Probing the internal heterogeneity of responsive microgels adsorbed

Subscriber access provided by UNIV OF SCIENCES PHILADELPHIA

Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Probing the internal heterogeneity of responsive microgels adsorbed to an interface by a sharp SFM tip – comparing core-shell and hollow microgels Marie Friederike Schulte, Andrea Scotti, Arjan P. H. Gelissen, Walter Richtering, and Ahmed Mourran Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03811 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Probing the internal heterogeneity of responsive microgels adsorbed to an interface by a sharp SFM tip – comparing core-shell and hollow microgels M. Friederike Schulte†§, Andrea Scotti†, Arjan P. H. Gelissen†, Walter Richtering*†§, Ahmed Mourran*§ †

Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany, European

Union §

DWI – Leibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52056 Aachen, Germany

Abstract Microgels composed of thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM) are interfacial active. Their adsorption leads to deformation causing conformational changes that have profound effects on the macroscopic properties of these films. Yet, methods to quantitatively probe the local density are lacking. We introduced scanning force microscopy (SFM) to quantitatively probe the internal structure of microgels physically adsorbed on a solid (SiO2) / water interface. Using a sharp SFM tip we investigated the two types of 1 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

microgels, (i) core-shell microgels featuring a hard silica core and a PNIPAM shell, and (ii) hollow microgels obtained by dissolution of the silica core. Thus, both systems have the same polymer network as peripheral structure but a distinctly different internal structure, i.e. a rigid core vs. a void. Evaluation of the force-distance curves, the force profile during insertion of the tip into the polymer network enables to determine a depth dependent contact resistance which closely correlates with the density profiles determined in solution by small-angle neutron scattering (SANS). We found that the cavity of the swollen hollow microgels is still present when adsorbed to the solid substrate.

Remarkably, while currently used techniques such as colloidal probe or reflectometry only provide an average of the z-profile, the methodology introduced herein actually probes the real three dimensional density profile, which is ultimately important to understand the macroscopic behavior of microgel films. This will bridge the gap between the colloidal probe experiments that deform the microgel globally and the insertion in which the disturbance is located near the tip.

2 ACS Paragon Plus Environment

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Introduction Microgels are macromolecular networks based on intramolecular chemical cross-links, highly swollen by a good solvent. Typically they have a spherical shape and their size ranges between several micrometers down to nanometers.1 Changes in their environment, such as temperature, pH or ionic strength can cause changes in the properties, dimensions, structure or interactions of the microgels.2 Microgels containing N-isopropylacrylamide (NIPAM) as main monomer show a volume phase transition temperature (VPTT) at 32 °C in water.1,

3-4

Below the VPTT, microgels exhibit a

unique open structure with a smooth decay of polymer segment density towards the periphery, caused by an inhomogeneous spatial distribution of the cross-linker.5 This results in a fuzzy appearance with dangling polymer chains reaching out of the highly cross-linked center. In contrast, an increase of temperature above the VPTT collapses the polymer network, resulting in a drastic decrease of the size. The water content is still significant but the structure is comparable to a homogeneous sphere.5 With these properties microgels cannot clearly be assigned to one of the major classes of colloids: rigid particles or flexible macromolecules. Their high interfacial activity, although not being amphiphilic, distinguishes them also from surfactants-based micellar aggregates. Microgels readily adsorb to liquid/liquid or liquid/air interfaces and deform, lower the interfacial tension, and stabilize emulsions.6-8 Combined with their stimulus-responsive character, microgels can be used for the modulation of emulsion stability.9-11 In addition, microgels spread at solid interfaces, balancing the adhesion energy, favoring high contact areas, and the elastic energy, opposing deformation.12 The deformation can be observed as the microgels show an expansion of 3 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 30

their lateral and a compression of their vertical dimensions.13 For microgels with low cross-link density, the stress due to deformation is so high that breaking of covalent bonds between the polymer segments could be observed.14 Their interfacial behavior makes microgels useful for many applications in the field of advanced polymeric coatings, such as antifouling15-16 or biosensing coatings.17 For the control of cell adhesion on microgel coated surfaces, not only the temperature-induced changes in topography but also of the mechanical properties of the microgels play an important role, e.g. on a microgel coated surface, fibroblast cells attach strongly when the temperature is above the VPTT and detach below their VPTT, respectively.18 Mechanical properties of microgels are governed by composition, chemical functionalization, and physical structure across multiple length scales. For instance, cross-linking density at the nanoscale is an important factor that helps determine the elastic modulus. However, the deformation of any microgel depends not only on its modulus but also on the shape and structure of the object. Although control of the size and shape of the microgel is possible, control of the internal structure remains difficult. That is mainly due to the spatial inhomogeneities introduced by the cross-links and to the finite size of the network, resulting in a high concentration of dangling-ends at the periphery of the microgel. The impact of these nanostructural heterogeneities on the microgel properties is peculiar and depends on the length and time scales at which the gels are probed. Scattering methods such as light scattering, small-angle x-ray and neutron scattering substantiate the heterogeneity of the density profile, but there is no consensus on the exact inner structure of the microgel.19-21 Information at the single microgel level is crucial.22-24 It remains even more challenging to visualize the network morphology of microgels in their native environment. A recent breakthrough in super-resolution microscopy offers 4 ACS Paragon Plus Environment

