A fast and simple label-free immunoassay based on a smartphone

Despite the continuous advancements in bio-molecular detection and fluidic systems integration, the realization of portable and high performance devices for diagnostic applications still presents major difficulties, mostly because of the need to combine adequate sensitivity with low cost of production and operational simplicity and speed. In this context, we propose 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 the previously proposed Reflective Phantom Interface that is based on measuring the intensity of light reflected by the surface of an amorphous fluoropolymer substrate, which has a refractive index very close to that of the aqueous sample solution and hosts various antibodies immobilized within spots. The reflectivity of dozens of spots is monitored in real time by the phone׳s camera using the embedded flash LED as the illumination source. We test the performance of the combined device targeting heterologous immunoglobulins and antigens commonly used as markers for diagnoses of hepatitis B and 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. The features of the proposed system enable the realization of novel handheld biosensing devices suitable for those applications where multiple targets have to be rapidly detected even without the presence of trained personnel.