Michael R. Shortreed

Development of a fluorescent optical potassium-selective ion sensor with ratiometric response for intracellular applications

Abstract A small, fluorescent fiber-optic potassium sensor is described with a working range which covers both intracellular cytosolic and blood levels of potassium at the respective physiological pH values. The highly potassium-selective ionophore, BME-44, is retained in a plasticized poly (vinyl chloride) (PVC) film which is placed on the end of an optical fiber. Three optode configurations are given, employing different lipophilic, fluorescent pH chromoionophores (Nile Blue derivatives), demonstrating the ability to improve the detection limit and tune the dynamic range to the desired region of interest. These optodes also have excellent selectivity over sodium, calcium and magnesium. Use of fluorescence (in contrast to absorbance) provides a means to reduce the sensor size dramatically while retaining the same signal-to-noise ratio. These optodes measure the samples potassium activity (rather than concentration) provided that the samples pH is measured simultaneously by another sensor, such as a glass pH electrode.

Anion-selective liquid-polymer optodes with fluorescent pH chromoionophores, tunable dynamic range and diffusion enhanced lifetimes

Abstract The present work demonstrates the ability to extend coextraction based anion optodes into the fluorescence regime for the purpose of miniaturization. These optodes utilize standard ion-selective electrode ionophores with universal pH chromoionophores that have been selected for their fluorescence efficiency, photo-stability and p K a based tunability of the optimal activity range. The miniaturization of the photo-excitation zone enhances the optode lifetime, due to fast diffusion of unbleached chromoionophores into this zone. Specifically, we provide here examples of charged and neutral-carrier ionophore based fluorescent optodes, one highly selective for nitride and the other selective for thiocyanate. The ability to tune the dynamic range of the nitrite-selective optode is also demonstrated.

Supermolecular excitation antenna: Ordered energy funnel

Abstract The development of exciton probes, including single molecule probes, inspired the design of a single molecule lens or energy antenna, based on phenylacetylene dendrimers, the largest so far synthesized structurally ordered molecules. To demonstrate the energy-funnel model we synthesized partial dendrimeric wedges with an excitation acceptor, a perylene derivative pendant, at the locus. As expected, the energy transfer from the large antenna (39 phenyl groups) increases the yield of the yellow perylenic emission by three orders of magnitude for a given excitation wavelength. Overall, such a photonic subwavelength antenna or nano-lens may play a role in developing molecular excitonics, including luminescent optical nanoprobes, scanning exciton tunneling microscopy and nanometer scale fiber-optic chemical and biochemical sensors.

Dendrimeric Antenna Supermolecules with Multistep Directed Energy Transfer

Photoinduced energy transfer in large molecular systems is a vital process in many biological systems. The primary step in these reactions, such as photosynthesis, is a multi-step process of energy flow from a highly absorbing antenna molecule or moiety to a reaction center.1–5 In general, however, this process is energetically disordered, and thus the energy transfer is dependent upon mechanisms such as random walk, thermal activation, exciton percolation, or a combination of these effects.6–8 In contrast, a large, well ordered molecule with organized local electronic excitation states may provide an efficient means of directed intramolecular energy transport. In this paper we examine a unique class of recently synthesized dendrimeric molecules which exhibit such desirable properties.9,10 These represent the largest purely hydrocarbon dendrimers which have been synthesized to date.

Anion selective optodes: development of a fluorescent fiber optic sensor for the determination of nitrite activity

The response of state of the art anion optodes often cannot be described in a thermodynamically exact manner because the ionic strength within the membrane phase of such optodes changes during the course of a titration. Incorporating lipophilic charge sites in the anion optode membranes provides a constant ionic strength in the membrane phase, the ability to measure anion activities, and a more thermodynamically describable system. This configuration has been used to create a micrometer-sized nitrite-selective optode. Recent elucidation of the many biological roles of nitric oxide (NO) has spurred interest in sensitive and selective detection of this molecule. In biological systems NO is converted to NO2- within 30 sec and the biological concentration of NO2- is normally on the micromolar level. The optode we have prepared contains a selective vitamin B12 derivative ionophore, a fluorescent chromoionophore (ETH 2439 or ETH 5350), and lipophilic charge sites. These components are entrapped in a highly plasticized PVC matrix which is placed on the distal end of the fiber. Sensor characteristics such as limit of detection and reversibility are presented.

Molecular Nano-Lenses: Directed Energy Migration and Back-Transfer in Dendrimeric Antenna Supermolecules

Abstract Experimental and theoretical evidence is presented for the process of directed, multistep energy transport in a unique class of fractal-like dendrimeric supermolecules. Due to the meta-position branching arrangement of these Cayley trees, tight excitation localization is observed at the branching nodes. Particular forms of these dendrimers have been synthesized with localized states of decreasing energy toward the molecular locus, creating a rapid and efficient exciton funnel. Spectroscppic data is presented illustrating nearly unit transfer efficiency from the dendrimer periphery to a specialized trap at the molecular center. In addition, the energetics and spatial arrangement of these molecules allow for the destruction of multiphoton excitations at the trap, providing protection from permanent photochemical bleaching.