Name: versatile technique to produce a hierarchical design in nanoporous gold
Uploaded: 2023-02-10T12:45:00
Description: Watch this Scientific Journal Video about Versatile Technique to Produce a Hierarchical Design in Nanoporous Gold at JoVE.com

This work was supported by an award from the NIGMS (GM111835).

1. Constructing a coating of nanoporous gold with hierarchical bimodal architecture on gold wires - Alloying
<ol>
	<li>Assemble an electrochemical cell in a 5 mL beaker. Use a Teflon-based lid with three holes to contain the three-electrode setup. 
	NOTE: Teflon is a popular material for making lids, as it does not react with other chemicals.</li>
	<li>Place a platinum wire counter-electrode, an Ag/AgCl (saturated KCl) reference electrode, and a gold wire with a diameter of 0.2 mm and a length of ...

<ol>
	<li>Fang, M., Dong, G., Wei, R., Ho, J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hierarchical+nanostructures:+design+for+sustainable+water+splitting.">Hierarchical nanostructures: design for sustainable water splitting.</a> Advanced Energy Materials. 7 (23), 1700559 (2017).</li><li>Inayat, A., Reinhardt, B., Uhlig, H., Einicke, W. -. D., Enke, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Silica+monoliths+with+hierarchical+porosity+obtained+from+porous+glasses.">Silica monoliths with hierarchical porosity obtained from porous glasses.</a> Chemical Society Reviews. 42 (9), 3753-3764 (2013).</li><li>Yang, X. -. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hierarchically+porous+materials:+synthesis+strategies+and+structure+design.">Hierarchically porous materials: synthesis strategies and structure design.</a> Chemical Society Reviews. 46 (2), 481-558 (2017).</li><li>Qi, Z., Weissmuller, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hierarchical+nested-network+nanostructure+by+dealloying.">Hierarchical nested-network nanostructure by dealloying.</a> ACS Nano. 7 (7), 5948-5954 (2013).</li><li>Sondhi, P., Stine, K. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Methods+to+generate+structurally+hierarchical+architectures+in+nanoporous+coinage+metals.">Methods to generate structurally hierarchical architectures in nanoporous coinage metals.</a> Coatings. 11 (12), 1440-1456 (2021).</li><li>Matharu, Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanoporous-gold-based+electrode+morphology+libraries+for+investigating+structure-property+relationships+in+nucleic+acid+based+electrochemical+biosensors.">Nanoporous-gold-based electrode morphology libraries for investigating structure-property relationships in nucleic acid based electrochemical biosensors.</a> ACS Applied Materials &amp; Interfaces. 9 (15), 12959-12966 (2017).</li><li>Bollella, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Porous+gold:+A+new+frontier+for+enzyme-based+electrodes.">Porous gold: A new frontier for enzyme-based electrodes.</a> Nanomaterials. 10 (4), 722-740 (2020).</li><li>Khan, R. K., Yadavalli, V. K., Collinson, M. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flexible+nanoporous+gold+electrodes+for+electroanalysis+in+complex+matrices.">Flexible nanoporous gold electrodes for electroanalysis in complex matrices.</a> ChemElectroChem. 6 (17), 4660-4665 (2019).</li><li>Sondhi, P., Stine, K. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrodeposition+of+nanoporous+gold+thin+films.">Electrodeposition of nanoporous gold thin films.</a> in Nanofibers-Synthesis, Properties and Applications. , 1-21 (2020).</li><li>Fujita, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hierarchical+nanoporous+metals+as+a+path+toward+the+ultimate+three-dimensional+functionality.">Hierarchical nanoporous metals as a path toward the ultimate three-dimensional functionality.</a> Science and Technology of Advanced Materials. 18 (1), 724-740 (2017).</li><li>Biener, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanoporous+plasmonic+metamaterials.">Nanoporous plasmonic metamaterials.</a> Advanced Materials. 20 (6), 1211-1217 (2008).</li><li>Zhang, Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+and+characterization+of+nanoporous+gold+composites+through+chemical+dealloying+of+two+phase+Al-Au+alloys.">Fabrication and characterization of nanoporous gold composites through chemical dealloying of two phase Al-Au alloys.</a> Journal of Materials Chemistry. 19 (33), 6042-6050 (2009).</li><li>Zhu, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Toward+digitally+controlled+catalyst+architectures:+Hierarchical+nanoporous+gold+via+3D+printing.">Toward digitally controlled catalyst architectures: Hierarchical nanoporous gold via 3D printing.</a> Science Advances. 4 (8), (2018).</li><li>Artymowicz, D. M., Erlebacher, J., Newman, R. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Relationship+between+the+parting+limit+for+de-alloying+and+a+particular+geometric+high-density+site+percolation+threshold.">Relationship between the parting limit for de-alloying and a particular geometric high-density site percolation threshold.</a> Philosophical Magazine. 89 (21), 1663-1693 (2009).</li><li>Sondhi, P., Neupane, D., Bhattarai, J. K., Demchenko, A. V., Stine, K. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Facile+fabrication+of+hierarchically+nanostructured+gold+electrode+for+bio-electrochemical+applications.">Facile fabrication of hierarchically nanostructured gold electrode for bio-electrochemical applications.</a> Journal of Electroanalytical Chemistry. 924, 116865 (2022).</li><li>Cerovic, K., Hutchison, H., Sandenbergh, R. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Kinetics+of+gold+and+a+gold-10%+silver+alloy+dissolution+in+aqueous+cyanide+in+the+presence+of+lead.">Kinetics of gold and a gold-10% silver alloy dissolution in aqueous cyanide in the presence of lead.</a> Minerals Engineering. 18 (6), 585-590 (2005).</li><li>Ciabatti, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gold+part-ing+with+nitric+acid+in+gold-silver+alloys.">Gold part-ing with nitric acid in gold-silver alloys.</a> Substantia. 3 (1), 53-60 (2019).</li><li>Reyes-Cruz, V., Ponce-de-Le&#243;n, C., Gonz&#225;lez, I., Oropeza, M. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+deposition+of+silver+and+gold+from+cyanide+leaching+solutions.">Electrochemical deposition of silver and gold from cyanide leaching solutions.</a> Hydrometallurgy. 65 (2-3), 187-203 (2002).</li></ol>

