SIMS proceedings paper

Received: 10 October 2011 Revised: 5 July 2012 Accepted: 6 July 2012 Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI 10.1002/sia.5141

Detection of micro- and nano-particles inanimal cells by ToF-SIMS 3D analysisBirgit Hagenhoff,a* Daniel Breitenstein,a Elke Tallarek,a Rudolf Möllers,b

Ewald Niehuis,b Michaela Sperber,c Barbara Goricnikc and JoachimWegenerc

The present study describes the detection and localization of silica particles with diameters between 2 mm and 150 nm within thecytoplasm of mammalian cells by means of ToF-SIMS 3D analysis. The particles were selected as model objects for non-luminescent,unlabeled particles that are hard to localize by other experimental approaches. ToF-SIMS analysis proved the uptake of the particlesinto the cell body, provided images of their distribution around the cell nucleus and indications that the cell membranes areundulated by the mm-sized particles beneath the membrane. Two ion sources (Cs+ and O2

+) were applied for sputtering the organicmaterial and expose deeper sections of the cells. The resulting images are presented and compared. Copyright © 2012 JohnWiley &Sons, Ltd.

Keywords: nanoparticle; ToF-SIMS 3D imaging; sputtering; cell-based bioanalytics; chemical contrast

* Correspondence to: Birgit Hagenhoff, Tascon GmbH, Heisenbergstr. 15, 48149Münster, Germany E-mail: Birgit.hagenhoff@tascon-gmbh.de

a Tascon GmbH, Heisenbergstr. 15, 48149, Münster, Germany

b ION-TOF GmbH, Heisenbergstr. 15, 48149, Münster, Germany

c Institut für Analytische Chemie, Chemo- und Biosensorik, Universitaet Regensburg,Universitaetstraße 31, 93053, Regensburg, Germany

Introduction

The interaction of nano- and mesoscale particles with livingcells is currently attracting an immense attention for two majorreasons: (i) the potentially toxic effects of nano-objects haveraised considerable health concerns and (ii) nanoscale objectsmay pave the way to new diagnostic, therapeutic or bioanalyticalmethods. Nanoparticles have indeed been found to be janus-faced when biomedical applications are concerned. The situationis, however, rather complex as a huge variety of particles –

differing in size, core material, surface and stability – have broughtin contact to an enormous number of different cell types from alltissues of the human body. The impact of one type of nanoparticleon one type of cell was often found to be rather unique forthis particular couple, and only a limited number of generalstatements could be deduced.[1] For instance, it was found thatthe number of particles that are taken up by the cells understudy determines the observed toxicity in most cases.[2] Toxicityis only very rarely caused by extracellular particles. Thus,measuring the number of particles inside the cells and identifyingtheir intracellular localization is one of the most importantanalytical problems to solve in this context. When the particlesunder study are luminescent either due to their own photophysi-cal properties (quantum dots) or via coupling to a suitable label,they can be nicely studied by optical microscopy even with alateral resolution beyond the limit of diffraction.[3] However, noinformation is gathered about the chemical composition of theparticle surface inside the cells.

This study describes the identification and localization ofmicro- and nanoparticles inside animal cells by means of 3DToF-SIMS analysis. As ToF-SIMS gives access to chemical contrastimaging, nanoparticle localization is not dependent on anylabel, and it can be applied to virtually any core material. Themanuscript will demonstrate the localization of micro- andnanoparticles made from silica inside animal cells by means ofalternating SIMS imaging and sputter erosion using O2

+ andCs+ ion beams.

Surf. Interface Anal. (2012)

Experimental

Sample preparation

The epithelial-like NRK cells (normal rat kidney, clone 52E)were grown in Dulbecco’s minimum essential medium(Biochrom, Berlin, Germany) supplemented with 10% fetal calfserum (Biochrom, Berlin, Germany), 2 mM L-glutamine, 100 mg/mlpenicillin and 100 mg/ml streptomycin (Biochrom, Berlin, Germany).Cultures were kept in incubators at 37 �C with 5% CO2

atmosphere. For ToF-SIMS experiments, the cells were seededon ordinary microscopy slides (sputter coated with Au(20 nm)/ITO(20 nm)). When the cell layer was fully established, silicananoparticles were added to the bathing fluid to allow forspontaneous uptake into the cells within 24 h after starting theincubation. Silica nanoparticles (Sigma, Deisenhofen, Germany)were used with three different diameters (2 mm, 500 nm and150 nm). The particles were diluted from an aqueous stocksolution into cell culture medium.

Prior to SIMS analysis, the culture medium was aspirated, thecell layers were washed two times with phosphate buffered salinecontaining 1 mM Ca2+ and 0.5 mM Mg2+ (PBS++) followed by anincubation with 2.5% (v/v) glutaraldehyde in PBS++ for 20 min atRT. The aldehyde solution was then replaced by water and thesamples were washed eight times. Finally, the water wascompletely aspirated, and the sample was allowed to dry at37 �C over night.

