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Article Cite This: ACS Omega 2018, 3, 18911−18916
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Fast and Green Synthesis of an Oligo-Hydrocaffeic Acid-Based Adhesive Ali Ghadban,† Harini Mohanram,† and Ali Miserez*,†,‡ †
Centre for Biomimetic Sensor Science, School of Materials Science and Engineering, Nanyang Technological University (NTU), 50 Nanyang Avenue, Singapore 637553 ‡ School of Biological Sciences, NTU, 60 Nanyang Drive, Singapore 637551
ACS Omega 2018.3:18911-18916. Downloaded from pubs.acs.org by 109.94.223.15 on 01/01/19. For personal use only.
S Supporting Information *
ABSTRACT: A green, mussel-inspired bioadhesive based on oligomerization of hydrocaffeic acid was synthesized in water by an ultrafast one-step reaction in the presence of 1-ethyl-3(3-dimethylaminopropyl)carbodiimide as an activating agent. The resulting oligomers exhibited strong wet adhesion when applied to different substrates including glass, stainless steel, and aluminum. Compared to most commercial adhesives, this bioinspired adhesive is produced via a sustainable and green process, i.e., aqueous-based synthesis, one-step reaction, and in the absence of any purification step to obtain the final functional adhesive.
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occurring polysaccharides20−26 or modified poly(ethylene glycol),27−31 enzymatic modification of tyrosine-bearing polymers,32 and using different polymerization techniques (conventional and controlled/living polymerizations).33−36 A number of reviews have documented the synthesis of catecholbased polymers and their adhesion mechanisms.11,37−41 To our knowledge, the only example of a polyester catecholic polymer synthesized by a condensation reaction is the work of Kaneko and co-workers.42,43 In their study, caffeic acid was copolymerized with 4-coumaric acid in the presence of acetic anhydride as a condensation agent and disodium hydrogen phosphate as a transesterification catalyst. The resulting hyperbranched polymer showed interesting adhesive properties in dry conditions when applied to copper and stainless steel surfaces, which nearly matched commercial cyanoacrylate glues. Other important requirements for streamlined production of adhesives are the synthesis process, the nature of the solvent, and a minimum number of purification steps to obtain the final product. Herein, we report a facile and fast (less than 1 min), one-step synthesis of a wet-resistant adhesive synthesized by oligomerization of the catecholic compound hydrocaffeic acid (HCA). We used a direct, water-based, and fast autocondensation reaction process, and the final functional adhesive could be obtained without any purification step.
INTRODUCTION Most known commercial adhesives are restricted to limited number of surfaces and are often not effective in wet environments.1,2 These limitations are circumvented by mussels, which secrete proteinaceous threads (byssus) containing adhesive proteins at the extremity of their byssus, allowing them to attach to a wide range of solid substrates (both inorganic and organic), and more remarkably, under a wet environment.3−6 To accomplish this feat, mussel adhesive proteins (MAPs) utilize exquisite biochemical and bioprocessing tricks. The most familiar is the incorporation of the posttranslated amino acid 3,4-dihydroxylphenyl-L-alanine (LDOPA) in MAPs,7 a catechol derivative of tyrosine which is able to form various intermolecular interactions that adapt to the surface chemistry, including coordination bonds,8,9 hydrogen bonds,10,11 as well as hydrophobic and hydrophilic interactions.12,13 More recently, additional characteristics of MAPs have been shown to enable their water-resistant adhesion capacity, including: (i) the use of redox coupling between MAPs to prevent premature oxidation of DOPA into their less-adhesive quinone form;14 (ii) their secretion as a nondispersible coacervate fluid that allows the adhesive to be delivered in high concentration at the attachment site;15,16 (iii) the synergistic effect of basic sidechains adjacent to L-DOPA in MAPs that displace hydrated cations from oxide surfaces;17 and (iv) their capacity to form π−cation interactions that can further enhance adhesion in immersed environments.18 Because characteristics (i−iv) have more recently been revealed, so far most efforts to duplicate the adhesive properties of MAPs have focused on strategies to synthesize catechol-containing adhesive polymers, including auto-oxidation of dopamine,19 pendant functionalization of naturally © 2018 American Chemical Society
Received: May 30, 2018 Accepted: December 11, 2018 Published: December 31, 2018 18911
DOI: 10.1021/acsomega.8b01181 ACS Omega 2018, 3, 18911−18916
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RESULTS AND DISCUSSION To synthesize the oligo(HCA) adhesive, HCA was reacted with itself in the presence of a concentrated solution of 1-ethyl3-(3-dimethylaminopropyl) carbodiimide (EDC) acting as an activating agent at room temperature and under vigorous stirring, leading to the immediate formation of a brown gluey precipitate (Figure 1). The exothermic nature of the reaction
Figure 1. Synthesis (A) and a possible structure (B) of oligo(HCA) adhesive showing the linear, dendritic, and terminal units. The drawing is for the oligomer with six repeating units.
