Distance Dependent Resonance Energy Transfer Between Molecular Machine and Plasmonic Nanostructure

poster abstractPhotoswitchable molecules (molecular machines) have attracted a great deal of attention over the past few years for the design of molecular sensors. Among photoswitchable molecules, azobenzene is widely studied due to its trans-cis photoisomerization, which produces a simple structure and optical and Raman spectra, and is photo and electrochemically active, and can be utilized for optical storage and other applications. Localized surface plasmon resonance (LSPR) properties of the metal nanostructures in conjunction with the photoswitching properties of the azobenzene molecules allow the nanoscale environment to be more controlled and to ultimately improve the sensing abilities of the metallic nanostructure. Herein, we develop an ultrasensitive molecular sensor by functionalizing the gold nanoprism with a selfassembled monolayer of alkanethiols containing azobenzenes. This is the first study where light-induced reversible switching of azobenzenes to cis and trans conformations was detected by monitoring the LSPR of gold nanoprisms-based sensing platforms. It was found that the LSPR red shift was observed as the light exposure was switched from UV to blue light due to the cis to trans isomerization of the azobenzene. This shift is consistent with the increase in thickness of the local dielectric environment (0.6 nm) surrounding the nanoprisms with perhaps a contribution from electronic interaction between the nanoprisms and azobenzene. We hypothesize that this electronic interaction is the nearfield resonance energy transfer (NF-RET). Changing the alkanethiol chain length altered the distance between the nanoprisms’ surface to the azobenzene. The LSPR red shift decreases as the distance between azobenzenes and nanoprisms increases due to the decrease in NF-RET. The LSPR shift was found to be reversible as the light source was switched back and forth several times from UV to blue light. The effects of the azobenzene conformational change and its photoreversibility were also probed through surface enhanced Raman spectroscopy (SERS) demonstrating that the NF-RET between the nanoprisms and bound azobenzenes in their cis conformation significantly enhances the intensity of the Raman bands of the azobenzenes and is highly dependent on the distance of azobenzene from the surface of the nanoprisms. The SERS data suggested that the isomerization was controlled by first-order kinetics with a rate constant of 1.0 x 10−4 s−1. Our demonstration of light-induced photoreversibility of this type of molecular machine is the first step toward eliminating current limitations on detection of molecular motion in solid-state devices using LSPR spectroscopy with nanoprisms. Modulating the LSPR position and controlling energy transfer across the nanostructure organic molecule interface are very important for the fabrication of plasmonic-based nanoscale devices

Achieving biosensing at attomolar concentrations of cardiac troponin T in human biofluids by developing a label-free nanoplasmonic analytical assay

Acute myocardial infarction (heart attack) is the fifth leading cause of death in the United States (Dariush et al., Circulation, 2015, 131, e29–e322). This highlights the need for early, rapid, and sensitive detection of its occurrence and severity through assaying cardiac biomarkers in human fluids. Herein we report chip-based fabrication of the first label-free, nanoplasmonic biosensor to assay cardiac troponin T (cTnT) in human biofluids (plasma, serum, and urine) with high specificity. The sensing mechanism is based on the adsorption model that measures the localized surface plasmon resonance (LSPR) wavelength shift of anti-cTnT functionalized gold triangular nanoprisms (Au TNPs) induced by a change of their local dielectric environment upon binding of cTnT. We demonstrate that controlled manipulation of the sensing volume and decay length of Au TNPs together with an appropriate surface functionalization and immobilization of anti-cTnT onto TNPs allows us to achieve a limit of detection (LOD) of our cTnT assay at attomolar concentration (∼15 aM) in human plasma. This LOD is at least 50-fold more sensitive than that of other label-free techniques. Furthermore, we demonstrate excellent sensitivity of our sensors in human serum and urine. Importantly, our chip-based fabrication strategy is extremely reproducible. We believe our powerful analytical tool for detection of cTnT directly in human biofluids using this highly reproducible, label-free LSPR sensor will have great potential for early diagnosis of heart attack and thus increase patients’ survival rate

Investigating the Effects of Size and Shape of Anisotropic Nanostructures on the Molecular Sensor Response

The photoreversiblity of molecular machine-attached onto anisotropic nanostructures have been studied using optical spectroscopy. For the first time, we have observed an unprecedented 21-nm shift of localized surface plasmon resonance (LSPR) peak of gold nanoprism upon cis to trans isomerization of azobenzenes. The observed shift was a combined effect of energy transfer across the nanostructure and azobenzene molecule and increase in the dielectric environment of the nanostructure. Furthermore, we also investigated the geometrical effects of plasmonic nanostructures by fine-tuning their size and shape on sensitivity of molecular sensors and determined the mechanism underlying LSPR peak shifts. Understanding such mechanism will aid in designing highly efficient sensing platforms for future optoelectronic device fabrication

