Gold Nanotechnology at Mintek - SAIMM

GOLD NANOTECHNOLOGY AT MINTEK 247

Gold nanotech—an introduction and overviewThe future of gold nanoscience and technology lookspromising. The preparation of metal nanoparticles (inparticular those of gold) has become an area of considerableinterest in both applied and fundamental research today,largely due to their fascinating properties and attractivepotential applications in electronics1, optics2–6, and thebiomedical and bioanalytical areas etc.7–10 An overview ofand introduction to gold nanoscience and technology issummarized in Figure 1.

Colloidal gold science perspective

Gold colloids have a long history11 and are particularly wellknown for their aesthetic appeal and their therapeuticproperties.12 Around the 4th century, gold colloids werefamously used for decorative purposes. An elegant exampleis the Lycurgus cup shown in Figure 1, exhibited in theBritish museum.13 The glass appears green in reflected lightand red in transmitted light. Chemical analysis revealed thatthe glass contains gold and silver nanoparticles ofapproximately 70 nm13. The colour change is due to thesmall fraction of gold nanoparticles contained in it.

Ever since Faraday14 published a report on the synthesisand properties of colloidal gold in 1857, a reasonableinsight into the nature of gold sols (nucleation, growth and

kinetics of coagulation) was developed by Turkevich,which in particular allowed the preparation of gold colloidsin aqueous media15. After Richard Feynman’s famouslecture titled ‘There is plenty of room at the bottom’ in1957, considerable attention has been drawn to thedevelopment and optimization of methods for thepreparation of gold colloids16. Well-documented use ofgold colloids in biological applications probably dates backto the invention of an immunogold staining proceduredeveloped by Faulk and Taylor in 197117. Since then, goldnanoparticles have been extensively used as labels oftargeting molecules, particularly proteins, in electronmicroscopy18–20 because they are much more electron-densethan organic molecules and hence yield excellent contrast tocells and tissues. Gold labeling technology was furtherimproved by the development of a silver staining procedure(autometallographic procedure) which relies on reducingsilver (I) ions deposited on colloidal gold to metallic silver,allowing the enlargement of stained regions, which can alsobe visualized in light microscopy as black spots.

The exploitation of gold nanoparticles as optical markersin the dark field microscopy of biological samples is a greatrevolution, since this area was previously dominated byfluorescent dyes, which have limitations such asphotobleaching, and often a relatively high detectionthreshold, whereas light scattering by gold nanoparticleseven has the potential for single particle detection8.

TSHIKHUDO, R.T., MDLULI, P.S, OSIBO, N., and VAN DER LINGEN, E. Gold Nanotechnology at Mintek. World Gold Conference 2009, The SouthernAfrican Institute of Mining and Metallurgy, 2009.

Gold Nanotechnology at Mintek

R.T. TSHIKHUDO, P.S. MDLULI, N. SOSIBO, and E. VAN DER LINGENAdvanced Materials Division, Mintek, South Africa

Nanotechnology can provide solutions to many of the world’s socio-economic challenges. Theapplication of nanomaterials for water treatment and analysis, point-of-care diagnostics andtargeted drug delivery systems or devices could improve the quality of life for people indeveloping countries. Gold colloids have a long history (dating back to the 4th century AD) andare particularly well known for their aesthetic appeal and their therapeutic properties. SinceFaraday published a report on the synthesis and properties of colloidal gold in 1857, a reasonableinsight into the nature of gold sols (nucleation, growth and kinetics of coagulation) has beendeveloped. Considerable attention has been drawn during the past few decades to the developmentand optimization of methods for the preparation of gold nanoparticles.