accelerated the rate of condensation between the OH groups of the benzene ring with the carboxylic group to form an ester bond. Hence a large amount of HCA could condense in a short amount of time, leading to a mixture of linear and branched oligomers with abundant catechol groups in the chain ends.43 The reaction mixture was decanted, and the adhesive was then washed and kept overnight in water to remove any unreacted reagents as well as any urea formed during the reaction, and then stored at −20 °C without drying until use. To assess the structure of the synthesized adhesive, 1H NMR, matrix-assisted laser desorption ionization time-of-flight (MALDI-ToF) mass spectrometry, and Fourier transform infrared (FTIR) spectroscopy measurements were done on the freeze-dried product (Figure 2). On the 1H NMR spectrum, characteristic peaks of the oligo(HCA) appeared between 6.5 and 7.2 ppm, which can be attributed to protons from the aromatic region as well as the aliphatic peaks at ∼2.8 ppm. Only traces of impurities (represented by the cross signs) were left in the adhesive after it was washed in water, which suggests that no purification step is necessary for the product to display its adhesive functionality. It is worth noting that the 1H NMR spectrum of the adhesive obtained directly after the reaction and without washing (see Supporting Information) indicated slight evidence of unreacted hydrocaffeic acid, with a 1:10 ratio of HCA to oligo(HCA) implying a high conversion of HCA to oligo(HCA) of ca. 90%. To calculate the degree of branching (DB), the peaks related to the terminal T (two phenolic OH), linear L (one phenolic OH), and dendritic D (no phenolic OH) protons of the aromatic ring were assigned (see Supporting Information for spectrum, Figure S1). The set of peaks between 7.0 and 7.3 ppm could be attributed to the dendritic units, whereas the peaks at 6.8 and 6.5 ppm were assigned to H5 and and H2/H6 linear units. Finally the peaks at 6.64 and 6.66 ppm were attributed to the terminal protons.44 DB was then calculated based on the study of Hawker et al.45 and was found to be ∼54% using the equation
Figure 2. 1H NMR spectra of (A) hydrocaffeic acid and (B) the synthesized adhesive oligo(HCA). Conditions: Oligo(HCA), 40 mg mL−1, dimethyl sulfoxide (DMSO)/CD3OD, 298 K, ns = 32, D1 = 3 s. The crosses represent impurities in the adhesive. Both spectra were referenced with respect to the DMSO peak at δppm = 2.5. Additional spectra with peaks attributed to dendritic, linear, and terminal units are shown in Supporting Information, Figure S1. (C) FTIR and (D) MALDI-ToF spectra of the synthesized oligo(HCA) adhesive. For the MALDI-ToF spectrum, the degree of polymerization (X) of the corresponding oligomers is shown. 164 Da corresponds to the repeating unit C9H8O3, whereas the 30 Da difference shoulder peak could not be attributed.
