Of polycrystalline, nanocrystalline, and amorphous structures, most materials are polycrystalline, made up of ordered crystals that meet at boundaries where, inevitably, there is disorder. Such materials have two structural length scales: that of the crystals and that of the atoms that make them up. The crystal size, typically, is between 0.1 mm and 1 mm. This means that the disordered material occupies only a tiny fraction of the volume-less than one part in a million. Nanocrystalline materials, have crystals that are much smaller, in the range of 10-100 nm. The smaller the crystals, the greater the fraction of disordered material, which now becomes large enough to influence mechanical and other properties. Surface effects are expected to have a prominent role in nanomaterials because of their large surface-to-volume ratio, and for that reason it can be expected that surface stress changes have pronounced effects on nanomaterials.
In this paper, we would highlight the synthesis of gold nanoparticles and the unique properties that have been brought about and discovered upon manipulating the size of the particles. A few popular methods would be highlighted. Besides introducing on their various compounds and methodology, the effects of the samples upon varying certain experimental parameters would be elaborated as well. We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are being regarded as one of the most stable metal nanoparticles. From the various synthesis techniques, AuNPs could also be manipulated to form highly ordered 1D,2D,or 3D nanonetworks and superstructures. (Figure 1) 
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Figure 1. AuNP superlattices. (Top) Packing sequences observed in AuNP superlattices: (a) hexagonal closepacking ABAB, (b) cubic close-packing ABC ABC, and (c) anomalous packing in which AuNPs sit on two fold saddle positions D. The packing becomes ADA or ADC. (Bottom) High-dispersion diffraction patterns from an FCC AuNP superlattice on the  zone axis. The reflections (220), corresponding to the second row of reflections, correspond to the void superlattice. 
They possess fascinating aspects which their assembly, behavior of the individual particles, size-related electronic, magnetic and optical properties and their applications to decorative purposes and biotechnology. For the past decade, gold colloids have been the subject of many, especially after the breakthroughs reported by Schmid and Brust et al.[3-4] The subject is now so intensively investigated, due to fundamental and applied aspects relevant to the quantum size effect, which would be discussed in the next section.
The reasons for the present excitement in AuNP research are also the stability of AuNPs on top of the extraordinary diversity of their modes of preparations involving many different materials, and their essential properties and role in nanoscience and future nanotechnology. After which, we would include properties that would be related to their applications, particularly the fascinating aspects are the optoelectronic properties of AuNPs related to the surface plasmon absorption, reflecting the collective oscillation of the conducting electrons of the gold core, a feature relevant to the quantum size effect. The study of the surface Plasmon band (SPB) has also remained an area of very active research from both scientific and technological standpoints, especially when the particles are embedded in ionic matrices and glasses . With regards to the SPB, it should also be noted that AuNP sols would be mentioned in the discussion and hence, the preparation of AuNPs dispersed within mesoporous silica will be made detailed as well.
After discussing about the synthesis, stabilization, and various types of assemblies, a detailed study of optical properties of AuNPs in solutions and its correlation with size, other parameters will be briefly discussed in this paper. After which we will conclude this paper with a on the perspectives of AuNPs in nanosciences and nanotechnology with a few of its applications.
Refraction index, Quantum Size Effect and Single-Electron Transitions
In general, spherical particles that are smaller than the wavelength of scattered light, they obey an equation proposed by Rayleigh which indicates that light scattering effect is very size dependent on the particle size.  To achieve almost perfect transparency and minimize light scattering, the particles should be as small as possible. Although this relationship mostly applies to composites, it should be noted that the optical properties of AuNPs are also size dependent and a more relevant theory, namely Mie theory would be applied in our case to discuss the optical behaviour of AuNPs, such as the phenomenon of SPB, adsorption behaviour and the corresponding red/blue shifts.
