Advances in Science

Advances in Science Volume 23 Number 1 June 2018

A special issue on Green & Sustainable Chemistry IFEATURES

Covalent organic frameworks are amazingChemistry – clean and leanTransformations using carbon materialsHigh performance pollutant detectors

Table of Contents

Advances in ScienceThe Faculty of Science conducts basic and applied experimental, theoretical and simulation research over a broad spectrum of science, mathematics and technology domains. We cover most of the key fields in biological sciences, chemistry, physics, pharmacy, mathematics and statistics.

Advances in Science is published online twice a year. It is written for a broad scientific audience interested to keep up with some of the key areas of science pioneered by researchers at the Faculty of Science.

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For the latest research news, please refer to:URL: www.science.nus.edu.sg/research/research-news

RESEARCH FEATURES2 Covalent organic frameworks are amazing4 Chemistry – clean and lean6 Transformations using carbon materials8 High performance pollutant detectors

NEWS ROUNDUP10 Effects of enzymes on selected fruit-based products10 Nanoscaled delivery for skin care applications

ADVANCES IN SCIENCE | VOLUME 23 | NUMBER 1 | JUNE 2018 1

On the cover: Green chemistry reduces environmental impact by developing scientific expertise that eliminates the use or generation of hazardous substances in the design, manufacture and application of products for sustainable development.

Covalent organic frameworks are amazingDesigning tailor-made polymers for advanced materials applications

Introduction

Chemistry is the central science that enables the design and synthesis of new molecules. It also explains the molecular origins of various physicochemical properties and functions at different structural levels and time scales. Creating molecular structures at both primary (single layer) and high-order levels (multi-layers) in a pre-designable and/ or tailor-made way is an ultimate goal of chemists. However, the covalent linking of organic molecules to form synthetic macromolecules with well-defined and precise primary and high-order structures, similar to those produced in biological polymers, such as protein, enzyme, DNA and RNA systems, is a bone-breaking task. Despite great advances in synthetic chemistry over the past 100 years, organic polymers are seldom designed and synthesised with ordering (periodic structure) at both primary and high-order structural levels. Highly ordered polymers are key to unique functions, as demonstrated by many artificial and biological systems.

Biological polymerisation systems involved in DNA, RNA and other proteins combine covalent bonds and non-covalent interactions to produce well-defined structures. In these structures, the covalent bonds determine the sequence of the primary-order chain structure, while the non-covalent forces are responsible for the high-order three-dimensional morphology. These coded reaction systems enable the sequential connection of building blocks onto covalently linked yet ordered chains, in which specific sites or segments are programmed and readily organised into high-order structures.

Although synthetic polymerisation systems can produce sequenced chain

propagation (a fixed pattern that is continuously regenerated during the course of a chemical reaction), it is almost impossible to create specific sites or segments that can direct hierarchical organisation (self-assembled structure with orderings from primary-order to high-order levels) in a polymer through non-covalent interactions. Pre-designable or programmable high-order structure formation is too difficult to achieve in real-world polymerisation systems. This situation can become worse when the polymerisation system involves the formation of more complex networks (e.g. amorphous porous polymers for carbon dioxide capture).

The combination of covalent bonds and non-covalent interactions in a synthetic polymer system to generate primary-order structures with specific sites or segments that are ready for guiding the formation of high-order structures seems to hold the key to developing such polymers.

Designing and preparing materials with a fun “Lego” element

Our basic approach to highly ordered polymers is based on confining the chain growth over a two-dimensional (2D) plane, in which the polymer skeleton is propagated and ordered [1]. The 2D polymers with extended polygon skeletons (network of polygons) stack to form layered frameworks, enabling the production of Covalent Organic Frameworks (COFs) that are well-defined in both the primary and high-order structure (Figure 1). A fundamental design strategy is to use a topology diagram that guides the connections of the building blocks into 2D polymers and COFs with a specific polygon topology based on how the blocks are combined (Figure 2). For each combination, both the polymer skeletons and polygon pores are pre-designed, with the scaffolds, pore size and pore shape determined solely by the building blocks. In this way, designing new 2D polymers and COFs is similar to “Lego” construction using blocks.


