Channel-fouling is a pervasive problem in continuous flow chemistry, causing poor product control and reactor failure. Droplet chemistry, in which the reaction mixture flows as discrete droplets inside an immiscible carrier liquid, prevents fouling by isolating the reaction from the channel walls. Unfortunately, the difficulty of controllably adding new reagents to an existing droplet stream has largely restricted droplet chemistry to simple reactions in which all reagents are supplied at the time of droplet formation. Here we describe an effective method for repeatedly adding controlled quantities of reagents to droplets. The reagents are injected into a multiphase fluid stream, comprising the carrier liquid, droplets of the reaction mixture and an inert gas that maintains a uniform droplet spacing and suppresses new droplet formation. The method, which is suited to many multistep reactions, is applied to a five-stage quantum dot synthesis wherein particle growth is sustained by repeatedly adding fresh feedstock.

Knowing how biomarker levels vary within biological fluids over time can produce valuable insight into tissue physiology and pathology, and could inform personalised clinical treatment. We describe here a wearable sensor for monitoring biomolecule levels that combines continuous fluid sampling with in situ analysis using wet-chemical assays (with the specific assay interchangeable depending on the target biomolecule). The microfluidic device employs a droplet flow regime to maximise the temporal response of the device, using a screw-driven push-pull peristaltic micropump to robustly produce nanolitre-sized droplets. The fully integrated sensor is contained within a small (palm-sized) footprint, is fully autonomous, and features high measurement frequency (a measurement every few seconds) meaning deviations from steady-state levels are quickly detected. We demonstrate how the sensor can track perturbed glucose and lactate levels in dermal tissue with results in close agreement with standard off-line analysis and consistent with changes in peripheral blood levels.

When making a droplet flow device what material should you use? What fabrication methods are available and will surface treatments be required? This review offers a guide, with examples, to making robust droplet flow devices.

Microfluidic reactors are emerging as a highly promising technology for quantum dot synthesis due to the unparalleled control they provide over particle properties. In this article, we review recent developments in the microfluidic synthesis of quantum dots, and discuss some of the advantages and challenges of preparing nanocrystalline materials in microscale fluidic channels. The relative merits of continuous-flow and segmented-flow reactors are considered, together with a number of outstanding issues that must be successfully addressed for microfluidics to become a truly viable technology for quantum dot synthesis.

We describe the controlled synthesis of dextran-coated superparamagnetic iron oxide nanoparticles (SPIONs) using a stable passively-driven capillary-based droplet reactor. High quality highly crystalline particles were obtained with a narrow size distribution of mean diameter 3.6 nm and standard deviation 0.8 nm. The particles were evaluated for use in MRI, and found to exhibit a large saturation magnetisation of 58 emu/g and a high T₂ relaxivity of 66 mM⁻¹s⁻¹ at 4.7 T, signifying good MRI contrast enhancement properties.