Hydration is crucial to the structure, conformation, and biological activity of proteins. Proteins without water molecules surrounding them would not have viable biological activity. Specifically, water molecules will interact with the surface and internal structure of proteins, and different hydration states of proteins make such interactions distinct. Thus, it is important to understand the hydration of proteins on surfaces, which can provide a fundamental understanding of the mechanism of their structure, conformation, and biological activity. Our group developed an important technique to study liquid surfaces and interfaces, namely System for Analysis at the Liquid Vacuum Interface (SALVI). It has been recently applied to study hydrated protein films. SALVI is a vacuum compatible microfluidic device that consists of a SiN window as the detection area and a microchannel made of polydimethylsiloxane (PDMS). The protein solution was introduced into the microchannel. After incubating for a period of time, a hydrated protein film formed on the back side of the SiN membrane. The information of hydrated proteins was collected using the time-of-flight secondary ion mass spectrometry (ToF-SIMS) in the SALVI device in the liquid state. Compared with previous results from dry protein samples, we not only confirmed the amino acid compositions of proteins, but also firstly discovered that the distribution of water molecules surrounding and inside proteins varied among different types of proteins. Our liquid ToF-SIMS results show that 1). The water clusters number and relative counts vary among the same hydrated proteins, which imply that the distribution of water molecules surrounding and inside a protein is inhomogeneous; 2). The same water clusters have varied content in different types of proteins, which indicate that the distribution of water molecules have a strong relationship with the structure and conformation of the proteins at the biointerface. These first observations of hydrated protein films on a surface will pave the investigation of the structure, conformation, and biological activity of proteins in the future.

Biofilms consist of a group of micro-organisms attached onto surfaces or interfaces and embedded with a self-produced extracellular polymeric substance (EPS) in natural environments. The EPS matrix, like the “house of the cells”, provides bacteria cells with a more stable environment and makes them physiologically different from planktonic cells. Shewanella oneidensis MR-1 is a metal-reducing bacterium, forming biofilms that can reduce toxic heavy metals. This capability makes S. oneidensis biofilms very attractive in environmental applications. To better understand the biofilm EPS matrix composition at the interface, in situ chemical imaging with higher spatial resolution and more molecular level chemical information is strongly needed. Traditionally, electron microscopy and fluorescence microscopy are common imaging tools in biofilm research. However, the bottlenecks in these imaging technologies face the limitations that it is difficult for them to provide chemical information of small molecules (e.g., molecule weight <200). In this study, we use an emerging technology liquid Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) to observe S. oneidensis biofilm cultured in a vacuum compatible microchannel of the System for Analysis at the Liquid Vacuum Interface (SALVI) device. Chemical spatial distributions of small organic molecules that are considered to be the main building components of EPS in live biofilms are obtained. Principal component analysis is used to determine differences among biofilms sampled along the microchannel. This new approach overcomes previous limitations in live biofilm analysis and provides more chemical information of the EPS relevant to biofilm formation. Better understanding of the biofilm matrix will potentially fill in the knowledge gap in biofilm surface attachment and detachment processes and improve the engineering and design of S. oneidensis biofilms with high efficiencies in heavy metal reduction.

The surfaces of aqueous phases and films have unique kinetics and thermodynamics, distinct from the bulk. However, major surface analytical techniques are mostly vacuum-based and direct applications for volatile liquid studies are difficult. We developed a vacuum compatible microfluidic interface, System for Analysis at the Liquid Vacuum Interface (SALVI), to enable direct observations of liquid surfaces and liquid-solid interactions using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The unique aspects of this R&D 100 award winner include the following: 1) the detection window is an aperture of 2-3 mm in diameter allowing direct imaging of the liquid surface, 2) surface tension is used to hold the liquid within the aperture, and 3) SALVI is portable among multiple analytical platforms. SALVI is composed of a silicon nitride (SiN) membrane as the detection area and a microchannel made of polydimethylsiloxane (PDMS). Its applications in ToF-SIMS as an analytical tool were evaluated using a variety of aqueous solutions and complex liquid mixtures, some of which contain nanoparticles. SALVI was also used to investigate the solvent structure of switchable ionic liquids. Recently, we demonstrated in situ probing of the electrode-electrolyte solution interface (or solid-electrolyte interface, SEI) using a new electrochemical SALVI. It provides the first direct observation of the surface and diffused layer of SEI in a liquid with chemical speciation using dynamic ToF-SIMS. Moreover, SALVI was extended for studying biofilm growth and single mammalian cells using correlative imaging by more than one spectroscopy and microscopy technique, each offering different spatial and temporal scales. That is, collecting data on different information level from an identical area in the same sample ideally could lead to a more holistic view of the hierarchical structural organization of complex systems in the real world. This capability is of interest in biological applications using liquid ToF-SIMS. Selected results from our latest development will be presented, showcasing new directions and applications of in situ chemical imaging of environmental surfaces and interfaces and studying chemistry from the bottom up, all based on microfluidics. SALVI, a portable microfluidic reactor, sets the analytical foundation toward chemical imaging of complex phenomena occurring in multiple time and length scales, or the mesoscale, underpinning chemical changes at the molecular level in the condensed phase.