Figure 1: The structural hierarchy of two-dimensional covalent polymers and COFs.

RESEARCH FEATURES

Donglin JIANG is a Professor with the Department of Chemistry, NUS. He received his Ph.D. degree from The University of Tokyo in 1998. He was appointed Assistant Professor at The University of Tokyo (1998-2000) and group leader of JST ERATO AIDA Nanospace project (2000-2005). He was an Associate Professor at the Institute for Molecular Science, National Institute of Natural Sciences (2005-2015), where he started research on 2D polymers and COFs. Prior to joining NUS, he was Professor at Japan Advanced Institute of Science and Technology (2016-2018).

References[1] Huang N; Wang P; Jiang D, “Covalent organic frameworks: A materials platform for structural and functional designs”, NATURE REVIEW MATERIALS, Volume 1 Article Number 16018 DOI: 10.1038/natrevmats.2016.68 Published in 2016.

[2] Xu H; Tao S; Jiang D, “Proton conductions in crystalline and porous covalent organic frameworks”, NATURE MATERIALS Volume 15 Pages722-727 DOI: 10.1038/NMAT4461 Published: 2016.

[3] Jin E; Asada M; Xu Q; Dalapati S; Addicoat MA; Brady MA; Xu H; Nakamura T; Heine T; Chen Q; Jiang D, “Two-dimensional sp2 carbon-conjugated covalent organic frameworks”, SCIENCE Volume 357 Pages 673-676 DOI: 10.1126/science.aan0202 Published: 2017.

COFs constitute ordered columnar arrays of building units and one-dimensional (1D) channels. These two structural features are unique to COFs and are distinct from other polymers and porous materials. They also form the basis of the structural design and functional exploration of COFs.

To produce COFs with ordered structures, we explored a variety of polycondensation reactions under solvothermal conditions (reaction process involving reactants dissolved in a solvent at relatively high temperatures). The objective was to develop reactions having certain levels of reversibility so that the system has the capability to self-heal mismatched structures. Using the topology diagram, we synthesised a broad variety of 2D polymers and COFs that are pre-designed at the primary and high-order levels (Figure 3).

A storehouse of functional design and application

The 2D polymers and COFs have unique structures of ordered π arrays and aligned 1D channels, which can find use in many applications. We focused on exploring unique functions and properties that are inherent to the COF architectures. By understanding the interactions between these well-defined frameworks and photons, excitons, electrical charges, magnetic spins and other molecules, we discovered many potential applications. These include p-type, n-type and ambipolar-type semiconductors, photoconductors, ion conductors [2], light emitters and biosensors, photocatalysts for hydrogen evolution, organocatalysts,

electrochemical catalysts for fuel cells, capacitors, batteries, carbon dioxide capture and separation, radioactive pollutant absorbers and, very recently, size sieving and spin coherence transition materials [3].

With their unique pre-designable characteristics coupled with the accessibility of topologies, the diversity

of building blocks and the availability of molecular linkages, 2D polymers and COFs represent a highly flexible materials development platform. This platform can enable innovations in materials development that can tackle challenging energy and environmental issues. My group endeavours to make and lead these breakthroughs to benefit society.


Figure 2: The “Lego”-like principle for designing two-dimensional covalent polymers and COFs.

Figure 3: Typical examples of COFs designed and synthesised by our group.

Chemistry – clean and leanHeterogeneous catalysts for environmentally friendly ways of producing chemical compounds

Introduction

The term catalysis has been known since 1835 when it was coined by Swedish chemist Jöns Jacob BERZELIUS following his review of observations made by other scientists on chemical changes in both homogeneous and heterogeneous systems. A number of reactions took place in the presence of a substance that remained unaffected. Berzelius attributed the chemical transformations to a “catalytic force” and came up with the term “catalysis” for the decomposition of bodies by this force.

