Enhanced Raman Sensing of Three-Dimensional Flexible AgNPs-ZIF-8 Composite Structures
Objective Surface-enhanced Raman scattering(SERS)is a highly specific and sensitive spectral detection technique with great potential,offering advantages such as non-destructiveness,high sensitivity,low detection limit,and high selectivity.SERS technology allows for rapid,non-destructive analysis based on the unique fingerprint characteristics of analytes,achieving single-molecule level sensitivity.It is widely applied in fields such as biology,chemistry,and life sciences.However,the detection of gas molecules using SERS is limited due to their low molecular densities,strong Brownian motion,and rapid diffusion,which hinder the adsorption of gas molecules onto the subwavelength"hot spots"on the surface of metal nanoparticles.To enhance the gas detection capabilities of SERS substrates,we propose a flexible SERS substrate,AgNPs@PDMS@ZIF-8,featuring a three-dimensional ordered nanostructure.This design utilizes the adsorption properties of polydimethylsiloxane(PDMS)and zeolitic imidazolate framework-8(ZIF-8)for the detection of benzaldehyde gas molecules.Methods First,AgCl colloidal solution is synthesized using PVP powder,AgNO3 solid,and NaCl solution under agitation.NaOH solution and AgCl colloidal solution are then sequentially added to the AA solution and stirred at room temperature under dark conditions.The resulting AgNPs solution is cleaned with deionized water to remove residual organic substances,such as PVP.The PDMS solution is thoroughly stirred and placed in a vacuum to remove air bubbles.It is then applied to an anodic aluminum oxide(AAO)template and left at 70℃for 4 h.After cooling,the AAO template is removed by placing the sample in NaOH solution.The AgNPs@PDMS composite structure is prepared by an interfacial self-assembly method.An oil-water interface is formed by adding n-hexane into the AgNPs colloid,and MPTMS is then added to the n-hexane layer solution.Ethanol is slowly added to the colloid,gradually absorbing the AgNPs onto the oil-water interface.When the n-hexane evaporates,a layer of AgNPs forms a reflective film above the solution.Finally,the PDMS is immersed in the AgNPs colloid and pulled out vertically,forming a large-area AgNPs layer on the PDMS.The ZIF-8 film is prepared by immersing the AgNPs@PDMS structure in a mixture of Zn2+and 2-methylimidazole,followed by drying to obtain the AgNPs@PDMS@ZIF-8 composite structure(Fig.1).Results and Discussions Using R6G at concentrations ranging from 10-8 mol/L to 10-12 mol/L as the probe molecule,Raman testing is conducted on the AgNPs@PDMS structure.The results demonstrate that the detection of R6G at a concentration of 10-12 mol/L,indicating excellent SERS sensitivity[Fig.2(a)].The analytical enhancement factors(AEFs)for SERS sensors at these concentrations are 1.57×107,7.90×107,2.82×108,1.84×109,and 5.09×109,respectively.The relative standard deviation(RSD)of the 10-8 mol/L sample is calculated to quantify substrate uniformity,with an RSD value of 5.09%at 611 cm-1,indicating good homogeneity[Fig.3(b)].A solution of 10-7 mol/L thiram in ethanol is prepared and sprayed onto the surface to be tested.After the AgNPs@PDMS composite structure fully contacts the surface,Raman testing confirms the SERS sensor's ability to detect real-world molecules[Fig.3(c)].The AgNPs@PDMS@ZIF-8 sensor is placed in a container with 1 mL benzaldehyde solution for 24 h,followed by 12 h of air exposure.Subsequent Raman testing detects three main characteristic peaks of benzaldehyde,demonstrating that the ZIF-8 thin film effectively adsorbs benzaldehyde molecules at the"hot spots"between silver nanoparticles,thus enhancing the Raman signal and allowing for gas detection.Conclusions Flexible SERS substrates represent a crucial development direction in SERS technology.In this paper,we propose a preparation method for an AgNPs@PDMS sensor,which is evaluated for SERS performance through Raman experiments.Detection of R6G probe molecules across a concentration gradient demonstrates that the sensor can detect R6G at a minimum concentration of 10-12 mol/L with an RSD of 5.09%[Fig.2(a)].The sensor exhibits good sensitivity and repeatability,with a maximum analytical enhancement factor of 5.09×109.The sensor also successfully detects a 10-7 mol/L thiram solution[Fig.3(c)].In addition,a thin ZIF-8 film on the AgNPs@PDMS sensor enables gas molecular detection,and FDTD simulations of the composite structure are used to analyze its electromagnetic field distribution.Simulation results show that when the average particle size of silver nanoparticles is about 50 nm,the sensor demonstrates superior Raman enhancement performance(Fig.6).The introduction of the ZIF-8 film slightly enhances the electric field intensity between the metal nanoparticles and delays its attenuation.Experimental results confirm the ZIF-8 film's effective adsorption of benzaldehyde molecules,allowing the sensor to effectively detect them(Fig.4).In conclusion,the sensor is low-cost,highly sensitive,and has significant potential for gas detection application.
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