全文获取类型
收费全文 | 2555篇 |
免费 | 80篇 |
国内免费 | 9篇 |
专业分类
化学 | 1691篇 |
晶体学 | 26篇 |
力学 | 58篇 |
数学 | 307篇 |
物理学 | 562篇 |
出版年
2023年 | 16篇 |
2022年 | 78篇 |
2021年 | 81篇 |
2020年 | 55篇 |
2019年 | 56篇 |
2018年 | 53篇 |
2017年 | 41篇 |
2016年 | 77篇 |
2015年 | 87篇 |
2014年 | 82篇 |
2013年 | 153篇 |
2012年 | 162篇 |
2011年 | 175篇 |
2010年 | 111篇 |
2009年 | 108篇 |
2008年 | 154篇 |
2007年 | 122篇 |
2006年 | 108篇 |
2005年 | 105篇 |
2004年 | 88篇 |
2003年 | 55篇 |
2002年 | 72篇 |
2001年 | 44篇 |
2000年 | 43篇 |
1999年 | 40篇 |
1998年 | 35篇 |
1997年 | 34篇 |
1996年 | 29篇 |
1995年 | 31篇 |
1994年 | 32篇 |
1993年 | 27篇 |
1992年 | 32篇 |
1991年 | 24篇 |
1990年 | 17篇 |
1989年 | 10篇 |
1988年 | 23篇 |
1987年 | 9篇 |
1986年 | 11篇 |
1985年 | 18篇 |
1984年 | 16篇 |
1983年 | 10篇 |
1982年 | 27篇 |
1981年 | 12篇 |
1980年 | 15篇 |
1978年 | 7篇 |
1977年 | 10篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1974年 | 9篇 |
1973年 | 6篇 |
排序方式: 共有2644条查询结果,搜索用时 15 毫秒
141.
142.
do Nascimento Ticiano Gomes Borges Arthur Luy Tavares Ferreira de Almeida Lara Mendes Ribeiro Êurica Adélia Nogueira Silva Fernanda Geny Calheiros da Costa Silva Valdemir do Nascimento Prata Ana Paula Basílio-Júnior Irinaldo Diniz Goulart Marília Oliveira Fonseca Morilla Demetrius Peireira de Freitas Johnnatan Duarte Aragão Cícero Flávio Soares 《Journal of Thermal Analysis and Calorimetry》2022,147(21):11949-11963
Journal of Thermal Analysis and Calorimetry - This work prepared and characterized microcapsule of Uncaria tomentosa (UT) in order to standardize a spray-dryer Uncaria tomentosa extract. The UT... 相似文献
143.
Fernanda das Neves Costa Gerold Jerz Peter Hewitson Fabiana de Souza Figueiredo Svetlana Ignatova 《Molecules (Basel, Switzerland)》2021,26(8)
The detailed metabolite profiling of Laguncularia racemosa was accomplished by high-performance countercurrent chromatography (HPCCC) using the three-phase system n-hexane–tert-butyl methyl ether–acetonitrile–water 2:3:3:2 (v/v/v/v) in step-gradient elution mode. The gradient elution was adjusted to the chemical complexity of the L. racemosa ethyl acetate partition and strongly improved the polarity range of chromatography. The three-phase solvent system was chosen for the gradient to avoid equilibrium problems when changing mobile phase compositions encountered between the gradient steps. The tentative recognition of metabolites including the identification of novel ones was possible due to the off-line injection of fractions to electrospray ionization mass spectrometry (ESI-MS/MS) in the sequence of recovery. The off-line hyphenation profiling experiment of HPCCC and ESI-MS projected the preparative elution by selected single ion traces in the negative ionization mode. Co-elution effects were monitored and MS/MS fragmentation data of more than 100 substances were used for structural characterization and identification. The metabolite profile in the L. racemosa extract comprised flavonoids, hydrolysable tannins, condensed tannins and low molecular weight polyphenols. 相似文献
144.
