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Fabrication of periodic nano-porous surface structure for highly sensitive gravimetric quartz crystal applications
Journal Pre-proof Fabrication of Periodic Nano-porous Surface Structure for Highly Sensitive Gravimetric Quartz Crystal Applications Deuk-Yong Shim, Woo-Sik Kim, Sang-Mok Chang, Jong Min Kim PII:
S0003-2670(20)30140-9
DOI:
https://doi.org/10.1016/j.aca.2020.01.071
Reference:
ACA 237431
To appear in:
Analytica Chimica Acta
Received Date: 26 December 2019 Revised Date:
28 January 2020
Accepted Date: 30 January 2020
Please cite this article as: D.-Y. Shim, W.-S. Kim, S.-M. Chang, J.M. Kim, Fabrication of Periodic Nanoporous Surface Structure for Highly Sensitive Gravimetric Quartz Crystal Applications, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2020.01.071. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Graphical abstract
A periodic nano-porous surface on quartz crystal electrodes was carefully fabricated for increasing the mass-sensitive areas. Detailed porous structures were prepared by analyzing Au electrochemical reduction process of PS layer coated quartz crystals. The sensitivity measurement of the porous quartz crystals was performed with several traditional methods, and an optimized reduction time for higher sensitivity was determined. The frequency shift of the nano-porous quartz crystals showed 10 times bigger change with the same concentration of target solutions in self-assembly procedures. In the procedures, the freshly increased surface portion did not produce additional molecular slip-effects on the measured resonant resistance values, thus, the periodic porous chips showed another side merit for the mass sensor applications. We propose a possible use of the current porous surface as a platform for developing other high-performance sensors and analyses.
Fabrication of Periodic Nano-porous Surface Structure for Highly Sensitive Gravimetric Quartz Crystal Applications Deuk-Yong Shim,a Woo-Sik Kim,b Sang-Mok Chang,c Jong Min Kimc* a
Major in Chemical Engineering, Dong-A University, 840 Hadan, Saha-gu, Busan 49315, Republic of Korea Department of Chemical Engineering, Kyung Hee University, Kyungki-do 17104, Republic of Korea c Department of Chemical Engineering, Dong-A University, 840 Hadan, Saha-gu, Busan 49315, Republic of Korea b
*Corresponding Author. Tel.: +82 51 200 7717; Fax: +82 51 200 7728. E-mail address: [email protected] (J.M. Kim).
Abstract A periodic nano-porous surface on quartz crystal electrodes was carefully fabricated for increasing the mass-sensitive areas. Detailed porous structures were prepared by analyzing Au electrochemical reduction process of PS layer coated quartz crystals. The sensitivity measurement of the porous quartz crystals was performed with several traditional methods, and an optimized reduction time for higher sensitivity was determined. The frequency shift of the nano-porous quartz crystals showed 10 times bigger change with the same concentration of target solutions in self-assembly procedures. In the procedures, the freshly increased surface portion did not produce additional molecular slip-effects on the measured resonant resistance values, thus, the periodic porous chips showed another side merit for the mass sensor applications. We propose a possible use of the current porous surface as a platform for developing other high-performance sensors and analyses. Keywords: Large surface area, Quartz crystal, Molecular slip-effect, High sensitivity, Polystyrene beads
1. Introduction Quartz crystals have been widely used as trustworthy mass sensors applying the correlation between the resonant frequency change and the adsorbed mass change on the electrode surface ever since Sauerbrey’s theoretical investigation. The well-known Sauerbrey’s relation is follows [1];
∆ = −
∆
(1)
where ∆F is the resonant frequency shift of quartz crystal; f0 is the resonant frequency of the quartz crystal;
∆m is the surface mass change; µ is the shear modulus of the quartz crystal; ρQ is the density of the quartz crystal and A is piezoelectrically active crystal area of the quartz crystal.
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The relation has enabled many developments in gas sensors, and the further development are still frequently reported [2-8]. Stable measurement of the sensors has been normally possible because the quality factor (the ratio of the wavelength to the wave height) of the quartz crystal is sufficiently high [9]. The use of quartz crystal in liquid phases has also been proved by Kanazawa et al., and the liquid phase applications have been numerously documented thought the relation between the resonant frequency change and the surface mass change has been recalibrated [10]. Furthermore, some analytical variables for understanding the interfacial and surface phenomena have been additionally conceptualized such as “resonant resistance”[11] and “dissipation factor”[12]. Many papers have been published for treating the resonant resistance and the dissipation factor changes, and most of them provides logical information or mechanism related to the resonant frequency change depending on the measured variables [13-16]. We also showed the resonant resistance and the quality factor could be measured simultaneously with the frequency measurement applying an impedance analyzer though we could not clearly reveal the difference between the two measured variables [17]. At anyhow, the embodiment of these concepts has also been a positive role for developing the quartz crystal applications not only for the specific sensors but also for the analytical devices. Thus, currently, the quartz crystal has also been known as a measuring device for the fine mass with the viscoelasticity, and various analyses using its measured variables have been reported in the areas of immune assay, gelation reactions, electrochemical experiments, viscoelastic measurements in thin films without careful consideration of the measuring conditions [13].
The sensor applications of the quartz crystal were normally made by a simple way. The target specific recognition layers were firstly coated on the electrode surface, and the frequency response was measured when the modified electrode interacted with target molecules. The development for the high performance quartz crystal chip had not been widely reported, and commercially available quartz crystals were still obtained with their primitive form. A few reports showed the increase of mass sensitivity in various quartz crystals and other gravimetric sensors applying porous structures or porous materials [1820]. These reports were well describing the merit of porous structures for measuring the interface interaction, but the increase of the sensitivity was not big compared with the increase of the surface areas because the obtained surfaces were random pore structures. The increase of the mass sensitivity was only about 2-fold for DNA detection with a nano-porous structure because the fabrication procedure was only based on a wet chemical etching, which produced random nano-porous structure [19]. Similarly, Terao et al. showed the fabrication of a nano-porous electrode, and the increase of the surface areas about 16-fold has been achieved by fabricating nano-needle shapes [20]. To enable the shapes, they added Pb2+ in the Au electrochemical reduction cell. This procedure enabled the increase of the surface area about 16-fold but the sensing gold area was not large by the contamination. Thus, the sensitivity increase of 4-fold was obtained in the IgG detection. Another method for increasing the surface area was utilizing nano-porous material coatings, but this method was only sufficiently helpful when the target component was not changed [6-8]. Thus, the porous platform usable for developing various different sensors is still required.
