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Flexible Humidity Sensor Based on Au Nanoparti-Cles/Organosilica-Containing Polyelectrolyte Composite

A peer-reviewed version of this preprint was published in:
Chemosensors 2023, 11(5), 291. https://doi.org/10.3390/chemosensors11050291

Submitted:

18 April 2023

Posted:

18 April 2023

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Abstract
A novel flexible humidity sensor, incorporating gold nanoparticles (Au NPs) and a trifunctional organosilica compound, has been developed through the integration of sol-gel processing, free radical polymerization, and self-assembly techniques. The trifunctional organosilica was initially synthesized by modifying (3-mercaptopropyl)trimethoxysilane (thiol-MPTMS) with 3-(trimethoxysilyl)propyl methacrylate (vinyl-TMSPMA). Subsequently, a hydrophilic polyelectrolyte, [3(methacryloylamino)propyl]trimethyl ammonium chloride (MAPTAC), was grafted onto the MPTMS-TMSPMA gel. The Au NPs were assembled onto the thiol groups present in the MPTMS-TMSPMA-MAPTAC gel network. The compositional and microstructural properties of the Au NPs/MPTMS-TMSPMA-MAPTAC composite film were investigated utilizing Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The presence of thiol groups and mesoporous silica skeletons ensured the stability of the humidity-sensing film on the substrate under highly humid conditions, while the hydrophilic groups functioned as humidity-sensitive sites. This innovative humidity sensor demonstrated high sensitivity, acceptable linearity, minimal hysteresis, and rapid response time across a broad range of working humidity levels. Based on the complex impedance spectra analysis, hydronium ions (H3O+) were determined to govern the conductance process of the flexible humidity sensor.
Keywords: 
Flexible humidity sensor
;  
organosilica
;  
Au NPs
;  
composite
;  
water-resistance
Subject: 
Chemistry and Materials Science  -   Analytical Chemistry

1. Introduction

Fabricating flexible sensors has emerged as a critical area of interest, particularly in the context of portable handheld consumer electronics, which are utilized in applications such as smart textiles, smart packaging, radio frequency identification (RFID) tags, and the Internet of Things (IoT) [1,2,3,4]. Humidity is a principal component integrated with RFID tags, expanding the scope of RFID technology in areas such as industrial process control, agriculture, health monitoring, and environmental monitoring [5,6,7]. Consequently, the development of flexible humidity sensors has gained significant traction [8,9,10]. To achieve high accuracy and precision in flexible humidity sensors that cater to the diverse needs of various applications, it is imperative to enhance the properties of humidity-sensing films in terms of cost-efficiency, ease of fabrication, sensitivity, hysteresis, linearity, mechanical stability, response/recovery times, and working range [7,11,12]. As a result, an array of materials, including polymers [13,14,15,16,17,18,19], carbon-based nanomaterials [20,21,22,23,24,25,26], ceramic materials [27,28,29], and diverse composite materials [30,31,32,33,34,35,36,37,38], have been employed in flexible humidity-sensing films. In comparison to polymers, ceramics demonstrate superior sensitivity, rapid response/recovery times, and enhanced stability under high humidity conditions.
Mesoporous silica has garnered considerable interest for humidity sensor applications due to its exceptional properties, such as non-toxicity, high internal specific surface area, and ordered pores [39,40,41,42]. However, a lack of sufficient hydrophilic groups and low conductivity restricts its humidity-sensing capabilities across a wide humidity range. To enhance the hydrophilic properties of silica materials, a common method involves modifying them with polymer electrolytes, as these exhibit significant conductivity changes when exposed to varying relative humidity (RH) environments [43,44,45,46,47]. Various researchers have developed resistive and capacitive-type humidity sensors using different materials and methods, such as in-situ polymerization and thiol-ene click reactions [43,44,45,46,47]. Additionally, incorporating materials like Au nanoparticles (Au NPs), single-walled carbon nanotubes (SWCNTs), or graphene oxide (GO) into silica has been used to improve conductivity and flexibility, thereby enhancing humidity-sensing properties [48,49,50,51]. In a previous study [48], a flexible humidity sensor comprising Au nanoparticles, GO, and silica was fabricated using a sol-gel process and self-assembly technique. However, there have been no attempts to utilize organosilica-containing polyelectrolyte-based materials in the fabrication of a flexible humidity sensor. In this study, Au NPs/organosilica-containing polyelectrolyte ((3-mercaptopropyl)trimethoxysilane (MPTMS)-3-(trimethoxysilyl)propyl methacrylate (TMSP-MA)-[3(methacryloylamino)propyl]trimethyl ammonium chloride (MAPTAC) (MPTMS-TMSPMA-MAPTAC) composite films were fabricated on a polyethylene terephthalate (PET) substrate to create flexible resistive-type humidity sensors by combining sol-gel processing, free radical polymerization, and self-assembly. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to analyze the composition and morphologies of the Au NPs/MPTMS-TMSPMA-MAPTAC composites. The electrical characteristics of the Au NPs/MPTMS-TMSPMA-MAPTAC composite films were investigated as functions of RH. The water resistance, flexibility, and humidity-sensing properties, encompassing sensitivity, hysteresis, influence of ambient temperature, effect of applied frequency, response/recovery times, and stability, were also examined. The humidity-sensing mechanism of the Au NPs/MPTMS-TMSPMA-MAPTAC composite film was studied by using complex impedance spectra.

