[Orthodontic retention: problems and problem-solving strategies]. / Orthodontische retentie: problemen en oplossingen.

The aim of orthodontic retention is to counteract post-treatment changes and thereby to preserve the result of active treatment. For active orthodontic treatment, a certain level of patient compliance is necessary and the same applies for the retention phase. Ideally, the retainer will never fail or get lost, the patient will adhere to all recommendations and will wear the retainer in accordance with the instructions, necessary precautions with the fixed retainer are followed, the patient reports a problem immediately, and appointments for retention check-ups will always be met. Unfortunately, the reality is often different. This article considers the need to provide the patient with information about retention before treatment and the problems that may arise during the retention phase. Recommendations are made on how to avoid these problems as much as possible, and solutions are offered for problems that do arise. Finally, it is made clear how the orthodontist, patient and dentist can be jointly responsible for the retention phase.

[Retention after orthodontic treatment]. / Retentie na de orthodontische behandeling.

In general, the result of orthodontic treatment is not stable. After active treatment, changes can occur as a result of a number of biological processes. The application of retention aims to counteract such changes and thereby preserve the result of orthodontic treatment. The way practitioners design the retention phase varies considerably. To reduce undesired variation in orthodontic retention between practices and to improve quality of care, clinical practice guidelines for retention were developed by the Dutch Association of Orthodontists. These guidelines contain recommendations for the application of retention. The duration of retention, additional techniques and retention after treatment of Class II malocclusions are discussed; consensus has not yet been reached on these subjects.

N-Heterocyclic Carbenes Reduce and Functionalize Copper Oxide Surfaces in One Pot.

Benzimidazolium hydrogen carbonate salts have been shown to act as N-heterocyclic carbene precursors, which can remove oxide from copper oxide surfaces and functionalize the resulting metallic surfaces in a single pot. Both the surfaces and the etching products were fully characterized by spectroscopic methods. Analysis of surfaces before and after NHC treatment by X-ray photoelectron spectroscopy demonstrates the complete removal of copper(II) oxide. By using 13 C-labelling, we determined that the products of this transformation include a cyclic urea, a ring-opened formamide and a bis-carbene copper(I) complex. These results illustrate the potential of NHCs to functionalize a much broader class of metals, including those prone to oxidation, greatly facilitating the preparation of NHC-based films on metals other than gold.

N-heterocyclic carbene-functionalized magic-number gold nanoclusters.

Magic-number gold nanoclusters are atomically precise nanomaterials that have enabled unprecedented insight into structure-property relationships in nanoscience. Thiolates are the most common ligand, binding to the cluster via a staple motif in which only central gold atoms are in the metallic state. The lack of other strongly bound ligands for nanoclusters with different bonding modes has been a significant limitation in the field. Here, we report a previously unknown ligand for gold(0) nanoclusters-N-heterocyclic carbenes (NHCs)-which feature a robust metal-carbon single bond and impart high stability to the corresponding gold cluster. The addition of a single NHC to gold nanoclusters results in significantly improved stability and catalytic properties in the electrocatalytic reduction of CO2. By varying the conditions, nature and number of equivalents of the NHC, predominantly or exclusively monosubstituted NHC-functionalized clusters result. Clusters can also be obtained with up to five NHCs, as a mixture of species.

N-Heterocyclic Carbene Self-Assembled Monolayers on Gold as Surface Plasmon Resonance Biosensors.

Surface plasmon resonance (SPR)-based biosensing is a powerful tool to study the recognition processes between biomolecules in real-time without need for labels. The use of thiol chemistry is a critical component in surface functionalization of various SPR biosensor surfaces on gold. However, its use is hampered by the high propensity for oxidation of the gold-thiol linkage even in ambient atmosphere, resulting in a short lifetime of SPR sensor chips unless strict precautions are taken. Herein, we describe an approach to overcome this limitation by employing highly robust self-assembled monolayers (SAMs) of alkylated N-heterocyclic carbenes (NHCs) on gold. An alkylated NHC sensor surface was developed and its biosensing capabilities were compared to a commercial thiol-based analogue-a hydrophobic association (HPA) chip-in terms of its ability to act as a reliable platform for biospecific interaction analysis under a wide range of conditions. The NHC-based SPR sensor outperforms related thiol-based sesnsors in several aspects, including lower nonspecific binding capacity, better chemical stability, higher reproducibility, shorter equilibration time, and longer life span. We also demonstrate that the NHC-based sensor can be used for rapid and efficient formation of a hybrid lipid bilayer for use in membrane interaction studies. Overall, this work identifies the great promise in designing NHC-based surfaces as a new technology platform for SPR-based biosensing.

