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Colloidal gold

December 06, 2023

Colloidal gold is a sol or colloidal suspension of nanoparticles of gold in a fluid, usually water.[1] The colloid is coloured usually either wine red (for spherical particles less than 100 nm) or blue-purple (for larger spherical particles or nanorods).[2] Due to their optical,[3] 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,[4] nanotechnology, materials science,[5] and biomedicine.[6][7][8][9]

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

The properties of colloidal gold nanoparticles, and thus their potential applications, depend strongly upon their size and shape.[10] For example, rodlike particles have both a transverse and longitudinal absorption peak, and anisotropy of the shape affects their self-assembly.[11]

History edit

 
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.[12][13]

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[14] (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 a book in 1656, Treatise of Aurum Potabile,[15] 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,[16] 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.[17][18] 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.[19] 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.[20][21] 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.[18]

In 1898, Richard Adolf Zsigmondy prepared the first colloidal gold in diluted solution.[22] 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.[11][23]

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 electronics.

Physical properties edit

Optical edit

 
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[24] resulting in colours ranging from vibrant reds (smaller particles) to blues to black and finally to clear and colorless (larger particles), 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, shape, composition and environment edit

As a general rule, the wavelength of light absorbed increases as a function of increasing nanoparticle size.[25] Both the surface plasmon resonance frequency and scattering intensity depend on the size, shape composition and environment of the nanoparticles. This phenomenon may be quantified by use of the Mie scattering theory for spherical nanoparticles. Nanoparticles with diameters of 30–100 nm may be detected easily by a microscope, and particles with a size of 40 nm may even be detected by the naked eye when the concentration of the particles is 10−4 M or greater. The scattering from a 60 nm nanoparticle is about 105 times stronger than the emission from a fluorescein molecule.[26]

Effect of local refractive index edit

Changes in the apparent color of a gold nanoparticle solution can also be caused by the environment in which the colloidal gold is suspended.[27][28] The optical properties of gold nanoparticles depend on the refractive index near the nanoparticle surface, so the molecules directly attached to the nanoparticle surface (i.e. nanoparticle ligands) and the nanoparticle solvent may both influence the observed optical features.[27] As the refractive index near the gold surface increases, the LSPR shifts to longer wavelengths.[28] In addition to solvent environment, the extinction peak can be tuned by coating the nanoparticles with non-conducting shells such as silica, biomolecules, or aluminium oxide.[29]

Effect of aggregation edit

When gold nanoparticles aggregate, the optical properties of the particle change, because the effective particle size, shape, and dielectric environment all change.[30]

Medical research edit

Electron microscopy edit

Main article: Immunogold labelling

Colloidal gold and various derivatives have long been among the most widely used labels for antigens in biological electron microscopy.[31][32][33][34][35] Colloidal gold particles can be attached to many traditional biological probes such as antibodies, lectins, superantigens, glycans, nucleic acids,[36] and receptors. Particles of different sizes are easily distinguishable in electron micrographs, allowing simultaneous multiple-labelling experiments.[37]

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).[38]

Drug delivery system edit

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.[39][40] 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).[41]

Gold nanoparticles are being investigated as carriers for drugs such as Paclitaxel.[42] 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 edit

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.[43]

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 administered spherical gold nanoparticles broadened the temporal profile of reflected optical signals and enhanced the contrast between surrounding normal tissue and tumors.[44]

 
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 edit

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 (single and double stranded DNA) by providing protection against intracellular nucleases and ease of functionalization for selective targeting.[45]

Photothermal agents edit

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.[46] 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.[47]

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.[48][49] 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.[50]

Radiotherapy dose enhancer edit

Considerable interest has been shown in the use of gold and other heavy-atom-containing nanoparticles to enhance the dose delivered to tumors.[51] 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.[52] This mechanism is the same as occurs in heavy ion therapy.

