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Thank you. Thank you for organizing the COST summer school. It was a nice experience.
Great. I would like to thank you and all the members of your team for the good work done at the COST school. The school was a great experience for me and I have learned much.
Thanks. Thanks for organizing such a great course!
Interesting People. I want to thank you very much for this nice week! I met many interesting people and enjoyed it really much.
Polymers and Surfaces. Can we treat polymers as 1D surface? We have surface charge density and also have linear charge density in polyelectrolyte. We have hydrophobic (hydrophilic) surface and we have polyelectrolyte with hydrophobic (hydrophilic) backbone. Surfaces regulate their surface charge as polymers do.
| There is analogy between adsorption of ions on a water-solid interface and binding of ions to a polyelectrolyte. Especially, binding of protons by ionizable interfaces (e.g. silica) and weak polyelectrolytes (e.g., polyacrylic acid) is surely similar. However, differences exist related the fact that a binding site at an interface a larger number of other binding sites in its neighborhood, and as a consequence, the binding curves become broad and featureless. |
Folded or Unfolded. What about denaturation of proteins when adsorbed on surfaces. Are there any experimental techniques what would allow you to tell if proteins are folded or unfolded once adsorbed?
| We have done some work on that but this is a difficult problem. Already in solution it is non-trivial to recognize folding and unfolding, but NMR techniques provide a possibility. On a surface, we do not know how question should be approached. Changes that occur on a surface are very much different on those happening in bulk systems where proteins are isolated. | |
| One possibility would be to use surface-enhanced FTIR spectroscopy and analyze shifts of absorption bands, for example in the C-O vibrational signature. These shifts may change with folding. | |
| Would it be possible to do circular dichroism, but surface enhanced? |
Desorbing Macromolecules. Sometimes you cannot desorb macromolecules by dilution, but by changing pH, for example, or by other means. Would you call this irreversible?
| It is very relevant problem, for example in chips making technology. What you see in similar fields in statistical mechanics is that you take a kind of local equilibrium approach, meaning that some aspects are static, meaning that they do not change over the timescales of experiments, and others are changing. Then you may calculate the effect of these changes of the fast degrees of freedom over the slow degrees of freedom (like the Born-Oppenheimer approach) and so to say you separate the timescales in a way. |
Soft Particles. Has anyone experience with the adsorption on surfaces of soft and charged particles? I am working with self-assembled copolymer particles, whereby one block is a hydrophobic polymer segment and the other a hydrophilic polyanion. Ideas are welcome.
| Soft particles (like microgels) will deform upon adsorption. The extent of the deformation will be determined by the attraction to the surface and the elasticity modulus of the particle. Their charge may modify their interactions with the interface, provided the interface is charged as well. |
Phase of light. Regarding the phase of light, is there a possibility for being partially in phase and partially out of phase?
| This is not possible. There are only these two possibilities. They can be out of phase with a different degree but there is going to be destructive interaction. If light is out of phase, you are going to have a destructive interference. |
Charge Regulation. When you fit the force curve, you use Poisson-Boltzmann equation. Why not use the charge regulation approximation?
| We did not consider surface charge regulation. We assumed that the surface charge was constant. | |
| Do you think it will make a difference? | |
| At closer distances it will. At close distances, with the spring constant that we used, we had a jump into contact. That means that we did not even measure the forces. There is an instability and there were discrepancies for sure but we did not measure them because at some point the two surfaces came into contact. |
Effective radius. Why does one scale measured forces with the effective radius?
| That way you can compare force measurements from different experiments and geometry. This is the commonly used normalization in all surface force experiments. | |
| What are then the typical forces? | |
| In the SFA measurement, the effective radius is about 20 mm. The normalized forces are few mN/m, so you obtain few tens of nN before the surfaces jump into contact. When you are pressing a polymer film adsorbed onto a surface, you can apply a very high force. | |
| In the colloidal probe experiments, the effective radii are few micrometers, and the force resolution typically 10-50 pN. |
Interferometry. Do you have to take the fact into consideration that as you approach the surface the refractive index might change due to the changing concentration of ions?