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

enormous potential for resolving the nanometric structure of soft matter, especially for microgels.25-30 Simultaneous high resolution imaging and nanomechanical profiling of the inner structure of microgels is of crucial importance. The scanning force microscope (SFM) excels at investigating the morphology and the nanomechanical properties of microgels at surfaces.31-40 An important parameter is the size of the probe compared to the dimensions of the microgel and two limiting cases can be considered:41 (i) Colloidal probe SFM where the probe dimensions are much larger than the microgel size and which provides information on the global mechanical properties of the microgel. (ii) A sharp tip, on the other hand, provides information with high lateral resolution but will not only deform but also penetrate the microgel. Various publications used a sharp tip and report an increase of the Young’s modulus of the surface-attached PNIPAM microgels by one order of magnitude upon heating (crossing the VPTT).18, 42-45 It was observed that the modulus varies laterally over the microgel, reflecting the hemispherical morphology of adsorbed microgels.18 The modulus that is determined from the force-distance curve, however, depends on the indentation depth.44 This is because of the inhomogeneous cross-linking of such PNIPAM microgels mentioned above. For that reason, the measurements were performed such that very low forces are applied and the outermost regions of the microgels are probed.18, 44 In this contribution, we explore if indentation experiments allow probing the internal structure, i.e. the inhomogeneous polymer density, in more detail. For this purpose, we first mapped the morphology of the microgels ‘in situ’ with a known indentation force through Peak Force Tapping mode. We employ two microgels with very different internal structure: (i) a core-shell 5 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 30

microgel consisting of a rigid silica core and a PNIPAM shell and (ii) the corresponding hollow microgel where the silica core was dissolved. The structure - i.e. the density profile - of these microgels in solution has been probed before by small angle neutron scattering (SANS).46 Here, we investigate the extent to which the scanning tip can correctly map the height profiles of swollen and collapsed microgels. In particular, because the force experienced by the microgel during scanning is known, we sought a correlation between height and force-distance curve profiles. This led us to the conclusion that the sharp tip, while allowing high-resolution imaging, is highly indenting and penetrates into the microgel. Based on this finding, we further investigate whether the internal structure of these microgels can be probed by SFM, as illustrated in Scheme 1. In the following we will demonstrate that indentation experiments using a sharp tip lead to penetration of the porous swollen microgel networks. Therefore, force-distance curve measurements enable to probe the absence or presence of the rigid silica core as well as the segment density distribution orthogonal to the solid support of adsorbed single microgels. We show that the cavity of the adsorbed hollow microgels can be detected and is still preserved in the swollen state.

Scheme 1. Schematic illustration of a sharp tip approaching the water/microgel interface, such that the density variation in the vicinity of the tip is probed, for a swollen core-shell (left) and hollow microgel (right).


Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Experimental Section Core-shell and hollow microgel synthesis. The core-shell and hollow microgels were synthesized by Karg as published previously.47-48 Briefly, the silica nanoparticles for the core were synthesized following the Stöber process of a condensation of TEOS (Merck) in ammonia (28% - 30%, Merck) and ethanol (Merck).49 The silica particle surface was afterwards modified with MPS (Sigma-Aldrich). The shell was polymerized around the cores by a reaction of NIPAM (1075 mg, Acros Organics), BIS (79 mg, Applichem), and the stabilizer PVP (100 mg, Merck) and was initiated with KPS (10 mg, Acros Organics). This resulted in core-shell microgels, which were cleaned by centrifugation and redispersion. The polymer shell contained 5 mol% BIS. The hollow microgels were prepared by stirring lyophilized core-shell microgels in highly diluted hydrofluoric acid (Merck) as reported previously.47, 50-52 The microgels were stirred overnight, dialyzed against bidistilled water, and lyophilized.

Sample preparation. For deposition on a solid substrate, a mixed microgel solution was prepared. The freeze-dried microgels were redispersed in water. The mass concentration of the core-shell microgel solution was 0.8 wt% and of the hollow microgel solution 0.4 wt%. Both aqueous solutions were mixed in a 1:1 volume ratio so that the concentration of the core-shell microgels was 0.4 wt% and of the hollow microgels was 0.2 wt%. Microscope glass coverslips (size: 2.2 × 2.2 cm2, Menzel-Gläser) were cleaned by ultrasonication in isopropanol (Sigma-Aldrich) for 15 minutes. The surface was afterwards activated by oxygen plasma treatment (in a PVA TePla plasma system 100) under 0.8 mbar oxygen 7 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 30

pressure at 200 W of microwave power for 20 minutes. Immediately, the deposition of the microgels was performed on a spin-coater (Convac 1001S). For the spin-coating, 120 µL of the mixed microgels solution was dropped onto the activated glass slide and it was rotated at a speed of 2500 rpm for 30 seconds.

SFM measurements. The measurements were performed on a Dimension Icon SFM with closed loop (Veeco Instruments Inc., software: Nanoscope 8.15 (Bruker Corporation)). They were analyzed by the Nanoscope Analysis 1.5 Software (Bruker Corporation). (i) Investigations at the solid/air interface: the microgels (deposited on the glass slide) were analyzed with PPP-FM-50 (Nanosensors) tips (resonance frequency 75 kHz, nominal spring constant 2.8 N/m) via tapping mode. (ii) Investigations at the solid/liquid interface: the measurements were conducted in a customized liquid cell on a heating stage (Dimension Icon Electrochemistry Chuck, Bruker Corporation) with

temperature

control

(Model

335

Cryogenic

Temperature

Controller,

Lake Shore Cryotronics). The glass slide coated with microgels was placed in the liquid-cell, with a 55 µm thick polyimide (PI) - foil beneath at the photo detector side, and the microgels were rehydrated by adding water. The microgels were analyzed at T = 27 °C and T = 34 °C. The temperatures were equilibrated for ca. 60 minutes. The measurements were recorded in the Peak Force QNM mode with modified MSNL-10-E (Bruker Corporation) tips (see below) with a nominal resonance frequency of 38 kHz in air and a nominal spring constant of 0.1 N/m of the cantilever (tip radius: 2 nm, semi-angle of the tip: 23°, assumed sample Possion’s ratio: 0.3). The SFM was operated with a Peak Force Frequency of 2 kHz. The integrated ScanAsyst was used to 8 ACS Paragon Plus Environment

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

control the feedback during imaging in terms of gain and Z limit except the Peak Force Setpoint was set manually. The Peak Force Setpoint was 1.8 nN at T = 27 °C, and 3.0 nN at T = 34 °C. The Peak Force Amplitude was 220 nm or 180 nm at T = 27 °C and T = 34 °C, respectively. Before images were taken and force-distance curve measurements were conducted, the tips used were calibrated. Modification of the SFM tip. SFM tips (MSNL-10, Bruker Corporation) were activated under oxygen flow with UV radiation for 15 minutes. Immediately, a droplet of an aqueous polyvinylpyrrolidone (PVP) - solution (0.5 wt%, PVP manufactured by Sigma Aldrich) was placed on the tip for physisorption of the polymer. Excess of the polymer was removed by rinsing with water. Subsequently, the tip was used for imaging. SFM tip calibration. The cantilever deflection sensitivity was determined from force-distance curves measured on the clean solid substrate. It corresponds to the slope after contact of the tip with the surface. The typical deflection sensitivity of the sharp microlever MSNL-10-E (Bruker Corporation) was 35 – 55 nm/V. The tip was withdrawn from the surface by 300 µm and the thermal noise was measured by the thermal tune of the Nanoscope 8.15 software. The mean displacement of the cantilever due to thermal noise was used to calculate the cantilever spring constant. The determined spring constants of the used tips varied between 0.17 – 0.19 N/m. Force-distance curve. Force-distance curve measurements were performed at the apex of the adsorbed microgels at the solid/liquid interface.

