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Communication Cite This: Chem. Mater. 2018, 30, 2198−2202
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Damage-Responsive Microcapsules for Amplified Photoacoustic Detection of Microcracks in Polymers JunLong Geng,*,‡,∥ Wenle Li,‡,§,∥ Lukas P. Smaga,†,‡ Nancy R. Sottos,‡,§ and Jefferson Chan*,†,‡ †
Department of Chemistry, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States § Department of Materials Science and Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States ‡
S Supporting Information *
T
specific enzymatic activities.45 Although various dye platforms can in theory be employed, we have identified aza-BODIPY dyes as an ideal scaffold for PA probe development.37,39,42,43 The wavelength of maximum absorbance (λabs) is in the nearinfrared (NIR) range (650−900 nm) which makes it less prone to light scattering. Moreover, this scaffold exhibits exceptional photostability, and its optical properties can be strategically tuned via chemical modifications to modulate the PA signal. Despite its tremendous potential in the biomedical arena, PA imaging has yet to be explored as an option in other fields, such as in materials science. In this study, we report the development of the first photoacoustic damage reporting probe (PDRP), a pHresponsive aza-BODIPY dye, which is loaded into microcapsules for material damage detection. The PDRP containing microcapsules are embedded in amine-cured epoxy to provide mechanically triggered damage indication (Scheme 1). When intact, the dye within the microcapsule is in its protonated form exhibiting a λabs value of 680 nm with no detectable absorbance at 760 nm. When released by mechanical rupture of the microcapsules, PDRPs interact with the residual amines from polyoxypropylene triamine (POPTA) in the epoxy matrices,
he development of polymeric and composite materials has had a profound impact in the automotive, construction, aerospace, and healthcare industries. The stability and structural integrity of these materials are important properties that are essential for these applications. However, all materials are inevitably susceptible to mechanical failure resulting from diverse damage modes, typically beginning with the appearance of microscopic fractures.1−3 Thus, methods to monitor such damage at an early stage can serve as a preventative measure to reduce maintenance costs and increase overall performance.4−7 Mechanical damage in polymeric materials is usually associated with local physical and/or chemical changes. One approach to detect these is based on small-molecule indicators which exhibit a change in the optical readout following mechanical damage (e.g., a scratch).8−10 These damage-responsive probes can be incorporated into the polymer via either covalent or noncovalent modifications.11−16 An alternative approach is the application of microcapsules or hollow fibers loaded with chromogenic and fluorogenic molecules, which are incorporated in polymeric materials during fabrication.17−23 When material damage occurs, the “microcontainers” are ruptured to release the encapsulated indicators. Subsequently, the interaction between the indicator and the matrix materials leads to an optical change that marks the damaged region.24−30 Unfortunately, optical approaches are only applicable to the detection of surface or open damage due to the limited depth penetration of light within translucent materials.27−29 Photoacoustic (PA) imaging is a hybrid imaging technology that overcomes these disadvantages by combining optical excitation with ultrasound detection. The underlying principle, the PA effect, was discovered by Bell in 1880 and relies on thermoelastic expansion following nonradiative relaxation of molecules.31,32 PA imaging enables superior imaging depths and greater spatial resolution than optical approaches, such as fluorescence imaging, because sound scatters several orders of magnitude less than light (for biological tissue). For this reason, PA imaging has emerged as a powerful approach for noninvasive imaging (∼8 cm depth) in both the preclinical and clinical settings.33−35 Recently, analyte-responsive PA probes have been developed enabling users to obtain molecular information within the tissue of interest.36 To date, PA probes have been developed for imaging metal ions (Cu2+37 and Ca2+38), tissue hypoxia,39,40 pH,41 and small-molecule analytes (nitric oxide43 and peroxynitrite43), as well as proteins44 and © 2018 American Chemical Society
Scheme 1. Schematic of PA Imaging of Microcracks in Polymeric Materialsa
a
PDRP microcapsules are embedded in the polymeric materials. The microcracks release PDRP, and the response of PDRP upon interaction with the matrix can be visualized.
Received: January 31, 2018 Revised: March 3, 2018 Published: March 22, 2018 2198
DOI: 10.1021/acs.chemmater.8b00457 Chem. Mater. 2018, 30, 2198−2202
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Chemistry of Materials leading to deprotonation of the dye and a concomitant bathochromic shift of λabs to 760 nm (Figure 1a). Since the
Figure 2. (a) Wavelength dependent PA spectra of PDRP in the absence and presence of POPTA (640 nM). (b) PA images of the PDRP upon POPTA treatment at different concentrations. The samples were placed in fluorinated ethylene propylene tubes to conduct the PA imaging tests at 760 nm. All measurements were acquired in EPA solutions.
