Scott A. Trammell

Quantum-dot/dopamine bioconjugates function as redox coupled assemblies for in vitro and intracellular pH sensing

The use of semiconductor quantum dots (QDs) for bioimaging and sensing has progressively matured over the past decade. QDs are highly sensitive to charge-transfer processes, which can alter their optical properties. Here, we demonstrate that QD-dopamine-peptide bioconjugates can function as charge-transfer coupled pH sensors. Dopamine is normally characterized by two intrinsic redox properties: a Nernstian dependence of formal potential on pH and oxidation of hydroquinone to quinone by O(2) at basic pH. We show that the latter quinone can function as an electron acceptor quenching QD photoluminescence in a manner that depends directly on pH. We characterize the pH-dependent QD quenching using both electrochemistry and spectroscopy. QD-dopamine conjugates were also used as pH sensors that measured changes in cytoplasmic pH as cells underwent drug-induced alkalosis. A detailed mechanism describing the QD quenching processes that is consistent with dopamines inherent redox chemistry is presented.

Interactions between Redox Complexes and Semiconductor Quantum Dots Coupled via a Peptide Bridge

Colloidal quantum dots (QDs) have a large fraction of their atoms arrayed on their surfaces and are capped with bifunctional ligands, which make their photoluminescence highly sensitive to potential charge transfer to or from the surrounding environment. In this report, we used peptides as bridges between CdSe-ZnS QDs and metal complexes to promote charge transfer between the metal complexes and QDs. We found that quenching of the QD emission is highly dependent on the relative position of the oxidation levels of QDs and metal complex used; it also traces the number of metal complexes brought in close proximity of the nanocrystal surface. In addition, partial bleaching of the absorption was measured for the QD-metal complex assemblies. These proximity driven interactions were further used to construct sensing assemblies to detect proteolytic enzyme activity.

Nanoporous Organosilicas as Preconcentration Materials for the Electrochemical Detection of Trinitrotoluene

We describe the use of nanoporous organosilicas for rapid preconcentration and extraction of trinitrotoluene (TNT) for electrochemical analysis and demonstrate the effect of template-directed molecular imprinting on TNT adsorption. The relative effects of the benzene (BENZ)- and diethylbenzene (DEB)-bridged organic-inorganic polymers, having narrow or broad pore size distributions, respectively, on electrochemical response and desorption behavior were examined. Sample volumes of 0.5-10 mL containing 5-1000 ppb TNT in a phosphate-buffered saline buffer were preconcentrated in-line before the detector using a microcolumn containing 10 mg of imprinted BENZ or DEB. Square-wave voltammetry was used to detect the first reduction peak of TNT in an electrochemical flow cell using a carbon working electrode and a Ag/AgCl reference electrode. Imprinted BENZ released TNT faster than imprinted DEB with considerably less peak tailing and displayed enhanced sensitivity and an improvement in the limit of detection (LOD) owing to more rapid elution of TNT from that material with increasing signal amplitude. For imprinted BENZ, the slope of signal versus concentration scaled linearly with increasing preconcentration volume, and for preconcentrating 10 mL of sample, the LOD for TNT was estimated to be 5 ppb. Template-directed molecularly imprinted DEB (TDMI-DEB) was 7-fold more efficient in adsorption of TNT from aqueous contaminated soil extract than nonimprinted DEB.

Multiplex Charge-Transfer Interactions between Quantum Dots and Peptide-Bridged Ruthenium Complexes

Simultaneous detection of multiple independent fluorescent signals or signal multiplexing has the potential to significantly improve bioassay throughput and to allow visualization of concurrent cellular events. Applications based on signal multiplexing, however, remain hard to achieve in practice due to difficulties in both implementing hardware and the photophysical liabilities associated with available organic dye and protein fluorophores. Here, we used charge-transfer interactions between luminescent semiconductor quantum dots (QDs) and proximal redox complexes to demonstrate controlled quenching of QD photoemission in a multiplexed format. In particular, we show that, because of the ability of the Ru complex to effectively interact with CdSe-ZnS QDs emitting over a broad window of the optical spectrum, higher orders of multiplexed quenching can be achieved in a relatively facile manner. Polyhistidine-appended peptides were site-specifically labeled with a redox-active ruthenium (Ru) phenanthroline complex and self-assembled onto QDs, resulting in controlled quenching of the QD emission. Different QD colors either alone or coupled to Ru-phen-peptide were then mixed together and optically interrogated. Composite spectra collected from mixtures ranging from four up to eight distinct QD colors were deconvoluted, and the individual QD photoluminescence (PL) loss due to charge transfer was quantified. The current multiplexing modality provides a simpler format for exploiting the narrow, size-tunable QD emissions than that offered by resonance energy transfer; for the latter, higher orders of multiplexing are limited by spectral overlap requirements.

Electrochemically controlled conductance switching in a single molecule: quinone-modified oligo(phenylene vinylene).

