One-pot synthesis of novel chitosan-salicylaldehyde polymer composites for ammonia sensing

Chitosan (Chs)-salicylaldehyde (Sal) polymer derivatives were formed via the reaction of Chs-Sal with zinc oxide nanoparticles (ZnO NPs) and beta-cyclodextrin (β-CD). These polymers were synthesized through inclusion with β-CD and doping with ZnO NPs to give pseudopolyrotaxane and Chs-Sal/ZnO NPs composite, respectively, for low-temperature detection and sensing of NH3 vapors as great significance in environmental control and human health. Additionally, the polymer (Chs-Sal/β-CD/ZnO NPs) was prepared via the insertion of generated composite (Chs-Sal/ZnO NPs) through β-cyclodextrin ring. The structural and morphological characterizations of the synthesized derivatives were confirmed by utilizing FTIR, XRD and, SEM, respectively. Also, the optical properties and thermal gravimetric analysis (TGA) of the synthesized polymers were explored. The obtained results confirmed that using β-CD or ZnO NPs for modification of polymer (Chs-Sal) dramatically enhanced thermal stability and optical features of the synthesized polymers. Investigations on the NH3-sensing properties of Chs-Sal/β-CD/ZnO NPs composite were carried out at concentrations down to 10 ppm and good response and recovery times (650 s and 350 s, respectively) at room temperature (RT) and indicated that modification by β-CD and doping with ZnO NPs effectively improves the NH3-sensing response of Chs-Sal from 712 to 6192 using Chs-Sal/β-CD/ZnO NPs, respectively, with low LOD and LOQ of 0.12 and 0.4 ppb, respectively.


Synthesis of Chs-Sal polymer
A solution of salicylaldehyde (0.40 g in 10 ml MeOH) was prepared at room temperature with stirring for 6 h before the temperature was raised to 100 °C for 1 h, according to Fig. 1a.Chitosan powder (1.0 g) in 50 ml of glacial acetic acid (1% v/v) was then added.The Chs-Sal polymer was obtained as a yellow powder by evaporating the solvent at room temperature.

Synthesis of pseudopolyrotaxane (Chs-Sal/β-CD)
A mixture of Chs-Sal polymer (1.0 g) and β-CD (2.0 g) in DMF (25 ml) was stirred at room temperature for 24 h, Fig. 1b.The obtained precipitate dried at room temperature to afford Chs-Sal/β-CD as a pale-yellow powder.

Synthesis of Chs-Sal/ZnO NPs composite
A mixture of ZnO NPs (0.10 g) and Chs-Sal polymer (1.0 g) in DMF (25 ml) was agitated for 30 h at room temperature.To obtain green crystals of Chs-Sal/ZnO NPs, the hydrogel that had formed was transferred into a Petri plate and allowed to dry at room temperature (Fig. 1c).

Synthesis of Chs-Sal/β-CD/ZnO NPs composite
For six hours, a solution containing 1.0 g of Chs-Sal polymer and 0.10 g of ZnO NPs in 25 ml of DMF was stirred at room temperature.Next, β-CD (2.0 g) was added to DMF (25 ml) and stirred while left at room temperature for 48 h (Fig. 1d).The formed hydrogel was poured into a Petri dish and dried at room temperature to afford Chs-Sal/β-CD/ZnO NPs composite as a pale-yellow crystal.

Characterizations
Fourier-transformation infrared (FTIR) FTIR spectroscopy was used to examine the structure of the generated derivatives at room temperature utilizing potassium bromide disc (KBr) (using an infrared spectrometer, a Jasco Model 4100 from Japan), specifically in the wavenumber range of 4000 to 400 cm −1 .

X-ray diffraction (XRD)
XRD measurements were performed at room temperature using a powder diffractometer (Brucker D8 Advance, Germany) fitted with a Cu K radiation source, yielding values of = 1.5406 and 2 in the range of (5°-80°) for the crystallite size and phase structure of the polymers

Scanning electron microscope (SEM)
A 20 kV accelerated voltage SEM (JEOL SEM model JSM-5500-Japan) was utilized to examine the morphological structures of the formed derivatives.

