Self-Assembled Gold Nanoparticle Cluster Array Onto Seeded Optical Fiber for Chemical Sensors
Authors: De Acha Nerea , López-Torres Diego , Delgado-Camón Arantzazu , Elía-Lorente Víctor , Elosúa César , Arregui Francisco J. 
Published: May 2020

Introduction
Gold nanoparticles (Au NP) exhibit interesting optical, physical and chemical features that can be tuned by modifying their morphology or by functionalizing them [1]. Therefore, a wide variety of sensing applications based on optical fiber have been developed using them, such as the colorimetric detection of toxic metal ions, cysteine or hydrogen peroxide. For those purposes, Au NP are typically embedded into polyelectrolyte multilayers or polymeric matrices, in order to be deposited onto optical fibers. Recent studies showed the possibility of depositing Au NP onto substrates seeded with block copolymers forming nanoparticle clusters [2]. The resulting array gave rise to a localized surface plasmon resonance, whose wavelength can be tailored, for instance, by modifying the amount of nanoparticles of each cluster or the distribution of the clusters. In order to check out the feasibility of the array as sensing structure, citrate-stabilized Au NPs have been self-assembled onto a polymeric template deposited along an optical fiber. The utilization of this device as a pH sensor opens the doors for the fabrication of sensors for different applications by controlling the seeding process and the morphology of Au NP clusters onto the surface of the optical fiber.

Self-assembly of patterned gold nanoparticle cluster array along the optical fiber

Au NP clusters were self-assembled by electrostatic interactions with a polymeric template [2] along the core of a 200 μm-core optical fiber. The optical fiber was first cleaned and activated by immersing it for 20 minutes in each one of the following solutions: soap, ultrapure water, potassium hydroxide (KOH) and ultrapure water. Subsequently, in order to build the template and so seeding the substrate, the fiber was dipped for 30 seconds in a 5 mg/mL solution of the cationic copolymer poly(styrene-b-2-vinylpyridine) in o-xylene. Finally, gold nanoparticles were self-assembled onto the template by immersing the fiber for 60 minutes into the anionic citrate-stabilized gold nanoparticles solution. The resulting coating was analyzed with images obtained by a Scanning Electronic Microscope (SEM).
The deposition of the Au NP cluster array along the fiber was controlled by analyzing the absorption spectra during the different steps of the process explained above. The experimental set up used to study the processes consisted of the connection of the optical fiber between a white light source in the Visible Spectra (VIS) and a spectrometer (see Figure 1).

Results

The averaged diameter of the clusters is 120nm whereas the distance between the centers is 140nm (see Figure 2). Due to their formation on the seeded surface of the optical fiber, an absorption peak was observed in the transmitted spectra: as the formation of the cluster array moved forward, the absorption peak broadened and red-shifted from 630 nm up to 700 nm, while the absorbance increased, as it is shown in Figure 3. These changes in the absorption spectra are characteristic of the formation of the cluster array along the surface of the optical fiber.
In order to evaluate the utilization of this Au NP cluster array as a possible sensing structure, the as-patterned optical fiber was exposed to different pH values ranging from 2 to 6. For each distinct pH value, the absorption peak was located at a different wavelength: as it can be observed in Figure 4, the absorption peak was red-shifted as the pH value decreased, showing a repetitive and reversible response.

Conclusions

The results exposed previously showed how the formation of a patterned gold nanoparticle cluster array yield to a localized surface plasmon resonance that could be used for measuring pH variations. The utilization of this array as sensing structure opens the possibility of utilizing this pattern for other sensing purposes by employing modified of functionalized Au NP. These parameters are currently been studied, together with different modifications of the patterned array, such as the distribution of the clusters and the amount of gold nanoparticles adsorbed in each cluster.

Acknowledgements

This research was supported by the Spanish State Research Agency (AEI) through the TEC2016-79367-C2-2-R project.

References

[1] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, The optical properties of metal nanoparticles: The influence of size, shape, and dielectric environment, J. Phys. Chem. B. 107 (2003) 668–677. doi:10.1021/jp026731y.
[2] F.L. Yap, P. Thoniyot, S. Krishnan, S. Krishnamoorthy, Nanoparticle cluster arrays for high-performance SERS through directed self-assembly on flat substrates and on optical fibers, ACS Nano. 6 (2012) 2056–2070. doi:10.1021/nn203661n.

Figure 1