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Recently, Kong et al. reported an impressive mass sensor application using surface area controllable porous structures [21]. The document focused on the molecular imprinting techniques, and did not treat the important merits of the periodic nano-porous structure. Here, we describes the conceptual difference in the sensing schemes, detailed manufacturing procedure of the nano-porous electrode, the improved sensitivity in a self-assembly process and further possibilities for the quartz crystal sensors as well as the fundamental characteristic of the nano-porous chip.
2. Materials and methods 2.1. Materials
Chloroauric acid (HAuCl4), ethylene diamine tetra acetic acid (EDTA), sodium sulfite (Na2SO3) and dipotassium phosphate (K2HPO4) were purchased from Sigma–Aldrich and used for the electrochemical reduction of Au ion. Poly styrene bead (PSB) colloids (750 nm) were obtained from Alfa Aesar and used as a template for fabricating periodic nano-porus gold structures. 3-Mercaptopropionic acid (3-MPA) and 3-(aminopropyl) triethoxysilane (3-APTES) were also purchased from Sigma-Aldrich and used for the performance test of the fabricated quartz crystal chips. All other chemicals used were analytical grades. 2.2. Preparing closely packed PS spheres
Detailed fabrication procedures were reported elsewhere [22]. Briefly, glass substrates (slide glass from matsunami) were rinsed with H2O and acetone. The substrates were dried with nitrogen gas and immersed in piranha solution (H2SO4:H2O2= 3:1, v/v) for 40 min (CAUTION: strong oxidizing solution). In the continuous step, a PSB stock solution (750 nm PSB suspension) was spread on the prepared substrates using micropipettes. Then, a spin coating was performed using a spin-coater (MS-A150, MikasaOpticoat Co., Ltd.) to spread colloidal suspension on the substrates. The operation was performed at 300 rpm for 5 min after dropping the colloidal solution on the substrates. Next, the substrates were carefully dipped in a fleshly prepared a Petri dish containing distilled water, and the PS layer was spread on the water surface. The colloid layer was normally not a closely packed form, and we used sodium dodecyl sulfate (SDS) solution to fabricate more condensed forms. Finally, the floating PS colloidal layer was transferred on a side of Au quartz crystal electrode (EG&G Co., Ltd.), and the crystal was dried for a day to remove moisture. 2.3. Fabrication nano-porous electrode surface
Potentiostatic electrochemical reduction of Au ion was performed to grow new gold areas from the gaps among the PS spheres (Scheme 1). Using a potentiostat (Par 273, EG&G Co., Ltd.), a reduction potential of − 0.7 V (vs. Ag/AgCl) was applied to the PSB modified quartz crystal in an aqueous electrolyte solution containing HAuCl4 (12 gL−1), EDTA (5 gL−1), Na2SO3 (160 gL−1), and K2HPO4 (30 gL−1). The procedure of the electrochemical reduction was monitored via QCM922A (EG&G Co., Ltd.) and WinEChem software (EG&G Co., Ltd.). After finishing the electrochemical reduction, the PS
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spheres on the quartz surface were carefully removed by rinsing with toluene for 2 h. The porous structure and the thickness of the gold film were determined by analyzing the electrochemical reduction data, and the deposition times were maintained as 80 s, 170 s, 250 s for comparing each difference. 2.4. Characterization
An atomic force microscope (Nanoscope IV, Digital Instruments) with commercially available tapping mode tips (Nano World, Al-coated silicon, f: 320 kHz) was used to investigate surface morphology. The AFM measurements were conducted with a scan rate of 1 Hz applying a large piezoscanner with a maximum scan area about 150 × 150 µm2. All AFM images were only flattened and other modifications were not performed using SPIP software (SPIP, Imagemet Co., Ltd.). An impedance analyzer (4294A, HP) was also used for the measurement of impedance spectra of the quartz chips. To perform self-assembly during the solvent evaporation, we used a thermohydrostat (MTH-2200, Sanyo Co., Ltd.) to maintain relative humidity (20 %) and temperature (20 °C). A micro syringe (2 µl, model 88411, Hamilton) was used for dropping small quantity of solution in the dropping and evaporation measurement. The resonant frequency and the resonant resistance of the fabricated chips were measured via QCM922A during the performance measurements using WinQCM software (EG&G, Co., Ltd.).
3. Results and discussion Typical response changes of the resonant frequency and the resonant resistance during the AuCl4electrochemical reduction on a PS array coated quartz crystal are presented in Figure 1(a). The resonant frequency was continuously decreased during the reduction process with minimum two breakpoints marked as arrows in the reduction time around 80-90 s and 170-180 s. The resonant frequency change were not only related with the accumulated mass but also related with other mass effects in the surface and interface. The change of the resonant resistance was more complicated, and showed three breakpoints at the least. The resonant resistance change normally means the change of viscoelascity on the surface and the interface as well as the mass loadings [23, 24]. The breakpoints toward the resistance increasing direction at the reduction times of 70 s and 200 s were located nearby or after the two breakpoints of the resonant frequency shift. In the current deposition condition, several surface phenomena should be also considered for understanding the resonant resistance change such as viscous attachment/detachment of the PS spheres and the electrolyte condition changes as well as the elastic Au deposition. In the case of a normal metal deposition without other surface phenomena, both the slopes showed a uniform [25]. For easy understanding, the relation between the accumulated charge values and the corresponding resonant frequency decreases (R2. Q1) was additionally presented in Figure 1(b). In Figure 1(b), two breakpoints were also observed nearby the frequency shifts of 40 kHz and 55 kHz. If Au ion reduction was the only mechanism for the frequency decrease, these points should not be observed. In addition, the first breakpoint, noted as ‘A’, showed the change to the bigger charge values (bigger ∆C/∆F or smaller ∆F/∆C),
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but that of the second breakpoint showed the change to the smaller values. The range between ‘A’ to ‘B’ was believed as most pure Au reduction region because the slope value was biggest. Other two areas were believed to have additional mass effects because these regions have small charge values with the same ∆F change (small ∆C/∆F). Sun et al. showed a possible reduction mechanism for the PS sphere coated electrodes [22]. They provided a two-step reduction mechanism by the function of the reduction time. In the mechanism, the horizontal Au reduction is only possible in the initial step, and the reduction is changed to both the horizontal and perpendicular directions considering the surface configuration. Thus, the reduction before the breakpoint ‘A’ was believed as the horizontal Au reduction, whereas the horizontal and perpendicular growth between the two breakpoints was estimated. In the horizontal reduction, the negatively charged HPO4- ions should be localized among the PSB gap areas [22]. These ions would be trapped in the growing Au atoms, and trapped ionic mass was assumed for the additional mass loading because the procedure was not related with electron exchanges. On the other hand, the growth after the breakpoints ‘B’ was difficult to clarify with only the proposed mechanism. Thus, assuming the reaction time of the breakpoints to 80 s and 170 s, we fabricated 2 additional chips and analyzed their properties with several methods. In the current paper, we calls P1 chip, P2 chip, P3 chip for 80 s, 170 s and 250 s deposited quartz crystals, respectively.