Thermogravimetric analysis (TGA)
Using a TGA (SDT Q600 V20.9 Build 20-Germany) with a 5 °C/min heating rate up to 400 °C and a 5 ml/min nitrogen gas flow, the thermal stability of the synthesized derivatives was assessed.A data collecting and handling mechanism is built into the thermal analyzer (TA-50WSI).

UV-visible spectroscopy
The optical characteristics of the produced polymers were determined using UV-visible spectroscopy.The UV-visible spectra were obtained by using a UV-visible spectrophotometer (PG Instruments, model T80, UK)

Gas sensing studies
The sensing properties of the synthesized polymer composites were studied by utilizing a homemade system comprises a 5 L glass chamber and a laptop interfaced digital multimeter for signal acquisition 36,43,44 .The sensor was fabricated by painting the composite paste on interdigital electrode interdigitated electrodes (5 cm × 4 cm × 2 cm) and dried at 45 °C for 24 h.
Different volumes of volatile liquids were injected into the bottle using a syringe to obtain the corresponding concentrations.The relation = Ra/Rg, where Ra and Rg stand for the sensor's resistances in the air and in the presence of the tested gas, respectively, was used to calculate the sensor response.The amount of time it takes to get to 90% of the final equilibrium value is defined as the response and recovery times.

SEM analysis
Figure 2 shows the SEM images and the surface morphology of chitosan, β-CD, ZnO NPs, and the generated chitosan polymers (Fig. 2a-g).In contrast to the fibrous nature of the chitosan surface, these images demonstrate the apparent differences between them and the surface appearances that were altered upon reaction.As rough, amorphous slides, the Chs-Sal polymer was visible (Fig. 2b).Additionally, the surface morphology of β-CD and pseudopolyrotaxane polymer showed a high difference between them due to the insertion of Chs-Sal polymer into the β-CD (Fig. 2d).Pseudopolyrotaxane polymer showed a scaly and slightly rough structure with a bigger random particle crystal size.SEM images of Chs-Sal/ZnO NPs composite showed smooth slides without pores surface.The pores of Chs-Sal polymer are considered a particle sink to get ZnO NPs inside it to form the soft surface of the composite (Fig. 2f).The surface morphology of pseudopolyrotaxane polymer was changed in Chs-Sal/β-CD/ZnO NPs composite (Fig. 2g).It has become uneven and aggregated in Chs-Sal/β-CD/ZnO NPs composite.

XRD analysis
The phase structure and crystallite size of chitosan (Chs), Chs-Sal polymer, β-CD, pseudopolyrotaxane polymer, ZnO NPs, Chs-Sal/ZnO NPs, Chs-Sal/β-CD/ZnO NPs composite, were accomplished using XRD analysis (Fig. 3a,b) at 25 °C, in the range of 5° to 80°.The diffraction peaks that are characteristic of Chs were observed at 2θ = 8.6° and 20°, indicating that it is semi-crystalline [45][46][47][48] .XRD analysis showed the formation of Schiff base (SB) (Chs-Sal polymer) via the changing of amino group nature.Moreover, the difference in the crystal size and crystallinity of Chs was indicated by disappearance peaks in 16.4°, 33.6°, and the appearance of a slight peak in 13.8°.Additionally, the crystallinity values of Chs and the formed Chs-Sal polymer showed 57.7 and 50.6%, respectively.The crystallinity value of Chs-Sal decreased due to the formation of SB and cleavage of hydrogen bonds 49 .The XRD pattern of pseudopolyrotaxane polymer showed a sharp diffraction angle at 14.2° and the crystallinity value was 48.3% and the crystal size increased compared to Chs-Sal polymer (crystal size values of Chs-Sal and pseudopolyrotaxane polymer were 2.4 and 3.2 nm, respectively).Decreased crystallinity in the case of the pseudopolyrotaxane polymer is due to the Chs-Sal chain being inserted into the electron-rich cavity of the cyclodextrin rings.After addition of ZnO NPs to the Chs-Sal polymer gave a different pattern in XRD analysis.
In the blind polymer, a broad peak decreased in intensity and some distinct peaks emerged at 15.9°, 17.9°, and 29.9°.Additionally, the cost of the crystallinity value (46.1%) this difference indicates the formation of hydrogen bonding between Chs-Sal and ZnO NPs.Finally, Chs-Sal/β-CD/ZnO NPs composite XRD result showed broad peaks at 13.3°, 13.7°, 14.2°, 14.6°, 17.6°, 18.2°, higher crystal size 5.78 nm and lower crystallinity value 45%.The Scherrer equation was used to calculate the crystal size of the synthesized polymers 50 .where λ is the wavelength of the X-ray; β, the half width of the diffraction peak; θ, diffraction angle; and k, constant.And the crystallinity values were calculated from this equation:

