STUDY ON SELECTED SYNTHESES OF GOLD NANOPARTICLES khóa luận tốt nghiệp tiếng anh

ACKNOWLEDGEMENT I would like to express my gratitude Assoc. Prof. Dr. Tran Thi Nhu Mai for her supervisor and guidance throughout all of my researches. A very special thanks goes out to Professor Catherine J. Murphy and her groups members, who gave truly help the progression and smoothness of the internship program in University of Illinois at Urbana Champaign. I am also thankful all of other members of Laboratory of Organic Catalyst for their helps during my working time. ABSTRACT Nanoparticles, already widely applied in diverse fields from catalysis to bioimaging, have undergone tremendous development in the past two decades, where size control of spherical nanocrystals of semiconductors, metals and insulators has been achieved for almost any material. The published results has proven that gold nanomaterials possess strong and unique characteristics being hopeful to apply on a variety of fields, especially in Chemistry for catalyst purposes or biochemical purposes. In this research, the seeded growth and hydrolysis Stobers method was applied to synthesize and overcoat gold nanorods aiming to biomedical purposes. Moreover, the hydrothermal method was used to nail gold nanoparticles on the Si template orienting to the green catalyst. The results showed by physical methods such as Zetapotential, transmission electron microscope (TEM) or UVVis. EDX and N2 adsorptiondesorption measurements have confirmed the effective of these methods.

VIETNAM NATIONAL UNIVERSITY, HANOI HANOI UNIVERSITY OF SCIENCE

FACULTY OF CHEMISTRY

Phan Thi Thanh Binh

STUDY ON SELECTED SYNTHESES OF GOLD

NANOPARTICLES

Submitted in partial fulfillment of the requirements for the degree of

Bachelor of Science in Chemistry

(Advanced Program)

Hanoi - 2012

VIETNAM NATIONAL UNIVERSITY, HANOI HANOI UNIVERSITY OF SCIENCE

FACULTY OF CHEMISTRY

Phan Thi Thanh Binh

STUDY ON SELECTED SYNTHESES OF GOLD

NANOPARTICLES

Submitted in partial fulfillment of the requirements for the degree of

Bachelor of Science in Chemistry

(Advanced Program)

Hanoi - 2012

I am also thankful all of other members of Laboratory of Organic Catalyst fortheir helps during my working time.

Nanoparticles, already widely applied in diverse fields from catalysis to imaging, have undergone tremendous development in the past two decades, where sizecontrol of spherical nanocrystals of semiconductors, metals and insulators has beenachieved for almost any material The published results has proven that gold nano-materials possess strong and unique characteristics being hopeful to apply on a variety

bio-of fields, especially in Chemistry for catalyst purposes or biochemical purposes

In this research, the seeded - growth and hydrolysis Stober's method was applied

to synthesize and overcoat gold nanorods aiming to biomedical purposes Moreover,the hydrothermal method was used to nail gold nanoparticles on the Si templateorienting to the green catalyst The results showed by physical methods such as Zeta-potential, transmission electron microscope (TEM) or UV-Vis EDX and N2adsorption/desorption measurements have confirmed the effective of these methods

Table of Contents

INTRODUCTION 1

-CHAPTER 1: OVERVIEW OF GOLD NANOPARTICLES 2

1.1 BACKGROUND OF METAL NANOPARTICLES 2

1.2 OVERVIEW OF GOLD NANOPARTICLES 2

1.2.1 Properties and Applications 4

1.2.2 Synthesis and Functionalization 12

1.3 GOLD NANORODS 20

1.3.1 Optical properties 20

1.3.2 Synthesis methods 22

1.3.3 Silica coating 26

1.3.4 Applications 26

CHAPTER 2: EXPERIMENT 27

2.1 Seeded growth method to synthesize gold nanorods 27

2.2 Hydrolysis Stober method to silica coating gold nanorods 27

2.3 Hydrothermal method to synthesize gold nanoparticles template 28

2.4 Characterization Methods 29

CHAPTER 3: RESULTS AND DISCUSSIONS 36

3.1 PURPOSES 36

3.2 SYTHESIS AND CHARACTERIZATION OF GOLD NANORODS 36

-3.2.1 The effects of AgNO 3 volume on obtained gold nanorods 37

3.2.2 Effects of PEG coating and silica coating on gold nanoparticles 40

3.3 SYNTHESIS AND CHARACTERIZATION OF AU/SI (Au/Si_01) 44

-3.2.1 N 2 adsorptiondesorption measurements 45

3.2.2 TEM, EDX and AAS methods 47

3.4 PROSPECTIVE APPLICATIONS 49

3.4.1 Gold nanorods 49

3.4.2 Au/Si material 50

CONCLUSION 52

REFERENCE 53

-LIST OF FIGURES

Figure 1: Exponential growth in the number of publication on gold nanotechnology and nano-medicine

over the two past decades 3

Figure 2: Conversion of glucose to gluconic acid in alkaline aqueous solution 4

Figure 3: Approaches of loading/unloading therapeutics 9

Figure 4: Loading drugs into the interior of gold nanoparticles 10

Figure 5: Scheme of sensing layer preparation using both peptide and antibody 11

Figure 6: Gold nanodendrites 12

-Figure 7: Gold Nanorods 13

Figure 8:Sharpened nanorods 13

Figure 9: Nanocages/nanoframes 14

Figure 10: Nanoshells 14

Figure 11: Hallow gold nanosphere 15

Figure 12: (a) tetrahedra/octahedra/cubes/icosahedra, (b) rhombic dodecahedra, (c) octahedra 15

