There are currently many cheap paper-based devices out there, one most notably being the $1 paper microscope. But paper devices are being researched more and more due to their ability to perform a variety of functions at low cost.
They have gained considerable attention in the sensing field and a team of researchers from Spain have now developed a paper-based sensing device with a smartphone readout, using graphene quantum dots (GQDs).
Paper-based devices are becoming increasingly common. This is especially true for sensing applications, where the ability to produce a simple, low-cost and rapid sensing device is the ideal outcome- especially when researchers look towards large scale and commercial production potentials.
Paper-sensing devices have been developed for nearly 10 years now, mainly in the optical and biosensing fields. Many paper devices to date either use gold nanoparticles or quantum dots composed of semiconducting, inorganic and biocomposite materials.
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Whilst many of these devices can provide efficient detection through photoluminescence mechanisms, for biosensing applications, they struggle in terms of biocompatibility and cytotoxicity.
However, graphene quantum dots (GQDs) are known to possess sensing properties towards many molecules, including heavy metals, biological macromolecules and small molecules. Given that GQDs are known to possess a low toxicity, high stability and good biocompatibility, it was only a matter of time before a simple and commercially viable sensor became available.
Most sensing devices to date require a complex output, generally involving the use of spectroscopic or complex fluorescence instrumentation. The team of Spanish researchers have created a simple, low-cost paper-based sensing device, embedded with GQDs, that requires no equipment other than a smartphone.
The researchers chose paper as the device material due to a few simple factors- it is of low cost, simple fabrication and paper can easily ‘pick up’ aqueous solvents through capillary action.
The sensor can detect two different kinds of phenolic compound. The screening tool utilises luminescent blue GQDs sensing probes embedded into a nitrocellulose matrix. The researchers fabricated the GQD probes through a pyrolysis approach using citric acid, whereupon they were physisorbed and confined within small wax-traced spots on the nitrocellulose substrate.
The researchers built a homemade 3D-printed dark chamber and excited the GQDs through an ultraviolet light emitting diode (UV-LED). The dark chamber was used to shield the paper from external light, to ensure that there was no interference and reproducible results could be produced. A mobile phone was used as both the energy source and as the colour imaging camera.
The sensing mechanism is due to a resonance energy transfer phenomenon. Upon excitation from the UV-LED, due to the smartphone, the GQDs fluorescence quenching occurs when there is an interaction between the GQDs and specific phenolic compounds.
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The mechanism is thought to occur due to the energy emission spectrum from the donor fluorophore and the energy absorption spectrum of the acceptor overlapping. It is also known as a FRET quenching mechanism.
The device, so far, has been useful for two scenarios- screening antioxidant capacity related to the flavonoid content in wine samples and screening 4-nitrophenol and paraoxon pollutants in seawater samples.
The sensor also shows a great selectivity. Due to differences in the amount of quenching exhibited by the two phenolic compounds, it is possible for the sensor to be able to detect each compound separately when both are present in the same mixture.
This paper-based device opens up new opportunities for simple and fast screening of organic compounds. Even though this device has been developed to be selective towards these two compounds, it offers more possibilities for versatile applications, especially as biosensors, due to its simplistic, tuneable and low cost nature. It is expected to be especially useful in remote settings where sophisticated instruments are not always readily available.
“Paper strip-embedded graphene quantum dots: a screening device with a smartphone readout”- Álvarez-Diduk et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-01134-3
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