Research
The aim of my research is the development of new methods and applications of ultra-sensitive laser spectroscopic techniques for analytical applications, including sensor applications of new light sources (DFB-QCL and diode lasers).
During my research activities in Zürich and Sheffield, I was able to develop new innovative experimental techniques for laser spectroscopy with great potential for analytical and fundamental applications.
In one experimental set up, a pulsed infrared (IR) laser excites vibrational levels of molecules which are then ionised by an ultraviolet laser. IR excitation is thus combined with extremely sensitive ionisation detection, e.g., using a mass spectrometer. This scheme is extremely sensitive and opens new possibilities for analytical applications, since it combines sensitive IR spectroscopy with highly selective mass spectrometry [3,13,15-17,21,28,31,41].
In another experiment, the extremely sensitive cavity-ring-down (CRD) spectroscopy (effective absorption path length of several km) is combined with the very high resolution (ca. 100 kHz) of cw-diode lasers [3,19,23,26,35]. Due to the high sensitivity and selectivity, cw-CRD is particularly well suited for analytical applications.
In an innovative laser experiment developed in Sheffield and partially sponsored by government departments and industry, highly sensitive resonant photoacoustic spectroscopy is combined with laser power build up of a cw-laser within an optical cavity, 'cavity enhanced resonant photoacoustic spectroscopy', CERPAS [38]. This new experimental technique is well suited for analytical and fundamental applications. At the moment, the technique is being developed further using quantum cascade lasers (QCL) in the 'fingerprint' region of the mid-IR for the ultra trace detection of illegal substances (explosives, humanitarian demining), and using near-IR diode lasers to detect toxic industrial gases.
Ultratrace Analytical Laser Spectroscopy for Humanitarian Demining, sponsored by the charity FABW (YouTube link)
In Sheffield, I also developed an innovative Raman spectrometer (stimulated Raman scattering with photoacoustic detection, PARS) [36,40,42]. Laser stimulation enables highly resolved Raman spectroscopy (ca. 0.05 cm-1) with high sensitivity. This method was characterised by a 5 ppm detection limit for molecular hydrogen in 1 atm of air [42].
In a project supported by the Nature and Environment Research Council (NERC), I developed a novel and innovative Raman spectrometer for the trace gas detection of Raman active gases (including N2, O2, H2, CH4) where Raman scattering is excited by a cw-diode laser which is enhanced by several orders of magnitude in an optical cavity: cavity enhanced Raman spectroscopy, CERS [43,45,48,53].
This method achieves detection limits in the ppm region with a simple low power 5 mW 635 nm diode laser; the analytical potential has been demonstrated by measuring the composition of natural gas samples (petrochemistry) and detecting hydrogen gas released by bacteria [43,45,48]. Patent applications and knowledge transfer activities are planned to exploit these new technologies.
With this method, I am currently branching into the field of molecular biology and biotechnology, to study bio-hydrogen production by bacteria, mechanisms of hydrogenase activity, and bacterial metabolism [48,53].
Cavity-Enhanced Raman Spectroscopy (CERS) in the Biosciences: Analytical Chemistry 89 (2017) 2147-2154. (open access) View the online article
In collaboration with members of the industry, I also introduced new schemes for sensitive detection of gases by resonant photoacoustic diode laser spectroscopy in advanced acoustic resonators (differential Helmholtz resonator, Herriott cell) for safety monitoring of toxic industrial gases (TICs) and for analysing natural gas samples in petrochemistry [51,52,53].
I am currently branching into the field of Surface Enhanced Raman Spectroscopy (SERS) with silver nanoparticles. We have succeeded in synthesizing nanoparticles and have obtained first SERS spectra of some interesting systems. Below TEM images of silver nanostars and nanospheres.
A further research interest involves the study of molecular association by intermolecular hydrogen bonding. At present, I employ spectroscopic experiments and quantum chemical calculations to investigate unusual hydrogen bonded systems in the gas phase [30,32,33,35,37,39, 40,47,53].
In addition, I also use more traditional spectroscopic methods such as ATR-FTIR (Attenuated Total Reflection FTIR), Raman microscopy and LIBS (Laser Induced Breakdown Spectroscopy) for analytical applications, sensing and imaging which I pursue in many collaborations with colleagues and industrial partners.