Reversible conductance switching in single quinone-oligo(phenylene vinylene) (Q-OPV) molecules was demonstrated using electrochemical STM. The switching was achieved by application of electrochemical potential to the substrate supporting the molecule. The ratio of conductances between the high- and low-conductivity states is over 40. The high-conductivity state is ascribed to strong electron delocalization of the fully conjugated hydroquinone-OPV structure, whereas the low-conductivity state is characterized by disruption of electron delocalization in the quinone-OPV structure.

Complex Förster Energy Transfer Interactions between Semiconductor Quantum Dots and a Redox-Active Osmium Assembly

The ability of luminescent semiconductor quantum dots (QDs) to engage in diverse energy transfer processes with organic dyes, light-harvesting proteins, metal complexes, and redox-active labels continues to stimulate interest in developing them for biosensing and light-harvesting applications. Within biosensing configurations, changes in the rate of energy transfer between the QD and the proximal donor, or acceptor, based upon some external (biological) event form the principle basis for signal transduction. However, designing QD sensors to function optimally is predicated on a full understanding of all relevant energy transfer mechanisms. In this report, we examine energy transfer between a range of CdSe-ZnS core-shell QDs and a redox-active osmium(II) polypyridyl complex. To facilitate this, the Os complex was synthesized as a reactive isothiocyanate and used to label a hexahistidine-terminated peptide. The Os-labeled peptide was ratiometrically self-assembled to the QDs via metal affinity coordination, bringing the Os complex into close proximity of the nanocrystal surface. QDs displaying different emission maxima were assembled with increasing ratios of Os-peptide complex and subjected to detailed steady-state, ultrafast transient absorption, and luminescence lifetime decay analyses. Although the possibility exists for charge transfer quenching interactions, we find that the QD donors engage in relatively efficient Förster resonance energy transfer with the Os complex acceptor despite relatively low overall spectral overlap. These results are in contrast to other similar QD donor-redox-active acceptor systems with similar separation distances, but displaying far higher spectral overlap, where charge transfer processes were reported to be the dominant QD quenching mechanism.

Increasing Efficiency of Photoelectronic Conversion by Encapsulation of Photosynthetic Reaction Center Proteins in Arrayed Carbon Nanotube Electrode

The construction of efficient light energy converting (photovoltaic and photoelectronic) devices is a current and great challenge in science and technology and one that will have important economic consequences. Here we show that the efficiency of these devices can be improved by the utilization of a new type of nano-organized material having photosynthetic reaction center proteins encapsulated inside carbon nanotube arrayed electrodes. In this work, a generically engineered bacterial photosynthetic reaction center protein with specifically synthesized organic molecular linkers were encapsulated inside carbon nanotubes and bound to the inner tube walls in unidirectional orientation. The results show that the photosynthetic proteins encapsulated inside carbon nanotubes are photochemically active and exhibit considerable improvement in the rate of electron transfer and the photocurrent density compared to the material constructed from the same components in traditional lamella configuration.

Fluorescence-based sensing of 2,4,6-trinitrotoluene (TNT) using a multi-channeled poly(methyl methacrylate) (PMMA) microimmunosensor.

Fluorescence immunoassays employing monoclonal antibodies directed against the explosive 2,4,6-trinitrotoluene (TNT) were conducted in a multi-channel microimmunosensor. The multi-channel microimmunosensor was prepared in poly (methyl methacrylate) (PMMA) via hot embossing from a brass molding tool. The multi-channeled microfluidic device was sol-gel coated to generate a siloxane surface that provided a scaffold for antibody immobilization. AlexaFluor-cadaverine-trinitrobenzene (AlexaFluor-Cad-TNB) was used as the reporter molecule in a displacement immunoassay. The limit of detection was 1–10 ng/mL (ppb) with a linear dynamic range that covered three orders of magnitude. In addition, antibody crossreactivity was investigated using hexahydro-1,3,5-triazine (RDX), HMX, 2,4-dinitrotoluene (DNT), 4-nitrotoluene (4-NT) and 2-amino-4,6-DNT.

A reagentless electrochemical biosensor based on a protein scaffoldElectronic supplementary information (ESI) available: details regarding protein engineering and purification. See http://www.rsc.org/suppdata/cc/b2/b209452e/

Apo-myoglobin, labeled with the environmentally sensitive redox probe RuII(NH3)4(1,10-phenanthroline-5-maleimide)2+, was immobilized onto gold electrodes modified with 11-mercaptoundecanoic acid and subsequently labeled with biotin; avidin binding to the immobilized biotin was specifically and quantitatively detected by a change in cyclic voltammetry of the co-immobilized probe.

Accelerating the initial rate of hydrolysis of methyl parathion with laser excitation using monolayer protected 10 nm Au nanoparticles capped with a Cu(bpy) catalyst

Using a low power green laser, we have demonstrated a rate acceleration of ~2-fold for the hydrolysis of methyl parathion by irradiating the plasmon absorption band of Au nanoparticles capped with a Cu(bpy) catalyst.