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   1563篇
  免费   52篇
  国内免费   5篇
化学   1326篇
晶体学   5篇
力学   13篇
数学   109篇
物理学   167篇
  2024年   3篇
  2023年   4篇
  2022年   87篇
  2021年   97篇
  2020年   37篇
  2019年   40篇
  2018年   36篇
  2017年   35篇
  2016年   65篇
  2015年   51篇
  2014年   60篇
  2013年   108篇
  2012年   122篇
  2011年   131篇
  2010年   67篇
  2009年   71篇
  2008年   98篇
  2007年   98篇
  2006年   64篇
  2005年   87篇
  2004年   61篇
  2003年   37篇
  2002年   42篇
  2001年   6篇
  2000年   11篇
  1999年   9篇
  1998年   7篇
  1997年   8篇
  1996年   8篇
  1995年   5篇
  1994年   4篇
  1993年   8篇
  1992年   4篇
  1991年   3篇
  1990年   2篇
  1989年   3篇
  1987年   5篇
  1986年   5篇
  1984年   5篇
  1982年   2篇
  1981年   3篇
  1979年   3篇
  1975年   3篇
  1974年   3篇
  1972年   1篇
  1969年   1篇
  1967年   2篇
  1964年   1篇
  1963年   1篇
  1962年   2篇
排序方式: 共有1620条查询结果,搜索用时 15 毫秒
31.
Epilobium angustifolium L. (EA) has been used as a topical agent since ancient times. There has been an increasing interest in applying EA as a raw material used topically in recent years. However, in the literature, there are not many reports on the comprehensive application of this plant to skin care and treatment. EA contains many valuable secondary metabolites, which determine antioxidant, anti-inflammatory, anti-aging, and antiproliferative activity effects. One of the most important active compounds found in EA is oenothein B (OeB), which increases the level of ROS and protects cells from oxidative damage. OeB also influences wound healing and reduces inflammation by strongly inhibiting hyaluronidase enzymes and inhibiting COX-1 and COX-2 cyclooxygenases. Other compounds that play a key role in the context of application to the skin are flavonoids, which inhibit collagenase and hyaluronidase enzymes, showing anti-aging and anti-inflammatory properties. While terpenes in EA play an important role in fighting bacterial skin infections, causing, among other things cell membrane, permeability increase as well as the modification of the lipid profiles and the alteration of the adhesion of the pathogen to the animal cells. The available scientific information on the biological potential of natural compounds can be the basis for the wider use of EA in skin care and treatment. The aim of the article is to review the existing literature on the dermocosmetic use of E. angustifolium.  相似文献   
32.
The aim of this study was to identify polyphenolic compounds contained in ethanol and water extracts of black alder (Alnus glutinosa L.) acorns and evaluate their anti-cancer and antimicrobial effects. The significant anti-cancer potential on the human skin epidermoid carcinoma cell line A431 and the human epithelial cell line A549 derived from lung carcinoma tissue was observed. Aqueous and ethanolic extracts of alder acorns inhibited the growth of mainly Gram-positive microorganisms (Staphylococcus aureus, Bacillus subtilis, Streptococcus mutans) and yeast-like fungi (Candida albicans, Candida glabrata), as well as Gram-negative (Escherichia coli, Citrobacter freundii, Proteus mirabilis, Pseudomonas aeruginosa) strains. The identification of polyphenols was carried out using an ACQUITY UPLC-PDA-MS system. The extracts were composed of 29 compounds belonging to phenolic acids, flavonols, ellagitannins and ellagic acid derivatives. Ellagitannins were identified as the predominant phenolics in ethanol and aqueous extract (2171.90 and 1593.13 mg/100 g DM, respectively) The results may explain the use of A. glutinosa extracts in folk medicine.  相似文献   
33.
34.
The present work focused on the qualitative and quantitative analysis of Cr detoxification strategy of aquatic cosmopolitan plant Callitriche cophocarpa. This plant species has just been described in the context of its unusual accumulation potential of Cr. The emphasis of the work was placed on the redox reaction Cr(VI)→Cr(III) which is considered to be remediation mechanism of highly reactive and mobile Cr(VI) ions. Plants were immersed for 5 days in 1 mM of Cr(VI) (potassium dichromate) or 1 mM of Cr(III) (chromium sulphate) solutions in semi-natural conditions. Cr was effectively removed from the solution up to the extent of ca.58% or 35% of the starting amount, in the case of Cr(III) and Cr(VI), respectively. No plant-induced Cr(VI) reduction accompanying Cr accumulation was observed in Cr(VI) solutions except from the apparent one, noticed at the fourth day of incubation. On the contrary to these results, according to the method of electron paramagnetic resonance spectroscopy (L-band EPR), biphasic signal of Cr(V) attending Cr(VI) to Cr(III) reduction was detected inside the plant tissue every day of investigations. Our results show that phytoextraction but not phytostabilization is the main strategy of Cr detoxification by C. cophocarpa in aquatic systems.   相似文献   
35.
