Facile Fabrication of Amorphous Molybdenum Oxide as a Sensitive and Stable SERS Substrate under Redox Treatment.

Amorphous MoO3-x nanosheets were fabricated by the room-temperature oxidation of molybdenum powder with H2 O2 , followed by light-irradiation reduction in methanol. When applied as a substrate for surface-enhanced Raman spectroscopy (SERS), these nanosheets exhibit higher sensitivity than the crystalline counterpart for a wide range of analytes. Moreover, the SERS activity remains stable on repeated oxygen insertion/extraction. In contrast, the performance of crystalline MoO3-x continuously deteriorates on successive redox treatments. This unique SERS activity allows the recycling of the substrate through an H2 O2 -based Fenton-like reaction. More importantly, the non-invasive SERS was unprecedentedly used for the self-diagnosis of amorphous MoO3-x as a more selective photocatalyst than its crystalline counterpart.

Ga-Doped and Pt-Loaded Porous TiO2-SiO2 for Photocatalytic Nonoxidative Coupling of Methane.

Photodriven nonoxidative coupling of CH4 (NOCM) is a potential alternative approach to clean hydrogen and hydrocarbon production. Herein, a Mott-Schottky photocatalyst for NOCM is fabricated by loading Pt nanoclusters on a Ga-doped hierarchical porous TiO2-SiO2 microarray with an anatase framework, which exhibits a CH4 conversion rate of 3.48 µmol g-1 h-1 with 90% selectivity toward C2H6. This activity is 13 times higher than those from microarrays without Pt and Ga. Moreover, a continuous H2 production (36 µmol g-1) with a high CH4 conversion rate of ∼28% can be achieved through a longtime irradiation (32 h). The influence of Ga on the chemical state of a surface oxygen vacancy (Vo) and deposited Pt is investigated through a combination of experimental analysis and first-principles density functional theory calculations. Ga substitutes for the five-coordinated Ti next to Vo, which tends to stabilize the single-electron trapped Vo and reduce the electron transfer from Vo to the adsorbed Pt, resulting in the formation of a higher amount of cationic Pt. The cationic Pt and electron-enriched metallic Pt form a cationic-anionic active pair, which is more efficient for the dissociation of C-H bonds. However, the presence of too much cationic Pt results in more C2+ product with a decrease in the CH4 conversion rate due to the reduced charge-carrier separation efficiency. This study provides deep insight into the effect of the doping/loading strategy on the photocatalytic NOCM reaction and is expected to shed substantial light on future structural design and modulation.

Noble-Metal-Free Materials for Surface-Enhanced Raman Spectroscopy Detection.

Surface-enhanced Raman spectroscopy (SERS) is an attractive tool for the sensing of molecules in the fields of chemical and biochemical analysis as it enables the sensitive detection of molecular fingerprint information even at the single-molecule level. In addition to traditional coinage metals in SERS analysis, recent research on noble-metal-free materials has also yielded highly sensitive SERS activity. This Minireview presents the recent development of noble-metal-free materials as SERS substrates and their potential applications, especially semiconductors and emerging graphene-based nanostructures. Rather than providing an exhaustive review of this field, possible contributions from semiconductor substrates, characteristics of graphene enhanced Raman scattering, as well as effect factors such as surface plasmon resonance, structure and defects of the nanostructures that are considered essential for SERS activity are emphasized. The intention is to illustrate, through these examples, that the promise of noble-metal-free materials for enhancing detection sensitivity can further fuel the development of SERS-related applications.

Carbon-Dot-Sensitized, Nitrogen-Doped TiO2 in Mesoporous Silica for Water Decontamination through Nonhydrophobic Enrichment-Degradation Mode.