Characterizing Substrate-Surface Interactions on Alumina-Supported Metal Catalysts by Dynamic Nuclear Polarization-Enhanced Double-Resonance NMR Spectroscopy.

The characterization of nanometer-scale interactions between carbon-containing substrates and alumina surfaces is of paramount importance to industrial and academic catalysis applications, but it is also very challenging. Here, we demonstrate that dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS) allows the unambiguous description of the coordination geometries and conformations of the substrates at the alumina surface through high-resolution measurements of 13C-27Al distances. We apply this new technique to elucidate the molecular-level geometry of 13C-enriched methionine and natural abundance poly(vinyl alcohol) adsorbed on γ-Al2O3-supported Pd catalysts, and we support these results with element-specific X-ray absorption near-edge measurements. This work clearly demonstrates a surprising bimodal coordination of methionine at the Pd-Al2O3 interface.

[Gingival recessions and orthodontics]. / Gingivarecessies en orthodontie.

Gingival recessions represent the most visible periodontal disease. The prevalence of gingival recessions is high. The root surface is literally exposed to negative influences such as erosion, abrasion, discoloration and decay. Moreover, gingival recessions can affect the quality of life by increased thermal sensitivity and reduced dento-gingival aesthetics. The aetiology of gingival recessions is complex and considered to be multifactorial. In order to prevent the development of gingival recessions during and after orthodontic treatment, several factors should be taken into account, among which maintenance of optimal oral hygiene and respect for the 'biological envelope' are decisive. Once gingival recessions have developed, orthodontic therapy can play a positive role in their treatment.

[Gingival recessions and periodontal plastic surgery]. / Gingivarecessies en parodontale plastische chirurgie.

Periodontal plastic surgery is defined as the set of surgical procedures that are performed to prevent or correct developmental disorders and anatomical, traumatic and pathological abnormalities of the gingiva, alveolar mucosa, and alveolar bone. Root coverage procedures fall under this term and have been applied for more than fifty years with varying degrees of success. There are several indications for the treatment of gingival recessions. When the treatment of choice - a conservative approach - offers no solace (any more), gingival recessions can be treated by applying periodontal plastic surgery. The goal of this surgery is complete recovery of the anatomical structures in the area of the recession. To this end several surgical techniques have been developed during the last decades. The choice of a particular technique depends on various factors, such as the number of defects, their size and the amount of keratinized gingiva around the defect.

The surface structure of silver-coated gold nanocrystals and its influence on shape control.

Understanding the surface structure of metal nanocrystals with specific facet indices is important due to its impact on controlling nanocrystal shape and functionality. However, this is particularly challenging for halide-adsorbed nanocrystals due to the difficulty in analysing interactions between metals and light halides (for example, chloride). Here we uncover the surface structures of chloride-adsorbed, silver-coated gold nanocrystals with {111}, {110}, {310} and {720} indexed facets by X-ray absorption spectroscopy and density functional theory modelling. The silver-chloride, silver-silver and silver-gold bonding structures are markedly different between the nanocrystal surfaces, and are sensitive to their formation mechanism and facet type. A unique approach of combining the density functional theory and experimental/simulated X-ray spectroscopy further verifies the surface structure models and identifies the previously indistinguishable valence state of silver atoms on the nanocrystal surfaces. Overall, this work elucidates the thus-far unknown chloride-metal nanocrystal surface structures and sheds light onto the halide-induced growth mechanism of anisotropic nanocrystals.

