Minimizing the photobleaching of self-assembled multilayers for sensor applications

Abstract

Layer-by-layer electrostatic self-assembled (ESA) multilayer nanostructures containing 1-hydroxypyrene-3,6,8-trisulfonate (HPTS), a fluorophore widely used for pH sensing, were built-up onto optical fiber substrates. The self-assembled structures are based on a basic matrix of alternating polyelectrolytes; poly(allylamine hydrochloride) (PAH) as polycation and poly(acrylic acid) (PAA) as polyanion. Doping the polyanionic solution with HPTS, the resultant (PAH/PAA + HTPS)_n multilayer coatings exhibited a pH-dependent fluorescencent behavior appropriate for optical sensing applications. Unfortunately, these sensitive coatings show short lifetimes due to their high rate of photobleaching. In order to improve the performance of the devices, the fabrication atmosphere and some post-treatments, like thermal curing and the addition of an antifading agent, 1,4-diazabicyclo[2.2.2]octane, have been experimentally studied. After these modifications were introduced in the fabrication routine, the photobleaching rate was drastically reduced: the initial sensitive coatings show a decrease in fluorescence intensity due to photobleaching of 58% after 90 min of continuous illumination and the optimized sensitive coatings show a decrease of only 4.7% after 3 days of continuous illumination.

Introduction

The world of optical fiber sensors is getting more and more competitive every day and some designs have already found a well-established niche in the market [1], [2]. A classical arrangement is based on a sensitive coating deposited onto the fiber, in this way if the target magnitude varies, the optical properties of the coating also change. This optical variation could be measured with the traditional benefits of the optical fiber sensors; capability of multiplexing signals, high bandwidth, electric and magnetic immunity, etc. The design and fabrication of the sensitive coatings is the main task in the creation of fast, reproducible, and stable optical devices. These features are strongly influenced by the composition and morphology of the active coatings. In order to improve these two aspects, homogeneous, thin, and reproducible coatings are desired onto a small and non-planar substrate as it is an optical fiber. Coating techniques like sol–gel [3] or cellulose acetate matrices [4] have been used for fabricating the active coatings, but their thickness and morphology was difficult to control; this aspect had negative effects in the reproducibility of the sensors. On the other hand the fabrication techniques related to nanotechnology have caused a revolution in many fields of science like photonics, biotechnology, electronics, materials, just to mention a few [5], [6], [7], [8]. More specifically, the utilization of nanostructured materials opens the possibility of fabricating ultra-thin sensitive coatings for optical fiber applications and in this work, some fluorescent pH sensitive coatings for the monitoring of pH are studied. In this particular field, the electrostatic self-assembly (ESA) technique [9] gives great advantages compared to other traditional techniques. The ESA method has been already demonstrated for the fabrication of magnetic, electrically conductive, non-linear optical and other thin-film coatings on substrates of various sizes, shapes and materials [10], [11], [12]. In the field of optical fiber sensors, our group has successfully applied the ESA technique for the fabrication of different devices [12], [13], [14], [15].

The ESA film deposition method is schematically described in Fig. 1. This method is based on the electrostatic attraction between oppositely charged molecules in each monolayer deposited, and involves several steps. First, a substrate (in this case the optical fiber) is cleaned and treated to create a charged surface. Then, the substrate is exposed to a solution of polyions of opposite charge for a short time (min) and by adsorption a monolayer of polyions is formed on the surface. In this way, the substrate is alternately dipped into solutions of cationic and anionic polymers (or appropriately charged inorganic clusters) to create a multilayer thin film, a polyanion–polycation multilayer. The molecular species of the cationic and the anionic components and the long-range physical order of the layers determine the resulting coating properties. It is important to notice that the polyanions and polycations overlap each other at the molecular level, and this produces a homogenous optical material [9]. The individual layer composition and thickness can be controlled and substrates may include metals, plastics, ceramics, and semiconductors (additional details concerning the deposition process can be found in the literature [9], [12], [16], [17], [18]).

Using the ESA technique under standard conditions several pH sensitive coatings based on fluorescence have been fabricated. Unfortunately, the photobleaching of the coatings fabricated under ESA standard conditions is so high that, although sensitive to pH, these coatings are useless for repetitive measurements. This work is focused on the study of some ESA fabrication parameters in order to minimize the photobleaching of the pH sensitive coatings. More specifically, the composition of the atmosphere of fabrication, a thermal curing process after the deposition or even the addition of an antifading agent to the polyelectrolyte multilayer structure are considered in order to enhance the response of these devices and minimize the photobleaching effect. To our knowledge this is the first time that this issue is studied in the literature.