Figure 2 shows AFM topography images for P1 (a), P2 (b) and P3 chips (c) within a scan area of 60×60 µm2. The height scales of the image were equally adjusted for easy comparison. The difference of the morphology could not be clearly distinguished between Figure 2(a) and Figure 2(b) except the height changes. In Figure 2(c), the bright areas were randomly located over the surface. The bright areas were not believed as non-dissolved PS spheres because the surface was carefully washed with toluene more than 2 h at 40 °C. In addition, we also tried many efforts to wash out the areas such as a long time washing (24 h) and washing at an elevated temperature (70 °C), but the bright areas were still remained. Thus, the random bright areas were believed as deposited gold layers. The line profiles in Figure 2(d) showed the surface roughness was critically increased in P3 chip. The explanation for the profile change was relatively simple considering the dimension of the PS spheres. The highest point in the line profile of P3 chip was reached to about 350 nm, which was similar with the mean radius of the PS spheres. We also added more expanded AFM images in Figure 3. In Figure 3, importantly, one can observe that the surface of the electrode bottom can be confirmed through the pores of P1 chip and P2 chip but that of P3 chip cannot be confirmed. i.e. one step height was confirmed from P1 and P2 chips whereas two step heights were confirmed in P3 chip, which implied the deep sites in the gold pores were not opened to the outer environment as shown in the line profiles of Figure 3. Thus, the direct access of analytic solution to the deep pore areas was relatively difficult for P3 chip. In addition, we added SEM images for the PS layer on the electrode as supporting information (Figure S1). As shown in Figure S1, the PS spheres showed closely packed surface structure in
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the large scale, but the gaps among the PS spheres were differently observed in the expanded view. Because the PS spheres had a weak negative charge, every PS sphere repelled neighbored PS spheres. Therefore, the gaps were normally configured, but the distance and the height of the gaps were differently produced by the local repulsive force difference and the irregular PS size. Thus, we believed, there were other growth mechanism when the obtained film thickness was reached up to the radius of the PS spheres, and the mechanism would be discussed at the end of the current paper.
The oscillation properties of the fabricated chips were examined by two methods. Figure 4 shows the result of impedance spectrum analyses nearby the oscillation frequencies obtained by the impedance analyzer. The result of a commercially available Au chip was also presented for comparisons. As shown in Figure 4, the diameters of the spectrum were continuously decreasing with increasing the reduction time. Here, 1/diameter was calculated as the resonant resistance value [11], thus, the resonant resistance was increasing with increasing the Au reduction time. The conductance maximum frequency of the 4 chips was nearly located in the real axis, and the quality factors [9] were 120,000(D factor: 8.33×10^-6), 116,000(D factor: 8.62×10^-6), 114,000(D factor: 8.77×10^-6) and 110,000(D factor: 9.09×10^-6) for the commercial, P1, P2 and P3 chips, respectively. For more detailed inspection of the oscillation properties, the stability measurement was performed in a room condition ((a) and (b), T: 20 °C, R.H.: 20 %) and a liquid phase ((c) and (d), distilled water) shown in Figure 5. Bonroy et al. showed that the Sauerbrey equation is valid in an electrodeposited porous gold quartz crystal [18]. Thus, the current frequency responses could be transformed to the mass changes with the same method used in commercial quartz crystal techniques [9]. The resonant frequency shift of the three chips (commercial, P1, P2) showed slight drifts of sub - 2 Hz within the measured time. P3 chip showed a little different tendency. In the initial step until 300 s, the resonant frequency was slightly decreased, and then, the frequency was stabilized within the drift of 1 Hz. P1 chip showed best stability, but all chips were reasonable stability in the atmosphere condition. In the case of the resonant resistance, the difference was only the absolute value. The commercial and P3 chips showed smallest and biggest values, which corresponded with the result obtained by the impedance analyses described in Figure 4. When the chips immersed in the distilled water, the frequency stabilities were greatly changed by the liquid mass loading. The commercial and P2 chips showed a similar frequency drift of about 5 Hz within the measured time scale. Unlikely with other chips, P3 chip showed a continuous frequency decrease, which implied continuous water penetration into the deep gold areas. This property was not desirable for the liquid phase operation because it would prohibit the precise analysis of target component quantities. The stability of the resonant resistance value was similar with that of the room condition except absolute value changes by the viscous liquid loading. Stability measurement showed little difference among the chips but the possibility as sensors was not clearly determinable. Thus, we performed two additional measurements.
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The first measurement was the observation of water dropping and evaporation procedure. As indicated in the support information (Figure S2), the water contact angles of the three chips were all over 80°. These surface properties enabled the evaporation measurement of a small solvent quantity because the water drop did not spread over the surface. Thus, we dropped 2 µl of distilled water in the center of the porous chips and monitored their evaporation procedures. For minimizing the temperature and humidity influence, the thermohydrostat was additionally used with the setting temperature and the relative humidity of 20 °C and 20 %, respectively. In Figure 6, the evaporation speed of the water drop was very slow for P3 chip. Reversely, it meant the delivery of water molecules to the deep gold area required a long time compared with other 2 chips. The slow evaporation in P3 chip was a coincident result with the continuous decreasing frequency of P3 chip in Figure 5(c).
