1 Introduction

With regards to environmental protection, volatile organic compounds (VOCs) refer to the types of volatile organic compounds that have a great impact on human health. VOCs can be classified into many types according to the chemical properties of the organic compounds, mainly benzene, organic chloride, organic ketone, aldehyde, amine, alcohol, and ether. When the gas concentration of a VOC exceeds the maximum safe indoor concentration, people experience headaches, nausea, vomiting, fatigue and other symptoms in a short period of time [1]. Among all VOCs, formaldehyde is one of the most common gases. Formaldehyde has been widely used in many areas of industrial production as an essential raw material, such as the timber industry, textile industry and other industries. However, this colourless VOC has an irritating odour that is highly poisonous to all humans, especially affecting the body’s blood system and nervous system [2]. That is, the more widely formaldehyde is used in our daily life, the more likely it is to cause harm. Concern is mounting regarding whether the indoor formaldehyde concentration exceeds the standard and whether it will endanger the health of the population. Therefore, a practical formaldehyde sensor is needed to assuage these concerns [3, 4].

Currently, there are many types of gas sensors with different working principles, and they exhibit varying performance. Here, we selected several of the most common sensors for a comparison of their parameters [5,6,7,8,9]. The results of the comparison are listed in Table 1. No single sensor has excellent performance, and the type of sensor that should be used depends on specific environmental conditions and requirements. However, it is undeniable that metal oxide semiconductor (MOS) gas sensors have the most potential for monitoring harmful VOCs due to advantages such as a high sensitivity, rapid response time, stable repeatability, ease of use, low cost and simplicity in fabrication [10]. For a MOS, as a gas-sensitive material, it is well known that its nanostructure is one of the most important parameters affecting its gas-sensing performance. For example, nano-lily bud garden-like ZnO nanostructure materials synthesized by Kumar et al. exhibited better gas-sensing properties towards H2 than the ZnO nanoparticle thin films fabricated by Drmosh et al. [11, 12]. Hwang et al. presented rice-like tellurium thin films that had better H2S-sensing properties than the tellurium nanoparticle thin films investigated by Sen et al. [13, 14]. Wang et al. described spindle-like In2O3 porous polyhedra that had a higher formaldehyde sensitivity than In2O3 nanoparticles [3]. Huang et al. synthesized CuO hollow microspheres with better ethanol-sensing properties than those of the 3D flower- and 2D sheet-like CuO nanostructures synthesized by Yang et al. [15, 16]. All of these researchers have synthesized high-performance gas-sensitive materials with distinct morphologies. From this background, we can see that one of the most common ways to improve the performance of sensors is to increase the contact area between the materials and target gases by changing the material morphology. This enhancement promotes the transduction reaction and accelerates electron mobility on the sensor surface. Therefore, a good morphology is one of the main contributors to improving the gas sensitivity of nanomaterials.

Table 1 Parameter comparison of several of the most common gas sensors (para: parameter)[5,6,7,8,9]
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Among the wide range of MOS gas-sensing materials, In2O3 is a typical n-type semiconductor with a direct band gap of 3.55–3.75 eV. In2O3 materials have been studied for years as one of the most promising gas-sensing materials due to their low resistivity, good conductivity and abundant defects on the sensing surface [17]. To date, to improve the gas-sensing properties of materials, many types of nanostructures of In2O3 have been developed, such as nanobricks [18], nanospheres [19], nanofibres [20], nanorods [21], and nanoplates [22]. The low dimensionality of nanomaterials is one of the future directions of gas-sensing material development. One-dimensional nanomaterials have advantages such as a high specific surface area, a large aspect ratio, and directional conduction of electrons, and they are difficult to agglomerate. One-dimensional nanomaterials play an important role in improving the sensitivity and response recovery characteristics of gas-sensitive materials, and thus, they have good application prospects in the field of sensors. As we all know, it is difficult to diversify one-dimensional nanomaterials. Here, the novelty of this paper lies in the addition of mineral oil into the precursor solution for electrospinning, which allowed the successful synthesis of one-dimensional nanomaterials with a new morphology, namely, PINTs. The results of subsequent measurements revealed that PINTs significantly increased the response value to formaldehyde compared with INTs and INFs that were prepared using a similar method but without adding mineral oil. The response of the PINT-based sensor to 100 ppm formaldehyde is 38.3, which is 2.1 times and 4.3 times those of INT-based and INF-based sensors to 100 ppm formaldehyde, respectively. In addition, the results showed that different morphologies of the same material, similar to In2O3, exert a profound influence the gas-sensing performance of the material. We also confirm that the new discovery of the one-dimensional nanostructure reported in this paper is of great significant to the studies of other gas-sensitive materials. This work will provide inspiration for synthesizing new one-dimensional nanomaterials with novel structures. Consequently, one-dimensional gas-sensing nanomaterials will experience more rapid development.