Mesoporous silica synthesized from the cocondensation of tetraethoxysilane and silylated carbon dots containing an amide group has been adopted as the carrier for the in situ growth of TiO2 through an impregnation-hydrothermal crystallization process. Benefitting from initial complexation between the titania precursor and carbon dot, highly dispersed anatase TiO2 nanoparticles can be formed inside the mesoporous channel. The hybrid material possesses an ordered hexagonal mesostructure with p6mm symmetry, a high specific surface area (446.27 m(2) g(-1) ), large pore volume (0.57 cm(3) g(-1) ), uniform pore size (5.11 nm), and a wide absorption band between λ=300 and 550 nm. TiO2 nanocrystals are anchored to the carbon dot through TiON and TiOC bonds, as revealed by X-ray photoelectron spectroscopy. Moreover, the nitrogen doping of TiO2 is also verified by the formation of the TiN bond. This composite shows excellent adsorption capabilities for 2,4-dichlorophenol and acid orange 7, with an electron-deficient aromatic ring, through electron donor-acceptor interactions between the carbon dot and organic compounds instead of the hydrophobic effect, as analyzed by the contact angle analysis. The composite can be photocatalytically recycled through visible-light irradiation after adsorption. The narrowed band gap, as a result of nitrogen doping, and the photosensitization effect of carbon dots are revealed to be coresponsible for the visible-light activity of TiO2 . The adsorption capacity does not suffer any clear losses after being recycled three times.

Multiple interactions steered high affinity toward PFAS on ultrathin layered rare-earth hydroxide nanosheets: Remediation performance and molecular-level insights.

The global occurrence of per- and polyfluoroalkyl substances (PFAS) in aquatic systems has raised concerns about their adverse effects on ecosystems and human health. Adsorption is a promising technique for the remediation of PFAS, yet effective adsorbents with rapid uptake kinetics and high adsorption capacity are still in high demand, and molecular-level understanding of the interfacial adsorption mechanisms is lacking. In this study, we developed a superior layered rare-earth hydroxide (LRH) adsorbent, ultrathin Y2(OH)4.86Cl1.44·1·07H2O (namely YOHCl) nanosheets, to achieve the effective removal of perfluorooctanoic acid (PFOA). YOHCl nanosheets exhibited ultra-high adsorption capacity toward PFOA (up to 957.1 mg/g), which is 1.9 times and 9.3 times higher than the state-of-the-art layered double hydroxides (MgAl-LDH) and benchmark granular activated carbon (GAC) under the same conditions, respectively. Furthermore, YOHCl nanosheets pose stable performance on the removal of PFOA under various water matrices with robust reusability. We also developed YOHCl-based continuous-flow column, demonstrating its promise in simultaneously removing multiple PFAS with wide range of chain lengths at environmentally relevant concentrations. With the molecular-level investigations, we have revealed that multi-mechanism, including ion exchange, electrostatic attraction and bidentate/bridging coordination, contributed to the strong PFOA-YOHCl affinity, leading to the ultra-high adsorption capacity of PFOA. We have provided emerging LRHs-based adsorbents for the effective remediation of PFAS with molecular-level insights on the interfacial mechanisms.

Secreted protein acidic and rich in cysteine (SPARC) induces apoptosis of human brain vascular smooth muscle cells through regulating HK2 in intracranial aneurysm.

Background: Vascular smooth muscle cell (VSMC) dysfunction is one of the crucial pathologic processes in the development of intracranial aneurysm (IA). Secreted protein acidic and rich in cysteine (SPARC), a multifunctional glycoprotein, is overexpressed in many tumor, but its underlying mechanism in vascular disease has not been elucidated. The aim of this study is to evaluate the potential function of SPARC in IA generation and regulation of mitochondrial function in VSMC. Methods: Human brain vascular smooth muscle cells were treated with recombinant SPARC to detect apoptosis-related markers. The downstream targets affecting mitochondrial dysfunction after SPARC treatment were explored by transcriptome sequencing and bioinformatics analysis, and verified using by immunohistochemistry and western blot. Further in vitro experiments verified the role of downstream targets in regulating VSMC mitochondrial function. Results: Secreted protein acidic and rich in cysteine (SPARC) expression was associated with the risk of IA rupture. SPARC induces mitochondrial pathway apoptosis in human brain VSMC. We screened 40 differentially expressed genes related to mitochondrial function after SPARC treatment. Hexokinase 2 (HK2) was identified as a downstream target of mitochondrial pathway apoptosis in VSMC induced by SPARC. In addition, immunohistochemical results confirmed that the difference between SPARC and HK2 expression is located mainly in the smooth muscle layer of IA. Overexpression of HK2 reversed the SPARC-induced increase in apoptosis and mitochondrial damage in VSMC. Conclusion: Secreted protein acidic and rich in cysteine (SPARC) regulated mitochondrial function in VSMC and induced apoptosis through HK2, which plays an important role in the formation and rupture of IA. Targeting SPARC may be a novel strategy to delay the development of intracranial aneurysms.

