Construction of Self-Assembled Polyelectrolyte/Cationic Microgel Multilayers and Their Interaction with Anionic Dyes Using Quartz Crystal Microbalance and Atomic Force Microscopy.

This study aimed to reveal the interaction between self-assembled multilayers and dye molecules in the environment, which is closely related to the multilayers' stable performance and service life. In this work, the pH-responsive poly (N-isopropylacrylamide-co-2-(dimethylamino) ethyl methacrylate) microgels were prepared by free-radical copolymerization and self-assembled with sodium alginate (SA) into multilayers by the layer-by-layer deposition method. Quartz crystal microbalance (QCM) and atomic force microscopy (AFM) results confirmed the construction of multilayers and the absorbed mass, resulting in a decrease in the frequency shift of the QCM sensor and the deposition of microgel particles on its surface. The interaction between the self-assembled SA/microgel multilayers and anionic dyes in the aqueous solution was further investigated by QCM, and it was found that the electrostatic attraction between dyes and microgels deposited on the QCM sensor surface was much larger than that of the microgels with SA in multilayers, leading to the release of the microgels from the self-assembled structure and a mass loss ratio of 27.6%. AFM observation of the multilayer morphology exposed to dyes showed that 29% of the microgels was peeled off, and the corresponding microgel imprints were generated on the surface. In contrast, the shape and size of the remaining self-assembled microgel particles did not change.

Introduction

Microgel comprises polymer colloidal particles with a three-dimensional cross-linked structure and exhibits adjustable size and strong environmental responsiveness. It can respond quickly to even weak external stimuli such as temperature, pH, light, ionic strength, and ultrasound.[1−9] Because of its rich functional sites, swelling performance in response to stimulation, and biocompatibility, microgel has been widely used in drug delivery and controlled release,[10−12] catalytic carriers,[13−15] and sensors.[16,51] Poly(N-isopropyl methacrylamide) (PNIMAM) copolymer microgel particles are used as adsorbents, which can effectively remove toxic dyes from aqueous solutions. It is worth noting that the PNIPAM copolymer microgel can also be reused with almost no loss of extraction efficiency.[17,18] For example, Jin et al.[19] found that polyvinylamine microgels functionalized by nanocompounds have significant effects on the adsorption of various anionic dyes. The interaction mechanism between microgels and dye molecules follows a pseudo-second-order model and can be understood with equilibrium adsorption isotherms. The interaction of the related materials with dye molecules,[20,21] ions,[22−24] proteins,[25−27] and biomolecules[28] has also been a research hotspot in recent years.

In addition to studying the functions of microgel particles dispersed in aqueous solutions, microgels can also be assembled to give them macro size and orderly structure, which will expand their application in the field of intelligent materials.[29−35] Zhang et al.[36] chose the poly(allylamine hydrochloride)-dextran (PAH-D) microgels and hyaluronic acid (HA) as building blocks to prepare highly adhesive films with drug-release capabilities via layer-by-layer (LbL) self-assembly technique, which are expected to be used in bio-adhesives. Serpe et al.[37] reported one alternate layer deposition method to prepare a heat-responsive microgel film made from poly(N-isopropylacrylamide-co-acrylic acid) microgel and PAH.

The phenomena of the rejection and adsorption on the surfaces and interfaces of the self-assembled multilayers driven by electrostatic interaction are closely related to their applications such as ion adsorption, separation, and desalination.[38−43] For example, Armstrong et al.[44] reported that a 30 nm-sized pore structure modified by poly(styrene sulfonate)/protonated poly(allylamine) (PAH) multilayers showed Br–/SO42– selectivity of 3.4 with a SO42– rejection rate of 85%. This is due to the selective electrostatic rejection of the negatively charged pore and the divalent anions. Once the self-assembled multilayer disintegrates or collapses under the influence of external ions or molecules, its stability and reliability will be decreased. Therefore, it is of great significance to study the adsorption and desorption phenomena on the interfaces and surfaces of the self-assembled multilayers to meet the application needs in water treatment and environmental protection.

