Solid-state NMR Applications

The solid-state NMR equipment was delivered in March 2015 and fully installed on 27 May 2015. The equipment installed included Avance I 300WB Solid-state NMR spectrometer, comprising: (1) a new shielded Ascend wide-bore (WB) magnet; (2) a refurbished two channel Avance I console; (3) a new 4 mm CPMAS probe BL4. In addition, we also have a 7 mm CPMAS probe BL7 from our previous spectrometer, Bruker MSL300, which was used with the new and refurbished equipment installed in May 2015.

The instrument is configured to run solid-state NMR spectra of nuclei falling in the frequency range between 15N (~30 MHz) and 31P (~121 MHz), as well as 1H NMR spectra (300 MHz). The spectrometer is operated manually (no sample changer).

Since the installation, the instrument has been used primarily for Physical Science research by groups from UCL Chemistry, Chemical Engineering, Physics and Astronomy departments and UCL School of Pharmacy. During the first year of operation, the estimated usage levels by different users were: departmental 70% and institutional 30%. Recently, the equipment was also used by the applied biotechnology research group from University of Westminster. The following research groups have used the solid-state NMR spectrometer since its installation: Prof G Sankar (UCL, Chemistry), Prof J Evans (UCL, Chemistry), Prof J Darr (UCL, Chemistry), Prof H C Hailes (UCL, Chemistry), Prof P MacMillan (UCL, Chemistry), Dr R Palgrave (UCL, Chemistry), Prof A Sella (UCL, Chemistry), Dr C Salzman (UCL, Chemistry), Prof X Guo (UCL, Chemistry), Prof A Beale (UCL, Chemistry), Prof R Catlow (UCL, Chemistry), Dr H Bronstein (UCL, Chemistry), Dr T D Sheppard (UCL, Chemistry), Prof G Bataglia (UCL, Chemistry), Prof N Skipper (UCL Physics and Astronomy), Dr J Tang (UCL, Chemical Engineering), Prof D Craig (UCL School of Pharmacy), Dr F Joubert (UCL School of Pharmacy), Prof I Roy University of Westminster).

During the first year of operation in 2015/2016, the instrument was employed to record multinuclear solid-state 7Li, 11B, 13C, 15N, 23Na, 27Al, 29Si, 31P and 79Br NMR spectra with magic angle spinning (MAS) and cross-polarisation (CP) from protons when needed. The demand for 13C, 15N, 27Al and 29Si measurements has been markedly higher than for other nuclei. Compared to our previous spectrometer, Bruker MSL300 (1985-2012), the sensitivity specifications of the current instrument are significantly better, allowing us to use the new 4 mm probe most of the time, including 15N CPMAS measurements of organic solids or 29Si MAS NMR of zeolites with relatively low Si content.

Examples of the usage of the new equipment by research students are included below.

Martin Rosillo-Lopez (supervisor Dr. Christoph G. Salzmann, UCL Chemistry) - The research carried out by Martin involves the structural characterisation of various oxidised graphenic nanomaterials. Solid-state 13C MAS NMR spectroscopy is paramount to the research as it is the sole method for differentiating between the many distinct oxidised (and non-oxidised) sp2 and sp3 carbon functional groups that exist in variety of graphenic materials such as graphene oxide. Graphene and graphene oxides are researched widely worldwide given their many promising applications. Martin has so far published two papers using the 13C solid-state spectra collected here at UCL to differentiate between the functional groups present on three graphenic analogues which are widely employed in the literature. The NMR data collected was used to explain why two of the graphenic analogues and not the third were able to react intramolecularly to generate highly reactive carboxylic anhydrides which was the first conclusive evidence of this elusive functional group in the literature. Other published research involving solid-state 13C NMR showed the deviations in functional groups that arise on scaling down from micron sized graphene oxide to nano graphene-oxide, and how this impacts the reactivity of these materials. Martin is also expected to publish two more papers with the remaining solid-state 13C NMR spectra he has obtained. It cannot be emphasized enough that graphene oxides are unstable in solution and the use of solid-state techniques is the only alternative.

