Total Internal Reflection Interrogates Nanoparticles in
Microfluidic Chips

David L. Shenkenberg

Whereas traditional methods for measuring nanoparticle dispersions involve time-consuming sample preparation and image analysis, a novel microfluidic device can characterize nanoparticle dispersions in real time. To do so, it employs an integrated diffraction grating operating under total internal reflection, a design conceived and developed by researchers from Technical University of Ilmenau in Germany, from the Bulgarian Academy of Sciences in Sofia and from the Slovak Academy of Sciences in Bratislava.

Figure 1. Each of these microfluidic chips can characterize nanoparticles in a microfluidic channel using an integrated diffraction grating under total internal reflection. Images reprinted with permission of Nano Letters.

The device consists of three layers of silicon wafers (Figure 2). The microchannel runs in a U-shape, up from an inlet in an outer wafer to the middle wafer, horizontally through the middle wafer and back through an outlet in the same outer wafer. To enable total internal reflection, the other outer wafer holds a prism made of polymethylmethacrylate, and an aluminum diffraction grating lies between the prism and the microchannel.

The device follows a previous design, but the researchers made several important changes: The wafers that contain the microchannel are thinner to allow for smaller input volumes; aluminum serves as the diffraction grating instead of chromium because it can achieve higher diffraction efficiencies; and the grating period is 2 μm instead of 10 μm to enable closer placement of the photodetector.

Figure 2. The structure of the microfluidic chip consists of three layers of silicon wafers, with the microfluidic channel in the middle layer and in an outer layer. The other outer layer contains a polymethylmethacrylate prism, and the aluminum diffraction grating rests between the prism and the microchannel. The beam of the diode laser travels to the diffraction grating and becomes totally internally reflected in the prism, and the resulting evanescent waves travel to the photodetector.

To measure the nanoparticles, the 633-nm beam of an external Hitachi red diode laser travels through the prism to the aluminum grating. The beam becomes totally internally reflected in the prism, creating evanescent waves that penetrate into the nanoparticle dispersion. Whereas the light reflected by the aluminum grating remains constant, the light that interacts with the nanoparticle dispersion causes periodic changes in the amplitude and the phase of the reflection at the prism base, resulting in interfering waves and a diffraction pattern.

In their work, the angle of incidence slightly exceeded the critical angle, such that it enabled total internal reflection across the range of the expected refractive indices and ensured linear detection. An external photodetector measured the changes of the diffraction efficiency of the zeroth order, enabling the use of one photodetector for simplicity and reducing relative error.

The researchers used the device to measure the diffraction efficiencies of various polymeric, metallic and ferromagnetic nanoparticles. From the diffraction efficiencies, they calculated the nanoparticle concentrations with resolutions of 0.3 to 0.5 percent by weight for polymeric nanoparticles, 0.03 to 0.05 percent by weight for metallic nanoparticles and 0.05 to 0.1 percent by weight for ferromagnetic nanoparticles. They calculated the effective refractive indices with accuracies of 7 × 10^–4 for polymeric dispersions and 2 × 10^–4 for the metallic and ferromagnetic dispersions.

The investigators concluded that the device can effectively analyze nanoparticle dispersions. They also pointed out that it can be mass-produced and that it is compatible with most existing systems based on micrototal analysis.

Nano Letters, February 2008, pp. 375-381.

Total Internal Reflection Interrogates Nanoparticles in Microfluidic Chips

Total Internal Reflection Interrogates Nanoparticles in
Microfluidic Chips