Colloidal gold

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Colloidal_gold 

 

Suspensions of gold nanoparticles of various sizes. The size difference causes the difference in colors.

 

Colloidal gold is a sol or colloidal suspension of nanoparticles of gold in a fluid, usually water. The colloid is usually either an intense red colour (for spherical particles less than 100 nm) or blue/purple (for larger spherical particles or nanorods). Due to their optical, electronic, and molecular-recognition properties, gold nanoparticles are the subject of substantial research, with many potential or promised applications in a wide variety of areas, including electron microscopy, electronics, nanotechnology, materials science, and biomedicine.

The properties of colloidal gold nanoparticles, and thus their potential applications, depend strongly upon their size and shape. For example, rodlike particles have both transverse and longitudinal absorption peak, and anisotropy of the shape affects their self-assembly.

History

This cranberry glass bowl was made by adding a gold salt (probably gold chloride) to molten glass.

Used since ancient times as a method of staining glass colloidal gold was used in the 4th-century Lycurgus Cup, which changes color depending on the location of light source.

During the Middle Ages, soluble gold, a solution containing gold salt, had a reputation for its curative property for various diseases. In 1618, Francis Anthony, a philosopher and member of the medical profession, published a book called Panacea Aurea, sive tractatus duo de ipsius Auro Potabili (Latin: gold potion, or two treatments of potable gold). The book introduces information on the formation of colloidal gold and its medical uses. About half a century later, English botanist Nicholas Culpepper published book in 1656, Treatise of Aurum Potabile, solely discussing the medical uses of colloidal gold.

In 1676, Johann Kunckel, a German chemist, published a book on the manufacture of stained glass. In his book Valuable Observations or Remarks About the Fixed and Volatile Salts-Auro and Argento Potabile, Spiritu Mundi and the Like, Kunckel assumed that the pink color of Aurum Potabile came from small particles of metallic gold, not visible to human eyes. In 1842, John Herschel invented a photographic process called chrysotype (from the Greek χρῡσός meaning "gold") that used colloidal gold to record images on paper.

Modern scientific evaluation of colloidal gold did not begin until Michael Faraday's work in the 1850s. In 1856, in a basement laboratory of Royal Institution, Faraday accidentally created a ruby red solution while mounting pieces of gold leaf onto microscope slides. Since he was already interested in the properties of light and matter, Faraday further investigated the optical properties of the colloidal gold. He prepared the first pure sample of colloidal gold, which he called 'activated gold', in 1857. He used phosphorus to reduce a solution of gold chloride. The colloidal gold Faraday made 150 years ago is still optically active. For a long time, the composition of the 'ruby' gold was unclear. Several chemists suspected it to be a gold tin compound, due to its preparation. Faraday recognized that the color was actually due to the miniature size of the gold particles. He noted the light scattering properties of suspended gold microparticles, which is now called Faraday-Tyndall effect.

In 1898, Richard Adolf Zsigmondy prepared the first colloidal gold in diluted solution. Apart from Zsigmondy, Theodor Svedberg, who invented ultracentrifugation, and Gustav Mie, who provided the theory for scattering and absorption by spherical particles, were also interested in the synthesis and properties of colloidal gold.

With advances in various analytical technologies in the 20th century, studies on gold nanoparticles has accelerated. Advanced microscopy methods, such as atomic force microscopy and electron microscopy, have contributed the most to nanoparticle research. Due to their comparably easy synthesis and high stability, various gold particles have been studied for their practical uses. Different types of gold nanoparticle are already used in many industries, such as medicine and electronics. For example, several FDA-approved nanoparticles are currently used in drug delivery.

Physical properties

Optical

The variation of scattering cross section of 100 nm-radius gold nanoparticle vs. the wavelength

 

Colloidal gold has been used by artists for centuries because of the nanoparticle’s interactions with visible light. Gold nanoparticles absorb and scatter light resulting in colours ranging from vibrant reds to blues to black and finally to clear and colorless, depending on particle size, shape, local refractive index, and aggregation state. These colors occur because of a phenomenon called localized surface plasmon resonance (LSPR), in which conduction electrons on the surface of the nanoparticle oscillate in resonance with incident light.

Effect of size

As a general rule, the wavelength of light absorbed increases as a function of increasing nano particle size. For example, pseudo-spherical gold nanoparticles with diameters ~ 30 nm have a peak LSPR absorption at ~530 nm.

Effect of local refractive index

Changes in the apparent color of a gold nanoparticle solution can also be caused by the environment in which the colloidal gold is suspended The optical properties of gold nanoparticles depends on the refractive index near the nanoparticle surface, therefore both the molecules directly attached to the nanoparticle surface (i.e. nanoparticle ligands) and/or the nanoparticle solvent both may influence observed optical features. As the refractive index near the gold surface increases, the NP LSPR will shift to longer wavelengths In addition to solvent environment, the extinction peak can be tuned by coating the nanoparticles with non-conducting shells such as silica, bio molecules, or aluminium oxide.

Effect of aggregation

When gold nano particles aggregate, the optical properties of the particle change, because the effective particle size, shape, and dielectric environment all change.

Medical research

Electron microscopy

Colloidal gold and various derivatives have long been among the most widely used labels for antigens in biological electron microscopy. Colloidal gold particles can be attached to many traditional biological probes such as antibodies, lectins, superantigens, glycans, nucleic acids, and receptors. Particles of different sizes are easily distinguishable in electron micrographs, allowing simultaneous multiple-labelling experiments.

In addition to biological probes, gold nanoparticles can be transferred to various mineral substrates, such as mica, single crystal silicon, and atomically flat gold(III), to be observed under atomic force microscopy (AFM).

Drug delivery system

Gold nanoparticles can be used to optimize the biodistribution of drugs to diseased organs, tissues or cells, in order to improve and target drug delivery. Nanoparticle-mediated drug delivery is feasible only if the drug distribution is otherwise inadequate. These cases include drug targeting of unstable (proteins, siRNA, DNA), delivery to the difficult sites (brain, retina, tumors, intracellular organelles) and drugs with serious side effects (e.g. anti-cancer agents). The performance of the nanoparticles depends on the size and surface functionalities in the particles. Also, the drug release and particle disintegration can vary depending on the system (e.g. biodegradable polymers sensitive to pH). An optimal nanodrug delivery system ensures that the active drug is available at the site of action for the correct time and duration, and their concentration should be above the minimal effective concentration (MEC) and below the minimal toxic concentration (MTC).

Gold nanoparticles are being investigated as carriers for drugs such as Paclitaxel. The administration of hydrophobic drugs require molecular encapsulation and it is found that nanosized particles are particularly efficient in evading the reticuloendothelial system.

Tumor detection

In cancer research, colloidal gold can be used to target tumors and provide detection using SERS (surface enhanced Raman spectroscopy) in vivo. These gold nanoparticles are surrounded with Raman reporters, which provide light emission that is over 200 times brighter than quantum dots. It was found that the Raman reporters were stabilized when the nanoparticles were encapsulated with a thiol-modified polyethylene glycol coat. This allows for compatibility and circulation in vivo. To specifically target tumor cells, the polyethylenegylated gold particles are conjugated with an antibody (or an antibody fragment such as scFv), against, e.g. epidermal growth factor receptor, which is sometimes overexpressed in cells of certain cancer types. Using SERS, these pegylated gold nanoparticles can then detect the location of the tumor.

