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   100495篇
  免费   962篇
  国内免费   1395篇
  102852篇
  2024年   21篇
  2023年   117篇
  2022年   283篇
  2021年   455篇
  2020年   300篇
  2019年   352篇
  2018年   12069篇
  2017年   10818篇
  2016年   7782篇
  2015年   1091篇
  2014年   927篇
  2013年   880篇
  2012年   4876篇
  2011年   13340篇
  2010年   12359篇
  2009年   8517篇
  2008年   10155篇
  2007年   11703篇
  2006年   535篇
  2005年   746篇
  2004年   1150篇
  2003年   1198篇
  2002年   904篇
  2001年   393篇
  2000年   287篇
  1999年   169篇
  1998年   95篇
  1997年   98篇
  1996年   76篇
  1995年   48篇
  1994年   62篇
  1993年   60篇
  1992年   69篇
  1991年   76篇
  1990年   30篇
  1989年   38篇
  1988年   39篇
  1987年   42篇
  1986年   13篇
  1985年   20篇
  1984年   19篇
  1983年   23篇
  1982年   7篇
  1975年   6篇
  1972年   246篇
  1971年   274篇
  1965年   14篇
  1962年   24篇
  1944年   12篇
  1940年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
Recent studies have demonstrated a marked decrease in peripheral lymphocyte levels in patients with coronavirus disease 2019 (COVID‐19) caused by severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2). Few studies have focused on the changes of NK, T‐ and B‐cell subsets, inflammatory cytokines and virus‐specific antibodies in patients with moderate COVID‐19. A total of 11 RT‐PCR‐confirmed convalescent patients with COVID‐19 and 11 patients with non‐SARS‐CoV‐2 pneumonia (control patients) were enrolled in this study. NK, CD8+ T, CD4+ T, Tfh‐like and B‐cell subsets were analysed using flow cytometry. Cytokines and SARS‐CoV‐2‐specific antibodies were analysed using an electrochemiluminescence immunoassay. NK cell counts were significantly higher in patients with COVID‐19 than in control patients (P = 0.017). Effector memory CD8+ T‐cell counts significantly increased in patients with COVID‐19 during a convalescent period of 1 week (P = 0.041). TIM‐3+ Tfh‐like cell and CD226+ Tfh‐like cell counts significantly increased (P = 0.027) and decreased (P = 0.022), respectively, during the same period. Moreover, ICOS+ Tfh‐like cell counts tended to decrease (P = 0.074). No abnormal increase in cytokine levels was observed. The high expression of NK cells is important in innate immune response against SARS‐CoV‐2. The increase in effector memory CD8+ T‐cell counts, the up‐regulation of inhibitory molecules and the down‐regulation of active molecules on CD4+ T cells and Tfh‐like cells in patients with COVID‐19 would benefit the maintenance of balanced cellular and humoural immune responses, may prevent the development of severe cases and contribute to the recovery of patients with COVID‐19.  相似文献   
932.
Zhang  Fan  Li  Chunbo  Deng  Kui  Wang  Zhuozhong  Zhao  Weiwei  Yang  Kai  Yang  Chunyan  Rong  Zhiwei  Cao  Lei  Lu  Yaxin  Huang  Yue  Han  Peng  Li  Kang 《Metabolomics : Official journal of the Metabolomic Society》2020,16(3):1-6
Introduction

Untargeted metabolomics intends to objectively analyze a wide variety of compounds. Their diverse physicochemical properties make it difficult to choose an appropriate reconstitution solvent after sample evaporation without influencing the chromatography or hamper column sorbent integrity.

Objectives

The study aimed to identify the most appropriate reconstitution solvent for blood plasma samples in terms of feature recovery, four endogenous compounds, and one selected internal standard.

Methods

We investigated several reconstitution solvent mixtures containing acetonitrile and methanol to resolve human plasma extract and evaluated them concerning the peak areas of tryptophan-d5, glucose, creatinine, palmitic acid, and the phophatidylcholine PC(P-16:0/P-16:0), as well as the total feature count

Results

Results indicated that acetonitrile containing 30% methanol was best suited to match all tested criteria at least for human blood plasma samples.

Conclusion

Despite identifying the mixture of acetonitrile and methanol being suitable as solvent for human blood plasma extracts, we recommend to systematically test for an appropriate reconstitution solvent for each analyzed biomatrix.

