Adsorption Thermodynamic Analysis of a Quartz Tuning Fork Based

Oct 23, 2017 - Due to its advantage in size, sensitivity, and cost, it is commonly used in watches for timing purposes, and miscellaneous applications...
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Adsorption Thermodynamic Analysis of a Quartz Tuning Fork Based Sensor for Volatile Organic Compounds (VOCs) Detection Yue Deng, Nai-Yuan Liu, Francis Tsow, Xiaojun Xian, and Erica S. Forzani ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00518 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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Adsorption Thermodynamic Analysis of a Quartz Tuning Fork Based Sensor for Volatile Organic Compounds (VOCs) Detection Yue Denga,b, Nai-Yuan Liua,b, Francis Tsow*b, Xiaojun Xianb, Erica S. Forzania,b,* a b

School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ, United States, 85287 Center for Bioelectronics and Biosensors, Biodesign Institute, Arizona State University, Tempe, AZ, United States, 85287

*Correspondence: E-mail address: [email protected] Tel.: +1 (480)965-9058.

KEYWORDS: Molecularly imprinted polymer, Quartz crystal tuning fork, VOC, Adsorption, Thermodynamic properties, Stability ABSTRACT A volatile organic compounds (VOC) sensor based on molecularly imprinted polymer (MIP) modified quartz tuning fork (QTF) has been developed. In this paper, the stability of the modified sensor as a function of the MIP composition, and the temperature effect of the analyte adsorption on the sensing transduction mechanism are evaluated. By mixing MIP and PS together, the stability was improved. A target analyte, o-xylene, was chosen as Volatile Organic Compound (VOC) model to study the sensor response in a temperature range of 6-40°C. Langmuir model fitted adsorption isotherms were used for thermodynamic analysis. The changes in the sensitivity of the QTF sensor to temperature rendered different behaviors. For a freshly modified QTF sensor, the adsorption response increased with increasing temperature, while for an aged QTF sensor, the adsorption response decreased with increasing temperature. The results indicated that the enthalpy change of the MIP and PS composition sensing material changes from positive to negative over the course of aging. The characterization of the reaction enabled the definition of sensor calibration conditions and stable sensor performance in field testing conditions.

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Volatile organic compounds (VOC) are known as one group of the major environmental gas pollutants from anthropogenic and biogenic sources 1. Most of VOC are harmful to human body, especially to the respiratory system 2. Therefore, it is of great significance to monitor real-time personal exposure level to VOC. Molecularly imprinted polymer (MIP), a synthetic polymer with specific cavities designed for target molecules, is very often used as selective material in analytical techniques 3. Molecular imprinting of synthetic polymers is a process where functional and cross-linking monomers are co-polymerized in the presence of the target analyte, which acts as a molecular template 3. The template is extracted afterwards, leaving a specific complementary cavity. The highly selective affinity to target analyte makes it attractive in many applications. Research of MIP applications in solid-phase extraction 4, 5, binding assays 6, and sensors 4, 7, 8 have been reported. Quartz tuning fork (QTF) is a mass sensitive piezoelectric resonator. Due to its advantage in size, sensitivity and cost, it is commonly used in watches for timing purposes, and miscellaneous applications 9. A VOC sensor based on MIP modified QTF has been developed. As described in previous publication 10, the polymer is synthesized using o-xylene as a template. The MIP-modified QTF sensors have been integrated in a wearable wireless gas monitor for VOC detection. Comparing with traditional VOC monitoring methods such as photo-ionization detector (PID) 11, the wireless VOC gas monitor with MIP modified QTF sensors demonstrates real-time VOC detection with high selectivity towards monoaromatic hydrocarbons, and alkyl hydrocarbons, fast response (~ 1 minute), and high sensitivity with detection limits in the range of part-perbillion volume (ppb) and part-per-million volume (ppm) 12, 13. Studies in our laboratory and field tests have validated the specifications of the VOC gas monitor based on QTF sensors 10, 12-14. This VOC gas monitor has shown the ability to provide timely response to VOCs exposure level, helping to identify potential health risks 15, 16. However, in the process of application of the VOC gas monitor, we found the MIP modified QTF sensor behavior to be dependent on temperature, and time. In fact, instabilities of MIPs have been reported before 10, 17 and they are a challenge when it comes to use MIPs in real sensing application conditions. In addition, there are not many studies reporting the mitigation of these challenges. In this current work, we present a study of MIP modified sensors and report the adsorption properties of the analyte as a function of temperature and aging. In addition, we present a solution to mitigate instabilities of MIP over time. The temperature influence on the analyteMIP adsorption is studied at environmental temperatures range from 6 to 39°C, and as a consequence, the entropy and enthalpy of the adsorption process are analyzed according to Van ’t Hoff and standard thermodynamic equations 18 to further characterize the MIP sensing material behavior. Material and methods Molecularly Imprinted Polymer (MIP) The MIP is synthesized using divinylbenzene (Sigma Aldrich) as sole component of the polymer, xylenes (o-, m-, p-) (Sigma Aldrich) as template and solvent, and azobisisobutyronitrile (AIBN, Sigma Aldrich) as initiator, under argon environment at 85ºC and overnight conditions. The

