全文获取类型
收费全文 | 29977篇 |
免费 | 163篇 |
国内免费 | 273篇 |
专业分类
化学 | 11501篇 |
晶体学 | 268篇 |
力学 | 1383篇 |
综合类 | 12篇 |
数学 | 9046篇 |
物理学 | 8203篇 |
出版年
2023年 | 35篇 |
2022年 | 105篇 |
2021年 | 126篇 |
2020年 | 81篇 |
2019年 | 65篇 |
2018年 | 1240篇 |
2017年 | 1471篇 |
2016年 | 789篇 |
2015年 | 592篇 |
2014年 | 531篇 |
2013年 | 731篇 |
2012年 | 3196篇 |
2011年 | 2394篇 |
2010年 | 1849篇 |
2009年 | 1580篇 |
2008年 | 627篇 |
2007年 | 652篇 |
2006年 | 646篇 |
2005年 | 4475篇 |
2004年 | 3945篇 |
2003年 | 2313篇 |
2002年 | 490篇 |
2001年 | 291篇 |
2000年 | 91篇 |
1999年 | 182篇 |
1998年 | 114篇 |
1997年 | 72篇 |
1996年 | 54篇 |
1995年 | 42篇 |
1994年 | 42篇 |
1992年 | 172篇 |
1991年 | 154篇 |
1990年 | 134篇 |
1989年 | 106篇 |
1988年 | 100篇 |
1987年 | 57篇 |
1986年 | 43篇 |
1985年 | 38篇 |
1984年 | 31篇 |
1979年 | 30篇 |
1976年 | 79篇 |
1975年 | 38篇 |
1974年 | 40篇 |
1973年 | 48篇 |
1972年 | 39篇 |
1970年 | 34篇 |
1969年 | 38篇 |
1968年 | 35篇 |
1967年 | 30篇 |
1966年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Bei?Xu RongLan?ZhangEmail author JiFeng?Wang JianShe?ZhaoEmail author 《Journal of Solid State Electrochemistry》2013,17(9):2391-2400
Three novel series of the binuclear metal phthalocyanines M2Pc2, M2Pc2Hc, and M2Nc2 (M?=?Mn(II), Fe(II), Co(II), Ni(II), and Cu(II)) were synthesized and characterized. The electrocatalytic performance of the binuclear compounds to lithium–thionyl chloride battery was evaluated by operating these compounds in the electrolyte of the battery. The results indicated that the binuclear metal phthalocyanines improved the capacity of the battery by an increase of approximately 30–58 %. Of all, Cu2Pc2Hc and Fe2Nc2 displayed the highest increments of 56 and 57 %, respectively. 相似文献
992.
Background
The regulation of lipid biosynthesis is essential in photosynthetic eukaryotic cells. This regulation occurs during the direct synthesis of fatty acids and triacylglycerols (TAGs), as well as during other controlling processes in the main carbon metabolic pathway.Results
In this study, the mRNA levels of Chlamydomonas citrate synthase (CrCIS) were found to decrease under nitrogen-limited conditions, which suggests suppressed gene expression. Gene silencing by RNA interference (RNAi) was conducted to determine whether CrCIS suppression affected the carbon flux in TAG biosynthesis. Results showed that the TAG level increased by 169.5%, whereas the CrCIS activities in the corresponding transgenic algae decreased by 16.7% to 37.7%. Moreover, the decrease in CrCIS expression led to the increased expression of TAG biosynthesis-related genes, such as acyl-CoA:diacylglycerol acyltransferase and phosphatidate phosphatase. Conversely, overexpression of CrCIS gene decreased the TAG level by 45% but increased CrCIS activity by 209% to 266% in transgenic algae.Conclusions
The regulation of CrCIS gene can indirectly control the lipid content of algal cells. Our findings propose that increasing oil by suppressing CrCIS expression in microalgae is feasible.993.
