Gold detector is cheap on paper: Detecting paracetamol on a paper-based microfluidic device

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  • Published: Apr 16, 2012
  • Author: Jon Evans
  • Channels: Detectors
thumbnail image: Gold detector is cheap on paper: Detecting paracetamol on a paper-based microfluidic device

Sputtering electrodes

Gold detector is cheap on paper: Detecting paracetamol on a paper-based microfluidic device  

By sputtering gold onto paper, Brazilian chemists led by Lauro Kubota at Unicamp's Institute of Chemistry in Campinas have come up with a cheap, easy-to-use microfluidic device for detecting analytes. What is more, unlike most other paper-based microfluidic devices, this one can be used more than once.

The sputtering process involves using energetic particles to blast atoms off a piece of gold and onto a sheet of paper. By utilising a copper mask, the gold atoms can be deposited on the paper as three thin electrodes, just 200nm thick. The electrodes are positioned at the far end of a flat channel formed by two lines of wax, each around 3cm long, separated by just 2mm.

Hooking the gold electrodes up to a power supply allows them to perform amperometric detection, in which analytes generate a detectable current when they are oxidised or reduced at the central working electrode. The idea is to introduce an analyte solution into the channel between the two wax lines; the solution then naturally travels along the channel by soaking into the paper while confined within the wax lines.

The analytes in the solution travel at different speeds depending on the extent to which they interact with the paper, potentially allowing them to be separated. Finally, the analytes are detected at the electrodes by amperometry.


Cation-exchange paper

To test whether this set-up actually works in practice, Kubota and his team tried using their paper-based microfluidic device to separate and detect the common pain-relief drug paracetamol and 4-aminophenol (4-AP), an important precursor chemical for paracetamol and thus a common impurity in paracetamol products.

In their first attempt, they fabricated the microfluidic device on standard chromatographic paper, but found that they were unable to separate the two analytes because both of them interacted with the paper in a similar way. So Kubota replaced the chromatographic paper with special cation-exchange paper covered in negatively-charged groups, because 4-AP has a positive charge whereas paracetamol doesn't, ensuring that 4-AP will interact more.

Using this cation-exchange paper, Kubota and his team were able to separate and detect paracetamol and 4-AP within 25 minutes. With limits of detection of 25µmol L-1 for paracetamol and 10µmol L-1 for 4-AP, the paper-based device is not actually that sensitive, at least compared with conventional analytical methods such as high performance liquid chromatography (HPLC) and capillary electrophoresis (CE). However, the device is still sensitive enough to determine whether 4-AP is present in paracetamol products at concentrations that exceed approved levels in the US and the UK.


Absorbent pads

Furthermore, the paper-based device has numerous other advantages over conventional analytical methods. It's obviously much cheaper than HPLC and CE, and doesn't require the use of pumps or high voltages, as the solution travels down the channel by nothing more than capillary action. Also, unlike some other methods, it doesn't require the paracetamol samples to be filtered to remove particles prior to analysis; the paracetamol products just need to be dissolved in acetate to produce a solution that can be injected into the channel.

Many other paper-based microfluidic devices share these advantages, including an earlier version developed by Kubota (see Follow the paper trail), but what makes Kubota's new device rather unique is that it can be reused. The reason other paper-based devices can't be reused is because there's often nowhere for the waste to go after the analysis is complete, which usually isn't a problem because the device is designed to be disposable.

To make their devices reusable, Kubota and his team simply placed an absorbent pad, consisting of lots of small squares of chromaography paper, at the far end of the channel after the electrodes. This pad soaks up all the waste, allowing the same device to be used up to 60 times, making it an even more cost-effective analytical option.

Related Links

Analytica Chimica Acta, 2012, 725, 44-50: "Separation and electrochemical detection of paracetamol and 4-aminophenol in a paper-based microfluidic device"

Article by Jon Evans

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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