Heterogeneous catalysts offer several advantages, including ease of separation from the reaction medium and recyclability. These are properties that are intrinsically aligned with green chemistry, which aims at reducing or eliminating the use and generation of hazardous substances by the invention, design and application of chemical products and processes. In our laboratory, we work on heterogeneous catalysts and the synthesis of materials for applications such as sorption, ion-exchange, catalyst supports, etc.

Catalysts for biomass conversion

The threat of future energy shortage and the depletion of fossil fuels has made utilisation of biomass (an energy source from organic matter) especially attractive to researchers and manufacturers. In a biorefinery, biofuels and value-added chemicals are produced using renewable bio-feedstock (biological material that can be used directly as a fuel, or converted to another form of energy product). Levulinic acid derived from various types of biomass can be hydrogenated to give γ-valerolactone (GVL), a liquid which can be used for fuel and to develop high value carbon-based chemicals. Our laboratory

has developed “zirconium zeolite beta” (Zr-Beta, Si/Zr 100), which is an excellent catalyst for Meerwein-Ponndorf-Verley (MPV) reactions. In MPV reactions, a secondary alcohol is used as the hydrogen donor instead of gaseous hydrogen. When Zr-Beta was used as a catalyst in the reduction of levulinic acid to GVL, it proved to be robust and highly efficient (Figure 1, [1]).

Quantitative conversion with more than 99% yield of GVL has been obtained with a steady generation rate of 0.46 molGVLgZr-1h-1 for up to 87 hours. The sustained productivity using Zr-Beta compares very well with reported values of 0.09 to 0.36 molGVLgZr-1h-1 for metal-based catalysts in the vapour phase. Thus, Zr-Beta offers an attractive alternative to precious metal catalysts for GVL formation from levulinic acid. As it has high thermal stability, Zr-Beta could be easily recalcined and reused.

Photocatalysts in organic synthesis

Photocatalysis involves the use of light instead of thermal energy to drive chemical reactions. It has drawn much attention in areas such as water remediation and water splitting. However, applications in organic synthesis are sparse. One example is the synthesis of imines, a class of important synthetic intermediates for pharmaceuticals and biologically active nitrogen containing organic compounds. Traditionally, the synthesis of secondary imines involves the condensation (a reaction which results in the release of a water molecule) of primary amines with carbonyl compounds (mostly aldehydes). A number of photocatalysts such as mesoporous carbon nitride (C3N4) and cadmium sulfide (CdS) have been reported, but they require pure oxygen at high pressure. Others such

as gold/ titanium dioxide (Au/TiO2) and bismuth vanadate (BiVO4) together with a copper complex can achieve 99% selectivity using atmospheric air as the oxygen source. However, the use of gold adds to materials cost while copper complex is non-reusable and also poses problems with product isolation.

Through in-depth studies on the influence of halides and facets, bismuth oxybromide (BiOBr) with (110) facets was found to exhibit the highest activity with 100% yield for the selective oxidation of imines [2]. This photocatalyst, which does not contain any precious metal, is able to achieve good performance under visible light using oxygen from atmospheric air. Scanning electron micrographs showed that BiOBr with (001) and (010) facets, prepared using water as a solvent, formed platelets (Figure 2). In contrast, BiOBr-(110) prepared by the solvothermal method is made of nanosheets aggregated into microspheres of two to seven micrometres in size. The latter shows a much higher absorption of light compared to BiOBr-(010) or BiOBr-(001) (Figure 3). This may be attributed to the higher surface area and unique hierarchical structure


Figure 1: Transformation of levulinic acid to γ-valerolactone (GVL) [1].

RESEARCH FEATURES

CHUAH Gaik Khuan is an Associate Professor with the Department of Chemistry, NUS. She received her Ph.D. from Texas A&M University in 1987. Her research interests are in heterogeneous catalysts and materials. Her group studies various catalytic reactions that are important in the production of fine chemicals and in environmental chemistry. Of particular interest are cascade reactions using catalysts with multifunctional groups, heteroatom-containing zeolites and zirconium compounds.

