全文获取类型
收费全文 | 1765篇 |
免费 | 36篇 |
国内免费 | 8篇 |
专业分类
化学 | 1413篇 |
晶体学 | 14篇 |
力学 | 25篇 |
数学 | 220篇 |
物理学 | 137篇 |
出版年
2023年 | 15篇 |
2022年 | 36篇 |
2021年 | 42篇 |
2020年 | 44篇 |
2019年 | 34篇 |
2018年 | 28篇 |
2017年 | 21篇 |
2016年 | 58篇 |
2015年 | 44篇 |
2014年 | 51篇 |
2013年 | 95篇 |
2012年 | 137篇 |
2011年 | 115篇 |
2010年 | 67篇 |
2009年 | 46篇 |
2008年 | 124篇 |
2007年 | 138篇 |
2006年 | 112篇 |
2005年 | 110篇 |
2004年 | 86篇 |
2003年 | 77篇 |
2002年 | 72篇 |
2001年 | 20篇 |
2000年 | 17篇 |
1999年 | 13篇 |
1998年 | 20篇 |
1997年 | 20篇 |
1996年 | 27篇 |
1995年 | 6篇 |
1994年 | 9篇 |
1993年 | 11篇 |
1992年 | 13篇 |
1991年 | 8篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1985年 | 11篇 |
1984年 | 13篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 7篇 |
1973年 | 3篇 |
1968年 | 1篇 |
1937年 | 1篇 |
排序方式: 共有1809条查询结果,搜索用时 31 毫秒
991.
Reynolds AJ Haines AH Russell DA 《Langmuir : the ACS journal of surfaces and colloids》2006,22(3):1156-1163
To mimic and measure calcium ion-mediated carbohydrate-carbohydrate interactions, four lactose derivatives have been synthesized for assembly on gold nanoparticles. The series of lactose derivatives varied by the length of the thiolated ethylene glycol anchor chain [O(CH2CH2O)(m)CH2CH2SH; where m = 0, 1, 2, and 3] used to self-assemble the carbohydrates to the preformed gold nanoparticles of ca. 16 nm diameter. Upon addition of calcium ions to the lactose-stabilized nanoparticles, rapid carbohydrate-carbohydrate interactions were visualized and subsequently measured using UV-visible spectrometry and transmission electron microscopy (TEM). The nanoparticle aggregates formed via metal-mediated carbohydrate-carbohydrate interactions could be readily redispersed through the addition of EDTA. Multiple reaggregation and redispersion cycles were achieved, confirming that the aggregation process was due to metal ion-mediated carbohydrate interactions rather than calcium chelation by residual citrate ions on the particle surface. The essential involvement of the lactose moiety in Ca2+ complexation was shown by control measurements on related D-glucose-derivatized nanoparticles, where a significantly reduced aggregation response was obtained only at high ion concentrations. Other group 2 metal ions with radii larger than that of calcium, viz., barium and strontium, were also shown to mediate the aggregation of the lactose-stabilized nanoparticles. The induced aggregation of the lactose nanoparticles was determined to be quantitatively dependent upon the calcium ion concentration. Furthermore, the analytical sensitivity of the calcium-induced aggregation and the linear dynamic range were dependent on the length of the ethylene glycol anchor chain. The shortest ethylene glycol chain (m = 0) gave the most sensitive response with the optimum limit of detection (0.8 mM Ca2+), whereas the longest ethylene glycol chain (m = 3) provides a measurement of calcium ion concentration over the largest linear dynamic range (10-35 mM Ca2+). This work has shown that the self-assembled deposition of lactose derivatives on gold nanoparticles provides multivalent carbohydrate surfaces that can be used as mimics for the measurement of biologically relevant carbohydrate-carbohydrate interactions. Additionally, this study has highlighted the importance of the structure and length of the ligand that anchors the carbohydrate sugar to the gold particle surface to facilitate such carbohydrate interactions and for "tuning" the analytical characteristics of bioassays developed using metal nanoparticle technology. 相似文献
992.
A graph of order n is said to be pancyclic if it contains cycles of all lengths from three to n. Let G be a Hamiltonian graph and let x and y be vertices of G that are consecutive on some Hamiltonian cycle in G. Hakimi and Schmeichel showed (J Combin Theory Ser B 45:99–107, 1988) that if d(x) + d(y) ≥ n then either G is pancyclic, G has cycles of all lengths except n − 1 or G is isomorphic to a complete bipartite graph. In this paper, we study the existence of cycles of various lengths in a Hamiltonian
graph G given the existence of a pair of vertices that have a high degree sum but are not adjacent on any Hamiltonian cycle in G. 相似文献
993.
994.
