Single particle studies of atmospherically-relevant aerosols
AFM experiments can be performed in a controlled relative humidity and temperature environment, allowing for the capability to study both the hygroscopicity and surface tension of substrate-deposited particles and nanometer-sized droplets. Particles with chemical compositions relevant to sea spray aerosol (salt, organic acids, and saccharides) are of interest. The goal of these studies is to understand the role of single particle aerosol in the world we live in.
Currently, the impact of various phase states of aerosols on the climate is not well understood, especially for submicrometer sized aerosol particles that typically have extended lifetime in the atmosphere. This is largely due to the inherent size limitations present in current experimental techniques that aim to directly assess the phase states of fine aerosol particles. Herein we present a technique that uses atomic force microscopy to probe directly for the phase states of individual, submicrometer particles by using nanoindentation and nano-Wilhelmy methodologies as a function of relative humidity (RH) and ambient temperature conditions. When using these methodologies for substrate deposited individual sucrose particles, Young’s modulus and surface tension can be quantified as a function of RH. We show that the force profiles collected to measure Young’s modulus and surface tension can also provide both qualitative and quantitative assessments of phase states that accompany solid, semisolid, and liquid particle phases. Specifically, we introduce direct measurements of relative indentation depth and viscoelastic response distance on a single particle basis at a given applied force to quantitatively probe for the phase state as a function of RH and corresponding viscosity. Thus, we show that the three phase states and phase state transitions of sucrose can be identified and ultimately propose that this technique may also be used to study other atmospherically relevant systems.
Lee, H. D.; Ray, K. K.; Tivanski, A. V., Solid, Semi-Solid, and Liquid Phases of Individual Submicrometer Particles Directly Probed Using Atomic Force Microscopy. Analytical Chemistry 2017, 89 (23), 12720–12726.
Understanding the role of sea spray aerosol (SSA) on climate and the environment is of great interest due to their high number concentration throughout the Earth’s atmosphere. Despite being of fundamental importance, direct surface tension measurements of SSA relevant sub-micrometer particles are rare, largely due to their extremely small volumes. Herein, atomic force microscopy (AFM) is used to directly measure the surface tension of individual sub-micrometer SSA particle mimics at ambient temperature and varying relative humidity (RH). Specifically, we probed both atmospherically relevant and fundamentally important model systems including electrolyte salts, dicarboxylic acids, and saccharides as single components and mixtures. Our results show that the single particle surface tension depends on RH or solute mole percentage and chemical composition. Moreover, for liquid droplets at and below 100 Pa s in viscosity, or at corresponding RH, we show good agreement between the AFM single particle and the bulk solution surface tension measurements at overlapping concentration ranges. Thus, direct surface tension measurements of individual particles using AFM is shown over a wide range of chemical systems as a function of RH, solute mole percentage, and viscosity than previously reported.
Lee, H. D.; Estillore, A. D.; Morris, H. S.; Ray, K. K.; Alejandro, A.; Grassian, V. H.; Tivanski, A. V., Direct Surface Tension Measurements of Individual Sub-Micrometer Particles Using Atomic Force Microscopy. The Journal of Physical Chemistry A 2017, 121 (43), 8296-8305.