References[1] Wang J; Jaenicke S; Chuah GK, “Zirconium-Beta zeolite as a robust catalyst for the transformation of levulinic acid to γ-valerolactone via Meerwein-Ponndorf-Verley reduction” RSC ADVANCES Volume: 4 Issue: 26 Pages: 13481-13489 DOI: 10.1039/c4ra01120a Published: 2014.

[2] Han AJ; Zhang HW; Chuah GK; Jaenicke S, “Influence of the halide and exposed facets on the visible-light photoactivity of bismuth oxyhalides for selective aerobic oxidation of primary amines” APPLIED CATALYSIS B-ENVIRONMENTAL Volume: 219 Pages: 269-275 DOI: 10.1016/j.apcatb.2017.07.050 Published: 2017.

[3] Cheng Y; Wang XD; Jaenicke S; Chuah GK, “Minimalistic liquid-assisted route to highly crystalline alpha-zirconium phosphate” CHEMSUSCHEM Volume: 10 Issue: 16 Pages: 3235-3242 DOI: 10.1002/cssc.201700885 Published: 2017.

of the BiOBr-(110), which facilitates multiple light reflections. The present work shows the potential of BiOBr as a photocatalyst, offering an economical, sustainable and green process for the synthesis of imines.

A minimalistic approach to materials synthesis

Zirconium phosphates (ZrP) find many uses in various applications, such as cation exchangers in the treatment of nuclear wastes, acid catalysts and catalyst supports, intercalation host for drugs or other molecules with desired performance, fast ion conductors and in chromatography. It can exist either as an amorphous gel or in crystalline

form. Crystalline ZrP is preferred when used as a catalyst because it has more reproducible performance. The positioning of the phosphate groups in the ZrP crystal lattice depends on the degree of crystallinity. With lower crystallinity (majority in amorphous form), more phosphate groups are irregularly positioned in the crystal lattice, resulting in cavities with a range of sizes. The crystalline compound, in contrast, has more uniform cavities which give a more consistent catalytic outcome.

Amorphous ZrP is easily obtained by precipitation of zirconium salts with phosphates or phosphoric acid. In contrast, to form crystalline ZrP, a

refluxing or hydrothermal treatment lasting for days to weeks is required. Alternatively, crystalline ZrP can be obtained by direct precipitation in the presence of complexing agents such as hydrofluoric acid or oxalic acid. However, this process is difficult to scale up and it generates large amounts of waste which require proper treatment and disposal.

Our group has developed a new approach for preparing crystalline ZrP having different phases (Figure 4, [3]). We term this a “minimal solvent” synthesis as the only solvent involved is water from the reaction system. This method using only zirconium oxychloride and concentrated phosphoric acid requires only light stirring followed by heating. The chemical transformation to the final products occurs within hours. Using this method, we are able to easily synthesise bulk quantities of crystalline ZrP for various applications.


Figure 2: Crystal structure of the exposed facets of BiOBr-(001), BiOBr-(010) and BiOBr-(110).

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Figure 3: Scanning electron microscopy images of (a) BiOBr-(001) (b) BiOBr-(010) (c) BiOBr-(110) and (d) light absorption by BiOBr with different facets.

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Figure 4: Different phases of crystalline zirconium phosphate (a) a-ZrP and (b) γ-ZrP.

Transformations using carbon materialsTwo-dimensional carbon-derived materials can provide extraordinary performance in heterogeneous catalysis and energy storage devices

Graphene catalyst for organic transformations

Metal catalysts often play an indispensable role in the chemical production industry. Despite many decades of optimisation, metal-based heterogeneous catalysts still suffer from two key limitations: (i) they often require resource-scarce metals such as platinum, which raises the cost for large-scale manufacturing, and (ii) even trace amounts of metal catalysts leaching into the reactant stream compromise product purity, especially in pharmaceutical applications.

