The use of polymer coated surface acoustic wave (SAW) sensor arrays is a very promising technique for highly sensitive and selective detection of volatile organic compounds (VOCs). We present new developments to achieve a low cost sensor setup with a sampling method enabling the highly reproducible detection of volatiles even in the ppb range. Since the VOCs of coffee are well known by gas chromatography (GC) research studies, the new sensor array was tested for an easy assessable objective: coffee ageing during storage. As reference method these changes were traced with a standard GC/FID set-up, accompanied by sensory panellists. The evaluation of GC data showed a non-linear characteristic for single compound concentrations as well as for total peak area values, disabling prediction of the coffee age. In contrast, the new SAW sensor array demonstrates a linear dependency, i.e. being capable to show a dependency between volatile concentration and storage time. (C) 2014 Elsevier Ltd. All rights reserved.

Arrays with polymer-coated acoustic sensors, such as surface acoustic wave (SAW) and surface transverse wave (STW) sensors, have successfully been applied for a variety of gas sensing applications. However, the stability of the sensors’ polymer coatings over a longer period of use has hardly been investigated. We used an array of eight STW resonator sensors coated with different polymers. This sensor array was used at semi-annual intervals for a three-year period to detect organic solvent vapors of three different chemical classes: a halogenated hydrocarbon (chloroform), an aliphatic hydrocarbon (octane), and an aromatic hydrocarbon (xylene). The sensor signals were evaluated with regard to absolute signal shifts and normalized signal shifts leading to signal patterns characteristic of the respective solvent vapors. No significant time-related changes of sensor signals or signal patterns were observed, i.e., the polymer coatings kept their performance during the course of the study. Therefore, the polymer-coated STW sensors proved to be robust devices which can be used for detecting organic solvent vapors both qualitatively and quantitatively for several years.

This study presents a sustainable approach for Methylene Blue (MB) dye removal using pristine, non-activated biochar derived from orange peel waste via Microwave-Assisted Pyrolysis (MAP). The key novelty lies in the systematic comparison of the biochar’s adsorption performance under both pH-controlled (constant pH 4) and unregulated pH conditions, demonstrating that pH regulation is essential for optimizing adsorption efficiency. The resulting biochar exhibited a high fixed carbon content (60.89%), an alkaline surface (Point of zero charge (pH_pzc) = 11.20, ZPotential = 0.1 mV @ pH 9), and oxygenated functional groups. Best MB removal of 83% was achieved at pH 4, despite the expected electrostatic repulsion. Kinetic studies showed the best fit with the Elovich model, indicating a heterogeneous surface. The Langmuir isotherm accurately described the equilibrium data, revealing a maximum adsorption capacity (q_max) of 20.57 mg g⁻¹ under pH-controlled conditions, representing an 83% increase over the 11.24 mg g⁻¹ obtained in the unregulated scenario. Thermodynamic analysis confirmed the process is spontaneous (ΔG° < 0), endothermic (ΔH° = + 4.88 kJ mol⁻¹ at constant pH), and governed by physisorption mechanisms, including hydrogen bonding and π-π interactions. This work demonstrates that pristine orange peel biochar generated via MAP is a highly effective adsorbent and highlights the critical impact of pH control on improving adsorption capacity and elucidating the dominant physisorption mechanisms.