Figure 13: Nanocubes 15

Figure 14: Complex nanostructures 16

Figure 15: (a) hydrophobic entrapment, (b) electronstatic adsorption 18

Figure 16: Silance conjugation of gold nanoparticle 19

Figure 17: Electron micrograph of a silica sphere sample 19

-Figure 18: UV- Vis spectra of gold nanorods with aspect ratios: (A) 1.42 ± 0.32, (B) 1.82 ± 0.49, (C) 2.31 ± 0.55, (D) 2.65 ± 0.43, and (E) 2.80 ± 0.37 21

Figure 19: Illustration of changes of gold nanorod colors due to aspect ratio 22

Figure 20: Scheme of template approach of gold nanorods 22

-Figure 21: a, Scheme of electrochemical approach of gold nanorods b, TEM of gold nanorods at other aspect ratios obtained by elec method 23

Figure 22: Illustration for gold nanorods growth in the absence of silver ion 24

Figure 23: Proposed formation of gold nanoparticles 24

Figure 24: Illustration: gold nanorods growth in the presence of silver ion 25

Figure 25: Scheme of seeded growth method procedure 36

Figure 26: UV VIS of rainbow solutions 37

Figure 27: The formation of gold nanorods in the presence of Ag 38

Figure 28: TEM image of 10Ag sample 39

Figure 29: Scheme of PEG, silica modification procedure 40

Figure 30: TEM of mPEG SH coated gold nanorods PEG6Ag 42

Figure 31: UV VIS of PEG6Ag and Sil6Ag 43

Figure 32: TEM of Silica coated nanorods Si6Ag 43

Figure 33: Scheme of synthesis template procedure 44

Figure 34: Scheme of preparing Au/Si procedure 45

Figure 35: BET of Au/Si_01 46

Figure 36: Pore distribution of Au/Si_01 47

Figure 37: TEM image of Au/Si_01 48

Figure 38: EDX spectrum of Au/Si 48

Figure 39: Typical properties and applications of gold nanorods 49

Figure 40: Products containing calcium gluconate 51

-LIST OF TABLES Table 1: Gold nanoparticles in photothermal therapy applications 8

Table 2: Summary of synthetic approaches to obtain various gold nanostructures 16

Table 3: Outline of relation between stability and zetapotential 31

Table 4: Data of rainbow gold nanorods 38

Table 5: BET data of Au/Si_01 46

Normally, we also know that pure gold is a transition metal in the group 11 ofperiodical table It has a very high melting point and shows the nature as one of theleast reactive metals However, it exhibits a beautiful appearance, amazing malleabilityand ductility, of course, good conductivity That means why gold have been morewidely used to craft expensive ornaments or gild electronic accessories than apply onchemical field

However, in the recent researches, the scientists have discovered that gold onnanoscale represents a huge number of predominant effects that are full of promises toplay the role of aurotherapy, photothermal argents, and especially catalysis, opticalmaterials and biomedicine such as drug therapy or biosensor

In fact, Murphy et al.[1] and Nikoobakht and El-Sayed[2] have been successful

to demonstrate a colloid method to synthesize mono - disperse nanorods at high yieldbased on seeded growth method After that, to increase the biological compatibility,nanorods will be coated and functionalized by other materials, generally for instance, avariety of silicates or polymers, which have the functional groups to be similar to acidamines, enzymes such as thiolate, amines, carboxylate etc In the first part of thisresearch, I have already prepared nanorods based on seeded - growth theory in thepresence of silver and cationic surfactant CTAB and Then the Stober method wasused to overcoat the nanorods by silica (i.e, TEOS)

On the other hands, although gold is the most inert of all metallic elements, butmany studies have proven that gold nanoparticles also have appropriate properties asheterogeneous catalysts The right explanation has been debated, however, onepossibility explaining this phenomenon might be because of the availability of surfacegold atoms with low coordination number and the associated electrons density forwhatever reactions is being catalyzed [3] But it cannot deny that nanogold has played

an important role in the Chemistry as the green catalyst for many reactions, especiallyfor Organic chemistry So we have built a process to nail the gold - nanoparticles onthe template reigned P123 and TEOS by hydrothermal method orienting to catalyze forreaction of producing calcium gluconate from natural D - Glucose

CHAPTER 1: OVERVIEW OF GOLD NANOPARTICLES 1.1 BACKGROUND OF METAL NANOPARTICLES

Just as in bulk metals, electrons in the conduction band of nanoscale metals arefree to oscillate upon excitation with incident radiation, referred to as the localizedsurface plasmon resonance (LSPR) However, on the nanoscale, the oscillationdistance is restricted by the nanoparticle size For gold nanoparticles, LSPRcorresponds to photon energies in the visible wavelength regime, giving rise tosignificant interest in their optical properties These optical characteristics includestrong plasmon absorption, resonant Rayleigh scattering, and localized electromagneticfields at the nanoparticle surface

Actually, plasmon absorption in metal nanoparticles is highly dependent onnanoparticle shape, size, and dielectric constant of the surrounding medium [4] Onearea of catalysis that is developing at a rapid pace is nanocatalysis Striking novelcatalytic properties including greatly enhanced reactivities and selectivities have beenreported for nanoparticle (NP) catalysts as compared to their bulk counterparts Inorder to harness the power of these nanocatalysts, a detailed understanding of theorigin of their enhanced performance is needed [5] Many experimental studies onnanocatalysts have focused on correlating catalytic activity with particle size Whileparticle size is an important consideration, many other factors such as geometry,composition, oxidation state, and chemical/physical environment can play a role indetermining NP reactivity However, the exact relationship between these parametersand NP catalytic performance may be system dependent, and is yet to be laid out formany nanoscale catalysts Clearly, a systematic understanding of the factors thatcontrol catalyst reactivity and selectivity is essential if trial and error methods are to beavoided

