Review: The Global Need of Quantum Dots in Point of Care Testing
Abstract
With the COVID-19 pandemic, point of care and rapid testing has been on the rise. Their continuous use brought new advancements in point of care testing causing the demand to increase in developing countries due to the urgent need for better disease diagnosis in those regions of the world. Current point of care tests require expensive and scarce materials causing difficulty when bringing them into both the developed and developing healthcare regions. However, with the use of an emerging field known as nanotechnology and quantum dots, point of care testing could become the new normal. This review will look at immediate testing and their applications within developing countries alongside using quantum dots to improve the analytical performance and to simplify the detection process.
Contents:
1. The Current Healthcare System: Developing vs. Developed
2. The Basics of Nanotechnology
2.1 Nanotechnology in Healthcare
3. Quantum Dots
3.1 Quantum Dots as Biomarkers
3.2 Properties
3.3 Synthesis Methods: Etching
3.3.1 Synthesis Methods: Bottom-up Synthesis and Nanoscale Fabrication
4. Subset of Quantum Dots — Graphene Quantum Dots: Properties and Characteristics
4.1 Synthesis Methods
4.2 Chemical Oxidation
4.3 Spin Polarisation
4.4 Absorption Peaks
4.5 Acid Oxidation
4.6 Hydrothermal Synthesis
5. The Quantum Confinement Effect
5.1 Understanding the Quantum Confinement Region
5.2 Particles in a Box Theory
6. Biosensors
7. Subset of Quantum Dots — Graphene Quantum Dots: Applications in POCT
7.1 Paper-strip Embedded GQDs
7.2 Paper-based IoT Device
8. Quantum Dot Applications
8.1 Point of Care Testing
8.2 Bioimaging
8.2.1 Vitro Bioimaging
8.3 Quantum Dot Biosensors
8.4 Clinical Applications
8.5 Barriers
9. Conclusion
10. Sources
- The Current Healthcare System: Developing vs. Developed
As humans, our first instinct is to survive and for survival we need nutrients, water and protection to help live a healthy life. As centuries passed, certain areas became better and better at gaining this security through their systems while others did not advance.
This is the gap between developed and developing countries and their overall access to healthcare. Several regions in developing countries remain vulnerable due to the lack of proper healthcare (in areas like Sri Lanka, Pakistan, Maldives, Nepal, etc.) where 25% of the world is concentrated in these regions, making it the globe’s most densely populated geographical locations. However, developed countries like the United States and Japan require the services of these foreign healthcare providers to meet the needs of their populations. The providers go to those countries as they can actually afford to pay for the services of the foreign healthcare providers. Thus, developing countries do not have healthcare providers as it is and the ones they do have are helping other countries causing the scarcity of doctors and experts in these fields to increase where the developed countries are contributing to the brain drain in developing countries.
Many foreign doctors in the United States come from Ethiopia, Pakistan and India where these countries may only have 1 doctor for every 1000 residents. However, Liberia is one of the countries with the worst data with only 10 doctors available for every 1 million people. Comparing this to developed countries like Canada, United Kingdom or United States where there are 3000 doctors for every 1 million people.
It is clear that more life threatening circumstances can occur when people are unable to access a physician. During desperate times, residents resort to alternative cures or reliefs and would rather use these alternative methods as opposed to spending the limited money they have on hospitals or to required medications.
2. The Basics of Nanotechnology
As mentioned in the abstract, point of care testing (POCT) is a technique growing in popularity which could help these countries. Successful POCT utilises quantum dots to increase sensitivity and speed. Quantum dots are a type of nanoparticle within the field of nanotechnology.
Nanotechnology is the study of manipulating materials on an atomic level called the nanoscale and because every material is investigated at the nanoscale, they become very small, changing their properties such as reactivity, conductivity and solubility as a function of the particle size. Keeping these materials on the nanoscale we can harness their properties and configuration to use them for the desired application. An important note is that on the nanoscale, the coulombic attraction becomes stronger (attraction between protons and electrons) which plays a large role in the behaviour of different nanomaterials.
A fascinating phenomenon about the nanoscale is when quantum effects are dominant on this scale with band gaps, S and P orbitals, etc. they change the electronic properties (section 5). The focus of this paper relies on the enhancement using quantum dots which exhibit unique optical properties due to the quantum confinement effect where higher energies occur as the particle decreases in size.