Quartz crystal microbalance (QCM) is a real-time technique at the nanogram level for measuring the adsorption interaction[45−52] of molecules or particles in the environment on the surface of different materials, including polymers, metals, and inorganic oxides. The structure and viscoelasticity of the adsorbed layer on QCM sensors are observed by monitoring the frequency changes of quartz crystals (Δf) and dissipation (ΔD).[53−56] Cao et al.[57] used QCM technology to study the interaction between the self-assembled polyelectrolyte multilayers containing guanidinium (Gu) moieties and different anions (Cl–, NO3–, SO42–, and H2PO4–) and found that the functional polyelectrolyte showed more adsorption to H2PO4– due to its strong binding affinity with Gu groups. Qin et al.[58] studied the interaction mechanism between lignin derivative dispersants and dyes using QCM with dissipation monitoring and atomic force microscopy (AFM). With the increase of the adsorption amount of hydroxypropyl sulfonated alkali lignin dispersant on the dye surface, a more viscoelastic adsorbed layer was obtained, compared to that of sodium lignosulfonate. This work provides a new method for the preparation of nanodispersed dyes. However, as far as we know, the use of QCM to study the stability of the self-assembled multilayers containing microgels under the action of dye molecules in water has not been reported in the literature. The study of this interaction is helpful to understand the stability and reliability of the microgel adsorbents in water environments containing small molecules of pollutants to elucidate the mechanism of interaction between the self-assembled microgels and dye molecules in solutions and to design and prepare new adsorbents and expand their application range.

Methyl blue is a hydrophilic and anionic dye and can be found in organic dye wastewater. Numerous literature studies have reported the adsorption and separation of methyl blue dye molecules by the adsorption materials, including microgel-core star polymers,[59] fibrous materials,[60] ionic covalent organic polymers,[61] and cross-linked ammonium-functionalized hollow polymer particles.[62] Methyl blue was chosen as the ideal model molecule for our research work. In this work, first of all, the P(NIPAM-co-DMAEMA) microgel was prepared by free-radical copolymerization, and the charged property of the copolymer microgels was adjusted by pH. The self-assembled multilayers of the positively charged P(NIPAM-co-DMAEMA) microgels and the negatively charged polyelectrolyte were constructed using the electrostatic-force-driven LbL self-assembly method. The built-up process of microgels and polyelectrolyte is monitored using QCM technology. The self-assembled multilayers on the gold surface of QCM sensors can be used as an ideal two-dimensional model for studying the interaction of methyl blue dye molecules in water. This work will help us to obtain better understanding of the interaction between self-assembled multilayers and molecules in the environment and improve the stable performance and service life of the self-assembled system.

Results and Discussion

Figure shows the schematic diagram of the preparation, Fourier transform infrared (FT-IR) spectra, morphology, and the charge distribution at different pH of the copolymer microgels. As shown in Figure a, in this study, first using N-isopropyl acrylamide (NIPAM) as the main monomer, 2-(dimethylamino) ethyl methacrylate (DMAEMA) as the comonomer, N, N′-methylene bisacrylamide (MBA) as the cross-linking agent, and KPS as the initiator, in the presence of Cetyltrimethylammonium Bromide (CTAB) surfactant, P(NIPAM-co-DMAEMA) copolymer microgels were prepared by free-radical copolymerization method. Since the amino group contained in the DMAEMA repeating unit in the copolymer microgel can undergo protonation, the protonated state of microgels was obtained under low pH conditions and form a quaternary ammonium, which increases the surface charge density and zeta potential of the microgel. By adjusting the pH value, microgel particles with different zeta potentials can be obtained. Figure b shows the FT-IR spectra of PNIPAM homopolymer, PDMAEMA homopolymer, and P(NIPAM-co-DMAEMA) copolymer microgels. According to the data analysis in the spectra, in the spectrum of PNIPAM, the peak at 1630 cm–1 is belonging to the vibration characteristic absorption of C=O in the amide bond.[63] In the PDMAEMA infrared spectrum, the peak at 1720 cm–1 is assigned to the C=O stretching of the ester group in DMAEMA.[64] In the spectrum of P(NIPAM-co-DMAEMA) microgels, the absorption band is observed at around 3440 cm–1, indicating the stretching of the N–H bond. In addition, the absorption bands appear at 2980, 1640, and 1560 cm–1, which are corresponding to the C–H bond stretching, the secondary amide C=O stretching (amide I bond), and the secondary amide C=O stretching (amide II bond). The results show that the final microgel is formed by copolymerization of two monomers, NIPAM and DMAEMA. Figure c,d indicates the transmission electron microscopy (TEM) image and particle size distribution diagram of the microgels prepared. It can be clearly seen that the microgels have a uniform spherical particle structure with an average particle size of about 160 nm. Figure e shows the zeta potential and potential distribution of cationic P(NIPAM-co-DMAEMA) microgels under different pH conditions. It can be seen from Figure e that the zeta potential of the microgel gradually decreases with the increase of pH, indicating that the P(NIPAM-co-DMAEMA) microgel is pH sensitive. When the pH value is low, the amino group contained in the DMAEMA repeating unit in the microgel is protonated to obtain quaternary ammonium, which increases the surface charge density of the microgel and the zeta potential. When the pH of the microgel solution was 7, the particle size of the microgels is found to increase sharply. The reason is that when the pH = 6 or above, the microgel will partially or completely flocculate.[65] When the solution is less than the pKa value of DMAEMA (8.1), in an acidic medium, the amino group in DMAEMA is protonated, generating strong electrostatic repulsion and forming stable microgels. Therefore, it can be said that dimethylamino [-N(CH3)2] is the decisive factor for the microgel to respond to pH changes.[65,66] Therefore, in this study, since the microgel contains plenty of positive charges at the zeta potential of 30.8 mV (pH = 2), it is beneficial for cationic microgels to have a strong electrostatic attraction for the anionic sodium alginate (SA) during the electrostatic self-assembly process, which will be used in the next self-assembly experiment.

