Plasmonic Optical Fiber-Grating Immunosensing: A Review

Abstract

Plasmonic immunosensors are usually made of a noble metal (in the form of a film or nanoparticles) on which bioreceptors are grafted to sense analytes based on the antibody/antigen or other affinity mechanism. Optical fiber configurations are a miniaturized counterpart to the bulky Kretschmann prism and allow easy light injection and remote operation. To excite a surface plasmon (SP), the core-guided light is locally outcoupled. Unclad optical fibers were the first configurations reported to this end. Among the different architectures able to bring light in contact with the surrounding medium, a great quantity of research is today being conducted on metal-coated fiber gratings photo-imprinted in the fiber core, as they provide modal features that enable SP generation at any wavelength, especially in the telecommunication window. They are perfectly suited for use with cost-effective high-resolution interrogators, allowing both a high sensitivity and a low limit of detection to be reached in immunosensing. This paper will review recent progress made in this field with different kinds of gratings: uniform, tilted and eccentric short-period gratings as well as long-period fiber gratings. Practical cases will be reported, showing that such sensors can be used in very small volumes of analytes and even possibly applied to in vivo diagnosis.

Keywords: fiber Bragg gratings; nanoparticles; optical fibers; plasmonics; sensing.

Figure 1

(a) Sketch of the Kretschmann prism configuration used for plasmonic sensing; and (b) its response to surrounding refractive index changes linked to biomolecules binding.

Figure 2

(a) Sketch of the light mode coupling in a uniform fiber Bragg grating (FBG); and (b) transmitted (black curve)/reflected (red curve) amplitude spectra of a 1 cm long uniform FBG.

Figure 3

(a) Sketch of the light mode coupling in a tilted-fiber Bragg grating (TFBG); and (b) transmitted amplitude spectrum of a 1 cm-long 10° TFBG.

Figure 4

(a) Sketch of the light mode coupling in an excessively tilted fiber grating (ETFG); and (b) transmitted amplitude spectrum of a 1 cm-long ETFG.

Figure 5

(a) Sketch of the light mode coupling in an eccentric fiber Bragg grating (EFBG); and (b) transmitted amplitude spectrum of a 1 cm long EFBG.

Figure 6

(a) Sketch of the light mode coupling in a long-period fiber grating (LPFG); and (b) transmitted amplitude spectrum of a 1 cm-long LPFG.

Figure 7

Sketch of the surface plasmon resonance (SPR) excitation around an optical fiber showing the required polarization of the electric field.

Figure 8

(a) Sketch of the sputtering (or vacuum evaporation) deposition process to obtain a uniform metal thickness all around the optical fiber cross-section; (b) principle of the double deposition process; and (c) microscope view of a gold-coated fiber surface.

Figure 9

Sketch of a typically biofunctionalized metal-coated optical fiber surface, showing the different strategies that are most often used to attract analytes.

Figure 10

Evolution of the SPR mode in the transmitted amplitude spectrum of a gold-coated 10° TFBG as a function of a change of the surrounding refractive index.

Figure 11

Scheme of the classic implementations of (a) spectral, and (b) intensity interrogation, of the sensors in transmission.

Figure 12

(a) Picture of the packaged plasmonic fiber-grating sensor; (b) biosensor inserted in a freshly biopsied tissue and its corresponding amplitude spectrum; and (c) packaged sensor inserted in the operating channel of an endoscope.