Motion-Based pH Sensing Based on the Cartridge-Case-like

Publication Date (Web): January 27, 2016. Copyright © 2016 American ... Micro- and nano-robots based sensing and biosensing. Lei Kong , Jianguo Guan ...
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Motion-Based pH Sensing Based on the Cartridge-Case-like Micromotor Yajun Su, Ya Ge, Limei Liu, Lina Zhang, Mei Liu, Yunyu Sun, Hui Zhang, and Bin Dong* Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices and Collaborative Innovation Center (CIC) of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123, People’s Republic of China S Supporting Information *

ABSTRACT: In this paper, we report a novel cartridge-case-like micromotor. The micromotor, which is fabricated by the template synthesis method, consists of a gelatin shell with platinum nanoparticles decorating its inner surface. Intriguingly, the resulting cartridge-case-like structure exhibits a pHdependent “open and close” feature, which originates from the pH responsiveness of the gelatin material. On the basis of the catalytic activity of the platinum nanoparticle inside the gelatin shell, the resulting cartridgecase-like structure is capable of moving autonomously in the aqueous solution containing the hydrogen peroxide fuel. More interestingly, we find out that the micromotor can be utilized as a motion-based pH sensor over the whole pH range. The moving velocity of the micromotor increases monotonically with the increase of pH of the analyte solution. Three different factors are considered to be responsible for the proportional relation between the motion speed and pH of the analyte solution: the peroxidase-like and oxidase-like catalytic behavior of the platinum nanoparticle at low and high pH, the volumetric decomposition of the hydrogen peroxide under the basic condition and the pH-dependent catalytic activity of the platinum nanoparticle caused by the swelling/deswelling behavior of the gelatin material. The current work highlights the impact of the material properties on the motion behavior of a micromotor, thus paving the way toward its application in the motion-based sensing field. KEYWORDS: micromotor, motion-based sensing, autonomous movement, hydrogen peroxide, self-propelling

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INTRODUCTION

Because the micromotor swims in the aqueous solution, the solutes may have a great influence on its motion behavior in a positive or negative way. For example, blood electrolytes,20 lead ions21 and thiol molecules22 can significantly decrease the micromotor’s velocity. The detection limit of the sensing micromotor is normally in the millimolar range. Recently, a few exceptions have been reported. For instance, Wang et al. have successfully realized the quantitative detection of trace silver ions with micromolar concentrations23 or DNAs down to the attomole level24 based on the Au/Pt micromotors. Wu et al. have developed a gold-nanoparticle-modified self-propelled polyaniline/Pt micromotor, which can quantitatively detect the protein target down to 1 ng/mL by utilizing the antigen− antibody interaction.25 These works indicate the micromotors are promising candidates for motion-based sensors. However, to the best of our knowledge, the motion-based pH sensing over the whole pH scale has not been realized. In this paper, we report a cartridge-case-like micromotor that exhibits the motion-based pH sensing property over the whole pH range. The structure of the micromotor, which is fabricated

Micro-objects that are capable of moving autonomously in a liquid environment by utilizing the on-board or off-board fuels have recently attracted more and more attention. The selfpropelling behavior can be realized based on a variety of mechanisms, which range from bubble propulsion1−3 to selfdiffusiophoresis4 or self-electrophoresis,5 etc. The unique motion behavior, together with several remote control techniques (such as light,6−8 ultrasound,9 magnetic field,10 etc.), makes the micromotor attractive for a wide range of applications. For example, the micromotors have been demonstrated to have potential applications in the biomedical field due to their capability to realize the cargo capture, transportation and release toward the target cancer cells in a controllable fashion.11,12 On the basis of the solution mixing phenomenon induced by the autonomous movement, the micromotors have been found to be a potentially attractive candidate for the environmental remediation (such as the detoxification13 or the removal of the contaminants14) or the separation and detection applications.15 In addition, micromotors have also been shown to be potentially useful for nanosurgery,16 energy devices,17 controlled self-assembly18 or sensing applications.19 © 2016 American Chemical Society

Received: January 1, 2016 Accepted: January 27, 2016 Published: January 27, 2016 4250

DOI: 10.1021/acsami.6b00012 ACS Appl. Mater. Interfaces 2016, 8, 4250−4257

Research Article

ACS Applied Materials & Interfaces by the template synthesis method, consists of two components (i.e., the gelatin shell with the inner platinum nanoparticle (PtNP)). When placed in the aqueous solution containing the hydrogen peroxide (H2O2) fuel, the cartridge-case-like catalytic micromotor can be self-propelled based on the bubble propulsion mechanism.26,27 Interestingly, we find out that its motion is pH-dependent. The pH-dependent motion may be ascribed to the synergistic interactions between the pHresponsive gelatin shell and PtNPs. In addition, the pHdependent catalytic behavior of PtNPs and the base catalyzed volumetric decomposition of H2O2 also contribute to the observed phenomenon. The facile fabrication and the motion readout shown in the current study make the cartridge-case-like micromotor an attractive candidate for pH sensing applications.

