全文获取类型
收费全文 | 1924篇 |
免费 | 47篇 |
国内免费 | 7篇 |
专业分类
化学 | 1388篇 |
晶体学 | 36篇 |
力学 | 26篇 |
数学 | 173篇 |
物理学 | 355篇 |
出版年
2023年 | 15篇 |
2022年 | 13篇 |
2021年 | 40篇 |
2020年 | 38篇 |
2019年 | 33篇 |
2018年 | 35篇 |
2017年 | 25篇 |
2016年 | 40篇 |
2015年 | 43篇 |
2014年 | 51篇 |
2013年 | 102篇 |
2012年 | 122篇 |
2011年 | 153篇 |
2010年 | 70篇 |
2009年 | 87篇 |
2008年 | 112篇 |
2007年 | 98篇 |
2006年 | 101篇 |
2005年 | 94篇 |
2004年 | 80篇 |
2003年 | 55篇 |
2002年 | 69篇 |
2001年 | 26篇 |
2000年 | 24篇 |
1999年 | 19篇 |
1997年 | 18篇 |
1996年 | 17篇 |
1995年 | 19篇 |
1994年 | 19篇 |
1993年 | 24篇 |
1992年 | 21篇 |
1991年 | 9篇 |
1989年 | 12篇 |
1988年 | 14篇 |
1986年 | 12篇 |
1985年 | 15篇 |
1984年 | 19篇 |
1983年 | 15篇 |
1982年 | 25篇 |
1981年 | 17篇 |
1980年 | 15篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1977年 | 13篇 |
1976年 | 16篇 |
1975年 | 12篇 |
1974年 | 15篇 |
1973年 | 12篇 |
1972年 | 11篇 |
1971年 | 9篇 |
排序方式: 共有1978条查询结果,搜索用时 46 毫秒
101.
Daniela Nunes da Silva Hanna Leijoto de Oliveira Keyller Bastos Borges Arnaldo César Pereira 《Electroanalysis》2021,33(2):506-514
The analysis of 17β-estradiol with high sensitivity and selectivity is extremely relevant to control the impacts that this compound can cause on health and the environment. Thus, we describe the development and application of a magneto carbon paste electrode based on magnetic molecularly imprinted polymer (MCPE-MMIP) for determination of 17β-estradiol. The analyte adsorbed on the MMIP was immobilized on the electrode surface by magnetic capture. The morphological and structural characterization of the obtained MMIP suggests that the material was effectively synthesized. MCPE-MMIP showed an improvement in the sensitivity for 17β-estradiol detection when compared to electrode configurations in the absence of this material. The optimum conditions (0.10 mol L−1 phosphate buffer pH 7.0) were reached by differential pulse adsorptive stripping voltammetry (DPAdSV), in which the method presented linearity ranged from 0.06 to 175 μmol L−1 with limits of detection and quantification of 0.02 and 0.06 μmol L−1, respectively. The proposed sensor was applied effectively in the analysis of 17β-estradiol in river water and raw milk samples, exhibiting excellent recovery values (between 96.20 and 104 %), which were confirmed by HPLC analysis. 相似文献
102.
A Combined Experimental and Theoretical Study on the Formation of Ag Filaments on β‐Ag2MoO4 Induced by Electron Irradiation 下载免费PDF全文
Juan Andrés Mateus M. Ferrer Lourdes Gracia Armando Beltran Valeria M. Longo Guilherme H. Cruvinel Ricardo L. Tranquilin Elson Longo 《Particle & Particle Systems Characterization》2015,32(6):646-651
A combined experimental and theoretical study is presented to understand the novel observed nucleation and early evolution of Ag filaments on β‐Ag2MoO4 crystals, driven by an accelerated electron beam from an electronic microscope under high vacuum. The growth process, chemical composition, and the element distribution in these filaments are analyzed in depth at the nanoscale level using field‐emission scanning electron microscopy (FE‐SEM) and transmission electron microscopy (TEM) with energy‐dispersive spectroscopy (EDS) characterization. To complement experimental results, chemical stability, structural and electronic aspects have been studied systematically using first‐principles electronic structure theory within a quantum theory of atoms in molecules (QTAIM) framework. The Ag nucleation and formation on β‐Ag2MoO4 are a result of structural and electronic changes of the AgO4 tetrahedral cluster as a constituent building block of β‐Ag2MoO4, consistent with Ag metallic formation. The formation of Ag filament transforms the β‐Ag2MoO4 semiconductor from n‐ to p‐type concomitant with the appearance of Ag defects. 相似文献
103.
