Petar Kassal

Miniaturised wireless smart tag for optical chemical analysis applications

A novel miniaturised photometer has been developed as an ultra-portable and mobile analytical chemical instrument. The low-cost photometer presents a paradigm shift in mobile chemical sensor instrumentation because it is built around a contactless smart card format. The photometer tag is based on the radio-frequency identification (RFID) smart card system, which provides short-range wireless data and power transfer between the photometer and a proximal reader, and which allows the reader to also energise the photometer by near field electromagnetic induction. RFID is set to become a key enabling technology of the Internet-of-Things (IoT), hence devices such as the photometer described here will enable numerous mobile, wearable and vanguard chemical sensing applications in the emerging connected world. In the work presented here, we demonstrate the characterisation of a low-power RFID wireless sensor tag with an LED/photodiode-based photometric input. The performance of the wireless photometer has been tested through two different model analytical applications. The first is photometry in solution, where colour intensity as a function of dye concentration was measured. The second is an ion-selective optode system in which potassium ion concentrations were determined by using previously well characterised bulk optode membranes. The analytical performance of the wireless photometer smart tag is clearly demonstrated by these optical absorption-based analytical experiments, with excellent data agreement to a reference laboratory instrument.

Hybrid sol–gel thin films doped with a pH indicator: effect of organic modification on optical pH response and film surface hydrophilicity

Optical sensors for application in innovative wearable sensing systems such as textile-integrated systems and wireless sensor platforms rely on the development of low-cost multifunctional materials compatible with standard fabrication technologies. We are developing optically responsive pH sensitive sol–gel coatings for integration with a mobile wireless smart tag sensing system. For this application, we have fabricated a range of thin pH sensitive films using bromocresol green (BCG) indicator immobilised in inorganic–organic silica hybrid matrices prepared by a sol–gel method and deposited by spin-coating onto glass substrates. The surface hydrophilicity of the films were varied by using the inorganic sol–gel precursor tetraethoxysilane together with either methyltriethoxysilane, ethyltriethoxysilane, phenyltrimethoxysilane or glycidoxypropyltrimethoxysilane as organically modified sol–gel precursors, co-polymerised in different ratios. Spectral characterisation of the films was performed using visible absorption spectroscopy. The shift in absorption maxima and other spectral changes of the different matrices have been identified, and the apparent pKapp values of the immobilised BCG pH indicator determined. The surface wettability properties of the films have been studied by measuring the contact angle of water, formamide and diiodomethane which has allowed the estimation of the surface free energy (SFE) using three different models: Owens–Wendt, Wu and van Oss-Chaudhury-Good. It is shown that the SFE of the hybrid films is directly related to the type and the degree of organic modification, which in turn has a significant effect on the pH response-time of these sensing films.

Paper-based ion-selective optodes for continuous sensing: Reversible potassium ion monitoring

A simple, low-cost paper-based potassium ion-selective optode that exhibits fully reversible sensing properties is presented. A classic optode formulation consisting of valinomycin as the ionophore and a lipophilic pH indicator has been successfully transferred to a paper substrate and characterised in a flow-through cell with reflectometric optical detection. The optode exhibits high repeatability, reversibility, and stability and can detect potassium in the physiologically relevant concentration range from 10-4 to 10-1 M. This new paper-based optode shows high potential for general application in paper microfluidic systems and for integration into wearable systems for perspiration monitoring due to its reversible and repeatable response. In general, we have demonstrated that ionophore-based optical sensors on paper can successfully be used for continuous ion-concentration monitoring.

Benzimidazole as a structural unit in fluorescent chemical sensors: the hidden properties of a multifunctional heterocyclic scaffold

Abstract Benzimidazole (BI) derivatives are structurally similar to naturally occurring nucleotides, a fact that enables their use as scaffolds in biologically active organic synthetic compounds. However, BI can serve as a multifunctional unit in heteroaromatic molecular systems for optoelectronics and non-linear optics, photovoltaics, and in sensing and bioimaging. Its many structure-related properties such as electron accepting ability, π-bridging, chromogenic pH sensitivity/switching and metal-ion chelating properties, also make BI an exceptional structural candidate for optical chemical sensors. The design of novel benzimidazole-based chemosensors is a challenging task. In this review, molecular sensing systems based on intramolecular charge transfer, photoinduced electron transfer and excited-state intramolecular proton transfer mechanisms are systematically examined and the role of the benzimidazole unit is discussed. Novel and emerging developments such as sensors based on aggregation-induced emission phenomena are also included. Conclusions are made on the advantages and disadvantages of existing fluorophores and chemosensors and guidelines are given for further design and development of new sensing systems based on benzimidazole.