Background
BTBD10 binds to Akt and protein phosphatase 2A (PP2A) and inhibits the PP2A-mediated dephosphorylation of Akt, thereby keeping Akt activated. Previous studies have suggested that BTBD10 plays an important role in preventing motor neuronal death and accelerating the growth of pancreatic beta cells. Because levels of BTBD10 expression are much lower in many non-nervous tissues than nervous tissues, there may be a relative of BTBD10 that has BTBD10-like function in non-neuronal cells.Results
A 419-amino-acid BTBD10-like protein, named KCTD20 (potassium channel tetramerization protein domain containing 20), was to found to bind to all Akt isoforms and PP2A. Overexpression of KCTD20 increased Akt phosphorylation at Thr308, as BTBD10 did, which suggests that KCTD20 as well as BTBD10 positively regulates the function of Akt. KCTD20 was ubiquitously expressed in non-nervous as well as nervous tissues.Conclusions
KCTD20 is a positive regulator of Akt and may play an important role in regulating the death and growth of some non-nervous and nervous cells.994.
Monique?SabatyEmail author Sandrine?Grosse Geraldine?Adryanczyk Séverine?Boiry Frédéric?Biaso Pascal?Arnoux David?Pignol 《BMC biochemistry》2013,14(1):28
Background
YedY, a molybdoenzyme belonging to the sulfite oxidase family, is found in most Gram-negative bacteria. It contains a twin-arginine signal sequence that is cleaved after its translocation into the periplasm. Despite a weak reductase activity with substrates such as dimethyl sulfoxide or trimethylamine N-oxide, its natural substrate and its role in the cell remain unknown. Although sequence conservation of the YedY family displays a strictly conserved hydrophobic C-terminal residue, all known studies on Escherichia coli YedY have been performed with an enzyme containing a 6 histidine-tag at the C-terminus which could hamper enzyme activity.Results
In this study, we demonstrate that the tag fused to the C-terminus of Rhodobacter sphaeroides YedY is detrimental to the enzyme’s reductase activity and results in an eight-fold decrease in catalytic efficiency. Nonetheless this C-terminal tag does not influence the properties of the molybdenum active site, as assayed by EPR spectroscopy. When a cleavable His-tag was fused to the N-terminus of the mature enzyme in the absence of the signal sequence, YedY was expressed and folded with its cofactor. However, when the signal sequence was added upstream of the N-ter tag, the amount of enzyme produced was approximately ten-fold higher.Conclusion
Our study thus underscores the risk of using a C-terminus tagged enzyme while studying YedY, and presents an alternative strategy to express signal sequence-containing enzymes with an N-terminal tag. It brings new insights into molybdoenzyme maturation in R. sphaeroides showing that for some enzymes, maturation can occur in the absence of the signal sequence but that its presence is required for high expression of active enzyme.995.
Alexander Boichenko Natalia Govorukhina Ate G. J. van der Zee Rainer Bischoff 《Analytical and bioanalytical chemistry》2013,405(10):3195-3203
Macroporous reversed-phase (mRP) chromatography was successfully used to develop an accurate and precise method for total protein in serum. The limits of detection (0.83 μg, LOD) and quantification (2.51 μg, LOQ) for the mRP method are comparable with those of the widely used micro BCA protein assay. The mRP method can be used to determine the total protein concentration across a wide dynamic range by detecting chromatographic peaks at 215 nm and 280 nm. The method has the added advantage of desalting and denaturing proteins, leading to more complete digestion by trypsin and to better LC–MS–MS identification in shotgun proteomics experiments. Figure
Simultaneous Serum Desalting and Total Protein Determination with Macroporous Reversed-Phase Chromatography: calibration plots 相似文献
996.
Mario?ThevisEmail author Andreas?Thomas Wilhelm?Sch?nzer 《Analytical and bioanalytical chemistry》2013,405(30):9655-9667
Urine samples have been the predominant matrix for doping controls for several decades. However, owing to the complementary information provided by blood (as well as serum or plasma and dried blood spots (DBS)), the benefits of its analysis have resulted in continuously increasing appreciation by anti-doping authorities. On the one hand, blood samples allow for the detection of various different methods of blood doping and the abuse of erythropoiesis-stimulating agents (ESAs) via the Athlete Biological Passport; on the other hand, targeted and non-targeted drug detection by means of chromatographic–mass spectrometric methods represents an important tool to increase doping control frequencies out-of-competition and to determine drug concentrations particularly in in-competition scenarios. Moreover, blood analysis seldom requires in-depth knowledge of drug metabolism, and the intact substance rather than potentially unknown or assumed metabolic products can be targeted. In this review, the recent developments in human sports drug testing concerning mass spectrometry-based techniques for qualitative and quantitative analyses of therapeutics and emerging drug candidates are summarized and reviewed. The analytical methods include both low and high molecular mass compounds (e.g., anabolic agents, stimulants, metabolic modulators, peptide hormones, and small interfering RNA (siRNA)) determined from serum, plasma, and DBS using state-of-the-art instrumentation such as liquid chromatography (LC)–high resolution/high accuracy (tandem) mass spectrometry (LC-HRMS), LC–low resolution tandem mass spectrometry (LC-MS/MS), and gas chromatography–mass spectrometry (GC-MS). 相似文献
997.