The development of carbon-based materials as metal-free catalysts for carbon-carbon (C-C) cross-coupling reactions (carbocatalysis) can enable the catalysis process to be operated under mild processing conditions. This is highly attractive because it bypasses the usage of costly or toxic metal catalysts which are often operated at highly pressurised oxidative conditions. The advantage of using pure carbon material is that it can be easily recovered from the reactant system. It is also highly insoluble in many liquids and thus will not contaminate the reactant stream. Carbocatalysis has become increasingly attractive in synthetic chemistry due to its potential to replace the use of noble metal catalysts. The use of graphene and its functionalised derivatives as carbocatalysts is of increasing interest as solution-processed graphene has entered the first phase of commercial production and the cost price of graphene is getting lower (less than US$10 per kilo).

Although the oxidised form of graphene (e.g. graphene oxide (GO)) has been widely used in various catalytic reactions, the range of feasible reactions reported by researchers is quite narrow. Also,

the relationships between the type of functional groups present on the reactant and the specific activity of the GO catalyst are not well understood. In our recent work, the effectiveness of GO as a catalyst for coupling (reacting) various electron-rich arenes (aromatic hydrocarbons) with xanthene was assessed [1]. A series of electron-rich arenes, such as meta- and para-dimethoxy-benzene and anisole were reacted with xanthene to produce the corresponding coupling products with isolated yields in the range of 45% to 74%. Acidic 2,6-xylenol and o-cresol can be smoothly coupled with xanthene to produce the corresponding products, with 80% and 52% yields, respectively. Our protocol was extended to the

direct coupling of xanthene with heteroaromatic rings. High yields with excellent regioselectivity (preference for a certain direction of chemical bond) can be achieved in the direct CH-CH (carbon-hydrogen to carbon-hydrogen) coupling of thioxanthene with a variety of electron-rich arenes including heteroaromatic compounds (Figure 1).

Our mechanistic investigations revealed a surprising finding. GO contains oxygen functional groups on its surface. When they are removed by heating to high temperatures, the catalytic efficiency only decreased marginally from 85% to 68% at the fifth run for these classes of reactions. The outcome suggests that


Figure 1: The reaction was carried out with xanthene or thioxanthene (0.5 mmol), arene (1 mmol), TsOH·H2O (20 mol%) and catalyst (20 mg) which were stirred at 100°C under solvent-free conditions. [a]Reaction at 60oC and [b] at 75oC.

RESEARCH FEATURES

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LOH Kian Ping is Provost’s Chair Professor with the Department of Chemistry, NUS. He received his B.Sc. (Hons) in chemistry from NUS (1994) and Ph.D. from Oxford University (1996). After his postdoctoral work at the National Institute for Material Science at Tsukuba, Japan, he joined NUS in 1998. He became an Associate Professor in 2005 and Professor in 2012.

Reference

[1] Wu H; Su C; Tandiana R; Liu C; Qiu C; Bao Y; Wu J; Xu Y; Lu J; Fan D; Loh KP, “Graphene oxide catalyzed direct CH−CH type cross-coupling: The intrinsic catalytic activities of zig-zag edges”, ANGEWANDTE CHEMIE INTERNATIONAL EDITION DOI: 10.1002/ange. 201802548 Published: 2018.

the oxygen functional groups on the GO were not essential for the catalysis process, and they were consumed as stoichiometric reactants during the chemical reactions. This explains the slight drop in catalytic efficiency when the GO catalyst was reused in subsequent cycles. Using scanning tunnelling microscopy and small molecule analogue studies, we found that the presence of zigzag edges around the pores of the GO plays a key role in the catalysis process. Theoretical calculations suggest that, due to the localised p states in the zigzag edges and their closeness to the Fermi level, these zigzag edges function like radicals and are potentially active catalytic sites. This study provided design guidelines for the microstructure of carbocatalysts to achieve effective C-C coupling reactions.

Wood-derived catalyst for energy conversion

An electrocatalyst is a type of catalyst used to promote electrochemical reactions. It has a highly accessible nanostructure with hierarchical pores that is ideal for maximising the exposure of the active sites on the catalyst to reactants. Wood fibres are a suitable natural material for this as they have an abundant hierarchically porous structure. They also have strong mechanical properties and are low cost.