Detection of toxic gas edit

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.[53]

Gold nanoparticle based biosensor edit

Gold nanoparticles are incorporated into biosensors to enhance its stability, sensitivity, and selectivity.[54] 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.[55][56] Main types of gold nanoparticle based biosensors are optical and electrochemical biosensor.

Optical biosensor edit

 
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 sensors in response to the change in the local refractive index. The angle of the incidence light for surface plasmon resonance, an interaction between light waves and conducting electrons in metal, changes when other substances are bounded to the metal surface.[57][58] Because gold is very sensitive to its surroundings' dielectric constant,[59][60] binding of an analyte significantly shifts the gold nanoparticle's SPR and therefore allows for more sensitive detection. Gold nanoparticle could also amplify the SPR signal.[61] When the plasmon wave pass through the gold nanoparticle, the charge density in the wave and the electron I the gold interact and result in a higher energy response, referred to as electron coupling.[54] When the analyte and bio-receptor both bind to the gold, the apparent mass of the analyte increases and therefore amplifies the signal.[54] These properties had been used to build a DNA sensor with 1000-fold greater sensitivity than without the Au NP.[62] Humidity sensors have also been 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.[63]

Electrochemical biosensor edit

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".[54] It transfers electron between the electrode and the active site of the enzyme.[64] 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.[65] 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.[54] 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.[66][67] Moreover, Au NP also catalyzes biological reactions.[68][69] Gold nanoparticle under 2 nm has shown catalytic activity to the oxidation of styrene.[70]

Immunological biosensor edit

Gold nanoparticles have been coated with peptides and glycans for use in immunological detection methods.[71] 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.[72]

Thin films edit

Gold nanoparticles capped with organic ligands, such as alkanethiol molecules, can self-assemble into large monolayers (>cm2). 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.[73] The mechanics of the membranes are guided by strong interactions between ligand shells on adjacent particles.[74] Upon fracture, the films crack perpendicular to the direction of strain at a fracture stress of 11   2.6 MPa, comparable to that of cross-linked polymer films.[75] Free-standing nanoparticle membranes exhibit bending rigidity on the order of 10  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.[76] 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.[77]

Surface chemistry edit

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.[78] 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 edit

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.[79] 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.[80] Ligand exchange can also be used to promote phase transfer of the colloidal particles.[78] 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.[81][82] 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.[83]

Ligand removal edit

In many cases, as in various high-temperature catalytic applications of Au, the removal of the capping ligands produces more desirable physicochemical properties.[84] 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.[85]

Surface structure and chemical environment edit

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.[78] 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 form “staple” motifs that have significant Thiyl-Au(0) character.[86][87] 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.[88]

Health and safety edit

See also: Health and safety hazards of nanomaterials and Nanotoxicology

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,[89] 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.[90] 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.[90] In vitro experiments are more popular than in vivo experiments because in vitro experiments are more simplistic to perform than in vivo experiments.[90]

Toxicity and hazards in synthesis edit

While AuNPs themselves appear to have low or negligible toxicity,[citation needed] 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.[91] The gold ions usually come from chloroauric acid, a potent acid.[92] 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 edit

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 .[92][93] 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.[92] 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.[94] Large AuNPs conjugated with biotin, cysteine, citrate, and glucose were not toxic in human leukemia cells (K562) for concentrations up to 0.25 M.[95] 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.[96] 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.[97] 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.[98] However, AuNPs conjugated with PEG copolymers showed negligible toxicity towards human colon cells (Caco-2).[99] 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.[94] 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.[100] 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.[101]

Toxicity due to size of nanoparticles edit

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.[94] 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.[102] Different sized AuNPs were found to buildup in the blood,[103][104] brain,[103] stomach,[103] pancreas,[103] kidneys,[103] liver,[103][104] and spleen.[103][104]

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.[105][106]

Synthesis edit

 
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[AuCl
4]). 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.[17]

Turkevich method edit

This simple method was pioneered by J. Turkevich et al. in 1951[107][108] and refined by G. Frens in the 1970s.[109][110] 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.[111]

Capping agents edit

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.[112] Citrate ions or tannic acid function both as a reducing agent and a capping agent.[113][114] Less sodium citrate results in larger particles.

