The resonance wavelength of collective surface plasmon polariton in a chain of 50 nm gold nanoparticles has been calculated and compared to experimental data. The distance between the nanoparticles in a chain was varied from 100 nm to 1000 nm, and the polarization of the incident light was gradually changed from parallel to perpendicular relative to the axis connecting the nanoparticles in the chain. The calculations explicitly included the near-, middle-, and far-field dipole coupling between the nanoparticles. The numerical results matched the experimental data with less than 2% error. Arrays of noble metal nanoparticles are of interest for plasmonics, nanooptics, photovoltaics, and biochemical applications. They are widely used as biosensors and molecular rulers. Over the last decade, interest has turned towards the localized surface plasmon resonance (LSPR) in single-nanoparticle sensors. Benefits of such an approach include simplicity (it does not require momentum-matching geometry), versatility on the nanoscale level, and the possibility of single-molecule detection. While single-nanoparticle sensors offer a better sensitivity down to a single protein-receptor binding, a high degree of sensor miniaturization tends to result in a worse detection limit because of limited surface coverage. A solution to this problem might be the use arrays of nanoplasmonic sensors, each of which is capable of resolving single protein binding events. Present study provides a background for bio-sensing, waveguiding, and molecular ruler applications.
We present experimental results on the multicolor (blue and green) photoluminescence from glycine-coated silver
nanoclusters and small nanoparticles which can be used as novel probes for bio-imaging. Glycine-coated silver
nanoclusters and nanoparticles were synthesized using thermal reduction of silver nitrate in a glycine matrix,
according to a modified procedure described in literature. The size characterization with mass spectrometry,
scanning electron microscopy and dynamic light scattering showed that the diameters of luminescent silver
nanoclusters and small nanoparticles vary from 0.5 nm to 17 nm. Extinction spectroscopy revealed that the
absorption band of the luminescent nanoclusters and nanoparticles was blue-shifted as compared to the nonluminescent
larger silver nanoparticles. This effect indicated the well-known size dependence of the surface
plasmon resonance in silver. The most pronounced photoluminescence peak was observed around 410 nm
(characteristic SPR wavelength for silver) which strongly suggests the enhancement of the photoluminescence from
silver nanoparticles by the SPR. The relative quantum yield of the photoluminescence of silver nanoclusters and
nanoparticles was evaluated to be 0.09.
In terms of their small size, brightness and photostability, noble metal nanoclusters and nanoparticles hold
the most promise as candidates for biological cell imaging, competing with commonly used semiconductor quantum
dots, fluorescent proteins and organic dyes. When applied to the problem of intracellular imaging, metal
nanoclusters and small nanoparticles offer advantages over their much larger sized semiconductor counterparts in
terms of ease of biological delivery. In addition, noble metal nanoparticles and nanoclusters are photostable. The
high quantum yield (QY) of the photoluminescence emission signal enables the isolation of their
photoluminescence from the cellular autofluorescence in cell imaging, improving the image contrast.