Electromagnetic waves in the terahertz (THz) band have a frequency range of 0.1–10 THz and a wavelength range of 0.03–3 mm. Studies have shown that the characteristic vibration modes of many biological macromolecules correspond to the THz band [1]; consequently, THz waves can be used to detect biological molecules [2-6]. Water plays an important role in the function of biomolecules, and thus, most experiments are performed in the solution state. Interactions among hydrated molecules in solution involve multiple biological phenomena [7], which are often denoted as hydration. Based on this principle, Yang et al. measured the spectrum of L-asparagine and L-asparagine monohydrate via THz time domain spectroscopy (THz-TDS) technology. In addition, he also observed dynamic changes in L-asparagine thermal dehydration in real time and found that THz waves are highly sensitive to changes in the crystal structure, the condition of the solution in which the crystal is contained and weak interactions between molecules [8]. Microfluidics, also known as lab-on-a-chip or microfluidic chip technology, enable researchers to precisely control and manipulate micro-scale fluids with submicron structures. Thus, microfluidics have practical value for distinguishing similar substances and detecting trace samples. Fisher et al. [9] measured the optical constants of four nucleotides in the THz band using THz technology and successfully identified these nucleotides. Serita et al. developed a THz microfluidic chip based on a nonlinear optical crystal, comprising an array of several split ring resonators, for the detection of ultra-trace biological samples [10] Baragwanath et al. used THz-TDS to examine a microfluidic chip and found that the time domain spectrum, refractive index and other parameters differed greatly for different concentrations and substances [11]. Thus, THz spectroscopy technology is a powerful tool for distinguishing substances and studying their structures.
Guar gum is an emulsifier, curing agent and stabiliser. Guam gum can act as a thickening liquid, can increase viscosity and reduce ice crystal formation, and is used in most processed foods. In this study, THz technology and microfluidic approaches were combined to study the THz absorption characteristics of guar gum for different concentrations, temperatures and electric field treatment durations. The method presented herein provides a feasible approach for rapidly identifying colloids.
Properties of guar gum
The molecular structure of guar gum is shown in Fig. 1 [12]. Guar gum is extracted from the endosperm of the leguminous plant guar bean as a free-flowing powder. The powder is generally white to light yellowish brown in colour and is nearly odourless. Guar gum is composed of approximately 75%–85% polysaccharide, 5%–6% protein, 2%–3% fibre and 1% ash.
Guar gum is a non-ionic neutral polysaccharide, with D-mannose linked by a β-1,4 bond as the main chain and D-galactose linked at the α-1,6 position as the branch chain. In general, the molar ratio of galactose to mannose is 1:2. The spatial structure of the plant is a curled spherical structure, with mannose on the inside and galactose on the outside. The ratio of mannose to galactose varies slightly for different plant varieties.
Guar gum [12] is a type of water-soluble natural polymer, which can be quickly hydrated in cold or hot water to obtain a translucent viscous liquid. The viscosity of a guar-gum-containing aqueous solution reflects the molecular weight of guar gum, while the swelling rate reflects the degree of difficulty that is encountered in swelling in guar gum applications. Higher temperatures can accelerate the swelling of guar gum, weaken the hydrogen bonding between molecular chains, increase the elongation of molecular chains and reduce the viscosity.
Notably, guar gum can form a high-viscosity aqueous solution at low concentrations [12] due to entanglement between polymer chains and intramolecular and intermolecular hydrogen bonding. The viscosity of 1% guar gum in aqueous solution is approximately 4000–6000 MPa∙s. In addition, guar gum aqueous solutions exhibit typical winding polymer characteristics, corresponding to the pseudoplastic fluid characteristics of a non-Newtonian fluid, with no yield stress. Because guar gum is widely utilized and exhibits a high viscosity and rheological properties in natural rubber, the THz absorption of guar gum was studied in this work, with guar gum acting as a representative colloid for further experimental research.
Experimental system
Experimental light path
An in-house-built THz-TDS system was employed in this work. The light source is a self-mode-locked fibre femtosecond laser purchased from Peking University, with a central wavelength of 1550 nm, pulse width of 75 fs, pulse repetition rate of 100 MHz and power of 130mW. After passing through a polarising beam splitter prism, the output laser is divided into two beams. One beam is used as the pump light, which is coupled into a fibre-optic photoconductive antenna (BPCA-100-05-10-1550-c-f, BATOP Company) through a mechanical translation stage to generate a THz wave. The other beam is used as a detection beam, which is coupled into another fibre-optic photoconductive antenna (BPCA-180-05-10-1550-c-f of BATOP Company) to detect THz waves. The fabricated microfluidic chip is placed between two off-axis parabolic mirrors. The THz wave emitted by the THz antenna passes through the microfluidic chip filled with guar gum, is received by the detection antenna and is then input into the lock-in amplifier for amplification. Next, the data are collected and processed by a computer. A schematic of the experimental optical path is shown in Fig. 2.
Fabrication of the microfluidic chip
THz microfluidic chip materials should exhibit high transmittance and allow for ease in processing. Currently, microfluidic chips are generally made of quartz glass, polydimethylsiloxane (PDMS) or polyethylene (PE). However, due to the low transmittance of quartz glass and PDMS for THz waves and the opacity of PE to visible light, these materials do not allow one to observe changes in liquid samples in a channel in real time. The transmittance of the cycloolefin copolymer Zeonor 1420R used in this study exceeds 90% and displays no obvious peaks, as shown in Fig. 3, which is suitable for the preparation of THz microfluidic chips.
The microfluidic chip in this study is a sandwich structure, with a 50-μm-thick strongly adhering double-sided adhesive as an interlayer and a concave microfluidic channel carved in the middle. The substrate and cover are composed of Zeonor 1420R material. A diagram of the process for preparing the microfluidic chip is shown in Fig. 4. In this study, we first measured the THz transmission characteristics of the microfluidic chip during continuous heating without liquid injection in order to assess the THz transmission characteristics of the chip under varying temperature. The results show that the transmission characteristics of the chip are not affected by temperature and that there is no leakage phenomenon. Therefore, the chip is suitable for the detection of liquid samples at different temperatures.
Temperature control system
To control the temperature of the guar gum, a high-precision temperature control system was designed. First, the fabricated microfluidic chip was bonded to a 2-mm-thick iron sheet by using a thermal conductive silica gel. A 6-mm-diameter hole was created on the iron sheet for THz waves to pass through. Second, a temperature sensor was secured to the same side of the iron sheet using thermal conductive silica gel. A circular alumina ceramic heating plate with holes (outer diameter: 40 mm; inner diameter: 10 mm) was glued to the other side of the iron sheet using thermal conductive silica gel in order to heat the microfluidic chip. The heating plate and temperature sensor were controlled by a temperature controller (ST700 intelligent PID temperature controller; temperature adjustment range: 0–400 °C; rated voltage: 220 V; working frequency: 50–60 Hz). The temperature controller, which is shown in Fig. 5, can adjust the temperature with an accuracy of 0.1 °C.
External electric field device
A schematic diagram of the external electric field device utilised in this experiment is shown in Fig. 6. A high-voltage power supply (dw-p153-05c51) was used to provide power. A uniform electric field was generated by two parallel metal plates, with a magnitude of approximately 2500 V/cm. The microfluidic chip was placed between the two metal plates for electric field treatment; subsequently, the two metal plates were removed, and the transmission of the microfluidic chip was measured by the THz-TDS system.