Piezoelectric effect enhanced photocatalysis in environmental remediation: State-of-the-art techniques and future scenarios.

Photocatalysis has been widely used as an advanced oxidation process to control pollutants effectively. However, environmental photocatalysis' decontamination efficiency is restricted to the photogenerated electron-hole pairs' rapid recombination. Recently, emerging investigations have been directed to generate internal electric field by piezoelectric effect to enhance the separation efficiency of photogenerated charge carriers for better photocatalytic performance; however, there are still huge knowledge gaps on the rational application of piezo-photocatalysis in environmental remediation and disinfection. Thus, we have conducted a comprehensive review to better understand the physicochemical properties of piezoelectric materials (non-centrosymmetric crystal structures, piezoelectric coefficient, Young's modulus, and etc.) and current study states. We also elucidated the strategy of piezo-photo catalysis system constructions (mono-component, core-shell structure, and etc.) and underlying mechanisms of enhanced remediation performance. Addressing the current challenges and future scenarios (degradation of organic pollutants, disinfection, and etc.), the present review would shed light on the advanced wastewater treatment development towards sustainable control of emerging containments.

Environmental implications of MoS2 nanosheets on rice and associated soil microbial communities.

Molybdenum disulfide (MoS2) is a transition metal dichalcogenides (TMDCs) material that is seeing rapidly increasing use. The wide range of applications will result in significant environmental release. Here, the impact of MoS2 nanosheets on rice and associated soil microbial communities was evaluated. Rice plants were grown for 4 weeks in a natural paddy soil amended with either 1T or 2H phase MoS2 nanosheets at 10 and 100 mg kg-1. The 1T MoS2 nanosheets have a significantly greater dissolution rate (58.9%) compared to 2H MoS2 (4.4%), indicating the instability of 1T MoS2 in environment. High dissolution rate resulted in a high Mo bioaccumulation in rice leaves (272 and 189 mg kg-1 under 1T and 2H exposure at 100 mg kg-1). However, this did not induce overt phytotoxicity, as indicated by a range of phenotypic or biochemical based determine endpoints, e.g., biomass, photosynthetic pigments, and malondialdehyde (MDA) content. Additionally, rice P uptake was significantly increased upon exposure to 1T and 2H MoS2 (10 mg kg-1). Gas chromatography-mass spectrometry (GC-MS) reveals that both phases of MoS2 in soil systematically enhanced the carbon and nitrogen related metabolic pathways in exposed plants. Soil 16S rRNA gene sequencing data show that soil microbial community structure was unchanged upon MoS2 exposure. However, both phases of MoS2 remarkably increased the relative abundance of N2-fixation cyanobacteria, and 2H MoS2 exposure increased a plant growth-promoting rhizobacteria-Bacillus. Overall, our results suggest that MoS2 nanosheets at tested doses did not exert negative impacts on rice plant and the associated soil microbial community.

Molecular-Level Insights on the Facet-Dependent Degradation of Perfluorooctanoic Acid.

Perfluorooctanoic acid (PFOA) has raised significant health concerns due to its high ecotoxicological risks and difficulties in removal by conventional water treatment process. Previous studies have demonstrated that photocatalytic techniques exhibit great potential in PFOA removal. However, the underlying mechanism of the degradation process has not been fully understood, particularly the contribution of the facet effects of catalysts. In this study, a combination of experiments and first-principles calculations were conducted to shed light on the facet-dependence of the interfacial interactions and oxidation during the PFOA degradation process. We proved that the interfacial interaction was essential in initiating the hole-dominated degradation process, and the {110}R3̅c facet of hexagonal In2O3 features the strongest interaction with PFOA. The overall defluorination rate was mainly controlled by the hole-dominated oxidation processes under UV irradiation, which were further attributed to the electronic structures and reaction site distributions of different In2O3 surfaces. This study provides molecular-level insights on the facet-dependent PFOA catalytic degradation process, which can guide the rational design of photocatalysts to achieve superior decontamination efficiency.

Aneurysm Wall Enhancement in Unruptured Intracranial Aneurysms: A Histopathological Evaluation.

