Ultrawideband and High-Resolution Terahertz Spectroscopy: Structural Identification of Glucose

Abstract

We report on the development of an ultrawideband and high-resolution THz spectroscopic system, which operates over a frequency range of 2–13.5 THz. This work goes beyond conventional THz spectroscopic measurements and demonstrates the capacity to differentiate D- and L-glucoses, which have the same molecular conformation and crystal structure. Furthermore, this system facilitates the structural identification of glucose microcrystals, which exists in α- and β-forms, revealing that L-glucose microcrystals are formed of a mixture of α- and β-D-glucose with approximately equal weight.

1. Introduction

Terahertz (THz) waves [1,2,3,4,5,6,7,8,9] enable the direct characterization of molecular aggregations and clusters via coupling with vibrational eigenmodes. Thus, this class of electromagnetic waves have the ability to revolutionize applications in fundamental sciences and enable advanced technologies, including structural analysis of crystalline objects [10,11,12,13,14], field-matter interaction [15], nonlinear optical responses [16,17,18], biological kinetics [19], THz comb spectroscopy [20], quantum sensing [21], scanning probe microscopy [22], coherence tomography [23], and novel imaging methods [24,25,26].

Fourier transform infrared spectroscopy (FTIR) and THz time-domain spectroscopy (TDS) techniques are well established, and commercial systems based on these techniques have been applied to the identification of molecules, such as nucleosides [27,28], amino acids [29,30], peptides [31], emerging materials [32], and sugars [33,34,35]. In each of these application cases, the materials under study have exhibited well-defined spectral features in the THz frequency range. Although systems based on THz-FTIR and THz-TDS have seen good commercial success, THz TDS systems, which utilize ultrashort pulse pump lasers, inherently have limited frequency bandwidth for spectroscopic measurements (with the frequency range typically limited to ~0.1–5 THz). FTIR methods enable a wider frequency range, however, they struggle to measure the properties of highly absorbing materials at a high signal-to-noise (S/N) ratio. These approaches also frequently require the use of attenuated total reflection (ATR) and waveguide configurations.

In this paper, we propose a frequency-domain THz spectroscopic system, based on difference frequency generation (DFG), that enables high S/N ratio measurements across a very broad THz frequency range of 2–13.5 THz. We utilize our system for the structural identification of glucose, one of the most prevalent biological molecules in nature.

2. Ultrawideband THz Spectroscopic System

A schematic diagram of the experimental ultrawideband frequency-domain THz spectroscopic system is shown in Figure 1. The frequency-tunable THz source was comprised of a 1 µm homemade picosecond pump source (average power 10 W, pulse width 7.4 ps, pulse repetition frequency 1 MHz), of which an injection seeded two 1.5 µm optical parametric amplifiers (OPAs) [7,36]. The output of the OPAs were then used to generate THz radiation in 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal via difference frequency generation (DFG). Most of the tunable THz sources are developed by employing a nanosecond laser pumping configuration, and their pulse repetition rates are typically limited to be less than 100 Hz [37,38,39,40]. Use of our THz source with an ultrahigh repetition rate of 1 MHz allows the detection with a high signal-to-noise (S/N) ratio and short acquisition time. Frequency tuning of the THz radiation was achieved by fixing the wavelength of the OPA1 output, λ₁, at 1.50 µm, and tuning the OPA2 output wavelength, λ₂, in the range 1.50–1.64 µm. The resulting THz output via the DFG process was linearly polarized and could be tuned across the range 2–13.5 THz. The THz output had a linewidth of ~0.12 THz at 2.6 THz (wavelength of ~116 μm), which is comparable to them of conventional nanosecond THz sources, as measured using a scanning etalon. The THz detection scheme was comprised of a cryogenic Si bolometric detector and a lock-in amplifier, and this enabled the high-resolution detection of the THz radiation with an ultra-high S/N ratio of >50 dB (Figure 1b).

3. THz Absorption Spectrum of Glucose Simulated with Density Functional Theory

Glucose (C₆H₁₂O₆), the most conventional monosaccharides, belongs to a subcategory of carbohydrates, and it acts as a source of energy, as well as a metabolic intermediary. Glucose with D- and L-enantiomers can further be classified into α and β forms, which arise from the difference in steric position of hydroxy groups to C-1 carbon.

