全文获取类型
收费全文 | 2717篇 |
免费 | 108篇 |
国内免费 | 9篇 |
专业分类
化学 | 2458篇 |
晶体学 | 6篇 |
力学 | 17篇 |
数学 | 212篇 |
物理学 | 141篇 |
出版年
2023年 | 13篇 |
2022年 | 38篇 |
2021年 | 80篇 |
2020年 | 37篇 |
2019年 | 48篇 |
2018年 | 29篇 |
2017年 | 41篇 |
2016年 | 68篇 |
2015年 | 93篇 |
2014年 | 83篇 |
2013年 | 165篇 |
2012年 | 201篇 |
2011年 | 212篇 |
2010年 | 105篇 |
2009年 | 131篇 |
2008年 | 169篇 |
2007年 | 180篇 |
2006年 | 206篇 |
2005年 | 204篇 |
2004年 | 147篇 |
2003年 | 129篇 |
2002年 | 121篇 |
2001年 | 26篇 |
2000年 | 23篇 |
1999年 | 16篇 |
1998年 | 20篇 |
1997年 | 29篇 |
1996年 | 22篇 |
1995年 | 20篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 16篇 |
1991年 | 15篇 |
1990年 | 14篇 |
1989年 | 10篇 |
1988年 | 3篇 |
1987年 | 12篇 |
1986年 | 11篇 |
1985年 | 14篇 |
1984年 | 12篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1970年 | 2篇 |
1927年 | 2篇 |
排序方式: 共有2834条查询结果,搜索用时 31 毫秒
991.
Sun beds and cod liver oil as vitamin D sources 总被引:1,自引:0,他引:1
Porojnicu AC Bruland OS Aksnes L Grant WB Moan J 《Journal of photochemistry and photobiology. B, Biology》2008,91(2-3):125-131
The objective of this study was to (1) to determine the contribution of moderate sun bed exposure to serum 25(OH)D(3) levels; (2) to estimate the decay time of a high 25(OH)D(3) level obtained by sun bed exposure; and (3) to evaluate if the recommended ingestion of vitamin D is sufficient to maintain the 25(OH)D(3) concentration obtained by sun bed exposure. Ten volunteers (20-35 y.o.), skin type I and II, living in Olso, Norway were whole body exposed twice per week to the radiation of a commercial and approved sun bed (Life Sun S 100 W, Wolff System), starting with 0.5 MED (minimal erythema dose) and escalating to up to 1 MED per exposure for 4 weeks. After that, half of the volunteers were given a daily supplement of 200 IU vitamin D in the form of cod liver oil capsules, while the other half of the persons received no supplements. Erythema did not occur at any time and a slight pigmentation was seen in most of the volunteers after the sun bed exposures. Serum level of 25(OH)D(3) increased by about 40% on the average. The initial serum 25(OH)D(3) level was different among the volunteers (40-100 nmol/L). Within eight weeks after the last exposure the 25(OH)D(3) level decreased to the initial value in all volunteers irrespective of vitamin D supplementation or not. 相似文献
992.
Díaz-Moscoso A Balbuena P Gómez-García M Ortiz Mellet C Benito JM Le Gourriérec L Di Giorgio C Vierling P Mazzaglia A Micali N Defaye J García Fernández JM 《Chemical communications (Cambridge, England)》2008,(17):2001-2003
Self-assembled cyclodextrin (CD)-DNA nanoparticles (CDplexes) exhibiting transfection efficiencies significantly higher than PEI-based polyplexes have been prepared from homogeneous seven-fold symmetric polyaminothiourea amphiphiles constructed on a beta-cyclodextrin scaffold. 相似文献
993.
Alajarín M Berná J López-Leonardo C Steed JW 《Chemical communications (Cambridge, England)》2008,(20):2337-2339
Macrobicyclic triphosphazides are able to reversibly exchange one of their tripodal components by means of a dynamic disassembly-reassembly process; surprisingly this strategy provides better yields of cage compounds than a direct tripod-tripod coupling. 相似文献
994.
Rodica Olar Mihaela Badea Maria Nicoleta Grecu Dana Marinescu Veronica Lazar Carmen Balotescu 《Journal of Thermal Analysis and Calorimetry》2008,92(1):239-243
The N,N-dimethylbiguanide (HDMBG) complexes [Cu2(HDMBG)2Cl4] (1) and respectively [Cu(HDMBG)2]Cl2·2H2O (2) exhibit in vitro antimicrobial activity. The complexes were characterised by IR, electronic as well as EPR spectra. The IR spectra of complexes show the pattern of N,N-dimethylbiguanide coordinated as chelate. The electronic and EPR data are in agreement with a square pyramidal stereochemistry for (1) and a square planar one for (2). The in vitro qualitative and quantitative antimicrobial activity assays showed that the complexes exhibited variable antimicrobial activity against Gram-negative strains (Escherichia coli, Klebsiella spp. and Enterobacter sp.) isolated from the hospital environment. The thermal analysis has evidenced the thermal intervals of stability and also the thermodynamic effects that accompany them. The thermal behaviour in nitrogen is complex according to TG and DTA curves including melting, dehydration as well as compounds decomposition. 相似文献
995.