2. Experimental Methods

2.1. Materials

Tetraethoxysilane (TEOS, 99%, Sigma-Aldrich), (3-mercaptopropyl)trimethoxysilane (MPTMS, 95%, Sigma-Aldrich), 3-(trimethoxysilyl)propyl methacrylate (TMSPMA, 98%, Sigma-Aldrich), [3(methacryloylamino)propyl] trimethyl ammonium chloride (MAPTAC, 50%, Sigma-Aldrich), ethanol (95%, Sigma-Aldrich), hydrochloric acid (0.1 mol/L, Sigma-Aldrich), sodium citrate (99%, Sigma-Aldrich) and hydrogen tetrachloroaurate(III) hydrate (HAuCl4·xH2O, 99.9%, , Sigma-Aldrich) were purchased. All used deionized water (DIW) was prepared using a Milli-Q Millipore (Bedford, MA, USA) purification system.

2.2. Fabrication of Au NPs

Au NPs were synthesized following the procedure described in our previous publication [48]. The colloidal Au NPs were prepared by incorporating 38.8 mM sodium citrate into 1 mM HAuCl4 aqueous solution, heated to boiling, and maintained at this temperature for 15 minutes with continuous vigorous stirring. Subsequently, the mixture was allowed to cool to room temperature and stored at 4°C prior to utilization.

2.3. Fabrication of flexible humidity sensor based on Au NPs/MPTMS-TMSPMA-MAPTAC composite

Figure 1 provides a schematic representation of the flexible humidity sensor structure. Interdigitated Au electrodes on a flexible PET substrate were fabricated by initially sputtering Cr (50 nm of thickness) followed by Au (250 nm of thickness) at 120 °C. The gap of electrode measured 0.25 mm. Figure 2 illustrates the fabrication process of a flexible resistance-type humidity sensor employing sol-gel processing, free radical polymerization, and self-assembly. Initially, 0.56 M MPTMS and 0.56 M TMSPMA were dissolved in 60 mL of 0.1 M HCl and ethanol solution under stirring at room temperature using a one-pot method. Subsequently, 0.08 M MAPTAC and 0.01 g AIBN were introduced into the as-prepared TMSPMA-MPTMS sol solution, and the mixture was heated at 70 °C for 24 hours with continuous stirring. The resulting MPTMS-TMSPMA-MAPTAC sol-gel solution was then deposited onto a PET substrate and dried at 60 °C for 4 hours. Finally, the MPTMS-TMSPMA-MAPTAC sol-gel was immersed in an Au NPs solution for 5 hours at 4 °C, yielding a flexible resistive-type humidity sensor. In comparison to the organosilica-based material, a pristine SiO2 film was also synthesized via the hydrolysis of TEOS.

2.4. Instruments and analysis

The infrared (IR) spectra of the MPTMS, MPTMS-TMSPMA, and MPTMS-TMSPMA-MAPTAC films were studied using a Fourier transform infrared spectrometer (Nicolet 380). The surface microstructures of the pristine SiO2, MPTMS-TMSPMA-MAPTAC, and Au NPs/MPTMS-TMSPMA-MAPTAC composite films coated onto PET substrates were studied by using a scanning electron microscope (SEM, Ibaraki, Japan). Figure 3 illustrates the humidity sensor measurement system within a temperature-controlled chamber. A divided humidity generator system was employed to produce the test humidity conditions by adjusting the proportion of dry and humid air under a total flow rate of 10 L/min. The required RH values were determined using a standard humidity hygrometer (with an accuracy of ±0.1% RH). An LCZ meter was utilized to measure the impedance values of the flexible humidity sensors as a function of RH. The maximum humidity atmosphere (97% RH) for water-resistance testing was controlled by a saturated K2SO4 solution. The measurement conditions for complex impedance analysis were set at an applied frequency range of 50 Hz to 100 kHz, an applied voltage of 1 V, and a humidity range from 20 to 90% RH at 25 °C.

3. Results

3.1. Preparation and characterization of Au NPs/MPTMS-TMSPMA-MAPTAC composite

Figure 2 illustrates the fabrication process of a flexible resistive-type humidity sensor composed of the Au NPs/MPTMS-TMSPMA-MAPTAC composite film. Initially, thiol-containing organosilica (thiol-MPTMS) functionalized with vinyl-containing organosilica (TMSPMA-vinyl) was synthesized using a hydrolyzed sol-gel process via crosslinkable silanol groups. Subsequently, a hydrophilic organic unit (MAPTAC) was chemically modified onto the thiol-MPTMS-TMSPMA-vinyl utilizing a free radical polymerization method. Finally, Au NPs were incorporated into the MPTMS-TMSPMA-MAPTAC using self-assembly. The as-prepared MPTMS-TMSPMA sol includes trifunctional groups, namely the vinyl head group, siloxane middle group, and thiol tail group. The vinyl head groups can be chemically modified with hydrophilic MAPTAC, generating a strong force to bind the host and guest materials together, thus enhancing the response of the MPTMS-TMSPMA-MAPTAC film to humidity. The siloxane middle groups form a three-dimensional network, providing not only stereo sites for Au NPs attachment but also improving stability in high humidity environments. The thiol tail groups serve dual purposes: they can chemisorb onto the Au electrode surface and participate in the assembly of Au NPs by forming Au-S bonds, ultimately enhancing the adhesion strength on the PET substrate, and improving the stability of the flexible sensors. Self-assembling Au NPs on the organosilica gel offers conduction paths that enhance the conductivity of the MPTMS-TMSPMA-MAPTAC film.

