Ru/CdS Quantum Dots Templated on Clay Nanotubes as Visible-Light-Active Photocatalysts: Optimization of S/Cd Ratio and Ru Content

Antimicrobial Nanomaterials Based on Halloysite Clay Mineral: Research Advances and Outlook

Bacterial infections represent one of the major causes of mortality worldwide. Therefore, over the years, several nanomaterials with antibacterial properties have been developed. In this context, clay minerals, because of their intrinsic properties, have been efficiently used as antimicrobial agents since ancient times. Halloysite nanotubes are one of the emerging nanomaterials that have found application as antimicrobial agents in several fields. In this review, we summarize some examples of the use of pristine and modified halloysite nanotubes as antimicrobial agents, scaffolds for wound healing and orthopedic implants, fillers for active food packaging, and carriers for pesticides in food pest control.

Prokaryotic and eukaryotic toxicity of halloysite decorated with photoactive nanoparticles

The development of new approaches to treat the growing antibiotic resistance of pathogenic bacterial species is an important task to ensure the future safety of society. Utilization of irradiation of different wavelengths together with nanostructured materials based on metal containing nanoparticles may result in synergetic antibacterial effects. In this paper we aim to show the main conceptions of light-assisted bacteria deactivation techniques and prospects of application of natural clay nanotubes as a carrier for scalable photoactive antibacterial nanomaterials. Halloysite aluminosilicate nanotubes (ca 50 nm diameter, ca. 1.0 μm length) are safe and biocompatible natural materials produced in tons. Their application as a template or a carrier for metal nanoparticles, QDs and organic compounds has already found application in biomedical research, cosmetics, polymers, coatings, catalysis and related applications. Here, we show the toxicity of halloysite decorated with photoactive nanoparticles on prokaryotic and eukaryotic cells. The formation of light active nanostructured materials with this clay as the base is a promising tool for solving the problem of the antibiotic resistance of microorganisms.

Architectural design of core-shell nanotube systems based on aluminosilicate clay

A nanoarchitectural approach to the design of functional nanomaterials based on natural aluminosilicate nanotubes and their catalysis, and practical applications are described in this paper. We focused on the buildup of hybrid core-shell systems with metallic or organic molecules encased in aluminosilicate walls, and nanotube templates for structured silica and zeolite preparation. The basis for such an architectural design is a unique Al₂O₃/SiO₂ dual chemistry of 50 nm diameter halloysite tubes. Their structure and site dependent properties are well combined with biocompatibility, environmental safety, and abundant availability, which makes the described functional systems scalable for industrial applications. In these organic/ceramic hetero systems, we outline drug, dye and chemical inhibitor loading inside the clay nanotubes, accomplished with their silane or amphiphile molecule surface modifications. For metal-ceramic tubule composites, we detailed the encapsulation of 2-5 nm Au, Ru, Pt, and Ag particles, Ni and Co oxides, NiMo, and quantum dots of CdZn sulfides into the lumens or their attachment at the outside surface. These metal-clay core-shell nanosystems show high catalytic efficiency with increased mechanical and temperature stabilities. The combination of halloysite nanotubes with mesoporous MCM-41 silica allowed for a synergetic enhancement of catalysis properties. Finally, we outlined the clay nanotubes' self-assembly into organized arrays with orientation and ordering similar to nematic liquid crystals, and these systems are applicable for life-related applications, such as petroleum spill bioremediation, antimicrobial protection, wound healing, and human hair coloring.

Photoinduced Antibacterial Activity and Cytotoxicity of CdS Stabilized on Mesoporous Aluminosilicates and Silicates

