全文获取类型
收费全文 | 256篇 |
免费 | 16篇 |
专业分类
电工技术 | 7篇 |
综合类 | 2篇 |
化学工业 | 96篇 |
机械仪表 | 5篇 |
建筑科学 | 9篇 |
矿业工程 | 3篇 |
能源动力 | 3篇 |
轻工业 | 48篇 |
水利工程 | 5篇 |
石油天然气 | 2篇 |
无线电 | 11篇 |
一般工业技术 | 35篇 |
冶金工业 | 13篇 |
原子能技术 | 3篇 |
自动化技术 | 30篇 |
出版年
2023年 | 4篇 |
2022年 | 11篇 |
2021年 | 20篇 |
2020年 | 6篇 |
2019年 | 11篇 |
2018年 | 6篇 |
2017年 | 8篇 |
2016年 | 8篇 |
2015年 | 10篇 |
2014年 | 10篇 |
2013年 | 23篇 |
2012年 | 30篇 |
2011年 | 20篇 |
2010年 | 9篇 |
2009年 | 11篇 |
2008年 | 11篇 |
2007年 | 15篇 |
2006年 | 10篇 |
2005年 | 12篇 |
2004年 | 8篇 |
2003年 | 6篇 |
2002年 | 7篇 |
2001年 | 2篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1982年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有272条查询结果,搜索用时 31 毫秒
261.
In this article, new tests for non‐parametric hypotheses in stationary processes are proposed. Our approach is based on an estimate of the L2‐distance between the spectral density matrix and its best approximation under the null hypothesis. We explain the main idea in the problem of testing for a constant spectral density matrix and in the problem of comparing the spectral densities of several correlated stationary time series. The method is based on direct estimation of integrals of the spectral density matrix and does not require the specification of smoothing parameters. We show that the limit distribution of the proposed test statistic is normal and investigate the finite sample properties of the resulting tests by means of a small simulation study. 相似文献
262.
Savić TD Janković IA Šaponjić ZV Čomor MI Veljković DŽ Zarić SD Nedeljković JM 《Nanoscale》2012,4(5):1612-1619
Surface modification of nanocrystalline TiO(2) particles (45 ?) with catecholate-type ligands consisting of an extended aromatic ring system, i.e., 2,3-dihydroxynaphthalene and anthrarobin, was found to alter the optical properties of the nanoparticles in a similar way to modification with catechol. The formation of inner-sphere charge-transfer (CT) complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites and the reduction of the band gap upon the increase of the electron delocalization on the inclusion of additional rings. The binding structures were investigated by FTIR spectroscopy. The investigated ligands have the optimal geometry for binding to surface Ti atoms, resulting in ring coordination complexes of catecholate type (binuclear bidentate binding-bridging) thus restoring the six-coordinated octahedral geometry of surface Ti atoms. From the Benesi-Hildebrand plot, stability constants in methanol/water = 90/10 solutions at pH 2 of the order 10(3) M(-1) have been determined. Quantum chemical calculations on model systems using density functional theory (DFT) were performed to obtain vibrational frequencies of charge transfer complexes, and the calculated values were compared with the experimental data. 相似文献
263.
Rosmarinic Acid Levels,Phenolic Contents,Antioxidant and Antimicrobial Activities of the Extracts from Salvia verbenaca L. Obtained with Different Solvents and Procedures
下载免费PDF全文
![点击此处可从《Journal of Food Biochemistry》网站下载免费的PDF全文](/ch/ext_images/free.gif)
264.
Abstract: The aim of this study is to investigate in vitro antioxidant, antimicrobial, and anticancer activity of the acetone extracts of the lichens Umbilicaria crustulosa, U. cylindrica, and U. polyphylla. Antioxidant activity was evaluated by 5 separate methods: free radical scavenging, superoxide anion radical scavenging, reducing power, determination of total phenolic compounds, and determination of total flavonoid content. Of the lichens tested, U. polyphylla had largest free radical scavenging activity (72.79% inhibition at a concentration of 1 mg/mL), which was similar as standard antioxidants in the same concentration. Moreover, the tested extracts had effective reducing power and superoxide anion radical scavenging. Total content of phenol and flavonoid in extracts was determined as pyrocatechol equivalent, and as rutin equivalent, respectively. The strong relationships between total phenolic and flavonoid contents and the antioxidant effect of tested extracts were observed. The antimicrobial activity was estimated by determination of the minimal inhibitory concentration by the broth microdilution method. The most active was extract of U. polyphylla with minimum inhibitory concentration values ranging from 1.56 to 12.5 mg/mL. Anticancer activity was tested against FemX (human melanoma) and LS174 (human colon carcinoma) cell lines using MTT method. All extracts were found to be strong anticancer activity toward both cell lines with IC50 values ranging from 28.45 to 97.82 μg/mL. The present study shows that tested lichen extracts demonstrated a strong antioxidant, antimicrobial, and anticancer effects. That suggests that lichens may be used as possible natural antioxidant, antimicrobial, and anticancer agents. 相似文献
265.
