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Two-dimensional Plasmonic Nanoparticle as a Nanoscale Sensor to Probe Polymer Brush Formation Assad Ullah Khan, Clayton Scruggs, David Hicks, and Guoliang Liu Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 20, 2017

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Analytical Chemistry

Two-dimensional Plasmonic Nanoparticle as a Nanoscale Sensor to Probe Polymer Brush Formation Assad U. Khan,† Clayton Scruggs,† David Hicks,‡ and Guoliang Liu†, §, * †

Department of Chemistry, ‡Materials Science and Engineering and §Macromolecules Innovation Institute, Virginia Tech, 800 West Campus Drive, Blacksburg, Virginia 24061, United States Corresponding Author: Tel. +1 540 231 8241. Email: [email protected].

ABSTRACT: Conventional analysis and characterization of polymer brush formation relies on laborious methods that use quartz crystal microbalance, atomic force microscope, microcantilever, or other tools that measure concentration change of solutions. Herein we develop a simple and easy method that utilizes intrinsically-flat two-dimensional (2D) plasmonic nanoparticles as sensors for unveiling the mechanism of polymer brush formation on surfaces. Via ultraviolet-visible spectroscopy, the plasmonic nanoparticles can be used to determine the amount of polymers near the surface in situ. As the amount of polymers increases near the surface, the nanoparticle characteristic localized surface plasmon resonance wavelength redshifts, and the shift amount corresponds linearly to the polymer density near the surface. By functionalizing the nanoparticles in solutions of thiolated polyethylene glycol (PEG-SH) with or without PEG disulfide (PEG-S-S-PEG), the three-regime kinetics of the polymer brush formation is confirmed. The fast adsorption and slow chain rearrangement in the first regime are found to be the causes of the latent regime. In the latent regime, the adsorbed polymer chains rearrange to anchor their ends onto the surface and contract to liberate space so that other polymer chains can graft onto the surface until saturation. The fundamental understanding gained herein enables the design of surfaces with complex chemistries and properties, which can find broad applications in responsive sensors, films, and coatings. Moreover, the novel analytical method of using 2D plasmonic nanoparticle as a sensor to understand the polymer brush formation is applicable to investigating the grafting of other molecules such as self-assembled monolayers, protein, and DNA.

Despite the significant advancement of preparing polymer brushes for a wide range of applications, a fundamental understanding of the polymer brush formation mechanism has been stagnant over the past decades due to lack of appropriate analytical methods to investigate the brush formation in situ.1-8 Since the first reports by Alexander1 and de Gennes,2 polymer brushes have been applied to a multitude of areas including surface patterning,9-11 responsive materials design,12-15 sensing,16 anti-corrosion,17 anti-fouling,18,19 polymer/nanoparticle self/directed-assembly,20-28 stabilization,29,30 lubrication,8,31-33 compatibilization of polymer composites,34-37 and modification of biological interfaces.38-40 Polymer brushes are densely tethered polymer chains with one end attached to a surface.2,41 To design polymer brushes effectively, it is crucial to understand the mechanism of polymer brush formation on surfaces. In the “graft-to” process, polymer chains first adsorb onto a surface and have a mushroom or pancake-like shape. At this stage, the distance between every two polymer chains is greater than twice the Flory radius. As the chain density increases, the mushroom-like polymer chains stretch perpendicularly to the surface and form a dense polymer brush layer, where the distance between every two polymer chains is less than twice the Flory radius.2,42,43 Recent advancement of understanding the transition from mushroom to brush shows that the polymer brush formation follows three-regime kinetics.3,4 Polymer chains quickly form a mushroom layer on a surface in the first regime (regime I), followed by a latent regime in which few polymer chains are grafted onto the surface (regime II). With

