It is known that luminescent optical sensors are perspective for detection towards nitroaromatic compounds that are a basis of a many kind of explosives. Operation of these sensors is based on quenching luminescence, which is caused by photo-induced electron transfer from a luminophore (donor) to a nitroaromatic molecule (acceptor). The conjugated polymers, small molecule dyes and metal-organic frameworks are used as a sensitive luminophores currently. One of the methods to improve these sensors is embedding the luminophore into porous matrix with properties of photonic crystal, which may be a porous silicon (pSi) Bragg mirror or a microcavity (MC). The PPV derivatives polymers are usually used as the sensitive luminophores for embedding into pSi matrix. However, there is a task to find an optimal set of luminophores to develop a highly sensitive and selective sensor. In this work we investigate embedding of 5- triphenylamino-4-(triphenylaminothiophen-2-yl)-pyrimidine (HEM-461) into pSi MC and examine the sensitivity of obtained structures. The pSi MC were fabricated using a standard electrochemical etching process. The eigenmode of the pSi MC had a width of 4-6 nm. The samples were oxidized to stabilize the surface chemical properties and to prevent quenching of luminescence of the embedded luminophores after fabrication. The embedding of the dye into the pSi MC was performed at excess pressure. Well known conjugated polymer MDMO-PPV was used as a reference. In this work, we compared the photophysical properties of MDMO-PPV and HEM-461 in solution and into pSi MC. The luminescence parameters and resistance to heat have been studied. Comparative studies of sensitivity of MDMO-PPV and HEM-461 to trinitrotoluene in liquid and gaseous phases have been carried out. It was concluded that pSi MC with embedded HEM-461 is a promising structure for developing sensors of nitroaromatic compounds.
It is known that development of optical sensors for explosives detection is currently of great interest. Among others sensors based on the luminescence quenching of conjugated polymers caused by photoinduced electron transfer have attracted considerable attention. Embedding such polymers into porous silicon (pSi) microcavity (MC) allows modify its luminescence spectrum and increase specific surface area and sensitivity of sensor. At the same time optimization of pSi MC structure and its mode of operation are important aspects of sensors design. This study presents the results of the structure and temperature optimization of pSi MC with embedded PPV derivatives polymers. The pSi MCs were fabricated using a standard electrochemical etching process. The luminescence spectra of polymers were drastically narrowed after embedding in pSi MC. It was experimentally found that optimal thickness of the front mirror is from 4 to 5 pairs of low and high porosity layers. The optimal thickness of the rear mirror is about 15 pairs of low and high porosity layers. We also discovered that temperature of pSi MC strongly influences on the rate of the polymer luminescence quenching under exposure to TNT vapors. In particular, it was shown that a decrease of MC temperature to 5° C leads to more than three times drop of quenching time. The obtained results can be applied for the design of optical sensors of explosives based on pSi MC.
We demonstrate a new way for detection ultralow concentration of explosives in this study. It combines an ion mobility spectrometry (IMS) and a promising method of laser desorption/ionization on silicon (DIOS). The DIOS is widely used in mass spectrometry due to the possibility of small molecule detection and high sensitivity. It is known that IMS based on laser ion source is a power method for the fast detection of ultralow concentration of organic molecules. However requirement of using high energy pulse ultraviolet laser increases weight and size of the device. The use of DIOS in an ion source of IMS could decrease energy pulse requirements and allows one to construct both compact and high sensitive device for analyzing gas and liquid probes. On the other hand mechanisms of DIOS in gas media is poorly studied, especially in case of nitroaromatic compounds. The investigation of the desorption/ionization on porous silicon (pSi) surface of nitroaromatic compounds has been carried out for 2,4,6-trinitrotoluene (TNT) using IMS and mass spectrometry (MS). It has been demonstrated that TNT ion formation in a gas medium is a complicated process and includes both an electron emission from the pSi surface with subsequent ion-molecular reactions in a gas phase and a proton transfer between pSi surface and TNT molecule.