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   37661篇
  免费   3731篇
  国内免费   2771篇
耳鼻咽喉   257篇
儿科学   489篇
妇产科学   291篇
基础医学   3911篇
口腔科学   601篇
临床医学   5288篇
内科学   4989篇
皮肤病学   362篇
神经病学   1603篇
特种医学   1035篇
外国民族医学   18篇
外科学   3034篇
综合类   8230篇
现状与发展   14篇
预防医学   3032篇
眼科学   959篇
药学   4420篇
  54篇
中国医学   2882篇
肿瘤学   2694篇
  2024年   173篇
  2023年   675篇
  2022年   1701篇
  2021年   2072篇
  2020年   1629篇
  2019年   1214篇
  2018年   1243篇
  2017年   1278篇
  2016年   1098篇
  2015年   1795篇
  2014年   2290篇
  2013年   2084篇
  2012年   3196篇
  2011年   3315篇
  2010年   2312篇
  2009年   1930篇
  2008年   2267篇
  2007年   2255篇
  2006年   2037篇
  2005年   1776篇
  2004年   1249篇
  2003年   1138篇
  2002年   931篇
  2001年   809篇
  2000年   691篇
  1999年   645篇
  1998年   368篇
  1997年   371篇
  1996年   281篇
  1995年   241篇
  1994年   232篇
  1993年   139篇
  1992年   127篇
  1991年   150篇
  1990年   113篇
  1989年   95篇
  1988年   67篇
  1987年   57篇
  1986年   44篇
  1985年   25篇
  1984年   13篇
  1983年   13篇
  1982年   7篇
  1981年   10篇
  1980年   3篇
  1979年   4篇
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
991.
目的:规范院内静脉血栓栓塞症(VTE)的防治护理工作,为住院患者提供规范、同质的VTE护理.方法:建立院内VTE防治护理专项小组,完善VTE防治护理程序,构建VTE管理信息化平台,采用多种形式的VTE知识培训,进行VTE防治质量监控.结果:组建VTE防治护理专项小组后,VTE高危患者健康教育知晓率为84.2%,高于组建...  相似文献   
992.
Objective:This study aimed to evaluate the global clinical practice guidelines on fever in children. We also aimed to select a guideline with good methodology and reporting quality to provide scientific reference for diagnosis and treatment of fever in children.Methods:The Chinese and English databases Embase, PubMed, Cochrane library, China National Knowledge Infrastructure, Wanfang database, clinical guides, and the website of the Department of Public Health Administration were retrieved up to January 2020. The clinical practice guidelines on fever in children were included. The AGREE II instrument and Reporting Items for Practice Guidelines in Healthcare statement were used to evaluate the methodology and reporting quality of the guidelines.Results:Eight clinical guidelines for fever in children were included. Methodological quality assessment showed that the recommendation level of ISP, South Africa, National Institute for Health and Care Excellence, China, and American College of Emergency Physicians were grade B (recommended with modification), while that of American Academy of Pediatrics, New South Wales, and South Australia was grade C recommendation (not recommended). No grade A recommendation guideline was found. The reporting quality from higher to lower was National Institute for Health and Care Excellence, the Chinese guideline, American College of Emergency Physicians, ISP, South Africa, New South Wales, South Australia, and American Academy of Pediatrics. The guideline recommendations were similar in various countries, but they were slightly different in various aspects, including body temperature measurement and the timing of drug administration.Conclusion:There are limitations in the methodology and reporting quality of all eight global guidelines on fever in children. For future development of these guidelines, attention should be paid to improving applicability of the guidelines in terms of methodology. Additionally, the principles and explanations for formation of recommendations should be described, as well as the limitations of the reporting guideline in detail in terms of the reporting quality. Treatments of fever in children are similar in different countries, but there are still differences that require further research.  相似文献   
993.
