Mass Spectrometry Research - Strouse Group Research

Building on our successful synthetic strategy, we have begun to analyze the growth of these materials using electrospray mass spectrometry methods (ESMS). This effort represents one of the first attempts to utilize a traditional biochemical analytical method to directly probe nano-scale inorganic structures. The initial results are allowing a broadening of the understanding of the role of kinetics and thermodynamics in nano-material growth.

Molecular clusters, as a class of materials, exist as discrete units with structures that are often related to a fragment of a bulk solid state lattice. These materials have long held promise in materials chemistry, biological chemistry, and catalysis, particularly as models in biological catalytic activity and more recently for understanding the effects of reduced dimensionality on bulk materials. Traditionally the structures and metal valence for metallo-clusters are probed by a correlation of solid state and solution phase physical techniques such as NMR, X-ray diffraction, XPS, UV/Visible, Infrared, Raman, and ESR spectroscopies. Recent efforts on electrospray ionization mass spectrometry (ESI-MS) of inorganic and bio-inorganic materials have yielded important insight into both the structure and valency of metallo-proteins and inorganic clusters in solution by analysis of the fragmentation behavior, parent ion m/z ratio and daughter ion analysis in tandem mass spectrometry studies. These early studies on inorganic materials suggest ESI-MS techniques may provide a unique platform for analysis of intact clusters under solution conditions without the need for multiple analytical techniques. The applicability of electrospray ionization (ESMS) methods to problems in materials chemistry is demonstrated by our results for mass spectrometry of these nano-precursor materials. Low-cone voltage ESMS is a useful technique for the rapid analysis of the organometallic precursor molecules when both positive and negative ionic modes are analyzed. This method shows promise as an alternative method of analysis of the dynamics of growth in nano-scale materials. The main advantage of ESMS over other mass spectrometry techniques is the capabilities for direct analyses of mixtures and solvated molecules.

The analysis of the change in physical properties of nanomaterials represents the transition from molecular to bulk behavior. The development of scaling laws for phase transitions, optical bandgaps, and melting points on CdSe and CdS have provided a significant body of knowledge on the size dependent behavior of inorganic semiconductors. Typically size and size dispersity in these materials are measured by TEM imaging, or estimates from the optical properties. With these techniques, specific distributions in the composition, surface compositions and structure of individual nano-materials are not addressed. The use of mass spectrometry is an alternative technique that provides exact mass and stability to fragmentation information by analysis of the mass/charge ratio of ions in the gas phase. While mass spectrometry is largely used in biological and polymeric tools, significant advances in the development of MS techniques have shown soft-ionization methods, such as electrospray, are ideal for studies of solution phase species in the gas phase without perturbation of the solution structures, particularly in biological and inorganic materials Electrospray ionization (ESI) mass spectrometry is a soft-ionization technique that allows analysis of exact structure, composition, and stability of solution phase materials by a gas phase technique.

We have described the first electrospray ionization mass spectrometry analysis of a 1.5 nm CdS cluster, [Cd32S14(SC6H5)36.DMF4]. The cluster [Cd32S14(SC6H5)36.DMF4] has been previously analyzed by X-ray diffraction providing evidence of the ~1.2 nm cluster core, and described in effect as the smallest nanocrystal for the CdS series. This size represents the boundary between molecular cluster and nanomaterial behavior. By using a nanospray mass spectrometry technique, we are able to carry out a careful mass spectroscopic analysis of the fragmentation behavior of an 82-atom CdS cluster that is a roughly spherical piece of cubic sphalerite lattice (~ 1.2 nm in diameter). With the capping of the four corners of the lattice by hexagonal wurtzite-like CdS units, the size of the cluster is ~ 1.5 nm in diameter. Analysis of the fragmentation, parent ion and daughter ion patterns allows the stability of the 1.5 nm CdS nanoparticle to be assessed, as well as evidence for cleavage planes in the crystallite. To our knowledge, this is the first time that mass spectrometry has given information on nano-sized metal chalcogenide clusters (“Cd32” is a 1.5 nm average diameter). Such results show the versatility of mass spectrometry analysis in determining the composition of nanomaterials. Specific information concerning the chemical composition of nanomaterials can be obtained with high accuracy. Parent ion fragmentation indicates cleavage along favorable planes in the CdS core of the cluster, that may be crucial in understanding the fragmentation behavior in ESI-MS /MS data of larger nanomaterials. We are continuing to apply ESI-MS and ESI-MS/MS techniques to a wide range of inorganic clusters including TiO2 and Au nanocrystals to probe the stability and fragmentation behavior in these materials.

or read the published articles:
"Mass Spectrometry Analysis of Organic-Inorganic Nanomaterials and their Precursors." Gaumet, J.J.; Strouse, G.F. Mat. Res. Soc. Symp. Proc., 726, Q10.6, 1-6 (2002). [ view article - PDF ]

"Mass Spectrometry Analysis of the 1.5 nm Sphalerite-CdS core of [Cd32S14(SC6H5)36*DMF4]." Gaumet, J.J.; Khitrov, G.A.; Strouse, G.F. Nano Lett., 2, 375-379 (2002). [ view article - PDF ]

"Nanospray Mass Spectrometry Technique for Analyzing Nanomaterials from Molecular Precursors up to 1.5 nm in Diameter Clusters." Gaumet, J.J.; Strouse, G.F. Mater. Sci. Eng. C, 19, 299-304 (2002). [ view article - PDF ]

"Power of Mass Spectrometry for Analyzing Organometallic Molecules as Precursors of Nanomaterials." Strouse, G.F.; Gaumet, J.J. Mat. Res. Soc. Symp. Proc., 581, 479-484 (2000). [ view article - PDF ]

"Electrospray Mass Spectrometry of Semiconductor Nanoclusters: Comparative Analysis of Positive and Negative Ion Mode". Gaumet, J.J.; Strouse, G.F. J. Amer. Soc. Mass. Spec., 11, 338-344 (2000). [ view article - PDF ]