Recently, my team has developed an approach to create nitrogen-doped hierarchically porous carbon from bulk raw wood material as a metal-free catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). By selectively carving the

celluloses using enzymes and keeping the cross-linked networks intact, we can produce a self-supporting, mechanically strong three-dimensional catalyst using biomass (Figure 2(a)). Using pyrolysis treatment (charring of the wood) with ammonium chloride (NH4Cl), additional nitrogen atoms can be incorporated into the catalyst, improving its ORR and OER activity in an alkaline electrolyte. This wood-derived catalyst can provide a low-cost and highly efficient cathode for energy conversion devices.

As a demonstration, an aqueous rechargeable zinc-air (Zn-air) battery was assembled using this wood-derived catalyst as the cathode (Figure 2(b)). The battery has an open circuit voltage of about 1.49 V together with a peak power density of 49.9 mW cm−2 at 0.7 V. The discharge profile showed

a specific capacity of 801 mA h g-1 at 10 mA cm−2, corresponding to an energy density of 955 W h kg−1. Both of these values are better than most of the recently reported Zn-air batteries using doped carbon catalysts.

The Zn-air battery was also cycled at 10 mA cm−2 for a total of 40 hours to examine its stability. Negligible voltage change was observed on both the charge and discharge segments (Figure 2(c)). A portable Zn-air battery demo unit was used to power a small toy car and it operated for more than 270 minutes (Figure 2(d)). The above results show the excellent activity and stability of the wood-derived catalyst when used in Zn-air batteries. This can be attributed to its abundant active sites for ORR and OER, and the hierarchically porous structure which facilitates the mass diffusion of reacting species.


Figure 2: (a) Schematic illustrating the processing of the wood-derived catalyst and a photo showing that it is able to support up to 667 times of its weight. (b) Zn-air battery prototype and (c) cycling stability profile. (d) Photograph of an electric toy car powered by the Zn-air battery.

High performance pollutant detectorsMolecular imprinting techniques can develop sensors to detect pollutants and pathogens in our environment for water security

Introduction

Certain contaminants found in surface water, groundwater and drinking water systems can adversely affect the health of plants, animals and humans who consume them. It is important to be able to detect these contaminants in a timely and cost-effective manner, without involving sophisticated equipment or tedious time-consuming procedures. We have developed low cost yet highly efficient sensor-based monitoring technologies for rapid detection of contaminants to improve the security of water supply.

Molecularly imprinted polymers (MIPs)-based sensors

Our highly selective and sensitive sensors can convert chemical or biological information into analytical signals to detect pollutants in the environment [1-3]. In particular, MIPs-based detection systems have been developed to replace the use of more expensive and environmentally susceptible sensors based on biosensing molecules (e.g. antibody-based sensors). MIPs-based sensors are also more flexible as they have

the added advantage of being able to detect a wider range of contaminants, making them suitable for water quality monitoring applications.

The MIPs-based sensors are produced using an imprinting technique at the molecular scale to create unique patterns of the molecule of interest (target or template molecule) on polymeric materials (Figure 1). During the synthesis process, monomers (small molecules that can be bonded to other identical molecules to form a longer molecular chain known as a polymer) with suitable functionalities and the template molecules (containing specific patterns that look like the target molecules) are mixed together. As a result of the interactions between their complementarily binding functionalities (e.g. dipole-dipole interactions, intermolecular hydrogen bonding interactions, hydrophobic interactions and van der Walls interactions), the mixture spontaneously forms an ordered polymeric network through a process known as self-assembly (Figure 2).

The template molecules are subsequently removed, leaving behind

molecular cavities in the polymer network. The binding sites in these molecular cavities are complementary in size, shape and orientation to those of the template molecule. These patterned polymers, known as MIPs, will then contain specific patterns with spaces that fit exactly with the target molecules. It is like an artificial tiny lock in which the target molecule or analyte serves as the miniature key. Even in the presence of similar structures in the environment, these cavities are capable of selectively binding to the target molecules.

