Demystifying the mechanism of spontaneous adsorption behavior of aromatic hydrocarbons on gold and silver colloidal nanoparticles

On July 11, 2022, Professor Steven E.J. Bell, a researcher at the School of Chemistry at Queen’s University (QUB) in the United Kingdom, and Professor Xu Xin from the School of Chemistry at Fudan University jointly published a joint publication in the journal Chem entitled “Uncovering strong π-metal interactions on Ag and Au nanosurfaces under ambient conditions via.” in-situ surface-enhanced Raman spectroscopy” important research results.

By using clean surfaced gold and silver nanoparticle interface assemblies and combined with surface enhanced Raman spectroscopy (SERS) and other means, the research group first revealed the spontaneous adsorption behavior of aromatic hydrocarbons and their derivatives on colloidal gold and silver nanoparticles, subverting the traditional understanding of the weak interaction between aromatic hydrocarbons and their derivatives and gold and silver nanoparticles for decades and the inability to spontaneously adsorb.

The first author of this work is Li Chunchun, postdoctoral researcher of QUB, chen zheng, postdoctoral researcher of Fudan University, the first corresponding author is Xu Yikai, researcher of QUB, and the first communication unit is Queen’s University Belfast (QUB), UK.

The interaction between aromatic hydrocarbons and their derivatives and metal nanoparticles radiates radiation in all aspects of nanotechnology. Specifically, the strength of π-metal interactions may directly affect the configuration, stability, and charge transfer characteristics of aromatic hydrocarbon-metal nanosystems. At present, aromatic hydrocarbons are known to have strong interactions with VIIIB group metals, and the corresponding research and application of π-metal interactions based on this type are also very extensive. In contrast, the research and application of aromatic hydrocarbons and IB group metals is very limited. Much of this stems from the traditional perception of the extremely weak π-metal interactions between IB group metals and aromatic hydrocarbon compounds. This has led to the current basic research on π-metal interactions in IB group metals around ideal or near-ideal systems (vacuum, solvent-free, extremely clean metal surfaces), while the practical application of π-metal interaction designs based on IB group metals has not yet been developed.

Figure 1: Schematic diagram of the experimental principle of nanoparticle assembly using surface clean oil-water interface combined with SERS to explore the π-metal interaction.

In 2019, Researcher Yikai Xu and Professor Steven E.J. Bell’s team reported the use of SERS to accidentally monitor the spontaneous adsorption of colloidal gold nanoparticles by aromatic hydrocarbon compounds (Analyst, 2019, 144, 448; inner cover). On the basis of this work, recently, the research group and the research group of Professor Xu Xin of Fudan University have confirmed and demonstrated for the first time that the IB group metals have an extremely considerable π-metal interaction and mechanism with aromatic hydrocarbons and their derivatives under real environmental conditions (normal temperature and pressure, the presence of solvents, free compounds and metal surface stabilizers). As shown in Figure 1, in the experiment, the QUB team first used the previously developed two-dimensional nanoparticle assembly (Nano Lett., 2016, 16, 5255) to enhance the substrate, combined with SERS, confirmed the universality of the π-metal interaction previously observed in the gold colloidal aqueous solution, and through a series of rigorous comparative tests, explored the relationship between its intensity of action and the solvent environment, metal surface stabilizer type, aromatic hydrocarbon π system size and other experimental conditions.

Figure 2: Schematic diagram of naphthalene molecules spontaneously adsorbing chloride-protected gold nanoparticles in aqueous solution.

Based on the above experiments, Professor Xu Xin’s research group of Fudan University used density functional theory (DFT) simulations to show that the experimentally observed π-IB metal interaction belongs to a dispersion force generated by instantaneous dipole. As shown in Figure 2, by introducing surface ligands and solvents into the computational model, the authors predicted at the atomic level that aromatic hydrocarbons were adsorbed on the surface of the IB metal by “interpolation” rather than “substitution”. This is due to the fact that the adsorption of the common anion stabilizer is accompanied by the accumulation of negative charges on the metal surface, which causes the stabilizer to reach a lower coverage level only on the metal surface, thus creating a favorable space for the adsorption of aromatic hydrocarbons.

Figure 3: Schematic diagram of (A) naphthalene interaction with silver colloidal nanoparticles of different oxidation degrees and SERS spectra. (B) Adsorption energy curves of oxygen atoms on gold and silver nanocolloidal particles covered with formic acid. (C) Adsorption energy curves of oxygen atoms on clean gold and silver nanoparticles. (D) Structure of the anticancer drug Pacificaxel, and SERS spectra of its interaction with colloidal gold and silver nanoparticles, respectively.

Although the surface chemistry of gold and silver has long been considered to be extremely similar, it is interesting that the QUB team has not previously observed strong π-metal interactions similar to gold nanoparticles in various types of colloidal silver nanoparticles (Analyst, 2019, 144, 448; inner cover). In this work, the authors also conducted a more in-depth and rational exploration of this phenomenon. As shown in Figure 3A-C, combined with experimental and theoretical simulations, the authors show that the spontaneous adsorption of IB metal by aromatic hydrocarbons is inhibited by the oxide layer on the surface of the nanoparticles, which takes less than 20 minutes to form in the newly formed colloidal silver nanoparticles, while not in the colloidal gold nanoparticles. Looking back at previous work using the new understanding of the π-IB metal interaction above, it is not difficult to find that the metal substrates used in the early adsorption studies of aromatic hydrocarbons and IB metals were silver or gold modified with strong adsorption groups, which led to the recognition that the π-IB metal interaction was extremely weak and could not be carried out spontaneously under conventional experimental conditions.

Finally, the authors use drug concentration monitoring as an example to demonstrate an application designed under real-world conditions based on π-IB metal interactions. As shown in Figure 3D, the molecular structure of the anticancer drug Pacificaxel lacks conventional chemical groups with strong adsorption of gold and silver nanoparticles, so in traditional cognition, the drug molecule is not easy to monitor with SERS. In fact, because of the presence of π-IB metal interactions, Paclitaxel can spontaneously adsorb to the surface of gold nanoparticles to generate a strong SERS signal. Correspondingly, SERS monitoring limits as low as 10-7 mol L-1 can be easily achieved using the most conventional colloidal gold nanoparticles. In contrast, because π-IB metal interactions in silver are inhibited by the oxide film on the surface of the nanoparticles, Paclitaxel adsorbs extremely weakly on the silver nanoparticles and cannot display the SERS signal.

The π-metal interaction forces of aromatic hydrocarbons and IB group metal nanomaterials have always been considered extremely weak and are therefore rarely taken into account in real-world systems. The above work breaks the deep-rooted cognition that π-IB metal interaction can be ignored, demonstrates the practicality and importance of π-IB metal interaction for rational design of functional nanomaterials, and provides a solid theoretical foundation for the rational design and efficient preparation of new π-IB metal nanosystems in the future.

This work has been supported by a number of projects such as the National Natural Science Foundation of China (21991130, 22103014), The Leverhulme Trust (ECF2020703), and the Royal Society of Chemistry (RSC) (RM1602-4142). (Source: Science Network)

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