  相似文献   
933.
934.
A water‐soluble polysaccharide (APP‐AW) was isolated from Agrimonia pilosa and prepared to three sulphated derivatives (S1, S2 and S3). The results showed that pre‐treatment with APP‐AW, S1, S2 and S3 each at the concentration of 50 μg/mL for 48 hours was able to prevent cytotoxicity induced by 1 μmol/L dexamethasone (Dex) in MC3T3‐E1 cells via inhibition of apoptosis, which is in line with the findings in flow cytometry analysis. Meanwhile, the decreased ALP activity, collagen content, mineralization, BMP2, Runx2, OSX and OCN protein expression in DEX‐treated MC3T3‐E1 cells were reversed by the addition of APP‐AW, S1, S2 and S3. Moreover, APP‐AW, S1, S2 and S3 rescued DEX‐induced increase of Bax, cytochrome c and caspase‐3 and decrease of Bcl‐2, Wnt3, β‐catenin and c‐Myc protein expression in MC3T3‐E1 cells. Our findings suggest that pre‐treatment with APP‐AW, S1, S2 and S3 could significantly protect MC3T3‐E1 cells against Dex‐induced cell injury via inhibiting apoptosis and activating Wnt/β‐Catenin signalling pathway, thus application of these polysaccharides may be a promising alternative strategy for steroid‐induced avascular necrosis of the femoral head (SANFH) therapy.  相似文献   
935.
Osteoarthritis (OA) is a common joint disease characterized by progressive cartilage degradation, in which elevated chondrocyte apoptosis and catabolic activity play an important role. MicroRNA‐155 (miR‐155) has recently been shown to regulate apoptosis and catabolic activity in some pathological circumstances, yet, whether and how miR‐155 is associated with OA pathology remain unexplored. We report here that miR‐155 level is significantly up‐regulated in human OA cartilage biopsies and also in primary chondrocytes stimulated by interleukin‐1β (IL‐1β), a pivotal pro‐catabolic factor promoting cartilage degradation. Moreover, miR‐155 inhibition attenuates and its overexpression promotes IL‐1β‐induced apoptosis and catabolic activity in chondrocytes in vitro. We also demonstrate that the PIK3R1 (p85α regulatory subunit of phosphoinositide 3‐kinase (PI3K)) is a target of miR‐155 in chondrocytes, and more importantly, PIK3R1 restoration abrogates miR‐155 effects on chondrocyte apoptosis and catabolic activity. Mechanistically, PIK3R1 positively regulates the transduction of PI3K/Akt pathway, and a specific Akt inhibitor reverses miR‐155 effects on promoting chondrocyte apoptosis and catabolic activity, phenocopying the results obtained via PIK3R1 knockdown, hence establishing that miR‐155 promotes chondrocyte apoptosis and catabolic activity through targeting PIK3R1‐mediated PI3K/Akt pathway activation. Altogether, our study discovers novel roles and mechanisms of miR‐155 in regulating chondrocyte apoptosis and catabolic activity, providing an implication for therapeutically intervening cartilage degradation and OA progression.  相似文献   
936.
Breast cancer is the most frequently diagnosed cancer and the leading cause of cancer‐related deaths in women worldwide. In this study, a large Chinese pedigree with breast cancer including a proband and two female patients was recruited and a familial history of breast cancer was collected by questionnaire. Clinicopathological assessments and neoadjuvant therapy‐related information were obtained for the proband. Blood samples were taken, and gDNA was extracted. The BRCA1/2 and PALB2 genes were screened using next‐generation sequencing by a targeted gene panel. We have successfully identified a novel, germline heterozygous, missense mutation of the gene BRCA2: c.7007G>T, p.R2336L, which is likely to be pathogenic in the proband and her elder sister who both had breast cancer. Furthermore, the risk factors for developing breast cancer in this family are discussed. Thus, genetic counselling and long‐term follow‐up should be provided for this family of breast cancer patients as well as carriers carrying a germline variant of BRCA2: c.7007G>T (p.R2336L).  相似文献   
937.
