The research is supported by the National Natural Science Foundation of China (Grant No. 61974004 and 61931018), as well as the National Key R&#38;D Program of China (Grant No. 2021YFB3200100). The study acknowledges the Electron Microscopy Laboratory of Peking University for providing access to electron microscopes. Additionally, the research extends thanks to Ying Cui and the School of Earth and Space Science of Peking University for their assistance in Raman measurements.

1. Synthesis of nanoparticles
<ol>
	<li>Preparation of Silver nitrate solution
	<ol>
		<li>Using a precision weighing balance, measure 0.0017 g of AR-grade silver nitrate (AgNO3, see Table of Materials) and add it to 10 mL of deionized (DI) water. Stir the mixture to create a 10-3 mol/L AgNO3 solution.</li>
	</ol>
	</li>
	<li>Preparation of the silver-ammonia complex
	<ol>
		<li>Take 1 mL of AR-grade ammonia water (NH3.H2O, see Table of Materials) using a syringe, and add it drop by drop into the silver nitrate solution while stirring. Stop the dropwise addition when the solution becomes clear.</li>
	</ol>
	</li>
	<li>Preparation of the glucose solution
	<ol>
		<li>Using a precise weighing balance, measure 0.36 g of AR-grade glucose powder (see Table of Materials) and add it to 10 mL of DI water. Stir the mixture thoroughly to create a 0.2 M glucose solution.</li>
	</ol>
	</li>
	<li>Synthesis of silver nanoparticles (AgNPs)
	<ol>
		<li>Employ a pipette gun to add 30 &#181;L of the silver-ammonia complex (prepared in step 1.2) to the glucose solution (prepared in step 1.3) at intervals of 30 min. Repeat this process 4-6 times while stirring until the solution turns yellow.</li>
	</ol>
	</li>
</ol>
2. Preparation of flexible substrates
<ol>
	<li>Preparation of PDMS substrate
	<ol>
		<li>To synthesize the PDMS substrate, take approximately 5 g of PDMS A solution and add B solution (from a commercially available kit, see Table of Materials) at a ratio of 1:10.</li>
		<li>Stir and thoroughly mix the PDMS A and B solutions.</li>
		<li>Transfer the mixed PDMS into a square dish and then bake it in an 80 &#176;C oven for 2 h.</li>
		<li>After curing through the above process, use a scalpel to cut the PDMS along the dark grid of the Petri dish, creating small PDMS cubes with dimensions of approximately 1 cm x 1 cm.</li>
	</ol>
	</li>
	<li>Surface modification
	<ol>
		<li>Subject the aforementioned small PDMS pieces to plasma treatment. Use a hand-held plasma processor (see Table of Materials) and move it back and forth approximately 5-10 cm above the PDMS surface to perform surface plasma treatment.</li>
		<li>Utilize the plasma processor to modify the surface, inducing the formation of hydroxyl groups on the PDMS surface, rendering it hydrophilic27.</li>
	</ol>
	</li>
	<li>Modification with APTES
	<ol>
		<li>Prepare a 10% APTES solution (see Table of Materials).</li>
		<li>Immerse the surface-modified PDMS obtained in step 2.2 into the APTES solution and allow it to sit for 10 h. This allows APTES to bind with the hydroxyl groups on the PDMS surface.</li>
	</ol>
	</li>
	<li>Self-assembly of AgNPs
	<ol>
		<li>Immerse the PDMS substrate obtained in step 2.3 into the AgNPs solution synthesized in step 1.4 for 10 h. This self-assembles the AgNPs onto the PDMS substrate, creating the final flexible SERS detection substrate.</li>
	</ol>
	</li>
</ol>

Fabrication of Flexible Substrate for SERS Detection of R6G and Thiram

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The authors declare no conflicts of interest.

In this study, a flexible SERS substrate was introduced, which bonded AgNPs to PDMS through chemical modification and achieved excellent performance. During particle synthesis, specifically in the silver ammonia complex synthesis (step 1.2), the color of the solution plays a crucial role. Adding too much ammonia water dropwise can adversely affect AgNPs synthesis quality, potentially leading to unsuccessful detection results. Attention should be paid to substrate modification (step 2.2) during the synthesis process; othe...