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Physicists predicted that nanoparticles in the diameter range between the size of small molecules and that of bulk metal, approximately 1-10 nm would display electronic structures, reflecting the electronic band structure of the nanoparticles, owing to quantum-mechanical rules. Properties thus strongly depend on the particle size, interparticle distance, nature of the protecting organic shell, and shape of the nanoparticles. The quantum size effect is involved when the de Broglie wavelength of the valence electrons is of the same order as the size of the particle itself. Thus, the particles would then behave electronically as zero-dimensional quantum dots (or quantum boxes). Freely mobile electrons that are trapped in such metal boxes would display a characteristic collective oscillation frequency of the plasma resonance, giving rise to the so-called plasmon resonance band observed near 530 nm in the 5-20-nm-diameter range.  In nanoparticles, there is a gap between the valence band and the conduction band, unlike in bulk metals. The size induced metal-insulator transition is observed when the metal particle is small enough for size-dependent quantization effects to occurLiterature review had stated the size is approximately 20nm and standing electron waves with discrete energy levels are formed as a result. [8-10]Interestingly, the absorption and emission of light are particle size- dependent as well. From DeBroglie relationship, we can conclude that phenomenon of blue shift for with decreasing size of the particle, which is also enforced by Pauli's Principle. For instance, blue shift may affect the adsorption or emission spectra; and if the latter is affected, the corresponding colour/ wavelength of the emitted light/photons would also experience a change. 
Single-electron transitions occur between a tip and a nanoparticle, causing the observation of so-called Coulomb blockades if the electrostatic energy, Eel = e2/2C, is larger than the thermal energy, ET = kT. Since capacitance C would decrease with particle size, single-electron transitions can be observed at a given temperature only if C is very small, i.e., for nanoparticles since they are small enough (C < 10-18 F). Large variations of electrical and optical properties are observed when the energy level spacing exceeds the temperature, and this flexibility is of great practical interest for applications (transistors, switches, electrometers, oscillators, biosensors, catalysis). [8-10] The number of atoms in these gold clusters are based on the dense packing of atoms taken as spheres, each atom being surrounded by 12 nearest neighbours. Thus, the smallest cluster contains 13 atoms, and the following layers contain 10n2 + 2 atoms, n being the layer number. For instance, the second layer contains 42 atoms, which leads to a total of 55 atoms for a gold cluster, and the compound [Au55(PPh3)12Cl6] has been well characterized by Schmid's group. [8-10]
Synthesis and Assembly- (a) Citrate Reduction
Conventional methods of synthesis of AuNPs is via the reduction of gold(III) derivatives. One of the most popular one would be employing the citrate reduction of HAuCl4 in water, proposed by Turkevitch et al, which leads to AuNPs of ca. 20 nm.  Litertature studies have revealed that Frens el al  were able obtain AuNPs of prechosen size (between 16 and 147 nm) via their controlled formation, a method was proposed where the ratio between the reducing/stabilizing agents (the trisodium citrate-to gold ratio) was varied. This method is very often used even now when a rather loose shell of ligands is required around the gold core in order to prepare a precursor to valuable AuNP-based materials. Recently, a practical preparation of sodium 3-mercaptopropionate- stabilized AuNPs was reported in which simultaneous addition of citrate salt and an amphiphile surfactant was adopted; the size could be controlled by varying the stabilizer/gold ratio (Figure 2).
Figure 2. Preparation Procedure of anionic mercaptoligand- stabilized AuNPs in water. [ 13 ]
Synthesis and Assembly- (b) The Brust-Schiffrin Method: Two-Phase Synthesis and Stabilization by Thiols
Schmid's cluster [Au55(PPh3)12Cl6] was reported in 1981 and was deemed to be unique with its narrow dispersity (1.4±0.4 nm) for the study of a quantum-dot nanomaterial. The Brust-Schiffrin method for AuNP synthesis, published in 1994, has had a considerable impact on the overall field because it allowed the synthesis of thermally stable and air-stable AuNPs of reduced dispersity and controlled size for the first time (ranging in diameter between 1.5 and 5.2 nm).  The technique of synthesis is inspired by Faraday's two-phase system and uses the thiol ligands that strongly bind gold due to the soft character of both Au and S.  AuCl4 - is transferred to toluene using tetraoctylammonium bromide as the phase-transfer reagent and reduced by NaBH4 in the presence of dodecanethiol (Figure 3).
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Figure 3. Formation of AuNPs coated with organic shells by reduction of AuIII compounds in the presence of thiols. 