| Yes, we do measure this and it changes. | |
| Then, how do you discriminate between the force and the change in the refractive index? | |
| The force is determined from the distance. The algorithm to analyze the fringes gives you two independent parameters, the distance and the refractive index. The latter you can use to estimate density changes when you confine a film. The distance is measured independently and thus the force does not influence the refractive index, but the refractive index of mica does change if you deform it. This is not considered so far and could affect the force. But these are very small changes, but they can affect the force measurement. |
Polymer Brushes. Will the pH influence the height of the polymer brushes?
| If you have an ionizable brush, for example, with carboxylic acid groups, the pH will directly affect the brush height. This effect is due to the dissociation of the carboxyl groups. At higher pH they will dissociate, increase the charge density, and lead to a larger thickness. |
Two Polymers. If you have two different polymer layers covering the two mica surfaces, would it be possible for you to characterize each one of these?
| These layers are going to have a different elastic modulus or viscoelasticity. From the slope of the curve you can calculate when the brushes are approaching and then when they come in contact, the moment in which they are compressing. | |
| An elegant way to study this situation is by neutron reflectivity. By selective deuteration, you can may make one or the other polymer invisible to neutrons, and study the properties like thickness and porosity of each polymer layer separately. | |
| Resonant X-ray reflectivity could also be used to achieve contrast variation to some extent in such a system. In this case, reflectivity spectra must be recorded at different wavelength. |
Approach Velocity. What is the range of approach velocity of the two surfaces in the SFA?
| Maximum 15 nm/s, minimum 0.05 nm/s, which is about two orders of magnitude. |
Forces and Images. Is it possible to measure forces without analyzing any images before? Sometimes, people do a scanning in one direction before coming back and then measuring forces on the surface. Is that what one should do?
| Yes, one should know the topography of the surface when forces are measured. But of course, one also can measure forces without imaging the surface. With colloidal probes, one normally does not image, since the probe is too large. On the other hand, for some surfaces it is interesting to have topographical information, for example, some of the work done in Geneva, macromolecules are adsorbed and the surface is imaged to know where macromolecules are. You can then then measure forces or pick up these molecules. | |
| You can also adsorb polymers on the tip and measure forces between the modified tip and opposite surface. Like that, no imaging is neccesary. But you would have to apply some statistics to be really sure that your results come from single molecules. |
Tapping mode. Does the tip touch the surface in tapping mode?
| Yes, it does. That's why this mode is called tapping. The tip taps the surface. This feature makes this mode also very stable. True non-contact modes are only sensitive to large features. Especially with soft samples, they can introduce artifacts. | |
| To hammer the surface, can one use the tapping mode? | |
| You can do this under certain conditions. But you may destroy your tip or surface in this way. If one is not careful, the tip might also get stuck to the surface, as might happen in humid air due to capillary forces. |
Approach and Retraction. Has anyone ever tried to use different velocities for approach and retraction? Is there any influence of the contact time?
| Some people use different dwelling times to leave the tip on the surface. During that time some reorganization processes might happen. | |
| What really matters is the velocity of retraction, approach does not change so much. Concerning the dwell time, there you can increase the probability that you attach a molecule to your tip and you may then remove this molecule from the surface more easily. |
SAMs on gold. Is one sure that salt ions are adsorbing at the top of the self-assembled monolayer (SAM)? Could they also not adsorb on the bare gold surface?
| One knows that ions do not adsorb on the bare gold surface since AFM can be used to detect any defects in the SAM. SAMs can be made to be almost perfect, almost without defects. Such SAMs are not penetrable to ions. | |
| What happens when one varies the length of chain length of the thiol molecules? | |
| The length has been varied, but then the problem with homogeneity and stability of the layers comes in. SAMs made with thiols with alkyl chains with 15 or 16 carbons are almost perfect, while those with 6 carbons have many defects. So one cannot control the thickness and thus the capacitance of the SAMs in this fashion too well. |
Water is not just Water. When there are no ions in water, what would then adsorb on the surface of SAMs?
| Water is composed mostly H2O molecules, but there are also some H+ and OH- ions. These ions may adsorb on the SAMs and induce a surface charge. In salt solutions, the salt ions will also adsorb on these surfaces. | |
| In a similar way, the air-water interface acquires a negative charge by adsorption of ions as discussed in the lecture by Hubert Motschmann. |
Adsorbed dendrimers. Regarding the increasing dendrimer dose in particle-particle interaction force measurement, did one start with a symmetric or an asymmetric system?
