Doped-Up: Bio-Inspired Assembly of Single Crystal Nanocomposites

Lead Research Organisation: University of Leeds
Department Name: Sch of Chemistry

Abstract

The ability to tune the physical properties of materials is extremely attractive. All too often, the performance of a material is a compromise between two important properties such as high transparency and high conductivity or low thermal conductivity and high electrical conductivity. The obvious solution to this problem is to combine materials to generate composite structures. However, the creation of a new hybrid material by simply mixing materials with complementary properties rarely results in a net advantage. The key is to exert control over the assembly of the component materials over multiple length scales.

The goal of this project is to develop a robust and general methodology for the synthesis of a unique class of functional nanocomposites - single crystals containing a uniform distribution of inorganic nanoparticles. Our approach takes its inspiration from biominerals, such as bones, teeth and seashells, where these are invariably inorganic/ organic composites with hierarchical structures. Indeed, even single crystal biominerals are composites in which organic molecules are embedded within the crystal lattice. Nature therefore demonstrates that although crystallisation is a common means of purification, it is entirely possible to occlude additives within a crystal lattice given the appropriate pairing of the crystal and additive. Using the biologically-important mineral calcite (calcium carbonate) as a test system, we have made the exiting discovery that this biogenic strategy can be translated to synthetic systems to achieve efficient nanoparticle occlusion in single crystals.

We now wish to build on these preliminary results to develop our bio-inspired crystallisation strategy - in which copolymer-stabilised nanoparticles are used as simple crystal growth additives - for the synthesis of functional nanoparticle/ single crystal nanocomposites. This strategy delivers a number of key features. We are creating nanocomposites in which the nanoparticles are embedded within a single crystal, rather than the typical amorphous or polycrystalline matrix, and the nanoparticles are not aggregated. This provides a unique structure where the absence of grain boundaries is expected to enhance many physical properties. It is experimentally straightforward and amenable to scale-up, and we can easily produce sufficient material to determine structure/property relationships. We also benefit from the vast knowledge that is available concerning the crystallisation of traditional ionic compounds to control the size, shape and porosity of the nanocomposites.

Judicious design of the copolymer will provide control over the structures of the nanocomposites at the nano- and meso- length scales, and we will establish a tool-kit for controlling the nanoparticle loading, the inter-particle separations and the interfaces between the nanoparticles and the crystal host. As a suitable test-system we will focus on functional metal oxides containing noble metal nanoparticles/ quantum dots and study their transport and photocatalytic properties. Particular emphasis will be placed on evaluating the structure/property relationships, where the absence of grain boundaries and our ability to tune the structures of our materials is expected to provide us with unique information about their material properties. Our synthetic method is quite general however, and it is envisaged that it can be used as a platform for creating a broad spectrum of materials including capacitors, batteries, thermoelectrics and electrochromics.

Finally, while significant efforts have been made to identify the strategies by which organisms control crystallisation, these have seldom been applied to functional materials. This project will demonstrate the feasibility and potential of this approach, and will hopefully inspire other researchers to use bio-inspired crystallisation strategies to control the structure and properties of advanced materials.

Planned Impact

This project will make an impact on UK industry, society and the economy in the principle areas of nanomaterials, composites, crystallisation and polymer science. Our overall goal is to develop a novel strategy for the formation of functional inorganic nanocomposites based on the polymer-directed occlusion of nanoparticles in single crystals. The work is undoubtedly fundamental science and highly adventurous. We therefore have no expectation of generating a product suitable for translation to industry within the 3 year time-frame of the project. However, the delivery of a new, flexible and scalable strategy for the creation of inorganic nanocomposites will undoubtedly have an impact on companies such as Toyota, Pilkington Glass and Johnson Matthey that generate advanced materials with eg. interesting optical, magnetic, conductivity, catalytic, thermoelectric and electrochromic properties.
As a suitable starting-point for our research the project will explore the formation of nanocomposites with photocatalytic properties, where these are relevant to companies such as BASF, CatalySystems, SA Evitech and KRONOS. Photocatalysis is a challenge of both national and global importance, driven by the need to reduce the consumption of fossil fuels and develop alternative energy sources with much smaller carbon footprints. Photocatalysts are also commonly employed for decontamination of water (e.g. harmful organic solutes), wastewate treatment, air treatment systems and in self-cleaning surfaces. However, the majority of photocatalysists on the market are based on UV light. The ability to extend further into the visible and improve quantum efficiency will be explored in our research.
Our research programme also relies upon cutting-edge synthetic polymer chemistry and the development of methods to control crystallisation processes. Both of these topics are hugely important to UK industry. There are many UK-based companies working in polymer manufacturing, including AkzoNobel, Lubrizol, Scott Bader, BP, and Ashland, and Armes has worked closely with 13 companies over the last 10 years. Crystallisation is fundamental to a wide range of technological processes, including the production of pharmaceuticals, foodstuffs and personal care products, as well as the synthesis of nanomaterials. It is also central to various natural phenomena, such as the formation of bones and teeth, the precipitation of ice in the atmosphere and the prevention of scale.
The relevance of our work to industry is also demonstrated through our industrial contacts. Armes is currently working with BASF, Lubrizol, P & G, AkzoNobel, Scott Bader, Ashland and GEO Specialty Chemicals, and sold a U. Sheffield patent application to DSM in 2007. Meldrum is/has worked with Unilever, Nexia Solutions, Imerys and P & G, while Critchley has worked with AstraZeneca, AF ChemPharm, Paraytech and Sekio-EPSON. He is also an inventor of a photochemically tunable surface system and also invented a method of monitoring UV degradation using photoelectron spectroscopy (see patents).