The water uptake behavior of atmospheric aerosol dictates their climate effects. In many studies, aerosol particles are deposited onto solid substrates to measure water uptake; however, the effects of the substrate are not well understood. Furthermore, in some cases, methods used to analyze and quantify water uptake of substrate deposited particles use a two-dimensional (2D) analysis to monitor growth by following changes in the particle diameter with relative humidity (RH). However, this 2D analysis assumes that the droplet grows equally in all directions. If particle growth is not isotropic in height and diameter, this assumption can cause inaccuracies when quantifying hygroscopic growth factors (GFs), where GF for a for a spherical particle is defined as the ratio of the particle diameter at a particular relative humidity divided by the dry particle diameter (typically about 5% RH). However, as shown here, anisotropic growth can occur in some cases. In these cases, a three-dimensional (3D) analysis of the growth is needed. This study introduces a way to quantify the hygroscopic growth of substrate deposited particles composed of model systems relevant to atmospheric aerosols using atomic force microscopy (AFM), which gives information on both the particle height and area and thus a three-dimensional view of each particle. In this study, we compare GFs of submicrometer sized particles composed of single component sodium chloride (NaCl) and malonic acid (MA), as well as binary mixtures of NaCl and MA, and NaCl and nonanoic acid (NA) determined by AFM using area (2D) equivalent diameters, similar to conventional microscopy methods, to GFs determined using volume (3D) equivalent diameter. We also compare these values to GFs determined by a hygroscopic tandem differential mobility analyzer (HTDMA; substrate free, 3D method). It was found that utilizing volume equivalent diameter for quantifying GFs with AFM agreed well with those determined by substrate-free HTDMA method, regardless of particle composition but area equivalent derived GFs varied for different chemical systems. Furthermore, the NaCl and MA mixture was substrate-deposited both wet and dry, revealing that the hydration state of the particle at the time of impaction influences how the particle grows on the substrate upon water uptake. Most importantly, for the binary mixtures it is shown here that different populations of particles can be distinguished with AFM, an individual particle method, whereas HTDMA sees the ensemble average. Overall, this study establishes the methodology of using AFM to accurately quantify the water uptake of individual submicrometer particles at ambient conditions over a wide range of RH values. Furthermore, the importance of single particle AFM analysis is demonstrated.
Morris, H. S.; Estillore, A. D.; Laskina, O.; Grassian, V. H.; Tivanski, A. V., Quantifying the Hygroscopic Growth of Individual Submicrometer Particles with Atomic Force Microscopy. Analytical Chemistry 2016, 88 (7), 3647-3654.
Surface tension, an important property of liquids, is easily measured for bulk samples. However, for droplets smaller than one micron in size, there are currently no reported measurements. In this study, atomic force microscopy (AFM) and force spectroscopy have been utilized to measure surface tension of individual submicron sized droplets at ambient pressure and controlled relative humidity (RH). Since the surface tension of atmospheric aerosols is a key factor in understanding aerosol climate effects, three atmospherically relevant systems (NaCl, malonic and glutaric acids) were studied. Single particle AFM measurements were successfully implemented in measuring the surface tension of deliquesced particles on the order of 200 to 500 nm in diameter. Deliquesced particles continuously uptake water at high RH, which changes the concentration and surface tension of the droplets. Therefore, surface tension as a function of RH was measured. AFM based surface tension measurements are close to predicted values based on bulk measurements and activities of these three chemical systems. Non-ideal behaviour in concentrated organic acid droplets is thought to be important and the reason for differences observed between bulk solution predictions and AFM data. Consequently, these measurements are crucial in order to improve atmospheric climate models as direct measurements hitherto have been previously inaccessible due to instrument limitations.
Morris, H. S.; Grassian, V. H.; Tivanski, A. V., Humidity-dependent surface tension measurements of individual inorganic and organic submicrometre liquid particles. Chemical Science 2015, 6 (5), 3242-3247.
Mechanical properties of nanomaterials
AFM nanoindentation method is employed to quantify the mechanical properties of millimeter and sub-micrometer sized organic organic cocrystals, capable of undergoing single crystal-to-crystal (SCSC) reaction in solid state. The effect of photoreaction on mechanical properties is studied for a variety of solids with the main goal to design advanced material with defined stiffness for device application.
A correlation between Young’s modulus, as determined by using nanoindentation atomic force microscopy (AFM), and atomic polarizability is observed for members of a series of cocrystals based on systematic changes to one cocrystal component. Time domain spectroscopy over terahertz frequencies (THz-TDS) is used for the first time to directly measure the polarizability of macro- and nanosized organic solids. Cocrystals of both macro- and nanodimensions with highly polarizable atoms result in softer solids and correspondingly higher polarizabilities.
