**1. Introduction**

Implementing new tools for bioanalytics is the priority goal for the modern diagnostics and medicine. This task can be efficiently fulfilled only by merging the expertise from various research fields, that is materials and nanoscience, bio- and electrical engineering, chemistry. In this respect, a microfluidic concept that groups the tools and techniques to study and manipulate fluids at a submillimeter length scale, represents an ideal solution for micro- and nanosensors-based platforms. The field of microfluidics has shown the power of downscaling techniques for the manipulation and analysis of fluids [1]. High-throughput characterization of processes in such applications is a key task, particularly in medical diagnostics, food processing and pharmaceutical industries [2]. Although downscaling is in principle compatible with parallelization to increase the throughput, implementing a large number of experiments would require a dramatic scaling-up of the overall platform size. The solution to this issue is found in the droplet microfluidics technology. Here, immiscible phases are used to create discrete volumes of confined solutions, maintaining the small footprint of the employed device [3].

The generated emulsions can be counted even up to thousands per second [4]. With each single droplet being an independent miniaturized laboratory for biochemical experiments, this technology allows highly parallelized and controlled measurements, surpassing the precision of conventional assays. Other advantages benefit the droplet generation systems, such as enhanced mixing and mass transfer efficiency leading to faster reaction times [5] or minimized sample absorption on channel walls [6].

Since the first demonstration in 2001 [7], this novel bioanalytic approach has found a wide range of applications: generation of drug-loaded particles for therapeutic agent delivery [8], fabrication of micromotors [9], confinement of particles [10] creation of synthetic cells [11], microbubbles as ultrasound or photoacoustic contrast agents [12], diagnostics [12], microcapsules for cell culture [12], etc. In particular, the approach has proved itself effective in microbial cell assays for both fundamental microbiology research and clinical studies, *i.e.* to determine the metabolic activity, division rate, and drug resistance [13–15]. Although important contribution have been made, the technology remains a work in progress. Strong efforts are still invested in the development of droplet tracking and analytical techniques. Real-time detection and analysis of the droplets is critical to monitor the dynamics of the ongoing confined events. Additionally, real-time analysis enables a timely response to the needs by tuning the properties or the direction of the generated droplets as needed. A simple photography or video acquisition process could provide access to basic features such as droplet size, morphology, color, or rough monitoring of microscopic objects inside. Here, the processing power may impose a limitation to the amount of droplets per second that can be analyzed. Implementing alternative transduction mechanisms could offer a much simpler setup with faster sampling rate. It could also give access to information on the nature of the droplet content, which cannot be easily obtained from an image.

Here, we provide an overview of the existing approaches for the real-time tracking of droplets in multiphase microfluidic devices. We make a distinction between optical and optics-less techniques, with electrical or electrochemical and magnetic techniques as part of the second group, which can find droplet properties that remain more hidden to the eye. While there are additional techniques already demonstrated such as mass spectrometry [16] or electrospray ionization mass spectrometry [17], we put the focus on the former two groups of techniques as more advantageous ones in terms of miniaturization possibilities in the spirit of the lab-on-a-chip purposes.