In nanoscience and nanotechnology today, gold nanomaterials appear to be the most widelyused material by both academics and industrialists. This is due to the extraordinary properties ofgold nanoparticles (e.g. optical, catalytic, and magnetic—they are also known to act as strongRaman enhancers and may also amplify fluorescence under certain circumstances). Mintek,through Project AuTEK and the DST/Mintek Nanotechnology Innovation Centre (NIC), has beenat the forefront of the development of new industrial uses for gold. Mintek’s gold nanotechnologyprogramme focuses on the development of gold-based nanostructured materials and theirapplications in health, water and other related areas. While few projects use gold nanoparticles forwater treatment and analysis, Mintek’s gold nanoparticles are widely used in the health sector. Forexample, gold nanoparticles are engineered and produced in large quantities as systems fortargeted drug delivery. They are also used as tools for advanced rapid diagnostic tests. Bothoptical and electrochemical rapid diagnostic methods are developed for the human and animalhealth sectors for point-of-care testing. This paper gives a brief introduction and overview of goldnanotechnology, and desrcibes the development of some of Mintek’s gold nanoparticle targeteddrug delivery systems and lateral flow point of care prototype devices.

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Ligand-stabilized gold nanoparticlesA shift from colloidal science to what is today termed‘chemical nanotechnology’ took place when ligand-stabilized gold nanoclusters were prepared at the Universityof Essen in Germany by Schmid et al. in 1981 for thesynthesis and characterization of Au55(PPh3)12Cl6 goldnanoclusters (Figure 1)21. These findings have led to thedevelopment and exploitation of ligand-stabilized metalnanoclusters in diverse areas including catalysis andelectronics. Ongoing research in this area has primarilybeen focused on developing stable monodisperse particlesof controllable size and shape, as it is generally believedthat current and novel potential applications of nano-structured materials depend on these properties. In fact, thiswork coincided with the invention of the scanningtunnelling microscope (STM) and atomic force microscopy(AFM), which are the major tools used today innanoscience and technology.

In 1994, the first facile synthesis of very stable, isolablegold clusters was developed at Liverpool by Brust et al.22.The evolution of this work has led to the preparation ofmetal nanoparticles of defined size and shape, and has beenexploited in diverse applications. Murray et al.demonstrated the usefulness of this method by carrying outextensive investigations, particularly to show that themethod tolerates various modification to include straight-chain alkanethiols, glutathione, tiopronin, thiolated PEG, p-mercaptophenol, aromatic alkane thiol, phenyl alkane thioland mercaptopropyl trimethoxysilane as stabilizingagents23. Murray’s group also characterized these materialsto gain insight into their physical and chemical properties.Upon understanding the 3D self-assembly process of thesematerials in contrast to the 2D self-assembled monolayers(SAMs) on the surface, Murray coined the name monolayerprotected clusters (MPCs) to distinguish them from 2DSAMs (Figure 1). Many laboratories today employ thisoriginal Brust 2-phase protocol for the size-selectiveformation of MPCs by varying the ratio between Au:HS-R,including the preparation of very small Au MPCs24.

The terms colloids and clusters are often usedinterchangeably in the literature. The criteria used todistinguish clusters (particles described here as ligand-stabilized or monolayer protected clusters or MPCs) fromcolloids (electrostatically-stabilized particles) are based on

the notion that clusters are isolable, reproducible moleculeswith a precise composition and structure, while colloids aremuch less defined and typically have a distribution ofsize25.

Biocompatible gold nanoparticle systemsIn the quest for the development of biocompatible goldnanoparticles, Mirkin from Northwestern Universityintroduced DNA-gold nanotechnology, whereby thiolatedDNA was attached to gold nanoparticles, which led to thedevelopment of gold nanoparticle-bioconjugate basedcolorimetric assay (Figure 1). In this case, when acomplementary strand was introduced to the solution, apolymeric network was formed due to Watson Crick basepairing, leading to reversible nanoparticle aggregation, witha concomitant colour change from red to blue26.

