全文获取类型
收费全文 | 1176篇 |
免费 | 22篇 |
国内免费 | 4篇 |
专业分类
化学 | 753篇 |
晶体学 | 33篇 |
力学 | 38篇 |
数学 | 87篇 |
物理学 | 291篇 |
出版年
2023年 | 7篇 |
2022年 | 34篇 |
2021年 | 28篇 |
2020年 | 21篇 |
2019年 | 38篇 |
2018年 | 31篇 |
2017年 | 35篇 |
2016年 | 31篇 |
2015年 | 29篇 |
2014年 | 48篇 |
2013年 | 64篇 |
2012年 | 65篇 |
2011年 | 63篇 |
2010年 | 54篇 |
2009年 | 51篇 |
2008年 | 44篇 |
2007年 | 41篇 |
2006年 | 46篇 |
2005年 | 36篇 |
2004年 | 25篇 |
2003年 | 23篇 |
2002年 | 20篇 |
2001年 | 17篇 |
2000年 | 19篇 |
1999年 | 16篇 |
1998年 | 15篇 |
1997年 | 13篇 |
1996年 | 12篇 |
1995年 | 12篇 |
1994年 | 9篇 |
1993年 | 14篇 |
1992年 | 19篇 |
1991年 | 13篇 |
1990年 | 10篇 |
1989年 | 11篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1985年 | 17篇 |
1984年 | 8篇 |
1983年 | 11篇 |
1982年 | 9篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1977年 | 13篇 |
1976年 | 13篇 |
1975年 | 6篇 |
1974年 | 9篇 |
1967年 | 5篇 |
排序方式: 共有1202条查询结果,搜索用时 0 毫秒
31.
Abstract— The binding of hematoporphyrin derivated (Hpd) to lipid vesicles and bacterial membranes was determined by fluorescence spectroscopy. The fluorescence measurements of Hpd in aqueous solutions showed two bands at 613 and 677 nm. In lipid environments of lecithin vesicles the fluorescence spectrum was shifted to 631 and 692 nm, respectively. Hpd was rapidly bound to the cell membrane of Staphylococcus aureus while much less binding occurred in the presence of Escherichia coli. At the same time, spheroplasts of both bacteria were shown to bind Hpd to a similar extent. These results are well correlated with the photoinactivation of the gram positive bacteria with Hpd while the gram negative cells were shown to be resistant. The pH dependence of both Hpd binding to S. aureus as well as the photodynamic inhibitory effect of the same bacteria are similar. It is concluded that the segregation of Hpd to the cell membrane is a prerequisite for its photodynamic effect. 相似文献
32.
Pawel J. Kulesza Silvia Zamponi Marcin A. Malik Krzysztof Miecznikowski Mario Berrettoni Roberto Marassi 《Journal of Solid State Electrochemistry》1997,1(1):88-93
Using Prussian blue (PB) electrodeposited on gold-covered foil as a model system, we have demonstrated the usefulness of
the time-derivative measurements of absorbance versus potential (linear potential-scan voltabsorptometry) for spectroelectrochemical
characterization of thin electrochromic films. The time-derivative signals were monitored for PB at 680 and 420 nm in potassium,
sodium and lithium electrolytes. Information obtained from cyclic voltabsorptometry is equivalent or complementary to that
from conventional cyclic voltammetry. In the case of PB films investigated in lithium electrolyte, the voltabsorptometric
time-derivative peaks are better defined than the respective voltammetric peaks. The combination of voltabsorptometry with
voltammetry enables molar absorptivity and/or film loading to be determined. Also, concentration changes of differently colored
mixed-valence redox centers can be monitored as a function of applied potential.
Received: 16 January 1997 / Accepted: 11 March 1997 相似文献
33.
Beh HK Seow LJ Asmawi MZ Abdul Majid AM Murugaiyah V Ismail N Ismail Z 《Natural product research》2012,26(16):1492-1497
Morinda citrifolia L. has been used for the treatment of a wide variety of diseases, including cancer. This study was undertaken to evaluate the anti-angiogenic effect of M. citrifolia fruits and leaves. Anti-angiogenic activity was evaluated in?vivo using the chick chorioallantoic membrane assay. Bioactivity-guided fractionation and isolation were performed to identify the active constituent, and high-performance liquid chromatography analysis was then used to quantify the amount of this active constituent in the active extracts and fraction. The methanol extracts of fruits and leaves of M. citrifolia and the subsequent chloroform fraction of the fruit methanolic extract were found to have potential anti-angiogenic activity and were more potent compared to suramin. Scopoletin was identified as one of the chemical constituents that may be partly responsible for the anti-angiogenic activity of M. citrifolia fruits. The present findings further support the use of M. citrifolia in cancer or other pathological conditions related to angiogenesis. 相似文献
34.
