Icon for: Thomas Wallin


Cornell University
Years in Grad School: 2

Judges’ Queries and Presenter’s Replies

  • Icon for: Jon Kellar

    Jon Kellar

    May 20, 2013 | 12:42 p.m.

    In the poster it is mentioned that NOHMs can be used for desalination. Please describe the mechanism by which this could be accomplished.

  • Icon for: Thomas Wallin

    Thomas Wallin

    May 20, 2013 | 03:45 p.m.

    Thank you for your interest in my work. Unfortunately due to constraints I was unable to outline all the cool applications of NOHMs, such as desalination.

    One thrust to improving membranes is to add a reactive barrier (such as a reactive nanoparticle) in addition to the selective barrier (size selective pores). Zero valent iron, Titanium dioxide and silver are examples of such nanoparticles that can react with and destroy contaminants. However, conventional methods of incorporating these nanoparticles present a fabrication challenge as they can alter the morphology, porosity, and mechanical properties of the membrane.

    Previous work has shown that these challenges can be overcome through NOHMs. With a silver core and a corona of polyethylene imine (PEI) molecules that link to the membrane, we can bind a high concentration of these biocidal (an inherent property of silver nanoparticles) NOHMs to the surface of polysufone ultrafiltration (UF) membranes. Thus, we have an active reactive barrier on the surface without modifying the inherent properties of the original membrane.

    The benefits of this approach include the wide range of available properties through different NOHM designs, ease of manufacturing, and reduction of costs by using a more efficient membrane.

    I hope that answers your question.

    Reference: Antifouling Ultrafiltration Membranes via Post-Fabrication Grafting of Biocidal Nanomaterials. Meagan S. Mauter, et al. ACS Applied Materials & Interfaces 2011 3 (8), 2861-2868
  • Icon for: Thomas Wallin

    Thomas Wallin

    May 22, 2013 | 02:30 p.m.

    Also, I’d like to add that NOHMs can be used to help keep the selective barrier clean of fouling organic species. Often times the membranes have organic species bind to them and this reduces the flux and performance. Recent work has shown that by adding polyamide-silica based NOHMs on to the membrane surface, we can drastically improve the hydrophilicity of the membrane. This hydrophilicity means that water can displace the organic foulants and keep membrane performance high.

    The reason why NOHMs are preferred to simply functionalizing the membrane itself with amide groups is the fact that this method decreases the performance of the membrane (similar to above). Additionally, in this case, the amide groups can be removed from the surface of the membrane where as they are more rigidly held to the nanoparticle in the case of the NOHM.

    Reference: Tiraferri, A., Kang, Y., Giannelis, E., and Elimelech, M. “Superhydrophilic Thin-Film Composite Forward Osmosis Membranes for Organic Fouling Control: Fouling Behavior and Antifouling Mechanisms”, Environmental Science & Technology, Volume 46, October 2012, pages 11135–11144.
  • Icon for: Marc Porter

    Marc Porter

    May 20, 2013 | 02:23 p.m.

    Why are the as prepared GNRs preferentially aligned in the TEM image?

  • Icon for: Thomas Wallin

    Thomas Wallin

    May 20, 2013 | 04:06 p.m.

    Thanks for taking an interest in my project.

    The image at the top of the poster depicts GNR that are coated with the surfactant CTAB as a result of the synthesis. These particles were originally in water and then drop casted on a TEM grid. The assembly you see is a result of the evaporation process and the interaction between the particles and surfactant. CTAB is comprised of long non-polar alkyl chains. It is my understanding that the particle alignment you notice stems from a CTAB bilayer forming between the two adjacent particles. Our microscopy has also shown that the distance between these adjacent gold particles to be relatively constant at about 4nm.

    A closer inspection of the poster will show a distinct difference in the particle alignment of that TEM image and the following TEM image of the GNR after the corona has been attached. This is because the organic molecules in the corona displace the CTAB and exhibit a different set of interactions. These particles were also dispersed in THF and not water so the evaporation process is different as well.

  • May 20, 2013 | 08:06 p.m.

    What specific structural changes are planned in the corona preparation? What is the rationale for these changes?

  • Icon for: Thomas Wallin

    Thomas Wallin

    May 20, 2013 | 09:01 p.m.

