Deposition Kinetics of Iron Oxide Nanoparticles on a Poly

Publication Date (Web): September 6, 2017. Copyright © 2017 American Chemical Society. *Address: JitKang Lim School of Chemical Engineering, Engineer...
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Deposition Kinetics of Iron Oxide Nanoparticles on a Poly(diallyldimethylammonium Chloride)-Coated Silica Surface: Influences on the Formation of a Softer Particle-Polyelectrolyte Layer Hui Xin Che,† Sim Siong Leong,† Wei Ming Ng,† Abdul Latif Ahmad,† and JitKang Lim*,†,‡ †

School of Chemical Engineering, Universiti Sains Malaysia, Nibong Tebal, Penang 14300, Malaysia Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States

‡

S Supporting Information *

ABSTRACT: Particle-polyelectrolyte layers with an open structure were constructed by using cationic poly(diallyldimethylammonium chloride) (PDDA) as a binder to promote the attachment of iron oxide nanoparticles (IONPs) onto silica colloids. The deposition kinetics of IONPs onto PDDA-coated silica surfaces were monitored by a quartz crystal microbalance with dissipation (QCM-D). Here, our experiments provide clear evidence of the direct influences of deposition kinetics on the structural properties of the formed PDDA/IONP layers. At low IONP deposition rates, “softer” particle-polyelectrolyte layers were formed with a great amount of IONPs attached to the outer compartment of these layers. This unique feature of the polymer−particle system can be further modulated by varying the ionic strength of the background medium, containing both PDDA and IONPs. As the ionic strength of the solution is increased, the PDDA/IONP assembled layers become more flexible, leading to a larger amount of deposited IONPs and reducing the characteristic rate constant of deposition kinetics. The ability to fine-tune the structural property of these layers by adjusting the ionic strength, however, is restricted by the critical coagulation concentration (CCC) of IONPs. To understand this threshold phenomenon, we employed dynamic light scattering (DLS) to determine the CCC value of IONPs as 50 mM. The result was further verified by extended Derjaguin−Landau−Verwey− Overbeek (DLVO) calculation. On the basis of our results, the adjustment of ionic strength below the CCC substantially influences the deposition kinetics of IONPs, where higher ionic strength gives faster deposition kinetics. Additionally, these PDDA/IONP assembled layers become rougher when a higher salt concentration is used, and this result was confirmed by a topography scan using atomic force microscopy (AFM). functionalize IONPs,14 mostly by serving as a coating layer to impart colloidal stability to the nanoparticles.15 By using a simple surface functionalization strategy, such as immobilizing IONPs onto a polyelectrolyte-modified surface, the nanoparticle arrangement on the surface can be altered by changing the conditions of the polyelectrolyte film.15 Here, we have chosen poly(diallyldimethylammonium chloride) (PDDA) as the binder because it adheres strongly to a charged solid surface, mainly through electrostatic interactions.16 Organization and deposition of IONPs onto a polyelectrolyte-coated surface can lead to new interesting properties in the design and synthesis of “soft” one-dimensional nanomaterials. This surface modification strategy further improves the physiochemical properties of polymer nanocomposites in (supra)molecular science.17 Unfortunately, the kinetic aspects of magnetic nanoparticle deposition on a charged surface

1. INTRODUCTION The deposition process of nanoparticles onto a charged surface has been actively investigated for various reasons, most notably, due to its influences on the surface functionalization scheme1 and sensor performance.2 Here, a polyelectrolyte is often employed as a binding agent to promote the fixation of suspended nanoparticles onto a charged surface for catalysis,3 ultrathin ion-selective membranes,4 chemical sensors,5 electrochromic devices,6 separation applications,7 polyelectrolyteprotein multilayered films,8 paper-making,9 and textile industries.10 It has been commonly illustrated that the underlying polyelectrolyte structure greatly influences the collective macroscopic behaviors and properties of the polymer− nanoparticle composite.11 Of all these nanocomposites, iron oxide nanoparticles (IONPs) are one of the popular building blocks chosen for chemical engineering applications due to their catalytic and magnetic properties.12 Combining IONPs with a polyelectrolyte allows for numerous possible constructions of a versatile nanocomposite with multifunctionalities.13 Traditionally, a polyelectrolyte has often been used to © XXXX American Chemical Society