2.1 Nanotechnology in Healthcare
The necessity to create healthcare that is more patient-centred rather than provider centred is now a global trend. The traditional forms of expensive care, centralised testing and making multiple visits for one assessment has adapted into a newer platform dependent on handheld devices. However, implementing this advancement is time consuming while the growth in increasing healthcare problems and infectious diseases are resulting in a significant mortality rate.
Effective diagnosis testing has been difficult to achieve as it traditionally requires regular monitoring, thus, the rising demand of home based point of care testing (POCT) has been defined as affordable, sensitive, user friendly, rapid and portable. It is estimated that these devices will be in developed and developing countries within the next decade and some of the most accurate POCT occurs with the enhancement of nanotechnology and quantum dots.
Nanotechnology is very effective for this type of testing since the first signs of any disease or medical problems occur at the nanometric scale. Trillions of cells begin with these diseases and the cells exchange DNA fragments. These fragments act as messages between the cell and are only a few nanometers (nm) in size and can be used as biomarkers. Thus, when a cell is sick, it sends out different messages which are molecular signs of disease and since each disease has its own unique protein marker, the POC tests can be used to determine the presence of the protein and in turn determine the disease while doing this very quickly and with high accessibility. With the use of nanotechnology, we can detect multiple different targets in the same reaction. For example, influenza broke out in 2009 and at the time, it would take up to 14 days to determine the presence of the pathogen however, with the use of nanotech, this could have been done in 3.5 hours.
3. Quantum Dots
Nanotechnology consists of various smart materials, one of which is known as quantum dots. Quantum dots (QDs) are artificial semiconducting nanoparticles which transport electrons. Semiconductors are materials with conductivity between the conductors and if the particles are small enough, the quantum effects come into play creating varying energies at electrons and holes (absence of electrons). Now, when ultraviolet light hits these semiconducting nanoparticles, they emit light of varying colours depending on the wavelength. These QDs are found in solar cells, fluorescent biological labels and even in quantum computation. The colour emitted relates to the wavelength of energy released which all goes back to two main factors: size and shape. Therefore, by controlling the size, the particles can emit or absorb particular wavelengths of light resulting in different colours.
3.1 Quantum Dots as Biomarkers
There are numerous applications of quantum dots ranging from photonics and imaging to sensing and computation however, the applications and scope within the healthcare field are unmatched. QDs in medicine are capable of molecular recognition and some can even be tuned to become self-assembly nanocrystals which are the building blocks for self assembled functional nanodevices; incredibly helpful for tracking a biological molecule.
QDs also allow for the study of cell processes at the level of a single molecule. These can be harnessed and used as high sensor elements as the fluorescence properties are changed upon a reaction with an analyte, or, when receptor molecules or antibodies are conjugated onto the surface of the dots. Although they have great potential, they come with their negatives (section 8.5).
3.2 Properties
The properties of all QDs depend on various factors: size, shape, composition, structure (solid or hollow) and the material (section 4). For example, specific electronic properties can be used as active materials in a single-electron transistor.
As mentioned before, QDs also have the ability to emit light of specific wavelengths if excited by light or electricity. The characteristics are determined by the size and shape, so, we can control their emission wavelength by tuning their size. Smaller QDs with a radius of 2–3 nm emit shorter wavelengths and generate cool colours such as violet, blue and green. On the other hand, larger QDs with a radius of 5–6 nm emit longer wavelengths and generate warmer colours such as red, orange and yellow.
3.3 Synthesis Methods: Etching
QDs can be synthesised in various ways, some more expensive than others and some more efficient than others. One common method is etching. This is where the bulk semiconductors are thinned to make the QD in top-down etching. This is a popular term when talking on the nanoscale. A dry etching is embedding a reactive gas into an etching chamber and applying radio frequency voltage to create plasma that breaks down this gas particle into more volatile parts.
3.3.1 Synthesis Methods: Bottom-up Synthesis and Nanoscale Fabrication
The bottom-up synthesis is known as wet-chemical methods typically used for colloidal QDs which dissolve in solvents. For more precise control there are parameters for simple or compounded solutions. The QDs of the desired shape and thickness are altered but stabilisation is mandatory in this process since it constructs the core shell.