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resulting structure of the MIP is a highly crossed linked structure with functional sites exclusively formed from divinylbenzene. After synthesizing, the polymer is milled into 1µm10µm particles, dissolved into xylene solvent and spin-coated (6700 Series, Specialty Coating Systems) onto the surface of QTF sensors. Sensors are dried under vacuum for one day after coating. Sensing Mechanism The sensing mechanism for VOC by the gas monitor is mainly divided into three steps: 1) Sample collection: The air sample first goes through a filtrating channel for 2 minutes (purging period), where a filter adsorbs particles, VOC, and interferents to generate a clean air flow, which defines a stable baseline. Then, the flow is switched to a sampling channel for 1 minute (sampling period), where a different filter is used to remove particles only. This generates a particle-free air sample for testing. Therefore, the total testing cycle involves 2-min purging and 1-min sampling, and it takes a total of 3 minutes. During the entire testing cycle, the sample passes through a dew line to avoid humidity interference. 2) Frequency recording: The resonant frequency of MIP modified QTF sensor is recorded, the change of frequency between the purging and sampling period is monitored, and the maximum frequency change is recorded to determine the sensor mass loading change. The mass change is proportional to the VOC concentration in the gas phase. In this paper, the response of the MIP-modified QTF sensor is defined as QTF resonant frequency change. 3) Data transmission: The real-time frequency change of the MIP-modified QTF sensor is transmitted from the VOC gas monitor via Bluetooth® to a smartphone or tablet. More detailed description of the testing mechanism can be found in published literatures 10, 12-14, 19. Mitigation of sensitivity drop on aged QTF sensors As presented in previous publication 10, a strong decrease of the MIP modified QTF sensor sensitivity was observed in first few days, with a decay of over 90% after seven days of fabrication. To overcome this issue, in the current work, polystyrene (PS) (Sigma Aldrich, Mw ~ 35,000) is introduced as “organic glue”. The stability of the MIP and PS mixture coated QTF sensors was studied as follows: 1) Two batches of QTF sensors (200 sensors for each batch) were modified with the mixture; 2) Each modified sensor was marked, packed, and sealed in a laminated bag (Fig. 1) after their test of initial response; 3) All sensors were stored in -4°C freezer; 4) One QTF sensor from each batch was randomly picked out from time to time to test the sensor response with 40 ppm o-xylene under room temperature. Each sensor was used for only one test to exclude potential impact due to handling and environment exposure. The test frequency was performed on weekly basis during the first four months, and biweekly after four months for a period of one year. Thermodynamic analysis of the adsorption process In order to study the effect of temperature on sensor sensitivity, the temperature effects on freshly modified QTF sensor and aged modified QTF sensor were also studied. O-xylene was chosen as calibration gas and vapor sample was generated from liquid o-xylene. All calibration tests were done in a sealed glovebox, where sample concentration and ambient temperature was controlled. A thermistor was implemented inside the device to monitor the environmental temperature. The sensitivity of the sensor was calibrated, using six different o-

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xylene concentrations over three different temperatures (279K (6°C), 297K (24°C), 312K (39°C). The testing sequence was from low temperature to high temperature. Sample concentrations were 0ppm, 10ppm, 20ppm, 50ppm and 100ppm. The reference concentrations were measured by a photo-ionization detector from RAE systems (RAE by Honeywell®). The sensor response as a function of concentration assessed at each temperature was fitted to a Langmuir adsorption isotherm. Langmuir Equation can be written as 20:  =

 ∗



Eq. 1

where R is the mass of adsorbed gas, which is proportional to the differential frequency change of QTF sensors, Rmax is the maximum amount of adsorbed gas that is represented by the maximum differential frequency change from the QTF sensors and proportional to the maximum amount of analyte binding sites, c is the o-xylene gas concentration, and KD is dissociation constant, which is defined as 20: K =

 

.