2 Experimental section

2.1 Chemical reagents

In(NO3)3 × H2O (99.9% metal basis), mineral oil (AR) and DMF (N,N-dimethylformamide, AR, 99.5%) were obtained from Aladdin (China). PVP (polyvinylpyrrolidone) was purchased from Sigma-Aldrich (China), and ethanol (AR) was obtained from Beijing Chemical Works (China).

None of the five chemical reagents used for the study underwent further purification.

2.2 Preparation of INFs, INTs and PINTs

Pearl-necklace-shaped In2O3 nanotubes were synthesized by a single-capillary electrospinning method. The specific process is as follows:

  1. 1.

    Dissolve 0.2 g of InN3O9 × H2O and 0.3 g of PVP into a mixed solution consisting of 4.0 g of ethanol, 2.0 g of DMF and 0.06 g of mineral oil and then stir for 10 h at room temperature.

  2. 2.

    Transfer the well-stirred precursor into an electrospinning syringe for spinning. The electrospinning device consists of a spinneret, a receiving screen and a high-voltage DC power supply connecting them. In this work, the spinneret was connected to the positive pole of the power supply, and the receiving screen was connected with the negative pole. The distance between the spinneret and receiving screen was 25 cm, and the voltage between them was 18 kV.

  3. 3.

    Transfer the polymer materials from the electrospinning receiving screen into a corundum crucible, calcine them in a muffle furnace at a heating rate of 7 °C per min to 550° C, and hold at this temperature for 2 h.

  4. 4.

    Take the annealed product out of the muffle furnace, which is the PINTs.

Fig. 1a shows a schematic diagram of the complete synthetic process for the PINTs. The INFs were synthesized following the above steps without mineral oil in the first step. The INTs were synthesized following the above steps without mineral oil in the first step, and the heating rate was adjusted from 7 to 3 °C per min in the third step.

Fig. 1
figure 1

a Schematic diagram of the complete process for synthesizing PINTs. b A schematic diagram of the fabrication of the sensors and the electrical circuit of the test system

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2.3 Material characterization

The morphologies and nanostructures of the PINTs, INTs and INFs were observed by scanning electron microscopy (SEM, FEI Magellan 400 FEI Corp., Hillsboro, Oregon, US). The composition of the PINTs was determined by an energy dispersive X-ray spectroscopy (EDS) attachment on the SEM instrument (OXFORD, X-MAX150, UK). The materials were characterized by X-ray diffraction (XRD, DX-2700, China) with a graphite monochromatized Cu Kα radiation source (λ = 1.54056 Å) with 2θ ranging from 15° to 85°.

2.4 Fabrication and measurement of the gas sensors

The gas sensors were fabricated as follows: we mixed the as-prepared powder with a proportionate amount of deionized water (resistivity = 18.0 MΩ/cm) in a weight ratio of 3:1 and then ground them evenly in an agate mortar to form a paste. Then, the paste was uniformly coated on an alumina ceramic tube by a spin coater (IC5000, at 1000 rpm for 20 s). The thickness of the formed sensing film was approximately 30 μm. The gold electrode components had previously been installed at each end of the ceramic tube, and two platinum wires were drawn from each electrode as measuring leads. A nickel-chromium alloy coil was placed inside the ceramic tube as a heating wire. Then, the ceramic tube with the heating wire was welded onto the hexagonal sensor base. Before the first test, the sensors were aged at a temperature of 210 °C for three days to ensure their measurement stability. In this work, the gas-sensing properties of the sensors were measured using a gas sensitivity testing instrument (CGS-8 intelligent gas-sensing analysis system, Beijing Elite Tech, China). The fabricated sensors were pre-heated at a temperature of 200 °C for approximately 30 mins in a reaction chamber whose volume is 20 L. When the resistance of the fabricated sensor was stable, the saturated solution of the target gas was injected into the reaction chamber via a microinjector through a rubber plug on the reaction chamber. For the target gases with a specific concentration, we obtained them using a solution of the target gases, and the volume of the solution of the target gases consumed can be calculated as follows:

