首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1464篇
  免费   96篇
  国内免费   21篇
化学   1164篇
晶体学   9篇
力学   29篇
数学   117篇
物理学   262篇
  2023年   21篇
  2022年   82篇
  2021年   61篇
  2020年   74篇
  2019年   86篇
  2018年   94篇
  2017年   76篇
  2016年   92篇
  2015年   67篇
  2014年   91篇
  2013年   145篇
  2012年   135篇
  2011年   111篇
  2010年   75篇
  2009年   71篇
  2008年   53篇
  2007年   32篇
  2006年   24篇
  2005年   27篇
  2004年   12篇
  2003年   17篇
  2002年   18篇
  2001年   8篇
  2000年   3篇
  1999年   2篇
  1998年   6篇
  1997年   5篇
  1996年   2篇
  1995年   4篇
  1992年   6篇
  1991年   3篇
  1990年   4篇
  1988年   3篇
  1987年   2篇
  1986年   5篇
  1985年   7篇
  1984年   4篇
  1983年   2篇
  1982年   6篇
  1981年   4篇
  1980年   6篇
  1979年   2篇
  1978年   2篇
  1977年   5篇
  1975年   3篇
  1974年   4篇
  1973年   5篇
  1970年   2篇
  1969年   2篇
  1968年   4篇
排序方式: 共有1581条查询结果,搜索用时 15 毫秒
991.
The acoustic characteristics of cries are an exhibition of an infant’s health condition and these characteristics have been acknowledged as indicators for various pathologies. This study focused on the detection of infants suffering from sepsis by developing a simplified design using acoustic features and conventional classifiers. The features for the proposed framework were Mel-frequency Cepstral Coefficients (MFCC), Spectral Entropy Cepstral Coefficients (SENCC) and Spectral Centroid Cepstral Coefficients (SCCC), which were classified through K-nearest Neighborhood (KNN) and Support Vector Machine (SVM) classification methods. The performance of the different combinations of the feature sets was also evaluated based on several measures such as accuracy, F1-score and Matthews Correlation Coefficient (MCC). Bayesian Hyperparameter Optimization (BHPO) was employed to tailor the classifiers uniquely to fit each experiment. The proposed methodology was tested on two datasets of expiratory cries (EXP) and voiced inspiratory cries (INSV). The highest accuracy and F-score were 89.99% and 89.70%, respectively. This framework also implemented a novel feature selection method based on Fuzzy Entropy (FE) as a final experiment. By employing FE, the number of features was reduced by more than 40%, whereas the evaluation measures were not hindered for the EXP dataset and were even enhanced for the INSV dataset. Therefore, it was deduced through these experiments that an entropy-based framework is successful for identifying sepsis in neonates and has the advantage of achieving high performance with conventional machine learning (ML) approaches, which makes it a reliable means for the early diagnosis of sepsis in deprived areas of the world.  相似文献   
992.
Wendlandia tinctoria var. grandis (Roxb.) DC. (Family: Rubiaceae) is a semi-evergreen shrub distributed over tropical and subtropical Asia. The present research intended to explore the pharmacological potential of the stem extract of W. tinctoria, focusing on the antioxidant, hypoglycemic, and antidiarrheal properties, and to isolate various secondary metabolites as mediators of such activities. A total of eight phenolic compounds were isolated from the dichloromethane soluble fraction of the stem extract of this plant, which were characterized by electrospray ionization (ESI) mass spectrometric and 1H NMR spectroscopic data as liquiritigenin (1), naringenin (2), apigenin (3), kaempferol (4), glabridin (5), ferulic acid (6), 4-hydroxybenzoic acid (7), and 4-hydroxybenzaldehyde (8). The dichloromethane soluble fraction exhibited the highest phenolic content (289.87 ± 0.47 mg of GAE/g of dried extract) and the highest scavenging activity (IC50 = 18.83 ± 0.07 µg/mL) against the DPPH free radical. All of the isolated compounds, except 4-hydroxybenzaldehyde, exerted a higher antioxidant effect (IC50 = 6.20 ± 0.10 to 16.11 ± 0.02 μg/mL) than the standard butylated hydroxytoluene (BHT) (IC50 = 17.09 ± 0.01 μg/mL). Significant hypoglycemic and antidiarrheal activities of the methanolic crude extract at both doses (200 mg/kg bw and 400 mg/kg bw) were observed in a time-dependent manner. Furthermore, the computational modeling study supported the current in vitro and in vivo findings, and the isolated constituents had a higher or comparable binding affinity for glutathione reductase and urase oxidase enzymes, glucose transporter 3 (GLUT 3), and kappa-opioid receptor, inferring potential antioxidant, hypoglycemic, and antidiarrheal properties, respectively. This is the first report of all of these phenolic compounds being isolated from this plant species and even the first demonstration of the plant stem extract’s antioxidant, hypoglycemic, and antidiarrheal potentials. According to the current findings, the W. tinctoria stem could be a potential natural remedy for treating oxidative stress, hyperglycemia, and diarrhea. Nevertheless, further extensive investigation is crucial for thorough phytochemical screening and determining the precise mechanisms of action of the plant-derived bioactive metabolites against broad-spectrum molecular targets.  相似文献   
993.
