Article pubs.acs.org/Langmuir
Characterizations of the Formation of Polydopamine-Coated Halloysite Nanotubes in Various pH Environments Junran Feng,† Hailong Fan,† Dao-an Zha,‡ Le Wang,† and Zhaoxia Jin*,† †
Department of Chemistry, Renmin University of China, 100872 Beijing, People’s Republic of China School of Science, Beijing Jiaotong University, No. 3 Shang Yuan Cun, Haidian District, Beijing 100044, People’s Republic of China
‡
S Supporting Information *
ABSTRACT: Recent studies demonstrated that polydopamine (PDA) coating is universal to nearly all substrates, and it endows substrates with biocompatibility, postfunctionality, and other useful properties. Surface chemistry of PDA coating is important for its postmodifications and applications. However, there is less understanding of the formation mechanism and surface functional groups of PDA layers generated in different conditions. Halloysite is a kind of clay mineral with tubular nanostructure. Water-swellable halloysite has unique reactivity. In this study, we have investigated the reaction of dopamine in the presence of waterswellable halloysite. We have tracked the reaction progresses in different pH environments by using UV−vis spectroscopy and surface-enhanced Raman spectroscopy (SERS). The surface properties of PDA on halloysite were clarified by X-ray photoelectron spectroscopy (XPS), SERS, Fourier transform infrared (FTIR) characterizations, zeta potential, surface wettability, and morphological characterizations. We noticed that the interaction between halloysite surface and dopamine strongly influences the surface functionality of coated PDA. In addition, pH condition further modulates surface functional groups, resulting in less content of secondary/aromatic amine in PDA generated in weak acidic environment. This study demonstrates that the formation mechanism of polydopamine becomes complex in the presence of inorganic nanomaterials. Substrate property and reaction condition dominate the functionality of obtained PDA together.
1. INTRODUCTION Marine mussel shows strong adhesive ability on wet rocks, and this interesting phenomenon inspires researchers to investigate.1−3 At the molecular level, catecholic amino acid, which is one important component in mussel adhesive proteins, contributes to the adhesive ability.4−7 The effect of catechol group is further confirmed by density state theory on atomic scale.8,9 New methodology has been developed for multifunctional coating on the basis of oxidative self-polymerization of dopamine.6,10 Numerous studies of multifunctional coatings based on dopamine polymerization were carried out.11−18 Because of the broadening application of polydopamine coating, exploring formation mechanism and chemical nature of polydopamine becomes urgently needed. Controlling surface chemistry of dopamine strongly depends on the understanding of formation mechanism of polydopamine, but that is still a debate in researcher’s community.19,20 Generally, polydopamine coatings are generated in weak alkaline conditions (pH > 8). However, the polymerization of dopamine can also be carried out in acidic or neutral conditions in the presence of some specific oxidants11,21,22 or by using specific reaction conditions.23 Using different oxidants and varied pH conditions further exacerbates the complexity of polymerization mechanism of dopamine.22,24,25 Researchers have recognized that the reaction conditions significantly influence the pathway of © 2016 American Chemical Society
polydopamine formation as well as the functional groups of polydopamine coatings.21,22,26 However, profound knowledge of the surface chemistry of polydopamine generated in different conditions is lacking yet. Halloysite (HNT) clay minerals (chemical composition: Al2Si2O5(OH)4·nH2O, 1:1 layer aluminosilicates) are natural tubular nanomaterials with high mechanical strength and good biocompatibility.27−29 HNT demonstrates a broad application as natural biomaterial, but the postmodification of HNT surface is difficult. Polydopamine-coated HNTs may divert the hard functionalization of natural halloysite to an easy pathway via the functional groups in polydopamine coating.13,30 On the other hand, vectorizing dopamine precursor ( L-3,4-dihydoxyphenylalanine, L-DOPA) or dopamine itself on inorganic matrices may result in controlled release of this important neurotransmitter, avoiding the side effects induced by the highdosage uptake of L-DOPA.31−34 Layered minerals like Mg/AlLDH,31 saponite,34 and laponite33 have been studied as loading matrixes for L-DOPA. Polymerization, even gelification of LDOPA, has been observed while it is loaded on inorganic nanomaterials.35 Jaber et al. have raised a question about the Received: August 8, 2016 Revised: September 19, 2016 Published: September 19, 2016 10377