Gold nanoparticles accumulate in tumors, due to the leakiness of tumor vasculature, and can be used as contrast agents for enhanced imaging in a time-resolved optical tomography system using short-pulse lasers for skin cancer detection in mouse model. It is found that intravenously administrated spherical gold nanoparticles broadened the temporal profile of reflected optical signals and enhanced the contrast between surrounding normal tissue and tumors.

Tumor targeting via multifunctional nanocarriers. Cancer cells reduce adhesion to neighboring cells and migrate into the vasculature-rich stroma. Once at the vasculature, cells can freely enter the bloodstream. Once the tumor is directly connected to the main blood circulation system, multifunctional nanocarriers can interact directly with cancer cells and effectively target tumors.

Gene therapy

Gold nanoparticles have shown potential as intracellular delivery vehicles for siRNA oligonucleotides with maximal therapeutic impact.

Multifunctional siRNA-gold nanoparticles with several biomolecules: PEG, cell penetration and cell adhesion peptides and siRNA. Two different approaches were employed to conjugate the siRNA to the gold nanoparticle: (1) Covalent approach: use of thiolated siRNA for gold-thiol binding to the nanoparticle; (2) Ionic approach: interaction of the negatively charged siRNA to the modified surface of the AuNP through ionic interactions.

 

Gold nanoparticles show potential as intracellular delivery vehicles for antisense oligonucleotides (ssDNA,dsDNA) by providing protection against intracellular nucleases and ease of functionalization for selective targeting.

Photothermal agents

Gold nanorods are being investigated as photothermal agents for in-vivo applications. Gold nanorods are rod-shaped gold nanoparticles whose aspect ratios tune the surface plasmon resonance (SPR) band from the visible to near-infrared wavelength. The total extinction of light at the SPR is made up of both absorption and scattering. For the smaller axial diameter nanorods (~10 nm), absorption dominates, whereas for the larger axial diameter nanorods (>35 nm) scattering can dominate. As a consequence, for in-vivo studies, small diameter gold nanorods are being used as photothermal converters of near-infrared light due to their high absorption cross-sections. Since near-infrared light transmits readily through human skin and tissue, these nanorods can be used as ablation components for cancer, and other targets. When coated with polymers, gold nanorods have been observed to circulate in-vivo with half-lives longer than 6 hours, bodily residence times around 72 hours, and little to no uptake in any internal organs except the liver.

Despite the unquestionable success of gold nanorods as photothermal agents in preclinical research, they have yet to obtain the approval for clinical use because the size is above the renal excretion threshold. In 2019, the first NIR-absorbing plasmonic ultrasmall-in-nano architecture has been reported, and jointly combine: (i) a suitable photothermal conversion for hyperthermia treatments, (ii) the possibility of multiple photothermal treatments and (iii) renal excretion of the building blocks after the therapeutic action.

Radiotherapy dose enhancer

Considerable interest has been shown in the use of gold and other heavy-atom-containing nanoparticles to enhance the dose delivered to tumors. Since the gold nanoparticles are taken up by the tumors more than the nearby healthy tissue, the dose is selectively enhanced. The biological effectiveness of this type of therapy seems to be due to the local deposition of the radiation dose near the nanoparticles. This mechanism is the same as occurs in heavy ion therapy.

Detection of toxic gas

Researchers have developed simple inexpensive methods for on-site detection of hydrogen sulfide H
₂S present in air based on the antiaggregation of gold nanoparticles (AuNPs). Dissolving H
₂S into a weak alkaline buffer solution leads to the formation of HS-, which can stabilize AuNPs and ensure they maintain their red color allowing for visual detection of toxic levels of H
₂S.

Gold nanoparticle based biosensor

Gold nanoparticles are incorporated into biosensors to enhance its stability, sensitivity, and selectivity. Nanoparticle properties such as small size, high surface-to-volume ratio, and high surface energy allow immobilization of large range of biomolecules. Gold nanoparticle, in particular, could also act as "electron wire" to transport electrons and its amplification effect on electromagnetic light allows it to function as signal amplifiers. Main types of gold nanoparticle based biosensors are optical and electrochemical biosensor.

Optical biosensor

Gold nanoparticle-based (Au-NP) biosensor for Glutathione (GSH). The AuNPs are functionalised with a chemical group that binds to GSH and makes the NPs partially collapse, and thus change colour. The exact amount of GSH can be derived via UV-vis spectroscopy through a calibration curve.

 

Gold nanoparticles improve the sensitivity of optical sensor by response to the change in local refractive index. The angle of the incidence light for surface plasmon resonance, an interaction between light wave and conducting electrons in metal, changes when other substances are bounded to the metal surface. Because gold is very sensitive to its surroundings' dielectric constant, binding of an analyte would significantly shift gold nanoparticle's SPR and therefore allow more sensitive detection. Gold nanoparticle could also amplify the SPR signal. When the plasmon wave pass through the gold nanoparticle, the charge density in the wave and the electron I the gold interacted and resulted in higher energy response, so called electron coupling. Since the analyte and bio-receptor now bind to the gold, it increases the apparent mass of the analyte and therefore amplified the signal. These properties had been used to build DNA sensor with 1000-fold sensitive than without the Au NP. Humidity senor was also built by altering the atom interspacing between molecules with humidity change, the interspacing change would also result in a change of the Au NP's LSPR.

Electrochemical biosensor

Electrochemical sensor convert biological information into electrical signals that could be detected. The conductivity and biocompatibility of Au NP allow it to act as "electron wire". It transfers electron between the electrode and the active site of the enzyme. It could be accomplished in two ways: attach the Au NP to either the enzyme or the electrode. GNP-glucose oxidase monolayer electrode was constructed use these two methods. The Au NP allowed more freedom in the enzyme's orientation and therefore more sensitive and stable detection. Au NP also acts as immobilization platform for the enzyme. Most biomolecules denatures or lose its activity when interacted with the electrode. The biocompatibility and high surface energy of Au allow it to bind to a large amount of protein without altering its activity and results in a more sensitive sensor. Moreover, Au NP also catalyzes biological reactions. Gold nanoparticle under 2 nm has shown catalytic activity to the oxidation of styrene.

Immunological biosensor

Gold nanoparticles have been coated with peptides and glycans for use in immunological detection methods. The possibility to use glyconanoparticles in ELISA was unexpected, but the method seems to have a high sensitivity and thus offers potential for development of specific assays for diagnostic identification of antibodies in patient sera 

Thin films

Gold nanoparticles capped with organic ligands, such as alkanethiol molecules, can self-assemble into large monolayers (>cm${\displaystyle ^{2}}$). The particles are first prepared in organic solvent, such as chloroform or toluene, and are then spread into monolayers either on a liquid surface or on a solid substrate. Such interfacial thin films of nanoparticles have close relationship with Langmuir-Blodgett monolayers made from surfactants.

The mechanical properties of nanoparticle monolayers have been studied extensively. For 5 nm spheres capped with dodecanethiol, the Young's modulus of the monolayer is on the order of GPa. The mechanics of the membranes are guided by strong interactions between ligand shells on adjacent particles. Upon fracture, the films crack perpendicular to the direction of strain at a fracture stress of 11 ${\displaystyle \pm }$ 2.6 MPa, comparable to that of cross-linked polymer films. Free-standing nanoparticle membranes exhibit bending rigidity on the order of 10${\displaystyle ^{5}}$ eV, higher than what is predicted in theory for continuum plates of the same thickness, due to nonlocal microstructural constraints such as nonlocal coupling of particle rotational degrees of freedom. On the other hand, resistance to bending is found to be greatly reduced in nanoparticle monolayers that are supported at the air/water interface, possibly due to screening of ligand interactions in a wet environment.