Brust-Schiffrin method edit

This method was discovered by Brust and Schiffrin in the early 1990s,[115] 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.[116] 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.[117][118] 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.[119] Tetra-dodecanthiol is a commonly used strong binding agent to synthesize smaller particles.[120] 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 edit

This approach, discovered by Perrault and Chan in 2009,[121] 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,[109][110] 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 edit

This simple method, discovered by Martin and Eah in 2010,[122] 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 edit

Bacillus licheniformis can be used in synthesis of gold nanocubes with sizes between 10 and 100 nanometres.[123] 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 edit

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 as the reducing agent and sodium citrate as the stabilizer.[124]

Sonolysis edit

Another method for the experimental generation of gold particles is by sonolysis. The first method of this type was invented by Baigent and Müller.[125] 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,[126] 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 edit

An economical, environmentally benign and fast synthesis methodology for gold nanoparticles using block copolymer has been developed by Sakai et al.[127] 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.[128] improved this synthesis method by enhancing the nanoparticle yield by manyfold at ambient temperature.

Applications edit

Antibiotic conjugated nanoparticle synthesis edit

Antibiotic functionalized metal nanoparticles have been widely studied as a mode to treat multi-drug resistant bacterial strains. For example, kanamycin capped gold-nanoparticles (Kan-AuPs) showed broad spectrum dose dependent antibacterial activity against both gram positive and gram negative bacterial strains in comparison to kanamycin alone.[129]

See also edit

References edit

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Further reading edit

  • Boisselier E, Astruc D (June 2009). "Gold nanoparticles in nanomedicine: preparations, imaging, diagnostics, therapies and toxicity". Chemical Society Reviews. 38 (6): 1759–82. doi:10.1039/b806051g. PMID 19587967.

External links edit

 
Wikimedia Commons has media related to Colloidal gold.
  • Moriarty, Philip. "Au – Gold Nanoparticle". Sixty Symbols. Brady Haran for the University of Nottingham.
  • Point-by-point methods for citrate synthesis and hydroquinone synthesis of gold nanoparticles are available .