It was mentioned that diameters of particles in the range of 1-3 nm, with a maximum in the particle size distribution at 2.0-2.5 nm had a preponderance of cuboctahedral and icosahedral structures.[16,18] Larger thiol/gold mole ratios is used to obtain smaller average core sizes, while fast reductant addition and cooled solutions produced smaller, more monodisperse particles. A higher abundance of small core sizes (≤2 nm) is obtained by quenching the reaction immediately following reduction or by using sterically bulky ligands.[16-17]
Brust et al. extended this synthesis to p-mercaptophenol-stabilized AuNPs in a singlephase system, which opened an avenue to the synthesis of AuNPs stabilized by a variety of functional thiol ligands.[16,18] Subsequently, many publications appeared describing the use of the Brust- Schiffrin procedure for the synthesis of other stable AuNPs, also sometimes called monolayer-protected clusters (MPCs), of this kind that contained functional thiols.[19-20] The proportion thiol:AuCl4- used in the synthesis controls the size of the AuNPs. Additionally, a small process during this set of synthesize, namely digestive ripening, which involves heating a colloidal suspension near the boiling point in the presence of alkanethiols also significantly reduced the average particle size and polydispersity in a convenient and efficient way, which led to the formation of 2D and 3D superlattices,
For instance, AuNPs obtained using acid-facilitated transfer are free of tetraalkylammonium impurity, are remarkably monodisperse, and form crystalline superstructures, which enhance physical properties such as hardness In addition, reduction of AuIVCl4 by NaBH4 in a mixture of tri-n-octylphosphine oxide (TOPO) and octadecylamine (1:0.57 molar ratio) at 190 °C resulted in the controlled growth of spherical AuNPs (8.59 ( 1.09 nm diameter) that are stable for months in toluene and were manipulated into crystals and 2D arrays (Figure 4).
Figure 4. 2D lattice of octadecylamine/TOPO-capped AuNPs spontaneously formed when the latter are deposited on a copper grid; bar ) 20 nm. (Inset) Scanning electron microscopy (SEM) image of a cubic colloidal crystal prepared from octadecylamine/TOPO-capped AuNPs (190 °C); bar = 80 µm. 
Synthesis and Assembly- (e) Physical Methods: Photochemistry (UV, Near-IR), Sonochemistry, Radiolysis, and Thermolysis
UV irradiation is a parameter that can improve the quality of the AuNPs, including when it is used in synergy with micelles[24-25] while Near-IR laser irradiation provokes an enormous size growth of thiol-stabilized AuNPs. Sonochemistry was also used for the synthesis of AuNPs within the pores of silica . Additionally, radiolysis had been used to control the AuNP size  or to synthesize them in the presence of specific radicals, and the mechanism of AuNP formation upon γ-irradiation has been carefully illustrated in the figure below (Figure 5).
Figure 5. Taping-mode AFM images of the species formed in a solution irradiated with γ rays (1.5 kGy) and then deposited on highly ordered pyrolytic graphite (HOPG) 30 days after the irradiation and dried under a mild N2 stream for visualization. The solution contains 10-3 mol/L AuIII and poly(vinyl alcohol) but no alcohol. (A) Height-mode (z range 80.0 nm) image and (B) phase (z range 42.2°) image, run simultaneously on the same area of the sample. (C, D) Close-up images showing the same area as in (A) and (B), respectively. 
It had been reported that thermolysis of [C14H29Me3N][Au(SC12H25)2] at 180 °C for 5 h under N2 produced alkyl-groups passivated AuNPs of 26 nm. while thermolysis of crude preparations of Brust's AuNPs without removing the phase-transfer reagent, tetraoctylammonium bromide, to 150-250 °C led to an increase of the particle sizes to 3.4-9.7 nm, causing the heat-treated AuNPs to formed 2D superlattices with hexagonal packing. Additionally, it was noted that the conformation of the alkanethiol is all-trans, and these ligands interpenetrate each other (Figure 6).
Figure 6. UV-vis spectra (A) and TEM images and size distributions (B) of (a) [AuCl4]- before reduction; dodecanethiol-AuNPs (b) as prepared and after heat treatment at (c) 150, (d) 190, and (e) 230 °C; and (f) octadecanethiol- AuNPs heat-treated at 250 °C. 
