Review Metal-Organic Framework-Based Stimuli-Responsive Polymers Menglian Wei 1,*, Yu Wan 2 and Xueji Zhang 1 1 Key Laboratory of Optoelectronic Devices and Systems, College of Physics and Optoeletronic Engineering, Shenzhen University, Shenzhen 518060, China; [email protected] 2 Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada; [email protected] * Correspondence: [email protected] Abstract: Metal-organic framework (MOF) based stimuli-responsive polymers (coordination poly- mers) exhibit reversible phase-transition behavior and demonstrate attractive properties that are capable of altering physical and/or chemical properties upon exposure to external stimuli, including pH, temperature, ions, etc., in a dynamic fashion. Thus, their conformational change can be imitated by the adsorption/desorption of target analytes (guest molecules), temperature or pressure changes, and electromagnetic field manipulation. MOF-based stimuli responsive polymers have received great attention due to their advanced optical properties and variety of applications. Herein, we summarized some recent progress on MOF-based stimuli-responsive polymers (SRPs) classified by physical and chemical responsiveness, including temperature, pressure, electricity, pH, metal ions, gases, alcohol and multi-targets. Keywords: coordination interaction; stimuli-responsive polymer; sensing 1. Introduction Citation: Wei, M.; Wan, Y.; Zhang, X. Metal-Organic Framework-Based Mother Nature has served as a great source of inspiration for the design and devel- Stimuli-Responsive Polymers. J. opment of intelligent materials. For example, the sunflower turns toward the direction of Compos. Sci. 2021, 5, 101. https:// sunlight, the Venus flytrap clamps immediately upon the touching of prey, and cephalopods doi.org/10.3390/jcs5040101 change their body pattern for camouflage [1]. Living creatures change their structures or behaviors adaptively according to the surrounding environment. Thus, by mimicking na- Academic Editor: ture, stimuli-responsive polymers (SRPs) are capable of adapting their physical or chemical Francesco Tornabene properties upon exposure to external stimuli, such as pH, temperature, electromagnetic field, humidity, analytes gradients, etc. [2]. SRPs have attracted significant attention in the Received: 2 March 2021 scientific community due to their novel properties and show wide applications, such as Accepted: 29 March 2021 sensing and biosensing [3], on demand drug or gene delivery [4], actuators [5], catalysts [6], Published: 7 April 2021 smart coatings [7], disease diagnoses and therapy [8], tissue engineering [9], etc. One category of SRPs is coordination polymer, namely metal-organic framework Publisher’s Note: MDPI stays neutral (MOF), that is formed by the linkage of metal entities and organic and inorganic lig- with regard to jurisdictional claims in ands [10]. Different from conventional stimuli responsive polymers in which the repeating published maps and institutional affil- unit (monomer) is connected through a covalent bond, MOF-based SRPs are constructed iations. via weak interactions, such as metal coordination, hydrophobic association, electrostatic interactions, van der Waals forces, etc. They have been widely used in sensing applica- tions due to their advanced optical properties, such as the intrinsic luminescent feature of sharp emission within a wide spectra range, long luminescence lifetime and large stokes Copyright: © 2021 by the authors. shift [11–13]. In addition, the features of flexible organic bonds, variable coordination Licensee MDPI, Basel, Switzerland. number/geometry and relatively weak metal-ligand interactions in MOFs make them great This article is an open access article candidates for sensing as their structure (or conformation) can be dynamically tuned by distributed under the terms and target analytes [10]. conditions of the Creative Commons In this review, we focused on the recent progress of MOF-based SRPs toward different Attribution (CC BY) license (https:// stimuli. First, the response mechanism/principles of MOF-based SRPs will be firstly creativecommons.org/licenses/by/ given, followed by a series of examples of MOFs in sensing applications. These examples 4.0/). J. Compos. Sci. 2021, 5, 101. https://doi.org/10.3390/jcs5040101 https://www.mdpi.com/journal/jcs J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 2 of 20 candidates for sensing as their structure (or conformation) can be dynamically tuned by target analytes [10]. J. Compos. Sci. 2021, 5, 101In this review, we focused on the recent progress of MOF-based SRPs toward differ- 2 of 19 ent stimuli. First, the response mechanism/principles of MOF-based SRPs will be firstly given, followed by a series of examples of MOFs in sensing