3.1.1. IR analysis

Figure 4a,b display the FT-IR spectra of the as-prepared MPTMS-TMSPMA and MPTMS-TMSPMA-MAPTAC sol-gel, respectively. The peak appearing at 852 cm−1 indicates the presence of the stretching of -C-S- group in the MPTMS structure. Two peaks emerging at 1014 and 1042 cm−1 correspond to the stretching vibrations of the cross-linked -Si-O-Si- framework. The peaks at 1084 and 1164 cm−1 signify the respective stretching of O-C-C and C-C-O of the ester groups in the TMSPMA structure. Another crucial band of TMSPMA is confirmed at 1634 cm−1, attributed to the mixture of stretching vibrations of C=O and C=C groups [52]. These results verify that the vinyl-containing organosilica (TMSPMA-vinyl) was successfully functionalized with thiol-containing organosilica (thiol-MPTMS). Regarding the IR spectrum of MPTMS-TMSPMA-MAPTAC (Fig. 4(b)), the characteristic peaks at 1650 and 1542 cm−1 correspond to the C=ONH and CON-H groups, respectively, in the MAPTAC structure. Additionally, the IR spectrum of MPTMS-TMSPMA-MAPTAC exhibits the same primary characteristic absorption peaks of MPTMS-TMSPMA in the 1200-800 cm−1 range. These results suggest that MAPTAC was successfully incorporated into the MPTMS-TMSPMA sol-gel.

3.1.2. SEM analyses

Figure 5 displays the SEM images of the pristine SiO2, MPTMS-TMSPMA-MAPTAC, and Au NPs/MAPTAC-TMSPMA-MPTMS composite films. Figure 5(a) reveals that the pristine SiO2 film exhibits a fractured and cracked surface, which can result in instability of the flexible properties. In contrast, the SEM of the MPTMS-TMSPMA-MAPTAC film (Fig. 5(b)) demonstrates a much smoother surface with numerous pores on the film's surface. This is because organosilica tends to produce more organic sites homogeneously branched to the pore walls, which better supports the film's cohesion on a PET substrate. Compared to the MPTMS-TMSPMA-MAPTAC film, the Au NPs/MPTMS-TMSPMA-MAPTAC composite film (Fig. 5(c)) indicates that the Au NPs are embedded within the MPTMS-TMSPMA-MAPTAC matrix with no uncovered areas present on its surface. The aggregation of Au NPs was observed. This phenomenon can be attributed to the thiol-containing organosilica gel providing stereo sites for anchoring Au NPs.

3.2. Water-resistance and flexibility properties of Au NPs/MPTMS-TMSPMA-MAPTAC composite film

Figure 6 illustrates the deviation of flexible humidity sensors based on pristine SiO2, MPTMS-TMSPMA-MAPTAC, and Au NPs/MPTMS-TMSPMA-MAPTAC composite films after being placed in a highly humid (97% RH) environment for 5 days. The deviation was determined by using the formula (logZ60% RH – logZ’60% RH)/logZ60% RH × 100%), where logZ60%RH and logZ’60%RH represent the impedance values of the flexible humidity sensors at 60% RH before and after the sensor was placed in the high humidity atmosphere (97% RH), respectively. In comparison to the pristine SiO2 film, no noticeable deviation was observed for the MPTMS-TMSPMA-MAPTAC and Au NPs/MPTMS-TMSPMA-MAPTAC composite films, indicating good water-resistance due to the silica skeletons ensuring stability in high humidity environments. Furthermore, the thiol-containing MPTMS-TMSPMA-MAPTAC film can be anchored onto a gold electrode, thus improving the stability of the sensing film in high humidity environments. Additionally, the incorporation of Au NPs did not reduce the water-resistance of the Au NPs/MPTMS-TMSPMA-MAPTAC composite film.
Figure 7 depicts the flexibility of humidity sensors based on pristine SiO2, MPTMS-TMSPMA-MAPTAC, and Au NPs/MPTMS-TMSPMA-MAPTAC composite films. At each bending angle, the sensor was exposed to 60% RH. The deviation of the impedance of the flexible humidity sensors was determined by using the formula (logZ60% RH, f –logZ60% RH, b)/logZ60% RH, f ×100%), where logZ60% RH, f and logZ60% RH, b represent the impedance of the flat and bent flexible humidity sensor at 60% RH, respectively. The deviation of the pristine SiO2 film reached up to 96% even at an angle of 5° due to its rigid nature. In contrast, both the MPTMS-TMSPMA-MAPTAC and Au NPs/MPTMS-TMSPMA-MAPTAC composite films exhibited strong flexibility at angles up to 60°, with deviations within 10%. This outcome can be attributed to their soft organosilica skeleton structure.
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Figure 6. Water-resistance of (a) pristine SiO2 film, (b) MPTMS-TMSPMA-MAPTAC film and (c) Au NPs/MPTMS-TMSPMA-MAPTAC composite film on high humidity, measured at 1 V, 1 kHz and 25 °C.
Figure 6. Water-resistance of (a) pristine SiO2 film, (b) MPTMS-TMSPMA-MAPTAC film and (c) Au NPs/MPTMS-TMSPMA-MAPTAC composite film on high humidity, measured at 1 V, 1 kHz and 25 °C.
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Figure 7. Flexibility of ■: pristine SiO2 film, ▲: MPTMS-TMSPMA-MAPTAC film and ●: Au NPs/MPTMS-TMSPMA-MAPTAC composite film on PET substrate, measured at 1 V, 1 kHz and 25 °C.
Figure 7. Flexibility of ■: pristine SiO2 film, ▲: MPTMS-TMSPMA-MAPTAC film and ●: Au NPs/MPTMS-TMSPMA-MAPTAC composite film on PET substrate, measured at 1 V, 1 kHz and 25 °C.
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3.3. Electrical and humidity-sensing properties of Au NPs/MPTMS-TMSPMA-MAPTAC composite film