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B. Hagenhoff et al.

ToF-SIMS analysis

3D imaging data were acquired using a TOF-SIMS.5 instrument(ION-TOF, Germany). Data were recorded in the non-interlacedmode,i.e sputter cycles during which a thin layer of sample materialwas removed were alternated with image acquisitions in therespective crater center. Thus, a 3D data set consists of (256 � 256 � n)data points (x,y,z) with n being the number of sputter cycles.Sputtering was performed using O2

+ (500 eV, detection ofpositively charged secondary ions) and Cs+ (1 keV, detection ofnegatively charged secondary ions) beams. The sputter currentswere 90 nA for O2

+ and 60 nA for Cs. The sputter areas were300 � 300 mm². Imaging of the crater centers was performedusing Bi3

+ ions at 25 keV energy. The imaged area was in theorder of 90 � 90 mm². Images were taken at nominal mass resolu-tion (focus size of beam: ca. 150 nm). Additional images taken in

Figure 1. Confluent NRK cells pre-loaded with silica particles of 2 mm diametcytoplasm by sputter ablation of the plasma membrane using Cs+-ions. The mAu (bottom, E, F) are shown. The individual caption indicates the number of ionpixel (maximum counts, MC).

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high mass resolution were used to identify and avoid critical peakinterferences in the low mass resolution data.

Results and Discussion

Cs+ sputtering, detection of negatively charged secondary ions

The chemical information obtained at the upper surface of theintact cell as well as in inner parts of the cell is shown in massresolved (x, y) images. Three different mass resolved images areprovided for the different samples:

1. CNO� (nitrogen containing organic components such as proteinsand nucleic acids)

2. SiO2� (nanoparticles)

3. Au� (growth substrate)

er. Cells are shown before (left, A, C, E) and after (right, B, D, F) opening theass resolved images attributed to SiO2 (top, A, B), CNO (middle, C, D) ands found in the entire image (total counts, TC) as well as in the most intense

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Detection of nanoparticles in cells by ToF-SIMS 3D analysis

At the membrane surface of those cells that were pre-loadedwith silica particles of 2 mm diameter (2 mm particles), neithergold (substrate) nor silicon signals (particles) are detected (Fig. 1,left panel) indicating that (i) the growth surface is entirelycovered with cells and (ii) that no particles are simply adheredextracellularly to the upper cell membrane. Thus, all particles thatare visible at any lower z-level must be inside the cell, a fact thatis not always easy to extract from optical images. The massresolved image of CNO�-ions at the cell surface shows a ratherhigh intensity of CNO� fragments originating from componentsof the plasma membrane. The presence of spherical particles of2 mm diameter beneath the plasma membrane inside the cellsprovides a topographic undulation of the membrane visible inthe CNO�-ion image. When the upper plasma membrane of the

Figure 2. Confluent NRK cells pre-loaded with silica particles of 500 nm diamthe cytoplasm by sputter ablation of the plasma membrane using Cs+-ions. Tand Au (bottom, E, F) are shown. The individual caption indicates the numberintense pixel (maximum counts, MC).

Surf. Interface Anal. (2012) Copyright © 2012 John Wiley

cells is removed by sputter erosion, the silica particles insidethe cells are imaged with bright contrast in the SiO2

� signaldistribution. In some cells, they are rather densely packed aroundthe nucleus that remains dark. Individual particles are easilyidentified. The particles can not enter the nucleus due to thesize limitations of the nuclear pore complex of app. 10 nm.[4]

Occasionally, a single particle has entered the nucleus anywayby an unknown mechanism. The xz distribution in the ToF-SIMSdata as well as light microscopic results confirmed the intracellu-lar distribution of the 2 mm particles (data not shown here). Theintense signals of Au�-secondary ions in the image obtainedfrom the center of the cells suggest that the sputter processreached the growth surface at thinner areas of the cell sample.The respective CNO�-ion image shows a bright intensity

eter. Cells are shown before (left, A, C, E) and after (right, B, D, F) openinghe mass resolved images attributed to SiO2 (top, A, B), CNO (middle, C, D)of ions found in the entire image (total counts, TC) as well as in the most

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B. Hagenhoff et al.

distribution due to the abundance of nitrogen-containingmolecules inside the cells. The cell nucleus can be identifiedby the homogeneous round appearance of the CNO� signalwhereas membrane areas show a more differentiated CNO�

distribution.Figure 2 compares the same set of secondary ion images for

cell layers that had been pre-loaded with silica particles of500 nm diameter. The left panel shows the secondary iondistribution at the cell surface, whereas the right panelprovides images from inside the cells. Again, the SiO2

�-signaldistribution before sputtering (Fig. 2a) and after removal ofthe uppermost organic layers confirms that the 500 nmparticles are inside the cells and do not adhere extracellularly

Figure 3. Confluent NRK cells that had been pre-loadedwith silica particles of 2after several sputter cycles (O2

+-sputtering) to remove the plasmamembrane. Thand Au+ In (bottom, G–I) are shown. The individual caption indicates the numbintense pixel (maximum counts, MC).

