Detection of Triclosan in Tuned Solutions by pH and Ionic Strength Using PAH/PAZO Thin Films ^†

Abstract

The electronic tongue concept based on layer-by-layer (LbL) films can be used to the detection in water of triclosan (TCS), a pernicious molecule used in personal care products and widely released in the environment. In this work, we analyzed the adsorption of TCS on poly(allylaminehydrochloride) (PAH) and poly[1-[4-(3-carboxy-4hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) layers of PAH/PAZO LbL films. We demonstrate that the adsorbed amount is strongly dependent of pH, the efficiency of adsorption of TCS on PAH layer is higher, and, when PAZO is the outmost layer, the electrical parameters can discriminate the ionic strength on solutions of TCS.

1. Introduction

Water is an essential and indispensable good to human health and nature’s wellbeing, and therefore it is of the utmost importance to preserve and maintain its quality and cleanliness. That is why the detection and monitorization of triclosan in water bodies, such as rivers, groundwater, as well as soils, remains as a paramount necessity due to it being present in a multitude of day-to-day products (dubbed PPCPs—pharmaceuticals and personal care products) and displaying fairly high levels of toxicity [1,2,3,4,5]. Therefore, and also related to the non-regulation of this compound, there is a growing concern about the presence and impact of PPCPs, as well as the need for reliable and effective water monitoring using sensors capable of detecting the target molecules in complex media [6]. One of the main challenges in sensors’ monitoring is the ensure analyte effective detection in environmental complex matrices that contain countless spurious molecules (potential interfering compounds), as well as microscopic life. These molecules, such as salts or even fluctuations of pH can jeopardize the analysis of a target molecule.

The purpose of this study was to assess the sensitivity of a sensor based on the electronic tongue concept [7] composed by an array of thin films prepared by layer-by-layer (LbL) technique [6]. In the present case, LbL films of poly(allylamine hydrochloride) (PAH) and poly[1-[4-(3-carboxy-4hydroxyphenylazo) benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) (PAH/PAZO) were prepared onto quartz supports or gold interdigitated electrode deposited on glass (DropSens) by alternated adsorbed layers using 10⁻² M aqueous solutions of these polyelectrolytes. In the present case, thin-films with 10 bilayers of PAH/PAZO ((PAH/PAZO)₁₀ and (PAH/PAZO)₁₀/PAH) were prepared. The amount of TCS adsorbed on these films was measured by ultraviolet-visible (UV-vis) spectroscopy using a UV 2101 PC Scanning Spectrophotometer while the characterization of electrical measurements when these films are immersed in 10⁻⁵ M aqueous solutions of TCS prepared with different salt concentrations was done using a Solartron 1260 Impedance Analyzer (frequency range of [1–1M] Hz, AC voltage of 25 mV).

2. Results

Figure 1a,b presents the absorbances at 232 nm and 280 nm of reused sensors based on (PAH/PAZO)₁₀ and (PAH/PAZO)₁₀/PAH LbL films after immersion in 10⁻⁴ M TCS solutions with decreasing pH of 9.04, 8.04, 7.0, 6.13, 4.68, 3.70 and 2.30. These absorbances were calculated by subtracting the UV-Vis spectrum after immersion from the initial one of each thin film.

From data presented in Figure 1 it is possible to observe that the adsorbed amounts of TCS, proportional to measured absorbances, are higher for all pHs when the TCS is adsorbed on (PAH/PAZO)₁₀/PAH films. Despite presenting different values of absorbances, the adsorption behaviour of TCS in both thin films is similar confirming that we are analyzing the same adsorbed amount. Starting at alkaline pH, as pH decreases the absorbances tend to increase until pH 6.13. Then, as pH decreases the absorbances tend to decrease. However, at the lowest pH of 2.30, the (PAH/PAZO)₁₀ presents a slight increment of absorbances. At acidic pH (lower than 6.13), the absorbance tends to decrease due to desorption phenomena, particularly of PAH/PAZO thin-films. Adsorption studies of PAZO onto PAH/PAZO LbL films revealed that the electrostatic forces, governing the interactions between PAH and PAZO layers, are strongly dependent on solutions pH [8]. It has been reported that the adsorbed amount of PAZO decreases as the pH of PAH aqueous solutions become more acidic, namely at pH 4 [9]. It is also important to note that at pH 2.30, the absorbance of (PAH/PAZO)₁₀ thin film revealed a slight increment. This fact is justified by a shift in the absorption bands of PAZO at acidic pH [10]. In fact, we also measured the spectra of 10⁻⁴ M PAZO solutions at pH 2.5 and pH 6, data not shown here, and an increase of absorbance, namely at 232 nm, was observed as well as a shift on the PAZO peak position which proves that the increase of absorbance at pH 2.30 is not related to adsorption of TCS.

The pH and ionic strength of the TCS solutions can also influence the electrical properties measured when these films are immersed in these solutions, namely, the ionic strength affects the impedance spectra at higher frequencies. In order to conduct this study, seven solutions were prepared, each with different concentrations of NaCl: 0 M, 10⁻³ M, 10^−2.5 M, 10⁻² M, 10^−1.5 M, 10⁻¹ M and 0.5 M. Figure 2a,b illustrate the principle components analysis (PCA) [11] of this study.

The obtained PCA plots, Figure 2a,b, demonstrate that the impedance spectra data obtained with both type of films allow the distinction between different salt concentrations. However, the one obtained with (PAH/PAZO)₁₀ LbL films has succeeded in distinguishing of the NaCl concentration, displaying a sequential-like pattern. For example, considering the obtained values of PC1, if one plots its values versus NaCl concentration, a function is obtained.

3. Conclusions

This study allowed to conclude that TCS adsorbs on both (PAH/PAZO)₁₀ and (PAH/PAZO)₁₀/ PAH LbL films but there is an adsorption increase of TCS when the outmost layer is positive. We also verified that the adsorbed amount of TCS increases when the pH decreases attaining a maximum and followed by a decrease as pH also decreases. We proved that this last decrease is not related to the decrease of TCS on the surface but to the change of PAZO spectra for lower pH. We also demonstrated that although pH and ionic strength have a strong influence in the impedance spectra of these films when immersed in solutions, one can find patterns that allow to distinguish between different ionic strengths. Interestingly, when we consider (PAH/PAZO)₁₀ films one can attain a pattern in which the values of PC1 follow a function when plotted versus the salt concentration. Finally, it should be referred that the characterization of adsorption variables are fundamental in the choice of the adequate LbL films that can be used in the development of electronic tongue to detect, in the present case, TCS.