Figure 1

Preparation and pH control schematic diagram of P(NIPAM-co-DMAEMA) copolymer microgels (a); FT-IR spectra of PNIPAM, PDMAEMA, and P(NIPAM-co-DMAEMA) copolymer microgels (b); TEM image of P(NIPAM-co-DMAEMA) copolymer microgels (c); particle size distribution of P(NIPAM-co-DMAEMA) copolymer microgels from TEM (d); zeta potential distribution of P(NIPAM-co-DMAEMA) copolymer microgels adjusted at different pH (e).

Figure shows the illustration of the LbL self-assembly of SA/P(NIPAM-co-DMAEMA) microgel multilayers on the gold surface of the QCM sensor and QCM signals including the frequency shifts (Δf/n) and the changes in half-band-half-width (ΔΓn) during self-assembly on the gold surface of the QCM sensor. As shown in Figure a, since the P (NIPAM-co-DMAEMA) microgel has a positive charge in an acidic (pH = 2) aqueous solution, it can be alternately assembled into a multilayer by electrostatic interaction with a negatively charged SA using LbL assembly technology. Because microgels and SA are both difficult to directly adsorb onto the gold surface of the QCM sensor, the gold surface is generally modified with a branched polyethyleneimine (PEI) layer first. PEI containing plenty of amino groups is easily protonated and acts as the positively charged building block for the LbL self-assembled multilayers. The mass-sensitive QCM technology was used to track the deposition of the microgels and SA on the gold surface of QCM sensors. Figure b,d shows the Δf3/3 and ΔΓ3/3 of the QCM sensor (n = 3, 15 MHz) as a function of time caused by the alternating self-assembly of PEI/SA/P(NIPAM-co-DMAEMA) microgels. The signal of frequency shift is related to the mass loaded on the surface of the QCM sensor, and the general adsorption of materials causes the mass on the QCM sensor to increase and the frequency of the QCM sensor to decrease. The signal of the change in half-band-half-width can reveal a change in the structure and viscoelastic property of thin polymer layers on the surface of the QCM sensor. The adsorbed thin films with rigid structures show lower ΔΓ values, while soft and hydrated films show significant viscoelastic behavior due to structural dissipation, showing larger ΔΓ values. First of all, the signals of the blank QCM sensor achieve stability in deionized water, obtaining a flat baseline as a reference. Then the deionized water is switched to a PEI solution. Because of the strong interaction between amino groups of PEI and gold of the QCM sensor surface, PEI molecular chains were gradually adsorbed onto the gold surface of the QCM sensor. The increase of the adsorption PEI mass led to the decrease of the frequency shift of the QCM sensor to −20.4 Hz, and finally, the frequency shift reached the absorption balance. Then the QCM surface was rinsed with deionized water to flush off unstable PEI chains. Alternately, SA solution was then passed into the QCM flow unit, and SA chains were slowly adsorbed onto the surface of the PEI layer due to the electrostatic attraction between the positively charged amino groups and the negatively charged carboxyl groups. After the adsorption saturation had been reached, the QCM sensor surface was rinsed with deionized water to flush off unstable SA chains. After the introduction of the solution containing positively charged P(NIPAM-co-DMAEMA) microgels, because of the electrostatic interaction, microgels were gradually adsorbed onto the negatively charged SA surface until adsorption equilibrium. During the self-assembly process, the ΔΓ3/3 value gradually increases, showing that the self-assembled SA/microgel multilayers adsorbed have strong viscoelastic property. SA is a sodium salt with strong hydrophilic and water-soluble properties. Microgel is composed of a kind of colloidal particles that are hydrophilic, water containing, and exhibit soft performance. The self-assembled multilayers prepared by the two substances on the surface of the QCM sensor also have strong viscoelasticity.

