  Exposure to bio-aerosols such as mold spores and pollen can lead to adverse
health effects. There is a need for a portable and cost-effective device for
long-term monitoring and quantification of various bio-aerosols. To address
this need, we present a mobile and cost-effective label-free bio-aerosol sensor
that takes holographic images of flowing particulate matter concentrated by a
virtual impactor, which selectively slows down and guides particles larger than
~6 microns to fly through an imaging window. The flowing particles are
illuminated by a pulsed laser diode, casting their inline holograms on a CMOS
image sensor in a lens-free mobile imaging device. The illumination contains
three short pulses with a negligible shift of the flowing particle within one
pulse, and triplicate holograms of the same particle are recorded at a single
frame before it exits the imaging field-of-view, revealing different
perspectives of each particle. The particles within the virtual impactor are
localized through a differential detection scheme, and a deep neural network
classifies the aerosol type in a label-free manner, based on the acquired
holographic images. We demonstrated the success of this mobile bio-aerosol
detector with a virtual impactor using different types of pollen (i.e.,
bermuda, elm, oak, pine, sycamore, and wheat) and achieved a blind
classification accuracy of 92.91%. This mobile and cost-effective device weighs
~700 g and can be used for label-free sensing and quantification of various
bio-aerosols over extended periods since it is based on a cartridge-free
virtual impactor that does not capture or immobilize particulate matter.
