Ultrafast Humidity Sensing Layers Made by Two-Photon Polymerization and Initiated Chemical Vapor Deposition

Abstract

Humidity sensors are used in many applications. The design of fast sensors that can operate in explosive environments is a difficult task. Therefore, current research efforts aim at combining reliability, sensitivity and high sensing speed. The use of structured ultrathin hydrogels perfectly meets these requirements. Nanostructures are directly fabricated with a two-photon-polymerisation (2PP) 3D printer to use them as templates for hydrogels. After the 3D printing multiple templates are coated with ultrathin films of poly(2-hydroxyethyl-methacrylate) (pHEMA) using initiated chemical vapor deposition (iCVD). p(HEMA) is a humidity responsive hydrogel which changes its thickness by orders of magnitude depending on the ambient conditions. The 3D printed structures are optimized to give both a fast response time, and an optical read-out method for visible wavelengths. Upon hydrogel swelling, the height of the nanostructure pillars increases, keeping their periodicity constant. This induces a change in intensity of the first -order refraction peak, which can be easily measured also at low humidity levels. The humidity response of the nanostructures is measured and an influence for different hydrogel thicknesses and humidity flow rates is observed. The ultrathin film with the lowest thickness of 50 nm shows the fastest response to relative humidity, which is much faster than commercial sensors with 8 s response time.