Name: controlling the size, shape and stability of supramolecular polymers in water
Uploaded: 2012-08-02T12:00:00
Duration: 16 min 24 s
Description: Watch this Scientific Journal Video about Controlling the Size, Shape and Stability of Supramolecular Polymers in Water at JoVE.com

The authors gratefully acknowledge Marko Nieuwenhuizen for assistance with the DOSY-NMR.

<img alt="Scheme 1" src="/files/ftp_upload/3975/3975scheme1.jpg" /> 
Scheme 1. Self-assembly of BTA-based discotics in citrate buffer into spherical aggregates showing diameters of about 5 nm, at millimolar concentrations of building block. Increasing the ionic strength by the addition of NaCl results in the formation of elongated rods with a diameter of around 3 nm and length &gt; 25 nm. <a href="https://www.jove.com/files/ftp_upload/3975/3975scheme1large.jpg" target="_blank"> Click here to view larger figure</a>.
1. Preparing a BTA-Gd(III)DTPA Solutions for CD Spectroscopy and Measurement of Temperature-dependent CD Spectra as a Function of NaCl Concentration
<ol>
 <li>Prepare a 100 mM citrate buffer (pH 6.0).</li>
 <li>Prepare a 100 mM citrate buffer (pH 6.0) with 2 M NaCl.</li>
 <li>Dissolve 0.254 mg of BTA-Gd(III)DTPA (MW = 3184 g&bull;mol-1) in 10 mL of 100 mM citrate buffer, target concentration 8&bull;10-3 mM of BTA-Gd(III)DTPA.</li>
 <li>Sonicate the solution for 5 minutes.</li>
 <li>Fill a 1 cm UV cuvet with the solution and measure a CD spectrum from 230 to 350 nm and a CD cooling curve at the highest intensity CD band (e.g. &lambda; = 269 nm) from 363 - 283 K at a rate of 1 K min-1.</li>
 <li>Add the same volume of 2 M NaCl buffered solution to the citrate buffered solution of BTA-Gd(III)DTPA in order to increase the ionic strength to 1 M NaCl, diluting the discotics to half the concentration, target concentration 4&bull;10-3 mM of BTA-Gd(III)DTPA.</li>
 <li>Vortex the solution with increased ionic strength for 5 minutes.</li>
 <li>Remeasure a CD spectrum from 230 to 350 nm and a CD cooling curve at the highest intensity CD band from 363 - 283 K at a rate of 1 K min-1.</li>
</ol>
2. Fitting the T-dependent CD Data to a Model for T-dependent Self-assembly
<ol>
 <li>The raw CD data were exported into Origin 8.5 and normalized. This was achieved by defining the CD effect at the highest measured temperature as equal to 0, and the CD effect at lowest measured temperature as equal to 1. Since the magnitude of the CD-effect is proportional to the degree of aggregation8, the normalized CD-curves are proportional to the degree of aggregation.</li>
 <li>The normalized data were fitted using the nonlinear curve fit option in OriginPro 8.5 using a T-dependent self-assembly model derived by van der Schoot8,9. In this model, a nucleation and an elongation regime are distinguished. First the degree of aggregation in the elongation regime (T &lt; Te) was fitted, using the following equation:</li>
</ol>
<img alt="Equation 1" src="/files/ftp_upload/3975/3975eq1.jpg" /> 
Above equation contains (next to the variable temperature, T, and the degree of aggregation, &Phi;n) three parameters; i.e. the enthalpy of elongation he, the elongation temperature Te (the temperature at which the self-assembly starts) and the parameter &Phi;SAT, which is introduced to ensure that &Phi;n/&Phi;SAT does not exceed unity, which follows from the constraint that the degree of aggregation cannot exceed unity.
Fitting renders the enthalpy of elongation he (J/mol) and the elongation temperature Te (K) that characterize the self-assembly of the molecules for a given concentration. When fitting, one restraint should be obeyed which is that only the degree of aggregation at temperatures below Te should be fitted, since equation 2.1 is only valid in the elongation regime.
Next, the experimentally found degree of aggregation in the nucleation regime can be fitted, using the following equation:
<img alt="Equation 2" src="/files/ftp_upload/3975/3975eq2.jpg" /> 
Above equation contains (next to the variables T and &Phi;n) four parameters of which already three were determined with equation 2.1; i.e. the enthalpy of elongation he, the elongation temperature Te and the parameter &Phi;SAT. The only unknown parameter is the Ka value -describing the cooperativity of the nucleation phase- which is found by fitting the experimentally found degree of aggregation for temperatures above Te.
3. Preparing BTA-Gd(III)DTPA Solutions for Transmission Electron Microscopy and Visualization of Supramolecular Polymers via Cryogenic TEM
<ol>
 <li>Prepare two buffers: a 100 mM citrate buffer (pH 6.0) and a 100 mM citrate buffer (pH 6.0) with 5 M NaCl.</li>
 <li>Dissolve 0.318 mg of BTA-Gd(III)DTPA (MW = 3184 g&bull;mol-1) in 0.1 mL of each of the prepared buffers, target concentration 1 mM of BTA-Gd(III)DTPA.</li>
 <li>The sample vitrification for cryogenic TEM is carried out using an automated vitrification robot (FEI Vitrobot Mark III). CryoTEM grids (R2/2 Quantifoil Jena grids from Quantifoil Micro Tools GmbH) are surface plasma treated prior to the vitrification procedure using a Cressington 208 carbon coater operating at 5 mA for 40 s. The aqueous solution is then applied on the grid during vitrification on an automated FEI Vitrobot. This involves the application of the sample on the grid, blotting of excess liquid to create a thin film of the aqueous solution on the grid and subsequent vitrification by dipping the grid very quickly in liquid ethane. After vitrification the sample is kept in liquid nitrogen and transferred manually on the autoloader cassette, also cooled with liquid nitrogen. The cassette is then inserted into the autoloader of the TEM. All this is done manually.</li>
