Gold Nanoparticles in Biodetection: Revolutionary Advances and Applications
Creators: name: Hussain, Zahid
Publisher: name: Zenodo
Year: 2024
Subjects: Gold Nanoparticles
Biodetection:
Revolutionary
Advances and Applications

description: Gold Nanoparticles in Biodetection: Revolutionary Advances and Applications

 

Abstract

Gold nanoparticles (AuNPs) have emerged as versatile tools in biodetection assays due to their unique optical and electronic properties, which can be tailored by manipulating their size, shape, surface chemistry, and aggregation state. These nanoparticles exhibit remarkable surface plasmon resonance (SPR), where oscillations of free electrons resonate with visible light, leading to distinct colorimetric changes. This phenomenon underpins their use in various high-sensitivity detection methods. Small gold nanoparticles (~30 nm) absorb blue-green light, reflecting red, while larger particles shift the SPR to longer wavelengths, producing colors ranging from purplish to translucent. Such tunable optical characteristics enable their application in organic photovoltaics, therapeutic agents, drug delivery, catalysis, and sensory probes. Specifically, in biodetection assays, AuNPs facilitate rapid, precise, and cost-effective detection of biological molecules, enhancing diagnostic capabilities in medical and biological research. This study explores the underlying mechanisms and highlights the potential of gold nanoparticles in advancing biodetection technologies.

 

 










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 INTRODUCTION

Colloidal gold nanoparticles have been used for centuries by artists because of the vibrant colors that are produced by their interaction with the visible light. Even more recently, these unique optical-electronic characteristics have been studied and used in high technological applications such as the organic photovoltaics, therapeutic agents, sensory probes drug delivery in the biological & medical applications, as well as electronic conductors and catalysis. Amazingly, the electronic and optical properties of gold nanoparticles are tunable by altering their size, shape, surface chemistry and aggregation state. (33)

The Optical and Electronics Properties of Gold Nanoparticles

Gold nanoparticles interact with light and this interaction is strongly controlled by their size, environment and the physical dimensions. The electric fields of a light ray propagating close to colloidal nanoparticles interacts with free electrons which causes a concerted oscillation of the electron charge which is in resonance with frequency of the  visible light. These resonant oscillations are called surface plasmons. (34) For small (~30nm) gold nanoparticles, the surface plasmon resonance causes an absorption of the light in blue-green portion of the spectrum (~450 nm) whereas the red light (~700 nm) is reflected, which yields a rich red color. As the particle size increases, the wavelength of the surface plasmon resonance related absorption shifts to the longer and redder wavelengths. Red light is absorbed, and the blue light is then reflected, giving solutions with a light blue or purplish color (as shown in Figure 1). As the particle size continues to increase towards the larger limit, surface plasmon resonance wavelengths shift into the Infrared part of spectrum and the most visible wavelengths are then reflected, hence this gives the nanoparticles a translucent or clear color. It is easy to alter surface plasmon resonance by altering the size or the shape of nanoparticles, which leads to particles with required optical properties for various applications. (6)(36)

 Figure 1: Colors of different sized mono-dispersed gold nanoparticles

(http://www.sigmaaldrich.com/materials-science/nanomaterials/gold-nanoparticles.html)

The aforementioned phenomenon is also seen if excess salt is added to gold solution. The surface charge of gold nanoparticle gets neutral, which causes the nanoparticles to aggregate. Hence, the solution color alters from red to blue. In order to minimize aggregation, versatile surface chemistry of gold nanoparticles permits them to get coated with small molecules, biological recognition molecules and polymers. This surface modification allows the gold nanoparticles to be utilized widely in chemical, bioengineering & medical applications. Some of the typical properties of gold nanoparticles are presented in table the below. (45)(47)

 






Diameter



Nanoparticles/ml



Peak SPR Wavelength



Molar Ext M-1cm-1)












2






  





