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Surface-supported metal-organic frameworks HKUST-1 (Hong Kong University of Science and Technology) were used as a model system for a development of a near ambient pressure (NAP) XPS based approach to investigate interaction with atmospheres of water, methanol or pyridine at pressures ranging from 1 to 4 mbar. The films were grown on a gold substrate functionalized with a COOH-terminated self-assembled monolayer using liquidphase epitaxy in a step-by-step fashion. Measurement protocols were developed and optimised for different gases in order to obtain spectra of similar quality in terms of signal intensity, noise and shape. Peak shapes were found to depend on the efficiency of charge compensation. Reference measurements in argon proved to be a useful strategy not only for the evaluation of the Cu(II)-fraction in pristine samples, but also to identify the contributions by the respective gas atmosphere to the C 1s and O 1s photoelectron spectra. Reduced copper was found during the exposition of HKUST-1 to water vapour and pyridine, but this effect was not observed in case of methanol. Additionally, it was established that there are no changes in relative Cu(II) percentage with increasing exposure time. This indicates that saturation was reached already at the lowest time of gas exposure. A detailed elucidation of the mechanism of Cu(II) reduction to Cu(I) in HKUST-1 mediated by water and pyridine is part of ongoing work and not in the scope of the present paper.
Near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at greater than 2500 Pa. With NAP-XPS, XPS can probe moderately volatile liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly. In this submission, we show survey, C 1s, O 1s, and N
1s narrow scans from an aqueous solution of a common protein, bovine serum albumin. The C 1s peak envelope is well fit to four symmetric peaks of equal width that correspond to carbon bonded to carbon and hydrogen (C-1), carbon singly bonded to oxygen (C-2), carbonyl and/or amide carbon (C-3), and carboxyl carbon (C-4). Two possible peak fits are considered for the N 1s and O 1s peak envelopes. The N 1s signal is fit to four peaks that correspond to amine (—NH2), Amide (OvCZNH2), ammonium (—NH3 +), and N2(g) nitrogen, and alternatively to three peaks that correspond to amine, amide, and N2(g) nitrogen. The O 1s peak envelope is similarly fit to three and four components.
A drawback for X-ray photoelectron spectroscopy is that the measurements must be performed under ultra-high vacuum, which limits the type of samples which can be studied. However, by applying a differentially pumped aperture positioned close to the surface, even wet samples can be measured at near ambient pressure while the energy analyser is still under ultra-high vacuum, as illustrated below. Successful XPS-measurements with pressure up to 30 mbar have been reported using this approach, which opens up a new world of possibilities for ambient pressure measurements with XPS.
While there are examples where NAP-XPS has been used to study electrochemical processes and heterogeneous catalysis, little attention has been paid to its potential use in biological materials. Until now, bacteria have only been characterised with conventional XPS, which requires tedious sample preparation usually involving freeze drying, a treatment that may degrade biological sample constituents. By studying biological samples in their native wet states, new insight about composition, absorption and transport of drugs through cell membranes and extracellular polymeric substance (EPS) layers can be obtained.
Both artificial model-films of exopolysaccharides and biofilms of Escherichia Coli have been characterised at pressures ranging from ultra-high vacuum to 15 mbar by using SPECS’ EnviroESCA NAP-XPS instrument and conventional XPS. By applying antimicrobials to model biofilms, some of which are known to be resistant towards the antimicrobial in question, the distribution of antimicrobials in biofilms has been studied. Measurement capabilities and limitations of the approach will be discussed.
Near Ambient Pressure XPS opens up a new world of possibilities for measurements with XPS. While there are examples where NAP-XPS has been used to study electrochemical processes and heterogeneous catalysis, little attention has been paid to its potential use in biological materials. Until now, bacteria have only been characterised with conventional XPS, which requires tedious sample preparation usually involving freeze drying, a treatment that may degrade biological sample constituents. By studying biological samples in their native wet states, new insight about composition, absorption and transport of drugs through cell membranes and extracellular polymeric substance (EPS) layers can be obtained. Both artificial model-films of exopolysaccharides and biofilms of Escherichia Coli have been characterised at pressures ranging from ultra-high vacuum to 15 mbar by using SPECS’ EnviroESCA NAP-XPS instrument and conventional XPS. By applying antimicrobials to model biofilms, some of which are known to be resistant towards the antimicrobial in question, the distribution of antimicrobials in biofilms has been studied. Capabilities and limitations of the approach will be discussed.
