S. Freitag, B. Baumgartner, S. Tauber, C. Gasser, S. Radel, A. Schwaighofer, B. Lendl:
"An Acoustic Trap for Bead Injection Attenuated Total Reflection Infrared Spectroscopy";
Analytical Chemistry, 91 (2019), 12; S. 7672 - 7678.

Kurzfassung englisch:
In this work, we introduce a system combining an acoustic trap for bead injection with attenuated total reflection
(ATR) infrared (IR) spectroscopy. By mounting an acoustofluidic cell hosting an ultrasound source on top of a custom-built
ATR fixture we were able to trap beads labeled with the enzyme alkaline phosphatase without requiring any mechanical
retention elements. Sequential injection analysis was employed for reproducible sample handling and bead injection into the
acoustic trap. To showcase potential applications of the presented setup for kinetic studies, we monitored the conversion of p-
nitrophenylphosphate into p-nitrophenol and phosphate via beads carrying the immobilized enzyme using ATR-IR
spectroscopy. Retaining the labeled beads via ultrasound particle manipulation resulted in excellent experimental reproducibility
(relative standard deviation, 3.91%). It was demonstrated that trapped beads remained stably restrained with up to eight cell
volumes of liquid passing through the acoustofluidic cell. Beads could be discarded in a straightforward manner by switching off
the ultrasound, in contrast to systems containing mechanical retention elements, which require backflushing. Multiple
experiments were performed by employing different substrate concentrations with the same batch of trapped beads as well as
varying the amount of enzyme present in the cell, enabling enzyme kinetic studies and emphasizing the application of the
proposed setup in studies where enzymatic reuse is desired. This proves the potential of the acoustic trap combined with ATR-
IR spectroscopy to monitor the activity of immobilized enzymes and its ability to perform complex bead-based assays.

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