Tweaking Photo CO2 Reduction by Altering Lewis Acidic Sites in Metalated-Porous Organic Polymer for Adjustable H2 /CO Ratio in Syngas Production

Herein, we have specifically designed two metalated porous organic polymers (Zn-POP and Co-POP) for syngas (CO+H2 ) production from gaseous CO2 . The variable H2 /CO ratio of syngas with the highest efficiency was produced in water medium (without an organic hole scavenger and photosensitizer) by utilizing the basic principle of Lewis acid/base chemistry. Also, we observed the formation of entirely different major products during photocatalytic CO2 reduction and water splitting with the help of the two catalysts, where CO (145.65 μmol g-1  h-1 ) and H2 (434.7 μmol g-1  h-1 ) production were preferentially obtained over Co-POP & Zn-POP, respectively. The higher electron density/better Lewis basic nature of Co-POP was investigated further using XPS, XANES, and NH3 -TPD studies, which considerably improve CO2 activation capacity. Moreover, the structure-activity relationship was confirmed via in situ DRIFTS and DFT studies, which demonstrated the formation of COOH* intermediate along with the thermodynamic feasibility of CO2 reduction over Co-POP while water splitting occurred preferentially over Zn-POP.

Elucidating the Mechanism of Large Phosphate Molecule Intercalation Through Graphene-Substrate Heterointerfaces

Intercalation is the process of inserting chemical species into the heterointerfaces of two-dimensional (2D) layered materials. While much research has focused on the intercalation of metals and small gas molecules into graphene, the intercalation of larger molecules through the basal plane of graphene remains challenging. In this work, we present a new mechanism for intercalating large molecules through monolayer graphene to form confined oxide materials at the graphene-substrate heterointerface. We investigate the intercalation of phosphorus pentoxide (P2O5) molecules directly from the vapor phase and confirm the formation of confined P2O5 at the graphene-substrate heterointerface using various techniques. Density functional theory (DFT) corroborates the experimental results and reveals the intercalation mechanism, whereby P2O5 dissociates into small fragments catalyzed by defects in the graphene that then permeates through lattice defects and reacts at the heterointerface to form P2O5. This process can also be used to form new confined metal phosphates (e.g., 2D InPO4). While the focus of this study is on P2O5 intercalation, the possibility of intercalation from predissociated molecules catalyzed by defects in graphene may exist for other types of molecules as well. This in-depth study advances our understanding of intercalation routes of large molecules via the basal plane of graphene as well as heterointerface chemical reactions leading to the formation of distinctive confined complex oxide compounds.

Selective and Stable CO2 Electroreduction to CH4 via Electronic Metal-Support Interaction upon Decomposition/Redeposition of MOF

The CO2 electroreduction to fuels is a feasible approach to provide renewable energy sources. Therefore, it is necessary to conduct experimental and theoretical investigations on various catalyst design strategies, such as electronic metal-support interaction, to improve the catalytic selectivity. Here a solvent-free synthesis method is reported to prepare a copper (Cu)-based metal-organic framework (MOF) as the precursor. Upon electrochemical CO2 reduction in aqueous electrolyte, it undergoes in situ decomposition/redeposition processes to form abundant interfaces between Cu nanoparticles and amorphous carbon supports. This Cu/C catalyst favors the selective and stable production of CH4 with a Faradaic efficiency of ≈55% at -1.4 V versus reversible hydrogen electrode (RHE) for 12.5 h. The density functional theory calculation reveals the crucial role of interfacial sites between Cu and amorphous carbon support in stabilizing the key intermediates for CO2 reduction to CH4 . The adsorption of COOH* and CHO* at the Cu/C interface is up to 0.86 eV stronger than that on Cu(111), thus promoting the formation of CH4 . Therefore, it is envisioned that the strategy of regulating electronic metal-support interaction can improve the selectivity and stability of catalyst toward a specific product upon electrochemical CO2 reduction.

