**5. Conclusions**

Point of care diagnostic devices reduce the distance that chemical signals must travel, and therefore, the dispersion, but for typical microchannels, it takes just millimeters for dispersion to be significant [69]. Furthermore, not all sensors can be brought to the sampling site. There have been many reviews of droplet-based microfluidics devices and droplet analysis methods. We focus on microfluidic slugs or droplets as a method of transporting chemical signals to overcome Taylor dispersion, and the devices use to capture those signals. We have surveyed examples of chemical signal sampling and delivery and classified them into three types: Aqueous phase sampling with downstream droplet formation; Preformed droplets for sampling; and Droplets formed near the analyte source. We pay particular attention to the temporal and spatial resolution of each system, and explain the features that limit each of these parameters. Strategies for downstream analysis have also been listed. We hope this review can help to broaden the use of droplet-based sampling probes for biochemical applications, enabling higher resolution study of fundamental processes.

**Author Contributions:** S.F. and D.W.I. collected and arranged the information; S.F., E.S. and D.W.I. wrote the paper. **Conflicts of Interest:** The authors declare no conflict of interest.