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Figure 1. (a−d) Schematic illustration showing the fabrication process of the cartridge-case-like structure.

EXPERIMENTAL SECTION

Materials. Gelatin and glutaraldehyde were purchased from SigmaAldrich Incorporation. The polycarbonate filter membrane with a pore size of 8.3 μm is purchased from Millipore Company. Three nm PtNP with surface polyvinylpyrrolidone ligand and polypyrrole (PPY in short) nanoparticles are synthesized according to the literature methods.28,29 Preparation of the Cartridge-Case-like Structure. 8 wt % gelatins are first dissolved in water at 50 °C. The polycarbonate filter membrane is then immersed into this solution. After sonication for 1 min to remove the trapped gas bubbles inside the pores of the filter membrane, the solution is placed on a heating plate at 50 °C for 30 min. During this process, the gelatin solution will infiltrate into the pores. The filter membrane is taken out from the gelatin solution, predried and then placed in an oven and dried at 45 °C for 10 min. After drying, the surface of the filter membrane is cleaned using a wet cotton swab. The filter membrane is then placed in the 0.5 wt % glutaraldehyde solution for 5 min to cross-link the gelatin. The membrane is removed from the solution and dried. It is then immersed in the methanol solution of the PtNP for 1 h to immobilize the PtNPs, after which, the membrane is placed in an oven until the methanol solvent is evaporated. Its surface is then cleaned with a methanol soaked cotton swab. Finally, the whole structure is dissolved in chloroform to release the cartridge-case-like structure. The synthesis of the PPY/PtNP cartridge-case-like structure follows the same procedure as shown above except that the 3 wt % aqueous solution of the PPY nanoparticle is applied at the room temperature instead of the 8 wt % gelatin solution at 50 °C. Characterizations. The scanning electron microscopy (SEM) experiment is carried out on a Carl Zeiss Supra 55 scanning electron microscope with an energy dispersive X-ray (EDX) analysis attachment. The SEM sample is prepared by drop-casting the cartridge-caselike structure on a cover glass. Solvent is allowed to evaporate before the SEM examination. A 6 nm thick gold is sputtered onto the structure prior to the SEM observation. The atomic force microscopy (AFM) measurement is performed on a Bruker Dimension scanning probe microscope. For the micromotor study, the cartridge-case-like structure is placed in the aqueous solution containing 15 wt % H2O2. The autonomous movement is observed and recorded by a Nikon Eclipse 80i microscope. The motion analysis for the captured video is performed using the PhysVis software. For the pH sensing experiment, the cartridge-case-like structure is first placed in the pH 0 solution (100 μL) containing 15 wt % H2O2. The solution with different pH (40 μL) is then directly added into this solution. The motion behavior of the micromotor is then observed and recorded.

