This paper deals with gold, which is described as a chemical element. Special attention is paid to its physical-chemical properties and, furthermore, where or in what form it can be found in nature. We discuss the role it has played through history and we inform how gold has been developed to the level it has reached today's value. Still more, when gold is broken into nanoparticles, this form could be highly useful for a wide range of processes, including general nanotechnology, electronics manufacturing and the synthesizing of different functional materials. It is important that we know that gold is also used in industry in many engineering applications (contacts in micro-electronics) and medicine (dental alloys, implants)

Anzel, M.

Marković, Evgenija

Rudolf, Rebeka

Stamenković, D.

Čolić, Miodrag

English

School of dental Medicine dIgitaL archivE

261METALURGIJA 51 (2012) 2, 261-264R. RUDOLF, M. ANŽEL, E. MARKOVIĆ, M. ČOLIĆ, D. STAMENKOVIĆGOLD IN THE PAST, TODAY AND FUTURE Received – Prispjelo: 2011-05-27Accepted – Prihvaćeno: 2011-07-10Review paper – Pregledni radISSN 0543-5846METABK 51(2) 261-264 (2012)UDC – UDK 669.14.018.298:669.18=111This paper deals with gold, which is described as a chemical element. Special attention is paid to its physical-chem-ical properties and, furthermore, where or in what form it can be found in nature. We discuss the role it has played through history and we inform how gold has been developed to the level it has reached today’s value. Still more, when gold is broken into nanoparticles, this form could be highly useful for a wide range of processes, including general nanotechnology, electronics manufacturing and the synthesizing of diff erent functional materials. It is im-portant that we know that gold is also used in industry in many engineering applications (contacts in micro-elec-tronics) and medicine (dental alloys, implants). Key words: gold, physical-chemical properties, nano-particles, applicationsZlato u prošlosti, sadašnjosti i budućnosti. Ovaj članak govori o zlatu kao kemijskom elementu. Posebna pažnja posvećena je njegovim fi zikalno-kemijskim svojstvima i gdje i u kojem obliku se može pronaći u prirodi. Razmatra se uloga koju je zlato odigralo u povijesti i donosi se informacije o tome kako je doseglo razinu vrijednosti koju ima danas. Zlato razlomljeno na nanočestice upotrebljivo je u širem spektru procesuiranja, uključujući opću nanoteh-nologiju, elektroničku prizvodnju i spajanju materijala raznih funkcionalnosti. Važno je znati da se zlato koristi i u industriji, mnogim inženjerskim procesima (kontakti u mikro-elektronici) i medicini (dentalne slitine, implatanti).Ključne riječi: zlato, fi zikalno-kemijska svojstva, nanočestice, primjeneINTRODUCTIONGold is a chemical element with the atomic number 79 and the symbol Au, which is derived from the Latin name “aurum” and means the morning dawn [1]. By co-agulation it crystallizes in a centred cubic grid and in the periodic table it is located between the transition metals. The relative atomic mass of gold is 196,97, the density is 19,32 g/cm³, the melting temperature is 1063 °C, the elastic modulus is 77 MPa and the coeffi cient tempera-ture of elongation (WAK) is 14,1 µm/m · K [1]. Gold is a very dispersed and rare element in the Earth’s crust. In the wild it is located in its native form or (less frequently) in the form of telluride, which contains the only Golden minerals (kalaverit, sylvanite, petcit) [2]. Native gold is located primarily in the veins of silica (SiO2), in the ores of other materials, or in deposits and is not clean; it con-tains 1–50 % mercury. The largest deposits of gold are in South Africa, the USA, Canada, Australia, Mexico and Russia. Approximately 20 % of world production is used commercially, while the rest is in gold reserves (the gold basis of paper money). R. Rudolf, M. Anžel, Faculty of Mechanical Engineering, University of Maribor, Maribor, Slovenia., E. Marković, D. Stamenković, High School of Dentistry, University of Belgrade, Belgrade, Serbia, M. Čolić, Univer-sity of Niš, Military Medical Institute Belgrade, SerbiaPHYSICAL-CHEMICAL PROPERTIES Gold has its own unique yellow colour. Its appear-ance is also infl uenced by its brilliance and refl ective ability. Because of these properties gold cannot be con-fused with any other metal. If gold contains an admix-ture of other elements it changes colour. Due to the ad-mixture of silver