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   804篇
  免费   18篇
  国内免费   1篇
化学   487篇
晶体学   1篇
力学   13篇
数学   55篇
物理学   267篇
  2023年   2篇
  2022年   16篇
  2021年   20篇
  2020年   22篇
  2019年   13篇
  2018年   8篇
  2017年   8篇
  2016年   20篇
  2015年   26篇
  2014年   32篇
  2013年   67篇
  2012年   43篇
  2011年   60篇
  2010年   44篇
  2009年   67篇
  2008年   44篇
  2007年   37篇
  2006年   41篇
  2005年   20篇
  2004年   35篇
  2003年   18篇
  2002年   18篇
  2001年   18篇
  2000年   16篇
  1999年   9篇
  1998年   4篇
  1997年   9篇
  1996年   10篇
  1995年   10篇
  1994年   17篇
  1993年   6篇
  1992年   13篇
  1991年   8篇
  1990年   3篇
  1989年   2篇
  1988年   5篇
  1987年   2篇
  1986年   3篇
  1984年   2篇
  1982年   2篇
  1980年   3篇
  1979年   2篇
  1978年   2篇
  1977年   3篇
  1976年   2篇
  1975年   1篇
  1974年   3篇
  1973年   2篇
  1971年   1篇
  1965年   1篇
排序方式: 共有823条查询结果,搜索用时 406 毫秒
61.
It remains unknown if the oxidative stress can be regulated by low‐level laser therapy (LLLT) in lung inflammation induced by intestinal reperfusion (i‐I/R). A study was developed in which rats were irradiated (660 nm, 30 mW, 5.4 J) on the skin over the bronchus and euthanized 2 h after the initial of intestinal reperfusion. Lung edema and bronchoalveolar lavage fluid neutrophils were measured by the Evans blue extravasation and myeloperoxidase (MPO) activity respectively. Lung histology was used for analyzing the injury score. Reactive oxygen species (ROS) was measured by fluorescence. Both expression intercellular adhesion molecule 1 (ICAM‐1) and peroxisome proliferator‐activated receptor‐y (PPARy) were measured by RT‐PCR. The lung immunohistochemical localization of ICAM‐1 was visualized as a brown stain. Both lung HSP70 and glutathione protein were evaluated by ELISA. LLLT reduced neatly the edema, neutrophils influx, MPO activity and ICAM‐1 mRNA expression. LLLT also reduced the ROS formation and oppositely increased GSH concentration in lung from i‐I/R groups. Both HSP70 and PPARy expression also were elevated after laser irradiation. Results indicate that laser effect in attenuating the acute lung inflammation is driven to restore the balance between the pro‐ and antioxidants mediators rising of PPARy expression and consequently the HSP70 production.  相似文献   
62.
A simple and straightforward synthesis of 3-aryl-5-hydroxyalkyl-1,2,4-oxadiazoles is described. The reaction among arylamidoximes, ethyl glycolate or ethyl lactate, and potassium carbonate in refluxing toluene afforded the desired 1,2,4-oxadiazoles in moderate to good yields. The synthesis has been accomplished in a single step, avoiding protection–deprotection protocols.

Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file.  相似文献   
63.
Glycodendrimers are neoglycoconjugates that can be considered as bioisosters of glycoproteins, since they can mimic the multivalent interactions of lectin-carbohydrate. The ability of glycodendrimers to present multivalent interactions with lectins as compared to a monovalent ligand is referred to as “cluster effect”. It is expected that, because of the cluster effect, glycodendrimers would result in a better association with lectins than mono-carbohydrate anchored systems. Radioisotopes are useful to evaluate biodistribution of molecules. This study is important to obtain information about molecule–receptor interactions. Indeed, such study can provide an exquisite tool to evaluate the affinity of certain molecules to specific areas in the body, leading to the development of new radiopharmaceuticals and/or drug delivery systems. Herein, we describe a d-galactose coated low molecular weight PAMAM G0 dendrimer that was successfully radiolabeled with technetium-99m. Biodistribution studies and scintigraphic images were performed in healthy mice. It was observed high liver uptake which was significantly reduced in blocking studies, indicating hepatic specificity. Therefore, low molecular weight glycodendrimer can be considered as useful platform for selective targeting of drugs to the liver and to assess hepatic function.  相似文献   
64.
A clean, efficient and facile one-pot protocol was developed for the synthesis of a series of new aminonaphthoquinones derived from 2-hydroxy-1,4-naphthoquinone (lawsone) by three-component Mannich reaction using catalytic amount of p-TsOH in CH3CN, at room temperature. At the present work, we improved the yield and significantly reduced the reaction time for several Mannich reactions with different amine and aromatic aldehydes using a non-expensive, mild catalyst and suitable solvent.  相似文献   
65.