Due to their polymeric nature, the MIPs can be tailored and designed to exhibit certain desirable properties (e.g. permeability) according to application requirements. This is achieved by selecting suitable reactants (monomers, crosslinkers, functional monomers, solvents and initiators) and controlling polymerisation conditions.

Advantages of MIPs-based sensors

The MIPs used in our sensor system are developed using relatively inexpensive materials and designed to function as artificial tailor-made receptors for


Figure 1: Illustration of the principle of MIPs-based sensors, in which unique patterns of the target molecules are imprinted on a polymer-based sensor film.

RESEARCH FEATURES

Sam LI is a Professor with the Department of Chemistry, NUS. He received his B.Sc., Ph.D. and D.Sc. degrees from Imperial College London, United Kingdom. His research interests include environmental science and technology, biosensors, metabolomics and nanotechnology. He has authored/ co-authored more than 380 publications in international peer review journals and has over 20 U.S. patents. He serves/ has served on editorial advisory boards of several international scientific journals.

References[1] Lin XH; Aik SXL; Angkasa J; Le QH; Chooi KS; Li SFY, “Selective and sensitive sensors based on molecularly imprinted poly(vinylidene fluoride) for determination of pesticides and chemical threat agent simulants” SENSORS AND ACTUATORS B-CHEMICAL Volume: 258 Pages: 228-237 DOI: 10.1016/j.snb.2017.11.070 Published: 2018.[2] Li PJ; Hong YY; Feng HT; Li SFY, “An efficient “off-on’’ carbon nanoparticle-based fluorescent sensor for recognition of chromium(VI) and ascorbic acid based on the inner filter effect” JOURNAL OF MATERIALS CHEMISTRY B Volume: 5 Issue: 16 Pages: 2979-2988 DOI: 10.1039/c7tb00017k Published: 2017.[3] Li PJ; Ang AN; Feng HT; Li SFY, “Rapid detection of an anthrax biomarker based on the recovered fluorescence of carbon dot-Cu(II) systems” JOURNAL OF MATERIALS CHEMISTRY C Volume: 5 Issue: 28 Pages: 6962-6972 DOI: 10.1039/c7tc01058c Published: 2017.

molecular recognition. They allow the detection of selected biological or chemical molecules, ranging from small molecules, such as those used in pesticides, to bigger structures such as proteins or cells. The advantage of these MIPs include:

(a) versatility for detecting a specific molecule of interest, or for a whole family of compounds; (b) outstanding physical and chemical robustness due to their polymeric nature;

(c) excellent mechanical strength for operation in harsh environments;

(d) high resistance to acids or bases;

(e) stability at elevated operating temperature and pressure, and

(f) long storage life with the potential to keep their recognition functionality for many years.

The signal from the sensor is generated by the chemical or biochemical interactions between the target analytes and a sensing layer consisting of a biomaterial or chemical compound, or a combination of both. It can determine the exact concentration of analytes, with typical detection limit of 20 parts per billion. For best performance, the sensors have to be optimised for each specific application so that they can generate the optimal detection signal to meet specific requirements.

Our methodologies can be used to prepare MIPs for specific and sensitive detection of a wide range of chemical and biological molecules.

These polymers are more robust than currently available antibodies-based sensors, which are susceptible to denaturation (the process in which the secondary and tertiary structures of proteins are disrupted) and degradation due to temperature and pH changes. Unlike biosensors, these MIPs-based sensors do not require special storage conditions. They can be stored in a wide temperature range of -80oC to 50oC, and hence can be used under various environmental conditions.

These sensors are expected to have applications in many fields, including environmental monitoring, biomedical analysis, homeland security, and food safety, among others. In particular, the sensors have significant competitive advantages in environmental monitoring and control applications. These include the detection of a specific analyte (e.g. a diagnostic tool for detecting a particular biomarker of a disease), or the detection of multiple analytes in an array (e.g. detecting many types of pesticides in a vegetable sample).

