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   10129篇
  免费   1574篇
  国内免费   1015篇
化学   7272篇
晶体学   108篇
力学   559篇
综合类   72篇
数学   1280篇
物理学   3427篇
  2024年   20篇
  2023年   179篇
  2022年   290篇
  2021年   331篇
  2020年   383篇
  2019年   379篇
  2018年   342篇
  2017年   302篇
  2016年   437篇
  2015年   462篇
  2014年   555篇
  2013年   766篇
  2012年   904篇
  2011年   906篇
  2010年   632篇
  2009年   605篇
  2008年   666篇
  2007年   543篇
  2006年   555篇
  2005年   451篇
  2004年   396篇
  2003年   304篇
  2002年   290篇
  2001年   220篇
  2000年   167篇
  1999年   179篇
  1998年   177篇
  1997年   180篇
  1996年   171篇
  1995年   127篇
  1994年   128篇
  1993年   99篇
  1992年   62篇
  1991年   59篇
  1990年   69篇
  1989年   47篇
  1988年   33篇
  1987年   22篇
  1986年   35篇
  1985年   28篇
  1984年   22篇
  1983年   20篇
  1982年   11篇
  1981年   13篇
  1980年   16篇
  1979年   10篇
  1978年   22篇
  1977年   10篇
  1976年   13篇
  1974年   12篇
排序方式: 共有10000条查询结果,搜索用时 381 毫秒
991.
Boronate affinity solid phase microextraction (BA-SPME) is a new format appeared recently with great potential for specific extraction of cis-diol-containing compounds. Unlike conventional SPME, BA-SPME relies on covalent interactions and thereby features with specific selectivity, eliminated matrix effect and manipulable capture/release. However, only on-fiber BA-SPME and its off-line combination with high performance liquid chromatography (HPLC) have been reported so far. In this study, we report on-line coupling of in-tube BA-SPME with HPLC-electrospray ionization tandem mass spectroscopy (in-tube BA-SPME-HPLC-ESI-MS/MS) for the specific and sensitive determination of cis-diol-containing biomolecules. A boronate affinity extraction phase was prepared onto the inner surface of the capillary by copolymerization of vinylphenylboronic acid (VPBA) and ethylene glycol dimethacrylate (EDMA). The extraction conditions were optimized by choosing appropriate extraction/desorption solutions and extraction time. The extraction capacity, linear range, reproducibility and life-time were investigated. The developed method was successfully applied for the determination of dopamine in urine samples. Since many cis-diol-containing compounds are of great biological importance, the in-tube BA-SPME-HPLC method can be a promising tool.  相似文献   
992.
From the whole plants of E. ritro L., the three new sesquiterpenoids (3α,4α,6α)‐3,13‐dihydroxyguaia‐7(11),10(14)‐dieno‐12,6‐lactone ( 1 ), (3α,4α,6α,11β)‐3‐hydroxyguai‐1(10)‐eno‐12,6‐lactone ( 2 ), and (11α)‐11,13‐dihydroarglanilic acid methyl ester (=(4β,6α,11α)‐4,6‐dihydroxy‐1‐oxoeudesm‐2‐en‐12‐oic acid methyl ester; 3 ), together with eight known sesquiterpenoids, were isolated. Their structures were elucidated through analysis of spectroscopic data including extensive 2D‐NMR.  相似文献   
993.
In the proposed method, an extraction solvent with a lower toxicity and density than the solvents typically used in dispersive liquid-liquid microextraction was used to extract seven polychlorinated biphenyls (PCBs) from aqueous samples. Due to the density and melting point of the extraction solvent, the extract which forms a layer on top of aqueous sample can be collected by solidifying it at low temperatures, which form a layer on top of the aqueous sample. Furthermore, the solidified phase can be easily removed from the aqueous phase. Based on preliminary studies, 1-undecanol was selected as the extraction solvent, and a series of parameters that affect the extraction efficiency were systematically investigated. Under the optimized conditions, enrichment factors for PCBs ranged between 494 and 606. Based on a signal-to-noise ratio of 3, the limit of detection for the method ranged between 3.3 and 5.4 ng L−1. Good linearity, reproducibility and recovery were also obtained.  相似文献   
994.
The interaction between 2,4-dichlorophenol (DCP) and bovine serum albumin (BSA) was investigated by fluorescence spectroscopy combined with UV-vis absorption and circular dichroism (CD) spectroscopy under simulative physiological conditions. The experiment results show that the fluorescence intensity of BSA is dramatically decreased owing to the formation of a DCP–BSA complex. The corresponding effective quenching constants (K a) between DCP and BSA at four different temperatures (292, 298, 304 and 310 K) were determined to be 10.08×104, 9.082×104, 8.177×104, and 7.260×104 L?mol?1, respectively. The thermodynamics parameters enthalpy change (ΔH) and entropy change (ΔS) were calculated to be ?13.64 kJ?mol?1 and 49.08 J?mol?1?K?1, which suggested that hydrophobic interaction was the predominant intermolecular force. Site marker competitive experiments indicated that the binding of DCP to BSA primarily takes place in subdomain IIA. The binding distance (r) between DCP and the tryptophan residue of BSA ias 4.09 nm according to Förster’s theory of non-radioactive energy transfer. The conformational investigation demonstrated that the presence of DCP decreased the α-helical content of BSA and induced a slight unfolding of the polypeptides of protein, which confirmed the occurrence some micro environmental and conformational changes of BSA molecules.  相似文献   
995.
