首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2582篇
  免费   152篇
  国内免费   19篇
儿科学   15篇
妇产科学   7篇
基础医学   399篇
口腔科学   19篇
临床医学   58篇
内科学   369篇
皮肤病学   14篇
神经病学   84篇
特种医学   347篇
外科学   47篇
综合类   102篇
预防医学   51篇
眼科学   11篇
药学   1023篇
  2篇
中国医学   165篇
肿瘤学   40篇
  2023年   14篇
  2022年   17篇
  2021年   31篇
  2020年   48篇
  2019年   58篇
  2018年   51篇
  2017年   59篇
  2016年   55篇
  2015年   87篇
  2014年   133篇
  2013年   182篇
  2012年   135篇
  2011年   156篇
  2010年   148篇
  2009年   129篇
  2008年   155篇
  2007年   117篇
  2006年   91篇
  2005年   115篇
  2004年   72篇
  2003年   100篇
  2002年   67篇
  2001年   65篇
  2000年   33篇
  1999年   49篇
  1998年   67篇
  1997年   52篇
  1996年   50篇
  1995年   71篇
  1994年   68篇
  1993年   75篇
  1992年   34篇
  1991年   33篇
  1990年   35篇
  1989年   13篇
  1988年   9篇
  1987年   10篇
  1986年   12篇
  1985年   18篇
  1984年   12篇
  1983年   13篇
  1982年   5篇
  1981年   2篇
  1980年   2篇
  1978年   3篇
  1977年   1篇
  1974年   1篇
排序方式: 共有2753条查询结果,搜索用时 15 毫秒
1.
Cynanchumpaniculatum(Bunge) Kitagawa is usually used as an herbal medicine for treating many diseases. Paeonol is the main active component, and its content is the key indicator for quality control of C.paniculatum. In the present study, we developed a rapid, accurate and precise method for quantitation of paeonol in C.paniculatum using 1H NMR spectra. The deuteratedsolvent of methanol-d4 enabled satisfactory separation of the signals to be integrated in 1H NMR spectrum. H-6 (δ 7.78) of1H NMR spectrum of C.paniculatum was selected as the feature signal for quantitation, and trimesic acid (TMA) was selected as an internal standard. Validation of the quantitative method was performed in terms of linearity, specificity, repeatability and stability. This is the first time to report quantitative 1H NMR(qHNMR) applied to determine the content of paeonol in C.paniculatum and showed a wider linearity range than the reported quantitation of paeonol in others. The simple extraction of paeonol from C.paniculatum was rapid and will prompt the application of the developed method. This work implied that qHNMR represented a feasible alternative to HPLC-based methods for quantitation of paeonol in C.paniculatum, and it was suitable for the quality control of C.paniculatum.  相似文献   
2.
Hypomineralized enamel may be found in connection with the condition molar incisor hypomineralization (MIH), which has a prevalence of around 15% in most parts of the world. Molar incisor hypomineralization is associated with extensive objective and subjective problems, such as hypersensitivity of the affected teeth, enamel breakdown, and problems with retention of restorations. The etiology behind MIH has not yet been elucidated, but a number of possible factors, which affect the same or different functions of ameloblasts during their different stages of maturation, have been suggested. The aim of this study was to utilize multi‐nuclear, solid‐state nuclear magnetic resonance (ss‐NMR) and time‐of‐flight secondary ion mass spectroscopy (ToF‐SIMS) to elucidate any differences, at a molecular level, between enamel powder prepared from normal, healthy teeth and enamel powder prepared from teeth diagnosed with MIH. 31P and 23Na ss‐NMR confirmed the presence of and two different Na+ sites in hypomineralized enamel, suggesting a heterogeneous chemical composition. The content of organic components was higher in hypomineralized enamel, as shown by both 13C ss‐NMR and ToF‐SIMS, indicating the presence of higher numbers of proteins and phospholipids. The interplay between both is necessary for the formation and mineralization of enamel, which might be disturbed or halted in hypomineralized enamel.  相似文献   
3.
4.