5 nm



5.47 x 1013



515-520 nm



1.10 x 107



Surfactant Stabilized





PBS





10 nm



5.98 x 1012



515-520 nm



1.01 x 108



Surfactant Stabilized





PBS





20 nm



6.54 x 1011



524 nm



9.21 x 108



Surfactant Stabilized





PBS





30 nm



1.79 x 1011



526 nm



3.36 x 109



Surfactant Stabilized





PBS





40 nm



7.15 x 1010



530 nm



8.42 x 109



Surfactant Stabilized





PBS





50 nm



3.51 x 1010



535 nm



1.72 x 1010



Surfactant Stabilized





PBS





60 nm



1.96 x 1010



540 nm



3.07 x 1010



Surfactant Stabilized





PBS





80 nm



7.82 x 109



553 nm



7.70 x 1010



Surfactant Stabilized





PBS





100 nm



3.84 x 109



572 nm



1.57 x 1011



Surfactant Stabilized





PBS





150 nm



3.60 x 109



Not Measured



-



Surfactant Stabilized





200 nm



1.9 x 109



Not Measured



-



Surfactant Stabilized





250 nm



7.1 x 108



Not Measured



-



Surfactant Stabilized





300 nm



4.5 x 108



Not Measured



-



Surfactant Stabilized





400 nm



1.9 x 108



Not Measured



-



Surfactant Stabilized





 










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 Gold nanoparticles mostly have sizes ranging from 1-100 nm. From the figure 2 given below, it can be seen that these dimensions are quite similar to those of many cellular objects, including DNA, viruses & cell surface receptors. (39)

 

Figure 2: Comparison of size of DNA and Nanoparticles

(neno-tech-views.com)

Metals (like gold) are good conductors due to the fact that their electrons are not bound to the atoms rather they form “clouds” around the atomic cores. This cloud of electrons is hence mobile and allows the metal to transport the charges/electrons easily. Metals are also shiny. This is due to the light being reflected off by their surfaces back to the eye. This reflection is due to the electron cloud which surrounds the metals. (38)

Photons of light can’t be absorbed by atomic cores because they are hindered by the electron clouds. As a result, these photons are reflected back to the eye and produce the shine that is associated with metals. (28)

From quantum mechanics, we know that the electrons can behave as a wave or a particle. If we compare electrons in the electron cloud to a wave with certain energy value, we can imagine a situation where it is possible for the light of the same wavelength to get absorbed by the electron cloud, which produces resonance. This is quite similar to what happens on stringed instruments as well when a vibration occurs which matches the natural length of either the string or to one of its harmonics. (17)(44)










4






  

Figure 3: Depiction of resonance in light

(http://en.wikipedia.org/wiki/Surface_plasmon_resonance)

Similarly, when a metal absorbs the light of a resonant wavelength, it causes electron clouds to vibrate, hence dissipating the energy. This process occurs usually at the surface of a material and therefore is called the surface plasmon resonance. Plasmons are actually the name for oscillations of electron clouds. (23)(28)

Hence, there are certain wavelengths for metals where the photons are not reflected, but rather they are absorbed & then converted into the surface plasmon resonance (i.e. electron cloud vibrations).  For “normal” metals, such as the gold jewelry, these wavelengths are in the IR part of the spectrum (Figure 4- wavelength > 800 nm).  So, these wavelengths fall outside the visible range (that can be seen with the eye), and the metal hence appears to reflect off most of the light and appears shiny. (8)

But contrarily nanoparticles have high proportions of their substance at the surface. More the surface area more would be the potential for surface plasmon resonance. (44)

 

Figure 4: Wavelengths and their frequencies (http://en.wikipedia.org/wiki/Image:Electromagnetic-Spectrum.png)

The small nanoparticles absorb light in blue-green part of the spectrum (about 400-500 nm) while the red light (~700 nm) is reflected off, which yields a rich red color.  








5

















6






  

Figure 5: Change in colours of gold solutions with increasing particle size (http://www.horiba.com/es/scientific/products/particle-characterization/applications/nanoparticles/gold-nanoparticles/)

These properties have since been used to make biosensors. The small gold nanoparticles appear to be red; however, when the particles aggregate the plasmon resonances combine. The particles hence appear as a large particle rather than separate ones. The plasmon resonance associated-absorption wavelengths shift from blue to red, & the reflected light shifts from red to blue colour. Hence particle color changes from red to blue upon aggregation. (31)

Gold nanoparticles can be conveniently made by the following reaction:

Aurochloric acid + sodium citrate → gold nanoparticles

  (Yellow solution) + (clear solution)

 

Shapes of Gold Nanoparticles:

Surface plasmon resonance peaks of the gold nanostructures can be changed from the visible to near IR region by controlling the structure (solid or hollow) & shape. There are four typical example of gold nanoparticle structures; nanospheres, nanorods, nanoshells, and nanocages. This optical tunability allows various biomedical applications. (42)

