ABSTRACT
The linker-free covalent immobilization of polymers on surfaces has the potential to impart new properties and functions to surfaces for a wide range of applications. However, most current methods for the production of these surfaces involve multiple chemical steps and do not have a high degree of control over the chemical functionalities at the surface. A comprehensive study detailing the facile two-step covalent grafting of the antimicrobial peptide nisin onto polystyrene surfaces is reported. Functionalization is achieved using an atmospheric pressure plasma jet, and the reaction is monitored and compared with a standard wet chemical functionalization approach using a variety of analytical techniques. The reactive species produced by the atmospheric pressure plasma jet were analyzed by mass spectrometry and optical emission spectroscopy. The surface chemistry and topography of the functionalized surfaces were determined using contact angle measurements, Fourier infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy and atomic force microscopy respectively. Following surface analysis, the antimicrobial efficacy of the covalently grafted nisin against two major food borne pathogens (Staphylococcus aureus and Listeria monocytogenes) was assessed at two different pHs. The results demonstrated that a post-plasma treatment step after nisin deposition is required to covalently graft the peptide onto the surface. The covalent immobilization of nisin resulted in a significant reduction in bacterial counts within a short 30 minutes contact time. These surfaces were also significantly more antimicrobial compared to those prepared via a more traditional wet chemical approach indicating that the reported method could be a less expensive and less time consuming alternative.
ABSTRACT
Results are presented on the characterisation and optimisation of a non-thermal atmospheric pressure micro-plasma ion source used for ambient mass spectrometry imaging. The geometry of the experiment is optimised to produce the most intense and stable ion signals. Signal stabilities (relative standard deviation) of 2.3-6.5% are achieved for total ion current measurements from chromatograms. Parameters are utilised to achieve MS imaging by raster scanning of PTFE/glass samples with a spatial resolution of 147 ± 31 µm. A systematic study of resolution as a function of acquisition parameters was also undertaken to underpin future technique development. Mass spectra are obtained from PTFE/glass sample edges in negative ion mode and used to construct images to calculate the spatial resolution. Images are constructed using the intensity variation of the dominant ion observed in the PTFE spectrum. Mass spectra originating from the polymer are dominated by three series of ions in a m/z spectral window from 200-500 Da. These ions are each separated by 50 Da and have the chemical formula [C2F + [CF2]n](-), [CF + [CF2]n + O](-) and [CF + [CF2]n + O3](-). The mechanism for the generation of these ions appears to be a polymer chain scission followed by ionisation by atmospheric ion adduction. Positive and negative ion mode mass spectra of personal care products, amino acids and pharmaceuticals, dominated by the proton abstracted/protonated molecular ion, highlight the potential areas of application for such a device. Further to this end a mass spectral image of cardamom seeds, constructed using the variation in intensity of possible fragments of the 1,8-cineole molecule, is included to reveal the potential application to the imaging of foods and other biological materials.
Subject(s)
Mass Spectrometry/methods , Plasma GasesABSTRACT
Plasma-based desorption/ionization sources are an important ionization technique for ambient surface analysis mass spectrometry. In this paper, we compare and contrast three competing plasma based desorption/ionization sources: a radio-frequency (rf) plasma needle, a dielectric barrier plasma jet, and a low-temperature plasma probe. The ambient composition of the three sources and their effectiveness at analyzing a range of pharmaceuticals and polymers were assessed. Results show that the background mass spectrum of each source was dominated by air species, with the rf needle producing a richer ion spectrum consisting mainly of ionized water clusters. It was also seen that each source produced different ion fragments of the analytes under investigation: this is thought to be due to different substrate heating, different ion transport mechanisms, and different electric field orientations. The rf needle was found to fragment the analytes least and as a result it was able to detect larger polymer ions than the other sources.
