Integrated 3D Hydrogel Waveguide Out-Coupler by Step-and-Repeat Thermal Nanoimprint Lithography: A Promising Sensor Device for Water and pH.

Achille Francone,Clivia Sotomayor Torres,Nikolaj Gadegaard,Timothy Kehoe,Claudia Simao,Isabel Obieta,Virginia Saez-Martinez,Nikolaos Kehagias,Ali Khokhar,Leire Bilbao

doi:10.3390/s18103240

Abstract

Hydrogel materials offer many advantages for chemical and biological sensoring due to their response to a small change in their environment with a related change in volume. Several designs have been outlined in the literature in the specific field of hydrogel-based optical sensors, reporting a large number of steps for their fabrication. In this work we present a three-dimensional, hydrogel-based sensor the structure of which is fabricated in a single step using thermal nanoimprint lithography. The sensor is based on a waveguide with a grating readout section. A specific hydrogel formulation, based on a combination of PEGDMA (Poly(Ethylene Glycol DiMethAcrylate)), NIPAAm (N-IsoPropylAcrylAmide), and AA (Acrylic Acid), was developed. This stimulus-responsive hydrogel is sensitive to pH and to water. Moreover, the hydrogel has been modified to be suitable for fabrication by thermal nanoimprint lithography. Once stimulated, the hydrogel-based sensor changes its topography, which is characterised physically by AFM and SEM, and optically using a specific optical set-up.

Highlights

Recent developments in optical sensors include the use of new materials which exhibit a particular behaviour in the presence of a stimulus
The first step consisted of UV photolithography exposure into a photoresist to define the waveguide area (Figure 1a) followed by reactive ion etching (RIE) (Figure 1b) to transfer the pattern into silicon
It is a relatively simple process, and in this work, we demonstrated that it is suitable to fabricate a 3D hydrogel-based sensor

Summary

Introduction

Recent developments in optical sensors include the use of new materials which exhibit a particular behaviour in the presence of a stimulus. Hydrogels have chemical groups that make them sensitive to changes in the external environment. Sensors 2018, 18, 3240 the environment, such as pH, temperature [5], or ionic strength, cause a reversible phase transition in the gel structure of these polymers. This sensitivity is expressed as a volume phase transition between a swollen and a collapsed state [6]. One of the most used sensing mechanisms relies on optical projectors

Methods

Results

Conclusion