Towards the early 2000s, many researchers becameinterested in developing novel gold nanoparticles, tools orsystems for biological applications. Halas developed metalnanoshells, a type of nanoparticle consisting of a silica-coreand a gold-shell, which acts as a near IR absorbers. Theirplasmon is tunable by controlling the thickness of core-shellmaterials27. Interestingly, nanoshell nanoparticles can beused in many applications by taking advantage of theirplasmonic properties. They have been used as heat deliverysystems28. The successful application of nanoshell NIRphotothermal tumour therapy for the destruction ofcarcinoma cells has been demonstrated in vitro. In addition,nanoshell nanoparticles have been used to trigger drugsystems photothermally when conjugated with specificbiomolecules29. In addition, El-Sayed from Georgia Tech30

showed that gold rods can be used for cancer diagnosis andtherapy, as they also absorb in the NIR region.

Mintek has been actively involved since the early 2000sin the development of gold nanoparticle-based catalysts,and in particular the development of biocompatible goldnanoparticle systems, which are briefly discussed below.Among the properties of gold nanoparticles that make themuseful tools today for various applications, the biologicalapplications mostly rely on the optical properties. Althoughgold is regarded as a biocompatible material, growingconcern about the toxicity of nanoparticles has led manyresearchers to focus on investigating the cytotoxity of goldnanoparticles.

Figure 1. Overview and introduction to gold nanoscience and technology

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Properties of gold nanoparticles

Optical propertiesThe optical properties of metal nanoparticles, in particularthose of gold, are dependent on size, shape, etc. (astheoretically explained by Mie in 1908)31. The d-orbitalelectrons of gold are free to travel through the nanoparticle.The average free path for gold is approximately 50 nm,therefore less scattering is expected for particles smallerthan this size, with all the interactions occurring at thesurface32.

The electrical field of an incoming light wave causespolarization of the free conduction electrons with respect tothe heavier ionic core of a spherical nanoparticle32b

(Figure 2). Due to the movement of electrons under theexternal field, there is a general displacement of negativecharge (electrons) from the positive, resulting in a netcharge difference at the nanoparticle boundary32,33. Adipolar oscillation of all the electrons with the same phaseis created in a process called surface plasmonoscillation33,34. When the incoming electromagnetic fieldbecomes resonant with the coherent electron motion, astrong absorption is observed in the spectrum32. Thefrequency and the surface plasmon absorption have beenfound to depend on the shape, size, and dielectric constantsof the metal nanoparticle and the solvent, and also the stateof aggregation33-36.

For non-spherical particles, such as nanorods andnanowires, the orientation of the electromagnetic fielddetermines the resonant wavelength, resulting in twodistinct oscillations, transverse and longitudinal33. Changesin shape and size cause changes in the oscillation frequencyof electrons, generating different cross-sections forabsorption and scattering properties33–34. The vivid colourof gold colloids of different sizes and morphology emanatesfrom this process. It has been shown that plasmons do existin bulk material and at the surface of larger particles, buttheir effect is reduced by the mismatch between theplasmon dispersion relation and that of a photon, meaningno plasmon excitation can be achieved by ordinary plain-wavelength light34.

In contrast, smaller particles do not require momentumconservation and their plasmons can be excited by ordinarylight. This observation results in the shift of the plasmonresonance towards the visible region of the electromagneticspectrum for the noble metal nanoparticles (gold, copperand silver), resulting in fascinating colloid colours36. Forsmaller clusters, the surface and the bulk plasmon arecoupled, and the charge density varies everywhere in thenanoparticle37. Generally, the majority of the transitionmetals show a broad and poorly resolved absorption band inthe ultraviolet (UV) region with the strong couplingbetween the plasmon transition and the interband excitationresponsible for such a difference37,38. Furthermore, theconduction-band electrons of the noble metals can beapproximated by the Drude free-electron model, whichassumes that the conduction-band electrons can be treatedindependently from the ionic background and can movefreely as some sort of an ‘electron gas’.