Eq. 2

where [A], [B], and [C] are concentration of species xylene analyte, MIP binding sites, and xylene-MIP complex, respectively. Four groups of modified QTF sensors were tested: Group 1 - Fresh MIP modified QTF sensor; Group 2 – Fresh MIP and PS mixture modified QTF sensor; Group 3 – Aged MIP modified QTF sensor; Group 4 – Aged MIP and PS mixture modified QTF sensor. Three sensors from each group were tested. The fresh condition was defined for tests performed one day after the QTF sensors were modified, and the aged condition was defined tests performed seven days after the QTF sensors were modified, during which the sensors were kept in a closed environment at 297K (24°C). Van ’t Hoff equation Under standard conditions, the Van’t Hoff equation for a dissociation of analyte-binding site complex (xylene-MIP complex) can be defined as 21:  

=

∆ ⊝

Eq. 3

 

where R is the ideal gas constant, ∆H⊝ is reaction enthalpy, KD is equilibrium dissociation constant, and T is absolute temperature. Assuming reaction enthalpy is constant in the tested temperature range of 6-39°C, the equation could be derived to estimate the relationship between temperature and equilibrium dissociation constant as follows 22: 9: ;< = −

∆ ⊝ 

+

∆? ⊝ 

Eq. 4

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where ∆S⊝ is the reaction entropy. Thus, a linear relationship between ln KD and 1/T could be used to assess ∆H⊝ for the xylene-MIP complex. Therefore, we should expect an exothermic reaction for a positive slope (−

∆ ⊝ 

@, and an endothermic reaction for a negative slope.

Results and Discussion Sensor fabrication and stability Fig. 1a shows the coating mass distribution of 200 QTF sensors coated with MIP and PS mixture, which were fabricated in one batch. The loading mass of mixture on the QTF prong is presented in terms of Hz with a mass sensitivity of 66 ng/Hz and a typical spring constant around 10 kN/m 13, 19, 23 . In addition, Fig. 1b shows the corresponding response to 40 ppm o-xylene for the same sensor batch. The black lines in both Fig. 1a and b represent the normal distribution curve of the designated data in each graph. As it can be seen, the distribution of coating mass has a standard deviation (SD) and a dispersion error defined as (mean/SD) × 100 of 10.5%, which indicates that the fabrication process was reproducible. In the other hand, the response to o-xylene has a dispersion error of 9.2%, which also indicates an acceptable reproducible sensor response.

Fig. 1. Reproducibility of sensor fabrication protocol on 200 QTF sensors. a) Distribution of coating mass on QTF modified with MIP – PS mixture; b) Distribution of the sensor reponse to 40 ppm xylene. As mentioned in the experimental part, our previous publication 10 reported an aging effect given by the QTF sensor response decrease over time. In this work, QTF sensors modified with MIP only showed the same behavior and a 90% response drop after seven days of sensor fabrication. This fact precluded the used of MIP QTF sensors from real-world applications (see Fig. 2a reproduced from reference 10).

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On the contrary, the decay of sensor response could be mitigated by adding PS into the MIP sensing material. As an example, Fig. 2b) shows the stability of the response to the analyte of MIP-PS mixture modified QTF sensors over time. As it can be seen, the sensors kept 60%-70% of the initial response for 4 months after fabrication. Furthermore, even after one year, the response still remained over 50%, which allowed for storage, and sensor calibration. Finally, the resulting stability of the response of MIP-PS mixture modified QTF sensors could be fitted with a linear function of log (t) (time logarithm). Intercept was fixed to 100 since the first day of test was considered as the full response. This linear function was used to predict the sensor response at the moment of use in the field application, enabling the use and deployment in the field.