Figure 8 displays the log-impedance of the pristine SiO2, MPTMS-TMSPMA, MPTMS-TMSPMA-MAPTAC, and Au NPs/MPTMS-TMSPMA-MAPTAC composite films as a function of RH. Table 1 summarizes the results about the sensitivities and linearities of the linear fitting curve. The pristine SiO2 film had only a small change in impedance with humidity over the studied range (20% to 90% RH) due to its insulating nature. The MPTMS-TMSPMA film displayed only a small change in impedance with humidity over the studied range (20% to 90% RH) because more crosslinkable silicol groups were modified onto the MPTMS-TMSPMA film, resulting in hydrophobic and low-conductance properties. The impedance of the MPTMS-TMSPMA-MAPTAC film decreased over a wider range of RH (20% to 90% RH) than the MPTMS-TMSPMA film, because the highly hydrophilic organic compound (MAPTAC) was chemically modified onto TMSPMA-MPTMS, thus improving its sensitivity. The impedance of the Au NPs/MPTMS-TMSPMA-MAPTAC film decreased more than the MPTMS-TMSPMA-MAPTAC film with humidity over the studied range (20% to 90% RH), indicating that the Au NPs were anchored into MPTMS-TMSPMA-MAPTAC, consequently generating a new conductive pathway, and enhancing the conductivity of this material. The flexible humidity sensor composed of the Au NPs/MPTMS-TMSPMA-MAPTAC composite film had the greatest sensitivity (0.0533 log Z/%RH), good linearity (R2 = 0.9562) (Table 1), the greatest flexibility (less 10% at an angle of up to 60°), and good stability in a high-humidity environment. Accordingly, its humidity-sensing properties were further studied.
Figure 9(a) illustrates the hysteresis phenomenon observed in the flexible humidity sensing device. The mean hysteresis remained within a 3.5% relative humidity (RH) margin throughout a dehydration-to-humidification cycle, which encompassed the complete spectrum of relative humidity, ranging from 20% to 90% RH. In Figure 9(b), the influence of the surrounding temperature on the log Z for the flexible humidity sensor as a function of RH is depicted. The temperature coefficient was approximately -0.55% RH/°C for temperatures between 15 and 35 °C, spanning a humidity range from 20% to 90% RH. Figure 9(c) presents the consequences of varying the applied frequency on the log Z for the flexible humidity sensor as a function of RH. Impedance measurements were conducted at frequencies of 1, 5, 11, and 100 kHz with an applied voltage of 1 V. A distinct decline in the impedance of the flexible humidity sensor was observed below 40% RH as the applied frequency escalated. The curve representing the log Z versus RH exhibited the highest degree of humidity-sensing linearity at a frequency of 1 kHz. Figure 9(d) illustrates the response and recovery times for the flexible humidity sensor, assessed at a temperature of 25 °C and a frequency of 1 kHz. The response time (Tres.90%) and recovery time (Trec.90%) are characterized as the durations necessary for the impedance of the flexible humidity sensor to undergo a 90% shift in its maximal variation, following humidification and desiccation processes ranging from 10% to 90% RH and 90% to 10% RH, respectively. Tres.90% and Trec.90% were determined to be 15 seconds and 100 seconds, respectively. Figure 9(e) displays the long-term stability of the flexible humidity sensor. No significant deviation in the log Z for the flexible humidity sensor was detected over a 29-day period at the examined RH values of 20%, 60%, and 90% RH. Table 2 juxtaposes the humidity-sensing characteristics of the current flexible humidity sensor with those of sensors documented in prior reports [43,44,45,46,47]. In comparison to organosilica-based materials, such as SBA-15 [45,46] and MCM-41 [47], the fabricated flexible humidity sensor, which utilizes an Au NPs/MPTMS-TMSPMA-MAPTAC composite film, demonstrated superior flexibility, commendable water resistance, and a comparable response time and hysteresis.
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Figure 9. Humidity-sensing properties of the flexible humidity sensor based on Au NPS/ MPTMS-TMSPMA-MAPTAC composite film. (a) Hysteresis of the sensor, (b) effect of ambient temperature, (c) effect of applied frequency, (d) response/recovery times, (e) long-term stability.
Figure 9. Humidity-sensing properties of the flexible humidity sensor based on Au NPS/ MPTMS-TMSPMA-MAPTAC composite film. (a) Hysteresis of the sensor, (b) effect of ambient temperature, (c) effect of applied frequency, (d) response/recovery times, (e) long-term stability.
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Table 1. Sensitivity and linearity of flexible humidity sensors based on pristine SiO2, MPTMS-TMSPMA, MPTMS-TMSPMA-MAPTAC and Au NPs/MPTMS-TMSPMA-MAPTAC composite films.
Table 1. Sensitivity and linearity of flexible humidity sensors based on pristine SiO2, MPTMS-TMSPMA, MPTMS-TMSPMA-MAPTAC and Au NPs/MPTMS-TMSPMA-MAPTAC composite films.
Sensing curve
Materials Sensitivity (log Z/%RH)a Linearity (R2)b
Pristine SiO2 –0.0003 0.9646
MPTMS-TMSPMA –0.0034 0.9609
MPTMS-TMSPMA-MAPTAC –0.0442 0.9694
Au NPs/MPTMS-TMSPMA-MAPTAC –0.0533 0.9562
aSensitivity was defined as the slope of the logarithmic impedance versus RH plot in the range from 20 to 90% RH. bLinearity was shown as the correlation coefficient of the logarithmic impedance versus RH plot in the range from 20 to 90% RH.
Table 2. Comparison of performance of flexible humidity sensor developed herein with the literature based on organicsilica materials and their composites.
Table 2. Comparison of performance of flexible humidity sensor developed herein with the literature based on organicsilica materials and their composites.
Sensing material Working range
(%RH)		 Sensitivity
−(log Z/% RH)		 Hysteresis
(% RH)		 Flexibility (%)
(log Z deviation)		 Response time (s) References
SiO2-poly-AMPS 30-90 0.0571 2% — — [43]
APTOSa/n-butyl bromide 11-97 — 1% — 13 [44]
SBA-15/MATMACb 11-95 — 2% — 11 [45]
SBA-15/sodium p-styrenesulfonate 11-95 — — — 5 [46]
MCM-41/PEDOT 11-95 — 6 — 165 [47]
Au NPs/MPTMS-TMSPMA-MAPTAC 20-90 0.0533 3.5% <10% 15 This work
aAPTOS: 3-aminopropyltriethoxysilane. b MATMAC: methacrylatoethyl trimethyl ammonium chloride.