applications. These examples were classified into three categories, including physical, chemical and multi-responsive were classified into three categories, including physical, chemical and multi-responsive SRPs. Specifically, physically responsive MOFs would change their properties according SRPs. Specifically, physically responsive MOFs would change their properties according to temperature, mechanical force and electricity. Chemically responsive MOFs, namely, to temperature, mechanical force and electricity. Chemically responsive MOFs, namely, would show responses to analytes, such as metal ions, anions, explosives, pH, gases, etc. would show responses to analytes, such as metal ions, anions, explosives, pH, gases, etc. The last categoryThe of MOFs last category is SRPs capable of MOFs of is altering SRPs capabletheir properties of altering under their multiple properties stim- under multiple uli. In addition, stimuli.there are In other addition, comprehensive there are reviews other comprehensive on the topic of reviewsMOFs in on a different the topic of MOFs in a perspective thatdifferent the readers perspective can refer that to [14–24]. the readers can refer to [14–24]. 2. Sensing Mechanism2. Sensing of MOF-Based Mechanism ofSRPs MOF-Based SRPs Generally, four Generally,different sensing four different principles sensing of MOF-based principles of SRP MOF-based sensors can SRP be sensors classi- can be classified fied based on thebased number on the of species number involved, of species including involved, metal including ions, metalligands, ions, auxiliary ligands, lig- auxiliary ligands ands and guest andmolecules guest molecules [16]. The direct [16]. The approach direct approach only involves only involvesmetal ions metal and ions ligands, and ligands, and the and the externalexternal stimuli stimuliimpact the impact coordinati the coordinationon environment environment of the metal of the ion metal and ion ligand, and ligand, leading leading to intrinsicto intrinsic optical opticalproperty property changes changes as shown as shown in Figure in Figure 1a. For1a. example, For example, the flu- the fluorescence of orescence of ZnZn coordinat coordinateded 9,10-Bis(pcarboxyphenyl) anthracene anthracene (BCPA) (BCPA) MOF MOF was was turned off upon the turned off uponpresence the presence of explosives, of explosives dinitrotoluene, dinitrotoluene (DNT) and(DNT) trinitrotoluene and trinitrotoluene (TNT) [25 ]. The second (TNT) [25]. Thetype second works type with works the with addition the a ofddition auxiliary of auxiliary ligands orligands metal or ions, metal which ions, compete for the which compete coordinationfor the coordination sites in MOFs sites in and MOFs play anand important play an important part in tuning part MOF in tuning properties, as shown MOF properties,in as Figure shown1b. in An Figure example 1b. An was example reported was by reported the Mao by group the Mao that group cerium that (Ce) coordinated cerium (Ce) coordinatedbi-ligands, bi-ligands, adenosine triphosphateadenosine triphosphate (ATP) and tris(hydroxymethyl)aminomethane (ATP) and tris(hydroxyme- (Tris), Ce- thyl)aminomethaneATP-Tris (Tris), can Ce-ATP-Tris serve as artificial can serv peroxidasee as artificial to detect peroxidase a low concentration to detect a low of H 2O2 selectively concentration ofdue H2 toO2the selectively fluorescent due property to the fluorescen switches att differentproperty states switches of Ce(III)/(IV) at different [26 ]. Introducing states of Ce(III)/(IV)guest [26]. molecules Introducing improves guest the selectivitymolecules towardimproves specific the selectivity targets, which toward are classified to the specific targets, thirdwhich cases are classified as shown to in the Figure third1c. cases Last butas shown not least, in Figure similar 1c. to theLast third but not case, the presence least, similar toof the guest third molecules case, the producespresence anof additionalguest molecules signal produces along with an that additional from the host, besides signal along withimproving that from the the selectivity. host, besides Thus, improving the target the breaks selectivity. the host–guest Thus, the interaction target and releases breaks the host–guestthe guest interaction molecules and into releas the environmentes the guest molecules and produces into secondthe environment signals, which ultimately and produces secondimproves signals, selectivity which andultimately reliability. improves selectivity and reliability. Figure 1. FourFigure different 1. Four sensing different mechanisms sensing mechanisms of metal-organic of metal framework-organic framework (MOF)-based (MOF)-based stimuli-responsive stim- polymer (SRP) sensors. (uli-responsivea) Direct sensing polymer approach (SRP) with