signal upon excitation at 760 nm (Figure S2), confirming that the PA enhancement is only attributed to the interaction between PDRP and POPTA. A 25-fold PA signal enhancement was observed at 10 nM of POPTA illustrating the high sensitivity of PDRP probe to the residual amine (Figure S1). A 300-fold PA enhancement was observed at 640 nM POPTA, demonstrating the potential for polymer damage detection with a strong contrast. In addition, PDRP exhibited high photostability since no observable PA intensity was lost after 1 h of continuous irradiation under standard measurement conditions. This indicates a high reliability of PDRP for the use in longterm polymeric structure monitoring (Figure S3). Next, we shifted our attention to the synthesis of PDRPcontaining microcapsules. Polyurethane/poly(urea-formaldehyde) (PU/UF) double-walled microcapsules containing 1 mg/mL solution of PDRP in EPA were prepared using a wellestablished emulsification polymerization method.46 SEM characterization revealed spherical microcapsules with a core− shell structure. The diameter of these capsules can be effectively adjusted. For our purposes we isolated capsules ranging from 45 to 75 μm (Figure 3a, Figure S4). Thermogravimetric analysis of the capsules showed excellent thermal stability up to 220 °C (Figure S5). Importantly, no PA signal was observed from the intact microcapsules upon excitation with 760 nm, indicating minimal background signal from intact capsules embedded in a polymeric material. In addition, storage of the PDRP microcapsules for more than eight months under ambient conditions did not result in any observable capsule degradation or false positive PA signals, suggesting high reliability in their application as a damage indicator. Epoxy specimens containing 10 wt % PDRP microcapsules were fabricated with EPON Resin 813 and POPTA (100:43 weight ratio, curing for 48 h at 35 °C) to investigate their damage detection capability via PA imaging. The epoxy exhibits a light green color corresponding to the protonated form of PDRP. When scratched with a razor blade, an immediate color change to red occurred exclusively in the damaged region due to the release of PDRP and subsequent deprotonation (Figure 3c). Optical micrographs of the scratched epoxy specimen showed that a 7.5 μm width scratch leads to rupturing of microcapsules and release of PDRP (Figure 3b). To further demonstrate the utility of PDRP microcapsules for detecting internal damage via PA imaging, we prepared an opaque polymer with a thickness of 0.5 cm to cover the damaged epoxy (Figure 3d). A strong PA signal from the damaged region was captured via PA imaging. In contrast, colorimetric detection of the damage with the naked eye was not possible due to the strong scattering of light within the
Figure 1. (a) Chemical structure of PDRP and depiction of the proposed sensing mechanism for material damage via phenol deprotonation. (b) UV−vis spectra of 2 μM PDRP in the presence of various concentrations of POPTA. Inset: 2 μM PDRP without (left) and with 0.64 μM POPTA (right). (c) Absorbance of PDRP at 760 nm in the presence of different concentrations of POPTA.
PA signal intensity is proportional to the extinction coefficient of the dye, excitation at 760 nm can be used to generate a PA readout that is indicative of damage within the material. PDRP was synthesized over two steps beginning from 4nitro-1,3-diphenylbutan-1-one precursors (Scheme S1).37,42 The identity and purity of the probe were verified via NMR spectroscopy and mass spectrometry. The apparent pKa value of the dichlorophenol group is ∼4.4.37 PDRP will therefore be fully protonated within the environment of the microcapsule. The O-propargyl functionality serves as a convenient conjugation site for potential covalent tethering onto other polymeric materials. Characterization of PDRP demonstrates that it exhibits a λabs at 680 nm in ethyl phenyl acetate (EPA) with a corresponding extinction coefficient of 5.64 × 104 M−1 cm−1. Upon addition of the amine-curing reagent POPTA, PDRP becomes deprotonated resulting in a shift of λabs to 760 nm with an extinction coefficient of 5.83 × 104 M−1 cm−1 (Figure 1b). We also observed a prominent change in the color of the solution from green to red (Figure 1b). The absorption peak at 760 nm increased gradually upon addition of POPTA, saturating at concentrations as low as 200 nM (Figure 1c). Lastly, no detectable fluorescence was observed for the deprotonated form of PDRP, indicating a high propensity to undergo nonradiative decay and therefore generate a strong PA signal. Inspired by these results, we next evaluated the PA response of PDRP in EPA solution. In the absence of POPTA, the maximum PA signal of PDRP was centered at ∼680 nm with essentially no signal at 760 nm. Following the exposure of PDRP to POPTA, the PA signal at 760 nm increased significantly (Figure 2a). To quantify the PA intensity change, the PDRP solutions were treated with various concentrations of POPTA, and the PA images upon excitation at 760 nm were acquired and quantified (Figure 2b, Figure S1). Much like the spectroscopic data, increasing POPTA concentrations resulted in concentration dependent increases in the PA signal (Figure 2b, Figure S1). In contrast, in the presence of EPON 813 epoxy resin (with no basic component) there was no significant PA 2199
