Characterizing the Adsorption of Poly(vinyl alcohol) on Colloidal Silica

Feb 22, 2016 - Poly(vinyl alcohol) (PVA) and colloidal silica (CS) were used as a model system. It was found that AIE molecules exhibited extremely lo...
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Characterizing the Adsorption of Poly(vinyl alcohol) on Colloidal Silica with Aggregation-Induced Emission Fluorophore Zhichao Zhu,†,∥ Xiaobiao Dong,‡,∥ Guanxin Zhang,‡ Junfeng Xiang,§ and Dong Qiu*,† †

State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Science, Beijing, 100190, China ‡ Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China § Analysis and Test Center, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China ∥ University of Chinese Academy of Sciences, Beijing, 100190, China S Supporting Information *

ABSTRACT: The adsorption of polymer on colloidal particle has significant influence on colloid structure and dynamics. Here we introduce a new method to monitor the adsorption in situ, based on the different emission behavior of aggregationinduced emission (AIE) luminogen in different micro environments. Poly(vinyl alcohol) (PVA) and colloidal silica (CS) were used as a model system. It was found that AIE molecules exhibited extremely low fluorescence intensity in water and PVA solution, while their emission efficiency was enhanced when adsorbed on CS, and became significantly boosted when PVA was adsorbed on CS at the same time. The fluorescence intensity increases with the amount of added PVA and reaches a saturation point, which is earlier than that obtained by the well-established solvent relaxation NMR method, due to their different sensitivities for adsorption segments in specific conformation. This new method is advantageous in quick response, where the measurement can be finished within 2 min, while others usually take hours. Therefore, it is expected that this new method may be used to monitor the dynamical adsorption process of polymer on colloidal particles.

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INTRODUCTION Polymer adsorption on a colloidal particle is of great importance in both fundamental research and industrial applications, such as protein separation, flow property modification, and cosmetics formulation.1−3The adsorption of polymer can be measured by various techniques. The essential requirement for such techniques is that they should be able to distinguish free polymers from those adsorbed; for example, the depletion method takes advantage of their different sedimentation coefficients in the centrifugation field, dynamic light scattering (DLS) distinguishes their different diffusion coefficients, while static scattering, i.e. small-angle neutron scattering (SANS), reads their different density profiles.4−9Alternatively, polymer adsorption can also be probed indirectly, for example, solvent relaxation nuclear magnetic resonance (NMR). In the presence of colloidal particles, the relaxation rate constant (i.e., spin−spin relaxation rate constant, R2) of solvent molecules increases due to the constraint by interface, and increases further when polymer adsorbs onto the colloidal particles; therefore, the changes in R2 can be used to dig out information on polymer adsorption.10−13More details on solvent relaxation NMR will be given in the Experimental Section. © XXXX American Chemical Society

In many cases, the adsorption of polymer will need to be measured at nonequilibrant conditions or under external fields, which raises new challenges on characterization techniques. For example, in order to monitor the adsorption as a function of time, higher time resolution will be required. Polymer adsorption under shear is also important within the context of colloidal drug carriers, which may face numerous adsorption events of proteins in the rapid blood flow, thus the adsorption could be completely different from that under the static condition. A typical system consists of small colloid-long chain polymer mixtures, which, in certain recipes, could form gel under shear, and return to the fluid state within around 10 min when shear is ceased.14−16 Changes in polymer adsorption have long been proposed as the key to understand such behavior. However, no direct experimental evidence exists on how the polymer adsorption is affected under shear at this time scale. Among the most used techniques in the characterization of polymer adsorption on colloidal particles, SANS has proved to be very successful in measuring the adsorption under Received: January 25, 2016 Revised: February 17, 2016

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DOI: 10.1021/acs.langmuir.6b00288 Langmuir XXXX, XXX, XXX−XXX

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relaxation time constant of water 1H nuclei at 25 °C. The time between the 90°and 180° pulse was set as 1.0 ms while the relaxation delay was about 5 times of the spin−lattice relaxation time constant. The fitting method could be found in the Supporting Information.