Radovanović AN Jovančićević BS Radovanović BC Mihajilov-Krstev T Zvezdanović JB 《Journal of the science of food and agriculture》2012,92(10):2154-2161
BACKGROUND: Antioxidant and antimicrobial potentials of Serbian red wines produced from different international Vitis vinifera grape varieties and their correlation with contents of phenolic compounds were studied by spectrophotometric and chromatographic methods. The antioxidant activity of red wines was estimated through their ability to scavenge 2,2′‐diphenyl‐1‐picrylhydrazyl free radical (DPPH?). The red wines, gallic acid, (+)‐catechin and quercetin were screened in vitro for antimicrobial activity against Gram‐positive and Gram‐negative strains using microdilution and disc diffusion techniques. RESULTS: Excellent correlations between the contents of quercetin‐3‐glucoside (R2 = 0.9463) and quercetin (R2 = 0.9337) and DPPH?‐scavenging ability of the red wines were found. Serbian red wines exhibited significant activity against Staphylococcus aureus, Listeria inocua, Micrococcus flavus, Sarcina lutea, Bacillus subtilis, Escherichia coli, Pseudomonas aeruginosa, Salmonella enteritidis and Shigella sonnei strains, which was in correlation with their phenolic composition and antioxidant activity. The compounds gallic acid, quercetin and (+)‐catechin showed high activity against B. subtilis, S. aureus, S. lutea and M. flavus Gram‐positive and S. enteritidis and P. aeruginosa Gram‐negative strains. CONCLUSION: The results show that quercetin‐3‐glucoside and quercetin concentrations can be used as markers for the determination of antioxidant and antimicrobial potentials of red wines. Copyright © 2012 Society of Chemical Industry 相似文献
266.
Markus Greule Luisa Dana Tumino Tatjana Kronewald Uwe Hener Jürgen Schleucher Armin Mosandl Frank Keppler 《European Food Research and Technology》2010,231(6):933-941
Vanilla still remains one of the most important and widely used flavours in the food industry and is also extensively employed
by fragrance and pharmaceutical manufacturing companies. Natural vanilla flavour, extracted from the pods of the tropic orchid
vanilla, is considerably more expensive than synthetic vanillin. The disparity of prices between natural vanillin and that derived
from other sources has given rise to many cases of fraudulent adulteration, and for more than 30 years, strenuous efforts
have been made to authenticate sources of vanillin. Stable isotope analysis is one of the most powerful analytical tools to
distinguish between natural vanillin and that originating from other sources. Recently, a rapid and precise method for analysis
of both δ13C and δ2H values of plant methoxyl groups has been published. Here, we report an application of the method for the control of authenticity
of vanillin. Carbon and hydrogen stable isotope values of the vanillin molecule and vanillin methoxyl groups of vanillin samples
of different origins including authentic and synthetic samples were measured. The results clearly show that use of this approach
provides a rapid and reliable authenticity assessment of vanillin. The technique used for these studies is robust and rapid,
involves minimum sample preparation and requires only a small amount of vanillin sample, usually 1 mg for stable carbon and
4 mg for stable hydrogen analysis. 相似文献
267.
