Monolithically integrated mid-infrared liquid sensing

Infrared radiation is the fraction of the electromagnetic spectrum spanning the wavelength range between 750 nanometer and 1 millimeter. It is invisible to the human eye (visible light lies between ca. 375 Nanometer and 750 Nanometer), but contains the main portion of thermal radiation, emitted by objects at room temperature (body heat). Especially the mid-infrared (MIR) light, located between 3 m and 30 m wavelength, is of particular interest, since it covers the characteristic molecular fingerprint region. It is the spectral range, where molecules show their by far strongest absorption of light and can therefore be readily detected. This includes important greenhouse-gases like CO2 or N2O. But also the liquid-phase detection of molecules becomes nowadays more and more relevant, as it is shown by different examples including the detection of cocaine in saliva or the analysis of different types of drugs and their precursors in the pharmaceutical industry. In contrast to gas-phase detection, the analysis of liquids in the MIR is a newly emerging field. This yields a strong demand for new concepts suitable in the design and development of highly integrated and versatile tools for sensitive liquid-phase detection. Within the project LIQI-Sense such a new concept will be investigated, based on special, guided surface- waves in the MIR, so called surface plasmon polaritons. They show particularly high overlap with their surrounding medium (96% and above) together with good wave-guiding properties and are therefore very well suitable for the analysis of liquids. They are joined with a new class of MIR-emitting semiconductor-lasers, so called quantum cascade lasers (QCLs). By applying the special polariton-based waveguide-approach together with a particular design of the QCLs light-generating active region, which can simultaneously emit and detect radiation of the same wavelength, a highly integrated scheme is realized, which will also be analyzed and compared to different already existing concepts. The LIQI-Sense project is conducted in the Intersubband Optoelectronics Group of Gottfried Strasser at TU Wien. The Strasser-group is specialized in the field of MIR-physics and engineering of new concepts which can be applied in analytical chemistry to detect different types and characteristics of molecules. As proof-of-concept and to analyze and optimize the proposed approach for molecules in aqueous solution, two spectroscopic experiments will be performed in the Process Analytics Group of Bernhard Lendl, also TU Wien: the analysis of the change in the local 3D-structure (secondary structure) of the 2 model-proteins -Chymotrypsin and poly-L-lysin. The Lendl-group is specialized in the detection of molecules in the MIR, gas- and liquid-phase, and has successfully developed multiple different experiments in the past.

Mid-infrared spectroscopy is a sensitive and selective technique for probing molecules in the gas or liquid phase. While mid-IR gas spectroscopy is nowadays well exploited for sensing applications, the research on liquid detection techniques is still in its infancy. Investigating chemical reactions in bio-medical applications such as drug production is recently gaining particular interest. However, the realization of high-performance sensors for real-time liquid analysis of dynamical processes is a long-standing challenge in analytical chemistry. It is commonly still limited to bulky systems and requires time-consuming offline analytics. Further development was long hampered by the lack of fundamental concepts, that could be matched with cutting-edge technology. This is due to the involved highly interdisciplinary research aspects in optoelectronics, semiconductor-based materials and (bio-)analytical chemistry. The Meitner project LIQI-Sense took the next major step in this development, by putting the spotlight onto a novel photonic integrated sensing scheme. It exploits pioneering quantum cascade (QC) technology combined with monolithic integration strategies, enabling fingertip-sized and robust lab-on-a-chip devices. For this, emitter (QC laser), interaction section (surface-sensitive plasmonic waveguide) and detector (QC detector) are integrated into a so-called QCLD-sensor. Calculations and simulations of the device performance were conducted within the project, demonstrating its excellent suitability for operation in a liquid matrix. In a proof-of-concept experiment, the thermal denaturation process of a protein is analyzed in real-time. This is performed in a specifically developed microliter-scale fluidic cell, but it is also possible to directly submerge (in-situ) the entire sensor into the liquid analyte. Monitoring the dynamical reaction as well as quantitative measurements reveal excellent device performance including sensor linearity, a coverage of 3 orders of magnitude of protein concentrations, a limit-of-detection in the ppm-range and 55 times higher absorbances than state-of-the-art bulky offline reference systems. Moreover, surface passivation techniques for improved sensor protection and functionalization schemes for enhanced sensor performance were analyzed within LIQI-Sense. Additional work revealed, that, by substituting one layer of the interaction section of the device, a wide range of new wavelengths is unlocked, as well as by implementing polymers like Polyethylene, on-chip beam guiding is enabled. Thus, LIQI-Sense opens up the field of real-time mid-infrared spectroscopy of liquids. Its impact reaches far beyond the protein monitoring and includes a wide variety of other time-dependent processes in liquids such as chemical reactions. The significant importance is further supported by the use of the QC platform, which can address the entire mid-IR spectral range (~3-12 m), triggering further activities in all related research fields.