The position of the nano-indentation was

determined by previous images. Ten measurements at the same spot were conducted. The measurements were done for one microgel of both kinds at each temperature. The received curves were first order baseline corrected. A value of 2.5 nN for the thus corrected force setpoint 9 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

was used to obtain force-distance curves. This force was the minimum force to allow imaging and furthermore allowed sufficient indentation to probe the properties of the polymer network. The contact-point of tip and sample was assigned to the value of the distance, at which the force was starting to increase above the noise level (≈ 50 pN). The contact-point-distance was subtracted from the previous distance values to obtain the corrected force-distance curves. Contact stiffness versus indentation depth. Contact stiffness versus indentation depth curves were obtained by calculating the first derivative of the force-distance curves shown in Figure 2. The first derivative was determined using differentiation options of the Origin software (OriginPro 2016G, OriginLab Corporation). The first derivative curves were smoothed using 300 points (lowest possible value to exclude strong fluctuations). The negative tip-sample-distances from Figure 2 were transferred into positive indentation depths.

Image analysis and profile extraction procedure. The height and elastic modulus53-54 images were analyzed with a MATLAB routine (MathWorks, 2016a), written by Gelissen (based on the routine published by Gelissen et al.30). By the routine the center of the microgels was determined. The microgel was partitioned into 100 ellipsoids with increasing radius around the center. The height resp. E–modulus was averaged for every ellipsoid. Height/ E-moduli as a function of the normalized radial position were received. The radial profiles were averaged for all microgels of the same species.

SANS measurements. For details regarding the SANS measurements see the publication by Schmid et al.46 10 ACS Paragon Plus Environment

Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Results and Discussion The solution properties of the microgels used in this study have been reported before.46 The coreshell microgel consists of a silica core with a radius of 63 nm and a total hydrodynamic radius of 267 nm in the swollen state. The increase of the temperature above the VPTT leads to a collapse of the microgel network. Their hydrodynamic radius is reduced to 150 nm in the collapsed state. The corresponding hollow microgels were obtained by dissolving the silica core, and have a hydrodynamic radius of 268 nm and 147 nm in swollen and collapsed state, respectively. In order to probe the structure of these two microgels adsorbed to an interface under identical conditions, we prepare a solution that contains both types of microgels and spin coat them on a glass substrate (SFM images of the microgels in the dry state are shown in the Supporting Information (Figure S1)). Afterwards the sample is rehydrated and probed at 27 °C and 34 °C where the microgels are swollen or collapsed, respectively. Imaging with a scanning force microscope provides insight into the morphology of microgels physically adsorbed at the SiO2/water interface. Height maps, resolved on the nanoscale, are obtained by operating the SFM in the Peak Force Tapping mode. Unlike conventional tapping mode imaging, where the tapping force is unknown, Peak Force Tapping enables mapping the topography of the microgel with a known indenting force.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 30

Figure 1. A: Height image of rehydrated core-shell and hollow microgels on a solid substrate at T = 27 °C obtained by application of a Peak Force of F = 1.8 nN. The graph below is a cross-section of the height image at T = 27 °C through a hollow and core-shell microgel (from left to right) at the position along the white line in the top image. B and C: Averaged height as a function of the normalized radial position starting from the apex of the microgels. The radial profiles are shown for core-shell microgels (CS, full lines, B), as well as for hollow microgels (HS, dashed lines, C) in dry state (green), at T = 27 °C (black, swollen), and T = 34 °C (red, collapsed).

Figure 1A shows the height image obtained by Peak Force Tapping mode at 27 °C, i.e. in the swollen state. Core-shell and hollow microgels are hardly distinguishable by their height. All imaged microgels show similar horizontal and vertical dimensions. Clear differentiation is only possible through modulus maps (see Figure S6 and S8). The cross-section height profile (Figure 1A, bottom) shows that the hollow microgels (left-hand side of cross-section) are slightly smaller in vertical direction as compared to the core-shell microgels (right-hand side of cross12 ACS Paragon Plus Environment

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

section). Both microgel types have nearly hemispherical shape. Height images in the dry state and in the collapsed state (at T = 34 °C) are given in the Supporting Information (Figure S1–2). The height images in the different swelling states are used to determine average height profiles. This was done for the two microgel species separately (see Experimental Section for more details) and results are shown in Figures 1B and 1C. The height profiles in the dry state of the core-shell and hollow microgels show significant differences, even though their lateral extension is similar. Considering the lateral morphology, the dried core-shell microgels adopt a “fried-egg” shape at the solid interface. A central region including the protruding silica core, further out a highly cross-linked polymer shell (ca. 40 nm), and a third molecular flat (1–2 nm) outer layer, resulting from the less cross-linked microgel periphery, is found. Height cross-section profiles of similar microgels revealed fried-egg shapes as well.55 In contrast, the dried hollow microgels are more flattened and resemble pancakes. ContrerasCáceres et al. observed hemispherical shapes of hollow adsorbed microgels,56 but for a decrease in cross-linker concentration of conventional microgels a transition from fried-egg shaped highly cross-linked to more pancake like loosely cross-linked microgels is reported.14 However, the lateral extension of the outer molecular layer remains comparable irrespective of the microgel type. Swelling the microgels with water expands the network. Compared to the dried state, the swollen microgels seem to shrink laterally and expand vertically. This leads to a hemispherical shape of the hydrated microgels as compared to the fried-egg/pancake morphology of the dry microgels.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

Hemispherical shapes of adsorbed hydrated microgels were already reported for conventional microgels.18, 44-45 When comparing the profiles at 27 °C and 34 °C, respectively, one notices that the core-shell microgels have nearly the same height (Figure 1B), and the hollow microgels appear to be more swollen in the vertical direction at 34 °C compared to 27 °C (Figure 1C). This is unexpected and indicates that the true dimensions of the hydrated networks are not detected in this type of measurement. As the images in the hydrated states are taken by operating in the Peak Force Tapping mode and thus applying a given force, the extracted height information has to be corrected, in particular in the swollen state where the sharp SFM tip can deform or even penetrate the microgel network. Therefore, to understand the impact of the Peak Force Tapping mode on the actual topography, we performed force-distance curve measurements that sense the network orthogonally to the interface. These force measurements can probe the weak interactions between the tip and sample that are inaccessible by imaging.


