全文获取类型
收费全文 | 475篇 |
免费 | 40篇 |
出版年
2023年 | 5篇 |
2022年 | 8篇 |
2021年 | 25篇 |
2020年 | 9篇 |
2019年 | 13篇 |
2018年 | 15篇 |
2017年 | 14篇 |
2016年 | 16篇 |
2015年 | 30篇 |
2014年 | 47篇 |
2013年 | 35篇 |
2012年 | 39篇 |
2011年 | 39篇 |
2010年 | 27篇 |
2009年 | 19篇 |
2008年 | 30篇 |
2007年 | 26篇 |
2006年 | 30篇 |
2005年 | 11篇 |
2004年 | 15篇 |
2003年 | 11篇 |
2002年 | 11篇 |
2001年 | 9篇 |
2000年 | 3篇 |
1999年 | 5篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1992年 | 3篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有515条查询结果,搜索用时 15 毫秒
81.
82.
The release kinetics of four model aroma compounds from coarse (d
32 = 1.0 μm) and fine (d
32 = 0.25 μm) eicosane and hydrogenated palm stearin (HPS) emulsions prepared with either solid or liquid lipid droplets were
measured using a model mouth instrument. For both lipids, the release of aroma compounds from emulsions with solid droplets
was higher than from emulsions with liquid droplets. This difference was greater for less polar aroma compounds. The release
from solid eicosane droplets increased with particle size but no such effect was observed for HPS emulsions, however, the
release from solid eicosane was higher than solid HPS. The initial aroma release profile of the solid droplet emulsion matches
that of a similar liquid oil emulsion but requires much less added aroma.
Meeting presentation: Presented at 98th AOCS Annual Meeting and Expo in Quebec City, Canada. 相似文献
83.
S. Devin McLennan Lauren A. Peterson Joan B. Rose 《Applied and environmental microbiology》2009,75(22):7283-7286
Four point-of-use disinfection technologies for treating sewage-contaminated well water were compared. Three systems, based on flocculant-disinfectant packets and N-halamine chlorine and bromine contact disinfectants, provided a range of 4.0 to >6.6 log10 reductions (LR) of naturally occurring fecal indicator and heterotrophic bacteria and a range of 0.9 to >1.9 LR of coliphage.Disasters and flooding can overwhelm sanitation infrastructure, leading to sewage contamination of potable waters. This may be routine during the wet season in many parts of the world and spreads numerous waterborne diseases (21). Point-of-use (POU) water treatment has reduced the incidence of diarrheal disease when used for household drinking water (3, 4, 6, 13) and is now being promoted for disaster relief. While POU systems have recently been reviewed (14), to our knowledge there has been no direct, experimental comparison for treating actual sewage-contaminated waters. In this study, the efficacies of four POU disinfection systems (based on sodium dichloroisocyanurate [NaDCC] tablets, a flocculent-disinfectant powder, and chlorine and bromine contact disinfectant cartridges) in reducing the concentrations of six microbial indicators in well water contaminated with raw sewage were compared.The NaDCC tablets (67 mg; Aquatabs; Medentech, Wexford, Ireland), used for disinfection in low-turbidity water, have shown preliminary efficacy for routine household drinking water treatment (3, 4). The flocculant-disinfectant packet (4 g; PUR; Procter & Gamble Co., Cincinnati, OH) includes Fe2(SO4)3, bentonite, Na2CO3, chitosan, polyacrylamide, KMnO4, and Ca(OCl)2 (13). It achieved >7.3 log10 reductions (LR) of 24 bacteria species; >4.6 LR of poliovirus and rotavirus in EPA no. 2 test water (turbidity, >30 nephelometric turbidity units [NTU]) (15); and reduced diarrheal illness in Guatemala, Liberia, Kenya, and Pakistan (6, 7, 11, 13).HaloPure canisters (Eureka Forbes, Mumbai, India) contain N-halamine polymer disinfectant beads, poly[1,2-dichloro-5-methyl-5-(4′-vinylphenyl)hydrantoin] for chlorine canisters, and poly[1,2-dibromo-5-methyl-5-(4′-vinylphenyl)hydrantoin] for bromine canisters. Seeded laboratory trials achieved >6.8 LR for Escherichia coli and Staphylococcus aureus as water was passed through the canisters (2). The Cl-contact (producing residuals ranging from 0 to 0.6 mg/liter) and Br-contact (with residuals of 0.68 to 1.8 mg/liter) disinfectants achieved 2.9 LR and 5.0 LR of the bacteriophage MS2, respectively, and 27.5% and 88.5% reductions of the algal toxin microcystin, respectively (5).Sewage-contaminated water was prepared by mixing 9 liters of potable, nonchlorinated well water (pH 7.8; turbidity, 0.33 NTU; Williamston, MI) with 1 liter of raw sewage (City of East Lansing Wastewater Treatment Plant, MI) with an average pH of 6.6 ± 0.1, a biochemical oxygen demand of 144 ± 36 mg/liter, a concentration of total suspended solids of 146 ± 31 mg/liter, and a turbidity of 132 ± 12 NTU. Three disinfection trials were conducted at room temperature for each POU system on three different days to allow for variance in sewage strength. The turbidities of 1:10 dilutions of raw sewage averaged 7.5 ± 2.0 NTU. Table Table11 lists the indicator microorganism concentrations in the influent and effluent for each system.