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was added in this mixture. The mixture was stirred at room temperature for 48 h. Then the suspension was centrifuged at 10 000 rpm for 20 min. The sediment halloysite nanotubes were washed several times by fresh water and then dried by freeze-drying. It was named as Tris-HNT. The obtained solid Tris-HNT was grinded into fine powder to conduct morphological and chemical characterizations. To avoid the possible influence from buffers, we have also conducted this experiment in alkaline condition by using NaOH solution to adjust solution pH to 8.5. The obtained product was named as NaOH-HNT. 2.3. Preparation of Au@P-HNT, Au@Tris-HNT, Ag@P-HNT, and Ag@Tris-HNT. To test the surface reactivity of P-HNT and TrisHNT, P-HNT and Tris-HNT (2 mg) were dispersed in HAuCl4/ ethanol solution (concentration 20 mM, 2 mL) separately and kept for 2 days at room temperature in dark. Then P-HNT and Tris-HNT were separated from unreacted HAuCl4 solution by centrifugation and cleaned by fresh water. AuCl4− ions formed complex with polydopamine on halloysite, and they were changed to gold nanoparticles by polydopamine. These hybrid nanostructures are named as Au@PHNT and Au@Tris-HNT. To improve the Raman signal of P-HNT and Tris-HNT, Ag nanoparticles were decorated on P-HNT and Tris-HNT for SERS measurements. P-HNT or Tris-HNT (2 mg) was dispersed in AgNO3 aqueous solution (concentration 50 mM, 2 mL) and kept for 15 min in the dark; then the mixture were centrifuged at 10 000 rpm for 20 min, and the obtained sediment was washed by water. NaBH4 (0.02 M, 2 mL) was added in the above sediment. These P-HNT and TrisHNT decorated with Ag nanoparticles (Ag@P-HNT and Ag@TrisHNT) were separated with NaBH4 solution by centrifugation and cleaned by fresh water. They were ready for SERS characterizations. Their morphologies have been characterized by transmission electron microscopy. 2.4. Characterizations of P-HNT and Tris-HNT. 2.4.1. Characterizations of the Formation Progress of P-HNT and Tris-HNT. To track the reaction progress, the mixtures of dopamine and halloysite in different conditions were separated by centrifugation (10 000 rpm, 20 min) to two parts, supernatant and sediment, after desired reaction times. UV−vis adsorption of these supernatant samples was measured by Varian Cary 50. Both two parts were characterized by using SERS. For supernatant samples, Ag nanoparticles (their size is around 5−10 nm) were first deposited on Si wafer. Supernatant solutions were dropped on these Ag nanoparticles, and to avoid the change of chemical composition in supernatants (because it may contain some reactive species), the measurement was conducted immediately. The sediment samples were decorated with Ag nanoparticles as described in the preparation part (section 2.3) and then characterized by SERS. These samples were measured by Raman spectroscopy using a laser (785 nm, 25 mW) in the range between 800 and 1900 cm−1 (Horiba Scientific XploRA PLUS). Spectra were calibrated using the 520 cm−1 line of a silicon wafer, and the laser beam was focused by an optical microscope with a 10× objective, 1200 grooves/mm grating, and 100 μm slit. 2.4.2. Characterizations of P-HNT and Tris-HNT Nanocomposites. For morphological characterizations, the samples of raw HNT, P-HNT, and Tris-HNT nanocomposites were coated with a thin layer of gold before characterizations by scanning electron microscopy (SEM, JEOL 7401). Au@P-HNT and Au@Tris-HNT samples were characterized directly by SEM without any pretreatment. P-HNT, Au@P-HNT, Ag@P-HNT, Tris-HNT, Au@Tris-HNT, Ag@TrisHNT, and raw HNT were dispersed in water to form suspensions after sonication. Then a drop of the above suspensions was placed onto a copper grid for transmission electron microscopy characterization (Hitachi TEM, H-7650B) at an accelerating voltage of 100 kV. The cleaned HNT, P-HNT, and Tris-HNT nanotubes were dispersed in KBr pellets to conduct FTIR measurements (Bruker Tensor 27) at a resolution of 1 cm−1 in the range from 500 to 4000 cm−1. The chemical functional groups of P-HNT and Tris-HNT were detected by XPS (Axis Ultra, Kratos Analytical, Ltd.) with monochromatic Al Kα (hν = 1486.7 eV) radiation as the excitation and X-ray power of 150 W. All spectra were calibrated using the