Received: June 1, 2017 Revised: September 2, 2017 Published: September 6, 2017 A

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Figure 1. Schematic illustration of different stages of QCM-D measurements used for investigating the adsorption of IONPs on a PDDA-coated silica surface. Unless otherwise stated, 6 mM PDDA solution and 100 ppm IONPs suspension were used in all experiments. For experiments involving ionic strength variations, the washing media was NaCl solution. At the final stage, the QCM-D sensor with deposited PDDA/IONP layers was removed from the chamber for AFM analysis.

covered by a polyelectrolyte layer are rarely reported. This is mainly due to the level of complexity involved with a system composed of solid spheres deposited onto a soft layer of polyelectrolyte. A true understanding of such a system requires investigating the highly interactive system composed of magnetic nanoparticles, polyelectrolytes, counterions, and a charged surface. Until now, most investigations of such a system mainly focused on the deposition kinetics of nonmagnetic nanomaterials. For instance, Chen and Elimelech used classical DLVO analysis to investigate the deposition kinetics of fullerene nanoparticles deposited onto a poly-Llysine-coated silica surface under the influences of monovalent and divalent salts.18 In a more complex colloidal system, the deposition kinetics of TiO2 nanoparticles onto a glass surface were evaluated in the presence of humic acid and bacteria under the influences of various external factors, such as solution pH and ionic strength.19 However, not much effort has been dedicated to relating deposition kinetics to the nanoparticle− polymer structure formed. Hence, the nature of deposition kinetics in affecting the structural properties of the PDDA/ IONP layers formed is worth investigating. In our study, we also observed that the effect of ionic strength plays an important role in dictating both the kinetics20 and conformation21 of the polyelectrolyte adsorbed onto a charged surface. We investigated the effect of ionic strength on this polyelectrolyte-nanoparticles system, specifically on the viscoelastic property of the final structure. Thus, we believe that the deposition kinetics of nanoparticles play a pivotal role in determining the formation of the nanoparticle−polymer matrix as an “open layer” structure. Here, open layer refers to the more loosely bound and softer structure of the adsorbed nanoparticle−polymer layer. A quartz crystal microbalance with dissipation (QCM-D) allows in situ and real-time monitoring of the adsorbed mass, the adsorption kinetics, and the conformational changes of the adsorbed layers simultaneously.22 For example, Alejo and coworkers used this approach to study the adsorption kinetics and adsorbed amount of CdSe quantum dots on a silica surface precoated with a film of polymer or surfactant.14 Thus, the deposition of IONPs on a dissipative polymeric layer can be monitored in situ by using QCM-D. In this case, data obtained from QCM-D allows the determination of the adlayer structure and deposition kinetics at a planar surface. In the works by Ikuma and co-workers, QCM-D was used to monitor the deposition of nonmagnetic hematite (α-Fe2O3) nanoparticles onto a polysaccharide coated surface. In accordance with their finding, hematite nanoparticles deposited fastest on a dextran sulfate-coated surface, with a deposition rate of approximately 120 ng/cm2 min, mainly due to electrosteric interactions and possible van der Waals attraction.23 More recently, Chen and co-workers monitored the deposition kinetics of graphene oxide on an N-(6-aminohexyl)aminopropyl-aminopropyltrimethoxysilane (AHAPS) decorated silica surface using QCM-D.24

Upon the basis of Gouy−Chapman theory and the additivity of van der Waals interactions, by taking into account the inclined configurations of graphene oxide nanoplatelet interactions with the flat surface, the authors explained the deposition trend of graphene oxide as a function of interaction potential. In complementing QCM-D monitoring, atomic force microscopy (AFM) is often used to provide additional surface topographic information about the polymer−nanoparticle assembled layers on charged surfaces.23,25 The present study investigated the deposition kinetics of IONPs deposited onto charged silica surfaces coated with PDDA by QCM-D. We hypothesized that nanoparticle deposition kinetics are dominant in governing the formation of an open layer structure. Further control of this special feature is possible by adjusting the ionic strength of the nanoparticle suspension prior to the deposition.

