Explore
Nanoparticles
Published July 22, 2022
Time and time again a fortress grows in silence. Spreading by the minute, a council of deranged elders orders soldiers to guard the curtain walls—walls conceived from the earth at which they grew.
The surrounding land litters with convoys filled with provisions and fire support. To many, the walls are impenetrable.
We can only slow the growth, they say.
Though others believe that one day, with the right set of tools, the walls will crack.
One day.
That is how Dr. Siegwart believes it. He sits at his lab desk, scrambling through his hard drive: an array of notes for hundreds of test subjects that have come and gone. To his left, a library of white mice lines a dozen or more shelves. Papered on the front of each cage reads the disease it carries. Ovarian cancer for some, liver cancer for others.
Molecular defenses line the outside of a malignant tumor. For many years researchers have attempted to break these defenses, but to no avail. Nothing seems to be enough.
CRISPR: the gene-editing tool that can edit the DNA inside living cells with astounding precision. Although marking a breakthrough in cancer research worldwide, the limitations of CRISPR are apparent; it lacks the efficiency to deliver select mRNA to cancer cells.
Lab coats rest on the backs of chairs, except for one. Dr. Siegwart shifts his eyes to the mice. Caged, numbered, hopeless.
One decade has passed, and still he’s utterly fascinated by the design of lipid nanoparticles. Inside several of the test subjects reside spherical lipid nanoparticles that hold a key weapon, siRNA, that functions to shut off the gene that holds together the defensive molecules along the walls of a tumor. After the barriers are broken down, the lipid nanoparticles act as a vessel as they enter the tumor, releasing a gene-editing system that alters the DNA of the cancer cells, blocking further growth. All while reactivating the body’s immune system to fight a formerly invulnerable disease.
Here he meets success. The tumors within the mice had shrunk to one-eighth their original size, and many had lived twice as long as their cancer-ridden counterparts without treatment.
No. 219 blinks, watches Dr. Siegwart as he lifts out from his seat, shutting off his computer. Shambling off as the blue screen fades to black.
Until tomorrow.
“Plenty of room at the bottom”
Nanoparticles can be of any material, shape, and size. For a particle to be classified as a nanoparticle, its structure must be within 1 to 100 nanometers (nm) for each of its three dimensions.
To give you some perspective on the size of a typical nanoparticle, a water molecule is approximately 0.275 nm in diameter, and a DNA strand is about 2 nm in width. The average diameter of a carbon nanotube is 4 nm.
A nanoparticle cannot simply be visualized as a whole unit. To understand its structure, consider its composition as two separate layers: the shell, and the core. In the case of a simple nanoparticle, the shell and the core are composed of the same material. Composite nanoparticles, otherwise known as core/shell nanoparticles, possess a core with a different material than its shell.
The shell also has a distinctive surface layer—the most reactive part of the nanoparticle. Metal ions, surfactant molecules, or polymers attach themselves to the nanoparticle’s surface. Based on the material attached to the surface, the nanoparticle is given a certain functionality.
Behind the magic
The properties of a certain material depend on a variety of factors, including its shape, chemical composition, and surface characteristics. But since the latter half of the 20th century, material size has gained the attention of numerous scientific fields, paving the way for nanoparticles to inherit physicochemical properties vastly different from their bulk form.
There are two reasons for material size to play such a crucial role in nanoparticles: increased relative surface area, and quantum size effects. These attributes determine properties such as reactivity, strength, melting and freezing point, electrical characteristics, optical characteristics, and thermal conductivity.
As you reduce the size of a material, the proportion of atoms at its surface increases. Atoms along the surface of the material have fewer neighboring atoms to bond together, and therefore form unsaturated bonds with the few atoms they are next to. These unsaturated bonds are responsible for the material’s reactivity. To summarize, the greater the proportion of atoms at a material’s surface, the greater its reactivity.
This is why nanoparticles can be so keen to react. Their higher surface area per unit mass dominates that of larger particles outside the nanoscale.
But why does reactivity matter?
It is the basis for almost all applications, particularly with catalysis. A catalyst will boost the rate of a chemical reaction by lowering its activation energy. Since the interaction between the catalyst and the reagent occurs at the catalyst’s surface, the reaction will occur at an even greater speed if the catalyst is in its nanoparticulate form.
Quantum size effects
Electron properties in solids vary depending on the size of a particle. It is on the nanoscale that quantum effects begin to dictate the behavior of matter. This is the point at which a material’s optical, electrical, and magnetic properties deviate from their values on the macroscale.
Understanding the reason behind the distinguishing characteristics of a nanoparticle requires a thorough understanding of quantum physics. Consider a compound in its bulk form. The average of all the quantum forces affecting each of the compound’s atoms determines its physicochemical properties.
As you reduce the size of this compound to its nanoparticulate form, the average of the quantum forces becomes obsolete, and the behavior of individual atoms becomes more apparent. Copper wire is pristine bronze, whereas copper nanowires are transparent. Platinum is an inert metal, but as a nanoparticle it serves as a catalyst in hydrogen fuel cells. Aluminum is a stable material for various products and appliances, however aluminum nanoparticles are spontaneously flammable.
Classification
Nanoparticles are commonly classified as either organic or inorganic. However, considering the variety of distinguishable properties, it’s more suitable to classify nanoparticles based on practicality.
The following list describes the types of nanoparticles based on morphology, size, and chemical properties:
Carbon-based
Nanoparticles with immense electrical conductivity, high strength, and high electron affinity. Examples include fullerenes, carbon nanotubes, graphene, and carbon dots.
