An experimental examine on the detection mechanism of 2-CEES utilizing SAW sensors underneath numerous temperature circumstances

Characterization

FT-IR information

Figure 2a presents the FT-IR spectra of the PPD-F2. A broad peak noticed round 3342 cm^− 1 corresponds to N–H stretching vibrations, suggesting the presence of amine teams, probably concerned within the formation of hydrogen bonds³⁴. Peaks at 3053 cm^− 1 and 2852 cm^− 1 are attributed to = C–H stretching of fragrant rings and aliphatic C–H stretching, respectively.

Strong absorption bands round 1612 cm^− 1 and 1502 cm^− 1 are assigned to fragrant C = C stretching vibrations, indicating the presence of conjugated or fragrant constructions throughout the polymer spine. Additionally, peaks within the area of 1247 cm^− 1 to 1049 cm^− 1 (at 1247 cm^− 1, 1203 cm^− 1, 1172 cm^− 1, and 1049 cm^− 1) are attributed stretching vibration of P = O group^35,36. Out-of-plane C–H bending vibrations noticed at 831 cm^− 1 and 700 cm^− 1 are indicative of substituted benzene rings^32,37.

XRD information

Figure 2b presents the XRD sample of the PPD-F2. The XRD sample of the polymer displays a broad halo centered round 2θ = 20°, indicative of an general amorphous construction. However, a number of distinct diffraction peaks are noticed at 2θ values of 13.06°, 14.86°, 15.72°, 17.22°, 18.62°, and 21.58°, which correspond to the (020), (201), (111), (211), (100), and (230) crystallographic planes, respectively³⁸. The presence of those sharp reflections suggests the existence of short-range ordering or partially crystalline domains embedded throughout the predominantly disordered polymer matrix. These options indicate that the polymer has a semi-crystalline nature, probably arising from the periodic packing of polymer chains or particular intermolecular interactions³⁹. Notably, no further diffraction peaks are noticed past 2θ = 50°, indicating the absence of long-range crystalline order at increased scattering angles.

Measured (a) FT-IR spectra of PPD-F2 and (b) XRD sample of PPD-F2.

XPS information

XPS was performed to investigate the floor chemical composition of the synthesized pattern. The high-resolution P2p spectrum (Fig. 3a) reveals a dominant peak at 144.08 eV. The N1s spectrum (Fig. 3b) presents a peak centered at 410.08 eV, whereas the C1s spectrum (Fig. 3c) displays a powerful peak at 298.08 eV. In the O1s area (Fig. 3d), a peak is noticed at 545.08 eV. The F1s spectrum shows a peak at 698.08 eV (Fig. 3e).

Quantitative evaluation based mostly on atomic percentages revealed that carbon (68.75%) and fluorine (16.3%) had been probably the most considerable parts, adopted by oxygen (8.41%), nitrogen (4.77%), and phosphorus (1.77%).

These outcomes point out that the XPS evaluation reveals the very best intensities for carbon and fluorine, which is in step with their excessive content material within the PPD-F2 pattern^40,41. The XPS survey spectrum (Fig. 3f) confirms the presence of carbon (C), nitrogen (N), oxygen (O), fluorine (F), and phosphorus (P) within the materials.

XPS evaluation of PPD-F2. (a) P2p, (b) N1s, (c) C1s, (d) O1s, (e) F1s, and (f) survey spectrum.

Selectivity check

In the selectivity check, an experiment was carried out to guage whether or not PPD-F2 might selectively detect a particular fuel by exhibiting a powerful response. PPD-F2 was uncovered to 2-CEES, dimethyl methyl phosphonate (DMMP), malathion, cyanogen chloride (CK), sulfur dioxide (SO₂), and ethylene oxide (EO) at a focus of 10 mg/m³. The outcomes revealed frequency shifts of 832.08 Hz for 2-CEES, 290.99 Hz for DMMP, 49.43 Hz for malathion, 47.63 Hz for CK, 41.82 Hz for SO₂, and 41.85 Hz for EO, as illustrated in Fig. 4. 2-CEES exhibited roughly 2.85 occasions increased reactivity in comparison with DMMP, the second most reactive substance, confirming that PPD-F2 reveals a considerably increased response particularly to 2-CEES.

To perceive this selectivity, it is very important contemplate the mechanism by which PPD-F2 interacts with goal gases. The goal gases are uncovered to a SAW sensor coated with PPD-F2, the place they work together with the fluorine (F) purposeful group of the sensing materials through their respective purposeful teams—hydrogen (2-CEES, DMMP, Malathion, EO), nitrogen (CK), and oxygen (SO₂). The bond energy between the purposeful group of the fuel and the fluorine atom of PPD-F2 performs an important position within the selectivity of the sensor.