Surface-Enhanced Raman Scattering (SERS), as a type of Raman scattering, offers the advantages of high sensitivity and gentle detection conditions, and can even achieve single molecule detection1,2,3,4. Metal nanostructures, such as gold and silver, are typically used as SERS substrates to enable substance detection5,6. Electromagnetic coupling enhancement on nanostructured surfaces plays a significant role in SERS applications. Metallic nanostructures with varying sizes, shapes, interparticle distances, and compositions can aggregate to create numerous &#34;hotspots&#34; generating intense electromagnetic fields due to localized surface plasmon resonances7,8. Many studies have developed metal nanoparticles with different morphologies as SERS substrates, demonstrating their effectiveness in achieving SERS enhancement9,10.
Flexible SERS substrates find wide applications, with nanostructures capable of producing SERS effects deposited on flexible substrates to facilitate direct detection on curved surfaces. Flexible SERS substrates are employed for detecting and collecting analytes on irregular, non-planar, or curved surfaces. Common flexible SERS substrates include fibers, polymer films, and graphene oxide films11,12,13,14. Among them, polydimethylsiloxane (PDMS) is one of the most widely used polymer materials and offers advantages such as high transparency, high tensile strength, chemical stability, non-toxicity, and adhesion15,16,17. PDMS has a low Raman cross-section, making its impact on the Raman signal negligible18. Since the PDMS prepolymer is in liquid form, it can be cured by heat or light, providing a high degree of controllability and convenience. PDMS-based SERS substrates are relatively common flexible SERS substrates, having been used in previous studies to embed various metal nanoparticles for detecting different biochemical substances with exemplary performance19,20.
In the preparation of SERS substrates, the fabrication of nanogap structures is crucial. Physical deposition technology offers advantages like high scalability, uniformity, and reproducibility but typically requires good vacuum conditions and specialized equipment, limiting its practical applications21. Additionally, fabricating nanostructures at the few-nanometer scale remains challenging with conventional deposition techniques22. Consequently, nanoparticles synthesized through chemical methods can be adsorbed onto flexible transparent films through various interactions, facilitating the self-assembly of metallic structures at the nanoscale. To ensure successful adsorption, interactions can be adjusted by physically or chemically modifying the film surface to alter its surface hydrophilicity23. Silver nanoparticles, compared to gold nanoparticles, exhibit better SERS performance, but their instability, particularly their susceptibility to oxidation in air, results in a rapid decrease in the SERS Enhancement Factor (EF), affecting substrate performance24. Hence, it is essential to develop a stable particle method.
The presence of pesticide residues has garnered significant attention, creating a pressing need for robust methods capable of rapidly detecting and identifying various classes of hazardous chemicals in food in the field25,26. Flexible SERS substrates offer unique advantages in practical applications, particularly in the realm of food safety. This article introduces a method for preparing a flexible SERS substrate by bonding synthesized glucose-coated silver nanoparticles (AgNPs) onto a PDMS substrate (Figure 1). The presence of glucose protects the AgNPs, mitigating silver oxidation in the air. The substrate demonstrates excellent detection performance, capable of detecting Rhodamine 6G (R6G) as low as 10-10 M and pesticide Thiram as low as 10-8 M, with good uniformity. Moreover, the flexible substrate can be employed for detection through bonding and sampling, with numerous potential application scenarios.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Ammonia (NH3.H2O, 25%)</td>
			<td>Beijing Chemical Works</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>APTES (98%)</td>
			<td>Beyotime</td>
			<td>ST1087</td>
			<td></td>
		</tr>
		<tr>
			<td>BD-20AC Laboratory Chrona Treater</td>
			<td>Electro-Technic Products Inc.</td>
			<td>12051A</td>
			<td></td>
		</tr>
		<tr>
			<td>D-glucose</td>
			<td>Beijing Chemical Works</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Environmental Scanning electron microscope (ESEM)</td>
			<td>FEI</td>
			<td>QUANTA 250</td>
			<td></td>
		</tr>
		<tr>
			<td>Raman microscope</td>
			<td>Horiba JY</td>
			<td>LabRAM HR Evolution</td>
			<td></td>
		</tr>
		<tr>
			<td>Rhodamine 6G</td>
			<td>Beijing Chemical Works</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Silicone Elastomer Base and Silicone Elastomer Curing Agent</td>
			<td>Dow Corning Corporation</td>
			<td>SYLGARD 184</td>
			<td></td>
		</tr>
		<tr>
			<td>Silver nitrate</td>
			<td>Beijing Chemical Works</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Thiram (C6H12N2S2, 99.9%)</td>
			<td>Beijing Chemical Works</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

fabrication of polydimethylsiloxane (pdms)-based flexible surface-enhanced raman scattering (sers) substrate for ultrasensitive detection

This article presents a fabrication method for a flexible substrate designed for Surface-Enhanced Raman Scattering (SERS). Silver nanoparticles (AgNPs) were synthesized through a complexation reaction involving silver nitrate (AgNO3) and ammonia, followed by reduction using glucose. The resulting AgNPs exhibited a uniform size distribution ranging from 20 nm to 50 nm. Subsequently, 3-aminopropyl triethoxysilane (APTES) was employed to modify a PDMS substrate that had been surface-treated with oxygen plasma. This process facilitated the self-assembly of AgNPs onto the substrate. A systematic evaluation of the impact of various experimental conditions on substrate performance led to the development of a SERS substrate with excellent performance and an Enhanced Factor (EF). Utilizing this substrate, impressive detection limits of 10-10 M for R6G (Rhodamine 6G) and 10-8 M for Thiram were achieved. The substrate was successfully employed for detecting pesticide residues on apples, yielding highly satisfactory results. The flexible SERS substrate demonstrates great potential for real-world applications, including detection in complex scenarios.