Impact of protecting ligands on surface structure and antibacterial activity of silver nanoparticles.

Silver nanoparticles (Ag NPs) have attracted much attention in the past decade because of their unique physicochemical properties and notable antibacterial activities. In particular, thiol-protected Ag NPs have come to the forefront of metal nanoparticle studies, as they have been shown to possess high stability and interesting structure-property relationships. However, a clear correlation between thiol-protecting ligands, the resulting Ag NP surface structure, and their antibacterial properties has yet to be determined. Here, a multielement (Ag and S), multi-edge (Ag K-edge, Ag L3-edge, S K-edge) X-ray absorption spectroscopy (XAS) methodology was used to identify the structure and composition of Ag NPs protected with cysteine. XAS characterization was carried out on similar-sized Ag NPs protected with poly(vinylpyrrolidone) (PVP), in order to provide a valid comparison of the ligand effect on surface structure. The PVP-Ag NPs showed a metallic Ag surface and composition, consistent with metal NPs protected by weak protecting ligands. On the other hand, the Cys-Ag NPs exhibited a distinct surface shell of silver sulfide, which is remarkably different than previously studied Cys-Ag NPs. The minimum inhibitory concentration (MIC) of both types of Ag NPs against Gram-positive (+) and Gram-negative (-) bacteria were tested, including Staphylococcus aureus (+), Escherichia coli (-), and Pseudomonas aeruginosa (-). It was found that the MICs of the Cys-Ag NPs were significantly lower than the PVP-Ag NPs for each bacteria, implicating the influence of the sulfidized surface structure. Overall, this work shows the effect of ligand on the surface structure of Ag NPs, as well as the importance of surface structure in controlling antibacterial activity.

Hepatic steatosis assessment with CT or MRI in patients with colorectal liver metastases after neoadjuvant chemotherapy.

PURPOSE: Preoperative radiological assessment of hepatic steatosis is recommended in patients undergoing a liver resection, but few studies investigated the diagnostic accuracy after neoadjuvant chemotherapy. The aim of this study was to compare diagnostic accuracy of preoperative CT or MRI measurements of steatosis in patients with colorectal liver metastases after induction chemotherapy. METHODS: MRI measurements (relative signal intensity decrease; RSID), N = 36, and CT scan measurements (Hounsfield units; HU), N = 32, were compared with histological steatosis assessment. Diagnostic accuracy was determined for detecting any (>5%) or marked macrovesicular steatosis (>33%). RESULTS: MRI showed the highest correlation with histology (r = 0.82, P < 0.001), compared to CT measurements (r = -0.65, P < 0.001). Based on linear regression analysis, radiological cut-off values for 5% and 33% macrovesicular steatosis, corresponded to 0.7% and 19.2% RSID in the MRI-group, and 60.4 and 54.2 HU in the CT-group, respectively. Sensitivity and specificity for the detection of any and marked macrovesicular steatosis using MRI was 87% and 69%, and 78% and 100%, respectively, and for CT, 83% and 64%, and 70% and 87%, respectively. CONCLUSION: In patients treated with neoadjuvant chemotherapy MRI measurements of steatosis showed the highest correlation coefficient and the best diagnostic accuracy, as compared to CT measurements.

Gold nanoparticles on titanium and interaction with prototype protein.