Surface chemistry

In many different types of colloidal gold syntheses, the interface of the nanoparticles can display widely different character – ranging from an interface similar to a self-assembled monolayer to a disordered boundary with no repeating patterns. Beyond the Au-Ligand interface, conjugation of the interfacial ligands with various functional moieties (from small organic molecules to polymers to DNA to RNA) afford colloidal gold much of its vast functionality.

Ligand exchange/functionalization

After initial nanoparticle synthesis, colloidal gold ligands are often exchanged with new ligands designed for specific applications. For example, Au NPs produced via the Turkevich-style (or Citrate Reduction) method are readily reacted via ligand exchange reactions, due to the relatively weak binding between the carboxyl groups and the surfaces of the NPs. This ligand exchange can produce conjugation with a number of biomolecules from DNA to RNA to proteins to polymers (such as PEG) to increase biocompatibility and functionality. For example, ligands have been shown to enhance catalytic activity by mediating interactions between adsorbates and the active gold surfaces for specific oxygenation reactions. Ligand exchange can also be used to promote phase transfer of the colloidal particles. Ligand exchange is also possible with alkane thiol-arrested NPs produced from the Brust-type synthesis method, although higher temperatures are needed to promote the rate of the ligand detachment. An alternative method for further functionalization is achieved through the conjugation of the ligands with other molecules, though this method can cause the colloidal stability of the Au NPs to breakdown.

Ligand removal

In many cases, as in various high-temperature catalytic applications of Au, the removal of the capping ligands produces more desirable physicochemical properties. The removal of ligands from colloidal gold while maintaining a relatively constant number of Au atoms per Au NP can be difficult due to the tendency for these bare clusters to aggregate. The removal of ligands is partially achievable by simply washing away all excess capping ligands, though this method is ineffective in removing all capping ligand. More often ligand removal achieved under high temperature or light ablation followed by washing. Alternatively, the ligands can be electrochemically etched off.

Surface structure and chemical environment

The precise structure of the ligands on the surface of colloidal gold NPs impact the properties of the colloidal gold particles. Binding conformations and surface packing of the capping ligands at the surface of the colloidal gold NPs tend to differ greatly from bulk surface model adsorption, largely due to the high curvature observed at the nanoparticle surfaces. Thiolate-gold interfaces at the nanoscale have been well-studied and the thiolate ligands are observed to pull Au atoms off of the surface of the particles to for “staple” motifs that have significant Thiyl-Au(0) character. The citrate-gold surface, on the other hand, is relatively less-studied due to the vast number of binding conformations of the citrate to the curved gold surfaces. A study performed in 2014 identified that the most-preferred binding of the citrate involves two carboxylic acids and the hydroxyl group of the citrate binds three surface metal atoms.

Health and safety

As gold nanoparticles (AuNPs) are further investigated for targeted drug delivery in humans, their toxicity needs to be considered. For the most part, it is suggested that AuNPs are biocompatible, but the concentrations at which they become toxic needs to be determined, and if those concentrations fall within the range of used concentrations. Toxicity can be tested in vitro and in vivo. In vitro toxicity results can vary depending on the type of the cellular growth media with different protein compositions, the method used to determine cellular toxicity (cell health, cell stress, how many cells are taken into a cell), and the capping ligands in solution. In vivo assessments can determine the general health of an organism (abnormal behavior, weight loss, average life span) as well as tissue specific toxicology (kidney, liver, blood) and inflammation and oxidative responses. In vitro experiments are more popular than in vivo experiments because in vitro experiments are more simplistic to perform than in vivo experiments.

Toxicity and hazards in synthesis

While AuNPs themselves appear to have low or negligible toxicity, and the literature shows that the toxicity has much more to do with the ligands rather than the particles themselves, the synthesis of them involves chemicals that are hazardous. Sodium borohydride, a harsh reagent, is used to reduce the gold ions to gold metal. The gold ions usually come from chloroauric acid, a potent acid. Because of the high toxicity and hazard of reagents used to synthesize AuNPs, the need for more “green” methods of synthesis arose.

Toxicity due to capping ligands

Some of the capping ligands associated with AuNPs can be toxic while others are nontoxic. In gold nanorods (AuNRs), it has been shown that a strong cytotoxicity was associated with CTAB-stabilized AuNRs at low concentration, but it is thought that free CTAB was the culprit in toxicity. Modifications that overcoat these AuNRs reduces this toxicity in human colon cancer cells (HT-29) by preventing CTAB molecules from desorbing from the AuNRs back into the solution. Ligand toxicity can also be seen in AuNPs. Compared to the 90% toxicity of HAuCl4 at the same concentration, AuNPs with carboxylate termini were shown to be non-toxic. Large AuNPs conjugated with biotin, cysteine, citrate, and glucose were not toxic in human leukemia cells (K562) for concentrations up to 0.25 M. Also, citrate-capped gold nanospheres (AuNSs) have been proven to be compatible with human blood and did not cause platelet aggregation or an immune response. However, citrate-capped gold nanoparticles sizes 8-37 nm were found to be lethally toxic for mice, causing shorter lifespans, severe sickness, loss of appetite and weight, hair discoloration, and damage to the liver, spleen, and lungs; gold nanoparticles accumulated in the spleen and liver after traveling a section of the immune system. There are mixed-views for polyethylene glycol (PEG)-modified AuNPs. These AuNPs were found to be toxic in mouse liver by injection, causing cell death and minor inflammation. However, AuNPs conjugated with PEG copolymers showed negligible toxicity towards human colon cells (Caco-2). AuNP toxicity also depends on the overall charge of the ligands. In certain doses, AuNSs that have positively-charged ligands are toxic in monkey kidney cells (Cos-1), human red blood cells, and E. coli because of the AuNSs interaction with the negatively-charged cell membrane; AuNSs with negatively-charged ligands have been found to be nontoxic in these species. In addition to the previously mentioned in vivo and in vitro experiments, other similar experiments have been performed. Alkylthiolate-AuNPs with trimethlyammonium ligand termini mediate the translocation of DNA across mammalian cell membranes in vitro at a high level, which is detrimental to these cells. Corneal haze in rabbits have been healed in vivo by using polyethylemnimine-capped gold nanoparticles that were transfected with a gene that promotes wound healing and inhibits corneal fibrosis.

Toxicity due to size of nanoparticles

Toxicity in certain systems can also be dependent on the size of the nanoparticle. AuNSs size 1.4 nm were found to be toxic in human skin cancer cells (SK-Mel-28), human cervical cancer cells (HeLa), mouse fibroblast cells (L929), and mouse macrophages (J774A.1), while 0.8, 1.2, and 1.8 nm sized AuNSs were less toxic by a six-fold amount and 15 nm AuNSs were nontoxic. There is some evidence for AuNP buildup after injection in in vivo studies, but this is very size dependent. 1.8 nm AuNPs were found to be almost totally trapped in the lungs of rats. Different sized AuNPs were found to buildup in the blood, brain, stomach, pancreas, kidneys, liver, and spleen.

Biosafety and biokinetics investigations on biodegradable ultrasmall-in-nano architectures have demonstrated that gold nanoparticles are able to avoid metal accumulation in organisms through escaping by the renal pathway.

Synthesis

Potential difference as a function of distance from particle surface.

Generally, gold nanoparticles are produced in a liquid ("liquid chemical methods") by reduction of chloroauric acid (H[AuCl₄]). To prevent the particles from aggregating, stabilizing agents are added. Citrate acts both as the reducing agent and colloidal stabilizer.

