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   19199篇
  免费   1777篇
  国内免费   1740篇
  22716篇
  2024年   49篇
  2023年   271篇
  2022年   575篇
  2021年   999篇
  2020年   763篇
  2019年   855篇
  2018年   810篇
  2017年   660篇
  2016年   898篇
  2015年   1182篇
  2014年   1454篇
  2013年   1475篇
  2012年   1789篇
  2011年   1519篇
  2010年   991篇
  2009年   816篇
  2008年   973篇
  2007年   934篇
  2006年   757篇
  2005年   646篇
  2004年   675篇
  2003年   563篇
  2002年   585篇
  2001年   447篇
  2000年   315篇
  1999年   310篇
  1998年   174篇
  1997年   150篇
  1996年   140篇
  1995年   100篇
  1994年   117篇
  1993年   86篇
  1992年   118篇
  1991年   85篇
  1990年   63篇
  1989年   76篇
  1988年   44篇
  1987年   47篇
  1986年   34篇
  1985年   41篇
  1984年   21篇
  1983年   32篇
  1982年   9篇
  1980年   11篇
  1979年   12篇
  1978年   6篇
  1977年   5篇
  1975年   6篇
  1974年   5篇
  1973年   6篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Microbial transformation of sulfate minerals plays an important role in controlling the behavior of heavy metals in mining areas. Here, the anaerobic reduction of Cr (VI)-loaded schwertmannite by Shewanella oneidensis MR-1 (S. oneidensis MR-1) was investigated. The release of ferrous iron (Fe(II)) to the solution demonstrated the microbial reduction of structural Fe(III) from the schwertmannite to Fe(II). The concentration of Cr in solution decreased in all treatments, indicating that no Cr was released to the solution during this bio-reduction process of schwertmannite. The incorporation of chromate into the mineral structure of schwertmannite increased the microbial stability of the mineral, retarding the formation of secondary phases during bio-reduction process. Analysis of the XRD, SEM and fourier transform infrared spectroscopy (FT-IR) results further showed that goethite formed after 3 or 7 days with a lower content (0.22% or 0.37%) of Cr in schwertmannite, while no secondary mineral was observed with a higher concentration of Cr (0.6 wt%) incorporated in schwertmannite until 22 days. These results imply that microbial reduction of Cr(VI)-loaded schwertmannite does not lead to the release of Cr to the solution, and the microbial stability of schwertmannite will be increased by the incorporation of chromate.  相似文献   
993.
Presently, commercialization of sodium‐ion batteries (SIBs) is still hindered by the relatively poor energy‐storage performance. In addition, low‐temperature (low‐T) Na storage is another principal concern for the wide application of SIBs. Unfortunately, the Na‐transfer kinetics is extremely sluggish at low‐T, as a result, there are few reports on low‐T SIBs. Here, an advanced low‐T sodium‐ion full battery (SIFB) assembled by an anode of 3D Se/graphene composite and a high‐voltage cathode (Na3V2(PO4)2O2F) is developed, exhibiting ultralong lifespan (over even 15 000 cycles, the capacity retention is still up to 86.3% at 1 A g?1), outstanding low‐T energy storage performance (e.g., all values of capacity retention are >75% after 1000 cycles at temperatures from 25 to ?25 °C at 0.4 A g?1), and high‐energy/power properties. Such ultralong lifespan signifies that the developed sodium‐ion full battery can be used for longer than 60 years, if batteries charge/discharge once a day and 80% capacity retention is the standard of battery life. As a result, the present study not only promotes the practicability and commercialization of SIBs but also points out the new developing directions of next‐generation energy storage for wider range applications.  相似文献   
994.
The roles of tumor necrosis factor alpha (TNF‐alpha) and its mediators in cellular processes related to intestinal diseases remain elusive. In this study, we aimed to determine the biological role of activated Cdc42‐associated kinase 1 (ACK1) in TNF‐alpha‐mediated apoptosis and proliferation in Caco‐2 cells. ACK1 expression was knocked down using ACK1‐specific siRNAs, and ACK1 activity was disrupted using a small molecule ACK1 inhibitor. The Terminal deoxynucleotidyl transferase biotin‐dUTP Nick End Labeling (TUNEL) and the BrdU incorporation assays were used to measure apoptosis and cell proliferation, respectively. ACK1‐specific siRNA and the pharmacological ACK1 inhibitor significantly abrogated the TNF‐alpha‐mediated anti‐apoptotic effects and proliferation of Caco‐2 cells. Interestingly, TNF‐alpha activated ACK1 at tyrosine 284 (Tyr284), and the ErbB family of proteins was implicated in ACK1 activation in Caco‐2 cells. ACK1‐Tyr284 was required for protein kinase B (AKT) activation, and ACK1 signaling was mediated through recruiting and phosphorylating the down‐stream adaptor protein AKT, which likely promoted cell proliferation in response to TNF‐alpha. Moreover, ACK1 activated AKT and Src enhanced nuclear factor‐кB (NF‐кB) activity, suggesting a correlation between NF‐кB signaling and TNF‐alpha‐mediated apoptosis in Caco‐2 cells. Our results demonstrate that ACK1 plays an important role in modulating TNF‐alpha‐induced aberrant cell proliferation and apoptosis, mediated in part by ACK1 activation. ACK1 and its down‐stream effectors may hold promise as therapeutic targets in the prevention and treatment of gastrointestinal cancers, in particular, those induced by chronic intestinal inflammation.  相似文献   
995.
PPARα and PPARγ have been the most widely studied Peroxisome proliferator-activated receptor (PPAR) subtypes due to their important roles in regulating glucose, lipids, and cholesterol metabolism. By combining the lowering serum triglyceride levels benefit of PPARα agonists (such as fibrates) with the glycemic advantages of the PPARγ agonists (such as TZD), the dual PPAR agonists approach can both improve the metabolic effects and minimize the side effects caused by either agent alone, and hence, has become a promising strategy for designing effective drugs against type-2 diabetes. In this study, by means of virtual screening, ADMET prediction and molecular dynamics (MD) simulations techniques, one compound-ASN15761007 with high binding score, low toxicity were gained. It was observed by MD simulations that ASN15761007 not only possessed the same function as AZ242 did in activating PPARα and BRL did in activating PPARγ, but also had more favorable conformation for binding to the two receptors. Our results provided an approach to rapidly produce novel PPARα/γ dual agonists which might be a potential lead compound to develop against insulin resistance and hyperlipidemia.  相似文献   
996.
Di Ye  Li Zhang  Weidong Fan 《Epigenetics》2018,13(3):310-317
Synthetic antidepressants in current use for the complex etiopathogeneses of depression have slow response and remission as well as various unpleasant side effects. As a result, it is imperative to develop new antidepressants with more effectiveness and less severe side effects. Recent studies demonstrated that genipin, the aglycon of geniposide, extracted from Gardenia jasminoides Ellis has antidepressive effects. However, knowledge regarding the molecular mechanisms of its antidepressant effects remains limited. Employing a depression-like mouse model, we confirmed that genipin is capable of correcting depressions-like behaviors induced by prenatal stress in offspring from prenatally stressed dams (defined as PRS mice). In further experiments, we found that the effect of genipin on PRS mice occurs through DNA demethylation by inhibiting DNA methyltransferase 1 (DNMT1), normalizing the expression of reduced brain-derived neurotrophic factor (BDNF) in the hippocampus.  相似文献   
997.