Open in a separate windowaValues shown are numbers of CFU/ml except those for coliphage, which are numbers of PFU/ml. The percentage of samples below the detection limit (n = 3 for all systems) is 0% if not shown.All systems were used in accordance with the manufacturer''s directions for 10 liters of water. For NaDCC trials, one tablet was added and allowed 30 min of contact time (total dose of 3.2 mg/liter of hypochlorite; in deionized water, one tablet produced 2.1 mg/liter free Cl residual). For flocculant-disinfectant trials, one packet was added, stirred vigorously for 5 min, strained through cheesecloth after 10 min, and allowed 20 min of further contact time. The amount of hypochlorite included in one packet was not indicated, but one packet provided 1.5 mg/liter free Cl residual in 10 liters of deionized water. Samples were taken at 1, 3, 5, 10, 15, and 30 min for both systems.For the Cl-contact and Br-contact trials, disinfectant cartridges were installed in AquaSure housings consisting of an upper reservoir for influent, which flows by gravity through the disinfectant cartridge to a lower reservoir with a tap for dispensing (Fig. (Fig.1).1). The housings usually include cloth and activated charcoal prefilters, but these were removed in order to directly evaluate the disinfectant. With the tap open, 10 liters of influent was added and samples were collected at first flow (6 to 12 min) and after 15 and 30 min of flow. A single chlorine canister was used for all trials; the bromine canister was replaced for the third trial because the original clogged.Open in a separate windowFIG. 1.Flow schematic for contact disinfectant cartridges. Arrows indicate the directions of water flow from the upper reservoir (U), through the halogen (chlorine or bromine) disinfectant cartridge (H) containing packed N-halamine beads (N), to the lower reservoir (L) and out through the open tap.Microbial indicators in the influent and effluent (collection tubes contained sodium thiosulfate) in triplicate were quantified as numbers of CFU/ml by using mENDO agar for total coliforms (9), mHPC agar for heterotrophic plate counts (8), mTEC medium for E. coli (19), mEI agar for the genus Enterococcus (18), and mCP agar for the genus Clostridium (1) (Becton, Dickinson and Co., Franklin Lakes, NJ). Coliphage (PFU/ml) were measured with a double agar overlay assay, EPA method 1601 (17). Residuals (mg/liter) were measured using a Hach chlorine (free and total) test kit, model CN66 (Hach Co., Loveland, CO) (used for bromine in accordance with Hach method 8016 [10], with the instrument reading multiplied by 2.25 [the ratio of the atomic weights of bromine and chlorine], as advised by Hach Co. technical support).Comparison of water quality levels was done at 30 minutes. LR were calculated, with zeros replaced with the detection limits (Fig. (Fig.2).2). All POU systems reduced microbial concentrations below the detection limit in some trials (Table (Table1),1), making the calculated reductions the lower bound for those trials.Open in a separate windowFIG. 