Marijana Kosanić Branislav Ranković Tatjana Stanojković Aleksandar Rančić Nedeljko Manojlović 《LWT》2014
The aim of this study is to investigate in vitro antioxidant, antimicrobial, and anticancer activities of the acetone extracts of the lichens Cladonia furcata, Cladonia pyxidata and Cladonia rangiferina and their atranorin and fumarprotocetraric acid constituents. Antioxidant activity was evaluated by free radical scavenging, superoxide anion radical scavenging, reducing power, and determination of total phenolic and flavonoid compounds. As a result of the study atranorin had largest free radical scavenging activity with IC50 values 131.48 μg/mL. Moreover, the tested samples had effective reducing power and superoxide anion radical scavenging. Total content of phenol and flavonoid in extracts was determined as pyrocatechol equivalent, and as rutin equivalent, respectively. The strong relationships between total phenolic and flavonoid contents and the antioxidant effect of tested extracts were observed. The antimicrobial activity was estimated by determination of the minimal inhibitory concentration by the broth microdilution method. The most active was fumarprotocetraric acid with minimum inhibitory concentration values ranging from 0.031 to 0.125 mg/mL. Anticancer activity was tested against FemX (human melanoma) and LS174 (human colon carcinoma) cell lines using MTT method. All samples were found to be strong anticancer activity toward both cell lines with IC50 values ranging from 10.97 to 41.23 μg/mL. 相似文献
268.
Ricarda Freke Dr. Björn Heinemann Samuel Edward Hakim Prof. Dr. Claus-Peter Witte Dr. Marco Herde Prof. Dr. Tatjana M. Hildebrandt Prof. Dr. Jakob Franke 《Chembiochem : a European journal of chemical biology》2023,24(10):e202300056
Plants of the genus Allium such as chives, onions or garlic produce S-alk(en)yl cysteine sulfoxides as flavor precursors. Two major representatives are S-propenyl cysteine sulfoxide (isoalliin) and S-propyl cysteine sulfoxide (propiin), which only differ by a double bond in the C3 side chain. The propenyl group of isoalliin is derived from the amino acid valine, but the source of the propyl group of propiin remains unclear. Here, we present an untargeted metabolomics approach in seedlings of chives (Allium schoenoprasum) to track mass features containing sulfur and/or 13C from labeling experiments with valine-13C5 guided by their isotope signatures. Our data show that propiin and related propyl-bearing metabolites incorporate carbon derived from valine-13C5, but to a much lesser extent than isoalliin and related propenyl compounds. Our findings provide new insights into the biosynthetic pathways of flavor precursors in Allium species and open new avenues for future untargeted labeling experiments. 相似文献
269.
Tatjana Lang Antonella Di Pizio Davide Risso Dennis Drayna Maik Behrens 《Molecular nutrition & food research》2023,67(11):2200775
Scope
To avoid ingestion of potentially harmful substances, humans are equipped with about 25 bitter taste receptor genes (TAS2R) expressed in oral taste cells. Humans exhibit considerable variance in their bitter tasting abilities, which are associated with genetic polymorphisms in bitter taste receptor genes. One of these variant receptor genes, TAS2R2, is initially believed to represent a pseudogene. However, TAS2R2 exists in a putative functional variant within some populations and can therefore be considered as an additional functional bitter taste receptor.Methods and results
To learn more about the function of the experimentally neglected TAS2R2, a functional screening with 122 bitter compounds is performed. The study observes responses with eight of the 122 bitter substances and identifies the substance phenylbutazone as a unique activator of TAS2R2 among the family of TAS2Rs, thus filling one more gap in the array of cognate bitter substances.Conclusions
The comprehensive characterization of the receptive range of TAS2R2 allows the classification into the group of TAS2Rs with a medium number of bitter agonists. The variability of bitter taste and its potential influences on food choice in some human populations may be even higher than assumed. 相似文献270.
Tatjana Schamber Oliver Binas Dr. Andreas Schlundt Anna Wacker Prof. Dr. Harald Schwalbe 《Chembiochem : a European journal of chemical biology》2022,23(3):e202100564
Riboswitches are regulatory RNA elements that undergo functionally important allosteric conformational switching upon binding of specific ligands. The here investigated guanidine-II riboswitch binds the small cation, guanidinium, and forms a kissing loop-loop interaction between its P1 and P2 hairpins. We investigated the structural changes to support previous studies regarding the binding mechanism. Using NMR spectroscopy, we confirmed the structure as observed in crystal structures and we characterized the kissing loop interaction upon addition of Mg2+ and ligand for the riboswitch aptamer from Escherichia coli. We further investigated closely related mutant constructs providing further insight into functional differences between the two (different) hairpins P1 and P2. Formation of intermolecular interactions were probed by small-angle X-ray scattering (SAXS) and NMR DOSY data. All data are consistent and show the formation of oligomeric states of the riboswitch induced by Mg2+ and ligand binding. 相似文献