Natalia P. Ivleva Susanne Huckele Bernadett Weinzierl Reinhard Niessner Christoph Haisch Thomas Baumann 《Analytical and bioanalytical chemistry》2013,405(28):9071-9084
We present for the first time the Raman microspectroscopic identification and characterization of individual airborne volcanic ash (VA) particles. The particles were collected in April/May 2010 during research aircraft flights, which were performed by Deutsches Zentrum für Luft- und Raumfahrt in the airspace near the Eyjafjallajökull volcano eruption and over Europe (between Iceland and Southern Germany). In addition, aerosol particles were sampled by an Electrical Low Pressure Impactor in Munich, Germany. As references for the Raman analysis, we used the spectra of VA collected at the ground near the place of eruption, of mineral basaltic rock, and of different minerals from a database. We found significant differences in the spectra of VA and other aerosol particles (e.g., soot, nitrates, sulfates, and clay minerals), which allowed us to identify VA among other atmospheric particulate matter. Furthermore, while the airborne VA shows a characteristic Raman pattern (with broad band from ca. 200 to ca. 700 cm?1 typical for SiO2 glasses and additional bands of ferric minerals), the differences between the spectra of aged and fresh particles were observed, suggesting differences in their chemical composition and/or structure. We also analyzed similarities between Eyjafjallajökull VA particles collected at different sampling sites and compared the particles with a large variety of glassy and crystalline minerals. This was done by applying cluster analysis, in order to get information on the composition and structure of volcanic ash. Figure
Images and Raman spectra of airborne volcanic ash 相似文献
998.
Manuel Sergi Natalia Battista Camilla Montesano Roberta Curini Mauro Maccarrone Dario Compagnone 《Analytical and bioanalytical chemistry》2013,405(2-3):785-793
Endocannabinoids (ECs) are endogenous compounds that interact with type-1 and type-2 cannabinoid receptors (CB1 and CB2), as well as non-cannabinoid receptors. The multitude of roles attributed to ECs makes them an emerging target of pharmacotherapy for a number of disparate diseases. Here a high-throughput bioanalytical method based on micro SPE (μ-SPE) followed by LC-MS/MS analysis for the simultaneous determination of the two major endocannabinoids 2-arachidonoylglycerol (2-AG) and N-arachidonoylethanolamine (anandamide, AEA) in human plasma is presented. The chromatographic conditions obtained with the fused-core column allowed a good separation in 10 min also of the AG isomers. A very simple and reliable extraction has been optimised by means of C18-modified tips: it requires only 100 μL of plasma and allows the use of minimal volumes of organic solvent. The present method allows a rapid and effective clean-up, which also minimises the isomerisation of 2-AG. The whole procedure has been validated following the FDA guidelines for bioanalytical methods validation: the satisfactory recovery values, the negligible matrix effect and the good values of accuracy and reproducibility make it a simple and high-throughput analytical tool for clinical and biochemical studies on endocannabinoid signaling in humans. Figure
Determination of the two major endocannabinoids in human plasma by μ-SPE followed by HPLC-MS/MS 相似文献
999.
Hamid?Mofakham Rahim?Ghadari Ahmad?ShaabaniEmail author Milad?Pedarpour Sabrieh?Ghasemi 《Journal of the Iranian Chemical Society》2013,10(2):307-317
In this work, functionalized pyrimidine-2,4-dione-, benzo[g]-, and dihydropyrano[2,3-g]chromene derivatives have been synthesized via a Michael addition of 2-hydroxy-1,4-naphthoquinone or 2,5-dihydroxy-1,4-benzoquinone to the Knoevenagel condensation product of an aldehyde with Meldrum’s acid, dimedone or barbituric acid in the presence of a catalytic amount of l-proline under refluxing conditions in water in good to excellent yields. 相似文献
1000.