Background Unruptured intracerebral aneurysm wall enhancement (AWE) on vessel wall magnetic resonance imaging scans may be a promising predictor for rupture-prone intracerebral aneurysms. However, the pathophysiology of AWE remains unclear. To this end, the association between AWE and histopathological changes was assessed in this study. Methods and Results A total of 35 patients with 41 unruptured intracerebral aneurysms who underwent surgical clipping were prospectively enrolled. A total of 27 aneurysms were available for histological evaluation. The macroscopic and microscopic features of unruptured intracerebral aneurysms with and without enhancement were assessed. The microscopic features studied included inflammatory cell invasion and vasa vasorum, which were assessed using immunohistochemical staining with CD68, CD3, CD20, and myeloperoxidase for the former and CD34 for the latter. A total of 21 (51.2%) aneurysms showed AWE (partial AWE, n=7; circumferential AWE, n=14). Atherosclerotic and translucent aneurysms were identified in 17 and 14 aneurysms, respectively. Aneurysm size, irregularity, and atherosclerotic and translucent aneurysms were associated with AWE on univariate analysis (P<0.05). Multivariate logistic regression analysis showed that atherosclerosis was the only factor significantly and independently associated with AWE (P=0.027). Histological assessment revealed that inflammatory cell infiltration, intraluminal thrombus, and vasa vasorum were significantly associated with AWE (P<0.05). Conclusions Though AWE on vessel wall magnetic resonance imaging scans may be associated with the presence of atherosclerotic lesions in unruptured intracerebral aneurysms, inflammatory cell infiltration within atherosclerosis, intraluminal thrombus, and vasa vasorum may be the main pathological features associated with AWE. However, the underlying pathological mechanism for AWE still needs to be further studied.

MoS2 Nanosheets-Cyanobacteria Interaction: Reprogrammed Carbon and Nitrogen Metabolism.

Fully understanding the environmental implications of engineered nanomaterials is crucial for their safe and sustainable use. Cyanobacteria, as the pioneers of the planet earth, play important roles in global carbon and nitrogen cycling. Here, we evaluated the biological effects of molybdenum disulfide (MoS2) nanosheets on a N2-fixation cyanobacteria (Nostoc sphaeroides) by monitoring growth and metabolome changes. MoS2 nanosheets did not exert overt toxicity to Nostoc at the tested doses (0.1 and 1 mg/L). On the contrary, the intrinsic enzyme-like activities and semiconducting properties of MoS2 nanosheets promoted the metabolic processes of Nostoc, including enhancing CO2-fixation-related Calvin cycle metabolic pathway. Meanwhile, MoS2 boosted the production of a range of biochemicals, including sugars, fatty acids, amino acids, and other valuable end products. The altered carbon metabolism subsequently drove proportional changes in nitrogen metabolism in Nostoc. These intracellular metabolic changes could potentially alter global C and N cycles. The findings of this study shed light on the nature and underlying mechanisms of bio-nanoparticle interactions, and offer the prospect of utilization bio-nanomaterials for efficient CO2 sequestration and sustainable biochemical production.

Induction of SPARC on Oxidative Stress, Inflammatory Phenotype Transformation, and Apoptosis of Human Brain Smooth Muscle Cells Via TGF-ß1-NOX4 Pathway.

Secreted protein acidic and rich in cysteine (SPARC) has a close association with inflammatory response and oxidative stress in tissues and is widely expressed in intracranial aneurysms (IAs), especially in smooth muscle cells. Therefore, it is inferred that SPARC might be involved in the formation and development of IAs through the inflammatory response pathway or oxidative stress pathway. The aim of this study is to investigate the pathological mechanism of SPARC in oxidative stress, inflammation, and apoptosis during the formation of IAs, as well as the involvement of TGF-ß1 and NOX4 molecules. Human brain vascular smooth muscle cells (HBVSMCs) were selected as experimental objects. After the cells were stimulated by recombinant human SPARC protein in vitro, the ROS level in the cells was measured using an ID/ROS fluorescence analysis kit combined with fluorescence microscope and flow cytometry. The related protein expression in HBVSMCs was measured using western blotting. The mitochondrial membrane potential change was detected using a mitochondrial membrane potential kit and laser confocal microscope. The mechanism was explored by intervention with reactive oxygen scavengers N-acetylcysteine (NAC), TGF-ß1 inhibitor (SD-208), and siRNA knockout. The results showed that SPARC upregulated the expression of NOX4 through the TGF-ß1-dependent signaling pathway, leading to oxidative stress and pro-inflammatory matrix behavior and apoptosis in HBVSMCs. These findings demonstrated that SPARC may promote the progression of IAs.

