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Mid-IR lab-on-a-chip allows rapid monitoring of reactions

31 Aug 2022

TU Wien device combines emitter, sensor and detector on a single chip.

A project at TU Wien has developed a chip-scale infrared sensor combining laser, interaction region, and detector on one chip.

The device is said to be significantly more sensitive than previous standard devices, and can operate directly on a liquid analyte to calculate a wide range of molecule concentrations.

Described in Nature Communications, the project's device utilizes quantum cascade technology to reduce the physical size of the device and speed up its analytical operation.

"We only need a few microliters of liquid for a measurement, and the sensor delivers data in real time, many times per second," said TU Wien's Borislav Hinkov.

"We can precisely monitor a change in concentration in real time and measure the current stage of a chemical reaction in the beaker. This is in strong contrast to other reference technologies, where you need to take a sample, analyze it and wait up to minutes for the result."

The breakthrough hinged on combining a suitable quantum cascade laser (QCL) source, a form of semiconductor laser characterized by relatively narrow line width and good wavelength tunability, with the use of quantum cascade detectors (QCDs), a class of tailorable IR photodetectors able to show reduced noise behavior and thermal load.

"Through the combination of the laser, interaction region, and detector on one chip, and avoiding typical diffraction limitations of conventional chip-scale photonic systems by exploiting plasmonic waveguides, we realize a fingertip sized (below 5 × 5 square millimeters) next-generation rapid liquid sensor," noted the project in its published paper.

New field in analytical chemistry

As a proof-of-concept trial, the project used its novel sensor to examine a familiar biochemistry reaction, the denaturation and change in shape from helical to flat of bovine serum albumin protein when heated in a heavy water matrix. This geometrical change also changes the particular mid-infrared fingerprint absorption spectrum of the protein.

Quantitative measurements revealed excellent performance characteristics in terms of sensor linearity, wide coverage of concentrations, and a 55-times higher absorbance than state-of-the-art bulky and offline reference systems, according to the TU Wien paper.

"We selected two suitable wavelengths and fabricated suitable quantum-cascade-based sensors, which we integrated onto a single chip," said Hinkov. "And indeed, it turns out: you can use this sensor to observe the so-called denaturation of the selected model protein with high sensitivity and in real time."

The next steps will involve modifying the sensor to work under biophysical conditions in normal water, requiring a redesigned and optimized plasmonic waveguide geometry, together with a careful selection of measurement wavelengths avoiding the highest absorption peaks in water. If this can be achieved, then the possible applications could be extremely diverse.

"This opens up a new field in analytical chemistry: real-time mid-infrared spectroscopy of liquids," said Borislav Hinkov. "Wherever there is the need to monitor the dynamics of chemical reactions in liquids, this new technique can bring important advantages."

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