Figure 2

Illustration of the LbL self-assembly of SA/P(NIPAM-co-DMAEMA) microgel multilayers on the gold surface of QCM sensor (a); frequency shift (Δf3/3) of the QCM sensor (n = 3, 15 MHz) as a function of time caused by the alternating self-assembly of SA/P(NIPAM-co-DMAEMA) microgels (b); frequency shift (Δf/n) of the QCM sensor (n = 3, 5, 7, 15, 25, 35 MHz) as a function of time caused by the alternating self-assembly of SA/P(NIPAM-co-DMAEMA) microgels (c); ΔΓ3/3 of the QCM sensor (n = 3, 15 MHz) as a function of time caused by the alternating self-assembly of SA/P(NIPAM-co-DMAEMA) microgels (d); ΔΓ/n of the QCM sensor (n = 3, 5, 7, 15, 25, 35 MHz) as a function of time caused by the alternating self-assembly of SA/P(NIPAM-co-DMAEMA) microgels (e).

Figure c,e shows the normalized Δf/n and ΔΓ/n during the self-assembly of multilayers at different harmonics (n = 3, 15 MHz; n = 5, 25 MHz; n = 7, 35 MHz). For a rigid thin film on the surface of the QCM sensor, the normalized frequency shifts obtained at 15, 25, and 35 MHz show the same trend of change, and the frequency change values are close to each other, with three change curves stacked together to show the rigid structure. If the surface of the QCM sensor absorbs films with soft hydration features and strong viscosity, these curves do not approach each other but rather show a tendency to deviate from each other. As can be seen from Figure c,e, Δf/n and ΔΓ/n that are corresponding to 15, 25, and 35 MHz, both indicate that at the beginning, the absorption of PEI and SA layers has the same trend. These curves wholly overlapped with each other, showing that the two polymer molecular chains form a more rigid film on the surface of the QCM sensors. After the adsorption of microgels onto the SA surface, ΔΓ/n values were increased obviously, and these curves were separated from each other, indicating that the structure of the self-assembled SA/microgels formed by electrostatic interaction is relatively loose, soft, and hydrated, with certain viscoelasticity. This result is consistent with the previous research report of the self-assembled film made from hydrophilic and hydrated polyelectrolyte chains and the soft and swelling microgels.[56]

