全文获取类型
收费全文 | 703篇 |
免费 | 11篇 |
国内免费 | 5篇 |
专业分类
化学 | 463篇 |
晶体学 | 8篇 |
力学 | 9篇 |
数学 | 73篇 |
物理学 | 166篇 |
出版年
2023年 | 4篇 |
2022年 | 32篇 |
2021年 | 27篇 |
2020年 | 11篇 |
2019年 | 21篇 |
2018年 | 11篇 |
2017年 | 10篇 |
2016年 | 20篇 |
2015年 | 17篇 |
2014年 | 39篇 |
2013年 | 30篇 |
2012年 | 42篇 |
2011年 | 48篇 |
2010年 | 38篇 |
2009年 | 32篇 |
2008年 | 34篇 |
2007年 | 37篇 |
2006年 | 42篇 |
2005年 | 26篇 |
2004年 | 33篇 |
2003年 | 16篇 |
2002年 | 20篇 |
2001年 | 8篇 |
2000年 | 10篇 |
1999年 | 14篇 |
1998年 | 6篇 |
1997年 | 15篇 |
1996年 | 7篇 |
1995年 | 8篇 |
1994年 | 14篇 |
1993年 | 8篇 |
1992年 | 7篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有719条查询结果,搜索用时 15 毫秒
701.
de Araújo ME Contesini FJ Franco YE Sawaya AC Alberto TG Dalfré N Carvalho Pde O 《Molecules (Basel, Switzerland)》2011,16(8):7171-7182
Response surface methodology (RSM) based on a five-level, three-variable central composite design (CCD) was employed for modeling and optimizing the conversion yield of the enzymatic acylation of hesperidin with decanoic acid using immobilized Candida antarctica lipase B (CALB) in a two-phase system containing [bmim]BF(4). The three variables studied (molar ratio of hesperidin to decanoic acid, [bmim]BF(4)/acetone ratio and lipase concentration) significantly affected the conversion yield of acylated hesperidin derivative. Verification experiments confirmed the validity of the predicted model. The lipase showed higher conversion degree in a two-phase system using [bmim]BF(4) and acetone compared to that in pure acetone. Under the optimal reaction conditions carried out in a single-step biocatalytic process when the water content was kept lower than 200 ppm, the maximum acylation yield was 53.6%. 相似文献
702.
703.
704.
Solvatochromic behavior of dyes with dimethylamino electron‐donor and nitro electron‐acceptor groups in their molecular structure 下载免费PDF全文
Carlos E. A. de Melo Leandro G. Nandi Moisés Domínguez Marcos C. Rezende Vanderlei G. Machado 《Journal of Physical Organic Chemistry》2015,28(4):250-260
Six dyes with N,N‐dimethylaminophenyl and 4‐nitrophenyl or 2,4‐dinitrophenyl groups in their molecular structures were prepared and characterized. These compounds have different conjugated bridges (C?C, C?N, and N?N) connecting the electron‐donor and the electron‐acceptor groups. All compounds are solvatochromic, with reverse solvatochromism occurring. The solvatochromic band observed in each spectrum for the dyes is due to a π ? π* transition, of an intramolecular charge transfer nature, which occurs from the electron‐donor N,N‐dimethylaminophenyl group to the electron‐acceptor group in the molecules, which is reinforced by the structures of the compounds optimized by applying density functional theory, which exhibit high planarity. The reverse solvatochromism was explained considering two resonance structures. The benzenoid form is better stabilized in less polar solvents and characterizes the region displaying positive solvatochromism, while the dipolar form is better stabilized in more polar solvents, in the region of negative solvatochromism. The Catalán multiparametric approach was used to study the contribution of solvent acidity, basicity, dipolarity, and polarizability to the solvatochromism exhibited by the compounds. These compounds are good candidates for the investigation of the polarizability and, to a lesser extent, the dipolarity of the medium, with very little interference from specific interactions of the solvent through hydrogen bonding. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
705.
María J. Tapia María Monteserín Hugh D. Burrows João S. Seixas de Melo Joan Estelrich 《Photochemistry and photobiology》2013,89(6):1471-1478
We have studied the effect of head group and alkyl chain length on β‐phase formation in poly(9,9‐dioctylfluorene) (PFO) solubilized in phospholipid liposomes. Systems studied have three different alkyl chain lengths (1,2‐dimyristoyl‐sn‐glycero‐3‐phosphatidylcholine [DMPC], 1,2‐didodecanoyl‐sn‐glycero‐3‐phosphatidylcholine [DLPC], 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphatidylcholine [DPPC]) and head groups (1,2‐dimyristoyl‐sn‐glycero‐3‐phosphate monosodium salt [DMPA], 1,2‐dimyristoyl‐sn‐glycero‐3‐phosphoethanolamine [DMPE] and 1,2‐dimyristoyl‐sn‐glycero‐3‐phospho‐l ‐serine sodium salt [DMPS]). Changes in liposome size upon addition of PFO are followed by dynamic light scattering. All the phospholipids induce the formation of PFO β‐phase, which is followed by the emission intensity and deconvolution of the absorption spectra. Both the head group and alkyl chain length affect the yield of β‐phase. The photophysics of PFO incorporated in liposomes is characterized by stationary and time‐resolved fluorescence, whereas the polymer‐phospholipid interactions have been studied by the effect of the PFO concentration on the phospholipid phase transitions (differential scanning calorimetry [DSC]). 相似文献
706.
707.