Copper nanoparticles (Cu-NPs) were incorporated into chitosan hydrogel to form a film on the surface of a glassy carbon electrode (GCE) leading to a sensing element for D-arabinitol with excellent oxidative catalytic activity. The electrochemical response to D-arabinitol was studied by cyclic voltammetry and differential pulse voltammetry. Operational parameters affecting the response were examined and optimized, and a simple and sensitive method was established for the determination of D-arabinitol. Response is linear in the concentration range from 10 μmol·L?1 to 10 mmol·L?1, and the limit of detection is 1.0 μmol·L?1. The method may be combined with separation techniques in order to analyze for the ratio of D- and L-arabinitol which is a diagnostic marker for candidiasis.  相似文献   
996.
Here we present a new bifunctional layer-by-layer (LbL) construct made by combining a permanent microbicidal polyelectrolyte multilayered (PEM) base film with a hydrolytically degradable PEM top film that offers controlled and localized delivery of therapeutics. Two degradable film architectures are presented: (1) bolus release of an antibiotic (gentamicin) to eradicate initial infection at the implant site, or (2) sustained delivery of an anti-inflammatory drug (diclofenac) to cope with inflammation at the site of implantation due to tissue injury. Each degradable film was built on top of a permanent base film that imparts the implantable device surface with microbicidal functionality that prevents the formation of biofilms. Controlled-delivery of gentamicin was demonstrated over hours and that of diclofenac over days. Both drugs retained their efficacy upon release. The permanent microbicidal base film was biocompatible with A549 epithelial cancer cells and MC3T3-E1 osteoprogenitor cells, while also preventing bacteria attachment from turbid media for the entire duration of the two weeks studied. The microbicidal base film retains its functionality after the biodegradable films have completely degraded. The versatility of these PEM films and their ability to prevent biofilm formation make them attractive as coatings for implantable devices.  相似文献   
997.
Dispersive liquid–liquid microextraction (DLLME) based on the solidification of floating organic droplets (DLLME-SFO) combined with gas chromatography-electron-capture detection (GC–ECD) has been developed for extraction and analysis of three dinitrobenzenes. The extraction conditions including extraction solvent, disperser solvent, extraction time, salt effect and temperature were investigated and optimized systematically. The limits of detection were 0.019 μg L?1 for 1,4-dinitrobenzene, 0.079 μg L?1 for 1,3-dinitrobenzene and 0.034 μg L?1 for 1,2-dinitrobenzene. Moreover, it offered good repeatability and high recovery. This method was successfully applied to monitor DNBs in different water samples.  相似文献   
998.
We describe here an analytical method of HEK293 α1A AR cell membrane chromatography (HEK293 α1A AR/CMC) combined with reverse phase liquid chromatography (RPLC) for recognition, separation and identification of target components from Traditional Chinese Medicines (TCMs) Radix Caulophylli. The HEK 293 α1A cells with high expressing α1A adrenergic receptors were used to prepare the stationary phase in the CMC model. Retention fractions on the α1A AR–CMC model were collected using an automated fraction collection and injection module (FC/I). And each fraction was analyzed by RPLC under optimized conditions. 5-Methylurapidil (5-MU) and tamsulosin hydrochloride were used as standard compounds to investigate the suitability and reliability of the HEK 293 α1A AR–CMC–RPLC method prior to screening target component from Radix Caulophylli total alkaloid. The results indicated that caulophine was the target component acting on the α1A AR. This method could be an efficient way in drug discovery using natural medicinal herbs as a source of novel compounds.  相似文献   
999.
The synthesis of butyl oleate was studied in this paper with immobilized lipase. Five types of membrane were used as support to immobilize Rhizopus arrhizus lipase by following a procedure combining filtration and protein cross-linking. Results showed that hydrophobic polytetrafluoroethene membrane with nonwoven fabric (HO-PTFE-NF) was the favorite choice in terms of higher protein loading, activity, and specific activity of immobilized lipase. The factors including solvent polarity, lipase dosage, concentration, and molar ratio of substrate and temperature were found to have significant influence on conversion. Results showed that hexane (logP = 3.53) was a favorable solvent for the biosynthesis of butyl oleate in our studies. The optimal conditions were experimentally determined of 50 U immobilized lipase, molar ratio of oleic acid to butanol of 1.0, substrate concentration of 0.12 mol/L, temperature of 37 °C, and reaction time of 2 h. The conversion was beyond 91% and decreased slightly after 18 cycles. Lipase immobilization can improve the conversion and the repeated use of immobilized lipase relative to free lipase.  相似文献   
1000.
The potential application of ordered mesoporous carbon (OMC)-modified glassy carbon electrode (OMC/GCE) in electrochemistry as a novel electrode material was investigated. X-ray diffraction, transmission electron micrographs, and cyclic voltammetry were used to characterize the structure and electrochemical behaviors of this material. Compared to GC electrode, the peak currents of potassium ferricyanide (K3[Fe(CN)6]) increase and the peak potential separation (ΔE p) decreases at the OMC/GC electrode. These phenomena suggest that OMC-modified GC electrode possesses larger electrode area and faster electron transfer rate, as compared with bare GC electrode. Furthermore, riboflavin was detected using OMC/GC electrode in aqueous solutions. The results showed that, under an optimum condition (pH 7.0), the OMC/GC electrode exhibited excellent response performance to riboflavin in the concentration range of 4.0 × 10−7 to 1.0 × 10−6 M with a high sensitivity of 769 μA mM−1. The detection limit was down to around 2 × 10−8 M. With good stability and reproducibility, the present OMC/GC electrode was applied in the determination of vitamin B2 content in vitamin tablets, and satisfactory results were obtained.  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