Plasmon-enhanced photocurrent generation from self-assembled monolayers of phthalocyanine by using gold nanoparticle films

Covalent immobilization of gold nanoparticles on a plastic substrate and subsequent immobilization of biomolecules

We propose a novel approach to stably immobilize gold nanoparticles (AuNPs) on a plastic substrate and demonstrate that the modified substrate is also capable of immobilizing biomolecules. To immobilize citrate-capped AuNPs, an acrylic substrate was simply dip-coated in a functional polymer solution to decorate the outermost surface with amino groups. Electrostatic interactions between AuNPs and the amino groups immobilized the AuNPs with a high density. The AuNP-modified acrylic substrate was transparent with a red tint. A heat treatment promoted the formation of amide bonds between carboxy groups on the AuNPs and amino groups on the substrate surface. These covalent bonds stabilized the immobilized AuNPs and the resulting substrate was resistant to washing with acid and thiol-containing solutions. The surface density of AuNPs was controlled by the surface density of amino groups on the substrate surface, which was in turn controlled by the dip-coating in the functional polymer solution. We attempted to immobilize functional biomolecules on the AuNPs-functionalized plastic surface by two different approaches. An enzyme (horseradish peroxidase) was successfully immobilized on the AuNPs through amide formation and 5′-thiolated DNA was also immobilized on the AuNPs through S–Au interactions. These chemistries allow for simultaneous immobilization of two different kinds of biomolecules on a plastic substrate without loss of their functional properties.

We propose a novel approach to stably immobilize gold nanoparticles (AuNPs) on a plastic substrate and demonstrate that the modified substrate is also capable of immobilizing biomolecules.

Efficient Photocurrent Enhancement from Porphyrin Molecules on Plasmonic Copper Arrays: Beneficial Utilization of Copper Nanoanntenae on Plasmonic Photoelectric Conversion Systems

We demonstrated the usefulness of Cu light-harvesting plasmonic nanoantennae for the development of inexpensive and efficient artificial organic photoelectric conversion systems. The systems consisted of the stacked structures of layers of porphyrin as a dye molecule, oxidation-suppressing layers, and plasmonic Cu arrayed electrodes. To accurately evaluate the effect of Cu nanoantenna on the porphyrin photocurrent, the production of Cu₂O by the spontaneous oxidation of the electrode surfaces, which can act as a photoexcited species under visible light irradiation, was effectively suppressed by inserting the ultrathin linking layers consisting of 16-mercaptohexadecanoic acid, titanium oxide, and poly(vinyl alcohol) between the electrode surface and porphyrin molecules. The reflection spectra in an aqueous environment of the arrayed electrodes, which were prepared by thermally depositing Cu on two-dimensional colloidal crystals of silica with diameters of 160, 260, and 330 nm, showed clear reflection dips at 596, 703, and 762 nm, respectively, which are attributed to the excitation of localized surface plasmon resonance (LSPR). While the first dip lies within the wavelengths where the imaginary part of the Cu dielectric function is moderately large, the latter two dips lie within a region of a quite small imaginary part. Consequently, the LSPR excited at the red region provided a particularly large enhancement of porphyrin photocurrent at the Q-band (ca. 59-fold), compared to that on a Cu planar electrode. These results strongly suggest that the plasmonic Cu nanoantennae contribute to the substantial improvement of photoelectric conversion efficiency at the wavelengths, where the imaginary part of the dielectric function is small.

Extraordinary enhancement of porphyrin photocurrent utilizing plasmonic silver arrays

We demonstrate up to ∼630-fold enhancement of the photocurrent from a porphyrin monolayer on a plasmonic Ag-array electrode showing plasmon absorption in the Q-band region relative to that on a planar Ag electrode. The photocurrent obtained by the Q-band excitation in the plasmonic electrodes even exceeded that obtained by the Soret-band excitation in a normal, nonplasmonic electrode.

Photocurrent enhancements in a porphyrin-viologen linked compound under plasmonic and magnetic fields

Zinc-porphyrin( ZnP )–viologen(V2+) linked compound containing six methylene group ( ZnP (6) V )–silver nanoparticle ( AgNP ) composite films was fabricated by combining electrostatic layer-by-layer adsorption and the Langmuir–Blodgett method. The incident photo to photocurrent efficiency (IPCE) values of the ZnP (6) V – AgNP composite films are higher than those of the ZnP (6) V films and much higher than those of ZnP derivative films without V2+moiety as a reference. The large increase in the IPCE values of the ZnP (6) V – AgNP composite films likely comes from a combination of localized surface plasmon resonance (LSPR) from AgNPs and photoinduced intramolecular electron-transfer upon linking to a V2+moiety. The photocurrents of the ZnP (6) V – AgNP composite films and the ZnP (6) V films increase upon application of a magnetic field. Magnetic field effects (MFEs) were clearly observed for both ZnP (6) V – AgNP composite films and the ZnP (6) V films. Photocurrents increase with magnetic field under low magnetic fields (B ≤ 150–300 mT) and are constant under high magnetic fields (B > 150–300 mT). MFEs can be explained by a radical pair mechanism. The magnitude of the MFEs in the ZnP (6) V – AgNP composite films is higher than that in the ZnP (6) V films. A remarkable increase in photocurrent for the ZnP (6) V – AgNP composite films was observed because of LSPR from the AgNPs in the presence of a magnetic field when compared with the ZnP (6) V films in the absence of a magnetic field.