Ana Margarida Silva Diana Pinto Iva Fernandes Victor de Freitas María de la Luz Cdiz-Gurrea Paulo Costa Cristina Delerue-Matos Francisca Rodrigues 《Molecules (Basel, Switzerland)》2021,26(8)
During kiwiberry production, different by-products are generated, including leaves that are removed to increase the fruit’s solar exposure. The aim of this work was to extract bioactive compounds from kiwiberry leaf by employing microwave-assisted extraction (MAE). Compatible food solvents (water and ethanol) were employed. The alcoholic extract contained the highest phenolic and flavonoid contents (629.48 mg of gallic acid equivalents (GAE) per gram of plant material on dry weight (dw) (GAE/g dw) and 136.81 mg of catechin equivalents per gram of plant material on dw (CAE/g dw), respectively). Oppositely, the hydroalcoholic extract achieved the highest antioxidant activity and scavenging activity against reactive oxygen and nitrogen species (IC50 = 29.10 μg/mL for O2•−, IC50 = 1.87 μg/mL for HOCl and IC50 = 1.18 μg/mL for •NO). The phenolic profile showed the presence of caffeoylquinic acids, proanthocyanidin, and quercetin in all samples. However, caffeoylquinic acids and quercetin were detected in higher amounts in the alcoholic extract, while proanthocyanidins were prevalent in the hydroalcoholic extract. No adverse effects were observed on Caco-2 viability, while the highest concentration (1000 µg/mL) of hydroalcoholic and alcoholic extracts conducted to a decrease of HT29-MTX viability. These results highlight the MAE potentialities to extract bioactive compounds from kiwiberry leaf. 相似文献
145.
Valeria Guarrasi Giacoma Cinzia Rappa Maria Assunta Costa Fabio Librizzi Marco Raimondo Vita Di Stefano Maria Antonietta German Silvia Vilasi 《Molecules (Basel, Switzerland)》2021,26(8)
Waste valorization represents one of the main social challenges when promoting a circular economy and environmental sustainability. Here, we evaluated the effect of the polyphenols extracted from apple peels, normally disposed of as waste, on the amyloid aggregation process of κ-casein from bovine milk, a well-used amyloidogenic model system. The effect of the apple peel extract on protein aggregation was examined using a thioflavin T fluorescence assay, Congo red binding assay, circular dichroism, light scattering, and atomic force microscopy. We found that the phenolic extract from the peel of apples of the cultivar “Fuji”, cultivated in Sicily (Caltavuturo, Italy), inhibited κ-casein fibril formation in a dose-dependent way. In particular, we found that the extract significantly reduced the protein aggregation rate and inhibited the secondary structure reorganization that accompanies κ-casein amyloid formation. Protein-aggregated species resulting from the incubation of κ-casein in the presence of polyphenols under amyloid aggregation conditions were reduced in number and different in morphology. 相似文献
146.
Lidiane Diniz do Nascimento Sebastio Gomes Silva Mrcia Moraes Cascaes Kauê Santana da Costa Pablo Luis Baia Figueiredo Cristiane Maria Leal Costa Eloisa Helena de Aguiar Andrade Lênio Jos Guerreiro de Faria 《Molecules (Basel, Switzerland)》2021,26(9)
Leaves of Lippia thymoides (Verbenaceae) were dried in an oven at 40, 50 and 60 °C and the kinetic of drying and the influence of the drying process on the chemical composition, yield, and DPPH radical scavenging activity of the obtained essential oils were evaluated. The composition of the essential oils was determined with gas chromatography-mass spectrometry and gas chromatography-flame ionization detection analyses. The influence of drying on the chemical composition of the essential oils of L. thymoides was evaluated by multivariate analysis, and their antioxidant activity was investigated via the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. The Midilli model was the most appropriate to describe the behavior of drying kinetic data of L. thymoides leaves. Thymol was the major compound for all analyzed conditions; the maximum content was obtained from fresh leaves (62.78 ± 0.63%). The essential oils showed DPPH radical scavenging activity with an average of 73.10 ± 12.08%, and the fresh leaves showed higher inhibition (89.97 ± 0.31%). This is the first study to evaluate the influence of drying on the chemical composition and antioxidant activity of L. thymoides essential oils rich in thymol. 相似文献
147.
Schneider MC Rosa SP Almeida MC Costa C Cella DM 《Micron (Oxford, England : 1993)》2007,38(6):590-598
The chromosome study of five species of the family Elateridae, belonging to the subfamilies Agrypninae and Elaterinae, and the analysis of the cytogenetic data previously recorded for this family permitted the establishment of the main strategies of karyotypic differentiation that has occurred in the elaterids. In Agrypninae, the three species studied (Conoderus fuscofasciatus, Conoderus rufidens, and Conoderus sp.) showed the male karyotype 2n=16+X0. This karyotypic uniformity detected in these Conoderus species has also been shared with other species of the same genus, differing considerably from chromosomal heterogeneity verified in the subfamily Agrypninae. The use of the C-banding technique in C. fuscofasciatus and Conoderus sp. revealed constitutive heterochromatin in the pericentromeric region of the majority of the chromosomes. In C. fuscofasciatus, additional constitutive heterochromatin were also observed in the long arm terminal region of almost all chromosomes. Among the representatives of Elaterinae, the karyotype 2n=18+Xy(p) of Pomachilius sp.2 was similar to that verified in the majority of the Coleoptera species, contrasting with the chromosomal formula 2n=18+X0 detected in Cardiorhinus rufilateris, which is most common in the species of Elaterinae. In the majority of the elaterids, the chromosomal differentiation has frequently been driven by reduction of the diploid number; but, among the four cytogenetically examined subfamilies, there are some differences in relation to the trends of karyotypic evolution. 相似文献
148.