Background

The efficacy of Helicobacter pylori (H. pylori) eradication has steadily declined, primarily because of antibiotic resistance. This study aimed to evaluate the efficacy and safety of furazolidone eradication therapies as initial treatments for H. pylori infection.

Methods

A national, multicenter, open‐label, randomized controlled trial was performed at 16 sites across 13 provinces in China to evaluate the efficacy and safety of furazolidone‐containing therapies for H. pylori infection. Treatment naïve patients were randomly assigned to: esomeprazole 20 mg, bismuth 220 mg, amoxicillin 1000 mg, and furazolidone 100 mg twice daily for 10 and 7 days (FAB 10 and FAB 7; the same therapy without bismuth (FA 10 and FA 7). The primary and secondary outcomes were the eradication rate and regimen safety, respectively. Treatment success was assessed by the 13C urea breath test at least 4 weeks after treatment completion.

Results

Overall, according to intention‐to‐treat (ITT) analysis, the eradication rates for FAB 10 and FAB 7 were 86.6% (95% confidence interval [CI], 79.9%‐93.2%) and 83.6% (95% CI, 76.3%‐90.9%) and for FA 10 and FA 7 were 82.4% (95% CI, 74.9%‐89.8%) and 77.6% (95% CI, 69.4%‐85.8%), respectively. According to per‐protocol analysis, the overall eradication rates for FAB 10 and FAB 7 were 94.7% (95% CI, 90.3%‐99.1%) and 90.8% (95% CI, 85.1%‐96.5%) and for FA 10 and FA 7 were 90.6% (95% CI, 84.9%‐96.3%) and 85.1% (95% CI, 78.2%‐92.1%), respectively. The overall prevalence of side effects was 8.1%.

Conclusions

Furazolidone‐containing therapies, particularly the tested 10‐day quadruple therapy, exhibited satisfactory efficacy and safety. This 10‐day quadruple therapy represents a promising initial treatment strategy for Chinese patients.  相似文献   
998.
999.
The synthesis of esters of norethisterone (17α-ethynyl-17β-hydroxy-estr-4-en-3-one) with acids containing a benzene ring is described, two methods of esterification being compared in terms of yield and convenience. The activities of these esters as long-acting contraceptive agents have been evaluated.  相似文献   
1000.
Many plant viruses with monopartite or bipartite genomes have been developed as efficient expression vectors of foreign recombinant proteins. Nonetheless, due to lack of multiple insertion sites in these plant viruses, it is still a big challenge to simultaneously express multiple foreign proteins in single cells. The genome of Beet necrotic yellow vein virus (BNYVV) offers an attractive system for expression of multiple foreign proteins owning to a multipartite genome composed of five positive‐stranded RNAs. Here, we have established a BNYVV full‐length infectious cDNA clone under the control of the Cauliflower mosaic virus 35S promoter. We further developed a set of BNYVV‐based vectors that permit efficient expression of four recombinant proteins, including some large proteins with lengths up to 880 amino acids in the model plant Nicotiana benthamiana and native host sugar beet plants. These vectors can be used to investigate the subcellular co‐localization of multiple proteins in leaf, root and stem tissues of systemically infected plants. Moreover, the BNYVV‐based vectors were used to deliver NbPDS guide RNAs for genome editing in transgenic plants expressing Cas9, which induced a photobleached phenotype in systemically infected leaves. Collectively, the BNYVV‐based vectors will facilitate genomic research and expression of multiple proteins, in sugar beet and related crop plants.  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