Citation: Masoud, A.-R.; Alakija, F.; Perves Bappy,M.J.; Mills, P.A.S.; Mills, D.K. Metallizing the Surface of Halloysite Nanotubes—A Review. Coatings 2023, 13, 542. https:// doi.org/10.3390/coatings13030542 Academic Editors: Callum Arthur Charles Stewart and Pierre-Yves Collart-Dutilleul Received: 27 December 2022 Revised: 17 February 2023 Accepted: 21 February 2023 Published: 2 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). coatings Review Metallizing the Surface of Halloysite Nanotubes—A Review Abdul-Razak Masoud 1 , Femi Alakija 1 , Mohammad Jabed Perves Bappy 2 , Patrick A. S. Mills 3 and David K. Mills 1,2,3, * 1 Molecular Science and Nanotechnology Program, College of Applied & Natural Sciences, Louisiana Tech University, Ruston, LA 71272, USA 2 Nanosystems Engineering Program, College of Engineering and Sciences, Louisiana Tech University, Ruston, LA 71272, USA 3 School of Biological Sciences, Louisiana Tech University, Ruston, LA 71272, USA * Correspondence: dkmills@latech.edu; Tel.: +318-257-2640; Fax: +318-257-4574 Abstract: Halloysite nanotubes (HNTs) have been shown to be ideal nanoparticles for polymer reinforcement, sustained drug release, nano-reactor synthesis, toxic material removal, regenerative medicine, and as a substrate material for nanostructured coatings. Metal and metal oxide nanoparti- cles have been used for centuries in various medical applications, primarily for their antimicrobial, antifungal, and antiviral properties. The focus of this review is the metallization of HNT surfaces. Different technologies use specific metal compounds and multi-step chemical reactions to metalize the HNT surface. This review begins with a brief overview of the current methods for metallizing the HNT surface. Our focus then provides a detailed study on specific applications of metal-coated HNTs (mHNTs) in the field of nanomedicine. The focus is on using mHNTs and Mhnt polymer composites in anti-infective therapy, immunotherapy, dentistry, regenerative medicine, and wound healing. The importance of HNTs in aerospace, defense, and industry has emerged, and the application poten- tial and enormous market value for metal oxide nanoparticles is apparent. The commercialization potential of metal-coated HNTs is also discussed. Keywords: coatings; halloysite; functionalization; metal nanoparticles; nanomedicine 1. Introduction 1.1. Structure and Properties of Halloysite Halloysite nanotubes (HNTs) exist worldwide and are mined from various mineral deposits, making them an easily accessible nanomaterial [1]. These clay nanotubes are struc- tured as a two-layered aluminosilicate, have predominantly hollow nanotubular structures in the submicron range, and are chemically identical to kaolin clay [1,2]. HNTs typically display an inner diameter between 15–50 nm and a length between 100–2000 nanometers (Figure 1). HNTs have a large surface area and can be loaded and coated with various materials, such as drugs, metals, and biomacromolecules [2,3]. A wide range of active agents, including antibiotics, cancer drugs, marine biocides, and biological molecules, can be entrapped within the inner lumen and void spaces within the aluminosilicate shells [3,4]. 1.2. Surface Modification of Halloysite Due to their tubular microstructure, biocompatibility, high mechanical strength, ther- mal stability, and potential for surface modification, HNTs have seen use as additives to inorganic/polymeric hybrid materials, including biofunctional composites, drug carriers, high-performance catalysts, nanoreactors, tissue engineering substrates, bone implants, and high-efficiency adsorbents [5–7] (Figure 2). Coatings 2023, 13, 542. https://doi.org/10.3390/coatings13030542 https://www.mdpi.com/journal/coatings