[INVITED] Cell sensing with near-infrared plasmonic optical fiber sensors

• Fiber optic plasmonic immunosensors based on gold-coated tilted fiber Bragg gratings, yielding operation in the near-IR wavelength range, which is compatible with telecommunication grade equipment and enhances the penetration depth of the sensing field in the surrounding medium, improving the sensing modalities for larger analytes, such as cells.
• Experimental evidence that the used surface plasmon resonance (SPR)/fiber-optic immunosensors can establish the contact with intact mammalian cells and monitor them in short time frames.
• Selective cellular detection achieved in complex samples, such as cell culture media with fetal bovine serum (FBS) with low noise, which suggests the potential for detection in blood samples.

Surface Plasmon resonance (SPR) optical fiber biosensors are a miniaturized counterpart to the bulky prism configuration that offer remote operation in very small volumes of analyte. They have the potential to yield in situ (or even possibly in vivo) molecular detection. They usually result from a gold-coated fiber segment for which the core-guided light is brought into contact with the surrounding medium. Recently, SPR excitation was achieved with tilted fiber Bragg gratings (TFBGs) photo-imprinted in the fiber core and surrounded by a thin gold layer. These gratings probe the surrounding medium with near-infrared narrowband (~100 pm linewidth) resonances, which enhances both the penetration depth of the evanescent field in the external medium and the wavelength resolution of the interrogation. They also constitute the unique configuration able to probe all the fiber cladding modes individually, with high Q-factors. We use these unique spectral features in this work to target and detect extracellular membrane receptors in native membranes of different human epithelial cell lines. A differential diagnosis has been demonstrated between two systems, a cell line with overexpressed membrane receptors (the positive control) and another cell line with a low number of these receptors (a negative control). Such results bring an important step towards the demonstration of in situ diagnosis.