H2 and O2 evolution activity of d0 and d10 metal oxide particulate catalysts under UV light irradiation.
\r\n\tThis volume presents the multifaceted aspects and should allow readers at all levels an entry into the exiting world of Chlamydomonas research.
\r\n\t
Hydrogen gas is one of the best alternatives to fossil fuels since it has a high gravimetric energy density (142 MJ/kg) and it produces zero carbon upon combustion. Hydrogen is also used as a major reactant in environmentally important reactions such as carbon dioxide hydrogenation to methanol [1] or ammonia production (Haber-Bosch reaction) [2]. For hydrogen to be used as a clean energy source, its production via renewable ways is of great importance. It is conventionally produced via steam reforming of methane and fossil fuels (energy intensive, ΔH0rxn = 206 kJ/mol, 700–1100°C [3]) and coal gasification, which results in significant amounts of carbon dioxide production. The renewable ways for carbon-free production include biological sources (microalgae and cyanobacteria) and electrolysis of water using wind energy and photovoltaic cells as electricity generation sources. In addition to the mentioned renewable ways, photocatalytic water splitting/oxidation is a promising alternative, in which solar energy is used as the driving force to split water molecules to hydrogen and oxygen on the surface of a catalyst. This renewable production method of hydrogen is advantageous over other renewable methods due to the free source of energy and lower cost of the photocatalysts when compared to that of photovoltaic cells or wind turbines. Solar-driven catalytic (photocatalytic) reactions are considered to be of fundamental importance to the catalysis community since the solar energy is inexhaustible; i.e., the solar energy absorbed by the lands and oceans on an hourly basis (432 EJ/h or 120,000 TW [4]) is comparable to the Earth’s yearly energy consumption (reaching 575 EJ/year or 18 TW in 2017). However, the solar-to-hydrogen energy conversion efficiency value for photocatalytic water splitting systems is much lower (targeted to be 10%, currently reaching 1% [5]) than that of photovoltaic-assisted electrolysis (reaching 30% [6]) due to the major drawbacks in the one-step photocatalytic water splitting systems. Herein, we firstly introduce photocatalytic water splitting systems and give the major developments in materials such as visible light utilization and corresponding H2 and O2 production activity values (in Section 2). Then in Section 3, we discuss the causes of the low efficiencies in photocatalytic water splitting systems and the recent approaches in preventing energy efficiency-lowering factors such as inefficient visible light utilization, charge recombination, back-oxidation reactions, and mass transfer limitations.
\nThe first report on water splitting via harvesting photon energy is authored by Fujishima and Honda using a photoelectrochemical cell with a TiO2 photoelectrode [7]. Following this first report suggesting the oxidation of water molecule via photo-generated holes on TiO2 surface with the aid of small electrical voltage, photocatalytic water splitting on powder photocatalyst particles is demonstrated by other authors in the late twentieth century [8, 9, 10, 11, 12, 13, 14, 15]. Metal-loaded semiconductors (such as Pt/TiO2) are described as “short-circuited photoelectrochemical cells” that provide both the oxidizing centers and the reduction centers on the same catalyst (see Figure 1) [16].
\nSchematic representation of photocatalytic water splitting on metal-loaded semiconductor particle systems: (1) light absorption and charge excitation from valence band to conduction band, (2) transfer of the photo-generated electrons and holes to the catalyst surface, (3) surface redox reactions, and (4) charge recombination.
Photocatalytic reactions are initiated by absorption of light having an energy higher than (or equal to) the bandgap of the photocatalysts that consist of semiconductor materials. This bandgap energy should be larger than 1.23 V for overall water oxidation reaction, for which the maximum of the valence band and the minimum of the conduction band should be located at proper potentials for the oxygen and hydrogen evolution reactions to occur. To illustrate, the minimum of the conduction band energy level should be located at a more negative potential than 0 V vs. NHE, at pH = 0 for H2 evolution (Eq. (1)), and the maximum of the valence band should be at a more positive potential than 1.23 V vs. NHE at pH = 0 for oxygen evolution reaction (Eq. (2)):
\nFollowing the light absorption, photoexcited electrons are transferred to the conduction band, while a positively charged charge carrier (hole) is generated at the valence band. These charge carriers are then transferred to the catalyst surface (step 2 in Figure 1) to be utilized in surface redox reactions, unless they recombine in the bulk or on the surface (step 4). Ultimately, electrons and holes reduce/oxidize the adsorbed species on the catalyst surface (step 3), the products of which should then be desorbed from the surface to complete the overall process.
\nTiO2, having a large bandgap (anatase: 3.2 eV), is the most commonly used photocatalyst due to its photostability, nontoxicity, and high activity (upon UV radiation λ < 387 nm). Following the report on water oxidation reaction [7], various photochemical reaction activities of TiO2 such as carbon dioxide reduction with H2O [17, 18, 19], alkene and alkyne hydrogenation [20, 21], CH3Cl oxidation [22], 1-octanol degradation [23], phenol degradation [24], surfactant degradation [25], and more have been reported. Detailed reviews on TiO2-based materials and photocatalytic performances can be found in literature [26, 27, 28].
\nAs photostable and active TiO2 is, UV light requirement to activate the large bandgap of TiO2 motivated research for visible light active semiconductors as well as bandgap engineering for TiO2 such as nonmetal ion doping (N [29], C [30], F [31], S [32]). Substitution of lattice oxygen atoms by these anions is reported to shift the valence band level upward and narrow the bandgap to as low as 2.25 eV (∼550 nm) with 16.5% N doping [33].
\nSimilar to TiO2, oxides of other transition metals with d0 (such as Ti4+, Zr4+, Nb5+, Ta5+, and W6+ [34, 35]) and d10 electronic configurations (such as Ga3+, In3+, Ge4+, Sn4+, and Sb5 [36, 37, 38]) are shown to possess large bandgap energies (>3 eV) due to the maximum valence band levels consisting O2p orbitals located near 3 V (vs. NHE at pH = 0). These d0 and d10 metal oxide catalysts are reported to show remarkable one-step photocatalytic water splitting activity under UV light irradiation [39] reaching 71% quantum yield with photocatalysts such as Al-doped SrTiO3 [40] or Zn-doped Ga2O3 [41]. The H2 and O2 evolution activity under UV radiation and the apparent quantum yields of some of these materials are given in Table 1. The apparent quantum yield is defined as the number of reacted electrons and holes divided by the number of incident photons on the photocatalysts. Table 1 is not intended to cover the whole range of particulate catalysts in literature but rather to give a selection of examples. A wider selection of d0 and d10 metal oxide particulate catalysts’ one-step water oxidation activity and apparent quantum yields can be found in the works of Kudo et al., Chen et al., and Domen et al. [39, 42, 43].
\nSemiconductor | \nCo-catalyst | \nBandgap (eV) | \nH2 activity (μmol/h) | \nO2 activity (μmol/h) | \nAQY (%) | \nReference | \n
---|---|---|---|---|---|---|
La2Ti2O7:Ba | \nNiOx | \n3.26 | \n5000 | \n\n | 50 | \n[44] | \n
SrTiO3:Al | \nRh2−yCryO3 | \n3.2 | \n550* | \n280* | \n30 at 300 nm | \n[35] | \n
SrTiO3:Al (200–500 nm) | \nRh2−yCryO3 | \n3.2 | \n1372* | \n683* | \n56 at 365 nm | \n[45] | \n
SrTiO3:Al | \nMoOy/RhCrOx | \n3.2 | \n1800* | \n900* | \n69 at 365 nm | \n[40] | \n
NaTaO3 | \nNiO | \n4.0 | \n3390 | \n1580 | \n20 at 270 nm | \n[46] | \n
NaTaO3:La | \nNiO | \n4.1 | \n19,800 | \n9700 | \n56 at 270 nm | \n[34] | \n
Ga2O3:Zn | \nNiO | \n4.4 | \n4100 | \n2200 | \n20 | \n[47] | \n
Ga2O3:Zn | \nRh0.5Cr1.5O3 | \n4.4 | \n32,000 | \n16,000 | \n71 at 254 nm | \n[41] | \n
H2 and O2 evolution activity of d0 and d10 metal oxide particulate catalysts under UV light irradiation.
0.1 g of photocatalyst is used instead of 1 g.
The most remarkable upgrades in the apparent quantum yields are achieved by material engineering such as (i) doping the metal oxides/perovskites with cations having lower valences, (ii) decreasing the crystal sizes to submicron levels, and (iii) loading with H2/O2 evolution co-catalysts.
\nAn important addition to the light-harvesting semiconductors is H2 evolution/O2 evolution co-catalysts on the surface. The early co-catalysts that have been widely used included the noble metals and transition metal oxides such as Pt [12, 13], Rh [10], Ru [48], Au [49, 50], and NiOx [11] that mainly promote the hydrogen evolution, and CoOx [51] and Fe [52], Mn [52], RuO2 [53], and IrO2 [54] that accelerate the oxygen evolution. These metals are considered to act as charge carrier sinks that suppress electron–hole pair recombination as well as increasing the reaction kinetics by lowering the activation energy of the redox reactions. Co-catalysts are also known to inhibit photodegradation of the photocatalysts such as oxysulfides and oxynitrides by generated holes due to the effective extraction of these holes by the co-catalysts [55, 56].
\nFollowing the works of noble metal co-catalysts, Domen et al. showed water splitting activity on SrTiO3 photocatalyst together with the co-catalyst NiO [57, 58], which became the choice of H2 evolution co-catalyst for many d0 and d10 metal oxides such as La2Ti2O7:Ba [44], La4CaTi5O17 [59], Rb4Nb6O17 [60], NaTaO3 [46], and Ga2O3:Zn [47]. The photocatalyst stability of NiO-loaded K2La2Ti3O10 is reported to increase by addition of a second co-catalyst, Cr, using a co-impregnation method [61]. Based on the promoting effect of Cr, a systematic study of Cr and various transition metals (such as Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Pt) on (Ga1-xZnx)(N1-xOx) has been conducted [62], from which core-shell structures of core Rh nanoislands and shell Cr2O3 structures (10–30 nm in size) are found to promote H2 and O2 evolution reactions to significant levels [63].
