Technical Note pubs.acs.org/ac
Simultaneous Nanomechanical and Electrochemical Mapping: Combining Peak Force Tapping Atomic Force Microscopy with Scanning Electrochemical Microscopy Peter Knittel, Boris Mizaikoff, and Christine Kranz* Institute of Analytical and Bioanalytical Chemistry, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany S Supporting Information *
ABSTRACT: Soft electronic devices play a crucial role in, e.g., neural implants as stimulating electrodes, transducers for biosensors, or selective drug-delivery. Because of their elasticity, they can easily adapt to their environment and prevent immunoreactions leading to an overall improved long-term performance. In addition, flexible electronic devices such as stretchable displays will be increasingly used in everyday life, e.g., for so-called electronic wearables. Atomic force microscopy (AFM) is a versatile tool to characterize these micro- and nanostructured devices in terms of their topography. Using advanced imaging techniques such as peak force tapping (PFT), nanomechanical properties including adhesion, deformation, and Young’s modulus can be simultaneously mapped along with surface features. However, conventional AFM provides limited laterally resolved information on electrical or electrochemical properties such as the activity of an electrode array. In this study, we present the first combination of AFM with scanning electrochemical microscopy (SECM) in PFT mode, thereby offering spatially correlated electrochemical and nanomechanical information paired with high-resolution topographical data under force control (QNM-AFM-SECM). The versatility of this combined scanning probe approach is demonstrated by mapping topographical, electrochemical, and nanomechanical properties of gold microelectrodes and of gold electrodes patterned onto polydimethylsiloxane.
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imaging experiments, and the electroactive area of the micro- or nanoelectrode is on the order of magnitude of the topological features of the sample surface. Several approaches have been developed for current-independent positioning of the probe in SECM, whereby combined AFM-SECM certainly is superior in terms of simultaneously obtaining high-resolution topographical information. AFM-SECM has developed into an attractive scanning probe technique used, e.g., for localized corrosion studies,11 characterizing nanoparticles,12 mapping of enzymatic activity,13 studying membrane transport,14 and biosensing.15 Apart from contact mode AFM-SECM imaging, dynamic modes have already been demonstrated16 and force spectroscopic measurements have readily been correlated with AFMSECM.17,18 Recently, nanomechanical AFM imaging techniques have been introduced, which offer quantitative nanomechanical mapping similar to force-volume imaging,19 yet at a speed comparable to contact or intermittent mode AFM.20 One of these techniques is the so-called peak force tapping (PFT) mode, where an AFM tip is oscillated with amplitudes ranging
ince its development in 1986 by Binnig, Quate, and Gerber,1 atomic force microscopy (AFM) has developed into an indispensable tool in materials and biological research. As an imaging tool, it is used on a routine basis to obtain highresolution topographical information on biological entities such as living cells,2 but it is also considered a standard tool for characterizing materials such as polymers,3 graphene,4 and nanoparticles.5 Because of its high force sensitivity, AFM is suitable for the investigation of mechanical properties on a wide range of materials, 6 for investigating single molecule interactions,7 and for mapping unfolding of proteins via force spectroscopic studies.8 Even though AFM was initially termed as “chemically blind”, a multitude of AFM-based methods for chemical mapping is known today. These advanced AFM techniques are frequently associated with an advanced probe design or involve instrumental combinations such as optical, electrochemical, or mass-sensitive techniques.9 Scanning electrochemical microscopy (SECM) is a versatile scanning probe technique, which not only provides information on electroactive processes at the sample surface but is frequently used for electroanalytical measurements at the micro and nanoscale. However, it may suffer from topographical artifacts of the electrochemical surface information,10 if the electrode is positioned at a constant height during © 2016 American Chemical Society