The self-assembled multilayers of P(NIPAM-co-DMAEMA) microgels and polyelectrolyte SA show very strong viscoelastic behavior from the previous results on QCM data including Δf3/3 and ΔΓ3/3. In this case, it is not accurate to quantify the mass change on the surface of the QCM sensor during the self-assembly using the Sauerbrey formula. However, it is still helpful to use the Sauerbrey formula for the semi-quantitative calculation and comparison, which is of great significance for us to understand the adsorption process of microgels and polyelectrolyte on the surface of the QCM sensor. Figure shows Δf3/3 (15 MHz) and the areal mass changes (Δm, ng/cm2) of the QCM sensor as a function of layer number and AFM images of the self-assembled SA/P(NIPAM-co-DMAEMA) microgels. As can be seen from Figure a, the areal mass of the first PEI layer on the gold surface of the QCM sensor is calculated to be about 360 ng/cm2. After the absorption of the SA layer, the areal mass of the PEI/SA bilayer becomes 679 ng/cm2. Then absorption of the SA/microgel multilayers on the gold surface of the QCM sensor leads to an areal mass of 1339 ng/cm2. It can be inferred that the areal mass of the third layer of microgel adsorbed in the self-assembled multilayers is 660 ng/cm2. It can also be calculated that the areal mass corresponding to the fifth and last layer of microgel absorbed in the self-assembled multilayers is 1195 and 397 ng/cm2, respectively. It can be seen that the areal mass of the microgel adsorbed first was increased and then decreased. This is because at the beginning, the SA layer on the gold surface of the QCM sensor was rather flat, and microgels with the opposite charges could be easily absorbed onto the SA layer by the electrostatic attraction. Once the microgels adsorbed onto the SA layer, the surface roughness of the modified QCM sensor became large, and the deposition of the separate microgels on the surface led to a particular spatial resistance, resulting in electrostatic rejection between the microgels on the surface and their counterpart in the solution, and the decreased areal mass of microgels absorbed. The P(NIPAM-co-DMAEMA) microgel and polyelectrolyte SA were alternately deposited on the QCM sensor with a total mass of 4255 ng/cm2, which could be characterized using AFM. Figure b–e shows AFM two-dimensional and three-dimensional images of the self-assembled multilayers at the layer number 5 and 7. As can be seen from Figure b,c, when the layer number of multilayers is 5, the P (NIPAM-co-DMAEMA) microgel particles were dispersed on the gold surface of the QCM sensor in the scanning area of 5 × 5 μm, and there is a certain distance between the microgel particles. When the layer number of multilayer was increased to 7, the number of P(NIPAM-co-DMAEMA) microgel particles attached on the gold surface of the QCM sensor was larger than that of microgel particles at the layer number of 5 in the same scanning area. However, microgels still did not completely cover the gold surface of the QCM sensor. One reason is that the concentration of the microgel solution used is only 6.8 mg/mL, which is far from sufficient to form a dense microgel film. On the other hand, the microgel at room temperature in the aqueous solution was in the swelling state with large volume. Less adsorption of microgel particles on the QCM sensor surface was due to their spatial resistance and electrostatic rejection.

Figure 3

Δf3/3 (15 M Hz) and the areal mass changes (Δm, ng/cm2) of the QCM sensor as a function of layer number during the self-assembly of SA/P(NIPAM-co-DMAEMA) microgels (a); AFM two-dimensional (b) and three-dimensional (c) images of the self-assembled multilayers at the layer number of 5; AFM two-dimensional (d) and three-dimensional (e) images of the self-assembled multilayers at the layer number of 7.

The SA/P(NIPAM-co-DMAEMA) microgel multilayers were constructed on the gold surface of the QCM sensor through electrostatic-force-driven LbL self-assembly technology. Note that the top layer of multilayers is the P(NIPAM-co-DMAEMA) microgel layer. We intend to absorb the dye molecules into the microgel contained in the multilayers using the electrostatic adsorption of the positively charged DMEMA unit in the microgel cross-linking network structure and for the negatively charged dyes from the water solution. However, since the microgel itself is also adsorbed onto the SA surface by the electrostatic attraction, there is a comparison between microgel-SA and microgel–dye interactions. We used mass-sensitive QCM technology to study the strength of these two interacting forces.

Figure shows Δf/n and ΔΓ/n (n = 3, 5, 7, 15, 25, and 35 MHz) of the self-assembled multilayer modified QCM sensor as a function of time after exposing to a series of methyl blue solutions (0.1, 0.3, 0.6, 0.9, and 1.2 mg/L) and schematic diagram of the disintegration of the self-assembled multilayers under the action of dye molecules. It can be seen from Figure a,b that the QCM sensor modified with the self-assembled multilayers was first exposed to the deionized water to obtain a stable signal baseline, which was set as the reference. Subsequently, the aqueous dye solution with a low concentration of 0.1 mg/L was introduced. It was found that Δf3/3 of the QCM sensor was gradually increased, indicating that the self-assembled multilayers had a mass loss. It was speculated that the negatively charged dye molecules in the water could interact with the oppositely charged groups attached in the microgel cross-linking networks deposited on the QCM surface. This strength of the microgel–dye interaction is much greater than that of the SA-microgel electrostatic attraction. It can be seen from Figure a that as the concentration of the dye molecule solution continues to increase to 1.2 mg/L, Δf3/3 continues to rise, indicating that the multilayers were decomposed under the action of the dye molecules. That is, there were microgels released from the multilayer structure into the solution, which further degraded the multilayers. Finally, the deionized water was used to rinse the surface of the QCM sensor deposited with the multilayers, and the frequency change was gradually stabilized. At the same time, it can be seen from Figure b that ΔΓ3/3 of the QCM sensor modified by the multilayer was gradually decreased with the addition of dye solutions with a series of concentrations. When the 0.1 mg/L dye aqueous solution was introduced, the QCM sensor modified with multilayers had a significant decrease in ΔΓ3/3 due to the electrostatic interaction between microgels in the multilayers and dyes. As the dye solution concentration was further increased to 1.2 mg/L, it could be found that ΔΓ3/3 decreased gradually, which also verified that the multilayers containing microgels and SA were peeled, leading to a thin multilayer and reduced viscoelastic property of the multilayers. From Figure c,d, after exposure to a series of methyl blue solutions (0.1, 0.3, 0.6, 0.9, and 1.2 mg/L), the self-assembled multilayers disintegrated and lost partial mass, resulting in a decrease in viscoelasticity. Based on the above analysis, the disintegration process of the self-assembled SA/microgel multilayers under the action of dye molecules was proposed and is shown in Figure e.