 <li>The cryoTEM experiments are performed on the TU/e cryoTITAN (FEI), (<a href="http://www.cryotem.nl" target="_blank">www.cryotem.nl</a>). The TU/e cryoTITAN is equipped with a field emission gun (FEG) operating at 300 kV. Images were recorded using a 2k x 2k Gatan CCD camera equipped with a post column Gatan Energy Filter (GIF).</li>
</ol>
4. 1H-DOSY NMR Measurements of Spherical Self-assembled BTA-Gd(III)DTPA at Low Ionic Strength
<ol>
 <li>Prepare a 50 mM d6-succinate buffer in D2O ('pH 6.0'); the buffer is prepared by dissolving d6-succinic acid in D2O, followed by adjusting the pH to 6.0 using 1 M ND4OD in D2O. The final concentration of 50 mM succinate was adjusted with additional D2O.</li>
 <li>Since Gd(III) is highly paramagnetic and 1H signals would thereby be broadened significantly, Gd(III) was substituted for Y(III).</li>
 <li>Dissolve 2.98 mg of BTA-Y(III)DTPA (MW = 2979 g&bull;mol-1) in 1 mL of a 50 mM d6-succinate buffer in D2O ('pH 6.0'), target concentration 1 mM BTA-Y(III)DTPA.</li>
 <li>The 1H-DOSY NMR measurements are carried out on a Varian Unity Inova 500 spectrometer equipped with a 5 mm ID-PFG probe from Varian. The DOSY experiments were carried out using the DOSY one-shot (Doneshot, Varian) pulse sequence. The 90 degree pulse und mixing times were adapted accordingly. Chemical shifts were referenced using the chemical shift of the 3-(trimethylsilyl)propionic-2,2,3,3-[D4] acid sodium salt (TMSP).</li>
 <li>The self-diffusion of HDO was used to calibrate the measurements; it is known from literature that the self-diffusion of HDO in D2O at 298 K is 19.0 &times; 10-9 m2&bull;s-1. As a reference, the self-diffusion of HDO in D2O was measured in a VARIAN 2 Hz D2O standard sample and calibrated to its standard value. The model used to calculate the hydrodynamic radii RH of the aggregates is the Stokes-Einstein relation for the diffusion of a spherical particle.</li>
</ol>
5. Representative Results
1H-DOSY NMR and SAXS measurements on BTA-M(III)-DTPA: spherical objects in citrate buffer
The ionic character of the peripheral Gd(III) complexes introduces frustration in the one-dimensional growth of the discotic monomers whose core is designed to polymerize into elongated rod-like aggregates. The balance between attractive and repulsive interactions controls the size and the shape of the aggregates (Scheme 2).
<img alt="Scheme 2" src="/files/ftp_upload/3975/3975scheme2.jpg" /> 
Scheme 2.
A powerful technique to determine the size and the shape of particles in solution is synchrotron source small angle X-ray scattering (SAXS). BTA-Gd(III)-DTPA was dissolved in a citrate buffer solution and the SAXS profiles were recorded and fitted in the region 0.01 &lt; q &lt; 0.1 &Aring;-1. A slope approaching zero in the low-q region (q &lt; 0.06 &Aring;-1) indicates a lack of shape anisotropy in the aggregate, suggesting the presence of spherical objects (Figure 1). The data measured at different concentrations were fitted using a homogeneous monodisperse spherical form factor leading to a calculated radius, R, of 3.2 nm. The calculated geometric radius of monomeric discotic BTA-Gd(III)-DTPA is 3.0 nm, which suggests the presence of aggregates with an aspect ratio close to 1.
<img alt="Figure 1" src="/files/ftp_upload/3975/3975fig1.jpg" /> 
Figure 1. SAXS profiles for BTA-Gd(III)-DTPA in citrate buffer (100 mM, pH 6) at 0.5 and 1.0 mM (top). DOSY NMR of BTA-Y(III)-DTPA in 50 mM d6-succinate buffer at 1.0 mM (bottom). <a href="https://www.jove.com/files/ftp_upload/3975/3975fig1large.jpg" target="_blank"> Click here to view larger figure</a>.
In order to provide further evidence for the spherical shape and nanometer-size of the self-assembled objects, we performed 1H diffusion-ordered NMR spectroscopy (1H-DOSY NMR) (Figure 1). DOSY-NMR allows determination of the diffusion coefficients of aggregates, from which the hydrodynamic radius (RH) can be calculated. Since Gd(III) is highly paramagnetic and 1H signals would thereby be broadened significantly, we changed Gd(III) for diamagnetic Y(III). The diffusion coefficients of the aggregated diamagnetic discotic amphiphile in a deuterated succinate buffer (50 mM, pH 6, c = 1 mM) was determined to be 0.69x10-10 m2s-1. Via the Stokes-Einstein relation, we calculate a hydrodynamic radius RH of 2.9 nm for the discrete objects of spherical size (Table 1). This size is in excellent agreement with value obtained from SAXS data for BTA-Gd(III)-DTPA.
<table border="1">
 <tbody>
 <tr>
 <td>BTA-M(III)-DTPA 
 [mM]</td>
 <td>Dt a 
 [10-10 m2s-1]</td>
 <td>RH a 
 [nm]</td>
 <td>R b 
 [nm]</td>
 </tr>
 <tr>
 <td>1</td>
 <td>0.69</td>
 <td>2.9</td>
 <td>3.2</td>
 </tr>
 </tbody>
</table>
a from DOSY; b from SAXS
Table 1. Results of SAXS and DOSY measurements for BTA-M(III)-DTPA.
Cryo-TEM on BTA-Gd(III)-DTPA: from spherical objects to elongated nanorods
Further evidence for successful control over one-dimensional stack length was obtained from cryo-TEM micrographs. Due to the vitrification of the aqueous solutions cryogenic TEM preserves the structural morphology of the self-assembled aggregates and avoids drying affects related to conventional TEM sample preparation. Figure 2 (left) shows that BTA-Gd(III)-DTPA produces the expected spherical objects with diameters close to 6 nm at a 1 mM concentration, which confirms the results from SAXS and DOSY measurements. According to these findings, we have been able to obtain self-assembled discrete objects that can be considered the supramolecular equivalent of dendritic macromolecules10.
