Introduction

Biosensors are special tools that can tell us what is in a substance. A biosensor combines biological sensing elements (receptors) with electronic components to detect substances, such as sugar levels in blood.¹ The transducer (tool that transforms the form of energy) then converts this binding event into a measurable signal, such as an electrical or optical signal.¹

Biosensors have gained significant attention due to their potential applications in various fields, particularly in:²

• Healthcare (including precision medicine)
• Environmental monitoring
• Food safety

In healthcare, biosensors offer rapid, accurate, and portable detection of clinically important biomarkers (the presence of a biological molecule which is used to determine if a normal or abnormal physiological process has taken place), enabling real-time monitoring of diseases and facilitating early intervention. For instance, glucose biosensors enable continuous monitoring of blood sugar levels in diabetic patients, providing valuable data for effective glycemic control (optimal blood glucose levels).² 

Moreover, biosensors hold great promise in environmental monitoring, offering efficient and affordable solutions for detecting pollutants, heavy metals, and pathogenic (cause harm) microorganisms in air, water, and soil. 

Lastly, biosensors play a crucial role in ensuring food safety by rapidly detecting harmful bacteria, allergens, and chemical residues in food products.²

The purpose of this article is to provide a comprehensive overview of biosensors, delving into their definition, working principle, applications, advantages, limitations, and future directions.

How biosensors work

Biosensors are innovative tools designed to detect specific substances or changes in substances. Let's break down how they're put together and how they work, without getting too technical.

Main parts of a biosensor

Biological receptor: This is the part of the biosensor that recognises the substance (or analyte). It could be anything from a piece of tissue to an enzyme or an antibody. This receptor binds with the target substance and produces a change (like a shift in light, heat, or electrical charge) that can be measured.²

Transducer: This component takes the change caused by the interaction of the biological receptor and the target substance and turns it into a signal we can measure, like an electrical signal.

• Example: In a glucose monitor (glucose biosensor), the enzyme glucose oxidase acts as the receptor, turning glucose into gluconic acid. A pH (acidity of a substance) sensor (transducer) then detects this acid production as a change in pH, converting it to an electrical signal

Signal Processing Unit: This unit takes the raw signal from the transducer and turns it into useful information. It filters out noise and amplifies the signal to make it clear and interpretable.²

Types of biosensors based on biological receptors

Biosensors are categorised based on the biological part that detects substances and the method they use to turn this detection into a signal we can read.

Enzyme-based biosensors (catalytic receptors):

• These are the most common and researched biosensors
• They can quickly measure changes in substances like glucose and urea in real-time
• Enzymes in these biosensors aren't used up, making the sensors reusable and reliable over time²

DNA-based biosensors (non-catalytic receptors):

• These biosensors use DNA to detect specific proteins or substances
• The key is how DNA pieces attach to their matches, which tells us if the target substance is present
• They're highly sensitive and selective, perfect for detecting low-level substances²

Types of biosensors based on how they work (transduction element)

Electrochemical biosensors:

• These are great for detecting DNA, glucose levels, and more
• They work by measuring electrical changes caused by the reaction between the substance and the biosensor
• They're popular for their stability, quick response, and high sensitivity²

Specific types of electrochemical biosensors:

• Potentiometric: Detects pH and ion changes. Used for detecting specific pathogens like Neisseria meningitides, Brucella melitensis, and Francisella tularensis
• Amperometric: Detects electroactive species, developed for the indirect detection of E. coli and Salmonella species
• Impedimetric: Monitors obstruction of the electron flow (impedance changes). Useful for the detection of bacteria in clinical specimens, food pathogens, and for controlling industrial microbial processes
• Conductometric: Used less commonly. Measures changes in electrical conductivity²

Mass-based biosensors (piezoelectric):

• These detect mass changes on a crystal surface, converting mechanical forces into electrical signals
• They're used in various applications, including electronic devices²

Optical biosensors:

• Work by detecting light interactions with substances
• Surface Plasmon Resonance (SPR) biosensors are a notable type, used for label-free and real-time detection of biomolecular interactions. May detect cancer biomarkers and infectious diseases^2,3,4,6

Summing up how biosensor works

The substance to be measured interacts with the biological receptor. This interaction causes a change (biological response) that the transducer converts into an electrical signal. This signal might be weak and noisy, so it's processed and amplified to make it clear and measurable. Finally, the processed signal provides us with information about the substance we're detecting, in a form we can understand (like a number on a display).²

In essence, biosensors turn biological reactions into understandable data, helping in areas like medical diagnosis, environmental monitoring, and food safety.

Applications of biosensors

From enzymes and DNA to how they convert detections into electrical or light signals, biosensors are vital in medical diagnostics, environmental monitoring, and much more.

Healthcare applications of biosensors

Biosensors can provide real-time physiological data that can help doctors and researchers better understand an individual's health status and response to therapy. By integrating this type of data into precision medicine approaches, clinicians can make more informed decisions about diagnosis, treatment, and prevention strategies in:

• Cancer detection and monitoring: They can spot early signs of cancer, track the disease's progress, and check if treatments are working
• Diabetes management: Biosensors monitor blood glucose levels, making it easier for people with diabetes to manage their condition
• Heart disease: Early detection of heart disease through biosensors can save lives by identifying risk factors and diseases sooner²

Detecting ovarian cancer with an SPR biosensor

Surface Plasmon Resonance (SPR) is an optical biosensor. It is the most accessible, key method used for detecting various substances without the need for special labels.⁴ It works by observing how a molecule stuck to the sensor's surface interacts with another molecule in a liquid.³

Lately, these sensors are being used more often to spot signals of diseases, including cancer.³

Researchers have come up with a special SPR biosensor designed to find CA 125, a marker that can indicate ovarian cancer.³ The tool offers a broad range of detection levels and selects what it’s looking for accurately, all while being affordable and straightforward to build.³

In short, SPR biosensors are groundbreaking tools for medical research, particularly in detecting important markers like CA 125 for ovarian cancer, offering hope for early detection and a better understanding of diseases.