    Hi Dr. Ludwick,
    I am not sure I fully understand your question.

    In the video I mention the all the changes we can make to the corona “hairs” such as the grafting density, the length of the polymer, etc. I can illustrate an example about how varying one of these design parameters will affect the resulting NOHMs material.

    Say we were to have a low grafting densities (less than the root of the polymer hair’s radius of gyration). Here the polymers would not have many neighbors and would be able to fold on itself and form a loose coil. Now as we increase the grafting density, the polymer chains in the corona would start to encroach upon eachother and the steric hindrance would force the chains to extend out. We could also attach a polymer with an ionic group at the end, this would add an electrostatic interaction to the above situation. These low to moderate grafting densities can be modeled like a polymer brush.

    Structurally these corona interactions have a lot of importance, particularly with regards to the dispersion state of the material. The steric repulsion at high grafting densities mentioned above can provide stabilization of the nanoparticles as it can keep the cores from agglomerating and phase separating. This is one of the key advantages of NOHMs over other organic-inorganic nanocomposites. By directly attaching the polymer to the inorganic core the only way to “demix” is by physically breaking this bond.

    A labmate of mine has recently studied the effect of corona design on the dispersion state of Silica-PEG NOHMs particles in a PEG matrix by systematically varying the grafting density on the particles. He found that at significantly high grafting densities, the polymers in the corona become significantly compressed and can actually jam the system. This is shown through a transition in the viscosity of the material. I highly recommend that you consult his work listed below. He also did a study on how the grafting density affects the structure factor, S(q).

    I hope the sufficiently answers your question and if I missed the mark, please let me know.

    Reference: Structure and Rheology of Nanoparticle-Polymer Suspensions. S. Srivastava, J. H. Shin and L. A. Archer Soft Matter 8, 4097 (2012)
  • May 21, 2013 | 07:40 p.m.

    I am unclear on what is meant by the statement regarding the recent discovery of NOHM being recently discovered at Cornell. Nanoparticles have been decorated with a variety of materials (corona) for a variety of reasons (sensors, drug delivery, others) and have been studied for at least 15 years. What distinguishes the NOHM that you have under investigation.

  • Icon for: Thomas Wallin

    Thomas Wallin

    May 21, 2013 | 09:47 p.m.

    Hi Dr. Gannett,

    Thank you for taking the time to review my presentation.

    You are correct that the notion of grafting polymers onto nanoparticles can be traced as far back as the 1950s and has been a growing area of research for the past few decades. I did not mean to imply that Cornell “invented the wheel” when it comes to organic-inorganic nanocomposites. I sincerely apologize if you feel my phrasing did not properly credit the appropriate researchers for their groundbreaking work in this field. However, to my knowledge, the concept of self-suspended nanoparticle-organic hybrid systems can be correctly attributed to Cornell university scientists (see reference below). I would say it is this solventless aspect that truly distinguishes the materials I’m talking about when I say “NOHMs” from the other materials you mentioned.

    There were a few reasons why I chose to use the term “recent.” First, I started my video with a reference to adobe bricks from thousands of years ago. With that time scale in mind, these nanocomposites can be considered “recent.” Further, I wanted to convey the sentiment that the full range of possible material properties in NOHMs are not fully known and additional research (like mine) is necessary. And as a consequence of this incomplete fundamental understanding, there exists a still greater potential for this class of materials to impact future technology. This motivates the work and conveys its importance to society as whole.

    Perhaps there is a better way to express these ideas than the phrase you quoted. Again, I apologize if you feel I misrepresented this work.

    Please feel free to ask any additional questions,
    T.J. Wallin

    Reference: A. B. Bourlinos, S. R. Chowdhury, R. Herrera, D. D. Jiang, Q. Zhang, L. A. Archer and E. P. Giannelis, Adv. Funct. Mater., 2005, 15, 1285.
  • Icon for: Thomas Wallin

    Thomas Wallin

    May 22, 2013 | 02:32 p.m.

    Further, the viscoelastic behavior of other nanoparticle-organic hybrids (star polymers, colloidal suspensions and granular particles) does not completely describe the viscoelastic behavior seen in our NOHMs. This points to NOHMs being a truly unique materials platform.