in gold (up to 30 % by weight) it turns silver-white [3]. If the surface is polished the sharp me-tallic shine is stressed. Gold products are usually made of an alloy in which you can fi nd behind the gold other alloying elements such as silver, copper, platinum, pal-ladium etc. in many cases where products contain cop-per, so-called oxidation zones are visible, which is the reason for the dark, unpolished look of the surface [4]. Gold has almost twice the density of lead and 19,3 times greater than that of water. The admixture of other metals also has an effect on the density. For example, if the mass fraction of silver in gold alloy is between 2 and 20 %, then consequently the alloy density is between 15,5 and 19,3 g/cm³ [1]. Other contaminants which may affect the density are copper, platinum, palladium, rho-dium, iridium and bismuth. In the chemical sense gold is the least active metal, since it burns in air, is not oxidized in water, does not change colour, does not react with strong alkaline solu-tions and all the pure acids (except for selenium acid). Gold dissolved in a solution which is a mixture of chlo-rine and nitric acid (HCl: HNO3 = 3:1); it dissolves in a 262  METALURGIJA 51 (2012) 2, 261-264R. RUDOLF et al.: GOLD IN THE PAST, TODAY AND FUTUREcyanide solution [5]. Gold dissolves in mercury to form amalgam. The gold compounds have one (+1) or three (+3) valence and they are not very persistent. Colloids are formed by adding a solution of formaldehyde or phenylhydrazine to gold. Gold is a soft and very dense metal with a strong yellow colour and has a high degree of cleavage. GOLD IN THE BANKING BUSINESSIn the fourth millennium (before Christ) the Egyp-tian workers found their fi rst silver and gold. This dis-covery formed the natural exchange system of the econ-omy at that time. Minted as indestructible silver and gold coins and precious metals they began to be used as currency. For security reasons, they began to be stored by the Goldsmith, and these in turn issued a Certifi cate of Safekeeping (CS), which was redeemable for silver and gold, and which later became the paper money used instead of silver and gold. On this basis they began sub-sequently to establish a private bank, and soon it was found to be in circulation longer than the CS as it was available in silver and gold. Later other currencies were developed which were also based on silver and gold, which corresponded to the supply of these precious metals as they were stable and not subject to infl ation.During the First World War huge fi nancial resources were required to fi nance military operations. Because the war lasted a full four years, they violated the gold stand-ard and printed more money than they had gold and sil-ver. As a result, the uncovered amount of money has in-creased and infl ation has started to show. In the fi fties of the last century they tried to get back the gold standard, so all currencies were tied to the dollar, and this contains an ounce of gold, which at the time was 35 dollars. To-day, gold stocks are at an end, the money in circulation is growing and is us only on the trust of its users.GOLD IN NANOTECHNOLOGYNanoparticles lie between bulk and atomic dimen-sions and are therefore endowed with special proper-ties, thereby making them starting material for many futuristic applications. The properties of nanoparticles depend on their crystallite sizes [6]. Therefore, control over the particle sizes and particle distributions (PSDs), is highly desirable. Of the different classes of organic and inorganic (metal, dielectric, and semiconductor) na-noparticles, noble metal nanoparticles have fascinated scientists’ from historic times because of their unique size- and shape-dependent optical properties [7]. Today the potential of Au nano- particles is recog-nised to derive from the addressability of their interesting optical properties via spectroscopic and photonic tech-niques [8]. For those spherical nanoparticles much small-er than the wavelength of light (diameter d << λ), an elec-tromagnetic fi eld at a certain frequency (ν) induces a resonant, coherent oscillation of the metal free electrons across the nano-particle (Figure 1). This oscillation is known as the surface Plasmon resonance [7,8]. The resonance lies at visible frequencies for Au metal. The surface Plasmon oscillation of the metal electrons results in a strong enhancement of absorption and scatter-ing of