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   14010篇
  免费   889篇
  国内免费   3篇
  14902篇
  2023年   130篇
  2022年   213篇
  2021年   367篇
  2020年   316篇
  2019年   367篇
  2018年   448篇
  2017年   393篇
  2016年   593篇
  2015年   788篇
  2014年   819篇
  2013年   988篇
  2012年   1193篇
  2011年   1067篇
  2010年   724篇
  2009年   603篇
  2008年   829篇
  2007年   767篇
  2006年   695篇
  2005年   597篇
  2004年   555篇
  2003年   456篇
  2002年   419篇
  2001年   144篇
  2000年   110篇
  1999年   113篇
  1998年   107篇
  1997年   74篇
  1996年   87篇
  1995年   86篇
  1994年   63篇
  1993年   51篇
  1992年   66篇
  1991年   52篇
  1990年   35篇
  1989年   46篇
  1988年   45篇
  1987年   26篇
  1986年   32篇
  1985年   36篇
  1984年   48篇
  1983年   23篇
  1982年   25篇
  1981年   36篇
  1980年   28篇
  1979年   32篇
  1978年   27篇
  1975年   17篇
  1974年   22篇
  1973年   24篇
  1968年   18篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Ethanol extract obtained from dried leaves of Acmella oleracea afforded after a liquid/liquid partition procedure a larvicidal hexane fraction (LC50 = 145.6 ppm) and a non larvicidal dichloromethane one. From the inactive fraction, three amides were identified, two new structures, named deca-6,9-dihydroxy-(2E,7E)-dienoic acid isobutylamide (1), deca-8,9-dihydroxy-(2E,6Z)-dienoic acid isobutylamide (2) and the known nona-2,3-dihydroxy-6,8-diynoic acid 2-phenylethylamide (3). Bioassay-guided chromatographic fractionation of the hexane partition led to the identification of an amide mixture, nona-(2Z)-en-6,8-diynoic acid 2-phenylethylamide (4) and deca-(2Z)-en-6,8-diynoic acid 2-phenylethlylamide (5). This mixture was active against Aedes aegypti larvae at LC50 = 7.6 ppm. Low toxicity of crude extracts and derived fractions on Artemia salina nauplies showed the possibility of using them to control the A. aegypti mosquito larvae. This is the first report on larvicidal activity of acetylenic 2-phenylethylamides and their identification in A. oleracea leaves.  相似文献   
993.
In a previous paper, we measured the affiliation between male individuals of Trinomys yonenagae and concluded that the intensity of affiliation was high and did not differ between animals from the same social group and from different social groups. In this paper, we report the results obtained with the same experimental procedure with female individuals. We also discuss sexual differences in the social interaction of this species. The experimental procedure was based on 40-min encounters between residents, which remained alone in an arena for 24 h, and introduced intruders, in a round-robin design. We quantified one variable indicative of activity level (number of squares crossed), one indicative of anxiety (time in marginal squares), three indicative of affiliation (number of physical contacts, mean distance between rodents, and total duration of physical contact), and the number of sound emissions. No aggressive behaviors were exhibited. The results indicate that there is a high level of affiliation mediated by acoustic communication both for males and females and that no anxiety is associated with social context, especially in females. The evolution of sociality in T. yonenagae was probably linked to an increase of tolerance especially among adult females. We also suggest that predation was a stronger selective pressure than resource availability in the evolution of sociality in this species.  相似文献   
994.
995.
A large amount of research within organic biosensors is dominated by organic electrochemical transistors (OECTs) that use conducting polymers such as poly(3,4-ethylene dioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS). Despite the recent advances in OECT-based biosensors, the sensing is solely reliant on the amperometric detection of the bioanalytes. This is typically accompanied by large undesirable parasitic electrical signals from the electroactive components in the electrolyte. Herein, we present the use of in situ resonance Raman spectroscopy to probe subtle molecular structural changes of PEDOT:PSS associated with its doping level. We demonstrate how such doping level changes of PEDOT:PSS can be used, for the first time, on operational OECTs for sensitive and selective metabolite sensing while simultaneously performing amperometric detection of the analyte. We test the sensitivity by molecularly sensing a lowest glucose concentration of 0.02 mM in phosphate-buffered saline solution. By changing the electrolyte to cell culture media, the selectivity of in situ resonance Raman spectroscopy is emphasized as it remains unaffected by other electroactive components in the electrolyte. The application of this molecular structural probe highlights the importance of developing biosensing probes that benefit from high sensitivity of the material's structural and electrical properties while being complimentary with the electronic methods of detection.  相似文献   
996.
997.

Objective

The aim of the present study is to determine if CEACAM6 can be detected in the bile of patients with biliary cancer and can serve as a diagnostic biomarker for cholangiocarcinoma.

Summary Background Data

Distinguishing bile duct carcinoma from other diagnoses is often difficult using endoscopic or percutaneous techniques. The cell surface protein CEACAM6 is over-expressed in many gastrointestinal cancers and may be selectively elevated in biliary adenocarcinoma.

Methods

Bile from patients with benign biliary disease and cholangiocarcinoma (hilar, intrahepatic and distal) was collected at the time of index operation. The concentration of CEACAM6 was quantified by sandwich enzyme-linked immunosorbent assay (ELISA) and correlated to pathologic diagnosis. Diagnostic capability of CEACAM6 was evaluated by Wilcoxon rank-sum, linear regression, multiple regression, and receiver operating characteristic (ROC) curve analysis.