Figure 4

Δf3/3 (a,b) ΔΓ3/3 of the 6-layer self-assembled multilayer modified QCM sensor as a function of time after exposing to a series of methyl blue solutions (0.1, 0.3, 0.6, 0.9, and 1.2 mg/L); Δf/n (c) and ΔΓ/n (n = 3, 5, 7, 15, 25, and 35 MHz) (d) of the self-assembled multilayer modified QCM sensor as a function of time after exposing to a series of methyl blue solutions (0.1, 0.3, 0.6, 0.9, and 1.2 mg/L); and schematic diagram of disintegration of the self-assembled SA/microgel multilayers under the action of dye molecules (e).

Similarly, in order to better understand the disintegration process of the self-assembled SA/P(NIPAM-co-DMAEMA) microgel multilayers, the QCM frequency change (Δf3/n) (15 MHz) and the corresponding areal mass loss (Δm, ng/cm2) calculated by the Sauerbrey formula were related to the concentration of methyl blue dye solutions. Figure shows Δf3/3 and the areal mass loss (Δm, ng/cm2) of the QCM sensor as a function of the dye concentration during the disintegration of the self-assembled SA/P(NIPAM-co-DMAEMA) microgel multilayers and AFM images of the self-assembled multilayers at the scanning area of 5 × 5 μm and 2 × 2 μm. It can be seen from Figure a that the mass loss of the self-assembled multilayer was 425 ng/cm2 when the modified sensor was exposed to dye solution with a low concentration of 0.1 mg/L. With the increase of the dye solution concentration to 1.2 mg/L, the mass loss of multilayers reached as high as 1175 ng/cm2. Combined with the data in Figure , the total mass adsorbed caused by the deposition of the self-assembled multilayers on the surface of the QCM sensor was about 4255 ng/cm2. It can be calculated that the mass of the remaining multilayers after the release of the microgel was approximately 3080 ng/cm2, and the mass loss ratio was about 27.6%. AFM was used to further characterize the multilayers on the gold surface of the QCM sensor after interacting with dye molecules (see Figure b–e). From Figure b,c, in the scanning area of 5 × 5 μm, it can be observed that most of the surface was covered with microgel particles, but a small number of holes can also be seen. For more details, in the scanning area of 2 × 2 μm, it is obvious that besides the microgel particles, a clear structure with some holes can be observed. Combined with QCM data results, it can be concluded that the holes on the surface are actually imprints left by the microgel after peeling off the multilayers. This can be explained by a few of the microgels in the multilayers being attracted by the dye molecules in the solution. The strength of microgel–dye interaction was much greater than that of the microgel-SA interaction, leading to the release of the microgels partially from the multilayers. However, most of the microgel particles can be stably attached to the surface of the QCM sensor through electrostatic attraction. AFM results confirmed that after the self-assembled SA/microgel multilayers were in contact with methyl blue, about 29% in quantity of the microgels were peeled off from the multilayer surface, the corresponding microgel imprints were left, and the morphology of the remaining self-assembled microgel particles hardly changed. This result is well consistent with the QCM data.