In the second measurement, each chip was initially dipped in the distilled water with a stirring rate of 100 rpm and waited until the chip reached to a stable frequency drift. In the continuous step, 3-APTES (100 µl, original stock solution) was dropped in the distilled water, and its final concentration was 10 mM. Figure 7 shows the resonant frequency and resonant resistance changes by the self-assembly on the active gold areas of the 4 chips. Overall tendency of the 4 chips was similar except the absolute value differences in the final frequency shifts. i.e. in the initial 10 min, fast and large decrease in the frequency was observed, and after that time, slow and small decrease in the frequency was observed. After 3 h of the assembly time, the final frequency shift was completely different among the chips as noted in Figure 7(a). The frequency shifts of P1 and P2 chips showed 10 times bigger value than that of the commercial chip. The difference clearly suggested the active gold areas of the two chips are larger about 10-fold than that of the commercial chip because 3-APTES only assembles on the gold surface through Au-Si bond [26]. Similar result was also obtained in the self-assembly of 3-MPA (data not shown or see Figure S3). Unexpected result was that of P3 chip because its surface area was assumed to be largest. Two possible explanations for the small frequency change were the contamination of the gold and the difficulty to deliver the 3-APTES into the deep gold pore areas. The second possibility was previously confirmed in the simple water dropping and evaporation measurement (Figure 6). The change of the resonant resistance was also interesting. The measured resonant resistance values would be influenced by the contacted liquid viscosity change, the slip-effect of adsorbed molecules, and molecular coupling effect on the surface as well as the surface mass change [11]. And therefore, in the current condition (same liquid viscosity, assembly of same molecules), the resonant resistance difference by the assembly should be proportionally increased with the magnitude of the frequency decreases when the self-assembly performed by the same molecule. In Figure 7(b), the resistance shifts of the 4 chips were similar in spite of the big differences in the frequency shift. A possible explanation was the direction of the gold film growth because the increased new gold area was the perpendicular direction for the oscillation. i.e. the horizontal surface area was not
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changed though horizontal reduction had occurred. When 3-APTES assembled on the new horizontal areas, the molecular slip effect on the resonant resistance was relatively small, i.e. the slip-effect is a horizontal mass effect [9], and thus, the resistance variation by the assembly was not big regardless of the assembled quantity. This was desirable properties for normal quartz crystal sensors because a small resonant resistance change implied the linear relation between the frequency shift and surface mass change in the Sauerbrey’s relation [9].
A summarized Au electrochemical reduction mechanism for the PS sphere modified electrodes was illustrated in Figure 8. Three steps were a definite feature in the electrochemical reduction process. The 1st and 2nd steps were the same procedure explained by Sun et al. The 3rd step would be happened when the new gold layer thickness reached over the radius of the PS spheres. At this moment, the growing gold portion exposed to the whole electrolyte solution, and started to cover the PS spheres. Thus, the exposed gold portion possibly pressed down the PS spheres, which produced additional mass effect without electron exchanges, and the mass-effect would increase the resonant resistance values by the contact with the viscous PS spheres. When the gold film covered the PS spheres, the removal of the PS spheres with the wet chemical method was also difficult because the solvent was difficult to access the whole PS sphere areas. In this case, the incomplete PS removal would be a contamination root for the active gold areas.
Finally, we demonstrated a novel application of the porous chip in the support information based on the hydrophobic nature of the surface using P2 chip (Figure S3). The application was based on the fact that the water drop shape was not seriously changed until it completely evaporated. The method suggested the best sensitive area of the quartz crystal could be used as a separate analyzing chip. In a simple method, 2 µl of 1 µM 3-MPA was dropped and evaporated, and the response depending on the dropping location was monitored. The Gaussian feature of the mass sensitivity by the sensing location was well-known in quartz crystal sensors [27]. The measurement showed possible use of the porous chip for a small quantity hydrophilic sample with using the best sensitive area. In Figure S3, the center position of the porous quartz crystal showed detailed 3-MPA self-assembly procedure compared with those of the other areas as well as the large frequency change.
4. Conclusion In the current paper, we showed the fabrication, the performance and the oscillation characteristic of the periodic nano-porous quartz crystals utilizing the 750 nm PS spheres as the template. In the fabrication procedure, the Au reduction process was important to obtain performance maximized porous chips, therefore, the process should be carefully investigated for avoiding undesirable chip properties. The result implied a different size of PS spheres could endow different sensitivity, thus, the mass sensitivity would be
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controllable to some extent though the possible gold film thickness for the maximum performance was influenced by the PS sphere sizes. In fact, the periodic structure also enabled a rapid detection because the 3-D detection scheme, which was comparable 2-D detection in the commercial plane chip (see Graphic Abstract). Furthermore, the analyses implied other material-template reduction procedures for fabricating large surface area would also have some potential problems for electrode contamination by the templates themselves. Thus, careful inspection for the reduction process would be required in normal template based reduction processes. Finally, the periodic structure could be fabricated on any electrode surface for improving any signal response because the fabrication was easily possible when an electrochemical reduction could be performed.
Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (No. 2017R1D1A1B03030262).
Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at http://.
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List of Figure Captions Scheme 1. Fabrication procedure of nano-porous electrodes using PSB as a template. Figure 1. Typical response changes by the Au electrochemical reduction process. The resonant frequency shift and the resonant resistance change (a), and the frequency shift by the function of charge accumulation (b). Figure 2. AFM topography images of Au deposited quartz crystal surfaces for P1 (a), P2 (b) and P3 chips (c). Line profile analyses for each location of the chips (d). Image areas are 60*60 µm2. Figure 3 Expanded views and line profiles for Figure 2. P1 and P2 chips show one step height line profiles ((a) and (b)), whereas P3 chip show clear two step height line profile as indicated with red arrows (c). Image areas are 20 × 10 µm2. Figure 4. Impedance spectra of the 4 chips. Figure 5. Resonant stability measurements in an atmosphere condition (a, b) and in the distilled water (c, d). Figure 6. Resonant frequency changes in the water dropping and evaporation measurement. In the measurement, 2 µl of the distilled water was dropped in the center of each chip. The inset graph shows an expanded view of the surface evaporation processes. Figure 7. Typical response differences of the 4 chips by the self-assembly of 3-APTES. The resonant frequency (a) and resonant resistance changes (b) by the assembly. Figure 8. Schematic illustration for the PSB templated electrochemical Au reduction process.
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Scheme 1
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Figure 8
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Highlights The mechanism for the PSB templated electrochemical gold deposition is clarified. The increased gold areas did not show the sensitivity decrease done by the molecular-slip effect. The self-assembly of 3-APTES showed 10-fold bigger frequency changes than a normal commercial quartz crystal. A novel application of the fabricated chip was suggested for readers.