1.2 OVERVIEW OF GOLD NANOPARTICLES

The first scientific report describing the production of colloidal goldnanoparticles was published in 1857 when Michael Faraday found that the ‘‘fineparticles’’ formed from the aqueous reduction of gold chloride by phosphorus could be

stabilized by the addition of carbon disulfide, resulting in a "beautiful ruby fluid".Actually, the Human has a huge step up to approach the gold nanoparticles Nowadays,most colloidal synthetic methods for obtaining gold nano particles follow a similarstrategy, whereby solvated gold salt is reduced in the presence of surface cappingligands which prevent aggregation of the particles by electrostatic and/or physicalrepulsion

Gold nanoparticles have been used

in biomedical applications since their first

colloidal syntheses more than three

centuries ago Actually, over the past two

decades, their beautiful colors and unique

electronic properties have also attracted

serious attention due to their historical

applications in art, medicine and current

applications in enhanced optoelectronics

and photovoltaics In spite of their modest

alchemical beginnings, gold nano-particles

possess physical properties that are

significantly different from both small

molecules and bulk materials, as well as

from other nano-scale particles -Moreover,

their unique combination of properties is

just beginning to be fully realized in range

of medical diagnostic and therapeutic

applications [6]

Figure 1: Exponential growth in the number of publication on gold nanotechnology and nano-medicine over the two past decades.[6]

To meet the variety of demands, many different methods have been discoveredand practised successfully to make a variety of gold nanoparticles forms such asnanorods, nanocages, pyramid etc Particle size is also adjusted by varying the goldion: reducing agent or gold ion : stabilizer ratio, with larger (and typically lessmonodisperse) sizes obtained from larger ratios

1.2.1 Properties and Applications

In his own paper on News & Views, using only one short sentence "Nanocystals

- Tiny seeds make a big difference"[7], Uri Banin of The Hewber University ofJelusalem has covered all valuable meanings of nanoparticles in general and goldnanoparticles in specific in the Human living Up to now, depending on the specificproperties, the applications of gold nanoparticles can divide in four main parts:catalysis, optical applications, biomedicine and sensors

Catalysis

Nanoparticulate Au catalysts are unique in their activity under mild conditions,even at ambient temperature or less When Au nanoparticles less than ~ 5 nm in sizeare supported on base metal oxides or carbon, very active catalysts are produced One

of the potential advantages that Au catalysts offer compared with other precious metalcatalysts is lower cost and greater price stability, Au being substantially cheaper (on aweight for weight basis) and considerably more plentiful than Pt A huge number ofpublications have exhibited the various application of gold nanoparticles as the role ofcatalyst such as: pollution and emission control, chemical processing of bulk andspecialty chemicals, clean hydrogen production for the emerging hydrogen economyincluding fuel cells, sensors for detecting pollutants [8]

Chemical Processes

Gluconic acid is an important food and beverage additive, and is also used as acleansing agent The German group of researchers has suggested that the oxidation ofglucose to gluconic acid can be maintained at high activity and selectivity using astirred tank reactor for up to 110 days with a nanoparticulate Au on alumina catalystprepared by deposition precipitation with urea and incipient wetness methods (Fig 2)[9]

Figure 2: Conversion of glucose to gluconic acid in alkaline aqueous solution

Another differential of glucose - methyl gluconate, which plays an importantrole as a solvent for semiconductor manufacturing processes, as a building block forcosmetics, and as a cleaner for boilers and metals, has been also demonstrated aprocess using Au catalyst with a capacity tons of month So the methyl gluconate can

be synthesized directly by one - step production:

The first application of Au/Pt catalyst in the vinyl acetate monomers (VAM)manufacturing has been established In the industrial scale, VAM is produced fromethene, acetic acid, and oxygen using Au–Pd catalysts:

In this case, the catalyst is durable and typically lasts for between one and two years.Additionally, the presence of Au leads to a significant increase in space-time yieldcompared with use of Pd alone, and the presence of Au clearly has commercialimportance

Besides, there are a variety of applications in this fields, but these are the mosttypical examples of using gold nanoparticles as catalysts for chemical processes

Pollution control

Au catalysts are highly active for the oxidation of many components in ambientair at low temperatures, particularly CO and nitrogen-containing malodorouscompounds, such as trimethylamine This ability offers scope for applications in airquality improvement and control of odors, be they in buildings, transport, or otherrelated applications such as gas masks

Fuel cells and hydrogen production

For use in a fuel cell, the remaining CO in this reversible reaction must beremoved to prevent it poisoning the Pt catalyst in the fuel cell Au catalysts have beenfound to be effective for this at room temperatureNanoparticulate Au on oxide can beused to catalyze the water gas shift to produce hydrogen from CO and steam:

The requirement of air-quality monitoring demands the development of sensorsthat are selective for the detection of individual pollutant gases It is particularlyconvenient that Au catalysts can operate at ambient temperatures Gas sensors based onnanoparticulate Au have therefore been developed for detecting a number of gases,including CO and NOx Au is also particularly promising for color-change sensors usedfor monitoring components of body liquids