DB =
AD + A T AD + A T + AL
where AD, AT, and AL are the integrated areas from the dendritic, terminal, and linear protons, respectively. The adhesive was further evaluated by FTIR spectroscopy, as shown in Figure 2C. The appearance of a characteristic peak at 1735 cm−1 can be attributed to the CO stretching of the ester group and indicated the formation of an ester bond. This was accompanied by broadening of the 1110 cm−1 band that could be attributed to the C−O bond of the ester group or to the C−O bond of the catechol ring, as well as by the disappearance of the CO stretching band of the carboxylic 18912
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group of HCA at 1669 cm−1.46 These features suggested the condensation of OH groups from the catechol ring with the carboxyl group (COOH) of HCA that led to the formation of an ester bond. Under these concentrated conditions, EDC acted as an activating agent and accelerated the process. The presence of two OH groups on the benzene ring likely led to the formation of a branched ester.43 It is worth noting that the reaction between dopamine (which does not contain carboxylic group) and EDC following the same reaction conditions did not result in the formation of the precipitate adhesive (data not shown), which supports the importance of the carboxylic groups during adhesive synthesis through the formation of the ester bond. From the MALDIToF analysis (Figure 2D), oligomers of 3−22 repeating units were observed, with the shorter oligomers as the dominant species. A difference of m/z ≅ 164 between two consecutive peaks was detected, confirming the mass of the simplest repeating unit with a chemical formula of C9H8O3 corresponding to two HCA molecules minus one H2O molecule formed during the condensation reaction. To evaluate the adhesive properties of the oligomers, the adhesive was spread from the hydrated state on one of the two rectangular surfaces of the selected substrates, i.e., glass, aluminum (Al), or stainless steel in wet conditions, clamped against the other surface, and dried at 60 °C for 12 h before testing. Two sets of mechanical testing were conducted using the lap-shear test,47 one with nontreated and the other with vacuum-plasma-treated substrates. For comparison, a commercial cyanoacrylate glue (Selley Pty limited) was used as a control for untreated Al and stainless steel substrates, and was applied in dry conditions as it did not display adhesion in the wet state. The failure stress from the lap-shear experiments are shown in Figure 3 for the three substrates.
compared to the glass substrate, for which a slight ductile behavior was noticed with relatively large failure strains and areas under the load/displacement curves (Supporting Information). The commercial glue showed more pronounced brittle failure for all tested substrates. To enhance the adhesive properties of the synthesized oligo(HCA), the substrates were vacuum-plasma treated for 10 min prior to the adhesion process to obtain a cleaner surface and introduce more free hydroxyl groups on the surface for enhanced molecular interactions. As expected, the failure stress increased with the treated surfaces for the three substrates (0.5 ± 0.03 MPa (glass), 1.8 ± 0.20 MPa (aluminum), and 1.7 ± 0.11 MPa (stainless steel)). Although the shear strength of oligo(HCA) is lower than that of the commercial cynaoacrylate, values exceeding 1 MPa can still be categorized as a robust adhesive, which would be adequate in several applications.47,49 On the other hand, the oligo(HCA) can be applied on wet surfaces, which is a critical advantage over other types of adhesives, and we attribute this capability to the presence of catecholic moieties in its chemical structure. In all substrates, we also observed a cohesive failure mode since remnants of the adhesive was observed on both sides of the substrate, which we attribute to the presence of mostly oligomeric chains in our adhesive that limit interchain entanglement interactions. We anticipate that higher chain entanglement may further enhance adhesion strength by increasing the interchain cohesion forces. For example, the incorporation of Fe3+ as complexing ions20,27,50,51 or periodate ions52,53 to oxidize the catechols and trigger covalent crosslinking may increase the molecular weight of the adhesive complex and enhance cohesive forces, although this may also come at the expense of the number of catechol units available for adhesive interfacial interactions. Therefore, it is likely that a compromise would exist between cohesive and adhesive forces to achieve the best adhesive properties. A comparative summary with bioinspired catecholic adhesives applied in both dry and wet conditions allows us to benchmark our adhesive. For example, the shear bond strength of poly(ethylene glycol) diacrylate-filled coacervate adhesive was found to be 1.2 MPa on wet glass substrates.54 Water-soluble polyoxetane adhesive decorated with catechol and bisphosphoric acid groups exhibited an adhesion strength of 0.35 MPa in humid conditions as measured by bulk lapshear joints between aluminum substrates.55 The bond strengths of a poly(vinylpyrrolidone) polymer functionalized with catechol residues50 and a catechol-functionalized polyacrylate56 applied to wet glass were found to be 1.3 and 1.6 MPa, respectively. More recently, a poly(catechol-styrene) adhesive led to shear bond strengths up to ∼3 MPa on aluminum substrates in wet environments.57 According to the authors, this represents the highest underwater adhesive developed to date. The same adhesive was also tested in dry conditions, and bond strengths of 4 MPa (without periodate) and 7 MPa (with periodate) were measured on aluminum surfaces, 5−6 MPa on steel surfaces, and up to 10 MPa on wood surfaces. Catechol-based adhesives applied in dry/ slightly wet states have also been reported. A co-polypeptide decorated with DOPA residues exhibited bond strengths of ∼2.5, ∼2.7, and ∼4.7 MPa on glass, steel, and aluminum substrates, respectively, after curing at high temperatures.58 In summary, bond strengths of our bioinspired adhesive reside between those previously obtained on wet and dry surfaces for catechol-based adhesives. However, the specific advantage of
Figure 3. Lap-shear adhesion measurements of the oligo(HCA) adhesive on different substrates and comparison with a commercial cyanoacrylate glue.
