by Dr. Edgar D. Goluch, Northeastern University, Boston, MA
A PDF version is available in the July 2011 AES Newsletter.
How do we know something is inside those tiny nanofluidic channels? Optical detection, and particularly fluorescent detection, is the dominant strategy for in situ monitoring in micro and nanofluidic systems, primarily utilizing fluorescent dyes, proteins, and beads to selectively visualize species and events . This is an ideal approach for many experiments, but optical detection runs into problems when the analyte of interest is about the same size as the fluorescent tag.
For certain systems specialized fluorescent dyes have been identified that only minimally interfere with the target analyte, such as intercalating dyes used to visualize DNA and pH sensitive dyes that map proton gradients. Specific markers, though, do not exist for every species, nor will it be practical to develop them. Also, attaching any sort of marker to small molecules, such as hormones or sugars, will significantly affect their transport and can also affect their uptake and function in living systems.
|Figure 1. Principle of device operation. Redox-active molecules undergoing Brownian motion are repeatedly oxidized and reduced at two parallel electrodes inside of a nanofluidic channel that is several micrometers long, leading to a measurable cur- rent. Height and length dimensions are not drawn to scale.|
Therefore, there is a significant need for developing label-free strategies for the slew of biomolecules and nanoparticles that researchers would like to study in micro and nanofluidic channels. The exciting development, though, is that some very interesting physics takes place at the nano scale that will lead to new analytical techniques and extend the capabilities of existing ones . A few approaches are described here. Surface plasmon resonance (SPR)  and surface enhanced Raman scattering (SERS)  are label-free techniques that rely on nanoscale phenoma. The detection mechanism exploits electronic interactions between the target species and the plasmons emitted by a gold, silver, or copper surface. These plasmons extend approximately 100 nm above the surface, allowing detection of molecules that reside in that space. SERS is particularly interesting because it provides a chemical fingerprint of the target molecule.
The most established label-free detection modality for micro and nanofluidics is electrochemical . Extensive cell and particle counting and characterization has been done with impedance cytometry . Impedance-based measurements are also routinely employed for DNA detection in nanopores and nanochannels .
Our group employs amperometric detection to determine the concentration of redox-active molecules, such as pyocyanin, in nanofluidic channels. The sensors we use are unique in that they contain two working electrodes inside a nanofluidic channel spaced approximated 100 nanometers apart. When a redox-active molecule enters the channel, it accepts an electron at one electrode and donates it at the other, as seen in Figure 1. It can repeat this proc- ess thousands of times before leaving the channel because it only takes a few microseconds for a molecule to diffuse between the two electrodes, while, on average, it takes a second to traverse the length of the channel. This type of device was recently shown to be able to detect when a single ferrocene molecule enters and leaves the sensor area inside the nanofluidic cavity . The limitation of this approach is that it is constrained to redox-active molecules and it is difficult to discern between different molecular species. None-the-less, bioanalytical measurements are possible with these devices in controlled environments [9, 10].
Overall, there are a growing number of ways to investigate the contents of nanofluidic channels, but the lab-on-a-chip community is still quite a bit away from demonstrating a universal label-free detection device.