<img alt="Figure 2" src="/files/ftp_upload/3975/3975fig2.jpg" /> 
Figure 2. Cryo-TEM images for BTA-Gd(III)-DTPA (left) 1 mM vitrified at 298 K in citrate buffer (100 mM, pH 6), scale bar represents 50 nm; (right) 1 mM vitrified at 298 K in citrate buffer (100 mM, pH 6) and an overall NaCl concentration of 5 M, scale bar represents 50 nm.
So far we have only worked in buffered solutions of low ionic strength. However, if electrostatic repulsive forces of the peripheral charged M(III)-DTPA complexes on BTA-Gd(III)-DTPA are at the origin of the frustrated one-dimensional growth, we expected that increasing the ionic strength of the buffered environment, using an inert 1:1 salt with highly hydrated counterions, should reduce the electrostatic interactions and hence a different type of self-assembled object should be formed. In citrate buffer comprising 5 M NaCl this effect was indeed observed (Figure 2, right). The formation of high aspect ratio rod-like supramolecular polymers is clearly observed in cryo-TEM micrographs at high ionic strength. Electrostatic screening is the most likely explanation for this finding. The shape changes from a spherical aggregate of around 6 nm in diameter to elongated rods with a diameter of 6 nm and length of up to several hundred nanometers.
CD measurements of BTA-Gd(III)-DTPA: switching on cooperative self-assembly by increasing the ionic strength
Circular dichroism (CD) spectroscopy measures the difference in absorption between left-handed and right-handed circularly polarized light. When a helical object has a preferred helical sense, left- and right-handed circularly polarized light will be absorbed to different extents, hence giving rise to a CD-effect. Since the intermolecular hydrogen bonds formed between consecutive BTA-Gd(III)-DTPA within the aggregates, are lined up in a helical fashion and the stereogenic centre at the L-phenylalanine moiety favors one helical sense over the other, we expect a clear CD spectrum from BTA-Gd(III)-DTPA based aggregates11,12. In addition, temperature-dependent CD spectroscopy is a powerful tool to assess the self-assembly mechanism of BTA-Gd(III)-DTPA polymerisation and allows to derive conclusions on the stability of the formed aggregates13.
As an example, the room temperature CD spectra of BTA-Gd(III)-DTPA (8x10-3 mM or 4x10-3 mM in a 100 mM citrate buffer) with increasing salt concentration (0 M NaCl to 1.0 M NaCl) are given in Figure 3A. Although a significantly lower concentration is applied for the CD measurements, the clear Cotton effect indicates the presence of intact aggregates, even at micromolar concentrations. The shape of the CD spectrum changes upon increasing the salt concentration, which is a good indication for reduced interactions at the periphery of the stacks and better packing of the discotics. In addition, the CD cooling curves of the same solutions (363 - 283 K, measured at &lambda; = 269 or 278 nm) show distinct differences in shape (Figure 3B). The apparent Te -the temperature at which aggregation starts- shifts to higher temperatures at higher salt concentration and an increasingly cooperative mechanism, characterized by a more abrupt increase in the CD-effect, becomes apparent. Whereas the cooling curve at 0 M NaCl is best described by an isodesmic self-assembly process, the cooling curve at 1.0 M NaCl is typical for a cooperative self-assembly process 14. In the former case, all association constants are assumed to be equal while in the latter case self-assembly occurs in at least two distinct stages. In the first step, a "nucleus" needs to be formed which is energetically highly unfavorable. After cooling below a critical polymerization temperature, elongation and exponential growth into supramolecular polymers of high molecular weight follows. Quantifying the thermodynamic parameters of the self-assembly of BTA-Gd(III)-DTPA at 0 and 1 M NaCl using a cooperative model clearly reveals the decrease in Ka, which is the dimensionless activation constant8. Lower values for Ka indicate a higher degree of cooperativity in the self-assembly process, which is expressed in the formation of highly elongated supramolecular polymers as observed in cryo-TEM.
<table border="1">
 <tbody>
 <tr>
 <td>BTA-Gd(III)-DTPA</td>
 <td>CNaCl</td>
 <td>Ka</td>
 </tr>
 <tr>
 <td>8 x10-3 mM</td>
 <td>0 M</td>
 <td>5 10-2</td>
 </tr>
 <tr>
 <td>4 x10-3 mM</td>
 <td>1 M</td>
 <td>1 10-4</td>
 </tr>
 </tbody>
</table>
Table 2. Degree of cooperativity expressed by Ka in the temperature-dependent self-assembly of BTA-Gd(III)-DTPA as a function of the NaCl concentration (CNaCl).
<img alt="Figure 3" src="/files/ftp_upload/3975/3975fig3.jpg" /> 
Figure 3. BTA-Gd(III)-DTPA in a 100 mM citrate buffer (c = 8 x10-3 mM at low ionic strength and 4 x10-3 mM at high ionic strength) A] CD spectra recorded at 293 K as a function of the ionic strength, cNaCl = 0 M - 1.0 M, the molar ellipticity &Delta;&#949; is calculated as follows: &Delta;&#949; = CD-effect/(cxl) in which c is the concentration of BTA in mol L-1 and l is the optical path length in cm; B] Corresponding CD cooling curves measured at &lambda; = 269 nm for 0 M NaCl and 278 nm for 1 M NaCl solutions expressed as the degree of aggregation &Phi;n as a function of the NaCl concentration cNaCl = 0 M - 1.0 M, &Phi;n is calculated by dividing the measured CD-effect by the maximal CD-effect.

