Anna Podolska

Ion versus pH sensitivity of ungated AlGaN/GaN heterostructure-based devices

We have investigated the pH and ion sensitivity of AlGaN/GaN heterostructure devices; these devices are sensitive to the ion concentration rather than to the pH of the solution. Sheet resistance as a function of pH for calibrated pH solutions and dilute NaOH, HCl, KOH, and NaCl showed an increase as a function of ionic concentration, regardless of whether the pH was acidic, basic, or neutral. An increase in resistance corresponds to accumulation of negative ions at the AlGaN surface, indicating device selectivity toward the negative ions. We attribute this to the formation of a double layer at the liquid/semiconductor interface.

Method to Predict and Optimize Charge Sensitivity of Ungated AlGaN/GaN HEMT-Based Ion Sensor Without Use of Reference Electrode

In this paper, we report on a methodology for theoretical prediction and optimization of charge sensitivity for ungated AlGaN/GaN high electron mobility transistor-based ion sensors operated in the reference electrode free configuration. We have performed numerical simulations of device sensitivity, specifically the change in channel electron concentration with the change in surface potential, for different Al mole fractions and AlGaN thicknesses. These results can be used for device optimization, signal analysis, and sensor calibration purposes. To validate the model, six ungated AlGaN/GaN transistor-based devices of different Al mole fractions and AlGaN thicknesses were fabricated. These devices were exposed to KOH solutions with different pH values, and the voltage change at the gate area was indirectly measured as a function of ionic concentration. The gain in conductivity across the measured range of pH values was experimentally extracted for each device and closely matched the sensitivity predicted by simulation.

Detection of biological reactions by AlGaN/GaN biosensor

We have investigated application of AlGaN/GaN cell-based biosensors for detection of biological reactions in response to the drug ionomycin. The device sensitivity was dependent on environmental factors such as temperature and atmospheric composition. A significant improvement in signal amplitude was obtained through stabilisation of measurement conditions. These results are an important step in development and optimisation of reliable AlGaN/GaN cell-based biosensors.

Aluminium Gallium Nitride/Gallium Nitride Transistor-Based Biosensor

Dye-sensitized solar cells (DSSCs), known as the third generation of solar cells, are being considered as a promising alternative to expensive conventional silicon-based photovoltaic devices. This interest relates to low material cost, easy and inexpensive methods of fabrication, and relatively high power conversion efficiency for DSSCs. Inspired by the breakthrough work of Gri?½tzel, much effort has been made to further improve the performance in the past decades. As one of the key components, the photoanode not only acts as the backbone for dye adsorption but also assumes the task of charge transport, which determines DSSC performance. The conventional TiO2 nanoparticle (NP)-based photoanode suffers from inefficient charge transfer and light harvesting. To optimize the photoanode architecture, we show the design of novel nanostructured photoanode materials featuring large dye uptake ability, efficient charge transfer, and improved light harvesting. In this chapter, the first part provides a brief introduction to various aspects of DSSCs, including device structure, working principles, and characterization techniques. The second part reviews recent advances in the engineering of photoanode architectures, followed by a brief discussion regarding the stability and commercialization of DSSC technology. Our recent work will then be presented in the next section, focusing on the design of new photoanode materials for enhanced DSSC application. In the end, a relevant conclusion and outlook will be addressed for the future development of low-cost and efficient solar cells.Nanosized particles are promising candidates as delivery vehicles in nanomedicine applications. They can be designed to be colloidally stable under diverse environmental conditions found in the body, they have an excellent drug-loading capacity, and their biodistribution can be controlled through size and shape [1]. One unique nanoparticle (NP) with a yolk-shell structure represents a new generation of smart NPs. The yolk-shell nanoparticles (YSNs) or “nanorattles” have a distinctive core-void-shell structure, generally indicated as A-B. The multilevel structure consists of a shell, a void, and a core and could be used for therapy and diagnosis, as first demonstrated by Xu et al. in 2007 [2]. Controlling the physical and chemical properties of the yolk and shell makes YSNs attractive in the delivery of therapeutics, targeting agents, imaging agents, and hyperthermia agents.

Optimisation studies for AlGaN/GaN-based nitrate sensors

AlGaN/GaN HEMT (high electron mobility transistor) sensors have been coated with a nitrate selective polymer membrane enabling nitrate ion sensing in water. In this study we optimise the thickness of the nitrate selective membrane to maximise sensor performance (sensitivity).

8.1.2 Recording of living cell membrane depolarisation with AlGaN/GaN sensor

This work reports on AlGaN/GaN-based sensor development and application to biological sensing. Living cells were seeded on the sensitive region of encapsulated devices for 30 minutes before membrane depolarisation with KCl. To confirm cell activity KCl was also added to devices in physiological salt solution with no cells. The resulting responses were recorded in both cases and demonstrated significant differences in profile and amplitude. Since the AlGaN/GaN structure itself is ion-sensitive, a chemical signal is expected even in the absence of cells. The depolarisation signal observed for the devices with cells is thus expected to incorporate this chemical response.

Ion sensitive AlGaN/GaN heterostructures for cell-based biosensor development

We have investigated the ion sensitivity of ungated AlGaN/GaN heterostructure-based devices and found that these devices are sensitive and selective to the negative ion concentration in the solution. Such selectivity towards negative ions can be employed in cell-based biosensor applications via detection of negative ion transport through the cell membrane/ion channels. Compatibility of living cells and AlGaN/GaN heterostructures for this application has therefore been investigated qualitatively and quantitatively by flow cytometry. Although the mortality rate increases marginally with Al composition, this effect is not strong. This provides much-needed flexibility in designing biosensors by enabling Al mole fraction to be selected on the basis of optimum heterostructure properties. The results of these investigations are very promising for cell-based biosensor development.