Cancer remains the topmost disorder of the mankind and number of cases is unceasingly growing at unprecedented rates. Although the synthetic anti-cancer compounds still hold the largest market in the modern treatment of cancer, natural agents have always been tried and tested for potential anti-cancer properties. Thymoquinone (TQ), a monoterpene and main ingredient in the essential oil of Nigella sativa L. has got very eminent rankings in the traditional systems of medicine for its anti-cancer pharmacological properties. In this review we summarized the diverse aspects of TQ including its chemistry, biosynthesis, sources and pharmacological properties with a major concern being attributed to its anti-cancer efficacies. The role of TQ in different aspects involved in the pathogenesis of cancer like inflammation, angiogenesis, apoptosis, cell cycle regulation, proliferation, invasion and migration have been described. The mechanism of action of TQ in different cancer types has been briefly accounted. Other safety and toxicological aspects and some combination therapies involving TQ have also been touched. A detailed literature search was carried out using various online search engines like google scholar and pubmed regarding the available research and review accounts on thymoquinone upto may 2019. All the articles reporting significant addition to the activities of thymoquinone were selected. Additional information was acquired from ethno botanical literature focusing on thymoquinone. The compound has been the centre of attention for a long time period and researched regularly in quite considerable numbers for its various physicochemical, medicinal, biological and pharmacological perspectives. Thymoquinone is studied for various chemical and pharmacological activities and demonstrated promising anti-cancer potential. The reviewed reports confirmed the strong anti-cancer efficacy of thymoquinone. Further in-vitro and in-vivo research is strongly warranted regarding the complete exploration of thymoquinone in ethnopharmacological context.  相似文献   
5.
During the past years, there has been an increasing focus on the presence of silicone oil as a contaminant in pharmaceutical formulations kept in prefilled syringes (PFSs). As the PFSs are coated on the inner wall with silicone oil (polydimethylsiloxane), there is a potential risk that the oil can migrate from the inner surface of the primary packing material into the aqueous solution. Several studies have demonstrated that presence of silicone oil as droplets in a high-concentrated protein formulation can cause protein aggregation. Hence, because the use of silicone-coated primary packing material for protein formulations are increasing, the call for an easy and quantitative method for determination of silicone oil and its degradation products in pharmaceutical formulations is therefore needed. Several analytical techniques have in the past been developed with the aim of detecting the presence of silicone oil and degradation products hereof. Most of these methods require hydrolyzation, derivatization, and extraction steps followed by, for example, gas chromatography-mass spectrometry analysis. Applying these methods can cause a loss in detection or an overestimation of the hydrolytic degradation products of silicone oil, that is, trimethylsilanol and dimethylsilanediol. The 2 silanols are highly hydrophilic and prefers the aqueous environment. Analysis of an aqueous formulation obtained from a PFS by 1H-NMR spectroscopy provides data about the content and levels of silicone oil and the 2 silanols even in levels below 10 ppm. The 1H-NMR method offers an easy and direct, quantitative measurement of samples intended for clinical use and samples kept at elevated temperature for a prolonged time (i.e., stability studies). The result of the study presented here showed dimethylsilanediol to be the main silicone compound present in the aqueous formulation when kept in baked-on PFSs. The degradation product dimethylsilanediol, in full accordance with expected hydrolytic degradation of silicone oil, increased during storage and with elevated temperature. In addition, the method can be applied to aqueous samples where polydimethylsiloxane has been added as, for example, the major constituent of antifoam.  相似文献   
6.
The molecular model is one of the most appealing to explain the peculiar optical properties of Carbon nanodots (CNDs) and was proven to be successful for the bottom up synthesis, where a few molecules were recognized. Among the others, citrazinic acid is relevant for the synthesis of citric acid-based CNDs. Here we report a combined experimental and computational approach to discuss the formation of different protonated and deprotonated species of citrazinic acid and their contribution to vibrational and magnetic spectra. By computing the free energy formation in water solution, we selected the most favoured species and we retrieved their presence in the experimental surface enhanced Raman spectra. As well, the chemical shifts are discussed in terms of tautomers and rotamers of most favoured species. The expected formation of protonated and de-protonated citrazinic acid ions under extreme pH conditions was proven by evaluating specific interactions with H2SO4 and NaOH molecules. The reported results confirm that the presence of citrazinic acid and its ionic forms should be considered in the interpretation of the spectroscopic features of CNDs.  相似文献   
7.
