全文获取类型
收费全文 | 1313篇 |
免费 | 22篇 |
国内免费 | 6篇 |
专业分类
化学 | 1042篇 |
晶体学 | 8篇 |
力学 | 9篇 |
数学 | 140篇 |
物理学 | 142篇 |
出版年
2023年 | 16篇 |
2022年 | 52篇 |
2021年 | 57篇 |
2020年 | 33篇 |
2019年 | 22篇 |
2018年 | 25篇 |
2017年 | 19篇 |
2016年 | 23篇 |
2015年 | 37篇 |
2014年 | 30篇 |
2013年 | 73篇 |
2012年 | 66篇 |
2011年 | 80篇 |
2010年 | 58篇 |
2009年 | 41篇 |
2008年 | 84篇 |
2007年 | 77篇 |
2006年 | 83篇 |
2005年 | 63篇 |
2004年 | 57篇 |
2003年 | 63篇 |
2002年 | 51篇 |
2001年 | 12篇 |
2000年 | 13篇 |
1999年 | 12篇 |
1998年 | 11篇 |
1997年 | 10篇 |
1996年 | 14篇 |
1995年 | 13篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 7篇 |
1991年 | 14篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1983年 | 4篇 |
1982年 | 11篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1972年 | 4篇 |
1959年 | 2篇 |
1940年 | 2篇 |
排序方式: 共有1341条查询结果,搜索用时 15 毫秒
991.
Dias RS Magno LM Valente AJ Das D Das PK Maiti S Miguel MG Lindman B 《The journal of physical chemistry. B》2008,112(46):14446-14452
The interactions between DNA and a number of different cationic surfactants, differing in headgroup polarity, were investigated by electric conductivity measurements and fluorescence microscopy. It was observed that, the critical association concentration (cac), characterizing the onset of surfactant binding to DNA, does not vary significantly with the architecture of the headgroup. However, comparing with the critical micelle concentration (cmc) in the absence of DNA, it can be inferred that the micelles of a surfactant with a simple quaternary ammonium headgroup are much more stabilized by the presence of DNA than those of surfactants with hydroxylated head-groups. In line with previous studies of polymer-surfactant association, the cac does not vary significantly with either the DNA concentration or its chain length. On the other hand, a novel observation is that the cac is much lower when DNA is denaturated and in the single-stranded conformation, than for the double-helix DNA. This is contrary to expectation for a simple electrostatically driven association. Thus previous studies of polyelectrolyte-surfactant systems have shown that the cac decreases strongly with increasing linear charge density of the polyion. Since double-stranded DNA (dsDNA) has twice as large linear charge density as single-stranded DNA (ssDNA), the stronger binding in the latter case indicates an important role of nonelectrostatic effects. Both a higher flexibility of ssDNA and a higher hydrophobicity due to the exposed bases are found to play a role, with the hydrophobic interaction argued to be more important. The significance of hydrophobic DNA-surfactant interaction is in line with other observations. The significance of nonelectrostatic effects is also indicated in significant differences in cac between different surfactants for ssDNA but not for dsDNA. For lower concentrations of DNA, the conductivity measurements presented an "anomalous" feature, i.e., a second inflection point for surfactant concentrations below the cac; this feature was not displayed at higher concentrations of DNA. The effect is attributed to the presence of a mixture of ss- and dsDNA molecules. Thus the stability of dsDNA is dependent on a certain ion atmosphere; at lower ion concentrations the electrostatic repulsions between the DNA strands become too strong compared to the attractive interactions, and there is a dissociation into the individual strands. Fluorescence microscopy studies, performed at much lower DNA concentrations, demonstrated a transformation of dsDNA from an extended "coil" state to a compact "globule" condition, with a broad concentration region of coexistence of coils and globules. The onset of DNA compaction coincides roughly with the cac values obtained from conductivity measurements. This is in line with the observed independence of cac on the DNA concentration, together with the assumption that the onset of binding corresponds to an initiation of DNA compaction. No major changes in either the onset of compaction or complete compaction were observed as the surfactant headgroup was made more polar. 相似文献
992.