Development of a Simple and Fast Electrochemical Method for Screening and Stoichiometric Determination of Dimenhydrinate 下载免费PDF全文
Jhonys Machado Freitas Thiago da Costa Oliveira Polyana Luiza Silva Denise Tofanello Gimenes Rodrigo Alejandro Abarza Munoz Eduardo Mathias Richter 《Electroanalysis》2014,26(9):1905-1911
Dimenhydrinate (DIM) is a salt composed by the combination of two active pharmaceutical ingredients: diphenhydramine (DIP) and 8‐chlorotheophylline (CTP). In this work, the use of batch injection analysis with multiple pulse amperometric detection (BIA‐MPA) was proposed for the first time for fast stoichiometric determination of DIM. DIP (cation) and CTP (anion) were determined simultaneously in pharmaceutical samples with a simple and fast injection procedure (70 injections h?1). Additional strategies were also proposed for rapid screening of samples containing the DIM salt. By a simple injection of a sample into the BIA system (without using of calibration curve), reliable information about stoichiometry of the DIM salt (1 : 1; DIP:CTP) and presence or absence of interfering species (electroactive) can be achieved. 相似文献
149.
Marina Nídia Ferreira dos Santos Costa Marcos Antônio Pena Muniz Charles Alberto Brito Negrão Carlos Emmerson Ferreira da Costa Maria Louze Nobre Lamarão Luiz Morais José Otávio Carréra Silva Júnior Roseane Maria Ribeiro Costa 《Journal of Thermal Analysis and Calorimetry》2014,115(3):2269-2275
The Pracaxi oil—(Pentaclethra macroloba) contains high concentrations of fatty acids with emollient action that contribute to skin hydration. The use of this oil is supported by the utilization of natural resources thus enabling regional development and social contribution. The objective of this study was to characterize the P. macroloba oil by thermogravimetry (TG, DTG, and DTA), gas chromatography, Fourier transform infrared spectroscopy (FT-IR), and oxidation stability—Rancimat, aiming at the quality control of plant raw material. Three samples of crude oil sold by Amazon Oil Industry (Ananindeua, Pará, Brazil) were studied. The analysis of these oil samples showed different fatty acids, especially the behenic, oleic, linoleic, and lignoceric acids totalizing approximately 96 % of the grease composition and in smaller percentage arachidic, lauric, myristic, palmitic, and linolenic acids were found. The major acids have wide medicinal use. According to the TG/DTG curve, thermal stability was observed up to 220 °C, indicating a greater mass loss related to the dehydration and elimination of volatile substances. The thermal decomposition process occurred in the range of 430–450° C according to the DTG curve. The absorption spectrum in the infrared region (FT-IR) showed well-defined bands confirming the presence of functional groups present in the oil. Tests in a Rancimat have shown an induction period between 8 and 10 h demonstrating that the samples are in agreement with the standards required by ANP No. 14/2012 which requires at least 6 h of testing. 相似文献
150.
Multiple Stable Conformations Account for Reversible Concentration‐Dependent Oligomerization and Autoinhibition of a Metamorphic Metallopeptidase 下载免费PDF全文
Mar López‐Pelegrín Dr. Núria Cerdà‐Costa Dr. Anna Cintas‐Pedrola Fátima Herranz‐Trillo Dr. Pau Bernadó Dr. Juan R. Peinado Dr. Joan L. Arolas Prof. Dr. F. Xavier Gomis‐Rüth 《Angewandte Chemie (International ed. in English)》2014,53(40):10624-10630
Molecular plasticity controls enzymatic activity: the native fold of a protein in a given environment is normally unique and at a global free‐energy minimum. Some proteins, however, spontaneously undergo substantial fold switching to reversibly transit between defined conformers, the “metamorphic” proteins. Here, we present a minimal metamorphic, selective, and specific caseinolytic metallopeptidase, selecase, which reversibly transits between several different states of defined three‐dimensional structure, which are associated with loss of enzymatic activity due to autoinhibition. The latter is triggered by sequestering the competent conformation in incompetent but structured dimers, tetramers, and octamers. This system, which is compatible with a discrete multifunnel energy landscape, affords a switch that provides a reversible mechanism of control of catalytic activity unique in nature. 相似文献