electromagnetic radiation in resonance with the surface Plasmon resonance (SPR) frequency of the Au nanoparticles, giving them intense colours and interest-ing optical properties. The frequency and cross section of SPR absorption and scattering is dependent on the metal composition, nanoparticle size and shape, dielectric prop-erties of surrounding medium/substrate and presence of inter-particle interactions. Au is the plasmonic metal of choice because of its much higher stability as compared to Cu and other metals. In addition, spherical Au colloids can easily be made in a wide range of sizes (4-80 nm) by facile chemistry involving the reductions of Au ions in solution [9]. Other interesting Au nanostructures (Figure 2) with modifi ed optical properties e.g. nanorods, nano-prisms, triangular nanoparticles, nanocubes, and com-posite silica core Au-shell particles, can be fabricated via different techniques, such as: wet synthesis techniques, electrochemistry, photochemical techniques, nano-li-thography, ultrasonic pyrolysis, etc.Due to the phenomenon of SPR, the absorption and scattering cross-section of Au nanoparticles are signifi -Figure 1  Schematic presentation of the interaction of a metal nano-sphere with lightFigure 2  a) Photographs of colloidal dispersions of Au nanoparticles with increasing sizes, b) Au nanospheres, c) Au nanorodsa)b) c)263METALURGIJA 51 (2012) 2, 261-264R. RUDOLF et al.: GOLD IN THE PAST, TODAY AND FUTUREcantly superior to the absorbing and fl uorescing dyes used conventionally in biological and biomedical imag-ing [10]. Researchers demonstrated that the optical cross-sections of the Au-nanospheres are typically four to fi ve orders of magnitude higher than those of conven-tional dyes. Current diagnostics and investigational techniques in molecular biology and biomedicine rely heavily on chemical contrast agents to stain/label spe-cifi c cells and tissues of interest [11] in order to over-come the problem of weak signals of endogenous chro-mospheres and the subtle spectral differences between normal and diseased cells and tissues. Colloidal Au na-noparticles with strong SPR enhanced absorption and scattering are an important addition to the toolbox of imaging labels and contrast agents. Au nanoparticles are not susceptible to photo-bleaching and they appear to be biocompatible and non-cytotoxic, as supported by recent experiments on cells [11]. Another factor motivating the use of Au nanoparti-cles is their facile bio conjugation and bio modifi cation [12]. The surface of Au nanoparticles has a strong bind-ing affi nity towards thiols, disulphides, and amines. In particular, the simple Au-thiol chemistry allows the sur-face conjugation of various peptides, proteins, and DNA. The electrostatic adsorption of biomolecules such as vitamin C and or large protein/enzyme molecules, to the nanoparticles` surface is a simple and commonly used technique for Au nanoparticles capped with citrate or similar carboxylic acid derivate.GOLD IN DENTISTRYGold is the oldest dental restorative material, having been used for dental repairs for more than 4 000 years. These early dental applications were based on aesthetics rather than masticatory ability. The use of gold in den-tistry remains signifi cant today, with typical annual con-sumption estimated to be approximately - 70 - tonnes - worldwide. However, with an increasingly wide range of alternative materials available for dental repairs, it is con-sidered appropriate to review the current gold based tech-nology available today and thereby highlight the excep-tional performance that competing materials must dem-onstrate if they are to displace gold from current usage. In conservative and restorative dentistry, as well as in orthodontics, gold is used either as a pure metal, or alloyed together with noble metals and base metals. This use of pure gold is limited to direct fi lling of small occlusive cavities and no standard exists for the appli-cation and properties of direct fi lling with gold. How-ever, pure gold used in this application is very soft (HV 25), has a very low 0,2 % proof stress (30 MPa), and a large elongation (45 %). As a result it can be cold worked very easily, a necessary requirement for fi lling a cavity precisely. Since gold fi llings do not have high mechanical resistance against masticatory forces, they are only suitable for very small cavities. In recent years, pure gold has also been