Fabrication of a Self-Assembled and Flexible SERS Nanosensor for Explosive Detection at Parts-Per-Quadrillion Levels from Fingerprints

Apart from high sensitivity and selectivity of surface-enhanced Raman scattering (SERS)-based trace explosive detection, efficient sampling of explosive residue from real world surfaces is very important for homeland security applications. Herein, we demonstrate an entirely new SERS nanosensor fabrication approach. The SERS nanosensor was prepared by self-assembling chemically synthesized gold triangular nanoprisms (Au TNPs), which we show display strong electromagnetic field enhancements at the sharp tips and edges, onto a pressure-sensitive flexible adhesive film. Our SERS nanosensor provides excellent SERS activity (enhancement factor = ∼6.0 × 106) and limit of detection (as low as 56 parts-per-quadrillions) with high selectivity by chemometric analyses among three commonly military high explosives (TNT, RDX, and PETN). Furthermore, the SERS nanosensors present excellent reproducibility (<4.0% relative standard deviation at 1.0 μM concentration) and unprecedentedly high stability with a “shelf life” of at least 5 months. Finally, TNT and PETN were analyzed and quantified by transferring solid explosive residues from fingerprints left on solid surfaces to the SERS nanosensor. Taken together, the demonstrated sensitivity, selectivity, and reliability of the measurements as well as with the excellent shelf life of our SERS nanosensors obviate the need for complicated sample processing steps required for other analytical techniques, and thus these nanosensors have tremendous potential not only in the field of measurement science but also for homeland security applications to combat acts of terror and military threats

Synthesis and Electrochemical Characterization of Ligand-Protected Molecule-Like Silver Nanoparticles

poster abstractThe focus of this research was to investigate the effects of the functional group of para-substituted thiophenol (X-TP) on the optical and electrochemical properties of the ultra stable silver nanoparticles (AgNPs), Ag44(X-TP)30. We have developed a simple experimental procedure to prepare AgNP-protected with various functional groups (X = F, CF3, H, CH3). These groups were varied from electron withdrawing to electron donating abilities. The synthesized AgNPs were characterized by UV-visible absorption spectroscopy, which showed highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) electronic transition at ~ 815 nm. This excitation energy was quantified and correlated with the potential difference between the lowest LUMO reduction and HOMO oxidation peaks obtained from cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Analysis of the potential difference from the varied functional groups gives important information concerning the dependence of the AgNPs electronic properties on composition and structure

Nanoplasmonic sensor for the detection of cardiac Troponin

poster abstractThe Isoform of troponin I is uniquely produce in the adult human myocardium and it overexpress at myocardial injury. Accordingly, Iso troponin 1 level in plasma and other biological fluids can serve as diagnostic and prognostic disease biomarkers. Our study focus on the design of a label free ultrasensitive nanoplasmonic sensor by utilizing unique localized surface Plasmon resonance (LSPR) property of highly sensitive gold nanoprisms. Herein our study reveals that chemically synthesized nanoprisms with 42 nm average edge lengths can be used in nanoplasmonic sensor fabrication for the troponin detection. The limit of detection has been found to be sub-picomolar concentrations in PBS buffer and we will explore this sensing mechanism to detect Troponin I of myocardial infarction patient’s samples

Unique Design of CuInSe2 Nanocrystal decorated Gold Nanoprism Hybrid Conjugates for Advanced Photocatalytic Application

poster abstractWe present CuInSe2 nanocrystal decorated gold nanoprism hybrid conjugates with advanced photocatalytic ability in order to offer a unique and environmentally sound solution to the current obstacles faced by photovoltaic device materials currently used. A search for clean and abundant energy sources is a major concern for the environmentally conscious scientist. Photocatalytic reactions can harness this energy and use it for a variety of applications including oxidation of organic contaminants, self-cleaning glass, conversion to water as hydrogen glass, and decomposition of crude oil. However solar absorption in these devices is lacking the efficiency needed to be cost effective. Choice of device material is pivotal in overcoming this large hurdle. Materials such as TiO2, the most commonly used semiconductor photocatalyst, for example only absorbs light in the ultraviolet region which accounts for less than 5% of total solar radiation. Hybrid conjugates, or nanomaterials combining semiconductor and metal materials, are a fast growing alternative to this problem. By incorporating localized surface plasmon resonance (LSPR) properties of the metal nanostructures with controllable band gaps of the semiconductor nanocrystals, the material can shift to the visible and near-infrared spectra thus allowing for greater solar absorbance. However, to the best of our knowledge, no reports are available in which plasmonic coupling occurs between a LSPR active metal nanostructures and the tailoring of the semiconductor nanocrystals’ band gap by a non-toxic, low temperature synthesis. Hybrid conjugates between LSPR active metal nanostructures and semiconductor nanostructures have been reported but suffer from cost effectiveness and often use environmentally unfriendly chemicals. We believe our unique hybrid nanomaterial will allow for further tuning of the LSPR peak position in order to extend light absorption to a more optimal window and further excite electron-hole pairs in order to provide the most photocatalytic activity to date while providing an environmentally friendly and cost-effective approach. This work has major implications in clean energy and more specifically the advancement of photocatalytic applications