<ol>
	<li>Palmer, L. C., Stupp, S. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+self-assembly+into+one-dimensional+nanostructures.">Molecular self-assembly into one-dimensional nanostructures.</a> Acc. Chem. Res. 41, 1674-1684 (2008).</li><li>Janssen, P. G. A., Vandenbergh, J., van Dongen, J. L. J., Meijer, E. W., Schenning, A. P. H. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=ssDNA+templated+self-assembly+of+chromophores.">ssDNA templated self-assembly of chromophores.</a> J. Am. Chem. Soc. 129, 6078-6079 (2007).</li><li>Bull, S. R., Palmer, L. C., Fry, N. J., Greenfield, M. A., Messmore, B. W., Meade, T. J., Stupp, S. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+templating+approach+for+monodisperse+self-assembled+organic+nanostructures.">A templating approach for monodisperse self-assembled organic nanostructures.</a> J. Am. Chem. Soc. 130, 2742-2743 (2008).</li><li>Lortie, F., Boileau, S., Bouteiller, L., Chassenieux, C., Laupr&#234;tre, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chain+stopper-assisted+characterization+of+supramolecular+polymers.">Chain stopper-assisted characterization of supramolecular polymers.</a> Macromolecules. 38, 5283-5287 (2005).</li><li>Wang, X., Guerin, G., Wang, H., Wang, Y., Manners, I., Winnik, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cylindrical+block+copolymer+micelles+and+co-micelles+of+controlled+length+and+architecture.">Cylindrical block copolymer micelles and co-micelles of controlled length and architecture.</a> Science. 317, 644-647 (2007).</li><li>Besenius, P., Portale, G., Bomans, P. H. H., Janssen, H. M., Palmans, A. R. A., Meijer, E. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Controlling+the+growth+and+shape+of+chiral+supramolecular+polymers+in+water.">Controlling the growth and shape of chiral supramolecular polymers in water.</a> Proc. Natl. Acad. Sci. U.S.A. 107, 17888-17893 (2010).</li><li>Besenius, P., van den Hout, K. P., Albers, H. M. H. G., de Greef, T. F. A., Olijve, L. L. C., Hermans, T. M., de Waal, B. F. M., Bomans, P. H. H., Sommerdijk, N. A. J. M., Portale, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Controlled+supramolecular+oligomerisation+of+C3-symmetrical+molecules+in+water:+the+impact+of+hydrophobic+shielding.">Controlled supramolecular oligomerisation of C3-symmetrical molecules in water: the impact of hydrophobic shielding.</a> Chem. Eur. J. 17, 5193-5203 (2011).</li><li>Smulders, M. M. J., Schenning, A. P. H. J., Meijer, E. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Insights+into+the+mechanisms+of+cooperative+self-assembly:+the+sergeants-and-soldiers+principle+of+chiral+and+achiral+C3-symmetrical+discotic+triamides.">Insights into the mechanisms of cooperative self-assembly: the sergeants-and-soldiers principle of chiral and achiral C3-symmetrical discotic triamides.</a> J. Am. Chem. Soc. 130, 606-611 (2008).</li><li>Jonkheijm, P., van der Schoot, P., Schenning, A. P. H. J., Meijer, E. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Probing+the+solvent-assisted+nucleation+pathway+in+chemical+self-assembly.">Probing the solvent-assisted nucleation pathway in chemical self-assembly.</a> Science. 313, 80-83 (2006).</li><li>Bosman, A. W., Janssen, H. M., Meijer, E. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=About+dendrimers:+structure,+physical+properties,+and+applications.">About dendrimers: structure, physical properties, and applications.</a> Chem. Rev. 99, 1665-1688 (1999).</li><li>Veld, M. A. J., Haveman, D., Palmans, A. R. A., Meijer, E. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sterically+demanding+benzene-1,3,5-tricarboxamides:+tuning+the+mechanisms+of+supramolecular+polymerization+and+chiral+amplification.">Sterically demanding benzene-1,3,5-tricarboxamides: tuning the mechanisms of supramolecular polymerization and chiral amplification.</a> Soft Matter. 7, 524-531 (2011).</li><li>Stals, P. J. M., Smulders, M. M. J., Mart&#237;n-Rap&#237;n, R., Palmans, A. R. A., Meijer, E. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Asymmetrically+substituted+benzene-1,3,5-tricarboxamides:+self-assembly+and+odd-even+effects+in+the+solid+state+and+in+dilute+solution.">Asymmetrically substituted benzene-1,3,5-tricarboxamides: self-assembly and odd-even effects in the solid state and in dilute solution.</a> Chem. Eur. J. 15, 2071-2080 (2009).</li><li>De Greef, T. F. A., Smulders, M. M. J., Wolffs, M., Schenning, A. P. H. J., Sijbesma, R. P., Meijer, E. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Supramolecular+polymerization.">Supramolecular polymerization.</a> Chem. Rev. 109, 5687-5754 (2009).</li><li>Smulders, M. M. J., Nieuwenhuizen, M. M. L., de Greef, T. F. A., Schoot, P. v. a. n. d. e. r., Schenning, A. P. H. J., Meijer, E. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=How+to+distinguish+isodesmic+from+cooperative+supramolecular+polymerization?+Chem.">How to distinguish isodesmic from cooperative supramolecular polymerization? Chem.</a> Eur. J. 16, 362-367 (2010).</li><li>Besenius, P., Heynens, J. L. M., Straathof, R., Nieuwenhuizen, M. M. L., Bomans, P. H. H., Terreno, E., Aime, S., Strijkers, G. J., Nicolay, K., Meijer, E. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Paramagnetic+self-assembled+nanoparticles+as+supramolecular+MRI+contrast+agents.">Paramagnetic self-assembled nanoparticles as supramolecular MRI contrast agents.</a> Contrast Media Mol. Imaging. , (2012).</li></ol>

