Silver nanofractals: electrochemical synthesis, XPS characterization and application in LDI-MS

Silver nanocluster-based fluorescent probes for detection of trace water in organic solvents with potential application in reaction monitoring

Confined silver nanoclusters within the pores of zeolites can bring about fascinating luminescent composites with high stability. The open structure of zeolites endues the materials with excellent fluorescence sensing performance, making them suitable for application in sensor fields. In LTA zeolites, Ag42+ clusters can form [Ag4(H2O)m]2+ in the hydrated state exhibiting green emission or form [Ag6(O)n]2+ with red emission in the dehydration state, leading to reversible emission. Herein, we prepared a luminescent detector based on Ag-zeolites for trace amounts of water in organic solvents through a simple, cost-effective and environmentally friendly method. With the incorporation of water, the materials display a remarkable reversible optical switch from red to green owing to the reversible Ag42+ cluster structure of [Ag4(H2O)m]2+ ↔ [Ag6(O)n]2+ in zeolites. The luminescent material features a low detection limit (0.2% v/v) for water and can be used for detecting trace amounts of water in organic solvents.

Advances in LDI-MS Analysis: The Role of Chemical Vapor Deposition-Synthesized Silver Nanoparticles in Enhancing Detection of Low-Molecular-Weight Biomolecules

In this investigation, we detail the synthesis of silver nanoparticles (AgNPs) via a precise chemical vacuum deposition (CVD) methodology, aimed at augmenting the analytical performance of laser desorption/ionization mass spectrometry (LDI-MS) for the detection of low-molecular-weight analytes. Employing a precursor supply rate of 0.0014 mg/s facilitated the formation of uniformly dispersed AgNPs, characterized by SEM and AFM to have an average diameter of 33.5 ± 1.5 nm and a surface roughness (Ra) of 11.8 nm, indicative of their homogeneous coverage and spherical morphology. XPS and SEM-EDX analyses confirmed the metallic silver composition of the nanoparticles with Ag peak splitting, reflecting the successful synthesis of metallic Ag. Comparative analytical evaluation with traditional MALDI matrices revealed that AgNPs significantly reduce signal suppression, thereby enhancing the sensitivity and specificity of LDI-MS for low-molecular-weight compounds such as triglycerides, saccharides, amino acids, and carboxylic acids. Notably, the application of AgNPs demonstrated a superior linear response for triglyceride signals with regression coefficients surpassing 0.99, markedly outperforming conventional matrices. The study further extends into quantitative analysis through nanoparticle-based laser desorption/ionization (NALDI), where AgNPs exhibited enhanced ionization efficiency, characterized by substantially lower limits of detection (LOD) and quantification (LOQ) for tested standards. Particular attention was paid to lipids with a detailed examination of their fragmentation pathways. These results highlight the significant potential of AgNPs synthesized via CVD to transform the analytical detection and quantification of low-molecular-weight compounds using NALDI. This approach offers a promising avenue for expanding the scope of analytical applications in mass spectrometry and introducing innovative methodologies for enhanced precision and sensitivity.

Effects of High-Quality Carbon Nanowalls Ionization-Assisting Substrates on Surface-Assisted Laser Desorption/Ionization Mass Spectrometry Performance

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) is performed using carbon nanowalls (CNWs) for ionization-assisting substrates. The CNWs (referred to as high-quality CNWs) in the present study were grown using a radical-injection plasma-enhanced chemical vapor deposition (RI-PECVD) system with the addition of oxygen in a mixture of CH₄ and H₂ gases. High-quality CNWs were different with respect to crystallinity and C-OH groups, while showing similar wall-to-wall distances and a wettability comparable to CNWs (referred to as normal CNWs) grown without O₂. The efficiency of SALDI was tested with both parameters of ion intensity and fragmental efficiency (survival yield (SY)) using N-benzylpyridinuim chloride (N-BP-CI). At a laser fluence of 4 mJ/cm², normal CNWs had an SY of 0.97 and an ion intensity of 0.13, while 5-sccm-O₂- high-quality CNWs had an SY of 0.89 and an ion intensity of 2.55. As a result, the sensitivity for the detection of low-molecular-weight analytes was improved with the high-quality CNWs compared to the normal CNWs, while an SY of 0.89 was maintained at a low laser fluence of 4 mJ/cm². SALDI-MS measurements available with the high-quality CNWs ionization-assisting substrate provided high ionization and SY values.