Inactivation of bacteria under the influence of visible light in presence of nanostructured materials is an alternative approach to overcome the serious problem of the growing resistance of pathogenic bacteria to antibiotics. Cadmium sulfide quantum dots are superefficient photocatalytic material suitable for visible light transformation. In this work, CdS nanoparticles with size of less than 10 nm (QDs) were synthesized on the surface of natural and synthetic mesoporous aluminosilicates and silicates (halloysite nanotubes, MCM-41, MCM-41/Halloysite, SBA-15). Materials containing 5–7 wt.% of CdS were characterized and tested as agents for photocatalytic bacteria degradation of Gram-positive S. aureus and Gram-negative E. coli with multiple antibiotic resistance. Eukaryotic cell viability tests were also conducted on the model cancer cells A 459. We found that the carrier affects prokaryotic and eukaryotic toxicity of CdS quantum dots. CdS/MCM-41/HNTs were assumed to be less toxic to eukaryotic cells and possess the most prominent photocatalytic antibacterial efficiency. Under visible light irradiation, it induced 100% bacterial growth inhibition at the concentration of 125 μg/mL and the bacteriostatic effect at the concentration of 63 μg/mL. CdS/MCM-41/HNTs showed 100% E. coli growth inhibition in the concentration of 1000 μg/mL under visible light irradiation.

Decoration of green synthesized S, N-GQDs and CoFe2O4 on halloysite nanoclay as natural substrate for electrochemical hydrogen storage application

Halloysite nanotubes (HNTs) with high active sites are used as natural layered mineral supports. Sulfur- and nitrogen-co doped graphene quantum dots (S, N-GQDs) as conductive additive and CoFe₂O₄ as the electrocatalyst was decorated on a HNT support to design an effective and environmentally friendly active material. Herein, an eco-friendly CoFe₂O₄/S, N-GQDs/HNTs nanocomposite is fabricated via a green hydrothermal method to equip developed hydrogen storage sites and to allow for quick charge transportation for hydrogen storage utilization. The hydrogen storage capacity of pure HNTs was 300 mAhg⁻¹ at a current density of 1 mA after 20 cycles, while that of S, N-GQD-coated HNTs (S, N-GQDs/HNTs) was 466 mAhg⁻¹ under identical conditions. It was also conceivable to increase the hydrogen sorption ability through the spillover procedure by interlinking CoFe₂O₄ in the halloysite nanoclay. The hydrogen storage capacity of the CoFe₂O₄/HNTs was 450 mAhg⁻¹, while that of the representative designed nanocomposites of CoFe₂O₄/S, N-GQDs/HNTs was 600 mAhg⁻¹. The halloysite nano clay and treated halloysite show potential as electrode materials for electrochemical energy storage in alkaline media; in particular, ternary CoFe₂O₄/S, N-GQD/HNT nanocomposites prove developed hydrogen sorption performance in terms of presence of conductive additive, physisorption, and spillover mechanisms.

Fabrication of Ni2P Cocatalyzed CdS Nanorods with a Well-Defined Heterointerface for Enhanced Photocatalytic H2 Evolution

Developing non-noble metal photocatalysts for efficient photocatalytic hydrogen evolution is crucial for exploiting renewable energy. In this study, a photocatalyst of Ni2P/CdS nanorods consisting of cadmium sulfide (CdS) nanorods (NRs) decorated with Ni2P nanoparticles (NPs) was fabricated using an in-situ solvothermal method with red phosphor (P) as the P source. Ni2P NPs were tightly anchored on the surface of CdS NRs to form a core-shell structure with a well-defined heterointerface, aiming to achieve a highly efficient photocatalytic H2 generation. The as-synthesized 2%Ni2P/CdS NRs photocatalyst exhibited the significantly improved photocatalytic H2 evolution rate of 260.2 μmol∙h−1, more than 20 folds higher than that of bare CdS NRs. Moreover, the as-synthesized 2%Ni2P/CdS NRs photocatalyst demonstrated an excellent stability, even better than that of Pt/CdS NRs. The photocatalytic performance enhancement was ascribed to the core-shell structure with the interfacial Schottky junction between Ni2P NPs and CdS NRs and the accompanying fast and effective photogenerated charge carriers’ separation and transfer. This work provides a new strategy for designing non-noble metal photocatalysts to replace the noble catalysts for photocatalytic water splitting.

