Imagine an ambulance kit that quickly and accurately checks unprepared blood for indications of a heart attack without making EMS personnel wait for lab results. And what if the device also lets clinicians accurately diagnose and differentiate common cold, influenza, and avian flu, or check for multiple genetic or protein markers for different diseases at the same time. Impossible? New biomarker detection systems based on nanotags are said to provide a diagnostics platform that is turning rapid, reliable, multiple biomarker measurement into reality for clinical lab and patient care at the bedside, clinic, ambulance, and home.

Biomarkers, typically proteins, are associated with the presence and severity of specific diseases. Biomarkers are detectable and measurable at the molecular level by laboratory assays and specific imaging technologies such as Raman spectroscopy, a technique in which scattered light, usually from a laser, quantifies and qualifies the structure of matter.

Early detection-tag technologies

The first widely-used tags to keep track of biomarkers used radioactive labels such as I¹²⁵ within assay formats such as radioimmunoassay (RIA) tests for protein and small molecules. The RIA procedure involves measuring minute amounts of a tagged substance by determining how much binds to an antibody. The tags withstood interference from other compounds, but concerns about their stability and disposal limited their use.

Current technologies include fluorescent and chemiluminescent tags, which emit light as a result of a chemical reaction. However, the tags, while potentially sensitive, are prone to interference from components that naturally fluoresce. Such tags lack the reliability required for for diagnostic systems used for near-patient care. In addition, for clinical diagnostics, the tags can't handle the simultaneous measurement of multiple biomarkers (multiplex). This is increasingly important for more accurate disease diagnosis and therapeutic monitoring.

Biomarker-detection systems

Current biomarker systems also required improvements to support advances in tag technology and wider testing. Optimal systems must combine flexibility with short time-to-results and high throughput. A system must work well in a wide variety of applications from rapid near-patient devices to advanced continuous random-access hospital laboratory systems.

Systems must also be reliable and provide sufficient sensitivity to detect diseases early when biomarker levels may still be low (at pico-molar amounts). Reliability and sensitivity are critical because biomarkers are measured in complex biological fluids which contain many naturally occurring substances, such as flurophores, that can interfere with assay results. In addition, many diseases produce an abnormally high number of compounds that may interfere with biomarker detection. These compounds can include bilirubin, a reddish-yellow bile pigment derived from a component of haemoglobin, and lipids, organic compounds such as cholesterol and triglycerides or fat-transport systems.

An optimal system must also measure low-volume samples including whole blood, which has a complex biological matrix. Taking lower-volume samples is less invasive and stressful for patients, important in the long-term monitoring of chronic diseases such as infection and cancer, particularly in infants having low blood volumes.

SERS Nanotags

New technology such as SERS Nanotags from Oxonica Healthcare, Oxford, UK, are said to address the requirements for such next-generation systems. The nanotags exploit Raman scattering, using laser excitation to cause a “reporter“ molecule to provide biomarker information. Raman scattering has been recognized and highly documented for many years, but it has not previously been used in clinical diagnostics because of its weak signal, which compromises detection sensitivity and dynamic range. However, Raman scattering is amplified when the reporter molecule is in a nanoscale-proximity to a roughened metal surface, a phenomenon is known as Surface-Enhanced Raman Scattering (SERS). Gold is ideal for this task.

The company says it exploits this phenomena, manufacturing 50-nm gold beads (tags) for SERS applications. The tags are coated with a reporter compound and encapsulated with silica, or glass. This protects them from the biological matrix to be analyzed and provides an ideal surface for biomarker attachment. Typical assay formats use antibodies attached to the silica surface as tools to bind to the target biomarker. When the bead is exposed to infra-red light, it gives a unique signal that indicates how much of the biomarker is present.

Similarly, DNA stands, which may contain disease-specific information, may be attached to facilitate DNA assays, also known as molecular assays. The company says the tiny nanotags also have potential for imaging applications within living organisms.

Each reporter molecule has a unique SERS signal so a mixture of nano tags containing different reporters can be measured simultaneously. This feature is possible by coating specific antibodies or DNA strands onto the nano tags. For example, reporter molecule 1 is associated with DNA strand x, reporter molecule 2 with DNA stand y, and so on.

The company is working to validate the application of nanotags in a biomarker-detection system with potential for measuring extremely low levels of multiple biomarkers in blood-collection tubes. This avoids complex assay procedures such as plasma separation and washing.