Stability of Polydopamine Coatings on Gold Substrates Inspected by

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Stability of Polydopamine Coatings on Gold Substrates Inspected by Surface Plasmon Resonance Imaging Wei Yang, Chanjuan Liu, and Yi Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03143 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Stability of Polydopamine Coatings on Gold Substrates Inspected by Surface Plasmon Resonance Imaging Wei Yang1, 2, Chanjuan Liu1, 2, Yi Chen1, 2, 3* 1 Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 Beijing National Laboratory for Molecular Sciences, Beijing 100190, China KEYWORDS: polydopamine, coating stability, pH, organic solvent, surface plasmon resonance imaging

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ABSTRACT

Polydopamine (PDA)-based surface modification has been used in a variety of fields. However, a vague impression on the stability of PDA still exists due to a lack of systematic studies. To ascertain the issue and make better use of this surface modification method, a technique of surface plasmon resonance imaging (SPRi) was exploited to study the stability of PDA coated on gold surface. The results showed that PDA-coating stability was largely dependent on pH of aqueous solutions, giving detachment ratios up to 66% and 80% at pH 1.0 and pH14.0, respectively. However, increasing the ionic strength of aqueous solutions can reduce the detachment of PDA in strongly acidic and strongly alkaline conditions. Besides, organic solvents also make a difference on the PDA-coating stability. Among the tested 10 kinds of organic solvents, including n-hexane, toluene, ethyl ether, tetrahydrofuran, ethyl acetate, iso-propanol, acetone, acetonitrile, dimethyl formamide (DMF) and dimethylsulfoxide (DMSO), DMSO caused the most serious detachment of PDA, up to 56%, followed by DMF with a detachment ratio of 31%. Ultrasonication caused less than 10% detachment of the coated PDA. It should be mentioned that the PDA coatings deposited on gold surface were not detached completely in all the test conditions, even at pH 14.0 (ca 20% PDA retained). In alkaline conditions, detachment competes with further polymerization, which made slight increase of the SPRi signals at pH 9.011.0. Based on the obtained information about PDA-coating stability, thickness-controllable and alkali-resistant PDA coatings were prepared. Moreover, the alkali-resistant PDA coatings remain reactive

to

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INTRODUCTION

Polydopamine (PDA) was first proposed by Lee et al. in 2007 inspired by the adhesion behavior of marine mussels.1 It can deposit spontaneously on nearly all materials (even including lowsurface-energy materials, such as Teflon2) through a simple dip-coating procedure. Furthermore, a variety of functional ad-layers can be created via electroless metallization, Michael addition or Schiff base reactions.1,3,4 Thus, such a facile and versatile surface modification technique quickly penetrates into a wide range of research fields, such as biomolecule immobilization,3 water treatment,5,6 antifouling surface,7 Li-ion batteries,8 controlled drug delivery,9-11 SPR-signal amplification,12 and others.4,13,14 The question is whether the PDA coatings and PDA-based ad-layers are stable enough in practical use, or in other word, what kind of conditions they can withstand. The stability impacts largely on the performance and lifespan of PDA-coated materials and PDA-based ad-layers. Unluckily, the stability information is quite deficient. Only several isolated conditions had been explored, such as 0.1 M HCl, 0.1 M NaOH, acetone and so on.15-18 Although we have known that PDA cannot endure the conditions of strongly acidic and alkaline solutions and polar organic solvents, it remains ambiguous about what conditions it can be used with confidence. This initiated us to carry out a systematic study to acquire more information on the stability of PDA coatings. PDA stability can be characterized by weighting the deposited and detached mass of PDA.17 However, the measured results are a bit of rough since the mass changes are tiny relative to the mass of substrates. Some researchers studied the PDA stability with UV-vis by comparing the absorbance of eluates at a certain wavelength.17,18 However, it may be biased since the