Using a multistep procedure involving alloying, partial dealloying, thermal treatment, and acid etching, fabricating hierarchically NPG with dual-sized pores and a higher active electrochemical surface area is demonstrated.
In alloying, the standard potential of metal precursors influences how reactive they are during electrodeposition. Au and Ag ions from liquid solutions are reduced during electrodeposition16,17.
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An erratum was issued for:&#160;<a href="https://www.jove.com/t/65065">Versatile Technique to Produce a Hierarchical Design in Nanoporous Gold</a>. The Authors section was updated from:
Palak Sondhi1 
Dharmendra Neupane2 
Jay K. Bhattarai3 
Hafsah Ali1 
Alexei V. Demchenko4 
Keith J. Stine1 
1Department of Chemistry and Biochemistry, University of Missouri-Saint Louis 
2Food and Drug Administration 
3Mallinckrodt Pharmaceuticals Company 
4Department of Chemistry, Saint Louis University
to:
Palak Sondhi1 
Dharmendra Neupane1 
Jay K. Bhattarai2 
Hafsah Ali1 
Alexei V. Demchenko3 
Keith J. Stine1 
1Department of Chemistry and Biochemistry, University of Missouri-Saint Louis 
2Mallinckrodt Pharmaceuticals Company 
3Department of Chemistry, Saint Louis University

An erratum was issued for:&#160;<a href="https://www.jove.com/t/65065">Versatile Technique to Produce a Hierarchical Design in Nanoporous Gold</a>. The Authors section was updated.

