Research

Postdoctoral Research Associate (Advisor:  Dr. Shelley Claridge)                       August, 2023 – Present

Kinetics of On-surface Topochemical Polymerization of Double-chain Phospholipids

In my Postdoctoral research, I have been studying on-surface topochemical polymerization (OSTP) kinetics of two double-chain phospholipids e.g., phosphocholine (PC) and phosphoethanolamine (PE). Our lab previously reported striped-phase monolayer of phospholipids containing diacetylene (DA) moiety using Langmuir-Schaefer technique.  Upon exposure to UV light (254 nm), the DA moiety undergoes on-surface topochemical polymerization, stitching the phospholipid monomers together to form a robust polydiacetylene (PDA) monolayer. This PDA monolayer is highly robust and can be transferred to pattern highly heterogeneous soft materials such as PDMS, hydrogels. Transfer efficiency of PDA monolayer from highly ordered pyrolytic graphite (HOPG) to soft materials depends on the OSTP reaction which in turn is influenced by pre-organization of phospholipids on HOPG. Therefore, understanding kinetics of OSTP reaction is crucial for optimizing the transfer of PDA monolayers to pattern soft materials effectively. During the kinetic study of double chain phospholipids, I discovered that PE exhibits significantly faster reaction kinetics compared to all single-chain fatty acids (e.g., TCDA, PCDA, TCD-NH2, PCD-NH2) even one order faster than PC, another double chain phospholipid. 

While the exact reason for PE’s faster polymerization is still under investigation, we hypothesize that strong electrostatic interactions between amphiphiles (due to their zwitterionic headgroups) and weak van-der-Walls interactions between the proximal chains and the substrate (HOPG) provide the necessary flexibility of the DA moiety to overcome crystal strain during the initiation and propagation step of OSTP reaction. In contrast, the bulky headgroups hinder the efficient packing of amphiphiles on HOPG surface, limiting polymerization efficiency. Several intriguing observations emerged during the study of polymerization kinetics. Notably, when the UV light intensity (254 nm) was reduced from 1.5 photon nm-2 s-1 to 0.2 photon nm-2 s-1, the reactions proceeded with more controlled manner and the lifted headgroup can be visible by AFM imaging. Additionally, depending upon the relative humidity, every two other rows of molecules can undergo OSTP reactions.  

Spectroscopic Characterization of Ultra-thin films of soft materials

Patterning Highly Stretchable Hydrogel formualation and charactrization for Tissue Regeneration

Double network (DN) hydrogels, which consists of a brittle, rigid network as the first network and a soft, ductile network as the second network, was first reported in 2003 by Prof. Gong. Because of its toughness (High fracture energy ~ 1000 J m-2) and compressive nominal strength 20-50 MPa, it has drawn our attention. Since then, people have been trying to use tough hydrogels for mimicking biomaterials such as scaffolds for tissue engineering to provide necessary stiffness. In 2012, Prof. David Mooney and Zhigang Suo from Harvard University reported highly stretchable (20 times of its original length) and tough (~ 9000 J m-2 fracture energy) hydrogels. In Claridge lab at Purdue University, we have expertise on patterning the interface of soft materials like PDMS and Hydrogel. I have been trying to utilize my expertise (patterning interface using Langmuir_Schaefer monolayers) to pattern this tough hydrogel for various applications such as tissue regeneration, saliva delivery, electrolytes for solid-state battery etc. We have made our initial breakthrough by being able to transfer monolayer of diynePE on PAAm-Alginate tough hydrogel. We are now trying to optimize the transfer efficiency as well as performing mechanical tests of PAAm-Alginate gel.

Graduate Research Associate (PhD advisor:  Dr. Gary J. Blanchard)                       2018 – July, 2023

Piezoelectric Effect of Room Temperature Ionic Liquids (RTILs): Structure-Function Relationship and Mechanistic Insights

Here, we reported the observation of the direct piezoelectric effect in room-temperature ionic liquids (RTILs). ​ The piezoelectric effect is the production of charge upon the application of force to a material, and it has been previously observed only in solid-phase materials. ​ 1-butyl-3-methyl imidazolium bis(trifluoromethyl-sulfonyl)imide (BMIM+TFSI-) and 1-hexyl-3-methyl imida-zolium bis(trifluoromethylsulfonyl) imide (HMIM+TFSI-) produce a potential upon the application of force when confined in a cell, with the magnitude of the potential being directly proportional to the force applied. ​ The effect is one order of magnitude smaller than that seen in quartz, a widely used piezoelectric material. ​ This is the first report of the direct piezoelectric effect in a neat liquid. ​ The discovery of the piezoelectric effect in a liquid has fundamental implications about the organization and dynamics in ionic liquids and invites further theoretical treatment. ​