Fig. 2. Stability test on QTF sensors. Response of QTF sensors to 40 ppm xylene. a) QTF sensors modified with MIP only; b) QTF sensors modified with MIP and PS mixture. The above comparison shows the enhancement of sensor stability by mixing PS with MIP as sensing material. In this regard, it is worthy to mention that this is the first time that a method to enhance the response stability of MIP over a year time period has been reported. In our previous publication, it was reported that a collapsing binding site effect could be responsible for the rapid loss of MIP modified QTF sensor response 10. A possible explanation for the relatively better response stability observed in the MIP-PS mixture modified QTF sensors is that PS scaffold could support the inner structure of MIP and prevents it from collapsing, keeping the binding sites’ structure relatively stable over time. To demonstrate the effectiveness of the imprinted polymer and to exclude the adsorption response from PS and non-imprinted polymer, control experiments were run (please refer to supporting information for more details). The experiments showed that the MIP system is selective to a family of aromatics and linear hydrocarbons in presence and absence of PS, and that the addition of PS does not affect the selectivity of the MIP. Furthermore, with the addition of PS, the specificity of the sensors was compared with the original MIP modified QTF sensors. In addition, sensors were tested with different hydrocarbons and other common interferents in the air (for more details of the selectivity test, please refer to the supporting information). Several important conclusions rendered from these comparisons. First, the significant detectable sensitivity of the QTF sensors is due to the MIP and its structure, rather than to PS composition or the polymer backbone components present in a non-imprinted

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polymer (NIP). Second, the selectivity of MIP + PS mixture is exclusively towards molecules capable to interact with the polymer via pi-pi and Van der Waals interactions. Analysis of temperature effect on the modified QTF sensors’ response Fresh polymer modified QTF sensors Fig. 3 shows the calibration curves of freshly polymer modified QTF sensors under different temperatures tested one day after fabrication. The calibration curves show the normalized sensor response as a function of o-xylene concentration. As the QTF sensors have mass sensitivity of 66 ng/Hz, the coating mass could be expressed in terms of Hz. The response is normalized with the ratio of raw response and coating mass. Fig. 3a and b show the results for group 1 (MIP only modified QTF sensors) and 2 (MIP+PS mixture modified QTF sensors), respectively. In addition, Langmuir isotherm fittings are applied to the calibration curves for each temperature. As it can be seen, Fig. 3a) and Table 1 shows that MIP modified QTF sensors’ response decreases with increasing temperature, rendering a decrease in both Rmax and KD. On the other hand, Fig. 3b) and Table 1 shows a MIP + PS modified QTF sensors’ response increase with increasing temperature, and both Rmax and KD values increase with increase in temperature. It is noticeable that the MIP modified QTF sensors have two times higher sensitivity than the MIP + PS mixture modified QTF sensors. This is because that the polystyrene could fill part of the cavities inside the polymer as a support, which will sacrifice part of the sensor sensitivity. But considering the big enhancement in terms of stability, we choose to tradeoff the sensitivity with stability. And for aged QTF sensors (shown in next part), the sensitivity will remain about the same level for the two coating materials. This also demonstrates that the MIP + PS modified QTF sensors are able to maintain the sensitivity for longer period of time. Details of the mixing ratio optimization are described in supporting information (Fig. S-2).

Fig. 3. Fresh modified QTF sensor calibration under different temperatures. a) Response on MIP modified QTF sensors; b) Response on MIP+PS mixture modified QTF sensors Aged polymer modified QTF sensors The above observations happened when the QTF sensors were prepared in fresh condition. In this section, the behavior of QTF polymer modified sensors stored at 297K for seven days after modification (Group 3 and 4) is reported. During the storage time, the QTF sensors were kept in

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closed clean air environment to prevent contamination from chemical species. Fig. 4 shows the results on MIP modified QTF sensor (Fig. 4a) and MIP - PS mixture modified QTF sensor (Fig. 4b). For aged MIP modified QTF sensor, the effect from temperature has similar trend as freshly modified QTF sensor (Fig. 3a). As summarized in Table 1, both Rmax and KD values decrease with increasing temperature. However, it is worthy to mention that the sensor response (as mentioned above) drops 90%. On the other hand, the MIP + PS mixture modified QTF sensor response (Fig. 4b) decreases with increasing temperature, rendering both Rmax and KD to decrease. This result is opposite to the one observed in freshly prepared MIP + PS mixture modified QTF sensors, and indicates that there is a change in the thermodynamics of the analyte-binding site complex dissociation reaction over time.

Fig. 4. Aged QTF sensor calibration under different temperature. a) Response on MIP modified QTF sensors; b) Response on MIP+PS mixture modified QTF sensors.

Table 1. Summary of KD and Rmax from fitted curves of four groups of QTF sensors Group 1* 279K 297K 312K KD (ppm) Rmax (a.u.)

Group 2* 279K 297K 312K

Group 3* 279K 297K 312K

Group 4* 279K 297K 312K

120

84

66

159

293

603

114

105

89

221

190

138

27

12

5.4

5.6

11

25

3.3

1.7

0.8

5.7

2.3

1.4

* Group 1 - Fresh MIP modified QTF sensor; Group 2 – Fresh MIP and PS mixture modified QTF sensor; Group 3 – Aged MIP modified QTF sensor; Group 4 – Aged MIP and PS mixture modified QTF sensor.

