity
0.25 LOD 42/91 Time 3.1 0.50 28/62 −1·L) 1.22(mg·L−1)7.2/17 constant/ 0.9 (Hz‧mg 0.4 2.86 6.7/15 t90 (s) 0.1 11.43 1.6/3.6
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This shows thatthe themolecular molecularweight weightof ofthe the analyte analyte seems seems to This shows that to affect affectthe thesensitivity. sensitivity.However, However,for isomeric alcohol of n-amyl alcohol and isoamyl alcohol, it is not the case. As pointed for isomeric alcohol of n-amyl alcohol and isoamyl alcohol, it is not the case. As pointedout outininthe theprevious previousstudies studies[22,23], [22,23],for forisomers isomersofofalcohol, alcohol,the theresponse responseofofthe theQCM QCMsensor sensordepended dependedononthe length of the alcohol chain. The highest sensitivity for n-amyl alcohol could be due linear the length of the alcohol chain. The highest sensitivity for n-amyl alcohol could be due toto itsits linear structure (Figure 5b). Another study shows that the hydrogen bond also influenced the sensitivity structure (Figure 5b). Another study shows that the hydrogen bond also influenced the sensitivity of of isomers alcohol [24]. isomers of of alcohol [24]. The hydrogen bond affects boiling point alcohol. The boiling point n-amyl alcohol The hydrogen bond affects thethe boiling point of of alcohol. The boiling point of of n-amyl alcohol is is higher than that isoamyl alcohol (see Table N-amyl alcohol gives a frequency response greater higher than that of of isoamyl alcohol (see Table 1).1). N-amyl alcohol gives a frequency response greater than the branched alcohol of isoamyl alcohol. The steric hindrance causes isoamyl alcohol to interact than the branched alcohol of isoamyl alcohol. The steric hindrance causes isoamyl alcohol to interact moderately with chitosan, which results a smaller frequency response than that n-amyl alcohol. moderately with chitosan, which results in in a smaller frequency response than that of of n-amyl alcohol. The limitofofdetection detection(LOD) (LOD) can can be standard deviation of blank QCM The limit be determined determinedbased basedononthe the standard deviation of blank measurement and the slope of the plot of the frequency shift versus analyte concentration of Figure 5 [25]. QCM measurement and the slope of the plot of the frequency shift versus analyte concentration –1), where σ is the standard deviation of the blank air The LOD may be expressed as 3.3 σ/S (mg L of Figure 5 [25]. The LOD may be expressed as 3.3 σ/S (mg·L–1 ), where σ is the standard deviation 0.47, 0.33,(0.34, and 0.47, 0.34 Hz), is the slope ofSthe calibration (4.4, 3.1, 0.9, of measurement the blank air (0.34, measurement 0.33,and andS 0.34 Hz), and is the slope ofcurve the calibration −1 ·L) forisoamyl alcohol, isoamyl n-propanol, andn-propanol, ethanol, respectively. The and(4.4, 0.4 Hz‧mg curve 3.1, 0.9,−1·L) andfor 0.4n-amyl Hz·mgalcohol, n-amyl alcohol, alcohol, and ethanol, calculated LOD of chitosan-based QCM sensing system can be seen in Table 1. respectively. The calculated LOD of chitosan-based QCM sensing system can be seen in Table 1. Figure 6 shows the frequencyresponse responseofofthe themodified modifiedQCM QCMsensing sensingsystem systemtoto77mg mg·L Figure 6 shows the frequency ·L−−11 of of namyl alcohol alcohol vapor. vapor.The Thefrequency frequencyshift shiftfollows followsthe thefirst firstorder orderand andfits fits the first-order instrument n-amyl toto the first-order instrument fitting. The time constant was achieved after s. The time constant defined time required fitting. The time constant was achieved after 42 42 s. The time constant (τ)(τ) is is defined as as thethe time required instrument reach 63.2% maximum response value [26]. Moreover, response value forfor anan instrument to to reach 63.2% of of itsits maximum response value [26]. Moreover, thethe response value equaling 0.9 of the maximum response (t 90 ) was achieved within 90 s, which determined the response equaling 0.9 of the maximum response (t90 ) was achieved within 90 s, which determined the response time device. time of of thethe device.
−1 Figure Thefrequency frequency shift shift of being exposed to 7tomg·L Figure 6. 6.The of chitosan-based chitosan-basedQCM QCMsensor sensorafter after being exposed 7 mgn-amyl ·L−1 alcohol. n-amyl alcohol.