Harry R. Hudson Rosalind J. Lee 《Phosphorus, sulfur, and silicon and the related elements》2014,189(7-8):1149-1155
AbstractReports on the anticancer activity of representative α-aminophosphonic acid derivatives are briefly reviewed, with comments where possible on modes of action. Preliminary in vitro screening results are also presented for selected dialkyl α-aryl (or heteroaryl)-α-(diphenylmethylamino)methanephosphonates against the National Cancer Institute (NCI) 60-cell line panel of human tumor cells, which showed average response parameters for active compounds of GI50 between 4.81 × 10?6 and 2.40 × 10?5 M, TGI between 1.88 and 6.28 × 10?5 M, and LC50 between 5.71 and 9.37 × 10?5 M. The highest activity was shown by the α-phenyl compound for which GI50 10?7 M was recorded against leukemia cell line MOLT-4. 相似文献
104.
Mônica Siqueira Ferreira Diogo Noin de Oliveira Rosimeire Nunes de Oliveira Silmara Marques Allegretti Aníbal Eugênio Vercesi Rodrigo Ramos Catharino 《Journal of mass spectrometry : JMS》2014,49(1):86-92
Schistosomiasis is a neglected disease with large geographic distribution worldwide. Among the several different species of this parasite, S. mansoni is the most common and relevant one; its pathogenesis is also known to vary according to the worms' strain. High parasitical virulence is directly related to granulomatous reactions in the host's liver, and might be influenced by one or more molecules involved in a specific metabolic pathway. Therefore, better understanding the metabolic profile of these organisms is necessary, especially for an increased potential of unraveling strain virulence mechanisms and resistance to existing treatments. In this report, MALDI‐MSI and the metabolomic platform were employed to characterize and differentiate two Brazilian S. mansoni strains: males and females from Belo Horizonte (BH) and from Sergipe (SE). By performing direct analysis, it is possible to distinguish the sex of adult worms, as well as identify the spatial distribution of chemical markers. Phospholipids, diacylglycerols and triacylglycerols were located in specific structures of the worms' bodies, such as tegument, suckers, reproductive and digestive systems. Lipid profiles were found to be different both between strains and males or females, giving specific metabolic fingerprints for each group. This indicates that biochemical characterization of adult S. mansoni may help narrowing‐down the investigation of new therapeutic targets according to worm composition, molecule distribution and, therefore, aggressiveness of disease. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
105.
S. C. Nunes V. T. Freitas R. A. S. Ferreira L. D. Carlos P. Almeida V. de Zea Bermudez 《Journal of Sol-Gel Science and Technology》2014,72(2):239-251
Ordered mono-amide cross-linked alkyl/siloxane hybrids (mono-amidosils) incorporating a Rhodamine (B) methyl ester perchlorate dye (Rh(B)CH3ClO4) have been synthesized through the sol–gel process and self-directed assembly. The host hybrid matrix m-A(14) is a lamellar bilayer hierarchically structured hybrid composed of short methyl-capped alkyl chains grafted to a siliceous framework through amide groups. At low dye concentration [n = 20, where n is the molar ratio of amide groups per Rh(B)CH3ClO4] a new lamellar structure with higher interlamellar distance than that of m-A(14) is formed, whereas at higher dye content (n = 5) this new lamellar structure coexists with that of m-A(14). The efficient encapsulation of Rh(B)CH3ClO4 provided by m-A(14) via hydrogen bonding interactions ensured the complete dissolution of the dye and induced a blue shift of the emission of the dye with respect to that of the isolated state, leading to an increase in the quantum yield from values below 0.01 % (measured for the isolated dye) to 4 % at n = 20. The formation of non-fluorescent H-type dimers in the sample with n = 5 accounts for the reduction of the quantum yield. The incorporation of Rh(B)CH3ClO4) into m-A(14) was clearly beneficial from the standpoint of the dye’s photostability, allowing to suppress photobleaching during the first 4 h. An intensification of the emission intensity by 50 and 25 % for the emission centered at 600 and 645 nm resulted, respectively, at n = 20. 相似文献
106.
Fernanda Cosme Alice Vilela Luís Moreira Carla Moura Jos A. P. Enríquez Luís Filipe-Ribeiro Fernando M. Nunes 《Molecules (Basel, Switzerland)》2020,25(24)
‘Mencía’/‘Jaen’ it’s an important red grape variety, exclusive of the Iberian Peninsula, used in wine production namely in Bierzo D.O. and Dão D.O., respectively. This work evaluates the effect of the two different “terroirs” on the phenolic composition and chromatic characteristics of ‘Mencía’/‘Jaen’ monovarietal wines produced at an industrial scale in the same vintage. Using Principal Component Analysis (PCA), Partial Least Squares-Discrimination Analysis (PLS-DA), and Orthogonal PLS-DA (OPLS-DA) it was found that peonidin-3-coumaroylglucoside, petunidin-3-glucoside, malvidin-3-coumaroylglucoside, peonidin-3-glucoside, malvidin-3-acetylglucoside, malvidin-3-glucoside, and ferulic acid were the phenolic compounds with the highest differences between the two regions. PLS regression allowed to correlate the differences in lightness (L*) and redness (a*) of wines from ‘Jaen’ and ‘Mencía’ to differences in colored anthocyanins, polymeric pigments, total pigments, total anthocyanins, cyanidin-3-acetylglucoside, delphinidin-3-acetylglucoside, delphinidin-3-glucoside, peonidin-3-coumaroylglucoside, petunidin-3-glucoside and malvidin-3-glucoside in wines, and the colorless ferulic, caffeic, and coutaric acids, and ethyl caffeate. The wines a* values were more affected by colored anthocyanins, ferulic acid, total anthocyanins, delphinidin-3-acetylglucoside, delphinidin-3-glucoside and petunidin-3-acetylglucoside, and catechin. The positive influence of ferulic acid in the a* values and ferulic, caffeic, coutaric acids, and ethyl caffeate on the L* values can be due to the co-pigmentation phenomena. The higher dryness and lower temperatures during the September nights in this vintage might explain the differences observed in the anthocyanin content and chromatic characteristics of the wines. 相似文献
107.