One‐stage synthesis of N‐substituted 2‐amino‐5‐(2,4‐dihydroxyphenyl)‐1,3,4‐thiadiazoles is described. The compounds were prepared by the reaction of the sulfinyl bis(2,4‐dihydroxythiobenzoyl) (STB) with 4‐substituted 3‐thiosemicarbazides. STB was obtained from 2,4‐dihydroxybenzenecarbodithioic acid and thionyl dichloride. The structure of compounds was confirmed by IR, 1H NMR, 13C NMR, and EI‐MS data.  相似文献   
36.
Ultraviolet (UV) reflection in the urban constructed environment is not well understood for topical issues such as measuring and modeling the received UV exposure due to that UV reflection for outdoor workers. Both predominantly specular and diffuse reflecting surface types have been identified and investigated for the erythemal UV reflection ratio variation due to solar zenith angle and orientation. This paper presents relationships between erythemal UV reflection ratios measured for non‐horizontal and horizontal surfaces, with predominantly specular surface types indicating stronger relationships with solar zenith angles than diffuse reflecting surfaces types. Erythemal UV exposures caused by the same reflecting surface types at three inclinations are also investigated. Non‐horizontal surfaces can increase erythemal UV exposures compared to erythemal UV exposures received from the same horizontal surface by factors of 1.07–1.46 for specific body sites and by 1.01–1.70 for averages of group body sites for zinc aluminium coated steel sheeting.  相似文献   
37.
Results of the dielectric studies of nine members of the nBT (4-n-alkyl-4'-thiocyanatobiphenyl, n= 2-10) homologous series in the crystal E (E) and isotropic (I) phases are presented. The dependence of the static permittivity ?s in the isotropic phase, the longitudinal relaxation times τis and τ, and the activation enthalpies ΔH is and ΔH on the number of carbon atoms in the alkyl chain n are analysed. A considerable increase in the retardation factor g= τis with decreasing n is observed. The results are compared with those obtained for similar two-ring homologous series. The parameters characterizing the molecular rotations around the short axis in the E phase (τ and ΔH ) indicate a hardening of this solid-like phase with shortening of the alkyl chain.  相似文献   
38.
39.
Alcohol ethoxylates (AEs) are a significant component of a stream of surfactants directed to the aquatic environment. The aim of this work was the investigation of the dependence of the analytical signals of homogeneous AE homologues on liquid chromatography with tandem mass spectrometry conditions, as well as the separation of AEs from the water matrix and, on this basis, the development of an analytical procedure suitable for the determination of AEs in environmental samples. Homogeneous homologues containing dodecyl moiety and 2–9 oxyethylene subunits were investigated. The analytical signals of the investigated homologues were optimized in terms of concentration of ammonium acetate in the mobile phase (optimum 5 mM) and a column temperature (optimum 35°C) of the liquid chromatography with tandem mass spectrometry system. A separation of AEs from the water matrix by liquid–liquid extraction (ethyl acetate, chloroform) or solid‐phase extraction (C18, styrene divinylbenzene, H‐RX) was investigated. In a model investigation, the best recoveries (>90%) were obtained with a styrene divinylbenzene cartridge eluted with a 1:1 mixture of chloroform and methanol. However, much worse recoveries were obtained from the river water sample. Better results were obtained for liquid–liquid extraction with ethyl acetate. Recoveries of 62–80% were obtained for homologues having 4–9 oxyethylene subunits, at the lowest spike.  相似文献   
40.
This study was focused on the analysis of the contents (by HPLC-DAD) and antioxidant activity of phenolic compounds of aquatic higher plant Callitriche cophocarpa. The novelty of the work was: 1) the study of the species which is almost unknown in the context of investigations of bio-active compounds and, 2) the application of a unique L-band electron resonance spectroscopy (L-band EPR) to measure antioxidant activity of samples. The results were compared with those obtained for microalgae Chlorella and Spirulina which are widely distributed as the diet supplements. The obtained results are very promising with respect to the freeze-dried material of Callitriche, which revealed both high level of phenolic compounds (33 mg g?1 dry weight) and antioxidant activity (0.14 g of reduced DPPH· g?1 dry weight). An isoform of sinapic acid appeared to be the main fraction in this sample. The average content of the total phenolic compounds in Callitriche freeze-dried samples was ca. 43 or 25 times higher than in the case of the Chlorella or Spirulina samples, respectively. These samples showed also 16 times lower scavenging activity in relation to freeze-dried Callitriche material.   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