Catalysis and Nanostructured Materials

In this field we have an interest in the study of nanostructured materials such as carbon nanotubes (CNTs) and carbon nanofibres (CNFs) which have applications as heterogeneous catalysts as well as in many other fields. Ultimately, our goal is to relate catalytic activity and selectivity to both electronic and physical characteristics of the catalysts.

Pharmaceutics, Coatings, Process Research

For pharmaceutical process research we apply terahertz radiation to non-destructively image film coatings and tablet surface density. By exploiting the short pulse duration and the ability to penetrate most polymers used as excipients for film coating we are able to spatially resolve the thickness and density of the coating over the surface of the tablet. Terahertz radiation can propagate more than 1 mm into the tablet and hence it is possible to measure through multiple coating layers without physically slicing the sample. Currently, one of our main interests is the development of online sensors which will allow the robust measurement of coating thickness in real-time in a film coating machine in situ.

The aim of our research activities in this field is to link the coating properties as measured by terahertz imaging with the drug release performance of the final dosage form. We use magnetic resonance imaging (MRI) along with other advanced techniques to interpret the terahertz results. Through the better understanding of the physical properties of the film coating and the tablet dissolution we open new opportunities for improved Process Analytical Technology (PAT) and Quality-by-Design (QbD).

Furthermore we develop novel sensor technology for particle size measurements and high speed tablet hardness testing.

Small Organic Molecular Crystals / Materials Properties (Glasses etc.)

Terahertz spectroscopy is a very exciting technique for the physico-chemical characterisation of organic molecular crystals (such as  typical drug molecules). By probing the hydrogen bonding networks and the phonon modes in molecular crystals the technique has a very high sensitivity to the supramolecular structure in solids. We study a range of solid state modifications such as polymorphs, hydrates, solvates and cocrystals. By exploiting the very short acquisition times of THz-TDS we are able to study phase transitions in real time which allows us a better understanding of the mechanism behind such phenomena.

In addition to our work on crystalline materials we explore the applications of terahertz spectroscopy to characterise amorphous materials. While glasses do not exhibit distinct spectral features at terahertz frequencies it is possible to extract very useful information from THz-TDS measurements, such as the charge distribution.

Understanding of Vibrational Modes at Terahertz Frequencies

In collaboration with the group of Dr Graeme Day at the Chemistry Department we work towards a better understanding of the spectral features which are observed in terahertz spectra. Using a state-of-the-art low temperature transmission THz-TDS setup we acquire spectra at 4 K. These spectra serve as the reference set for the calculations in Dr Day's group which cover a range of computational methods, including both atomistic and solid state periodic Density Functional Theory (DFT) simulations in the harmonic and quasi-harmonic approximation.

Our work aims to advance the application of molecular simulations in understanding far infrared/THz spectra of molecular crystals. We are currently developing a critical analysis of the various computational methods, their ability to model the range of interactions in organic molecular crystals and the validity of harmonic calculations for such systems at low and ambient temperatures.

Quantum Cascade Laser Applications

We are currently working on novel experimental techniques and reconstruction algorithms to produce high quality tomographic images using quantum cascade lasers (QCL) as a terahertz source. A custom algebraic reconstruction technique is being used that allows accurate tomographic reconstruction from non-refractive index matched samples. This allows a wider range of samples, particularly those of industrial, pharmaceutical, and medical interest to be studied. By using multicolour QCLs chemical maps of the samples can be obtained.

Work has also been conducted using a 2D bolometric array to detect the terahertz radiation produced by a QCL. By using a wide collimated beam to illuminate a sample it is possible to obtain real-time reflection or transmission mode images of the sample. This is useful when studying rapidly changing systems. 

Implementation of Novel Sensing Paradigms

The field of terahertz remained largely within the domains of the laboratory environment as the technique matured with much fundamental work undertaken to improve generation and detection capability and also to understand results from the many different potential application areas as discussed above.  However, in order to fully realise the potential of the technique in terms of real world applications novel setups are required to tailor the technique to a particular problem.  In TAG we have been looking at how sensing technologies such as QCLs may be incorporated into robust online sensing paradigms within industry. Some examples of potential applications include real-time measurements of emulsion compositions, the study of polymer gels, and mapping of dynamic changes in moisture content in materials.

In addition we have also been looking at how to maximise the extraction of information from a terahertz pulse by using novel algorithms to help untangle the spectroscopic information recorded.

Dynamics in Biomolecules

Computational studies predict that the collective vibrational modes of biomolecules fall in the terahertz frequency range.  Therefore, THz-TDS is regarded as an extremely promising technique with which to study the dynamics and conformational changes of biomolecules such as proteins and peptides in aqueous environments.  Rather than merely the chemical properties, the secondary physical structure and the dynamics of transitions between them are known to be key to understanding the biological function.