Received: March 18, 2016 Accepted: May 20, 2016 Published: May 20, 2016 6174
DOI: 10.1021/acs.analchem.6b01086 Anal. Chem. 2016, 88, 6174−6178
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Technical Note
from less than 100 up to 3 000 nm at frequencies of 0.25−2.0 kHz.21 This technique has already been used to investigate structural and physical properties of, e.g., amyloid fibrils, membrane proteins, and cells.22 Despite the high speed, PFT can be used to image chemical and biological interactions at live cells.23 The obtained force curves provide information on tip− sample adhesion force and energy, deformation, and Young’s modulus using the Derjaguin-Muller-Toporov (DMT) model for the latter.24 Another advantage for topographical imaging, especially for soft samples, is the direct force control, which impedes high lateral forces. Nanomechanical properties play a crucial role particularly in implant materials including neural electrodes. Elasticity mismatch can lead to significantly lower long-term performance.25 Thus, soft polymers, e.g., conductive polymers,26 electrode arrays on polydimethylsiloxane (PDMS),27 or PDMS-based conducting composites28 are promising candidates for implants as stimulating electrodes or in drug delivery and may readily be modified (e.g., by electrochemical deposition, etc.). For example, in order to characterize soft electronic devices in terms of topography, structure, and electrochemical properties, as well as elasticity and adhesion at the nanoscale, we present here AFM-SECM in combination with PFT. Electrochemical and quantitative nanomechanical mapping (QNM-AFM-SECM) allows AFM-SECM imaging with precise force control. Orchestrated electrochemical and PFT measurements are demonstrated at a gold microelectrode sealed in glass and at a gold electrode patterned onto a soft polymer (i.e., PDMS) substrate.
Figure 1. (a) AFM-SECM probe mounted on the custom-made probe holder (inset shows an exemplary SEM image of a tip-integrated frame electrode with a diameter of approximately 1 μm). (b) Electrochemical characterization after mounting in 2.5 mM Fc(MeOH)2 recording 50 consecutive cyclic voltammograms: 1st (black), 10th (red), and 50th (blue) cycle are shown. (c) Side view SEM image of an AFM-SECM probe.
QNM-AFM-SECM Measurements. Imaging was performed in 2.5 mM Fc(MeOH)2/0.1 M KCl (2% ethanolic solution) with the AFM-SECM electrode biased at a potential of 0.5 V vs Ag/AgCl and a scan rate of 0.25 Hz. The PFT frequency was 1 kHz, the amplitude 80−100 nm with a peak force of 2−5 nN. To evaluate the influence of the amplitude on the obtained faradaic current signal, a 20 μm × 1 μm image at the interface between gold and glass of a disc microelectrode (data not shown) was recorded in 5 mM Fc(MeOH)2/0.1 M KCl at amplitudes of 100, 150, 200, 300, 500, and 800 nm (Note for 500 and 800 nm, the frequency is 0.5 kHz). Prior to all measurements, CVs were recorded to ensure insulation of the electrical contact of the AFM-SECM probe. For quantitative data evaluation, the force constant of the modified probe was determined using the thermal noise method.30 Apart from determining the spring constant, for the structured PDMS, a sample with known elastic modulus was scanned to determine accurately the tip curvature radius needed for the DMT fit. A PDMS sample (E = 3.5 MPa, Bruker, Germany) served as a model sample, and a tip curvature radius of 50 nm was determined, which is in good agreement with the SEM image (Figure 1c). The structured, gold-modified PDMS sample was prepared as schematically shown in Figure S3. Briefly, a 10:1 mixture of PDMS (base, curing agent, Sylgard 184, Dow Corning, USA) was spin-coated onto a silicon wafer and cured. The gold structure was patterned onto the PDMS using two photoresists and UV photolithography. Poor resist and metal adhesion was compensated for using gentle plasma treatment.31