Deuk-Yong Shim: Data curation, Software, Methodology. Woo-Sik Kim: Data discussion, Methodlogy. SangMok Chang: Data discussion, Methodlogy. Jong Min Kim: Supervision, Writing- Reviewing and Editing.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0003-2670(20)30140-9
DOI:
https://doi.org/10.1016/j.aca.2020.01.071
Reference:
ACA 237431
To appear in:
Analytica Chimica Acta
Received Date: 26 December 2019 Revised Date:
28 January 2020
Accepted Date: 30 January 2020
Please cite this article as: D.-Y. Shim, W.-S. Kim, S.-M. Chang, J.M. Kim, Fabrication of Periodic Nanoporous Surface Structure for Highly Sensitive Gravimetric Quartz Crystal Applications, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2020.01.071. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Graphical abstract
A periodic nano-porous surface on quartz crystal electrodes was carefully fabricated for increasing the mass-sensitive areas. Detailed porous structures were prepared by analyzing Au electrochemical reduction process of PS layer coated quartz crystals. The sensitivity measurement of the porous quartz crystals was performed with several traditional methods, and an optimized reduction time for higher sensitivity was determined. The frequency shift of the nano-porous quartz crystals showed 10 times bigger change with the same concentration of target solutions in self-assembly procedures. In the procedures, the freshly increased surface portion did not produce additional molecular slip-effects on the measured resonant resistance values, thus, the periodic porous chips showed another side merit for the mass sensor applications. We propose a possible use of the current porous surface as a platform for developing other high-performance sensors and analyses.
Fabrication of Periodic Nano-porous Surface Structure for Highly Sensitive Gravimetric Quartz Crystal Applications Deuk-Yong Shim,a Woo-Sik Kim,b Sang-Mok Chang,c Jong Min Kimc* a
Major in Chemical Engineering, Dong-A University, 840 Hadan, Saha-gu, Busan 49315, Republic of Korea Department of Chemical Engineering, Kyung Hee University, Kyungki-do 17104, Republic of Korea c Department of Chemical Engineering, Dong-A University, 840 Hadan, Saha-gu, Busan 49315, Republic of Korea b
*Corresponding Author. Tel.: +82 51 200 7717; Fax: +82 51 200 7728. E-mail address: [email protected] (J.M. Kim).
Abstract A periodic nano-porous surface on quartz crystal electrodes was carefully fabricated for increasing the mass-sensitive areas. Detailed porous structures were prepared by analyzing Au electrochemical reduction process of PS layer coated quartz crystals. The sensitivity measurement of the porous quartz crystals was performed with several traditional methods, and an optimized reduction time for higher sensitivity was determined. The frequency shift of the nano-porous quartz crystals showed 10 times bigger change with the same concentration of target solutions in self-assembly procedures. In the procedures, the freshly increased surface portion did not produce additional molecular slip-effects on the measured resonant resistance values, thus, the periodic porous chips showed another side merit for the mass sensor applications. We propose a possible use of the current porous surface as a platform for developing other high-performance sensors and analyses. Keywords: Large surface area, Quartz crystal, Molecular slip-effect, High sensitivity, Polystyrene beads
1. Introduction Quartz crystals have been widely used as trustworthy mass sensors applying the correlation between the resonant frequency change and the adsorbed mass change on the electrode surface ever since Sauerbrey’s theoretical investigation. The well-known Sauerbrey’s relation is follows [1];
∆ = −
∆
(1)
where ∆F is the resonant frequency shift of quartz crystal; f0 is the resonant frequency of the quartz crystal;
∆m is the surface mass change; µ is the shear modulus of the quartz crystal; ρQ is the density of the quartz crystal and A is piezoelectrically active crystal area of the quartz crystal.
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The relation has enabled many developments in gas sensors, and the further development are still frequently reported [2-8]. Stable measurement of the sensors has been normally possible because the quality factor (the ratio of the wavelength to the wave height) of the quartz crystal is sufficiently high [9]. The use of quartz crystal in liquid phases has also been proved by Kanazawa et al., and the liquid phase applications have been numerously documented thought the relation between the resonant frequency change and the surface mass change has been recalibrated [10]. Furthermore, some analytical variables for understanding the interfacial and surface phenomena have been additionally conceptualized such as “resonant resistance”[11] and “dissipation factor”[12]. Many papers have been published for treating the resonant resistance and the dissipation factor changes, and most of them provides logical information or mechanism related to the resonant frequency change depending on the measured variables [13-16]. We also showed the resonant resistance and the quality factor could be measured simultaneously with the frequency measurement applying an impedance analyzer though we could not clearly reveal the difference between the two measured variables [17]. At anyhow, the embodiment of these concepts has also been a positive role for developing the quartz crystal applications not only for the specific sensors but also for the analytical devices. Thus, currently, the quartz crystal has also been known as a measuring device for the fine mass with the viscoelasticity, and various analyses using its measured variables have been reported in the areas of immune assay, gelation reactions, electrochemical experiments, viscoelastic measurements in thin films without careful consideration of the measuring conditions [13].
The sensor applications of the quartz crystal were normally made by a simple way. The target specific recognition layers were firstly coated on the electrode surface, and the frequency response was measured when the modified electrode interacted with target molecules. The development for the high performance quartz crystal chip had not been widely reported, and commercially available quartz crystals were still obtained with their primitive form. A few reports showed the increase of mass sensitivity in various quartz crystals and other gravimetric sensors applying porous structures or porous materials [1820]. These reports were well describing the merit of porous structures for measuring the interface interaction, but the increase of the sensitivity was not big compared with the increase of the surface areas because the obtained surfaces were random pore structures. The increase of the mass sensitivity was only about 2-fold for DNA detection with a nano-porous structure because the fabrication procedure was only based on a wet chemical etching, which produced random nano-porous structure [19]. Similarly, Terao et al. showed the fabrication of a nano-porous electrode, and the increase of the surface areas about 16-fold has been achieved by fabricating nano-needle shapes [20]. To enable the shapes, they added Pb2+ in the Au electrochemical reduction cell. This procedure enabled the increase of the surface area about 16-fold but the sensing gold area was not large by the contamination. Thus, the sensitivity increase of 4-fold was obtained in the IgG detection. Another method for increasing the surface area was utilizing nano-porous material coatings, but this method was only sufficiently helpful when the target component was not changed [6-8]. Thus, the porous platform usable for developing various different sensors is still required.