Gold nanospheres

Nanoparticles that are prepared in a solution phase tend to being spherical shape because this shape has the smallest surface area relative to the objects of other shapes. They are multiply twinned particles with almost rounded profile and facets on their surface. In a way these particles can also be called quasi-spheres. One of the most famous protocols for making gold nanospheres is the reduction of HAuCl4 by citrate in water, that was first described by Turkevitchin 1951 and is commonly called the ‘‘Turkevitch method’’. In this method, the citrate serves both as a reducing agent as well as an anionic stabilizer. It gives 15 nm nanospheres with narrow size distribution by the following reaction: (44)










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 6AuCl4- + C6H8O7 + 5H2O →6CO2 + 24Cl - + 6Au 0 + 18H+

Gold nanorods

The shape of a particle can greatly change its surface plasmon resonance properties. For the nanorods, the boundary conditions given in the Mie theory have to be modified in order to describe their plasmonic properties. But, it is not possible to obtain the analytic expressions for particles with such an arbitrary shape. Gans altered the Mie theory in 1912, and predicted that for a rod-shaped particle, the SPR band should split into 2 modes because of the different orientations of the rod shape with respect to electric field of the incident light.(40)

Gold nanoshells

In 1989, based on the calculation, Neeves and Birnboim hypothesized that a spherical particle consisting of metallic shell & a dielectric core can give rise to SPR modes with the wavelengths tunable over a wide range of electromagnetic spectrum.(44)

The SPR peaks of gold nanoparticles were first produced by Halas and co-workers in the near-IR region by the coating of silica beads with the gold shells of different thickness. Contrarily to gold nanorods, the plasmonic properties of gold nanoshells are extremely sensitive to small changes in size (i.e., shell thickness). The dependence of the position of SPR peak on the shell’s thickness becomes weaker when the shells get thicker. Gold nanoshells are usually made by directly depositing gold onto the silica colloidal spheres. (44) (31)

Gold nanocages

Gold nanocages give a novel class of the nanostructures that developed by the Xia group recently. Their SPR peaks can be altered to the near-IR region by controlling the porosity & thickness of the shells. They can be utilized as the carriers for drugs and hence have applications in drug delivery and controlled release of drug. Moreover, the hollow interiors can hold very minute substances such as the magnetic nanoparticles used to construct multifunctional hybrid nanostructures. (31)

Though the gold nanocages may have a complex structure, they can be easily made by templating against the silver nanocubes through the famous galvanic replacement reaction in a solution. The silver nanocubes are first synthesized using a polyol process, where a polyol (e.g. ethylene glycol) serves as a solvent as well as a reducing agent. In a typical reaction, AgNO3 is reduced by ethylene glycol at a high temperature to make silver nanocrystals/seeds. More Ag atoms are then added to these seeds as the AgNO3 is continuously reduced. (42)

Technical problems associated with gold solutions










8






 The availability of lateral-flow assays is greatly restricted by the availability of gold particles that are coated with appropriate receptors. Without going into much technical detail, this availability problem arises from two issues: firstly the chemistry involved in dressing up the gold, and secondly the problems that are associated with the scale-up of gold solutions into reliable industrial production. Another closely associated problem is the making of the solution that carries the Au particles along the membrane of nitro-cellulose in a way that reduces the background noise and enhances the gold reaction.(42)

The storage, shipping, packaging & handling can cause problems with current gold solutions that are of 1-OD concentrations. Stability over time also turns out to be an issue. Gold also loses its red color when its reduced gold chloride comes out of metallic lattice. A variety of reactions can cause this problem to happen. One of the possibilities is that gold chloride continues to be reduced, such that the gold-chloride becomes metallic gold plus HCl. A second possibility is that a metal chelating agent, (such as EDTA), takes the gold ions out of gold lattice. Trace contaminants on the glass’s surface may possess the reduction properties. Contrarily, the glass lattice may also serve to chelate the gold ions. Moreover, air itself may also participate in the decomposition of gold. The macromolecules can also suddenly lose the activity as their concentration approaches to zero. (41)

APPLICATIONS

DIAGNOSTICS:

