Gold Nanoparticles in Biodetection: Revolutionary Advances and Applications

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. 1 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 3 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