Cardiac and hepatotoxicities are major concerns in the development of new drugs. Better alternatives to other treatments are being sought to protect these vital organs from the toxicities of these pharmaceuticals. In this regard, a preclinical study is designed to investigate the histopathological effects of a new succinimide derivative (Comp-1) on myocardial and liver tissues, and the biochemical effects on selected cardiac biomarkers, hepatic enzymes, and lipid profiles. For this, an initially lethal/toxic dose was determined, followed by a grouping of selected albino rats into five groups (each group had n = 6). The control group received daily oral saline for 8 days. The 5-FU (5-Fluorouracil) group received oral saline daily for 8 days, added with the administration of a single dose of 5-FU (150 mg/kg I.P.) on day 5 of the study. The atenolol group received oral atenolol (20 mg/kg) for 8 days and 5-FU (150 mg/kg I.P.) on day 5 of the protocol. Similarly, two groups of rats treated with test compound (Comp-1) were administered with 5 mg/kg I.P. and 10 mg/kg I.P. for 8 days, followed by 5-FU (150 mg/kg I.P.) on day 5. Toxicity induced by 5-FU was manifested by increases in the serum creatinine kinase myocardial band (CK-MB), troponin I (cTnI) and lactate dehydrogenase (LDH), lipid profile, and selected liver enzymes, including ALP (alkaline phosphatase), ALT (alanine transaminase), AST (aspartate aminotransferase), BT (bilirubin total), and BD (direct bilirubin). These biomarkers were highly significantly decreased after the administration of the mentioned doses of the test compound (5 mg/kg and 10 mg/kg). Similarly, histological examination revealed cardiac and hepatic tissue toxicity by 5-FU. However, those toxic effects were also significantly recovered/improved after the administration of Comp-1 at the said doses. This derivative showed dose-dependent effects and was most effective at a dose of 10 mg/kg body weight. Binding energy data computed via docking simulations revealed that our compound interacts toward the human beta2-adrenergic G protein-coupled receptor (S = −7.89 kcal/mol) with a slight stronger affinity than the calcium channel T-type (S = −7.07 kcal/mol). In conclusion, the histological and biochemical results showed that the test compound (Comp-1) had prominent cardioprotective, hepatoprotective, and lipolytic effects against 5-FU-induced toxicity in the subjected animal model.  相似文献   
994.
The by-product of the previous transesterification, glycerol was utilised as an acid catalyst precursor for biodiesel production. The crude glycerol was treated through the sulfonation method with sulfuric acid and chlorosulfonic acid in a reflux batch reactor giving solid glycerol-SO3H and glycerol-ClSO3H, respectively. The synthesised acidic glycerol catalysts were characterised by various analytical techniques such as thermalgravimetric analyser (TGA), infrared spectroscopy, surface properties adsorption-desorption by nitrogen gas, ammonia-temperature programmed desorption (NH3-TPD), X-ray diffraction spectroscopy (XRD), elemental composition analysis by energy dispersive spectrometer (EDX) and surface micrographic morphologies by field emission electron microscope (FESEM). Both glycerol-SO3H and glycerol-ClSO3H samples exhibited mesoporous structures with a low surface area of 8.85 mm2/g and 4.71 mm2/g, respectively, supported by the microscopic image of blockage pores. However, the acidity strength for both catalysts was recorded at 3.43 mmol/g and 3.96 mmol/g, which is sufficient for catalysing PFAD biodiesel at the highest yield. The catalytic esterification was optimised at 96.7% and 98.2% with 3 wt.% of catalyst loading, 18:1 of methanol-PFAD molar ratio, 120 °C, and 4 h of reaction. Catalyst reusability was sustained up to 3 reaction cycles due to catalyst deactivation, and the insight investigation of spent catalysts was also performed.  相似文献   
995.