4. Subset of Quantum Dots — Graphene Quantum Dots: Properties and Characteristics
Graphene quantum dots (GQDs) are a type of quantum dots with similar properties and synthesis methods however, they vary in material and their scope in each field. GQDs differ from QDs since they contain graphene. Graphene is an unrolled planar form of carbon nanotubes which is very useful for nanoscale electronics. These GQDs can carve out nanoscale transistors from single graphene crystals and have good potential in biosensing, bioimaging (due to their unique photoluminescent properties), however have challenges with their high toxicity and biocompatibility. The GQDs omit different colours due to varying photoluminescent (PL) properties alongside changes in the bandgap and size.
Photoluminescence properties are one of the outcomes of the strong quantum properties and edge effects that GQDs possess. GQDs range from 3–20 nm in size and contain 1+ layers of graphene where the thickness is less than 5 nm. They typically have a circular and elliptical composition however with recent advancements, a hexagon and quadrilateral shapes have been synthesised.
The ideal GQDs only have one atomic layer of carbon atoms however when synthesised they can contain over three functional groups and multiple atomic layers with each less than 10 nm in size. This monoatomic later typically conjugates the structure, has large specific surface area and can hold oxygen functional groups resulting in many active loading and absorption sights with a spacious environment. GQDs can also bind with many aromatic molecules through electrostatic bonding (π — π stacking). π — π stacking results in non-contact forces, essentially having push and pull forces without touching.
The main concerns with GQDs rely on their biocompatibility and toxicity. hGQDs (HO — GQD) are the most toxic and result in immediate cell death while cGQDs (HOOC — GQD) and aGQDs (H2N — GQD) have no cytotoxicity within a measurable concentration range. However, all three of these can cause autophagy where the body consumes its own tissues, normally occurring during malnutrition and in certain diseases.
4.1 Synthesis Methods
When forming any nanoparticles, there are two main groups of synthesis methods: top-down and bottom-up.
For GQDs, top-down synthesis is simply where graphene sheets are cut and work from the functional groups to the core. Bottom-up synthesis is done through cutting graphene sheets and then generating small precursors of the aromatic molecules. Precursors allow us to find which chemicals can be paired and are useful for a chemical reaction.
Between the two, the top-down method is more abundant and can be synthesised at a larger scale. The graphene sheets are negatively charged and contain hydrophilic oxygen groups (groups soluble in solvents such as water and ethanol). This method can also cut large blocks of carbon material into smaller pieces and since the materials are abundant carbon materials, they are cheap and easy to maintain.
4.2 Chemical Oxidation
Chemical oxidation is one of the many synthesis methods for GQDs. Typically, the carbon bonds of graphene or carbon nanotubes are destroyed by H4SO4, HNO3 and other oxidants. To avoid the use of concentrated acids and bringing more metal impurities, the GQDs are synthesised with black carbon and H2O2 (hydrogen peroxide) as an oxidant requiring no further purification steps. The diameter of these nanoparticles are 3–4.5 nm and take around 90 minutes. They result in good light stability, salt tolerance, low toxicity and strong biocompatibility while staying fast. However, chemical oxidation is not very safe as it creates chemical waste, polluting the environment.
4.3 Spin Polarisation
Graphene is a bidimensional material that is a single atom thick sheet of a honeycomb like structure arranged in sp2 bonded carbon atoms (understanding the carbon bonds are important since they are applied in the GQDs). This is a new hybridization type which is where two atomic orbitals are mixed to create a new type of hybridised orbitals resulting in different energies and shapes. Sp2 bonds are a result of mixing 1 s-orbital and two p-orbitals. This results in the trigonal planar shape of the graphene where the molecular shape and electron-pair geometry are both 120 degrees. Each of the carbon atoms are attached to three other carbon atoms by making three sp2 bonds where one is out of the plane p-orbital known as the electron delocalization network. The network is electrons in the molecules ion where the metals are not associated with a single atom or covalent bond. This is molecular orbital electrons that have extended over several adjacent atoms.
The s-orbitals are spherically symmetric around the nucleus of the atom while the p-orbitals are a lobe shaped region describing where exactly the electrons can be found. However, the scale of this depends on the quantum confinement effect and the associated energy state.
Due to these bonds, these nanoparticles are able to cross biological barriers and target desired regions that are not normally accessible. All of these bonds and their configurations affect the quantum confinement effect and lead to spin polarisation which is the degree at which the spin of the particle is aligned with a given direction.
4.4 Absorption Peaks
The uniqueness with GQDs is that their absorption peaks are comparable to other graphene based materials allowing for us to measure the absorption of radiation as wavelengths. The features which determine the absorption and PL properties of the graphene based materials allow us to measure the absorption of radiation as a wavelength. These properties involve the size of the molecule, sp2-hybridised configurations, functional groups (specifically aldehyde (-O) and amide (-N)) and the synthesis method as it plays a crucial role in the PL.