Fourier-transform infrared spectroscopy (FT-IR)
FT-IR was used to confirm the formation of the components.Figure 4a shows the difference between the Chs and Chs-Sal polymer as a SB.The hydrocarbon bond C-H appeared at 2978 cm −1 , and the -OH group characteristic peak in Chs appeared at 3334 cm −1 , superimposed over the N-H stretching band.In addition, the peaks of -C=O amide and -NH 2 were observed at 1657 cm −1 and 1598 cm −1 .The spectrum of SB polymer showed C=N band at 1631 cm −1 which confirmed the Chs-Sal polymer formation.Additionally, C-C bonds stretching and bending vibrations on the aromatic ring of the aldehyde and the stretching vibration of -C-O of Sal appeared at 1490, 820, and 1250 cm −1 , respectively [51][52][53] .There is no detectable residual of free Sal, as evidenced by the absence of the salicylaldehyde characteristic band in the area 1660-1730 cm −1 in the SB spectrum.Table 1 provides a summary of the differences between the absorbance bands of chitosan and Chs-Sal polymer.It was noticed that the absorption band of the hydroxyl group is slightly high and less intensity than the β-CD sample (Fig. 4a).Also, the ν[OH] symmetric stretching was shifted to a higher frequency and ν[CH-aliphatic] was shifted to a lower frequency compared to those in β-CD.Furthermore, the ν[C-O-C] and ν[CH 2 -O] bending vibrations were shifted to lower frequencies at 1027 and 1158 cm −1 , respectively.These results confirmed the formation of pseudopolyrotaxane polymer through the reaction of Chs-Sal polymer with β-CD.The increase in frequency is due to the Chs-Sal chain being inserted into the electron-rich cavity of the cyclodextrin rings, which accounts for the observation 54 .Conversely, the drop in frequency can be attributed to the formation of hydrogen bonds and Vander Waals forces between the β-CDs and the hydroxyl groups of Chs and Sal molecules, as well as between β-CDs themselves (Fig. 4a).The absorbance bands of pure β-CD, Chs-Sal polymer, and pseudopolyrotaxane polymer are shown to vary in Table 2.
The FT-IR spectrum of the Chs-Sal/ZnO NPs polymer (Fig. 4b), showed a broad absorption band at 3408 cm −1 corresponding to the stretching vibrations of hydroxyl (OH) groups.The absorption band at 2945 cm −1 is attributed to symmetric stretching of aliphatic C-H groups of Chs in polymer blend 14 , which is markedly shifted and decreased in intensity (2930 cm −1 ) upon doping of ZnO NPs.The absorption band at 1660 cm −1 is assigned to free C=O stretching vibration 55 .The absorption bands at 1560 and 1413 cm −1 corresponding to the polymer (C=N) group bending 56 and stretching vibration of (CH 2 -OH), were shifted towards higher wave numbers and decreased in their intensities, because of the interaction between the polymer blend chains and the ZnO NPs 57 .The band at 1073 cm −1 which is attributed to C-O-C stretching became less intense and was shifted to lower wavenumber.Table 3 summarizes the variations in absorption bands between Chs-Sal polymer and Chs-Sal polymer doped with ZnO NPs.
These changes (shift and decreased intensity) indicated the strong interaction between these functional groups in the polymer blend and ZnO NPs.The absorption band at 675 cm −1 appeared due to the stretching mode of the amide groups attached to ZnO NPs 58,59 (Fig. 4b).
Figure 4c shows the difference between Chs-Sal polymer, Chs-Sal/ZnO NPs composite, Chs-Sal/β-CD, and Chs-Sal/β-CD/ZnO NPs composite.The FT-IR spectrum of the Chs-Sal/β-CD/ZnO NPs composite showed a broad absorption band of OH group at 3375 cm −1 with increasing intensity compared to the last polymers.www.nature.com/scientificreports/ The absorption band of aliphatic C-H groups at 2927 cm −1 , was markedly shifted and increased in intensity.
The absorption band at 1663 cm −1 was assigned to free C=O stretching vibration, and C-N group appeared at 1663 cm −1 with high intensity.The absorption bands at 1158 and 1028 cm −1 correspond to the stretching vibration of (CH 2 -OH) group and C-O-C stretching became more intense and shifted to a lower wavenumber.The strong interaction between these functional groups in the polymer blend of pseudopolyrotaxane and ZnO NPs is indicated by these changes (shift and increase in intensity).The absorption band at 756 cm −1 appeared due to the stretching mode of the amide groups attached to ZnO NPs.Table 4 summarizes the differences between the absorption bands of the Chs-Sal/ZnO NPs polymer, Chs-Sal/β-CD polymer, and Chs-Sal/β-CD/ZnO NPs polymer.

