Innovative utilization of molecular imprinting technology for selective adsorption and (photo)catalytic eradication of organic pollutants

Highlights

• The preparing process, recognition mechanism and common imprinted systems of molecular imprinting technology (MIT) are summarized.

• Insights of their specific binding affinities are demonstrated for removing target molecules.

• The innovative utilization of MIT in catalytic eradication of pollutants is overviewed.

• Perspectives in guiding the future exploitation on MIT for environmental protection are provided.

Abstract

The rapid development of industrialization and urbanization results in a numerous production of various organic chemicals to meet the increasing demand in high-quality life. During the synthesis and utilization of these chemical products, their residues unavoidably emerged in environments to severely threaten human’s health. It is thus urgent to exploit effective technology for readily removing the organic pollutants with high selectivity and good reusability. As one of the most promising approaches, molecular imprinting technology (MIT) employs a chemically synthetic route to construct artificial recognition sites in highly-crosslinked matrix with complementary cavity and functional groups to target species, which have been attracting more and more interest for environmental remediation, such as the selective adsorption/separation and improved catalytic degradation of pollutants. In this review, MIT is first introduced briefly to understand their preparing process, recognition mechanism and common imprinted systems. Then, their specific binding affinities are demonstrated for selectively adsorbing and removing target molecules with a large capacity. Furthermore, the innovative utilization of MIT in catalytic eradication of pollutants is comprehensively overviewed to emphasize their enhanced efficiency and improved performances, which are classified by the used catalytically-active nanocrystals and imprinted systems. After summarizing recent advances in these fields, some limitations are discussed and possible suggestions are given to guide the future exploitation on MIT for environmental protection.

Introduction

In modern world, industrialization and urbanization are rapidly developed to cause an exponential growth in population, which necessarily consumes more and more industrial chemicals for remaining/improving human’s life, such as pharmaceuticals, pesticides, dyes, organic compounds, etc (Huang et al., 2015; Shen et al., 2012). In the synthesized and used processes of the chemical products, large amounts of organic contaminants are unavoidably disposed into and emerged in natural environment to cause a toxicity to living organisms, and it is difficult or impossible to eliminate the toxicity via natural degradation (Xie et al., 2016). Due to their persistence in environment and high toxicity to human health, the excessive organic chemicals have recently become a major issue around the world (Wu et al., 2018). It is thus urgent to properly control and treat the persistent organic pollutants (POPs) for remediating environment to protect human beings against the pollutants’ toxicity (Bhatnagar and Anastopoulos, 2017; Rodriguez-Narvaez et al., 2017). In this field, various graphene-involved materials with unique physicochemical properties have exhibited superb performance in photocatalytic degradation of POPs through their efficient utilization of solar energy, including gel (Zhang et al., 2019), aerogels (Lu et al., 2018), and composite photocatalysts with organics, semiconductor, metal (Zhang et al., 2015a). In addition of the photocatalytic degradation, other kinds of treatment techniques have been demonstrated for effectively eliminating organic pollutants from wastewater, such as precipitation, coagulation, adsorption, biodegradation, and so on (Huang et al., 2015; Rodriguez-Narvaez et al., 2017; Ndunda and Mizaikoff, 2016). As indicated, these techniques are applicable for simultaneously removing various pollutants with high concentrations, however they are not efficient for specific pollutants at a low concentration in the complicated system due to their lack of selectivity and high affinity towards target molecules (Shen et al., 2012; Ndunda and Mizaikoff, 2016). With the emergence of more and more highly toxic pollutants in environment, the regular treatments are indeed difficult to address the current requirements for eliminating the highly toxic micropollutants. To this end, new functional materials with high selectivity need to be further exploited for significantly improving the performance of the existing treatment strategies (Huang et al., 2015).

Aiming at the urgent requirements on molecular selectivity, scientists have started to employ molecular imprinting technology (MIT) for chemically synthesizing molecular recognition components, which can specifically generate molecular cavity with high binding affinity and prominent selectivity towards target molecules (Wackerlig and Schirhagl, 2016). Moreover, the resultant molecular cavities are surrounded with highly-crosslinked materials to ensure their physical/chemical stability in harsh conditions and good reusability with cost-effectiveness. As shown in Fig. 1, the imprinted site is generated through cross-linking a complex of target molecule with functional monomers and subsequently removing the target molecule (i.e. molecular template) (Zimmerman and Lemcoff, 2004). Owing to the complementary shape, size and functional groups, the resultant site can specifically and strongly bind the used template molecules and its structural analogue, similar with natural recognition entity (e.g. antibody) (Chen et al., 2016). Based on the process of molecular imprinting, various material’s systems have been developed to provide a wealth of precursors for effectively imprinting diversified molecules of interest in synthetic polymers, macromolecules or inorganic xerogel (BelBruno, 2019). Benefiting from easy preparation and facile hybridization, molecularly imprinted materials with salient advantages have been widely employed as recognition components in various fields, including solid phase extraction, sensor and environmental remediation (Huang et al., 2015; Speltini et al., 2017; Yang et al., 2018).

Very significantly, molecularly imprinted materials have exhibited outstanding performances in selective adsorption/separation and enhanced catalytic degradation of organic pollutants, which will be more promising for environmental remediation after further combined with colloidal particles or other functional nanostructures as composite materials (Lofgreen and Ozin, 2014; Muratsugu et al., 2020; Shen et al., 2012). Typically, molecular imprinting on TiO₂ readily achieves the selective photocatalytic degradation of highly toxic organic pollutants with low concentrations to less toxic chemicals even to CO₂ and H₂O, which is a green and environmentally friendly solution for eliminating pollutants (Lai et al., 2018; Sajini et al., 2019; Sun et al., 2018; Suyana et al., 2017). In this review, MIT is specially focused on innovative utilization for selectively and rapidly adsorbing/removing target pollutants and photocatalytically degrading organic chemicals with enhanced efficiency and improved performances.