Figure 5

Δf3/3 and the areal mass loss (Δm, ng/cm2) of the QCM sensor as a function of dye concentration during the disintegration of the 6-layer self-assembled SA/P(NIPAM-co-DMAEMA) microgel multilayers (a); AFM two-dimensional (b) and three-dimensional (c) images of the self-assembled multilayers at the scanning area of 5 × 5 μm; AFM two-dimensional (d) and three-dimensional (e) images of the self-assembled multilayers at the scanning area of 2 × 2 μm.

Conclusions

Using electrostatic interaction, the cationic P(NIPAM-co-DMAEMA) microgels and anionic SA were LbL self-assembled on the gold surface of the QCM sensor. From the changes in Δf and ΔΓ signals of QCM, the self-assembly process of SA/microgel and the viscoelastic structure of the resulting self-assembled multilayers were confirmed. According to the Sauerbrey formula, the total mass of the multilayers formed by the alternate deposition of P(NIPAM-co-DMAEMA) microgels and polyelectrolyte SA on the gold surface of the QCM sensor was calculated to be approximately 4255 ng/cm2. AFM observation also confirmed the deposition of multilayers containing microgel particles separately dispersed on the gold surface of QCM sensors. Using the multilayers as an ideal two-dimensional model, the interaction between the cationic microgels and the anionic dye molecule methyl blue was also studied using QCM. The QCM data and AFM images confirmed that the interaction between the microgels with positively charged groups and anionic dyes was much greater than that between the microgel and SA, leading to the release of microgel particles from the multilayers and the “disintegration” of the multilayers. Calculating by the Sauerbrey equation, the mass loss ratio caused by the desorption of the microgels from the surface is about 27.6%. At the same time, it can be seen from the AFM image of the QCM sensor modified with multilayers after exposure to dye solutions that the microgel imprints remained on the surface due to the desorption of 29% of the microgels. There was no significant change in the morphology of the other microgel particles left, which were still deposited on the gold surface of the QCM sensor. In this work, first, multilayers containing microgels were constructed using LbL self-assembly technology driven by electrostatic force. Then, the interaction mechanism between the microgel-contained multilayers and dye molecules was studied through the mass-sensitive QCM technology. This study helps the future development of stable and reliable adsorption materials and QCM-based sensors for detecting dye contaminants in aqueous solution.

Experimental Section

Chemicals and Materials

NIPAM (99%), N,N′-methylene bisacrylamide (MBA, 98%), and potassium persulfate (K2S2O8, KPS, 99%) are all purchased from J&K Scientific Ltd (Beijing, China). CTAB was purchased from Shanghai Runjie Chemical Reagent Co., Ltd (Shanghai, China). DMAEMA (98%), branched PEI, (Mw = 10,000 g/mol, 99%) were purchased from Shanghai Aladdin Reagent Co., Ltd (Shanghai, China). Methyl blue, SA, sodium hydroxide, hydrochloric acid, hydrogen peroxide, and sulfuric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents are of analytical grade, and all experimental water is deionized water.

Synthesis of P(NIPAM-co-DMAEMA) Microgels

The P(NIPAM-co-DMAEMA) microgel was synthesized by free-radical emulsion polymerization. NIPAM (1 g), DMAEMA (0.11 g), MBA (0.096 g), and CTAB (0.025 g) were dissolved in 95 mL of deionized water and transferred to a 250 mL three-neck round bottom flask with a condenser, magnetic stirring, and the gas inlet/outlet device. After the mixture was changed into a transparent solution under continuous stirring at 400 rpm, nitrogen was introduced for 30 min to remove oxygen in the solution. Then the mixture was allowed to react at 70 °C. Under the same experimental conditions for 1 h, 5 mL of KPS solution (68 mg) was added to the reaction mixture to initiate polymerization. The reaction was continued for 6 h to complete the synthesis of the microgel. Finally, the microgel was purified by dialysis to remove unreacted monomer molecules and the surfactant CTAB (dialysis membrane, molecular weight cutoff = 14,000 g/mol). The pH value of the microgel solution was adjusted by dropping 1 M HCl solution.