fashion. In the current study, we apply the polymer solution directly. The template (i.e., the polycarbonate filter membrane with a pore diameter of approximately 8.3 μm) is first immersed in the aqueous solution of gelatin. After drying, a layer of gelatin is formed inside the pores (Figure 1b). Glutaraldehyde is utilized to cross-link the gelatin through its reaction with the pendant amino groups. PtNPs are then immobilized onto the inner surface of gelatin by the selfassembly technique (Figure 1c). PtNP is selected because of the higher catalytic activity (as compared to the bulk counterpart) originating from its higher surface area to volume ratio.32,33 In addition, the self-assembly technique is a solutionbased method that does not require the utilization of expensive equipment, which can greatly facilitate the fabrication. Finally, the structure is released from the template by dissolving the polycarbonate filter membrane away in chloroform (Figure 1d). As demonstrated previously in the literature, the combination of the template synthesis and the thermal or electrochemical deposition often results in two different micromotor structures (i.e., the segmented rod made of a variety of metals/conducting polymers34 or the tube-like structure consisting of different layers of functional materials30). However, after examining the resulting microstructure by utilizing the SEM, we have found out that it possesses a unique cartridge-case-like shape, as can be seen from the large area image shown in Figure S1 in the Supporting Information. Figure 2a shows a schematic illustration of the cartridge-case-like structure. The opening and the sealed end are illustrated in the SEM images shown in Figure 2b,c, respectively. The resulting cartridge-case-like structure is approximately 15 μm long with the inner, outer diameter and wall thickness of approximately 4.3, 8.3 and 2 μm, respectively. We attribute the formation of the cartridge-caselike structure to the utilization of the polymeric solution during the fabrication process. For the normal template synthesis method, different functional materials are directly formed in the pores. However, in our current case, the polymeric solution is in its liquid form during the deposition process. Upon drying, the polymer solution is capable of flowing under the influence of the gravitational force. As a result, they accumulate at the bottom part of the filter membrane and form a film, leading to the formation of the sealed end of the cartridge-case-like structure. Note that, because of this mechanism, the resulting cartridge-like-structure has a length of ∼15 μm, which is shorter than the thickness of the filter membrane (∼20 μm). Furthermore, we have studied the detailed structures of the

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RESULTS AND DISCUSSION The cartridge-case-like structure is constructed by the template synthesis method, as illustrated in Figure 1. To obtain the micromotor structure, the previously developed template synthesis method normally involves the thermal30 or electrochemical deposition31 of various metals in a step by step 4251

DOI: 10.1021/acsami.6b00012 ACS Appl. Mater. Interfaces 2016, 8, 4250−4257

Research Article

ACS Applied Materials & Interfaces

shown in Figure 2d, confirms the composition (i.e., a gelatin shell with PtNPs decorating the inner surface). The polymeric solution concentration has a great influence on the final structure. At the high concentration (>10 wt %), the gelatins fill the pores completely, leading to the formation of the solid rodlike structure (Figure S2a in the Supporting Information, similar to that reported in the literature). On the contrary, when the concentration is low ( 7). It can not be utilized in the acidic region because of the dissolution of silver microparticle at low pH. The Cu/Pt tubular micromotor exhibits stable reading from pH 0−12. Because of the structural changes through corrosive effect of the acid, the velocity of the Cu/Pt tubular micromotor does not increase monotonically with the pH of the solution. However, the cartridge-case-like micromotor in this study is sensitive over the whole pH scale and there is a proportional relationship between the micromotor velocity and the solution pH. Furthermore, due to the differences in the velocities under different conditions, the distance it can travel is also dependent on the pH of the analyte solution. Figure S3 in the Supporting Information illustrates the calibration curve showing the travel distance of the micromotor during 5 s after the addition of solutions with different pH, which increases gradually with the increasing pH of the analyte solution. To elucidate the motion-based pH sensing behavior, we have performed a few control experiments. Because the current cartridge-case-like micromotor consists of only two components (i.e., the gelatin and the PtNP), we have first studied the pH effect on the decomposition of the H2O2 fuel catalyzed by PtNPs. To this end, we have performed a control experiment by substituting the gelatin with another non-pH-responsive polymer (i.e., PPY). By utilizing the same fabrication method, we are capable of fabricating a similar cartridge-case-like structure. Figure S4 in the Supporting Information shows the typical SEM image of the cartridge-case-like structure made of the PPY and PtNPs. This PPY/PtNP structure appears to be black under the optical microscope (Figure 6 inset), which is in