  • Icon for: Antal Jakli

    Antal Jakli

    May 22, 2013 | 09:37 p.m.

    Do you have an idea why the recovery is not good for 572/669nm and the 475/669 nm cases?

  • Icon for: Thomas Wallin

    Thomas Wallin

    May 22, 2013 | 10:34 p.m.

    Hi Dr. Jakli,

    Thanks for your interest!

    That is a very good question and one that I can not definitively answer currently. However, I can provide an explanation which will shed light on this probelm.

    The optical spectrum of these films is determined by the localized surface plasmon resonance (LSPR) which depends on many things including the interparticle orientation and spacing (For those of you unfamiliar with LSPR, please see my response to Eammon Walker in the discussion tab for a full explanation).

    A single gold nanorod with no neighbors will show two distinct peaks when dispersed in THF: a transverse (about 570 nm) and longitudinal peak (about 750nm) correspondiing to electron oscillation across the diameter and across the length of the rod respectively. However, when we move from one isolated rod to a collection of rods, the interactions between neighboring nanorods gives rise to changes in the LSPR. It is the coupling of the transverse and longitudinal peaks of adjacent rods that is giving rise to the spectra changes at the observed wavelengths. For example, if we just consider two neighboring rods we find that a head-to-head interaction will have a certain shift that depends on, among other things, how far apart the two heads are. Similarly, a different shift would be seen for a head-to-tail or tail-to-tail interaction. It’s not hard to see that as we move to multiple neighboring rods, these interactions and spectra shifts become incredibly complex (head-to-tail-to-tail, etc.).

    The simple answer is to say that whatever geometric orientation of a collection of rods that leads to one (or multiple ones) of the peaks you mention is affected by shearing and not reversible upon recovery. I should add that I repeated the measurements 20 minutes afterwards and the spectra was unchanged from immediately after recovery. This means that there was no relaxation back to an equilibrium orientation in this time.

    With Scanning Electron Microscopy, I tried to view the interparticle orientation before and after shearing. Unfortunately this was not elucidating and did not reveal any specific particle interaction that was present before shearing but not after. I am fortunate enough to have access to Grazing Incident Small Angle X-Ray Scattering here at Cornell. This experimental technique might be able to yield data about the interparticle correlation before and after shearing and help answer your question.

    I hope that provides some insight, but you asked a tough question and one that my future work aims to resolve.

    T.J. Wallin

  • Further posting is closed as the competition has ended.

Presentation Discussion
  • Icon for: Eamonn Walker

    Eamonn Walker

    May 20, 2013 | 03:49 p.m.

    Hi, Thomas.
    Very interesting and informative video. I thought the application as a shear sensor was particularly cool. Is this a transition between two states, or is there a continuous change that could potentially be used to measure the stress level?
    Also, if I understand correctly, the NOHM material acts as a liquid, correct? Is the change related to the shear, or the shear rate? It sounds like the former, but I wanted to be sure.
    Finally, you mentioned that one of your goals was to study the not-fully understood behaviour of the nanoparticles. I realise you appear to be more involved in the experimental side of things, but have you considered using Molecular Dynamics simulations of the NOHMs to better investigate this? I’m not sure how much computational power it would take to simulate your corona strands, but it might be worth looking into, if you haven’t already.


  • Icon for: Thomas Wallin

    Thomas Wallin

    May 20, 2013 | 06:04 p.m.

    Hi Eamonn,

    Thanks for your questions. Constraints on the video prevented me from truly explaining how localized surface plasmon resonance (LSPR) causes the chromatic shift.

    I don’t know how familiar you are with localized surface plasmon resonance, but it is a quantum mechanical phenomenon that occurs in some nanoparticles. Incoming light interacts with electrons in the conduction band of the gold nanorod and causes them to collectively oscillate, while a restorative force from the nuclei also acts on the electrons. Like in classical physics, resonance occurs at frequencies (of light) that maximize the amplitude of (electron) oscillation.

    Now in the case of the gold nanorods, you generally see two frequencies that cause resonance in the UV/Vis spectrum. One of these frequencies occurs at around 500 nm and corresponds to electrons oscillating across the diameter of the rod. The other frequency occurs at longer wavelengths and corresponds to oscillations along the length of the rod (the longer the rod, the longer the wavelength).