interaction of L-DOPA and smectites (a kind of swelling phyllosilicate clay): vectorization or composite formation via polymerization?35 HNTs have many structural features including curvature, nanoscale size, surface charge, layered tube wall, inner lumen, and large surface area.27 They have been studied as carriers for various drugs,36−39 so they may also be a possible deliverer for dopamine or its precursor. However, swellable halloysite clays show unique reactivity.34 The hydrated clays are more easily intercalated by small molecules which will further assemble or even form oligomers in clay’s interlayers.40−42 We need to fully understand the interaction between halloysite and dopamine before developing the delivering system for neurotransmitter based on halloysite nanotubes. Aiming to explore the potential of halloysite as carriers for dopamine and the reaction mechanism of dopamine in the presence of water-swellable halloysite, we have studied the transformation of dopamine with water-swellable halloysite. In this article, we demonstrated a systematical investigation of polydopamine@halloysite hybrid nanocomposites generated in various pH environments. Surface-enhanced Raman spectroscopy (SERS) was first used to track the formation progress of polydopamine@halloysite hybrids in weak acidic or weak alkaline conditions. Surface functionality of polydopamine coating generated in different conditions were compared by a combining characterization of X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), and SERS. We observed that the existence of water-swellable halloysite and pH condition influence the functional groups of PDA together. Revealing the special interaction between dopamine and HNTs will help us understand the transformation mechanism of organic molecules in confined space, which is an important knowledge for choosing inorganic nanomaterials as carriers for drugs. Moreover, because of the inherent biocompatibility of halloysite and the additional functionality of polydopamine coatings, PDA@HNT nanocomposites will present broad applications in the biomedical field.
2. EXPERIMENTAL SECTION 2.1. Materials. Water-swellable halloysite mineral (HNT) was purchased from Zhengzhou Jinyangguang Ceramics Co. Ltd. Pristine HNT was cleaned by fresh water, dried in freeze-drier, and then grinded into a fine powder by mortar before treatment. Dopamine hydrochloride (purity 98%) was obtained from Sigma-Aldrich. Tris(hydroxymethyl)aminomethane (Tris)−HCl buffer (1.5 M, pH = 8.8) was obtained from Shanghai BioScience Co., Ltd. Chloroauric acid (HAuCl4), silver nitrate (AgNO3), sodium borohydride (NaBH4), sodium hydroxide (NaOH), and ethanol (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. All solutions were prepared by using Millipore water. 2.2. Preparation of PDA@HNT Hybrid Nanocomposites. Cleaned halloysite nanotubes (50 mg) were dispersed in dopamine aqueous solution (4 mL). The concentrations for dopamine are varied from 3 to 5 to 8 mg mL−1. The pH value of these suspensions was in the range of pH 6.1−6.5 depending on the concentrations of dopamine solutions. These suspensions were magnetically stirred for 48 h at room temperature. The black products were centrifuged at 10 000 rpm for 20 min to separate the halloysite sediments and supernatant. The sediment was washed several times with fresh water and then dried by freeze-drying. It was named as P-HNT. The obtained solid P-HNT was grinded into fine powder by mortar before conducting morphological and chemical characterizations. To prepare PDA@HNT in weak alkaline condition, dopamine (20 mg) and halloysite nanotubes (50 mg) were mixed in 4 mL of deionized water, and then Tris-HCl buffer (26.67 μL, 1.5 M, pH = 8.8) 10378
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Langmuir hydrocarbon C 1s peak (284.4 eV). High resolution scans were acquired for C 1s and N 1s regions. Three batches of samples have been measured. Wide-angle X-ray diffraction was performed with a XRD-7000 diffractomer (Shimadzu) in the reflection mode using Cu target (λ = 0.154 18 nm) as incident X-ray. Raw HNTs, dried and wet waterswollen halloysite (water-swollen halloysite was named as W-HNTs), and dried and wet P-HNTs were measured in the same apparatus. The scan step was 0.01°, and the scan scope was from 5° to 50°. The dried powders of HNT, P-HNT, and Tris-HNT were further characterized using a TGA Q50 (TA Instruments) under nitrogen flow of 40 cm3 min−1. The explored temperature range was from 25 to 750 °C at a heating rate of 10 °C min−1. The zeta-potential values of P-HNT, TrisHNT, and raw HNT were measured using a Brookheaven ZetaPALS (Brookheaven Instrument, USA) at 25 °C. Contact angle measurements for raw HNT, P-HNT, and Tris-HNT were performed by using an optical contact angle apparatus (15 μL droplet) (KRÜ SS drop shape analyzer DSA30).
conditions was used to identify the reaction progress of dopamine in the starting stage under different oxidants.22 Figure 2 demonstrates UV−vis spectra of the supernatant solutions of dopamine−halloysite mixtures after varied reaction times in different pH conditions (with or without Tris buffer). We noticed that when dopamine solution was mixed with halloysite in the absence of any buffer solution, their supernatant solutions showed no clear absorption peak in the focused range of UV−vis spectra (Figure 2a), which is similar to the case of copper sulfate as oxidant for dopamine at pH 5, in which the Cu2+−dopamine complex is generated.22 In the halloysite case, the formation of hydrogen bond or coordination between dopamine and halloysite surface dominates in weak acidic conditions. Previous studies demonstrated that some molecules like urea, hydrazine, and acetamide can form a strong hydrogen bond with a silicon tetrahedral sheet;27 catechol groups are also anchored on the saponite layers via the hydrogen bond.35 The broken edges of enrolled halloysite layers may expose a large amount of silicate and aluminate sites which have a strong tendency to form hydrogen bonds or coordinate bonds30,43 with catechol groups or NH2. On the other hand, the transformation of dopamine to dopaminechrome is well documented by the appearance of peaks at 305 and 480 nm in the UV−vis spectrum.44,45 However, in the TrisHNT case, two peaks are observed at 420 and 488 nm (Figure 2b), which is similar to that of semiquinone dimer.46 Therefore, these UV−vis spectra reflect that the presence of halloysite in the oxidative self-polymerization of dopamine influences the transformation of dopamine at the early stage in both two cases. Moreover, we have carefully characterized the reaction progress by using SERS. After a desired reaction period, the mixture of dopamine and halloysite was separated to supernatant and sediment parts by centrifugation, and then these two parts were characterized individually. Compared with small molecules in the supernatant which easily contact with silver nanoparticles and whose signals are easily enhanced by SERS, identifying chemical structures of treated halloysite (the sediments) by SERS characterization is much harder. To utilize the enhancement effect of silver nanoparticles,47 treated HNTs in sediments were changed to Ag@HNTs as described in the Experimental Section. Figure S2 demonstrates that silver nanoparticles are loaded on dopamine-treated halloysite nanotubes no matter with or without Tris buffer. The close contact of the coating layer on halloysite and silver nanoparticles ensures the efficiency of SERS characterizations. If halloysite nanotubes have not been treated by dopamine, it is hard to anchor silver nanoparticles on halloysite, giving a low Raman intensity. SERS characterizations of the supernatant and the sediment of dopamine−HNT mixtures after different reaction periods are shown in Figure 3, in weak acidic conditions (Figure 3a,c) and weak alkaline conditions (Figure 3b,d). In the Raman spectrum of dopamine, the bands at 1425 and 1559 cm−1 are identified as benzene ring stretching modes.48 The peak at 1471 cm−1 belongs to the ring stretching vibration contributed mainly from the stretching of the C−C bond to which the oxygens are attached.48,49 The peak at 1254 cm−1 is assigned to in-plane CH2 bending, and the other peak at 1594 cm−1 is assigned to H−N−C in-plane bending vibration.50 It is really surprising to observe that the supernatant presents different signals only after 30 min reaction time. The pristine dopamine solution shows no identical peak in the range of 1350−1300 cm−1; therefore, the appearance of new SERS peak
3. RESULTS AND DISCUSSION Figure 1 presents the color change of the mixture of halloysite and dopamine in weak acidic conditions (a) and in weak
Figure 1. (a) Color changed while halloysite was mixed with dopamine hydrochloride and stirred for 2 days, with (named as Tris-HNT) or without Tris-buffer (named as P-HNT). (b) Color of original halloysite (HNT) has been changed to dark gray (P-HNT) or black (Tris-HNT).
alkaline conditions (b). If no Tris-buffer was added, the starting pH value of the mixture is 6.3. After 2 days reaction, the pH of mixture solution went down to 4.7. In the period of stirring, we noticed that the color of both HNT suspensions gradually changed from original colorless to deep gray (Figure 1a). The color change in weak alkaline conditions is faster than that in weak acidic conditions. The color change is a sign of the transformation of dopamine that happens in both conditions. On the other hand, the clear volume expansion of halloysite after mixing with dopamine solution indicated that these halloysite were swollen in reaction. Wide-angle XRD characterizations confirmed that these halloysite can be swollen by water and dopamine aqueous solution (Figure S1). Apart from the layered distance (d001) of 7 Å (2θ ∼ 11.85) in the dried HNTs, other peaks at 2θ ∼ 8.79 in water-swollen HNT or 2θ ∼ 8.73 in dopamine-solution-swollen HNT are observed. These new peaks are assigned to 10 Å interlayer distance based on the literature.27 The reaction progress of dopamine and halloysite in different pH conditions was tracked by UV−vis spectroscopy and surface-enhanced Raman spectroscopy. The evolution of the UV−vis spectra of dopamine solution in different reaction 10379
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Figure 2. UV−vis spectra of supernatant solutions of dopamine−halloysite mixtures after different reaction periods in two conditions. (a) Reaction was conducted without using buffer solution. (b) Reaction was conducted in the presence of Tris buffer solution.
Figure 3. SERS characterizations of the reaction progress of dopamine and halloysite in different pH conditions. (a, b) SERS of supernatant solutions of dopamine and halloysite after different reaction periods: 0.5, 1, 2, 5, and 11 h. Dopamine hydrochloride aqueous solution was tested as contrast. (a) Without buffer and (b) with Tris-buffer. (c, d) SERS of reacted halloysite after varied reaction periods: 2, 6, 10, 16, 24, and 30 h. Pristine halloysite was measured as contrast. (c) Without buffer and (d) with Tris-buffer.
(1312 cm−1) in this range is remarkable. On the basis of the literature, we assigned this peak to the indole ring vibrations or C−N stretching.47 The appearance of indole ring is an important signal showing cyclization step in the reaction progress of dopamine. Such a cyclization step is nearly confirmed in the oxidative self-polymerization of dopamine in weak alkaline conditions. In acidic conditions, the cyclization
(i.e., the formation of indole) has been observed in the presence of Fe3+ ion as oxidant.51 In halloysite case, the cyclization may relate to the charge transfer while dopamine was anchored on halloysite surface by monodentate−bidentate bond.30 On the other hand, in these sediment samples (Figure 3c,d), another peak at 1344 cm−1 (1325 cm−1 in Figure 3d) has appeared, which is assigned to the bonds connecting phenyl 10380
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Langmuir groups in polydopamine in the literature.52 It may indicate that the connection was formed between the phenyl groups of dopamine while they are adsorbed on HNTs. The peaks around 950−940 cm−1 are related to C−H out-of-plane deformation mode and the O−H out-of-plane deformation mode.53 These peaks in 1250−1000 cm−1 spectral range can be assigned to C− H and N−H in-plane deformation vibrations.53 These features can be identified in both sediment and supernatant samples generated in two conditions. In particular, the peaks at 1145 cm−1 is assigned to the N−H in-plane deformation based on the literature.47 We can clearly observe it in both supernatant samples (Figure 3a and 1153 cm−1 in Figure 3b) and even in the sediment in weak acidic conditions (1136 cm−1 in Figure 3c). But it merges into the broad peak around 1200−1100 cm−1 of sediment sample produced in weak alkaline conditions. We summarize these SERS information in Table 1, and the
Figure 4. TEM images of polydopamine-coated HNTs in different pH conditions: (a) in weak acidic conditions; (b) in weak alkaline conditions.
HNT samples endows their surface reactivity similar to polydopamine, which was verified by the formation of gold nanoparticles on polydopamine layer via coordination with AuCl4− ions. Au@P-HNT and Au@Tris-HNT were characterized by TEM (Figure 5). We noticed that a thin layer
Table 1. SERS Characterizations of the Supernatant and the Sediment of Dopamine/Halloysite Mixtures Obtained in Different pH Conditions P-HNT Raman (cm−1) 960
in supernatant 956
1145
1153
1254 1312
1254 1312
1415 1471 1574
1430 1471 1557
1594
1594 in sediment 951 1185
946 1136, 1118
a
Tris-HNT Raman (cm−1)
1344
1325
1485 1585
1587
assignmentsa pyrrole C−H out-of-plane deformation pyrrole N−H in-plane deformation/ ring breathing in-plane CH2 bending indole ring vibrations or C−N stretching pyrrole ring stretching stretching of the C−C bond CC aromatic/pyrrole ring stretching H−N−C in-plane bending vibration. C−H out-of-plane deformation C−H and N−H in-plane deformation vibrations indole ring vibration or C−N stretching pyrrole ring vibrations CC aromatic/pyrrole ring stretching
The assignments were conducted based on literatures.47,52,53
Figure 5. TEM images of (a, b) Au@P-HNT and (c, d) Au@TrisHNT.
tentative assignments of all peaks were conducted based on literatures.47,52,53 The deposition of Ag nanoparticles on polydopamine layers has shown excellent performance as surface-enhanced Raman scatter substrate.54,55 In our characterizations, we observed that Ag nanoparticles can also enhance the Raman signals of polydopamine itself. The SERS characterization of P-HNT and Tris-HNT products after 2 days reaction is shown in Figure S3. They are nearly same as that of sediment samples that reacted 30 h. We further conducted characterizations for P-HNT and TrisHNT. Figure 4 presents TEM images of P-HNT and TrisHNT. Thin coating layer can be observed on the tube surface of P-HNT and Tris-HNT. Polydopamine layer has successfully coated on inner surface and outer surface of halloysite either in weak acidic or in weak alkaline conditions. The thickness of coating layer on outer surface of halloysite is ca. 5 nm. Although the coating layer is very thin, the chemical property of polydopamine persists. The existence of coating layers on both
covered on gold nanoparticles can be observed in both Au@PHNT and Au@Tris-HNT cases (Figures 5b and 5d). Some gold nanoparticles are even trapped inside halloysite (Figure S4). The successful coating of polydopamine on HNT nanotubes in both conditions can also be confirmed by the change of surface charge of P-HNT and Tris-HNT. The inner surface of original HNT consists of Al−OH groups, whereas their external surface is composed of siloxane groups (Si−O− Si), which allow them a positively charged lumen and negatively charged outer surface in pH 2−8.29 The pristine HNT in water shows zeta-potential value of −23 mV (pH ∼ 6), which is similar to that reported in the literature (−19 mV) (Figure 6).56 Zeta-potential values are clearly changed after HNT mixed with dopamine solutions. P-HNT and Tris-HNT show isoelectric point at pH ∼ 4 or pH ∼ 4.5, respectively, which are similar to pure PDA and PDA modified surface.57 Furthermore, we have checked the wettability of P-HNT and Tris-HNT by using 10381
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Figure 6. Varied zeta-potential values of P-HNT, Tris-HNT, and raw HNT in the pH range of 2−10.
Figure 8. TGA curves of pristine halloysite, P-HNT, and Tris-HNT.
and Tris-HNT (Figure 9, Figure S7, and Table 2). We chose one batch of samples as an example. The survey scan shows
pristine halloysite as a contrast (Figure 7). Water contact angle was measured to illustrate the surface properties. It is ∼30° for P-HNT and ∼46.9° for Tris-HNT; the initial HNT is more hydrophilic (water contact angle ∼15°). The larger change of contact angle of Tris-HNT may be due to thicker coating on Tris-HNT. The different coating amount of polydopamine on P-HNT and Tris-HNT can be measured by using thermogravimetric analysis (Figure 8). Pristine HNTs showed small weight loss at 200−250 °C due to the leaving of H2O in layers and the second weight loss from 400 to 500 °C because of the loss of structural water.58,59 Compared with raw HNTs, in the range of 200−250 °C, P-HNTs and Tris-HNTs showed more weight loss (ca. 0.8% and 2.7%, respectively). It indicated that except the leaving of interlayer water in HNT, some physically adsorbed dopamine was decomposed. In the range of 300−500 °C, the loss of structural water of HNT accompanies with the decomposition of polydopamine, resulting in a comparatively higher weight loss in P-HNT and Tris-HNT, ca. 16.1%(PHNT) and 17.6% (Tris-HNT) vs 13.8% (HNT). The amount of PDA in Tris-HNT (∼3.8 wt %) is more than that in P-HNT (∼2.3 wt %). The variation of dopamine concentration showed no distinct influence to the coating amount of P-HNT and Tris-HNT (Figure S5). The chemistry features of P-HNT and Tris-HNT were analyzed based on FTIR and XPS characterizations. In FTIR spectra (Figure S6), original HNT shows no observed peak in the range of 3400−3500 cm−1; however, both P-HNT and TrisHNT have similar peaks at 3488 and 3452 cm−1 (3487 and 3455 cm−1 for P-HNT). They are assigned to the stretching mode of N−H in polydopamine coating. Other representative peaks of PDA for indole or indoline structures60,61 are unobserved because they merged into strong peaks of HNT itself (Figure S6). XPS characterization gives clear information on elements and functional groups of coating layers on P-HNT
Figure 9. High resolution XPS spectra of C 1s and N 1s core level in P-HNT and Tris-HNT. (a) XPS of P-HNT, C 1s core-level. (b) XPS of P-HNT, N 1s core level. (c) C 1s core-level of Tris-HNT. (d) N 1s core-level of Tris-HNT.
atomic percentages of all elements in P-HNT and Tris-HNT, in which carbon and nitrogen originated from the coating layer (Figure S7 and Table S1). The values of N/C are 0.144 in PHNT and 0.150 in Tris-HNT, which are in the range of reported N/C values of polydopamine.7 In addition, the atomic percentage of C 1s and N 1s in Tris-HNT is nearly doubled compared with that in P-HNT, indicating the coating amount of Tris-HNT is higher than that of P-HNT, which coincides with TGA result. The C 1s high-resolution spectrum of P-HNT can be peak-fitted into three components corresponding to
Figure 7. Contact angle measurements of raw HNT, P-HNT, and Tris-HNT. 10382
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± 4.9% for P-HNT and 32.8 ± 6.3% for Tris-HNT, Table S2) compared with that in PDA (10%),61 only 2.3% in P-HNT and 8.3% in Tris-HNT. We speculated this may relate with the bonding mode of dopamine on HNT, which is the bidentate or monodentate coordination or hydrogen bond via catechol groups.30 Anchoring on HNT will inhibit the change of dopamine from phenol to quinone. In case of Tris-HNT, the increased amount of CO may originate from polydopamine generated in solution in the beginning and then adhere to HNT.62 The N 1s peak is fitted with three components that are primary (R−NH2, 56.7%), secondary (R1−NH−R2, 32.1%), and tertiary/aromatic (CN−R, 11.2%) amine functionalities; those for Tris-HNT are 40.2%, 57.3%, and 2.5%, respectively. We noticed that in three batches of P-HNT and Tris-HNT the contribution of primary amine group is surprisingly high (53.0
Figure 10. (a) Molecular structures of intermediate species and their tautomers in oxidative polymerization of dopamine.40 (b) Illustration of proposed polymerization and connection modes of dopamine on halloysite. 10383
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our knowledge of interface interaction.69,70 Clay minerals abound on earth. Hydrate minerals with specific interlayer region show great potential as catalysts for organic synthesis.42 Investigation combining in situ surface characterizations and simulations will deepen our understanding of this complex system, guiding its applications in various fields.
the thermal decomposition of polydopamine. Therefore, we believe that the high content of primary amine in P-HNT and Tris-HNT is not only due to the physical adsorption of dopamine but also from the polymerized dopamine. We suppose that part of immobilized dopamine may take polymerization on HNT, either in the case of P-HNT or Tris-HNT. The content of primary amine in Tris-HNT (or NaOH-HNT) is lower than that in P-HNT, indicating the reaction medium also contributes to its surface functionality. The relatively higher sum of secondary and aromatic amine in Tris-HNT and NaOH-HNT is in accordance with its reaction condition (i.e., the weak alkaline condition). It leads to a question: why the weak acidic condition will induce polydopamine coating on P-HNT with higher primary amine? Chemical functionality is a clue to discover possible formation pathway. Della Vecchia et al. have indicated that the uncyclized catecholamine favors a dimerization via covalent bond in high concentration of dopamine, but cyclized units (indole) pass another route through the coupling and polymerization of DHI.66 In our SERS characterizations, we observed the formation of indole or pyrrole in these supernatant samples and sediment samples, no matter in weak acidic or alkaline conditions, indicating that cyclization of dopamine on halloysite happened, at least in part of adsorbed dopamine. (These indole species observed in supernatant samples may be originated from desorbed molecules from HNT.) In weak acidic condition, the number of uncyclized dopamine should be higher than that in weak alkaline condition due to the lower cyclization speed in acidic environment.67 The uncyclized dopamine can carry out reaction via dimerization, which is favored in high concentration of dopamine.66 As we know, the concentration of dopamine on the surface of halloysite is much higher than that in solution because of the strong adsorption of halloysite. The study of the organization of catecholate monolayer on the anatase surface has observed that it is energetically convenient for adsorption of catechol to pair up in nearest-neighbor positions,68 showing a self-organizing tendency in the adsorption of catecholate. Such progress may stimulate the adsorbed dopamine to assemble or polymerize, resulting in catecholamine-based oligomers product. A similar reaction pathway has been reported in the L-DOPA polymerization in the presence of laponite which shows significant differences due to “adsorbed phase” on mineral surface, resulting in the polymerized product before indole ring formation.33,35 On the basis of the characterizations mentioned above, we observed that PDA covers HNT in both weak acidic and weak alkaline conditions. The strong adsorption of halloysite influences the reaction of dopamine, resulting in relative lower content of secondary and aromatic amine in polydopamine coated on halloysite. The high concentration of adsorbed dopamine on halloysite favors a catecholamine-based polymerization pathway that is more significant in the weakly acidic condition. We tentatively proposed the polymerization progress and connection mode of dopamine on HNT surface in Figure 10b. The scheme in Figure 10b is only concerned with the first layer of polydopamine adhering tightly on the halloysite surface. Although there are still undiscovered questions in the dopamine/halloysite system, this study reveals the complexity of reaction mechanism of dopamine in the presence of reactive clay minerals and in various pH conditions. Recently, the investigations of adhesive interaction between catechol and various solid surfaces at the single molecule scale largely enrich
4. CONCLUSION In conclusion, we have demonstrated the comparative studies of polydopamine formation on water-swellable halloysite nanotubes in different pH environments. We first tracked the reaction progress by using UV−vis and SERS characterizations. We also conducted morphological, surface charge, surface wettability, and chemical characterizations of P-HNT which is generated in weak acidic condition and Tris-HNT (or NaOHHNT) which is obtained in weak alkaline condition. The thickness of the coating layer on the outer surface of halloysite is ca. 5 nm, but that on the inner surface is thinner. The difference in the thickness of outer-surface coating and innersurface coating of halloysite may be due to the blocking effect of small tube mouth (∼15 nm). We noticed that the chemistry of polydopamine coating on halloysite is different from free polydopamine, showing low contents of CO and secondary/ aromatic amine functional groups, which may be because of the strong binding between halloysite and dopamine (or its derivatives). In addition, weak acidic environment further enhances the primary amine amount due to the increasing possibility of catecholamine-based polymerization in adsorbed dopamine on HNT. This study not only deepens our understanding of polydopamine chemistry but also benefits the application of PDA@halloysite nanocomposites.
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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.6b02948. Tables S1−S3 and Figures S1−S10 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*(Z.J.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grants 21374132) for financial support.
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REFERENCES
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