Cell/particle concentration inside droplets holds great potential in extending lab-in-a-droplet applications, typically ranging from biological and chemical assays. Herein, we present a universal, massive and versatile technique, namely, alternating current electrothermal-flow field-effect transistor (ACET-FFET) to accomplish in-droplet cell/synthetic particle concentration on demand. Three parallel planar electrodes are utilized to generate an artificially reorderable electric field inside droplets by tuning the gate voltage through field-effect control, which results in a reshapable ACET-based microvortices pattern for in-droplet concentration. A downstream Y-shaped junction promotes the mother droplet splitting into two daughter droplets containing highly and poorly concentrated cells/particles, respectively. Fluorescent polystyrene (PS) nanoparticles are used to characterize the variations of ACET-microvortices flow pattern formation within droplets. Moreover, the concentration performance is demonstrated using PS microparticles and Neurospora crassa cells. We show that particles/cells can flexibly accumulate into any daughter droplet or be equally concentrated in both daughter droplets by conveniently regulating the gate voltage. The highly concentrated cells at the entrance of the concentrator show an instantaneous response performance to the external electric field. Further, online simultaneous particle synthesis and concentration inside droplets are proposed and implemented for the first time, demonstrated by efficient in-droplet micromixing and Prussian blue (PB) reaction. The accompanying synthetic PB particles are highly concentrated into either daughter droplet, thereby extending the versatility of the platform. The presented in-droplet concentration strategy, together with its unique features of simple geometric configuration, facile operation and broad applicability can broaden utility in droplet microfluidics.