The adhesive exhibited higher failure stress with the stainless steel (1.27 ± 0.21 MPa) substrate than Al (1.1 ± 0.28 MPa) and glass (0.3 ± 0.08 MPa) substrates. This could be attributed to stronger H-bonding and coordination bond interactions between the catechol residues with Al and stainless steel surfaces than the glass substrate, where only H-bonding would predominate.11,39,48 A similar trend was observed with the commercial cyanoacrylate adhesive, which showed higher failure stress than our synthesized adhesive on both Al (1.1 vs 2.2 MPa) and stainless steel (1.27 vs 2.8 MPa) substrates. It should also be noted that the oligo(HCA) adhesive exhibited a brittle type of failure on Al and stainless steel substrates 18913
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using a Kratos Axima Performance (Kratos-Shimadzu Biotech, Manchester, U.K.) equipped with a pulsed N2 laser (λ = 337 nm) and a time-delayed extraction ion source. An accelerating voltage of 20 kV was used. The spectrometer was operated in positive ion linear mode, and mass spectra were obtained by accumulating 1000 laser shots for each analysis at a power of 110 with Rastering mode on. The matrix, 2,5-dihydroxybenzoic acid (DHB), was dissolved in tetrahydrofuran (20 mg mL−1). Sample was solubilized in dimethylformamide (8 mg L−1) and mixed with DHB (1:10 ratio). The mixture was then deposited onto a MALDI plate and dried at room temperature before analysis. Fourier Transform Infrared (FTIR) Spectroscopy. A Vertex 70 instrument (Bruker) was employed in the attenuated total reflection mode, with 64 scans and a resolution of 4 cm−1 in the 4000−400 cm−1 region. The solid freeze-dried product was directly analyzed. The data were processed using OPUS 6.5 software (Bruker) and replotted on origin 8. Lap-Shear Adhesion Measurements. Lap-shear adhesion measurements were performed using an Instron 5567 testing system equipped with 500 N load cell (unless specified) at RT. Adherends were pulled apart at a rate of 1 mm min−1. The dimensions of Al and stainless steel rectangular plates were 80 mm long and 12 mm wide, and the overlap bonded region was ≅ 10−15 mm long. For the glass plates, standard microscopic slides (25 mm × 75 mm) were used with an overlap bonded region of ≅12−19 mm long. The failure stress was obtained by dividing the maximum load (N) by the area of the adhesive overlap, giving the lap-shear adhesion in Pa (Pa = N m−2). To this end, the wet oligo(HCA) adhesive was smeared on one of the two rectangular surfaces of a selected material (glass, aluminum, or stainless steel), clamped to the other surface, and maintained at 60 °C for 12 h before analysis. In all experiments, the adhesive was applied directly from the hydrated state. For each material, 10 experiments were conducted. Cleaning of Substrates. All substrates (treated or nontreated) were washed with ethanol and water, then flushed with air prior to adhesion to remove any dust or other contaminants on the surface. Following this, the surfaces were hydrated to conduct testing corresponding to a wet application. Vacuum Plasma Treatment. Vacuum plasma treatments were conducted using a plasma cleaner from Harrick Plasma. The substrate was placed inside a plasma chamber, vacuum was reduced to 200 mTorr, and then O2 was flown inside the chamber to insure a vacuum of 700 mbar during the plasma treatment.
our adhesive is its simple synthesis and the green processing (from synthesis to purification) by which it is obtained, and we also envisage that its performance could be enhanced by promoting intermolecular cohesive forces within the complex.
