Poly(glycidyl ether)-Based Monolayers on Gold Surfaces: Control of

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Poly(glycidyl ether)-based monolayers on gold surfaces: Control of grafting density and chain conformation by grafting procedure, surface anchor, and molecular weight Silke Heinen, and Marie Weinhart Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03927 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 13, 2017

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Poly(glycidyl ether)-based monolayers on gold surfaces: Control of grafting density and chain conformation by grafting procedure, surface anchor, and molecular weight Silke Heinen, Marie Weinhart* Institute of Chemistry and Biochemistry, Freie Universitaet Berlin, Takustr. 3, 14195 Berlin, Germany * Corresponding author: email [email protected], phone: +49 30 838 75050

Self-assembled monolayers, polymer brushes, gold surface, grafting density, sulfur-containing anchor, QCM-D, ellipsometry

ACS Paragon Plus Environment

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ABSTRACT For a meaningful correlation of surface coatings with their respective biological response reproducible coating procedures, well-defined surface coatings, and thorough surface characterization with respect to layer thickness and grafting density are indispensable. The same applies to polymeric monolayer coatings which are intended to be used for, e.g., fundamental studies on the volume phase transition of surface end-tethered thermoresponsive polymer chains. Planar gold surfaces are frequently used as model substrates, since they allow a variety of straightforward surface characterization methods. Herein we present reproducible grafting-to procedures performed with thermoresponsive poly(glycidyl ether) copolymers composed of glycidyl methyl ether (GME) and ethyl glycidyl ether (EGE). The copolymers feature different molecular weights (2 kDa, 9 kDa, 24 kDa) and are equipped with varying sulfur-containing anchor groups in order to achieve adjustable grafting densities on gold surfaces and hence control the tethered polymers’ chain conformation. We determined "wet" and "dry" thicknesses of these coatings by QCM-D and ellipsometry measurements and deduced anchor distances and degrees of chain overlap of the polymer chains assembled on gold. Grafting under cloud point conditions allowed for higher degrees of chain overlap compared to grafting from a good solvent like ethanol, independent of the used sulfur-containing anchor group for polymers with low (2 kDa) and medium (9 kDa) molecular weights. In contrast, the achieved grafting densities and thus chain overlaps of surface-tethered polymers with high (24 kDa) molecular weights were identical for both grafting methods. Monolayers prepared from an ethanolic solution of poly(glycidyl ether)s equipped with sterically demanding disulfidecontaining anchors revealed the lowest degrees of chain overlap. The ratio of the radius of gyration to the anchor distance (2 Rg/l) of the latter coating was found to be lower than 1.4,


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indicating that the assembly was rather in the mushroom-like than in the brush regime. Polymer chains with thiol-containing anchors of different alkyl chain lengths (C11SH vs. C4SH) formed assemblies with comparable degrees of chain overlap with 2 Rg/l values above 1.4 and are thus in the brush regime. Molecular weights influenced the achievable degree of chain overlap on the surface. Coatings prepared with the medium molecular weight polymer (9 kDa) resulted in the highest chain packing density. Control of grafting density and thus chain overlap in different regimes (brush vs. mushroom) on planar gold substrates are attainable for monolayer coatings with poly(GME-ran-EGE) by adjusting the polymer’s molecular weight and anchor group as well as the conditions for the grafting-to procedure.

INTRODUCTION Well-defined and thoroughly characterized surfaces are required for a meaningful correlation of bioactive or bioinert coatings with their biological response.1, 2 Reproducible coating protocols for repeated, reliable fabrication of such surfaces with defined and controlled surface parameters are therefore indispensable. This is particularly mandatory when the surfaces are intended to be used in in vitro cell culture experiments that commonly require several experiment repetitions in order to allow statistically significant results. Although published monolayer coatings on model substrates are often thoroughly characterized, the reproducibility of their fabrication by a reliable coating procedure is rarely demonstrated.3, 4 Grafting-to and grafting-from methods are commonly applied to generate polymer brush coatings.5-8 Planar, smooth surfaces such as silicon or gold surfaces thereby offer plenty of methods for the physical characterization of brush coatings, e.g., layer thickness, hydrophilicity, grafting density, and degree of chain overlap. An advantage of the grafting-to over the grafting-