Erratum: Versatile Technique to Produce a Hierarchical Design in Nanoporous Gold

Often seen in nature, hierarchical porous architectures have been imitated at the nanoscale to alter the physical characteristics of materials for improved performance1. Interconnected structural elements of various scales of length are a characteristic of the hierarchical architecture of porous materials2. Dealloyed nanoporous metals typically have unimodal pore size distributions; hence, multiple techniques have been devised to produce hierarchically bimodal porous structures with two separate pore size ranges3. The two fundamental objectives of the material design approach, namely the large specific surface area for functionalization and rapid transport pathways, which are distinct and inherently in conflict with one another, are fulfilled by functional materials possessing structural hierarchy4,5. 
Performance of the electrochemical sensor is determined by the electrode morphology, since the nanomatrix's pore size is crucial for molecular transport and capture. Small pores have been found to aid in target identification in complicated samples, whereas bigger pores enhance the target molecule's accessibility, increasing the sensor's detection range6. The template-based fabrication, electroplating, bottom-up synthetic chemistry, thin film sputtering deposition7, complex flexible matrices based on polydimethylsiloxane support8, alloying of various metals followed by selective etching of the less noble metal, and electrodeposition are some of the methods that are frequently used to introduce nanostructures into the electrode. One of the best methods for creating porous structures is the dealloying procedure. Due to the disparity in dissolution rates, the sacrificial metal, which is the less noble metal, significantly influences the final morphology of the electrode. An interconnected network of pores and ligaments results from the effective process of creating nanoporous gold (NPG) structures, in which the less noble component selectively dissolves out of the starting alloy, and the remaining atoms reorganize and consolidate9. 
The method of dealloying/plating/re-dealloying used by Ding and Erlebacher to make these nanostructures involved first subjecting the precursor alloy composed of gold and silver to chemical dealloying using nitric acid, followed by heating at a higher temperature with a single pore size distribution to create the upper hierarchical level, and removing the remaining silver using a second dealloying to produce the lower hierarchical level. This method was applicable to thin films10. Using ternary alloys, which are comprised of two comparatively more reactive noble metals that are eroded away one at a time, was advised by Biener et al; Cu and Ag were initially removed from the Cu-Ag-Au material, leaving behind bimodally structured, low-density NPG samples11. Long-range ordered structures are not produced by the procedures outlined utilizing ternary alloys. Bigger pores were produced by extracting away one of the phases of the master alloy of Al-Au employed by Zhang et al., which produced the bimodal structure with a minimal degree of order12. An ordered hierarchical structure has reportedly been created by controlling several length scales, through the use of processing pathways that include disassembling bulk materials and putting basic components together into larger structures. In this case, a hierarchical NPG structure was made via direct ink writing (DIW), alloying, and dealloying13. 
Here, a two-step dealloying method for fabricating a hierarchical bimodal nanoporous gold (hb-NPG) structure employing various Au-Ag alloy compositions is presented. The amount of reactive element below which dealloying stops is, in theory, the parting limit. The surface diffusion kinetics is slightly impacted by the parting limit or dealloying threshold, which is typically between 50 and 60 atomic percentage for electrolytic dissolution of the more reactive component from a binary alloy. A large atomic fraction of Ag in the Au:Ag alloy is necessary for the successful synthesis of hb-NPG, since both the electrochemical and chemical dealloying processes cannot be successfully completed at low concentrations near the parting limit14.