Optical properties - Chemical sensors and Imaging

Strong plasmon absorption and sensitivity to local environment have mademetal nanoparticles attractive candidates as colorimetric sensors for analytes includingDNA, metal ions, and antibodies [10] These visible color changes are due to metalnanoparticle aggregation, which in turn affects the plasmon coupling and induceddipoles Taking the example of use of gold nanoparticles as selective sensors for Li+[11], this solution-based sensor utilizes nanoparticles functionalized with a ligand thatbinds to gold via a thiol at its back end, and a phenanthroline derivative at the front end

to selectively bind to Li+ as a bidentate ligand

Resonant Rayleigh scattering from metallic nanoparticles is a uniquecharacteristic of nanoscale metals Due to the fact of the sensitivity of these plasmonresonant particles (PRPs) to local chemical environment, refractive index, andnanoparticle size and shape, resonant Rayleigh scattering from gold nanoparticles,made by colloidal or lithographic techniques, has been utilizing for biological andchemical analyses [12] Moreover, the applications have been straighten in the elasticlight scattering from metallic nanoparticles that is measured to infer nanoparticleposition, local environment, or (in the case of nanorods) relative orientation

Inelastic visible light scattering from metal nanoparticles is also a useful means

to gain chemical information about the nanoparticle’s environment Surface-enhancedRaman scattering (SERS) is a powerful analytical tool for determining chemicalinformation for molecules on metallic substrates on the 10 - 200 nm size scale Ramanvibrations of molecules are in general very weak; but in the presence of metals (copper,silver, gold) with nanoscale roughness, the molecular Raman vibrations excited byvisible light are enhanced by orders of magnitude

In addition to SERS, surface enhanced fluorescence has also been reported formolecules near the surfaces of metallic nanoparticles While molecular fluorescence isquenched within ~5 nm of the metal nanoparticle surface, at distances of ~10 nm orgreater, fluorescence is enhanced up to 100 - fold by the localized electric field andincreased intrinsic decay of the fluorphore [13].

Golden ages of biomedicine

Gold nanoparticles as intrinsic drug agents

Gold nanoparticles of very small diameters (less than 2 nm) are able to penetratecells and cellular compartments (such as the nucleus) and can be extremely toxic [14].Interestingly, larger sizes of gold nanoparticles with the same surface capping agent,were found to be non-toxic under the same dosing conditions Recently, it was foundthat gold nano-particles (5 nm in diameter) exhibit anti-angiogenic properties (inhibit

the tumorigentic growth of new blood vessels) in both in vitro and in vivo studies.

Because of their comparable size relative to biomolecules and proteins, goldnanoparticles can also interact with and modify physiological processes whenspecifically localized within cells and tissues They have explored similar strategieswhereby gold nanoparticles were found to selectively exert anti-proliferative andradiosensitization effects

Gold nanoparticles in photothermal therapy

Photothermal therapy is a central application of gold nano-particles in medicine.The ability of gold nano-particles to absorb light and convert it to heat is a fascinatingproperty and has been employed to destroy cancer cells, bacteria, and viruses Thus,laser-exposed gold nanoparticles could act as therapeutic agents by themselves andwithout the need for co-conjugated drugs

Gold nanoparticles absorb light with high efficiency (extinction coefficient B10)

in the near-infrared (NIR) region of the electromagnetic spectrum, where attenuation

by biological fluids and tissues is minimal Gold nanoparticles have the advantage ofhigher absorption cross section, higher solubility, efficient absorption at longerwavelengths, and facile conjugation with targeting molecules and drugs Theseproperties make gold nanoparticles promising candidates for photothermal therapy of

cancer and various pathogenic diseases The common use of gold nanoparticles inphotothermal therapy are abundant in the literature is sum up in Table 1.

Table 1: Gold nanoparticles in photothermal therapy applications

Nanoform Particle size (nm) Available region Applications

Gold - silica

nanoshells

110 - 150 Vis - NIR ablating various cancerous

cell lines in vitro and treating

of cancer in animal models in

ablate tumors in mousemodels of colon cancer andsquamous cell carcinoma

agents was demonstrated both

in vitro and in vivo.

nanoshells

therapeutics in both in vitro and in vivo models.

Gold - gold sulfide

nanoparticles

melanoma tumors in vivo.

Gold nanoparticles as drug delivery vehicles

Nanoparticles have been used in exploratory drug delivery applications due tothe following 5 main reasons: (i) the high surface area of nanoparticles provides sitesfor drug loading and enhances solubility and stability of loaded drugs, (ii) the ability tofunctionalize nanoparticles with targeting ligands to enhance therapeutic potency anddecrease side effects, (iii) the advantage of multivalent interactions with cell surfacereceptors or other biomolecules, (iv) enhanced pharmacokinetics and tumor tissueaccumulations compared to free drugs, and (v) biological selectivity which allowsnanoscale drugs to preferentially accumulate at tumor sites due to their ‘‘leaky’’ bloodvessels - the so - called enhanced permeability and retention (EPR) effect Figure 3

focuses and expounds upon the use of gold nanoparticles of different shape/size in drugdelivery applications, each categorized by the previous methods in which their activeagent is loaded and/or released.

Figure 3: Approaches of loading/unloading therapeutics

into/from gold nanoparticles

The illustration describes the partitioning and diffusion-driven release of hydrophobicdrug molecules in (a) a surfactant bilayer or (b) an amphiphilic corona layer Thedriving force for the observed release is re-partitioning of the drug from the polymermonolayer on the gold nano-particles to hydrophobic domains in the cellularmembrane [15] In other case drugs was anchored directly to the surfaces of goldnanoparticles through Au–S or Au–N bonds (c) (capping agent in blue is hydrophilicpolymer,e.g PEG, to enhance the overall solubility of the system) Release is triggered

by the photothermal effect, thiol exchange (e.g glutathione exchange), or simplediffusion to the cell membranes (in the case of Au–N) Additionally, (d–e) is using Au–