2. EXPERIMENTAL SECTION 2.1. Materials. All the reagents used in this work were of analytical grade and used as received without further purification. Sodium hydroxide (NaOH) was bought from Fisher Scientific (M) Sdn. Bhd. Poly(diallyldimethylammonium chloride) (PDDA of 100−200 kg/mol and 20 w% in water) was obtained from Aldrich Chemistry. Iron(III) chloride (FeCl3, 98% pure, anhydrous) and iron(II) chloride (FeCl2·4H2O, 99%) were obtained from Acros Organics. A QCM-D silica quartz crystal was supplied by Q-Sense. Sodium dodecyl sulfate (SDS), approximately 95% based on total alkyl sulfate content, was purchased from Sigma-Aldrich. Deionized water with a resistivity of 18 MΩ cm, supplied by Pure-lab Option-Q, was used for all experiments. 2.2. Synthesis of Fe3O4 Nanoparticles. IONPs were synthesized by coprecipitation of ferrous chloride and ferric chloride as iron precursors in the standard 2:1 mol ratio.16 The reaction was conducted in a nonoxidizing environment. Briefly, the reactants were mixed with 60 mL deionized water and thoroughly degassed by purging with argon gas. After 30 min, the temperature was raised to 80 °C under vigorous magnetic stirring at 500 rpm to allow the coprecipitation process to continue for 30 min.16 At the end of the reaction, the mixture turned blackish, indicating the formation of IONPs. The mixture was cooled down to room temperature, and the black precipitate was collected using a permanent magnet. The synthesized IONPs remained colloidally stable for months without any agglomeration most likely due to their high zeta potential of −30.4 mV (measured by a Zetasizer Nanoseries). It has been suggested that nanoparticles with a zeta potential greater than |30 mV| have no tendency to aggregate due to interparticle electrostatic repulsion.26 A transmission electron microscopy (TEM) image of the resulting particles can be found in Figure S1. The absolute size of individual IONPs (in contrast to the hydrodynamic size) was measured by performing image analysis with ImageJ on 150 particles from TEM micrographs, where the average particle B

DOI: 10.1021/acs.jpcc.7b05358 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C diameter was 11.8 ± 3.59 nm (see Figure S2). The absolute size of the IONPs is reported here, as it is used to estimate the magnetic volume of one IONP, which is needed for our extended DLVO analysis discussed in section 3.3. 2.3. Growth of Polyelectrolyte and IONP Bilayer Films. The silica-coated quartz crystal was cleaned with SDS for 15 min before being rinsed thoroughly with a running stream of deionized water and blow-dried with nitrogen gas. A quartz crystal resonator with a fundamental resonance frequency of 4.95 MHz was then placed in a fluid cell with one side exposed to the solution.27 All the sample solutions were channeled through the surface by a peristaltic pump operating at a constant flow rate of 20 μL/min. All the QCM-D measurements were conducted at 25 ± 0.1 °C. The steps taken in the formation of PDDA/IONP layers during the QCM-D measurement performed with Q-Sense E1 are shown in Figure 1. Initially, background medium used to disperse the adsorbed substrates was injected to develop a stable baseline for QCM-D measurements. Later, a PDDA solution in the same background medium, with a concentration of 6 mM (based on the monomeric unit of 161.5 g/mol) was introduced into the system to promote the formation of a cationic PDDA layer on the silica quartz surface. The QCM-D signal was continuously recorded until the responses reached a plateau, and the inlet was switched back to pure dispersion medium to remove the unbound or loosely bound PDDA before introducing the IONP suspension. By doing so, we completed the adsorption step of the PDDA adlayer. We proceeded to particle deposition by injecting the negatively charged IONP solution into the flow cell. After the deposition of IONPs, the flow chamber was flushed with pure dispersion medium (without particles) again to remove unbound moieties. For the experiment adjusting the ionic strength of the dispersion medium, NaCl solution at different concentrations (0−100 mM) was pumped into the QCM-D system before introduction of the IONP suspension (at the same ionic strength). Here, the signal of the raw data from all measurements was monitored at the third overtone (n = 3). From the changes in frequency and dissipation factors normalized with overtone n = 3, one can obtain information about the mass and conformational change of adsorbed layers.28 The change of resonance frequency is directly proportional to the added mass for flat and rigidly adsorbed films according to the Sauerbrey relationship, given as27 Δm = −

