A Study On Novel Chemical Sensors Based On Luminescence Quenching Of Cryptophane-functionalized SiO_x Nanowires

Gas sensors detect different gas types and transform current gas concentration in an electrical signal (or non-electrical signal) which can be read by indicators, regulators, alarm systems and other another analysis systems. Recently, optical fiber gas sensors have been attracting attention owing to several advantages over conventional electricity-based gas sensors. Methane (CH₄) is extremely flammable and may form explosive mixtures with air. As one of chlorinated derivatives of methane, chloroform (CHCl₃) is harmful to both human health and the environment. Inhaling its vapors depresses the central nervous system and can cause dizziness, fatigue, and headache. Chronic exposure may damage the liver and kidneys, and some people have an allergic reaction to it. Thus, developing sensors for the detection of methane and volatile chloroform is gaining interest in fields related to coal mine production and industrial and environmental applications.Therefore, this paper presents two novel quenched-luminescence gas sensors based on the binding properties of cryptophanes for detection of methane and volatile chloroform, respectively. They are optical methane sensor based on luminescence quenching of silica nanowires (SiO_xNWs) modified with cryptophane-A and volatile chloroform sensor based on cryptophane-E-(OEt)₆@SiO_xNWs. The detailed contents are as follow:â‘ The operation principle of luminescence-based fibre-optical sensor with methane-sensing element comprising cryptophane-functionalized SiO_x nanowires immobilized on the silicon substrate was analysed. A general mathematical model, which describes the integral luminescent intensity signal of the quenched-luminescence methane sensor, was applied in the continuous excitation condition.â‘¡Cryptophane-A and cryptophane-E-(OEt)₆ were synthesized from the starting materials vanillin and ethyl vanillin, respectively, according to the well-known"direct method"with modified procedures. A quantum chemical study was devoted to the complexation of methane by cryptophane-A. The spectral studies indicate that cryptophane-E-(OEt)₆ is able to selectively encapsulate chloroform. The complexation of a nonpolar substrate by a cryptophane host depends mainly on the size of the guest with respect to the size of the cavity, and on the size of the portals through which the guest can enter and leave the cavity. The association is stabilized mainly through van der Waals forces. This knowledge has been gained from recent endeavors, which allow sensitive investigation of cryptophane-based sensor and other devices using cryptophanes, likely to develop in the field of environmental chemistry.â‘¢Based on the thermal evaporation of silicon monoxide at high temperature, an improved method has been developed for large-scale synthesis of ultralong amorphous silica sub-micron wires using polished p-Si wafers as substrates. The synthesis was done with and without thermite. Silica nanowires as excellent candidate materials for chemical sensors and biosensors, have attracted wide attention due to their intrinsic vast surface-to-bulk ratio, good reversibility, quick response, and oxide-coated or H-terminated surface, which allows easy attachment to various functional groups.â‘£An optical sensor based on luminescence quenching of cryptophane-A@silica nanowires was successfully constructed and used to dynamically monitor methane gas at low concentration below 3.5% (v/v). The sensing element shows an intensive and stable blue luminescence when excited by UV light source at wavelength of 380 nm, and it is efficiently quenched by molecular methane. The response of the sensing element demonstrates excellent linear Stern-Volmer behavior at the fixed wavelength 439 nm within the methane concentration range between 0.1% and 3.5% (v/v). A detection limit of below 0.1% (v/v) is estimated for the methane sensing element. This newly developed methane sensing element has significant advantages over the currently available methane sensors such as fast response and recovery (within seconds), good repeatability, selectivity, and long-term stability. On the other hand, experimental investigations of the methane sensing performance of the fabricated cryptophane-E-(OEt)₆@SiO_xNWs material show that there was a downward curvature of the Stern-Volmer plots (i.e. turning nonlinear) especially at higher methane concentrations (above 0.5% v/v). This methane sensor will also have good selectivity in the mine environment.⑤According a general mathematical model suggested by V.I. Ogurtsov et al., just considering the continuous excitation, the integral luminescent intensity signal of the quenched-luminescence methane sensor was described in the case of non-uniform distribution of the main parameters inside active medium, namely the quenching constant, methane concentration and cryptophane distribution and intensity of excitation. Firstly, discrete single-exponential model and Rayleigh and Maxwell distributions (positively defined) were analyzed. For both Rayleigh and Maxwell distributions approximation errors were smaller than for the discrete single-exponential model. The model with Rayleigh distribution provided the best agreement between experimental and calculated intensity data (δin = 0.34 for the sensor based on cryptophane-A; andδin = 1.66 for the sensor based on cryptophane-E-(OEt)₆). Average of distributed quenching constant was larger than for the single-exponential model. On the other hand, double-exponential model fδ(k-k₁) + (1-f)δ(k-k₂), which belongs to three-parametric model, was analyzed. This model provided better approximation than above one-parametric models. Approximation errorδin reduces to 1.14 for the sensor based on cryptophane-E-(OEt)₆.â‘¥A quenched-luminescence sensor for chloroform vapor detection was designed and implemented successfully through the employment of silica nanowires as a substrate for the immobilization of the cryptophane-E-(OEt)₆ transducer, coupled with a fiber optical device, which was designed to operate via luminescence reflection. The sensing material shows a stable blue luminescence, and it is efficiently quenched by chloroform vapor. The prepared optical sensor was highly sensitive to 52.4 ppm of chloroform vapor and the response time was very fast within 80 s. There is almost no interference from CCl4 and CH2Cl2 on the detection. This novel efficient chloroform vapor sensor has significant advantages over gas chromatography and might have application potential.