between O−O, resulting in the formation of two hydroxyl radicals (·OH) based on the following equation: H2O2 → 2· OH. This peroxidase-like activity has been previously demonstrated by utilizing the electron spin resonance spectroscopy.43 Under other conditions, the PtNPs possess the catalase-like activity, which can directly decompose the hydrogen peroxide to water and oxygen based on the following equation: H2O2 → 2H2O+O2. The gas bubble formation can be directly visualized during this process.43 In the strong acid region (pH 1), PPY/PtNP moves very slowly. For the catalytic micromotors based on the heavy metal catalysts, there are three different mechanisms, i.e. the bubble recoil, the self-electrophoresis and the self-diffusiophoresis. The bubble recoil mechanism requires the generation of the gas bubbles; whereas the self-electrophoresis mechanism is based on the electric field generated by the bimetallic structures. Because no gas bubbles are observed under the strong acid condition and the current micromotor contains only one metal, the propulsion mechanism at low pH is likely due to the self-diffusiophoresis. For the self-diffusiophoresis, the motion is driven by the concentration gradient of solutes. The cartridge-case-like structure is asymmetric and the catalytic reactions can only happen near the opening, leading to the concentration gradient between the open and closed end, which propels the structure. In addition, the cartridge-case-like micromotor under this situation has a velocity of approximately 5 μm/s. This velocity falls in the range between 1 and 10 μm/s, which is typical for a micromotor propelled based on the self-diffusiophoresis mechanism.44 Under the weak acidic condition, the concentration of the generated hydroxyl radicals is much lower than the case of pH 1,43 while the oxygen bubble formation starts to dominate (as can be seen from Video S2 in the Supporting Information). Because the bubble recoil is a more efficient propulsion mechanism, the velocity of the micromotor thus becomes much faster. In the neutral solution, the PPY/PtNP is propelled by the bubble recoil mechanism as well with a velocity similar to that in the weak acid solution. Under the basic condition (pH > 7), in addition to the oxygen produced by the catalytic decomposition of the H2O2, there is an additional OH− catalyzed volumetric decomposition reaction of H2O2. As a consequence, more oxygen bubbles are generated and the micromotor moves faster with the increasing pH. On the other hand, we have examined the influence of the pH on the gelatin. It has been reported that the swelling behavior of the gelatin materials differs under different pH conditions. It may swell under the acidic environment due to the ionization of the amine groups and shrink when the pH is greater than 7.45 To study the influence of the pH on the morphology of the cartridge-case-like structure, we have examined its morphology at different pH under the SEM. To facilitate the observation, we directly inspect the structure while it is still inside the pore of the polycarbonate filter membrane. To this end, we have prepared the samples under different pH conditions, while the other parameters (such as the drying time) remain the same. As illustrated in Figure 7a−c, the cartridge-case-like structure exhibits a pH-dependent swelling behavior. In the acidic environment (e.g., pH 1), the cartridgecase-like structure is in its swollen state. The wall thickness of the cartridge-case-like structure is around 3.3 μm, which is close to the half of the pore diameter of the filter membrane (∼4.15 μm). As a consequence, the opening of the cartridge-case-like structure is the smallest, leaving just a tiny hole that is around 1.7 μm in size (Figure 7a). When the pH is increased to the

Figure 6. Speed of the PPY/PtNP cartridge-case-like micromotor in H2O2 versus the pH of the analyte solution. Inset: the optical microscopic image showing the black PPY/PtNP structure.

contrast to the transparent gelatin counterpart shown in Figure 3b−e. On the basis of this structure, we have studied its motion-based pH sensing behavior. As can be seen from Figure 6, the motion of the PPY/PtNP micromotor has three different regions (i.e., very slow motion around pH 1, steady moving speed for pH from 3 to 7 and gradually increasing moving velocity at pH > 7). The catalytic behavior of the PtNP is different under different pH conditions. Under the strong acid conditions, PtNPs are capable of directly fragmenting the bond 4254

DOI: 10.1021/acsami.6b00012 ACS Appl. Mater. Interfaces 2016, 8, 4250−4257

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Figure 7. SEM images of the cartridge-case-like structure under different pH conditions (a) pH 1, (b) pH 7 and (c) pH 12. The influence of pH on (d) the wall thickness and (e) the size of the opening. (f) The bubble generation frequency of the micromotor after the addition of analyte solutions with different pH.

have performed another control experiment. We have utilized the AFM to measure the morphology of the gelatin film with surface PtNPs under different pH conditions. As can be seen from Figure S5 in the Supporting Information, the gelatin surface is decorated with PtNPs in all three cases. However, the surface roughness differs significantly. The root-mean-square roughness (Rq) decreases gradually from 1.73 nm in the case of pH 12 to 0.524 nm in the case of pH 2. This may be due to the swelled gelatin encapsulates more surfaces of PtNPs, thus lowering the surface roughness. On the basis of this control experiment, we propose the following tentative explanation for the pH-dependent motion behavior of the gelatin-based cartridge-case-like structure: with the decrease of the pH, the gelatin material gradually swells, which encapsulates more surfaces of the PtNP, leading to the decrease of the exposed surface area of the PtNP and thus lower catalytic activity. This, together with the above-mentioned different catalytic behavior of the PtNP under different pH conditions and base-catalyzed volumetric decomposition of H2O2, accounts for the pHdependent motion observed in Figure 5.

