全文获取类型
收费全文 | 1202篇 |
免费 | 96篇 |
专业分类
1298篇 |
出版年
2023年 | 7篇 |
2022年 | 22篇 |
2021年 | 35篇 |
2020年 | 18篇 |
2019年 | 19篇 |
2018年 | 33篇 |
2017年 | 18篇 |
2016年 | 45篇 |
2015年 | 68篇 |
2014年 | 77篇 |
2013年 | 74篇 |
2012年 | 98篇 |
2011年 | 91篇 |
2010年 | 60篇 |
2009年 | 45篇 |
2008年 | 54篇 |
2007年 | 55篇 |
2006年 | 52篇 |
2005年 | 46篇 |
2004年 | 52篇 |
2003年 | 35篇 |
2002年 | 47篇 |
2001年 | 13篇 |
2000年 | 11篇 |
1999年 | 9篇 |
1998年 | 11篇 |
1997年 | 5篇 |
1996年 | 5篇 |
1995年 | 5篇 |
1993年 | 7篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1987年 | 13篇 |
1986年 | 12篇 |
1985年 | 23篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1978年 | 8篇 |
1977年 | 8篇 |
1975年 | 5篇 |
1973年 | 5篇 |
1972年 | 3篇 |
1971年 | 5篇 |
1966年 | 3篇 |
1959年 | 2篇 |
排序方式: 共有1298条查询结果,搜索用时 0 毫秒
131.
An alpha,omega-functionalized polymenthide was synthesized by the ring-opening polymerization of menthide in the presence of diethylene glycol with diethyl zinc as the catalyst. Termination with water afforded the dihydroxy polymenthide. The reaction of this telechelic polymer with triethylaluminum formed the corresponding aluminum alkoxide macroinitiator that was used for the controlled polymerization of lactide to yield biorenewable polylactide-b-polymenthide-b-polylactide triblock copolymers. The molecular weight and chemical composition were easily adjusted by the monomer-to-initiator ratios. Microphase separation in these triblock copolymers was confirmed by small-angle X-ray scattering and differential scanning calorimetry. A representative triblock was prepared with a hexagonally packed cylindrical morphology as determined by small-angle X-ray scattering, and tensile testing was employed to assess the mechanical behavior. On the basis of the ultimate elongations and elastic recovery, these triblock copolymers behaved as thermoplastic elastomers. 相似文献
132.
Leisher Craig Robinson Nathaniel Brown Matthew Kujirakwinja Deo Schmitz Mauricio Castro Wieland Michelle Wilkie David 《Biodiversity and Conservation》2022,31(4):1329-1343
Biodiversity and Conservation - Sub-Saharan Africa receives large investments in biodiversity conservation, and if these investments can be concentrated on the highest threats to biodiversity, the... 相似文献
133.
134.
The basal ganglia, in particular the striatum, are central to theories of behavioral control, and often identified as a seat of action selection. Reinforcement learning (RL) models--which have driven much recent experimental work on this region--cast striatum as a dynamic controller, integrating sensory and motivational information to construct efficient and enriching behavioral policies. Befitting this informationally central role, the BG sit at the nexus of multiple anatomical 'loops' of synaptic projections, connecting a wide range of cortical and subcortical structures. Numerous pioneering anatomical studies conducted over the past several decades have meticulously catalogued these loops, and labeled them according to the inferred functions of the connected regions. The specific cotermina of the projections are highly localized to several different subregions of the striatum, leading to the suggestion that these subregions perform complementary but distinct functions. However, until recently, the dominant computational framework outlined only a bipartite, dorsal/ventral, division of striatum. We review recent computational and experimental advances that argue for a more finely fractionated delineation. In particular, experimental data provide extensive insight into unique functions subserved by the dorsomedial striatum (DMS). These functions appear to correspond well with theories of a 'model-based' RL subunit, and may also shed light on the suborganization of ventral striatum. Finally, we discuss the limitations of these ideas and how they point the way toward future refinements of neurocomputational theories of striatal function, bringing them into contact with other areas of computational theory and other regions of the brain. 相似文献
135.
136.