    However, surface plasmon resonance for a collection of particles becomes complicated and depends on many factors such as the local electron density, the dimensions of the rods, the dielectric constant of the dispersing medium, interparticle spacing, orientation, etc.

    The chromatic shift arises from changes in the relative orientation of the GNRs. In the initial state of the video, the particles are randomly oriented. As we shear the particles, they tend to align with the shear and develop a nonrandom orientation (Small angle x-ray scattering was used to probe the interparticle correlation). This change in orientation shifts the LSPR of the material and causes different frequencies of light to resonate. Thus changing the spectrum of the material. We are not directly measuring shear or shear rate, but instead the interparticle orientation. I would imagine the shear and shear rate would both be a factor in how the interparticle spacing and orientation changes over time.

    Also, this system is not “jumping” between two states from sheared to unsheared. It is a continuous transition in the LSPR as the liquid-like particles move relative to each other and occurs on a similar time scale. When you see the sample go back to its original color in the video, you are seeing the particles relaxing and going back to a random orientation.

    And finally, you are correct that I am more of an experimentalist. However, this is an active research thrust at Cornell and there are other groups (particularly Professor Donald Koch) who have taken a more theoretical approach to modelling NOHMs.

    I hope that answers your questions and feel free to ask more! If you’re interested in learning more, I would recommend searching “plasmon ruler” as this is another cool application of LSPR.

    Reference: Bhattacharjee, Rama Ranjan, Ruipeng Li, Luis Estevez, Detlef-M. Smilgies, and A. “A plasmonic fluid with dynamically tunable optical properties.” Journal of material 19, no. 46 (2009): 8728-8731
  • Icon for: Eamonn Walker

    Eamonn Walker

    May 20, 2013 | 06:54 p.m.

    OK, thanks. I actually knew pretty much nothing about plasmon resonance, but your explanation was excellent.

  • Icon for: Robert Opila

    Robert Opila

    May 22, 2013 | 11:57 p.m.

    Good job—I had only been vaguely familiar with NOHMs before—these composites on the mess-scale are one of the important advances going on in materials science. Liked the use of platoons

  • Icon for: Thomas Wallin

    Thomas Wallin

    May 23, 2013 | 01:38 a.m.

    Thanks Dr. Opilia. I felt this medium lent itself well to actually illustrating some of the examples.

  • Icon for: Tony Reames

    Tony Reames

    May 23, 2013 | 01:11 a.m.

    Great video Thomas. Completely outside my wheelhouse, but very understandable delivery. Is NOHM discovery a product of your current research or previous research at Cornell?

  • Icon for: Thomas Wallin

    Thomas Wallin

    May 23, 2013 | 01:36 a.m.

    Thanks Tony! My goal was to make the material accessible to everyone and I’m glad someone from another field could understand it.

    I am in no way the discoverer of this class of nanoparticle-organic composite materials (that happened before my arrival in the 2000s). However, I mentioned in the video there is a wide range of possible properties in NOHMs because of the many ways in which we can modify the design. So in this regard, my research can still be considered important.

    For example, my particles have an unusual shape (rods). How self-suspended rod-shaped NOHMs flow and how different factors affect their flow behavior is unknown in the field. My research, in addition to the shear sensor application, looks to probe this behavior on a fundamental level.

    Another unknown variable my research looks to study from a fundamental standpoint is the type of bond that attaches the hairs to the core. I am using a thiol-gold bond which is a different type of covalent bond than the previous studied NOHMs materials by my group.

    Finally, the cool optical properties are a result of an intrinsic feature of gold nanoparticles called localized surface plasmon resonance. There has been little work done on NOHM cores that have this feature, so my work can be considered somewhat cutting edge in that regard.

    I really think the strength of NOHMs lies in their tunability-we don’t know the limit of the possible properties this class of materials can exhibit. So while I am far removed from the discovery, I still feel this work is necessary and innovative (but I’m a little biased).

  • Icon for: Terri La Count

    Terri La Count

    May 23, 2013 | 09:50 a.m.

    Interesting video and work; I am positive your work will impart great insight in the NOHM field.

  • Further posting is closed as the competition has ended.