$$ V = \frac{{20 * c * M * 10^{ - 6} }}{22.4 * \rho * \omega t\% } $$

where V (ml) is the required volume of the solution of the target gas, c (ppm) is the required concentration of the target gas, M (g·mol−1) is the molecular weight of the target gas molecule, ρ (g·ml−1) is the density of the aqueous solution of the target gas, and ωt% is the weight percentage of the solution of target gas. All solutions we used were purchased from Beijing Chemical Works, and all solutions have AR purity. Two fans were used to rapidly mix the saturated target gas with air in the reaction chamber. When the resistance of the sensor was stabilized at a new constant value, the reaction chamber was opened so that the sensor could return to its normal value in ambient air. Finally, the real-time resistance and response of the sensor was obtained by a gas-sensitive analysis system. A schematic diagram of the fabrication of the sensors and the electrical circuit of the test system is displayed in Fig. 1b. In the electrical circuit, the working temperature of the sensor is controlled by the heating power supply (Vheat). Then, the resistance of the sensor is obtained by the output voltage (Vout) and reference resistance (Rref).

In addition, for a reducing gas such as formaldehyde, the response value of the sensor is defined as Ra/Rg, where Ra is the resistance of the sensor in air and Rg is the resistance of the sensor in the target gas.

3 Results and discussion

3.1 Characterization

The nanostructures and morphologies of the PINTs, INTs and INFs characterized by SEM are presented in Fig. 2. As shown in Fig. 2a–d, the average diameters of the INTs and INFs are 80 nm and 100 nm, respectively. Furthermore, Fig. 2e, f clearly shows that the PINTs have a novel pearl-necklace-like shape. The average diameters of the thin and thick sections of the PINTs are 105 nm and 160 nm, respectively. From the SEM images, we can clearly observe that the homogeneity of the materials is quite satisfactory. The formation of the PINT nanostructures can be explained as follows. First, we added a quantity of mineral oil into the electrospinning precursor solution and uniformly dispersed it by magnetic stirring for several hours. In this way, the mineral oil molecules were also evenly dispersed in the nanomaterials obtained by electrospinning. By controlling the calcination conditions, we could mediate the expansion of mineral oil in the one-dimensional material to prevent the material from excessive expansion before the mineral oil reacted and decomposed at high temperature. Finally, nanotubes with pearl necklace shapes were obtained after annealing.

Fig. 2
figure 2

High-definition SEM images showing the morphology and good uniformity of the synthesized samples: a, b the images of the INFs; c, d the images of the INTs; and e, f the images of the PINTs

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EDS analysis was used to determine the chemical composition of the as-prepared sample, and the spectrum is presented in Fig. 3. We found that the molar ratio of indium to oxygen is approximately 2:3, which coincides with that of the standard oxide In2O3. This result indicates that the PINTs consist of an In2O3 material with high purity. In addition, the elements Si and Al were detected from the aluminium platforms and polished silicon wafers that held the samples.

Fig. 3
figure 3

EDS spectrum of PINTs with clear characteristic peaks and the corresponding relative contents of each element

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To investigate the structural properties of the PINTs, we also carried out XRD analysis and identified distinct characteristic peaks. Fig. 4 shows the XRD patterns of the PINTs; the diffraction peaks of the synthesized PINTs are highly consistent with those of typical cubic In2O3 nanomaterials (JCPDS-ICDD Card No. 06-0416)[23]. In addition, no other impure peaks were observed in the XRD patterns, which indicated that the obtained In2O3 samples have relatively high purity. The average crystallite size of the PINTs was estimated by the Scherrer formula[24]:

Fig. 4
figure 4

X-ray diffraction spectrum of pearl necklace-shaped In2O3 nanotubes with characteristic peaks in accordance with the JCPDS-ICDD card

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$$ D = \frac{0.89\lambda }{\beta * \cos \left( \theta \right)} $$
(1)

where D is the mean crystallite size, the constant of 0.89 is the shape factor, λ is the wavelength of the X-ray radiation (λ = 1.5406 Å for Cu Kα radiation), β is the full width at half maximum of the diffraction peak, and θ is the Bragg angle. As a result, the average crystallite size of the PINTs is approximately 17.1 nm. It could also be calculated from the XRD analysis that the PINTs consist of a cubic crystal material with lattice parameters of a=b=c= 10.128 Å, which is in good agreement with the recorded XRD data.

