What Determines the Detection Limit during Impedance Biosensing?

Impedance biosensors have been widely studied for applications to biomedicine, environmental monitoring, food, and agriculture.  The most common format for AC impedance biosensors involves surface immobilization of an antibody, receptor protein, DNA strand, or other species capable of bio-recognition, and AC impedance detection of the binding event. 

For an electrode coated with a polymer film, electron transfer to a redox probe such as Fe(CN)₆^3-/4- is essentially an electron tunneling process, so one expects exponential decline in the rate of electron transfer (k_ET) with film thickness (x) according to the equation below, where α is a constant for a given redox couple and l is the Marcus reorganization energy.  This relationship has been quantified most accurately through studies of Au-thiol self-assembled monolayers (SAM) of variable chain length.  Therefore, fundamental electrochemistry predicts that the detection limit should decline with increasing analyte molecular weight.   However, this functional dependence is complicated, since it is related to the different between two terms that depend exponentially on film thickness. 

Literature reports of detection limits during impedance biosensing will be summarized and discussed. Our results for impedance biosensing as a function of analyte molecular weight will also be discussed for impedance biosensing of peanut protein Ara h 1, cortisol, and endocrine-disrupting chemicals such as norfluoxetine and BDE-47.  In addition, detection of BDE-47 using an alpaca single domain antibody results in a lower detection limit (0.79 ng/ml) than use of a rabbit polyclonal antibody (1.3 ng.ml).  Possible electrostatic effects on detection limit are discussed through the use of oppositely charged Fe(CN)₆^3-/4- and Ru(NH₃)₆^3+/4+ redox probes.