全文获取类型
收费全文 | 2365篇 |
免费 | 95篇 |
国内免费 | 12篇 |
专业分类
化学 | 2025篇 |
晶体学 | 10篇 |
力学 | 16篇 |
数学 | 245篇 |
物理学 | 176篇 |
出版年
2023年 | 17篇 |
2022年 | 43篇 |
2021年 | 61篇 |
2020年 | 62篇 |
2019年 | 56篇 |
2018年 | 29篇 |
2017年 | 43篇 |
2016年 | 89篇 |
2015年 | 75篇 |
2014年 | 84篇 |
2013年 | 126篇 |
2012年 | 163篇 |
2011年 | 179篇 |
2010年 | 100篇 |
2009年 | 102篇 |
2008年 | 143篇 |
2007年 | 136篇 |
2006年 | 149篇 |
2005年 | 120篇 |
2004年 | 104篇 |
2003年 | 95篇 |
2002年 | 86篇 |
2001年 | 31篇 |
2000年 | 20篇 |
1999年 | 21篇 |
1998年 | 19篇 |
1997年 | 12篇 |
1996年 | 23篇 |
1995年 | 21篇 |
1994年 | 13篇 |
1993年 | 9篇 |
1992年 | 11篇 |
1991年 | 13篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 8篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 6篇 |
1983年 | 13篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 9篇 |
1978年 | 3篇 |
1976年 | 5篇 |
1973年 | 3篇 |
1958年 | 55篇 |
1957年 | 20篇 |
1934年 | 4篇 |
排序方式: 共有2472条查询结果,搜索用时 0 毫秒
81.
Harth EM Hecht S Helms B Malmstrom EE Fréchet JM Hawker CJ 《Journal of the American Chemical Society》2002,124(15):3926-3938
The influence of macromolecular architecture on the physical properties of polymeric materials has been studied by comparing poly(benzyl ether) dendrons with their exact linear analogues. The results clearly confirm the anticipation that dendrimers are unique when compared to other architectures. Physical properties, from hydrodynamic volume to crystallinity, were shown to be different, and in a comparative study of core encapsulation in macromolecules of different architecture, energy transduction from the polymer backbone to a porphyrin core was shown to be different for dendrimers as compared to that of isomeric four- or eight-arm star polymers. Fluorescence excitation revealed strong, morphology dependent intramolecular energy transfer in the three macromolecular isomers investigated. Even at high generations, the dendrimers exhibited the most efficient energy transfer, thereby indicating that the dendritic architecture affords superior site isolation to the central porphyrin it surrounds. 相似文献
82.
83.
de Rijke E Zappey H Ariese F Gooijer C Brinkman UA 《Analytical and bioanalytical chemistry》2004,378(4):995-1006
Reversed-phase LC on C-18 bonded silica with a methanol–ammonium formate gradient was used to determine the main flavonoids in leaves of four species of the Leguminosae family. The detection modes were diode-array UV absorbance, fluorescence, and (tandem) mass spectrometry. LC–UV was used for a general screening, sub-classification, and the calculation of total flavonoid contents. LC–FLU was included to identify isoflavones on the basis of their native fluorescence. Most structural information regarding aglycons, sugar moieties, and acidic groups was derived from LC–MS in both the full-scan and extracted-ion mode, using negative-ion atmospheric pressure chemical ionization. MS/MS did not provide much additional information, because the same fragments were observed as in full-scan MS.In T. pratense and T. repens, the main constituents were flavonoid glucoside–(di)malonates, while T. dubium and L. corniculatus mainly contained flavonoid (di)glycosides. Satellite sets comprising an aglycon, the glucoside and glucoside–malonates or –acetates, were abundantly present only in T. pratense. Generally speaking, the main aglycons and sugars in the four plant species are surprisingly different. In addition, while the results for T. pratense are similar to those reported in the literature, there is little agreement in the case of the other species. Finally, total flavonoid contents ranged from 50–65 mg/g for L. corniculatus and T. dubium, to 15 mg/g for T. pratense and only 1 mg/g for T. repens. 相似文献
84.
