Hydrogel-based plasmonic sensor system

Fast and reliable evaluation of liquid properties such as pH,temperature, ethanol- or carbon dioxide concentrations is of extreme importancefor the in-line process monitoring in the fields of water treatment,... [ view full abstract ]

Fast and reliable evaluation of liquid properties such as pH,temperature, ethanol- or carbon dioxide concentrations is of extreme importancefor the in-line process monitoring in the fields of water treatment, biotechnology,food, and pharmacy industry. We undertook the challenge of the in-linemeasurements by integrating hydrogels on optical sensors. At the FraunhoferInstitute of Ceramic Technologies and Systems a robust optical sensor, based ona plasmonic gold nanostructure, is being developed. The nanostructure is fabricatedby means of nanoimprint lithography (NIL). The use of polymer foils as acarrier material improves biocompatibility and reduces production costs of thesensor substrate as it allows roll-to-roll fabrication. The patterning of thesensor substrate allows for excitation of surface plasmons in the metal layerwith light under normal incidence.

It is well known that the spectral transmission properties of the goldnanostructure depend on the refractive index of the surrounding. Hydrogelschange their swelling state with varying values of the parameters listed above.Swelling and deswelling causes a change in the effective refractive index. Asvery thin hydrogel layers (well below 1 µm thickness) can be applied on thegold nanostructure, the diffusion-controlled hydrogel swelling process will befast and, hence, the sensor response will be sufficiently short (in the rangeof minutes). The concept of using plasmonic sensors with analytically sensitivehydrogels offers an opportunity for an inexpensive and robust system forin-line measurements.

Initial studies for this concept were carried out with both a pH- and a carbondioxide sensitive hydrogel and results demonstrating the working principle willbe presented. Furthermore, we will present the opportunities and challenges of thefabrication process for the foil-based sensor substrates.