Flow Diverter-Assisted Coil Embolization of Blood Blister-Like Aneurysm Using Semi-deploying Technique.

Despite many therapeutic methods were utilized to treat blood blister-like aneurysms (BBAs), the optimal treatment approach has not yet been defined. This study presents the single center experience with BBAs treated with flow diverter-assisted coiling using semi-deploying technique, and discusses the efficacy and safety of the method. The patients with subarachnoid hemorrhages (SAH) due to BBAs and treated with Pipeline Flex Embolization Device (PED) between November 2015 and February 2019 in our hospital were retrospectively reviewed. Patient demographic data, timing of treatment, angiographic details, treatment techniques, clinical outcomes and follow-up results were recorded. Ten cases (6 women and 4 men) were enrolled. The mean age of patients was 50.7 years (range 40-61 years). The aneurysm size ranged from 2 × 1.7 mm to 4.5 × 3.8 mm. Seven patients were treated with PED assisted coil embolization using semi-deploying technique, and all of the aneurysms were totally obliterated at the follow up. One patient treated with PED assisted coil embolization suffered from parenchymal hemorrhage 3 days after the treatment, and another one patient also treated with PED and coil died of severe vasospasm 10 days after the treatment. There was no reruptured cases during the follow-up. Here we showed that PED assisted coil embolization using semi-deploying technique could be a technically safe and effective treatment for BBAs.

The Clinical and Morphologic Features Related to Aneurysm Wall Enhancement and Enhancement Pattern in Patients with Anterior Circulation Aneurysms.

BACKGROUND: Aneurysm wall enhancement (AWE) may predict rupture-prone intracranial aneurysms (IAs). However, the clinical and morphologic risk factors related to AWE have not been well described. Furthermore, the risk factors related to enhancement patterns have never been studied, especially in patients with anterior circulation aneurysms. Therefore, we aimed to investigate the risk factors related to wall enhancement and the enhancement patterns in anterior circulation unruptured aneurysms. METHODS: One hundred patients (median age, 59 years; 68% female) with 113 anterior circulation unruptured aneurysms were included in this prospective study. Clinical and morphologic risk factors related to wall enhancement and circumferential enhancement were analyzed using univariate and multivariate analyses. RESULTS: There were 33 symptomatic unruptured IAs (29.2%) and 50 IAs with AWE (44.2%) (partial [n = 16] and circumferential [n = 34]). Univariate analysis showed that symptomatic IAs and morphologic factors (irregular shape, size, width, dome depth, size ratio, aspect ratio, and bottleneck) correlated with wall enhancement. Furthermore, female sex, blood parameters (cholesterol and low-density lipoprotein), and morphologic factors (size and dome depth) correlated with wall enhancement patterns (P <0.05). Multivariate analysis showed that size was the most important factor in wall enhancement (P = 0.06; odds ratio, 3.758) and a trend for symptomatic IAs (P = 0.033; odds ratio, 2.426). Female sex was the most important factor in circumferential enhancement (P = 0.017; odds ratio, 7.276). CONCLUSIONS: AWE was strongly associated with aneurysm size and was observed more frequently in symptomatic unruptured IAs. Sex hormones and atherosclerotic factors may be involved in circumferential enhancement. However, further studies should be performed to investigate the pathologic mechanisms for pattern of enhancement.

SPARC induces phenotypic modulation of human brain vascular smooth muscle cells via AMPK/mTOR-mediated autophagy.