3.4. Humidity-sensing mechanism

Impedance spectroscopy is employed to explicate the ion transport as the humidity-sensing mechanism for the flexible humidity sensor, which is made of an Au NPs/MPTMS-TMSPMA-MAPTAC composite film. Figure 10 exhibits the complex impedance spectra for the flexible humidity sensor across an RH range from 30% to 90% RH. The impedance evaluations were performed at frequencies between 50 Hz and 100 kHz, an alternating current (AC) voltage of 1 V, and a temperature of 25 °C. At a low RH of 30% (Figure 10(a)), a discernible semicircular curve was observed, primarily attributable to the intrinsic impedance of the film. As the RH increased to 40% (Figure 10(b)), a distorted and diminishing semicircle at elevated frequencies became evident. This suggests that some of the physisorbed water molecules on the Au NPs/MPTMS-TMSPMA-MAPTAC composite film started to gradually dissociate, forming H3O+ ions, resulting in diffusing from the humidity-sensing film to the Au electrode. At a high RH of 90% (Figure 10(c)), only a straight line is apparent, indicating the H3O+ ions predominate the conductance process within the Au NPs/MPTMS-TMSPMA-MAPTAC composite film. According to the obtained complex impedance spectra, the H3O+ ions are the dominant factor influencing the conduction characteristics of the flexible humidity sensor based on the Au NPs/MPTMS-TMSPMA-MAPTAC composite film [53,54].

4. Conclusions

A novel flexible impedance-based humidity sensor utilizing an Au NPs/MPTMS-TMSPMA-MAPTAC composite film was engineered by integrating sol-gel processing, free radical polymerization, and self-assembly techniques. Thiol groups and mesoporous silica skeletons can be anchored to the Au electrode, creating a three-dimensional network that offers stereo sites for the attachment of Au NPs, thereby enhancing the stability of the humidity-sensing film in high-humidity environments. The hydrophilic organic units, MAPTAC, underwent chemical modification on the mesoporous organosilica, augmenting the sensitivity of humidity sensing. Furthermore, the self-assembled Au NPs supplied conduction pathways, ameliorating the conductivity of the humidity-sensing film, and consequently improving its sensitivity. The flexible humidity sensor, composed of the Au NPs/MPTMS-TMSPMA-MAPTAC composite film, displayed excellent water resistance, superior sensitivity, and satisfactory linearity (Y = -0.0533X + 8.2654; R² = 0.9562) between the log Z and the RH range from 20% to 90% RH. Additionally, it exhibited low hysteresis (less 3.5% RH) and a small temperature coefficient (-0.55% RH/°C). Analysis of the complex impedance spectra for this flexible humidity sensor, which utilized the Au NPs/MPTMS-TMSPMA-MAPTAC composite film, revealed a transition from a semicircular curve at 30% RH to a straight line at 90% RH. This observation underscores the dominant role of H3O+ ions in governing the conductivity variations within this humidity sensor.

Author Contributions

Conceptualization, P.G. Su; Methodology, P.G. Su and C.C. Hsu; Investigation, P.G. Su and C.C. Hsu; Writing—original draft preparation, P.G. Su and C.C. Hsu; Writing—review and editing, P.G. Su.; Supervision, P.G. Su.; Project administration, P.G. Su.; Funding acquisition, P.G. Su.

