Size‐fractionated aerosol particles were collected with a MOUDI 10‐stage cascade impactor from an urban roadside place in a downtown area of Hong Kong. Fine aerosol particulate samples from stage 6 ...(aerodynamic particle diameter between 0.56 and 1 μm) and stage 9 (aerodynamic particle diameter between 0.10 and 0.18 μm) were pretreated at a chosen temperature, including −100°C, −50°C, 25°C, and 60°C, in a load lock chamber and then analyzed using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) at the same temperature (−100°C). Principal component analysis (PCA) was applied to further analyze ToF‐SIMS spectra of aerosol particles with different pretreatment temperatures from two selected stages. ToF‐SIMS results showed that the intensities of aliphatic hydrocarbon ions such as C4H7+ and C4H9+ and amine ions such as C2H8N+ and C4H12N+ decreased with an increase of the pretreatment temperature under ultrahigh vacuum conditions. We have shown that analyses of this type of aerosol particles using ToF‐SIMS should not be conducted at ambient temperature but at low temperature (eg, −50°C). In addition, we also developed a procedure that can be used to analyze aerosol particle samples under ultrahigh vacuum environment.
Adhesive hydrogels are attractive biomaterials for various applications, such as electronic skin, wound dressing, and wearable devices. However, fabricating a hydrogel with both adequate adhesiveness ...and excellent mechanical properties remains a challenge. Inspired by the adhesion mechanism of mussels, we used a two-step process to develop an adhesive and tough polydopamine-clay-polyacrylamide (PDA-clay-PAM) hydrogel. Dopamine was intercalated into clay nanosheets and limitedly oxidized between the layers, resulting in PDA-intercalated clay nanosheets containing free catechol groups. Acrylamide monomers were then added and in situ polymerized to form the hydrogel. Unlike previous single-use adhesive hydrogels, our hydrogel showed repeatable and durable adhesiveness. It adhered directly on human skin without causing an inflammatory response and was easily removed without causing damage. The adhesiveness of this hydrogel was attributed to the presence of enough free catechol groups in the hydrogel, which were created by controlling the oxidation process of the PDA in the confined nanolayers of clay. This mimicked the adhesion mechanism of the mussels, which maintain a high concentration of catechol groups in the confined nanospace of their byssal plaque. The hydrogel also displayed superior toughness, which resulted from nanoreinforcement by clay and PDA-induced cooperative interactions with the hydrogel networks. Moreover, the hydrogel favored cell attachment and proliferation, owning to the high cell affinity of PDA. Rat full-thickness skin defect experiments demonstrated that the hydrogel was an excellent dressing. This free-standing, adhesive, tough, and biocompatible hydrogel may be more convenient for surgical applications than adhesives that involve in situ gelation and extra agents.
Polystyrene (PS)‐dioctyl phthalate (DOP) films were prepared by spin coating solutions of a mixture of PS and DOP on Si wafers and characterized by time‐of‐flight secondary ion mass spectrometry ...(ToF‐SIMS) and X‐ray photoelectron spectroscopy (XPS). ToF‐SIMS depth profile results showed that DOP mainly segregated to the top 10 nm of the surface of the PS‐DOP film as well as to the interface between the film and the substrate. The normalized intensities of the ions corresponding to DOP on the surface increased as the annealing temperature increased and reached a maximum at the annealing temperature of 60°C, and then decreased as the annealing temperature increased. XPS results indicate that the maximum amount of DOP segregated to the top surface of the PS‐DOP film can reach to 39 wt% at the annealing temperature of 60°C. In addition, DOP evaporated at 25°C under ultra‐high vacuum conditions; hence, ToF‐SIMS and XPS analyses need to be carried out at low temperatures (e.g., −40°C).
Surface-adherent polydopamine (PDA) films as multifunctional coatings can be easily deposited onto a wide range of materials through dopamine self-polymerization. However, a lack of in-depth ...understanding of PDA aggregation and deposition processes and definite structure elucidation of PDA make it challenging to tailor the surface characteristic and functionality of the PDA films. Herein, we demonstrate that the surface characteristics of the PDA films can be readily tuned by controlling the competitive interplay between PDA aggregation in solution and deposition on the substrate. Moreover, a structural investigation of the PDA films using analytical tools such as X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) allows us to propose a new structure model for the PDA building block. The (DHI)2/PCA trimer complex, which consists of two 5,6-dihydroxyindole (DHI) units and one pyrrolecarboxylic acid (PCA) moiety, is definitely identified as a primary building block of PDA, and its formation is steered by covalent interactions in the initial stages of polymerization. In latter stages, the (DHI)2/PCA trimer complexes are further linked primarily through noncovalent interactions to build up the supramolecular structure of PDA. This study provides new insights into the mechanisms of PDA buildup.
Graphene, as well as other two-dimensional materials, is a promising candidate for use in bioimaging, therapeutic drug delivery, and bio-sensing applications. Here, we developed a protocol to ...functionalize graphene with recombinant proteins using genetically encoded SpyTag-SpyCatcher chemistry. SpyTag forms a covalent isopeptide bond with its genetically encoded partner SpyCatcher through spontaneous amidation under physiological conditions. The functionalization protocol developed is based on the use of short proteins as a linker, where two graphene-binding-peptides (GBPs) are attached to both ends of SpyTag (referred to as GStG), followed by the covalent conjugation with SpyCatcher-fusion proteins. The proposed method enables the decoration of crystalline graphene with various proteins, such as fluorescent proteins and affibody molecules that bind to cancerous cells. This scheme, which takes advantage of the cleanness of single-crystal graphene and the robustness of SpyTag-SpyCatcher chemistry, provides a versatile platform on which to study the biomolecule-surface and cell-substrate interactions and, indeed, may lead to a new way of designing biomedical devices. The interaction between peptides and graphene was clearly shown using molecular dynamics simulation and proven using specially designed experiments.