2.Average LR of naturally occurring microorganisms at 30 min for sewage-contaminated well water (1:10 dilution of raw sewage in well water) with the use of four POU disinfection systems (error bars represent 1 standard error). * indicates that effluent was below the limit of detection for all samples. Limit of detection was substituted to calculate LR and actual reductions may be greater than shown.Average LR for each POU system were compared using two-way analysis of variance with post hoc least-significant-difference (LSD) tests, performed with SPSS 11.0.1 (SPSS, Inc.). LR at 30 min differed significantly between systems (analysis of variance; F3,5 = 20.6; P < 0.001). There was no significant difference between the LR achieved by flocculant-disinfectant and contact disinfectants (LSD; mean difference, 0.2 to 0.5 LR; P > 0.05), while the NaDCC tablets induced significantly lower reductions (LSD; mean difference, 1.5 to 2.0 LR; P < 0.001).There was detectable residual free chlorine after 30 min for one NaDCC trial (0.4 mg/liter) and two flocculant-disinfectant trials (0.1 and 0.4 mg/liter). No contact disinfectant trial produced a measurable residual.No system in this study reliably produced residuals for safe storage after POU treatment or ideal virus reduction. Except for the NaDCC system, the POU systems achieved approximately 5.5 LR for E. coli and coliforms, 4.5 LR for enterococci, 4.0 LR for heterotrophs, 2.5 LR for clostridia, and 1.0 LR for coliphage. Coliphage was reduced below detection limits in all trials with Br-contact, similar to what was found in previous research (5). Bromine disinfection has proved safe and effective for large-scale maritime applications, like U.S. Navy vessels (20), and appears promising for household treatment. Further assessment of the Br-contact system is warranted, as is field comparison of POU systems in disaster relief. 相似文献
TABLE 1.
Concentrations of influent and 30-min-effluent microorganisms for POU disinfectant systems treating sewage-contaminated waterMicroorganism group | Geometric mean concn (range) [% of samples below detection limit]a | |||||||
---|---|---|---|---|---|---|---|---|
NaDCC | Flocculant-disinfectant | Cl-contact | Br-contact | |||||
Influent | Effluent at 30 min | Influent | Effluent at 30 min | Influent | Effluent at 30 min | Influent | Effluent at 30 min | |
Total coliforms | 2.7 × 104 (6.7 × 103 to 7.6 × 104) | 4.3 (4.0 × 10−2 to 1.6 × 102) | 1.7 × 104 (1.2 × 104 to 2.7 × 104) | 4.0 × 10−2 (<1.0 × 10−2 to 2.4 × 10−1) [33] | 2.9 × 104 (2.3 × 104 to 4.0 × 104) | <1.0 × 10−2 [100] | 4.5 × 104 (1.9 × 104 to 7.2 × 104) | 1.1 × 10−2 (<1.0 × 10−2 to 1.3 × 10−2) [66] |
Heterotrophic plate counts | 8.7 × 104 (2.7 × 104 to 1.8 × 105) | 6.4 × 101 (2.1 × 101 to 4.5 × 102) | 8.9 × 104 (2.9 × 104 to 4.3 × 105) | 8.5 (4.7 to 2.7 × 101) | 6.6 × 104 (3.5 × 104 to 1.1 × 105) | 3.9 (3.5 to 4.2) | 8.3 × 104 (2.4 × 104 to 2.0 × 105) | 4.6 (2.2 to 7.7) |
E. coli | 3.3 × 103 (7.7 × 102 to 1.1 × 104) | 1.8 × 101 (9.0 × 10−1 to 5.3 × 102) | 6.7 × 103 (2.3 × 103 to 4.3 × 104) | 1.1 × 10−2 (<1.0 × 10−2 to 1.3 × 10−2) [66] | 4.7 × 103 (2.3 × 103 to 1.1 × 104) | <1.0 × 10−2 [100] | 1.5 × 104 (6.3 × 103 to 4.6 × 104) | <1.0 × 10−2 [100] |
Enterococci | 8.8 × 102 (5.7 × 102 to 1.3 × 103) | 2.3 (<1.0 × 10−2 to 4.9 × 101) [33] | 6.3 × 102 (5.0 × 102 to 8.7 × 102) | <1.0 × 10−2 [100] | 9.9 × 102 (5.3 × 102 to 1.7 × 103) | <1.0 × 10−2 [100] | 1.3 × 103 (7.3 × 102 to 2.3 × 103) | <1.0 × 10−2 [100] |
Clostridia | 1.6 × 102 (6.0 × 101 to 3.0 × 102) | 6.4 (6.7 × 10−1 to 7.7 × 101) | 2.0 × 102 (7.0 × 101 to 6.0 × 102) | 7.9 × 10−1 (4.5 × 10−1 to 1.4) | 3.4 × 101 (2.0 × 101 to 6.3 × 101) | 2.4 × 10−2 (<1.0 × 10−2 to 6.0 × 10−2) [33] | 4.4 × 101 (2.7 × 101 to 9.3 × 101) | 7.4 × 10−2 (<1.0 × 10−2 to 3.6 × 10−1) [33] |
Coliphage | 1.5 × 102 (1.2 × 102 to 2.2 × 102) | 3.1 × 101 (<1.0 to 1.8 × 102) [33] | 1.4 × 102 (1.3 × 102 to 1.4 × 102) | 1.9 × 101 (<1.0 to 1.1 × 102) [33] | 9.4 × 101 (4.3 × 101 to 1.6 × 102) | 7.3 (1.3 to 4.7 × 101) | 7.7 × 101 (4.0 × 101 to 1.2 × 102) | <1.0 [100] |
84.