Gabriela Medina-Prez Jos Antonio Estefes-Duarte Laura N. Afanador-Barajas Fabin Fernndez-Luqueo Andrea Paloma Zepeda-Velzquez Melitn Jesús Franco-Fernndez Armando Pelez-Acero Rafael Germn Campos-Montiel 《Molecules (Basel, Switzerland)》2020,25(23)
Cactus acid fruit (Xoconostle) has been studied due its content of bioactive compounds. Traditional Mexican medicine attributes hypoglycemic, hypocholesterolemic, anti-inflammatory, antiulcerogenic and immunostimulant properties among others. The bioactive compounds contained in xoconostle have shown their ability to inhibit digestive enzymes such as α-amylase and α-glucosidase. Unfortunately, polyphenols and antioxidants in general are molecules susceptible to degradation due to storage conditions, (temperature, oxygen and light) or the gastrointestinal tract, which limits its activity and compromises its potential beneficial effect on health. The objectives of this work were to evaluate the stability, antioxidant and antidiabetic activity of encapsulated extract of xoconostle within double emulsions (water-in-oil-in-water) during storage conditions and simulated digestion. Total phenols, flavonoids, betalains, antioxidant activity, α-amylase and α-glucosidase inhibition were measured before and after the preparation of double emulsions and during the simulation of digestion. The ED40% (treatment with 40% of xoconostle extract) treatment showed the highest percentage of inhibition of α-glucosidase in all phases of digestion. The inhibitory activity of α-amylase and α-glucosidase related to antidiabetic activity was higher in microencapsulated extracts than the non-encapsulated extracts. These results confirm the viability of encapsulation systems based on double emulsions to encapsulate and protect natural antidiabetic compounds. 相似文献
108.
Ruben Elvas-Leito Filomena Martins Leonor Borbinha Catarina Marranita Angela Martins Nelson Nunes 《Molecules (Basel, Switzerland)》2020,25(23)
Attempts to optimize heterogeneous catalysis often lack quantitative comparative analysis. The use of kinetic modelling leads to rate (k) and relative sorption equilibrium constants (K), which can be further rationalized using Quantitative Structure-Property Relationships (QSPR) based on Multiple Linear Regressions (MLR). Friedel-Crafts acylation using commercial and hierarchical BEA zeolites as heterogeneous catalysts, acetic anhydride as the acylating agent, and a set of seven substrates with different sizes and chemical functionalities were herein studied. Catalytic results were correlated with the physicochemical properties of substrates and catalysts. From this analysis, a robust set of equations was obtained allowing inferences about the dominant factors governing the processes. Not entirely surprising, the rate and sorption equilibrium constants were found to be explained in part by common factors but of opposite signs: higher and stronger adsorption forces increase reaction rates, but they also make the zeolite active sites less accessible to new reactant molecules. The most relevant parameters are related to the substrates’ molecular size, which can be associated with different reaction steps, namely accessibility to micropores, diffusion capacity, and polarizability of molecules. The relatively large set of substrates used here reinforces previous findings and brings further insights into the factors that hamper/speed up Friedel-Crafts reactions in heterogeneous media. 相似文献
109.
Cesar Garcías-Morales Selene H. Martínez-Salas Armando Ariza-Castolo 《Tetrahedron letters》2012,53(26):3310-3315
The effect of the nitrogen nonbonding electron pair on the 1JC,H values of 1,3-diazaheterocycles was analyzed and compared to 1,5-diazabiciclo[3.2.1]octanes, which have a restricted conformation. The 1JC,H values were measured by observing the 13C satellites in the 1H NMR spectra and then determining the 1H-coupled 13C NMR spectra. The 1JC,H values are 10 Hz larger when the α-hydrogen is synperiplanar rather than antiperiplanar to the nonbonding electron pair on the nitrogen, which serves as experimental evidence of the orbital nN→σ1C,Hap interactions. In addition, the homoanomeric effect from the interactions of the nitrogen lone pair with the antibonding orbital of the equatorial hydrogen, which was in the β position, was discussed (nN→σ1C(β),Heq). 相似文献
110.