Experimental Validation and Computational Predictions Join Forces to Map Catalytic C-H Activation in Ferrocene Metalated Porous Organic Polymers

In recent times, a self-complementary balanced characteristic feature with the combination of both covalent bonds (structural stability) and open metal sites (single-site catalysis) introduced an advanced emerging functional nanoarchitecture termed metalated porous organic polymers (M-POPs). However, the development of M-POPs in view of the current interest in catalysis has been realized still in its infancy and remains a challenge for the years to come. In this work, we built benzothiazole-linked Fe-metalated porous organic polymer (Fc-Bz-POP) using ferrocene dicarboxaldehyde (FDC), 1,3,5-tris(4-aminophenyl) benzene (APB), and elemental sulfur (S₈) via a template-free, multicomponent, cost-effective one-pot synthetic approach. This Fc-Bz-POP is endowed with unique features including an extended network unit, isolated active sites, and catalytic pocket with a possible local structure, in which convergent binding sites are positioned in such a way that substrate molecules can be held in close proximity. Prospective catalytic application of this Fc-Bz-POP has been explored in executing catalytic allylic "C-H" bond functionalization of cyclohexene (CHX) in water at room temperature. Catalytic screening results identified that a superior performance with a CHX conversion of 95% and a 2-cyclohexene-1-ol selectivity (COL) of 80.8% at 4 h and 25 °C temperature has been achieved over Fc-Bz-POP, thereby addressing previous shortcomings of the other conventional catalytic systems. Comprehensive characterization understanding with the aid of synchrotron-based extended X-ray absorption fine structure (EXAFS) analysis manifested that the Fe atom with an oxidation state of +2 in our Fc-Bz-POP catalytic system encompasses a sandwich structural environment with the two symmetrical eclipsed cyclopentadienyl (Cp) rings, featuring nearest-neighbor (NN) Fe-C (≈2.05 Å) intramolecular bonds, as validated by the Fe L₃-edge EXAFS fitting result. Furthermore, in situ attenuated total reflection-infrared spectroscopy (ATR-IR) analysis data for liquid-phase oxidation of cyclohexene allow for the formulation of a molecular-level reaction mechanistic pathway with the involvement of specific reaction intermediates, which is initiated by the radical functionalization of the allyl hydrogen. A deep insight investigation from density functional theory (DFT) calculations unambiguously revealed that the dominant pathway from cyclohexene to 2-cyclohexene-1-ol is initiated by an allyl-H functionalization step accompanied by the formation of 2-cyclohexene-1-hydroperoxide species as the key reaction intermediate. Electronic properties obtained from DFT simulations via the charge density difference plot, Bader charge, and density of state (DOS) demonstrate the importance of the organic polymer frame structure in altering the electronic properties of the Fe site in Fc-Bz-POP, resulting in its high activity. Our contribution has great implications for the precise design of metalated porous organic polymer-based robust catalysts, which will open a new avenue to get a clear image of surface catalysis.

SERS Chemical Enhancement of 2,4,5-Trichlorophenoxyacetic Acid Adsorbed on Silver Substrate

Surface-enhanced Raman spectroscopy (SERS) was employed to gain an understanding of the chemical enhancement mechanism of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), an Agent Orange, adsorbed on a silver substrate surface. Experimental measurements were performed using a micro-Raman spectrophotometer with an excitation wavelength of 532 nm and successfully detected 2,4,5-T at a relatively low concentration of 0.4 nM. Density functional theory (DFT) calculations on the interactions of the 2,4,5-T molecule with some small silver clusters, Ag_n with n = 4, 8, and 20, as well as with extended Ag surfaces, demonstrate that the most stable adsorption configuration is formed via coordination of Cl9 sites and carbonyl C═O group on the 2,4,5-T ligand to the Ag atoms on surfaces. Analyses of charge transfer mechanism and frontier orbitals distributions show an electron transfer from 2,4,5-T to the cluster in the ground state, and an inversed trend occurs for the excited singlet state process, consequently leading to a chemical enhancement of SERS signals. The obtained results are of importance for subsequent work in guiding the design of mobile sensors specifically used for services of rapid screening and detection of these toxic compounds present in the environment, as well as agricultural and food products. Extensive computations pointed out that small silver clusters, in particular of Ag₂₀ size, can be used as appropriate models for a metal nanoparticle surface.

Strong Metal-Support Interaction for 2D Materials: Application in Noble Metal/TiB2 Heterointerfaces and their Enhanced Catalytic Performance for Formic Acid Dehydrogenation