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EXPERIMENTAL SECTION Instrumental Modifications. All QNM-AFM-SECM measurements were performed with a Bioscope Catalyst AFM (Bruker, Germany). For adding electrochemical functionality, the system was modified including a custom-made electrochemical cell. The cell allows the integration of a platinum wire as a counter electrode, an Ag/AgCl reference electrode (DRIREF-2SH, WPI, Germany), and a second working electrode (Figure S1). The electrochemical cell was housed in a Faraday cage, and a shielded connection of an Ivium CompactStat (Ivium, The Netherlands) was readily achieved using BNC connectors at the cage. Probe Mounting. AFM-SECM probes were fabricated from silicon nitride probes with a nominal force constant of 0.05 N/m (Olympus RC800PSA, Japan), which were modified with a Ti/Au coating and insulated with mixed silicon nitride/ silicon dioxide layers according to a procedure described elsewhere.18,29 The tip-integrated frame-shaped gold microelectrodes were obtained by focused ion beam milling (FEI Quanta 3D FEG dual-beam FIB/SEM, The Netherlands). The AFM-SECM probes had a 500−800 nm long thorn-shaped tip surrounded by the frame electrode (approximately 1 μm in diameter) at a spring constant of 0.7−1 N/m, which is about 10 times higher than the initial value. Contact to the tip-integrated electrode of the AFM-SECM probe was obtained using a custom-made probe holder fabricated from polyether ether ketone (PEEK) with a small transparent polyethylene terephthalate (PET) window. After mounting the probe, all electrical contacts were sealed with UV curable glue (see scheme and detailed description, Figure S2). Before and after mounting, all probes were characterized by cyclic voltammetry in 2.5 mM ferrocenedimethanol (Fc(MeOH)2)/0.1 M KCl (Figure 1).
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RESULTS AND DISCUSSION AFM measurements on soft substrates are prone to imaging artifacts due to possible high adhesive and lateral forces, which also may lead to tip contamination. Hence, accurate control of the applied forces during imaging reduces such problems. To demonstrate the feasibility of combined nanomechanical and electrochemical mapping, a bare unpolished 25 μm gold disc microelectrode sealed into glass was imaged as model sample to ensure that the instrumental modifications were successfully implemented. Figure 2a shows the topography of the gold disc microelectrode sealed in glass with scratches from the grinding process clearly evident. The overlaid simultaneously recorded electrochemical image and the extracted profile provides 6175
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Figure 2. QNM-AFM-SECM measurement of a bare, unpolished gold microelectrode (diameter, 25 μm) recorded in 2.5 mM Fc(MeOH)2/0.1 M KCl. (a) Topographical image of the sealed disc microelectrode overlaid with the simultaneously mapped faradaic current with the AFM-SECM probe biased at 0.5 V vs Ag/AgCl (white line shows region of extracted profiles), (b) the recorded tip−sample adhesion (probe thorn length, 750 nm; frame electrode width, 1 μm). (c) Approach curves with a representative force curve on the gold electrode (green) and the glass (red dashed) showing a positive and a negative feedback as expected (measurement regions are denoted in part a with the corresponding color). For the amplitude used, the oscillation region of the probe is shown in blue. The graph in part d shows the decreasing faradaic current difference between a gold and a glass surface observed with increasing amplitude in 5 mM Fc(MeOH)2/0.1 M KCl.