Now, the dimension of the GQD is comparable to the exciton diameter (distance between electrons and holes; section 5). These particles are treated as particles-in-a-box since their band gap energy is inversely proportional to the size of the potential well (section 5.2).
The absorbance on a vertical axis is just the measure of the amount of light absorbed and the higher the value, the more of a specific wavelength is being absorbed; called the absorption peak. Absorption peaks go quite higher than expected where the GQDs with lateral dimensions from 1–4 n have an absorption peak at 270 nm while when the diameter is 7–10 nm, the maximum is 330 nm.
This occurs when considering the sp2 structures within a molecule. The number of controlled sp2 sites, the band gap energy resulted from those structures resulting in PL peaks. For example, a single aromatic ring (benzene) has a band gap of 4eV (measured in electron volts). When increasing the number of aromatic rings, there’s a progressive reduction in the energy gap. So, graphene-oxide (GO) with 20 aromatic rings has a band gap of 2eV and 100 rings with a size of 3 nm has an energy band gap of 0.5eV. This concludes that the higher number of rings, the less the energy gap and the higher the wavelength of the emission.
4.5 Acid Oxidation
Acidic oxidation is an acidic treatment of the carbon fibres. The GQDs are polar due to the edge functionalized groups like hydroxyl (OH), carboxylic acid (COOH), epoxy (C-O-C) and carbonyl (CO) groups allowing them to be highly soluble in water. The band gaps of these nanoparticles are controlled by changing temperatures.
This approach can also be combined with the wet chemistry method. An inexpensive way to change the temperatures are with bituminous coal which is the most abundant and readily available coal, causing chemical oxidation. The coal is then sonicated (subjected to sound waves) in a mixture of concentrated sulfuric and nitric acid followed by a heat treatment (80–120 degrees celsius) for 24 hours. This in turn creates 2–3 layers of GO type structures.
4.6 Hydrothermal Synthesis
Hydrothermal synthesis depends on GO sheets. These sheets are oxidised under acidic treatment (HNO3), creating a line of rupture along the C-C bonds because of the formation of the epoxy groups. Epoxy is a functional group with an oxygen atom joined by 2 single bonds to 2 adjacent carbon atoms which creates a tri-membered epoxide ring. The conditions have to be quite specific for the correct outcome where the temperature must be 200 degrees celsius and the ion and alkaline medium must be basic (pH = 8). The epoxy groups are stabilised in carbonyl groups making the GQDs soluble in water. However, depending on the desired GQD, the situation is catered that way. For example, when creating green luminescent GQDs, the GO sheets are reduced into graphene sheets with a temperature of 600 degrees celsius, completing the process within two hours. The alkaline is set to a pH of 12 and the green GQDs are formed.
The hydrothermal synthesis can also be done through the microwave assisted hydrothermal method (MAH). The microwave heating is a fast and even heating process where glucose molecules are dehydrated to form the nucleus of GQDs and are composed of C = C bonds. When increasing the heat, the growth of the GQD will occur at the peripheral surface (edge growth) by the formation of the C = C bonds in between the glucose molecules. The molecules then reach the surface and generate new double bonds by dehydration. Overall the GQD will become larger as the heating time increases.
5. The Quantum Confinement Effect
The quantum confinement effect is vital to understand the umbrella of quantum dots. This theory describes electrons with energy labels, potential wells, valence and conduction bands and electron energy band gaps. These properties are observed when the particle size is too small to be comparable to the wavelength of the electron representing the energy of the electron. The confinement of electrons depends on material properties revolving around the Bohr radius (ab). The key rule to this effect is that the band gaps of groups II — VI semiconductors become more and more narrower as the atoms become heavier.