Optical properties
UV-visible spectroscopy of Chs-Sal polymer, Chs-Sal/ZnO NPs composite, Chs-Sal/β-CD, and Chs-Sal/β-CD/ ZnO NPs composite were recorded in the range of 200-800 nm at 25 °C (Fig. 5).The UV-visible spectrum of Chs-Sal polymer showed a broad peak at the region 327-360 nm due to the presence of π bond (-N=CH-).It's interesting to note that adding ZnO NPs and β-CD to the Chs-Sal polymer caused the characteristic peak to slightly shift blue, from 360 to 340 and 356 nm, respectively.Also, in the case of the addition of β-CD and  ZnO NPs into Chs-Sal as a one pot-reaction gave a sharp peak in 439 nm.These data approved the formation of Chs-Sal/ZnO NPs, Chs-Sal/β-CD, and Chs-Sal/β-CD/ZnO NPs composite.Additionally, the energy gap Eg of the prepared polymers was calculated depending on the UV-visible absorption spectra according to Tauc's formula.The values of Eg for Chs-Sal, Chs-Sal/β-CD, Chs-Sal/ZnO NPs composite and Chs-Sal/β-CD/ZnO NPs composite were 4.4, 3.55, 2.65, and 3.11 eV, respectively.The lower Eg of Chs-Sal/β-CD/ZnO NPs composite (3.11 eV) compared to Chs-Sal (4.4 eV) makes the resistance of the sensor more likely to change in the presence of gas due to electron transition which leads to gas sensitivity improvement 37,[60][61][62][63] .

Thermal analysis
The thermal stability and heat resistance of the synthesized polymers were investigated using thermogravimetric analysis (TGA).TGA curves for Chs-Sal, Chs-Sal/ZnO NPs composite, Chs-Sal/β-CD, and Chs-Sal/β-CD/ ZnO NPs composite are displayed in Fig. 6. Figure 6 clearly shows that, overall, the thermal stability of the Chs-Sal/β-CD composite was lower than that of the Chs-Sal polymer, while the weight loss at 350 °C was 40% and 34%, respectively.In the case of Chs-Sal was an organic polymer directly exposed to heat.But when the polymer was inserted into β-CD, it was preserved from the heat more and the loss of weight was less.The heat resistance of Chs-Sal/ZnO composite was improved by the incorporation of ZnO NPs because of the strong bond between zinc and oxygen in ZnO NPs, whereas the weight loss of Chs-Sal/ZnO at 350 °C was 17%.Conversely, when compared to Chs-Sal/β-CD, the thermal stability of the ZnO NPs composite increased by 55% at the same temperature.This was because the polymer's stability was increased by the addition of ZnO NPs.After addition of ZnO NPs into Chs-Sal/β-CD gave a more stable polymer, whereas the weight loss of Chs-Sal/β-CD/ZnO at 350 °C was 5%.The order of thermal stability for the four samples is based on the results shown in Fig. 6 and was as follow: Chs-Sal/β-CD/ZnO NPs ˃ Chs-Sal/ZnO NPs ˃ Chs-Sal/β-CD ˃ Chs-Sal.