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from approach is the possibility to fully characterize the polymers that are intended to be tethered to the surface with respect to molecular weight, chemical composition, polydispersity, and a surface-reactive anchor group prior to the grafting process.7 In contrast, when applying graftingfrom approaches, e.g., surface-initiated atom transfer radical polymerization (SI-ATRP), the molecular weight and polydispersity of the tethered polymers can only be indirectly investigated by assuming similar polymerization kinetics on the surface and in solution. Alternatively, cleavable surface-bound initiators can be applied that allow the characterization of the generated, surface-tethered polymers after surface cleavage, e.g., via hydrolysis.7 Grafting-from approaches generally yield polymer brushes of higher grafting densities, especially for sterically-demanding high molecular weight chains compared to brushes prepared by grafting-to methods.6 The resulting polymer-grafting density, however, is often only approximated by the preassembled initiator density on the surface. This results in rather vague chain density assumptions for surfaces coated via grafting-from methods, which hardly allow a comparison with other coatings generated by a different approach or originating from another study.7 Nevertheless, correlations of polymer brush-coated surfaces with approximated grafting densities with their biological response, e.g., protein adsorption or cell interaction, are commonly found in the literature.9-12 Bioactive coatings of thermoresponsive polymers, for instance, are known to trigger protein adsorption as well as cell adhesion and detachment depending on the applied temperature.12-17 This is due to the phase transition of thermoresponsive polymers from a well hydrated, swollen state to a less hydrated, collapsed state in aqueous solutions or when tethered to surfaces. The phase transition is induced by an increase in temperature from below to above the polymer’s cloud point temperature.18, 19 Poly(N-isopropyl acryl amide) (PNIPAM) is the most intensively studied thermoresponsive polymer, which is investigated in both solution and on


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surfaces and has a lower critical solution temperature (LCST) of 32 °C in aqueous solutions.18, 20, 21

An alternative to PNIPAM are thermoresponsive poly(glycidyl ether) copolymers of glycidyl

methyl ether (GME) and ethyl glycidyl ether (EGE).22-24 The cloud point temperature of these copolymers in solution can be adjusted from 15 °C to 60 °C by the comonomer ratio, the polymer concentration, and the molecular weight of the copolymer.23,

25

Reliable coating

procedures and a thorough characterization are needed to investigate these thermoresponsive polymer coatings of various molecular weights and with different grafting densities for their physical properties or interactions with biological entities. Herein we report our work on the fabrication of thermoresponsive polymer-coated gold surfaces via a grafting-to approach. We compared different grafting-to conditions and established reproducible coating procedures, which allowed us to generate polymer brushes with adjustable grafting densities by varying the surface-reactive anchor group, the molecular weight of the polymer, and the grafting conditions. Quartz crystal microbalance with dissipation (QCM-D) was used to investigate the coating process in real-time and deduce the mass of the polymeric coating and the solvated “wet” layer thickness. The resulting polymeric coatings were further characterized by water contact angle measurements in order to determine the hydrophilicity and by ellipsometry in order to determine the “dry” layer thickness of the coatings. The degree of solvation of the coatings was deduced and the grafting densities and anchor distances of the endtethered polymer chains were calculated from the “dry” and “wet” layer thicknesses. The degree of chain overlap was estimated from the ratio of the polymers’ radius of gyration to the respective anchor distance (2 Rg/l). From this ratio the conformation of the tethered polymers can be derived, assuming there was a homogenous distribution of the tethered chains on the surfaces. Different polymers and polymers of various molecular weights can then be compared regarding


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their conformation on the surface within the brush-, mushroom- or pancake-like regime, respectively, because the degree of chain overlap is a normalized parameter.

EXPERIMENTAL SECTION All materials, analytical methods as well as full characterization of the final polymers and intermediates are given in the Supporting Information (SI).

Polymer Synthesis and Post-modification. Polymer synthesis via a non-activated or monomeractivated ring-opening polymerization (ROP) and characterization of the resulting polymers has been previously described.24 Briefly, comonomer ratios of GME to EGE were adjusted to 1:3 for all copolymers. Low (L) molecular weight (2-3 kDa) copolymers with 11-benzylmercapto-1undecanolate or methanolate as initiator, respectively, were prepared by a non-activated anionic ROP, followed by deprotection of the thiol group as published previously.22, 26 Medium (M) and high (H) molecular weight copolymers (9 kDa, 24 kDa) were synthesized by a monomeractivated anionic ROP with bromide or azide initiators and triisobutylaluminum as an activator.24,

27, 28

After the polymerization, terminal bromide, azide or hydroxyl groups of the

polymer were transformed to amine groups that were suitable for surface-reactive anchor attachment as described in detail in the Supporting Information.

