Common framework for surface plasmon binding and voltage sensing and microscopy with transmission line representation

Surface plasmon imaging and sensing is a well-established and important technology for the detection of minute binding events in, for instance, antibody/antigen reactions. More recently it has been realized that surface plasmon effects can be used to measure voltages as well as electrical impedance. At first sight the physical mechanisms for binding and voltage sensing appear very different; however, we develop a transmission line and impedance representation of the sensing process which clearly shows that binding and voltage sensing can be conveniently represented in a common framework. Our transmission line model shows graphically how the gold layer amplifies reflectivity changes resulting in optimum sensitivity at around 48 nm gold thickness. The other elegant feature of this representation is that the model clearly shows the role of the change in amplitude and phase in the sensing process; indeed it reveals their relative contribution to the output of the sensor. The graphical representation is also very suggestive of a model to quantify the performance of different detection strategies. This model provides a framework to describe these strategies without reference to any specific noise mechanisms. The results of the model definitively support previous assertions that phase imaging gives better sensitivity compared to intensity measurement. Moreover, we show that measurement of the complex amplitude containing both amplitude and phase of the detected signal performs even better than phase only detection. This opens the way for further enhancements of detection sensitivity.