Sensor de umidade quimiorresistivo nanoestruturado impresso em 3D baseado em ABS/NTCS com revestimento de fécula de mandioca (Manihot esculenta)/NTCs

Abstract

With the growth of the world population, estimates indicate that food demand will increase by 35% to 56% by 2050. Consequently, production in the food industry will need to rise accordingly. In addition, the climate crisis is disrupting the planet’s water cycle, leading to longer droughts and changes in rainfall patterns. In this context, the development of nanostructured sensors for soil-moisture monitoring is valuable. These sensors consist of 3D-printed devices based on a poly(acrylonitrile–butadiene–styrene)/carboxylic-acid-functionalized multiwalled carbon nanotube nanocomposite (ABS/f-MWCNTs) and coated with cassava-starch films nanostructured with f-MWCNTs (CS/f-MWCNTs). Substrates were prototyped and printed by fused deposition modeling (FDM) and nanostructured by immersion in a 7:1 H₂O/N,N-dimethylformamide solution containing 1 mg mL-1 f-MWCNTs under ultrasonic bath. Five sample groups were obtained: nanostructured ABS (n-ABS), n-ABS/CS, n-ABS/n1-CS (1 wt% f-MWCNTs), n-ABS/n2-CS (2 wt% f-MWCNTs), and n-ABS/n3-CS (3 wt% f-MWCNTs). The devices were morphologically characterized by Scanning Electron Microscopy (SEM) and Raman spectroscopy, as well as by electrical characterization and testing, and contact-angle measurements. SEM results confirmed the surface impregnation of f-MWCNTs in the polymer matrix and the formation of films with an average thickness of 1.645 ± 0.236 μm. Raman spectroscopy revealed the characteristic vibrational modes of CNTs (D, G, and 2D bands), cassava starch, and ABS, indicating that coating with cassava starch and f-MWCNTs modifies the electronic structure of the nanotubes in a concentration-dependent manner. Humidity tests in a chamber, compared with the conventional HW-390 sensor, resulted in average sensitivities ranging from 0,00152 ± 0,00059 %⋅%RH-1 (n-ABS) to 0.03409 ± 0.02829 %⋅%RH-1 (n-ABS/n3-CS). The contact angle decreased from 89.87 ± 0.91° for neat ABS to 45.58 ± 1.33° for n-ABS/CS, indicating increased surface hydrophilicity of the sensor due to the presence of starch and f-MWCNTs. The results indicate that the sensitivity increase with f-MWCNT concentration in the starch film is associated with the formation of conductive networks in the active layer, whose swelling upon hydration of the coating amplifies the change in electrical resistance per unit humidity. The device enables integration into an IoT platform using an ESP32 microcontroller, demonstrating its applicability for remote soil-moisture monitoring.