Laser textured superhydrophilic silicon for uniform solidification and sensitive detection of water based samples using laser induced breakdown spectroscopy

In this report, we have evaluated the use of laser textured silicon substrates for uniform solidification of water based samples for their analysis using laser induced breakdown spectroscopy (LIBS) with good repeatability. The laser texturing imparted stable superhydrophilicity to the silicon surfaces and this superhydrophilicity resulted in uniform spreading and solidification of the water based samples over the textured silicon surfaces. Uniformity of the samples solidified upon evaporation of water was assessed by using the shot-to-shot relative standard deviation (RSD) of the LIBS spectra recorded from the central region of the textured area. The RSD for the solidified water samples containing Cr, Cd and Sr was found to be in the range of similar to 6.4-7.8% and that for the blank laser textured silicon i.e. without any sample, was found to be similar to 4.5%. These results suggested that the water based samples got solidified with good uniformity within the central region of the superhydrophilic textured area. The laser textured area also served to confine the liquid samples within the textured area owing to its superhydrophilic properties, and due to this confinement, a liquid volume as small as 3 mu L was found to result in a ppm or sub-ppm level limit of detection depending upon the sensitivity of the LIBS spectral line. Furthermore, the limit of detection could be improved to ppb level by repeating the process of placing and drying small droplets over the textured area without significantly affecting the shot-to-shot RSD of LIBS spectral peaks for up to a few 100's of such repetitions.