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Colour changes of gold nanoparticles solution from dispersion form (red solution) to aggregation form (purple solution).

‘Invisible tattoo’ made of gold nanoparticles could aid in medical diagnostics

Image credit: Sarayut Watchasit/Dreamstime

Scientists in Germany have developed an implantable sensor made of gold nanoparticles that resembles an ‘invisible tattoo’ and can reveal concentration changes of substances in the body.

The idea of implantable sensors that continuously transmit information on vital values and concentrations of substances or drugs in the body has been on scientists’ radar for quite some time. Such sensors enable the constant monitoring of disease progression and therapeutic success.

But until now implantable sensors have not been suitable to remain in the body permanently but had to be replaced after a few days or weeks. One problem is implant rejection, because the body recognises the sensor as a foreign object. Meanwhile, the sensor’s colour, which indicates concentration changes, has been unstable so far and faded over time.

To tackle this, scientists at Johannes Gutenberg University Mainz (JGU) have developed a novel type of implantable sensor that can operate in the body for several months, and is based on colour-stable gold nanoparticles that are modified with receptors for specific molecules. Embedded into an artificial polymeric tissue, the nanogold is implanted under the skin, where it reports changes in drug concentrations by changing its colour.

Gold nanoparticles act like small antennas for light: they strongly absorb and scatter it and, therefore, appear colourful, and react to alterations in their surroundings by changing colour. With this knowledge, Professor Carsten Sönnichsen’s research group at JGU has exploited this concept for implanted medical sensing.

To prevent the tiny particles from swimming away or being degraded by immune cells, the team embedded them in a porous hydrogel with a tissue-like consistency. Once implanted under the skin, small blood vessels and cells grow into the pores.

“Our sensor is like an invisible tattoo, not much bigger than a penny and thinner than one millimetre,” said Professor Sönnichsen. And since the gold nanoparticles are infrared, they are not visible. However, the team said a special measurement device can detect their colour non-invasively through the skin.

Gold nanoparticles embedded in a porous hydrogel can be implanted under the skin and used as medical sensors. The sensor is like an invisible tattoo revealing concentration changes of substances in the blood by colour change.

Gold nanoparticles embedded in a porous hydrogel can be implanted under the skin and used as medical sensors. The sensor is like an invisible tattoo revealing concentration changes of substances in the blood by colour change.

Image credit: Nanobiotechnology Group, JGU Department of Chemistry

In their study, the JGU researchers implanted their gold nanoparticle sensors under the skin of hairless rats. They then monitored colour changes in these sensors following the administration of various doses of an antibiotic.

The researchers transported drug molecules to the sensor via the bloodstream. By binding to specific receptors on the surface of the gold nanoparticles, they induce colour change that is dependent on drug concentration.

According to the researchers, the sensor remained mechanically and optically stable over several months with the help of the colour-stable gold nanoparticles and the tissue-integrating hydrogel.

“We are used to coloured objects bleaching over time. Gold nanoparticles, however, do not bleach but keep their colour permanently. As we can easily coat them with different receptors, they are an ideal platform for implantable sensors,” explained Dr Katharina Kaefer, first author of the study.

In the future, the researchers said the gold nanoparticle-based implantable sensors could observe concentrations of different biomarkers or drugs in the body simultaneously. Such sensors could also find application in drug development, medical research, or personalised medicine, such as the management of chronic diseases.

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