Glyoxal, a ubiquitous water-soluble gas-phase oxidation product in the atmosphere, is an important source of oxalic acid, a precursor to aqueous secondary organic aerosol (SOA) formation. Many recent laboratory experiments and field observations suggest that more complex chemical reactions can occur in the aqueous aerosol surface; however, direct probing of aqueous surface changes is a challenging task using surface sensitive techniques. The ability to map the molecular distribution of reactants, reaction intermediates, and products at the aqueous surface are highly important to investigate surface chemistry driven by photochemical aging. In this study, photochemical reactions of glyoxal and hydrogen peroxide (H₂O₂) were studied by a microfluidic reactor, System for Analysis at the Liquid Vacuum Interface (SALVI), coupled with Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Aqueous surfaces containing glyoxal and hydrogen peroxide were exposed to UV light at variable lengths of time and were immediately analyzed in the SALVI microchannel by in situ liquid ToF-SIMS. In addition, various control samples were conducted to ensure that our findings were reliable. Compared with previous results of bulk solutions using ESI-MS, our unique liquid surface molecular imaging approach provided observations of glyoxal hydrolysis (i.e., first and secondary products, dimers, trimers, and other oligomers) and oxidation products (i.e., glyoxylic acid, oxalic acid and formic acid) with sub-micrometer spatial resolution. We potentially provide a new perspective and solution to study aqueous surface chemistry as an important source of aqueous SOA formation of relevance to atmospheric chemistry known to the community.

Dynamic molecular information at electrode-electrolyte interface is critical for a range of scientific fields, including electrocatalysis, electrodeposition, and rechargeable battery. However, it has been a challenge to directly monitor the molecular process at the electrode-electrolyte interface under reaction conditions. For example, the mechanism of electro-oxidation of vitamin C on the anode surface has been hypothesized for quite a few years;[1] however, no direct experimental evidence has been reported to support such hypothesis. We recently developed a self-contained microfluidic device (i.e., System for Analysis at the Liquid Vacuum Interface, SALVI) for probing solid-liquid interfaces and demonstrated its feasibility in ToF-SIMS and SEM.[2,3] An electrochemical version or the E-cell was also developed by enclosing a three-electrode system in the microreactor. We demonstrated that this new E-cell can be used in in situ study of I^- oxidation at the anode-electrolyte solution interface while supplying current using cyclic voltammetry. The intermediate products (e.g., I₂^-, I₃^-, and AuI₂^-) and the final product (IO₃^-) can be successfully detected dynamically.[4] Our latest results show that not only the oxidation process of I^- can be monitored, but also the oxidation-reduction cycles in real time. Most recently, we successfully observed the short-lived chemical reaction intermediate of the electro-oxidation of vitamin C on the anode surface; and our new observation provides the first direct experimental evidence to the hypothesized oxidation mechanism.

[1] Erdurak-Kilic, C. S.; Uslu, B.; Dogan, B.; Ozgen, U.; Ozkan, S. A.; Coskun, M. J. Anal. Chem. 2006, 61, 1113.

[2] L. Yang, X. Y. Yu, Z. Zhu, M. J. Iedema, J. P. Cowin, Lab Chip, 2011, 11, 2481-2484.

[3] L. Yang, X. Y. Yu, Z. Zhu, S. Thevuthasan, J. P. Cowin, J. Vac. Sci. Technol. A, 2011, 29(6),061101.

[4] B. Liu, X. Y. Yu, Z. Zhu, X. Hua, L. Yang, Z. Wang, Lab Chip, 2014, 14, 855.

In situ liquid SIMS has proven to be a very promising new tool to provide molecular information at solid/liquid interfaces.[1,2] However, the initial data showed that signals of secondary positive ions were too low to be usable in some cases.[2,3] In addition, it was difficult to obtain strong negative molecular ion signals with m/z > 100.[2] These two drawbacks make SIMS community wonder the potential applications of this new analytical approach. In this presentation, we report that strong positive and negative molecular signals are achievable after we optimize the SIMS experimental conditions. Our results show that both beam current and primary ion species (e.g., Bi⁺, Bi₃⁺, Bi₃²⁺) play important roles in achieving optimal molecular signals at the liquid interface. Data sets from three model systems, including an ionic liquid, water, and several liposome solutions, will be presented. In addition, beam damage at the liquid surface will also be discussed.