Gas sensing properties
Figure 7b,c show the response of the polymer composite as a sensor towards various concentrations of NH 3 at RT.The Chs-Sal/β-CD/ZnO NPs composite sensor exhibited high sensing characteristics toward NH 3 down to 10 ppm concentration.It was observed that the sensor response was increased by increasing NH 3 concentration and recovered after purging the chamber with air, indicating reversible response characteristics of the composite (Fig. 7b).The sensor response and recovery times were 650 s and 350 s, respectively, indicating that Chs-Sal/ β-CD/ZnO NPs composite is suitable for gas sensing applications.Besides, the selectivity of the Chs-Sal/β-CD/ ZnO NPs composite sensor has also been studied at room temperature upon exposure to various 100 ppm gases/ vapors (NH 3 , chloroform, ethanol, and methanol) as depicted in Fig. 7e.The results indicate that the Chs-Sal/ β-CD/ZnO NPs composite sensor exhibits the highest response when exposed to 100 ppm NH 3 which may to be due to its highest electron-donating ability compared to the other analytes 64 .Furthermore, the selective response of the composite toward NH 3 might be related to ZnO (transition metal oxide) which has more than one oxidation state and d 10 electronic configuration that could retain cations with its filled orbital and be reduced when it reacted with reducing analyte like NH 3 to generate free electrons at RT 65 .Another reason for the high selectivity towards NH 3 may be due to the lone pair of electrons on the N atom, which are more likely to occupy the vacant orbital on Zn 2+ , leading to the formation of a stronger binding energy 37 , suggesting that the Chs-Sal/β-CD/ZnO NPs composite has excellent selectivity in NH 3 -monitoring application.Figure 7a  www.nature.com/scientificreports/representing the sensing properties of the sensors, a high sensitivity usually has a low limit of detection.The calculations were achieved using the variation in the gas response at baseline (without analyte gas) using the root-mean-square deviation (rms).The rms deviation was calculated on 50 points at baseline 66 using RMS noise = i (Si − S) 2 /N , where S i is the experimental data point, S is the corresponding value calculated from the curve-fitting, and N is the number of data points, respectively.The LOD is calculated as 3(RMS noise /slope) (according to the IUPAC definition), where the slope is obtained from linear fitting of the response vs concentration (Fig. 7d).LOD of the prepared composite Chs-Sal/β-CD/ZnO NPs was 0.12 ppb surpassing the threshold value for NH 3 gas indicated by the National Institute for Occupational Safety and Health (NIOSH) (total weight average (TWA) permissible ammonia exposure limit is 25 ppm and the short-term exposure limit (ST) (for 15 min) is 35 ppm) 67 .Furthermore, this LOD is less than other NH 3 sensors such as 0.4 ppm 64 , 11 ppm 68 , and 0.78 ppb 69 , respectively.The limit of quantification (LOQ) was calculated using 10(RMS noise /slope) 70 and was 0.4 ppb which is lower than that reported in other studies [71][72][73] .
Gas sensing behavior of the semiconductor is based on the strong interaction between the analyte gas molecules and the adsorbed oxygen species at the surface of the sensing material.Initially, the adsorbed oxygen molecules attract electrons from the ZnO NPs surface because they have high electron affinity 74 , so, it is believed that the sensing mechanism occurs at the surface of the ZnO NPs.It was reported that defects such as Zn interstitials and oxygen vacancies that are found on ZnO NPs surface enable oxygen molecules in the ambient atmosphere to be adsorbed easily due to the differences in chemical potential energy 75 .When the Chs-Sal/β-CD/ZnO NPs composite is exposed to atmospheric air, the oxygen molecules trap the electrons from the conduction band of ZnO NPs and chemisorbed on the surface forming O 2− , O − and O 2− by increasing depletion layer width.This leads to increasing the surface resistance of the sensor.Upon injecting ammonia, the adsorbed oxygen species react with the NH 3 molecules forming H 2 O, N 2 , and three electrons which are injected into the conduction band of ZnO NPs by decreasing the depletion layer width and decreasing sensor resistance as a result.Upon the recovery of the sensor, the adsorbed ammonia molecules get desorbed from the ZnO NPs surface and increase the depletion layer width again and the resistance of the sensor is increased and returns to the initial baseline value 38 .The mechanism of the process is expressed by the following equations: www.nature.com/scientificreports/ In addition, The presence of OH groups of β-CD may also participate in the adsorption of NH 3 molecules through hydrogen bonding (hydrophilic outer side), and thus provides a favorable environment and more active sites for the adsorption of more NH 3 molecules [76][77][78][79][80] .Moreover, many vapor channels in the cavities of β-CD in addition to the inclusion of Ch-Sal-ZnO into these cavities may enhance the sensor response of Chs-Sal/β-CD/ ZnO NPs composite 81 .Also, to further improve the sensitivity, it is necessary to introduce a conductive substrate such as ZnO NPs to increase the specific surface area of the composite 77 .Furthermore, the inner cavity of β-CD may occupied by traces of water vapor from the atmospheric air which may also interact with NH 3 vapor 82,83 , leading to higher response value in the Chs-Sal/β-CD/ZnO NPs composite.

