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   222篇
  免费   10篇
化学   180篇
晶体学   1篇
力学   3篇
数学   12篇
物理学   36篇
  2023年   3篇
  2022年   2篇
  2021年   9篇
  2020年   4篇
  2019年   8篇
  2018年   5篇
  2017年   3篇
  2016年   12篇
  2015年   2篇
  2014年   7篇
  2013年   18篇
  2012年   19篇
  2011年   14篇
  2010年   3篇
  2009年   12篇
  2008年   12篇
  2007年   10篇
  2006年   20篇
  2005年   15篇
  2004年   2篇
  2003年   4篇
  2002年   6篇
  2001年   8篇
  2000年   7篇
  1999年   5篇
  1998年   3篇
  1997年   1篇
  1996年   4篇
  1995年   1篇
  1994年   1篇
  1993年   3篇
  1991年   3篇
  1988年   1篇
  1986年   1篇
  1970年   2篇
  1967年   1篇
  1966年   1篇
排序方式: 共有232条查询结果,搜索用时 750 毫秒
41.
The antioxidant activity, total phenolic content, and total flavonoid content of 10 edible mushrooms species, including cultivated (Pleurotus ostreatus, Agaricus bisporus white and brown) and wild (P. ostreatus, Macrolepiota procera, Cantharellus cibarius, Russula vesca, Russula alutacea, Boletus edulis and Agaricus campestris), were determined. The extraction was performed using water and 50% water–ethanol and the caps and stipe were investigated separately. Water was the most appropriate solvent for phenolic compounds regardless of the sample. In contrast, the presence of ethanol in the solvent increased the extraction of flavonoids for cultivated P. ostreatus (caps and stipe), and wild R. alutacea, R. vesca, A. campestris, P. ostreatus (only caps) and C. cibarius, M. procera (only stipe). Significant differences between the antioxidant activities of the samples were registered in relationship with the different solvents. The antioxidant activity of water extract of dried A. bisporus brown (cultivated) showed the strongest 1,1-diphenyl-2-picrylhydrazyl radicals-scavenging assay (88.64%), while the B. edulis hydroalcoholic extract contained 74.93%. A detailed investigation into the functional group of phenolics and other organic compounds responsible with the antioxidant activity has been performed using Fourier transform infrared spectroscopy. The infrared spectra showed that the solvent-type directly influences the extraction process and, hence, the antioxidant activity. The present study contributes to information concerning mushrooms as sources of biologically active compounds. To investigate the correlations between phytochemical characteristics (i.e., phenolics, flavonoids and antioxidant activity) closely related to nature of solvents, the statistical analysis was performed.  相似文献   
42.

Aniline derivatives were phosphorylated in biphase systems using three methods. A comparative study was performed. The best results were obtained when a solid–liquid system was used. This method is the easiest and lead to higher yields (54–81%).  相似文献   
43.
44.
Abstract

Polyphosphonates were synthesized by interfacial alkaline polycondensation of bispheml A (BA) with phenyldichlorophosphonate (PPD) and cyclohexyl-dichlorophosphonate (CPD) in liquid-liquid[1] (1–1) and Iiquid-vapoR[2] (I-v) systems.  相似文献   
45.
Relatively monodisperse and highly luminescent Mn(2+)-doped zinc blende ZnSe nanocrystals were synthesized in aqueous solution at 100 °C using the nucleation-doping strategy. The effects of the experimental conditions and of the ligand on the synthesis of nanocrystals were investigated systematically. It was found that there were significant effects of molar ratio of precursors and heating time on the optical properties of ZnSe:Mn nanocrystals. Using 3-mercaptopropionic acid as capping ligand afforded 3.1 nm wide ZnSe:Mn quantum dots (QDs) with very low surface defect density and which exhibited the Mn(2+)-related orange luminescence. The post-preparative introduction of a ZnS shell at the surface of the Mn(2+)-doped ZnSe QDs improved their photoluminescence properties, resulting in stronger emission. A 2.5-fold increase in photoluminescence quantum yield (from 3.5 to 9%) and of Mn(2+) ion emission lifetime (from 0.62 to 1.39 ms) have been observed after surface passivation. The size and the structure of these QDs were also corroborated by using transmission electron microscopy, energy dispersive spectroscopy, and X-ray powder diffraction.  相似文献   
46.