\nThe alterations to the semiconductors such as doping with low-valence cations, reducing particle sizes to submicron levels and obtaining a high degree of crystallinity help with the overall water splitting activity. However, activation of these photocatalysts uses a narrow portion of the solar spectrum (4%); i.e., the UV light sustains as a problem due to the large bandgap energies of these materials. To enable visible light utilization of the d0- and d10-type oxide semiconductors and to split water into H2 and O2 via one-step excitation, the valence band levels should be shifted upward without changing the conduction band potentials. One approach to do this is to use oxynitrides to make use of N2p states that lie at a more negative potential than O2p states. Emerging LaMgTa1−xO1+3xN2−3x [64] and Ga1−xZnxN1−xOx [65] oxynitrides are representatives of visible light active overall water splitting catalysts. Using N doping, the absorbed light wavelength can be increased up to 500 nm on solid solutions of GaN:ZnO (Ga1−xZnxN1−xOx) [66] and up to 600 nm on solid solutions of LaTaON2 and LaMg2/3Ta1/3O3 (LaMgTa1−xO1+3xN2−3x) [64]. Other examples include LaScxTa1−xO1+2xN2−2x [67] and CaTaO2N [68] in which La or Ta sites are replaced by Ca and Sc that alters O/N ratios due to charge compensation, which in turn results in valence band energy level shift.
\nSome examples of visible light active photocatalyst and their H2 and O2 evolution activity are given in Table 2. As it can be seen from the table, one-step water splitting quantum yields are quite lower when compared to those of the UV-activated photocatalysts (Table 1). The exceptions to the low activity are reported by Rh2-yCryO3 (Rh 1.0 wt%, Cr 1.5 wt%)-loaded (Ga1−xZnx)(N1−xOx) photocatalyst [63], multiband InGaN/GaN nanowire arrays [69], and monodisperse 4 nm graphite nanoparticle-deposited C3N4 catalysts [70].
\nSemiconductor | \nCo-catalyst | \nBandgap (eV) | \nH2 activity (μmol/h) | \nO2 activity (μmol/h) | \nAQY (%) | \nReference | \n
---|---|---|---|---|---|---|
SrTiO3:Rh,Sb | \nIrO2 | \n\n | 4.4 | \n1.9 | \n0.1 at 420 nm | \n[71] | \n
g-C3N4 | \nPt-CoOx | \n2.8 | \n∼8.5 | \n∼3.5 | \n0.3 at 405 nm | \n[72] | \n
CDots-C3N4 | \n\n | 2.74 | \n46 | \n\n | 16 at 420 nm | \n[70] | \n
Bi1-xInxV1-xMoxO4 | \nRuO2 | \n2.5 | \n17 | \n\n | 3.2 at 420 nm | \n[73] | \n
BiYWO6 | \nRuO2 | \n2.7 | \n4.1 | \n1.8 | \n0.17 at 420 nm | \n[74] | \n
LaMg1/3Ta2/3O2N | \nRhCrOx | \n\n | 22 | \n11 | \n0.18 at 440 nm | \n[75] | \n
(Zn0.18Ga0.82)(N0.82O0.18) | \nRh2-yCryO3 | \n2.64 | \n927 | \n460 | \n5.9 at 420 nm | \n[76] | \n
GaN:Mg/InGaN:Mg | \nRh/Cr2O3 | \n2.22 | \n38 | \n21 | \n12.3 at 400 nm | \n[69] | \n
H2 and O2 evolution activity of one-step water splitting catalysts under visible light irradiation.
An alternative way to cover both oxidation and reduction reactions with semiconductors that could be activated under visible light radiation is to utilize two individual photocatalysts with an electron transfer mediator to obtain two-step excitation known as the two-step water oxidation (“Z-scheme system,” see Figure 2). In this system, O2 evolution photocatalysts oxidize the water molecules to O2, while the photo-generated electron is transferred to the mediator to reduce the electron acceptor (such as Fe3+ ions or IO3− ions). Then, the reduced mediator is oxidized by donating its electron to the H2 evolution photocatalyst. At the same time, the photo-generated electrons in the H2 evolution photocatalyst reduce H+s to H2.
\nSchematic diagram for photocatalytic water splitting using a two-step photoexcitation system.
The semiconductors used in this two-step water splitting process should be selected based on the energy levels of their corresponding valence or conduction band maximum/minimum that would enable O2/H2O oxidation and H+/H2 reduction. As H2 evolution and O2 evolution reactions are realized at separate photocatalysts, these semiconductors could have bandgap energy values lower than 3 eV that would enable visible light utilization such as Pt- or RuO2-loaded WO3 (Eg ∼ 2.8 eV) or oxynitrides such as TaON (Eg ∼ 2.4 eV) or Rh-doped SrTiO3 (Eg ∼ 2.4 eV). Examples of these materials and systems can be seen in Table 3. The detailed reviews on two-step photocatalytic water splitting can be found elsewhere [83].
\nH2 photocatalyst | \nO2 photocatalyst | \nMediator | \nH2 activity (μmol/h) | \nO2 activity (μmol/h) | \nAQY (%) | \nReference | \n
---|---|---|---|---|---|---|
Pt/SrTiO3:Rh | \nBiVO4 | \nFe3+/Fe2+ | \n15 | \n7.2 | \n0.4 at 420 nm | \n[77] | \n
Pt/SrTiO3:Cr/Ta | \nPtOx/WO3 | \nIO3−/I− | \n∼16 | \n∼8 | \n1 at 420 nm | \n[78] | \n
Pt/TaON | \nPtOx/WO3 | \nIO3−/I− | \n∼16.5 | \n∼8 | \n0.5 at 420 nm | \n[79] | \n
Pt/ZrO2/TaON | \nPtOx/WO3 | \nIO3−/I− | \n52 | \n27 | \n6.3 at 420 nm | \n[80] | \n
Ru/SrTiO3:Rh | \nBiVO4 | \nFe3+/Fe2+ | \n88 | \n44 | \n4.2 at 420 nm | \n[81] | \n
Pt/MgTa2O6-xNy/ TaON | \nPtOx/WO3 | \nIO3−/I− | \n108 | \n55 | \n6.8 at 420 nm | \n[82] | \n
Z-scheme-type photocatalysts for water splitting without sacrificial agents.
There are numerous and challenging processes that need to be realized for photocatalytic evolution of H2 and O2 (Table 4) via a thermodynamically unfavorable reaction (Eq. (3)):
\nThese processes include (i) excitation of the semiconductor photocatalyst with photon having higher energies than the bandgap energy of the material, (ii) transfer of the photo-generated electrons and holes to the reaction sites on the surface, (iii) utilization of these charge carriers in the oxidation/reduction reactions, and (iv) desorption of the products from the surface of the photocatalyst to the liquid/gas medium.
\nAs the timescale of these processes varies, recombination of the electrons and holes in the bulk or on the surface happens more frequently than the rate of the chemical oxidation/reduction reactions. Recombination is therefore considered to be one of the main reasons limiting the photocatalytic activity. Together with the recombination events, realization of back-oxidation reactions (Eq. (4)) on noble metals and the rate-limiting mass transfer events are the major drawbacks in an efficient photocatalytic process:
\nNatural photosynthesis yields a much higher rate of O2 evolution (see Table 4) when compared to artificial water splitting due to improved charge carrier and mass transfer events. From this comparison, it is clear that the photocatalytic systems still need to be perfected to compete with the nature’s intricate design.
\nDue to the presence of the multiple processes, the overall photocatalytic reactions are extremely complicated. In order to obtain an efficient photocatalytic performance, the photo-generated charges must be transferred to the surface reaction sites as rapidly as possible while preventing recombination or trapping of these charge carriers. It is reported by Leytner and Hupp that 60% of the trapped electron–hole pairs recombine with a timescale of about 25 ns while releasing heat of 154 kJ/mol [85]. As the defects such as vacancies and dislocations are considered as recombination sites, higher crystallinity of the photocatalysts is often aimed to decrease the recombination rates. From diffusion point of view, the shorter distances for the charge carriers to the surface reaction centers are also aimed to prevent the recombination. Shorter pathways are achieved via smaller crystal/particle sizes of the photocatalysts. More than two times of increase in the H2 and O2 evolution rates on Al-doped SrTiO3 photocatalyst (reaching an apparent quantum yield of 56% [45]) as the particle size drops from few micrometers to 200 nm is a direct evidence of the effect of the particle size. Another method for reducing the charge recombination is to make use of phase junctions. One example is the α-β-phase junction of Ga2O3, which results in enhanced interfacial charge transfer, charge separation, and therefore enhanced water splitting activity [86]. Loading the photocatalysts with co-catalysts such as noble metals or transition metal oxides to accelerate the reduction/oxidation reactions is a commonly employed method. These co-catalysts are known to enhance the charge migration from the semiconductor depending on the alignment of the potentials of the semiconductor and the co-catalyst. As these co-catalysts accelerate the desired H2 evolution and O2 evolution reactions, they can also increase the rates of undesired secondary reactions such as hydrogen oxidation or oxygen reduction to water reactions.
\nIntroduction of one-step photocatalysts for overall water splitting combined the H2 evolution and O2 evolution sites on the same catalyst surface. This design of a photocatalytic system that realizes both charge trapping and reduction/oxidation reactions on the same surface not only accelerated the charge recombination but also allowed secondary reactions on these reduction/oxidation centers. When fast removal of the products, i.e., H2 and O2, is not provided and there are no barriers that prevent interaction of these products with highly active sites, the reaction of H2 and O2 on the photocatalyst surface to produce H2O (2H2 + O2 → H2O) is highly probable. And back-oxidation of the produced H2 is considered to be one of the main reasons for observed low photocatalytic water splitting activity values.