General Procedure 1 (GP1). Polymer end-group modification to azides. Azidation29 of the terminal bromide group of polymers was performed by dissolving the respective copolymer (1 eq.) and sodium azide (10 eq.) in dimethylformamide (DMF). The reaction mixture was heated to 60 °C and stirred for three days. DMF was removed by high


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vacuum (HV) distillation at 30 °C, the crude product was dissolved in diethyl ether (Et2O) in order to precipitate residual salts. The polymer solution was centrifuged, decanted, concentrated at 40 °C, and dialyzed in methanol for further purification. After concentration of the purified product under reduced pressure and drying in HV, the product was obtained as a clear, viscous liquid. Successful end-group modification with an azide group was proven via infrared (IR) spectroscopy.

General Procedure 2 (GP2). Polymer end-group modification to amines. For reduction of a terminal azide group30 of the polymer, the respective copolymer (1 eq.) was dissolved in tetrahydrofuran (THF) with 0.5% water followed by addition of triphenylphosphine (PPh3, 5 eq.). After stirring the reaction mixture for three days at room temperature, water was added until the mixture turned turbid. THF was removed under reduced pressure; the remaining aqueous solution was cooled to 4 °C and centrifuged in order to precipitate and remove major amounts of triphenylphosphine oxide. Sodium hydrogen carbonate was added to the separated supernatant in order to keep the obtained amines deprotonated. The polymer was extracted from the aqueous solution with Et2O (3x) in order to purify it from inorganic salts. The purified product was concentrated at 40 °C and dried in HV in order to obtain a clear viscous oil. Copolymers with terminal amine groups were used for further attachment of surface reactive anchor groups and are indicated by the annotation NH2 after the respective polymer number.

Surface Reactive Anchor Attachment. Terminal amine group-bearing copolymers were further modified with Traut’s reagent (2-iminothiolane·HCl) or thioctic acid, respectively, in order to introduce sulfur-containing anchoring groups.


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General procedure 3 (GP 3): functionalization with Traut’s reagent. Terminal amine group-bearing polymers were dissolved in the appropriate solvent and the solution was degassed by three freeze-thaw cycles. Traut’s reagent (1.2-2.5 eq.) was added and the reaction mixture was stirred for varying time periods (1 to 21 h) depending on the used solvent. Individual work-up procedures were conducted depending on the applied solvent. More detailed information can be found in the SI. Traut’s functionalized polymers are indicated by the annotation C4SH.

General procedure 4 (GP 4): functionalization with thioctic acid. Thioctic acid-NHS ester (2 eq.) and the amine-terminated polymer (1 eq.) were dissolved separately in dry dichloromethane (DCM) (1.5 mL and 2 mL, respectively). Both solutions were mixed together under inert gas atmosphere and stirred for 4 h at room temperature. The product was purified by dialysis in DCM, concentrated under reduced pressure, and dried in HV to yield a clear, viscous oil. The thioctic acid-functionalized polymer is annotated with the anchor abbreviation C8SS-.

Gold Surface Modification with a Thermoresponsive Monolayer and its Characterization. Cleaning procedure for gold-coated QCM-D sensors QCM-D sensors were ozone-treated for 5 min in an UV/Ozone ProCleaner from BioForce Nanosciences, Inc. (Ames, USA). Further chemical cleaning was accomplished by immersing the gold-coated sensors in a mixture of Milli-Q water (MQ), ammonia, and hydrogen peroxide (ratio 5:1:1) at 80 °C for 5 min and rinsing with MQ and ethanol, followed by drying in a stream of nitrogen and a subsequent 5 min treatment in the UV/Ozone oven. The cleaning of the gold


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surface was performed in order to remove adsorbed organic residues which would have affected the self-assembly of the monolayer.

Online Monitoring of the Coating Process by Quartz Crystal Microbalance Measurements The gold surface modification was performed online in a QCM-D in order to distinguish the different coating approaches with respect to the deposited mass. The deposited, solvated polymer mass of sulfur-functionalized polymers assembled on gold surfaces was determined by monitoring the change in resonance frequency (∆f) and dissipation (∆D) of a piezoelectric quartz crystal over time (Fig. S1).31 Changes in the fundamental frequency (4.95 MHz) and in overtones 3 to 13 were measured. For calculation of the layer thickness, the Voigt model was chosen as it is valid for viscoelastic systems.32 The viscoelasticity of a system is indicated by non-overlapping harmonics, dissipation values larger than zero and ∆D larger than 5% of ∆f.33 Calculations were conducted using the software package QTools® considering the 3rd, 5th, 7th, 9th, and 11th overtones. Further prerequisites for applying the Voigt model are a laterally homogeneous and evenly distributed film, a Newtonian bulk fluid, and a non-slippery coupling of the adsorbed layer to the sensor. We applied this simplified model, which assumes a frequency-independent complex shear modulus, because it corrects the estimated adsorbed mass for viscosity losses.34 In order to generate a reliable and comparable baseline, all measurements were started and finished with a flow of ethanol over the sensor surface, respectively. Therefore, the fluid density was set to 789 kg m-3, fluid viscosity to 0.0012 kg m- 1 s-1, and the layer density was estimated to be 1000 kg m-3. All solvents used during the measurements were degassed for 20 min in an ultrasonic bath. Ethanol was distilled before use and phosphate buffered saline (PBS) tablets were dissolved


