(443a) Electrophoresis of Biospecific Microparticles for Label-Free Biomarker Detection

Detection of biomarkers, including proteins, small molecules, and other physiologically relevant molecules, is essential for patient diagnosis and disease management. Approaches to detect and quantify biomarkers incorporate three main elements: a target biomarker being detected, a recognition element that specifically identifies the target biomarker, and a transducer that converts biomarker recognition into a measurable output signal. Recently, numerous active and inactive nano- and microparticle-based biomarker detection assays have been demonstrated. Compared to non-particle-based biomarker detection assays, such as enzyme-linked immunosorbent assays (ELISAs), particle-based assays can enhance biomarker detection by improving sample mixing, reducing required sample volumes, and imparting additional assay tunability. Generally, particle-based assays are distinguished by their output signal, with the most commonly demonstrated classes being electrochemical and optical; thus, biomarker concentration is coupled with changes in electrical signal, solution color, or fluorescence. Reliance on such signals often necessitates use of labeling molecules, as measured signals typically do not directly originate from the target biomarkers. Introduction of these additional labeling components inherently complicates assays by introducing new sources of error and requiring extra steps that increase the time to results. Moreover, because these signals often require complex or nonstandard equipment for signal measurement (e.g., ultraviolet−visible spectrophotometers and fluorimeters), systems that generate simplified and easily measured signals are of great interest.

Here, we demonstrate electrokinetic microparticle-based approaches for label-free, simplified biosensing using both induced-charge electrophoresis (ICEP) and dielectrophoresis (DEP). By leveraging the innate changes to the electrical properties of particle surfaces upon specific capture of target biomolecules, we show direct signal transduction that manifests in a change in electrokinetic particle motion, which can be easily measured by optical microscopy without the use of secondary labels. More specifically, we show preparation of induced-charge electrophoretic microsensors (ICEMs) by functionalization of gold-polystyrene Janus particles with biospecific recognition elements on their gold hemispheres. Upon application of a high frequency (i.e., ~10 kHz) alternating current (AC) electric field to ICEMs suspended in custom electrokinetic propulsion chambers, asymmetric electroosmotic flows are established over the particles, leading to ICEM propulsion perpendicular to the applied field. When biomarkers are captured on the gold hemisphere of the ICEMs, the biomarkers cause a change in particle polarizability that leads to a decrease in the asymmetry of the electroosmotic flows, resulting in ICEM speed suppression. We show the functionalization of ICEMs with both biotin and anti-ovalbumin IgG antibodies for the specific capture of two model biomolecules, streptavidin and ovalbumin, respectively. We demonstrate that the capture of biomolecules leads to direct signal transduction through ICEM speed suppression; at 100 nM SA, ICEM speed is reduced ~46%, as measured by analysis of videos captured by optical microscopy. We additionally show that by tuning the number of ICEMs used in assay, SA can be detected at concentrations as low as 0.1 nM. Relative to assays like ELISA, which can require over 3 hours to conduct, the ICEM-based assay requires only ~1 hour to yield results. We further explore the use of negative dielectrophoretic (nDEP) trapping of biospecific microparticles in interdigitated electrode chambers as a novel label-free biomarker detection approach. Again, the electrical properties of the surfaces of prepared particles change upon capture of biomarkers, leading to changes in the nDEP trapping force on particles at high AC field frequencies (i.e., 1 MHz) and, consequently, changes in the probability distribution of nDEP trapped particles. These two approaches, which leverage innate properties of target biomolecules and their interactions with particle surfaces, form the foundations for new paradigms of rapid, simple, and label-free biomarker detection with optical microscopy.