Capturing particles includes introducing a fluid sample, which includes particles of a first type, into a first channel of a microfluidic device and flowing the fluid sample past a porous or partially porous membrane. The pores fluidly connect the first channel to a second channel, and the device further includes multiple binding moieties on a first side of the porous membrane adjacent to the first channel. The binding moieties are capable of binding to the first type of particles. Capturing particles also includes creating a pressure difference between the first and second channels to enable the fluid sample to flow from the first channel through the porous membrane into the second channel and to direct the particles toward the binding moieties, thereby capturing the first type of particles. In addition, by creating a modified capture surface that is impermeable near the walls of the channels, capture efficiencies and throughput can be increased.