\nAs early as 1985, Sato and coworkers realized the importance of back-oxidation of H2 with O2 to produce H2O. They have realized that the metal-loaded photocatalysts, mainly Pt- or Pd-loaded TiO2, can oxidize H2 with O2 easily under the same photocatalytic water oxidation conditions. They have reported first-order reaction rate constants in the range of 0.23–0.51 h−1 for Pt, 0.32–1.8 h−1 for Pd, and 0.2–0.3 h−1 for Rh, suggesting the least active metal for back-oxidation reaction to be Rh [10]. Later in 2000, Anpo and coworkers investigated back-oxidation reaction on Pt/TiO2 systems under dark conditions and observed increased back-oxidation rate with increasing Pt loading (up to 0.1 wt.% [87]). While Pt is active for H2 evolution (Eq. (1)), it is also notoriously active for dark H2–O2 recombination reaction (Eq. (4)) even at room temperature [88]. In order to prevent H2–O2 recombination reaction, the Pt surface is modified with F ions for Pt/TiO2 catalyst, and the reaction rate decreased from 2 to 0.3 h−1 upon F− modification [89]. The inhibition mechanism is suggested to be due to the occupation of the H2 surface adsorption sites on Pt by F atoms.
\nAnother modification to the noble metal surfaces is reported by Lercher et al., in which CO is chemisorbed on the Rh co-catalyst for GaN:ZnO semiconductor. Chemisorbed molecular layer of CO suppressed the back-oxidation reaction by selective metal poisoning of the back-oxidation sites by CO. While H2 evolution rates of 28 μmol/h are achieved (75 mg photocatalyst, 300 W Xe lamp [90]), significant CO oxidation to CO2 is also observed.
\nThe back-oxidation reaction-inhibiting effects of the nanolayer coating on noble metals are shown on Rh/Cr2O3-loaded GaN:ZnO photocatalysts. Rh/Cr2O3 core-shell structure [91] is formed by photodeposition of Rh and reduction of CrO42− by electrons coming from Rh upon radiation, resulting in few nanometer thickness of Cr2O3 layer (2–3 nm, see Figure 3). Hydrated Cr2O3 nanolayer is reported to selectively permeate protons for H2 evolution reaction [92], whereas it hinders O2 permeation from the layer inhibiting O2 reduction reaction (Eq. (5)) on Rh sites [93]. The same effect is also valid for Cr2O3-coated Pt catalyst (GaN:ZnO). Back-oxidation rates on Pt-loaded GaN:ZnO photocatalyst decreased significantly from ∼105 × 10−12 molecules/s to ∼8 × 10−12 molecules/s, while photocatalytic H2 evolution rate increased from ∼5 × 10−12 molecules/s to ∼30 × 10−12 molecules/s upon Cr2O3 coating [93]. Apart from the oxygen-blocking role of the Cr2O3 nanolayer, much lower back-oxidation rate of Rh-loaded GaN:ZnO when compared to Pt-loaded GaN:ZnO (11 × 10−12 molecules/s vs. 105 × 10−12 molecules/s) explains the significant photocatalytic H2 evolution activity on Rh/Cr2O3-loaded GaN:ZnO (130 × 10−12 molecules/s). Lower back-oxidation rate of Rh-loaded GaN:ZnO could be related to the low-oxygen reduction reaction (Eq. 5) activity of Rh when compared to Pt [94]:
\nHR-TEM images of GaN:ZnO photodeposited with (A) Rh and (B) Rh/Cr2O3. Reprinted with permission from [55]. Copyright 2007 American Chemical Society. (C) Schematic representation of O2 and H2 evolution reactions with inhibited O2 permeation and O2 reduction reaction on core-shell-type co-catalysts.
Similar selective permeability concept is considered to be the case for Ni/NiO core-shell structures deposited on various photocatalysts such as SrTiO3 or NaTaO3 [34, 57]. In these systems, in addition to the back-oxidation reaction impeding effect of NiO layer on Ni [58], low-oxygen reduction activity of NiOx catalysts when compared to Pt can also be considered to be effective for improved water splitting activity.
\nCoatings of the whole photocatalyst instead of the co-catalyst by oxyhydroxides of Ti, Nb, and Ta are reported on Rh-loaded SrTiO3:Sc photocatalyst. Surface nanolayer not only suppressed back-oxidation reactions but also prevented the access of sacrificial agents such as ethanol to the photocatalyst surface, resulting in nearly stoichiometric H2/O2 ratios [95]. Surface nanolayer coatings on the whole photocatalysts have proven to also prevent photodecomposition (N2 evolution) of oxynitride photocatalysts while increasing the overall water splitting activity [64, 68].
\nPrevention of the secondary reactions such as H2 oxidation or O2 reduction reaction to H2O is found to be essential for improving the overall water splitting activity and the apparent quantum yield values (reaching apparent quantum yield value of 69% under irradiation at 365 nm [40]). In addition to the reduced back-oxidation rates, complementary measures such as decreasing the charge recombination rates and enhancing the product transfer rates away from the surface (increasing the mass transfer rates) are necessary for increased photocatalytic water splitting activity.
\nMass transfer limitations especially in the slurry photocatalytic systems can be the most overlooked problem in the photocatalytic field. To complete the photocatalytic reaction cycle, adsorption of the reactants, reduction/oxidation of the reactants, desorption of the products, and transfer of the products from the photocatalyst surface to the gas phase need to be realized. When the rates of the mass transfer of the products from the surface are slower than the reduction/oxidation rates, produced H2 and O2 would stay longer on the surface, resulting in promotion of back-oxidation reactions. Moreover, when the mass transfer rates are slower than the reaction kinetics, the apparent H2 and O2 evolution rates in the gas phase will be limited by the mass transfer rates.
\nExperimental evidence for mass transfer limitations in agitated systems is presented in a previous publication [96]. In a batch slurry reactor, where the catalyst particles are suspended via agitation, observed H2 evolution rates for UV-irradiated Pt/TiO2 photocatalyst showed improvement with increasing stirring rates up to 900 rpm (Figure 4a). This improvement is a direct indication of mass transfer limitations on the solid–liquid and gas–liquid interfaces as the turbulence in the liquid and therefore boundary layers are affected by increasing stirring rates. In another experiment, the effect of liquid volume is investigated by varying catalyst weight and liquid volumes (keeping the catalyst concentration constant). H2 evolution rates on an hour basis (μmol H2/h) are found the same regardless of the liquid volume (or catalyst weight) above 62.5 ml (Figure 4b) as the H2 evolution rate per gram and hour basis decreased as liquid volume increased. Similar H2 evolution rates regardless of the catalyst weight indicate significant mass transfer limitations in the liquid–gas interface.
\n(a) Effect of stirring rate on photocatalytic hydrogen evolution with methanol as sacrificial agent, with 0.5 wt% Pt/TiO2, 250 ml deionized water, 2 ml methanol, (■) 900 rpm and (●) 350 rpm. (b) Observed hydrogen evolution rates in the gas phase with changing liquid volume, CH3OH/H2O:1/125 (v/v) and Ccatalyst: 1 g/L for each case. Adapted from [96].
Mass transfer limitations for different photocatalytic reaction systems are analyzed by different groups. For immobilized photocatalyst systems, the importance of internal mass transfer resistance is emphasized [97]. In another investigation, severe mass transfer limitations are observed in the product separator (liquid–gas interface) for a fluidized bed/separator system, in which modification of the liquid–gas surface area enhanced the H2 evolution rates by 350% [98].
\nTo prevent mass transfer limitations in the photocatalytic tests and to report actual kinetic rates; stirring rates, liquid levels, and mass transfer areas should be designed carefully. To design these parameters, approximate mass transfer rates should be known. Here, we present a sample calculation for H2 mass transfer rate in a slurry reactor containing 0.5 g TiO2 photocatalyst having a surface area of 40 m2/g inside an agitated glass reactor having 200 ml liquid volume and a tank diameter of 7 cm.
\nMass transfer resistances in a gas–liquid–solid multiphase photocatalytic systems involve the internal mass transfer, mass transfer from the solid catalyst particles to liquid (Eq. 6), transfer from the liquid bulk to the liquid interface (Eq. 7), and transfer from the liquid–gas interface to the gas phase (Eq. 8). Photocatalysts such as perovskites and TiO2 are known to be nonporous (unless mesoporous versions are prepared on purpose [99, 100]) and have surface area values between 5 and 50 m2/g. For nonporous photocatalysts, the internal mass transfer limitations can be discarded (Eq. 9). Hence, the H2 mass transfer rate equation will have a form containing the mass transfer resistances from the solid–liquid and liquid–gas interfaces as seen in Eq. 8:
\nThe mass transfer limitations coming from the solid–liquid and liquid–gas interfaces may play important role depending on the photocatalytic reactor type. The most often used photocatalytic reactor systems such as slurry reactors have solid–liquid and liquid–gas phase interfaces that suspend its catalysts by agitation using an impeller or a magnetic stirrer. The convection mass transfer coefficient for solid–liquid interface of such a system could be estimated using Eq. 10 suggested by Armenante and Kirwan for agitated tanks using Kolmogorov’s theory for Reynold’s number calculation to consider the effect of solid particle size [101]:
\nwhere \n
Process | \n\n | Timescale | \n
---|---|---|
Light absorption and electron and hole generation | \n\n\n | \nfs | \n
Photo-generated electron and hole transfer to the surface and trapping | \n\n\n \n\n | \n200 fs 50 ps | \n
Recombination of charge carriers | \n\n\n | \n>20 ns | \n
Interfacial charge transfer | \n\n\n | \n10–100 μs | \n
Observed O2 evolution* | \n\n\n | \n37 s* | \n
Photosynthesis of H2O oxidation | \n\n\n | \n1.59 ms [84] | \n
The processes occurring in photocatalytic water splitting on TiO2 and their timescales [27] and the references therein.