3.2 Gas-sensing properties

To indicate the excellent performance of the sensor based on PINTs, several important parameters are necessary. First, to systematically study the sensor performance more conveniently, the optimal operating temperature (OOT) is a parameter that must be determined before the other parameters. The reason for the importance of the OOT is as follows: as a resistive gas sensor material, In2O3 absorbs oxygen molecules and generates chemisorbed oxygen species on the surface. The formation of oxygen ions increases with increasing temperature, leading to a change in the resistance. In addition, the spontaneous chemisorption of oxygen also occurs on the surface of the sensor. As oxygen adsorption is an exothermic reaction, it decreases with increasing temperature [25]. Considering these two points, the sensitivity curve of the sensor shows a trend of first increasing and then decreasing, and then the optimal operating temperature appears. Fig. 5 presents the optimal operating temperature for the sensors based on the as-obtained samples. We observed that they all have the highest sensitivity at the temperature of 220 °C in the range from 200 to 250 °C. Then, the response value is defined as the ratio of the resistance of the sensor in air to that in the target gas, i.e., Ra/Rg. As shown in Fig. 5, the PINT-based sensor has the highest response to 100 ppm formaldehyde (38.3) at the OOT, which is 4.25 times that of the INFs (9.0) and 2.07 times that of the INTs (18.5). Then, the subsequent sensor performance tests were conducted under the OOT of 220 °C.

Fig. 5
figure 5

Responses of the sensors based on INFs, INTs and PINTs to 100 ppm formaldehyde at temperatures ranging from 200 to 250 °C

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The response and recovery properties are important parameters for gas sensors. When the sensor is exposed to formaldehyde gas, it takes some time to minimize the resistance. Moreover, when the sensor is separated from formaldehyde gas and placed in contact with air, it also takes some time to recover to its normal resistance. These two periods are defined as the response time and recovery time, as quantified by the time required to attain 90% of the final change. In practical applications, gas sensors must have fast response times and recovery times. In particular, a fast response time is needed to reflect the real-time function of the sensor. Under a formaldehyde gas environment of 100 ppm, the response and recovery time curve images of sensors based on INFs, INTs and PINTs are displayed in Fig. 6. From the images, it is clearly seen that all of the sensors based on the three as-obtained samples have an excellent response time and recovery time. Among them, the PINT-based sensor has the fastest response and recovery times, which are 6 s and 16 s, respectively. The response and recovery times of the sensors based on the INFs are 8 s and 27 s, and those of the INT-based sensor are 8 s and 22 s, respectively.

Fig. 6
figure 6

a The overall trends of the response time (Tres) and recovery time (Trec) curves of the sensors based on INFs, INTs and PINTs. b, c, d Detailed images of the response time (Tres) and recovery time (Trec) curves to more clearly reflect the performance differences between the INFs, INTs, and PINTs

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To realize the relationship between the response and concentration of the PINT-based sensor, we measured its sensing performance in different concentrations of formaldehyde gas and compared it with those of the INF-based sensors and INT-based sensors. To better observe the comparison of these sensors, we plotted the results of the measurement in Fig. 7a–d presents the responses of the sensors to different formaldehyde concentrations from 1 ppm to 200 ppm. As shown in Fig. 7d, the responses of the PINT-based sensor towards 1, 2, 5, 10, 20, 50, 100 and 200 ppm of formaldehyde are approximately 2.1, 3.5, 5.0, 6.5, 10.3, 19.3, 38.3 and 68.1, respectively. The above results were also fitted by a line. If we define x as the formaldehyde concentration and y as the response, the fitted equation is:

Fig. 7
figure 7

a The responses of the sensors based on INFs, INTs and PINTs to different concentrations in the range of 1–200 ppm. b, c, d Detailed response-concentration diagrams of the sensors based on INFs, INTs and PINTs, respectively, with fitting lines, revealing a set of favourably high R2 correlation coefficients

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$$ y = 0.33x + 3.13 $$
(2)

The correlation coefficient R2 is 0.9974, which shows a good linear dependence and indicates that PINTs may be a promising candidate for a formaldehyde-sensitive material. The minimum detectable concentration of formaldehyde of the PINT-based sensor cannot be neglected. The sensor can detect formaldehyde gas at 1 ppm with a response value of 2.1. The low minimum detectable concentration also shows the excellent sensitivity of the PINT-based sensor to formaldehyde gas. The measurement results of the sensors based on INFs and INTs are also fitted by the lines displayed in Fig. 7b, c, respectively.