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in MQ (1 tablet per 200 mL), the solution was filtered (0.22 µm), and the pH was adjusted to 7.4 adding NaOH (1 M) or HCl (1 M), respectively. Cleaned sensor chips were inserted into the flow chamber and equilibrated under ethanol flow (0.1 mL min-1) until the baseline was constant. The flow rate for all steps within the experiments was set to 0.1 mL min-1, unless stated otherwise. Four different online coating procedures were performed according to the following protocols: Grafting from ethanol (1): (i) ethanol (0.2 mL), (ii) 0.5 mM solution of polymer with sulfurcontaining anchor group in ethanol (0.8 mL), (iii) flow stopped (180 min), and (iv) ethanol (2.5 mL). Grafting from PBS buffer at 20 °C and 32 °C, respectively, (2) and (3): (i) ethanol (0.2 mL), (ii) PBS (0.5 mL), (iii) 0.5 mM solution of polymer with a sulfur-containing anchor group in PBS (0.8 mL), (iv) flow stopped (180 min), (v) PBS (0.5 mL), and (vi) ethanol (1.5 mL). Three consecutive grafting steps from PBS buffer at 32 °C for 1h each, with ethanol washing steps in between, that were abbreviated as cloud point grafting (CPG) in the following (4): (i) ethanol (0.2 mL), (ii) PBS (0.5 mL), (iii) 0.5 mM solution of polymer with sulfurcontaining anchor group in PBS (0.8 mL), (iv) flow stopped (60 min), (v) ethanol (till constant), (vi) PBS (0.5 mL), (vii) 0.5 mM solution of polymer with sulfur-containing anchor group in PBS (0.8 mL), (viii) flow stopped (60 min), (ix) ethanol (till constant), (x) PBS (0.5 mL), (xi) 0.5 mM solution of polymer with sulfur-containing anchor group in PBS (0.8 mL), (xii) flow stopped (60 min), (xiii) PBS (1 mL), and (xiv) ethanol (till constant). The volume of ethanol varied from 1.5 mL to 6 mL to reach a constant ∆f value after static grafting from PBS. Polymers 5 (C4SH, H) and 6 (C4SH; PNIPAM) were used at lower (0.1 mM and 0.25 mM, respectively) but there was still sufficient concentration to form a dense monolayer on the gold surface.


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Contact Angle Measurements The sessile drop method was applied to determine the static water contact angle on cleaned and polymer-coated gold surfaces at room temperature. Therefore, a drop of MQ (2 µL) was placed onto the respective surface and a photo was taken. Contact angles were determined with an ellipse-fitting model. For each substrate, contact angles were measured on three different spots to determine the homogeneity of the coating and at least three independent substrates (n = 3) were investigated to determine reproducibility. Mean contact angles of each substrate were averaged.

Ellipsometry Measurements to Determine Dry Thickness of Thermoresponsive Coatings Dry layer thicknesses of polymer coatings were determined by multi-angle spectroscopic ellipsometry at 50° and 70°. Parameters of the cleaned gold-coated QCM-D sensors were determined and taken as fixed values for the following polymer layer thickness modeling. After online coating in the QCM-D flow chamber, the layer thickness was measured at wavelengths from 370 nm to 1050 nm and fitted using a model consisting of the previously measured gold layer with fixed parameters. There was a Cauchy layer with fixed refractive index of n=1.495 and air as the surrounding medium.

Statistical Evaluation Graphical illustration of data and statistical analysis was performed with OriginPro® and Excel®. For all the experiments that were operated with small numbers of independent samples (n = 3), all data points are presented together with their respective mean values and their 90% confidence interval (CI). In some cases, numeric values are presented within the graphs that specify the


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mean value together with the standard error of the mean (SEM). As no distribution assumption can be made due to small sample size, no assumptions were made about the significance levels.

