Exceptionality Of Metal Nanoparticles

Different Types Of Nanoparticles

Nanoparticles are tiny particles with sizes between 1 and 100 nanometers. There are various types of nanoparticles, including:

· Metal nanoparticles

· Semiconductor nanoparticles

· Magnetic nanoparticles

· Carbon nanoparticles

· Polymeric nanoparticles

· Lipid nanoparticles

· Ceramic nanoparticles

Each type has unique properties that make them useful in various fields of science and technology.

Metal nanoparticles: These are nanoparticles made from metals such as gold, silver, platinum, and copper. Metal nanoparticles have unique physical and chemical properties that make them useful in a wide range of applications such as medical diagnostics, drug delivery, and catalysis.

Semiconductor nanoparticles: e.g., cadmium selenide, zinc oxide, and silicon. used in electronics, solar cells, and biological imaging.

Magnetic nanoparticles:

o Made from magnetic materials such as iron oxide and cobalt,

o used in biomedical applications such as drug delivery and magnetic resonance imaging (MRI)

Carbon nanoparticles:

o e.g., carbon nanotubes and graphene,

o used in electronics, energy storage, and water purification.

Polymeric nanoparticles:

o e.g., synthetic or natural polymers such as polystyrene, polyethylene glycol, and chitosan,

o used in drug delivery and gene therapy.

Lipid nanoparticles: phospholipids and cholesterol

Ceramic nanoparticles:

o Ceramic materials such as alumina, silica, and titania,

o used in catalysis, electronic materials, and biomedical applications.

Metal nanoparticles overview

Metal nanoparticles (MNPs) are tiny particles made from metal atoms with dimensions in the range of 1 to 100 nanometers. These particles have unique physical, chemical, and electronic properties that differ from those of the bulk metal material. The surface area-to-volume ratio of metal nanoparticles is very high, which makes them highly reactive and can enhance their properties in certain applications such as electronics, catalysis, biomedical engineering, and materials science.

MNPs can be made from a variety of metals, including gold, silver, platinum, copper, iron, and more. In recent years, the use of metal nanoparticles in medicine has received significant attention due to their unique properties, such as high biocompatibility, tunable surface chemistry, which make them useful for drug delivery, medical imaging, and cancer therapy.

Physical properties of metal nanoparticles

MNPs have unique physical properties that differ from their bulk metal counterparts since their size and shape can be precisely controlled during synthesis. The physical properties of metal nanoparticles are highly tunable. The size and shape of the nanoparticles affect their electronic, optical, and magnetic properties.

Because of their higher surface area-to-volume ratio they become more reactive and enhance their properties in specific applications.

MNPs can exhibit unique colors and fluorescence properties. for example, Au NPs exhibit a characteristic red color due to their plasmonic properties. Some MNPs, such as iron, cobalt, and nickel, exhibit magnetic properties that can be exploited in applications such as magnetic data storage and biomedical imaging.

The melting point of metal nanoparticles can be lower than that of the bulk metal due to the presence of surface defects and lattice distortions.

The surface chemistry of MNPs can be tuned by modifying their surface with ligands or coatings, which can affect their properties in applications such as catalysis and drug delivery.

Synthesis/Preparation of metal nanoparticles

Chemical reduction method: In this method, a reducing agent is used to reduce metal ions into nanoparticles. For example, sodium borohydride is commonly used to reduce metal ions such as gold, silver, and platinum to form nanoparticles.

In Electrochemical method, metal ions are reduced electrochemically to form nanoparticles. The size and shape of the nanoparticles can be controlled by adjusting the voltage and current during the electrochemical process.

In Microwave-assisted method: In this method, metal ions are mixed with a reducing agent and then exposed to microwave radiation. The heat generated by the microwave radiation promotes the reduction of the metal ions to form nanoparticles.

Green synthesis method, here the plant extracts or other natural sources are used as reducing agents to prepare metal nanoparticles. This method is considered environmentally friendly as it avoids the use of harsh chemicals and solvents.