Immediate impact will be achieved through the training of early-stage researchers in synthetic polymer chemistry (Sheffield PDRA) and crystallisation/nanoscience (Leeds PDRA) for future careers in either academia or industry. The interdisciplinary nature of the project will be particularly effective in helping these two researchers to develop a flexible approach to solving problems and to working within a close-knit team. This project also lends itself to impact through public engagement and outreach. Leeds and Sheffield each have well-established outreach programmes and we will participate in a range of activities including the annual "Festivals of Science" where local schools take part in educational workshops related to science and engineering. Meldrum's group also has an "outreach" team, which is active in organising events, while Critchley's group contributes to the annual Leeds Light festival.

Publications

10 25 50
 
Description We are using a novel, bio-inspired strategy to create a new class of materials - single crystal nanocomposites. Nature shows us that - with the correct pairing of the additive and crystals, and optimised crystallisation conditions - it is possible to intentionally occlude additives within single crystals. This provides a way of enhancing the properties of the host crystal, for example improving mechanical properties, or even creating materials with new properties. We have been investigating this strategy by occluding additives ranging from molecules (amino acids and dyes) to organic and inorganic nanoparticles within crystals. This approach has given us a new understanding of the occlusion mechanism, of the effect of the occluded additives on the crystal lattice, and has enabled us to generate materials with new properties.

(1) Biomineralisation processes invariably occur in the presence of multiple organic additives, which act in combination to give exceptional control over structures and properties. However, few synthetic studies have investigated the cooperative effectives of soluble additives. We have addressed this challenge by studying the combined effects of amino acids and coloured dye molecules. The experiments demonstrated that strongly coloured calcite crystals only form in the presence of Brilliant Blue R (BBR) and four of the seventeen soluble amino acids, as compared with almost colourless crystals using the dye alone. The active amino acids are identified as those which themselves effectively occluded in calcite, suggesting a mechanism where they can act as chaperones for individual molecules or even aggregates of dyes molecules. These results provided new insight into crystal-additive interactions and suggest a novel strategy for generating materials with target properties.

(2) While many techniques are available for analyzing particle shape and structure, it remains challenging to characterize the structural inhomogeneities and defects introduced into individual crystals by the occluded additives, where these govern many important material properties. We have therefore used Bragg coherent diffraction imaging to visualize the effects of soluble additives on the internal structures of individual crystals on the nanoscale. Investigation of bio-inspired calcite crystals grown in the presence of lysine or magnesium ions revealed that while a single dislocation is observed in calcite crystals grown in the presence of lysine, magnesium ions generate complex strain patterns. Indeed, in addition to the expected homogeneous solid solution of Mg ions in the calcite lattice, two zones comprising alternating lattice contractions and relaxation were observed, where comparable alternating layers of high magnesium calcite have been observed in many magnesium calcite biominerals. Such insight into the structures of nanocomposite crystals will ultimately enable us to understand and control their properties.

(3) We also demonstrated the use of ptychographic X-ray computed tomography to visualize the three-dimensional structures of two nanocomposite crystals - single crystals of calcite occluding diblock copolymer worms and vesicles. This provides unique information about the distribution of the copolymer nano-objects within entire, micron-sized crystals with nanometer spatial resolution and reveals how occlusion is governed by factors including the supersaturation and calcium concentration. Both nanocomposite crystals are seen to exhibit zoning effects that are governed by the solution composition and interactions of the additives with specific steps on the crystal surface. Additionally, the size and shape of the occluded vesicles varies according to their location within the crystal, and therefore the solution composition at the time of occlusion.