S bonding for double-stranded DNA-loaded gold nanoparticles The release of double(d) or single (e) stranded DNA is controlled by an applied laser Using another way,Therapeutic agents are coupled/complexed to terminal functional groups of the cappingagent via a cleavable linker (f) In this case, the gold surface is already passivated withvarious functional groups and the drug attachment proceeds to the outermost layer ontop of the particles Release can be triggered by hydrolysis, light, heat, and/or pHchanges But in the case of (g), charged biomolecules ( e.g DNA or siRNA) can beeasily attached to the surfaces of complementary charged gold nanoparticles byelectrostatic-conjugation or the related layer-by-layer (LbL) coating Release ofpayload can be triggered by the use of charge-reversal polyelectrolytes combined with

pH change Finally, drug molecules are incorporated into the matrix of a sensitive, crosslinked polymer (h) Then, release can be triggered by the photothermalheating by gold nanoparticles also incorporated into the matrix.

thermo-Furthermore, under the appropriate excitement, the loading inside thenanoparticles could be happened by some ways

Figure 4: Loading drugs into the interior of gold nanoparticles

In the detail:

(a) Gold nanocages (hollow gold cubes with porous walls) are functionalizedwith a thermosensitive polymer brush layer at their exterior surface to cage drugmolecules in their interior Laser irradiation induces local heat flux and thus, collapse

of the thermo-sensitive polymer to release the caged drug molecules

(b) Gold nanocages with the drugs dispersed into a thermosensitive material inthe interior of the nanoparticles Laser irradiation results in phase-change (melting) ofthe thermosensitive ‘‘filler’’ and thus enhances drug release

(c) A gold nanoshell covers a liposome carrying drugs in its interior Goldnanoshells absorb light and convert it to heat and these events result in disintegrationand clearance of the carrier, as well as release of its encapsulated drugs

Optional applications

Besides the great potential for gold nanoparticles as drug delivery carriers, theyhave also been used to stabilize and enhance the efficiency of other drug deliverycarriers such as liposomes and microcapsules Moreover, gold nanoparticles wereincorporated in various types of materials to fabricate gold-containing devices for drug

delivery such as thermo-sensitive microcapsules, films, and hydrogels to use as acomponent in composite materials for controlled drug delivery applications The goldparticles were also aiming to target to diseased sites as well

In the conclusion, with their exceptional properties, gold nanoparticles havebeen played one of the most important roles in improving biomedicine In the future,they are also full of promising to apply in developing and optimizing biologicalchemistry

Applications of gold nanoparticles in Vietnam

In fact, the study of gold nanoparticles has not been truly developed in Vietnamdue to a lot of difficulties in required facilities and expenditure However, recently,Institute of Chemistry - Vietnam Academy of Science and Technology has beensuccessful to make electrochemical sensors based on gold nanopartcles In the firstpublication in 2009, they represented electrochemical method - square wavevoltammetric detection using gold biosensors for selective recognition of a proteinmarker The idea was that a short chain peptide would be utilized for the selectiveelectrochemical detection of the protein biomarker, protective antigen (PA), for thediagnosis of Anthrax [16]

Figure 5: Scheme of sensing layer preparation using both peptide and antibody

Actually, the major motivation of using a peptide instead of an antibody for thedevelopment of a biosensor is that there are advantages associated with the smallersize, better biological stability and easy synthesizability of a peptide In theexperiment, PA-selective peptide was synthesized and conjugated on a binding layerpreviously immobilized onto gold electrode.

After that, in 2011, Huan at al continued to published a paper related to gold

nanoparticles But this time, he used 3D gold nanodendrite applying on anelectrochemical sensing for detection of As (III) in ultra low concentration range Theresults showed that the network porous structure of three - dimentional Aunanodendrite could be greatly promising for high sensitive and selective detection ofdiverse biomolecules when a target - specific sensing layer is formed in the surface ofstructure [17]

Figure 6: Gold nanodendrites

In addition, the Au microelectrode sensor has been developed for detection of Hg (II)

in a low concentration as well In short speaking, a simple and reproductile carbonmicroelectrode array (CMA), designed to eliminate diffusive interference among themicroelectrodes, has been fabricated and used as a frame to build a gold nicroelectrodearray (GMA) sensor [18]

1.2.2 Synthesis and Functionalization

A variety of synthetic methods and gold nanoparticles forms

In historical sequence, interest in the shape-controlled synthesis of gold

nanostructures began booming in the early 1990's when Masuda et al and Martin [19]

developed techniques to prepare gold nanorods by electrochemical reduction intonanoporous aluminium oxide membranes However, in the beginning, the obtainednanorods had mono - disperse structures relatively, but because of the low yield andlarge diameter (>100nm), the optical response is difficult to discern and largelydominated by multipolar plasmon resonance modes [20] This negative effects hadbeen improved by Wang and coworker later by electrochemical oxidation of a goldplate electrode in the presence of cationic, quaternary ammonium surfactants (CTAB

or TOAB) The resulting particles synthesized by this method has only ~ 10nm indiameter It also existed many disadvantages needed to upgrade, though

In general based on previous

achievements and in particularly seeded growth

method, Murphy et al and Nikoobakht and

El-Sayed later demonstrated a colloidal growth

method to produce mono - disperse gold

nanorods in high yield (Fig 7) In this method,

small (~1.5 nm diameter) single-crystal seed

particles, produced from the reduction of

chloroauric acid by borohydride in the presence

of CTAB, are aliquoted into Au(I) growth

solution prepared from the mild reduction of

Figure 7: Gold Nanorods [21]

chloroauric acid by ascorbate and the addition of AgNO3 and CTAB As the result,gold nanorods ca 10–20 nm in diameter and up to 300 nm in length can be obtained inrelatively high yield Furthermore, nanorod aspect ratio can be controlled by theseed/gold salt ratio or by the relative concentration of additive impurity ions

Figure 8:Sharpened nanorods [22]

Liz-Marzan and coworkers also showed thatspherically-capped colloidal gold nanorods could bereshaped to form single-crystal octahedra, usingpoly(vinylpyrrolidone) (PVP) functionalized goldnanorods as seeds for the ultrasound-induced reduction

of chloroauric acid by N, N-dimethylformamide (DMF)

in the presence of PVP [22] The authors showed

that by increasing the ratio of gold salt to nanorod seeds, the subsequent morphology

varies from sharpened (octagonal) rods to tetragons to octahedra (Fig.8).