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Recently, Kong et al. reported an impressive mass sensor application using surface area controllable porous structures [21]. The document focused on the molecular imprinting techniques, and did not treat the important merits of the periodic nano-porous structure. Here, we describes the conceptual difference in the sensing schemes, detailed manufacturing procedure of the nano-porous electrode, the improved sensitivity in a self-assembly process and further possibilities for the quartz crystal sensors as well as the fundamental characteristic of the nano-porous chip.
2. Materials and methods 2.1. Materials
Chloroauric acid (HAuCl4), ethylene diamine tetra acetic acid (EDTA), sodium sulfite (Na2SO3) and dipotassium phosphate (K2HPO4) were purchased from Sigma–Aldrich and used for the electrochemical reduction of Au ion. Poly styrene bead (PSB) colloids (750 nm) were obtained from Alfa Aesar and used as a template for fabricating periodic nano-porus gold structures. 3-Mercaptopropionic acid (3-MPA) and 3-(aminopropyl) triethoxysilane (3-APTES) were also purchased from Sigma-Aldrich and used for the performance test of the fabricated quartz crystal chips. All other chemicals used were analytical grades. 2.2. Preparing closely packed PS spheres
Detailed fabrication procedures were reported elsewhere [22]. Briefly, glass substrates (slide glass from matsunami) were rinsed with H2O and acetone. The substrates were dried with nitrogen gas and immersed in piranha solution (H2SO4:H2O2= 3:1, v/v) for 40 min (CAUTION: strong oxidizing solution). In the continuous step, a PSB stock solution (750 nm PSB suspension) was spread on the prepared substrates using micropipettes. Then, a spin coating was performed using a spin-coater (MS-A150, MikasaOpticoat Co., Ltd.) to spread colloidal suspension on the substrates. The operation was performed at 300 rpm for 5 min after dropping the colloidal solution on the substrates. Next, the substrates were carefully dipped in a fleshly prepared a Petri dish containing distilled water, and the PS layer was spread on the water surface. The colloid layer was normally not a closely packed form, and we used sodium dodecyl sulfate (SDS) solution to fabricate more condensed forms. Finally, the floating PS colloidal layer was transferred on a side of Au quartz crystal electrode (EG&G Co., Ltd.), and the crystal was dried for a day to remove moisture. 2.3. Fabrication nano-porous electrode surface
Potentiostatic electrochemical reduction of Au ion was performed to grow new gold areas from the gaps among the PS spheres (Scheme 1). Using a potentiostat (Par 273, EG&G Co., Ltd.), a reduction potential of − 0.7 V (vs. Ag/AgCl) was applied to the PSB modified quartz crystal in an aqueous electrolyte solution containing HAuCl4 (12 gL−1), EDTA (5 gL−1), Na2SO3 (160 gL−1), and K2HPO4 (30 gL−1). The procedure of the electrochemical reduction was monitored via QCM922A (EG&G Co., Ltd.) and WinEChem software (EG&G Co., Ltd.). After finishing the electrochemical reduction, the PS
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spheres on the quartz surface were carefully removed by rinsing with toluene for 2 h. The porous structure and the thickness of the gold film were determined by analyzing the electrochemical reduction data, and the deposition times were maintained as 80 s, 170 s, 250 s for comparing each difference. 2.4. Characterization
An atomic force microscope (Nanoscope IV, Digital Instruments) with commercially available tapping mode tips (Nano World, Al-coated silicon, f: 320 kHz) was used to investigate surface morphology. The AFM measurements were conducted with a scan rate of 1 Hz applying a large piezoscanner with a maximum scan area about 150 × 150 µm2. All AFM images were only flattened and other modifications were not performed using SPIP software (SPIP, Imagemet Co., Ltd.). An impedance analyzer (4294A, HP) was also used for the measurement of impedance spectra of the quartz chips. To perform self-assembly during the solvent evaporation, we used a thermohydrostat (MTH-2200, Sanyo Co., Ltd.) to maintain relative humidity (20 %) and temperature (20 °C). A micro syringe (2 µl, model 88411, Hamilton) was used for dropping small quantity of solution in the dropping and evaporation measurement. The resonant frequency and the resonant resistance of the fabricated chips were measured via QCM922A during the performance measurements using WinQCM software (EG&G, Co., Ltd.).
3. Results and discussion Typical response changes of the resonant frequency and the resonant resistance during the AuCl4electrochemical reduction on a PS array coated quartz crystal are presented in Figure 1(a). The resonant frequency was continuously decreased during the reduction process with minimum two breakpoints marked as arrows in the reduction time around 80-90 s and 170-180 s. The resonant frequency change were not only related with the accumulated mass but also related with other mass effects in the surface and interface. The change of the resonant resistance was more complicated, and showed three breakpoints at the least. The resonant resistance change normally means the change of viscoelascity on the surface and the interface as well as the mass loadings [23, 24]. The breakpoints toward the resistance increasing direction at the reduction times of 70 s and 200 s were located nearby or after the two breakpoints of the resonant frequency shift. In the current deposition condition, several surface phenomena should be also considered for understanding the resonant resistance change such as viscous attachment/detachment of the PS spheres and the electrolyte condition changes as well as the elastic Au deposition. In the case of a normal metal deposition without other surface phenomena, both the slopes showed a uniform [25]. For easy understanding, the relation between the accumulated charge values and the corresponding resonant frequency decreases (R2. Q1) was additionally presented in Figure 1(b). In Figure 1(b), two breakpoints were also observed nearby the frequency shifts of 40 kHz and 55 kHz. If Au ion reduction was the only mechanism for the frequency decrease, these points should not be observed. In addition, the first breakpoint, noted as ‘A’, showed the change to the bigger charge values (bigger ∆C/∆F or smaller ∆F/∆C),
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but that of the second breakpoint showed the change to the smaller values. The range between ‘A’ to ‘B’ was believed as most pure Au reduction region because the slope value was biggest. Other two areas were believed to have additional mass effects because these regions have small charge values with the same ∆F change (small ∆C/∆F). Sun et al. showed a possible reduction mechanism for the PS sphere coated electrodes [22]. They provided a two-step reduction mechanism by the function of the reduction time. In the mechanism, the horizontal Au reduction is only possible in the initial step, and the reduction is changed to both the horizontal and perpendicular directions considering the surface configuration. Thus, the reduction before the breakpoint ‘A’ was believed as the horizontal Au reduction, whereas the horizontal and perpendicular growth between the two breakpoints was estimated. In the horizontal reduction, the negatively charged HPO4- ions should be localized among the PSB gap areas [22]. These ions would be trapped in the growing Au atoms, and trapped ionic mass was assumed for the additional mass loading because the procedure was not related with electron exchanges. On the other hand, the growth after the breakpoints ‘B’ was difficult to clarify with only the proposed mechanism. Thus, assuming the reaction time of the breakpoints to 80 s and 170 s, we fabricated 2 additional chips and analyzed their properties with several methods. In the current paper, we calls P1 chip, P2 chip, P3 chip for 80 s, 170 s and 250 s deposited quartz crystals, respectively.
