During the course of nanomaterials for thernostics gold nanorods were studied. During this generation of gold nanorods with already established protocol is done along with chitosan modified for functionalizing gold nanorods. Also During the semester several papers were studied for literature review on gold nanorods production and applications.
The importance of small particles and studying processes at small time frame, lead many important discoveries. Development of wet chemical synthesis over last 2 decades is outstanding considering the discovery/ first use of it reported in 1923. As we go from macro to nano level the physical as well as chemical properties of metals changes drastically. These changes in the properties could be exploited in various fields. In this report we are going to discuss gold nanoparticles or in particular gold nanorods. Specific chemical and physical properties of the gold nanorods make it very useful in the field of chemical sensing and imaging, drug delivery, and photo thermal destruction of cells. Along with this the gold nanorods are also used in different fields like nanocomposites. Optical properties of gold nanorods make it more attractive.
Usually electronic, crystallographic, mechanical or catalytic properties differ in accordance with size and shape. Such differences can be detected by means of optical measurement, spectroscopic measurement. In addition to this several other methods, interested in single molecule detection and identification are under scrutiny. Also we know that every element and particle size has its distinct surface Plasmon.
The optical property possesses by gold nanorods makes it Interesting. Nanoparticles/nanorods contain metal conduction electrons. These electrons when come in contact with electric field component of electromagnetic radiation leads to strong, characteristic absorption bands in the visible part of the spectrum. These bands are characteristic to the size of the particle. In this section we will discuss how absorption, aspect ratio, shell layer, orientation affects this surface Plasmon of gold nanorods.
In a dilute colloidal solution the rate of loss of proton is directly proportional to the light intensity at a distance x in to the medium, and also to the number density of light extinguishing particles. This relation gives rise to absorbance. Nanorods are modeled by using ellipsoids. There are many numerical solutions discussed for different shapes in review articles used. By studying these articles one can say that absorbance ultimately depends on size shape and surface properties. Thus one can use this property to understand the controlling parameter in synthesis. This can give a control over the aspect ratio of the nanorods; also it can give important insight in the process mechanism.
Absorbance depends on the extension cross section of a single particle along with no of particles per unit volume. The extension cross section of single particle in turns depends on dielectric function, wavelength and dimensions of the particle. When we speak about aspect ratio, it gives us information about dimensions. Thus one can say the absorbance and hence surface Plasmon depends on dimensions. The value of extension scales cubic in radius considering a nanoparticle. Thus a small change in aspect ratio can result in big change in resonance.
One of the important parameter is dielectric properties of the medium as well as the layer. Different medium has different refractive indices. A change in refractive index results a change in dielectric properties of the medium. This change results in dramatic shifts in spectra. When using these nanorods for biosensors or as targeted delivery the coating of the nanorods plays important role. The coating usually shifts the spectra thus changing the optical properties. The reason for this is the same as that for medium. The coat of the nanorods usually has different dielectric properties.
In addition to above parameters orientation of the nanorods can also affect the resonance.
Synthesis and Mechanism:
Synthesis of gold nanorods can be done through various processes. Out of them wet chemical synthesis offers great control and mono-dispersity. Wet chemical synthesis has three main types, viz. template method, electrochemical method and seed mediated synthesis method. Seeded growth synthesis method is the one which is used during this semester.
Template method uses nano-porous membrane (polycarbonate or alumina) to electrochemically deposit the gold nanoparticles in its pores. The size and shape can be controlled by controlling porosity of the membrane. The nanorods thus obtained can be dispersed by dissolving membrane in to proper solvent and then stabilizing with the help of appropriate solvent. The problem with this process was yield of nanorods was low as only a monolayer of rods is formed every time.
The problem of yield is solved by electrochemical method. Electrochemical method essentially uses a electrochemical cell to produce nanorods. Sacrifice electrode (Au electrode) and a platinum electrode is dipped in to electrolyte solution. Electrolyte solution contains cationic surfactant (CTAB), small amount of tetradodecyleammonium bromide (much more hydrophobic that CTAB).
Name
Reference
Comment
One step photochemical strategy
Ahmed2010
Wet chemical synthesis, uses photo initiator for reduction of Au, takes about a minute for the production of GNR
X-ray irradiation
Cai2010
Uses X-rays GNR production, The shape is controlled by modulation of intensity of x-ray irradiation.
Direct high yield synthesis
Wu2006
Uses CTAB in seed solution as capping agent and nitric acid in the growth solution, avg. aspect ratio-19
One pot synthesis by ultrasonic irradiation
Okitsu2009
Under low pH GNR is achieved by sono-chemical reactions.
The TCAB acts as rod reducing co-surfactant whereas CTAB serves as supporting electrolyte as well as stabilizer. Sonication is important part of the process. In addition to this to control the aspect ratio sliver rod is gradually immersed in the electrolyte behind platinum electrode. The concentration of silver ions determines the aspect ratio of gold nanorods.