Susannah S. French Timothy J. Greives Devin A. Zysling Emily M. Chester Gregory E. Demas 《Proceedings. Biological sciences / The Royal Society》2009,276(1675):4003-4011
The primary goal of virtually all organisms is to produce genetic offspring, thereby passing on their genes to future generations. Offspring production, however, is limited by available resources within an environment. Moreover, distributing sufficient energy among competing physiological systems is challenging and can result in trade-offs between self-maintenance and offspring investment when resources are limited. In the current study, we tested the hypothesis that the adipose hormone leptin is involved in mediating energetic trade-offs between competing physiological systems. Specifically, we tested the effects of elevated maternal leptin on investment into offspring production versus self maintenance (immune function), in the Siberian hamster (Phodopus sungorus). The current study provides the first evidence that leptin serves as a signal to mothers of available energy resulting in epigenetic effects. Therefore, elevated leptin allows females to retain more embryos to parturition, and rear more offspring to weaning via reduced maternal infanticide. Innate immune response was suppressed seemingly as a result of these enlarged litters, suggesting that the observed fitness increase is not without costs to the mother. Collectively, these findings suggest that leptin plays a critical role in allowing mothers to determine how much energy to invest in the production and care of young versus self-maintenance. 相似文献
85.
Range expansion by invasive marine algae: rates and patterns of spread at a regional scale 总被引:1,自引:0,他引:1
Aim Introduced macroalgae are widespread in the world's oceans and, despite increasing awareness and attempts to limit the phenomenon, the number of species introductions in coastal waters has increased exponentially over time. Little is known about the rates and mechanisms of spread, even among species that have received the most attention. We compare patterns of range expansion for nine species of invasive algae across eight geographic regions.
Location World-wide.
Methods We compiled records of introduced algae from the scientific literature, herbaria, and by contacting experts to reconstruct chronologies for 22 algal invasions. These were used to map patterns of spread at a regional scale (thousands of km).
Results Range size tended to increase linearly with time, often after an initial lag. Range expansion occurred at rates of tens to hundreds of kilometres per year, often with large infrequent increases. Rates of range expansion differed significantly between species within the same region, and between regions for the same species.
Main conclusions Our results suggest that anthropogenic vectors likely play a key role in the spread of introduced macroalgae at a regional scale, although natural long-distance dispersal also may be important for some species. The lack of consistency in the rates within individual species and regions suggests that multiple interacting factors (e.g. algal traits, characteristics of invaded communities, environmental conditions and anthropogenic activities) determine where propagules of introduced algae are delivered and whether they become established. 相似文献
Location World-wide.
Methods We compiled records of introduced algae from the scientific literature, herbaria, and by contacting experts to reconstruct chronologies for 22 algal invasions. These were used to map patterns of spread at a regional scale (thousands of km).