Zhen G Eggli V Vörös J Zammaretti P Textor M Glockshuber R Kuennemann E 《Langmuir : the ACS journal of surfaces and colloids》2004,20(24):10464-10473
Understanding the conformation, orientation, and specific activity of proteins bound to surfaces is crucial for the development and optimization of highly specific and sensitive biosensors. In this study, the very efficient enzyme beta-lactamase is used as a model protein. The wild-type form was genetically engineered by site-directed mutagenesis to introduce single cysteine residues on the surface of the enzyme. The cysteine thiol group is subsequently biotinylated with a dithiothreitol (DTT)-cleavable biotinylation reagent. beta-Lactamase is then immobilized site-specifically via the biotin group on neutral avidin-covered surfaces with the aim to control the orientation of the enzyme molecule at the surface and study its effect on enzymatic activity using Nitrocefin as the substrate. The DTT-cleavable spacer allows the release of the specifically bound enzyme from the surface. Immobilization of the enzyme is performed on a monolayer of the polycationic, biotinylated polymer PLL-g-PEG/PEG-biotin assembled on niobium oxide (Nb2O5) surfaces via neutral avidin as the docking site. Two different assembly protocols, the sequential adsorption of avidin and biotinylated beta-lactamase and the immobilization of preformed complexes of beta-lactamase and avidin, are compared in terms of immobilization efficiency. In situ optical waveguide lightmode spectroscopy and colorimetric analysis of enzymatic activity were used to distinguish between specific and unspecific enzyme adsorption, to sense quantitatively the amount of immobilized enzyme, and to determine Michaelis-Menten kinetics. All tested enzyme variants turned out to be active upon immobilization at the polymeric surface. However, the efficiency of immobilized enzymes relative to the soluble enzymes was reduced about sevenfold, mainly because of impaired substrate (Nitrocefin) diffusion or restricted accessibility of the active site. No significant effect of different enzyme orientations could be detected, probably because the enzymes were attached to the surface through long, flexible PEG chain linkers. 相似文献
85.
86.
Bittrich E Burkert S Müller M Eichhorn KJ Stamm M Uhlmann P 《Langmuir : the ACS journal of surfaces and colloids》2012,28(7):3439-3448
Temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm) brushes with different molecular weights M(n) and grafting densities σ were prepared by the "grafting-to" method. Changes in their physicochemical properties according to temperature were investigated with the help of in situ spectroscopic ellipsometry and in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Brush criteria indicate a transition between a brush conformation below the lower critical solution temperature (LCST) and an intermediate to mushroom conformation above the LCST. By in situ ellipsometry distinct changes in the brush layer parameters (wet thickness, refractive index, buffer content) were observed. A broadening of the temperature region with maximum deswelling occurred with decreasing grafting density. The brush layer properties were independent of the grafting density below the LCST, but showed a virtually monotonic behavior above the LCST. The midtemperature ?(half) of the deswelling process increased with increasing grafting density. Thus grafting density-dependent design parameters for such functional films were presented. For the first time, ATR-FTIR spectroscopy was used to monitor segment density and hydrogen bonding changes of these very thin PNIPAAm brushes as a function of temperature based on significant variations of the methyl stretching, Amide I, as well as Amide II bands with respect to intensity and wavenumber position. No dependence on M(n) and σ in the wavenumber shift of these bands above the LCST was found. The temperature profile of these band intensities and thus segment density was found to be rather step-like, exceeding temperatures around the LCST, while the respective profile of their wavenumber positions suggested continuous structural and hydration processes. Remaining buffer amounts and residual intermolecular segment/water interaction in the collapsed brushes above the LCST could be confirmed by both in situ methods. 相似文献
87.