·         Determination of interleukin-6 

An electrochemical immunoassay flow-injection system based on a disposable immunosensor is used for the determination of interleukin-6 (IL-6). This immunosensor was prepared by entrapping an antibody, horse radish peroxidase (HRP)-labeled IL-6, into gold nanoparticles-modified composite membrane at a screen-printed electrode composed of graphite. The immunosensor is inserted in the flow system, without any sample, and then the injected sample, having IL-6 antigen, exhibiting immunoaffinity reaction with the immobilized HRP-labeled IL-6 antibody. As a result, the antigen-antibody complex is formed, which would be partly inhibited by the active center of HRP, & would decrease the immobilized HRP to H2O2 reduction. The flow-injection immunoassay system could automatically control the period of incubation, its washing as well as measurement steps with adequate reproducibility and efficient stability. Furthermore, the suggested immunosensors were used to analyze IL-6 in the human serum samples. Analytical results obtained from the clinical samples show a developed immunoassay having a promising alternative approach for detecting IL-6 in the clinical diagnosis.(9) (25)

·         Immunoassays pregnancy test

The most familiar example of nanoparticles in sensing is the home pregnancy test. Some brands are using a gold nanoparticle aggregation, for creating a colorimetric response. Nanoparticles (< 50 nm) are bound to antibodies complementary to a hormone produced by pregnant women. Latex microspheres are also bound to antibodies for the hormone. When the stick is submerged in urine flow, if the respective hormone is present it would bind to the microspheres (~ 500 μm) and nanoparticles would cause an aggregate formation. Then the solution would pass through a filter paper. If the hormone for pregnancy is present, the aggregates would be obtained as a residue producing a colored product. If the hormone for pregnancy is absent the nanoparticles would pass through the filter, due to their small size. (46)










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 Although this test does not rely explicitly on plasmon resonance to create the signal, the deep red color of the nanoparticles employed results directly from this feature. Other tests that do take advantage of surface plasmon resonance changes include tests for the detection of DNA. For example, one of the DNA test looks for some bases. In that test, nanoparticles begin as large aggregates, which are blue. If complementary DNA base appears, the nanoparticles bind to that base instead of each other and the aggregates dissolve producing a deep red color. (46)

 

Figure 6: Positive appearance of bands in test strips

(homehealth-uk.com)

·         Detection of biomarkers in the diagnosis of cancers

Feasibility studies of a new test for quick cancer detection that were developed at the University of Central Florida, have made it apparent that the test is greatly susceptible, simple, and is able to provide major developments in early diagnosis of cancer. This test uses the principle of nanotechnology called nanoDLSAY, and it is known to be the first test to use dynamic light scattering and gold nanoparticle probes to detect cancer biomarkers. The light scattering properties of gold nanoparticles are much more than the biomolecules & the polymer particles, and hence they can be found by the dynamic light scattering with high sensitivity at minute levels. Thus this type of highly sensitive tests would be very valuable for an early cancer diagnosis and as well as for monitoring cancer recurrence after surgery. (22)(37)

The in vitro procedure takes in the exposing of sample of the patient's blood or serum to the gold nanospheres covered with antibodies against a particular biomarker for cancer. These nanospheres adhere to and gather around the biomarkers in the sample. (4) A beam of light comes from the sample & by making the measurements of the scattered light can help to calculate the size of nanospheres in solution, and also the concentration of the cancer biomarker. The test is simple & involves a single-step procedure which requires only a very small amount of sample. It is not much expensive and is also very time-saving, usually within a few minutes to half an hour, in either the point-of-care facilities such as in the hospitals & clinics central clinical or the testing labs.(10)(22)










10






  

 

Figure 7: Gold nanoparticles accumulating in cancerous tissue

(http://www.nanomedicine.dtu.dk/Research/Gold.aspx)

 

In in vitro diagnostics, the platform technology is also used for research, drug screenings and developments by the pharmaceutical industries. Although the method was developed at first for screening prostate and other cancers, but it can also serve for other diseases such as infectious & sexually passed on diseases, neurodegenerative disorders, and cardiovascular diseases; and for unlawful performance drug monitoring and biodefense.(14) (29)

The use of gold in cancer treatment is reported back to about 1955. Recently, some new treatment technologies that are under development actually rely on ability of gold nanoparticles to aggregate in a cancerous tumor by passing through the leaky vessels of the blood that attaches with the tumor. So, when injected to a patient, there is a way, through which an intoxicating anti-cancer compound adhered to a gold nanoparticle, can be properly and directly delivered to tumor, leaving the healthy body tissue. Such efficient drug delivery system with less toxicity is considered to be a major innovation, restraining the side effects such as lowered immunology and hair loss.(22)

