Nanopillars – a State-of-the-art Optical Sensor

Reliable, cheap and quick recognition of molecules at challenging concentrations of ~1 ppb (parts per billion) or less is now possible thanks to a sensor developed at DTU Nanotech.

nanopillars

Potential analytes include toxic food additives, chemical warfare agents, hazardous building materials and human disease markers.

The sensor consists of nanopillars with heights of 600 – 800 nm. These pillars are able to enhance significantly the spectroscopic fingerprints of the target molecules nearby, making them distinguishable at ultra-low concentrations. The technique is called surface-enhanced Raman spectroscopy (SERS).

Postdoc Kaiyu Wu from DTU Nanotech says that “an ideal sensor for SERS should exhibit reproducibly high enhancement over macroscopic areas and be cost effective. It is extremely difficult to fulfil both.”

The nanopillar sensor is among the very few throughout the world known to achieve both standards.

Just like ‘pixels’

“Hot spots are the key elements in a SERS sensor, as they resolve the spectroscopic fingerprints of the target molecules. They are like pixels in a display that resolve different parts of an image. However, hot spots cannot be seen with the naked eye since their dimensions are only several to tens of nanometers”, says Kaiyu Wu.

The initial nanofabrication process of the nanopillar sensors was developed by Senior Researcher Michael Stenbæk Schmidt at DTU Nanotech. The process does not require any lengthy and expensive lithographic steps, thus it provides possibilities for the produced nanopillars to be used as cheap and expendable consumables.

Nanopillars
Nanopillars under a scanning electron microscope.

The aim of Kaiyu Wu’s PhD project, which finished recently in June 2016, was to engineer ‘pixels’, i.e. hot spots in the nanopillars in order to take their sensing performance to the next level. This goal was achieved with the help of advanced spectroscopic and theoretical tools, as well as state-of-the-art nanofabrication techniques at DTU Nanotech and DTU Danchip.

The improved sensors now possess high-quality, high-density and reproducible nanoscale hot spots over macroscopic areas, as well as enhancement factors ultimately eclipsing the 100 million mark for molecular fingerprints.

The nanopillar sensors can be easily paired with portable read-out systems to enable on-site detection of analytes. Furthermore, their high-quality and high-density hot spots guarantee an extra-high sensitivity which reduces the time of detection to just several seconds for regular targets.

Kaiyu Wu’s research is part of the ‘NAPLAS – NAnoPLAsmonic Sensors’ project funded by The Danish Council for Independent Research, and the ‘HERMES – High Exponential Rise in Miniaturized cantilever-like Sensing’ project funded by the European Research Council.

Kaiyu Wu is part of the Nanoprobes research group at DTU Nanotech led by Professor Anja Boisen.