time, the grafting resumes and more polymer chains are tethered onto the surface until saturation, which is termed the third regime (regime III). Despite numerous efforts on investigating the three-regime kinetics, the cause of the latent regime and what happens in the latent regime remain challenging fundamental questions. Answering these fundamental questions demands new analytical methods for investigating the polymer brush formation in situ. Most previous investigations have used ex-situ analytical techniques such as atomic force microscope (AFM),4 concentration-change measurement in solutions,3,4,44 and microcantilevers42 to investigate the kinetics of polymer brush formation. While AFM can be used to image polymer chains on a surface at various stages of functionalization, it cannot provide realtime polymer conformation changes during functionalization. In addition, the polymer chain morphologies imaged by AFM under dried conditions differ from that in solutions because of the evaporation of solvents. Other ex-situ analytical techniques cannot probe the latent regime with a high precision due to insufficient data points. Liu et al. for the first time used quartz crystal microbalance with dissipation (QCM-D) to reveal the three-regime kinetics in real time,45 which was later confirmed by Penn and coworkers.5 QCM-D, which measures the changes in vibrational frequency as a function of mass, can analyse the total weight of polymer molecules on a surface. However, it cannot measure the density of polymer molecules or the polymer volume fraction near the surface, not to mention the complicated operational procedure that hinders precise evalua1

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tion of the kinetics. More importantly, none of these techniques can probe what happens in the latent regime or the cause of the latent regime. Plasmonic nanoparticles, whose localized surface plasmon resonance (LSPR) wavelength, λLSPR, strongly depends on the refractive index of the surrounding medium, have been used to detect zeptomole concentrations of molecules around single nanoparticles.46,47 λLSPR redshifts to longer wavelengths as the refractive index increases.48 The refractive index, which is correlated to the polymer concentration near the nanoparticles following the Lorentz-Lorenz relationship, can reveal the polymer density and volume fraction on the nanoparticle surface (Figure S1).49,50 Based on the previous reports,51,52 plasmonic nanoparticles are insensitive to the surroundings far away from the nanoparticles but can detect changes within a short distance from the nanoparticle surface. The distance is in the order of a few nanometers, and according to the previous studies on the fluorophore quenching of Au nanoparticles, the distance should be no more than 20 nm52 or 30 nm.51 In this work, the physisorbed or chemically bonded polymer chains are in close proximity to the Ag nanoparticles, and thus the kinetics of polymer brush formation can be revealed.

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orously stirred throughout the synthesis process. The assynthesized Ag nanoparticles were prismatic in shape, but they were aged in the dark for 2–3 months to convert the shape into nanodisks (Figure S2), which are termed 2D Ag nanoparticles. Functionalization of 2D Ag nanoparticles and UV-visible Spectroscopy. To functionalize 2D Ag nanoparticles, aqueous solutions of PEG-SH with or without PEG-S-S-PEG at various concentrations (0.40, 0.30, 0.25, 0.20, 0.175, 0.12, 0.125, 0.10, 0.08, 0.075, and 0.05 wt. %) were prepared and then mixed with colloidal suspensions of 2D Ag nanoparticles. For example, to functionalize 2D Ag nanoparticles at a polymer concentration of 0.10 wt.%, a polymer solution at a concentration of 0.20 wt.% was added to a suspension of Ag nanoparticles at a volume ratio of 1:1. Upon mixing, the extinction spectra of the combined solutions were immediately measured using UV-Vis spectroscopy (Agilent Cary 60 UV-Vis or Cary 5000 UV-Vis-NIR spectrophotometer) to in situ monitor the functionalization of 2D Ag nanoparticles. The extinction spectra were collected every 2 min in the first 3 h and every 15 min in the next 21 h. Afterwards, the interval was kept at 30 min in the case where more extinction spectra were collected. The peak positions (λLSPR) were determined by identifying the maximum of a second order polynomial fitting to the spectra. Transmission Electron Microscopy (TEM). TEM samples were prepared by drop-casting aliquots (5–10 µL) of Ag nanoparticles colloidal suspension on copper grids and drying at room temperature for 4 h. TEM images were obtained on a Philips EM420 at an electron accelerating voltage of 120 keV.