Results Range size tended to increase linearly with time, often after an initial lag. Range expansion occurred at rates of tens to hundreds of kilometres per year, often with large infrequent increases. Rates of range expansion differed significantly between species within the same region, and between regions for the same species.
Main conclusions Our results suggest that anthropogenic vectors likely play a key role in the spread of introduced macroalgae at a regional scale, although natural long-distance dispersal also may be important for some species. The lack of consistency in the rates within individual species and regions suggests that multiple interacting factors (e.g. algal traits, characteristics of invaded communities, environmental conditions and anthropogenic activities) determine where propagules of introduced algae are delivered and whether they become established. 相似文献
86.
Angela Schoolmeesters Teresa Eklund Devin Leake Annaleen Vermeulen Queta Smith Shelley Force Aldred Yuriy Fedorov 《PloS one》2009,4(5)
Background
Mesenchymal stem (MS) cells are excellent candidates for cell-based therapeutic strategies to regenerate injured tissue. Although human MS cells can be isolated from bone marrow and directed to differentiate by means of an osteogenic pathway, the regulation of cell-fate determination is not well understood. Recent reports identify critical roles for microRNAs (miRNAs), regulators of gene expression either by inhibiting the translation or by stimulating the degradation of target mRNAs.Methodology/Principal Findings
In this study, we employed a library of miRNA inhibitors to evaluate the role of miRNAs in early osteogenic differentiation of human MS cells. We discovered that miR-148b, -27a and -489 are essential for the regulation of osteogenesis: miR-27a and miR-489 down-regulate while miR-148b up-regulates differentiation. Modulation of these miRNAs induced osteogenesis in the absence of other external differentiation cues and restored osteogenic potential in high passage number human MS cells.Conclusions/Significance
Overall, we have demonstrated the utility of the functional profiling strategy for unraveling complex miRNA pathways. Our findings indicate that miRNAs regulate early osteogenic differentiation in human MS cells: miR-148b, -27a, and -489 were found to play a critical role in osteogenesis. 相似文献87.
Swanson DM Wilson SJ Boggs JD Xiao W Apodaca R Barbier AJ Lovenberg TW Carruthers NI 《Bioorganic & medicinal chemistry letters》2006,16(4):897-900
Aplysamine-1 (1), a marine natural product, was synthesized and screened for in vitro activity at the human and rat histamine H3 receptors. Aplysamine-1 (1) was found to possess a high binding affinity for the human H3 receptor (Ki = 30+/-4 nM). Synthetic analogs of 1, including des-bromoaplysamine-1 (10) and dimethyl-{2-[4-(3-piperidin-1-yl-propoxy)-phenyl]-ethyl}-amine (13), were potent H3 antagonists. 相似文献
88.
89.
Finger joint coordination during tapping 总被引:1,自引:0,他引:1
We investigated finger joint coordination during tapping by characterizing joint kinematics and torques in terms of muscle activation patterns and energy profiles. Six subjects tapped with their index finger on a computer keyswitch as if they were typing on the middle row of a keyboard. Fingertip force, keyswitch position, kinematics of the metacarpophalangeal (MCP) and the proximal and distal interphalangeal (IP) joints, and intramuscular electromyography of intrinsic and extrinsic finger muscles were measured simultaneously. Finger joint torques were calculated based on a closed-form Newton–Euler inverse dynamic model of the finger. During the keystroke, the MCP joint flexed and the IP joints extended before and throughout the loading phase of the contact period, creating a closing reciprocal motion of the finger joints. As the finger lifted, the MCP joint extended and the interphalangeal (IP) joints flexed, creating an opening reciprocal motion. Intrinsic finger muscle and extrinsic flexor activities both began after the initiation of the downward finger movement. The intrinsic finger muscle activity preceded both the IP joint extension and the onset of extrinsic muscle activity. Only extrinsic extensor activity was present as the finger was lifted. While both potential energy and kinetic energy are present and large enough to overcome the work necessary to press the keyswitch, the motor control strategies utilize the muscle forces and joint torques to ensure a successful keystroke. 相似文献
90.
Jarrod J. Scott Kevin J. Budsberg Garret Suen Devin L. Wixon Teri C. Balser Cameron R. Currie 《PloS one》2010,5(3)