Eldar N. Khaliullin

Low-threshold lasing and broad-band multiphoton-excited light emission from Ag aggregate-adsorbate complexes in microcavity

A novel class of composites for optics, microcavities doped with metal fractal aggregates, is studied. Lasing and broad-band Stokes and anti-Stokes emission from (Ag colloidal aggregates)/(adsorbed molecules)/(micro-cavity) composite at low-intensity cw and pulse laser excitation has been found. At 633 nm cw excitation wavelength the emission spectrum contains many peaks, spanning a range from wavelength 200 nm to 800 nm. Experiments with pulse excitation of Ag/dye/microcavity composite show that the duration of the observed broad-band anti-Stokes emission significantly exceeds the pump pulse duration, dye molecule fluorescence time, and relaxation times in silver particles. It may be interpreted as a luminescence governed by long-living triplet states of dye molecules. These observations were made possible by use of a fractal-microcavity composite, where coupling the localized plasmon modes in fractal aggregates with microcavity resonances is provided. The important role of multiphoton resonant transitions between discrete states of a finite-size metal particle in enhanced local fields is shown. Analysis, based on the model of a spherical potential well, shows that the observed spectra contain fingerprints of the quantum size effect.

Semicontinuous silver films for protein sensing with SERS

High SERS sensitivity for protein detection has been accomplished with semicontinuous silver films. Specifically, an insulin surface density as low as 80 fmol/mm2 and 25 amol in a probed area has been readily detected.

Adaptive SERS substrates for protein sensing

Adaptive surface-enhanced Raman scattering (SERS) substrates exhibit unique properties which make them well suited for SERS studies of proteins on surfaces. Specifically, adaptive silver films (ASFs) allow nanoscale restructuring of metal particles during protein deposition which yields a three-fold benefit of soft protein adsorption, protein-metal complex stabilization, and increased SERS signal. In this work ASF fabrication and characterization methods are introduced, with special attention paid to characterization methods that provide insight into the adaptive nature of the substrates, such as UV-vis spectrophotometry, field-emission scanning electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. These ASF substrates show SERS enhancement factors in the range of 106 for an area-averaged signal, and have been successfully used for sub-monolayer protein detection. The addition of a thick metal layer in the ASF fabrication structure typically increases the SERS signal by a factor of four or five. Finally, several examples of current SERS protein studies using ASF substrates are provided.

Semicontinuous silver films as an ultra-sensitive Raman biosensor for recognition of insulin variants

Protein analysis is playing an important role in various biomedical applications. Performing such analysis at the molecular level is critical for further advancements in diagnostics. New instruments for measuring protein primary and secondary structures are in great demand for synthesis, purification, and recognition of proteins. Human insulin (humulin) and its artificial substitute humalog used for people with diabetes, are among particularly important proteins. There is a tiny difference in structures of these two proteins: the humalog structure can be obtained from humulin by interchanging lysine and proline amino acids at the B28 and B29 positions. These two proteins have the same mass, but because of the small structural difference, rather different properties, including their medical activity. Raman spectroscopy is known to have a high sensitivity in detecting bond structures of molecules, and thus it can be a powerful means for detection even smallest differences. Furthermore, surface-enhanced Raman scattering (SERS), providing a dramatic increase for Raman signals, makes possible analysis even at very minute protein quantities. In our experiments, we demonstrated that SERS substrates based on semicontinuous silver films [1] can be successfully used as highly sensitive substrates for distinguishing Humalog and Humulin at the sub-monolayer density, 80 fmol/mm2.

Discrete spectrum of anti-Stokes emission from metal particle-adsorbate complexes in a microcavity

This report presents the discovery of greatly enhanced, broad-range, multiphoton excited emission from Ag aggregate- adsorbate complexes seeded into a cylindrical microcavity. The emission spectrum contains descrete peaks spanning the wavelength range from the 632 nm HeNe laser exciting wavelength down to 200 nm. Observation of multiphoton processes at the low exciting light intensity (20 W/cm2) became possible due to using a fractal-microcavity composite, where coupling the localized plasmon modes in fractal aggregates with microcavity resonances is provided. The important role of the multiphoton resonant transitions between discrete states of a finite-size metal particle in enhanced local fields is shown. Analysis, based on the model of a spherical potential well, shows that the observed spectra contain fingerprints of the quantum size effect.