No conflicts of interest declared.

The self-assembling discotic amphiphiles discussed in this contribution containing the Gd(III)-DTPA complex are currently under investigation as novel magnetic resonance imaging (MRI) agents that combine high contrast with tunable excretion times.15 Hence, the details of their self-assembling behavior and their stability in different conditions are of critical importance. The combination of spectroscopic (CD and NMR), scattering (SAXS) and microscopy (cryo-TEM) techniques allows visualization of the formed structures and the quantification of their thermodynamic parameters. This combination of techniques is generally applicable for self-assembling molecules as long as a preferential helical sense in the studied system allows a difference in the absorption of left- and right-handed circularly polarized light.

<table border="1">
<tbody>
 <tr>
 <td>Name of the reagent/equipment</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>BTA-Gd(III)-DTPA</td>
 <td>&nbsp;</td>
 <td>&nbsp;</td>
 <td>Made in-house</td>
 </tr>
 <tr>
 <td>BTA-Y(III)-DTPA</td>
 <td>&nbsp;</td>
 <td>&nbsp;</td>
 <td>Made in-house</td>
 </tr>
 <tr>
 <td>CD spectroscopy</td>
 <td>Jasco</td>
 <td>Jasco J-815 spectropolarimeter</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>NMR</td>
 <td>Varian</td>
 <td>Varian Unity Inova 			500 spectrometer</td>
 <td>5-mm ID-PFG probe of Varian</td>
 </tr>
 <tr>
 <td>cryo-TEM</td>
 <td>FEI</td>
 <td>cryoTITAN TEM</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>SAXS</td>
 <td>Dutch-Belgian beamline (BM26B) at 			the European 			Synchotron Radiation Facility (ESRF) in 			Grenoble, France</td>
 <td>&nbsp;</td>
 <td>&nbsp;</td>
 </tr>
 </tbody>
</table>

controlling the size, shape and stability of supramolecular polymers in water

For aqueous based supramolecular polymers, the simultaneous control over shape, size and stability is very difficult1. At the same time, the ability to do so is highly important in view of a number of applications in functional soft matter including electronics, biomedical engineering, and sensors. In the past, successful strategies to control the size and shape of supramolecular polymers typically focused on the use of templates2,3, end cappers4 or selective solvent techniques5. 
Here we disclose a strategy based on self-assembling discotic amphiphiles that leads to the control over stack length and shape of ordered, chiral columnar aggregates. By balancing electrostatic repulsive interactions on the hydrophilic rim and attractive non-covalent forces within the hydrophobic core of the polymerizing building block, we manage to create small and discrete spherical objects6,7. Increasing the salt concentration to screen the charges induces a sphere-to-rod transition. Intriguingly, this transition is expressed in an increase of cooperativity in the temperature-dependent self-assembly mechanism, and more stable aggregates are obtained. 
For our study we select a benzene-1,3,5-tricarboxamide (BTA) core connected to a hydrophilic metal chelate via a hydrophobic, fluorinated L-phenylalanine based spacer (Scheme 1). The metal chelate selected is a Gd(III)-DTPA complex that contains two overall remaining charges per complex and necessarily two counter ions. The one-dimensional growth of the aggregate is directed by &pi;-&pi; stacking and intermolecular hydrogen bonding. However, the electrostatic, repulsive forces that arise from the charges on the Gd(III)-DTPA complex start limiting the one-dimensional growth of the BTA-based discotic once a certain size is reached. At millimolar concentrations the formed aggregate has a spherical shape and a diameter of around 5 nm as inferred from 1H-NMR spectroscopy, small angle X-ray scattering, and cryogenic transmission electron microscopy (cryo-TEM). The strength of the electrostatic repulsive interactions between molecules can be reduced by increasing the salt concentration of the buffered solutions. This screening of the charges induces a transition from spherical aggregates into elongated rods with a length &gt; 25 nm. Cryo-TEM allows to visualise the changes in shape and size. In addition, CD spectroscopy permits to derive the mechanistic details of the self-assembly processes before and after the addition of salt. Importantly, the cooperativity -a key feature that dictates the physical properties of the produced supramolecular polymers- increases dramatically upon screening the electrostatic interactions. This increase in cooperativity results in a significant increase in the molecular weight of the formed supramolecular polymers in water.