Metals
Nanoparticles with great optoelectrical properties. They have a significant role in medical applications like bacterial resistance, drug delivery, and cancer treatment. Examples include simple nanoparticles such as gold, platinum, silver, and iron, as well as composites in the form of metal sulfides (AgS, CuS, FeS) and metal oxides (TiO2, AgO, ZnO).
Ceramics
Nanoparticles classified as inorganic, nonmetallic solids. The structure of ceramic nanoparticles can take on a number of forms, be it amorphous, polycrystalline, dense, porous, or hollow. Their functionalities prove useful in catalysis, photocatalysis, photodegradation of dyes, imaging, and drug delivery. Examples include silica (SiO2), silicon carbide (SiC), and alumina (Al2O3).
Semiconductors
Nanoparticles with properties between metals and non-metals. By reason of their wide band gaps, the efficacy of their electrical conductivity is better than an insulator, but less than that of a metal. They are used in photocatalysis, photo-optics, and electronic devices. Examples include GaN, GaP, InP, InAs, ZnS, CdS, CdSe, and CdTe.
Polymers
Organic nanoparticles in a spherical or capsular structure. Polymeric nanoparticles are used for surface coating, sensor technology, and catalysis. They are also useful for several medical applications due to their biocompatibility, such as controlled release, protection of drug molecules, and specific targeting. Examples of polymeric nanoparticles are poly-ɛ-caprolactone (PCL), PGA-co-PDL, and poly(lactic-glycolic acid) (PLGA).
Lipids
Nanoparticles containing lipid moieties. Lipid nanoparticles possess a spherical structure, with diameters ranging between 10 to 1,000 nm. The core is composed of solid lipid matter, surrounded by a matrix of soluble lipophilic molecules. They act as drug carriers in biomedical applications, deliver nucleic acids in gene therapy, and release RNA in cancer therapy. Examples of lipid nanoparticles are liposomes, micelles, amphotericin B, and doxorubicin.
Applications
Since the early 1980s, research and development of nanoparticles have soared into the fields of chemistry, biology, physics, materials science, and engineering. After so much contribution to the thorough understanding of nanoparticles, they have successfully reached commercialization in several applications. Some of which are electronics, energy, environmental remediation, sensor technology, biotechnology, aerospace, medicine, coatings, food and packaging, plasmonics, and displays.
Here I will cover the applications that have seen unimaginable breakthroughs with nanoparticles in recent years.
Energy
Solar cells are the reservoirs in solar panels that absorb sunlight. Conventionally, these solar cells are made up of crystalline silicon. With the help of nanotechnology, anti-reflection layers can be added to achieve a greater absorption yield.
Current research focuses on thin-layer solar cells made from alternative materials to crystalline silicon, such as copper, indium, selenium, dyes, and various polymers. Polymer solar cells can theoretically produce a high light absorption yield, but are still under ongoing research.
As of the year 2022, solar cell efficiency is around 15-20%. However, quantum dots and nanowires could dramatically raise solar cell efficiencies to over 60%.
Nanoparticles have also breached hydrogen fuel cell innovation through the inclusion of nanostructured electrodes, catalysts, and membranes. In particular, semiconductor nanoparticles are efficient in photocatalytic applications—photoelectrochemical and electrochemical water splitting—because of their fitting band gaps and bandedge positions.
What I find most intriguing by far with the extent nanoparticles have on the energy sector is thermoelectric energy conversion. Semiconductor nanoparticles could utilize waste heat for electricity, such as capturing the heat from vehicles for more mileage, or absorbing human body heat to charge portable electronics.
Medicine
Nanoparticles can deliver drugs into the body at the optimum dosage with astounding efficiency. For instance, a particular iron oxide particle, magnetite (Fe3O4), is commonly used for MRI contrast enhancement, tissue repair, immunoassay, detoxification of biological fluids, hyperthermia, drug delivery, and cell separation. These applications deem iron oxide so favorable because of its high magnetization value, small size, and narrow particle size distribution.
Liposomes serve a practical use in protecting drugs from degradation, however they have several limitations, such as squat encapsulation efficiency, water leakage in blood components, and poor storage. In contrast to liposomes, polymeric nanoparticles increase the rateability of drugs and inherit controlled drug-release properties superior to liposomes.
Semiconductor and metallic nanoparticles have great potential for cancer diagnosis and therapy due to their surface plasmon resonance, enhanced light scattering, and absorption. For example, gold nanoparticles can effectively absorb light and convert it into localized heat for photothermal cancer therapy.
Sensor Technology
Sensor technology is an expansive field that scientists use extensively in the medical sector. In recent years, nanosensors have gained more attention due to their capacity to detect chemicals, biological variables, and electromagnetic radiation.
A sensing device is considered a nanosensor when at least one of its dimensions is smaller than 100 nm. Its purpose is to collect and transfer information on the nanoscale for further analysis.
In the medical field, scientists produce nanosensors through self-assembled nanostructures—typically with liposomes—that can send electrical signals whenever detecting certain biochemicals.
Nanowires and nanofibers have use in chemiresistive sensors for the diagnosis of diseases. Even graphene nanosensors can be used to monitor real-time concentrations of insulin in the pancreas.
Truly remarkable, the age of diagnosis has come to the point of a machine analyzing the biochemicals in your body, to evaluate traces of diabetes, halitosis, kidney malfunction, lung cancer. I would be remiss to disregard the extent of medical achievement in the coming decades. Nanoparticles, and their role in sensor technology, is a field of sprouting fortune we cannot neglect.