Abstract: A novel technique for label-free, rapid detection of ultra-low concentrations of virus specific antibodies is described. We have developed a simple, robust capacitive biosensor using microwires coated with Zika or Chikungunya virus envelope antigen. With little discernable nonspecific binding, the sensor can detect as few as 10 antibody molecules in a small volume (10 molecules/30 ?L) within minutes. It can also be used to rapidly, specifically, and accurately determine the isotype of antigen-specific antibodies. Finally, we demonstrate that anti-Zika virus antibody can be sensitively and specifically detected in dilute mouse serum and can be isotyped using the sensor. Overall, our findings indicate that our microwire sensor platform can be used as a reliable, sensitive, and inexpensive diagnostic tool to detect immune responses at the point of care.