Based on 16,000 μmol O2/g/h O2 evolution rate on Rh0.5Cr1.5O3-doped Ga2O3:Zn upon illumination at 254 nm [41], assuming 10 m2/g surface area and 1015 sites/cm2 site density.
Parameter | \nUnit | \nValue | \n
---|---|---|
Particle size, \n | \nμm | \n1 | \n
Density of water, \n | \nkg/m3 | \n997 at 25°C | \n
Viscosity of water, \n | \nPa s | \n890 × 10−6 at 25°C | \n
Kinematic viscosity, \n | \nm2/s | \n8.93 × 10−7 | \n
Schmidt number \n | \n\n | 141 | \n
Energy density, \n | \nm2/s3 | \n25 for 5 W stirrer, 200 g solution | \n
Reynold’s number \n | \n\n | 0.033 | \n
Sherwood number, \n | \n\n | 2.46 | \n
Diffusion coefficient of H2 in water | \nm2/s | \n6.30*10−9 at 25 °C | \n
Convection mass transfer coefficient for solid, \n | \nm/s | \n0.015 | \n
\n\n | \nm3/s | \n0.31 | \n
Convection mass transfer coefficient calculation for solid–liquid transfer and parameters used in the calculation.
The creation of air/inert bubbles in the continuous phase (water) due to agitation could be considered as the transfer mechanism of produced H2 from the liquid phase to the gas phase. In such systems, comparing the mass transfer resistance from liquid to interface and interface to gas, it can be assumed that nearly all of the mass transfer resistance comes from the liquid side of the interface [102], leaving Eq. 9 as Eq. 11:
\nThe liquid side mass transfer coefficient for such a system could then be calculated using Calderbank and Moo-Young correlation for rising small bubbles of gas in continuous liquid phase (Eq. 12) [103]:
\nThe first term on Eq. 12 is the molecular diffusion term, whereas the second term is for the rise of the bubbles due to gravitational forces independent of the agitation. With estimations on the bubble size and gas holdup of such a system (given in Table 6), the mass transfer coefficient and \n
Parameter | \nUnit | \nValue | \n
---|---|---|
Bubble size, \n | \nμm | \n700 | \n
Density of water, \n | \nkg/m3 | \n997 at 25°C | \n
Density of air, \n | \nkg/m3 | \n1.18 at 25°C | \n
Viscosity of water, \n | \nPa s | \n890 × 10−6 at 25°C | \n
Raleigh number \n | \n\n | 5.97 × 105 | \n
Sherwood number, \n | \n\n | 28.1 | \n
Diffusion coefficient of H2 in water | \nm2/s | \n6.3 × 10−9 at 25°C | \n
Convection mass transfer coefficient for liquid, \n | \nm/s | \n2.5 × 10−4 | \n
Liquid–gas bubble contact area, \n | \nm2 | \n0.008 (gas holdup, \n | \n
\n\n | \nm3/s | \n2.2 × 10−6 | \n
Convection mass transfer coefficient calculation for liquid–gas transfer and parameters used in the calculation.
The overall mass transfer coefficient and the mass transfer rate from solid to the gas phase can be calculated with the estimated \n
Calculated H2 mass transfer rate values (μmol H2/h) for gas holdup values of 0.001 and 0.005 and H2 adsorption capacity values in the range of 50–400 μmol/g. The liquid volume is taken as 200 ml and catalyst weight is taken as 0.5 g.
The H2 concentration on the solid surface and the gas–liquid contact area are not easy to estimate. However, for the limited gas–liquid area, the photocatalytic reaction rates above the calculated mass transfer rate will be suppressed due to the limiting mass transfer rates. Therefore, special care must be given for the UV-irradiated photocatalytic systems, in which observed H2 and O2 evolution rates are found to be close to the calculated mass transfer rates here (see Table 1).
\nThese studies show that for each type of photocatalytic system that contains limited gas–liquid contact area or immobilized photocatalyst, mass transfer limitations should not be underestimated, and not only the materials but also the systems should be improved for better photocatalytic efficiencies.
\nThe literature examples of photocatalytic water splitting activities show improvements in visible light utilization, charge separation, and prevention of back-oxidation reactions via fine tuning of photocatalyst materials that enabled more efficient water splitting systems. The efficiency of these systems working under sunlight is better defined with solar-to-hydrogen energy conversion efficiency (STH), i.e., hydrogen production rate times the Gibbs free energy for generating 1 mole of H2 divided by the power of incident sunlight (Eq. 13):
\nThe estimated STH required for the particulate photocatalytic systems to be economically compatible with current H2 production technologies is 10% [104]. However, the highest STH values obtained with current developed photocatalysts are in the order of 1–2% (1.8% at 400–475 nm using Rh/Cr2O3-loaded GaN:Mg/InGaN:Mg photocatalyst [69] and 2% at 420 nm using CDots-C3N4 [70]). The STH conversion efficiency depends both on the catalytic activity and the extent of the utilization of sunlight that depends on the bandgap of the semiconductor. With current photocatalysts having absorption edges around 500 nm, even 100% apparent quantum yields would not guarantee 10% STH values [106]. For a photocatalyst to show 10% STH values, it should have absorption edges at least at 600 nm with apparent quantum yields around 60%. Under the light of these calculations, it can be said that the present photocatalysts having adsorption edge values around 450 nm and quantum yields around 10% are far from being utilized in commercial systems. In order to achieve targeted STH values, the photocatalysts with lower bandgap energies such as (oxy)nitrides and (oxy)sulfides should be improved for H2 evolution activities while ensuring their thermal stability and photostability.
\nLarge-scale photocatalytic water splitting reactors are implemented with current low STH values as of 2015. The first example of large-scale photocatalytic water splitting utilized Pt-loaded C3N4 photocatalyst with sacrificial electron donor triethanolamine in a flat-panel-type photocatalytic reactor system in 2015 [107]. The solar-to-hydrogen conversion efficiency is reported to be 0.12%, for which the photocatalytic activity is monitored for 30 days. In such systems, where a sacrificial reagent such as triethanolamine or methanol is irreversibly oxidized at a more negative potential than water (thermodynamically more favorable) at the oxidation centers, the photo-generated charges can be more efficiently separated, thus increasing H2 evolution rates. However, in those systems, hydrogen production is not solely due to the water splitting; as the carbon- and hydrogen-containing “sacrificial agents” are being oxidized at the oxidation centers, they produce hydrogen as well as aldehydes, carboxylic acid, and carbon dioxide [108].
\nAnother large-scale photoreactor is reported by Domen et al., who used Al-doped and RhCrOx-loaded SrTiO3 photocatalyst sheets in their 1 × 1 m water splitting panel [45]. The achieved STH value under simulated sunlight is 0.6% at 331 K and limited to a maximum value of 1.4% due to the large bandgap energy of the photocatalyst (3.2 eV). Improved STH values (reaching 1.1%) are shown to be possible on a two-step excitation system in 2016 using photocatalyst sheets having smaller bandgap energy values such as Mo-doped BiVO4 (2.4 eV) and La- and Rh-doped SrTiO3 [5].
\nThe developments in the photocatalytic water splitting reactions are explained here with the emphasis on the one-step photocatalysis systems. The early photocatalyst improvements with bandgap engineering, co-catalyst usage, and size reductions are shown to contribute to the increased visible light-driven H2 evolution activity values. The main drawbacks in the present systems are discussed to be the charge recombination, back-oxidation reactions of the products into water, and mass transfer limitations especially in the three-phase systems. Using defect-free small crystals of photocatalysts and making use of phase junctions or metal co-catalysts are suggested to decrease charge recombination rates. Back-oxidation of H2 into water or oxygen reduction reaction to water is expected in many noble metal-containing particulate photocatalyst systems. The prevention of these unwanted secondary reactions is shown to be possible to some extent by modification of the noble metal surfaces. Some examples of these modifications are anion coating, partial adsorption of a poison, or nanolayer coating of the co-catalyst or the whole photocatalyst. Selective permeation property of the nanolayer coatings such as Cr2O3 is reported to suppress the back-oxidation rates, resulting in enhanced H2 and O2 evolution rates. Possible mass transfer limitations, limiting the observed rates in three-phase systems, are predicted especially in the liquid–gas interfaces. The literature examples attracted attention for the liquid–gas interfaces in suspended systems and internal mass transfer limitations for the immobilized photocatalyst systems. It is concluded that, in addition to the required developments in activities with suppression of charge recombination, back-oxidation, and mass transfer limitations, future of the photocatalytic systems would necessitate active and stable photocatalysts with narrower bandgap energies (to be activated at >600 nm) for achieving targeted 10% solar-to-hydrogen energy conversion efficiency value.
\nCoumarins are members of the benzopyrone class of organic compounds that are found in many plants [1] and possess a variety of pharmacological properties such as antimicrobial, anti-inflammatory, antidiabetic, and antioxidant activity, as well as a significant influence on physiological processes like enzyme inhibitory activity [2]. Despite the wide availability of coumarins and their lead compounds and metabolites in natural products [3], their application up till now has been mostly limited to the anticoagulant activity of warfarin derived from dicoumarol and its analogues [4]. The mechanism of action of these anticoagulants lies in the competitive antagonism of vitamin K, through which they inhibit coagulation of blood in the body by preventing the production of prothrombin and several other coagulation factors [5]. Due to this action, these coumarins are a major group of oral drugs with anticoagulant activity. Anticoagulants reduce the risks of undesirable blood clots leading to myocardial infarction, pulmonary embolism, and ischemic stroke among others. This chapter gives an overview of medical applications of coumarins, in particular the history and evolution of warfarin and related compounds as important anticoagulant agents.