Self-Standing Bioinspired Polymer Films Doped with Ultrafine Silver Nanoparticles as Innovative Antimicrobial Material

Thin self-standing films with potential antimicrobial synergistic activity have been produced by a simple green chemical synthesis with overnight thermal treatment. Their properties have been studied by scanning electron microscopy, X-ray photoelectron spectroscopy and other techniques to understand their potential range of applications. In this work, the focus was set on the development of a potential novel and effective alternative to conventional antimicrobial materials. By creating an antimicrobial polymer blend, and using it to develop and immobilize fine (~25 nm) silver nanophases, we further aimed to exploit its film-forming properties and create a solid composite material. The resulting polymer matrix showed improved water uptake percentage and better stability in the presence of water. Moreover, the antimicrobial activity of the films, which is due to both organic and inorganic components, has been evaluated by Kirby-Bauer assay against common foodborne pathogens (Staphylococcus aureus and Salmonella enterica) and resulted in a clear inhibition zone of 1.2 cm for the most complex nanocomposition. The excellent performance against bacteria of fresh and 6-month-old samples proves the prospects of this material for the development of smart and biodegradable food packaging applications.

Isolation and Identification of Lactococcus lactis and Weissella cibaria Strains from Fermented Beetroot and an Investigation of Their Properties as Potential Starter Cultures and Probiotics

The presence of certain microorganisms in dairy products or silage is highly desirable. Among them are probiotic strains of lactic acid bacteria (LAB), which show many beneficial features, including antimicrobial properties that support the development of beneficial microflora; in addition, owing to their biochemical activity, they influence the nutritional, dietary, and organoleptic properties of food products. Before being placed on the market, each strain requires separate testing to determine its probiotic properties and effectiveness. The aim of this study was to isolate LAB strains from a pickled beetroot sample that could be used in the dairy industry and with the potential to be considered as a probiotic in the future. Two strains identified using the MALDI technique were selected—Lactococcus lactis and Weissella cibaria. The optimal growth conditions of the strains were determined, and their proteolytic properties were assessed with the use of the o-PA reagent and spectrophotometry. The lipid profile was analyzed using the SALDI (surface-assisted laser desorption/ionization) technique and silver nanoparticles. High-performance liquid chromatography was used to assess the ability of the strains to synthesize beneficial metabolites, such as B vitamins (B2, B3, and B9) or lactic acid, and gas chromatography was used to analyze the substances responsible for organoleptic properties. Moreover, the ability to inhibit the growth of pathogenic strains was also tested in the selected strains. Both tested strains demonstrated the desired properties of starter cultures for future use in functional food production, showing that fermented plant products can serve as valuable potential probiotic sources.

Nanostructured Layer of Silver for Detection of Small Biomolecules in Surface-Assisted Laser Desorption Ionization Mass Spectrometry

A facile approach for the synthesis of a silver nanostructured layer for application in surface-assisted laser desorption/ionization mass spectrometry of low-molecular-weight biomolecules was developed using electrochemical deposition. The deposition was carried out using the following silver salts: trifluoroacetate, acetate and nitrate, varying the voltage and time. The plate based on trifluoroacetate at 10 V for 15 min showed intense SALDI-MS responses for standards of various classes of compounds: fatty acids, cyclitols, saccharides and lipids at a concentration of 1 nmol/spot, with values of the signal-to-noise ratio ≥50. The values of the limit of detection were 0.71 µM for adonitol, 2.08 µM for glucose and 0.39 µM for palmitic acid per spot. SEM analysis of the plate showed anisotropic flower-like microstructures with nanostructures on their surface. The reduced chemical background in the low-mass region can probably be explained by the absence of stabilizers and reducing agents during the synthesis. The plate synthesized with the developed approach showed potential for future use in the analysis of low-molecular-weight compounds of biological relevance. The absence of the need for the utilization of sophisticated equipment and the synthesis time (10 min) may benefit large-scale applications of the layer for the detection of various types of small biomolecules.