DNA DETECTION:

A very sensitive but yet uncomplicated way for detection of differences in DNA strands using the metal nanoparticle solutions has been successfully developed by Roejarek Kanjanawarut and Xiaodi Su from the Institute of Materials Research and Engineering, Singapore. This method needs no alteration of the surfaces of nanoparticles; this makes it on the whole fast, effective and flexible to apply. The work of the researchers is based on the principle that aggregated and dispersed nanoparticles have varying optical properties which make the solutions of different colors. Thus consequently, the test results can be shown in minutes, and can be recorded easily by the naked eye for proper quantification by a standard spectrometer. (3)(49)










11






  

Figure 8: The schematic diagram shows the change of the PNA to a PNA–DNA complex (on top) and photographs (at bottom) of Au nanoparticle solutions with addition of a PNA–DNA complex (left) and PNA (right).The nature of nanoparticles to aggregate when in a solution is considered a drawback, & in the previous approaches for using them for sensing, DNA strands were directly linked to the particles’ surfaces to prevent them clumping together. (http://www.sciencemag.org/content/289/5485/1757.short))

These researchers utilized the gold and silver nanoparticles which have a coating composed of negatively charged molecules. The charges then pushed one another in opposite direction, to prevent the nanoparticles from forming aggregates in a solution. Then they added the peptide nucleic acid (PNA) probes to this solution, which attached to the negative surfaces, protecting the charge, as well as enabling the nanoparticles to form aggregates.(24) (43)The solution containing gold nanoparticles changed from red to a dark purple. PNA can be changed to bind selectively to the DNA strand which is the target for recognition. After addition of a target DNA strand to the solution, it adhered to the PNA, which had made a PNA–DNA complex. The resulting solution would stay bright red, which would be clearly different from aggregated dark purple solution. (20) In this way even a single mismatched DNA base when compared to the target strand can be detected. This color difference results from the production of the PNA–DNA complex on the surface of the nanoparticle, which is actually negatively charged, and due to the negative charge borne by  DNA chains; this causes the nanoparticles to repel one another and hence take much longer to aggregate. (12) (18)










12






  

Figure 9: Changing colour with increased binding of targets

(http://www.nature.com/nprot/journal/v2/n6/full/nprot.2007.201.html)

SENSORS:

·         Gold Nanoparticles used to detect Contamination of Milk

 

Figure 10

(http://onlinelibrary.wiley.com/doi/10.1002/anie.201202350/abstract)

 

In Australia, researchers have developed a biosensor that can detect very small traces of contaminants in solutions within just about 40 min. This is known as a biochemiresistor, and it meets a challenge to make a sensor which isn’t only hyper-sensitive to the presence of the chemical compounds but also responds quickly. It’s most likely uses are the detecting drugs, toxins, and pesticides for the analysis of medical or environmental problems.(5)

In a paper published in the journal AngewandteChemie, the researchers described how effectively they tested the new sensors, by detecting the tiny traces in the milk of a veterinary antibiotic, such as enrofloxacin. The antibiotic Enrofloxacin, is utilized in agricultural industry which can be transferred to the food chains. The biochemiresistor was capable of detecting enrofloxacin in milk, in about 40 minutes, only about one nanogram in about a liter of milk. (32)










13






 A biosensor is simply a portable analytical device which uses the biological molecules to detect selectively a single compound within a mixture of many others. Such small biosensors are being used in daily testing the portability of water, and also for checking the blood-sugar levels of a diabetic, as well as for pregnancy tests.(30) (32)

 

 

Figure 11: Detecting traces of veterinary antibiotic enrofloxacin in milk

http://onlinelibrary.wiley.com/doi/10.1002/anie.201202350/abstract

 

Magnets are utilized to assemble the nanoparticles, coated with the antibodies complementary to enrofloxacin, between the two electrodes: when the antibodies would leave the nanoparticles, the electrical resistance would fall down. The biochemiresistor brings in use the Au-coated magnetic nanoparticles altered with the antibodies that are selective for the analyte of particular interest. These nanoparticles are dispersed in the sample for analysis & if the analyte is present then some antibodies detach from the nanoparticles.(32)