Figure 2 shows AFM topography images for P1 (a), P2 (b) and P3 chips (c) within a scan area of 60×60 µm2. The height scales of the image were equally adjusted for easy comparison. The difference of the morphology could not be clearly distinguished between Figure 2(a) and Figure 2(b) except the height changes. In Figure 2(c), the bright areas were randomly located over the surface. The bright areas were not believed as non-dissolved PS spheres because the surface was carefully washed with toluene more than 2 h at 40 °C. In addition, we also tried many efforts to wash out the areas such as a long time washing (24 h) and washing at an elevated temperature (70 °C), but the bright areas were still remained. Thus, the random bright areas were believed as deposited gold layers. The line profiles in Figure 2(d) showed the surface roughness was critically increased in P3 chip. The explanation for the profile change was relatively simple considering the dimension of the PS spheres. The highest point in the line profile of P3 chip was reached to about 350 nm, which was similar with the mean radius of the PS spheres. We also added more expanded AFM images in Figure 3. In Figure 3, importantly, one can observe that the surface of the electrode bottom can be confirmed through the pores of P1 chip and P2 chip but that of P3 chip cannot be confirmed. i.e. one step height was confirmed from P1 and P2 chips whereas two step heights were confirmed in P3 chip, which implied the deep sites in the gold pores were not opened to the outer environment as shown in the line profiles of Figure 3. Thus, the direct access of analytic solution to the deep pore areas was relatively difficult for P3 chip. In addition, we added SEM images for the PS layer on the electrode as supporting information (Figure S1). As shown in Figure S1, the PS spheres showed closely packed surface structure in
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the large scale, but the gaps among the PS spheres were differently observed in the expanded view. Because the PS spheres had a weak negative charge, every PS sphere repelled neighbored PS spheres. Therefore, the gaps were normally configured, but the distance and the height of the gaps were differently produced by the local repulsive force difference and the irregular PS size. Thus, we believed, there were other growth mechanism when the obtained film thickness was reached up to the radius of the PS spheres, and the mechanism would be discussed at the end of the current paper.
The oscillation properties of the fabricated chips were examined by two methods. Figure 4 shows the result of impedance spectrum analyses nearby the oscillation frequencies obtained by the impedance analyzer. The result of a commercially available Au chip was also presented for comparisons. As shown in Figure 4, the diameters of the spectrum were continuously decreasing with increasing the reduction time. Here, 1/diameter was calculated as the resonant resistance value [11], thus, the resonant resistance was increasing with increasing the Au reduction time. The conductance maximum frequency of the 4 chips was nearly located in the real axis, and the quality factors [9] were 120,000(D factor: 8.33×10^-6), 116,000(D factor: 8.62×10^-6), 114,000(D factor: 8.77×10^-6) and 110,000(D factor: 9.09×10^-6) for the commercial, P1, P2 and P3 chips, respectively. For more detailed inspection of the oscillation properties, the stability measurement was performed in a room condition ((a) and (b), T: 20 °C, R.H.: 20 %) and a liquid phase ((c) and (d), distilled water) shown in Figure 5. Bonroy et al. showed that the Sauerbrey equation is valid in an electrodeposited porous gold quartz crystal [18]. Thus, the current frequency responses could be transformed to the mass changes with the same method used in commercial quartz crystal techniques [9]. The resonant frequency shift of the three chips (commercial, P1, P2) showed slight drifts of sub - 2 Hz within the measured time. P3 chip showed a little different tendency. In the initial step until 300 s, the resonant frequency was slightly decreased, and then, the frequency was stabilized within the drift of 1 Hz. P1 chip showed best stability, but all chips were reasonable stability in the atmosphere condition. In the case of the resonant resistance, the difference was only the absolute value. The commercial and P3 chips showed smallest and biggest values, which corresponded with the result obtained by the impedance analyses described in Figure 4. When the chips immersed in the distilled water, the frequency stabilities were greatly changed by the liquid mass loading. The commercial and P2 chips showed a similar frequency drift of about 5 Hz within the measured time scale. Unlikely with other chips, P3 chip showed a continuous frequency decrease, which implied continuous water penetration into the deep gold areas. This property was not desirable for the liquid phase operation because it would prohibit the precise analysis of target component quantities. The stability of the resonant resistance value was similar with that of the room condition except absolute value changes by the viscous liquid loading. Stability measurement showed little difference among the chips but the possibility as sensors was not clearly determinable. Thus, we performed two additional measurements.
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The first measurement was the observation of water dropping and evaporation procedure. As indicated in the support information (Figure S2), the water contact angles of the three chips were all over 80°. These surface properties enabled the evaporation measurement of a small solvent quantity because the water drop did not spread over the surface. Thus, we dropped 2 µl of distilled water in the center of the porous chips and monitored their evaporation procedures. For minimizing the temperature and humidity influence, the thermohydrostat was additionally used with the setting temperature and the relative humidity of 20 °C and 20 %, respectively. In Figure 6, the evaporation speed of the water drop was very slow for P3 chip. Reversely, it meant the delivery of water molecules to the deep gold area required a long time compared with other 2 chips. The slow evaporation in P3 chip was a coincident result with the continuous decreasing frequency of P3 chip in Figure 5(c).