used through the electroforming process. Electroformed inlays and onlays are suitable to be cemented into cavities after they have been veneered with porcelain. Tooth restorations such as porcelain ve-neered copings for crowns and bridgework can be elec-troformed with pure gold. Unfortunately, no standard yet exists for this process, which is rapidly becoming an established mainstream technique in modern dentistry. A more common technique in conservative dentistry is cementing investment cast gold alloy inlays and onlays into cavities. Gold alloys are used in dentistry, not only for their preferred golden colour [13], but also because they maintain an extremely high chemical stability in the mouth. They also possess several desirable mechanical properties such as high strength, ductility and elasticity [14]. From among the various types of alloys used for porcelain fused to metal (PFM) restorations, Au-Pt based high noble alloys have had the advantage of being around for some considerable time. They are part of clinical experience and are extremely successful [13]. High Au-content dental alloys, including Au-Pt alloys, show good biocompatibility due to the corrosion resist-ance of high noble elements [13]. Besides functional performance and aesthetics, biocompatibility is a third important requirement for dental restorative materials. Where dental restorations are based on the use of differ-ent alloys, there is the obvious potential for oral poly-metallism. Indeed, two alloys of different composition have different electrochemical potentials and inevitably induce corrosion, with the subsequent release of metal ions into the tissue. The ion release from metallic pros-theses and implants is the main cause of any unwanted primary and secondary reactions in the human body. This phenomenon may lead to the deterioration of den-tal devices and, in the worst case, may cause prosthesis failure or fracture.CONCLUSIONS The Future of Gold is in the Biomedical Industry and Dentistry and is connected to the historical devel-opment work concerning the properties of gold. If lon-gevity, functionality, aesthetics and biocompatibility, together with ease of manufacture, are considered as the most important requirements, the optimum material for biomedical applications and dental restorations is still a well-approved high gold alloy. On the other hand there is the application of gold in nano-technology. Gold nanoparticles in the 1-10 nm size range have special physic-chemical properties that are useful in a wide variety of applications such as can-cer diagnostics, catalysis, Raman and fl uorescence spectroscopy selective, ionisation of biomolecules, and single electronics. In the future commercial realisation of such applications relies on economical synthesis of size-controlled nanoparticles, which requires the devel-opment of continuous-fl ow versions of lab-scale proc-esses.264  METALURGIJA 51 (2012) 2, 261-264R. RUDOLF et al.: GOLD IN THE PAST, TODAY AND FUTUREACKNOWLEDGEMENTS This paper is part of a Slovenian Research Pro-gramme No. P2-0120, Bilateral Project SLO/SR BI-SR/10-10-022, and EUREKA Programme E!4213 NANO-FOILS. The authors thank Zlatarna Celje d.d., Celje, Slovenia, working on the Au-development.REFERENCES [1] D. Williams (ed.), Medical & Dental materials, (1990), Oxford OX3, England.[2] Y. Ning, Gold Bull., 39 (2006) 4, 220-225.[3] J. H. Park, M. H. Cho, M. G. Parka, Y. H. Kwon, H. Kim and H. J. Seol, Gold Bull., 43 (2010) 4, 316-323.[4] S. K. Gade, K. E. Coulter and J. D. Way, Gold Bull., 43 (2010) 4, 287-297.[5] A. Hugon, L. Delannoy, C. Louis, Gold Bull., 41 (2008) 2, 127-138.[6] H. Gleiter, Acta Mater., 18 (2000), 1, 1-29.[7] M. A. El-Sayed, Acc. Chem. Res., 34 (2001) 4, 257-264.[8] E. Hutter, J. H. Fendler, Adv. Mater., 16 (2004) 19, 1685-1706.[9] P. K. Jain, et al., J. Phys. Chem. B, 110 (2006) 37, 18243-18253.[10] W. Huang, et al., J. Phys. Chem. B, 109 (2005) 40, 18881-18888.[11] C. Loo, et al., Techol. Cancer Res. Treatment, 3 (2004) 1, 33-40.[12] E. Katz, I. Willner, Chem. Int. Ed., 43 (2004) 3, 6042-6108.[13] M. Čolić, D. Stamenković, I. Anžel, G. Lojen, R. Rudolf, Gold Bull., 42 (2009) 1, 34-47.[14] R. Rudolf, I. Anžel, L. Gusel, D. Stamenković, A. Todorović, M. Čolić. Met. Mater. Int., 16 (2010) 6, 931-940.Note:  The responsible translator for English language is mag. Shelagh Hedges Baker, Faculty of Mechanical Engineering, University of Maribor, Slovenia.