pq tq Δf f0 n

= −CQCM − D

Δf n

information (with a density of 1000 kg/m3 and a viscosity of 0.001 kg/ms) for the Voigt model calculation. Information about the viscoelastic properties of the adsorbed film can be obtained from the dissipation of the crystal oscillation during the QCM-D measurement. The data used for viscoelastic modeling can be found in the Supporting Information. Briefly, by terminating the electric current to the QCM-D cell, the decay in the amplitude of the oscillating crystal is measured as a function of time, which in turn defines the dissipation factor as31

D=

Edissipated 2πEstored

(2)

where Edissipated is the energy dissipated during one oscillation and Estored is the energy stored in the oscillating system. For a rigid adlayer with little to no viscoelastic coupling, the decay time is longer than the measuring time. Thus, no significant change in dissipation has been observed for rigid films. On the other hand, the adsorbed viscoelastic layer is highly dissipative, displaying significant viscoelastic coupling and a considerable decrease in the decay time. Hence, a qualitative measurement of the relative stiffness or conformation of an adsorbed layer can be determined by observing the change in dissipation.30 Following the QCM-D experiment, the used QCM-D crystal with deposited PDDA/IONP layers was further examined with atomic force microscopy (AFM) (Park System XE-100 AFM) operated in noncontact mode using a NSC15 10 M cantilever. This topography study revealed the surface smoothness and homogeneity of the PDDA/IONP layer formation. The rootmean-square (RMS) of surface roughness measured for a typical silica surface on a 500 × 500 nm surface area was approximately 0.2 nm.32 Silica surfaces have a negative charge of −30.0 mV at 1.0 mM KCl due to the dissociation of silanol groups in contact with water.33 2.4. Dynamic Light Scattering (DLS) and Zeta Potential Measurements. The particle size distributions in terms of the hydrodynamic diameter of the nanoparticles were determined by dynamic light scattering (DLS) (Malvern Instruments, Zetasizer Nano-ZS). The CONTIN algorithm was employed to fit the light scattering intensity autocorrelation function to provide an intensity-weighted distribution of hydrodynamic diameter. In measuring the particle size, we used low particle concentrations within the range of 10−20 ppm in deionized water without adjusting the pH. The zeta potential was measured by electrophoretic mobility (EPM) based on the Helmholtz-Smoluchowski approximation. All the zeta potential measurements were performed in 0.1 mM NaCl within a flow cell equipped with a gold-plated electrode. The zeta potential is reported as the averaged value from 3 runs, and each run consisted of 15 measurements at 25 °C.

(1)

where f 0 is the resonance frequency of the AT-cut quartz crystal, pq is the specific density of quartz, tq is the thickness of the crystal, CQCM‑D is the mass sensitivity constant of 17.7 ng cm−2 Hz1−, n is the overtone number, Δf is the frequency change, and Δm is the mass change. However, the Sauerbrey relationship is not valid for polyelectrolyte adsorption due to the viscoelastic property of the adsorbed layers, whereby the adsorbed layers do not adsorb as a “dead” rigid mass but undergo deformation associated with their oscillatory motion.29 In such a situation, the Voigt-based model is used to describe the relationship by taking into account the viscosity and complex shear modulus of the layers by using Q-Tools software (Q-Sense).30 By taking the adsorbed PDDA film as a homogeneous layer, with the assumption of only water contributing to its layer density, we used water relevant