Herein we take advantage of the sensitivity of plasmonic nanoparticles and use them as sensors to probe the kinetics of polymer brush formation. Among many nanoparticle candidates, we choose 2D Ag nanoparticles for the following reasons: (i) they are highly sensitive to minute changes in refractive index as shown in our previous report53 and (ii) the top and bottom surfaces of 2D Ag nanoparticles are atomically flat54 so that we can investigate the polymer brush formation on absolutely flat surfaces. Specifically, we use thiolated polyethylene glycol (PEG-SH) as a model material to functionalize 2D Ag nanoparticles and employ ultraviolet-visible (UV-Vis) spectroscopy to monitor the shift of λLSPR in real time. EEXPERIMENTAL SECTION Materials. Poly(ethylene glycol) methyl ether thiol (PEGSH, average Mn ̴ 6 kDa, Mw/Mn < 1.2) (729159), silver nitrate (≥99.9999%) (204390), sodium borohydride (≥99.99%) (480886), sodium citrate tribasic dihydrate (≥99.0%) (S4641), poly(sodium 4-styrenesulfonate) (PSSS) (average Mw ̴ 1,000 kg mol-1) (434574), and ascorbic acid (≥99.0%) (A5960) were purchased from Sigma-Aldrich and used as received. Nanoparticle synthesis and polymer brush functionalization were carried out in ultrapure deionized (DI) water obtained from Thermo ScientificTM BarnsteadTM GenPureTM Pro water purification system at a resistance above 17.60 MΩ-cm. Synthesis and aging of 2D Ag nanoparticles. Ag nanoparticles were prepared using the seed-mediated synthesis method as reported previously.53,55 To prepare the seeds, 0.25 mL of 0.5 mg/mL concentrated PSSS was mixed with 5 mL of 2.5 mM sodium citrate and 0.3 mL of 10 mM ice-cold NaBH4. Afterwards, 5 mL of 0.5 mM AgNO3 was added to the mixture at a rate of 2 mL/min using a syringe pump. All procedures were performed while the solution was stirred vigorously. The seed solution was kept stirring at room temperature for 2 h. To synthesize Ag nanoprisms, 3 mL of ascorbic acid was added to 508 mL of ultra-pure water followed by 0.8 mL of seeds. Afterwards, 12 mL of 5 mM AgNO3 was added to the growth solution at a rate of 4 mL/min. The color of the growth solution changed indicating that the small seeds grew into larger Ag nanoprisms. Finally, 20 mL of 25 mM sodium citrate was added to stabilize the Ag nanoprisms. The solutions were vig-

Size Exclusion Chromatography (SEC). Water Breeze 1515 HPLC Pump Aqueous SEC with a miniDAWN Treos light scattering detector and Waters 2414 differential refractive index detector was used to characterize the polymer after partial oxidation of PEG-SH into PEG-S-S-PEG. The operational temperature was 30 °C and flow rate was 1.0 mL/min using Agilent PL Aquagel-OH 30 and PL Aquagel-OH 40 columns in series. RESULTS AND DISCUSSION Based on the theoretical work in the literature,3,4 we propose a mechanism for PEG brush formation on 2D Ag nanoparticle surfaces and aim to disclose the underlying fundamentals in the three regimes (Figure 1a). Water is used as the solvent. The polymer chains are at a semi-collapsed state because water is a poor solvent for PEG and the short-range intrachain interactions are more prominent than the long-range interchain interactions.56,57 First, polymer chains are adsorbed onto the Ag nanoparticle surfaces and adopt a mushroom shape (regime I: ①→②). Once the polymer chains are adsorbed, the surroundings near the nanoparticles change immediately and λLSPR should redshift to a longer wavelength than the original wavelength. Afterwards, the polymer chains gradually relax and extend so that the thiol end groups can chemically bind onto the Ag nanoparticle surfaces (regime II: ②→③). During this regime, the mushroom-like polymer chains rearrange and form extended mushrooms so that there is sufficient space for other polymer chains to reach the nanoparticle surface. The shifting rate of λLSPR should slow down in this regime. As the mushrooms extend and more space is created, more polymer chains can reach the surface and the polymer grafting resumes (regime III: ③→④). The functionalization of Ag nanoparti2