Seeded growth method is reported back in 1920s. The process is developed to give mono-disperse nanorods with high yield. In the process seed solution is prepared by reducing Au ions. This seed solution is then used with growth solution to produce gold nanorods by slow reduction. The seed solution can be reduced by using a strong reducing agent such as hydroxyl amine, sodium citrate. It is found that when hydroxyl amine is used 5-10% of colloidal gold rods are obtained. It is important to avoid additional nucleation in growth solution. The parameters governs the nucleation is rate of addition of reducing agent to metal seeds, metal salt solution and chemical reduction potential of reducing agent. During secondary nucleation in growth phase the growth conditions are controlled by using ascorbic acid as reducing agent. Ascorbic acid does not start reduction unless there are no seeds present. This makes it possible to produce nanorods of tunable aspect ratio. Further presence of silver ion in small amount ensures small uniform gold nanorods with high yields. It’s said that silver nitrate affects yield, crystal structure, morphology and optical properties. While doing synthesis without silver nitrate pH of the solution governs aspect ratio and yield. Also the process is step by step. The growth solution is added in steps to increase yield, aspect ratio.
The mechanism for the production of gold nanorods is zipping mechanism according to Murphy and coworkers. The zipping mechanism suggests that van der Waals interaction between surfactant tails within the surfactant bilayer on the gold surface may promote the formation of longer nanorods from more stable bilayers. The yield in this case depends on stability of seed, temperature and nature of concentration. Also the length of the chain/tail in the CTAB matters in the formation of gold nanorods in terms of yield and length of nanorods.
Perez-Juste et al. proposed an electric field directed growth of gold nanorods in aqueous solution. It is observed that colloidal stability of the seeds increases the yield meaning coalescence is not the phenomena by which the nanorods form. Also addition of NaCl, NaNO3 or NaBr reduces the aspect ratio of nanorods thus it is concluded that increase in the ionic strength produces decrease in the yield of rods. Extra seed solution decreases aspect ratio. Higher temperatures decreases yield gradually. Also presence of CTAB directs Au to tip and drastically retards the rate of metallic gold formation. Based on these observations theory postulates that during typical microelectrode deposition, electrons are transferred to the gold particle while adsorbed AuCl2- ions may pic up electrons at any favorable adsorption site. The rate of nanorods formation depends on the frequency of collision of AuCl2- laden cationic micelles with cationic gold seed particles. The interaction is controlled by electric double layer.
The effect of the addition of silver ions on the synthesis of gold nanorods is profound. The mechanism through which silver ions work is unknown. There are two different theories which try to explain the role of silver ion. Silver ions can only be reduced in basic environment. As the solution is acidic in nature the silver ions tends to form pair with bromide ion giving Ag-Br pairs. This pair reduces charge density on bromide ion. The repulsion between neighboring head groups on the gold surface results in elongation. The second mechanism supposes the CTAB structure to be rigid. This helps in maintaining one dimensional growth and slower gold reduction. This slower growth induces single crystalline growth of nanorods.
Along with the methods discussed above there is continuous search for new methods. One reason behind searching for new method is cytotoxicity of the CTAB bilayer. Thus there is focus on creating nanorods with biocompatible surfactants like CTAB, Surface modification of the gold nanorods or using methods different than wet chemical synthesis. Some of the methods are given in the table 1 along with references and comments.
Applications:
While going through properties of gold nanorods we have learned about surface Plasmon possessed by them. This resonance gives the distinct property to the nanorods. Gold nanorods have strong surface Plasmon near infrared region. And infrared light penetrates deeply in the tissue. This property is exploited for bioimaging and nanomedicines. But for using GNR for these purposes it isalso important to functionalize and detoxify them. The functionalization can give GNR a power to be region specific. It is usually done by using thiolated chitosan or PEG glyalation. In the experiment conducted in the lab thiolated chitosan is used for functionalizing the GNR. Affinity of Au for sulfur plays important role. To these functionalize GNR a specific antibody can be attached which is usually target specific. This method is usually used for photothermal therapy of cancer and for imaging. With the help of the GNR it is possible to image single molecules too. For photothermal therapy the GNR are excited with the help of Laser.
The detoxification is also major issue as it is found that CTAB stabilized GNR shows high cytotoxicity. To overcome the problem usually synthesis method of GNR is changed. There is also interest in developing surfactant similar to CTAB but nontoxic. Murphy and coworkers have found one such surfactant. Also to get rid of this problem chitosan or PEG partially replaces the CTAB making it more biocompatible, but this rises the problem with the stability of GNR in solution. One other approach found was using polyelectrolyte coating.
To use the GNR it is essential to use different coatings, functionalisation according to the uses of GNR. In addition to these particular applications GNR are also used in optics, surface coatings and electronics. In surface coating a coat of gold nanorods can be used to filter out the radiations of desired frequency. This is particularly useful in optics. Following table shows different applications.
Name
Reference
Remarks
Glucose and biotin functionalized GNR
Basiruddin2009
Detection of proteins
Silica coated GNR, PEG peptide modified GNR,
Coumarin-PEG-Thiol GNR
Chen2010, T Niidome 2010, Shenoy 2005
Photo acoustic imaging, Image guided photothermal therapy
Polyelectrolyte coated GNR
Huang 2009
Induces stability and low toxicity, good platform for Theranostics
Biosensors by using antibody (e.g. For hepatitis B HBsAb antibody is used)
Wang 2010
Detection of Hepatitis B in Blood serum, plasma and buffer