Anish Niall Goodeal (supervisor Dr Hugo Bronstein, UCL Chemistry) - Covalent organic frameworks (COFs) are a novel class of porous materials which offer low densities, high thermal stabilities and tuneable structures and properties. However, COFs are typically obtained as microcrystalline, insoluble powders which limits the number of techniques which can be used to analyse these materials. Niall’s project involves the synthesis of many different porphyrin based COFs with minor adjustments made to each structure in order to improve their crystallinity and stability. Solid state NMR spectroscopy remains one of the few techniques that can be used to probe the chemical bonding and structure of these insoluble powders. Of particular importance is the ability to fingerprint each structure using solid state 13C and 15N NMR. This provides a quick way to confirm that polymerisation has taken place and also gives an indication of the quality of each sample. Deviations from the fingerprint spectra will be used to track COF degradation over a period of time and will also become increasingly important in the near future as the research project moves towards post-synthetic modification and functionalisation of the existing COF structures.

Charlotte Maughan (supervisor Prof Jawwad Darr, UCL Chemistry) - Charlotte’s project is based on synthesising inorganic compounds for the use in vaccine adjuvants, therefore, composition of the materials is required to understand their properties, i.e., their adsorption capacity for antigenic particles within the vaccine formulation. Solid-state NMR was thus required to determine the structure of the amorphous, aluminium-containing samples to ascertain which polymorph [either Al(OH)3 or AlO(OH)] to help identify the materials. This was successfully achieved using solid-state 27Al MAS NMR spectra recorded at an MAS frequency of 12 kHz using the new 4 mm probe.

Miriam Flores-Merino (supervisor Prof Giuseppe Bataglia, UCL Chemistry) - The nature of the polymeric hydrogel networks of poly(ethylenglycol) and chitosan was investigated using solid-state NMR. Since chemical polymeric networks are not soluble, 13C CPMAS solid-state NMR serves as a powerful chemical analysis tool. For example, NMR is used to study the cross-linking of polymeric networks, which is a key characteristic for their properties (i.e. mechanical properties), and therefore for their performance in contact with cells. Polymeric biomaterials studied are intended for tissue engineering and the overall objective of the project is study the relationship between their biophysical and chemical properties and cell behaviour.

Glen Smales (supervisor Prof. Gopinathan Sankar, UCL Chemistry) - Solid state NMR was used to check the coordination of aluminium atoms present in zeolite samples made from a novel silica/alumina sources. Two samples were analysed, one on the silica/alumina precursor and one on the zeolite formed from this precursors. From the analysis of 27Al MAS NMR spectra, it was found that the silica/alumina precursor had Al present in 3 different sites, whereas the Al in the zeolites was present in a single tetrahedral site, indicating that aluminium was incorporated into the zeolite framework.

Alexis Schmidt (supervisor Prof. Gopinathan Sankar, UCL Chemistry) - Alexis uses solid-state NMR to determine the coordination environment of Al, Si, and P in zeolites consisting of silicoaluminophosphate frameworks. Al, Si and P which were incorporated into the zeolite framework are found on tetrahedral sites. The Si/Al ratio in the framework was also established using solid-state NMR spectra. In addition, 1H solid-state NMR was employed to find out to what extent Si-OH-Al groups and Si-OH units were present in the framework. The samples studied differ in their crystallisation time during hydrothermal synthesis, product recovery route, and whether they were calcined or not.

Theo Suter (supervisor Prof. Paul McMillan, UCL Chemistry) - The aim of Theo’s project is to manipulate the electronic properties of the layered carbon nitride material by removing and changing the intercalants held between the layers. Solid-state 7Li MAS NMR was used to detect the known lithium intercalated into the structure, and was useful because of the very poor sensitivity XPS has for lithium. 7Li MAS NMR has allowed not only detect lithium, but also determine relative amounts of it in different samples.

Peixi Cong (supervisor Prof. Andrew Beale, UCL Chemistry) - ZSM-5 zeolite is a typical example of a shape selective acid catalyst. The acidity of ZSM5 depends upon the Silicon/Aluminium ratio. The framework Si/Al atomic ratio of zeolites is an important parameter that exerts a strong influence on the properties such as maximum ion exchange capacity, thermal and hydrothermal stability, hydrophobicity, and concentration and strength of acid sites of the Bronsted type. Thus, the acid catalytic properties of zeolites can be controlled by manipulating the Si/Al ratio either during synthesis or by post-synthetic dealumination treatment. The extent of dealumination treatment cannot be judged solely on the basis of bulk elemental analysis of the product because extraframework aluminium is usually formed during dealumination. Therefore, spectroscopic technique such as solid-state NMR is used for better understanding and optimisation of the dealumination treatment.