Watch this Scientific Journal Video about Controlling the Size, Shape and Stability of Supramolecular Polymers in Water at JoVE.com

Controlling the Size, Shape and Stability of Supramolecular Polymers in Water

The goal of this experiment is to determine and control the size, shape and stability of self-assembled discotic amphiphiles in water. For aqueous based supramolecular polymers such level of control is very difficult. We apply a strategy using both repulsive and attractive interactions. The experimental techniques applied to characterize this system are broadly applicable.

For aqueous based supramolecular polymers, the simultaneous control over shape, size and stability is very difficult1. At the same time, the ability to do ...

controlling-size-shape-stability-supramolecular-polymers

Research

JoVE Journal

Engineering

Fine-tuning the Size and Minimizing the Noise of Solid-state Nanopores

Solid-state nanopores have emerged as a versatile tool for the characterization of single biomolecules such as nucleic acids and proteins1. However, the creation of a nanopore in a thin insulating membrane remains challenging. Fabrication methods involving specialized focused electron beam systems can produce well-defined nanopores, but yield of reliable and low-noise nanopores in commercially available membranes remains low2,3 and size control is nontrivial4,5. Here, the application of high electric fields to fine-tune the size of the nanopore while ensuring optimal low-noise performance is demonstrated. These short pulses of high electric field are used to produce a pristine electrical signal and allow for enlarging of nanopores with subnanometer precision upon prolonged exposure. This method is performed in situ in an aqueous environment using standard laboratory equipment, improving the yield and reproducibility of solid-state nanopore fabrication.

A methodology for preparing solid-state nanopores in solution for biomolecular translocation experiments is presented. By applying short pulses of high electric fields, the nanopore diameter can be controllably enlarged with subnanometer precision and its electrical noise characteristics significantly improved. This procedure is performed in situ using standard laboratory equipment under experimental conditions.

Solid-state nanopores have emerged as a versatile tool for the characterization of single biomolecules such as nucleic acids and proteins1. However, the ...

The Preparation of Electrohydrodynamic Bridges from Polar Dielectric Liquids

Horizontal and vertical liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields (8-20 kV/cm) and polar dielectric liquids. These bridges are unique from capillary bridges in that they exhibit extensibility beyond a few millimeters, have complex bi-directional mass transfer patterns, and emit non-Planck infrared radiation. A number of common solvents can form such bridges as well as low conductivity solutions and colloidal suspensions. The macroscopic behavior is governed by electrohydrodynamics and provides a means of studying fluid flow phenomena without the presence of rigid walls. Prior to the onset of a liquid bridge several important phenomena can be observed including advancing meniscus height (electrowetting), bulk fluid circulation (the Sumoto effect), and the ejection of charged droplets (electrospray). The interaction between surface, polarization, and displacement forces can be directly examined by varying applied voltage and bridge length. The electric field, assisted by gravity, stabilizes the liquid bridge against Rayleigh-Plateau instabilities. Construction of basic apparatus for both vertical and horizontal orientation along with operational examples, including thermographic images, for three liquids (e.g., water, DMSO, and glycerol) is presented.

Horizontal and vertical electrohydrodynamic liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields and polar dielectric liquids. The construction of basic apparatus and operational examples, including thermographic images, for three liquids (e.g., water, DMSO, and glycerol) is presented.

Horizontal and vertical liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields (8-20 kV/cm) and ...

Experimental Methods for Investigation of Shape Memory Based Elastocaloric Cooling Processes and Model Validation

Shape Memory Alloys (SMA) using elastocaloric cooling processes have the potential to be an environmentally friendly alternative to the conventional vapor compression based cooling process. Nickel-Titanium (Ni-Ti) based alloy systems, especially, show large elastocaloric effects. Furthermore, exhibit large latent heats which is a necessary material property for the development of an efficient solid-state based cooling process. A scientific test rig has been designed to investigate these processes and the elastocaloric effects in SMAs. The realized test rig enables independent control of an SMA&#39;s mechanical loading and unloading cycles, as well as conductive heat transfer between SMA cooling elements and a heat source/sink. The test rig is equipped with a comprehensive monitoring system capable of synchronized measurements of mechanical and thermal parameters. In addition to determining the process-dependent mechanical work, the system also enables measurement of thermal caloric aspects of the elastocaloric cooling effect through use of a high-performance infrared camera. This combination is of particular interest, because it allows illustrations of localization and rate effects &#8212; both important for efficient heat transfer from the medium to be cooled.
The work presented describes an experimental method to identify elastocaloric material properties in different materials and sample geometries. Furthermore, the test rig is used to investigate different cooling process variations. The introduced analysis methods enable a differentiated consideration of material, process and related boundary condition influences on the process efficiency. The comparison of the experimental data with the simulation results (of a thermomechanically coupled finite element model) allows for better understanding of the underlying physics of the elastocaloric effect. In addition, the experimental results, as well as the findings based on the simulation results, are used to improve the material properties.