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cles continues until saturation, and similarly, λLSPR redshifts continuously until saturation. To better reveal what happens in regime II, we deliberately introduce PEG-S-S-PEG into the PEG-SH solutions (Figure 1b). The disulfide groups are embedded in the middle of PEG-S-S-PEG polymer chains and cannot functionalize Ag nanoparticles easily. Therefore, in regime II we expect the PEG-S-S-PEG chains to come off the Ag nanoparticle surfaces after initial adsorption. As a result, there is a short time period in which PEG-S-S-PEG will be replaced by PEG-SH, and the polymer density near the surface decreases, inducing λLSPR to blueshift. Overall, based on the plasmonic properties of Ag nanoparticle, λLSPR should redshift, blueshift, and then redshift to saturation.

ticles were then functionalized with polymer brushes. The shape of Ag nanoparticles did not change significantly after functionalization (Figure 2). The polymer brush cannot be distinguished under transmission electron microscope (TEM), suggesting that it is difficult to use TEM to monitor the polymer brush formation on Ag nanoparticles.

Figure 2. TEM images of 2D Ag nanoparticles a) before and b) after functionalization with PEG-SH.

Since TEM cannot directly monitor the polymer brush formation on nanoparticles, we instead take advantage of the plasmonic properties of 2D Ag nanoparticles to probe the polymer brush formation. The localized surface plasmon resonance wavelength of plasmonic nanoparticles depends strongly on the refractive index of the surroundings, which is determined by the concentrations of polymers and solvents following the Lorentz-Lorenz equation.53 Therefore, λLSPR can indicate the polymer concentration near the nanoparticle surface during the functionalization process. PEG has a higher refractive index than water (nPEG = 1.46 and nwater = 1.33).59 When the Ag nanoparticles were functionalized with PEG-SH at 0.10 wt.%, λLSPR redshifted as shown in Figure 3a. Note that the redshift amount of λLSPR was more than that of nanoparticles immersed in a solution of PEG without thiol end groups at the same concentration of 0.10 wt.% (Figure S1). This phenomenon suggests that PEG-SH chains were adsorbed onto the nanoparticle surfaces and the local polymer concentration near the nanoparticles was significantly higher than that in the solution. The decreasing intensity of the extinction peaks is attributed to the development of metal core and dielectric shell structures that partially inactivate the Ag nanoparticle surface plasmon.60-62 As suggested in the previous reports,3-5 the tethering of polymer chains follows a log(time) relation. The plot of λLSPR as a function of log(time) confirms the three-regime kinetics for PEG brush formation on 2D Ag nanoparticles (Figure 3c), which is similar to the polymer brush formation on macroscopic surfaces in the previous reports.3-5,45,63 To evaluate the change in polymer brush grafting rate in the three regimes, the linear-log plot of λLSPR versus time was fitted by straight lines to show the rate in each regime (Figure 3c inset). The slope decreased from regime I to regime II. In regime I, the polymer chains were quickly adsorbed onto the Ag nanoparticle surfaces. In regime II, the rate of polymer adsorption decreased due to a decreasing amount of available sites on the Ag nanoparticles. The polymer chains started to rearrange themselves so that the thiol end groups could find a way to be grafted onto the nanoparticle surface. Once all the adsorbed polymer chains were rearranged and straightened up to a sufficient level, more polymer chains penetrated the initial layer and reached the nanoparticle surface, inducing the slope to increase again. At a higher polymer concentration of 0.30

Figure 1. Schematic illustration of using 2D Ag plasmonic nanoparticles as sensors to in situ monitor the polymer brush formation and the expected λLSPR evolution profiles. The 2D Ag nanoparticles are functionalized with a) PEG-SH and b) PEG-SH/PEG-S-S-PEG mixtures. The round disks represent Ag nanoparticles. The black and red chains are PEG-SH and PEG-S-S-PEG, respectively. ①, ②, ③, and ④ denote different functionalization stages.