Particle size dependence of the surface-enhanced Raman scattering properties of densely arranged two-dimensional assemblies of Au(core)-Ag(shell) nanospheres

We investigated the dependence of the surface-enhanced Raman scattering (SERS) activity of densely arranged two-dimensional assemblies of spherical Au(core)-Ag(shell) nanoparticles (Au/AgNSs) on the nanoparticle diameter. The size-controlled Au/AgNSs were synthesized using the Au nanosphere seed-mediated growth method without any bulky stabilizers. The diameters of the Au/AgNSs were 38, 53, and 90 nm and the ratio of the total diameter to the Au core diameter was adjusted to ca. 2.0. Extinction spectra of the colloidal solutions of these nanoparticles exhibited the prominent peak of the localized surface plasmon resonance (LSPR) of Ag and therefore the Au/AgNSs exhibited LSPR properties almost the same as Ag nanospheres. It was confirmed from SEM observation that the organic solvent-mediated liquid-liquid interface assembly technique easily generated densely arranged two-dimensional assemblies of the nanospheres. The extinction spectra of all the assemblies exhibited a prominent broad peak ranging from 500 nm to the near-infrared region, which is assigned to the longitudinal LSPR mode of the coupling nanospheres. The extinction intensity increased with increasing nanosphere diameter. The SERS activities of these assemblies were investigated using p-aminothiophenol as a probe molecule. The result revealed that the enhancement factor (EF) of the Raman signal dramatically increased upon increasing the particle diameter. The maximum EF obtained with a laser excitation wavelength of 785 nm was 1.90 × 10(6) for a nanosphere diameter of 90 nm. This renders the two-dimensional assemblies of the plasmonic Au/AgNSs promising for the development of highly sensitive SERS sensor platforms due to their strong electromagnetic effect.

Densely arranged two-dimensional silver nanoparticle assemblies with optical uniformity over vast areas as excellent surface-enhanced Raman scattering substrates

Dense two-dimensional assemblies of silver nanoparticles were fabricated over vast areas (~19 cm(2)) by utilizing a simple liquid-liquid interface assembly technique. The Raman signal for p-aminothiophenol immobilized on the assemblies was drastically enhanced by plasmon coupling-induced hot spots, and the enhanced signal showed high spatial reproducibility.

Site-selective assembly and reorganization of gold nanoparticles along aminosilane-covered nanolines prepared on indium-tin oxide

We have fabricated gold nanoparticle (AuNP) arrays on indium-tin oxide (ITO) substrates in a nearly one-dimensional fashion. AuNPs were site-selectively immobilized on ITO of which the surface had been patterned by a nanolithography process based on scanning probe microscopy. The fabricated nanoscale lines covered with aminosilane self-assembled monolayer served as chemisorption sites for citrate-stabilized AuNPs of 20 nm in diameter, accordingly, AuNP nanolines with a thickness of single nanoparticle diameter were spontaneously assembled on the lines. In this 1D array, the AuNPs were almost separated from each other due to the electrostatic repulsion between their negatively charged surface layers. Furthermore, a reorganization process of the immobilized AuNP arrays has been successfully demonstrated by replacing each AuNP's surface layer from citric acid to dodecanethiol. By this process, the AuNPs lost their electrostatic repulsion and became hydrophobic so as to be attracted to each other through hydrophobic interaction, resulting in reorganization of the AuNP array. By repeating the deposition and reorganization cycle, AuNPs were more densely packed. The optical absorption peak of the arrays due to their plasmonic resonance was found to shift from 526 to 590 nm in wavelength with repeating cycles, indicating that the resonance manner was changed from the single nanoparticle mode to the multiple particle mode with interparticle coupling.

Enormous enhancement in photocurrent generation using electrochemically fabricated gold nanostructures

A self-assembled monolayer of porphyrins fabricated on the surface of an electrochemically deposited gold nanostructure exhibits enormous enhancement of photocurrent due to porphyrin excitation, especially in the near-infrared region, where localized surface plasmon resonance was responsible.