Funding

This research was founded by Ministry of Science and Technology of Taiwan, grant no. MOST 111-2113-M-034-001.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Cochrane, C.; Hertleer, C.; Schwarz-Pfeiffer, A. Smart textiles in health: an overview, in Smart textiles and their applications, ed. Vladan Koncar, 2016, 178, 9-32. [CrossRef]
  2. Wasson, T.; Choudhury, T.; Sharma, S.; Kumar, P. Integration of RFID and sensor in agriculture using IOT. International Conference On Smart Technologies For Smart Nation (SmartTechCon). IEEE. 2017, 217-222.
  3. Mezzanotte, P.; Palazzi, V.; Alimenti, F.; Roselli, L. Innovative RFID Sensors for Internet of Things Applications. IEEE J. Microwaves 2021, 1, 55–65, . [CrossRef]
  4. Zuo, J.; Feng, J.; Gameiro, M.G.; Tian, Y.; Liang, J.; Wang, Y.; Ding, J.; He, Q. RFID-based sensing in smart packaging for food applications: A review. Futur. Foods 2022, 6, 100198, . [CrossRef]
  5. Amin, E.M.; Bhuiyan, S.M.; Karmakar, N.C.; Winther-Jensen, B. Development of a Low Cost Printable Chipless RFID Humidity Sensor. IEEE Sens. J. 2013, 14, 140–149, . [CrossRef]
  6. Feng, Y.; Xie, L.; Chen, Q.; Zheng, L.-R. Low-Cost Printed Chipless RFID Humidity Sensor Tag for Intelligent Packaging. IEEE Sensors J. 2014, 15, 3201–3208, . [CrossRef]
  7. Delipinar, T.; Shafique, A.; Gohar, M.S.; Yapici, M.K. Fabrication and Materials Integration of Flexible Humidity Sensors for Emerging Applications. ACS Omega 2021, 6, 8744–8753, . [CrossRef]
  8. Alrammouz, R.; Podlecki, J.; Abboud, P.; Sorli, B.; Habchi, R. A review on flexible gas sensors: From materials to devices. Sensors Actuators A: Phys. 2018, 284, 209–231, . [CrossRef]
  9. Costa, J.C.; Spina, F.; Lugoda, P.; Garcia-Garcia, L.; Roggen, D.; Munzenrieder, N. Flexible Sensors—From Materials to Applications. Technologies 2019, 7, 35, . [CrossRef]
  10. Kuzubasoglu, B.A. Recent Studies on the Humidity Sensor: A Mini Review. ACS Appl. Electron. Mater. 2022, 4, 4797–4807, . [CrossRef]
  11. Su, P.-G.; Shiu, W.-L.; Tsai, M.-S. Flexible humidity sensor based on Au nanoparticles/graphene oxide/thiolated silica sol–gel film. Sensors Actuators B: Chem. 2015, 216, 467–475, . [CrossRef]
  12. Jeong, H.; Noh, Y.; Lee, D. Highly stable and sensitive resistive flexible humidity sensors by means of roll-to-roll printed electrodes and flower-like TiO2 nanostructures. Ceram. Int. 2018, 45, 985–992, . [CrossRef]
  13. Najeeb, M.A.; Ahmad, Z.; Shakoor, R.A. Organic Thin-Film Capacitive and Resistive Humidity Sensors: A Focus Review. Adv. Mater. Interfaces 2018, 5, . [CrossRef]
  14. Zhao, G.; Huang, Y.; Mei, C.; Zhai, S.; Xuan, Y.; Liu, Z.; Pan, M.; Rojas, O.J. Chiral Nematic Coatings Based on Cellulose Nanocrystals as a Multiplexing Platform for Humidity Sensing and Dual Anticounterfeiting. Small 2021, 17, 2103936, . [CrossRef]
  15. da Silva, E.A.; Duc, C.; Redon, N.; Wojkiewicz, J. Humidity Sensor Based on PEO/PEDOT:PSS Blends for Breath Monitoring. Macromol. Mater. Eng. 2021, 306, 2100489, . [CrossRef]
  16. Sobhanimatin, M.; Pourmahdian, S.; Tehranchi, M. Fast inverse opal humidity sensor based on acrylamide/AMPS hydrogel. Mater. Today Commun. 2020, 26, 101997, . [CrossRef]
  17. Zhang, Z.; Chen, M.; Alem, S.; Tao, Y.; Chu, T.-Y.; Xiao, G.; Ramful, C.; Griffin, R. Printed flexible capacitive humidity sensors for field application. Sensors Actuators B: Chem. 2022, 359, 131620, . [CrossRef]
  18. Tekcin, M.; Sayar, E.; Yalcin, M.K.; Bahadir, S.K. Wearable and Flexible Humidity Sensor Integrated to Disposable Diapers for Wetness Monitoring and Urinary Incontinence. Electronics 2022, 11, 1025, . [CrossRef]
  19. Cheng, Y.; Xiong, Y.; Pan, M.; Li, L.; Dong, L. A flexible, sensitive and stable humidity sensor based on an all-polymer nanofiber film. Mater. Lett. 2023, 330, . [CrossRef]
  20. Li, X.; Chen, X.; Chen, X.; Ding, X.; Zhao, X. High-sensitive humidity sensor based on graphene oxide with evenly dispersed multiwalled carbon nanotubes. Mater. Chem. Phys. 2018, 207, 135–140, . [CrossRef]
  21. Wu, Y.; Huang, Q.; Nie, J.; Liang, J.; Joshi, N.; Hayasaka, T.; Zhao, S.; Zhang, M.; Wang, X.; Lin, L. All-carbon based flexible humidity sensor. Journal of Nanoscience and Nanotechnology 2019, 19, 5310-5316.
  22. Tulliani, J.-M.; Inserra, B.; Ziegler, D. Carbon-Based Materials for Humidity Sensing: A Short Review. Micromachines 2019, 10, 232, . [CrossRef]
  23. Liang, R.; Luo, A.; Zhang, Z.; Li, Z.; Han, C.; Wu, W. Research Progress of Graphene-Based Flexible Humidity Sensor. Sensors 2020, 20, 5601, . [CrossRef]
