Amplitude Holographic Interference-Based Microfluidic Colorimetry at the Micrometer Scale.

Quantitative molecular analysis is usually based on spectrophotometric methods using colorimetric assay. Conventional methods, however, rely on the direct uniform absorption of the sample under test, and the detection sensitivity is strictly limited by the length of the absorption cell at the millimeter scale. Here, we report a new methodology for colorimetric assay based on the amplitude holographic interference (AHI) caused by nonuniform absorption of light, with detection sensitivity at the micrometer scale. In our method, the curved surface of the microfluidics results in a phase profile with a high diffraction efficiency, and the nonuniform absorption of samples exactly matches with the amplitude modulation in the holographic interference. The signal intensity is affected by not only direct sample absorption but also the sequential optical interference behind the liquid level. Both single- and multiple-wavelength colorimetric analyses of the Griess-Saltzman dye (GSD) were carried out using this method, and we found that the sensitivity can be improved by approximately 2-fold in comparison to the conventional method. This interference-based method would be a useful tool for the colorimetric assay of chemical samples in highly integrated systems with better performance.