Thermodynamic analysis of polymer modified QTF sensors’ response In summary, for MIP modified QTF sensors, the temperature dependence of the response do not change the trend with aging process, although a significant response decrease is observed. On the contrary, the MIP+PS mixture modified QTF sensors change the temperature dependence of the response over time. This temperature-dependence change observed on fresh vs. aged MIP+PS

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mixture modified QTF sensors could be attributed intrinsic changes in the nature of the binding sites due to internal diffusion of PS lineal polymer chains inside the MIP which is a highly crosslinked polymer, exclusively formed by crosslinker divinylbenzene. From the calibration data, the relationship between reaction equilibrium constant KD and temperature can be plotted according to Van’t Hoff Equation (Ln KD vs. 1/T), as shown in Fig. 5a and b. Furthermore, it is important to mention, that the above-mentioned observations are reproducible across different batches of prepared sensors. Fig. 5a and b shows the average trend of data for three independently-prepared batches of fresh and aged MIP and MIP+PS mixture modified QTF sensors, respectively.

Fig. 5. Van ’t Hoff plot (ln KD vs. 1/T) of: a) Fresh modified QTF sensors; b) Aged modified QTF sensors. Error bars in a) and b) represent the standard deviation of the average of KD assessed for the three batches. Table 2 summarizes the thermodynamic constants for each group of polymer modified QTF sensors calculated from the averaged KD values assessed for the three analysis temperatures. Table 2. Thermodynamic constants of polymer modified QTF sensors from this study

∆HΘ/ kJ ∆SΘ/J·K

Fresh MIP modified -15.5 (± 0.2) 6.8 (± 0.2)

Fresh mixture modified 25.7 (± 2.7) 156.8 (± 116.2)

Aged MIP modified -7.8 (± 0.8) -129.3 (± 2.9)

Aged mixture modified -12.4 (± 2.5) -140.2 (± 8.6)

According to basic thermodynamic principle, in an exothermic reaction, the potential energy of products is lower than the reactants, and on the contrary, for an endothermic reaction, the potential energy of the products is higher than the reactants to products 24. From Table 1, the standard enthalpy change (∆HΘ) of pure MIP modified QTF sensors is less negative after they are aged, indicating that at the aged state for the dissociation reaction, the potential energy of the product (analyte + MIP polymer) is closer to the potential energy of the reagent (analyte- MIP polymer complex). For theMIP+PS mixture modified QTF sensors, the ∆HΘ changed from positive to negative during aging, which demonstrate the dissociation reaction changed from endothermic to exothermic, and aging induced theMIP+PS mixture to adopt a similar energy distribution than the aged pure MIP. This means that the potential energy of the product (analyte

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+ (MIP + PS) mixture complex) has lower potential energy than the reagent (analyte + (MIP + PS) mixture complex). This could be explained by the hypothesis that with aging MIP+PS mixture complex reaches more stable energy levels due to relaxation of the structure. This result fits the expectation of adding PS into the sensing material to improve the stability issue: PS will slow down MIP inner structure collapsing, which makes the lifetime of the sensor longer and the sensor practical use feasible.

Conclusion In this paper, stability of a QTF based gas sensor for environmental VOCs detection was tested. By mixing MIP and PS together, the stability was improved and the calibration under different temperature on fresh/aged sensing coatings of QTFs made of pure MIP modified or MIP and PS mixture is presented. Results show that there is a significant temperature dependent response difference between a freshly modified and an aged sensor. Thermodynamic analysis on the calibration data shows that dissociation reaction on the mixture modified QTF sensors goes through a transformation from endothermic reaction to exothermic reaction. This is the first time that this kind of behavior has been reported. The observed phenomenon is of great significance in MIP applications. Supporting Information The sensitivity and selectivity of the QTF sensors modified with MIP+PS mixture; Control experiments to validate the characteristics of the sensors; The effects of time and addition of polystyrene on the selectivity of the MIP+PS mixture - QTF sensor. Acknowledgement The authors would like to thank NIH/NIEHS (GEI, #5U01ES016064 and SBIR 1R44ES021678 program) for their financial support in the development of the quartz tuning fork chemical sensor. Dr. NJ Tao for his support to our work.

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For TOC only

ACS Paragon Plus Environment

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