Conclusion
Stirring at room temperature of Chs polymer as an amine with Sal as a carbonyl component afforded Chs-Sal polymer.The obtained Chs-Sal was added to β-CD to form pseudopolyrotaxane inclusion complex and doped with ZnO NPs to produce the corresponding composite in a one-pot, adequate, simple, cost-effective, and ecofriendly environmentally method.Chs, Sal, ZnO NPs, and β-CD were reacted as one pot reaction to form Chs-Sal/β-CD/ZnO NPs composite polymer.These derivatives were confirmed by FT-IR and XRD analyses, while SEM analysis indicated remarkable morphological changes between them.UV-visible results confirmed the improvement of the optical properties of the prepared samples, as well as the decreasing of Eg of Chs-Sal/β-CD/ ZnO NPs composite (3.11 eV) compared to Chs-Sal (4.4 eV) which enhances the sensitivity.The thermal stability of Chs-Sal polymer was enhanced by doping with ZnO NPs.Gas sensing results showed that the modification of chitosan-salicylaldehyde polymer through inclusion into β-CDs and doping with ZnO nanoparticles (NPs) showed high sensitivity (from 712 for Chs-Sal to 6192 for Chs-Sal/β-CD/ZnO NPs), good response and recovery times (650 s and 350 s, respectively) and low LOD and LOQ of 0.12 and 0.4 ppb, respectively, in addition to high selectivity towards NH 3 vapors which is related to its highest electron-donating ability compared to the other analytes, making them have a potential application prospect in NH 3 monitoring.

Figure 6 .
Figure7b,cshow the response of the polymer composite as a sensor towards various concentrations of NH 3 at RT.The Chs-Sal/β-CD/ZnO NPs composite sensor exhibited high sensing characteristics toward NH 3 down to 10 ppm concentration.It was observed that the sensor response was increased by increasing NH 3 concentration and recovered after purging the chamber with air, indicating reversible response characteristics of the composite (Fig.7b).The sensor response and recovery times were 650 s and 350 s, respectively, indicating that Chs-Sal/ β-CD/ZnO NPs composite is suitable for gas sensing applications.Besides, the selectivity of the Chs-Sal/β-CD/ ZnO NPs composite sensor has also been studied at room temperature upon exposure to various 100 ppm gases/ vapors (NH 3 , chloroform, ethanol, and methanol) as depicted in Fig.7e.The results indicate that the Chs-Sal/ β-CD/ZnO NPs composite sensor exhibits the highest response when exposed to 100 ppm NH 3 which may to be due to its highest electron-donating ability compared to the other analytes64 .Furthermore, the selective response of the composite toward NH 3 might be related to ZnO (transition metal oxide) which has more than one oxidation state and d 10 electronic configuration that could retain cations with its filled orbital and be reduced when it reacted with reducing analyte like NH 3 to generate free electrons at RT65 .Another reason for the high selectivity towards NH 3 may be due to the lone pair of electrons on the N atom, which are more likely to occupy the vacant orbital on Zn 2+ , leading to the formation of a stronger binding energy37 , suggesting that the Chs-Sal/β-CD/ZnO NPs composite has excellent selectivity in NH 3 -monitoring application.Figure7ashows the response comparison of Chs-Sal, Chs-Sal/β-CD, Chs-Sal/ZnO NPs, and Chs-Sal/β-CD/ZnO NPs composite as sensors to 100 ppm of NH 3 at RT.The obtained observations indicated that Chs-Sal/β-CD/ZnO NPs composite exhibited improved NH 3 sensing response compared to other polymers (712 for Chs-Sal, and 6192 for Chs-Sal/β-CD/ZnO NPs, respectively).The limit of detection (LOD) is used as an important parameter for