Detection of Bio-analytes with Streaming Current : From Fundamental Principles to Novel Applications

Sammanfattning: A biosensor based on streaming current is a new and relatively unexplored subject with significant potential. This thesis attempts to gain a deeper understanding of the governing principles, and then exploit them to further improve its performance as well as develop novel applications. To this end, the underlying theoretical frameworks were examined and two critical parameters of the target: its size and electric charge, influencing the sensor’s sensitivity were identified. This was followed by experimental evaluation of the parameters, using a set of tailor-made proteins, aiming to understand the nature and extent of their influence on the sensor response in relation to simulation performed following an established model.The dependence of the sensor response on the charge of an analyte, or specifically the charge contrast between the sensor surface and an analyte, opens a new avenue to improve the sensitivity and also to develop novel functionality. First, this aspect was exploited to improve the sensitivity by optimizing the surface functionalization strategy. Three such methods were compared in terms of the resulting zeta potential of the surface. The sensitivity was the highest when the charge contrast was maximum. The optimal functionalization strategy was then used for highly sensitive detection of extracellular vesicles (EVs), where an improvement in the limit of detection by two orders of magnitude over the previously reported results was demonstrated. Two applications of the improved method were then demonstrated: monitoring the effectiveness of targeted cancer medicines and analysis of liquid biopsy of cancer patients via sensitive profiling of EV-membrane proteins.Improvement in the detection specificity is a critical aspect of biosensing. This was achieved by implementing a sandwich immunoassay and demonstrating the proof of concept using trastuzumab as the target and Z-domain as both the capture and detection probes. Although the improved selectivity came at the cost of a lower sensitivity, this could be mitigated via DNA-conjugation with the detection probes, a novel electrostatic labelling strategy that allows for improvement of the sensitivity by exploiting the electrostatic influence. An application of this method was then demonstrated by detecting the target from a complex medium of E. coli cell lysate. Continuing the prospect of charge engineering of antibodies, a set of positively and negatively charged antibodies were synthesized by conjugating poly-lysine and DNA oligonucleotides, respectively. This enabled stepwise, multiplexed membrane protein analysis of EVs using the alternating charge-labelled antibodies. The method was then applied to investigate EV-heterogeneity.