nanomaterials Review Experimental Advances in Nanoparticle-Driven Stabilization of Liquid-Crystalline Blue Phases and Twist-Grain Boundary Phases George Cordoyiannis 1,2, * , Marta Lavriˇ c 1 , Vasileios Tzitzios 3 , Maja Trˇ cek 1 , Ioannis Lelidis 4 , George Nounesis 5 , Samo Kralj 6 , Jan Thoen 7 and Zdravko Kutnjak 1   Citation: Cordoyiannis, G.; Lavriˇ c, M.; Tzitzios, V.; Trˇ cek, M.; Lelidis, I.; Nounesis, G.; Kralj, S.; Thoen, J.; Kutnjak, Z. Experimental Advances in Nanoparticle-Driven Stabilization of Liquid-Crystalline Blue Phases and Twist-Grain Boundary Phases. Nanomaterials 2021, 11, 2968. https:// doi.org/10.3390/nano11112968 Academic Editors: Domenico Lombardo and Angelina Angelova Received: 7 October 2021 Accepted: 3 November 2021 Published: 5 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Condensed Matter Physics Department, Jožef Stefan Institute, 1000 Ljubljana, Slovenia; marta.lavric@ijs.si (M.L.); maja.trcek@ijs.si (M.T.); zdravko.kutnjak@ijs.si (Z.K.) 2 Faculty of Mechanical Engineering, Czech Technical University in Prague, 16600 Prague 6, Czech Republic 3 Institute of Nanoscience and Nanotechnology, National Centre for Scientific Research “Demokritos”, Aghia Paraskevi, 15310 Athens, Greece; v.tzitzios@inn.demokritos.gr 4 Faculty of Physics, National and Kapodistrian University of Athens, Zografou, 15784 Athens, Greece; ilelidis@phys.uoa.gr 5 Institute of Nuclear and Radiological Sciences and Technology, National Centre for Scientific Research “Demokritos”, Aghia Paraskevi, 15310 Athens, Greece; nounesis@rrp.demokritos.gr 6 Faculty of Natural Sciences, University of Maribor, 2000 Maribor, Slovenia; samo.kralj@um.si 7 Department of Physics and Astronomy, KU Leuven, 3001 Leuven, Belgium; jan.thoen@kuleuven.be * Correspondence: georgios.kordogiannis@ijs.si Abstract: Recent advances in experimental studies of nanoparticle-driven stabilization of chiral liquid-crystalline phases are highlighted. The stabilization is achieved via the nanoparticles’ as- sembly in the defect lattices of the soft liquid-crystalline hosts. This is of significant importance for understanding the interactions of nanoparticles with topological defects and for envisioned techno- logical applications. We demonstrate that blue phases are stabilized and twist-grain boundary phases are induced by dispersing surface-functionalized CdSSe quantum dots, spherical Au nanoparticles, as well as MoS 2 nanoplatelets and reduced-graphene oxide nanosheets in chiral liquid crystals. Phase diagrams are shown based on calorimetric and optical measurements. Our findings related to the role of the nanoparticle core composition, size, shape, and surface coating on the stabilization effect are presented, followed by an overview of and comparison with other related studies in the literature. Moreover, the key points of the underlying mechanisms are summarized and prospects in the field are briefly discussed. Keywords: liquid crystals; nanoparticles; quantum dots; reduced-graphene oxide; calorimetry; microscopy; blue phases; twist-grain boundary phases; disclination lines; screw dislocations 1. Introduction Liquid crystals (LCs) are soft materials exhibiting many intermediate phases, the so-called mesophases, with structures in between the high-symmetry disordered liquid and the low-symmetry ordered crystal phases. Upon reducing the temperature, they undergo several symmetry-braking phase transitions and gradually acquire orientational and partial positional order. The first liquid-crystalline material, cholesteryl benzoate, was experimentally discovered by the Austrian botanist F. Reinitzer towards the end of the 19th century [1]. However, it was exactly one century ago when G. Friedel [2] contributed the nomenclature of the first liquid-crystalline mesophases (nematic, smectic) and about half a century ago when liquid crystals found their inroads into optical display applications [3]. LCs respond strongly to even weak perturbations due to their soft, fluid-like character. In the past, the feature mentioned above as well as the various types of transitions occurring between mesophases, led to the choice of LCs and nanoparticles (NPs) mixtures as model Nanomaterials 2021, 11, 2968. https://doi.org/10.3390/nano11112968 https://www.mdpi.com/journal/nanomaterials