Effects of Carbon Nanowalls (CNWs) Substrates on Soft Ionization of Low-Molecular-Weight Organic Compoundsin Surface-Assisted Laser Desorption/Ionization Mass Spectrometry (SALDI-MS)

Carbon nanowalls (CNWs), which are vertically oriented multi-layer graphene sheets, were employed in surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) measurements to detect low-molecular-weight organic compounds. CNWs substrates with widely different wall-to-wall distances from 142 to 467 nm were synthesized using a radical-injection plasma-enhanced chemical vapor deposition (RI-PECVD) system with nanosecond pulse biasing to a sample stage. When survival yield (SY) values of N-benzylpyridinium chloride (N-BP-Cl) were examined, which is commonly used to evaluate desorption/ionization efficiency, a narrower wall-to-wall distance presented a higher SY value. The highest SY value of 0.97 was realized at 4 mJ/cm² for the highest-density CNWs with a wall-to-wall distance of 142 nm. The laser desorption/ionization effect of arginine, an amino acid, was also investigated. When CNWs with a narrower wall-to-wall distance were used, the signal-to-noise (SN) ratios of the arginine signals were increased, while the intensity ratios of fragment ions to arginine signals were suppressed. Therefore, the CNWs nanostructures are a powerful tool when used as a SALDI substrate for the highly efficient desorption/ionization of low-molecular-weight biomolecules.

The Pros and Cons of the Use of Laser Ablation Synthesis for the Production of Silver Nano-Antimicrobials

Silver nanoparticles (AgNPs) are well-known for their antimicrobial effects and several groups are proposing them as active agents to fight antimicrobial resistance. A wide variety of methods is available for nanoparticle synthesis, affording a broad spectrum of chemical and physical properties. In this work, we report on AgNPs produced by laser ablation synthesis in solution (LASiS), discussing the major features of this approach. Laser ablation synthesis is one of the best candidates, as compared to wet-chemical syntheses, for preparing Ag nano-antimicrobials. In fact, this method allows the preparation of stable Ag colloids in pure solvents without using either capping and stabilizing agents or reductants. LASiS produces AgNPs, which can be more suitable for medical and food-related applications where it is important to use non-toxic chemicals and materials for humans. In addition, laser ablation allows for achieving nanoparticles with different properties according to experimental laser parameters, thus influencing antibacterial mechanisms. However, the concentration obtained by laser-generated AgNP colloids is often low, and it is hard to implement them on an industrial scale. To obtain interesting concentrations for final applications, it is necessary to exploit high-energy lasers, which are quite expensive. In this review, we discuss the pros and cons of the use of laser ablation synthesis for the production of Ag antimicrobial colloids, taking into account applications in the food packaging field.

Inhibiting P. fluorescens biofilms with fluoropolymer-embedded silver nanoparticles: an in-situ spectroscopic study

Surface colonization by microorganisms leads to the formation of biofilms, i.e. aggregates of bacteria embedded within a matrix of extracellular polymeric substance. This promotes adhesion to the surface and protects bacterial community, providing an antimicrobial-resistant environment. The inhibition of biofilm growth is a crucial issue for preventing bacterial infections. Inorganic nanoparticle/Teflon-like (CF_x) composites deposited via ion beam sputtering demonstrated very efficient antimicrobial activity. In this study, we developed Ag-CF_x thin films with tuneable metal loadings and exceptional in-plane morphological and chemical homogeneity. Ag-CF_x antimicrobial activity was studied via mid-infrared attenuated total reflection spectroscopy utilizing specifically adapted multi-reflection waveguides. Biofilm was sampled by carefully depositing the Ag-CF_x film on IR inactive regions of the waveguide. Real-time infrared spectroscopy was used to monitor Pseudomonas fluorescens biofilm growth inhibition induced by the bioactive silver ions released from the nanoantimicrobial coating. Few hours of Ag-CF_x action were sufficient to affect significantly biofilm growth. These findings were corroborated by atomic force microscopy (AFM) studies on living bacteria exposed to the same nanoantimicrobial. Morphological analyses showed a severe bacterial stress, leading to membrane leakage/collapse or to extended cell lysis as a function of incubation time.

Spectroscopic Characterization and Nanosafety of Ag-Modified Antibacterial Leather and Leatherette

The development of antibacterial coatings is of great interest from both industry and the consumer's point of view. In this study, we characterized tanned leather and polyurethane leatherette, typically employed in the automotive and footwear industries, which were modified by photo-deposition of antibacterial silver nanoparticles (AgNPs). Material surface chemical composition was investigated in detail by X-ray photoelectron spectroscopy (XPS). The material's antibacterial capability was checked against Escherichia coli and Staphylococcus aureus, as representative microorganisms in cross transmissions. Due to the presence of silver in a nanostructured form, nanosafety issues were considered, as well. Ionic release in contact media, as well as whole nanoparticle release from treated materials, were quantitatively evaluated, thus providing specific information on potential product nanotoxicity, which was further investigated through cytocompatibility MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assays, also after surface abrasion of the materials. The proved negligible nanoparticle release, as well as the controlled release of antibacterial ions, shed light on the materials' potentialities, in terms of both high activity and safety.

