Materials
the sensing relies on a chemical reaction, the large surface area translates to high reactivity and, consequently, faster reaction times. This allows for quick detection of analytes like ions, lipids and proteins, among others. Additionally, nanomaterials exhibit unique mechanical and optical properties. “The effect of the nanoscale introduces a whole new set of properties,” says Tony Cass, professor of chemical biology at Imperial College London. “Which can either improve the sensitivity of existing materials or open up completely new ways of making measurements.”
The gold standard
Rapid testing kits are the most commonly available sensors that use nanomaterials for biosensing. These kits use lateral flow assays to detect pregnancies, Covid-19 or allergen sensitivity, among others. They use nanoparticles – typically, gold nanoparticles (GNPs) – to achieve high sensitivity and specificity in analyte detection. In clinical settings, similar GNP- based biosensors are used to identify biomarkers for different diseases like cancers and cardiovascular diseases. “We use gold nanoparticles to either increase the surface area [for signal detection] or to enhance the signal,” says Pedro Estrela, director of the Centre for Bioengineering and Biomedical Technologies at the University of Bath. In the latter case, the gold nanoparticles tug at the target molecules to make them more detectable.
Gold nanoparticles are the gold standard for medical biosensors for good reason: they have unique optical and electrochemical properties that make them suitable for rapid visual biosensing. When electrons hit gold nanoparticles, they emit scattered light at particular wavelengths – a phenomenon known as surface plasmon resonance. Modifying the shape and size of gold nanoparticles changes the emitted spectra, allowing design of biosensors that detect specific molecules and can change in colour in response. Plus, the high conductivity of gold and the nanoparticle structure facilitate ultrafast electron transport between different electrodes and materials. GNP-based biosensors typically leverage one or both of these properties to detect analytes. Researchers are also developing biosensors that tap into other interesting properties of GNPs. For example, piezoelectric biosensors convert stress or strain into electric currents and can detect even the slightest changes in mass during biomolecular interactions. Another approach under investigation is hybridising gold nanoparticles with other materials or nanomaterials to boost the chemical stability or versatility of GNP biosensors.
In addition to using GNPs for detecting different analytes, researchers are looking for ways to optimise and automate their synthesis. A key aim here is to lower
the cost of nanoparticle synthesis, which is often the limiting step in the fabrication of GNP biosensors.
Carbon-based biosensing
In recent years, graphene has received a lot of attention in the field of nanomaterial-based biosensing. “Whether it’s for sensing infectious disease agents or monitoring biomarkers associated with diabetes, cardiovascular disease, or cancer, it is one of the first go-to materials in many different biosensors,” says Cass. Graphene is an allotrope of carbon that exists as a sheet of a single layer of carbon atoms arranged in a honeycomb lattice. The distinct composition of graphene gives it interesting properties. Mechanically, it is incredibly strong, despite being an atom thick, yet flexible. That means that you can add sensors to its surface without hurting its structural integrity. Optically, it exhibits plasmon surface resonance and is sensitive enough to detect single cells. The 2D material combines extremely high conductivity and low charge carrier resistance (meaning electrical current passes through easily) which enables ultrasensitive detection of biological signals. Estrela also sees great potential in using arrays of graphene-based sensors, most notably in field effect transistors (FET). These are semiconductor biosensors that do not need fluorescent probes or expensive optical instruments. This convenience, together with the ease of integration with other technologies and the flexibility of graphene, makes them suitable for use in wearables. Because of the thinness and high chemical stability of graphene, graphene FET biosensors have superior electrical performance compared to other FET biosensors. However, despite the hype around it, graphene has its limitations. For instance, the process of synthesising graphene is complicated, while the surface chemistry between graphene and different probes is not as well understood as that for nanoparticles and, as a result, graphene biosensing is not as robust and reproducible. Though graphene is highly sensitive to environmental fluctuations and, consequently, can pick up extremely weak signals amidst biological noise.
Another approach to carbon-based nanomaterials is to roll carbon, typically graphene sheets, into nanotubes. While the electrical and optical properties of carbon nanotubes are comparable to graphene’s, the former’s geometry permits greater electron transfer and signal transduction. Beyond carbon, scientists are exploring nanotubes made from silicon, nitrides, and peptides – each offering different opportunities for what they could detect.
Other advances
Although metal nanoparticles, graphene and nanotubes remain the most studied nanomaterials for biosensing, scientists are testing a range of new options, too. Some
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