This perspective reviews recent developments in the synthesis, electrochemistry, and optical properties of gold nanoparticles, with emphasis on papers initiating the developments and with an eye to their consequences. Key aspects of Au nanoparticle synthesis have included the two-phase synthesis of thiolated nanoparticles, the sequestration and reduction of Au salts within dendrimers, the controlled growth of larger particles of well-defined shapes via the seeded approach, and the assembling of a variety of nanoparticle networks and nanostructures. The electrochemistry of thiolated Au nanoparticles is systemized as regions of bulk-continuum voltammetry, voltammetry reflective of quantized double-layer charging, and molecule-like voltammetry reflective of molecular energy gaps. These features are principally determined by the nanoparticle core. Interesting multielectron Au nanoparticle voltammetry is observed when the thiolate ligand shell has been decorated with redox groupings. Another development is that Au nanoparticles were discovered to exhibit unanticipated properties as heterogeneous catalysts, starting with the low-temperature oxidation of CO. Substantial progress has also been made in understanding the surface plasmon spectroscopy of Au nanoparticles and nanorods. The need to investigate the optical properties of metal particles of a single, well-defined shape and size has motivated the development of a number of new techniques, leading to the study of electron transfer and redox catalysis on single nanoparticles.

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MicroRNAs are short noncoding RNAs consisting of 18–25 nucleotides that target specific mRNA moieties for translational repression or degradation, thereby modulating numerous biological processes. Although microRNAs have the ability to behave like oncogenes or tumor suppressors in a cell-autonomous manner, their exact roles following release into the circulation are only now being unraveled and it is important to establish sensitive assays to measure their levels in different compartments in the circulation. Here, an ultrasensitive localized surface plasmon resonance (LSPR)-based microRNA sensor with single nucleotide specificity was developed using chemically synthesized gold nanoprisms attached onto a solid substrate with unprecedented long-term stability and reversibility. The sensor was used to specifically detect microRNA-10b at the attomolar (10–18 M) concentration in pancreatic cancer cell lines, derived tissue culture media, human plasma, and media and plasma exosomes. In addition, for the first time, our label-free and nondestructive sensing technique was used to quantify microRNA-10b in highly purified exosomes isolated from patients with pancreatic cancer or chronic pancreatitis, and from normal controls. We show that microRNA-10b levels were significantly higher in plasma-derived exosomes from pancreatic ductal adenocarcinoma patients when compared with patients with chronic pancreatitis or normal controls. Our findings suggest that this unique technique can be used to design novel diagnostic strategies for pancreatic and other cancers based on the direct quantitative measurement of plasma and exosome microRNAs, and can be readily extended to other diseases with identifiable microRNA signatures.

X-ray Photoelectron Spectroscopic (XPS) Analysis. All XPS spectra were collected on a Kratos Axis Ultra DLD system with an Mg anode at 1253.6 eV and X-ray power of 150 W. A charge neutralizer was used to prevent charging. The survey scans were collected from binding energies of 0-1000 eV with a 80 eV pass energy. For high resolution scans of Cd(3d), Se(3d), C(1s), and N(1s), a 20 eV pass energy was used. All data were collected so that the C 1s line was shifted to 284.6 eV. The measurements were conducted at a pressure <5 x10 -9 Torr. For XPS analysis, the samples were drop-casted on a piranha-cleaned silicon wafer inside a N 2 filled glove box and the solution was allowed to evaporate at room temperature. The piranha-cleaned silicon wafer was washed with copious amount of Nanopure water and ethanol, and then dried under high vacuum over night. The XPS analysis was performed for four different batches of CdSe nanocrystal and two different sample concentrations (3.0 and 6.0 mg/mL). (Warning: piranha solution is very corrosive and must be handled with extreme caution. It reacts violently with organic materials and may not be stored in tightly closed vessels).

This Letter describes an unprecedentedly large and photoreversible localized surface plasmon resonance (LSPR) wavelength shift caused by photoisomerization of azobenzenes attached to gold nanoprisms that act as nanoantennas. The blue light-induced cis to trans azobenzene conformational change occurs in the solid state and controls the optical properties of the nanoprisms shifting their LSPR peak up to 21 nm toward longer wavelengths. This shift is consistent with the increase in thickness of the local dielectric environment (0.6 nm) surrounding the nanoprism and perhaps a contribution from plasmonic energy transfer between the nanoprism and azobenzenes. The effects of the azobenzene conformational change and its photoreversibility were also probed through surface-enhanced Raman spectroscopy (SERS) showing that the electronic interaction between the nanoprisms and bound azobenzenes in their cis conformation significantly enhances the intensity of the Raman bands of the azobenzenes. The SERS data suggests that the isomerization is controlled by first-order kinetics with a rate constant of 1.0 × 10(-4) s(-1). Our demonstration of light-induced photoreversibility of this type of molecular machine is the first-step toward removing present limitations on detection of molecular motion in solid-state devices using LSPR spectroscopy with nanoprisms. Modulating the LSPR peak position and controlling energy transfer across the nanostructure-organic molecule interface are very important for the fabrication of plasmonic-based nanoscale devices.