全文获取类型
收费全文 | 6362篇 |
免费 | 272篇 |
国内免费 | 14篇 |
专业分类
6648篇 |
出版年
2023年 | 64篇 |
2022年 | 164篇 |
2021年 | 327篇 |
2020年 | 140篇 |
2019年 | 205篇 |
2018年 | 232篇 |
2017年 | 169篇 |
2016年 | 253篇 |
2015年 | 294篇 |
2014年 | 359篇 |
2013年 | 495篇 |
2012年 | 470篇 |
2011年 | 454篇 |
2010年 | 270篇 |
2009年 | 234篇 |
2008年 | 302篇 |
2007年 | 272篇 |
2006年 | 239篇 |
2005年 | 224篇 |
2004年 | 174篇 |
2003年 | 150篇 |
2002年 | 132篇 |
2001年 | 68篇 |
2000年 | 69篇 |
1999年 | 54篇 |
1998年 | 35篇 |
1997年 | 19篇 |
1996年 | 29篇 |
1995年 | 22篇 |
1994年 | 15篇 |
1993年 | 29篇 |
1992年 | 36篇 |
1991年 | 49篇 |
1990年 | 48篇 |
1989年 | 41篇 |
1988年 | 33篇 |
1987年 | 46篇 |
1986年 | 33篇 |
1985年 | 45篇 |
1984年 | 30篇 |
1983年 | 22篇 |
1982年 | 21篇 |
1981年 | 19篇 |
1980年 | 15篇 |
1979年 | 24篇 |
1978年 | 28篇 |
1977年 | 16篇 |
1976年 | 16篇 |
1975年 | 15篇 |
1973年 | 22篇 |
排序方式: 共有6648条查询结果,搜索用时 15 毫秒
91.
Ahsan Mushir Shemsi Firdous Ahmad Khanday Ahsanulhaq Qurashi Amjad Khalil Gea Guerriero Khawar Sohail Siddiqui 《Biotechnology advances》2019,37(3):357-381
Numerous enzymes of biotechnological importance have been immobilized on magnetic nanoparticles (MNP) via random multipoint attachment, resulting in a heterogeneous protein population with potential reduction in activity due to restriction of substrate access to the active site. Several chemistries are now available, where the modifier can be linked to a single specific amino acid in a protein molecule away from the active-site, thus enabling free access of the substrate. However, rarely these site-selective approaches have been applied to immobilize enzymes on nanoparticles. In this review, for the first time, we illustrate how to adapt site-directed chemical modification (SDCM) methods for immobilizing enzymes on iron-based MNP. These strategies are mainly chemical but may additionally require genetic and enzymatic methods. We critically examine each method and evaluate their scope for simple, quick, efficient, mild and economical immobilization of enzymes on MNP. The improvements in the catalytic properties of few available examples of immobilized enzymes are also discussed. We conclude the review with the applications and future prospects of site-selectively modified magnetic enzymes and potential benefits of this technology in improving enzymes, including cold-adapted homologues, modular enzymes, and CO2-sequestering, as well as non-iron based nanomaterials. 相似文献
92.
Since 1960, magnetic fields have been discussed as Zeitgebers for circadian clocks, but the mechanism by which clocks perceive and process magnetic information has remained unknown. Recently, the radical-pair model involving light-activated photoreceptors as magnetic field sensors has gained considerable support, and the blue-light photoreceptor cryptochrome (CRY) has been proposed as a suitable molecule to mediate such magnetosensitivity. Since CRY is expressed in the circadian clock neurons and acts as a critical photoreceptor of Drosophila's clock, we aimed to test the role of CRY in magnetosensitivity of the circadian clock. In response to light, CRY causes slowing of the clock, ultimately leading to arrhythmic behavior. We expected that in the presence of applied magnetic fields, the impact of CRY on clock rhythmicity should be altered. Furthermore, according to the radical-pair hypothesis this response should be dependent on wavelength and on the field strength applied. We tested the effect of applied static magnetic fields on the circadian clock and found that flies exposed to these fields indeed showed enhanced slowing of clock rhythms. This effect was maximal at 300 μT, and reduced at both higher and lower field strengths. Clock response to magnetic fields was present in blue light, but absent under red-light illumination, which does not activate CRY. Furthermore, cryb and cryOUT mutants did not show any response, and flies overexpressing CRY in the clock neurons exhibited an enhanced response to the field. We conclude that Drosophila's circadian clock is sensitive to magnetic fields and that this sensitivity depends on light activation of CRY and on the applied field strength, consistent with the radical pair mechanism. CRY is widespread throughout biological systems and has been suggested as receptor for magnetic compass orientation in migratory birds. The present data establish the circadian clock of Drosophila as a model system for CRY-dependent magnetic sensitivity. Furthermore, given that CRY occurs in multiple tissues of Drosophila, including those potentially implicated in fly orientation, future studies may yield insights that could be applicable to the magnetic compass of migratory birds and even to potential magnetic field effects in humans. 相似文献
93.
94.