Gold nanocages and nanoframes which

possess desirable optical properties and potentially

cargo-holding hollow structures recently developed by

Xia and coworkers (Fig 9) In this case, gold

nanocages/frames are produced by reacting Au(III)

with silver nanocubes produced from the polyol

reduction of silver nitrate, based on a phenomenon

known as galvanic replacement, whereby more noble

metal ions (e.g Au, Pt) spontaneously oxidize the

surface atoms of a less noble metal (e.g Ag, Cu) with

concomitant reduction of the more noble metal [23]

Figure 9:

Nanocages/nanoframes

Figure 10: Nanoshells [24]

Silica-gold coreshell nanoparticles, or

gold nanoshells (Fig 10), have recently attracted

much attention, for example, Aden and Kerber(1951) or Halas and his coworkers (1998), due totheir interesting optical properties and numerousbiomedical applications In a typical synthesis,silica nanoparticle cores are synthesized by thebase-catalyzed condensation of orthosilicate (i.e Stober hydrolysis) and functionalizedwith an amine - terminal silane Small, anionic gold nanoparticles synthesized from theaqueous reduction of chloroauric acid by tetrakis (hydroxymethyl) phosphonium

chloride (THPC) are electrostatically adsorbed onto the surfaces of the silica cores and added to a solution of mildly reduced chloroauric acid When formaldehyde is added tothe solution, the adsorbed gold particles serve as nucleation sites for the furtherreduction of gold around the silicacore, subsequently forming a conformal nanoshell

Caruso and coworkers obtained hollow gold nanospheres by calcination or

dissolution of polystyrene–gold core–shell nanoparticles (Fig.11) In this case,polystyrene nanospheres were bounded by polyelectrolyte multilayer films and 4 -

(dimethylamino) pyridine (DMAP) stabilized

gold nanospheres (approx 6 nm diameter) were

electrostatically adsorbed to the polyelectrolyte

surface After that, Liang et al showed that

similar structures could be obtained by galvanic

replacement with citrate- stabilized cobalt

nanospheres synthesized from the reduction of

CoCl2 by borohydride under anaerobic

conditions [25] Figure 11: Hallow gold nanosphere

Other reports showed that more geometrically complex gold nanostructures

(100–300 nm in size) could be produced by a modified polyol process ( Fig 12a)[26]

Figure 12: (a) tetrahedra/octahedra/cubes/icosahedra, (b) rhombic dodecahedra,

(c) octahedra

Murphy and Sau later built the high yield synthesis of complex gold nanostructures viaseed-mediated growth methods [31] in which by varying the concentrations of Au(III),ascorbic acid, and presence of silver nitrate in the growth solution, as well as thequantity of added seeds, rectangular, hexagonal, cubic, triangular, and star-likenanoparticles were obtained Moreover, other methods also led to obtained a variety offorms of gold nanoparticles, for instant, rhombic dodecahedral morphology (Fig 12b)and octahedral geometries (Fig 12c)

Figure 13: Nanocubes

Recently, a seeded growth technique analogous

to that used to produce nanorod have been applied byMirkin and coworkers to develop a method to

synthesize monodisperse gold nanocubes (Fig 13), but

using the chlorideanalog of CTAB: cetyltrimethyl-ammonium chloride, CTAC

[28] The advantage ofthis method is that nanocube size could be adjusted simply by varying the amount ofseeds added to the growth solution, obtaining cubes with edge lengths ranging from 38

± 7 nm to 269 ± 18 nm width at high yield up to 95%

Besides, nano - gold has been found in other various shapes and sizes such astetrahexahedra, rhombic dodecahedra, obtuse triangular bipyramids (Fig 14)synthesized by a number of methods

Figure 14: Complex nanostructures

In brief, Table 2 covered approaching methods to make gold nanoparticles:

Table 2: Summary of synthetic approaches to obtain various gold nanostructures

Au nanoform Approached methods Authors

G.Frens (1973)

B Nikoobak & El-Sayed (2003)

Hollow nanospheres

Overgrowth of core - bound particles,

H - P Liang & L Jiang (2005) Nanocages/frames PVP - stabilized polyol, galvanic Y Xia (2002, 2006)

Nanocubes/octahedra

PVP - stabilized polyol, seeded

Icosahedra/tetrahedra

PVP - stabilized polyol, seeded

where: CTAB: cetyltrimethylammonium bromide, PVP : poly(vinylpyrrolidone),CTAC: cetyltrimethylammonium chloride, CPC: cetylpyridinium chloride

The data in table 1 shows the fact that the seeded growth method is one of themost popular methods to prepare gold nanoparticles In this case, instead of directlymaking gold nanoparticles, the nanoseeds, of course with the smaller size, would beprepared before used for making the real size nanoparticles The major reason is thatthe advantages of this method is significant possibility of success, high yield and theability to apply in large scale

