Advanced preparation technologies and strategies for molecularly imprinted materials

Abstract

Molecularly imprinted polymers (MIPs) prepared by molecular imprinting technology (MIT) are polymers with specific recognition sites matching the shape, size and functional groups of template molecules, which can selectively identify and enrich target analytes (template molecules), and have been widely used in sample pretreatment, chemical/biological sensing and other fields. However, in the processes of preparation and applications of MIPs, there are still some problems, such as difficult elution of template molecules, fewer effective recognition sites, low binding capacity, low mass transfer rate and poor recognition in aqueous media. As a multidisciplinary technology, MIT has developed rapidly by borrowing and integrating related advanced technologies/strategies of other fields. Consequently, a variety of new imprinting technologies and strategies have continuously emerged, which not only effectively solve the abovementioned problems but also push forwards the development of novel MIPs and widen their applications. In this paper, orienting the applications of MIPs in sample pretreatment, sensors and stimuli responses, some advanced preparation technologies and strategies for MIPs materials are highlighted, including ingenious imprinting technologies (surface imprinting, nanoimprinting; controlled/living polymerization, solid-phase synthesis, etc.), special imprinting strategies (multi-template/monomer imprinting, dummy imprinting, boronate affinity imprinting, etc.), and stimuli-responsive imprinting (magnetic, temperature, pH responsive, etc.). Fundamental features of the advanced imprinting technologies/strategies and their utilizations for MIPs preparations along with representative applications are described in details, involving important issues and research challenges. Firstly, a comprehensive overview of main imprinting technologies and strategies for MIPs preparation in sample pretreatment application is provided. In this regard, MIPs are used as selective adsorbents of various extraction technologies such as solid-phase extraction (SPE), dispersive SPE and magnetic SPE. Aiming at high selectivity and high adsorption capacity, the MIPs should have ideal morphology, uniform size and excellent surface properties. Besides conventional preparative methods, it is required to introduce new imprinting technologies and strategies, mainly including the ingenious imprinting technologies of surface imprinting, nanoimprinting, controlled/living free radical polymerization (CLRP), click chemistry, hollow porous polymer synthesis technology and solid-phase synthesis, and the special imprinting strategies of multi-template/monomer imprinting, dummy/segment imprinting, magnetic material and boronate affinity imprinting. Surface imprinting and nanoimprinting technologies are usually adopted by coupling with the abovementioned imprinting technologies and strategies. Secondly, advanced imprinting technologies and strategies for the construction of MIPs-based sensors are summarized. For the sensors, the MIPs as recognition elements can specifically bind target analytes and as transduction elements can generate output signals for detection. Typically, the output detection signals can be classified into three types, electrochemical, optical and piezoelectric types according to the transduction mechanism; molecular imprinting based electrochemistry, fluorescence and surface enhanced Raman scattering sensors are the research hotspots. For sensing applications, it is necessary to consider the main parameters such as response time, linear dynamic range, sensitivity, selectivity and reproducibility. Therefore, the MIPs should have excellent interface properties by employing appropriate preparative technologies and strategies. Nanoimprinting, surface imprinting and composite material imprinting strategy have become the preferences. Herein, constructions of molecular imprinting fluorescence sensors are emphatically introduced, especially ratiometric fluorescence ones. Thirdly, the imprinting technologies and strategies orienting stimuli-responsive application are briefly introduced, for preparing stimuli-responsive MIPs (SR-MIPs) with specific recognition ability toward targeted molecules under stimuli-regulation and thereby achieving intelligent control. SR-MIPs are able to sensitively respond to specific external physicochemical/biological stimuli with a considerable and reversible change in their properties, such as molecular chain structure, solubility, swelling or dissociation behavior, resulting in regular changes of imprinting properties. The most reported magnetic, temperature, photonic and pH sensitive SR-MIPs and their dual or multi stimuli responsive SR-MIPs are reviewed. The rapid development of smart ecofriendly SR-MIPs and their stimuli-responsive application will accelerate the drug delivery application and assist the widely carried out sample pretreatment and sensors applications. Lastly, future perspectives of MIT and MIPs are proposed. In order to solve the core issues of selectivity, mass transfer rate and adsorption capacity of MIPs, it is imperative to rationally combine various imprinting technologies and strategies. The ingenious fusion of MIT and various advanced technologies should be continuously strengthened to promote the preparation of MIPs materials.