MIPs-based sensors for water systems

The primary market of interest for MIPs-based sensors is in the monitoring of small systems used for supplying drinking water. According to the United States Environmental Protection Agency (USEPA), there are approximately 54,000 community water systems in the United States of America (USA) and 85% of these are small (cater to less than 3,300 people) and very small (cater to less than 500 people) drinking water systems. This would mean that in the USA alone, there are approximately 45,900 such

systems. In other large markets, where the population size is larger and water supply infrastructure is less developed, we expect that there will be at least as many, or even more systems. Other potential areas include agricultural water systems and wastewater treatment systems.

Future plans

The following international patents have been filed:

• A method of making a molecularly imprinted polymer sensor (PCT/SG2017/050340).

• A molecularly imprinted polymer sensor (PCT/SG2017/050342).

We plan to partner with water treatment and environmental monitoring companies to transform our scientific innovation into commercial solutions that benefit industry and society.


Figure 2: Scanning electron microscopy micrograph of the prepared ready-to-use MIPs sensor surface, in which spherical polymer particles form abundant micron-sized pores containing specific patterns at the molecular scale to bind with the target molecules.


Flavour and texture are two of the most important sensory attributes for fruit-based products such as purees and juices. There are a number of different factors, which could be biological (e.g. enzymes) or environmental (e.g. weather conditions) that affect the flavour and texture of whole fruits or those which have been mashed into purees and juices. Depending on the type of fruit, enzymes with the ability to transform food molecules, such as glycosides, pectin, cellulose and starch, play a crucial role in influencing the resulting flavour and texture.

Commercial enzyme preparations are increasingly becoming an important aspect in the food industry to enhance the sensory quality of fruit-based products. Partnering Kikkoman Singapore R&D Laboratory Pte Ltd, a part of the Kikkoman Corporation, a global leader in traditional Asian foods and ingredients, Prof LIU Shao Quan and Prof HUANG Dejian from the Food Science and Technology Programme at the Department of Chemistry, NUS, are gaining deeper insights into the effects

of commercial enzyme preparations on the physical, nutritional and flavour qualities of fruit-based items to develop better tasting products.

Prof Huang said, “This project continues to build on our strong relationship with Kikkoman and will involve students and scientists from NUS working closely with researchers from the research and development teams in Singapore

and Japan.”

“Through closer collaboration, we will develop new tomato and tropical fruit- based products and beverages with improved texture and flavour, in response to consumer demand. This partnership also provides an opportunity for our students to network and gain working experience in the food industry,” added Prof Liu.

Effects of enzymes on selected fruit-based products

NEWS ROUNDUP

Bioactive molecules need to reach target cells and tissues in sufficient concentrations to produce the desired effects. However, there are various barriers within the human body that interfere with the delivery process, limiting their effectiveness.

Nanoscaled delivery systems involving the use of liposomes represent an intriguing option to improve the delivery, especially through the impermeable outermost layer of the skin, which acts as a mechanical and physical barrier. Liposomes are made of biocompatible lipid (fats) molecules in the body that form a capsule structure, which is 50 nm to 200 nm in size. This lipid capsule, which is usually biocompatible, can be used to encapsulate active ingredients for

consumer care applications, protecting its content from damage until it reaches its intended destination. The contents are then released in a timed and well controlled manner.

Prof Gigi CHIU from the Department of Pharmacy, NUS, is collaborating with Sunstar Singapore Pte Ltd to develop nanoscaled liposome formulations for the delivery of a range of active ingredients for skin care applications. Sunstar Singapore Pte Ltd is part of the Sunstar Group with businesses in oral care, health & beauty and safety & environment. This research collaboration will bring together NUS scientists and students with researchers from Sunstar to develop nanoscaled delivery techniques of natural agents based on liposome technology.

Prof Chiu said, “Through this research partnership, we can transfer our expertise on liposome technology to our industry collaborators and work together with them towards improving existing delivery techniques through the skin. This enables consumers to benefit from more effective and better skin care products.”

Nanoscaled delivery for skin care applications

Liposome, a versatile nanoscaled delivery system for a variety of bioactive molecules.

Enzymes that are naturally present or added can modify the macromolecules in tomato-based products, making them sweeter and more aromatic.

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Advances in Science