Rodríguez-Fariñas N Gomez-Gomez MM Camara-Rica C 《Analytical and bioanalytical chemistry》2008,390(1):29-35
Oral administration of sodium tungstate is an effective treatment for type 1 and 2 diabetes in animal models; it does not
incur significant side effects, and it may constitute an alternative to insulin. However, the mechanism by which tungstate
exerts its observed metabolic effects in vivo is still not completely understood. In this work, serum-containing proteins
which bind tungstate have been characterized. Size exclusion chromatography (SEC) coupled to inductively coupled plasma mass
spectrometry (ICP-MS) with a Phenomenex Bio-Sep-S 2000 column and 20 mM HEPES and 150 mM NaCl at pH 7.4 as the mobile phase
was chosen as the most appropriate methodology to screen for tungsten–protein complexes. When human serum was incubated with
tungstate, three analytical peaks were observed, one related to tungstate–albumin binding, one to free tungstate, and one
to an unknown protein binding (MW higher than 300 kDa). Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF)
mass spectrometric analysis of the tungsten-containing fractions collected from SEC–ICP-MS chromatograms, after desalting
and preconcentration processes, confirmed the association of tungstate with albumin and the other unknown protein.
Figure SEC-ICP-MS // MALDI-TOF 相似文献
996.
Goenaga Infante H Ovejero Bendito Mdel C Cámara C Evans L Hearn R Moesgaard S 《Analytical and bioanalytical chemistry》2008,390(8):2099-2106
A method for the accurate determination of ultratrace selenium species of relevance to cancer research, such as gamma-glutamyl-Se-methylselenocysteine
(γ-glutamyl-SeMC), using species-specific double isotope dilution analysis (IDA) with HPLC–ICP–MS is reported for the first
time. The 77Se-enriched γ-glutamyl-SeMC spike was produced in-house by collecting the fraction at the retention time of the γ-glutamyl-SeMC
peak from a chromatographed aqueous extract of 77Se-enriched yeast, pooling the collected fractions and freeze-drying the homogenate. The Se content of this spike was characterised
using reverse isotope dilution mass spectrometry (IDMS) and the isotopic composition of this spike was checked prior to quantification
of the natural abundance dipeptide species in garlic using speciated IDA. The extraction of the γ-glutamyl-SeMC species in
water was performed in a sonication bath for 2 h after adding an appropriate quantity of 77Se-enriched γ-glutamyl-SeMC to 50 mg of garlic to give optimal 78Se/77Se and 82Se/77Se ratios of 1.5 and 0.6, respectively. The effect of ultrasonic nebulisation, in comparison with the loading of the ICP with
carbon (through the addition of methane gas on-line), on the detection of Se associated with γ-glutamyl-SeMC using collision/reaction
cell ICP–MS with hydrogen as collision gas was investigated. Sensitivity enhancements of approximately fourfold and twofold
were achieved using USN and methane mixed plasma, respectively, in comparison with conventional nebulisation and conventional
Ar ICP–MS. However, an approximately twofold improvement in the detection limit was achieved using both approaches (42 ng
kg−1 for 78Se using peak height measurements). The use of species-specific IDMS enabled quantification of the dipeptide species at ng
g−1 levels (603 ng g−1 Se) in the complex food matrix with a relative standard deviation (RSD, n = 4) of 4.5%, which was approximately half that obtained using standard addition as a confirmatory technique. Furthermore,
good agreement was found between the γ-glutamyl-SeMC species concentrations obtained using both calibration methods. 相似文献
997.
Limban C Chifiriuc MC Missir AV Chiriţă IC Bleotu C 《Molecules (Basel, Switzerland)》2008,13(3):567-580
We report here the characterisation of eight newly synthesized thioureides of 2-(4-chlorophenoxymethyl)-benzoic acid and the evaluation of the in vitro antimicrobial activity of the new compounds against Gram-positive [Listeria monocytogenes,Staphylococcus aureus, Bacillus subtilis], Gram-negative [Psedomonas aeruginosa,Escherichia coli, Salmonella enteritidis], as well as Candida spp., using both reference and clinical multidrug resistant strains to establish the minimal inhibitory concentration (MIC)values. Our results showed that the tested compounds exhibited specific antimicrobial activities, both concerning the spectrum of antimicrobial activity and the corresponding MIC values, which ranged widely between 1024 and 32 mug/mL, depending on the nature and position of the substituents on the benzene ring. The most active compounds were N-[2-(4-chlorophenoxymethyl)-benzoyl]-N'-(2,6-dichlorophenyl)-thiourea (5 g) and N-[2-(4-chlorophenoxymethyl)-benzoyl]-N'-(4-bromophenyl)-thiourea (5h), which showed a broad spectrum of antimicrobial activity against enterobacterial strains (E. coli and S. enteritidis),P. aeruginosa, S. aureus and Candida spp. All the tested compounds except 5f were highly active against S. aureus (MIC=32 mug/mL), suggesting their possible use in the treatment of MRSA infections. Four of compounds also exhibited antifungal activity (MIC =256-32 microg/mL) against C. albicans, but L. monocytogenes as well as B. subtilis were resistant to all tested compounds. Our studies thus demonstrated that among other biological activities,the thioureides of 2-(4-chlorophenoxymethyl)-benzoic acid also exhibit selective and effective antimicrobial properties that could lead to the selection and use of these compounds as efficient antimicrobial agents, especially for the treatment of multidrug resistant infections. 相似文献
998.
Olga S. Finikova Dr. Artem Y. Lebedev Dr. Alexey Aprelev Dr. Thomas Troxler Dr. Feng Gao Dr. Carmen Garnacho Dr. Silvia Muro Prof. Robin M. Hochstrasser Prof. Sergei A. Vinogradov Prof. 《Chemphyschem》2008,9(12):1673-1679
High‐resolution images of oxygen distributions in microheterogeneous samples are obtained by two‐photon laser scanning microscopy (2P LSM), using a newly developed dendritic nanoprobe with internally enhanced two‐photon absorption (2PA) cross‐section. In this probe, energy is harvested by a 2PA antenna, which passes excitation onto a phosphorescent metalloporphyrin via intramolecular energy transfer. The 2P LSM allows sectioning of oxygen gradients with near diffraction‐limited resolution, and lifetime‐based acquisition eliminates dependence on the local probe concentration. The technique is validated on objects with a priori known oxygen distributions and applied to imaging of pO2 in cells. 相似文献
999.
Muñoz-Moreno BK Claver C Ruiz A Bianchini C Meli A Oberhauser W 《Dalton transactions (Cambridge, England : 2003)》2008,(20):2741-2750
The diphosphine 3,5-dideoxy-1,2-O-isopropylidene-3,5-bis(di(2-methoxyphenyl)phosphanyl)-alpha-D-xylofuranose (o-MeO-xylophos), which differs from the known 3,5-dideoxy-1,2-O-isopropylidene-3,5-bis(diphenylphosphanyl)-alpha-D-xylofuranose (xylophos) by the presence of 2-methoxy substituents on the P-aryl rings, has been synthesized and characterized. These two ligands have been employed to stabilize the Pd(II) complexes [PdCl2(o-MeO-xylophos)] (1a), [PdCl2(xylophos)] (2a), [PdClMe(o-MeO-xylophos)] (1b), [PdClMe(xylophos)] (2b), [Pd(OTs)(H2O)(o-MeO-xylophos)](OTs) (1c) and [Pd(OTs)(H2O)(xylophos)](OTs) (2c). All complexes have been characterized by multinuclear-NMR spectroscopy. The solid-state structure of 1a has been determined by a single crystal X-ray analysis. The Pd-aqua complexes 1c and 2c have been employed to catalyse the CO-ethene and CO-propene copolymerization as well as the CO-ethene-propene terpolymerization reaction in MeOH. The catalytic activity and the molecular weight of the polyketones have been compared to those of the products obtained with analogous catalysts, [Pd(H2O)2(o-MeO-dppp)](OTs)2 (3c) and [Pd(H2O)(OTs)(dppp)](OTs) (4c), bearing the classical 1,3-bis(diphenylphoshino)propane ligand (dppp). Under comparable catalytic conditions, all catalysts produce structurally similar polymeric materials, with 1c yielding the largest propene incorporation as well as the highest productivity of low-molecular-weight terpolymers. 相似文献
1000.
González-Alvarez MJ Balbuena P Mellet CO Fernández JM Mendicuti F 《The journal of physical chemistry. B》2008,112(44):13717-13729
Steady-state and time-resolved fluorescence techniques were used to study the behavior of 2I,3I-O-(o-xylylene)-per-O-Me-alpha- and -beta-cyclodextrins in aqueous solution, based on the fluorescence of the bidentate xylylene moiety. Fluorescence decay profiles obtained upon excitation of the xylylene group were fitted to three-exponential decay functions. In addition to a fast component due to stray and/or scattered light, two other components ascribed to the monomer and dimer species, respectively, were identified. The dimer/monomer ratio increases with concentration and decreases with temperature, which is in agreement with an enthalpy-driven association process. The corresponding dimerization equilibrium constants (KD) were obtained from nonlinear regression analysis of the plots of tau against [CD] in the 5-45 degrees C range. A linear van't Hoff analysis for KD allows us to obtain the DeltaH and DeltaS associated to dimer formation. Molecular mechanics as well as molecular dynamics calculations in the presence of water were also employed to study the conformational behavior of such secondary-face-substituted cyclodextrins and rationalize the dimerization processes. 相似文献