| The system is always symmetric. The bare latex particles are negatively charged, and the forces are strongly repulsive. When positively charged dendrimers adsorb, the forces are less and less repulsive until the negative charge of the latex particles are compensated by the dendrimers. At this point van der Waals forces are dominant and the forces are attractive. In addition, there is also an electrostatic force operating due to the patchy charge distribution on the surface. When even more dendrimers adsorb, the overall charge is reversed, and the interaction becomes repulsive again. |
Bubbles and DNA. How can one induce or prevent bubble formation in DNA?
| DNA from bacteria is not relaxed but it has torsion against the direction of the double helix. It has a linking number that is lower than that it should have if there was no torsion. When it is deposited on a surface, because of fluctuations, there can be some writhe introduced. When then we take it off the surface, torsion of the molecule can be induced due to the writhing and the bubble will open up. | |
| Is it the interaction with the surface that forms the bubble? | |
| There might be some interaction with the surface but this effect seems to be happening in solution and not due to the surface interaction. | |
| How many molecules do you need to analyze in order to some bubbles? | |
| Say 50 molecules. | |
| Does the probability of finding a bubble increase if you increase the temperature? | |
| Of course. A theory by Metzler explains the experimental results quantitatively by introducing topology, the energy involved in torsion, in bending, and into opening the molecule. The energies involved do influence this phenomenon. |
DNA and polyacrylic acid. How far does the analogy go between DNA and polyacrylic acid?
| Both are flexible and charged polymers. However, the synthetic polymers have very short persistence length and they are much more flexible than DNA. So they will much more coiled up than DNA. |
Single and double. How do single and double stranded DNA compare?
| Single stand DNA has a much smaller persistence length and it can form intramolecular bonds. Otherwise, it behaves similarly to double stranded DNA. Some substrates can prevent single stranded DNA to form intra-polymer bonds, thus formation of loops. | |
| Why is single stranded DNA sometimes cross-linked? | |
| Because the bases are exposed. One A-base at an end can bind to a T-base on another end, and you can form intra-polymer connections. One can use a substance that prevents this interaction between bases, and then it behaves similarly to double strand DNA. |
Film Thickness. Can you determine the thickness of a polymer brush using optical techniques?
| If you make a careful optical analysis, you can work out the thickness and refractive index of the layer independently. | |
| Aren't the errors of the refractive index and of the distance correlated? | |
| They are, especially in thin layers, say below a thickness of 10 nm. You can get the product of the two parameters easily, but you cannot easily get the value of each of them. Both parameters can be obtained only for thicker layers independently. | |
| Why do I have problems with the signal of my polymer brush? | |
| It might be related to the molecular mass of the polymer. While you get a large signal at high molecular mass, but at low molecular mass the signal may be low. One also needs good optical contrast. |
Good Signal. What is the density of molecules or particles to obtain a good signal?
| The sensitivity is about a hundredth of a monolayer. | |
| For polymeric layers in water or air, ellipsometry has a typical sensitivity of about 10 µg/m2, while SPR is about factor of 10-20 lower. |
Particulate Films. Can we measure the thickness of monolayer of silica particles even if this layer has gaps?
| The technique would be to characterize the layer step by step (bare silicon, oxidized silicon, and covered wafer). If I want to extrapolate some information about packing density, then one has to assume a model (like an effective medium model). There is a lot of theory out there but in order to use such a model, one has to assume many parameters and make averages. In the end, the details of this model might not be that relevant. | |
| Provided the film is thin (few nm), one can use an effective medium (homogeneous slab) model. The particulate nature of the film has then no effect. |
Imaging Ellipsometry. Why do you to superpose microscopic images in imaging ellipsometry?
| Since you take the image at the reflection angle, only a central stripe of the image is in focus. But one can perform a scan, and take different images of the sample with different focus each time. Then one selects the stripes in focus, superimposes these stripes, and in this way one can obtain an entire image that is in focus. |
Synchrotron versus Lab Sources. What are the advantages to do X-ray reflectivity at a synchrotron versus a lab source?
| With the lab source one is restricted to a single wavelength. This situation does not allow measurements at, for example, liquid-liquid interfaces. At synchrotron you can go up to higher energy and get access to these interfaces. |
SPR. What are the advantages of surface plasmon resonance (SPR)?