Mintek’s targeted drug delivery systems

Generic structureThe gold nanoparticle targeted drug delivery systems atMintek are designed and developed with a wide range of

capabilities, making them versatile systems for diagnosticsand drug delivery. This is shown in Figure 3. Thebiomolecular recognition capability coupled with theirspectroscopic sensitivity and efficient carrier ability hasrendered these systems a route of choice in life scienceapplications. Importantly, the Mintek gold monolayerprotected clusters offer the ability to incorporate specifictargeting ligands on the surface, allowing for multiplexedbiocompatible tools for various applications. The choice oftargeting ligands can be made such that they evadeimmunogenicity, a common response known to impedeother delivery avenues such as viral-based deliverysystems. In addition to addressing key issues such asstability, solubility and biocompatibility of the resultingsystems, which can be accomplished through flexible placeexchange reactions, various functional groups such ascarboxyl, amine, azide, biotin, nitrilotriacetic acid (NTA)are incorporated on the surface of gold nanoparticles toallow advanced bioconjugation strategies. Variousbioconjugation strategies such as carbodiimide, biotin-avidin, click chemistry, etc. are employed to attachbiomolecular functionalities of choice such as peptides,proteins, nucleic acids, aptamers, synthetic drugs and other

Figure 2. Origin of the surface plasmon resonance caused by theinteractions of the nanoparticle electron in the conduction band

with electromagnetic radiation. (A) A dipole is induced,oscillating in phase with the electric of the incoming light.

(B) Transverse and longitudinal oscillation of electrons in a metal nanorod33

Figure 3. Mintek’s versatile gold nanoparticle toolbox fordiagnostics and targeted drug delivery applications



small molecules on nanoparticles. The resulting hybridsystem, as shown in Figure 3, presents a multifunctionaltoolbox needed to address the specific delivery of specifictherapeutic models into biological environments, with thesubsequent processes both spatially and temporallytraceable and controllable. The application of such targeteddrug delivery systems is currently exploited in manybiological areas such as in cancer therapy, obesity research,optical diagnostics (lateral flow) and many other subsets ofthese platforms.

Gold nanoparticle systems: synthesis andcharacterizationA typical example for the synthesis and functionalisation ofgold nanoparticles produced at Mintek is shown in Figure 4. Basically, Mintek produces gold nanoparticles ofvarious sizes, ranging from 2–100 nm. These particles arestabilized by readily functionalized biocompatible ligandsto produce the desired functionalized gold nanoparticlescontaining specific functional groups, which are then usedas products and for further biomolecular functionalization.

Importantly, these nanoparticles are purified by standardchromatographic techniques such as size exclusionchromatography, and characterized by agarose gelelectrophoresis and transmission electron microscopy(TEM). Figure 5a, shows how gold nanoparticles (14 nm),are purified (to remove any contaminants and excessligands) by size exclusion chromatography, which leads tothe production of highly pure gold nanoparticles. In fact,Mintek’s gold nanoparticle products can be dried, storedand redispersed in organic and aqueous media withoutlosing their properties. Figure 5b shows how agarose gelelectrophoresis, an important qualitative tool used tocharacterize gold systems, is used to characterize thesematerials. The gold nanoparticle systems migratedifferently on the gel depending on molecular weight andcharge. The TEM image of 14 nm gold nanoparticlesdepicted in Figure 5c clearly shows that these materials arepure and monodisperse. While it is important andchallenging to scale up nanomaterials, Mintek’s goldsystems are produced in large quantities to meet industryrequirements. Mintek’s facility for large-scale production ofgold nanoparticles is shown in Figure 6.

Cytotoxicity evaluationBefore gold nanoparticles can be used as tools for targeteddrug delivery, it is important to evaluate their cytotoxicity.Generally, the cytotoxicity of the nanomaterials has beenassessed in a three-pronged strategy39:

Composition of the nanoparticlesThe nanomaterials may be composed of toxic materials thatupon corrosion inside an organism can release toxic ionsthat will eventually poison the cell40. Compared to thecorresponding bulk materials, partial decomposition andrelease of ions is highly likely for nanosized materials dueto their large surface-to-volume ratio.

