全文获取类型
收费全文 | 687篇 |
免费 | 38篇 |
国内免费 | 2篇 |
专业分类
化学 | 592篇 |
晶体学 | 2篇 |
力学 | 7篇 |
数学 | 74篇 |
物理学 | 52篇 |
出版年
2024年 | 2篇 |
2023年 | 7篇 |
2022年 | 40篇 |
2021年 | 45篇 |
2020年 | 14篇 |
2019年 | 15篇 |
2018年 | 13篇 |
2017年 | 7篇 |
2016年 | 31篇 |
2015年 | 23篇 |
2014年 | 22篇 |
2013年 | 35篇 |
2012年 | 44篇 |
2011年 | 43篇 |
2010年 | 27篇 |
2009年 | 31篇 |
2008年 | 39篇 |
2007年 | 49篇 |
2006年 | 39篇 |
2005年 | 39篇 |
2004年 | 29篇 |
2003年 | 34篇 |
2002年 | 24篇 |
2001年 | 9篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 8篇 |
1997年 | 3篇 |
1996年 | 14篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有727条查询结果,搜索用时 125 毫秒
701.
702.
Riassunto Si prova la rigidità di una certa classe di algebre integre di Cohen-Macaulay.
Lavoro svolto nell'ambito del G.N.S.A.G.A., sez. n. 3, del C.N.R. 相似文献
Summary We prove the rigidity of a certain class of algebras which are Cohen-Macaulay integral domains.
Lavoro svolto nell'ambito del G.N.S.A.G.A., sez. n. 3, del C.N.R. 相似文献
703.
Donato Donati Serena Ferrini Stefania Fusi Fabio Ponticelli 《Journal of heterocyclic chemistry》2004,41(5):761-766
Ring opening with molybdenum hexacarbonyl of functionalized isoxazoles is a valuable synthetic process. Tetrazolopyridine 4 and pyrazolopyridine 9 were obtained from isoxazolopyridines 3 and 6 , respectively, whereas the isoxazole 14 gave ketone 16 through the intermediate 17. 相似文献
704.
Omar Benhar Valeria Ferrari Leonardo Gualtieri Stefania Marassi 《General Relativity and Gravitation》2007,39(9):1323-1330
We discuss the possibility that the detection of gravitational waves emitted by compact stars may allow to constrain the MIT
bag model of quark matter equation of state. Our results show that the combined knowledge of the frequency of the emitted
gravitational wave and of the mass, or the radiation radius, of the source allows one to discriminate between strange stars
and neutron stars and set stringent bounds on the bag constants. 相似文献
705.
Stefano DallAcqua Kouadio Ibrahime Sinan Irene Ferrarese Stefania Sut Kouadio Bene Mohamad Fawzi Mahomoodally Nabeelah Bibi Sadeer Gunes Ak Gokhan Zengin 《Molecules (Basel, Switzerland)》2020,25(23)
Breynia retusa (Dennst.) Alston (also known as Cup Saucer plant) is a food plant with wide applications in traditional medicine, particularly in Ayurveda. Extracts obtained with four solvents (dichloromethane, methanol, ethyl acetate and water), from three plant parts, (fruit, leaf and bark) were obtained. Extracts were tested for total phenolic, flavonoid content and antioxidant activities using a battery of assays including 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), ferric reducing antioxidant power (FRAP), cupric reducing antioxidant capacity (CUPRAC), total antioxidant capacity (TAC) (phosphomolybdenum) and metal chelating. Enzyme inhibitory effects were investigated using acetylcholinesterase (AChE), butyrylcholinesterase (BChE), tyrosinase, α-amylase and α-glucosidase as target enzymes. Results showed that the methanolic bark extract exhibited significant radical scavenging activity (DPPH: 202.09 ± 0.15; ABTS: 490.12 ± 0.18 mg Trolox equivalent (TE)/g), reducing potential (FRAP: 325.86 ± 4.36: CUPRAC: 661.82 ± 0.40 mg TE/g) and possessed the highest TAC (3.33 ± 0.13 mmol TE/g). The methanolic extracts were subjected to LC-DAD-MSn and NMR analysis. A two-column LC method was developed to separate constituents, allowing to identify and quantify forty-four and fifteen constituents in bark and fruits, respectively. Main compound in bark was epicatechin-3-O-sulphate and isolation of compound was performed to confirm its identity. Bark extract contained catechins, procyanidins, gallic acid derivatives and the sulfur containing spiroketal named breynins. Aerial parts mostly contained flavonoid glycosides. Considering the bioassays, the methanolic bark extract resulted a potent tyrosinase (152.79 ± 0.27 mg kojic acid equivalent/g), α-amylase (0.99 ± 0.01 mmol acarbose equivalent ACAE/g) and α-glucosidase (2.16 ± 0.01 mmol ACAE/g) inhibitor. In conclusion, methanol is able to extract the efficiently the phytoconstituents of B. retusa and the bark is the most valuable source of compounds. 相似文献
706.
In this study, a RP-HPLC method for the analysis of polyacetylenes and polyenes in Echinacea pallida roots and phytopharmaceuticals was developed. The reference compounds used for quantification were isolated from the plant material and their structures were determined on the basis of the analysis of UV, IR, NMR and MS data. The complete structure elucidation of three compounds, namely 8-hydroxy-tetradec-(9E)-ene-11,13-diyn-2-one (1), tetradec-(8Z)-ene-11,13-diyn-2-one (6) and pentadec-(8Z)-en-2-one (9) is described. In the analysis of the n-hexane extracts of E. pallida roots, the comparison between conventional and monolithic columns showed that the elution order in both cases is identical and the selectivity is equivalent. However, the retention times achieved by the monolithic column are shorter, resulting in a faster separation (20 min). Therefore, the analyses were carried out on a Chromolith Performance RP-18e (100 mm x 4.6 mm i.d.), with a gradient mobile phase composed by H(2)O and ACN at the flow rate of 2 mL/min. The column was thermostatted at 20 degrees C. The photodiode array detector monitored the eluent at 210 nm. The validation procedure confirmed that this technique affords reliable analysis of these components and is appropriate for the quality control of complex matrices, such as E. pallida roots and phytopharmaceuticals. 相似文献
707.
Di Giambattista L Pozzi D Grimaldi P Gaudenzi S Morrone S Castellano AC 《Analytical and bioanalytical chemistry》2011,399(8):2771-2778
Fourier transform infrared spectroscopy in attenuated total reflection can be used to discriminate the necrotic from the apoptotic
cell death in a tumoral T cell line irradiated by a UV source able to induce both apoptosis and necrosis. Using Jurkat cells
as the model system, significant spectral differences in the irradiated cells vs. time were observed in the lipid–proteins
ratio absorbance band at 1,397 cm−1 and in lactic acid IR band at 1,122 cm−1; these spectral features are inversely correlated with the percentage of apoptotic cells assessed by flow cytometry. From
the analysis of second derivatives in the IR spectral region between 1,800 and 900 cm−1, we have detected two significant spectral changes: the first centered at 1,621 cm−1 by analyzing the components of the amide I band and the second centered at 1,069 cm−1 due to C–O stretching vibration of the DNA backbone sensitive to the dehydrated state of DNA; these identified differences
in the intracellular biomolecules have been allowed to monitor the necrotic process. The variations in the spectral data set
have been identified by the Kruskal–Wallis test and confirmed by the hierarchical cluster analysis. 相似文献
708.
Gemma S Colombo L Forloni G Savini L Fracasso C Caccia S Salmona M Brindisi M Joshi BP Tripaldi P Giorgi G Taglialatela-Scafati O Novellino E Fiorini I Campiani G Butini S 《Organic & biomolecular chemistry》2011,9(14):5137-5148
Here we describe the identification and preliminary characterization of a new class of pyrrolo(imidazo)quinoxaline hydrazones as florescent probes for Aβ(1-42) fibrils. All the newly developed compounds were able to bind amyloid fibrils formed in vitro and some of them displayed an increase of their fluorescence upon binding. When tested on brain tissue preparations presenting Aβ deposits, the described hydrazones selectively stained amyloid structures and did not display aspecific binding. The hydrazones did not show antifibrillogenic activity and electron microscopy analysis revealed that they do not interfere with fibrils structure. The described pyrrolo(imidazo)quinoxalines could be useful for studying amyloid structures in vitro. Moreover, their experimentally proven ability to cross the blood-brain barrier in mouse opens the possibility of developing these compounds as potential amyloid imaging agents for in vivo applications. 相似文献
709.
De Giglio E Trapani A Cafagna D Sabbatini L Cometa S 《Analytical and bioanalytical chemistry》2011,400(7):1997-2002
The formulation and characterization of dopamine (DA)-loaded chitosan nanoparticles (CSNPs) are described as preliminary steps
for the development of potential DA carrier systems intended for Parkinson’s disease treatment. For this purpose, CSNPs were
firstly produced and, afterwards, they were incubated in a DA aqueous solution to promote neurotransmitter loading. The characterization
of the resulting nanoparticles started with Fourier transform infrared spectroscopy analysis to ascertain the presence of
DA in the nanocarrier, whereas X-ray photoelectron spectroscopy analysis provided evidence of the localization of DA on the
nanoparticle surface. A quartz crystal microbalance with dissipation monitoring (QCM-D) was then exploited to investigate
both swelling of CSNPs and interaction of DA with CSNPs. In particular, the QCM-D revealed that this interaction is fast and
so this allows a stable nanostructured system to be obtained. 相似文献
710.
Dr. Matteo Amelia Stefania Impellizzeri Simone Monaco Dr. Ibrahim Yildiz Dr. Serena Silvi Prof. Françisco M. Raymo Prof. Alberto Credi 《Chemphyschem》2011,12(12):2280-2288
Two series of CdSe quantum dots (QDs) with different diameters are prepared, according to frequently used protocols of the same synthetic procedure. For each sample the photophysical properties and the potentials for the first reduction and oxidation processes in organic solution are determined. The band gap obtained from electrochemical experiments is compared with that determined from the absorption and luminescence spectra. While the optical band gap decreases upon increasing the nanocrystal diameter, as expected on the basis of quantum confinement, the redox potentials and the electrochemical band gap are not monotonously related to the QD size. For both series, the smallest and largest QDs are both easier to oxidize and reduce than mid‐sized QDs. In fact, the latter samples exhibit very broad voltammetric profiles, which suggests that the heterogeneous electron‐transfer processes from/to the electrode are kinetically hindered. Conversely, the electrochemical band gap for the smallest and largest particles of each series is somewhat smaller than the optical band gap. These results indicate that, while the optical band gap depends on the actual electron–hole recombination within the nanocrystal, and therefore follows the size dependence expected from the particle‐in‐a‐box model, the electrochemical processes of these QDs are strongly affected by other factors, such as the presence of surface defects. The investigations suggest that the influence of these defects on the potential values is more important for the smallest and largest QDs of each series, as confirmed by the respective luminescence bands and quantum yields. An interpretation for the size‐dependent evolution of the surface defects in these nanocrystals is proposed based on the mechanism of their formation and growth. 相似文献