To ensure the accuracy of the sensors in this work, repeatability is also one of the factors that must be considered. We recorded the response and recovery curves of the sensors based on INFs, INTs and PINTs to 100 ppm formaldehyde over 500 s with four cycles. The results are presented in Fig. 8. The responses of the sensors did not decrease significantly during the measurement period, proving that the materials have good repeatability.

Fig. 8
figure 8

The response and recovery curves of the sensors based on INFs, INTs and PINTs to 100 ppm formaldehyde over 8.5 minutes with four cycles

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Moreover, stability and selectivity are also important parameters for evaluating whether a sensor is practical. From the perspective of practical applications, gas sensors should have good long-term stability and excellent selectivity to ensure the accuracy of detection [2]. The long-term stability of the sensor can guarantee its long-term operation. Fig. 9a records the responses of the sensors based on INFs, INTs and PINTs to 100 ppm formaldehyde on the 1st, 5th, 10th, 15th, 20th, 25th and 30th days. We found that the response of the PINT-based sensor maintained a constant value over one month, whereas the responses of the sensors based on INFs and INTs showed slight declines. This result suggested the excellent long-term stability of the PINT-based sensor. Sensor selectivity is the ability to distinguish the target gas from other gases. The responses of the above sensors to 100 ppm formaldehyde, methanol, toluene, acetone and m-xylene were measured at the OOC, and the results are presented in Fig. 9b. Obviously, the response of the PINT-based sensor displayed excellent selectivity towards 100 ppm formaldehyde. PINT-based sensors can also improve the performance for other gases, as shown in Fig. 9b, but not to the same extent as that for formaldehyde detection. In addition, an excellent formaldehyde sensor can accurately distinguish formaldehyde from a mixture, and thus, we also measured the responses of the sensors in a gas environment filled with multiple VOCs. The results show that the PINT-based sensor receives little interference from the other gases when detecting formaldehyde gas.

Fig. 9
figure 9

a Responses of the sensors based on INFs, INTs and PINTs in an environment of 100 ppm formaldehyde recorded over one month for determining the stability. b Responses of the sensors based on INFs, INTs and PINTs towards 100 ppm of other detected VOCs and mixtures containing the above gases

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The formaldehyde gas-sensing performance of PINTs in this work and other various nanomaterials and are summarized in Table 2. In comparison with the In2O3 architectures, Y-doped SnO2 nanoflowers, In2O3 porous polyhedra, SnO2 hollow microspheres, BiFeO3 microspheres, In2O3 nanoparticles, SnO/SnO2 nanoflowers, rGO/SnO2 nanosheets and Ag@WO3 nanocomposite, the PINTs exhibited a competitive formaldehyde gas-sensing performance. Compared with outstanding microstructure gas sensors, the prepared PINT-based sensors still have some limitations. For example, in terms of reducing energy consumption, the optimum operating temperature of the PINT-based sensor is not low enough. Nevertheless, the PINTs reported in this work still have room for improvement, and we believe that PINTs as a formaldehyde gas-sensing material has good prospects for development. As a next step, we will synthesize PINT-based composite nanomaterials or fabricate microstructure sensors based on PINTs to achieve a lower optimal operating temperature and higher sensitivity to formaldehyde.