The nanoparticles are then assembled into a film, using a magnet, between two electrodes & the electrical resistance is then measured. The more the amount of analyte is there, the more the antibodies would detach from the nanoparticles, & the lower would be the resistance in the nanoparticle film. (5)

·         Use of gold nanoparticles in biodetection of Pathogens

Gold nanoparticle-based probes have been used in the identification of many pathogenic bacteria in a system that is chip-based. (1)The bioassay contains a captured DNA strand that is ineffective on a glass chip, and it then recognizes the DNA that is required. A different sequence on target strand that has been captured is then labeled with an oligonucleotide-functionalized probe of gold nanoparticles. After the catalytic reduction of the silver onto the gold nanoparticle surfaces, for amplifying the target signal, the captured target nanoparticle sandwich can be seen with a flatbed scanner.(26) Detection of genomic DNA from Staphylococcus aureus has also utilized gold nanoparticles, without enzymatic amplification, at 33fM concentrations. The assay actually depends on the target-selective binding of the gold nanoparticle probes to the respective regions of the target DNA sequence in solution that would lead to the transfer of a vanishing illuminated glass microscope slide for light scattering dimensions. (50)While the gold particle probes disperse the orange light in the presence of target, and the probes disperse the green light if the target is missing, the target concentration can thus be calculated by measuring the intensity of the light that is dispersed. The detection of different viruses and bacteria that is meant to be a multiplexed system uses the gold nanoparticles surface functionalized with an oligonucleotide that is target-specific and is also programmed with a Raman-active dye molecule, is also been reported. While the complementary sequence of probe gives the specificity for target sequence, that is required, the target presence can be shown by silver staining and the characteristics of the target are revealed by observing the surface-enhanced Raman scattering (SERS) of Raman dye close to the nanoparticle surface. This particular assay demonstrated a sensitivity of about 1 fM target concentration.(31) (48)










14






 Gold nanoparticles have also been used in Mycobacterium tuberculosis detection. The nanoprobe solution shows up in a red color due to the surface plasmon resonance at absorbance peak of about 526 nm. At high NaCl concentrations, the nanoprobe’s aggregation in absence of complementary DNA sequence makes the solution purple (which indicates an absorbance peak shift towards the longer wavelength). In the process of a specific probe’s hybridization to its complementary sequence (such as DNA from M. tuberculosis), no nanoprobe aggregation occurs, and hence the solution remains red. (21)

·         Detection of Pollutants using gold nanoparticles

Gold nanoparticles (AuNP)-based sensors have the potential to detect toxins, heavy metals, and inorganic or organic pollutants in water rapidly with high sensitivity, and they are expected to play an important role in environmental monitoring. Perchlorate, is a chemical that poses serious health effects, and is also been detected in ground water, surface-level water, soils, and also in plants. (2) At present, perchlorate detection has been done through ion chromatography or IC-mass spectrometry, a process in which a long sample preparation and analytical time is required, in a laboratory. The latest ORNL method for the surface improved Raman spectroscopy (SERS) involves manufacturing gold metal nanoparticles that are customized with specific amine groups. This method can be used in the respective field, alongwith portable Raman spectroscopy.This method is not only sensitive but highly specific because of the unique Raman vibrational bands of perchlorate and other environmental pollutants. The gold nanoparticles are customized with amine groups such as 2-(dimethylamino) ethanethol or trimethylaminethiol and then are used to interact with, and also identify the specific pollutants of interest e.g. perchlorate, nitrate, or a radioactive sample such as uranium. The strength of the dispersed signal is measured, and the amount of pollutant material present is detected. (2) (46)

Benefits:










15






 • It greatly reduces the analytical time and cost • It is highly sensitive for routine monitoring of perchlorate • It has quite good selectivity • It requires in situ, real-time monitoring in the field with no necessary pretreatment of samples • It has a rapid turnaround of data and decision making applications and industries

• It monitors pollutants, such as perchlorate 

PROBE:

Gold nanoparticles for Surface Enhanced Raman Spectroscopy (SERS) Applications 

Raman spectroscopy identifies & detects the molecules based on the vibrational energy they possess. Generally, Raman scattering is a weak technique that confines the sensitivity of this particular technique. However, when the molecules are placed close or on the gold nanoparticles, signals from Raman scattering can increase by factor of 105-1015. This increase is considered to be due to contact between the surface plasmon of gold nanoparticles &incident light from Raman active molecule. Signal augmentation relies upon the shape and size of the nanoparticles. Gold nanoparticles coated with silver may further increase the sensitivity of this method. Detection of a single molecule is possible using the SERS & several proteins, nucleic acid biosensors that are based upon this principle.(7)(16)

