In this project, funded by the Macromolecular, Supramolecular, and Nanochemistry Program of the Division of Chemistry, Professor Scott Anderson of the University of Utah will develop new techniques for studying single, isolated nanoparticles. The approach will measure the optical properties and surface chemical reactions of trapped nanoparticles while simultaneously allowing the particle mass-to-charge ratio to be measured with precision of one part per million, via measurement of the particle motional frequency in the trap. By also determining the absolute charge state, the absolute mass will be measured. These new capabilities will allow, for the first time, correlations between particle size, charge state, and optical and chemical properties to be studied with high precision. The techniques will initially be applied to semiconductor quantum dots, allowing measurements of spectral properties, including fluorescence lifetimes, to be correlated with particle size, composition, charge state, and the nature of the charging species. Experiments directed at extending the technique to study surface chemistry of non-fluorescent nanoparticles will also be carried out. In these experiments, two particles are trapped simultaneously, and the mass/charge ratio of the non-fluorescent particle is inferred by its effects on the motion of a co-trapped fluorescent particle.

Nanoparticles in the 1-10 nanometer diameter range have electronic properties that depend strongly on size. Such particles are used extensively in applications ranging from fluorescent tags for biosensing, to heterogeneous catalysts, to nanomaterials, and in each application, the effects of electronic properties on optical or chemical properties is critical. Because nanoparticles generally are produced with significant size distributions, it is difficult to unravel size-property relationships in any detail. Professor Anderson, along with a graduate student and undergraduate student, will develop a single nanoparticle trapping approach to studying optical properties of semiconductor nanoparticles (quantum dots), with simultaneous high precision mass measurement. The techniques will be extended to metal and other nanoparticles. Prof. Anderson will continue his efforts at recruiting minority students, and will continue to host high school and undergraduate students for research that expose them to sophisticated physical and mathematical concepts.

Project Report

Intellectual Merit The goal of this project was to develop Nanoparticle Mass Spectrometry (NPMS) as a tool to study the optical and surface chemistry properties of single nanoparticles (NPs), trapped in controlled atmospheres. A third generation NPMS instrument was built, and new methods for NP mass and charge measurements were developed, improving the speed and precision of the method by two orders of magnitude. NPMS involves trapping a charged NP in a quadrupole trap, determining the mass and charge on the NP non-destructively by monitoring the frequency (ωz) of the NP's motion in the trap. The first figure shows an example where a new beat frequency method was used to measure the frequency to a few ppm precision. Other methods, providing a range of precisions and measurement speeds, were also developed. Detailed studies of the optical properties of semiconductor nanocrystal quantum dots (QDs) were carried out, revealing for the first time, how the absorption and emission properties change for single QDs with 5 - 20 positive charges, and also the effects of heating with both visible and infrared lasers on the optical properties. The second figure shows example emission spectra for 7 nm diameter CdSe/ZnS core/shell QDs excited at 532 nm. As shown, the spectrum for as-trapped QDs is identical to that measured in solution, but after brief heating at 10.6 µm, the spectrum changes dramatically. The integrated intensity increases by a factor of ~1800, and there are new features both to the blue and to the red of the original peak. Much of the work was directed toward developing methods to allow small non-luminescent particles to be detected optically, so that their mass and charge can be monitored continuously, allowing studies of surface chemical kinetics. The approach is to trap the dark NP of interest with a single bright NP, and detect the dark particle by its influence on the motion of the bright particle. Laser-heated QDs were found to be excellent candidates for this non-contact probe application, as shown in the third figure. Here, a bright QD was first trapped and characterized, and then a single dark NP was injected into the trap. As the figure shows, the 2nd NP had no effect on the frequency of the bright NP, however, a new peak appeared at the frequency for motion of the dark NP, due to the influence of the dark NP motion on the bright NP. As shown in the fourth figure, we have demonstrated the ability to monitor multiple particles simultaneously in this way. The top inset to this figure shows the frequency spectrum for three trapped particles, and the top frame shows how the "secular" frequencies varied with time for three hours, as the particles were heated by laser irradiation at 40 or 60 W/cm2. The steps in the frequencies are due to collisions with background argon ions or electrons that change the charge on the particles by one or two elementary charges. From the size of the steps, the exact charge on each particle can be determined, and once the charges are known, the masses can be determined as well. The masses are plotted in the lower frame. It can be seen that the three NPs have masses in the 1 - 2.5 mega Dalton (MDa) range, slowly declining due to sublimation driven by the laser. The inset to the lower frame shows the frequency peak for particle 1 at higher resolution. Broader Impacts: Products/Science Infrastructure: The NPMS method has potential applications from nanoscience to virology, and our work has substantially improved usability of the method as a practical tool. We also continued to develop the "Utah RF source", which has been implemented in at least 49 research groups in 16 countries. We developed two new sources for the 80 kHz - 3 MHz range, and the 15 kHz – 50 kHz range (frequency and amplitude stabilized). The design for the former has been disseminated via email lists, and we will publish the latter after additional testing. 8 papers were submitted/published. Educational and Outreach Activities: The main activity linked to my NSF research was getting young scientists involved. Connections with local high schools and programs at the University of Utah focusing on women/minority undergraduates were used to recruit students. The original instrument was constructed by undergraduate Darby Lewis, working with high school (HS) student Emma van Burns and two graduate students. The work has continued with Ryan Johnson (W. Jordan HS), Kevin Furukawa (Juan Diego HS), and Zhiyuan Feng (female undergraduate). Four HS/undergraduate students are co-authors on one or more papers. NPMS provides excellent training for graduate students, currently Collin Howder and Bryan Long. Dave Bell’s Ph.D. thesis on NPMS won the Giddings prize for best thesis in 2014.

National Science Foundation (NSF)
Division of Chemistry (CHE)
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Program Officer
Timothy E. Patten
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University of Utah
Salt Lake City
United States
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