White light microscope

Silver nanoparticles that can generate white light could improve microscopy in research into cancer and bone diseases according to a paper in the March issue of Nano Letters.

Optical microscopy is commonly thought to use optical transmission measurements, whereas most modern microscopists working with contemporary high-resolution microscopes use fluorescence. But, fluorescence has its limitations.

Powerful as it is, fluorescence microscopy has a fatal flaw - the need to treat any specimen with fluorescent dyes or stains. The application of these dyes often alters or harms tissues and can kill living cells. So, although fluorescence microscopy is unsurpassed in acquisition speed and spatial resolution it is severely limited in this sense.

John Lupton and colleagues, Debansu Chaudhuri, Jeremy Galushs, Manfred Walter, Nicholas Borys, and Michael Bartl in the Department of Physics and Department of Chemistry, at the University of Utah, Salt Lake City, hope to circumvent this problem without resorting to old-style microscopy.

They have worked clusters of silver nanoparticles. Silver nanoparticle films have been used for single molecule surface-enhanced Raman scattering (SERS) studies before with one- or two-photon excitation. They can thus act as a tuneable beacon of light for transmission microscopy carried out at resolutions below the diffraction limit. This limit normally stymies conventional microscopy. The researchers suggest that their success could allow researchers to investigate cancerous tissues, diseased bone and other diffuse materials in unprecedented detail and without dyes to damage the sample.

For some applications, microscopists might turn to near-field scanning microscopy to side step the diffraction limit of light. However, that technique is not always viable with soft, thick and heterogeneous biological samples that may be thick, highly scattering, and so appear opaque in transmission.

The team adds that their method might also have applications in engineering, allowing investigators to identify structural damage in a wide variety of materials, including the carbon-fibre composites often used in aerospace applications, the researchers say.

The technique works by exciting clusters of silver nanoparticles with infrared laser light. If this silver beacon is placed below a sample, then the resulting intense beam of light that emerges following excitation shines up through the sample. This allows the researchers to reveal latent information about the composition and structure of the substance.

Technically, they have demonstrated the phenomenon of surface-enhanced two-photon luminescence at hotspots in their silver nanoparticle clusters. This produces a broad spectrum of light that is confined spatially to a narrow beam.

"We demonstrated high resolution transmission microscopy in a conventional two-photon wide-field fluorescence microscope by exploiting nonlinear white light generation from clusters of silver nanoparticles placed beneath the specimen," the team explains.

So far, the team has carried investigations of the iridescent green scales of the so-called "photonic beetle" - Lamprocyphus augustus. The beetle displays a green iridescence almost regardless of viewing angle because of its internal diamond-like photonic crystal structure, which is organized in differently oriented single-crystalline, pixel-type, domains.

These findings may provide clues to designing new materials that emulate the intriguing properties of such natural crystals and may have potential for building more powerful solar cells or even optical computer chips. In this proof of principle, the team has observed individual crystalline domains within the beetle's scales.

"We can identify sub-micrometre changes between photonic crystal facets as well as the occurrence of stacked domains invisible to surface-sensitive methods," the researchers add.

They point out that by controlling the wavelength, polarization, and pulse shape of the luminescence from the silver nanoparticle clusters they might also selectively address specific hotspots in the clusters for different materials and applications. Nanoparticle hotspot transmission spectroscopy opens up the world of mesoscopic analysis of biological and synthetic materials.

The researchers explain that overall morphology is crucial in tumour malignancy tests and in spotting stress and thermal degradation in amorphous materials. "With the facile applicability to conventional multiphoton wide-field imaging microscopes," the researchers explain, "we expect to provide a crucial addition to the ever-expanding palette of high-performance microscopy techniques."