Functionalizing

In order to increase the activity and stretching the application range, especially

in biomedicine or biochemical, gold nanoparticles have been studied to be coated andfunctionalized by a variety of materials in which the anchoring groups utilized forattachment of these molecules to the gold surface generally include: thiolate, dithiolate,dithiocarbamate, amine, carboxylate, selenide, isothiocyanate, or phosphine [6]

Depending on desired functional groups, it is noticeable that the bond strengthbetween anchoring groups and the gold surface plays a critical role in determining thesubsequent functionality Besides, packing density and surface energetics also makeequally important contributions

Functionalization of gold nanoparticles follows largely on work initiallyconducted on the formation of self-assembled monolayers (SAMs) [29] of molecules

on planar gold and later in studying the dynamics and conformations of theseassemblies by electrochemical , scanning probe, and mass spectrometric methods [21].Besides, cationic surfactants which are seriously important to control gold nanoparticleshape (e.g CTAB, CPC, CTAC, etc ), appear to adsorb by a different mechanism: as abilayer on the nano-particle surface [30] In the detail , the quaternary ammoniumgroups face the solvent, with a postulated chemisorbed bromide at the gold surface.The obtained data of mass spectrometry and vibrational spectroscopy lead to thesuggestion that Au–Br is present at the surface In fact, this surfactant bilayer can beeasily rests on either replacement or overcoating by other biomolecules Moreover,gold nanoparticles are also coated based on simple physical methods such ashydrophobic–hydrophobic interaction and charge-pairing (Fig 15)

Figure 15: (a) hydrophobic entrapment, (b) electronstatic adsorption[6]

In biochemical, inorganic complexes such as cisplatin or its prodrug forms canalso be datively bound to gold nanoparticle ligands by way of appropriate ligands

Another well - known mechanism is layer - by - layer assemblies It means that thegold nanoparticles will be coated with alternative anionic and cationic materials Theresulting nanoparticles can exhibit tunable surface charge and permit the electrostaticadsorption over a wide range of isoelectric points and solvent pH values.

Silicate coating and Stober's method

In the mid 1990’s, gold

nanoparticles have been proven that

they can be fully encapsulated by

silica (glass) shell by vitreophobic

surface conjugation and facile silane

chemistry (Fig 16) This prospect is

prefered to use with gold nanorods,

where compelling evidence showing

complete removal/displacement of

CTAB molecules from the sides of

the rods has yet to be demonstrated

Figure 16: Silance conjugation of gold

nanoparticle

The further processing of silica nanoparticles allows their use in many fieldsincluding ceramics, chromatography, catalysis, and chemical mechanical polishing[31] Additionally, precursor silica particles have been used in stabilizers, coatings,glazes, emulsifiers, strengtheners, and binders Since the first publication in 1968, up tonow, the main method used to coat gold nanoparticles (of course, mostly goldnanorods) has been Stober colloid synthesis in which the ammonia - catalyzedreactions of silicates [Si(OR)4] with water in low - molecular - weight alcohols [32]

As the content of Stober's publication,

the silica spheres had been synthesized in the

high concentration of ammonia because of the

formation of spherical particles Initially, pure

alcohol or alcohol mixtures, saturated alcoholic

ammonia solution, ammonium hydroxide, and

water were mixed well at strictly definite

concentrations Subsequently, an exact amount

of alkyl silicate was added andthen the mixture was agitated.Normally, after 120 minutes, the

Figure 17: Electron micrograph

of a silica sphere sample

sample was taken for electron microscopic investigation The obtained particles hadsizes in the range from less than 0.5µm to 2µm in diameter The results showed 2 mainconclusions that: first, reaction rates were fastest with methanol and lowest with n -butanol, and likewise, the particles sizes were smallest in methanol and biggest in n -butanol; second, the particle sizes proportionally increased regarding to the reactionrates when comparing results with different alkyl silicates [33]

Based on this research, many studies on Stober particles have empiricallyallowed to predict the final particle size from the range of the initial reactantconcentrations Moreover, recently, two models (monomer addition [34] and controlledaggregation [35]) have been proposed to elucidate the chemical/physical growthmechanism of silica dividing the formation of silica into two events: nucleation andgrowth The monomer addition model supports a mechanism where an initial burst ofnucleation, growth occurs through the addition of hydrolyzed monomers to theparticles surface Besides, the controlled aggregation claims that nucleation happensthroughout the reactions continuously In fact, they are two different approaches todescribe particle growth Taking the example of TEOS, the process to hydrolyze andform silica can be simplified by following scheme:[32]

First, the singly-hydrolyzed form of TEOS [Si(OH)4] is produced in thesolution:

Then the real silica is formed:

1.3 GOLD NANORODS

1.3.1 Optical properties

Gold nanorods also possess all of optical properties of metal nanopaticles whichare characterized by Raman scattering, LSPR, SERS etc In short, an understanding ofthe basic optical properties is important for several distinct reasons On the one hand,small particles may have electronic, crystallographic, mechanical or catalytic propertiesthat are different to the bulk material Such differences may be probed through opticalmeasurements On the other hand, spectroscopic measurements are often the easiestmethods for monitoring surface processes such as dissolution and precipitation,adsorption and electron transfer [36]