Preparation of the Self-Assembled SA/Microgel Multilayers

The QCM gold-coated sensor was soaked in the mixture of 30% hydrogen peroxide and 98% concentrated sulfuric acid (volume ratio of 3:7) piranha solution for 30 min and then washed with deionized water and blow-dried with nitrogen. These treated gold-coated QCM sensors served as a substrate for the preparation of microgel/SA multilayers. The iQCM (using a QCM based on admittance analysis (QCM-A DBY-17, Hangzhou Longqin Advanced Materials Sci & Tech Co., Ltd., Hangzhou, China) with a control program “QCM-DBY” written by Prof. Binyang Du, Zhejiang University.) was applied to study the LbL self-assembly of cationic microgels and anionic SA on the gold surface of QCM sensors.

The fundamental frequency of the gold-coated QCM sensor (AT-cut) sensor is 5 MHz, and the gold electrode thickness is 100 nm. First of all, the deionized water was entered into the QCM sensor flow unit with a flow rate of 100 μL/min. The frequency shift (Δf) and half-band half-width shift (ΔΓ) of the gold-coated QCM sensor were recorded in the deionized water as the baseline. The branched PEI solution (5 mg/mL) was then flowed into the QCM flow unit at the same flow rate, and a layer of PEI was adsorbed on the gold electrode surface of the QCM sensor through the strong interaction between amino groups of PEI and gold. After the equilibrium was achieved, the surface of the QCM sensor was rinsed with deionized water. A solution of the negatively charged SA (2 mg/mL) was switched into the flow unit, and the second layer was deposited on the PEI surface due to the electrostatic attraction. The surface was also continued to rinse with the deionized water after the adsorption saturation was obtained. The next solution containing the positively charged microgels (6.8 mg/mL) was introduced, and the microgel layer was adsorbed on the previous SA layer. After the alternating self-assembly, the multilayers containing SA and microgels were obtained on the surface of QCM sensors, rinsed with the deionized water, and blow-dried with nitrogen. Note that the label “SA/microgel” means the multilayers containing SA and microgel layers.

The changes in frequency and structural viscoelasticity (Δf and ΔΓ) can be monitored in real time by QCM. Mass changes on the surface of the QCM sensor during the self-assembly can be calculated using the Sauerbrey equation[67]In the equation, Δm is the areal mass density (mass per unit area, ng/cm–2) of the absorbed film, C is 17.7 ng cm–2 Hz–1, and Δf is the frequency shift at overtone order n (3, 5, 7). Note that ΔΓ, the change in half-band-half-width of the QCM sensor, is used to evaluate the dissipation and viscoelastic property of the adsorbed layer. In our work, the QCM data were acquired at the third (15 MHz), fifth (25 MHz), and seventh (35 MHz) overtones. Since the Sauerbrey equation is only applicable to the mass change of the rigid thin film on the surface of the quartz crystal in vacuum or air and the SA/microgel multilayers obtained in the study are a soft, water-containing, and viscoelastic material, the mass change obtained by this formula is a semi-quantitative value. However, this parameter is still of great importance, which can be used to analyze the amount of adsorption materials on the QCM sensor surface.

Interaction between the Self-Assembled SA/Microgel Multilayers and Dyes

First, the QCM sensor modified with the P(NIPAM-co-DMAEMA) microgels and polyelectrolytes was immersed in deionized water to obtain a baseline and set it as the reference frequency. Then a 0.1 mg/L dye molecule aqueous solution was introduced. The frequency changes and dissipation changes caused by the interaction between the self-assembled microgel layers and the dye molecules were recorded by QCM. The methyl blue aqueous solution with a series of concentration (0.3, 0.6, 0.9, and 1.2 mg/L) was introduced as well. After reaching the equilibrium, the surface was rinsed with deionized water, and finally, the self-assembled microgel multilayers on the gold surface of the QCM sensor were obtained.

Characterization of P(NIPAM-co-DMAEMA) Microgels and the Assembled Multilayers

The FT-IR spectra of the P(NIPAM-co-DMAEMA) microgels were measured by a Nicolet AVATAR 370 spectrometer (Nicolet, USA). The particle size and zeta potential of the microgels in the aqueous solutions were measured at 25 oC by a Malvern Zetasizer Nano ZS. The size and morphology of the microgel prepared were tested by a Hitachi HT7700 TEM, and the microgel dispersion was drop-cast onto the copper grid covered with amorphous carbon. The shape and height of microgels deposited on the gold surface of QCM sensors can be observed using AFM (NanoMan VS, Veeco Instrument Inc., USA) with a tapping mode.