Study of a Polymeric Nanofilm Formed By PSP/PAH (Poly(Sodium Phosphate) and Poly(Allylamine Hydrochloride)) Based on Its Roughness for Optical Fiber Sensors Development
Authors: Lopez-Torres Diego , De Acha Nerea , Delgado-Camón Arantzazu , Elía-Lorente Víctor , Elosúa César , Arregui Francisco J. 
Published: May 2020

Introduction

Nanofilms surfaces with a high roughness are relevant to develop optical fiber sensors (OFS). It has been reported, in previous publications [1], that this type of structures has been used in several applications to detect parameters as for example, gases and VOCs. One of the most important features of these sensors, based on this type of nanofilms, is the high surface/volume ratio; thanks to this fact, the molecules of the parameter to be measured can interact in a better way, with the rough surface improving the final sensitivity. Moreover, in most of the cases, these structures can be functionalized to make the final sensor more selective.
This is the case of the structure proposed in this work formed by PAH (Poly(Allylamine Hydrochloride)) / PSP (Poly(Sodium Phosphate)) when it is deposited along the optical fiber by means of the technique Layer-by-Layer (LbL) nano assembly. The main idea of LbL technique consists of the nanoassembly of oppositely electrically charged polyelectrolytes: PAH is the material used as polycation and PSP is the inorganic compound employed as polyanion. It has been studied and exposed in [2] that the final nanofilm obtained with both polymers yields into an increasing roughness as the thickness does so making it use very interesting for humidity sensing.

Nano Film Construction and experimental set up

As it can be mentioned above, LbL nano assembly technique was employed with the goal of obtaining a specific roughness for the PSP/PAH nanofilm (Sensor A). The roughness surface developed enables the interaction between the PSP/PAH thin film and the evanescent field of the light guided and, due to the surface/volume ratio obtained, the molecules of water can penetrate through the nanofilm deposited improving in this manner the sensitivity. Different constructions with a distinct number of pair of layers (15 for sensor A and 100 for sensor B) and were carried out with polyelectrolyte concentrations of 10-3M along the core of a 200 μm-core optical fiber using a robotic arm (Nadetech S.L.). Another construction with a polyelectrolyte concentration of 10-2 M was made (Sensor C: 15 pair of layers). Firstly, the optical fibers were cleaned by immersing them for 20 minutes in each one of the following solutions: soap, ultrapure water, potassium hydroxide (KOH) and ultrapure water. After that, in order to facilitate the deposition of the first layer of PSP, an anchoring layer of Poly(ethyleneimine) (PEI) was deposited during another 20 minutes. Every sensor was exposed to different concentrations of relative humidity (RH) and their responses were recorded and studied.
For sensor characterization, a transmission architecture was employed. For humidity sensing, the interrogation was carried out with a halogen broadband source (HL-2000-FHSA) connected to one end of the optical fiber and a spectrometer (USB2000+XR1) connected to the other one (see Figure 1). The sensors were placed in a climatic chamber where changes in the RH, from 20% to 90%, were performed. This cycle of RH variations was repeated several times and the responses of the sensors were recorded every minute.

Results and Conclusions

The responses of the sensors towards RH were carried out by monitoring the signal power level at 600 nm. As Figures 2 and 3 show, the optical power variations of Sensor A (0,6 dB), when the RH changes from 20% to 90%, are higher than Sensor B variations (0,18 dB); this is because with a low number of pair of layers, the final nanofilm thickness enables a better interaction between the evanescent field of the optical fiber and Sensor A nanofilm. Both factors lead to increased sensitivity. On the other hand, the value of the nanofilm roughness, as with the thickness, of Sensor B is higher than Sensor A; thanks to this fact, the molecules of H2O can penetrate and spread more in its structure. As it can be appreciated in Figure 3, the consequence of this fact is the reduction of the noise component of the sensor B response. Finally, Figure 4 shows the response of the Sensor C towards HR. In this case, due to the polyelectrolyte concentration used (10-2M), the roughness obtained was the lowest and consequently, the response of the sensor C is the worst. Therefore, this fact proves that roughness is a very important factor in the sensitivity optimization process of this type of nanofilms.

Acknowledgements

This work was carried out with the financial support of MINECO (Spain) through TEC2016-79367-C2-2-R (AEI/FEDER, UE) as well as Public University of Navarre PhD grants program.

References

[1] N. Cini, T. Tulun, G. Decher, and V. Ball, “Step-by-step assembly of self-patterning polyelectrolyte films violating (Almost) all rules of layer-by-layer deposition,” J. Am. Chem. Soc., vol. 132, no. 24, pp. 8264–8265, 2010.
[2] C. Elosua, D. Lopez-Torres, M. Hernaez, I. R. Matias, and F. J. Arregui, “Comparative study of layer-by-layer deposition techniques for poly(sodium phosphate) and poly(allylamine hydrochloride).,” Nanoscale Res. Lett., vol. 8, p. 539, 2013.

Figure 1