| It is a quick and easy method, and one can do time-dependent measurements. Main disadvantage is that you need a special gold-coated substrate to create the plasmon. | |
| It is the most sensitive optical surface sensitive technique commercially available. | |
| You should always use simple methods like SPR or ellipsometry to characterize you samples before you move on to more complex and cumbersome techniques like X-ray or neutron reflectivity. Do not use these methods if other techniques are available and can perform the same measurement for you. You need to try everything else, and if it does not work, then you should try complicated techniques. |
Network Analyzer. People used to use a network analyzer to analyze frequency shifts of the quartz crystal with moderate success, and now you use an internal crystal as an internal reference. Why does your method work better?
| The answer probably is the following. Only when one disconnects the excitation field, the quartz starts to oscillate at exactly the resonance frequency. Disconnecting it completely from external electronics and having a free oscillator is probably the trick. The only connection is with a very delicate probe. We either probe at very high impedance or in short-circuit with very low impedence, either the current or the potential mode. |
Defects. How do you make sure there are no defects in the crystal?
| At times, the crystals have defects. One must probe the frequency and the dissipation of the clean crystal before each experiment and make sure it is consistent with expectations. Often, one can see from this check that the quartz is actually to be thrown away. |
Different components. Can one differentiate between different adsorbed components if one is adsorbing and the other desorbing?
| If you now the frequency to dissipation relation of one and of the other, you could try to work out the two different contributions by coupling the behavior of frequency and dissipation that you measure. |
Sensor Crystal. How large is a typical sensor crystal?
| The crystal is 1 cm diameter, and it is in contact with a solution with a minimal volume of 50 µL. |
Changing the Solvent. If you change the solvent, do you have to consider the parameters of the solvent and where do they come into play in your calculations?
| We tend to keep the solvent constant so the parameters remain the same. Otherwise, you need to change the theoretical analysis of your results. | |
| At an experimental level, one should record different baselines with different solvents at the beginning of the experiment, and then use the different baselines as needed. |
Nano-antenna. Regarding the nano-antenna, how do you bring the protein into gap between two particles? The efficiency might be lower rather than when you bind proteins directly to gold.
| Diffusion is fast at reasonable concentrations, proteins will find their way because distances are relatively small. | |
| Is it suitable for the principle of QCM to attach Au nanoparticles to the surface of the crystal to increase the signal? | |
| You can do this as an enhancement for the signal. |
Washing Protocol. How important is the washing protocol for the formation of layer-by-layer (LBL) films. Is continuous washing in streaming solvent preferable or washing in a beaker?
| Both possible, problems come from the fact that ultrapure water costs money and streaming flow might be expensive. To save money, you can dip it in a solvent; when you take it out there will be a film of water on the surface that is about one millimeter thick. You can calculate the volume of this adhered water and then you want to dilute this volume of a factor of about 1 million. However, the washing is extremely system-dependent and you should always optimize the washing specifically for the targeted properties of your LBL. |
Divalent Ions. Have you ever used divalent salt?
| Of course. However, sometimes when you move up with the valency, polyelectrolytes can precipitate. Magnesium ions work better than calcium ions for some polyelectrolytes, for example with polystyrene sulfonate. Magnesium sulfate will reduce the size of this polyelectrolyte coil and the films will be smaller than at the equivalent ionic strength achieved using monovalent salts. |
Scratch Resistance. In everyday life, conditions change, such as temperature variations, mechanical wear, scratching, etc. Are LBL films stable under such conditions?
| One can make scratch resistant coatings. It is not necessary for them to be very thick. For example, LBL coatings can be on the inner part of a window screen, not necessarily on the outside. As for scratch resistance, you need to engineer quite a bit. For example, some groups use particle coatings for spectacles based on LBL approach, where larger and larger particles are deposited as you move away from the lens. This approach creates a capillary force such that when water is in contact, it is immediately channeled towards the inside and you never get fogging of your spectacles. Additionally, you can add a hydrothermal baking process after the LBL deposition, which maintains the capillarity and the voids, but which connects the particles, thus providing mechanical resistance for such a layer of 100 nm or less. So it can be done, but at the present time scratch resistance remains a weakness. |