Page 15 of 30

2.5

A

Force [nN]

2.0

T = 34 °C

1.5 1.0

T = 27 °C 0.5 0.0 -300 -250 -200 -150 -100 -50

0

50

100

Tip-Sample-Distance [nm] 0.9

B

0.8

0.7

C

0.15

T = 34 °C

0.10 0.05

T = 27 °C

0.00 -0.05

0.6 -230

0.20

Force [nN]

Force [nN]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

-220

-210

-200

Tip-Sample-Distance [nm]

-190

-0.10 -150

-100

-50

0

50

100

Tip-Sample-Distance [nm]

Figure 2. Force-distance curves corrected by the contact-point for the vertical approach of an SFM tip towards the solid substrate at the apex of a core-shell microgel (CS, filled circles) and hollow microgel (HS, unfilled circles) at T = 27 °C (CS: black, HS: light gray) and T = 34 °C (CS: red, HS: light red); A: total force scale; B: zoom into the kink within a range of tip-sample distances of -205 nm till -215 nm: the force is nearly constant before it starts to increase again; C: zoom into lower force region. For the maximal load (2.5 nN) the cantilever deflection δ is only

δ = 12.5 nm (F/k = 2.5 nN/0.2 Nm-1). It is rather small and within the range of linear elasticity response. C highlights the noise level which is within 50 pN enabling to measure cantilever deflections of 1 nm equivalent to 0.2 nN load.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

Figure 2 shows the force-distance curves measured at the apex of a core-shell and hollow microgel in the swollen and collapsed state. At positive tip-sample-distances the force is zero because no interaction between sample and tip occurs. As the tip is moving towards the substrate, SFM tip and microgel come into contact (zero tip-sample-distance) and the force increases with the indentation into the microgel (negative tip-sample-distances). The sharp SFM tip indents 75 nm into the core-shell microgel in the collapsed state by applying a force of 2.5 nN. This indentation depth is within the range of the microgel shell thickness obtained from SANS measurements (dshell(CS, 40 °C) = 67 nm)46. However, indentation of a collapsed hollow microgel with a similar force results in an indentation depth of 110 nm. This is significantly larger compared to the shell thickness (dshell(HS, 40 °C) = 67 nm)46. As the shell structure is similar for both microgel types, this variation must result from the difference in the internal structure. The core-shell microgel shows a higher indentation force that reflects the underlying rigid silica core. On the other hand, the hollow shape is easier to deform, which allows a greater indentation depth by applying a similar force. Indentation can be expected to be even more significant below the VPTT where the network is swollen. Indeed, the indentation depth for the core-shell microgel in the swollen state is 210 nm. We note that this is in agreement with the microgel shell thickness around the rigid, impenetrable silica core as determined in bulk solution by SANS.46 This value further indicates a swelling ratio perpendicular to the solid support of about 2.8. Remarkably, with a similar force, the sharp tip effortless penetrates through the hollow microgel to a depth of 300 nm. In addition, the corresponding force-distance curve shows a kink (Figure 2B). The kink is only observable for the hollow microgel and occurs at an indentation depth of 210 nm, very close to the shell thickness as observed in Ref. [46] using SANS (dshell(HS, 20 °C) = 220 nm)46. The force is nearly 16 ACS Paragon Plus Environment

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

constant within tip-sample-distances of -205 till -215 nm. This strongly suggests that the adsorbed microgel is not only compressed, but that the tip is mostly penetrating into the fuzzy network and even detects the presence of the cavity. These experiments thus indicate that a cavity is retained when the hollow microgels deform during adsorption to the solid substrate. It suggests a cavity with a thickness of about 10 nm perpendicular to the substrate. This is smaller than the void diameter in bulk solution (57.2 nm)46. Therefore, the microgel deforms upon adsorption leading to an anisometric cavity as depicted in Scheme 1. This information is difficult to obtain by other means and is relevant for applications that rely on functional species encapsulated in the cavity of hollow microgels, e.g. in sensors or functional coatings.

18, 57-58

In

addition, it should be noted that the kink is not observed above the VPTT. This indicates that the penetration is limited by the dense network at high temperatures and the probe mostly compresses the collapsed microgels. The slope of the force-distance curves is not constant after the point of contact in a force range between 0 and 2.5 nN (Figure 2A). An extraction of the elastic modulus at different force ranges results in an increase of Young’s modulus with increasing indentation. This was already observed for conventional microgels by Burmistrova et al.44 Models used to determine a modulus are typically only applicable for a compression; in our case where the SFM probe is sharp and clearly penetrates the swollen network, the models are not applicable.41 An interaction between sample and tip is already evident for forces below 0.2 nN as shown in Figure 2C. This rather small indentation force still provides signals well above the noise (the noise level is within 50 pN), and is in the range of steric forces.59 Both types of microgels respond similarly to weak indenting forces. Only the microgel shell is probed which is similar for both types. However, the curves reveal a clear difference between the swollen and the collapsed 17 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

state. The force-distance curves at T = 34 °C show a constant and steep slope. In contrast, at T = 27 °C the slope shows a non-monotonic increase with the indentation depth. These changes in the slope as a function of indentation depth must result from an increase in polymer density as the tip penetrates into the swollen network and approaches the microgel center. Swollen microgels are characterized by an inhomogeneous distribution of cross-links.5 The density of cross-links decreases from the center to the periphery, which leads to a smoothly decaying density profile. It is tempting to analyze the first derivative of the force-distance curve which reflects the variation of the contact stiffness on the indentation depth. The contact stiffness variation arises from the gradual increase in the network density. As the indentation experiments on the swollen microgels are clearly dominated by a penetration of the SFM tip, this provides information on the polymer density distribution orthogonal to the interface and, therefore, it can be compared to the density profiles in bulk solution.


Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Collapsed State Core-Shell

Hollow

Swollen State

Core-Shell

Hollow

Figure 3. Contact stiffness (first derivative of the force-distance curves (Figure 2)) versus indentation depth plotted on the left and bottom axis (black), and polymer density from SANS measurements (green curves, A and B: T = 20 °C, C and D: T = 40 °C )46 versus radial position within the microgel plotted on the right and upper axis (green). Zero radial position signifies the microgels’ periphery. The contact stiffness curves are shown for the core-shell (full lines, A and C) and hollow microgels (dashed lines, B and D) at T = 34 °C (red, A and B) and T = 27 °C (black, C and D). Note that the contact stiffness axis has a different scale in the collapsed state and swollen state, respectively.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

Figure 3 shows the contact stiffness as a function of the indentation depth. The contact stiffness is zero prior to contact (i.e. for "negative" indentation depths). As the tip starts to indent the microgel, the contact stiffness rises up. The increase is stronger in the collapsed than in the swollen state. In addition, the curves are clearly different considering the microgel species. The contact stiffness curves of the core-shell microgels end after a plateau is reached (Figures 3A and 3C, ∆d(34 °C) ≈ 70 nm, ∆d(27 °C) ≈ 200 nm), whereas the curves of the hollow microgels

show

a

further

increase

(Figures 3B and 3D,

∆d(34 °C) > 70 nm,

∆d(27 °C) > 200 nm). The comparison between the radial polymer density profile obtained by SANS and the contact stiffness reveals that the indentation depth, where the plateau appears (200 nm/ 70 nm), is at the same value where the rigid silica core is detected by SANS (Figures 3A and 3C). Additional penetration is not possible due to the impenetrable rigid core. In contrast, the absence of a silica core leads to a higher deformation, as evidenced by the deep indentation with increasing contact stiffness (Figures 3B and 3D). Therefore, SFM-based indentation measurements can discriminate between these different microgel architectures. In addition to the information whether the impenetrable silica core is present or not, SFM can probe a significant difference between the two swelling states before these plateaus are reached. The contact stiffness increases monotonically and strongly i.e. with a steep slope, for the microgels in the collapsed state for indentation depths between -25 and 50 nm and changes into a plateau for indentation depths between 50 and 75 nm (Figures 3A and 3B). According to SANS measurements, these two regions correspond to the box-like polymer density profile of the microgels in bulk solution.46 Thus SFM does not only detect the sharp surface of collapsed


Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

microgels but can also provide information on the internal density profiles below the surface due to compression. In contrast to the collapsed state, the contact stiffness in the swollen state shows a nonmonotonic increase with the indentation depth down to 200 nm (Figures 3C and 3D), and is always smaller than in the collapsed state. This region is consistent with the polymer density profiles in bulk solution. The microgels are characterized by an inhomogeneous and decaying polymer density from the center to the periphery. This shows that this fuzzy surface of the microgels, which is characterized by dangling polymer chains, and their heterogeneities in internal network density can be sensed by force-distance curve measurements. In addition, the presence of a cavity in the case of the swollen hollow microgel is very marked by the decrease in contact stiffness at an indentation depth of 200 nm (Figure 3D). At this point it is interesting to note that regardless of the state of the microgel, for a higher indentation depth, the pyramidal shaped tip does not only penetrate and deform the network locally but begins to compress the microgel. We also attempted to fit the force distance-curve data to a ‘brush-like’ model without success. The result of the fit was erroneous indicating that the brush thickness was comparable to the microgel dimensions, regardless of the internal structure. However, the very low deflection of the cantilever (1 nm) and thus the inferred force sensitivity range (50 pN) comply with steric forces arising for the loose peripheral network of the microgel. Moreover, the agreement of SFM and SANS data indicates that compression is negligible with respect to penetration in the swollen state. In the collapsed state, the microgels mostly deform by compression as the dense network opposes tip penetration. This can be supported by the SANS measurements, which showed a strong decrease of the mesh size from 10 nm at 20 °C to 1 nm at 40 °C.46 Quantitative assessment of the contribution of shape 21 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

deformation with respect to network deformation would require needle like probe geometries that deserves further studies. As already mentioned above, SFM has been widely employed to study adsorbed PNIPAM microgels. On the one hand, studies focused on extracting topographic structure information obtained from scanning the surfaces covered by microgels.14, 18, 42-45, 55-56 Furthermore, employing low forces, force-distance curve measurements were used to gain information on the elastic modulus of the outermost region of the adsorbed microgels.18, 42-45 In our current contribution, by applying quantifiable forces, we show that the swollen microgels are penetrated strongly by a sharp SFM tip. This allows extracting structural information beyond probing the periphery of the microgels. Our comparison of the force profiles to density profiles obtained by SANS shows that information on the internal structure can be obtained by SFM.


Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Summary and Conclusion In summary, we introduced a method to probe the density profiles of microgels at interfaces using a sharp SFM tip. This enables us to study the structure of core-shell and corresponding hollow microgels at the solid/liquid interface. Force-distance curve measurements show that i) core-shell and hollow microgels can be clearly distinguished, ii) the tip penetrates the swollen microgel network, and iii) the non-monotonic increase of the force results from the local segment density of the microgel. Computing the first derivative of the force-distance curves provided the polymer density distribution orthogonal to the interface. The obtained structural information agrees with the density distributions determined by data fitting of SANS scattered intensities. The acquisition of a topographic map of swollen microgels adsorbed on a substrate requires the application of a force, which causes the sharp SFM tip to penetrate into the open microgel network. Therefore, it is inaccurate to interpret the height images as a true outer surface of the microgel. We further argue that the height profile results from a force balance between the stiffness of the probe and the insertion resistance, defined by the network density and the geometry of the probe. It is this mechanical balance that we attempted to quantify, based on the variation of the internal structure of the microgel, and derive an insertion resistance profile which we then compared with the SANS density profile. The experimental results demonstrate that one is able to probe the internal heterogeneity of the adsorbed swollen microgels. Compared to, e.g., reflectometry, where the internal structure information is averaged over the whole substrate,60-61 SFM provides spatially resolved internal structure information of individual microgels. Interestingly, the hollow microgels still exhibit a cavity even though they deform at the solid substrate. While the comparison between SANS profile and force profile is qualitative, further 23 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

studies call for theoretical efforts to link polymer density profiles to the insertion force. More generally we have shown that sharp tip force-distance curve measurements can be used as a fast and reliable method to determine the internal structure of soft, porous materials at solid interfaces.