The medical application of plants containing coumarins probably started long before the isolation of this chemical compound from the Tonka bean in 1820 by Nicholas Jean Baptiste Gaston Guibourt [6]. Ancient Romans produced a cough syrup from the marshmallow (Althea officinalis) [7], which contains the coumarin scopoletin [8]. This coumarin demonstrated inhibition of leucocyte migration in mice [9], a process that can be linked to the alleged antitussive effect. Cough is a result of the reaction of the airways to leukotrienes and other factors secreted by leucocytes [10, 11, 12]. Inhibition of the migration of these to the affected region consequently reduces the availability of these paracrine factors. In addition, at least one of the herbs mentioned by the famous Roman General Pliny in his pharmacopeia [13] contain coumarins with proven action. For example, the extracts of the common rue or herb-of-grace Ruta graveolens contain xanthotoxin [14], a coumarin that reduces the mobility of human spermatozoa possibly through inhibition of membranaeous potassium channels [15].
The application of coumarin and its derivatives in current western medicine dates to the fifties of the past century with the clinical recognition of coumarins as anticoagulant agents. This event was the result of observations of poisoning of animals with coumarin derivatives that led to massive internal organ bleeding [16]. Soon it became clear that the substance that was responsible for the deadly internal bleeding of cattle was dicoumarol [17]. Shortly after this, the proposal was made to develop a coumarin derivative with rodent killing ability and gradually warfarin found its application as a potent rodenticide [18]. When it became clear that this substance also led to bleeding disorders after poisoning in human beings [19], its application as a therapeutic anticoagulant found its way in medicine [20, 21]. Seven decades later, warfarin is still in use as an anticoagulant [22]. In the meantime, several other coumarins with anticoagulant properties like acenocoumarol, phenprocoumon, and fluindione have been developed, and they are used in a variety of clinical settings [23, 24, 25].
Hemostasis in mammals and humans is the result of three sequential processes. The first of these is the acute vasoconstriction within seconds after damaged arteries and veins, by local activity of the potent vasoconstrictor thromboxane among others [26]. The second step is the formation of a blood clot through the entrapment of platelets by fibrin within hours and finally followed by the organization of the fibrin mesh into an adhesive structure on the vessel wall [27]. In one of the last steps of the coagulation, prothrombin converts to thrombin, an enzyme that converts the plasma protein fibrinogen to fibrin monomers and activates factor XIII of the coagulation cascade. Activated factor XIII synthesizes fibrin from these monomers. The acquired fibrin molecules then trap the platelets and eventually form the blood clot [27, 28]. The coagulation process has both an intrinsic and an extrinsic pathway. The difference is that the intrinsic pathway only requires ionized calcium to be activated while the extrinsic pathway requires both calcium and tissue factor that is released with trauma [28]. Figure 1 gives an overview of the coagulation process along with the interference sites of warfarin.
Overview of the coagulation process along with the interference sites of warfarin.
Warfarin is highly lipid soluble [29, 30, 31]. Between 70 and 100% of the oral intake is rapidly absorbed by the intestines with a maximum plasma concentration within 2 h after oral intake [30]. The half-life of the drug is generally more than 20 h, with a large individual variety [32]. Due to the overt lipid solubility, the major part of the drug is protein bound with less than 3% being biologically available [29]. Consequently, the agent has a slow onset of action and a long duration of activity [33]. In fact, the optimal effect is delayed for a few days, until all remaining activated factors II, VII, IX, and X are depleted from the liver and the circulation [33]. Warfarin accumulates in the liver where it exerts its effect and is inactivated through oxidative metabolism by cytochromes P450 to several isomers of water-soluble hydroxywarfarin with negligible anticoagulant activity [34, 35]. These metabolites are almost completely cleared by the kidneys [36]. The hepatic accumulation and relative easy absorption in the intestines result in an enterohepatic circulation of the drug [37]. Enterohepatic circulation is a process in which substances are secreted by the liver with bile to the intestines and subsequently absorbed again by the latter [28]. This results in recycling of the product with very little elimination.
Warfarin inhibits the enzyme vitamin K epoxide reductase that recycles oxidized vitamin K [38]. Vitamin K activates the coagulating factors prothrombin (factor II) and the structurally related serine proteases known as factors VII, IX, and X in the liver cells [27]. Decreasing the biological availability of vitamin K inhibits the synthesis of these essential factors and eventually leads to inhibition of the coagulation process. This means that this compound affects both the intrinsic as well as the extrinsic cascade of coagulation since prothrombin plays a central role in both of these pathways [28] and renders it a highly effective anticoagulant drug.
Hemostasis is an essential process to prevent significant external as well as internal blood loss after injury. However, under certain circumstances, it is not desirable to activate or continue this homeostatic process like in disorders with spontaneous thrombosis such as deep venous thrombosis in the legs often resulting in pulmonary embolism [39]. In addition, there are conditions that are prone to a reasonable chance of forming a blood clot during stasis of the blood circulation like in atrial fibrillation and in the limbs of patients with prolonged immobility after surgery [39]. Moreover, conditions like myocardial infarction or ischemic stroke form a preventable group of disorders with inhibition of the thrombotic process [5]. Based on its anticoagulant properties, warfarin is thus an ideal compound for the treatment and prevention of these thromboembolic conditions [5].
Based on the pharmacokinetic properties and the challenges they present, dosing of warfarin is not simple, and a careful approach is necessary. On one hand, a low plasma concentration will not achieve the effect of sufficient anticoagulation and, on the other side of the spectrum, there is the constant chance of overdosing with potential lethal internal or external bleeding. Another problem is the great variety of absorption, body distribution, and metabolism of the agent with individual patients based on the pharmacokinetic properties of warfarin [30]. Frequent monitoring of therapeutic efficiency with adequate laboratory tools like prothrombin time (PT) or international normalized ratio (INR) is absolutely necessary [40, 41] and a fixed or constant dose is close to impossible. Nevertheless, warfarin is highly effective in anticoagulation regiments when carefully dosing and assessing the potential bleeding sites as well as other potential side effects. Warfarin is initially dosed at 5–10 mg daily [42]. Subsequent doses depend on the international normalized ratio, with a therapeutic value between 2 and 4. Concomitant administration of heparins like fraxiparine is necessary when fast anticoagulant activity is desirable [43].
Warfarin is not an ideal agent in conditions when immediate treatment of thromboembolism is imminent due to the long time of onset. In cases of pulmonary embolism and acute ischemic stroke, it is desirable to start with both the oral anticoagulant and fast-acting agent like heparins [44]. The long duration of action harbors another challenge. When acute termination of anticoagulation is necessary with unwanted bleeding like in menstruating women and after blunt and sharp trauma leading to hemorrhage, it could take days before the process of coagulation completely restores after quitting oral administration [27, 30] due to the depletion of coagulant factors in liver and blood. In these cases, intravenous administration of prothrombin complex, fresh frozen plasma with coagulation factors, and high doses of vitamin K may be helpful [45].
Warfarin readily passes the placenta and may result in spontaneous abortion due to retroplacental bleeding [46], as well as prematurity [47], fetal deformity [48], stillbirth [48], and fetal cranial bleeding [49]. Administration during the first trimester of pregnancy has a high risk of embryopathy [50]. This is accompanied by deformities of bone and cartilage [51], blindness, mental retardation, and other neurologic abnormalities [52]. The occurrence of these complications and defects seem to be dose dependent [47, 53] and are most probably the result of the interference with vitamin K-dependent coagulation [38] and bone formation [54]. The effects of the central nervous system and the blindness are probably the result of microhemorrhages in the developing brain as a result of the anticoagulant activity [55]. Clotting factors are easily depleted in the fetus due to the immature liver and small circulating volume [46]. Warfarin does not enter breastmilk and is thus completely safe during lactation [56].
In conclusion, warfarin must be administered with great caution to women in their child-bearing age [57]. Therapy with this agent must be ceased immediately when it becomes clear that the patient is pregnant. Low-molecular weight heparins are a good alternative, since they do not cross the placenta and have been proven to be safe for mother, embryo, and fetus [58].
To say that anticoagulant coumarins have only a few side effects is an absolute understatement. Warfarin is one of the leading drugs with adverse effects requiring hospital admission [59]. Most of all, there is the constant chance of severe bleeding [60]. This can include internal hemorrhagic conditions in the head, gastrointestinal tract, female genitalia, the bladder and urethra or skeletal joints and muscles [40, 61]. They generally present as severe headache, stomach pain, and black or bloody stool, heavier than normal menstrual bleeding, discoloration of urine, and pain and swelling of the joints or muscles. Prolonged bleeding from external sharp or blunt wounds is always present [61]. All these conditions are the result of inability of the affected tissues to initiate and continue the process of hemostasis after damage to the epithelial barrier [62].
Patients suffering from hypertension, disorders of the liver, bleeding lesions, and the elderly and patients using drugs and substances that affect coagulation are at higher risk to suffer from bleeding when using warfarin [63]. Hypertension poses mechanical defects in the blood vessels, especially the arteries. Disorders of the liver reduce the ability of the body to eliminate the warfarin and thus make it more biologically available. In bleeding lesions, warfarin inhibits hemostasis. Among substances that can lead to bleeding when used with warfarin are steroidal and non-steroidal anti-inflammatory drugs, antibiotics, and alcohol. These potentiate the activity, interfere with the protein binding, and reduce the metabolism of warfarin, respectively [63]. Other side effects include injury to the kidneys with potential nephritis [64, 65, 66], inflammation of the skin [67] and blood vessels [66], and potentiation of rhabdomyolysis by simvastatin [64].
Due to resistance of rodents against warfarin, superwarfarins have been created [68]. These have a much longer time of activity and hence need only to be consumed once by the rodents, contrary to warfarin. The result however is that their effect persists much longer when deliberately or accidently consumed by humans [69] and treatment of this intoxication is a more challenging enterprise.
The efficacy of the anticoagulant treatment with warfarin highly depends on its bioavailability, since inhibition of the target (epoxy reductase) enzyme depends on direct binding of the drug to this protein [38]. In addition, vitamin K from external sources does not rely on recycling through this enzyme [38]. Hence, the absorption, transport, delivery, and elimination of warfarin as well as the external availability of vitamin K are potential sites of interaction with other drugs and with food and dietary supplements.
