Intracellular compound delivery is vital for therapeutic applications like genome editing. For in vitro or ex vivo intracellular compound delivery, membrane disruption methods are widely used, with photoporation using photothermal nanoparticles (NPs) emerging as a versatile technique. In this method, NPs bind to the cell membrane and, upon laser irradiation, create pores that allow effector molecules to enter the cell. Developing an efficient photoporation method requires quantifying the number of NPs bound to membranes of individual cells, which is typically done through manual counting in confocal microscopy images. This process is time-consuming, prone to bias, and has a low sample throughput (typically based on a few dozen cells only). [1]
In this context, single-cell ICP-MS (SC-ICP-MS) is presented as a fast and reliable alternative, requiring less manual labour while providing much higher throughput (thousands of cells per minute). However, introducing human cells in suspension into the ICP as intact entities is challenging [2], leading to low transport efficiencies (TE), especially with large cells such as the A549 cell line ($\sim$20 μm), for which a broad range of TEs is typically reported at 0.2–5%. [3,4]
In this work, we address the issue of low transport efficiency for large human cells using a custom-built heated spray chamber. We found that applying high temperatures (>100°C) significantly improved TE across various human cell lines (red blood cells, Raji cells, and A549). Notably, the A549 cell line exhibited an increase in TE from 0.27% to 10%, highlighting the potential of SC-ICP-MS for photoporation applications. In a subsequent step, coupling the optimized setup to an ICP-TOF-MS instrument enabled simultaneous monitoring of endogenous elements (e.g., ³¹P, ⁶⁴Zn) from the cells, ¹⁹³Ir as a DNA intercalator to tag the cells, and ¹⁹⁷Au from the NPs. This allowed us to distinguish between cells with bound AuNPs (cells:xAuNPs), cells without bound AuNPs, and free AuNPs (see Figure 1).
Finally, we present results from a case study with 50 nm AuNPs bound to A549 cells, showing a mixture of normal distributions, each representing a specific number of AuNPs per cell (A549:xAuNPs). Unlike traditional methods like confocal microscopy, our approach can analyse thousands of cells in a time span of minutes, offering valuable insights into the mechanism of photoporation.

References
[1] J. Ramon et al., Current Opinion in Colloid and Interface Science, 2021, 54, 101453-101473.
[2] M. Corte-Rodríguez et al., Trends in Analytical Chemistry, 2020, 132, 116042-116058.
[3] H. Wang et al., Analyst, 2015, 140, 523-531.
[4] L. Hendriks et al., Enviromelntal Science Nano, 2023, 10, 3439-3449.