The development of compact optical and electromagnetic sensors is a key step in developing suites of on-chip measurement capabilities. While a wide variety of quantities can potentially be measured, many fundamental issues such as accuracy, calibration, reproducibility, and interoperability must be carefully addressed in high-throughput, compact, on-chip environments.
Electromagnetic and optical techniques can be exploited to develop rapid, accurate measurements for a wide range of complex fluids, such as cell suspensions, biomolecules, and nanoparticles in solution. Many properties of biological samples can be revealed by exposing them to high-frequency (microwave or millimeter-wave) electromagnetic stimuli, and NIST is devising on-chip microwave standards that will provide NIST-traceable power measurements for frequencies as high as 100 GHz.
For example, the ways in which cells and other objects become charged and polarized in response to electric fields (their dielectric properties) reveal important information about cell membranes as well as external and internal chemistry. By simply varying the measurement frequency from the kHz regime to hundreds of GHz, one can discriminate between membrane properties, intracellular properties, and nuclear properties. NIST researchers are developing such metrology for on-chip, broadband dielectric measurements.
Similarly, measuring response to magnetic fields also yields key information about biological and other kinds of samples. One way to study that response, called electron paramagnetic resonance (EPR), detects signals from unpaired electrons in a fluid sample in somewhat the same way that MRI scans detect signals from protons in the body. EPR spectroscopy allows “spin labeling” of proteins, high-frequency bio-sensing, and microwave chemistry and processing. NIST scientists are at work on creating on-chip EPR sensors for use in highly miniaturized devices.
Beyond directly measuring electromagnetic properties, electrical and electromagnetic-based sensors can be used to detect and measure other fluid physical properties, such as heat capacity and thermal conductivity, speed of sound, flow, pressure, and temperature. The rapid response times of electrical-based measurements and the potential for integration with microfluidics in the on-chip environment will continue to drive the interest in these areas.
Many microfluidic measurement devices need some form of optical access for microscopy. But there are other kinds of optically-based measurements -- of pressure, temperature, and other properties -- that can be made in extremely small confines with high accuracy. NIST scientists are experimenting with photonic sensors as sensitive, localized probes of fluid properties. Early efforts involve placing optical fibers that have embedded photonic elements, called fiber bragg gratings, inside microfluidic devices. The gratings block or transmit certain wavelengths traveling down the fiber depending upon the physical characteristics of the grating. Placing the fiber adjacent to a microfluidic channel – by sandwiching inside layers of double-sided silicone tape – brings it in contact to flowing fluids and enables detection of changes in fluid properties. For example, if the fiber is attached to a flexible membrane, increasing the pressure of the fluid will deform the membrane and strain the fiber, changing the shape of the grating and in turn changing its optical output. That makes the grating function as a pressure or strain gauge. Similarly, different temperatures in the fluid channel will cause thermal expansion or contraction that will cause a change in the properties of the grating, also altering the light output and allowing the grating to act as a thermometer. To date, these embedded gratings can measure <1 °C change in <2 µL volume.