Drugs and food that influence the enterohepatic circulation can all affect the absorption of warfarin. Examples of these are the drug cholestyramine [37] and the avocado fruit [70], which prevent the reabsorption of warfarin in the intestines Concomitant administration of other protein-bound drugs may lead to greater amounts of circulating warfarin and increased risks of bleeding. Valproate sodium increases the bioavailability of warfarin through dislocation of its protein-binding sites [71]. Interference with the metabolism of warfarin is a potential of most drugs that are eliminated by hepatic metabolism. Among these are aspirin [72], nonsteroidal anti-inflammatory drugs [72], serotonin reuptake inhibitors [49], antiplatelet agents and some antibiotics [72]. It can go both ways with the metabolism. Induction of the cytochromes will increase the elimination, while occupation of the binding sites by the drugs will increase the availability of warfarin.
Since warfarin acts through elimination of available bioactive vitamin K, variations of the net intake of this vitamin will certainly interfere with the drug action. A high intake of the vitamin will keep the coagulant factors at a higher level and thus inhibit the anticoagulant activity. Likewise, a lower intake will potentiate the effect of warfarin. The vitamin occurs in food in the form of phylloquinone and menaquinone. Phylloquinone is the form mostly found in plants and is also the most abundant form in food [73]. Menaquinones are mainly the product of bacterial production or conversion [74]. Consequently, simple multivitamin and other supplements, food with high vitamin K content [74] as well as antibiotics are sources of fluctuation in vitamin K intake since intestinal bacteria significantly contribute to the production of menaquinones [75].
Recently, another source of interference came into focus. In addition to the previous mentioned parameters, genetic variation in the expression of cytochrome P450 seems to play a role in the metabolism of warfarin [76], thus influencing the availability of the drug [77]. All these considerations make it clear that close monitoring of the individual coagulation ability is necessary for a successful therapy with this agent.
The abovementioned interactions are just a few of the many that are possible. Table 1 gives examples of a variety of interactions with drugs, food, natural products, and supplements. This is only to underscore the cautious approach patients should practice when taking warfarin.
Agent | Category | Possible mechanism | Effect | Reference | |
---|---|---|---|---|---|
Allopathic medications | Amiodarone | Antiarrhythmic | Inhibition of hepatic metabolism | [78] | |
Ciprofloxacin | Antibiotic | Reduction of vitamin K synthesis by intestinal bacteria | Increased bleeding | [79] | |
Paroxetine | Antidepressant | Inhibition of hepatic metabolism | Potentiation | [80] | |
Citalopram | Antidepressant | Inhibition of hepatic metabolism | Potentiation | [80] | |
Clopidogrel | Antiplatelet medication | Inhibition of coagulation cascade | Potentiation | [81] | |
Dipyridamole | Antiplatelet medication | Inhibition of coagulation cascade | Potentiation | [81] | |
Diclofenac | NSAID | Inhibition of coagulation cascade | Potentiation | [82] | |
Naproxen | NSAID | Inhibition of coagulation cascade | Potentiation | [82] | |
Acetaminophen | Analgesic | Interference with hepatic metabolism | Increased bleeding | [83] | |
Food supplements | Fish oil | Lipid profile improvement | Inhibition of coagulation cascade? | Potentiation | [84] |
Pomegranate juice | Antioxidant | Interference with hepatic metabolism | Potentiation | [85] | |
Glucosamine | Cartilage improvement | Unknown | Potentiation | [86] | |
Traditional medications | Chamomile | Medicinal herbal tea | Unknown | Increased bleeding | [87] |
Ginseng | Improving cognitive functions | Unknown | Inhibition | [88] | |
St John’s wort | Against depression | Induction of metabolism | Inhibition | [89] |
Brief overview of possible interactions with warfarin.
Today, coumarins find their application predominantly as anticoagulants in medicine. The narrow therapeutic index of warfarin and related compounds sometimes limit their applicability and consequently there is a constant search for more safe agents in this drug class [90]. Unfortunately, the development of these will probably limit the use of these oral anticoagulants.
Aside from these developments, coumarins with several applications in medical practice are progressively being introduced. Investigators found that coumarin-3-carboxylic acid could be utilized as a dosimeter for radiotherapy. This substance converts to the highly fluorescent 7-hydroxy-coumarin-3-carboxylic acid, with a near perfect linear correlation upon irradiation [91].
The coumarin 2-hydroxycinnamic acid demonstrated inhibitive properties on the enzyme carbonic anhydrase [92]. Inhibition of this enzyme leads to diuresis [93] and decreases intraocular pressure in glaucoma patients [93] with clear therapeutic potential and clinical perspective.
Furano(pyrano)coumarins found in the roots of the Korean angelica (Angelica gigas) showed antibacterial activity in hay bacillus (Bacillus subtilis) cultures [94]. The coumarine derivative cloricromene reduced the inflammatory parameters in rats subjected to collagen-induced arthritis [95]. In addition, several studies found that coumarins may be useful as anti-tumor agents [4, 96].
Probably since ancient times, coumarins found their application in medicine. Currently, however, coumarins with predominantly anticoagulant properties are applied in daily medical practice. These have been developed from the initial discovery of a cattle killing weed more than six decades ago. Initially applied as a rodenticide, soon a therapeutic usable oral anticoagulant was developed, and slowly other agents entered the market. They have a small therapeutic index, rendering them toxic in a number of circumstances. The search for more safe agents with anticoagulant effects is ongoing and this may result in a decline of the use of coumarins in this field. Nevertheless, coumarins gradually find their way in other fields of medicine. Nevertheless, all these developments promise a bright future for coumarins in medical applications.
The author declares no conflict of interest.
My gratitude goes out to my colleague Prof. Dr Dennis Mans, who encouraged me to write this chapter. I also want to thank the Anton de Kom University of Suriname for allowing me to do this on top of my regular job.
I want to thank my lovely wife and children who understand why I spend so much time in front of my PC. You are all such a supporting force for me.
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\n'}]},successStories:{items:[]},authorsAndEditors:{filterParams:{sort:"featured,name"},profiles:[{id:"105746",title:"Dr.",name:"A.W.M.M.",middleName:null,surname:"Koopman-van Gemert",slug:"a.w.m.m.-koopman-van-gemert",fullName:"A.W.M.M. Koopman-van Gemert",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/105746/images/5803_n.jpg",biography:"Dr. Anna Wilhelmina Margaretha Maria Koopman-van Gemert MD, PhD, became anaesthesiologist-intensivist from the Radboud University Nijmegen (the Netherlands) in 1987. She worked for a couple of years also as a blood bank director in Nijmegen and introduced in the Netherlands the Cell Saver and blood transfusion alternatives. She performed research in perioperative autotransfusion and obtained the degree of PhD in 1993 publishing Peri-operative autotransfusion by means of a blood cell separator.\nBlood transfusion had her special interest being the president of the Haemovigilance Chamber TRIP and performing several tasks in local and national blood bank and anticoagulant-blood transfusion guidelines committees. Currently, she is working as an associate professor and up till recently was the dean at the Albert Schweitzer Hospital Dordrecht. She performed (inter)national tasks as vice-president of the Concilium Anaesthesia and related committees. \nShe performed research in several fields, with over 100 publications in (inter)national journals and numerous papers on scientific conferences. \nShe received several awards and is a member of Honour of the Dutch Society of Anaesthesia.",institutionString:null,institution:{name:"Albert Schweitzer Hospital",country:{name:"Gabon"}}},{id:"83089",title:"Prof.",name:"Aaron",middleName:null,surname:"Ojule",slug:"aaron-ojule",fullName:"Aaron Ojule",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:{name:"University of Port Harcourt",country:{name:"Nigeria"}}},{id:"295748",title:"Mr.",name:"Abayomi",middleName:null,surname:"Modupe",slug:"abayomi-modupe",fullName:"Abayomi Modupe",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/no_image.jpg",biography:null,institutionString:null,institution:{name:"Landmark University",country:{name:"Nigeria"}}},{id:"94191",title:"Prof.",name:"Abbas",middleName:null,surname:"Moustafa",slug:"abbas-moustafa",fullName:"Abbas