The benefit of this method is that the structure and pore size can be tightly controlled. Each step in the protocol is crucial for fine-tuning the typical porosity length scale and the typical distance between ligaments15. To regulate the rate of ion interfacial diffusion and dissolution, the applied voltage is carefully calibrated. To prevent cracking during dealloying, the Ag dissolution rate is controlled.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Argon gas compressed</td>
			<td>Fisher Scientific Compay</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Bovine serum albumin (BSA)</td>
			<td>Sigma-Aldrich (St.Louis, MO)</td>
			<td>A9418</td>
			<td>&#62; 98% purity</td>
		</tr>
		<tr>
			<td>Counter electrode (Platinum wire)</td>
			<td>Alfa Aesar</td>
			<td>43288-BU</td>
			<td>0.5 mm diameter</td>
		</tr>
		<tr>
			<td>Digital Lab furnace</td>
			<td>Barnstead Thermolyne 47,900</td>
			<td>F47915</td>
			<td>used for annealing at high temperatures</td>
		</tr>
		<tr>
			<td>Digital Potentiostat/galvanostat</td>
			<td>EG&#38;G Princeton Applied Research</td>
			<td>273A</td>
			<td>PowerPULSE software</td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>Sigma-Aldrich (St.Louis, MO)</td>
			<td>CAS-64-17-5</td>
			<td>HPLC/spectrophotometric grade</td>
		</tr>
		<tr>
			<td>Fetuin from fetal calf serum</td>
			<td>Sigma-Aldrich (St.Louis, MO)</td>
			<td>F2379</td>
			<td>lyophilized powder</td>
		</tr>
		<tr>
			<td>Gold wire roll</td>
			<td>Electron Microscopy Sciences (Fort Washington, PA)</td>
			<td>73100</td>
			<td>0.2 mm diameter, 10 ft, 99.9%</td>
		</tr>
		<tr>
			<td>Hydrochloric acid</td>
			<td>Fisher Chemical</td>
			<td>A144C-212</td>
			<td>36.5-38%</td>
		</tr>
		<tr>
			<td>Hydrogen peroxide</td>
			<td>Fisher Scientific (Pittsburg, PA)</td>
			<td>CAS-7732-18-5</td>
			<td>30%</td>
		</tr>
		<tr>
			<td>Kimwipes</td>
			<td>KIMTECH Science brand, Kimberly-Clark professional</td>
			<td>34120</td>
			<td>4.4 x 8.2 in</td>
		</tr>
		<tr>
			<td>Nitric acid</td>
			<td>Fisher Scientific (Pittsburg, PA)</td>
			<td>A2008-212</td>
			<td>trace metal grade</td>
		</tr>
		<tr>
			<td>Parafilm</td>
			<td>Bemis PM996</td>
			<td>13-374-10</td>
			<td>4 IN. x 125 FT.</td>
		</tr>
		<tr>
			<td>Peroxidase from horseradish (HRP)</td>
			<td>Sigma-Aldrich (St.Louis, MO)</td>
			<td>9003-99-0</td>
			<td></td>
		</tr>
		<tr>
			<td>PharMed silicone tubing</td>
			<td>Norton</td>
			<td>AY242606&#160;</td>
			<td>1/32&#34; Inner Diameter, 5/32&#34; Outer Diameter, 1/16&#34; Wall Thickness, 25&#39; Length</td>
		</tr>
		<tr>
			<td>Potassium dicyanoargentate</td>
			<td>Sigma-Aldrich (St.Louis, MO)</td>
			<td>379166</td>
			<td>99.96%, 10 G</td>
		</tr>
		<tr>
			<td>Potassium dicyanoaurate</td>
			<td>Sigma-Aldrich (St.Louis, MO)</td>
			<td>389867</td>
			<td>99.98%, 1 G</td>
		</tr>
		<tr>
			<td>PowerSuite software</td>
			<td>EG&#38;G Princeton Applied Research</td>
			<td></td>
			<td>comes with the instrument</td>
		</tr>
		<tr>
			<td>PTFE tape</td>
			<td>Fisherbrand</td>
			<td>15-078-261</td>
			<td>1&#34; wide 600&#34; long</td>
		</tr>
		<tr>
			<td>Reference electrode (Ag/AgCl)</td>
			<td>Princeton Applied Research&#160;</td>
			<td>K0265</td>
			<td></td>
		</tr>
		<tr>
			<td>Scanning Electron Microscopy (SEM) Apreo 2C</td>
			<td>ThermoFisher scientific</td>
			<td>APREO 2 SEM</td>
			<td>equipped with Color SEM technology</td>
		</tr>
		<tr>
			<td>Simplicity UV system</td>
			<td>Millipore corporation, Boston, MA, USA</td>
			<td>SIMSV00WW</td>
			<td>for generating Milli-Q water(18.2 M&#937; cm at 25 &#176;C)&#160;</td>
		</tr>
		<tr>
			<td>Sodium Borohydride</td>
			<td>Sigma-Aldrich (St.Louis, MO)</td>
			<td>213462</td>
			<td>100 G</td>
		</tr>
		<tr>
			<td>Sodium Carbonate</td>
			<td>Sigma-Aldrich (St.Louis, MO)</td>
			<td>452882</td>
			<td>enzyme grade, &#62;99%, 100 G</td>
		</tr>
		<tr>
			<td>Stir bar</td>
			<td>Fisherbrand</td>
			<td>14-512-153</td>
			<td>5 x 2 mm</td>
		</tr>
		<tr>
			<td>Sulphuric acid</td>
			<td>Fisher Scientific (Pittsburg, PA)</td>
			<td>A300C-212</td>
			<td>certified ACS plus</td>
		</tr>
		<tr>
			<td>Supracil quartz cuvette</td>
			<td>Fisher Scientific (Pittsburg, PA)</td>
			<td>14-385-902C</td>
			<td>10 mm light path, volume capacity 1 mL</td>
		</tr>
		<tr>
			<td>UV-Visible Spectrophotometer</td>
			<td>Varian Cary 50</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