5.1 Understanding the Quantum Confinement Region
Colloidal semiconductor nanoparticles exhibit a distinctive set of optical and transportation properties due to the spatial confinement regime known as the quantum confinement effect (section 5). These bulk semiconductors are characterised by composition dependent band gap energy. This band gap is the minimum energy required to excite the electron from the ground state valence energy band into the vacant conduction energy band. Now, the presence of multiple atoms causes the splitting of electrons when grouped, forming the energy band. The most filled band is known as the valence band containing lower energy levels whereas the most empty band is known as the conduction band containing higher energy levels. The valence and conduction bands are forbidden by an energy gap known as the band gap. To excite the electrons on the valence band to the conduction band, the applied radiation of energy radiation must be equivalent to the forbidden energy of the band gap. After the absorption of the suitable energy, the electrons can jump into the conduction band from the valence band causing the formation of vacant space in the valence band known as a hole (h+). When the electrons and holes are paired, they create a hydrogen-like specific known as an exciton. When the excitons for a certain semiconductor have separation between the electron and holes, it’s called the exciton Bohr radius.
Electrons (e-) and holes (h+) attract each other because of Electrostatic Coulomb Force meaning they attract due to their opposing charges. Once they attract, the electrons and holes go into a bound state which is one way that the energy can take in the potential given by a figure. This bound state occurs when the particle cannot travel to infinity meaning that it is confined to a potential well (section 5.2) causing the particle to remain localised. The bound state where the electrons and holes attract are called excitons. This results in an electrically neutral quasiparticle existing within semiconductors which is where a lattice has momentum and position to be considered a particle.
In QDs, the excitons are confined to a smaller volume of semiconductor materials which results in less splitting of the energy bands leading to the quantum confinement region. This region, with the electron hole pairs in different dimensions, are discrete and quantized. The size and composition of the QDs can be altered to allow energy levels and band gaps to be fine-tuned to the desired energies meaning they can be engineered to meet the specific biomedical applications.
An important characteristic is that the conductive region creates the regulated quantity of excess electrons which are constrained to a small region by the application of electromagnetic additions. The parameters can be altered to achieve the desired configuration for the needed application.
5.2 Particles in a Box Theory
The particle in a box theory is where quantum mechanics come into play. This is known as an infinite potential well and describes the particle’s free ability to move in small spaces surrounded with impenetrable barriers. These particles can occupy positive energy levels where higher band gaps result in smaller emission wavelengths.
Overall, these particles in a box are when the exciton diameter are isolated by the bound state and can be measured through:
Where h represents planck’s constant related the energy of one quantum (photon) of electromagnetic radiation to the frequency of that radiation. m = mass of the particle and L = length of the box. This equation proves that quantum dots and electrons can only occupy certain discrete energy levels which is why they are called artificial atoms.
As a whole, the particle in a box theory is an application of Schrödinger’s equation. This equation describes how a physical system, when subjected to certain forces, will change overtime.
The equation resulted out of the way that e- behave like particles in some situations but act as waves in others. We know that the particle is somewhere in that potential well at all times but we don’t know where exactly it is until we measure it (almost like superposition). A common myth is that the wave function describes a physical wave in space where the particle is spread out like “goo.” We never see this particle goo yet it somehow contracts into one point where we make a measurement.
However, this is a large misconception because the wave does not describe a physical wave since it is not a function defined on a physical space. Instead, it is designed on configuration space. This configuration space takes all the input as possible configurations of the locations the particles could be in and returns a value related to the probability that you will find the particles in a given configuration at a given time.
6. Biosensors
Although quantum dots become complex when considering quantum mechanics, it is these exact complexities which allow for unique application, one of which is biosensors. Biosensors allow for accurate and early detection of analytes which is a major and attainable goal for point of care testing (POCT). These QDs are ideal candidates as they have a high density of activated sights on the surface.They display a wide range of electronics semiconducting to insulating properties. The surface groups can also be modified with defect engineering and functionalizations which can selectively respond to specific target materials.
There are different types of wearable biosensors, Vivo and Vitro. Vivo sensors are typically invasive and are performed within an organism causing them to be invasive as they require body fluids obtained by injecting into the body. In Vitro sensors, the sensing is performed outside an organism causing them to be non-invasive.
7. Subset of Quantum Dots — Graphene Quantum Dots: Applications in POCT
7.1 Paper-strip Embedded GQDs
GQDs are heavily used in biosensing since they have a high stability, PL quantum yield and high biocompatibility (ability to be compatible with living tissue and have no toxic response to the body and body fluid). These nanoparticles can detect heavy metals, small molecules and biomacromolecules (large biological polymers — nucleic acids, proteins, carbohydrates, etc). However, when GQDs and paper merge, we are given a paper-based platform.
This tool is where blue luminescent GQD sensing probes are embedded into a nitrocellulose matrix. These probes are synthesised from citric acid by the pyrolysis procedure (section 4.1). Essentially, it’s the thermal decomposition of materials at elevated temperatures in inert atmospheres.