Strong metal-support interaction (SMSI) is a phenomenon commonly observed on heterogeneous catalysts. Here, direct evidence of SMSI between noble metal and 2D TiB₂ supports is reported. The temperature-induced TiB₂ overlayers encapsulate the metal nanoparticles, resulting in core-shell nanostructures that are sintering-resistant with metal loadings as high as 12.0 wt%. The TiO_x -terminated TiB₂ surfaces are the active sites catalyzing the dehydrogenation of formic acid at room temperature. In contrast to the trade-off between stability and activity in conventional SMSI, TiB₂ -based SMSI promotes catalytic activity and stability simultaneously. By optimizing the thickness and coverage of the overlayer, the Pt/TiB₂ catalyst displays an outstanding hydrogen productivity of 13.8 mmol g^-1 _cat h^-1 in 10.0 m aqueous solution without any additive or pH adjustment, with >99.9% selectivity toward CO₂ and H₂ . Theoretical studies suggest that the TiB₂ overlayers are stabilized on different transition metals through an interplay between covalent and electrostatic interactions. Furthermore, the computationally determined trends in metal-TiB₂ interactions are fully consistent with the experimental observations regarding the extent of SMSI on different transition metals. The present research introduces a new means to create thermally stable and catalytically active metal/support interfaces for scalable chemical and energy applications.

gPROMS-driven modeling and simulation of fixed bed adsorption of heavy metals on a biosorbent: benchmarking and case study

Adsorptive separation of heavy metals from wastewater is a viable approach to reuse it and avoid environmental pollution. The productive employment of adsorptive separation at a commercial scale, however, relies on the optimized conditions of an adsorber bed holding maximum and selective isolation of the heavy metals. The experimental route includes a significant trial and error approach, is time-consuming, involves operating cost, and remains economically unattractive. Contrarily, simulation of a mathematical model mimicking the adsorption system along with experimental validation can significantly minimize optimization efforts and suggests the best conditions of separation. In this work, a convective-dispersive model and adsorption model for fixed bed adsorption of copper (Cu), chromium (Cr), and cadmium (Cd) metals over wheat bran biosorbent are simulated using the gPROMS tool for benchmarking. The influence of feed flow rate, bed height, and metal concentration is studied, and breakthrough profiles of all heavy metals are predicted and matched with the literature. The error values (R² and RMSE) and Chi-squared values determined from gPROMS simulations matched well with the previously available MATLAB-simulated data. After a successful benchmarking, we modeled pilot-scale adsorption of Cr on coconut coir (or Biosorbent) in a gPROMS simulation environment. A detailed method and algorithm of gPROMS simulation for Cr isolation is provided. The influence of feed flow rate, bed height, and initial metal concentration is studied on the breakthrough curves of the Cr. The optimum operating condition for the pilot-scale isolation of Cr from the water is suggested. The parameters, such as the axial dispersion coefficient and distribution coefficient, are determined.

Realizing Catalytic Acetophenone Hydrodeoxygenation with Palladium-Equipped Porous Organic Polymers

Porous organic polymers (POPs) constructed through covalent bonds have raised tremendous research interest because of their suitability to develop robust catalysts and their successful production with improved efficiency. In this work, we have designed and explored the properties and catalytic activity of a template-free-constructed, hydroxy (-OH) group-enriched porous organic polymer (Ph-POP) bearing functional Pd nanoparticles (Pd-NPs) by one-pot condensation of phloroglucinol (1,3,5-trihydroxybenzene) and terephthalaldehyde followed by solid-phase reduction with H₂. The encapsulated Pd-NPs rested within well-defined POP nanocages and remained undisturbed from aggregation and leaching. This polymer hybrid nanocage Pd@Ph-POP is found to enable efficient liquid-phase hydrodeoxygenation (HDO) of acetophenone (AP) with high selectivity (99%) of ethylbenzene (EB) and better activity than its Pd@Al₂O₃ counterpart. Our investigation demonstrates a facile, scalable, catalyst-template-free methodology for developing novel porous organic polymer catalysts and next-generation efficient greener chemical processes from platform molecules to produce value-added chemicals. With the aid of comprehensive in situ ATR-IR spectroscopy experiments, it is suggested that EB can be more easily desorbed in a solution, reflecting from the much weaker but better-resolved signal at 1494 cm^-1 in Pd@Ph-POP compared to that in Pd@Al₂O₃, which is the key determining factor in favoring an efficient catalytic mechanism. Density functional theory (DFT) calculations were performed to illustrate the detailed reaction network and explain the high catalytic activity observed for the fabricated Pd@Ph-POP catalyst in the HDO conversion of AP to EB. All of the hydrogenation routes, including direct hydrogenation by surface hydrogen, hydrogen transfer, and the keto-enol pathway, are evaluated, providing insights into the experimental observations. The presence of phenolic hydroxyl groups in the Ph-POP frame structure facilitates the hydrogen-shuttling mechanism for dehydration from the intermediate phenylethanol, which was identified as a crucial step for the formation of the final product ethylbenzene. Besides, weaker binding of the desired product ethylbenzene and lower coverage of surface hydrogen atoms on Pd@Ph-POP both contributed to inhibiting the overhydrogenation reaction and explained well the high yield of EB produced during the HDO conversion of AP on Pd@Ph-POP in this study.