PPD-F2 varieties a very sturdy bond with 2-CEES in comparison with different gases as a result of excessive bond dissociation power and the sturdy electronegativity of sulfur (S) in 2-CEES. The hydrogen-containing gases (2-CEES, DMMP, Malathion, EO) exhibit a bond dissociation power of 569.4 kJ/mol with fluorine, whereas the nitrogen-containing fuel CK has a bond dissociation power of 280.5 kJ/mol, and the oxygen-containing fuel SO₂ has a bond dissociation power of 191.7 kJ/mol⁴². This distinction in bond dissociation power explains why 2-CEES varieties a stronger and extra steady interplay with PPD-F2, resulting in considerably bigger frequency shifts.

Furthermore, the bond energy will be in contrast based mostly on the electronegativity of the central atoms within the purposeful teams. The central atom of 2-CEES is sulfur (S) with an electronegativity of two.58, whereas DMMP and Malathion have phosphorus (P) at 2.19, and EO has carbon (C) at 2.55. Due to the upper electronegativity of sulfur, PPD-F2 varieties a stronger bond with 2-CEES in comparison with different gases, ensuing within the highest frequency shifts. Therefore, the selectivity of PPD-F2 for 2-CEES is attributed to each the bond dissociation power and the electronegativity of the purposeful group’s central atom. Hence, these supplies can be utilized as potential candidates for 2-CEES sensing⁴³.

Selectivity check of PPD-F2 when uncovered to 6 goal gases at mounted focus of 10 mg/m³: (a) 2-CEES, (b) DMMP, (c) Malathion, (d) CK, (e) SO₂, and (f) EO. Significant frequency shifts are noticed from (a) and (b).

2-CEES sensing performances underneath totally different temperature ranges

Figure 5a-h present the real-time frequency response and repeatability analysis of PPD-F2-coated SAW sensors uncovered to 2-CEES vapor at a focus of 5 mg/m³ underneath temperatures starting from – 20 °C to 50 °C. The noticed traits will be defined by the interplay between the carboxyl teams of the PPD-F2 sensing materials and the carbonyl purposeful teams of the goal fuel. The repeatability check outcomes confirmed that the common ∆f was 723.12 Hz at – 20 °C, 500.19 Hz at – 10 °C, 310.19 Hz at 0 °C, 220.33 Hz at 10 °C, 205.36 Hz at 20 °C, 162.21 Hz at 30 °C, 131.35 Hz at 40 °C, and 91.21 Hz at 50 °C. Since there have been no vital variations between the frequency shift values and their averages at every temperature, the sensors demonstrated glorious repeatability. Figure 6a-h present the response time and restoration time of SAW sensors coated with PPD-F2 for detecting 2-CEES vapor at a focus of 5 mg/m³ underneath environmental circumstances starting from – 20 °C to 50 °C. The response occasions required to succeed in 90% of the ultimate equilibrium worth upon publicity to 2-CEES at numerous temperatures are summarized as follows. The shortest response time was roughly 69 s at 40 °C, adopted by 71 s at 50 °C. The shortest restoration time was 82 s at 50 °C, with 113 s at 30 °C. These outcomes point out that because the environmental temperature will increase, each the adsorption and desorption reactions turn into extra lively, resulting in longer response and restoration occasions⁴⁴.

To be sure that the frequency shift noticed within the above experiments is solely as a result of interplay between 2-CEES and the sensing layer, and never a results of the intrinsic thermal sensitivity of the SAW sensor itself, management measurements had been performed utilizing a naked (un-coated) SAW machine underneath equivalent temperature circumstances. As proven in Fig. 7, the resonance frequency of the naked sensor shows a slight drift relying on the ambient temperature however stays steady over time after reaching thermal equilibrium. In Fig. 7-a reveals absolutely the resonance frequency throughout the temperature vary of – 20 °C to 50 °C, and 7-b presents the ∆f relative to the preliminary frequency at every temperature. These outcomes point out that the frequency drift as a consequence of temperature alone was minimal in comparison with the shifts noticed with 2-CEES publicity.

Consequently, baseline correction was utilized the place acceptable, confirming that the frequency responses described in Fig. 7-a and -b had been primarily pushed by the chemical interplay between PPD-F2 and 2-CEES reasonably than the thermal sensitivity of the sensor substrate.

Repeatability of SAW sensing efficiency at working temperature (a) – 20 °C, (b) – 10 °C, (c) 0 °C, (d) 10 °C, (e) 20 °C, (f) 30 °C, (g) 40 °C, and (h) 50 °C for five mg/m³ of 2-CEES.