information on the electrochemical activity of the scanned substrate, which in contrast to the topography does not reveal the grinding artifacts, as expected.32 The recorded current difference between the nonconducting glass and the sealed electroactive area amounts to approximately 10 pA (i.e., negative feedback current, red spot; positive feedback current, green spot).10 Although the tip is oscillating at 1 kHz with an amplitude of 100 nm, the electrochemical signal remains stable, and the positive and negative current feedback currents are clearly evident, as anticipated from contact mode AFM-SECM measurements (see profiles in Figure 2a). Additional information on the sample adhesion is shown in Figure 2b). The sharp tip only shows adhesion forces below 1 nN for both, the glass and the sealed gold microwire; however, a minute difference between the two surfaces could be observed mainly attributed to mechanical adhesion at the rather corrugated electrode surface. Approach curves obtained at the gold and the glass surface and an exemplary force curve is shown in Figure 2c. The faradaic current difference between gold and glass in contact mode is approximately the same, as obtained during the measurement oscillating the probe with an amplitude of 100 nm (see blue region in Figure 2c). Depending on the height of the structures and the adhesion between the tip and the sample, higher amplitudes may be needed for imaging. As the current signal is depending on the surface distance, the effect of higher amplitudes has been investigated in 5 mM Fc(MeOH)2)/0.1 M KCl. With increasing amplitudes, the current difference between the gold surface and the bare glass surface is constantly decreasing. This is expected, as the potentiostat records only 500 data points/s, and therefore only 1 point per 2 force curves (AFM operated at 1 kHz) is obtained. Although the scan is recorded at a scan rate of 0.25 Hz (resolution, 512 × 512; 2 force curves/pixel), the application of higher amplitudes will decrease the contact time of the probe, and hence, likewise the recorded feedback current, which is obtained as an averaged signal over the whole distance. For higher amplitudes, the
feedback signal can drop significantly and the current difference between conductor and insulator drops as shown in Figure 2d. In the future, using a high-speed potentiostat for such measurements may allow that the time for recording electrochemical signal may be reduced to the contact time of the probe or even individual approach curves for each sample point may be obtained. Given the low spring constant of the used AFM-SECM probes (0.7−1.0 N/m) apart from adhesion force and energy, no additional information on Young’s modulus could be obtained from this rather stiff substrate. For microelectrode arrays that are in mechanical compliance with neural tissue, soft polymeric substrates with thin metal electrodes are commonly applied.33 PDMS is a widely used material for that purpose owing to its biocompatibility. The elasticity of PDMS can be readily tuned via the amount of cross-linking during the curing process.34 A gold electrode (thickness 95 nm with 5 nm Ti adhesion layer) 2 μm in width was patterned onto a PDMS substrate with a mixture of 10:1 (base, curing agent) using photolithography and electron beam evaporation (process flowchart is shown in Figure S3). An exemplary QNM-AFM-SECM measurement (peak force 2 nN; amplitude, 80 nm) of the fabricated soft gold microelectrode is shown in Figure 3. Although only 100 nm Ti/Au (5 nm/95 nm, respectively) was evaporated, the obtained topographical data reveal that the height of the electrode is about twice as high (250 nm) (see also zoomed view in Figure 4b). Thermal stress during metal coating35 may lead to swelling effects affecting the area below the patterned gold electrode and hence may lead to an increase in height as observed in the AFM topography. Also, the elasticity image calculated from a DMT fit shows an expected increase of the Young’s modulus to 5.6 ± 1.7 MPa (N = 2300) at the gold pattern. However, for the surrounding PDMS (green dashed area) material, approximately the same modulus of 6.2 ± 0.9 MPa (N = 4400) is observed (see also Figure S4). The PDMS substrate not directly adjacent to the electrode array reveals a quite low modulus of 4.6 ± 0.5 MPa (N = 12 500). 6176
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Interestingly, the elasticity image exhibits cracks in the PDMS substrate. The modulus within these cracks is 2.5 ± 0.6 MPa (N = 200), which is in excellent agreement with the modulus for a 10:1 PDMS of around 2.6 MPa as reported in literature (see also histograms in Figure 4a).34 Evaluating the topography data, the cracks exhibit depths of 2−5 nm and widths of 150−250 nm (see inset of Figure 3a). From the recorded SECM image, it can be concluded that the gold electrode is conductive and only the desired structure was coated with gold (Figure 3b). For this measurement, the current image was flattened with a zeroth order fit to compensate for current drifts. The green-dashed area does not show higher conductivity although revealing a higher modulus. This indicates, that the increase in modulus around the gold electrode is mainly related to the plasma treatment of the surface. Additionally, the adhesion force image