Table 2 Comparison of the properties towards HCHO between sensors based on various materials in other reports and the PINT-based sensor in this work
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4 Mechanism

The working mechanism of the PINT-based sensor can be explained as follows: it is well known that In2O3 is a typical n-type semiconductor material that exhibits a chemiresistance gas-sensing capability. For this type of material, the change in conductivity may be primarily caused by the interaction between atmospheric oxygen and the target gas molecules on the surface of the gas-sensing materials [34]. More specifically, when the PINTs are surrounded by the air, oxygen molecules may adsorb on the surface of the In2O3 materials and capture free electrons from the conduction band of In2O3. Then, chemisorbed oxygen species such as O2−, O2− and O− are generated. A depletion layer is formed by the generated chemisorbed oxygen species, which causes a decrease in the conductivity of the sensor and presents a high resistance. The process can be described by the following equations [27, 30, 35]:

$$ {\text{O}}_{2} \left( {\text{gas}} \right) \to {\text{O}}_{2} \left( {\text{ads}} \right) $$
(3)
$$ {\text{O}}_{2} \left( {\text{ads}} \right) + {\text{e}}^{ - } \to {\text{O}}_{2}^{ - } \left( {\text{ads}} \right)\,\,\,\left( {{\text{T}} < 100\,^\circ {\text{C}}} \right) $$
(4)
$$ {\text{O}}_{ 2}^{ - } \left( {\text{ads}} \right) \, + {\text{ 2e}}^{ - } \to 2 {\text{O}}^{ - } \left( {\text{ads}} \right) \, \left( { 100^\circ {\text{C }} < {\text{ T }} < { 3}00\,^\circ {\text{C}}} \right) $$
(5)
$$ {\text{O}}^{ - } \left( {\text{ads}} \right) + {\text{e}}^{ - } \to {\text{O}}^{2 - } \left( {\text{ads}} \right)\,\,\,\,\left( {{\text{T}} > 300\,^\circ {\text{C}}} \right) $$
(6)

Because the optimum operating temperature of the PINT-based sensors is 220 °C, chemisorbed oxygen species on the surface of the formaldehyde-sensitive materials mainly exist in the form of O- at this temperature. When the sensor is exposed to formaldehyde, the following reaction occurs between the target gas molecules and the adsorbed oxygen species [36]:

$$ {\text{HCHO}}\left( {\text{gas}} \right) + 2{\text{O}}^{ - } \left( {\text{ads}} \right) \to {\text{CO}}_{2} \left( {\text{gas}} \right) + {\text{H}}_{2} {\text{O}}\left( {\text{vap}} \right) + 2{\text{e}}^{ - } $$
(7)

Then, the captured electrons are released back into the conduction band of the In2O3 materials. Although oxygen molecules still react with electrons to produce oxygen species at this time, the reactions between the formaldehyde and oxygen groups are more intense and rapid. As a result, more electrons are released back onto the surface of the material than are consumed, resulting in a thinner depletion layer as well as a lower potential barrier and, thus, a rapid reduction in resistance. A schematic illustration of the formaldehyde gas-sensing process is presented in Fig. 10.

Fig. 10
figure 10

Schematic illustration of the formaldehyde gas-sensing process of the PINTs

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In this work, the enhancement of the gas-sensing properties of the PINT-based sensor should be attributed to the unique pearl necklace shape of the In2O3 nanotubes and the relatively small average crystallite size. Due to the novel nanostructure of the material and its own excellent structural properties, atmospheric oxygen and the target gas molecules have more opportunities to contact the surface of the material. All of the reactions mentioned above are facilitated. In addition, when the average crystallite size of the semiconductor material is less than or equal to twice the Debye length, the response value obtained is inversely proportional to the crystallite size. That is, the smaller the average crystallite size is, the higher the response value of the material will be [37]. Therefore, the excellent gas sensitivity of PINTs can also be attributed to their relatively small average crystallite size.

5 Conclusion

In summary, in order to meet the increasing demand for high-performance sensors, novel 1D PINTs have been successfully synthesized via a single-nozzle electrospinning method followed by subsequent thermal treatment and further developed for formaldehyde detection. Due to the physical reactions of mineral oil during electrospinning and subsequent thermal treatment, the as-prepared novel In2O3 nanotubes appear to assemble one by one into countless hollow pearls. Compared with the sensors based on INFs and INTs, whose responses towards 100 ppm formaldehyde were 9 and 18.5, respectively, gas sensors based on PINTs exhibited the best response (38.3). Moreover, the sensors based on PINTs also exhibited a short response time (6 s) and recovery time (16 s), good linear dependence, repeatability, stability and selectivity towards 100 ppm formaldehyde at the low optimum operating temperature of 220 °C. Due to the facile green synthetic method of PINTs and the unique tubular structure with enhanced formaldehyde-sensing performance, PINTs have potential for practical application as a formaldehyde-sensing material. This work will also provide inspiration for the future research and development of gas-sensing materials.