To test the proposed mechanism, we have synthesized 2D Ag nanoparticles. The as-synthesized Ag nanoparticles had sharp-tips and were mostly prismatic in shape (Figure S2). The sharp tips, however, were susceptible to chemical etching due to a high surface energy.58 As a result, the Ag nanoparticles became rounded with time and their localized surface plasmon resonance wavelength (λLSPR) gradually blueshifted. To avoid the interference between the shift of λLSPR caused by nanoparticle shape change and that caused by nanoparticle functionalization, we allowed the Ag nanoparticles to stabilize for at least one month and up to three months. After stabilization, the nanoparticle shape did not change any more and λLSPR no longer blueshifted (Figure S3). The stabilized Ag nanopar3



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have a high diffusion rate following the Fick’s first law64 and can penetrate the initial layer during chain rearrangement, and therefore suppress the latent regime.

wt.%, however, the regime II was quenched and only the regimes I and III were apparent (Figure 3b and 3d). This is because, at high polymer concentrations, the polymer chains

Figure 3. In situ monitoring of the polymer brush formation on 2D Ag plasmonic nanoparticles. (a, b) Evolution of the UV-Vis extinction spectra of Ag nanoparticles during functionalization with PEG-SH. The polymer brush was formed by immersing Ag nanoparticles in solutions of PEG-SH at concentrations of 0.10 and 0.30 wt.%. (c, d) The corresponding λLPSR as a function of time for Ag nanoparticles functionalized with PEG-SH. (e, f) λLSPR as a function of time for Ag nanoparticles functionalized with PEG-SH containing PEG-S-S-PEG at a total polymer concentration of (e) 0.15 and (f) 0.40 wt.%. (Insets) Fitted straight lines are used to determine the slopes in the first (kI), second (kII) and third (kIII) regimes.

Interestingly, when PEG-S-S-PEG was added to the polymer solutions, the behavior of the regime II changed drastically. The affinity for Ag enables the initial physical adsorption of PEG-S-S-PEG on the Ag nanoparticles, however, the disulfide groups are embedded in the center of the semi-collapsed polymer chains, and they cannot react with the Ag nanoparticles to form chemical bonds. The inability of PEG disulfide to functionalize Ag nanoparticles was confirmed by immersing Ag nanoparticles in pure PEG-S-S-PEG solutions, which did not show any shift of λLSPR after the initial adsorption of poly-

mer chains on the nanoparticles (Figure S4). PEG-S-S-PEG was introduced to the solutions by partial oxidization of PEGSH to PEG-S-S-PEG. When a PEG-SH solution was left under ambient conditions for weeks, some thiol groups were oxidized to disulfides and the solution contained PEG-S-S-PEG, while the rest of the polymers in the solution were still PEGSH. The amount of PEG-S-S-PEG was estimated to be ~32 %, as analyzed by SEC (Figure S5). When the Ag nanoparticles were functionalized with mixtures of PEG-SH and PEG-S-SPEG at a total concentration of 0.15 wt.%, it appeared that 4


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there was a small dip of λLSPR in regime II, that is, λLSPR blueshifted slightly and redshifted afterwards (Figure 3e). The blueshift of λLSPR differed significantly from the functionalization of Ag nanoparticles with pure PEG-SH. At high polymer concentrations, regime II was quenched, similar to the functionalization of Ag nanoparticles in pure PEG-SH (Figure 3f). It should be noted that we have investigated a wide range of polymer concentrations. All functionalization processes showed similar behaviors: at low polymer concentrations, the latent regime could be distinguished while at high polymer concentrations it was quenched (Figures S6 and S7). At significantly lower polymer concentrations, however, λLSPR blueshifted continuously without any redshift (Figure S8), probably due to etching of Ag nanoparticles as reported by Liu et al.65 To further gain insight into the polymer brush formation in regimes I and III, we have calculated the slopes of the fitted lines and analyzed their dependence on the concentration of the polymer solution. When Ag nanoparticles were functionalized with PEG-SH, the slope of regime I (kI) remained at a constant of 13.1 ± 1.4 for all investigated concentrations (Figure 4). The weak dependence of kI on polymer concentration suggests that regime I follows zero-order kinetics because there are excess polymer chains to cover the Ag nanoparticle surfaces and form a mushroom layer. A similar weak dependence of kI on polymer concentration was evident when Ag nanoparticles were functionalized with mixtures of PEG-SH and PEG-S-S-PEG.

