Sensors and Actuators B: Chemical
A Lossy Mode Resonance optical sensor using silver nanoparticles-loaded films for monitoring human breathing


This work is focused on the fabrication of a human breathing sensor based on the in situ synthesis of silver nanoparticles (Ag-NPs) inside a polymeric coating previously deposited on an optical fiber core by means of the Layer-by-Layer self-assembly. The Ag-NPs were created using a synthesis protocol consisting of a loading step of the Ag+ cations into the polymeric film and a further reduction step using dimethylamine borane (DMAB). The morphology and distribution of the Ag-NPs inside the polymeric coating have been studied using atomic force microscopy (AFM). Furthermore, UV–VIS spectroscopy and energy dispersive X-ray (EDX) were also used to confirm the synthesis of the Ag-NPs within the resultant coating. The amount of Ag-NPs increases when the number of loading/reduction cycles is higher. Therefore the incorporation of the Ag-NPs affects the refractive index of the overlay promoting the observation of a resonant attenuation band in the infrared region (NIR), known as Lossy Mode Resonance (LMR), which can be used as a sensing signal to monitor the human breathing. The quality of the device has been experimentally tested with good sensitivity (0.455 nm per RH%) and fast response time (692 ms and 839 ms for rise/fall).

Fabrication of the polymeric matrix

… A polymeric matrix has been synthesized using the Layer-by-Layer technique (LbL) by sequentially exposing the optical fiber core to the cationic polyelectrolyte poly(allylamine hydrochloride) (PAH) and to the anionic polyelectrolyte poly(acrylic acid, sodium salt) (PAA) with an immersion time of 2 min in each solution. A rinsing step in deionized water was performed between the two polyelectrolytes baths in order to remove the excess of material adsorbed. After this sequence, it has been fabricated the [PAH/PAA] basic structure, known as bilayer. This process was carried out using a 3-axis robot (Nadetech Innovations) and was repeated until reaching a [PAH/PAA] structure of 40 bilayers.


Keywords: Silver nanoparticles (Ag-NPs); Lossy Mode Resonances (LMR); Humidity sensing