Objective:The newly defined cancer-testis(CT)gene,MEIOB,was previously found to play key roles in DNA double-strand break(DSB)repair.In this study,we aimed to investigate the effects and mechanisms of MEIOB in the carcinogenesis of triple-negative breast cancers(TNBCs).Methods:The Cancer Genome Atlas database was used to quantify the expression of MEIOB.Cox regression analysis was used to evaluate the association between MEIOB expression and the prognosis of human TNBC.The effects of MEIOB on cell proliferation and migration in TNBCs were also assessed in vitro.Patient-derived xenograft(PDX)models were used to assess the sensitivity of breast cancers with active MEIOB to PARP1 inhibitors.Results:We confirmed MEIOB as a CT gene whose expression was restricted to the testes and breast tumors,especially TNBCs.Its activation was significantly associated with poor survival in breast cancer patients[overall,hazard ratio(HR)=1.90(1.16–2.06);TNBCs:HR=7.05(1.16–41.80)].In addition,we found that MEIOB was oncogenic and significantly promoted the proliferation of TNBC cells.Further analysis showed that MEIOB participated in DSB repair in TNBCs.However,in contrast to its function in meiosis,it mediated homologous recombination deficiency(HRD)through the activation of poly ADP-ribose polymerase(PARP)1 by interacting with YBX1.Furthermore,activated MEIOB was shown to confer sensitivity to PARP inhibitors,which was confirmed in PDX models.Conclusions:MEIOB played an oncogenic role in TNBC through its involvement in HRD.In addition,dysregulation of MEIOB sensitized TNBC cells to PARP inhibitors,so MEIOB may be a therapeutic target of PARP1 inhibitors in TNBC.  相似文献   
994.
We report a copper-catalyzed alkoxycyclization of tryptamine derivatives with O2 as the sole oxidant, leading to a variety of C3a-alkoxypyrroloindolines in good yields with high diastereoselectivities. This reaction involves an interesting double catalytic cycle in which copper-catalyzed carboamination cyclization is favored to form the C-3 radical pyrrolidinoindoline intermediate, then a copper-catalytic radical alkoxylation reaction proceeds smoothly.

An oxazoline/copper-catalyzed cascade carboamination alkoxylation of substituted tryptamine under mild eco-friendly O2 oxidation conditions was reported.

Pyrrolidino[2,3-b]indoline is an important heterocyclic core skeleton that exists in numerous biologically active natural products and pharmaceutical molecules.1 Cyclotryptamine type molecules which are oxygenated at the C3a position are especially outstanding due to their prominent bioactivity profiles,2 various applications in biological probes3 and chiral catalysts.4As direct access to these complex products, the development of C3a-oxygenation/cyclization reactions of tryptamine or tryptophan derivatives has attracted extensive interest from synthetic chemists. Recently, some remarkable efforts have contributed to the one-step assembly of 3-hydroxyl,5 acetoxyl,6 peroxyl7 and other oxygenated8 pyrroloindolines through oxidative cyclization of tryptophan substrates. However, by utilizing a similar strategy, the direct synthesis of 3-alkoxyl pyrroloindolines remains less developed. In 2020, Zhong et al.9 reported the first example of alkoxycyclization of tryptamine derivatives using molecular iodine catalyst with tert-butyl hydroperoxide as the oxidant. None of the other studies, like using transition-metal catalysts, have been described yet.Copper salts, which are inexpensive and easily accessible, have been widely used in organic synthesis as catalysts. Copper(ii)-promoted radical intramolecular carboamination of alkene has proven to be an effective means toward the synthesis of N-fused heterocycles.10 Recent reports have utilized this strategy toward the cyclization and radical alkylation, aromatization and aminooxygenation of alkene.10 However, due to the difficulty in homolytic breakage of the oxygen–hydrogen bond in alcohols with a high bond dissociation energy (BDE is ca.105 kcal mol−1),11 the related direct cyclization and radical alkoxylation of carbon–carbon double bond with copper catalysts is still unknown. Inspired by the relevant research of copper-catalyzed radical alkoxylation reaction,12 we assume that if the catalytic carboamination and radical alkoxylation tandem reaction could be realized by a single copper catalyst, which will represent as a new effective protocol for the direct construction of alkoxyl-containing N-fused heterocycles. Herein, we report an oxazoline/copper-catalyzed cascade carboamination alkoxylation of substituted tryptamine under mild eco-friendly O2 oxidation conditions, which facilitate the construction of the 3-alkoxyl pyrroloindolinese motif in good yield with good to excellent levels of diastereoselectivity (Scheme 1).Open in a separate windowScheme 1Copper-catalyzed cyclization and alkoxylation of tryptamines.In our studies, the commercial easily available N-methyl tryptamine 1a was chosen as model substrate. Initially, 10 mol% of metal salt CuBr2 was used as catalyst, the 3-alkoxylation product 2a was obtained as 38% yield with 14/1 dr (
EntryMetal saltsLigandYieldb (%)Drc
1CuBr23814/1
2CuBr2L128>20/1
3CuBr2L245>20/1
4CuBr2L32413/1
5CuBr2L4358/1
6Cu(OTf)2L2Trace
7CuOL2nr
8Cu(OAc)2L2nr
9Cu(ClO4)2L2nr
10CuCl2L2158/1
11dCuBr2L271>20/1
12d,eCuBr2L246>20/1
Open in a separate windowaCarried out under oxygen atmosphere: metal salt (0.02 mmol, 10 mol%), 1a (0.2 mmol), 2 mL MeOH.bIsolated yields.cdr was determined by 1H NMR.d4 mL methanol was used.eAir atmosphere; nr: not reaction.With the optimized reaction conditions in hand, we continued to investigate the substrate scope of the reaction ( Open in a separate windowIn order to gain insight into the mechanism of the methodology, several control experiments were carried out. As shown in Scheme 2, the radical scavenger, 2,2,6,6-tetramethylpiperidine1-oxyl (TEMPO), inhibited the alkoxycyclization process completely, suggesting that a radical process might be involved in this reaction (Scheme 2: eqn (1)).13 When the nucleophilic substrate 1-methyl indole was involved in the standard conditions (Scheme 2: eqn (2)), trace amount of 3-indole pyrrolidinoindoline adduct 4 was detected by HRMS, suggesting that the exposed carbocation intermediate may be the precursor for the formation of the 3-alkoxylation product. Besides, the amidyl radical addition process has been ruled out by the substrate scope investigation of 1n (Scheme 2: eqn (3)), which indicated that this reaction proceeded via an intramolecular collaborative tandem process.Open in a separate windowScheme 2Control experiments.Combining with the previous reports about copper-catalyzed carboamination,10 alkoxylation12 of alkene, a possible reaction pathway is proposed in Scheme 3. Initially, a ligand–exchange reaction of Cu(ii) species with substrate 1a proceeds to form the chelation intermediate A. Subsequent nitrogen intramolecular addition–cyclization forms the C3a Cu(ii) pyrrolidinoindoline intermediate B, Then, homolytic cleavage of carbon–Cu(ii) bond to generate the Cu(i) species and C3a radical intermediate C. The C3a radical could be oxidized by CuII species to generate the C3a cation intermediate D. Subsequent nucleophilic attack of alcohol delivers the product 2a. Meanwhile, CuII complex was produced in situ through the reaction of Ln–CuI complex with O2 on the basis of the previous reports,14 completing the catalytic cycle.Open in a separate windowScheme 3Plausible reaction pathway.In conclusion, we have successfully developed copper-catalyzed alkoxycyclization of tryptamine under mild O2 oxidation conditions, affording C3a-alkoxylation pyrrolidinoindolines in good yields with high diastereoselectivities. This protocol was proved practicable and useful by the rapid concise total synthesis of natural product CPC-1. Mechanistic studies illustrated that the copper-catalyzed carboamination cyclization was favored to form the C-3 radical pyrrolidinoindoline intermediate, then a copper-catalyzed radical alkoxylation reaction proceeded to deliver the desired product. The extension of the present catalytic protocol to other useful reactions and biological evaluation of these products are undergoing in our laboratory.  相似文献   
995.
High-performance soy protein-based films from cellulose nanofibers and graphene oxide constructed synergistically via hydrogen and chemical bonding     
Ningsi Wei  Murong Liao  Kaijie Xu  Zhiyong Qin 《RSC advances》2021,11(37):22812
Soybean protein isolate (SPI) shows a broad application prospect in the food and packaging industry. However, its inferior mechanical properties and water resistance limit its application. In this work, a series of SPI-based composite films were prepared by combining with cellulose nanofiber (CNF), graphene oxide (GO), GO/CNF, ethylene glycol diglycidyl ether (EDGE) or GO/CNF/EGDE. The results show that by adding a small amount of reinforced materials (3%), the water resistance, hydrophilicity, mechanical properties and thermal stability of composite films were improved. The filling effect and hydrogen bonding of the reinforcing materials contribute to the formation of film structure. EGDE cross-link SPI with CNF and GO build a chemical network to improve the properties of the film. In addition, they could make a synergistic effect to better enhance the performance of a protein film. Therefore, the tensile strength and elastic modulus of the SGCE film reached 469.21% and 367.58%, respectively.

Soybean protein isolate (SPI) shows a broad application prospect in the food and packaging industry.  相似文献   
996.
Dual-wavelength visible photodetector based on vertical (In,Ga)N nanowires grown by molecular beam epitaxy     
Jianya Zhang  Min Zhou  Dongmin Wu  Lifeng Bian  Yukun Zhao  Hua Qin  Wenxian Yang  Yuanyuan Wu  Zhiwei Xing  Shulong Lu 《RSC advances》2021,11(26):15632
Due to the wide applications of blue and red photodetectors, dual-wavelength (blue/red) photodetectors are promising for future markets. In this work, a dual-wavelength photodetector based on vertical (In,Ga)N nanowires and graphene has been fabricated successfully. By using the transparent graphene, both blue and red responses can be clearly detected. The rise time of response can reach 3.5 ms. Furthermore, the underlying mechanism of double responses has also been analyzed. The main reason contributing to the dual-wavelength response could be the different diameters of nanowires, leading to different In components within (In,Ga)N sections.

A dual-wavelength (blue/red) photodetector based on vertical (In,Ga)N nanowires and graphene has been fabricated successfully, which is promising for wide applications. The rise time of the response can reach 3.5 ms.  相似文献   
997.
Correction: Roles of water in the formation and preparation of graphene oxide     
Qiang Zhang  Yuying Yang  Huiqing Fan  Liu Feng  Guangwu Wen  Lu-Chang Qin 《RSC advances》2021,11(30):18321
Correction for ‘Roles of water in the formation and preparation of graphene oxide’ by Qiang Zhang et al., RSC Adv., 2021, 11, 15808–15816, DOI: 10.1039/D0RA10026A.

The authors regret that there was an error in the author affiliations in the original article. The correct affiliations are given here.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.  相似文献   
998.
Polyvinylpyrrolidone-assisted synthesis of highly water-stable cadmium-based metal–organic framework nanosheets for the detection of metronidazole     
Guoxu Qin  Duojun Cao  Xinjun Wan  Xinyun Wang  Yaqiong Kong 《RSC advances》2021,11(55):34842
Recently, much effort has been dedicated to ultra-thin two-dimensional metal–organic framework (2D MOF) nanosheets due to their outstanding properties, such as ultra-thin morphology, large specific surface area, abundant modifiable active sites, etc. However, the preparation of high-quality 2D MOF nanosheets in good yields still remains a huge challenge. Herein, we report 2D cadmium-based metal–organic framework (Cd-MOF) nanosheets prepared in a one-pot polyvinylpyrrolidone (PVP)-assisted synthesis method with high yield. The Cd-MOF nanosheets were characterized with good stability and dispersion in aqueous systems, and were highly selective and sensitive to the antibiotic metronidazole (MNZ) with low limit of detection (LOD: 0.10 μM), thus providing a new and promising fluorescent sensor for rapid detection of MNZ in aqueous solution.

Except PVP was added for Cd-MOF nanosheets, the preparation process of bulk and Cd-MOF nanosheets was similar.  相似文献   
999.
Polypyrrole nanoparticles embedded nitrogen-doped graphene composites as novel cathode for long life cycles and high-power zinc-ion hybrid supercapacitors     
Prasit Pattananuwat  Rojana Pornprasertsuk  Jiaqian Qin  Suchittra Prasertkaew 《RSC advances》2021,11(56):35205
The well-designed network structure of synthetic polypyrrole (PPy) nanoparticles embedded on a nitrogen-doped graphene (N-rGO) surface was utilized as a cathode for aqueous zinc-ion hybrid supercapacitors. Owing to the combination of the redox surface of PPy and the two-dimensional network structure of N-rGO, the PPy/N-rGO cathode affords rapid transport channels for Zn2+ ion adsorption/desorption and a faradaic reaction toward the synergistic composite materials. Subsequently, the constructed zinc-ion hybrid supercapacitors with the optimized PPy/N-rGO cathode composites deliver the highest capacity of 145.32 mA h g−1 at 0.1 A g−1 and the maximum energy density of 232.50 W h kg−1 at a power density of 160 W kg−1. Besides this, excellent cycling stability of 85% retention after 10 000 charge–discharge cycles at 7.0 A g−1 was achieved. The high-rate capabilities with long life cycle performance of these novel zinc-ion hybrid supercapacitors could find practical use in a wide range of applications, ranging from next-generation electronic devices to large-scale stationary energy storage.

The well-designed network structure of synthetic polypyrrole (PPy) nanoparticles embedded on a nitrogen-doped graphene (N-rGO) surface was utilized as a cathode for aqueous zinc-ion hybrid supercapacitors.  相似文献   
1000.
Effect of porous structural properties on lithium-ion and sodium-ion storage: illustrated by the example of a micro-mesoporous graphene1−x(MoS2)x anode     
Yige Sun  Jie Tang  Kun Zhang  Xiaoliang Yu  Jinshi Yuan  Da-Ming Zhu  Kiyoshi Ozawa  Lu-Chang Qin 《RSC advances》2021,11(54):34152
In this work, we synthesized micro-mesoporous graphene1−x(MoS2)x with different compositional ratios via co-reduction of graphite oxide and exfoliated MoS2 platelets. We systematically studied the performance of the micro-mesoporous graphene1−x(MoS2)x as anodes in lithium-ion batteries and sodium-ion batteries. The results show that the specific surface areas of the composites decrease with introducing MoS2. The irreversible capacitance, which is related to the formation of solid electrolyte interphases, also decreases. Besides specific surface area, we found that micropores can benefit the lithiation and sodiation. We demonstrated that a specific charge capacity of 1319.02 mA h g−1 can be achieved at the 50th cycle for the graphene½(MoS2)½ anode in lithium-ion batteries. Possible relationships between such a high specific capacity and the micro-mesoporous structure of the graphene1−x(MoS2)x anode are discussed. This work may shed light on a general strategy for the structural design of electrode materials in lithium-ion batteries and sodium-ion batteries.

In this work, we systematically studied the effect of porous structural properties on performance of the micro-mesoporous graphene1−x (MoS2)x as anodes in lithium-ion and sodium-ion batteries.  相似文献   
[首页] « 上一页 [95] [96] [97] [98] [99] 100 下一页 » 末  页»
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