Prodrug based on halloysite delivery systems to improve the antitumor ability of methotrexate in leukemia cell lines

The prodrug approach, as well as the development of specific systems able to deliver a chemotherapeutic agent in the target site, decreasing the side effects often associated with its administration, are still a challenging. In this context, both methotrexate drug molecules (MTX) and biotin ligand moieties, whose receptors are overexpressed on the surface of several cancer cells, were loaded on halloysite nanotubes (HNTs) to develop nanomaterial based on multifunctional and "smart" delivery systems. To highlight the crucial role played by biotin, carrier systems based on HNTs and MTX were also synthetized. In detail, several approaches were envisaged: i) a supramolecular interaction between the clay and the drug; ii) a covalent grafting of the drug onto the HNTs external surface and, iii) a combination of both approaches. The nanomaterials obtained were characterized by thermogravimetric analysis, FT-IR, and UV-vis spectroscopies, DLS and ζ-potential measurements and the morphologies were imaged by HAADF/STEM investigations. Kinetic release experiments at different pH conditions were also performed. Finally, as a proof-of-concept application of our pro-drug delivery systems based on HNTs in cancer therapy, the cytotoxic effects were evaluated on acute myeloid leukemia cell lines, HL60 and its multidrug resistance variant, HL60R. The obtained results showed that both the MTX prodrug system and the biotinylated ones played a crucial role in the biological activity and, they are promising agents for the cancer treatments.

Coupling Natural Halloysite Nanotubes and Bimetallic Pt-Au Alloy Nanoparticles for Highly Efficient and Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid

The aerobic oxidation of 5-hydroxymethylfurfural (HMF), a key platform compound derived from biomass, to 2,5-furandicarboxylic acid (FDCA) is a highly important reaction in the production of green and sustainable chemicals. Here, we developed a highly efficient and stable halloysite-supported Pt-Au alloy catalyst for the selective oxidation of HMF to FDCA. The catalyst was synthesized through the organosilane functionalization of halloysite nanotubes, followed by the in situ formation and dispersion of Pt-Au alloy nanoparticles on the internal and external surfaces of nanotubes. The composition, morphology, and structure of the prepared catalyst were characterized. The catalyst with the optimal composition of Pt/Au molar ratio of 1/4 and metal loading of 1.5 wt % exhibited outstanding catalytic activity for the oxidation of HMF to FDCA using O₂ as an oxidant with 100% conversion of HMF and 99% selectivity of FDCA. This excellent catalytic performance is mainly attributed to the high dispersion and alloying effects of bimetallic nanoparticles, which promoted the activation of reactants or intermediates and further improved FDCA selectivity. Furthermore, the halloysite-supported Pt/Au bimetallic catalyst showed high stability and reusability. This study provides a promising strategy by combining clay mineral halloysite and bimetallic alloys for developing efficient catalysts with high FDCA selectivity and stability for the oxidation of HMF to FDCA.

Halloysite Nanotubes: Smart Nanomaterials in Catalysis

The use of clay minerals as catalyst is renowned since ancient times. Among the different clays used for catalytic purposes, halloysite nanotubes (HNTs) represent valuable resources for industrial applications. This special tubular clay possesses high stability and biocompatibility, resistance against organic solvents, and most importantly be available in large amounts at a low cost. Therefore, HNTs can be efficiently used as catalysts themselves or supports for metal nanoparticles in several catalytic processes. This review reports a comprehensive overview of the relevant advances in the use of halloysite in catalysis, focusing the attention on the last five years.

Natural aluminosilicate nanotubes loaded with RuCo as nanoreactors for Fischer-Tropsch synthesis

Following nanoarchitectural approach, mesoporous halloysite nanotubes with internal surface composed of alumina were loaded with 5-6 nm RuCo nanoparticles by sequential loading/reduction procedure. Ruthenium nanoclusters were loaded inside clay tube by microwave-assisted method followed by cobalt ions electrostatic attraction to ruthenium during wetness impregnation step. Developed nanoreactors with bimetallic RuCo nanoparticles were investigated as catalysts for the Fischer-Tropsch process. The catalyst with 14.3 wt.% of Co and 0.15 wt.% of Ru showed high activity (СO conversion reached 24.6%), low selectivity to methane (11.9%), CO₂ (0.3%), selectivity to C₅₊ hydrocarbons of 79.1% and chain growth index (α) = 0.853. Proposed nanoreactors showed better selectivity to target products combined with high activity in comparison to the similar bimetallic systems supported on synthetic porous materials. It was shown that reducing agent (NaBH₄ or H₂) used to obtain Ru nanoclusters at first synthesis step played a very important role in the reducibility and selectivity of resulting RuCo catalysts.