The response time is crucial gas sensing, particularly that rapid detection. this study, The response time is crucial forfor gas sensing, particularly in in that forfor rapid detection. InIn this study, the response times increased in the following order: ethanol, n-propanol, isoamyl alcohol, and n-amyl the response times increased in the following order: ethanol, n-propanol, isoamyl alcohol, and n-amyl alcohol.The The response time related well different vapor pressures analytes. The vapor alcohol. response time is is related well to to thethe different vapor pressures of of analytes. The vapor pressure analyte is related capability liquid evaporate a given temperature. pressure of of thethe analyte is related to to thethe capability of of thethe liquid to to evaporate at at a given temperature. Analytes with high vapor pressure evaporate readily at room temperature. Therefore, the response Analytes with high vapor pressure evaporate readily at room temperature. Therefore, the response time time will beThe short. Thepressure vapor pressure and response time of the analytes areinshown inThis Table 1. This will be short. vapor and response time of the analytes are shown Table 1. shows shows that the response time decreases with theinincrease in the vapor pressure. that the response time decreases with the increase the vapor pressure. Figure 7a shows frequency movement the prepared sensing first 10 Figure 7a shows the the frequency movement of theofprepared QCMQCM sensing systemsystem for the for firstthe 10 days of measurement. The resonant frequency shift isafter steady after of 10operation. days of operation. The variation of days measurement. The resonant frequency shift is steady 10 days The variation in the in the resonant frequency be attributed onlychange to the change in the and temperature resonant frequency could becould attributed only to the in the RH andRH temperature (Figure(Figure 7b) [27].7b) [27].
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Figure 7. (a) Long-term resonant frequency shift of blank QCM and chitosan-based QCM sensing Figurefor7.the (a) first Long-term frequency of blank QCM chitosan-based system 10 days resonant of measurement, andshift (b) change in RH andand temperature for theQCM first 10sensing days system for the first 10 days of measurement, and (b) change in RH and temperature for the first 10 of measurement. days of measurement.
In the QCM sensing system, the decrease in the resonant frequency is proportional to the mass In on thethe QCM sensing system, thesubstrate. decrease in themass resonant frequency is proportional to the loading surface of the QCM The adsorbed on the QCM substrate canmass be loading on the surface of the QCM substrate. The mass adsorbed on the QCM substrate can be calculated by use of the Sauerbrey equation [28], as expressed in Equation (1): calculated by use of the Sauerbrey equation [28], as expressed in Equation 1: 2f2 ∆ f = − √20f 2 ∆m (1) Δf = −A ρq µ0 q Δm (1)
A ρ q μq
where ∆ f is the resonant frequency change (Hz), f 0 is the base resonant frequency (10 MHz), ∆m is the mass change (g), A is the electrode surface area (in thisf case 0.283 cm2 ), µq and ρq are the quartz shear where Δf is the resonant frequency change (Hz), 0 is the base resonant frequency (10 MHz), Δm modulus (2.947 × 1011 g·cm−1 ·s−2 ) and density of quartz crystal (2.648 g·cm−3 ), respectively. μ q and ρdecreases is the electrode area (in system, this casethe 0.283 cm2), frequency is the mass (g), A equation, Based onchange the Sauerbrey for thesurface 10-MHz QCM resonant q are the by 1 Hz shear for each 1.15 ng additional theofmass of crystal the compound adsorbed by the −3), respectively. cm−1·s−2Therefore, ) and density quartz (2.648 g·cm quartz modulus (2.947 × 1011 g·mass. QCM membrane could be calculated using the Sauerbrey equation. The analyte mass loading for the Based on the Sauerbrey equation, for the 10-MHz QCM system, the resonant frequency prepared QCM to be mass. 45.1, 27.7, 5.7, andthe 3.4mass ng when it compound was in contact with decreases by 1sensor Hz forchip eachwas 1.15estimated ng additional Therefore, of the adsorbed −1 of n-amyl alcohol, isoamyl alcohol, n-propanol, and ethanol, respectively. the vapor of 7 mg · L by the QCM membrane could be calculated using the Sauerbrey equation. The analyte mass loading Figure 8 shows thesensor interaction model between modified and the in analyte for the prepared QCM chip was estimated to bethe 45.1, 27.7, 5.7, QCM and 3.4surface ng when it was contact molecules. The high frequency response of theisoamyl QCM sensing after being exposedrespectively. to alcohol with the vapor of 7 mg ·L−1 of n-amyl alcohol, alcohol,system n-propanol, and ethanol, vapor Figure is believed to be the dueinteraction to hydrogen bondbetween formation between the QCM hydroxyl group (-OH) of the 8 shows model the modified surface and the analyte alcohol molecules and the -OH group of the chitosan. The frequency shift for n-amyl alcohol is the molecules. The high frequency response of the QCM sensing system after being exposed to alcohol highest. couldtobe to the high of the n-amyl alcohol. Although vapor isThis believed beattributed due to hydrogen bondmolecular formationweight between hydroxyl group (-OH) ofthe the sensitivity of the chitosan-based QCM sensor for n-amyl alcohol is the highest, its response time is alcohol molecules and the -OH group of the chitosan. The frequency shift for n-amyl alcohol is the the longestThis of all because of its low to vapor N-amyl alcohol isoamyl alcohol have the highest. could be attributed the pressure. high molecular weight of and n-amyl alcohol. Although the
sensitivity of the chitosan-based QCM sensor for n-amyl alcohol is the highest, its response time is the longest of all because of its low vapor pressure. N-amyl alcohol and isoamyl alcohol have the
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same molecular weight. Please note that the linear n-amyl alcohol interacts with chitosan better than branched isoamyl alcohol.
