Directional Excitation of Surface Plasmon Polaritons via Molecular Through-Bond Tunneling across Double-Barrier Tunnel Junctions.

Directional excitation of surface plasmon polaritons (SPPs) by electrical means is important for the integration of plasmonics with molecular electronics or steering signals toward other components. We report electrically driven SPP sources based on quantum mechanical tunneling across molecular double-barrier junctions, where the tunneling pathway is defined by the molecules' chemical structure as well as by their tilt angle with respect to the surface normal. Self-assembled monolayers of S(CH2)nBPh (BPh = biphenyl, n = 1-7) on Au, where the alkyl chain and the BPh units define two distinct tunnel barriers in series, were used to demonstrate and control the geometrical effects. The tilt angle of the BPh unit with respect to the surface normal depends on the value of n, and is 45° when n is even and 23° when n is odd. The tilt angle of the alkyl chain is fixed at 30° and independent of n. For values of n = 1-3, SPPs are directionally launched via directional tunneling through the BPh units. For values of n > 3, tunneling along the alkyl chain dominates the SPP excitation. Molecular level control of directionally launching SPPs is achieved without requiring additional on-chip optical elements, such as antennas, or external elements, such as light sources. Using the molecular tunneling junctions, we provide the first direct experimental demonstration of molecular double-barrier tunneling junctions.

Rectification Ratio and Tunneling Decay Coefficient Depend on the Contact Geometry Revealed by in Situ Imaging of the Formation of EGaIn Junctions.

This paper describes how the intensive (tunneling decay coefficient ß and rectification ratio R) and extensive (current density J) properties of Ag-S(CH2) n-1CH3//GaO x/EGaIn junctions ( n = 10, 14, 18) and molecular diodes of the form of Ag-S(CH2)11Fc//GaO x/EGaIn depend on Ageo, the contact area between the self-assembled monolayer and the cone-shaped EGaIn tip. Large junctions with Ageo ≥ 1000 µm2 are unreliable and defects, such as pinholes, dominate the charge transport characteristics. For S(CH2)11Fc SAMs, R decreases from 130 to unity with increasing Ageo due to an increase in the leakage current (the current flowing across the junction at reverse bias when the diodes block current flow). The value of ß decreases from 1.00 ± 0.06 n-1 to 0.70 ± 0.03 n-1 with increasing Ageo which also indicates that large junctions suffer from defects. Small junctions with Ageo ≤ 300 µm2 are not stable due to the high surface tension of the bulk EGaIn resulting in unstable EGaIn tips. In addition, the contact area for such small junctions is dominated by the rough tip apex reducing the effective contact area and reproducibility significantly. The contact area of very large junctions is dominated by the relatively smooth side walls of the tips. Our findings show that there is an optimum range for the value of Ageo between 300-500 µm2 where the electrical properties of the junctions are dominated by molecular effects. In this range of Ageo, the value of J (defined by I/ Ageo where I is the measured current) increases with Ageo until it plateaus for junctions with Ageo > 1000 µm2 in agreement with recently reported findings by the Whitesides group. In this regime reproducible measurements of J can be obtained provided Ageo is kept constant.

Aptamer/Polydopamine Nanospheres Nanocomplex for in Situ Molecular Sensing in Living Cells.

A nanocomplex was developed for molecular sensing in living cells, based on the fluorophore-labeled aptamer and the polydopamine nanospheres (PDANS). Due to the interaction between ssDNA and PDANS, the aptamer was adsorbed onto the surface of PDANS forming the aptamer/PDANS nanocomplex, and the fluorescence was quenched by PDANS through Förster resonance energy transfer (FRET). In vitro assay, the introduction of adenosine triphosphate (ATP) led to the dissociation of the aptamer from the PDANS and the recovery of the fluorescence. The retained fluorescence of the nanocomplex was found to be linear with the concentration of ATP in the range of 0.01-2 mM, and the nanocomplex was highly selective toward ATP. For the strong protecting capability to nucleic acids from enzymatic cleavage and the excellent biocompatibility of PDANS, the nanocomplex was transported into cells and successfully realized "signal on" sensing of ATP in living cells; moreover, the nanocomplex could be employed for ATP semiquantification. This design provides a strategy to develop biosensors based on the polydopamine nanomaterials for intracellular molecules analysis. For the advantages of polydopamine, it would be an excellent candidate for many biological applications, such as gene and drug delivery, intracellular imaging, and in vivo monitoring.

Silver decahedral nanoparticles-enhanced fluorescence resonance energy transfer sensor for specific cell imaging.