Vito Rizzi Pinalysa Cosma Ramona Abbattista Paola Fini Angela Agostiano Tommaso R.I. Cataldi Ilario Losito 《Journal of mass spectrometry : JMS》2019,54(5):389-401
The reactivity of the sulfur‐containing nucleoside 4‐thio‐(2′‐deoxy)‐thymidine usually abbreviated as 4‐thio‐thymidine, (S4‐TdR) under Fenton conditions, ie, in the presence of H2O2 and catalytic amounts of Fe(II), was investigated by UV‐vis spectroscopy and electrospray ionization single and tandem mass spectrometry (ESI‐MS and MS/MS). S4‐TdR hydroxylated on the S atom was found to be a key reaction intermediate, ultimately leading to (2′‐deoxy)‐thymidine usually abbreviated as thymidine, (TdR) as the main reaction product. This finding was in accordance with the outcome of the reaction between S4‐TdR and H2O2, previously investigated in our laboratory. On the other hand, the additional presence of ?OH radicals, induced by the Fe(II)/H2O2 combination, led to the increased generation of another interesting S4‐TdR product, already observed after its reaction with H2O2 alone, ie, the covalent dimer including a S? S bridge between two S4‐TdR molecules. More importantly, multihydroxylated derivatives of S4‐TdR and TdR were detected as peculiar products obtained under Fenton conditions. Among them, a product bearing an OH group both on the methyl group linked to the thymine ring and on the C5 atom of the ring was found to prevail. The results obtained during this study, integrated by those found previously in our laboratory, indicate 4‐thiothymidine as a promising molecular probe for the recognition, through a careful characterization of its reaction products, of the prevailing species among reactive oxygen species (ROS) corresponding to singlet‐state oxygen, hydrogen peroxide, and hydroxylic radical. 相似文献
995.
Dr. Tiago Zaminelli Dr. Elisa Magli Prof. Francesco Frecentese Dr. Caroline H. Lescano Dr. Rafael Campos Dr. Irene Saccone Dr. Angela Corvino Dr. Paola Di Vaio Dr. Flavia Giordano Dr. Paolo Luciano Prof. Ferdinando Fiorino Prof. Elisa Perissutti Prof. Vincenzo Santagada Prof. Beatrice Severino Prof. Giuseppe Caliendo Prof. Gilberto De Nucci 《ChemistryOpen》2019,8(4):464-475
The increased levels of cyclic nucleotides (cGMP and cAMP) in enterocytes trigger intracellular mechanisms of ion and fluid secretion into the lumen, causing secretory diarrhea. Twelve novel pyridopyrimidines derived from 5-(3,5-bistrifluoromethylphenyl)-1,3-dimethyl-5,11-dihydro-1H-indeno[2,1 : 5,6]pyrido[2,3-d]pyrimidine-2,4,6-trione (FPIPP) were synthesized and evaluated on intracellular cyclic nucleotide accumulation. All compounds had no effect on either cyclic nucleotide basal levels or on pre-contracted aortic rings. The metabolic activity and viability in T84 cells, assessed by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) and the LDH (lactate dehydrogenase) assays, respectively, were not affected by incubation with the compounds (50 μM). Compound VI almost abolished cGMP accumulation (94 % inhibition) induced by STa toxin in T834 cells and significantly reduced (69 %) forskolin-induced cAMP accumulation in Jurkat cells. Compound VI was active in an in vivo model for diarrhea in rabbits. These results prompted us to perform a microscopic histopathological analysis of intestinal tissues, showing that only compound VI preserves the intestine without significant pathological changes and with a decreased inflammatory pattern in comparison to FPIPP. In vitro stability test revealed that compound VI is resistant to oxidation promoted by atmospheric oxygen. 相似文献
996.
Michael Schneider Thomas Ballweg Lennart Groß Carsten Gellermann Angela Sanchez‐Sanchez Vanessa Fierro Alain Celzard Karl Mandel 《Particle & Particle Systems Characterization》2019,36(6)
Herein, the development of a novel composite particle system prepared from superparamagnetic Fe3O4@SiO2 microparticles as core and coated with a carbon shell is presented. The coating with carbon is done by pyrolyzing furan resin, which was previously deposited on the particle surface by poly‐merization of furfuryl alcohol. The novel composite material thus combines magnetic and sorptive properties. Upon pyrolysis the magnetic material is converted from magnetite to α‐iron and fayalite, changing its behavior from superparamagnetic to ferromagnetic with nearly zero remanence. The magnetic properties can be utilized to magnetically collect the particles dispersed in fluids, whereas the sorptive properties of the carbon shell can be used to remove organic contaminants from these fluids. The adsorption behavior of the composite particles and a potential electrochemical regeneration route are investigated, using the model dye methylene blue. 相似文献
997.
Angela Vasilescu L. Craciun O. Constantinescu Catalina Chiojdeanu A. Kiss P.M. Racolta 《Applied Surface Science》2008,255(1):46-49
A new positron gun (PG) will enable high sensitivity measurements in applications of positron annihilation spectroscopy in Romania. Some data concerning the design of a modular system for focussing, transport and acceleration of mono-energetic positrons in the range 0.8-50 keV have been obtained and experimenting on moderators and CDBS was performed. We present a short overview of the present status of the project and preliminary results from Coincidence Doppler Broadening Spectroscopy with a 22NaCl source, on Al samples. The entire positron gun system will be designed as a high-vacuum dedicated extension operating with two options: a 50 mCi 22NaCl source and in-line with the NIPNE cyclotron or a new intense compact cyclotron. 相似文献
998.
Guy Boillat Angela Giannone 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》1989,40(2):285-289
In a non linear electromagnetic isotropic medium the wave, radial and shock velocities do not exceed light speed iff some simple conditions are satisfied.
Sunto Si danno condizioni necessarie e sufficienti par la limitatezza delle vélocitá d'onda, radiale e d'urto nei materiali elettromagnetici isotropi non lineari.相似文献
999.
1000.
Bengt Mattson Anna Kron Torbjrn Reitberger Angela Y. Craig Ronald H. Fleming 《Polymer Testing》1992,11(5):357-372
A chemiluminescence (CL) instrument designed for the imaging of degradation processes is described and applications in polymer technology are given. The possibility of assessing heterogeneously distributed ageing effect is shown to be an important advantage of imaging-CL compared to ordinary non-imaging CL techniques. 相似文献