versatile technique to produce a hierarchical design in nanoporous gold

The potential to generate variable pore sizes, simplistic surface modification, and a breadth of commercial uses in the fields of biosensors, actuators, drug loading and release, and the development of catalysts have unquestionably accelerated the usage of nanoporous gold (NPG)-based nanomaterials in research and development. This article describes the process of the generation of hierarchical bimodal nanoporous gold (hb-NPG) by employing a step-wise procedure involving electrochemical alloying, chemical dealloying techniques, and annealing to create both macro- and mesopores. This is done to improve the utility of NPG by creating a bicontinuous solid/void morphology. The area available for surface modification is enhanced by smaller pores, while molecular transport benefits from the network of larger pores. The bimodal architecture, which is the result of a series of fabrication steps, is visualized using scanning electron microscopy (SEM) as a network of pores that are less than 100 nm in size and connected by ligaments to larger pores that are several hundred nanometers in size. The electrochemically active surface area of the hb-NPG is assessed using cyclic voltammetry (CV), with a focus on the critical roles that both dealloying and annealing play in creating the necessary structure. The adsorption of different proteins is measured by solution depletion technique, revealing the better performance of hb-NPG in terms of protein loading. By changing the surface area to volume ratio, the created hb-NPG electrode offers tremendous potential for biosensor development. The manuscript discusses a scalable method to create hb-NPG surface structures, as they offer a large surface area for the immobilization of small molecules and improved transport pathways for faster reactions.

The ligament size and inter-ligament gap adjustments are of utmost significance for the manufactured electrode. Creating a structure with dual-sized pores by optimizing the Au/Ag ratios is the first step in this study, along with the characterization utilizing surface morphology, roughness factor, and loading capacity. Compared to conventional NPG, the bimodal pore structure has demonstrated a higher electrochemical surface area, roughness factor, and protein loading capacity15.
<p class="jove...

Watch this Scientific Journal Video about Versatile Technique to Produce a Hierarchical Design in Nanoporous Gold at JoVE.com

Versatile Technique to Produce a Hierarchical Design in Nanoporous Gold

Nanoporous gold with a hierarchical and bimodal pore size distribution can be produced by combining electrochemical and chemical dealloying. The composition of the alloy can be monitored via EDS-SEM examination as the dealloying process advances. The material's loading capacity can be determined by studying protein adsorption onto the material.