We report on a silver decahedral nanoparticles (Ag10NPs)-based FRET (fluorescence resonance energy transfer) sensor for target cell imaging. Fluorophores-functionalized aptamers (Sgc8-FITC) were bound with Ag10NPs via the SH group on the aptamer to form Ag10-Sgc8-FITC. Then, quencher-carrying strands (BHQ-1) were hybridized with Sgc8-FITC to form a Ag10NPs-based FRET sensor (Ag10-Sgc8-F/Q). The sensor interacted with membrane protein tyrosine kinase-7 (PTK-7) on the CCRF-CEM (CCL-119, T-cell line, human acute lymphoblastic leukemia) cell surface to attain fluorescence imaging of CCRF-CEM cells. The addition of CCRF-CEM cells resulted in many sensors binding with cells membrane and the displacement of BHQ-1, thus disrupting the FRET effect and the enhanced fluorescence intensity of FITC. It was found that Ag10NPs largely enhanced the fluorescence intensity of FITC. The results also showed that the Ag10NPs-based FRET sensor (Ag10-Sgc8-F/Q) was not only superior to the bare FRET sensor (Sgc8-F/Q) and sensor Ag-Sgc8-F/Q but also highly sensitive and specific for CCRF-CEM cells imaging.

Multifunctional aptamer-silver conjugates as theragnostic agents for specific cancer cell therapy and fluorescence-enhanced cell imaging.

We fabricated a multifunctional theragnostic agent Ag-Sgc8-FAM for apoptosis-based cancer therapy and fluorescence-enhanced cell imaging. For cancer therapy, aptamers Sgc8 and TDO5 acted as recognizing molecules to bind CCRF-CEM and Ramos cells specifically. It was found that aptamer-silver conjugates (Ag-Sgc8, Ag-TDO5) could be internalized into cells by receptor-mediated endocytosis, inducing specific apoptosis of CCRF-CEM and Ramos cells. The apoptosis of cells depended on the concentration of aptamer-silver conjugates, as well as the incubation time between cells and aptamer-silver conjugates. The apoptotic effects on CCRF-CEM and Ramos cells were different. Annexin V/PI staining, AO/PI staining, MTT assays and ROS (reactive oxygen species) detection demonstrated the specific apoptosis of CCRF-CEM and Ramos cells. For fluorescence-enhanced cell imaging, Ag-Sgc8-FAM was prepared. Compared to Sgc8-FAM molecules, Ag-Sgc8-FAM was an excellent imaging agent as numerous Sgc8-FAM molecules were enriched on the surface of AgNPs for multiple binding with CCRF-CEM cells and signal amplification. Moreover, AgNPs could increase the fluorescence intensity of FAM by metal-enhanced fluorescence (MEF) effect. Therefore, aptamer-silver conjugates can be potential theragnostic agents for inducing specific apoptosis of cells and achieving cells imaging in real time.

Aptamer-functionalized silver nanoparticles for scanometric detection of platelet-derived growth factor-BB.

In this work, we reported a scanometric assay system based on the aptamer-functionalized silver nanoparticles (apt-AgNPs) for detection of platelet-derived growth factor-BB (PDGF-BB) protein. The aptamer and ssDNA were bound with silver nanoparticles by self-assembly of sulfhydryl group at 5' end to form the apt-AgNPs probe. The apt-AgNPs probe can catalyze the reduction of metallic ions in color agent to generate metal deposition that can be captured both by human eyes and a flatbed scanner. Two different color agents, silver enhancer solution and color agent 1 (10 mM HAuCl4+2 mM hydroquinone) were used to develop silver and gold shell on the surface of AgNPs separately. The results demonstrated that the formation of Ag core-Au shell structure had some advantages especially in the low concentrations. The apt-AgNPs probe coupled with color agent 1 showed remarkable superiority in both sensitivity and detection limit compared to the apt-AuNPs system. The apt-AgNPs system also produced a wider linear range from 1.56 ng mL(-1) to 100 ng mL(-1) for PDGF-BB with the detection limit lower than 1.56 ng mL(-1). The present strategy was applied to the determination of PDGF-BB in 10% serum, and the results showed that it had good specificity in complex biological media.

Metal-enhanced fluorescent detection for protein microarrays based on a silver plasmonic substrate.

This paper presents an ultrasensitive fluorescent detection method through fabricating a silver microarray substrate. Silver nanoparticles (AgNPs) and Ag@Au core-shell nanoparticles with different sizes were first synthesized by a seed-mediated growth method and the metal-enhanced fluorescence of these nanoparticles on different fluorescent dyes was investigated. The results indicated that AgNPs could act as a versatile and effective metal-enhanced fluorescence material for various fluorophores, whereas the enhanced fluorescence from Ag@Au was limited only to certain fluorophores. When the AgNPs were functionalized with aptamers and fluorescent dyes, a good analytical performance for simultaneous detection of human IgE and platelet-derived growth factor-BB (PDGF-BB) could be obtained. AgNPs were not only used as detection tags but also used to fabricate the plasmonic microarray substrate to further enhance the sensitivity of fluorescent detection. As a result, a linear response to PDGF-BB concentration was obtained in the concentration range of 16 pg mL(-1) to 50 ng mL(-1), and the detection limit was 3.2 pg mL(-1). In addition, the AgNP modified plasmonic microarrays showed remarkable recovery and no significant interference from human serum when applied to 2 ng mL(-1) PDGF-BB concentration. The plasmonic microarray substrate demonstrated both high specificity and sensitivity for protein microarray detection and this novel approach has great potential for ultrasensitive detection of protein biomarkers in the bio-medical field.