It is several decades since the speech given by Dr. Feynmann at Caltech in 1959 where he pointed out the possibilities of nanotechology [1]. Since then, nanotechnology has provided many new solutions and unexpected advances in lots of scientific areas such as medicine, biology, defense or environment and it still applies in many others. In particular, the ability to create new particles of nanometric size (nanoparticles) and their utilization can result on better or improved performance for the devices they are included in. With this idea in mind, the scientific community has dedicated great efforts in this area.
The sensors research community has been especially benefited from the utilization of nanoparticles, resulting in more efficient and accurate sensors, or even reaching applications not possible until now. A particular case consists of the utilization of nanoparticles for optical fiber sensing applications [2–3] as well as their combination with different micro and nanostructures described in a different blog post. With the only difference in size from bulk materials, nanoparticles permit to observe extraordinary interactions with light depending on the nature of the materials, such as localized surface plasmon resonance (LSPR) or surface-enhanced Raman scattering (SERS), quantum confinement, etc. and have been used for the fabrication of hundreds of devices and applications.
In the next paragraphs we will briefly introduce some of the main nanoparticles or nanometer-size structures used for the fabrication of optical fiber sensors, such as metallic nanoparticles, semiconductor nanocrystals (Quantum Dots), magnetic nanoparticles or silica nanoparticles.
Metalic nanoparticles
Metals are widely used among the scientific community since ancient times and they are involved in almost any engineering application (buildings, transportation, aeronautics, etc.) thanks to their exceptional mechanical properties and their thermal conductivities. Their counterpart metallic nanoparticles have been studied not only attending to their mechanical or thermal properties but also looking at their interactions with electromagnetic waves. This particular case has enabled the observation of interesting phenomena, such as surface plasmon resonance (SPR) in the case of metallic thin-films (see our previous post) and localized surface plasmon resonance (LSPR) in the case of metallic nanoparticles. These phenomena consist in a resonant coupling between an incident electromagnetic wave at the surface of a metallic thin-film, where some of the energy of the incident electromagnetic wave is transferred to the surface free electrons of the metal, causing a collective oscillation at the interface between the metal and the dielectric. LSPR-based devices are extremely sensitive to minute variations of the environment and can produce a measurable signal with interaction of only a few target molecules, being suitable for biosensing applications as well as being also helpful to promote SERS in the nearby of metallic nanoparticles.
Silica nanoparticles
Silica nanoparticles are highly attractive for optical fiber sensing applications because of their similarity with standard silica fibers as well as their inherent properties, such as biocompatibility (silica is generally recognized as safe by the US Food and Drug Administration), transparency, chemical stability, uniformity and solubility. Silica particles can be found as mesoporous silica and silica shells and they permit to modify their surface with a wide range of functional groups in order to enable easy molecule conjugation and multi-target applications.
Magnetic nanoparticles
The utilization of magnetic particles dates back to computer or automotive science. These materials have permitted the development of an enormous variety of technologies in the sensing field, such as magnetoresistive sensors, giant magnetoresistance (GMR), magnetic tunnel junction sensors (MJT), extraordinary magnetoresistance and ballistic magnetoresistance, giant magnetoimpedance (GMI) sensors, magnetodiodes, magnetotransistors, magnetostricitive sensors, magnetooptic sensors, magnetic resonance imaging (MRI) or Single Photon Emission Computed Tomography (SPECT) among others. These technologies are also associated to magnetic nanoparticles and can be exploited in many different applications. We can differentiate between biological and non-biological application of previous technologies. However, the utilization of magnetic nanoparticles aims always to obtain improved performances.
Finally, it is important to remark that previously described nanoparticles are only a few remarkable examples of nanoparticles and their utilization and combination between them as well as with other micro and nanostructures consist of a step forward to the development of what is known as lab-on-fiber technology as it is schematically represented in Figure 1.
Bibliography
[1] There’s Plenty of Room at the Bottom in February 1960 Engineering and Science Caltech Magazine