Novel Architecture Titanium Carbide (Ti3C2Tx) MXene Cocatalysts toward Photocatalytic Hydrogen Production: A Mini-Review

Low dimensional transition metal carbide and nitride (MXenes) have been emerging as frontier materials for energy storage and conversion. Ti3C2Tx was the first MXenes that discovered and soon become the most widely investigated among the MXenes family. Interestingly, Ti3C2Tx exhibits ultrahigh catalytic activity towards the hydrogen evolution reaction. In addition, Ti3C2Tx is electronically conductive, and its optical bandgap is tunable in the visible region, making it become one of the most promising candidates for the photocatalytic hydrogen evolution reaction (HER). In this review, we provide comprehensive strategies for the utilization of Ti3C2Tx as a catalyst for improving solar-driven HER, including surface functional groups engineering, structural modification, and cocatalyst coupling. In addition, the reaming obstacle for using these materials in a practical system is evaluated. Finally, the direction for the future development of these materials featuring high photocatalytic activity toward HER is discussed.

Silk Fibroin-Based Biomaterials for Biomedical Applications: A Review

Since it was first discovered, thousands of years ago, silkworm silk has been known to be an abundant biopolymer with a vast range of attractive properties. The utilization of silk fibroin (SF), the main protein of silkworm silk, has not been limited to the textile industry but has been further extended to various high-tech application areas, including biomaterials for drug delivery systems and tissue engineering. The outstanding mechanical properties of SF, including its facile processability, superior biocompatibility, controllable biodegradation, and versatile functionalization have allowed its use for innovative applications. In this review, we describe the structure, composition, general properties, and structure-properties relationship of SF. In addition, the methods used for the fabrication and modification of various materials are briefly addressed. Lastly, recent applications of SF-based materials for small molecule drug delivery, biological drug delivery, gene therapy, wound healing, and bone regeneration are reviewed and our perspectives on future development of these favorable materials are also shared.

Occurrence and risk assessment of multiple classes of antibiotics in urban canals and lakes in Hanoi, Vietnam

Very little information on the occurrence and risk assessment of antibiotics in the aquatic environment is reported for Vietnam, where antibiotics are assumed to be omnipresent in urban canals and lakes at high concentrations due to the easy accessibility of antibiotics without doctor prescription. This study provides comprehensive analysis of the occurrence of 23 antibiotics in urban canals (To Lich and Kim Nguu) and lakes (West Lake, Hoan Kiem, and Yen So) in Hanoi, Vietnam. Of these 23 antibiotics, 18 were detected in urban canals at above 67.9% detection frequency (DF). The concentrations of detected antibiotics were in the range from below quantification limit (MQL) to almost 50,000 ng/L, depending on the compound and sampling site. In urban canals, median concentration of amoxicillin, erythromycin, and sulfamethoxazole was >1000 ng/L while other antibiotics such as ampicillin, chloramphenicol, clindamycin, sulfamethazine, tetracycline, tylosin and vancomycin were detected at median concentrations of <100 ng/L. Similarly, 16 target antibiotics were also detected in urban lakes. Macrolides (azithromycin, clarithromycin, and erythromycin-H₂O), fluoroquinolones (enrofloxacin and ofloxacin), lincosamides (clindamycin and lincomycin), and trimethoprim were ubiquitously detected in urban lakes (DF = 100%). In this study, potential risks of antibiotics in the investigated urban canals and lakes were assessed based on the predicted no-effect concentration (PNEC) from the existing literature for antibiotic resistance selection (PNEC_ARM) and ecological toxicity to aquatic organisms (PNEC_Ecotox). Ampicillin, amoxicillin, azithromycin, ciprofloxacin, clarithromycin, enrofloxacin, erythromycin, ofloxacin, tetracycline, and trimethoprim were found in the investigated urban canals at concentrations exceeding their PNEC_ARM and PNEC_Ecotox. Similarly, most of the target antibiotics (i.e. amoxicillin, ciprofloxacin, clarithromycin, clindamycin, enrofloxacin, erythromycin, lincomycin, ofloxacin, sulfamethoxazole, tetracycline, trimethoprim and tylosin) were detected in the investigated urban lakes at concentrations close to or exceeding PNEC_Ecotox for aquatic organisms. Further investigations on the occurrence and fate of antibiotic residues and antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) in surface waters are recommended.