(4) Our strategy was used to create materials within luminescent properties, where we have shown that it is possible to tune these properties according to the selection of the host crystal. There is a significant drive to identify new materials that exhibit room temperature phosphorescence for technologies including bio-imaging, photodynamic therapy and organic light-emitting diodes. Ideally, these materials should be non-toxic and cheap, and it will be possible to control their photoluminescent properties. Here, this was achieved by embedding carbon nanodots within crystalline particles of alkaline earth carbonates, sulfates and oxalates. The resultant nanocomposites are luminescent and exhibit a bright, sub-second lifetime afterglow. Importantly, the excited state lifetimes, and steady-state and afterglow colours can all be systematically controlled by varying the cations and anions in the host inorganic phase, due to the influence of the cation size and material density on emissive and non-emissive electronic transitions. This simple strategy provides a flexible route for generating materials with specific, phosphorescent properties and is an exciting alternative to approaches relying on the synthesis of custom-made luminescent organic molecules.

(5) Our approach has also provided a unique opportunity to explore the activity of additives. Acidic macromolecules are traditionally considered key to calcium carbonate biomineralisation and have long been first choice in the bio-inspired synthesis of crystalline materials. Our work has challenged this view and demonstrated that low-charge macromolecules can actually vastly outperform their acidic counterparts in the synthesis of nanocomposites. Using gold nanoparticles functionalised with low charge, hydroxyl-rich proteins and homopolymers as growth additives, we have shown that extremely high concentrations of nanoparticles can be incorporated within calcite single crystals, while maintaining the continuity of the lattice and the original rhombohedral morphologies of the crystals. The nanoparticles are perfectly dispersed within the host crystal and at high concentrations are so closely apposed that they exhibit plasmon coupling and induce an unexpected contraction of the crystal lattice. The versatility of this strategy was then demonstrated by extension to alternative host crystals. This simple and scalable occlusion approach opens the door to a novel class of single crystal nanocomposites.

(6) It is well known that oil and water do not mix. Similarly, the incorporation of oil droplets within inorganic crystals is highly counter-intuitive because such components are normally considered to be mutually incompatible. We have used our approach to achieve efficient occlusion of nano-sized oil droplets within calcite crystals, where the oil droplets were stabilized using various amphiphilic diblock copolymer emulsifiers. Both copolymer concentration and diblock compositions affect the extent of occlusion, with optimized conditions producing calcite crystals containing up to 30% volume oil. This protocol enables the incorporation of hydrophobic dyes, drugs and nanoparticles within calcite, where the single crystal nature of the host lattice ensures efficient retention of such guests, while lowering the solution pH leads to triggered release via acid dissolution.
Exploitation Route Nanocomposites are attracting enormous interest for the potential to tune their properties. Our results therefore give researchers the opportunity to use this approach to make new materials by combining materials with dissimilar properties, and to tune the properties of the host phase. We can also use this approach to encapsulate liquids within single crystals. This approach can potentially be used as a new delivery system (eg drugs, agrochemicals) and to protect sensitive compounds within a host crystal.
Sectors Agriculture

Food and Drink

Environment

Healthcare

Manufacturing

including Industrial Biotechology

URL http://www1.chem.leeds.ac.uk/FCM/
 
Description This project established our occlusion strategy as a universal means of generating composite crystals containing second phase objects as diverse as oli, metal nanoparticles, drug-loaded nanogels and dyes.
First Year Of Impact 2019
Impact Types Societal

 
Description ERC Advanced Grant
Amount € 2,630,000 (EUR)
Funding ID 788968 
Organisation European Research Council (ERC) 
Sector Public
Country Belgium
Start 08/2018 
End 08/2023
 
Description Exhibition of Images 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact 6 week exhibition of images of crystals at the North Bar Leeds
Year(s) Of Engagement Activity 2017
 
Description Exhibition of scientific images 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact 6 week exhibition of images of crystals at the North Bar Leeds
Year(s) Of Engagement Activity 2018,2019
 
Description Presentation at 2021 BCA Meeting 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Talk at British Association of Crystallography 2021 Autumn meeting
Year(s) Of Engagement Activity 2021