They can be functionalized with various organic ligands to create organic-inorganic hybrids with advanced functionality.

Turkevich method

This simple method was pioneered by J. Turkevich et al. in 1951 and refined by G. Frens in the 1970s. It produces modestly monodisperse spherical gold nanoparticles of around 10–20 nm in diameter. Larger particles can be produced, but at the cost of monodispersity and shape. In this method, hot chloroauric acid is treated with sodium citrate solution, producing colloidal gold. The Turkevich reaction proceeds via formation of transient gold nanowires. These gold nanowires are responsible for the dark appearance of the reaction solution before it turns ruby-red.

Capping agents

A capping agent is used during nanoparticle synthesis to inhibit particle growth and aggregation. The chemical blocks or reduces reactivity at the periphery of the particle—a good capping agent has a high affinity for the new nuclei. Citrate ions or tannic acid function both as a reducing agent and a capping agent. Less sodium citrate results in larger particles.

Brust-Schiffrin method

This method was discovered by Brust and Schiffrin in the early 1990s, and can be used to produce gold nanoparticles in organic liquids that are normally not miscible with water (like toluene). It involves the reaction of a chlorauric acid solution with tetraoctylammonium bromide (TOAB) solution in toluene and sodium borohydride as an anti-coagulant and a reducing agent, respectively.

Here, the gold nanoparticles will be around 5–6 nm. NaBH₄ is the reducing agent, and TOAB is both the phase transfer catalyst and the stabilizing agent. 

TOAB does not bind to the gold nanoparticles particularly strongly, so the solution will aggregate gradually over the course of approximately two weeks. To prevent this, one can add a stronger binding agent, like a thiol (in particular, alkanethiols), which will bind to gold, producing a near-permanent solution. Alkanethiol protected gold nanoparticles can be precipitated and then redissolved. Thiols are better binding agents because there is a strong affinity for the gold-sulfur bonds that form when the two substances react with each other. Tetra-dodecanthiol is a commonly used strong binding agent to synthesize smaller particles. Some of the phase transfer agent may remain bound to the purified nanoparticles, this may affect physical properties such as solubility. In order to remove as much of this agent as possible, the nanoparticles must be further purified by soxhlet extraction.

Perrault method

This approach, discovered by Perrault and Chan in 2009, uses hydroquinone to reduce HAuCl₄ in an aqueous solution that contains 15 nm gold nanoparticle seeds. This seed-based method of synthesis is similar to that used in photographic film development, in which silver grains within the film grow through addition of reduced silver onto their surface. Likewise, gold nanoparticles can act in conjunction with hydroquinone to catalyze reduction of ionic gold onto their surface. The presence of a stabilizer such as citrate results in controlled deposition of gold atoms onto the particles, and growth. Typically, the nanoparticle seeds are produced using the citrate method. The hydroquinone method complements that of Frens, as it extends the range of monodispersed spherical particle sizes that can be produced. Whereas the Frens method is ideal for particles of 12–20 nm, the hydroquinone method can produce particles of at least 30–300 nm.

Martin method

This simple method, discovered by Martin and Eah in 2010, generates nearly monodisperse "naked" gold nanoparticles in water. Precisely controlling the reduction stoichiometry by adjusting the ratio of NaBH₄-NaOH ions to HAuCl₄-HCl ions within the "sweet zone," along with heating, enables reproducible diameter tuning between 3–6 nm. The aqueous particles are colloidally stable due to their high charge from the excess ions in solution. These particles can be coated with various hydrophilic functionalities, or mixed with hydrophobic ligands for applications in non-polar solvents. In non-polar solvents the nanoparticles remain highly charged, and self-assemble on liquid droplets to form 2D monolayer films of monodisperse nanoparticles.

Nanotech studies

Bacillus licheniformis can be used in synthesis of gold nanocubes with sizes between 10 and 100 nanometres. Gold nanoparticles are usually synthesized at high temperatures in organic solvents or using toxic reagents. The bacteria produce them in much milder conditions.

Navarro et al. method

For particles larger than 30 nm, control of particle size with a low polydispersity of spherical gold nanoparticles remains challenging. In order to provide maximum control on the NP structure, Navarro and co-workers used a modified Turkevitch-Frens procedure using sodium acetylacetonate Na(acac) as the reducing agent and sodium citrate as the stabilizer.

Sonolysis

Another method for the experimental generation of gold particles is by sonolysis. The first method of this type was invented by Baigent and Müller. This work pioneered the use of ultrasound to provide the energy for the processes involved and allowed the creation of gold particles with a diameter of under 10 nm. In another method using ultrasound, the reaction of an aqueous solution of HAuCl₄ with glucose, the reducing agents are hydroxyl radicals and sugar pyrolysis radicals (forming at the interfacial region between the collapsing cavities and the bulk water) and the morphology obtained is that of nanoribbons with width 30–50 nm and length of several micrometers. These ribbons are very flexible and can bend with angles larger than 90°. When glucose is replaced by cyclodextrin (a glucose oligomer), only spherical gold particles are obtained, suggesting that glucose is essential in directing the morphology toward a ribbon.

Block copolymer-mediated method

An economical, environmentally benign and fast synthesis methodology for gold nanoparticles using block copolymer has been developed by Sakai et al. In this synthesis methodology, block copolymer plays the dual role of a reducing agent as well as a stabilizing agent. The formation of gold nanoparticles comprises three main steps: reduction of gold salt ion by block copolymers in the solution and formation of gold clusters, adsorption of block copolymers on gold clusters and further reduction of gold salt ions on the surfaces of these gold clusters for the growth of gold particles in steps, and finally its stabilization by block copolymers. But this method usually has a limited-yield (nanoparticle concentration), which does not increase with the increase in the gold salt concentration. Ray et al. improved this synthesis method by enhancing the nanoparticle yield by manyfold at ambient temperature.

Pandemic prevention

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Pandemic_prevention 

 

Pandemic prevention is the organization and management of preventive measures against pandemics. Those include measures to reduce causes of new infectious diseases and measures to prevent outbreaks and epidemics from becoming pandemics.

History

The 2003 SARS-CoV virus was prevented from causing a pandemic. Rapid action by national and international health authorities such as the World Health Organization helped to slow transmission and eventually broke the chain of transmission, which ended the localized epidemics before they could become a pandemic. However, the disease has not been eradicated and could re-emerge. This warrants monitoring and reporting of suspicious cases of atypical pneumonia. Effective isolation of patients was enough to control spread because infected individuals usually not transmitting the virus until several days after symptoms began and were most infectious only after developing severe symptoms.

Measures

Infrastructure and international development

Robust, collaborating public health systems may be required to be able stop contagion promptly. After an outbreak there is a certain window of time during which a pandemic can still be stopped by the competent authorities isolating the first infected and/or fighting the pathogen. A good global infrastructure, consequent information exchange, short ways in bureaucracy and effective, targeted treatment measures can be prepared. 2012 it has been proposed to consider pandemic prevention as an aspect of international development in terms of health-care infrastructure and changes to the pathogen-related dynamics between humans and their environment including animals. Often local authority carers or doctors in Africa, Asia or Latin America register uncommon accumulations (or clusterings) of symptoms but lack options for more detailed investigations. Scientists state that "research relevant to countries with weaker surveillance, lab facilities and health systems should be prioritized" and that "in those regions, vaccine supply routes should not rely on refrigeration, and diagnostics should be available at the point of care".