The THz absorption spectra of glucose crystals were numerically computed by employing density functional theory (DFT) with the PBE exchange–correlation potential as implemented in the VASP code [41,42]. The plane-wave energy cut-off was set to 700 eV, which gave converged results as checked by comparison with a value of 600 eV. Firstly, the crystal structures of α-D-glucose [43] and β-D-glucose [44] were optimized until all atomic forces were below 1 meV/Angs. The phonons at the Γ-point, Born effective charges and the infrared spectra were then computed by using density functional perturbation theory [45] (VASP settings IBRION = 7, LEPSILON = T). It is worth mentioning that the computed spectra of single α- and β-D-glucose molecules did not support the experimental data well, manifesting that the intermolecular interactions play an important role for an accurate description of the THz spectra. The computed spectra of α- and β-D-glucose crystals are shown in Figure 2.

β-D-glucose has a strong absorption band at around 10 THz, while absorption peaks in α-D-glucose can be observed at around 6 THz and its corresponding overtone at 12 THz. Conventional THz-TDS methods, which have a relatively narrow spectral bandwidth, would struggle to detect these features and thus differentiate these two types/forms of glucose.

4. Experiments

4.1. α- and β-D-Glucose

α- or β-D-glucose and polypropylene (SD-PPW5) (host material) powders were mixed with a 3:2 mass ratio (60 wt%), and they were pressed to shape a pellet sample with approximately 1 mm thickness and 12 mm in diameter. The α- and β-D-glucose powders were purchased from Sigma Aldrich Chemistry Co., Inc. (CAS No. 492-62-6), Milwaukee, WI, USA and Tokyo Chemical Industry Co., Ltd. (CAS No. 492-61-5) Tokyo, Japan, respectively. As a reference, a pure pellet of the host material was also prepared. Starch and sucrose exist in large amounts in natural plants, and they are formed of a α-D-glucose polymer and linear condensation of β-D-glucose, thus, non-destructive identification of α- and β-D-glucoses is of great interest in food science and biology. Figure 3a shows the experimentally obtained THz spectra of α- and β-D-glucoses (with each plot being the average of 10 scans) at a frequency step resolution of 0.05 THz. As above mentioned, the spectral resolution is approximately 0.12 THz. Figure 3b also shows the measured absorption spectra by using a conventional FTIR method (frequency resolution: ~4 cm⁻¹ (~0.13 THz)). There is an adequate consistence between them in the absorption peaks and depths. However, the FTIR is still difficult to assign accurately the absorption peaks at 8–10 THz, owing to its low S/N ratio (most 10⁻³). The α-D-glucose sample exhibited absorption features at 6 and 12 THz, while the β-D-glucose sample exhibited absorption at around 10 THz (Figure 3). Our system allows the full assignment of the absorption peaks predicted by the DFT calculations in the entire THz frequency region.

These results indicate that this spectroscopic system is capable of resolving and assigning the THz spectral features of anomers of glucose, which are nearly identical stereoisomers and differ only in anomeric carbon.

4.2. D- and L-Glucose

The optical isomers, D- and L-glucose used in this work were obtained from Kanto Chemical Co., Inc. (CAS No. 50-99-7) Tokyo, Japan and Tokyo Chemical Industry Co., Ltd. (CAS No. 921-60-8), Tokyo, Japan, respectively. It should be noted that D-glucose is a naturally occurring substance, while L-glucose is artificially synthesized. Both glucoses should in theory exhibit identical THz absorption characteristics, however, this was not observed in this work. In this work, it was found that L-glucose exhibited a strong absorption feature at 10 THz, which was not observed in D-glucose, as shown in Figure 4. This absorption feature in L-glucose arises from the presence of β-form glucose. As mentioned above, the FTIR was difficult to assign several absorption peaks below a few percent transmission.

We observed that a fabricated sample, made of α- and β-D-glucoses with a mixing ratio of 1:1, exhibited nearly the same absorption characteristics as that of L-glucose (Figure 5). D-glucose, as a naturally occurring substance, has a well-defined molecular form and can clearly be identified using our THz spectroscopy system. L-glucose, on the other hand, is an artificially synthesized substance, which based on our work, typically contains both α- and β-forms. The results demonstrated here, show that our THz spectroscopy system has the potential to identify whether a target material is natural or synthesized.

5. Conclusions

We have demonstrated the application of an ultrawideband frequency-domain, high-resolution THz spectroscopy system to the differentiation and identification of a range of glucose microcrystals. The results are highlighted by the fact that the THz spectroscopy system operates over a very broad THz frequency range with a signal-to-noise ratio, which far-exceeds that of conventional TDS and FTIR systems. We believe that these characteristics may make it the ideal tool for the identification of a broad range of biological materials and may also be used to differentiate whether substances are natural or artificially synthesized.