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American scholars first proposed bubble nucleation electrochemical method to quantitatively detect surfactants.

2019-07-11 来源:亚科官网
Surfactant refers to a substance which is added in a small amount to cause a significant change in the interface state of the solution system. It can reduce surface tension, has many properties such as dispersing, wetting, penetrating, solubilizing, emulsifying, foaming, etc. It is a kind of fine chemical industry important product, and is known as “industrial MSG”. However, in actual use, a large amount of wastewater and waste residue containing surfactants are inevitably discharged into the water, soil and so on. Undegraded components may cause soil and water pollution, and even cause harm to human health.
Recently, Professor Luo Long of Wayne State University in the United States first proposed an electrochemical method based on bubble-nucleation for high-selectivity and high-sensitivity quantitative detection of surfactants. Because of the high surface activity of the surfactant, the surfactant concentration can be determined by observing its effect on the formation of bubbles on the surface of the electrode, and converting the change in the nucleation conditions of the bubble into an electrochemical signal. The method is convenient to operate and the device is simple. Perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA) were used as targets, and the detection limits were 80 μg/L and 30 μg/L, respectively. And the experimental results are consistent with the theoretical model results derived from classical nucleation theory.
According to the classical bubble nucleation theory, the energy barrier existing in the solution due to the formation of the gas-liquid interface during bubble formation requires the dissolved gas in the solution to reach a supersaturated state. In the presence of surfactant molecules, the surface tension of the gas-liquid interface is reduced, which helps to stabilize the gas core, thereby reducing the degree of supersaturation of the dissolved gas required for bubble nucleation. The authors hope to convert the change in the degree of supersaturation required for bubble nucleation into an electrochemical signal based on this principle to achieve high sensitivity and specific detection of surfactants. Based on previous work (Langmuir, 2013, 29, 11169), the authors found that surfactants have a high surface activity and can nucleate bubbles when the concentration of hydrogen in the solution is very low. The generation of electroreduction of a single hydrogen gas bubble at the platinum/gas/liquid interface can be monitored on the platinum nanodisk electrode by a decrease in diffusion limited current. During the potential scanning process, the current rapidly decreases when the bubble nucleates, and a current peak occurs. The authors consider the detection of this current peak to achieve quantitative detection of surfactants in water.
In order to verify the feasibility of the method, the authors used PFOS and PFOA as models. When the platinum nanoelectrode is in the negative sweep process, the current gradually increases to a peak and then decreases to a minimum, which is consistent with the process of nucleation and formation of bubbles on the electrode surface. Because the hydrogen gas forms bubbles on the surface of the electrode and adsorbs on the surface of the electrode as the hydrogen evolution reaction proceeds, the electrical resistance is greatly increased. Among them, the degree of supersaturation required for nucleation of hydrogen bubbles in the solution is proportional to the peak current.
In addition, the authors also studied the effects of perfluoroinic carboxylic acids (PFCA) with different carbon chain lengths on the results (n=3, 5, 6 and 7). The results show that the electrochemical method is very good. The PFCA surfactants with different chain lengths are distinguished and quantitative detection can be achieved. The author also uses classical nucleation theory to prove the result from theoretical calculations. The authors then demonstrated the selectivity of the assay by adding polyethylene glycol (PEG).
The gas nucleation and formation process in solution is a common phenomenon. In this work, the author skillfully combines it with electrochemistry to achieve selective and sensitive detection of surfactants in water. The linear range is more than three orders of magnitude, and the minimum detection limit is ∼30 μg/L, which is much higher than the detection limit of the inhibitory conductivity detection method (∼2 mg/L), which is worse than the liquid chromatography-mass spectrometry (0.5 μg/L). However, the method is simple in operation and inexpensive, and provides a new idea for detecting surfactants.