Moustafa",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/94191/images/96_n.jpg",biography:"Prof. Moustafa got his doctoral degree in earthquake engineering and structural safety from Indian Institute of Science in 2002. He is currently an associate professor at Department of Civil Engineering, Minia University, Egypt and the chairman of Department of Civil Engineering, High Institute of Engineering and Technology, Giza, Egypt. He is also a consultant engineer and head of structural group at Hamza Associates, Giza, Egypt. Dr. Moustafa was a senior research associate at Vanderbilt University and a JSPS fellow at Kyoto and Nagasaki Universities. He has more than 40 research papers published in international journals and conferences. He acts as an editorial board member and a reviewer for several regional and international journals. His research interest includes earthquake engineering, seismic design, nonlinear dynamics, random vibration, structural reliability, structural health monitoring and uncertainty modeling.",institutionString:null,institution:{name:"Minia University",country:{name:"Egypt"}}},{id:"84562",title:"Dr.",name:"Abbyssinia",middleName:null,surname:"Mushunje",slug:"abbyssinia-mushunje",fullName:"Abbyssinia Mushunje",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:{name:"University of Fort Hare",country:{name:"South Africa"}}},{id:"202206",title:"Associate Prof.",name:"Abd Elmoniem",middleName:"Ahmed",surname:"Elzain",slug:"abd-elmoniem-elzain",fullName:"Abd Elmoniem Elzain",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:{name:"Kassala University",country:{name:"Sudan"}}},{id:"98127",title:"Dr.",name:"Abdallah",middleName:null,surname:"Handoura",slug:"abdallah-handoura",fullName:"Abdallah Handoura",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:{name:"École Supérieure des Télécommunications",country:{name:"Morocco"}}},{id:"91404",title:"Prof.",name:"Abdecharif",middleName:null,surname:"Boumaza",slug:"abdecharif-boumaza",fullName:"Abdecharif Boumaza",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:{name:"Abbès Laghrour University of Khenchela",country:{name:"Algeria"}}},{id:"105795",title:"Prof.",name:"Abdel Ghani",middleName:null,surname:"Aissaoui",slug:"abdel-ghani-aissaoui",fullName:"Abdel Ghani Aissaoui",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/105795/images/system/105795.jpeg",biography:"Abdel Ghani AISSAOUI is a Full Professor of electrical engineering at University of Bechar (ALGERIA). He was born in 1969 in Naama, Algeria. He received his BS degree in 1993, the MS degree in 1997, the PhD degree in 2007 from the Electrical Engineering Institute of Djilali Liabes University of Sidi Bel Abbes (ALGERIA). He is an active member of IRECOM (Interaction Réseaux Electriques - COnvertisseurs Machines) Laboratory and IEEE senior member. He is an editor member for many international journals (IJET, RSE, MER, IJECE, etc.), he serves as a reviewer in international journals (IJAC, ECPS, COMPEL, etc.). He serves as member in technical committee (TPC) and reviewer in international conferences (CHUSER 2011, SHUSER 2012, PECON 2012, SAI 2013, SCSE2013, SDM2014, SEB2014, PEMC2014, PEAM2014, SEB (2014, 2015), ICRERA (2015, 2016, 2017, 2018,-2019), etc.). His current research interest includes power electronics, control of electrical machines, artificial intelligence and Renewable energies.",institutionString:"University of Béchar",institution:{name:"University of Béchar",country:{name:"Algeria"}}},{id:"99749",title:"Dr.",name:"Abdel Hafid",middleName:null,surname:"Essadki",slug:"abdel-hafid-essadki",fullName:"Abdel Hafid Essadki",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:{name:"École Nationale Supérieure de Technologie",country:{name:"Algeria"}}},{id:"101208",title:"Prof.",name:"Abdel Karim",middleName:"Mohamad",surname:"El Hemaly",slug:"abdel-karim-el-hemaly",fullName:"Abdel Karim El Hemaly",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/101208/images/733_n.jpg",biography:"OBGYN.net Editorial Advisor Urogynecology.\nAbdel Karim M. A. El-Hemaly, MRCOG, FRCS � Egypt.\n \nAbdel Karim M. A. El-Hemaly\nProfessor OB/GYN & Urogynecology\nFaculty of medicine, Al-Azhar University \nPersonal Information: \nMarried with two children\nWife: Professor Laila A. Moussa MD.\nSons: Mohamad A. M. El-Hemaly Jr. MD. Died March 25-2007\nMostafa A. M. El-Hemaly, Computer Scientist working at Microsoft Seatle, USA. \nQualifications: \n1.\tM.B.-Bch Cairo Univ. June 1963. \n2.\tDiploma Ob./Gyn. Cairo Univ. April 1966. \n3.\tDiploma Surgery Cairo Univ. Oct. 1966. \n4.\tMRCOG London Feb. 1975. \n5.\tF.R.C.S. Glasgow June 1976. \n6.\tPopulation Study Johns Hopkins 1981. \n7.\tGyn. Oncology Johns Hopkins 1983. \n8.\tAdvanced Laparoscopic Surgery, with Prof. Paulson, Alexandria, Virginia USA 1993. \nSocieties & Associations: \n1.\t Member of the Royal College of Ob./Gyn. London. \n2.\tFellow of the Royal College of Surgeons Glasgow UK. \n3.\tMember of the advisory board on urogyn. FIGO. \n4.\tMember of the New York Academy of Sciences. \n5.\tMember of the American Association for the Advancement of Science. \n6.\tFeatured in �Who is Who in the World� from the 16th edition to the 20th edition. \n7.\tFeatured in �Who is Who in Science and Engineering� in the 7th edition. \n8.\tMember of the Egyptian Fertility & Sterility Society. \n9.\tMember of the Egyptian Society of Ob./Gyn. \n10.\tMember of the Egyptian Society of Urogyn. \n\nScientific Publications & Communications:\n1- Abdel Karim M. El Hemaly*, Ibrahim M. Kandil, Asim Kurjak, Ahmad G. Serour, Laila A. S. Mousa, Amr M. Zaied, Khalid Z. El Sheikha. \nImaging the Internal Urethral Sphincter and the Vagina in Normal Women and Women Suffering from Stress Urinary Incontinence and Vaginal Prolapse. Gynaecologia Et Perinatologia, Vol18, No 4; 169-286 October-December 2009.\n2- Abdel Karim M. El Hemaly*, Laila A. S. Mousa Ibrahim M. Kandil, Fatma S. El Sokkary, Ahmad G. Serour, Hossam Hussein.\nFecal Incontinence, A Novel Concept: The Role of the internal Anal sphincter (IAS) in defecation and fecal incontinence. Gynaecologia Et Perinatologia, Vol19, No 2; 79-85 April -June 2010.\n3- Abdel Karim M. El Hemaly*, Laila A. S. Mousa Ibrahim M. Kandil, Fatma S. El Sokkary, Ahmad G. Serour, Hossam Hussein.\nSurgical Treatment of Stress Urinary Incontinence, Fecal Incontinence and Vaginal Prolapse By A Novel Operation \n"Urethro-Ano-Vaginoplasty"\n Gynaecologia Et Perinatologia, Vol19, No 3; 129-188 July-September 2010.\n4- Abdel Karim M. El Hemaly*, Ibrahim M. Kandil, Laila A. S. Mousa and Mohamad A.K.M.El Hemaly.\nUrethro-vaginoplasty, an innovated operation for the treatment of: Stress Urinary Incontinence (SUI), Detursor Overactivity (DO), Mixed Urinary Incontinence and Anterior Vaginal Wall Descent. \nhttp://www.obgyn.net/urogyn/urogyn.asp?page=/urogyn/articles/ urethro-vaginoplasty_01\n\n5- Abdel Karim M. El Hemaly, Ibrahim M Kandil, Mohamed M. Radwan.\n Urethro-raphy a new technique for surgical management of Stress Urinary Incontinence.\nhttp://www.obgyn.net/urogyn/urogyn.asp?page=/urogyn/articles/\nnew-tech-urethro\n\n6- Abdel Karim M. El Hemaly, Ibrahim M Kandil, Mohamad A. Rizk, Nabil Abdel Maksoud H., Mohamad M. Radwan, Khalid Z. El Shieka, Mohamad A. K. M. El Hemaly, and Ahmad T. El Saban.\nUrethro-raphy The New Operation for the treatment of stress urinary incontinence, SUI, detrusor instability, DI, and mixed-type of urinary incontinence; short and long term results. \nhttp://www.obgyn.net/urogyn/urogyn.asp?page=urogyn/articles/\nurethroraphy-09280\n\n7-Abdel Karim M. El Hemaly, Ibrahim M Kandil, and Bahaa E. El Mohamady. Menopause, and Voiding troubles. \nhttp://www.obgyn.net/displayppt.asp?page=/English/pubs/features/presentations/El-Hemaly03/el-hemaly03-ss\n\n8-El Hemaly AKMA, Mousa L.A. Micturition and Urinary\tContinence. Int J Gynecol Obstet 1996; 42: 291-2. \n\n9-Abdel Karim M. El Hemaly.\n Urinary incontinence in gynecology, a review article.\nhttp://www.obgyn.net/urogyn/urogyn.asp?page=/urogyn/articles/abs-urinary_incotinence_gyn_ehemaly \n\n10-El Hemaly AKMA. Nocturnal Enuresis: Pathogenesis and Treatment. \nInt Urogynecol J Pelvic Floor Dysfunct 1998;9: 129-31.\n \n11-El Hemaly AKMA, Mousa L.A.E. Stress Urinary Incontinence, a New Concept. Eur J Obstet Gynecol Reprod Biol 1996; 68: 129-35. \n\n12- El Hemaly AKMA, Kandil I. M. Stress Urinary Incontinence SUI facts and fiction. Is SUI a puzzle?! http://www.obgyn.net/displayppt.asp?page=/English/pubs/features/presentations/El-Hemaly/el-hemaly-ss\n\n13-Abdel Karim El Hemaly, Nabil Abdel Maksoud, Laila A. Mousa, Ibrahim M. Kandil, Asem Anwar, M.A.K El Hemaly and Bahaa E. El Mohamady. \nEvidence based Facts on the Pathogenesis and Management of SUI. http://www.obgyn.net/displayppt.asp?page=/English/pubs/features/presentations/El-Hemaly02/el-hemaly02-ss\n\n14- Abdel Karim M. El Hemaly*, Ibrahim M. Kandil, Mohamad A. Rizk and Mohamad A.K.M.El Hemaly.\n Urethro-plasty, a Novel Operation based on a New Concept, for the Treatment of Stress Urinary Incontinence, S.U.I., Detrusor Instability, D.I., and Mixed-type of Urinary Incontinence.