Wearable biosensors

Wearable biosensors are becoming more popular because they can constantly track our health in real-time without being invasive. This means they can measure things like sweat, tears, saliva, and the fluid that fills the spaces between cells in our body known as interstitial fluid (ISF), all without having to pierce the skin or draw blood. These biosensors, especially those that use electrochemical and optical methods, are making big strides in monitoring everything from body metabolism products to bacteria and hormones right through to our skin.⁵

Here are examples of commercially available non-invasive or minimally-invasive wearable biosensors:⁵

• Watches
• Wrist straps
• Contact lenses
• Mouthguards
• Finger clips
• Patches

They collect samples in a way that's easy and painless, minimising the risk of causing infections or damage, making them very user-friendly.⁵

Wearable biosensors can act like personal health assistants that offer many advantages for people wanting to keep an eye on their health. Here's a look at the benefits they may bring:

• Stay informed about your health: Wearable biosensors can let you track your health status all the time, right as it happens. This means you could get to know your body better and can take charge of your health more effectively
• Catch health issues early: These devices may warn you about health problems before they become serious. Catching issues early can mean easier and more successful treatment
• Care from home: For those with long-term health conditions or older adults who find it hard to get around, wearable biosensors may mean fewer trips to the hospital. You can get the monitoring you need without leaving home, which also helps hospitals by keeping beds available for those who need them most
• Help your healthcare team help you: The data from wearable biosensors may give doctors a clearer picture of your health. This can lead to better advice, adjustments to your treatment if needed, and quick action if your health changes. Over time, the information gathered can help spot health trends, which might prevent problems before they start⁵

Environmental applications of biosensors

Pollution affects our health and quality of life. Biosensors come in handy for:

• Detecting toxins: They identify harmful chemicals and pollutants in the environment, like pesticides, which can harm both people and wildlife
• Spotting pathogens: Biosensors can detect dangerous bacteria and viruses in our surroundings, helping to prevent the spread of diseases²

Food safety applications of biosensors

Here's a look at how biosensors are making a difference in food safety: 

• Quality control: Biosensors check the quality of food during production and processing. This helps ensure the food is safe to eat and meets high standards
• Detecting contaminants: They can identify harmful substances in food, like chemicals or biological agents. This detection is crucial for preventing foodborne illnesses
• Monitoring cross-contamination: Biosensors track possible contamination on food surfaces and products, helping to keep our food safe²

Wearable biosensor challenges

Wearable biosensors are changing healthcare but face some hurdles:⁵

• Power issues
• Device communication challenges
• Data security and privacy
• Contamination

To tackle these issues, several strategies are being explored:⁵

• Energy-saving strategies: Using longer-lasting batteries, exploring new energy sources like biofuel or solar cells, and improving how sensors use power can help
• Smart usage: Changing how often the biosensor checks and sends data, especially during less active times, saves battery
• Wireless approach: Biosensors must communicate with the wearer, other sensors, and external devices efficiently. Bluetooth Low Energy (BLE) and Near Field Communication (RFID) are used but have limits. Research is ongoing to improve data transmission without draining the battery and to allow for communication over greater distances
• Protecting health info: Using strong authentication and encryption helps protect against data breaches. Monitoring for suspicious activity can also enhance security
• Reducing contamination: Possibility of using disposable elements for skin surface devices

Overall, while wearable biosensors hold great promise for advancing healthcare, addressing these power, communication, and security challenges is crucial for their successful integration into everyday health management.

Future directions of biosensor technology

Recent studies have highlighted significant advancements in materials used in biosensors, specifically piezoelectric and magnetostrictive materials.⁶ These materials have shown considerable promise in identifying different viruses, including the novel coronavirus (COVID-19), directly from the air.⁶

This research is crucial, as it paves the way for creating more sensitive, accurate, compact, and affordable virus detection sensors, preparing us for future pandemics. The goal is for these sensors to be integrated into everyday items like smart clothing, making virus detection more accessible to everyone.^4,6

There is a notable shift towards incorporating wearable biosensors into textiles and everyday accessories.⁵ This integration will make health monitoring seamless in our daily routines. The field of wearable biosensors is on the brink of significant breakthroughs, thanks to the ongoing collaborative efforts across engineering, science, and medicine.⁵

Summary

Biosensors are advanced tools that help us detect specific substances using biological elements combined with electronic components. They play a crucial role in healthcare, environmental monitoring, and food safety by providing quick, accurate, and portable detection capabilities.

• In healthcare: They enable real-time tracking of diseases and conditions like diabetes and heart disease, improving early diagnosis and treatment
• Environmental monitoring: Biosensors detect pollutants and pathogens, aiding in the protection of our health and environment
• Food safety: They ensure the quality of food by detecting contaminants and monitoring for cross-contamination

Wearable biosensors, which are part of this technology, offer continuous health monitoring in a non-invasive way, making health management more convenient and effective. Despite their potential, challenges related to power consumption, communication, and data security need to be addressed for their broader adoption.

Future developments in biosensor technology are focusing on creating more sensitive, compact, and affordable devices that can be integrated into everyday items for widespread health and environmental monitoring. This promising direction points to a future where monitoring and managing health and safety become seamlessly integrated into our daily lives.