Humidity sensors have potential in many fields, including health care, industrial processes, and flexible electronics. The high sensitivity of humidity sensors with high performance and stability are super important in the sensing process. To fulfill these needs, significant efforts are needed to create various types of humidity sensors. Therefore, in this chapter, a variety of materials-based humidity sensors and their sensing mechanisms such as surface acoustic wave, quartz crystal microbalance, optical fiber humidity sensors, as well as field-effect transistors are discussed. Moreover, it covers a wide range of humidity sensor applications.

The photo-degradation of methylene blue (MB) dye solution is studied under UV light irradiation using a TiO₂-Mica composite calcined at 450 °C (TiO₂-M 450). X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy (FTIR), and UV spectroscopy are used to study the photocatalyst after it is made. XRD shows TiO₂ nanoparticles (NPs) are in the anatase and rutile phases. MB degradation is evaluated by different parameters, including the concentration of MB dye, pH, the amount of photocatalyst used, and the effect of the oxidant (hydrogen peroxide, H₂O₂). Under moderate exposure to UV light, the TiO₂-M 450 composite can remove approximately 99.2% of MB dye (1.0 × 10⁻⁴ M) for 80 min. The optimal amount of TiO₂-M 450 composite is 5.0 g L⁻¹, corresponding to 2.0 g L⁻¹ of TiO₂ NPs. MB dyes can be treated with a simple, successful, and effective process. The nanocomposite is thought to be a good candidate for a photocatalyst that could be used to clean up the environment.

Novel biomass-derived carbon dots co-doped with nitrogen and sulfur were fabricated through facile and simple synthetic method from manufactured milk powder and methionine as precursors. The as-fabricated platform was used for ratiometric fluorescence sensing of Cu (II) and bisphosphonate drug risedronate sodium. The sensing platform is based on oxidation of o-phenylenediamine by Cu (II) to form 2, 3-diaminophenazine (oxidized product) with an emission peak at 557 nm. The resultant product quenched the fluorescence emission of as-fabricated carbon dots at 470 nm through Förster resonance energy transfer (FRET) and inner-filter effect (IFE). Upon addition of risedronate sodium, the formation of 2, 3-diaminophenazine was decreased as a result of Cu (II) chelation with risedronate sodium, recovering the fluorescence emission of carbon dots. The ratio of fluorescence at 470 nm and 557 nm was measured as a function of Cu (II) and risedronate sodium concentrations. The proposed sensing platform sensitively detected Cu (II) and risedronate sodium in the range of 0.01–55 μM and 5.02–883 µM with LODs (S/N = 3) of 0.003 μM and 1.48 µM, respectively. The sensing platform exhibited a good selectivity towards Cu (II) and risedronate sodium. The sensing system was used to determine Cu (II) and risedronate sodium in different sample matrices with recoveries % in the range of 99–103 % and 97.4–103.8 %, and RSDs % in the range of 1.5–3.0 % and 1.8–3.6 %, respectively.

Herein, a novel ultrasensitive lacidipine (LCDP) electrochemical sensing platform was constructed based on functionalized glassy carbon (FGC) microspheres anchored with ceria nanoparticles (CeO₂NPs@FGC, ratio 2:1 w/w). The XRD, SEM, HRTEM, and EDX results confirmed the formation of the hybrid CeO₂NPs@FGC composite. A molecular wire (MW) modified CeO₂NPs@FGC electrode (CeO₂NPs@FGC-MWE) was firstly fabricated by mixing CeO₂NPs@FGC composite with diphenylacetylene (DPA). The fabricated CeO₂NPs@FGC-MWE sensor exhibited high electro-catalytic activity towards the oxidation of antihypertensive drug LCDP. Quantitative LCDP detection was achieved using adsorptive anodic stripping square-wave voltammetry (AdAS-SWV). The LOD and the LOQ are estimated to be 2.3 and 7.6 nM, respectively, with a dynamic range of 7.0–2680 nM and a sensitivity of 206 μA μM⁻¹ cm⁻². Moreover, the fabricated sensor also shows good selectivity, favorable reproducibility, good antifouling activity, and long-term stability. The practical utility of the sensor for LCDP monitoring in biological fluids, patient plasma and pharmaceutical formulation was evaluated with satisfactory results. The UHPLC technique was utilized to verify the accuracy of the proposed method.

The main interest of this work is to construct surface family pair with the symmetry of
Bertrand pair in Euclidean 3-space E3. Then, by employing the Serret-Frenet frame, we conclude
the sucient and necessary conditions of surface family pair interpolating Bertrand pair as mutual
geodesic curves. Moreover, the conclusion to ruled surface family pair is also obtained. Meanwhile,
this work is demonstrated through several examples.