Author Spotlight: Development and Application of SERS Flexible Substrates Using Synthesized AgNPs 

Fabrication of polydimethylsiloxane (PDMS)-Based Flexible Surface-Enhanced Raman Scattering (SERS) Substrate for Ultrasensitive Detection

This protocol describes preparing SERS flexible substrates, applying them to actual detection scenarios. The SERS flexible substrate offered by this protocol is environmental friendly and achieves good detection performance. The silver nanoparticles can be synthesized easily by simple vessels, and do not require complicated experimental conditions and environment.In our laboratory, we plan to focus on researching the silver nanoparticles synthesized through the vessel used in this study. We'll explore methods to enhance their SERS detection capabilities, and leverage their unique properties and characteristics to design better performing SERS detection substrates.

In this study, a flexible SERS substrate composed of synthetic AgNPs wrapped in glucose and self-assembled on PDMS using APTES was developed, achieving excellent detection performance for practical pesticide detection applications. The detection limits for R6G and Thiram were both reached at 10-10 M and 10-8 M, respectively, with an Enhancement Factor (EF) of 1 x 105. Furthermore, the substrate demonstrated uniformity.
The AgNPs wrapped in glucose were synthesi...

Watch this Scientific Journal Video about Development and Application of SERS Flexible Substrates Using Synthesized AgNPs at JoVE.com

This protocol describes a fabrication method for a flexible substrate for surface-enhanced Raman scattering. This method has been used in the successful detection of low concentrations of R6G and Thiram.

This article presents a fabrication method for a flexible substrate designed for Surface-Enhanced Raman Scattering (SERS). Silver nanoparticles (AgNPs) ...

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JoVE Journal

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2.0K Views.  Peking University. This protocol describes preparing SERS flexible substrates, applying them to actual detection scenarios. The SERS flexible substrate offered by this protocol is environmental friendly and achieves good detection performance. The silver nanoparticles can be synthesized easily by simple vessels, and do not require complicated experimental conditions and environment.In our laboratory, we plan to focus on researching the silver nanoparticles synthesized through the vessel used in this stud...

Video: Author Spotlight: Development and Application of SERS Flexible Substrates Using Synthesized AgNPs 

Synthesis of Silver Nanoparticles and Preparation of Flexible SERS Detection Substrate

To prepare silver nitrate solution, measure 0.0017 grams of AR grade silver nitrate on a weighing balance. Add the weighed powder to 10 milliliters of deionized water and stir the mixture. Draw one milliliter of AR grade ammonia water using a syringe and add it to the silver nitrate solution while stirring, until the solution becomes clear.To prepare 0.2 molar glucose solution, weigh 0.36 grams of glucose powder and add it to 10 milliliters of deionized water. Stir the mixture thoroughly. Next, using a pipette gun, add 30 microliters of the silver ammonia complex into the glucose solution at 30 minute intervals.To synthesize yellow colored silver nanoparticles. To synthesize the PDMS substrate, take five grams of PDMS A solution and add B solution into it, at a ratio of one to 10. Stir the mixture thoroughly.Transfer the mixed PDMS into a square Petri dish and bake it in an 80 degree Celsius oven for two hours. Once cured, use a scalpel to cut the PDMS along the dark grid of the dish, creating small cubes. Next, move the handheld plasma processor back and forth over the PDMS piece for surface plasma treatment.For APTES modification, immerse the surface modified PDMS pieces into 10%APTES solution for 10 hours. Finally, immerse the PDMS substrate into the silver nanoparticle solution for 10 hours. ESEM images showed uniformity in synthesized nanoparticles with diameters between 20 and 50 nanometers.The Surface-Enhanced Raman Scattering signals for fabricated PDMS substrates displayed excellent detection capability, reaching a limit of 10 to the negative 10th molar for R6G with notable enhancement effect. The substrate also showed Thiram detection with clear characteristic peaks, up to 10 to the minus eighth molar. The paste and peel method on the apple surface show the detection of Thiram up to 10 to the minus seventh molar, indicating the effectiveness of the substrate for identifying pesticide residues on fruit surfaces.