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to the upper membrane. In agreement with the diameter ofthe nuclear pore complex, even particles of this smaller diame-ter are unable to enter the nucleus. They arrange around thenucleus instead. Individual particles are hardly identifiedunequivocally as the pixel size of 300 nm is close to the particlediameter of 500 nm. However, in areas of the sample with lowparticle density, individual particles are observable. It is importantto note that the unlabeled 500 nm particles are hardly visible intransmission light microscopy and their intracellular distributioncan only be guessed (data not shown). Hence, the contrast-richimages provided by 3D-ToF-SIMS may become an importanttool to clarify the fate of nanoparticles when they encounterintracellular structures.

mm (left, A, D, G), 500 nm (middle, B, E, H) and 150 nm (right, C, F, I) diametere mass resolved images attributed to Si (top, A–C), amino acids (middle, D–F)er of ions found in the entire image (total counts, TC) as well as in the most

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Detection of nanoparticles in cells by ToF-SIMS 3D analysis

O2+ sputtering, detection of positively charged secondary ions

In contrast to the data processing discussed above, here not justone single ion (CNO-) but a number of ions resulting from proteinfragmentation are pooled to provide a group signal for amino acids.In specific, the signals m/z= 70 (C4H8N

+; proline), m/z= 102(C4H8NO2

+; glutamic acid) and m/z= 110 (C5H8N3+; histidine) were

used for this purpose.[5] The growth surface is represented by theintegrated signal of Au+ and In+, whereas particle localization isimaged via the distribution of Si+ ions. Figure 3 compares thesecondary ion distribution for cell layers that had been pre-loadedwith silica particles of three different diameters: 2 mm (left),500 nm (middle) and 150 nm (right). The medium-sized particleswere applied in higher concentrations than the biggest particlesto image particle distribution inside the cells under these condi-tions. The smallest particles were applied in significantly lowerconcentration in order to allow for single particle detection. TheSi+ ion distribution shows the lateral distribution of 2 mm and500 nm silica particles as described above. They arrange aroundthe nucleus that they cannot enter. The smallest particles of150 nm are visible at individual spots even though they wereapplied in lower concentrations and they have a diameter thatis only half the pixel size. It remains to be studied whether theobserved signals originate from individual particles with asecondary ion signal blurred over several pixels or small particleaggregates. Correlation experiments with fluorescence-labelednanoparticles will be conducted in order to unravel the minimumsize of nanoparticles to be detected by 3D-ToF-SIMS.

The group signal for amino acids shows a characteristic patternthat we have reported elsewhere before.[6,7] The amino acidssignal is highest in the nucleus which is biologically meaningfulas the nucleus is densely packed with histone proteins.

Conclusions

The present study proves that ToF-SIMS is capable of detectingmicro- and nanoparticles insidemammalian cells down to a particlediameter of 150 nm. Thus, it is possible to locate non-luminescent,unlabeled nanoparticles in biological specimens just by chemicalcontrast. This experimental option is particularly interestingwhen the fate of ‘non-intended use’ particles, which are typically

Surf. Interface Anal. (2012) Copyright © 2012 John Wiley

non-luminescent, is addressed. The localization of particles withinthe organic cell components as well as the identification of theparticles’ chemical composition – with the potential for parallelidentification of different particle species – is now possible. TheToF-SIMS approach allows discriminating between extracellularand intracellular localization of the particles – the key question inmany biomedical projects. Using Cs+ and O2

+ sputtering, it waspossible to locate the nucleus as the most prominent intracellularcompartment by the detection of small protein fragment ionslike CNO�. Further details on the intracellular architecture werenot obtained. Here, the use of Ar-cluster ions for sputtering willimprove the situation and allow imaging of the lipid structures.First results of this approach (not shown here) indicated that itwill be possible to detect not only inorganic but also organicxenobiotica within cells.

Acknowledgement

This work is supported by the European Regional DevelopmentFund (ERDF) and the Ziel-2-NRW-Project of the federal state ofNordrhein Westfalen (Grand-No.: W0804nm011a). Further supportby the Deutsche Forschungsgemeinschaft (DFG) within the priorityprogram SPP1313 to JW is gratefully acknowledged.

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