We conducted experiments to measure the direct piezoelectric effect in the RTILs. ​ We used a cell made of steel and Delrin, with an O-ring to ensure a tight seal. ​ The force-induced charge was measured as an open circuit potential difference between the cell body and the piston center electrode. ​ The force applied was measured with a digital force gauge. ​ The potential vs force data for both BMIM+TFSI- and HMIM+TFSI- showed a linear dependence, indicating the direct piezoelectric effect. ​ The magnitude of the effect was determined to be 16 ± 1 mV/N for BMIM+TFSI- and 17 ± 1 mV/N for HMIM+TFSI-. ​ These values correspond to piezoelectric constants, d33, of 0.34 ± 0.02 pC/N and 0.36 ± 0.02 pC/N, respectively. ​

The observation of the direct piezoelectric effect in RTILs suggests the existence of organization being induced in these liquids that lifts the bulk center of symmetry. ​ This is significant because bulk liquids are typically centrosymmetric. RTILs exhibit local and long-range organization, and the existence of a charge density gradient in these materials suggests that charge can be used to produce the same structural effect. ​ The current theory for the piezoelectric effect in solids may require modification to account for the experimental observations in RTILs. ​

In conclusion, the discovery of the direct piezoelectric effect in room-temperature ionic liquids opens up new possibilities for applications that were previously not accessible with solid-state materials. ​ RTILs are more readily recyclable and pose fewer environmental issues compared to many currently used piezoelectric materials. ​ Further research is needed to better understand the organization and dynamics in RTILs and to develop a theoretical framework to explain the piezoelectric effect in these liquids. 

In another study, we discuss the structure-dependence and mechanistic insights into the piezoelectric effect in ionic liquids (RTILs). ​ We conducted experiments to investigate the generality and mechanism of the piezoelectric effect in RTILs by studying several RTILs with different cations and anions. ​ We found that all the RTILs studied exhibited the direct piezoelectric effect, with the magnitude of the effect (d33) and the threshold force depending on the structures of both the cation and anion. ​ We also observed a pressure-induced liquid-to-crystalline solid phase transition in the RTILs, which is consistent with the existence of a threshold force for the piezoelectric effect. ​ The X-ray diffraction (XRD) data showed discrete peaks associated with the RTILs, indicating the formation of crystalline domains under pressure. ​ We concluded that the direct piezoelectric effect in RTILs is likely due to the presence of crystalline domains in the RTILs. ​ The study provides insights into the structural dependence of the piezoelectric effect in RTILs and highlights the potential applications of RTILs in piezo-pneumatic devices and liquid-phase optics. ​

Relating the Free Charge Density Gradient to Molecular-scale Organization of RTILs

Our paper titled "Relating the Induced Free Charge Density Gradient in a Room-Temperature Ionic Liquid to Molecular-Scale Organization" explores the relationship between the induced free charge density gradient (ρf) in room-temperature ionic liquids (RTILs) and the molecular-scale organization within these liquids. ​ The study focuses on a binary system consisting of the RTIL 1-decyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (DMPyrr+TFSI-) and the molecular solvent 1-decanol. ​ Two fluorescent probes, perylene (nonpolar) and cresyl violet (CV+) (polar), were used to investigate the local environments and reorientation dynamics within the binary system. ​

The results show that both perylene and CV+ experience changes in their local environments with increasing 1-decanol content. ​ Perylene, which resides in the nonpolar regions, exhibits a change in its rotational diffusion dynamics at a lower 1-decanol concentration compared to CV+, which is localized in the polar/ionic regions. ​ Perylene reorients as an oblate rotor, while CV+ reorients as an oblate rotor for Xdecanol ≤ 0.75 and as a prolate rotor for higher values of Xdecanol. ​ This difference in reorientation behavior allows for the extraction of detailed information about the chromophore's local environment. ​