Electrochemical preparation of Metal nanoparticles

Electrochemical methods are widely used for the preparation of metal nanoparticles due to their simplicity, scalability, and ability to control the size and shape of the nanoparticles. The electrochemical methods for the preparation of metal nanoparticles can be classified into two categories: anodic and cathodic methods.

Anodic methods: In anodic methods, the metal ions are reduced at the electrode surface to form metal nanoparticles. Anodic stripping voltammetry (ASV) is an example of an anodic method used to prepare metal nanoparticles. In ASV, the metal ions are first adsorbed onto the electrode surface and then reduced to form metal nanoparticles.

Cathodic methods: In cathodic methods, the metal ions are reduced at the cathode surface to form metal nanoparticles. Cathodic reduction can be carried out in the presence of a stabilizing agent, such as a surfactant or polymer, to prevent the aggregation of nanoparticles. Electrochemical deposition (ECD) is an example of a cathodic method used to prepare metal nanoparticles. In ECD, a metal salt solution is electrolyzed using an appropriate cathode material to form metal nanoparticles.

Both anodic and cathodic methods can be used to prepare metal nanoparticles with a high degree of control over their size and shape. The electrochemical methods also offer the advantage of being scalable and easy to perform. The choice of method depends on the specific metal, the desired size and shape of the nanoparticles, and the intended application.

Photochemical Methods for the Preparation Of Metal Nanoparticles

Photochemical methods are another popular approach for the preparation of metal nanoparticles. These methods involve the use of light to drive the reduction of metal ions to form nanoparticles. The photochemical methods can be divided into two categories: direct and indirect photochemical methods.

In direct photochemical methods, the metal ions are reduced (in the presence of a reducing agent) directly by a photochemical reaction induced by UV light Which lead to the formation of metal nanoparticles.

In indirect photochemical methods, a photosensitive metal precursor is first synthesized, which upon exposure to light, undergoes a chemical transformation to form the metal nanoparticles.

Photochemical methods offer several advantages over other synthesis methods, including the ability to prepare metal nanoparticles at low temperatures, high yields, and good control over the size and shape of the nanoparticles. Moreover, photochemical methods are eco-friendly as they do not require any harsh reducing agents or high temperatures. However, these methods require a high-intensity light source, and the synthesis process can be time-consuming.

Surface Plasmon Resonance

Surface plasmon resonance (SPR) is a phenomenon that occurs when light hits a metal surface and creates oscillations of electrons on the surface called surface plasmons. The surface plasmons interact with the incident light and cause a reduction in the reflected light intensity at a specific angle, known as the resonance angle.

Origin Of SPR

MNPs exhibit surface plasmon resonance because of their unique optical properties, which arise due to the interaction of light with free electrons on the surface of the nanoparticles.

In a metal nanoparticle, the electrons are confined to a small volume, and their energy levels are quantized, leading to discrete energy levels. When light is incident on the nanoparticle, the electrons absorb photons and get excited to higher energy levels. This excitation leads to a collective oscillation of the free electrons on the surface of the nanoparticle, known as a surface plasmon.

The surface plasmon resonance of a metal nanoparticle occurs when the frequency of the incident light matches the resonant frequency of the collective oscillation of the electrons. This leads to a sharp peak in the absorption or scattering spectrum of the nanoparticle at a particular wavelength, which is characteristic of the size, shape, and composition of the nanoparticle.

Metal nanoparticles are commonly used in biosensing applications because of their unique optical properties. The localized surface plasmon resonance (LSPR) of metal nanoparticles can be tuned by changing their size, shape, and composition, allowing for sensitive detection of biomolecules such as proteins, DNA, and viruses. LSPR-based biosensors are highly sensitive and specific, making them useful for a wide range of applications in medical diagnostics, environmental monitoring, and food safety. SPR is commonly used as a label-free analytical technique to study the interaction between biomolecules such as proteins, DNA, and antibodies. It is particularly useful for determining the binding affinity and kinetics of biomolecular interactions.

In an SPR experiment, one of the interacting partners is immobilized on a metal surface, typically gold, while the other partner is injected in solution over the surface. As the molecules bind and dissociate, the refractive index at the metal surface changes, resulting in a shift in the resonance angle. By measuring the shift in the resonance angle, the binding kinetics and affinity can be calculated.

