Hydrogel-based plasmonic sensor system

For in-line process monitoring in the fields of water treatment, biotechnology, food and pharmacy industry it is necessary to evaluate several parameters in liquids such as pH, temperature, ethanol-, sugar- or salt... [ view full abstract ]

For in-line process monitoring in the fields of water treatment, biotechnology, food and pharmacy industry it is necessary to evaluate several parameters in liquids such as pH, temperature, ethanol-, sugar- or salt concentrations. One approach to tackle the challenge of in-line measurements is the integration of hydrogels on optical sensors. At the Fraunhofer Institute of Ceramic Technologies and Systems a robust optical sensor, based on a plasmonic gold nanostructure, is currently under development. The nanostructure is fabricated by means of nanoimprint-lithography (NIL). The patterning of the sensor substrate makes it possible to excite surface plasmons in the metal layer with light under normal incidence.

It is well known that the spectral transmission properties of the gold nanostructure depends on the refractive index of the surrounding. The concept of using plasmonic sensors with analytically sensitive hydrogels offers an opportunity for an inexpensive and robust system for in-line measurements. Hydrogels change their swelling state with varying values of the parameters listed above. Swelling and deswelling results in a change of the (effective) refractive index. As very thin hydrogel layers (well below 1 µm thickness) can be applied on the gold nanostructure, the diffusion-controlled hydrogel swelling process can fast and, hence, the sensor response time will be short (in the range of seconds).

Initial studies for this concept were carried out with pH-sensitive hydrogels and   first results on the pH sensitivity are presented in our contribution which allow us to demonstrate the working principle.