Gold in the past, today and future

This paper deals with gold, which is described as a chemical element. Special attention is paid to its physical-chemical properties and, furthermore, where or in what form it can be found in nature. We discuss the role it has played through history and we inform how gold has been developed to the level it has reached today’s value. Still more, when gold is broken into nanoparticles, this form could be highly useful for a wide range of processes, including general nanotechnology, electronics manufacturing and the synthesizing of different functional materials. It is important that we know that gold is also used in industry in many engineering applications (contacts in micro-electronics) and medicine (dental alloys, implants)

R. Rudolf

M. Anžel

E. Marković

M. Čolić

D. Stamenković

Directory of Open Access Journals

261 METALURGIJA 51 (2012) 2, 261-264R. RUDOLF, M. ANŽEL, E. MARKOVIĆ, M. ČOLIĆ, D. STAMENKOVIĆGOLD IN THE PAST, TODAY AND FUTURE Received – Prispjelo: 2011-05-27Accepted – Prihvaćeno: 2011-07-10Review paper – Pregledni radISSN 0543-5846METABK 51(2) 261-264 (2012)UDC – UDK 669.14.018.298:669.18=111This paper deals with gold, which is described as a chemical element. Special attention is paid to its physical-chem-ical properties and, furthermore, where or in what form it can be found in nature. We discuss the role it has played through history and we inform how gold has been developed to the level it has reached today’s value. Still more, when gold is broken into nanoparticles, this form could be highly useful for a wide range of processes, including general nanotechnology, electronics manufacturing and the synthesizing of diff  erent functional materials. It is im-portant that we know that gold is also used in industry in many engineering applications (contacts in micro-elec-tronics) and medicine (dental alloys, implants). Key words: gold, physical-chemical properties, nano-particles, applicationsZlato u prošlosti, sadašnjosti i budućnosti. Ovaj članak govori o zlatu kao kemijskom elementu. Posebna pažnja posvećena je njegovim fi  zikalno-kemijskim svojstvima i gdje i u kojem obliku se može pronaći u prirodi. Razmatra se uloga koju je zlato odigralo u povijesti i donosi se informacije o tome kako je doseglo razinu vrijednosti koju ima danas. Zlato razlomljeno na nanočestice upotrebljivo je u širem spektru procesuiranja, uključujući opću nanoteh-nologiju, elektroničku prizvodnju i spajanju materijala raznih funkcionalnosti. Važno je znati da se zlato koristi i u industriji, mnogim inženjerskim procesima (kontakti u mikro-elektronici) i medicini (dentalne slitine, implatanti).Ključne riječi: zlato, fi  zikalno-kemijska svojstva, nanočestice, primjeneINTRODUCTIONGold is a chemical element with the atomic number 79 and the symbol Au, which is derived from the Latin name “aurum” and means the morning dawn [1]. By co-agulation it crystallizes in a centred cubic grid and in the periodic table it is located between the transition metals. The relative atomic mass of gold is 196,97, the density is 19,32 g/cm³, the melting temperature is 1063 °C, the elastic modulus is 77 MPa and the coefﬁ  cient tempera-ture of elongation (WAK) is 14,1 µm/m  ·  K [1]. Gold is a very dispersed and rare element in the Earth’s crust. In the wild it is located in its native form or (less frequently) in the form of telluride, which contains the only Golden minerals (kalaverit, sylvanite, petcit) [2]. Native gold is located primarily in the veins of silica (SiO2), in the ores of other materials, or in deposits and is not clean; it con-tains 1–50 % mercury. The largest deposits of gold are in South Africa, the USA, Canada, Australia, Mexico and Russia. Approximately 20 % of world production is used commercially, while the rest is in gold reserves (the gold basis of paper money). R. Rudolf, M. Anžel, Faculty of Mechanical Engineering, University of Maribor, Maribor, Slovenia., E. Marković, D. Stamenković, High School of Dentistry, University of Belgrade, Belgrade, Serbia, M. Čolić, Univer-sity of Niš, Military Medical Institute Belgrade, SerbiaPHYSICAL-CHEMICAL PROPERTIES Gold has its own unique yellow colour. Its appear-ance is also inﬂ  uenced by its brilliance and reﬂ  ective ability. Because of these properties gold cannot be con-fused with any other metal. If gold contains an admix-ture of other elements it changes colour. Due to the ad-mixture of silver in gold (up to 30 % by