When it comes to preparation, characterization, and study of AuNPs dispersed within mesoporous silica, AuSiO2, various aspects such as the influence of size, the crystal growth, the influence of radiations on the nucleation and the sol-gel approach have been examined in the preparations of AuSiO2. Possible and common synthetic methods to form solgel matrices of AuNPs are the citrate route mentioned earlier and followed by stabilization by a (3-aminopropyl)trimethoxysilane (APTMS)-derived aminosilicate and the sol-gel processing in inverse micelles (Figure 7). Also, the sonochemical approach of HAuCl4 reduction leads to the deposition of AuNPs on the surface of the silica spheres or within the pores of mesoporous silica.
AuNP-containing materials, whose assembly are mentioned earlier are behind many color based biosensor applications based on the phenomenon of surface plasmon resonance which is closely to the optical property for the AuNPs and would be discussed in the following section.
Figure 7. Schematic illustration of AuNPs in a silicate matrix. 
Optical Property-The Surface Plasmon Band (SPB)
Studies of the optical properties of gold nanoparticles indicate that a change in absorbance or wavelength of the surface plasmon (SP) resonance band provides a measure of particle size, shape, concentration, and dielectric medium properties. The size controllability and high monodispersity of the nanoparticles allowed us to investigate the correlation of the size with the SP resonance optical properties. SPB provides a considerable body of information on the development of the band structure in metals and has been the subject of extensive study of optical spectroscopic properties of AuNPs, in which the parameters such as size controllability and surface tunability are essential. .
The nature of the SPB was rationalized into a theory authored by Mie in 1908 as mentioned in literature studies  According to Mie theory, the total cross section composed of the SP absorption and scattering is given as a summation over all electric and magnetic oscillations. The resonances denoted as surface plasmons were described quantitatively by solving Maxwell's equations for spherical particles with the appropriate boundary conditions. Mie theory attributes the plasmon band of spherical particles to the dipole oscillations of the free electrons in the conduction band occupying the energy states immediately above the Fermi energy level. The phenomenon of SPB is due to the collective oscillations of the electron gas at the surface of nanoparticles. The 6s electrons of the conduction band for AuNPs that are responsible for the electron gas correlate with the electromagnetic field of the incoming light and hence inducing the excitation of the coherent oscillation of the conduction band.
Gold nanoparticles in solution are well-known to display visible colors and SP resonance bands in the visible spectral region. For instance, formation of large aggregates caused a reversible change in color of the AuNP suspension from red to violet due to coupling to surface plasmons in aggregated colloids. The explanation above serves the purpose to account for various SPB observations, such as the deep-red color of AuNP sols in water and glasses reflecting the surface plasmon band, a broad absorption band in the visible region at approximately 520 nm.
SPB also decreases with decreasing core size for AuNPs with 1.4-3.2-nm core diameters due to the onset of quantum size effects that become significant for particles with core sizes <3 nm in diameter and also cause a slight blue shift, observed as the damping of the SP mode follows a 1/radius dependence due to surface scattering of the conduction electrons. On the contrary, studies has recently revealed that the SP band undergoes a red shift as the particle size increases and the substrate-particle distance decreases. This red shift is also accompanied by a small broadening of the SP band in the long wavelength region, which would be explained subsequently.
The wavelength of the SP bands is clearly dependent on the particle size. AuNPs of increasing mean diameters would also correspond to the observation of SPB maximum λmax in increasing magnitude and in the order of nm when the particles are in aqueous media. (Figure 8) Usually, a decrease of intensity of the SPB as particle size decreases is accompanied by broadening of the Plasmon bandwidth; and steplike spectral structures indicating transitions to the discrete unoccupied levels of the conduction band with monodispersed AuNPs with core diameters between 1.1 and 1.9 nm. Thus, the SPB is absent for AuNPs with core diameter less than 2 nm, as well as for bulk gold.
To account for the for the association between the particle sizes and the λmax of the SP band, an equation from the Mie theory is used.
γ is the extinction coefficient while NV is the volume concentration of the assembled nanoparticles. É›α is the dielectric medium constant, which is related to the refractive index (i.e., (neff + ikeff)2 = É› eff, where É› eff is the effective dielectric constant, n eff is the real part of the effective complex index of refraction for the nanocomposite, and k eff is the imaginary part of the effective complex index of refraction.) É›1 and É›2 represent the real and complex part of Au The particle sizes can be considered to be a result of the increase in the effective refractive index for nanoparticles. 
Figure 8. Relationship between λmax of SP band and particle size. 