Experimental methods for investigation of solid state cooling processes and characterization of elastocaloric material properties of Shape Memory Alloys (SMA) are presented. A custom-built test rig has been designed for controlling and comprehensive monitoring of elastocaloric cooling processes. Furthermore, it provides a validation platform for thermomechanically coupled modeling approaches.

Shape Memory Alloys (SMA) using elastocaloric cooling processes have the potential to be an environmentally friendly alternative to the conventional vapor ...

Capillary-based Centrifugal Microfluidic Device for Size-controllable Formation of Monodisperse Microdroplets

Here, we demonstrate a simple method for the rapid production of size-controllable, monodisperse, W/O microdroplets using a capillary-based centrifugal microfluidic device. W/O microdroplets have recently been used in powerful methods that enable miniaturized chemical experiments. Therefore, developing a versatile method to yield monodisperse W/O microdroplets is needed. We have developed a method for generating monodisperse W/O microdroplets based on a capillary-based centrifugal axisymmetric co-flowing microfluidic device. We succeeded in controlling the size of microdroplets by adjusting the capillary orifice. Our method requires equipment that is easier-to-use than with other microfluidic techniques, requires only a small volume (0.1-1 &#181;l) of sample solution for encapsulation, and enables the production of hundreds of thousands number of W/O microdroplets per second. We expect this method will assist biological studies that require precious biological samples by conserving the volume of the samples for rapid quantitative analysis biochemical and biological studies.

Here, we demonstrate a simple production method for size-controllable, monodisperse, water-in-oil (W/O) microdroplets using a capillary-based centrifugal microfluidic device. This method requires only a small sample volume and enables high-yield production. We expect this method will be useful for rapid biochemical and cellular analyses.

Here, we demonstrate a simple method for the rapid production of size-controllable, monodisperse, W/O microdroplets using a capillary-based centrifugal ...

Ice Generation and the Heat and Mass Transfer Phenomena of Introducing Water to a Cold Bath of Brine

We demonstrate a method for the study of the heat and mass transfer and of the freezing phenomena in a subcooled brine environment. Our experiment showed that, under the proper conditions, ice can be produced when water is introduced to a bath of cold brine. To make ice form, in addition to having the brine and water mix, the rate of heat transfer must bypass that of mass transfer. When water is introduced in the form of tiny droplets to the brine surface, the mode of heat and mass transfer is by diffusion. The buoyancy stops water from mixing with the brine underneath, but as the ice grows thicker, it slows down the rate of heat transfer, making ice more difficult to grow as a result. When water is introduced inside the brine in the form of a flow, a number of factors are found to influence how much ice can form. Brine temperature and concentration, which are the driving forces of heat and mass transfer, respectively, can affect the water-to-ice conversion ratio; lower bath temperatures and brine concentrations encourage more ice to form. The flow rheology, which can directly affect both the heat and mass transfer coefficients, is also a key factor. In addition, the flow rheology changes the area of contact of the flow with the bulk fluid.

Here, we present a protocol to demonstrate the generation of ice when water is introduced to a cold bath of brine, as a secondary refrigerant, at a range of temperatures well below the freezing point of water. It can be used as an alternative way of producing ice for industry.

We demonstrate a method for the study of the heat and mass transfer and of the freezing phenomena in a subcooled brine environment. Our experiment showed ...

Detecting the Water-soluble Chloride Distribution of Cement Paste in a High-precision Way

To improve the accuracy of the chloride distribution along the depth of cement paste under cyclic wet-dry conditions, a new method is proposed to obtain a high-precision chloride profile. Firstly, paste specimens are molded, cured, and exposed to cyclic wet-dry conditions. Then, powder samples at different specimen depths are grinded when the exposure age is reached. Finally, the water-soluble chloride content is detected using a silver nitrate titration method, and chloride profiles are plotted. The key to improving the accuracy of the chloride distribution along the depth is to exclude the error in the powderization, which is the most critical step for testing the distribution of chloride. Based on the above concept, the grinding method in this protocol can be used to grind powder samples automatically layer by layer from the surface inward, and it should be noted that a very thin grinding thickness (less than 0.5 mm) with a minimum error less than 0.04 mm can be obtained. The chloride profile obtained by this method better reflects the chloride distribution in specimens, which helps researchers to capture the distribution features that are often overlooked. Furthermore, this method can be applied to studies in the field of cement-based materials, which require high chloride distribution accuracy.

A protocol for obtaining a water-soluble chloride profile by using a high precision milling method is presented.

To improve the accuracy of the chloride distribution along the depth of cement paste under cyclic wet-dry conditions, a new method is proposed to obtain a ...