Site-specific halloysite functionalization by polydopamine: A new synthetic route for potential near infrared-activated delivery system

Halloysite nanotubes (HNTs) represent a versatile core structure for the design of functional nanosystems of biomedical interest. However, the development of selective methodologies for the site-controlled functionalization of the nanotubes at specific sites is not an easy task. This study aims to accomplish a procedure for the site-selective/specific, "pin-point", functionalization of HNTs with polydopamine (HNTs@PDA). This goal was achieved, at pH 6.5, by exploiting the basicity of ZnO nanoparticles anchored on the HNTs external surface (HNTs@ZnO) to induce a punctual polydopamine polymerization and coating. The morphology and the chemical composition of the nanomaterial was demonstrated by several techniques. Turbidimetric analysis showed that PDA coating affected the aqueous stability of HNTs@PDA compared to both HNTs@ZnO and HNTs. Notably, hyperthermia studies revealed that the nanomaterial induced a local thermic rise, up to 50 °C, under near-infrared (NIR) irradiation. Furthermore, secondary functionalization of HNTs@PDA by selective grafting of biotin onto the PDA coating followed by avidin binding was also accomplished.

Biodistribution of Quantum Dots-Labelled Halloysite Nanotubes: A Caenorhabditis elegans In Vivo Study

Halloysite is a promising building block in nanoarchitectonics of functional materials, especially in the development of novel biomaterials and smart coatings. Understanding the behavior of materials produced using halloysite nanotubes within living organisms is essential for their safe applications. In this study, quantum dots of different compositions were synthesized on the surface of modified clay nanotubes, and the biodistribution of this hybrid material was monitored within Caenorhabditis elegans nematodes. The influence of the modification agent as well as the particles’ composition on physicochemical properties of hybrid nanomaterials was investigated. Several microscopy techniques, such as fluorescence and dark-field microscopy, were compared in monitoring the distribution of nanomaterials in nematodes’ organisms. The effects of QDs-halloysite composites on the nematodes’ life cycle were investigated in vivo. Our fluorescent hybrid probes induced no acute toxic effects in model organisms. The stable fluorescence and low toxicity towards the organisms suggest that the proposed synthesis procedure yields safe nanoarchitectonic materials that will be helpful in monitoring the behavior of nanomaterials inside living cells and organisms.

Sustainable Hydrogen Production from Starch Aqueous Suspensions over a Cd0.7Zn0.3S-Based Photocatalyst

We explored the photoreforming of rice and corn starch with simultaneous hydrogen production over a Cd0.7Zn0.3S-based photocatalyst under visible light irradiation. The photocatalyst was characterized by UV–vis diffuse reflectance spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The influence of starch pretreatment conditions, such as hydrolysis temperature and alkaline concentration, on the reaction rate was studied. The maximum rate of H2 evolution was 730 μmol·h−1·g−1, with AQE = 1.8% at 450 nm, in the solution obtained after starch hydrolysis in 5 M NaOH at 70 °C. The composition of the aqueous phase of the suspension before and after the photocatalytic reaction was studied via high-performance liquid chromatography, and such products as glucose and sodium gluconate, acetate, formate, glycolate, and lactate were found after the photocatalytic reaction.

Clay nanotube-metal core/shell catalysts for hydroprocesses

Catalytic hydroprocesses play a significant role in oil refining and petrochemistry. The tailored design of new metal nanosystems and optimization of their support, composition, and structure is a prospective strategy for enhancing the efficiency of catalysts. Mesoporous support impacts the active component by binding it to the surface, which leads to the formation of tiny highly dispersed catalytic particles stabilized from aggregation and with minimized leaching. The structural and acidic properties of the support are crucial and determine the size and dispersion of the active metal phase. Currently, research efforts are shifted toward the design of nanoscale porous materials, where homogeneous catalysts are displaced by heterogeneous. Ceramic materials, such as 50 nm diameter natural halloysite nanotubes, are of special interest for this. Much attention to halloysite clay is due to its tubular structure with a hollow 10-15 nm diameter internal cavity, textural characteristics, and different chemical compositions of the outer/inner surfaces, allowing selective nanotube modification. Loading halloysite with metal particles or placing them outside the tubes provides stable and efficient mesocatalysts. The low cost of this abundant nanoclay makes it a good choice for the scaled-up architectural design of core-shell catalysts, containing active metal sites (Au, Ag, Pt, Ru, Co, Mo, Fe₂O₃, CdS, CdZnS, Cu-Ni) located inside or outside the tubular template. These alumosilicate nanotubes are environment-friendly and are available in thousands of tons. Herein, we summarized the advances of halloysite-based composite materials for hydroprocesses, focusing on the selective binding of metal particles. We analyze the tubes' morphology adjustments and size selection, the physicochemical properties of pristine and modified halloysite (e.g., acid-etched or silanized), the methods of metal clusters formation, and their localization. We indicate prospective routes for the architectural design of stable and efficient nanocatalysts based on this safe and natural clay material.

