Water as a Universal Infrared Probe for Bioanalysis in Aqueous Solution by Attenuated Total Reflection-Surface Enhanced Infrared Absorption Spectroscopy

A surface-enhanced infrared absorption spectroscopy (SEIRA) multivariate approach for atrazine detection

A green, fast and effective multivariate method for the determination of atrazine (ATZ) was developed using conventional infrared equipment furnished with an attenuated total reflectance module (ATR-IR), providing limit of detection (LOD) and limit of quantification (LOQ) in the ranges from 1.9 to 4.6 µg/mL and from 5.6 to 14 µg/mL, respectively. Furthermore, the surface-enhanced infrared absorption (SEIRA) approach was investigated to improve the sensitivity of the measurements and detect ATZ at low concentrations, addressing the compatibility with reference methods. To this end, a substrate formed by silver selenide quantum dots stabilized with mercaptopropionic acid (Ag2Se/MPA), synthesized in aqueous medium by an one-pot synthesis, was used. The spectral data were investigated by univariate and multivariate calibrations, allowing to calculate the enhancement factor (EF) and the multivariate enhancement factor (MEF), respectively. The SEIRA strategy proved to be able to enhance the atrazine signal up to 86-fold, allowing the detection of ATZ at concentrations as low as 0.001 µg/mL.

Thermally Driven Transformation of Water Clustering Structures at Self-Assembled Monolayers

Water clustering structures are considered to play key roles in various temperature-dependent life activities. However, our fundamental understanding of the temperature-dependent water structures remains murky because of the limits of the real-time and real-condition monitoring techniques at the molecular level. We propose an in situ ATR-IR approach combining Gaussian fitting to qualitatively and quantitatively explore the temperature-dependent structural stability and transformation of the three water components, multimer water (MW), intermediate water (IW), and network water (NW), on interfaces with different wettabilities. Our results show that the transformation between NW and IW/MW will occur with a change in temperature. This conversion process shows reversibility on hydrophilic Au NPs film/ZnSe, while it is irreversible on a hydrophobic mercaptohexane self-assembled monolayer due to the irreversibility of the monolayer structure and the hydrophobic confinement effect. The established approach enables us to explore the change in the water properties at any interfaces upon external stimuli.

In situ monitoring of the selective adsorption mechanism of small environmental pollutant molecules on aptasensor interface by attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS)

In situ monitoring of the interactions and properties of pollutant molecules at the aptasensor interface is being a very hot and interesting topic in environmental analysis since its charming molecule level understanding of the mechanism of environmental biosensors. Attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) provides a unique and convenient technique for the in situ analysis, but is not easy for small molecules. Herein, an ATR-SEIRAS platform has been successfully developed to in situ monitor the selective adsorption mechanism of small pollutant molecule atrazine (ATZ) on the aptasensor interface by characteristic N‒H peak of ATZ for the first time. Based on the constructed ATR-SEIRAS platform, a thermodynamics model is established for the selective adsorption of ATZ on the aptasensor interface, described with Langmuir adsorption with a dissociation constant of 1.1 nM. The adsorption kinetics parameters are further obtained with a binding rate constant of 8.08×10⁵ M^-1 s^-1. A promising and feasible platform has therefore successfully provided for the study of the selective sensing mechanism of small pollutant molecules on biosensors interfaces, further broadening the application of ATR-SEIRAS technology in the field of small pollutant molecules.

Dynamic surface properties of PEG-coated CuS nanoparticles alter their interaction with cells as revealed by surface-enhanced infrared spectroscopy

Characterization of the dynamic changes of the basic surface properties of nanoparticles is of great significance to reveal the interaction mechanism between nanoparticles and cells; however, it is often neglected due to the limitations of existing analytical methods. This knowledge has been renewed by using surface enhanced infrared absorption spectroscopy (SEIRAS) to study the interaction between PEG-CuS nanoparticles and living cells attached to rGO-Au modified Au films. Based on the difference spectra of cell membranes and the associated water, we clearly revealed that Cu²⁺ ions produced by the degradation of PEG-CuS can coordinate with PEG, thus changing the interaction between nanoparticles and cells including how and how many nanoparticles enter the cells and the sequential photothermal effect, which breaks through the limitations of the present analytical methods.

End Group Properties of Thiols Affecting the Self-Assembly Mechanism at Gold Nanoparticles Film As Evidenced by Water Infrared Probe

Water infrared probe has been employed for in situ monitoring of the detailed self-assembly processes of four thiol molecules with different end groups (-CH₃, -NH₂, -COOH, and -OH) on gold nanoparticles (Au NPs) film in aqueous solution. Based on the change of water IR signal, the significant influence of end group properties on the kinetics and thermodynamics of thiols self-assembly can be estimated. It is found that the assembly kinetics of thiols decreases with the increase of the hydrophobicity of the end groups. In addition, the charges carried by the end groups (-COOH and -NH₂ terminated thiols) will also slow down the self-assembly kinetics owing to the electrostatic repulsions. However, the isothermal adsorption is only affected by the wettability of the end groups of thiols. The higher hydrophilicity of the end groups results in larger equilibrium constant of the self-assembly process. Results show that water infrared probe offers an additional approach to the monitoring of thiols self-assembly processes with higher sensitivity and more detailed information as compared to traditional molecule fingerprints.