controlled process and the diffusion barrier is to stretch the polymer chains so that they can penetrate the initially grafted polymer layer. This contrasts with the grafting in regime I, whereas the polymer chains do not have to be stretched and they can reach the surface easily. Moreover, there is an excess of polymer chains in the solution for adsorption onto the surface. As a result, polymer concentration is irrelevant in regime I. For PEG-SH with PEG disulfide, some PEG-SH molecules were oxidized to disulfides. The effective PEG-SH concentration was lower than that in pure PEG-SH given the same total polymer concentration. As a result, we observed consistently lower kIII values for Ag nanoparticles functionalized in PEGSH/PEG-S-S-PEG mixtures (hexagons in Figure 4) than in pure PEG-SH solutions (triangles in Figure 4). The shift behaviors of λLSPR provide insight into the kinetics of polymer brush formation, which is impossible by other methods. First, our data suggest that the thiol end groups of most polymer chains cannot immediately react with Ag in regime I. The majority of the polymer chains are physisorbed on the surface. The affinity of PEG for Ag allows for the adsorption of the semi-collapsed PEG chains on the Ag nanoparticles.49,56,66 After adsorption, the contact surface area between a polymer chain and the Ag nanoparticle is expected to increase and the polymer chain forms a pancake-like structure to minimize the surface energy. The attachment of the polymer chains on the nanoparticle surface induces a decrease in conformational entropy. The decrease in surface energy, however, is insufficient to compensate the loss of the polymer chain conformational entropy, especially when there is an excess of PEG chains available for further adsorption. As a result, the initially adsorbed polymer chains rearrange so that other polymer chains can reach the surface.66 Penn and coworkers have attributed the adsorption of more polymer chains to spontaneous contraction of the initially adsorbed polymer chains, which provides more space for the stretched polymer chains in the solution to reach the surface.3 Besides, when nanoparticles are functionalized in PEGSH/PEG-S-S-PEG mixtures, the blueshift of λLSPR suggests that the PEG-S-S-PEG molecules depart from the Ag nanoparticle surfaces and create void space, which cause the polymer concentration and the refractive index to decrease near the nanoparticles. The amount of created void space can be estimated by the Flory radius of the polymer chains, RF = αN3/5, in which α is the effective monomer length and N is the degree of polymerization. For PEG-SH and PEG-S-S-PEG, the monomer length is 0.35 nm,67,68 and their Flory radii are estimated to be 6.67 and 10.11 nm, respectively. Therefore, more space is occupied by PEG-S-S-PEG than by PEG-SH after the initial adsorption on the Ag nanoparticles.67,68 When the PEGS-S-PEG chains leave the Ag nanoparticle surfaces, the local refractive index near the nanoparticles decreases significantly and λLSPR blueshifts. In addition, it is highly plausible that even the PEG-SH chains are not fully reacted with Ag in regime I due to steric hindrance. Some polymer chains are simply physisorbed on the Ag nanoparticles and the reaction between the thiol end groups and the Ag nanoparticles are postponed to regime II. Consequently, the polymer chains have to rearrange in regime II so that the thiol end groups can anchor onto the surface. As a result, the polymer brush formation has to go through a latent regime. Second, the slow chain rearrangement and fast chain adsorption are the fundamental causes of the latent regime; what

Figure 4. Slopes of regime I (kI) and regime III (kIII) as a function of polymer concentration.

In contrast, the slope in regime III (kIII) depended strongly on the polymer concentration. As the polymer concentration increased, kIII increased regardless of whether PEG-S-S-PEG was present or not (Figure 4). Penn et al has described regime III as “layer-assisted tethering”.3,5 In the “layer-assisted tethering” process, the tethered chains contract to occupy less surface area so that more polymer chains can reach the surface. Correspondingly, the incoming polymer chains are stretched so that they can penetrate the initially tethered layer and be grafted onto the surface. The stretched polymer chains, however, are only a small fraction of the polymer chains in the solution. The amount is small and depends on the total polymer concentration. As a result, the zero-order kinetics is lost and the grafting rate shows a strong dependence on the polymer concentration. In other words, regime III is a diffusion5



