Disclosed are systems, devices, methods, and other implementations, including a device to detect at least one target species in a sample, with the device including a microfluidic channel configured to receive the sample containing the at least one target species and a biocompatible ferrofluid in which the at least one target species is suspended, a detector to determine the at least one target species in the sample, and at least two of electrodes positioned proximate the microfluidic channel, the at least two electrodes configured to generate controllable magnetic forces in the sample containing the ferrofluid when a controllable at least one electrical current is applied to the at least two electrodes. The generated controllable magnetic forces causes the at least one target species to be directed towards the detector. Also disclosed are devices for separating target species in a ferrofluid, and for focusing target species suspended in a ferrofluid.