shear.17−21 However, even on the most powerful SANS instrument, a typical SANS measurement on the scattering from adsorbed polymer will take around 60 min, and thus could not monitor the dynamical changes of polymer adsorption under shear. Additionally, resources for SANS are still rather limited. Therefore, a quick and versatile method for the characterization of polymer adsorption on colloidal particles is still highly demanded. Aggregation-induced emission (AIE) fluorophores may serve as a solution for the above challenge. AIE fluorophores are nonemissive in solution but highly luminescent in aggregated states,22,23 owing to the restriction of intramolecular rotation (RIR) in the aggregates. Similarly, RIR can also be realized when AIE molecules are associated with some less mobile species, such as DNA strands or micelles.24,25 We therefore propose that AIE molecules may also be used to monitor the adsorption of polymer on colloidal particles, because colloidal particle and adsorbed polymer can both serve as anchors to immobilize AIE molecules. This rationale is somehow similar to solvent relaxation NMR, where the mobility of solvent molecules is used to probe polymer adsorption. However, when AIE molecules are used as probes, the adsorption can be evaluated by measuring the fluorescence intensity, which is very sensitive and relatively quick. In this work, we aim to provide a conceptual study on whether this principle would work. Solvent relaxation NMR, due to its similar working rationale, is used for comparison.

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RESULTS AND DISCUSSION

First, the absorption and fluorescence spectra of AIE luminogen (0.1 mM) were measured. Obviously, the AIE luminogen exhibited a strong absorption band in the range of 350−450 nm, whereas a weak fluorescence emerged at ∼632 nm as shown in Figure 1b. Since the solution of AIE luminogen in good solvent such as CH2Cl2 and CH3OH is almost nonemissive,26 such fluorescence enhancement at ∼632 nm might be attributed to the addition of poor solvent (H2O) and the heavy atom effect of iodide anion.30 In the following, we investigated the effect of free PVA molecules on the fluorescence intensity of AIE luminogen solution (0.1 mM) with PVA aqueous solution (0.24 wt %) for comparison (Figure 2a,b). PVA aqueous solution showed no UV−vis absorption and was almost nonemissive. The mixture of PVA and AIE luminogen (PVA+AIE) showed almost the same absorption spectrum as AIE luminogen solution, indicating that AIE molecules are the only luminogen in the system (Figure 2b). Figure 2c,d shows a very small fluorescence enhancement for AIE luminogen solution after the addition of PVA, suggesting that PVA molecules would contribute very little to the AIE behavior. Differently from AIE luminogen, CS exhibited a new absorption band at 290−320 nm, whereas a weak fluorescence appeared at ∼488 nm (Figure 2a,d). However, CS+AIE showed almost the same absorption spectra as AIE solution (Figure 2b), indicating that CS particles were not the main luminogen in CS+AIE, which is further confirmed by the emission behavior (Figure 2c). Interestingly, the fluorescence intensity of AIE luminogen at ∼632 nm was enhanced more than 10-fold in the presence of CS particles, which should be ascribed to the electrostatic interaction between CS particles (negatively charged) and AIE ions (positively charged). In other words, the adsorption of AIE luminogen on the surface of CS particles enhances the AIE effect, which may affect subsequent adsorption of PVA on CS. (Figure S1) The emission behavior for the mixture of AIE luminogen, CS and PVA (CS+PVA+AIE) was studied to explore the role of adsorption in the fluorescence intensity. As shown in Figure 2a, the main emission group is still AIE luminogen, evidenced by their nearly the same absorption spectra. As expected, the fluorescence intensity of CS+PVA+AIE was greatly enhanced, about twice of that in the CS+AIE system and 20 times of that in the PVA+AIE system. Although the difference between CS +PVA+AIE and CS+AIE is only the addition of PVA, the increase in fluorescence intensity is far more than the value of the PVA+AIE system, suggesting that there must be interactions between PVA and CS. As well-known, PVA would adsorb on CS particles, thus this enhancement in fluorescence intensity may reflect the event of such adsorption. These observations suggest that through the different emission behaviors, AIE luminogen could distinguish free PVA molecules from those adsorbed on CS particles, which forms the foundation of using this phenomenon to monitor the PVA adsorption process. Since the emission intensity is positively correlated to the amount of AIE molecules aggregated, a quantitative adsorption curve of the CS−PVA system (i.e., an adsorption isotherm)

EXPERIMENTAL SECTION

Materials and Characterization. The purified water used here was generated by an Elga PureLab system (18.2 MΩ·cm−1). The AIE luminogen used in this work was synthesized according to the reported procedures26 and the structure was shown in Figure 1a. The colloidal