Associated Content Supporting Information includes: List of abbreviations, height maps in dry (Figure S1) and collapsed state (Figure S2), volume estimation, correction of hydrated height profiles by indentation depth and swelling ratios of adsorbed microgels (Equation S1-4, Figure S3-5, Table S1-3), Young’s modulus maps and profiles (Figure S6-8), bulk structure from SANS measurements (Figure S9), description of profile extraction procedure (Figure S10), contact stiffness versus indentation depth (Figure S11). The data are available upon request at: https://hdl.handle.net/21.11102/539ec614-10c9-11e880f7-e41f1366df48.

Author Information Corresponding Author Walter Richtering*†§ (E-Mail): [email protected] Ahmed Mourran*§ (E-Mail): [email protected]


Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Acknowledgments We thank Katja Nothdurft, Janine Dubbert, Andreas J. Schmid and Matthias Karg for synthesis and characterization of the microgels. Andrea Scotti thanks the Alexander von Humboldt Foundation. The Deutsche Forschungsgemeinschaft (DFG) is acknowledged for financial support within the Sonderforschungsbereich SFB 985 “Functional Microgels and Microgel Systems” (Projects A7 and B8).


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

References 1. Pelton, R., Temperature-sensitive aqueos microgels. Adv. Colloid Interface Sci. 2000, 85, 1-33. 2. Plamper, F. A.; Richtering, W., Functional Microgels and Microgel Systems. Acc. Chem. Res. 2017, 50 (2), 131-140. 3. Heskins, M.; Guillet, J. E., Solution Properties of Poly(N-isopropylacrylamide). J. Macromol. Sci. Chem. A2 1968, 8, 1441-1455. 4. Schild, H. G., Poly(N-isopropylacrylamide): experiment, theory and application. Prog. Polym. Sci. 1992, 17, 163-249. 5. Stieger, M.; Richtering, W.; Pedersen, J. S.; Lindner, P., Small-angle neutron scattering study of structural changes in temperature sensitive microgel colloids. J. Chem. Phys. 2004, 120 (13), 6197-206. 6. Destribats, M.; Lapeyre, V.; Wolfs, M.; Sellier, E.; Leal-Calderon, F.; Ravaine, V.; Schmitt, V., Soft microgels as Pickering emulsion stabilisers: role of particle deformability. Soft Matter 2011, 7 (17), 7689-7698. 7. Deshmukh, O. S.; Maestro, A.; Duits, M. H. G.; van den Ende, D.; Stuart, M. C.; Mugele, F., Equation of state and adsorption dynamics of soft particles at an air-water interface. Soft Matter 2014, 10, 7045-7050. 8. Rumyantsev, A. M.; Gumerov, R. A.; Potemkin, I. I., A polymer microgel at a liquidliquid interface: theory vs. computer simulations. Soft Matter 2016, 12 (32), 6799-6811. 9. Brugger, B.; Richtering, W., Magnetic, Thermosensitive Microgels as StimuliResponsive Emulsifiers Allowing for Remote Control of Separability and Stability of Oil in Water-Emulsions. Adv. Mater. 2007, 19 (19), 2973-2978. 10. Monteillet, H.; Workamp, M.; Li, X.; Schuur, B.; Kleijn, J. M.; Leermakers, F. A.; Sprakel, J., Multi-responsive ionic liquid emulsions stabilized by microgels. Chem. Commun. 2014, 50 (81), 12197-12200. 11. Picard, C.; Garrigue, P.; Tatry, M. C.; Lapeyre, V.; Ravaine, S.; Schmitt, V.; Ravaine, V., Organization of Microgels at the Air-Water Interface under Compression: Role of Electrostatics and Cross-Linking Density. Langmuir 2017, 33 (32), 7968-7981. 12. Carrillo, J. M.; Raphael, E.; Dobrynin, A. V., Adhesion of nanoparticles. Langmuir 2010, 26 (15), 12973-12979. 13. Cors, M.; Wrede, O.; Genix, A. C.; Anselmetti, D.; Oberdisse, J.; Hellweg, T., Core-Shell Microgel-Based Surface Coatings with Linear Thermoresponse. Langmuir 2017, 33 (27), 68046811. 14. Mourran, A.; Wu, Y.; Gumerov, R. A.; Rudov, A. A.; Potemkin, I. I.; Pich, A.; Möller, M., When Colloidal Particles Become Polymer Coils. Langmuir 2016, 32 (3), 723-30. 15. Lynch, I.; Miller, I.; Gallagher, W. M.; Dawson, K. A., Novel Method to Prepare Morphologically Rich Polymeric Surfaces for Biomedical Applications via Phase Separation and Arrest of Microgel Particles. J. Phys. Chem. B 2006, 110, 14581-14589. 16. South, A. B.; Whitmire, R. E.; Garcia, A. J.; Lyon, L. A., Centrifugal Deposition of Microgels for the Rapid Assembly of Nonfouling Thin Films. ACS Appl. Mater. Interfaces 2009, 1, 2747-2754.


Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

17. Sigolaeva, L. V.; Gladyr, S. Y.; Mergel, O.; Gelissen, A. P. H.; Noyong, M.; Simon, U.; Pergushov, D. V.; Kurochkin, I. N.; Plamper, F. A.; Richtering, W., Easy-Preparable Butyrylcholinesterase/Microgel Construct for Facilitated Organophosphate Biosensing. Anal. Chem. 2017, 89 (11), 6091-6098. 18. Schmidt, S.; Zeiser, M.; Hellweg, T.; Duschl, C.; Fery, A.; Möhwald, H., Adhesion and Mechanical Properties of PNIPAM Microgel Films and Their Potential Use as Switchable Cell Culture Substrates. Adv. Funct. Mater. 2010, 20 (19), 3235-3243. 19. Fernandez-Barbero, A.; Fernandez-Nieves, A.; Grillo, I.; Lopez-Cabarcos, E., Structural modifications in the swelling of inhomogeneous microgels by light and neutron scattering. Phys. Rev. E 2002, 66 (5 Pt 1), 051803 1-10. 20. Gawlitza, K.; Turner, S. T.; Polzer, F.; Wellert, S.; Karg, M.; Mulvaney, P.; von Klitzing, R., Interaction of gold nanoparticles with thermoresponsive microgels: influence of the crosslinker density on optical properties. Phys. Chem. Chem. Phys. 2013, 15 (37), 15623-15631. 21. Dulle, M.; Jaber, S.; Rosenfeldt, S.; Radulescu, A.; Förster, S.; Mulvaney, P.; Karg, M., Plasmonic gold-poly(N-isopropylacrylamide) core-shell colloids with homogeneous density profiles: a small angle scattering study. Phys. Chem. Chem. Phys. 2015, 17 (2), 1354-1367. 22. Plamper, F. A.; Gelissen, A. P.; Timper, J.; Wolf, A.; Zezin, A. B.; Richtering, W.; Tenhu, H.; Simon, U.; Mayer, J.; Borisov, O. V.; Pergushov, D. V., Spontaneous assembly of miktoarm stars into vesicular interpolyelectrolyte complexes. Macromol. Rapid. Commun. 2013, 34 (10), 855-860. 23. Proetto, M. T.; Rush, A. M.; Chien, M. P.; Abellan Baeza, P.; Patterson, J. P.; Thompson, M. P.; Olson, N. H.; Moore, C. E.; Rheingold, A. L.; Andolina, C.; Millstone, J.; Howell, S. B.; Browning, N. D.; Evans, J. E.; Gianneschi, N. C., Dynamics of soft nanomaterials captured by transmission electron microscopy in liquid water. J. Am. Chem. Soc. 2014, 136 (4), 1162-1165. 24. Schulte, B.; Rahimi, K.; Keul, H.; Demco, D. E.; Walther, A.; Möller, M., Blending of reactive prepolymers to control the morphology and polarity of polyglycidol based microgels. Soft Matter 2015, 11 (5), 943-953. 25. Ullal, C. K.; Primpke, S.; Schmidt, R.; Böhm, U.; Egner, A.; Vana, P.; Hell, S. W., Flexible Microdomain Specific Staining of Block Copolymers for 3D Optical Nanoscopy. Macromolecules 2011, 44 (19), 7508-7510. 26. Tian, Z.; Li, A. D.; Hu, D., Super-resolution fluorescence nanoscopy applied to imaging core-shell photoswitching nanoparticles and their self-assemblies. Chem. Commun. 2011, 47 (4), 1258-1260. 27. Gramlich, M. W.; Bae, J.; Hayward, R. C.; Ross, J. L., Fluorescence imaging of nanoscale domains in polymer blends using stochastic optical reconstruction microscopy (STORM). Opt. Express 2014, 22 (7), 8438-8450. 28. Yan, J.; Zhao, L. X.; Li, C.; Hu, Z.; Zhang, G. F.; Chen, Z. Q.; Chen, T.; Huang, Z. L.; Zhu, J.; Zhu, M. Q., Optical nanoimaging for block copolymer self-assembly. J. Am. Chem. Soc. 2015, 137 (7), 2436-2439. 29. Conley, G. M.; Nöjd, S.; Braibanti, M.; Schurtenberger, P.; Scheffold, F., Superresolution microscopy of the volume phase transition of pNIPAM microgels. Colloids Surf. A 2016, 499, 18-23. 30. Gelissen, A. P. H.; Oppermann, A.; Caumanns, T.; Hebbeker, P.; Turnhoff, S. K.; Tiwari, R.; Eisold, S.; Simon, U.; Lu, Y.; Mayer, J.; Richtering, W.; Walther, A.; Wöll, D., 3D Structures of Responsive Nanocompartmentalized Microgels. Nano Lett. 2016, 16 (11), 7295-7301.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