Pilet G Tommasino JB Fenain F Matrak R Médebielle M 《Dalton transactions (Cambridge, England : 2003)》2008,(41):5621-5626
The syntheses of three original trifluoromethylated enaminones L1H-L3H, an unexplored type of ligand with possible multiple coordination centres, their redox properties and explorative coordination chemistry with copper(II) are presented. The ability of these ligands to coordinate copper(II) and then to form new mono- and dinuclear complexes is presented and discussed. The consequences of this metal coordination on redox properties are also explored. 相似文献
993.
Trombino S Cassano R Bloise E Muzzalupo R Leta S Puoci F Picci N 《Macromolecular bioscience》2008,8(1):86-95
In this study we report the synthesis and characterisation of cellulose ferulate, lipoate and alpha-tocopherulate, and their ability to inhibit lipid peroxidation in rat-liver microsomal membranes, induced in vitro by two different sources of free radicals: tert-butyl hydroperoxide and 2,2'-azobis-(2-amidinopropane). We also compared the antioxidant efficiency of the ferulate derivatives obtained through two different synthetic runs, and of a tocopherulate derivative prepared from 6-carboxycellulose. This study showed that the designed systems, preserving the antioxidant activity of the free substrates, are more effective in protecting from tert-butyl hydroperoxide than from 2,2'-azobis-(2-amidinopropane). Moreover, the cellulose ferulate with the higher degree of substitution acted as the best antioxidant. 相似文献
994.
Recently, deep eutectic systems (DESs) as extraction techniques for bioactive compounds have surfaced as a greener alternative to common organic solvents. In order to study the effect of these systems on the extraction of phenolic compounds from different natural sources, a comprehensive review of the state of the art was carried out. In a first approach, the addition of water to these systems and its effect on DES physicochemical properties such as polarity, viscosity, and acidity was investigated. This review studied the effect of the hydrogen bond donor (HBD) on the nature of the extracted phenolics. The effects of the nature of the HBD, namely carbon chain length as well as the number of hydroxyl, methyl, and carbonyl groups, have shown to play a critical role in the extraction of different phenolic compounds. This review highlights the differences between DES systems and systematizes the results published in the literature, so that a more comprehensive evaluation of the systems can be carried out before any experimental trial. 相似文献
995.
Bruna R. Silva Rita Rebelo Juliana M. Rodrigues Cristina P. R. Xavier M. Helena Vasconcelos Maria-Joo R. P. Queiroz 《Molecules (Basel, Switzerland)》2021,26(6)
A series of novel functionalized methyl 3-(hetero)arylthieno[3,2-b]pyridine-2-carboxylates 2a–2h were synthesized by C-C Pd-catalyzed Suzuki-Miyaura cross-coupling of methyl 3-bromothieno[3,2-b]pyridine-2-carboxylate with (hetero)aryl pinacol boranes, trifluoro potassium boronate salts or boronic acids. Their antitumoral potential was evaluated in two triple negative breast cancer (TNBC) cell lines—MDA-MB-231 and MDA-MB-468, by sulforhodamine B assay. Their effects on the non-tumorigenic MCF-12A cells were also evaluated. The results demonstrated that three compounds caused growth inhibition in both TNBC cell lines, with little or no effect against the non-tumorigenic cells. The most promising compound was further studied concerning possible effects on cell viability (by trypan blue exclusion assay), cell proliferation (by bromodeoxyuridine assay) and cell cycle profile (by flow cytometry). The results demonstrated that the GI50 concentration of compound 2e (13 μM) caused a decreased in MDA-MB-231 cell number, which was correlated with a decreased in the % of proliferating cells. Moreover, this compound increased G0/G1 phase and decreased S phases, when compared to control cells (although was not statistic significant). Interestingly, compound 2e also reduced tumor size using an in ovo CAM (chick chorioallantoic membrane) model. This work highlights the potential antitumor effect of a novel methyl 3-arylthieno[3,2-b]pyridine-2-carboxylate derivative. 相似文献
996.