weight) it turns silver-white [3]. If the surface is polished the sharp me-tallic shine is stressed. Gold products are usually made of an alloy in which you can ﬁ  nd behind the gold other alloying elements such as silver, copper, platinum, pal-ladium etc. in many cases where products contain cop-per, so-called oxidation zones are visible, which is the reason for the dark, unpolished look of the surface [4]. Gold has almost twice the density of lead and 19,3 times greater than that of water. The admixture of other metals also has an effect on the density. For example, if the mass fraction of silver in gold alloy is between 2 and 20 %, then consequently the alloy density is between 15,5 and 19,3 g/cm³ [1]. Other contaminants which may affect the density are copper, platinum, palladium, rho-dium, iridium and bismuth. In the chemical sense gold is the least active metal, since it burns in air, is not oxidized in water, does not change colour, does not react with strong alkaline solu-tions and all the pure acids (except for selenium acid). Gold dissolved in a solution which is a mixture of chlo-rine and nitric acid (HCl: HNO3 = 3:1); it dissolves in a 262   METALURGIJA 51 (2012) 2, 261-264R. RUDOLF et al.: GOLD IN THE PAST, TODAY AND FUTUREcyanide solution [5]. Gold dissolves in mercury to form amalgam. The gold compounds have one (+1) or three (+3) valence and they are not very persistent. Colloids are formed by adding a solution of formaldehyde or phenylhydrazine to gold. Gold is a soft and very dense metal with a strong yellow colour and has a high degree of cleavage. GOLD IN THE BANKING BUSINESSIn the fourth millennium (before Christ) the Egyp-tian workers found their ﬁ  rst silver and gold. This dis-covery formed the natural exchange system of the econ-omy at that time. Minted as indestructible silver and gold coins and precious metals they began to be used as currency. For security reasons, they began to be stored by the Goldsmith, and these in turn issued a Certiﬁ  cate of Safekeeping (CS), which was redeemable for silver and gold, and which later became the paper money used instead of silver and gold. On this basis they began sub-sequently to establish a private bank, and soon it was found to be in circulation longer than the CS as it was available in silver and gold. Later other currencies were developed which were also based on silver and gold, which corresponded to the supply of these precious metals as they were stable and not subject to inﬂ  ation.During the First World War huge ﬁ  nancial resources were required to ﬁ  nance military operations. Because the war lasted a full four years, they violated the gold stand-ard and printed more money than they had gold and sil-ver. As a result, the uncovered amount of money has in-creased and inﬂ  ation has started to show. In the ﬁ  fties of the last century they tried to get back the gold standard, so all currencies were tied to the dollar, and this contains an ounce of gold, which at the time was 35 dollars. To-day, gold stocks are at an end, the money in circulation is growing and is us only on the trust of its users.GOLD IN NANOTECHNOLOGYNanoparticles lie between bulk and atomic dimen-sions and are therefore endowed with special proper-ties, thereby making them starting material for many futuristic applications. The properties of nanoparticles depend on their crystallite sizes [6]. Therefore, control over the particle sizes and particle distributions (PSDs), is highly desirable. Of the different classes of organic and inorganic (metal, dielectric, and semiconductor) na-noparticles, noble metal nanoparticles have fascinated scientists’ from historic times because of their unique size- and shape-dependent optical properties [7]. Today the potential of Au nano- particles is recog-nised to derive from the addressability of their interesting optical properties via spectroscopic and photonic tech-niques [8]. For those spherical nanoparticles much small-er than the wavelength of light (diameter d << λ), an elec-tromagnetic ﬁ  eld at a certain frequency (ν) induces a resonant, coherent oscillation of the metal free electrons across the nano-particle (Figure 1). This oscillation is known as the surface Plasmon resonance [7,8]. The resonance lies at visible frequencies for Au metal. The surface Plasmon oscillation of the metal electrons results in a strong enhancement of absorption and scatter-ing of electromagnetic radiation in resonance with the