Study On The Preparation Of SERS Gas-sensor Based On Layered Double Hydroxides And Noble Metal And Its Application Of VOCs Detection

Volatile organic compounds(VOCs)are a class of important environmental pollutants which directly affect human health,even at a very low concentration in the air.In recent years,VOCs have also become a research hot topic as the biomarkers of lung cancer or other diseases,showing their potential value in the early disease diagnosis.Surface-enhanced Raman spectroscopy(SERS),as an efficient,nondestructive and highly sensitive detection method,has been widely used in trace analysis of various molecules,but its application in trace detection of VOCs remains to be developed.Since gas molecules are difficult to adsorb and be detected on traditional SERS substrates,it is still a challenging task to apply SERS to gas detection.To fabricate a SERS sensor for the detection of VOCs,especially for gaseous aldehydes,we proposed a facial strategy to prepare a novel composite Ag nanocubes@hollow Co-Ni layered double hydroxide(AgNCs@Co-Ni LDH)as a SERS probe with a strong adsorption function and an ultrahigh sensitivity to gaseous aldehydes.The SERS probe consists of Ag nanocubes as the core and hierarchical hollow structures of Co-Ni LDHs as the shell produced by ion etching of metal-organic framework(MOF)materials(Zeolitic Imidazolate framework-67,ZIF-67),as well as 4-Aminophenol(4-ATP)that serves as the gas-capturing agent.Co-Ni LDH with a large specific surface area and hollow structure enhances the adsorption and enrichment of gas molecules,improves the sensitivity,and also enhances the stability of the SERS substrate.The adsorption properties of LDH for benzaldehyde in terms of general kinetics(pseudo-first-order and pseudo-second-order model)and adsorption isotherm(Langmuir and Freundlich model)were thus investigated.The kinetic adsorption process could be better fitted by the pseudo-first-order kinetic model with a higher correlation coefficient(R2=0.932)than the pseudo-second-order model,and the adsorption rate of 0.0308 min-1 was obtained from the fitting curve.The adsorption isotherm fits the Langmuir isotherm model and its adsorption constant is 6.25×106 L/mol,from which we derived that the adsorption sites of the composite were uniform where the monolayer chemisorption dominated.Taking the composite effect of the good adsorptive performance and "hot spots",AgNCs@Co-Ni LDH-based SERS protocol with ultrahigh sensitivity was thus developed to detect gaseous benzaldehyde.It has a good linearity of dynamic concentration from 5 to 100 ppb with a correlation coefficient(R2)of 0.986.The limit of detection(LOD)reaches 1.83ppb,which is lower than those achieved by the previously proposed SERS substrates.In addition,the accuracy,repeatability,stability and selectivity of the sensor were also verified.Combined with principal component analysis(PCA),the qualitative identification of different aldehydes was realized.The established method can be used to detect and identify air samples containing aldehydes.Benzaldehyde is a VOC that has a potential threat to human health,and it is also a biomarker of lung cancer,so the SERS sensor can be employed for the detection of the pollutants in the air or the lung cancer biomarkers in exhaled breath test.This thesis work promotes the application of SERS in gas sensing based on the newly constructed SERS substrate with improved sensitivity owing to the introduction of LDH adsorption material,thus in general further expanding the scope for the practical application of SERS technology.Moreover,this work established a new measurement method for probing adsorption process with extremely low consumption of both adsorbates and adsorbents.