Birut Sapijanskait-Banevi
Vykintas Palskys Rita Vaickelionien Jrat iugdait Povilas Kavaliauskas Birut Grybait Vytautas Mickevi
ius 《Molecules (Basel, Switzerland)》2021,26(9)
The p-aminobenzoic acid was applied for the synthesis of substituted 1-phenyl-5-oxopyrrolidine derivatives containing benzimidazole, azole, oxadiazole, triazole, dihydrazone, and dithiosemicarbazide moieties in the structure. All the obtained compounds were evaluated for their in vitro antimicrobial activity against Staphylococcus aureus, Bacillus cereus, Listeria monocytogenes, Salmonella enteritidis, Escherichia coli, and Pseudomonas aeruginosa by using MIC and MBC assays. This study showed a good bactericidal activity of γ-amino acid and benzimidazoles derivatives. The antimicrobial activity of the most promising compounds was higher than ampicillin. Furthermore, two benzimidazoles demonstrated good antimicrobial activity against L. monocytogenes (MIC 15.62 µg/mL) that was four times more potent than ampicillin (MIC 65 µg/mL). Further studies are needed to better understand the mechanism of the antimicrobial activity as well as to generate antimicrobial compounds based on the 1-phenyl-5-oxopyrrolidine scaffold. 相似文献
997.
Karpan VM Giovannetti G Khomyakov PA Talanana M Starikov AA Zwierzycki M van den Brink J Brocks G Kelly PJ 《Physical review letters》2007,99(17):176602
Based upon the observations (i) that their in-plane lattice constants match almost perfectly and (ii) that their electronic structures overlap in reciprocal space for one spin direction only, we predict perfect spin filtering for interfaces between graphite and (111) fcc or (0001) hcp Ni or Co. The spin filtering is quite insensitive to roughness and disorder. The formation of a chemical bond between graphite and the open d-shell transition metals that might complicate or even prevent spin injection into a single graphene sheet can be simply prevented by dusting Ni or Co with one or a few monolayers of Cu while still preserving the ideal spin-injection property. 相似文献
998.
999.
The System Gd/Co/B: Preparation and Characterization by X‐ray Diffraction of GdCo4B, Gd3Co11B4, GdCoB4, and GdCo12B6 The compounds GdCo4B, Gd3Co11B4, GdCoB4, and GdCo12B6 were characterized by X‐ray investigations of single crystals. GdCo4B (P 6/mmm, a = 505.9(1) pm, c = 690.1(1) pm) crystallizes with the CeCo4B structure type; Gd3Co11B4 (P 6/mmm, a = 508.7(1) pm, c = 982.9(9) pm) with the Ce3Co11B4 stucture type; GdCo12B6 ( , a = 949.5(1) pm, c = 747.4(1) pm) with the SrNi12B6 structure type and GdCoB4 (P bam, a = 591.3(9) pm, b = 1145.1(6) pm, c = 346.2(3) pm) with the YbCoB4 structure type. 相似文献
1000.
Muzzalupo R Nicoletta FP Trombino S Cassano R Iemma F Picci N 《Colloids and surfaces. B, Biointerfaces》2007,58(2):197-202
Niosomes have shown promise as cheap and chemically stable drug delivery systems. In this paper a novel crown ether amphiphile, 1,16-hexadecanoyl-bis-(2-aminomethyl)-18-crown-6 (Bola A-16), has been synthesized with the aim of developing a long time stable controlled release system. Niosomes have been prepared with different molar ratios of amphiphile and cholesterol and their morphological properties have been determined by quasi-elastic light scattering and transmission electron microscopy. The composition of niosomes affects the entrapment efficiency and the release rate of 5-fluorouracil, a well-known antineoplastic molecule. In addition, other two known azacrown ether amphiphiles (4,7,10,13-pentaoxa-16-aza-cyclooctadecane)-hexadecanedioc acid diamide (Bola D-16) and ,ω-(4,7,10,13-pentaoxa-16-aza-cyclooctadecane)-hexadecane (Bola C-16), have been synthesized and the obtained vesicles have been characterized for comparison. Furthermore, the release profile of 5-fluorouracil in vitro, from these niosomes, has been studied over a period of 6 h in order to simulate a hematic adsorption. 相似文献