Secondary Ion Mass Spectrometry for Biological Applications

With this brief editorial, I want to introduce the themed collection on Secondary Ion Mass Spectrometry (SIMS) for Biological Applications. The articles selected for this collection highlight one major feature of SIMS applied to biology: the ability to acquire high-resolution element- and molecule-specific images of cells and tissues. Many readers of JAAS are probably more familiar with the technique of Laser Ablation (LA) ICP-MS for the generation of elemental images from biological tissues; therefore, this themed collection aims to introduce SIMS, which is an atomic spectrometry technique, too, as important tool for bioimaging.

Imaging and imaging techniques are a steadily growing field of high interest in science. This great success is surely conditioned by the fact that we as humans are much more perceptive when information is visualized. For example, this is expressed by the well-known saying that “a picture is worth a thousand words”. Indeed, images allow the quick absorption of large amounts of data. Even at the dawn of humankind, people used images such as cave paintings as an effective way to communicate abstract and concrete information. In science, visual observation has always been an important way to acquire new knowledge, followed by its transmission with the help of images. Therefore, the development of light microscopes in the 16^th and 17^th centuries was a major milestone in order to get insight into the micro-world. From the beginning, researchers used these new tools for the observation of biological organisms. This first bioimaging led to the early description of biological cells, recorded in Robert Hook’s drawings in Microscopia (1665). Nowadays, powerful electron microscopes, notably transmission electron microscopes (TEM), allow the imaging of subcellular structures and small objects such as viruses. High-resolution TEM with correction of spherical aberrations can achieve a resolution down to 0.08 nm, even enabling the visualization of atoms.

Although light and electron microscopy already give fascinating insights into small biological structures on a micro- and nanometer level, there is considerable interest in combining this structural information with chemical information. The question is: How to obtain information on the presence and the quantity of chemical elements, including their isotopes, as well as molecules in each pixel of an image? For example, TEM can be combined with energy-dispersive X-ray spectroscopy (TEM/X-EDS), almost preserving the high resolution of TEM alone, but with element sensitivity only in the percentage range and no possibility of isotope analysis. On the other hand, LA ICP-MS provides high element sensitivity at a low μg kg⁻¹ level, but with only low spatial resolution at the micrometer level, which is not sufficient for subcellular imaging. The gap between these two opposite possibilities is filled by SIMS techniques. In SIMS imaging, a primary ion beam scans the sample surface and generates secondary ions of the chemical elements or molecules, which are subsequently analyzed by a mass spectrometer. SIMS provides high spatial resolution down to 50–100 nm, which is sufficient for the observation of subcellular structures, combined with element sensitivity at the mg kg⁻¹ level, as well as the possibility of isotope analysis.

The present themed collection provides the reader with three review articles and six research papers from different application fields of SIMS bioimaging. Notably, the tutorial and critical reviews in this collection present and compare different SIMS techniques. The first tutorial review introduces the two different main SIMS configurations, time-of-flight SIMS (ToF-SIMS) and nanoscale SIMS (NanoSIMS), as well as some new developments, correlative imaging with other imaging techniques, and applications in neurobiology and cell biology. Briefly, ToF-SIMS uses a pulsed primary ion beam, which ionizes the first atomic layers of the sample surface. It allows the detection of elements and molecular ions separated by a time-of-flight mass analyzer. In contrast, NanoSIMS employs a continuous high energy and highly focused ion beam that ionizes several nanometers of the sample surface but produces only atomic ions, which are separated with a magnetic sector mass analyzer. The second tutorial review focuses on ToF-SIMS in more detail, regarding different primary ion beams, signal enhancement methods, identification of biomolecules (lipids, peptides, proteins, and DNA), single cell imaging, and the possibilities to generate three-dimensional images. The critical review explores the applications of NanoSIMS in biology and biomedical research. This includes the detection of non-metals such as N, P, S, and Br in cells and tissues. A special application of NanoSIMS is the use of stable isotopes such as ²H, ¹³C, ¹⁵N, and ¹⁸O for tracking intracellular metabolic processes. Examples include the investigation of nutrient fluxes, nucleic acid synthesis, lipid and glucose metabolism, and protein turnover.

Three research papers focus on ToF-SIMS applications. One paper deals with the detection of zinc in the hippocampus of rat brain after traumatic brain injury. Two papers explore the possibilities of three-dimensional ToF-SIMS imaging; one demonstrates the localization of titanium dioxide nanoparticles in algal biofilm with high lateral resolution of about 100 nm, and the other paper presents a study of lipid composition and 3D distribution in the ovaries of mosquitos.

Although the number of NanoSIMS instruments worldwide that are dedicated to biological applications is relatively limited, two research papers with NanoSIMS applications are included in this collection. One paper presents the development of fluorinated nanobodies for specific labeling of cellular proteins. Via¹⁹F detection in NanoSIMS, the distribution of various target proteins could be visualized in cells. The other paper shows the combination of subcellular arsenic imaging in seaweed with arsenic speciation analysis by coupling techniques. Surprisingly, most arsenic was located in the cell walls and no arsenic was detected inside the cells. In a further paper an IMS-3f SIMS instrument was used to investigate intracellular calcium stores in pig kidney cells. In addition, the use of the stable ⁴⁴Ca isotope allowed the imaging of calcium influx in cells.

In summary, this themed collection encompasses the presentation of different SIMS techniques for bioimaging and shows a variety of different biological applications. All the papers demonstrate that SIMS bioimaging is not a stand-alone technique but in most cases is combined with other imaging tools such as light, fluorescence, and electron microscopy. This reveals the need for correlative imaging and sophisticated data treatment. Other common analytical challenges in SIMS bioimaging concern the development of dedicated sample preparation methods and precise quantification methods. The latter is still difficult due to matrix effects and a lack of matrix-matched standards. The application of chemical imaging by SIMS techniques to biological questions is a rapidly growing field. In the future, many more research papers with applications in medicine, pharmacology, toxicology, and environmental sciences are expected.

Finally, I wish to thank all the authors who contributed to this themed collection, the reviewers for their time evaluating the manuscripts, and the JAAS editorial staff for their work on this collection. I hope the readers will enjoy the articles and that they find many new ideas and stimulation for their own research.

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