Mechanisms of Nanophase-Induced Desorption in LDI-MS. A Short Review

Nanomaterials are frequently used in laser desorption ionization mass spectrometry (LDI-MS) as DI enhancers, providing excellent figures of merit for the analysis of low molecular weight organic molecules. In recent years, literature on this topic has benefited from several studies assessing the fundamental aspects of the ion desorption efficiency and the internal energy transfer, in the case of model analytes. Several different parameters have been investigated, including the intrinsic chemical and physical properties of the nanophase (chemical composition, thermal conductivity, photo-absorption efficiency, specific heat capacity, phase transition point, explosion threshold, etc.), along with morphological parameters such as the nanophase size, shape, and interparticle distance. Other aspects, such as the composition, roughness and defects of the substrate supporting the LDI-active nanophases, the nanophase binding affinity towards the target analyte, the role of water molecules, have been taken into account as well. Readers interested in nanoparticle based LDI-MS sub-techniques (SALDI-, SELDI-, NALDI- MS) will find here a concise overview of the recent findings in the specialized field of fundamental and mechanistic studies, shading light on the desorption ionization phenomena responsible of the outperforming MS data offered by these techniques.

Coherent-Interface-Assembled Ag2O-Anchored Nanofibrillated Cellulose Porous Aerogels for Radioactive Iodine Capture

Nanofibrillated cellulose (NFC) has received increasing attention in science and technology because of not only the availability of large amounts of cellulose in nature but also its unique structural and physical features. These high-aspect-ratio nanofibers have potential applications in water remediation and as a reinforcing scaffold in composites, coatings, and porous materials because of their fascinating properties. In this work, highly porous NFC aerogels were prepared based on tert-butanol freeze-drying of ultrasonically isolated bamboo NFC with 20-80 nm diameters. Then nonagglomerated 2-20-nm-diameter silver oxide (Ag₂O) nanoparticles (NPs) were grown firmly onto the NFC scaffold with a high loading content of ∼500 wt % to fabricate Ag₂O@NFC organic-inorganic composite aerogels (Ag₂O@NFC). For the first time, the coherent interface and interaction mechanism between the cellulose I_β nanofiber and Ag₂O NPs are explored by high-resolution transmission electron microscopy and 3D electron tomography. Specifically, a strong hydrogen between Ag₂O and NFC makes them grow together firmly along a coherent interface, where good lattice matching between specific crystal planes of Ag₂O and NFC results in very small interfacial straining. The resulting Ag₂O@NFC aerogels take full advantage of the properties of the 3D organic aerogel framework and inorganic NPs, such as large surface area, interconnected porous structures, and supreme mechanical properties. They open up a wide horizon for functional practical usage, for example, as a flexible superefficient adsorbent to capture I^- ions from contaminated water and trap I₂ vapor for safe disposal, as presented in this work. The viable binding mode between many types of inorganic NPs and organic NFC established here highlights new ways to investigate cellulose-based functional nanocomposites.

Functionalization of silicon nanowire arrays by silver nanoparticles for the laser desorption ionization mass spectrometry analysis of vegetable oils

In this work, novel hybrid nanostructured surfaces, consisting of dense arrays of silicon nanowires (SiNWs) functionalized by Ag nanoparticles (AgNP/SiNWs), were used for the laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) analysis of some typical unsaturated food components (e.g. squalene, oleic acid) to assess their MS performance. The synthesis of the novel platforms is an easy, cost-effective process based on the maskless wet-etching preparation at room temperature of SiNWs followed by their decoration with AgNPs, produced by pulsed laser deposition. No particular surface pretreatment or addition of organic matrixes/ionizers was necessary. Moreover, oil extracts (e.g. extra virgin olive oil, peanut oil) could be investigated on AgNP/SiNWs surfaces, revealing their different MS profiles. It was shown that such substrates operate at reduced laser energy, typically generating intense silver cluster ions and analyte adducts. A comparison with bare SiNWs was also performed, indicating the importance of AgNP density on NW surface. In this case, desorption/ionization on silicon was invoked as probable LDI mechanism. Finally, the influence of SiNW length and surface composition on MS results was assessed. The combination of typical properties of SiNWs (hydrophobicity, antireflectivity) with ionization ability of metal NPs can be a valid methodology for the further development of nanostructured surfaces in LDI-TOF MS applications. Copyright © 2016 John Wiley & Sons, Ltd.