Generation of Size-controlled Poly (ethylene Glycol) Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

Uniform and size-controllable poly (ethylene glycol) diacrylate (PEGDA) droplets could be produced via the flow focusing process in a microfluidic device. This paper proposes a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation. The polydimethylsiloxane (PDMS) chip was fabricated using the multi-layer soft lithography method. Hexadecane containing surfactant was used as the continuous phase, and PEGDA with the ultraviolet (UV) photo-initiator was the dispersed phase. Surfactants allowed the local surface tension to drop and formed a more cusped tip which promoted breaking into tiny micro-droplets. As the pressure of dispersed phase was constant, the size of droplets became smaller with increasing continuous phase pressure before dispersed phase flow was broken off. As a result, droplets with size variation from 1 &#181;m to 80 &#181;m in diameter could be selectively achieved by changing the pressure ratio in two inlet channels, and the average coefficient of variation was estimated to be below 7%. Furthermore, droplets could turn into micro-beads by UV exposure for photo-polymerization. Conjugating biomolecules on such micro-beads surface have many potential applications in the fields of biology and chemistry.

Generation of Size-controlled Poly ethylene Glycol Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

Here, we present a protocol to illustrate the fabrication processes and verifying experiments of a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation.

Uniform and size-controllable poly (ethylene glycol) diacrylate (PEGDA) droplets could be produced via the flow focusing process in a microfluidic device. ...

Near-Infrared Temperature Measurement Technique for Water Surrounding an Induction-heated Small Magnetic Sphere

A technique to measure the temperature of water and non-turbid aqueous media surrounding an induction-heated small magnetic sphere is presented. This technique utilizes wavelengths of 1150 and 1412 nm, at which the absorption coefficient of water is dependent on temperature. Water or a non-turbid aqueous gel containing a 2.0-mm- or 0.5-mm-diameter magnetic sphere is irradiated with 1150 nm or 1412 nm incident light, as selected using a narrow bandpass filter; additionally, two-dimensional absorbance images, which are the transverse projections of the absorption coefficient, are acquired via a near-infrared camera. When the three-dimensional distributions of temperature can be assumed to be spherically symmetric, they are estimated by applying inverse Abel transforms to the absorbance profiles. The temperatures were observed to consistently change according to time and the induction heating power.

A technique utilizing wavelengths of 1150 and 1412 nm to measure the temperature of water surrounding an induction-heated small magnetic sphere is presented.

A technique to measure the temperature of water and non-turbid aqueous media surrounding an induction-heated small magnetic sphere is presented. This ...

Fabricating Reactive Surfaces with Brush-like and Crosslinked Films of Azlactone-Functionalized Block Co-Polymers

In this paper, fabrication methods that generate novel surfaces using the azlactone-based block co-polymer, poly (glycidyl methacrylate)-block-poly (vinyl dimethyl azlactone) (PGMA-b-PVDMA), are presented. Due to the high reactivity of azlactone groups towards amine, thiol, and hydroxyl groups, PGMA-b-PVDMA surfaces can be modified with secondary molecules to create chemically or biologically functionalized interfaces for a variety of applications. Previous reports of patterned PGMA-b-PVDMA interfaces have used traditional top-down patterning techniques that generate non-uniform films and poorly controlled background chemistries. Here, we describe customized patterning techniques that enable precise deposition of highly uniform PGMA-b-PVDMA films in backgrounds that are chemically inert or that have biomolecule-repellent properties. Importantly, these methods are designed to deposit PGMA-b-PVDMA films in a manner that completely preserves azlactone functionality through each processing step. Patterned films show well-controlled thicknesses that correspond to polymer brushes (~90 nm) or to highly crosslinked structures (~1-10 &#956;m). Brush patterns are generated using either the parylene lift-off or interface directed assembly methods described and are useful for precise modulation of overall chemical surface reactivity by adjusting either the PGMA-b-PVDMA pattern density or the length of the VDMA block. In contrast, the thick, crosslinked PGMA-b-PVDMA patterns are obtained using a customized micro-contact printing technique and offer the benefit of higher loading or capture of secondary material due to higher surface area to volume ratios. Detailed experimental steps, critical film characterizations, and trouble-shooting guides for each fabrication method are discussed.

Surface fabrication methods for patterned deposition of nanometer thick brushes or micron thick, crosslinked films of an azlactone block co-polymer are reported. Critical experimental steps, representative results, and limitations of each method are discussed. These methods are useful for creating functional interfaces with tailored physical features and tunable surface reactivity.

In this paper, fabrication methods that generate novel surfaces using the azlactone-based block co-polymer, poly (glycidyl methacrylate)-block-poly (vinyl ...

Design and Fabrication of an Optical Fiber Made of Water

In this report, an optical fiber of which the core is made solely of water, while the cladding is air, is designed and manufactured. In contrast with solid-cladding devices, capillary oscillations are not restricted, allowing the fiber walls to move and vibrate. The fiber is constructed by a high direct current (DC) voltage of several thousand volts (kV) between two water reservoirs that creates a floating water thread, known as a water bridge. Through the choice of micropipettes, it is possible to control the maximal diameter and length of the fiber. Optical fiber couplers, at both sides of the bridge, activate it as an optical waveguide, allowing researchers to monitor the water fiber capillary body waves through transmission modulation and, therefore, deducing changes in surface tension.
Co-confining two important wave types, capillary and electromagnetic, opens a new path of research in the interactions between light and liquid-wall devices. Water-walled microdevices are a million times softer than their solid counterparts, accordingly improving the response to minute forces.

This protocol describes the design and manufacture of a water bridge and its activation as a water fiber. The experiment demonstrates that capillary resonances of the water fiber modulate its optical transmission.

In this report, an optical fiber of which the core is made solely of water, while the cladding is air, is designed and manufactured. In contrast with ...