Mechanistic Study of the Formation of Bright White Light-Emitting Ultrasmall CdSe Nanocrystals: Role of Phosphine Free Selenium Precursors

We have designed a new nonphosphinated reaction pathway, which includes synthesis of a new, highly reactive Se-bridged organic species (chalcogenide precursor), to produce bright white light-emitting ultrasmall CdSe nanocrystals of high quality under mild reaction conditions. The detailed characterization of structural properties of the selenium precursor through combined 77Se NMR and laser desorption ionization–mass spectrometry (LDI-MS) provided valuable insights into Se release and delineated the nanocrystal formation mechanism at the molecular level. The 1H NMR study showed that the rate of disappearance of Se precursor maintained a single-exponential decay with a rate constant of 2.3 × 10–4 s–1 at room temperature. Furthermore, the combination of LDI-MS and optical spectroscopy was used for the first time to deconvolute the formation mechanism of our bright white light-emitting nanocrystals, which demonstrated initial formation of a smaller key nanocrystal intermediate (CdSe)19. Application of thermal driving force for destabilization resulted in (CdSe)n nanocrystal generation with n = 29–36 through continuous dissolution and addition of monomer onto existing nanocrystals while maintaining a living-polymerization type growth mode. Importantly, our ultrasmall CdSe nanocrystals displayed an unprecedentedly large fluorescence quantum yield of ∼27% for this size regime (<2.0 nm diameter). These mixed oleylamine and cadmium benzoate ligand-coated CdSe nanocrystals showed a fluorescence lifetime of ∼90 ns, a significantly large value for such small nanocrystals, which was due to delocalization of the exciton wave function into the ligand monolayer. We believe our findings will be relevant to formation of other metal chalcogenide nanocrystals through expansion of the understanding and manipulation of surface ligand chemistry, which together will allow the preparation of “artificial solids” with high charge conductivity and mobility for advanced solid-state device applications

Pure white‐light emitting ultrasmall organic‐inorganic hybrid perovskite nanoclusters

Organic–inorganic hybrid perovskites, direct band-gap semiconductors, have shown tremendous promise for optoelectronic device fabrication. We report the first colloidal synthetic approach to prepare ultrasmall (∼1.5 nm diameter), white-light emitting, organic–inorganic hybrid perovskite nanoclusters. The nearly pure white-light emitting ultrasmall nanoclusters were obtained by selectively manipulating the surface chemistry (passivating ligands and surface trap-states) and controlled substitution of halide ions. The nanoclusters displayed a combination of band-edge and broadband photoluminescence properties, covering a major part of the visible region of the solar spectrum with unprecedentedly large quantum yields of ∼12% and photoluminescence lifetime of ∼20 ns. The intrinsic white-light emission of perovskite nanoclusters makes them ideal and low cost hybrid nanomaterials for solid-state lighting applications

Synthesis of PEG-Thiolate Monolayer Protected CdSe Nanoclusters with Unique Solubility Properties

poster abstractLigands protected metal chalcogenides have shown potential applications in bionanotechnology and device fabrication due to their unique optical properties. However, most metal chalcogenides suffer from solubility problems, which hinders their applications. To overcome the solubility issue of metal chalcogenide nanoclusters, we have demonstrated the aqueous phase synthesis of polyethylene glycol thiolate (PEG-S-) protected CdSe nanoclusters for the first time. The CdSe nanoclusters displayed a first absorption peak ~430 nm, which indicated formation of magic-sized nanoclusters with possible composition of (CdSe)33,34. The PEG-thiolate protected CdSe nanoclusters demonstrated unique solubility properties. The resulting nanoclusters can easily be transferred to organic solvents from an aqueous medium by a simple solvent extraction method. The organic-phase extracted CdSe nanoclusters can readily be redispersed in a wide array of organic solvents such as CH3CN, CH2Cl2, DMF, THF, and CH3Cl. Most importantly, the CdSe nanoclusters, soluble in organic solvents, can also be redispersed in aqueous medium as well. We investigated different chain length PEGn-thiols, e.g., PEG4-SH, PEG6-SH, PEG12-SH, and PEG18-SH and found that the PEG-chain length significantly influenced the aqueous to organic phase transfer properties. Successful transfers were accomplished for PEGn-SH (n = 6, 12, 18). Future studies will be performed on the synthesis of PEG-SH stabilized various metal chalcogenide nanoclusters (CdS, CdTe, ZnS, ZnSe, and CdSe/ZnS nanoclusters)