December 06, 2023
colloidal, gold, this, scientific, article, needs, additional, citations, secondary, tertiary, sourcessuch, review, articles, monographs, textbooks, please, also, establish, relevance, primary, research, articles, cited, unsourced, poorly, sourced, material, c. This scientific article needs additional citations to secondary or tertiary sourcessuch as review articles monographs or textbooks Please also establish the relevance for any primary research articles cited Unsourced or poorly sourced material may be challenged and removed August 2017 Learn how and when to remove this template message Colloidal gold is a sol or colloidal suspension of nanoparticles of gold in a fluid usually water 1 The colloid is coloured usually either wine red for spherical particles less than 100 nm or blue purple for larger spherical particles or nanorods 2 Due to their optical 3 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 4 nanotechnology materials science 5 and biomedicine 6 7 8 9 Gold Colloid of varying sizeSuspensions of gold nanoparticles of various sizes The size difference causes the difference in colors The properties of colloidal gold nanoparticles and thus their potential applications depend strongly upon their size and shape 10 For example rodlike particles have both a transverse and longitudinal absorption peak and anisotropy of the shape affects their self assembly 11 Contents 1 History 2 Physical properties 2 1 Optical 2 1 1 Effect of size shape composition and environment 2 1 2 Effect of local refractive index 2 1 3 Effect of aggregation 3 Medical research 3 1 Electron microscopy 3 2 Drug delivery system 3 3 Tumor detection 3 4 Gene therapy 3 5 Photothermal agents 3 6 Radiotherapy dose enhancer 3 7 Detection of toxic gas 3 8 Gold nanoparticle based biosensor 3 8 1 Optical biosensor 3 8 2 Electrochemical biosensor 3 8 3 Immunological biosensor 3 9 Thin films 4 Surface chemistry 4 1 Ligand exchange functionalization 4 2 Ligand removal 4 3 Surface structure and chemical environment 5 Health and safety 5 1 Toxicity and hazards in synthesis 5 2 Toxicity due to capping ligands 5 3 Toxicity due to size of nanoparticles 6 Synthesis 6 1 Turkevich method 6 2 Capping agents 6 3 Brust Schiffrin method 6 4 Perrault method 6 5 Martin method 6 6 Nanotech studies 6 7 Navarro et al method 6 8 Sonolysis 6 9 Block copolymer mediated method 7 Applications 7 1 Antibiotic conjugated nanoparticle synthesis 8 See also 9 References 10 Further reading 11 External linksHistory edit nbsp 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 12 13 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 14 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 a book in 1656 Treatise of Aurum Potabile 15 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 16 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 xrῡsos 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 17 18 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 19 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 20 21 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 18 In 1898 Richard Adolf Zsigmondy prepared the first colloidal gold in diluted solution 22 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 11 23 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 electronics Physical properties editOptical edit nbsp The variation of scattering cross section of 100 nm radius gold nanoparticle vs the wavelengthColloidal gold has been used by artists for centuries because of the nanoparticle s interactions with visible light Gold nanoparticles absorb and scatter light 24 resulting in colours ranging from vibrant reds smaller particles to blues to black and finally to clear and colorless larger particles 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 shape composition and environment edit As a general rule the wavelength of light absorbed increases as a function of increasing nanoparticle size 25 Both the surface plasmon resonance frequency and scattering intensity depend on the size shape composition and environment of the nanoparticles This phenomenon may be quantified by use of the Mie scattering theory for spherical nanoparticles Nanoparticles with diameters of 30 100 nm may be detected easily by a microscope and particles with a size of 40 nm may even be detected by the naked eye when the concentration of the particles is 10 4 M or greater The scattering from a 60 nm nanoparticle is about 105 times stronger than the emission from a fluorescein molecule 26 Effect of local refractive index edit Changes in the apparent color of a gold nanoparticle solution can also be caused by the environment in which the colloidal gold is suspended 27 28 The optical properties of gold nanoparticles depend on the refractive index near the nanoparticle surface so the molecules directly attached to the nanoparticle surface i e nanoparticle ligands and the nanoparticle solvent may both influence the observed optical features 27 As the refractive index near the gold surface increases the LSPR shifts to longer wavelengths 28 In addition to solvent environment the extinction peak can be tuned by coating the nanoparticles with non conducting shells such as silica biomolecules or aluminium oxide 29 Effect of aggregation edit When gold nanoparticles aggregate the optical properties of the particle change because the effective particle size shape and dielectric environment all change 30 Medical research editThis section needs more reliable medical references for verification or relies too heavily on primary sources Please review the contents of the section and add the appropriate references if you can Unsourced or poorly sourced material may be challenged and removed Find sources Colloidal gold news newspapers books scholar JSTOR August 2017 nbsp Electron microscopy edit Main article Immunogold labelling Colloidal gold and various derivatives have long been among the most widely used labels for antigens in biological electron microscopy 31 32 