90% response/restoration occasions of SAW sensor at working temperature (a) – 20 °C, (b) – 10 °C, (c) 0 °C, (d) 10 °C, (e) 20 °C, (f) 30 °C, (g) 40 °C, and (h) 50 °C for five mg/m³ of 2-CEES.

Temperature-dependent conduct of a naked SAW sensor with none sensing layer. (a) Resonance frequency variation as a perform of temperature from – 20 °C to 50 °C. (b) Frequency shift (Δf) over time relative to the preliminary worth at every temperature, demonstrating thermal stability after equilibration.

Effect of relative humidity

Figure 8a reveals the response of the SAW sensor coated with PPD-F2 underneath various humidity circumstances of 30% RH, 50% RH, and 70% RH when uncovered to 25 mg/m³ of 2-CEES. The sensor exhibited frequency shifts of 2218.13 Hz at 30% RH, 2553.95 Hz at 50% RH, and 2640.88 Hz at 70% RH, indicating enhanced responsiveness with rising humidity. This pattern is attributed to the adsorption of water molecules on the sensing layer, which will increase the floor mass and consequently reduces the speed of the floor acoustic wave. In our sensor system, this leads to a larger ∆f, which instantly displays improved sensitivity. Therefore, the elevated humidity facilitates mass loading and enhances the detection efficiency of the PPD-F2-coated SAW sensor underneath humid circumstances^45,46.

Limit of detection (LOD) check

Figure 8b illustrates the investigation of linearity to find out the LOD of the goal fuel. PPD-F2-coated SAW sensors had been uncovered to varied focus of 2-CEES underneath managed environmental circumstances at a temperature of – 20 °C. The concentrations of 2-CEES had been set to 1/2, 2/5, 3/10, 1/5, and 1/10 of the beforehand examined concentrations, equivalent to 2.5, 2.0, 1.5, 1.0, and 0.5 mg/m³, respectively. The imply and normal deviation (SD) values for every focus had been calculated from three repeated experiments, and the outcomes are as follows: for two.5 mg/m³, the imply is 60 with SD of three; for two.0 mg/m³, the imply is 94 with SD of two; for 1.5 mg/m³, the imply is 137 with SD of three; for 1.0 mg/m³, the imply is 165 with SD of 11; and for 0.5 mg/m³, the imply is 197 with SD of 10. Error bars had been added to the information factors to signify the variability of the measurements. The experimental information had been used to derive a linearity curve, with coefficient of dedication (R²) of 0.99289. High R² signifies glorious linearity between the fuel focus and the sensor’s frequency response, demonstrating the reliability and accuracy of the PPD-F2-coated SAW sensor in detecting 2-CEES.

Based on this linear relationship, the LOD is estimated because the lowest fuel focus at which the sensor can reliably distinguish a sign from baseline noise. Based on this linear relationship, the LOD is estimated because the lowest fuel focus at which the sensor can reliably distinguish a sign from baseline noise. To decide the theoretical LOD, the usual deviation σ_clean of the frequency shift on the lowest examined focus (0.5 mg/m³) was used, which was 2.7517 Hz. The slope of the calibration curve was 74.0123, derived from the linear regression of the experimental information.

The LOD is then calculated utilizing the usual method:

.

Although this theoretically derived LOD signifies the minimal detectable focus underneath very best circumstances, our sensible experiments confirmed that the sensor begins to provide a clearly distinguishable sign from 0.5 mg/m³. Therefore 0.5 mg/m³ is reported as the sensible LOD, reflecting the precise detection functionality of the sensor⁴⁷.

These outcomes spotlight the sensor’s excessive sensitivity and its potential for detecting low concentrations of 2-CEES, confirming its suitability for functions requiring exact fuel detection⁴⁸.

Stability check

Figure 8c presents the frequency shift information collected over 7 days. The imply and SD values for every day had been calculated from three repeated experiments. For day 1, the imply is 428 with an SD of 18; for day 4, the imply is 336 with an SD of 19; and for day 7, the imply is 294 with an SD of 33. All sensors confirmed a ~ 30% discount in sign over 7 days. The outcomes present a gradual lower in frequency shift over time, suggesting a decline in sensor sensitivity presumably as a consequence of chemical degradation on the sensing floor⁴⁹. A linear regression evaluation was carried out, and the fitted line reveals a adverse slope. Notably, the regression yielded a excessive R² worth is 0.953, which signifies a powerful linear relationship. This excessive coefficient of dedication means that the efficiency degradation follows a predictable pattern, which is useful for long-term functions the place calibration or sign correction could also be crucial to keep up dependable sensor efficiency⁵⁰.

