An Acoustic Trap for Bead Injection Attenuated Total Reflection Infrared Spectroscopy

Abstract

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 changes 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.

3D Printing for Low-Cost and Versatile Attenuated Total Reflection Infrared Spectroscopy

“…To determine the source for this intriguingly reproducible deviation, the same ATR crystals and ATR fixture were used in the ATR setup built from commercial available optomechanics including xyz-stages and rotation mounts allowing for precisely adjusting the angle of incident, used in our previous studies. 8,10 From this experiment, we found that the d e,exp values for both crystals in combination with the 3-D printed liquid compartment exactly match the theoretically obtained values (absorbance for the Si ATR is given in Table 1). For this reason and due to the exact same deviation for both ATR materials, we assumed the small inaccuracies in dimension of the 3D-printed mirror mounts to result in a deviation of the angle of incidence, which highly affects the effective pathlength.…”

“…3 These advantages of ATR spectroscopy have been exploited in a wide variety of fields including forensic science, 4 on-line monitoring 5 and biofilm studies. 6 Combination of ATR with novel high power IR light sources, 5,7 optimized liquid handling, 8,9 as well as divers modification of the ATR element indicates the potential of this technique. [10][11][12][13] Complementary to commercial, costly ATR prisms, crystals prepared from double-side polished Silicon wafer have emerged.…”

A thermoelectrically stabilized aluminium acoustic trap combined with attenuated total reflection infrared spectroscopy for detection of Escherichia coli in water

“…Classic acoustic traps are mostly made of glass; 17,26 however, recent acoustofluidic devices based on aluminium have shown promising results for particle manipulation. 13,14,34 Thus, aluminium-based acoustic traps represent a low-cost alternative that can be easily manufactured in comparison to glassware. We took advantage of the thermal conductivity of aluminium to build a TE-controlled trap, which allowed us to perform our assay at the desired 37 °C and which further counteracted thermal drifts induced by the high gain of the piezo driving signal or environmental influences.…”

“…Please do not adjust margins Please do not adjust margins research introduced an acoustic trap for bead injection combined with ATR-IR spectroscopy, 34 which has been used in a wide variety of applications due to its robustness. 32,33 Here, total reflection of the IR light within the ATR element generates an evanescent field that penetrates the sample on top of the ATR element to a depth of up to 2 µm.…”

A pocket-sized 3D-printed attenuated total reflection-infrared filtometer combined with functionalized silica films for nitrate sensing in water

“…These spectrometers are still considered as gold-standard for IR spectroscopy. The broad spectral coverage and robustness of FTIR spectrometers enabled applications in a variety of analytical tasks including quantitative analysis of liquids in off-line as well as in-line applications, [15][16][17][18][19][20] studying reactions at surfaces, [21][22][23] as well as clinical diagnositics. 24 An emerging, cost-effective alternative to FTIR spectrometers are filtometers that, in comparison with benchtop spectrometers, allow on-site measurements due to their small size and low energy consumption.…”