Comments on "Antibiotic pollution in surface fresh waters: Occurrence and effects", Science of the Total Environment, 664, 793-804 (2019)

This article found mistakes and misunderstandings regarding the collection of occurrence data of antibiotics in surface freshwater from the previous literature. These mistakes and misunderstandings were corrected to avoid the propagation of inaccurate information in the scientific literature.

Insights into biofilm carriers for biological wastewater treatment processes: Current state-of-the-art, challenges, and opportunities

Biofilm carriers play an important role in attached growth systems for wastewater treatment processes. This study systematically summarizes the traditional and novel biofilm carriers utilized in biofilm-based wastewater treatment technology. The advantages and disadvantages of traditional biofilm carriers are evaluated and discussed in light of basic property, biocompatibility and applicability. The characteristics, applications performance, and mechanism of novel carriers (including slow-release carriers, hydrophilic/electrophilic modified carriers, magnetic carriers and redox mediator carriers) in wastewater biological treatment were deeply analyzed. Slow release biofilm carriers are used to provide a solid substrate and electron donor for the growth of microorganisms and denitrification for anoxic and/or anaerobic bioreactors. Carriers with hydrophilic/electrophilic modified surface are applied for promoting biofilm formation. Magnetic materials-based carriers are employed to shorten the start-up time of bioreactor. Biofilm carriers acting as redox mediators are used to accelerate biotransformation of recalcitrant pollutants in industrial wastewater.

Interface Engineering of Graphene-Supported Cu Nanoparticles Encapsulated by Mesoporous Silica for Size-Dependent Catalytic Oxidative Coupling of Aromatic Amines

In this study, graphene nanosheet-supported ultrafine Cu nanoparticles (NPs) encapsulated with thin mesoporous silica (Cu-GO@m-SiO₂) materials are fabricated with particle sizes ranging from 60 to 7.8 nm and are systematically investigated for the oxidative coupling of amines to produce biologically and pharmaceutically important imine derivatives. Catalytic activity remarkably increased from 76.5% conversion of benzyl amine for 60 nm NPs to 99.3% conversion and exclusive selectivity of N-benzylidene-1-phenylmethanamine for 7.8 nm NPs. The superior catalytic performance along with the outstanding catalyst stability of newly designed catalysts are attributed to the easy diffusion of organic molecules through the porous channel of mesoporous SiO₂ layers, which not only restricts the restacking of the graphene nanosheets but also prevents the sintering and leaching of metal NPs to an extreme extent through the nanoconfinement effect. Density functional theory calculations were performed to shed light on the reaction mechanism and to give insight into the trend of catalytic activity observed. The computed activation barriers of all elementary steps are very high on terrace Cu(111) sites, which dominate the large-sized Cu NPs, but are significantly lower on step sites, which are presented in higher density on smaller-sized Cu NPs and could explain the higher activity of smaller Cu-GO@m-SiO₂ samples. In particular, the activation barrier for the elementary coupling reaction is reduced from 139 kJ/mol on flat terrace Cu(111) sites to the feasible value of 94 kJ/mol at step sites, demonstrating the crucial role of the step site in facilitating the formation of secondary imine products.

Capability of Aganonerion polymorphum leaf-water extract in protecting hydrochloric acid induced steel corrosion

A new green water extract as an environmentally friendly and high-performing inhibitor for steel corrosion.

Integrated Experimental and Theoretical Study of Shape-Controlled Catalytic Oxidative Coupling of Aromatic Amines over CuO Nanostructures

We have synthesized CuO nanostructures with flake, dandelion-microsphere, and short-ribbon shapes using solution-phase methods and have evaluated their structure-performance relationship in the heterogeneous catalysis of liquid-phase oxidative coupling reactions. The formation of nanostructures and the morphological evolution were confirmed by transmission electron microscopy, scanning electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, energy-dispersive X-ray spectroscopy, elemental mapping analysis, and Fourier transform infrared spectroscopy. CuO nanostructures with different morphologies were tested for the catalytic oxidative coupling of aromatic amines to imines under solvent-free conditions. We found that the flake-shaped CuO nanostructures exhibited superior catalytic efficiency compared to that of the dandelion- and short-ribbon-shaped CuO nanostructures. We also performed extensive density functional theory (DFT) calculations to gain atomic-level insight into the intriguing reactivity trends observed for the different CuO nanostructures. Our DFT calculations provided for the first time a detailed and comprehensive view of the oxidative coupling reaction of benzylamine over CuO, which yields N-benzylidene-1-phenylmethanamine as the major product. CuO(111) is identified as the reactive surface; the specific arrangement of coordinatively unsaturated Cu and O sites on the most stable CuO(111) surface allows N-H and C-H bond-activation reactions to proceed with low-energy barriers. The high catalytic activity of the flake-shaped CuO nanostructure can be attributed to the greatest exposure of the active CuO(111) facets. Our finding sheds light on the prospective utility of inexpensive CuO nanostructured catalysts with different morphologies in performing solvent-free oxidative coupling of aromatic amines to obtain biologically and pharmaceutically important imine derivatives with high selectivity.