We employ low‐temperature single‐molecule spectroscopy combined with pattern recognition techniques for data analysis on a methyl‐substituted ladder‐type poly(para‐phenylene) (MeLPPP) to investigate the electron–phonon coupling to low‐energy vibrational modes as well as the origin of the strong spectral diffusion processes observed for this conjugated polymer. The results indicate weak electron–phonon coupling to low‐frequency vibrations of the surrounding matrix of the chromophores, and that low‐energy intrachain vibrations of the conjugated backbone do not couple to the electronic transitions of MeLPPP at low temperatures. Furthermore, these findings suggest that the main line‐broadening mechanism of the zero‐phonon lines of MeLPPP is fast, unresolved spectral diffusion, which arises from conformational fluctuations of the side groups attached to the MeLPPP backbone as well as of the surrounding host material.  相似文献   
47.
The Suzuki-type cross-coupling reaction of 2,7-dihalofluorenes with 1 equiv of arylboronic acid and Pd2(dba)3/P(t-Bu3) as a catalyst system is investigated. The exclusive formation of the diarylated coupling product demonstrates that "preferential oxidative addition" is also applicable to fluorene monomers due to a controlled intramolecular motion of the regenerated Pd(0) catalyst across the "large" distance between the 2- and the 7-position of one fluorene monomer.  相似文献   
48.
We investigate the coupling of a single molecule to a single spherical gold nanoparticle acting as a nanoantenna. Using scanning probe technology, we position the particle in front of the molecule with nanometer accuracy and measure a strong enhancement of more than 20 times in the fluorescence intensity simultaneous to a 20-fold shortening of the excited state lifetime. Comparisons with three-dimensional calculations guide us to decipher the contributions of the excitation enhancement, spontaneous emission modification, and quenching. Furthermore, we provide direct evidence for the role of the particle plasmon resonance in the molecular excitation and emission processes.  相似文献   
49.
A new efficient approach for the synthesis of functionalized dihydro-pyrimido[4,3-d]coumarins is described. Use of benzyl- or tert-butyl acetoacetate and various salicyl aldehydes in typical Biginelli multicomponent reactions provides esters that can be easily deprotected to yield the corresponding hydroxy acids. Annelation of the latter leads to the target dihydro-pyrimido[4,3-d]coumarins. Formation of some unexpected oxo-bridged tetrahydro-pyrimidines is also described. Spectral properties of the synthesized dihydro-pyrimido[4,3-d]coumarins indicate the necessity to revisit some compounds previously reported as products of direct one-pot Biginelli condensation involving 4-hydroxycoumarin as dicarbonyl component.  相似文献   
50.
Novel complexes of type [Cu(N-N)(dmtp)2(OH2)](ClO4)2·dmtp ((1) N-N: 2,2′-bipyridine; (2) L: 1,10-phenantroline and dmtp: 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine) were designed in order to obtain biologically active compounds. Complexes were characterized as mononuclear species that crystallized in the space group P-1 of the triclinic system with a square pyramidal geometry around the copper (II). In addition to the antiproliferative effect on murine melanoma B16 cells, complex (1) exhibited low toxicity on normal BJ cells and did not affect membrane integrity. Complex (2) proved to be a more potent antimicrobial in comparison with (1), but both compounds were more active in comparison with dmtp—both against planktonic cells and biofilms. A stronger antimicrobial and antibiofilm effect was noticed against the Gram-positive strains, including methicillin-resistant Staphylococcus aureus (MRSA). Both electron paramagnetic resonance (EPR) and Saccharomyces cerevisiae studies indicated that the complexes were scavengers rather than reactive oxygen species promoters. Their DNA intercalating capacity was evidenced by modifications in both absorption and fluorescence spectra. Furthermore, both complexes exhibited nuclease-like activity, which increased in the presence of hydrogen peroxide.  相似文献   
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