3. RESULTS AND DISCUSSION 3.1. Kinetics of IONP Immobilization. The growth of bilayer films composed of a cationic polyelectrolyte and anionic IONPs, on a negatively charged silica quartz crystal surface, was monitored in real-time by QCM-D. These layers composed of PDDA and IONPs were deposited in sequence, with the process mainly driven by electrostatic interactions between the building blocks. The negatively charged IONPs did not deposit onto the clean silica quartz crystal surface, as both of these were negatively charged. The EPM for IONPs was recorded as −2.96 μm cm/(V s), and in the case of silica colloid used as a C

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The Journal of Physical Chemistry C surrogate to construct the QCM-D crystal, it was previously reported by Yi and Chen as −0.17 μm cm/(V s).34 However, successful detection of IONP deposition can only be accomplished after the QCM-D quartz crystal is coated with positively charged PDDA [soluble PDDA, which are freely suspended in the solution, is having an EPM value at +1.67 μm cm/(V s)]. It is worth noting that these EPMs can serve as indirect references to indicate the overall charge of each surface, either positive or negative, confirming our argument that the electrostatic interaction is the driving force for IONP attachment. After a stable layer of PDDA was formed, the loosely bound polyelectrolyte was removed by rinsing the surface with deionized water. After this rinsing step, the IONP suspension was introduced into the system. The raw QCM-D data, including both frequency (f) and dissipation (D) factors, during the successful sequential deposition of the polyelectrolyte and nanoparticles are presented in Figure 2. By using 6

mM PDDA as a binder, a continuous decrease of frequency and an increase of dissipation factors in response to the deposition of IONPs were recorded at different particle concentrations (see Figure 2). During the deposition process of PDDA, the response of the frequency and dissipation is not significant, but after the formation of the subsequent IONP layer, a prominent change in frequency from −2.55 to −180.88 Hz and in dissipation from 3.69 × 10−6 to 17.35 × 10−6 was detected. We ascribed this drastic change in frequency (Δf = 178.33 Hz) and dissipation (ΔD = 13.66 × 10−6) to the swelling of adsorbed layers resulting from the deposition of considerable amounts of IONPs onto the polymer layers,14 whereby the high hydrophilicity of PDDA and the electrostatic attraction between PDDA and IONPs are responsible for the swollen structure.35 In addition, at higher particle concentrations, more drastic decrease and increase of frequency and dissipation shifts were recorded, accompanied by lower constant values of frequency and dissipation shifts (see Figure 2b). By changing the IONP concentration from 20 to 500 ppm, the rate of IONPs deposited on PDDA increased with a steeper slope, as recorded in the frequency plot. From Figure 2a, the deposition rate of 20 ppm IONPs at saturation was 0.11 Hz s−1 (−2.55 to −180.88 Hz within 3000 to 4600 s). On the other hand, the deposition rate of 500 ppm IONPs was 0.35 Hz s−1 (−1.88 to −71.84 Hz within 3600 to 3800 s), as shown in Figure 2b. In addition, a smaller-deposited mass was recorded with a lower frequency shift value of 71.66 Hz compared to 178.33 Hz. Moreover, the lower dissipation shift value recorded for the experiment using 500 ppm IONPs of 9.71 × 10−6 (compared to 13.66 × 10−6 recorded at 20 ppm) indirectly indicates the formation of stiffer particle-polyelectrolyte layers. The surface mass density of IONPs deposited onto a silica quartz surface, which had been precoated with PDDA, over a time course of 16000 s is illustrated in Figure 2c. Here, two different IONP concentrations of 20 and 500 ppm were used. As illustrated in Figure 2c, the deposition rate of IONPs changed significantly with the IONP concentration. In the case of 500 ppm IONPs, a deposition rate of 16.57 × 10−8 kg m−2 s−1 (1657.3 kg m−2/100 s) was recorded, and this system reached full saturation in ∼100 s, whereas when using an initial IONP concentration of 20 ppm IONPs, saturation took much longer (∼7760 s) for the PDDA/IONP system with the slow deposition rate of 0.80 × 10−8 kg m−2 s−1 (6197.1 kg m−2/7760 s). Another observation from Figure 2c is that by increasing the initial IONP concentration, the deposited IONP mass decreased. This result contradicts the conventional understanding that the adsorbed amount increases as the given adsorbate concentration increases.36,37 This peculiar result was verified further by expanding the working range of particle concentration during the deposition experiment with QCM-D (see Figure 3). To better quantify the deposition rate, we fitted the experimental data for the IONP deposition process to a firstorder equation described by36 q(t ) = qe[1 − exp(−kobst )]

