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Because current homogeneous immunoassays show some limitations, particularly low sensitivity, we developed a new immunoassay to overcome these limitations. The approach was based on magnetic nanoparticles with a thermoresponsive polymer layer, a negatively charged polymer, and streptavidin-biotin-based antibody-antigen detection and yielded higher sensitivity than commonly used heterogeneous immunoassays. Because no special equipment is needed, it can be applied to currently available absorbance-based systems for high-throughput assays.

This correspondence presents a new strategy for detecting biological molecules that relies on competitive exchange interactions of an analyte with two-component molecular tethers attaching superparamagnetic microspheres (4 microm in diameter) to a sensor surface. The individual tethers consist of an antibody-antigen complex and are designed to selectively detect antigenic proteins in a sensitive reagentless fashion. In order to impart a driving force to the otherwise free energy neutral antibody-antigen exchange equilibrium, a small mechanical force of approximately 10 pN was applied to stretch the antibody-antigen tethers using a massively parallel magnetic tweezers device. The experimental work was carried out with human cardiac troponin I. This serum heart attack marker was used as an example of analytes of credible relevance to biomedical diagnostics. The initial results illustrate the functioning of a cardiotroponin sensor and offer a preliminary estimate of its sensitivity of 16 pM.