An ideal hydrogel for biomedical engineering should mimic the intrinsic properties of natural tissue, especially high toughness and self-healing ability, in order to withstand cyclic loading and ...repair skin and muscle damage. In addition, excellent cell affinity and tissue adhesiveness enable integration with the surrounding tissue after implantation. Inspired by the natural mussel adhesive mechanism, we designed a polydopamine-polyacrylamide (PDA-PAM) single network hydrogel by preventing the overoxidation of dopamine to maintain enough free catechol groups in the hydrogel. Therefore, the hydrogel possesses super stretchability, high toughness, stimuli-free self-healing ability, cell affinity and tissue adhesiveness. More remarkably, the current hydrogel can repeatedly be adhered on/stripped from a variety of surfaces for many cycles without loss of adhesion strength. Furthermore, the hydrogel can serve as an excellent platform to host various nano-building blocks, in which multiple functionalities are integrated to achieve versatile potential applications, such as magnetic and electrical therapies.
•Review the recent applications of ToF-SIMS in heterogeneous catalysis.•Demonstrate the power and versatility of ToF-SIMS using typical examples published during the last decade.•ToF-SIMS provides ...more precise characterization of heterogeneous catalysts.
This paper provides a critical review on the applications of time-of-flight secondary ion mass spectrometry (ToF-SIMS) in heterogeneous catalysis, with a particular emphasis on the examples published during the last decade. The covered areas include supported metal oxide catalysts, supported metal catalysts, electrocatalysts for oxygen-reduction reaction and organometallic clusters as precursors for the preparation of heterogeneous catalysts. The molecular specificity and surface sensitivity of ToF-SIMS have been shown to be extremely useful in the surface characterization of heterogeneous catalysts, in particular in the areas of assessing the formation of new compounds or interactions between different components (e.g. active phase–active phase or active phase–support), providing more precision on the structure of surface species, monitoring the different steps of catalyst preparation and/or activation, etc. In some cases, ToF-SIMS is able to provide unique molecular information that is unattainable with other conventional techniques and thus give a more precise characterization of the heterogeneous catalysts. Finally, the advantages and limitations of ToF-SIMS with respect to other more conventional techniques such as XPS are also discussed.
The time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) positive and negative ion spectra of poly(2‐vinylpyridine) (P2VP) and poly(4‐vinylpyridine) (P4VP) were analyzed using density functional ...theory calculations. Most of the ions from these structural isomers shared the same accurate mass, but had different relative abundance. This could be attributed to the fact that from a thermodynamics perspective, the disparity in the molecular structures can affect the ion stability if we assume that they shared the same mechanistic pathway of formation with similar reaction kinetics. The molecular structures of these ions were assigned, and their stability was evaluated based on calculations using the Kohn‐Sham density functional theory with Becke's 3‐parameter Lee‐Yang‐Parr exchange‐correlation functional and a correlation‐consistent, polarized, valence, double‐zeta basis set for cations and the same basis set with a triple‐zeta for anions. The computational results agreed with the experimental observations that the nitrogen‐containing cations such as C5H4N+ (m/z = 78), C8H7N+· (m/z = 117), C8H8N+ (m/z = 118), C9H8N+ (m/z = 130), C13H11N2+ (m/z = 195), C14H13N2+ (m/z = 209), C15H15N2+ (m/z = 223), and C21H22N3+ (m/z = 316) ions were more favorably formed in P2VP than in P4VP due to higher ion stability because the calculated total energies of these cations were more negative when the nitrogen was situated at the ortho position. Nevertheless, our assumption was invalid in the formation of positive ions such as C6H7N+˙ (m/z = 93) and C8H10N+ (m/z = 120). Their formation did not necessarily depend on the ion stability. Instead, the transition state chemistry and the matrix effect both played a role. In the negative ion spectra, we found that nitrogen‐containing anions such as C5H4N− (m/z = 78), C6H6N− (m/z = 92), C7H6N− (m/z = 104), C8H6N− (m/z = 116), C9H10N− (m/z = 132), C13H11N2− (m/z = 195), and C14H13N2− (m/z = 209) ions were more favorably formed in P4VP, which is in line with our computational results without exception. We speculate that whether anions would form from P2VP and P4VP is more dependent on the stability of the ions.
A novel calcium hydroxide nanospherulite (CNS) strengthened super elastic hydrogel with excellent mechanical properties has been successfully invented and investigated. The CNSs are one major ...hydration product of cement hydrated at low temperature. They are incorporated into the hydrogel polymer network through the Ca 2+ ions diffusing from the cement particles into the hydrogel matrix first and then forming calcium hydroxide nanospherulites with diameters <4 nm uniformly in the matrix. This develops an innovative method to form such nanometer-sized calcium hydroxide. The uniformly distributed 4 nm-sized spherulites formed by such a method can act as a crosslinker and hence enhance the properties of the hydrogel remarkably. By incorporating about 40 ppm of 4 nm-sized calcium hydroxide spherulites, the modified hydrogel sample can be stretched to 112 times of its initial length without breaking and withstand a maximum stress of 400 kPa. The strain recovery rate R r , which is defined as the ratio of recovered strain to the maximum strain, increases from 18.0% for the original hydrogel to 96.7% for the hydrogel with incorporation of around 200 ppm 4 nm-sized calcium hydroxide spherulites. This research contributes to the field with a unique formulation method of uniformly distributed 4 nm or smaller nanoparticles acting as crosslinkers of a hydrogel, achieving enhancement of the excellent overall mechanical properties for the hydrogel. It opens a new direction for nanoparticle strengthened material development by controlling the nanoparticle size from hydrolyzing the matrix.
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