全文获取类型
收费全文 | 18020篇 |
免费 | 1658篇 |
国内免费 | 866篇 |
专业分类
化学 | 3710篇 |
晶体学 | 85篇 |
力学 | 7627篇 |
综合类 | 169篇 |
数学 | 3741篇 |
物理学 | 5212篇 |
出版年
2024年 | 8篇 |
2023年 | 164篇 |
2022年 | 242篇 |
2021年 | 413篇 |
2020年 | 490篇 |
2019年 | 379篇 |
2018年 | 424篇 |
2017年 | 533篇 |
2016年 | 651篇 |
2015年 | 592篇 |
2014年 | 753篇 |
2013年 | 1294篇 |
2012年 | 924篇 |
2011年 | 1139篇 |
2010年 | 843篇 |
2009年 | 1078篇 |
2008年 | 1037篇 |
2007年 | 1033篇 |
2006年 | 941篇 |
2005年 | 813篇 |
2004年 | 843篇 |
2003年 | 703篇 |
2002年 | 638篇 |
2001年 | 475篇 |
2000年 | 433篇 |
1999年 | 350篇 |
1998年 | 371篇 |
1997年 | 348篇 |
1996年 | 323篇 |
1995年 | 323篇 |
1994年 | 292篇 |
1993年 | 248篇 |
1992年 | 236篇 |
1991年 | 188篇 |
1990年 | 166篇 |
1989年 | 131篇 |
1988年 | 128篇 |
1987年 | 89篇 |
1986年 | 86篇 |
1985年 | 98篇 |
1984年 | 85篇 |
1983年 | 41篇 |
1982年 | 87篇 |
1981年 | 24篇 |
1980年 | 17篇 |
1979年 | 16篇 |
1978年 | 15篇 |
1976年 | 7篇 |
1971年 | 7篇 |
1957年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
Immunoglobulins in bovine colostrum were separated and fractionated from other proteins using the method and instrumentation developed in our laboratory. The proposed separation was based on bidirectional isotachophoresis/moving boundary electrophoresis with electrofocusing of the analytes in a pH gradient from 3.9 to 10.1. The preparative instrumentation included the trapezoidal non-woven fabric that served as separation space with divergent continuous flow. The defatted and casein precipitate-free colostrum supernatant was loaded directly into the instrument without any additional colostrum pre-preparation. Immunoglobulin G was fractionated from other immune proteins such as bovine serum albumin, β-lactoglobulin, and α-lactalbumin, and was continuously collected in separated fractions over 3 h. The fractions were further processed, and isolated immunoglobulin G in the liquid fractions was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by re-focusing in gel isoelectric focusing. Separated immunoglobulin G was detected in seven fractions by sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a gradually decreased concentration in the fractions. Re-focusing of the proteins in the fractions by gel isoelectric focusing revealed multiple separated zones of immunoglobulin G with the isoelectric point values covering the range from 5.4 to 7.2. Each fraction contained distinct zones with gradually increased isoelectric point values and decreased concentrations from fraction to fraction. 相似文献
82.
建立了一种快速、灵敏且高精度的流动注射-分光光度法用于研究天然产物对酪氨酸酶活性的影响。此方法基于一定介质条件下L-3,4-二羟基苯丙氨酸(L-DOPA)在酪氨酸酶的作用下,发生氧化生成棕褐色多巴醌,其最大吸收峰位于475 nm处,而酪氨酸酶抑制剂能降低酶促反应速度,减少多巴醌形成的量,从而降低475 nm下的吸光度值,形成倒峰。在优化实验条件下,半抑制浓度的曲酸抑制酪氨酸酶活性测定相对标准偏差(n=10)为0.036%,与分光光度法和酶标仪微量法相比,精密度提高了10倍以上。咖啡酸对酪氨酸酶有较强的激活作用,对二酚酶相对激活率达到50%时的浓度(IC50)为0.170 mmol/L;薰衣草花水相提取物具有较强的抑制活性,其IC50值为0.96 mg/m L。与分光光度法及酶标仪微量法相比,流动注射-分光光度法精密度高,重复性好,适用于天然产物对酪氨酸酶活性影响的测定。 相似文献
83.
84.
