**1. Introduction**

In 2010, a pioneering researcher Radhakrishnan developed the concept of "dip catalyst", and successively presented an Ag nanoparticle-embedded PVA thin film for the reduction of 4-nitrophenol by sodium borohydride [1]. Since then, the dip catalyst, which can switch the reaction on and off instantaneously, by merely dipping in and out of the reaction vessels, has drawn increasing attention for its ease of fabrication, excellent catalytic performance, convenient separation and reusability, and environmental friendliness [2]. To date, the dip catalyst has emerged as one of the most powerful tools in catalysis, synthetic methodology, materials science, and environmental science. A considerable number of dip catalysts have been well documented in the last dozen years, and the range of reported dip catalysts includes Pt@GS (dip coating) [3], CMC-Ni-BC (coating) [4], gold nanoparticleloaded filter paper (impregnated into the filter paper) [5], palladium nanoparticle-loaded cellulose paper (dip coating) [6], Pd@filter paper (dip coating) [7], Pt-PVA thin film (spin coating) [8], Pd-PVA (spin coating) [9], AgNPs@NH2-CP (chemical modification) [10],

**Citation:** Xu, Z.; Xiao, L.; Fan, X.; Lin, D.; Ma, L.; Nie, G.; Li, Y. Spray-Assisted Interfacial Polymerization to Form CuII/I@CMC-PANI Film: An Efficient Dip Catalyst for A<sup>3</sup> Reaction. *Nanomaterials* **2022**, *12*, 1641. https:// doi.org/10.3390/nano12101641

Academic Editor: Francisco Alonso

Received: 31 March 2022 Accepted: 2 May 2022 Published: 11 May 2022

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Cu@CS-FP (layer coating) [11], Ag/CH-FP (layer coating) [12], and so on. Common strategies for the fabrication of a dip catalyst previously included dip-coating [3–7] and spincoating technology [8,9], chemical modification of thin films with a variety of functional units [10], and layer coating over thin films with functional material [11,12]. However, these preparation procedures suffered from one or more drawbacks, such as tedious operation, being time consuming, involving multiple step reactions, and possessing low catalytic performance. Therefore, searching for novel and convenient protocols for the fabrication of a dip catalyst with excellent catalytic activity and reusability is still in grea<sup>t</sup> demand.

Copper is a low-toxicity, economical, sustainable, and readily available metal elemental, belonging to the [Ar]3d9 transition metal, with various oxidation states, such as Cu(0), Cu(I), Cu(II), and Cu(III). Depending on its unique properties and characteristics, copper can effectively catalyze various organic reactions, such as Suzuki–Miyaura crosscoupling reactions [13,14], azide–alkyne cycloadditions [15], and Ullmann-type coupling reactions [16,17]. Performing the transition metal-catalyzed three-component reactions of aldehydes, alkynes, and amines (commonly called A<sup>3</sup> reactions) in an atom-economic way is of importance, as it produces valuable N-containing heteroatom propargylamine products [18]. A grea<sup>t</sup> number of transition metal catalysts, including Cu, Ag, Au, and so on, have been exploited for the A<sup>3</sup> coupling process. Several reviews have outlined the latest progress in A<sup>3</sup> couplings well, and a large variety of copper catalysts have been criticized in these surveys [18–20]. However, there are no pertinent reports of a copperbased dip catalyst being applied to a one-pot A<sup>3</sup> coupling reaction for the synthesis of versatile propargylamines.

We envisaged that polymer thin films generated in situ by interfacial polymerization, initiated by metal salts and embedded metal catalysts simultaneously, would be a class of easily fabricated, efficient, and reusable dip catalysts. Interfacial polymerization is a process that utilizes the interfacial layer (liquid–liquid layer or liquid–gas layer, etc.) to provide a unique space to constrain the polymerization for the preparation of polymer films or membranes [21]. Polymer materials generated in situ by interfacial polymerization have multiple advantages, including mild production conditions, high molecular weight, excellent uniformity [21], and high permeation rates [22]. The interfacial polymerization method also permits the creation of films that are highly resistant to destruction by exposure to harsh environments [22]. These merits of the films prepared by interfacial polymerization make them beneficial to be a dip catalyst.

Polyaniline exhibits good affinity to metal ions, through the unique coordination with its nitrogen atoms [23] and delocalized π-π conjugate system [24]; thus, it is a useful metal supportive material to form PANI-based metal catalysts (Metal@PANI), such as Pd@PANI [25,26], Fe@PANI [27], and Ag@PANI [28]. However, as most developed catalysts were reported in the preparation of powder-based PANI hybrids, using PANI to form a film is difficult, due to its strong rigidity, even when using an advanced method, such as the interfacial polymerization method mentioned above. The traditional method for the preparation of PANI films is the solution casting technique, using various solvents, such as *m*-cresol solution [29], to dissolve PANIs. To develop a new approach, a template method was developed by using synthetic polymer film as a soft template for the polymerization of aniline monomer on its surface, and, thus, assisted the formation of PANI film [30]. Carboxymethylcellulose (CMC) is non-toxic, biodegrade, economic, and eco-friendly, and it bears a grea<sup>t</sup> number of carboxymethyl (–CH2COO−) and free hydroxyl (–OH) groups on its glucose-unit chain, providing it with a splendid capacity to coordinate with various metal cations [31], and to form H-bonds with molecules containing active groups, such as hydroxy-, amino-, and carboxylic groups.

Based on the properties of PANI and CMC, in our previous work, we fabricated CMC-PANI hybrids via the one-pot and one-step oxidative polymerization of aniline, with CMC as the soft template and CuSO4 as the initiator [32]. However, the CuSO4NPs@CMC/PANI hybrids were obtained as dark green powders, which are not appropriate for application in dip catalysis. Interfacial polymerization may be an alternative approach to address the

PANI shaping issue. Unfortunately, many attempts made on interfacial polymerization by various conventional methods, including pouring, dropping, immersing, and so on, failed to obtain the desired CMC-PANI hybrid film. It was reported that the occurrence of interfacial polymerization needs an interface between two phases to provide a unique reaction space [33,34]. However, the interfacial polymerization of aniline monomer onto the surface of the CMC-Na macromolecular chain is hard to perform using CuSO4 solution as an initiator, by conventional approaches, such as pouring, dropping, or immersing methods, as these two phases are both water soluble. Creatively, the spraying method was introduced into the process to solve this problem. By spraying copper sulfate solution onto the surface of the CMC and aniline mixture, aniline monomers polymerized immediately to rapidly form an extremely thin layer of CMC-PANI film. This thin layer served as an interface to provide a suitable space for further interfacial polymerization. Concerning the above facts, innovatively, we have attempted to fabricate carboxymethylcellulose–polyaniline film-supported copper catalyst (CuII/I@CMC-PANI) in situ via spray-assisted interfacial polymerization, and further explored it as a dip catalyst for A<sup>3</sup> reactions.