After this procedure, they were then physisorbed which is the bonding of gas molecules to the surface of a solid or liquid that gases come in contact with at low temperatures. These quantum dots are confined within small wax-traced spots on a nitrocellulose substrate which creates a resonance energy transfer, acting as the sensing mechanism. The bonding of gas molecules to the solid/liquid surface creates Van der Waals forces between the gas molecules. LED light from a smartphone then excites the GQDs which are then placed within a 3D printed dark chamber as it isolates the paper from external lights which could also excite the GQDs. The smartphone is used as an energy source and the digital colour imaging capture. This is an easy to use, low cost and disposable paper-based sensing device for chemical screenings with a phone readout. In this solution, the dark chamber is a vital part of the solution. There is a strip hole where the paper strip goes through where each spot is processed one at a time to reach the LED light given by the smartphone. The light will excite the confined GQDs at 365 nm. The phone then captures the light and PL properties emitted by the GQDs through the camera at 460 nm and the amount captured by the fluorescent light will be quantified through the phone displaying the results.
The use of paper is what allows for a cheaper and disposable solution. Paper has been introduced in the labs for centuries where they discovered the valuable substrate for biosensors one decade ago. Using the smartphone increases accessibility and potential for use as they allow for quick data processing and collecting, displaying real-time quantitative information.
7.2 Paper-based IoT Device
Using a paper-based device for POCT means its cost effective, handy, environmentally friendly, easy and quick. µPADs are microfabricated paper-based analytical devices where the sensitivity can be tuned by incorporating active catalysts such as silver nanoparticles, gold nanoparticles, QDs, etc. Since the detection can be fine-tuned, this can be calculated by dividing the slope of the linear range by the surface area using the projection area.
As seen in section 7.1, this device has a phone readout where the smartphone displays and reads out the results of the test. This readout has a barcode-like design which was developed for easier analysis of length-based information through smartphones. The results are then generated as electronic data and can be directly transferred through a text message to the patient and doctor.
This particle µPAD uses Whatman-4 paper used for distinct applications as they contain a larger pore size and higher retention rate. The nitrocellulose membranes are used due to the functional groups and allows for covalent immobilisation of biomolecules, weak hydrogen bonding, charge to charge interaction and protein-based substrates with Van der Waals interaction.
µPADs are incredible for on-site screening of overall human health and provide minute modification to the surface to tune the screening based on the patient. These devices can be modified to do numerous things. For example, they can detect and isolate bio-targets and pathogens in plasma, blood and fluid. These can be done through simple surface modification via nano functionalization of electrodes and detection zones. This functionalization is done through smart materials and can capture and release specific components at varying conditions by isolating the analyte.
8. Quantum Dot Applications
8.1 Point of Care Testing
Microfluidic devices are made with PDMS (polydimethylsiloxane: a type of silicon-based polymer) and thin glass slides. More specifically, PDMS is a non-toxic dimethyl silicone-based oil with low viscosity, high molecular weight and distinctive flow properties. When creating the device the two main methods consistent are photolithography and the replica modelling technique, where the number of chambers and microchannels differ based on the user’s needs. Using QDs for bioimaging only requires a little bit of fine-tuning and can do incredible things in each application. For example, experts used QD-IgA as a conjugate of the QD with the microfluidic chip for cancer detection. It was shown that the microfluidic systems improve the detection limit up to 4 orders in magnitude compared to the organic dyes.
These small scale integrated devices can be used for high throughput bioassays of DNA, bacteria, protein and viruses. The QDs have high potential in bioanalytical applications due to their distinctive PL properties and ability to fabricate them as a biocompatibile system with the conjugation to the varied biomolecules. The easy availability of different types of QDs for clinical use allow for the advancement in clinical use and can even create an entire system running off of QD applications as follows: bioimaging, biosensing, immunoassays, drug delivery, detection of genetic disease, clinical application. Read more in the following sections.
8.2 Bioimaging
Bioimaging is vital to understand the issue that needs solving in the body. Current techniques come at high costs requiring long imaging time and are poor at early stage detection causing many drawbacks. QDs however have a probe which can overcome all of these barriers. Conventional bioimaging is based on the biological specific conjugated with organic dyes to enhance the target. The biggest disadvantage is the high photosensitivity, broad emission bands and narrowed absorption.