Secreted protein acidic and rich in cysteine (SPARC) was widely expressed in VSMCs of human IAs and could reduce the capability of self-repair. This indicates that SPARC may play a role in the promotion of IAs formation and progression, but the mechanism remains unclear. In this study, we further investigated whether SPARC could induce phenotypic modulation of Human Brain Vascular Smooth Muscle Cells (HBVSMCs) and sought to elucidate the role of SPARC-mediated autophagy involved in it. The results demonstrated that SPARC inhibited the expression of contractile genes in HBVSMCs and induced a synthetic phenotype. More importantly, SPARC significantly up-regulated multiple proteins including autophagy marker microtubule-associated protein light chain 3-II (LC3-II), Beclin-1, and autophagy-related gene 5(ATG5). Furthermore, SPARC could promote p62 degradation. The autophagy inhibitor 3- methyladenine (3-MA) significantly blocked SPARC-induced phenotypic modulation of HBVSMCs. We further sought to elucidate the molecular mechanism involved in SPARC-induced autophagy, and found that SPARC could activate the AMPK/mTOR signaling pathway in HBVSMCs. AMPK could be pharmacologically inhibited by Compound C (CC), which significantly decreased the phosphorylation of AMPK into p-AMPK, increased the phosphorylation of mTOR into p-mTOR, and decreased LC3-II, Beclin-1 and ATG5 levels. This suggested that activated AMPK/ mTOR signaling is related to SPARC-mediated autophagy. These results indicated that SPARC plays a role in the phenotypic modulation of HBVSMCs through autophagy activation by AMPK/mTOR signaling pathway.

Nitrogen-Doped Mesoporous Carbon-Encapsulated MoO2 Nanobelts as a High-Capacity and Stable Host for Lithium-Ion Storage.

N-doped mesoporous carbon-capped MoO2 nanobelts (designated as MoO2 @NC) were synthesized and applied to lithium-ion storage. Owing to the stable core-shell structural framework and conductive mesoporous carbon matrix, the as-prepared MoO2 @NC shows a high specific capacity of around 700 mA h g-1 at a current of 0.5 A g-1 , excellent cycling stability up to 100 cycles, and superior rate performance. The N-doped mesoporous carbon can greatly improve the conductivity and provide uninhibited conducting pathways for fast charge transfer and transport. Moreover, the core-shell structure improved the structural integrity, leading to a high stability during the cycling process. All of these merits make the MoO2 @NC to be a suitable and promising material for lithium ion battery.

Surface-Enhanced Raman Spectroscopy Assisted by Radical Capturer for Tracking of Plasmon-Driven Redox Reaction.

The deep understanding about the photocatalytic reaction induced by the surface plasmon resonance (SPR) effect is desirable but remains a considerable challenge due to the ultrafast relaxation of hole-electron exciton from SPR process and a lack of an efficient monitoring system. Here, using the p-aminothiophenol (PATP) oxidation SPR-catalyzed by Ag nanoparticle as a model reaction, a radical-capturer-assisted surface-enhanced Raman spectroscopy (SERS) has been used as an in-situ tracking technique to explore the primary active species determining the reaction path. Hole is revealed to be directly responsible for the oxidation of PATP to p, p'-dimercaptoazobenzene (4, 4'-DMAB) and O2 functions as an electron capturer to form isolated hole. The oxidation degree of PATP can be further enhanced through a joint utilization of electron capturers of AgNO3 and atmospheric O2, producing p-nitrothiophenol (PNTP) within 10 s due to the improved hole-electron separation efficiency.

Plasmonic MoO3-x@MoO3 nanosheets for highly sensitive SERS detection through nanoshell-isolated electromagnetic enhancement.

Plasmonic MoO3-x@MoO3 nanosheets obtained from surface oxidation of MoO3-x were employed as a SERS substrate for methylene blue detection. They exhibit extraordinary sensitivity comparable to noble metals, which is attributed to shell-isolated electromagnetic enhancing by the plasmonic MoO3-x core and elimination of the photocatalytic degradation by the MoO3 shell.

Experimental Investigation on the Mechanical Behavior of Bovine Bone Using Digital Image Correlation Technique.

In order to understand the fracture mechanisms of bone subjected to external force well, an experimental study has been performed on the bovine bone by carrying out the three-point bending test with 3D digital image correlation (DIC) method, which provides a noncontact and full field of displacement measurement. The local strain and damage evolution of the bone has been recorded real time. The results show that the deflection measured by DIC agrees well with that obtained by the displacement sensor of the mechanical testing machine. The relationship between the deflection and the force is nearly linear prior to reaching the peak strength which is about 16 kN for the tested bovine tibia. The full-field strain contours of the bone show that the strain distribution depends on not only the force direction, but also the natural bone shape. The natural arched-shape bovine tibia bone could bear a large force, due to the tissue structure with high strength, and the fracture propagation process of the sample initiates at the inner side of the bone first and propagates along the force direction.