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happen in the latent regime are predominantly polymer chain rearrangement and negligible penetration of the initial layer by incoming polymer chains. Once the surface is fully covered with semi-collapsed polymer chains, few polymer chains can be adsorbed onto the surface and the polymer brush formation enters the latent regime (regime II). In this regime, the amount of polymer chains reaching the surface is negligible; therefore, we see a noticeable drop in the slope. If no polymer chains anchor onto the surface, λLSPR should remain at a constant. Instead we have observed a small shift in λLSPR for nanoparticle immersed in PEG-SH. The slopes of the latent regime, albeit substantially lower than those in regimes I and III, was not zero (Figure 3c) indicating that the functionalization does not stop completely in the latent regime. The extremely slow functionalization is due to the slow chain rearrangement, which provides space for incoming polymer chains. If the chain rearrangement is faster or at least not slower than the adsorption, the polymer chain grafting should be continuous and no latent regime can be observed, regardless of the concentration of the polymer solution. The rearrangement rate is expected to be affected by the polymer molecular weight. With a high molecular weight, the polymer chains take a long time to rearrange from the mushroom state to the fully extended brush state.3,4,69 Another parameter that affects the kinetics is temperature. In this work, all the experiments were performed at room temperature, which is below the Θ– temperature of PEG in water (~330 K),56 and the polymer chains were all in a semi-collapsed state. We expected that at a high temperature, both the solubility of PEG in water and the rearrangement rate of the chains on the Ag nanoparticles surface increase, which can result in fast grafting kinetics. Lastly, at the end of the latent regime, the polymer chains stretch to an extended state and liberate space for incoming polymer chains (Figure 1). Grafting resumes and continues until the whole surface is covered with sufficient polymer chains to reach a saturation point. We distinguish the three regimes by the slopes of the functionalization curves (Figure 3 insets). Although the polymer chains have saturated on the 2D Ag nanoparticle surface, there should be still citrate molecules remaining on the surface based on previous reports.70,71 The ligand exchange—either citrate-to-thiol on Au nanoparticles71 and hexadecyltrimethylammonium bromide (CTAB)-tothiol on Au nanorods70—cannot reach 100%. Similarly, herein we do not expect PEG-SH to fully replace all citrate molecules on the 2D Ag nanoparticles. Since the polymer chains have a much larger size than the citrate molecules, there should be sufficient space near the nanoparticles that allows for the citrate molecules to remain on the surface. Nevertheless, the polymer chain density has reached a thermodynamic equilibrium under the functionalization conditions. CONCLUSIONS In conclusion, we have used 2D Ag nanoparticles as in situ plasmonic sensors to probe the polymer brush formation on surfaces. The initial grafting of polymer chains onto a surface is predominantly physisorption. The physisorption rate is independent of the concentration of the polymer solution, given that there are sufficient polymer molecules available for functionalization. Afterwards, the polymer brush formation goes through a latent regime, in which the physisorbed polymer chains rearrange and react with the surface to form chemical bonds. The chemically bonded polymer chains contract to liberate space for incoming chains. Depending on the concen-

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tration of the polymer solutions, the latent regime may or may not be apparent. The third regime, however, strongly depends on the polymer solution concentration and the rate increases as the concentration increases. Utilizing the unique plasmonic properties of 2D Ag nanoparticles, we can reveal information about the polymer chains near the surfaces, which is otherwise impossible using all previous methods. The approach herein can be extended to other investigations, for example, monitoring functionalization of surfaces with natural polymers such as DNA and proteins.72 Moreover, the insight into the mechanism of polymer brush formation, especially the cause of the latent regime and what happens in the latent regime, enables the design of surfaces with complex chemistries and properties. By selectively functionalizing a surface with different molecules in different regimes, the resulting multi-functional surface can find broad applications in responsive sensors, films, coatings, textiles, and drug delivery vehicles.

ASSOCIATED CONTENT Supporting Information Additional TEM, UV-Vis-NIR characterizations, SEC and other plots are available. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by the Virginia Tech Department of Chemistry and the Virginia Tech ICTAS JFC Award. This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-17-10112. The authors acknowledge use of facilities within Nanoscale Characterization and Fabrication Laboratory (NCFL) and Macromolecular Materials Discovery Center (MMDC) of the Macromolecular Innovation Institute (MII) at Virginia Tech.

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