The Detection of Hg2+ Ions by Fluorescence Polarization Enhanced with Gold Nanoparticles

Thymine–Hg2+–thymine coordination is used as basis for a selective assay which detects Hg2+ ions with a limit of up to 0.2 parts per/billion. This detection is based on the enhancement of fluorescence polarization (FP) by the gold nanoparticles, Figure 12. The overall assay can hence be carried out at room temperature within only 10 minutes, making it suitable for the high-throughput routine applications in food samples & environment.(11)

 

Figure 12: Enhancement of fluorescence polarization by gold nanoparticles

(blog.sciencenet.cn/upload/blog/file/2008/.../2008105945876325.pdf)

As well as chip based scanometric method for detection of Hg2+ has been developed. It takes the advantage of the cooperative binging and the catalytic properties of DNA functionalized gold nanoparticles and selective binding of T-T (i.e. Thymine) mismatch for Hg2+. This method has high throughput, portability and easy readout.(13) (35)










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 PROBLEMS AND SAFETY ISSUES



Safety Issues of Nanoparticles for Diagnostics

The potentially toxic effects are a concern with the in vivo use of nanoparticles but not in the in vitro diagnostics, that forms the major portion of the laboratory diagnostics. There are also environmental concerns regarding the release of the nanoparticles during manufacturing of the nanoparticles & environmental effects. These are hence studied along with the naturally present nanoparticles in atmosphere.(19)

There are many questions still unanswered about the fate of the nanoparticles that are introduced into a living body. Due to the largevariety of materials used and wide range in sizes of nanoparticles, these effects will vary. It is hence conceivable that particular sizes of materials may have a bearing on the toxic effects. A number of studies have been conducted, but at this stage, there can be no conclusions drawn about the safety of nanoparticles. Concern basically centers on nanoparticles that are smaller than 20 nm in diameter, which can hence penetrate the cells. One vitallimitation in approval of the in vivo nanomaterials for human diagnostics is that the demonstration of the safety of these nanoparticles would be essentially required.(16) (27)


Identification of potential biomarkers of gold nanoparticle causing toxicity 

Studies have shown that 15–50-nm AuNPs can cross blood–brain barrier, which results in their accumulation in brain. The biosafety of metallic gold is well known, and it has been used in vivo since the 1950s. However, it is important to understand the interaction of gold nanoparticles with the vital organs such as the heart, liver, kidneys, brain, etc. Gold nanoparticles (also written as AuNPs) have found increased utilization in therapeutics & imaging. However, the toxic effects they possess have still to be elucidated. Hence this study was conducted to study biochemical effects of the AuNPs on brain of rats & to identify the potential biomarkers of toxicity caused by AuNP. (27)

Rats (Male Wisters) that weighed 150–200 g were injected with 20 μg/kg of body weight of the 20-nm gold nanoparticles for 3 days through intraperitoneal route.24 h after last dose of the gold nanoparticle injection; the rats were killed by CO2 asphyxiation. The parameters that were studied include lipid peroxidation, glutathione peroxidase, caspase-3, heat shock protein70, serotonin, dopamine, 8- hydroxydeoxyguanosine, gamma amino-butyric acid and interferon-γ. (27)

In this study, the AuNPs caused the generation of oxidative stress &decrease of antioxidant enzyme, i.e. glutathione peroxidase activity in the rats’ brain. This was accompanied with an increase in heat shock protein70, caspase-3 and8-hydroxydeoxyguanosine, which might lead to DNA damage as well as cell death. Gold nanoparticles also resulted in a significant decrease in levels of neurotransmitters like serotonin and dopamine, indicating a possible change in behavior of the treated animals. There was also a significant increase in cerebral levels of IFN-γ in animals that were treated.(27)










17






 The study hence concluded that AuNPs result in the generation of oxidative stress & an impairment of the antioxidant enzyme glutathione peroxidase in the brain of rats. AuNPs also resulted in generation of 8-hydroxydeoxyguanosine (8OHdG), and heat shock protein70 (Hsp70),IFN-γ and caspase-3, which may result in inflammation &DNA damage.(27)