SPR is widely used in drug discovery, biomolecular interaction studies, and medical diagnostics. It is a powerful tool for understanding the molecular mechanisms of biological processes and for developing new drugs and diagnostic tests.

Quantum Dots

Quantum dots are tiny semiconductor nano crystals, typically only a few nanometers in size (2-7nm), that have unique optical and electronic properties which are dependent on their size and composition. The size and composition of quantum dots can be precisely controlled, which allows their emission wavelength to be tuned across a wide range of the electromagnetic spectrum. QDs are very bright and can emit light with high efficiency, making them useful for a range of applications in lighting and displays. They are very stable and can maintain their properties for long periods of time, which is important for many practical applications. QDs can be easily incorporated into a range of materials, including polymers, glasses, and semiconductors, which makes them useful for a variety of applications in fields such as medicine, energy, and electronics. Because of these and other advantages, quantum dots are being studied and developed for a wide range of applications, including in solar cells, LEDs, biological imaging, and quantum computing, among others. They are typically synthesized using one of several methods, including colloidal synthesis, epitaxial growth, and lithographic techniques.

Preparation Of QDs Using Colloidal Synthesis Method

The first step in the colloidal synthesis of quantum dots involves the preparation of precursor solutions containing the appropriate chemical compounds. These compounds typically include a metal salt and a stabilizing ligand. The precursor solutions are then heated to a specific temperature, causing nucleation to occur. At this stage, small clusters of atoms begin to form. As the precursor solution continues to heat up, the clusters of atoms grow into larger nuclei. The size and shape of the nuclei can be controlled by adjusting the reaction conditions such as temperature, concentration, and pH. The larger nuclei eventually begin to coalesce with each other, forming larger particles. During this stage, some particles will dissolve, and others will continue to grow by absorbing atoms from the surrounding solution. This process is called ripening. The final step in the synthesis process is to add a stabilizing agent to the solution to prevent the particles from agglomerating or growing any larger. The stabilizing agent, such as an organic ligand, will bind to the surface of the quantum dot, passivating the surface and stabilizing the structure. The resulting quantum dots can be separated by size and purified for use in various applications such as solar cells, bioimaging, and optoelectronic devices.

Optical Properties Of QDs:

As the QDS size is very smaller, the surface defect density reduces, and the band gap broadens (shows blue shift with respect to the bulk material). Emission color of QDs can be easily tuned by adjusting their size, with smaller QDs emitting at shorter wavelengths (blue) and larger QDs emitting at longer wavelengths (red). i.e., quantum size effect.

Have narrow emission linewidth, which makes them ideal for use in imaging and sensing applications. This is due to the discrete energy levels that are characteristic of quantum confinement.

High photoluminescence quantum yield, that is a larger proportion of the absorbed photons are emitted as light which are more useful in the development of efficient optoelectronic devices.

Also, have a broad absorption spectrum, which allows them to absorb light over a wide range of wavelengths. This property is suitable for applications such as solar cells, where broad spectral coverage is required.

Size-dependent extinction coefficient (the measure of their ability to absorb light), which is more important for the design of QD-based devices, as it affects the device performance. Consequently, quantum dots have an attractive wide range of applications, including biological labeling, light-emitting diodes (LEDs), photovoltaics, and quantum computing.

Metal Oxide Quantum Dots

Metal oxide(MO) quantum dots are nanoscale particles made of metal and oxygen atoms (metal cations and O anions) that exhibit unique physical and chemical properties due to their small size and high surface-to-volume ratio. In MO nanostructures, closely packed Oxygen anions form interstitial sites which are occupied by metal cations. The partially d-shells of transition metal ions facilitate enhanced electronic transitions, wide band gaps, superior electrical characteristics, and high dielectric constants. They have been studied and developed for a range of applications in fields such as energy, catalysis, sensing, and electronics. The size and shape of metal oxide quantum dots can be precisely controlled, which allows their properties to be tailored for specific applications such as solar cells. Metal oxide quantum dots are highly stable, and biocompatible, which makes them useful for medical applications such as imaging and drug delivery. Some examples of metal oxide quantum dots include titanium dioxide (TiO2), zinc oxide (ZnO), and iron oxide (Fe2O3) quantum dots. These materials have been studied for a range of applications, such as photocatalysis, gas sensing, environmental remediation, and bioimaging, among others.