(3) 2

where q(t) is the adsorbed mass in kg/m at time t, qe is the adsorbed amount at saturation, and kobs is the characteristic rate constant. The qe and kobs values obtained from fitting our experimental data to a first-order deposition process are summarized in Table 1. From Table 1, the same observation is made again whereby with the increment of IONP concentration, the deposited mass

Figure 2. Time-dependence frequency (gray line) and dissipation shift (black line) for a salt-free solution of 6 mM PDDA at IONP concentrations of (a) 20 ppm and (b) 500 ppm deposited onto a QCM-D silica quartz crystal. (c) Changes of deposited surface mass density of IONPs at 20 and 500 ppm over time by evaluating both frequency and dissipation shifts simultaneously with the Voigt model using data from (a) and (b). D

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IONPs. For the low IONP concentration of 20 ppm, the particle deposition rate was recorded as 0.80 × 10−8 kg m−2 s−1. This low deposition rate is very likely due to the need for IONPs to diffuse into the polyelectrolyte matrix formed on the QCM-D quartz cell surface, whereas in the case of the high IONP concentration of 500 ppm, the deposition rate became much higher due to higher particle-to-absorbed surface collision frequency (please refer to the supporting document for sample calculation).39 This conceptual model is based on the simple fact that adsorption cannot occur unless there is collision. By considering diffusion-limited flux of particles on the quartz crystal surface, we could assume that each approaching diffusive particle adsorbs irreversibly on the surface. In this case, the surface acts as a sink for particle diffusion with a Dirichlet boundary condition in which the time-averaged surface concentration of particles at the charged surface is equal to zero.40 Hence, the collision frequency of a particle is proportional to the diffusion coefficient of the particle and to the averaged bulk concentration of particles. At 500 ppm IONP concentration, the particle-to-surface collision frequency was determined as 1.51 × 1012 particles s−1, which is 1 order of magnitude higher than that for 20 ppm (5.99 × 1010 particles s−1). With this high deposition rate, IONPs have little chance to penetrate into the preadsorbed PDDA matrix. This circumstance causes most of the particle deposition to occur on the outermost region of the PDDA layer, leading to suppression of the particle-polyelectrolyte layers. The immobilized IONPs further prohibit the incoming particles from penetrating into the polymeric matrix. As a consequence, stiffer layers of PDDA/IONPs are formed (see Figure 4). By evaluating the dissipation (D) and frequency ( f) factors of QCM-D simultaneously, energy dissipation as a consequence of the adsorption process by a unit mass can be compared. In this regard, we investigated the influence of the IONPs deposition on damping of the crystal resonance by correlating the nanostructure of the PDDA/IONP layer and its viscoelastic properties. Since the distribution of particles on the surface, as well as their absolute number, influences the magnitude of the QCM-D signal,41 a plot of the ΔD/Δf relationship is more

Figure 3. Specific amount of IONPs deposited on a 14 mm diameter silica surface as a function of initial IONP concentration in salt-free solution at pH 6.3−6.8. This result is obtained from the Voigt model based on the QCM-D measurement.