References

[1] B. Liu, X. Y. Yu, Z. Zhu,X. Hua, L. Yang, Z. Wang, Lab Chip, 2014, 14, 855.

[2] X. Hua, X. Y. Yu, Z. Wang, L. Yang, B. Liu, Z. Zhu, A. E. Tucker, W. B. Chrisler, E. A. Hill, S. Thevuthasan, Y. Lin, S. Liu, and M. J. Marshall, Analyst, 2014, 139, 1609.

[3] L. Yang, Z. Zhu, X. Y. Yu, S. Thevuthasan, J. P. Cowin, Anal. Methods,2013, 5, 2515.

Li-ion batteries are now indispensably used as energy storage devices for portable electronics, electric vehicles, and are starting to enter the market of the renewable energies. The rechargeable capacity and the battery life depends critically on the structural stability of the electrodes themselves, the electrolyte degradation rate, and the electrode-electrolyte interaction layer-the so called solid electrolyte interphase (SEI) layer. Over the last few years tremendous progress has been made towards direct in-situ TEM observation of structural and chemical evolution of electrodes used for lithium ion batteries. However, capturing of molecular information across the solid-liquid interface has never been possible. Here we report the development of an in-situ liquid SIMS and its usage for the first time to directly observe the molecular structural evolution on the electrode and within the liquid electrolyte for lithium ion battery under dynamic operating condition. We observed that, upon charging of the battery, PF₆^- anions were repelled from the anode side, Li⁺ ions were reduced at the anode, and a liquid layer with a significantly low concentration of Li⁺ and PF₆^- was formed around the anode. Formation of the lean electrolyte layer around the electrode will lead to reduced ionic conductivity and therefore contributing to the overpotential of the battery. The present work opens the door for in-situ SIMS studies of both dynamic structural and chemical evolution of the electrodes and the SEI layer formation in batteries using real battery relevant electrolytes.

As a high voltage cathode material with good rate capability and relatively low cost, lithium manganese oxide spinel (LMO) has been widely studied and commercially used in lithium ion batteries. Despite of its good electrochemical performance at room temperature, spinel based battery usually suffers from degradation at high temperature and prolonged operation, which significantly limits its application as power source in Hybrid Electric Vehicles (HEVs) and Electric Vehicles (EVs).

In the spinel based Li-ion batteries, Mn dissolution from the spinel cathode is a critical issue, which can cause severe battery capacity and power fade. It changes the cathode surface properties as well as the cathode material. When the dissolved Mn reaches the anode, it damages the solid electrolyte interphase (SEI), which is a native protective layer on the anode formed by electrolyte reduction. These changes in electrodes and electrolyte interfaces are major causes for battery degradation.^1,2 Therefore, to improve battery performance, Mn dissolution and its impacts on electrode materials and the electrode/electrolyte interfaces needs to be understood.

Developing an effective analytical technique for this purpose is challenging. Firstly, the thickness of the SEI surface layer is only in the order of nanometers and exhibits very complex chemistry. Secondly, most surface analysis techniques cannot reach the deeper internal surface layer inside the electrode materials. By leveraging the surface analysis and cross section capabilities of FIB TOF SIMS (Focus-Ion-Beam Time-of-Flight Secondary-Ion-Mass-Spectrometer) these challenges were overcome and a powerful tool for Li-ion battery analysis was developed. The chemistry and thickness changes in SEI caused by Mn were identified with TOF SIMS and XPS. The Mn dissolved from the LMO cathode was found to mix in the anode SEI, and resulted in further electrolyte decomposition at the outer electrode surface layer. 3D chemical imaging of the LMO and graphite was obtained using FIB and ion milling cross sectioning and showed electrolyte decomposition at the internal surfaces inside and between graphite particles,.

(1) Delacourt, C.; Kwong, a.; Liu, X.; Qiao, R.; Yang, W. L.; Lu, P.; Harris, S. J.; Srinivasan, V. J. Electrochem. Soc. 2013, 160 (8), A1099–A1107.

(2) Pieczonka, N. P. W.; Liu, Z.; Lu, P.; Olson, K. L.; Moote, J.; Powell, B. R.; Kim, J. J. Phys. Chem. C 2013, 117, 15947–15957.