Technologies

Pathogen detection and prediction

In a 2012 study it is claimed that "new mathematical modelling, diagnostic, communications, and informatics technologies can identify and report hitherto unknown microbes in other species, and thus new risk assessment approaches are needed to identify microbes most likely to cause human disease". The study investigates challenges in moving the global pandemic strategy from response to pre-emption. Some scientists are screening blood samples from wildlife for new viruses. The international Global Virome Project (GVP) aims to identify the causes of fatal new diseases before emergence in human hosts by genetically characterizing viruses found in wild animals. Edward Rubin notes that after sufficient data has been gathered artificial intelligence could be used to identify common features and develop countermeasures and vaccines against whole categories of viruses.^[10] It might be possible to predict viral evolution using machine learning. Funding for the United States' PREDICT government research program that sought to identify animal pathogens that might infect humans and to prevent new pandemics was cut in 2019. Funding for United States' CDC programs that trained workers in outbreak detection and strengthened laboratory and emergency response systems in countries where disease risks are greatest to stop outbreaks at the source was cut by 80% in 2018.

CRISPR-based immune subsystems

In March 2020 scientists of Stanford University presented a CRISPR-based system, called PAC-MAN (Prophylactic Antiviral Crispr in huMAN cells), that can find and destroy viruses in vitro. However, they weren't able to test PAC-MAN on the actual SARS-CoV-2, use a targeting-mechanism that uses only a very limited RNA-region, haven't developed a system to deliver it into human cells and would need a lot of time until another version of it or a potential successor system might pass clinical trials. In the study published as a preprint they write that it could be used prophylactically as well as therapeutically. The CRISPR-Cas13d-based system could be agnostic to which virus it's fighting so novel viruses would only require a small change. In an editorial published in February 2020 another group of scientists claimed that they have implemented a flexible and efficient approach for targeting RNA with CRISPR-Cas13d which they have put under review and propose that the system can be used to also target SARS-CoV-2 in specific. There have also been earlier successful efforts in fighting viruses with CRISPR-based technology in human cells.

Testing and containment

A SARS-CoV-2 laboratory test kit by the CDC

Timely use and development of quick testing systems for novel virus in combination with other measures might make it possible to end transmission lines of outbreaks before they become pandemics. A high discovery-rate is important for tests. For instance this is the reason why no thermal scanners with a low discovery-rate were used in airports for containment during the 2009 swine flu pandemic. The German program InfectControl 2020 seeks to develop strategies for prevention, early recognition and control of infectious diseases. In one of its projects "HyFly" partners of industry and research work on strategies to contain chains of transmission in air traffic, to establish preventive countermeasures and to create concrete recommendations for actions of airport operators and airline companies. One approach of the project is to detect infections without molecular-biological methods during passenger screening. For this researchers of the Fraunhofer-Institut for cell therapy and immunology are developing a non-invasive procedure based on ion-mobility spectrometry (IMS).

Surveillance and mapping

Monitoring people who are exposed to animals in viral hotspots – including via virus monitoring stations – can register viruses at the moment they enter human populations - this might enable prevention of pandemics. The most important transmission pathways often vary per underlying driver of emerging infectious diseases such as the vector-borne pathway and direct animal contact for land-use change – the leading driver for emerging zoonoses by number of emergence events as defined by Jones et al. (2008). Zoonoses account for 75% of the reviewed 1415 species of infectious organisms known to be pathogenic to humans until 2001. Genomics could be used to precisely monitor virus evolution and transmission in real time across large, diverse populations by combining pathogen genomics with data about host genetics and about the unique transcriptional signature of infection. The "Surveillance, Outbreak Response Management and Analysis System" (SORMAS) of the German Helmholtz-Zentrum für Infektionsforschung (HZI) and Deutsches Zentrum für Infektionsforschung (DZIF), who collaborate with Nigerian researchers, gathers and analyzes data during an outbreak, detects potential threats and allows to initiate protective measures early. It's meant specifically for poorer regions and has been used for the fight against a monkeypox outbreak in Nigeria.

Policy and economics

A 2014 analysis asserts that "the window of opportunity to deal with pandemics as a global community is within the next 27 years. Pandemic prevention therefore should be a critical health policy issue for the current generation of scientists and policymakers to address. A 2007 study warns that "the presence of a large reservoir of SARS-CoV-like viruses in horseshoe bats, together with the culture of eating exotic mammals in southern China, is a time bomb. The possibility of the reemergence of SARS and other novel viruses from animals or laboratories and therefore the need for preparedness should not be ignored". The US' National Security Council Directorate for Global Health Security and Biodefense, which worked on preparing for the next disease outbreak and preventing it from becoming an epidemic or pandemic, was closed in 2018.

Environmental policy and economics

Some experts link pandemic prevention with environmental policy and caution that environmental destruction as well as climate change drives wildlife to live close to people. For instance the WHO projects that climate change will also affect infectious disease occurrence. A 2016 study reviews literature on the evidences for the impact of climate change on human infectious disease, suggests a number of proactive measures for controlling health impacts of climate change and finds that climate change impacts human infectious disease via alterations to pathogen, host and transmission. Studies have shown that the risk of disease outbreaks can increase substantially after forests are cleared. Stanford biological anthropologist James Holland Jones notes that humanity has "engineer[ed] a world where emerging infectious diseases are both more likely and more likely to be consequential", referring to the modern world's prevalent highly mobile lifestyles, increasingly dense cities, various kinds of human interactions with wildlife and alterations of the natural world.

Biotechnology research and development regulation

Toby Ord puts into question whether the current public health and international conventions, and self-regulation by biotechnology companies and the scientific community are adequate. In the context of the 2019–2020 coronavirus pandemic Neal Baer writes that the "public, scientists, lawmakers, and others" "need to have thoughtful conversations about gene editing now".

Food markets and wild animal trade

Fowl cages at wet market in Shenzhen, China

In January 2020 during the SARS-CoV 2 outbreak experts in and outside China warned that wild animal markets, where the virus originated from, should be banned worldwide. On January 26 China banned the trade of wild animals until the end of the coronavirus epidemic at the time. On February 24 China announced a permanent ban on wildlife trade and consumption with some exceptions. Some scientists point out that banning informal wet markets worldwide isn't the appropriate solution as fridges aren't available in many places and because much of the food for Africa and Asia is provided through such traditional markets. Some also caution that simple bans may force traders underground, where they may pay less attention to hygiene and some state that it's wild animals rather than farmed animals that are the natural hosts of many viruses.

International coordination

The Global Health Security Agenda (GHSA) a network of countries, international organizations, NGOs and companies that aim to improve the world's ability to prevent, detect, and respond to infectious diseases. Sixty-seven countries have signed onto the GHSA framework. Funding for the GHSA has been reduced since the launch in 2014, both in the US and globally. In a 2018 lecture in Boston Bill Gates called for a global effort to build a comprehensive pandemic preparedness and response system.

Containment and prevention by artificial induction of immunity and/or biocides

Outbreaks could be contained or delayed – to enable other containment-measures – or prevented by artificial induction of immunity and/or biocides in combination with other measures that include prediction or early detection of infectious human diseases.

In a preprint published on March 24, 2020 researchers suggested that the unique transcriptional signature of SARS-CoV-2 in the human immune system may be responsible for the development of COVID-19: SARS-CoV-2 did not induce the antiviral genes that code for type I and type III interferons. This could be relevant for the development or repurposing of treatments.

Vaccination

Development and provision of new vaccines usually takes years. The Coalition for Epidemic Preparedness Innovations, which was launched in 2017, works on reducing the time of vaccine-development.

