Electron spin resonance microfluidics with subnanoliter liquid samples

Microfluidics is a well-established technique to synthesize, process, and analyze small amounts of materials for chemical, biological, medical, and environmental applications. Typically, it involves the use of reagents with a volume smaller than similar to 1 micro-l -ideally even nano-or picoliters. When the sample of interest contains para-magnetic species, it can in principle be quantified and analyzed by electron spin resonance (ESR) spectroscopy. However, conventional ESR is typically carried out with a sample volume of similar to 1 ml, thereby making it incompat-ible with most microfluidics applications. Here we show that by using a new class of miniature surface resonators combined with photolithography to prepare microfluidic patterns, ESR can be applied to measure small liquid samples, down to picoliter volumes, without considerably sacrificing concentration sensitivity. Our experiments, carried out with resonators whose mode volumes range from similar to 1 to 3.6 nL, showed that with a sample volume of similar to 0.25 nL good signals could be obtained from solutions with spin concentrations of less than 0.1 mu M. The advantage of using microfluidics ESR is evident in our work, not only because it facilitates the use of a very small sample volume, but also because it makes it possible to apply huge ( similar to 1000 T/m) and fast ( similar to 1 mu s) pulsed magnetic field gradients to the sample. This is a key capability to measuring unique properties such as nanoscale real-space diffusion and quantum spin diffusion. All our experiments are performed at room temperature, making our technique compatible with future microfluidics applications that might employ a complete system of compact resonators, microfluidic chips, miniature magnets, and a compact ESR-on-a-chip spectrometer. This could result in a completely new approach to processing and measuring paramagnetic liquid samples for use in a variety of chemical, biological, medical, and environmental applications.