Abstract— Human cells of the line NHIK 3025 were incubated with Photofrin II (PII) and exposed to light. Fluorescence- and absorption spectra of PII in the cells were measured. Light exposure resulted in a degradation of PII in the cells and changes in the shape of the fluorescence spectra. These changes are probably partly due to a photochemical modification of PII and to a relocalization of PII in the cells. Notably, a destruction of binding sites for PII on or close to proteins was caused by the light exposure. The rate of the light-induced decay of the porphyrin fluorescence intensity was only slightly increasing with the PII concentration, indicating that each porphyrin molecule is mainly degraded by photoproducts originating from itself. On the other hand, the rate of the degradation of porphyrin binding sites on the proteins increased with increasing PII concentrations.
The excitation spectrum of PII in cells has a peak at285–290 nm attributed to energy transfer from proteins to porphyrins located close to the proteins. The intensity of this peak relative to the intensity of the Soret band increases with decreasing porphyrin concentrations. This might indicate that some of the binding sites close to proteins have a higher affinity for the porphyrin than binding sites at longer distances from the proteins. 相似文献
The excitation spectrum of PII in cells has a peak at285–290 nm attributed to energy transfer from proteins to porphyrins located close to the proteins. The intensity of this peak relative to the intensity of the Soret band increases with decreasing porphyrin concentrations. This might indicate that some of the binding sites close to proteins have a higher affinity for the porphyrin than binding sites at longer distances from the proteins. 相似文献
35.
Noeen Malik Wolfgang Voelter Hans-Jürgen Machulla Christoph Solbach 《Journal of Radioanalytical and Nuclear Chemistry》2011,287(1):287-292
In homoaromatic systems, isotopic exchange (18F/19F) was previously (J Label Compd Radiopharm 18(12):1721–1730 [2], J Chem Soc Perkin Trans 1(3):295–298 [3]) proven to be advantageous, yet in general specific activity is thought to be low. For heteroaromatic systems, in particular,
very few examples are published regarding the 18F-labelling of 2-substituted pyridines (J Label Compd Radiopharm 42:975–985 [9]). Therefore, in 2-fluoropyridines, we decided to study the 18F labelling by isotopic exchange (18F/19F). The radiochemical yield for 2-fluoropyridine was 90 ± 2%. Even if 2-fluoropyridine was substituted by an electron-donating
group such as a methyl or a methoxy group, radiochemical yields were 80 ± 1 and 78 ± 1%, respectively. Although in benzenes,
these substituents are known to decrease nucleophilic substitutions by 18F-Fluoride significantly. Moreover, by choosing appropriate concentrations of 2-fluoropyridines, reasonably high specific
activities up to 10 GBq/μmol were obtained. 相似文献
36.
Sarova G Roussanova D Jeliazkova B Yordanov ND Malik MA O'Brien P 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2000,(2):351-356
The photochemical reactions of bis(diethyl-diselenocarbamato)copper(II), Cu(Et2dsc)2, complex have been studied in toluene, CH2Cl2, CHCl3 and chloroalkane/EtOH mixed solvents. Charge-transfer irradiation induces intramolecular oxidation of the ligand and reduction of copper(II) to copper(I) as evidenced by EPR and UV-Vis spectra of the complex as well as quantum yield results. When photolysis is carried out in CHCl3 or CH2Cl2 or in the solvent mixture CHCl3/EtOH resp. CH2Cl2/EtOH of lower than 1:1 EtOH content, the primary photoproduct CuI(Et2dsc) is further oxidised in a dark reaction with the chloroalkane producing the corresponding paramagnetic mixed-ligand CuII(Et2dsc)Cl complex in equilibrium with its chloride-bridged and EPR silent, dimeric form Cu2(Et2dsc)2Cl2. At low concentration of EtOH the equilibrium is shifted to the dimeric form whereas at higher than 1:1 EtOH content in the mixed solvent CHCl3/EtOH it is shifted to CuII(Et2dsc)Cl. A reaction mechanism is proposed and the role of ethanol is discussed. 相似文献
37.
