Rapid and label-free metamaterial-based biosensor for fatty acid detection with terahertz time-domain spectroscopy.

A rapid method for detecting fatty acids (FAs) using terahertz time-domain spectroscopy (THz-TDS) technology combined with a metamaterial-based THz sensor was developed. We measured the THz responses to oleic acid, linoleic acid and α-linoleic acid with different numbers of double-bond, α-linoleic acid and γ-linoleic acid with different conformations. In addition, in order to explore the reason for the observed redshifts of the resonance frequencies of the four FAs, the dielectric constants of the FAs were measured in the THz region. Furthermore, the four fatty acids were also attempted to be identified by Raman spectroscopy, which was difficult to accomplish unambiguously because of the effect of fluorescence. This result thus demonstrates the power and usefulness of metamaterial-assisted THz-TDS in the rapid determination of the FAs, and its potential as a versatile tool for investigation of biological metabolism, and for food product quality, safety inspection and control.

Low-cost and broadband terahertz antireflection coatings based on DMSO-doped PEDOT/PSS.

We report the potential application of 6% dimethylsulfoxide (DMSO)-doped poly (3, 4-ethylenedioxythiophene)/poly (4-styrenesulfonate) (PEDOT/PSS) as a low cost and broadband terahertz (THz) antireflection coating based on the impedance matching effect. The reflected pulses from the quartz and silicon substrates are observed to change with the thickness of the PEDOT/PSS layer. Theoretical analysis based on an equivalent transmission line circuit model and FDTD computational simulations have been used to understand the experimental results. Excellent impedance matching is achieved by a ∼39-nm-thick 6% DMSO-doped PEDOT/PSS layer on quartz, and a ∼101-nm-thick 6% DMSO-doped PEDOT/PSS layer on silicon due to the almost-frequency-independent conductivity of the thin film between 0.3 and 2.5 THz. In the critical conditions, the normalized main pulse transmission remains as high as 74% and 64%, for the quartz and silicon substrates, respectively, significantly higher than the existing state of the art THz antireflection coatings.

A novel method of terahertz spectroscopy and imaging in reflection geometry.

In reflection geometry of terahertz spectroscopy, the biological sample is usually placed on a sample window. This paper presents a novel method for eliminating the effect of the ringing, i.e., the interference between reflections of the reference and the sample, and from the air-window and sample-window interfaces, respectively. In the proposed method, a special thickness of substrate is designed to acquire an accurate reference reflection. The reflections of the samples of deionized water and ethanol were examined, and the calculation of optical properties of the samples by using our proposed method agrees with standard data. The main advantages of this method are simplicity, accuracy, and ease of application for reflection systems with different incident angles.

Quantitative evaluation of mefenamic acid polymorphs by terahertz-chemometrics.

The purpose of the present study is to measure polymorphic content in a bulk powder, mefenamic acid polymorph of pharmaceuticals, as a model drug by THz-spectrometer using frequency-tunable THz wave generators based on difference-frequency generation in gallium phosphate crystals. Mefenamic acid polymorphic forms I and II were obtained by recrystallisation. Eleven standard samples varying a various polymorphic form I content (0-100%) were prepared by physical mixing. After smoothing and area normalising, the THz-spectra of all standard samples showed an isosbestic point at 3.70 THz. After the THz-spectral data sets were arranged into five frequency ranges, and pretreated using various functions, calibration models were calculated by the partial least square regression method. The effect of spectral data management on the chemometric parameters of the calibration models was investigated. The relationship between predicted and actual form I content was the best linear plot. On the regression vector (RV) that corresponded to absorption THz-spectral data, the peak at 1.45 THz was the highest value, and the peak at 2.25 THz was the lowest on RV. THz-spectroscopy with chemometrics would be useful for the quantitative evaluation of mefenamic acid polymorphs in the pharmaceutical industry. This method is expected to provide a rapid and nondestructive quantitative analysis of polymorphs.