Month: March 2024

Essential oils have been employed for centuries in various cultures for their therapeutic properties. Beyond their delightful fragrances, essential oils are gaining increasing recognition for their potent antimicrobial activity. These volatile and concentrated extracts derived from plants possess a remarkable ability to combat a wide array of harmful microorganisms, including bacteria, viruses, fungi, and even some parasites. The exploration of essential oils as natural alternatives to synthetic antimicrobial agents has sparked significant interest in recent years, fuelled by the growing awareness of the limitations and potential risks associated with conventional antibiotics and disinfectants. Among the wide variety of essential oils available, thymol, menthol, eugenol, and carvacrol stand out for their remarkable potency in terms of antimicrobial properties. In this comprehensive exploration of essential oils for antimicrobial activity, RELIANCE is currently in progress, merging the potential of nanomaterials with essential oils to develop a highly promising antimicrobial surface. Given that one method depends on the gradual release of essential oils from loaded nanoparticles, monitoring the amount of essential oils present on the surface in real-time can serve as a valuable, fast analytical approach. In this context, it has recently been demonstrated that the paper can be used as support for the development of electrochemical devices useful for this purpose.

Paper-based sensors

Paper-based electrochemical (bio)sensors have emerged as highly attractive analytical devices for their superior sustainable features, such as avoiding the use of polyester as support and the reduction of waste, being incinerated after use. However, paper-based electrochemical (bio)sensors have recently demonstrated further advantages, including the simple combination with vertical microfluidics and their use as a reservoir to deliver smart electrochemical (bio)sensors that are able to i) contain the reagents, ii) preconcentrate the target analyte, and iii) synthesize the nanomaterials inside the paper network. Furthermore, these devices have shown ability to overcome the limitations of the other printed electrochemical sensors in the measurement of entirely liquid samples by detecting the target analyte in the aerosol phase or solid sample, without the additional sampling system. Herein, we developed in RELIANCE a paper-based device for the detection of essential oils on the surface, as a smart tool to evaluate the availability of these compounds on the functionalized surface.

Preparation of paper-based sensor

Filter paper-based screen-printed electrodes were home-produced. Filter paper sheets were firstly modified with an ad hoc designed wax pattern in order to delimitate the hydrophilic area in which liquid samples were dropped, avoiding them to reach the electrical contacts through capillary permeation. The wax pattern was printed onto filter paper by means of a special printer and treated at 100 °C for 1 min in order to allow the wax to homogeneously permeate through the paper network. Then, conductive inks were used to print a three-electrode system onto wax-modified filter paper sheets. The working and counter-electrodes were obtained using a graphite-based ink and the pseudo-reference electrode was printed using Ag/AgCl-based ink (Fig. 1).

Electrochemical detection of essential oils

Different pulse voltammetry was selected for essential oils detection by connecting the paper-based sensor to a portable potentiostat connected to a laptop to easily manage the data. The modification of the working electrode surface with nanomaterial dispersion in order to improve the analytical performance of the sensor was demonstrated to be critical for the sensitive detection of essential oils. The electrochemical measurements were performed by sampling the biological compounds putting the paper strip in contact with the site wetted by oils. The calibration curves were carried out by analyzing different solutions of thymol, eugenol, and carvacrol in the concentration range comprised between 2-16 ppm.

Conclusion

The results obtained have demonstrated the ability to detect essential oils such as thymol, eugenol, and carvacrol on the surface using a paper-based device. This method offers the advantage of simplicity, sustainability and provides broad and promising applications for the detection of these substances.

Contributor: University of Rome Tor Vergata

The goal of RELIANCE project is to design and develop smart response self-disinfectant antimicrobial nanocoatings based on a new range of antimicrobial nanoparticles. The framework, on which the actual functionalities that bring out the antimicrobial effects are being built, comprises of mesoporous silica nanoparticles (SMIN). The silica particles are obtained via modification of the well-established Stöber process, using a soft-template method. Although the process to prepare these particles is relatively simple per se, the upscale of chemical processes or reactions using larger quantities of reactants rarely goes smoothly giving comparable results to laboratory scale. The upscale of mesoporous silica nanoparticles is no exception. Furthermore, with reference to the myriad of scientific articles dealing with the silica nanoparticle synthesis, it is evident that even minute changes in some parameter may cause a significant effect on the result. Therefore, to obtain conditions that are robust enough to enable the synthesis in a reproducible manner, would be of paramount importance from the upscaling point of view.

In RELIANCE, SMIN are prepared in batch processes. Unfortunately, there is no real practical way to monitor the progress of the reaction during the synthesis itself and to do adjustments as necessary to steer the reaction into the wanted direction. Therefore, the evaluation of the outcome will be done only after the process is completed and the product isolated and purified. The method of “trial and error” is both laborious and time consuming.

One of the key targets in RELIANCE is to produce particles with defined particle size distribution. When going to ever smaller particle sizes (i.e. nanoparticles), the task becomes more challenging. Multiple experiments have been carried out do date, to close the gap between the results obtained at lab-scale and the initial upscaled trials. The figure below shows a scanning electron microscope image of the product obtained in very recent upscaled experiment (50x reactant amounts compared to labscale). The particle size distribution measured from the image, and further supported by an independent laser diffraction analysis, are in agreement of a result very close to the targeted values. In the near future, the quest will focus on fine tuning of the current experimental parameters and set-up to still improve the particle size distribution.

When speaking about true colloidal solutions where the particles (with their size characterised in the nanometer scale, ≈ 100 nm and below) are separate and dispersed throughout the media, their isolation from the reaction mixture is not possible by simple filtration techniques and more advanced methods need to be utilised, like for example centrifugation. Preparing SMIN in quantities as set up by RELIANCE would require upscaling to volumes of tens of liters and that would no longer be viable to be processed by centrifugation. By changing the synthesis parameters, it has been observed that the formed particles start to assemble into larger aggregated structures. Formation of aggregates has a huge effect on the following step: when forming larger sized aggregates, simple vacuum or pressure filtration of the particles is again possible, therefore simplifying the work-up remarkably.

Although the aggregation is beneficial to ease the product isolation process, there is a downside. For the final coatings, particles need to be separated from each other to give well-dispersed and homogenous coating solutions. The experiments done so far demonstrate that using suitable techniques together with possible processing aids can lead to obtaining good dispersions of the particles.

Contributor: MILLIDYNE