Probing ND7/23 Neuronal Cells Before and After Differentiation with SERS Using Sharp-Tipped Au Nanopyramid Arrays

The development of local plasmonic nanosensors which can investigate cellular processes, whilst remaining non-invasive, with high sensitivity is still challenging but of great interest in the field of sensing.^[1,2] The use of plasmonic nanostructures for Surface-enhanced Raman spectroscopy (SERS) measurements is one promising route for diagnostics and the measurement of cell activity and cell constituents. SERS allows for highly specific optical data in much shorter acquisition windows for the detection of molecules and markers of interest.<br/>In this work we show the development of a SERS based sensor, which was conceived with the aim of monitoring cell activity and to discriminate cells before and after differentiation.^[3] The SERS sensor is composed of a platform of label-free sharp-tipped Au nanopyramids. To create the array we demonstrate a fabrication technique based on colloidal lithography. We have however altered the technique by using surface charged PS spheres and controlling the pH of the water. This allows us to tune the size of the nanoparticle interstices with sharp definition before metal depositions and achieve tip curvatures of 10nm without the use/cost of mask etching (i.e., reactive ion or plasma etching) or other expensive fabrication routes. Characterisation of the array shows a good matching of both the experimental and simulated optical data, with a plasmonic hotspot confined at the tip of the structure.<br/>The focus of this study was ND7/23 neuronal cells where we performed SERS measurements of the cells both before and after differentiation. We found good success in discriminating the two groups’ data by multivariate analysis. From the SERS data we were able to identify fingerprints of both the cell membrane, such as lipids and proteins, and intracellular components, such as RNA/DNA. This shows that the sharp tips of our structure are also able to penetrate the cell membrane and spontaneous poration occurs. A viability test of the cells showed high viability on the sensor and the cells were still found to be in good condition after hours of measurements.<br/>This plasmonic array shows the realisation of a sensing platform which greatly enhances the SERS, with a high signal to noise ratio and reproducibility. This is a structure of interest as we show the development of an efficient SERS sensor based upon a novel facile low-cost fabrication route. The structure is also found to be a good bio-interface with high cell viability and the ability to spontaneously porate the cell. Taking in mind the potential of nanostructured colloidal templates, our fabrication technique is compatible for use with other substrates of interest, including flexible substrates for in-vivo studies. Moreover, the fabrication is very controllable and can be easily tuned to produce NPs with resonances in other wavelength windows of interest.<br/><br/>[1] <i>Royal Society of Chemistry</i>, <b>2018,</b> 143, 17, 3990; [2] <i>Nano Lett. <b>2013</b>, 13, 6, 2463</i>; [3] <i>Sensors Actuators B Chem. </i><b>2022</b>, 361, 131724.