  24. Anichini, C.; Aliprandi, A.; Gali, S.M.; Liscio, F.; Morandi, V.; Minoia, A.; Beljonne, D.; Ciesielski, A.; Samorì, P. Ultrafast and Highly Sensitive Chemically Functionalized Graphene Oxide-Based Humidity Sensors: Harnessing Device Performances via the Supramolecular Approach. ACS Appl. Mater. Interfaces 2020, 12, 44017–44025, . [CrossRef]
  25. Songkeaw, P.; Onlaor, K.; Thiwawong, T.; Tunhoo, B. Transparent and flexible humidity sensor based on graphene oxide thin films prepared by electrostatic spray deposition technique. J. Mater. Sci. Mater. Electron. 2020, 31, 12206–12215, . [CrossRef]
  26. Hajian, S.; Zhang, X.; Khakbaz, P.; Tabatabaei, S. M.; Maddipatla, D.; Narakathu, B. B.; Blar, R. G.; Atashbar, M. Z. Development of a fluorinated graphene-based resistive humidity sensor. IEEE Sensors Journal 2020, 20, 7517-7524. [CrossRef]
  27. Gu, Y.; Jiang, H.; Ye, Z.; Sun, N.; Kuang, X.; Liu, W.; Li, G.; Song, X.; Zhang, L.; Bai, W.; et al. Impact of Size on Humidity Sensing Property of Copper Oxide Nanoparticles. Electron. Mater. Lett. 2019, 16, 61–71, . [CrossRef]
  28. Li, P.; Yu, S.; Zhang, H. Preparation and Performance Analysis of Ag/ZnO Humidity Sensor. Sensors 2021, 21, 857, . [CrossRef]
  29. Nitta, R.; Lin, H.-E.; Kubota, Y.; Kishi, T.; Yano, T.; Matsushita, N. CuO nanostructure-based flexible humidity sensors fabricated on PET substrates by spin-spray method. Appl. Surf. Sci. 2022, 572, . [CrossRef]
  30. Zhang, D.; Chang, H.; Li, P.; Liu, R.; Xue, Q. Fabrication and characterization of an ultrasensitive humidity sensor based on metal oxide/graphene hybrid nanocomposite. Sensors Actuators B: Chem. 2016, 225, 233–240, . [CrossRef]
  31. Zhang, D.; Zong, X.; Wu, Z. Fabrication of tin disulfide/graphene oxide nanoflower on flexible substrate for ultrasensitive humidity sensing with ultralow hysteresis and good reversibility. Sensors Actuators B: Chem. 2019, 287, 398–407, . [CrossRef]
  32. Zhang, D.; Xu, Z.; Yang, Z.; Song, X. High-performance flexible self-powered tin disulfide nanoflowers/reduced graphene oxide nanohybrid-based humidity sensor driven by triboelectric nanogenerator. Nano Energy 2019, 67, 104251, . [CrossRef]
  33. Chaloeipote, G.; Samarnwong, J.; Traiwatcharanon, P.; Kerdcharoen, T.; Wongchoosuk, C. High-performance resistive humidity sensor based on Ag nanoparticles decorated with graphene quantum dots. R. Soc. Open Sci. 2021, 8, 210407. [CrossRef]
  34. Moustafa, H.; Morsy, M.; Ateia, M.A.; Abdel-Haleem, F.M. Ultrafast response humidity sensors based on polyvinyl chloride/graphene oxide nanocomposites for intelligent food packaging. Sensors Actuators A: Phys. 2021, 331, 112918, . [CrossRef]
  35. Cai, C.; Zhao, W.; Yang, J.; Zhang, L. Sensitive and flexible humidity sensor based on sodium hyaluronate/MWCNTs composite film. Cellulose 2021, 28, 1–11, . [CrossRef]
  36. Singh, A.; Singh, P. K.; Mathuriya, A. S.; Sachdeva, A. Synthesis, characterisation and humidity sensing properties of PVA-NaI composite. Mater. Today Proc. 2022, 49, 3365−3369. [CrossRef]
  37. Afnas, V.M.; Unnikrishnan, G.; Budhe, S.; Manaf, O.; Ameen, J. PVA/gelatin/chitin ternary blend as a humidity sensing material. J. Mater. Sci. Mater. Electron. 2021, 33, 2031–2043, . [CrossRef]
  38. Luo, M.; Liu, Z.; Wang, Q.; Liu, R.; Xu, Y.; Wang, K.; Shi, X.; Ye, S. Surface engineering on polyimide−silver films in low-cost, flexible humidity sensors. ACS Appl. Mater. Interfaces 2022, 14, 16621−16630.
  39. Kim, S.-J.; Park, J.-Y.; Lee, S.-H.; Yi, S.-H. Humidity sensors using porous silicon layer with mesa structure. J. Phys. D: Appl. Phys. 2000, 33, 1781–1784, . [CrossRef]
  40. Fürjes, P.; Kovács, A.; Dücso, C.; Ádám, M.; Müller, B.; Mescheder, U. Porous silicon-based humidity sensor with interdigital electrodes and internal heaters. Sensors Actuators B: Chem. 2003, 95, 140–144, . [CrossRef]
  41. Harraz, F.A. Porous silicon chemical sensors and biosensors: A review. Sensors Actuators B: Chem. 2014, 202, 897–912, . [CrossRef]
  42. He, X. W.; Geng, W. C.; Zhang, B. L.; Jia, L. M.; Duan, L. B.; Zhang, Q. Y. Ultrahigh humidity sensitivity of NaCl-added 3D mesoporous silica KIT-6 and its sensing mechanism, RSC Adv. 2016, 6, 38391–38398. [CrossRef]
  43. Su, P.-G.; Chen, I.; Wu, R.-J. Use of poly(2-acrylamido-2-methylpropane sulfonate) modified with tetraethyl orthosilicate as sensing material for measurement of humidity. Anal. Chim. Acta 2001, 449, 103–109, . [CrossRef]
  44. Lv, X.; Li, Y.; Hong, L.; Luo, D.; Yang, M. A highly water-resistive humidity sensor based on silicon-containing polyelectrolytes prepared by one-pot method. Sensors Actuators B: Chem. 2007, 124, 347–351, . [CrossRef]
