Measurements of IgG Protein at Ultralow and Ultrawide Concentration Ranges Exploiting Spoon-Shaped Waveguides

In this work, a surface plasmon resonance (SPR) sensor based on plastic spoon-shaped waveguides and plastic optical fibers (POFs) was tested in a novel experimental configuration. The latter was achieved by launching the light from the bowl of the spoon and collecting it from its neck. First, the sensor was optically tested, thus obtaining a 55% improvement in terms of bulk sensitivity with respect to the better spoon-shaped sensor configuration presented in the literature. Then, binding tests were performed by functionalizing the neck of the SPR spoon-shaped probe with an anti-IgG-based recognition layer. The SPR biosensor tested via the novel experimental configuration was tested in IgG detection, denoting an ultralow limit of detection (LOD) equal to about 60 fM. Moreover, by keeping the measurement setup unchanged and by a 180° rotation of the biosensor chip position, it was possible to achieve an LOD of about 50 pM. Therefore, the proposed sensing approach can be used to span a higher concentration range, thus paving the way to the possibility to detect the IgG in an ultrawide concentration range over four orders of magnitude via a simple rotation of the chip. These findings highlight the adaptability of the spoon-shaped SPR sensor to provide tailored sensitivity through the variation of the interrogation configuration, thus broadening its potential use for various point-of-care test (POCT) applications.