Figure Figure 8. 8. Schematic Schematic of of the the interaction interaction mechanism mechanism between between alcohol alcohol and chitosan layer.
4. Conclusions Conclusions 4. A new new chitosan-based chitosan-based QCM QCM sensing sensing system system for for vapor vapor analysis analysis aa short-chain short-chain alcohols alcohols has has been been A developed using an active layer of chitosan covalently attached to the self-assembly monolayer of developed using an active layer of chitosan covalently attached to the self-assembly monolayer of LL-cysteine. The QCM sensing systemyields yieldsreproducible reproducibleand andlinear linearresonant resonantfrequency frequency responses responses for for cysteine. The QCM sensing system ethanol, n-propanol, isoamyl alcohol, and n-amyl alcohol. The resonant frequency response correlates ethanol, n-propanol, isoamyl alcohol, and n-amyl alcohol. The resonant frequency response well with the of the the size alcohol molecules, in which n-amyl alcohol produces biggestthe sensitivity correlates wellsize with of the alcohol molecules, in which n-amyl alcoholthe produces biggest − 1 − 1 of 4.4 Hz · mg · L, whereas ethanol produces the smallest sensitivity of 0.4 Hz · mg · L. The response −1 −1 sensitivity of 4.4 Hz·mg ·L, whereas ethanol produces the smallest sensitivity of 0.4 Hz·mg ·L. The time of the proposed sensing system also affected the vapor pressure of the analytes. Ethanol response time of the proposed sensingissystem is also by affected by the vapor pressure of the analytes. has the shortest responseresponse time of 15 s, whereas n-amyl n-amyl alcohol alcohol has the has longest response time oftime 91 s. Ethanol has the shortest time of 15 s, whereas the longest response The resonant frequency shift of the chitosan-based QCM sensing system is relatively stable after a test of 91 s. The resonant frequency shift of the chitosan-based QCM sensing system is relatively stable of 10 days of consecutive measurement. The developed system shows good sensitivity, reproducibility, after a test of 10 days of consecutive measurement. The developed system shows good sensitivity, and short response forresponse the detection of alcohols in theof air. The proposed device be a device useful reproducibility, andtime short time for the detection alcohols in the air. The could proposed technology for the rapid and in situ analysis of the vapors of short-chain alcohols and could be an could be a useful technology for the rapid and in situ analysis of the vapors of short-chain alcohols alternative to conventional instrumental methods. and could be an alternative to conventional instrumental methods. Author Contributions: K.T. conceived and designed the experiments, calibrated the frequency measurement Author K.T.and conceived andpaper. designed experiments, calibrated the frequency measurement system, Contributions: analyzed the data, wrote the A.S.the performed the experiments, wrote the paper. A.R. and system, analyzed the data, and wrote the paper. A.S. performed the experiments, wrote the paper. A.R. and S.N.H. analyzed the data and wrote the paper. R.R., and T.J. designed and developed the frequency measurement S.N.H. analyzed the data and wrote A.K., the paper. and T.J. the designed developed the frequency system and performed the experiments. I.S., andR.R., R.R. analyzed data andand contributed reagent/materials, and wrote the system paper. and performed the experiments. A.K., I.S., and R.R. analyzed the data and contributed measurement reagent/materials, and wrote the would paper.like to thank the Directorate of Research and Community Service, Ministry Acknowledgments: The authors of Research, Technology and Higher Education, the Republic of Indonesia for providing research grants with the Acknowledgments: The authors would like to thank the Directorate of Research and Community Service, contract numbers 1884/UN1/DITLIT/DIT-LIT/LT/2018 (PTUPT) and 2034/UN1/DITLIT/DIT-LIT/LT/2018 Ministry of Research, Technology and Higher Education, the Republic of Indonesia for providing research grants (PMDSU). with the contract numbers 1884/UN1/DITLIT/DIT-LIT/LT/2018 (PTUPT) and 2034/UN1/DITLIT/DITConflicts of Interest: The authors declare no conflict of interest. LIT/LT/2018 (PMDSU).
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