In the second measurement, each chip was initially dipped in the distilled water with a stirring rate of 100 rpm and waited until the chip reached to a stable frequency drift. In the continuous step, 3-APTES (100 µl, original stock solution) was dropped in the distilled water, and its final concentration was 10 mM. Figure 7 shows the resonant frequency and resonant resistance changes by the self-assembly on the active gold areas of the 4 chips. Overall tendency of the 4 chips was similar except the absolute value differences in the final frequency shifts. i.e. in the initial 10 min, fast and large decrease in the frequency was observed, and after that time, slow and small decrease in the frequency was observed. After 3 h of the assembly time, the final frequency shift was completely different among the chips as noted in Figure 7(a). The frequency shifts of P1 and P2 chips showed 10 times bigger value than that of the commercial chip. The difference clearly suggested the active gold areas of the two chips are larger about 10-fold than that of the commercial chip because 3-APTES only assembles on the gold surface through Au-Si bond [26]. Similar result was also obtained in the self-assembly of 3-MPA (data not shown or see Figure S3). Unexpected result was that of P3 chip because its surface area was assumed to be largest. Two possible explanations for the small frequency change were the contamination of the gold and the difficulty to deliver the 3-APTES into the deep gold pore areas. The second possibility was previously confirmed in the simple water dropping and evaporation measurement (Figure 6). The change of the resonant resistance was also interesting. The measured resonant resistance values would be influenced by the contacted liquid viscosity change, the slip-effect of adsorbed molecules, and molecular coupling effect on the surface as well as the surface mass change [11]. And therefore, in the current condition (same liquid viscosity, assembly of same molecules), the resonant resistance difference by the assembly should be proportionally increased with the magnitude of the frequency decreases when the self-assembly performed by the same molecule. In Figure 7(b), the resistance shifts of the 4 chips were similar in spite of the big differences in the frequency shift. A possible explanation was the direction of the gold film growth because the increased new gold area was the perpendicular direction for the oscillation. i.e. the horizontal surface area was not
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changed though horizontal reduction had occurred. When 3-APTES assembled on the new horizontal areas, the molecular slip effect on the resonant resistance was relatively small, i.e. the slip-effect is a horizontal mass effect [9], and thus, the resistance variation by the assembly was not big regardless of the assembled quantity. This was desirable properties for normal quartz crystal sensors because a small resonant resistance change implied the linear relation between the frequency shift and surface mass change in the Sauerbrey’s relation [9].
A summarized Au electrochemical reduction mechanism for the PS sphere modified electrodes was illustrated in Figure 8. Three steps were a definite feature in the electrochemical reduction process. The 1st and 2nd steps were the same procedure explained by Sun et al. The 3rd step would be happened when the new gold layer thickness reached over the radius of the PS spheres. At this moment, the growing gold portion exposed to the whole electrolyte solution, and started to cover the PS spheres. Thus, the exposed gold portion possibly pressed down the PS spheres, which produced additional mass effect without electron exchanges, and the mass-effect would increase the resonant resistance values by the contact with the viscous PS spheres. When the gold film covered the PS spheres, the removal of the PS spheres with the wet chemical method was also difficult because the solvent was difficult to access the whole PS sphere areas. In this case, the incomplete PS removal would be a contamination root for the active gold areas.
Finally, we demonstrated a novel application of the porous chip in the support information based on the hydrophobic nature of the surface using P2 chip (Figure S3). The application was based on the fact that the water drop shape was not seriously changed until it completely evaporated. The method suggested the best sensitive area of the quartz crystal could be used as a separate analyzing chip. In a simple method, 2 µl of 1 µM 3-MPA was dropped and evaporated, and the response depending on the dropping location was monitored. The Gaussian feature of the mass sensitivity by the sensing location was well-known in quartz crystal sensors [27]. The measurement showed possible use of the porous chip for a small quantity hydrophilic sample with using the best sensitive area. In Figure S3, the center position of the porous quartz crystal showed detailed 3-MPA self-assembly procedure compared with those of the other areas as well as the large frequency change.
4. Conclusion In the current paper, we showed the fabrication, the performance and the oscillation characteristic of the periodic nano-porous quartz crystals utilizing the 750 nm PS spheres as the template. In the fabrication procedure, the Au reduction process was important to obtain performance maximized porous chips, therefore, the process should be carefully investigated for avoiding undesirable chip properties. The result implied a different size of PS spheres could endow different sensitivity, thus, the mass sensitivity would be
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controllable to some extent though the possible gold film thickness for the maximum performance was influenced by the PS sphere sizes. In fact, the periodic structure also enabled a rapid detection because the 3-D detection scheme, which was comparable 2-D detection in the commercial plane chip (see Graphic Abstract). Furthermore, the analyses implied other material-template reduction procedures for fabricating large surface area would also have some potential problems for electrode contamination by the templates themselves. Thus, careful inspection for the reduction process would be required in normal template based reduction processes. Finally, the periodic structure could be fabricated on any electrode surface for improving any signal response because the fabrication was easily possible when an electrochemical reduction could be performed.
Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (No. 2017R1D1A1B03030262).
Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at http://.
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List of Figure Captions Scheme 1. Fabrication procedure of nano-porous electrodes using PSB as a template. Figure 1. Typical response changes by the Au electrochemical reduction process. The resonant frequency shift and the resonant resistance change (a), and the frequency shift by the function of charge accumulation (b). Figure 2. AFM topography images of Au deposited quartz crystal surfaces for P1 (a), P2 (b) and P3 chips (c). Line profile analyses for each location of the chips (d). Image areas are 60*60 µm2. Figure 3 Expanded views and line profiles for Figure 2. P1 and P2 chips show one step height line profiles ((a) and (b)), whereas P3 chip show clear two step height line profile as indicated with red arrows (c). Image areas are 20 × 10 µm2. Figure 4. Impedance spectra of the 4 chips. Figure 5. Resonant stability measurements in an atmosphere condition (a, b) and in the distilled water (c, d). Figure 6. Resonant frequency changes in the water dropping and evaporation measurement. In the measurement, 2 µl of the distilled water was dropped in the center of each chip. The inset graph shows an expanded view of the surface evaporation processes. Figure 7. Typical response differences of the 4 chips by the self-assembly of 3-APTES. The resonant frequency (a) and resonant resistance changes (b) by the assembly. Figure 8. Schematic illustration for the PSB templated electrochemical Au reduction process.
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Scheme 1
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 8
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Highlights The mechanism for the PSB templated electrochemical gold deposition is clarified. The increased gold areas did not show the sensitivity decrease done by the molecular-slip effect. The self-assembly of 3-APTES showed 10-fold bigger frequency changes than a normal commercial quartz crystal. A novel application of the fabricated chip was suggested for readers.
Deuk-Yong Shim: Data curation, Software, Methodology. Woo-Sik Kim: Data discussion, Methodlogy. SangMok Chang: Data discussion, Methodlogy. Jong Min Kim: Supervision, Writing- Reviewing and Editing.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: