全文获取类型
收费全文 | 8227篇 |
免费 | 1239篇 |
国内免费 | 749篇 |
专业分类
化学 | 5708篇 |
晶体学 | 132篇 |
力学 | 485篇 |
综合类 | 75篇 |
数学 | 786篇 |
物理学 | 3029篇 |
出版年
2024年 | 35篇 |
2023年 | 182篇 |
2022年 | 311篇 |
2021年 | 306篇 |
2020年 | 377篇 |
2019年 | 351篇 |
2018年 | 300篇 |
2017年 | 254篇 |
2016年 | 414篇 |
2015年 | 402篇 |
2014年 | 529篇 |
2013年 | 606篇 |
2012年 | 673篇 |
2011年 | 693篇 |
2010年 | 461篇 |
2009年 | 376篇 |
2008年 | 499篇 |
2007年 | 427篇 |
2006年 | 382篇 |
2005年 | 324篇 |
2004年 | 216篇 |
2003年 | 218篇 |
2002年 | 167篇 |
2001年 | 124篇 |
2000年 | 156篇 |
1999年 | 151篇 |
1998年 | 130篇 |
1997年 | 133篇 |
1996年 | 153篇 |
1995年 | 110篇 |
1994年 | 108篇 |
1993年 | 89篇 |
1992年 | 78篇 |
1991年 | 69篇 |
1990年 | 72篇 |
1989年 | 47篇 |
1988年 | 49篇 |
1987年 | 37篇 |
1986年 | 37篇 |
1985年 | 26篇 |
1984年 | 22篇 |
1983年 | 15篇 |
1982年 | 19篇 |
1981年 | 15篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1970年 | 12篇 |
1937年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Yan-Ping Liu Li-Li Sun Xiang-Lin Zhang Hai-Yuan Niu Zheng-Hong Pan 《Natural product research》2020,34(16):2295-2300
Abstract A phytochemical investigation on the stems of Mappianthus iodoides led to the isolation of a new naturally occurring prenylated isoflavone, mappianthone A (1), together with seven known analogues (2?8). The structure of 1 was elucidated by extensive spectroscopic methods and the known compounds were identified by comparison with data reported in the literature. All isolated compounds were evaluated for their antiproliferative activities against five human cancer cell lines: HL-60, SMMC-7721, A-549, MCF-7 and SW480 in vitro. Compounds 1?8 showed significant antiproliferative effects against several human cancer cell lines with IC50 values ranging from 0.16 to 12.68?μM. 相似文献
102.
Dan Lu Weiyang Zhang Yingnan Jiang Yu Zhang Dabo Pan Dan Zhang 《Natural product research》2019,33(19):2789-2794
A new cycloartane triterpenoid, named gardenolic acid C (1), a new ursane triterpenoid, named 3β,16β,21β,23,24-pentahydroxy urs-12,18,20-trien-28-oic acid γ-lactone (2), together with three know triterpenoids, gardenolic acid A (3), gardenolic acid B (4), and 3α,16β,23,24-tetrahydroxy-28-nor-ursane-12,17,19,21-tetraen (5) were isolated from the fruits of Gardenia jasminoides Ellis. The structures of these compounds were elucidated by analyses of spectroscopic data. All isolates were evaluated for their neuroprotective effects in vitro.
103.
E. M. F. of the Cell, Cd-Hg (2-phase)/CdAc2(m), Hg2Ac2(s)/Hg was measured at 20°, 25°, 30° and 40°C. The standard e. m. f. of the cell, Cd/CdAc3(m), Hg2Ac2(c)/Hg was evaluated as E°=1.1500?11.09×10?4T+1.06×10?8T2 The thermodynamic data of the reaction, Cd(c) + Hg2Ac2(c)=2Hg(l)+Cd++(aq)+2Ac?(aq) at 25°C were estimated as ΔF°=?42,139, ΔH°=?48,698 cal mole?1 and ΔS°=?22.0 cal deg?1 mole?1 at 25°C. The thermodynamic data for the formation of Hg2Ac2(s) were evaluated as ΔFf°=?202.3, ΔHf°=?154.5 Kcal mole?1 and S°=72.9 cal deg?1 mole?1. From measurements of the heats of solution of CdAc2·2H2O in aqueous solution, the relative partial molal enthalpies of cadmium acetate in aqueous solution were estimated. 相似文献
104.
Lin Jia Hui Yang Jun Deng Junmei Chen Yuan Zhou Pan Ding Leigang Li Na Han Yanguang Li 《中国化学》2019,37(5):497-500
Electrocatalytic carbon dioxide reduction holds great promise for reducing the atmospheric CO2 level and alleviating the energy crisis. High‐performance electrocatalysts are often required in order to lower the high overpotential and expedite the sluggish reaction kinetics of CO2 electroreduction. Copper is a promising candidate metal. However, it usually suffers from the issues of poor stability and low product selectivity. In this work, bimetallic Cu‐Bi is obtained by reducing the microspherical copper bismuthate (CuBi2O4) for selectively catalyzing the CO2 reduction to formate (HCOO–). The bimetallic Cu‐Bi electrocatalyst exhibits high activity and selectivity with the Faradic efficiency over 90% in a wide potential window. A maximum Faradaic efficiency of ~95% is obtained at –0.93 V versus reversible hydrogen electrode. Furthermore, the catalyst shows high stability over 6 h with Faradaic efficiency of ~95%. This study provides an important clue in designing new functional materials for CO2 electroreduction with high activity and selectivity. 相似文献
105.
The reaction of 1-alkynylcyclobutanols with aryl iodides in the presence of Pd(OAc)2 and Et3N in acetonitrile at 80°C for 24 h gives 2-disubstituted methylenecyclopentan-1-ones in modest to good yields. The tandem insertion-ring expansion process proceeds via the formation of an alkynyl π-complex, followed by migration of a carbon-carbon bond of the tert-alkanol to form the cyclopentanones stereoselectively. 相似文献
106.
Pan A Yang H Liu R Yu R Zou B Wang Z 《Journal of the American Chemical Society》2005,127(45):15692-15693
High-quality CdSxSe1-x nanobelts of variable composition (0 = x = 1) were fabricated by a simple one-step thermal evaporation route. Photoluminescence measurements showed that all the CdSxSe1-x nanobelts have a single near-bandedge emission band, and their spectral maximum positions can be tuned from approximately 508 nm (for pure CdS) to approximately 705 nm (for pure CdSe). These kinds of nanobelts may have potential applications in adjustable nano/micro photoelectric devices in the visible region. 相似文献
107.
108.
Solution self-assembly, spontaneous deprotonation, and crystal structures of bipyrazolate-bridged metallomacrocycles with dimetal centers 总被引:5,自引:0,他引:5
Yu SY Huang HP Li SH Jiao Q Li YZ Wu B Sei Y Yamaguchi K Pan YJ Ma HW 《Inorganic chemistry》2005,44(25):9471-9488
A series of nanosized cavity-containing bipyrazolate-bridged metallomacrocycles with dimetal centers, namely, {[(bpy)M]8L4}(NO3)8 [L=3,3',5,5'-tetramethyl-4,4'-bipyrazolyl, Pd, Pt; 1,4-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene), Pd; and 1,4-bis-4'-(3',5'-dimethyl)-pyrazolylbiphenyl, Pd], {[(phen)M]8L4}(NO3)8 [L=3,3',5,5'-tetramethyl-4,4'-bipyrazolyl, Pd, Pt; 1,4-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene, Pd; and 1,4-bis-4'-(3',5'-dimethyl)-pyrazolylbiphenyl, Pd], {[(bpy)Pd]6L3}(NO3)6 [L=1,4-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene,], {[(phen)Pd]6L3}(NO3)6 [L=1,4-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene,], {[(bpy)Pd]4L2}(NO3)4 [L=1,3-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene, and 1,2-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene,], and {[(phen)Pd]4L2}(NO3)4 [L=1,3-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene, and 1,2-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene,] (where bpy=2,2'-bipyridine and phen=1,10-phenanthroline) have been synthesized through a directed self-assembly approach that involves spontaneous deprotonation of the 1H-bipyrazolyl ligands in aqueous solution. These complexes, with weak Pd(II)...Pd(II) or Pt(II)...Pt(II) interactions, have been characterized by elemental analysis, 1H and 13C NMR, cold-spray ionization or electrospray ionization mass spectrometry, UV-visible spectroscopy, and luminescence spectroscopy. Complexes and have also been characterized by single-crystal X-ray diffraction analysis. 相似文献
109.
Qunying Jiang Min Pan Jialing Hu Junlin Sun Lei Fan Zhiqiao Zou Jianshuang Wei Xiaoquan Yang Xiaoqing Liu 《Chemical science》2021,12(1):148
Many cancer treatments including photodynamic therapy (PDT) utilize reactive oxygen species (ROS) to kill tumor cells. However, elevated antioxidant defense systems in cancer cells result in resistance to the therapy involving ROS. Here we describe a highly effective phototherapy through regulation of redox homeostasis with a biocompatible and versatile nanotherapeutic to inhibit tumor growth and metastasis. We systematically explore and exploit methylene blue adsorbed polydopamine nanoparticles as a targeted and precise nanocarrier, oxidative stress amplifier, photodynamic/photothermal agent, and multimodal probe for fluorescence, photothermal and photoacoustic imaging to enhance anti-tumor efficacy. Remarkably, following the glutathione-stimulated photosensitizer release to generate exogenous ROS, polydopamine eliminates the endogenous ROS scavenging system through depleting the primary antioxidant, thus amplifying the phototherapy and effectively suppressing tumor growth in vitro and in vivo. Furthermore, this approach enables a robust inhibition against breast cancer metastasis, as oxidative stress is a vital impediment to distant metastasis in tumor cells. Innovative, safe and effective nanotherapeutics via regulation of redox balance may provide a clinically relevant approach for cancer treatment.Amplified oxidative stress achieved by modulating redox homeostasis with PDA–MB for highly effective synergistic phototherapy to inhibit primary tumors and metastases. 相似文献
110.
Yang M Wang GJ Wang SJ Li XT Xu YP Wang SP Xiang JD Pan SR Cao GX Ye WC 《Rapid communications in mass spectrometry : RCM》2005,19(12):1619-1623
23-Hydroxybetulinic acid is a newly isolated derivative of betulinic acid. The agent exhibits potential anti-tumor activity and functions in this regard via apoptosis. In support of pharmacokinetic and toxicological evaluations, a new assay based on liquid chromatography/mass spectrometry (LC/MS) was developed for the quantitative analysis of 23-hydroxybetulinic acid. Sample preparation consisted of extraction of the plasma by the addition of methylene chloride followed by centrifugation. Aliquots of the supernatant were analyzed using an isocratic reversed-phase high-performance liquid chromatography (HPLC) system coupled to a negative ion electrospray mass spectrometer. Molecules of 23-hydroxybetulinic acid and the internal standard limonin were detected using selected ion monitoring at m/z 471 and 469, respectively. The limit of detection of 23-hydroxybetulinic acid was 0.05 pg (0.11 fmol) injected on-column (10 pg/mL, 5 microL injection volume), and the limit of quantitation was 10 pg (21.19 fmol, 2 ng/mL, 5 muL injection volume). 23-Hydroxybetulinic acid was stable in plasma samples at -20 degrees C for at least 3 weeks. The intra-day and inter-day coefficients of variation of the assay were 3.0 and 4.8%, respectively. The utility of the assay was demonstrated by measuring 23-hydroxybetulinicacid in mouse plasma following intragastric administration (IG) in vivo. Pharmacokinetic parameters were calculated using the 3P97 pharmacokinetic software package. A two-compartment, first-order model was selected for pharmacokinetic modeling. The result showed that after IG of 200 mg/kg 23-hydroxybetulinic acid, the plasma concentrations reached peaks at 2 h with C(max) of 3.1 microg/mL. The 200 mg/kg 23-hydroxybetulinic acid suspension IG doses were found to have long elimination half-lives of 25.6 h and low bioavailability of 2.3%. No interference was noted due to endogenous substances. These analytical methods should be of value in future studies related to the development and characterization of 23-hydroxybetulinic acid. 相似文献