Measured outcomes of SAW sensor (a) Effect of relative humidity on the sensing performances in detection of 2-CEES, (b) Linearity check for detection of 2-CEES at low focus starting from 0.5 to 2.5 mg/m³ on the temperature of – 20 °C, and (c) Long time period stability outcomes for detection of 2-CEES for seven days at three-day interval.

Freezing level check

To examine the underlying reason for the very best reactivity noticed at – 20 °C, an experimental evaluation was performed to discover the likelihood that 2-CEES has a freezing-point increased than – 20 °C, amongst numerous elements. The experiment was performed in separate vials containing 2-CEES and water, adopted by a gradual discount in temperature utilizing an environmental chamber. The temperature was decreased from 50 to – 20 °C at a fee of two °C per minute^51,52.

In Fig. 9, the outcomes of the freezing level experiments for 2-CEES and water are proven. As the environmental temperature decreased, the temperatures of each the 2-CEES and water additionally decreased. In the case of water, the temperature dropped after super-cooling and the section change course of. Super-cooling is a phenomenon wherein a liquid stays unfrozen regardless of being under its freezing level. To provoke the section change of water into ice, a course of generally known as nucleation is critical, throughout which water molecules organize themselves in a particular configuration. If this nucleation is inadequate, super-cooling happens. Various elements contribute to this phenomenon, similar to an absence of nucleation websites, a fast cooling fee, and the presence of impurities.

During the phase-change course of, as water molecules come nearer to one another and kind an elevated variety of hydrogen bonds, the interconnection between water molecules strengthens, in the end forming a strong ice construction. Through this course of, we outlined a temperature of 0 °C because the freezing level of water, the place a section change happens⁵³. However, within the case of 2-CEES, there is no such thing as a observable section change area or super-cooling vary just like that of water. This signifies that the freezing level of 2-CEES is decrease than – 20 °C. Therefore, the experiment confirmed that the rise in responsiveness was not as a consequence of a rise in reactivity as 2-CEES solidified.

Freezing level check outcomes of (a) 2-CEES and (b) H₂O.

Adsorption principle

The following investigates the connection between temperature and adsorption fee because the noticed trigger for the very best reactivity at – 20 °C. The adsorption response refers back to the course of wherein atoms, molecules, or ions within the gaseous, liquid, or dissolved state adhere to the floor of a strong or liquid and includes simultaneous and fast adsorption and desorption reactions. The adsorption course of will be categorized into bodily and chemical, relying on the kind of bonding concerned. During bodily adsorption, bodily binding happens by way of Van der Waals forces, releasing a small quantity of warmth. In chemical adsorption, chemical bonds (ionic and covalent bonds) are fashioned between the molecules and the adsorbent, ensuing within the launch of a better warmth of adsorption in comparison with bodily adsorption. The adsorption reactions are spontaneous processes with a adverse Gibbs free power worth (∆G = ∆H – T × ∆S < 0, the place ∆H < T × ∆S), the place H is the enthalpy, T is absolutely the temperature, and S is the entropy in the course of the course of. Furthermore, the adsorption response is exothermic, leading to a adverse enthalpy change in worth (∆H < 0). When fuel molecules are adsorbed onto the adsorbent, the diploma of molecular freedom decreases, resulting in a lower in entropy (∆S < 0). As the adsorption capability will increase, ∆H approaches zero, and when ∆G turns into zero with the change in adsorption capability, the adsorption and desorption reactions attain the identical response fee and set up equilibrium⁵⁴.

Le Chatelier’s precept (equilibrium regulation) states that when circumstances similar to focus, strain, and temperature change, the equilibrium state shifts to counteract the modifications in these circumstances. According to the above precept, with rising temperature, the equilibrium of the response shifts in the direction of desorption reasonably than adsorption, resulting in a lower within the adsorption quantity. Therefore, based on Le Chatelier’s precept, because the temperature decreases, the response equilibrium shifts in the direction of adsorption, rising the adsorption quantity^55,56.

In addition to the thermodynamic concerns, the adsorption conduct will also be interpreted from a kinetic perspective. The pseudo-second-order kinetic mannequin is usually employed to explain chemisorption processes, the place the adsorption fee is proportional to the sq. of the distinction between the equilibrium adsorption capability (q_e) and the quantity adsorbed (q_t) at time t^57,58. This relationship is expressed as:

the place y₂ is the speed fixed, A is Arrhenius pre-exponential issue, E_a is activation power, R is fuel fixed, and T is absolute temperature. The fee fixed itself is temperature-dependent and follows the Arrhenius equation:

This implies that though increased temperatures could speed up the adsorption fee as a consequence of elevated molecular power, they could concurrently cut back the general adsorption capability as a result of exothermic nature of the response. Thus, the noticed lower in sensor response at elevated temperatures could also be attributed to this kinetic-thermodynamic trade-off relation.