Table 1. Characteristic Rate Constant kobs and Equilibrium Deposited Mass qe,fitted with Standard Deviations Acquired by Fitting the Experimental Data of IONPs Deposited onto a PDDA Pre-Adsorbed Silica Surface as a Function of IONP Concentration IONP concentration (ppm) 20 80 100 200 500

kobs × 10−4 ± standard deviation × 10−4 (s−1) 3 40 37 320 340

± ± ± ± ±

0.25 1.53 0.49 4.04 4.36

qe,fitted ± standard deviation (1 × 10−8 kg/m2) 6045.08 3873.45 3781.97 2390.36 819.91

± ± ± ± ±

57.56 25.02 25.32 26.53 15.17

of IONPs detected by QCM-D measurement decreased, accompanied by a higher value of the adsorption rate constant. It has been reported that the initial particle coverage on a surface is dependent on interparticle diffusion. For later stages after the particle adsorption, in-plane repulsion between the adsorbed particles further drives the deposited particle layer to reach an equilibrium condition, which determines the final surface coverage.38 Hence, there is a need to investigate how the adsorbed PDDA layer influences the particle distribution on the surface, which subsequently affects the deposited amount of

Figure 4. Schematic of the suggested effect of the initial IONP concentration on the PDDA/IONP system. Having (a) low versus (b) high initial IONP concentrations drastically changes the conformation of PDDA/IONP layers. It is noted that a stiff layer of PDDA/IONPs (b) prohibits further registration of incoming IONPs into the PDDA/IONP matrix. Hence, this layer accommodates a lesser amount of IONPs. In contrast, more IONPs can be accommodated by the softer layer of the PDDA/IONP matrix shown in (a). E

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salt concentrations, shown in Figure 6a, a linear association between the dissipation shift and frequency shift is observed.

appropriate to qualitatively compare the adsorption dynamics independent of the spatial distribution of the particles on the surface.24 It was found that increasing the initial IONP concentration used to construct PDDA/IONP layers increases the stiffness (with decrement of the ΔD/Δf ratio) of the assembled films (see Figure 5). This result has confirmed our

Figure 5. Layer stiffness of the assembled PDDA/IONP system on the silica quartz crystal surface (denoted by the ΔD/Δf relationship) as a function of IONP concentration in a salt-free solution.

hypothesis about the high deposition kinetics suppressing the dissipative polyelectrolyte layer, leading to a stiffer structure. Further, a low initial IONP concentration with lower deposition kinetics allows diffusion of IONPs into the polymeric matrix, hence preserving a soft and diffusive particle−polymeric structure. This soft layer also accommodates more IONPs with more extended structures, as shown by our measurement. Here, an understanding of the deposition process kinetic factors has provided better insight into the construction of multilayer particle-polyelectrolyte structures. In addition, this finding also offers an exciting possibility for the development of IONP deposition schemes in which the deposition kinetics can be further stipulated to custom design a particle-polyelectrolyte structure for engineering applications.14,42 3.2. Modulation of IONP Immobilization Rate by Ionic Strength Adjustment. In addition to the observation that IONP attachment to the polyelectrolyte layer is limited by deposition kinetics, it is equally important to demonstrate how this effect can be modulated. Since, electrostatic attraction between the IONPs and PDDA is the main driving force for the construction of PDDA/IONP layers,16 the easiest way to govern this process is by controlling the ionic strength of the surrounding medium.43 In addition, altering the flexibility of the polyelectrolyte matrix with freely suspended ionic species44 has also allowed us to investigate the structural effect of the PDDA layer on the deposition kinetics of IONPs and thus its loading capacity. With an open polymeric structure (with a large ΔD/ Δf ratio), mass transfer of IONPs through the ramified polyelectrolyte layer(s) occurs more freely. From the previous section, by changing the initial particle concentration, we observe that the deposited mass of IONPs is inversely proportional to the particle deposition rate. Hence, the primary goal here is to control the deposition rate associated with the enhancement of IONP loading by adjusting the ionic strength. By referring to the time-dependent frequency and dissipation profile of a QCM-D measurement, it is clear that increasing the ionic strength of the particle suspension during IONP deposition causes more IONPs to deposit onto the PDDA layer and at the same time increases the energy dissipation in the adsorbed layer (see Figure S3 for full QCM-D measurement data).45 By inspecting the ΔD/Δf relationship at different