neutral condition (pH 7), the opening extends to around 4.3 μm because of the dramatically decreased wall thickness of approximately 2 μm, which is caused by the shrinkage of the gelatin materials at high pH (Figure 7b). When the pH is further increased to the basic environment (pH 12), the gelatin material further shrinks. As is shown in Figure 7c, the wall thickness in this case is estimated from the SEM image to be approximately 0.7 μm, leading to the formation of the largest opening (approximately 6.9 μm). It can thus be concluded that, due to the swelling behavior of the gelatin material at different pH, the cartridge-case-like structure exhibits the pH-dependent “open and close” property. Figure 7d,e summarizes the changes of the wall thickness and the size of the opening under different pH conditions. On the one hand, the wall thickness of the cartridge-case-like structure increases as the pH decreases (Figure 7d). On the other hand, the size of the opening decreases correspondingly with the decreasing pH (Figure 7e). Because the motion of the current micromotor is based on the catalytic decomposition of the H2O2 fuel near the opening area, we have studied whether the size of the opening has an influence on the monotonic velocity increase observed in Figure 5. For the tube-like microengine, the bubble generation frequency is dependent on the tube diameter. As previously demonstrated in the literature,46 it decreases with the increasing tube radius if the reaction rate per unit time is constant. However, in our current case, the bubble generation frequency of the cartridge-case-like structure becomes faster (from ∼3 Hz to ∼16 Hz) with the increasing opening size, as can be seen from Figure 7f. Therefore, the observed opening size change is not the determining parameter for the proportional relation between the motion speed and pH of the analyte solution. By comparing Figures 5 and 6, the main difference is the monotonic increase of the motion velocity with the increasing pH in the case of the gelatin-based micromotor. For the heterogeneous catalysis, the reaction occurs on the surface of the catalyst, the catalytic activity of the catalyst is proportional to its exposed surface area.47 On the other hand, it is known that the pH-responsive polymer stabilized PtNPs exhibit tunable catalytic activity due to the volume changes of the capping agents at different pH.48 In the gelatin/PtNP case, we

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CONCLUSIONS In conclusion, we have reported a cartridge-case-like micromotor fabricated by the template synthesis method, which consists of two components (i.e., a gelatin shell with the inner PtNPs). Because of the structural asymmetry, it can be selfpropelled toward the sealed end based on the bubble propulsion mechanism. Intriguingly, this autonomous micromotor exhibits the motion-based pH sensing property over the whole pH range. By studying the morphology and motion differences under different pH conditions, we find out that the unique sensing phenomenon is possibly due to the pHdependent catalytic activity of the PtNPs caused by the swelling/deswelling behavior originating from the gelatin material at different pH. The base-catalyzed volumetric decomposition of H2O2 and the pH-dependent catalytic behavior of PtNPs also contribute to the motion-based pH sensing property. Our current work not only illustrates how the property of the constituent parts may influence the motion 4255

DOI: 10.1021/acsami.6b00012 ACS Appl. Mater. Interfaces 2016, 8, 4250−4257

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behavior of a micromotor under different conditions but also paves the way toward the further development of motion-based sensors.

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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/acsami.6b00012. SEM image showing several cartridge-case-like structures, SEM images of (a) the rod-like and (b) tube-like microstructures, calibration curve showing the travel distance over 5 s period of the cartridge-case-like structure versus the pH of the analyte solution, SEM image showing the PPY/PtNP cartridge-case-like structure, AFM images of the gelatin film with surface PtNPs at different pH (PDF). Video S1: Autonomous movement of the cartridge-caselike micromotor in the aqueous solution containing H2O2 (AVI). Video S2: Motion-based pH sensing based on the cartridge-case-like micromotor for pH 3, 5, 8 and 13 (AVI).

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AUTHOR INFORMATION

Corresponding Author

*B. Dong, E-mail: [email protected]. Author Contributions

The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 21574094, 21304064), the Natural Science Foundation of Jiangsu Province (Grant No. BK20130292, BK20150314), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Fund for Excellent Creative Research Teams of Jiangsu Higher Education Institutions and the project-sponsored by SRF for ROCS, SEM.

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DOI: 10.1021/acsami.6b00012 ACS Appl. Mater. Interfaces 2016, 8, 4250−4257

Research Article

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DOI: 10.1021/acsami.6b00012 ACS Appl. Mater. Interfaces 2016, 8, 4250−4257