When gold nanorods are illuminated at the proper optical frequencies, theconduction band electrons in the gold are excited, resulting in a resonant, coherentoscillation of these electrons This resonance condition leads to extensive lightextinction (the sum of optical absorption and scattering) [37] For gold nanorods, theLSPR band also splits into two, corresponding to a transverse (short axis) band and alongitudinal (long axis) band, whose position depends on aspect ratio (length/width) ofthe particles Previously, by mathematics, small changes in aspect ratio lead to drasticchanges in transmitted colors or the plasmon band appears to be drastically red-shiftedfrom the positions were predicted by the Gans model [36] Then experimentally, as theaspect ratio increases, the position of the longitudinal plasmon band red-shifts, and thetransverse plasmon band position stays relatively constant at ~ 500 - 520 nm ( Fig 18).Thus, the particle shape dictates what wavelengths of light can be absorbed, andelastically scattered; gold nanorods of moderate aspect ratio (2 –5) display plasmonbands with tunable maxima from ~ 700 to 900 nm, and high-aspect ratio nanorodsexhibit a longitudinal plasmon band past 1500 nm

Figure 18: UV- Vis spectra of gold nanorods with aspect ratios: (A) 1.42 ± 0.32,

(B) 1.82 ± 0.49, (C) 2.31 ± 0.55, (D) 2.65 ± 0.43, and (E) 2.80 ± 0.37.

Figure 19: Illustration of changes of gold nanorod colors due to aspect ratio

In general, smaller nanoparticles absorb more light than elastically scatter it; but as theparticle grows in size, it scatters more light than it absorbs (although the absoluteamount of light absorbed increases with particle size as well)

1.3.2 Synthesis methods

Template method

The template method for the synthesis of gold nanorods based on theelectrochemical deposition of Au within the pores of nanoporous polycarbonate or alu-mina template membranes It was showed that the Au/alumina composites can beoptically transparent in the visible and also that by changing the aspect ratio of theprepared nanocylinders; the color of the composite membrane can be varied.Schematically, the method can be explained as follows: initially a small amount of Ag

or Cu is sputtered onto the alumina template

membrane to provide a conductive film for

electrodeposition This is then used as a foundation

onto which the Au nanoparticles can be

electrochemically grown (stage I) Subsequently,

Au is electrodeposited within the nanopores of

alumina (stage II) The next stage involves the

selective dissolution of both the alumina membrane

and the copper or silver film, in the presence of a

polymeric stabilizer such as poly(vinylpyrrolidone)

(PVP) (III and IV) In the last stage, the rods are

dispersed either in water or in organic solvents by

means of sonication or agitation [36]

Figure 20: Scheme of template approach of gold

nanorods

In the same way, though using different membranes, have been successfullyapplied to the synthesis of gold nanotubes , and nanostructured composites, includingtubular composites, which comprise coaxial nanotubes made of different materials[38]

Electrochemical methods

The synthesis is conducted within a simple two-electrode-type electrochemicalcell, as shown in the schematic diagram in Fig 21 A gold metal plate is used as asacrificial anode whilst the cathode is a platinum plate with similar dimensions Bothelectrodes are immersed in an electrolytic solution containing a cationic surfactant,hexadecyltrimethy-lammonium bromide (C16TAB), and a small amount of a muchmore hydrophobic cationic surfactant, tetradodecylam-monium bromide (TC12AB),which acts as a rod-inducing co-surfactant The C16TAB serves not only as thesupporting electrolyte but also as the stabilizer for the nanoparticles, to prevent theirfurther aggregation

Figure 21: a, Scheme of electrochemical approach of gold nanorods b, TEM of

gold nanorods at other aspect ratios obtained by elec method

During the synthesis, the bulk gold metal anode is initially consumed, formingAuBr4- These anions are complexed to the cationic surfactants and migrate to thecathode where re-duction occurs It is unclear at present whether nucleation occurs onthe cathode surface or within the micelles But, actually, the complete mechanism, aswell as the role of the silver ions, is still unknown

Seeded - mediate growth method

Proposed mechanism of gold nanorods growth in the absence of silver ions

Figure 22: Illustration for gold nanorods growth in the absence of silver ion

The mechanism proposes a differential adsorption of CTAB to different crystalfaces followed by thermodynamically favorable intermolecular interactions of the 16-carbon cetyl chains to promote surface adhesion (Fig 22)

More specifically, it was postulated that CTAB adsorbs preferentially to the side

of rods, Au{100} or Au{110}, via chemisorbed bromide counterions, over the ends ofthe rods, which display Au{111} faces This preferential binding regime agrees withthe size of the quaternary ammonium head group relative to the larger binding sitesavailable on the {100} and {110} faces of the crystalline rods The binding of CTAB,ultimately as a bilayer [12], on the side of the rod blocks the deposition of further gold

to the side and promotes the growth from the ends

Figure 23: Proposed formation of gold nanoparticles

It was predicted that the CTAB micellar structure also promoted the deposition

of metal at the tips of gold seed particles that are also surrounded by CTAB [39].Specifically, AuCl4- ions first displace Br- ions and then tightly bind to CTA+ micelles.Addition of ascorbic acid then reduced AuCl4- to AuCl2- at the micelle surface, and therate of growth of the different nanorod facets would be determined by the approach ofthe micelle and thus gold species toward the facets of the gold seed particles that arealso covered with CTAB

Proposed mechanism of gold nanorods growth in the presence of silver

Using a one-step seeding method, which includes AgNO3, one can prepare lowaspect ratio (~ 1–6) gold nanorods starting from 1.5 nm seeds prepared by sodiumborohydride reduction of gold salt in the presence of CTAB It is suggested that therelatively slow growth of rods during Ag+ assisted growth uniquely allows gold atoms

to deposit at energetically favorable locations with no defect character, maintaining thesingle crystalline or twinned structure of the seeds

One suggestion for the growth is that silver bromide complexes play criticalroles during nanorod formation [40] In this scenario, the deposition of AgBr on aspecific facet of gold nanorods during seed-mediated growth stabilizes the rods anddirects their growth by allowing more rapid incorporation of gold on less hindered