Selective ablation of biological tissue and single cells on a glass substrate by controlling the laser energy density of nanosecond 193 nm laser radiation†
This paper describes the possibility of controlling and reducing the laser energy density of nanosecond 193 nm laser radiation in order to selectively ablate biological material from a glass substrate for LA-ICP-MS bioimaging applications. Atomic force microscopy and optical profilometry were used to study the shape of single-shot craters in dried gelatin droplets, ablated at low energy (<500 mJ cm−2). These craters were characterized by straight walls and a ‘flat bottom’. Based on the ablated volume per pulse and corresponding ablation depth, the ablation threshold of the dried gelatin material was estimated at 44 mJ cm−2 by relying on the Beer model derived from the Beer–Lambert law. Three different glass substrates, i.e. a microscope slide and coverslip of soda–lime–silica (SLS) glass with a slightly different bulk elemental composition and a borosilicate glass coverslip, were ablated using laser energy densities ranging from 150–730 mJ cm−2 and typical instrument settings applied during LA-ICP-MS bioimaging. The signal intensity of 29Si, a minor isotope of Si which is a major matrix component of the glass surface, was monitored as a measure of material removal and was used to estimate ablation thresholds for the three glass substrates at 262, 181 and 104 mJ cm−2, respectively. As a proof-of-concept, kidney tissue mounted onto a SLS glass microscope slide substrate and MDA-MB-231 tumor cells seeded on a SLS glass coverslip were selectively ablated by controlling the laser energy density to ensure soft ablation of the glass substrate, but provide hard ablation of the biological material.