Turkish researchers develop low-cost, flexible device using simple methods
Bisphenol A (BPA), also known as 2,2′-bis(4hydroxyphenyl)propone, is widely exploited as an additive in polycarbonate and epoxy resins, which in turn are extensively used in the production of food and beverage containers. In April 2023, the European Food Safety Authority declared that dietary exposure to BPA – through leaching from containers – posed health risks to humans. After their re-evaluation, the EFSA reduced the tolerable daily intake for BPA by four orders of magnitude (from 4µg to 0.2ng per kilogram of body weight per day). So, a new paper from researchers at Necmettin Erbakan University in Turkey, reporting on a low-cost, high sensitivity BPA sensor, seems very timely indeed. Published in Carbon [DOI: 10.1016/j.carbon.2023.118362], the paper brings together electrochemistry, multi-walled carbon nanotubes (MWCNTs), and inkjet printing.
The first step involved developing an electrochemically-active ink that could be successfully printed using a basic commercial printer. Carbon nanotubes have repeatedly been proposed as an electrically-conductive filler for inks, but challenges with their dispersibility have limited their widespread use. Printer inks should have low viscosity and surface tension. They should be stable over time, and not form flocs or aggregates that could clog the printing nozzle. The Turkish team chose to use shortened, nitrogen-doped multi-walled carbon nanotubes (MWCNTs). These were produced by mixing MWCNTs, melamine, and hardened stainless steel balls in high energy ball milling setup, with milling time varied from 0 to 60 minutes. After heating to 600 °C, the nanotubes were then modified with sulfonate (SO3H-) groups to further improve their dispersibility.
The resulting SO3H/N-SMWCNT samples were then dispersed in deionized water in varying concentrations. The ink formulations were centrifuged, before small quantities of sodium dodecyl sulfate were added to reduce the ink’s viscosity. Structural characterisation of the inks showed that the high energy ball milling step not only shortened the length of the CNT bundles, it also created a large proportion of defects on the CNTs – together, these features enhanced their dispersibility. The addition of melamine as a nitrogen source was also confirmed to have played a role in retaining the tubular structure of the MWCNTs. Zeta potential and average particle size measurements were used to evaluate the stability of the ink samples. This analysis showed that using their fabrication approach, it was possible to produce stable, printable CNT inks with a concentration of up to 20 mg/ml.
The SO3H/N-SMWCNT inks that had been ball-milled for 15 minutes were loaded into commercial ink cartridges, and used to print electrode patterns on flexible, plasma-treated PET (polyethylene tetraflate) surfaces in multiple passes. By continuously monitoring the resistivity of the printed electrodes, they found that 70 passes provided optimal performance. Bending tests confirmed that the electrodes were mechanically stable and robust.
The fabricated flexible electrodes were used as the working electrode in an electrochemical cell, and their performance was tested against bisphenol A (BPA). The authors write, “The electrodes did not show any obvious electrocatalytic peak between the potentials of 0.0 and 1.0V. Upon the addition of BPA into the [PBS] electrolyte, additional oxidation peaks were observed ascribing to BPA oxidation.” They also found that pH had an effect on the electrocatalytic behaviour of the printed sensors, with the highest oxidation currents obtained at a pH of 5.0.
Chronoamperometry (CA) and differential pulse voltammetry (DPV) were used to further evaluate the performance of the sensor. Results showed that the sensor has a wide linear range of 5–100 (CA) and 60–700 (DPV), and a limit of detection of 0.7 μM, outperforming “conventional …. glassy carbon, screen-printed electrode-based BPA sensors.” The BPA detection process was also found to be unaffected by the presence of interferants such as glucose, dopamine and uric acid. And finally, when tested in a real medium – cow’s milk – the sensor’s behaved linearly, matching its performance in PBS.
They conclude, “Inkjet-printed BPA sensors were successfully developed by printing SO3H-functionalized N-doped shortened MWCNTs on flexible substrates……inkjet printing technology has not been implemented to develop flexible/nonflexible electrochemical BPA sensors before.” Given that all components of the sensors, including the functional inks, were made using very standard, simple preparation methods, it suggests that these sensors show potential for industrial scale-up.
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Sumeyye Sarikaya, Hasan Huseyin Ipekci, Hasan Kotan, Aytekin Uzunoglu. “Inkjet printing of highly dispersed, shortened, and defect-rich MWCNTs to construct flexible electrochemical sensors for the detection of bisphenol A in milk samples,” Carbon 214 (2023) 118362. DOI: 10.1016/j.carbon.2023.118362