The SPB maximum and bandwidth are also influenced by the particle shape, dielectric medium properties, and temperature. Additionally, the refractive index of the solvent has been also shown to induce a shift of the SPB, as predicted by Mie theory.[35,41] Since AuNPs require stabilizing ligands or polymer in a aqueous solution, an agreement with Mie theory is obtained only when the shift induced by ligand shells are taken into account. For instance, a shift is especially significant with thiolate ligands, which are responsible for a strong ligand field interacting with the surface electron cloud. Otherwise, the band energy is rarely exactly as predicted by Mie theory if the shift of this stabilizer is not considered.
When the spacing between particles is reduced such as in the case of elliptical particles, the SPB would be shifted to higher wavelength described as an exponential function of the gap between the two particles. It becomes negligible when the gap is larger than about 2.5 times the short-axis length.  A red shift for a polarization parallel to the long particle axis and a blue shift for the orthogonal polarization were reported and rationalized by a dipolar interaction mechanism.
Another influential parameter is the core charge. Excess electronic charge causes shifts to higher energy, whereas electron deficiency causes shifts to lower energy.[42-44] A convenient theoretical expression has been derived for the SPB position as a function of the changes in free electron concentration: (λfinal/λìinitial ) (Ninitial/Nfinal)1/2 Explanations from literature involve a much reduced number of free electrons and large shift amplification by the thiolate ligand shell.
In AuSiO2core-shell particles (AuSiO2), varying the SiO2 shell thickness and the refractive index of the solvent allowed control over the optical properties of the dispersions, and the optical spectra were in good agreement with Mie theory. A near-field optical antenna effect was used to measure the line shape of the SPB in single AuNPs, and the results were found to be in agreement with Mie theory; double-peak shapes caused by electromagnetic coupling between close-lying particles were observed.
From the stabilization techniques and the establishment of the correlation between the optical properties and size particles, the following discussion below illustrates the feasibility of size variation of cluster size, in particular coagulation and Oswald ripening of AuNPs and the effects to the optical properties such as adsorption that comes along with it from literature. Coagulation, along with Oswald ripening of AuNPs dispersed in organic liquids could be accelerated by visible light, and the process was shown to be wavelength dependent; UV irradiation caused coalescence. Laser irradiation at the SPB of suspended AuNPs in 2-propanol causes coagulation or dispersion due to electron transfer from a solvent molecule to the AuNP. AuNPs could also be pulverized into smaller AuNPs with a desired average diameter and a narrow distribution by a suitable selection of laser irradiation. When a molecular linker, 4-aminobenzenethiol, attached several AuNPs together, the optical absorption differed. (Figure 9).
Figure 9. (a) UV/vis solution spectra of 4-mercaptobenzoic acid-capped AuNPs as a function of the addition of NaOH(aq) and (b) the variation of the intensity of the plasmon absorbance, at 525 nm, as a function of the pH of the solution. Points i, iii, v, vii, ix, and xi in (b) were obtained from curves i, iii, v, vii, ix, and xi in (a), respectively. 
Literature studies had indicated that the usage of Rhodamine 6G provoked morphological changes and particle growth upon laser irradiation of the SPB as a result of melting and fusion of AuNPs, and the multiphoton process leading to the fusion process has been elucidated using picosecond laser flash photolysis (Figure 10).
Figure 10. Schematic diagram illustrating the possible morphological changes associated with laser irradiation of the AuNP-dye assembly. AuSCN, AuNPs obtained by reduction of AuCl4- with SCN-. 
As mentioned above, AuNPs are very likely to be in a medium or solvents when being studied. Hence it is also to be noted that surface interaction of AuNPs with the medium could change the optical properties too. For instance, when a molecular linker, 4-aminobenzenethiol, attached several AuNPs together, the optical absorption differed.
Applications of the sensitivity of the position of the SPB are known, especially in the fields of sensors and biology. These findings serve as the basis of size correlation for exploiting the optical and spectroscopic properties of gold nanoparticles of different sizes in aqueous solutions, which has become increasingly important in analytical and bioanalytical applications. Another obvious application from the dependence of the optical properties of gold would be also to use gold o change the colour of glass to red.
AuNPs, which have been known for 2500 years, are the subject of an exponentially increasing number of reports and are full of promises for optical, electronic, magnetic, catalytic, and biomedical applications in the 21st century.