Culling

Experts warned that depleting the numbers of species by culling to forestall human infections reduces genetic diversity and thereby puts future generations of the animals as well as people at risk while others contend that it's still the best, practical way to contain a virus of livestock.

Prevention versus mitigation

Pandemic prevention seeks to prevent pandemics while mitigation of pandemics seeks to reduce their severity and negative impacts. Some have called for a shift from a treatment-oriented society to a prevention-oriented one. Authors of a 2010 study write that contemporary "global disease control focuses almost exclusively on responding to pandemics after they have already spread globally" and argue that the "wait-and-respond approach is not sufficient and that the development of systems to prevent novel pandemics before they are established should be considered imperative to human health". Nathan Wolfe criticizes that "our current global public health strategies are reminiscent of cardiology in the 1950s when doctors focused solely on responding to heart attacks and ignored the whole idea of prevention".

Antimicrobial properties of copper

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Antimicrobial_properties_of_copper 

 

Copper and its alloys (brasses, bronzes, cupronickel, copper-nickel-zinc, and others) are natural antimicrobial materials. Ancient civilizations exploited the antimicrobial properties of copper long before the concept of microbes became understood in the nineteenth century. In addition to several copper medicinal preparations, it was also observed centuries ago that water contained in copper vessels or transported in copper conveyance systems was of better quality (i.e., no or little visible slime or biofouling formation) than water contained or transported in other materials.

The antimicrobial properties of copper are still under active investigation. Molecular mechanisms responsible for the antibacterial action of copper have been a subject of intensive research. Scientists are also actively demonstrating the intrinsic efficacy of copper alloy "touch surfaces" to destroy a wide range of microorganisms that threaten public health.

Mechanisms of action

In 1852 Victor Burq discovered those working with copper had far fewer deaths to cholera than anyone else, and did extensive research confirming this. In 1867 he presented his findings to the French Academies of Science and Medicine, informing them that putting copper on the skin was effective at preventing someone from getting cholera.

The oligodynamic effect was discovered in 1893 as a toxic effect of metal ions on living cells, algae, molds, spores, fungi, viruses, prokaryotic, and eukaryotic microorganisms, even in relatively low concentrations. This antimicrobial effect is shown by ions of copper as well as mercury, silver, iron, lead, zinc, bismuth, gold, and aluminium. 

In 1973, researchers at Battelle Columbus Laboratories conducted a comprehensive literature, technology and patent search that traced the history of understanding the "bacteriostatic and sanitizing properties of copper and copper alloy surfaces", which demonstrated that copper, in very small quantities, has the power to control a wide range of molds, fungi, algae and harmful microbes. Of the 312 citations mentioned in the review across the time period 1892–1973, the observations below are noteworthy:

• Copper inhibits Actinomucor elegans, Aspergillus niger, Bacterium linens, Bacillus megaterium, Bacillus subtilis, Brevibacterium erythrogenes, Candida utilis, Penicillium chrysogenum, Rhizopus niveus, Saccharomyces mandshuricus, and Saccharomyces cerevisiae in concentrations above 10 g/L.
• Candida utilis (formerly, Torulopsis utilis) is completely inhibited at 0.04 g/L copper concentrations.
• Tubercle bacillus is inhibited by copper as simple cations or complex anions in concentrations from 0.02 to 0.2 g/L.
• Achromobacter fischeri and Photobacterium phosphoreum growth is inhibited by metallic copper.
• Paramecium caudatum cell division is reduced by copper plates placed on Petri dish covers containing infusoria and nutrient media.
• Poliovirus is inactivated within 10 minutes of exposure to copper with ascorbic acid.

A subsequent paper probed some of copper's antimicrobial mechanisms and cited no fewer than 120 investigations into the efficacy of copper's action on microbes. The authors noted that the antimicrobial mechanisms are very complex and take place in many ways, both inside cells and in the interstitial spaces between cells. 

Examples of some of the molecular mechanisms noted by various researchers include the following:

• The 3-dimensional structure of proteins can be altered by copper, so that the proteins can no longer perform their normal functions. The result is inactivation of bacteria or viruses.
• Copper complexes form radicals that inactivate viruses.
• Copper may disrupt enzyme structures, and functions by binding to sulfur- or carboxylate-containing groups and amino groups of proteins.
• Copper may interfere with other essential elements, such as zinc and iron.
• Copper facilitates deleterious activity in superoxide radicals. Repeated redox reactions on site-specific macromolecules generate HO• radicals, thereby causing "multiple hit damage" at target sites.
• Copper can interact with lipids, causing their peroxidation and opening holes in the cell membranes, thereby compromising the integrity of cells. This can cause leakage of essential solutes, which in turn, can have a desiccating effect.
• Copper damages the respiratory chain in Escherichia coli cells. and is associated with impaired cellular metabolism.
• Faster corrosion correlates with faster inactivation of microorganisms. This may be due to increased availability of cupric ion, Cu2+, which is believed to be responsible for the antimicrobial action.
• In inactivation experiments on the flu strain, H1N1, which is nearly identical to the H5N1 avian strain and the 2009 H1N1 (swine flu) strain, researchers hypothesized that copper's antimicrobial action probably attacks the overall structure of the virus and therefore has a broad-spectrum effect.
• Microbes require copper-containing enzymes to drive certain vital chemical reactions. Excess copper, however, can affect proteins and enzymes in microbes, thereby inhibiting their activities. Researchers believe that excess copper has the potential to disrupt cell function both inside cells and in the interstitial spaces between cells, probably acting on the cells' outer envelope.

Currently, researchers believe that the most important antimicrobial mechanisms for copper are as follows:

• Elevated copper levels inside a cell causes oxidative stress and the generation of hydrogen peroxide. Under these conditions, copper participates in the so-called Fenton-type reaction — a chemical reaction causing oxidative damage to cells.
• Excess copper causes a decline in the membrane integrity of microbes, leading to leakage of specific essential cell nutrients, such as potassium and glutamate. This leads to desiccation and subsequent cell death.
• While copper is needed for many protein functions, in an excess situation (as on a copper alloy surface), copper binds to proteins that do not require copper for their function. This "inappropriate" binding leads to loss-of-function of the protein, and/or breakdown of the protein into nonfunctional portions.

These potential mechanisms, as well as others, are the subject of continuing study by academic research laboratories around the world.

Antimicrobial efficacy of copper alloy touch surfaces

Copper alloy surfaces have intrinsic properties to destroy a wide range of microorganisms. In the interest of protecting public health, especially in healthcare environments with their susceptible patient populations, an abundance of peer-reviewed antimicrobial efficacy studies have been conducted in the past 10 years regarding copper's efficacy to destroy E. coli O157:H7, methicillin-resistant Staphylococcus aureus (MRSA), Staphylococcus, Clostridium difficile, influenza A virus, adenovirus, and fungi. Stainless steel was also investigated because it is an important surface material in today's healthcare environments. The studies cited here, plus others directed by the United States Environmental Protection Agency, resulted in the 2008 registration of 274 different copper alloys as certified antimicrobial materials that have public health benefits.

E. coli

E. coli O157:H7 is a potent, highly infectious, ACDP (Advisory Committee on Dangerous Pathogens, UK) Hazard Group 3 foodborne and waterborne pathogen. The bacterium produces potent toxins that cause diarrhea, severe aches and nausea in infected persons. Symptoms of severe infections include hemolytic colitis (bloody diarrhea), hemolytic uremic syndrome (kidney disease), and death. E. coli O157:H7 has become a serious public health threat because of its increased incidence and because children up to 14 years of age, the elderly, and immunocompromised individuals are at risk of incurring the most severe symptoms.