Figure 1. UV Absorption and florescent (FL) emission spectra of 1.0 × 10−4 mol L−1 (0.1 mM) AIE luminogen in a methanol/water mixture (1/99 by volume) at 398 nm light irradiation. Inset is the chemical structure of the AIE luminogen. silica dispersions was LUDOX-TM40 purchased form Sigma-Aldrich with a quoted average diameter of 25 nm and a specific surface area of 120 m2 g−1. Poly(vinyl alcohol) (PVA) (Mw of 57−66 kDa, degree of hydrolysis of 0.86−0.89) was purchased from Alfa Aesar. Sample preparation was provided in Supporting Information. Absorption spectra were recorded on a Jasco V-570 spectrophotometer while fluorescence spectra were collected on a Hitachi FP-6000 spectrophotometer at 25 °C. Solvent Relaxation NMR. A Bruker AVIII 500WB NMR spectrometer, using the Carr−Purcell−Meiboom−Gill (CPMG) pulse gradient sequence,27−29 was used to measure the spin−spin B

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Figure 2. (a) UV absorption and (c) FL emission spectra for AIE luminogen, PVA, CS, the mixture of PVA and AIE luminogen (PVA+AIE), the mixture of CS and AIE luminogen (CS+AIE), and that of CS, PVA and AIE luminogen (CS+PVA+AIE). (b,d) Partial magnification of panels a and c, respectively (CS: 1.0 wt %, PVA: 0.24 wt %, AIE luminogen: 0.1 mM in a methanol/water mixture (1/99 by volume)).

Figure 3. (a) FL emission spectra of CS+PVA+AIE with different nominal adsorption amounts of PVA (0.5−30 mg.m−2). (b) Partial magnification of panel a, to further exhibit detailed information. (c) Adsorption isotherm of PVA on CS established using an AIE luminogen probe and the solvent relaxation NMR method. (d) Relative FL emission and specific relaxation rate as a function of absolute adsorption amount (obtained by depletion method). The lines are guides for the eyes.

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Figure 4. (a) The average fluorescence spectra and (b) the standard error of a CS+PVA+AIE system (PVA 0.48 wt %, CS 1 wt % and AIE 0.1 mM) for single, 5, 10, 15, and 20 times repeated scans.

Figure 5. Schematic illustration of AIE molecules FL emission behavior in four different situations in the CS−PVA system.

and small “loops” of adsorbed polymer, which can restrain the diffusion of solvent molecules, it normally reaches a plateau earlier than the depletion method or PCS, etc. The AIE probe method is sensitive to those segments that can restrain the intramolecular rotation of AIE molecules, and thus requires even stronger interactions between them. Therefore, it might be only sensitive to trains, and not small loops. Thus, it becomes reasonable that the fluorescence emission intensity is not strictly linear to the adsorption amount since not all adsorbed polymer segments can satisfy the requirement for a strong interaction (just like R2sp does). The absolute adsorption amount of PVA was also independently determined by the depletion method, i.e., by measuring the supernatant after centrifugation. When plotting FL emission intensity against the absolute adsorption amount, one can see that at lower adsorption amount, a linear relationship exists, thus the FL emission intensity can be used to generate quantitative information on the adsorption amount (Figure 3d). While at higher adsorption amounts, FL emission intensity reaches a plateau, thus becoming independent of the adsorption amount. This is expected because FL emission is only sensitive to those polymer segments tightly attached to the surface (e.g., train

could be established through measuring the AIE luminogen emission intensity. Solvent relaxation NMR was used as a comparison, in which specific spin−spin relaxation rate constant, R2sp, is the parameter to describe the adsorption state in the system, similar to the fluorescence intensity when using AIE luminogen as the probe. R2sp is inversely related to the mobility of water molecules at interface since the contribution of free water molecules is eliminated. When adsorbed polymer layers are present, the water molecules near the surface will exhibit a higher R2sp to reflect the adsorption state at the interface until saturation since the motion of water molecules at the interface is further constrained.9,11−13 Samples with different amounts of added PVA on CS were measured. It can be seen that with the increase in the amount of added PVA, the FL emission increases (Figure 3a,b). The PVA adsorption isotherm was obtained by plotting the FL emission intensity as a function of the total amount of added PVA, with data from the solvent relaxation NMR method as a comparison (Figure 3c). The results showed that the fluorescence intensity rose faster than R2sp with the amount of added PVA (GPVA) and reached a plateau when GPVA approached ∼10 mg.m−2, earlier than the saturation point determined by the solvent relaxation NMR experiment. Since R2sp is believed to be related to “trains” D