Dark-field/hyperspectral microscopy for detecting nanoscale particles in environmental nanotoxicology research

Nanoscale contaminants (including engineered nanoparticles and nanoplastics) pose a significant threat to organisms and environment. Rapid and non-destructive detection and identification of nanosized materials in cells, tissues and organisms is still challenging, although a number of conventional methods exist. These approaches for nanoparticles imaging and characterisation both inside the cytoplasm and on the cell or tissue outer surfaces, such as electron or scanning probe microscopies, are unquestionably potent tools, having excellent resolution and supplemented with chemical analysis capabilities. However, imaging and detection of nanomaterials in situ, in wet unfixed and even live samples, such as living isolated cells, microorganisms, protozoans and miniature invertebrates using electron microscopy is practically impossible, because of the elaborate sample preparation requiring chemical fixation, contrast staining, matrix embedding and exposure into vacuum. Atomic force microscopy, in several cases, can be used for imaging and mechanical analysis of live cells and organisms under ambient conditions, however this technique allows for investigation of surfaces. Therefore, a different approach allowing for imaging and differentiation of nanoscale particles in wet samples is required. Dark-field microscopy as an optical microscopy technique has been popular among researchers, mostly for imaging relatively large specimens. In recent years, the so-called "enhanced dark field" microscopy based on using higher numerical aperture light condensers and variable numerical aperture objectives has emegred, which allows for imaging of nanoscale particles (starting from 5 nm nanospheres) using almost conventional optical microscopy methodology. Hyperspectral imaging can turn a dark-field optical microscope into a powerful chemical characterisation tool. As a result, this technique is becoming popular in environmental nanotoxicology studies. In this Review Article we introduce the reader into the methodology of enhanced dark-field and dark-field-based hyperspectral microscopy, covering the most important advances in this rapidly-expanding area of environmental nanotoxicology.

Comparative Toxicity of Fly Ash: An In Vitro Study

Fly ash produced during coal combustion is one of the major sources of air and water pollution, but the data on the impact of micrometer-size fly ash particles on human cells is still incomplete. Fly ash samples were collected from several electric power stations in the United States (Rockdale, TX; Dolet Hill, Mansfield, LA; Rockport, IN; Muskogee, OK) and from a metallurgic plant located in the Russian Federation (Chelyabinsk Electro-Metallurgical Works OJSC). The particles were characterized using dynamic light scattering, atomic force, and hyperspectral microscopy. According to chemical composition, the fly ash studied was ferro-alumino-silicate mineral containing substantial quantities of Ca, Mg, and a negligible concentration of K, Na, Mn, and Sr. The toxicity of the fly ash microparticles was assessed in vitro using HeLa cells (human cervical cancer cells) and Jurkat cells (immortalized human T lymphocytes). Incubation of cells with different concentrations of fly ash resulted in a dose-dependent decrease in cell viability for all fly ash variants. The most prominent cytotoxic effect in HeLa cells was produced by the ash particles from Rockdale, while the least was produced by the fly ash from Chelyabinsk. In Jurkat cells, the lowest toxicity was observed for fly ash collected from Rockport, Dolet Hill and Muscogee plants. The fly ash from Rockdale and Chelyabinsk induced DNA damage in HeLa cells, as revealed by the single cell electrophoresis, and disrupted the normal nuclear morphology. The interaction of fly ash microparticles of different origins with cells was visualized using dark-field microscopy and hyperspectral imaging. The size of ash particles appeared to be an important determinant of their toxicity, and the smallest fly ash particles from Chelyabinsk turned out to be the most cytotoxic to Jukart cells and the most genotoxic to HeLa cells.