Metal Sulfide Quantum Dots (MS-QDs)

A class of quantum dots that are composed of metal cations and sulfur anions. MS-QDs exhibit unique optical and electronic properties due to their quantum confinement effect, which occurs when the size of the QD is smaller than the exciton Bohr radius. Some examples of commonly studied MS-QDs include cadmium sulfide (CdS), zinc sulfide (ZnS), and lead sulfide (PbS) quantum dots. The emission color of MS-QDs can be tuned by varying their size, with smaller QDs emitting at shorter wavelengths and larger QDs emitting at longer wavelengths. They have high photoluminescence quantum yields, making them useful for applications such as biological imaging and sensing. Their broad absorption spectrum makes them useful for solar energy conversion and photocatalysis. In addition, MS-QDs have excellent electron transport properties due to their high electron mobility and large surface area, making them useful in electronic and optoelectronic applications. The other MS-QDs like ZnS and CuS, are relatively non-toxic and biocompatible, making them attractive for biomedical applications. However, the toxicity of some MS-QDs, such as CdS and PbS, remains a concern and efforts are being made to develop alternative, non-toxic materials for use in these applications.

Photocatalytic nanomaterials

Photocatalytic nanomaterials

Photocatalyst nanomaterials are a class of materials that have gained significant attention in recent years due to their unique properties and potential applications in a wide range of fields. These materials are typically composed of nanoparticles with sizes ranging from a few nanometers to several hundred nanometers and are designed to absorb light and convert it into chemical energy.

One of the key advantages of photocatalyst nanomaterials is their high surface area-to-volume ratio, which makes them highly effective at catalyzing chemical reactions. This property is particularly useful in applications such as water purification, where photocatalysts can be used to break down organic contaminants and other pollutants.

Some common examples of photocatalyst nanomaterials include titanium dioxide (TiO2), zinc oxide (ZnO), and cadmium sulfide (CdS). These materials can be synthesized using a variety of methods, including sol-gel, hydrothermal, and microwave-assisted synthesis, and their properties can be tuned by adjusting factors such as particle size, shape, and composition.

While photocatalyst nanomaterials have shown great promise in a range of applications, there are also some potential concerns associated with their use since these materials may be toxic to certain organisms, and their long-term environmental impact is not yet fully understood. As such, further research is needed to fully understand the potential benefits and risks associated with the use of photocatalyst nanomaterials.

Photocatalytic mechanism

The photocatalytic mechanism is a complex process that involves several steps. The basic principle behind photocatalysis is that when a photocatalyst material is exposed to light, the energy from the light can excite electrons in the material, creating electron-hole pairs. Such excited electrons or holes can diffuse to the surface and create two oxidation reactants – hydroxyl radicals .OH and superoxide anion O2-. These reactants decompose toxic organic substances to non-toxic organic compounds such as carbon, water, ammonium, nitrates and chloride ions. The specific steps involved in the photocatalytic mechanism depend on the material being used, as well as the type of reaction being catalyzed.

Steps involved in the photocatalytic process:

Absorption of light: The photocatalyst material absorbs light energy, which excites electrons in the material to higher energy levels.

Generation of electron-hole pairs: The excited electrons leave behind positively charged "holes" in the material, creating electron-hole pairs.

Surface reactions: The electron-hole pairs can then participate in surface reactions with adsorbed molecules on the surface of the photocatalyst material. For example, in the case of a pollutant such as organic contaminants, the electron-hole pairs can react with the adsorbed molecules to break them down into smaller, less harmful compounds.

Electron transfer: In some cases, the electron-hole pairs can transfer electrons to or from other molecules in the system, such as in a redox reaction.

