Bio-chemical Process Monitoring with Terahertz Sensor
An in-house developed Label-free Immobilization-free Terahertz (THz) sensor has shown its potential in various applications . In this paper, we present the experimental results in the field of High Performance Liquid Chromatography (HPLC) and protein crystallization. Ultra-Violet (UV) detectors are very commonly used as a detection technique in HPLC systems. However, it relies on labelling when target substances do not absorb UV light. Therefore, it has the drawback of increased cost and time consumption. Our experimental result shows that THz sensor has the potential to replace UV sensor integrated on the HPLC machine as it is able to detect both UV and non-UV absorbing substances without labelling. Meanwhile, the THz sensor is also deployed for the first time to follow a protein crystallization process.
KeywordsMillimeter wave HPLC Protein crystallization THz Dielectric permittivity
In biological solutions, the presence of bio-molecules alters the hydrogen bond network dynamics of water molecules in its vicinity. These molecules, also known as hydration shell, exhibit different dielectric permittivity than normal bulk water molecules in the THz frequency range (30 GHz–1 THz) . The amount of hydration shell water molecules in liquid is closely related to the amount of bio-molecules, their shape, conformation flexibility and the distribution of hydrophobic and hydrophilic surface. In other words, the change in concentration or conformation of these bio-molecules will translate to dielectric permittivity change in the THz frequency range. In previous work , a THz sensor operating @ 270 GHz has been made to probe permittivity changes in aqueous solutions. It is a label-free, immobilization-free real-time sensor that features very high sensitivity. In this study, another sensor topology operating @ 60 GHz is developed for the same purpose. High performance liquid chromatography (HPLC) and protein crystallization measurements are chosen as proof-of-concept studies to validate the sensor design.
4.2 THz Sensor & Setup
Vector network analyzer that produces and detects 60GHz signal
Directional coupler that separates reflected wave from the forward propagating wave
THz sensor that is very sensitive to dielectric change of liquid under test.
Such measurement set-up has shown its capability in the previous work . When compared to other THz sensor designs by bench marking water-ethanol contrast measurements (Fig. 4.2b), it shows superior sensitivity. Note that the THz sensor used in this paper is denoted as A1 in the graph which is 30 dB more sensitive than the closest K .
4.2.1 High Performance Liquid Chromatography System Setup & Measurements
HPLC pump(s) to keep mobile phase flowing through the entire system
An injector to inject samples onto the mobile phase
A stationary phase to separate individual substances from the sample injected
Detector to register chromatograph in order to measure the concentration of each component in the sample. There exists various techniques . In our case, UV detector is integrated on the available machine.
Three substances as candidates have been chosen for this proof-of-concept study. Two UV-absorbing substances are: Trans-Stilbene-Oxide (TSO) and Praziquantel (PRA). One non-UV-absorbing substance is: Sorbitol. Since TSO and PRA only dissolve poorly in water, acetonitrile (ACN) is chosen as mobile phase in such case. Water is chosen to be the mobile phase for the measurement with sorbitol for the same reason.
As shown in Fig. 4.5a, very little signal variation can be observed from both sensors with a blank injection. For THz sensor, since the impedance of the entire system is matched with ACN, the injected ACN does not introduce any permittivity change. The UV detector has a flat line response since ACN is not UV absorbing. As PRA and TSO are injected (Fig. 4.5b, c), both detectors respond to the substances. The time delay also stays constant around 1.3 min. Since sorbitol is non-UV absorbing, the UV detector was not able to detect it (Fig. 4.5d). However, the THz sensor gave a very obvious response.
4.2.2 Protein Crystallization Measurements
Protein crystallization plays a vital role in understanding the structure of proteins . Though it has been extensively studied, the mechanism of this process is still not clear. UV-VIS and Dynamic light scattering are very commonly used to follow this process among various other techniques. In this study, the capability of THz sensor in this field will be tested.
When the concentrations of lysozyme (40, 50 &60 mg/ml) and NaCl (40 mg/ml) are properly chosen in a mixture(0.1 M Sodium Acetate as buffer), crystallization process can be established at an predictable speed. Increasing the concentration of either lysozyme or NaCl will leads to reduction of time needed for such process. In each measurement, the freshly made crystalizing mixture is chosen as reference liquid. The sensor system is tuned so that very low S11 parameter is achieved. This reference is then compared to the same process at different times (Fig. 4.6b).
As shown in Fig. 4.6c, with increasing lysozyme concentration, the time needed to observe a rise in the THz signal shortens. This corresponds to the accelerated crystallization process.
Although further studies are still required to validate the THz sensor for the two applications demonstrated above, through our proof-of-concept study, it is already clear that this sensor can track protein crystallization processes. The label-free feature also gives it an advantage over the UV detector in the field of High-performance Liquid Chromatography. In the near future, the focus will be shifted to quantitative measurements of HPLC so that the concentration of injected substances can be determined from measurement curves. For protein crystallization, efforts need to be made to increase the time resolution of the measurements.
The authors acknowledge the Vrije Universiteit Brussel (VUB) through the SRP-project M3D2, the FWO-Vlaanderen through FWOAL682 “Building blocks of lab-on-chip system for label-free monitoring of bio-molecular interactions” and the FWOAL611 “Millimeter wave sensor solutions for chromatographic analyzer systems of today and tomorrow”, the, the COST-action MP1204, TERAMIR the NATO support for the Workshop on THz Diagnostics of CBRN effects and Detection of Explosives & CBRN.
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