Conifers have long been recognized for their therapeutic potential in different disorders. Alkaloids, terpenes and polyphenols are the most abundant naturally occurring phytochemicals in these plants. Here, we provide an overview of the phytochemistry and related commercial products obtained from conifers. The pharmacological actions of different phytochemicals present in conifers against bacterial and fungal infections, cancer, diabetes and cardiovascular diseases are also reviewed. Data obtained from experimental and clinical studies performed to date clearly underline that such compounds exert promising antioxidant effects, being able to inhibit cell damage, cancer growth, inflammation and the onset of neurodegenerative diseases. Therefore, an attempt has been made with the intent to highlight the importance of conifer-derived extracts for pharmacological purposes, with the support of relevant in vitro and in vivo experimental data. In short, this review comprehends the information published to date related to conifers’ phytochemicals and illustrates their potential role as drugs.  相似文献   
66.
Propolis is a balsamic product obtained from vegetable resins by exotic Africanized bees Apis mellifera L., transported and processed by them, originating from the activity that explores and maintains these individuals. Because of its vegetable and natural origins, propolis is a complex mixture of different compound classes; among them are the volatile compounds present in the aroma. In this sense, in the present study we evaluated the volatile fraction of propolis present in the aroma obtained by distillation and simultaneous extraction, and its chemical composition was determined using coupled gas chromatography, mass spectrometry, and flame ionization detection. The majority of compounds were sesquiterpene and hydrocarbons, comprising 8.2–22.19% α-copaene and 6.2–21.7% β-caryophyllene, with additional compounds identified in greater concentrations. Multivariate analysis showed that samples collected from one region may have different chemical compositions, which may be related to the location of the resin’s production. This may be related to other bee products.  相似文献   
67.
Journal of Radioanalytical and Nuclear Chemistry - This study has evaluated chemical, radiological composition and determined the cation exchange capacity (CEC) for three commercially available...  相似文献   
68.
69.
Cellulose nanofibrils (CNF) were isolated from cotton microfibrils (CM) by acid hydrolysis and coated with polyaniline (PANI) by in situ polymerization of aniline onto CNF in the presence of hydrochloride acid and ammonium peroxydisulfate to produce CNF/PANI. Nanocomposites of natural rubber (NR) reinforced with CNF and CNF/PANI were obtained by casting/evaporation method. TG analyses showed that coating CNF with PANI resulted in a material with better thermal stability since PANI acted as a protective barrier against cellulose degradation. Nanocomposites and natural rubber showed the same thermal profiles to 200 °C, partly due to the relatively lower amount of CNF/PANI added as compared to conventional composites. On the other hand, mechanical properties of natural rubber were significantly improved with nanofibrils incorporation, i.e., Young’s modulus and tensile strength were higher for NR/CNF than NR/CNF/PANI nanocomposites. The electrical conductivity of natural rubber increased five orders of magnitude for NR with the addition of 10 mass% CNF/PANI. A partial PANI dedoping might be responsible for the low electrical conductivity of the nanocomposites.  相似文献   
70.
The use of lipases in industrial processes can result in products with high levels of purity and at the same time reduce pollutant generation and improve both selectivity and yields. In this work, lipase from Thermomyces lanuginosus was immobilized using two different techniques. The first involves the hydrolysis/polycondensation of a silica precursor (tetramethoxysilane (TMOS)) at neutral pH and ambient temperature, and the second one uses tetraethoxysilane (TEOS) as the silica precursor, involving the hydrolysis and polycondensation of the alkoxide in appropriate solvents. After immobilization, the enzymatic preparations were dried using the aerogel and xerogel techniques and then characterized in terms of their hydrolytic activities using a titrimetric method with olive oil and by the formation of 2-phenylethyl acetate in a transesterification reaction. The morphological properties of the materials were characterized using scanning electron microscopy, measurements of the surface area and pore size and volume, thermogravimetric analysis, and exploratory differential calorimetry. The results of the work indicate that the use of different silica precursors (TEOS or TMOS) and different drying techniques (aerogel or xerogel) can significantly affect the properties of the resulting biocatalyst. Drying with supercritical CO2 provided higher enzymatic activities and pore sizes and was therefore preferable to drying, using the xerogel technique. Thermogravimetric analysis and differential scanning calorimetry analyses revealed differences in behavior between the two biocatalyst preparations due to the compounds present.  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