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This new method is advantageous in its much shorter measuring time (2 min vs 20 min). Therefore, it is expected that this new method can be used to monitor the dynamical adsorption process of polymer on colloidal particles. Additionally, by plotting against the absolute adsorption amount determined by the depletion method, it was found that the quantitative adsorption amount can be obtained at a relatively low adsorption amount.

layers), and the train layer for a nonionic polymer usually saturates well before the full adsorption is achieved. As shown above, the isotherm of PVA adsorption on CS can be successfully established by using an AIE luminogen probe. As we mentioned in the Introduction, we were aiming to set up not only a new method, but also a faster method. In the solvent relaxation NMR method, a typical measurement of a datum in the adsorption isotherm would take ∼20 min. The corresponding minimal time for such a datum in the AIE luminogen probe method was also determined for comparison. One full spectrum of AIE fluorescence emission takes ∼23 s. As shown in Figure 4a, the FL emission spectrum for a single scan is almost identical to that obtained by the average of 20 repeated scans. Better evaluation can be obtained by comparing the noise level of their baselines. As is well known, the noise level can be reduced by repeated scans. It can be seen from Figure 4b that the standard error approaches a fixed value when the repeated scanning time is more than 5, suggesting that 5 scans are enough to obtain a high quality spectrum. Thus, the measuring time for one datum in the isotherm of PVA adsorption by the AIE luminogen probe method can be obtained by at most 5 scans (115 s or ∼2 min), which is much faster than the solvent relaxation NMR or SANS method. This new method could thus serve as a quick measuring tool to monitor the polymer dynamical adsorption process. In principle, it can also be combined with external fields, for example, the shearing field, when special flow cells are used, which will be explored in the future. It has now become clear that AIE molecules could well distinguish the four different situations in a CS−PVA system: free AIE molecules, free PVA molecules, free CS particle,s and PVA adsorbed on the surface of CS particles (Figure 5). The solution of AIE luminogen in good solvent such as CH2Cl2 and CH3OH is almost nonemissive; however, the fluorescence of AIE luminogen can be switched on after aggregation induced by the addition of poor solvent (H2O) or the presence of heavy atoms (Figure 5a).30 PVA chains have very weak interaction with AIE molecules. Consequently, the fluorescence intensity of PVA+AIE only increased a little compared to AIE solution in water (Figure 5b). On the contrary, negatively charged CS particles have strong electrostatic interactions with positively charged AIE ions and restrain their intramolecular rotation, thus enhance the FL emission significantly (Figure 5c). In the mixture of CS and PVA, the adsorbed PVA further restrains the intramolecular rotation of AIE at the interface, and thus greatly promotes the AIE effect (Figure 5d). The emission intensity is positively correlated to the adsorption amount of PVA in CS− PVA system until a saturation point, forming the foundation of establishing PVA adsorption isotherm by this method.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00288. Sample preparation, the fitting method of solvent relaxation NMR, control experiment of the effect of added AIE molecules on adsorption behavior, and solvent relaxation NMR experiments for CS+PVA and CS+PVA+AIE systems (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Prof. Dong Qiu, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Science, Beijing, 100190, China. Tel: +86-10-82618476; Fax: +86-10-82618476; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21474122), State Key Development Program of Basic Research of China (2012CB933200), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020300).

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CONCLUSION In this work, we carried out a conceptual study on whether AIE luminogen can be used to monitor the adsorption of PVA on CS particles. It was found that AIE luminogen could well distinguish free PVA molecules and those adsorbed on CS particles according to their rather different fluorescence emission behaviors. The FL emission intensity of AIE luminogen increased with PVA adsorption amount, and an adsorption isotherm was successfully established using the FL emission intensity as an index. The saturation point in this adsorption isotherm appears earlier than that obtained by the solvent relaxation NMR method, due to their different sensitivities for adsorption segments in specific conformation. E

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