Chemical properties of the nanoparticlesNanoparticles have been shown to adhere to cellmembranes41 and also be ingested by cells42. The breachingof the cell membrane and the intracellular storage may havea negative effect on the cells regardless of the toxicity ofthe particles and their subsequent functionality.

Figure 4. One-step synthesis and functionalization of goldnanoparticles

Figure 5. Purification and characterization of gold nanoparticles or systems, (a) 14 nm gold nanoparticles purified by size exclusionchromatography, (b) migration of 14 nm gold nanoparticles on agarose gel eletrophoresis and (c) TEM image of the 14 nm gold nanoparticles

Figure 6. Large-scale production of Mintek’s gold nanoparticles(20 L)



Nanoparticle morphologyDistinctly shaped materials can rip cells like needles43. Thisin turn suggests that nanomaterials of the same compositionwould have different biologic responses for differentmorphologies.

Mintek’s gold nanoparticle systems have been evaluatedfor their cytotoxicity in two different cell lines, both forapoptosis and necrosis induction. Importantly, it was foundthat these systems do not show any cytotoxicity effectindicating that they are the required tools for biologicalapplications. However, more studies are needed toinvestigate fully the cytotoxicity of these materials.

Mintek’s optical diagnosis (lateral flow)development

The optical diagnosis platform develops simple, accurate,and inexpensive immuno-chromatographic or lateral flowdiagnostic prototypes for both animal and human healthtests, ideally for point-of-care (POC) or home use. POCdiagnostic tests are suitable for resource-limited areas asthey can be conducted without specialized equipment andwith limited experience. The availability of rapid diagnostictests is very important to control diseases at their earlystages, before they develop sufficiently to have fatalconsequences. Through Project AuTEK and the DST/Mintek Nanotechnology Innovation centre (NIC), aplatform for the development of rapid diagnostic deviceshas been established at Mintek. This entails thedevelopment of the immune-based or nucleic acid-basedtests for the detection of various human and animaldiseases. The rapid diagnostic tests for human health testingcovers diseases such as tuberculosis (TB) and malaria,while diseases targeted under animal health testing includefoot and mouth.

In human health, prototype devices have been fabricatedas proof of concept for the development of immuno-chromatographic tests for the detection of diseases such asTB and malaria. Figure 7 shows some of the lateral flowprototype devices produced for human health testing atMintek.

ConclusionsThe future of gold nanotechnology looks promising.Various gold nanoparticles containing specific functionalgroup or biomolecular functionality of choice can today beprepared in large quantities. The optical (light scattering)

properties of gold nanoparticles, which can be detecteddown to single particle level, opens the ability to tracksingle molecules within the cell and hence to monitormolecular binding events in biological systems. Eventhough cytotoxicity evaluation studies indicate that variousgold nanoparticle systems do not show any cytotoxicityeffect, further investigations are required in this regard.However, it appears that the future of cancer diagnosticsand therapy will rely on gold nanotechnology. Thebiolabelling industry in general (which is currentlydominated by the use of fluorescent dyes) will depend ongold nanotechnology, in particular in the whole area ofrapid diagnostics.

AcknowledgementsThe authors acknowledge Mintek, Project AuTEK-GoldFields, DST/Mintek Nanotechnology Innovation Centre(NIC), for permission to publish this paper.

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Figure 7. Typical lateral flow diagnostic prototype devices atMintek



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R.T. TshikhudoAdvanced Materials Division, Mintek, South Africa

Robert is the head of nanoscience and nanotechnology at Mintek (Advanced Materials Division).

He holds a MSc (Chemistry) degree from Rhodes University and a PhD in Chemistry from the

University of Liverpool (UK). His research interests are the preparation of metal nanoparticles and

their applications in health (diagnostics and therapeutics), water (monitoring and remediation) and

other related areas


Gold Nanotechnology at Mintek - SAIMM

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