Theoretical Research And Detection Exploration Of Trimethylamine Based On Surface-enhanced Raman Spectroscopy

Trimethylamine(TMA)is an important indicator to measure the degree of spoilage in livestock and aquatic products.Surface-enhanced Raman spectroscopy technology has extremely high detection sensitivity on rough structure of nano-scale precious metals,and has become an efficient and practical tool for rapid detection and analysis.At present,the surface-enhanced Raman spectroscopy research on TMA has not been reported and the vibration has not been assigned,so it is of great significance to calculate surface-enhanced Raman spectra of TMA and compare them with experimental results.The density functional theory B3LYP method was used to explore the enhanced effect of TMA on silver nanoparticles,gold nanoparticles and graphene substrates.The behaviors of TMA adsorbed on silver surface were studied by surface-enhanced Raman spectroscopy.In addition,glass capillary was used to prepare a SERS substrate for trimethylamine detection,and the performance of the substrate was tested and the surface-enhanced Raman spectroscopy of TMA was obtained.The main research contents are as follows:(1)Interaction and Raman enhancement of trimethylamine with silver nanoparticles.Firstly optimized the geometric configuration of the TMA molecule,and analyzed the front orbit and electrostatic potential of the TMA molecule.It showed that the electron density at the N atom was large and the electronegativity was strong.Secondly the adsorption behavior of TMA on the silver cluster was studied.According to the symmetry of the TMA molecule,the Up terminus,Down terminus,and N terminus were respectively the binding sites for Ag clusters.In all adsorption structures,TMA formed a stablest complex through N-terminus adsorption which had larger bonding energy and closer interaction distance with the Ag atom.Therefore,the N-terminus was the best binding site for TMA and silver clusters.Then choose different numbers and structures of silver clusters to simulate the interaction with TMA and calculated the surface-enhanced Raman spectroscopy.The surface-enhanced Raman spectroscopy of the composite was analyzed and the vibration attribution was assigned.The results showed that TMA and silver clusters formed a relatively stable composite,and the N atom in TMA preferentially adsorbed on the surface of the silver clusters and N atom was perpendicular to the silver surface,conforming to the chemical enhancement mechanism.Comparing the effects of silver clusters of different sizes on the surface enhanced Raman spectrum of TMA,it was suggested that the Raman intensity increased with the increase of Ag atoms in a certain range and reached the maximum value of 39.75 at the Ag₃-TMA,then the increase of silver clusters size inhibited the surface-enhanced Raman effect and the intensity decreased gradually.(2)Interaction and Raman enhancement of trimethylamine with gold nanoparticles.TMA molecule was adsorbed with gold clusters at the N-terminus,and the complex formed by TMA and Aun(n=1,3,5,8,10,20)were calculated.The results showed that the change of gold clusters had little effect on the peak shape,but the influence on the Raman intensity showed a certain regularity.Within a certain range,the Raman intensity increased with the increase of Au atoms in the gold cluster,Au₈-TMA reached the maximum value(365.06).When the number of gold atoms was greater than 8,the enhancement decreased.It was due to the fact that in all the Aun(n=1,3,5,8,10,20)-TMA complexs,the bond length of C-N in the Au₈-TMA complex was longer,and the N-Au interaction distance was the closest.At the same time,polarization and natural bond orbital analysis also supported this conclusion.The charges at the N atom in Au₈-TMA was the largest,When the electronegativity of the N atom increased,the electron density was greater,and it was easier to lead chemical enhancement.Due to the role of gold clusters,the bond angle and bond length of TMA would change,the C-N-C angle becomed smaller,the atomic groups were tight,and coordination was easy to occur.The bond length of C-N becomed longer,and the polarization rate changed accordingly.Raman scattering was enhanced,which in line with the chemical enhancement mechanism.(3)Raman enhancement effect of trimethylamine on graphene substrates.Analyzed the surface-enhanced Raman spectroscopy and vibration attribution of the TMA/graphene composite systems.The results showed that the enhancement effect of monovacancy graphene and graphene oxide was significantly higher than that of B/N doped graphene and perfect graphene.The surface enhancement effect of TMA-Au/GD was the best among all composites,followed by TMA-Au/GOO.The enhancement effect of TMA-GD and TMA-GOO was better than that of the B/N doped graphene system with gold atoms.The enhancement effect of perfect graphene was the weakest.The calculation of the adsorption energy,the polarizability,and the distance between the N and the Au in TMA confirmed this conclusion.(4)Exploringly detection of trimethylamine by surface-enhanced Raman spectroscopy.After the capillary undergoes hydroxylation and silanization,the Ag are coordinated to the capillary wall to obtain capillary SERS substrates.The experimental results showed that the performance of the substrates were the best when the Ag NPs was concentrated 25 times and the modification time was set to 30 min.The capillary SERS substrates were used for obtaining surface-enhanced Raman spectroscopy of TMA.Comparing the experimental spectrum with the theoretical spectrum,the peaks were basically the same,the vibration of TMA were assigned in detail.Finally,capillary SERS substrates were applied to the actual samples.Raman intensity increased with the increase of TMA solution concentration.When the concentration of pork extract was as low as 100mg/L,Raman intensity of TMA could still be detected at 817 cm^-1,the detection limit of TMA in pork extract could be as low as 100 mg/L.The peaks at 817 cm^-1corresponding Raman intensity had a good linear relationship when the concentration of TMA in port extract(R²=0.9655).The recovery was 94.6%?111.8%which indicated the feasibility of qualitative and semi-quantitative detection of TMA using SERS.