In this subproject, our goal is to develop a miniaturized mid-IR spectrometer to realize the real-time detection of the photochemical reaction inside the microreactor.

The example photochemical reaction for study is 2,1-Benzisoxazolonon Synthesis (provided by Prof. Bräse group).

The concept of the miniaturized spectrometer (Fig. 1) is to combine a broadband IR light source, a window for IR and UV light on microreactor, wavelength selective components (Fabry-Perot optical filters), and a IR detector array.

Fig. 1: Concept of the miniaturized spectrometer integrated with microreactor.

The light source and the IR detector array are commercially available. We focused on the development of the optical filter. Two different approaches were developed in Phase 1: the static multi-step Fabry-Perot (FP) filter array and the tunable FP filter. Without any moving part, the static filter is able to realize a fast and stable detection, while the tunable one gives us the possibility to obtain the continuous spectrum and is more flexible for different chemical measurements.

Development of static multi-step Fabry-Perot filter array

According to the spectrum change of the chemical before and after reaction, 4-step FP filter with the output wavelength range between 3.8 µm and 4.5 µm was designed.
The designed filter was successfully fabricated with MEMS technology (PEVCD and ICP).
The transmission spectrum was characterized with FTIR (Fig. 2). Full-width-half-maximum of the filter better than 25 nm (step 3 and 4) was realized.

Fig. 2: Transmission spectrum of fabricated 4-step FP filter measured with FTIR.

Optimization of filter was achieved to focus the output wavelengths at 4.3 µm and 4.7 µm, which match the shift of absorption peaks during the reaction (Fig. 3). The 2-step filter was also applied in array (pixel size: 2mm*2mm) to realize the spatial resolution.

Fig. 3: Transmission spectrum of the optimized 2-step filter array. The two peaks match the absorption peak shift of the chemical reaction.

Development of tunable FP filter with LCE actuator

Liquid crystal elastomer (LCE) is a new kind of material with the potential of larger actuation compared with conventional approaches of actuation of FP filter with electro-static or piezo.
Structure of the tunable filter with LCE actuators was designed (Fig. 4).

Fig. 4: Designed structure of the tunable FP filter with LCE actuators.

Tunable filter sample was fabricated and assembled.
Wavelength tuning range of 100 nm was realized (Fig. 5).

Fig. 5: Transmission peak wavelength of fabricated tunable filter with LCE actuators.

Design of a microreactor

Materials were selected regarding the requirements of high transparency in both IR and UV range: NaCl or Sapphire for window of the microreactor; Teflon or PDMS for the spacer.
Different patterns of micro-channel on the microreactor were designed and fabricated by laser cutting of the spacer (Fig. 6).

Fig. 6: Microreactor with flow channel and fabricated 4-step filter.

Proof of concept: experiments with designed detection system

Experiments of chemical reaction in designed microreactor was performed and the real-time detection was realized with the fabricated 4-step filter. The relative transmission intensity change of different steps was observed during the chemical reaction (Fig.8).

Fig. 7: Images from the IR detector at different time points during the reaction.

Fig. 8: Detected intensity ratio (step 3 / step 1 of the 4-step filter) during the reaction (based on the data from Fig. 7).

Chemical spectroscopy of photochemical organic reactions in micro reactors

Development of static multi-step Fabry-Perot filter array

Development of tunable FP filter with LCE actuator

Design of a microreactor

Proof of concept: experiments with designed detection system