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CONCLUSIONS In conclusion, we have prepared a bioinspired catechol-based adhesive using a simple, one-pot reaction methodology synthesized from the aqueous soluble precursor HCA, and the entire process of adhesive synthesis takes less than 1 min. After oligomerization, oligo(HCA) exhibits adhesive properties that can be used to bond different types of substrates, including Al, stainless steel, and glass. Importantly, the adhesive can be applied on wet surfaces. We consider that the lower adhesion strength of our oligo(HCA) compared to commercial glues such as cyanoacrylate represents an acceptable compromise in the light of the sustainable process by which the adhesive is produced, notably water-based synthesis, fast one-step reaction, and the absence of any purification step before final usage as a multipurpose glue.
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EXPERIMENTAL SECTION
Materials. The following chemicals were of reagent grade and were used as received. Hydrocaffeic acid (HCA, 98%, Alfa Aesar) and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC, 98%, Gl biochemicals) were used. Deionized water was produced in-house with a Milli-Q apparatus (Millipore) and used for all experiments. Aluminum and stainless steel rectangular plates were purchased from Kim Hwee Industrial Technology, and the glass plates (microscope slides) were provided by Biomedia. General Methods. Accurate volumes (10−1000 μL) were measured with calibrated automatic pipettes (Eppendorf Research). Synthesis of oligo(HCA). In a typical experiment, hydrocaffeic acid HCA (1 g, 5.48 mmol, 1 equiv) and EDC (1.05 g, 5.48 mmol, 1 equiv) solutions were prepared by dissolving the corresponding reagents in 3 and 1 mL water, respectively. To the HCA solution, the EDC solution was added in one portion under vigorous stirring at RT. In less than 1 min, the oligo(HCA) precipitated out of the solution. The product was recovered, kept overnight in water, and stored in a freezer without drying until use. The reader is referred to Supporting InformationVideo 1 as an illustration of the reaction, demonstrating the ease and fast speed of synthesis. We also stored the adhesive at room temperature in water for up to 2 months. The oligomeric adhesive remained in the precipitate state and did not resolubilize, thus demonstrating that the ester bond of the oligomeric adhesive did not hydrolyze and remained stable. Analytical Techniques. Nuclear Magnetic Resonance (NMR). Spectra were acquired on a Bruker Avance I spectrometer equipped with a variable temperature module (resonance frequency of 400.13 MHz for 1H) and a QNP probe. Unless otherwise specified, for 1H experiments, 90° pulses and pulse sequence recycle times of 3 s were used. Onedimensional 1H spectra were obtained with 32 scans and 32 K data points and were reprocessed using MestReNova software (v6.2). Chemical shifts were referenced with respect to the DMSO peak (δDMSO = 2.5 ppm at 298 K). Matrix-Assisted Laser Desorption Ionization Time-ofFlight (MALDI-ToF). MALDI-ToF analyses were performed
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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/acsomega.8b01181. 1 H NMR for the crude adhesive and some selected lapshear spectra (PDF)
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Synthesis of the adhesive (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 18914
DOI: 10.1021/acsomega.8b01181 ACS Omega 2018, 3, 18911−18916
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ORCID
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Ali Miserez: 0000-0003-0864-8170 Author Contributions
A.G. designed the synthesis method and conducted all experiments. H.M. helped with NMR data interpretation. A.G. and A.M. conceived the study and wrote the manuscript. Funding
This research was supported by the Singapore National Research Foundation (NRF) through an NRF Fellowship awarded to A.M., and by a Singapore Ministry of Education (MOE) Academic Research Fund (AcR) Tier 2 grant # MOE2015-T2-1-062. Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acsomega.8b01181 ACS Omega 2018, 3, 18911−18916