Rupasinghe, T. P.; Hutchins, K. M.; Bandaranayake, B. S.; Ghorai, S.; Karunatilake, C.; Bučar, D.-K.; Swenson, D. C.; Arnold, M. A.; MacGillivray, L. R.; Tivanski, A. V., Mechanical Properties of a Series of Macro- and Nanodimensional Organic Cocrystals Correlate with Atomic Polarizability. Journal of the American Chemical Society 2015, 137 (40), 12768-12771.
AFM nanoindentation was used to demonstrate that millimeter- and nanometer-sized organic cocrystals exhibit changes in the mechanical properties before, during, and after a single-crystal-to-single-crystal [2+2] photodimerization of olefins. Millimeter-sized crystals become 40 % softer, whereas nanocrystals of the same material become 40 % harder following the photoreaction.
Karunatilaka, C.; Bučar, D.-K.; Ditzler, L. R.; Friščić, T.; Swenson, D. C.; MacGillivray, L. R.; Tivanski, A. V., Softening and Hardening of Macro- and Nano-Sized Organic Cocrystals in a Single-Crystal Transformation. Angewandte Chemie International Edition 2011, 50 (37), 8642-8646.
Electrical properties of nanomaterials
Conductive Probe Atomic Force Microscopy (CP-AFM) is utilized to study electrical properties of organic materials at the nanoscale level. The approach uniquely offers spatially resolved (nm) measurements of electrical current (pA) with outstanding control of applied forces (pN). The goal of these studies is to develop fundamental understanding of factors that control electrical conduction at the nanoscale, potentially leading to development of novel molecular devices.
Ag(I) is used to form a π-stacked metal–organic solid that exhibits remarkably high electrical conductivity. The solid undergoes a single-crystal-to-single-crystal [2+2] photodimerization to generate a 1D coordination polymer with over 40% higher conductivity. The Ag(I) complex represents the first example of an increase in conductivity resulting from a [2+2] photodimerization. Density of states calculations show a higher contribution from Ag(I) ions to the valence band in the photodimerized solid, supporting the increase in conductivity.
Hutchins, K. M.; Rupasinghe, T. P.; Ditzler, L. R.; Swenson, D. C.; Sander, J. R. G.; Baltrusaitis, J.; Tivanski, A. V.; MacGillivray, L. R., Nanocrystals of a Metal–Organic Complex Exhibit Remarkably High Conductivity that Increases in a Single-Crystal-to-Single-Crystal Transformation. Journal of the American Chemical Society 2014, 136 (19), 6778-6781.
Enzymatic studies using force spectroscopy
Molecular Recognition Force Microscopy (MRFM) is used to study the forces required to rupture the bond between enzyme and an inhibitor. The ability of AFM to resolve forces with pN resolution allows quantification of interactions between other cofactors, as well as study enzyme dynamics. The goal of these studies is to develop a technique to reliably study enzyme dynamics on a single molecule.
Escherichia coli dihydrofolate reductase (ecDHFR) has one surface cysteine, C152, located opposite and distal to the active site. Here, we show that the enzyme spontaneously assembles on an ultraflat gold surface as a homogeneous, covalently bound monolayer. Surprisingly, the activity of the gold-immobilized ecDHFR as measured by radiographic analysis was found to be similar to that of the free enzyme in solution. Molecular recognition force spectroscopy was used to study the dissociation forces involved in the rupture of AFM probe-tethered methotrexate (MTX, a tight-binding inhibitor of DHFR) from the gold-immobilized enzyme. Treatment of the ecDHFR monolayer with free MTX diminished the interaction of the functionalized tip with the surface, suggesting that the interaction was indeed active-site specific. These findings demonstrate the viability of a simple and direct enzymatic surface-functionalization without the use of spacers, thus, opening the door to further applications in the area of biomacromolecular force spectroscopy.
Ditzler, L. R.; Sen, A.; Gannon, M. J.; Kohen, A.; Tivanski, A. V., Self-Assembled Enzymatic Monolayer Directly Bound to a Gold Surface: Activity and Molecular Recognition Force Spectroscopy Studies. Journal of the American Chemical Society 2011, 133 (34), 13284-13287.