Azhdarinia A Wilganowski N Robinson H Ghosh P Kwon S Lazard ZW Davis AR Olmsted-Davis E Sevick-Muraca EM 《Bioorganic & medicinal chemistry》2011,19(12):3769-3776
Optical imaging possesses similar sensitivity to nuclear imaging and has led to the emergence of multimodal approaches with dual-labeled nuclear/near-infrared (NIR) agents. The growing impact of (68)Ga (t(1/2)=68 min) labeled peptides on preclinical and clinical research offers a promising opportunity to merge the high spatial resolution of NIR imaging with the clinically-accepted positron emission tomography (PET). Previously, dual-labeled agents have been prepared with longer-lived radiometals and showed no detrimental effects on optical properties as a result of radiolabeling. In this study, we selected a peptide (M(2)) that targets MMP-2/9 and is dual-labeled with IRDye 800 CW and (68)Ga. Since (68)Ga chelation typically requires low pH (3.5-4) and elevated heating temperatures (95 °C), we sought to evaluate the impact of (68)Ga labeling on the optical properties of M(2). An efficient method for preparation of (68)Ga-M(2) was developed and reaction conditions were optimized. Stability studies in PBS, DTPA, and serum were performed and high levels of intact agent were evident under each condition. The addition of multiple reporters to a targeting agent adds further complexity to the characterization and validation and thus requires not only testing to ensure the agent is stable chemically and radiochemically, but also optically. Therefore, fluorescence properties were evaluated using a spectrofluorometer as well as by fluorescence detection via HPLC. It was determined that (68)Ga-labeling conditions did not impair the fluorescent properties of the agent. The agent was then used for in vivo imaging in a mouse model of heterotopic ossification (HO) with activated MMP-9 expression as an early biomarker which precedes mineralization. Although (68)Ga-complexation greatly reduced binding affinity of the peptide and negated tracer uptake on PET, NIR imaging showed consistent fluorescent signal that correlated to MMP-9 expression. This attests to the feasibility of using (68)Ga/NIR for dual-labeling of other peptides or small molecules for multimodality molecular imaging. 相似文献
137.
Cornelis MC Monda KL Yu K Paynter N Azzato EM Bennett SN Berndt SI Boerwinkle E Chanock S Chatterjee N Couper D Curhan G Heiss G Hu FB Hunter DJ Jacobs K Jensen MK Kraft P Landi MT Nettleton JA Purdue MP Rajaraman P Rimm EB Rose LM Rothman N Silverman D Stolzenberg-Solomon R Subar A Yeager M Chasman DI van Dam RM Caporaso NE 《PLoS genetics》2011,7(4):e1002033
We report the first genome-wide association study of habitual caffeine intake. We included 47,341 individuals of European descent based on five population-based studies within the United States. In a meta-analysis adjusted for age, sex, smoking, and eigenvectors of population variation, two loci achieved genome-wide significance: 7p21 (P = 2.4 × 10(-19)), near AHR, and 15q24 (P = 5.2 × 10(-14)), between CYP1A1 and CYP1A2. Both the AHR and CYP1A2 genes are biologically plausible candidates as CYP1A2 metabolizes caffeine and AHR regulates CYP1A2. 相似文献
138.
Stimulus properties, attention, and behavioral context influence correlations between the spike times produced by a pair of neurons. However, the biophysical mechanisms that modulate these correlations are poorly understood. With a combined theoretical and experimental approach, we show that the rate of balanced excitatory and inhibitory synaptic input modulates the magnitude and timescale of pairwise spike train correlation. High rate synaptic inputs promote spike time synchrony rather than long timescale spike rate correlations, while low rate synaptic inputs produce opposite results. This correlation shaping is due to a combination of enhanced high frequency input transfer and reduced firing rate gain in the high input rate state compared to the low state. Our study extends neural modulation from single neuron responses to population activity, a necessary step in understanding how the dynamics and processing of neural activity change across distinct brain states. 相似文献
139.
Global patterns of leaf mechanical properties 总被引:1,自引:0,他引:1
Onoda Y Westoby M Adler PB Choong AM Clissold FJ Cornelissen JH Díaz S Dominy NJ Elgart A Enrico L Fine PV Howard JJ Jalili A Kitajima K Kurokawa H McArthur C Lucas PW Markesteijn L Pérez-Harguindeguy N Poorter L Richards L Santiago LS Sosinski EE Van Bael SA Warton DI Wright IJ Wright SJ Yamashita N 《Ecology letters》2011,14(3):301-312
Leaf mechanical properties strongly influence leaf lifespan, plant-herbivore interactions, litter decomposition and nutrient cycling, but global patterns in their interspecific variation and underlying mechanisms remain poorly understood. We synthesize data across the three major measurement methods, permitting the first global analyses of leaf mechanics and associated traits, for 2819 species from 90 sites worldwide. Key measures of leaf mechanical resistance varied c. 500-800-fold among species. Contrary to a long-standing hypothesis, tropical leaves were not mechanically more resistant than temperate leaves. Leaf mechanical resistance was modestly related to rainfall and local light environment. By partitioning leaf mechanical resistance into three different components we discovered that toughness per density contributed a surprisingly large fraction to variation in mechanical resistance, larger than the fractions contributed by lamina thickness and tissue density. Higher toughness per density was associated with long leaf lifespan especially in forest understory. Seldom appreciated in the past, toughness per density is a key factor in leaf mechanical resistance, which itself influences plant-animal interactions and ecosystem functions across the globe. 相似文献
140.
van Bemmel DM Boffetta P Liao LM Berndt SI Menashe I Yeager M Chanock S Karami S Zaridze D Matteev V Janout V Kollarova H Bencko V Navratilova M Szeszenia-Dabrowska N Mates D Slamova A Rothman N Han SS Rosenberg PS Brennan P Chow WH Moore LE 《PloS one》2011,6(7):e20432