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

Summary

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.

Abstract

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.

Introduction

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 detection^1,2,3,4. Metal nanostructures, such as gold and silver, are typically used as SERS substrates to enable substance detection^5,6. 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 "hotspots" generating intense electromagnetic fields due to localized surface plasmon resonances^7,8. Many studies have developed metal nanoparticles with different morphologies as SERS substrates, demonstrating their effectiveness in achieving SERS enhancement^9,10.

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 films^11,12,13,14. 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 adhesion^15,16,17. 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 performance^19,20.

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 field^25,26. 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.

Protocol

1. Synthesis of nanoparticles

1. Preparation of Silver nitrate solution
   1. 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.
2. Preparation of the silver-ammonia complex
   1. 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.
3. Preparation of the glucose solution
   1. 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.
4. Synthesis of silver nanoparticles (AgNPs)
   1. Employ a pipette gun to add 30 µL 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.

2. Preparation of flexible substrates

1. Preparation of PDMS substrate
   1. 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.
   2. Stir and thoroughly mix the PDMS A and B solutions.
   3. Transfer the mixed PDMS into a square dish and then bake it in an 80 °C oven for 2 h.
   4. 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.
2. Surface modification
   1. 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.
   2. Utilize the plasma processor to modify the surface, inducing the formation of hydroxyl groups on the PDMS surface, rendering it hydrophilic²⁷.
3. Modification with APTES
   1. Prepare a 10% APTES solution (see Table of Materials).
   2. 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.
4. Self-assembly of AgNPs
   1. 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.

Results

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...

Discussion

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...

Materials

Name	Company	Catalog Number	Comments
Ammonia (NH3.H2O, 25%)	Beijing Chemical Works
APTES (98%)	Beyotime	ST1087
BD-20AC Laboratory Chrona Treater	Electro-Technic Products Inc.	12051A
D-glucose	Beijing Chemical Works
Environmental Scanning electron microscope (ESEM)	FEI	QUANTA 250
Raman microscope	Horiba JY	LabRAM HR Evolution
Rhodamine 6G	Beijing Chemical Works
Silicone Elastomer Base and Silicone Elastomer Curing Agent	Dow Corning Corporation	SYLGARD 184
Silver nitrate	Beijing Chemical Works
Thiram (C6H12N2S2, 99.9%)	Beijing Chemical Works