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polymerization degree and composition of the eluates vary with elution conditions. Besides, Shao et al. used thermal gravimetric analyzer and transmission electron microscopy to characterize the stability of PDA coated on nanoparticles.19 In general, the stability is investigated by ellipsometry15,16 and atomic force microscopy (AFM)20 according to the thickness changes of PDA coatings before and after being treated by each test condition. Surface plasmon resonance imaging (SPRi) is another potential choice since it can sensitively perceive the variation of membrane thickness in the vicinity of thin metal layers.21 Moreover, it facilitates to monitor the formation and detachment process of PDA coatings in real time, offering a new way to have an insight into the stability of PDA coatings. In this paper, we proposed a SPRi approach to inspect the stability of PDA coated on gold substrates. A series of conditions, including aqueous solutions at pH from 1 to 14, different ionic strengths, ten kinds of organic solvents and ultrasonication, the stability of PDA coatings in were measured by the proposed method. Quite systematic and detailed information was provided on the stability of PDA coatings and their reaction behaviors. A competition process between the detachment and further polymerization of PDA on gold substrates was unexpectedly observed in alkaline environments. At last, thickness-controllable and alkali-resistant PDA coatings were prepared. EXPERIMENTAL SECTION Chemicals and solutions Tris(hydroxylmethyl)aminomethane (Tris), HCl (36%-38%), NaOH, sodium chloride, n-hexane, toluene, ethyl ether, tetrahydrofuran (THF), ethyl acetate, iso-propanol, acetone, acetonitrile, N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were obtained from Beijing


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Chemical Works (Beijing, China). Dopamine hydrochloride (99%) was purchased from Alfa Aesar (MA, USA). Bovine serum albumin (BSA) was from Amresco (OH, USA) and anti-BSA was from Beijing Solarbio Technology Co., Ltd (Beijing, China). Dopamine solutions were prepared immediately before use by dissolving 2 mg/mL dopamine hydrochloride in 10 mM Tris-HCl at pH 8.5. Phosphate buffers (PB, 10 mM) were prepared in water and adjusted to pH 1-14 with HCl or NaOH. The water used was purified by a Milli-Q water purification system (Millipore, Milford, MA, USA). SPRi apparatus All SPRi experiments were conducted on a laboratory-built SPRi apparatus cooperated with a laboratory-edited workstation.22 The apparatus was fabricated based on Kretschmann configuration. Briefly, a beam of p-polarized light was collimated and directed toward a prism attached with a gold chip via refraction index matching oil (i.e. cedar oil). The reflected light filtered through a narrow bandpass filter (λ = 645 nm) was collected by a CCD camera (WAT902B, Watec Co., Ltd., Japan) and analyzed by the workstation (Figure S1).23-26 Deposition of PDA on gold sensing chips The gold sensing chips were prepared by depositing 48 nm-thick gold film on top of 2 nm-thick chromium adherent layer on glass slides (2.4 cm ×2.4 cm, Shitai Co., Ltd., China) by vapordeposition in a JEE-420 vacuum evaporator (JEOL Ltd., Tokyo, Japan). They were cut into 8 smaller pieces (1.2 cm× 0.6 cm) for later use. Before performing deposition experiments, the gold chips were ultrasonically cleaned in acetone, water and ethanol successively, dried by nitrogen gas stream, and then installed on SPRi analyzer as shown in Figure S1. PDA deposition