The study also reveals that ρf, induced by placing the RTIL in contact with a charged surface, persists in the DMPyrr+TFSI-−1-decanol binary system up to high 1-decanol content (Xdecanol = 0.75). ​ The depth-dependent changes in the reorientation dynamics of CV+ provide insight into the structural details of the induced charge density gradient. ​ The data show that rotational motion about the x-axis becomes less hindered with distance from the charged support surface, while rotational motion about the z-axis becomes more hindered. ​ This suggests a change in the local environment from lamellar organization near the surface to a more spherically symmetric organization further away. ​

Furthermore, the study highlights the decoupling of the rotational diffusion constants (DROT) of perylene and CV+ from the bulk viscosity of the binary system. ​ This decoupling indicates the presence of microheterogeneity within the RTIL medium, where the chromophores are confined within different nanoscale domains. ​ The viscosity sensed by the reorientation measurements is reflective of the immediate environment of the chromophores, while the bulk viscosity represents interactions between larger assemblies in which the chromophores are confined. ​

Overall, the study provides valuable insights into the molecular-scale organization and dynamics within RTIL-molecular solvent binary systems. ​ The findings contribute to a better understanding of the properties and behavior of RTILs from a fundamental perspective. ​

Effect of Dilution on Induced Free Charge Density Gradients in RTILs: Persistent compositional heterogeneity and the importance of dipolar interactions

The ability to establish free charge density gradients (pf) in RTILs depends on two conditions: (1) charge screening in RTILs is sufficient to isolate charges effectively, and (2) low mobility of discrete ions, such that, once free charge density gradient is established, it is not compromised by diffusion. To evaluate the role of ion mobility on the persistence of free charge density gradients, we diluted RTILs with organic solvents.  The addition of diluents can alter and diminish the organized structure of RTILs and enhance ion mobility by virtue of reducing viscosity.  We found that free charge density gradients persists upon RTIL dilution to ca. 30 mol-% of molecular solvent diluent and is not seen for higher dilution.

Three different RTILs, 1-butyl-3-methylimidazoliumtetrafluoroborate (BMIM+BF4-), 1-butyl-3-methylimidazoliumbistriflimide (BMIM+TFSI-), 1-hexyl-3-methylimidazoliumbistriflimide (HMIM+TFSI-) and two different diluents, acetonitrile and methanol have been investigated. We used depth- and time-resolved fluorescence anisotropy decay measurements to characterize the rotational diffusion dynamics of Cresyl violet (CV+) in the diluted RTILs.

The experimental data demonstrate a reduction in the spatial extent of  over macroscopic distances that does not depend linearly on amount of diluent. We also find that RTIL domain size in the diluted systems increases with increasing diluent. The binary mixtures of RTILs produced a heterogeneous system even at a high dilution (80 mol-% diluent), consistent with model predictions. These findings provide insight into the nature of compositional heterogeneity in RTIL/solvent binary systems. We found that the gradient persists in RTILs to varying degrees depending on the RTIL and diluent identity. The functional form of the gradient is not a smooth diminution with increasing diluent, but rather a stepwise collapse. ​ This collapse is correlated with the onset of RTIL ion pair dimer formation. ​ The formation of molecular-scale aggregates in the RTIL solutions, even at high dilution, which suggests the existence of persistent compositional heterogeneity in diluted RTIL systems. ​ The findings of this study provide insights into the short-range organization in RTILs and the relationship between local and longer-range organization and the induced free charge density gradient. 

Then, we extended our work to understand the importance of solvent dielectric constant and dipole moment in the absence of hydrogen-bonding processes. The study focuses on two imidazolium-based RTILs with different anions (BF4- and TFSI-) to understand the role of anion in RTIL organization, and two different polar aprotic diluents, dimethyl sulfoxide (DMSO) and propylene carbonate (PC) have been investigated. Our findings suggest that binary mixtures of RTILs behave in the same manner qualitatively, irrespective of diluent or RTIL identity. The dependence of the induced free charge density gradient on dilution does not vary linearly with dilution. ​ Instead, there are discontinuous changes in qf magnitude and persistence length with increasing dilution, and these changes depend on the identity of the diluent and RTIL anion. ​ We also observe the formation of aggregated domains of RTIL in the binary systems, with the size of the domains increasing with increasing dilution. ​ The study suggests that the magnitude and persistence of qf in RTILs are not solely determined by ion mobility, but also by dipolar interactions and the properties of the RTIL ion pairs. ​ The findings highlight the importance of dipolar interactions in RTIL behavior and the ability of these materials to support a charge density gradient over macroscopic distances. ​