A Three‐Minute Synthesis and Purification of Ibuprofen: Pushing the Limits of Continuous‐Flow Processing 下载免费PDF全文
Dr. David R. Snead Prof. Dr. Timothy F. Jamison 《Angewandte Chemie (International ed. in English)》2015,54(3):983-987
In a total residence time of three minutes, ibuprofen was assembled from its elementary building blocks with an average yield of above 90 % for each step. A scale‐up of this five‐stage process (3 bond‐forming steps, one work‐up, and one in‐line liquid–liquid separation) provided ibuprofen at a rate of 8.09 g h?1 (equivalent to 70.8 kg y?1) using a system with an overall footprint of half the size of a standard laboratory fume hood. Aside from the high throughput, several other aspects of this synthesis expand the capabilities of continuous‐flow processing, including a Friedel–Crafts acylation run under neat conditions and promoted by AlCl3, an exothermic in‐line quench of high concentrations of precipitation‐prone AlCl3, liquid–liquid separations run at or above 200 psi to provide solvent‐free product, and the use of highly aggressive oxidants, such as iodine monochloride. The use of simple, inexpensive, and readily available reagents thus affords a practical synthesis of this important generic pharmaceutical. 相似文献
85.
Rapid and Efficient Copper‐Catalyzed Finkelstein Reaction of (Hetero)Aromatics under Continuous‐Flow Conditions 下载免费PDF全文
Dr. Mao Chen Saki Ichikawa Prof. Dr. Stephen L. Buchwald 《Angewandte Chemie (International ed. in English)》2015,54(1):263-266
A general, rapid, and efficient method for the copper‐catalyzed Finkelstein reaction of (hetero)aromatics has been developed using continuous flow to generate a variety of aryl iodides. The described method can tolerate a broad spectrum of functional groups, including N‐H and O‐H groups. Additionally, in lieu of isolation, the aryl iodide solutions were used in two distinct multistep continuous‐flow processes (amidation and Mg–I exchange/nucleophilic addition) to demonstrate the flexibility of this method. 相似文献
86.
Effects of GC temperature and carrier gas flow rate on on‐line oxygen isotope measurement as studied by on‐column CO injection 下载免费PDF全文
Although deemed important to δ18O measurement by on‐line high‐temperature conversion techniques, how the GC conditions affect δ18O measurement is rarely examined adequately. We therefore directly injected different volumes of CO or CO–N2 mix onto the GC column by a six‐port valve and examined the CO yield, CO peak shape, CO–N2 separation, and δ18O value under different GC temperatures and carrier gas flow rates. The results show the CO peak area decreases when the carrier gas flow rate increases. The GC temperature has no effect on peak area. The peak width increases with the increase of CO injection volume but decreases with the increase of GC temperature and carrier gas flow rate. The peak intensity increases with the increase of GC temperature and CO injection volume but decreases with the increase of carrier gas flow rate. The peak separation time between N2 and CO decreases with an increase of GC temperature and carrier gas flow rate. δ18O value decreases with the increase of CO injection volume (when half m/z 28 intensity is <3 V) and GC temperature but is insensitive to carrier gas flow rate. On average, the δ18O value of the injected CO is about 1‰ higher than that of identical reference CO. The δ18O distribution pattern of the injected CO is probably a combined result of ion source nonlinearity and preferential loss of C16O or oxygen isotopic exchange between zeolite and CO. For practical application, a lower carrier gas flow rate is therefore recommended as it has the combined advantages of higher CO yield, better N2–CO separation, lower He consumption, and insignificant effect on δ18O value, while a higher‐than‐60 °C GC temperature and a larger‐than‐100 µl CO volume is also recommended. When no N2 peak is expected, a higher GC temperature is recommended, and vice versa. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
87.
Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All‐Organic Redox Flow Battery 下载免费PDF全文
Dr. Xiaoliang Wei Dr. Wu Xu Dr. Jinhua Huang Dr. Lu Zhang Dr. Eric Walter Dr. Chad Lawrence Dr. M. Vijayakumar Dr. Wesley A. Henderson Dr. Tianbiao Liu Dr. Lelia Cosimbescu Dr. Bin Li Dr. Vincent Sprenkle Dr. Wei Wang 《Angewandte Chemie (International ed. in English)》2015,54(30):8684-8687
Nonaqueous redox flow batteries hold the promise of achieving higher energy density because of the broader voltage window than aqueous systems, but their current performance is limited by low redox material concentration, cell efficiency, cycling stability, and current density. We report a new nonaqueous all‐organic flow battery based on high concentrations of redox materials, which shows significant, comprehensive improvement in flow battery performance. A mechanistic electron spin resonance study reveals that the choice of supporting electrolytes greatly affects the chemical stability of the charged radical species especially the negative side radical anion, which dominates the cycling stability of these flow cells. This finding not only increases our fundamental understanding of performance degradation in flow batteries using radical‐based redox species, but also offers insights toward rational electrolyte optimization for improving the cycling stability of these flow batteries. 相似文献
88.