Modifying titanium (Ti) implant surfaces with functional proteins can strengthen the interface between prosthesis and bone. A prototype system was developed using gold nanoparticles (AuNPs) to immobilize proteins onto Ti. An electroless (galvanic displacement) deposition method was first used to form AuNPs of controlled size and coverage on commercial Ti foil (giving Ti-AuNPs). Parameters were then modified to create two groups of discs (n = 26) with different average AuNP diameters. Scanning electron microscopy and X-ray photoelectron spectroscopy were used to characterize the morphology and surface structure of Ti-AuNPs. To study the interaction of Ti-AuNPs with proteins, Ti discs (n = 8) modified with plain AuNPs and discs (n = 8) modified with thiol (HS--R--COOH)-functionalized AuNPs were treated with lysozyme solution. The amount and activity of the lysozyme on the discs were examined with Micro-BCA and enzymatic assays. Lysozyme was immobilized onto the discs, and the assays showed that the discs with thiol-functionalized AuNPs, discs with bare AuNPs, and Ti controls had average lysozyme adsorptions of 23 x 10(4), 2.3 x 10(4), and 5.7 x 10(4) microg/m2, respectively. The activity assays showed that 21.5, 18.4, and 12.5% of the adsorbed lysozyme was active on the discs with thiol-functionalized AuNPs, discs with bare AuNPs, and Ti controls, respectively. This technique holds promise for binding functional biomolecules to surgical implants, hence possibly creating implant surfaces that react to their local environment.

Structural and electronic properties of protein/thiolate-protected gold nanocluster with "staple" motif: A XAS, L-DOS, and XPS study.

Following the recent breakthrough of total structural determination of a Au-thiolate nanocluster [P. Jadzinsky et al., Science 318, 430 (2007)], extensive interests have been stimulated to unveil (or revisit) the structure-property relationship of various thiolate-Au nanostructures in light of the new finding of -SR-(Au-SR)(x)- "staple" motif. Here, we present experimental x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS) results on the local structure and electronic properties of thiolate-protected Au nanocluster encapsulated in bovine serum albumin (Au-BSA) together with theoretical calculation of projected local density of states (l-DOS) of Au(25)(SR)(18) model cluster. Analysis of the Au L(3)-edge extended x-ray absorption fine structure (EXAFS) of Au-BSA suggested that the nanocluster is Au(25) with Au-thiolate "staple" motif. X-ray absorption near-edge structure (XANES) and Au 4f XPS were used to probe the electronic behavior of Au-BSA. The Au d-electron density of Au-BSA was found to decrease by 0.047 e(-) relative to that of the bulk. A self-consistent real space Green's function approach implemented in ab initio FEFF8 program was used to calculate the l-DOS of Au(25)(SR)(18) and other model clusters from a site-specific perspective. The theoretical results are in good agreement with the experimental d-DOS data of Au-BSA and, importantly, systematically illustrate the effect of Au-thiolate "staple" motif on the electronic behavior of Au(25)(SR)(18). The present work sheds light on the structure-property relationship of thiolate-protected Au(25) from both experimental and theoretical perspectives and illustrates the usefulness of XAS/l-DOS method in such studies.

Surface structural characteristics and tunable electronic properties of wet-chemically prepared Pd nanoparticles.

The ligand substitution reaction, Pd L(3,2,1)-edge and S K-edge x-ray absorption fine structure (XAFS), XAFS simulations, and valence-band and core-level x-ray photoelectron spectroscopy (XPS) have been used to systematically study the surface chemical and electronic properties of wet-chemically prepared Pd nanoparticles of varied size, molecular capping, and metal composition. It was found that the replacement of weakly interacting capping molecules (amine and tetra-alkylphosphonium bromide) with strongly binding thiols caused a considerable change in the surface bonding of Pd nanoparticles. However, the Pd d-electron counts (number of d electrons) remained almost unchanged before and after ligand substitution, which is unexpected since Pd atoms normally lose electrons to the more electronegative S atoms. XAFS results and simulations provided useful insights into the surface structural characteristics of Pd nanoparticles and satisfactorily accounted for the unexpected d-electron behavior involved in the ligand substitution process. XPS valence and core-level spectra further revealed a size-dependent d-band narrowing and presented complementary information to XAFS about the surface electronic properties of Pd atoms. The small weakly bound Pd nanoparticles seem inevitably to have a net d-electron depletion due to the influence of the surface effect (chemical adsorption by oxygen), which is more significant than the d-electron enriching nanosize effect. However, it was demonstrated that by forming Pd-Ag alloy nanoparticles, a net increase of the Pd d-electron counts can be realized. Therefore, it is illustrated that by manipulating the surface, size, and alloying effects, the electronic properties of Pd nanoparticles can be possibly tuned.