Wang Y Corbitt TS Jett SD Tang Y Schanze KS Chi EY Whitten DG 《Langmuir : the ACS journal of surfaces and colloids》2012,28(1):65-70
The bactericidal mechanisms of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPE) and oligo-phenylene ethynylenes (OPE) were investigated using electron/optical microscopy and small-angle X-ray scattering (SAXS). The ultrastructural analysis shows that polymeric PPE-Th can significantly remodel the bacterial outer membrane and/or the peptidoglycan layer, followed by the possible collapse of the bacterial cytoplasm membrane. In contrast, oligomeric end-only OPE (EO-OPE) possesses potent bacteriolysis activity, which efficiently disintegrates the bacterial cytoplasm membrane and induces the release of bacterial cell content. Using single giant vesicles and SAXS, we demonstrated that the membrane perturbation mechanism of EO-OPE against model bacterial membranes results from a 3D membrane phase transition or perturbation. 相似文献
88.
Santini E Guzmán E Ravera F Ferrari M Liggieri L 《Physical chemistry chemical physics : PCCP》2012,14(2):607-615
The properties and structure of different types of interfacial layers obtained from aqueous dispersions of nanometric silica and palmitic acid (PA) have been studied and characterized by different diagnostics and measurements. The investigations concern PA monolayers spread on the silica dispersions, dispersions in contact with PA solutions in oil and silica dispersions containing PA, aiming at elucidating the role of the PA interaction with the particles and investigating the surface-activity of the originated silica-PA complexes. Drop shape tensiometry was utilized to measure the dynamic surface and interfacial tension while a Langmuir trough apparatus was used to obtain compression isotherms of the spread PA layers and to measure the dilational viscoelasticity according to the oscillating barrier method. Brewster angle microscopy and ellipsometry were utilized to investigate the lateral and vertical structure of the interfacial layers. From this multifold approach emerges a complex picture of the features of these interfacial layers that can be rationalized on the basis of the adsorption of PA on the particle surface. The results evidence a threshold in PA adsorption above which particles change from hydrophilic to partially hydrophobic, promoting their incorporation into the interfacial layer. 相似文献
89.
Chen D Heberling MS Nakata M Hough LE Maclennan JE Glaser MA Korblova E Walba DM Watanabe J Clark NA 《Chemphyschem》2012,13(1):155-159
The B4 liquid crystal phase of bent-core molecules, a smectic phase of helical nanofilaments, is one of the most complex hierarchical self-assemblies in soft materials. We describe the layer topology of the B4 phase of mesogens in the P-n-OPIMB homologous series near the liquid crystal/glass interface. Freeze-fracture transmission electron microscopy reveals that the twisted layer structure of the bulk is suppressed, the layers instead forming a structure with periodic layer undulations, with the topography depending on the distance from the glass. The surface layer structure is modeled as parabolic focal conic arrays generated by equidistant parabolas whose foci are defect lines along the glass surface. Nucleation and growth of toric focal conics near the glass substrate is also observed. Although the growth of twisted nanofilaments, the usual manifestation of structural chirality in the B4 phase, is suppressed near the surface, the smectic layers are intrinsically chiral, and the helical filaments that form on top of them grow with specific handedness. 相似文献
90.
Beurer E Venkataraman NV Sommer M Spencer ND 《Langmuir : the ACS journal of surfaces and colloids》2012,28(6):3159-3166
An orthogonal, charge-density-versus-net-charge, surface-chemical gradient, composed of ternary mixed self-assembled monolayers, has been prepared from three hydrophilic components: positively chargeable amine-terminated, negatively chargeable carboxylic-acid-terminated, and hydroxyl-terminated alkanethiols, with the latter bearing a slight negative charge in electrolytes. The chemical composition and its distribution have been monitored by X-ray photoelectron spectroscopy. The adsorption behavior of negatively charged SiO(2) nanoparticles and positively charged amine-modified SiO(2) nanoparticles has been studied. Additionally, negatively charged proteins (bovine serum albumin and fibrinogen) and positively charged proteins (lysozyme) were adsorbed on the gradients. Negatively charged nanoparticles and proteins adsorb mainly in the positively charged region and vice versa, illustrating that the adsorption behavior is mainly influenced by electrostatic interactions, and showing the potential of the gradient for sorting applications. Despite literature reports to the contrary, no area was found that was completely resistant to protein adsorption. 相似文献