Heteroatom doping is widely recognized as an appealing strategy to break the capacitance limitation of carbonaceous materials toward sodium storage. However, the concrete effects, especially for heteroatomic phase transformation, during the sodium storage reaction remain a confusing topic. Here, a novel hypercrosslinked polymerization approach is demonstrated to fabricate pyrrole/thiophene hypercrosslinked microporous copolymer and further give porous carbonaceous materials with accurately regulated N/S dual doping corresponding to starting feeding ratios. Significantly, the N doping contributes to the conductivity and surface wettability, while the S doping is bridged to build stable active sites, which can be electrochemically converted into mercaptan anions via faraday reaction and further enhancing reversible capacities. Meanwhile, the abundant S doping can also conduce to the expanded interlayer spacing to shorten the ions diffusion distance, thus optimizing the reaction kinetic. As a result, the N0.2S0.8‐micro‐dominant porous carbon delivers the highest reversible capacity of 521 mAh g?1 at 100 mA g?1 and excellent cyclic stability over 2000 cycles at 2000 mA g?1 with a capacity decay of 0.0145 mAh g?1 per cycle. This work is anticipated to provide an in‐depth understanding of capacitance contribution and illuminate the heteroatomic phase transformation during sodium storage reactions for doping carbonaceous anodes.  相似文献   
938.
Functional nanomaterials are playing a crucial role in the emerging field of energy‐related devices. Recently, as a novel synthesis method, high‐temperature shock (HTS), which is rapid, low cost, eco‐friendly, universal, scalable, and controllable, has provided a promising option for the rational design and synthesis of various high‐quality nanomaterials. In this report, the HTS technique, including the equipment setup and operating principle, is systematically introduced, and recent progress in the synthesis of nanomaterials for energy storage and conversion applications using this HTS method is summarized. The growth mechanisms of nanoparticles and carbonaceous nanomaterials are thoroughly discussed, followed by the summary of the characteristic advantages of the HTS strategy. A series of nanomaterials prepared by the HTS method, including carbon‐based films, metal nanoparticles and compound nanoparticles, show high performance in the diverse applications of storage energy batteries, highly active catalysts, and smart energy devices. Finally, the future perspectives and directions of HTS in nanomanufacturing for broader applications are presented.  相似文献   
939.
In this review, the recent progress in using transient absorption microscopy to image charge transport and dynamics in semiconducting hybrid organic–inorganic perovskites is discussed. The basic principles, instrumentation, and resolution of transient absorption microscopy are outlined. With temporal resolution as high as 10 fs, sub‐diffraction‐limit spatial resolution, and excited‐state structural resolution, these experiments have provided crucial details on charge transport mechanisms that have been previously obscured in conventional ultrafast spectroscopy measurements. Morphology‐dependent mapping unveils spatial heterogeneity in carrier recombination and cooling dynamics. By spatially separating the pump and probe beams, carrier transport across grain boundaries has been directly visualized. Further, femtosecond temporal resolution allows for the examination of nonequilibrium transport directly, revealing extraordinarily long‐range hot carrier migration. The application of transient absorption microscopy is not limited to hybrid perovskites but can also be useful for other polycrystalline materials in which morphology plays an important role in carrier transport.  相似文献   
940.
The lithium–sulfur (Li–S) battery is a next generation high energy density battery, but its practical application is hindered by the poor cycling stability derived from the severe shuttling of lithium polysulfides (LiPSs). Catalysis is a promising way to solve this problem, but the rational design of relevant catalysts is still hard to achieve. This paper reports the WS2–WO3 heterostructures prepared by in situ sulfurization of WO3, and by controlling the sulfurization degree, the structure is controlled, which balances the trapping ability (by WO3) and catalytic activity (by WS2) toward LiPSs. As a result, the WS2–WO3 heterostructures effectively accelerate LiPS conversion and improve sulfur utilization. The Li–S battery with 5 wt% WS2–WO3 heterostructures as additives in the cathode shows an excellent rate performance and good cycling stability, revealing a 0.06% capacity decay each cycle over 500 cycles at 0.5 C. By building an interlayer with such heterostructure‐added graphenes, the battery with a high sulfur loading of 5 mg cm?2 still shows a high capacity retention of 86.1% after 300 cycles at 0.5 C. This work provides a rational way to prepare the metal oxide–sulfide heterostructures with an optimized structure to enhance the performance of Li–S batteries.  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