Atomic force microscopy (AFM) measurements show that the Ag nanoparticles that yield surface-enhanced Raman scattering (SERS) of single molecules of Rhodamine (R6G) are all compact aggregates consisting of a minimum of two individual particles. Comparison of 514.5 and 632.8 nm excitation shows that the single molecule R6G signal is significantly higher when the excitation wavelength is resonant with the absorption band of R6G and suggests that the Raman excitation spectrum follows the absorption profile for R6G. We have also observed an interesting superlinear power dependence of the SERS signal. On average, by increasing the incident power by 2 orders of magnitude and decreasing the integration time by the same factor to maintain constant fluence, increases of 4 to 6 times were observed in the SERS intensity. We discuss these results in terms of model where the R6G molecule that yields single molecule SERS signals is located at the junction of two Ag nanoparticles. We have also modeled the system using molecular resonance Raman theory to provide further insight into the enhancement mechanism.

Molecular surface enhanced Raman scattering (SERS) in compact clusters of 30-70 nm Ag nanocrystals has shown single molecule Raman scattering cross sections that are orders of magnitude larger than free space single molecule luminescence cross sections. We analyze certain aspects of this phenomenon with new numerical electromagnetic calculations, and we also present new spectral depolarization data for single molecule rhodamine 6G scattering. We stress the central role of the Ag femtosecond radiative lifetime, and the spatial distribution of the excited Ag electrons, in the near field and far field optical properties. The fundamental nature of the Ag plasmon excited-electronic-state changes from a volume excitation to a surface junction excitation as two particles approach each other within 1 nm. Adsorbed molecules in the junction interact directly with the metallic excited-state wave function, showing electron-transfer-initiated photochemistry as well as enhanced Raman scattering. Depolarization studies show an uniaxial local electromagnetic symmetry at the junction site. Simultaneous intensity fluctuations in both the R6G molecular lines and the accompanying Ag electronic Raman continuum appear to reflect R6G adsorption-desorption kinetics. We outline the wavelength-dependent properties of a hybrid molecular-metallic wave function as the Raman resonant state.

Enhanced near-field at noble metal nanoparticle surfaces due to localized surface plasmon resonance (LSPR) has been researched in fields ranging from biomedical to photoelectrical applications. However, it is rarely explored on nonmetallic nanomaterials discovered in recent years, which can also support LSPR by doping-induced free charge carriers, let alone the investigation of an intricate system involving both. Here we construct a dual plasmonic hybrid nanosystem Au-Cu9S5 with well controlled interfaces to study the coupling effect of LSPR originating from the collective electron and hole oscillations. Cu9S5 LSPR is enhanced by 50% in the presence of Au, and the simulation results confirm the coupling effect and the enhanced local field as well as the optical power absorption on Cu9S5 surface. This enhanced optical absorption cross section, high photothermal transduction efficiency (37%), large light penetration depth at 1064 nm, excellent X-ray attenuation ability, and low cytotoxicity enable Au-Cu9S5 hybrids for robust photothermal therapy in the second near-infrared (NIR) window with low nanomaterial dose and laser flux, making them potential theranostic nanomaterials with X-ray CT imaging capability. This study will benefit future design and optimization of photoabsorbers and photothermal nanoheaters utilizing surface plasmon resonance enhancement phenomena for a broad range of applications.

Dendritic spines receive most synaptic inputs in the forebrain. Their morphology, with a spine head isolated from the dendrite by a slender neck, indicates a potential role in isolating inputs. Indeed, biochemical compartmentalization occurs at spine heads because of the diffusional bottleneck created by the spine neck. Here we investigate whether the spine neck also isolates inputs electrically. Using two-photon uncaging of glutamate on spine heads from mouse layer-5 neocortical pyramidal cells, we find that the amplitude of uncaging potentials at the soma is inversely proportional to neck length. This effect is strong and independent of the position of the spine in the dendritic tree and size of the spine head. Moreover, spines with long necks are electrically silent at the soma, although their heads are activated by the uncaging event, as determined with calcium imaging. Finally, second harmonic measurements of membrane potential reveal an attenuation of somatic voltages into the spine head, an attenuation directly proportional to neck length. We conclude that the spine neck plays an electrical role in the transmission of membrane potentials, isolating synapses electrically.second harmonic ͉ electrical isolation ͉ uncaging ͉ cortex ͉ glutamate T he dendritic spine is a ubiquitous feature in the nervous system, whose function is still poorly understood and heavily investigated (1). Spines are recipients of excitatory inputs in many neurons, including pyramidal cells (2), but excitatory inputs in nonspiny neurons contact dendritic shafts. Therefore, rather than just serving as recipients of inputs, spines likely perform a specific function with those inputs. Indeed, spines are calcium compartments and can therefore restrict local biochemical reactions to single inputs (3, 4). Nevertheless, nonspiny neurons can also perform similar calcium compartmentalization (5, 6), so it is conceivable that spines could implement an additional function.Theoretical work, spanning several decades, has proposed that spines could play an important role in altering synaptic potentials (7-12) (for a recent review, see ref. 13). Because of the resistance of the spine neck, spines could electrically isolate inputs and thus prevent input resistance variations in the dendrite during synaptic transmission (8). Thus, excitatory synaptic potentials could be filtered when they reach the dendrite (8-10, 12).The resistance of the spine neck, a crucial variable in ascertaining the electrical function of the spine, has never been measured. Estimates made from passive cable models (14) or diffusional coupling (15) would make its value too low to significantly filter synaptic potentials. At the same time, recent diffusional estimates indicate that neck resistances could be higher (16). Indeed, in our recent work examining input integration, we found that potentials onto spines sum linearly, whereas depolarizations on dendritic shafts shunt each other (R.A., K.B.E., and R.Y., unpublished work). Thus, our data would imply that spines isolate input...