  45. Zhao, H.; Zhang, T.; Dai, J.; Jiang, K.; Fei, T. Preparation of hydrophilic organic groups modified mesoporous silica materials and their humidity sensitive properties. Sensors Actuators B: Chem. 2017, 240, 681–688, . [CrossRef]
  46. Zhao, H.; Zhang, T.; Qi, R.; Dai, J.; Liu, S.; Fei, T.; Lu, G. Organic-inorganic hybrid materials based on mesoporous silica derivatives for humidity sensing. Sensors Actuators B: Chem. 2016, 248, 803–811, . [CrossRef]
  47. Qi, R., Zhang, T., Guan, X., Dai, J., Liu, S., Zhao, H., & Fei, T. (2020). Capacitive humidity sensors based on mesoporous silica and poly (3, 4-ethylenedioxythiophene) composites. Journal of colloid and interface science 2020, 565, 592-600. [CrossRef]
  48. Su, P.-G.; Shiu, W.-L.; Tsai, M.-S. Flexible humidity sensor based on Au nanoparticles/graphene oxide/thiolated silica sol–gel film. Sensors Actuators B: Chem. 2015, 216, 467–475, . [CrossRef]
  49. Su, P. G.; Huang, S. C. Electrical and humidity sensing properties of carbon nanotubes-SiO2-poly(2-acrylamido-2-methylpropane sulfonate) composite material. Sens. Actuators B 2006, 113, 142–149.
  50. Ma, M.; Li, H.; Xiong, Y.; Dong, F. Rational design, synthesis, and application of silica/graphene-based nanocomposite: A review. Mater. Des. 2020, 198, 109367, . [CrossRef]
  51. Zhu, J., Zhang, N., Yin, Y., Xu, B., Zhang, W., & Wang, C. (2022). High-sensitivity and low-hysteresis GO-NH2/mesoporous SiO2 nanosphere-fabric-based humidity sensor for respiratory monitoring and noncontact sensing. Advanced Materials Interfaces 2022, 9, 2101498. [CrossRef]
  52. Zhang, J.; Zou, H.; Qing, Q.; Yang, Y.; Li, Q.; Liu, Z.; Guo, X.; Du, Z. Effect of Chemical Oxidation on the Structure of Single-Walled Carbon Nanotubes. J. Phys. Chem. B 2003, 107, 3712–3718, . [CrossRef]
  53. Casalbore-Miceli, G.; Yang, M.; Camaioni, N.; Mari, C.-M.; Li, Y.; Sun, H.; Ling, M. Investigations on the ion transport mechanism in conducting polymer films. Solid State Ionics 2000, 131, 311–321, . [CrossRef]
  54. Wang, J.; Xu, B. K.; Ruan, S. P.; Wang, S. P. Preparation and electrical properties of humidity sensing films of BaTiO3/polystrene sulfonic sodium. Mater. Chem. Phys. 2003, 78, 746-750. [CrossRef]
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Figure 1. Schematic diagram of flexible humidity sensor.
Figure 1. Schematic diagram of flexible humidity sensor.
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Figure 2. Fabrication of a flexible humidity sensor based on Au NPs/MPTMS-TMSPMA-MAPTAC film by combining the sol-gel process, free radical polymerization, and self-assembly method.
Figure 2. Fabrication of a flexible humidity sensor based on Au NPs/MPTMS-TMSPMA-MAPTAC film by combining the sol-gel process, free radical polymerization, and self-assembly method.
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Figure 3. Schematically plots the humidity atmosphere controller system and impedance measurement of flexible humidity sensors.
Figure 3. Schematically plots the humidity atmosphere controller system and impedance measurement of flexible humidity sensors.
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Figure 4. IR spectra of (a) MPTMS-TMSPMA and (b) MPTMS-TMSPMA-MAPTAC sol-gel.
Figure 4. IR spectra of (a) MPTMS-TMSPMA and (b) MPTMS-TMSPMA-MAPTAC sol-gel.
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Figure 5. SEM images of (a) pristine SiO2 film, (b) MPTMS-TMSPMA-MAPTAC film and (c) Au NPs/MPTMS-TMSPMA-MAPTAC composite film. (White arrows indicated the Au NPs).
Figure 5. SEM images of (a) pristine SiO2 film, (b) MPTMS-TMSPMA-MAPTAC film and (c) Au NPs/MPTMS-TMSPMA-MAPTAC composite film. (White arrows indicated the Au NPs).
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Figure 8. Impedance versus RH for flexible humidity sensors based on pristine SiO2, MPTMS-TMSPMA, MPTMS-TMSPMA-MAPTAC and Au NPs/MPTMS-TMSPMA-MAPTAC composite films, measured at 1 V, 1 kHz and 25 °C.
Figure 8. Impedance versus RH for flexible humidity sensors based on pristine SiO2, MPTMS-TMSPMA, MPTMS-TMSPMA-MAPTAC and Au NPs/MPTMS-TMSPMA-MAPTAC composite films, measured at 1 V, 1 kHz and 25 °C.
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Figure 10. Complex impedance plots of the flexible humidity sensor based on Au NPs/MPTMS-TMSPMA-MAPTAC composite film at (a) 30% RH, (b) 40% RH (c) 90% RH.
Figure 10. Complex impedance plots of the flexible humidity sensor based on Au NPs/MPTMS-TMSPMA-MAPTAC composite film at (a) 30% RH, (b) 40% RH (c) 90% RH.
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