Efficacy on copper surfaces

Recent studies have shown that copper alloy surfaces kill E. coli O157:H7. Over 99.9% of E. coli microbes are killed after just 1–2 hours on copper. On stainless steel surfaces, the microbes can survive for weeks.

Results of E. coli O157:H7 destruction on an alloy containing 99.9% copper (C11000) demonstrate that this pathogen is rapidly and almost completely killed (over 99.9% kill rate) within ninety minutes at room temperature (20 °C). At chill temperatures (4 °C), over 99.9% of E. coli O157:H7 are killed within 270 minutes. E. coli O157:H7 destruction on several copper alloys containing 99%–100% copper (including C10200, C11000, C18080, and C19700) at room temperature begins within minutes. At chilled temperatures, the inactivation process takes about an hour longer. No significant reduction in the amount of viable E. coli O157:H7 occurs on stainless steel after 270 minutes.

Studies have been conducted to examine the E. coli O157:H7 bactericidal efficacies on 25 different copper alloys to identify those alloys that provide the best combination of antimicrobial activity, corrosion/oxidation resistance, and fabrication properties. Copper's antibacterial effect was found to be intrinsic in all of the copper alloys tested. As in previous studies, no antibacterial properties were observed on stainless steel (UNS S30400). Also, in confirmation with earlier studies the rate of drop-off of E. coli O157:H7 on the copper alloys is faster at room temperature than at chill temperature.

For the most part, the bacterial kill rate of copper alloys increased with increasing copper content of the alloy. This is further evidence of copper's intrinsic antibacterial properties.

Efficacy on brass, bronze, copper-nickel alloys

Brasses, which were frequently used for doorknobs and push plates in decades past, also demonstrate bactericidal efficacies, but within a somewhat longer time frame than pure copper. All nine brasses tested were almost completely bactericidal (over 99.9% kill rate) at 20 °C within 60–270 minutes. Many brasses were almost completely bactericidal at 4 °C within 180–360 minutes.

The rate of total microbial death on four bronzes varied from within 50–270 minutes at 20 °C, and from 180 to 270 minutes at 4 °C. 

The kill rate of E. coli O157 on copper-nickel alloys increased with increasing copper content. Zero bacterial counts at room temperature were achieved after 105–360 minutes for five of the six alloys. Despite not achieving a complete kill, alloy C71500 achieved a 4-log drop within the six-hour test, representing a 99.99% reduction in the number of live organisms.

Efficacy on stainless steel

Unlike copper alloys, stainless steel (S30400) does not exhibit any degree of bactericidal properties against E. coli O157:H7. This material, which is one of the most common touch surface materials in the healthcare industry, allows toxic E. coli O157:H7 to remain viable for weeks. Near-zero bacterial counts are not observed even after 28 days of investigation. Epifluorescence photographs have demonstrated that E. coli O157:H7 is almost completely killed on copper alloy C10200 after just 90 minutes at 20 °C; whereas a substantial number of pathogens remain on stainless steel S30400.

MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is a dangerous bacteria strain because it is resistant to beta-lactam antibiotics. Recent strains of the bacteria, EMRSA-15 and EMRSA-16, are highly transmissible and durable. This is of extreme importance to those concerned with reducing the incidence of hospital-acquired MRSA infections.

In 2008, after evaluating a wide body of research mandated specifically by the United States Environmental Protection Agency (EPA), registration approvals were granted by EPA in 2008 granting that copper alloys kill more than 99.9% of MRSA within two hours.

Subsequent research conducted at the University of Southampton (UK) compared the antimicrobial efficacies of copper and several non-copper proprietary coating products to kill MRSA. At 20 °C, the drop-off in MRSA organisms on copper alloy C11000 is dramatic and almost complete (over 99.9% kill rate) within 75 minutes. However, neither a triclosan-based product nor two silver-based antimicrobial treatments (Ag-A and Ag-B) exhibited any meaningful efficacy against MRSA. Stainless steel S30400 did not exhibit any antimicrobial efficacy.

In 2004, the University of Southampton research team was the first to clearly demonstrate that copper inhibits MRSA. On copper alloys — C19700 (99% copper), C24000 (80% copper), and C77000 (55% copper) — significant reductions in viability were achieved at room temperatures after 1.5 hours, 3.0 hours and 4.5 hours, respectively. Faster antimicrobial efficacies were associated with higher copper alloy content. Stainless steel did not exhibit any bactericidal benefits.

Leyland Nigel S., Podporska-Carroll Joanna, Browne John, Hinder Steven J., Quilty Brid, Pillai Suresh C. (2016). "Highly Efficient F, Cu doped TiO2 anti-bacterial visible light active photocatalytic coatings to combat hospital-acquired infections". Scientific Reports. 6. doi:10.1038/srep24770.

Clostridium difficile

Clostridium difficile, an anaerobic bacterium, is a major cause of potentially life-threatening disease, including nosocomial diarrheal infections, especially in developed countries. C. difficile endospores can survive for up to five months on surfaces. The pathogen is frequently transmitted by the hands of healthcare workers in hospital environments. C. difficile is currently a leading hospital-acquired infection in the UK, and rivals MRSA as the most common organism to cause hospital acquired infections in the US. It is responsible for a series of intestinal health complications, often referred to collectively as Clostridium difficile Associated Disease (CDAD).

The antimicrobial efficacy of various copper alloys against Clostridium difficile was recently evaluated. The viability of C. difficile spores and vegetative cells were studied on copper alloys C11000 (99.9% copper), C51000 (95% copper), C70600 (90% copper), C26000 (70% copper), and C75200 (65% copper). Stainless steel (S30400) was used as the experimental control. The copper alloys significantly reduced the viability of both C. difficile spores and vegetative cells. On C75200, near total kill was observed after one hour (however, at 6 hours total C. difficile increased, and decreased slower afterwards). On C11000 and C51000, near total kill was observed after 3 hours, then total kill in 24 hours on C11000 and 48 hours on C51000. On C70600, near total kill was observed after 5 hours. On C26000, near total kill was achieved after 48 hours. On stainless steel, no reductions in viable organisms were observed after 72 hours (3 days) of exposure and no significant reduction was observed within 168 hours (1 week).

Influenza A

Influenza, commonly known as flu, is an infectious disease from a viral pathogen different from the one that produces the common cold. Symptoms of influenza, which are much more severe than the common cold, include fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort. Influenza can cause pneumonia, which can be fatal, particularly in young children and the elderly.

After incubation for one hour on copper, active influenza A virus particles were reduced by 75%. After six hours, the particles were reduced on copper by 99.999%. Influenza A virus was found to survive in large numbers on stainless steel.

Once surfaces are contaminated with virus particles, fingers can transfer particles to up to seven other clean surfaces. Because of copper's ability to destroy influenza A virus particles, copper can help to prevent cross-contamination of this viral pathogen.

Adenovirus

Adenovirus is a group of viruses that infect the tissue lining membranes of the respiratory and urinary tracts, eyes, and intestines. Adenoviruses account for about 10% of acute respiratory infections in children. These viruses are a frequent cause of diarrhea.

In a recent study, 75% of adenovirus particles were inactivated on copper (C11000) within one hour. Within six hours, 99.999% of the adenovirus particles were inactivated. Within six hours, 50% of the infectious adenovirus particles survived on stainless steel.

Fungi

The antifungal efficacy of copper was compared to aluminium on the following organisms that can cause human infections: Aspergillus spp., Fusarium spp., Penicillium chrysogenum, Aspergillus niger and Candida albicans. An increased die-off of fungal spores was found on copper surfaces compared with aluminium. Aspergillus niger growth occurred on the aluminium coupons growth was inhibited on and around copper coupons.