《Vaccine》2020,38(31):4853-4860
Aluminum-adjuvanted vaccines are freeze-sensitive products that require attentive cold chain adherence. Freeze/thaw events can be tested using “The World Health Organization Shake Test”, a qualitative test whereby a vial from the batch suspected to have been frozen is checked to infer whether the whole batch has been frozen. In this paper, we present a noninvasive and quantitative method to detect whether a vial of liquid vaccine has experienced freeze/thaw using the water proton transverse relaxation rate by Nuclear Magnetic Resonance relaxometry (wNMR relaxometry). Importantly, wNMR relaxometry does not compromise the vial’s integrity so the analyzed vial can be used for vaccination if it meets the quality specifications. Vial-to-vial variability in freezing susceptibility within a single carton of vaccine vials was also detected, both by visual observation and concurrently by wNMR relaxometry. This variability brings into question the practice of using one or a few vials in a batch of vaccines to infer about the quality of the whole batch.  相似文献   
8.
Four new C20-diterpenoid alkaloids, rotundifosines D-G (1–4), along with eight known ones (5–12) were isolated from the whole plant of Aconitum rotundifolium Kar. & Kir. The structures of the compounds were elucidated on the basis of spectroscopic analyses, including HR-ESI-MS and 1D, 2D NMR. Rotundifosine F (3) is a rare C20-diterpenoid alkaloid with quaternary ammonium salt. Alkaloids 1–4, 5, 6, 9, and 12 were evaluated for cytotoxicity against MCF-7, HCT-116 and HepG2 human cancer cell lines.  相似文献   
9.
The purpose of this study is to assess some of the variables determining the aldol-like condensation of pyruvic acid (1), a peroxide scavenger, in aqueous solution to parapyruvic acid and higher oligomers. Its stability is compared to 3 other α-keto carboxylic acids, 2 with sterically hindered methylene groups alpha to the keto functionality (2-3) and phenylglyoxylic acid (4) with no methylene group. High-performance liquid chromatography, nuclear magnetic resonance, and liquid chromatography mass spectroscopy techniques are used in the kinetics and product analyses. 1 condensation is concentration dependent and base catalyzed above pH 7, consistent with the reaction mechanism proceeding through the attack of the fraction of the methylene group, alpha to the keto group, in its anionic form, at the keto group of a second molecule of 1. The major product is confirmed to be parapyruvic acid, but higher-order oligomers are also observed. All 3 of the other α-keto carboxylic acids 2-4 are considerably less reactive, with 4 being completely stable. Stable solutions of 1 can be prepared by the use of relatively dilute solutions maintained at slightly acidic pH values. 1 prevents the oxidation of methionine on addition of hydrogen peroxide.  相似文献   
10.
The aim of this study was to synthetize cocrystals of nateglinide, an antidiabetic agent of biopharmaceutics classification system Class IIa, as a strategy to improve both the solubility and the dissolution rate of the drug. Benzamide was selected by a screening procedure as a suitable coformer, and binary mixtures with different compositions were prepared and analyzed by differential scanning calorimetry (DSC). An in-depth analysis of DSC data allowed obtaining both the eutectic mixture and cocrystal compositions. The rationale of such an analysis was highlighted and explained. Cocrystals were prepared by kneading and solvent evaporation. Their formation was proved by DSC and confirmed by X-ray powder diffraction, solid-state nuclear magnetic resonance, and Fourier-transform infrared spectroscopy. The functional groups involved in the interaction leading to cocrystals formation were investigated by spectroscopic techniques. The in vitro dissolution profiles show that cocrystals have definite better pharmaceutical performances than the pure drug.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号