The QDs especially are a great candidate for bioimaging because they control the size, composition and one coating can tune the desired emission needed with the corresponding absorption wavelengths. They have a high quantum yield and brightness allowing for multitarget bioimaging. The quantum yield (denoted by Φ) is vital and determines the efficiency of release of electrons for a minimum of Φ1 or 100%.
8.2.1 Vitro Bioimaging
Vitro bioimaging is non-invasive but allows for in depth analysis of the site. The diameter of the QDs range from 2–20 nm allowing for unique metallic and the smallest nanoparticle with only 200–10000 atoms. This produces enough electron density and high molar extinction coefficient which is the measure of how strongly a substance can absorb light at a specific wavelength; leading to a higher contrast in bioimaging.
The conjugation of the surface coatings let them effectively be incorporated by the cells and are utilised as tags and labels. They are used to see cellular receptors and structures to mimic the properties of organic fluorophores. Since the conjugation is a large part of the QDs, there are two main methods to the antibody conjugation. Firstly, the QDs antigen are labelled as primary and secondary antibodies. The primary is attached to the antigen and it is recognized by the biotinylated secondary antibody which is further attached to the coated QDs. Biotinylated is the process of attaching biotin to proteins where biotin maintains many major body systems. The second method is where the biotinylated primary antibody is conjugate onto the coated QDs and is directly recognized and targeted by the antigen.
8.3 Quantum Dot Biosensors
Quantum dots are incredibly helpful for biosensors since they can attach various biomolecules, both covalently or noncovalently to their surface which is further capped by the QDs through cross-linker molecules without negatively impacting the properties. Crosslinking is when the two molecules are joined by a covalent bond and the cross linkers are molecules containing two or more reactive ends which chemically attach functional groups on proteins and molecules. These reactive groups which end up on the hydrophilic structure help with attachment through ionic strength, surface charge, pH and temperature. An example of this crosslinker is EDC (ethyl-3-carbodiimide) which links NH2 and COOH.
Now, the pro with using QDs over fluorescent dyes is that they include a higher quantum yield, hyper photostability and size-tunable advantages for the fluorescent emission therefore, QD based sensors are highly targeted, sensitive and stable. The QDs also contain a broad and narrow excitation spectra, great for donors for FRET (fluorescence resonance energy transfer) based biosensors where the interactions between the fluorophores of two molecules are measured. For example, there’s more rapid and sensitive methods for detection of nucleic acids by using 2-colour quantum dots based on a single molecule detection. Compared to the traditional QD based assay, this method is high in detection efficiency and high hybridization efficiency with a short analysis time.
8.4 Clinical Applications
QDs are more than 100x stable and 20x brighter in comparison to traditional reporters inducing fluorescence. They act as promising antimicrobial agents as well as novel sensors for antigens and allergens and are used for the detection of protein.
More specifically, further lateral flow tests are some of the most common POCT and are used for the detection of various analytes (nucleic acids, antibodies, biomarkers, viruses). They are easy to use, give rapid results and are portable. They work differently for every disease and are personalised rapid tests. For example, there was a POCT created for the detection of syphilis. SPA (staphylococcal) protein A is conjugated to the QD. The QD labelled SPA is then mixed with the sample and the QD based lateral strips contain a mixture of the syphilis antigen (TP15, TP15, TP47). The QD-based POCT is then suitable for rapid on-the-site testing of syphilis.
8.5 Barriers
Although QDs hold great potential, they also have various issues and barriers stopping them from advancing, including: biocompatibility, short term stability, chronic toxicity and long term breakdown. The cytotoxicity levels are specifically determined by various factors and conditions such as: size, colour, dose of QDs surface chemistry, etc. However, due to this toxicity, they are restricted to certain Vivo applications. Most applications from the QDs also come in bulk solutions which limit the development of reusable sensors. The surface chemistry on the QDs are also poorly understood in these varying circumstances causing a large concern for the potential use.
9. Conclusion
The future prospect of using quantum dots in point of care testing allows for the fast track detection and quantification of diseases. With a cost effective, rapid, accurate and portable solution, this is to combat the need for healthcare providers during the detection process. It’s also helpful that nanotechnology is already being implemented in various fields in up to 62 countries and it’s different from previous technological revolutions, since these characteristics would place developing countries in a more favourable position to face this revolution and benefit from it.
Nanotechnology could remodel the healthcare system in developing countries and make treatments and detection more readily available for diseases that claim millions of lives around the world each year.
10. Sources