Taraxacum officinale (TO) has been historically used for medicinal purposes due to its biological activity against specific disorders. To investigate the antioxidant and the antiproliferativepotential of TO essential oil in vitro and in vivo, the chemical composition of the essential oil was analyzed by GC-MS. The in vivo antioxidant capacity was assessed on liver and kidney homogenate samples from mice subjected to acetaminophen-induced oxidative stress and treated with TO essential oil (600 and 12,000 mg/kg BW) for 14 days. The in vitro scavenging activity was assayed using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) and the reducing power methods. The cytotoxic effects against the HeLa cancer cell line were analyzed. The GC-MS analysis showed the presence of 34 compounds, 8 of which were identified as major constituents. The TO essential oil protected mice’s liver and kidneys from acetaminophen-induced oxidative stress by enhancing antioxidant enzymes (catalase, superoxide dismutase, and glutathione) and lowering malondialdehyde levels. In vitro, the TO essential oil demonstrated low scavenging activity against DPPH (IC50 = 2.00 ± 0.05 mg/mL) and modest reducing power (EC50 = 0.963 ± 0.006 mg/mL). The growth of the HeLa cells was also reduced by the TO essential oil with an inhibition rate of 83.58% at 95 µg/mL. Current results reveal significant antioxidant and antiproliferative effects in a dose-dependent manner and suggest that Taraxacum officinale essential oil could be useful in formulations for cancer therapy.  相似文献   
996.
Molecular Diversity - In this research we investigated the preparation of new (5Z, 8Z)-7H-pyrido[2,3-d]azepine derivatives in high yields via multicomponent reaction of isatins, alkyl bromides,...  相似文献   
997.

Quinoxaline derivatives are well-known N-heterocycles with pharmacological and fluorescence activities. Almost all quinoxaline derivatives with extensive π-conjugation have been introduced as fluorophores which emit blue and green light. For the first time, we designed and synthesized 6-chloro-2,3 di(Pyridine-2yl) quinoxaline (2-CPQ) as a pink fluorophore in acetonitrile medium by simple route at room temperature whitin 30 min. The synthesized quinoxaline was identified using 1H, 13C NMR, MS, and FT-IR spectroscopy. Our results showed that the iodine-catalyzed method for both oxidation and cyclization during the synthesis of quinoxaline from pyridine 2-carbaldehyde was straightforward, efficient, and clean. All of the mentioned characterization devices confirmed the synthesis of 2-CPQ.

Moreover, we studied the photophysical properties of the synthesized fluorophore in which The UV–Vis absorption spectrum of 2-CPQ in DMF were three peaks at 451, 518 and 556 nm. Based on photophysical properties investigation, 2-CPQ shows good fluorescence with maximum peaks 607 and 653 nm in DMF as solvent (фF?=?0.21). Hence, the fluorophore was applied in the peroxyoxalate chemiluminescence system. The reaction of imidazole, H2O2, and bis (2,4,6-trichlorophenyl) oxalate (TCPO) can transfer energy to a 6-chloro-2,3 di(pyridine-2yl) quinoxaline. In this process, dioxetane was synthesized, which chemically initiated the electron exchange luminescence (CIEEL) mechanism and led to pink light emission. We anticipate our synthesized fluorophores 2-CPQ will have great potential applications in imaging and medical markers.

  相似文献   
998.
Plasma Chemistry and Plasma Processing - This research explored the capability of seed priming with the non-thermal plasma to modify reactions of Melissa officinalis, an important medicinal plant,...  相似文献   
999.
Novel heterogeneous catalysts were prepared using immobilization of bis(2‐decylsulfanylethyl)amine–CrCl3 (Cr‐SNS‐D) on various supports, namely commercial TiO2, Al2O3 and magnetic Fe3O4@SiO2 nanoparticles, to yield solid catalysts denoted as support@Cr‐SNS‐D. The structure of the catalysts was confirmed on the basis of spectroscopic analyses, N2 adsorption–desorption and inductively coupled plasma (ICP) analysis. The surface areas of Al2O3@Cr‐SNS‐D, Fe3O4@SiO2@Cr‐SNS‐D and TiO2@Cr‐SNS‐D catalysts were determined to be 70, 23 and 41 m2 g?1, respectively. A decrease in surface area from that of the supports clearly establishes accurate immobilization of Cr‐SNS‐D catalyst on the surface of the parent carriers. The loading of Cr was determined to be 0.02, 0.16 and 0.11 mmol g?1 for Cr‐SNS‐D supported on TiO2, Al2O3 and Fe3O4@SiO2, respectively, using ICP analysis. After preparation and full characterization of the catalysts, ethylene trimerization reaction was accomplished in 40 ml of dry toluene, at 80°C and 25 bar ethylene pressure and in the presence of methylaluminoxane (Al/Cr = 700) within 30 min. The supported chromium catalysts were found to be efficient and selective for the ethylene trimerization reaction. The highest activity (74 650 g1‐hexene gCr?1 h?1), as well as no polyethylene formation during reaction processes, was observed when TiO2 was used as the catalyst support.  相似文献   
1000.
Rashid  Faraz  Baghla  Rahul  Kale  Prashant  Shah  Milin  Malakar  Dipankar  Pillai  Manoj 《Chromatographia》2021,84(8):793-802
Chromatographia - In vitro quantification of follicle stimulating hormone (a Macroheterogeneity protein) in human serum using mass spectrometry is challenging and as yet unreported due to its...  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号