surface Plasmon resonance (SPR) frequency of the Au nanoparticles, giving them intense colours and interest-ing optical properties. The frequency and cross section of SPR absorption and scattering is dependent on the metal composition, nanoparticle size and shape, dielectric prop-erties of surrounding medium/substrate and presence of inter-particle interactions. Au is the plasmonic metal of choice because of its much higher stability as compared to Cu and other metals. In addition, spherical Au colloids can easily be made in a wide range of sizes (4-80 nm) by facile chemistry involving the reductions of Au ions in solution [9]. Other interesting Au nanostructures (Figure 2) with modiﬁ  ed optical properties e.g. nanorods, nano-prisms, triangular nanoparticles, nanocubes, and com-posite silica core Au-shell particles, can be fabricated via different techniques, such as: wet synthesis techniques, electrochemistry, photochemical techniques, nano-li-thography, ultrasonic pyrolysis, etc.Due to the phenomenon of SPR, the absorption and scattering cross-section of Au nanoparticles are signiﬁ  -Figure 1   Schematic presentation of the interaction of a metal nano-sphere with lightFigure 2   a) Photographs of colloidal dispersions of Au nanoparticles with increasing sizes, b) Au nanospheres, c) Au nanorodsa)b) c)263 METALURGIJA 51 (2012) 2, 261-264R. RUDOLF et al.: GOLD IN THE PAST, TODAY AND FUTUREcantly superior to the absorbing and ﬂ  uorescing dyes used conventionally in biological and biomedical imag-ing [10]. Researchers demonstrated that the optical cross-sections of the Au-nanospheres are typically four to ﬁ  ve orders of magnitude higher than those of conven-tional dyes. Current diagnostics and investigational techniques in molecular biology and biomedicine rely heavily on chemical contrast agents to stain/label spe-ciﬁ  c cells and tissues of interest [11] in order to over-come the problem of weak signals of endogenous chro-mospheres and the subtle spectral differences between normal and diseased cells and tissues. Colloidal Au na-noparticles with strong SPR enhanced absorption and scattering are an important addition to the toolbox of imaging labels and contrast agents. Au nanoparticles are not susceptible to photo-bleaching and they appear to be biocompatible and non-cytotoxic, as supported by recent experiments on cells [11]. Another factor motivating the use of Au nanoparti-cles is their facile bio conjugation and bio modiﬁ  cation [12]. The surface of Au nanoparticles has a strong bind-ing afﬁ  nity towards thiols, disulphides, and amines. In particular, the simple Au-thiol chemistry allows the sur-face conjugation of various peptides, proteins, and DNA. The electrostatic adsorption of biomolecules such as vitamin C and or large protein/enzyme molecules, to the nanoparticles` surface is a simple and commonly used technique for Au nanoparticles capped with citrate or similar carboxylic acid derivate.GOLD IN DENTISTRYGold is the oldest dental restorative material, having been used for dental repairs for more than 4 000 years. These early dental applications were based on aesthetics rather than masticatory ability. The use of gold in den-tistry remains signiﬁ  cant today, with typical annual con-sumption estimated to be approximately - 70 - tonnes - worldwide. However, with an increasingly wide range of alternative materials available for dental repairs, it is con-sidered appropriate to review the current gold based tech-nology available today and thereby highlight the excep-tional performance that competing materials must dem-onstrate if they are to displace gold from current usage. In conservative and restorative dentistry, as well as in orthodontics, gold is used either as a pure metal, or alloyed together with noble metals and base metals. This use of pure gold is limited to direct ﬁ  lling of small occlusive cavities and no standard exists for the appli-cation and properties of direct ﬁ  lling with gold. How-ever, pure gold used in this application is very soft (HV 25), has a very low 0,2 % proof stress (30 MPa), and a large elongation (45 %). As a result it can be cold worked very easily, a necessary requirement for ﬁ  lling a cavity precisely. Since gold ﬁ  llings do not have high mechanical resistance against masticatory forces, they are only suitable for very small cavities. In recent years, pure gold has also been used through the electroforming process. Electroformed inlays and onlays are