Bartholomus Pieber Jamal A. Malik Cristian Cavedon Sebastian Gisbertz Aleksandr Savateev Daniel Cruz Tobias Heil Guigang Zhang Peter H. Seeberger 《Angewandte Chemie (International ed. in English)》2019,58(28):9575-9580
Cross‐coupling reactions mediated by dual nickel/photocatalysis are synthetically attractive but rely mainly on expensive, non‐recyclable noble‐metal complexes as photocatalysts. Heterogeneous semiconductors, which are commonly used for artificial photosynthesis and wastewater treatment, are a sustainable alternative. Graphitic carbon nitrides, a class of metal‐free polymers that can be easily prepared from bulk chemicals, are heterogeneous semiconductors with high potential for photocatalytic organic transformations. Here, we demonstrate that graphitic carbon nitrides in combination with nickel catalysis can induce selective C?O cross‐couplings of carboxylic acids with aryl halides, yielding the respective aryl esters in excellent yield and selectivity. The heterogeneous organic photocatalyst exhibits a broad substrate scope, is able to harvest green light, and can be recycled multiple times. In situ FTIR was used to track the reaction progress to study this transformation at different irradiation wavelengths and reaction scales. 相似文献
38.
Ramandeep Kaur Heena Ripneel Kaur Aman Grover Susheela Rani Ashok Kumar Malik Abuzar Kabir Kenneth G. Furton 《Journal of separation science》2020,43(13):2626-2635
A simple, fast, and sensitive analytical protocol using fabric‐phase sorptive extraction followed by high performance liquid chromatography with ultraviolet detection has been developed and validated for the extraction of five parabens including methylparaben, ethylparaben, propylparaben, butylparaben, and benzylparaben. In the present work, sol‐gel polyethylene glycol coated fabric‐phase sorptive extraction membrane is used for the preconcentration of parabens (polar) from complex matrices. The use of fabric‐phase sorptive extraction membrane provides a high surface area which offers high sorbent loading, shortened equilibrium time, and overall decrease in the sample preparation time. Various factors affecting the performance of fabric‐phase sorptive extraction, including extraction time, eluting solvent, elution time, and pH of the sample matrix, were optimized. Separation was performed using a mobile phase consisting of water:acetonitrile (63:37; v/v) at an isocratic elution mode at a flow rate of 0.9 mL/min with wavelength at 254 nm. The calibration curves of the target analytes were prepared with good correlation coefficient values (r2 > 0.9955). The limit of detection values range from 0.252 to 0.580 ng/mL. Finally, the method was successfully applied to various cosmetics and personal care product samples such as rose water, deodorant, hair serum, and cream with extraction recoveries ranged between 88 and 122% with relative standard deviation <5%. 相似文献
39.
Noeen Malik Xian Lin Dirk Löffler Bin Shen Christoph Solbach Gerald Reischl Wolfgang Voelter Hans-Jürgen Machulla 《Journal of Radioanalytical and Nuclear Chemistry》2012,292(3):1025-1033
For detection of hypoxic tumor tissue, all radiotracers synthesized until now, are based on the concept that cellular uptake
is being controlled by diffusion. As a new approach, we chose the concept to have the tracer hypothetically transported into
the cells by well known carrier systems like the amino acid transporters. For this purpose, radiosynthesis of O-[2-[18F]fluoro-3-(2-nitro-1H-imidazole-1yl)propyl]tyrosine ([18F]FNT]) was carried out from methyl 2-(benzyloxycarbonyl)-3-(4-3-(2-nitro-1H-imidazol-1-yl)-2-(tosyloxy)propoxy) phenyl)propanoate via no-carrier-added nucleophilic aliphatic substitution. After labelling,
81 ± 0.9% of labelled intermediate i.e. methyl 2-(benzyloxycarbonyl)-3-(4-(2-[18F]fluoro-3-(2-nitro-1H-imidazole-1-yl)propoxy) phenyl)propanoate was obtained at 140 °C. At the end of radiosynthesis, [18F]FNT was obtained in an overall radiochemical yield of 40 ± 0.9% (not decay corrected) within 90 min in a radiochemical purity
of >98% in a formulation ready for application in the clinical studies for PET imaging of hypoxia. 相似文献
40.