Potential of levitated drops to serve as microreactors for biophysical measurements

Microreactors are desirable for exploring chemical and biological processes, as reactant consumption is minimal and safety issues are easily managed. Levitated drops are a class of microreactors for which mixing is continuous and solid/liquid interfaces are absent or of lesser importance than in channeled microfabricated flow reactors. Thus, reactant adsorption or wall catalysis possibly of importance in ordinary microfluidic systems is absent in levitated drops. Transport of gaseous reactants or products is facile. Levitated drop microreactors are amenable to batch or continuous flow study of biochemical reactions. The possibility of studying oscillatory enzyme-catalyzed reactions in drops is apparent. This review explains the physics and chemistry of levitated drop microreactors and describes practical aspects of their design, fabrication, implementation, and optimization. Such considerations as drop evaporation, thermal control, protein behavior at the gas/liquid interface, and observation with spectroscopic and electrochemical probes are discussed.

Highlights► Ultrasonically-levitated drops show promise as reactors for studying biochemical kinetics. ► Such drops are most interesting for systems in which free radicals are reactants or products. ► Fluid handling and diagnostics are sufficiently developed for near-term application. ► Rate constants ≤ 1 s− 1 (first order) and ≤ 106 M− 1 s− 1 (second order) should be measurable. ► Complex dynamics such as chaotic oscillations should be well-suited for study in levitated drops.