Nonanal Sensor Fabrication Using Aldol Condensation Reaction Inside Alkali-Resistant Porous Glass

Abstract

Monitoring the gases released during breathing or via the skin has gained significance towards diagnosing diseases. In this study, a sensor chip capable of detecting nonanal gas, which is known to be a marker of lung cancer, was developed. The gas detection agent used was vanillin, which underwent aldol condensation with nonanal in the presence of a basic catalyst, resulting in the formation of an unsaturated aldehyde. Porous glass was used as the reaction field to carry the detection agent. Alkali-resistant porous glass was chosen because conventional porous glasses show low durabilities under basic conditions, as they primarily consist of SiO 2 . Nonanal can be detected through changes in the absorption spectrum of the sensor. An accumulatetype sensor was used in this study, which exhibited a linear relationship between the degree of absorption changes at 470 nm and nonanal concentration in the 60 ppb-1.3 ppm range. Therefore, this biological marker gas sensor is effective for the early diagnosis of diseases. The alkali-resistant porous glass sensor chip exhibited a higher degree of absorption change than the conventional porous glass sensor chip.

Nonanal Gas Sensors Using Porous Glass as a Reaction Field for Ammonia-Catalyzed Aldol Condensation

“…Notably, if the pore size decreases, the transmittance increases; however, it then becomes difficult for the gas to diffuse into the pores. The calculated mean free path of nonanal was 9 nm according to the Maxwell–Boltzmann approximation . If the pore size is reduced further to increase the transmittance, Knudsen diffusion occurs, which causes the rate of gas diffusion into the pores to become very low as the mean free path and pore size become equivalent …”

“…Ammonia was used in a solution system to confirm its role as a catalyst in the reaction between nonanal and vanillin, which was used as a nonanal-detecting reagent in our previous study . The changes in the absorbance spectra of different reagent mixtures due to heating (when applied to promote the aldol condensation reaction) are illustrated in Figure .…”

“…Porous glass was prepared by a phase-separation method, as shown in Figure S1 . Soda borosilicate glass was subjected to heat treatment at the glass transition temperature (or higher) and separated into silica and soda boron oxide phases.…”

“…For this purpose, we had prepared alkali-resistant porous glass using sodium hydroxide as an alkali catalyst. The limit of detection (LOD) was found to be in the order of 10 2 ppb; however, the gas sensor exhibited a low transmittance . Nevertheless, the sensor was able to detect changes in absorbance caused by the aldol condensation of vanillin and nonanal within the inner surfaces of the porous glass.…”

“…We have previously reported a nonanal sensor utilizing porous glass and an aldol condensation reaction between nonanal and vanillin under alkaline conditions . Because porous glass contains translucent nanopores, changes in the reactant color due to the chemical reactions occurring inside the pores can be easily detected. − In addition, porous glass has an inner-wall surface functionalized with silanol groups, which retains a thin water film because of its hydrophilicity. Thus, porous glass is a suitable nanoporous material for homogeneously supporting condensation reagents.…”