Yiou Wang (supervisor Dr. Junwang Tang, UCL Chemical Engineering) - The work done by Yiou on the solid-state NMR instrument included 13C and 15N MAS NMR of polymeric graphitic carbon nitride (GCN), which is one of the most promising organic polymer photocatalysts for solar fuel synthesis including water splitting and CO2 conversion, as GCN is low cost, robust and relatively efficient. One of the most important advantages of polymeric photocatalysts is that their properties such as charge separation and light absorption could be enhanced by carefully tuning its structure. In order to identify the structural change, solid-state 13C and 15N MAS NMR spectra were measured and analysed.

Dan Kong (supervisor Dr. Junwang Tang, UCL Chemical Engineering) - Dan’s work on the solid-state NMR instrument included running 13C and 15N CPMAS NMR spectra for characterisation of covalent triazine frameworks (CTFs), which are one of the most promising organic polymer photocatalysts for visible-light-driven water splitting. The most important advantage of polymeric photocatalysts is that their properties such as charge separation and light absorption could be enhanced by carefully tuning their structure, which in turn requires solid-state 13C and 15N CPMAS NMR spectra for monitoring structural changes introduced.

Ingram David (supervisor Prof Neal Skipper, UCL Physics and Astronomy) - The work done by David on the solid-state NMR instrument included 11B MAS NMR of ammonia borane (AB, NH3BH3), which is a very promising material for hydrogen storage for green fuel purposes due to its safe and light-weight nature, as well as the high hydrogen content. A problem with AB is that the polymeric waste after dehydrogenation is badly characterised and hard to regenerate back to AB. Solid-state 11B MAS NMR spectra of some of this dehydrogenated waste were measured. Pilot studies revealed that it is a good technique for looking at this system. David’s group is likely to use this technique to determine the efficiency of any regeneration procedures in order to quantify the amount of AB in the post-regeneration samples. 11B SSNMR will be used for a variety of the wastes treated with ammonia and hydrazine. This solids NMR technique is likely to make a large impact on David’s research as the waste substances he is working with are insoluble in virtually all solvents, due to the similarity in their structure with inert ceramic boron nitride. Solid-state NMR is therefore one of the only techniques that can actually be useful for these materials, and probably the one that provides the most useful information.

Stefania Marano (supervisor Prof Duncan Craig, UCL School of Pharmacy) - Stefania’s study explores the use a solvent-free temperature-controlled centrifugal spinning process as a means of rapid production of amorphous solid dispersions in the form of drug-loaded sucrose microfibers. The microfibers formed have shown significant dissolution improvements for the model drugs olanzapine (an anti-psychotic drug), piroxicam (a non-steroidal anti-inflammatory) and itraconazole (triazole antifungal agent). However, because of the particularly hygroscopic nature of amorphous sucrose, the shelf life of microfibers produced using this technique was relatively short. When exposed to high relative humidity conditions (75% RH/25 °C), drug-loaded microfibers were shown to collapse into a powder. Therefore, particular attention was paid to the characterization of the drug physical state as any potential transitions of the drug solid state can have a significant effect on the final drug dissolution performance and stability. Since the drug loading in the microfibres was kept low (10% w/w), the characterization of the drug physical state using common techniques, such as differential scanning calorimetry (DSC) and X-ray powder diffraction (XRPD), was unsuccessful due to poor sensitivity of these techniques in detecting components present in relatively lower ratios in a two phase system. On the other hand, solid-state NMR spectroscopy was found to be particularly selective and highly sensitive in identifying small quantities of drug in the formulations, both in the crystalline and amorphous states.

Barbara Lukasiewicz (supervisor Prof Ipsita Roy, University of Westminster, Applied Biotechnology Research Group) - The main aim of the project is to develop novel natural origin material suitable for coronary stent application. Polymers are produced by bacterial fermentation using various types of carbon sources. Waste products and renewable materials have been used to lower the total cost of polymer production. Obtained polymers were fully identified and characterized. Polymers were characterized using solution and solid state NMR techniques. Materials with the most suitable properties will be selected for tube manufacturing and final stent prototype.