33 34 35 Colloidal gold particles can be attached to many traditional biological probes such as antibodies lectins superantigens glycans nucleic acids 36 and receptors Particles of different sizes are easily distinguishable in electron micrographs allowing simultaneous multiple labelling experiments 37 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 38 Drug delivery system edit 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 39 40 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 41 Gold nanoparticles are being investigated as carriers for drugs such as Paclitaxel 42 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 edit 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 43 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 administered spherical gold nanoparticles broadened the temporal profile of reflected optical signals and enhanced the contrast between surrounding normal tissue and tumors 44 nbsp 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 edit Gold nanoparticles have shown potential as intracellular delivery vehicles for siRNA oligonucleotides with maximal therapeutic impact nbsp 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 single and double stranded DNA by providing protection against intracellular nucleases and ease of functionalization for selective targeting 45 Photothermal agents edit 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 gt 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 46 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 47 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 48 49 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 50 Radiotherapy dose enhancer edit Considerable interest has been shown in the use of gold and other heavy atom containing nanoparticles to enhance the dose delivered to tumors 51 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 52 This mechanism is the same as occurs in heavy ion therapy Detection of toxic gas edit Researchers have developed simple inexpensive methods for on site detection of hydrogen sulfide H2 S present in air based on the antiaggregation of gold nanoparticles AuNPs Dissolving H2 S 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 H2 S 53 Gold nanoparticle based biosensor edit Gold nanoparticles are incorporated into biosensors to enhance its stability sensitivity and selectivity 54 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 55 56 Main types of gold nanoparticle based biosensors are optical and electrochemical biosensor Optical biosensor edit nbsp 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 sensors in response to the change in the local refractive index The angle of the incidence light for surface plasmon resonance an interaction between light waves and conducting electrons in metal changes when other substances are bounded to the metal surface 57 58 Because gold is very sensitive to its surroundings dielectric constant 59 60 binding of an analyte significantly shifts the gold nanoparticle s SPR and therefore allows for more sensitive detection Gold nanoparticle could also amplify the SPR signal 61 When the plasmon wave pass through the gold nanoparticle the charge density in the wave and the electron I the gold interact and result in a higher energy response referred to as electron coupling 54 When the analyte and bio receptor both bind to the gold the apparent mass of the analyte increases and therefore amplifies the signal 54 These properties had been used to build a DNA sensor with 1000 fold greater sensitivity than without the Au NP 62 Humidity sensors have also been 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 63 Electrochemical biosensor edit 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 54 It transfers electron between the electrode and the active site of the enzyme 64 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 65 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 54 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 66 67 Moreover Au NP also catalyzes biological reactions 68 69 Gold nanoparticle under 2 nm has shown catalytic activity to the oxidation of styrene 70 Immunological biosensor edit Gold nanoparticles have been coated with peptides and glycans for use in immunological detection methods 71 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 72 Thin films edit Gold nanoparticles capped with organic ligands such as alkanethiol molecules can self assemble into large monolayers gt cm2 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 73 The mechanics of the membranes are guided by strong interactions between ligand shells on adjacent particles 74 Upon fracture the films crack perpendicular to the direction of strain at a fracture stress of 11 displaystyle pm nbsp 2 6 MPa comparable to that of cross linked polymer films 75 Free standing nanoparticle membranes exhibit bending rigidity on the order of 105 displaystyle 5 nbsp 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 76 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 77 Surface chemistry editIn 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 78 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 edit 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 79 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 80 Ligand exchange can also be used to promote phase transfer of the colloidal particles 78 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 81 82 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 83 Ligand removal edit In many cases as in various high temperature catalytic applications of Au the removal of the capping ligands produces more desirable physicochemical properties 84 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 85 Surface structure and chemical environment edit 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 78 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 form staple motifs that have significant Thiyl Au 0 character 86 87 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 