Portable, multispot, label-free immunoassay on a phantom perfluorinated plastic

Despite the continuous advancements in bio-molecular detection methods and fluidic integration approaches, the realization of portable and high performance devices for diagnostic applications still presents major difficulties, mostly due to the need of combining adequate sensitivity with low cost of production, operational simplicity and rapidity. In this context, we have previously proposed a compact device composed of a smartphone and a custom-designed cradle, containing only a disposable sensing cartridge, a tiny magnetic stirrer and a few passive optical components. The detection principle is named Reflective Phantom Interface and is based on measuring the intensity of light reflected by the surface of an amorphous fluoropolymer substrate having a refractive index very similar to that of water. The reflectivity of dozens of spots is monitored in real time by the smartphone’s camera using the embedded flash LED as the illumination source. We tested the sensitivity and the repeatability of the combined device analyzing multiple spots of antibodies targeting an antigen commonly used as marker for diagnoses of HIV. Target concentrations as low as a few ng/ml can be rapidly and robustly determined by comparing the rate of increase of the signal after the addition of the sample with that measured after the subsequent addition of a standard solution with known concentration.