A new type of glucose sensor that works using a magnetically polarizable nanoemulsion could help change the way blood sugar is measured. The new device does not rely on glucose oxidase enzymes, unlike conventional glucometers, but instead simply changes colour when it comes into contact with glucose.
A team of researchers, led by John Philip at the Indira Gandhi Centre for Atomic Research in India, made the new sensor using a magnetically polarizable oil-in-water nanoemulsion of droplets that have a radius of around 100 nm. They made the emulsion by mixing together ferrimagnetic nanoparticles of iron oxide (around 10 nm across) with oil, a surfactant and water.
When the solution is exposed to glucose and a magnetic field applied, its colour simply changes.
“We stumbled on this effect quite by accident while working with magnetically polarizable nanoemulsions for fundamental physics studies,” explains Philip. “We then measured the colour (or diffracted light wavelength) of the nanoemulsion using a spectrograph and noticed that the shift (or change) in the diffracted wavelength (Δλmax) was quite high and that it varied linearly with glucose concentration.”
To our surprise, the Δλmax value at 30 mM glucose concentrations was as high as around 69 nm in the system under study, but this shift could be even larger with more suitably tailored emulsions, says Philip. “Since the Δλmax varies linearly with glucose concentration, we realized that the emulsion itself could be used as a biosensor,” he tells nanotechweb.org.
The new device could help change the way diabetics monitor their blood sugar levels. Most existing glucometers are based on glucose oxidase enzyme platforms coupled to electromechanical systems in which the device response depends on enzyme activity or glucose mass transport. These techniques take a relatively long time to produce results and require quite complicated apparatus.
Label free and fast
“The novelty of our technique is that it is label (or enzyme) free and fast (it works within just milliseconds rather than minutes),” says Philip. “It also allows us to detect glucose concentrations visually without any electronic equipment.”
The device is also portable. “For qualitative glucose testing, you simply need to look at the colours in the nanoemulsion upon mixing with a fraction of blood or urine under a magnetic field that you might generate with a tiny magnet or solenoid. For quantitative sensing, all you would need is about 200 microlitres of nanoemulsion and a pocket sized fibre-optic spectrograph for testing your samples.”
How it works
So how does the sensor actually work? At a constant applied magnetic field, the nanoemulsion droplets form 1D chain-like structures that diffract light in the visible region of the electromagnetic spectrum, explains Philip. “The diffracted wavelength depends on the distance between the droplets. When glucose concentrations in a sample reach the 1–30 mM range, the diffracted wavelength shifts and since it varies linearly with glucose concentration, we can accurately determine this concentration using a calibration curve.”
Without an external magnetic field, the nanoemulsion droplets move about randomly (thanks to ordinary Brownian motion) but an applied magnetic field induces a dipole moment in each droplet, orienting it along the field direction. “Linear chain-like structures are formed along the field direction when the repulsive forces between the droplets exactly balance the attractive forces between them,” says Philip. “For perfectly aligned droplets spaced a distance d apart, the so-called first order Bragg condition is 2d = λmax/n, where λmax is the Bragg peak wavelength and n is the refractive index of water.”
As the droplets and the spaces between the droplets are about the same size as the wavelength of visible light, we see a Bragg peak in the visible wavelength range – which manifests itself as a colour change in these fluids that we can actually see with naked eye.”
The researchers are now busy trying to improve the sensitivity of their device. “We also need to work with companies that are interested in developing the sensor into a marketable product,” adds team member Vellaichamy Mahendran.
The current work is published in Applied Physics Letters.
Article by Belle Dumé