shows a higher adhesion of 0.75 ± 0.15 nN (N = 48 000) on the PDMS substrate compared to the gold (Figure 3c). This may be explained with the higher penetration depth of the tip into this soft substrate and due to van der Waals interactions. The cracks observed in the topographical and Young’s modulus images are not evident in the adhesion plot. Also, the green-dashed area with increased modulus does not reveal a decrease in adhesion. Because of the processing steps including oxygen plasma treatment to increase metal and especially resist adhesion,31 a hydrophilic surface layer is created, which is characterized by increased stiffness. This layer is clearly observable in the elasticity image reflected by the cracks and also in the related topography. Because of surface amorphization, the structure height of the gold pattern is significantly increased, and also the edges of the gold structure appear approximately 30 nm higher, which may be related to either poor surface adhesion or thermal stress during metallization (Figure 4b, red arrow). However, the plasma treatment appears to have a more significant impact on the overall Young’s modulus compared to the thin metal coating. Plasma treatment of PDMS can be used for controlled structuring,36 which is in agreement with the observed topographical changes. Within the cracks, the modulus of the PDMS is still low and fits values reported in the literature as well as the data recorded prior to the treatment. Two photoresists were applied to facilitate the lift-off procedure after metal coating, which results in a gap surrounding the metal coating. The second plasma treatment clearly contributes to this effect (see Figure S3). The green dashed area in Figure 3a) is most likely originating from this gap yet may also be related to thermal stress during metal coating, such as the observed PDMS swelling underneath the gold electrode. As stated above, this swelling leads to a structure height of 250 nm although a metal coating of only 100 nm was deposited.
Figure 3. QNM-AFM-SECM measurement of a gold electrode patterned onto PDMS recorded in 2.5 mM Fc(MeOH)2/0.1 M KCl. Peak force, 2 nN; amplitude, 80 nm. (a) 3D topography of the structure overlaid with the Young’s modulus (calculated with a DMT fit, green mark shows increased stiffness of the PDMS surrounding the gold pattern, inset shows topography of the cracked PDMS); (b) faradaic current image obtained with the AFM-SECM probe biased at 0.5 V vs Ag/AgCl; and (c) tip−sample adhesion image.
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CONCLUSIONS In the present study, we demonstrate that AFM-SECM imaging may be readily combined with nanomechanical mapping providing quantitative nanomechanical information simultaneously with electrochemical information. The obtained results establish an innovative instrumental combination, QNM-AFMSECM, for studying complex functional structures with variations in surface features, electrochemical activity, and mechanical properties across the surface. This is particularly interesting for soft electronic devices (e.g., in neural implants), as processing steps during microfabrication may lead to significant changes in local elasticity, which may induce
Figure 4. (a) Histograms showing the different Young’s modulus recorded on the patterned PDMS substrate. Cracks on the surface with 2.5 ± 0.6 MPa (N = 200) (red), the PDMS substrate with 4.6 ± 0.5 MPa (N = 12 500) (blue), the PDMS near the gold structure with 6.2 ± 0.9 MPa (N = 4400) (green), and the gold structure with 5.6 ± 1.7 MPa (N = 2300) (yellow). (b) A zoomed view of the topography of the gold structure with corresponding extracted profile (white line). The sputtered metal height is indicated in the profile in yellow, and the red arrow indicates a gap in the center part of the structure.
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mismatches. However, the developed approach may also be of substantial interest for studies of complex biological samples such as bacterial cells where advanced control of the exerted force and the mapping of nanomechanical properties along with recording of electroactive molecules is important. Future advancements will lead to simultaneously recording force− distance curves and electrochemical feedback curves at the speed of conventional contact mode AFM, which then provides valuable information on kinetic data.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b01086. Photo of electrochemical cell, mounting and insulation of the cantilever, fabrication flow chart of the goldstructured PDMS substrate, and QNM-AFM-SECM measurement at the center region of the deposited gold structure (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +49-731-5022749. Fax: +49-731-50-22763. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the Boehringer Ingelheim Ulm University BioCenter (BIU) and the German Federal Ministry of Education and Research (BMBF - NanoMatFutur: Grant 13N12545) for financial support. Alexander Eifert is thanked for his assistance with CAD drawings. Also, the FIB Center UUlm at the Institute of Analytical and Bioanalytical Chemistry and the Cleanroom & Workshop at Ulm University are acknowledged.
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REFERENCES
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