RESULTS AND DISCUSSION Synthesis,

Functionalization,

and

Characterization

of

Linear,

Thermoresponsive

Poly(glycidyl ether)s and PNIPAM. Thermoresponsive poly(glycidyl ether)s with low (L, 23 kDa), medium (M, 9 kDa) and high (H, 24 kDa) molecular weight, and three different sulfurcontaining anchoring groups were synthesized according to Scheme 1 in order to investigate monolayer coatings on gold surfaces. Non-activated anionic ROP of GME and EGE was applied to prepare low molecular weight copolymers.22,

23, 25

Medium and high molecular weight

copolymers were easily accessible via monomer-activated anionic ROP at low temperature.24, 27, 28


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Scheme 1. (a) Synthesis of linear thermoresponsive poly(GMEx-ran-EGEy) via anionic ringopening polymerization. (b) Polymers with sulfur-containing anchor groups C11SH, C4SH, and C8SS- after deprotection or post-modification for surface immobilization.

Polymers obtained via a non-activated polymerization that was initiated with potassium 11benzylmercapto-1-undecanolate directly exhibited a surface-reactive anchor group (C11SH) after the benzyl group was removed for subsequent monolayer formation. In contrast, polymers initiated with either methanolate, bromide or azide required post-modification of the respective end group (OH, Br or N3) to generate a reactive terminal amine group which was then used to attach either 2-iminothiolane (C4SH) or thioctic acid (C8SS-) as a sulfur-containing surfacereactive anchor group. In order to generate terminal amine groups, the respective bromides or mesylates, the latter resulting from activation of a terminal hydroxyl group, were converted into azides, which were further reduced to amines. Both, the successful conversion into azides and


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their reduction to amines were proven via IR spectroscopy, tracking the presence and absence of an azide peak at 2100 cm-1. Further modification with sulfur-containing anchor groups was proven by 1H and

13

C NMR spectroscopy. However, the 1H NMR spectra of 2-iminothiolane-

modified polymers showed broad peaks and poorly resolved signals of the anchor group, which was attributed to the low solubility of this sulfur-containing anchor in organic deuterated solvents. In order to further confirm the successful attachment of 2-iminothiolane to the polymers, we performed QCM-D measurements to provide an additional indirect proof (Fig. S2) as well as MALDI-TOF-MS to directly proof the end-group modification (Fig. S3). In QCM-D the online monitoring of the assembly of polymer 4 (C4SH, M) and its amine-terminated precursor 4b (NH2, M) without surface-reactive anchor group as a control revealed that stable polymer attachment on gold substrates was accomplished only in the presence of sulfurcontaining anchors. From the fact that we were able to obtain stable coatings with reproducible layer thicknesses, we could indirectly prove that the anchor unit was covalently attached to the amine-terminated polymer. In addition, MALDI-TOF-MS measurements directly revealed the presence of the sulfur-containing anchor in polymer 3 (C4SH, L) (Fig. S3). Unfortunately, polyglycidyl ethers of higher molecular weights could not be transferred into the gas phase and thus were not detected. In Table 1 specifications of the synthesized random copolymers with sulfur-containing anchor groups are summarized. For all p(GME-ran-EGE)s the obtained comonomer ratios matched the monomer feed ratio adjusted to 1:3 which is known to yield a phase transition temperature in aqueous solution between 16 °C and 36 °C depending on the molecular weight and concentration.23,

24

Molecular weights were close to the theoretically

expected values of 2 or 3 kDa, 10 kDa, and 25 kDa, respectively, and polydispersity indices (PDI)s were narrow and ranged from 1.05 to 1.23.


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Table 1. Specification of thermoresponsive poly(glycidyl ether) copolymers and PNIPAM with surface-reactive anchor groups.

Formula

Polymer

Mn (g mol-1)a

PDIa

GME:EGEb

p(GME6-ran-EGE20) OC11H22SH

1 (C11SH)

2500 (L)

1.16

1:3.5

p(GME7-ran-EGE22) NH(O)C8H13SS-

2 (C8SS-)

3000 (L)

1.23

1:3.1

p(GME5-ran-EGE14) NHC(NH)C3H6SH

3 (C4SH)

2000 (L)

1.23

1:3.1


4 (C4SH)

9000 (M)

1.23

1:3.2


5 (C4SH)

24,000 (H)

1.05

1:2.9

PNIPAM NHC(NH)C3H6SHc

6 (C4SH)

2500 (L)

Poly(glycidyl ether)-Based Monolayers on Gold Surfaces: Control of

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