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was performed in on-line SPRi by first pumping in water till stable to obtain initial signals. Then 2 mg/mL dopamine monomer solution was flowed through the sensing surface for a duration time of 1 h to deposited PDA coatings. Finally the formed PDA coatings were rinsed with water for 10 min to replace the dopamine solution and remove unfixed PDA. The flow rate was kept constantly at 60 µL/min. SPRi signals were recorded in real time. Detachment of PDA coatings The in situ deposited PDA coatings were subjected to detaching experiments under various conditions, including aqueous solutions at pH from 1 to 14, aqueous solutions at pH 1.0 and pH 13.0 with different ionic strength, ten kinds of organic solvents (n-hexane, toluene, ethyl ether, THF, ethyl acetate, iso-propanol, acetone, acetonitrile, DMF and DMSO) and ultrasonication. For conditions of aqueous solutions and iso-propanol, DMSO, the detaching experiments were performed online continually by pumping in each tested solution for 1 h, followed by washing with water till signal stable. For the rest organic solvents, detaching experiments were performed offline because they can dissolve and/or swell the plastic elements (e.g., polymethyl methacrylate (PMMA) flow cell and polydimethylsiloxane (PDMS) spacer) used in the flow system. Briefly, the PDA-modified chips were taken from the SPRi apparatus and immersed in the tested organic solvents (2 mL) for 1 h under shaking (MS 3 digital, IKA®-Webke GmbH & Co.KG, Staufen, Germany). Then, they were dried by nitrogen gas stream and installed back on the SPRi analyzer to gather signals against water. The off-line protocol was also used in checking the stability of PDA coatings under ultrasonication. Instead of organic solvents, the chip modified by PDA was submerged in 2 mL water and subjected to ultrasonication at 200 W for 1 h with a KQ5200DE ultrasonic cleaner from Kun Shan Ultrasonic Instruments Co., Ltd. (Jiangsu, China). Each condition was performed in triplicate.


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The morphology of PDA coatings was characterized by atomic force microscope (AFM, Nanoscope V, Bruker). PDA coatings were deposited for 1 h by on-line SPRi modification. For alkali-treated PDA, the PDA coatings were rinsed with PB at pH 10.0, 11.0 and 12.0 for 1 h, respectively. RESULTS AND DISCUSSION SPRi method to determine the PDA-coating stability SPRi is commonly used to detect biomolecules in the field of biosensing.27 SPRi signal responds to the change of refractive index near the sensing metal (e.g., gold) surface in theory.27,28 All processes that alter the refractive index near the sensing surface may induce a response of SPRi signals. Therefore, the processes of PDA depositing on and removing from the sensing surface can be observed with SPRi. Figure 1 illustrates the complete experimental process and a realtime SPRi intensity curve measured at the condition of pH 13.0. Firstly, water is pumped through the gold sensing surface until signals become stable to obtain initial signal values. Then, water is replaced by dopamine monomer solution. The sharp signal increase in the beginning is due to the difference of refractive index between water and dopamine solution. Soon, the signals turn to increase nearly linearly with time, indicating that PDA starts to deposit on the gold surface. After 1 h of modification, the formed PDA is rinsed with water, giving a net signal increment of ∆i, which represents the amount of PDA deposited. Later, the deposited PDA is subjected to detachment experiment under various conditions we interested in. The detachment of PDA, if happens, will lead to a signal drop (in the premise of without any structure change). Finally, the sensing surface is washed with water again. The net signal reduction, ∆r, represents the detached amount of PDA. Accordingly, detachment ratio of PDA can be calculated by ∆r/∆i:


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Detachment Ratio = (∆r/∆i)×100% A high detachment ratio indicates low stability of PDA coatings. (Refer to Supporting Information for a more detailed calculation process.)

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30

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Figure 1. (A) Schematic illustration of the experimental process. Bare gold chips were first modified by PDA, and then they were subjected to detaching test conditions we interested in. (B) Typical real-time SPRi curve measured at the condition of pH 13.0, which is used to show the calculation method of detachment ratios. ∆i is the signal increment after PDA deposition; ∆r is the signal reduction after PDA contacting the detaching test condition. In the above discussion, a prerequisite of the SPRi method is that the SPRi signal variation is proportional to the deposited or detached amount of PDA, at least in the range of signal variation we involved in. To prove it, we design an experiment of multiple PDA deposition, namely, alternating pumping in 15 min of dopamine monomer solutions and 5 min of water. The real-