\nhttp://www.obgyn.net/urogyn/urogyn.asp?page=/urogyn/articles/urethro-plasty_01\n\n15-Ibrahim M. Kandil, Abdel Karim M. El Hemaly, Mohamad M. Radwan: Ultrasonic Assessment of the Internal Urethral Sphincter in Stress Urinary Incontinence. The Internet Journal of Gynecology and Obstetrics. 2003. Volume 2 Number 1. \n\n\n16-Abdel Karim M. El Hemaly. Nocturnal Enureses: A Novel Concept on its pathogenesis and Treatment.\nhttp://www.obgyn.net/urogynecolgy/?page=articles/nocturnal_enuresis\n\n17- Abdel Karim M. El Hemaly. Nocturnal Enureses: An Update on the pathogenesis and Treatment.\nhttp://www.obgyn.net/urogynecology/?page=/ENHLIDH/PUBD/FEATURES/\nPresentations/ Nocturnal_Enuresis/nocturnal_enuresis\n\n18-Maternal Mortality in Egypt, a cry for help and attention. The Second International Conference of the African Society of Organization & Gestosis, 1998, 3rd Annual International Conference of Ob/Gyn Department � Sohag Faculty of Medicine University. Feb. 11-13. Luxor, Egypt. \n19-Postmenopausal Osteprosis. The 2nd annual conference of Health Insurance Organization on Family Planning and its role in primary health care. Zagaziz, Egypt, February 26-27, 1997, Center of Complementary Services for Maternity and childhood care. \n20-Laparoscopic Assisted vaginal hysterectomy. 10th International Annual Congress Modern Trends in Reproductive Techniques 23-24 March 1995. Alexandria, Egypt. \n21-Immunological Studies in Pre-eclamptic Toxaemia. Proceedings of 10th Annual Ain Shams Medical Congress. Cairo, Egypt, March 6-10, 1987. \n22-Socio-demographic factorse affecting acceptability of the long-acting contraceptive injections in a rural Egyptian community. Journal of Biosocial Science 29:305, 1987. \n23-Plasma fibronectin levels hypertension during pregnancy. The Journal of the Egypt. Soc. of Ob./Gyn. 13:1, 17-21, Jan. 1987. \n24-Effect of smoking on pregnancy. Journal of Egypt. Soc. of Ob./Gyn. 12:3, 111-121, Sept 1986. \n25-Socio-demographic aspects of nausea and vomiting in early pregnancy. Journal of the Egypt. Soc. of Ob./Gyn. 12:3, 35-42, Sept. 1986. \n26-Effect of intrapartum oxygen inhalation on maternofetal blood gases and pH. Journal of the Egypt. Soc. of Ob./Gyn. 12:3, 57-64, Sept. 1986. \n27-The effect of severe pre-eclampsia on serum transaminases. The Egypt. J. Med. Sci. 7(2): 479-485, 1986. \n28-A study of placental immunoreceptors in pre-eclampsia. The Egypt. J. Med. Sci. 7(2): 211-216, 1986. \n29-Serum human placental lactogen (hpl) in normal, toxaemic and diabetic pregnant women, during pregnancy and its relation to the outcome of pregnancy. Journal of the Egypt. Soc. of Ob./Gyn. 12:2, 11-23, May 1986. \n30-Pregnancy specific B1 Glycoprotein and free estriol in the serum of normal, toxaemic and diabetic pregnant women during pregnancy and after delivery. Journal of the Egypt. Soc. of Ob./Gyn. 12:1, 63-70, Jan. 1986. Also was accepted and presented at Xith World Congress of Gynecology and Obstetrics, Berlin (West), September 15-20, 1985. \n31-Pregnancy and labor in women over the age of forty years. Accepted and presented at Al-Azhar International Medical Conference, Cairo 28-31 Dec. 1985. \n32-Effect of Copper T intra-uterine device on cervico-vaginal flora. Int. J. Gynaecol. Obstet. 23:2, 153-156, April 1985. \n33-Factors affecting the occurrence of post-Caesarean section febrile morbidity. Population Sciences, 6, 139-149, 1985. \n34-Pre-eclamptic toxaemia and its relation to H.L.A. system. Population Sciences, 6, 131-139, 1985. \n35-The menstrual pattern and occurrence of pregnancy one year after discontinuation of Depo-medroxy progesterone acetate as a postpartum contraceptive. Population Sciences, 6, 105-111, 1985. \n36-The menstrual pattern and side effects of Depo-medroxy progesterone acetate as postpartum contraceptive. Population Sciences, 6, 97-105, 1985. \n37-Actinomyces in the vaginas of women with and without intrauterine contraceptive devices. Population Sciences, 6, 77-85, 1985. \n38-Comparative efficacy of ibuprofen and etamsylate in the treatment of I.U.D. menorrhagia. Population Sciences, 6, 63-77, 1985. \n39-Changes in cervical mucus copper and zinc in women using I.U.D.�s. Population Sciences, 6, 35-41, 1985. \n40-Histochemical study of the endometrium of infertile women. Egypt. J. Histol. 8(1) 63-66, 1985. \n41-Genital flora in pre- and post-menopausal women. Egypt. J. Med. Sci. 4(2), 165-172, 1983. \n42-Evaluation of the vaginal rugae and thickness in 8 different groups. Journal of the Egypt. Soc. of Ob./Gyn. 9:2, 101-114, May 1983. \n43-The effect of menopausal status and conjugated oestrogen therapy on serum cholesterol, triglycerides and electrophoretic lipoprotein patterns. Al-Azhar Medical Journal, 12:2, 113-119, April 1983. \n44-Laparoscopic ventrosuspension: A New Technique. Int. J. Gynaecol. Obstet., 20, 129-31, 1982. \n45-The laparoscope: A useful diagnostic tool in general surgery. Al-Azhar Medical Journal, 11:4, 397-401, Oct. 1982. \n46-The value of the laparoscope in the diagnosis of polycystic ovary. Al-Azhar Medical Journal, 11:2, 153-159, April 1982. \n47-An anaesthetic approach to the management of eclampsia. Ain Shams Medical Journal, accepted for publication 1981. \n48-Laparoscopy on patients with previous lower abdominal surgery. Fertility management edited by E. Osman and M. Wahba 1981. \n49-Heart diseases with pregnancy. Population Sciences, 11, 121-130, 1981. \n50-A study of the biosocial factors affecting perinatal mortality in an Egyptian maternity hospital. Population Sciences, 6, 71-90, 1981. \n51-Pregnancy Wastage. Journal of the Egypt. Soc. of Ob./Gyn. 11:3, 57-67, Sept. 1980. \n52-Analysis of maternal deaths in Egyptian maternity hospitals. Population Sciences, 1, 59-65, 1979. \nArticles published on OBGYN.net: \n1- Abdel Karim M. El Hemaly*, Ibrahim M. Kandil, Laila A. S. Mousa and Mohamad A.K.M.El Hemaly.\nUrethro-vaginoplasty, an innovated operation for the treatment of: Stress Urinary Incontinence (SUI), Detursor Overactivity (DO), Mixed Urinary Incontinence and Anterior Vaginal Wall Descent. \nhttp://www.obgyn.net/urogyn/urogyn.asp?page=/urogyn/articles/ urethro-vaginoplasty_01\n\n2- Abdel Karim M. El Hemaly, Ibrahim M Kandil, Mohamed M. Radwan.\n Urethro-raphy a new technique for surgical management of Stress Urinary Incontinence.\nhttp://www.obgyn.net/urogyn/urogyn.asp?page=/urogyn/articles/\nnew-tech-urethro\n\n3- Abdel Karim M. El Hemaly, Ibrahim M Kandil, Mohamad A. Rizk, Nabil Abdel Maksoud H., Mohamad M. Radwan, Khalid Z. El Shieka, Mohamad A. K. M. El Hemaly, and Ahmad T. El Saban.\nUrethro-raphy The New Operation for the treatment of stress urinary incontinence, SUI, detrusor instability, DI, and mixed-type of urinary incontinence; short and long term results. \nhttp://www.obgyn.net/urogyn/urogyn.asp?page=urogyn/articles/\nurethroraphy-09280\n\n4-Abdel Karim M. El Hemaly, Ibrahim M Kandil, and Bahaa E. El Mohamady. Menopause, and Voiding troubles. \nhttp://www.obgyn.net/displayppt.asp?page=/English/pubs/features/presentations/El-Hemaly03/el-hemaly03-ss\n\n5-El Hemaly AKMA, Mousa L.A. Micturition and Urinary\tContinence. Int J Gynecol Obstet 1996; 42: 291-2. \n\n6-Abdel Karim M. El Hemaly.\n Urinary incontinence in gynecology, a review article.\nhttp://www.obgyn.net/urogyn/urogyn.asp?page=/urogyn/articles/abs-urinary_incotinence_gyn_ehemaly \n\n7-El Hemaly AKMA. Nocturnal Enuresis: Pathogenesis and Treatment. \nInt Urogynecol J Pelvic Floor Dysfunct 1998;9: 129-31.\n \n8-El Hemaly AKMA, Mousa L.A.E. Stress Urinary Incontinence, a New Concept. Eur J Obstet Gynecol Reprod Biol 1996; 68: 129-35. \n\n9- El Hemaly AKMA, Kandil I. M. Stress Urinary Incontinence SUI facts and fiction. Is SUI a puzzle?! http://www.obgyn.net/displayppt.asp?page=/English/pubs/features/presentations/El-Hemaly/el-hemaly-ss\n\n10-Abdel Karim El Hemaly, Nabil Abdel Maksoud, Laila A. Mousa, Ibrahim M. Kandil, Asem Anwar, M.A.K El Hemaly and Bahaa E. El Mohamady. \nEvidence based Facts on the Pathogenesis and Management of SUI. http://www.obgyn.net/displayppt.asp?page=/English/pubs/features/presentations/El-Hemaly02/el-hemaly02-ss\n\n11- Abdel Karim M. El Hemaly*, Ibrahim M. Kandil, Mohamad A. Rizk and Mohamad A.K.M.El Hemaly.\n Urethro-plasty, a Novel Operation based on a New Concept, for the Treatment of Stress Urinary Incontinence, S.U.I., Detrusor Instability, D.I., and Mixed-type of Urinary Incontinence.\nhttp://www.obgyn.net/urogyn/urogyn.asp?page=/urogyn/articles/urethro-plasty_01\n\n12-Ibrahim M. Kandil, Abdel Karim M. El Hemaly, Mohamad M. Radwan: Ultrasonic Assessment of the Internal Urethral Sphincter in Stress Urinary Incontinence. The Internet Journal of Gynecology and Obstetrics. 2003. Volume 2 Number 1. \n\n13-Abdel Karim M. El Hemaly. Nocturnal Enureses: A Novel Concept on its pathogenesis and Treatment.\nhttp://www.obgyn.net/urogynecolgy/?page=articles/nocturnal_enuresis\n\n14- Abdel Karim M. El Hemaly. 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