88 Health and safety editSee also Health and safety hazards of nanomaterials and Nanotoxicology 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 89 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 90 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 90 In vitro experiments are more popular than in vivo experiments because in vitro experiments are more simplistic to perform than in vivo experiments 90 Toxicity and hazards in synthesis edit While AuNPs themselves appear to have low or negligible toxicity citation needed 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 91 The gold ions usually come from chloroauric acid a potent acid 92 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 edit 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 92 93 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 92 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 94 Large AuNPs conjugated with biotin cysteine citrate and glucose were not toxic in human leukemia cells K562 for concentrations up to 0 25 M 95 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 96 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 97 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 98 However AuNPs conjugated with PEG copolymers showed negligible toxicity towards human colon cells Caco 2 99 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 94 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 100 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 101 Toxicity due to size of nanoparticles edit 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 94 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 102 Different sized AuNPs were found to buildup in the blood 103 104 brain 103 stomach 103 pancreas 103 kidneys 103 liver 103 104 and spleen 103 104 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 105 106 Synthesis edit nbsp 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 17 Turkevich method edit This simple method was pioneered by J Turkevich et al in 1951 107 108 and refined by G Frens in the 1970s 109 110 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 111 Capping agents edit 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 112 Citrate ions or tannic acid function both as a reducing agent and a capping agent 113 114 Less sodium citrate results in larger particles Brust Schiffrin method edit This method was discovered by Brust and Schiffrin in the early 1990s 115 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 116 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 117 118 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 119 Tetra dodecanthiol is a commonly used strong binding agent to synthesize smaller particles 120 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 edit This approach discovered by Perrault and Chan in 2009 121 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 109 110 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 edit This simple method discovered by Martin and Eah in 2010 122 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 edit Bacillus licheniformis can be used in synthesis of gold nanocubes with sizes between 10 and 100 nanometres 123 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 edit 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 as the reducing agent and sodium citrate as the stabilizer 124 Sonolysis edit Another method for the experimental generation of gold particles is by sonolysis The first method of this type was invented by Baigent and Muller 125 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 126 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 edit An economical environmentally benign and fast synthesis methodology for gold nanoparticles using block copolymer has been developed by Sakai et al 127 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 128 improved this synthesis method by enhancing the nanoparticle yield by manyfold at ambient temperature 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Nanocarriers with ultrahigh chromophore loading for fluorescence bio imaging and photodynamic therapy Biomaterials 34 33 8344 51 doi 10 1016 j biomaterials 2013 07 032 PMID 23915950 Baigent CL Muller G 1980 A colloidal gold prepared using ultrasonics Experientia 36 4 472 473 doi 10 1007 BF01975154 S2CID 32998274 Zhang J Du J Han B Liu Z Jiang T Zhang Z February 2006 Sonochemical formation of single crystalline gold nanobelts Angewandte Chemie 45 7 1116 9 Bibcode 2006AngCh 118 1134Z doi 10 1002 ange 200503762 PMID 16389606 Sakai T Alexandridis P April 2005 Mechanism of gold metal ion reduction nanoparticle growth and size control in aqueous amphiphilic block copolymer solutions at ambient conditions The Journal of Physical Chemistry B 109 16 7766 77 doi 10 1021 jp046221z PMID 16851902 Ray D Aswal VK Kohlbrecher J March 2011 Synthesis and Characterization of High Concentration Block Copolymer Mediated Gold Nanoparticles Langmuir 27 7 4048 56 doi 10 1021 la2001706 PMID 21366279 Payne NJ Waghwani HK Connor MG Hamilton W Tockstein S Moolani H Chavda F Badwaik VD Lawrenz MB Dakshinamurthy R May 2016 Novel Synthesis of Kanamycin Conjugated Gold Nanoparticles with Potent Antibacterial Activity Frontiers in Microbiology 7 607 doi 10 3389 fmicb 2016 00607 PMC 4908860 PMID 27330535 Further reading editBoisselier E Astruc D June 2009 Gold nanoparticles in nanomedicine preparations imaging diagnostics therapies and toxicity Chemical Society Reviews 38 6 1759 82 doi 10 1039 b806051g PMID 19587967 External links edit nbsp Wikimedia Commons has media related to Colloidal gold Moriarty Philip Au Gold Nanoparticle Sixty Symbols Brady Haran for the University of Nottingham Point by point methods for citrate synthesis and hydroquinone synthesis of gold nanoparticles are available here Retrieved from https en wikipedia org w index php title Colloidal gold amp oldid 1186730293, wikipedia, wiki, book, books, library,

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