*3.4. Doping with SiO2 Particles*

Incorporation of silica nanoparticles may further enhance the sensing properties of studied PVA–Ac copolymer thin films. The silanol groups on the particles' surface can develop intermolecular bonds with PVA hydroxyl groups and may assist improving the sensing performance and reducing hysteresis. This approach was demonstrated by implementing in situ sol-gel reaction of TEOS in Ac24 copolymer solutions prior to the thin film deposition.

The influence of SiO2 particles doping on humidity thickness change and percentage of hysteresis is illustrated in Figure 5a. The comparison between undoped (Ac24) and differently doped samples (Ac24p1, 20% and Ac24p2, 50%) shows that the swelling (Δ*d*/*d*) due to humidity exposure decreases with doping from 14.5% for undoped sample to 6.6% and 4.2% for 20% and 50% doped samples, respectively. The possible reason is the rigidity of the films that increases when SiO2 particles are incorporated in the polymer matrix. However, the doping has a positive effect on percentage of hysteresis: H decreases more than twice for 20% doped sample (Ac24p1): from 3.8% (undoped film) to 1.7% (20% doped film). In this case, the increased rigidity of the doped film contributes positively because it prevents fast shrinking of polymer films during humidity desorption in high range (RH > 70%) where the hysteresis is commonly observed.

**Figure 5.** (**a**) Percentage of hysteresis, *H* (Equation (3)) and relative change of film thickness Δ*d*/*d* due to humidity exposure from 5% to 95% RH for polymer films (about 80 nm in thickness) with 24% acetal content (Ac24) doped with 20% (Ac24p1) and 50% (Ac24p2) SiO2 particles; (**b**) Reflectance versus relative humidity curve for films with 24% acetal content doped with 20% and 50% SiO2 particles measured for increasing (solid black symbols) and decreasing (open blue symbols) humidity.

The *R*-vs-RH curves for the doped films are plotted in Figure 5b. A very good linear dependence of measured reflectivity is observed in the whole RH range for both samples, mostly pronounced for heavily doped one (Ac24p2) sample. This means that the acetal modified PVA films doped with appropriate amount of SiO2 particles are very suitable for optical sensing of humidity offering linearity in the entire humidity range.

Results summary is presented in Table 2. From all studied samples the smallest hysteresis is observed for Ac24 polymer film doped with 20% SiO2 particles. The sample has full dynamic range and linear dependence of the measured signal as a function of relative humidity. The accuracy of sensing is 10% RH but could be substantially increased if transmittance measurements are implemented. More studies are underway to optimize the sensing behavior of doped polymer samples.

It is seen from Table 2 that another good option for optical sensing of humidity is Ac24 polymer film (PVA with 24% acetal content) deposited on glass substrate with Au–Pd overlayer (30 nm). At RH > 70% the accuracy is less than 1% RH, the dynamic range is wide, and the hysteresis is acceptable.