Antimicrobial copper-alloy touch surfaces

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Antimicrobial_copper-alloy_touch_surfaces 

 

Antimicrobial copper-alloy touch surfaces can prevent frequently touched surfaces from serving as reservoirs for the spread of pathogenic microbes. This is especially true in healthcare facilities, where harmful viruses, bacteria, and fungi colonize and persist on doorknobs, push plates, railings, tray tables, tap (faucet) handles, IV poles, HVAC systems, and other equipment. These microbes can sometimes survive on surfaces for more than 30 days.

The surfaces of copper and its alloys, such as brass and bronze, are antimicrobial. They have an inherent ability to kill a wide range of harmful microbes relatively rapidly – often within two hours or less – and with a high degree of efficiency. These antimicrobial properties have been demonstrated by an extensive body of research. The research also suggests that if touch surfaces are made with copper alloys, the reduced transmission of disease-causing organisms can reduce patient infections in hospital intensive care units (ICU) by as much as 58%.

Evidence

As of 2019 a number of studies have found that copper surfaces may help prevent infection in the healthcare environment.

Microorganisms are known to survive on inanimate surfaces for extended periods of time. Hand and surface disinfection practices are a primary measure against the spread of infection. Since approximately 80% of infectious diseases are known to be transmitted by touch, and pathogens found in healthcare facilities can survive on inanimate surfaces for days or months, the microbial burden of frequently touched surfaces is believed to play a significant role in infection causality.

EPA registrations

On February 29, 2008, the United States Environmental Protection Agency (EPA) approved the registrations of five different groups of copper alloys as "antimicrobial materials" with public health benefits. The EPA registrations now cover 479 different compositions of copper alloys within six groups (an up-to-date list of all approved alloys is available). All of the alloys have minimum nominal copper concentrations of 60%. The results of the EPA-supervised antimicrobial studies demonstrating copper's strong antimicrobial efficacies across a wide range of alloys have been published.

Microbes tested and killed in EPA laboratory tests

The bacteria destroyed by copper alloys in the EPA-supervised antimicrobial efficiency tests include:

• Escherichia coli O157:H7, a foodborne pathogen associated with large-scale food recalls.
• Methicillin-resistant Staphylococcus aureus (MRSA), one of the most virulent strains of antibiotic-resistant bacteria and a common culprit of hospital- and community-acquired infections.
• Staphylococcus aureus, the most common of all bacterial staphylococcus (i.e., Staph) infections that cause life-threatening disease, including pneumonia and meningitis.
• Enterobacter aerogenes, a pathogenic bacterium commonly found in hospitals that causes opportunistic skin infections and impacts other body tissues.
• Pseudomonas aeruginosa, a bacterium in immunocompromised individuals that infects the pulmonary and urinary tracts, blood and skin.
• Vancomycin-resistant Enterococcus (VRE), a pathogenic bacterium that is the second leading cause of hospital-acquired infections.

EPA test protocols for copper alloy surfaces

The registrations are based on studies supervised by EPA which found that copper alloys kill more than 99.9% of disease-causing bacteria within just two hours when cleaned regularly (i.e., the metals are free of dirt or grime that may impede the bacteria's contact with the copper surface). 

To attain the EPA registrations, the copper alloy groups had to demonstrate strong antimicrobial efficacies according to all of the following rigorous tests:

• efficiency as a sanitizer: This test protocol measures surviving bacteria on alloy surfaces after two hours.
• Residual self-sanitizing activity: This test protocol measures surviving bacteria on alloy surfaces before and after six wet and dry wear cycles over 24 hours in a standard wear apparatus.
• Continuous reduction of bacterial contamination: This test protocol measures the number of bacteria that survive on a surface after it has been re-inoculated eight times over a 24-hour period without intermediate cleaning or wiping.

EPA registered antimicrobial copper alloys

The alloy groups tested and approved were C11000, C51000, C70600, C26000, C75200, and C28000. 

The EPA registration numbers for the six groups of alloys are as follows:

Group	Copper %	EPA registration number
I	95.2 to 99.99	82012-1
II	87.3 to 95.0	82012-2
III	78.1 to 87.09	82012-3
IV	68.2 to 77.5	82012-4
V	65.0 to 67.8	82012-5
VI	60.0 to 64.5	82012-6

Claims granted by EPA in antimicrobial copper alloy registrations

The following claims are now legally permitted when marketing EPA-registered antimicrobial copper alloys in the U.S.:

Laboratory testing has shown that when cleaned regularly:

• Antimicrobial Copper Alloys continuously reduce bacterial contamination, achieving 99.9% reduction within two hours of exposure.
• Antimicrobial Copper Alloy surfaces kill greater than 99.9% of Gram-negative and Gram-positive bacteria within two hours of exposure.
• Antimicrobial Copper Alloy surfaces deliver continuous and ongoing antibacterial action, remaining effective in killing greater than 99% of bacteria within two hours.
• Antimicrobial Copper Alloys surfaces kill greater than 99.9% of bacteria within two hours, and continue to kill 99% of bacteria even after repeated contamination.
• Antimicrobial Copper Alloys surfaces help inhibit the buildup and growth of bacteria within two hours of exposure between routine cleaning and sanitizing steps.
• Testing demonstrates effective antibacterial activity against Staphylococcus aureus, Enterobacter aerogenes, Methicillin-resistant Staphylococcus aureus (MRSA), Escherichia coli O157:H7, and Pseudomonas aeruginosa

The registrations state that “antimicrobial copper alloys may be used in hospitals, other healthcare facilities, and various public, commercial and residential buildings.”

Product stewardship requirements of EPA

As a condition of registration established by EPA, the Copper Development Association (CDA) in the U.S. is responsible for the product stewardship of antimicrobial copper alloy products. CDA must ensure that manufacturers promote these products in an appropriate manner. Manufacturers must only promote the proper use and care of these products and must specifically emphasize that the use of these products is a supplement and not a substitute to routine hygienic practices.

EPA mandated that all advertising and marketing materials for antimicrobial copper products contain the following statement:

The use of a Copper Alloy surface is a supplement to and not a substitute for standard infection control practices; users must continue to follow all current infection control practices, including those practices related to cleaning and disinfection of environmental surfaces. The Copper Alloy surface material has been shown to reduce microbial contamination, but it does not necessarily prevent cross-contamination.

Antimicrobial copper alloys are intended to provide supplemental antimicrobial action in between routine cleaning of environmental or touch surfaces in healthcare settings, as well as in public buildings and the home. Users must also understand that in order for antimicrobial copper alloys to remain effective, they cannot be coated in any way. 

CDA is currently implementing an outreach program through written communications, a product stewardship website, and through a Working Group which meets periodically to expand educational efforts.

More than 100 different potential product applications were cited in the registrations for their potential public health benefits.

EPA warranty statement

The EPA warranty statement is worded as follows:

If used as intended, ANTIMICROBIAL COPPER ALLOYS are wear-resistant and the durable antibacterial properties will remain effective for as long as the product remains in place and is used as directed.

Note: With the exception of the product name and the percentage of active ingredient, the EPA-approved Master Labels for the six groups of registered alloys are identical.

Antimicrobial copper products

Many antimicrobial copper alloy products have been approved for registration in healthcare facilities, public and commercial buildings, residences, mass transit facilities, laboratories, and play area equipment in the US. A complete list of registered products is available from EPA.