Results

Bile from 83 patients was analyzed: 42 with benign disease and 41 with cholangiocarcinoma. Patients in the benign cohort were younger, predominantly female, and had lower median biliary CEACAM6 levels than patients in the malignant cohort (7.5 ng/ml vs. 40 ng/ml; p = <.001). ROC curve analysis determined CEACAM6 to be a positive predictor cholangiocarcinoma with a CEACAM6 level >14 ng/ml associated with 87.5% sensitivity, 69.1% specificity, and a likelihood ratio of 2.8 (AUC 0.74). Multiple regression analysis suggested elevated alkaline phosphatase and the presence of biliary endoprostheses may influence CEACAM6 levels.

Conclusion

Biliary CEACAM6 can identify patients with extrahepatic cholangiocarcinoma with a high degree of sensitivity and should be investigated further as a potential screening tool.  相似文献   
998.
This study aims to demonstrate how the state of chronic hyperglycemia from experimental Diabetes Mellitus can influence the homeostatic imbalance of tendons and, consequently, lead to the characteristics of tendinopathy. Twenty animals were randomly divided into two experimental groups: control group, consisting of healthy rats and diabetic group constituted by rats induced to Diabetes Mellitus I. After twenty-four days of the induction of Diabetes type I, the Achilles tendon were removed for morphological evaluation, cellularity, number and cross-sectional area of blood vessel, immunohistochemistry for Collagen type I, VEGF and NF-κB nuclear localization sequence (NLS) and nitrate and nitrite level. The Achilles tendon thickness (µm/100g) of diabetic animals was significantly increased and, similarly, an increase was observed in the density of fibrocytes and mast cells in the tendons of the diabetic group. The average number of blood vessels per field, in peritendinous tissue, was statistically higher in the diabetic group 3.39 (2.98) vessels/field when compared to the control group 0.89 (1.68) vessels/field p = 0.001 and in the intratendinous region, it was observed that blood vessels were extremely rare in the control group 0.035 (0.18) vessels/field and were often present in the tendons of the diabetic group 0.89 (0.99) vessels/field. The immunohistochemistry analysis identified higher density of type 1 collagen and increased expression of VEGF as well as increased immunostaining for NFκB p50 NLS in the nucleus in Achilles tendon of the diabetic group when compared to the control group. Higher levels of nitrite/nitrate were observed in the experimental group induced to diabetes. We conclude that experimental DM induces notable structural, inflammatory and vascular changes in the Achilles tendon which are compatible with the process of chronic tendinopathy.  相似文献   
999.
Amplified fragment length polymorphism (AFLP) analysis was used to assess the biodiversity of one of the most important dipteran pests of cereals, the Asian rice gall midge (Orseolia oryzae Wood Mason). Larvae and pupae were collected at 15 locations in five Asian countries and preserved in 95% ethanol for storage, shipment, and DNA extraction using cetyltrimethylammonium bromide (CTAB). Although only approximately 1 microg of DNA was extracted from a single pupa or larva, the use of several AFLP primers in various combinations meant that this amount of DNA was sufficient to allow many DNA fingerprints to be made per individual. Fingerprints were sufficiently reproducible, especially during selective amplification, to allow the genetic diversity within a field population to be characterized. Extraction of DNA from a pool of 20 insects yielded AFLP fingerprints in which variation among individuals was sacrificed in favor of detecting differences among populations. For each location, pooled DNA was amplified with three primer pairs. A total of 261 distinct AFLP bands were identified for the 45 fingerprints. Cluster analysis, performed by the unweighted pair-group method (UPGMA), separated the populations into two distinct groups. Group I included two populations from Guangdong province of southern China and one each from Laos and Imphal in northeastern India, while group II was comprised of eleven populations from elsewhere in India (Assam, Orissa, Madhya Pradesh, Andhra Pradesh, and Kerala) and from Nepal and Sri Lanka. AFLP analysis provided insight into the origins of gall midge biotypes. In 1992, the prevailing biotype in Imphal changed from Indian biotype 3 to a new biotype 3M. Our data show that biotype 3M belongs to group I and did not arise by a recent mutation from biotype 3, which belongs to group II. By contrast, Indian biotypes 2 and 4 are likely to have diverged through recent mutation and selection, as are Chinese biotypes 1 and 4. The almost simultaneous emergence of new biotypes in Kerala and Sri Lanka during 1985-1988 was most probably coincidental, because these biotypes are not closely related. AFLP fingerprints were also able to detect sexual dimorphism in the DNA of adult gall midges and to distinguish gall midge from its major parasite Platygaster oryzae.  相似文献   
1000.

Background

Hepatorenal syndrome (HRS) is a severe and progressive functional renal failure occurring in patients with cirrhosis and ascites. Terlipressin is recognized as an effective treatment of HRS, but it is expensive and not widely available. Norepinephrine could be an effective alternative. This systematic review and meta-analysis aimed to evaluate the efficacy and safety of norepinephrine compared to terlipressin in the management of HRS.

Methods

We searched the Medline, Embase, Scopus, CENTRAL, Lilacs and Scielo databases for randomized trials of norepinephrine and terlipressin in the treatment of HRS up to January 2014. Two reviewers collected data and assessed the outcomes and risk of bias. The primary outcome was the reversal of HRS. Secondary outcomes were mortality, recurrence of HRS and adverse events.

Results

Four studies comprising 154 patients were included. All trials were considered to be at overall high risk of bias. There was no difference in the reversal of HRS (RR = 0.97, 95% CI = 0.76 to 1.23), mortality at 30 days (RR = 0.89, 95% CI = 0.68 to 1.17) and recurrence of HRS (RR = 0.72; 95% CI = 0.36 to 1.45) between norepinephrine and terlipressin. Adverse events were less common with norepinephrine (RR = 0.36, 95% CI = 0.15 to 0.83).

Conclusions

Norepinephrine seems to be an attractive alternative to terlipressin in the treatment of HRS and is associated with less adverse events. However, these findings are based on data extracted from only four small studies.  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