Size-Dependent Catalytic Activity of Palladium Nanoparticles Fabricated in Porous Organic Polymers for Alkene Hydrogenation at Room Temperature

Ultrafine palladium nanoparticles (Pd NPs) with 8 and 3 nm sizes were effectively fabricated in triazine functionalized porous organic polymer (POP) TRIA that was developed by nonaqueous polymerization of 2,4,6-triallyoxy-1,3,5-triazine. The Pd NPs encapsulated POP (Pd-POP) was fully characterized using several techniques. Further studies revealed an excellent capability of Pd-POP for catalytic transfer hydrogenation of alkenes at room temperature with superior catalytic performance and high selectivity of desired products. Highly flammable H2 gas balloon at high pressure and temperature used in conventional hydrogenation reactions was not needed in the present synthetic system. Catalytic activity is strongly dependent on the size of encapsulated Pd NPs in the POP. The Pd-POP catalyst with Pd NPs of 8 nm in diameter exhibited higher catalytic activity for alkene hydrogenation as compared with the Pd-POP catalyst encapsulating 3 nm Pd NPs. Computational studies were undertaken to gain insights into different catalytic activities of these two Pd-POP catalysts. High reusability and stability as well as no Pd leaching of these Pd-POP catalysts make them highly applicable for hydrogenation reactions at room temperature.

Insights into the synergistic role of metal–lattice oxygen site pairs in four-centered C–H bond activation of methane: the case of CuO

Of the three mechanisms for activation of methane on copper and copper oxide surfaces, the under-coordinated Cu–O site pair mediated mechanism on CuO surfaces has the lowest activation energy barriers.

Mechanistic insights into the catalytic elimination of tar and the promotional effect of boron on it: first-principles study using toluene as a model compound

The incorporation of a monolayer subsurface B into the Ni catalyst results in a corrugated Ni top surface and the activation of toluene is significantly promoted on B–Ni.

Crystallization-induced red emission of a facilely synthesized biodegradable indigo derivative

A new crystallization-induced red-emitting luminogen based on a biodegradable indigo derivative forms well-defined microcrystals in tetrahydrofuran–water mixtures upon ultrasonication.

Enzyme/prodrug gene therapy: comparison of cytosine deaminase/5-fluorocytosine versus thymidine kinase/ganciclovir enzyme/prodrug systems in a human colorectal carcinoma cell line

We have been developing an enzyme/prodrug gene therapy approach for the treatment of primary and metastatic tumors in the liver. This system uses the cytosine deaminase/5-fluorocytosine (CD/5-FCyt) enzyme/prodrug combination. Another system that has received considerable attention is the herpes simplex virus thymidine kinase/ganciclovir (HSV-TK/GCV) enzyme/prodrug combination. The purpose of the present study was to compare these two enzyme/prodrug systems. The human colorectal tumor cell line, WiDr, was genetically modified to express either the CD gene (WiDr/CD) or the HSV-TK gene (WiDr/TK). The IC50 (concentration of drug producing 50% inhibition of cell growth) for GCV was approximately 3.4 microM in WiDr/TK cells, while the IC50 for 5-FCyt was approximately 27 microM in WiDr/CD cells. In vivo antitumor studies were conducted using high but nontoxic levels of GCV (50 mg/kg/day) or 5-FCyt (500 mg/kg/day). When tumor xenografts were composed of 100% of cells expressing either HSV-TK or CD, 100% tumor-free animals were observed after GCV or 5-FCyt treatment, respectively. However, when only 10% of the tumor cells expressed HSV-TK, no antitumor effect by GCV treatment could be observed. In contrast, when tumors were composed of 4% of the cells expressing CD, 60% of the animals were tumor-free after 5-FCyt treatment. Transmission electron microscopy of the WiDr solid tumors revealed the presence of desmosomes but no gap junctions.