suitable to be cemented into cavities after they have been veneered with porcelain. Tooth restorations such as porcelain ve-neered copings for crowns and bridgework can be elec-troformed with pure gold. Unfortunately, no standard yet exists for this process, which is rapidly becoming an established mainstream technique in modern dentistry. A more common technique in conservative dentistry is cementing investment cast gold alloy inlays and onlays into cavities. Gold alloys are used in dentistry, not only for their preferred golden colour [13], but also because they maintain an extremely high chemical stability in the mouth. They also possess several desirable mechanical properties such as high strength, ductility and elasticity [14]. From among the various types of alloys used for porcelain fused to metal (PFM) restorations, Au-Pt based high noble alloys have had the advantage of being around for some considerable time. They are part of clinical experience and are extremely successful [13]. High Au-content dental alloys, including Au-Pt alloys, show good biocompatibility due to the corrosion resist-ance of high noble elements [13]. Besides functional performance and aesthetics, biocompatibility is a third important requirement for dental restorative materials. Where dental restorations are based on the use of differ-ent alloys, there is the obvious potential for oral poly-metallism. Indeed, two alloys of different composition have different electrochemical potentials and inevitably induce corrosion, with the subsequent release of metal ions into the tissue. The ion release from metallic pros-theses and implants is the main cause of any unwanted primary and secondary reactions in the human body. This phenomenon may lead to the deterioration of den-tal devices and, in the worst case, may cause prosthesis failure or fracture.CONCLUSIONS The Future of Gold is in the Biomedical Industry and Dentistry and is connected to the historical devel-opment work concerning the properties of gold. If lon-gevity, functionality, aesthetics and biocompatibility, together with ease of manufacture, are considered as the most important requirements, the optimum material for biomedical applications and dental restorations is still a well-approved high gold alloy. On the other hand there is the application of gold in nano-technology. Gold nanoparticles in the 1-10 nm size range have special physic-chemical properties that are useful in a wide variety of applications such as can-cer diagnostics, catalysis, Raman and ﬂ  uorescence spectroscopy selective, ionisation of biomolecules, and single electronics. In the future commercial realisation of such applications relies on economical synthesis of size-controlled nanoparticles, which requires the devel-opment of continuous-ﬂ  ow versions of lab-scale proc-esses.264   METALURGIJA 51 (2012) 2, 261-264R. RUDOLF et al.: GOLD IN THE PAST, TODAY AND FUTUREACKNOWLEDGEMENTS This paper is part of a Slovenian Research Pro-gramme No. P2-0120, Bilateral Project SLO/SR BI-SR/10-10-022, and EUREKA Programme E!4213 NANO-FOILS. The authors thank Zlatarna Celje d.d., Celje, Slovenia, working on the Au-development.REFERENCES [1]  D. Williams (ed.), Medical & Dental materials, (1990), Oxford OX3, England.[2]  Y. Ning, Gold Bull., 39 (2006) 4, 220-225.[3]  J. H. Park, M. H. Cho, M. G. Parka, Y. H. Kwon, H. Kim and H. J. Seol, Gold Bull., 43 (2010) 4, 316-323.[4]  S. K. Gade, K. E. Coulter and J. D. Way, Gold Bull., 43 (2010) 4, 287-297.[5]  A. Hugon, L. Delannoy, C. Louis, Gold Bull., 41 (2008) 2, 127-138.[6]  H. Gleiter, Acta Mater., 18 (2000), 1, 1-29.[7]  M. A. El-Sayed, Acc. Chem. Res., 34 (2001) 4, 257-264.[8]  E. Hutter, J. H. Fendler, Adv. Mater., 16 (2004) 19, 1685-1706.[9]  P. K. Jain, et al., J. Phys. Chem. B, 110 (2006) 37, 18243-18253.[10]  W. Huang, et al., J. Phys. Chem. B, 109 (2005) 40, 18881-18888.[11]  C. Loo, et al., Techol. Cancer Res. Treatment, 3 (2004) 1, 33-40.[12]  E. Katz, I. Willner, Chem. Int. Ed., 43 (2004) 3, 6042-6108.[13]  M. Čolić, D. Stamenković, I. Anžel, G. Lojen, R. Rudolf, Gold Bull., 42 (2009) 1, 34-47.[14] R. Rudolf, I. Anžel, L. Gusel, D. Stamenković, A. Todorović, M. Čolić. Met. Mater. Int., 16 (2010) 6, 931-940.Note:   The responsible translator for English language is mag. Shelagh Hedges Baker, Faculty of Mechanical Engineering, University of Maribor, Slovenia.

Gold in the past, today and future

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