Mingkun Zhang

Molecular dynamics simulations of conformation and chain length dependent terahertz spectra of alanine polypeptides

Terahertz absorption spectra of alanine polypeptides in water were simulated with classical molecular dynamics at 310 K. Vibrational modes and oscillator strengths were calculated based on a quasi-harmonic approximation. Absorption spectra of Alan (n = 5, 15, 30) with different chain lengths and Ala15 in coiled and helical conformations were studied in 10–40 cm− 1 bandwidth. Simulation results indicated both the chain length and the conformation have significant influences on THz spectra of alanine polypeptides. With the increase of chain length, the average THz absorption intensity increases. Compared with the helical Ala15 polypeptide, the THz spectra of coiled one shows stronger absorption peaks. These results were explained from different numbers of hydrogen bonds formed between polypeptides and the surrounding water molecules.

Rapid and label-free detection and assessment of bacteria by terahertz time-domain spectroscopy

Here we demonstrated the potential and applicability of terahertz (THz) spectroscopy to detect four commonly found bacteria in the infectious diseases. Besides the different spectral characteristics between bacterial species, THz absorption differences for living bacteria, dead bacteria and bacterial powder of the same species were also investigated. Our results revealed that small differences in water contents between bacterial cells account for distinct discrepancies of the absorption coefficients, which can be used for bacterial species identification. Furthermore, living and dead bacteria showed different absorption coefficients as a result of their different hydration levels, suggesting that THz spectroscopy can be used to rapidly assess the living state of bacteria under test. Our results clearly demonstrated the ability of THz spectroscopy for time-saving and label-free detection of bacteria with minimal sample preparation, potentially to be utilized for point-of-care tests in the near future. Schematic representation of bacterial detection by THz spectroscopy. Different bacteria have distinctive absorption coefficients as a result of their different water contents.

Determination of Critical Micelle Concentrations of Surfactants by Terahertz Time-Domain Spectroscopy

A novel detection method is presented to determine the critical micelle concentrations (CMCs) of surfactants by using a simple and convenient terahertz time-domain spectroscopy (THz-TDS) technology which has been successfully used to study the picosecond dynamics of water in reverse micelles. Absorption coefficients or refractive indices of aqueous solutions of an ionic surfactant sodium dodecyl sulfate and a nonionic surfactant nonaethylene glycol monododecyl ether in 0.2-1.8 THz frequency range at different concentrations are measured through the THz-TDS. A clear intersection of THz absorption coefficient or refractive index caused by the micelle formation is observed. This phenomenon is strongly associated with the change of component ratio between surfactant, bulk water and hydration water during the formation of micelles. The corresponding concentration is determined as the CMC of the surfactant which is in good agreement with the most existing results using traditional methods. For comparison, absorption coefficients of the two surfactants in hydrochloric acid solutions are also measured and differences in contrast to those in aqueous solutions are observed and explained. Our findings indicate that THz spectroscopy technology can be applied to determine the CMCs of surfactants in a sensitive, accurate, and label-free way.

Label-free sensing of the binding state of MUC1 peptide and anti-MUC1 aptamer solution in fluidic chip by terahertz spectroscopy

The aptamer and target molecule binding reaction has been widely applied for construction of aptasensors, most of which are labeled methods. In contrast, terahertz technology proves to be a label-free sensing tool for biomedical applications. We utilize terahertz absorption spectroscopy and molecular dynamics simulation to investigate the variation of binding-induced collective vibration of hydrogen bond network in a mixed solution of MUC1 peptide and anti-MUC1 aptamer. The results show that binding-induced alterations of hydrogen bond numbers could be sensitively reflected by the variation of terahertz absorption coefficients of the mixed solution in a customized fluidic chip. The minimal detectable concentration is determined as 1 pmol/μL, which is approximately equal to the optimal immobilized concentration of aptasensors.

A microfluidic chip for terahertz spectral detection

Fluidic chips as sample storage devices have been widely used to control and analyze traces of solutions by terahertz time domain spectroscopy (THz-TDS). This paper presents a fluidic chip fabricated with ultra-thick micro-fluidic structures utilizing SU-8 photoresist for THz spectral detection. The chip consists of 2 pieces of quartz windows and a photoresist structure serves as liquid cell between the windows. In order to reduce the attenuation of THz energy by the windows and guarantee measurement stability, an infrared quartz with thickness of 1mm is adopted as the windows. For the purpose of improving the signal-to-noise ratio (SNR) in the frequency band between 1.0 THz-1.5 THz, and making the liquid film thickness match the THz optical path, the structure depth is designed to be 50μm. Water absorption coefficient at 0.1THz-1.5THz was detected with this chip. The result indicates that the chip has a good stability for terahertz spectral detection.

The properties, preparation approaches and uses of microfluidic channels for terahertz absorption signatures detection in aqueous

A microfluidic channels chip and its processing method and application are introduced for Terahertz (THz) absorption signatures detection of biomolecules in aqueous solution. Thousands of rectangular microchannels with a characteristic dimension of 2 μm and their reservoirs were precisely etched in a high resistivity silicon wafer by lithography and etching process, then the silicon wafer and a glass sheet were bonded to form a sealed microfluidic chip by anodic bonding technology. The chip can be sampled through the capillary action and the microfluidic pressure system. Three absorption signatures around 850, 928, 950 GHz of dissolved λ-DNA were obtained in 0.7∼1.0 THz band by using this chip and the coherent photomixing spectrometer. It is indicated that dispersion of biomolecular solution in 2 μm spaces between the tines of the grating in microchannels can reduce the water component in THz transmission detection, thereby reducing the water background absorption, effectively improve the signal strength of THz characteristic attenuation.

Detection of oligonucleotides in aqueous solution based on terahertz spectroscopy

A terahertz spectroscopic study is carried out to analyze DNA mutations in a label-free manner. The 5 base mutations from the 3′-terminus on single-stranded 17nt oligonucleotides has been detected in aqueous solution.