Bacteria generally interact with the environment via processes involving their cell-envelope. Thus, techniques that may shed light on their surface chemistry are attractive tools for providing an understanding of bacterial interactions. One of these tools is Al Kα-excited photoelectron spectroscopy (XPS) with its estimated information depth of <10 nm. XPS-analyses of bacteria have been performed for several decades on freeze-dried specimens in order to be compatible with the vacuum in the analysis chamber of the spectrometer. A limitation of these studies has been that the freeze-drying method may collapse cell structure as well as introduce surface contaminants. However, recent developments in XPS allow for analysis of biological samples at near ambient pressure (NAP-XPS) or as frozen hydrated specimens (cryo-XPS) in vacuum. In this work, we have analyzed bacterial samples from a reference strain of the Gram-negative bacterium Pseudomonas fluorescens using both techniques. We compare the results obtained and, in general, observe good agreement between the two techniques. Furthermore, we discuss advantages and disadvantages with the two analysis approaches and the output data they provide. XPS reference data from the bacterial strain are provided, and we propose that planktonic cells of this strain (DSM 50090) are used as a reference material for surface chemical analysis of bacterial systems.
The degradation of the chemotherapy drug 5-fluorouracil by a non-pristine metal surfaces is studied.Using density functional theory, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy we show that the drug is entirely degraded by medical-grade silver surfaces, already at body temperature,and that all of the fluorine has left the molecule, presumably as HF. Remarkably, this degradation is even more severe than that reported previously for 5-fluorouracil on a pristine monocrystalline silver surface(in which case 80% of the drug reacted at body temperature) [1]. We conclude that the observed reaction is due to a reaction pathway, driven by H to F attraction between molecules on the surface, which results in the direct formation of HF; a pathway which is favoured when competing pathways involving reactive Ag surface sites are made unavailable by environmental contamination. Our measurements indicate that realistically cleaned, non-pristine silver alloys, which are typically used in medical applications, can result in severe degradation of 5-fluorouracil, with the release of HF – a finding which may have important implications for the handling of chemotherapy drugs.
Synchrotron XPS in the soft-X-ray regime is suitable for the detection of light elements commonly found in biological samples. Various model systems of biofilms have been developed and characterised at synchrotron- and lab-based facilities. By obtaining the chemical composition at various information depths, the vertical distribution of iodine in an artificial biofilm have been determined.
Detection of suspended nanoparticles with near-ambient pressure x-ray photoelectron spectroscopy
(2017)
Two systems of suspended nanoparticles have been studied with near-ambient pressure x-ray photoelectron spectroscopy: silver nanoparticles in water and strontium fluoride—calcium fluoride core-shell nanoparticles in ethylene glycol. The corresponding dry samples were measured under ultra high vacuum for comparison. The results obtained under near-ambient pressure were overall comparable to those obtained under ultra high vacuum, although measuring silver nanoparticles in water requires a high pass energy and a long acquisition time. A shift towards higher binding energies was found for the silver nanoparticles in aqueous Suspension compared to the corresponding dry sample, which can be assigned to a change of surface potential at the water-nanoparticle interface. The shell-thickness of the core-shell nanoparticles was estimated based on simulated spectra from the National Institute of Standards and Technology database for simulation of electron spectra for surface analysis. With the instrumental set-up presented in this paper, nanoparticle suspensions in a suitable Container can be directly inserted into the analysis chamber and measured without prior sample preparation.
Near-ambient pressure XPS makes it possible to characterise samples not compatible to ultra-high vacuum, and enables the study of liquid-solid, gas-liquid and gas-solid interfaces. NAP-XPS meas-urements of biofilms, suspended nanoparticles and metal-organic frameworks were performed with EnviroESCA developed by SPECS.
An interesting application is surface characterisation of biofilms, which are bacterial communities embedded in a self-produced polysaccharide matrix. Various model systems ranging from pure polysaccharides of alginate to biofilms harvested directly from the growth medium have been char-acterised in humid conditions[1].
NAP-XPS also makes it possible to characterise nanoparticles in solution. Silver nanoparticles in aqueous solution were characterised and the Ag 3d-spectrum compared to spectra obtained of dried nanoparticles in UHV-conditions[2]. The binding energy of the Ag 3d-core level peak was shifted by 0,6 eV towards higher binding energy for suspended nanoparticles compared to the dried sample measured in UHV. This can be assigned to a change in surface potential at the water-nanoparticle interface.
Metal-organic frameworks (MOFs) are suitable materials for gas storage of small molecules due to their nanoporous, crystalline structure. However, instability in humidity remains an issue for many types of MOFs. XPS-measurements of the MOF-structure HKUST-1 were performed in various NAP-conditions to assess the stability of the sample and its interaction with the gas molecules as water, methanol and pyridine.