31. Lee, G. U.; Kidwell, D. A.; Colton, R. J., Sensing Discrete Streptavidin-Biotin Interactions with Atomic Force Microscopy. Langmuir 1994, 10, 354-357. 32. Moy, V. T.; Florin, E. L.; Gaub, H. E., Intermolecular Forces and Energies Between Ligands and Receptors. Science 1994, 266 (5183), 257-259. 33. Radmacher, M.; Fritz, M.; Hansma, P. K., Imaging soft samples with the atomic force microscope: gelatin in water and propanol. Biophys. J. 1995, 69 (1), 264-270. 34. Verbelen, C.; Gruber, H. J.; Dufrene, Y. F., The NTA-His6 bond is strong enough for AFM single-molecular recognition studies. J. Mol. Recognit. 2007, 20 (6), 490-494. 35. Höfl, S.; Zitzler, L.; Hellweg, T.; Herminghaus, S.; Mugele, F., Volume phase transition of “smart” microgels in bulk solution and adsorbed at an interface: A combined AFM, dynamic light, and small angle neutron scattering study. Polymer 2007, 48 (1), 245-254. 36. Hashmi, S. M.; Dufresne, E. R., Mechanical properties of individual microgel particles through the deswelling transition. Soft Matter 2009, 5 (19), 3682-3688. 37. Puntheeranurak, T.; Neundlinger, I.; Kinne, R. K.; Hinterdorfer, P., Single-molecule recognition force spectroscopy of transmembrane transporters on living cells. Nat. Protoc. 2011, 6 (9), 1443-1452. 38. Morsi, S. M.; Pakzad, A.; Amin, A.; Yassar, R. S.; Heiden, P. A., Chemical and nanomechanical analysis of rice husk modified by ATRP-grafted oligomer. J. Colloid Interface Sci. 2011, 360 (2), 377-85. 39. Chyasnavichyus, M.; Young, S. L.; Tsukruk, V. V., Mapping micromechanical properties of soft polymer contact lenses. Polymer 2014, 55 (23), 6091-6101. 40. Li, M.; Bresson, B.; Cousin, F.; Fretigny, C.; Tran, Y., Submicrometric Films of SurfaceAttached Polymer Network with Temperature-Responsive Properties. Langmuir 2015, 31 (42), 11516-11524. 41. Halperin, A.; Zhulina, E. B., Atomic force microscopy of polymer brushes: colloidal versus sharp tips. Langmuir 2010, 26 (11), 8933-40. 42. Wiedemair, J. S., Michael J.; Kim, Jongseong; Masson, Jean-Francois; Lyon, L. Andrew; Mizaikoff, Boris; Kranz, Christine, In-Situ AFM Studies of the Phase-Transition Behavior of Single Thermoresponsive Hydrogel Particles. Langmuir 2007, 23, 130-137. 43. Tagit, O.; Tomczak, N.; Vancso, G. J., Probing the morphology and nanoscale mechanics of single poly(N-isopropylacrylamide) microgels across the lower-critical-solution temperature by atomic force microscopy. Small 2008, 4 (1), 119-26. 44. Burmistrova, A.; Richter, M.; Üzüm, C.; Klitzing, R. v., Effect of cross-linker density of P(NIPAM-co-AAc) microgels at solid surfaces on the swelling/shrinking behaviour and the Young’s modulus. Colloid Polym. Sci. 2011, 289 (5-6), 613-624. 45. Burmistrova, A.; Richter, M.; Eisele, M.; Üzüm, C.; von Klitzing, R., The Effect of CoMonomer Content on the Swelling/Shrinking and Mechanical Behaviour of Individually Adsorbed PNIPAM Microgel Particles. Polymers 2011, 3 (4), 1575-1590. 46. Schmid, A. J.; Dubbert, J.; Rudov, A. A.; Pedersen, J. S.; Lindner, P.; Karg, M.; Potemkin, I. I.; Richtering, W., Multi-Shell Hollow Nanogels with Responsive Shell Permeability. Sci. Rep. 2016, 6, 22736. 47. Dubbert, J.; Honold, T.; Pedersen, J. S.; Radulescu, A.; Drechsler, M.; Karg, M.; Richtering, W., How Hollow Are Thermoresponsive Hollow Nanogels? Macromolecules 2014, 47 (24), 8700-8708.


Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

48. Dubbert, J.; Nothdurft, K.; Karg, M.; Richtering, W., Core-shell-shell and hollow doubleshell microgels with advanced temperature responsiveness. Macromol. Rapid Commun. 2015, 36 (2), 159-64. 49. Stöber, W.; Fink, A.; Bohn, E., Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62-69. 50. Zha, L.; Zhang, Y.; Yang, W.; Fu, S., Monodisperse Temperature-Sensitive Microcontainers. Adv. Mater. 2002, 14, 1090-1092. 51. Lapeyre, V.; Renaudie, N.; Dechezelles, J. F.; Saadaoui, H.; Ravaine, S.; Ravaine, V., Multiresponsive hybrid microgels and hollow capsules with a layered structure. Langmuir 2009, 25 (8), 4659-4667. 52. Xing, Z.; Wang, C.; Yan, J.; Zhang, L.; Li, L.; Zha, L., Dual stimuli responsive hollow nanogels with IPN structure for temperature controlling drug loading and pH triggering drug release. Soft Matter 2011, 7, 7992-7997. 53. Sneddon, I. N., The relation between loas and penetration in the axisymmetric boussinesq problem for a punch of arbitrary profile. Int. J. Engng. Sci. 1965, 3, 47-57. 54. Sneddon, I. N., Fourier Transforms. Dover Publications Ink: New York, 1995; p 542. 55. Karg, M.; Wellert, S.; Pastoriza-Santos, I.; Lapp, A.; Liz-Marzan, L. M.; Hellweg, T., Thermoresponsive core-shell microgels with silica nanoparticle cores: size, structure, and volume phase transition of the polymer shell. Phys. Chem. Chem. Phys. 2008, 10 (44), 67086716. 56. Contreras-Caceres, R.; Schellkopf, L.; Fernandez-Lopez, C.; Pastoriza-Santos, I.; PerezJuste, J.; Stamm, M., Effect of the cross-linking density on the thermoresponsive behavior of hollow PNIPAM microgels. Langmuir 2015, 31 (3), 1142-1149. 57. Sigolaeva, L. V.; Mergel, O.; Evtushenko, E. G.; Gladyr, S. Y.; Gelissen, A. P. H.; Pergushov, D. V.; Kurochkin, I. N.; Plamper, F. A.; Richtering, W., Engineering Systems with Spatially Separated Enzymes via Dual-Stimuli-Sensitive Properties of Microgels. Langmuir 2015, 31 (47), 13029-13039. 58. Saxena, S.; Spears, M. W., Jr.; Yoshida, H.; Gaulding, J. C.; Garcia, A. J.; Lyon, L. A., Microgel film dynamics modulate cell adhesion behavior. Soft Matter 2014, 10 (9), 1356-1364. 59. Oesterhelt, F.; Rief, M.; Gaub, H. E., Single molecule force spectroscopy by AFM indicates helical structure of poly(ethylene-glycol) in water. New J. Phys. 1999, 1, 6.1-6.11. 60. Wellert, S.; Richter, M.; Hellweg, T.; von Klitzing, R.; Hertle, Y., Responsive Microgels at Surfaces and Interfaces. Zeitschrift für Physikalische Chemie 2015, 229 (7-8). 61. Wellert, S.; Hertle, Y.; Richter, M.; Medebach, M.; Magerl, D.; Wang, W.; Deme, B.; Radulescu, A.; Muller-Buschbaum, P.; Hellweg, T.; von Klitzing, R., Inner structure of adsorbed ionic microgel particles. Langmuir 2014, 30 (24), 7168-76.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

Table of Contents Graphic


Probing the internal heterogeneity of responsive microgels adsorbed

Recommend Documents