Embedment of Quantum Dots and Biomolecules in a Dipeptide Hydrogel Formed In Situ Using Microfluidics

As low-molecular-weight hydrogelators, dipeptide hydrogel materials are suited for embedding multiple organic molecules and inorganic nanoparticles. Herein, a simple but precisely controllable method is presented that enables the fabrication of dipeptide-based hydrogels by supramolecular assembly inside microfluidic channels. Water-soluble quantum dots (QDs) as well as premixed porphyrins and a dipeptide in dimethyl sulfoxide (DMSO) were injected into a Y-shaped microfluidic junction. At the DMSO/water interface, the confined fabrication of a dipeptide-based hydrogel was initiated. Thereafter, the as-formed hydrogel flowed along a meandering microchannel in a continuous fashion, gradually completing gelation and QD entrapment. In contrast to hydrogelation in conventional test tubes, microfluidically controlled hydrogelation led to a tailored dipeptide hydrogel regarding material morphology and nanoparticle distribution.

Natural clay nanotube supported Mo and W catalysts for exhaustive oxidative desulfurization of model fuels

Oxidative desulfurization is a promising way to produce, under mild conditions, clean ecological fuels with ultra-low sulfur content. Herein, we present for the first time heterogeneous catalysts based on natural aluminosilicate nanotubes (halloysite) loaded with transition metal oxides for oxidative sulfur removal using hydrogen peroxide as environmentally safe oxidant. The halloysite nanotubes (HNTs) provide acid sites for C–S bond scission, while the Mo and W oxides act as hydrogen peroxide activators. The structure and acidity of both the clay support and catalysts were investigated by low-temperature nitrogen adsorption/desorption, Fourier-transform infrared spectroscopy, X-ray fluorescence analysis, and transmission electron microscopy techniques. These clay-based catalysts revealed the high activity in the oxidation of various classes of sulfur-containing compounds (sulfides, heteroatomic sulfur compounds) under mild reaction conditions. The conversion of various substrates decreases in the following trend: MeSPh > Bn2S > DBT > 4-MeDBT > BT, which deals with substrate electron density and steric hindrance. The influence of the temperature, oxidant to sulfur molar ratio, and reaction time on catalytic behavior was evaluated for Mo- and W-containing systems with various metal content. The complete oxidation of the most intractable dibenzothiophene to the corresponding sulfone was achieved at 80 °C and H2O2:S = 6:1 (molar) for 2 h both for Mo- and W-containing systems. These transition metal oxides HNTs supported catalysts are stable for 10 cycles of dibenzothiophene oxidation, which makes them promising systems for clean fuel production.

Comparative Study of the Photocatalytic Hydrogen Evolution over Cd1-xMnxS and CdS-β-Mn3O4-MnOOH Photocatalysts under Visible Light

A series of solid solutions of cadmium and manganese sulfides, Cd_1−xMn_xS (x = 0–0.35), and composite photocatalysts, CdS-β-Mn₃O₄-MnOOH, were synthesized by precipitation with sodium sulfide from soluble cadmium and manganese salts with further hydrothermal treatment at 120 °C. The obtained photocatalysts were studied by the X-ray diffraction method (XRD), UV-vis diffuse reflectance spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N₂ low temperature adsorption. The photocatalysts were tested in hydrogen production using a Na₂S/Na₂SO₃ aqueous solution under visible light (λ = 450 nm). It was shown for the first time that both kinds of photocatalysts possess high activity in hydrogen evolution under visible light. The solid solution Cd_0.65Mn_0.35S has an enhanced photocatalytic activity due to its valence and conduction band position tuning, whereas the CdS-β-Mn₃O₄-MnOOH (40–60 at% Mn) samples were active due to ternary heterojunction formation. Further, the composite CdS-β-Mn₃O₄-MnOOH photocatalyst had much higher stability in comparison to the Cd_0.65Mn_0.35S solid solution. The highest activity was 600 mmol g⁻¹ h⁻¹, and apparent quantum efficiency of 2.9% (λ = 450 nm) was possessed by the sample of CdS-β-Mn₃O₄-MnOOH (40 at% Mn).