Colorimetric Sensors Based On Silver Deposition On The Surface Of Gold Nanorods For The Detection Of Enzyme Activity

Colorimetric sensors are optical sensors that change the color when they are encountered target analytes.Colorimetric optical sensing is becoming increasingly popular due to its high accessibility,low cost,and high sensitivity.The unique optical properties generated by the excitation of local surface plasmon resonance（LSPR）in the visible to near-infrared range have become a powerful tool for manufacturing many chemical/biological sensors.Gold nanorods（Au NRs）are widely used in the field of colorimetric sensing due to their shape-dependent local surface plasmon resonance characteristics.Therefore,different colorimetric sensors based on gold nanorods were constructed for the detection of xanthine oxidase（XOD）activity,screening of XOD activity inhibitors and detection of Salmonella.1.A colorimetric sensor for detecting xanthine oxidase activity was successfully constructed based on the deposition of Ag NPs onto the surface of Au NRs by uric acid reducing Ag⁺.In this method,xanthine was used as the enzyme substrate and was hydrolyzed by XOD to produce uric acid,which had a weak reductive property.Under heating and alkaline conditions,Ag⁺can be reduced to Ag NPs by uric acid and deposited on the surface of Au NRs,resulting in a change in the aspect ratio of Au NRs,leading to a blue shift in their longitudinal LSPR absorption peak.Meanwhile,the color of Au NRs solution changed from blue-purple to red,enabling the detection of XOD activity.According to the experimental results,when the concentration of XOD was in the range of 4-40 U/L,there was a good linear relationship between the shift value of the longitudinal plasmon resonance absorption peak of Au NRs and the concentration of XOD.The linear equation was y=0.61x-1.42（R²=0.99）,and the detection limit was 2.3 U/L.This method has the advantages of low cost,high selectivity,and simple steps,and has been successfully applied to the detection of XOD activity in serum and the screening of XOD activity inhibitors.2.A colorimetric sensor for detecting Salmonella concentration by detecting octanoate esterase activity was successfully constructed based on hydroquinone reducing Ag⁺and depositing Ag NPs on the surface of Au NRs.The octanoate esterase in Salmonella was released through ultrasonic fragmentation,and 4-hydroxyphenyl octanoate was used as the hydrolysis substrate of the octanoate esterase.After hydrolysis by the octanoate esterase,hydroquinone was produced.Hydroquinone had a strong reducing property,which can reduce Ag⁺to Ag NPs and deposit on the surface of Au NRs to form a silver shell Ag@Au NRs To change the morphology of Au NRs and cause their longitudinal LSPR absorption peak to shift red,thereby realizing the detection of Salmonella.There was a good linear relationship between the shift value of the longitudinal plasmon resonance absorption peak of Au NRs and the logarithmic value of Salmonella concentration in the range of 10¹-10⁷CFU/m L.The obtained linear equation was y=6.04x+6.71（R²=0.98）.Using this sensor,Salmonella in water samples has been successfully detected.Besides,by recycling Ag@Au NRs,a colorimetric sensor for the detection of sodium nitrite was constructed,used for the detection of Na NO₂in water samples,demonstrating the reusability of Au NRs.