A Babaei BD Ward S Ahmad A Patel A Nencka SJ Li J Hyde R Shaker 《American journal of physiology. Gastrointestinal and liver physiology》2012,303(5):G600-G609
Functional MRI (fMRI) studies have demonstrated that a number of brain regions (cingulate, insula, prefrontal, and sensory/motor cortices) display blood oxygen level-dependent (BOLD) positive activity during swallow. Negative BOLD activations and reproducibility of these activations have not been systematically studied. The aim of our study was to investigate the reproducibility of swallow-related cortical positive and negative BOLD activity across different fMRI sessions. We studied 16 healthy volunteers utilizing an fMRI event-related analysis. Individual analysis using a general linear model was used to remove undesirable signal changes correlated with motion, white matter, and cerebrospinal fluid. The group analysis used a mixed-effects multilevel model to identify active cortical regions. The volume and magnitude of a BOLD signal within each cluster was compared between the two study sessions. All subjects showed significant clustered BOLD activity within the known areas of cortical swallowing network across both sessions. The cross-correlation coefficient of percent fMRI signal change and the number of activated voxels across both positive and negative BOLD networks were similar between the two studies (r ≥ 0.87, P < 0.0001). Swallow-associated negative BOLD activity was comparable to the well-defined "default-mode" network, and positive BOLD activity had noticeable overlap with the previously described "task-positive" network. Swallow activates two parallel cortical networks. These include a positive and a negative BOLD network, respectively, correlated and anticorrelated with swallow stimulus. Group cortical activity maps, as well as extent and amplitude of activity induced by volitional swallowing in the cortical swallowing network, are reproducible between study sessions. 相似文献
95.
Yosra Mohammed Yaghmour Shraim Ala'a Said Abbas Manal Ahmad 《Journal of Medical Biochemistry》2023,42(1):35
BackgroundCOVID-19 is a new pandemic that has infected millions of people worldwide and caused a high morbidity and mortality rate. COVID-19 may have a harmful effect on organs, especially the kidneys. Aims: The main aim of our research is to study the association between the severity of COVID-19 disease and biochemical parameters related to kidney function and to investigate certain risk factors of COVID-19-associated kidney disease.MethodsA total of 174 individuals, 121 COVID-19 positive and 53 COVID-19 negative, were enrolled in this study. The relation between COVID-19 infection, severity, kidney function test, and hematological indicators were examined.ResultsThe most prominent symptoms among COVID-19 were fever (95% ) and fatigue (92%). Regarding biochemical parameters, median creatinine, MPV, and CRP were significantly higher in COVID-19 patients, whereas median eGFR, Na+, WBC, MCH, MCHC, and eosinophil percentages were significantly lower in this group. Severely infected patients were observed to have higher urea, creatinine, neutrophils, and NLR. However, median sodium, eGFR, hemoglobin, hematocrit, RBC, lymphocytes, and platelet count were significantly lower in the severe group. Urine examination of the severe group showed a significantly lower specific gravity, while urine pH, protein, and glucose were significantly higher.ConclusionsOur analysis indicates that COVID-19 infection affects kidney function, mainly creatinine level, urea, eGFR, Na+ and urine protein. Additionally, comorbidities such as older age (>65), hypertension, taking medications, and CRP (>33.55 mg/L) are considered risk factors that are more likely to contribute to kidney impairment in COVID-19 positive patients. 相似文献
96.
Removal efficiency of gold from a solution of pure tetrachloroaurate ions was investigated using microbial fuel cell (MFC) technology. The effects of type of catholyte solution and initial gold concentration on the removal efficiency were considered. Due to its presence at high levels in the gold wastewater, the effect of copper ions on the removal efficiency of the gold ions was also studied. The effects of pH and initial biomass concentration on the gold removal efficiency was also determined. The results showed that after 5 h contact time, 95% of gold removal efficiency from a wastewater containing 250 ppm of initial gold ions at ambient temperature using 80 g/L yeast concentration was achieved. After 48 h of the cell''s operation under the same condition, 98.86% of AuCl4 – ions were successfully removed from the solution. At initial gold concentration in the waste solution of 250 ppm, pH 2, and initial yeast concentration of 80 g/L, 100% removal efficiency of the gold was achieved. On the other hand, the most suitable condition for copper removal was found at a pH of 5.2, where 53% removal efficiency from the waste solution was accomplished. 相似文献
97.
98.
99.
100.
We previously reported the antifungal properties of a monoterpene phenol “Eugenol” against different Candida strains and have observed that the addition of methyl group to eugenol drastically increased its antimicrobial potency. Based on the results and the importance of medicinal synthetic chemistry, we synthesized eugenol-tosylate and its congeners (E1-E6) and tested their antifungal activity against different clinical fluconazole (FLC)- susceptible and FLC- resistant C. albicans isolates alone and in combination with FLC by determining fractional inhibitory concentration indices (FICIs) and isobolograms calculated from microdilution assays. Minimum inhibitory concentration (MIC) results confirmed that all the tested C. albicans strains were variably susceptible to the semi-synthetic derivatives E1-E6, with MIC values ranging from 1–62 μg/ml. The test compounds in combination with FLC exhibited either synergy (36%), additive (41%) or indifferent (23%) interactions, however, no antagonistic interactions were observed. The MICs of FLC decreased 2–9 fold when used in combination with the test compounds. Like their precursor eugenol, all the derivatives showed significant impairment of ergosterol biosynthesis in all C. albicans strains coupled with down regulation of the important ergosterol biosynthesis pathway gene-ERG11. The results were further validated by docking studies, which revealed that the inhibitors snugly fitting the active site of the target enzyme, mimicking fluconazole, may well explain their excellent inhibitory activity. Our results suggest that these compounds have a great potential as antifungals, which can be used as chemosensitizing agents with the known antifungal drugs. 相似文献