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from 0.5 to 1.0 cm with the light transmittance ranging from 4% to 2% at 760 nm, respectively. It is important to note that the speed of sound in PDMS is approximately 1000 m/s, which is significantly different from that of water (∼1500 m/s).47 This requires a manual adjustment of the value for the speed of sound that is used in the filtered back projection algorithm of the image reconstruction process to guarantee high quality images (details in the Supporting Information).48,49 For example, 1 cm thick PDMS required a value of 1400 m/s (Figures S10 and S11). In agreement with the data collected through the opaque agarose-based gels, comparable PA signals were detected through PDMS (Figure 3g and Figure S9), illustrating the great potential of our approach for the detection of microscopic damages in deep layers of polymeric materials. In closing, we have developed a facile and robust approach for the detection of internal damage in polymers with micrometer resolution based on the synergistically combined technologies of PDRP microcapsules and PA imaging. PDRP exhibits a pKa value of ∼4.4, rendering it sensitive to the basic amines in epoxy materials. A large extinction coefficient change from nearly zero to 5.83 × 104 M−1 cm−1 at 760 nm leads to a large PA turn-on response associated with the deprotonation. Using microcapsules containing a solution of PDRP in EPA, the PA signal develops rapidly following mechanical damage to the polymeric structures. In contrast to previously reported colorimetric and fluorescent damage detection approaches, the current technique provides outstanding resolution in the micrometer range at centimeter depths with high contrast. Due to the generalizability of this approach, we envision the development of PA probes with specific responses toward a variety of chemical groups commonly existing in polymeric materials, which will considerably expand this damage detection strategy to a broad range of applications. Moreover, advances in PA instrumentation and reconstruction methods49 tailored for this application (e.g., portable PA setups)50 will extend the utility of PA imaging beyond our initial report.
Figure 3. Optical images of (a) intact microcapsules and (b) the mechanical rupture in epoxy. Photographs of polymer films with 7.5 μm cracks in the (c) absence and (d) presence of translucent PDMS films. (e) PA images of polymer damage with different scratch widths through a 0.5 cm thick agarose gel. PA images of the polymeric damage with 7.5 μm scratch width through (f) agarose and (g) translucent PDMS film with different thicknesses. All the images (e, f, g) share the same scale bar of 200 μm.
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opaque gel (Figure 3d, Figure S6). This result highlights the superior performance of PA imaging relative to colorimetric indication for subsurface damage detection. A series of scratches with various widths from 1 to 7.5 μm were then introduced by a test panel scratcher (Corrocutter 639, Erichsen) to investigate the relationship between the PA response and damage size. Notably, a scratch as narrow as 1 μm was clearly detected by PA imaging (Figure 3e) through a 0.5 cm thick opaque gel. Both the area and the intensity of PA signals increased proportionately with the scratch width (Figure 3e and Figure S7). Stereomicroscopy analysis confirmed that the PA signal was spatially correlated with the damaged regions, including areas that experienced shear forces upon creation of the primary scratch (Figure 3e and Figure S8). To demonstrate that our detection method is compatible at greater depths, we studied the performance of microcrack detection with opaque gels with various thicknesses. As anticipated, the PA signal was slightly attenuated as the gel thickness increased. However, even at a thickness of 1.0 cm, significant PA signal was captured that clearly visualized the damage (Figure 3f and Figure S9). 3D reconstruction of the imaging data allowed us to pinpoint the damage with high lateral and axial resolution (Movie S1). Finally, to probe the damage detection capabilities in engineered polymeric materials, we examined damage through translucent polydimethylsiloxane (PDMS). The thicknesses of PDMS films were varied
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00457. Movie S1 (AVI) Schematic synthesis route of PDRP, stability, SEM and theromgravimetric analysis, optical images of damaged film, stereomicroscopy images of the epoxy films, reconstructed PA images of the damaged film at different sound speeds, and the correlation of PDMS film thickness with sound speeds (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(J.C.) E-mail: jeff
[email protected]. *(J.G.) E-mail:
[email protected]. ORCID
Nancy R. Sottos: 0000-0002-5818-520X Jefferson Chan: 0000-0003-4139-4379 Author Contributions ∥
J.G. and W.L. contributed equally to this work.
Notes
The authors declare no competing financial interest. 2200
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ACKNOWLEDGMENTS J.G. acknowledges support from the Beckman Institute for Advanced Science and Technology in the form of a postdoctoral fellowship. This work was supported by the Alfred P. Sloan Fellowship (FG-2017-8964 to J.C.). Funding for the 500 MHz Bruker CryoProbe was provided to the School of Chemical Sciences NMR Lab by the Roy J. Carver Charitable trust (Muscatine, Iowa; Grant No. 15-4521). Purchase of the Q-Tof Ultima mass spectrometer was made possible by a grant from the National Science Foundation, Division of Biological Infrastructure (DBI-0100085). We thank Dr. Jun Xia and Mr. Dapeng Wang for the helpful discussions. We also thank Mr. Christopher Reinhardt for help with editing the manuscript. Graphic support from Dorothy Loudermilk in the Department of Chemistry at the UIUC is greatly appreciated. Microscopy was performed using facilities in the Imaging Technology Group at the Beckman Institute at UIUC.
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