Surface chemical and biological characterization of flax fabrics modified with silver nanoparticles for biomedical applications

Silver nanophases are increasingly used as effective antibacterial agent for biomedical applications and wound healing. This work aims to investigate the surface chemical composition and biological properties of silver nanoparticle-modified flax substrates. Silver coatings were deposited on textiles through the in situ photo-reduction of a silver solution, by means of a large-scale apparatus. The silver-coated materials were characterized through X-ray Photoelectron Spectroscopy (XPS), to assess the surface elemental composition of the coatings, and the chemical speciation of both the substrate and the antibacterial nanophases. A detailed investigation of XPS high resolution regions outlined that silver is mainly present on nanophases' surface as Ag2O. Scanning electron microscopy and energy dispersive X-ray spectroscopy were also carried out, in order to visualize the distribution of silver particles on the fibers. The materials were also characterized from a biological point of view in terms of antibacterial capability and cytotoxicity. Agar diffusion tests and bacterial enumeration tests were performed on Gram positive and Gram negative bacteria, namely Staphylococcus aureus and Escherichia coli. In vitro cytotoxicity tests were performed through the extract method on murine fibroblasts in order to verify if the presence of the silver coating affected the cellular viability and proliferation. Durability of the coating was also assessed, thus confirming the successful scaling up of the process, which will be therefore available for large-scale production.

Silver nanostructures in laser desorption/ionization mass spectrometry and mass spectrometry imaging

Silver nanoparticles have been successfully applied as a matrix replacement for the laser desorption/ionization time-of-flight mass spectrometry (LDI-ToF-MS).

Proteomic analysis of complex protein samples by MALDI-TOF mass spectrometry

MALDI MS has become a technique of considerable impact on many fields, from proteomics to lipidomics, including polymer analysis and, more recently, even low molecular weight analytes due to the introduction of matrix-less ionization techniques (e.g., DIOS) or new matrices such as ionic liquids, proton sponges, and metal nanoparticles. However, protein identification by peptide mass fingerprint (PMF) still remains the main routine application. In the last few years, MALDI MS has played an emerging role in food chemistry especially in detection of food adulterations, characterization of food allergens, and investigation of protein structural modifications, induced by various industrial processes that could be detrimental for food quality and safety. Sample handling and pretreatment can be very different depending on the physical state, liquid or solid, of the analyzed matrices. Here, we describe simple protocols for protein extraction and MALDI MS analysis of liquid (milk) and solid (hazelnuts) samples taken as model. A classic approach based on a preliminary SDS gel electrophoresis separation followed by in-gel digestion and a faster approach based on in-solution digestion of whole samples are described and compared.

1H-pteridine-2,4-dione (lumazine): a new MALDI matrix for complex (phospho)lipid mixtures analysis

Nowadays, matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry represents an emerging and versatile tool for analysis of lipids. However, direct (i.e., with no previous separation of lipid classes) analysis of crude extracts containing a complex mixture of lipids (a problem typically encountered in shotgun lipidomics) is still a quite challenging task using a conventional MALDI matrix such as 2,5-dihydroxybenzoic acid (DHB). Indeed, in the presence of phospholipids containing quaternary ammonium groups, such as phosphatidylcholines and sphingomyelins, strong ionization-suppression effects are experienced especially in positive ion mode. To overcome this limitation, lumazine (1H-pteridine-2,4-dione) was evaluated as an alternative matrix. Lumazine in the solid state showed an absorption maximum at 350 nm, ionizes/desorbs without appreciable decomposition and extensive cluster formation, and can be used in both ion modes. In positive ion mode, the main species were M(*+) and 2M(*+) radical cations and cationized species ([M+H](+), [M+Na](+), [M+2Na+2Li-3H](+)). In negative ion mode, the main signals observed were the deprotonated molecular ion and the radical anion. The signal-to-noise ratio for phosphatidylglycerols and phosphatidylethanolamines using lumazine was almost 1 order of magnitude higher than that observed for DHB. Lumazine was successfully used for MALDI analysis (positive and negative ion modes) of crude lipid extracts of milk, soymilk, and hen egg, where phosphatidylethanolamines, phosphatidylserines, and phosphatidylinositols could additionally be detected.