Recombination: Eventually, the electron-hole pairs will recombine, releasing energy in the form of heat or light. This can limit the efficiency of the photocatalytic process, and researchers often aim to design materials that can minimize recombination and maximize the number of active electron-hole pairs.

Thus, the basic idea is to use light energy to drive chemical reactions and break down pollutants, making it a promising approach for environmental remediation and other applications.

There are many examples of nano photocatalyst materials that have been developed and studied for various applications. Some common examples include:

Titanium dioxide (TiO2): TiO2 is one of the most widely studied photocatalyst materials and is used in applications such as air and water purification, self-cleaning surfaces, and solar cells. TiO2 nanoparticles can be synthesized in a variety of sizes and shapes, and their photocatalytic properties can be tuned by controlling the crystal structure and surface area.

Zinc oxide (ZnO): ZnO is another commonly studied photocatalyst material, with applications in areas such as wastewater treatment, air purification, and photovoltaic devices. Like TiO2, the photocatalytic properties of ZnO nanoparticles can be modified by controlling different factors such as crystal structure, doping, and surface modification.

Cadmium sulfide (CdS): CdS is a semiconductor material with photocatalytic properties that has been studied for applications such as hydrogen production and environmental remediation.

Graphene oxide (GO): GO is a 2D material with unique properties that make it an attractive photocatalyst material. GO has been studied for applications such as water purification, air purification, and photocatalytic degradation of organic pollutants.

Metal-organic frameworks (MOFs): MOFs are a class of materials that consist of metal ions or clusters coordinated with organic ligands. MOFs have been studied as photocatalysts for applications such as hydrogen production, carbon dioxide reduction, and water purification.

Each material has its own unique properties and advantages, and the choice of material will depend on the specific application and desired performance characteristics.

TiO2 photocatalyst

The transition metal oxide semiconductor TiO2 photocatalyst is one of the most popular and widely studied photocatalytic materials for several reasons. TiO2 is an essential and a widely used material in most industrial products like sunscreen, paints, coatings, and self-cleaning glasses because of its high photoactive ability. Besides, more research works are going on TiO2materials because of their abundant, non-toxic, economically cheap, and thermally and chemically stable in nature. Catalytic oxidation and reduction processes are one of the possible remediations to solve the environmental pollution issues. TiO2 semiconductor photocatalyst is a well-known commercially available catalyst material. Still, the wider bandgap (3.2eV), high energy requirement for exciton generation & faster rate of recombination of excitons of TiO2 photocatalyst hold back its extended benefits in the fields of photocatalytic and photovoltaic applications.

Some of the key advantages of TiO2 photocatalyst include:

High efficiency: TiO2 has a high quantum yield, meaning that it can convert a large proportion of absorbed photons into chemical energy. This makes it an efficient photocatalytic material for a variety of applications.

A stable material that is resistant to corrosion and degradation, making it suitable for use in harsh environments and over long periods of time.

Biocompatible material, due to its non-toxicity, is more useful in biomedical applications, such as drug delivery and tissue engineering.

Abundant and low-cost material thus becomes an attractive option for large-scale applications.

Easily synthesized using a variety of methods, including sol-gel, hydrothermal, and microwave-assisted synthesis.

Band structure of Cu doped TiO2

Photocatalytic applications

Photocatalysis has a wide range of potential applications in areas such as environmental remediation, energy production, and biomedical engineering.

Degrades organic pollutants and disinfects wastewater and air. i.e., Environmental remediation

These materials can be used in applications such as water treatment plants, air purifiers, and self-cleaning surfaces.

Used to generate hydrogen fuel through the water splitting reaction. This process involves using sunlight to split water molecules into hydrogen and oxygen. E.g., CdS and TiO2

In Self-cleaning surfaces, it breaks down organic contaminants and maintains their cleanliness. These surfaces can be used in applications such as building exteriors, car paints, and clothing.

Used to make more efficient solar cells, like dye-sensitized solar cells.

Also, used for drug delivery, tissue engineering, and sterilization. Basically, it is used to trigger drug release or degrade unwanted biomolecules.