Gabriela Furlan Giordano Luis Carlos Silveira Vieira Angelo Luiz Gobbi Renato Sousa Lima Lauro Tatsuo Kubota 《Analytica chimica acta》2015
An integrated platform was developed for point-of-use determination of ethanol in sugar cane fermentation broths. Such analysis is important because ethanol reduces its fuel production efficiency by altering the alcoholic fermentation step when in excess. The custom-designed platform integrates gas diffusion separation with voltammetric detection in a single analysis module. The detector relied on a Ni(OH)2-modified electrode. It was stabilized by uniformly depositing cobalt and cadmium hydroxides as shown by XPS measurements. Such tests were in accordance with the hypothesis related to stabilization of the Ni(OH)2 structure by insertion of Co2+ and Cd2+ ions in this structure. The separation step, in turn, was based on a hydrophobic PTFE membrane, which separates the sample from receptor solution (electrolyte) where the electrodes were placed. Parameters of limit of detection and analytical sensitivity were estimated to be 0.2% v/v and 2.90 μA % (v/v)−1, respectively. Samples of fermentation broth were analyzed by both standard addition method and direct interpolation in saline medium based-analytical curve. In this case, the saline solution exhibited ionic strength similar to those of the samples intended to surpass the tonometry colligative effect of the samples over analyte concentration data by attributing the reduction in quantity of diffused ethanol vapor majorly to the electrolyte. The approach of analytical curve provided rapid, simple and accurate analysis, thus contributing for deployment of point-of-use technologies. All of the results were accurate with respect to those obtained by FTIR method at 95% confidence level. 相似文献
89.
Minghui Chen Zhibiao Yu Daofeng Liu Tao Peng Kun Liu Shuying Wang Yonghua Xiong Hua Wei Hengyi Xu Weihua Lai 《Analytica chimica acta》2015
Two patterns of signal amplification lateral flow immunoassay (LFIA), which used anti-mouse secondary antibody-linked gold nanoparticle (AuNP) for dual AuNP-LFIA were developed. Escherichia coli O157:H7 was selected as the model analyte. In the signal amplification direct LFIA method, anti-mouse secondary antibody-linked AuNP (anti-mouse-Ab-AuNP) was mixed with sample solution in an ELISA well, after which it was added to LFIA, which already contained anti-E. coli O157:H7 monoclonal antibody-AuNP (anti-E. coli O157:H7-mAb-AuNP) dispersed in the conjugate pad. Polyclonal antibody was the test line, and anti-mouse secondary antibody was the control line in nitrocellulose (NC) membrane. In the signal amplification indirect LFIA method, anti-mouse-Ab-AuNP was mixed with sample solution and anti-E. coli O157:H7-mAb-AuNP complex in ELISA well, creating a dual AuNP complex. This complex was added to LFIA, which had a polyclonal antibody as the test line and secondary antibody as the control line in NC membrane. The detection sensitivity of both LFIAs improved 100-fold and reached 1.14 × 103 CFU mL−1. The 28 nm and 45 nm AuNPs were demonstrated to be the optimal dual AuNP pairs. Signal amplification LFIA was perfectly applied to the detection of milk samples with E. coli O157:H7 via naked eye observation. 相似文献
90.
Human pluripotent stem cells (hPSCs), such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), provide a powerful model system for studies of cellular identity and early mammalian development, which hold great promise for regenerative medicine. It is necessary to develop a convenient method to discriminate hPSCs from other cells in clinics and basic research. Herein, a simple and reliable biosensor for stem cell detection was established. In this biosensor system, stage-specific embryonic antigen-3 (SSEA-3) and stage-specific embryonic antigen-4 (SSEA-4) were used to mark human pluripotent stem cells (hPSCs). Antibody specific for SSEA-3 was coated onto magnetic beads for hPSCs enrichment, and antibody specific for SSEA-4 was conjugated with carboxyl-modified tDNA sequence which was used as template for strand displacement amplification (SDA). The amplified single strand DNA (ssDNA) was detected with a lateral flow biosensor (LFB). This biosensor is capable of detecting a minimum of 19 human embryonic stem cells by a strip reader and 100 human embryonic stem cells by the naked eye within 80 min. This approach has also shown excellent specificity to distinguish hPSCs from other types of cells, showing that it is promising for specific and handy detection of human pluripotent stem cells. 相似文献