The potential to generate variable pore sizes, simplistic surface modification, and a breadth of commercial uses in the fields of biosensors, actuators, ...

Our protocol provides a method to produce nanoporous gold electrodes with hierarchical pore size distribution of larger pores for enhanced transport of molecules and smaller pores for increased surface area. The main advantage of the stepwise protocol lies in the strict control over the silver dissolution rate during de-alloying, which determines the electrode's final morphology. The healthcare system may benefit from the produced electrode design.A faster and more precise diagnosis will be possible as the bimodal porous structure provides large surface area and easy movement of molecules. To begin, assemble an electrochemical cell in a five milliliter beaker. Use a Teflon-based lid with three holes to contain the three-electrode setup.Place a platinum wire as a counter electrode, silver chloride as a reference electrode, and a gold wire as a working electrode in each lid hole maintaining a distance of 0.7 centimeters between the working and counter electrode. Prepare 50 millimolar solutions of each potassium silver cyanide and potassium gold cyanide in water. Add 0.5 milliliters of potassium gold cyanide solution and 4.5 milliliters of potassium silver cyanide salt solution in the five milliliter beaker.Insert the magnetic stir bar into the electrochemical cell and mix the solution at 300 RPM stirring speed until the bubbling of argon gas is observed. Circulate argon gas through the electrolyte solution to remove dissolved oxygen using a silicone tube. Once the electrochemical cell is assembled, connect the potentiostat with alligator clips attached to the appropriate electrodes.After turning on the potentiostat, use the software to perform electrode deposition utilizing chronoamperometry. Configure the software with the desired parameters. Set the potential at a fixed value of minus one volts for 600 seconds.Press Run to complete the alloy deposition on the working electrode. For de-alloying, configure the electrochemical cell as previously demonstrated and use four milliliters of one normal nitric acid as the electrolyte solution for partial de-alloying. Once the solution is evenly circulated and the potentiostat is attached to the correct electrode, in the chronoamperometry software, set a potential of 0.6 volts for 600 seconds.Press Run to finish de-alloying the deposited alloy on the working electrode. For the annealing process, keep the de-alloyed wires in a glass vial. Turn on the furnace, place the glass vial inside the furnace, and set the temperature at 600 degrees Celsius for three hours.Once the process is finished, turn off the furnace, remove the vial, and let it cool to room temperature. For complete de-alloying, immerse the partially de-alloyed annealed wire in four milliliters of concentrated nitric acid and leave it in the fume hood overnight. The next day, remove the nitric acid from the vial.Then prepare the hierarchical bimodal nanoporous gold or hierarchical bimodal MPG-coated wires by rinsing them with deionized water, followed by ethanol. After drying, visualize the wire using scanning electron microscopy. The scanning electron micrographs of hierarchical bimodal MPG demonstrated an open linked network of ligaments and pores following chemical de-alloying.The larger holes were indicated by an upper hierarchy and the lower hierarchy indicated smaller pores. Color-coded elemental mapping for each step of the creation of hierarchical bimodal MPG revealed the presence of silver and gold. The cyclic voltammogram shown as an inset depicts the 10%gold in 90%silver alloy.The structure created via chemical de-alloying showed a small gold oxide reduction. The bimodal structure incorporating chemical and electrochemical de-alloying showed a more pronounced gold oxide reduction peak, indicating an increase in the surface area. It is important to follow the sequential order of the protocol starting from alloying, de-alloying, annealing, chemical de-alloying, and strict control over the time and potential during alloying and de-alloying is equally important.This method has made it possible to create electrochemically hierarchical designs and it can be expanded in the future to turn into a monolith for industrial use and create electrochemical biosensors for glycoproteins.

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