Stella Gomes Rodrigues Izabel de Souza Chaves Nathalie Ferreira Silva de Melo Marcelo Bispo de Jesus Leonardo Fernandes Fraceto S��rgio Ant?nio Fernandes Eneida de Paula Matheus Puggina de Freitas Luciana de Matos Alves Pinto 《Journal of inclusion phenomena and macrocyclic chemistry》2011,70(1-2):19-28
Schistosomiasis is still an endemic disease in many regions, with 250 million people infected with Schistosoma and about 500,000 deaths per year. Praziquantel (PZQ) is the drug of choice for schistosomiasis treatment, however it is classified as Class II in the Biopharmaceutics Classification System, as its low solubility hinders its performance in biological systems. The use of cyclodextrins is a useful tool to increase the solubility and bioavailability of drugs. The aim of this work was to prepare an inclusion compound of PZQ and methyl-??-cyclodextrin (MeCD), perform its physico-chemical characterization, and explore its in vitro cytotoxicity. SEM showed a change of the morphological characteristics of PZQ:MeCD crystals, and IR data supported this finding, with changes after interaction with MeCD including effects on the C?CH of the aromatic ring, observed at 758 cm?1. Differential scanning calorimetry measurements revealed that complexation occurred in a 1:1 molar ratio, as evidenced by the lack of a PZQ transition temperature after inclusion into the MeCD cavity. In solution, the PZQ UV spectrum profile in the presence of MeCD was comparable to the PZQ spectrum in a hydrophobic solvent. Phase solubility diagrams showed that there was a 5.5-fold increase in PZQ solubility, and were indicative of a type AL isotherm, that was used to determine an association constant (Ka) of 140.8 M?1. No cytotoxicity of the PZQ:MeCD inclusion compound was observed in tests using 3T3 cells. The results suggest that the association of PZQ with MeCD could be a good alternative for the treatment of schistosomiasis. 相似文献
708.
Optimization of the extraction conditions of the volatile compounds from chili peppers by headspace solid phase micro-extraction 总被引:2,自引:0,他引:2
Junior SB de Marchi Tavares de Melo A Zini CA Godoy HT 《Journal of chromatography. A》2011,1218(21):3345-3350
A method involving headspace-solid phase micro-extraction (HS-SPME), gas chromatography with flame ionization detection (GC-FID) and gas chromatography with mass spectrometry (GC-MS) was developed and optimized to investigate the volatile composition of Capsicum chili peppers. Five SPME fibers were tested for extraction: carboxen/polydimethylsiloxane (CAR/PDMS-75μm), polydimethylsiloxane (PDMS-100μm), divinylbenzene/polydimethylsiloxane (DVB/PDMS-65μm), carbowax/divinylbenzene (CW/DVB-70μm), and divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS-50/30μm), the last of which was shown to be the most efficient fiber to trap the volatile compounds. Optimization of the extraction conditions was carried out using multivariate strategies such as factorial design and response surface methodology. Eighty three compounds were identified by GC-MS when using the optimized extraction conditions, the majority of which were esters. 相似文献
709.
Drio Rodrigues do Nascimento Junior Antonio Tabernero Elaine Christine de Magalhes Cabral Albuquerque Silvio Alexandre Beisl Vieira de Melo 《Molecules (Basel, Switzerland)》2021,26(13)
As an alternative to synthetic pesticides, natural chemistries from living organisms, are not harmful to nontarget organisms and the environment, can be used as biopesticides, nontarget. However, to reduce the reactivity of active ingredients, avoid undesired reactions, protect from physical stress, and control or lower the release rate, encapsulation processes can be applied to biopesticides. In this review, the advantages and disadvantages of the most common encapsulation processes for biopesticides are discussed. The use of supercritical fluid technology (SFT), mainly carbon dioxide (CO2), to encapsulate biopesticides is highlighted, as they reduce the use of organic solvents, have simpler separation processes, and achieve high-purity particles. This review also presents challenges to be surpassed and the lack of application of SFT for biopesticides in the published literature is discussed to evaluate its potential and prospects. 相似文献
710.
M. A. S. Melo D. M. de-Paula J. P. M. Lima F. M. C. Borges C. Steiner-Oliveira M. Nobre-dos-Santos I. C. J. Zanin E. B. Barros L. K. A. Rodrigues 《Laser Physics》2010,20(6):1504-1513
The development of a method to ensure bacterial-free substrates without extensive cavity preparation would be highly useful
to dentistry, since there is no currently available effective method for killing residual bacteria in dentinal tissue. This
randomized in vitro study determined parameters for using toluidine blue O (TBO) with a light-emitting diode (LED) for dentine
caries disinfection and monitored intrapulpal/periodontal temperatures during irradiation. Occlusal human dentine slabs were
immersed in Streptococcus mutans culture for demineralization induction. Slabs were allocated to 10 groups (n = 15), which were treated with 0.1 mg ml−1 TBO with 5 min of incubation time or 0.9% NaCl solution for 5, 10 or 15 min, and submitted or not to irradiation for 5, 10
or 15 min (47, 94, and 144 J/cm2). Before and after treatments, dentine samples were analyzed with regard to S. mutans counts. In whole teeth, temperature in pulp and periodontium was measured by thermocouples during irradiation. Kruskal-Wallis/Student-Newman-Keuls,
and ANOVA/Tukey test were respectively utilized to compare log reductions and temperature rises between groups. Bacterial
reduction was observed when dentine was exposed to both TBO and LED at all irradiation times, as well as to LED alone for
10 and 15 min. Temperature increases lower than 2°C were observed for either pulp or periodontium. Concluding, LED combined
with TBO is a safe and effective approach for dentine caries disinfection. Nevertheless, additional studies should be conducted
to determine the influence of the irradiation in S. mutans viability in dentinal surface/tubules. 相似文献