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time SPRi signal vs. time curve for 10 cycles is plotted in Figure 2. Dopamine solutions were prepared immediately before use. Since dopamine concentration, deposition time and other reaction conditions were unchanged, the deposited amounts of PDA are same for each modification step. If SPRi signal variation is proportional to the deposited amount of PDA, the signal increments are equal for each modification steps. Indeed, the total signal increments (SPRi signal values after each PDA deposition cycle minus initial signal values before PDA deposition) have a linear relationship with the number of modifications (inset in Figure 2), with a linear correlation coefficient of 0.999. The signal variations we used in PDA-stability inspection (deposition for 1 h) completely locates in the linear range. Therefore, the calculated results of detachment ratios based on the SPRi method are reliable.

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Figure 2. Real-time SPRi curve of stepwise PDA deposition by alternative pumping in 15-min dopamine monomer solutions and 5-min water. The dopamine monomer solutions were prepared just before use in each deposition step. The inset shows the regression line of total SPRi signal increments to the number of modifications. R2=0.999 for the total 10 time modifications. pH-caused detachment


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PDA-coating stability had been inspected only in strongly acidic and alkaline conditions previously, such as typical 0.1 M HCl and 0.1 M NaOH,17,18 but its stability in other commonly used pH conditions remains unclear. To gain an overall insight into the issue, we systematically checked the stability of PDA in the whole pH range for the first time, to the best of our knowledge. To avoid the impact of buffer reagents on PDA stability, phosphate buffers were first used throughout whole the pH range of 1.0-14.0. Figure S3 displays the real-time SPRi curves measured at each pH condition and Figure 3 collectively illustrates the variation of PDA stability with pH. Serious detachment happens at both strongly acidic and strongly alkaline conditions and the detachment ratios are higher in strongly alkaline conditions than in strongly acidic conditions (e.g., the detachment ratio is (66±9)% at pH 1.0, and (75±10)% at pH 13.0), which agrees well with the previous report.17 The detachment ratios are much lower in the near neutral range, below 15% at pH 4-11. The rationale is that PDA is assembled primarily by noncovalent interactions29,30 and an abundant of amino and catechol groups are incorporated in the PDA structure. Each structural unit of PDA carries one positive charge in strongly acidic conditions where the amine group is protonated, and in strongly basic conditions, each structural unit carries two negative charges because of the deprotonated catechol group. Therefore a strong electrostatic repulsion is produced under extreme pH environments and the PDA aggregates are dissociated.17


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Detachment Ratios/ %

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80 60 40 20 0 -20

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Figure 3. Detachment ratios of PDA coatings in the whole pH range. At the acidic side, detachment ratios decrease quickly from pH 1.0 to pH 4.0 and are almost unchanged from pH 5.0 to pH 7.0, which is parallel to the weakening electrostatic repulsion. However, the case is somewhat complicated at the basic side. The detachment ratios are very tiny for pH 8.0-10.0, but turn to negative at pH 11.0 and increase dramatically after pH 11.0. The unexpected negative detachment ratio is caused by the resumed polymerization reaction. Different from the cases in acidic conditions, PDA coatings can resume the polymerization reaction in alkaline conditions because the freshly deposited PDA coatings are not oxidized and polymerized completely,17,31,32 which was indirectly confirmed by checking the eluates. Figure S4 illustrates that all the eluted solutions continue to react in the alkaline conditions, accompanied with color changes from light to dark. Morphologies of original PDA coating and PDA coatings after being treated with PB at pH 10.0, pH 11.0 or pH 12.0 were characterized with AFM (Figure 4A). It can be seen that the small granules of deposited PDA tend to aggregate into larger ones after being treated, which is more apparent at pH 10.0 and pH 11.0. Further polymerization may produce a more compact PDA coating, which may be the reason of signals


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increase. Because pH 9-11 is outside the buffer windows of phosphate, the polymerization of PDA may be impeded by the produced protons during PDA polymerization due to the weak buffer capacity. If PB was replaced by other buffers, for example Tris-HCl at pH 9.0 and carbonate at pH 10.0/11.0, which all locate in the buffer windows of the selected buffers, negative detachment ratios were also obtained, the same as that of PB at pH 11.0 (Figure 4B). In addition to obtaining the final results of detachment ratios, SPRi is also convenient to monitor the detaching process of PDA in real time, which cannot be realized by AFM and ellipsometry. Generally, SPRi signals increase with the deposition of PDA on the gold sensing surface, and oppositely, the signals decrease along with detachment of PDA from the surface. Figure 4B and Figure S3 plot the real-time SPRi curves, which present the dynamic depositing and detaching processes of PDA at each test condition. For example, at pH 1.0 and pH 13.0, SPRi signals drop sharply once the deposited PDA coatings contact the detaching solutions, which suggest that the detaching processes are very fast. At pH 3.0, the curve descends slowly, which indicates that part of PDA is removed slowly from the gold sensing surface.


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Figure 4 (A) AFM images of original PDA coatings and PDA coatings after being treated by solutions at pH 10.0, pH 11.0 or pH 12.0. (B) Real-time SPRi curves measured at the conditions of Tris-HCl at pH 9.0, carbonate at pH 10.0/11.0 and phosphate at pH 11.0. The signals increase is a result of further polymerization of PDA in these alkaline solutions. Ionic strength effect The results of pH effect on PDA stability reveal that serious detachment occurs in both strongly acidic and alkaline conditions due to electrostatic repulsion. Because ions in solutions have an ability to shield electrostatic repulsion, it initiates us to wonder whether the detachment will reduce in high ionic strength solutions. To check it, two typical conditions of pH 1.0 and pH 13.0


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were chosen, representing strongly acidic and strongly alkaline conditions respectively. The stability of PDA in 10 mM PB at pH 1.0 and pH 13.0 has been studied in the above section. Herein, 0.1 M HCl and 0.1 M NaOH, acting as lower ionic strength conditions, and 10 mM PB containing 300 mM NaCl at pH 1.0 and pH 13.0, acting as higher ionic strength conditions, were also tested. The results are plotted in Figure 5. As we imagined, detachment ratios decrease reversely with the ionic strength increasing. It can be interpreted by the shielding effect of ions on electrostatic repulsion inside PDA aggregates. The PDA-coating stability in extreme pH conditions is enhanced by increasing the ionic strength of solutions.

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0.1 M HCl or 0.1 M NaOH 10 mM PB 10 mM PB+300 m M NaCl

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Figure 5. Impact of ionic strength on the stability of PDA coatings. Organic solvent caused detachment The PDA-coating stability in organic solvents was seldom discussed. Only several organic solvents had been studied, such as acetone, DMF and DMSO. Xu et al.18 found that PDA was not stable in polar organic solvents. To have a broader knowledge about the PDA-coating stability in organic solvents, ten kinds of routinely used organic solvents were systematically tested,


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including n-hexane, toluene, ethyl ether, THF, ethyl acetate, iso-propanol, acetone, acetonitrile, DMF and DMSO, which cover a variety of classifications of organic solvents. Considering that the organic solvents except iso-propanol and DMSO are able to dissolve and/or swell the PMMA-made flow cell and PDMS spacer, the detaching experiments of these organic solvents were performed offline by direct submerging the PDA-modified gold chips into target organic solvents for 1 h under shaking. As seen in Figure 6, DMF and DMSO both cause serious loss of PDA, with detachment ratios of up to 31% and 56% respectively, which are consistent with the reported results of Xu et al. In addition, detachment ratios of other tested organic solvents are all below 15%. Two of them, n-hexane and ethyl acetate, even obtained negative detachment ratios, maybe induced by a slight swelling of the deposited PDA coatings. 100


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80 60 40 20 0 -20 F te ol ne ile F O ne ne er ex a lue eth yTl Ha ceptaropaanceteotonitr DMDMS n-h toethyl ac e th iso-

Figure 6. Detachment ratios of PDA coatings in the ten kinds of tested organic solvents. Structural studies reveal that PDA is not a real polymer but aggregate of dopamine monomers and oligomers assembled together by hydrogen bond, charge transfer, π-stacking, and so on.29 The deposited PDA may be dissociated if the organic solvents have an ability to break the


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noncovalent force existing in PDA structure. It has been known that DMSO is able to disrupt hydrogen bonds,30,33 which may contribute to the highest detachment ratio of PDA in DMSO. Ultrasonication-caused detachment The stability of PDA in ultrasonication was studied by inserting a PDA-coated chip in 2 mL water and sonicating for 1 h at 200 W. The obtained detachment ratio is 6.17% ± 9.66%. Only a negligible amount of PDA was detached, which agrees well with the results of Ellison et al. measured by AFM.20 Preparation of thickness-controllable, alkali-resistant PDA coatings Scrutiny of Figure S3, it can be found that the SPRi signals after detaching treatment do not drop back to the initial levels for all the tested conditions, implying that the deposited PDA coatings are not detached completely. For the most serious detachment at pH 14.0, around 20% signal increment is reserved. The remained is alkali-resistant stable PDA coating. This phenomenon occurred not only on bare gold surface, but also on PDA-modified gold surface.34 By repeating the modification-detaching cycles of PDA deposition for 30 min and detaching by 1 M NaOH for 15 min, SPRi signals after each detaching step increase gradually (Figure S5), indicating a layer-by-layer deposition of stable PDA coatings. Figure S6 shows that the thickness of remained stable PDA coatings is 0.9 nm per cycle. The thickness of alkali-resistant PDA coatings can be well controlled by tailoring the number of deposition-detachment cycles. If deposited PDA coatings were pretreated with moderate alkaline solutions, for example PB at pH 11.0 or carbonate at pH 10.0, before rinsing with 1 M NaOH, the retained amount of alkaline-resistant PDA increased dramatically (Figure 7A). This is because the stability of original PDA coatings


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was enhanced due to further polymerization in moderate alkaline solutions. By this method, the time needed to acquire a certain thickness of stable PDA coatings was shortened greatly.


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Figure 7. (A) Real-time SPRi curves recording the forming processes of original PDA and stabilized PDA coatings. PDA coatings were all deposited for 1 h. To obtain stabilized PDA coatings, original PDA coatings were first treated with pH 11.0 of phosphate or pH 10.0 of carbonate for 1 h and then rinsed with 1 M NaOH for 30 min. (B) Real-time SPRi curves of original PDA coatings and stabilized PDA coatings by functionalizing with BSA, immune recognition with anti-BSA, and then regenerating with Gly-HCl. (PB, 10 mM phosphate buffer at pH7.4; BSA and anti-BSA, 0.2 mg/mL; Gly-HCl, 10 mM glycine adjusted to pH2.0 with HCl.)


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Immobilizing biomolecule is one of the attracting features of PDA.1,4 Bovine serum albumin (BSA), as a model molecule, was used to testify whether the stable PDA coatings still preserve the ability to immobilize biomolecule.35 BSA solutions (0.2 mg/mL) were flowed through the original PDA coatings and stabilized PDA coatings prepared with the simplified method (Figure 7B). It shows that the signal increments of stabilized PDA coatings are larger than that of untreated original PDA after BSA immobilization, implying that the stabilized PDA coatings have an even better reactivity to BSA than original PDA. Next, to highlight the superiority of stabilized PDA in practical usage, anti-BSA was first immune recognized by BSA and then the chip was regenerated with glycine-HCl at pH 2.0 (Figure 7B).36 Anti-BSA was dissociated immediately from the BSA molecules anchored on PDA layer once regenerating with Gly-HCl. After regeneration, the signals of original PDA drop to a level lower than that after BSA immobilized, while the SPRi signals of stable PDA are slightly elevated. It may be explained as a part of BSA is released along with the detachment of PDA in the highly acidic solution of GlyHCl for untreated original PDA coatings. However, the stabilized PDA coatings can endure the harsh regeneration condition. The slight signal elevation may be ascribed to partial anti-BSA bound to the stable PDA coatings directly. CONCLUSIONS In this paper, we established a SPRi approach to investigate the PDA-coating stability. Meantime, it is convenient to monitor the dynamic detachment process of PDA in each test condition in real time. PDA-coating stability is shown to depend much on the pH values of aqueous solutions. The most serious detachment happens at both ends of the whole pH range, giving a relatively stable pH window at pH 4.0-11.0. However, high ionic strength is able to stabilize the PDA coatings in extreme pH conditions. Among the tested organic solvents, DMF


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and DMSO also cause much detachment of the deposited PDA. Ultrasonication has a negligible impact on the deposited PDA. Therefore, it is better to avoid using PDA in extreme pH conditions and highly polar organic solvents. It should be noted that PDA coatings are not detached completely in all the tested conditions, even including the harshest condition of pH 14.0. Based on this, alkali-resistant and thickness-controllable PDA coatings were prepared by multiple repeating the coating-detaching cycles, or it can be simplified by pre-treating the PDA coatings with moderate alkaline solutions for further polymerization to shorten the preparation time. Moreover, the obtained highly stable alkali-resistant PDA coatings still preserve the reactivity to biomolecule and support further functionalization. ASSOCIATED CONTENT Supporting Information. Schematic diagram of SPRi apparatus, detailed calculation process of detachment ratios, realtime SPRi curves, photographs and UV-vis spectra of eluates and several AFM images. AUTHOR INFORMATION Corresponding Author *Tel.:+86-10-62618240. Fax: +86-10-62559373. E-mail: [email protected] (Y. Chen). Funding Sources The work was financially supported by NSFC (Nos. 21235007 & 21621062) and CAS (No. QYZDJ-SSW-SLH034). Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support from NSFC (Nos. 21235007 & 21621062) and CAS (No. QYZDJ-SSW-SLH034). ABBREVIATIONS PDA, polydopamine; SPRi, surface plasmon resonance imaging; AFM, atomic force microscopy; DMF, dimethyl formamide; DMSO, dimethylsulfoxide; Tris, Tris(hydroxylmethyl)aminomethane; PB, phosphate buffer; BSA, bovine serum albumin; PMMA, polymethyl methacrylate; PDMS, polydimethylsiloxane. REFERENCES (1) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-Inspired Surface Chemistry for Multifunctional Coatings. Science 2007, 318, 426-430. (2) Kim, B. H.; Lee, D. H.; Kim, J. Y.; Shin, D. O.; Jeong, H. Y.; Hong, S.; Yun, J. M.; Koo, C. M.; Lee, H.; Kim, S. O. Mussel-Inspired Block Copolymer Lithography for Low Surface Energy Materials of Teflon, Graphene, and Gold. Adv. Mater. 2011, 23, 5618-5622. (3) Lee, H.; Rho, J.; Messersmith, P. B. Facile Conjugation of Biomolecules onto Surfaces via Mussel Adhesive Protein Inspired Coatings. Adv. Mater. 2009, 21, 431-434. (4) Chien, H. W.; Kuo, W. H.; Wang, M. J.; Tsai, S. W.; Tsai, W. B. Tunable Micropatterned Substrates Based on Poly(dopamine) Deposition via Microcontact Printing. Langmuir 2012, 28, 5775-5782.


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For Table of Contents Only


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Stability of Polydopamine Coatings on Gold Substrates Inspected by

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