Figure 6. (a) Dissipation versus frequency shift of the PDDA/IONP layers on a silica surface in (i) 0, (ii) 1, and (iii) 10 mM NaCl solutions. (b) Surface mass density of IONPs deposited on a PDDAsilica surface over time. Time-resolved Voigt IONP mass calculated from data shown in (a). (c) Kinetic rate constants for IONP deposition plotted as a function of solution ionic strength based on the results in (b) fitted to first-order kinetics. For all these measurements, the PDDA layer was constructed using 6 mM PDDA solution and a 100 ppm IONP suspension.

The linearity of the ΔD/Δf relationship reveals, via a constant slope at any point on the curve, that the same degree of dissipation change is caused by a unit frequency (adsorbed mass) change. This scenario directly implies that there is no structural change(s) during the assembly process of the PDDA/ IONP layers on the silica surface since the same amount of adsorbed mass is causing the same degree of dissipation.29 As ionic strength increases, a structure with higher flexibility forms, judging by the large ΔD/Δf ratio, which is consistent with a “less stiff” layer postulation.46 Again, we observed an out-of-thenorm scenario here in which a PDDA/IONPs layer with higher IONP (hard sphere) loading exhibited a “softer” behavior. To further rationalize our observation and to verify its consistency with our observations in the previous section, we calculated the deposited mass after IONP immobilization using the Voigt model.46 After IONP deposition, the total mass of PDDA/IONP layers increased with ionic strength, following different deposition rates (see Figure 6b). Figure 6c shows the values of kobs at different ionic strengths obtained by fitting the experimental data (from Figure 6b) to first-order kinetics described in the previous section. From these results, the fitted F

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The Journal of Physical Chemistry C kobs values decrease drastically from 5.2 to 1.4 s−1 as the ionic strength increases from 0 to 10 mM. Therefore, the deposition rate for IONPs decreases with increasing ionic strength. Here, the presence of free ionic species imposes a Debye screening effect on the electrostatic attraction between the IONPs and PDDA-silica surface (see Table 2 for the Debye length). At

Lp = Lo +

Debye length (nm)

qe,exp (1 × 10−8 kg/m2)

thickness (nm)

0 0.1 0.5 1 10 15

− 30.00 13.42 9.49 3.00 2.45

2330.72 2732.24 4221.18 4965.79 6876.88 7040.61

19.22 22.84 41.22 48.28 66.54 71.22

4κ 2

(4)

where Lo corresponds to the rigidity of an uncharged chain independent of the salt concentration, lB represents the Bjerrum length, Γ is the linear charge density of the polyelectrolyte, and κ−1 is the Debye length.49 By referring to the working range of ionic strength, I (mol/L), the Debye length for a monovalent salt is calculated as k−1 = 0.3/ I ,50 and the values are tabulated in Table 2. Here, the entire second term on the right-hand side of eq 4 denotes the electrostatic repulsion between identically charged groups from the same chain and is dominated by the external salt concentration.51 By increasing the ionic strength, the Debye length decreases, indicating that the electrostatic interaction is screened at a length scale smaller than the diameter of a sodium ion.52 Reduction of κ−1 implies the simultaneous shortening of Lp. This phenomenon indicates that the ionic strength increment promotes coiling of the polyelectrolyte and at the same time reduces the effective rigidity of its chains at the polyelectrolyte− solution interface. Both of these features are the consequences of free ionic species weakening the electrostatic repulsion between identically charged groups within the polyelectrolyte.53 Under this circumstance, a more coiled and less rigid PDDA layer has the capability to accommodate a higher loading of IONPs and allows the slow deposition of IONPs into its matrix. From a structural point of view, the coil-like and loopy conformation of a polyelectrolyte has been observed upon adsorption with increasing NaCl concentration in the